Regulation of endoplasmic reticulum Ca2+ oscillations in mammalian eggs

Abstract

Changes in the intracellular concentration of free calcium ([Ca]i) regulate diverse cellular processes including fertilization. In mammalian eggs, the [Ca]i changes induced by the sperm unfold in a pattern of periodical rises, also known as [Ca ]i oscillations. The source of Ca during oscillations is the endoplasmic reticulum ([Ca]ER), but it is presently unknown how [Ca ]ER is regulated. Here, we show using mouse eggs that [Ca]i oscillations induced by a variety of agonists, including PLCf, SrCl2 and thimerosal, provoke simultaneous but opposite changes in [Ca]ER and cause differential effects on the refilling and overall load of [Ca ]ER. We also found that Ca influx is required to refill [Ca]ER, because the loss of [Ca ]ER was accelerated in medium devoid of Ca . Pharmacological inactivation of the function of the mitochondria and of the Ca-ATPase pumps PMCA and SERCA altered the pattern of oscillations and abruptly reduced [Ca]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]i rises, promotes initiation of oscillations and enhances refilling of [Ca]ER. Collectively, our results provide novel insights on the regulation of [Ca ]ER oscillations, which underlie the unique Ca-signalling system that activates the developmental program in mammalian eggs.