ROCK mediates thrombin's endothelial barrier dysfunction

Innovative and potential treatments for fungal central nervous system infections

Fungal infections of the central nervous system (FI-CNS) are a problematic and important medical challenge considering that those most affected are immunocompromised. Individuals with systemic cryptococcosis (67-84%), candidiasis (3-64%), blastomycosis (40%), coccidioidomycosis (25%), histoplasmosis (5-20%), mucormycosis (12%), and aspergillosis (4-6%) are highly susceptible to develop CNS involvement, which often results in high mortality (15-100%) depending on the mycosis and the affected immunosuppressed population. Current antifungal drugs are limited, prone to resistance, present host toxicity, and show reduced brain penetration, making FI-CNS very difficult to treat. Given these limitations and the rise in FI-CNS, there is a need for innovative strategies for therapeutic development and treatments to manage FI-CNS in at-risk populations. Here, we discuss standards of care, antifungal drug candidates, and novel molecular targets in the blood-brain barrier, which is a protective structure that regulates movement of particles in and out of the brain, to prevent and combat FI-CNS.

Edema and lymphatic clearance: molecular mechanisms and ongoing challenges

Resolution of edema remains a significant clinical challenge. Conditions such as traumatic shock, sepsis, or diabetes often involve microvascular hyperpermeability, which leads to tissue and organ dysfunction. Lymphatic insufficiency due to genetic causes, surgical removal of lymph nodes, or infections, leads to varying degrees of tissue swelling that impair mobility and immune defenses. Treatment options are limited to management of edema as there are no specific therapeutics that have demonstrated significant success for ameliorating microvascular leakage or impaired lymphatic function. This review examines current knowledge about the physiological, cellular, and molecular mechanisms that control microvascular permeability and lymphatic clearance, the respective processes for interstitial fluid formation and removal. Clinical conditions featuring edema, along with potential future directions are discussed.

Beyond epithelial damage: vascular and endothelial contributions to idiopathic pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a progressive scarring disease of the lung with poor survival. The incidence and mortality of IPF are rising, but treatment remains limited. Currently, two drugs can slow the scarring process but often at the expense of intolerable side effects, and without substantially changing overall survival. A better understanding of mechanisms underlying IPF is likely to lead to improved therapies. The current paradigm proposes that repetitive alveolar epithelial injury from noxious stimuli in a genetically primed individual is followed by abnormal wound healing, including aberrant activity of extracellular matrix-secreting cells, with resultant tissue fibrosis and parenchymal damage. However, this may underplay the importance of the vascular contribution to fibrogenesis. The lungs receive 100% of the cardiac output, and vascular abnormalities in IPF include (a) heterogeneous vessel formation throughout fibrotic lung, including the development of abnormal dilated vessels and anastomoses; (b) abnormal spatially distributed populations of endothelial cells (ECs); (c) dysregulation of endothelial protective pathways such as prostacyclin signaling; and (d) an increased frequency of common vascular and metabolic comorbidities. Here, we propose that vascular and EC abnormalities are both causal and consequential in the pathobiology of IPF and that fuller evaluation of dysregulated pathways may lead to effective therapies and a cure for this devastating disease.

Glucuronoxylomannan intranasal challenge prior to Cryptococcus neoformans pulmonary infection enhances cerebral cryptococcosis in rodents

The encapsulated fungus Cryptococcus neoformans is the most common cause of fungal meningitis, with the highest rate of disease in patients with AIDS or immunosuppression. This microbe enters the human body via inhalation of infectious particles. C. neoformans capsular polysaccharide, in which the major component is glucuronoxylomannan (GXM), extensively accumulates in tissues and compromises host immune responses. C. neoformans travels from the lungs to the bloodstream and crosses to the brain via transcytosis, paracytosis, or inside of phagocytes using a "Trojan horse" mechanism. The fungus causes life-threatening meningoencephalitis with high mortality rates. Hence, we investigated the impact of intranasal exogenous GXM administration on C. neoformans infection in C57BL/6 mice. GXM enhances cryptococcal pulmonary infection and facilitates fungal systemic dissemination and brain invasion. Pre-challenge of GXM results in detection of the polysaccharide in lungs, serum, and surprisingly brain, the latter likely reached through the nasal cavity. GXM significantly alters endothelial cell tight junction protein expression in vivo, suggesting significant implications for the C. neoformans mechanisms of brain invasion. Using a microtiter transwell system, we showed that GXM disrupts the trans-endothelial electrical resistance, weakening human brain endothelial cell monolayers co-cultured with pericytes, supportive cells of blood vessels/capillaries found in the blood-brain barrier (BBB) to promote C. neoformans BBB penetration. Our findings should be considered in the development of therapeutics to combat the devastating complications of cryptococcosis that results in an estimated ~200,000 deaths worldwide each year.

Molecular Mechanisms Controlling Lymphatic Endothelial Junction Integrity

The lymphatic system is essential for lipid absorption/transport from the digestive system, maintenance of tissue fluid and protein homeostasis, and immune surveillance. Despite recent progress toward understanding the cellular and molecular mechanisms underlying the formation of the lymphatic vascular system, the nature of lymphatic vessel abnormalities and disease in humans is complex and poorly understood. The mature lymphatic vasculature forms a hierarchical network in which lymphatic endothelial cells (LECs) are joined by functionally specialized cell-cell junctions to maintain the integrity of lymphatic vessels. Blind-ended and highly permeable lymphatic capillaries drain interstitial fluid via discontinuous, button-like LEC junctions, whereas collecting lymphatic vessels, surrounded by intact basement membranes and lymphatic smooth muscle cells, have continuous, zipper-like LEC junctions to transport lymph to the blood circulatory system without leakage. In this review, we discuss the recent advances in our understanding of the mechanisms by which lymphatic button- and zipper-like junctions play critical roles in lymphatic permeability and function in a tissue- and organ-specific manner, including lacteals of the small intestine. We also provide current knowledge related to key pathways and factors such as VEGF and RhoA/ROCK signaling that control lymphatic endothelial cell junctional integrity.

Biased signaling in platelet G-protein coupled receptors

Platelets are small megakaryocyte-derived, anucleate, disk-like structures that play an outsized role in human health and disease. Both a decrease in the number of platelets and a variety of platelet function disorders result in petechiae or bleeding that can be life threatening. Conversely, the inappropriate activation of platelets, within diseased blood vessels, remains the leading cause of death and morbidity by affecting heart attacks and stroke. The fine balance of the platelet state in healthy individuals is controlled by a number of receptor-mediated signaling pathways that allow the platelet to rapidly respond and maintain haemostasis. G-protein coupled receptors (GPCRs) are particularly important regulators of platelet function. Here we focus on the major platelet-expressed GPCRs and discuss the roles of downstream signaling pathways (e.g., different G-protein subtypes or β-arrestin) in regulating the different phases of the platelet activation. Further, we consider the potential for selectively targeting signaling pathways that may contribute to platelet responses in disease through development of biased agonists. Such selective targeting of GPCR-mediated signaling pathways by drugs, often referred to as biased signaling, holds promise in delivering therapeutic interventions that do not present significant side effects, especially in finely balanced physiological systems such as platelet activation in haemostasis.

Shear stimulation of FOXC1 and FOXC2 differentially regulates cytoskeletal activity during lymphatic valve maturation

Mutations in the transcription factor FOXC2 are predominately associated with lymphedema. Herein, we demonstrate a key role for related factor FOXC1, in addition to FOXC2, in regulating cytoskeletal activity in lymphatic valves. FOXC1 is induced by laminar, but not oscillatory, shear and inducible, endothelial-specific deletion impaired postnatal lymphatic valve maturation in mice. However, deletion of Foxc2 induced valve degeneration, which is exacerbated in Foxc1; Foxc2 mutants. FOXC1 knockdown (KD) in human lymphatic endothelial cells increased focal adhesions and actin stress fibers whereas FOXC2-KD increased focal adherens and disrupted cell junctions, mediated by increased ROCK activation. ROCK inhibition rescued cytoskeletal or junctional integrity changes induced by inactivation of FOXC1 and FOXC2 invitro and vivo respectively, but only ameliorated valve degeneration in Foxc2 mutants. These results identify both FOXC1 and FOXC2 as mediators of mechanotransduction in the postnatal lymphatic vasculature and posit cytoskeletal signaling as a therapeutic target in lymphatic pathologies.

