Leveraging chromatin state transitions for the identification of regulatory networks orchestrating heart regeneration.

The limited regenerative capacity of the human heart contributes to high morbidity and mortality worldwide. In contrast, zebrafish exhibit robust regenerative capacity, providing a powerful model for studying how to overcome intrinsic epigenetic barriers maintaining cardiac homeostasis and initiate regeneration. Here, we present a comprehensive analysis of the histone modifications H3K4me1, H3K4me3, H3K27me3 and H3K27ac during various stages of zebrafish heart regeneration. We found a vast gain of repressive chromatin marks one day after myocardial injury, followed by the acquisition of active chromatin characteristics on day four and a transition to a repressive state on day 14, and identified distinct transcription factor ensembles associated with these events. The rapid transcriptional response involves the engagement of super-enhancers at genes implicated in extracellular matrix reorganization and TOR signaling, while H3K4me3 breadth highly correlates with transcriptional activity and dynamic changes at genes involved in proteolysis, cell cycle activity, and cell differentiation. Using loss- and gain-of-function approaches, we identified transcription factors in cardiomyocytes and endothelial cells influencing cardiomyocyte dedifferentiation or proliferation. Finally, we detected significant evolutionary conservation between regulatory regions that drive zebrafish and neonatal mouse heart regeneration, suggesting that reactivating transcriptional and epigenetic networks converging on these regulatory elements might unlock the regenerative potential of adult human hearts.

Natural genetic variation quantitatively regulates heart rate and dimension.

The polygenic contribution to heart development and function along the health-disease continuum remains unresolved. To gain insight into the genetic basis of quantitative cardiac phenotypes, we utilize highly inbred Japanese rice fish models, Oryzias latipes, and Oryzias sakaizumii. Employing automated quantification of embryonic heart rates as core metric, we profiled phenotype variability across five inbred strains. We observed maximal phenotypic contrast between individuals of the HO5 and the HdrR strain. HO5 showed elevated heart rates associated with embryonic ventricular hypoplasia and impaired adult cardiac function. This contrast served as the basis for genome-wide mapping. In a segregation population of 1192 HO5 x HdrR F2 embryos, we mapped 59 loci (173 genes) associated with heart rate. Experimental validation of the top 12 candidate genes in loss-of-function models revealed their causal and distinct impact on heart rate, development, ventricle size, and arrhythmia. Our study uncovers new diagnostic and therapeutic targets for developmental and electrophysiological cardiac diseases and provides a novel scalable approach to investigate the intricate genetic architecture of the vertebrate heart.

Identification and Tissue-Specific Characterization of Novel SHOX-Regulated Genes in Zebrafish Highlights SOX Family Members Among Other Genes.

SHOX deficiency causes a spectrum of clinical phenotypes related to skeletal dysplasia and short stature, including Léri-Weill dyschondrosteosis, Langer mesomelic dysplasia, Turner syndrome, and idiopathic short stature. SHOX controls chondrocyte proliferation and differentiation, bone maturation, and cellular growth arrest and apoptosis via transcriptional regulation of its direct target genes NPPB, FGFR3, and CTGF. However, our understanding of SHOX-related pathways is still incomplete. To elucidate the underlying molecular mechanisms and to better understand the broad phenotypic spectrum of SHOX deficiency, we aimed to identify novel SHOX targets. We analyzed differentially expressed genes in SHOX-overexpressing human fibroblasts (NHDF), and confirmed the known SHOX target genes NPPB and FGFR among the most strongly regulated genes, together with 143 novel candidates. Altogether, 23 genes were selected for further validation, first by whole-body characterization in developing shox-deficient zebrafish embryos, followed by tissue-specific expression analysis in three shox-expressing zebrafish tissues: head (including brain, pharyngeal arches, eye, and olfactory epithelium), heart, and pectoral fins. Most genes were physiologically relevant in the pectoral fins, while only few genes were also significantly regulated in head and heart tissue. Interestingly, multiple sox family members (sox5, sox6, sox8, and sox18) were significantly dysregulated in shox-deficient pectoral fins together with other genes (nppa, nppc, cdkn1a, cdkn1ca, cyp26b1, and cy26c1), highlighting an important role for these genes in shox-related growth disorders. Network-based analysis integrating data from the Ingenuity pathways revealed that most of these genes act in a common network. Our results provide novel insights into the genetic pathways and molecular events leading to the clinical manifestation of SHOX deficiency.

