Duchenne muscular dystrophy and malignant hyperthermia: A genetic study of the ryanodine receptor in 47 patients

Signaling through the dystrophin glycoprotein complex affects the stress-dependent transcriptome in Drosophila

Deficiencies in the human dystrophin glycoprotein complex (DGC), which links the extracellular matrix with the intracellular cytoskeleton, cause muscular dystrophies, a group of incurable disorders associated with heterogeneous muscle, brain and eye abnormalities. Stresses such as nutrient deprivation and aging cause muscle wasting, which can be exacerbated by reduced levels of the DGC in membranes, the integrity of which is vital for muscle health and function. Moreover, the DGC operates in multiple signaling pathways, demonstrating an important function in gene expression regulation. To advance disease diagnostics and treatment strategies, we strive to understand the genetic pathways that are perturbed by DGC mutations. Here, we utilized a Drosophila model to investigate the transcriptomic changes in mutants of four DGC components under temperature and metabolic stress. We identified DGC-dependent genes, stress-dependent genes and genes dependent on the DGC for a proper stress response, confirming a novel function of the DGC in stress-response signaling. This perspective yields new insights into the etiology of muscular dystrophy symptoms, possible treatment directions and a better understanding of DGC signaling and regulation under normal and stress conditions.

Leaky ryanodine receptors delay the activation of store overload-induced Ca2+ release, a mechanism underlying malignant hyperthermia-like events in dystrophic muscle

The mouse model of Duchenne muscular dystrophy, the mdx mouse, displays changes in Ca(2+)homeostasis that may lead to the pathology of the muscle. Here we examine the activation of store overload-induced Ca(2+)release (SOICR) in mdx muscle. The activation of SOICR is associated with the depolymerization of the sarcoplasmic reticulum (SR) Ca(2+)buffer calsequestrin and the reduction of SR Ca(2+)buffering power (BSR). The role of SOICR in healthy and dystrophic muscle is unclear. Using skinned fibers we show that lowering the Mg(2+)concentration can activate discrete Ca(2+)release events that did not necessarily lead to activation of SOICR. However, SOICR waves could propagate into these fiber segments. The average delay to activation of SOICR in mdx fibers was longer than in wild-type (WT) fibers. In the lowered Ca(2+)-buffered environment following large SOICR events, brief waves in mdx fibers displayed a low amplitude and propagation rate, in contrast to WT fibers that showed a range of amplitudes correlated with wave propagation rate. The distinct properties of SOICR in mdx fibers were consistent with a ryanodine receptor (RyR) that was leakier to Ca(2+)than in WT. The consequence of delayed SOICR and leaky RyRs is prolonged high BSRand a reduction in free Ca(2+)concentration inside the SR as total SR calcium drops. We present a hypothesis that SOICR activation is required in healthy muscle and that this mechanism works suboptimally in mdx fibers to fail to limit the activation of store-operated Ca(2+)entry.