Autophagy in Schwann cells: A potential pharmacotherapeutic target in diabetic peripheral neuropathy

Abstract

Diabetic peripheral neuropathy (DPN) is a common complication of diabetes and is characterized by sensory and motor impairments resulting from neural injury. Schwann cells (SCs), which are important for peripheral nerve function, are compromised under hyperglycemic conditions, leading to impaired axonal regeneration and demyelination. Autophagy, a cellular degradation process, is essential for SC function and significantly influences DPN progression. This article highlights the significance of autophagy in SCs and its potential as a pharmacotherapeutic target in DPN. We discuss the mechanisms of autophagy in SCs, including the mammalian target of rapamycin, adenosine monophosphate-activated protein kinase, and phosphatase and tensin homolog-induced putative kinase/parkin pathways, and their dysregulation in DPN. This article also examines various natural products and chemical agents that modulate autophagy and enhance the efficacy of DPN treatment. These agents target key signaling pathways, such as adenosine monophosphate-activated protein kinase/mammalian target of rapamycin and demonstrate potential in promoting nerve regeneration and restoring SC function. The roles of exosomes, long non-coding RNA, and proteins in the regulation of autophagy have also been explored. In conclusion, targeting autophagy in SCs is a promising strategy for DPN treatment and offers new insights into therapeutic interventions. Further research is warranted to fully exploit these targets for clinical applications.

Key Words: Autophagy; Schwann cells; Diabetic peripheral neuropathy; Adenosine monophosphate-activated protein kinase; Mammalian target of rapamycin; Agent

Core Tip: This article underscores the critical role of autophagy in Schwann cells in the progression of diabetic peripheral neuropathy progression, highlight its potential as a pharmacotherapeutic target. It explored autophagy mechanisms (mammalian target of rapamycin, adenosine monophosphate-activated protein kinase, and phosphatase and tensin homolog-induced putative kinase/parkin pathways), their dysregulation in diabetic peripheral neuropathy, and various autophagy-modulating agents (natural products and chemical drugs) that promote nerve regeneration and recovery of Schwann cell function.

INTRODUCTION

Diabetic peripheral neuropathy (DPN) is one of the most prevalent chronic complications of diabetes, encountered in clinical practice. Surveys indicate that DPN accounts for approximately 50% of all diabetic complications[1]. Patients with DPN experience abnormal temperature or pain sensations, as well as diminished conduction function in both sensory and motor neurons during clinical evaluations. The symptoms included numbness, pain, sensory disturbances, and limb muscle weakness. These manifestations significantly compromise the quality of life of patients with diabetes and represent risk factors for disability and mortality in this population[2]. Research has shown that DPN development is linked to various types of injury, including axonal atrophy, demyelination, loss of nerve fiber function, and delayed regeneration under hyperglycemic conditions[3]. The essential process of nerve repair and regeneration after injury involves several key steps: The formation of axonal regeneration pathways and microcirculation, extension and growth of axonal sprouts, myelination of newly regenerated axons, and reinnervation of target cells. This process transitioned from structural to functional recovery[4]. Schwann cells (SCs), the primary glial cells in the peripheral nerves, are responsible for myelin sheath formation. SCs are essential for maintaining the stability of the axonal microenvironment, providing support and protection to axons, facilitating nutrient metabolism, and improving signal conduction[5]. Following peripheral nerve injury (PNI), SCs contribute significantly to nerve repair and regeneration through various mechanisms.

Hyperglycemia has been shown to increases aldose reductase activity and polyol pathway metabolism in SCs. This leads to the accumulation of abnormal metabolites, which in turn causes organelle damage and morphological changes, including swelling and vacuolization[6]. Furthermore, autophagy serves as a crucial protective mechanism in nerve tissues that helps eliminate harmful substances by degrading protein aggregates and removing damaged organelles, thereby mitigating their detrimental effects on nerve cells. Impairment of the autophagy pathway is a well-established contributing factor in DPN progression[7]. Consequently, restoration of autophagy is vital for the effective treatment of DPN. In recent years, the use of medications that affect autophagy has increased significantly. Recent studies have highlighted the potential of various agents to improve PNI and DPN outcomes by altering autophagic processes. This article discusses the modern studies on autophagy in SCs and investigates the mechanisms by which these agents enhance the efficacy of DPN treatment by modulating autophagy modulation. This study highlights the potential of autophagy-modulating agents as viable therapeutic agents for the management of DPN.

