Omecamtiv mecarbil lowers the contractile deficit in a mouse model of nebulin-based nemaline myopathy.

Johan Lindqvist,Eun-Jeong Lee,Esmat Karimi,Justin Kolb,Henk Granzier,Agustín Guerrero-Hernandez

doi:10.1371/journal.pone.0224467

Abstract

Nemaline myopathy (NEM) is a congenital neuromuscular disorder primarily caused by nebulin gene (NEB) mutations. NEM is characterized by muscle weakness for which currently no treatments exist. In NEM patients a predominance of type I fibers has been found. Thus, therapeutic options targeting type I fibers could be highly beneficial for NEM patients. Because type I muscle fibers express the same myosin isoform as cardiac muscle (Myh7), the effect of omecamtiv mecarbil (OM), a small molecule activator of Myh7, was studied in a nebulin-based NEM mouse model (Neb cKO). Skinned single fibers were activated by exogenous calcium and force was measured at a wide range of calcium concentrations. Maximal specific force of type I fibers was much less in fibers from Neb cKO animals and calcium sensitivity of permeabilized single fibers was reduced (pCa50 6.12 ±0.08 (cKO) vs 6.36 ±0.08 (CON)). OM increased the calcium sensitivity of type I single muscle fibers. The greatest effect occurred in type I fibers from Neb cKO muscle where OM restored the calcium sensitivity to that of the control type I fibers. Forces at submaximal activation levels (pCa 6.0-6.5) were significantly increased in Neb cKO fibers (~50%) but remained below that of control fibers. OM also increased isometric force and power during isotonic shortening of intact whole soleus muscle of Neb cKO mice, with the largest effects at physiological stimulation frequencies. We conclude that OM has the potential to improve the quality of life of NEM patients by increasing the force of type I fibers at submaximal activation levels.

Highlights

Nebulin is a giant filamentous protein, located in the skeletal muscle sarcomere[1] where it winds around the actin filament, from the Z-disk to near the pointed end of the actin filament [2]
We investigated whether Omecamtiv mecarbil (OM) has a beneficial effect on slow skeletal muscle and used a conditional nebulin knockout mouse model (Neb cKO) which expresses low levels of nebulin and has severe weakness in peripheral and respiratory muscles[25], phenocopying Nemaline myopathy (NEM)[9, 16, 25, 26]
We performed studies to establish the OM dosage for our experiments and measured the OM effect in intact Neb cKO soleus muscle activated with 20 Hz stimulation

Summary

Introduction

Nebulin is a giant filamentous protein, located in the skeletal muscle sarcomere[1] where it winds around the actin filament, from the Z-disk to near the pointed end of the actin filament [2]. Nebulin is believed to play an important role in muscle contraction[3,4,5], it maintains myofibrillar alignment[6] and functions as a thin filament stabilizer that regulates thin filament length[7]. Nebulin deficiency due to mutations in the nebulin gene (NEB) is an important cause of Nemaline Myopathy (NEM)[8,9,10,11].

Methods

Results

Conclusion