Notch signaling in cardiac development and disease

In-Depth Genomic Analysis: The New Challenge in Congenital Heart Disease

The use of next-generation sequencing has provided new insights into the causes and mechanisms of congenital heart disease (CHD). Examinations of the whole exome sequence have detected detrimental gene variations modifying single or contiguous nucleotides, which are characterised as pathogenic based on statistical assessments of families and correlations with congenital heart disease, elevated expression during heart development, and reductions in harmful protein-coding mutations in the general population. Patients with CHD and extracardiac abnormalities are enriched for gene classes meeting these criteria, supporting a common set of pathways in the organogenesis of CHDs. Single-cell transcriptomics data have revealed the expression of genes associated with CHD in specific cell types, and emerging evidence suggests that genetic mutations disrupt multicellular genes essential for cardiogenesis. Metrics and units are being tracked in whole-genome sequencing studies.

Molecular Pathways and Animal Models of Tricuspid Atresia and Univentricular Heart

The process of valve formation is a complex process that involves intricate interplay between various pathways at precise times. Although we have not completely elucidated the molecular pathways that lead to normal valve formation, we have identified a few major players in this process. We are now able to implicate TGF-ß, BMP, and NOTCH as suspects in tricuspid atresia (TA), as well as their downstream targets: NKX2-5, TBX5, NFATC1, GATA4, and SOX9. We know that the TGF-ß and the BMP pathways converge on the SMAD4 molecule, and we believe that this molecule plays a very important role to tie both pathways to TA. Similarly, we look at the NOTCH pathway and identify the HEY2 as a potential link between this pathway and TA. Another transcription factor that has been implicated in TA is NFATC1. While several mouse models exist that include part of the TA abnormality as their phenotype, no true mouse model can be said to represent TA. Bridging this gap will surely shed light on this complex molecular pathway and allow for better understanding of the disease process.

Capturing heart valve development with Gene Ontology

Introduction: The normal development of all heart valves requires highly coordinated signaling pathways and downstream mediators. While genomic variants can be responsible for congenital valve disease, environmental factors can also play a role. Later in life valve calcification is a leading cause of aortic valve stenosis, a progressive disease that may lead to heart failure. Current research into the causes of both congenital valve diseases and valve calcification is using a variety of high-throughput methodologies, including transcriptomics, proteomics and genomics. High quality genetic data from biological knowledge bases are essential to facilitate analyses and interpretation of these high-throughput datasets. The Gene Ontology (GO, http://geneontology.org/) is a major bioinformatics resource used to interpret these datasets, as it provides structured, computable knowledge describing the role of gene products across all organisms. The UCL Functional Gene Annotation team focuses on GO annotation of human gene products. Having identified that the GO annotations included in transcriptomic, proteomic and genomic data did not provide sufficient descriptive information about heart valve development, we initiated a focused project to address this issue. Methods: This project prioritized 138 proteins for GO annotation, which led to the curation of 100 peer-reviewed articles and the creation of 400 heart valve development-relevant GO annotations. Results: While the focus of this project was heart valve development, around 600 of the 1000 annotations created described the broader cellular role of these proteins, including those describing aortic valve morphogenesis, BMP signaling and endocardial cushion development. Our functional enrichment analysis of the 28 proteins known to have a role in bicuspid aortic valve disease confirmed that this annotation project has led to an improved interpretation of a heart valve genetic dataset. Discussion: To address the needs of the heart valve research community this project has provided GO annotations to describe the specific roles of key proteins involved in heart valve development. The breadth of GO annotations created by this project will benefit many of those seeking to interpret a wide range of cardiovascular genomic, transcriptomic, proteomic and metabolomic datasets.

Zinc Finger 521 Modulates the Nrf2-Notch Signaling Pathway in Human Ovarian Carcinoma

The human zinc finger protein 521 (ZNF521) is a co-transcriptional factor with multiple recognized regulatory functions in a range of normal, cancer and stem cell compartments. ZNF521 regulates proliferation, progression and CSC (cancer stem cell) compartments in human ovarian cancer (hOC), which is a very aggressive and late-diagnosed female tumor. Two other important regulators of hOC are the NRF2 and NOTCH signaling pathways. In the present paper, the mRNA and protein levels of ZNF521 were correlated with those of the NRF2-NOTCH signaling components in two different hOC cell lines and in a public dataset of 381 hOC patients. The data show that high levels of ZNF521 significantly increase NRF2-NOTCH signaling expression; conversely, the silencing of ZNF521 impairs NRF2-NOTCH signaling. This experimental work shows that, in hOC, different levels of ZNF521 modulate the NRF2-NOTCH signaling pathway and also influences hOC CSC properties.

Exposure to environmental concentrations of glyphosate induces cardiotoxicity through cellular senescence and reduced cell proliferation capacity

Glyphosate (GLY), the preeminent herbicide utilized globally, is known to be exposed to the environment and population on a chronic basis. Exposure to GLY and the consequent health risks are alarming public health problems that are attracting international attention. However, the cardiotoxicity of GLY has been a matter of dispute and uncertainty. Here, AC16 cardiomyocytes and zebrafish were exposed to GLY. This study found that low concentrations of GLY lead to morphological enlargement of AC16 human cardiomyocytes, indicating a senescent state. The increased expression of P16, P21, and P53 following exposure to GLY demonstrated that GLY causes senescence in AC16. Moreover, it was mechanistically confirmed that GLY-induced senescence in AC16 cardiomyocytes was produced by ROS-mediated DNA damage. In terms of in vivo cardiotoxicity, GLY decreased the proliferative capacity of cardiomyocytes in zebrafish through the notch signaling pathway, resulting in a reduction of cardiomyocytes. It was also found that GLY caused zebrafish cardiotoxicity associated with DNA damage and mitochondrial damage. KEGG analysis after RNA-seq shows a significant enrichment of protein processing pathways in the endoplasmic reticulum (ER) after GLY exposure. Importantly, GLY induced ER stress in AC16 cells and zebrafish by activating PERK-eIF2α-ATF4 pathway. Our study has thus provided the first novel insights into the mechanism underlying GLY-induced cardiotoxicity. Furthermore, our findings emphasize the need for increased attention to the potential cardiotoxic effects of GLY.

Integrative Analyses of Genes Associated With Right Ventricular Cardiomyopathy Induced by Tricuspid Regurgitation

