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Myotubularin, a protein tyrosine phosphatase mutated in myotubular myopathy, dephosphorylates the lipid second messenger, phosphatidylinositol 3-phosphate.

ABSTRACT: The lipid second messenger phosphatidylinositol 3-phosphate [PI(3)P] plays a crucial role in intracellular membrane trafficking. We report here that myotubularin, a protein tyrosine phosphatase required for muscle cell differentiation, is a potent PI(3)P phosphatase. Recombinant human myotubularin specifically dephosphorylates PI(3)P in vitro. Overexpression of a catalytically inactive substrate-trapping myotubularin mutant (C375S) in human 293 cells increases PI(3)P levels relative to that of cells overexpressing the wild-type enzyme, demonstrating that PI(3)P is a substrate for myotubularin in vivo. In addition, a Saccharomyces cerevisiae strain in which the myotubularin-like gene (YJR110w) is disrupted also exhibits increased PI(3)P levels. Both the recombinant yeast enzyme and a human myotubularin-related protein (KIAA0371) are able to dephosphorylate PI(3)P in vitro, suggesting that this activity is intrinsic to all myotubularin family members. Mutations in the MTM1 gene that cause human myotubular myopathy dramatically reduce the ability of the phosphatase to dephosphorylate PI(3)P. Our findings provide evidence that myotubularin exerts its effects during myogenesis by regulating cellular levels of the inositol lipid PI(3)P.

SUBMITTER: Taylor GS

PROVIDER: S-EPMC16795 | biostudies-literature | 2000 Aug

REPOSITORIES: biostudies-literature

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Myotubularin, a protein tyrosine phosphatase mutated in myotubular myopathy, dephosphorylates the lipid second messenger, phosphatidylinositol 3-phosphate.

Taylor G S GS Maehama T T Dixon J E JE

Proceedings of the National Academy of Sciences of the United States of America 20000801 16

The lipid second messenger phosphatidylinositol 3-phosphate [PI(3)P] plays a crucial role in intracellular membrane trafficking. We report here that myotubularin, a protein tyrosine phosphatase required for muscle cell differentiation, is a potent PI(3)P phosphatase. Recombinant human myotubularin specifically dephosphorylates PI(3)P in vitro. Overexpression of a catalytically inactive substrate-trapping myotubularin mutant (C375S) in human 293 cells increases PI(3)P levels relative to that of c ...[more]

PMID: 10900271

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Project description:Myotubularin is a lipid phosphatase implicated in endosomal trafficking in vitro, but with an unknown function in vivo. Mutations in myotubularin cause myotubular myopathy, a devastating congenital myopathy with unclear pathogenesis and no current therapies. Myotubular myopathy was the first described of a growing list of conditions caused by mutations in proteins implicated in membrane trafficking. To advance the understanding of myotubularin function and disease pathogenesis, we have created a zebrafish model of myotubular myopathy using morpholino antisense technology. Zebrafish with reduced levels of myotubularin have significantly impaired motor function and obvious histopathologic changes in their muscle. These changes include abnormally shaped and positioned nuclei and myofiber hypotrophy. These findings are consistent with those observed in the human disease. We demonstrate for the first time that myotubularin functions to regulate PI3P levels in a vertebrate in vivo, and that homologous myotubularin-related proteins can functionally compensate for the loss of myotubularin. Finally, we identify abnormalities in the tubulo-reticular network in muscle from myotubularin zebrafish morphants and correlate these changes with abnormalities in T-tubule organization in biopsies from patients with myotubular myopathy. In all, we have generated a new model of myotubular myopathy and employed this model to uncover a novel function for myotubularin and a new pathomechanism for the human disease that may explain the weakness associated with the condition (defective excitation-contraction coupling). In addition, our findings of tubuloreticular abnormalities and defective excitation-contraction coupling mechanistically link myotubular myopathy with several other inherited muscle diseases, most notably those due to ryanodine receptor mutations. Based on our findings, we speculate that congenital myopathies, usually considered entities with similar clinical features but very disparate pathomechanisms, may at their root be disorders of calcium homeostasis.

