Maturation-associated increase in IP 3 receptor type 1: role in conferring increased IP 3 sensitivity and Ca 2+ oscillatory behavior in mouse eggs

Abstract

Maturation of mouse oocytes is accompanied by an increase in sensitivity to inositol 1,4,5-trisphosphate (IP 3)-mediated release of intracellular calcium. To test the hypothesis that the maturation-associated 1.5- to 2.0-fold increase in the mass of the type 1 IP 3 receptor (IP 3R-1) confers this increase in IP 3 sensitivity, we employed RNA interference to prevent this change in IP 3R-1 protein level. Microinjection into germinal vesicle (GV)-intact oocytes of dsRNA corresponding to the IP 3R-1 sequence resulted in a >90% reduction in the amount of maternal IP 3R-1 mRNA and prevented the maturation-associated increase in the mass of the IP 3R-1 protein. These injected oocytes matured to metaphase II, and there was no effect on the maturation-associated increases in p34 cdc2/cyclin B kinase and MAP kinase activities or the global pattern of protein synthesis. IP 3-induced cortical granule exocytosis was significantly decreased in these eggs when compared with controls previously injected with enhanced green fluorescent protein (EGFP) dsRNA. Following insemination, the IP 3R-1 dsRNA-injected eggs displayed significantly fewer Ca 2+ transients than controls, and the duration of the first Ca 2+ transient was about half that of controls. These results support the hypothesis that the maturation-associated increase in the mass of IP 3R-1 confers the increase in IP 3-sensitivity that is observed following oocyte maturation and is necessary for the proper Ca 2+ oscillatory pattern following insemination.