Coordinating the timing of cardiac precursor development during gastrulation: A new role for Notch signaling

Nodal and churchill1 position the expression of a notch ligand during Xenopus germ layer segregation

Churchill and Nodal signaling, which participate in vertebrates’ germ layer induction, position a domain of Delta/Notch activity, which refines germ layer boundaries during frog gastrulation.

In vertebrates, Nodal signaling plays a major role in endomesoderm induction, but germ layer delimitation is poorly understood. In avian embryos, the neural/mesoderm boundary is controlled by the transcription factor CHURCHILL1, presumably through the repressor ZEB2, but there is scarce knowledge about its role in other vertebrates. During amphibian gastrulation, Delta/Notch signaling refines germ layer boundaries in the marginal zone, but it is unknown the place this pathway occupies in the network comprising Churchill1 and Nodal. Here, we show that Xenopus churchill1 is expressed in the presumptive neuroectoderm at mid-blastula transition and during gastrulation, upregulates zeb2, prevents dll1 expression in the neuroectoderm, and favors neuroectoderm over endomesoderm development. Nodal signaling prevents dll1 expression in the endoderm but induces it in the presumptive mesoderm, from where it activates Notch1 and its target gene hes4 in the non-involuting marginal zone. We propose a model where Nodal and Churchill1 position Dll1/Notch1/Hes4 domains in the marginal zone, ensuring the delimitation between mesoderm and neuroectoderm.

Heart organoids and tissue models for modeling development and disease

Organoids, or miniaturized organs formed in vitro, hold potential to revolutionize how researchers approach and answer fundamental biological and pathological questions. In the context of cardiac biology, development of a bona fide cardiac organoid enables study of heart development, function, and pathogenesis in a dish, providing insight into the nature of congenital heart disease and offering the opportunity for high-throughput probing of adult heart disease and drug discovery. Recently, multiple groups have reported novel methods for generating in vitro models of the heart; however, there are substantial conceptual and methodological differences. In this review we will evaluate recent cardiac organoid studies through the lens of the core principles of organoid technology: patterned self-organization of multiple cell types resembling the in vivo organ. Based on this, we will classify systems into the following related types of tissues: developmental cardiac organoids, chamber cardiac organoids, microtissues, and engineered heart tissues. Furthermore, we highlight the interventions which allow for organoid formation, such as modulation of highly conserved cardiogenic signaling pathways mediated by developmental morphogens. We expect that consolidation and categorization of existing organoid models will help eliminate confusion in the field and facilitate progress towards creation of an ideal cardiac organoid.

Notch and interacting signalling pathways in cardiac development, disease, and regeneration

Cardiogenesis is a complex developmental process involving multiple overlapping stages of cell fate specification, proliferation, differentiation, and morphogenesis. Precise spatiotemporal coordination between the different cardiogenic processes is ensured by intercellular signalling crosstalk and tissue-tissue interactions. Notch is an intercellular signalling pathway crucial for cell fate decisions during multicellular organismal development and is aptly positioned to coordinate the complex signalling crosstalk required for progressive cell lineage restriction during cardiogenesis. In this Review, we describe the role of Notch signalling and the crosstalk with other signalling pathways during the differentiation and patterning of the different cardiac tissues and in cardiac valve and ventricular chamber development. We examine how perturbation of Notch signalling activity is linked to congenital heart diseases affecting the neonate and adult, and discuss studies that shed light on the role of Notch signalling in heart regeneration and repair after injury.

Notch signaling in the division of germ layers in bilaterian embryos

Bilaterian embryos are triploblastic organisms which develop three complete germ layers (ectoderm, mesoderm, and endoderm). While the ectoderm develops mainly from the animal hemisphere, there is diversity in the location from where the endoderm and the mesoderm arise in relation to the animal-vegetal axis, ranging from endoderm being specified between the ectoderm and mesoderm in echinoderms, and the mesoderm being specified between the ectoderm and the endoderm in vertebrates. A common feature is that part of the mesoderm segregates from an ancient bipotential endomesodermal domain. The process of segregation is noisy during the initial steps but it is gradually refined. In this review, we discuss the role of the Notch pathway in the establishment and refinement of boundaries between germ layers in bilaterians, with special focus on its interaction with the Wnt/β-catenin pathway.

Transient HES5 Activity Instructs Mesodermal Cells toward a Cardiac Fate

Notch signaling plays a role in specifying a cardiac fate but the downstream effectors remain unknown. In this study we implicate the Notch downstream effector HES5 in cardiogenesis. We show transient Hes5 expression in early mesoderm of gastrulating embryos and demonstrate, by loss and gain-of-function experiments in mouse embryonic stem cells, that HES5 favors cardiac over primitive erythroid fate. Hes5 overexpression promotes upregulation of the cardiac gene Isl1, while the hematopoietic regulator Scl is downregulated. Moreover, whereas a pulse of Hes5 instructs cardiac commitment, sustained expression after lineage specification impairs progression of differentiation to contracting cardiomyocytes. These findings establish a role for HES5 in cardiogenesis and provide insights into the early cardiac molecular network.

