Rasip1 regulates vertebrate vascular endothelial junction stability through Epac1-Rap1 signaling

Ras interacting protein 1 facilitated proliferation and invasion of diffuse large B-cell lymphoma cells

A GTPase binding protein, Ras interacting protein 1 (RASIP1), has been reported with a tumor-promoting role in lung cancer cells, and its role in lymphoma remains unknown. The analysis of medical databank shows that RASIP1 is upregulated in diffuse large B-cell lymphoma (DLBCL) specimens. In this article, we demonstrated that RASIP1 is highly expressed in DLBCL cell lines, compared with primary B cells. The gain- and loss-of-function experiments were performed to investigate the effects of RASIP1 on DLBCL cells. CCK-8, flow cytometry, western blot, and transwell assays demonstrated that silence of RASIP1 inhibited proliferation, cell cycle transition, and invasion and induced significant apoptosis in DLBCL cells, and ectopic expression of RASIP1 played opposite roles. Xenograft results revealed that RASIP1 facilitated the growth of DLBCL cells in vivo. These findings suggest that RASIP1 may be required for malignancy of DLBCL cells. In addition, we also found that the expression of RASIP1 was negatively regulated by forkhead box O3 (FOXO3), which has been reported to suppress the proliferation of DLBCL cells. Our results indicate that FOXO3 is bound to the promoter sequence of RASIP1 and inhibits its transcription. The suppressive effects of FOXO3 on proliferation and invasion of DLBCL cells were neutralized by RASIP1. In conclusion, we demonstrate that FOXO3 negatively regulated RASIP1 facilitates growth and invasion of DLBCL cells, provides novel diagnostic markers and therapeutic targets for DLBCL in clinic.

Rap1 small GTPase is essential for maintaining pulmonary endothelial barrier function in mice

Vascular permeability is dynamically but tightly controlled by vascular endothelial (VE)-cadherin-mediated endothelial cell-cell junctions to maintain homeostasis. Thus, impairments of VE-cadherin-mediated cell adhesions lead to hyperpermeability, promoting the development and progression of various disease processes. Notably, the lungs are a highly vulnerable organ wherein pulmonary inflammation and infection result in vascular leakage. Herein, we showed that Rap1, a small GTPase, plays an essential role for maintaining pulmonary endothelial barrier function in mice. Endothelial cell-specific Rap1a/Rap1b double knockout mice exhibited severe pulmonary edema. They also showed vascular leakage in the hearts, but not in the brains. En face analyses of the pulmonary arteries and 3D-immunofluorescence analyses of the lungs revealed that Rap1 potentiates VE-cadherin-mediated endothelial cell-cell junctions through dynamic actin cytoskeleton reorganization. Rap1 inhibits formation of cytoplasmic actin bundles perpendicularly binding VE-cadherin adhesions through inhibition of a Rho-ROCK pathway-induced activation of cytoplasmic nonmuscle myosin II (NM-II). Simultaneously, Rap1 induces junctional NM-II activation to create circumferential actin bundles, which anchor and stabilize VE-cadherin at cell-cell junctions. We also showed that the mice carrying only one allele of either Rap1a or Rap1b out of the two Rap1 genes are more vulnerable to lipopolysaccharide (LPS)-induced pulmonary vascular leakage than wild-type mice, while activation of Rap1 by administration of 007, an activator for Epac, attenuates LPS-induced increase in pulmonary endothelial permeability in wild-type mice. Thus, we demonstrate that Rap1 plays an essential role for maintaining pulmonary endothelial barrier functions under physiological conditions and provides protection against inflammation-induced pulmonary vascular leakage.

Syndapin and GTPase RAP-1 control endocytic recycling via RHO-1 and non-muscle myosin II

After endocytosis, many plasma membrane components are recycled via membrane tubules that emerge from early endosomes to form recycling endosomes, eventually leading to their return to the plasma membrane. We previously showed that Syndapin/PACSIN-family protein SDPN-1 is required in vivo for basolateral endocytic recycling in the C. elegans intestine. Here, we document an interaction between the SDPN-1 SH3 domain and a target sequence in PXF-1/PDZ-GEF1/RAPGEF2, a known exchange factor for Rap-GTPases. We found that endogenous mutations engineered into the SDPN-1 SH3 domain, or its binding site in the PXF-1 protein, interfere with recycling in vivo, as does the loss of the PXF-1 target RAP-1. In some contexts, Rap-GTPases negatively regulate RhoA activity, suggesting a potential for Syndapin to regulate RhoA. Our results indicate that in the C. elegans intestine, RHO-1/RhoA is enriched on SDPN-1- and RAP-1-positive endosomes, and the loss of SDPN-1 or RAP-1 elevates RHO-1(GTP) levels on intestinal endosomes. Furthermore, we found that depletion of RHO-1 suppressed sdpn-1 mutant recycling defects, indicating that control of RHO-1 activity is a key mechanism by which SDPN-1 acts to promote endocytic recycling. RHO-1/RhoA is well known for controlling actomyosin contraction cycles, although little is known about the effects of non-muscle myosin II on endosomes. Our analysis found that non-muscle myosin II is enriched on SDPN-1-positive endosomes, with two non-muscle myosin II heavy-chain isoforms acting in apparent opposition. Depletion of nmy-2 inhibited recycling like sdpn-1 mutants, whereas depletion of nmy-1 suppressed sdpn-1 mutant recycling defects, indicating that actomyosin contractility controls recycling endosome function.

Systematic Mendelian randomization study of the effect of gut microbiome and plasma metabolome on severe COVID-19

Background

COVID-19 could develop severe respiratory symptoms in certain infected patients, especially in the patients with immune disorders. Gut microbiome and plasma metabolome act important immunological modulators in the human body and could contribute to the immune responses impacting the progression of COVID-19. However, the causal relationship between specific intestinal bacteria, metabolites and severe COVID-19 remains not clear.

Methods

Based on two-sample Mendelian randomization (MR) framework, the causal effects of 131 intestinal taxa and 452 plasma metabolites on severe COVID-19 were evaluated. Single nucleotide polymorphisms (SNPs) strongly associated with the abundance of intestinal taxa and the concentration of plasma metabolites had been utilized as the instrument variables to infer whether they were causal factors of severe COVID-19. In addition, mediation analysis was conducted to find the potential association between the taxon and metabolite, and further colocalization analysis had been performed to validate the causal relationships.

Results

MR analysis identified 13 taxa and 53 metabolites, which were significantly associated with severe COVID-19 as causal factors. Mediation analysis revealed 11 mediated relationships. Myo-inositol, 2-stearoylglycerophosphocholine, and alpha-glutamyltyrosine, potentially contributed to the association of Howardella and Ruminiclostridium 6 with severe COVID-19, respectively. Butyrivibrio and Ruminococcus gnavus could mediate the association of myo-inositol and N-acetylalanine, respectively. In addition, Ruminococcus torques abundance was colocalized with severe COVID-19 (PP.H4 = 0.77) and the colon expression of permeability related protein RASIP1 (PP.H4 = 0.95).

Conclusions

Our study highlights the potential causal relationships between gut microbiome, plasma metabolome and severe COVID-19, which potentially serve as clinical biomarkers for risk stratification and prognostication and benefit the mechanism mechanistic investigation of severe COVID-19.

A Sox17 downstream gene Rasip1 is involved in the hematopoietic activity of intra-aortic hematopoietic clusters in the midgestation mouse embryo

BACKGROUND

During mouse embryonic development, definitive hematopoiesis is first detected around embryonic day (E) 10.5 in the aorta-gonad-mesonephros (AGM) region. Hematopoietic stem cells (HSCs) arise in the dorsal aorta's intra-aortic hematopoietic cell clusters (IAHCs). We have previously reported that a transcription factor Sox17 is expressed in IAHCs, and that, among them, CD45lowc-Kithigh cells have high hematopoietic activity. Furthermore, forced expression of Sox17 in this population of cells can maintain the formation of hematopoietic cell clusters. However, how Sox17 does so, particularly downstream signaling involved, remains poorly understood. The purpose of this study is to search for new Sox17 targets which contribute to cluster formation with hematopoietic activity.

