Role of PTGS2-generated PGE2 during gonadotrophin-induced bovine oocyte maturation and cumulus cell expansion.

Prostaglandin E2 (PGE2) is an autocrine/paracrine factor which mediates gonadotrophin (Gn) stimulation of cumulus expansion and oocyte maturation in rodents. Its role in bovine oocyte maturation is less characterized. This study detected PTGS2 (COX2) and PGE synthases (PTGES1, PTGES2 and PTGES3) in bovine cumulus-oocyte complexes (COC). Only PTGS2 and PTGES1 expression changed during maturation. In Gn-free media, no cumulus expansion and ∼45% nuclear maturation was achieved, while Gn-induced maturation showed full cumulus expansion (score 3) and ∼87% maturation. PGE2 supplementation without Gn induced mild cumulus expansion (score 0.5-1) but increased nuclear maturation to levels similar to those obtained with Gn alone. In the presence of Gn, exogenous PGE2 did not affect expansion or nuclear maturation and subsequent embryo development. Treatment with PTGS2 selective inhibitor (NS398), PTGS2-specific siRNA or PTGER2-receptor antagonist (AH6809) resulted in ∼20-25% reduction in nuclear maturation. NS398 and AH6809 did not affect cumulus expansion. Most oocytes not reaching metaphase of second meiosis (MII) following NS398, AH6809 and PTGS2-specific siRNA treatments were at MI. After longer maturation, NS398-treated oocytes had normal MII rate and uncompromised embryo development. PGE2 has a limited role in cumulus expansion in bovine COC but is important for the timing of Gn-induced nuclear maturation. We confirmed that genes involved in the synthesis of prostaglandin E2 (PGE2) are expressed by cumulus-oocyte complexes (or eggs) of cows and that PGE2 is synthesized during oocyte maturation in the presence of gonadotrophin hormones. When we inhibited synthesis of PGE2 or blocked its receptors, oocyte maturation, but not cumulus expansion, was compromised. Further investigation showed that oocyte maturation is delayed but not arrested when PGE2 synthesis is inhibited. On the other hand, addition of exogenous PGE2 induced a high maturation rate and mild cumulus expansion only in the absence of gonadotrophin stimulation, and had no effect in the presence of gonadotrophin.