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

Imaging and Analysis of the Dynamics of Filamentous Actin Structures in Live Endothelial Cells

The ability to view and record the movements of subcellular structures is a powerful tool that has accelerated the discovery and understanding of signaling mechanisms that control microvascular functions such as the control of endothelial permeability. Advances in molecular biology over the past few decades have facilitated the generation of fusion proteins in which fluorescent reporters based upon the structure of green fluorescent protein can be linked to proteins found in human endothelial cells, such as VE-cadherin or β-actin. These fusion proteins have been found to incorporate into structures alongside their native protein counterparts, allowing the dynamic visualization of how these subcellular structures are modified when cells are challenged with stimuli such as inflammatory mediators. The result of such studies has been a much more advanced view of the complex mechanisms by which endothelial cells maintain barrier properties than previously obtained by only viewing fixed cells labeled by immunofluorescence. Here, we describe our protocols that we have used to view the dynamics of actin filaments using time-lapse microscopy to record endothelial cells expressing GFP-actin and the analysis tools available to quantify dynamics of subcellular structures.

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.

Inflammatory Immune Cytokine TNF-α Modulates Ezrin Protein Activation via FAK/RhoA Signaling Pathway in PMVECs Hyperpermeability

Background: One of the important pathogenesis of acute respiratory distress syndrome (ARDS) is the dysfunction of pulmonary microvascular endothelial barrier induced by a hyperinflammatory immune response. However, the potential mechanisms of such an imbalance in pulmonary microvascular endothelial cells (PMVECs) are not yet understood. Purpose: Explore the molecular mechanism of endothelial barrier dysfunction induced by inflammatory immune cytokines in ARDS, and find a therapeutic target for this syndrome. Methods: Rat PMVECs were cultured to form a monolayer. Immunofluorescence, flow cytometry, and Western blotting were selected to detect the distribution and the expression level of phosphorylated Ezrin protein and Ezrin protein. Transendothelial electrical resistance (TER) and transendothelial fluxes of fluorescein isothiocyanate (FITC)-labeled bovine serum albumin (BSA) were utilized to measure the permeability of the cell monolayer. Ezrin short hairpin RNA (shRNA) and Ezrin 567-site threonine mutant (Ezrin^T567A) were used to examine the role of Ezrin protein and phosphorylated Ezrin protein in endothelial response induced by tumor necrosis factor-alpha (TNF-α), respectively. The function of focal adhesion kinase (FAK) and Ras homolog gene family, member A (RhoA) signaling pathways were estimated by inhibitors and RhoA/FAK shRNA in TNF-α-stimulated rat PMVECs. The activation of FAK and RhoA was assessed by Western blotting or pull-down assay plus Western blotting. Results: The TER was decreased after TNF-α treatment, while the Ezrin protein phosphorylation was increased in a time- and dose-dependent manner. The phosphorylated Ezrin protein was localized primarily at the cell periphery, resulting in filamentous actin (F-actin) rearrangement, followed by a significant decrease in TER and increase in fluxes of FITC-BSA. Moreover, FAK and RhoA signaling pathways were required in the phosphorylation of Ezrin protein, and the former positively regulated the latter. Conclusion: The phosphorylated Ezrin protein was induced by TNF-α via the FAK/RhoA signaling pathway leading to endothelial hyperpermeability in PMVECs.

Autoregulatory "Multitasking" at Endothelial Cell Junctions by Junction-Associated Intermittent Lamellipodia Controls Barrier Properties

Vascular endothelial cell (EC) junctions are key structures controlling tissue homeostasis in physiology. In the last three decades, excellent studies have addressed many aspects of this complex and highly dynamic regulation, including cell signaling, remodeling processes of the proteins of tight junctions, adherens junctions, and gap junctions, the cytoskeleton, and post-transcriptional modifications, transcriptional activation, and gene silencing. In this dynamic process, vascular endothelial cadherin (VE-cadherin) provides the core structure of EC junctions mediating the physical adhesion of cells as well as the control of barrier function and monolayer integrity via remodeling processes, regulation of protein expression and post-translational modifications. In recent years, research teams have documented locally restricted dynamics of EC junctions in which actin-driven protrusions in plasma membranes play a central role. In this regard, our research group showed that the dynamics of VE-cadherin is driven by small (1-5 μm) actin-mediated protrusions in plasma membranes that, due to this specific function, were named "junction-associated intermittent lamellipodia" (JAIL). JAIL form at overlapping, adjacent cells, and exactly at this site new VE-cadherin interactions occur, leading to new VE-cadherin adhesion sites, a process that restores weak or lost VE-cadherin adhesion. Mechanistically, JAIL formation occurs locally restricted (1-5 μm) and underlies autoregulation in which the local VE-cadherin concentration is an important parameter. A decrease in the local concentration of VE-cadherin stimulates JAIL formation, whereas an increase in the concentration of VE-cadherin blocks it. JAIL mediated VE-cadherin remodeling at the subjunctional level have been shown to be of crucial importance in angiogenesis, wound healing, and changes in permeability during inflammation. The concept of subjunctional regulation of EC junctions is strongly supported by permeability assays, which can be employed to quantify actin-driven subjunctional changes. In this brief review, we summarize and discuss the current knowledge and concepts of subjunctional regulation in the endothelium.

