Mechanically Activated snai1b Coordinates the Initiation of Myocardial Delamination for Trabeculation

During development, myocardial contractile force and intracardiac hemodynamic shear stress coordinate the initiation of trabeculation. While Snail family genes are well-recognized transcription factors of epithelial-to-mesenchymal transition (EMT), snai1b-positive (+) cardiomyocytes (CMs) are sparsely distributed in the ventricle of zebrafish at 4 days post-fertilization (dpf). Isoproterenol (ISO) treatment significantly increased the number of snai1b+ CMs, of which 80% were Notch-negative. CRISPR-activation of snai1b led to 51.6% CMs forming trabeculae, whereas CRISPR-repression reduced trabecular CMs to 6.7% under ISO. In addition, 36.7% of snai1b-repressed CMs underwent apical delamination. 4-D strain analysis demonstrated that ISO increased the myocardial strain along radial trabecular ridges in alignment with the snai1b expression and Notch-ErbB2-mediated trabeculation. Single-cell and spatial transcriptomics revealed that these snai1b+ CMs were devoid of some EMT-related phenotypes, such as collagen 1a2 production and induction by ErbB2 or TGF-b. Thus, we uncovered snai1b+ CMs that are mechanically activated to initiate delamination for cardiac trabeculation.