Type V Collagen in Scar Tissue Regulates the Size of Scar after Heart Injury

Scar tissue size following myocardial infarction is an independent predictor of cardiovascular outcomes, yet little is known about factors regulating scar size. We demonstrate that collagen V, a minor constituent of heart scars, regulates the size of heart scars after ischemic injury. Depletion of collagen V led to a paradoxical increase in post-infarction scar size with worsening of heart function. A systems genetics approach across 100 in-bred strains of mice demonstrated that collagen V is a critical driver of postinjury heart function. We show that collagen V deficiency alters the mechanical properties of scar tissue, and altered reciprocal feedback between matrix and cells induces expression of mechanosensitive integrins that drive fibroblast activation and increase scar size. Cilengitide, an inhibitor of specific integrins, rescues the phenotype of increased post-injury scarring in collagen-V-deficient mice. These observations demonstrate that collagen V regulates scar size in an integrin-dependent manner.

Enhancement of β-catenin/T-cell factor 4 signaling causes susceptibility to cardiac arrhythmia by suppressing NaV1.5 expression in mice

BACKGROUND

β-Catenin/T-cell factor 4 (TCF4) signaling is enhanced in ischemic heart disease in which ventricular tachycardia (VT)/ventricular fibrillation occurs frequently. How this signaling links to arrhythmogenesis remains unclear.

OBJECTIVE

The purpose of this study was to investigate the role of β-catenin gain of function in the development of arrhythmia.

METHODS

A mouse model with a conditional deletion of CTNNB1 exon 3 resulting in cardiac exon 3-deleted and stabilized β-catenin (β-catΔE3) was used to determine the role of β-catenin gain of function in the regulation of cardiac rhythm.

RESULTS

Western blotting showed β-catΔE3 expression and significantly decreased NaV1.5 protein in CTNNB1 E3-/- and CTNNB1 E3+/- mouse hearts. Real-time qRT-PCR revealed significantly decreased NaV1.5 messenger RNA with no changes in Na+ channel β1 to β4 expression in these hearts. Immunofluorescence revealed accumulation of β-catΔE3 in the nuclei of CTNNB1 E3-/- cardiomyocytes. Immunohistochemistry demonstrated nuclear localization of β-catenin in cardiomyocytes, which was associated with significantly decreased NaV1.5 messenger RNA in human ischemic hearts. Immunoprecipitation revealed that β-catΔE3 interacted with TCF4 in CTNNB1 E3-/- cardiomyocytes. Whole-cell recordings showed that Na+ currents and depolarization and amplitude of action potentials were significantly decreased in CTNNB1 E3-/- ventricular myocytes. Electrocardiographic recordings demonstrated that in mice with cardiac CTNNB1 E3-/-, the QRS complex was prolonged and VT was induced by the Na+ channel blocker flecainide. However, cardiac function, as determined by echocardiography and heart/body weight ratios, remained unchanged.

CONCLUSION

Enhancement of β-catenin/TCF4 signaling led to the prolongation of the QRS complex and increase in susceptibility to VT by suppression of NaV1.5 expression and Na+ channel activity in mice.

Endocardially Derived Macrophages Are Essential for Valvular Remodeling

During mammalian embryogenesis, de novo hematopoiesis occurs transiently in multiple anatomical sites including the yolk sac, dorsal aorta, and heart tube. A long-unanswered question is whether these local transient hematopoietic mechanisms are essential for embryonic growth. Here, we show that endocardial hematopoiesis is critical for cardiac valve remodeling as a source of tissue macrophages. Colony formation assay from explanted heart tubes and genetic lineage tracing with the endocardial specific Nfatc1-Cre mouse revealed that hemogenic endocardium is a de novo source of tissue macrophages in the endocardial cushion, the primordium of the cardiac valves. Surface marker characterization, gene expression profiling, and ex vivo phagocytosis assay revealed that the endocardially derived cardiac tissue macrophages play a phagocytic and antigen presenting role. Indeed, genetic ablation of endocardially derived macrophages caused severe valve malformation. Together, these data suggest that transient hemogenic activity in the endocardium is indispensable for the valvular tissue remodeling in the heart.

Topological Arrangement of Cardiac Fibroblasts Regulates Cellular Plasticity

RATIONALE

Cardiac fibroblasts do not form a syncytium but reside in the interstitium between myocytes. This topological relationship between fibroblasts and myocytes is maintained throughout postnatal life until an acute myocardial injury occurs, when fibroblasts are recruited to, proliferate and aggregate in the region of myocyte necrosis. The accumulation or aggregation of fibroblasts in the area of injury thus represents a unique event in the life cycle of the fibroblast, but little is known about how changes in the topological arrangement of fibroblasts after cardiac injury affect fibroblast function.