Cardiac Regeneration and Tumor Growth-What Do They Have in Common?

Acute myocardial infarction is a leading cause of death. Unlike most adult mammals, zebrafish have the capability to almost fully regenerate their hearts after injury. In contrast, ischemic damage in adult human and mouse hearts usually results in scar tissue. mRNA-Sequencing (Seq) and miRNA-Seq analyses of heart regeneration in zebrafish over time showed that the process can be divided into three phases: the first phase represents dedifferentiation and proliferation of cells, the second phase is characterized by migration, and in the third phase cell signals indicate heart development and differentiation. The first two phases seem to share major similarities with tumor development and growth. To gain more insight into these similarities between cardiac regeneration and tumor development and growth, we used patient matched tumor normal ("healthy") RNA-Seq data for several tumor entities from The Cancer Genome Atlas (TCGA). Subsequently, RNA data were processed using the same pipeline for both the zebrafish samples and tumor datasets. Functional analysis showed that multiple Gene Ontology terms (GO terms) are involved in both early stage cardiac regeneration and tumor development/growth across multiple tumor entities. These GO terms are mostly associated with cell cycle processes. Further analysis showed that orthologous genes are the same key players that regulated these changes in both diseases. We also observed that GO terms associated with heart development in the third late phase of cardiac regeneration are downregulated in the tumor entities. Taken together, our analysis illustrates similarities between cardiac remodeling and tumor progression.

miR-103/107 regulates left-right asymmetry in zebrafish by modulating Kupffer's vesicle development and ciliogenesis.

In zebrafish, cilia movement within the Kupffer's vesicle (KV) generates a fluid flow responsible for accumulating nodal signals exclusively in the left lateral plate mesoderm, thereby initiating left-right patterning (LRP). Defects in LRP cause devastating congenital disorders including congenital heart malformations due to organ mis-positioning. We identified the miR-103/107 family to be involved in regulating LRP. Depletion of miR-103/107 in zebrafish embryos resulted in malpositioned and malformed visceral organs and hearts due to disturbed LRP gene expression, indicating early defects in LRP. Additionally, loss of miR-103/107 affected KV morphogenesis and cilia formation without disturbing endoderm development. Human fibroblasts depleted of miR-103a/107 often failed to extend cilia or developed shorter cilia, indicating functional conservation between species. We identified arl6, araf and foxH1 as direct targets of miR-103/107 providing a mechanistic link to cilia development and nodal signal titration. We describe a new microRNA family controlling KV development and hence influencing establishment of internal organ asymmetry.

The histone demethylase JMJD2B regulates endothelial-to-mesenchymal transition.

Endothelial cells play an important role in maintenance of the vascular system and the repair after injury. Under proinflammatory conditions, endothelial cells can acquire a mesenchymal phenotype by a process named endothelial-to-mesenchymal transition (EndMT), which affects the functional properties of endothelial cells. Here, we investigated the epigenetic control of EndMT. We show that the histone demethylase JMJD2B is induced by EndMT-promoting, proinflammatory, and hypoxic conditions. Silencing of JMJD2B reduced TGF-ß2-induced expression of mesenchymal genes, prevented the alterations in endothelial morphology and impaired endothelial barrier function. Endothelial-specific deletion of JMJD2B in vivo confirmed a reduction of EndMT after myocardial infarction. EndMT did not affect global H3K9me3 levels but induced a site-specific reduction of repressive H3K9me3 marks at promoters of mesenchymal genes, such as Calponin (CNN1), and genes involved in TGF-ß signaling, such as AKT Serine/Threonine Kinase 3 (AKT3) and Sulfatase 1 (SULF1). Silencing of JMJD2B prevented the EndMT-induced reduction of H3K9me3 marks at these promotors and further repressed these EndMT-related genes. Our study reveals that endothelial identity and function is critically controlled by the histone demethylase JMJD2B, which is induced by EndMT-promoting, proinflammatory, and hypoxic conditions, and supports the acquirement of a mesenchymal phenotype.

miR-128a Acts as a Regulator in Cardiac Development by Modulating Differentiation of Cardiac Progenitor Cell Populations.

