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The BMP pathway regulates developmental processes including angiogenesis, yet its signaling outputs are complex and context-dependent. Recently, we showed that SMAD6, an intracellular BMP inhibitor expressed in endothelial cells, decreases vessel sprouting and branching both in vitro and in zebrafish. Genetic deletion of SMAD6 in mice results in poorly characterized cardiovascular defects and lethality. Here, we analyzed the effects of SMAD6 loss on vascular function during murine development. SMAD6 was expressed in a subset of blood vessels throughout development, primarily in arteries, while expression outside of the vasculature was largely confined to developing cardiac valves with no obvious embryonic phenotype. Mice deficient in SMAD6 died during late gestation and early stages of postnatal development, and this lethality was associated with vessel hemorrhage. Mice that survived past birth had increased branching and sprouting of developing postnatal retinal vessels and disorganized tight and adherens junctions. In vitro, knockdown of SMAD6 led to abnormal endothelial cell adherens junctions and increased VE-cadherin endocytosis, indicative of activated endothelium. Thus, SMAD6 is essential for proper blood vessel function during murine development, where it appears to stabilize endothelial junctions to prevent hemorrhage and aberrant angiogenesis.

The BMP pathway regulates developmental processes including angiogenesis, yet its signaling outputs are complex and context-dependent. Recently, we showed that SMAD6, an intracellular BMP inhibitor expressed in endothelial cells, decreases vessel sprouting and branching both in vitro and in zebrafish. Genetic deletion of SMAD6 in mice results in poorly characterized cardiovascular defects and lethality. Here, we analyzed the effects of SMAD6 loss on vascular function during murine development. SMAD6 was expressed in a subset of blood vessels throughout development, primarily in arteries, while expression outside of the vasculature was largely confined to developing cardiac valves with no obvious embryonic phenotype. Mice deficient in SMAD6 died during late gestation and early stages of postnatal development, and this lethality was associated with vessel hemorrhage. Mice that survived past birth had increased branching and sprouting of developing postnatal retinal vessels and disorganized tight and adherens junctions. In vitro, knockdown of SMAD6 led to abnormal endothelial cell adherens junctions and increased VE-cadherin endocytosis, indicative of activated endothelium. Thus, SMAD6 is essential for proper blood vessel function during murine development, where it appears to stabilize endothelial junctions to prevent hemorrhage and aberrant angiogenesis.

Functional blood vessel growth depends on generation of distinct but coordinated responses from endothelial cells. Bone morphogenetic proteins (BMP), part of the TGFβ superfamily, bind receptors to induce phosphorylation and nuclear translocation of SMAD transcription factors (R-SMAD1/5/8) and regulate vessel growth. However, SMAD1/5/8 signalling results in both pro- and anti-angiogenic outputs, highlighting a poor understanding of the complexities of BMP signalling in the vasculature. Here we show that BMP6 and BMP2 ligands are pro-angiogenic in vitro and in vivo, and that lateral vessel branching requires threshold levels of R-SMAD phosphorylation. Endothelial cell responsiveness to these pro-angiogenic BMP ligands is regulated by Notch status and Notch sets responsiveness by regulating a cell-intrinsic BMP inhibitor, SMAD6, which affects BMP responses upstream of target gene expression. Thus, we reveal a paradigm for Notch-dependent regulation of angiogenesis: Notch regulates SMAD6 expression to affect BMP responsiveness of endothelial cells and new vessel branch formation., The mechanism underlying endothelial cell responses to BMP signals is unknown. Here, the authors show that the endothelial response to pro-angiogenic BMP ligands is regulated by Notch via its effect on SMAD6, a known inhibitor of BMP intracellular signaling cascade.

Arterial and venous endothelial cells exhibit distinct molecular characteristics at early developmental stages. These lineage-specific molecular programs are instructive to the development of distinct vascular architectures and physiological conditions of arteries and veins, but their roles in angiogenesis remain unexplored. Here, we show that the caudal vein plexus in zebrafish forms by endothelial cell sprouting, migration and anastomosis, providing a venous-specific angiogenesis model. Using this model, we have identified a novel compound, aplexone, which effectively suppresses venous, but not arterial, angiogenesis. Multiple lines of evidence indicate that aplexone differentially regulates arteriovenous angiogenesis by targeting the HMG-CoA reductase (HMGCR) pathway. Treatment with aplexone affects the transcription of enzymes in the HMGCR pathway and reduces cellular cholesterol levels. Injecting mevalonate, a metabolic product of HMGCR, reverses the inhibitory effect of aplexone on venous angiogenesis. In addition, aplexone treatment inhibits protein prenylation and blocking the activity of geranylgeranyl transferase induces a venous angiogenesis phenotype resembling that observed in aplexone-treated embryos. Furthermore, endothelial cells of venous origin have higher levels of proteins requiring geranylgeranylation than arterial endothelial cells and inhibiting the activity of Rac or Rho kinase effectively reduces the migration of venous, but not arterial, endothelial cells. Taken together, our findings indicate that angiogenesis is differentially regulated by the HMGCR pathway via an arteriovenous-dependent requirement for protein prenylation in zebrafish and human endothelial cells.