MECHANISM AND REGULATION OF AUTOPHAGY IN SCs

Autophagy is a crucial physiological process that functions as the body’s primary defense mechanism in response to stressors, such as nutrient or energy deprivation. Autophagy can be typically divided into three distinct forms: Macroautophagy, microautophagy, and chaperone-mediated autophagy. Macroautophagy, simply referred to as autophagy, represents the most significant pathway, and entails a complex self-degradative process characterized by multiple vesicle fusion events[8]. When a cell detects an autophagy signal, it initiates formation of a membrane-like structure within the cytoplasm. This structure continues to expand and, gradually develops into a crescent-shaped lipid bilayer known as the autophagic vacuole. The membrane extends further, enveloping surrounding organelles, such as the endoplasmic reticulum and mitochondria, ultimately forming an autophagosome with a circular double-membrane architecture. Subsequently, autophagosomes fuse with lysosomes to form autolysosomes, in which lysosomal enzymes degrade the encapsulated contents, yielding amino and fatty acids for cellular utilization[9,10]. Under normal physiological conditions, cells maintain a certain level of autophagy to remove excess metabolites and mitigate the damage caused by cellular stressors, thereby preserving intracellular homeostasis. However, under pathological conditions, insufficient autophagic function can lead to the abnormal accumulation of senescent organelles and damaged proteins, resulting in cellular dysfunction. Conversely, autophagic overload induces apoptosis by removing key cellular components[11].

SCs originate from neural crest cells as precursors, undergo a series of developmental stages, and eventually reach maturity. In the peripheral nervous system, mature SCs are classified into two main types: Myelinated SCs, which form myelin sheaths, and non-myelinated SCs, which do not[12,13]. The myelinated SCs encase individual axons in a multilayered myelin sheath, whereas non-myelinated SCs associate with multiple axons, particularly C-fibers, to form a fascicle. Both types of SCs work together to sheathe the axons of peripheral nerves, offering protection, facilitating efficient nerve signal conduction, and delivering crucial nutritional support[14,15]. Under normal physiological conditions, SCs exhibit high autophagic activity, which is essential for the removal of damaged myelin fragments and the promotion of nerve regeneration and repair[16]. Research has demonstrated that SCs can eliminate up to 60% of damaged fragments within 72 hours of injury[17]. Knockdown of autophagy-related genes, such as autophagy related 7 (ATG7) exacerbates the fragmentation of the myelin sheath surrounding nerve fibers and impedes axonal regeneration following injury[18]. Another study found that SCs activate autophagy to degrade myelin debris, which create a favorable environment for axonal regeneration after nerve injury. This process is regulated by signaling pathways such as the p75 neurotrophin receptor/adenosine monophosphate-activated protein kinase (AMPK)/mammalian target of rapamycin (mTOR) pathway. Nerve growth factor, acting through p75 neurotrophin receptor, enhances SCs autophagy and accelerates nerve regeneration[19]. SC autophagy also supports neuronal survival by clearing myelin debris that can otherwise inhibit axonal growth[20].

Autophagy plays a crucial role in maturation and structural plasticity of SCs and oligodendrocytes[21]. The major signaling cascades regulating autophagy include the mTOR, AMPK, and phosphatase and tensin homolog-induced putative kinase 1 (PINK1) pathways. In DPN, autophagy is influenced by autophagy-related factors, noncoding RNAs, and oxidative stress (Figure 1). Autophagy supports SC maturation and myelin integrity by regulating lipid metabolism (e.g., cholesterol supply) and clearing misfolded proteins via the p62/sequestosome 1 pathway, thereby preventing proteotoxicity. Autophagy also maintains homeostasis by removing damaged mitochondria and oxidative stressors, thereby safeguarding myelin stability. In DPN, autophagy impairment, as evidenced by reduced light chain 3-II (LC3-II) conversion, p62 accumulation, and decreased beclin-1 - disrupts expression, disrupts these processes, which leads to aberrant lipid metabolism, protein aggregation, and oxidative stress. This dysfunction promoted myelin degradation, apoptosis, and degeneration. Thus, the restoration of autophagy may represent a therapeutic strategy for preserving myelination in patients with DPN.

