Enteric nervous system as a therapeutic target in gastrointestinal disorders

Abstract

BACKGROUND

The enteric nervous system (ENS), often referred to as the "second brain", plays a vital role in regulating gastrointestinal (GI) functions such as motility, secretion, and immune responses. Located within the walls of the gastrointestinal tract, the ENS is involved in several GI disorders, including functional gastrointestinal disorders like irritable bowel syndrome (IBS), motility disorders such as gastroparesis, and conditions like inflammatory bowel diseases (IBD) and congenital aganglionosis. Understanding the mechanisms and therapeutic potential of the ENS is crucial for advancing treatment options for these conditions.

AIM

To review the therapeutic targeting of the ENS in treating gastrointestinal disorders, with a focus on the mechanisms of action, current therapies, and emerging treatment strategies.

METHODS

This review adhered to Preferred Reporting Items for Systematic Reviews and Meta-analyses 2009 guidelines and included a comprehensive literature search across PubMed, Scopus, and Google Scholar from 2010 onwards. Studies were selected based on their exploration of the structure, function, and pathology of the ENS in relation to gastrointestinal diseases, particularly motility disorders and inflammation. Articles were evaluated for therapeutic strategies, including pharmacological, surgical, and gene-based interventions.

RESULTS

The ENS is found to regulate essential functions like peristalsis, secretion, and immune responses, and its dysfunction leads to a variety of gastrointestinal diseases. Recent studies highlight several promising therapies targeting ENS neurotransmitters, including serotonin receptor modulators, prokinetic agents, and neuroprotective strategies aimed at enteric glial cells. These approaches have shown promise in treating conditions like IBS, gastroparesis, and IBD. Additionally, bioelectronic therapies and stem cell-based treatments are emerging as potential therapies for ENS regeneration.

CONCLUSION

Targeting the ENS offers novel therapeutic avenues for treating gastrointestinal disorders. Advances in pharmacological treatments, gene therapy, and neuromodulation techniques show great promise in restoring ENS function and improving clinical outcomes. While current research remains in early stages, future studies focusing on personalized medicine and the gut-brain axis could lead to more effective treatments for complex gastrointestinal diseases.

Key Words: Enteric nervous system; Gastrointestinal motility disorders; Irritable bowel syndrome; Inflammatory bowel diseases; Neuroprotection

Core Tip: The enteric nervous system (ENS) serves as a pivotal regulatory system within the gastrointestinal tract. Dysfunction in the ENS has been linked to a range of gastrointestinal disorders, including irritable bowel syndrome, gastroparesis, and inflammatory bowel diseases. Advances in therapeutic approaches targeting ENS pathways—ranging from neurotransmitter modulation to gene therapy—hold great promise for improving patient outcomes. By leveraging the gut-brain axis and focusing on restoring neural and glial cell function, these therapies represent a new frontier in gastrointestinal medicine. Targeting the ENS could offer a more effective, individualized approach to managing complex gastrointestinal diseases, providing significant improvements in symptom management and quality of life for affected patients.

INTRODUCTION

The enteric nervous system (ENS) is a vast network of neurons embedded in the walls of the gastrointestinal (GI) tract, often referred to as the "second brain" due to its ability to function independently of the central nervous system (CNS)[1]. This extensive neural structure contains approximately 500 million neurons, which is more than the spinal cord, and is organized into two major plexuses: The myenteric (Auerbach’s) plexus and the submucosal (Meissner’s) plexus[2]. These two plexuses coordinate critical functions such as motility, secretion, blood flow, and immune responses within the GI tract, allowing for complex physiological processes like peristalsis and digestive enzyme secretion to occur with minimal input from the brain[3]. The therapeutic potential of targeting the ENS in the treatment of GI disorders is gaining considerable attention. This is primarily due to the growing recognition of the ENS's essential role in maintaining GI homeostasis and the pathological involvement of ENS dysfunction in a wide range of GI diseases. Conditions such as congenital aganglionosis [Hirschsprung’s disease (HSCR)] and acquired inflammatory neuropathies [e.g., inflammatory bowel diseases (IBD) or functional GI disorders like irritable bowel syndrome (IBS)] have highlighted the importance of understanding the ENS’s structure and function. These disorders often involve disturbances in neural signalling or structural integrity of the ENS, leading to abnormal motility, secretion, and inflammation in the GI tract, contributing to the clinical symptoms of these diseases[4]. Recent advances in research have deepened our understanding of the ENS, especially its role in both the normal and pathological states of the GI tract. Understanding how the ENS interacts with the CNS and other components of the GI system has opened new avenues for therapeutic interventions. Investigating the mechanisms underlying ENS dysfunction is essential for developing targeted treatments that can modulate its activity, potentially offering more precise and effective therapies for GI disorders. This research highlights the promise of targeting the ENS directly through pharmacological, bioengineering, and neuromodulation strategies, thus paving the way for novel treatments that could improve outcomes for patients suffering from chronic and debilitating GI diseases[5]. In this review, we will explore the current understanding of the ENS's role in GI disorders, emphasizing its structure, function, and potential therapeutic targeting. By investigating both the physiological mechanisms and the pathophysiological changes in ENS function, we aim to provide a comprehensive perspective on the future prospects for therapeutic interventions in GI diseases.

MATERIALS AND METHODS

This review followed the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) 2009 guidelines to ensure a transparent and thorough analysis of the literature on the ENS as a therapeutic target for GI disorders. A broad search was conducted across databases like PubMed, Scopus, and Google Scholar, using keywords such as "enteric nervous system", "gastrointestinal disorders", "therapeutic targets", and "ENS dysfunction". Studies published in English from 2010 onward were included to reflect the latest research. The focus was on studies examining the structure, function, and pathology of the ENS in relation to GI diseases, especially motility disorders, inflammatory conditions, and congenital abnormalities. Studies that explored therapeutic strategies targeting the ENS, including pharmacological, surgical, and gene-based approaches, were also included. Abstracts, full-text articles, and Supplementary materials were reviewed for relevant data on methodology, outcomes, and conclusions. The quality of the studies was assessed using standardized tools, and the results were synthesized to provide a clear overview of current evidence and emerging treatment options. This approach aimed to enhance understanding of the ENS's role in GI disorders and its potential for therapeutic targeting.

RESULTS

The initial search yielded 1224 records. After removing duplicates and screening abstracts, 464 full-text articles were reviewed. Two hundred forty-two studies met inclusion criteria (Figure 1). The literature review found increasing evidence supporting the ENS as a therapeutic target for GI disorders. Research emphasized the ENS's key role in regulating motility, secretion, and immune responses. Dysfunction in the ENS is linked to conditions such as IBS, IBD, and congenital aganglionosis. Promising therapeutic approaches, including pharmacological treatments, neural modulation, and gene therapy, were identified. Studies showed that modulating enteric neurotransmitters could improve motility and reduce inflammation. Gene therapy aimed at restoring ENS function also showed early promise in preclinical models. While much of the research is still in early stages, the findings suggest that targeting the ENS could lead to better treatments for GI diseases.

Figure 1  PRISMA 2009 flow diagram.

DISCUSSION

Anatomical and physiological foundation of the ENS

The ENS is organized along the GI tract from the oesophagus to the anal canal. It includes two main plexuses: The myenteric plexus, which controls motility, and the submucosal plexus, which regulates secretion, blood flow, and epithelial transport[6]. This organization enables the ENS to integrate motor and secretory functions, vital for digestion. The ENS contains several types of neurons: Motor neurons, interneurons, and sensory neurons. Motor neurons control smooth muscle, secretion, and blood vessels. Interneurons transmit signals between different parts of the gut[7]. Sensory neurons detect mechanical and chemical changes and initiate reflex responses. Enteric glial cells support and regulate the ENS. They maintain the structure of enteric ganglia, manage neurotransmission, and contribute to the blood-brain barrier in the gut. Recent studies suggest these cells may also play a role in inflammation, making them potential targets for therapeutic strategies[8]. The ENS uses various neurotransmitters and neuromodulators, which provide therapeutic targets. Acetylcholine is the primary excitatory neurotransmitter, acting through nicotinic and muscarinic receptors to stimulate muscle contractions and secretion[9]. Cholinesterase inhibitors, like itopride, help treat conditions like functional dyspepsia and gastroparesis. Serotonin is essential for ENS function, with over 90% produced in the GI tract. The 5-HT3 and 5-HT4 receptors regulate motility, secretion, and sensory transmission. Serotonergic agents, such as 5-HT4 agonists like mosapride, help treat functional bowel disorders[10]. Dopamine inhibits motility and secretion through D2 receptors. Dopamine receptor antagonists, like metoclopramide, can increase GI motility but may have CNS side effects, especially with prolonged use[11]. Neuropeptides such as substance P, VIP, neuropeptide Y, and calcitonin gene-related peptide also play roles in GI function. These neuropeptides offer additional therapeutic targets, particularly for conditions with altered sensory processing or inflammation[12]. The ENS operates independently with intrinsic reflex circuits that manage peristaltic movements, secretory responses, and vascular adjustments[13]. These circuits are complex and allow sophisticated information processing, similar to the CNS. The ENS also communicates with the central autonomic control through vagal and sympathetic pathways[14]. Understanding these interactions is critical for developing therapies targeting both central and peripheral mechanisms.

Current therapeutic interventions targeting the ENS conventional

Pharmacological approaches targeting the ENS involve various drug classes that affect enteric neurotransmission. Prokinetic agents, the largest group, aim to boost GI motility through different mechanisms. These agents are selective, acting on the gastric, small bowel, or colonic functions based on their receptor targets. Dopamine D2 receptor antagonists, including metoclopramide, domperidone, and levosulpiride, enhance motility by blocking inhibitory dopaminergic signals. Metoclopramide is the only the United States Food and Drug Administration (FDA) approved treatment for gastroparesis in the United States, but its use is limited to three months due to risks of extrapyramidal side effects. Domperidone, with fewer side effects due to limited blood-brain barrier penetration, is available in many countries through special access programs[15]. Serotonin 5-HT4 receptor agonists, such as mosapride and prucalopride, stimulate enteric cholinergic neurons, boosting motility across the GI tract. Prucalopride is particularly effective for chronic constipation and shows promise for other motility disorders. Ongoing research aims to develop new 5-HT4 agonists with better selectivity and fewer cardiovascular risks[16]. Cholinesterase inhibitors, like itopride, offer prokinetic and antiemetic effects by increasing acetylcholine levels and blocking dopamine D2 receptors. This dual action is helpful for conditions with both motility issues and nausea, such as functional dyspepsia[17].

Emerging pharmacological targets

Recent advancements in ENS pharmacology have introduced new therapeutic targets that may offer advantages over traditional treatments. Ghrelin receptor agonists, a promising class of prokinetic agents, enhance gastric motility by activating motilin-like receptors. These agents have shown effectiveness in gastroparesis models and may benefit patients who do not respond to conventional therapies[18]. Neurokinin NK1 receptor antagonists, initially developed for chemotherapy-induced nausea, are being explored for gastroparesis treatment. Aprepitant, an NK1 antagonist, has shown efficacy in relieving some gastroparesis symptoms, though the exact mechanism is still under investigation[19]. Modulating gut peptide systems provides further opportunities for therapeutic intervention in the ENS. GLP-1 agonists, often used for diabetes management, impact ENS function through multiple pathways, including direct effects on enteric neurons and modulation of gastric emptying. Understanding these effects is crucial for optimizing their use and reducing GI side effects[20]. Short-chain fatty acids (SCFAs), produced by intestinal microbiota, naturally modulate ENS function. These metabolites influence enteric neurotransmission, inflammation, and metabolic signaling, indicating that microbiome-targeted therapies could indirectly affect ENS function. Developing prebiotic and probiotic treatments to boost SCFA production is an emerging approach for ENS-targeted therapy[21].

