Schwann cells and the pathogenesis of inherited motor and sensory neuropathies (Charcot-Marie-Tooth disease)

Mutations in the SPTLC1 protein cause mitochondrial structural abnormalities and endoplasmic reticulum stress in lymphoblasts

Mutations in serine palmitoyltransferase long chain subunit 1 (SPTLC1) cause the typical length-dependent axonal degeneration hereditary sensory neuropathy type 1 (HSN1). Transmission electron microscopy studies on SPTLC1 mutant lymphoblasts derived from patients revealed specific structural abnormalities of mitochondria. Swollen mitochondria with abnormal cristae were clustered around the nucleus, with some mitochondria being wrapped in rough endoplasmic reticulum (ER) membranes. Total mitochondrial counts revealed a significant change in mitochondrial numbers between healthy and diseased lymphocytes but did not reveal any change in length to width ratios nor were there any changes to cellular function. However, there was a notable change in ER homeostasis, as assessed using key ER stress markers, BiP and ERO1-Lα, displaying reduced protein expression. The observations suggest that SPTLC1 mutations cause mitochondrial abnormalities and ER stress in HSN1 cells.

Global gene expression analysis reveals a link between NDRG1 and vesicle transport

N-myc downstream-regulated gene 1 (NDRG1) is induced by cellular stress such as hypoxia and DNA damage, and in humans, germ line mutations cause Charcot-Marie-Tooth disease. However, the cellular roles of NDRG1 are not fully understood. Previously, NDRG1 was shown to mediate doxorubicin resistance under hypoxia, suggesting a role for NDRG1 in cell survival under these conditions. We found decreased apoptosis in doxorubicin-treated cells expressing NDRG1 shRNAs under normoxia, demonstrating a requirement for NDRG1 in apoptosis in breast epithelial cells under normal oxygen pressure. Also, different cellular stress regimens, such as hypoxia and doxorubicin treatment, induced NDRG1 through different stress signalling pathways. We further compared expression profiles in human breast epithelial cells ectopically over-expressing NDRG1 with cells expressing NDRG1 shRNAs in order to identify biological pathways where NDRG1 is involved. The results suggest that NDRG1 may have roles connected to vesicle transport.

Distribution and development of peripheral glial cells in the human fetal cochlea

The adult human cochlea contains various types of peripheral glial cells that envelop or myelinate the three different domains of the spiral ganglion neurons: the central processes in the cochlear nerve, the cell bodies in the spiral ganglia, and the peripheral processes in the osseous spiral lamina. Little is known about the distribution, lineage separation and maturation of these peripheral glial cells in the human fetal cochlea. In the current study, we observed peripheral glial cells expressing SOX10, SOX9 and S100B as early as 9 weeks of gestation (W9) in all three neuronal domains. We propose that these cells are the common precursor to both mature Schwann cells and satellite glial cells. Additionally, the peripheral glial cells located along the peripheral processes expressed NGFR, indicating a phenotype distinct from the peripheral glial cells located along the central processes. From W12, the spiral ganglion was gradually populated by satellite glial cells in a spatiotemporal gradient. In the cochlear nerve, radial sorting was accomplished by W22 and myelination started prior to myelination of the peripheral processes. The developmental dynamics of the peripheral glial cells in the human fetal cochlea is in support of a neural crest origin. Our study provides the first overview of the distribution and maturation of peripheral glial cells in the human fetal cochlea from W9 to W22.

The Gdap1 knockout mouse mechanistically links redox control to Charcot-Marie-Tooth disease

Mutations in the mitochondrial fission factor GDAP1 are associated with severe peripheral neuropathies, but why the CNS remains unaffected is unclear. Using a Gdap1^−/− mouse, Niemann et al. demonstrate that a CNS-expressed Gdap1 paralogue changes its subcellular localisation under oxidative stress conditions to also act as a mitochondrial fission factor.

The ganglioside-induced differentiation-associated protein 1 (GDAP1) is a mitochondrial fission factor and mutations in GDAP1 cause Charcot–Marie–Tooth disease. We found that Gdap1 knockout mice (Gdap1^−/−), mimicking genetic alterations of patients suffering from severe forms of Charcot–Marie–Tooth disease, develop an age-related, hypomyelinating peripheral neuropathy. Ablation of Gdap1 expression in Schwann cells recapitulates this phenotype. Additionally, intra-axonal mitochondria of peripheral neurons are larger in Gdap1^−/− mice and mitochondrial transport is impaired in cultured sensory neurons of Gdap1^−/− mice compared with controls. These changes in mitochondrial morphology and dynamics also influence mitochondrial biogenesis. We demonstrate that mitochondrial DNA biogenesis and content is increased in the peripheral nervous system but not in the central nervous system of Gdap1^−/− mice compared with control littermates. In search for a molecular mechanism we turned to the paralogue of GDAP1, GDAP1L1, which is mainly expressed in the unaffected central nervous system. GDAP1L1 responds to elevated levels of oxidized glutathione by translocating from the cytosol to mitochondria, where it inserts into the mitochondrial outer membrane. This translocation is necessary to substitute for loss of GDAP1 expression. Accordingly, more GDAP1L1 was associated with mitochondria in the spinal cord of aged Gdap1^−/− mice compared with controls. Our findings demonstrate that Charcot–Marie–Tooth disease caused by mutations in GDAP1 leads to mild, persistent oxidative stress in the peripheral nervous system, which can be compensated by GDAP1L1 in the unaffected central nervous system. We conclude that members of the GDAP1 family are responsive and protective against stress associated with increased levels of oxidized glutathione.

New mutation in periaxin gene causing Charcot Marie Tooth disease in a Puerto Rican young male

Charcot-Marie-Tooth (CMT) disease is an inherited peripheral neuropathy caused by mutations in more than 30 different genes. One of the genes encodes for periaxin (PRX) protein, which is required for the maintenance of peripheral nerve myelin. Individuals with PRX gene mutations have been described to present early-onset, autosomal recessive, demyelinating CMT disease or CMT4F subtype. Only 23 mutations involving the PRX gene have been reported in patients throughout the world. We describe a case of a Puerto Rican adolescent with history, neurologic examination, electromyographic data, and laboratory tests consistent with CMT4F. Genetic analysis of this individual showed a heterozygous transversion resulting in amino acid change from arginine to glycine in the PRX gene, suggesting CMT4F. We report this novel PRX mutation to expand the clinical spectrum of CMT disease.

Bves and NDRG4 regulate directional epicardial cell migration through autocrine extracellular matrix deposition

The Bves and NDRG4 proteins interact to regulate directional cell movement by mediating cell surface fusion of internalized fibronectin for resecretion. This provides the first evidence of Bves/NDRG4 protein function within subcellular trafficking pathways and explains how the Bves complex diversely influences development, cancer, and repair.