METHODS

RNA-sequencing (RNA-seq) analysis was done to identify genes that are upregulated in Sox17-expressing IAHCs as compared with Sox17-negative ones. Among the top 7 highly expressed genes, Rasip1 which had been reported to be a vascular-specific regulator was focused on in this study, and firstly, the whole-mount immunostaining was done. We conducted luciferase reporter assay and chromatin immunoprecipitation (ChIP) assay to examine whether Sox17 regulates Rasip1 gene expression via binding to its enhancer element. We also analyzed the cluster formation and the multilineage colony-forming ability of Rasip1-transduced cells and Rasip1-knockdown Sox17-transduced cells.

RESULTS

The increase of the Rasip1 expression level was observed in Sox17-positive CD45lowc-Kithigh cells as compared with the Sox17-nonexpressing control. Also, the expression level of the Rasip1 gene was increased by the Sox17-nuclear translocation. Rasip1 was expressed on the membrane of IAHCs, overlapping with the endothelial cell marker, CD31, and hematopoietic stem/progenitor marker (HSPC), c-Kit. Rasip1 expression was observed in most part of c-Kit+Sox17+ cells in IAHCs. Luciferase reporter assay and ChIP assay indicated that one of the five putative Sox17-binding sites in the Rasip1 enhancer region was important for Rasip1 expression via Sox17 binding. Rasip1 knockdown in Sox17-transduced cells decreased the cluster formation and diminished the colony-forming ability, while overexpression of Rasip1 in CD45lowc-Kithigh cells led to a significant but transient increase in hematopoietic activity.

CONCLUSIONS

Rasip1 knockdown in Sox17-transduced CD45lowc-Kithigh cells displayed a significant decrease in the multilineage colony-forming ability and the cluster size. Rasip1 overexpression in Sox17-untransduced CD45lowc-Kithigh cells led to a significant but transient increase in the multilineage colony-forming ability, suggesting the presence of a cooperating factor for sustained hematopoietic activity.

cAMP: A master regulator of cadherin-mediated binding in endothelium, epithelium and myocardium

Regulation of cadherin-mediated cell adhesion is crucial not only for maintaining tissue integrity and barrier function in the endothelium and epithelium but also for electromechanical coupling within the myocardium. Therefore, loss of cadherin-mediated adhesion causes various disorders, including vascular inflammation and desmosome-related diseases such as the autoimmune blistering skin dermatosis pemphigus and arrhythmogenic cardiomyopathy. Mechanisms regulating cadherin-mediated binding contribute to the pathogenesis of diseases and may also be used as therapeutic targets. Over the last 30 years, cyclic adenosine 3',5'-monophosphate (cAMP) has emerged as one of the master regulators of cell adhesion in endothelium and, more recently, also in epithelial cells as well as in cardiomyocytes. A broad spectrum of experimental models from vascular physiology and cell biology applied by different generations of researchers provided evidence that not only cadherins of endothelial adherens junctions (AJ) but also desmosomal contacts in keratinocytes and the cardiomyocyte intercalated discs are central targets in this scenario. The molecular mechanisms involve protein kinase A- and exchange protein directly activated by cAMP-mediated regulation of Rho family GTPases and S665 phosphorylation of the AJ and desmosome adaptor protein plakoglobin. In line with this, phosphodiesterase 4 inhibitors such as apremilast have been proposed as a therapeutic strategy to stabilize cadherin-mediated adhesion in pemphigus and may also be effective to treat other disorders where cadherin-mediated binding is compromised.

Syndapin Regulates the RAP-1 GTPase to Control Endocytic Recycling via RHO-1 and Non-Muscle Myosin II

After endocytosis, many plasma membrane components are recycled via narrow-diameter membrane tubules that emerge from early endosomes to form recycling endosomes, eventually leading to their return to the plasma membrane. We previously showed that the F-BAR and SH3 domain Syndapin/PACSIN-family protein SDPN-1 is required in vivo for basolateral endocytic recycling in the C. elegans intestine. Here we sought to determine the significance of a predicted interaction between the SDPN-1 SH3 domain and a target sequence in PXF-1/PDZ-GEF1/RAPGEF2, a known exchange factor for Rap-GTPases. We found that endogenous mutations we engineered into the SDPN-1 SH3 domain, or its binding site in the PXF-1 protein, interfere with recycling in vivo , as does loss of the PXF-1 target RAP-1. Rap-GTPases have been shown in several contexts to negatively regulate RhoA activity. Our results show that RHO-1/RhoA is enriched on SDPN-1 and RAP-1 positive endosomes in the C. elegans intestine, and loss of SDPN-1 or RAP-1 elevates RHO-1(GTP) levels on intestinal endosomes. Furthermore, we found that depletion of RHO-1 suppressed sdpn-1 mutant recycling defects, indicating that control of RHO-1 activity is a key mechanism by which SDPN-1 acts to promote endocytic recycling. RHO-1/RhoA is well-known for controlling actomyosin contraction cycles, although little is known of non-muscle myosin II on endosomes. Our analysis found that non-muscle myosin II is enriched on SDPN-1 positive endosomes, with two non-muscle myosin II heavy chain isoforms acting in apparent opposition. Depletion of nmy-2 inhibited recycling like sdpn-1 mutants, while depletion of nmy-1 suppressed sdpn-1 mutant recycling defects, indicating actomyosin contractility in controlling recycling endosome function.

Rap1 Small GTPase Regulates Vascular Endothelial-Cadherin-Mediated Endothelial Cell-Cell Junctions and Vascular Permeability

The vascular permeability of the endothelium is finely controlled by vascular endothelial (VE)-cadherin-mediated endothelial cell-cell junctions. In the majority of normal adult tissues, endothelial cells in blood vessels maintain vascular permeability at a relatively low level, while in response to inflammation, they limit vascular barrier function to induce plasma leakage and extravasation of immune cells as a defense mechanism. Thus, the dynamic but also simultaneously tight regulation of vascular permeability by endothelial cells is responsible for maintaining homeostasis and, as such, impairments of its underlying mechanisms result in hyperpermeability, leading to the development and progression of various diseases including coronavirus disease 2019 (COVID-19), a newly emerging infectious disease. Recently, increasing numbers of studies have been unveiling the important role of Rap1, a small guanosine 5'-triphosphatase (GTPase) belonging to the Ras superfamily, in the regulation of vascular permeability. Rap1 enhances VE-cadherin-mediated endothelial cell-cell junctions to potentiate vascular barrier functions via dynamic reorganization of the actin cytoskeleton. Importantly, Rap1 signaling activation reportedly improves vascular barrier function in animal models of various diseases associated with vascular hyperpermeability, suggesting that Rap1 might be an ideal target for drugs intended to prevent vascular barrier dysfunction. Here, we describe recent progress in understanding the mechanisms by which Rap1 potentiates VE-cadherin-mediated endothelial cell-cell adhesions and vascular barrier function. We also discuss how alterations in Rap1 signaling are related to vascular barrier dysfunction in diseases such as acute pulmonary injury and malignancies. In addition, we examine the possibility of Rap1 signaling as a target of drugs for treating diseases associated with vascular hyperpermeability.

Control of dynamic cell behaviors during angiogenesis and anastomosis by Rasip1

Organ morphogenesis is driven by a wealth of tightly orchestrated cellular behaviors, which ensure proper organ assembly and function. Many of these cell activities involve cell-cell interactions and remodeling of the F-actin cytoskeleton. Here, we analyze the requirement for Rasip1 (Ras-interacting protein 1), an endothelial-specific regulator of junctional dynamics, during blood vessel formation. Phenotype analysis of rasip1 mutants in zebrafish embryos reveals distinct functions of Rasip1 during sprouting angiogenesis, anastomosis and lumen formation. During angiogenic sprouting, loss of Rasip1 causes cell pairing defects due to a destabilization of tricellular junctions, indicating that stable tricellular junctions are essential to maintain multicellular organization within the sprout. During anastomosis, Rasip1 is required to establish a stable apical membrane compartment; rasip1 mutants display ectopic, reticulated junctions and the apical compartment is frequently collapsed. Loss of Ccm1 and Heg1 function mimics the junctional defects of rasip1 mutants. Furthermore, downregulation of ccm1 and heg1 leads to a delocalization of Rasip1 at cell junctions, indicating that junctional tethering of Rasip1 is required for its function in junction formation and stabilization during sprouting angiogenesis.

