Mechanisms of weakness in Mdx muscle following in vivo eccentric contractions

Eccentric contraction-induced strength loss in dystrophin-deficient muscle: Preparations, protocols, and mechanisms

The absence of dystrophin hypersensitizes skeletal muscle of lower and higher vertebrates to eccentric contraction (ECC)-induced strength loss. Loss of strength can be accompanied by transient and reversible alterations to sarcolemmal excitability and disruption, triad dysfunction, and aberrations in calcium kinetics and reactive oxygen species production. The degree of ECC-induced strength loss, however, appears dependent on several extrinsic and intrinsic factors such as vertebrate model, skeletal muscle preparation (in vivo, in situ, or ex vivo), skeletal muscle hierarchy (single fiber versus whole muscle and permeabilized versus intact), strength production, fiber branching, age, and genetic background, among others. Consistent findings across research groups show that dystrophin-deficient fast(er)-twitch muscle is hypersensitive to ECCs relative to wildtype muscle, but because preparations are highly variable and sensitivity to ECCs are used repeatedly to determine efficacy of many preclinical treatments, it is critical to evaluate the impact of skeletal muscle preparations on sensitivity to ECC-induced strength loss in dystrophin-deficient skeletal muscle. Here, we review and discuss variations in skeletal muscle preparations to evaluate the factors responsible for variations and discrepancies between research groups. We further highlight that dystrophin-deficiency, or loss of the dystrophin-glycoprotein complex in skeletal muscle, is not a prerequisite for accelerated strength loss-induced by ECCs.

Proteomic profiling of impaired excitation-contraction coupling and abnormal calcium handling in muscular dystrophy

The X-linked inherited neuromuscular disorder Duchenne muscular dystrophy is characterised by primary abnormalities in the membrane cytoskeletal component dystrophin. The almost complete absence of the Dp427-M isoform of dystrophin in skeletal muscles renders contractile fibres more susceptible to progressive degeneration and a leaky sarcolemma membrane. This in turn results in abnormal calcium homeostasis, enhanced proteolysis and impaired excitation-contraction coupling. Biochemical and mass spectrometry-based proteomic studies of both patient biopsy specimens and genetic animal models of dystrophinopathy have demonstrated significant changes in the concentration and/or physiological function of essential calcium-regulatory proteins in dystrophin-lacking voluntary muscles. Abnormalities include dystrophinopathy-associated changes in voltage sensing receptors, calcium release channels, calcium pumps and calcium binding proteins. This review article provides an overview of the importance of the sarcolemmal dystrophin-glycoprotein complex and the wider dystrophin complexome in skeletal muscle and its linkage to depolarisation-induced calcium-release mechanisms and the excitation-contraction-relaxation cycle. Besides chronic inflammation, fat substitution and reactive myofibrosis, a major pathobiochemical hallmark of X-linked muscular dystrophy is represented by the chronic influx of calcium ions through the damaged plasmalemma in conjunction with abnormal intracellular calcium fluxes and buffering. Impaired calcium handling proteins should therefore be included in an improved biomarker signature of Duchenne muscular dystrophy.