OBJECTIVE

The objective of the study was to investigate how changes in topological states of cardiac fibroblasts (such as after cardiac injury) affect cellular phenotype.

METHODS AND RESULTS

Using 2 and 3-dimensional (2D versus 3D) culture conditions, we show that simple aggregation of cardiac fibroblasts is sufficient by itself to induce genome-wide changes in gene expression and chromatin remodeling. Remarkably, gene expression changes are reversible after the transition from a 3D back to 2D state demonstrating a topological regulation of cellular plasticity. Genes induced by fibroblast aggregation are strongly associated and predictive of adverse cardiac outcomes and remodeling in mouse models of cardiac hypertrophy and failure. Using solvent-based tissue clearing techniques to create optically transparent cardiac scar tissue, we show that fibroblasts in the region of dense scar tissue express markers that are induced by fibroblasts in the 3D conformation. Finally, using live cell interferometry, a quantitative phase microscopy technique to detect absolute changes in single cell biomass, we demonstrate that conditioned medium collected from fibroblasts in 3D conformation compared with that from a 2D state significantly increases cardiomyocyte cell hypertrophy.

CONCLUSIONS

Taken together, these findings demonstrate that simple topological changes in cardiac fibroblast organization are sufficient to induce chromatin remodeling and global changes in gene expression with potential functional consequences for the healing heart.

WNT Signaling in Cardiac and Vascular Disease

WNT signaling is an elaborate and complex collection of signal transduction pathways mediated by multiple signaling molecules. WNT signaling is critically important for developmental processes, including cell proliferation, differentiation and tissue patterning. Little WNT signaling activity is present in the cardiovascular system of healthy adults, but reactivation of the pathway is observed in many pathologies of heart and blood vessels. The high prevalence of these pathologies and their significant contribution to human disease burden has raised interest in WNT signaling as a potential target for therapeutic intervention. In this review, we first will focus on the constituents of the pathway and their regulation and the different signaling routes. Subsequently, the role of WNT signaling in cardiovascular development is addressed, followed by a detailed discussion of its involvement in vascular and cardiac disease. After highlighting the crosstalk between WNT, transforming growth factor-β and angiotensin II signaling, and the emerging role of WNT signaling in the regulation of stem cells, we provide an overview of drugs targeting the pathway at different levels. From the combined studies we conclude that, despite the sometimes conflicting experimental data, a general picture is emerging that excessive stimulation of WNT signaling adversely affects cardiovascular pathology. The rapidly increasing collection of drugs interfering at different levels of WNT signaling will allow the evaluation of therapeutic interventions in the pathway in relevant animal models of cardiovascular diseases and eventually in patients in the near future, translating the outcomes of the many preclinical studies into a clinically relevant context.

Prognostic Significance of Left Ventricular Fibrosis in Patients With Congenital Bicuspid Aortic Valve

This study sought to evaluate the prognostic value of left ventricular (LV) fibrosis assessed by late gadolinium enhancement (LGE) of the myocardium during cardiac magnetic resonance (CMR) imaging in patients with bicuspid aortic valve (BAV), which is associated with early aortic valve fibrosis and calcification. To what degree the LV myocardial wall is affected by fibrosis and its prognostic value is currently unknown. This is a retrospective, single-center study evaluating all adult patients with BAV who had CMR and followed from March 2002 to March 2016. CMR and transthoracic echocardiogram images were reviewed. Clinical data were abstracted from the electronic medical record. A total of 29 patients were included in the study, of which 11 (38%) had CMR studies that demonstrated the presence of LGE. Patients with LGE had significantly higher aortic valve mean gradients by echocardiography when compared with LGE-negative patients (30.3 ± 7.2 mm Hg vs 14.7 ± 3.6 mm Hg, p = 0.049). They were also more likely to have LV hypertrophy. Patients with LGE were 10 times more likely to need aortic valve replacement within 1 year of the CMR study than did patients without LGE (55% vs 5.5%, p = 0.0028). In conclusion, evaluation of LGE by CMR as a marker of LV myocardial fibrosis can have additional prognostic value when evaluating patients with aortic stenosis secondary to BAV.

