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.

Trophoblast lineage specific expression of the alternative splicing factor RBFOX2 suggests a role in placental development.

INTRODUCTION: RBFOX2, an RNA-binding protein, controls tissue-specific alternative splicing of exons in diverse processes of development. The progenitor cytotrophoblast of the human placenta differentiates into either the syncytiotrophoblast, formed via cell fusion, or the invasive extravillous trophoblast lineage. The placenta affords a singular system where a role for RBFOX2 in both cell invasion and cell fusion may be studied. We investigated a role for RBFOX2 in trophoblast cell differentiation, as a foundation for investigations of RBFOX2 in embryo implantation and placental development. METHODS: Immunohistochemistry of RBFOX2 was performed on placental tissue sections from three trimesters of pregnancy and from pathological pregnancies. Primary trophoblast cell culture and immunofluorescence were employed to determine RBFOX2 expression upon cell fusion. Knockdown of RBFOX2 expression was performed with ßhCG and syncytin-1 as molecular indicators of fusion. RESULTS: In both normal and pathological placentas, RBFOX2 expression was confined to the cytotrophoblast and the extravillous trophoblast, but absent from the syncytiotrophoblast. Additionally, we showed that primary trophoblasts that spontaneously fused in cell culture downregulated RBFOX2 expression. In functional experiments, knockdown expression of RBFOX2 significantly upregulated ßhCG, while the upregulation of syncytin-1 did not reach statistical significance. DISCUSSION: RBFOX2, by conferring mRNA diversity, may act as a regulator switch in trophoblast differentiation to either the fusion or invasive pathways. By studying alternative splicing we further our understanding of placental development, yielding possible insights into preeclampsia, where expression of antiangiogenic isoforms produced through alternative splicing play a critical role in disease development and severity.