Sh3tc2 deficiency affects neuregulin-1/ErbB signaling

Peripheral Nervous System (PNS) Myelin Diseases

This is a review of inherited and acquired causes of human demyelinating neuropathies and a subset of disorders that affect axon-Schwann cell interactions. Nearly all inherited demyelinating neuropathies are caused by mutations in genes that are expressed by myelinating Schwann cells, affecting diverse functions in a cell-autonomous manner. The most common acquired demyelinating neuropathies are Guillain-Barré syndrome and chronic, inflammatory demyelinating polyneuropathy, both of which are immune-mediated. An additional group of inherited and acquired disorders affect axon-Schwann cell interactions in the nodal region. Overall, these disorders affect the formation of myelin and its maintenance, with superimposed axonal loss that is clinically important.

Mitf is a Schwann cell sensor of axonal integrity that drives nerve repair

Schwann cells respond to acute axon damage by transiently transdifferentiating into specialized repair cells that restore sensorimotor function. However, the molecular systems controlling repair cell formation and function are not well defined, and consequently, it is unclear whether this form of cellular plasticity has a role in peripheral neuropathies. Here, we identify Mitf as a transcriptional sensor of axon damage under the control of Nrg-ErbB-PI3K-PI5K-mTorc2 signaling. Mitf regulates a core transcriptional program for generating functional repair Schwann cells following injury and during peripheral neuropathies caused by CMT4J and CMT4D. In the absence of Mitf, core genes for epithelial-to-mesenchymal transition, metabolism, and dedifferentiation are misexpressed, and nerve repair is disrupted. Our findings demonstrate that Schwann cells monitor axonal health using a phosphoinositide signaling system that controls Mitf nuclear localization, which is critical for activating cellular plasticity and counteracting neural disease.

AAV9-mediated SH3TC2 gene replacement therapy targeted to Schwann cells for the treatment of CMT4C

Type 4C Charcot-Marie-Tooth (CMT4C) demyelinating neuropathy is caused by autosomal recessive SH3TC2 gene mutations. SH3TC2 is highly expressed in myelinating Schwann cells. CMT4C is a childhood-onset progressive disease without effective treatment. Here, we generated a gene therapy for CMT4C mediated by an adeno-associated viral 9 vector (AAV9) to deliver the human SH3TC2 gene in the Sh3tc2-/- mouse model of CMT4C. We used a minimal fragment of the myelin protein zero (Mpz) promoter (miniMpz), which was cloned and validated to achieve Schwann cell-targeted expression of SH3TC2. Following the demonstration of AAV9-miniMpz.SH3TC2myc vector efficacy to re-establish SH3TC2 expression in the peripheral nervous system, we performed an early as well as a delayed treatment trial in Sh3tc2-/- mice. We demonstrate both after early as well as following late treatment improvements in multiple motor performance tests and nerve conduction velocities. Moreover, treatment led to normalization of the organization of the nodes of Ranvier, which is typically deficient in CMT4C patients and Sh3tc2-/- mice, along with reduced ratios of demyelinated fibers, increased myelin thickness and reduced g-ratios at both time points of intervention. Taken together, our results provide a proof of concept for an effective and potentially translatable gene replacement therapy for CMT4C treatment.

Imbalance of NRG1-ERBB2/3 signalling underlies altered myelination in Charcot-Marie-Tooth disease 4H

Charcot-Marie-Tooth (CMT) disease is one of the most common inherited neurological disorders, affecting either axons from the motor and/or sensory neurons or Schwann cells of the peripheral nervous system (PNS) and caused by more than 100 genes. We previously identified mutations in FGD4 as responsible for CMT4H, an autosomal recessive demyelinating form of CMT disease. FGD4 encodes FRABIN, a GDP/GTP nucleotide exchange factor, particularly for the small GTPase Cdc42. Remarkably, nerves from patients with CMT4H display excessive redundant myelin figures called outfoldings that arise from focal hypermyelination, suggesting that FRABIN could play a role in the control of PNS myelination. To gain insights into the role of FGD4/FRABIN in Schwann cell myelination, we generated a knockout mouse model (Fgd4SC-/-), with conditional ablation of Fgd4 in Schwann cells. We show that the specific deletion of FRABIN in Schwann cells leads to aberrant myelination in vitro, in dorsal root ganglia neuron/Schwann cell co-cultures, as well as in vivo, in distal sciatic nerves from Fgd4SC-/- mice. We observed that those myelination defects are related to an upregulation of some interactors of the NRG1 type III/ERBB2/3 signalling pathway, which is known to ensure a proper level of myelination in the PNS. Based on a yeast two-hybrid screen, we identified SNX3 as a new partner of FRABIN, which is involved in the regulation of endocytic trafficking. Interestingly, we showed that the loss of FRABIN impairs endocytic trafficking, which may contribute to the defective NRG1 type III/ERBB2/3 signalling and myelination. Using RNA-Seq, in vitro, we identified new potential effectors of the deregulated pathways, such as ERBIN, RAB11FIP2 and MAF, thereby providing cues to understand how FRABIN contributes to proper ERBB2 trafficking or even myelin membrane addition through cholesterol synthesis. Finally, we showed that the re-establishment of proper levels of the NRG1 type III/ERBB2/3 pathway using niacin treatment reduces myelin outfoldings in nerves of CMT4H mice. Overall, our work reveals a new role of FRABIN in the regulation of NRG1 type III/ERBB2/3 NRG1signalling and myelination and opens future therapeutic strategies based on the modulation of the NRG1 type III/ERBB2/3 pathway to reduce CMT4H pathology and more generally other demyelinating types of CMT disease.

