Small Rho GTPase-mediated actin dynamics at endothelial adherens junctions

Off-targets of BRAF inhibitors disrupt endothelial signaling and vascular barrier function

Targeted therapies against mutant BRAF are effectively used in combination with MEK inhibitors (MEKi) to treat advanced melanoma. However, treatment success is affected by resistance and adverse events (AEs). Approved BRAF inhibitors (BRAFi) show high levels of target promiscuity, which can contribute to these effects. The blood vessel lining is in direct contact with high plasma concentrations of BRAFi, but effects of the inhibitors in this cell type are unknown. Hence, we aimed to characterize responses to approved BRAFi for melanoma in the vascular endothelium. We showed that clinically approved BRAFi induced a paradoxical activation of endothelial MAPK signaling. Moreover, phosphoproteomics revealed distinct sets of off-targets per inhibitor. Endothelial barrier function and junction integrity were impaired upon treatment with vemurafenib and the next-generation dimerization inhibitor PLX8394, but not with dabrafenib or encorafenib. Together, these findings provide insights into the surprisingly distinct side effects of BRAFi on endothelial signaling and functionality. Better understanding of off-target effects could help to identify molecular mechanisms behind AEs and guide the continued development of therapies for BRAF-mutant melanoma.

Molecular Mechanisms Regulating Vascular Endothelial Permeability

Vascular endothelial cells form a monolayer in the vascular lumen and act as a selective barrier to control the permeability between blood and tissues. To maintain homeostasis, the endothelial barrier function must be strictly integrated. During acute inflammation, vascular permeability temporarily increases, allowing intravascular fluid, cells, and other components to permeate tissues. Moreover, it has been suggested that the dysregulation of endothelial cell permeability may cause several diseases, including edema, cancer, and atherosclerosis. Here, we reviewed the molecular mechanisms by which endothelial cells regulate the barrier function and physiological permeability.

N-Cadherin Targeted Melanin Nanoparticles Reverse the Endothelial-Mesenchymal Transition in Vascular Endothelial Cells to Potentially Slow the Progression of Atherosclerosis and Cancer

Endothelial-mesenchymal transition (EndoMT) of vascular endothelial cells has recently been considered as a key player in the early progression of a variety of vascular and nonvascular diseases, including atherosclerosis, cancer, and organ fibrosis. However, current strategies attempting to identify pharmacological inhibitors to block the regulatory pathways of EndoMT suffer from poor selectivity, unwanted side effects, and a heterogeneous response from endothelial cells with different origins. Furthermore, EndoMT inhibitors focus on preventing EndoMT, leaving the endothelial cells that have already undergone EndoMT unresolved. Here, we report the design of a simple but powerful nanoparticle system (i.e., N-cadherin targeted melanin nanoparticles) to convert cytokine-activated, mesenchymal-like endothelial cells back to their original endothelial phenotype. We term this process "Reversed EndoMT" (R-EndoMT). R-EndoMT allows the impaired endothelial barriers to recover their quiescence and intactness, with significantly reduced leukocyte and cancer cell adhesion and transmigration, which could potentially stop atheromatous plaque formation and cancer metastasis in the early stages. R-EndoMT is achieved on different endothelial cell types originating from arteries, veins, and capillaries, independent of activating cytokines. We reveal that N-cadherin targeted melanin nanoparticles reverse EndoMT by downregulating an N-cadherin dependent RhoA activation pathway. Overall, this approach offers a different prospect to treat multiple EndoMT-associated diseases by designing nanoparticles to reverse the phenotypical transition of endothelial cells.

Tankyrase inhibition interferes with junction remodeling, induces leakiness, and disturbs YAP1/TAZ signaling in the endothelium

Tankyrase inhibitors are increasingly considered for therapeutic use in malignancies that are characterized by high intrinsic β-catenin activity. However, how tankyrase inhibition affects the endothelium after systemic application remains poorly understood. In this study, we aimed to investigate how the tankyrase inhibitor XAV939 affects endothelial cell function and the underlying mechanism involved. Endothelial cell function was analyzed using sprouting angiogenesis, endothelial cell migration, junctional dynamics, and permeability using human umbilical vein endothelial cells (HUVEC) and explanted mouse retina. Underlying signaling was studied using western blot, immunofluorescence, and qPCR in HUVEC in addition to luciferase reporter gene assays in human embryonic kidney cells. XAV939 treatment leads to altered junctional dynamics and permeability as well as impaired endothelial migration. Mechanistically, XAV939 increased stability of the angiomotin-like proteins 1 and 2, which impedes the nuclear translocation of YAP1/TAZ and consequently suppresses TEAD-mediated transcription. Intriguingly, XAV939 disrupts adherens junctions by inducing RhoA-Rho dependent kinase (ROCK)-mediated F-actin bundling, whereas disruption of F-actin bundling through the ROCK inhibitor H1152 restores endothelial cell function. Unexpectedly, this was accompanied by an increase in nuclear TAZ and TEAD-mediated transcription, suggesting differential regulation of YAP1 and TAZ by the actin cytoskeleton in endothelial cells. In conclusion, our findings elucidate the complex relationship between the actin cytoskeleton, YAP1/TAZ signaling, and endothelial cell function and how tankyrase inhibition disturbs this well-balanced signaling.

