Abstract
Oocyte in vitro maturation (IVM) is still a major challenge in human and animal assisted reproduction. Gradual instead of abrupt activation of the ovulatory cascade during IVM has been proposed to enhance nuclear-cytoplasmic synchrony and cumulus-oocyte communication, thus favoring oocyte developmental competence. Herein, we assessed the effects of neuregulin 1 (NRG1), an EGF-like factor that modulates EGFR signaling, on oocyte nuclear maturation dynamics, cumulus expansion and expression of mRNAs regulating these processes during IVM, as well as on post-IVF embryo development following AREG-stimulated IVM in cattle. In experiment 1, cumulus-oocyte complexes (COCs) were subjected to IVM with graded doses of NRG1 (1, 10 or 100 ng/mL) for 6, 9, 12, 20, and 24 h, after which oocyte nuclear status and cumulus mRNA expression were assessed. At 6 h of IVM, NRG1 at 1 ng/mL significantly decreased the percentage of GVBD (germinal vesicle breakdown) oocytes without altering later meiotic dynamics or the percentage of oocytes achieving meiosis II. In experiment 2, adding NRG1 (1 ng/mL) to the IVM medium did not affect cumulus expansion but increased the percentage of expanded and hatched blastocysts, and blastocyst total cell number following IVF/IVC. NRG1 decreased EGFR mRNA abundance while increasing NPR2 and PTX3 mRNA levels at 9 h, and TNFAIP6 mRNA abundance at 20 h of IVM. This is the first study that reports the modulatory effect of NGR1 during oocyte maturation in a mono-ovulatory species and demonstrates that this action may be applied during IVM to improve post-IVF embryo development.