Bioelectronic and neuromodulation approaches

Bioelectronic medicine, which uses electrical stimulation to modulate the ENS, is an emerging treatment option. These techniques can directly activate or inhibit enteric neural circuits, offering precise control over GI functions without systemic drug exposure. Approaches range from non-invasive external stimulation to implantable devices for chronic neuromodulation[22]. Gastric electrical stimulation has shown effectiveness in some gastroparesis patients, especially those with a diabetic cause. High-frequency stimulation appears to influence gastric motility by targeting enteric pacemaker cells and neural circuits, though the exact mechanisms are still being explored. New technologies, such as responsive stimulation based on physiological feedback, may improve treatment outcomes[23]. Sacral nerve stimulation, initially developed for urological conditions, has proven effective for severe constipation and faecal incontinence by modulating colonic ENS function. It affects both parasympathetic and enteric neural pathways, helping restore normal bowel movements. Ongoing improvements in electrode design and stimulation parameters continue to enhance patient outcomes and expand its use[24]. Abramson et al[25] developed an ingestible device that autonomously anchors to the stomach wall using electrically conductive, hooked probes. This device delivers timed electrical pulses to stimulate gastric tissue, offering a non-invasive approach to treat motility disorders like gastroparesis. Their innovation paves the way for targeted GI therapies. Emerging bioelectronic techniques, such as targeted stimulation of specific enteric neural populations using optogenetics or focused ultrasound, promise even greater precision in modulating ENS function. These methods may enable new therapeutic interventions that were not possible with traditional pharmacological approaches.

Regenerative medicine and tissue engineering approaches stem cell-based therapies

Stem cell-based therapies show promise for treating intestinal failure by targeting the ENS, which regulates key GI functions. Conditions like short bowel syndrome (SBS), where large portions of the intestine are removed, impair ENS function. Stem cells may help regenerate the ENS, restore neural activity, and promote intestinal adaptation[3]. Enteric neural stem cells (ENSCs) are among the most promising for ENS regeneration. These cells, naturally found in the intestine, can form neurospheres and integrate into tissue to restore motility. Transplanting ENSCs has been shown to improve gut motility in conditions like HSCR without causing tumor formation[26]. Human pluripotent stem cells (hPSCs), including embryonic and induced pluripotent stem cells, can differentiate into ENS progenitors. These cells offer an unlimited supply and can generate enteric neurons and glia, improving gut motility in human tissue models[27]. Neural crest-derived cells (ENCCs), which develop into the entire ENS during embryonic stages, are another valuable regenerative cell source. Enhancing ENCC differentiation with magnesium ions and targeted signaling pathways can improve cell survival and integration[28]. Mesenchymal stem cells (MSCs), though not naturally differentiating into enteric neurons, support ENS regeneration by secreting neurotrophic factors. These cells promote the survival and expansion of ENSCs, showing promise in applications like reducing rejection rates in intestinal transplants[29]. Stem cells aid ENS restoration by directly replacing neurons, including motor, sensory, and interneurons. They also secrete factors like heparin-binding epidermal growth factor to support ENS cell growth and survival. Stem cell therapy can also activate the ENS’s natural repair mechanisms[30]. Advances in stem cell therapies, such as improved cell culture techniques and biomaterial scaffolds, are enhancing the potential for treating conditions like SBS, HSCR, and necrotizing enterocolitis[31].

Bioengineering and scaffold technologies

Tissue engineering strategies for ENS regeneration focus on creating biocompatible scaffolds that support cell survival, growth, and differentiation while providing the right mechanical and biochemical cues. These scaffolds must match the complex three-dimensional structure of the ENS and integrate well with existing neural networks[32]. Natural biomaterials like collagen, alginate, and decellularized extracellular matrix are highly biocompatible and promote cell adhesion and growth. Alginate microfibers are particularly useful for ENS applications, as they can encapsulate cells while allowing nutrient diffusion and waste removal. These materials can be shaped to meet the specific needs of different GI tract regions[33]. Synthetic polymers such as polycaprolactone and polylactic-co-glycolic acid provide more control over the mechanical properties and degradation rates of scaffolds[34]. These polymers can be formed into complex three-dimensional structures through electrospinning, creating nanofiber networks that replicate the natural extracellular matrix[35]. Incorporating bioactive molecules into scaffolds enhances ENS regeneration. Growth factors, neurotrophic proteins, and small molecules can be integrated to promote cell survival, proliferation, and differentiation[36]. Controlled release systems allow for consistent delivery of these factors, improving the chances of successful tissue recovery[37]. Recent advancements, including endoscopic delivery methods and the isolation of stem cells from adult human tissue, have demonstrated functional restoration in preclinical studies. MSCs are the most clinically advanced, showing efficacy in treating IBD with neuroprotective effects. Clinical trials highlight the potential for MSCs to reduce inflammation and promote tissue regeneration. Bioelectronic therapies, including gastric electrical stimulation (GES) and vagus nerve stimulation (VNS), have shown encouraging results in treating GI disorders. GES has been FDA-approved for gastroparesis, improving symptoms like vomiting frequency. VNS, currently under investigation for IBD, has demonstrated anti-inflammatory effects and a favourable safety profile. However, these treatments face limitations and risks. Stem cell therapies encounter challenges in cell survival, integration, and long-term safety in inflammatory environments. Bioelectronic devices require further optimization of stimulation parameters and patient selection. Additionally, both therapies are hindered by regulatory hurdles, inconsistent clinical results, and potential adverse effects like infections or immune responses. Combining these therapies may hold promise, but further research is needed to overcome these barriers and ensure safety and efficacy.

Pathophysiology and therapeutics of ENS dysfunction in GI disorders

Functional GI disorders functional GI disorders (FGIDs) such as IBS and functional dyspepsia affect millions of people worldwide, severely disrupting daily life[38]. These disorders are frequently associated with dysfunctions in the ENS. The pathophysiology of FGIDs is complex and involves changes in ENS structure and function, along with factors like inflammation and psychosocial stress[39]. Alterations in enteric neurons, glial cells, and neurotransmitter levels are central to the onset of these conditions. Patients with overlapping FGIDs often experience more severe symptoms, suggesting that ENS dysfunction may predispose individuals to develop multiple GI disorders[3]. Medications such as 5-HT4 receptor agonists, including prucalopride and tegaserod, stimulate cholinergic pathways in the ENS to enhance motility and relieve constipation-predominant symptoms[40]. For diarrhea-predominant IBS, μ-opioid receptor antagonists like eluxadoline help regulate motility and reduce gut hyperactivity without affecting the CNS. Alongside pharmacological options, herbal remedies like ginger, peppermint oil, and licorice root have demonstrated effectiveness in alleviating symptoms by targeting the ENS through their antispasmodic and anti-inflammatory actions[41]. Non-pharmacological treatments are also gaining traction in managing FGIDs. Transcutaneous vagus nerve stimulation (tVNS) is one such intervention that restores parasympathetic tone and offers relief to patients with persistent symptoms[42]. Research shows that tVNS improves gut motility and reduces visceral hypersensitivity. Electroacupuncture, which modulates neurotransmitters such as nitric oxide and vasoactive intestinal peptide, also plays a role in regulating the brain-gut axis and alleviating GI symptoms[43]. The gut microbiome significantly influences ENS function. Probiotics like Bifidobacterium infantis and Lactobacillus rhamnosus help restore balance in the ENS by reducing inflammation and improving gut barrier integrity[44]. These probiotics produce SCFAs that activate enteric neurons, promoting overall gut health[45]. Dietary changes, such as following a low- fermentable oligosaccharides, disaccharides, monosaccharides, and polyols diet, have been shown to reduce fermentable substrates that aggravate symptoms and overstimulate the ENS[46]. Psychological therapies also contribute to the management of FGIDs[47]. Cognitive behavioural therapy has proven effective in addressing stress-induced gut hypersensitivity, helping to reduce ENS hyperactivity and improve both motility and pain perception[48]. Antidepressants such as tricyclics and selective serotonin reuptake inhibitors modulate serotonin signalling in the ENS, providing symptom relief and enhancing GI function[49]. Looking toward the future, stem cell-based therapies hold promise for patients with chronic FGIDs[50]. ENSCs may help regenerate damaged tissue and restore normal motility in preclinical models[51]. Researchers are also exploring nanoparticle delivery systems that target ENS-specific agents like glial cell line-derived neurotrophic factor (GDNF), which promotes neuronal survival and reduces inflammation[52]. Additionally, targeting cellular senescence in enteric glial cells could offer a new approach to reversing ENS dysfunction in chronic FGID patients[53]. Recent research provides new insights into potential therapeutic strategies for FGIDs. Baj et al[54] reviewed the therapeutic potential of marine toxins such as tetrodotoxin and saxitoxin, which target ion channels involved in pain pathways. These toxins could offer targeted treatments for managing chronic abdominal pain. Bosi et al[55] investigated hyaluronan’s role in regulating neuronal and immune functions in the rat small intestine after ischemia/reperfusion injury. Their findings suggest that hyaluronan may serve as a therapeutic agent for GI injury by modulating gut inflammation. Chai et al[56] explored the role of stem cell growth factor/c-Kit in IBS, revealing that stem cell growth factor and its receptor play a significant role in gut motility, highlighting this pathway as a potential target for managing IBS symptoms. Chang et al[57] focused on enteric glial CRF-R1/Cx43 to alleviate stress-induced colonic motility and found that modulating this signalling pathway may benefit those with stress-related GI dysfunction. Chen et al[58] discovered that Patchouli alcohol restores gut homeostasis in IBS with diarrhoea through neurotransmitter regulation, suggesting its potential as a novel therapeutic agent for managing IBS with predominant diarrhoea (IBS-D). Cheng et al[59] reviewed the calcium-sensing receptor (CaSR) as a target for managing diarrhoea, highlighting its role in motility and the potential for CaSR antagonists to treat various GI disorders. Del Colle et al[60] reviewed enteric serotonergic signalling, emphasizing serotonin's role in regulating motility and its therapeutic implications for conditions like IBS. Dong et al[61] identified the Na(+)/Ca(2+) exchanger 1 as a key regulator of oesophageal motility, suggesting its potential as a therapeutic target for oesophageal disorders. Guarino et al[62] explored the influence of gut microbiota on the GI neuromuscular apparatus, proposing microbiota-targeted therapies for treating motility disorders like constipation and IBS. Gulbransen and Christofi et al[63] discussed the role of enteric glial cells in GI diseases and motility disorders, suggesting that modulating glial activity could improve motility outcomes. Holland et al[64] examined how microRNAs regulate ENS development and disease, indicating that microRNA modulation could play a crucial role in treating neuro-GI disorders like IBS. Holzer and Holzer-Petsche et al[65] found that anti-calcitonin gene-related peptide migraine therapeutics could exacerbate constipation by antagonizing motor-stimulating and prosecretory functions in the intestine. Kimono et al[66] reviewed GI issues in Gulf War illness and proposed therapeutic strategies to address gut dysbiosis and improve motility. Linan-Rico et al[67] focused on the therapeutic implications of modulating enteric glial cells in GI diseases, motility disorders, and abdominal pain, suggesting that targeting glial cells could offer novel treatments for chronic GI dysfunction. Mace et al[68] reviewed the pharmacology and physiology of GI enteroendocrine cells, emphasizing their role in regulating GI function and their potential as therapeutic targets for disorders like IBS and obesity. Morales-Soto and Gulbransen et al[69] explored the contribution of enteric glial cells to abdominal pain in GI disorders, proposing that targeting glial activity could offer new treatment avenues for conditions like IBS. O'Mahony et al[70] investigated the relationship between IBS and stress-related psychiatric comorbidities, suggesting that targeting stress pathways in the brain-gut axis could provide new therapeutic options for IBS patients with comorbid psychiatric disorders. Tait and Sayuk et al[71] examined the brain-gut-microbiota axis, proposing that understanding the role of microbiota in modulating gut function could lead to new therapeutic approaches for disorders like IBS and functional dyspepsia. Weber et al[72] reviewed the latest treatment options for IBS-D, highlighting therapies targeting gut motility, including bile acid sequestrants and serotonin antagonists, as promising treatments. Zhang et al[73] investigated EphrinB2/ephB2 signalling in I IBS, suggesting that this signalling pathway contributes to visceral hypersensitivity and could be targeted to manage IBS-related pain (Table 1). Despite advances in understanding and treating FGIDs, challenges remain in their clinical application. Personalized treatments guided by biomarkers, such as heart rate variability and microbiota profiles, could help optimize patient outcomes and provide more tailored therapies for these complex and often debilitating disorders[74-76].