TIMP2 ameliorates blood-brain barrier disruption in traumatic brain injury by inhibiting Src-dependent VE-cadherin internalization

Blood-brain barrier (BBB) disruption is a serious pathological consequence of traumatic brain injury (TBI), for which there are limited therapeutic strategies. Tissue inhibitor of metalloproteinase-2 (TIMP2), a molecule with dual functions of inhibiting MMP activity and displaying cytokine-like activity through receptor binding, has been reported to inhibit VEGF-induced vascular hyperpermeability. Here, we investigate the ability of TIMP2 to ameliorate BBB disruption in TBI and the underlying molecular mechanisms. Both TIMP2 and AlaTIMP2, a TIMP2 mutant without MMP-inhibiting activity, attenuated neurological deficits and BBB leakage in TBI mice; they also inhibited junctional protein degradation and translocation to reduce paracellular permeability in human brain microvascular endothelial cells (ECs) exposed to hypoxic plus inflammatory insult. Mechanistic studies revealed that TIMP2 interacted with α3β1 integrin on ECs, inhibiting Src activation-dependent VE-cadherin phosphorylation, VE-cadherin/catenin complex destabilization, and subsequent VE-cadherin internalization. Notably, localization of VE-cadherin on the membrane was critical for TIMP2-mediated EC barrier integrity. Furthermore, TIMP2-mediated increased membrane localization of VE-cadherin enhanced the level of active Rac1, thereby inhibiting stress fiber formation. All together, our studies have identified an MMP-independent mechanism by which TIMP2 regulates EC barrier integrity after TBI. TIMP2 may be a therapeutic agent for TBI and other neurological disorders involving BBB breakdown.

Lipocalin 10 is essential for protection against inflammation-triggered vascular leakage by activating LDL receptor-related protein 2-slingshot homologue 1 signalling pathway

AIMS

Systemic inflammation occurs commonly during many human disease settings and increases vascular permeability, leading to organ failure, and lethal outcomes. Lipocalin 10 (Lcn10), a poorly characterized member of the lipocalin family, is remarkably altered in the cardiovascular system of human patients with inflammatory conditions. Nonetheless, whether Lcn10 regulates inflammation-induced endothelial permeability remains unknown.

METHODS AND RESULTS

Systemic inflammation models were induced using mice by injection of endotoxin lipopolysaccharide (LPS) or caecal ligation and puncture (CLP) surgery. We observed that the expression of Lcn10 was dynamically altered only in endothelial cells (ECs), but not in either fibroblasts or cardiomyocytes isolated from mouse hearts following the LPS challenge or CLP surgery. Using in vitro gain- and loss-of-function approaches and an in vivo global knockout mouse model, we discovered that Lcn10 negatively regulated endothelial permeability upon inflammatory stimuli. Loss of Lcn10 augmented vascular leakage, leading to severe organ damage and higher mortality following LPS challenge, compared to wild-type controls. By contrast, overexpression of Lcn10 in ECs displayed opposite effects. A mechanistic analysis revealed that both endogenous and exogenous elevation of Lcn10 in ECs could activate slingshot homologue 1 (Ssh1)-Cofilin signalling cascade, a key axis known to control actin filament dynamics. Accordingly, a reduced formation of stress fibre and increased generation of cortical actin band were exhibited in Lcn10-ECs, when compared to controls upon endotoxin insults. Furthermore, we identified that Lcn10 interacted with LDL receptor-related protein 2 (LRP2) in ECs, which acted as an upstream factor of the Ssh1-Confilin signalling. Finally, injection of recombinant Lcn10 protein into endotoxic mice showed therapeutic effects against inflammation-induced vascular leakage.

CONCLUSION

This study identifies Lcn10 as a novel regulator of EC function and illustrates a new link in the Lcn10-LRP2-Ssh1 axis to controlling endothelial barrier integrity. Our findings may provide novel strategies for the treatment of inflammation-related diseases.

Low-dose albumin-coated gold nanorods induce intercellular gaps on vascular endothelium by causing the contraction of cytoskeletal actin

Cytotoxicity of nanoparticles, typically evaluated by biochemical-based assays, often overlook the cellular biophysical properties such as cell morphology and cytoskeletal actin, which could serve as more sensitive indicators for cytotoxicity. Here, we demonstrate that low-dose albumin-coated gold nanorods (HSA@AuNRs), although being considered noncytotoxic in multiple biochemical assays, can induce intercellular gaps and enhance the paracellular permeability between human aortic endothelial cells (HAECs). The formation of intercellular gaps can be attributed to the changed cell morphology and cytoskeletal actin structures, as validated at the monolayer and single cell levels using fluorescence staining, atomic force microscopy, and super-resolution imaging. Molecular mechanistic study shows the caveolae-mediated endocytosis of HSA@AuNRs induces the calcium influx and activates actomyosin contraction in HAECs. Considering the important roles of endothelial integrity/dysfunction in various physiological/pathological conditions, this work suggests a potential adverse effect of albumin-coated gold nanorods on the cardiovascular system. On the other hand, this work also offers a feasible way to modulate the endothelial permeability, thus promoting drug and nanoparticle delivery across the endothelium.

Cellular crosstalk in atherosclerotic plaque microenvironment

Atherosclerosis is an underlying pathology of many vascular diseases as a result of cellular, structural and molecular dysfunctions within the sub-endothelial space. This review deals with the events involved in the formation, growth and remodeling of plaque, including the cell recruitment, cell polarization, and cell fat droplets. It also describes cross talking between endothelial cells, macrophages, and vascular smooth muscle cells, as well as the cellular pathways involved in plaque development in the plaque microenvironment. Finally, it describes the plaque structural components and the role of factors involved in the rupture and erosion of plaques in the vessel. Video Abstract.

Microvascular Leakage as Therapeutic Target for Ischemia and Reperfusion Injury

Reperfusion injury is a very common complication of various indicated therapies such as the re-opening of vessels in the myocardium or brain as well as reflow in hemodynamic shutdown (cardiac arrest, severe trauma, aortic cross-clamping). The treatment and prevention of reperfusion injury has therefore been a topic of immense interest in terms of mechanistic understanding, the exploration of interventions in animal models and in the clinical setting in major prospective studies. While a wealth of encouraging results has been obtained in the lab, the translation into clinical success has met with mixed outcomes at best. Considering the still very high medical need, progress continues to be urgently needed. Multi-target approaches rationally linking interference with pathophysiological pathways as well as a renewed focus on aspects of microvascular dysfunction, especially on the role of microvascular leakage, are likely to provide new insights.

