Caspase 9 is constitutively activated in mouse oocytes and plays a key role in oocyte elimination during meiotic prophase progression.

In many mammalian species, more than half of the initial oocyte population is eliminated by neonatal life, thus limiting the oocyte reserve for reproduction. The cause or mechanism of this major oocyte loss remains poorly understood. We examined the apoptotic pathway involved in oocyte elimination in wild-type mouse ovaries as well as in Msh5 -/- ovaries, in which all oocytes were eliminated due to a lack of double strand break repair. Immunoblot and immunofluorescence staining showed that an initiator caspase 9 and an effector caspase 7 were constitutively activated in almost all oocytes in fetal ovaries regardless of their genotypes. In caspase 9 -/- ovaries, the total number of oocytes remained high while that in wild-type ovaries steadily declined during ovarian development. Therefore, the activation of caspase 9 was required for but did not immediately lead to oocyte demise. We found that XIAP, an endogenous inhibitor of apoptosis, was also abundant in oocytes during meiotic prophase progression. On the other hand, a cleaved form of PARP1, a target of effector caspases, was localized to the nuclei of a limited number of oocytes, and the frequency of cleaved PARP1-positive oocyte nuclei increased significantly higher before all oocytes disappeared in Msh5 -/- ovaries. We conclude that the mitochondrial apoptotic pathway mediated by caspase 9 is constitutively activated in oocytes and renders the elimination of oocytes with meiotic errors, which can be captured by the cleavage of PARP1.