Egg activation in physiologically polyspermic newt eggs: involvement of IP₃ receptor, PLCγ, and microtubules in calcium wave induction.

Abstract

The egg of the polyspermic newt is activated by Ca(2+) waves induced by several sperm at fertilization. A major component of the sperm factor for egg activation is the sperm-specific citrate synthase (CS), which is introduced into the egg cytoplasm after sperm-egg fusion. We tried to clarify the mechanism for sperm-specific CS to induce [Ca(2+)]i increase in egg cytoplasm. The injection of the sperm factor into the unfertilized egg induces a [Ca(2+)]i increase that propagates over the whole egg surface as a Ca(2+) wave. The propagation of the Ca(2+) wave is inhibited by depolymerization of microtubules in the egg cytoplasm. The sperm-specific CS is highly phosphorylated and binds the component containing microtubules and the IP3 receptor. The sperm CS localized in the midpiece region was dispersed in the egg cytoplasm, but most of the CS accumulates at the sperm entry site and is distributed in association with the microtubules around the midpiece region and the nucleus. Phospholipase C (PLC) γ in egg cytoplasm also accumulates around the midpiece region in association with the sperm CS. Thus, CS at the initiation site of the Ca(2+) wave forms a complex of microtubules and endoplasmic reticulum (ER) with the IP3 receptor, in addition to PLCγ, indicating close involvement of those complexes in Ca(2+) releases from the ER by the sperm factor.