Targeting Rho kinase to restore endothelial barrier function following vascular scaffold implantation

Vascular scaffold implantation induces injury to the intimal layer and causes discontinuity of the regenerated endothelial monolayer, compromising barrier integrity, increasing permeability, and allowing the transmigration of leukocytes and lipoproteins into the subendothelial space. Mechanical vascular wall stretching triggers Ras homolog family member A (RhoA)/Rho kinase-mediated actomyosin contractility and destabilization of adherens junctions, leading to endothelial barrier dysfunction. Assembly of intercellular adhesion and actin cytoskeletal organization of interendothelial junctions are controlled by downregulation of RhoA guanosine triphosphatase (GTPase)-mediated barrier-disruptive activity and upregulation of repressor-activator protein 1 (Rap1) and Ras-related C3 botulinum toxin substrate 1 (Rac1) GTPase-mediated cytoskeletal reorganization, leading to endothelial barrier stabilization. This review highlights the involvement of Rho GTPases in the disruption of endothelial barrier integrity following vascular scaffold implantation and the targeting of downstream Rho-associated protein kinases, which signal the network to restore endothelial barrier integrity and stability.

Schisandrin A ameliorates increased pulmonary capillary endothelial permeability accompanied with sepsis through inhibition of RhoA/ROCK1/MLC pathways

BACKGROUND

Sepsis is a systemic inflammatory response, and vascular leakage associated with acute lung injury (ALI) is an important pathophysiological process during sepsis. Schisandrin A (SchA) is a bioactive lignan which has been reported to have the anti-inflammatory effects in many studies, while whether SchA can ameliorate ALI-related vascular leakage caused by sepsis is unknown.

OBJECTIVE

To evaluate the role and the underlying mechanism of SchA in increase of pulmonary vascular permeability induced by sepsis.

METHODS

The effect of SchA on pulmonary vascular permeability was examined in rat acute lung injury model. The effect of SchA on skin vascular permeability of mice was investigated through Miles assay. MTT assay was performed to detect the cell activity, and transwell assay was used to detect the effect of SchA on cell permeability. The effects of SchA on junction proteins and RhoA/ROCK1/MLC signaling pathway were manifested by immunofluorescence staining and western blot.

RESULTS

The administration of SchA alleviated rat pulmonary endothelial dysfunction, relieved increased permeability in the mouse skin and HUVECs induced by lipopolysaccharide (LPS). Meanwhile, SchA inhibited the formation of stress fibers, reversed the decrease of expression of ZO-1 and VE-cadherin. Subsequent experiments confirmed that SchA inhibited RhoA/ROCK1/MLC canonical pathway in rat lungs and HUVECs induced by LPS. Moreover, overexpression of RhoA reversed the inhibitory effect of SchA in HUVECs, which suggested that SchA protected the pulmonary endothelial barrier by inhibiting RhoA/ROCK1/MLC pathway.

CONCLUSION

In summary, our results indicate that SchA ameliorates the increase of pulmonary endothelial permeability induced by sepsis through inhibition of RhoA/ROCK1/MLC pathway, providing a potentially effective therapeutic strategy for sepsis.

Interferon-gamma (IFN-γ): Reviewing its mechanisms and signaling pathways on the regulation of endothelial barrier function

Interferon-gamma (IFN-γ) is a pleiotropic cytokine that plays a critical role in mediating an array of immune responses including promotes antiviral activity, facilitates macrophage activation, controls Th1/Th2 balance, and regulates cellular apoptosis and proliferation. A few articles have previously reviewed the effects of IFN-γ in the regulation of barrier permeability, but none of these articles focuses on barrier function of endothelial cells. This review aims to discuss the regulatory mechanisms of IFN-γ on endothelial barrier function and its underlying signaling pathways. Articles were retrieved from electronic databases such as PubMed and Google Scholar using keywords "Interferon-gamma", "endothelial cells", "barrier function", and "signaling pathway". The articles published between 2000 and 2022 that are related to the aforementioned topics were selected. A few journals published beyond this period were also included due to limited information available. The results showed that IFN-γ modulates endothelial barrier function, mainly involves small GTPases, STAT1-dependent pathway, p38 MAPK and nitric oxide. In conclusion, more in depth cellular and molecular studies are needed to elucidate the pathways of IFN-γ in the regulation of endothelial barrier function.

Phosphorylation/dephosphorylation of PTP-PEST at Serine 39 is crucial for cell migration

We investigated the molecular details of the role of protein tyrosine phosphatase (PTP)-PEST in cell migration. PTP-PEST knockout mouse embryonic fibroblasts (KO MEFs) and MEF cells expressing a dominant-negative mutant of PTP-PEST showed significant suppression of cell migration compared to MEF cells expressing wild-type PTP-PEST (WT MEFs). Moreover, MEF cells harbouring a constitutively active mutant of PTP-PEST (S39A MEFs) showed a marked decrease in cell migration. In addition, MEF cells with no PTP-PEST or little PTP activity rapidly adhered to fibronectin and made many focal adhesions compared to WT MEF cells. In contrast, S39A MEF cells showed weak adhesion to fibronectin and formed a few focal adhesions. Furthermore, investigating the subcellular localization showed that Ser39-phosphorylated PTP-PEST was favourably situated in the adherent area of the pseudopodia. Therefore, we propose that suppression of PTP-PEST enzyme activity due to Ser39-phosphorylation in pseudopodia and at the leading edge of migrating cells induces rapid and good adherence to the extracellular matrix. Thus, suppression of PTP activity by Ser39-phosphorylation is critical for cell migration. Three amino acid substitutions in human PTP-PEST have been previously reported to alter PTP activity. These amino acid substitutions in mouse PTP-PEST altered the migration of MEF cells in a positive correlation.