Directional cell movement is universally required for tissue morphogenesis. Although it is known that cell/matrix interactions are essential for directional movement in heart development, the mechanisms governing these interactions require elucidation. Here we demonstrate that a novel protein/protein interaction between blood vessel epicardial substance (Bves) and N-myc downstream regulated gene 4 (NDRG4) is critical for regulation of epicardial cell directional movement, as disruption of this interaction randomizes migratory patterns. Our studies show that Bves/NDRG4 interaction is required for trafficking of internalized fibronectin through the “autocrine extracellular matrix (ECM) deposition” fibronectin recycling pathway. Of importance, we demonstrate that Bves/NDRG4-mediated fibronectin recycling is indeed essential for epicardial cell directional movement, thus linking these two cell processes. Finally, total internal reflectance fluorescence microscopy shows that Bves/NDRG4 interaction is required for fusion of recycling endosomes with the basal cell surface, providing a molecular mechanism of motility substrate delivery that regulates cell directional movement. This is the first evidence of a molecular function for Bves and NDRG4 proteins within broader subcellular trafficking paradigms. These data identify novel regulators of a critical vesicle-docking step required for autocrine ECM deposition and explain how Bves facilitates cell-microenvironment interactions in the regulation of epicardial cell–directed movement.

Mitochondrial dysfunction in neuromuscular disorders

This review deciphers aspects of mitochondrial (mt) dysfunction among nosologically, pathologically, and genetically diverse diseases of the skeletal muscle, lower motor neuron, and peripheral nerve, which fall outside the traditional realm of mt cytopathies. Special emphasis is given to well-characterized mt abnormalities in collagen VI myopathies (Ullrich congenital muscular dystrophy and Bethlem myopathy), megaconial congenital muscular dystrophy, limb-girdle muscular dystrophy type 2 (calpainopathy), centronuclear myopathies, core myopathies, inflammatory myopathies, spinal muscular atrophy, Charcot-Marie-Tooth neuropathy type 2, and drug-induced peripheral neuropathies. Among inflammatory myopathies, mt abnormalities are more prominent in inclusion body myositis and a subset of polymyositis with mt pathology, both of which are refractory to corticosteroid treatment. Awareness is raised about instances of phenotypic mimicry between cases harboring primary mtDNA depletion, in the context of mtDNA depletion syndrome, and established neuromuscular disorders such as spinal muscular atrophy. A substantial body of experimental work, derived from animal models, attests to a major role of mitochondria (mt) in the early process of muscle degeneration. Common mechanisms of mt-related cell injury include dysregulation of the mt permeability transition pore opening and defective autophagy. The therapeutic use of mt permeability transition pore modifiers holds promise in various neuromuscular disorders, including muscular dystrophies.

SWI/SNF enzymes promote SOX10- mediated activation of myelin gene expression

SOX10 is a Sry-related high mobility (HMG)-box transcriptional regulator that promotes differentiation of neural crest precursors into Schwann cells, oligodendrocytes, and melanocytes. Myelin, formed by Schwann cells in the peripheral nervous system, is essential for propagation of nerve impulses. SWI/SNF complexes are ATP dependent chromatin remodeling enzymes that are critical for cellular differentiation. It was recently demonstrated that the BRG1 subunit of SWI/SNF complexes activates SOX10 expression and also interacts with SOX10 to activate expression of OCT6 and KROX20, two transcriptional regulators of Schwann cell differentiation. To determine the requirement for SWI/SNF enzymes in the regulation of genes that encode components of myelin, which are downstream of these transcriptional regulators, we introduced SOX10 into fibroblasts that inducibly express dominant negative versions of the SWI/SNF ATPases, BRM or BRG1. Dominant negative BRM and BRG1 have mutations in the ATP binding site and inhibit gene activation events that require SWI/SNF function. Ectopic expression of SOX10 in cells derived from NIH 3T3 fibroblasts led to the activation of the endogenous Schwann cell specific gene, myelin protein zero (MPZ) and the gene that encodes myelin basic protein (MBP). Thus, SOX10 reprogrammed these cells into myelin gene expressing cells. Ectopic expression of KROX20 was not sufficient for activation of these myelin genes. However, KROX20 together with SOX10 synergistically activated MPZ and MBP expression. Dominant negative BRM and BRG1 abrogated SOX10 mediated activation of MPZ and MBP and synergistic activation of these genes by SOX10 and KROX20. SOX10 was required to recruit BRG1 to the MPZ locus. Similarly, in immortalized Schwann cells, BRG1 recruitment to SOX10 binding sites at the MPZ locus was dependent on SOX10 and expression of dominant negative BRG1 inhibited expression of MPZ and MBP in these cells. Thus, SWI/SNF enzymes cooperate with SOX10 to directly activate genes that encode components of peripheral myelin.

Similarities between inherited demyelinating neuropathies and Wallerian degeneration: an old repair program may cause myelin and axon perturbation under nonlesion conditions

Wallerian degeneration (WD) and inherited demyelinating neuropathies of the Charcot-Marie-Tooth type 1 (CMT1) appear to represent completely distinct events. CMT1-like diseases are chronic disorders of peripheral nerves that are genetically caused and lead to secondary neurodegenerative events, resulting in usually non-treatable disabilities, whereas WD is an acute, usually transient, reaction on injuries, aiming to allow peripheral nerve regeneration. Despite these differences, there are some striking similarities regarding molecular characteristics of neural cells in the affected peripheral nerves. The most conspicuous similarities might comprise the inflammatory component in both situations, as identified in appropriate mouse models. However, although inflammation is a beneficial component in WD, leading to removal of regrowth-repellent myelin debris, inflammation in CMT1 mouse models causes damage of initially intact nerve fibers. We hypothesize that, in CMT1 models, molecular pathways are activated that are shared with an important repair program after peripheral nerve injury, but lead to neural perturbation when activated under nonlesion conditions, as is the case in CMT1. These novel insights into the pathogenesis of CMT1 might be instrumental for the development of new therapeutic options in humans.

Myelin-specific proteins: a structurally diverse group of membrane-interacting molecules

The myelin sheath is a multilayered membrane in the nervous system, which has unique biochemical properties. Myelin carries a set of specific high-abundance proteins, the structure and function of which are still poorly understood. The proteins of the myelin sheath are involved in a number of neurological diseases, including autoimmune diseases and inherited neuropathies. In this review, we briefly discuss the structural properties and functions of selected myelin-specific proteins (P0, myelin oligodendrocyte glycoprotein, myelin-associated glycoprotein, myelin basic protein, myelin-associated oligodendrocytic basic protein, P2, proteolipid protein, peripheral myelin protein of 22 kDa, 2',3'-cyclic nucleotide 3'-phosphodiesterase, and periaxin); such properties include, for example, interactions with lipid bilayers and the presence of large intrinsically disordered regions in some myelin proteins. A detailed understanding of myelin protein structure and function at the molecular level will be required to fully grasp their physiological roles in the myelin sheath.

In vivo expression of the Arf6 Guanine-nucleotide exchange factor cytohesin-1 in mice exhibits enhanced myelin thickness in nerves

The myelin sheath consists of a unique multiple layer structure that acts as an insulator between neuronal axons to enhance the propagation of the action potential. In neuropathies such as demyelinating or dismyelinating diseases, chronic demyelination and defective remyelination occur repeatedly, leading to more severe neuropathy. As yet, little is known about the possibility of drug target-specific medicine for such diseases. In the developing peripheral nervous system (PNS), myelin sheaths form as Schwann cells wrap individual axons. It is thought that the development of a drug promoting myelination by Schwann cells would provide effective therapy against peripheral nerve disorders: to test such treatment, genetically modified mice overexpressing the drug target molecules are needed. We previously identified an Arf6 activator, the guanine-nucleotide exchange factor cytohesin-1, as the signaling molecule controlling myelination of peripheral axons by Schwann cells; yet, the important issue of whether cytohesin-1 itself promotes myelin thickness in vivo has remained unclear. Herein, we show that, in mouse PNS nerves, Schwann cell-specific expression of wild-type cytohesin-1 exhibits enhanced myelin thickness. Downstream activation of Arf6 is also seen in these transgenic mice, revealing the involvement of the cytohesin-1 and Arf6 signaling unit in promoting myelination. These results suggest that cytohesin-1 may be a candidate for the basis of a therapy for peripheral neuropathies through its enhancement of myelin thickness.