Figure 1 Autophagic imbalance leads to Schwann cell dysfunction. High glucose-induced oxidative stress triggers reactive oxygen species release, activates protein kinase B via phosphatidylinositol 3-kinase, inhibits tuberous sclerosis complex, and promotes mammalian target of rapamycin phosphorylation. Subsequently mRNA translation is enhanced and autophagosome formation is inhibited. Lysosomal damage induces transforming growth factor-β-activated protein kinase 1-galectin-9 binding, which activates adenosine monophosphate-activated protein kinase and unc-51-like kinase 1 to promote autophagy. Mitochondrial damage under high glucose leads to phosphatase and tensin homolog induced putative kinase 1 accumulation and parkin phosphorylation. AMPK: Adenosine monophosphate-activated protein kinase; mTOR: Mammalian target of rapamycin; PI3K: Phosphatidylinositol 3-kinase; ULK1: Unc-51-like kinase 1; ATG13: Autophagy related 13; FIP200: Focal adhesion kinase family kinase-interacting protein 200kD; ROS: Reactive oxygen species; SQSTM1: Sequestosome 1; LC3: Light chain 3; PINK1: Phosphatase and tensin homolog-induced putative kinase 1.

MTOR signaling pathway

As a negative regulator of autophagy, mTOR plays a crucial role in neural cell autophagy and mediates multiple signaling pathways involved in this process[22]. MTOR is a 289-kDa serine/threonine protein kinase that comprises two distinct protein complexes: MTORC1 and mTORC2. Notably, mTORC1 is highly sensitive to rapamycin and inhibits the autophagy initiation kinase ATG1. In addition, the inhibition of mTORC2 by rapamycin was caused by the blockage of mTORC2 assembly. Inhibition of unc-51-like kinase 1 (ULK1), a key inducer of autophagy at the ser757 site, effectively suppresses autophagy onset[23]. Studies have shown that high glucose levels upregulate mTOR expression, leading to phosphorylation of its downstream effector, p70 ribosomal S6 kinase. This process enhances mRNA translation while inhibiting autophagosome formation by preventing the detachment of endoplasmic reticulum membranes[24]. MTOR phosphorylation is closely associated with the phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt) signaling pathway. High glucose-induced oxidative stress triggers the release of substantial reactive oxygen species (ROS), which stimulates cell proliferation. This process leads to Akt phosphorylation via PI3K activation, inhibits the activation of the downstream tuberous sclerosis complex, and promotes the phosphorylation of downstream factors, including mTOR. This cascade effectively regulates the PI3K/Akt/mTOR signaling pathway. Furthermore, it mitigates excessive autophagy in SCs induced by high glucose conditions, slows cellular apoptosis, and ultimately improves DPN[25].

AMPK signaling pathway

In contrast to the inhibitory effect of the mTOR signaling pathway on autophagy, the activation of AMPK signaling promotes autophagy. AMPK serves as an intracellular sensor of adenosine nucleotide levels, maintaining cellular energy homeostasis by promoting catabolic pathways that generate adenosine triphosphate, while reducing adenosine triphosphate consumption. Prolonged exposure to high glucose environments causes lysosomal damage, triggering the binding of transforming growth factor-β-activated protein kinase 1 with galectin-9, which subsequently phosphorylates AMPK. AMPK activates ULK1 to induce autophagy[26]. Abdelkader et al[27] have demonstrated a reduction in AMPK phosphorylation in a model of streptozotocin-induced DPN. The sodium-dependent glucose transporter 2 inhibitor empagliflozin enhances AMPK phosphorylation and upregulates the expression of autophagy proteins, specifically beclin-1 and LC3-II. This intervention significantly alleviates high glucose-induced injury in peripheral nerve cells[27].

PINK/parkin signaling pathway

The PINK/parkin signaling pathway plays a key role in diabetic neuropathic pain by mediating mitophagy[28]. PINK1 is predominantly expressed in mitochondria, where it undergoes degradation and cleavage in the inner mitochondrial membrane. PINK1 is imported into the inner membrane and cleaved in healthy mitochondria, whereas in the event of a potential loss in the inner mitochondrial membrane, PINK1 is unable to enter the inner membrane and thus accumulates in the outer mitochondrial membrane, which in turn recruits parkin. Under sustained high glucose conditions, mitochondrial damage occurs alongside inhibited protease hydrolysis activity, leading to the accumulation of PINK1 in the outer mitochondrial membrane and the subsequent phosphorylation of parkin, a key regulator of mitochondrial autophagy. This initiates mitochondrial autophagy[29]. He et al[30] demonstrated that an increase in parkin levels within the spinal cord of diabetic mice correlated with enhanced mitochondrial autophagy. Additionally, hypoxia-inducible factor-1 ameliorates mitochondrial autophagy and hyperalgesia in diabetic mice by mediating parkin-related mitochondrial dysfunction[30]. Therefore, parkin may exert its neuroprotective effects by promoting mitochondrial autophagy.