MicroRNAs (miRs) appear to be major, yet poorly understood players in regulatory networks guiding cardiogenesis. We sought to identify miRs with unknown functions during cardiogenesis analyzing the miR-profile of multipotent Nkx2.5 enhancer cardiac progenitor cells (NkxCE-CPCs). Besides well-known candidates such as miR-1, we found about 40 miRs that were highly enriched in NkxCE-CPCs, four of which were chosen for further analysis. Knockdown in zebrafish revealed that only miR-128a affected cardiac development and function robustly. For a detailed analysis, loss-of-function and gain-of-function experiments were performed during in vitro differentiations of transgenic murine pluripotent stem cells. MiR-128a knockdown (1) increased Isl1, Sfrp5, and Hcn4 (cardiac transcription factors) but reduced Irx4 at the onset of cardiogenesis, (2) upregulated Isl1-positive CPCs, whereas NkxCE-positive CPCs were downregulated, and (3) increased the expression of the ventricular cardiomyocyte marker Myl2 accompanied by a reduced beating frequency of early cardiomyocytes. Overexpression of miR-128a (4) diminished the expression of Isl1, Sfrp5, Nkx2.5, and Mef2c, but increased Irx4, (5) enhanced NkxCE-positive CPCs, and (6) favored nodal-like cardiomyocytes (Tnnt2+, Myh6+, Shox2+) accompanied by increased beating frequencies. In summary, we demonstrated that miR-128a plays a so-far unknown role in early heart development by affecting the timing of CPC differentiation into various cardiomyocyte subtypes.

The Zebrafish (Danio rerio) Is a Relevant Model for Studying Sex-Specific Effects of 17ß-Estradiol in the Adult Heart.

Cardiovascular diseases are a major cause of morbidity and mortality, and there are significant sex differences therein. However, the underlying mechanisms are poorly understood. The steroid hormone 17ß-estradiol (E2) is thought to play a major role in cardiovascular sex differences and to be protective, but this may not hold true for males. We aimed at assessing whether the zebrafish is an appropriate model for the study of E2 effects in the heart. We hypothesized that E2 regulates the cardiac contractility of adult zebrafish in a sex-specific manner. Male and female zebrafish were treated with vehicle (control) or E2 and the cardiac contractility was measured 0, 4, 7 and 14 days after treatment initiation using echocardiography. There was no significant effect on the heart rate by E2. Notably, there was a significant decrease in the ejection fraction of male zebrafish treated with E2 compared with controls. By contrast, there was no major difference in the ejection fraction between the two female groups. The dramatic effect in male zebrafish occurred as early as 4 days post treatment initiation. Although there was no significant difference in stroke volume and cardiac output between the two male groups, these were significantly higher in female zebrafish treated with E2 compared with controls. Gene expression analysis revealed that the levels of estrogen receptors were comparable among all groups. In conclusion, our data demonstrate that the adult zebrafish heart robustly responds to E2 and this occurs in a sex-specific manner. Given the benefits of using zebrafish as a model, new targets may be identified for the development of novel cardiovascular therapies for male and female patients. This would contribute towards the realization of personalized medicine.

Biallelic Variants in ASNA1, Encoding a Cytosolic Targeting Factor of Tail-Anchored Proteins, Cause Rapidly Progressive Pediatric Cardiomyopathy.

