Phosphorylation of inositol 1,4,5-triphosphate receptor 1 during in vitro maturation of porcine oocytes

During fertilization in mammalian species, a sperm-induced intracellular Ca2+ signal ([Ca2+](i)) mediates both exit of meiosis and oocyte activation. Recently, we demonstrated in mouse oocytes that the phosphorylation levels of inositol 1,4,5 trisphosphate receptor type1 (IP(3)R1), the channel responsible for Ca2+ release and oscillations during fertilization, changed during maturation and fertilization. Therefore, we examined the expression and phosphorylation of IP(3)R1 during in vitro maturation of pig oocytes. Here, our present study shows that expression of IP(3)R1 protein did not change during maturation, although the phosphorylation status of the receptor, specifically at an MPM-2 epitope, did. We found that while at the beginning of maturation IP(3)R1 lacked MPM-2 immunoreactivity, it became MPM-2 reactive by 24 h and reached maximal reactivity by 36 h. Interestingly, the acquisition of MPM-2 reactivity coincided with the activation of p34(cdc2) kinase and mitogen-activated protein kinase (MAPK), which are involved in meiotic progression. Following completion of maturation, inactivation of MAPK by U0126 did not affect IP(3)R1 phosphorylation, although inactivation of p34(cdc2) kinase by roscovitine dramatically reduced IP(3)R1 phosphorylation. Neither inhibitor affected total expression of IP(3)R1. Altogether, our results show that IP(3)R1 undergoes dynamic phosphorylation during maturation and this might underlie the generation of oscillations at fertilization.