Elevated estradiol-17β levels inhibit final oocyte maturation via G protein-coupled estrogen receptor (Gper) in yellowfin porgy, Acanthopagrus laws

Yellowfin porgy a protandrous teleost, exhibits asynchronous oocyte development and multiple spawning. Seasonal profiles of plasma estradio1-17 beta (E2) levels showed a peak in three-year-old females during the spawning season, when batches of fully-grown oocytes undergo final oocyte maturation (FOM). Because E2 has been shown to inhibit FOM via the G protein-coupled estrogen receptor (Gper) in several teleost species, we investigated the role of this "paradoxical" increase in E2 during FOM in yellowfin porgy. In vivo treatment with a GnRH-agonist stimulated germinal vesicle breakdown (GVBD) and increased E2 plasma levels, and ovarian cyp19a1a transcripts, confirming the increase in E2 production at the time of FOM. Ovarian transcripts of gper peaked at the time of FOM, indicating an increase in ovarian responsiveness to Gper-mediated E2 effects. In vitro, E2 and the Gper agonist, G-1, inhibited the stimulatory effect of maturation-inducing steroids (MIS) on GVBD, while an aromatase inhibitor enhanced the MIS effect, in agreement with a physiological inhibitory role of E2 on FOM via Gper. Immunohistological studies showed that the Gper protein was specifically located on the oocyte plasma membrane. Ovarian membranes displayed high-affinity and limited-capacity specific [H-3]-E2 receptor binding which was displaced by G-1, characteristic of Gper. Expression of gper increased at the time of FOM in mid-vitellogenic oocytes, but not in larger oocytes undergoing GVBD. These results suggest increases in both E2 production and E2 responsiveness via Gper upregulation in mid-vitellogenic oocytes, may maintain meiotic arrest in this oocyte stage class during the period when full-grown oocytes are undergoing FOM. This study indicates a critical involvement of E2 in the control of asynchronous oocyte maturation and the multiple spawning pattern in Sparidae.