Tricuspid regurgitation (TR) induces right ventricular cardiomyopathy, a common heart disease, and eventually leads to severe heart failure and serious clinical complications. Accumulating evidence shows that long non-coding RNAs (lncRNAs) are involved in the pathological process of a variety of cardiovascular diseases. However, the regulatory mechanisms and functional roles of RNA interactions in TR-induced right ventricular cardiomyopathy are still unclear. Accordingly, we performed integrative analyses of genes associated with right ventricular cardiomyopathy induced by TR to study the roles of lncRNAs in the pathogenesis of this disease. In this study, we used high-throughput sequencing data of tissue samples from nine clinical cases of right ventricular myocardial cardiomyopathy induced by TR and nine controls with normal right ventricular myocardium from the Genotype-Tissue Expression database. We identified differentially expressed lncRNAs and constructed a protein-protein interaction and lncRNA-messenger RNA (mRNA) co-expression network. Furthermore, we determined hub lncRNA-mRNA modules related to right ventricular myocardial disease induced by TR and constructed a competitive endogenous RNA network for TR-induced right ventricular myocardial disease by integrating the interaction of lncRNA-miRNA-mRNA. In addition, we analyzed the immune infiltration using integrated data and the correlation of each immune-related gene with key genes of the integrated expression matrix. The present study identified 648 differentially expressed mRNAs, 201 differentially expressed miRNAs, and 163 differentially expressed lncRNAs. Protein-protein interaction network analysis confirmed that ADRA1A, AVPR1B, OPN4, IL-1B, IL-1A, CXCL4, ADCY2, CXCL12, GNB4, CCL20, CXCL8, and CXCL1 were hub genes. CTD-2314B22.3, hsa-miR-653-5p, and KIF17ceRNA; SRGAP3-AS2, hsa-miR-539-5p, and SHANK1; CERS6-AS1, hsa-miR-497-5p, and OPN4; INTS6-AS1, hsa-miR-4262, and NEURL1B; TTN-AS1, hsa-miR-376b-3p, and TRPM5; and DLX6-AS1, hsa-miR-346, and BIRC7 axes were obtained by constructing the ceRNA networks. Through the immune infiltration analysis, we found that the proportion of CD4 and CD8 T cells was about 20%, and the proportion of fibroblasts and endothelial cells was high. Our findings provide some insights into the mechanisms of RNA interaction in TR-induced right ventricular cardiomyopathy and suggest that lncRNAs are a potential therapeutic target for treating right ventricular myocardial disease induced by TR.

The roles of signaling pathways in cardiac regeneration

In recent years, knowledge of cardiac regeneration mechanisms has dramatically expanded. Regeneration can replace lost parts of organs, common among animal species. The heart is commonly considered an organ with terminal development, which has no reparability potential during post-natal life; however, some intrinsic regeneration capacity has been reported for cardiac muscle, which opens novel avenues in cardiovascular disease treatment. Different endogenous mechanisms were studied for cardiac repairing and regeneration in recent decades. Survival, proliferation, inflammation, angiogenesis, cell-cell communication, cardiomyogenesis, and anti-aging pathways are the most important mechanisms that have been studied in this regard. Several in vitro and animal model studies focused on proliferation induction for cardiac regeneration reported promising results. These studies have mainly focused on promoting proliferation signaling pathways and demonstrated various signaling pathways such as Wnt, PI3K/Akt, IGF-1, TGF-β, Hippo, and VEGF signaling cardiac regeneration. Therefore, in this review, we intended to discuss the connection between different critical signaling pathways in cardiac repair and regeneration.

NOTCH Signaling in Aortic Valve Development and Calcific Aortic Valve Disease

NOTCH intercellular signaling mediates the communications between adjacent cells involved in multiple biological processes essential for tissue morphogenesis and homeostasis. The NOTCH1 mutations are the first identified human genetic variants that cause congenital bicuspid aortic valve (BAV) and calcific aortic valve disease (CAVD). Genetic variants affecting other genes in the NOTCH signaling pathway may also contribute to the development of BAV and the pathogenesis of CAVD. While CAVD occurs commonly in the elderly population with tri-leaflet aortic valve, patients with BAV have a high risk of developing CAVD at a young age. This observation indicates an important role of NOTCH signaling in the postnatal homeostasis of the aortic valve, in addition to its prenatal functions during aortic valve development. Over the last decade, animal studies, especially with the mouse models, have revealed detailed information in the developmental etiology of congenital aortic valve defects. In this review, we will discuss the molecular and cellular aspects of aortic valve development and examine the embryonic pathogenesis of BAV. We will focus our discussions on the NOTCH signaling during the endocardial-to-mesenchymal transformation (EMT) and the post-EMT remodeling of the aortic valve. We will further examine the involvement of the NOTCH mutations in the postnatal development of CAVD. We will emphasize the deleterious impact of the embryonic valve defects on the homeostatic mechanisms of the adult aortic valve for the purpose of identifying the potential therapeutic targets for disease intervention.

miR-106a-363 cluster in extracellular vesicles promotes endogenous myocardial repair via Notch3 pathway in ischemic heart injury

Endogenous capability of the post-mitotic human heart holds great promise to restore the injured myocardium. Recent evidence indicates that the extracellular vesicles (EVs) regulate cardiac homeostasis and regeneration. Here, we investigated the molecular mechanism of EVs for self-repair. We isolated EVs from human iPSC-derived cardiomyocytes (iCMs), which were exposed to hypoxic (hEVs) and normoxic conditions (nEVs), and examined their roles in in vitro and in vivo models of cardiac injury. hEV treatment significantly improved the viability of hypoxic iCMs in vitro and cardiac function of severely injured murine myocardium in vivo. Microarray analysis of the EVs revealed significantly enriched expression of the miR-106a-363 cluster (miR cluster) in hEVs vs. nEVs. This miR cluster preserved survival and contractility of hypoxia-injured iCMs and maintained murine left-ventricular (LV) chamber size, improved LV ejection fraction, and reduced myocardial fibrosis of the injured myocardium. RNA-Seq analysis identified Jag1-Notch3-Hes1 as a target intracellular pathway of the miR cluster. Moreover, the study found that the cell cycle activator and cytokinesis genes were significantly up-regulated in the iCMs treated with miR cluster and Notch3 siRNA. Together, these results suggested that the miR cluster in the EVs stimulated cardiomyocyte cell cycle re-entry by repressing Notch3 to induce cell proliferation and augment myocardial self-repair. The miR cluster may represent an effective therapeutic approach for ischemic cardiomyopathy.

Control of sinus venous valve and sinoatrial node development by endocardial NOTCH1

AIMS

Sinus venous valve (SVV) and sinoatrial node (SAN) develop together at the sinoatrial junction during embryogenesis. SVV ensures unidirectional cardiac input and SAN generates sinus rhythmic contraction, respectively; both functions are essential for embryonic survival. We aim to reveal the potential role of endocardial NOTCH signalling in SVV and SAN formation.

METHODS AND RESULTS

We specifically deleted Notch1 in the endocardium using an Nfatc1Cre line. This deletion resulted in underdeveloped SVV and SAN, associated with reduced expression of T-box transcription factors, Tbx5 andTbx18, which are essential for the formation of SVV and SAN. The deletion also led to decreased expression of Wnt2 in myocardium of SVV and SAN. WNT2 treatment was able to rescue the growth defect of SVV and SAN resulted from the Notch1 deletion in whole embryo cultures. Furthermore, the Notch1 deletion reduced the expression of Nrg1 in the SVV myocardium and supplement of NRG1 restored the growth of SVV in cultured Notch1 knockout embryos.

CONCLUSION

Our findings support that endocardial NOTCH1 controls the development of SVV and SAN by coordinating myocardial WNT and NRG1 signalling functions.

The roles and activation of endocardial Notch signaling in heart regeneration

As a highly conserved signaling pathway in metazoans, the Notch pathway plays important roles in embryonic development and tissue regeneration. Recently, cardiac injury and regeneration have become an increasingly popular topic for biomedical research, and Notch signaling has been shown to exert crucial functions during heart regeneration as well. In this review, we briefly summarize the molecular functions of the endocardial Notch pathway in several cardiac injury and stress models. Although there is an increase in appreciating the importance of endocardial Notch signaling in heart regeneration, the mechanism of its activation is not fully understood. This review highlights recent findings on the activation of the endocardial Notch pathway by hemodynamic blood flow change in larval zebrafish ventricle after partial ablation, a process involving primary cilia, mechanosensitive ion channel Trpv4 and mechanosensitive transcription factor Klf2.

