Molecular mechanisms of inherited demyelinating neuropathies

Characterization of sciatic nerve myelin sheath during development in C57BL/6 mice

Myelin sheath plays important roles in information conduction and nerve injury repair in the peripheral nerve system (PNS). Enhancing comprehension of the structure and components of the myelin sheath in the PNS during development would contribute to a more comprehensive understanding of the developmental and regenerative processes. In this research, the structure of sciatic nerve myelin sheath in C57BL/6 mice from embryonic day 14 (E14) to postnatal 12 months (12M) was observed with transmission electron microscopy. Myelin structure appeared in the sciatic nerve as early as E14, and the number and thickness of myelin lamellar gradually increased with the development until 12M. Transcriptome analysis was performed to show the expressions of myelin-associated genes and transcriptional factors involved in myelin formation. The genes encoding myelin proteins (Mag, Pmp22, Mpz, Mbp, Cnp and Prx) showed the same expression pattern, peaking at postnatal day 7 (P7) and P28 after birth, whereas the negative regulators of myelination (c-Jun, Tgfb1, Tnc, Cyr61, Ngf, Egr1, Hgf and Bcl11a) showed an opposite expression pattern. In addition, the expression of myelin-associated proteins and transcriptional factors was measured by Western blot and immunofluorescence staining. The protein expressions of MAG, PMP22, MPZ, CNPase and PRX increased from E20 to P14. The key transcriptional factor c-Jun co-localized with the Schwann cells Marker S100β and decreased after birth, whereas Krox20/Egr2 increased during development. Our data characterized the structure and components of myelin sheath during the early developmental stages, providing insights for further understanding of PNS development.

TEAD1 is crucial for developmental myelination, Remak bundles, and functional regeneration of peripheral nerves

Previously we showed that the hippo pathway transcriptional effectors, YAP and TAZ, are essential for Schwann cells (SCs) to develop, maintain and regenerate myelin . Although TEAD1 has been implicated as a partner transcription factor, the mechanisms by which it mediates YAP/TAZ regulation of SC myelination are unclear. Here, using conditional and inducible knockout mice, we show that TEAD1 is crucial for SCs to develop and regenerate myelin. It promotes myelination by both positively and negatively regulating SC proliferation, enabling Krox20/Egr2 to upregulate myelin proteins, and upregulating the cholesterol biosynthetic enzymes FDPS and IDI1. We also show stage-dependent redundancy of TEAD1 and that non-myelinating SCs have a unique requirement for TEAD1 to enwrap nociceptive axons in Remak bundles. Our findings establish TEAD1 as a major partner of YAP/TAZ in developmental myelination and functional nerve regeneration and as a novel transcription factor regulating Remak bundle integrity.

Activation of XBP1s attenuates disease severity in models of proteotoxic Charcot-Marie-Tooth type 1B

Mutations in myelin protein zero (MPZ) are generally associated with Charcot-Marie-Tooth type 1B (CMT1B) disease, one of the most common forms of demyelinating neuropathy. Pathogenesis of some MPZ mutants, such as S63del and R98C, involves the misfolding and retention of MPZ in the endoplasmic reticulum (ER) of myelinating Schwann cells. To cope with proteotoxic ER-stress, Schwann cells mount an unfolded protein response (UPR) characterized by activation of the PERK, ATF6 and IRE1α/XBP1 pathways. Previous results showed that targeting the PERK UPR pathway mitigates neuropathy in mouse models of CMT1B; however, the contributions of other UPR pathways in disease pathogenesis remains poorly understood. Here, we probe the importance of the IRE1α/XBP1 signalling during normal myelination and in CMT1B. In response to ER stress, IRE1α is activated to stimulate the non-canonical splicing of Xbp1 mRNA to generate spliced Xbp1 (Xbp1s). This results in the increased expression of the adaptive transcription factor XBP1s, which regulates the expression of genes involved in diverse pathways including ER proteostasis. We generated mouse models where Xbp1 is deleted specifically in Schwann cells, preventing XBP1s activation in these cells. We observed that Xbp1 is dispensable for normal developmental myelination, myelin maintenance and remyelination after injury. However, Xbp1 deletion dramatically worsens the hypomyelination and the electrophysiological and locomotor parameters observed in young and adult CMT1B neuropathic animals. RNAseq analysis suggested that XBP1s exerts its adaptive function in CMT1B mouse models in large part via the induction of ER proteostasis genes. Accordingly, the exacerbation of the neuropathy in Xbp1 deficient mice was accompanied by upregulation of ER-stress pathways and of IRE1-mediated RIDD signaling in Schwann cells, suggesting that the activation of XBP1s via IRE1 plays a critical role in limiting mutant protein toxicity and that this toxicity cannot be compensated by other stress responses. Schwann cell specific overexpression of XBP1s partially re-established Schwann cell proteostasis and attenuated CMT1B severity in both the S63del and R98C mouse models. In addition, the selective, pharmacologic activation of IRE1α/XBP1 signaling ameliorated myelination in S63del dorsal root ganglia explants. Collectively, these data show that XBP1 has an essential adaptive role in different models of proteotoxic CMT1B neuropathy and suggest that activation of the IRE1α/XBP1 pathway may represent a therapeutic avenue in CMT1B and possibly for other neuropathies characterized by UPR activation.

TEAD1 is crucial for developmental myelination, Remak bundles, and functional regeneration of peripheral nerves

Previously we showed that the hippo pathway transcriptional effectors, YAP and TAZ, are essential for Schwann cells (SCs) to develop, maintain and regenerate myelin (Grove et al., 2017; Grove, Lee, Zhao, & Son, 2020). Although TEAD1 has been implicated as a partner transcription factor, the mechanisms by which it mediates YAP/TAZ regulation of SC myelination are unclear. Here, using conditional and inducible knockout mice, we show that TEAD1 is crucial for SCs to develop and regenerate myelin. It promotes myelination by both positively and negatively regulating SC proliferation, enabling Krox20/Egr2 to upregulate myelin proteins, and upregulating the cholesterol biosynthetic enzymes FDPS and IDI1. We also show stage-dependent redundancy of TEAD1 and that non-myelinating SCs have a unique requirement for TEAD1 to enwrap nociceptive axons in Remak bundles. Our findings establish TEAD1 as a major partner of YAP/TAZ in developmental myelination and functional nerve regeneration and as a novel transcription factor regulating Remak bundle integrity.

EBP50 is a key molecule for the Schwann cell-axon interaction in peripheral nerves

Peripheral nerve injury disrupts the Schwann cell-axon interaction and the cellular communication between them. The peripheral nervous system has immense potential for regeneration extensively due to the innate plastic potential of Schwann cells (SCs) that allows SCs to interact with the injured axons and exert specific repair functions essential for peripheral nerve regeneration. In this study, we show that EBP50 is essential for the repair function of SCs and regeneration following nerve injury. The increased expression of EBP50 in the injured sciatic nerve of control mice suggested a significant role in regeneration. The ablation of EBP50 in mice resulted in delayed nerve repair, recovery of behavioral function, and remyelination following nerve injury. EBP50 deficiency led to deficits in SC functions, including proliferation, migration, cytoskeleton dynamics, and axon interactions. The adeno-associated virus (AAV)-mediated local expression of EBP50 improved SCs migration, functional recovery, and remyelination. ErbB2-related proteins were not differentially expressed in EBP50-deficient sciatic nerves following injury. EBP50 binds and stabilizes ErbB2 and activates the repair functions to promote regeneration. Thus, we identified EBP50 as a potent SC protein that can enhance the regeneration and functional recovery driven by NRG1-ErbB2 signaling, as well as a novel regeneration modulator capable of potential therapeutic effects.

