Gonadotropin-Releasing Hormone and Chorionic Gonadotropin Gene Expression in Human Placental Development

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Abstract
Control of human chorionic gonadotropin (hCG) synthesis during pregnancy is poorly understood, although in vitro data suggest a role for placental gonadotropin releasing hormone (GnRH) in its regulation. To study GnRH regulation during placental development, placental tissue of different gestational ages was analyzed for GnRH and βhCG mRNA content. cRNA probes to exonic/intronic sequences of GnRH and βhCG transcripts were constructed and used to perform solution hybridization/nuclease protection and in situ hybridization assays. The levels of GnRH mRNA were approximately 0.1–1% of that of βhCG mRNA, in agreement with its suggested paracrine, rather than endocrine, role. While βhCG mRNA content decreased significantly from first trimester to term (643 to 21.6 pg/μg RNA), there was no significant change in GnRH mRNA (0.179 to 0.155 pg/μg RNA). While βhCG mRNA was localized almost exclusively in syncytiotrophoblasts, GnRH mRNA was present in all cell types of the placenta, including the stroma. In the course of performing sense-strand controls in the in situ hybridization, we noted that the placenta appeared to express more antisense GnRH than sense GnRH mRNA, again, in all cell types. Solution hybridization/nuclease protection analysis with exon 1 and exon 3 probes confirmed this observation, showing that there is two to three times more antisense GnRH RNA than sense GnRH mRNA. These studies suggest that GnRH gene expression and its role in regulating hCG production in human placenta is complex and does not fit a simple model for paracrine regulation of hCG.