Functional characteristics of membrane progestin receptor alpha (mPRα) subtypes: A review with new data showing mPRα expression in seatrout sperm and its association with sperm motility

A novel cDNA with characteristics of the membrane progestin receptor regulating induction of oocyte maturation by a non-genomic mechanism, named st-mPRα, was recently discovered in seatrout. Subsequently, both recombinant and native mPRαs have been localized to the plasma membrane in several vertebrate models where they have been shown to bind progestins specifically, resulting in activation of G proteins. Non-genomic actions of progestins to stimulate hypermotility and the acrosome reaction in sperm have been identified, but the receptors which mediate these processes are unknown. Here, we demonstrate progestin stimulation of sperm motility in seatrout and expression of st-mPRα mRNA and protein in sperm with the receptor localized on the plasma membrane. Immunocytochemical staining of non-permeabilized sperm shows st-mPRα is localized to the midpiece with an extracellular N-terminal region, indicating its likely role in progestin regulation of sperm motility. Moreover, the abundance of the st-mPRα protein on sperm membranes from seatrout donors with low motility was significantly reduced compared to that of normal motile sperm. Finally, progestin treatment of sperm membranes caused activation of G proteins. These results suggest that st-mPRα is an intermediary in progestin stimulation of sperm motility in seatrout by an unknown mechanism involving G protein activation.