Mutations in cardiac actin (ACTC) have been associated with different cardiac abnormalities in humans including dilated cardiomyopathy and septal defects. However, it is still poorly understood how altered ACTC structure affects cardiovascular physiology and results in the development of distinct congenital disorders. A zebrafish mutant (s434) was identified that displays blood regurgitation in a dilated heart and lacks endocardial cushion (EC) formation. We identified the mutation as a single nucleotide change in Alpha-Cardiac Actin (actc1a), resulting in a Y169S amino acid substitution. This mutation locates at the W-loop of actin, which has been implicated in nucleotide sensing. Consequently, s434 mutants show loss of polymerized cardiac actin. An analogous mutation in yeast actin results in rapid depolymerization of F-actin into fragments that cannot reanneal. This polymerization defect can be partially rescued by phalloidin treatment which stabilizes F-actin. In addition, actc1a mutants show defects in cardiac contractility and altered blood flow within the heart tube. This leads to downregulation or mislocalization of EC-specific gene expression and results in the absence of EC development. Our study underscores the importance of the W-loop for actin functionality and will help us to understand structural and physiological consequences of ACTC mutations in human congenital disorders.

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