snRNAs are reduced in the syncytiotrophoblast: a possible mechanism for regulation of human placental protein production.

The multinucleate syncytiotrophoblast of the human placenta is formed by fusion of the underlying cytotrophoblast progenitor cells. The large surface area of the syncytiotrophoblast is necessary for transport functions while it also serves as the site of synthesis of hormones and steroids. Studies of syncytiotrophoblast transcription are puzzling, demonstrating that many of the nuclei in the multinucleated syncytium are transcriptionally inactive. To further elucidate RNA activity in the syncytiotrophoblast, we investigated expression of snRNAs involved in RNA splicing. Using RNA in situ hybridization, we observed that snRNAs were markedly reduced in the syncytium throughout the course of pregnancy. Recapitulating these results in primary trophoblasts and in trophoblast cell lines in vitro, we found, using qRT-PCR and RNA in situ hybridization, that snRNA expression is reduced in trophoblasts cultured under fusion conditions. Our finding that snRNA is markedly reduced in the syncytiotrophoblast suggests that the placenta has evolved a balance between the large surface area essential for its transport function and the need to regulate protein production in the multinucleated syncytium.