In ventricular myocytes, the physiological function of stromal interaction molecule 1 (STIM1), an endo/sarcoplasmic reticulum (ER/SR) Ca(2+) sensor, is unclear with respect to its cellular localization, its Ca(2+)-dependent mobilization, and its action on Ca(2+) signaling. Confocal microscopy was used to measure Ca(2+) signaling and to track the cellular movement of STIM1 with mCherry and immunofluorescence in freshly isolated adult rat ventricular myocytes and those in short-term primary culture. We found that endogenous STIM1 was expressed at low but measureable levels along the Z-disk, in a pattern of puncta and linear segments consistent with the STIM1 localizing to the junctional SR (jSR). Depleting SR Ca(2+) using thapsigargin (2-10 µM) changed neither the STIM1 distribution pattern nor its mobilization rate, evaluated by diffusion coefficient measurements using fluorescence recovery after photobleaching. Two-dimensional blue native polyacrylamide gel electrophoresis and coimmunoprecipitation showed that STIM1 in the heart exists mainly as a large protein complex, possibly a multimer, which is not altered by SR Ca(2+) depletion. Additionally, we found no store-operated Ca(2+) entry in control or STIM1 overexpressing ventricular myocytes. Nevertheless, STIM1 overexpressing cells show increased SR Ca(2+) content and increased SR Ca(2+) leak. These changes in Ca(2+) signaling in the SR appear to be due to STIM1 binding to phospholamban and thereby indirectly activating SERCA2a (Sarco/endoplasmic reticulum Ca(2+) ATPase). We conclude that STIM1 binding to phospholamban contributes to the regulation of SERCA2a activity in the steady state and rate of SR Ca(2+) leak and that these actions are independent of store-operated Ca(2+) entry, a process that is absent in normal heart cells.
Keywords: STIM1; diffusion coefficient; heart; phospholamban; store-operated calcium entry.