Autophagy-related protein factors

Autophagy is a complex and dynamic process tightly regulated by various autophagy-related factors. Beclin-1, a gene homologous to the yeast autophagy gene ATG/vps30, plays a crucial role in autophagosome and autophagic vesicle formation, and its expression levels are positively correlated with the extent of autophagy[31]. LC3 serves as a marker protein for autophagy and undergoes mutual transformation between the two forms. Outside of the autophagic membrane, LC3 covalently binds to phosphatidylethanolamine on the surface of autophagosome catalysis, modification, and processing by various autophagy-related enzymes, resulting in the formation of LC3-I and LC3-II. The conversion of LC3-I to LC3-II signifies the formation of autophagosomes, and is an important marker of autophagic activity. p62 protein (also known as sequestosome 1) acts as an autophagy receptor and is characterized by its ubiquitin-binding structural domain. It interacts with LC3-II through specific amino acid sequences to form a complex, which is subsequently degraded during lysosomal digestion and is associated with autophagosome formation. Consequently, p62 levels were inversely correlated with the degree of autophagic activity[32]. Numerous studies involving animal models and cell cultures have demonstrated that beclin-1 expression is significantly downregulated in DPN models. This decline correlated with reduced levels of LC3-II and increased p62 levels.

Oxidative stress

ROS induced by glycolipid toxicity serve as critical signaling molecules involved in autophagy induction. ROS produced during starvation inactivates Atg4 by oxidizing a critical, catalytic cysteine residue (C81). This prevents Atg4-mediated cleavage of LC3-II, which leads to the accumulation of LC3-II on the phagophore membrane and the formation of autophagosomes[33]. Conversely, ROS induce poly (adenosine diphosphate-ribose) polymerase 1 and activate AMPK through AMP activation, promoting autophagy[22]. The ULK-ATG13-focal adhesion kinase family kinase-interacting protein 200kD complex plays a vital role in the formation of autophagosomes and double-membrane autophagic vacuoles[34]. Furthermore, ROS can inhibit the mTOR signaling pathway, leading to ATG13 dephosphorylation, ULK activation, and focal adhesion kinase family kinase-interacting protein 200kD modulation, thereby regulating autophagy. Zhou et al[35] demonstrated that ROS-induced c-Jun N-terminal kinase activation leads to both autophagy and apoptosis, and is associated with caspase-independent cell death via autophagy.

TARGETS AND AGENTS REGULATING AUTOPHAGY IN THE PREVENTION AND TREATMENT OF DPN

The field of peripheral neuropathy research has witnessed remarkable advancements in recent years, particularly concerning nerve regeneration and recovery of SCs function. Autophagy, a fundamental cellular mechanism, has gained increasing attention because of its pivotal role in nerve repair following injury. Recent studies have focused on elucidating the mechanism by which various compounds and molecules enhance nerve regeneration by modulating autophagy (Table 1).

Table 1 Targets and agents for preventing diabetic peripheral neuropathy by regulating Schwann cell autophagy.