Summary:In vivo analysis of rasip1 mutants reveals multiple roles for Rasip1 during angiogenic sprouting, anastomosis and lumen formation, including stabilization of tricellular junctions to permit coordinated cell rearrangements and multicellular tube formation.

Endothelial cell mechanics and blood flow forces in vascular morphogenesis

The vertebrate cardiovascular system is made up by a hierarchically structured network of highly specialised blood vessels. This network emerges during early embryogenesis and evolves in size and complexity concomitant with embryonic growth and organ formation. Underlying this plasticity are actin-driven endothelial cell behaviours, which allow endothelial cells to change their shape and move within the vascular network. In this review, we discuss the cellular and molecular mechanisms involved in vascular network formation and how these intrinsic mechanisms are influenced by haemodynamic forces provided by pressurized blood flow. While most of this review focusses on in vivo evidence from zebrafish embryos, we also mention complementary findings obtained in other experimental systems.

Therapeutic Hypothermia Mitigates the Sepsis-Increased Permeability in EA. hy926 Cells by Preserving Rap1 Expression

To determine the effect and potential mechanisms of therapeutic hypothermia (TH) on the permeability of septic cells. Human EA. hy926 cells were transfected with, or without, control or ras-proximate-1 (Rap1)-specific siRNA and treated with 2 μg/mL of lipopolysaccharide (LPS). The cells were cultured in normal temperature (NT) or a temporary TH for 10 hours. The cellular permeability of each group of cells was determined by transwell permeability assay. The relative levels of Rap1, RhoA (a small GTP enzyme of the Rho family), VE-cadherin expression, and myosin light chain (MLC) phosphorylation were quantified by Western blot and immunofluorescent assays. Compared with the control group, LPS stimulation increased cellular permeability in EA. hy926 cells under an NT condition, but significantly mitigated by TH. The effect of TH decreased after Rap1 silencing. Furthermore, LPS upregulated RhoA expression and MLC phosphorylation, but reduced Rap1 and VE-cadherin expression, which were also enhanced by Rap1 silencing, but significantly mitigated by TH. Immunofluorescent analyses indicated that LPS significantly increased phosphorylated MLC, but decreased VE-cadherin expression, which were further deteriorated by Rap1 silencing, but significantly mitigated by TH in EA. hy926 cells. TH significantly mitigated the sepsis-increased permeability of EA. hy926 cells by enhancing the Rap1 expression to attenuate the RhoA/MLC signaling.

Double-Hit-Induced Leukocyte Extravasation Driven by Endothelial Adherens Junction Destabilization

During inflammation, endothelial cells are bombarded with cytokines and other stimuli from surrounding cells. Leukocyte extravasation and vascular leakage are both prominent but believed to be uncoupled as they occur in separate spatiotemporal patterns. In this study, we investigated a "double-hit" approach on primary human endothelial cells primed with LPS followed by histamine. Using neutrophil transendothelial migration (TEM) under physiological flow assays, we found that an LPS-primed endothelium synergistically enhanced neutrophil TEM when additionally treated with histamine, whereas the effects on neutrophil TEM of the individual stimuli were moderate to undetectable. Interestingly, the double-hit-induced TEM increase was not due to decreased endothelial barrier, increased adhesion molecule expression, or Weibel-Palade body release. Instead, we found that it was directly correlated with junctional remodeling. Compounds that increased junctional "linearity" (i.e., stability) counteracted the double-hit effect on neutrophil TEM. We conclude that a compound, in this case histamine (which has a short primary effect on vascular permeability), can have severe secondary effects on neutrophil TEM in combination with an inflammatory stimulus. This effect is due to synergic modifications of the endothelial cytoskeleton and junctional remodeling. Therefore, we hypothesize that junctional linearity is a better and more predictive readout than endothelial resistance for compounds aiming to attenuate inflammation.

Cerebral Cavernous Malformation Proteins in Barrier Maintenance and Regulation

Cerebral cavernous malformation (CCM) is a disease characterized by mulberry shaped clusters of dilated microvessels, primarily in the central nervous system. Such lesions can cause seizures, headaches, and stroke from brain bleeding. Loss-of-function germline and somatic mutations of a group of genes, called CCM genes, have been attributed to disease pathogenesis. In this review, we discuss the impact of CCM gene encoded proteins on cellular signaling, barrier function of endothelium and epithelium, and their contribution to CCM and potentially other diseases.

Genome-Wide Mapping Defines a Role for C/EBPβ and c-Jun in Non-Canonical Cyclic AMP Signalling

The novel exchange protein activated by cyclic AMP (EPAC1) activator, I942, induces expression of the suppressor of cytokine signalling 3 (SOCS3) gene, thereby inhibiting interleukin 6 (IL6) inflammatory processes in human umbilical vein endothelial cells (HUVECs). Here we use RNA-SEQ and ChIP-SEQ to determine global gene responses to I942, in comparison with cyclic AMP production promoted by forskolin and rolipram (F/R). We found that I942 promoted significant changes in the RNA expression of 1413 genes, largely associated with microtubule stability and cell cycle progression, whereas F/R regulated 197 genes linked to endothelial cell function, including chemokine production and platelet aggregation. A further 108 genes were regulated by both treatments, including endothelial regulatory genes involved in purinergic signalling and cell junction organization. ChIP-SEQ demonstrated that F/R induced genome-wide recruitment of C/EBPβ and c-Jun transcription factors, whereas I942 promoted recruitment of c-Jun to genes associated with IL6 signalling, with little effect on C/EBPβ activation. Despite this, certain key inflammatory genes, including IL6, VEGF, CCL2/MCP1, VCAM1, SELE and ICAM1 were regulated by I942 without significant c-Jun recruitment, suggesting an additional, indirect mode of action for I942. In this regard, SOCS3 induction by I942 was found to require c-Jun and was associated with suppression of IL6-promoted ERK MAP kinase and AKT activity and induction of ICAM1. Pharmacological inhibition of ERK and AKT also potentiated ICAM1 induction by I942. We therefore propose that c-Jun activation by I942 regulates endothelial gene expression in HUVECs through direct mechanisms, involving recruitment of c-Jun or, as for ICAM1, through indirect regulation of tertiary regulators, including SOCS3.

Passing the Vascular Barrier: Endothelial Signaling Processes Controlling Extravasation

A central function of the vascular endothelium is to serve as a barrier between the blood and the surrounding tissue of the body. At the same time, solutes and cells have to pass the endothelium to leave or to enter the bloodstream to maintain homeostasis. Under pathological conditions, for example, inflammation, permeability for fluid and cells is largely increased in the affected area, thereby facilitating host defense. To appropriately function as a regulated permeability filter, the endothelium uses various mechanisms to allow solutes and cells to pass the endothelial layer. These include transcellular and paracellular pathways of which the latter requires remodeling of intercellular junctions for its regulation. This review provides an overview on endothelial barrier regulation and focuses on the endothelial signaling mechanisms controlling the opening and closing of paracellular pathways for solutes and cells such as leukocytes and metastasizing tumor cells.

ASK1-p38 cascaded signal mediates pulmonary microvascular endothelial barrier injury induced by the return of PHSML in rats

The return of post-hemorrhagic shock mesenteric lymph (PHSML) induces pulmonary vascular endothelial barrier dysfunction, which results in acute lung injury. Activation of the apoptosis signal-regulated kinase 1 (ASK1) and p38 mitogen-activated protein kinase (p38 MAPK) pathway has been shown to trigger inflammatory responses. However, whether the ASK1-p38 MAPK pathway is involved in the PHSML-induced pulmonary endothelial barrier dysfunction remains unclear. In the present study, permeability changes of pulmonary capillaries were found in vivo, and activation of the ASK1-p38 MAPK pathway was determined in vitro. PMVEC barrier dysfunction was determined by measuring TEER. Furthermore, junctional and cytoskeletal protein expressions were analyzed. The results showed that hemorrhagic shock led to a marked increase in the permeability of pulmonary capillaries in vivo, which was markedly alleviated by PHSML drainage. In cultured pulmonary microvascular endothelial cells (PMVECs), PHSML reduced the endothelial barrier function accompanied by upregulated p-ASK1 and p-p38 MAPK protein expression, impaired the cytoskeletal protein structure, and down-regulated junction protein expression. These adverse effects were eliminated by applying either Trx1 (ASK1 inhibitor) or SB203580 (p38 MAPK inhibitor). These results indicated that the ASK1-p38 MAPK pathway mediates PHSML-induced pulmonary vascular endothelial barrier dysfunction during hemorrhagic shock.

