Muscles organise pseudo-crystalline arrays of actin, myosin and titin filaments to build force-producing sarcomeres. To study sarcomerogenesis, we have generated a transcriptomics resource of developing Drosophila flight muscles and identified 40 distinct expression profile clusters. Strikingly, most sarcomeric components group in two clusters, which are strongly induced after all myofibrils have been assembled, indicating a transcriptional transition during myofibrillogenesis. Following myofibril assembly, many short sarcomeres are added to each myofibril. Subsequently, all sarcomeres mature, reaching 1.5 µm diameter and 3.2 µm length and acquiring stretch-sensitivity. The efficient induction of the transcriptional transition during myofibrillogenesis, including the transcriptional boost of sarcomeric components, requires in part the transcriptional regulator Spalt major. As a consequence of Spalt knock-down, sarcomere maturation is defective and fibers fail to gain stretch-sensitivity. Together, this defines an ordered sarcomere morphogenesis process under precise transcriptional control – a concept that may also apply to vertebrate muscle or heart development.
Post-transcriptional gene regulation is prevalent in the nervous system, where multiple tiers of regulatory complexity contribute to the development and function of highly specialized cell types. Whole-genome studies in Drosophila have identified several hundred genes containing long 39 extensions in neural tissues. We show that ELAV (embryonic-lethal abnormal visual system) is a key mediator of these neural-specific extensions. Misexpression of ELAV results in the ectopic synthesis of long messenger RNAs (mRNAs) in transgenic embryos. RNA immunoprecipitation assays suggest that ELAV directly binds the proximal polyadenylation signals of many target mRNAs. Finally, ELAV is sufficient to suppress 39 end formation at a strong polyadenylation signal when tethered to a synthetic RNA. We propose that this mechanism for coordinating 39 UTR extension may be generally used in a variety of cellular processes.
SUMMARY Alternative polyadenylation (APA) has been implicated in a variety of developmental and disease processes. A particularly dramatic form of APA occurs in the developing nervous system of flies and mammals, whereby various developmental genes undergo coordinate 3′ UTR extension. In Drosophila, the RNA-binding protein ELAV inhibits RNA processing at proximal polyadenylation sites, thereby fostering the formation of exceptionally long 3′ UTRs. Here, we present evidence that paused Pol II promotes recruitment of ELAV to extended genes. Replacing promoters of extended genes with heterologous promoters blocks normal 3′ extension in the nervous system, while extension-associated promoters can induce 3′ extension in ectopic tissues expressing ELAV. Computational analyses suggest that promoter regions of extended genes tend to contain paused Pol II and associated cis-regulatory elements such as GAGA. ChIP-Seq assays identify ELAV in the promoter regions of extended genes. Our study provides evidence for a regulatory link between promoter-proximal pausing and APA.
Muscles are the major force producing tissue in the human body. While certain muscle types specialize in producing maximum forces, others are very enduring. An extreme example is the heart, which continuously beats for the entire life. Despite being specialized, all body muscles share similar contractile mini-machines called sarcomeres that are organized into regular higher order structures called myofibrils. The major sarcomeric components and their organizational principles are conserved throughout most of the animal kingdom. In this review, we discuss recent progress in the understanding of myofibril and sarcomere development largely obtained from in vivo models. We focus on the role of mechanical forces during muscle and myofibril development and propose a tension driven self-organization mechanism for myofibril formation. We discuss recent technological advances that allow quantification of forces across tissues or molecules in vitro and in vivo. Although their application towards muscle development is still in its infancy, these technologies are likely to provide fundamental new insights into the mechanobiology of muscle and myofibril development in the near future.
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