STIM1 is required for Ca2+ signaling during mammalian fertilization

During fertilization in mammals, a series of oscillations in the oocyte's intracellular free Ca2+ concentration is responsible for oocyte activation and stimulation of embryonic development. The oscillations are associated with influx of Ca2+ across the plasma membrane that is probably triggered by the depletion of the intracellular stores, a mechanism known as store-operated Ca2+ entry. Recently, STIM1 has been identified in oocytes as a key component of the machinery that generates the Ca2+ influx after store depletion. In this study, the involvement of STIM1 in the sperm-induced Ca2+ oscillations and its significance in supporting subsequent embryo development were investigated. Downregulation of STIM1 levels in pig oocytes by siRNA completely inhibited the repetitive Ca2+ signal triggered by the fertilizing sperm. In addition, a significantly lower percentage of oocytes cleaved or formed blastocysts when STIM1 was downregulated prior to fertilization compared to the control groups. Restoring STIM1 levels after fertilization in such oocytes by means of mRNA injection could not rescue embryonic development that in most cases was arrested at the 2-cell stage. On the other hand, STIM1 overexpression prior to fertilization did not alter the pattern of sperm-induced Ca2+ oscillations and development of these fertilized oocytes up to the blastocyst stage was also similar to that registered in the control group. Finally, downregulation of STIM1 had no effect on oocyte activation when activation was stimulated artificially by inducing a single large elevation in the oocyte's intracellular free Ca2+ concentration. These findings suggest that STIM1 is essential for normal fertilization as it is involved in the maintenance of the long-lasting repetitive Ca2+ signal.

► The signaling protein STIM1 is essential for proper mammalian fertilization.
► Sperm-induced Ca2+ transients in oocytes are sustained through the action of STIM1.
► Oocytes with downregulated STIM1 levels cannot display the repetitive Ca2+ signal.
► STIM1 downregulation before fertilization also leads to impaired embryo development.