SUMMARYHox transcription factors control many aspects of animal morphogenetic diversity. The segmental pattern of Drosophila larval muscles shows stereotyped variations along the anteroposterior body axis. Each muscle is seeded by a founder cell and the properties specific to each muscle reflect the expression by each founder cell of a specific combination of 'identity' transcription factors. Founder cells originate from asymmetric division of progenitor cells specified at fixed positions. Using the dorsal DA3 muscle lineage as a paradigm, we show here that Hox proteins play a decisive role in establishing the pattern of Drosophila muscles by controlling the expression of identity transcription factors, such as Nautilus and Collier (Col), at the progenitor stage. Highresolution analysis, using newly designed intron-containing reporter genes to detect primary transcripts, shows that the progenitor stage is the key step at which segment-specific information carried by Hox proteins is superimposed on intrasegmental positional information. Differential control of col transcription by the Antennapedia and Ultrabithorax/Abdominal-A paralogs is mediated by separate cis-regulatory modules (CRMs). Hox proteins also control the segment-specific number of myoblasts allocated to the DA3 muscle. We conclude that Hox proteins both regulate and contribute to the combinatorial code of transcription factors that specify muscle identity and act at several steps during the muscle-specification process to generate muscle diversity.

SUMMARYThe LIM-homeodomain transcription factor Tailup/Islet1 (Tup) is a key component of cardiogenesis in Drosophila and vertebrates. We report here an additional major role for Drosophila Tup in specifying dorsal muscles. Tup is expressed in the four dorsal muscle progenitors (PCs) and tup-null embryos display a severely disorganized dorsal musculature, including a transformation of the dorsal DA2 into dorsolateral DA3 muscle. This transformation is reciprocal to the DA3 to DA2 transformation observed in collier (col) mutants. The DA2 PC, which gives rise to the DA2 muscle and to an adult muscle precursor, is selected from a cluster of myoblasts transiently expressing both Tinman (Tin) and Col. The activation of tup by Tin in the DA2 PC is required to repress col transcription and establish DA2 identity. The transient, partial overlap between Tin and Col expression provides a window of opportunity to distinguish between DA2 and DA3 muscle identities. The function of Tup in the DA2 PC illustrates how single cell precision can be reached in cell specification when temporal dynamics are combined with positional information. The contributions of Tin, Tup and Col to patterning Drosophila dorsal muscles bring novel parallels with chordate pharyngeal muscle development.

The T-box transcription factor Tbx1 and the LIM-homeodomain transcription factor Islet1 are key components in regulatory circuits that generate myogenic and cardiogenic lineage diversity in chordates. We show here that Org-1 and Tup, the Drosophila orthologs of Tbx1 and Islet1, are co-expressed and required for formation of the heart-associated alary muscles (AMs) in the abdomen. The same holds true for lineage-related muscles in the thorax that have not been described previously, which we name thoracic alary-related muscles (TARMs). Lineage analyses identified the progenitor cell for each AM and TARM. Three-dimensional highresolution analyses indicate that AMs and TARMs connect the exoskeleton to the aorta/heart and to different regions of the midgut, respectively, and surround-specific tracheal branches, pointing to an architectural role in the internal anatomy of the larva. Org-1 controls tup expression in the AM/TARM lineage by direct binding to two regulatory sites within an AM/TARM-specific cis-regulatory module, tupAME. The contributions of Org-1 and Tup to the specification of Drosophila AMs and TARMs provide new insights into the transcriptional control of Drosophila larval muscle diversification and highlight new parallels with gene regulatory networks involved in the specification of cardiopharyngeal mesodermal derivatives in chordates.

Highlights d Notch promotes tumor growth through inter-tissue communication d Cancerous epithelial cells hijack the normal mesenchymal cells by Notch signaling d Delta from epithelial cells activates Notch in the mesenchyme through cytonemes d Zfh1/ZEB is upregulated by Notch and prevents mesenchymal differentiation

Adult stem cells are important for tissue maintenance and repair. One key question is how such cells are specified and then protected from differentiation for a prolonged period. Investigating the maintenance of Drosophila muscle progenitors (MPs) we demonstrate that it involves a switch in zfh1/ZEB1 RNA-isoforms. Differentiation into functional muscles is accompanied by expression of miR-8/miR-200, which targets the major zfh1-long RNA isoform and decreases Zfh1 protein. Through activity of the Notch pathway, a subset of MPs produce an alternate zfh1-short isoform, which lacks the miR-8 seed site. Zfh1 protein is thus maintained in these cells, enabling them to escape differentiation and persist as MPs in the adult. There, like mammalian satellite cells, they contribute to muscle homeostasis. Such preferential regulation of a specific RNA isoform, with differential sensitivity to miRs, is a powerful mechanism for maintaining a population of poised progenitors and may be of widespread significance.

