Effect of levosimendan on the contractility of muscle fibers from nemaline myopathy patients with mutations in the nebulin gene

Josine M De Winter,Nigel F Clarke,Ger Jm Stienen,Alan H Beggs,Jolanda Van Der Velden,Vasco Sequeira,Barbara Joureau,Henk Granzier,Coen Ac Ottenheijm

doi:10.1186/s13395-015-0037-7

Abstract

BackgroundNemaline myopathy (NM), the most common non-dystrophic congenital myopathy, is characterized by generalized skeletal muscle weakness, often from birth. To date, no therapy exists that enhances the contractile strength of muscles of NM patients. Mutations in NEB, encoding the giant protein nebulin, are the most common cause of NM. The pathophysiology of muscle weakness in NM patients with NEB mutations (NEB-NM) includes a lower calcium-sensitivity of force generation. We propose that the lower calcium-sensitivity of force generation in NEB-NM offers a therapeutic target. Levosimendan is a calcium sensitizer that is approved for use in humans and has been developed to target cardiac muscle fibers. It exerts its effect through binding to slow skeletal/cardiac troponin C. As slow skeletal/cardiac troponin C is also the dominant troponin C isoform in slow-twitch skeletal muscle fibers, we hypothesized that levosimendan improves slow-twitch muscle fiber strength at submaximal levels of activation in patients with NEB-NM.MethodsTo test whether levosimendan affects force production, permeabilized slow-twitch muscle fibers isolated from biopsies of NEB-NM patients and controls were exposed to levosimendan and the force response was measured.ResultsNo effect of levosimendan on muscle fiber force in NEB-NM and control skeletal muscle fibers was found, both at a submaximal calcium level using incremental levosimendan concentrations, and at incremental calcium concentrations in the presence of levosimendan. In contrast, levosimendan did significantly increase the calcium-sensitivity of force in human single cardiomyocytes. Protein analysis confirmed that the slow skeletal/cardiac troponin C isoform was present in the skeletal muscle fibers tested.ConclusionsThese findings indicate that levosimendan does not improve the contractility in human skeletal muscle fibers, and do not provide rationale for using levosimendan as a therapeutic to restore muscle weakness in NEB-NM patients. We stress the importance of searching for compounds that improve the calcium-sensitivity of force generation of slow-twitch muscle fibers. Such compounds provide an appealing approach to restore muscle force in patients with NEB-NM, and also in patients with other neuromuscular disorders.Electronic supplementary materialThe online version of this article (doi:10.1186/s13395-015-0037-7) contains supplementary material, which is available to authorized users.

Highlights

Nemaline myopathy (NM), the most common non-dystrophic congenital myopathy, is characterized by generalized skeletal muscle weakness, often from birth
Recent work revealed that skeletal muscle fibers of NM patients with NEB mutations (NEB-NM) develop muscle weakness due to loss of these functions of nebulin; their myofibers contain thin filaments of shorter length, they show altered actomyosin crossbridge kinetics [22,23,24], and they have a lower calciumsensitivity of force generation [23]
The contractile performance of permeabilized muscle fibers from nemaline myopathy patients with nebulin mutations Muscle contractility experiments The force-generating capacity of permeabilized muscle fiber bundles isolated from nemaline myopathy biopsies with mutations in the nebulin gene (NEB-NM) was lower compared to muscle fibers from healthy controls at both maximal and submaximal calcium levels

Summary

Introduction

Nemaline myopathy (NM), the most common non-dystrophic congenital myopathy, is characterized by generalized skeletal muscle weakness, often from birth. The pathophysiology of muscle weakness in NM patients with NEB mutations (NEB-NM) includes a lower calcium-sensitivity of force generation. Recent work revealed that skeletal muscle fibers of NM patients with NEB mutations (NEB-NM) develop muscle weakness due to loss of these functions of nebulin; their myofibers contain thin filaments of shorter length, they show altered actomyosin crossbridge kinetics [22,23,24], and they have a lower calciumsensitivity of force generation [23]

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