BACKGROUND: Pediatric cardiomyopathies are a clinically and genetically heterogeneous group of heart muscle disorders associated with high morbidity and mortality. Although knowledge of the genetic basis of pediatric cardiomyopathy has improved considerably, the underlying cause remains elusive in a substantial proportion of cases. METHODS: Exome sequencing was used to screen for the causative genetic defect in a pair of siblings with rapidly progressive dilated cardiomyopathy and death in early infancy. Protein expression was assessed in patient samples, followed by an in vitro tail-anchored protein insertion assay and functional analyses in zebrafish. RESULTS: We identified compound heterozygous variants in the highly conserved ASNA1 gene (arsA arsenite transporter, ATP-binding, homolog), which encodes an ATPase required for post-translational membrane insertion of tail-anchored proteins. The c.913C>T variant on the paternal allele is predicted to result in a premature stop codon p.(Gln305*), and likely explains the decreased protein expression observed in myocardial tissue and skin fibroblasts. The c.488T>C variant on the maternal allele results in a valine to alanine substitution at residue 163 (p.Val163Ala). Functional studies showed that this variant leads to protein misfolding as well as less effective tail-anchored protein insertion. Loss of asna1 in zebrafish resulted in reduced cardiac contractility and early lethality. In contrast to wild-type mRNA, injection of either mutant mRNA failed to rescue this phenotype. CONCLUSIONS: Biallelic variants in ASNA1 cause severe pediatric cardiomyopathy and early death. Our findings point toward a critical role of the tail-anchored membrane protein insertion pathway in vertebrate cardiac function and disease.

Rapamycin attenuates pathological hypertrophy caused by an absence of trabecular formation.

Cardiac trabeculae are mesh-like muscular structures within ventricular walls. Subtle perturbations in trabeculation are associated with many congenital heart diseases (CHDs), and complete failure to form trabeculae leads to embryonic lethality. Despite the severe consequence of an absence of trabecular formation, the exact function of trabeculae remains unclear. Since ErbB2 signaling plays a direct and essential role in trabecular initiation, in this study, we utilized the erbb2 zebrafish mutant as a model to address the function of trabeculae in the heart. Intriguingly, we found that the trabeculae-deficient erbb2 mutant develops a hypertrophic-like (HL) phenotype that can be suppressed by inhibition of Target of Rapamycin (TOR) signaling in a similar fashion to adult mammalian hearts subjected to mechanical overload. Further, cell transplantation experiments demonstrated that erbb2 mutant cells in an otherwise wildtype heart did not undergo hypertrophy, indicating that erbb2 mutant HL phenotypes are due to a loss of trabeculae. Together, we propose that trabeculae serve to enhance contractility and that defects in this process lead to wall-stress induced hypertrophic remodeling.

Functional missense and splicing variants in the retinoic acid catabolizing enzyme CYP26C1 in idiopathic short stature.

Height is a complex quantitative trait with a high heritability. Short stature is diagnosed when height is significantly below the average of the general population for that person's age and sex. We have recently found that the retinoic acid degrading enzyme CYP26C1 modifies SHOX deficiency phenotypes toward more severe clinical manifestations. Here, we asked whether damaging variants in CYP26C1 alone could lead to short stature. We performed exome and Sanger sequencing to analyze 856 individuals with short stature where SHOX deficiency was previously excluded. Three different damaging missense variants and one splicing variant were identified in six independent individuals; the functional significance of the identified variants was tested in vitro or in vivo using zebrafish as a model. The genetic and functional data reported here indicate that CYP26C1 represents a novel gene underlying growth disorders and that damaging variants in the absence of SHOX variants can lead to short stature.

Delineating the Dynamic Transcriptome Response of mRNA and microRNA during Zebrafish Heart Regeneration.