TIMP1 preserves the blood-brain barrier through interacting with CD63/integrin 1 complex and regulating downstream FAK/RhoA signaling

Blood-brain barrier (BBB) breakdown and the associated microvascular hyperpermeability are hallmark features of several neurological disorders, including traumatic brain injury (TBI). However, there is no viable therapeutic strategy to rescue BBB function. Tissue inhibitor of metalloproteinase-1 (TIMP1) has been considered to be beneficial for vascular integrity, but the molecular mechanisms underlying the functions of TIMP1 remain elusive. Here, we report that TIMP1 executes a protective role on neuroprotective function via ameliorating BBB disruption in mice with experimental TBI. In human brain microvessel endothelial cells (HBMECs) exposed to hypoxia and inflammation injury, the recombinant TIMP1 (rTIMP1) treatment maintained integrity of junctional proteins and trans-endothelial tightness. Mechanistically, TIMP1 interacts with CD63/integrin β1 complex and activates downstream FAK signaling, leading to attenuation of RhoA activation and F-actin depolymerization for endothelial cells structure stabilization. Notably, these effects depend on CD63/integrin β1 complex, instead of the MMP-inhibitory function. Together, our results identified a novel MMP-independent function of TIMP1 in regulating endothelial barrier integrity. Therapeutic interventions targeting TIMP1 and its downstream signaling may be beneficial to protect BBB function following brain injury and neurological disorders.

The Rho Kinase Isoforms ROCK1 and ROCK2 Each Contribute to the Development of Experimental Pulmonary Fibrosis

Pulmonary fibrosis is thought to result from dysregulated wound repair after repetitive lung injury. Many cellular responses to injury involve rearrangements of the actin cytoskeleton mediated by the two isoforms of the Rho-associated coiled-coil-forming protein kinase (ROCK), ROCK1 and ROCK2. In addition, profibrotic mediators such as transforming growth factor-β, thrombin, and lysophosphatidic acid act through receptors that activate ROCK. Inhibition of ROCK activation may be a potent therapeutic strategy for human pulmonary fibrosis. Pharmacological inhibition of ROCK using nonselective ROCK inhibitors has been shown to prevent fibrosis in animal models; however, the specific roles of each ROCK isoform are poorly understood. Furthermore, the pleiotropic effects of this kinase have raised concerns about on-target adverse effects of ROCK inhibition such as hypotension. Selective inhibition of one isoform might be a better-tolerated strategy. In the present study, we used a genetic approach to determine the roles of ROCK1 and ROCK2 in a mouse model of bleomycin-induced pulmonary fibrosis. Using ROCK1- or ROCK2-haploinsufficient mice, we found that reduced expression of either ROCK1 or ROCK2 was sufficient to protect them from bleomycin-induced pulmonary fibrosis. In addition, we found that both isoforms contribute to the profibrotic responses of epithelial cells, endothelial cells, and fibroblasts. Interestingly, ROCK1- and ROCK2-haploinsufficient mice exhibited similar protection from bleomycin-induced vascular leak, myofibroblast differentiation, and fibrosis; however, ROCK1-haploinsufficient mice demonstrated greater attenuation of epithelial cell apoptosis. These findings suggest that selective inhibition of either ROCK isoform has the potential to be an effective therapeutic strategy for pulmonary fibrosis.

Stiff Substrates Enhance Endothelial Oxidative Stress in Response to Protein Kinase C Activation

Arterial stiffness, which increases with aging and hypertension, is an independent cardiovascular risk factor. While stiffer substrates are known to affect single endothelial cell morphology and migration, the effect of substrate stiffness on endothelial monolayer function is less understood. The objective of this study was to determine if substrate stiffness increased endothelial monolayer reactive oxygen species (ROS) in response to protein kinase C (PKC) activation and if this oxidative stress then impacted adherens junction integrity. Porcine aortic endothelial cells were cultured on varied stiffness polyacrylamide gels and treated with phorbol 12-myristate 13-acetate (PMA), which stimulates PKC and ROS without increasing actinomyosin contractility. PMA-treated endothelial cells on stiffer substrates increased ROS and adherens junction loss without increased contractility. ROS scavengers abrogated PMA effects on cell-cell junctions, with a more profound effect in cells on stiffer substrates. Finally, endothelial cells in aortae from elastin haploinsufficient mice (Eln+/-), which were stiffer than aortae from wild-type mice, showed decreased VE-cadherin colocalization with peripheral actin following PMA treatment. These data suggest that oxidative stress may be enhanced in endothelial cells in stiffer vessels, which could contribute to the association between arterial stiffness and cardiovascular disease.

Epac agonist improves barrier function in iPSC-derived endothelial colony forming cells for whole organ tissue engineering

Whole organ engineering paradigms typically involve repopulating acellular organ scaffolds with recipient-compatible cells, to generate a neo-organ that may provide key physiological functions. In the case of whole lung engineering, functionally endothelialized pulmonary vasculature is critical for establishing a fluid-tight barrier at the level of the alveolus, so that oxygen and carbon dioxide can be exchanged in the organ. We have previously developed a protocol to efficiently seed endothelial cells into the microvascular channels of decellularized lung scaffolds, but fully functional endothelial coverage, in terms of barrier function and resistance to thrombosis, was not achieved. In this study, we investigated whether various small molecules could favorably impact endothelial functionality after seeding into decellularized lung scaffolds. We demonstrated that the Epac-selective cAMP analog 8CPT-2Me-cAMP improves endothelial barrier function in repopulated lung scaffolds. When treated with the Epac agonist, barrier function of human umbilical vein endothelial cells (HUVECs) improved, and was maintained for at least three days, whereas the effect of other tested molecules lasted for only 5 h. Treatment with the Epac agonist re-organized actin structure, and appeared to increase the continuity of junction proteins such as VE-cadherin and ZO1. Blockade of actin polymerization abolished the effect of the Epac agonist on barrier function and actin reorganization, confirming a strong actin-mediated effect. Similarly, after treatment with Epac agonist, the barrier function in iPSC-derived endothelial colony forming cells (ECFCs) was increased and the enhanced barrier was maintained for at least 60 h. After culture in lung scaffolds for 5 days, iPSC-ECFCs maintained their phenotype by expressing CD31, eNOS, vWF, and VE-Cadherin. Treatment with the Epac agonist significantly improved the barrier function of iPSC-ECFC-repopulated lung for at least 6 h. Taken together, these findings demonstrated that Epac-selective 8CPT-2Me-cAMP activation enhanced vascular barrier in iPSC-ECFC-engineered lungs, and may be useful to improve endothelial functionality for whole organ tissue engineering.

Multiplexed, high-throughput measurements of cell contraction and endothelial barrier function

Vascular leakage, protein exudation, and edema formation are events commonly triggered by inflammation and facilitated by gaps that form between adjacent endothelial cells (ECs) of the vasculature. In such paracellular gap formation, the role of EC contraction is widely implicated, and even therapeutically targeted. However, related measurement approaches remain slow, tedious, and complex to perform. Here, we have developed a multiplexed, high-throughput screen to simultaneously quantify paracellular gaps, EC contractile forces, and to visualize F-actin stress fibers, and VE-cadherin. As proof-of-principle, we examined barrier-protective mechanisms of the Rho-associated kinase inhibitor, Y-27632, and the canonical agonist of the Tie2 receptor, Angiopoietin-1 (Angpt-1). Y-27632 reduced EC contraction and actin stress fiber formation, whereas Angpt-1 did not. Yet both agents reduced thrombin-, LPS-, and TNFα-induced paracellular gap formation. This unexpected result suggests that Angpt-1 can achieve barrier defense without reducing EC contraction, a mechanism that has not been previously described. This insight was enabled by the multiplex nature of the force-based platform. The high-throughput format we describe should accelerate both mechanistic studies and the screening of pharmacological modulators of endothelial barrier function.

Endothelial Protrusions in Junctional Integrity and Barrier Function

Endothelial cells of the microcirculation form a semi-permeable diffusion barrier between the blood and tissues. This permeability of the endothelium, particularly in the capillaries and postcapillary venules, is a normal physiological function needed for blood-tissue exchange in the microcirculation. During inflammation, microvascular permeability increases dramatically and can lead to tissue edema, which in turn can lead to dysfunction of tissues and organs. The molecular mechanisms that control the barrier function of endothelial cells have been under investigation for several decades and remain an important topic due to the potential for discovery of novel therapeutic strategies to reduce edema. This review highlights current knowledge of the cellular and molecular mechanisms that lead to endothelial hyperpermeability during inflammatory conditions associated with injury and disease. This includes a discussion of recent findings demonstrating temporal protrusions by endothelial cells that may contribute to intercellular junction integrity between endothelial cells and affect the diffusion distance for solutes via the paracellular pathway.

