Guanine Nucleotides in the Meiotic Maturation of Starfish Oocytes: Regulation of the Actin Cytoskeleton and of Ca2+ Signaling

Abstract

BACKGROUND: Starfish oocytes are arrested at the first prophase of meiosis until they are stimulated by 1-methyladenine (1-MA). The two most immediate responses to the maturation-inducing hormone are the quick release of intracellular Ca(2+) and the accelerated changes of the actin cytoskeleton in the cortex. Compared with the later events of oocyte maturation such as germinal vesicle breakdown, the molecular mechanisms underlying the early events involving Ca(2+) signaling and actin changes are poorly understood. Herein, we have studied the roles of G-proteins in the early stage of meiotic maturation. METHODOLOGY/PRINCIPAL FINDINGS: By microinjecting starfish oocytes with nonhydrolyzable nucleotides that stabilize either active (GTPgammaS) or inactive (GDPbetaS) forms of G-proteins, we have demonstrated that: i) GTPgammaS induces Ca(2+) release that mimics the effect of 1-MA; ii) GDPbetaS completely blocks 1-MA-induced Ca(2+); iii) GDPbetaS has little effect on the amplitude of the Ca(2+) peak, but significantly expedites the initial Ca(2+) waves induced by InsP(3) photoactivation, iv) GDPbetaS induces unexpectedly striking modification of the cortical actin networks, suggesting a link between the cytoskeletal change and the modulation of the Ca(2+) release kinetics; v) alteration of cortical actin networks with jasplakinolide, GDPbetaS, or actinase E, all led to significant changes of 1-MA-induced Ca(2+) signaling. CONCLUSIONS/SIGNIFICANCE: Taken together, these results indicate that G-proteins are implicated in the early events of meiotic maturation and support our previous proposal that the dynamic change of the actin cytoskeleton may play a regulatory role in modulating intracellular Ca(2+) release.