Ca2+ oscillation-inducing phospholipase C zeta expressed in mouse eggs is accumulated to the pronucleus during egg activation

Sperm-specific phospholipase C zeta (PLCzeta) is known to induce intracellular Ca2+ oscillations and egg, activation when expressed in mouse eggs by injection of RNA encoding PLCzeta. We investigated the expression level and spatial distribution of PLCzeta in the egg in real time and in relation to the initiation and termination of Ca2+ oscillations by monitoring fluorescence of a yellow fluorescent protein 'Venus' fused with PLCzeta. Ca2+ oscillations similar to those at fertilization were induced at 40-50 min after RNA injection, when expressed PLCzeta reached 10-40 x 10(-15) g in the egg. PLCzeta-Venus increased up to 3 h and attained a steady level at 4-5 h. Interestingly, PLCzeta-Venus is accumulated to the pronucleus (PN) formed at 5-6 h and continuously increased there. Ca2+ oscillations stopped in most eggs before initiation of the accumulation. A variant of PLCzeta that lacks three 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 the accumulation to the pronucleus qualifies PLCzeta for a strong candidate of the Ca2+ oscillation-inducing sperm factor, which is introduced into the ooplasm upon sperm-egg fusion and concentrated to the pronucleus after inducing egg activation. (C) 2004 Elsevier Inc. All rights reserved.