p90Rsk is required for G1 phase arrest in unfertilized starfish eggs.
The cell cycle in oocytes generally arrests at a particular meiotic stage to await fertilization. This arrest occurs at metaphase of meiosis II (meta-II) in frog and mouse, and at G1 phase after completion of meiosis II in starfish. Despite this difference in the arrest phase, both arrests depend on the same Mos-MAPK (mitogen-activated protein kinase) pathway, indicating that the difference relies on particular downstream effectors. Immediately downstream of MAPK, Rsk (p90 ribosomal S6 kinase, p90(Rsk)) is required for the frog meta-II arrest. However, the mouse meta-II arrest challenges this requirement, and no downstream effector has been identified in the starfish G1 arrest. To investigate the downstream effector of MAPK in the starfish G1 arrest, we used a neutralizing antibody against Rsk and a constitutively active form of Rsk. Rsk was activated downstream of the Mos-MAPK pathway during meiosis. In G1 eggs, inhibition of Rsk activity released the arrest and initiated DNA replication without fertilization. Conversely, maintenance of Rsk activity prevented DNA replication following fertilization. In early embryos, injection of Mos activated the MAPK-Rsk pathway, resulting in G1 arrest. Moreover, inhibition of Rsk activity during meiosis I led to parthenogenetic activation without meiosis II. We conclude that immediately downstream of MAPK, Rsk is necessary and sufficient for the starfish G1 arrest. Although CSF (cytostatic factor) was originally defined for meta-II arrest in frog eggs, we propose to distinguish ;G1-CSF' for starfish from ;meta-II-CSF' for frog and mouse. The present study thus reveals a novel role of Rsk for G1-CSF.
Laboratory of Cell and Developmental Biology, Graduate School of Bioscience, Tokyo Institute of Technology, Nagatsuta, Midoriku, Yokohama 226-8501, Japan.
Masashi
M
Mori
Masatoshi
M
Hara
Kazunori
K
Tachibana
Takeo
T
Kishimoto
D000818:Animals; D047288:Asterina / Q000502:physiology*; D001757:Blastomeres / Q000166:cytology / Q000201:enzymology; D004789:Enzyme Activation; D005260:Female; D016193:G1 Phase; D005982:Glutathione Transferase / Q000378:metabolism; D006639:Histidine / Q000737:chemistry; D007700:Kinetics; D008540:Meiosis; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism; D008954:Models, Biological; D010063:Ovum / Q000166:cytology / Q000201:enzymology* / Q000502:physiology*; D010312:Parthenogenesis; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D038744:Ribosomal Protein S6 Kinases, 90-kDa / Q000032:analysis / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*
D011994:Recombinant Proteins; D006639:Histidine; D005982:Glutathione Transferase; D038744:Ribosomal Protein S6 Kinases, 90-kDa; D020928:Mitogen-Activated Protein Kinases