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1 July 2005 Fertilization and Inositol 1,4,5-Trisphosphate (IP3)-Induced Calcium Release in Type-1 Inositol 1,4,5-Trisphosphate Receptor Down-Regulated Bovine Eggs
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It is widely believed that stimulation of the phosphoinositide pathway and production of 1,4,5-inositol trisphosphate (IP3) underlies the oscillatory changes in the concentration of intracellular free calcium ions ([Ca2 ]i) seen during mammalian fertilization. IP3 promotes Ca2 release in eggs by binding to its receptor, the type-1 IP3 receptor (IP3R-1, also known as ITPR1), a ligand-gated Ca2 channel located in the membrane of the endoplasmic reticulum, the main Ca2 store of the cell. While IP3R-1 has been shown to mediate all Ca2 release during mouse fertilization, whether or not it plays such an essential role in fertilization-induced Ca2 release in large domestic species such as bovine and porcine is presently not known. Accordingly, we have generated metaphase II bovine eggs with a ∼70%–80% reduction in the number of intact IP3R-1 by inducing receptor down-regulation during oocyte maturation. We did so by injecting the nonhydrolyzable IP3 analogue, adenophostin A. Functional Ca2 release analysis revealed that IP3R-1 is the predominant Ca2 release channel in bovine eggs, requiring as little as 20% of total intact receptor to mount persistent [Ca2 ]i oscillations in response to fertilization, expression of PLCζ (also known as PLCZ1), and adenophostin A. However, lower concentrations of IP3 and near-physiological concentrations of porcine sperm extract were unable to trigger [Ca2 ]i oscillations in this reduced IP3R-1 model. Furthermore, we present evidence that the sensitivity of bovine IP3R-1 is impaired at the first embryonic interphase. Together, these results demonstrate the essential role of IP3R-1-mediated Ca2 release during fertilization in bovine eggs, and identify cell cycle regulatory mechanisms of [Ca2 ]i oscillations at the level of IP3R-1.

Christopher Malcuit, Jason G. Knott, Changli He, Tara Wainwright, Jan B. Parys, James M. Robl, and Rafael A. Fissore "Fertilization and Inositol 1,4,5-Trisphosphate (IP3)-Induced Calcium Release in Type-1 Inositol 1,4,5-Trisphosphate Receptor Down-Regulated Bovine Eggs," Biology of Reproduction 73(1), 2-13, (1 July 2005).
Received: 22 October 2004; Accepted: 1 February 2005; Published: 1 July 2005

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