Ref.	Agents	Target	Functions	Dose in vitro
[37]	Lycorine	AMPK/MMP9	Decrease MMP9 expression and induce autophagy (increase)	2.87 μg/mL
[38]	Resveratrol	N/A	Promote autophagy (increase)	10 μM
[25]	Muscone	Akt/mTOR	Inhibit p-AKT/p-mTOR expression and mitigates autophagy (decrease)	10 μm
[40]	Astragaloside IV	MicroRNA-155	Reduce apoptosis by promoting autophagy (increase)	15 μg/mL
[42]	Salvianolic acid B	JNK	Prevent autophagic cell death by activating the JNK pathway (decrease)	0.1-10 μM
[43]	Quercetin	N/A	Alleviate high glucose-induced damage by autophagy (decrease)	25 μM
[45]	Curcumin	Erk1/2 and Akt	Accelerate cell autophagy and prevent cell apoptosis (increase)	30 μM
[46]	Lentinan	AMPK/mTOR	Promote autophagic flux (increase)	100 μg/mL
[48]	Docosahexaenoic acid	AMPK	DHA attenuates excessive autophagy (decrease)	10 μM
[50]	Imidazole propionate	Erk/mTOR	The reduction of imidazole propionate promotes migration of Schwan cells through enhancing autophagy (increase)	100 μM
[51]	Epothilone B	PI3K	Enhance migration and autophagy (increase)	1 nM
[52]	5-azacytidine	TXNIP	Reduce TXNIP protein expression, improve autophagy and inhibite apoptosis (increase)	10 μM
[53]	AG490	STAT3	Enhance P62 expression in high glucose-stimulated RSC96 cells (increase)	N/A
[54]	4’-chlorodiazepam (Ro5- 4864)	Translocator protein	Improve Schwann cells function by activating autophagy (increase)	10-5 M
[57]	Healthy plasma-derived exosomes (miR-20b-3p)	Stat3	Alleviate autophagy impairment (increase)	20 μg/mL
[58]	Exosomes produced by adipose-derived stem cells (miRNA-26b)	Kpna2	Promote the regeneration of the myelin sheath by moderately reducing autophagy (decrease)	50 μg/mL
[60]	Long non-coding RNA XIST	MiR-30d-5p/SIRT1	Induce autophagy (increase)	N/A
[62]	LV-lipin1 overexpressing virus	N/A	Decreased the autophagic hyperactivity and apoptosis (decrease)	1.0 × 109 TU/mL
[64]	Plasmid-UNC5B	AMPK/ULK1	Facilitates autophagic flux in SCs (increase)	N/A
[66]	Methyltransferase-like protein 3	MiR-192-5p/ATG7	Stimulated miR-192-5p maturation to depress ATG7 and SCs autophagy (decrease)	N/A
[70]	Fibroblast growth factor 21	ERK/Nrf-2	Suppresses autophagy-induced cell death (decrease)	250 ng/mL
[71]	Basic fibroblast growth factor	TEFB	Activate autophagy (decrease)	100 ng/mL
[73]	protein inhibitor of activated STAT1	MiR-124	Inhibit apoptosis and promote autophagy of Schwann cells (increase)	N/A
[75]	Triggering receptor expressed on myeloid cells-2	N/A	TREM2 deficiency induces mitochondrial damage and autophagy of Schwann cells (increase)	N/A
[77]	Rat p66shc siRNA	N/A	Recovers the high glucose-induced impairment of autophagy and ER stress in SCs (increase)	N/A
[79]	Mst1 siRNA	N/A	Promoted parkin recruitment to mitochondria in SCs (increase)	N/A

AMPK: Adenosine monophosphate-activated protein kinase; mTOR: Mammalian target of rapamycin; MMP9: Matrix metallopeptidase 9; JNK: C-Jun N-terminal kinase; Erk1/2: Extracellular signal-regulated kinase 1/2; DHA: Docosahexaenoic acid; PI3K: Phosphatidylinositol 3-kinase; TXNIP: Thioredoxin interacting protein; STAT3: Signal transducer and activator of transcription 3; Kpna2: Karyopherin subunit alpha 2; XIST: X-inactive specific transcript; SIRT1: Sirtuin 1; ULK1: Unc-51-like kinase 1; UNC5B: Uncoordinated gene 5 homolog B; ATG7: Autophagy related 7; Nrf-2: Nuclear factor-erythroid 2 related factor 2; TEFB: Transcription elongation factor B; TREM2: Triggering receptor expressed on myeloid cells 2; ER: Endoplasmic reticulum; SCs: Schwann cells; Mst1: Mammalian ste20-like kinase 1; N/A: Not applicable.