Expression and Role of Toll-like Receptors in Facial Nerve Regeneration after Facial Nerve Injury

Facial nerve palsy directly impacts the quality of life, with patients with facial nerve palsy showing increased rates of depression and limitations in social activities. Although facial nerve palsy is not life-threatening, it can devastate the emotional and social lives of affected individuals. Hence, improving the prognosis of patients with this condition is of vital importance. The prognosis of patients with facial nerve palsy is determined by the cause of the disease, the degree of damage, and the treatment provided. The facial nerve can be easily damaged by middle ear and temporal bone surgery, trauma or infection, and tumors of the peripheral facial nerve or tumors surrounding the nerve secondary to systemic disease. In addition, idiopathic, acquired immunodeficiency syndrome and autoimmune diseases may damage the facial nerve. The treatment used for facial paralysis depends on the cause. Treatment of facial nerve amputation injury varies depending on the degree of facial nerve damage, comorbidities, and duration of injury. Recently, interest has increased in Toll-like receptors (TLRs) related to innate immune responses, as these receptors are known to be related to nerve regeneration. In addition to innate immune cells, both neurons and glia of the central nervous system (CNS) and peripheral nervous system (PNS) express TLRs. A comprehensive literature review was conducted to assess the expression and role of TLRs in peripheral nerve injury and subsequent regeneration. Studies conducted on rats and mice have demonstrated the expression of TLR1-13. Among these, TLR2-5 and TLR7 have received the most research attention in relation to facial nerve degeneration and regeneration. TLR10, TLR11, and TLR13 increase during compression injury of the facial nerve, whereas during cutting injury, TLR1-5, TLR8, and TLR10-13 increase, indicating that these TLRs are involved in the degeneration and regeneration of the facial nerve following each type of injury. Inadequate TLR expression or absence of TLR responses can hinder regeneration after facial nerve damage. Animal studies suggest that TLRs play an important role in facial nerve degeneration and regeneration.

Pharmacologic Targeting of the C-Terminus of Heat Shock Protein 90 Improves Neuromuscular Function in Animal Models of Charcot Marie Tooth X1 Disease

Charcot-Marie-Tooth X1 (CMTX1) disease is an inherited peripheral neuropathy that arises from loss-of-function mutations in the protein connexin 32 (Cx32). CMTX1 currently lacks a pharmacologic approach toward disease management, and we have previously shown that modulating the expression of molecular chaperones using novologue therapy may provide a viable disease-modifying approach to treat metabolic and demyelinating neuropathies. Cemdomespib is an orally bioavailable novologue that manifests neuroprotective activity by modulating the expression of heat shock protein 70 (Hsp70). We examined if 1 to 5 months of daily cemdomespib therapy may improve neuropathic symptoms in three mouse models of CMTX1 (Cx32 deficient (Cx32def), T55I-Cx32def, and R75W-Cx32 mice). Daily drug therapy significantly improved motor nerve conduction velocity (MNCV) and grip strength in all three models, but the compound muscle action potential was only improved in Cx32def mice. Drug efficacy required Hsp70 as improvements in MNCV, and the grip strength was abrogated in Cx32def × Hsp70 knockout mice. Five months of novologue therapy was associated with improved neuromuscular junction morphology, femoral motor nerve myelination, reduction in foamy macrophages, and a decrease in Schwann cell c-jun levels. To determine if c-jun may be downstream of Hsp70 and necessary for drug efficacy, c-jun expression was specifically deleted in Schwann cells of Cx32def mice. While the deletion of c-jun worsened the neuropathy, cemdomespib therapy remained effective in improving MNCV and grip strength. Our data show that cemdomespib therapy improves CMTX1-linked neuropathy in an Hsp70-dependent but a c-jun-independent manner and without regard to the nature of the underlying Cx32 mutation.

Knock-in mouse models for CMTX1 show a loss of function phenotype in the peripheral nervous system

The X-linked form of Charcot-Marie-Tooth disease (CMTX1) is the second most common form of CMT. In this study we used CRISPR/Cas9 to develop new "knock-in" models of CMTX1 that are more representative of the spectrum of mutations seen with CMTX1 than the Cx32 knockout (KO) mouse model used previously. We compared mice of four genotypes - wild-type, Cx32KO, p.T55I, and p.R75W. Sciatic motor conduction velocity slowing was the most robust electrophysiologic indicator of neuropathy, showing reductions in the Cx32KO by 3 months and in the p.T55I and p.R75W mice by 6 months. At both 6 and 12 months, all three mutant genotypes showed reduced four limb and hind limb grip strength compared to WT mice. Performance on 6 and 12 mm width balance beams revealed deficits that were most pronounced at on the 6 mm balance beam at 6 months of age. There were pathological changes of myelinated axons in the femoral motor nerve in all three mutant lines by 3 months of age, and these became more pronounced at 6 and 12 months of age; sensory nerves (femoral sensory and the caudal nerve of the tail) appeared normal at all ages examined. Our results demonstrate that mice can be used to show the pathogenicity of human GJB1 mutations, and these new models for CMTX1 should facilitate the preclinical work for developing treatments for CMTX1.

A new mouse model of Charcot-Marie-Tooth 2J neuropathy replicates human axonopathy and suggest alteration in axo-glia communication

Myelin is essential for rapid nerve impulse propagation and axon protection. Accordingly, defects in myelination or myelin maintenance lead to secondary axonal damage and subsequent degeneration. Studies utilizing genetic (CNPase-, MAG-, and PLP-null mice) and naturally occurring neuropathy models suggest that myelinating glia also support axons independently from myelin. Myelin protein zero (MPZ or P0), which is expressed only by Schwann cells, is critical for myelin formation and maintenance in the peripheral nervous system. Many mutations in MPZ are associated with demyelinating neuropathies (Charcot-Marie-Tooth disease type 1B [CMT1B]). Surprisingly, the substitution of threonine by methionine at position 124 of P0 (P0T124M) causes axonal neuropathy (CMT2J) with little to no myelin damage. This disease provides an excellent paradigm to understand how myelinating glia support axons independently from myelin. To study this, we generated targeted knock-in MpzT124M mutant mice, a genetically authentic model of T124M-CMT2J neuropathy. Similar to patients, these mice develop axonopathy between 2 and 12 months of age, characterized by impaired motor performance, normal nerve conduction velocities but reduced compound motor action potential amplitudes, and axonal damage with only minor compact myelin modifications. Mechanistically, we detected metabolic changes that could lead to axonal degeneration, and prominent alterations in non-compact myelin domains such as paranodes, Schmidt-Lanterman incisures, and gap junctions, implicated in Schwann cell-axon communication and axonal metabolic support. Finally, we document perturbed mitochondrial size and distribution along MpzT124M axons suggesting altered axonal transport. Our data suggest that Schwann cells in P0T124M mutant mice cannot provide axons with sufficient trophic support, leading to reduced ATP biosynthesis and axonopathy. In conclusion, the MpzT124M mouse model faithfully reproduces the human neuropathy and represents a unique tool for identifying the molecular basis for glial support of axons.

Recognising the potential of large animals for modelling neuromuscular junction physiology and disease

The aetiology and pathophysiology of many diseases of the motor unit remain poorly understood and the role of the neuromuscular junction (NMJ) in this group of disorders is particularly overlooked, especially in humans, when these diseases are comparatively rare. However, elucidating the development, function and degeneration of the NMJ is essential to uncover its contribution to neuromuscular disorders, and to explore potential therapeutic avenues to treat these devastating diseases. Until now, an understanding of the role of the NMJ in disease pathogenesis has been hindered by inherent differences between rodent and human NMJs: stark contrasts in body size and corresponding differences in associated axon length underpin some of the translational issues in animal models of neuromuscular disease. Comparative studies in large mammalian models, including examination of naturally occurring, highly prevalent animal diseases and evaluation of their treatment, might provide more relevant insights into the pathogenesis and therapy of equivalent human diseases. This review argues that large animal models offer great potential to enhance our understanding of the neuromuscular system in health and disease, and in particular, when dealing with diseases for which nerve length dependency might underly the pathogenesis.

Activation of the unfolded protein response by Connexin47 mutations associated with Pelizaeus-Merzbacher-like disease

Pelizaeus-Merzbacher-like disease type 1 (PMLD1) is a hypomyelinating disorder arising in patients with mutations in GJC2, encoding Connexin47 (Cx47). PMLD1 causes nystagmus, cerebellar ataxia, spasticity and changes in CNS white matter detected by MRI. At least one mutation (p.I33M) yields a much milder phenotype, spastic paraplegia type 44 (SPG44). Cx47 contributes to gap junction communication channels between oligodendrocytes (OLs), the myelinating cells in the central nervous system (CNS), and between OLs and astrocytes. Prior studies in cell lines have shown that PMLD1 mutants such as p.P87S display defective protein trafficking, intracellular retention in the ER and loss-of-function. Here we show that when expressed in primary OLs, three PMLD1 associated mutants (p.P87S, p.Y269D and p.M283T) show ER retention of Cx47 and evidence of activation of the cellular stress (unfolded protein response, UPR) and apoptotic pathways. On the other hand, the milder SPG44 associated mutation p.I33M shows a wild-type-like subcellular distribution and no activation of the UPR or apoptotic pathways. These studies provide new insight into a potential element of toxic gain of function underlying the mechanism of PMLD1 that should help guide future therapeutic approaches.

