Therapeutic potential of mesenchymal stem cells in neurodegenerative diseases

Abstract

Neurodegenerative diseases, such as Alzheimer’s disease, Parkinson’s disease, and Huntington’s disease, are characterized by the progressive loss of neuronal function and structure, leading to severe morbidity and mortality. Current therapeutic approaches are ineffective at stopping or reversing disease progression. Stem cell therapy has emerged as a promising candidate in research and treatment. Mesenchymal stem cells (MSCs) are considered ideal candidates for regenerative medicine because of their high proliferation rate and multi-differentiation potential. MSCs can differentiate into neurons and glial cells, modulate immune responses, and reduce inflammation, and their exosomes can promote neural repair and regulate neuronal function; thus, MSCs offer unique advantages for treating neurodegenerative diseases. However, challenges remain in optimizing cell delivery methods, ensuring the long-term survival and integration of transplanted cells, and fully understanding their therapeutic effects. This article primarily outlines the functions of MSCs in neurodegenerative diseases, with the intention that further research will fully harness their potential and translate these findings into clinical applications, offering new hope for patients suffering from neurodegenerative diseases.

Key Words: Mesenchymal stem cells; Neurodegenerative disease; Stem cell therapy; Immunomodulation; Paracrine

Core Tip: This review emphasizes the incidence and social burden of neurodegenerative diseases, focusing on disease pathogenesis and available therapeutic approaches. We have thoroughly investigated the potential of mesenchymal stem cells in treating neurodegenerative diseases and explored their therapeutic effects on different diseases. This review aims to elucidate how mesenchymal stem cells can alleviate neurodegenerative disease progression by modulating molecular pathways related to abnormal protein accumulation, neuroinflammation, oxidative stress, and neurodegeneration.

INTRODUCTION

Neurodegenerative diseases are disorders characterized by the progressive loss of neuronal function and structure in the central nervous system, and they are often accompanied by myelin damage and synaptic dysfunction[1].These conditions tend to gradually worsen over time, resulting in severe neurological deficits. These diseases include Alzheimer’s disease (AD), Parkinson’s disease (PD), Huntington’s disease (HD), amyotrophic lateral sclerosis (ALS) and Niemann-Pick disease type C (NPC). Neurodegenerative diseases are associated with aging; as age increases, misfolded protein accumulation in the brain eventually leads to progressive loss of cognitive and/or motor functions in patients[2]. Figure 1 summarizes the main pathogenic mechanisms and clinical manifestations of five neurodegenerative diseases. With the increasing aging of the global population, the incidence and prevalence of neurodegenerative diseases have significantly increased[3]. Neuronal degeneration not only severely affects patients’ quality of life, but has a series of secondary impacts, imposing a heavy burden on society[4]. The etiology of neurodegenerative diseases is multifaceted, with certain conditions such as AD, HD, and ALS being precipitated by genetic mutations. Moreover, a lack of exercise and unhealthy diet can also increase the risk of disease onset. Currently, interventions for neurodegenerative diseases such as AD, PD, HD, ALS, and NPC primarily focus on symptomatic treatment and non-pharmacological therapies, such as lifestyle modifications and peer and caregiver support[5].

Figure 1 Pathogenesis and clinical manifestations of five neurodegenerative diseases. This diagram summarizes the complex etiologies and phenotypic spectra of the five neurodegenerative diseases. AD: Alzheimer’s disease; PD: Parkinson’s disease; ALS: Amyotrophic lateral sclerosis; NPC: Niemann-Pick disease type C; HD: Huntington’s disease; Aβ: Amyloid-β; mHTT: Mutant Huntingtin; TDP-43: Transactive response DNA-binding protein 43.

The therapeutic management of AD primarily relies on pharmacological interventions, with cholinesterase inhibitors often used in combination with memantine. Notably, phase III trials of anti-amyloid-β (anti-Aβ) monoclonal antibodies, such as lecanemab and donanemab, have validated the clinical viability of disease-modifying therapies that target pathological proteins[6,7]. In PD, therapeutic strategies primarily focus on modulating α-synuclein (αSN) pathology, complemented by a well-established dopaminergic treatment system including levodopa, dopamine receptor agonists, and monoamine oxidase B inhibitors. The drug development pipeline for PD remains robust, with a wide variety of approaches being developed and evaluated in phase I and II. However, only a limited number of disease-modifying therapies have advanced to phase III[8]. Therapeutic options for ALS are currently relatively limited, and only three disease-modifying drugs are currently approved by the United States Food and Drug Administration (FDA). These agents target various pathological mechanisms, including excitotoxicity, oxidative stress, mitochondrial dysfunction, protein homeostasis, nucleocytoplasmic transport, and neuroinflammation. However, the therapeutic efficacy of these agents against this devastating disease remains modest[9]. This has stimulated a rapidly expanding pipeline of investigational drugs, and superoxide dismutase 1 (SOD1)-targeting antisense oligonucleotides have established a precedent for gene-specific therapies[10]. Tetrabenazine and deutetrabenazine are currently approved for treating HD-associated chorea, whereas several other investigational drugs are in clinical trials[11]. Therapeutic options for NPC remain limited, with only two United States FDA-approved disease-modifying drug currently available (Miplyffa and Aqneursa). Nevertheless, several promising investigational therapies, including intravenous cyclodextrin and acetyl-L-leucine, have demonstrated encouraging preliminary results in late-stage clinical trials[12]. However, no approaches have halted or reversed disease progression[4].

Stem cells are endowed with self-renewal capacity and the potential to differentiate into multiple lineages. Stem cell transplantation represents a promising and rapidly advancing therapeutic approach for several neurological disorders[13]. Stem cell-derived extracellular vesicles (EVs) are considered natural therapeutic agents and innate drug delivery systems for the treatment of brain diseases because of their low immunogenicity, high biosafety, ability to effectively penetrate the blood-brain barrier (BBB), and anti-inflammatory and antioxidant effects[14]. Furthermore, engineered EVs obtained by modifying natural EVs via gene editing, chemical modification or bioengineering techniques can achieve precise targeting of neurodegenerative disease lesions by decoration of their surfaces with targeting peptides, nanobodies, or other molecules[15,16]. Mesenchymal stem cells (MSCs) are adult stem cells that originate from the mesoderm and can self-renew and undergo multipotent differentiation into various lineages[17]. MSCs are easy to collect, isolate and culture, they exhibit low immunogenicity, and perform immunomodulatory functions; furthermore, they have the potential to differentiate into neurons and glial cells[18]. Therefore, MSCs and their derivatives hold great promise as therapeutic approaches for treating neurodegenerative diseases.

NEURODEGENERATIVE DIEASES

Although neurodegenerative diseases exhibit significant differences in clinical manifestations and pathological features, they share several common biological mechanisms. Protein misfolding and aggregation represent one of the core pathological hallmarks of neurodegenerative diseases[2]. Together with mechanisms such as mitochondrial dysfunction, oxidative stress, neuroinflammation, and synaptic dysfunction, protein misfolding and aggregation jointly drive neuronal degeneration and death[4]. Here, we provide an overview of the characteristics and traditional treatment methods for AD, PD, HD, ALS, and NPC.

AD

AD is a highly prevalent neurodegenerative disease characterized as progressive and irreversible[19]. It leads to varying degrees of mental and cognitive impairments, including impairments in attention, social cognition, processing speed, working memory, verbal learning, and visual learning, and poses a serious threat to human health[20,21]. In the United States, approximately 1 in 9 people aged 65 years and above currently have AD[22]. It is projected that the number of individuals worldwide with AD will exceed 100 million by 2050[23]. People 65 years and older can survive an average of 4-8 years after being diagnosed with AD[19].

The etiology of AD is complex and is influenced by a combination of factors, including age, genetics, and the environment[24]. Currently, the mainstream pathophysiology of AD is believed to involve the progressive accumulation of amyloid plaques (also known as senile plaques) and neurofibrillary tangles, which lead to the loss of neurons related to cognition and memory in the brain[25-27]. This results in brain atrophy, depletion of cholinergic neurons and acetylcholine, and, consequently, synaptic dysfunction[28-31].

The approved drugs for AD can be categorized into three main classes: Acetylcholinesterase inhibitors, N-methyl-D-aspartate receptor modulators (memantine), and anti-Aβ monoclonal antibodies[32-34]. However, these available drugs can only provide symptomatic relief for AD patients and are not capable of effectively curing the disease. In the past few decades, most clinical drugs have been discontinued because of their limited efficacy or adverse reactions[35]. Although the anti-Aβ monoclonal antibodies abucanumab, which was approved by the United States FDA in June 2021, and lecanemab, which was approved in July 2023, have shown good therapeutic effects, their long-term effectiveness and safety require further validation[21]. The multifactorial nature of AD onset, compounded by the intricate interplay among various factors, presents formidable challenges for drug development and substantially raises the bar for the next generation of AD therapeutics[21].