Oxidized 1-palmitoyl-2-arachidonoyl- sn -glycero-3-phosphorylcholine (OxPAPC), which has been shown to accumulate in atherosclerotic lesions and other sites of chronic inflammation, activates endothelial cells (EC) to bind monocytes by activation of endothelial β1 integrin and subsequent deposition of fibronectin on the apical surface. Our previous studies suggest this function of OxPAPC is mediated via a Gs protein–coupled receptor (GPCR). PEIPC (1-palmitoyl-2-epoxyisoprostane E2- sn -glycero-3-phosphorylcholine) is the most active lipid in OxPAPC that activates this pathway. We screened a number of candidate GPCRs for their interaction with OxPAPC and PEIPC, using a reporter gene assay; we identified prostaglandin E2 receptor EP2 and prostaglandin D2 receptor DP as responsive to OxPAPC. We focused on EP2, which is expressed in ECs, monocytes, and macrophages. OxPAPC component PEIPC, but not POVPC, activated EP2 with an EC 50 of 108.6 nmol/L. OxPAPC and PEIPC were also able to compete with PGE2 for binding to EP2 in a ligand-binding assay. The EP2 specific agonist butaprost was shown to mimic the effect of OxPAPC on the activation of β1 integrin and the stimulation of monocyte binding to endothelial cells. Butaprost also mimicked the effect of OxPAPC on the regulation of tumor necrosis factor-α and interleukin-10 in monocyte-derived cells. EP2 antagonist AH6809 blocked the activation of EP2 by OxPAPC in HEK293 cells and blocked the interleukin-10 response to PEIPC in monocytic THP-1 cells. These results suggest that EP2 functions as a receptor for OxPAPC and PEIPC, either as the phospholipid ester or the released fatty acid, in both endothelial cells and macrophages.

Real-time RT-PCR using SYBR Green chemistry uses a green fluorescence dye, SYBR Green I, that binds to double stranded DNA (dsDNA) and exhibits enhancement of fluorescence upon binding to the DNA. The indiscriminate binding ability of SYBR Green I dye to dsDNA often results in non-specific products. We have shown that increasing the amplicon size from approximately 50 to approximately 75-100 bp increases the specificity due to higher melting temperature of the amplicon and also enhances the sensitivity of detection of real-time RT-PCR using SYBR Green chemistry while detecting two RNA viruses in laboratory-challenged shrimp, the Taura syndrome virus (TSV), and yellow head virus (YHV). The increased sensitivity of the larger amplicon over the smaller amplicon varied from 1.6 to 6.82-fold (with a median value of 4-fold) for the TSV-infected samples, and 1.80-10.27-fold (with a median value of 4-fold) for the YHV-infected samples. The longer amplicon also has a higher Tm value compared with the shorter amplicon (75.6 vs. 72.0 degrees C for TSV, and 81.3 vs. 72.5 degrees C for YHV). The increased melting temperature of the longer amplicon compared with the shorter amplicon will enable easier discrimination of a specific product from a primer dimer or other non-specific products. The improved method for the detection of TSV and YHV will be applicable not only to the detection of other viral pathogens but also to the quantitative measurement of cellular gene expression by real-time SYBR Green RT-PCR.

