Genetic evidence in the mouse solidifies the calcium hypothesis of myofiber death in muscular dystrophy

Abstract

Muscular dystrophy (MD) refers to a clinically and genetically heterogeneous group of degenerative muscle disorders characterized by progressive muscle wasting and often premature death. Although the primary defect underlying most forms of MD typically results from a loss of sarcolemmal integrity, the secondary molecular mechanisms leading to muscle degeneration and myofiber necrosis is debated. One hypothesis suggests that elevated or dysregulated cytosolic calcium is the common transducing event, resulting in myofiber necrosis in MD. Previous measurements of resting calcium levels in myofibers from dystrophic animal models or humans produced equivocal results. However, recent studies in genetically altered mouse models have largely solidified the calcium hypothesis of MD, such that models with artificially elevated calcium in skeletal muscle manifest fulminant dystrophic-like disease, whereas models with enhanced calcium clearance or inhibited calcium influx are resistant to myofiber death and MD. Here, we will review the field and the recent cadre of data from genetically altered mouse models, which we propose have collectively mostly proven the hypothesis that calcium is the primary effector of myofiber necrosis in MD. This new consensus on calcium should guide future selection of drugs to be evaluated in clinical trials as well as gene therapy-based approaches.

Figure 1

Schematic of the calcium handling proteins and downstream calcium-regulated effectors that are involved in calcium dysregulation in MD, leading to myofiber necrosis. Elevations in resting calcium has been associated with increased store-operated calcium entry (SOCE), increased stretch-activated calcium entry, increased calcium leak, and increased receptor-operated calcium entry (ROCE), attributed to the activity of transient receptor potential canonical (TRPC) and vanilloid (TRPV) family members, as well as by Stim and Orai family member proteins that can directly generate a store-operated calcium entry event. The L-type calcium channel might also be responsible for some content of pathologic calcium influx, as well as leak from the RyR1 in dystrophic skeletal muscle. In addition to elevations in calcium, sodium is increased in the cytosol of dystrophic myofibers owing to increased activity of TRPC channels, sodium channels (Na_v), or possibly in conjunction with less effective sodium extrusion by the sodium–potassium ATPase (NKA) pump. Elevated intracellular sodium can secondarily increase resting calcium levels by causing reverse-mode calcium influx through the sodium–calcium exchanger (NCX) as well as by altering NHE1 activity. Sarcoplasmic reticulum (SR) calcium reuptake is also reduced in MD with decreased function of the SERCA pump. Finally, pathologic calcium may also arise owing to increased IP₃R activity. In response to this pathologic profile of elevated intracellular calcium, the mitochondria (mito) can swell and rupture owing to MPTP activation, and intracellular proteins can be degraded by the calpains (CAPN)

Figure 2

Schematic of the pharmacologic agents that have been or could be used to address a profile of elevated calcium in dystrophic muscle. Drugs previously tested in dystrophic mouse models are shown in blue text, whereas those that are more experimental are shown in red text