Relationship between toll-like receptor expression in the distal facial nerve and facial nerve recovery after injury

Objectives: This study aimed to determine whether toll-like receptor expression patterns differ in the distal facial nerve during recovery after crushing and cutting injuries. Methods: Adult male Sprague-Dawley rats underwent crushing or cutting injury of the unilateral facial nerve. Their whisker movement and blink reflex were examined. Western blotting was performed with the normal nerve on the left side and the damaged nerve on the right side, four days, 14 days, and 3 months after injury. Results: The scores of whisker movements and blink reflex in the crushing group showed improvements, while the score of the cutting group was significantly lower at 14 days and 3 months (p < 0.05). Western blotting showed that TLRs 11 and 13 increased in the crushing group, and TLRs 1, 2, 3, 4, 5, 8, 10, 11, 12, and 13 increased in the cutting group after 14 days (p < 0.05). After 3 months, TLRs 10 and 11 increased in the crushing group, and TLRs 1, 4, 5, 8, 11, and 12 increased in the cutting group (p < 0.05). Conclusion: TLRs 1, 4, 5, 8, and 12 are related to nerve degeneration after facial nerve injury, and TLRs 10, 11, and 13 are related to recovery from facial palsy.

Structure and Functions of Gap Junctions and Their Constituent Connexins in the Mammalian CNS

Numerous data obtained in the last 20 years indicate that all parts of the mature central nervous system, from the retina and olfactory bulb to the spinal cord and brain, contain cells connected by gap junctions (GJs). The morphological basis of the GJs is a group of joined membrane hemichannels called connexons, the subunit of each connexon is the protein connexin. In the central nervous system, connexins show specificity and certain types of them are expressed either in neurons or in glial cells. Connexins and GJs of neurons, combining certain types of inhibitory hippocampal and neocortical neuronal ensembles, provide synchronization of local impulse and rhythmic activity, thalamocortical conduction, control of excitatory connections, which reflects their important role in the processes of perception, concentration of attention and consolidation of memory, both on the cellular and at the system level. Connexins of glial cells are ubiquitously expressed in the brain, and the GJs formed by them provide molecular signaling and metabolic cooperation and play a certain role in the processes of neuronal migration during brain development, myelination, tissue homeostasis, and apoptosis. At the same time, mutations in the genes of glial connexins, as well as a deficiency of these proteins, are associated with such diseases as congenital neuropathies, hearing loss, skin diseases, and brain tumors. This review summarizes the existing data of numerous molecular, electrophysiological, pharmacological, and morphological studies aimed at progress in the study of the physiological and pathophysiological significance of glial and neuronal connexins and GJs for the central nervous system.

Characterizing the molecular etiology of arthrogryposis multiplex congenita in patients with LGI4 mutations

Disruption of axon-glia interactions in the peripheral nervous system has emerged as a major cause of arthrogryposis multiplex congenita (AMC), a condition characterized by multiple congenital postural abnormalities involving the major joints. Several genes crucially important to the biology of Schwann cells have now been implicated with AMC. One such gene is LGI4 which encodes a secreted glycoprotein. LGI4 is expressed and secreted by Schwann cells and binds its receptor ADAM22 on the axonal membrane to drive myelination. Homozygous mutations in LGI4 or ADAM22 results in severe congenital hypomyelination and joint contractures in mice. Recently bi-allelic LGI4 loss of function mutations has been described in three unrelated families with severe AMC. Two individuals in a fourth, non-consanguineous family were found to be compound heterozygous for two LGI4 missense mutations. It is not known how these missense mutations affect the biology of LGI4. Here we investigated whether these missense mutations affected the secretion of the protein, its ADAM22 binding capacity, or its myelination-promoting function. We demonstrate that the mutations largely affect the progression of the mutant protein through the endomembrane system resulting in severely reduced expression. Importantly, binding to ADAM22 and myelination-promoting activity appear largely unaffected, suggesting that treatment with chemical chaperones to improve secretion of the mutant proteins might prove beneficial.

Mek/ERK1/2-MAPK and PI3K/Akt/mTOR signaling plays both independent and cooperative roles in Schwann cell differentiation, myelination and dysmyelination

Multiple signals are involved in the regulation of developmental myelination by Schwann cells and in the maintenance of a normal myelin homeostasis throughout adult life, preserving the integrity of the axons in the PNS. Recent studies suggest that Mek/ERK1/2-MAPK and PI3K/Akt/mTOR intracellular signaling pathways play important, often overlapping roles in the regulation of myelination in the PNS. In addition, hyperactivation of these signaling pathways in Schwann cells leads to a late onset of various pathological changes in the sciatic nerves. However, it remains poorly understood whether these pathways function independently or sequentially or converge using a common mechanism to facilitate Schwann cell differentiation and myelin growth during development and in causing pathological changes in the adult animals. To address these questions, we analyzed multiple genetically modified mice using simultaneous loss- and constitutive gain-of-function approaches. We found that during development, the Mek/ERK1/2-MAPK pathway plays a primary role in Schwann cell differentiation, distinct from mTOR. However, during active myelination, ERK1/2 is dependent on mTOR signaling to drive the growth of the myelin sheath and regulate its thickness. Finally, our data suggest that peripheral nerve pathology during adulthood caused by hyperactivation of Mek/ERK1/2-MAPK or PI3K is likely to be independent or dependent on mTOR-signaling in different contexts. Thus, this study highlights the complexities in the roles played by two major intracellular signaling pathways in Schwann cells that affect their differentiation, myelination, and later PNS pathology and predicts that potential therapeutic modulation of these pathways in PNS neuropathies could be a complex process.

Emerging Therapies for Charcot-Marie-Tooth Inherited Neuropathies

Inherited neuropathies known as Charcot-Marie-Tooth (CMT) disease are genetically heterogeneous disorders affecting the peripheral nerves, causing significant and slowly progressive disability over the lifespan. The discovery of their diverse molecular genetic mechanisms over the past three decades has provided the basis for developing a wide range of therapeutics, leading to an exciting era of finding treatments for this, until now, incurable group of diseases. Many treatment approaches, including gene silencing and gene replacement therapies, as well as small molecule treatments are currently in preclinical testing while several have also reached clinical trial stage. Some of the treatment approaches are disease-specific targeted to the unique disease mechanism of each CMT form, while other therapeutics target common pathways shared by several or all CMT types. As promising treatments reach the stage of clinical translation, optimal outcome measures, novel biomarkers and appropriate trial designs are crucial in order to facilitate successful testing and validation of novel treatments for CMT patients.

Pain Phenotypes in Rare Musculoskeletal and Neuromuscular Diseases

For patients diagnosed with a rare musculoskeletal or neuromuscular disease, pain may transition from acute to chronic; the latter yielding additional challenges for both patients and care providers. We assessed the present understanding of pain across a set of ten rare, noninfectious, noncancerous disorders; Osteogenesis Imperfecta, Ehlers-Danlos Syndrome, Achondroplasia, Fibrodysplasia Ossificans Progressiva, Fibrous Dysplasia/McCune-Albright Syndrome, Complex Regional Pain Syndrome, Duchenne Muscular Dystrophy, Infantile- and Late-Onset Pompe disease, Charcot-Marie-Tooth Disease, and Amyotrophic Lateral Sclerosis. Through the integration of natural history, cross-sectional, retrospective, clinical trials, & case studies we described pathologic and genetic factors, pain sources, phenotypes, and lastly, existing therapeutic approaches. We highlight that while rare diseases possess distinct core pathologic features, there are a number of shared pain phenotypes and mechanisms that may be prospectively examined and therapeutically targeted in a parallel manner. Finally, we describe clinical and research approaches that may facilitate more accurate diagnosis, monitoring, and treatment of pain as well as elucidation of the evolving nature of pain phenotypes in rare musculoskeletal or neuromuscular illnesses.