Three-dimensional forces exerted by leukocytes and vascular endothelial cells dynamically facilitate diapedesis

Leukocyte transmigration across vessel walls is a critical step in the innate immune response. Upon their activation and firm adhesion to vascular endothelial cells (VECs), leukocytes preferentially extravasate across junctional gaps in the endothelial monolayer (paracellular diapedesis). It has been hypothesized that VECs facilitate paracellular diapedesis by opening their cell-cell junctions in response to the presence of an adhering leukocyte. However, it is unclear how leukocytes interact mechanically with VECs to open the VEC junctions and migrate across the endothelium. In this study, we measured the spatial and temporal evolution of the 3D traction stresses generated by the leukocytes and VECs to elucidate the sequence of mechanical events involved in paracellular diapedesis. Our measurements suggest that the contractile stresses exerted by the leukocytes and the VECs can separately perturb the junctional tensions of VECs to result in the opening of gaps before the initiation of leukocyte transmigration. Decoupling the stresses exerted by the transmigrating leukocytes and the VECs reveals that the leukocytes actively contract the VECs to open a junctional gap and then push themselves across the gap by generating strong stresses that push into the matrix. In addition, we found that diapedesis is facilitated when the tension fluctuations in the VEC monolayer were increased by proinflammatory thrombin treatment. Our findings demonstrate that diapedesis can be mechanically regulated by the transmigrating leukocytes and by proinflammatory signals that increase VEC contractility.

Dasatinib inhibits actin fiber reorganization and promotes endothelial cell permeability through RhoA-ROCK pathway

Treatment with dasatinib, a tyrosine kinase inhibitor, is associated with edema, pleural effusion, and pulmonary edema. We investigated the effect of dasatinib on the barrier function of human microvascular endothelial cells‐1 (HMEC‐1) in vitro and in vivo. The permeability of HMEC‐1 to fluorescein isothiocyante (FITC)‐dextran increased in Transwell chambers within 5 min following the addition of therapeutic concentrations of dasatinib. The change in permeability was associated with increased activation of RhoA GTPase and its effector Rho‐associated coiled‐coil kinase 1(ROCK1). RhoA inhibitor C3 transferase almost completely inhibited dasatinib‐induced increase in permeability. Under similar conditions, imatinib had no effect on permeability or activation of RhoA. Since integrin‐induced cell spreading suppresses RhoA activation, we examined the effect of dasatinib on cell spreading on fibronectin substrate. Dasatinib impaired endothelial cell spreading in a concentration‐dependent manner and induced disorganization of actin fibers. Tyrosine kinases play an essential role in transmitting signals from integrins to RhoA and we examined tyrosine phosphorylation of several cytoskeletal proteins. Dasatinib markedly inhibited tyrosine phosphorylation of p130 Crk‐associated substrate (p130cas), paxillin and vinculin. These results suggest that the inhibition of tyrosine phosphorylation of the focal adhesion plaque components by dasatinib may alter the assembly of actin fibers resulting in the activation of RhoA/ROCK pathway. Consistent with these findings, dasatinib‐induced increase in the permeability was blocked by ROCK inhibitor y27632. In vivo administration of y27632, significantly inhibited the dasatinib‐induced extravasation of Evans blue in mice and dasatinib‐induced increase in microvascular permeability was attenuated in ROCK1‐deficient mice. These findings suggest that ROCK inhibitors could serve as therapeutic modalities to ameliorate the dasatinib‐induced pulmonary changes.

Low tidal volume ventilation preconditioning ameliorates lipopolysaccharide-induced acute lung injury in rats

BACKGROUND

Effects of low tidal volume (LTV) ventilation preconditioning in endotoxin-induced acute lung injury (ALI) have not been studied. We investigated the effect of LTV ventilation pre-treatment on ALI induced by lipopolysaccharide (LPS) in rats.

METHODS

Male Sprague-Dawley rats were assigned to four groups (n = 8 each): (1) sham rats injected (i.p.) with 0.9% (physiologic) saline; sham rats pre-treated with tidal volume 6 ml/kg ventilation for 1 h followed by injection (i.p.) of physiologic saline (mechanical ventilation; MV-saline group); (2) LPS group (rats injected with LPS (i.p.); rats pre-treated with tidal volume 6 ml/kg ventilation for 1 h before injection (i.p.) with LPS (MV-LPS group). Animals were observed for 6 h. ALI extent was evaluated by lung wet-to-dry ratio, Evans Blue Dye extravasation, and histologic examination. We measured levels of tumor necrosis factor (TNF)-α, interleukin (IL)-1β, and IL-6. Apoptotic index (AI) and the expression of pulmonary RhoA, ROCK2 mRNA, and ROCK1 protein in lung alveolar cells was determined.

RESULTS

Lipopolysaccharide caused severe ALI, as evidenced by increases in ALI extent, impairment of pulmonary functions, and increases in pulmonary levels of TNF-α, IL-1β, IL-6, and AI. LTV ventilation preconditioning mitigated LPS-induced increases in release of pulmonary pro-inflammatory cytokines and AI of alveolar cells. Expression of pulmonary RhoA, ROCK2 mRNA, and ROCK1 protein was upregulated by LPS and reduced by LTV ventilation pre-treatment.

CONCLUSION

Low tidal volume ventilation preconditioning can attenuate release of pulmonary pro-inflammatory cytokines and decrease the AI induced by severe sepsis. Early protection seems to be mediated partly through inhibition of activation of a Rho pathway.

Wnt5A/Ryk signaling critically affects barrier function in human vascular endothelial cells

Satisfactory therapeutic strategies for septic shock are still missing. Previously we found elevated levels of Wnt5A in patients with severe sepsis and septic shock. Wnt5A is released by activated macrophages but knowledge of its effects in the vascular system remains scant. Here we investigate the response of human coronary artery endothelial cells (HCAEC) to Wnt5A. We used a genome-wide differential expression approach to define novel targets regulated by Wnt5A. Gene ontology analysis of expression profiles revealed clusters of genes involved in actin cytoskeleton remodeling as the predominant targets of Wnt5A. Wnt5A targeted Rho-associated protein serine/threonine kinase (ROCK), leading to phosphorylation of LIM kinase-2 (LIMK2) and inactivation of the actin depolymerization factor cofilin-1 (CFL1). Functional experiments recording cytoskeletal rearrangements in living cells showed that Wnt5A enhanced stress fiber formation as a consequence of reduced actin depolymerization. The antagonist Wnt inhibitory factor 1 (WIF1) that specifically interferes with the WIF domain of Ryk receptors prevented actin polymerization. Wnt5A disrupted β-catenin and VE-cadherin adherens junctions forming inter-endothelial gaps. Functional experiments targeting the endothelial monolayer integrity and live recording of trans-endothelial resistance revealed enhanced permeability of Wnt5A-treated HCAEC. Ryk silencing completely prevented Wnt5A-induced endothelial hyperpermeability. Wnt5A decreased wound healing capacity of HCAEC monolayers; this was restored by the ROCK inhibitor Y-27632. Here we show that Wnt5A acts on the vascular endothelium causing enhanced permeability through Ryk interaction and downstream ROCK/LIMK2/CFL1 signaling. Wnt5A/Ryk signaling might provide novel therapeutic strategies to prevent capillary leakage in systemic inflammation and septic shock.

Rnd3 as a Novel Target to Ameliorate Microvascular Leakage

Background

Microvascular leakage of plasma proteins is a hallmark of inflammation that leads to tissue dysfunction. There are no current therapeutic strategies to reduce microvascular permeability. The purpose of this study was to identify the role of Rnd3, an atypical Rho family GTPase, in the control of endothelial barrier integrity. The potential therapeutic benefit of Rnd3 protein delivery to ameliorate microvascular leakage was also investigated.

Methods and Results

Using immunofluorescence microscopy, Rnd3 was observed primarily in cytoplasmic areas around the nuclei of human umbilical vein endothelial cells. Permeability to fluorescein isothiocyanate–albumin and transendothelial electrical resistance of human umbilical vein endothelial cell monolayers served as indices of barrier function, and RhoA, Rac1, and Cdc42 activities were determined using G‐LISA assays. Overexpression of Rnd3 significantly reduced the magnitude of thrombin‐induced barrier dysfunction, and abolished thrombin‐induced Rac1 inactivation. Depleting Rnd3 expression with siRNA significantly extended the time course of thrombin‐induced barrier dysfunction and Rac1 inactivation. Time‐lapse microscopy of human umbilical vein endothelial cells expressing GFP‐actin showed that co‐expression of mCherry‐Rnd3 attenuated thrombin‐induced reductions in local lamellipodia that accompany endothelial barrier dysfunction. Lastly, a novel Rnd3 protein delivery method reduced microvascular leakage in a rat model of hemorrhagic shock and resuscitation, assessed by both intravital microscopic observation of extravasation of fluorescein isothiocyanate–albumin from the mesenteric microcirculation, and direct determination of solute permeability in intact isolated venules.

