OCT4-mediated inflammation induces cell reprogramming at the origin of cardiac valve development and calcification.

Cell plasticity plays a key role in embryos by maintaining the differentiation potential of progenitors. Whether postnatal somatic cells revert to an embryonic-like naïve state regaining plasticity and redifferentiate into a cell type leading to a disease remains intriguing. Using genetic lineage tracing and single-cell RNA sequencing, we reveal that Oct4 is induced by nuclear factor κB (NFκB) at embyronic day 9.5 in a subset of mouse endocardial cells originating from the anterior heart forming field at the onset of endocardial-to-mesenchymal transition. These cells acquired a chondro-osteogenic fate. OCT4 in adult valvular aortic cells leads to calcification of mouse and human valves. These calcifying cells originate from the Oct4 embryonic lineage. Genetic deletion of Pou5f1 (Pit-Oct-Unc, OCT4) in the endocardial cell lineage prevents aortic stenosis and calcification of ApoE−/− mouse valve. We established previously unidentified self-cell reprogramming NFκB- and OCT4-mediated inflammatory pathway triggering a dose-dependent mechanism of valve calcification.

Mutations in DCHS1 cause mitral valve prolapse.

Mitral valve prolapse (MVP) is a common cardiac valve disease that affects nearly 1 in 40 individuals. It can manifest as mitral regurgitation and is the leading indication for mitral valve surgery. Despite a clear heritable component, the genetic aetiology leading to non-syndromic MVP has remained elusive. Four affected individuals from a large multigenerational family segregating non-syndromic MVP underwent capture sequencing of the linked interval on chromosome 11. We report a missense mutation in the DCHS1 gene, the human homologue of the Drosophila cell polarity gene dachsous (ds), that segregates with MVP in the family. Morpholino knockdown of the zebrafish homologue dachsous1b resulted in a cardiac atrioventricular canal defect that could be rescued by wild-type human DCHS1, but not by DCHS1 messenger RNA with the familial mutation. Further genetic studies identified two additional families in which a second deleterious DCHS1 mutation segregates with MVP. Both DCHS1 mutations reduce protein stability as demonstrated in zebrafish, cultured cells and, notably, in mitral valve interstitial cells (MVICs) obtained during mitral valve repair surgery of a proband. Dchs1(+/-) mice had prolapse of thickened mitral leaflets, which could be traced back to developmental errors in valve morphogenesis. DCHS1 deficiency in MVP patient MVICs, as well as in Dchs1(+/-) mouse MVICs, result in altered migration and cellular patterning, supporting these processes as aetiological underpinnings for the disease. Understanding the role of DCHS1 in mitral valve development and MVP pathogenesis holds potential for therapeutic insights for this very common disease.

miR-21 represses Pdcd4 during cardiac valvulogenesis.

The discovery of small non-coding microRNAs has revealed novel mechanisms of post-translational regulation of gene expression, the implications of which are still incompletely understood. We focused on microRNA 21 (miR-21), which is expressed in cardiac valve endothelium during development, in order to better understand its mechanistic role in cardiac valve development. Using a combination of in vivo gene knockdown in zebrafish and in vitro assays in human cells, we show that miR-21 is necessary for proper development of the atrioventricular valve (AV). We identify pdcd4b as a relevant in vivo target of miR-21 and show that protection of pdcd4b from miR-21 binding results in failure of AV development. In vitro experiments using human pulmonic valve endothelial cells demonstrate that miR-21 overexpression augments endothelial cell migration. PDCD4 knockdown alone was sufficient to enhance endothelial cell migration. These results demonstrate that miR-21 plays a necessary role in cardiac valvulogenesis, in large part due to an obligatory downregulation of PDCD4.

Identification of heart rate-associated loci and their effects on cardiac conduction and rhythm disorders.

Elevated resting heart rate is associated with greater risk of cardiovascular disease and mortality. In a 2-stage meta-analysis of genome-wide association studies in up to 181,171 individuals, we identified 14 new loci associated with heart rate and confirmed associations with all 7 previously established loci. Experimental downregulation of gene expression in Drosophila melanogaster and Danio rerio identified 20 genes at 11 loci that are relevant for heart rate regulation and highlight a role for genes involved in signal transmission, embryonic cardiac development and the pathophysiology of dilated cardiomyopathy, congenital heart failure and/or sudden cardiac death. In addition, genetic susceptibility to increased heart rate is associated with altered cardiac conduction and reduced risk of sick sinus syndrome, and both heart rate-increasing and heart rate-decreasing variants associate with risk of atrial fibrillation. Our findings provide fresh insights into the mechanisms regulating heart rate and identify new therapeutic targets.

