Myostatin Inhibition as a Therapeutic Strategy for Skeletal Muscle Myopathies–Effects on Nuclear Organisation

Abstract

Skeletal myopathies are a broad spectrum of genetic disorders that affect muscle, and are caused by mutations in a variety of genes required for muscle function and cellular homeostasis. Actin is an essential component of the contraction machinery. Mutations in actin‐associated proteins cause nemaline myopathy, which is characterized by the presence of protein aggregates, as well as muscle fiber atrophy, contractile dysfunction and weakness. At present, there are no effective therapies for nemaline myopathy.Myostatin is a secreted factor responsible for negatively regulating muscle growth. Inhibition of myostatin (MI) results in muscle fiber hypertrophy, and has received much interest as a possible therapeutic strategy for a variety of muscle diseases. Previous work has been carried out in two mouse models of nemaline myopathy with mutations in ACTA1 (skeletal muscle actin): an H40Y mutant, in which MI confers only modest therapeutic benefits; and a D286G mutant, in which MI is able to induce marked hypertrophy and functional improvements. We aim to investigate the reasons for this differing response.Muscle fibers are giant cells which require hundreds of internal nuclei to generate sufficient gene products. It has been hypothesized that insufficient myonuclear density is a pathological factor in a variety of muscle diseases, leading to reduced expression of proteins required for e.g. contraction and energy metabolism. We hypothesized that nuclear density might also be a pathological factor in ACTA1 mutants, and that differences in this parameter might explain the variable responses to MI. Thus, using confocal microscopy and single muscle fibers, we assessed nuclear number and organization in 3 dimensions, in mutants with and without MI treatment.We find that nuclei are positioned irregularly in the severe H40Y mutant. MI treatment is able to rescue this defect, and cause insertion of extra nuclei; yet these effects are not accompanied by significant hypertrophy. No underlying nuclear defects were observed in the milder D286G mutant; MI treatment did not alter nuclear arrangement but caused marked hypertrophy. Nuclear organisation defects will also be discussed in light of our findings in patients with ACTA1 mutations. These findings suggest that aberrant nuclear arrangement may be a pathological factor in diseases of muscle contraction. MI treatment may not be an effective treatment when force production and/or pathology is too severe. Unravelling the mechanisms of myostatin action at the subcellular level is of importance for the design, modulation and improvement of therapies.Support or Funding InformationMedical Research Council, UK