Broad tissue expression of membrane progesterone receptor Alpha in normal mice

The broad tissue distribution of membrane progesterone receptor alpha (mPR alpha) in vertebrates suggests multiple physiological functions of the receptor. Current knowledge regarding the receptor distribution, however, is largely obtained via non-histological assays. In this study, the tissue distribution of mPR alpha in mice of both sexes was described using both histological and non-histological methods. Immunohistochemical analysis revealed that abundant expression of mPR alpha was consistently detected in the cytoplasm and membrane of smooth muscles in vasculatures, gastro-intestines, and uterus. It was also observed in myoepithelial cells of mammary gland and intra-ovarian myofibroblasts. These findings suggest that mPR alpha may function as a mediator of P4 in regulating function of smooth muscles or smooth muscle-like cells in numerous physiological processes such as vasodilation, transportation of contents within luminary organs, relaxation of the uterine myometrium during pregnancy, release of oocytes, and milk secretion. In addition, strong mPR alpha expression was identified in the parietal cells of gastric glands, indicating the potential roles of P4/mPR alpha signaling in the modulation of gastric acid secretion. Surprisingly, in the testis of male mice mPR alpha was mainly seen in the nuclei, rather than cytoplasm and/or membrane, of the primary and secondary spermatocytes, suggesting a direct role of the receptor in gene regulation. Our results indicate that mPR alpha may function as a key modulator of P4 in the modulation of multiple physiological functions in normal mice.