PD

PD is the second most prevalent neurodegenerative disorder in the world and affects more than 6 million people globally, posing a significant threat to human health[36-39]. It is characterized by the progressive degeneration of dopaminergic neurons and the formation of αSN protein aggregates in the substantia nigra, which are known as Lewy bodies and Lewy neurites[40-42]. These pathological changes lead to motor symptoms, such as bradykinesia, muscle stiffness, resting tremor, and postural instability, as well as nonmotor symptoms such as hyposmia, depression, sleep disturbance, and cognitive deficits[36,43,44]. The exact mechanisms that lead to neuronal death in PD are not fully understood, but they are likely multifactorial and involve neuroinflammation, mitochondrial dysfunction, oxidative stress, genetic factors, glial dysfunction, excitotoxicity and other factors[45,46].

To date, there are no effective treatments to reverse the pathological progression of dopaminergic neurodegeneration[47]. Traditional treatment methods primarily include pharmacological therapy, surgical therapy, and physical therapy[48]. Levodopa, dopamine agonists, monoamine oxidase B inhibitors, and catechol-O-methyltransferase inhibitors are common drug treatments for PD, but long-term administration may reduce their efficacy and lead to side effects, including involuntary motor movements and movement disorders, affecting patients’ quality of life[49]. Deep brain stimulation (DBS), including subthalamic nucleus DBS and medial globus pallidus DBS, is used to treat motor fluctuations and dyskinesia. This treatment can reduce the dependence of patients on drugs such as levodopa, reduce side effects caused by drugs and improve patient quality of life. However, the destruction of neural nuclei caused by DBS surgery may lead to irreversible nerve injury[50,51]. Physical therapy plays an important role in the treatment of PD, helping to increase muscle strength and flexibility of patients[52]. Nevertheless, the effect of physical therapy is limited and requires continuous investment. In general, the treatment of PD faces many challenges, and new therapeutic methods and strategies to improve patient quality of life are needed.

HD

HD is a fatal, incurable autosomal dominant inherited neurodegenerative disorder caused by an autosomal dominant mutation in the Huntingtin (HTT) gene[53]. The molecular pathology of HD is complex and involves aggregate formation, transcriptional dysregulation, synaptic plasticity alterations, and glial dysfunction[54]. Clinically, HD patients exhibit uncontrolled movements, mood disorders, and progressive cognitive dysfunction[54,55]. As the disease progresses, it may develop into dystonia, rigidity, and ataxia[56].

Treatment for HD primarily focuses on the symptomatic management of chorea, as well as psychiatric and cognitive symptoms. Tetrabenazine and deuterated tetrabenazine, which are United States FDA-approved medications, are effective in reducing HD-associated chorea[57,58]. HD-associated dementia symptoms can be treated with clozapine or acetylcholinesterase inhibitors, albeit with limited success[11]. For patients who exhibit suboptimal responses to pharmacological treatments or who experience severe adverse drug reactions, DBS may present an effective therapeutic option. However, current therapies are primarily aimed at alleviating symptoms rather than reversing or halting neuronal damage. Consequently, more clinical trials and research studies are needed to further explore treatment options for HD[59].

ALS

ALS is a severe neurodegenerative disease characterized by progressive muscle paralysis due to motor neuron degeneration in the motor cortex of the brainstem and spinal cord[60]. The primary clinical symptoms include progressive muscle weakness, muscle atrophy, dysphagia, and dysarthria, and in rare cases, episodes of respiratory failure[61,62]. As a heterogeneous syndrome, ALS pathogenesis is very complex and involves numerous different cellular mechanisms such as oxidative stress, mitochondrial dysfunction, glutamate excitotoxicity, axonal transport, impaired protein degradation, hypermetabolism, and RNA defects[63]. The global prevalence and incidence rates of ALS are estimated at 4.42 cases per 100000 people and 1.59 cases per 100000 person-years, respectively[64]. Most patients die within 2-5 years after initial symptom onset[65]. The United States FDA has approved three drugs for the treatment of ALS, namely riluzole, edaravone, and tofersen injection[9]. However, the contributions of these drugs to extending patient survival time and improving quality of life are limited, and they also have side effects on other organs[66-68].

NPC

NPC is a neurodegenerative lysosomal cholesterol storage disorder caused by mutations in the NPC1 or NPC2 genes. NPC is characterized by the accumulation of cholesterol and glycolipids within the endosomal/Lysosomal system, leading to hepatosplenomegaly and progressive neurodegeneration[12]. The disease is classified into the following categories according to the age of symptom onset: Early infantile form (onset before 2 years), late infantile form (onset between 2 and 6 years), juvenile form (onset between 6 and 15 years), and adult form (onset after 15 years)[69].

The first approved therapeutic agent with the potential to modify NPC progression was miglustat, an inhibitor of glycosphingolipid synthesis[70]. Miglustat has several limitations in the treatment of NPC, as it cannot completely prevent disease progression and does not easily restore damaged tissues and organs to a normal state. Furthermore, its applicability is limited to a subset of patients and is accompanied by numerous adverse effects. Recently, Miplyffa (arimoclomol) and Aqneursa (levacetylleucine) have been approved by the United States FDA for the treatment of NPC[71,72]. When used in combination with miglustat, Miplyffa can improve the neurological symptoms associated with NPC in children aged 2 years and older and in adults[73]. However, the drug may cause allergic reactions such as urticaria and angioedema, as well as side effects including respiratory tract infections, diarrhea, and weight loss. Early diagnosis and treatment are crucial for improving patient outcomes. Future research is necessary to gain a better understanding of the pathomechanism of these diseases and to explore new therapeutic options[12].

MSCS AND THEIR MECHANISMS IN NEURODEGENERATIVE DISEASE TREATMENT

MSCs are a class of adult stem cells with multidirectional differentiation potential, self-renewal capacity, and immunomodulatory properties, that are widely present in a variety of tissues[17]. Many preclinical studies and clinical trials have demonstrated that MSCs have great potential in the treatment of neurodegenerative diseases.

MSC classification and characteristics

MSCs can be extracted from a variety of tissues, such as the bone marrow, umbilical cord, placenta, adipose tissue, and dental pulp (Table 1). MSCs from different sources have tissue-specific differentiation potential. To describe MSCs more specifically, the abbreviation of the source tissue is typically added before their name[74]. The Mesenchymal and Tissue Stem Cell Committee of the International Society for Cellular Therapy has established the minimal criteria for defining human MSCs. First, MSCs must be plastic-adherent under standard culture conditions. Second, MSCs must express CD105, CD73, and CD90, but lack expression of the CD45, CD34, CD14 or CD11b, CD79a or CD19, and HLA-DR surface molecules. Third, MSCs must be able to differentiate into osteoblasts, adipocytes, and chondroblasts in vitro[75].

Table 1 Classification and characteristics of mesenchymal stem cells.

Type	Advantage	Limitations	Ref.
BM-MSCs	First to be discovered, abundant sources, possesses good differentiation capacity and immune regulatory functions	Invasive collection process, age-related decline in proliferation capacity and differentiation potential	[76-79]
ADSCs	Abundant sources, relatively easy collection process	Cell quality influenced by the age, weight, and disease status of donor	[80,81]
UC-MSCs	Convenient collection, no ethical issues, strong proliferative capacity, high differentiation potential, and low immunogenicity	Requires establishment of cord blood banks for preservation and management	[82,83]
PMSCs	Easily accessible, convenient collection, limited ethical concerns and low immunogenicity	Placental tissue has complex origin and requires strict screening and processing, needs to be collected from a healthy mother giving birth to a healthy baby	[84]
DPSCs	No ethical concerns, low immunogenicity, and high regenerative potential	Limited opportunities for collection	[85,86]
MenSCs	No ethical concerns, low immunogenicity, and strong cell proliferation capacity	Proliferation capacity influenced by donor age, passage number, and storage time	[87,88]

BM-MSCs: Bone marrow-derived mesenchymal stem cells; ADSCs: Adipose-derived mesenchymal stem cells; UC-MSCs: Umbilical cord-derived mesenchymal stem cells; PMSCs: Placenta-derived mesenchymal stem cells; DPSCs: Dental pulp-derived mesenchymal stem cells; MenSCs: Menstrual blood-derived endometrial stem cells.

Bone marrow-derived MSCs (BM-MSCs) were the first adult MSCs to be discovered and have become one of the most widely used sources of MSCs[76]. BM-MSCs exhibit well-defined differentiation potential, robust immunomodulatory capacity and broad clinical applicability. BM-MSCs are typically isolated from bone marrow aspirates obtained via iliac crest or sternal puncture[77]. Despite the invasive harvesting procedure, limited initial cell yield, and age-dependent decline in donor cells, BM-MSCs possess remarkable expansion capacity and have tremendous potential in bone repair, cartilage regeneration, and immunomodulation[78,79]. The primary advantages of BM-MSCs over other types of MSCs include the abundance of tissue sources for isolation, the ease of tissue collection and cell isolation, and their therapeutic potential.

Adipose-derived MSCs (ADSCs) are predominantly derived from adipose tissue within the human body. ADSCs have the advantages of abundant sources, minimally invasive harvesting procedures, high cell yields, and rapid proliferation rates. ADSCs also possess superior adipogenic differentiation ability and are more suitable for autologous transplantation. ADSC quality is influenced by donor age, weight, and disease status[80]. ADSCs do not express the adhesion marker CD106 or podocalyxin-like, which distinguishes them from BM-MSCs[81].