Pan-Cancer Analysis Reveals SH3TC2 as an Oncogene for Colorectal Cancer and Promotes Tumorigenesis via the MAPK Pathway

SH3 domain and tetrapeptide repeat 2 (SH3TC2) is a protein-encoding gene and has previously been described as a critical signaling hub for neurological disorders. Although increasing evidence supports a vital role of SH3TC2 in the tumorigenesis of various kinds of cancer, no systematic analysis of SH3TC2 is available. The function and mechanism of SH3TC2 in other cancers remain unknown. Thus, this study aimed to analyze SH3TC2 in various kinds of cancer to find its tumorigenic role in one or more specific cancers. In the current study, we analyzed the expression level and prognostic value of SH3TC2 in different tumors in the TCGA-GTEx pan-cancer dataset. Subsequently, the prognostic role and mechanism of SH3TC2 in colorectal cancer (CRC) were further explored via clinical samples and in vitro and in vivo experiments. We observed differential expression of SH3TC2 in colon adenocarcinoma (COAD), acute myeloid leukemia (LAML), READ (rectum adenocarcinoma), SKCM (skin cutaneous melanoma), and TGCT (testicular germ cell tumors). Subsequently, SH3TC2 showed a significant effect on the clinical stage and prognostic value in CRC, LAML, and SKCM. Moreover, we found in the TCGA database and seven GEO datasets that SH3TC2 was significantly highly expressed in tumor tissue. Through enrichment analysis of SH3TC2 and its co-expressed genes, we found that SH3TC2 may play a role in the MAPK signaling pathway. Correlation analysis indicated that SH3TC2 was significantly associated with multiple key factors in the MAPK signaling pathway. Additionally, higher expression of SH3TC2 was found in tumor tissue in our cohort including 40 CRC patients. Overexpression of SH3TC2 may imply poor prognosis. Knockdown of SH3TC2 significantly inhibited tumor invasion, migration, and proliferation. More importantly, knockdown of SH3TC2 inhibited tumor growth in a CRC mouse model. The study preliminarily conducted a pan-cancer study of SH3TC2 and further explored the mechanism of SH3TC2 in CRC. Our research revealed that higher expression of SH3TC2 may promote CRC progression and invasion via the MAPK signaling pathway.

Protein misfolding and clearance in the pathogenesis of a new infantile onset ataxia caused by mutations in PRDX3

Peroxiredoxin 3 (PRDX3) encodes a mitochondrial antioxidant protein which is essential for the control of reactive oxidative species (ROS) homeostasis. So far, PRDX3 mutations are involved in mild-to-moderate progressive juvenile onset cerebellar ataxia. We aimed to unravel the molecular bases underlying the disease in an infant suffering from cerebellar ataxia that started at 19 months old and presented severe cerebellar atrophy and peripheral neuropathy early in the course of disease. By whole exome sequencing, we identified a novel homozygous mutation, PRDX3 p.D163E, which impaired the mitochondrial ROS defense system. In mouse primary cortical neurons, the exogenous expression of PRDX3 p.D163E was reduced and triggered alterations in neurite morphology and in mitochondria. Mitochondrial computational parameters showed that p.D163E led to serious mitochondrial alterations. In transfected HeLa cells expressing the mutation, mitochondria accumulation was detected by correlative light electron microscopy (CLEM). Mitochondrial morphology showed severe changes, including extremely damaged outer and inner membranes with a notable cristae disorganization. Moreover, spherical structures compatible with lipid droplets were identified, which can be associated with a generalized response to stress and can be involved in the removal of unfolded proteins. In the patient's fibroblasts, PRDX3 expression was nearly absent. The biochemical analysis suggested that the mutation p.D163E would result in an unstable structure tending to form aggregates that trigger unfolded protein responses via mitochondria and endoplasmic reticulum. Altogether, our findings broaden the clinical spectrum of the recently described PRDX3-associated neurodegeneration and provide new insight into the pathological mechanisms underlying this new form of cerebellar ataxia.