Sigma-1 receptor activation-induced glycolytic ATP production and endothelial barrier enhancement

OBJECTIVE

We tested the hypothesis that σ1 modulates endothelial barrier function due to its influence on endothelial bioenergetics.

METHODS

Cultured HUVEC monolayers were used to model the endothelial barrier. ECIS, Transwell assays, and immunofluorescence labeling of junctional proteins were used to evaluate endothelial barrier function. Endothelial cell bioenergetics was determined using extracellular flux analysis and direct ATP level measurements. The endothelial-specific contribution of σ1 was tested using the σ1-selective agonist, PRE-084, and with targeted knockdown of σ1 expression using siRNA.

RESULTS

Activation of σ1 with PRE-084 significantly enhanced endothelial barrier function and decreased permeability to albumin and dextran. Knockdown of σ1 with siRNA reduced barrier function and abolished PRE-084-induced endothelial barrier enhancement. PRE-084 upregulated endothelial glycolysis and glycolytic ATP production, but this response was abolished by siRNA-mediated knockdown of σ1 expression. PRE-084 also reduced the degree of endothelial barrier dysfunction caused by the mitochondrial oxidative phosphorylation uncoupler CCCP.

CONCLUSION

Activation of σ1 enhances endothelial barrier function and modulates the ratio of glycolytic versus mitochondrial ATP production. These novel findings suggest that endothelial σ1 may prove beneficial as a novel therapeutic target for reducing microvascular hyperpermeability and counteracting mitochondrial dysfunction.

Histamine H3 Inverse Agonist BF 2649 or Antagonist with Partial H4 Agonist Activity Clobenpropit Reduces Amyloid Beta Peptide-Induced Brain Pathology in Alzheimer's Disease

Alzheimer's disease (AD) is one of the leading causes for disability and death affecting millions of people worldwide. Thus, novel therapeutic strategies are needed to reduce brain pathology associated with AD. In view of increasing awareness regarding involvement of histaminergic pathways in AD, we explored the role of one H3 receptor inverse agonist BF 2649 and one selective H3 receptor antagonist with partial H4 agonist activity in amyloid beta peptide (AβP) infusion-induced brain pathology in a rat model. AD-like pathology was produced by administering AβP (1-40) intracerebroventricular (i.c.v.) in the left lateral ventricle (250 ng/10 μl, once daily) for 4 weeks. Control rats received saline. In separate group of rats, either BF 2649 (1 mg/kg, i.p.) or clobenpropit (1 mg/kg, i.p.) was administered once daily for 1 week after 3 weeks of AβP administration. After 30 days, blood-brain barrier (BBB) breakdown, edema formation, neuronal, glial injuries, and AβP deposits were examined in the brain. A significant reduction in AβP deposits along with marked reduction in neuronal or glial reactions was seen in the drug-treated group. The BBB breakdown to Evans blue albumin and radioiodine in the cortex, hippocampus, hypothalamus, and cerebellum was also significantly reduced in these drug-treated groups. Clobenpropit showed superior effects than the BF2649 in reducing brain pathology in AD. Taken together, our observations are the first to show that blockade of H3 and stimulation of H4 receptors are beneficial for the treatment of AD pathology, not reported earlier.

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.

Cortical Actin Dynamics in Endothelial Permeability

The pulmonary endothelial cell forms a critical semi-permeable barrier between the vascular and interstitial space. As part of the blood-gas barrier in the lung, the endothelium plays a key role in normal physiologic function and pathologic disease. Changes in endothelial cell shape, defined by its plasma membrane, determine barrier integrity. A number of key cytoskeletal regulatory and effector proteins including non-muscle myosin light chain kinase, cortactin, and Arp 2/3 mediate actin rearrangements to form cortical and membrane associated structures in response to barrier enhancing stimuli. These actin formations support and interact with junctional complexes and exert forces to protrude the lipid membrane to and close gaps between individual cells. The current knowledge of these cytoskeletal processes and regulatory proteins are the subject of this review. In addition, we explore novel advancements in cellular imaging that are poised to shed light on the complex nature of pulmonary endothelial permeability.

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.

