Type 4B1 Research Articles

An animal model for Charcot–Marie–Tooth disease type 4B1

Charcot-Marie-Tooth disease (CMT) comprises a family of clinically and genetically very heterogeneous hereditary peripheral neuropathies and is one of the most common inherited neurological disorders. We have generated a mouse model for CMT type 4B1 using embryonic stem cell technology. To this end, we introduced a stop codon into the Mtmr2 locus within exon 9, at the position encoding amino acid 276 of the MTMR2 protein (E276X). Concomitantly, we have deleted the chromosomal region immediately downstream of the stop codon up to within exon 13. The resulting allele closely mimics the mutation found in a Saudi Arabian CMT4B1 patient. Animals homozygous for the mutation showed various degrees of complex myelin infoldings and outfoldings exclusively in peripheral nerves, in agreement with CMT4B1 genetics and pathology. Mainly, paranodal regions of the myelin sheath were affected, with a high degree of quantitative and qualitative variability between individuals. This pathology was progressive with age, and axonal damage was occasionally observed. Distal nerve regions were more affected than proximal parts, in line with the distribution in CMT. However, we found no significant electrophysiological changes, even in aged (16-month-old) mice, suggesting that myelin infoldings and outfoldings per se are not invariably associated with detectable electrophysiological abnormalities. Our animal model provides a basis for future detailed molecular and cellular studies on the underlying disease mechanisms in CMT4B1. Such an analysis will reveal how the disease develops, in particular, the enigmatic myelin infoldings and outfoldings as well as axonal damage, and provide mechanistic insights that may aid in the development of potential therapeutic approaches.

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.

Disruption of Mtmr2 produces CMT4B1-like neuropathy with myelin outfolding and impaired spermatogenesis

Mutations in MTMR2, the myotubularin-related 2 gene, cause autosomal recessive Charcot-Marie-Tooth (CMT) type 4B1, a demyelinating neuropathy with myelin outfolding and azoospermia. MTMR2 encodes a ubiquitously expressed phosphatase whose preferred substrate is phosphatidylinositol (3,5)-biphosphate, a regulator of membrane homeostasis and vesicle transport. We generated Mtmr2-null mice, which develop progressive neuropathy characterized by myelin outfolding and recurrent loops, predominantly at paranodal myelin, and depletion of spermatids and spermatocytes from the seminiferous epithelium, which leads to azoospermia. Disruption of Mtmr2 in Schwann cells reproduces the myelin abnormalities. We also identified a novel physical interaction in Schwann cells, between Mtmr2 and discs large 1 (Dlg1)/synapse-associated protein 97, a scaffolding molecule that is enriched at the node/paranode region. Dlg1 homologues have been located in several types of cellular junctions and play roles in cell polarity and membrane addition. We propose that Schwann cell–autonomous loss of Mtmr2–Dlg1 interaction dysregulates membrane homeostasis in the paranodal region, thereby producing outfolding and recurrent loops of myelin.

Membrane association of myotubularin-related protein 2 is mediated by a pleckstrin homology-GRAM domain and a coiled-coil dimerization module.

Mutations in the myotubularin (MTM)-related protein 2 (MTMR2) gene are responsible for the severe autosomal recessive neuropathy Charcot-Marie-Tooth disease type 4B1. MTMR2 belongs to the MTM family of dual-specific phosphatases that use phosphatidylinositol (PI) 3,5-bisphosphate [PI(3,5)P2] and PI 3-phosphate [PI(3)P] as their substrate. Because these substrates are localized in the membrane bilayer, membrane targeting of Mtmr2 is an important regulatory mechanism. In hypoosmotically stressed COS cells with increased levels of PI(3,5)P2, Mtmr2 is bound to the membrane of vacuoles formed under these conditions. Using several mutant forms of Mtmr2, we identified two domains that are necessary for membrane association: (i) A pleckstrin homology-GRAM domain; and (ii) a coiled-coil module. Protein-lipid overlay assays show that the pleckstrin homology-GRAM domain binds to PI(3,5)P2 and PI(5)P, a substrate and a product of the Mtmr2 enzyme, respectively. We also demonstrate that Mtmr2 forms a dimer and that the C-terminal coiled-coil is responsible for homodimerization, in addition to membrane association. Our data indicate that phosphoinositide-protein interactions, as well as protein-protein interactions, are necessary for the correct regulation of MTMR2.