The return of post-hemorrhagic shock mesenteric lymph (PHSML) induces pulmonary vascular endothelial barrier dysfunction, which results in acute lung injury.

Rasip1 is a RUNX1 target gene and promotes migration of NSCLC cells

Background

Runt-related transcription factor 1 (RUNX1), an essential regulator of hematopoiesis, is overexpressed in patients with nonsmall-cell lung cancer (NSCLC) and is correlated with enhanced metastatic ability. Ras-interacting protein 1 (Rasip1), a potential oncogene, is required for blood vessel formation, and recently, it has been shown that Rasip1 is widely expressed in NSCLC patients. We noticed that Rasip1 promoter contains several potential RUNX1-binding sequences. However, the relationship between Rasip1 and RUNX1 in NSCLC is still unknown. In this study, the potential function of RUNX1 involving in Rasip1 expression and the potential role of Rasip1 in lung cancer cells were investigated.

Materials and methods

Rasip1 and RUNX1 expressions were analyzed by quantitative reverse transcription polymerase chain reaction (qRT-PCR) and Western blotting in NSCLC cells lines. A549 and H1299 cells were transfected with plasmids or interfering RNA (siRNA) to upregulate or downregulate the expression of Rasip1 and RUNX1. Cell motility was assessed by transwell and wound-healing assay. Location of Rasip1 and RUNX1 was detected via immunofluorescence. Meanwhile, chromatin immunoprecipitation was done using an anti-RUNX1 antibody. Rasip1 promoter was constructed, and cells were lysed for the analysis of luciferase activity.

Results

In this study, we showed that ectopic expression or knockdown of RUNX1 resulted in a significant increase or reduction in Rasip1 expression, respectively. RUNX1 bound directly to a specific DNA sequence within Rasip1 promoter and modulated its transcription. Furthermore, silencing of Rasip1 inhibited the migration of RUNX1-overexpressing NSCLC cells through inactivation of Rac1 pathway. Moreover, we found that Rasip1 was expressed ubiquitously in NSCLC cells lines and enhanced cell migration. In addition, EGFR signaling was involved both in the expression and the subcellular localization of Rasip1.

Conclusion

Our data indicated that Rasip1 is regulated in part by the transcription factor RUNX1 and might be developed as a therapeutic target for NSCLC.

Vascular Endothelial (VE)-Cadherin, Endothelial Adherens Junctions, and Vascular Disease

Endothelial cell-cell adherens junctions (AJs) supervise fundamental vascular functions, such as the control of permeability and transmigration of circulating leukocytes, and the maintenance of existing vessels and formation of new ones. These processes are often dysregulated in pathologies. However, the evidence that links dysfunction of endothelial AJs to human pathologies is mostly correlative. In this review, we present an update of the molecular organization of AJ complexes in endothelial cells (ECs) that is mainly based on observations from experimental models. Furthermore, we report in detail on a human pathology, cerebral cavernous malformation (CCM), which is initiated by loss-of-function mutations in the genes that encode the three cytoplasmic components of AJs (CCM1, CCM2, and CCM3). At present, these represent a unique example of mutations in components of endothelial AJs that cause human disease. We describe also how studies into the defects of AJs in CCM are shedding light on the crucial regulatory mechanisms and signaling activities of these endothelial structures. Although these observations are specific for CCM, they support the concept that dysfunction of endothelial AJs can directly contribute to human pathologies.

Rasip1 controls lymphatic vessel lumen maintenance by regulating endothelial cell junctions

Although major progress in our understanding of the genes and mechanisms that regulate lymphatic vasculature development has been made, we still do not know how lumen formation and maintenance occurs. Here, we identify the Ras-interacting protein Rasip1 as a key player in this process. We show that lymphatic endothelial cell-specific Rasip1-deficient mouse embryos exhibit enlarged and blood-filled lymphatics at embryonic day 14.5. These vessels have patent lumens with disorganized junctions. Later on, as those vessels become fragmented and lumens collapse, cell junctions become irregular. In addition, Rasip1 deletion at later stages impairs lymphatic valve formation. We determined that Rasip1 is essential for lymphatic lumen maintenance during embryonic development by regulating junction integrity, as Rasip1 loss results in reduced levels of junction molecules and defective cytoskeleton organization in vitro and in vivo We determined that Rasip1 regulates Cdc42 activity, as deletion of Cdc42 results in similar phenotypes to those seen following the loss of Rasip1 Furthermore, ectopic Cdc42 expression rescues the phenotypes in Rasip1-deficient lymphatic endothelial cells, supporting the suggestion that Rasip1 regulates Cdc42 activity to regulate cell junctions and cytoskeleton organization, which are both activities required for lymphatic lumen maintenance.

Hold Me, but Not Too Tight-Endothelial Cell-Cell Junctions in Angiogenesis

Endothelial cell-cell junctions must perform seemingly incompatible tasks during vascular development-providing stable connections that prevent leakage, while allowing dynamic cellular rearrangements during sprouting, anastomosis, lumen formation, and functional remodeling of the vascular network. This review aims to highlight recent insights into the molecular mechanisms governing endothelial cell-cell adhesion in the context of vascular development.

Multiple Rap1 effectors control Epac1-mediated tightening of endothelial junctions

Epac1 and Rap1 mediate cAMP-induced tightening of endothelial junctions. We have previously found that one of the mechanisms is the inhibition of Rho-mediated tension in radial stress fibers by recruiting the RhoGAP ArhGAP29 in a complex containing the Rap1 effectors Rasip1 and Radil. However, other mechanisms have been proposed as well, most notably the induction of tension in circumferential actin cables by Cdc42 and its GEF FGD5. Here, we have investigated how Rap1 controls FGD5/Cdc42 and how this interconnects with Radil/Rasip1/ArhGAP29. Using endothelial barrier measurements, we show that Rho inhibition is not sufficient to explain the barrier stimulating effect of Rap1. Indeed, Cdc42-mediated tension is induced at cell-cell contacts upon Rap1 activation and this is required for endothelial barrier function. Depletion of potential Rap1 effectors identifies AF6 to mediate Rap1 enhanced tension and concomitant Rho-independent barrier function. When overexpressed in HEK293T cells, AF6 is found in a complex with FGD5 and Radil. From these results we conclude that Rap1 utilizes multiple pathways to control tightening of endothelial junctions, possibly through a multiprotein effector complex, in which AF6 functions to induce tension in circumferential actin cables.

Dynamic Regulation of Vascular Permeability by Vascular Endothelial Cadherin-Mediated Endothelial Cell-Cell Junctions

Endothelial cells lining blood vessels regulate vascular barrier function, which controls the passage of plasma proteins and circulating cells across the endothelium. In most normal adult tissues, endothelial cells preserve basal vascular permeability at a low level, while they increase permeability in response to inflammation. Therefore, vascular permeability is tightly controlled by a number of extracellular stimuli and mediators to maintain tissue homeostasis. Accordingly, impaired regulation of endothelial permeability causes various diseases, including chronic inflammation, asthma, edema, sepsis, acute respiratory distress syndrome, anaphylaxis, tumor angiogenesis, and diabetic retinopathy. Vascular endothelial (VE)-cadherin, a member of the classical cadherin superfamily, is a component of cell-to-cell adherens junctions in endothelial cells and plays an important role in regulating vascular permeability. VE-cadherin mediates intercellular adhesion through trans-interactions formed by its extracellular domain, while its cytoplasmic domain is anchored to the actin cytoskeleton via α- and β-catenins, leading to stabilization of VE-cadherin at cell-cell junctions. VE-cadherin-mediated cell adhesions are dynamically, but tightly, controlled by mechanisms that involve protein phosphorylation and reorganization of the actomyosin cytoskeleton. Phosphorylation of VE-cadherin, and its associated-catenins, results in dissociation of the VE-cadherin/catenin complex and internalization of VE-cadherin, leading to increased vascular permeability. Furthermore, reorganization of the actomyosin cytoskeleton by Rap1, a small GTPase that belongs to the Ras subfamily, and Rho family small GTPases, regulates VE-cadherin-mediated cell adhesions to control vascular permeability. In this review, we describe recent progress in understanding the signaling mechanisms that enable dynamic regulation of VE-cadherin adhesions and vascular permeability. In addition, we discuss the possibility of novel therapeutic approaches targeting the signaling pathways controlling VE-cadherin-mediated cell adhesion in diseases associated with vascular hyper-permeability.