Conclusions

The data suggest that Rnd3 can shift the balance of RhoA and Rac1 signaling in endothelial cells. In addition, our findings suggest the therapeutic, anti‐inflammatory potential of delivering Rnd3 to promote endothelial barrier recovery during inflammatory challenge.

LIM Kinase, a Newly Identified Regulator of Presynaptic Remodeling by Rod Photoreceptors After Injury

PURPOSE

Rod photoreceptors retract their axon terminals and develop neuritic sprouts in response to retinal detachment and reattachment, respectively. This study examines the role of LIM kinase (LIMK), a component of RhoA and Rac pathways, in the presynaptic structural remodeling of rod photoreceptors.

METHODS

Phosphorylated LIMK (p-LIMK), the active form of LIMK, was examined in salamander retina with Western blot and confocal microscopy. Axon length within the first 7 hours and process growth after 3 days of culture were assessed in isolated rod photoreceptors treated with inhibitors of upstream regulators ROCK and p21-activated kinase (Pak) (Y27632 and IPA-3) and a direct LIMK inhibitor (BMS-5). Porcine retinal explants were also treated with BMS-5 and analyzed 24 hours after detachment. Because Ca2+ influx contributes to axonal retraction, L-type channels were blocked in some experiments with nicardipine.

RESULTS

Phosphorylated LIMK is present in rod terminals during retraction and in newly formed processes. Axonal retraction over 7 hours was significantly reduced by inhibition of LIMK or its regulators, ROCK and Pak. Process growth was reduced by LIMK or Pak inhibition especially at the basal (axon-bearing) region of the rod cells. Combining Ca2+ channel and LIMK inhibition had no additional effect on retraction but did further inhibit sprouting after 3 days. In detached porcine retina, LIMK inhibition reduced rod axonal retraction and improved retinal morphology.

CONCLUSIONS

Thus structural remodeling, in the form of either axonal retraction or neuritic growth, requires LIMK activity. LIM kinase inhibition may have therapeutic potential for reducing pathologic rod terminal plasticity after retinal injury.

Pitfalls in assessing microvascular endothelial barrier function: impedance-based devices versus the classic macromolecular tracer assay

The most frequently used parameters to describe the barrier properties of endothelial cells (ECs) in vitro are (i) the macromolecular permeability, indicating the flux of a macromolecular tracer across the endothelium, and (ii) electrical impedance of ECs grown on gold-film electrodes reporting on the cell layer’s tightness for ion flow. Due to the experimental differences between these approaches, inconsistent observations have been described. Here, we present the first direct comparison of these assays applied to one single cell type (human microvascular ECs) under the same experimental conditions. The impact of different pharmacological tools (histamine, forskolin, Y-27632, blebbistatin, TRAP) on endothelial barrier function was analyzed by Transwell^® tracer assays and two commercial impedance devices (xCELLigence^®, ECIS^®). The two impedance techniques provided very similar results for all compounds, whereas macromolecular permeability readings were found to be partly inconsistent with impedance. Possible reasons for these discrepancies are discussed. We conclude that the complementary combination of both approaches is highly recommended to overcome the restrictions of each assay. Since the nature of the growth support may contribute to the observed differences, structure-function relationships should be based on cells that are consistently grown on either permeable or impermeable growth supports in all experiments.

The Rho kinases: critical mediators of multiple profibrotic processes and rational targets for new therapies for pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is characterized by progressive lung scarring, short median survival, and limited therapeutic options, creating great need for new pharmacologic therapies. IPF is thought to result from repetitive environmental injury to the lung epithelium, in the context of aberrant host wound healing responses. Tissue responses to injury fundamentally involve reorganization of the actin cytoskeleton of participating cells, including epithelial cells, fibroblasts, endothelial cells, and macrophages. Actin filament assembly and actomyosin contraction are directed by the Rho-associated coiled-coil forming protein kinase (ROCK) family of serine/threonine kinases (ROCK1 and ROCK2). As would therefore be expected, lung ROCK activation has been demonstrated in humans with IPF and in animal models of this disease. ROCK inhibitors can prevent fibrosis in these models, and more importantly, induce the regression of already established fibrosis. Here we review ROCK structure and function, upstream activators and downstream targets of ROCKs in pulmonary fibrosis, contributions of ROCKs to profibrotic cellular responses to lung injury, ROCK inhibitors and their efficacy in animal models of pulmonary fibrosis, and potential toxicities of ROCK inhibitors in humans, as well as involvement of ROCKs in fibrosis in other organs. As we discuss, ROCK activation is required for multiple profibrotic responses, in the lung and multiple other organs, suggesting ROCK participation in fundamental pathways that contribute to the pathogenesis of a broad array of fibrotic diseases. Multiple lines of evidence therefore indicate that ROCK inhibition has great potential to be a powerful therapeutic tool in the treatment of fibrosis, both in the lung and beyond.

Histamine activates p38 MAP kinase and alters local lamellipodia dynamics, reducing endothelial barrier integrity and eliciting central movement of actin fibers

The role of the actin cytoskeleton in endothelial barrier function has been debated for nearly four decades. Our previous investigation revealed spontaneous local lamellipodia in confluent endothelial monolayers that appear to increase overlap at intercellular junctions. We tested the hypothesis that the barrier-disrupting agent histamine would reduce local lamellipodia protrusions and investigated the potential involvement of p38 mitogen-activated protein (MAP) kinase activation and actin stress fiber formation. Confluent monolayers of human umbilical vein endothelial cells (HUVEC) expressing green fluorescent protein-actin were studied using time-lapse fluorescence microscopy. The protrusion and withdrawal characteristics of local lamellipodia were assessed before and after addition of histamine. Changes in barrier function were determined using electrical cell-substrate impedance sensing. Histamine initially decreased barrier function, lamellipodia protrusion frequency, and lamellipodia protrusion distance. A longer time for lamellipodia withdrawal and reduced withdrawal distance and velocity accompanied barrier recovery. After barrier recovery, a significant number of cortical fibers migrated centrally, eventually resembling actin stress fibers. The p38 MAP kinase inhibitor SB203580 attenuated the histamine-induced decreases in barrier function and lamellipodia protrusion frequency. SB203580 also inhibited the histamine-induced decreases in withdrawal distance and velocity, and the subsequent actin fiber migration. These data suggest that histamine can reduce local lamellipodia protrusion activity through activation of p38 MAP kinase. The findings also suggest that local lamellipodia have a role in maintaining endothelial barrier integrity. Furthermore, we provide evidence that actin stress fiber formation may be a reaction to, rather than a cause of, reduced endothelial barrier integrity.

Elevated cytokines, thrombin and PAI-1 in severe HCPS patients due to Sin Nombre virus

Sin Nombre Hantavirus (SNV, Bunyaviridae Hantavirus) is a Category A pathogen that causes Hantavirus Cardiopulmonary Syndrome (HCPS) with case fatality ratios generally ranging from 30% to 50%. HCPS is characterized by vascular leakage due to dysregulation of the endothelial barrier function. The loss of vascular integrity results in non-cardiogenic pulmonary edema, shock, multi-organ failure and death. Using Electric Cell-substrate Impedance Sensing (ECIS) measurements, we found that plasma samples drawn from University of New Mexico Hospital patients with serologically-confirmed HCPS, induce loss of cell-cell adhesion in confluent epithelial and endothelial cell monolayers grown in ECIS cultureware. We show that the loss of cell-cell adhesion is sensitive to both thrombin and plasmin inhibitors in mild cases, and to thrombin only inhibition in severe cases, suggesting an increasing prothrombotic state with disease severity. A proteomic profile (2D gel electrophoresis and mass spectrometry) of HCPS plasma samples in our cohort revealed robust antifibrinolytic activity among terminal case patients. The prothrombotic activity is highlighted by acute ≥30 to >100 fold increases in active plasminogen activator inhibitor (PAI-1) which, preceded death of the subjects within 48 h. Taken together, this suggests that PAI-1 might be a response to the severe pathology as it is expected to reduce plasmin activity and possibly thrombin activity in the terminal patients.

