Ca2+ influx and the store-operated Ca2+ entry pathway undergo regulation during mouse oocyte maturation

In preparation for fertilization, mammalian oocytes undergo optimization of the mechanisms that regulate calcium homeostasis. Among these changes is the increase in the content of the Ca2+ stores ([Ca2+](ER)), a process that requires Ca2+ influx. Nevertheless, the mechanism(s) that mediates this influx remains obscure, although is known that [Ca2+](ER) can regulate Ca2+ influx via store-operated Ca2+ entry (SOCE). We find that during maturation, as [Ca2+](ER) increases, Ca2+ influx decreases. We demonstrate that mouse oocytes/eggs express the two molecular components of SOCE-stromal interaction molecule 1 (Stim1) and Orai1- and expression of human (h) Stim1 increases Ca2+ influx in a manner that recapitulates endogenous SOCE. We observe that the cellular distribution of hStim1 and hOrai1 during maturation undergoes sweeping changes that curtail their colocalization during the later stages of maturation. Coexpression of hStim1 and hOrai1 enhances influx throughout maturation but increases basal Ca2+ levels only in GV oocytes. Further, expression of a constitutive active form of hStim1 plus Orai1, which increases basal Ca2+ throughout maturation, disturbs resumption of meiosis. Taken together, our results demonstrate that Ca2+ influx and SOCE are regulated during maturation and that alteration of Ca2+ homeostasis undermines maturation in mouse oocytes.