Table 1 Studies showing therapeutic role of enteric nervous system on functional bowel disorders.

No.	Ref.	Study characteristics	Key role of ENS
1	Baj et al[54]	Investigates the potential therapeutic use of marine toxins in treating visceral pain associated with gastrointestinal disorders	Focuses on the role of ENS in visceral pain perception and potential modulation via marine toxins
2	Bosi et al[55]	Examines how hyaluronan regulates neuronal and immune function in the rat small intestine and microbiota following ischemia/reperfusion injury	Highlights the ENS's involvement in gut motility and immune responses, particularly in the context of injury
3	Chai et al[56]	Studies the role of stem cell growth factor/c-Kit in the pathogenesis of IBS	Focuses on ENS's involvement in the pathophysiology of IBS, particularly in neuroimmune interactions
4	Chang et al[57]	Investigates how targeting enteric glial CRF-R1/Cx43 attenuates stress-induced accelerated colonic motility	Demonstrates the role of enteric glia and receptors in stress-induced gastrointestinal dysfunction
5	Chen et al[58]	Explores how patchouli alcohol restores gut homeostasis in IBS with diarrhea through neurotransmitter regulation via myosin Va	ENS plays a key role in restoring intestinal balance and regulating gut motility in IBS through neurotransmission
6	Cheng[59]	Identifies calcium-sensing receptor as a new therapeutic target for diarrhea	Focuses on how ENS receptors can be targeted to improve gastrointestinal disorders, specifically diarrhea
7	Del Colle et al[60]	Explores novel aspects of enteric serotonergic signaling in both health and brain-gut diseases	Examines serotonergic signaling in the ENS and its implications for gut function and diseases like IBS
8	Dong et al[61]	Investigates the role of Na+/Ca2+ exchanger 1 as a mechanosensitive molecule in esophageal myenteric neurons	Highlights the mechanosensitive role of ENS neurons in regulating esophageal motility and responsiveness
9	Guarino et al[62]	Explores the role of the gastrointestinal neuromuscular apparatus as an underestimated target of gut microbiota	Focuses on the interaction between ENS, microbiota, and neuromuscular function in gastrointestinal health
10	Gulbransen and Christofi[63]	Investigates the potential of targeting enteric glia in gastrointestinal diseases and motility disorders	Highlights the emerging therapeutic role of enteric glia in modulating motility disorders and disease states
11	Holland et al[64]	Discusses the regulation of microRNA in enteric nervous system development and its role in disease	Emphasizes the role of microRNAs in ENS development, function, and the onset of gastrointestinal diseases
12	Holzer and Holzer-Petsche[65]	Studies how anti-calcitonin gene-related peptide migraine therapeutics may cause constipation by antagonizing the motor functions of the intestine	Examines the ENS's involvement in gastrointestinal motility affected by treatments for other conditions
13	Kimono[66]	Discusses gastrointestinal issues and therapeutic approaches in Gulf War illness	Focuses on the complex interplay between the ENS and the broader neurological and immune systems in disease contexts
14	Linan-Rico et al[67]	Investigates the role of enteric glial cells in inflammation and their potential therapeutic implications for GI diseases	Highlights the critical role of ENS glial cells in inflammation and their therapeutic potential in gastrointestinal disorders
15	Mace et al[68]	Provides insights into the pharmacology and physiology of gastrointestinal enteroendocrine cells	Explores the interaction between ENS and enteroendocrine cells in regulating gut physiology and responses
16	Morales-Soto and Gulbransen[69]	Discusses how enteric glia significantly contributes to abdominal pain in gastrointestinal discomfort	Sheds light on the role of ENS glial cells in mediating abdominal pain and their emerging role in therapeutic strategies
17	O'Mahony et al[70]	Examines the connection between early-life stress and the development of irritable bowel syndrome and psychiatric comorbidities	Investigates the role of the ENS in the brain-gut axis, highlighting its importance in IBS and related conditions
18	Tait and Sayuk[71]	Studies the brain-gut-microbiota axis in functional gastrointestinal illnesses and potential therapeutic interventions	Explores the role of the ENS as a mediator between the brain, gut, and microbiota in functional gastrointestinal disorders
19	Weber[72]	Discusses new treatment options for irritable bowel syndrome with predominant diarrhea	Focuses on therapeutic targets within the ENS to treat IBS with diarrhea
20	Zhang et al[73]	Explores how EphrinB2/ephB2 activation facilitates colonic synaptic potentiation and contributes to long-term visceral hypersensitivity in IBS	Focuses on the role of synaptic plasticity in the ENS, contributing to visceral hypersensitivity in IBS

ENS: Enteric nervous system; IBS: Irritable bowel syndrome; GI: Gastrointestinal; RNA: Ribonucleic acid.

IBD

IBD, including Crohn's disease and ulcerative colitis, are chronic conditions that cause ongoing inflammation in the GI tract, which leads to significant damage to the ENS. This damage disrupts GI motility, increases visceral hypersensitivity, and impairs the gut barrier[77]. The altered ENS function in IBD is a result of changes in enteric neurons, glial cells, and neurotransmitter signalling, contributing to motor and sensory dysfunction seen in these patients[78]. The inflammatory processes in IBD create a self-perpetuating cycle that further exacerbates ENS dysfunction, reinforcing disease chronicity. Addressing this dysfunction could offer a novel therapeutic approach, targeting both the symptoms and the underlying causes of the disease[79]. The changes in the ENS observed in IBD include both enteric neuronal loss and the activation of enteric glial cells, which play a key role in maintaining gut homeostasis. Inflammation in the GI tract triggers an increase in enteric glial cells, particularly in areas like Peyer's patches, which are involved in immune responses[80]. This activation of glial cells contributes to both inflammation and dysfunction in the gut. Additionally, glial cell senescence, where cells adopt a secretory phenotype that promotes inflammation, plays a critical role in disease progression[81]. Dysfunctional glial cells lead to reduced secretion of neurotrophic factors like GDNF, which are essential for the survival and function of enteric neurons[82]. Inflammatory mediators such as cytokines further damage enteric neurons, exacerbating ENS dysfunction. Muscularis macrophages, which reside in the muscular layer of the gut, play a central role in IBD pathology. These macrophages switch between pro-inflammatory and anti-inflammatory states, influencing both immune responses and ENS function. They interact with enteric neurons and glial cells, thus perpetuating the inflammatory cycle in IBD. Additionally, disruptions in the gut-brain axis, the connection between the ENS and the immune system, result in altered gut motility and increased intestinal permeability, further promoting inflammation[83]. Several therapeutic strategies aim to target the ENS to treat IBD. One approach focuses on modulating the P2X7 receptor, a receptor that plays a critical role in inflammation and is expressed in enteric cells. Targeting this receptor could help reduce inflammation and preserve ENS function. The cholinergic anti-inflammatory pathway, mediated by acetylcholine release via the vagus nerve, is another potential therapeutic target[84]. Chronic vagus nerve stimulation has been shown to reduce inflammation in experimental colitis models, providing a promising approach for alleviating both inflammation and GI dysfunction in IBD[85]. The STING (Stimulator of Interferon Genes) pathway is another emerging target in IBD treatment. STING activation in enteric neurons and glial cells leads to the production of inflammatory mediators. Modulating this pathway could help reduce inflammation, improve motility, and restore gut barrier function in IBD patients. Microbiome-based interventions, including probiotic therapy, also show promise. Probiotics such as Bifidobacterium longum can influence enteric glial cells, promoting the secretion of neurotrophic factors like GDNF, which support neuronal health and reduce inflammation. Faecal microbiota transplantation (FMT) has also shown potential in inducing remission in ulcerative colitis and improving symptoms in Crohn's disease, although further research is needed to optimize protocols and assess long-term efficacy[86]. Regenerative medicine holds additional promise for treating ENS dysfunction in IBD. Enteric glial cells produce neurotrophic factors like nerve growth factor (NGF), which protect intestinal cells and promote repair. In IBD, reduced NGF expression contributes to epithelial damage and colonic inflammation. Exogenous NGF administration has shown potential in reversing this damage, highlighting the importance of glial cell-derived therapies in treating IBD. Enteric glial cell-based therapies could also regenerate and protect the ENS, restoring normal GI function in these patients[87]. Phyto-cannabinoids, such as THC, have demonstrated anti-inflammatory effects in macrophages and enteric glial cells. By modulating immune responses and reducing the secretion of pro-inflammatory cytokines, cannabinoids may provide dual benefits in controlling inflammation and protecting ENS function. Studies suggest that Phyto-cannabinoids can influence signalling pathways like mTOR and IL-6, making them a promising addition to IBD treatment regimens[88]. Bioelectronic medicine, particularly VNS, is an emerging non-pharmacological treatment. VNS operates through the cholinergic anti-inflammatory pathway, influencing immune responses and modulating intestinal permeability[89]. Non-invasive methods such as transcutaneous auricular VNS have shown promise in experimental models by reducing intestinal permeability and controlling inflammation. This suggests that VNS could serve as a potential therapy for managing IBD symptoms and improving gut function[90]. Recent studies further support the potential of targeting the ENS in IBD treatment. Belarif et al[91] identified a strong association between high expression of IL-7 receptor (IL-7R) and non-responsiveness to anti-TNF therapy in IBD. They found that IL-7R blockade reduced T cell homing to the gut and colonic inflammation in vivo, suggesting IL-7R as a relevant therapeutic target and a potential biomarker in clinical IBD detection and treatment. Boldyreva et al[92] examined the gut-brain axis in IBD and highlighted the role of gut microbiota interactions and immune responses, providing new insights into how these interactions could inform novel therapeutic strategies for IBD. Bosáková et al[93] demonstrated that serotonin helps attenuate TNF-induced intestinal inflammation by interacting with human mucosal tissue, highlighting serotonin's potential as a therapeutic agent in IBD. Hawker et al[94] reviewed the role of Mas-related G protein-coupled receptors in GI dysfunction and IBD, suggesting that these receptors could be targeted to manage inflammation and other GI disorders. Lê et al[95] examined the therapeutic effects of prebiotics, probiotics, synbiotics, and postbiotics in IBD treatment, emphasizing the need for individualized approaches to restore gut homeostasis. Magalhães et al[17] discussed the role of P2X7 receptor signalling in the ENS and its potential as a target for IBD treatment. Ochoa-Cortes et al[96] reviewed enteric glial cells as a therapeutic target for IBD, suggesting that modulating glial cell activity could help manage IBD symptoms and promote healing. Robinson et al[97] studied the neuroprotective effects of human bone marrow MSCs in a TNBS colitis model, showing that stem cell transplantation reduced colonic inflammation and protected myenteric neurons, highlighting a promising therapeutic approach. Sahakian et al[98] identified molecular targets such as redox signalling and neuroinflammation to improve treatment outcomes for IBD-related GI dysfunction. Stavely et al[99] examined neuronal damage in IBD and suggested that targeting neuronal pathways could provide novel treatments for motility disorders associated with the disease. His research emphasized that targeting ENS dysfunction could offer a therapeutic strategy to alleviate IBD-related motility and pain disorders, suggesting a more focused approach to managing symptoms (Table 2). Targeting the ENS in IBD offers a multifaceted treatment approach that combines pharmacological, bioelectronic, microbiome-based, and regenerative strategies. These therapies hold promise in improving patient outcomes and could significantly change how IBD is managed. As research into ENS dysfunction in IBD continues to evolve, the development of targeted therapies will be essential in addressing the complexities of this disease and improving patients' quality of life.

