Expression of gap junction protein connexin-43 in the human and baboon (Papio anubis) corpus luteum

In the nonhuman primate and human corpora lutea, gap junctions have been identified by means of electron microscopy. Gap junctions are formed by connexons, which consist of a multigene family of tissue-specific connexins. In the ovarian follicle, the gap junction protein connexin-43 is present and hormonally regulated. However, there is little evidence indicating the type of connexin present in the corpus luteum. Therefore, the aim of this study was to demonstrate the presence of gap junctions by electron microscopy and the presence of connexin-43 and messenger ribonucleic acid (mRNA) for this protein. Using immunocytochemical procedures, we have shown the presence of connexin-43 in baboon and human midluteal phase corpora lutea and in the atretic corpora lutea of the baboon. The intensity of immunoreactivity was lower in atretic corpora lutea than in the midluteal phase corpora lutea. Western analysis indicates the presence of two bands at 43-45 kDa, and that the levels of connexin-43 protein are abundant in the midluteal phase. The two bands suggest the presence of the protein in a phosphorylated or a nonphosphorylated form. Ribonuclease protection assay suggests that the mRNA levels of connexin-43 remain constant throughout the luteal phase. mRNA for connexin-43 was not detectable in atretic corpora lutea. Thus, connexin-43 is one of the connexin family of proteins forming the connexon of gap junctions in the baboon and human corpus luteum. The expression of the protein may be hormonally regulated by locally produced factors, such as estradiol and progesterone. We suggest that gap junctional communication between the cells of the primate and human corpus luteum may be important in hormone synthesis and secretion and may be involved in the process of luteolysis through luteal cell apoptosis.