Protein phosphorylation and oocyte maturation. I. Induction of starfish oocyte maturation by intracellular microinjection of a phosphatase inhibitor, alpha-naphthylphosphate.

Oocyte maturation (meiosis re-initiation) in starfish is induced by the natural hormone 1-methyladenine (1-MeAde). Following hormonal stimulation of the oocyte, an intracellular Maturation Promoting Factor (MPF) appears in the cytoplasm which triggers nuclear envelope breakdown and maturation divisions. alpha-Naphthylphosphate (alpha-NP), a widely used phosphatase inhibitor/substrate, was found to induce oocyte maturation when microinjected intracellularly (50% maturation of 3.5 mM; 100% above 6mM, final intracellular concentration) into oocytes of Marthasterias and Asterias but not of Astropecten. As 1-MeAde, alpha-NP triggers a complete maturation, i.e. germinal vesicle breakdown, extrusion of the two polar bodies and formation of the female pronucleus. The kinetics of alpha-NP-induced maturation (35-45 min) is, however, longer than the kinetics of 1-MeAde-induced maturation (18-20 min). The addition of alpha-NP externally to oocytes does not trigger maturation. Among several reported phosphatase inhibitors, including two natural protein phosphatase inhibitors and several products structurally related to alpha-NP, only alpha-NP was found capable of inducing maturation when microinjection into oocytes. alpha-NP triggers the appearance of MPF activity in the cytoplasm of oocytes into which it has been injected. Although alpha-NP-induced maturation is insensitive to inhibitors whose action is known to be restricted to the hormone-dependent period (such as the protease inhibitor leupeptin), it is blocked by inhibitors of MPF action (such as nicotinamide and lithium). Finally it was found that alpha-NP-induced maturation is inhibited by simultaneous microinjection of protein phosphatase-2A; also, alpha-NP, classically used as an inhibitor of acid and alkaline phosphatases, is able to inhibit protein phosphatases, is able to inhibit protein phosphatases 1 and 2 A. The addition of alpha-NP to oocytes increases the level of phosphorylated proteins. These results constitute direct evidence that an elevated level of phosphorylated proteins is sufficient to trigger MPF activity and to induce maturation.