Effect of M-phase kinase phosphorylations on type 1 inositol 1,4,5-trisphosphate receptor-mediated Ca2+ responses in mouse eggs
Zhang, Nan Yoon, Sook Young Parys, Jan Fissore, Rafael A. # ×
Cell Calcium vol:58 issue:5 pages:476-488
The type 1 inositol 1,4,5-trisphosphate 1 receptor (IP3R1) mediates increases in the intracellular concentration of Ca2+ ([Ca2+]i) during fertilization in mammalian eggs. The activity of IP3R1 is enhanced during oocyte maturation, and phosphorylations by M-phase kinases are thought to positively regulate the activity of IP3R1. Accordingly, we and others have found that IP3R1 is phosphorylated at S421, T799 (by Cdk1) and at S436 (by ERK). Nevertheless, the effects of these phosphorylations on the function of the receptor and their impact on [Ca2+]i oscillations in eggs have not been clearly examined. To address this, we expressed in mouse oocytes an IP3R1 variant with the three indicated phosphorylation sites replaced by acidic residues, IIIE-IP3R1, such that it would act like a constitutively phosphorylated IP3R1, and examined [Ca2+]i parameters in response to stimuli. We found that overexpression of wild type (wt-IP3R1) or IIIE-IP3R1 in oocytes containing endogenous receptors caused dominant negative-like effects on Ca2+ release and oscillations. Therefore, we first selectively removed the endogenous IP3R1, and subsequently expressed the exogenous receptors. We found that in response to injection of PLCzeta cRNA, eggs without endogenous IP3R1 failed to mount persistent Ca2+ oscillations, although expression of wt-IP3R1 restored their [Ca2+]i oscillatory activity. We also observed that the Ca2+ oscillatory ability and the sensitivity to IP3 in eggs expressing IIIE-IP3R1 were greater than in those expressing wt-IP3R1. Lastly, we found that exogenous IP3R1s are resistant to downregulation and support longer oscillations and of higher amplitude. Altogether, our results show that phosphorylations by Cdk1 and MAPK enhance the activity of IP3R1, which is consistent with its maximal activity observed at the time of fertilization and the role of Ca2+ release in egg activation.