Aberrant Neuregulin 1/ErbB Signaling in Charcot-Marie-Tooth Type 4D Disease

Charcot-Marie-Tooth type 4D (CMT4D) is an autosomal recessive demyelinating form of CMT characterized by progressive motor and sensory neuropathy. N-myc downstream regulated gene 1 (NDRG1) is the causative gene for CMT4D. Although more CMT4D cases have been reported, the comprehensive molecular mechanism underlying CMT4D remains elusive. Here, we generated a novel knockout mouse model in which the fourth and fifth exons of the Ndrg1 gene were removed. Ndrg1-deficient mice develop early progressive demyelinating neuropathy and limb muscle weakness. The expression pattern of myelination-related transcriptional factors, including SOX10, OCT6, and EGR2, was abnormal in Ndrg1-deficient mice. We further investigated the activation of the ErbB2/3 receptor tyrosine kinases in Ndrg1-deficient sciatic nerves, as these proteins play essential roles in Schwann cell myelination. In the absence of NDRG1, although the total ErbB2/3 receptors expressed by Schwann cells were significantly increased, levels of the phosphorylated forms of ErbB2/3 and their downstream signaling cascades were decreased. This change was not associated with the level of the neuregulin 1 ligand, which was increased in Ndrg1-deficient mice. In addition, the integrin β4 receptor, which interacts with ErbB2/3 and positively regulates neuregulin 1/ErbB signaling, was significantly reduced in the Ndrg1-deficient nerve. In conclusion, our data suggest that the demyelinating phenotype of CMT4D disease is at least in part a consequence of molecular defects in neuregulin 1/ErbB signaling.

Disrupted myelination network in the cingulate cortex of Parkinson's disease

The cingulate cortex is part of the conserved limbic system, which is considered as a hub of emotional and cognitive control. Accumulating evidence suggested that involvement of the cingulate cortex is significant for cognitive impairment of Parkinson's disease (PD). However, mechanistic studies of the cingulate cortex in PD pathogenesis are limited. Here, transcriptomic and regulatory network analyses were conducted for the cingulate cortex in PD. Enrichment and clustering analyses showed that genes involved in regulation of membrane potential and glutamate receptor signalling pathway were upregulated. Importantly, myelin genes and the oligodendrocyte development pathways were markedly downregulated, indicating disrupted myelination in PD cingulate cortex. Cell‐type‐specific signatures revealed that myelinating oligodendrocytes were the major cell type damaged in the PD cingulate cortex. Furthermore, downregulation of myelination pathways in the cingulate cortex were shared and validated in another independent RNAseq cohort of dementia with Lewy bodies (DLB). In combination with ATACseq data, gene regulatory networks (GRNs) were further constructed for 32 transcription factors (TFs) and 466 target genes among differentially expressed genes (DEGs) using a tree‐based machine learning algorithm. Several transcription factors, including Olig2, Sox8, Sox10, E2F1, and NKX6‐2, were highlighted as key nodes in a sub‐network, which control many overlapping downstream targets associated with myelin formation and gliogenesis. In addition, the authors have validated a subset of DEGs by qPCRs in two PD mouse models. Notably, seven of these genes,TOX3, NECAB2 NOS1, CAPN3, NR4A2, E2F1 and FOXP2, have been implicated previously in PD or neurodegeneration and are worthy of further studies as novel candidate genes. Together, our findings provide new insights into the role of remyelination as a promising new approach to treat PD after demyelination.

Distinct roles for the Charcot-Marie-tooth disease-causing endosomal regulators Mtmr5 and Mtmr13 in axon radial sorting and Schwann cell myelination

The form of Charcot-Marie-Tooth type 4B (CMT4B) disease caused by mutations in myotubularin-related 5 (MTMR5; also called SET Binding Factor 1; SBF1) shows a spectrum of axonal and demyelinating nerve phenotypes. This contrasts with the CMT4B subtypes caused by MTMR2 or MTMR13 (SBF2) mutations, which are characterized by myelin outfoldings and classic demyelination. Thus, it is unclear whether MTMR5 plays an analogous or distinct role from that of its homolog, MTMR13, in the peripheral nervous system (PNS). MTMR5 and MTMR13 are pseudophosphatases predicted to regulate endosomal trafficking by activating Rab GTPases and binding to the phosphoinositide 3-phosphatase MTMR2. In the mouse PNS, Mtmr2 was required to maintain wild type levels of Mtmr5 and Mtmr13, suggesting that these factors function in discrete protein complexes. Genetic elimination of both Mtmr5 and Mtmr13 in mice led to perinatal lethality, indicating that the two proteins have partially redundant functions during embryogenesis. Loss of Mtmr5 in mice did not cause CMT4B-like myelin outfoldings. However, adult Mtmr5-/- mouse nerves contained fewer myelinated axons than control nerves, likely as a result of axon radial sorting defects. Consistently, Mtmr5 levels were highest during axon radial sorting and fell sharply after postnatal day seven. Our findings suggest that Mtmr5 and Mtmr13 ensure proper axon radial sorting and Schwann cell myelination, respectively, perhaps through their direct interactions with Mtmr2. This study enhances our understanding of the non-redundant roles of the endosomal regulators MTMR5 and MTMR13 during normal peripheral nerve development and disease.