Involvement of local lamellipodia in endothelial barrier function

Recently we observed that endothelial cells cultured in tightly confluent monolayers display frequent local lamellipodia, and that thrombin, an agent that increases endothelial permeability, reduces lamellipodia protrusions. This led us to test the hypothesis that local lamellipodia contribute to endothelial barrier function. Movements of subcellular structures containing GFP-actin or VE-cadherin-GFP expressed in endothelial cells were recorded using time-lapse microscopy. Transendothelial electrical resistance (TER) served as an index of endothelial barrier function. Changes in both lamellipodia dynamics and TER were assessed during baseline and after cells were treated with either the barrier-disrupting agent thrombin, or the barrier-stabilizing agent sphingosine-1-phosphate (S1P). The myosin II inhibitor blebbistatin was used to selectively block lamellipodia formation, and was used to test their role in the barrier function of endothelial cell monolayers and isolated, perfused rat mesenteric venules. Myosin light chain (MLC) phosphorylation was assessed by immunofluorescence microscopy. Rac1 and RhoA activation were evaluated using G-LISA assays. The role of Rac1 was tested with the specific inhibitor NSC23766 or by expressing wild-type or dominant negative GFP-Rac1. The results show that thrombin rapidly decreased both TER and the lamellipodia protrusion frequency. S1P rapidly increased TER in association with increased protrusion frequency. Blebbistatin nearly abolished local lamellipodia protrusions while cortical actin fibers and stress fibers remained intact. Blebbistatin also significantly decreased TER of cultured endothelial cells and increased permeability of isolated rat mesenteric venules. Both thrombin and S1P increased MLC phosphorylation and activation of RhoA. However, thrombin and S1P had differential impacts on Rac1, correlating with the changes in TER and lamellipodia protrusion frequency. Overexpression of Rac1 elevated, while NSC23766 and dominant negative Rac1 reduced barrier function and lamellipodia activity. Combined, these data suggest that local lamellipodia, driven by myosin II and Rac1, are important for dynamic changes in endothelial barrier integrity.

Effective dose of the combination of salvianolic acids and tanshinones against inflammatory injury in human umbilical vein endothelial cells injured by thrombin

OBJECTIVE

To investigate the pharmacological effective doses of Chinese Danshen components, (salvianolic acids and tanshinones) combinations on vascular endothelia cells against inflammatory injury.

METHODS

Thrombin (50 U/mL) was incubated in human umbilical vein endothelia cells (HUVECs) with 50 and 100 μg/L (1:0, 0:1, 3:1, 2:1, 1:1, and 1:2) salvianolic acids and tanshinones or without the components of Danshen extract. Cell viability was confirmed in (4, 5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium (MTT) assay and cell death was confirmed in lactate dehydrogenase (LDH) released into the cell culture supernatant.

RESULTS

Six hours treatment of HUVECs with 50 or 100 μg/L salvianolic acids and tanshinones combinations at ratios 1:0, 0:1, 3:1, 2:1, 1:1, and 1:2 significantly (P<0.05 or P<0.01) improved cell proliferation in vitro compared with model group when assayed in MTT, whereas 100 μg/L groups displayed trend toward upshift in potency compared with 50 μg/L groups (2:1, 1:1, and 1:2), respectively, in addition to inhibition of cell death confirmed in reduced LDH released into the culture medium.

CONCLUSIONS

These findings provided some useful information for safe and effective use of Danshen preparations in clinical practice. We made observations regarding the protective effect of Danshen components combinations in vascular endothelial against inflammatory injury caused by thrombin activation of endothelial cell.

Rho Kinase and Dopaminergic Degeneration

The small GTP-binding protein Rho plays an important role in several cellular functions. RhoA, which is a member of the Rho family, initiates cellular processes that act on its direct downstream effector Rho-associated kinase (ROCK). ROCK inhibition protects against dopaminergic cell death induced by dopaminergic neurotoxins. It has been suggested that ROCK inhibition activates neuroprotective survival cascades in dopaminergic neurons. Axon-stabilizing effects in damaged neurons may represent another mechanism of neuroprotection of dopaminergic neurons by ROCK inhibition. However, it has been shown that microglial cells play a crucial role in neuroprotection by ROCK inhibition and that activation of microglial ROCK mediates major components of the microglial inflammatory response. Additional mechanisms such as interaction with autophagy may also contribute to the neuroprotective effects of ROCK inhibition. Interestingly, ROCK interacts with several brain factors that play a major role in dopaminergic neuron vulnerability such as NADPH-oxidase, angiotensin, and estrogen. ROCK inhibition may provide a new neuroprotective strategy for Parkinson’s disease. This is of particular interest because ROCK inhibitors are currently used against vascular diseases in clinical practice. However, it is necessary to develop more potent and selective ROCK inhibitors to reduce side effects and enhance the efficacy.

Titanium dioxide nanomaterials cause endothelial cell leakiness by disrupting the homophilic interaction of VE-cadherin

The use of nanomaterials has raised safety concerns, as their small size facilitates accumulation in and interaction with biological tissues. Here we show that exposure of endothelial cells to TiO₂ nanomaterials causes endothelial cell leakiness. This effect is caused by the physical interaction between TiO₂ nanomaterials and endothelial cells' adherens junction protein VE-cadherin. As a result, VE-cadherin is phosphorylated at intracellular residues (Y658 and Y731), and the interaction between VE-cadherin and p120 as well as β-catenin is lost. The resulting signalling cascade promotes actin remodelling, as well as internalization and degradation of VE-cadherin. We show that injections of TiO₂ nanomaterials cause leakiness of subcutaneous blood vessels in mice and, in a melanoma-lung metastasis mouse model, increase the number of pulmonary metastases. Our findings uncover a novel non-receptor-mediated mechanism by which nanomaterials trigger intracellular signalling cascades via specific interaction with VE-cadherin, resulting in nanomaterial-induced endothelial cell leakiness.

ROCK mediates the inflammatory response in thrombin induced microglia

To investigate whether the ROCK pathway is involved in thrombin-induced microglial inflammatory response, thrombin-induced microglia were pretreated with the thrombin inhibitor argatroban or a ROCK inhibitor Y-27632. Microglial inflammatory response was evaluated by phagocytosis of fluorescein labeled latex beads analyses and inflammatory mediators' expression such as nitric oxide (NO) and tumor necrosis factor-alpha (TNF-а). Compared to non-induced microglia, thrombin-induced microglia show significantly enhanced phagocytotic capacity and increased ROCK, NO and TNF-а expression. Pretreatment of thrombin-induced microglia with argatroban or Y-27632 significantly decreased phagocytotic capacity and reduced ROCK, NO and TNF-α expression. Therefore, the ROCK pathway may play a vital role in the mechanisms by which thrombin induces microglia in the inflammatory response.

Endothelial and epithelial barriers in graft-versus-host disease

Endothelial and epithelial cells form selectively permeable barriers that separate tissue compartments. These cells coordinate movement between the lumen and tissue via the transcellular and paracellular pathways. The primary determinant of paracellular permeability is the tight junction, which forms an apical belt-like structure around endothelial and epithelial cells. This chapter discusses endothelial and epithelial barriers in graft-versus-host disease after allogeneic bone marrow transplantation, with a focus on the tight junction and its role in regulating paracellular permeability. Recent studies suggest that in graft-versus-host disease, pathological increases in paracellular permeability, or barrier dysfunction, are initiated by pretransplant conditioning and sustained by alloreactive cells and the proinflammatory milieu. The intestinal epithelium is a significant focus, as it is a target organ of graft-versus-host disease, and the mechanisms of barrier regulation in intestinal epithelium have been well characterized. Finally, we propose a model that incorporates endothelial and epithelial barrier dysfunction in graft-versus-host disease and discuss modulating barrier properties as a therapeutic approach.

Platelet-mediated vascular dysfunction during acute lung injury

CONTEXT

Platelets have significant roles in initiating and mediating reduced alveolar blood flow, microvascular leak, and ventilation/perfusion mismatch caused by metabolic changes and altered signal transduction caused by ischemia-reperfusion.

OBJECTIVE

This review focuses on platelet mechanisms of vascular dysfunction in the lung and presents a hypothesis for interplay between platelet activation, endothelial damage and fibrinogen. The purpose is to discuss current knowledge regarding mechanisms of platelet-mediated endothelial injury and implications for new strategies to treat vascular dysfunction associated with acute lung injury (ALI).

METHODS

Literature from a number of fields was searched using Medline and Google Scholar.