Nature product

Lycorine is an alkaloid derived from species of the Amaryllidaceae family that exhibits a range of biological activities, including antitumor, antiviral, and anti-inflammatory effects[36]. Yuan et al[37] indicated that lycorine enhances SCs autophagy by activating the AMPK pathway and downregulating MMP9, leading to LC3-II transformation in DPN. Resveratrol, a natural compound extracted from various food sources, exhibits antioxidant properties. Zhang et al[38] suggested that resveratrol facilitates recovery from sciatic nerve crush injuries by promoting SC autophagy, thereby accelerating myelin clearance. Muscone, a primary active compound extracted from traditional Chinese herbs, has been reported to exhibit several beneficial effects. Muscones exert anti-apoptotic effects during myocardial remodeling post-myocardial infarction in murine models[39]. Dong et al[25] found that muscone ameliorates high glucose-induced apoptosis and autophagy in RSC96 cells by modulating the Akt/mTOR signaling pathway. Astragaloside IV is an important component of Astragalus membranaceus and is known for its efficacy in the treatment of DPN. One study indicated that AS-IV reduced apoptosis in RSC96 cells and promoted autophagy through the regulation of the miR-155-mediated PI3K/Akt/mTOR signaling pathways[40]. Salvianolic acid B, extracted from the roots of “Salviae miltiorrhizae (Danshen)”, represent a new class of natural water-soluble bioactive compounds[41]. Recent findings have demonstrated that salvianolic acid B effectively inhibits c-Jun N-terminal kinase activation, thereby reducing apoptosis and excessive autophagy in SCs by downregulating the caspase-dependent pathway and enhancing B cell leukemia/lymphoma 2 expression[42]. Quercetin, a flavonoid widely found in traditional Chinese medicine and various foods, was recently shown to enhance the proliferation of neurons and glial cells, which are typically suppressed in high glucose environments. Additionally, quercetin has been shown to mitigate oxidative stress-mediated damage and protect SCs from high glucose-induced damage by influencing autophagic processes[43]. Curcumin, a natural compound extracted from “Curcuma longa”, promotes spinal cord repair and recovery of neural function by inhibiting glial scar formation and reducing inflammation[44]. Another study suggested that curcumin enhances autophagy in RSCs by modulating the extracellular signal-regulated kinase 1/2 (Erk1/2) and Akt pathways related to autophagy, thereby preventing RSCs apoptosis[45]. Lentinan, a bioactive polysaccharide isolated from shiitake mushrooms, has various biochemical and pharmacological properties. Findings from one study suggested that lentinan enhances nerve regeneration by promoting the autophagic elimination of myelin debris in SCs, and that this process is potentially regulated by the AMPK/mTOR signaling pathway[46].

Chemical drugs

Docosahexaenoic acid, an n-3 polyunsaturated fatty acid, induces the expression of antioxidant enzymes, thereby protecting immortalized adult mouse SCs (IMS32) from oxidative stress[47]. Research has demonstrated that docosahexaenoic acid can suppress excessive autophagy resulting from oxidative stress in SCs, suggesting that it may be beneficial for preventing or minimizing cell death in vitro[48]. Imidazole propionate, a microbial metabolite derived from histidine, is elevated in the portal and peripheral blood of individuals with type 2 diabetes[49]. A previous study demonstrated that a reduction in imidazole propionate promotes SC migration by enhancing autophagy mediated through the MAPK/Erk/mTOR signaling pathway[50]. Epothilone B, an anti-tumor agent approved by the Food and Drug Administration, is well-known for promoting alpha-tubulin polymerization and enhancing the stability of microtubules. Studies have shown that epothilone B significantly improves axonal regeneration and functional recovery and augments the migratory and autophagic capabilities of SCs[51]. 5-azacytidine is a cytosine analog that functions as a DNA methyltransferase inhibitor (DNMT inhibitor). It can be incorporated into DNA/RNA, where it interacts with DNA methyltransferase, inhibiting its activity and subsequent DNA demethylation within cells. A previous study indicated that 5-azacytidine enhances thioredoxin interacting protein expression by downregulating DNMT1 and DNMT3a, thereby influencing apoptosis and autophagy in RSC96 cells[52]. AG490 is identified as a tyrosine kinase inhibitor that targets epidermal growth factor receptor, signal transducer and activator of transcription 3 (Stat3), and Janus kinase 2/3. Research conducted by Du et al[53] demonstrated that AG490 effectively inhibited STAT3 phosphorylation, which promoted autophagy by modulating both the histone deacetylase 1-Atg3-LC3-II axis and P62. 4’-chlorodiazepam (Ro5-4864), a derivative of diazepam, functions as an effective ligand for the mitochondrial translocation protein 18 kDa, exhibiting notable neuroprotective effects. Research has demonstrated that Ro5-4864 can alleviate allodynia and enhances SC function and regeneration in sciatic nerves via autophagy activation and the nuclear factor-erythroid 2 related factor 2-driven antioxidant system[54].

