Abstract
In response to the gonadotropin surge, the compact cumulus-oocyte complex (COC) undergoes expansion by synthesis of the mucopolysaccharide hyaluronan (HA) accompanying oocyte maturation. The objective of the present study was to quantify mRNA transcripts of the HA synthase (HAS) 1, HAS2, and HAS3 and the HA-receptors CD44 and RHAMM (receptor for HA-mediated motility). Additionally, we determined the histological localization of HA and its receptor, CD44, in maturing bovine COCs and cultured granulosa cells (GCs). Full-length transcript of bovine HAS2 and a part of the bovine RHAMM sequence has been made available. Real-time reverse transcriptase-polymerase chain reaction was used for individual mRNA expressions of bovine COCs in comparison to follicular GC gonadotropin treatment. Localization of CD44 and HA were done by immunohistochemistry and biotinylated HA-binding protein, respectively. Gonadotropins caused a rapid, 120-fold increase of HAS2 mRNA, whereas a delayed, 2-fold up-regulation of HAS3 mRNA was observed. The HAS1 transcripts were barely detected. Expression of CD44 mRNA greatly increased during in vitro maturation of COCs, indicating an important role when compared to an unchanged, steady-state RHAMM expression. As a consequence, HA was locally enriched after COC expansion, but only limited change was observed in the GCs. In cultured GCs, HAS2 expression was stimulated through FSH application, followed by the effective treatments of FSH+LH and LH. Treatment with LH induced the highest increase of the CD44 receptor, followed by FSH and FSH+LH treatments. These results suggest that HAS2 is mainly responsible for rapid HA synthesis in bovine COCs and GCs. In bovine COCs, the transcriptional up-regulation of both HAS2 and the receptor CD44 appear to be important prerequisites for initiating HA-mediated effects during final oocyte development and sperm-egg interaction.
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