Myocyte hypertrophy antecedent to heart failure involves changes in global gene expression, although the preceding mechanisms to coordinate DNA accessibility on a genomic scale are unknown. Chromatin-associated proteins alter chromatin structure by changing their association with DNA, thereby altering the gene expression profile. Little is known about the global changes in chromatin subproteomes that accompany heart failure, and the mechanisms by which these proteins alter chromatin structure. The present study tests the fundamental hypothesis that cardiac growth and plasticity in the setting of disease recapitulates conserved developmental chromatin remodeling events. We used quantitative proteomics to identify chromatin-associated proteins extracted via detergent and to quantify changes in their abundance during disease. Our study identified 321 proteins in this subproteome, demonstrating it to have modest conservation (37%) with that revealed using strong acid. Of these proteins, 176 exhibited altered expression during cardiac hypertrophy and failure; we conducted extensive functional characterization of one of these proteins, Nucleolin. Morpholino-based knockdown of nucleolin nearly abolished protein expression but surprisingly had little impact on gross morphological development. However, hearts of fish lacking Nucleolin displayed severe developmental impairment, abnormal chamber patterning and functional deficits, ostensibly due to defects in cardiac looping and myocyte differentiation. The mechanisms underlying these defects involve perturbed bone morphogenetic protein 4 expression, decreased rRNA transcription, and a shift to more heterochromatic chromatin. This study reports the quantitative analysis of a new chromatin subproteome in the normal and diseased mouse heart. Validation studies in the complementary model system of zebrafish examine the role of Nucleolin to orchestrate genomic reprogramming events shared between development and disease. © 2013 the American Physiological Society.

Objective— Vascular endothelial growth factor (VEGF) signaling induces Notch signaling during angiogenesis. Flt-1/VEGF receptor-1 negatively modulates VEGF signaling. Therefore, we tested the hypothesis that disrupted Flt-1 regulation of VEGF signaling causes Notch pathway defects that contribute to dysmorphogenesis of Flt-1 mutant vessels. Approach and Results— Wild-type and flt-1 −/− mouse embryonic stem cell–derived vessels were exposed to pharmacological and protein-based Notch inhibitors with and without added VEGF. Vessel morphology, endothelial cell proliferation, and Notch target gene expression levels were assessed. Similar pathway manipulations were performed in developing vessels of zebrafish embryos. Notch inhibition reduced flt-1 −/− embryonic stem cell–derived vessel branching dysmorphogenesis and endothelial hyperproliferation, and rescue of flt-1 −/− vessels was accompanied by a reduction in elevated Notch targets. Surprisingly, wild-type vessel morphogenesis and proliferation were unaffected by Notch suppression, Notch targets in wild-type endothelium were unchanged, and Notch suppression perturbed zebrafish intersegmental vessels but not caudal vein plexuses. In contrast, exogenous VEGF caused wild-type embryonic stem cell–derived vessel and zebrafish intersegmental vessel dysmorphogenesis that was rescued by Notch blockade. Conclusions— Elevated Notch signaling downstream of perturbed VEGF signaling contributes to aberrant flt-1 −/− blood vessel formation. Notch signaling may be dispensable for blood vessel formation when VEGF signaling is below a critical threshold.

OBJECTIVE: Vascular endothelial growth factor (VEGF) signaling induces Notch signaling during angiogenesis. Flt-1/VEGF receptor-1 negatively modulates VEGF signaling. Therefore, we tested the hypothesis that disrupted Flt-1 regulation of VEGF signaling causes Notch pathway defects that contribute to dysmorphogenesis of Flt-1 mutant vessels. APPROACH AND RESULTS: Wild-type and flt-1(-/-) mouse embryonic stem cell-derived vessels were exposed to pharmacological and protein-based Notch inhibitors with and without added VEGF. Vessel morphology, endothelial cell proliferation, and Notch target gene expression levels were assessed. Similar pathway manipulations were performed in developing vessels of zebrafish embryos. Notch inhibition reduced flt-1(-/-) embryonic stem cell-derived vessel branching dysmorphogenesis and endothelial hyperproliferation, and rescue of flt-1(-/-) vessels was accompanied by a reduction in elevated Notch targets. Surprisingly, wild-type vessel morphogenesis and proliferation were unaffected by Notch suppression, Notch targets in wild-type endothelium were unchanged, and Notch suppression perturbed zebrafish intersegmental vessels but not caudal vein plexuses. In contrast, exogenous VEGF caused wild-type embryonic stem cell-derived vessel and zebrafish intersegmental vessel dysmorphogenesis that was rescued by Notch blockade. CONCLUSIONS: Elevated Notch signaling downstream of perturbed VEGF signaling contributes to aberrant flt-1(-/-) blood vessel formation. Notch signaling may be dispensable for blood vessel formation when VEGF signaling is below a critical threshold.