RESULTS

Activated platelets contribute to redox imbalance through reactive oxygen species production, pro-leak molecules such as PAF and serotonin, and recruitment of inflammatory cytokines and leukocytes to the damaged endothelium.

CONCLUSION

Platelets are a critical component of pulmonary ALI, acting in conjunction with fibrinogen to mediate endothelial damage through multiple signal transduction pathways.

Hydroxyfasudil-mediated inhibition of ROCK1 and ROCK2 improves kidney function in rat renal acute ischemia-reperfusion injury

Renal ischemia-reperfusion (IR) injury (IRI) is a common and important trigger of acute renal injury (AKI). It is inevitably linked to transplantation. Involving both, the innate and the adaptive immune response, IRI causes subsequent sterile inflammation. Attraction to and transmigration of immune cells into the interstitium is associated with increased vascular permeability and loss of endothelial and tubular epithelial cell integrity. Considering the important role of cytoskeletal reorganization, mainly regulated by RhoGTPases, in the development of IRI we hypothesized that a preventive, selective inhibition of the Rho effector Rho-associated coiled coil containing protein kinase (ROCK) by hydroxyfasudil may improve renal IRI outcome. Using an IRI-based animal model of AKI in male Sprague Dawley rats, animals treated with hydroxyfasudil showed reduced proteinuria and polyuria as well as increased urine osmolarity when compared with sham-treated animals. In addition, renal perfusion (as assessed by ¹⁸F-fluoride Positron Emission Tomography (PET)), creatinine- and urea-clearances improved significantly. Moreover, endothelial leakage and renal inflammation was significantly reduced as determined by histology, ¹⁸F-fluordesoxyglucose-microautoradiography, Evans Blue, and real-time PCR analysis. We conclude from our study that ROCK-inhibition by hydroxyfasudil significantly improves kidney function in a rat model of acute renal IRI and is therefore a potential new therapeutic option in humans.

Role of RhoA and its effectors ROCK and mDia1 in the modulation of deformation-induced FAK, ERK, p38, and MLC motogenic signals in human Caco-2 intestinal epithelial cells

Repetitive deformation enhances intestinal epithelial migration across tissue fibronectin. We evaluated the contribution of RhoA and its effectors Rho-associated kinase (ROK/ROCK) and mammalian diaphanous formins (mDia1) to deformation-induced intestinal epithelial motility across fibronectin and the responsible focal adhesion kinase (FAK), extracellular signal-regulated kinase (ERK), p38, and myosin light chain (MLC) signaling. We reduced RhoA, ROCK1, ROCK2, and mDia1 by smart-pool double-stranded short-interfering RNAs (siRNA) and pharmacologically inhibited RhoA, ROCK, and FAK in human Caco-2 intestinal epithelial monolayers on fibronectin-coated membranes subjected to 10% repetitive deformation at 10 cycles/min. Migration was measured by wound closure. Stimulation of migration by deformation was prevented by exoenzyme C3, Y27632, or selective RhoA, ROCK1, and ROCK2 or mDia1 siRNAs. RhoA, ROCK inhibition, or RhoA, ROCK1, ROCK2, mDia1, and FAK reduction by siRNA blocked deformation-induced nuclear ERK phosphorylation without preventing ERK phosphorylation in the cytoplasmic protein fraction. Furthermore, RhoA, ROCK inhibition or RhoA, ROCK1, ROCK2, and mDia1 reduction by siRNA also blocked strain-induced FAK-Tyr(925), p38, and MLC phosphorylation. These results suggest that RhoA, ROCK, mDia1, FAK, ERK, p38, and MLC all mediate the stimulation of intestinal epithelial migration by repetitive deformation. This pathway may be an important target for interventions to promote mechanotransduced mucosal healing during inflammation.

Cross talk between focal adhesion kinase and cadherins: role in regulating endothelial barrier function

A layer of endothelial cells attached to their underlying matrices by complex transmembrane structures termed focal adhesion (FA) proteins maintains the barrier property of microvascular endothelium. FAs sense the physical properties of the extracellular matrix (ECM) and organize the cytoskeleton accordingly. The close association of adherens junction (AJ) protein, cadherin, with the cytoskeleton is known to be essential in coordinating the appropriate mechanical properties to cell-cell contacts. Recently, it has become clear that a crosstalk exists between focal adhesion kinase (FAK) and cadherin that regulates signaling at intercellular endothelial junctions. This review discusses recent advances in our understanding of the dynamic regulation of the molecular connections between FAK and the cadherin complex and cadherin-catenin-actin interaction-dependent changes as well as the role of small GTPases in endothelial barrier regulation. This review also discusses how a signaling network regulates a range of cellular processes important for barrier function and diseases.

ROCK and cAMP promote lymphatic endothelial cell barrier integrity and modulate histamine and thrombin-induced barrier dysfunction

BACKGROUND

There is recent evidence that inflammatory signals can modulate lymphatic vessel permeability, but current understanding of the mechanisms regulating lymphatic endothelial barrier function is limited. The objectives of this study were to 1) investigate whether inflammatory mediators that increase microvascular permeability also cause barrier dysfunction of lymphatic endothelial cell monolayers, and 2) determine the roles of signaling pathways that affect intercellular junctions and cell contraction in lymphatic endothelial barrier function.

METHODS AND RESULTS

Transendothelial electrical resistance (TER) of confluent adult human microlymphatic endothelial cells of dermal origin (HMLEC-d) served as an indicator of lymphatic endothelial barrier function. Human umbilical vein endothelial cells (HUVEC) were used to model blood-tissue barrier function. The inflammatory mediators histamine and thrombin each caused a decrease in TER of HMLEC-d and HUVEC monolayers, with notable differences between the two cell types. Treatment with 8-Br-cAMP enhanced HMLEC-d barrier function, which limited histamine and thrombin-induced decreases in TER. Blockade of myosin light chain kinase (MLCK) with ML-7 did not affect histamine or thrombin-induced decreases in TER. Treatment with the Rho kinase (ROCK) inhibitor Y-27632 caused a decrease in HMLEC-d barrier function.

CONCLUSIONS

These data show that inflammatory mediators can cause lymphatic endothelial barrier dysfunction, although the responses are not identical to those seen with blood endothelial cells. ROCK and cAMP both promote lymphatic endothelial barrier function, however ROCK appears to also serve as a mediator of histamine and thrombin-induced barrier dysfunction.

Role of RhoA/ROCK signaling in endothelial-monocyte-activating polypeptide II opening of the blood-tumor barrier: role of RhoA/ROCK signaling in EMAP II opening of the BTB

The purpose of the present study was to determine the potential for RhoA/ROCK signaling to play a role in endothelial-monocyte-activating polypeptide (EMAP) II-induced increase in blood-tumor barrier (BTB) permeability in rat brain microvascular endothelial cells (RBMECs). In the present study, we used an in vitro BTB model, a RhoA inhibitor (C3 exoenzyme) and a ROCK inhibitor (Y27632) to determine whether RhoA/ROCK pathway play a role in the process of TJ disassembly, stress fiber formation, MLC and cofilin phosphorylation, as well as increase of BTB permeability induced by EMAP II. The results revealed that BTB permeability was increased by EMAP II induction, and C3 exoenzyme or Y27632 could partially inhibit the EMAP II-induced increase of BTB permeability. The significant down-regulations in tight junction (TJ)-associated proteins occludin, claudin-5 and ZO-1 and stress fiber formation by EMAP II administration were observed, which were partly prevented by C3 exoenzyme or Y27632 pretreatment. Moreover, the significant increases in RhoA activity, myosin light chain (MLC) and cofilin phosphorylation by EMAP II administration were observed, MLC and cofilin phosphorylation were partly inhibited by C3 exoenzyme or Y27632 pretreatment. The present study demonstrates that the activation of RhoA/ROCK signaling in RBMECs was required for the increase of BTB permeability and these effects are related with the ability for RhoA/ROCK to mediate TJ disassembly and stress fiber formation by phosphorylating cofilin and MLC.

Heterotrimeric G proteins, focal adhesion kinase, and endothelial barrier function

Ligands by binding to G protein coupled receptors (GPCRs) stimulate dissociation of heterotrimeric G proteins into Gα and Gβγ subunits. Released Gα and Gβγ subunits induce discrete signaling cues that differentially regulate focal adhesion kinase (FAK) activity and endothelial barrier function. Activation of G proteins downstream of receptors such as protease activated receptor 1 (PAR1) and histamine receptors rapidly increases endothelial permeability which reverses naturally within the following 1-2 h. However, activation of G proteins coupled to the sphingosine-1-phosphate receptor 1 (S1P1) signal cues that enhance basal barrier endothelial function and restore endothelial barrier function following the increase in endothelial permeability by edemagenic agents. Intriguingly, both PAR1 and S1P1 activation stimulates FAK activity, which associates with alteration in endothelial barrier function by these agonists. In this review, we focus on the role of the G protein subunits downstream of PAR1 and S1P1 in regulating FAK activity and endothelial barrier function.