Rap1B promotes VEGF-induced endothelial permeability and is required for dynamic regulation of the endothelial barrier

Vascular endothelial growth factor (VEGF), a key angiogenic and permeability factor, plays an important role in new blood vessel formation. However, abnormal VEGF-induced VEGFR2 signaling leads to hyperpermeability. We have shown previously that Rap1, best known for promoting cell adhesion and vessel stability, is a critical regulator of VEGFR2-mediated angiogenic and shear-stress EC responses. To determine the role of Rap1 role in endothelial barrier dynamics, we examined vascular permeability in EC-specific Rap1A- and Rap1B-knockout mice, cell-cell junction remodeling and EC monolayer resistivity in Rap1-deficient ECs under basal, inflammatory or elevated VEGF conditions. Deletion of either Rap1 isoform impaired de novo adherens junction (AJ) formation and recovery from LPS-induced barrier disruption in vivo However, only Rap1A deficiency increased permeability in ECs and lung vessels. Interestingly, Rap1B deficiency attenuated VEGF-induced permeability in vivo and AJ remodeling in vitro Therefore, only Rap1A is required for the maintenance of normal vascular integrity. Importantly, Rap1B is the primary isoform essential for normal VEGF-induced EC barrier dissolution. Deletion of either Rap1 isoform protected against hyper permeability in the STZ-induced diabetes model, suggesting clinical implications for targeting Rap1 in pathologies with VEGF-induced hyperpermeability.

Molecular Regulation of Sprouting Angiogenesis

The term angiogenesis arose in the 18th century. Several studies over the next 100 years laid the groundwork for initial studies performed by the Folkman laboratory, which were at first met with some opposition. Once overcome, the angiogenesis field has flourished due to studies on tumor angiogenesis and various developmental models that can be genetically manipulated, including mice and zebrafish. In addition, new discoveries have been aided by the ability to isolate primary endothelial cells, which has allowed dissection of various steps within angiogenesis. This review will summarize the molecular events that control angiogenesis downstream of biochemical factors such as growth factors, cytokines, chemokines, hypoxia-inducible factors (HIFs), and lipids. These and other stimuli have been linked to regulation of junctional molecules and cell surface receptors. In addition, the contribution of cytoskeletal elements and regulatory proteins has revealed an intricate role for mobilization of actin, microtubules, and intermediate filaments in response to cues that activate the endothelium. Activating stimuli also affect various focal adhesion proteins, scaffold proteins, intracellular kinases, and second messengers. Finally, metalloproteinases, which facilitate matrix degradation and the formation of new blood vessels, are discussed, along with our knowledge of crosstalk between the various subclasses of these molecules throughout the text. Compr Physiol 8:153-235, 2018.

The EPAC-Rap1 pathway prevents and reverses cytokine-induced retinal vascular permeability

Increased retinal vascular permeability contributes to macular edema, a leading cause of vision loss in eye pathologies such as diabetic retinopathy, age-related macular degeneration, and central retinal vein occlusions. Pathological changes in vascular permeability are driven by growth factors such as VEGF and pro-inflammatory cytokines such as TNF-α. Identifying the pro-barrier mechanisms that block vascular permeability and restore the blood-retinal barrier (BRB) may lead to new therapies. The cAMP-dependent guanine nucleotide exchange factor (EPAC) exchange-protein directly activated by cAMP promotes exchange of GTP in the small GTPase Rap1. Rap1 enhances barrier properties in human umbilical endothelial cells by promoting adherens junction assembly. We hypothesized that the EPAC-Rap1 signaling pathway may regulate the tight junction complex of the BRB and may restore barrier properties after cytokine-induced permeability. Here, we show that stimulating EPAC or Rap1 activation can prevent or reverse VEGF- or TNF-α-induced permeability in cell culture and in vivo Moreover, EPAC activation inhibited VEGF receptor (VEGFR) signaling through the Ras/MEK/ERK pathway. We also found that Rap1B knockdown or an EPAC antagonist increases endothelial permeability and that VEGF has no additive effect, suggesting a common pathway. Furthermore, GTP-bound Rap1 promoted tight junction assembly, and loss of Rap1B led to loss of junctional border organization. Collectively, our results indicate that the EPAC-Rap1 pathway helps maintain basal barrier properties in the retinal vascular endothelium and activation of the EPAC-Rap1 pathway may therefore represent a potential therapeutic strategy to restore the BRB.

Endothelial cell disease: emerging knowledge from cerebral cavernous malformations

PURPOSE OF REVIEW

Endothelial cells dysfunctions are crucial determinants of several human diseases. We review here the most recent reports on endothelial cell defects in cerebral cavernous malformations (CCMs), particularly focusing on adherens junctions. CCM is a vascular disease that affects specifically the venous microvessels of the central nervous system and which is caused by loss-of-function mutation in any one of the three CCM genes (CCM1, 2 or 3) in endothelial cells. The phenotypic result of these mutations are focal vascular malformations that are permeable and fragile causing neurological symptoms and occasionally haemorrhagic stroke.

RECENT FINDINGS

CCM is still an incurable disease, as no pharmacological treatment is available, besides surgery. The definition of the molecular alterations ensuing loss of function mutation of CCM genes is contributing to orientate the testing of targeted pharmacological tools.Several signalling pathways are altered in the three genotypes in a similar way and concur in the acquisition of mesenchymal markers in endothelial cells. However, also genotype-specific defects are reported, in particular for the CCM1 and CCM3 mutation.

SUMMARY

Besides the specific CCM disease, the characterization of endothelial alterations in CCM has the potentiality to shed light on basic molecular regulations as the acquisition and maintenance of organ and vascular site specificity of endothelial cells.

Rap1 in endothelial biology

PURPOSE OF REVIEW

Ubiquitously-expressed small GTPase Rap1 is a key modulator of integrin- and cadherin-regulated processes. In endothelium, Rap1 promotes angiogenesis and endothelial barrier function, acting downstream from cAMP-activated Rap1GEF, Epac. Recent in-vivo studies in mouse models have provided more information about the physiological role of Rap1 in vessel development and after birth under normal and pathologic conditions. Important molecular details of dynamic regulation of endothelial barrier are uncovered.

RECENT FINDINGS

Rap1 is not essential for initial vessel formation but is critical for vessel stabilization, as double knockout of the two Rap1 isoforms leads to hemorrhage and embryonic lethality. After development, Rap1 is not required for endothelial barrier maintenance but is critical for nitric oxide production and endothelial function. Radil and Afadin mediate Rap1 effects on endothelial barrier function by regulating connection with Rho GTPases, actomyosin cytoskeleton, and cell-cell adhesion receptors.

SUMMARY

Rap1 is critically required for nitric oxide release and normal endothelial function in vivo. Mechanistic studies lead to a novel paradigm of Rap1 as a critical regulator of endothelial cell shear stress responses and endothelial homeostasis. Increased understanding of molecular mechanisms underlying endothelial barrier regulation may identify novel pharmacological targets for retinopathies and conditions with altered endothelial barrier function or when increased endothelial barrier is desired.

Endothelial cell-cell adhesion and signaling

Endothelial cells line blood vessels and provide a dynamic interface between the blood and tissues. They remodel to allow leukocytes, fluid and small molecules to enter tissues during inflammation and infections. Here we compare the signaling networks that contribute to endothelial permeability and leukocyte transendothelial migration, focusing particularly on signals mediated by small GTPases that regulate cell adhesion and the actin cytoskeleton. Rho and Rap GTPase signaling is important for both processes, but they differ in that signals are activated locally under leukocytes, whereas endothelial permeability is a wider event that affects the whole cell. Some molecules play a unique role in one of the two processes, and could therefore be targeted to selectively alter either endothelial permeability or leukocyte transendothelial migration.

