The myogenic repressor gene Holes in muscles is a direct transcriptional target of Twist and Tinman in the Drosophila embryonic mesoderm

Assessing the Roles of Potential Notch Signaling Components in Instructive and Permissive Pathways with Two Drosophila Pericardial Reporters

The highly conserved Notch signaling pathway brings about the transcriptional activation of target genes via either instructive or permissive mechanisms that depend on the identity of the specific target gene. As additional components of the Notch signaling pathway are identified, assessing whether each of these components are utilized exclusively by one of these mechanisms (and if so, which), or by both, becomes increasingly important. Using RNA interference-mediated knockdowns of the Notch component to be tested, reporters for two Notch-activated pericardial genes in Drosophila melanogaster, immunohistochemistry, and fluorescence microscopy, we describe a method to determine the type of signaling mechanism-instructive, permissive, or both-to which a particular Notch pathway component contributes.

Extremely rapid and reversible optogenetic perturbation of nuclear proteins in living embryos

Many developmental regulators have complex and context-specific roles in different tissues and stages, making the dissection of their function extremely challenging. As regulatory processes often occur within minutes, perturbation methods that match these dynamics are needed. Here, we present the improved light-inducible nuclear export system (iLEXY), an optogenetic loss-of-function approach that triggers translocation of proteins from the nucleus to the cytoplasm. By introducing a series of mutations, we substantially increased LEXY's efficiency and generated variants with different recovery times. iLEXY enables rapid (t1/2 < 30 s), efficient, and reversible nuclear protein depletion in embryos, and is generalizable to proteins of diverse sizes and functions. Applying iLEXY to the Drosophila master regulator Twist, we phenocopy loss-of-function mutants, precisely map the Twist-sensitive embryonic stages, and investigate the effects of timed Twist depletions. Our results demonstrate the power of iLEXY to dissect the function of pleiotropic factors during embryogenesis with unprecedented temporal precision.

Genome-wide identification and characterization of basic helix-loop-helix genes in nine molluscs

The basic helix-loop-helix (bHLH) transcription factors form a large superfamily that plays an important role in numerous physiological processes, including development and response to environmental stresses. In this study, the distribution of bHLH genes in nine molluscs was systematically investigated (including five bivalves, three gastropods and one cephalopod). Finally, 53-85 bHLH genes were identified from each genome and classified into corresponding families by using phylogenetic analysis. The results of gene structure and conserved motif analysis illustrated the hereditary conservation of bHLH transcription factors during evolution but showed low similarity in group C. Through transcription profile analysis of C. gigas and T. granosa, we found a important role of bHLH genes in responding to multiple external challenges and development; meanwhile, they also exhibited tissue-specific expression. Interestingly, we were also surprised to find different bHLH genes from the same group generally possess similar patterns expression that tends to simultaneously present high or lower expression of multiple challenges and different tissues in this study. In summary, this study lays the foundation for further investigation of the biological functions and evolution of molluscan bHLH genes.

Three distinct mechanisms, Notch instructive, permissive, and independent, regulate the expression of two different pericardial genes to specify cardiac cell subtypes

The development of a complex organ involves the specification and differentiation of diverse cell types constituting that organ. Two major cell subtypes, contractile cardial cells (CCs) and nephrocytic pericardial cells (PCs), comprise the Drosophila heart. Binding sites for Suppressor of Hairless [Su(H)], an integral transcription factor in the Notch signaling pathway, are enriched in the enhancers of PC-specific genes. Here we show three distinct mechanisms regulating the expression of two different PC-specific genes, Holes in muscle (Him), and Zn finger homeodomain 1 (zfh1). Him transcription is activated in PCs in a permissive manner by Notch signaling: in the absence of Notch signaling, Su(H) forms a repressor complex with co-repressors and binds to the Him enhancer, repressing its transcription; upon alleviation of this repression by Notch signaling, Him transcription is activated. In contrast, zfh1 is transcribed by a Notch-instructive mechanism in most PCs, where mere alleviation of repression by preventing the binding of Su(H)-co-repressor complex is not sufficient to activate transcription. Our results suggest that upon activation of Notch signaling, the Notch intracellular domain associates with Su(H) to form an activator complex that binds to the zfh1 enhancer, and that this activator complex is necessary for bringing about zfh1 transcription in these PCs. Finally, a third, Notch-independent mechanism activates zfh1 transcription in the remaining, even skipped-expressing, PCs. Collectively, our data show how the same feature, enrichment of Su(H) binding sites in PC-specific gene enhancers, is utilized by two very distinct mechanisms, one permissive, the other instructive, to contribute to the same overall goal: the specification and differentiation of a cardiac cell subtype by activation of the pericardial gene program. Furthermore, our results demonstrate that the zfh1 enhancer drives expression in two different domains using distinct Notch-instructive and Notch-independent mechanisms.

Muscle defects due to perturbed somite segmentation contribute to late adult scoliosis

Scoliosis is an abnormal bending of the body axis. Truncated vertebrae or a debilitated ability to control the musculature in the back can cause this condition, but in most cases the causative reason for scoliosis is unknown (idiopathic). Using mutants for somite clock genes with mild defects in the vertebral column, we here show that early defects in somitogenesis are not overcome during development and have long lasting and profound consequences for muscle fiber organization, structure and whole muscle volume. These mutants present only mild alterations in the vertebral column, and muscle shortcomings are uncoupled from skeletal defects. None of the mutants presents an overt musculoskeletal phenotype at larval or early adult stages, presumably due to compensatory growth mechanisms. Scoliosis becomes only apparent during aging. We conclude that adult degenerative scoliosis is due to disturbed crosstalk between vertebrae and muscles during early development, resulting in subsequent adult muscle weakness and bending of the body axis.

Adult Drosophila muscle morphometry through microCT reveals dynamics during ageing

Indirect flight muscles (IFMs) in adult Drosophila provide the key power stroke for wing beating. They also serve as a valuable model for studying muscle development. An age-dependent decline in Drosophila free flight has been documented, but its relation to gross muscle structure has not yet been explored satisfactorily. Such analyses are impeded by conventional histological preparations and imaging techniques that limit exact morphometry of flight muscles. In this study, we employ microCT scanning on a tissue preparation that retains muscle morphology under homeostatic conditions. Focusing on a subset of IFMs called the dorsal longitudinal muscles (DLMs), we find that DLM volumes increase with age, partially due to the increased separation between myofibrillar fascicles, in a sex-dependent manner. We have uncovered and quantified asymmetry in the size of these muscles on either side of the longitudinal midline. Measurements of this resolution and scale make substantive studies that test the connection between form and function possible. We also demonstrate the application of this method to other insect species making it a valuable tool for histological analysis of insect biodiversity.