Abstract 466: Cardiac Activation of β-catenin in Mice Leads to Prolongation of QRS and Susceptibility to Arrhythmia by Inhibiting Na + Channel Activity Through Suppression of Na V 1.5 Expression

Introduction: In vitro studies showed that activation of β-catenin suppresses Na V 1.5 expression by inhibiting SCN5a promoter activity, leading to a decrease of Na + channel activity. How β-catenin regulates cardiac electrophysiological phenotype is unknown.

Hypothesis: We hypothesized that cardiac activation of β-catenin regulates electrophysiological phenotype by suppressing Na V 1.5 expression.

Methods: Adult mice with cardiac-specific, tamoxifen-induced deletion of β-catenin exon3 , leading to cardiac activation of β-catenin (β-catenin exon3 -/- ) were generated, and the effects of cardiac activation of β-catenin on the electrophysiological remodeling were assessed by electrocardiogram (ECG) recording. Class Ic antiarrhythmic reagent, flecainide, was administered to evaluate susceptibility to ventricular tachycardia (VT). Cardiac structure and function were evaluated by histologic and echocardiographic examinations, respectively. Western blot and qRT-PCR were performed to determine the levels of Na V 1.5 and β-catenin expression in mouse hearts. Whole-cell recording technique was utilized to record Na + currents and action potentials (APs) from ventricular myocytes.

Results: Histologic and echocardiographic examinations showed that β-catenin exon3 -/- mice had normal cardiac structure and function. Compared to wild type (WT) mice, the ratio of heart/body weight was not changed and the duration of QRS was significantly prolonged in β-catenin exon3 -/- mice. VT was induced by flecainide in 60% of β-catenin exon3 -/- mice but not in WT mice. Western blot and qRT-PCR showed that Na V 1.5 protein and mRNA were significantly decreased in β-catenin exon3 -/- hearts, compared to WT hearts. Maximal upstroke velocity and amplitude of APs and Na + currents were significantly decreased in β-catenin exon3 -/- ventricular myocytes, compared to WT cells.

Conclusion: Cardiac activation of β-catenin leads to prolongation of QRS and susceptibility to VT by decreasing Na V 1.5 expression and Na + channel activity.

Exogenous miR-29B Delivery Through a Hyaluronan-Based Injectable System Yields Functional Maintenance of the Infarcted Myocardium

Myocardial infarction (MI) results in debilitating remodeling of the myocardial extracellular matrix (ECM). In this proof-of-principle study it was sought to modulate this aggressive remodeling by injecting a hyaluronic acid-based reservoir delivering exogenous microRNA-29B (miR-29B). This proof-of-principal study was executed whereby myocardial ischemia/reperfusion was performed on C57BL/6 mice for 45 min after which five 10 μL boluses of a hydrogel composed of thiolated hyaluronic acid cross-linked with poly (ethylene glycol) diacrylate, containing exogenous miR-29B as an active therapy, were injected into the border zone of the infarcted myocardium. Following surgery, the myocardial function of the animals was monitored up to 5 weeks. Delivering miR-29B locally using an injectable hyaluronan-based hydrogel resulted in the maintenance of myocardial function at 2 and 5 weeks following MI in this proof-of-principle study. In addition, while animals treated with the control of a nontargeting miR delivered using the hyaluronan-based hydrogel had a significant deterioration of myocardial function, those treated with miR-29B did not. Histological analysis revealed a significantly decreased presence of elastin and significantly less immature/newly deposited collagen fibers at the border zone of the infarct. Increased vascularity of the myocardial scar was also detected and Raman microspectroscopy discovered significantly altered ECM-specific biochemical signals at the border zone of the infarct. This preclinical proof-of-principle study demonstrates that an injectable hyaluronic acid hydrogel system could be capable of delivering miR-29B toward maintaining cardiac function following MI. In addition, Raman microspectroscopy revealed subtle, yet significant changes in ECM organization and maturity. These findings have great potential with regard to using injectable biomaterials as a local treatment for ischemic tissue and exogenous miRs to modulate tissue remodeling.

