Optogenetic-Programmable Human Embryo Models Elucidate the Interplay Between Tissue Architecture and BMP4 Signaling at the Onset of Gastrulation

The spatiotemporal regulation of morphogenetic signals, along with their interaction with local tissue mechanics, guides morphogenesis and determines the shape and form of the embryo (Valet et al., 2022 PMID: 34754086; Pfeifer et al., 2024 PMID: 38181658). However, how these signals precisely integrate into developmental circuits remains poorly understood. Here, we took advantage of a light-inducible strategy (De Santis et al., 2021 PMID: 34799555) to induce BMP4 signaling with precise spatial coordinates to study epiblast symmetry breaking and its interplay with tissue mechanics at the onset of gastrulation. Light-controlled BMP4 induced SMAD1-5 phosphorylation, resulting in the differentiation of extra-embryonic amnion cells and relied on a tension-dependent induction of the WNT (Muncie et al., 2020 PMID: 33207224) and NODAL pathways for mesoderm and endoderm differentiation. In micropattern gastruloids (Warmflash et al., 2014), tissue tension signals through YAP1 (Totaro et al., 2018, PMID: 30050119). In response to BMP4 stimulation, YAP1 localizes to the nucleus where it represses WNT3 mRNA (Estras et al., 2017, PMID: 29269485), thereby regulating the induction of the three germ layers. Based on these findings, we developed a new mathematical model that integrates tissue mechanics into morphogen dynamics, quantitatively explaining cellular and tissue-scale responses to BMP4 signaling. In all, light induction of the morphogen BMP4 in human embryo models unveiled how embryonic patterning is organized in relation to specific tissue architecture and mechanics at the onset of gastrulation. Opto-BMP4 hESCs were light induced in micropatterns (edge-activated) or in regular culture in absence of geometrical confinment (center-activated) and analyzed 48 hours later by scRNA-seq