Expression and gonadotropin regulation of membrane progestin receptor alpha in Atlantic croaker ( Micropogonias undulatus) gonads: Role in gamete maturation

Abstract

Recent results suggest that membrane progestin receptor alpha (mPRα) mediates nongenomic actions of progestin hormones to induce oocyte maturation and sperm hypermotility in several teleost species. The role of mPRα in gamete and gonadal physiology was further evaluated in the present study by examining gonadal expression of mPRα during gamete maturation in Atlantic croaker ( Micropogonias undulatus), a well-characterized teleost model of oocyte maturation and sperm motility. Sequencing of the croaker mPRα gene isolated from croaker ovaries showed it is 98% homologous at the nucleotide level to spotted seatrout mPRα. The mPRα mRNA and protein were detected in both somatic and gonadal tissues. In croaker ovaries, the mPRα protein was present throughout the gonadal cycle and was upregulated by gonadotropin in vitro, coincident with the acquisition of oocyte maturational competence (i.e., ability to respond to progestin hormones and complete oocyte maturation). Both mPRα mRNA and protein were also expressed in croaker testes throughout the gonadal cycle. Expression of mPRα protein was weakly upregulated in testes after 18 h of in vitro gonadotropin treatment. Immunocytochemical staining showed mPRα was localized to both germ and interstitial cells. Finally, elevated levels of mPRα protein in croaker sperm were associated with high sperm motility. Taken together, these data strongly support the hypothesis that mPRα mediates progestin induction of oocyte maturation and upregulation of sperm motility in teleosts.