A systems genetics approach identifies Trp53inp2 as a link between cardiomyocyte glucose utilization and hypertrophic response

Cardiac failure has been widely associated with an increase in glucose utilization. The aim of our study was to identify factors that mechanistically bridge this link between hyperglycemia and heart failure. Here, we screened the Hybrid Mouse Diversity Panel (HMDP) for substrate-specific cardiomyocyte candidates based on heart transcriptional profile and circulating nutrients. Next, we utilized an in vitro model of rat cardiomyocytes to demonstrate that the gene expression changes were in direct response to substrate abundance. After overlaying candidates of interest with a separate HMDP study evaluating isoproterenol-induced heart failure, we chose to focus on the gene Trp53inp2 as a cardiomyocyte glucose utilization-specific factor. Trp53inp2 gene knockdown in rat cardiomyocytes reduced expression and protein abundance of key glycolytic enzymes. This resulted in reduction of both glucose uptake and glycogen content in cardiomyocytes stimulated with isoproterenol. Furthermore, this reduction effectively blunted the capacity of glucose and isoprotereonol to synergistically induce hypertrophic gene expression and cell size expansion. We conclude that Trp53inp2 serves as regulator of cardiomyocyte glycolytic activity and can consequently regulate hypertrophic response in the context of elevated glucose content.NEW & NOTEWORTHY Here, we apply a novel method for screening transcripts based on a substrate-specific expression pattern to identify Trp53inp2 as an induced cardiomyocyte glucose utilization factor. We further show that reducing expression of the gene could effectively blunt hypertrophic response in the context of elevated glucose content.

Cardiac Fibroblasts Adopt Osteogenic Fates and Can Be Targeted to Attenuate Pathological Heart Calcification

Mammalian tissues calcify with age and injury. Analogous to bone formation, osteogenic cells are thought to be recruited to the affected tissue and induce mineralization. In the heart, calcification of cardiac muscle leads to conduction system disturbances and is one of the most common pathologies underlying heart blocks. However the cell identity and mechanisms contributing to pathological heart muscle calcification remain unknown. Using lineage tracing, murine models of heart calcification and in vivo transplantation assays, we show that cardiac fibroblasts (CFs) adopt an osteoblast cell-like fate and contribute directly to heart muscle calcification. Small-molecule inhibition of ENPP1, an enzyme that is induced upon injury and regulates bone mineralization, significantly attenuated cardiac calcification. Inhibitors of bone mineralization completely prevented ectopic cardiac calcification and improved post injury heart function. Taken together, these findings highlight the plasticity of fibroblasts in contributing to ectopic calcification and identify pharmacological targets for therapeutic development.

The long noncoding RNA Chaer defines an epigenetic checkpoint in cardiac hypertrophy

Epigenetic reprogramming is a critical process of pathological gene induction during cardiac hypertrophy and remodeling, but the underlying regulatory mechanisms remain to be elucidated. Here we identified a heart-enriched long noncoding (lnc)RNA, named cardiac-hypertrophy-associated epigenetic regulator (Chaer), which is necessary for the development of cardiac hypertrophy. Mechanistically, Chaer directly interacts with the catalytic subunit of polycomb repressor complex 2 (PRC2). This interaction, which is mediated by a 66-mer motif in Chaer, interferes with PRC2 targeting to genomic loci, thereby inhibiting histone H3 lysine 27 methylation at the promoter regions of genes involved in cardiac hypertrophy. The interaction between Chaer and PRC2 is transiently induced after hormone or stress stimulation in a process involving mammalian target of rapamycin complex 1, and this interaction is a prerequisite for epigenetic reprogramming and induction of genes involved in hypertrophy. Inhibition of Chaer expression in the heart before, but not after, the onset of pressure overload substantially attenuates cardiac hypertrophy and dysfunction. Our study reveals that stress-induced pathological gene activation in the heart requires a previously uncharacterized lncRNA-dependent epigenetic checkpoint.

Skeletal and cardiac muscle pericytes: Functions and therapeutic potential

Pericytes are periendothelial mesenchymal cells residing within the microvasculature. Skeletal muscle and cardiac pericytes are now recognized to fulfill an increasing number of functions in normal tissue homeostasis, including contributing to microvascular function by maintaining vessel stability and regulating capillary flow. In the setting of muscle injury, pericytes contribute to a regenerative microenvironment through release of trophic factors and by modulating local immune responses. In skeletal muscle, pericytes also directly enhance tissue healing by differentiating into myofibers. Conversely, pericytes have also been implicated in the development of disease states, including fibrosis, heterotopic ossication and calcification, atherosclerosis, and tumor angiogenesis. Despite increased recognition of pericyte heterogeneity, it is not yet clear whether specific subsets of pericytes are responsible for individual functions in skeletal and cardiac muscle homeostasis and disease.

