SFRP4 Is Dispensable for Female Fertility.

Abstract

Canonical WNT signal transduction occurs via Frizzled (Fzd) receptors and results in the stabilization of CTNNB1 (beta-catenin), which then enters the cell nucleus to modulate the transcriptional activity of target genes. During ovarian development, a canonical WNT signal is initiated by WNT4 and modulated by RSPO1, the absence of which results in developmental defects including partial sex-reversal. WNT4 also plays a role in the adult ovary by regulating follicle development and modulating the expression of a variety of ovarian genes, including several involved in steroid hormone biosynthesis. Secreted Frizzled Related Proteins (SFRPs) are antagonists of WNT signaling that act by directly binding WNTs and preventing their interaction with Fzd receptors. In particular, SFRP4 is induced during luteinization and its expression is maintained in the corpus luteum during gestation. Moreover, a study suggested that SFRP4 expression is associated with the apoptosis of luteinized granulosa cells. Together, these results suggest that SFRP4 could serve to antagonize WNT4-mediated survival signaling or steroidogenesis in the corpus luteum. Our objective was thus to characterize the ovarian role of SFRP4 during luteinization. To this end, we created a Sfrp4 KO mouse strain by conventional gene targeting. The deletion of SFRP4 did not result in any observable developmental or functional defects in non-reproductive tissues, and male fertility appeared to be normal. Unexpectedly, a long-term mating trial comparing Sfrp4 KO mice and control WT littermates revealed no significant differences in numbers of litters or average pups/litter produced after five months (WT: 4.8 ± 0.8 litters, 8.24 ± 1.1 pups/litter; KO: 4.8 ± 0.7 litters, 7.6 ± 1.4 pups/litter; P > 0.05). We then did a preliminary gene expression analysis using qRT-PCR to assess if the absence of SFRP4 would affect the expression of WNT4-regulated steroidogenic enzymes. At 24h hours post-hCG, a time point at which SFRP4 is normally strongly expressed, there were no differences in the expression of Cyp19 or StAR between Sfrp4 KO and WT ovaries (P > 0.05). We conclude that SFRP4 is dispensable for female fertility, and does not appear to modulate WNT4 signaling in the ovary. Ongoing experiments are aimed at assessing the effects of SFRP4 deficiency on the expression of other ovarian genes at different times during the ovarian cycle and gestation. (poster)