Functional disparity between human pawpand plcζ in the generation of Ca²⁺ oscillations for oocyte activation

Mammalian oocyte activation is mediated by cytosolic calcium (Ca²⁺) oscillations initiated upon delivery of a putative 'sperm factor' by the fertilizing sperm. Previous studies suggest the identity of this sperm factor as the testis-specific phospholipase C-zeta (PLCz). Recently, a post-acrosomal sheathWWdomain-binding protein (PAWP) has been proposed as an alternative sperm factor candidate, following a report that human PAWPprotein and cRNA elicited Ca²⁺ oscillations in mouse and human oocytes. Those Ca²⁺ oscillations were inhibited by a PAWP-derived peptide corresponding to a functionalPPGYbinding motif. Herein, using a series of human PAWPexpression constructs,we demonstrate that both human PAWP protein and cRNA are, in our experiments, unable to elicit Ca²⁺ release following microinjection into mouse oocytes. Parallel experiments performed with human PLCz elicited the characteristic Ca²⁺ oscillations present at mammalian fertilization, which produced oocyte activation and embryo development. Furthermore, sperm-induced Ca²⁺ oscillations were not inhibited by the PAWP-derived PPGY peptide following in vitro fertilization or intracytoplasmic sperm injection. Thus, the functional disparity with PLCz leads us to conclude that human PAWP is neither sufficient nor necessary for the Ca²⁺ oscillations that initiate mammalian oocyte activation at fertilization.