Hwan Tae Park

Mutations in Rab7a GTPase (RAB7A) cause the development of hereditary peripheral axonal neuropathy, referred to as Charcot-Marie-Tooth disease 2B (CMT2B). Disturbances in cellular organelle dynamics, such as autophagy, endosomal trafficking, and mitochondrial homeostasis, are related to the pathogenesis of CMT2B. However, the mechanism by which these processes evoke axonal neuropathy in peripheral sensory neurons remains unclear. Here, we demonstrated that RAB7A ^L129F elicited mitochondrial oxidative stress-mediated activation of Sterile alpha and TIR motif-containing protein 1 (SARM1), the critical executor of Wallerian degeneration, in dorsal root ganglion (DRG) neurons. In addition, SARM1 exacerbated mitochondrial ROS generation, indicating the presence of a feedback loop of the mitochondrial-SARM1 axis. However, activation of the mitochondrial stress-SARM1 axis by RAB7A ^L129F only caused metastable axons, structurally unstable axons that did not proceed to axonal fragmentation, due to NAD ⁺ decrease without intra-axonal ATP depletion. We observed that RAB7A ^L129F -induced autophagy activation inhibited the progression of metastable axons to axon degeneration by inhibiting mitochondrial-SARM1 feedback loop in DRG neurons. Finally, peripheral sensory neuron-specific transduction of adeno-associated virus of RAB7A ^L129F in mice developed abnormal pain sensation with the degeneration of intraepidermal nerve fibers in footpads with a SARM1-dependent manner. Our research identified a mitochondrial-SARM1 positive feedback loop driving a metastable axonal state in a cellular model of CMT2B-type neuropathy and showed that autophagy plays a negative regulatory role in the feedback loop to prevent the development of severe axonal degeneration.

Schwann cells (SCs) are required for supporting axons, forming myelin, and facilitating repair through remyelination after injury in the peripheral nervous system (PNS). Processes of differentiation, myelination, and remyelination of SCs are tightly modulated by a complex network of transcription factors and coregulators, including Sox10, Oct6/Pou3f1, Krox20/Egr2, Nab1/2, YY1, COUP‐TFII/NR2F2, YAP/TAZ‐TEAD1, c‐Jun, Sox2, Zeb2, and Etv1/Er81. These factors can regulate the expression of essential target genes such as Mpz and Mbp in SC myelination and repair. Genetic mutations or dysregulation within this network can lead to peripheral neuropathies such as Charcot–Marie–Tooth disease. However, the transcriptional regulatory network of differentiation, myelination, and remyelination of SCs has not been fully understood yet. Thus, this review briefly introduces processes of differentiation, myelination, and remyelination of SCs and explores the role and molecular mechanisms of each transcription factor and coregulator in differentiation and myelination of SCs and their remyelination following nerve injury. Clinical implications for peripheral neuropathies associated with specific gene mutations and variations of transcription factors and coregulators affecting SC biology are also discussed.

Autophagy is a self-degradation system for recycling to maintain homeostasis. p62/sequestosome-1 (p62) is an autophagy receptor that accumulates in neuroglia in neurodegenerative diseases. The objective of this study was to determine the elevation of plasma p62 protein levels in patients with Charcot–Marie–Tooth disease 1A (CMT1A) for its clinical usefulness to assess disease severity. We collected blood samples from 69 CMT1A patients and 59 healthy controls. Plasma concentrations of p62 were analyzed by ELISA, and we compared them with Charcot-Marie-Tooth neuropathy score version 2 (CMTNSv2). A mouse CMT1A model (C22) was employed to determine the source and mechanism of plasma p62 elevation. Plasma p62 was detected in healthy controls with median value of 1978 pg/ml, and the levels were significantly higher in CMT1A (2465 pg/ml, p < 0.001). The elevated plasma p62 levels were correlated with CMTNSv2 (r = 0.621, p < 0.0001), motor nerve conduction velocity (r = − 0.490, p < 0.0001) and disease duration (r = 0.364, p < 0.01). In C22 model, increased p62 expression was observed not only in pathologic Schwann cells but also in plasma. Our findings indicate that plasma p62 measurement could be a valuable tool for evaluating CMT1A severity and Schwann cell pathology.

AAV: adeno-associated virus; ATF3: activating transcription factor 3; ATG7: autophagy related 7; AVIL: advillin; cADPR: cyclic ADP ribose; CALC: calcitonin/calcitonin-related polypeptide; CMT: Charcot-Marie-Tooth disease; cKO: conditional knockout; DEG: differentially expressed gene; DRG: dorsal root ganglion; FE-SEM: field emission scanning electron microscopy; IF: immunofluorescence; NCV: nerve conduction velocity; PVALB: parvalbumin; RAG: regeneration-associated gene; ROS: reactive oxygen species; SARM1: sterile alpha and HEAT/Armadillo motif containing 1; SYN1: synapsin I.