Human umbilical cord-derived MSCs (hUC-MSCs) and placenta-derived MSCs (PMSCs) are isolated from the umbilical cord and placenta, respectively. Both types of MSCs are characterized by noninvasive harvesting techniques, low immunogenicity, robust proliferative capacity, and fewer ethical issues. These features make them suitable for allogeneic transplantation and endow them with unique advantages in clinical applications[82-84]. However, the placenta is a complex tissue, and it is important to obtain it from healthy mothers who have given birth to healthy babies. Dental pulp-derived MSCs (DPSCs) are isolated from the pulp of exfoliated deciduous teeth or wisdom teeth. Although the opportunities for acquisition are limited, DPSCs raise no ethical concerns and possess low immunogenicity and high regenerative potential[85,86]. Additionally, MSCs can also be isolated from synovial tissue and menstrual blood, expanding the range of accessible sources for regenerative therapies[87]. Menstrual blood-derived endometrial stem cells (MenSCs) have increased proliferation rates and low immunogenicity with few ethical issues, and they can be obtained noninvasively; therefore, MenSCs have been extensively studied in recent years[88].

Therapeutic mechanisms of MSCs

The therapeutic mechanisms of MSCs involve several complex and interrelated biological processes, including homing, differentiation capacity, paracrine effects, and neuroprotective actions (Figure 2).

Figure 2 The therapeutic mechanisms of mesenchymal stem cells. Mesenchymal stem cells (MSCs) migrate to the injury or inflammation sites through the homing effect. MSCs possess multidirectional differentiation potential and can differentiate into both mesodermal and non-mesodermal lineages. Moreover, MSCs promote tissue repair by secreting exosomes and bioactive molecules. Furthermore, MSCs demonstrate significant neuroprotective properties through multiple pathways. MSCs: Mesenchymal stem cells; CXCR: C-X-C chemokine receptor; SDF-1: Stromal cell-derived factor 1; Exos: Exosomes; IL: Interleukin; TGF: Transforming growth factor; MAC: Membrane attack complex; TNF: Tumor necrosis factor.

Homing: The homing mechanism of MSCs involves multiple steps and the interaction of various molecules[89]. First, when MSCs enter the bloodstream, CD44 on their surface binds transiently and reversibly to selectins, such as E-selectin, P-selectin, and L-selectin on endothelial cell surfaces, resulting in a rolling phenomenon. During this rolling process, MSCs encounter chemokines, such as stromal cell-derived factor 1 produced by local tissues. These chemokines bind to G-protein-coupled chemokine receptors on the surface of MSCs, activating intracellular signaling pathways. MSCs express various chemokine receptors, such as C-X-C chemokine receptor type 4 (CXCR4) and CXCR7. After binding to their receptors, chemokines activate signaling pathways within MSCs, leading to conformational changes that activate integrins, enhancing MSC adhesion to endothelial cells[90]. After activation, the integrins on MSCs bind to their corresponding ligands on endothelial cell surfaces, forming a strong adhesion that allows MSCs to stably attach to endothelial cells. MSCs secrete various proteases, such as matrix metalloproteinases, which can degrade the extracellular matrix and the connections between endothelial cells. Through these proteases, MSCs can break through the endothelial layer and enter the tissue interstitium. Subsequently, MSCs migrate along the chemokine concentration gradient and eventually reach the site of injury or disease. These MSCs then exert their therapeutic effects, promote tissue regeneration, and regulate the immune response[91].

Differentiation: MSCs possess multipotent differentiation capacity, enabling their commitment to both mesodermal and non-mesodermal lineages. In addition to their well-documented osteogenic, adipogenic, and chondrogenic potential, MSCs can differentiate into myocytes, cardiomyocytes, fibroblasts, myofibroblasts, epithelial cells, and neuron-like cells, highlighting their remarkable plasticity[92]. MSCs isolated from different tissues exhibit strong phenotypic similarity in their differentiation potential, while their lineage commitment is significantly influenced by in vitro culture conditions, particularly growth factor stimulation[93]. Culturing conditions, such as passage number, cell density, and oxygen concentration, can affect MSC differentiation efficiency. MSC multipotency has broad prospects for regenerative medicine. By optimizing induction conditions and the microenvironment, the differentiation efficiency of MSCs can be further increased, providing new strategies for clinical treatment[94].

Paracrine signaling: MSCs modulate the microenvironment and promote tissue repair by secreting exosomes (Exos) and bioactive molecules, including cytokines, chemokines, growth factors and EVs. MSCs produce extensive secretemany factors, such as interleukin (IL)-10, stromal cell-derived factor 1, IL-6, transforming growth factor-β (TGF-β) and some growth factors and chemokine ligands[95]. Although initial studies suggested that MSCs may play a pivotal role in tissue repair, the paracrine efficacy of MSCs can be affected by in vitro factors, including number of passages, culture conditions and donor source, as well as in vivo administration methods. Accumulating evidence has revealed low survival rate and engraftment potential in damaged tissue regions, significantly limiting their therapeutic efficacy in regenerative applications[96-98]. Further research indicates that the therapeutic benefits of MSCs in tissue repair are primarily due to their paracrine signaling, which involves EV secretion[99]. Emerging studies have demonstrated that Exos can function as substitutes for whole MSC-based therapies in various injury and disease models, offering a cell-free alternative with comparable therapeutic effects[100].

Neuroprotection: In neurodegenerative diseases and neural injuries, MSCs perform significant anti-inflammatory functions. MSCs exert neuroprotective effects by secreting TGF-β and IL-10 to suppress the microglial release of proinflammatory factors (e.g., tumor necrosis factor-α and IL-6) and promote microglial polarization toward the anti-inflammatory M2 phenotype, thereby mitigating neuronal damage[101]. MSCs also express CD59, which inhibits the formation of the membrane attack complex, thereby modulating the complement system to protect neurons[102]. Through paracrine secretion of neurotrophic factors and direct differentiation, MSCs promote axonal regeneration and synaptic remodeling[103,104]. In addition, MSCs can regulate neuronal cell survival and function through secreted Exos and microRNAs[105], and MSCs can further improve neuronal function through electrophysiological modulation and metabolic regulation[106].

PRECLINICAL RESEARCH ON MSCS AND THEIR DERIVATIVES IN TREATING NEURODEGENERATIVE DISEASES

The bioactive substances released by MSCs through paracrine action, collectively known as MSC derivatives, possess immunoregulatory and regenerative repair potential similar to that of MSCs[107-109]. Owing to their low immunogenicity, low risk of tumor formation, and favorable safety profile, MSC derivatives may be superior to MSC transplantation in disease treatment[110-112]. Herein, we summarize the preclinical findings of MSCs and their derivatives in AD, PD, HD, ALS and NPC.

MSCs and their derivatives in the treatment of AD

In recent years, substantial advances have been achieved in the application of MSCs and their derivatives for the treatment of AD. A multitude of preclinical studies utilizing diverse animal models have consistently demonstrated the neuroprotective potential of MSCs and their derivatives. These effects include reducing Aβ deposition, modulating neuroinflammation, and promoting neuroregeneration, ultimately leading to improvements in AD-associated cognitive impairment. Table 2 provides a comprehensive summary of the current major studies on MSCs and their derivatives in the context of AD treatment.

Table 2 Summary of therapeutic efficacy and safety of mesenchymal stem cells and their derivatives in Alzheimer’s disease.

Animal model	Source	Type	Administration method	Main outcomes	Ref.
C57BL/6 mice with Aβ25-35	Mice	BM-MSCs	IV	Upregulate BDNF expression, downregulate GSK-3β activity, improve cognitive dysfunction	[116]
C57BL/6 mice with Aβ25-35	Mice	BM-MSCs	IV	Inhibit microglial activation, improve behavioral deficits, reduce neuroinflammatory cytokines	[115]
APP/PS1 mice	Human	hUC-MSCs, SHED, ADSCs	IV	Reduce amyloid plaques, improve behavioral deficits, and increase neuronal and Nissl body density in brain regions	[117]
APPswe/PS1dE9 mice	Human	OM-MSCs	SI	Alleviate AD symptoms and promote Aβ clearance	[118]
C57BL/6 mice with Aβ1-42	Human	BM-MSC-EVs	SI	Stimulate neurogenesis in the subventricular zone, alleviate cognitive impairment	[121]
C57BL/6 mice with STZ	Human	iPSC-MSC-sEVs	SI	Alleviate neuroinflammation, reduce amyloid deposition and neuronal apoptosis, and mitigate cognitive dysfunction	[120]
C57BL/6 mice with AlCl3	Mice	BM-MSC-EVs	IP	Regulate autophagy through the PI3K/AKT/mTOR pathway, promote Aβ degradation, modulate immunity, and improve memory and neurological dysfunction	[122]
C57BL/6 mice with AlCl3	Mice	ADSC	IV	Reduce amyloid deposition, mitigate cognitive dysfunction	[119]
Zebrafish with LPS	Human	ADSC-EVs	IV	Reduce LPS-induced inflammatory cytokines	[123]
C57BL/6 mice with Aβ1-42	Mice	BM-MSCs	IV	Induce mitophagy in neuronal cells, alleviating mitochondrial damage-mediated apoptosis and NLRP3 inflammasome activation	[125]
C57BL/6 mice with STZ	Mice	BM-MSC-Exos	SI	Modulate hippocampal glial cell activation, alleviate neuroinflammation, mitigate cognitive dysfunction	[127]
APP/PS1 mice	Human	hUC-MSCs	IV	Inhibit glial cell activation and oxidative stress	[128]
APP/PS1 mice	Mice	BM-MSCs	HIPP injection	Reduce number of Aβ plaques and increase M2 microglial polarization	[101]
C57BL/6 mice	Mice	BM-MSCs	SI	Stimulate endogenous neurogenesis	[130]
C57BL/6 mice with Aβ1-42	Human	OE-MSCs	IN	Upregulate BDNF and NMDAR; reduce neuronal loss	[132]
C57BL/6 mice with STZ	Human	hUC-MSC-Exos	IV	Increase adiponectin levels and protect neurons	[133]
C57BL/6 mice with AlCl3	Mice	ADSCs	IV	Reduce amyloid deposition, mitigate cognitive dysfunction	[119]
SH-SY5Y cell with Aβ1-40	Human	Exosomes derived from the serum of AD patients	Coculture	Reduce apoptosis through the PI3K/AKT signaling pathway	[134]
APP/PS1 mice	Human	hUC-MSCs	IV	Enhance targeting ability of hUC-MSCs and promote production of neuroprotective factors; improve cognitive function	[136]