Heart diseases are the leading cause of death for the vast majority of people around the world, which is often due to the limited capability of human cardiac regeneration. In contrast, zebrafish have the capacity to fully regenerate their hearts after cardiac injury. Understanding and activating these mechanisms would improve health in patients suffering from long-term consequences of ischemia. Therefore, we monitored the dynamic transcriptome response of both mRNA and microRNA in zebrafish at 1⁻160 days post cryoinjury (dpi). Using a control model of sham-operated and healthy fish, we extracted the regeneration specific response and further delineated the spatio-temporal organization of regeneration processes such as cell cycle and heart function. In addition, we identified novel (miR-148/152, miR-218b and miR-19) and previously known microRNAs among the top regulators of heart regeneration by using theoretically predicted target sites and correlation of expression profiles from both mRNA and microRNA. In a cross-species effort, we validated our findings in the dynamic process of rat myoblasts differentiating into cardiomyocytes-like cells (H9c2 cell line). Concluding, we elucidated different phases of transcriptomic responses during zebrafish heart regeneration. Furthermore, microRNAs showed to be important regulators in cardiomyocyte proliferation over time.

Induction of cardiac dysfunction in developing and adult zebrafish by chronic isoproterenol stimulation.

Zebrafish is a widely used model to evaluate genetic variants and modifiers that can cause heart muscle diseases. Surprisingly, the ß-adrenergic receptor (ß-AR) pathway in zebrafish is not well characterized, although abnormal ß-AR signaling is a major contributor to human heart failure (HF). Chronic ß-AR activation in the attempt to normalize heart function in the failing heart results in a reduction of the ß-ARs expression and receptor desensitization, largely mediated through G-protein coupled receptor kinase 2 (GRK2) upregulation. This in turn leads to further deterioration of heart function and progression towards HF. This study seeks to systematically characterize the function of the ß-AR signaling in developing and adult zebrafish to ultimately assess the ability to induce HF through chronic ß-AR activation by isoproterenol (ISO) as established in the mouse model. Larval hearts first responded to ISO by 3dpf, in concordance with robust expression of key components of the ß-AR signaling pathway. Although ISO-induced ß1-AR and ß2-AR isoform upregulation persisted, chronic ISO stimulation for 5d caused systolic cardiac dysfunction concurrently with maximal expression of G-protein-coupled receptor kinase-2 (GRK2). More consistent to mammalians, adult zebrafish developed significant heart failure in concert with ß1-AR downregulation, and GRK2 and brain natriuretic peptide (BNP) upregulation in response to prolonged, 14d ISO-stimulation. This was accompanied by significant cell death and inflammation without detectable fibrosis. Our study unveils important characteristics of larvae and adult zebrafish hearts pertaining to ß-AR signaling. A lack of ß-AR responsiveness and atypical ß-AR/GRK2 ratios in larval zebrafish should be considered. Adult zebrafish resembled the mammalian situation on the functional and molecular level more closely, but also revealed differences to dysfunctional mammalian hearts, i.e. lack of fibrosis. Our study establishes the first ISO-inducible HF model in adult zebrafish and present critical characteristics of the zebrafish heart essential to be considered when utilizing the zebrafish as a human disease and future drug discovery model.

miR-19b Regulates Ventricular Action Potential Duration in Zebrafish.

Sudden cardiac death due to ventricular arrhythmias often caused by action potential duration (APD) prolongation is a common mode of death in heart failure (HF). microRNAs, noncoding RNAs that fine tune gene expression, are frequently dysregulated during HF, suggesting a potential involvement in the electrical remodeling process accompanying HF progression. Here, we identified miR-19b as an important regulator of heart function. Zebrafish lacking miR-19b developed severe bradycardia and reduced cardiac contractility. miR-19b deficient fish displayed increased sensitivity to AV-block, a characteristic feature of long QT syndrome in zebrafish. Patch clamp experiments from whole hearts showed that miR-19b deficient zebrafish exhibit significantly prolonged ventricular APD caused by impaired repolarization. We found that miR-19b directly and indirectly regulates the expression of crucial modulatory subunits of cardiac ion channels, and thereby modulates AP duration and shape. Interestingly, miR-19b knockdown mediated APD prolongation can rescue a genetically induced short QT phenotype. Thus, miR-19b might represent a crucial modifier of the cardiac electrical activity, and our work establishes miR-19b as a potential candidate for human long QT syndrome.

