Regulation of Inositol 145-Trisphosphate Receptor Function During Mouse Oocyte Maturation

摘要

At the time of fertilization, an increase in the intracellular Ca²⁺ concentration ([Ca²⁺]i) underlies egg activation and initiation of development in all species studied to date. The inositol 1,4,5-trisphosphate receptor (IP3R1), which is mostly located in the endoplasmic reticulum (ER) mediates the majority of this Ca²⁺ release. The sensitivity of IP3R1, i.e. its Ca²⁺ releasing capability, is increased during oocyte maturation so that the optimum [Ca²⁺]i response concurs with fertilization, which in mammals occurs at metaphase of second meiosis. Multiple IP3R1 modifications affect its sensitivity, including phosphorylation, sub-cellular localization and ER Ca²⁺ concentration ([Ca²⁺]ER). Here we evaluated using mouse oocytes how each of these factors affected IP3R1 sensitivity. The capacity for IP3-induced Ca²⁺ release markedly increased at the germinal vesicle breakdown stage, although oocytes only acquire the ability to initiate fertilization-like oscillations at later stages of maturation. The increase in IP3R1 sensitivity was underpinned by an increase in [Ca²⁺]ER and receptor phosphorylation(s) but not by changes in IP3R1 cellular distribution, as inhibition of the former factors reduced Ca²⁺ release, whereas inhibition of the latter had no impact. Therefore, the results suggest that the regulation of [Ca²⁺]ER and IP3R1 phosphorylation during maturation enhance IP3R1 sensitivity rendering oocytes competent to initiate oscillations at the expected time of fertilization. The temporal discrepancy between the initiation of changes in IP3R1 sensitivity and acquisition of mature oscillatory capacity suggest that other mechanisms that regulate Ca²⁺ homeostasis also shape the pattern of oscillations in mammalian eggs.