Date Published: January 25, 2017
Publisher: Public Library of Science
Author(s): Guo Zhenhua, Sandeep K. Rajput, Joseph K. Folger, Liu Di, Jason G. Knott, George W. Smith, Peter J. Hansen.
Our previous studies demonstrated that maternal (oocyte derived) follistatin (FST) expression is positively associated with bovine oocyte competence and exogenous follistatin treatment during the pre-compaction period of development (d 1–3 post insemination) is stimulatory to bovine early embryogenesis in vitro [blastocyst rates and cell numbers/allocation to trophectoderm (TE)]. In the present study, bovine embryos were treated with exogenous follistatin during d 1–3, d 4–7 and d 1–7 post insemination to test the hypothesis that embryotropic effects of exogenous follistatin are specific to the pre-compaction period (d 1–3) of early embryogenesis. Follistatin treatment during d 4–7 (peri-/post-compaction period) of embryo culture increased proportion of embryos reaching blastocyst and expanded blastocyst stage and total cell numbers compared to controls, but blastocyst rates and total cell numbers were lower than observed following d 1–3 (pre-compaction) follistatin treatment. Follistatin supplementation during d 1–7 of embryo culture increased development to blastocyst and expanded blastocyst stages and blastocyst total cell numbers compared to d 1–3 and d 4–7 follistatin treatment and untreated controls. A similar increase in blastocyst CDX2 mRNA and protein (TE cell marker) was observed in response to d 1–3, d 4–7 and d 1–7 follistatin treatment. However, an elevation in blastocyst BMP4 protein (TE cell regulator) was observed in response to d 1–3 and d 1–7, but not d 4–7 (peri-/post-compaction) follistatin treatment. In summary, our study revealed the potential utility of follistatin treatment for increasing the success rate of in vitro embryo production in cattle. Such results also expand our understanding of the embryotropic actions of follistatin and demonstrate that follistatin actions on blastocyst development and cell allocation to the TE layer are not specific to the pre-compaction period.
Poor oocyte quality is recognized as a key limiting factor in improving the current low rate of successful assisted reproductive technologies and development of new options in infertility treatments. Over the last decades, understanding of the molecular determinants of oocyte quality has been a subject of intense study and still remains a major challenge in the field of reproductive biology especially in single ovulatory species. A large body of evidence suggests that maternal transcripts and proteins accumulated during oogenesis are crucial in determining competence of the oocyte to resume meiosis and develop into a viable embryo after fertilization [1–3].
All chemicals and reagents were purchased from Sigma unless otherwise stated.
Previous results from our laboratory demonstrated positive effects of follistatin treatment during the pre-compaction period on indices of embryo developmental progression . Results from the present study revealed that (1). Embryotropic effects of exogenous follistatin treatment are not specific to exposure during the pre-compaction period (d 1–3), but are also observed when supplemented during d 4–7 of culture (after embryonic genome activation) (2). Early exposure (d1-3) to follistatin had some advantage over d 4–7 follistatin treatment as an increase in 8–16 cell embryos and blastocysts was observed (3). Embryotropic effects of follistatin on blastocyst rates and cell numbers (total, TE) were additive and maximal following follistatin exposure during the entire window of in vitro culture (d 1–7) (4). Elevated blastocyst BMP4 protein abundance was observed in response to follistatin treatment and response is stage dependent and not observed in response to d 4–7 (peri-/post-compaction treatment). While previous studies established a link between maternal (oocyte derived) follistatin and oocyte quality/embryo developmental progression , follistatin mRNA is detectable in bovine embryos from 1C stage to the blastocyst stage and elevated in 16-cell embryos . This suggests that follistatin could have a potential regulatory role both during pre-compaction and peri-/post-compaction stages of embryo development. Collectively, these results support dynamic embryotropic actions of follistatin and TGF-beta superfamily signaling during multiple stages of bovine early embryo development.