Early gestational expression of retinol-binding protein mRNA by the ovine conceptus and endometrium

Communication between the mother and the early developing embryo is mediated by a variety of signals secreted by either the uterus or the embryo to elicit a response from the other. These signals include prostaglandins, proteins, and steroids. Recently, retinol-binding protein (RBP) has been described as a product of both the conceptus and endometrium in several species. Utilizing a cDNA clone to bovine RBP, we have described RBP mRNA expression in the endometrium, early conceptus, and extraembryonic membranes of sheep. Endometrial RBP mRNA expression did not differ between samples collected on day 13 of the estrous cycle and early pregnancy. In cyclic animals, RBP mRNA expression decreased two-fold between days 13 and 16, presumably a result of luteal regression and the consequential withdrawal of progesterone. In pregnant animals, endometrial RBP mRNA expression likewise decreased between days 13 and 16 and remained at this reduced level through day 30, despite the presence of a functional corpus luteum. Initiation of embryonic RBP expression appeared to coincide with early stages of blastocyst elongation at day 13. Levels of expression increased dramatically with conceptus development, peaked on day 23, and declined afterwards. Results from restriction enzyme analysis of genomic DNA indicated that RBP was encoded by a single gene per haploid genome. Differences in the temporal and tissue-specific expression of the protein, despite the apparent utilization of a single gene, suggest complex regulation of RBP gene expression.