Potential negative effects of anti-histamines on male reproductive function

Histamine (HA) is a pleiotropic biogenic amine synthesized exclusively by histidine decarboxylase (HDC) in most mammalian tissues. The literature on the role of HA within the male gonad has expanded over the last years, attracting attention to potential unexpected side-effects of anti-histamines on testicular function. In this regard, HA receptors (HRH1, HRH2 and HRH4) have been described in Leydig cells of different species, including human. Via these receptors, HA has been reported to trigger positive or negative interactions with the LH/hCG signaling pathway depending upon its concentration, thereby contributing to the local control of testicular androgen levels. It should then be considered that anti-histamines may affect testicular homeostasis by increasing or decreasing steroid production. Additionally, HRH1 and HRH2 receptors are present in peritubular and germ cells, and HRH2 antagonists have been found to negatively affect peritubular cells and reduce sperm viability. The potential negative impact of anti-histamines on male reproduction becomes even more dramatic if we consider that HA has also been associated with human sexual behavior and penile erection. What is more, although testicular mast cells are the major source of locally produced HA, recent studies have described HDC expression in macrophages, Leydig cells and germ cells, revealing the existence of multiple sources of HA within the testis. Undoubtedly, the more we learn about the testicular histaminergic system, the more opportunities there will be for rational design of drugs aimed at treating HA-related pathologies, with minimum or nule negative impact on fertility.

Cellular cross talk, inflammatory signals, and enhanced microvascular permeability

Microvascular leakage remains a significant clinical problem for which there is no specific therapeutic available. This Special Topics issue of the journal Microcirculation features reviews pertaining to the cellular and molecular mechanisms that control the integrity of the microvascular barrier. (Meegan et al., Microcirculation, 2017) summarize recent research findings about how neutrophil extracellular traps and how these can cause injury to the microvascular barrier.( Zhang et al. Microcirculation, 2017) highlight the microvascular response to advanced glycation end-products that are formed as part of the pathophysiology of diabetes mellitus. These ongoing investigations are helping to clarify the mechanisms by which endothelial cells receive information and integrate it to generate cellular responses that fine-tune barrier function. Better understanding of these mechanisms is needed for the rational development of therapeutic strategies to reduce excessive microvascular leakage.

The p38 pathway, a major pleiotropic cascade that transduces stress and metastatic signals in endothelial cells

By gating the traffic of molecules and cells across the vessel wall, endothelial cells play a central role in regulating cardiovascular functions and systemic homeostasis and in modulating pathophysiological processes such as inflammation and immunity. Accordingly, the loss of endothelial cell integrity is associated with pathological disorders that include atherosclerosis and cancer. The p38 mitogen-activated protein kinase (MAPK) cascades are major signaling pathways that regulate several functions of endothelial cells in response to exogenous and endogenous stimuli including growth factors, stress and cytokines. The p38 MAPK family contains four isoforms p38α, p38β, p38γ and p38δ that are encoded by four different genes. They are all widely expressed although to different levels in almost all human tissues. p38α/MAPK14, that is ubiquitously expressed is the prototype member of the family and is referred here as p38. It regulates the production of inflammatory mediators, and controls cell proliferation, differentiation, migration and survival. Its activation in endothelial cells leads to actin remodeling, angiogenesis, DNA damage response and thereby has major impact on cardiovascular homeostasis, and on cancer progression. In this manuscript, we review the biology of p38 in regulating endothelial functions especially in response to oxidative stress and during the metastatic process.

Hydrogen sulfide metabolism regulates endothelial solute barrier function

Hydrogen sulfide (H₂S) is an important gaseous signaling molecule in the cardiovascular system. In addition to free H₂S, H₂S can be oxidized to polysulfide which can be biologically active. Since the impact of H₂S on endothelial solute barrier function is not known, we sought to determine whether H₂S and its various metabolites affect endothelial permeability. In vitro permeability was evaluated using albumin flux and transendothelial electrical resistance. Different H₂S donors were used to examine the effects of exogenous H₂S. To evaluate the role of endogenous H₂S, mouse aortic endothelial cells (MAECs) were isolated from wild type mice and mice lacking cystathionine γ-lyase (CSE), a predominant source of H₂S in endothelial cells. In vivo permeability was evaluated using the Miles assay. We observed that polysulfide donors induced rapid albumin flux across endothelium. Comparatively, free sulfide donors increased permeability only with higher concentrations and at later time points. Increased solute permeability was associated with disruption of endothelial junction proteins claudin 5 and VE-cadherin, along with enhanced actin stress fiber formation. Importantly, sulfide donors that increase permeability elicited a preferential increase in polysulfide levels within endothelium. Similarly, CSE deficient MAECs showed enhanced solute barrier function along with reduced endogenous bound sulfane sulfur. CSE siRNA knockdown also enhanced endothelial junction structures with increased claudin 5 protein expression. In vivo, CSE genetic deficiency significantly blunted VEGF induced hyperpermeability revealing an important role of the enzyme for barrier function. In summary, endothelial solute permeability is critically regulated via exogenous and endogenous sulfide bioavailability with a prominent role of polysulfides.