Aβ: Amyloid-β; BM-MSCs: Bone marrow-derived mesenchymal stem cells; IV: Intravenous injection; BDNF: Brain-derived neurotrophic factor; GSK-3β: Glycogen synthase kinase-3 beta; APP: Amyloid precursor protein; hUC-MSCs: Human umbilical cord-derived mesenchymal stem cells; SHED: Stem cells from human exfoliated deciduous teeth; ADSCs: Adipose-derived mesenchymal stem cells; OM-MSCs: Olfactory mucosa mesenchymal stem cells; SI: Stereotaxic injection; AD: Alzheimer’s disease; BM-MSC-EVs: Bone marrow mesenchymal stem cell-derived extracellular vesicles; STZ: Streptozotocin; iPSC-MSC-sEVs: Induced pluripotent stem cells-derived mesenchymal stem cell-derived small extracellular vesicles; AlCl₃: Aluminum chloride; IP: Intraperitoneal injection; PI3K: Phosphatidylinositol 3-kinase; AKT: Proteinkinase B; mTOR: Mechanistic target of rapamycin; LPS: Lipopolysaccharide; ADSC-EVs: Adipose-derived mesenchymal stem cell-derived extracellular vesicles; BM-MSC-Exos: Bone marrow-derived mesenchymal stem cells-derived exosomes; NLRP3: NOD-like receptor family, pyrin domain containing 3; HIPP: Hippocampus; IN: Intranasal administration; OE-MSCs: Olfactory ecto-mesenchymal stem cells; NMDAR: N-methyl-D-aspartate receptor; hUC-MSC-Exos: Human umbilical cord-derived mesenchymal stem cell-derived exosomes.

Remove abnormally accumulated proteins: Aβ accumulation is a general feature of AD[113,114]. Intravenous injection of BM-MSCs during pregnancy can reduce the Aβ burden induced by Aβ25-35 Levels in female mice, thereby improving cognitive dysfunction[115,116]. The administration of MSCs derived from the umbilical cord, dental pulp, and adipose tissue via the tail vein can significantly reduce Aβ levels, behavioral deficits, and phosphorylated tau in the hippocampus and frontal cortex of APP/PS1 model mice, while increasing neuronal count and Nissl body density in brain regions[117]. Olfactory mucosa MSC (OM-MSC) transplantation through brain stereotaxic injection can alleviate AD symptoms and promote Aβ clearance[118]. Intravenous injection of ADSCs may inhibit amyloid production induced by intraperitoneal aluminum chloride injection in AD rats by activating of the sirtuin-1 signaling pathway, thereby alleviating cognitive dysfunction[119]. Therefore, MSCs can improve AD-associated cognitive impairment by reducing Aβ deposition.

MSC derivatives can also reduce Aβ accumulation in AD. For example, the intracisternal administration of BM-MSC-derived Exos or EVs can mitigate the cognitive dysfunction induced by Aβ1-42 or streptozotocin (STZ)[120,121]. In addition, intraperitoneal injection of BM-MSC-derived EVs(BM-MSC-EVs) can regulate autophagy through the phosphatidylinositol 3-kinase/protein kinase B/mechanistic target of rapamycin (PI3K/AKT/mTOR) axis, promote Aβ degradation, regulate immunity, and ameliorate memory and neurological dysfunction[122]. Compared to stem cell therapies, EV therapies exhibit superior translational potential. EVs circumvent critical limitations of cell-based approaches, including tumorigenic risks, immunogenicity, and pulmonary entrapment after systemic administration. These attributes position EVs as promising next-generation alternatives to stem cell therapies.

Neuroinflammation reduction: Persistent reactive oxygen species (ROS) and neuroinflammation are key factors that contribute to AD[123]. Intravenous injection of ADSC-derived EVs into the zebrafish larval brain significantly decreased the lipopolysaccharide-induced levels of proinflammatory cytokines and oxidative stress[123]. The NOD-like receptor family, pyrin domain containing 3 (NLRP3) inflammasome is a multimeric protein complex that participates in the innate immune system and plays a vital role in inflammatory reactions[124]. The intracisternal administration of BM-MSC-EVs inhibits neuroinflammation and mitigates cognitive dysfunction in AD mice by suppressing the NLRP3/gasdermin D pathway mediated by miR-223-3p[120]. Nanoscale EVs prepared from BM-MSCs with high tyrosine phosphatase-2 expression can significantly induce mitophagy in neuronal cells, thereby alleviating mitochondrial damage-mediated apoptosis and NLRP3 inflammasome activation[125]. Therefore, MSC-derived EVs inhibit neuroinflammation primarily through the NLRP3-mediated signaling pathway and restore the damaged neuronal microenvironment[126].

Activated microglia are among the key sources of neuroinflammation. BM-MSCs not only reduce the number and size of activated microglia, but also markedly increase dendrite length and reverse proinflammatory cytokine elevation in offspring[116]. Furthermore, lateral ventricle administration of BM-MSC-derived Exos can regulate hippocampal glial cell activation and related neuroinflammation in an STZ-induced sporadic AD mouse model, thus improving the cognitive ability of AD mice[127]. Intravenous injection of hUC-MSCs inhibits glial cell activation and oxidative stress by activating nuclear factor erythroid 2-related factor signaling[128]. Stereotaxic injection of OM-MSCs or BM-MSCs into the brain can downregulate the inflammatory response by increasing the ratio of M2/M1 microglia[101,118]. Therefore, MSCs and their derivatives can reduce neuroinflammation by inhibiting proinflammatory cytokines, ameliorating oxidative stress, and reducing glial cell activation.

Promotion of neurogenesis: Progressive neuronal loss in the brain tissue of AD patients leads to cognitive deficits[121,129]. Stereotaxic injection of BM-MSCs into the subventricular zone of mice can stimulate endogenous neurogenesis and exert a neuroprotective effect on the hippocampus of AD rats[130,131]. Intranasal administration of olfactory ecto-MSCs (OE-MSCs) significantly reduced neuronal loss by increasing the levels of brain-derived neurotrophic factor (BDNF) and N-methyl-D-aspartate receptor in an AD rat model[132]. Intravenous injection of hUC-MSC-derived Exos into an STZ-induced AD mouse model increased adiponectin levels in the peripheral central nervous system and promoted neuroprotective function by improving adult neurogenesis[133]. Additionally, this treatment reduced the apoptotic response in AD through the PI3K/AKT/mTOR signaling pathway[134]. hUC-MSC-derived EVs can treat the pathological characteristics of the APP/PS1 mouse model by regulating the synaptic vesicle cycle signaling pathway[135]. When coated with Fe₃O₄@PDA, hUC-MSC targeting efficiency can be increased and the memory and cognitive ability of AD mice can be improved via the excessive generation of neuroprotective factors[136]. Therefore, the transplantation of MSCs and their derivatives can induce the sustained production of neurotrophic factors such as BDNF and nerve growth factor, making them excellent therapeutic agents for neuronal repair and regeneration[137,138].

MSCs and their derivatives in treating PD

There has been a recent surge of interest in exploring the therapeutic potential of MSCs and their derivatives for the treatment of PD. A growing body of evidence suggests that MSCs exert neuroprotective effects through multiple mechanisms, such as promotion of dopaminergic neuron survival, suppression of neuroinflammation, reduction of αSN aggregation, and increased secretion of neurotrophic factors. Moreover, MSCs derived from different sources have varying degrees of therapeutic efficacy in animal models of PD. Table 3 provides a systematic summary of the key studies investigating MSC-based therapies for PD.