SARS-CoV-2-Encoded Proteome and Human Genetics: From Interaction-Based to Ribosomal Biology Impact on Disease and Risk Processes

SARS-CoV-2 (COVID-19) has infected millions of people worldwide, with lethality in hundreds of thousands. The rapid publication of information, both regarding the clinical course and the viral biology, has yielded incredible knowledge of the virus. In this review, we address the insights gained for the SARS-CoV-2 proteome, which we have integrated into the Viral Integrated Structural Evolution Dynamic Database, a publicly available resource. Integrating evolutionary, structural, and interaction data with human proteins, we present how the SARS-CoV-2 proteome interacts with human disorders and risk factors ranging from cytokine storm, hyperferritinemic septic, coagulopathic, cardiac, immune, and rare disease-based genetics. The most noteworthy human genetic potential of SARS-CoV-2 is that of the nucleocapsid protein, where it is known to contribute to the inhibition of the biological process known as nonsense-mediated decay. This inhibition has the potential to not only regulate about 10% of all biological transcripts through altered ribosomal biology but also associate with viral-induced genetics, where suppressed human variants are activated to drive dominant, negative outcomes within cells. As we understand more of the dynamic and complex biological pathways that the proteome of SARS-CoV-2 utilizes for entry into cells, for replication, and for release from human cells, we can understand more risk factors for severe/lethal outcomes in patients and novel pharmaceutical interventions that may mitigate future pandemics.

Designing Biomaterials to Modulate Notch Signaling in Tissue Engineering and Regenerative Medicine

The design of cell-instructive biomaterials for tissue engineering and regenerative medicine is at a crossroads. Although the conventional tissue engineering approach is top-down (cells seeded to macroporous scaffolds and mature to form tissues), bottom-up tissue engineering strategies are becoming appealing. With such developments, we can study cell signaling events, thus enabling functional tissue assembly in physiologic and diseased models. Among many important signaling pathways, the Notch signaling pathway is the most diverse in its influence during tissue morphogenesis and repair following injury. Although Notch signaling is extensively studied in developmental biology and cancer biology, our knowledge of designing biomaterial-based Notch signaling platforms and incorporating Notch signaling components into engineered tissue systems is limited. By incorporating Notch signaling to tissue engineering scaffolds, we can direct cell-specific responses and improve engineered tissue maturation. This review will discuss recent progress in the development of Notch signaling biomaterials as a promising target to control cellular fate decisions, including the influences of ligand identity, biophysical material cues, ligand presentation strategies, and mechanotransduction. Notch signaling is consequently of interest to direct, control, and reprogram cellular behavior on a biomaterial surface. We anticipate that discussions in this article will allow for enhanced knowledge and insight into designing Notch targeted biomaterials for various tissue engineering and cell fate determinations. Impact statement Notch signaling is recognized as an important pathway in tissue engineering and regenerative medicine; however, there is no systematic review on this topic. The comprehensive review and perspectives presented here provide an in-depth discussion on ligand presentation strategies both in 2D and in 3D cell culture environments involving biomaterials/scaffolds. In addition, this review article provides insight into the challenges in designing cell surrogate biomaterials capable of providing Notch signals. To the best of the authors' knowledge, this is the first review relevant to the fields of tissue engineering.

Intrinsic Endocardial Defects Contribute to Hypoplastic Left Heart Syndrome

Hypoplastic left heart syndrome (HLHS) is a complex congenital heart disease characterized by abnormalities in the left ventricle, associated valves, and ascending aorta. Studies have shown intrinsic myocardial defects but do not sufficiently explain developmental defects in the endocardial-derived cardiac valve, septum, and vasculature. Here, we identify a developmentally impaired endocardial population in HLHS through single-cell RNA profiling of hiPSC-derived endocardium and human fetal heart tissue with an underdeveloped left ventricle. Intrinsic endocardial defects contribute to abnormal endothelial-to-mesenchymal transition, NOTCH signaling, and extracellular matrix organization, key factors in valve formation. Endocardial abnormalities cause reduced cardiomyocyte proliferation and maturation by disrupting fibronectin-integrin signaling, consistent with recently described de novo HLHS mutations associated with abnormal endocardial gene and fibronectin regulation. Together, these results reveal a critical role for endocardium in HLHS etiology and provide a rationale for considering endocardial function in regenerative strategies.

Notch dimerization and gene dosage are important for normal heart development, intestinal stem cell maintenance, and splenic marginal zone B-cell homeostasis during mite infestation

Cooperative DNA binding is a key feature of transcriptional regulation. Here we examined the role of cooperativity in Notch signaling by CRISPR-mediated engineering of mice in which neither Notch1 nor Notch2 can homo- or heterodimerize, essential for cooperative binding to sequence-paired sites (SPS) located near many Notch-regulated genes. Although most known Notch-dependent phenotypes were unaffected in Notch1/2 dimer-deficient mice, a subset of tissues proved highly sensitive to loss of cooperativity. These phenotypes include heart development, compromised viability in combination with low gene dose, and the gut, developing ulcerative colitis in response to 1% dextran sulfate sodium (DSS). The most striking phenotypes-gender imbalance and splenic marginal zone B-cell lymphoma-emerged in combination with gene dose reduction or when challenged by chronic fur mite infestation. This study highlights the role of the environment in malignancy and colitis and is consistent with Notch-dependent anti-parasite immune responses being compromised in Notch dimer-deficient animals.

Delta-like ligand 4-mediated Notch signaling controls proliferation of second heart field progenitor cells by regulating Fgf8 expression

The role played by the Notch pathway in cardiac progenitor cell biology remains to be elucidated. Delta-like ligand 4 (Dll4), the arterial-specific Notch ligand, is expressed by second heart field (SHF) progenitors at time-points that are crucial in SHF biology. Dll4-mediated Notch signaling is required for maintaining an adequate pool of SHF progenitors, such that Dll4 knockout results in a reduction in proliferation and an increase in apoptosis. A reduced SHF progenitor pool leads to an underdeveloped right ventricle (RV) and outflow tract (OFT). In its most severe form, there is severe RV hypoplasia and poorly developed OFT resulting in early embryonic lethality. In its milder form, the OFT is foreshortened and misaligned, resulting in a double outlet right ventricle. Dll4-mediated Notch signaling maintains Fgf8 expression by transcriptional regulation at the promoter level. Combined heterozygous knockout of Dll4 and Fgf8 demonstrates genetic synergy in OFT alignment. Exogenous supplemental Fgf8 rescues proliferation in Dll4 mutants in ex-vivo culture. Our results establish a novel role for Dll4-mediated Notch signaling in SHF biology. More broadly, our model provides a platform for understanding oligogenic inheritance that results in clinically relevant OFT malformations.

Cardiopharyngeal Progenitor Specification: Multiple Roads to the Heart and Head Muscles

During embryonic development, the heart arises from various sources of undifferentiated mesodermal progenitors, with an additional contribution from ectodermal neural crest cells. Mesodermal cardiac progenitors are plastic and multipotent, but are nevertheless specified to a precise heart region and cell type very early during development. Recent findings have defined both this lineage plasticity and early commitment of cardiac progenitors, using a combination of single-cell and population analyses. In this review, we discuss several aspects of cardiac progenitor specification. We discuss their markers, fate potential in vitro and in vivo, early segregation and commitment, and also intrinsic and extrinsic cues regulating lineage restriction from multipotency to a specific cell type of the heart. Finally, we also discuss the subdivisions of the cardiopharyngeal field, and the shared origins of the heart with other mesodermal derivatives, including head and neck muscles.

