Endometrial connexin expression in the mare and pig: evidence for the suppression of cell-cell communication in uterine luminal epithelium.
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This investigation examines the relationship between implantation strategy and gap junction protein expression in uterine endometrium. The pattern of gap junction and connexin protein expression was analyzed in porcine and equine endometrium from cycling and pregnant animals using electron microscopy and immunocytochemistry. Functional analysis of cell-cell communication was also monitored by laser cytometry in primary cultures of endometrial epithelial cells. Gap junctions were detected in endometrial stroma of cycling and pregnant animals, which was correlated with immunoreactive Cx43 within stromal fibroblasts and vascular elements. No Cx26, Cx32, or Cx43 immunostaining was detected in luminal endometrial epithelium in either the mare or the pig at any stage of the estrous cycle or pregnancy. In contrast, endometrial glands of the mare exhibited a spatiotemporal pattern of Cx43 expression in the apicolateral plasma membrane which, when present, colocalized with the tight junction-associated protein, ZO-1. Uterine glandular Cx43 expression in mares was present from day 3 postovulation through day 14 of diestrus and until day 23 of pregnancy, whereas Cx43 was absent within uterine glands during seasonal anestrus, estrus, and after day 30 of pregnancy. Primary cultures of equine endometrial epithelial cells expressed both immunoreactive Cx43 and significant gap junction-mediated intercellular communication (GJIC) which was rapidly upregulated by 1.0 mM 8-bromo-cAMP or blocked with 1.0 mM octanol. No GJIC or connexin protein was detected in cultured porcine epithelial cells despite incubation with a variety of agents, including 8-bromo-cAMP, steroid hormones, retinoic acid, and/or prolactin. Junctional communication in endometrial epithelium of domestic farm animals is different than that reported for species exhibiting invasive implantation. The absence of GJIC in uterine luminal epithelium of the gilt and mare may be involved in limiting trophoblast invasiveness.