Regulation of endoplasmic reticulum Ca2+ oscillations in mammalian eggs

Changes in the intracellular concentration of free calcium ([Ca2+]i) regulate diverse cellular processes including fertilization. In mammalian eggs, the [Ca2+]i changes induced by the sperm unfolds in a pattern of periodical rises, also known as [Ca2+]i oscillations. The source of Ca2+ during oscillations is the endoplasmic reticulum ([Ca2+]ER), but is presently unknown how [Ca2+]ER is regulated. Here we show using mouse eggs that [Ca2+]i oscillations induced by a variety of agonists, including PLCζ, SrCl2 and thimerosal, provoked simultaneous but opposite changes in [Ca2+]ER and caused differential effects on the refilling and overall load of [Ca2+]ER. We also found that Ca2+ influx is required to refill [Ca2+]ER, as in media devoid of Ca2+, the loss of [Ca2+]ER was accelerated. Pharmacological inactivation of the function of the mitochondria and of the Ca2+-ATPase pumps PMCA and SERCA altered the pattern of oscillations and abruptly reduced [Ca2+]ER, especially after inactivation of mitochondria and SERCA functions. We also examined the expression of SERCA2b protein and found it expressed throughout oocyte maturation and attaining a conspicuous cortical cluster organization in mature eggs. We showed that its overexpression reduces the duration of IP3-induced [Ca2+]i rises, promoted initiation of oscillations and enhanced refilling of [Ca2+]ER. Collectively, our results provide novel insights on the regulation of [Ca2+]ER oscillations, which underlie the unique Ca2+ signalling system that activates the developmental program in mammalian eggs.