Vasodilator-stimulated phosphoprotein is involved in stress-fiber and membrane ruffle formation in endothelial cells

Membrane Ruffles: Composition, Function, Formation and Visualization

Membrane ruffles are cell actin-based membrane protrusions that have distinct structural characteristics. Linear ruffles with columnar spike-like and veil-like structures assemble at the leading edge of cell membranes. Circular dorsal ruffles (CDRs) have no supporting columnar structures but their veil-like structures, connecting from end to end, present an enclosed ring-shaped circular outline. Membrane ruffles are involved in multiple cell functions such as cell motility, macropinocytosis, receptor internalization, fluid viscosity sensing in a two-dimensional culture environment, and protecting cells from death in response to physiologically compressive loads. Herein, we review the state-of-the-art knowledge on membrane ruffle structure and function, the growth factor-induced membrane ruffling process, and the growth factor-independent ruffling mode triggered by calcium and other stimulating factors, together with the respective underlying mechanisms. We also summarize the inhibitors used in ruffle formation studies and their specificity. In the last part, an overview is given of the various techniques in which the membrane ruffles have been visualized up to now.

Adenylate cyclase type 9 antagonizes cAMP accumulation and regulates endothelial signaling involved in atheroprotection

AIMS

The adenylate cyclase type 9 (ADCY9) gene appears to determine atherosclerotic outcomes in patients treated with dalcetrapib. In mice, we recently demonstrated that Adcy9 inactivation potentiates endothelial function and inhibits atherogenesis. The objective of this study was to characterize the contribution of ADCY9 to the regulation of endothelial signaling pathways involved in atherosclerosis.

METHODS AND RESULTS

We show that ADCY9 is expressed in the endothelium of mouse aorta and femoral arteries. We demonstrate that ADCY9 inactivation in cultured endothelial cells paradoxically increases cAMP accumulation in response to the adenylate cyclase activators forskolin and vasoactive intestinal peptide (VIP). Reciprocally, ADCY9 overexpression decreases cAMP production. Using mouse femoral artery arteriography, we show that Adcy9 inactivation potentiates VIP-induced endothelial-dependent vasodilation. Moreover, Adcy9 inactivation reduces mouse atheroma endothelial permeability in different vascular beds. ADCY9 overexpression reduces forskolin-induced phosphorylation of Ser157-vasodilator-stimulated phosphoprotein (VASP) and worsens thrombin-induced fall of RAP1 activity, both leading to increased endothelial permeability. ADCY9 inactivation in thrombin-stimulated human coronary artery endothelial cells results in cAMP accumulation, increases p-Ser157-VASP and inhibits endothelial permeability. MLC2 phosphorylation and actin stress fiber increases in response to thrombin were reduced by ADCY9 inactivation, suggesting actin cystoskeleton regulation. Finally, using the Miles assay, we demonstrate that Adcy9 regulates thrombin-induced endothelial permeability in vivo in normal and atherosclerotic animals.

CONCLUSION

Adcy9 is expressed in endothelial cells and regulates local cAMP and endothelial functions including permeability relevant to atherogenesis.

The Adhesome Network: Key Components Shaping the Tumour Stroma

Beyond the conventional perception of solid tumours as mere masses of cancer cells, advanced cancer research focuses on the complex contributions of tumour-associated host cells that are known as "tumour microenvironment" (TME). It has been long appreciated that the tumour stroma, composed mainly of blood vessels, cancer-associated fibroblasts and immune cells, together with the extracellular matrix (ECM), define the tumour architecture and influence cancer cell properties. Besides soluble cues, that mediate the crosstalk between tumour and stroma cells, cell adhesion to ECM arises as a crucial determinant in cancer progression. In this review, we discuss how adhesome, the intracellular protein network formed at cell adhesions, regulate the TME and control malignancy. The role of adhesome extends beyond the physical attachment of cells to ECM and the regulation of cytoskeletal remodelling and acts as a signalling and mechanosensing hub, orchestrating cellular responses that shape the tumour milieu.

Disruption of profilin1 function suppresses developmental and pathological retinal neovascularization

Angiogenesis-mediated neovascularization in the eye is usually associated with visual complications. Pathological angiogenesis is particularly prominent in the retina in the settings of proliferative diabetic retinopathy, in which it can lead to permanent loss of vision. In this study, by bioinformatics analyses, we provide evidence for elevated expression of actin-binding protein PFN1 (profilin1) in the retinal vascular endothelial cells (VECs) of individuals with proliferative diabetic retinopathy, findings further supported by gene expression analyses for PFN1 in experimentally induced abnormal retinal neovascularization in an oxygen-induced retinopathy murine model. We observed that in a conditional knockout mouse model, postnatal deletion of the Pfn1 gene in VECs leads to defects in tip cell activity (marked by impaired filopodial protrusions) and reduced vascular sprouting, resulting in hypovascularization during developmental angiogenesis in the retina. Consistent with these findings, an investigative small molecule compound targeting the PFN1-actin interaction reduced random motility, proliferation, and cord morphogenesis of retinal VECs in vitro and experimentally induced abnormal retinal neovascularization in vivo In summary, these findings provide the first direct in vivo evidence that PFN1 is required for formation of actin-based protrusive structures and developmental angiogenesis in the retina. The proof of concept of susceptibility of abnormal angiogenesis to small molecule intervention of PFN1-actin interaction reported here lays a conceptual foundation for targeting PFN1 as a possible strategy in angiogenesis-dependent retinal diseases.

RIAM-VASP Module Relays Integrin Complement Receptors in Outside-In Signaling Driving Particle Engulfment

The phagocytic integrins and complement receptors α_Mβ₂/CR3 and α_Xβ₂/CR4 are classically associated with the phagocytosis of iC3b-opsonized particles. The activation of this receptor is dependent on signals derived from other receptors (inside-out signaling) with the crucial involvement of the Rap1-RIAM-Talin-1 pathway. Here, we analyze the implication of RIAM and its binding partner VASP in the signaling events occurring downstream of β₂ integrins (outside-in) during complement-mediated phagocytosis. To this end, we used HL-60 promyelocytic cell lines deficient in RIAM or VASP or overexpressing EGFP-tagged VASP to determine VASP dynamics at phagocytic cups. Our results indicate that RIAM-deficient HL-60 cells presented impaired particle internalization and altered integrin downstream signaling during complement-dependent phagocytosis. Similarly, VASP deficiency completely blocked phagocytosis, while VASP overexpression increased the random movement of phagocytic particles at the cell surface, with reduced internalization. Moreover, the recruitment of VASP to particle contact sites, amount of pSer157-VASP and formation of actin-rich phagocytic cups were dependent on RIAM expression. Our results suggested that RIAM worked as a relay for integrin complement receptors in outside-in signaling, coordinating integrin activation and cytoskeletal rearrangements via its interaction with VASP.

