Critical role of hyaluronidase-2 during preimplantation embryo development

Biological functions of hyaluronan (HA) depend on its molecular size. Small size HAs are known to regulate cell proliferation; however, the expression of HA synthases (HASs) and hyaluronidase-2 (HYAL2) and their role during early embryo development have not been previously identified. In this paper, we have shown by immunostaining that HA is produced by bovine in vitro-produced embryos at all stages of early development to blastocyst. Addition of HA-synthesis inhibitor (4-methylumbelliferone; 4MU) to in vitro embryo culture inhibited blastocyst formation. HASs HAS2 and HAS3 mRNA were expressed at all stages of embryo development; however, relative mRNA expression of HAS2 was significantly reduced as the embryos develop to the blastocyst stage. HAS1 was detected during 2- and 4-cell stages but was barely detectable in subsequent stages. HYAL2 mRNA expression was detected in oviducts at the early luteal phase but was only detected in the embryos at morula and blastocyst stages (Day 6 and 7 post-fertilization). Addition of HYAL2 to embryo culture media at Day 2 post-fertilization increased phosphorylated mitogen-activated protein kinases (MAPK1 and 3) in the embryos and improved development to the blastocyst stage and increased embryo cell numbers. Addition of an anti-CD44 antibody or an MAPK inhibitor (U0126) abrogated the positive effects of HYAL2 on blastocyst rates. In conclusion, we demonstrate that the expression of different HAS genes and HYAL2 in bovine embryos varies according to the stage of development and that the supplementation of HYAL2 in vitro mimics oviductal conditions and is shown to improve the blastocyst rate and embryo quality, an effect which requires CD44 activity and MAPK signalling.