Expression data from progesterone receptor knockout versus heterozygous mouse ovaries: cumulus oocyte complexes

Ovulation requires sequential molecular events and structural remodeling in the ovarian follicle for the successful release of a mature oocyte capable of being fertilised. Critical to this process is progesterone receptor (PGR), a transcription factor highly yet transiently expressed in granulosa cells of preovulatory follicles. Progesterone receptor knockout (PRKO) mice are anovulatory, with a specific and complete defect in follicle rupture. Therefore, this model was used to examine the critical molecular and biochemical mechanisms necessary for successful ovulation. Although PGR is not expressed in the cumulus cells or oocyte of the preovulatory cumulus oocyte complex (COC), it is well known that the COC responds to the cascade of gene expression changes that occurs in preovulatory granulosa cells. We used microarrays to identify putative ‘ovulation’ genes in preovulatory COCs at a time when PGR expression is maximal in granulosa cells (eCG + 8h hCG) and the preovulatory COC and follicle are undergoing the final changes necessary for successful ovulation. The mutant strain used in this experiment were PRlacZ knock-in mice which originated from Assoc Prof John Lydon, Baylor College of Medicine, Houston TX, USA. The lacZ insertion results in disruption of transcription of both isoforms of PGR (Ismail et al, 2002, Mol Endocrinol 16:2475-2489), and therefore mice homozygous for the lacZ insertion are a phenocopy of the knockout strain described by Lydon et al. (1995, Genes Dev. 9:2266-2278) and are hereafter referred to as PRKO. Heterozygous mice (PR+/-) are a phenocopy of WT and have normal fertility (Ismail et al, 2002) and are therefore appropriate controls. Cumulus oocyte complexes (COCs) were collected by puncturing antral follicles from the ovaries of pre-pubertal PRKO and PR+/- mice 8 h after a standard protocol for hormonal induction of ovulation. Day 21-23 old mice were injected i.p. with 5IU of equine chorionic gonadotropin (eCG) to stimulate follicle growth, followed 44-47 h later by i.p. injection of 5 IU of human chorionic gonadotropin (hCG) to trigger ovulatory processes. COCs from 12 animals were collected per genotype, with cells from 3 animals pooled per sample for a total of n = 4 samples per genotype.