Extracellular matrix regulates force transduction at VE-cadherin junctions

Increased tension on VE-cadherin (VE-cad) complexes activates adaptive cell stiffening and local cytoskeletal reinforcement--two key signatures of intercellular mechanotransduction. Here we demonstrate that tugging on VE-cad receptors initiates a cascade that results in downstream integrin activation. The formation of new integrin adhesions potentiates vinculin and actin recruitment to mechanically reinforce stressed cadherin adhesions. This cascade differs from documented antagonistic effects of integrins on intercellular junctions. We identify focal adhesion kinase, Abl kinase, and RhoA GTPase as key components of the positive feedback loop. Results further show that a consequence of integrin involvement is the sensitization of intercellular force transduction to the extracellular matrix (ECM) not by regulating junctional tension but by altering signal cascades that reinforce cell-cell adhesions. On type 1 collagen or fibronectin substrates, integrin subtypes α2β1 and α5β1, respectively, differentially control actin remodeling at VE-cad adhesions. Specifically, ECM-dependent differences in VE-cad force transduction mirror differences in the rigidity sensing mechanisms of α2β1 and α5β1 integrins. The findings verify the role of integrins in VE-cad force transduction and uncover a previously unappreciated mechanism by which the ECM impacts the mechanical reinforcement of interendothelial junctions.

BRAF increases endothelial cell stiffness through reorganization of the actin cytoskeleton

The dynamics of the actin cytoskeleton and its connection to endothelial cell-cell junctions determine the barrier function of endothelial cells. The proper regulation of barrier opening/closing is necessary for the normal function of vessels, and its dysregulation can result in chronic and acute inflammation leading to edema formation. By using atomic force microscopy, we show here that thrombin-induced permeability of human umbilical vein endothelial cells, associated with actin stress fiber formation, stiffens the cell center. The depletion of the MEK/ERK kinase BRAF reduces thrombin-induced permeability prevents stress fiber formation and cell stiffening. The peripheral actin ring becomes stabilized by phosphorylated myosin light chain, while cofilin is excluded from the cell periphery. All these changes can be reverted by the inhibition of ROCK, but not of the MEK/ERK module. We propose that the balance between the binding of cofilin and myosin to F-actin in the cell periphery, which is regulated by the activity of ROCK, determines the local dynamics of actin reorganization, ultimately driving or preventing stress fiber formation.

Respiratory syncytial virus disrupts the airway epithelial barrier by decreasing cortactin and destabilizing F-actin

Respiratory syncytial virus (RSV) infection is the leading cause of acute lower respiratory tract infection in young children worldwide. Our group recently revealed that RSV infection disrupts the airway epithelial barrier in vitro and in vivo. However, the underlying molecular pathways were still elusive. Here, we report the critical roles of the filamentous actin (F-actin) network and actin-binding protein cortactin in RSV infection. We found that RSV infection causes F-actin depolymerization in 16HBE cells, and that stabilizing the F-actin network in infected cells reverses the epithelial barrier disruption. RSV infection also leads to significantly decreased cortactin in vitro and in vivo. Cortactin-knockout 16HBE cells presented barrier dysfunction, whereas overexpression of cortactin protected the epithelial barrier against RSV. The activity of Rap1 (which has Rap1A and Rap1B forms), one downstream target of cortactin, declined after RSV infection as well as in cortactin-knockout cells. Moreover, activating Rap1 attenuated RSV-induced epithelial barrier disruption. Our study proposes a key mechanism in which RSV disrupts the airway epithelial barrier via attenuating cortactin expression and destabilizing the F-actin network. The identified pathways will provide new targets for therapeutic intervention toward RSV-related disease. This article has an associated First Person interview with the first author of the paper.

In vivo dissection of Rhoa function in vascular development using zebrafish

The small monomeric GTPase RHOA acts as a master regulator of signal transduction cascades by activating effectors of cellular signaling, including the Rho-associated protein kinases ROCK1/2. Previous in vitro cell culture studies suggest that RHOA can regulate many critical aspects of vascular endothelial cell (EC) biology, including focal adhesion, stress fiber formation, and angiogenesis. However, the specific in vivo roles of RHOA during vascular development and homeostasis are still not well understood. In this study, we examine the in vivo functions of RHOA in regulating vascular development and integrity in zebrafish. We use zebrafish RHOA-ortholog (rhoaa) mutants, transgenic embryos expressing wild type, dominant negative, or constitutively active forms of rhoaa in ECs, pharmacological inhibitors of RHOA and ROCK1/2, and Rock1 and Rock2a/b dgRNP-injected zebrafish embryos to study the in vivo consequences of RHOA gain- and loss-of-function in the vascular endothelium. Our findings document roles for RHOA in vascular integrity, developmental angiogenesis, and vascular morphogenesis in vivo, showing that either too much or too little RHOA activity leads to vascular dysfunction.