Table 3 Summary of therapeutic efficacy and safety of mesenchymal stem cells and their derivatives in Parkinson’s disease.

Animal model	Source	Type	Administration method	Main outcomes	Ref.
C57BL/6 mice with MPTP	Human	hUC-MSCs	IN	Inhibit activated glial cells, repair dopamine neuron degeneration, improve motor behavior in mice	[158]
C57BL/6 mice with 6-OHDA	Human	hUC-MSCs	IN	Upregulate SATB1, activate Wnt/β-catenin pathway, improve motor behavior, reduce neuronal damage	[159]
C57BL/6 mice with 6-OHDA	Human	hUC-MSCs	ST	Increase dopamine levels and improve motor dysfunction	[160]
C57BL/6 mice with 6-OHDA	Human	hUC-MSCs	ST	Improve rotational behavior, provide neuroprotection, exhibit anti-neuroinflammatory effects	[161]
C57BL/6 mice with 6-OHDA	Human	hUC-MSCs-BDNF	IV	Increase neuronal survival rate	[162]
C57BL/6 mice with 6-OHDA	Human	DPSCs	SI	Promote recovery of dopaminergic neurons, reduce dopaminergic neuron loss, improve motor behavior	[163]
C57BL/6 mice with 6-OHDA	Human	DPSCs	IV	Improve rotational and forelimb asymmetry behaviors, enhance anti-apoptotic Bcl-2/Bax axis	[164]
Zebrafish with 6-OHDA	Human	DPSCs	Yolk sac injection	Improve motor dysfunction	[166]
Zebrafish with rotenone	Mice	BM-MSCs	IV	Improve motor and behavioral performance	[167]
C57BL/6 mice with MPTP	Mice	BM-MSCs	IN	Restore dopaminergic neurons in the substantia nigra and nerve terminals in the striatum, improve motor deficits	[168]
C57BL/6 mice with 6-OHDA	Human	OE-MSCs	IN	Improve motor dysfunction	[169]
C57BL/6 mice with MnCl2	Human	hnmMSC-sEVs	IN	Restore motor dysfunction and enhance neurogenesis	[170]
C57BL/6 mice with MPTP	Human	hUC-MSC-Exos	IN	Increase number of dopaminergic neurons in the SNPC region, rescue death of substantia nigra dopaminergic neurons, alleviate inflammatory responses, and improve local microenvironment	[149]
C57BL/6 mice with 6-OHDA	Human	hUC-MSC-Exos	IV and LV	Repair damage to the nigrostriatal dopamine system, inhibit microglial activation	[150]
C57BL/6 mice with MPTP	Human	hUC-MSCs	IV	Alleviate dopaminergic neuron degeneration, exhibit anti-inflammatory effects	[151]
PD patients	Human	OM-MSCs	IT	Promote recovery of neural function, modulate neuroinflammation	[152]
C57BL/6 mice and cells with MPTP	Human	T-MSC-Exos	IV	Protect DA neurons through the Nox4-ROS-Nrf2 axis, maintain function of the nigrostriatal system, improve motor deficits, and reduce oxidative stress	[153]
C57BL/6 mice with MPTP	Human	BM-MSCs	IV	Reduce neuronal loss, damage, and inflammatory responses, inhibit cell apoptosis	[154]
C57BL/6 mice with MPTP	Human	ADSCs	SI	Secrete neuroprotective factors to prevent neuronal damage, reduce dopaminergic neuron loss and alleviate neuroinflammation; protect dopaminergic neurons	[155]
SH-SY5Y cells with rotenone	Human	NI-hADSC-CM	Coculture	Provide neuroprotection, alleviate αSN aggregation	[143]
Moderate PD patients	Human	BM-MSCs	IV	Safe, well tolerated, and not immunogenic	[142]
C57BL/6 mice with AAV 1/2A 53 T-a-syn	Human	BM-MSCs	SI	Reduce αSN levels; protect dopaminergic neurons, modulate microglial cells	[145]
C57BL/6 mice with rotenone	Mice	BM-MSCs	SI, IV	Improve motor function, protection of the nigrostriatal system, and improve striatal dopamine release	[146]

MPTP: 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine; hUC-MSCs: Human umbilical cord-derived mesenchymal stem cells; IN: Intranasal administration; 6-OHDA: 6-hydroxydopamine; SATB1: Special AT-rich sequence-binding protein-1; ST: Striatal transplantation; BDNF: Brain-derived neurotrophic factor; IV: Intravenous injection; DPSCs: Dental pulp-dirived mesenchymal stem cells; SI: Stereotaxic injection; Bcl-2: B-cell lymphoma 2; Bax: B-cell lymphoma 2-associated X protein; BM-MSCs: Bone marrow-derived mesenchymal stem cells; OE-MSCs: Olfactory ecto-mesenchymal stem cells; hnmMSC-sEVs: Human nasal mucosal mesenchymal stem cells derived small extracellular vesicles; SNPC: Substantia nigra pars compacta; hUC-MSCs-Exos: Human umbilical cord-derived mesenchymal stem cells derived exosomes; LV: Lateral ventricle injection; PD: Parkinson’s disease; OM-MSCs: Olfactory mucosa mesenchymal stem cells; T-MSC-Exos: Trophoblast stage-derived mesenchymal stem cell-derived exosomes; DA: DArgic; Nox4: NADPH oxidase 4; ROS: Reactive oxygen species; Nrf2: Nuclear factor erythroid 2-related factor 2; SH-SY5Y: Human neuroblastoma cell line SH-SY5Y; NI-hADSC-CM: Neural-induced human adipose tissue-derived stem cell-conditioned medium; αSN: α-synuclein.

Reduction in αSN aggregate accumulation: One of the main causes of PD is the accumulation of αSN protein aggregates[139]. αSN aggregates can be transmitted to neighboring cells in the form of Lewy bodies and Lewy neurites, thus spreading to a large area of the brain with severe damage[140,141]. Therefore, inhibiting aggregation or directing self-assembly to less toxic aggregates could be therapeutic. In fact, numerous experiments have shown that the secretome of MSCs can trigger neuronal differentiation and protect dopaminergic neurons in vitro and in vivo, reversing the PD phenotype in 6-hydroxydopamine (6-OHDA)-induced PD models[142,143]. The hybrid of Exos from hUC-MSCs and nanoliposomes containing baicalein and oleuropein decreased αSN pathogenicity and increased drug internalization into cells, surpassing the efficacy of nanoliposomes alone; additionally, this formulation crossed the BBB in an in vitro cellular model[144].

Injection of BM-MSCs into the substantia nigra pars compacta (SNPC) and striatum significantly reduced αSN levels in the SNPC in AAV 1/2A 53 T-a-syn-induced neurodegeneration[145]. In addition, intracerebral and tail vein injection of BM-MSCs ameliorated the motor function of PD model rats induced by subcutaneous injection of rotenone and normalized tyrosine hydroxylase, 3,4-dihydroxyphenylalanine decarboxylase, and αSN expression to levels comparable to controls in the nigral and striatal tissues of male rats[146].

Curcumin (Cur) can effectively reduce αSN aggregates in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced PD mice[147,148]. The intranasal administration of a microfluidic electroporation chip prepared with MSC-derived neuron-like cell membrane-coated Cur nanoparticles can significantly increase the levels of several neurotransmitters and ameliorate movement disorders in PD mice[106]. A self-oriented nanocarrier termed PR-EXO/PP@Cur, which combines therapeutic MSC-derived Exos with Cur, can improve the motor and coordination abilities of mice with MPTP-induced PD and promote neuronal function recovery following intranasal administration[147].

Alleviation of inflammatory responses: Intranasal administration of hUC-MSC-derived Exos reverses dopaminergic neuron death in mice with MPTP-induced PD and reduces the inflammatory response[149]. hUC-MSC-derived Exos can also be absorbed by dopaminergic neurons and microglia in the substantia nigra, repairing nigral-striatal dopamine system damage and inhibiting microglial activation[150]. Tail vein injection of hUC-MSCs attenuate MPTP-induced microglial activation in the striatum and NLRP3 inflammasome levels[151]. In addition, trophoblast stage-derived MSCs, BM-MSC-derived Exos, ADSCs, and MenSC conditioned medium can also inhibit MPP⁺-induced cytotoxicity and ameliorate neuroinflammation through different reaction mechanisms[152-156].

Restoration of dopaminergic neurons: hUC-MSCs can significantly alleviate motion defects in PD. Dopaminergic neurons in the SNPC play central roles in motor control. The loss of dopaminergic neurons and the presence of cytoplasmic protein aggregates known as Lewy bodies in the remaining dopaminergic neurons contribute to PD[157]. The intranasal administration of hUC-MSCs significantly alleviates locomotor deficits and rescues dopaminergic neurons by inhibiting neuroinflammation in a MPTP-induced PD mouse model[158]. In addition, intranasal administration of hUC-MSC-derived EVs mitigates motor deficits in a 6-OHDA-induced PD model by upregulating special AT-rich sequence-binding protein-1 and activating the Wnt/β-catenin signaling pathway[159]. Moreover, hUC-MSCs can be efficiently differentiated into dopaminergic neurons by a one-step differentiation system using a YFBP cocktail (Y27632, forskolin, SB431542, and SP600125), which increases dopamine levels in the striatum and significantly ameliorates motor impairments in PD model mice by transiently activating Wnt/β-catenin signaling[160]. hUC-MSCs modified with BDNF and transplanted into the right striatum can improve the rotational behavior of PD model rats induced by 6-OHDA, exerting neuroprotective and anti-neuroinflammatory effects[161]. Similarly, tail vein injection of BDNF-loaded Exos derived from hUC-MSCs can significantly increase neuronal survival in PD models[162].

