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 unfold 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 it 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 PLCf, SrCl2 and thimerosal, provoke simultaneous but opposite changes in [Ca²⁺]ER and cause differential effects on the refilling and overall load of [Ca²⁺]ER. We also found that Ca²⁺ influx is required to refill [Ca²⁺]ER, because the loss of [Ca²⁺]ER was accelerated in medium devoid of Ca²⁺. 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 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 [Ca²⁺]i rises, promotes initiation of oscillations and enhances 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.