Project description:Centronuclear myopathy (CNM) is an inherited neuromuscular disorder characterised by clinical features of a congenital myopathy and centrally placed nuclei on muscle biopsy.The incidence of X-linked myotubular myopathy is estimated at 2/100000 male births but epidemiological data for other forms are not currently available.The clinical picture is highly variable. The X-linked form usually gives rise to a severe phenotype in males presenting at birth with marked weakness and hypotonia, external ophthalmoplegia and respiratory failure. Signs of antenatal onset comprise reduced foetal movements, polyhydramnios and thinning of the ribs on chest radiographs; birth asphyxia may be the present. Affected infants are often macrosomic, with length above the 90th centile and large head circumference. Testes are frequently undescended. Both autosomal-recessive (AR) and autosomal-dominant (AD) forms differ from the X-linked form regarding age at onset, severity, clinical characteristics and prognosis. In general, AD forms have a later onset and milder course than the X-linked form, and the AR form is intermediate in both respects.Mutations in the myotubularin (MTM1) gene on chromosome Xq28 have been identified in the majority of patients with the X-linked recessive form, whilst AD and AR forms have been associated with mutations in the dynamin 2 (DNM2) gene on chromosome 19p13.2 and the amphiphysin 2 (BIN1) gene on chromosome 2q14, respectively. Single cases with features of CNM have been associated with mutations in the skeletal muscle ryanodine receptor (RYR1) and the hJUMPY (MTMR14) genes.Diagnosis is based on typical histopathological findings on muscle biopsy in combination with suggestive clinical features; muscle magnetic resonance imaging may complement clinical assessment and inform genetic testing in cases with equivocal features. Genetic counselling should be offered to all patients and families in whom a diagnosis of CNM has been made.The main differential diagnoses include congenital myotonic dystrophy and other conditions with severe neonatal hypotonia.Management of CNM is mainly supportive, based on a multidisciplinary approach. Whereas the X-linked form due to MTM1 mutations is often fatal in infancy, dominant forms due to DNM2 mutations and some cases of the recessive BIN1-related form appear to be associated with an overall more favourable prognosis.

Project description:X-linked myotubular myopathy (XLMTM) is a congenital disorder caused by mutations of the myotubularin gene, MTM1. Myotubularin belongs to a large family of conserved lipid phosphatases that include both catalytically active and inactive myotubularin-related proteins (i.e., "MTMRs"). Biochemically, catalytically inactive MTMRs have been shown to form heteroligomers with active members within the myotubularin family through protein-protein interactions. However, the pathophysiological significance of catalytically inactive MTMRs remains unknown in muscle. By in vitro as well as in vivo studies, we have identified that catalytically inactive myotubularin-related protein 12 (MTMR12) binds to myotubularin in skeletal muscle. Knockdown of the mtmr12 gene in zebrafish resulted in skeletal muscle defects and impaired motor function. Analysis of mtmr12 morphant fish showed pathological changes with central nucleation, disorganized Triads, myofiber hypotrophy and whorled membrane structures similar to those seen in X-linked myotubular myopathy. Biochemical studies showed that deficiency of MTMR12 results in reduced levels of myotubularin protein in zebrafish and mammalian C2C12 cells. Loss of myotubularin also resulted in reduction of MTMR12 protein in C2C12 cells, mice and humans. Moreover, XLMTM mutations within the myotubularin interaction domain disrupted binding to MTMR12 in cell culture. Analysis of human XLMTM patient myotubes showed that mutations that disrupt the interaction between myotubularin and MTMR12 proteins result in reduction of both myotubularin and MTMR12. These studies strongly support the concept that interactions between myotubularin and MTMR12 are required for the stability of their functional protein complex in normal skeletal muscles. This work highlights an important physiological function of catalytically inactive phosphatases in the pathophysiology of myotubular myopathy and suggests a novel therapeutic approach through identification of drugs that could stabilize the myotubularin-MTMR12 complex and hence ameliorate this disorder.

Dataset Information

Myotubularin, a protein tyrosine phosphatase mutated in myotubular myopathy, dephosphorylates the lipid second messenger, phosphatidylinositol 3-phosphate.

Publications

Myotubularin, a protein tyrosine phosphatase mutated in myotubular myopathy, dephosphorylates the lipid second messenger, phosphatidylinositol 3-phosphate.

Similar Datasets

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