Dlg1, Sec8, and Mtmr2 regulate membrane homeostasis in Schwann cell myelination.

How membrane biosynthesis and homeostasis is achieved in myelinating glia is mostly unknown. We previously reported that loss of myotubularin-related protein 2 (MTMR2) provokes autosomal recessive demyelinating Charcot-Marie-Tooth type 4B1 neuropathy, characterized by excessive redundant myelin, also known as myelin outfoldings. We generated a Mtmr2-null mouse that models the human neuropathy. We also found that, in Schwann cells, Mtmr2 interacts with Discs large 1 (Dlg1), a scaffold involved in polarized trafficking and membrane addition, whose localization in Mtmr2-null nerves is altered. We here report that, in Schwann cells, Dlg1 also interacts with kinesin 13B (kif13B) and Sec8, which are involved in vesicle transport and membrane tethering in polarized cells, respectively. Taking advantage of the Mtmr2-null mouse as a model of impaired membrane formation, we provide here the first evidence for a machinery that titrates membrane formation during myelination. We established Schwann cell/DRG neuron cocultures from Mtmr2-null mice, in which myelin outfoldings were reproduced and almost completely rescued by Mtmr2 replacement. By exploiting this in vitro model, we propose a mechanism whereby kif13B kinesin transports Dlg1 to sites of membrane remodeling where it coordinates a homeostatic control of myelination. The interaction of Dlg1 with the Sec8 exocyst component promotes membrane addition, whereas with Mtmr2, negatively regulates membrane formation. Myelin outfoldings thus arise as a consequence of the loss of negative control on the amount of membrane, which is produced during myelination.

Heat shock protein 27 R127W mutation: evidence of a continuum between axonal Charcot-Marie-Tooth and distal hereditary motor neuropathy.

BACKGROUND: Heat shock protein 27 (HSP27) mutations have been reported to cause both Charcot-Marie-Tooth disease (CMT) type 2F and distal hereditary motor neuropathy (dHMN) although never previously in a single family. OBJECTIVE: To analyse clinical and electrophysiological findings obtained in a single large Sardinian family bearing the HSP27 R127W mutation. METHODS: Twenty-one members of a five generation Sardinian family have been studied, including thirteen members affected by peroneal muscular atrophy and proved heterozygous for the known HSP27 R127W mutation. Twelve patients and eight unaffected relatives were subjected to clinical examination. A standardised electrophysiological study was performed in eleven patients and six unaffected relatives. RESULTS: Mean age at onset (+/-SD) was 31.2+/-7.2 years. Mean age at investigation was 45.2+/-12.9 years and mean disease duration at the time of investigation was 14+/-12.9 years. According to current criteria for CMT2 and dHMN, of the 10 patients who had undergone both clinical and neurophysiological examination, five were diagnosed as CMT2, two as dHMN and a further two patients were labelled as an intermediate type. Finally, due to the presence of spastic paraplegia, the index patient did not meet established criteria for the diagnosis of CMT or dHMN. DISCUSSION: Findings obtained in the present study, broadening the spectrum of clinical manifestations of disorders associated with HSP27 mutations, support the hypothesis of a continuum between CMT2 and dHMN forms and suggest a possible common spectrum between these entities and several forms of CMT plus pyramidal features (HMSN V), providing important implications for molecular genetic testing.

Myotubularin-related (MTMR) phospholipid phosphatase proteins in the peripheral nervous system.