Mechanisms and Treatments in Demyelinating CMT

Demyelinating forms of Charcot-Marie-Tooth disease (CMT) are genetically and phenotypically heterogeneous and result from highly diverse biological mechanisms including gain of function (including dominant negative effects) and loss of function. While no definitive treatment is currently available, rapid advances in defining the pathomechanisms of demyelinating CMT have led to promising pre-clinical studies, as well as emerging clinical trials. Especially promising are the recently completed pre-clinical genetic therapy studies in PMP-22, GJB1, and SH3TC2-associated neuropathies, particularly given the success of similar approaches in humans with spinal muscular atrophy and transthyretin familial polyneuropathy. This article focuses on neuropathies related to mutations in PMP-22, MPZ, and GJB1, which together comprise the most common forms of demyelinating CMT, as well as on select rarer forms for which promising treatment targets have been identified. Clinical characteristics and pathomechanisms are reviewed in detail, with emphasis on therapeutically targetable biological pathways. Also discussed are the challenges facing the CMT research community in its efforts to advance the rapidly evolving biological insights to effective clinical trials. These considerations include the limitations of currently available animal models, the need for personalized medicine approaches/allele-specific interventions for select forms of demyelinating CMT, and the increasing demand for optimal clinical outcome assessments and objective biomarkers.

Held Up in Traffic-Defects in the Trafficking Machinery in Charcot-Marie-Tooth Disease

Charcot-Marie-Tooth disease (CMT), also known as motor and sensory neuropathy, describes a clinically and genetically heterogenous group of disorders affecting the peripheral nervous system. CMT typically arises in early adulthood and is manifested by progressive loss of motor and sensory functions; however, the mechanisms leading to the pathogenesis are not fully understood. In this review, we discuss disrupted intracellular transport as a common denominator in the pathogenesis of different CMT subtypes. Intracellular transport via the endosomal system is essential for the delivery of lipids, proteins, and organelles bidirectionally to synapses and the soma. As neurons of the peripheral nervous system are amongst the longest neurons in the human body, they are particularly susceptible to damage of the intracellular transport system, leading to a loss in axonal integrity and neuronal death. Interestingly, defects in intracellular transport, both in neurons and Schwann cells, have been found to provoke disease. This review explains the mechanisms of trafficking and subsequently summarizes and discusses the latest findings on how defects in trafficking lead to CMT. A deeper understanding of intracellular trafficking defects in CMT will expand our understanding of CMT pathogenesis and will provide novel approaches for therapeutic treatments.

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.

Mutations in GDAP1 Influence Structure and Function of the Trans-Golgi Network

Charcot-Marie-Tooth disease (CMT) is a heritable neurodegenerative disease that displays great genetic heterogeneity. The genes and mutations that underlie this heterogeneity have been extensively characterized by molecular genetics. However, the molecular pathogenesis of the vast majority of CMT subtypes remains terra incognita. Any attempts to perform experimental therapy for CMT disease are limited by a lack of understanding of the pathogenesis at a molecular level. In this study, we aim to identify the molecular pathways that are disturbed by mutations in the gene encoding GDAP1 using both yeast and human cell, based models of CMT-GDAP1 disease. We found that some mutations in GDAP1 led to a reduced expression of the GDAP1 protein and resulted in a selective disruption of the Golgi apparatus. These structural alterations are accompanied by functional disturbances within the Golgi. We screened over 1500 drugs that are available on the market using our yeast-based CMT-GDAP1 model. Drugs were identified that had both positive and negative effects on cell phenotypes. To the best of our knowledge, this study is the first report of the Golgi apparatus playing a role in the pathology of CMT disorders. The drugs we identified, using our yeast-based CMT-GDAP1 model, may be further used in translational research.

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.

Novel Mutations Involved in Charcot-Marie-Tooth 4C and Intrafamilial Variability: Let's Not Miss the Forest for the Trees

Charcot-Marie-Tooth 4C is characterized by early-onset, rapid progression, and mainly associated with SH3TC2 gene mutations. We reported a male patient carrying a novel heterozygous nonsense mutation in SH3TC2 gene along with a heterozygous known pathogenic mutation. Symptoms began at 15 months and by 14 years, he presented significant motor impairment. Both parents exhibited one of the mutations in the heterozygous state, while his 8-year-old brother carried the same compound heterozygosity, showing only a mild phenotype. In our case, we discussed the contribution of compound heterozygosity to intrafamilial variability in Charcot-Marie-Tooth and the role of modifying genes.

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.