AKAP12 deficiency impairs VEGF-induced endothelial cell migration and sprouting

Aim

Protein kinase (PK) A anchoring protein (AKAP) 12 is a scaffolding protein that anchors PKA to compartmentalize cyclic AMP signalling. This study assessed the consequences of the downregulation or deletion of AKAP12 on endothelial cell migration and angiogenesis.

Methods

The consequences of siRNA‐mediated downregulation AKAP12 were studied in primary cultures of human endothelial cells as well as in endothelial cells and retinas from wild‐type versus AKAP12^−/− mice. Molecular interactions were investigated using a combination of immunoprecipitation and mass spectrometry.

Results

AKAP12 was expressed at low levels in confluent endothelial cells but its expression was increased in actively migrating cells, where it localized to lamellipodia. In the postnatal retina, AKAP12 was expressed by actively migrating tip cells at the angiogenic front, and its deletion resulted in defective extension of the vascular plexus. In migrating endothelial cells, AKAP12 was co‐localized with the PKA type II‐α regulatory subunit as well as multiple key regulators of actin dynamics and actin filament‐based movement; including components of the Arp2/3 complex and the vasodilator‐stimulated phosphoprotein (VASP). Fitting with the evidence of a physical VASP/AKAP12/PKA complex, it was possible to demonstrate that the VEGF‐stimulated and PKA‐dependent phosphorylation of VASP was dependent on AKAP12. Indeed, AKAP12 colocalized with phospho‐Ser157 VASP at the leading edge of migrating endothelial cells.

Conclusion

The results suggest that compartmentalized AKAP12/PKA signalling mediates VASP phosphorylation at the leading edge of migrating endothelial cells to translate angiogenic stimuli into altered actin dynamics and cell movement.

Role of NO/VASP Signaling Pathway against Obesity-Related Inflammation and Insulin Resistance

Obesity has quickly become a worldwide pandemic, causing major adverse health outcomes such as dyslipidemia, type 2 diabetes mellitus, cardiovascular disease and cancers. Obesity-induced insulin resistance is the key for developing these metabolic disorders, and investigation to understand the molecular mechanisms involved has been vibrant for the past few decades. Of these, low-grade chronic inflammation is suggested as a critical concept in the development of obesity-induced insulin resistance, and the anti-inflammatory effect of nitric oxide (NO) signaling has been reported to be linked to improvement of insulin resistance in multiple organs involved in glucose metabolism. Recently, a body of evidence suggested that vasodilatory-stimulated phosphoprotein (VASP), a downstream mediator of NO signaling plays a crucial role in the anti-inflammatory effect and improvement of peripheral insulin resistance. These preclinical studies suggest that NO/VASP signaling could be an ideal therapeutic target in the treatment of obesity-related metabolic dysfunction. In this review, we introduce studies that investigated the protective role of NO/VASP signaling against obesity-related inflammation and insulin resistance in various tissues.

Anti-angiogenic effects of resveratrol in combination with 5-fluorouracil on B16 murine melanoma cells

5-Fluorouracil (5-FU) has been used as a chemotherapeutic drug for various types of cancer, although the development of resistance remains a major limitation for its use in clinical settings. In the present study, the anti-angiogenic effects of resveratrol and 5-FU either alone or in combination were examined in a B16 murine melanoma model. Co-treatment using resveratrol and 5-FU inhibited cell proliferation more efficiently compared with use of either drug alone and the antiproliferative effect coincided with changes in the expression levels of AMP-activated protein kinase (AMPK), cyclooxygenase-2, vasodilator-stimulated phosphoprotein (VASP) and vascular endothelial growth factor (VEGF). Furthermore, co-treatment with resveratrol and 5-FU reduced tumor growth compared with that in the control group and this growth-inhibitory effect was associated with changes in the expression levels of AMPK, VASP and VEGF. Immunohistochemical staining for angiogenesis demonstrated that co-treatment with resveratrol and 5-FU reduced the number of microvascular vessels compared with that in the control group. These results suggested that co-treatment with resveratrol and 5-FU suppressed cell growth and angiogenesis in B16 murine melanoma tumors.

Barrier enhancing signals in pulmonary edema

Increased endothelial permeability and reduction of alveolar liquid clearance capacity are two leading pathogenic mechanisms of pulmonary edema, which is a major complication of acute lung injury, severe pneumonia, and acute respiratory distress syndrome, the pathologies characterized by unacceptably high rates of morbidity and mortality. Besides the success in protective ventilation strategies, no efficient pharmacological approaches exist to treat this devastating condition. Understanding of fundamental mechanisms involved in regulation of endothelial permeability is essential for development of barrier protective therapeutic strategies. Ongoing studies characterized specific barrier protective mechanisms and identified intracellular targets directly involved in regulation of endothelial permeability. Growing evidence suggests that, although each protective agonist triggers a unique pattern of signaling pathways, selected common mechanisms contributing to endothelial barrier protection may be shared by different barrier protective agents. Therefore, understanding of basic barrier protective mechanisms in pulmonary endothelium is essential for selection of optimal treatment of pulmonary edema of different etiology. This article focuses on mechanisms of lung vascular permeability, reviews major intracellular signaling cascades involved in endothelial monolayer barrier preservation and summarizes a current knowledge regarding recently identified compounds which either reduce pulmonary endothelial barrier disruption and hyperpermeability, or reverse preexisting lung vascular barrier compromise induced by pathologic insults.

Guiding cell migration by tugging

The ability of cells to move directionally toward areas of stiffer extracellular matrix (ECM) via a process known as 'durotaxis' is thought to be critical for development and wound healing, but durotaxis can also drive cancer metastasis. Migration is driven by integrin-mediated focal adhesions (FAs), protein assemblies that couple contractile actomyosin bundles to the plasma membrane, transmit force generated by the cytoskeleton to the ECM, and convert the mechanical properties of the microenvironment into biochemical signals. To probe the stiffness of the ECM, motile fibroblasts modulate FA mechanics on the nanoscale and exert forces that are reminiscent of repeated tugging on the ECM. Within a single cell, all FAs tug autonomously and thus act as local rigidity sensors, allowing discernment of differences in the extracellular matrix rigidity at high spatial resolution. In this article, we review current advances that may shed light on the mechanism of traction force fluctuations within FAs. We also examine plausible downstream effectors of tugging forces which may regulate cytoskeletal and FA dynamics to guide cell migration in response to ECM stiffness gradients.