DPSCs can effectively promote the recovery of dopaminergic neurons. Transplanting DPSCs into the SNPC of PD model mice via stereotaxic brain injection of 6-OHDA can reduce the loss of dopaminergic neurons in the SNPC and improve dopaminergic function[163]. Intravenous injection of human gingival-derived MSCs improves rotational behavior and forelimb misalignment in PD model rats[164]. This finding is consistent with the results of Fujii et al[165]. Chen et al[166] injected stem cells from human exfoliated deciduous teeth (SHED)-conditioned medium at different doses into the yolk sac of zebrafish with 6-OHDA-induced PD. They reported that SHED-conditioned medium repaired motor deficits in a zebrafish PD model similar to the drug nomifensin, which has potential in treating PD. In addition, BM-MSCs and OE-MSCs transplanted into different PD models can restore dopaminergic neurons in the substantia nigra and nerve terminals in the striatum, improve PD-like motor deficits, and enhance neurogenesis[167-170].

MSCs and their derivatives in treating HD

HD is a progressive neurodegenerative disorder characterized primarily by the loss of striatal neurons and the development of motor and cognitive dysfunction. Emerging evidence suggests that MSCs may help alleviate the pathological features of HD through multiple mechanisms, including reducing mutant HTT protein aggregation, modulating synaptic plasticity, suppressing neuroinflammation, and providing neurotrophic support. Table 4 summarizes the major research advances in MSC-based therapies for HD, providing a valuable reference for the development of stem cell treatment strategies.

Table 4 Summary of therapeutic efficacy and safety of mesenchymal stem cells and their derivatives in Huntington’s disease.

Animal model	Source	Type	Administration method	Main outcomes	Ref.
R62 mice	Human	hUC-MSCs	IP	Regulate microglia; improve neurological dysfunction	[171]
R62 mice	Mice	BM-MSCs	IN	Improve neuroinflammation and dopaminergic signaling, increase survival rate	[172]
C57BL/6 mice with 3-NP	Human	hUC-MSCs	SI	Decrease gliosis, ameliorate motor coordination and muscle activity, along with an increase in striatal volume and dendritic length of the striatum	[173]
C57BL/6 mice with 3-NP	Mice	BM-MSCs	IV	Inhibit 3-NP-induced neurological insults via modulation of the Ca2+/CaN/NFATc4 and Wnt/β-catenin signaling pathways	[174]
C57BL/6 mice with 3-NP	Human	DPSCs	IV	Neuroprotection	[175]

R62 mice: HTT exon1-62Q knock-in mice; hUC-MSCs: Human umbilical cord-derived mesenchymal stem cells; IP: Intraperitoneal injection; BM-MSCs: Bone marrow-derived mesenchymal stem cells; IN: Intranasal administration; 3-NP: 3-nitropropionic acid; SI: Stereotaxic injection; IV: Intravenous injection; CaN: Calcineurin; NFATc4: Nuclear factor of activated t-cells c4; DPSCs: Dental pulp-derived mesenchymal stem cells.

Amelioration of neuroinflammation: Mutation of the HTT protein triggers neuroinflammation, activating microglia and other cells to release inflammatory factors, thereby significantly influencing HD progression. Human full-term placental amniotic MSC-conditioned medium can ameliorate neurological dysfunction in R6/2 mice by regulating microglia[171]. Intranasal administration of BM-MSCs can improve neuroinflammation and dopaminergic signaling in R6/2 mice and increase their survival rates[172].

Neuroprotective effects: The striatum contains various types of neurons and plays a pivotal role in motor control. MSCs can participate in neuroprotection through mechanisms such as the regulation of neurotransmitters. hUC-MSC and hUC-MSC-conditioned medium transplantation into the bilateral striatum can ameliorate motor dysfunction and striatal atrophy in rats with 3-nitropropionic acid (3-NP)-induced PD[173]. The intravenous injection of BM-MSCs in combination with lercanidipine can inhibit 3-NP-induced neurotoxicity by modulating the Ca²⁺/CaN/NFATc₄ and Wnt/β-catenin signaling pathways. This treatment also increases BDNF, forkhead box protein 3, Wnt, and β-catenin expression in the striatum. These findings indicate that the combination of lercanidipine and BM-MSCs has neuroprotective effects and may represent a feasible approach to increasing the beneficial effects of stem cell therapy for treating HD[174]. Furthermore, intravenous administration of DPSCs can exert neuroprotective effects by restoring BDNF expression in the cortex and striatum of 3-NP-induced rats[175].

MSCs and their derivatives in treating ALS

MSCs and their derivatives have shown significant potential for neuroprotection and anti-inflammatory effects in the treatment of ALS. The pathophysiology of ALS is characterized by motor neuron degeneration, neuroinflammation, and glutamate excitotoxicity. Current research suggests that MSCs may slow disease progression through multiple mechanisms, including the secretion of neurotrophic factors, immunomodulation of the neural microenvironment, and reduction of oxidative stress. Table 5 provides a summary of pivotal studies evaluating MSC-based therapies for ALS, highlighting their therapeutic potential.

Table 5 Summary of therapeutic efficacy and safety of mesenchymal stem cells and their derivatives in amyotrophic lateral sclerosis.

Animal model	Source	Type	Administration method	Main outcomes	Ref.
In vitro SIM-A9 hSOD1 (G93A) microglial cells	Mice	ADSCs	Coculture	Reduce metabolic activity of microglia, decrease iNOS+ cells, and increase CD206+ cells	[176]
BV-2 cells with SOD 1-G93 A	Human	hUC-MSC-CM	IV	Extend lifespan and reduce expression of pro-inflammatory cytokines and iNOS	[178]
SOD 1G 93 A mice	Human	BM-MSCs	IV	Mediate anti-inflammatory responses through the CX3CL1/CX3CR1 axis	[179]
SOD 1G 93 A mice	Mice	BM-MSCs	IV	Upregulate expression of Nrf2 and NQO1, promote antioxidant responses, and reduce accumulation of reactive oxygen species	[180]
ALS patients	Human	BM-MSCs	IT	Feasible and safe	[181]
ALS patients	Human	BM-MSCs	IT, IV	Feasible and safe	[182]
ALS patients	Human	BM-MSCs	IT	Feasible and safe	[183]
hSOD 1 G93 A mice	Human	hUC-MSCs	IM	Extend lifespan; enhance motor function	[184]
ALS cell model expressing TDP-43 mutant M337V	Human	hUC-MSCs	Coculture	Activate the Nrf-2/HO-1 axis to exert antioxidant and neuroprotective effects	[185]
SOD 1G 93 A-NSC 34 cell model	Human	hUC-MSCs	Coculture	Inhibit the NF-κB/Bcl-2 signaling pathway; reduce cell apoptosis	[186]
ALS patients	Human	hUC-MSCs	IT	Feasible and safe	[187]
ALS patients	Human	ADSCs	LP	Feasible and safe	[188]
ALS patients	Human	hUC-MSCs	IN	Feasible and safe, lifespan extended by 2-fold	[189]
SOD 1G 93 A rat	Human	BM-MSCs	IT, IM	Increase survival of motor neurons	[190]
SOD 1G 93 A mice	Mice	BM-MSCs	IV	Reduce astrocyte activation and expression of neuroinflammatory factors	[180]
SOD 1G 93 A rat	Not mentioned	BM-MSCs	IV	Prolong survival period and protect the motor function	[191]
SOD 1G 93 A mice	Human	ADSCs	IV	Delay disease progression, prolong survival rate, and enhance neuron survival	[192]
SOD 1G 93 A mice	Human	iPSC-sEVs	IN	Improve motor performance and survival time	[193]

SOD1: Superoxide dismutase 1; ADSCs: Adipose-derived mesenchymal stem cells; iNOS: Inducible nitric oxide synthase; CD206: Cluster of differentiation 206; SOD 1-G93 A: Superoxide dismutase 1 with glycine 93 to alanine mutation; hUC-MSC-CM: Human umbilical cord-derived mesenchymal stem cell-conditioned medium; IV: Intravenous injection; BM-MSCs: Bone marrow-derived mesenchymal stem cells; CX3CL1: C-X3-C motif chemokine ligand 1; CX3CR1: C-X3-C motif chemokine receptor 1; Nrf2: Nuclear factor erythroid 2-related factor; NQO1: NAD(P)H quinone dehydrogenase 1; ALS: Amyotrophic lateral sclerosis; IT: Intrathecal injection; hUC-MSCs: Human umbilical cord-derived mesenchymal stem cells; IM: Intramuscular injection; TDP-43: Transactive response DNA-binding protein 43; HO-1: Heme oxygenase-1; NF-κB: Nuclear factor-kappa B; Bcl-2: B-cell lymphoma 2; LP: Lumbar puncture; iPSC-sEVs: Induced pluripotent stem cell-derived small extracellular vesicles.

