Semaphorin3f as an intrinsic regulator of chamber-specific heart development

During development a pool of precursors form a heart with atrial and ventricular chambers that exhibit distinct transcriptional and electrophysiological properties. Normal development of these chambers is essential for full term survival of the fetus, and deviations result in congenital heart defects. The large number of genes that may cause congenital heart defects when mutated, and the genetic variability and penetrance of the ensuing phenotypes, reveals a need to understand the molecular mechanisms that allow for the formation of chamber-specific cardiomyocyte differentiation. We find that in the developing zebrafish heart, mRNA for a secreted Semaphorin (Sema), Sema3fb, is expressed by all cardiomyocytes, whereas mRNA for its receptor Plexina3 (Plxna3) is expressed by ventricular cardiomyocytes. In sema3fb CRISPR zebrafish mutants, ventricular chamber development is impaired; the ventricles of mutants are smaller in size than their wild type siblings, apparently because of differences in cell size and not cell numbers, with ventricular cardiomyocytes failing to undergo normal developmental hypertrophy. Analysis of chamber differentiation indicates defects in chamber specific gene expression at the border between the ventricular and atrial chambers, with spillage of ventricular chamber genes into the atrium, and vice versa, and a failure to restrict bmp4a mRNA to the atrioventricular canal. The disrupted atrioventricular border region in mutants is accompanied by hypoplastic heart chambers and impaired cardiac function. These data suggest a model whereby cardiomyocytes secrete a Sema cue that, through spatially restricted expression of the receptor, signals in a ventricular chamber-specific manner to establish a distinct border between atrial and ventricular chambers that is important for functional development of the heart.