Membrane and Nuclear Progestin Receptors in Oocyte Maturation of Zebrafish

Abstract

Progestins induce final oocyte maturation in fish and amphibian species, but the identity of the receptor(s) mediating these progestin actions has been a subject of heated debate. We have examined the functions and signaling of zebrafish progestin receptors in both mammalian cell lines and zebrafish germ cells, in order to establish models for studying the physiological actions and molecular mechanisms of nuclear progestin (nPR) and membrane progestin receptors (mPRα and mPRα). Rapid, progestin mediated signaling via inhibition of cAMP and activation of MAPK was observed in mPRα and mPRα transfected mammalian cell lines deficient for nuclear progesterone receptors, while overexpression of nPR increased progestin induced transcriptional activities as measured by a progesterone responsive reporter assay. Immunohistochemical analysis with progestin receptor specific antibodies revealed both similar and distinct localization of mPRs and nPR in the major reproductive tissues of the ovary, testis and pituitary. In the ovary, mPRα and mPRα expression is found at or near the oocyte membrane and in the follicle layer surrounding oocytes, while nPR expression is constricted to the follicle layer. Microinjection of either mPRα or nPR into late stage oocytes significantly accelerated the rate of oocyte maturation, implying a potential role of both receptors in induction of final oocyte maturation. However, the increase in oocyte maturation induced by overexpression of nPR was blocked by the transcriptional inhibitor Actinomycin D, while the mPRα mediated increase was unaffected. Our findings suggest that actions of mPRs and nPR in zebafish oocytes are distinct and mediated by two separate signaling pathways (genomic for nPR and rapid non‐genomic signaling for mPRs).