Myotubularin-related 2 protein phosphatase and neurofilament light chain protein, both mutated in CMT neuropathies, interact in peripheral nerve.

Charcot-Marie-Tooth disease type 4B1, CMT4B1, is a severe, autosomal-recessive, demyelinating peripheral neuropathy, due to mutations in the Myotubularin-related 2 gene, MTMR2. MTMR2 is widely expressed and encodes a phosphatase whose substrates include phosphoinositides. However, this does not explain how MTMR2 mutants specifically produce demyelination in the peripheral nerve. Therefore, we analysed the cellular and subcellular distribution of Mtmr2 in nerve. Mtmr2 was detected in all cytoplasmic compartments of myelin-forming Schwann cells, as well as in the cytoplasm of non-myelin-forming Schwann cells and both sensory and motorneurons. In contrast, Mtmr2 was detected in the nucleus of Schwann cells and motorneurons, but not in the nucleus of sensory neurons. As Mtmr2 is diffusely present also within the nerve, a specific function could derive instead from nerve-specific interacting proteins. Therefore, we performed two yeast two-hybrid screenings, using either fetal brain or peripheral nerve cDNA libraries. The neurofilament light chain protein, NF-L, was identified repeatedly in both screenings, and found to interact with MTMR2 in both Schwann cells and neurons. Interestingly, NF-L, encoding NF-L, is mutated in CMT2E. These data may provide a basis for the nerve-specific pathogenesis of CMT4B1, and further support for the notion that hereditary demyelinating and axonal neuropathies may represent different clinical manifestations of a common pathological mechanism.

Loss of Phosphatase Activity in Myotubularin‐related Protein 2 is Associated with Charcot‐marie‐tooth Disease Type 4B1

Mutations in the gene encoding myotubularin‐related protein 2 (MTMR2) are responsible for autosomal recessive Charcot‐Marie‐Tooth disease type 4B1 (CMT4B1), a severe hereditary motor and sensory neuropathy characterized by focally folded myelin sheaths and demyelination. MTMR2 belongs to the myotubularin family, which is characterized by the presence of a phosphatase domain. Myotubularin (MTM), the archetype member of this family, is mutated in X‐linked myotubular myopathy. Although MTMR2 and MTM are closely related, they are likely to have different functions. Recent studies revealed that MTM dephosphorylates specifically phosphatidylinositol 3‐phosphate. Here we analyze the biochemical properties of the mouse MTMR2 protein, which shares 97% amino acid identity with human MTMR2. We show that phosphatidylinositol‐3‐phosphate is also a substrate for MTMR2, but, unlike myotubularin, MTMR2 dephosphorylates phosphatidylinositol 3,5‐bisphosphate with high efficiency and peak activity at neutral pH. We demonstrate that the known disease‐associated MTMR2 mutations lead to dramatically reduced phosphatase activity, suggesting that the MTMR2 phosphatase activity is crucial for the proper function of peripheral nerves in CMT4B1. Expression analysis of MTMR2 suggests particularly high levels in neurons. Thus, the demyelinating neuropathy CMT4B1 might be triggered by the malfunction of neural membrane recycling, membrane trafficking, and/or endocytic or exocytotic processes, combined with altered axon‐Schwann cell interactions. Furthermore, the different biochemical properties of MTM and MTMR2 offer a potential explanation for the different human diseases caused by mutations in their respective genes.