SIMPLE: A new regulator of endosomal trafficking and signaling in health and disease

SIMPLE, also known as LITAF, EET1 and PIG7, was originally identified based on its transcriptional upregulation by estrogen, p53, lipopolysaccharide or a microbial cell-wall component. Missense mutations in SIMPLE cause Charcot-Marie-Tooth disease (CMT), and altered SIMPLE expression is associated with cancer, obesity and inflammatory bowel diseases. Despite increasing evidence linking SIMPLE to human diseases, the biological function of SIMPLE is unknown and the pathogenic mechanism of SIMPLE mutations remains elusive. Our recent study reveals that SIMPLE is a functional partner of the endosomal sorting complex required for transport (ESCRT) machinery in the regulation of endosome-to-lysosome trafficking and intracellular signaling. Our results indicate that CMT-linked SIMPLE mutants are loss-of-function mutants which act dominantly to impair endosomal trafficking and signaling attenuation. We propose that endosomal trafficking and signaling dysregulation is a key pathogenic mechanism in CMT and other diseases that involve SIMPLE dysfunction.

Resetting translational homeostasis restores myelination in Charcot-Marie-Tooth disease type 1B mice

Reduction of the CHOP target Gadd34 restores motor function in P0S63del mice with demyelinating neuropathy.

P0 glycoprotein is an abundant product of terminal differentiation in myelinating Schwann cells. The mutant P0S63del causes Charcot-Marie-Tooth 1B neuropathy in humans, and a very similar demyelinating neuropathy in transgenic mice. P0S63del is retained in the endoplasmic reticulum of Schwann cells, where it promotes unfolded protein stress and elicits an unfolded protein response (UPR) associated with translational attenuation. Ablation of Chop, a UPR mediator, from S63del mice completely rescues their motor deficit and reduces active demyelination by half. Here, we show that Gadd34 is a detrimental effector of CHOP that reactivates translation too aggressively in myelinating Schwann cells. Genetic or pharmacological limitation of Gadd34 function moderates translational reactivation, improves myelination in S63del nerves, and reduces accumulation of P0S63del in the ER. Resetting translational homeostasis may provide a therapeutic strategy in tissues impaired by misfolded proteins that are synthesized during terminal differentiation.

Organization and maintenance of molecular domains in myelinated axons

Over a century ago, Ramon y Cajal first proposed the idea of a directionality involved in nerve conduction and neuronal communication. Decades later, it was discovered that myelin, produced by glial cells, insulated axons with periodic breaks where nodes of Ranvier (nodes) form to allow for saltatory conduction. In the peripheral nervous system (PNS), Schwann cells are the glia that can either individually myelinate the axon from one neuron or ensheath axons of many neurons. In the central nervous system (CNS), oligodendrocytes are the glia that myelinate axons from different neurons. Review of more recent studies revealed that this myelination created polarized domains adjacent to the nodes. However, the molecular mechanisms responsible for the organization of axonal domains are only now beginning to be elucidated. The molecular domains in myelinated axons include the axon initial segment (AIS), where various ion channels are clustered and action potentials are initiated; the node, where sodium channels are clustered and action potentials are propagated; the paranode, where myelin loops contact with the axolemma; the juxtaparanode (JXP), where delayed-rectifier potassium channels are clustered; and the internode, where myelin is compactly wrapped. Each domain contains a unique subset of proteins critical for the domain's function. However, the roles of these proteins in axonal domain organization are not fully understood. In this review, we highlight recent advances on the molecular nature and functions of some of the components of each axonal domain and their roles in axonal domain organization and maintenance for proper neuronal communication.

Motor and sensory neuropathy due to myelin infolding and paranodal damage in a transgenic mouse model of Charcot-Marie-Tooth disease type 1C

Charcot-Marie-Tooth disease type 1C (CMT1C) is a dominantly inherited motor and sensory neuropathy. Despite human genetic evidence linking missense mutations in SIMPLE to CMT1C, the in vivo role of CMT1C-linked SIMPLE mutations remains undetermined. To investigate the molecular mechanism underlying CMT1C pathogenesis, we generated transgenic mice expressing either wild-type or CMT1C-linked W116G human SIMPLE. Mice expressing mutant, but not wild type, SIMPLE develop a late-onset motor and sensory neuropathy that recapitulates key clinical features of CMT1C disease. SIMPLE mutant mice exhibit motor and sensory behavioral impairments accompanied by decreased motor and sensory nerve conduction velocity and reduced compound muscle action potential amplitude. This neuropathy phenotype is associated with focally infolded myelin loops that protrude into the axons at paranodal regions and near Schmidt-Lanterman incisures of peripheral nerves. We find that myelin infolding is often linked to constricted axons with signs of impaired axonal transport and to paranodal defects and abnormal organization of the node of Ranvier. Our findings support that SIMPLE mutation disrupts myelin homeostasis and causes peripheral neuropathy via a combination of toxic gain-of-function and dominant-negative mechanisms. The results from this study suggest that myelin infolding and paranodal damage may represent pathogenic precursors preceding demyelination and axonal degeneration in CMT1C patients.

Recessively transmitted predominantly motor neuropathies

Recessively transmitted predominantly motor neuropathies are rare and show a severe phenotype. They are frequently observed in populations with a high rate of consanguineous marriages. At least 15 genes and six loci have been found to be associated with autosomal recessive CMT (AR-CMT) and X-linked CMT (AR-CMTX) and also distal hereditary motor neuronopathy (AR-dHMN). These disorders are genetically heterogeneous but the clinical phenotype is relatively homogeneous. Distal muscle weakness and atrophy predominating in the lower extremities, diminished or absent deep tendon reflexes, distal sensory loss, and pes cavus are the main clinical features of this disorder with occasional cranial nerve involvement. Although genetic diagnosis of some of subtypes of AR-CMT are now available, rapid advances in the molecular genetics and cell biology show a great complexity. Animal models for the most common subtypes of human AR-CMT disease provide clues for understanding the pathogenesis of CMT and also help to reveal possible treatment strategies of inherited neuropathies. This chapter highlights the clinical features and the recent genetic and biological findings in these disorders based on the current classification.

Evaluating dermal myelinated nerve fibers in skin biopsy

Although there has been extensive research on small, unmyelinated fibers in the skin, little research has investigated dermal myelinated fibers in comparison. Glabrous, nonhairy skin contains mechanoreceptors that afford a vantage point for observation of myelinated fibers that have previously been seen only with invasively obtained nerve biopsies. This review discusses current morphometric and molecular expression data of normative and pathogenic glabrous skin obtained by various processing and analysis methods for cutaneous myelinated fibers. Recent publications have shed light on the role of glabrous skin biopsy in identifying signs of peripheral neuropathy and as a potential biomarker of distal myelin and mechanoreceptor integrity. The clinical relevance of a better understanding of the role of dermal myelinated nerve terminations in peripheral neuropathy will be addressed in light of recent publications in the growing field of skin biopsy.