Gene expression profiles of human adipose-derived mesenchymal stem cells dynamically seeded on clinically available processed nerve allografts and collagen nerve guides

It was hypothesized that mesenchymal stem cells (MSCs) could provide necessary trophic factors when seeded onto the surfaces of commonly used nerve graft substitutes. We aimed to determine the gene expression of MSCs when influenced by Avance® Nerve Grafts or NeuraGen® Nerve Guides. Human adipose-derived MSCs were cultured and dynamically seeded onto 30 Avance® Nerve Grafts and 30 NeuraGen® Nerve Guides for 12 hours. At six time points after seeding, quantitative polymerase chain reaction analyses were performed for five samples per group. Neurotrophic [nerve growth factor (NGF), glial cell line-derived neurotrophic factor (GDNF), pleiotrophin (PTN), growth associated protein 43 (GAP43) and brain-derived neurotrophic factor (BDNF)], myelination [peripheral myelin protein 22 (PMP22) and myelin protein zero (MPZ)], angiogenic [platelet endothelial cell adhesion molecule 1 (PECAM1/CD31) and vascular endothelial cell growth factor alpha (VEGFA)], extracellular matrix (ECM) [collagen type alpha I (COL1A1), collagen type alpha III (COL3A1), Fibulin 1 (FBLN1) and laminin subunit beta 2 (LAMB2)] and cell surface marker cluster of differentiation 96 (CD96) gene expression was quantified. Unseeded Avance® Nerve Grafts and NeuraGen® Nerve Guides were used to evaluate the baseline gene expression, and unseeded MSCs provided the baseline gene expression of MSCs. The interaction of MSCs with the Avance® Nerve Grafts led to a short-term upregulation of neurotrophic (NGF, GDNF and BDNF), myelination (PMP22 and MPZ) and angiogenic genes (CD31 and VEGFA) and a long-term upregulation of BDNF, VEGFA and COL1A1. The interaction between MSCs and the NeuraGen® Nerve Guide led to short term upregulation of neurotrophic (NGF, GDNF and BDNF) myelination (PMP22 and MPZ), angiogenic (CD31 and VEGFA), ECM (COL1A1) and cell surface (CD96) genes and long-term upregulation of neurotrophic (GDNF and BDNF), angiogenic (CD31 and VEGFA), ECM genes (COL1A1, COL3A1, and FBLN1) and cell surface (CD96) genes. Analysis demonstrated MSCs seeded onto NeuraGen® Nerve Guides expressed significantly higher levels of neurotrophic (PTN), angiogenic (VEGFA) and ECM (COL3A1, FBLN1) genes in the long term period compared to MSCs seeded onto Avance® Nerve Grafts. Overall, the interaction between human MSCs and both nerve graft substitutes resulted in a significant upregulation of the expression of numerous genes important for nerve regeneration over time. The in vitro interaction of MSCs with the NeuraGen® Nerve Guide was more pronounced, particularly in the long term period (> 14 days after seeding). These results suggest that MSC-seeding has potential to be applied in a clinical setting, which needs to be confirmed in future in vitro and in vivo research.

AAV9-mediated Schwann cell-targeted gene therapy rescues a model of demyelinating neuropathy

Mutations in the GJB1 gene, encoding the gap junction (GJ) protein connexin32 (Cx32), cause X-linked Charcot-Marie-Tooth disease (CMT1X), an inherited demyelinating neuropathy. We developed a gene therapy approach for CMT1X using an AAV9 vector to deliver the GJB1/Cx32 gene under the myelin protein zero (Mpz) promoter for targeted expression in Schwann cells. Lumbar intrathecal injection of the AAV9-Mpz.GJB1 resulted in widespread biodistribution in the peripheral nervous system including lumbar roots, sciatic and femoral nerves, as well as in Cx32 expression in the paranodal non-compact myelin areas of myelinated fibers. A pre-, as well as post-onset treatment trial in Gjb1-null mice, demonstrated improved motor performance and sciatic nerve conduction velocities along with improved myelination and reduced inflammation in peripheral nerve tissues. Blood biomarker levels were also significantly ameliorated in treated mice. This study provides evidence that a clinically translatable AAV9-mediated gene therapy approach targeting Schwann cells could potentially treat CMT1X.

Genetic mechanisms of peripheral nerve disease

Peripheral neuropathies of genetic etiology are a very diverse group of disorders manifesting either as non-syndromic inherited neuropathies without significant manifestations outside the peripheral nervous system, or as part of a systemic or syndromic genetic disorder. The former and most frequent group is collectively known as Charcot-Marie-Tooth disease (CMT), with prevalence as high as 1:2,500 world-wide, and has proven to be genetically highly heterogeneous. More than 100 different genes have been identified so far to cause various CMT forms, following all possible inheritance patterns. CMT causative genes belong to several common functional pathways that are essential for the integrity of the peripheral nerve. Their discovery has provided insights into the normal biology of axons and myelinating cells, and has highlighted the molecular mechanisms including both loss of function and gain of function effects, leading to peripheral nerve degeneration. Demyelinating neuropathies result from dysfunction of genes primarily affecting myelinating Schwann cells, while axonal neuropathies are caused by genes affecting mostly neurons and their long axons. Furthermore, mutation in genes expressed outside the nervous system, as in the case of inherited amyloid neuropathies, may cause peripheral neuropathy resulting from accumulation of β-structured amyloid fibrils in peripheral nerves in addition to various organs. Increasing insights into the molecular-genetic mechanisms have revealed potential therapeutic targets. These will enable the development of novel therapeutics for genetic neuropathies that remain, in their majority, without effective treatment.

Axo-glial interaction in the injured PNS

Axons share a close relationship with Schwann cells, their glial partners in peripheral nerves. An intricate axo-glia network of signals and bioactive molecules regulates the major aspects of nerve development and normal functioning of the peripheral nervous system. Disruptions to these complex axo-glial interactions can have serious neurological consequences, as typically seen in injured nerves. Recent studies in inherited neuropathies have demonstrated that damage to one of the partners in this symbiotic unit ultimately leads to impairment of the other partner, emphasizing the bidirectional influence of axon to glia and glia to axon signaling in these diseases. After physical trauma to nerves, dramatic alterations in the architecture and signaling environment of peripheral nerves take place. Here, axons and Schwann cells respond adaptively to these perturbations and change the nature of their reciprocal interactions, thereby driving the remodeling and regeneration of peripheral nerves. In this review, we focus on the nature and importance of axon-glia interactions in injured nerves, both for the reshaping and repair of nerves after trauma, and in driving pathology in inherited peripheral neuropathies.

A Yeast-Based Model for Hereditary Motor and Sensory Neuropathies: A Simple System for Complex, Heterogeneous Diseases

Charcot–Marie–Tooth (CMT) disease encompasses a group of rare disorders that are characterized by similar clinical manifestations and a high genetic heterogeneity. Such excessive diversity presents many problems. Firstly, it makes a proper genetic diagnosis much more difficult and, even when using the most advanced tools, does not guarantee that the cause of the disease will be revealed. Secondly, the molecular mechanisms underlying the observed symptoms are extremely diverse and are probably different for most of the disease subtypes. Finally, there is no possibility of finding one efficient cure for all, or even the majority of CMT diseases. Every subtype of CMT needs an individual approach backed up by its own research field. Thus, it is little surprise that our knowledge of CMT disease as a whole is selective and therapeutic approaches are limited. There is an urgent need to develop new CMT models to fill the gaps. In this review, we discuss the advantages and disadvantages of yeast as a model system in which to study CMT diseases. We show how this single-cell organism may be used to discriminate between pathogenic variants, to uncover the mechanism of pathogenesis, and to discover new therapies for CMT disease.

