Two P90Ribosomal S6Kinase Isoforms Are Involved In The Regulation Of Mitotic And Meiotic Arrest In Artemia

Artemia, a small crustacean that can survive in extreme conditions such as high salinity, low levels of oxygen, and large change in temperature. To cope with such harsh habitats and widely different environmental conditions, Artemia have two independent reproductive pathways. Under unfavorable environmental conditions, mature Artemia produce and release encysted gastrula embryos that follow the diapause-destined developmental pathway and thereby enter diapause (an obligate dormancy). When environmental conditions are favorable, embryonic development precedes uninterrupted, and mature Artemia release swimming nauplius larvae.Artemia is a useful model in which to study cell cycle arrest because these animals undergo prolonged diapauses, a state of obligate dormancy.In most metazoa, unfertilized oocytes are arrested at meiotic metaphase owing to a cytoplasmic activity called cytostatic factor (CSF). This ensures that oocytes are released into the ovisac (uterus) in an optimal physiological state, which reduces the risk of insemination failing, such as occurs in polyspermy. The E3ubiquitin ligase anaphase-promoting complex/cyclosome (APC/C) can degrade the components of CSF, after which meiosis resumes. The MAPK-Rsk pathway is required to maintain CSF-dependent meiotic arrest. Little is known about CSF-dependent meiotic arrest in invertebrate species. In Artemia, oocytes are arrested in metaphase I until fertilization occurs.There are multiple isoforms of p90ribosomal S6kinase (RSK), which regulate diverse cellular functions such as cell growth, proliferation, maturation, and motility. However, the relationship between the structures and functions of RSK isoforms remains undetermined.A novel RSK isoform was identified in Artemia, which was termed Ar-Rsk2. This isoform was compared to an RSK isoform that we previously identified in Artemia, termed Ar-Rskl. Ar-Rsk2has an ERK-docking motif, whereas Ar-Rskl does not. Western blot analysis revealed that Ar-Rskl was activated by phosphorylation, which blocked meiosis in oocytes. Knockdown of Ar-Rskl reduced the level of phosphorylated cdc2and thereby suppressed cytostatic factor activity. This indicates that Ar-Rskl regulates cytostatic factor in meiosis. Expression of Ar-Rsk2was downregulated in Artemia cysts in which mitosis was arrested. Knockdown of Ar-Rsk2resulted in decreased levels of cyclin D3and phosphorylated histone H3, and the production of pseudo-diapause cysts. This indicates that Ar-Rsk2regulates mitotic arrest. PLK and ERK RNAi showed that Ar-Rsk2, but not Ar-Rskl, could be activated by PLK-ERK in Artemia.This is the first study to report that RSK isoforms with and without an ERK-docking motif regulate mitosis and meiosis, respectively. This study provides insight into the relationship between the structures and functions of RSK isoforms.