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Pleiotropic effect of okadaic acid on maturing mouse oocytes
H Alexandre1, A Van Cauwenberge, Y Tsukitani
1Laboratoire de Cytologie et Embryologie Moléculaires, Université Libre de Bruxelles, Rhode-St-Genèse, Belgium.
Abstract:
Okadaic acid (OA), a potent inhibitor of types 1 and 2A protein phosphatases, was shown recently to induce chromatin condensation and germinal vesicle breakdown (GVBD) in mouse oocytes arrested at the dictyate stage by dibutyryl cAMP (dbcAMP), isobutyl methylxanthine (IBMX) and 12,13-phorbol dibutyrate (PDBu). We confirm these results using IBMX and another phorbol diester, 12-O-tetradecanoylphorbol-13-acetate (TPA) and show that OA also bypasses the inhibitory effect of 6-dimethylaminopurine (6-DMAP). It has been concluded that protein phosphatases 1 and/or 2A (PP1, 2A), involved in the negative control of MPF activation, are thus operating downstream from both the protein kinase A and protein kinase C catalysed phosphorylation steps that prevent the breakdown of GV. Similar enzymatic activities are also able to counteract the general inhibition of protein phosphorylation. However, PP1 and/or PP2A are positively involved in the activation of pericentriolar material (PCM) into microtubule organizing centres (MTOCs). This explains the inhibitory effect of OA on spindle assembly. Finally, OA interferes with the integrity and/or function of actomyosin filaments. This results in a dramatic ruffling of the plasma membrane leading to the internalization of large vacuoles, the inhibition of chromosome centrifugal displacement and, consequently, the prevention of polar body extrusion.
Insights
Okadaic acid (OA) triggers mouse oocyte maturation by inhibiting protein phosphatases 1 and 2A (PP1, 2A). This process bypasses inhibitory signals and affects spindle assembly and polar body extrusion.
Area of Science:
- Cell Biology
- Developmental Biology
- Biochemistry
Background:
- Okadaic acid (OA) is a known inhibitor of protein phosphatases 1 and 2A (PP1, 2A).
- Previous studies indicated OA induces chromatin condensation and germinal vesicle breakdown (GVBD) in mouse oocytes arrested by specific inhibitors.
Purpose of the Study:
- To confirm and extend previous findings on OA's effects on mouse oocyte maturation.
- To investigate the role of PP1 and PP2A in regulating key meiotic events.
Main Methods:
- Mouse oocytes were arrested at the dictyate stage using dibutyryl cAMP (dbcAMP), isobutyl methylxanthine (IBMX), or 12-O-tetradecanoylphorbol-13-acetate (TPA).
- Okadaic acid (OA) was applied to assess its effects on GVBD, MPF activation, spindle assembly, and polar body extrusion.
- The influence of OA on protein phosphorylation and actomyosin filament integrity was examined.
Main Results:
- OA confirmed to induce GVBD and chromatin condensation, bypassing inhibitory signals like 6-dimethylaminopurine (6-DMAP).
- PP1 and/or 2A phosphatases act downstream of protein kinase A and C pathways in preventing GV breakdown.
- OA inhibits spindle assembly by affecting pericentriolar material (PCM) activation into microtubule organizing centers (MTOCs).
- OA disrupts actomyosin filaments, leading to plasma membrane ruffling, vacuole internalization, and prevention of polar body extrusion.
Conclusions:
- PP1 and/or 2A phosphatases are crucial negative regulators of MPF activation and GV breakdown.
- These phosphatases are also essential for the proper organization of the cytoskeleton, specifically MTOCs.
- OA's inhibition of PP1/2A disrupts cytoskeletal dynamics and cytokinesis, ultimately preventing oocyte maturation and polar body extrusion.