Gene replacement therapy in a model of Charcot-Marie-Tooth 4C neuropathy

Charcot-Marie-Tooth disease type 4C is the most common recessively inherited demyelinating neuropathy that results from loss of function mutations in the SH3TC2 gene. Sh3tc2-/- mice represent a well characterized disease model developing early onset progressive peripheral neuropathy with hypo- and demyelination, slowing of nerve conduction velocities and disturbed nodal architecture. The aim of this project was to develop a gene replacement therapy for treating Charcot-Marie-Tooth disease type 4C to rescue the phenotype of the Sh3tc2-/- mouse model. We generated a lentiviral vector LV-Mpz.SH3TC2.myc to drive expression of the human SH3TC2 cDNA under the control of the Mpz promoter specifically in myelinating Schwann cells. The vector was delivered into 3-week-old Sh3tc2-/- mice by lumbar intrathecal injection and gene expression was assessed 4-8 weeks after injection. Immunofluorescence analysis showed presence of myc-tagged human SH3TC2 in sciatic nerves and lumbar roots in the perinuclear cytoplasm of a subset of Schwann cells, in a dotted pattern co-localizing with physiologically interacting protein Rab11. Quantitative PCR analysis confirmed SH3TC2 mRNA expression in different peripheral nervous system tissues. A treatment trial was initiated in 3 weeks old randomized Sh3tc2-/- littermate mice which received either the full or mock (LV-Mpz.Egfp) vector. Behavioural analysis 8 weeks after injection showed improved motor performance in rotarod and foot grip tests in treated Sh3tc2-/- mice compared to mock vector-treated animals. Moreover, motor nerve conduction velocities were increased in treated Sh3tc2-/- mice. On a structural level, morphological analysis revealed significant improvement in g-ratios, myelin thickness, and ratios of demyelinated fibres in lumbar roots and sciatic nerves of treated Sh3tc2-/- mice. Finally, treated mice also showed improved nodal molecular architecture and reduction of blood neurofilament light levels, a clinically relevant biomarker for axonal injury/degeneration. This study provides a proof of principle for viral gene replacement therapy targeted to Schwann cells to treat Charcot-Marie-Tooth disease type 4C and potentially other similar demyelinating inherited neuropathies.

Neuromuscular Junction Changes in a Mouse Model of Charcot-Marie-Tooth Disease Type 4C

The neuromuscular junction (NMJ) appears to be a site of pathology in a number of peripheral nerve diseases. Charcot-Marie-Tooth (CMT) 4C is an autosomal recessive, early onset, demyelinating neuropathy. Numerous mutations in the SH3TC2 gene have been shown to underlie the condition often associated with scoliosis, foot deformities, and reduced nerve conduction velocities. Mice with exon 1 of the Sh3tc2 gene knocked out demonstrate many of the features seen in patients. To determine if NMJ pathology is contributory to the pathomechanisms of CMT4C we examined NMJs in the gastrocnemius muscle of SH3TC2-deficient mice. In addition, we performed proteomic assessment of the sciatic nerve to identify protein factors contributing to the NMJ alterations and the survival of demyelinated axons. Morphological and gene expression analysis of NMJs revealed a lack of continuity between the pre- and post-synaptic apparatus, increases in post-synaptic fragmentation and dispersal, and an increase in expression of the gamma subunit of the acetylcholine receptor. There were no changes in axonal width or the number of axonal inputs to the NMJ. Proteome investigations of the sciatic nerve revealed altered expression of extracellular matrix proteins important for NMJ integrity. Together these observations suggest that CMT4C pathology includes a compromised NMJ even in the absence of changes to the innervating axon.

Clinical and mutational spectrum of Japanese patients with recessive variants in SH3TC2

SH3TC2, known as the causative gene of autosomal recessive demyelinating Charcot-Marie-Tooth type 4C (CMT4C), was also found linked to a mild mononeuropathy of the median nerve with an autosomal dominant inheritance pattern. Using DNA microarray, Illumina MiSeq, and Ion proton, we carried out gene panel sequencing among 1483 Japanese CMT patients, containing 397 patients with demyelinating CMT. From seven patients with demyelinating CMT, we identified eight recessive variants in the SH3TC2 gene, consisting of five novel (pathogenic/likely pathogenic) and three reported variants. Additionally, from two patients with axonal CMT, we detected a reported recessive variant, p.Arg77Trp, which was herein reclassified as variant with unknown significance. Of the seven CMT4C patients (six females and one male), 2/7 patients developed symptoms at their first decade, and 5/7 patients lost their ambulation around age 50. Scoliosis was observed from more than half (4/7) of these patients, whereas hearing loss is the most common symptom of central nervous system (6/7). No median nerve mononeuropathy was recorded from their family members. We identified recessive variants in SH3TC2 from 1.76% of demyelinating CMT patients. An uncommon gender difference was recognized and the wild spectrum of these variants suggests mutational diversity of SH3TC2 in Japan.

Charcot-Marie-Tooth Disease type 4C: Novel mutations, clinical presentations, and diagnostic challenges

INTRODUCTION

This study analyzes and describes atypical presentations of Charcot-Marie-Tooth disease type 4C (CMT4C).

METHODS

We present clinical and physiologic features of 5 patients with CMT4C caused by biallelic private mutations of SH3TC2.

RESULTS

All patients manifested scoliosis, and nerve conduction study indicated results in the demyelinating range. All patients exhibited signs of motor impairment within the first years of life. We describe 2 or more different genetic diseases in the same patient, atypical presentations of CMT, and 3 new mutations in CMT4C patients.

