Fhod3 in zebrafish supports myofibril stability during growth of embryonic skeletal muscle

BACKGROUND: Actin filament organization in cardiomyocytes critically depends on the formin Fhod3, but a role for Fhod3 in skeletal muscle development has not yet been described. RESULTS: We demonstrate here that in zebrafish mutated for one of two fhod3 paralog genes, fhod3a, skeletal muscle of the trunk appears normal through 2 days post-fertilization, but afterward exhibits myofibril damage, including gaps between myofibrils and myofibril fragmentation. Despite the progressive nature of the myofibril damage, fhod3a mutants differ from muscular dystrophy models in that damage is exacerbated by inhibition of muscle activity, and fhod3a mutants show no evidence of sarcolemma disruption. Rather, myofibril damage appears to coincide with growth of the contractile apparatus. We find neither the second fhod3 paralog, fhod3b, nor the related fhod1 contribute to embryonic skeletal muscle development, but fish individually mutated for fhod3a, fhod3b, or fhod1 are viable and appear grossly normal as adults. This may reflect redundancy in adults, as all three are expressed in many adult organs. CONCLUSIONS: These results indicate a fhod3-encoded formin is dispensable for initial myofibril assembly in skeletal muscle but promotes myofibril stability during muscle fiber growth. This is the first demonstration in a vertebrate that Fhod3 contributes to skeletal muscle development.