Constant or fluctuating hyperglycemias increases cytomembrane stiffness of human umbilical vein endothelial cells in culture: roles of cytoskeletal rearrangement and nitric oxide synthesis

Background

Previous studies have implicated continuous or intermittent hyperglycemia in altered endothelium-derived nitric oxide (NO) synthesis. NO can regulate both the F-actin cytoskeleton and endothelial cell membrane stiffness. Atomic force microscopy (AFM) is a powerful tool that can be used to study plasma membrane deformability at the single cell level. As membrane stiffness is partially dependent on filamentous F-actin, the interdependence of these parameters can be studied through the combined approaches of AFM and laser scanning confocal microscopy (LSCM). In the present study, we evaluated the effects of constant or fluctuating hyperglycemia on endothelial-derived NO synthesis, the cytoskeletal contribution and endothelial cell membrane stiffness.

Results

Compared to control cells cultured in low glucose (5 mM), constant (25 mM) or fluctuating (25/5 mM) high glucose significantly decreased NO release along with stiffening of endothelial cell membranes and F-actin rearrangement. The non-selective nitric oxide synthase (NOS) inhibitor, N^G-nitro-_L-arginine methyl ester (_L-NAME) exerted similar effects on endothelial cells. Increasing concentrations of _L-NAME (from 0.1 to 1 mM) exacerbated these effects in a concentration-dependent manner.

Conclusions

Result from the present study suggest that stiffening endothelial cell membranes are associated with decreased NO synthesis, which was established through the F-actin cytoskeletal redistribution. The precise mechanisms of hyperglycemia-induced endothelial dysfunction require further investigation.

Vasodilator phosphostimulated protein (VASP) protects endothelial barrier function during hypoxia

The endothelial barrier controls the passage of solutes from the vascular space. This is achieved through active reorganization of the actin cytoskeleton. A central cytoskeletal protein involved into this is vasodilator-stimulated phosphoprotein (VASP). However, the functional role of endothelial VASP during hypoxia has not been thoroughly elucidated. We determined endothelial VASP expression through real-time PCR (Rt-PCR), immunhistochemistry, and Western blot analysis during hypoxia. VASP promoter studies were performed using a PGL3 firefly luciferase containing plasmid. Following approval by the local authorities, VASP ( -/- ) mice and littermate controls were subjected to normobaric hypoxia (8% O(2), 92% N(2)) after intravenous injection of Evans blue dye. In in vitro studies, we found significant VASP repression in human microvascular and human umbilical vein endothelial cells through Rt-PCR, immunhistochemistry, and Western blot analysis. The VASP promoter construct demonstrated significant repression in response to hypoxia, which was abolished when the binding of hypoxia-inducible factor 1 alpha was excluded. Exposure of wild-type (WT) and VASP ( -/- ) animals to normobaric hypoxia for 4 h resulted in an increase in Evans blue tissue extravasation that was significantly increased in VASP ( -/- ) animals compared to WT controls. In summary, we demonstrate here that endothelial VASP holds significant importance for endothelial barrier properties during hypoxia.

Investigation of in vivo microtubule and stress fiber mechanics with laser ablation

Laser ablation has emerged as a useful technique to study the mechanical properties of the cytoskeleton in living cells. Laser ablation perturbs the force balance in the cytoskeleton, resulting in a dynamic response which can be imaged. Quantitative measurement of the dynamic response allows the testing of mechanical theories of the cytoskeleton in living cells. This review discusses recent work in applying laser ablation to study stress fiber and microtubule mechanics in living endothelial cells. These studies reveal that molecular motors are major determinants of the mechanical properties of the cytoskeleton in cells.

Cytoplasmic Ig-domain proteins: cytoskeletal regulators with a role in human disease

Immunoglobulin domains are found in a wide variety of functionally diverse transmembrane proteins, and also in a smaller number of cytoplasmic proteins. Members of this latter group are usually associated with the actin cytoskeleton, and most of them bind directly to either actin or myosin, or both. Recently, studies of inherited human disorders have identified disease-causing mutations in five cytoplasmic Ig-domain proteins: myosin-binding protein C, titin, myotilin, palladin, and myopalladin. Together with results obtained from cultured cells and mouse models, these clinical studies have yielded novel insights into the unexpected roles of Ig domain proteins in mechanotransduction and signaling to the nucleus. An emerging theme in this field is that cytoskeleton-associated Ig domain proteins are more than structural elements of the cell, and may have evolved to fill different needs in different cellular compartments. Cell Motil. Cytoskeleton 2009. (c) 2009 Wiley-Liss, Inc.

Differential VASP phosphorylation controls remodeling of the actin cytoskeleton

Proteins of the Enabled/vasodilator-stimulated phosphoprotein (Ena/VASP) family link signal transduction pathways to actin cytoskeleton dynamics. VASP is substrate of cAMP-dependent, cGMP-dependent and AMP-activated protein kinases that primarily phosphorylate the sites S157, S239 and T278, respectively. Here, we systematically analyzed functions of VASP phosphorylation patterns for actin assembly and subcellular targeting in vivo and compared the phosphorylation effects of Ena/VASP family members. Methods used were the reconstitution of VASP-null cells with ;locked' phosphomimetic VASP mutants, actin polymerization of VASP mutants in vitro and in living cells, site-specific kinase-mediated VASP phosphorylation, and analysis of the endogenous protein with phosphorylation-status-specific antibodies. Phosphorylation at S157 influenced VASP localization, but had a minor impact on F-actin assembly. Phosphorylation of the S157-equivalent site in the Ena/VASP family members Mena and EVL had no effect on the ratio of cellular F-actin to G-actin. By contrast, VASP phosphorylation at S239 (and the equivalent site in Mena) or T278 impaired VASP-driven actin filament formation. The data show that VASP functions are precisely regulated by differential phosphorylation and provide new insights into cytoskeletal control by serine/threonine kinase-dependent signaling pathways.