DISCUSSION

A new era of unbiased genetic testing has led to this small case series of individuals with CMT4C and highlights the recognition of different genetic diseases in CMT4C patients for accurate diagnosis, genetic risk identification, and therapeutic intervention. The phenotype of CMT4C, in addition, appears to be enriched by a number of features unusual for the broad CMT category. Muscle Nerve 57: 749-755, 2018.

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.

The topology, structure and PE interaction of LITAF underpin a Charcot-Marie-Tooth disease type 1C

BACKGROUND

Mutations in Lipopolysaccharide-induced tumour necrosis factor-α factor (LITAF) cause the autosomal dominant inherited peripheral neuropathy, Charcot-Marie-Tooth disease type 1C (CMT1C). LITAF encodes a 17 kDa protein containing an N-terminal proline-rich region followed by an evolutionarily-conserved C-terminal 'LITAF domain', which contains all reported CMT1C-associated pathogenic mutations.

RESULTS

Here, we report the first structural characterisation of LITAF using biochemical, cell biological, biophysical and NMR spectroscopic approaches. Our structural model demonstrates that LITAF is a monotopic zinc-binding membrane protein that embeds into intracellular membranes via a predicted hydrophobic, in-plane, helical anchor located within the LITAF domain. We show that specific residues within the LITAF domain interact with phosphoethanolamine (PE) head groups, and that the introduction of the V144M CMT1C-associated pathogenic mutation leads to protein aggregation in the presence of PE.

CONCLUSIONS

In addition to the structural characterisation of LITAF, these data lead us to propose that an aberrant LITAF-PE interaction on the surface of intracellular membranes contributes to the molecular pathogenesis that underlies this currently incurable disease.

Niacin-mediated Tace activation ameliorates CMT neuropathies with focal hypermyelination

Charcot–Marie–Tooth (CMT) neuropathies are highly heterogeneous disorders caused by mutations in more than 70 genes, with no available treatment. Thus, it is difficult to envisage a single suitable treatment for all pathogenetic mechanisms. Axonal Neuregulin 1 (Nrg1) type III drives Schwann cell myelination and determines myelin thickness by ErbB2/B3‐PI3K–Akt signaling pathway activation. Nrg1 type III is inhibited by the α‐secretase Tace, which negatively regulates PNS myelination. We hypothesized that modulation of Nrg1 levels and/or secretase activity may constitute a unifying treatment strategy for CMT neuropathies with focal hypermyelination as it could restore normal levels of myelination. Here we show that in vivo delivery of Niaspan, a FDA‐approved drug known to enhance TACE activity, efficiently rescues myelination in the Mtmr2^−/− mouse, a model of CMT4B1 with myelin outfoldings, and in the Pmp22^+/− mouse, which reproduces HNPP (hereditary neuropathy with liability to pressure palsies) with tomacula. Importantly, we also found that Niaspan reduces hypermyelination of Vim (vimentin)^−/− mice, characterized by increased Nrg1 type III and Akt activation, thus corroborating the hypothesis that Niaspan treatment downregulates Nrg1 type III signaling.

Akt Regulates Axon Wrapping and Myelin Sheath Thickness in the PNS

The signaling pathways that regulate myelination in the PNS remain poorly understood. Phosphatidylinositol-4,5-bisphosphate 3-kinase 1A, activated in Schwann cells by neuregulin and the extracellular matrix, has an essential role in the early events of myelination. Akt/PKB, a key effector of phosphatidylinositol-4,5-bisphosphate 3-kinase 1A, was previously implicated in CNS, but not PNS myelination. Here we demonstrate that Akt plays a crucial role in axon ensheathment and in the regulation of myelin sheath thickness in the PNS. Pharmacological inhibition of Akt in DRG neuron-Schwann cell cocultures dramatically decreased MBP and P0 levels and myelin sheath formation without affecting expression of Krox20/Egr2, a key transcriptional regulator of myelination. Conversely, expression of an activated form of Akt in purified Schwann cells increased expression of myelin proteins, but not Krox20/Egr2, and the levels of activated Rac1. Transgenic mice expressing a membrane-targeted, activated form of Akt under control of the 2',3'-cyclic nucleotide 3'-phosphodiesterase promoter, exhibited thicker PNS and CNS myelin sheaths, and PNS myelin abnormalities, such as tomacula and myelin infoldings/outfoldings, centered around the paranodes and Schmidt Lanterman incisures. These effects were corrected by rapamycin treatmentin vivo Importantly, Akt activity in the transgenic mice did not induce myelination of nonmyelinating Schwann cells in the sympathetic trunk or Remak fibers of the dorsal roots, although, in those structures, they wrapped membranes redundantly around axons. Together, our data indicate that Akt is crucial for PNS myelination driving axonal wrapping by unmyelinated and myelinated Schwann cells and enhancing myelin protein synthesis in myelinating Schwann cells.