Patterning and development of the atrioventricular canal in zebrafish.

Proper atrioventricular canal (AVC) patterning and subsequent valvulogenesis is a complex process, and defects can result in disease or early death. The zebrafish Danio rerio has become a useful model system for studying AVC development, and much progress has been made in dissecting out the critical steps. Here, we review the recent advances in the field and highlight the cellular and molecular changes observed during zebrafish AVC development.

Novel chemical suppressors of long QT syndrome identified by an in vivo functional screen.

BACKGROUND: Genetic long QT (LQT) syndrome is a life-threatening disorder caused by mutations that result in prolongation of cardiac repolarization. Recent work has demonstrated that a zebrafish model of LQT syndrome faithfully recapitulates several features of human disease, including prolongation of ventricular action potential duration, spontaneous early afterdepolarizations, and 2:1 atrioventricular block in early stages of development. Because of their transparency, small size, and absorption of small molecules from their environment, zebrafish are amenable to high-throughput chemical screens. We describe a small-molecule screen using the zebrafish KCNH2 mutant breakdance to identify compounds that can rescue the LQT type 2 phenotype. METHODS AND RESULTS: Zebrafish breakdance embryos were exposed to test compounds at 48 hours of development and scored for rescue of 2:1 atrioventricular block at 72 hours in a 96-well format. Only compounds that suppressed the LQT phenotype in 3 of 3 fish were considered hits. Screen compounds were obtained from commercially available small-molecule libraries (Prestwick and Chembridge). Initial hits were confirmed with dose-response testing and time-course studies. Optical mapping with the voltage-sensitive dye di-4 ANEPPS was performed to measure compound effects on cardiac action potential durations. Screening of 1200 small molecules resulted in the identification of flurandrenolide and 2-methoxy-N-(4-methylphenyl) benzamide (2-MMB) as compounds that reproducibly suppressed the LQT phenotype. Optical mapping confirmed that treatment with each compound caused shortening of ventricular action potential durations. Structure activity studies and steroid receptor knockdown suggest that flurandrenolide functions via the glucocorticoid signaling pathway. CONCLUSIONS: Using a zebrafish model of LQT type 2 syndrome in a high-throughput chemical screen, we have identified 2 compounds, flurandrenolide and the novel compound 2-MMB, as small molecules that rescue the zebrafish LQT type 2 syndrome by shortening the ventricular action potential duration. We provide evidence that flurandrenolide functions via the glucocorticoid receptor-mediated pathway. These 2 molecules and future discoveries from this screen should yield novel tools for the study of cardiac electrophysiology and may lead to novel therapeutics for human LQT patients.

Small molecule screening in zebrafish.

Small molecule screens have become useful tools in ongoing efforts to understand complex biologic systems and disease states. These screens can identify compounds that specifically affect signaling pathways, development, and disease processes. The zebrafish Danio rerio has increasingly been used as a whole organism model in which to perform such functional small molecule screens. Here, we review recent advances in screening approaches that focus on modulation of developmental processes and signaling pathways, along with suppressor screens of disease models. The identification of small molecule targets continues to be a challenge, and several recent approaches to this problem are discussed. The promise of chemical screens in zebrafish to identify novel biologic probes and therapeutic compounds continues to drive this rapidly growing field.

Chondroitin sulfate expression is required for cardiac atrioventricular canal formation.

Defects in cardiac valvulogenesis are a common cause of congenital heart disease, and the study of this process promises to provide mechanistic insights and lead to novel therapeutics. Normal valve development involves multiple signaling pathways, and recently roles have been identified for extracellular matrix components, including glycosaminoglycans. We, therefore, explored the role of the glycosaminoglycan chondroitin sulfate during zebrafish cardiac development. Beginning at 33 hr, there is a distinct zone of chondroitin sulfate expression in the atrioventricular (AV) boundary, in the cardiac jelly between the endocardium and myocardium. This expression is both spatially and temporally restricted, and is undetectable after 48 hr. Chemical as well as genetic inhibition of chondroitin synthesis results in AV canal (AVC) defects, including loss of the atrioventricular constriction, blood regurgitation, and failure of circulation. Lack of chondroitin disrupts a marker of cell migration, results in a loss of myocardial and endothelial markers of valvulogenesis, and misregulates bone morphogenetic protein expression, supporting an early role in AVC development. In summary, we have defined a requirement for chondroitin sulfate expression in the normal patterning of the AV boundary, suggesting that this component of the cardiac jelly provides a necessary signal in this critical transition in vertebrate cardiogenesis.