Inflammation-associated repression of vasodilator-stimulated phosphoprotein (VASP) reduces alveolar-capillary barrier function during acute lung injury

Acute lung injury (ALI) is an inflammatory disorder associated with reduced alveolar-capillary barrier function, increased pulmonary vascular permeability, and infiltration of leukocytes into the alveolar space. Pulmonary function might be compromised, its most severe form being the acute respiratory distress syndrome. A protein central to physiological barrier properties is vasodilator-stimulated phosphoprotein (VASP). Given the fact that VASP expression is reduced during periods of cellular hypoxia, we investigated the role of VASP during ALI. Initial studies revealed reduced VASP expressional levels through cytokines in vitro. Studies in the putative human VASP promoter identified NF-kappaB as a key regulator of VASP transcription. This VASP repression results in increased paracellular permeability and migration of neutrophils in vitro. In a model of LPS-induced ALI, VASP(-/-) mice demonstrated increased pulmonary damage compared with wild-type animals. These findings were confirmed in a second model of ventilator-induced lung injury. Studies employing bone marrow chimeric animals identified tissue-specific repression of VASP as the underlying cause of decreased barrier properties of the alveolar-capillary barrier during ALI. Taken together these studies identify tissue-specific VASP as a central protein in the control of the alveolar-capillary barrier properties during ALI.

Cyclic GMP signaling in cardiovascular pathophysiology and therapeutics

Cyclic guanosine 3',5'-monophosphate (cGMP) mediates a wide spectrum of physiologic processes in multiple cell types within the cardiovascular system. Dysfunctional signaling at any step of the cascade - cGMP synthesis, effector activation, or catabolism - have been implicated in numerous cardiovascular diseases, ranging from hypertension to atherosclerosis to cardiac hypertrophy and heart failure. In this review, we outline each step of the cGMP signaling cascade and discuss its regulation and physiologic effects within the cardiovascular system. In addition, we illustrate how cGMP signaling becomes dysregulated in specific cardiovascular disease states. The ubiquitous role cGMP plays in cardiac physiology and pathophysiology presents great opportunities for pharmacologic modulation of the cGMP signal in the treatment of cardiovascular diseases. We detail the various therapeutic interventional strategies that have been developed or are in development, summarizing relevant preclinical and clinical studies.

Plasma membrane cholesterol content affects nitric oxide diffusion dynamics and signaling

Nitric oxide (NO) signaling is inextricably linked to both its physical and chemical properties. Due to its preferentially hydrophobic solubility, NO molecules tend to partition from the aqueous milieu into biological membranes. We hypothesized that plasma membrane ordering provided by cholesterol further couples the physics of NO diffusion with cellular signaling. Fluorescence lifetime quenching studies with pyrene liposome preparations showed that the presence of cholesterol decreased apparent diffusion coefficients of NO approximately 20-40%, depending on the phospholipid composition. Electrochemical measurements indicated that the diffusion rate of NO across artificial bilayer membranes were inversely related to cholesterol content. Sterol transport-defective Niemann-Pick type C1 (NPC1) fibroblasts exhibited increased plasma membrane cholesterol content but decreased activation of both intracellular soluble guanylyl cyclase and vasodilator-stimulated phosphoprotein (VASP) phosphorylation at Ser(239) induced by exogenous NO exposure relative to their normal human fibroblast (NHF) counterparts. Augmentation of plasma membrane cholesterol in NHF diminished production of both cGMP and VASP phosphorylation elicited by NO to NPC1-comparable levels. Conversely, decreasing membrane cholesterol in NPC1 resulted in the augmentation in both cGMP and VASP phosphorylation to a level similar to those observed in NHF. Increasing plasma membrane cholesterol contents in NHF, platelets, erythrocytes and tumor cells also resulted in an increased level of extracellular diaminofluorescein nitrosation following NO exposure. These findings suggest that the impact of cholesterol on membrane fluidity and microdomain structure contributes to the spatial heterogeneity of NO diffusion and signaling.

Role of vasodilator-stimulated phosphoprotein in cGMP-mediated protection of human pulmonary artery endothelial barrier function

Increased pulmonary endothelial cGMP was shown to prevent endothelial barrier dysfunction through activation of protein kinase G (PKG(I)). Vasodilator-stimulated phosphoprotein (VASP) has been hypothesized to mediate PKG(I) barrier protection because VASP is a cytoskeletal phosphorylation target of PKG(I) expressed in cell-cell junctions. Unphosphorylated VASP was proposed to increase paracellular permeability through actin polymerization and stress fiber bundling, a process inhibited by PKG(I)-mediated phosphorylation of Ser(157) and Ser(239). To test this hypothesis, we examined the role of VASP in the transient barrier dysfunction caused by H(2)O(2) in human pulmonary artery endothelial cell (HPAEC) monolayers studied without and with PKG(I) expression introduced by adenoviral infection (Ad.PKG). In the absence of PKG(I) expression, H(2)O(2) (100-250 microM) caused a transient increased permeability and pSer(157)-VASP formation that were both attenuated by protein kinase C inhibition. Potentiation of VASP Ser(157) phosphorylation by either phosphatase 2B inhibition with cyclosporin or protein kinase A activation with forskolin prolonged, rather than inhibited, the increased permeability caused by H(2)O(2). With Ad.PKG infection, inhibition of VASP expression with small interfering RNA exacerbated H(2)O(2)-induced barrier dysfunction but had no effect on cGMP-mediated barrier protection. In addition, expression of a Ser-double phosphomimetic mutant VASP failed to reproduce the protective effects of activated PKG(I). Finally, expression of a Ser-double phosphorylation-resistant VASP failed to interfere with the ability of cGMP/PKG(I) to attenuate H(2)O(2)-induced disruption of VE-cadherin homotypic binding. Our results suggest that VASP phosphorylation does not explain the protective effect of cGMP/PKG(I) on H(2)O(2)-induced endothelial barrier dysfunction in HPAEC.

Cytoskeleton assembly at endothelial cell-cell contacts is regulated by alphaII-spectrin-VASP complexes

Directed cortical actin assembly is the driving force for intercellular adhesion. Regulated by phosphorylation, vasodilator-stimulated phosphoprotein (VASP) participates in actin fiber formation. We screened for endothelial proteins, which bind to VASP, dependent on its phosphorylation status. Differential proteomics identified αII-spectrin as such a VASP-interacting protein. αII-Spectrin binds to the VASP triple GP₅-motif via its SH3 domain. cAMP-dependent protein kinase–mediated VASP phosphorylation at Ser157 inhibits αII-spectrin–VASP binding. VASP is dephosphorylated upon formation of cell–cell contacts and in confluent, but not in sparse cells, αII-spectrin colocalizes with nonphosphorylated VASP at cell–cell junctions. Ectopic expression of the αII-spectrin SH3 domain at cell–cell contacts translocates VASP, initiates cortical actin cytoskeleton formation, stabilizes cell–cell contacts, and decreases endothelial permeability. Conversely, the permeability of VASP-deficient endothelial cells (ECs) and microvessels of VASP-null mice increases. Reconstitution of VASP-deficient ECs rescues barrier function, whereas αII-spectrin binding-deficient VASP mutants fail to restore elevated permeability. We propose that αII-spectrin–VASP complexes regulate cortical actin cytoskeleton assembly with implications for vascular permeability.

