Cloning, pharmacological characterization and expression analysis of Atlantic cod (Gadus morhua, L.) nuclear progesterone receptor.

To better understand the role(s) of progesterone in fish spermatogenesis, we cloned the nuclear progesterone receptor (Pgr) of Atlantic cod. The open-reading frame of the cod pgr consists of 2076 bp, coding for a 691-amino acids-long protein that shows the highest similarity with other piscine Pgr proteins. Functional characterization of the receptor expressed in mammalian cells revealed that the cod Pgr exhibited progesterone-specific, dose-dependent induction of reporter gene expression, with 17α,20β-dihydroxy-4-pregnen-3-one (DHP), a typical piscine progesterone, showing the highest potency in activating the receptor. During ontogenesis, the pgr mRNA was undetectable in embryo's 24 h after fertilization, but became detectable 4 days after fertilization. During the larval stage, the expression levels increased steadily with the development of the larvae. In adult fish, pgr was predominantly expressed in gonads of both sexes. During the onset of puberty, testicular pgr transcript levels started to increase during rapid spermatogonial proliferation, and peaked when spermiation started. In situ hybridization studies using testis tissue during the rapid growth phase containing all germ cell stages indicated that in cod, pgr mRNA is predominantly located in Sertoli cells that are in contact with proliferating spermatogonia. Taken together, our data suggests that the Pgr is involved in mediating progestagen stimulation of the mitotic expansion of spermatogonia, and in processes associated with the spermiation/spawning period in Atlantic cod.