Longitudinal genomic alternations and clonal dynamics analysis of primary malignant melanoma of the esophagus

Primary malignant melanoma of the esophagus (PMME) is a rare gastrointestinal melanoma with a high rate of recurrence and metastasis. The standard of care for PMME has not been established yet due to a lack of understanding of its clinical and molecular pathogenesis. Thus, we performed genomic profiling on a recurrent PMME case to seek novel opportunities for the management of this rare disease. Between 2013 and 2016, 6 tissue samples including 3 from the primary tumors, 2 from the relapsed tumors, and 1 from a normal control were collected from a patient diagnosed with PMME and were subjected to whole-exome sequencing to track the dynamic genetic changes. Additionally, we also analyzed a cohort of 398 samples obtained from the TCGA skin cutanesous melanoma (TCGA-SKCM) dataset to assess the frequency and determine the clinical implications of genomic events found in the presented study. ARHGAP35 (p.L1022M) was the only mutation shared across temporal PMME lesions. The PMME samples showed higher levels of genetic instability and intra-tumor heterogeneity. They also shared several concordant copy number variations (CNV). All lesions were concordant with the evolution trajectory, and shrinkage of the founding clone caused the subclonal population to become dominant in PT1c, which was likely the reason behind metastatic seeding. ARHGAP35 mutations were found in 6% of the TCGA-SKCM cohort samples. The presence of the mutations was associated with poor progression-free survival (PFS) by both univariate and multivariate Cox regression analyses. Our study showed that the primary tumor clone disseminates earlier in PMME. This highlights the need to understand the mechanism involved in the early PMME recurrence to optimize treatment.

Canthin-6-one (CO) from Picrasma quassioides (D.Don) Benn. ameliorates lipopolysaccharide (LPS)-induced astrocyte activation and associated brain endothelial disruption

BACKGROUND

Canthin-6-one (CO) is an active ingredient found in Picrasma quassioides (D.Don) Benn. (PQ) that displays various biological activities including anti-inflammatory properties. Several studies reported PQ displayed neuroprotective activities, but its effects on astrocytes have not yet been investigated. Astrocytes are crucial regulators of neuroinflammatory responses under pathological conditions in the central nervous system (CNS). Proinflammatory astrocytes can induce the blood-brain barrier (BBB) breakdown, which plays a key role in the progression of neurodegenerative disorder (ND).

PURPOSE

This study aims to investigate the anti-neuroinflammatory effects of CO in LPS-induced astrocyte activation and its underlying mechanisms in protecting the blood-brain barrier (BBB) in vitro.

METHODS

Mouse astrocytes (C8-D1A) were activated with lipopolysaccharide (LPS) with or without CO pretreatment. Effects of CO on astrocyte cell viability, secretions of tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6), interleukin-1β (IL-1β) and nitric oxide (NO) were determined. Intracellular transcriptions and translations of proinflammatory mediators, molecular signaling, [Ca²⁺] and the levels of reactive oxygen species (ROS) were evaluated by RT-PCR, western blotting, and flow cytometry, respectively. Astrocyte-conditioned medium (ACM) was further prepared for incubating endothelial monolayer (bEnd.3) grown on transwell. Endothelial disruptions were evaluated by transendothelial electrical resistance (TEER), FITC-dextran permeability and monocyte adhesion assays. Endothelial tight junctions (TJs) and molecular signaling pathways were evaluated by immunofluorescence staining and western blotting.

RESULTS

CO attenuated LPS-induced expression of astrocytic proinflammatory mediators (TNF-α, IL-1β, IL-6, NO) and inhibited deleterious molecular activities including inducible nitric oxide synthase (iNOS), p-NFκB and p-STAT3 in astrocytes. Incubation of ACM collected from CO-treated astrocytes significantly ameliorated endothelial disruptions, reduced expressions of endothelial cytokine receptors (IL-6R, gp130 (IL-6RB), TNFR and IL-1R), suppressed proinflammatory pathways, MAPKs (p-AKT, p-MEK, p-ERK, p-p38, p-JNK) and p-STAT3, restored endothelial stabilizing pathways (p-Rac 1) and upregulated beneficial endothelial nitric oxide synthase (eNOS).

CONCLUSION

Our study demonstrates for the first time CO exhibited potent protective effects against astrocyte-mediated proinflammatory responses and associated endothelial barrier disruptions.

Vascular Leakage Prevention by Roundabout 4 under Pathological Conditions

Vascular permeability is regulated mainly by the endothelial barrier and controls vascular homeostasis, proper vessel development, and immune cell trafficking. Several molecules are involved in regulating endothelial barrier function. Roundabout 4 (Robo4) is a single-pass transmembrane protein that is specifically expressed in vascular endothelial cells. Robo4 is an important regulator of vascular leakage and angiogenesis, especially under pathological conditions. The role of Robo4 in preventing vascular leakage has been studied in various disease models, including animal models of retinopathy, tumors, diabetes, and endotoxemia. The involvement of Robo4 in vascular endothelial growth factor and inflammation-mediated signaling pathways has been well studied, and recent evidence suggests that Robo4 modulates endothelial barrier function via distinct mechanisms. In this review, we discuss the role of Robo4 in endothelial barrier function and the underlying molecular mechanisms.

Mechanisms Governing Anaphylaxis: Inflammatory Cells, Mediators, Endothelial Gap Junctions and Beyond

