Reciprocal coupling between EMT and BMP morphogen signalling drives collective cell invasion

During collective cell invasion, epithelial-to-mesenchymal transition (EMT) and morphogen signalling-mediated cell fate specification are traditionally viewed as a linear cascade: morphogens drive cell fates that activate EMT gene expression programs. Here, we uncover reciprocal coupling between EMT initiation and BMP signalling mediated by SNAI2 and SMAD1. Using ex vivo chick tissue explants, we demonstrate that substrate-induced migration upregulates SMAD1 expression, activating BMP signalling across germ layers. Single-cell RNA sequencing reveals SNAI2 and SMAD1 co-expression in EMT initiation regions, and SNAI2 overexpression is sufficient to induce ectopic SMAD1 expression in vivo. Ex vivo CRISPR-Cas9 experiments further demonstrate that SNAI2 is required for SMAD1 expression and BMP signalling. While BMP signalling is dispensable for EMT initiation, it regulates cell fate proportions, dispersal dynamics, precursor region depletion rates, and migration directionality. This coupling provides a mechanism for synchronising cell fate specification with invasion progression during axis elongation, positioning SNAI2-SMAD1 interaction as mechanism to coordinate cell fate specification and EMT during collective cell invasion.