Astrocytes Can Adopt Endothelial Cell Fates in a p53-Dependent Manner

Astrocytes respond to a variety of CNS injuries by cellular enlargement, process outgrowth, and upregulation of extracellular matrix proteins that function to prevent expansion of the injured region. This astrocytic response, though critical to the acute injury response, results in the formation of a glial scar that inhibits neural repair. Scar-forming cells (fibroblasts) in the heart can undergo mesenchymal-endothelial transition into endothelial cell fates following cardiac injury in a process dependent on p53 that can be modulated to augment cardiac repair. Here, we sought to determine whether astrocytes, as the primary scar-forming cell of the CNS, are able to undergo a similar cellular phenotypic transition and adopt endothelial cell fates. Serum deprivation of differentiated astrocytes resulted in a change in cellular morphology and upregulation of endothelial cell marker genes. In a tube formation assay, serum-deprived astrocytes showed a substantial increase in vessel-like morphology that was comparable to human umbilical vein endothelial cells and dependent on p53. RNA sequencing of serum-deprived astrocytes demonstrated an expression profile that mimicked an endothelial rather than astrocyte transcriptome and identified p53 and angiogenic pathways as specifically upregulated. Inhibition of p53 with genetic or pharmacologic strategies inhibited astrocyte-endothelial transition. Astrocyte-endothelial cell transition could also be modulated by miR-194, a microRNA downstream of p53 that affects expression of genes regulating angiogenesis. Together, these studies demonstrate that differentiated astrocytes retain a stimulus-dependent mechanism for cellular transition into an endothelial phenotype that may modulate formation of the glial scar and promote injury-induced angiogenesis.

Cardiac fibroblast in development and wound healing

Cardiac fibroblasts are the most abundant cell type in the mammalian heart and comprise approximately two-thirds of the total number of cardiac cell types. During development, epicardial cells undergo epithelial-mesenchymal-transition to generate cardiac fibroblasts that subsequently migrate into the developing myocardium to become resident cardiac fibroblasts. Fibroblasts form a structural scaffold for the attachment of cardiac cell types during development, express growth factors and cytokines and regulate proliferation of embryonic cardiomyocytes. In post natal life, cardiac fibroblasts play a critical role in orchestrating an injury response. Fibroblast activation and proliferation early after cardiac injury are critical for maintaining cardiac integrity and function, while the persistence of fibroblasts long after injury leads to chronic scarring and adverse ventricular remodeling. In this review, we discuss the physiologic function of the fibroblast during cardiac development and wound healing, molecular mediators of activation that could be possible targets for drug development for fibrosis and finally the use of reprogramming technologies for reversing scar. This article is part of a Special Issue entitled "Myocyte-Fibroblast Signalling in Myocardium."

Cell-cell interaction in the heart via Wnt/β-catenin pathway after cardiac injury

The adult mammalian heart predominantly comprises myocytes, fibroblasts, endothelial cells, smooth muscle cells, and epicardial cells arranged in a precise three-dimensional framework. Following cardiac injury, the spatial arrangement of cells is disrupted as different populations of cells are recruited to the heart in a temporally regulated manner. The alteration of the cellular composition of the heart after cardiac injury thus enables different phenotypes of cells to interact with each other in a spatio-temporal-dependent manner. It can be argued that the integrated study of such cellular interactions rather than the examination of single populations of cells can provide more insights into the biology of cardiac repair especially at an organ-wide level. Many signalling systems undoubtedly mediate such cross talk between cells after cardiac injury. The Wnt/β-catenin system plays an important role during cardiac development and disease. Here, we describe how cell populations in the heart after cardiac injury mediate their interactions via the Wnt/β-catenin pathway, determine how such interactions can affect a cardiac repair response and finally suggest an integrated approach to study cardiac cellular interactions.

Abstract 33: Cardiac Fibroblasts Reprogram Into Endothelial Like Cells in a p53 Dependent Manner After Acute Ischemic Cardiac Injury

