Effect of Bcl-2 on the Primordial Follicle Endowment in the Mouse Ovary1

Abstract

Little is known about the embryonic factors that regulate the size of the primordial follicle endowment at birth. A few studies suggest that members of the B-cell lymphoma/leukemia-2 (bcl-2) family of protooncogenes may be important determinants. Thus, the purpose of this study was to test whether bcl-2 regulates the size of the primordial follicle pool at birth. To test this hypothesis, three lines of transgenic mice (c-kit/bcl-2 mice) were generated that overexpress human bcl-2 in an effort to reduce prenatal oocyte loss. The overexpression was targeted to the ovary and appropriate embryonic time period with the use of a 4.8-kilobase c-kit promoter. This promoter provided two to three times more expression of bcl-2 in the ovaries with minimal or no overexpression in most nongonadal tissues. On Postnatal Days 8-60, ovaries were collected from homozygous c-kit/bcl-2 and nontransgenic littermates (controls) and processed for histological evaluation of follicle numbers. All lines of c-kit/bcl-2 mice were born with significantly more primordial follicles than control mice (P < or = 0.05). By Postnatal Days 30-60, however, there were no significant differences in follicle numbers between c-kit/bcl-2 and control mice. These results indicate that bcl-2 overexpression increases the number of primordial follicles at birth, but that the surfeit of primordial follicles is not maintained in postnatal life. These data suggest that it is possible that the ovary may contain a census mechanism by which excess numbers of primordial follicles at birth are detected and removed from the ovary by adulthood.

Follicular assembly: mechanisms of action

“…Adult female Bcl2 knockout mice have fewer oocytes than wild-type mice, suggesting that BCL2 is important for oocyte survival though it is not known when the oocytes are lost in the knockout mice (Ratts et al 1995). Overexpression of BCL2 in germ cells results in ovaries with more oocytes at PND8; however, by PND60, transgenic ovaries have the same number of oocytes as wildtype ovaries, suggesting that there is a mechanism to detect excess oocytes (Flaws et al 2001). There is also evidence that the pro-apoptotic family member BAX is involved in regulating oocyte numbers (Perez et al 1999 apoptosis and cyst breakdown (Greenfeld et al 2007).…”

Apoptosis in the germ line

“…Developmentally programmed cell death during ovarian development: BCL2-mediated apoptosis There is mounting evidence to suggest that the death of primordial germ cells, oogonia, and oocytes, which occurs during the ovarian development in both mice and humans, is mediated through the actions of the BCL2 family of proteins (Ratts et al 1995, Rucker et al 2000, Flaws et al 2001, Jefferson et al 2006, Greenfeld et al 2007, Lobascio et al 2007b, Albamonte et al 2008, De Felici et al 2008.…”

“…If the antiapoptotic protein BCL2L1 is deleted, the neonatal ovaries have a deficiency of oocytes apparently due to a failure to survive during embryonic life (Rucker et al 2000). Deletion (Ratts et al 1995) or overexpression (Flaws et al 2001(Flaws et al , 2006 of BCL2 leads to either reduced oocyte endowment or enhanced oocyte endowment respectively. Collectively, these studies provide clear evidence of the role of the BCL2-regulated apoptotic pathway in establishing the pool of primordial follicles.…”

In vitro maturation, apoptotic gene expression and incidence of numerical chromosomal abnormalities following cryotop vitrification of sheep cumulus-oocyte complexes

“…The Bcl-2 family consists of two types of members: anti-apoptotic such as Bcl-2 and Bcl-XL, and pro-apoptotic such as Bax and Bid [17,20]. Bcl-2 is a membrane-associated protein present in the nuclear envelope and mitochondria and is fully capable of promoting germ cell survival in females irrespective to the developmental status of the oocyte [18]. The pro survival function of Bcl-2 is exerted by first modulating the mitochondrial release of cytochrome c causing the interaction of Apaf-1 with caspase 9 and secondly through binding to Bax, and finally blocking apoptosis induced by C-myc [19,20].…”