Approximately 30% of tumor endothelial cells have over-duplicated (>2) centrosomes, which may contribute to abnormal vessel function and drug resistance. Elevated levels of vascular endothelial growth factor A induce excess centrosomes in endothelial cells, but how other features of the tumor environment affect centrosome over-duplication is not known. To test this, we treated endothelial cells with tumor-derived factors, hypoxia, or reduced p53, and assessed centrosome numbers. We found that hypoxia and elevated levels of bone morphogenetic protein 2, 6 and 7 induced excess centrosomes in endothelial cells through BMPR1A and likely via SMAD signaling. In contrast, inflammatory mediators IL-8 and lipopolysaccharide did not induce excess centrosomes. Finally, down-regulation in endothelial cells of p53, a critical regulator of DNA damage and proliferation, caused centrosome over-duplication. Our findings suggest that some tumor-derived factors and genetic changes in endothelial cells contribute to excess centrosomes in tumor endothelial cells.

Cardiac hypertrophy is a common precursor to heart failure, during which cardiomyocytes grow to compensate for an increased workload. Hypertrophic cardiomyocytes undergo significant changes in cellular plasticity by adopting the expression profile and some phenotypic aspects of more primitive (embryonic or fetal) cardiac cells. These global changes in gene expression, conserved between humans and animal models of heart failure, must be preceded by structural alterations to the chromatin. Specifically, chromatin regions must be architecturally modified to allow or deny access for transcriptional machinery. However, the proteins responsible for remodeling chromatin to accomplish these gene expression changes during cardiac hypertrophy and failure are largely unknown. We used a proteomics approach to identify proteins bound to cardiac chromatin and to quantify changes in their abundance during disease. Quantitative mass spectrometry and bioinformatics revealed that 366 of the chromatin-bound proteins detected in this study displayed altered expression in a mouse model of pressure overload cardiac hypertrophy and failure. This included the chromatin remodeling protein Nucleolin (Ncl), which exhibited increased association with chromatin in the hypertrophic heart. To examine its role in regulating cardiac morphology and function we performed morpholino based knockdown of Ncl in zebrafish embryos. Ncl knockdown promoted the expression of bmp4 (a fetal marker), inhibited normal cardiomyocyte differentiation and resulted in abnormal heart chamber formation and looping. Hearts in surviving fish exhibited functional deficits as measured by fluorescence imaging and line-scanning analysis. To investigate the actions of Ncl in the mammalian cardiomyocyte, knockdown was carried out in isolated rat ventricular myocytes using siRNA. Loss of Ncl induced heterochromatin formation (increased Histone H3 K9-trimethylation), suppressed rDNA transcription (52% decrease in pre-rRNA via qPCR) and promoted fetal gene expression (65% increase in ANF; 41% increase in β-MHC transcripts). Overall, this study identifies Ncl as a regulator of chromatin structure, cell growth via ribosome biogenesis and cellular plasticity in the cardiomyocyte.

Oxidation products of 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphatidylcholine (Ox-PAPC) are found in atherosclerotic lesions, apoptotic cells, and oxidized LDL and stimulate human aortic endothelial cells (HAECs) to produce inflammatory cytokines, leukocyte chemoattractants, and coagulation factors. This regulation is thought to be a receptor-mediated process in which oxidized phospholipids activate specific receptors on HAECs to evoke an inflammatory response. To characterize the HAEC proteins with which oxidized phospholipids interact, a biotinylated PAPC analog, 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphatidyl-(N-biotinylethanolamine) (PAPE-N-biotin), was synthesized. Oxidation of PAPE-N-biotin in air generated a mixture of biotin-labeled oxidized lipids analogous to Ox-PAPC. Ox-PAPE-N-biotin, like Ox-PAPC, induced interleukin-8 (IL-8) protein synthesis and stimulated IL-8, low density lipoprotein receptor, heme oxygenase-1, and activating transcription factor-3 mRNA expression in HAECs. After treatment of HAECs with Ox-PAPE-N-biotin, the cellular proteins were isolated and separated by SDS-PAGE. Western analysis with streptavidin-HRP demonstrated at least 20 different biotinylated HAEC proteins to which the Ox-PAPE-N-biotin was associated, which were not detected with unoxidized PAPE-N-biotin treatment. This work suggests that oxidized phospholipids, such as those found in oxidized LDL, apoptotic cells, and atherosclerotic lesions, form tight interactions with specific endothelial cell proteins, which may be responsible for the inflammatory response. Identification of these putative oxidized phospholipid targets may reveal therapeutic targets to modulate inflammation and atherosclerosis.

