Phospholipase C-zeta has the ability of Ca²⁺ oscillation induction and nuclear translocation in mouse eggs

Abstract

Ca²⁺ oscillations due to IP₃ receptor-mediated repetitive Ca²⁺ release occur at fertilization of mammalian eggs and trigger egg activation. They are caused by a sperm factor driven into the egg upon sperm-egg fusion. It has recently shown that a novel sperm-specific phospholipase C isoform, PLCζ, generates Ca²⁺ oscillations when expressed in mouse eggs, probably via continuously produced IP₃. We injected RNA encoding PLCζ fused with a fluorescent protein Venus into mouse eggs to monitor expressed PLCζ by fluorescence. Ca²⁺ oscillations similar to those at fertilization appeared at 40-50 min after RNA injection when expressed PLCζ reached 10-40x10⁻¹⁵ g in the egg, comparable to estimated content of PLCζ in a single mouse sperm. PLCζ-Venus increased up to 3 h and attained a steady level at 4-5 h. Interestingly, PLCζ-Venus translocated into the pronucleus that was formed at 5-6 h, and continuously increased there. A splicing variant of PLCζ that lacks three of four EF hand domains was much less effective in induction of Ca²⁺ oscillations and little accumulated in the pronucleus, indicating a critical role of those domains. The ability of nuclear translocation qualifies PLCζ for a strong candidate of the Ca²⁺ oscillation-inducing sperm factor which is known to be concentrated to the pronucleus after fertilization and egg activation. [Jpn J Physiol 54 Suppl:S223 (2004)]