The role of small GTPases and EPAC-Rap signaling in the regulation of the blood-brain and blood-retinal barriers

Maintenance and regulation of the vascular endothelial cell junctional complex is critical for proper barrier function of the blood-brain barrier (BBB) and the highly related blood-retinal barrier (BRB) that help maintain proper neuronal environment. Recent research has demonstrated that the junctional complex is actively maintained and can be dynamically regulated. Studies focusing on the mechanisms of barrier formation, maintenance, and barrier disruption have been of interest to understanding development of the BBB and BRB and identifying a means for therapeutic intervention for diseases ranging from brain tumors and dementia to blinding eye diseases. Research has increasingly revealed that small GTPases play a critical role in both barrier formation and disruption mechanisms. This review will summarize the current data on small GTPases in barrier regulation with an emphasis on the EPAC-Rap1 signaling pathway to Rho in endothelial barriers, as well as explore its potential involvement in paracellular flux and transcytosis regulation.

Quantitative assessment of angiogenesis and pericyte coverage in human cell-derived vascular sprouts

Background

Pericytes, surrounding the endothelium, fulfill diverse functions that are crucial for vascular homeostasis. The loss of pericytes is associated with pathologies, such as diabetic retinopathy and Alzheimer's disease. Thus, there exists a need for an experimental system that combines pharmacologic manipulation and quantification of pericyte coverage during sprouting angiogenesis. Here, we describe an in vitro angiogenesis assay that develops lumenized vascular sprouts composed of endothelial cells enveloped by pericytes, with the additional ability to comparatively screen the effect of multiple small molecules simultaneously. For automated analysis, we also present an ImageJ plugin tool we developed to quantify sprout morphology and pericyte coverage.

Methods

Human umbilical vein endothelial cells and human brain vascular pericytes were coated on microcarrier beads and embedded in fibrin gels in a 96-well plate to form lumenized vascular sprouts. After treatment with pharmacologic compounds, sprouts were fixed, stained, and imaged via optical z-sections over the area of each well. The maximum intensity projections of these images were stitched together to form montages of the wells, and those montages were processed and analyzed.

Results

Vascular sprouts formed within 4-12 days and contained a patent lumen surrounded by a layer of human endothelial cells and pericytes. Using our workflow and image analysis, pericyte coverage after treatment with various compounds was successfully quantified.

Conclusions

Here we present a robust in vitro assay using primary human vascular cells that allows researchers to analyze the effects of multiple compounds on sprouting angiogenesis and pericyte coverage. Our ImageJ plugin offers automated evaluation across multiple different vascular parameters, such as sprout length, cell density, branch points, and pericyte coverage.

Engulfed cadherin fingers are polarized junctional structures between collectively migrating endothelial cells

The development and maintenance of tissues requires collective cell movement, during which neighbouring cells coordinate the polarity of their migration machineries. Here, we ask how polarity signals are transmitted from one cell to another across symmetrical cadherin junctions, during collective migration. We demonstrate that collectively migrating endothelial cells have polarized VE-cadherin-rich membrane protrusions, 'cadherin fingers', which leading cells extend from their rear and follower cells engulf at their front, thereby generating opposite membrane curvatures and asymmetric recruitment of curvature-sensing proteins. In follower cells, engulfment of cadherin fingers occurs along with the formation of a lamellipodia-like zone with low actomyosin contractility, and requires VE-cadherin/catenin complexes and Arp2/3-driven actin polymerization. Lateral accumulation of cadherin fingers in follower cells precedes turning, and increased actomyosin contractility can initiate cadherin finger extension as well as engulfment by a neighbouring cell, to promote follower behaviour. We propose that cadherin fingers serve as guidance cues that direct collective cell migration.

Structural Basis of Dimeric Rasip1 RA Domain Recognition of the Ras Subfamily of GTP-Binding Proteins

Ras-interacting protein 1 (Rasip1) is an endothelial-specific Rap1 and Ras effector, important for vascular development and angiogenesis. Here, we report the crystal structure of the Rasip1 RA domain (RRA) alone, revealing the basis of dimerization, and in complex with Rap1 at 2.8 Å resolution. In contrast to most RA domains, RRA formed a dimer that can bind two Rap1 (K_D = 0.9 μM) or Ras (K_D = 2.2 μM) molecules. We solved the Rap1-RRA complex and found that Rasip1 binds Rap1 in the Switch I region, and Rap1 binding induces few conformation changes to Rasip1 stabilizing a β strand and an unstructured loop. Our data explain how Rasip1 can act as a Rap1 and Ras effector and show that Rasip1 defines a subgroup of dimeric RA domains that could mediate cooperative binding to membrane-associated Ras superfamily members.

Rasip1-Mediated Rho GTPase Signaling Regulates Blood Vessel Tubulogenesis via Nonmuscle Myosin II

RATIONALE

Vascular tubulogenesis is essential to cardiovascular development. Within initial vascular cords of endothelial cells, apical membranes are established and become cleared of cell-cell junctions, thereby allowing continuous central lumens to open. Rasip1 (Ras-interacting protein 1) is required for apical junction clearance, as well as for regulation of Rho GTPase (enzyme that hydrolyzes GTP) activity. However, it remains unknown how activities of different Rho GTPases are coordinated by Rasip1 to direct tubulogenesis.

OBJECTIVE

The aim of this study is to determine the mechanisms downstream of Rasip1 that drive vascular tubulogenesis.

METHODS AND RESULTS

Using conditional mouse mutant models and pharmacological approaches, we dissect GTPase pathways downstream of Rasip1. We show that clearance of endothelial cell apical junctions during vascular tubulogenesis depends on Rasip1, as well as the GTPase Cdc42 (cell division control protein 42 homolog) and the kinase Pak4 (serine/threonine-protein kinase 4). Genetic deletion of Rasip1 or Cdc42, or inhibition of Pak4, all blocks endothelial cell tubulogenesis. By contrast, inactivation of RhoA (Ras homologue gene family member A) signaling leads to vessel overexpansion, implicating actomyosin contractility in control of lumen diameter. Interestingly, blocking activity of NMII (nonmuscle myosin II) either before, or after, lumen morphogenesis results in dramatically different tubulogenesis phenotypes, suggesting time-dependent roles.

CONCLUSIONS

Rasip1 controls different pools of GTPases, which in turn regulate different pools of NMII to coordinate junction clearance (remodeling) and actomyosin contractility during vascular tubulogenesis. Rasip1 promotes activity of Cdc42 to activate Pak4, which in turn activates NMII, clearing apical junctions. Once lumens open, Rasip1 suppresses actomyosin contractility via inhibition of RhoA by Arhgap29, allowing controlled expansion of vessel lumens during embryonic growth. These findings elucidate the stepwise processes regulated by Rasip1 through downstream Rho GTPases and NMII.

Cyclic AMP Sensor EPAC Proteins and Their Role in Cardiovascular Function and Disease

cAMP is a universal second messenger that plays central roles in cardiovascular regulation influencing gene expression, cell morphology, and function. A crucial step toward a better understanding of cAMP signaling came 18 years ago with the discovery of the exchange protein directly activated by cAMP (EPAC). The 2 EPAC isoforms, EPAC1 and EPAC2, are guanine-nucleotide exchange factors for the Ras-like GTPases, Rap1 and Rap2, which they activate independently of the classical effector of cAMP, protein kinase A. With the development of EPAC pharmacological modulators, many reports in the literature have demonstrated the critical role of EPAC in the regulation of various cAMP-dependent cardiovascular functions, such as calcium handling and vascular tone. EPAC proteins are coupled to a multitude of effectors into distinct subcellular compartments because of their multidomain architecture. These novel cAMP sensors are not only at the crossroads of different physiological processes but also may represent attractive therapeutic targets for the treatment of several cardiovascular disorders, including cardiac arrhythmia and heart failure.