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Polysulfide from cystathionine γ-lyase (CSE) and exogenous polysulfide donors increases endothelial permeability.

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The ability of polysulfide to increase permeability is associated with junction disruption and stress fiber formation.

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CSE expression in vivo regulates VEGF induced hyper-permeability.

Activation of RhoA, but Not Rac1, Mediates Early Stages of S1P-Induced Endothelial Barrier Enhancement

Compromised endothelial barrier function is a hallmark of inflammation. Rho family GTPases are critical in regulating endothelial barrier function, yet their precise roles, particularly in sphingosine-1-phosphate (S1P)-induced endothelial barrier enhancement, remain elusive. Confluent cultures of human umbilical vein endothelial cells (HUVEC) or human dermal microvascular endothelial cells (HDMEC) were used to model the endothelial barrier. Barrier function was assessed by determining the transendothelial electrical resistance (TER) using an electrical cell-substrate impedance sensor (ECIS). The roles of Rac1 and RhoA were tested in S1P-induced barrier enhancement. The results show that pharmacologic inhibition of Rac1 with Z62954982 failed to block S1P-induced barrier enhancement. Likewise, expression of a dominant negative form of Rac1, or knockdown of native Rac1 with siRNA, failed to block S1P-induced elevations in TER. In contrast, blockade of RhoA with the combination of the inhibitors Rhosin and Y16 significantly reduced S1P-induced increases in TER. Assessment of RhoA activation in real time using a fluorescence resonance energy transfer (FRET) biosensor showed that S1P increased RhoA activation primarily at the edges of cells, near junctions. This was complemented by myosin light chain-2 phosphorylation at cell edges, and increased F-actin and vinculin near intercellular junctions, which could all be blocked with pharmacologic inhibition of RhoA. The results suggest that S1P causes activation of RhoA at the cell periphery, stimulating local activation of the actin cytoskeleton and focal adhesions, and resulting in endothelial barrier enhancement. S1P-induced Rac1 activation, however, does not appear to have a significant role in this process.

Melatonin Preserves Blood-Brain Barrier Integrity and Permeability via Matrix Metalloproteinase-9 Inhibition

Microvascular hyperpermeability that occurs at the level of the blood-brain barrier (BBB) often leads to vasogenic brain edema and elevated intracranial pressure following traumatic brain injury (TBI). At a cellular level, tight junction proteins (TJPs) between neighboring endothelial cells maintain the integrity of the BBB via TJ associated proteins particularly, zonula occludens-1 (ZO-1) that binds to the transmembrane TJPs and actin cytoskeleton intracellularly. The pro-inflammatory cytokine, interleukin-1β (IL-1β) as well as the proteolytic enzymes, matrix metalloproteinase-9 (MMP-9) are key mediators of trauma-associated brain edema. Recent studies indicate that melatonin a pineal hormone directly binds to MMP-9 and also might act as its endogenous inhibitor. We hypothesized that melatonin treatment will provide protection against TBI-induced BBB hyperpermeability via MMP-9 inhibition. Rat brain microvascular endothelial cells grown as monolayers were used as an in vitro model of the BBB and a mouse model of TBI using a controlled cortical impactor was used for all in vivo studies. IL-1β (10 ng/mL; 2 hours)-induced endothelial monolayer hyperpermeability was significantly attenuated by melatonin (10 μg/mL; 1 hour), GM6001 (broad spectrum MMP inhibitor; 10 μM; 1 hour), MMP-9 inhibitor-1 (MMP-9 specific inhibitor; 5 nM; 1 hour) or MMP-9 siRNA transfection (48 hours) in vitro. Melatonin and MMP-9 inhibitor-1 pretreatment attenuated IL-1β-induced MMP-9 activity, loss of ZO-1 junctional integrity and f-actin stress fiber formation. IL-1β treatment neither affected ZO-1 protein or mRNA expression or cell viability. Acute melatonin treatment attenuated BBB hyperpermeability in a mouse controlled cortical impact model of TBI in vivo. In conclusion, one of the protective effects of melatonin against BBB hyperpermeability occurs due to enhanced BBB integrity via MMP-9 inhibition. In addition, acute melatonin treatment provides protection against BBB hyperpermeability in a mouse model of TBI indicating its potential as a therapeutic agent for brain edema when established in humans.

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.