Peripheral thickening of the sarcomeres and pointed end elongation of the thin filaments are both promoted by SALS and its formin interaction partners

During striated muscle development the first periodically repeated units appear in the premyofibrils, consisting of immature sarcomeres that must undergo a substantial growth both in length and width, to reach their final size. Here we report that, beyond its well established role in sarcomere elongation, the Sarcomere length short (SALS) protein is involved in Z-disc formation and peripheral growth of the sarcomeres. Our protein localization data and loss-of-function studies in the Drosophila indirect flight muscle strongly suggest that radial growth of the sarcomeres is initiated at the Z-disc. As to thin filament elongation, we used a powerful nanoscopy approach to reveal that SALS is subject to a major conformational change during sarcomere development, which might be critical to stop pointed end elongation in the adult muscles. In addition, we demonstrate that the roles of SALS in sarcomere elongation and radial growth are both dependent on formin type of actin assembly factors. Unexpectedly, when SALS is present in excess amounts, it promotes the formation of actin aggregates highly resembling the ones described in nemaline myopathy patients. Collectively, these findings helped to shed light on the complex mechanisms of SALS during the coordinated elongation and thickening of the sarcomeres, and resulted in the discovery of a potential nemaline myopathy model, suitable for the identification of genetic and small molecule inhibitors. Sarcomeres are the smallest structural and functional units of muscles, characterized by a well-defined length and width in most muscle types. These two parameters are critically important to determine the strength output of a muscle. Although previous studies described the contribution of several proteins involved in the regulation of sarcomere elongation and width, the mechanisms of these processes remained largely unclear. In this study we show that, beyond its well established role in sarcomere elongation, the SALS protein is involved in Z-disc formation and peripheral growth of the sarcomeres. Our protein localization data and loss-of-function studies in the Drosophila indirect flight muscle strongly suggest that radial sarcomere growth is initiated at the Z-disc. In addition, we demonstrate that the roles of SALS in sarcomere elongation and radial growth are both dependent on DAAM and Fhos, two formin type of actin assembly factors. Remarkably, we found that excess amounts of SALS promotes the formation of actin aggregates, similar to the ones described in nemaline myopathy patients. We expect that our studies will open new avenues of research to gain deeper insights into several key aspects of sarcomerogenesis, and might pave the way towards the development of novel myopathy models.