Upregulation of store-operated Ca2+ entry in dystrophic mdx mouse muscle

Abstract

Store-operated Ca2+ entry (SOCE) is an important mechanism in virtually all cells. In adult skeletal muscle, this mechanism is highly specialized for the rapid delivery of Ca2+ from the transverse tubule into the junctional cleft during periods of depleting Ca2+ release. In dystrophic muscle fibers, SOCE may be a source of Ca2+ overload, leading to cell necrosis. However, this possibility is yet to be examined in an adult fiber during Ca2+ release. To examine this, Ca2+ in the tubular system and cytoplasm were simultaneously imaged during direct release of Ca2+ from sarcoplasmic reticulum (SR) in skeletal muscle fibers from healthy (wild-type, WT) and dystrophic mdx mouse. The mdx fibers were found to have normal activation and deactivation properties of SOCE. However, a depression of the cytoplasmic Ca2+ transient in mdx compared with WT fibers was observed, as was a shift in the SOCE activation and deactivation thresholds to higher SR Ca2+ concentrations ([Ca2+]SR). The shift in SOCE activation and deactivation thresholds was accompanied by an approximately threefold increase in STIM1 and Orai1 proteins in dystrophic muscle. While the mdx fibers can introduce more Ca2+ into the fiber for an equivalent depletion of [Ca2+]SR via SOCE, it remains unclear whether this is deleterious.