TFEB controls expression of human syncytins during cell-cell fusion

During human development, a temporary organ is formed, the placenta, which invades the uterine wall to support nutrient, oxygen, and waste exchange between the mother and fetus until birth. Most of the human placenta is formed by a syncytial villous structure lined by syncytialized trophoblasts, a specialized cell type that forms via cell-cell fusion of underlying progenitor cells. Genetic and functional studies have characterized the membrane protein fusogens Syncytin-1 and Syncytin-2, both of which are necessary and sufficient for human trophoblast cell-cell fusion. However, identification and characterization of upstream transcriptional regulators regulating their expression have been limited. Here, using CRISPR knockout in an in vitro cellular model of syncytiotrophoblast development (BeWo cells), we found that the transcription factor TFEB, mainly known as a regulator of autophagy and lysosomal biogenesis, is required for cell-cell fusion of syncytiotrophoblasts. TFEB translocates to the nucleus, exhibits increased chromatin interactions, and directly binds the Syncytin-1 and Syncytin-2 promoters to control their expression during differentiation. Although TFEB appears to play a critical role in syncytiotrophoblast differentiation, ablation of TFEB largely does not affect lysosomal gene expression or lysosomal biogenesis in differentiating BeWo cells, suggesting a previously uncharacterized role for TFEB in controlling the expression of human syncytins.