Long-term, in toto live imaging of cardiomyocyte behaviour during mouse ventricle chamber formation at single-cell resolution

Mapping of the holistic cell behaviours sculpting the four-chambered mammalian heart has been a goal or previous studies, but so far only success in transparent invertebrates and lower vertebrates with two-chambered hearts has been achieved. Using a live-imaging system comprising a customized vertical light-sheet microscope equipped with a mouse embryo culture module, a heartbeat-gated imaging strategy and a digital image processing framework, we realized volumetric imaging of developing mouse hearts at single-cell resolution and with uninterrupted cell lineages for up to 1.5 d. Four-dimensional landscapes of Nppa⁺ cardiomyocyte cell behaviours revealed a blueprint for ventricle chamber formation by which biased outward migration of the outermost cardiomyocytes is coupled with cell intercalation and horizontal division. The inner-muscle architecture of trabeculae was developed through dual mechanisms: early fate segregation and transmural cell arrangement involving both oriented cell division and directional migration. Thus, live-imaging reconstruction of uninterrupted cell lineages affords a transformative means for deciphering mammalian organogenesis.

Congenital heart diseases: genetics, non-inherited risk factors, and signaling pathways

Background

Congenital heart diseases (CHDs) are the most common congenital anomalies with an estimated prevalence of 8 in 1000 live births. CHDs occur as a result of abnormal embryogenesis of the heart. Congenital heart diseases are associated with significant mortality and morbidity. The damage of the heart is irreversible due to a lack of regeneration potential, and usually, the patients may require surgical intervention. Studying the developmental biology of the heart is essential not only in understanding the mechanisms and pathogenesis of congenital heart diseases but also in providing us with insight towards developing new preventive and treatment methods.

Main body

The etiology of congenital heart diseases is still elusive. Both genetic and environmental factors have been implicated to play a role in the pathogenesis of the diseases. Recently, cardiac transcription factors, cardiac-specific genes, and signaling pathways, which are responsible for early cardiac morphogenesis have been extensively studied in both human and animal experiments but leave much to be desired. The discovery of novel genetic methods such as next generation sequencing and chromosomal microarrays have led to further study the genes, non-coding RNAs and subtle chromosomal changes, elucidating their implications to the etiology of congenital heart diseases. Studies have also implicated non-hereditary risk factors such as rubella infection, teratogens, maternal age, diabetes mellitus, and abnormal hemodynamics in causing CHDs.

These etiological factors raise questions on multifactorial etiology of CHDs. It is therefore important to endeavor in research based on finding the causes of CHDs. Finding causative factors will enable us to plan intervention strategies and mitigate the consequences associated with CHDs. This review, therefore, puts forward the genetic and non-genetic causes of congenital heart diseases. Besides, it discusses crucial signaling pathways which are involved in early cardiac morphogenesis. Consequently, we aim to consolidate our knowledge on multifactorial causes of CHDs so as to pave a way for further research regarding CHDs.

Conclusion

The multifactorial etiology of congenital heart diseases gives us a challenge to explicitly establishing specific causative factors and therefore plan intervention strategies. More well-designed studies and the use of novel genetic technologies could be the way through the discovery of etiological factors implicated in the pathogenesis of congenital heart diseases.

Notch and Delta are required for survival of the germline stem cell lineage in testes of Drosophila melanogaster

In all metazoan species, sperm is produced from germline stem cells. These self-renew and produce daughter cells that amplify and differentiate dependent on interactions with somatic support cells. In the male gonad of Drosophila melanogaster, the germline and somatic cyst cells co-differentiate as cysts, an arrangement in which the germline is completely enclosed by cytoplasmic extensions from the cyst cells. Notch is a developmentally relevant receptor in a pathway requiring immediate proximity with the signal sending cell. Here, we show that Notch is expressed in the cyst cells of wild-type testes. Notch becomes activated in the transition zone, an apical area of the testes in which the cyst cells express stage-specific transcription factors and the enclosed germline finalizes transit-amplifying divisions. Reducing the ligand Delta from the germline cells via RNA-Interference or reducing the receptor Notch from the cyst cells via CRISPR resulted in cell death concomitant with loss of germline cells from the transition zone. This shows that Notch signaling is essential for the survival of the germline stem cell lineage.

The Significance of NOTCH Pathway in the Development of Fibrosis in Systemic Sclerosis

Systemic sclerosis (SSc) is an autoimmune disorder characterized by the fibrosis of skin, heart, lung, and kidney as well. Excessive activation of fibroblasts is associated with higher expression of Notch1 and/or Notch3 genes. The constitutive expression of NOTCH genes was described in epithelial cells: epidermal keratinocytes, hair follicle cells and sebaceous glands. The NOTCH signalling pathway may be involved in the development of fibrosis, myofibroblast formation and the process of epithelial-mesenchymal transition. Activation of the NOTCH pathway leads to morphological, phenotypic and functional changes in epithelial cells. Furthermore, inhibition of Notch signalling prevent the development of fibrosis in different models, among them, bleomycin-induced fibrosis and in the Task-1 mause model. Molecular mechanisms, including the role of NOTCH signaling pathway, associated with fibrosis in SSc have not been completely recognized.

Exposure to acrylamide disrupts cardiomyocyte interactions during ventricular morphogenesis in zebrafish embryos

Acrylamide (AA), a ubiquitous chemical that is present in surrounding environment and baked or fried carbohydrate-rich food, has recently been linked to cardiac developmental toxicity. However, the toxicological role of AA exposure in the cardiac development remains largely unknown. Here we showed the cardiotoxicity of AA and its role in cardiomyocyte interactions in zebrafish embryos during ventricular morphogenesis. Using the embryo model of transgenic zebrafish Tg(Tp1:d2GFP;myl7:mCherry), we found AA interfered the dynamics of Notch signaling in the endocardium during early cardiogenesis. Prolonged exposure to AA thickened the chamber wall and prevented the trabeculae from extending into the lumen of ventricular chamber. As a result, AA reduced the ventricular shortening fraction and spatial dimension via excessively activating the Notch signal in myocardium during cardiac maturation. Moreover, exposure to AA inhibited the re-distribution of N‑cadherin and failed to coordinate cardiomyocyte interactions between the myocardium layers due to the lack of delaminated cardiomyocytes. Therefore, AA-treated embryos exhibited subcellular pathological states including disarrayed myofibrils and abnormal morphology of mitochondria despite normal proliferation of cardiomyocytes. In addition, we found overexpression of some cardiac-specific transcription factors, such as hand2 and nkx2.5, in hearts of AA-treated embryos compared with those in control group. Our study provided the evidence that the period of ventricular chamber morphogenesis might be a vulnerable window in zebrafish, and revealed new insights into how AA might exert cardiac developmental toxicity.

