Crosstalk between tissue mechanics and BMP4 signaling regulates symmetry breaking in human gastrula models

The spatiotemporal regulation of morphogenetic signals, along with local tissue mechanics, guides morphogenesis and determines the shape of the embryo. However, how these signals integrate into developmental circuits remains poorly understood. Here, we developed a light-inducible strategy to induce BMP4 signaling with precise spatial coordinates in human pluripotent stem cells. Light-controlled BMP4 induces SMAD1-5 phosphorylation, resulting in amnion differentiation, and relies on a tension-dependent induction of WNT and NODAL for mesoderm differentiation. In response to BMP4 signaling, the mechanosensitive transcription factor YAP1 accumulates in the nucleus, where it represses WNT3 mRNA, regulating the induction of the three germ layers. Based on these findings, we developed a mathematical model that integrates tissue mechanics into morphogen dynamics, quantitatively explaining tissue-scale responses to BMP4 signaling. Thus, light induction of the morphogen BMP4 in human stem cell models elucidated the interplay between tissue mechanics and signaling at the onset of gastrulation.