SIGNIFICANCE STATEMENT

Although the role of the key serine/threonine kinase Akt in promoting CNS myelination has been demonstrated, its role in the PNS has not been established and remains uncertain. This work reveals that Akt controls several key steps of the PNS myelination. First, its activity promotes membrane production and axonal wrapping independent of a transcriptional effect. In myelinated axons, it also enhances myelin thickness through the mTOR pathway. Finally, sustained Akt activation in Schwann cells leads to hypermyelination/dysmyelination, mimicking some features present in neuropathies, such as hereditary neuropathy with liability to pressure palsies or demyelinating forms of Charcot-Marie-Tooth disease. Together, these data demonstrate the role of Akt in regulatory mechanisms underlying axonal wrapping and myelination in the PNS.

Chronic stress and excessive glucocorticoid exposure both lead to altered Neuregulin-1/ErbB signaling in rat myocardium

Exposure to chronic stress or excess glucocorticoids is associated with the development of depression and heart disease, but the underlying mechanisms remain equivocal. While recent evidence has indicated that Neuregulin-1 (NRG1) and its ErbB receptors play an essential role in cardiac function, much is still unknown concerning the biological link between NRG1/ErbB pathway and the stress-induced comorbidity of depression and cardiac dysfunction. Therefore, we examined the protein expression of NRG1 and ErbB receptors in the myocardium of rats following chronic unpredictable mild stress (CUMS) or rats treated with two different doses (0.2 and 2mg/kg/day, respectively) of dexamethasone (Dex). The stressed rats showed elevated expression of NRG1 and phosphorylated ErbB4 (pErbB4) in the myocardium, whereas ErbB2 and pErbB2 were inhibited. The lower dose of Dex enhanced myocardial NRG1/ErbB signaling, but as the dose is increased, while ErbB4 remained activated, the expression of ErbB2 and pErbB2 became compromised. Both CUMS and 2mg/kg of Dex suppressed the downstream Akt and ERK phosphorylation. Although the lower dose of Dex increased myocardial antiapoptotic Bcl-xl expression, a significant decrease of Bcl-xl expression was found in rats treated with the higher dose. Meanwhile, both CUMS and two different doses of Dex induced proapoptotic Bax level. Combined, our data firstly showed (mal)adaptive responses of NRG1/ErbB system in the stressed heart, indicating the potential involvement of NRG1/ErbB pathway in the stress-induced cardiac dysfunction.

Exclusive expression of the Rab11 effector SH3TC2 in Schwann cells links integrin-α6 and myelin maintenance to Charcot-Marie-Tooth disease type 4C

Charcot-Marie-Tooth disease type 4C (CMT4C) is one of the commonest autosomal recessive inherited peripheral neuropathies and is associated with mutations in the Rab11 effector, SH3TC2. Disruption of the SH3TC2-Rab11 interaction is the molecular abnormality underlying this disease. However, why SH3TC2 mutations cause an isolated demyelinating neuropathy remains unanswered. Here we show that SH3TC2 is an exclusive Schwann cell protein expressed late in myelination and is downregulated following denervation suggesting a functional role in myelin sheath maintenance. We support our data with an evolutionary cell biological analysis showing that the SH3TC2 gene, and its paralogue SH3TC1, are derived from an ancestral homologue, the duplication of which occurred in the common ancestor of jawed vertebrates, coincident with the appearance of Schwann cells and peripheral axon myelination. Furthermore, we report that SH3TC2 associates with integrin-α6, suggesting that aberrant Rab11-dependent endocytic trafficking of this critical laminin receptor in myelinated Schwann cells is connected to the demyelination seen in affected nerves. Our study therefore highlights the inherent evolutionary link between SH3TC2 and peripheral nerve myelination, pointing also towards a molecular mechanism underlying the specific demyelinating neuropathy that characterizes CMT4C.

Dysregulation of ErbB Receptor Trafficking and Signaling in Demyelinating Charcot-Marie-Tooth Disease

Charcot-Marie-Tooth (CMT) disease is the most common inherited peripheral neuropathy with the majority of cases involving demyelination of peripheral nerves. The pathogenic mechanisms of demyelinating CMT remain unclear, and no effective therapy currently exists for this disease. The discovery that mutations in different genes can cause a similar phenotype of demyelinating peripheral neuropathy raises the possibility that there may be convergent mechanisms leading to demyelinating CMT pathogenesis. Increasing evidence indicates that ErbB receptor-mediated signaling plays a major role in the control of Schwann cell-axon communication and myelination in the peripheral nervous system. Recent studies reveal that several demyelinating CMT-linked proteins are novel regulators of endocytic trafficking and/or phosphoinositide metabolism that may affect ErbB receptor signaling. Emerging data have begun to suggest that dysregulation of ErbB receptor trafficking and signaling in Schwann cells may represent a common pathogenic mechanism in multiple subtypes of demyelinating CMT. In this review, we focus on the roles of ErbB receptor trafficking and signaling in regulation of peripheral nerve myelination and discuss the emerging evidence supporting the potential involvement of altered ErbB receptor trafficking and signaling in demyelinating CMT pathogenesis and the possibility of modulating these trafficking and signaling processes for treating demyelinating peripheral neuropathy.