Table 2 Studies showing therapeutic role of enteric nervous system on inflammatory bowel disease.

No.	Ref.	Study characteristics	Key role of ENS
1	Belarif et al[91]	Investigates the influence of the IL-7 receptor on anti-TNF responsiveness and T-cell gut homing in IBD	Focuses on how the IL-7 receptor affects the ENS by influencing T-cell migration and responsiveness to anti-TNF therapy
2	Boldyreva et al[92]	Highlights the gut-brain axis and non-obvious factors involved in IBD	Explores the communication between the ENS and brain, and how it impacts the development and progression of IBD
3	Bosáková et al[93]	Studies the role of serotonin in attenuating TNF-induced intestinal inflammation through interaction with human mucosal tissue	Investigates serotonin's effects on the ENS and its potential to modulate inflammation in the gut via interaction with mucosal tissue
4	Hawker et al[94]	Explores the role of Mas-related G protein-coupled receptors in gastrointestinal dysfunction and IBD	Examines the involvement of Mas-related receptors in ENS-mediated gastrointestinal functions, particularly in IBD
5	Lê et al[95]	Reviews therapeutic strategies for IBD involving modulation of the microbiota and the introduction of pre-, pro-, syn-, or postbiotics	Focuses on how microbiota manipulation impacts ENS function, providing new therapeutic strategies for IBD management
6	Ochoa-Cortes et al[96]	Discusses enteric glial cells as a new frontier in neurogastroenterology and a potential clinical target for IBD	Highlights the growing importance of enteric glial cells in IBD, providing potential new targets for clinical treatment
7	Robinson et al[97]	Investigates the neuroprotective effects of bone marrow mesenchymal stem cells in TNBS-induced colitis	Demonstrates how stem cells interact with the ENS to protect against damage and restore function in gastrointestinal disorders
8	Sahakian et al[98]	Identifies molecular targets to alleviate enteric neuropathy and gastrointestinal dysfunction	Focuses on targeting specific molecules in the ENS to relieve neuropathy and restore normal gastrointestinal function
9	Stavely et al[99]	Explores strategies for targeting enteric neurons and plexitis as a way to manage IBD	Emphasizes the role of enteric neurons and the plexuses in managing inflammation and dysfunction in IBD

ENS: Enteric nervous system; IBD: Inflamatory bowel disease; TNF: Tumor necrosis factor; TNBS: Trinitrobenzenesulfonic acid.

Gastroparesis and other motility disorders

Gastroparesis is a chronic condition characterized by delayed gastric emptying without any mechanical obstruction, leading to symptoms such as nausea, vomiting, early satiety, abdominal pain, and bloating[100]. The ENS plays a pivotal role in the development and progression of gastroparesis, with dysfunction of neural control mechanisms being central to the disease. This dysfunction is caused by several factors, including vagal dysfunction, impaired activity of interstitial cells of Cajal (ICC), reduced neural nitric oxide synthase activity, and increased oxidative stress[101]. These disruptions contribute to the motor and sensory disturbances that are characteristic of gastroparesis. The pathogenesis of gastroparesis is multifactorial and involves autonomic neuropathy, vagus nerve dysfunction, and disturbances in ICC, which are crucial for gastric pacemaker activity[102]. In conditions like Parkinson’s disease (PD), motility issues are not primarily due to defects in gastric pacemaker cells, but rather disruptions in neurohumoral signalling involving the vagus nerve and myenteric plexus. This evolving understanding has pointed to the autonomic nervous system as a critical therapeutic target[103]. Current treatments for gastroparesis include prokinetic agents that target neurotransmitter systems within the ENS. Metoclopramide, a dopamine D2 receptor antagonist, is the only FDA-approved medication for gastroparesis in the United States. Its use, however, is limited due to the risk of extrapyramidal side effects[104]. Domperidone, another D2 receptor antagonist, has a better safety profile as it does not readily cross the blood-brain barrier and has shown effectiveness in long-term treatment. Serotonin receptor modulators, particularly 5-HT4 receptor agonists such as velusetrag, felcisetrag, and prucalopride, have demonstrated efficacy in clinical trials, improving symptoms and gastric emptying by stimulating enteric cholinergic neurons to enhance motility throughout the GI tract. Motilin receptor agonists, like erythromycin and azithromycin, have proven effective in treating acute gastroparesis exacerbations, although their long-term use is hindered by antibiotic resistance and tachyphylaxis[105]. Emerging therapies for gastroparesis include ghrelin receptor agonists and NK1 receptor antagonists. Ghrelin, a hormone that stimulates appetite, has shown promise in improving symptoms, particularly in diabetic gastroparesis. Ghrelin agonists have been well-tolerated and significantly reduce symptoms. Tradipitant, a novel NK1 receptor antagonist, has been found to reduce nausea and vomiting by blocking NK1 receptors, which are involved in the emetic pathway, offering a new treatment option for gastroparesis patients[106]. Bioelectronic medicine, particularly GES, has shown efficacy in refractory gastroparetic patients. Although GES reduces nausea and vomiting, it has not conclusively improved gastric emptying. New advancements in closed-loop gastric pacemakers aim to work in conjunction with ICC to sense abnormal gastric patterns and modulate activity to restore normal gastric conduction[107]. VNS, especially non-invasive transcutaneous auricular VNS, is emerging as a promising alternative. Pilot studies indicate that VNS can improve both symptoms and gastric emptying, offering a non-invasive option with fewer off-target effects compared to other interventions[108]. Endoscopic treatments such as gastric peroral endoscopic pyloromyotomy (G-POEM) have proven effective in managing refractory gastroparesis. With a high success rate, G-POEM provides direct access to the gastric muscle and pacemaker cells, offering potential for new insights into the pathogenesis of gastroparesis while improving symptoms[109]. Looking to the future, personalized medicine tailored to individual patient characteristics and the underlying cause of gastroparesis appears to be a promising approach. The integration of novel pharmacological agents, endoscopic interventions, and lifestyle modifications can significantly enhance patient outcomes[110]. Furthermore, combining traditional Chinese medicine with Western pharmacological treatments may hold potential, though more research is needed to understand the mechanisms involved. Advances in diagnostic tools, such as meal “stress” tests and companion diagnostics for ENS dysfunction, are expected to improve patient selection and enable more targeted treatment. These innovations will optimize gastroparesis management, leading to better outcomes and a higher quality of life for patients[111]. Recent research continues to shed light on the role of the ENS in gastroparesis and motility disorders. Bessard et al[112] examined alterations in prostanoid expression and intestinal epithelial barrier functions in ileus, suggesting that targeting prostanoid signalling could offer therapeutic opportunities. Bonaz et al[113] reviewed the role of enteric neuropathy and vagus nerve dysfunction in gut disorders, highlighting vagus nerve stimulation as a potential therapy for gastroparesis. Brun et al[114] investigated the role of toll-like receptor-2 in the production of glial-derived neurotrophic factors, suggesting that modulating neurotrophic factors could help regulate motility. Ferreira-Duarte et al[115] explored the interaction between the renin-angiotensin system and enteric neurotransmission, proposing new therapeutic strategies for colonic dysmotility in inflammatory conditions. Filpa et al[116] reviewed glutamatergic neurotransmission in the ENS, concluding that targeting glutamatergic pathways could provide new treatment options for GI disorders. He also highlighted the potential of modulating ENS function to treat motility disorders, offering promising therapeutic avenues. Floettmann et al[117] characterized naloxegol, a μ-opioid receptor antagonist for opioid-induced constipation, finding it to be effective without impacting opioid analgesia, thus offering a novel solution for opioid-induced GI dysfunction. Further research from Gao et al[118] on μ-opioid receptor-mediated enteric glial activation in morphine-induced constipation suggests that targeting these receptors could help manage opioid-related constipation. Sanger et al[119] explored ghrelin and motilin control systems in GI physiology, emphasizing their therapeutic applications in treating motility disorders. Similarly, Stakenborg et al[120] discussed postoperative ileus and the role of neuroinflammation, proposing modulation of neuroinflammation and gut motility as a treatment strategy for improved recovery (Table 3). Targeting the ENS provides a comprehensive approach to treating gastroparesis[121]. Despite progress with prokinetic agents and bioelectronic therapies, optimal results for all patients remain challenging. The integration of precision medicine, combination therapies, and advanced neuromodulation techniques holds the potential to improve gastroparesis management. Ongoing research into ENS dysfunction and targeted interventions will be crucial for advancing treatments and enhancing patient outcomes[122-127].

Table 3 Studies showing therapeutic role of enteric nervous system on gastrointestinal motility disorders.

No.	Ref.	Study characteristics	Key role of ENS
1	Bessard et al[112]	Investigates alterations in prostanoid expression and intestinal epithelial barrier functions that contribute to ileus	Highlights how disruptions in ENS signaling and epithelial function lead to impaired motility and ileus development
2	Bonaz[113]	Discusses the role of enteric neuropathy and the vagus nerve in gastrointestinal disorders, with therapeutic implications	Explores how enteric neuropathy and vagal signaling impact gut function, offering insights into treatment strategies
3	Brun et al[114]	Studies how Toll-like receptor-2 regulates glial-derived neurotrophic factor production in intestinal smooth muscle cells	Focuses on how glial cells in the ENS influence smooth muscle function and gut motility via neurotrophic factor production
4	Ferreira-Duarte et al[115]	Explores the interaction between the renin-angiotensin system and enteric neurotransmission in colonic dysmotility in TNBS-induced colitis	Highlights the interplay between systemic signaling and the ENS, leading to dysmotility in inflammatory conditions like colitis
5	Filpa et al[116]	Investigates the role of glutamatergic neurotransmission in the ENS and its role in the brain-gut axis in health and disease	Explores how glutamate signaling in the ENS influences gut-brain communication and impacts gastrointestinal disorders
6	Floettmann et al[117]	Provides the pharmacological profile of Naloxegol, a peripherally acting μ-opioid receptor antagonist for opioid-induced constipation	Focuses on the interaction between opioid receptors in the ENS and peripheral opioid antagonism to treat constipation
7	Gao et al[118]	Investigates μ-opioid receptor-mediated enteric glial activation in morphine-induced constipation	Highlights the activation of enteric glia through opioid receptors, contributing to constipation in opioid use
8	Sanger et al[119]	Discusses the role of ghrelin and motilin control systems in gastrointestinal physiology and therapeutics	Explores the control of gastrointestinal motility via ghrelin and motilin systems, which interact with the ENS for motility regulation
9	Stakenborg et al[120]	Reviews pathophysiology and current therapeutic approaches to postoperative ileus	Discusses how the ENS is involved in the pathophysiology of ileus and how therapies targeting ENS dysfunction can improve recovery

ENS: Enteric nervous system; TNBS: Trinitrobenzenesulfonic acid.