Enhanced axonal neuregulin-1 type-III signaling ameliorates neurophysiology and hypomyelination in a Charcot-Marie-Tooth type 1B mouse model

Charcot-Marie-Tooth (CMT) neuropathies are a group of genetic disorders that affect the peripheral nervous system with heterogeneous pathogenesis and no available treatment. Axonal neuregulin 1 type III (Nrg1TIII) drives peripheral nerve myelination by activating downstream signaling pathways such as PI3K/Akt and MAPK/Erk that converge on master transcriptional regulators of myelin genes, such as Krox20. We reasoned that modulating Nrg1TIII activity may constitute a general therapeutic strategy to treat CMTs that are characterized by reduced levels of myelination. Here we show that genetic overexpression of Nrg1TIII ameliorates neurophysiological and morphological parameters in a mouse model of demyelinating CMT1B, without exacerbating the toxic gain-of-function that underlies the neuropathy. Intriguingly, the mechanism appears not to be related to Krox20 or myelin gene upregulation, but rather to a beneficial rebalancing in the stoichiometry of myelin lipids and proteins. Finally, we provide proof of principle that stimulating Nrg1TIII signaling, by pharmacological suppression of the Nrg1TIII inhibitor tumor necrosis factor-alpha-converting enzyme (TACE/ADAM17), also ameliorates the neuropathy. Thus, modulation of Nrg1TIII by TACE/ADAM17 inhibition may represent a general treatment for hypomyelinating neuropathies.

Gene therapy approaches targeting Schwann cells for demyelinating neuropathies

Charcot-Marie-Tooth disease (CMT) encompasses numerous genetically heterogeneous inherited neuropathies, which together are one of the commonest neurogenetic disorders. Axonal CMT types result from mutations in neuronally expressed genes, whereas demyelinating CMT forms mostly result from mutations in genes expressed by myelinating Schwann cells. The demyelinating forms are the most common, and may be caused by dominant mutations and gene dosage effects (as in CMT1), as well as by recessive mutations and loss of function mechanisms (as in CMT4). The discovery of causative genes and increasing insights into molecular mechanisms through the study of experimental disease models has provided the basis for the development of gene therapy approaches. For demyelinating CMT, gene silencing or gene replacement strategies need to be targeted to Schwann cells. Progress in gene replacement for two different CMT forms, including CMT1X caused by GJB1 gene mutations, and CMT4C, caused by SH3TC2 gene mutations, has been made through the use of a myelin-specific promoter to restrict expression in Schwann cells, and by lumbar intrathecal delivery of lentiviral viral vectors to achieve more widespread biodistribution in the peripheral nervous system. This review summarizes the molecular-genetic mechanisms of selected demyelinating CMT neuropathies and the progress made so far, as well as the remaining challenges in the path towards a gene therapy to treat these disorders through the use of optimal gene therapy tools including clinically translatable delivery methods and adeno-associated viral (AAV) vectors.

Investigation of Mutations in Exon 14 of SH3TC2 Gene and Exon 7 of NDRG1 Gene in Iranian Charcot-Marie-Tooth Disease Type 4 (CMT4D) Patients

OBJECTIVES

Charcot-Marie-tooth disease type 4 (CMT4D) is an autosomal recessive form of Charcot-Marie-tooth disease with an earlier age of onset and greater severity, compared to other types of this disease. CMT4C and CMT4D are the most prevalent subtypes in Mediterranean countries due to the higher rate of consanguineous marriage. In this study, we aimed to identify p.R148X mutation in NDRG1 gene and p.R1109X mutation in SH3TC2 gene (responsible for CMT4D and CMT4C, respectively) and to investigate other possible nucleotide changes in exon 14 of SH3TC2 gene and exon 7 of NDRG1 gene in an Iranian population.

MATERIALS & METHODS

A total of 24 CMT4D patients, who were referred to Iran Special Medical Center, were clinically and electrophysiologically evaluated in this study. DNA was extracted from the patients' blood samples. Next, polymerase chain reaction (PCR) assay was carried out, and the products were sequenced and analyzed in FinchTV software.

RESULTS

None of the target mutations were found in this study. Sequencing of SH3TC2 gene showed SNP rs1025476 (g.57975C>T) in 21 (87.5%) patients, including 7 homozygous and 14 heterozygous individuals.

CONCLUSION

Despite the high rate of mutations in some populations, it seems that they are very rare in Iranian CMT4D patients. Regarding the association of SNP rs1025476 with CMT4D, further assessments are needed to reach a better understanding of genetic markers and their genetic features and to propose better diagnostic and treatment plans for the Iranian population.

Gene replacement therapy after neuropathy onset provides therapeutic benefit in a model of CMT1X

X-linked Charcot-Marie-Tooth disease (CMT1X), one of the commonest forms of inherited demyelinating neuropathy, results from GJB1 gene mutations causing loss of function of the gap junction protein connexin32 (Cx32). The aim of this study was to examine whether delayed gene replacement therapy after the onset of peripheral neuropathy can provide a therapeutic benefit in the Gjb1-null/Cx32 knockout model of CMT1X. After delivery of the LV-Mpz.GJB1 lentiviral vector by a single lumbar intrathecal injection into 6-month-old Gjb1-null mice, we confirmed expression of Cx32 in lumbar roots and sciatic nerves correctly localized at the paranodal myelin areas. Gjb1-null mice treated with LV-Mpz.GJB1 compared with LV-Mpz.Egfp (mock) vector at the age of 6 months showed improved motor performance at 8 and 10 months. Furthermore, treated mice showed increased sciatic nerve conduction velocities, improvement of myelination and reduced inflammation in lumbar roots and peripheral nerves at 10 months of age, along with enhanced quadriceps muscle innervation. Plasma neurofilament light (NEFL) levels, a clinically relevant biomarker, were also ameliorated in fully treated mice. Intrathecal gene delivery after the onset of peripheral neuropathy offers a significant therapeutic benefit in this disease model, providing a proof of principle for treating patients with CMT1X at different ages.

FGF10 Enhances Peripheral Nerve Regeneration via the Preactivation of the PI3K/Akt Signaling-Mediated Antioxidant Response

The process of axonal regeneration after peripheral nerve injury (PNI) is slow and mostly incomplete. Previous studies have investigated the neuroprotective effects of fibroblast growth factor 10 (FGF10) against spinal cord injury and cerebral ischemia brain injury. However, the role of FGF10 in peripheral nerve regeneration remains unknown. In this study, we aimed to investigate the underlying therapeutic effects of FGF10 on nerve regeneration and functional recovery after PNI and to explore the associated mechanism. Our results showed that FGF10 administration promoted axonal regeneration and functional recovery after nerve damage. Moreover, exogenous FGF10 treatment also prevented SCs from excessive oxidative stress-induced apoptosis, which was probably related to the activation of phosphatidylinositol-3 kinase/protein kinase B (PI3K/Akt) signaling. The inhibition of the PI3K/Akt pathway by the specific inhibitor LY294002 partially reversed the therapeutic effects of FGF10 both in vivo and in vitro. Thus, from our perspective, FGF10 may be a promising therapeutic drug for repairing sciatic nerve damage through countering excessive oxidative stress-induced SC apoptosis.

An In Vitro Model of Charcot-Marie-Tooth Disease Type 4B2 Provides Insight Into the Roles of MTMR13 and MTMR2 in Schwann Cell Myelination

Charcot-Marie-Tooth Disorder Type 4B (CMT4B) is a demyelinating peripheral neuropathy caused by mutations in myotubularin-related (MTMR) proteins 2, 13, or 5 (CMT4B1/2/3), which regulate phosphoinositide turnover and endosomal trafficking. Although mouse models of CMT4B2 exist, an in vitro model would make possible pharmacological and reverse genetic experiments needed to clarify the role of MTMR13 in myelination. We have generated such a model using Schwann cell-dorsal root ganglion (SC-DRG) explants from Mtmr13^−/− mice. Myelin sheaths in mutant cultures contain outfoldings highly reminiscent of those observed in the nerves of Mtmr13^−/− mice and CMT4B2 patients. Mtmr13^−/− SC-DRG explants also contain reduced Mtmr2, further supporting a role of Mtmr13 in stabilizing Mtmr2. Elevated PI(3,5)P₂ has been implicated as a cause of myelin outfoldings in Mtmr2^−/− models. In contrast, the role of elevated PI3P or PI(3,5)P₂ in promoting outfoldings in Mtmr13^−/− models is unclear. We found that over-expression of MTMR2 in Mtmr13^−/− SC-DRGs moderately reduced the prevalence of myelin outfoldings. Thus, a manipulation predicted to lower PI3P and PI(3,5)P₂ partially suppressed the phenotype caused by Mtmr13 deficiency. We also explored the relationship between CMT4B2-like myelin outfoldings and kinases that produce PI3P and PI(3,5)P₂ by analyzing nerve pathology in mice lacking both Mtmr13 and one of two specific PI 3-kinases. Intriguingly, the loss of vacuolar protein sorting 34 or PI3K-C2β in Mtmr13^−/− mice had no impact on the prevalence of myelin outfoldings. In aggregate, our findings suggest that the MTMR13 scaffold protein likely has critical functions other than stabilizing MTMR2 to achieve an adequate level of PI 3-phosphatase activity.

