SUMMARY Sarcomeres, the fundamental contractile units of muscles, are conserved structures composed of actin thin filaments and myosin thick filaments. How sarcomeres are formed and maintained is not well understood. Here, we show that knockdown of Drosophila cofilin ( DmCFL ), an actin depolymerizing factor, disrupts both sarcomere structure and muscle function. The loss of DmCFL also results in the formation of sarcomeric protein aggregates and impairs sarcomere addition during growth. The activation of the proteasome delays muscle deterioration in our model. Furthermore, we investigate how a point mutation in CFL2 that causes nemaline myopathy (NM) in humans affects CFL function and leads to the muscle phenotypes observed in vivo . Our data provide significant insights to the role of CFLs during sarcomere formation, as well as mechanistic implications for disease progression in NM patients.

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In the skeletal muscle, nuclei are positioned at the periphery of each myofiber and are evenly distributed along its length. Improper positioning of myonuclei has been correlated with muscle disease and decreased muscle function. Several mechanisms required for regulating nuclear position have been identified using the fruit fly, Drosophila melanogaster. The conservation of the myofiber between the fly and vertebrates, the availability of advanced genetic tools, and the ability to visualize dynamic processes using fluorescent proteins in vivo makes the fly an excellent system to study myonuclear positioning. This chapter describes time-lapse and fixed imaging methodologies using both the Drosophila embryo and the larva to investigate mechanisms of myonuclear positioning.

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Sarcomeres, the fundamental contractile units of muscles, are conserved structures composed of actin thin filaments and myosin thick filaments. How sarcomeres are formed and maintained is not well understood. Here, we show that knockdown of Drosophila Cofilin (DmCFL), an actin depolymerizing factor, leads to the progressive disruption of sarcomere structure and muscle function in vivo. Loss of DmCFL also results in the formation of sarcomeric protein aggregates and impairs sarcomere addition during growth. Strikingly, activation of the proteasome delayed muscle deterioration in our model. Further, we investigate how a point mutation in CFL2 that causes nemaline myopathy (NM) in humans, affects CFL function and leads to the muscle phenotypes observed in vivo. Our data provide significant insights to the role of CFLs during sarcomere formation as well as mechanistic implications for disease progression in NM patients.Thick filaments interact with thin filaments, and are crosslinked at the M-line, which is composed of proteins like obscurin (Henderson et al., 2017). While many conserved sarcomeric proteins have been identified, their incorporation into and function within the sarcomere are not completely understood. Moreover, it is still unclear as to how sarcomere size is regulated and maintained during muscle homeostasis. Understanding these fundamental areas of muscle biology is crucial, as mutations in sarcomeric proteins have been implicated in several muscle diseases.

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Myogenesis in Drosophila relies on the activity of the transcription factor Twist during several key events of mesoderm differentiation. To identify the mechanism(s) by which Twist establishes a unique gene expression profile in specific spatial and temporal locales, we employed a yeast-based double interaction screen to discover new Twist-interacting proteins (TIPs) at the myocyte enhancer factor 2 (mef2) and tinman (tinB) myogenic enhancers. We identified a number of proteins that interacted with Twist at one or both enhancers, and whose interactions with Twist and roles in muscle development were previously unknown. Through genetic interaction studies, we find that Twinstar (Tsr), and its regulators are required for muscle formation. Loss of function and null mutations in tsr and its regulators result in missing and/or misattached muscles. Our data suggest that the yeast double interaction screen is a worthy approach to investigate spatial-temporal mechanisms of transcriptional regulation in muscle and in other tissues.

In this issue of Developmental Cell, Tan et al. (2018) show how a novel player in myonuclear positioning, the ubiquitin ligase Ari-1, regulates levels of Koi, a member of the LINC mechanosensing complex, and affects nuclear morphology and positioning in both Drosophila muscles and human vascular smooth muscle cells.