Rasip1 is essential to blood vessel stability and angiogenic blood vessel growth

Cardiovascular function depends on patent, continuous and stable blood vessel formation by endothelial cells (ECs). Blood vessel development initiates by vasculogenesis, as ECs coalesce into linear aggregates and organize to form central lumens that allow blood flow. Molecular mechanisms underlying in vivo vascular 'tubulogenesis' are only beginning to be unraveled. We previously showed that the GTPase-interacting protein called Rasip1 is required for the formation of continuous vascular lumens in the early embryo. Rasip1(-/-) ECs exhibit loss of proper cell polarity and cell shape, disrupted localization of EC-EC junctions and defects in adhesion of ECs to extracellular matrix. In vitro studies showed that Rasip1 depletion in cultured ECs blocked tubulogenesis. Whether Rasip1 is required in blood vessels after their initial formation remained unclear. Here, we show that Rasip1 is essential for vessel formation and maintenance in the embryo, but not in quiescent adult vessels. Rasip1 is also required for angiogenesis in three models of blood vessel growth: in vitro matrix invasion, retinal blood vessel growth and directed in vivo angiogenesis assays. Rasip1 is thus necessary in growing embryonic blood vessels, postnatal angiogenic sprouting and remodeling, but is dispensable for maintenance of established blood vessels, making it a potential anti-angiogenic therapeutic target.

Heart of glass anchors Rasip1 at endothelial cell-cell junctions to support vascular integrity

Heart of Glass (HEG1), a transmembrane receptor, and Rasip1, an endothelial-specific Rap1-binding protein, are both essential for cardiovascular development. Here we performed a proteomic screen for novel HEG1 interactors and report that HEG1 binds directly to Rasip1. Rasip1 localizes to forming endothelial cell (EC) cell-cell junctions and silencing HEG1 prevents this localization. Conversely, mitochondria-targeted HEG1 relocalizes Rasip1 to mitochondria in cells. The Rasip1-binding site in HEG1 contains a 9 residue sequence, deletion of which abrogates HEG1’s ability to recruit Rasip1. HEG1 binds to a central region of Rasip1 and deletion of this domain eliminates Rasip1’s ability to bind HEG1, to translocate to EC junctions, to inhibit ROCK activity, and to maintain EC junctional integrity. These studies establish that the binding of HEG1 to Rasip1 mediates Rap1-dependent recruitment of Rasip1 to and stabilization of EC cell-cell junctions.

DOI:http://dx.doi.org/10.7554/eLife.11394.001

Blood vessels are lined with cells known as vascular endothelial cells. These cells are connected to each other at junctions that consist of several different proteins. The junctions help to control how the blood vessel develops and provide a barrier that controls the movement of water and certain other molecules through the vessel wall. This barrier becomes weakened in diseases like sepsis and atherosclerosis.

Two proteins that are essential for the heart and blood vessels to develop correctly are called “Heart of Glass” (HEG1) and Rasip1. Although a protein has been identified that binds to HEG1 at the cell junctions, this binding only involves a small region of HEG1. This led de Kreuk, Gingras et al. to look for other proteins that interact with HEG1 and that might be important for controlling the development of the blood vessels. This revealed that HEG1 binds directly to Rasip1.

Further experiments revealed that HEG1 is essential for targeting Rasip1 to the junctions between the endothelial cells, and that this helps to stabilize the cell junctions. Mutant forms of Rasip1 that lacked a particular sequence in the middle of the protein were unable to bind to HEG1 and did not localize to the cell junctions. These studies open the door to future work to define how the interaction of Rasip1 and HEG1 is controlled and how Rasip1 stabilizes blood vessels.

DOI:http://dx.doi.org/10.7554/eLife.11394.002

Small GTPase Rap1 Is Essential for Mouse Development and Formation of Functional Vasculature

Background

Small GTPase Rap1 has been implicated in a number of basic cellular functions, including cell-cell and cell-matrix adhesion, proliferation and regulation of polarity. Evolutionarily conserved, Rap1 has been studied in model organisms: yeast, Drosophila and mice. Mouse in vivo studies implicate Rap1 in the control of multiple stem cell, leukocyte and vascular cell functions. In vitro, several Rap1 effectors and regulatory mechanisms have been proposed. In particular, Rap1 has been implicated in maintaining epithelial and endothelial cell junction integrity and linked with cerebral cavernous malformations.

Rationale

How Rap1 signaling network controls mammalian development is not clear. As a first step in addressing this question, we present phenotypes of murine total and vascular-specific Rap1a, Rap1b and double Rap1a and Rap1b (Rap1) knockout (KO) mice.

Results and Conclusions

The majority of total Rap1 KO mice die before E10.5, consistent with the critical role of Rap1 in epithelial morphogenesis. At that time point, about 50% of Tie2-double Rap1 KOs appear grossly normal and develop normal vasculature, while the remaining 50% suffer tissue degeneration and show vascular abnormalities, including hemorrhages and engorgement of perineural vessels, albeit with normal branchial arches. However, no Tie2-double Rap1 KO embryos are present at E15.5, with hemorrhages a likely cause of death. Therefore, at least one Rap1 allele is required for development prior to the formation of the vascular system; and in endothelium–for the life-supporting function of the vasculature.

A local VE-cadherin and Trio-based signaling complex stabilizes endothelial junctions through Rac1

Endothelial cell-cell junctions maintain a restrictive barrier that is tightly regulated to allow dynamic responses to permeability-inducing angiogenic factors, as well as to inflammatory agents and adherent leukocytes. The ability of these stimuli to transiently remodel adherens junctions depends on Rho-GTPase-controlled cytoskeletal rearrangements. How the activity of Rho-GTPases is spatio-temporally controlled at endothelial adherens junctions by guanine-nucleotide exchange factors (GEFs) is incompletely understood. Here, we identify a crucial role for the Rho-GEF Trio in stabilizing junctions based around vascular endothelial (VE)-cadherin (also known as CDH5). Trio interacts with VE-cadherin and locally activates Rac1 at adherens junctions during the formation of nascent contacts, as assessed using a novel FRET-based Rac1 biosensor and biochemical assays. The Rac-GEF domain of Trio is responsible for the remodeling of junctional actin from radial into cortical actin bundles, a crucial step for junction stabilization. This promotes the formation of linear adherens junctions and increases endothelial monolayer resistance. Collectively, our data show the importance of spatio-temporal regulation of the actin cytoskeleton through Trio and Rac1 at VE-cadherin-based cell-cell junctions in the maintenance of the endothelial barrier.

Rap1 signaling in endothelial barrier control

The small G-protein Rap1 plays an important role in the regulation of endothelial barrier function, a process controlled largely by cell–cell adhesions and their connection to the actin cytoskeleton. During the various stages of barrier dynamics, different guanine nucleotide exchange factors (GEFs) control Rap1 activity, indicating that Rap1 integrates multiple input signals. Once activated, Rap1 induces numerous signaling cascades, together responsible for the increased endothelial barrier function. Most notably, Rap1 activation results in the inhibition of Rho to decrease radial stress fibers and the activation of Cdc42 to increase junctional actin. This implies that Rap regulates endothelial barrier function by dual control of cytoskeletal tension. The molecular details of the signaling pathways are becoming to be elucidated.

Regulation of vascular endothelial junction stability and remodeling through Rap1-Rasip1 signaling

The ability of blood vessels to sense and respond to stimuli such as fluid flow, shear stress, and trafficking of immune cells is critical to the proper function of the vascular system. Endothelial cells constantly remodel their cell-cell junctions and the underlying cytoskeletal network in response to these exogenous signals. This remodeling, which depends on regulation of the linkage between actin and integral junction proteins, is controlled by a complex signaling network consisting of small G proteins and their various downstream effectors. In this commentary, we summarize recent developments in understanding the small G protein RAP1 and its effector RASIP1 as critical mediators of endothelial junction stabilization, and the relationship between RAP1 effectors and modulation of different subsets of endothelial junctions.   The vasculature is a dynamic organ that is constantly exposed to a variety of signaling stimuli and mechanical stresses. In embryogenesis, nascent blood vessels form via a process termed vasculogenesis, wherein mesodermally derived endothelial precursor cells aggregate into cords, which subsequently form a lumen that permits trafficking of plasma and erythrocytes. (1)(,) (2) Angiogenesis occurs after establishment of this primitive vascular network, where new vessels sprout from existing vessels, migrate into newly expanded tissues, and anastomose to form a functional and complex circulatory network. (1)(,) (2) In the mouse, this process occurs through the second half of embryogenesis and into postnatal development in some tissues, such as the developing retinal vasculature. (3) Further, angiogenesis occurs in a variety of pathological conditions, such as diabetic retinopathy, age-related macular degeneration, inflammatory diseases such as rheumatoid arthritis, wound healing, and tumor growth. (1)(,) (2)(,) (4) Both vasculogenesis and angiogenesis are driven through signaling by vascular endothelial growth factor (VEGF), and therapeutic agents targeting this pathway have shown efficacy in a number of diseases. (5)(-) (9) Blood vessels must have a sufficient degree of integrity so as to not allow indiscriminate leak of plasma proteins and blood cells into the underlying tissue. However, vessels must be able to sense their environment, respond to local conditions, and mediate the regulated passage of protein, fluid, and cells. For example, endothelial cells are the primary point of attachment for immune cells leaving the blood stream and entering tissue, and leukocytes subsequently migrate either through the endothelial cell body itself (the transcellular route), or through transient disassembly of cell-cell junctions (the paracellular route). (10) Precise regulation of endothelial junctions is critical to the proper maintenance of vascular integrity and related processes, and disruption of vascular cell-cell contacts is an underlying cause or contributor to numerous pathologies such as cerebral cavernous malformations (CCM) and hereditary hemorrhagic telangiectasia (HHT). (11)(-) (13) Understanding the basic mechanisms of endothelial junction formation and maintenance will therefore lead to a greater chance of success of therapeutic intervention in these pathologic conditions, especially in instances where targeting of VEGF signaling is insufficient to resolve vascular abnormalities.