Exosomes and long non-coding RNA

Exosomes are extracellular vesicles with a lipid bilayer and sizes ranging from 30 to 200 nm that, are produced by virtually all cell types in both physiological and pathological contexts[55]. MiRNAs within these vesicles can be transported to adjacent or distant cells, facilitating subsequent signaling[56]. Li et al[57] revealed that miR-20b-3p is enriched in plasma-derived exosomes from healthy rats and can regulate SC autophagy under pathological conditions by targeting Stat3, thereby inhibiting DPN progression. Distinct stressors or treatment durations alter autophagic responses. Another study suggested that exosomes isolated from adipose-derived stem cells support myelin sheath regeneration by inhibiting nerve injury-induced autophagy in injured SCs via the action of miRNA-26b, which targets karyopherin subunit alpha 2[58]. Long non-coding RNA, defined as transcripts longer than 200 nucleotides, can directly target autophagy-related genes and enhance post-transcription[59]. Liu et al[60] reported that the long non-coding RNA X-inactive specific transcript promoted autophagy in SCs cultured under high-glucose conditions via the miR-30d-5p/sirtuin 1 axis, thereby preventing cell apoptosis and reducing DPN progression.

Protein factors

Previous studies have indicated that Lipin1, an enzyme essential for glycolipid metabolism encoded by LIPIN, is necessary to ensure normal peripheral nerve conduction[61]. Studies have also revealed that Lipin1 overexpression in RSC96 cells significantly diminishes hyperglycemia-induced excessive autophagy and apoptosis[62]. Uncoordinated gene 5 homolog B, recognized as the netrin-1 receptor, is specifically present in SCs and is involved in neuropathic pain mechanisms[63]. Xiao et al[64] indicated that uncoordinated gene 5 homolog B may preserve myelin by increasing autophagic flux in SCs via the phosphorylation of AMPK and ULK1, a process dependent on its ligand, netrin-1. The modification of m6A, influenced by methyltransferase-like protein 3, is crucial for regulating RNA cleavage, stability, transport, localization, and post-transcriptional translation[65]. Liu et al[66] demonstrated that methyltransferase-like protein 3 enhances the maturation of miR-192-5p via m6A methylation, suppressing ATG7 and SCs autophagy while exacerbating PNI. Molecular therapies utilizing fibroblast growth factors (FGFs) are widely employed to promote peripheral nerve myelination and regeneration[67]. FGFs are a class of signaling molecules that regulate cell differentiation, proliferation, and metabolism[68]. Among them, FGF21 is a key metabolic regulator that modulates blood glucose and lipid homeostasis[69]. Research suggests that FGF21 facilitates remyelination and nerve regeneration following PNI by inhibiting excessive activation of the ERK/nuclear factor-erythroid 2 related factor 2 signaling pathway, which regulates oxidative stress and autophagy-related cell death[70]. Basic FGF, a powerful neurotrophic molecule produced by SCs and diverse neuronal groups, plays a significant role in the initial stages of peripheral nerve regeneration. According to Jiang et al[71], this function is intricately associated with clearance of myelin debris by SCs, potentially triggering SCs-mediated autophagy. Protein inhibitor of activated STAT1 functions as a small ubiquitin-like modifier E3 ligase, significantly influencing multiple cellular processes, including cell growth regulation, DNA damage response, and inflammatory modulation[72]. Moreover, studies by Hou et al[73] offer further evidence that protein inhibitor of activated STAT1 can enhance autophagy and suppress apoptosis in SCs via the peroxisome proliferator-activated receptors-γ-miR-124- enhancer of zeste homolog 2/STAT3 signaling pathway. Triggering receptor expressed on myeloid cells-2 is a membrane protein primarily expressed in immune cells of the central nervous system, particularly microglia. This protein is essential for the regulation of microglial proliferation, migration, and phagocytic activity[74]. Zhang et al[75] showed that the absence of triggering receptor expressed on myeloid cells-2 leads to impaired glycolytic flux and oxidative metabolism in SCs, thereby hindering cell proliferation. The resulting energy deficit triggers the activation of AMPK and disrupts the PI3K/AKT/mTOR signaling pathway, ultimately causing mitochondrial dysfunction and inducing autophagy[75]. The Src homology and collagen A family comprises of three isoforms: P46shc, p52shc, and p66shc. Among these, p66shc acts as a redox enzyme that modulates intracellular ROS levels in response to cellular stress[76]. Research has shown that siRNA-mediated inhibition of p66shc gene expression can reverse high glucose-induced oxidative stress, autophagy dysfunction, and early apoptosis in SCs[77]. Mammalian ste20-like kinase 1 is a key serine/threonine kinase that plays an essential role in hippo signaling. This pathway governs various cellular processes, including proliferation, survival, migration, stem cell properties, and differentiation[78]. Huang et al[79] discovered that mammalian ste20-like kinase 1 knockdown alleviates chronic constriction injury-induced neuropathic pain by enhancing the recruitment of parkin to mitochondria and promoting mitophagic degradation. This process ultimately preserves mitochondrial function and myelin integrity.

