Immobilized methylglyoxal-bis(guanylhydrazone) induces starfish oocyte maturation

Contrasting effects of fatty acids on oocyte maturation in several starfish species

Oocyte maturation (meiosis reinitiation) in starfish is induced by the natural hormone 1-methyladenine. In some species (group 2) oocyte maturation can be induced by micromolar concentrations of a few fatty acids such as arachidonic and eicosapentaenoic acids or by nanomolar concentrations of hydroxyeicosatetraenoic acid. Complete maturation is triggered: increased protein phosphorylation, appearance of the cytoplasmic "maturation-promoting factor", germinal vesicle breakdown, emission of the two polar bodies and formation of the female pronucleus. In other species (group 1), however, no maturation can be induced by the fatty acids active in the species of group 2, despite a large variety of experimental conditions.

Protein phosphorylation and oocyte maturation. II. Inhibition of starfish oocyte maturation by intracellular microinjection of protein phosphatases 1 and 2A and alkaline phosphatase

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. Microinjection of pure preparations of the catalytic subunits of protein phosphatases 1 and 2A inhibits 1-MeAde-induced maturation in a dose-dependent manner. Calmodulin-dependent protein phosphatase 2B is inefficient. Maturation induced by mimetics of 1-MeAde, such as dithiothreitol (DTT), methylglyoxal-bis(guanylhydrazone) (MGBG), 8-hydroxyeicosatetraenoic acid (8 HETE) and arachidonic acid (AA) is also inhibited by these protein phosphatases. In all cases inhibition can be reversed by increasing the concentration of 1-Me-Ade or of mimetic. Alkaline phosphatase also inhibits maturation in a dose-dependent way and in a reversible manner. Microinjection of protein phosphatase is still effective when preformed long after the end of the hormone-dependent period, and can even be effective a few minutes before the breakdown of the nuclear envelope. No detectable MPF activity is found in 1-MeAde-treated phosphatase-injected oocytes. However, microinjection of phosphatase 2A simultaneously with MPF (obtained from 1-MeAde-treated donors) does not result in inhibition. These results constitute direct evidence for the necessity of an elevated level of phosphorylated proteins for MPF activity and maturation. The mode of action of 1-MeAde in inducing starfish oocyte maturation is discussed in relation to protein phosphorylation.

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.