Regulation of endoplasmic reticulum Ca2+ oscillations in mammalian eggs

Changes in the intracellular concentration of free calcium (Ca(2+)i) regulate diverse cellular processes including fertilization. In mammalian eggs, the Ca(2+)i changes induced by the sperm unfold in a pattern of periodical rises, also known as Ca(2+)i oscillations. The source of Ca(2+) during oscillations is the endoplasmic reticulum (Ca(2+)ER), but it is presently unknown how Ca(2+)ER is regulated. Here, we show using mouse eggs that Ca(2+)i oscillations induced by a variety of agonists, including PLCζ, SrCl2 and thimerosal, provoke simultaneous but opposite changes in Ca(2+)ER and cause differential effects on the refilling and overall load of Ca(2+)ER. We also found that Ca(2+) influx is required to refill Ca(2+)ER, because the loss of Ca(2+)ER was accelerated in medium devoid of Ca(2+). Pharmacological inactivation of the function of the mitochondria and of the Ca(2+)-ATPase pumps PMCA and SERCA altered the pattern of oscillations and abruptly reduced Ca(2+)ER, especially after inactivation of mitochondria and SERCA functions. We also examined the expression of SERCA2b protein and found that it was expressed throughout oocyte maturation and attained a conspicuous cortical cluster organization in mature eggs. We show that its overexpression reduces the duration of inositol-1,4,5-trisphosphate-induced Ca(2+)i rises, promotes initiation of oscillations and enhances refilling of Ca(2+)ER. Collectively, our results provide novel insights on the regulation of Ca(2+)ER oscillations, which underlie the unique Ca(2+)-signalling system that activates the developmental program in mammalian eggs.