STIM1 Enhances SR Ca2+ Refilling through Activating SERCA2a in Rat Ventricular Myocytes

In cardiac ventricular myocytes, the function of “stromal interaction molecule 1” (STIM1), an ER/SR Ca2+ sensor, is still a mystery regarding its localization, mobilization, and Ca2+ signaling regulation. Here, adult rat ventricular myocytes freshly isolated or in primary culture were examined. Endogenous STIM1 was found by immunofluorescence and shown to be distributed mainly within the Z-disk. Using mcherry tagged expression of STIM1, we found that re-distribution of STIM1 does not occur after SR Ca2+ depletion (using thapsigargin, 2 μM) nor does SR Ca2+ depletion affect STIM1 movement within the SR when evaluated with fluorescence recovery after photobleaching (FRAP). Consistent with this result, native protein electrophoresis showed that STIM1 exists mainly as an oligomer, which is not altered upon SR Ca2+ depletion. Additionally, no store-operated Ca2+ entry (SOCE) or Ca2+ release-activated Ca2+ current (ICRAC) was observed in control or STIM1 overexpressing ventricular myocytes. The overexpression of STIM1 did, however, have dramatic consequences in ventricular myocytes: The SR Ca2+ content did increase as did SR Ca2+ leak. Parallel investigations indicated that STIM1 physically binds to phospholamban (PLN), suggesting that overexpressed STIM1 may activate SERCA2a by regulating PLN and this may underlie the increase in SR Ca2+ content. The molecular signaling and regulatory pathways that may be involved in these pathways are discussed.