Anaphylaxis is a severe, acute, life-threatening multisystem allergic reaction resulting from the release of a plethora of mediators from mast cells culminating in serious respiratory, cardiovascular and mucocutaneous manifestations that can be fatal. Medications, foods, latex, exercise, hormones (progesterone), and clonal mast cell disorders may be responsible. More recently, novel syndromes such as delayed reactions to red meat and hereditary alpha tryptasemia have been described. Anaphylaxis manifests as sudden onset urticaria, pruritus, flushing, erythema, angioedema (lips, tongue, airways, periphery), myocardial dysfunction (hypovolemia, distributive or mixed shock and arrhythmias), rhinitis, wheezing and stridor. Vomiting, diarrhea, scrotal edema, uterine cramps, vaginal bleeding, urinary incontinence, dizziness, seizures, confusion, and syncope may occur. The traditional (or classical) pathway is mediated via T cells, Th2 cytokines (such as IL-4 and 5), B cell production of IgE and subsequent crosslinking of the high affinity IgE receptor (FcεRI) on mast cells and basophils by IgE-antigen complexes, culminating in mast cell and basophil degranulation. Degranulation results in the release of preformed mediators (histamine, heparin, tryptase, chymase, carboxypeptidase, cathepsin G and tumor necrosis factor alpha (TNF-α), and of de novo synthesized ones such as lipid mediators (cysteinyl leukotrienes), platelet activating factor (PAF), cytokines and growth factors such as vascular endothelial growth factor (VEGF). Of these, histamine, tryptase, cathepsin G, TNF-α, LTC₄, PAF and VEGF can increase vascular permeability. Recent data suggest that mast cell-derived histamine and PAF can activate nitric oxide production from endothelium and set into motion a signaling cascade that leads to dilatation of blood vessels and dysfunction of the endothelial barrier. The latter, characterized by the opening of adherens junctions, leads to increased capillary permeability and fluid extravasation. These changes contribute to airway edema, hypovolemia, and distributive shock, with potentially fatal consequences. In this review, besides mechanisms (endotypes) underlying IgE-mediated anaphylaxis, we also provide a brief overview of IgG-, complement-, contact system-, cytokine- and mast cell-mediated reactions that can result in phenotypes resembling IgE-mediated anaphylaxis. Such classifications can lead the way to precision medicine approaches to the management of this complex disease.

Primaquine Diphosphate, a Known Antimalarial Drug, Blocks Vascular Leakage Acting Through Junction Stabilization

Endothelial barrier integrity is important for vascular homeostasis, and hyperpermeability participates in the progression of many pathological states, such as diabetic retinopathy, ischemic stroke, chronic bowel disease, and inflammatory disease. Here, using drug repositioning, we discovered that primaquine diphosphate (PD), previously known as an antimalarial drug, was a potential blocker of vascular leakage. PD inhibited the linear pattern of vascular endothelial growth factors (VEGF)-induced disruption at the cell boundaries, blocked the formation of VEGF-induced actin stress fibers, and stabilized the cortactin actin rings in endothelial cells. PD significantly reduced leakage in the Miles assay and mouse model of streptozotocin (STZ)-induced diabetic retinopathy. Targeted prediction programs and deubiquitinating enzyme activity assays identified a potential mechanism of action for PD and demonstrated that this operates via ubiquitin specific protease 1 (USP1). USP1 inhibition demonstrated a conserved barrier function by inhibiting VEGF-induced leakage in endothelial permeability assays. Taken together, these findings suggest that PD could be used as a novel drug for vascular leakage by maintaining endothelial integrity.

Sodium Glucose Co-Transporter 2 Inhibitors Ameliorate Endothelium Barrier Dysfunction Induced by Cyclic Stretch through Inhibition of Reactive Oxygen Species

SGLT-2i's exert direct anti-inflammatory and anti-oxidative effects on resting endothelial cells. However, endothelial cells are constantly exposed to mechanical forces such as cyclic stretch. Enhanced stretch increases the production of reactive oxygen species (ROS) and thereby impairs endothelial barrier function. We hypothesized that the SGLT-2i's empagliflozin (EMPA), dapagliflozin (DAPA) and canagliflozin (CANA) exert an anti-oxidative effect and alleviate cyclic stretch-induced endothelial permeability in human coronary artery endothelial cells (HCAECs). HCAECs were pre-incubated with one of the SGLT-2i's (1 µM EMPA, 1 µM DAPA and 3 µM CANA) for 2 h, followed by 10% stretch for 24 h. HCAECs exposed to 5% stretch were considered as control. Involvement of ROS was measured using N-acetyl-l-cysteine (NAC). The sodium-hydrogen exchanger 1 (NHE1) and NADPH oxidases (NOXs) were inhibited by cariporide, or GKT136901, respectively. Cell permeability and ROS were investigated by fluorescence intensity imaging. Cell permeability and ROS production were increased by 10% stretch; EMPA, DAPA and CANA decreased this effect significantly. Cariporide and GKT136901 inhibited stretch-induced ROS production but neither of them further reduced ROS production when combined with EMPA. SGLT-2i's improve the barrier dysfunction of HCAECs under enhanced stretch and this effect might be mediated through scavenging of ROS. Anti-oxidative effect of SGLT-2i's might be partially mediated by inhibition of NHE1 and NOXs.

Regulatory T Cells Prevent Neutrophilic Infiltration of Skin during Contact Hypersensitivity Reactions by Strengthening the Endothelial Barrier

The healing phase of contact hypersensitivity reactions is critically dependent on regulatory T cells (Tregs), but even the early inflammatory phase, that is, 6-24 hours after induction of a contact hypersensitivity reaction, is susceptible to Treg-mediated suppression. To investigate the underlying mechanisms, we injected Tregs before the challenge and analyzed the skin-infiltrating cells as early as 6 hours later. Early on, we found mainly neutrophils in the challenged skin, but only a few T cells. This influx of neutrophils was blocked by the injection of Tregs, indicating that they were able to prevent the first wave of leukocytes, which are responsible for starting an immune reaction. As an underlying mechanism, we identified that Tregs can tighten endothelial junctions by inducing intracellular cAMP, leading to protein kinase A-RhoA‒dependent signaling. This eventually reorganizes endothelial junction proteins, such as Notch3, Nectin 2, Filamin B, and VE-cadherin, all of which contribute to the tightening of the endothelial barrier. In summary, Tregs prevent the leakage of proinflammatory cells from and into the tissue, which establishes a mechanism to downregulate immune reactions.

