On cyclins, oocytes, and eggs

Abstract

Oocyte and egg are suitable model systems for studying cell division since meiotic maturation resembles a G2/M transition and early embryonic divisions are precisely timed and occur without zygotic transcription. The analysis of oocytes and eggs from different species provides the opportunity to understand the roles of proteins that are critical to the progression and maintenance of the cell cycle. Among them, cyclins are certainly worthy of investigation. Mitotic cyclins (cyclins A and B) are clearly implicated in meiosis and early embryonic cell cycles. More recent studies have revealed that G1‐type cyclins (cyclins E and D) could also play a role in both processes and cyclin H has been suggested to participate to CAK activity (cdc2‐activating kinase) in oocytes. The study of cyclins in oocytes and eggs clearly offer insights into their roles during the cell cycle. Mol. Reprod. Dev. 48:397–411, 1997. © 1997 Wiley‐Liss, Inc.

Upregulation of eIF5B controls cell-cycle arrest and specific developmental stages

“…The transition from immature (G0-like) to mature oocytes has been compared to the entry of G0 cells into the cell cycle (8), while serum starvation of proliferating mammalian cells induces an early phase of transient stress: Within 1 d of serum starvation, the cells either adapt to these nonproliferative conditions and enter G0 arrest or undergo cell death (9). Cells that have been serum-starved for 1 d represent a transient and heterogeneous state, with gene expression that is distinct from cells that have experienced prolonged (>1 d) serum starvation and cells that have entered G0 and late G0 (9).…”

“…These data suggest that eIF5B negatively regulates G0 and G0-like (immature oocyte; ref. 8) states. Conversely, depletion of eIF5B promotes the G0-like, immature oocyte state ( Fig.…”

“…The transition from immature to mature oocytes shows some features similar to the entry into mammalian G1/cell cycle from quiescence (G0) (8), an assortment of reversible, cell-cyclearrested states that can withstand unfavorable environments (9). Serum deprivation of proliferating mammalian cells induces an early stage of transient stress that alters gene expression; cells subjected to such stress either adapt to these nonproliferative conditions and proceed further into G0 or alternatively undergo cell death (9).…”

“…Importantly, we find that eIF5B overexpression promotes the transition away from G0 or G0-like states (immature oocytes; ref. 8), enhancing the maturation of late G0-like immature oocytes and causing cell death, the alternative to G0, upon serum deprivation of proliferating mammalian cells. These data suggest that eIF5B promotes translation in these specific conditions and is an antagonist of G0 and G0-like states; consistently, eIF5B depletion decreases translation, promotes the immature state of oocytes, and enables earlier G0 arrest upon serum starvation of proliferating mammalian cells.…”

Xenopus H-RasV12 promotes entry into meiotic M phase and cdc2 activation independently of Mos and p42MAPK

“…To evaluate cdc2 activation, we analysed Cyclin B2 phosphorylation state by Western blotting (Figure 3b). Cdc2 activation is correlated with a change in the electrophoretic mobility of its regulatory partner, Cyclin B2 (Taieb et al, 1997). An electrophoretic shift of Cyclin B2 was observed after Xe H-RasV12 injection and progesterone addition but not after Xe H-RasV12/S186 injection ( Figure 3b).…”