Role of Ena/VASP proteins in homeostasis and disease

The actin cytoskeleton is required for many important processes during embryonic development. In later stages of life, important homeostatic processes depend on the actin cytoskeleton, such as immune response, haemostasis and blood vessel preservation. Therefore, the function of the actin cytoskeleton must be tightly regulated, and aberrant regulation may cause disease. A growing number of proteins have been described to bind and regulate the actin cytoskeleton. Amongst them, Ena/VASP proteins function as anti-capping proteins, thereby directly modulating the actin ultrastructure. Ena/VASP function is regulated by their recruitment into protein complexes downstream of plasma membrane receptors and by phosphorylation. As regulators of the actin ultrastructure, Ena/VASP proteins are involved in crucial cellular functions, such as shape change, adhesion, migration and cell-cell interaction and hence are important targets for therapeutic intervention. In this chapter, we will first describe the structure, function and regulation of Ena/VASP proteins. Then, we will review the involvement of Ena/VASP proteins in the development of human diseases. Growing evidence links Ena/VASP proteins to important human diseases, such as thrombosis, cancer, arteriosclerosis, cardiomyopathy and nephritis. Finally, present and future perspectives for the development of therapeutic molecules interfering with Ena/VASP-mediated protein-protein interactions are presented.

Atrial natriuretic peptide-initiated cGMP pathways regulate vasodilator-stimulated phosphoprotein phosphorylation and angiogenesis in vascular endothelium

Nitric oxide (NO)- and atrial natriuretic peptide (ANP)-initiated cGMP signaling cascades are important in the maintenance of cardiovascular homeostasis. The molecular signaling mechanisms downstream of cGMP are not well understood, however. We have used small interfering RNA (siRNA) approaches to specifically knock down a series of signaling proteins in bovine aortic endothelial cells, and we have combined biochemical analyses with physiological assays to investigate cGMP-mediated signal transduction pathways. Activation of particulate guanylate cyclase (GC-A) by ANP leads to a substantial, dose-dependent, rapid, and sustained increase in intracellular cGMP. In contrast, stimulation of soluble guanylate cyclase by NO yields only a weak and transient increase in cGMP. ANP-induced cGMP production is selectively suppressed by siRNA-mediated knockdown of GC-A. ANP greatly enhances the phosphorylation at Ser-239 of the vasodilator-stimulated phosphoprotein (VASP), a major substrate of cGMP-dependent protein kinase (PKG) that significantly influences actin dynamics. Moreover, the ANP-induced phosphorylation of VASP at Ser-239 is accompanied by increased actin stress fiber formation and enhanced endothelial tube formation. siRNA-mediated knockdown of GC-A, VASP, or PKG abolishes ANP-induced VASP Ser-239 phosphorylation, stress fiber formation, and endothelial tube formation. We have demonstrated similar findings in human umbilical vein endothelial cells, where ANP substantially enhances intracellular cGMP content, phosphorylation of VASP at Ser-239, and endothelial tube formation. Taken together, our findings suggest that ANP-mediated cGMP signal transduction pathways regulate PKG phosphorylation of VASP Ser-239 in endothelial cells, resulting in reorganization of the actin cytoskeleton and enhancement of angiogenesis.

Ena/VASP is required for endothelial barrier function in vivo

Enabled/vasodilator-stimulated phosphoprotein (Ena/VASP) proteins are key actin regulators that localize at regions of dynamic actin remodeling, including cellular protrusions and cell–cell and cell–matrix junctions. Several studies have suggested that Ena/VASP proteins are involved in the formation and function of cellular junctions. Here, we establish the importance of Ena/VASP in endothelial junctions in vivo by analysis of Ena/VASP-deficient animals. In the absence of Ena/VASP, the vasculature exhibits patterning defects and lacks structural integrity, leading to edema, hemorrhaging, and late stage embryonic lethality. In endothelial cells, we find that Ena/VASP activity is required for normal F-actin content, actomyosin contractility, and proper response to shear stress. These findings demonstrate that Ena/VASP is critical for actin cytoskeleton remodeling events involved in the maintenance of functional endothelia.

Regulated proteolytic processing of Tie1 modulates ligand responsiveness of the receptor-tyrosine kinase Tie2

Regulated ectodomain shedding followed by intramembrane proteolysis has recently been recognized as important in cell signaling and for degradation of several type I transmembrane proteins. The receptor-tyrosine kinase Tie1 is known to undergo ectodomain cleavage generating a membrane-tethered endodomain. Here we show Tie1 is a substrate for regulated intramembrane proteolysis. After Tie1 ectodomain cleavage the newly formed 45-kDa endodomain undergoes additional proteolytic processing mediated by gamma-secretase to generate an amino-terminal-truncated 42-kDa fragment that is subsequently degraded by proteasomal activity. This sequential processing occurs constitutively and is stimulated by phorbol ester and vascular endothelial growth factor. To assess the biological significance of regulated Tie1 processing, we analyzed its effects on angiopoietin signaling. Activation of ectodomain cleavage causes loss of phosphorylated Tie1 holoreceptor and generation of phosphorylated receptor fragments in the presence of cartilage oligomeric protein angiopoietin 1. A key function of gamma-secretase is in preventing accumulation of these phosphorylated fragments. We also find that regulated Tie1 processing modulates ligand responsiveness of the Tie-1-associated receptor Tie2. Activation of Tie1 ectodomain cleavage increases cartilage oligomeric protein angiopoietin 1 activation of Tie2. This correlates with increased ability of Tie2 to bind ligand after shedding of the Tie1 extracellular domain. A similar enhancement of ligand activation of Tie2 is seen when Tie1 expression is suppressed by RNA interference. Together these data indicate that Tie1, via its extracellular domain, limits the ability of ligand to bind and activate Tie2. Furthermore the data suggest that regulated processing of Tie1 may be an important mechanism for controlling signaling by Tie2.