The mammalian heart exhibits poor regenerative capacity after acute ischemic injury and heals primarily by fibrosis. Recently, several groups have demonstrated that cardiac fibroblasts can be reprogrammed to adopt myogenic fates using exogenous transcription factors. However, the ability of cardiac fibroblasts to adopt specific cellular fates in the absence of exogenous factors is unclear. Here, we demonstrate that a subset of cardiac fibroblasts adopt endothelial characteristics after ischemic cardiac injury in the absence of any added factors. Using mice harboring genetically labeled fibroblasts (Col1a2CreERT:R26RTdTomato), we show that 34 +/- 3% (mean, SEM) of labeled cardiac fibroblasts in the injury border zone express endothelial markers such as VE-cadherin. Fibroblast derived endothelial cells comprised 25 +/- 2% of total and 8 +/- 2% of luminal endothelial cells at the border zone 3 days after injury. To better understand fibroblast-endothelial reprogramming we subjected cardiac fibroblasts to cellular stress (serum starvation) and found that they formed tubes on Matrigel and up-regulated endothelial specific genes (e.g. VE-cadherin, Flk1, Flt1) 6-20 fold. We show that reprogramming of fibroblasts to endothelial like cells ex vivo is p53 dependent. Inhibiting p53 activity by pharmacological means (Pifithrin-α) or genetic deletion in fibroblasts (Col1a2CreERT:p53fl/fl) led to a 94% decrease in Matrigel tube formation and 90% reduction in endothelial gene expression. Moreover, we observed that p53 levels in cardiac fibroblasts were more than 10-fold higher at the injury border zone using semi-quantitative immunofluorescent staining. Injection of a p53 activator after injury doubled p53 levels in cardiac fibroblasts and increased the rate of fibroblast-endothelial reprogramming by 43%. Enhanced fibroblast-endothelial reprogramming was also associated with decreased collagen deposition 3 days post injury. In summary, we show that cardiac fibroblasts are able to adopt endothelial cell like fates both in vivo and ex vivo in a p53 dependent manner. Manipulation of fibroblast to endothelial reprogramming could represent a novel therapeutic strategy to increase post infarct angiogenesis and enhance function in the injured heart.

Rib fractures and death from deletion of osteoblast βcatenin in adult mice is rescued by corticosteroids

Ribs are primarily made of cortical bone and are necessary for chest expansion and ventilation. Rib fractures represent the most common type of non-traumatic fractures in the elderly yet few studies have focused on the biology of rib fragility. Here, we show that deletion of βcatenin in Col1a2 expressing osteoblasts of adult mice leads to aggressive osteoclastogenesis with increased serum levels of the osteoclastogenic cytokine RANKL, extensive rib resorption, multiple spontaneous rib fractures and chest wall deformities. Within days of osteoblast specific βcatenin deletion, animals die from respiratory failure with a vanishing rib cage that is unable to sustain ventilation. Increased bone resorption is also observed in the vertebrae and femur. Treatment with the bisphosphonate pamidronate delayed but did not prevent death or associated rib fractures. In contrast, administration of the glucocorticoid dexamethasone decreased serum RANKL and slowed osteoclastogenesis. Dexamethasone preserved rib structure, prevented respiratory compromise and strikingly increased survival. Our findings provide a novel model of accelerated osteoclastogenesis, where deletion of osteoblast βcatenin in adults leads to rapid development of destructive rib fractures. We demonstrate the role of βcatenin dependent mechanisms in rib fractures and suggest that glucocorticoids, by suppressing RANKL, may have a role in treating bone loss due to aggressive osteoclastogenesis.

Wnt1 is a proangiogenic molecule, enhances human endothelial progenitor function, and increases blood flow to ischemic limbs in a HGF-dependent manner

Human endothelial progenitor cells (hEPCs) participate in neovascularization of ischemic tissues. Function and number of hEPCs decline in patients with cardiovascular disease, and therapeutic strategies to enhance hEPC function remain an important field of investigation. The Wnt signaling system, comprising 19 lipophilic proteins, regulates vascular patterning in the developing embryo. However, the effects of Wnts on hEPCs and the adult vasculature remain unclear. We demonstrate here that Wnt1 is expressed in a subset of endothelial cells lining the murine embryonic dorsal aorta and is reactivated in malignant angiosarcoma, suggesting a strong association of Wnt1 with angiogenesis. We investigate the effects of Wnt1 in enhancing hEPC function and blood flow to ischemic tissues and show that Wnt1 enhances the proliferative and angiogenic functions of hEPCs in a hepatocyte growth factor (HGF)-dependent manner. Injection of Wnt1-expressing hEPCs increases blood flow and capillary density in murine ischemic hindlimbs. Furthermore, injection of Wnt1 protein alone similarly increases blood flow and capillary density in ischemic hindlimbs, and this effect is associated with increased HGF expression in ischemic muscle. These findings demonstrate that Wnt1, a marker of neural crest cells and hitherto unknown angiogenic function, is a novel angiogenic protein that is expressed in developing endothelial cells, exerts salutary effects on postnatal hEPCs, and can be therapeutically deployed to increase blood flow and angiogenesis in ischemic tissues.