Editors' ChoiceCalcium signaling

STIM-ulating SR calcium uptake

Sci. Signal.  01 Sep 2015:
Vol. 8, Issue 392, pp. ec247
DOI: 10.1126/scisignal.aad3268

In neonatal cardiac myocytes, depletion of Ca2+ from the sarcoplasmic reticulum (SR) stimulates store-operated calcium entry (SOCE) mediated by the Ca2+-sensing protein STIM1 and the calcium channel ORAI1. SOCE would not be necessary in adult ventricular myocytes because of a large Ca2+influx current mediated by L-type Ca2+ channels. Zhao et al. found an indirect ORAI-independent mechanism by which STIM1 enhanced SR Ca2+ stores in adult rat ventricular myocytes. ORAI1 was undetectable in the adult ventricular myocytes, but STIM1 was present in puncta and linear patterns at the Z-disk, consistent with localization in the junctional SR that is in close proximity to the sarcolemma (plasma membrane). STIM1 had this localization both in control cells and cells exposed to thapsigargin to deplete SR Ca2+ stores, but under no conditions was a current consistent with SOCE detectable. Fluorescence recovery after photobleaching (FRAP) experiments with tagged STIM1 revealed that the diffusion coefficient of STIM1 was slower in adult myocytes than in neonatal myocytes and both were slower than the reported mobility of STIM1 in HeLa or rat basophilic leukemia cells. Native gel electrophoresis indicated that STIM1 in the adult myocytes was present in large complexes, with the smallest one at a molecular weight consistent with STIM1 dimers. Overexpression of STIM1 in adult cardiomycytes increased the number of cells exhibiting propagating Ca2+ waves and resulted in an increase in SR Ca2+ concentration and in the amplitude of intracellular Ca2+ transients with faster decay kinetics, consistent with more rapid uptake into the SR and enhanced activity of the SR Ca2+ pump SERCA. Phospholamban (PLN) is an SR protein that in its dephosphorylated form inhibits SERCA. Nonphosphorylated PLN had a stronger interaction with STIM1 than did phosphorylated PLN, as detected by coimmunoprecipitation and in vitro binding assays. Coimmunoprecipitation assays indicated that STIM1 reduced the interaction between SERCA and PLN, which would result in increased SERCA activity, increased SR Ca2+, and enhanced intracellular Ca2+ transients. Thus, in adult rodent ventricular myocytes, STIM1 sequesters PLN, rather than activates SOCE, to maintain SR Ca2+ stores.

G. Zhao, T. Li, D. X. P. Brochet, P. B. Rosenberg, W. J. Lederer, STIM1 enhances SR Ca2+ content through binding phospholamban in rat ventricular myocytes. Proc. Natl. Acad. Sci. U.S.A. 112, E4792–E4801 (2015). [PubMed]