Reduction in neuroinflammation: Recently, the role of neuroinflammation in ALS pathogenesis has been emphasized. In particular, numerous studies have shown that microglia are highly activated in ALS mouse models and are closely associated with the severity of motor neuron degeneration in ALS[176,177]. ADSCs were cocultured with SIM-A9 microglia expressing the G93A mutant human Cu/Zn SOD1 protein for 24 hours. This coculture significantly reduced the metabolic activity of microglia by decreasing ROS levels, polarizing the microglia toward an anti-inflammatory phenotype[176]. The intravenous administration of hUC-MSC-conditioned medium can prolong the lifespan of hSOD1-G93A mice. In an ALS cell model of BV-2 cells overexpressing hSOD1-G93A, hUC-MSC-conditioned medium suppressed the lipopolysaccharide-induced inflammatory response, including a reduction in proinflammatory cytokine and inducible nitric oxide synthase expression[178]. When cocultured with primary motor neurons from female hSOD1-G93A mice, BM-MSCs and their conditioned medium mediated anti-inflammatory responses through the C-X3-C motif chemokine ligand 1/C-X3-C motif chemokine receptor 1 axis[179]. Single intravenous injection of BM-MSCs in hSOD1-G93A mice can promote an antioxidant response by increasing the expression of nuclear factor erythroid 2-related factor and NAD(P)H quinone dehydrogenase 1, thereby reducing ROS accumulation[180].

Moreover, the safety of BM-MSC therapy has been validated in ALS patients. Repeated intrathecal injections of autologous BM-MSCs in ALS patients have significantly improved cerebrospinal biomarkers of neuroinflammation, neurodegeneration, and neurotrophic factors[181]. Current research indicates that the simultaneous intrathecal and intravenous administration of autologous BM-MSCs to ALS patients is feasible and safe. During the follow-up period, mild headache was the most common treatment-related adverse reaction[182]. A potential mechanism is that BM-MSCs mediate the conversion of proinflammatory cells to anti-inflammatory cells. Additionally, transplanting autologous BM-MSCs into the lumbar spinal cord region is feasible[182,183].

Extension of lifespan and promotion of neuronal survival: As potential therapeutic agents for ALS, hUC-MSCs prolong lifespan and increase motor function in hSOD1-G93A mice[184]. Treatment of an ALS cell model expressing the transactive response DNA-binding protein 43 mutant M337V with hUC-MSC-conditioned medium for 24 hours had antioxidant effects by activating the nuclear factor erythroid 2-related factor/heme oxygenase-1 axis, confirming that hUC-MSC-conditioned medium has potential neuroprotective effects[185]. Ginsenoside-Rg1 combined with hUC-MSC-conditioned medium effectively reduces cell apoptosis in a hSOD1-G93A-NSC34 cell model after 24 hours of treatment by inhibiting the nuclear factor-kappa B/B-cell lymphoma 2 signaling pathway[186]. ALS patients have shown good tolerance to intrathecal injection of hUC-MSCs and ADSCs[187,188].The survival period was extended in all treatment groups, and no serious adverse reactions were observed[189].

The combined administration of BM-MSCs via lumbar puncture and intramuscular (quadriceps) injection can increase motor neuron survival in the hSOD1-G93A mouse model[190]. In vitro BM-MSC-derived EV exposure reverses the neurotoxicity of ALS astrocytes from both humans and mice toward motor neurons[180]. Moreover, repeated administration of MSCs (four times weekly) increases survival times, protects motor functions, and reduces the deterioration of locomotor activity[191].

Furthermore, intravenous injection of ADSCs via the dorsal tail vein delays disease progression in hSOD1-G93A mice, significantly increasing survival by 20 days, increasing neuronal survival, and promoting a less degenerative neuronal microenvironment[192]. The intranasal delivery of EVs derived from human induced pluripotent stem cells (iPSCs) significantly improves motor performance and survival time and alleviates motor neuron-related pathological changes in the hSOD1-G93A mouse model[193].

MSCs and their derivatives in treating NPC

NPC is a rare lysosomal storage disorder caused by mutations in the NPC1 or NPC2 genes, resulting in impaired cholesterol metabolism that triggers neuronal apoptosis and cerebellar degeneration. Emerging experimental studies suggest that MSCs may help attenuate disease progression through four principal mechanisms: Restoring cholesterol homeostasis, reducing oxidative stress, providing neurotrophic support, and promoting neuronal repair. Table 6 compiles representative studies investigating MSC-derived therapies for NPC, offering critical insights for translational research.

Table 6 Summary of therapeutic efficacy and safety of mesenchymal stem cells and their derivatives in Niemann-Pick disease type C.

Animal model	Source	Type	Administration method	Main outcomes	Ref.
NP-C GsbsGFP mice	Mice	BM-MSCs	IV	Improve degenerative loss of Purkinje neurons	[194]
BALB/c npcnih (NPC) mice	Mice	BM-MSCs	Cerebellum injection	Promote neuronal networks with functional synaptic transmission	[195]
BALB/c npcnih (NPC) mice	Mice	BM-MSCs	Cerebellum injection	Upregulate fusion ability of Purkinje neurons and donor-derived BM-MSCs	[196]
BALB/c npcnih (NPC) mice	Mice	BM-MSCs	Cerebrum injection	Modulate endogenous NPC NSCs, stimulate NSC proliferation and neuronal differentiation	[199]
BALB/c npcnih (NPC) mice	Mice	BM-MSCs	Cerebellum injection	Inhibit activation of astrocytes and microglia, reduce inflammation	[197]
BALB/c npcnih (NPC) mice	Mice	BM-MSCs	Cerebellum injection	Release bioactive neurotrophic factors, modulate sphingolipid metabolism of endogenous NPC Purkinje neurons	[198]
BALB/c npcnih (NPC) mice	Mice	ADSCs	Cerebellum injection	Rescue Purkinje neurons, restore motor coordination, and alleviate inflammatory responses	[201]
BALB/c npcnih (NPC) mice	Human	hUC-MSCs	HIPP injection	Stimulate endogenous neurogenesis, diminish intracellular cholesterol accumulation, and safeguard motor functionality	[200]
BALB/c npcnih (NPC) mice	Human	hUC-MSCs	IN	Reduce cholesterol levels, decrease loss of Purkinje cells in the cerebellum, delay motor dysfunction	[202]
Npc1KO N2a cells	Human	MenSCs-CM	Coculture	Increase survival rate, significantly alleviate inflammatory responses and apoptosis	[203]

NPC: Nimman-Pick disease type C; BM-MSCs: Bone marrow-derived mesenchymal stem cells; IV: Intravenous injection; NSCs: Neural stem cells; ADSCs: Adipose-derived mesenchymal stem cells; hUC-MSCs: Human umbilical cord-derived mesenchymal stem cells; HIPP: Hippocampus; IN: Intranasal administration; Npc1^KO N2a cells: Npc1 gene knockout neuro-2a cells; MenSC-CM: Menstrual blood-derived endometrial stem cell conditioned medium.

Purkinje cell loss is a prominent feature of NPC. Tail vein injection of BM-MSCs ameliorates the degenerative loss of Purkinje cells in the cerebellum of NPC mice[194]. In addition, cerebellar transplantation of BM-MSCs promotes the formation of neuronal networks with functional synaptic transmission in NPC mice[195,196]. Moreover, BM-MSCs can reduce inflammatory responses by inhibiting astrocyte and microglia activation[197,198]. Similarly, transplanting BM-MSCs into the brain can promote the neural stem cell proliferation and neuronal differentiation in the subventricular zone[199]. The transplantation of ADSCs and hUC-MSCs can also rescue damaged Purkinje cells in NPC mice and alleviate inflammatory responses[200,201]. Additionally, hUC-MSCs can reduce intracellular cholesterol accumulation, activate cholesterol transporters to improve cholesterol metabolism, and delay motor dysfunction in NPC mice[202]. Our previous study revealed that conditioned medium of MenSCs can also ameliorate cholesterol accumulation and inhibit neuroinflammation and apoptosis in Npc1 knockout neuro-2a cells[203].

CHALLENGES AND FUTURE OF CLINICAL TRANSLATION OF MSCS FOR NEURODEGENERATIVE DISEASES

Currently, human embryonic stem cells and human iPSCs are commonly employed in the clinical translation research of stem cell therapy for neurodegenerative diseases[204]. An open-label Phase I clinical trial (NCT04802733) transplanted bemdaneprocel, a dopaminergic neuron progenitor cell product derived from human embryonic stem cells, bilaterally into the putamen of patients with PD[205]. ¹⁸Fluoro-DOPA positron emission tomography (PET) scans indicated graft survival, and patients receiving high-dose therapy showed improvements in motor dysfunction. In a phase I/II clinical trial (jRCT2090220384), iPSC-derived dopaminergic progenitors were bilaterally transplanted into the putamen of patients with PD, leading to varying degrees of motor improvement without evidence of tumor formation[206]. Although MSCs have numerous advantages in preclinical studies of neurodegenerative diseases, significant challenges remain regarding their source and quality control; delivery methods; long-term survival, differentiation and functional integration in vivo; and safety and ethical concerns in clinical translational research.

