Zinc supports transcription and improves meiotic competence of growing bovine oocytes.

In the last years, many studies focused on the understanding of the possible role of zinc in the control of mammalian oogenesis, mainly on oocyte maturation and fertilization. However, little is known about the role of zinc at earlier stages, when the growing oocyte is actively transcribing molecules that will regulate and sustain subsequent stages of oocyte and embryonic development. In this study, we used the bovine model to gain insights into the possible involvement of zinc in oocyte development. We first mined the EmbryoGENE transcriptomic dataset, which revealed that several zinc transporters and methallothionein are impacted by physiological conditions throughout the final phase of oocyte growth and differentiation. We then observed that zinc supplementation during in vitro culture of growing oocytes is beneficial to the acquisition of meiotic competence when subsequently subjected to standard in vitro maturation. Furthermore, we tested the hypothesis that zinc supplementation might support transcription in growing oocytes. This hypothesis was indirectly confirmed by the experimental evidence that the content of labile zinc in the oocyte decreases when a major drop in transcription occurs in vivo. Accordingly, we observed that zinc sequestration with a zinc chelator rapidly reduced global transcription in growing oocytes, which was reversed by zinc supplementation in the culture medium. Finally, zinc supplementation impacted the chromatin state by reducing the level of global DNA methylation, which is consistent with the increased transcription. In conclusion, our study suggests that altering zinc availability by culture-medium supplementation supports global transcription, ultimately enhancing meiotic competence.

Steroid hormones interact with natriuretic peptide C to delay nuclear maturation, to maintain oocyte-cumulus communication and to improve the quality of in vitro-produced embryos in cattle.

In vivo, oocyte maturation is triggered by the ovulatory LH surge, whereas in vitro it is precociously induced when the cumulus-oocyte complex is removed from the follicle. Natriuretic peptide C (NPPC) delays germinal vesicle breakdown (GVBD) while increasing oocyte-cumulus communication during in vitro maturation (IVM) in cattle. In the present study we first tested the hypothesis that steroids secreted by the follicle (17ß-oestradiol, progesterone and androstenedione) interact with NPPC to delay GVBD and to maintain oocyte-cumulus communication as assessed by transfer of a dye (Lucifer Yellow) from the oocyte to cumulus cells. Then, we assessed the effects of steroid hormones and NPPC, alone and in combination in a pre-IVM culture, on embryo production. The combination of NPPC with steroids delayed GVDB, increased natriuretic peptide receptor 2 (NPR2) mRNA abundance in cumulus cells during culture, and maintained oocyte-cumulus communication at levels not different from non-cultured controls. The addition of steroids and/or NPPC to a pre-IVM culture did not alter blastocyst rates after IVF, but supplementation with steroids increased blastocyst total cell number. The present study provides evidence, for the first time in cattle, that steroids interact with NPPC to regulate oocyte nuclear maturation and oocyte-cumulus communication, and improve oocyte developmental competence.

Effect of oral administration of low-dose follicle stimulating hormone on hyperandrogenized mice as a model of polycystic ovary syndrome.

BACKGROUND: Polycystic Ovary Syndrome (PCOS) is a widespread reproductive disorder characterized by a disruption of follicular growth and anovulatory infertility. In women with PCOS, follicular growth and ovulation can be induced by subcutaneous injections of low doses of follicle stimulating hormone (FSH). The aim of this study was to determine the effect of oral administration of recombinant human FSH (rhFSH) on follicle development in a PCOS murine model. Moreover, since it is unlikely that intact rhFSH is present into the circulation after oral administration, the biological activity of a peptide fragment, derived from the predicted enzymatic cleavage sites with the FSH molecule, was investigated in vitro on cumulus-enclosed oocytes (COCs). METHODS: Female peripubertal mice were injected with dehydroepiandrosterone (DHEA) diluted in sesame oil for 20 consecutive days and orally treated with a saline solution of rhFSH. A control group received only sesame oil and saline solution. At the end of treatments, blood was analyzed for hormone concentrations and ovaries were processed for morphological analysis. The presumptive bioactive peptide was added during in vitro maturation of bovine COCs and the effects on cumulus expansion and on maturation rate were evaluated. RESULTS: DHEA treatment increased serum levels of testosterone, estradiol and progesterone as well as the percentage of cystic follicles. Orally administered rhFSH restored estradiol level and reduced the percentage of cystic follicles. Despite these results indicating a reduction of the severity of PCOS in the mouse model, the presumptive bioactive peptide did not mimic the effect of rhFSH and failed to induce bovine cumulus expansion and oocyte maturation in vitro. CONCLUSIONS: Although further studies are needed, the present data supports the concept that orally administrated FSH could attenuate some of the characteristic of PCOS in the mouse model.