Pre-assembled Ca(2+) entry units and constitutively active Ca(2+) entry in skeletal muscle of calsequestrin-1 knockout mice

Store-operated Ca(2+) entry (SOCE) is a ubiquitous Ca(2+) influx mechanism triggered by depletion of Ca(2+) stores from the endoplasmic/sarcoplasmic reticulum (ER/SR). We recently reported that acute exercise in WT mice drives the formation of Ca(2+) entry units (CEUs), intracellular junctions that contain STIM1 and Orai1, the two key proteins mediating SOCE. The presence of CEUs correlates with increased constitutive- and store-operated Ca(2+) entry, as well as sustained Ca(2+) release and force generation during repetitive stimulation. Skeletal muscle from mice lacking calsequestrin-1 (CASQ1-null), the primary Ca(2+)-binding protein in the lumen of SR terminal cisternae, exhibits significantly reduced total Ca(2+) store content and marked SR Ca(2+) depletion during high-frequency stimulation. Here, we report that CEUs are constitutively assembled in extensor digitorum longus (EDL) and flexor digitorum brevis (FDB) muscles of sedentary CASQ1-null mice. The higher density of CEUs in EDL (39.6 ± 2.1/100 µm(2) versus 2.0 ± 0.3/100 µm(2)) and FDB (16.7 ± 1.0/100 µm(2) versus 2.7 ± 0.5/100 µm(2)) muscles of CASQ1-null compared with WT mice correlated with enhanced constitutive- and store-operated Ca(2+) entry and increased expression of STIM1, Orai1, and SERCA. The higher ability to recover Ca(2+) ions via SOCE in CASQ1-null muscle served to promote enhanced maintenance of peak Ca(2+) transient amplitude, increased dependence of luminal SR Ca(2+) replenishment on BTP-2-sensitive SOCE, and increased maintenance of contractile force during repetitive, high-frequency stimulation. Together, these data suggest that muscles from CASQ1-null mice compensate for the lack of CASQ1 and reduction in total releasable SR Ca(2+) content by assembling CEUs to promote constitutive and store-operated Ca(2+) entry.