Mutations in MTMR13, a New Pseudophosphatase Homologue of MTMR2 and Sbf1, in Two Families with an Autosomal Recessive Demyelinating Form of Charcot-Marie-Tooth Disease Associated with Early-Onset Glaucoma

Charcot-Marie-Tooth disease (CMT) with autosomal recessive (AR) inheritance is a heterogeneous group of inherited motor and sensory neuropathies. In some families from Japan and Brazil, a demyelinating CMT, mainly characterized by the presence of myelin outfoldings on nerve biopsies, cosegregated as an autosomal recessive trait with early-onset glaucoma. We identified two such large consanguineous families from Tunisia and Morocco with ages at onset ranging from 2 to 15 years. We mapped this syndrome to chromosome 11p15, in a 4.6-cM region overlapping the locus for an isolated demyelinating ARCMT (CMT4B2). In these two families, we identified two different nonsense mutations in the myotubularin-related 13 gene, MTMR13. The MTMR protein family includes proteins with a phosphoinositide phosphatase activity, as well as proteins in which key catalytic residues are missing and that are thus called "pseudophosphatases." MTM1, the first identified member of this family, and MTMR2 are responsible for X-linked myotubular myopathy and Charcot-Marie-Tooth disease type 4B1, an isolated peripheral neuropathy with myelin outfoldings, respectively. Both encode active phosphatases. It is striking to note that mutations in MTMR13 also cause peripheral neuropathy with myelin outfoldings, although it belongs to a pseudophosphatase subgroup, since its closest homologue is MTMR5/Sbf1. This is the first human disease caused by mutation in a pseudophosphatase, emphasizing the important function of these putatively inactive enzymes. MTMR13 may be important for the development of both the peripheral nerves and the trabeculum meshwork, which permits the outflow of the aqueous humor. Both of these tissues have the same embryonic origin.

Regulation of myotubularin-related (MTMR)2 phosphatidylinositol phosphatase by MTMR5, a catalytically inactive phosphatase

The myotubularin (MTM) family constitutes one of the most highly conserved protein-tyrosine phosphatase subfamilies in eukaryotes. MTM1, the archetypal member of this family, is mutated in X-linked myotubular myopathy, whereas mutations in the MTM-related (MTMR)2 gene cause the type 4B1 Charcot-Marie-Tooth disease, a severe hereditary motor and sensory neuropathy. In this study, we identified a protein that specifically interacts with MTMR2 but not MTM1. The interacting protein was shown by mass spectrometry to be MTMR5, a catalytically inactive member of the MTM family. We also demonstrate that MTMR2 interacts with MTMR5 via its coiled-coil domain and that mutations in the coiled-coil domain of either MTMR2 or MTMR5 abrogate this interaction. Through this interaction, MTMR5 increases the enzymatic activity of MTMR2 and dictates its subcellular localization. This article demonstrates an active MTM member being regulated by an inactive family member.

Loss of phosphatase activity in myotubularin-related protein 2 is associated with Charcot-Marie-Tooth disease type 4B1.

Mutations in the gene encoding myotubularin-related protein 2 (MTMR2) are responsible for autosomal recessive Charcot-Marie-Tooth disease type 4B1 (CMT4B1), a severe hereditary motor and sensory neuropathy characterized by focally folded myelin sheaths and demyelination. MTMR2 belongs to the myotubularin family, which is characterized by the presence of a phosphatase domain. Myotubularin (MTM), the archetype member of this family, is mutated in X-linked myotubular myopathy. Although MTMR2 and MTM are closely related, they are likely to have different functions. Recent studies revealed that MTM dephosphorylates specifically phosphatidylinositol 3-phosphate. Here we analyze the biochemical properties of the mouse Mtmr2 protein, which shares 97% amino acid identity with human MTMR2. We show that phosphatidylinositol-3-phosphate is also a substrate for Mtmr2, but, unlike myotubularin, Mtmr2 dephosphorylates phosphatidylinositol 3,5-bisphosphate with high efficiency and peak activity at neutral pH. We demonstrate that the known disease-associated MTMR2 mutations lead to dramatically reduced phosphatase activity, suggesting that the MTMR2 phosphatase activity is crucial for the proper function of peripheral nerves in CMT4B1. Expression analysis of Mtmr2 suggests particularly high levels in neurons. Thus, the demyelinating neuropathy CMT4B1 might be triggered by the malfunction of neural membrane recycling, membrane trafficking, and/or endocytic or exocytotic processes, combined with altered axon-Schwann cell interactions. Furthermore, the different biochemical properties of MTM and MTMR2 offer a potential explanation for the different human diseases caused by mutations in their respective genes.