Polarization and myelination in myelinating glia

Myelinating glia, oligodendrocytes in central nervous system and Schwann cells in peripheral nervous system, form myelin sheath, a multilayered membrane system around axons enabling salutatory nerve impulse conduction and maintaining axonal integrity. Myelin sheath is a polarized structure localized in the axonal side and therefore is supposed to be formed based on the preceding polarization of myelinating glia. Thus, myelination process is closely associated with polarization of myelinating glia. However, cell polarization has been less extensively studied in myelinating glia than other cell types such as epithelial cells. The ultimate goal of this paper is to provide insights for the field of myelination research by applying the information obtained in polarity study in other cell types, especially epithelial cells, to cell polarization of myelinating glia. Thus, in this paper, the main aspects of cell polarization study in general are summarized. Then, they will be compared with polarization in oligodendrocytes. Finally, the achievements obtained in polarization study for epithelial cells, oligodendrocytes, and other types of cells will be translated into polarization/myelination process by Schwann cells. Then, based on this model, the perspectives in the study of Schwann cell polarization/myelination will be discussed.

Systematic approaches to central nervous system myelin

Rapid signal propagation along vertebrate axons is facilitated by their insulation with myelin, a plasma membrane specialization of glial cells. The recent application of 'omics' approaches to the myelinating cells of the central nervous system, oligodendrocytes, revealed their mRNA signatures, enhanced our understanding of how myelination is regulated, and established that the protein composition of myelin is much more complex than previously thought. This review provides a meta-analysis of the > 1,200 proteins thus far identified by mass spectrometry in biochemically purified central nervous system myelin. Contaminating proteins are surprisingly infrequent according to bioinformatic prediction of subcellular localization and comparison with the transcriptional profile of oligodendrocytes. The integration of datasets also allowed the subcategorization of the myelin proteome into functional groups comprising genes that are coregulated during oligodendroglial differentiation. An unexpectedly large number of myelin-related genes cause-when mutated in humans-hereditary diseases affecting the physiology of the white matter. Systematic approaches to oligodendrocytes and myelin thus provide valuable resources for the molecular dissection of developmental myelination, glia-axonal interactions, leukodystrophies, and demyelinating diseases.

The CMT4B disease-causing proteins MTMR2 and MTMR13/SBF2 regulate AKT signalling

Charcot-Marie-Tooth disease type 4B is caused by mutations in the genes encoding either the lipid phosphatase myotubularin-related protein-2 (MTMR2) or its regulatory binding partner MTMR13/SBF2. Mtmr2 dephosphorylates PI-3-P and PI-3,5-P2 to form phosphatidylinositol and PI-5-P, respectively, while Mtmr13/Sbf2 is an enzymatically inactive member of the myotubularin protein family. We have found altered levels of the critical signalling protein AKT in mouse mutants for Mtmr2 and Mtmr13/Sbf2. Thus, we analysed the influence of Mtmr2 and Mtmr13/Sbf2 on signalling processes. We found that overexpression of Mtmr2 prevents the degradation of the epidermal growth factor receptor (EGFR) and leads to sustained Akt activation whereas Erk activation is not affected. Mtmr13/Sbf2 counteracts the blockage of EGFR degradation without affecting prolonged Akt activation. Our data indicate that Mtmr2 and Mtmr13/Sbf2 play critical roles in the sorting and modulation of cellular signalling which are likely to be disturbed in CMT4B.

Genetic disruption of Pten in a novel mouse model of tomaculous neuropathy

‘Tomacula’ and myelin outfoldings are striking neuropathological features of a diverse group of inherited demyelinating neuropathies. Whereas the underlying genetic defects are well known, the molecular mechanisms of tomacula formation have remained obscure. We hypothesized that they are caused by uncontrolled, excessive myelin membrane growth, a process, which is regulated in normal development by neuregulin-1/ErbB2, PI3 Kinase signalling and ERK/MAPK signalling. Here, we demonstrate by targeted disruption of Pten in Schwann cells that hyperactivation of the endogenous PI3 Kinase pathway causes focal hypermyelination, myelin outfoldings and tomacula, even when induced in adult animals by tamoxifen, and is associated with progressive peripheral neuropathy. Activated AKT kinase is associated with PtdIns(3,4,5)P₃ at paranodal loops and Schmidt–Lanterman incisures. This striking myelin pathology, with features of human CMT type 4B1 and HNPP, is dependent on AKT/mTOR signalling, as evidenced by a significant amelioration of the pathology in mice treated with rapamycin. We suggest that regions of non-compact myelin are under lifelong protection by PTEN against abnormal membrane outgrowth, and that dysregulated phosphoinositide levels play a critical role in the pathology of tomaculous neuropathies.

Charcot-Marie-Tooth disease and intracellular traffic

Mutations of genes whose primary function is the regulation of membrane traffic are increasingly being identified as the underlying causes of various important human disorders. Intriguingly, mutations in ubiquitously expressed membrane traffic genes often lead to cell type- or organ-specific disorders. This is particularly true for neuronal diseases, identifying the nervous system as the most sensitive tissue to alterations of membrane traffic. Charcot-Marie-Tooth (CMT) disease is one of the most common inherited peripheral neuropathies. It is also known as hereditary motor and sensory neuropathy (HMSN), which comprises a group of disorders specifically affecting peripheral nerves. This peripheral neuropathy, highly heterogeneous both clinically and genetically, is characterized by a slowly progressive degeneration of the muscle of the foot, lower leg, hand and forearm, accompanied by sensory loss in the toes, fingers and limbs. More than 30 genes have been identified as targets of mutations that cause CMT neuropathy. A number of these genes encode proteins directly or indirectly involved in the regulation of intracellular traffic. Indeed, the list of genes linked to CMT disease includes genes important for vesicle formation, phosphoinositide metabolism, lysosomal degradation, mitochondrial fission and fusion, and also genes encoding endosomal and cytoskeletal proteins. This review focuses on the link between intracellular transport and CMT disease, highlighting the molecular mechanisms that underlie the different forms of this peripheral neuropathy and discussing the pathophysiological impact of membrane transport genetic defects as well as possible future ways to counteract these defects.

Common variants near TARDBP and EGR2 are associated with susceptibility to Ewing sarcoma

Ewing sarcoma, a pediatric tumor characterized by EWSR1-ETS fusions, is predominantly observed in populations of European ancestry. We performed a genome-wide association study (GWAS) of 401 French individuals with Ewing sarcoma, 684 unaffected French individuals and 3,668 unaffected individuals of European descent and living in the United States. We identified candidate risk loci at 1p36.22, 10q21 and 15q15. We replicated these loci in two independent sets of cases and controls. Joint analysis identified associations with rs9430161 (P = 1.4 × 10(-20); odds ratio (OR) = 2.2) located 25 kb upstream of TARDBP, rs224278 (P = 4.0 × 10(-17); OR = 1.7) located 5 kb upstream of EGR2 and, to a lesser extent, rs4924410 at 15q15 (P = 6.6 × 10(-9); OR = 1.5). The major risk haplotypes were less prevalent in Africans, suggesting that these loci could contribute to geographical differences in Ewing sarcoma incidence. TARDBP shares structural similarities with EWSR1 and FUS, which encode RNA binding proteins, and EGR2 is a target gene of EWSR1-ETS. Variants at these loci were associated with expression levels of TARDBP, ADO (encoding cysteamine dioxygenase) and EGR2.

