Functional Characterization of the Intact Diaphragm in a Nebulin-Based Nemaline Myopathy (NM) Model-Effects of the Fast Skeletal Muscle Troponin Activator tirasemtiv.

Eun-Jeong Lee,Henk L Granzier,Fady I Malik,Darren T Hwee,Justin Kolb

doi:10.3390/ijms20205008

Abstract

Respiratory failure due to diaphragm dysfunction is considered a main cause of death in nemaline myopathy (NM) and we studied both isometric force and isotonic shortening of diaphragm muscle in a mouse model of nebulin-based NM (Neb cKO). A large contractile deficit was found in nebulin-deficient intact muscle that is frequency dependent, with the largest deficits at low–intermediate stimulation frequencies (e.g., a deficit of 72% at a stimulation frequency of 20 Hz). The effect of the fast skeletal muscle troponin activator (FSTA) tirasemtiv on force was examined. Tirasemtiv had a negligible effect at maximal stimulation frequencies, but greatly reduced the force deficit of the diaphragm at sub-maximal stimulation levels with an effect that was largest in Neb cKO diaphragm. As a result, the force deficit of Neb cKO diaphragm fell (from 72% to 29% at 20 Hz). Similar effects were found in in vivo experiments on the nerve-stimulated gastrocnemius muscle complex. Load-clamp experiments on diaphragm muscle showed that tirasemtiv increased the shortening velocity, and reduced the deficit in mechanical power by 33%. Thus, tirasemtiv significantly improves muscle function in a mouse model of nebulin-based nemaline myopathy.

Highlights

Nemaline myopathy (NM) causes weakness and poor tone in the muscles of the face, neck, and upper limbs, and often greatly affects the respiratory muscles [1,2]
The specific force of diaphragm muscle strips was lower in nebulin knockout (Neb cKO) mice than in ctrl mice (Figure 1A, Supplementary Table S1)
The Hill-fits to the relative force–frequency relation showed in cKO diaphragm a significantly higher frequency needed to elicit half-maximal force (F50 frequency) 33 vs 38 Hz and a Hill coefficient that was significantly higher than in ctrl, 1.5 vs 2.5 (Table 1) for ctrl and Neb cKO, respectively

Summary

Introduction

Nemaline myopathy (NM) causes weakness and poor tone (hypotonia) in the muscles of the face, neck, and upper limbs, and often greatly affects the respiratory muscles [1,2]. NM is a congenital neuromuscular disorder that is caused by mutations in genes that play critical roles in muscle contraction, in particular genes related to the structure and function of the thin filament [3,4]. We studied how the function of the diaphragm is altered in a mouse model of NM, the recently developed tissue-specific and conditional nebulin KO model, the of nebulin-based NM (Neb cKO) mouse, that phenocopies a severe form of NM [13]

Methods

Results

Discussion

Conclusion