Rap1 Spatially Controls ArhGAP29 To Inhibit Rho Signaling during Endothelial Barrier Regulation

The small GTPase Rap1 controls the actin cytoskeleton by regulating Rho GTPase signaling. We recently established that the Rap1 effectors Radil and Rasip1, together with the Rho GTPase activating protein ArhGAP29, mediate Rap1-induced inhibition of Rho signaling in the processes of epithelial cell spreading and endothelial barrier function. Here, we show that Rap1 induces the independent translocations of Rasip1 and a Radil-ArhGAP29 complex to the plasma membrane. This results in the formation of a multimeric protein complex required for Rap1-induced inhibition of Rho signaling and increased endothelial barrier function. Together with the previously reported spatiotemporal control of the Rap guanine nucleotide exchange factor Epac1, these findings elucidate a signaling pathway for spatiotemporal control of Rho signaling that operates by successive protein translocations to and complex formation at the plasma membrane.

MAP4K4 regulates integrin-FERM binding to control endothelial cell motility

Cell migration is a stepwise process that coordinates multiple molecular machineries. Using in vitro angiogenesis screens with short interfering RNA and chemical inhibitors, we define here a MAP4K4-moesin-talin-β1-integrin molecular pathway that promotes efficient plasma membrane retraction during endothelial cell migration. Loss of MAP4K4 decreased membrane dynamics, slowed endothelial cell migration, and impaired angiogenesis in vitro and in vivo. In migrating endothelial cells, MAP4K4 phosphorylates moesin in retracting membranes at sites of focal adhesion disassembly. Epistasis analyses indicated that moesin functions downstream of MAP4K4 to inactivate integrin by competing with talin for binding to β1-integrin intracellular domain. Consequently, loss of moesin (encoded by the MSN gene) or MAP4K4 reduced adhesion disassembly rate in endothelial cells. Additionally, α5β1-integrin blockade reversed the membrane retraction defects associated with loss of Map4k4 in vitro and in vivo. Our study uncovers a novel aspect of endothelial cell migration. Finally, loss of MAP4K4 function suppressed pathological angiogenesis in disease models, identifying MAP4K4 as a potential therapeutic target.

The future of EPAC-targeted therapies: agonism versus antagonism

•

Although tractable to drug development, targeting of cAMP signalling has side effects.

•

Selectively targeting EPAC1 and EPAC2 cAMP sensor enzymes may limit some of these off-target effects.

•

EPAC agonists could be used to treat vascular inflammation (EPAC1) or type 2 diabetes (EPAC2).

•

EPAC1 and EPAC2 antagonists could be used to treat heart disease.

Pharmaceutical manipulation of cAMP levels exerts beneficial effects through the regulation of the exchange protein activated by cAMP (EPAC) and protein kinase A (PKA) signalling routes. Recent attention has turned to the specific regulation of EPAC isoforms (EPAC1 and EPAC2) as a more targeted approach to cAMP-based therapies. For example, EPAC2-selective agonists could promote insulin secretion from pancreatic β cells, whereas EPAC1-selective agonists may be useful in the treatment of vascular inflammation. By contrast, EPAC1 and EPAC2 antagonists could both be useful in the treatment of heart failure. Here we discuss whether the best way forward is to design EPAC-selective agonists or antagonists and the current strategies being used to develop isoform-selective, small-molecule regulators of EPAC1 and EPAC2 activity.

The role of actin-binding proteins in the control of endothelial barrier integrity

The endothelial barrier of the vasculature is of utmost importance for separating the blood stream from underlying tissues. This barrier is formed by tight and adherens junctions (TJ and AJ) that form intercellular endothelial contacts. TJ and AJ are integral membrane structures that are connected to the actin cytoskeleton via various adaptor molecules. Consequently, the actin cytoskeleton plays a crucial role in regulating the stability of endothelial cell contacts and vascular permeability. While a circumferential cortical actin ring stabilises junctions, the formation of contractile stress fibres, e. g. under inflammatory conditions, can contribute to junction destabilisation. However, the role of actin-binding proteins (ABP) in the control of vascular permeability has long been underestimated. Naturally, ABP regulate permeability via regulation of actin remodelling but some actin-binding molecules can also act independently of actin and control vascular permeability via various signalling mechanisms such as activation of small GTPases. Several studies have recently been published highlighting the importance of actin-binding molecules such as cortactin, ezrin/radixin/moesin, Arp2/3, VASP or WASP for the control of vascular permeability by various mechanisms. These proteins have been described to regulate vascular permeability under various pathophysiological conditions and are thus of clinical relevance as targets for the development of treatment strategies for disorders that are characterised by vascular hyperpermeability such as sepsis. This review highlights recent advances in determining the role of ABP in the control of endothelial cell contacts and vascular permeability.

Blood and lymphatic vascular tube formation in mouse

The blood and lymphatic vasculatures are essential for nutrient delivery, gas exchange and fluid homeostasis in all tissues of higher vertebrates. They are composed of a hierarchical network of vessels, which are lined by vascular or lymphatic endothelial cells. For blood vascular lumen formation to occur, endothelial cell cords polarize creating apposing apical cell surfaces, which repulse each other and give rise to a small intercellular lumen. Following cell shape changes, the vascular lumen expands. Various junctional proteins, polarity complexes, extracellular matrix binding and actin remodelling molecules are required for blood vascular lumen formation. In contrast, little is known regarding the molecular mechanisms leading to lymphatic vascular tube formation. Current models agree that lymphatic vessels share a blood vessel origin, but they differ in identifying the mechanism by which a lymphatic lumen is formed. A ballooning mechanism was proposed, in which lymph sacs are connected via their lumen to the cardinal veins. Alternatively, a mechanism involving budding of streams of lymphatic endothelial cells from either the cardinal veins or both the cardinal veins and the intersomitic vessels, and subsequent assembly and lumenisation was recently described. Here, we discuss what is currently known about the molecular and cellular machinery that guides blood and lymphatic vascular tube formation in mouse.

Mechanical control of the endothelial barrier

The integrity of the endothelial barrier is controlled by the combined action of chemical and mechanical signaling systems. Permeability-regulating factors signal through small GTPases to regulate the architecture of the cytoskeleton and this has a strong impact on the morphology and stability of VE-cadherin-based cell-cell junctions. The details of how structural and mechanical properties of the actin cytoskeleton influence cell-cell adhesion and how this impacts the dynamic regulation of the endothelial barrier, are beginning to be elucidated. In this review, we discuss the physical and regulatory interactions between the VE-cadherin complex and the actomysoin cytoskeleton, as they are the main determinants of cell-cell adhesion and the mechanical architecture of the cytoskeleton. We discuss, based on recent in vitro data, how a balance between Linear Adherens Junctions, paralleled by cortical actin bundles and Focal Adherens Junctions, connected to radial action bundles, determines endothelial barrier function. We discuss how small GTPases control this balance by regulating the spatial organization and mechanics of actomyosin. We propose a hypothetical model of how biochemical and mechanical signals cooperate locally, at the actomyosin-adhesion interface to open and re-seal the barrier in a rapid and controlled manner.