Measurement and analysis of traction force dynamics in response to vasoactive agonists

Mechanical traction forces exerted by adherent cells on their surroundings serve an important role in a multitude of cellular and physiological processes including cell motility and multicellular rearrangements. For endothelial cells, contraction also provides a means to disrupt cell-cell junctions during inflammation to increase permeability between blood and interstitial tissue compartments. The degree of contractility exhibited by endothelial cells is influenced by numerous soluble factors, such as thrombin, histamine, lysophosphatidic acid, sphingosine-1-phosphate, and vascular endothelial growth factor (VEGF). Upon binding to cell surface receptors, these agents trigger changes in cytoskeletal organization, adhesion and myosin II activity to varying degrees. While conventional antibody-based biochemical assays are suitable for detecting relatively large changes in biomarkers of contractility in an end-point format, they cannot resolve subtle or rapid changes in contractility and cannot do so noninvasively. To overcome these limitations, we developed an approach to measure the contractile response of single cells exposed to contractility agonists with high spatiotemporal resolution. A previously developed traction force sensor, comprised of dense arrays of elastomeric microposts on which cells are cultured, was combined with custom, semi-automated software developed here to extract strain energy measurements from thousands of time-lapse images of micropost arrays deformed by adherent cells. Using this approach we corroborated the differential effects of known agonists of contractility and characterized the dynamics of their effects. All of these agonists produced a characteristic first-order rise and plateau in forces, except VEGF, which stimulated an early transient spike in strain energy followed by a sustained increase. This novel, two-phase contractile response was present in a subpopulation of cells, was mediated through both VEGFR2 and ROCK activation, and its magnitude was modulated by receptor internalization. Interestingly, the concentration of VEGF could shift the proportion of cells that responded with a spike versus only a gradual increase in forces. Furthermore, cells repeatedly exposed to VEGF were found to contract with different dynamics after pretreatment, suggesting that exposure history can impact the mechanical response. These studies highlight the importance of direct measurements of traction force dynamics as a tool for studies of mechanotransduction.

Substrate stiffening promotes endothelial monolayer disruption through enhanced physical forces

A hallmark of many, sometimes life-threatening, inflammatory diseases and disorders is vascular leakage. The extent and severity of vascular leakage is broadly mediated by the integrity of the endothelial cell (EC) monolayer, which is in turn governed by three major interactions: cell-cell and cell-substrate contacts, soluble mediators, and biomechanical forces. A potentially critical but essentially uninvestigated component mediating these interactions is the stiffness of the substrate to which the endothelial monolayer is adherent. Accordingly, we investigated the extent to which substrate stiffening influences endothelial monolayer disruption and the role of cell-cell and cell-substrate contacts, soluble mediators, and physical forces in that process. Traction force microscopy showed that forces between cell and cell and between cell and substrate were greater on stiffer substrates. On stiffer substrates, these forces were substantially enhanced by a hyperpermeability stimulus (thrombin, 1 U/ml), and gaps formed between cells. On softer substrates, by contrast, these forces were increased far less by thrombin, and gaps did not form between cells. This stiffness-dependent force enhancement was associated with increased Rho kinase activity, whereas inhibition of Rho kinase attenuated baseline forces and lessened thrombin-induced inter-EC gap formation. Our findings demonstrate a central role of physical forces in EC gap formation and highlight a novel physiological mechanism. Integrity of the endothelial monolayer is governed by its physical microenvironment, which in normal circumstances is compliant but during pathology becomes stiffer.

Adherens junctions connect stress fibres between adjacent endothelial cells

Background

Endothelial cell-cell junctions maintain endothelial integrity and regulate vascular morphogenesis and homeostasis. Cell-cell junctions are usually depicted with a linear morphology along the boundaries between adjacent cells and in contact with cortical F-actin. However, in the endothelium, cell-cell junctions are highly dynamic and morphologically heterogeneous.

Results

We report that endothelial cell-cell junctions can attach to the ends of stress fibres instead of to cortical F-actin, forming structures that we name discontinuous adherens junctions (AJ). Discontinuous AJ are highly dynamic and are increased in response to tumour necrosis factor (TNF)-α, correlating with the appearance of stress fibres. We show that vascular endothelial (VE)-cadherin/β-catenin/α-catenin complexes in discontinuous AJ are linked to stress fibres. Moreover, discontinuous AJ connect stress fibres from adjacent cells independently of focal adhesions, of which there are very few in confluent endothelial cells, even in TNF-α-stimulated cells. RNAi-mediated knockdown of VE-cadherin, but not zonula occludens-1, reduces the linkage of stress fibres to cell-cell junctions, increases focal adhesions, and dramatically alters the distribution of these actin cables in confluent endothelial cells.

Conclusions

Our results indicate that stress fibres from neighbouring cells are physically connected through discontinuous AJ, and that stress fibres can be stabilized by AJ-associated multi-protein complexes distinct from focal adhesions.

Rho-kinase inhibitors as therapeutics: from pan inhibition to isoform selectivity

The emerging critical implications of Rho/Rho-kinase (ROCK) signaling in neurodegenerative diseases, glaucoma, renoprotection, diabetes and cancer have sparked growing interest in the pharmacological potential of ROCK inhibitors beyond their current application in cardiovascular disease. This article discusses the therapeutic benefits of novel ROCK inhibitors in development, and highlights the recent advances in the current understanding of disease-dependent and isoform-specific functions of ROCK and their potential impact on future therapeutic strategies.

Direct activation of RhoA by reactive oxygen species requires a redox-sensitive motif

BACKGROUND

Rho family GTPases are critical regulators of the cytoskeleton and affect cell migration, cell-cell adhesion, and cell-matrix adhesion. As with all GTPases, their activity is determined by their guanine nucleotide-bound state. Understanding how Rho proteins are activated and inactivated has largely focused on regulatory proteins such as guanine nucleotide exchange factors (GEFs) and GTPase activating proteins (GAPs). However, recent in vitro studies have indicated that GTPases may also be directly regulated by redox agents. We hypothesized that this redox-based mechanism occurs in cells and affects cytoskeletal dynamics, and in this report we conclude this is indeed a novel mechanism of regulating the GTPase RhoA.

METHODOLOGY/PRINCIPAL FINDINGS

In this report, we show that RhoA can be directly activated by reactive oxygen species (ROS) in cells, and that this requires two critical cysteine residues located in a unique redox-sensitive motif within the phosphoryl binding loop. First, we show that ROS can reversibly activate RhoA and induce stress fiber formation, a well characterized readout of RhoA activity. To determine the role of cysteine residues in this mechanism of regulation, we generated cysteine to alanine RhoA mutants. Mutation of these cysteines abolishes ROS-mediated activation and stress fiber formation, indicating that these residues are critical for redox-regulation of RhoA. Importantly, these mutants maintain the ability to be activated by GEFs.

CONCLUSIONS/SIGNIFICANCE

Our findings identify a novel mechanism for the regulation of RhoA in cells by ROS, which is independent of classical regulatory proteins. This mechanism of regulation may be particularly relevant in pathological conditions where ROS are generated and the cellular redox-balance altered, such as in asthma and ischemia-reperfusion injury.

c-Jun DNAzymes Inhibit Myocardial Inflammation, ROS Generation, Infarct Size, and Improve Cardiac Function After Ischemia-Reperfusion Injury

Objectives— Coronary reperfusion has been the mainstay therapy for reduced infarct size after a heart attack. However, this intervention also results in myocardial injury by initiating a marked inflammatory reaction, and new treatments are keenly sought.

Methods and Results— The basic-region leucine zipper protein, c-Jun is poorly expressed in the normal myocardium and is induced within 24 hours after myocardial ischemia-reperfusion injury. Synthetic catalytic DNA molecules (DNAzymes) targeting c-Jun (Dz13) reduce infarct size in the area-at-risk (AAR) regardless of whether it is delivered intramyocardially at the initiation of ischemia or at the time of reperfusion. Dz13 attenuates neutrophil infiltration, c-Jun and ICAM-1 expression in vascular endothelium, cardiomyocyte apoptosis, and the generation of reactive oxygen species in the reperfused myocardium. It inhibits infiltration into the AAR of complement 3 (C3), C3a receptor (C3aR), membrane attack complex-1 (Mac-1), or matrix metalloproteinase-2 (MMP-2) positive inflammatory cells. Dz13 also improves cardiac function without influencing myocardial vascularity or fibrosis.

