Abnormal actin binding of aberrant β-tropomyosins is a molecular cause of muscle weakness in TPM2-related nemaline and cap myopathy

Abstract

NM (nemaline myopathy) is a rare genetic muscle disorder defined on the basis of muscle weakness and the presence of structural abnormalities in the muscle fibres, i.e. nemaline bodies. The related disorder cap myopathy is defined by cap-like structures located peripherally in the muscle fibres. Both disorders may be caused by mutations in the TPM2 gene encoding β-Tm (tropomyosin). Tm controls muscle contraction by inhibiting actin-myosin interaction in a calcium-sensitive manner. In the present study, we have investigated the pathogenetic mechanisms underlying five disease-causing mutations in Tm. We show that four of the mutations cause changes in affinity for actin, which may cause muscle weakness in these patients, whereas two show defective Ca2+ activation of contractility. We have also mapped the amino acids altered by the mutation to regions important for actin binding and note that two of the mutations cause altered protein conformation, which could account for impaired actin affinity.

Aberrant movement of β-tropomyosin associated with congenital myopathy causes defective response of myosin heads and actin during the ATPase cycle

“…Thus, thin filaments reconstituted with R91G and E139del mutant TM species show an increased Ca 2+ sensitivity and a slightly higher maximum sliding speed in an in vitro motility assay [37,79]. Conversely, the E41K mutation is associated with a hypocontractile phenotype that is manifested both clinically (hypotonia and muscle weakness in patients [34]) and biochemically (a reduced Ca 2+ sensitivity and cross-bridge cycling rate in vitro [35]). The phenotype associated with each TPM2 mutation is likely related, at least in part, to the specific effects of the mutation on protein function but our understanding of these relationships is far from complete [26,25].…”

“…Thus, the substitution of the Arg91 with Gly may cause a disruption of the salt bridge through amino acid charge reversal induced by the mutation (the positively charged residue is substituted by the uncharged one). The E139del and E41K mutations are in the ''f'' position, and may affect the binding of actin and other thin filament proteins [34]. These substitutions are likely to cause changes in the TM conformation, resulting in a deformation of the TM molecule, which can be a structural basis for alteration in the position and flexibility of TM strands.…”

“…Some tropomyosin mutations produced a higher Ca 2+ sensitivity and a slightly higher maximum sliding speed in motility assay to give a hypercontractile phenotype [36]. The examples of these mutations are R91G and E139del that have been found to cause arthrogryposis [37] and cap myopathy [34,38].…”

“…The majority of these mutations demonstrated a hypocontractile phenotype showing itself in lower Ca 2+ sensitivity, lower sliding speeds in motility assay and reduced cross-bridge cycling rate. The examples of these mutations are E41K and E117K linked to nemaline myopathy or cap disease [34,35]. Some tropomyosin mutations produced a higher Ca 2+ sensitivity and a slightly higher maximum sliding speed in motility assay to give a hypercontractile phenotype [36].…”

Investigating the effects of tropomyosin mutations on its flexibility and interactions with filamentous actin using molecular dynamics simulation

“…Extensive structure-function analyses have been conducted for Tpm mutations that are linked to hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy (DCM) (Marston 2011), and skeletal muscle myopathy (Marston et al 2013;Marttila et al 2012;Yuen et al 2015). As revealed by electron microscopy (EM) studies of Tpmdecorated F-actin, Tpm adopts three average azimuthal positions (i.e.…”