Hippo signaling cofactor, WWTR1, at the crossroads of human trophoblast progenitor self-renewal and differentiation.

Healthy progression of human pregnancy relies on cytotrophoblast (CTB) progenitor self-renewal and its differentiation toward multinucleated syncytiotrophoblasts (STBs) and invasive extravillous trophoblasts (EVTs). However, the underlying molecular mechanisms that fine-tune CTB self-renewal or direct its differentiation toward STBs or EVTs during human placentation are poorly defined. Here, we show that Hippo signaling cofactor WW domain containing transcription regulator 1 (WWTR1) is a master regulator of trophoblast fate choice during human placentation. Using human trophoblast stem cells (human TSCs), primary CTBs, and human placental explants, we demonstrate that WWTR1 promotes self-renewal in human CTBs and is essential for their differentiation to EVTs. In contrast, WWTR1 prevents induction of the STB fate in undifferentiated CTBs. Our single-cell RNA sequencing analyses in first-trimester human placenta, along with mechanistic analyses in human TSCs revealed that WWTR1 fine-tunes trophoblast fate by directly regulating WNT signaling components. Importantly, our analyses of placentae from pathological pregnancies show that extreme preterm births (gestational time ≤28 wk) are often associated with loss of WWTR1 expression in CTBs. In summary, our findings establish the critical importance of WWTR1 at the crossroads of human trophoblast progenitor self-renewal versus differentiation. It plays positive instructive roles in promoting CTB self-renewal and EVT differentiation and safeguards undifferentiated CTBs from attaining the STB fate.