The TSC1-mTOR-PLK axis regulates the homeostatic switch from Schwann cell proliferation to myelination in a stage-specific manner

Proper peripheral myelination depends upon the balance between Schwann cell proliferation and differentiation programs. The serine/threonine kinase mTOR integrates various environmental cues to serve as a central regulator of cell growth, metabolism, and function. We report here that tuberous sclerosis complex 1 (TSC1), a negative regulator of mTOR activity, establishes a stage-dependent program for Schwann cell lineage progression and myelination by controlling cell proliferation and myelin homeostasis. Tsc1 ablation in Schwann cell progenitors in mice resulted in activation of mTOR signaling, and caused over-proliferation of Schwann cells and blocked their differentiation, leading to hypomyelination. Transcriptome profiling analysis revealed that mTOR activation in Tsc1 mutants resulted in upregulation of a polo-like kinase (PLK)-dependent pathway and cell cycle regulators. Attenuation of mTOR or pharmacological inhibition of polo-like kinases partially rescued hypomyelination caused by Tsc1 loss in the developing peripheral nerves. In contrast, deletion of Tsc1 in mature Schwann cells led to redundant and overgrown myelin sheaths in adult mice. Together, our findings indicate stage-specific functions for the TSC1-mTOR-PLK signaling axis in controlling the transition from proliferation to differentiation and myelin homeostasis during Schwann cell development.

Characterization of Naïve and Vitamin C-Treated Mouse Schwann Cell Line MSC80: Induction of the Antioxidative Thioredoxin Related Transmembrane Protein 1

Schwann cells (SCs) are essential in the production of the axon-wrapping myelin sheath and provide trophic function and repair mechanisms in the peripheral nerves. Consequently, well-characterized SC in vitro models are needed to perform preclinical studies including the investigation of the complex biochemical adaptations occurring in the peripheral nervous system (PNS) under different (patho)physiological conditions. MSC80 cells represent a murine SC line used as an in vitro system for neuropathological studies. Here, we introduce the most abundant 9532 proteins identified via mass spectrometry-based protein analytics, and thus provide the most comprehensive SC protein catalogue published thus far. We cover proteins causative for inherited neuropathies and demonstrate that in addition to cytoplasmic, nuclear and mitochondrial proteins and others belonging to the protein processing machinery are very well covered. Moreover, we address the suitability of MSC80 to examine the molecular effect of a drug-treatment by analyzing the proteomic signature of Vitamin C-treated cells. Proteomic findings, immunocytochemistry, immunoblotting and functional experiments support the concept of a beneficial role of Vitamin C on oxidative stress and identified TMX1 as an oxidative stress protective factor, which might represent a promising avenue for therapeutic intervention of PNS-disorders with oxidative stress burden such as diabetic neuropathy.

Pathological classification of equine recurrent laryngeal neuropathy

Recurrent Laryngeal Neuropathy (RLN) is a highly prevalent and predominantly left-sided, degenerative disorder of the recurrent laryngeal nerves (RLn) of tall horses, that causes inspiratory stridor at exercise because of intrinsic laryngeal muscle paresis. The associated laryngeal dysfunction and exercise intolerance in athletic horses commonly leads to surgical intervention, retirement or euthanasia with associated financial and welfare implications. Despite speculation, there is a lack of consensus and conflicting evidence supporting the primary classification of RLN, as either a distal ("dying back") axonopathy or as a primary myelinopathy and as either a (bilateral) mononeuropathy or a polyneuropathy; this uncertainty hinders etiological and pathophysiological research. In this review, we discuss the neuropathological changes and electrophysiological deficits reported in the RLn of affected horses, and the evidence for correct classification of the disorder. In so doing, we summarize and reveal the limitations of much historical research on RLN and propose future directions that might best help identify the etiology and pathophysiology of this enigmatic disorder.

MicroRNA Mediated Regulation of Schwann Cell Migration and Proliferation in Peripheral Nerve Injury

Schwann cells (SCs) contribute to nerve repair following injury; however, the underlying molecular mechanism is poorly understood. MicroRNAs (miRNAs), which are short noncoding RNAs, have been shown to play a role in neuronal disease. In this work, we show that miRNAs regulate the peripheral nerve system by modulating the migration and proliferation of SCs. Thus, miRNAs expressed in peripheral nerves may provide a potential therapeutic target for peripheral nerve injury or repair.

HERC1 Ubiquitin Ligase Is Required for Normal Axonal Myelination in the Peripheral Nervous System

A missense mutation in HERC1 provokes loss of cerebellar Purkinje cells, tremor, and unstable gait in tambaleante (tbl) mice. Recently, we have shown that before cerebellar degeneration takes place, the tbl mouse suffers from a reduction in the number of vesicles available for release at the neuromuscular junction (NMJ). The aim of the present work was to study to which extent the alteration in HERC1 may affect other cells in the nervous system and how this may influence the motor dysfunction observed in these mice. The functional analysis showed a consistent delay in the propagation of the action potential in mutant mice in comparison with control littermates. Morphological analyses of glial cells in motor axons revealed signs of compact myelin damage as tomacula and local hypermyelination foci. Moreover, we observed an alteration in non-myelinated terminal Schwann cells at the level of the NMJ. Additionally, we found a significant increment of phosphorylated Akt-2 in the sciatic nerve. Based on these findings, we propose a molecular model that could explain how mutated HERC1 in tbl mice affects the myelination process in the peripheral nervous system. Finally, since the myelin abnormalities found in tbl mice are histological hallmarks of neuropathic periphery diseases, tbl mutant mice could be considered as a new mouse model for this type of diseases.

miR-221-3p Inhibits Schwann Cell Myelination

Following peripheral nerve injury, Schwann Cells (SCs) undergo dedifferentiation, proliferation, migration, and remyelination. Recent works demonstrated the importance of the short non-coding RNA (miRNAs) in SC dedifferentiation and remyelination after nerve injury. Previously, we found some miRNAs like miR-9, miR-221, miR-222 and miR-182 could regulate the proliferation and migration of SCs. Therefore, it is imperative to ask whether these miRNAs could regulate the myelination of SCs. Here we demonstrated that miR-221-3p could inhibit the myelination of SCs when co-cultured with dorsal root ganglion cells in vitro. In addition, NGF1-A binding protein 1 (Nab1) which was essential for SCs myelination could be downregulated by miR-221-3p. Suppressing the expression of Nab1 could reverse the promotion of miR-221-3p antagomir on SC myelination. The effects of miR-221-3p on SC myelination might be used to improve peripheral nerve regeneration, thus offering a new approach to peripheral nerve repair.

A histone deacetylase 3-dependent pathway delimits peripheral myelin growth and functional regeneration

Deficits in Schwann cell-mediated remyelination impair functional restoration after nerve damage, contributing to peripheral neuropathies. The mechanisms mediating block of remyelination remain elusive. Here, through small-molecule screening focusing on epigenetic modulators, we identified histone deacetylase 3 (HDAC3; a histone-modifying enzyme) as a potent inhibitor of peripheral myelinogenesis. Inhibition of HDAC3 enhanced myelin growth and regeneration and improved functional recovery after peripheral nerve injury in mice. HDAC3 antagonizes the myelinogenic neuregulin-PI3K-AKT signaling axis. Moreover, genome-wide profiling analyses revealed that HDAC3 represses promyelinating programs through epigenetic silencing while coordinating with p300 histone acetyltransferase to activate myelination-inhibitory programs that include the HIPPO signaling effector TEAD4 to inhibit myelin growth. Schwann cell-specific deletion of either Hdac3 or Tead4 in mice resulted in an elevation of myelin thickness in sciatic nerves. Thus, our findings identify the HDAC3-TEAD4 network as a dual-function switch of cell-intrinsic inhibitory machinery that counters myelinogenic signals and maintains peripheral myelin homeostasis, highlighting the therapeutic potential of transient HDAC3 inhibition for improving peripheral myelin repair.