Oxidized 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphorylcholine (Ox-PAPC) and its component phospholipid, 1-palmitoyl-2-epoxyisoprostane-sn-glycero-3-phosphorylcholine, induce endothelial cells (EC) to synthesize chemotactic factors, such as interleukin 8 (IL-8). Previously, we demonstrated a role for c-Src kinase activation in Ox-PAPC-induced IL-8 transcription. In this study, we have examined the mechanism regulating IL-8 transcription by Ox-PAPC downstream of c-Src. Our findings demonstrate an important role for JAK2 in the regulation of IL-8 transcription by Ox-PAPC. Treatment of human aortic EC with Ox-PAPC and 1-palmitoyl-2-epoxyisoprostane-sn-glycero-3-phosphorylcholine induced a rapid yet sustained activation of JAK2; activation of JAK2 by Ox-PAPC was dependent on c-Src kinase activity. Furthermore, pretreatment with selective JAK2 inhibitors significantly reduced Ox-PAPC-induced IL-8 transcription. In previous studies, we also demonstrated activation of STAT3 by Ox-PAPC. Here we provide evidence that STAT3 activation by Ox-PAPC is dependent on JAK2 activation and that STAT3 activation regulates IL-8 transcription by Ox-PAPC in human EC. Transfection with small interfering RNA against STAT3 significantly reduced Ox-PAPC-induced IL-8 transcription. Using chromatin immunoprecipitation assays, we demonstrated binding of activated STAT3 to the sequence flanking the consensus gamma-interferon activation sequence (GAS) in the IL-8 promoter; site-directed mutagenesis of GAS inhibited IL-8 transcription by Ox-PAPC. Finally, these studies demonstrate a role for STAT3 activation in atherosclerosis in vivo. We found increased staining for activated STAT3 in the inflammatory regions of human atherosclerotic lesions and reduced fatty streak formation in EC-specific STAT3 knock-out mice on the atherogenic diet. Taken together, these data demonstrate an important role for the JAK2/STAT3 pathway in Ox-PAPC-induced IL-8 transcription in vitro and in atherosclerosis in vivo.

Objective— Previous studies have shown that oxidized products of PAPC (Ox-PAPC) regulate cell transcription of interleukin-8, LDL receptor, and tissue factor. This upregulation takes place in part through the activation of sterol regulatory element-binding protein (SREBP) and Erk 1/2. The present studies identify vascular endothelial growth factor receptor 2 (VEGFR2) as a major regulator in the activation of SREBP and Erk 1/2 in endothelial cells activated by Ox-PAPC. Methods and Results— Ox-PAPC induced the phosphorylation of VEGFR2 at Tyr 1175 in human aortic endothelial cells. Inhibitors and siRNA for VEGFR2 decreased the transcription of interleukin-8, LDL receptor, and tissue factor in response to Ox-PAPC and the activation of SREBP and Erk 1/2, which mediate this transcription. We provide evidence that the activation of VEGFR2 is rapid, sustained, and c-Src–dependent. Conclusions— These data point to a major role of VEGFR2 in endothelial regulation by oxidized phospholipids which accumulate in atherosclerotic lesions and apoptotic cells.

Oxidized-1-palmitoyl-2-arachidonyl- sn -glycero-3-phosphorylcholine (Ox-PAPC), found in atherosclerotic lesions and other sites of chronic inflammation, activates endothelial cells (EC) to synthesize chemotactic factors, such as interleukin (IL)-8. Previously, we demonstrated that the sustained induction of IL-8 transcription by Ox-PAPC was mediated through the activation of sterol regulatory element–binding protein (SREBP). We now present evidence for the role of endothelial nitric oxide synthase (eNOS) in the activation of SREBP by Ox-PAPC. Ox-PAPC treatment of EC induced a dose- and time-dependent activation of eNOS, as measured by phosphorylation of serine 1177, dephosphorylation of threonine 495, and the conversion of l -arginine to l -citrulline. Activation of eNOS by Ox-PAPC was regulated through a phosphatidylinositol-3-kinase/Akt-mediated mechanism. These studies also demonstrated that pretreatment of EC with NOS inhibitor, N ω -nitro- l -arginine-methyl ester (L-NAME), significantly inhibited Ox-PAPC–induced IL-8 synthesis. Because SREBP activation had been previously shown to regulate IL-8 transcription by Ox-PAPC, we examined the effects of L-NAME on Ox-PAPC–induced SREBP activation. Our data demonstrated that Ox-PAPC–induced SREBP activation and expression of SREBP target genes were significantly reduced by pretreatment with L-NAME. Interestingly, treatment of EC with NO donor, S -nitroso- N -acetylpenicillamine, did not activate SREBP, suggesting that NO alone was not sufficient for SREBP activation. Rather, our findings indicated that superoxide (O 2 ·− ), in combination with NO, regulated SREBP activation by Ox-PAPC. We found that Ox-PAPC treatment generated O 2 ·− through an eNOS-mediated mechanism and that mercaptoethylguanidine, a peroxynitrite scavenger, reduced SREBP activation by Ox-PAPC. Taken together, these findings propose a novel role for eNOS in the activation of SREBP and SREBP-mediated inflammatory processes.