miRNA-584-5p exerts tumor suppressive functions in human neuroblastoma through repressing transcription of matrix metalloproteinase 14

Matrix metalloproteinase 14 (MMP-14) is a membrane-anchored MMP crucial for tumorigenesis and aggressiveness, and is highly expressed in neuroblastoma (NB), the most common extracranial solid tumor in childhood. Recent evidence shows the emerging roles of endogenous promoter-targeting microRNAs (miRNAs) in regulating gene transcription. However, the roles of miRNAs in the transcription of MMP-14 still remain largely unknown. In this study, through mining computational algorithm program and Argonaute-chromosome interaction dataset, we identified one binding site of miRNA-584-5p (miR-584-5p) within the MMP-14 promoter. In NB tissues, miR-584-5p was under-expressed and inversely correlated with MMP-14 expression, and was an independent prognostic factor for favorable outcome of patients. miR-584-5p precursor attenuated the expression of MMP-14 in a Dicer-dependent manner, resulting in decreased levels of vascular endothelial growth factor, in cultured NB cell lines. In addition, miR-584-5p suppressed the promoter activity of MMP-14, and mutation of miR-584-5p binding site abolished these effects. Mechanistically, miR-584-5p recruited Argonaute 2 to facilitate the enrichment of enhancer of zeste homolog 2, histone H3 lysine 27 trimethylation, and histone H3 lysine 9 dimethylation on MMP-14 promoter in NB cells, which was abolished by repressing the miR-584-5p-promoter interaction. Gain- and loss-of-function studies demonstrated that miR-584-5p suppressed the growth, invasion, metastasis, and angiogenesis of NB cells in vitro and in vivo. Moreover, restoration of MMP-14 expression rescued the NB cells from changes in these biological features. Taken together, these results indicate that promoter-targeting miR-584-5p exerts tumor suppressive functions in NB through repressing the transcription of MMP-14.

Sox4 participates in the modulation of Schwann cell myelination

In order to identify new regulators of Schwann cell myelination potentially playing a role in peripheral nervous system (PNS) pathologies, we analysed gene expression profiling data from three mouse models of demyelinating neuropathies and from the developing PNS. This analysis revealed that Sox4, which encodes a member of the Sry-related high-mobility group box protein family, was consistently upregulated in all three analysed models of neuropathy. Moreover, Sox4 showed a peak in its expression during development that corresponded with the onset of myelination. To gain further insights into the role of Sox4 in PNS development, we generated a transgenic mouse that specifically overexpresses Sox4 in Schwann cells. Sox4 overexpression led to a temporary delay in PNS myelination without affecting axonal sorting. Importantly, we observed that, whereas Sox4 mRNA could be efficiently overexpressed, Sox4 protein expression in Schwann cells was strictly regulated. Finally, our data showed that enforced expression of Sox4 in the mouse model for Charcot-Marie-Tooth 4C aggravated its neuropathic phenotype. Together, these observations reveal that Sox4 contributes to the regulation of Schwann cell myelination, and also indicates its involvement in the pathophysiology of peripheral neuropathies.

Haplotype-specific modulation of a SOX10/CREB response element at the Charcot-Marie-Tooth disease type 4C locus SH3TC2

Loss-of-function mutations in the Src homology 3 (SH3) domain and tetratricopeptide repeats 2 (SH3TC2) gene cause autosomal recessive demyelinating Charcot-Marie-Tooth neuropathy. The SH3TC2 protein has been implicated in promyelination signaling through axonal neuregulin-1 and the ERBB2 Schwann cell receptor. However, little is known about the transcriptional regulation of the SH3TC2 gene. We performed computational and functional analyses that revealed two cis-acting regulatory elements at SH3TC2-one at the promoter and one ∼150 kb downstream of the transcription start site. Both elements direct reporter gene expression in Schwann cells and are responsive to the transcription factor SOX10, which is essential for peripheral nervous system myelination. The downstream enhancer harbors a single-nucleotide polymorphism (SNP) that causes an ∼80% reduction in enhancer activity. The SNP resides directly within a predicted binding site for the transcription factor cAMP response element binding protein (CREB), and we demonstrate that this regulatory element binds to CREB and is activated by CREB expression. Finally, forskolin induces Sh3tc2 expression in rat primary Schwann cells, indicating that SH3TC2 is a CREB target gene. These findings prompted us to determine if SNP genotypes at SH3TC2 are associated with differential phenotypes in the most common demyelinating peripheral neuropathy, CMT1A. Interestingly, this revealed several associations between SNP alleles and disease severity. In summary, our data indicate that SH3TC2 is regulated by the transcription factors CREB and SOX10, define a regulatory SNP at this disease-associated locus and reveal SH3TC2 as a candidate modifier locus of CMT disease phenotypes.