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Loss of myotubularin function results in T-tubule disorganization in zebrafish and human myotubular myopathy.

Dowling JJ, Vreede AP, Low SE, Gibbs EM, Kuwada JY, Bonnemann CG, Feldman EL - PLoS Genet. (2009)

Bottom Line: Zebrafish with reduced levels of myotubularin have significantly impaired motor function and obvious histopathologic changes in their muscle.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.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.

View Article: PubMed Central - PubMed

Affiliation: Department of Pediatrics, University of Michigan Medical Center, Ann Arbor, Michigan, USA. jamedowl@umich.edu

ABSTRACT
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.

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Related in: MedlinePlus

Excitation-Contraction Coupling abnormalities in myotubularin morphants.Excitation-contraction (E–C) coupling. Control morphant myofibers could respond to 15 ms depolarizing current injections to 0 mV from 0 to 30 Hz (average max frequency = 27.0+/−0.9 Hz, N = 5). In contrast, myotubularin morphant muscle progressively failed to contract to stimuli above 10 Hz (average maximum frequency = 11.5+/−1.5 Hz, N = 5, p<0.0001).
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pgen-1000372-g010: Excitation-Contraction Coupling abnormalities in myotubularin morphants.Excitation-contraction (E–C) coupling. Control morphant myofibers could respond to 15 ms depolarizing current injections to 0 mV from 0 to 30 Hz (average max frequency = 27.0+/−0.9 Hz, N = 5). In contrast, myotubularin morphant muscle progressively failed to contract to stimuli above 10 Hz (average maximum frequency = 11.5+/−1.5 Hz, N = 5, p<0.0001).

Mentions: We then proceeded to study excitation-contraction coupling (Figure 10). This was accomplished by measuring the ability of myofibers to contract when exposed to depolarizing stimuli of progressively higher frequencies [27]. Employing this technique, we found that control myofibers consistently contracted at all stimuli up to 30 Hz, with the average maximal frequency equaling 27.0 Hz. Conversely, myofibers from myotubularin morphants exhibited increasing abnormalities above 10 Hz, with no myofibers able to contract to stimuli at 25 Hz and the average maximal frequency equaling only 11.5 Hz. This result is consistent with a defect in excitation-contraction coupling, and provides functional evidence to support the morphologic abnormalities observed in the T-tubules.


Loss of myotubularin function results in T-tubule disorganization in zebrafish and human myotubular myopathy.

Dowling JJ, Vreede AP, Low SE, Gibbs EM, Kuwada JY, Bonnemann CG, Feldman EL - PLoS Genet. (2009)

Excitation-Contraction Coupling abnormalities in myotubularin morphants.Excitation-contraction (E–C) coupling. Control morphant myofibers could respond to 15 ms depolarizing current injections to 0 mV from 0 to 30 Hz (average max frequency = 27.0+/−0.9 Hz, N = 5). In contrast, myotubularin morphant muscle progressively failed to contract to stimuli above 10 Hz (average maximum frequency = 11.5+/−1.5 Hz, N = 5, p<0.0001).
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2631153&req=5

pgen-1000372-g010: Excitation-Contraction Coupling abnormalities in myotubularin morphants.Excitation-contraction (E–C) coupling. Control morphant myofibers could respond to 15 ms depolarizing current injections to 0 mV from 0 to 30 Hz (average max frequency = 27.0+/−0.9 Hz, N = 5). In contrast, myotubularin morphant muscle progressively failed to contract to stimuli above 10 Hz (average maximum frequency = 11.5+/−1.5 Hz, N = 5, p<0.0001).
Mentions: We then proceeded to study excitation-contraction coupling (Figure 10). This was accomplished by measuring the ability of myofibers to contract when exposed to depolarizing stimuli of progressively higher frequencies [27]. Employing this technique, we found that control myofibers consistently contracted at all stimuli up to 30 Hz, with the average maximal frequency equaling 27.0 Hz. Conversely, myofibers from myotubularin morphants exhibited increasing abnormalities above 10 Hz, with no myofibers able to contract to stimuli at 25 Hz and the average maximal frequency equaling only 11.5 Hz. This result is consistent with a defect in excitation-contraction coupling, and provides functional evidence to support the morphologic abnormalities observed in the T-tubules.

Bottom Line: Zebrafish with reduced levels of myotubularin have significantly impaired motor function and obvious histopathologic changes in their muscle.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.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.

View Article: PubMed Central - PubMed

Affiliation: Department of Pediatrics, University of Michigan Medical Center, Ann Arbor, Michigan, USA. jamedowl@umich.edu

ABSTRACT
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.

Show MeSH
Related in: MedlinePlus