Identification of vasodilator-stimulated phosphoprotein (VASP) as an HIF-regulated tissue permeability factor during hypoxia

Increased tissue permeability is commonly associated with hypoxia of many origins. Since hypoxia-inducible factor (HIF) represents a predominant hypoxia signaling mechanism, we compared hypoxia-elicited changes in tissue barrier function in mice conditionally lacking intestinal epithelial hypoxia-inducible factor-1alpha (hif1a). Somewhat surprisingly, these studies revealed that mutant hif1a mice were protected from hypoxia-induced increases in intestinal permeability in vivo. Guided by microarray analysis of tissues derived from these mutant hif1a mice, we identified HIF-1-dependent repression of vasodilator-stimulated phosphoprotein (VASP), a molecule known to be important in the control of cytoskeletal dynamics, including barrier function. Studies at the mRNA and protein level confirmed hypoxia-elicited repression of VASP in murine tissue, cultured epithelia and endothelia, as well as human saphenous vein ex vivo. Targeted repression of VASP by siRNA recapitulated our findings with hypoxia and directed overexpression of VASP abolished hypoxia-induced barrier dysfunction. Studies in the cloned human VASP promoter revealed hypoxia-dependent transcriptional repression, and functional studies by chromatin immunoprecipitation (ChIP) and site-directed mutagenesis revealed hypoxia-dependent binding of HIF-1alpha to the human VASP promoter. These studies identify HIF-1-dependent repression of VASP as a control point for hypoxia-regulated barrier dysfunction.

AMP-activated protein kinase impairs endothelial actin cytoskeleton assembly by phosphorylating vasodilator-stimulated phosphoprotein

Vasodilator-stimulated phosphoprotein (VASP) is an actin regulatory protein that links signaling pathways to remodeling of the cytoskeleton. VASP functions are modulated by protein kinases, which phosphorylate the sites Ser-157, Ser-239, and Thr-278. The kinase responsible for Thr-278 phosphorylation, biological functions of the phosphorylation, and association with disease states have remained enigmatic. Using VASP phosphorylation status-specific antibodies, we identified AMP-activated protein kinase (AMPK), a serine-threonine kinase and fundamental sensor of energy homeostasis, in a screen for kinases that phosphorylate the Thr-278 site of VASP in endothelial cells. Pharmacological AMPK inhibitors and activators and AMPK mutants revealed that the kinase specifically targets residue Thr-278 but not Ser-157 or Ser-239. Quantitative fluorescence-activated cell sorter analysis and serum response factor transcriptional reporter assays, which quantify the cellular F-/G-actin equilibrium, indicated that AMPK-mediated VASP phosphorylation impaired actin stress fiber formation and altered cell morphology. In the Zucker Diabetic Fatty (ZDF) rat model for type II diabetes, AMPK activity and Thr-278 phosphorylation were substantially reduced in arterial vessel walls. These findings suggest that VASP is a new AMPK substrate, that VASP Thr-278 phosphorylation translates metabolic signals into actin cytoskeleton rearrangements, and that this signaling system becomes down-regulated in diabetic vessels.

Role of protein kinase G in barrier-protective effects of cGMP in human pulmonary artery endothelial cells

Increases in endothelial cGMP prevent oxidant-mediated endothelial barrier dysfunction, but the downstream mechanisms remain unclear. To determine the role of cGMP-dependent protein kinase (PKG)(I), human pulmonary artery endothelial cells (HPAEC) lacking PKG(I) expression were infected with a recombinant adenovirus encoding PKG(Ibeta) (Ad.PKG) and compared with uninfected and control-infected (Ad.betagal) HPAEC. Transendothelial electrical resistance (TER), an index of permeability, was measured after H(2)O(2) (250 microM) exposure with or without pretreatment with 8-(4-chlorophenylthio)guanosine 3',5'-cyclic monophosphate (CPT-cGMP). HPAEC infected with Ad.PKG, but not Ad.betagal, expressed PKG(I) protein and demonstrated Ser(239) and Ser(157) phosphorylation of vasodilator-stimulated phosphoprotein after treatment with CPT-cGMP. Adenoviral infection decreased basal permeability equally in Ad.PKG- and Ad.betagal-infected HPAEC compared with uninfected cells. Treatment with CPT-cGMP (100 microM) caused a PKG(I)-independent decrease in permeability (8.2 +/- 0.6%). In all three groups, H(2)O(2) (250 microM) caused a similar approximately 35% increase in permeability associated with increased actin stress fiber formation, intercellular gaps, loss of membrane VE-cadherin, and increased intracellular Ca(2+) concentration ([Ca(2+)](i)). In uninfected and Ad.betagal-infected HPAEC, pretreatment with CPT-cGMP (100 microM) partially blocked the increased permeability induced by H(2)O(2). In Ad.PKG-infected HPAEC, CPT-cGMP (50 microM) prevented the H(2)O(2)-induced TER decrease, cytoskeletal rearrangement, and loss of junctional VE-cadherin. CPT-cGMP attenuated the peak [Ca(2+)](i) caused by H(2)O(2) similarly (23%) in Ad.betagal- and Ad.PKG-infected HPAEC, indicating a PKG(I)-independent effect. These data suggest that cGMP decreased HPAEC basal permeability by a PKG(I)-independent process, whereas the ability of cGMP to prevent H(2)O(2)-induced barrier dysfunction was predominantly mediated by PKG(I) through a Ca(2+)-independent mechanism.

VASP-dependent regulation of actin cytoskeleton rigidity, cell adhesion, and detachment

Enabled/vasodilator-stimulated phosphoprotein (Ena/VASP) proteins are established regulators of actin-based motility, platelet aggregation, and growth cone guidance. However, the molecular mechanisms involved essentially remain elusive. Here we report on a novel mechanism of VASP action, namely the regulation of tensile strength, contractility, and rigidity of the actin cytoskeleton. Compared to wild-type cells fibroblasts derived from VASP-deficient mice have thicker and more stable actin stress fibres. Furthermore focal adhesions are enlarged, myosin light chain phosphorylation is increased, and the rigidity of the filament-supported plasma membrane is elevated about three- to fourfold, as is evident from atomic force microscopy. Moreover, fibronectin-coated beads adhere stronger to the surface of VASP-deficient cells. The resistance of these beads to mechanical displacement by laser tweezers is dramatically increased in an F-actin-dependent mode. Cytoskeletal stabilization coincides with slower cell adhesion and detachment, while overall adhesion is increased. Interestingly, many of these effects observed in VASP (-/-) cells are recapitulated in VASP-overexpressing cells, hinting towards a balanced stoichiometry necessary for appropriate VASP function. Taken together, our results suggest that VASP regulates surface protrusion formation and cell adhesion through modulation of the mechanical properties of the actin cytoskeleton.