Sources and quality control of MSCs

MSCs derived from different tissue sources all exhibit multipotent differentiation capacity, have low immunogenicity, and perform immunomodulatory functions, but have heterogeneous biological characteristics that may influence their therapeutic effects. Compared to those isolated from adult tissues, fetal MSCs exhibit superior proliferative capacity in vitro[207,208]. For example, compared to BM-MSCs and ADSCs, hUC-MSCs demonstrate superior migration[209,210]. However, the proliferative potential of ADSCs is greater than that of BM-MSCs[211]. In addition, studies have revealed that SHEDs exhibit greater proliferative capacity than hUC-MSCs[212]. MSCs from different tissue sources exhibit distinct immunogenic and immunomodulatory properties. Specifically, compared to BM-MSCs, hUC-MSCs demonstrate lower immunogenicity and enhanced immunomodulatory capacity[115,212]. Compared with BM-MSCs, AD-MSCs exert superior immunomodulatory effects[211]. Moreover, the secretory profiles of MSCs vary significantly across tissue sources. Compared with BM-MSCs, hUC-MSCs demonstrate substantially higher cytokine levels (granulocyte colony-stimulating factor, granulocyte-macrophage colony-stimulating factor, hepatocyte growth factor, IL-6, IL-8, and IL-11) secretion. In contrast, BM-MSCs exhibit greater vascular endothelial growth factor production than hUC-MSCs, suggesting their greater potential for promoting angiogenesis[213]. Consequently, the therapeutic efficacy of MSCs varies significantly depending on their tissue origin, requiring further preclinical and clinical studies for comprehensive validation.

Quality control is critical during MSC manufacturing. However, non-standardized isolation and culture techniques pose challenges to the production and quality management of MSC-based products[214]. Variations in donor age and culture passage number contribute to batch-to-batch differences in cellular senescence levels among MSCs, ultimately leading to inconsistent immunomodulatory potency[215]. Wang et al[216] developed a microcarrier-based stirred bioreactor system for hUC-MSC expansion, which enhanced the proliferative capacity and delayed the process of cell aging while preserving the essential stem cell properties of hUC-MSCs compared with conventional planar culture. Therefore, establishing standardized isolation and culture protocols for MSCs from different tissue sources will increase their therapeutic efficacy.

Delivery methods for MSCs

The in vivo fate and therapeutic efficacy of MSCs are critically influenced by the delivery method, and distinct pharmacokinetic and pharmacodynamic outcomes are associated with each administration route. Systemic administration delivers MSCs via the circulatory system, offering whole-body distribution with clinical practicality. Although intravenous injection is the clinically preferred administration route, the average diameter of MSCs exceeds that of pulmonary capillaries, leading to pulmonary retention and limited dose delivery to target organs[217]. Arterial injection of MSCs bypasses pulmonary circulation but carries risks of cellular accumulation in cerebral arteries[218]. Intraperitoneal administration of MSCs prevents their entrapment in capillary networks, although its therapeutic application is anatomically restricted to the treatment of peritoneal and pelvic cavity disorders due to localized biodistribution[219]. Localized administration through direct MSC injection into target tissues or lesion sites significantly increases homing efficiency and therapeutic efficacy. This approach is suitable for treating localized injuries or pathologies, such as osteoarthritis, and offers targeted therapy with minimal systemic exposure[220]. Intrathecal, intracerebral, and endocardial injection methods constitute invasive surgical interventions that carry inherent risks of iatrogenic tissue damage and procedure-related complications. Intranasal administration represents a noninvasive delivery route whereby transplanted cells can migrate to the brain via the olfactory epithelium, although its bioavailability may be constrained by multiple physiological factors, particularly the absorptive capacity of the nasal mucosa. Therefore, selecting the appropriate administration route should be based on the specific disease type, therapeutic objectives, and individual patient characteristics. Moreover, combining multiple delivery methods may further enhance therapeutic efficacy.

Long-term survival, differentiation, and functional integration of MSCs

The in vivo survival of MSCs is influenced by multiple factors, including donor source, delivery route, and host immune status. Typically, MSC viability progressively decreases within days to weeks post-transplantation[221,222]. MSCs derived from different tissue sources exhibit variations in immunogenicity and viability. For example, compared with BM-MSCs, hUC-MSCs may exhibit stronger immune-privileged properties and longer survival times[223]. Through genetic modification, MSCs can produce or overexpress functional genes, enabling these cells to withstand hostile microenvironments and resist apoptosis, increasing their migration and homing capacity and potentiating paracrine effects. Zhu et al[224] proposed a novel in situ empowerment of autologous MSC strategies in which bone morphogenetic protein 2-loaded bioactive materials are implanted to establish bone-like tissue in vivo to induce the immunomodulatory functions of MSCs with the help of the inflammatory microenvironment in early developmental stages. The autologous MSCs obtained through this method have reduced immunogenicity, which promotes their long-term survival in the host while maintaining their immunomodulatory capacity longer. The in vivo persistence of MSCs depends on the administration route. MSCs disappear within a few days after intravenous injection, while MSCs are detected within 3 to 4 weeks after intraperitoneal or subcutaneous injection[222]. However, after intramuscular injection, the hUC-MSCs and BM-MSCs survived in situ for more than 5 months[222]. MSCs transplanted into the host may encounter hostile microenvironment conditions including hypoxia, oxidative stress, and chronic inflammation, ultimately leading to cellular apoptosis[225]. Because MSCs can recognize stimuli in the microenvironment and remember them, preconditioning MSCs can achieve the desired effects and reverse their inactivation. Pretreatment with hypoxia, inflammatory factors, and bioactive compounds can enhance MSC proliferation, survival, and paracrine function[226,227].

MSC differentiation in vivo is profoundly influenced by the microenvironment, making it challenging to precisely direct their differentiation into specific cell lineages. For example, an inflammatory environment may induce MSCs to differentiate into unexpected cell types, thus altering the therapeutic effect. Furthermore, even in an ideal microenvironment, the differentiation efficiency of MSCs in vivo may be relatively low, which can limit their role in tissue repair and regeneration. Even when MSCs successfully differentiate into the desired cell types, their functionality often remains incomplete, resulting in failure to fully recapitulate native tissue performance. For example, in neural regeneration, MSC-derived neurons frequently exhibit inferior electrophysiological properties and synaptic transmission capacity than endogenous neural cells.

Safety and ethical considerations of MSCs

The most common adverse reaction following MSC therapy is fever. Although MSCs exhibit low immunogenicity, the febrile response may be associated with acute inflammatory reactions triggered by allogeneic MSC infusion. Notably, patients with MSC-induced pyrexia typically recover spontaneously without requiring specific treatment. Multiple studies have demonstrated that MSCs may acquire chromosomal abnormalities during in vitro expansion; however, these genetic alterations do not lead to tumor growth. For example, studies have revealed that even when MSCs develop chromosomal abnormalities and undergo senescence due to prolonged culture, they still fail to form tumors upon transplantation into mouse models[228]. MSCs that are derived from different sources present distinct ethical considerations. For example, the collection of hUC-MSCs and MenSCs is a noninvasive, painless, and safe procedure, posing no significant ethical concerns[82]. In contrast, BM-MSCs require invasive harvesting procedures, which may involve more ethical considerations. Consequently, stem cell secretory products, including the secretome, Exos, and EVs, offer superior safety and ethical profiles compared with whole-cell therapies[229].

CONCLUSION

MSCs demonstrate great potential in neurodegenerative diseases due to their self-renewal capacity, multipotent differentiation potential, low immunogenicity, immune regulatory functions, and minimal ethical concerns. Moreover, derivatives of MSCs, such as the secretome, Exos, and EVs, show even greater advantages in terms of safety and ethics. Preclinical studies have shown that MSCs and their derivatives can ameliorate neuroinflammation in AD, PD, HD, ALS, and NPC. MSCs and their derivatives can reduce Aβ accumulation and promote neurogenesis, thereby improving cognitive deficits in AD. They can also decrease αSN aggregation and restore dopaminergic neuron function, thus alleviating motor impairments in PD. In HD, MSCs and their derivatives exert neuroprotective effects, thereby mitigating motor dysfunction. In ALS, they increase neuronal survival, thereby prolonging survival and protecting motor function. MSCs and their derivatives can reduce cholesterol accumulation and increase Purkinje cell numbers, thereby delaying motor dysfunction in NPC. Although there are still challenges related to the sources, delivery methods, safety and ethics considerations of MSCs, an increasing number of studies are addressing these limitations. For example, given that the paracrine effects of MSCs are widely recognized as playing a crucial role in disease therapy, the study of secretome, Exos, and EVs will likely be a key focus for future research in neurodegenerative diseases. In conclusion, MSCs have remarkable promise for treating neurodegenerative diseases. As research advances and technologies evolve, MSC-based therapies are expected to emerge as clinically viable treatment options.