HSCR and congenital aganglionosis

HSCR is a severe congenital disorder marked by the absence of enteric ganglia in the distal colon, leading to functional intestinal obstruction and megacolon[3]. This disease is caused by defects in the migration, proliferation, or differentiation of neural crest cells during embryonic development. These cells fail to reach the gut wall, particularly in the distal colon, preventing the formation of enteric neurons. The lack of these neurons leads to an aganglionic segment that remains contracted, causing functional bowel obstruction[128]. The pathophysiology of HSCR is rooted in the abnormal development of neural crest cells, which normally migrate from the neural tube to the gut to form the ENS. In HSCR, incomplete migration results in a lack of enteric ganglia essential for peristalsis. As a result, the aganglionic region of the colon cannot relax, causing a functional blockage[129]. Several key genes, including RET, GDNF, EDNRB, and SOX10, are commonly implicated in HSCR. Mutations in these genes affect the development of the ENS, particularly during the migration and differentiation of neural crest cells. The RET gene, in particular, is associated with 12%-50% of HSCR cases. RET plays a crucial role in activating signalling pathways necessary for neurogenesis in the ENS. Mutations in RET disrupt these pathways, preventing proper development and leading to aganglionic regions in the colon[130]. Surgical resection of the aganglionic bowel remains the primary treatment for HSCR. However, this procedure carries risks, such as incontinence and Hirschsprung-associated enterocolitis (HAEC), a severe complication characterized by bowel inflammation and infection. HAEC is thought to be related to abnormal cholinergic activity in the aganglionic segments, were acetylcholine, secreted by tuft cells, may influence inflammatory responses by promoting M2 macrophage polarization[131]. Recent advances in stem cell-based therapies present a promising alternative to traditional surgery. hPSCs have been studied for their potential to differentiate into enteric neurons, offering the possibility of restoring function to aganglionic segments. In animal models, transplanted hPSCs have shown promise in reducing mortality and restoring some GI function. Schwann cells from aganglionic segments have also been investigated for their regenerative potential, with studies indicating that their transplantation promotes neuronal differentiation and restores motor function in affected smooth muscle[132]. Gene editing technologies, such as CRISPR-Cas9, offer new possibilities for directly correcting the genetic mutations responsible for HSCR[133]. Although still in the early stages, this technology has the potential to target specific mutations in the RET gene and other related genes, providing a more direct treatment option[134]. Organoid technology and tissue engineering have also contributed to new methods for studying HSCR and testing therapies. Organoids, 3D cultures that replicate the gut environment, have been useful for examining how genetic mutations affect ENS formation. These models serve as valuable platforms for testing new treatments and improving our understanding of disease mechanisms[135]. Despite these advancements, challenges remain in translating these therapies to clinical practice. The ENS is complex, and effective treatment requires that transplanted cells survive and integrate with existing neural circuits. There is also safety concerns related to the use of pluripotent stem cells, including the risk of tumorigenesis. Furthermore, understanding the cellular niche, which regulates ENS development, is crucial for improving the effectiveness of stem cell therapies[136]. Recent research has advanced our understanding of HSCR and potential treatments. Jonscher and Belkind-Gerson et al[137] derived human ENS lineages for drug discovery and cell therapy in HSCR, and reviewed injury-induced neurogenesis in postnatal enteric tissues, suggesting potential for reparative therapies. Morikawa et al[138] demonstrated how mesenchymal cells regulate neural crest cell migration, proposing new strategies for enhancing neurogenesis. Windster et al[139] identified diverse subtypes of enteric glial cells and their roles in gut function, highlighting the potential for targeting glial cell diversity in HSCR treatment. Despite surgical resection being the current standard, stem cell therapies, gene editing, and bioengineering are progressing rapidly. These emerging approaches could restore ENS function and provide long-term solutions for HSCR. However, further research is necessary to better understand ENS development and disease progression, and to address safety concerns for clinical application. As research continues, these new strategies offer promising alternatives for treating HSCR[140,141].

SBS and intestinal failure

SBS and intestinal failure are critical conditions in which the GI tract is unable to maintain adequate nutrition, fluid, and electrolyte balance, often requiring long-term parenteral nutrition. SBS specifically arises from extensive surgical resection of the small intestine, leading to malabsorption and malnutrition. The ENS, which regulates GI motility, secretion, blood flow, and immune responses, plays an essential role in both conditions, contributing significantly to intestinal adaptation following gut loss. Recent research has highlighted the significant involvement of the ENS in SBS and intestinal failure, suggesting that targeting the ENS could improve therapeutic approaches by promoting intestinal adaptation, optimizing motility, and enhancing overall GI function[142]. Following major intestinal resection, the remaining bowel undergoes substantial structural and functional changes, with the ENS being a key player in these adaptations. Studies have shown that muscle tissue in the remaining intestine, particularly in the jejunum and ileum, undergoes hypertrophy as a compensatory mechanism to offset the lost absorptive surface area. Increased expression of nestin, a marker for neuronal plasticity, in the myenteric plexus suggests active neuroplasticity in response to the loss of intestinal segments. In SBS patients, a twofold increase in stem cells in the myenteric plexus further supports the involvement of the ENS in intestinal adaptation after resection. This process compensates for the lost absorptive surface area and involves mechanisms regulated by the ENS, such as blood flow, motility, secretory functions, and immune responses[143]. Therapeutic strategies targeting the ENS in both SBS and intestinal failure aim to promote intestinal adaptation and optimize motility. One promising agent is glucagon-like peptide-2 (GLP-2). Teduglutide, the first GLP-2 analogue approved for clinical use, has been shown to reduce reliance on parenteral nutrition by promoting intestinal growth and function. Newer, long-acting GLP-2 analogues, such as apraglutide and glepaglutide, are advancing through clinical trials and offer potential benefits, such as improved dosing schedules and enhanced safety profiles. Apraglutide has shown efficacy in treating steroid-refractory GI acute graft-versus-host disease, and glepaglutide has completed phase 2 trials and is moving toward phase 3 studies. These GLP-2 analogues promote the regeneration of Paneth cells and intestinal stem cells, which are essential for ENS function and intestinal adaptation[144]. Combination therapies involving multiple growth factors are also under investigation. Hepatocyte growth factor has shown promise in preclinical models by enhancing intestinal adaptation, leading to increased body weight, villus height, and crypt depth. When combined with GLP-2, these therapies activate complementary signalling pathways in both the ENS and intestinal epithelium, potentially offering superior therapeutic outcomes[145]. Neurotrophic factors, such as GDNF and its receptor RET, are crucial for ENS function. GDNF supplementation has been found to reverse age-related changes in intestinal motility, suggesting its potential application in intestinal failure. Sacral nerve stimulation, which activates the GDNF-PI3K/Akt pathway, has also emerged as a promising neuromodulation technique. Animal studies have demonstrated that sacral nerve stimulation increases GDNF expression and phosphorylated AKT levels in colon tissue, improving enteric neuron function and motility[146]. L-Fucose, a dietary sugar, has demonstrated potential in promoting ENS regeneration by inhibiting the SMAD2 signalling pathway in enteric neural precursor cells. In diabetic mouse models, L-Fucose treatment improved GI motility, increased the number of neurons and glial cells, and promoted the differentiation of enteric neural precursor cells. This approach modulates the TGF-β pathway, which regulates neurogenesis, suggesting that L-Fucose could be a beneficial adjunct in nutritional therapies for patients with SBS or intestinal failure[147]. Stem cell-based therapies show significant potential for ENS regeneration and improving GI motility in SBS and intestinal failure. While much research has focused on HSCR, the principles of stem cell therapy could also apply to SBS and intestinal failure. Studies have shown that human pluripotent stem cell-derived ENS progenitors can integrate into human colon tissue, form neurons and glial cells, and enhance responses to electrical stimulation. Additionally, tissue engineering approaches, including biocompatible scaffolds, may support ENS regeneration by providing mechanical and biochemical cues to promote cell survival and differentiation[148]. ENSCs have also shown promise for restoring GI function. In preclinical models, ENSC transplantation has led to the formation of neural networks, rescuing colonic motility and restoring nitrergic responses. Improvements in cryopreservation techniques have made it easier to store ENSCs long-term without significant loss of function, enhancing their clinical feasibility[149]. Microbiome-based interventions are gaining attention for their potential to modulate ENS function. Studies have shown that maternal gut microbiota plays a crucial role in ENS development, and dysbiosis in the gut microbiome can affect ENS plasticity in offspring. Supplementation with specific bacterial strains, such as Limosilactobacillus reuteri, and metabolites like propionate has been shown to impact ENS development, offering a non-invasive strategy for improving intestinal health. SCFAs, produced by gut microbiota, influence enteric neurotransmission, inflammatory responses, and metabolic signalling, making them potential candidates for restoring ENS function in conditions like SBS and intestinal failure[150]. Bioelectronic medicine, particularly VNS, has been shown to improve GI function in conditions like opioid-induced constipation. Enhancing vagal nerve activity through neuromodulation may help restore motility in SBS and intestinal failure patients. Advanced bioelectronic systems are being developed to monitor and control ENS function in real-time. Closed-loop systems, which adjust stimulation parameters based on physiological feedback, could offer precise control over ENS activity, optimizing therapeutic outcomes while minimizing side effects[150]. Pharmacological therapies targeting serotonin receptors also hold promise. The 5-HT4 receptor agonist prucalopride has demonstrated potential for stimulating ENS regeneration in animal models, improving motility and promoting neuronal regeneration, both crucial for ENS function in intestinal failure and SBS. Despite significant advances in understanding ENS function and its role in SBS and intestinal failure, several challenges remain. The complexity of the ENS, with its diverse neuronal populations and intricate connectivity, complicates the prediction of therapeutic interventions' effects. Moreover, current animal models may not fully replicate human ENS physiology, limiting the translation of preclinical findings to clinical practice. The development of reliable biomarkers for ENS function and appropriate outcome measures for ENS-targeted therapies remains ongoing[151]. Modulating ENS plasticity represents a promising approach to treating both SBS and intestinal failure. Advances in GLP-2 analogues, neurotrophic factors, stem cell therapies, microbiome-based interventions, and neuromodulation techniques offer new opportunities to enhance ENS function and promote intestinal adaptation. As research continues to evolve, these therapies may revolutionize the management of SBS and intestinal failure, improving the quality of life for affected patients[152-156].

Colorectal cancers

Colorectal cancer (CRC) is the third most commonly diagnosed cancer globally, with 1.9 million new cases each year, and is the second leading cause of cancer-related deaths. Despite advancements in treatment, prognosis for many patients remains poor. Recent studies have highlighted the significant role of the ENS in CRC progression, suggesting that targeting the ENS could provide new therapeutic opportunities. The ENS interacts with cancer cells and plays a crucial role in both the prevention and progression of CRC[157]. In CRC, tumor invasion disrupts the normal architecture of the large intestine, leading to damage in nerve fibres and neurons within the ENS. This damage causes atrophy of the myenteric and submucosal plexuses, which are essential for normal ENS function. As the tumor grows, the size of these plexuses diminishes, and the tumor boundary is surrounded by additional nervous elements, which contribute to disease progression[158]. Research has shown that neurotransmitters, such as galanin, are elevated in areas near tumours. Galanin may inhibit apoptosis and promote cancer cell survival, aiding in tumor growth. Similarly, beta-2 adrenergic receptors are associated with tumor grade, size, and metastasis, further emphasizing their role in CRC progression. Genetic studies have identified chromosomal deletions, such as on chromosome 1p, linking the ENS to CRC. Cancer cells can migrate along ENS neurons, potentially acquiring mutations from these cells during recombination. This suggests a deeper molecular relationship between the ENS and CRC development, solidifying the ENS as a critical target for therapeutic strategies[159]. Therapeutic strategies targeting the ENS in CRC include neurotransmitter receptor modulators. Beta-blockers, such as propranolol, have shown promising results in delaying tumor progression. Propranolol reduces tumor angiogenesis, fosters an anti-tumoral microenvironment, and enhances T cell infiltration while limiting myeloid-derived suppressor cells' infiltration. These effects suggest that beta-blockers may improve cancer treatment outcomes by modulating the immune response. Additionally, neurokinin receptor antagonists, particularly those targeting substance P (SP) and the neurokinin-1 receptor (NK1R), may help reduce tumor progression. High expression of SP and NK1R has been linked to lymph node metastasis, and blocking this pathway could potentially reduce CRC progression[160]. Dopamine receptors, particularly dopamine receptor D4 (DRD4), are also implicated in CRC. DRD4 is upregulated in CRC and is associated with poor prognosis. DRD4 interacts with transforming growth factor beta receptors, promoting epithelial-mesenchymal transition, which enhances CRC's metastatic potential. Inhibiting DRD4 could therefore be a promising strategy for reducing the metastatic potential of CRC[161]. Emerging strategies also focus on perineural invasion (PNI), where tumor cells invade nerve sheaths, contributing to tumor invasion and metastasis. PNI is linked to poor clinical outcomes, and therapies targeting the communication between nerves and cancer cells could improve survival by preventing tumor progression[162]. Galanin, a neurotransmitter released by ENS neurons, has been shown to play a role in CRC. Elevated levels of galanin receptor 1 (GALR1) and GALR3 proteins have been detected in CRC tissues. Increased GALR3 expression is linked to better prognosis, while higher GALR1 expression near tumors correlates with poor prognosis and shorter survival. This suggests that galanin and its receptors may promote tumor proliferation and metastasis, making them potential therapeutic targets[163]. Enteric glial cells (EGCs), which support ENS function, are also involved in the tumor microenvironment of CRC. Chemotherapy drugs, such as oxaliplatin, cause significant damage to EGCs, leading to GI toxicity. Research into proteins such as AmotL2, which is involved in cell survival and inflammation, suggests potential targets for reducing chemotherapy-induced neurotoxicity in CRC patients[164]. Oxaliplatin-induced peripheral neuropathy (OIPN) is a significant challenge in CRC treatment, limiting the use of this chemotherapy drug. Molecular signatures associated with OIPN, such as the downregulation of proteins involved in axon excitability, provide opportunities for developing treatments to prevent or manage OIPN, thus improving the quality of life for CRC patients[165]. Promoting ENS regeneration in CRC patients, particularly after chemotherapy or surgery, is another area of interest. Studies have shown that oxaliplatin treatment reduces the number of neurons and glial cells in patients. Strategies to regenerate these cells, including neurotrophic factors, stem cell therapies, or pharmacological agents, could help protect and restore ENS function in CRC patients. Precision medicine approaches that tailor treatments to individual patients based on genetic and molecular profiles offer promising opportunities for ENS-targeted therapies. Customizing treatments could improve patient outcomes, and combining multiple ENS-targeted therapies could enhance efficacy. For example, combining propranolol with immune checkpoint inhibitors, such as anti-CTLA4, has shown potential for improving cancer treatment outcomes in preclinical models[160]. Recent research continues to explore the role of the ENS in CRC and suggests new therapeutic avenues. Kmiec et al[166] reviewed the role of galanin in CRC progression, suggesting that targeting galanin signalling could be an effective therapeutic approach. Li et al[167] examined the neuroimmune axis in CRC, focusing on neural circuitry and its potential therapeutic implications. They identified that altering neuroimmune signalling could influence cancer progression and metastasis, further highlighting this pathway as a target for CRC treatment. Targeting the ENS in CRC presents a promising direction for cancer therapy. Therapies that focus on neurotransmitter receptors, PNI, and enteric glial cells offer multiple approaches to improve treatment outcomes. As research continues to uncover the role of the ENS in CRC, these strategies could transform the treatment landscape, offering new hope for patients with this challenging disease.

