Gamete Biology

Fertilization and Inositol 1,4,5-Trisphosphate (IP₃)-Induced Calcium Release in Type-1 Inositol 1,4,5-Trisphosphate Receptor Down-Regulated Bovine Eggs

Abstract
It is widely thought 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 the 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 the bovine and porcine is presently not known. Accordingly, we have generated metaphase II (MII) bovine eggs with an ~70-80% reduction in the numbers of intact IP₃R-1 by inducing receptor down regulation during oocyte maturation by injecting the non-hydrolyzable IP₃ analog, 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 PLC (also known as PLCZ1), and adenophostin A. However, lower concentrations of IP₃) and near-physiological concentrations of porcine sperm extracts 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.

Key words: Gamete Biology • Calcium • Fertilization • In vitro fertilization

This article has been cited by other articles:

				S.-Y. Yoon and R. A Fissore Release of phospholipase C {zeta}and [Ca2+]i oscillation-inducing activity during mammalian fertilization Reproduction, November 1, 2007; 134(5): 695 - 704. [Abstract] [Full Text] [PDF]