DAAM Is Required for Thin Filament Formation and Sarcomerogenesis during Muscle Development in Drosophila

During muscle development, myosin and actin containing filaments assemble into the highly organized sarcomeric structure critical for muscle function. Although sarcomerogenesis clearly involves the de novo formation of actin filaments, this process remained poorly understood. Here we show that mouse and Drosophila members of the DAAM formin family are sarcomere-associated actin assembly factors enriched at the Z-disc and M-band. Analysis of dDAAM mutants revealed a pivotal role in myofibrillogenesis of larval somatic muscles, indirect flight muscles and the heart. We found that loss of dDAAM function results in multiple defects in sarcomere development including thin and thick filament disorganization, Z-disc and M-band formation, and a near complete absence of the myofibrillar lattice. Collectively, our data suggest that dDAAM is required for the initial assembly of thin filaments, and subsequently it promotes filament elongation by assembling short actin polymers that anneal to the pointed end of the growing filaments, and by antagonizing the capping protein Tropomodulin. Author Summary Sarcomeres, the smallest contractile units of muscle, are formed by two major filament systems, the myosin containing thick and the actin containing thin filaments. Although it is well established that sarcomerogenesis involves the formation of novel actin filaments, so far it remained largely unclear how these filaments form. In this study, we show that the Drosophila and mouse members of the DAAM formin subfamily are sarcomere associated actin assembly factors. Genetic analysis revealed that dDAAM plays an essential role in thin filament formation and sarcomere organization. In addition, we demonstrate that mDaam1 is an early determinant of myofibrillogenesis. Our data suggest that besides a role at the barbed end of the thin filaments, dDAAM also functions at the pointed end where it antagonizes the capping protein Tropomodulin. Based on these observations, we propose that DAAM family formins are very good candidates for being the long sought-after muscle actin nucleators, that also promote filament elongation by assembling short actin polymers that anneal to the Z-disc anchored growing filament. Given that cardiomyopathies, muscular dystrophies and the cardiovascular disease related heart muscle degenerations belong to the major health problems worldwide, understanding the mechanism of how muscles normally form is of immense biomedical relevance.