Mammalian heart development requires precise allocation of cardiac progenitors. The existence of a multipotent progenitor for all anatomic and cellular components of the heart has been predicted but its identity and contribution to the two cardiac progenitor ‘fields’ has remained undefined. Here we show, using clonal genetic fate mapping, that Mesp1+ cells in gastrulating mesoderm are rapidly specified into committed cardiac precursors fated for distinct anatomic regions of the heart. We identify Smarcd3 as a marker of early specified cardiac precursors and identify within these precursors a compartment boundary at the future junction of the left and right ventricles that arises prior to morphogenesis. Our studies define the timing and hierarchy of cardiac progenitor specification and demonstrate that the cellular and anatomical fate of mesoderm-derived cardiac cells is specified very early. These findings will be important to understand the basis of congenital heart defects and to derive cardiac regeneration strategies.DOI: http://dx.doi.org/10.7554/eLife.03848.001

Summary Vegf signaling specifies arterial fate during early vascular development by inducing the transcription of Delta-like 4 (Dll4), the earliest Notch ligand gene expressed in arterial precursor cells (aPCs). Dll4 expression precedes that of Notch receptors in arteries, and factors that direct its arterial-specific expression are not known. To identify the transcriptional program that initiates arterial Dll4 expression we characterized an arterial-specific and Vegf-responsive enhancer of Dll4. Our findings demonstrate that Notch signaling is not required for initiation of Dll4 expression in arteries, and suggest that Notch instead functions as a maintenance factor. Importantly, we find that Vegf signaling activates MAP kinase (MAPK)-dependent ETS factors in the arterial endothelium to drive expression of Dll4, as well as Notch4. These findings identify a Vegf/MAPK-dependent transcriptional pathway that specifies arterial identity by activating Notch signaling components, and illustrate how signaling cascades can modulate broadly expressed transcription factors to achieve tissue-specific transcriptional outputs.

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Many organs are composed of branched networks of epithelial tubes that transport vital fluids or gases. The proper size and shape of tubes are crucial for their transport function, but the molecular processes that govern tube size and shape are not well understood. Here we show that three genes required for tracheal tube morphogenesis in Drosophila melanogaster encode proteins involved in the synthesis and accumulation of chitin, a polymer of N-acetyl-␤-D-glucosamine that serves as a scaffold in the rigid extracellular matrix of insect cuticle. In all three mutants, developing tracheal tubes bud and extend normally, but the epithelial walls of the tubes do not expand uniformly, and the resultant tubes are grossly misshapen, with constricted and distended regions all along their lengths. The genes are expressed in tracheal cells during the expansion process, and chitin accumulates in the lumen of tubes, forming an expanding cylinder that we propose coordinates the behavior of the surrounding tracheal cells and stabilizes the expanding epithelium. These findings show that chitin regulates epithelial tube morphogenesis, in addition to its classical role protecting mature epithelia.branching morphogenesis ͉ tracheal system ͉ Drosophila ͉ tube shape ͉ mandril M any organs, including the lungs, kidney, liver, and vascular system, are composed of branched networks of epithelial (or endothelial) tubes that transport vital fluids or gases, and the proper size and shape of the tubes are crucial for their transport function. Although there has been progress recently in elucidating the molecular basis of some of the early steps in the development of branched tubular networks, including branch budding and tube formation (1-4), very little is understood about the molecular processes that govern tube size and shape. The Drosophila tracheal (respiratory) system, with its simple cellular structure, an epithelial monolayer without surrounding support cells and accessible development and genetics, provides a valuable model system to address fundamental questions of branching morphogenesis, including how tube size and shape are controlled (5).During development of the Drosophila tracheal system, branches bud sequentially from an epithelial sac consisting of Ϸ80 cells in each hemisegment (6). Two primary branches extend toward and fuse with branches from neighboring segments, forming the dorsal trunk that spans the length of the animal, whereas others extend and ramify on internal tissues. Once the network is established, tubes expand postmitotically to reach their characteristic lengths and diameters (7). A genetic screen identified eight genes required for tube expansion (7), four of which have been cloned (8-11). All four encode claudins or other components of septate junctions, the insect equivalent of vertebrate tight junctions (12, 13). Mutations in seven other septate junction genes also affect tube expansion (9-11, 13, 14), demonstrating that these junctions not only seal the tracheal epithelium but also regulate its size a...

