Progesterone receptors on human spermatozoa.

Progesterone, primarily recognized as a female steroid hormone, is reported to affect several sperm functions especially capacitation, motility and acrosome reaction. These effects of progesterone on the spermatozoa are mediated via the progesterone binding sites/progesterone receptor (PR) on the acrosomal membrane. These receptors in response to progesterone increase the intercellular Ca2+ levels and stimulate Ca2+ influx in the mature human spermatozoa via non-genomic mode of actions. Characterization of this receptor reveals that the sperm PR is masked protein and is exposed to the surface by some non-ionic detergents. Localized on to the acrosome region of the spermatozoa, these receptors are recognized by most antibodies directed towards the C-terminal region of the conventional PR. The estimated molecular weight of PR on spermatozoa varies from 27 kDa to 85 kDa. At the molecular level, sequences encoding for the entire DNA and hormone binding domains of the conventional PR are detected in the mRNA derived from spermatozoa. No insertions, deletions or mutations are detected in this region. These results are suggestive of the fact that at least the C terminal region of the conventional PR is conserved in the sperm. It is hypothesized that post-translational modifications or peptide splicing of the conventional PR in spermatozoa may possibly lead to the variant of the steroid hormone receptor. Detailed characterization of the sperm PR will be important in understanding the alternate non-genomic mode of action of steroid hormone receptors.