Oocyte-derived Smad4 is not required for development of the oocyte or the preimplantation embryo

Abstract

The generation of a competent egg requires complex molecular interactions between the oocyte and the ovary, and transforming growth factor β (TGF-β) is a major signaling pathway. Smad4 is a central regulator of the TGF-β signaling pathway as it mediates gene expression triggered by activation of TGF-β receptors. Deletion of Smad4 in granulosa cells disrupts follicle development; however, the role of Smad4 in the oocyte has not been confirmed. Furthermore, the role of Smad4 in embryo development has not been confirmed because previous studies of Smad4del/del embryos were generated from heterozygous parents, and thus it is possible that maternal transcripts rescue development before embryonic day 6.5 (E6.5) when Smad4del/del embryos die. To determine the role of TGF-β signaling in oocyte and embryo development, mice with oocyte-specific deletion of Smad4 were studied. Fertility was evaluated in Mutant (Smad4F/F:ZP3Cre) and Control (Smad4F/F) females mated continuously with control males during a 6-month period. Surprisingly, Mutant females were fertile with the same litter size (Mutants, 9.23 ± 0.4; Controls, 9.42 ± 0.4) and interlitter period as Controls. Ovulation rate induced using a superovulation regime did not differ between Controls and Mutants at both 6 weeks and 6 months. Embryo development was assessed at E6.5 using Control and Mutant females mated with heterozygous males. Development of Smad4del/del embryos at E6.5 was retarded consistent with previous studies of embryos generated from heterozygous parents indicating that there is no rescue of preimplantation development by maternal transcripts. The numbers of implanted embryos at 6.5 dpc also did not differ (Control: 9.1 ± 0.4; Mutant: 7.0 ± 0.9). However, only 26.3% of E6.5 embryos carried by Mutant females were Smad4del/del compared with the expected ratio of 50%. Since litter size was not decreased, this indicates that either the number of Smad4del sperm fertilizing the oocytes is reduced or implantation of Smad4del/del embryos is suboptimal. In summary, we have shown that Smad4 in the oocyte, and thus TGF-β signaling, is not required for oocyte or follicle development, ovulation, fertilization, preimplantation development, or implantation.