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Hes5 is expressed in the nascent mesoderm of gastrulating mouse embryos

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Hes5 knockdown enhances primitive erythropoiesis in mESCs

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A stage-specific pulse of Hes5 instructs preferential cardiac fate in mESCs

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Sustained Hes5 activation impairs differentiation to contracting cardiomyocytes

Early stages of induction of anterior head ectodermal properties in Xenopus embryos are mediated by transcriptional cofactor ldb1

BACKGROUND

Specific molecules involved in early inductive signaling from anterior neural tissue to the placodal ectoderm to establish a lens-forming bias, as well as their regulatory factors, remain largely unknown. In this study, we sought to identify and characterize these molecules.

RESULTS

Using an expression cloning strategy to isolate genes with lens-inducing activity, we identified the transcriptional cofactor ldb1. This, together with evidence for its nuclear dependence, suggests its role as a regulatory factor, not a direct signaling molecule. We propose that ldb1 mediates induction of early lens genes in our functional assay by transcriptional activation of lens-inducing signals. Gain-of-function assays demonstrate that the inductive activity of the anterior neural plate on head ectodermal structures can be augmented by ldb1. Loss-of-function assays show that knockdown of ldb1 leads to decreased expression of early lens and retinal markers and subsequently to defects in eye development.

CONCLUSIONS

The functional cloning, expression pattern, overexpression, and knockdown data show that an ldb1-regulated mechanism acts as an early signal for Xenopus lens induction.

Role of Notch signaling in the mammalian heart

Notch signaling is an evolutionarily ancient, highly conserved pathway important for deciding cell fate, cellular development, differentiation, proliferation, apoptosis, adhesion, and epithelial-to-mesenchymal transition. Notch signaling is also critical in mammalian cardiogenesis, as mutations in this signaling pathway are linked to human congenital heart disease. Furthermore, Notch signaling can repair myocardial injury by promoting myocardial regeneration, protecting ischemic myocardium, inducing angiogenesis, and negatively regulating cardiac fibroblast-myofibroblast transformation. This review provides an update on the known roles of Notch signaling in the mammalian heart. The goal is to assist in developing strategies to influence Notch signaling and optimize myocardial injury repair.

Genetic and functional genomics approaches targeting the Notch pathway in cardiac development and congenital heart disease

The Notch signalling pathway plays crucial roles in cardiac development and postnatal cardiac homoeostasis. Gain- and loss-of-function approaches indicate that Notch promotes or inhibits cardiogenesis in a stage-dependent manner. However, the molecular mechanisms are poorly defined because many downstream effectors remain to be identified. Genome-scale analyses are shedding light on the genes that are regulated by Notch signalling and the mechanisms underlying this regulation. We review the functional data that implicates Notch in cardiac morphogenetic processes and expression profiling studies that enlighten the regulatory networks behind them. A recurring theme is that Notch cross-talks reiteratively with other key signalling pathways including Wnt and Bmp to coordinate cell and tissue interactions during cardiogenesis.

Use of small molecule inhibitors of the Wnt and Notch signaling pathways during Xenopus development

Small molecule inhibitors of growth factor signaling pathways are extremely convenient reagents for investigation of embryonic development. The chemical may be introduced at a precise time, the dose can be altered over a large range and the chemical may be removed simply by replacing the medium surrounding the embryo. Because small molecule modulators are designed to target conserved features of a protein, they are usually effective across species. Ideally the chemicals offer remarkable specificity for a particular signaling pathway and exhibit negligible off-target effects. In this study we examine the use of small molecules to modulate the Wnt and Notch signaling pathways in the Xenopus embryo. We find that IWR-1 and XAV939 are effective inhibitors of the canonical Wnt signaling pathway while BIO is an excellent activator. For Notch signaling, we find that both DAPT and RO4929097 are effective inhibitors, but that RO4929097 is the more potent reagent. This report provides researchers with useful working concentrations of reagents and a small series of genetic and biological assays that may be used to characterize the role of Wnt and Notch signaling during embryonic development.

RITA, a novel modulator of Notch signalling, acts via nuclear export of RBP-J

The evolutionarily conserved Notch signal transduction pathway regulates fundamental cellular processes during embryonic development and in the adult. Ligand binding induces presenilin-dependent cleavage of the receptor and a subsequent nuclear translocation of the Notch intracellular domain (NICD). In the nucleus, NICD binds to the recombination signal sequence-binding protein J (RBP-J)/CBF-1 transcription factor to induce expression of Notch target genes. Here, we report the identification and functional characterization of RBP-J interacting and tubulin associated (RITA) (C12ORF52) as a novel RBP-J/CBF-1-interacting protein. RITA is a highly conserved 36 kDa protein that, most interestingly, binds to tubulin in the cytoplasm and shuttles rapidly between cytoplasm and nucleus. This shuttling RITA exports RBP-J/CBF-1 from the nucleus. Functionally, we show that RITA can reverse a Notch-induced loss of primary neurogenesis in Xenopus laevis. Furthermore, RITA is able to downregulate Notch-mediated transcription. Thus, we propose that RITA acts as a negative modulator of the Notch signalling pathway, controlling the level of nuclear RBP-J/CBF-1, where its amounts are limiting.