Background and purpose: Elevated plasma concentrations of neural cell adhesion molecule 1 (NCAM1) and p75 neurotrophin receptor (p75) in patients with peripheral neuropathy have been reported. This study aimed to determine the specificity of plasma concentration elevation of either NCAM1 or p75 in a subtype of Charcot- Marie- Tooth disease (CMT) and its correlation with pathologic nerve status and disease severity.Methods: Blood samples were collected from 138 patients with inherited peripheral neuropathy and 51 healthy controls. Disease severity was measured using Charcot- Marie- Tooth Neuropathy Score version 2 (CMTNSv2), and plasma concentrations of NCAM1 and p75 were analyzed by enzyme-linked immunosorbent assay. Eight sural nerves from CMT patients were examined to determine the relation of histopathology and plasma NCAM1 levels.Results: Plasma concentration of NCAM1, but not p75, was specifically increased in demyelinating subtypes of CMT (median = 7100 pg/mL, p < 0.001), including CMT1A, but not in axonal subtype (5964 pg/mL, p > 0.05), compared to the control (3859 pg/mL). CMT1A patients with mild or moderate severity (CMTNSv2 < 20) showed higher levels of plasma NCAM1 than healthy controls. Immunofluorescent NCAM1 staining for the sural nerves of CMT patients showed that NCAM1-positive onion bulb cells and possible demyelinating Schwann cells might be associated with the specific increase of plasma NCAM1 in demyelinating CMT.Conclusions: The plasma NCAM1 levels in demyelinating CMT might be a surrogate bio- marker reflecting pathological Schwann cell status and disease progression.

The myelin sheath is an essential structure for the rapid transmission of electrical impulses through axons, and peripheral myelination is a well-programmed postnatal process of Schwann cells (SCs), the myelin-forming peripheral glia. SCs transdifferentiate into demyelinating SCs (DSCs) to remove the myelin sheath during Wallerian degeneration after axonal injury and demyelinating neuropathies, and macrophages are responsible for the degradation of myelin under both conditions. In this study, the mechanism by which DSCs acquire the ability of myelin exocytosis was investigated. Using serial ultrastructural evaluation, we found that autophagy-related gene 7-dependent formation of a “secretory phagophore (SP)” and tubular phagophore was necessary for exocytosis of large myelin chambers by DSCs. DSCs seemed to utilize myelin membranes for SP formation and employed p62/sequestosome-1 (p62) as an autophagy receptor for myelin excretion. In addition, the acquisition of the myelin exocytosis ability of DSCs was associated with the decrease in canonical autolysosomal flux and was demonstrated by p62 secretion. Finally, this SC demyelination mechanism appeared to also function in inflammatory demyelinating neuropathies. Our findings show a novel autophagy-mediated myelin clearance mechanism by DSCs in response to nerve damage.

Vincristine (VCR) is a chemotherapeutic agent widely used in treatment of malignancies. However, VCR has a limitation in use since it commonly causes a painful neuropathy (VCR-induced peripheral neuropathy, VIPN). Inflammatory cytokines secreted by immune cells such as macrophages can exacerbate allodynia and hyperalgesia, because inhibiting the inflammatory response is a treatment target for VIPN. In this study, we investigated whether Trichinella spiralis, a widely studied helminth for its immunomodulatory abilities, can alleviate VCR-induced allodynia. Von Frey test showed that T. spiralis infection improved mechanical allodynia at 10 days after VCR injection. We further observed whether the difference was due to mitigated axon degeneration, but no significant difference between the groups in axonal degeneration in sciatic nerves and intra-epidermal nerve fibers was found. Conversely, we observed that number of infiltrated macrophages was decreased in the sciatic nerves of the T. spiralis infected mice. Moreover, treatment of T. spiralis excretory-secretory products caused peritoneal macrophages to secrete decreased level of IL-1β. This study suggests that T. spiralis can relieve VCR-induced mechanical allodynia by suppressing neuroinflammation and that application of controllable degree of helminth may prove beneficial for VIPN treatment.

Although microRNAs (miRNAs or miRs) have been studied in the peripheral nervous system, their function in Schwann cells remains elusive. In this study, we performed a microRNA array analysis of cyclic adenosine monophosphate (cAMP)-induced differentiated primary Schwann cells. KEGG pathway enrichment analysis of the target genes showed that upregulated miRNAs (mR212-5p, miR335, miR20b-5p, miR146b-3p, and miR363-5p) were related to the calcium signaling pathway, regulation of actin cytoskeleton, retrograde endocannabinoid signaling, and central carbon metabolism in cancer. Several key factors, such as purinergic receptors (P2X), guanine nucleotide-binding protein G(olf) subunit alpha (GNAL), P2RX5, P2RX3, platelet-derived growth factor receptor alpha (PDGFRA), and inositol 1,4,5-trisphosphate receptor type 2 (ITPR2; calcium signaling pathway) are potential targets of miRNAs regulating cAMP. Our analysis revealed that miRNAs were differentially expressed in cAMP-treated Schwann cells; miRNA363-5p was upregulated and directly targeted the P2X purinoceptor 4 (P2RX4)-UTR, reducing the luciferase activity of P2RX4. The expression of miRNA363-5p was inhibited and the expression of P2RX4 was upregulated in sciatic nerve injury. In contrast, miRNA363-5p expression was upregulated and P2RX4 expression was downregulated during postnatal development. Of note, a P2RX4 antagonist counteracted myelin degradation after nerve injury and increased pERK and c-Jun expression. Interestingly, a P2RX4 antagonist increased the levels of miRNA363-5p. This study suggests that a double-negative feedback loop between miRNA363-5p and P2RX4 contributes to the dedifferentiation and migration of Schwann cells after nerve injury.