The biphasic increase of PIP2 in the fertilized eggs of starfish: new roles in actin polymerization and Ca2+ signaling

Background

Fertilization of echinoderm eggs is accompanied by dynamic changes of the actin cytoskeleton and by a drastic increase of cytosolic Ca²⁺. Since the plasma membrane-enriched phospholipid phosphatidylinositol 4,5-bisphosphate (PIP2) serves as the precursor of inositol 1,4,5 trisphosphate (InsP₃) and also regulates actin-binding proteins, PIP2 might be involved in these two processes.

Methodology/Principal Findings

In this report, we have studied the roles of PIP2 at fertilization of starfish eggs by using fluorescently tagged pleckstrin homology (PH) domain of PLC-δ1, which has specific binding affinity to PIP2, in combination with Ca²⁺ and F-actin imaging techniques and transmission electron microscopy. During fertilization, PIP2 increased at the plasma membrane in two phases rather than continually decreasing. The first increase was quickly followed by a decrease about 40 seconds after sperm-egg contact. However, these changes took place only after the Ca²⁺ wave had already initiated and propagated. The fertilized eggs then displayed a prolonged increase of PIP2 that was accompanied by the appearance of numerous spikes in the perivitelline space during the elevation of the fertilization envelope (FE). These spikes, protruding from the plasma membrane, were filled with microfilaments. Sequestration of PIP2 by RFP-PH at higher doses resulted in changes of subplasmalemmal actin networks which significantly delayed the intracellular Ca²⁺ signaling, impaired elevation of FE, and increased occurrences of polyspermic fertilization.

Conclusions/Significance

Our results suggest that PIP2 plays comprehensive roles in shaping Ca²⁺ waves and guiding structural and functional changes required for successful fertilization. We propose that the PIP2 increase and the subsequent formation of actin spikes not only provide the mechanical supports for the elevating FE, but also accommodate increased membrane surfaces during cortical granule exocytosis.