title: Cyclin B1 turnover and the mechanism causing insensitivity of fully grown mouse oocytes to cycloheximide inhibition of meiotic resumption.
abstract: Cyclin B1 turnover and the insensitivity of fully-grown mouse oocytes to cycloheximide (CHX) inhibition of germinal vesicle breakdown (GVBD) were examined by assaying GVBD and cyclin B1 levels after treatment of oocytes with various combinations of eCG and CHX. Whereas over 95% of oocytes underwent GVBD after culture for 24 h with CHX alone, only 10% did so after culture with CHX + eCG (P < 0.05). In addition, preculture with eCG alone had no effect, but preculture with eCG + CHX prevented GVBD during a second culture with CHX alone. Therefore, we inferred that eCG delayed GVBD long enough for CHX inhibition of protein synthesis to allow cyclin B1 to decrease below a threshold where GVBD became dependent upon its de novo synthesis. However, western blot revealed no cyclin B1 synthesis, but cyclin B1 degradation, as long as GVs were maintained intact with eCG. Regarding the function of CHX in preculture without protein synthesis to block subsequent GVBD, whereas eCG delayed GVBD for only 3 h, CHX had an ongoing effect that further postponed GVBD, thus allowing cyclin B1 to decrease below the threshold. When oocytes precultured with eCG + CHX were further cultured without eCG and CHX, cyclin B1 first decreased but then, because of the ongoing effects of CHX, increased to a level sufficient to induce GVBD. The content of P34Cdc2 was not altered under any of the culture conditions (P > 0.05). We concluded that insensitivity of mouse germinal vesicle (GV) oocytes to CHX was due to the presence of sufficient cyclin B1, and that cyclin B1 level in such oocytes was maintained by an equilibrium between synthesis and degradation.
authors.affiliation: College of Animal Science and Veterinary Medicine, Shandong Agricultural University, Tai-an City, China.
authors.forename: Dong
authors.initials: D
authors.lastname: Han
authors.forename: Xin-Yong
authors.initials: XY
authors.lastname: Liu
authors.forename: Guang-Zhong
authors.initials: GZ
authors.lastname: Jiao
authors.forename: Bo
authors.initials: B
authors.lastname: Liang
authors.forename: Nan
authors.initials: N
authors.lastname: He
authors.forename: W-Q
authors.initials: WQ
authors.lastname: Gao
authors.forename: Jing-He
authors.initials: JH
authors.lastname: Tan
mesh_terms: D000818:Animals; D015153:Blotting, Western; D006063:Chorionic Gonadotropin / Q000494:pharmacology; D054885:Cumulus Cells / Q000502:physiology; D056744:Cyclin B1 / Q000378:metabolism*; D003513:Cycloheximide / Q000494:pharmacology*; D059471:In Vitro Oocyte Maturation Techniques; D008540:Meiosis / Q000187:drug effects* / Q000502:physiology; D051379:Mice; D009865:Oocytes / Q000166:cytology / Q000187:drug effects* / Q000502:physiology*; D013997:Time Factors
chemical_list: D006063:Chorionic Gonadotropin; D056744:Cyclin B1; D003513:Cycloheximide