MIB1 mutations reduce Notch signaling activation and contribute to congenital heart disease

Congenital heart disease (CHD) is one of the most common birth defects in humans, but its genetic etiology remains largely unknown despite decades of research. The Notch signaling pathway plays critical roles in embryonic cardiogenesis. Mind bomb 1 (Mib1) is a vital protein that activates the Notch signaling pathway through promoting ubiquitination, endocytosis and subsequent activation of Notch ligands. Previous studies show that Mib1 knockout in mice completely abolishes Notch signaling, leading to cardiac deformity. However, the function of MIB1 and its potential disease-causing mutations are poorly studied in human CHD. In this research, we identified four novel non-synonymous heterozygous rare mutations of MIB1 from 417 Han Chinese CHD patients. The following biochemical analyses revealed that mutations p.T312K fs*55 and p.W271G significantly deplete MIB1’s function, resulting in a lower level of JAGGED1 (JAG1) ubiquitination and Notch signaling induction. Our results suggest that pathologic variants in MIB1 may contribute to CHD occurrence, shedding new light on the genetic mechanism of CHD in the context of the Notch signaling pathway.

NOTCH maintains developmental cardiac gene network through WNT5A

NOTCH and WNT signaling pathways play critical roles in cardiac chamber formation. Here we explored the potential interactions between the two pathways in this developmental process by using genetically modified mouse models and whole embryo culture systems. By deletion of Notch1 to inactivate NOTCH1 signaling in the endocardium in vivo and ex vivo rescue experiments, we showed that myocardial WNT5A mediated endocardial NOTCH1 signaling to maintain the gene regulatory network essential for cardiac chamber formation. Furthermore, genetic deletion of β-catenin in the myocardium and inhibition of the WNT/Ca²⁺ signaling by FK506 resulted in a similar disruption of the gene regulatory network as inactivation of endocardial NOTCH1 signaling. Together, these findings identify WNT5A as a key myocardial factor that mediates the endocardial NOTCH signaling to maintain the gene regulatory network essential for cardiac chamber formation through WNT/β-catenin and WNT/Ca²⁺ signaling pathways.

Defining the earliest step of cardiovascular lineage segregation by single-cell RNA-seq

Mouse heart development arises from Mesp1-expressing cardiovascular progenitors (CPs) that are specified during gastrulation. The molecular processes that control early regional and lineage segregation of CPs have been unclear. We performed single-cell RNA sequencing of wild-type and Mesp1-null CPs in mice. We showed that populations of Mesp1 CPs are molecularly distinct and span the continuum between epiblast and later mesodermal cells, including hematopoietic progenitors. Single-cell transcriptome analysis of Mesp1-deficient CPs showed that Mesp1 is required for the exit from the pluripotent state and the induction of the cardiovascular gene expression program. We identified distinct populations of Mesp1 CPs that correspond to progenitors committed to different cell lineages and regions of the heart, identifying the molecular features associated with early lineage restriction and regional segregation of the heart at the early stage of mouse gastrulation.

Integration of Drosophila and Human Genetics to Understand Notch Signaling Related Diseases

Notch signaling research dates back to more than one hundred years, beginning with the identification of the Notch mutant in the fruit fly Drosophila melanogaster. Since then, research on Notch and related genes in flies has laid the foundation of what we now know as the Notch signaling pathway. In the 1990s, basic biological and biochemical studies of Notch signaling components in mammalian systems, as well as identification of rare mutations in Notch signaling pathway genes in human patients with rare Mendelian diseases or cancer, increased the significance of this pathway in human biology and medicine. In the 21st century, Drosophila and other genetic model organisms continue to play a leading role in understanding basic Notch biology. Furthermore, these model organisms can be used in a translational manner to study underlying mechanisms of Notch-related human diseases and to investigate the function of novel disease associated genes and variants. In this chapter, we first briefly review the major contributions of Drosophila to Notch signaling research, discussing the similarities and differences between the fly and human pathways. Next, we introduce several biological contexts in Drosophila in which Notch signaling has been extensively characterized. Finally, we discuss a number of genetic diseases caused by mutations in genes in the Notch signaling pathway in humans and we expand on how Drosophila can be used to study rare genetic variants associated with these and novel disorders. By combining modern genomics and state-of-the art technologies, Drosophila research is continuing to reveal exciting biology that sheds light onto mechanisms of disease.

2,3,5,4'-Tetrahydroxystilbene-2-O-β-D-glucoside protects murine hearts against ischemia/reperfusion injury by activating Notch1/Hes1 signaling and attenuating endoplasmic reticulum stress

2,3,5,4'-Tetrahydroxystilbene-2-O-β-D-glucoside (TSG) is a water-soluble active component extracted from Polygonum multiflorum Thunb. A number of studies demonstrate that TSG exerts cardioprotective effects. Since endoplasmic reticulum (ER) stress plays a key role in myocardial ischemia/reperfusion (MI/R)-induced cell apoptosis, we sought to determine whether modulation of the ER stress during MI/R injury was involved in the cardioprotective action of TSG. Male mice were treated with TSG (60 mg·kg^-1·d^-1, ig) for 2 weeks and then were subjected to MI/R surgery. Pre-administration of TSG significantly improved post-operative cardiac function, and suppressed MI/R-induced myocardial apoptosis, evidenced by the reduction in the myocardial apoptotic index, serum levels of LDH and CK after 6 h of reperfusion. TSG (0.1-1000 μmol/L) did not affect the viability of cultured H9c2 cardiomyoblasts in vitro, but pretreatment with TSG dose-dependently decreased simulated ischemia/reperfusion (SIR)-induced cell apoptosis. Furthermore, both in vivo and in vitro studies revealed that TSG treatment activated the Notch1/Hes1 signaling pathway and suppressed ER stress, as evidenced by increasing Notch1, Notch1 intracellular domain (NICD), Hes1, and Bcl-2 expression levels and by decreasing p-PERK/PERK ratio, p-eIF2α/eIF2α ratio, and ATF4, CHOP, Bax, and caspase-3 expression levels. Moreover, the protective effects conferred by TSG on SIR-treated H9c2 cardiomyoblasts were abolished by co-administration of DAPT (the Notch1 signaling inhibitor). In summary, TSG ameliorates MI/R injury in vivo and in vitro by activating the Notch1/Hes1 signaling pathway and attenuating ER stress-induced apoptosis.

Notch3 Ameliorates Cardiac Fibrosis After Myocardial Infarction by Inhibiting the TGF-β1/Smad3 Pathway

Notch3 and TGF-β1 signaling play a key role in the pathogenesis and progression of chronic cardiovascular disease. However, whether Notch3 protects against myocardial infarction (MI) and the underlying mechanisms remains unknown. C57BL/6 mice were randomized to be treated with Notch3 siRNA (siNotch3) or lentivirus carrying Notch3 cDNA (Notch3) before coronary artery ligation. Four weeks after constructing MI model, cardiac function and fibrosis were compared between groups. The cardiac fibroblast cells (CFs) were isolated from newborn C57BL/6 mice (1-3 days old) and transfected with lentivirus carrying Notch3 cDNA. TGF-β1 (5 ng/ml), a well-known pro-fibrotic factor, was administered 72 h after Notch3 cDNA administration in CFs. The related proteins of fibrosis such as a-smooth muscle actin (a-SMA), Type I collagen, metalloprotease (MMP)-9 and the tissue inhibitor of metalloproteinases (TIMP)-2 were examined by western blot analysis. Notch3 cDNA treatment attenuated cardiac damage and inhibited fibrosis in mice with MI. Meanwhile, Notch3 siRNA administration aggravated cardiac function damage and markedly enhanced cardiac fibrosis in mice with MI. Overexpression of Notch3 inhibited TGF-β1-induced fibroblast-myofibroblast transition of mouse cardiac fibroblast cells, as evidenced by down-regulating a-SMA and Type I collagen expression. Notch3 cDNA treatment also increased MMP-9 expression and decreased TIMP-2 expression in the TGF-β1-stimulated cells. This study indicates that Notch3 is an important protective factor for cardiac fibrosis in a MI model, and the protective effect of Notch3 is attributable to its action on TGF-β1/Smad3 signaling.

