Mechanisms of progestin-stimulated sperm hypermotility in two teleosts: the Atlantic croaker (Micropogonias undulatus) and the southern flounder (Platylicthys lethigstomata)
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The goal of this research was to examine the role of the novel membrane progestin receptor alpha (mPR[alpha]) in the stimulation of sperm hypermotility by the progestin 17,20[beta],21-trihydroxy-4-pregnen-3-one (20[beta]-S) in two teleosts; the Atlantic croaker (Micropogonias undulatus) and the southern flounder (Platylicthys lethigstomata). In croaker, the expression, localization and hormonal regulation of mPR[alpha] in testis and sperm were investigated, as were the intracellular signaling pathways activated by 20[beta]-S and mPR[alpha] to induce croaker sperm hypermotility. In flounder, stimulation of sperm hypermotility by 20[beta]-S and binding of this steroid to flounder sperm membranes were examined. Finally, expression of mPR[alpha] was investigated in flounder testes and the expression and localization of this receptor in flounder testis and sperm was examined. In croaker sperm, mPR[alpha] was expressed on the plasma membrane and localized to the midpiece. Expression of mPR[alpha] was also shown to be associated with high sperm motility and regulated by gonadotropin. The signaling pathways activated by 20[beta]-S in croaker sperm were shown to involve activation of olfactory G-proteins (Golf). Subsequent activation of membrane adenylyl cyclases was also demonstrated and shown to be necessary for 20[beta]-S-stimulated cAMP production and 20[beta]-S-induction of sperm hypermotility. Furthermore, co-immunoprecipitation studies show mPR[alpha] and Golf physically associate with one another, establishing mPR[alpha] as the mediator of 20[beta]-S actions in croaker sperm. Finally, evidence was obtained for progestin-stimulation of sperm hypermotility and the presence of mPR[alpha] on sperm membranes in another marine teleost species belonging to a different family, the southern flounder. In addition, mPR[alpha] was shown to be expressed on flounder sperm membranes and also localized to the sperm midpiece. Results from the following studies support the hypothesis that mPR[alpha] is the mediator of 20[beta]S-stimulated sperm hypermotility in croaker and is a likely intermediary in southern flounder. Furthermore, these data provide a plausible mechanism by which 20[beta]-S and mPR[alpha] stimulate croaker sperm hypermotility. In addition, these results provide the first evidence of hormonal activation of Golf proteins for any species. Finally, mPR[alpha]-mediated mechanisms to increase sperm motility are suggested to be evolutionarily conserved in teleosts since they also likely exist in a non-sciaenid species, the southern flounder.