Oxidized 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphorylcholine (Ox-PAPC) and its component phospholipids 1-palmitoyl-2-epoxyisoprostane-sn-glycero-3-phosphorylcholine (PEIPC) and 1-palmitoyl-2-(5-oxovaleroyl)-sn-glycero-3-phosphorylcholine induce endothelial cells to synthesize chemotactic factors, such as interleukin 8 (IL-8). We have shown recently that Ox-PAPC-mediated induction of IL-8 transcription is independent of NF-kappaB activation, a major transcription factor utilized by cytokines and lipopolysaccharide for the induction of IL-8 transcription. In this study, we provide evidence for the role of c-src in Ox-PAPC and, specifically, PEIPC-mediated IL-8 induction. Ox-PAPC and its component phospholipids induced a rapid and transient phosphorylation of c-src Tyr418, a hallmark of c-src activation, in human aortic endothelial cells (HAEC). Ox-PAPC-mediated IL-8 protein synthesis in HAEC was inhibited by Src family kinase inhibitors, PP1 and PP2, but not by an inactive analog, PP3. Transient expression of plasmids containing C-terminal Src kinase or kinase-deficient dominant-negative c-src resulted in a 72 and 50% reduction in Ox-PAPC-induced IL-8 promoter activation in human microvascular endothelial cells, respectively. In contrast, overexpression of v-src kinase resulted in a 4-fold increase in IL-8 promoter activation, without inducing NF-kappaB promoter activation. Furthermore, treatment of HAEC with Ox-PAPC and its component PEIPC induced the activation of STAT3 by phosphorylating Tyr705, a feature of STAT3 activation. STAT3 is a known downstream effector of c-src. Ox-PAPC-induced activation of STAT3 resulted in the translocation of STAT3 from the cytoplasm of HAEC into their nuclear compartment. Transient expression of a dominant-negative STAT3beta construct in HMEC strongly inhibited IL-8 induction by Ox-PAPC. Taken together, these data demonstrate the role of the c-src kinase/STAT3 pathway in Ox-PAPC-mediated IL-8 expression in endothelial cells.

Background: Angiogenesis is the process by which the highly branched and functional vasculature arises from the major vessels, providing developing tissues with nutrients, oxygen, and removing metabolic waste. During embryogenesis, vascular patterning is dependent on a tightly regulated balance between pro- and anti-angiogenic signals, and failure of angiogenesis leads to embryonic lethality. Using the zebrafish as a model organism, we sought to identify genes that influence normal vascular patterning. Methodology and Principal Findings: In a forward genetic screen, we identified mutant LA1908, which manifests massive apoptosis during early embryogenesis, abnormal expression of several markers of arterial-venous specification, delayed angiogenic sprouting of the intersegmental vessels (ISV), and malformation of the caudal vein plexus (CVP), indicating a critical role for LA1908 in cell survival and angiogenesis. Genetic mapping and sequencing identified a G to A transition in the splice site preceding exon 11 of utp15 in LA1908 mutant embryos. Overexpression of wild type utp15 mRNA suppresses all observed mutant phenotypes, demonstrating a causative relationship between utp15 and LA1908. Furthermore, we found that injecting morpholino oligonucleotides inhibiting p53 translation prevents cell death and rescues the vascular abnormalities, indicating that p53 is downstream of Utp15 deficiency in mediating the LA1908 phenotypes. Conclusions and Significance: Taken together, our data demonstrate an early embryonic effect of Utp15 deficiency on cell survival and the normal patterning of the vasculature and highlight an anti-angiogenic role of p53 in developing embryos.