Regulation of inositol 1,4,5‐trisphosphate receptor function during mouse oocyte maturation

Abstract

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.

Down-regulation of store-operated Ca2+ entry during mammalian meiosis is required for the egg-to-embryo transition

“…Orai1 distribution was described as patchy membrane localized in MII eggs. We do not understand the reasons for these discrepancies, however the unusual distribution of STIM1 described by Gomez-Fernandez et al is quite distinct from the spatial distribution of the ER viewed with traditional ER markers such as the lipophilic dicarbocyanine dye DiI (FitzHarris et al, 2003;Mehlmann et al, 1995), or after direct staining for the IP 3 receptor, a resident ER protein (Wakai et al, 2012). These studies show a diffuse ER similar to what we observe in STIM1 expressing cells (Fig.…”

“…3). Furthermore, in pig oocytes STIM1 exhibits a more diffuse ER-like distribution and clustering following store depletion (Wakai et al, 2012). This brings into question the physiological significance of the unusual distribution of STIM1 observed by Gomez-Fernandez et al (Gómez-Fernández et al, 2009).…”

Protein phospholipase C Zeta1 expression in patients with failed ICSI but with normal sperm parameters

“…Samples were counterstained with 5 μg/ml Hoechst 33342 (Sigma-Aldrich) and mounted using mounting media (Vector Laboratories, Burlingame, CA). Preparation of mouse eggs MII eggs were obtained from superovulated 6-to 10-week-old B6D2F1 (C57BL/6 J× DBA/2 J) female mice [20]. Eggs were collected in a HEPES-buffered Tyrode's lactate solution (TL-HEPES) supplemented with 5 % heat-treated FCS (GIBCO; Invitrogen, Carlsbad, CA).…”

Regulation of endoplasmic reticulum Ca2+ oscillations in mammalian eggs

“…In fact, in-vitrofertilized GV oocytes show fewer [Ca 2+ ] i oscillations and each [Ca 2+ ] i rise exhibits lesser duration and amplitude than those observed in fertilized MII oocytes (henceforth referred to as eggs) (Jones et al, 1995b;Mehlmann et al, 1996). Given that several of the parameters of Ca 2+ homeostasis progressively change during maturation, including the steady increase of [Ca 2+ ] ER (Jones et al, 1995b;Mehlmann and Kline, 1994;Wakai et al, 2012), it is possible that the molecules responsible for these adjustments in Ca 2+ homeostasis, which are mostly unknown in mammalian oocytes, experience dynamic modifications such that some of the mechanisms active at the GV stage might not be so at the MII stage and vice versa.…”