Inactivation of mitogen‐activated protein kinase is neither necessary nor sufficient for the onset of pronuclear formation in mouse oocytes

Abstract

Mammalian oocytes are arrested at metaphase II due to high MAP kinase activity. After fertilization, oocytes resume meiosis, leading to female chromosome segregation, polar body emission and pronuclear (PN) formation. Previous biochemical studies showed that MAP kinase activity remained high for several hours after fertilization and began to decrease in parallel with PN formation. It has been thought that MAP kinase activity is incompatible with PN formation, and its inactivation is required for the initiation of PN formation in mammalian oocytes. In this study, we revisited this hypothesis by examining MAP kinase activity and PN formation in individual mouse oocytes using cytological analysis. We showed that MAP kinase activity in oocytes could be evaluated using phospho-ERK1/2 immunofluorescent staining. Co-immunofluorescent staining of phospho-ERK1/2 and nuclear pore components showed that PN formation preceded MAP kinase inactivation and could be initiated while MAP kinase activity was still high. Moreover, artificial inactivation of MAP kinase or its downstream target, ribosomal S6 kinase, accelerated but did not rapidly induce PN formation. Our results show that although the MAP kinase pathway negatively regulates PN formation, its inactivation is neither necessary nor sufficient for PN formation. These results suggest the involvement of other essential factor(s) in this process.