Permeability of the Endothelial Barrier: Identifying and Reconciling Controversies

Leakage from blood vessels into tissues is governed by mechanisms that control endothelial barrier function to maintain homeostasis. Dysregulated endothelial permeability contributes to many conditions and can influence disease morbidity and treatment. Diverse approaches used to study endothelial permeability have yielded a wealth of valuable insights. Yet, ongoing questions, technical challenges, and unresolved controversies relating to the mechanisms and relative contributions of barrier regulation, transendothelial sieving, and transport of fluid, solutes, and particulates complicate interpretations in the context of vascular physiology and pathophysiology. Here, we describe recent in vivo findings and other advances in understanding endothelial barrier function with the goal of identifying and reconciling controversies over cellular and molecular processes that regulate the vascular barrier in health and disease.

Irreversible disruption of the cytoskeleton as induced by non-cytotoxic exposure to titanium dioxide nanoparticles in lung epithelial cells

Exposure to TiO₂ NPs induces several cellular alterations after NPs uptake including disruption of cytoskeleton that is crucial for lung physiology but is not considered as a footprint of cell damage. We aimed to investigate cytoskeleton disturbances and the impact on cell migration induced by an acute TiO₂ NPs exposure (24 h) and the recovery capability after 6 days of NPs-free treatment, which allowed investigating if cytoskeleton damage was reversible. Exposure to TiO₂ NPs (10 μg/cm²) for 24 h induced a decrease 20.2% and 25.1% in tubulin and actin polymerization. Exposure to TiO₂ NPs (10 μg/cm²) for 24 h followed by 6 days of NPs-free had a decrease of 26.6% and 21.3% in tubulin and actin polymerization, respectively. The sustained exposure for 7 days to 1 μg/cm² and 10 μg/cm² induced a decrease of 22.4% and 30.7% of tubulin polymerization respectively, and 28.7% and 46.2% in actin polymerization. In addition, 24 h followed 6 days of NPs-free exposure of TiO₂ NPs (1 μg/cm² and 10 μg/cm²) decreased cell migration 40.7% and 59.2%, respectively. Cells exposed (10 μg/cm²) for 7 days had a decrease of 65.5% in cell migration. Ki67, protein surfactant B (SFTPB) and matrix metalloprotease 2 (MMP2) were analyzed as genes related to lung epithelial function. The results showed a 20% of Ki67 upregulation in cells exposed for 24 h to 10 μg/cm² TiO₂ NPs while a downregulation of 20% and 25.8% in cells exposed to 1 μg/cm² and 10 μg/cm² for 24 h followed by 6 days of NPs-free exposure. Exposure to 1 μg/cm² and 10 μg/cm² for 24 h and 7 days upregulates SFTPB expression in 53% and 59% respectively, MMP2 expression remain unchanged. In conclusion, exposure of TiO₂ NPs affected cytoskeleton of lung epithelial cells irreversibly but this damage was not cumulative.

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.

Small GTPase Rap1A/B Is Required for Lymphatic Development and Adrenomedullin-Induced Stabilization of Lymphatic Endothelial Junctions

Objective- Maintenance of lymphatic permeability is essential for normal lymphatic function during adulthood, but the precise signaling pathways that control lymphatic junctions during development are not fully elucidated. The G_s-coupled AM (adrenomedullin) signaling pathway is required for embryonic lymphangiogenesis and the maintenance of lymphatic junctions during adulthood. Thus, we sought to elucidate the downstream effectors mediating junctional stabilization in lymphatic endothelial cells. Approach and Results- We knocked-down both Rap1A and Rap1B isoforms in human neonatal dermal lymphatic cells (human lymphatic endothelial cells) and genetically deleted the mRap1 gene in lymphatic endothelial cells by producing 2 independent, conditional Rap1a/b knockout mouse lines. Rap1A/B knockdown caused disrupted junctional formation with hyperpermeability and impaired AM-induced lymphatic junctional tightening, as well as rescue of histamine-induced junctional disruption. Less than 60% of lymphatic- Rap1a/b knockout embryos survived to E13.5 exhibiting interstitial edema, blood-filled lymphatics, disrupted lymphovenous valves, and defective lymphangiogenesis. Consistently, inducible lymphatic- Rap1a/b deletion in adult animals prevented AM-rescue of histamine-induced lymphatic leakage and dilation. Conclusions- Rap1 (Ras-related protein) serves as the dominant effector downstream of AM to stabilize lymphatic junctions. Rap1 is required for maintaining lymphatic permeability and driving normal lymphatic development.

Structural Features of Actin Cytoskeleton Required for Endotheliocyte Barrier Function

Cytoplasmic actin structures are essential components of the eukaryotic cytoskeleton. According to the classic concepts, actin structures perform contractile and motor functions, ensuring the possibility of cell shape changes during cell spreading, polarization, and movement both in vitro and in vivo, from the early embryogenesis stages and throughout the life of a multicellular organism. Intracellular organization of actin structures, their biochemical composition, and dynamic properties play a key role in the realization of specific cellular and tissue functions and vary in different cell types. This paper is a review of recent studies on the organization and properties of actin structures in endotheliocytes, interaction of these structures with other cytoskeletal components and elements involved in cell adhesion, as well as their role in the functional activity of endothelial cells.

Gliotoxin Induces Cofilin Phosphorylation to Promote Actin Cytoskeleton Dynamics and Internalization of Aspergillus fumigatus Into Type II Human Pneumocyte Cells

Aspergillus fumigatus is able to internalize into lung epithelial cells to escape from immune attack for further dissemination. We previously reported that gliotoxin, a major mycotoxin of A. fumigatus, promotes this internalization; however, the mechanism remained unclear. Here, we report that gliotoxin is able to induce cofilin phosphorylation in A549 type II human pneumocytes. Either too high or too low a level of cofilin phosphorylation blocked the gliotoxin-induced actin cytoskeleton rearrangement and A. fumigatus internalization. LIM domain kinase 1 (LIMK1) and its upstream small GTPases (Cdc42 and RhoA, but not Rac1) predominantly mediated the gliotoxin-induced cofilin phosphorylation and A. fumigatus internalization. Simultaneously, gliotoxin significantly stimulated an increase in cAMP; however, adding an antagonist of PKA did not block gliotoxin-induced A. fumigatus internalization. In vivo, exogenous gliotoxin helped gliotoxin synthesis deficient strain gliPΔ invade into the lung tissue and the lung fungal burden increased markedly in immunosuppressed mice. In conclusion, these data revealed a novel role of gliotoxin in inducing cofilin phosphorylation mostly through the Cdc42/RhoA-LIMK1 signaling pathway to promote actin cytoskeleton rearrangement and internalization of A. fumigatus into type II human pneumocytes.