Myotubularin phosphoinositide phosphatases in human diseases

The level and turnover of phosphoinositides (PIs) are tightly controlled by a large set of PI-specific enzymes (PI kinases and phosphatases). Mammalian PI phosphatases are conserved through evolution and among this large family the dual-specificity phosphatase (PTP/DSP) are metal-independent enzymes displaying the amino acid signature Cys-X5-Arg-Thr/Ser (CX5RT/S) in their active site. Such catalytic site characterizes the myotubularin 3-phosphatases that dephosphorylate PtdIns3P and PtdIns(3,5)P₂ and produce PtdIns5P. Substrates of myotubularins have been implicated in endocytosis and membrane trafficking while PtdIns5P may have a role in signal transduction. As a paradox, 6 of the 14 members of the myotubularin family lack enzymatic activity and are considered as dead phosphatases. Several myotubularins have been genetically linked to human diseases: MTM1 is mutated in the congenital myopathy X-linked centronuclear or myotubular myopathy (XLCNM) and MTMR14 (JUMPY) has been linked to an autosomal form of such disease, while MTMR2 and MTMR13 are mutated in Charcot-Marie-Tooth (CMT) neuropathies. Furthermore, recent evidences from genetic association studies revealed that several other myotubularins could be associated to chronic disorders such as cancer and obesity, highlighting their importance for human health. Here, we discuss cellular and physiological roles of myotubularins and their implication in human diseases, and we present potential pathological mechanisms affecting specific tissues in myotubularin-associated diseases.

Structural basis for the Trembler-J phenotype of Charcot-Marie-Tooth disease

Mutations in peripheral myelin protein 22 (PMP22) can result in the common peripheral neuropathy Charcot-Marie-Tooth disease (CMTD). The Leu16Pro mutation in PMP22 results in misassembly of the protein, which causes the Trembler-J (TrJ) disease phenotype. Here we elucidate the structural defects present in a partially folded state of TrJ PMP22 that are decisive in promoting CMTD-causing misfolding. In this state, transmembrane helices 2-4 (TM2-4) form a molten globular bundle, while transmembrane helix 1 (TM1) is dissociated from this bundle. The TrJ mutation was seen to profoundly disrupt the TM1 helix, resulting in increased backbone dynamics and changes in the tertiary interactions of TM1 with the PMP22 TM2-4 core in the folded state. Consequently, TM1 undergoes enhanced dissociation from the other transmembrane segments in TrJ PMP22, becoming available for recognition and sequestration by protein-folding quality control, which leads to loss of function and toxic accumulation of aggregates that result in CMTD.

Charcot-Marie-Tooth caused by a copy number variation in myelin protein zero

INTRODUCTION

Charcot-Marie-Tooth disease (CMT) is the most common inherited disorder of the peripheral nervous system. The majority has a duplication of the peripheral myelin protein 22. CMT is otherwise caused by point mutations or small insertions/deletions in one of the 44 known CMT genes.

METHODS AND RESULTS

Conventional sequencing of six CMT genes were followed by Multiplex Ligation-dependent Probe Amplification (MLPA), array Comparative Genomic Hybridization (aCGH) and breakpoint analysis in a large Norwegian CMT pedigree. Affected had an extra copy of the myelin protein zero (MPZ) gene.

CONCLUSION

To our knowledge this is the first non-peripheral myelin protein 22 copy number variation to cause Charcot-Marie-Tooth disease.

Reduction of Dicer impairs Schwann cell differentiation and myelination

The process of Schwann cell myelination requires precisely coordinated gene expression. At the onset of myelination, there is an increase in the expression of differentiation-promoting transcription factors that regulate key Schwann cell genes. Further control of myelin gene expression occurs at the posttranscriptional level and, in part, is mediated by RNA binding proteins and micro-RNAs (miRNAs). miRNAs are small, endogenously derived RNA molecules that repress gene expression by specifically binding to their mRNA targets. In the experiments described here, we tested whether miRNAs were essential in controlling myelination by reducing the levels of Dicer, an essential endoribonuclease in miRNA biogenesis. We decreased the expression of Dicer by about 60% within Schwann cells using a lentiviral vector expressing an shRNA against Dicer. The reduced levels of Dicer led to a decrease in the steady-state expression of selected miRNAs and of the transcription factors Oct6 and Egr2/Krox20, both of which are critical for Schwann cells differentiation and myelination. In contrast, the levels of c-jun and Sox2 were up-regulated by the reduction in Dicer and were associated with an increase in Schwann cell proliferation. In dorsal root ganglion cocultures, Schwann cells transduced with Dicer shRNA synthesized less myelin, which was accompanied by significant reductions in the levels of myelin basic protein and protein zero. These findings support a critical role for Dicer and miRNAs in Schwann cell differentiation and myelination.

New insights into signaling during myelination in zebrafish

Myelin is a vertebrate adaptation that allows for the rapid propagation of action potentials along axons. Specialized glial cells-oligodendrocytes in the central nervous system (CNS) and Schwann cells in the peripheral nervous system (PNS)-form myelin by repeatedly wrapping axon segments. Debilitating diseases result from the disruption of myelin, including multiple sclerosis and Charcot-Marie-Tooth peripheral neuropathies. The process of myelination involves extensive communication between glial cells and the associated neurons. The past few years have seen important progress in understanding the molecular basis of the signals that coordinate the development of these fascinating cells. This review highlights recent advances in myelination deriving from studies in the zebrafish model system, with a primary focus on the PNS. While Neuregulin1-ErbB signaling has long been known to play important roles in peripheral myelin development, work in zebrafish has elucidated its roles in Schwann cell migration and radial sorting of axons in vivo. Forward genetic screens in zebrafish have also uncovered new genes required for development of myelinated axons, including gpr126, which encodes a G-protein coupled receptor required for Schwann cells to progress beyond the promyelinating stage. In addition, work in zebrafish uncovered new roles for Schwann cells themselves, including in regulating the boundary between the PNS and CNS and positioning a nerve after its initial outgrowth.

A novel deletion mutation in GJB1 causes X-linked Charcot-Marie-Tooth disease in a Han Chinese family

X-linked Charcot-Marie-Tooth disease CMT (CMTX) is predominantly caused by mutations in the GJB1 gene that encode connexin32. We describe the clinical findings and the identification of a novel mutation in GJB1 in a large Han Chinese family with CMTX. Linkage to GJB1 was determined by genotyping five polymorphic markers flanking GJB1. Sequence alterations were determined by directly sequencing the coding region of the GJB1 gene. The affected members have variable clinical manifestations. Linkage analysis confirmed the cosegregation of the disease with the GJB1 locus. Sequencing of the GJB1 gene revealed a 1-basepair deletion (c.110delT) in the coding region. The frameshift begins at amino acid 37 and generates a premature stop codon at position 83. The shortened peptide is unlikely to be functional, as it lacks most of the functional domains. The CMTX in this family is caused by a novel loss of function mutation.

