Activation of oocytes by latrunculin A.

Actin depolymerization by latrunculin A (LAT-A) in mature starfish oocytes induces a massive calcium mobilization that results in the discharge of the cortical granules and in the elevation of the fertilization envelope. The Ca2+ liberation starts as a circumscribed subplasma membrane hotspot, which is followed by a flash of Ca2+ increase restricted to the cortical layer. Ca2+ propagates rapidly from these peripheral regions to the center of the oocyte, initiating calcium oscillations. Blockade of the inositol 1,4,5-trisphosphate receptors with heparin does not affect the liberation of Ca2+ at the initial hotspot or the cortical flash, but abolishes the centripetal spreading of the wave and the Ca2+ oscillations. In Ca2+-free medium, LAT-A also initiates Ca2+ release at a discrete cortical point, but then propagates throughout the cell without first forming the uniform cortical flash. The latter is thus linked to the influx of external Ca2+, somehow promoted by the depolymerization of cortical (microvillar) actin. The Ca2+ response to spermatozoa (i.e., peripheral hotspot, cortical flash, globalization of the signal) closely mimics that promoted by LAT-A. Thus, the initial cortical release of Ca2+ promoted by the sperm may be due to the depolymerization of actin.