Bim Mediates Germ Cell Death During Ovarian Development.

Abstract

Normal ovarian development in mice is characterized by the initial expansion of germ cells followed by the death of more than two thirds of their maximum number. The final result of germ cell mitosis and apoptosis during embryogenesis is a fixed number of oocytes stored within the ovary as primordial follicles just after birth, from which all ovulated oocytes are derived. Thus, apoptosis plays a critical role in determining the number of oocytes endowed within the ovary and the fertile lifespan of the female. In this study, we investigated the role of Bim (Bcl-2 interacting mediator of cell death), a pro-apoptotic protein belonging to the BH3-only subgroup of the Bcl-2 family of apoptotic regulators, in mediating cell death during the establishment and maintenance of the primordial follicle pool. Immunohistochemical (IHC) staining revealed that Bim protein is expressed within a small subset of mouse oocytes in meiotic prophase at embryonic day (E) 15.5 to E17.5, with minimal expression detected in oocytes once primordial follicle assembly is complete at post natal (PN) day 5. Analysis of follicle numbers at PN10 showed that bim-/- mice had significantly more primordial follicles than WT (2.76 ± 0.25 vs 1.51 ± 0.23 follicles/104μm2, P<0.05), supporting an important role for Bim in determining the initial size of the primordial follicle pool. Furthermore, while quantitative real-time polymerase chain reaction (QRT-PCR), in situ hybridisation (ISH) and IHC analysis revealed the induction of Bim expression within primordial follicle oocytes following ?-irradiation-induced DNA damage, Bim deficiency could not rescue oocytes from ?-irradiation induced apoptosis, probably due to the presence of other BH3-only family members, most likely Puma and Noxa. In summary, this study supports a non-redundant and essential role for pro-apoptotic Bim in the elimination of germ cells during ovarian development and in the establishment of the primordial follicle reserve. This work was supported by the NHMRC of Australia (Program Grants #494802 and #257502, Fellowships JKF (#441101), KJH (#494836), CLS (#406675), AS (#461299); Victorian Government's Operational Infrastructure Support Program.