When a sperm joins with an egg, development is begun by a signal encoded in oscillations in the concentration of intracellular free calcium ([Ca2+]i) in the cytoplasm of the egg. Precisely how the interaction triggers these oscillations has been a matter of debate, but soluble extracts of sperm cells can reproduce the effect. The Ca2+ is released from intracellular stores in response to increased concentrations of inositol 1,4,5-trisphosphate (IP3), suggesting that a phosphatidylinositol-specific phospholipase C (PLC), which generates IP3 from phosphoinositides, could be the culprit. But known PLCs can't substitute for sperm in egg activation.
Saunders et al. now report that mouse sperm produce an isoform of PLC, PLCξ. Eggs microinjected with complementary RNA encoding PLCξ showed oscillations in [Ca2+]i with similar temporal characteristics to those in normally fertilized eggs. Furthermore, the amount of PLCξ from just one sperm cell (about 50 fg) mimicked activation by a sperm, and depletion of PLCξ from sperm extracts by immunoprecipitation removed egg-activating activity. Thus, PLCξ from the sperm cytoplasm appears to be the key molecular determinant of egg activation. These findings may be important in future treatments of male infertility or in the activation of eggs for the production of stem cells. — LBR
Development129, 3533 (2002).