Ca(2+) homeostasis and regulation of ER Ca(2+) in mammalian oocytes/eggs

Abstract

The activation of the developmental program in mammalian eggs relies on the initiation at the time of fertilization of repeated rises in the intracellular concentration of free calcium ([Ca(2+)](i)), also known as [Ca(2+)](i) oscillations. The ability to mount the full complement of oscillations is only achieved at the end of oocyte maturation, at the metaphase stage of meiosis II (MII). Over the last decades research has focused on addressing the mechanisms by which the sperm initiates the oscillations and identification of the channels that mediate intracellular Ca(2+) release. This review will describe the up-to-date knowledge of other aspects of Ca(2+) homeostasis in mouse oocytes, such as the mechanisms that transport Ca(2+) out of the cytosol into the endoplasmic reticulum (ER), the Ca(2+) store of the oocyte/egg, into other organelles and also those that extrude Ca(2+). Evidence pointing to channels in the plasma membrane that mediate Ca(2+) entry from the extracellular milieu, which is required for the persistence of the oscillations, is also discussed, along with the modifications that these mechanisms undergo during maturation. Lastly, we highlight areas where additional research is needed to obtain a better understating of the molecules and mechanisms that regulate Ca(2+) homeostasis in this unique Ca(2+) signaling system.

Fig. 1

[Ca²⁺]_ER and [Ca²⁺]_i undergo simultaneous but opposite changes in concentration during oscillations in mouse MII eggs. Ca²⁺ responses were induced by injection of 0.05 μg/μl mouse PLCζ cRNA into MII eggs. In vitro transcribed D1ER RNA was injected into eggs 5 hr before the initiation of [Ca²⁺]_i measurements. The emission ratio of D1ER (YFP/CFP) was used to estimate relative changes in [Ca²⁺]_ER (right axis, blue trace). [Ca²⁺]_i (left axis, red trace) was recorded using Rhod-2. Rhod-2 is generally used to measure mitochondrial Ca²⁺, although given that it fails to target into mitochondria in mouse eggs, it is possible to use it as a reported of [Ca²⁺]_i (Dumollard, R. et al.).

Fig. 2

[Ca²⁺]_ER content increases during mouse oocyte maturation. [Ca²⁺]_ER was estimated from the [Ca²⁺]_i responses induced by the addition of 2 μM ionomycin under Ca²⁺ free conditions. The mean fluorescent peak (Fura-2) was compared at 0 (red), 4 (blue), 8 (green) and 12 (black) hr of in vitro maturation, which corresponded with GV, GVBD, MI and MII stages of meiotic progression (n=5).