Mitochondrial dynamics in cell death and neurodegeneration

Mitochondria are highly dynamic organelles that continuously undergo two opposite processes, fission and fusion. Mitochondrial dynamics influence not only mitochondrial morphology, but also mitochondrial biogenesis, mitochondrial distribution within the cell, cell bioenergetics, and cell injury or death. Drp1 mediates mitochondrial fission, whereas Mfn1/2 and Opa1 control mitochondrial fusion. Neurons require large amounts of energy to carry out their highly specialized functions. Thus, mitochondrial dysfunction is a prominent feature in a variety of neurodegenerative diseases. Mutations of Mfn2 and Opa1 lead to neuropathies such as Charcot-Marie-Tooth disease type 2A and autosomal dominant optic atrophy. Moreover, both Aβ peptide and mutant huntingtin protein induce mitochondrial fragmentation and neuronal cell death. In addition, mutants of Parkinson's disease-related genes also show abnormal mitochondrial morphology. This review highlights our current understanding of abnormal mitochondrial dynamics relevant to neuronal synaptic loss and cell death in neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease and Huntington's disease.

Attenuation of MCP-1/CCL2 expression ameliorates neuropathy in a mouse model for Charcot-Marie-Tooth 1X

The chemokine monocyte chemoattractant protein-1 (MCP-1/CCL2) has been previously shown to be an important mediator of macrophage-related neural damage in models of two distinct inherited neuropathies, Charcot-Marie-Tooth (CMT) 1A and 1B. In mice deficient in the gap junction protein connexin 32 (Cx32def), an established model for the X-chromosome-linked dominant form of CMT (CMT1X), we investigated the role of the chemokine in macrophage immigration and neural damage by crossbreeding the Cx32def mice with MCP-1 knockout mutants. In Cx32def mutants typically expressing increased levels of MCP-1, macrophage numbers were strongly elevated, caused by an MCP-1-mediated influx of haematogenous macrophages. Curiously, the complete genetic deletion of MCP-1 did not cause reduced macrophage numbers in the nerves due to compensatory proliferation of resident macrophages. In contrast, and as already seen in other CMT models, heterozygous deletion of MCP-1 led to reduced numbers of phagocytosing macrophages and an alleviation of demyelination. Whereas alleviated demyelination was transient, axonal damage was persistently improved and even robust axonal sprouting was detectable at 12 months. Other axon-related features were alleviated electrophysiological parameters, reduced muscle denervation and atrophy, and increased muscle strength. Similar to models for CMT1A and CMT1B, we identified MEK-ERK signalling as mediating MCP-1 expression in Cx32-deficient Schwann cells. Blocking this pathway by the inhibitor CI-1040 caused reduced MCP-1 expression, attenuation of macrophage increase and amelioration of myelin- and axon-related alterations. Thus, attenuation of MCP-1 upregulation by inhibiting ERK phosphorylation might be a promising approach to treat CMT1X and other so far untreatable inherited peripheral neuropathies in humans.

Describing the hexapeptide identity platform between the influenza A H5N1 and Homo sapiens proteomes

We searched the primary sequence of influenza A H5N1 polyprotein for hexamer amino acid sequences shared with human proteins using the Protein International Resource database and the exact peptide matching analysis program. We find that the viral polyprotein shares numerous hexapeptides with the human proteome. The human proteins involved in the viral overlap are represented by antigens associated with basic cell functions such as proliferation, development, and differentiation. Of special importance, many human proteins that share peptide sequences with influenza A polyprotein are antigens such as reelin, neurexin I-α, myosin-IXa, Bardet–Biedl syndrome 10 protein, Williams syndrome transcription factor, disrupted in schizophrenia 1 protein, amyotrophic lateral sclerosis 2 chromosomal region candidate gene 17 protein, fragile X mental retardation 2 protein, and jouberin. That is, the viral-vs-human overlap involves human proteins that, when altered, have been reported to be potentially associated with multiple neurological disorders that can include autism, epilepsy, obesity, dystonia, ataxia–telangiectasia, amyotrophic lateral sclerosis, sensorineural deafness, sudden infant death syndrome, Charcot-Marie-Tooth disease, and myelination. The present data are discussed as a possible molecular basis for understanding influenza A viral escape from immunosurveillance and for defining anti-influenza immune-therapeutic approaches devoid of collateral adverse events.

Defective adult oligodendrocyte and Schwann cell development, pigment pattern, and craniofacial morphology in puma mutant zebrafish having an alpha tubulin mutation

The processes of myelination remain incompletely understood but are of profound biomedical importance owing to the several dysmyelinating and demyelinating disorders known in humans. Here, we analyze the zebrafish puma mutant, isolated originally for pigment pattern defects limited to the adult stage. We show that puma mutants also have late-arising defects in Schwann cells of the peripheral nervous system, locomotor abnormalities, and sex-biased defects in adult craniofacial morphology. Using methods of positional cloning, we identify a critical genetic interval harboring two alpha tubulin loci, and we identify a chemically induced missense mutation in one of these, tubulin alpha 8-like 3a (tuba8l3a). We demonstrate tuba8l3a expression in the central nervous system (CNS), leading us to search for defects in the development of oligodendrocytes, the myelinating cells of the CNS. We find gross reductions in CNS myelin and oligodendrocyte numbers in adult puma mutants, and these deficits are apparent already during the larval-to-adult transformation. By contrast, analyses of embryos and early larvae reveal a normal complement of oligodendrocytes that nevertheless fail to localize normal amounts of myelin basic protein (mbp) mRNA in cellular processes, and fail to organize these processes as in the wild-type. This study identifies the puma mutant as a valuable model for studying microtubule-dependent events of myelination, as well as strategies for remyelination in the adult.

Xenomitochondrial mice: investigation into mitochondrial compensatory mechanisms

Xenomitochondrial mice, harboring evolutionarily divergent Mus terricolor mitochondrial DNA (mtDNA) on a Mus musculus domesticus nuclear background (B6NTac(129S6)-mt(M. terricolor)/Capt; line D7), were subjected to molecular and phenotypic analyses. No overt in vivo phenotype was identified in contrast to in vitro xenomitochondrial cybrid studies. Microarray analyses revealed differentially expressed genes in xenomitochondrial mice, though none were directly involved in mitochondrial function. qRT-PCR revealed upregulation of mt-Co2 in xenomitochondrial mice. These results illustrate that cellular compensatory mechanisms for mild mitochondrial dysfunction alter mtDNA gene expression at a proteomic and/or translational level. Understanding these mechanisms will facilitate the development of therapeutics for mitochondrial disorders.

Phosphoinositide phosphatases in cell biology and disease

Phosphoinositides are essential signaling molecules linked to a diverse array of cellular processes in eukaryotic cells. The metabolic interconversions of these phospholipids are subject to exquisite spatial and temporal regulation executed by arrays of phosphatidylinositol (PtdIns) and phosphoinositide-metabolizing enzymes. These include PtdIns- and phosphoinositide-kinases that drive phosphoinositide synthesis, and phospholipases and phosphatases that regulate phosphoinositide degradation. In the past decade, phosphoinositide phosphatases have emerged as topics of particular interest. This interest is driven by the recent appreciation that these enzymes represent primary mechanisms for phosphoinositide degradation, and because of their ever-increasing connections with human diseases. Herein, we review the biochemical properties of six major phosphoinositide phosphatases, the functional involvements of these enzymes in regulating phosphoinositide metabolism, the pathologies that arise from functional derangements of individual phosphatases, and recent ideas concerning the involvements of phosphoinositide phosphatases in membrane traffic control.