Vasodilator-Stimulated Phosphoprotein–Deficient Mice Demonstrate Increased Platelet Activation but Improved Renal Endothelial Preservation and Regeneration in Passive Nephrotoxic Nephritis

Vasodilator-stimulated phosphoprotein (VASP), an actin cytoskeletal protein, is expressed in various cell types including renal cells. In vitro studies provide evidence for a role of VASP regarding platelet activation, cell adhesion, migration, and capillary formation. The in vivo role of VASP was investigated in experimental inflammatory renal disease. Kidneys of healthy VASP deficient (-/-) and wild-type (wt) mice were compared regarding morphology and functional parameters. Passive nephrotoxic nephritis was induced in 28 VASP -/- and 28 wt mice; kidneys were harvested; and tissues were analyzed by morphometric, immunohistochemical, and electron microscopic techniques on days 3, 7, 14, and 28. The time course of disease in VASP -/- mice differed substantially and biphasically from that in wt controls. Early on, VASP -/- mice demonstrated increased platelet influx associated with augmented glomerular and tubulointerstitial inflammation and sclerosis. Whereas renal disease continuously worsened up to day 28 in wt controls, renal disease in VASP -/- mice hardly progressed after day 3 as assessed by various injury indices. This long-term improvement of renal histology in VASP -/- compared with wt mice was associated with remarkable capillary preservation/regeneration up to day 28 mediated via an increased proliferative and a reduced apoptotic activity of VASP-negative peritubular endothelial cells. Despite an enhanced injury response early on, VASP -/- mice are protected from long-term progression of nephrotoxic nephritis, which is associated with improved renal endothelial cell preservation and regeneration.

The palladin/myotilin/myopalladin family of actin-associated scaffolds

The dynamic remodeling of the actin cytoskeleton plays a critical role in cellular morphogenesis and cell motility. Actin-associated scaffolds are key to this process, as they recruit cohorts of actin-binding proteins and associated signaling complexes to subcellular sites where remodeling is required. This review is focused on a recently discovered family of three proteins, myotilin, palladin, and myopalladin, all of which function as scaffolds that regulate actin organization. While myotilin and myopalladin are most abundant in skeletal and cardiac muscle, palladin is ubiquitously expressed in the organs of developing vertebrates. Palladin's function has been investigated primarily in the central nervous system and in tissue culture, where it appears to play a key role in cellular morphogenesis. The three family members each interact with specific molecular partners: all three bind to alpha-actinin; in addition, palladin also binds to vasodilator-stimulated phosphoprotein (VASP) and ezrin, myotilin binds to filamin and actin, and myopalladin also binds to nebulin and cardiac ankyrin repeat protein (CARP). Since mutations in myotilin result in two forms of muscle disease, an essential role for this family member in organizing the skeletal muscle sarcomere is implied.

Palladin is a novel binding partner for Ena/VASP family members

Palladin is an actin-associated protein that contains proline-rich motifs within its amino-terminal sequence that are similar to motifs found in zyxin, vinculin, and the Listeria protein ActA. These motifs are known to be potential binding sites for the Vasodilator-Stimulated Phosphoprotein (VASP). Here, we demonstrate that palladin is an additional direct binding partner for VASP, by using co-immunoprecipitation and blot overlay techniques with both endogenous palladin and recombinant myc-tagged palladin. These results show that VASP binds to full-length palladin and also to the amino-terminal half of palladin, where the polyproline motifs are located. Using a synthetic peptide array, two discrete binding sites for VASP were identified within palladin's proline-rich amino-terminal domain. Using double-label immunofluorescence staining of fully-spread and actively-spreading fibroblasts, the extent of co-localization of palladin and VASP was explored. These proteins were found to strongly co-localize along stress fibers, and partially co-localize in focal adhesions, lamellipodia, and focal complexes. These results suggest that the recently described actin-associated protein palladin may play an important role in recruiting VASP to sites of actin filament growth, anchorage, and crosslinking.

Endoglin regulates cytoskeletal organization through binding to ZRP-1, a member of the Lim family of proteins

Endoglin is a component of the transforming growth factor-beta receptor complex abundantly expressed at the surface of endothelial cells and plays an important role in cardiovascular development and vascular remodeling. By using the cytoplasmic domain of endoglin as a bait for screening protein interactors, we have identified ZRP-1 (zyxin-related protein 1), a 476-amino acid member that belongs to a family of LIM containing proteins that includes zyxin and lipoma-preferred partner. The endoglin interacting region was mapped within the three double zinc finger LIM domains of the ZRP-1 C terminus. Analysis of the subcellular distribution of ZRP-1 demonstrated that in the absence of endoglin, ZRP-1 mainly localizes to focal adhesion sites, whereas in the presence of endoglin ZRP-1 is found along actin stress fibers. Because the LIM family of proteins has been shown to associate with the actin cytoskeleton, we investigated the possibility of a regulatory role for endoglin with regard to this structure. Expression of endoglin resulted in a dramatic reorganization of the actin cytoskeleton. In the absence of endoglin, F-actin was localized to dense aggregates of bundles, whereas in the presence of endoglin, expressed in endothelial cells, F-actin was in stress fibers and colocalized with ZRP-1. Furthermore, small interfering RNA-mediated suppression of endoglin or ZRP-1, or clustering of endoglin in endothelial cells, led to mislocalization of F-actin fibers. These results suggest a regulatory role for endoglin, via its interaction with ZRP-1, in the actin cytoskeletal organization.

Function and regulation of Ena/VASP proteins

Regulation of cytoskeletal dynamics is required to coordinate cell movement, adhesion and shape change. The Ena/VASP protein family is thought to play an important role in linking signaling pathways to remodeling of the actin cytoskeleton. This review will examine the mechanisms by which Ena/VASP function might control actin dynamics and how these proteins are linked to various signaling pathways.