Impairment of protein degradation and proteasome function in hereditary neuropathies

In several neurodegenerative diseases in which misfolded proteins accumulate there is impairment of the ubiquitin proteasome system (UPS). We tested if a similar disruption of proteostasis occurs in hereditary peripheral neuropathies. In sciatic nerves from mouse models of two human neuropathies, Myelin Protein Zero mutation (S63del) and increased copy number (P0 overexpression), polyubiquitinated proteins accumulated, and the overall rates of protein degradation were decreased. 26S proteasomes affinity-purified from sciatic nerves of S63del mice were defective in degradation of peptides and a ubiquitinated protein, unlike proteasomes from P0 overexpression, which appeared normal. Nevertheless, cellular levels of 26S proteasomes were increased in both, through the proteolytic-activation of the transcription factor Nrf1, as occurs in response to proteasome inhibitors. In S63del, increased amounts of the deubiquitinating enzymes USP14, UCH37, and USP5 were associated with proteasomes, the first time this has been reported in a human disease model. Inhibitors of USP14 increased the rate of protein degradation in S63del sciatic nerves and unexpectedly increased the phosphorylation of eIF2α by Perk. Thus, proteasome content, composition and activity are altered in these diseases and USP14 inhibitors have therapeutic potential in S63del neuropathy.

Strength of ERK1/2 MAPK Activation Determines Its Effect on Myelin and Axonal Integrity in the Adult CNS

Myelin growth is a tightly regulated process driven by multiple signals. ERK1/2-MAPK signaling is an important regulator of myelin thickness. Because, in demyelinating diseases, the myelin formed during remyelination fails to achieve normal thickness, increasing ERK1/2 activity in oligodendrocytes is of obvious therapeutic potential for promoting efficient remyelination. However, other studies have suggested that increased levels of ERK1/2 activity could, in fact, have detrimental effects on myelinating cells. Because the strength, duration, or timing of ERK1/2 activation may alter the biological outcomes of cellular responses markedly, here, we investigated the effect of modulating ERK1/2 activity in myelinating cells using transgenic mouse lines in which ERK1/2 activation was upregulated conditionally in a graded manner. We found enhanced myelin gene expression and myelin growth in the adult CNS at both moderate and hyperactivated levels of ERK1/2 when upregulation commenced during developmental myelination or was induced later during adulthood in quiescent preexisting oligodendrocytes, after active myelination is largely terminated. However, a late onset of demyelination and axonal degeneration occurred at hyperelevated, but not moderately elevated, levels regardless of the timing of the upregulation. Similarly, myelin and axonal pathology occurred with elevated ERK1/2 activity in Schwann cells. We conclude that a fine tuning of ERK1/2 signaling strength is critically important for normal oligodendrocyte and Schwann cell function and that disturbance of this balance has negative consequences for myelin and axonal integrity in the long term. Therefore, therapeutic modulation of ERK1/2 activity in demyelinating disease or peripheral neuropathies must be approached with caution.

SIGNIFICANCE STATEMENT

ERK1/2-MAPK activation in oligodendrocytes and Schwann cells is an important signal for promoting myelin growth during developmental myelination. Here, we show that, when ERK1/2 are activated in mature quiescent oligodendrocytes during adulthood, new myelin growth is reinitiated even after active myelination is terminated, which has implications for understanding the mechanism underlying plasticity of myelin in adult life. Paradoxically, simply increasing the "strength" of ERK1/2 activation changed the biological outcome from beneficial to detrimental, adversely affecting myelin and axonal integrity in both the CNS and PNS. Therefore, this study highlights the complexity of ERK1/2-MAPK signaling in the context of oligodendrocyte and Schwann cell function in the adult animal and emphasizes the need to approach potential therapeutic modulation of ERK1/2 activity with caution.

Crucial motifs and residues in the extracellular loops influence the formation and specificity of connexin docking

Most of the early studies on gap junction (GJ) channel function and docking compatibility were on rodent connexins, while recent research on GJ channels gradually shifted from rodent to human connexins largely due to the fact that mutations in many human connexin genes are found to associate with inherited human diseases. The studies on human connexins have revealed some key differences from those found in rodents, calling for a comprehensive characterization of human GJ channels. Functional studies revealed that docking and formation of functional GJ channels between two hemichannels are possible only between docking-compatible connexins. Two groups of docking-compatible rodent connexins have been identified. Compatibility is believed to be due to their amino acid residue differences at the extracellular loop domains (E1 and E2). Sequence alignment of the E1 and E2 domains of all connexins known to make GJs revealed that they are highly conserved and show high sequence identity with human Cx26, which is the only connexin with near atomic resolution GJ structure. We hypothesize that different connexins have a similar structure as that of Cx26 at the E1 and E2 domains and use the corresponding residues in their E1 and E2 domains for docking. Based on the Cx26 GJ structure and sequence analysis of well-studied connexins, we propose that the E1-E1 docking interactions are staggered with each E1 interacting with two E1s on the docked connexon. The putative E1 docking residues are conserved in both docking-compatible and -incompatible connexins, indicating that E1 does not likely serve a role in docking compatibility. However, in the case of E2-E2 docking interactions, the putative docking residues are only conserved within the docking-compatible connexins, suggesting the E2 is likely to serve the function of docking compatibility. Docking compatibility studies on human connexins have attracted a lot of attention due to the fact that putative docking residues are mutational hotspots for several connexin-linked human diseases. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

Schwann cell-specific deletion of the endosomal PI 3-kinase Vps34 leads to delayed radial sorting of axons, arrested myelination, and abnormal ErbB2-ErbB3 tyrosine kinase signaling

The PI 3-kinase Vps34 (Pik3c3) synthesizes phosphatidylinositol 3-phosphate (PI3P), a lipid critical for both endosomal membrane traffic and macroautophagy. Human genetics have implicated PI3P dysregulation, and endosomal trafficking in general, as a recurring cause of demyelinating Charcot-Marie-Tooth (CMT) peripheral neuropathy. Here, we investigated the role of Vps34, and PI3P, in mouse Schwann cells by selectively deleting Vps34 in this cell type. Vps34-Schwann cell knockout (Vps34^SCKO ) mice show severe hypomyelination in peripheral nerves. Vps34^-/- Schwann cells interact abnormally with axons, and there is a delay in radial sorting, a process by which large axons are selected for myelination. Upon reaching the promyelinating stage, Vps34^-/- Schwann cells are significantly impaired in the elaboration of myelin. Nerves from Vps34^SCKO mice contain elevated levels of the LC3 and p62 proteins, indicating impaired autophagy. However, in the light of recent demonstrations that autophagy is dispensable for myelination, it is unlikely that hypomyelination in Vps34^SCKO mice is caused by impaired autophagy. Endosomal trafficking is also disturbed in Vps34^-/- Schwann cells. We investigated the activation of the ErbB2/3 receptor tyrosine kinases in Vps34^SCKO nerves, as these proteins, which play essential roles in Schwann cell myelination, are known to traffic through endosomes. In Vps34^SCKO nerves, ErbB3 was hyperphosphorylated on a tyrosine known to be phosphorylated in response to neuregulin 1 exposure. ErbB2 protein levels were also decreased during myelination. Our findings suggest that the loss of Vps34 alters the trafficking of ErbB2/3 through endosomes. Abnormal ErbB2/3 signaling to downstream targets may contribute to the hypomyelination observed in Vps34^SCKO mice.

mTORC1 promotes proliferation of immature Schwann cells and myelin growth of differentiated Schwann cells

The myelination of axons in peripheral nerves requires precisely coordinated proliferation and differentiation of Schwann cells (SCs). We found that the activity of the mechanistic target of rapamycin complex 1 (mTORC1), a key signaling hub for the regulation of cellular growth and proliferation, is progressively extinguished as SCs differentiate during nerve development. To study the effects of different levels of sustained mTORC1 hyperactivity in the SC lineage, we disrupted negative regulators of mTORC1, including TSC2 or TSC1, in developing SCs of mutant mice. Surprisingly, the phenotypes ranged from arrested myelination in nerve development to focal hypermyelination in adulthood, depending on the level and timing of mTORC1 hyperactivity. For example, mice lacking TSC2 in developing SCs displayed hyperproliferation of undifferentiated SCs incompatible with normal myelination. However, these defects and myelination could be rescued by pharmacological mTORC1 inhibition. The subsequent reconstitution of SC mTORC1 hyperactivity in adult animals resulted in focal hypermyelination. Together our data suggest a model in which high mTORC1 activity promotes proliferation of immature SCs and antagonizes SC differentiation during nerve development. Down-regulation of mTORC1 activity is required for terminal SC differentiation and subsequent initiation of myelination. In distinction to this developmental role, excessive SC mTORC1 activity stimulates myelin growth, even overgrowth, in adulthood. Thus, our work delineates two distinct functions of mTORC1 in the SC lineage essential for proper nerve development and myelination. Moreover, our studies show that SCs retain their plasticity to myelinate and remodel myelin via mTORC1 throughout life.

