Are Xenopus oocytes the exception or the rule?

Abstract

Two universal features of oocyte meiotic maturation are activation of mitogen-activated protein kinase (MAPK) and of maturation-promoting factor (MPF) just prior to germinal vesicle breakdown (GVBD) and entry into the cell cycle. While it is clear that MPF is the pivotal molecule driving cells into M-phase, the role of MAPK in meiosis varies among organisms. In most systems, MAPK is not required for MPF activation. The exception is in Xenopus oocytes, where extensive studies have shown that MAPK is both necessary and sufficient for activating MPF.Fisher and colleagues1xDissociation of MAP kinase activation and MPF activation in hormone-stimulated maturation of Xenopus oocytes. Fisher, D.L. et al. Development. 1999; 126: 4537–4546PubMedSee all References1 recently reported an interesting twist on the MAPK–MPF relationship in Xenopus. Their most exciting finding comes from using a sensitive assay for MAPK activation. They show that MAP kinase is activated several hours before MPF and GVBD, whereas it was accepted previously that MAPK activation just barely precedes MPF activation. They go on to ask, using an Hsp90 inhibitor to prevent MAPK activation indirectly, whether MAPK activation is required for MPF activation. When the inhibitor is added before progesterone stimulation, MAPK is not activated, yet a small percentage of the cells after extended incubation undergo GVBD and activate a low level of MPF. The percentage of cells entering the cell cycle increases if MAPK activity is allowed for a short period just after progesterone stimulation but is then inhibited prior to GVBD, and well in advance of any detectable MPF activity, suggesting that active MAPK does not need to be present in order for MPF to become activated.The somewhat difficult to swallow conclusion drawn from their combination of results is that MAPK is not required for MPF activation in Xenopus oocytes but instead might enhance already activated MPF. This conclusion contradicts extensive data obtained from many other labs, and therefore it is imperative to demonstrate, using other, rigorous methods, that these results are repeatable. In addition, it remains to be explained why, in other species, MPF activation is disconnected completely from MAPK, whereas, in Xenopus, there exists some link between these two molecules.