Putting VE-cadherin into JAIL for junction remodeling

Junction dynamics of endothelial cells are based on the integration of signal transduction, cytoskeletal remodeling and contraction, which are necessary for the formation and maintenance of monolayer integrity, but also enable repair and regeneration. The VE-cadherin–catenin complex forms the molecular basis of the adherence junctions and cooperates closely with actin filaments. Several groups have recently described small actin-driven protrusions at the cell junctions that are controlled by the Arp2/3 complex, contributing to cell junction regulation. We identified these protrusions as the driving force for VE-cadherin dynamics, as they directly induce new VE-cadherin-mediated adhesion sites, and have accordingly referred to these structures as junction-associated intermittent lamellipodia (JAIL). JAIL extend over only a few microns and thus provide the basis for a subcellular regulation of adhesion. The local (subcellular) VE-cadherin concentration and JAIL formation are directly interdependent, which enables autoregulation. Therefore, this mechanism can contribute a subcellularly regulated adaptation of cell contact dynamics, and is therefore of great importance for monolayer integrity and relative cell migration during wound healing and angiogenesis, as well as for inflammatory responses. In this Review, we discuss the mechanisms and functions underlying these actin-driven protrusions and consider their contribution to the dynamic regulation of endothelial cell junctions.

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.

Mind the gap: mechanisms regulating the endothelial barrier

The endothelial barrier consists of intercellular contacts localized in the cleft between endothelial cells, which is covered by the glycocalyx in a sievelike manner. Both types of barrier-forming junctions, i.e. the adherens junction (AJ) serving mechanical anchorage and mechanotransduction and the tight junction (TJ) sealing the intercellular space to limit paracellular permeability, are tethered to the actin cytoskeleton. Under resting conditions, the endothelium thereby builds a selective layer controlling the exchange of fluid and solutes with the surrounding tissue. However, in the situation of an inflammatory response such as in anaphylaxis or sepsis intercellular contacts disintegrate in post-capillary venules leading to intercellular gap formation. The resulting oedema can cause shock and multi-organ failure. Therefore, maintenance as well as coordinated opening and closure of interendothelial junctions is tightly regulated. The two principle underlying mechanisms comprise spatiotemporal activity control of the small GTPases Rac1 and RhoA and the balance of the phosphorylation state of AJ proteins. In the resting state, junctional Rac1 and RhoA activity is enhanced by junctional components, actin-binding proteins, cAMP signalling and extracellular cues such as sphingosine-1-phosphate (S1P) and angiopoietin-1 (Ang-1). In addition, phosphorylation of AJ components is prevented by junction-associated phosphatases including vascular endothelial protein tyrosine phosphatase (VE-PTP). In contrast, inflammatory mediators inhibiting cAMP/Rac1 signalling cause strong activation of RhoA and induce AJ phosphorylation finally leading to endocytosis and cleavage of VE-cadherin. This results in dissolution of TJs the outcome of which is endothelial barrier breakdown.

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.

Cytotoxic necrotizing factor 1 promotes prostate cancer progression through activating the Cdc42-PAK1 axis

Uropathogenic Escherichia coli (UPEC) is the leading cause of urinary tract infections and plays a role in prostatic carcinogenesis and prostate cancer (PCa) progression. However, the mechanisms through which UPEC promotes PCa development and progression are unclear. Cytotoxic necrotizing factor 1 (CNF1) is one of the most important UPEC toxins and its role in PCa progression has never been studied. We found that UPEC-secreted CNF1 promoted the migration and invasion of PCa cells and PCa metastasis. In vitro studies showed that CNF1 promotes pro-migratory and pro-invasive activity through entering PCa cells and activating Cdc42, which subsequently induced PAK1 phosphorylation and up-regulation of MMP-9 expression. CNF1 also promoted pulmonary metastasis in a xenograft mouse model through these mechanisms. PAK1 phosphorylation correlated with advanced grades of PCa in human clinical PCa tissues. These results suggest that CNF1 derived from UPEC plays an important role in PCa progression through activating a Cdc42-PAK1 signal axis and up-regulating the expression of MMP-9. Therefore, surveillance for and treatment of cnf1-carrying UPEC strains may diminish PCa progression and thus have an important clinical therapeutic impact. Copyright © 2017 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Parallels between single cell migration and barrier formation: The case of RhoB and Rac1 trafficking

The appearance of multicellularity implied the adaptation of signaling networks required for unicellular life to new functions arising in this remarkable evolutionary transition. A hallmark of multicellular organisms is the formation of cellular barriers that compartmentalize spaces and functions. Here we discuss recent findings concerning the role of RhoB in the negative control of Rac1 trafficking from endosomes to the cell border, in order to induce membrane extensions to restore endothelial barrier function after acute contraction. This role closely resembles that proposed for RhoB in controlling single cell migration through Rac1, which has also been observed in cancer cell invasion. We highlight these similarities as a signaling paradigm that shows that endothelial barrier integrity is controlled not only by the formation of cell-cell junctions, but also by a balance between ancestral mechanisms of cell spreading and contraction conserved from unicellular organisms and orchestrated by Rho GTPases.