The syncytiotrophoblast is formed by fusion of the underlying cytotrophoblasts, and among its many tasks synthesizes hormones and steroids. Yet, studies of syncytiotrophoblast transcription are puzzling, demonstrating that most of the nuclei in the multinucleated syncytium are transcriptionally inactive. To further elucidate RNA activity in the sycytiotrophoblast we investigated spliceosome snRNAs, U6 and U2. We found, using RNA in situ hybridization with U6 and U2 LNA DIG labeled probes, that snRNAs were downregulated in the syncytium throughout the span of pregnancy as well as in placental pathologies such as complete hydatidiform mole, persistent trophoblastic disease, preeclampsia and choriocarcinoma. U2 was downregulated temporally as the villi differentiated.

Using the choriocarcinoma cell line, JEG-3, we established a triple fluorescent staining assay, detecting cell membrane E-Cadherin, the nucleus and the snRNAs in situ. U2 and U6 appeared in a speckled nuclear pattern with nucleoli sparing and differential distribution during the cell cycle. This speckled pattern is very similar to the diffusely distributed speckles of interchromatin granule clusters (IGCs). The IGCs contain splicing factors and functionally are known as centers for the storage, assembly and modification of the splicing machinery necessary to accommodate active transcription sites which organize at the periphery of the speckles.

Our findings of downregulation of snRNA in the syncytiotrophoblast adds another dimension to the enigma of a transcriptionally inactive but translationally active syncytium. Furthermore, the distribution of the nuclear speckles in trophoblasts may shed light on dynamic assembly of nuclear organelles during cytotrophoblast fusion and multinucleated syncytium differentiation.

Authors

Debra Goldman-Wohl, Caryn Greenfield, Galia Skarzinski#, Ronit Haimov-Kochman, Tal Imbar, Ilana Ariel^# and Simcha Yagel
Center for Human Placenta Research, Department of Obstetrics and Gynecology and Department of Pathology^#, Hadassah University Hospital, Mt. Scopus, Jerusalem