Cyclin D2 ArrestsXenopusEarly Embryonic Cell Cycles

Abstract

Xenopuscyclin D2 mRNA is a member of the class of maternal RNAs. It is rare and stable during early embryonic development. To investigate the potential role of cyclin D2 during early embryonic cell cycles, cyclin D2 was injected into one blastomere of a two-cell embryo. This injection induced a cell cycle arrest in the injected blastomere. To analyze more precisely the mechanism of this arrest, we took advantage of cycling egg extracts that recapitulate major events of the cell cycle when supplemented with demembranated sperm heads. WhenXenopuscyclin D2 is added to egg extracts, the first round of DNA replication occurs as in control extracts. However,Xenopuscyclin D2 blocks subsequent rounds of DNA replication and the oscillations of histone H1 kinase activity associated with cdc2 kinase, indicating that the cell cycle is arrested after the first S-phase. The block induced byXenopuscyclin D2 is not due to a lack of the mitotic cyclin B2 that accumulates normally. RadiolabeledXenopuscyclin D2 enters nuclei after completion of the first S-phase and remains stable over the entire period of the arrest. These features suggest thatXenopuscyclin D2 could play an original role during early development, controlling the G2-phase and/or the G2/M transition.