Mice lacking calsequestrin-1 have reduced levels of releasable Ca 2+ in the sarcoplasmic reticulum of their skeletal muscles. Michelucci et al. reveal that this is compensated by constitutive assembly of STIM1 and Orai1 into Ca 2+ entry units, promoting both constitutive and store-operated Ca 2+ entry.
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.