The intracellular domains of Notch1 and Notch2 are functionally equivalent during development and carcinogenesis

Although Notch1 and Notch2 are closely related paralogs and function through the same canonical signaling pathway, they contribute to different outcomes in some cell and disease contexts. To understand the basis for these differences, we examined in detail mice in which the Notch intracellular domains (N1ICD and N2ICD) were swapped. Our data indicate that strength (defined here as the ultimate number of intracellular domain molecules reaching the nucleus, integrating ligand-mediated release and nuclear translocation) and duration (half-life of NICD-RBPjk-MAML-DNA complexes, integrating cooperativity and stability dependent on shared sequence elements) are the factors that underlie many of the differences between Notch1 and Notch2 in all the contexts we examined, including T-cell development, skin differentiation and carcinogenesis, the inner ear, the lung and the retina. We were able to show that phenotypes in the heart, endothelium, and marginal zone B cells are attributed to haploinsufficiency but not to intracellular domain composition. Tissue-specific differences in NICD stability were most likely caused by alternative scissile bond choices by tissue-specific γ-secretase complexes following the intracellular domain swap. Reinterpretation of clinical findings based on our analyses suggests that differences in outcome segregating with Notch1 or Notch2 are likely to reflect outcomes dependent on the overall strength of Notch signals.

Notch1 Mutation Leads to Valvular Calcification Through Enhanced Myofibroblast Mechanotransduction

OBJECTIVE

Calcific aortic valve disease (CAVD) is a significant cardiovascular disorder, and controversy exists as to whether it is primarily a dystrophic or osteogenic process in vivo. In this study, we sought to clarify the mechanism of CAVD by assessing a genetic mutation, Notch1 heterozygosity, which leads to CAVD with 100% penetrance in humans.

APPROACH AND RESULTS

Murine immortalized Notch1(+/-) aortic valve interstitial cells (AVICs) were isolated and expanded in vitro. Molecular signaling of wild-type and Notch1(+/-) AVICs were compared to identify changes in pathways that have been linked to CAVD-transforming growth factor-β1/bone morphogenetic protein, mitogen-activated protein kinase, and phosphoinositide 3-kinase/protein kinase B-and assessed for calcification potential. Additionally, AVIC mechanobiology was studied in a physiologically relevant, dynamic mechanical environment (10% cyclic strain) to investigate differences in responses between the cell types. We found that Notch1(+/-) AVICs resembled a myofibroblast-like phenotype expressing higher amounts of cadherin-11, a known mediator of dystrophic calcification, and decreased Runx2, a known osteogenic marker. We determined that cadherin-11 expression is regulated by Akt activity, and inhibition of Akt phosphorylation significantly reduced cadherin-11 expression. Moreover, in the presence of cyclic strain, Notch1(+/-) AVICs exhibited significantly upregulated phosphorylation of Akt at Ser473 and smooth muscle α-actin expression, indicative of a fully activated myofibroblast. Finally, these Notch1-mediated alterations led to enhanced dystrophic calcific nodule formation.

CONCLUSIONS

This study presents novel insights in our understanding of Notch1-mediated CAVD by demonstrating that the mutation leads to AVICs that are fully activated myofibroblasts, resulting in dystrophic, but not osteogenic, calcification.

Maternal and zygotic Zfp57 modulate NOTCH signaling in cardiac development

Zfp57 is a maternal-zygotic effect gene that maintains genomic imprinting. Here we report that Zfp57 mutants exhibited a variety of cardiac defects including atrial septal defect (ASD), ventricular septal defect (VSD), thin myocardium, and reduced trabeculation. Zfp57 maternal-zygotic mutant embryos displayed more severe phenotypes with higher penetrance than the zygotic ones. Cardiac progenitor cells exhibited proliferation and differentiation defects in Zfp57 mutants. ZFP57 is a master regulator of genomic imprinting, so the DNA methylation imprint was lost in embryonic heart without ZFP57. Interestingly, the presence of imprinted DLK1, a target of ZFP57, correlated with NOTCH1 activation in cardiac cells. These results suggest that ZFP57 may modulate NOTCH signaling during cardiac development. Indeed, loss of ZFP57 caused loss of NOTCH1 activation in embryonic heart with more severe loss observed in the maternal-zygotic mutant. Maternal and zygotic functions of Zfp57 appear to play redundant roles in NOTCH1 activation and cardiomyocyte differentiation. This serves as an example of a maternal effect that can influence mammalian organ development. It also links genomic imprinting to NOTCH signaling and particular developmental functions.

A detailed comparison of mouse and human cardiac development

BACKGROUND

Mouse mutants are used to model human congenital cardiovascular disease. Few studies exist comparing normal cardiovascular development in mice vs. humans. We carried out a systematic comparative analysis of mouse and human fetal cardiovascular development.

METHODS

Episcopic fluorescence image capture (EFIC) was performed on 66 wild-type mouse embryos from embryonic day (E) 9.5 to birth; 2-dimensional and 3-dimensional datasets were compared with EFIC and magnetic resonance images from a study of 52 human fetuses (Carnegie stage 13-23).

RESULTS

Time course of atrial, ventricular, and outflow septation were outlined and followed a similar sequence in both species. Bilateral venae cavae and prominent atrial appendages were seen in the mouse fetus; in human fetuses, atrial appendages were small, and a single right superior vena cava was present. In contrast to humans with separate pulmonary vein orifices, a pulmonary venous confluence with one orifice enters the left atrium in mice.

CONCLUSION

The cardiac developmental sequences observed in mouse and human fetuses are comparable, with minor differences in atrial and venous morphology. These comparisons of mouse and human cardiac development strongly support that mouse morphogenesis is a good model for human development.

Timely inhibition of Notch signaling by DAPT promotes cardiac differentiation of murine pluripotent stem cells

The Notch signaling pathway plays versatile roles during heart development. However, there is contradictory evidence that Notch pathway either facilitates or impairs cardiomyogenesis in vitro. In this study, we developed iPSCs by reprogramming of murine fibroblasts with GFP expression governed by Oct4 promoter, and identified an effective strategy to enhance cardiac differentiation through timely modulation of Notch signaling. The Notch inhibitor DAPT (N-[N-(3,5-difluorophenacetyl)-l-alanyl]-S-phenylglycine t-butyl ester) alone drove the iPSCs to a neuronal fate. After mesoderm induction of embryoid bodies initiated by ascorbic acid (AA), the subsequent treatment of DAPT accelerated the generation of spontaneously beating cardiomyocytes. The timed synergy of AA and DAPT yielded an optimal efficiency of cardiac differentiation. Mechanistic studies showed that Notch pathway plays a biphasic role in cardiomyogenesis. It favors the early–stage cardiac differentiation, but exerts negative effects on the late-stage differentiation. Therefore, DAPT administration at the late stage enforced the inhibition of endogenous Notch activity, thereby enhancing cardiomyogenesis. In parallel, DAPT dramatically augmented the expression of Wnt3a, Wnt11, BMP2, and BMP4. In conclusion, our results highlight a practicable approach to generate cardiomyocytes from iPSCs based on the stage-specific biphasic roles of Notch signaling in cardiomyogenesis.