Retinoic acid catabolizing enzyme CYP26C1 is a genetic modifier in SHOX deficiency.

Mutations in the homeobox gene SHOX cause SHOX deficiency, a condition with clinical manifestations ranging from short stature without dysmorphic signs to severe mesomelic skeletal dysplasia. In rare cases, individuals with SHOX deficiency are asymptomatic. To elucidate the factors that modify disease severity/penetrance, we studied a three-generation family with SHOX deficiency. The variant p.Phe508Cys of the retinoic acid catabolizing enzyme CYP26C1 co-segregated with the SHOX variant p.Val161Ala in the affected individuals, while the SHOX mutant alone was present in asymptomatic individuals. Two further cases with SHOX deficiency and damaging CYP26C1 variants were identified in a cohort of 68 individuals with LWD The identified CYP26C1 variants affected its catabolic activity, leading to an increased level of retinoic acid. High levels of retinoic acid significantly decrease SHOX expression in human primary chondrocytes and zebrafish embryos. Individual morpholino knockdown of either gene shortens the pectoral fins, whereas depletion of both genes leads to a more severe phenotype. Together, our findings describe CYP26C1 as the first genetic modifier for SHOX deficiency.

Essential light chain S195 phosphorylation is required for cardiac adaptation under physical stress.

AIMS: Regulatory proteins of the sarcomere are pivotal for normal heart function and when affected by mutations are frequently causing cardiomyopathy. The exact function of these regulatory proteins and how mutations in these translate into distinct cardiomyopathy phenotypes remains poorly understood. Mutations in the essential myosin light chain (ELC) are linked to human cardiomyopathy characterized by a marked variability in disease phenotypes and high incidences of sudden death. Here we studied the role of the highly conserved S195 phosphorylation site of ELC using heterozygous adult zebrafish lazy susan (laz(m647)) in regulating contractile function in normal physiology and disease. METHODS AND RESULTS: Echocardiography revealed signs of systolic dysfunction in otherwise phenotypically unremarkable heterozygote mutants. However, after physical stress, heart function of laz heterozygous zebrafish severely deteriorated causing heart failure and sudden death. Mechanistically, we show that upon physical stress, ELCs become phosphorylated and lack of S195 dominant-negatively impairs ELC phosphorylation. In vitro motility analysis with native myosin from adult heterozygous hearts demonstrates that S195 loss, specifically following physical stress, results in altered acto-myosin sliding velocities and myosin binding cooperativity, causing reduced force generation and organ dysfunction. CONCLUSION: Using adult heterozygous zebrafish, we show that ELC S195 phosphorylation is pivotal for adaptation of cardiac function to augmented physical stress and we provide novel mechanistic insights into the pathogenesis of ELC-linked cardiomyopathy.

Advanced echocardiography in adult zebrafish reveals delayed recovery of heart function after myocardial cryoinjury.

Translucent zebrafish larvae represent an established model to analyze genetics of cardiac development and human cardiac disease. More recently adult zebrafish are utilized to evaluate mechanisms of cardiac regeneration and by benefiting from recent genome editing technologies, including TALEN and CRISPR, adult zebrafish are emerging as a valuable in vivo model to evaluate novel disease genes and specifically validate disease causing mutations and their underlying pathomechanisms. However, methods to sensitively and non-invasively assess cardiac morphology and performance in adult zebrafish are still limited. We here present a standardized examination protocol to broadly assess cardiac performance in adult zebrafish by advancing conventional echocardiography with modern speckle-tracking analyses. This allows accurate detection of changes in cardiac performance and further enables highly sensitive assessment of regional myocardial motion and deformation in high spatio-temporal resolution. Combining conventional echocardiography measurements with radial and longitudinal velocity, displacement, strain, strain rate and myocardial wall delay rates after myocardial cryoinjury permitted to non-invasively determine injury dimensions and to longitudinally follow functional recovery during cardiac regeneration. We show that functional recovery of cryoinjured hearts occurs in three distinct phases. Importantly, the regeneration process after cryoinjury extends far beyond the proposed 45 days described for ventricular resection with reconstitution of myocardial performance up to 180 days post-injury (dpi). The imaging modalities evaluated here allow sensitive cardiac phenotyping and contribute to further establish adult zebrafish as valuable cardiac disease model beyond the larval developmental stage.