BackgroundA stereotyped array of body wall muscles enables precision and stereotypy of animal movements. In Drosophila, each syncytial muscle forms via fusion of one founder cell (FC) with multiple fusion competent myoblasts (FCMs). The specific morphology of each muscle, i.e. distinctive shape, orientation, size and skeletal attachment sites, reflects the specific combination of identity transcription factors (iTFs) expressed by its FC. Here, we addressed three questions: Are FCM nuclei naive? What is the selectivity and temporal sequence of transcriptional reprogramming of FCMs recruited into growing syncytium? Is transcription of generic myogenic and identity realisation genes coordinated during muscle differentiation?ResultsThe tracking of nuclei in developing muscles shows that FCM nuclei are competent to be transcriptionally reprogrammed to a given muscle identity, post fusion. In situ hybridisation to nascent transcripts for FCM, FC-generic and iTF genes shows that this reprogramming is progressive, beginning by repression of FCM-specific genes in fused nuclei, with some evidence that FC nuclei retain specific characteristics. Transcription of identity realisation genes is linked to iTF activation and regulated at levels of both transcription initiation rate and period of transcription. The generic muscle differentiation programme is activated independently.ConclusionsTranscription reprogramming of fused myoblast nuclei is progressive, such that nuclei within a syncytial fibre at a given time point during muscle development are heterogeneous with regards to specific gene transcription. This comprehensive view of the dynamics of transcriptional (re)programming of post-mitotic nuclei within syncytial cells provides a new framework for understanding the transcriptional control of the lineage diversity of multinucleated cells.Electronic supplementary materialThe online version of this article (doi:10.1186/s12915-017-0386-2) contains supplementary material, which is available to authorized users.

Most eukaryotic proteins are N-terminally acetylated, but the functional impact on a global scale has remained obscure. Using genome-wide CRISPR knockout screens in human cells, we reveal a strong genetic dependency between a major N-terminal acetyltransferase and specific ubiquitin ligases. Biochemical analyses uncover that both the ubiquitin ligase complex UBR4-KCMF1 and the acetyltransferase NatC recognize proteins bearing an unacetylated N-terminal methionine followed by a hydrophobic residue. NatC KO-induced protein degradation and phenotypes are reversed by UBR knockdown, demonstrating the central cellular role of this interplay. We reveal that loss of Drosophila NatC is associated with male sterility, reduced longevity, and age-dependent loss of motility due to developmental muscle defects. Remarkably, muscle-specific overexpression of UbcE2M, one of the proteins targeted for NatC KO-mediated degradation, suppresses defects of NatC deletion. In conclusion, NatC-mediated N-terminal acetylation acts as a protective mechanism against protein degradation, which is relevant for increased longevity and motility.

Mitochondrial disorders associated with genetic defects of the ATP synthase are among the most deleterious diseases of the neuromuscular system that primarily manifest in newborns. Nevertheless, the number of established animal models for the elucidation of the molecular mechanisms behind such pathologies is limited. In this paper, we target the Drosophila melanogaster gene encoding for the ATP synthase subunit c, ATPsynC, in order to create a fruit fly model for investigating defects in mitochondrial bioenergetics and to better understand the comprehensive pathological spectrum associated with mitochondrial ATP synthase dysfunctions. Using P-element and EMS mutagenesis, we isolated a set of mutations showing a wide range of effects, from larval lethality to complex pleiotropic phenotypes encompassing developmental delay, early adult lethality, hypoactivity, sterility, hypofertility, aberrant male courtship behavior, locomotor defects and aberrant gonadogenesis. ATPsynC mutations impair ATP synthesis and mitochondrial morphology, and represent a powerful toolkit for the screening of genetic modifiers that can lead to potential therapeutic solutions. Furthermore, the molecular characterization of ATPsynC mutations allowed us to better understand the genetics of the ATPsynC locus and to define three broad pathological consequences of mutations affecting the mitochondrial ATP synthase functionality in Drosophila: i) pre-adult lethality; ii) multi-trait pathology accompanied by early adult lethality; iii) multi-trait adult pathology. We finally predict plausible parallelisms with genetic defects of mitochondrial ATP synthase in humans.