Loss of calpain 3 dysregulates store-operated calcium entry and its exercise response in mice.

Limb-Girdle Muscular Dystrophy 2A (LGMD2A) is caused by mutations in the gene encoding Calpain 3, a skeletal-muscle specific, Ca-dependent protease. Localization of Calpain 3 within the triad suggests it contributes to Ca homeostasis. Through live-cell Ca measurements, muscle mechanics, immunofluorescence, and electron microscopy (EM) in deficient (C3KO) and wildtype (WT) mice, we determined if loss of Calpain 3 altered Store-Operated Calcium Entry (SOCE) activity. Direct Ca influx measurements revealed loss of elicits elevated resting SOCE and increased resting cytosolic Ca, supported by high incidence of calcium entry units (CEUs) observed by EM. C3KO and WT mice were subjected to a single bout of treadmill running to elicit SOCE. Within 1HR post-treadmill running, C3KO mice exhibited diminished force production in muscles and a greater decay of Ca transients in muscle fibers during repetitive stimulation. Striking evidence for impaired exercise-induced SOCE activation in C3KO mice included poor colocalization of key SOCE proteins, stromal-interacting molecule 1 (STIM1) and ORAI1, combined with disappearance of CEUs in C3KO muscles. These results demonstrate that Calpain 3 is a key regulator of SOCE in skeletal muscle and identify SOCE dysregulation as a contributing factor to LGMD2A pathology.