Myotubularin-related proteins (MTMRs) constitute a broad family of ubiquitously expressed phosphatases with 14 members in humans, of which eight are catalytically active phosphatases, while six are catalytically inactive. Active MTMRs possess 3-phosphatase activity toward both PtdIns3P and PtdIns(3, 5)P 2 poliphosphoinositides (PPIn), suggesting an involvement in intracellular trafficking and membrane homeostasis. Among MTMRs, catalytically active MTMR2 and inactive MTMR13 have a nonredundant function in nerve. Loss of either MTMR2 or MTMR13 causes Charcot-Marie-Tooth type 4B1 and B2 neuropathy, respectively, characterized by demyelination and redundant loops of myelin known as myelin outfoldings. In Mtmr2-null mouse nerves, these aberrant foldings occur at 3-4 weeks after birth, a time when myelination is established, and Schwann cells are still elongating to reach the final internodal length. Moreover, Mtmr2-specific ablation in Schwann cells is both sufficient and necessary to provoke CMT4B1 with myelin outfoldings. MTMR2 phospholipid phosphatase might regulate intracellular trafficking events and membrane homeostasis in Schwann cells during postnatal nerve development. In this review, we will discuss recent findings on the MTMR family with a major focus on MTMR2 and MTMR13 and their putative role in Schwann cell biology.

Loss of Mtmr2 phosphatase in Schwann cells but not in motor neurons causes Charcot-Marie-Tooth type 4B1 neuropathy with myelin outfoldings.

Mutations in MTMR2, the myotubularin-related 2 gene, cause autosomal recessive Charcot-Marie-Tooth type 4B1 (CMT4B1). This disorder is characterized by childhood onset of weakness and sensory loss, severely decreased nerve conduction velocity, demyelination in the nerve with myelin outfoldings, and severe functional impairment of affected patients, mainly resulting from loss of myelinated fibers in the nerve. We recently generated Mtmr2-null(neo) mice, which show a dysmyelinating neuropathy with myelin outfoldings, thus reproducing human CMT4B1. Mtmr2 is detected in both Schwann cells and neurons, in which it interacts with discs large 1/synapse-associated protein 97 and neurofilament light chain, respectively. Here, we specifically ablated Mtmr2 in either Schwann cells or motor neurons. Disruption of Mtmr2 in Schwann cells produced a dysmyelinating phenotype very similar to that of the Mtmr2-null(neo) mouse. Disruption of Mtmr2 in motor neurons does not provoke myelin outfoldings nor axonal defects. We propose that loss of Mtmr2 in Schwann cells, but not in motor neurons, is both sufficient and necessary to cause CMT4B1 neuropathy. Thus, therapeutical approaches might be designed in the future to specifically deliver the Mtmr2 phospholipid phosphatase to Schwann cells in affected nerves.

Analysis of 13 tetrameric short tandem repeat loci in a population of Tuscany (Central Italy) performed by means of an automated infrared sequencer.

Allele frequencies for the 13 STRs of the Combined DNA Index System (CODIS) core were obtained from a sample of 188 unrelated individuals living in the area of Florence, Prato and Pistoia (Tuscany, Central Italy).

Analyzing histopathological features of rare charcot-marie-tooth neuropathies to unravel their pathogenesis.

BACKGROUND: Charcot-Marie-Tooth (CMT) neuropathies are very heterogeneous disorders from both a clinical and genetic point of view. The CMT genes identified so far encode different proteins that are variably involved in regulating Schwann cells and/or axonal functions. However, the function of most of these proteins still remains to be elucidated. OBJECTIVE: To characterize a large cohort of patients with demyelinating, axonal, and intermediate forms of CMT neuropathy. DESIGN: A cohort of 131 unrelated patients were screened for mutations in 12 genes responsible for CMT neuropathies. Demyelinating, axonal, and intermediate forms of CMT neuropathy were initially distinguished as usual on the basis of electrophysiological criteria and clinical evaluation. A sural nerve biopsy was also performed for selected cases. Accordingly, patients underwent first-level analysis of the genes most frequently mutated in each clinical form of CMT neuropathy. RESULTS: Although our cohort had a particularly high percentage of cases of rare axonal and intermediate CMT neuropathies, we found mutations in 40% of patients. Among identified changes, 7 represented new mutations occurring in the MPZ, GJB1, EGR2, MFN2, NEFL, and HSBP1/HSP27 genes. Histopathological analysis performed in selected cases revealed morphological features, which correlated with the molecular diagnosis and provided evidence of the underlying pathogenetic mechanism. CONCLUSION: Clinical and pathological analysis of patients with CMT neuropathies contributes to our understanding of the molecular mechanisms of CMT neuropathies.