Conclusion— These findings demonstrate the regulatory role of c-Jun in the pathogenesis of myocardial inflammation and infarction following ischemia-reperfusion injury, and inhibition of this process using catalytic DNA.

Agonist-biased signaling via proteinase activated receptor-2: differential activation of calcium and mitogen-activated protein kinase pathways

We evaluated the ability of different trypsin-revealed tethered ligand (TL) sequences of rat proteinase-activated receptor 2 (rPAR(2)) and the corresponding soluble TL-derived agonist peptides to trigger agonist-biased signaling. To do so, we mutated the proteolytically revealed TL sequence of rPAR(2) and examined the impact on stimulating intracellular calcium transients and mitogen-activated protein (MAP) kinase. The TL receptor mutants, rPAR(2)-Leu(37)Ser(38), rPAR(2)-Ala(37-38), and rPAR(2)-Ala(39-42) were compared with the trypsin-revealed wild-type rPAR(2) TL sequence, S(37)LIGRL(42)-. Upon trypsin activation, all constructs stimulated MAP kinase signaling, but only the wt-rPAR(2) and rPAR(2)-Ala(39-42) triggered calcium signaling. Furthermore, the TL-derived synthetic peptide SLAAAA-NH2 failed to cause PAR(2)-mediated calcium signaling but did activate MAP kinase, whereas SLIGRL-NH2 triggered both calcium and MAP kinase signaling by all receptors. The peptides AAIGRL-NH2 and LSIGRL-NH2 triggered neither calcium nor MAP kinase signals. Neither rPAR(2)-Ala(37-38) nor rPAR(2)-Leu(37)Ser(38) constructs recruited beta-arrestins-1 or -2 in response to trypsin stimulation, whereas both beta-arrestins were recruited to these mutants by SLIGRL-NH2. The lack of trypsin-triggered beta-arrestin interactions correlated with impaired trypsin-activated TL-mutant receptor internalization. Trypsin-stimulated MAP kinase activation by the TL-mutated receptors was not blocked by inhibitors of Galpha(i) (pertussis toxin), Galpha(q) [N-cyclohexyl-1-(2,4-dichlorophenyl)-1,4-dihydro-6-methylindeno[1,2-c]pyrazole-3-carboxamide (GP2A)], Src kinase [4-amino-5-(4-methylphenyl)-7-(t-butyl)pyrazolo[3,4-d]-pyrimidine (PP1)], or the epidermal growth factor (EGF) receptor [4-(3'-chloroanilino)-6,7-dimethoxy-quinazoline (AG1478)], but was inhibited by the Rho-kinase inhibitor (R)-(+)-trans-N-(4-pyridyl)-4-(1-aminoethyl)-cyclohexanecarboxamide, 2HCl (Y27362). The data indicate that the proteolytically revealed TL sequence(s) and the mode of its presentation to the receptor (tethered versus soluble) can confer biased signaling by PAR(2), its arrestin recruitment, and its internalization. Thus, PAR(2) can signal to multiple pathways that are differentially triggered by distinct proteinase-revealed TLs or by synthetic signal-selective activating peptides.

Multifaceted role of Rho, Rac, Cdc42 and Ras in intercellular junctions, lessons from toxins

Tight junctions (TJs) and adherens junctions (AJs) are dynamic structures linked to the actin cytoskeleton, which control the paracellular permeability of epithelial and endothelial barriers. TJs and AJs are strictly regulated in a spatio-temporal manner by a complex signaling network, including Rho/Ras-GTPases, which have a pivotal role. Rho preferentially regulates TJs by controlling the contraction of apical acto-myosin filaments, whereas Rac/Cdc42 mainly coordinate the assembly-disassembly of AJ components. However, a subtle balance of Rho/Ras-GTPase activity and interplay between these molecules is required to maintain an optimal organization and function of TJs and AJs. Conversely, integrity of intercellular junctions generates signals through Rho-GTPases, which are involved in the regulation of multiple cellular processes. Rho/Ras-GTPases and the control of intercellular junctions are the target of various bacterial toxins responsible for severe diseases in man and animals, and are part of their mechanism of action. This review focuses on the regulation of TJs and AJs by Rho/Ras-GTPases through molecular approaches and bacterial toxins.

Activated protein C protects against ventilator-induced pulmonary capillary leak

The coagulation system is central to the pathophysiology of acute lung injury. We have previously demonstrated that the anticoagulant activated protein C (APC) prevents increased endothelial permeability in response to edemagenic agonists in endothelial cells and that this protection is dependent on the endothelial protein C receptor (EPCR). We currently investigate the effect of APC in a mouse model of ventilator-induced lung injury (VILI). C57BL/6J mice received spontaneous ventilation (control) or mechanical ventilation (MV) with high (HV(T); 20 ml/kg) or low (LV(T); 7 ml/kg) tidal volumes for 2 h and were pretreated with APC or vehicle via jugular vein 1 h before MV. In separate experiments, mice were ventilated for 4 h and received APC 30 and 150 min after starting MV. Indices of capillary leakage included bronchoalveolar lavage (BAL) total protein and Evans blue dye (EBD) assay. Changes in pulmonary EPCR protein and Rho-associated kinase (ROCK) were assessed using SDS-PAGE. Thrombin generation was measured via plasma thrombin-antithrombin complexes. HV(T) induced pulmonary capillary leakage, as evidenced by significant increases in BAL protein and EBD extravasation, without significantly increasing thrombin production. HV(T) also caused significant decreases in pulmonary, membrane-bound EPCR protein levels and increases in pulmonary ROCK-1. APC treatment significantly decreased pulmonary leakage induced by MV when given either before or after initiation of MV. Protection from capillary leakage was associated with restoration of EPCR protein expression and attenuation of ROCK-1 expression. In addition, mice overexpressing EPCR on the pulmonary endothelium were protected from HV(T)-mediated injury. Finally, gene microarray analysis demonstrated that APC significantly altered the expression of genes relevant to vascular permeability at the ontology (e.g., blood vessel development) and specific gene (e.g., MAPK-associated kinase 2 and integrin-beta(6)) levels. These findings indicate that APC is barrier-protective in VILI and that EPCR is a critical participant in APC-mediated protection.

Thrombin-mediated increases in cytosolic [Ca2+] involve different mechanisms in human pulmonary artery smooth muscle and endothelial cells

Thrombin is a procoagulant inflammatory agonist that can disrupt the endothelium-lumen barrier in the lung by causing contraction of endothelial cells and promote pulmonary cell proliferation. Both contraction and proliferation require increases in cytosolic Ca(2+) concentration ([Ca(2+)](cyt)). In this study, we compared the effect of thrombin on Ca(2+) signaling in human pulmonary artery smooth muscle (PASMC) and endothelial (PAEC) cells. Thrombin increased the [Ca(2+)](cyt) in both cell types; however, the transient response was significantly higher and recovered quicker in the PASMC, suggesting different mechanisms may contribute to thrombin-mediated increases in [Ca(2+)](cyt) in these cell types. Depletion of intracellular stores with cyclopiazonic acid (CPA) in the absence of extracellular Ca(2+) induced calcium transients representative of those observed in response to thrombin in both cell types. Interestingly, CPA pretreatment significantly attenuated thrombin-induced Ca(2+) release in PASMC; this attenuation was not apparent in PAEC, indicating that a PAEC-specific mechanism was targeted by thrombin. Treatment with a combination of CPA, caffeine, and ryanodine also failed to abolish the thrombin-induced Ca(2+) transient in PAEC. Notably, thrombin-induced receptor-mediated calcium influx was still observed in PASMC after CPA pretreatment in the presence of extracellular Ca(2+). Ca(2+) oscillations were triggered by thrombin in PASMC resulting from a balance of extracellular Ca(2+) influx and Ca(2+) reuptake by the sarcoplasmic reticulum. The data show that thrombin induces increases in intracellular calcium in PASMC and PAEC with a distinct CPA-, caffeine-, and ryanodine-insensitive release existing only in PAEC. Furthermore, a dynamic balance between Ca(2+) influx, intracellular Ca(2+) release, and reuptake underlie the Ca(2+) transients evoked by thrombin in some PASMC. Understanding of such mechanisms will provide an important insight into thrombin-mediated vascular injury during hypertension.