CONCLUSION

Emerging evidence indicates that autophagy is a key regulator of DPN development. The dual roles of SCs autophagy, protective and pathogenic, in diabetes must be discussed. Under stress conditions, such as nerve injury or metabolic dysfunction, AMPK compensates for mTOR suppression to maintain autophagy, whereas the PINK1/parkin pathway ensures mitochondrial integrity. However, dysregulation of these pathways, such as mTOR overactivation in diabetes, impairs autophagy, thus leading to SC dysfunction and neuropathy. Based on available data, it can be posited that a moderate level of constructive autophagy is protective; conversely, hyperactivation of autophagy appears to be detrimental to DPN. Therefore, when considering autophagy as a potential therapeutic target, it is crucial to accurately assess its status in the SCs. In early DPN, moderate autophagy (LC3-II/LC3-I ratio increase by 2-3 folds)[80] helps clear damaged mitochondria and protein aggregates to maintain myelin integrity. This phase is characterized by sustained AMPK activation, partial mTOR inhibition, and coordinated lysosomal biogenesis. However, when chronic hyperglycemia exceeds 12 months, autophagy becomes excessive (LC3-II accumulation > 5-fold with paradoxical p62 elevation), accompanied by lysosomal dysfunction (cathepsin B activity decrease > 40%, pH elevation > 5.2) and mitophagic failure (parkin translocation impairment > 60%, mitochondrial ROS surge > 2.5-fold), shifting from protective to detrimental effects[81,82]. Therapeutic strategies that target AMPK activation, mTOR inhibition, or PINK1/parkin enhancement can restore autophagy and improve SC health under disease conditions.

Several pharmacotherapeutic agents have demonstrated their efficacy in alleviating DPN by modulating SCs autophagy via various mechanisms. For instance, numerous growth factors, plant extracts, and compounds have been identified to expedite autophagy after nerve injury, thereby facilitating the reconstruction and functional recovery of the nerve myelin sheath. Moreover, some studies have elucidated how oxidative stress, high-glucose environments, and other factors influence the autophagic function of SCs, and subsequently contribute to neuropathy. Conversely, exosomes exhibit considerable potential for repairing nerve injuries; they are thought to transport essential signaling molecules that regulate both autophagy and other functions within receptor cells. This article integrates the latest findings on autophagy mechanisms in SCs and their potential as pharmacotherapeutic targets in DPN. While previous reviews have touched upon autophagy in SCs or DPN treatment strategies, our study comprehensively explored the intricate interplay between autophagy pathways (such as mTOR, AMPK, and PINK/parkin) and DPN progression. Thus, we provided a novel perspective on how autophagy dysregulation underlies DPN pathogenesis and how it can be targeted therapeutically.

In conclusion, these observations indicate that the regulation of autophagy in SCs may represent a promising therapeutic strategy for DPN. However, autophagy within SCs is a critical factor in determining appropriate therapeutic agents. Numerous pharmacotherapeutic agents that mitigate DPN through mechanisms involving autophagy regulation and target either specific or multiple pathways have been identified in preclinical studies. However, clinical success with a specific autophagy regulator in DPN remains unclear. While some of these agents have demonstrated the potential for advancement in clinical trials against DPN, it is essential to gain a comprehensive understanding of the interplay between the signaling events involved in their autophagy regulatory mechanisms. In the future, it will be essential to conduct a more in-depth exploration of the mechanisms regulating autophagy and the associated signaling pathways, as this may lead to the identification of novel therapeutic targets. Secondly, advancements in technology, including the development of robust biomarkers to monitor autophagy in vivo and the creation of targeted delivery systems to enhance drug efficacy, could offer new insights into the study of neuropathies. Further investigations should focus on personalized medical approaches to tailor therapeutic interventions based on individual patient characteristics and autophagy status. Finally, additional clinical trials are required to validate the therapeutic potential of these agents. By addressing these issues, we can move closer to develop more effective treatments for DPN.