A reciprocal regulatory loop between TAZ/YAP and G-protein Gαs regulates Schwann cell proliferation and myelination

Schwann cell (SC) myelination in the peripheral nervous system is essential for motor function, and uncontrolled SC proliferation occurs in cancer. Here, we show that a dual role for Hippo effectors TAZ and YAP in SC proliferation and myelination through modulating G-protein expression and interacting with SOX10, respectively. Developmentally regulated mutagenesis indicates that TAZ/YAP are critical for SC proliferation and differentiation in a stage-dependent manner. Genome-wide occupancy mapping and transcriptome profiling reveal that nuclear TAZ/YAP promote SC proliferation by activating cell cycle regulators, while targeting critical differentiation regulators in cooperation with SOX10 for myelination. We further identify that TAZ targets and represses Gnas, encoding Gαs-protein, which opposes TAZ/YAP activities to decelerate proliferation. Gnas deletion expands SC precursor pools and blocks peripheral myelination. Thus, the Hippo/TAZ/YAP and Gαs-protein feedback circuit functions as a fulcrum balancing SC proliferation and differentiation, providing insights into molecular programming of SC lineage progression and homeostasis.

The Hippo pathway has recently been implicated in Schwann cell (SC) development and myelination. Here the authors reveal mechanistic insights into how TAZ and YAP regulate and interact with target genes; they further identify a negative feedback loop between TAZ/YAP and G protein Gαs that balances SC proliferation and differentiation.

Role of Schwann cells in the regeneration of penile and peripheral nerves

Schwann cells (SCs) are the principal glia of the peripheral nervous system. The end point of SC development is the formation of myelinating and nonmyelinating cells which ensheath large and small diameter axons, respectively. They play an important role in axon regeneration after injury, including cavernous nerve injury that leads to erectile dysfunction (ED). Despite improvement in radical prostatectomy surgical techniques, many patients still suffer from ED postoperatively as surgical trauma causes traction injuries and local inflammatory changes in the neuronal microenvironment of the autonomic fibers innervating the penis resulting in pathophysiological alterations in the end organ. The aim of this review is to summarize contemporary evidence regarding: (1) the origin and development of SCs in the peripheral and penile nerve system; (2) Wallerian degeneration and SC plastic change following peripheral and penile nerve injury; (3) how SCs promote peripheral and penile nerve regeneration by secreting neurotrophic factors; (4) and strategies targeting SCs to accelerate peripheral nerve regeneration. We searched PubMed for articles related to these topics in both animal models and human research and found numerous studies suggesting that SCs could be a novel target for treatment of nerve injury-induced ED.

De novo PMP2 mutations in families with type 1 Charcot-Marie-Tooth disease

We performed whole exome sequencing on a patient with Charcot-Marie-Tooth disease type 1 and identified a de novo mutation in PMP2, the gene that encodes the myelin P2 protein. This mutation (p.Ile52Thr) was passed from the proband to his one affected son, and segregates with clinical and electrophysiological evidence of demyelinating neuropathy. We then screened a cohort of 136 European probands with uncharacterized genetic cause of Charcot-Marie-Tooth disease and identified another family with Charcot-Marie-Tooth disease type 1 that has a mutation affecting an adjacent amino acid (p.Thr51Pro), which segregates with disease. Our genetic and clinical findings in these kindred demonstrate that dominant PMP2 mutations cause Charcot-Marie-Tooth disease type 1.

Differential Sox10 genomic occupancy in myelinating glia

Myelin is formed by specialized myelinating glia: oligodendrocytes and Schwann cells in the central and peripheral nervous systems, respectively. While there are distinct developmental aspects and regulatory pathways in these two cell types, myelination in both systems requires the transcriptional activator Sox10. Sox10 interacts with cell type-specific transcription factors at some loci to induce myelin gene expression, but it is largely unknown how Sox10 transcriptional networks globally compare between oligodendrocytes and Schwann cells. We used in vivo ChIP-Seq analysis of spinal cord and peripheral nerve (sciatic nerve) to identify unique and shared Sox10 binding sites and assess their correlation with active enhancers and transcriptional profiles in oligodendrocytes and Schwann cells. Sox10 binding sites overlap with active enhancers and critical cell type-specific regulators of myelination, such as Olig2 and Myrf in oligodendrocytes, and Egr2/Krox20 in Schwann cells. Sox10 sites also associate with genes critical for myelination in both oligodendrocytes and Schwann cells and are found within super-enhancers previously defined in brain. In Schwann cells, Sox10 sites contain binding motifs of putative partners in the Sp/Klf, Tead, and nuclear receptor protein families. Specifically, siRNA analysis of nuclear receptors Nr2f1 and Nr2f2 revealed downregulation of myelin genes Mbp and Ndrg1 in primary Schwann cells. Our analysis highlights different mechanisms that establish cell type-specific genomic occupancy of Sox10, which reflects the unique characteristics of oligodendrocyte and Schwann cell differentiation. GLIA 2015;63:1897-1914.

Targeting the colony stimulating factor 1 receptor alleviates two forms of Charcot-Marie-Tooth disease in mice

See Scherer (doi:10.1093/awv279) for a scientific commentary on this article.Charcot-Marie-Tooth type 1 neuropathies are inherited disorders of the peripheral nervous system caused by mutations in Schwann cell-related genes. Typically, no causative cure is presently available. Previous preclinical data of our group highlight the low grade, secondary inflammation common to distinct Charcot-Marie-Tooth type 1 neuropathies as a disease amplifier. In the current study, we have tested one of several available clinical agents targeting macrophages through its inhibition of the colony stimulating factor 1 receptor (CSF1R). We here show that in two distinct mouse models of Charcot-Marie-Tooth type 1 neuropathies, the systemic short- and long-term inhibition of CSF1R by oral administration leads to a robust decline in nerve macrophage numbers by ∼70% and substantial reduction of the typical histopathological and functional alterations. Interestingly, in a model for the dominant X-linked form of Charcot-Marie-Tooth type 1 neuropathy, the second most common form of the inherited neuropathies, macrophage ablation favours maintenance of axonal integrity and axonal resprouting, leading to preserved muscle innervation, increased muscle action potential amplitudes and muscle strengths in the range of wild-type mice. In another model mimicking a mild, demyelination-related Charcot-Marie-Tooth type 1 neuropathy caused by reduced P0 (MPZ) gene dosage, macrophage blockade causes an improved preservation of myelin, increased muscle action potential amplitudes, improved nerve conduction velocities and ameliorated muscle strength. These observations suggest that disease-amplifying macrophages can produce multiple adverse effects in the affected nerves which likely funnel down to common clinical features. Surprisingly, treatment of mouse models mimicking Charcot-Marie-Tooth type 1A neuropathy also caused macrophage blockade, but did not result in neuropathic or clinical improvements, most likely due to the late start of treatment of this early onset disease model. In summary, our study shows that targeting peripheral nerve macrophages by an orally administered inhibitor of CSF1R may offer a highly efficacious and safe treatment option for at least two distinct forms of the presently non-treatable Charcot-Marie-Tooth type 1 neuropathies.