New Insights into the Structural Roles of Nebulin in Skeletal Muscle

One important feature of muscle structure and function that has remained relatively obscure is the mechanism that regulates thin filament length. Filament length is an important aspect of muscle function as force production is proportional to the amount of overlap between thick and thin filaments. Recent advances, due in part to the generation of nebulin KO models, reveal that nebulin plays an important role in the regulation of thin filament length. Another structural feature of skeletal muscle that is not well understood is the mechanism involved in maintaining the regular lateral alignment of adjacent sarcomeres, that is, myofibrillar connectivity. Recent studies indicate that nebulin is part of a protein complex that mechanically links adjacent myofibrils. Thus, novel structural roles of nebulin in skeletal muscle involve the regulation of thin filament length and maintaining myofibrillar connectivity. When these functions of nebulin are absent, muscle weakness ensues, as is the case in patients with nemaline myopathy with mutations in nebulin. Here we review these new insights in the role of nebulin in skeletal muscle structure

Respiratory muscle function in patients with nemaline myopathy

Publisher Copyright: © 2022 The AuthorsIn this cross-sectional study, we comprehensively assessed respiratory muscle function in various clinical forms of nemaline myopathy (NM) including non-volitional tests for diaphragm function. Forty-two patients with NM were included (10 males (25-74 y/o); 32 females (11-76 y/o)). The NM forms were typical (n=11), mild (n=7), or childhood-onset with slowness of movements (n=24). Forced vital capacity (FVC) and maximal inspiratory pressure were decreased in typical NM in comparison with childhood-onset NM with slowness (32.0 [29.0-58.5] vs 81.0 [75.0-87.0]%, p<0.01, and 35.0 [24.0-55.0] vs 81.0 [65.0-102.5] cmH2O, p<0.01). Eight patients with childhood-onset NM with slowness had respiratory muscle weakness. There was a low correlation between FVC and Motor Function Measure scores (r=0.48, p<0.01). End-inspiratory diaphragm thickness and twitch mouth pressure were decreased in patients requiring home mechanical ventilation compared to non-ventilated patients with normal lung function (1.8 [1.5-2.4] vs 3.1 [2.0-4.6] mm, p=0.049, and -7.9 [-10.9- -4.0] vs -14.9 [-17.3- -12.6], p=0.04). Our results show that respiratory muscle weakness is present in all NM forms, including childhood-onset NM with slowness, and may be present irrespective of the degree of general motor function impairment. These findings highlight the importance for screening of respiratory function in patients with NM to guide respiratory management.Peer reviewe

Nebulin nemaline myopathy recapitulated in a compound heterozygous mouse model with both a missense and a nonsense mutation in Neb

Nemaline myopathy (NM) caused by mutations in the gene encoding nebulin (NEB) accounts for at least 50% of all NM cases worldwide, representing a significant disease burden. Most NEB-NM patients have autosomal recessive disease due to a compound heterozygous genotype. Of the few murine models developed for NEB-NM, most are Neb knockout models rather than harbouring Neb mutations. Additionally, some models have a very severe phenotype that limits their application for evaluating disease progression and potential therapies. No existing murine models possess compound heterozygous Neb mutations that reflect the genotype and resulting phenotype present in most patients. We aimed to develop a murine model that more closely matched the underlying genetics of NEB-NM, which could assist elucidation of the pathogenetic mechanisms underlying the disease. Here, we have characterised a mouse strain with compound heterozygous Neb mutations; one missense (p.Tyr2303His), affecting a conserved actin-binding site and one nonsense mutation (p.Tyr935*), introducing a premature stop codon early in the protein. Our studies reveal that this compound heterozygous model, Neb(Y2303H, Y935X), has striking skeletal muscle pathology including nemaline bodies. In vitro whole muscle and single myofibre physiology studies also demonstrate functional perturbations. However, no reduction in lifespan was noted. Therefore, Neb(Y2303H,Y935X) mice recapitulate human NEB-NM and are a much needed addition to the NEB-NM mouse model collection. The moderate phenotype also makes this an appropriate model for studying NEB-NM pathogenesis, and could potentially be suitable for testing therapeutic applications.Peer reviewe

Nebulin nemaline myopathy recapitulated in a compound heterozygous mouse model with both a missense and a nonsense mutation in Neb

Nemaline myopathy (NM) caused by mutations in the gene encoding nebulin (NEB) accounts for at least 50% of all NM cases worldwide, representing a significant disease burden. Most NEB-NM patients have autosomal recessive disease due to a compound heterozygous genotype. Of the few murine models developed for NEB-NM, most are Neb knockout models rather than harbouring Neb mutations. Additionally, some models have a very severe phenotype that limits their application for evaluating disease progression and potential therapies. No existing murine models possess compound heterozygous Neb mutations that reflect the genotype and resulting phenotype present in most patients. We aimed to develop a murine model that more closely matched the underlying genetics of NEB-NM, which could assist elucidation of the pathogenetic mechanisms underlying the disease. Here, we have characterised a mouse strain with compound heterozygous Neb mutations; one missense (p.Tyr2303His), affecting a conserved actin-binding site and one nonsense mutation (p.Tyr935*), introducing a premature stop codon early in the protein. Our studies reveal that this compound heterozygous model, Neb(Y2303H, Y935X), has striking skeletal muscle pathology including nemaline bodies. In vitro whole muscle and single myofibre physiology studies also demonstrate functional perturbations. However, no reduction in lifespan was noted. Therefore, Neb(Y2303H,Y935X) mice recapitulate human NEB-NM and are a much needed addition to the NEB-NM mouse model collection. The moderate phenotype also makes this an appropriate model for studying NEB-NM pathogenesis, and could potentially be suitable for testing therapeutic applications.Peer reviewe

Leiomodin-3 dysfunction results in thin filament disorganization and nemaline myopathy

Peer reviewe