Information processing in the nervous system depends on the creation of specific synaptic connections between neurons and targets during development. The homeodomain transcription factor Otx1 is expressed in early-generated neurons of the developing cerebral cortex. Within layer 5, Otx1 is expressed by neurons with subcortical axonal projections to the midbrain and spinal cord. Otx1 is also expressed in the precursors of these neurons, but is localized to the cytoplasm. Nuclear translocation of Otx1 occurs when layer 5 neurons enter a period of axonal refinement and eliminate a subset of their long-distance projections. Otx1 mutant mice are defective in the refinement of these exuberant projections, suggesting that Otx1 is required for the development of normal axonal connectivity and the generation of coordinated motor behavior.

AimsA unique fibrosarcoma‐like tumour of the uterine cervix harbouring a rearrangement of a neurotrophic tyrosine kinase receptor (NTRK) gene (NTRK1 or NTRK3) has recently been described in 11 young women, some with recurrence and/or metastasis. The aims of this study were to expand the morphological spectrum of this tumour by reporting three additional cases that showed adenosarcoma‐like features not previously described, one of which is the first reported to respond to targeted therapy, and to evaluate 19 conventional uterine adenosarcomas for evidence of NTRK rearrangement.Methods and resultsThree patients presented with a polyp or mass confined to the cervix. The constellation of polypoid growth, spindle cell morphology, entrapped endocervical glands and intraglandular stromal projections raised diagnostic consideration for adenosarcoma with stromal overgrowth. Deep cervical wall invasion was present in two cases at hysterectomy, and the third was removed by polypectomy. All three stained for S100 and pan‐Trk, but were negative for a spectrum of other diagnostic markers. All three harboured NTRK rearrangements (TPM3–NTRK1, TPR–NTRK1, and SPECC1L–NTRK3). One patient developed pleural metastases at 16 months, received the NTRK inhibitor larotrectinib, and is free of disease 15 months later. Two others are alive without disease. None of the uterine adenosarcomas showed any S100 or pan‐Trk staining, or rearrangement of NTRK1, NTRK2 or NTRK3 on next‐generation sequencing.ConclusionsUnusual adenosarcoma‐like spindle cell neoplasms of the cervix may represent an NTRK fusion sarcoma, which can be detected by S100 and pan‐Trk staining and confirmed by NTRK molecular testing. Conventional uterine adenosarcomas do not harbour NTRK rearrangements.

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Haploinsufficiency of transcriptional regulators causes human congenital heart disease (CHD). However, underlying CHD gene regulatory network (GRN) imbalances are unknown.Here, we define transcriptional consequences of reduced dosage of the CHD-linked transcription factor, TBX5, in individual cells during cardiomyocyte differentiation from human induced pluripotent stem cells (iPSCs). We discovered highly sensitive dysregulation of TBX5dependent pathways-including lineage decisions and genes associated with cardiomyocyte function and CHD genetics-in discrete subpopulations of cardiomyocytes. GRN analysis identified vulnerable nodes enriched for CHD genes, indicating that cardiac network stability is sensitive to TBX5 dosage. A GRN-predicted genetic interaction between Tbx5 and Mef2c was validated in mouse, manifesting as ventricular septation defects. These results demonstrate exquisite sensitivity to TBX5 dosage by diverse transcriptional responses in heterogeneous subsets of iPSC-derived cardiomyocytes. This predicts candidate GRNs for human CHDs, with implications for quantitative transcriptional regulation in disease.