The gut-brain axis, systemic diseases and therapeutic implications bidirectional communication pathways

The gut-brain axis has changed our understanding of how the ENS affects systemic physiology and behavior. This network includes neural, hormonal, and immune pathways that connect the GI tract to the CNS. The ENS integrates local gut signals with central neural control[168]. Vagal afferent pathways are the main route for gut-to-brain communication, transmitting information about intestinal contents, mechanical distension, and inflammation to the brainstem. Interventions targeting these pathways could influence mood, behavior, and cognitive function. Recent studies show that serotonin produced in the gut affects emotional behavior via vagal pathways, suggesting a potential therapeutic target for gut-based serotonin[169]. The GI tract also produces neurotransmitters like serotonin, dopamine, and GABA. Enteric bacteria synthesize these neurotransmitters, while intestinal cells release hormones and peptides that influence the CNS. Targeting these systems could offer new treatments for neuropsychiatric disorders through gut-based interventions[170]. The gut microbiome plays a key role in regulating ENS function, making it a target for treating ENS-related disorders. Bacterial metabolites, especially SCFAs, influence enteric neurotransmission, neuroplasticity, and neural circuit development. Dysbiosis, or microbial imbalance, is linked to GI disorders involving ENS dysfunction, prompting microbiome-targeted therapies[171]. Probiotics, which produce neuroactive compounds, reduce inflammation, and improve gut barrier function, show promise in modulating ENS function. Prebiotics that promote beneficial bacteria can boost SCFA production and reduce harmful bacteria, improving ENS function. Personalized prebiotic therapies based on individual microbiome profiles may enhance treatment outcomes[172]. ENS dysfunction also contributes to conditions like diabetes, obesity, and mood disorders. Modulating ENS function could offer new approaches to managing these diseases. Targeting serotonin in the ENS may provide antidepressant effects without the systemic side effects of traditional medications. Additionally, ENS modulation could aid in treating inflammatory diseases like multiple sclerosis (MS) and rheumatoid arthritis (RA), underscoring the ENS's role in managing diverse health conditions[173].

Obesity

Obesity has become a global epidemic, affecting over a billion people worldwide. It is a complex metabolic disorder, and recent research highlights the ENS as a critical therapeutic target[174]. The ENS regulates motility, secretion, and visceral sensitivity, and has been identified as a key player in obesity, particularly in regulating nutrient absorption and energy metabolism. As obesity alters the structure and function of the ENS, targeting these changes may offer new strategies for managing the condition[175]. Obesity-related ENS dysfunction involves structural and functional changes. High-fat diet models show that obesity impairs the sensitivity of enteric neurons, with delayed responses to nutrients like glucose and oleic acid[176]. This dysfunction includes alterations in neuroplasticity, glial cell activation, and inflammation, which contribute to metabolic dysregulation. Early shifts in gut microbiota, such as increases in Clostridia and Proteobacteria, are linked to ENS remodelling and impaired motility, exacerbating neuronal dysfunction[177]. Inflammation plays a significant role in obesity-related ENS dysfunction, particularly through the NLRP3 inflammasome. This complex trigger neurogenic inflammation in the ENS, releasing pro-inflammatory cytokines that disrupt gut-brain communication[178]. Reducing inflammation via calorie restriction or exercise can alleviate these effects and improve insulin sensitivity, making NLRP3 inflammasome inhibition a promising therapeutic approach[179]. Enteric glial cells, crucial for GI function, undergo gliosis in obesity, worsening motility and gut-brain axis dysfunction. Targeting sirtuin proteins, such as SIRT1 and SIRT3, may restore ENS function, as these proteins regulate stress responses and nutrient sensing[180]. Therapies targeting the ENS in obesity include both pharmacological and non-pharmacological approaches[181]. GLP-1 receptor agonists, like semaglutide, modulate appetite and energy metabolism through the gut-brain axis. When combined with NLRP3 inflammasome inhibitors, their efficacy may improve significantly. Probiotics and prebiotics are also being explored for their potential to modulate gut motility and reduce inflammation, while SCFAs produced by the microbiota influence ENS function and offer therapeutic potential[182]. Bioelectronic medicine, such as VNS, has shown promise in improving GI function in obesity[183]. VNS may help restore motility and enhance the adaptive response in the gut-brain axis. Additionally, the modulation of the sirtuin pathway is being investigated as a way to mimic calorie restriction effects, providing a sustainable approach to obesity management[184]. Emerging NLRP3 inflammasome inhibitors show significant promise in reversing diet-induced obesity in animal models, targeting inflammation as a root cause of metabolic dysfunction[185]. Precision medicine is also evolving to offer tailored treatments based on individual gut microbiome profiles and ENS function, helping to guide personalized therapies[186]. Bariatric surgery remains the most effective treatment for morbid obesity, though its impact on ENS function continues to be explored[187]. Post-surgical outcomes, including reductions in inflammatory markers and improvements in gut-brain communication, suggest that bariatric surgery may offer benefits beyond weight loss by modulating ENS-related pathways[188]. Targeting the ENS in obesity offers a revolutionary approach to managing this global health crisis. With diverse strategies such as GLP-1 receptor agonists, NLRP3 inflammasome inhibitors, microbiome-based therapies, and bioelectronic interventions, significant progress is being made[189-191]. As our understanding of the ENS in obesity deepens, these therapies will provide more effective, personalized, and sustainable solutions for treating obesity and its complications[192].

Diabetes mellitus

The ENS is gaining attention as a critical target for therapeutic interventions in diabetes mellitus, offering a novel approach to manage complications related to the disease. Diabetes often causes significant damage to the ENS, resulting in GI dysfunctions that greatly affect the quality of life of patients. The ENS, composed of about 500 million neurons, plays a key role in regulating essential functions such as motility, secretion, blood flow, and immune responses. In the context of diabetes, alterations in the ENS contribute to complications like gastroparesis, constipation, and other motility disorders, making it a key area of focus for potential therapeutic strategies[193]. Diabetes mellitus induces both structural and functional changes in the ENS, which contribute to GI dysfunction. Hyperglycaemia causes an increase in the number of neurons in the myenteric plexus that are immunoreactive to various neurotransmitters and neuropeptides, including nitric oxide synthase, vasoactive intestinal peptide, galanin, calcitonin gene-related peptide, and cocaine amphetamine-regulated transcript. These changes are considered compensatory responses, aimed at maintaining GI function despite the damage caused by diabetes. Alongside these neuronal changes, EGCs are particularly susceptible to diabetic damage. Research has shown that circular RNA-VPS13A is downregulated in hyperglycaemia-treated EGCs, and its overexpression appears to protect these cells. This suggests that targeting the circular RNA-VPS13A pathway could offer a promising therapeutic approach for addressing ENS dysfunction in diabetes[194]. Inflammation plays a significant role in the dysfunction of the ENS in diabetes. The gut-brain axis, which regulates glycaemic control by communicating nutritional signals to the hypothalamus, is disrupted in type 2 diabetes, contributing to insulin resistance and hyperglycaemia. Inflammatory factors from both host cells and gut microbiota target ENS neurons, worsening metabolic dysfunction and diabetic complications. Targeting this disruption in the gut-brain axis holds potential for improving diabetes management[195]. Various therapeutic strategies are being explored to target the ENS in diabetes, including both pharmacological and non-pharmacological approaches. For example, prucalopride, a selective 5-HT4 receptor agonist, has been shown to promote ENS regeneration in diabetic models[196]. Treatment with prucalopride reduces colonic transit time and increases neuronal markers, suggesting both immediate prokinetic effects and long-term regenerative benefits for the ENS. Electroacupuncture is another promising treatment, particularly for diabetic gastroparesis. By modulating ENS neurotransmitters, electroacupuncture has been shown to improve glucose tolerance, insulin resistance, and gastric emptying, highlighting its potential in managing obesity and related diabetic complications[197]. Regenerative medicine also shows promise for restoring ENS function in diabetes. L-fucose, a dietary sugar, has been found to promote ENS regeneration by inhibiting the SMAD2 signalling pathway in diabetic mice, facilitating the differentiation of enteric neural precursor cells into neurons and glial cells. Bone marrow-derived MSCs (BMSCs) are another potential therapeutic option. Modifying BMSCs with CD44 fucosylation enhances their ability to target the GI tract, promoting ENS remodelling and improving GI motility. These stem cells have been shown to restore colonic contractions and increase the expression of key neuronal markers, indicating their potential for treating ENS dysfunction in diabetes[198]. Advancements in drug delivery systems are also improving the specificity and efficacy of ENS-targeted therapies. Nanoparticle-based and exosome-based delivery systems offer targeted drug delivery, controlled release, and enhanced biocompatibility. These systems have shown promise in animal models for improving insulin-producing beta-cell regeneration and reducing extracellular matrix degradation. Engineered exosomes, in particular, provide high bioavailability and specificity, making them suitable for long-term circulation and efficient intracellular drug release in ENS interventions[199]. The gut microbiome plays a critical role in ENS function, and microbiome-based interventions are emerging as a therapeutic strategy for managing diabetes. SCFAs, produced by intestinal microbiota, influence ENS activity, inflammatory responses, and metabolic signalling. SCFAs stimulate the secretion of GLP-1 and insulin, offering multiple benefits for diabetes management. Probiotics, including Lactobacillus and Bifidobacterium, have shown promise in modulating the gut microbiota and improving motility disorders associated with diabetes[200]. Biomarkers for ENS dysfunction in diabetes are also being actively investigated. Advanced glycation end products (AGEs), which accumulate in diabetes, have been identified as potential biomarkers for ENS damage. Elevated serum levels of AGEs, such as N-epsilon-carboxymethyl-lysine, provide valuable insights into the extent of ENS dysfunction and treatment efficacy. Muscularis macrophages, which are involved in muscle repair in the GI tract, may also serve as biomarkers for diabetic gastroparesis and targets for therapeutic intervention[201]. Precision medicine, which tailor treatments to individual patient characteristics, is becoming more important in diabetes care. Co-culture systems that integrate the ENS with intestinal organoids are helping to better understand ENS interactions in the context of diabetes, leading to more effective treatment strategies. Combining pharmacological agents, regenerative therapies, microbiome interventions, and neuromodulation may provide superior outcomes compared to single-agent therapies[202]. While promising, challenges remain in translating ENS-targeted therapies into clinical practice. The complexity of the ENS, with its diverse neuronal populations and intricate connectivity, complicates predictions of therapeutic outcomes. Furthermore, current animal models may not fully replicate human ENS physiology, limiting the translation of preclinical findings. Manufacturing challenges, especially for regenerative medicine, require specialized facilities to ensure the consistent production of safe and effective treatments. Additionally, the development of appropriate outcome measures for ENS-targeted therapies remains a significant challenge, as traditional endpoints may not capture the full scope of ENS dysfunction[202]. Abot et al[203] examined how gut-derived peptides influence the ENS to regulate glucose metabolism and food intake. They found that these peptides can modulate enteric neurons, affecting local physiology and the gut-brain axis. Their research suggests that targeting this interaction could provide novel therapeutic options for metabolic disorders. He also explored the crosstalk between intestinal immune cells and the ENS in type 2 diabetes. They discovered that inflammation disrupts gut-brain communication, contributing to insulin resistance and metabolic disturbances. Their study indicates that targeting this pathway could improve the management of type 2 diabetes. Jiang et al[204] investigated oxidative damage to enteric glial cells under hyperglycaemic stress. They identified that redoxosomes/p66SHC activation plays a key role in this process, highlighting redox signalling as a potential therapeutic target for GI complications in diabetes. He explored the role of SCFAs in type 2 diabetes and their potential to restore gut homeostasis. Their findings suggest that SCFAs could be targeted to treat metabolic diseases by influencing gut health. Richards et al[205] reviewed therapeutic approaches targeting the gut-brain axis in type 2 diabetes, obesity, and related disorders. They highlighted several promising signalling pathways for improving metabolic outcomes, focusing on microbiome modulation and neural signalling. They investigated circular RNA-VPS13A's role in protecting enteric glial cells from diabetes-induced damage by targeting the miR-182/GDNF axis. Their study suggested that targeting circular RNA-VPS13A could be a promising strategy for treating GI dysfunction in diabetes. Targeting the ENS represents a promising new approach in diabetes management, particularly in addressing GI complications and potentially modifying disease progression. With various therapeutic strategies, including pharmacological treatments, regenerative medicine, drug delivery systems, and microbiome-based interventions, diabetes care could be significantly improved. As research on the ENS in diabetes continues to progress and new technologies emerge, these therapies have the potential to transform diabetes treatment and improve patient outcomes.