Manipulation of a VEGF-Notch signaling circuit drives formation of functional vascular endothelial progenitors from human pluripotent stem cells

Human pluripotent stem cell (hPSC)-derived endothelial lineage cells constitutes a promising source for therapeutic revascularization, but progress in this arena has been hampered by a lack of clinically-scalable differentiation protocols and inefficient formation of a functional vessel network integrating with the host circulation upon transplantation. Using a human embryonic stem cell reporter cell line, where green fluorescent protein expression is driven by an endothelial cell-specific VE-cadherin (VEC) promoter, we screened for > 60 bioactive small molecules that would promote endothelial differentiation, and found that administration of BMP4 and a GSK-3β inhibitor in an early phase and treatment with VEGF-A and inhibition of the Notch signaling pathway in a later phase led to efficient differentiation of hPSCs to the endothelial lineage within six days. This sequential approach generated > 50% conversion of hPSCs to endothelial cells (ECs), specifically VEC⁺CD31⁺CD34⁺CD14⁻KDR^high endothelial progenitors (EPs) that exhibited higher angiogenic and clonogenic proliferation potential among endothelial lineage cells. Pharmaceutical inhibition or genetical knockdown of Notch signaling, in combination with VEGF-A treatment, resulted in efficient formation of EPs via KDR⁺ mesodermal precursors and blockade of the conversion of EPs to mature ECs. The generated EPs successfully formed functional capillary vessels in vivo with anastomosis to the host vessels when transplanted into immunocompromised mice. Manipulation of this VEGF-A-Notch signaling circuit in our protocol leads to rapid large-scale production of the hPSC-derived EPs by 12- to 20-fold vs current methods, which may serve as an attractive cell population for regenerative vascularization with superior vessel forming capability compared to mature ECs.

Notch signaling is required for the formation of mesangial cells from a stromal mesenchyme precursor during kidney development

Mesangial cells are specialized pericyte/smooth muscle cells that surround and constrain the vascular network within the glomerulus of the kidney. They are derived from the stromal mesenchyme, a progenitor population distinct from nephron stem cells. Whether mesangial cells have a distinct origin from vascular smooth muscle cells (VSMCs) and the pathways that govern their specification are unknown. Here we show that Notch signaling in stromal progenitors is essential for mesangial cell formation but is dispensable for the smooth muscle and interstitial cell lineages. Deletion of RBPjk, the common DNA-binding partner of all active Notch receptors, with Foxd1(tgCre) results in glomerular aneurysm and perinatal death from kidney failure. This defect occurs early in glomerular development as stromal-derived, desmin-positive cells fail to coalesce near forming nephrons and thus do not invade the vascular cleft of the S-shaped body. This is in contrast to other mutants in which the loss of the mesangium was due to migration defects, and suggests that loss of Notch signaling results in a failure to specify this population from the stroma. Interestingly, Pdgfrb-positive VSMCs do not enter the vascular cleft and cannot rescue the mesangial deficiency. Notch1 and Notch2 act redundantly through γ-secretase and RBPjk in this process, as individual mutants have mesangial cells at birth. Together, these data demonstrate a unique origin of mesangial cells and demonstrate a novel, redundant function for Notch receptors in mesangial cell specification, proliferation or survival during kidney development.

From zebrafish heart jogging genes to mouse and human orthologs: using Gene Ontology to investigate mammalian heart development

For the majority of organs in developing vertebrate embryos, left-right asymmetry is controlled by a ciliated region; the left-right organizer node in the mouse and human, and the Kuppfer's vesicle in the zebrafish. In the zebrafish, laterality cues from the Kuppfer's vesicle determine asymmetry in the developing heart, the direction of 'heart jogging' and the direction of 'heart looping'.  'Heart jogging' is the term given to the process by which the symmetrical zebrafish heart tube is displaced relative to the dorsal midline, with a leftward 'jog'. Heart jogging is not considered to occur in mammals, although a leftward shift of the developing mouse caudal heart does occur prior to looping, which may be analogous to zebrafish heart jogging. Previous studies have characterized 30 genes involved in zebrafish heart jogging, the majority of which have well defined orthologs in mouse and human and many of these orthologs have been associated with early mammalian heart development.    We undertook manual curation of a specific set of genes associated with heart development and we describe the use of Gene Ontology term enrichment analyses to examine the cellular processes associated with heart jogging.  We found that the human, mouse and zebrafish 'heart jogging orthologs' are involved in similar organ developmental processes across the three species, such as heart, kidney and nervous system development, as well as more specific cellular processes such as cilium development and function. The results of these analyses are consistent with a role for cilia in the determination of left-right asymmetry of many internal organs, in addition to their known role in zebrafish heart jogging.    This study highlights the importance of model organisms in the study of human heart development, and emphasises both the conservation and divergence of developmental processes across vertebrates, as well as the limitations of this approach.

From zebrafish heart jogging genes to mouse and human orthologs: using Gene Ontology to investigate mammalian heart development

For the majority of organs in developing vertebrate embryos, left-right asymmetry is controlled by a ciliated region; the left-right organizer node in the mouse and human, and the Kuppfer’s vesicle in the zebrafish. In the zebrafish, laterality cues from the Kuppfer’s vesicle determine asymmetry in the developing heart, the direction of ‘heart jogging’ and the direction of ‘heart looping’.  ‘Heart jogging’ is the term given to the process by which the symmetrical zebrafish heart tube is displaced relative to the dorsal midline, with a leftward ‘jog’. Heart jogging is not considered to occur in mammals, although a leftward shift of the developing mouse caudal heart does occur prior to looping, which may be analogous to zebrafish heart jogging. Previous studies have characterized 30 genes involved in zebrafish heart jogging, the majority of which have well defined orthologs in mouse and human and many of these orthologs have been associated with early mammalian heart development. 

 

We undertook manual curation of a specific set of genes associated with heart development and we describe the use of Gene Ontology term enrichment analyses to examine the cellular processes associated with heart jogging.  We found that the human, mouse and zebrafish ‘heart jogging orthologs’ are involved in similar organ developmental processes across the three species, such as heart, kidney and nervous system development, as well as more specific cellular processes such as cilium development and function. The results of these analyses are consistent with a role for cilia in the determination of left-right asymmetry of many internal organs, in addition to their known role in zebrafish heart jogging. 

 

This study highlights the importance of model organisms in the study of human heart development, and emphasises both the conservation and divergence of developmental processes across vertebrates, as well as the limitations of this approach.

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.

Embryonic cardiac chamber maturation: Trabeculation, conduction, and cardiomyocyte proliferation

Congenital heart diseases are some of the most common human birth defects. Though some congenital heart defects can be surgically corrected, treatment options for other congenital heart diseases are very limited. In many congenital heart diseases, genetic defects lead to impaired embryonic heart development or growth. One of the key development processes in cardiac development is chamber maturation, and alterations in this maturation process can manifest as a variety of congenital defects including non-compaction, systolic dysfunction, diastolic dysfunction, and arrhythmia. During development, to meet the increasing metabolic demands of the developing embryo, the myocardial wall undergoes extensive remodeling characterized by the formation of muscular luminal protrusions called cardiac trabeculae, increased cardiomyocyte mass, and development of the ventricular conduction system. Though the basic morphological and cytological changes involved in early heart development are clear, much remains unknown about the complex biomolecular mechanisms governing chamber maturation. In this review, we highlight evidence suggesting that a wide variety of basic signaling pathways and biomechanical forces are involved in cardiac wall maturation.