Resistance to thienopyridines: clinical detection of coronary stent thrombosis by monitoring of vasodilator-stimulated phosphoprotein phosphorylation

We carried out a prospective evaluation of a new vasodilator-stimulated phosphoprotein (VASP) phosphorylation assay in order to detect patients with high-risk coronary subacute stent thrombosis (SAT) despite thienopyridine regimen. Twenty healthy donors (group 1) without any medication were compared to 16 stented patients (group 2) treated by ticlopidin or clopidogrel initiated 2 days before stenting and aspirin (250 mg/day). No difference in platelet reactivity was noted between group 1 and group 2 treated only with aspirin (72.00% +/- 4.17% vs. 69.73% +/- 5.62%, respectively; P = NS). Significant differences were found between patients of group 2 treated with aspirin alone (69.73% +/- 5.62%), after 2.0 days (60.14% +/- 9.60%; P < 0.05), and after 4.8 +/- 1.3 days (48.37% +/- 11.19%; P < 0.05) with thienopyridine-aspirin. Among 1,684 consecutive stented patients, 16 patients who presented an SAT (group 3) were compared with 30 other stented patients free of SAT (group 4). We found a significant difference between group 3 (63.28% +/- 9.56%) and group 4 (39.80% +/- 10.9%; P < 0.0001). VASP phosphorylation analysis may be useful for the detection of coronary SAT.

A role for VASP in RhoA-Diaphanous signalling to actin dynamics and SRF activity

Vasodilator-stimulated phosphoprotein (VASP) is involved in multiple actin-mediated processes, including regulation of serum response factor (SRF) activity. We used the SRF transcriptional assay to define functional domains in VASP and to show that they coincide with those required for F-actin accumulation, as determined by a quantitative FACS assay. We identified inactive VASP mutants that can interfere both with F-actin assembly and with SRF activation by wild-type VASP. These VASP mutants also inhibit actin-based motility of Vaccinia virus and Shigella flexneri. VASP-induced F-actin accumulation and SRF activation require both functional Rho and its effector mDia, and conversely, mDia-mediated SRF activation is critically dependent on functional VASP. VASP and mDia also associate physically in vivo. These findings show that VASP and mDia function cooperatively downstream of Rho to control F-actin assembly and SRF activity.

Linked regulation of motility and integrin function in activated migrating neutrophils revealed by interference in remodelling of the cytoskeleton

Neutrophils migrate rapidly by co-ordinating regulation of their beta2-integrin adhesion with turnover of filamentous F-actin. The seven-protein Arp2/3 complex regulates actin polymerisation upon activation by proteins of the WASP-family. To investigate links between actin polymerisation, adhesion, and migration, we used a novel osmotic-shock method to load neutrophils with peptides: (1). WASP-WA and Scar-WA (which incorporate the actin- and Arp2/3-binding regions of WASP and Scar1), to compete with endogenous WASP-family members; (2). proline rich motifs (PRM) from the ActA protein of L. monocytogenes or from vinculin, which bind vasodilator-stimulated phosphoprotein (VASP), a regulator of cytoskeleton assembly. In a flow system, rolling-adherent neutrophils were stimulated with formyl tri-peptide. This caused rapid immobilisation, followed by migration with increasing velocity, supported by activated beta2-integrin CD11b/CD18. Loading ActA PRM (but not vinculin PRM) caused concentration-dependent reduction in migration velocity. At the highest concentration, unstimulated neutrophils had elevated F-actin and were rigid, but could not change their F-actin content or shape upon stimulation. Scar-WA also caused marked reduction in migration rate, but WASP-WA had a lesser effect. Scar-WA did not modify activation-dependent formation of F-actin or change in shape. However, a reduction in rate of downregulation of integrin adhesion appeared to contribute to impaired migration. These studies show that interference in cytoskeletal reorganisation that follows activation in neutrophils, can impair regulation of integrin function as well as motility. They also suggest a role of the Arp2/3 complex and WASP-family in co-ordinating actin polymerisation and integrin function in migrating neutrophils.

Critical roles of phosphorylation and actin binding motifs, but not the central proline-rich region, for Ena/vasodilator-stimulated phosphoprotein (VASP) function during cell migration

The Ena/vasodilator-stimulated phosphoprotein (VASP) protein family is implicated in the regulation of a number of actin-based cellular processes, including lamellipodial protrusion necessary for whole cell translocation. A growing body of evidence derived largely from in vitro biochemical experiments using purified proteins, cell-free extracts, and pathogen motility has begun to suggest various mechanistic roles for Ena/VASP proteins in the control of actin dynamics. Using complementation of phenotypes in Ena/VASP-deficient cells and overexpression in normal fibroblasts, we have assayed the function of a panel of mutants in one member of this family, Mena, by mutating highly conserved sequence elements found in this protein family. Surprisingly, deletion of sites required for binding of the actin monomer-binding protein profilin, a known ligand of Ena/VASP proteins, has no effect on the ability of Mena to regulate random cell motility. Our analysis revealed two features essential for Ena/VASP function in cell movement, cyclic nucleotide-dependent kinase phosphorylation sites and an F-actin binding motif. Interestingly, expression of the C-terminal EVH2 domain alone is sufficient to complement loss of Ena/VASP function in random cell motility.

Role of vasodilator-stimulated phosphoprotein in PKA-induced changes in endothelial junctional permeability

At sites of ongoing inflammation, polymorphonuclear leukocytes (PMN, neutrophils) migrate across vascular endothelia, and such transmigration has the potential to disturb barrier properties and can result in intravascular fluid loss and edema. It was recently appreciated that endogenous pathways exist to dampen barrier disruption during such episodes and may provide an important anti-inflammatory link. For example, during transmigration, PMN-derived adenosine activates endothelial adenosine receptors and induces a cAMP-dependent resealing of endothelial barrier function. In our study reported here, we sought to understand the link between cyclic nucleotide elevation and increased endothelial barrier function. Initial studies revealed that adenosine-induced barrier function is tightly linked to activation of protein kinase A (PKA). Because PKA selectively phosphorylates serine and threonine residues, we screened zonula occludens-1 (ZO-1) immunoprecipitates for the existence of such phosphorylated proteins as targets for barrier regulation. This analysis revealed a dominantly phosphorylated band at 50 kDa. Microsequencing identified this protein as vasodilator-stimulated phosphoprotein (VASP), an actin binding protein with multiple serine/threonine phosphorylation sites. Immunofluorescent microscopy revealed that VASP localizes to endothelial junctional complexes and colocalizes with ZO-1, occludin, and junctional adhesion molecule-1 (JAM-1). To address the role of phospho-VASP in regulation of barrier function, we generated a phosphospecific VASP antibody targeting the Ser157 residue phosphorylation site, the site preferred by PKA. Immunolocalization studies with this antibody revealed that upon PKA activation, phospho-VASP appears at cell-cell junctions. Transient transfection of truncated VASP fragments revealed a parallel increase in barrier function. Taken together, these studies reveal a central role for phospho-VASP in the coordination of PKA-regulated barrier function, such as occurs during episodes of inflammation.