Debra Goldman-Wohl¹, Caryn Greenfield¹, Iris Eisenberg¹, Galia Skarzinski², Tal Imbar¹, Ronit Haimov-Kochman¹, Polina Denichenko³, Rotem Karni³, Yaron Hamani¹, Ilana Ariel² and Simcha Yagel¹

¹The Magda and Richard Hoffman Center for Human Placenta Research, Department of Obstetrics and Gynecology;²Department of Pathology and ³Department of Biochemistry and Molecular Biology,IMRIC,Hadassah-Hebrew University Medical Center, Jerusalem, Israel.

Introduction: The successful establishment and maintenance of pregnancy relies on a series of complex interactions between the developing fetus and maternal environment. Among these, placentation is a key process supporting the survival and growth of the fetus during gestation. The progenitor cytotrophoblast of the human placenta can differentiates into one of two main pathways forming either the syncytiotrophoblast or the invasive extravillous trophoblast (EVT). The diversity of mRNA essential for cell differentiation and development is mediated in part by post transcriptional alternative splicing coding for protein isoforms. Rbfox2, an alternative splicing RNA binding protein, plays a major role in controlling myoblast fusion. Interestingly, Rbfox2 also regulates invasive EMT (epithelial to mesenchymal transition). We suggest that Rbfox2 may act as a molecular switch directing cytotrophoblast differentiation to either syncytiotrophoblst formation or EVT.
Methods: We screened publicly available data for trophoblast cell lineage expression of RNA binding proteins, splicing and alternative splicing proteins. Immunohistochemistry for Rbfox2 was performed on three trimesters of normal placenta sections, preeclampsia, complete hydatidiform mole and placenta accreta.  Rbfox2 immunofluorescence was performed on isolated primary trophoblasts differentiated to syncytiotrophoblast.
Results: Rbfox2 is expressed in a trophoblast lineage specific manner in both normal and pathological placentas. Rbfox2 is expressed in the progenitor epithelial cell cytotrophoblast, reduced in the terminally differentiated syncytiotrophoblast (formed by cell fusion) and conversely, highly expressed in the invasive trophoblasts, known to express mesenchymal cell markers.  Primary trophoblast differentiated to syncytium in cell culture, produced bHcg, lost e-cadherin expression at cell borders as an indicator of cell fusion and displayed reduced Rbfox2 expression.
Conclusions: Rbfox2, by conferring mRNA diversity, may act as a master regulator switch in trophoblast differentiation to either the fusion or invasive pathways. Hence, understanding the functional role of Rbfox2 during placenta development will enable us to propose therapeutic strategies aiming at improving reproductive success.