PD and other neuropsychiatric disorders

PD is a progressive neurodegenerative disorder, traditionally recognized for its motor symptoms, such as tremors, bradykinesia, and rigidity. However, it also includes significant non-motor symptoms, particularly GI dysfunction, which are closely linked to the ENS. Recent studies have shown that alterations in the ENS occur early in PD, often before the CNS becomes involved. This discovery positions the ENS as not only a potential early diagnostic marker but also as a promising target for disease-modifying therapies[206]. PD pathophysiology is characterized by the aggregation of alpha-synuclein (αS), a protein that accumulates in both the CNS and the ENS. This aggregation is a hallmark of the disease, observed in certain subtypes of enteric neurons. Post-mortem studies of PD patients have shown α-synuclein deposits in the ENS, which spread retrogradely through preganglionic vagal fibres to the dorsal motor nucleus of the vagus nerve and other central nervous structures. This prion-like spread suggests that targeting α-synuclein aggregation in the ENS may help slow disease progression. Additionally, the aggregation of α-synuclein increases with age, creating a reservoir of pathogenic protein that perpetuates disease progression[207]. Alterations in the ENS occur early in PD, contributing to GI dysfunction before motor symptoms emerge. Animal models, including those exposed to rotenone, have shown that changes in the ENS precede CNS involvement. These changes include a reduction in enteric neuron numbers, increased neurotransmitter receptor expression, and altered responses to electrical stimulation. In the A53T mouse model of PD, deficits in colonic motility were observed before motor impairments, providing strong evidence that ENS dysfunction plays a role in the early GI issues seen in PD. This makes the ENS a potential early target for intervention[208]. Several therapeutic strategies targeting the ENS in PD are under investigation, with a focus on preventing α-synuclein aggregation, enhancing autophagy, and improving gut microbiome health. One promising pharmacological approach is the use of α-synuclein aggregation inhibitors[209]. Squalamine, an aminosterol derived from dogfish shark liver, has been shown to displace membrane-bound α-synuclein and restore ENS function. Studies using the A53T human α-synuclein mutant model demonstrated that squalamine rapidly restored colonic motility and improved gut propulsive activity by enhancing the excitability of intrinsic primary afferent neurons in the ENS[210]. Other compounds, such as synthetic protein mimetics and foldamers, have also shown potent effects against both native and phosphorylated α-synuclein aggregation, suggesting they could slow disease progression[211]. Enhancing autophagy, the process by which the body clears damaged proteins and organelles, is another potential therapeutic approach. Electroacupuncture has shown promise in promoting the clearance of α-synuclein and damaged mitochondria in the ENS by modulating autophagy pathways[212]. In rotenone-induced PD models, electroacupuncture alleviated constipation symptoms and reversed the downregulation of autophagy-related proteins in the colon, such as ATG5, LC3II, and Parkin. These findings suggest that autophagy promotion in the ENS may help clear pathological protein aggregates and improve GI function[213]. The gut microbiome plays a pivotal role in PD, and microbiome-based interventions show potential in modulating ENS function. Probiotics have been demonstrated to improve both motor and non-motor symptoms of PD[214]. A meta-analysis of eleven clinical trials involving 840 participants revealed significant improvements in motor and non-motor symptoms, including depression, following probiotic treatment. Probiotic strains, such as Bifidobacterium animalis subsp. lactis, have been particularly effective in enhancing motor function and GI health while reducing anxiety[215]. Probiotics work by promoting beneficial bacterial populations, reducing harmful species, and increasing the production of neurotransmitters like dopamine and SCFAs, which support the ENS[216]. FMT has emerged as another promising therapeutic strategy for modulating the gut-brain axis in PD. Randomized controlled trials have demonstrated FMT’s safety and efficacy in improving motor symptoms, autonomic dysfunction, and GI problems in Parkinson’s patients[217]. A study involving 56 patients showed significant improvements in MDS-UPDRS scores and enhanced microbiome complexity following FMT. Mechanistic studies suggest that FMT works by reducing systemic inflammation and suppressing the lipopolysaccharide-TLR4 signalling pathway, which plays a key role in the inflammatory response in PD[218]. Neuromodulation approaches, such as transcutaneous vagus nerve stimulation (tVNS), are also being explored to alleviate GI symptoms in PD. Clinical trials have shown that tVNS significantly improves Gastrointestinal Symptom Rating Scale scores, suggesting it as a viable therapeutic option for treating gastroenteric symptoms in PD[219]. tVNS works by modulating brain activity in regions linked to motor and sensory functions, potentially restoring communication between the brain and the gut[220]. Acupuncture has demonstrated promise in modulating ENS function and improving PD symptoms[221]. Studies suggest that acupuncture can reduce α-synuclein aggregation in the ENS, restore intestinal microbial balance, and mitigate neuroinflammatory responses and oxidative stress. These benefits make acupuncture a potential adjunctive therapy for improving GI function in PD[222]. The ENS also offers a window into PD pathology, providing opportunities for early detection. Alterations in the ENS, such as changes in enteric neuron density, glial cell activation, and neurotransmitter expression, may serve as biomarkers for disease progression[223]. Functional assessments of GI motility and neurotransmitter expression patterns could offer objective measures of ENS dysfunction, facilitating early diagnosis and monitoring[224]. Agrawal et al[225] explored the role of serotonin 4 receptors (5-HT₄R) in chronic depression and its GI comorbidities. They suggested that modulation of 5-HT₄R could influence the brain-gut axis, offering potential therapeutic strategies for treating depression and related gut disorders. Alam et al[226] reviewed the role of gut microbiota in PD, discussing mechanisms like α-synuclein aggregation, intestinal permeability, and inflammation. They highlighted therapies targeting the microbiome, including probiotics, prebiotics, synbiotics, and faecal microbiota transplantation, as promising treatments for PD. Campagnolo et al[227] investigated immune cell alterations in the ENS of Parkinson’s patients and suggested that immune modulation in the ENS could offer therapeutic targets for the disease. Chalazonitis and Rao et al[228] reviewed enteric nervous system dysfunction in neurodegenerative diseases like Parkinson’s, proposing that targeting the ENS could help manage GI symptoms and potentially improve overall disease outcomes. Targeting the ENS in PD represents a revolutionary approach to treating this neurodegenerative disorder. Therapies like α-synuclein aggregation inhibitors, microbiome-based interventions, and neuromodulation approaches are making significant strides in modulating ENS function for the benefit of Parkinson’s patients. As research continues to evolve, these therapies hold great potential for improving patient outcomes and potentially modifying the disease course through comprehensive targeting of the gut-brain axis[229,230].