Regulation of ß1 integrin-Klf2-mediated angiogenesis by CCM proteins.

Mechanotransduction pathways are activated in response to biophysical stimuli during the development or homeostasis of organs and tissues. In zebrafish, the blood-flow-sensitive transcription factor Klf2a promotes VEGF-dependent angiogenesis. However, the means by which the Klf2a mechanotransduction pathway is regulated to prevent continuous angiogenesis remain unknown. Here we report that the upregulation of klf2 mRNA causes enhanced egfl7 expression and angiogenesis signaling, which underlies cardiovascular defects associated with the loss of cerebral cavernous malformation (CCM) proteins in the zebrafish embryo. Using CCM-protein-depleted human umbilical vein endothelial cells, we show that the misexpression of KLF2 mRNA requires the extracellular matrix-binding receptor ß1 integrin and occurs in the absence of blood flow. Downregulation of ß1 integrin rescues ccm mutant cardiovascular malformations in zebrafish. Our work reveals a ß1 integrin-Klf2-Egfl7-signaling pathway that is tightly regulated by CCM proteins. This regulation prevents angiogenic overgrowth and ensures the quiescence of endothelial cells.

Fibronectin mediates mesendodermal cell fate decisions.

Non-cell-autonomous signals often play crucial roles in cell fate decisions during animal development. Reciprocal signaling between endoderm and mesoderm is vital for embryonic development, yet the key signals and mechanisms remain unclear. Here, we show that endodermal cells efficiently promote the emergence of mesodermal cells in the neighboring population through signals containing an essential short-range component. The endoderm-mesoderm interaction promoted precardiac mesoderm formation in mouse embryonic stem cells and involved endodermal production of fibronectin. In vivo, fibronectin deficiency resulted in a dramatic reduction of mesoderm accompanied by endodermal expansion in zebrafish embryos. This event was mediated by regulation of Wnt signaling in mesodermal cells through activation of integrin-ß1. Our findings highlight the importance of the extracellular matrix in mediating short-range signals and reveal a novel function of endoderm, involving fibronectin and its downstream signaling cascades, in promoting the emergence of mesoderm.

MicroRNA-10 regulates the angiogenic behavior of zebrafish and human endothelial cells by promoting vascular endothelial growth factor signaling.

RATIONALE: Formation and remodeling of the vasculature during development and disease involve a highly conserved and precisely regulated network of attractants and repellants. Various signaling pathways control the behavior of endothelial cells, but their posttranscriptional dose titration by microRNAs is poorly understood. OBJECTIVE: To identify microRNAs that regulate angiogenesis. METHODS AND RESULTS: We show that the highly conserved microRNA family encoding miR-10 regulates the behavior of endothelial cells during angiogenesis by positively titrating proangiogenic signaling. Knockdown of miR-10 led to premature truncation of intersegmental vessel growth in the trunk of zebrafish larvae, whereas overexpression of miR-10 promoted angiogenic behavior in zebrafish and cultured human umbilical venous endothelial cells. We found that miR-10 functions, in part, by directly regulating the level of fms-related tyrosine kinase 1 (FLT1), a cell-surface protein that sequesters vascular endothelial growth factor, and its soluble splice variant sFLT1. The increase in FLT1/sFLT1 protein levels upon miR-10 knockdown in zebrafish and in human umbilical venous endothelial cells inhibited the angiogenic behavior of endothelial cells largely by antagonizing vascular endothelial growth factor receptor 2 signaling. CONCLUSIONS: Our study provides insights into how FLT1 and vascular endothelial growth factor receptor 2 signaling is titrated in a microRNA-mediated manner and establishes miR-10 as a potential new target for the selective modulation of angiogenesis.