Genetic interaction between pku300 and fbn2b controls endocardial cell proliferation and valve development in zebrafish

Abnormal cardiac valve morphogenesis is a common cause of human congenital heart disease. The molecular mechanisms regulating endocardial cell proliferation and differentiation into cardiac valves remain largely unknown, although great progress has been made on the endocardial contribution to the atrioventricular cushion and valve formation. We found that scotch tape(te382) (sco(te382)) encodes a novel transmembrane protein that is crucial for endocardial cell proliferation and heart valve development. The zebrafish sco(te382) mutant showed diminished endocardial cell proliferation, lack of heart valve leaflets and abnormal common cardinal and caudal veins. Positional cloning revealed a C946T nonsense mutation of a novel gene pku300 in the sco(te382) locus, which encoded a 540-amino-acid protein on cell membranes with one putative transmembrane domain and three IgG domains. A known G3935T missense mutation of fbn2b was also found ∼570 kb away from pku300 in sco(te382) mutants. The genetic mutant sco(pku300), derived from sco(te382), only had the C946T mutation of pku300 and showed reduced numbers of atrial endocardial cells and an abnormal common cardinal vein. Morpholino knockdown of fbn2b led to fewer atrial endocardial cells and an abnormal caudal vein. Knockdown of both pku300 and fbn2b phenocopied these phenotypes in sco(te382) genetic mutants. pku300 transgenic expression in endocardial and endothelial cells, but not myocardial cells, partially rescued the atrial endocardial defects in sco(te382) mutants. Mechanistically, pku300 and fbn2b were required for endocardial cell proliferation, endocardial Notch signaling and the proper formation of endocardial cell adhesion and tight junctions, all of which are crucial for cardiac valve development. We conclude that pku300 and fbn2b represent the few genes capable of regulating endocardial cell proliferation and signaling in zebrafish cardiac valve development.

Mutations in the NOTCH pathway regulator MIB1 cause left ventricular noncompaction cardiomyopathy

Left ventricular noncompaction (LVNC) causes prominent ventricular trabeculations and reduces cardiac systolic function. The clinical presentation of LVNC ranges from asymptomatic to heart failure. We show that germline mutations in human MIB1 (mindbomb homolog 1), which encodes an E3 ubiquitin ligase that promotes endocytosis of the NOTCH ligands DELTA and JAGGED, cause LVNC in autosomal-dominant pedigrees, with affected individuals showing reduced NOTCH1 activity and reduced expression of target genes. Functional studies in cells and zebrafish embryos and in silico modeling indicate that MIB1 functions as a dimer, which is disrupted by the human mutations. Targeted inactivation of Mib1 in mouse myocardium causes LVNC, a phenotype mimicked by inactivation of myocardial Jagged1 or endocardial Notch1. Myocardial Mib1 mutants show reduced ventricular Notch1 activity, expansion of compact myocardium to proliferative, immature trabeculae and abnormal expression of cardiac development and disease genes. These results implicate NOTCH signaling in LVNC and indicate that MIB1 mutations arrest chamber myocardium development, preventing trabecular maturation and compaction.

The Notch pathway controls fibrotic and regenerative repair in the adult heart

Aims

In the adult heart, Notch signalling regulates the response to injury. Notch inhibition leads to increased cardiomyocyte apoptosis, and exacerbates the development of cardiac hypertrophy and fibrosis. The role of Notch in the mesenchymal stromal cell fraction, which contains cardiac fibroblasts and cardiac precursor cells, is, however, largely unknown. In the present study, we evaluate, therefore, whether forced activation of the Notch pathway in mesenchymal stromal cells regulates pathological cardiac remodelling.

Methods and results

We generated transgenic mice overexpressing the Notch ligand Jagged1 on the surface of cardiomyocytes to activate Notch signalling in adjacent myocyte and non-myocyte cells. In neonatal transgenic mice, activated Notch sustained cardiac precursor and myocyte proliferation after birth, and led to increased numbers of cardiac myocytes in adult mice. In the adult heart under pressure overload, Notch inhibited the development of cardiomyocyte hypertrophy and transforming growth factor-β/connective tissue growth factor-mediated cardiac fibrosis. Most importantly, Notch activation in the stressed adult heart reduced the proliferation of myofibroblasts and stimulated the expansion of stem cell antigen-1-positive cells, and in particular of Nkx2.5-positive cardiac precursor cells.

Conclusions

We conclude that Notch is pivotal in the healing process of the injured heart. Specifically, Notch regulates key cellular mechanisms in the mesenchymal stromal cell population, and thereby controls the balance between fibrotic and regenerative repair in the adult heart. Altogether, these findings indicate that Notch represents a unique therapeutic target for inducing regeneration in the adult heart via mobilization of cardiac precursor cells.

Partitioning the heart: mechanisms of cardiac septation and valve development

Heart malformations are common congenital defects in humans. Many congenital heart defects involve anomalies in cardiac septation or valve development, and understanding the developmental mechanisms that underlie the formation of cardiac septal and valvular tissues thus has important implications for the diagnosis, prevention and treatment of congenital heart disease. The development of heart septa and valves involves multiple types of progenitor cells that arise either within or outside the heart. Here, we review the morphogenetic events and genetic networks that regulate spatiotemporal interactions between the cells that give rise to septal and valvular tissues and hence partition the heart.

Wnt/β-catenin and Bmp signals control distinct sets of transcription factors in cardiac progenitor cells

Progenitor cells of the first and second heart fields depend on cardiac-specific transcription factors for their differentiation. Using conditional mutagenesis of mouse embryos, we define the hierarchy of signaling events that controls the expression of cardiac-specific transcription factors during differentiation of cardiac progenitors at embryonic day 9.0. Wnt/β-catenin and Bmp act downstream of Notch/RBPJ at this developmental stage. Mutation of Axin2, the negative regulator of canonical Wnt signaling, enhances Wnt and Bmp4 signals and suffices to rescue the arrest of cardiac differentiation caused by loss of RBPJ. Using FACS enrichment of cardiac progenitors in RBPJ and RBPJ/Axin2 mutants, embryo cultures in the presence of the Bmp inhibitor Noggin, and by crossing a Bmp4 mutation into the RBPJ/Axin2 mutant background, we show that Wnt and Bmp4 signaling activate specific and nonoverlapping cardiac-specific genes in the cardiac progenitors: Nkx2-5, Isl1 and Baf60c are controlled by Wnt/β-catenin, and Gata4, SRF, and Mef2c are controlled by Bmp signaling. Our study contributes to the understanding of the regulatory hierarchies of cardiac progenitor differentiation and outflow tract development and has implications for understanding and modeling heart development.

Notch and disease: a growing field

Signals through the Notch receptors are used throughout development to control cellular fate choices. Our intention here is to provide an overview of the involvement of Notch signaling in human disease, which, keeping pace with the known biology of the pathway, manifests itself in a pleiotropic fashion. A pathway with such broad action in normal development, a profound involvement in the biology of adult stem cells and intricate and complex controls governing its activity, poses numerous challenges. We provide an overview of Notch related pathologies identified thus far and emphasize aspects that have been modeled in experimental systems in order to understand the underlying pathobiology and, hopefully, help the definition of rational therapeutic avenues.