Autoimmune disorders

MS is an inflammatory disease of the CNS that often leads to GI dysmotility, particularly constipation, a common but poorly understood symptom. Research indicates that MS patients have serum antibodies targeting both CNS myelin and neuronal proteins present in the ENS[231]. These antibodies specifically target ENS cells more intensely than in healthy age- and gender-matched controls, with distinct staining observed on enteric neurons and glial cells. Studies in experimental autoimmune encephalomyelitis (EAE), an animal model of MS, reveal significant impairment of colonic motility, suggesting that ENS dysfunction contributes to MS-related GI symptoms. Further analysis of neurotransmitter alterations in the ENS of MS patients and EAE mice has provided deeper insights into the disease's pathophysiology[232]. Ghezzi et al[233] explored the potential of targeting the gut to treat MS, discovering that modulating the gut microbiota could influence neuroinflammation and improve outcomes in MS patients. This approach opens new avenues for managing neurodegenerative diseases. Weier et al[234] studied the effects of siponimod on both the enteric and CNS pathology in late-stage EAE, finding that siponimod reduced inflammation and improved GI function. This suggests it may be a potential treatment for both neurological and GI dysfunctions in MS. Wunsch et al[235] reviewed the possibility of targeting the ENS in MS, proposing that focusing on the ENS could help manage the neurological dysfunction and GI symptoms associated with the disease. The STING pathway, present in immune cells and ENS neurons, has been implicated in both MS and other neurological disorders. Activation of the STING pathway leads to the production of interferons and other inflammatory mediators, which contribute to inflammation and motility dysfunction in autoimmune diseases like MS. This highlights the potential for targeting the STING pathway as a therapeutic strategy for MS and related conditions[236]. RA, another autoimmune disorder, also impacts the ENS. Studies have shown reduced neuronal density in the ileum of affected individuals, alongside changes in the size of varicosities and neuronal areas when compared to healthy controls. These findings demonstrate the systemic impact of RA on the ENS and highlight its role in contributing to GI symptoms. Combination therapies involving quercetin and ibuprofen have been shown to reduce inflammation and preserve neuronal density, outperforming individual treatments in RA models[237]. One promising therapeutic strategy for autoimmune diseases involves targeting the cholinergic anti-inflammatory pathway, which regulates immune cell activity, inhibits cell proliferation, and suppresses cytokine release. This pathway, involving the vagus nerve and acetylcholine receptors, plays a critical role in diseases such as RA. Non-invasive VNS has proven to be effective in managing inflammation in autoimmune diseases like RA and IBD. Clinical applications of VNS have demonstrated reductions in inflammatory markers such as C-reactive protein and interferon-γ in patients with high disease activity[238]. VNS has also shown promise in managing systemic lupus erythematosus (SLE), a chronic autoimmune disorder. In experimental SLE models, chronic VNS reduced renal inflammation, lowered blood pressure, and attenuated pro-inflammatory cytokine production, supporting its potential as a treatment strategy for autoimmune diseases affecting multiple organ systems[239]. The gut microbiota plays a pivotal role in the ENS and the development of autoimmune diseases. FMT has emerged as a promising therapeutic approach to modulate the microbiome and restore healthy gut-brain axis communication. FMT has shown efficacy in MS and RA, helping modulate the immune response and alleviate disease symptoms. In SLE models, FMT reduced blood pressure and decreased autoimmune-associated B cells, suggesting that microbiota manipulation could be an effective strategy for managing autoimmune diseases[240]. MSCs are another potential therapeutic option for autoimmune diseases affecting the ENS. Known for their immunomodulatory properties, MSCs have demonstrated effectiveness in regulating immune responses, promoting tissue repair, and enhancing regeneration. Clinical applications of MSCs have been extended to treat autoimmune diseases such as RA and SLE. Evidence suggests that MSCs could restore ENS function and improve gut-brain axis communication. MSC-derived exosomes, which carry bioactive molecules, are also being explored as a cell-free alternative for treating autoimmune and neurodegenerative disorders. These exosomes have shown potential in reducing inflammation, decreasing demyelination, and upregulating regulatory T cells, offering a promising therapeutic approach for modulating the ENS in autoimmune diseases[241]. Targeting the ENS offers a revolutionary approach to treating autoimmune diseases, with multiple therapeutic avenues emerging, ranging from pharmacological interventions to neuromodulation, microbiome-based therapies, and stem cell treatments. As our understanding of ENS pathophysiology in autoimmune disorders evolves, these interventions hold the potential to improve patient outcomes by targeting the gut-brain axis. The future of autoimmune disease management lies in personalized, multi-modal approaches that address the underlying neurobiological mechanisms of these complex diseases[242,243].

Future directions and emerging technologies precision medicine approaches

The future of ENS-targeted therapies lies in precision medicine, which tailors treatments to individual patient characteristics and pathophysiology. This approach requires a deep understanding of the molecular mechanisms behind ENS dysfunction and diagnostic tools to identify specific therapeutic targets for each patient[173]. Advances in genomics and proteomics are shedding light on the molecular basis of ENS disorders and uncovering potential therapeutic targets. Single-cell RNA sequencing of enteric neurons is revealing previously unknown cellular diversity and functional specialization within the ENS. These insights may help identify specific neuronal populations to target for various diseases[244]. Artificial intelligence and machine learning are being used to analyze complex clinical, molecular, and physiological data to predict therapeutic responses and optimize treatment choices. These tools can identify patterns in patient data that traditional analysis might miss, allowing for more effective personalized treatment strategies[245]. The development of companion diagnostics, which identify patients most likely to benefit from specific ENS-targeted therapies, is crucial for precision medicine. These tools must be validated in large patient populations and integrated into clinical practice to ensure their effectiveness[246].

Advanced drug delivery systems

Innovative drug delivery systems are being developed to improve the specificity and effectiveness of ENS-targeted therapies while reducing systemic exposure and side effects. These systems must address challenges in delivering drugs to specific regions of the GI tract, ensuring drug stability and bioavailability[247]. pH-responsive drug delivery systems use the natural pH gradient from the stomach to the colon to target specific areas of the GI tract. These systems allow for region-specific drug release, enhancing therapeutic efficacy and minimizing systemic exposure. Advanced formulations with multiple pH-sensitive polymers can provide complex release profiles tailored to different therapeutic needs[248]. Nanoparticle-based delivery systems improve drug stability, enable controlled release, and offer the potential for targeted cellular delivery. These systems protect drugs from degradation in the GI tract and provide sustained release over longer periods. Surface modifications allow for the specific targeting of enteric neurons or other cells within the ENS[249]. Mucoadhesive drug delivery systems increase the residence time of drugs in targeted GI regions, raising local drug concentrations and improving therapeutic efficacy. These systems are especially useful for targeting the ENS, ensuring drugs remain available at sites where enteric neural activity occurs[249].

Bioelectronic medicine and neural interfaces

The field of bioelectronic medicine is progressing quickly with the development of advanced neural interfaces for precise, real-time modulation of ENS function. These technologies integrate microelectronics, materials science, and neuroscience to create implantable devices that can both monitor and modulate neural activity[250]. Closed-loop neuromodulation systems represent the next generation of ENS therapies. These devices continuously monitor physiological parameters and adjust stimulation settings automatically to maintain optimal effects. Closed-loop systems have the potential to offer better clinical outcomes than open-loop devices by adapting to changing physiological conditions[251]. Advances in wireless power transfer and communication technologies are enabling fully implantable ENS neuromodulation devices that do not require battery replacements. These innovations could improve patient acceptance and long-term outcomes by reducing the risks linked to device maintenance. Optogenetic approaches, though still experimental, show promise for precise neural modulation with minimal tissue damage. This technique involves modifying target neurons to express light-sensitive ion channels, allowing optical control over neural activity. The clinical use of optogenetics will depend on safe gene delivery methods and biocompatible optical interfaces[24].

Integration with digital health technologies

The integration of ENS-targeted therapies with digital health technologies is a growing trend that could improve treatment outcomes and patient engagement. These approaches combine traditional medical treatments with digital tools for monitoring, feedback, and behaviour modification. Smartphone apps and wearable devices allow for continuous monitoring of GI symptoms and physiological parameters, enabling real-time adjustments to ENS-targeted therapies. These tools can identify patterns in symptom occurrence and treatment responses that may not be detected through traditional clinical assessments[252]. Telemedicine platforms offer a way to remotely monitor and adjust ENS therapies, which is especially useful for patients with chronic conditions needing long-term management. These platforms improve access to specialized care and can help reduce healthcare costs[253]. Artificial intelligence algorithms can analyze large datasets from digital health tools to identify optimal treatment protocols and predict therapeutic responses. These tools could enable more precise dosing and timing of ENS-targeted treatments, further improving their effectiveness[254].

Advantages, challenges, and innovations

One of the key advancements in therapeutic strategies targeting the ENS lies in the development of pharmacological, bioelectronic, and regenerative therapies. Each approach brings its own set of advantages and challenges. Pharmacological treatments, such as serotonin receptor modulators and prokinetic agents, are effective in improving GI motility and addressing conditions like IBS and gastroparesis. However, their use is often limited by side effects, such as GI discomfort or CNS-related complications. On the other hand, bioelectronic therapies, including gastric electrical stimulation and sacral nerve stimulation, offer a promising non-pharmacological alternative, allowing for more targeted and less invasive interventions. While these therapies have shown potential in improving motility and reducing inflammation, their long-term effectiveness and safety remain uncertain, with issues such as device malfunction and patient response variability still under investigation. Regenerative therapies, including stem cell-based treatments and gene therapy, represent the most innovative approach, aiming to restore ENS function and address underlying pathologies. However, these therapies are still largely in the preclinical or early clinical stages, and concerns over their long-term safety, tumorigenic potential, and ethical implications must be carefully considered. Despite these challenges, the innovations brought by these therapies are transformative. They hold the potential not only to provide symptom relief but also to restore normal GI function by targeting the root causes of ENS dysfunction. With further clinical validation and refinement, these approaches could revolutionize the treatment of GI disorders, offering personalized and more effective therapeutic options for patients.

Challenges and limitations

The clinical translation of regenerative ENS therapies faces significant challenges despite promising preclinical results. The ENS's complexity requires transplanted cells to survive and integrate with existing neural circuits, from synaptic connections to coordinated reflex patterns. Ensuring safety is critical, especially with pluripotent cell therapies, as they may have tumorigenic potential. Rigorous testing and long-term follow-ups are essential to assess safety and therapy durability[255]. Regulatory pathways for regenerative ENS therapies are complex, involving biological products, medical devices, and growth factors. Coordinating across multiple regulatory frameworks is necessary, and international harmonization of standards will be key to advancing clinical development[256]. Clinical investigations for ENS-targeted therapies currently include pharmacological agents, bioelectronic devices, and regenerative medicine. Clinical trials face challenges, including the need for appropriate outcome measures that reflect both objective physiological parameters and subjective symptom improvements. Traditional endpoints, like gastric emptying studies, may not capture the full clinical benefits, highlighting the need for patient-reported outcomes that address quality of life[257]. Patient heterogeneity adds complexity to trial design, particularly for functional GI disorders with varying phenotypes. Stratifying patients based on pathophysiology, biomarkers, or genetic factors could optimize trial outcomes[258]. Biomarker development is crucial for advancing ENS-targeted therapies. Reliable biomarkers reflecting GI motility, neurotransmitter levels, inflammatory markers, and genetic variants could improve clinical management and enable personalized treatments. Technologies like wireless motility capsules and high-resolution manometry offer objective measures of ENS function, while molecular biomarkers may allow for less invasive monitoring[259]. Regulatory challenges for bioelectronic devices and regenerative medicine require long-term safety demonstrations and coordination across regulatory frameworks. Combination products integrating therapies could enhance outcomes but introduce additional regulatory complexities[260]. Despite progress, technical challenges remain in predicting therapeutic outcomes due to the ENS's complexity. Current animal models may not fully replicate human ENS physiology, limiting the translation of preclinical findings. Developing standardized manufacturing processes and quality control measures is crucial for ensuring safe and effective therapies[261]. Economic challenges also pose barriers to the development of ENS-targeted therapies. The high cost of regenerative medicine and specialized healthcare training may limit accessibility, particularly in resource-limited settings. Health economic studies proving cost-effectiveness will be crucial for securing reimbursement and improving patient access. Figure 2 demonstrates and summarizes the role of ENS as a therapeutic target.

Figure 2  Enteric nervous system as a target for various disorders.

CONCLUSION

The ENS plays a pivotal role in maintaining the homeostasis of the GI tract, and its dysfunction is increasingly recognized as a key contributor to several GI disorders. This review highlights the growing body of research surrounding therapeutic strategies targeting the ENS, with an emphasis on pharmacological, bioelectronic, and regenerative approaches. These therapies hold significant promise for addressing complex conditions such as IBS, gastroparesis, IBD, and congenital aganglionosis, among others. However, despite the encouraging preclinical findings, several challenges remain in the clinical translation of these therapies. The limited availability of large-scale clinical trial data for emerging treatments, such as stem cell therapies and bioelectronic devices, calls for a more robust evidence base to confirm their efficacy and long-term safety. Additionally, the complexity of the ENS and its intricate interactions with the CNS and gut microbiome pose significant hurdles in developing targeted therapies that are both effective and personalized. The potential of precision medicine in tailoring therapeutic strategies to individual patients based on genetic, microbiome, and ENS characteristics is an exciting avenue for future research. Advances in understanding the gut-brain axis and how it influences ENS function will also provide crucial insights into novel treatment paradigms. Furthermore, as we continue to explore the regenerative capacity of stem cells and bioelectronic interventions, it is essential to consider the ethical implications, risks, and potential side effects of these therapies. In conclusion, while the field of ENS-targeted therapies is still in its early stages, it represents a promising frontier in the treatment of GI disorders. The combination of innovative therapeutic modalities, the increasing understanding of ENS biology, and the integration of personalized approaches will likely lead to more effective, individualized treatment options in the near future. However, to fully realize the therapeutic potential of the ENS, ongoing clinical trials, long-term follow-up studies, and interdisciplinary collaboration are necessary to address the remaining scientific and clinical challenges.