The control of cyclin B1 mRNA translation during mouse oocyte maturation.

In maturing mouse oocytes, protein synthesis is required for meiotic maturation subsequent to germinal vesicle breakdown (GVBD). While the number of different proteins that must be synthesized for this progression to occur is unknown, at least one of them appears to be cyclin B1, the regulatory subunit of M-phase-promoting factor. Here, we investigate the mechanism of cyclin B1 mRNA translational control during mouse oocyte maturation. We show that the U-rich cytoplasmic polyadenylation element (CPE), a cis element in the 3' UTR of cyclin B1 mRNA, mediates translational repression in GV-stage oocytes. The CPE is also necessary for cytoplasmic polyadenylation, which stimulates translation during oocyte maturation. The injection of oocytes with a cyclin B1 antisense RNA, which probably precludes the binding of a factor to the CPE, delays cytoplasmic polyadenylation as well as the transition from GVBD to metaphase II. CPEB, which interacts with the cyclin B1 CPE and is present throughout meiotic maturation, becomes phosphorylated at metaphase I. These data indicate that CPEB is involved in both the repression and the stimulation of cyclin B1 mRNA and suggest that the phosphorylation of this protein could be involved in regulating its activity.

Tay J ，Hodgman R ，Richter JD 《DEVELOPMENTAL BIOLOGY》

Meiotic maturation of the mouse oocyte requires an equilibrium between cyclin B synthesis and degradation.

Among the proteins whose synthesis and/or degradation is necessary for a proper progression through meiotic maturation, cyclin B appears to be one of the most important. Here, we attempted to modulate the level of cyclin B1 and B2 synthesis during meiotic maturation of the mouse oocyte. We used cyclin B1 or B2 mRNAs with poly(A) tails of different sizes and cyclin B1 or B2 antisense RNAs. Oocytes microinjected with cyclin B1 mRNA showed two phenotypes: most were blocked in MI, while the others extruded the first polar body in advance when compared to controls. Moreover, these effects were correlated with the length of the poly(A) tail. Thus it seems that the rate of cyclin B1 translation controls the timing of the first meiotic M phase and the transition to anaphase I. Moreover, overexpression of cyclin B1 or B2 was able to bypass the dbcAMP-induced germinal vesicle block, but only the cyclin B1 mRNA-microinjected oocytes did not extrude their first polar body. Oocytes injected with the cyclin B1 antisense progressed through the first meiotic M phase but extruded the first polar body in advance and were unable to enter metaphase II. This suggested that inhibition of cyclin B1 synthesis only took place at the end of the first meiotic M phase, most likely because the cyclin B1 mRNA was protected. The injection of cyclin B2 antisense RNA had no effect. The life observation of the synthesis and degradation of a cyclin B1-GFP chimera during meiotic maturation of the mouse oocyte demonstrated that degradation can only occur during a given period of time once it has started. Taken together, our data demonstrate that the rates of cyclin B synthesis and degradation determine the timing of the major events taking place during meiotic maturation of the mouse oocyte.

Ledan E ，Polanski Z ，Terret ME ，Maro B ... -  《DEVELOPMENTAL BIOLOGY》

Translational control of cyclin B1 mRNA during meiotic maturation: coordinated repression and cytoplasmic polyadenylation.

Translational control is prominent during meiotic maturation and early development. In this report, we investigate a mode of translational repression in Xenopus laevis oocytes, focusing on the mRNA encoding cyclin B1. Translation of cyclin B1 mRNA is relatively inactive in the oocyte and increases dramatically during meiotic maturation. We show, by injection of synthetic mRNAs, that the cis-acting sequences responsible for repression of cyclin B1 mRNA reside within its 3'UTR. Repression can be saturated by increasing the concentration of reporter mRNA injected, suggesting that the cyclin B1 3'UTR sequences provide a binding site for a trans-acting repressor. The sequences that direct repression overlap and include cytoplasmic polyadenylation elements (CPEs), sequences known to promote cytoplasmic polyadenylation. However, the presence of a CPE per se appears insufficient to cause repression, as other mRNAs that contain CPEs are not translationally repressed. We demonstrate that relief of repression and cytoplasmic polyadenylation are intimately linked. Repressing elements do not override the stimulatory effect of a long poly(A) tail, and polyadenylation of cyclin B1 mRNA is required for its translational recruitment. Our results suggest that translational recruitment of endogenous cyclin B1 mRNA is a collaborative effect of derepression and poly(A) addition. We discuss several molecular mechanisms that might underlie this collaboration.

Barkoff AF ，Dickson KS ，Gray NK ，Wickens M ... -  《DEVELOPMENTAL BIOLOGY》

Expression of cyclin B1 messenger RNA isoforms and initiation of cytoplasmic polyadenylation in the bovine oocyte.

Tremblay K ，Vigneault C ，McGraw S ，Sirard MA ... -  《BIOLOGY OF REPRODUCTION》

Dissolution of the maskin-eIF4E complex by cytoplasmic polyadenylation and poly(A)-binding protein controls cyclin B1 mRNA translation and oocyte maturation.

Cao Q ，Richter JD 《-》