Mpz gene suppression by shRNA increases Schwann cell apoptosis in vitro

We investigated the effects of short hairpin RNA (shRNA) on myelin protein zero (MPZ) gene expression in Schwann cells (SCs) in vitro and determined the effects of the MPZ gene suppression on the survival of SCs. The MPZ-specific shRNA was introduced into a lentiviral vector for expression under the U6 promoter, and the viral vector-based shRNAs were used to infect cultured SCs. The efficiency of MPZ knockdown was analyzed by real time-PCR (RT-PCR) and western blotting. Flow cytometric analysis and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) were used to determine the cell cycle and the amount of apoptosis of SCs. We found that MPZ shRNAs significantly inhibited the expression of the MPZ gene and induced SC apoptosis in vitro. These results provided interesting experimental evidence for understanding the mechanism of demyelinating neuropathies caused by MPZ gene malfunction.

The phosphoinositide 3-phosphatase MTMR2 interacts with PSD-95 and maintains excitatory synapses by modulating endosomal traffic

MTMR2 is a 3-phosphatase specific for the phosphoinositides PI(3)P and PI(3,5)P(2), which are mainly present on endosomes. Mutations in the MTMR2 gene in Schwann cells lead to a severe demyelinating peripheral neuropathy known as Charcot-Marie-Tooth disease type 4B1. MTMR2 expression is also detected in peripheral and central neurons, but neural functions of MTMR2 remain unclear. Here, we report that MTMR2 is localized to excitatory synapses of central neurons via direct interaction with PSD-95, a postsynaptic scaffolding protein abundant at excitatory synapses. Knockdown of MTMR2 in cultured neurons markedly reduces excitatory synapse density and function. This effect is rescued by wild-type MTMR2 but not by a mutant MTMR2 lacking PSD-95 binding or 3-phosphatase activity. MTMR2 knockdown leads to a decrease in the intensity of EEA1-positive early endosomes in dendrites but increases the intensity in the cell body region. Moreover, MTMR2 suppression promotes endocytosis, but not recycling, of the GluR2 subunit of AMPA receptors, which is an endosomal cargo. In addition, colocalization of internalized GluR2 with Lamp1-positive late endosomes/lysosomes is enhanced in the cell body area but not in dendrites. These results suggest that PSD-95-interacting MTMR2 contributes to the maintenance of excitatory synapses by inhibiting excessive endosome formation and destructive endosomal traffic to lysosomes.

Calnexin deficiency leads to dysmyelination

Calnexin is a molecular chaperone and a component of the quality control of the secretory pathway. We have generated calnexin gene-deficient mice (cnx(-/-)) and showed that calnexin deficiency leads to myelinopathy. Calnexin-deficient mice were viable with no discernible effects on other systems, including immune function, and instead they demonstrated dysmyelination as documented by reduced conductive velocity of nerve fibers and electron microscopy analysis of sciatic nerve and spinal cord. Myelin of the peripheral and central nervous systems of cnx(-/-) mice was disorganized and decompacted. There were no abnormalities in neuronal growth, no loss of neuronal fibers, and no change in fictive locomotor pattern in the absence of calnexin. This work reveals a previously unrecognized and important function of calnexin in myelination and provides new insights into the mechanisms responsible for myelin diseases.

Sodium-dependent vitamin C transporter 2 (SVCT2) is necessary for the uptake of L-ascorbic acid into Schwann cells

Ascorbic acid has been shown to be an essential component for in vitro myelination and to improve the clinical and pathological phenotype of a mouse model of Charcot-Marie-tooth disease 1A. The mechanism of ascorbic acid uptake into peripheral nerves, however, has not been addressed so far. Hence, we studied the expression and activity of sodium-dependent vitamin C transporters 1 and 2 (SVCT1 and 2) in the peripheral nervous system. Using immunohistochemistry, immunoblotting, and reverse transcription PCR, we could show that SVCT1 and 2 were differentially expressed in myelinated peripheral nerve fibers and Schwann cell (SC) cultures. SVCT1 was expressed at very low levels confined to the axons, whereas SVCT2 was highly expressed both in the axons and in the SCs. SVCT2 was localized particularly in SC compartments of uncompacted myelin. Uptake assays using (14)C-labeled ascorbic acid showed transport of ascorbic acid into SC cultures. Ascorbic acid transport was dependent on the concentration of sodium, magnesium, and calcium in the extracellular medium. Treatment with the flavonoid phloretin, a known inhibitor of SVCT1 and 2, and specific RNA interference with SVCT2 caused significant reductions in ascorbic acid uptake into SCs. Phloretin-inhibited uptake of ascorbic acid was further shown in freshly dissected, cell-culture-naïve rat sciatic nerves. These results provide evidence for the first time that uptake of ascorbic acid in the peripheral nervous system is crucially dependent on the expression and activity of SVCT2.

Inactivation of fibroblast growth factor receptor signaling in myelinating glial cells results in significant loss of adult spiral ganglion neurons accompanied by age-related hearing impairment

Hearing loss has been attributed to many factors, including degeneration of sensory neurons in the auditory pathway and demyelination along the cochlear nerve. Fibroblast growth factors (FGFs), which signal through four receptors (Fgfrs), are produced by auditory neurons and play a key role in embryonic development of the cochlea and in neuroprotection against sound-induced injury. However, the role of FGF signaling in the maintenance of normal auditory function in adult and aging mice remains to be elucidated. Furthermore, the contribution of glial cells, which myelinate the cochlear nerves, is poorly understood. To address these questions, we generated transgenic mice in which Fgfr1 and Fgfr2 were specifically inactivated in Schwann cells and oligodendrocytes but not in neurons. Adult mutant mice exhibited late onset of hearing impairment, which progressed markedly with age. The hearing impairment was accompanied by significant loss of myelinated spiral ganglion neurons. The pathology extended into the cochlear nucleus, without apparent loss of myelin or of the deletion-bearing glial cells themselves. This suggests that perturbation of FGF receptor-mediated glial function leads to the attenuation of glial support of neurons, leading to their loss and impairment of auditory functions. Thus, FGF/FGF receptor signaling provides a potentially novel mechanism of maintaining reciprocal interactions between neurons and glia in adult and aging animals. Dysfunction of glial cells and FGF receptor signaling may therefore be implicated in neurodegenerative hearing loss associated with normal aging.

