310 COBALAMIN SUPPLEMENTATION DURING IN VITRO MATURATION IMPROVES PREIMPLANTATION DEVELOPMENT OF SHEEP EMBRYOS

Abstract

Oocyte maturation process includes the establishment of proper epigenetics marks, fundamental to ensure successful pregnancy. Epigenetic maturation of the oocyte depends on one-carbon-metabolism (OCM), whose key elements (cobalamin, folate) are crucial cofactors for the transfer of methyl groups onto chromatin. Commercially available IVM-media only partially supports oocyte metabolic requirements, and thus may negatively affect epigenetic maturation. Of relevance, cobalamin, one of the OCM cofactors normally present in follicular fluid, is missing in IVM-media. We investigated if cobalamin supplementation of IVM media affects sheep oocyte developmental competence in term of subsequent embryo development, epigenetic pattern, and fetal survival. Briefly, sheep oocytes were isolated from ovaries and divided into 2 groups: an untreated control (in vitro CTR group) and oocytes in vitro matured in medium containing cobalamin at 200 p.m. (Cobalamin group). Following maturation, MII oocytes were in vitro fertilized and cultured until blastocyst stage. A cohort of blastocysts was surgically transferred to recipient ewes and then conceptuses were collected at 20 days of pregnancy. Naturally mated conceptuses were also collected (in vivo CTR group). In vitro-matured oocytes and -derived blastocysts were analysed (i) by immunofluorescence anti-5-methylcitydine and (ii) by qRT-PCR for the expression of a panel of genes (MATb, ACHY, CBS, MTHFR, DNMT1) involved in OCM pathway. Immunofluorescence results were analysed by Image J software. Decimal variables were analysed using the Mann-Whitney test, while variables were expressed as percentage with the Fisher exact test. Cobalamin exposure during IVM significantly increased (i) cleavage rate (cobalamin 130/220 v. in vitro CTR 134/191, P < 0.02) following in vitro fertilization and (ii) global DNA methylation in blastocyst-stage embryos (P < 0.05). Then, qRT-PCR analysis of a select panel of OCM genes following IVM supplemented with cobalamin revealed a downregulation of MATb, ACHY, and DNMT1 in cobalamin-treated oocytes v. in vitro CTR group (P < 0.05), while no differences were observed at blastocyst stage. Finally, we found that the survival rate at 20 days of pregnancy was comparable in in vitro-produced (IVP) embryos from cobalamin and in vitro CTR oocytes, but reduced compared to in vivo CTR (cobalamin 60%, in vitro CTR 72%, and in vivo CTR 100%). Altogether, our results showed that cobalamin supplementation in IVM medium positively affects competence of oocytes and methylation profile at the blastocyst stage, presumably through the OCM pathway. Moreover, postimplantation survival of IVP conceptus, derived from untreated and treated oocytes, was reduced compared to naturally mated ones. Further investigation will clarify whether cobalamin supplementation influences fetal and placental development of IVP conceptus.