MCP-1/CCL2 modifies axon properties in a PMP22-overexpressing mouse model for Charcot-Marie-tooth 1A neuropathy

Charcot-Marie-Tooth 1A (CMT1A) neuropathy, the most common inherited peripheral neuropathy, is primarily caused by a gene duplication for the peripheral myelin protein-22 (PMP22). In an accordant mouse model, we investigated the role of monocyte chemoattractant protein-1 (MCP-1/CCL2) as a regulator of nerve macrophages and neural damage including axonopathy and demyelination. By generating PMP22tg mice with reduced levels or lack of MCP-1/CCL2, we found that MCP-1/CCL2 is involved in the increase of macrophages in mutant nerves. PMP22tg mice with wild-type levels of MCP-1/CCL2 showed strong macrophage increase in the diseased nerves, whereas either 50% reduction or total absence of MCP-1/CCL2 led to a moderate or a strong reduction of nerve macrophages, respectively. Interestingly, MCP-1/CCL2 expression level and macrophage numbers were correlated with features indicative of axon damage, such as maldistribution of K+ channels, reduced compound muscle action potentials, and muscle weakness. Demyelinating features, however, were most highly reduced when MCP-1/CCL2 was diminished by 50%, whereas complete lack of MCP-1/CCL2 showed an intermediate demyelinating phenotype. We also identified the MEK1/2-ERK1/2-pathway as being involved in MCP-1/CCL2 expression in the Schwann cells of the CMT1A model. Our data show that, in a CMT1A model, MCP-1/CCL2 activates nerve macrophages, mediates both axon damage and demyelination, and may thus be a promising target for therapeutic approaches.

[Predominant parasympathetic involvement in a patient with Charcot-Marie-Tooth disease caused by the MPZ Thr124Met mutation]

Erectile dysfunction, dysuria, photophobia, and chronic cough developed insidiously in a 49-year-old man from his third decade. Severe difficulty of urination resulted in intermittent catheterization. He had six family members who had suffered similar autonomic symptoms with or without motor deficits. He presented asymmetrical tonic pupils, a neurogenic bladder, and mild sensory impairment in the distal parts of the bilateral lower limbs without orthostatic hypotension and motor deficits. Nerve conduction studies revealed mild axonal changes with slightly reduced conduction velocities in the lower limbs. His left pupil over-responded to instillation with 0.125% pilocarpine. Functional bladder tests showed an atonic bladder, suggesting postganglionic parasympathetic involvement. Autonomic evaluation for sympathetic components including head-up tilt, beat to beat responses to Valsalva's maneuver, cardiac MIBG imaging, plasma catecholamine levels and sweat tests were all normal. A genetic test disclosed a heterozygous mutation of myelin protein zero (MPZ); p.Thr124Met. Selectively distributed dysautonomia in this pedigree may indicate parasympathetic postganglionic components including the ganglion as the primary target of this mutated MPZ in the autonomic nervous system.

Peripheral myelin protein 22 is regulated post-transcriptionally by miRNA-29a

Peripheral myelin protein 22 (PMP22) is a dose-sensitive, disease-associated protein primarily expressed in myelinating Schwann cells. Either reduction or overproduction of PMP22 can result in hereditary neuropathy, suggesting a requirement for correct protein expression for peripheral nerve biology. PMP22 is post-transcriptionally regulated and the 3'untranslated region (3'UTR) of the gene exerts a negative effect on translation. MicroRNAs (miRNAs) are small regulatory molecules that function at a post-transcriptional level by targeting the 3'UTR in a reverse complementary manner. We used cultured Schwann cells to demonstrate that alterations in the miRNA biogenesis pathway affect PMP22 levels, and endogenous PMP22 is subjected to miRNA regulation. GW-body formation, the proposed cytoplasmic site for miRNA-mediated repression, and Dicer expression, an RNase III family ribonuclease involved in miRNA biogenesis, are co-regulated with the differentiation state of Schwann cells. Furthermore, the levels of Dicer inversely correlate with PMP22, while the inhibition of Dicer leads to elevated PMP22. Microarray analysis of actively proliferating and differentiated Schwann cells, in conjunction with bioinformatics programs, identified several candidate PMP22-targeting miRNAs. Here we demonstrate that miR-29a binds and inhibits PMP22 reporter expression through a specific miRNA seed binding region. Over-expression of miR-29a enhances the association of PMP22 RNA with Argonaute 2, a protein involved in miRNA function, and reduces the steady-state levels of PMP22. In contrast, inhibition of endogenous miR-29a relieves the miRNA-mediated repression of PMP22. Correlation analyses of miR-29 and PMP22 in sciatic nerves reveal an inverse relationship, both developmentally and in post-crush injury. These results identify PMP22 as a target of miRNAs and suggest that myelin gene expression by Schwann cells is regulated by miRNAs.

SH3TC2/KIAA1985 protein is required for proper myelination and the integrity of the node of Ranvier in the peripheral nervous system

Charcot-Marie-Tooth disease type 4C (CMT4C) is an early-onset, autosomal recessive form of demyelinating neuropathy. The clinical manifestations include progressive scoliosis, delayed age of walking, muscular atrophy, distal weakness, and reduced nerve conduction velocity. The gene mutated in CMT4C disease, SH3TC2/KIAA1985, was recently identified; however, the function of the protein it encodes remains unknown. We have generated knockout mice where the first exon of the Sh3tc2 gene is replaced with an enhanced GFP cassette. The Sh3tc2(DeltaEx1/DeltaEx1) knockout animals develop progressive peripheral neuropathy manifested by decreased motor and sensory nerve conduction velocity and hypomyelination. We show that Sh3tc2 is specifically expressed in Schwann cells and localizes to the plasma membrane and to the perinuclear endocytic recycling compartment, concordant with its possible function in myelination and/or in regions of axoglial interactions. Concomitantly, transcriptional profiling performed on the endoneurial compartment of peripheral nerves isolated from control and Sh3tc2(DeltaEx1/DeltaEx1) animals uncovered changes in transcripts encoding genes involved in myelination and cell adhesion. Finally, detailed analyses of the structures composed of compact and noncompact myelin in the peripheral nerve of Sh3tc2(DeltaEx1/DeltaEx1) animals revealed abnormal organization of the node of Ranvier, a phenotype that we confirmed in CMT4C patient nerve biopsies. The generated Sh3tc2 knockout mice thus present a reliable model of CMT4C neuropathy that was instrumental in establishing a role for Sh3tc2 in myelination and in the integrity of the node of Ranvier, a morphological phenotype that can be used as an additional CMT4C diagnostic marker.

The role of CNS glia in preserving axon function

Axons are the physical conduits by which information is relayed within the nervous system and as such, are essential for normal neurological function. In the central nervous system (CNS), axons comprise the bulk of the white matter, where they are closely associated with glial cells. Primary alterations of glial cell functions can have detrimental secondary consequences for axons, demonstrating that white matter glia are important custodians of axonal integrity. For example, genetic ablation of key oligodendroglial molecules abrogates the oligodendrocytes' supportive function, while expression of mutant super oxide dismutase in astrocytes expedites progression of motor neuron disease. Here we review some of the recent literature on the role of CNS glia in axonal health.

Reconstruction of multiplanar deformity of the hindfoot and midfoot with internal fixation techniques

Reconstruction surgery of the midand hindfoot is a demanding challenge for foot surgeons. Satisfactory results depend not only on surgical technique and skills but also on the knowledge of underlying disorders, pathomechanics, and indication criteria. The cavovarus foot, the planovalgus foot, and Charcot's foot are some of the most challenging foot deformities, requiring different surgical strategies for their correction. Most of the osteotomies and fusions in children and adults can be fixed with transcutaneous Kirschner wires, which are inexpensive and easy to use and remove. The use of alternative fixation systems such as cannulated screws, compression screws, or angle-stable locking plates depends on patient age, vascular situation, risk for nonunion, and underlying pathology.