H2O2‐induced Microvessel Barrier Dysfunction: The Interplay Between Reactive Oxygen Species, Nitric Oxide, and Peroxynitrite

Elevated H2O2 is implicated in many cardiovascular diseases. We previously demonstrated that H2O2-induced endothelial nitric oxide synthase (eNOS) activation and excessive NO production contribute to vascular cell injury and increases in microvessel permeability. However, the mechanisms of excessive NO-mediated vascular injury and hyperpermeability remain unknown. This study aims to examine the functional role of NO-derived peroxynitrite (ONOO) in H2O2-induced vascular barrier dysfunction by elucidating the interrelationships between H2O2-induced NO, superoxide, ONOO, and changes in endothelial [Ca2+ ]i and microvessel permeability. Experiments were conducted on intact rat mesenteric venules. Microvessel permeability was determined by measuring hydraulic conductivity (Lp). Endothelial [Ca2+ ]i, NO, and O2 were assessed with fluorescence imaging. Perfusion of vessels with H2O2 (10 µmol/L) induced marked productions of NO and O2, resulting in extensive protein tyrosine nitration, a biomarker of ONOO. The formation of ONOO was abolished by inhibition of NOS with NG-Methyl-L-arginine. Blocking NO production or scavenging ONOO by uric acid prevented H2O2- induced increases in endothelial [Ca2+ ]i and Lp. Additionally, the application of exogenous ONOO to microvessels induced delayed and progressive increases in endothelial [Ca2+ ]i and microvessel Lp, a pattern similar to that observed in H2O2-perfused vessels. Importantly, ONOO caused further activation of eNOS with amplified NO production. We conclude that the augmentation of NOderived ONOO is essential for H2O2-induced endothelial Ca2+ overload and progressively increased microvessel permeability, which is achieved by self-promoted amplifications of NO-dependent signaling cascades. This novel mechanism provides new insight into the reactive oxygen and/or reactive nitrogen species-mediated vascular dysfunction in cardiovascular diseases

Spent Culture Medium from Virulent Borrelia burgdorferi Increases Permeability of Individually Perfused Microvessels of Rat Mesentery

Background Lyme disease is a common vector-borne disease caused by the spirochete Borrelia burgdorferi(Bb), which manifests as systemic and targeted tissue inflammation. Both in vitro and in vivostudies have shown that Bb-induced inflammation is primarily host-mediated, via cytokine or chemokine production that promotes leukocyte adhesion/migration. Whether Bb produces mediators that can directly alter the vascular permeability in vivo has not been investigated. The objective of the present study was to investigate if Bb produces a mediator(s) that can directly activate endothelial cells resulting in increases in permeability in intact microvessels in the absence of blood cells. Methodology/Principal Findings The effects of cell-free, spent culture medium from virulent (B31-A3) and avirulent (B31-A) B. burgdorferi on microvessel permeability and endothelial calcium concentration, [Ca2+]i, were examined in individually perfused rat mesenteric venules. Microvessel permeability was determined by measuring hydraulic conductivity (Lp). Endothelial [Ca2+]i, a necessary signal initiating hyperpermeability, was measured in Fura-2 loaded microvessels. B31-A3 spent medium caused a rapid and transient increase in Lp and endothelial [Ca2+]i. Within 2–5 min, the mean peak Lp increased to 5.6±0.9 times the control, and endothelial [Ca2+]i increased from 113±11 nM to a mean peak value of 324±35 nM. In contrast, neither endothelial [Ca2+]i nor Lp was altered by B31-A spent medium. Conclusions/Significance A mediator(s) produced by virulent Bb under culture conditions directly activates endothelial cells, resulting in increases in microvessel permeability. Most importantly, the production of this mediator is associated with Bb virulence and is likely produced by one or more of the 8 plasmid(s) missing from strain B31-A

Reduction of Endothelial Nitric Oxide Increases the Adhesiveness of Constitutive Endothelial Membrane ICAM-1 through Src-Mediated Phosphorylation

Nitric oxide (NO) is a known anti-adhesive molecule that prevents platelet aggregation and leukocyte adhesion to endothelial cells (ECs). The mechanism has been attributed to its role in the regulation of adhesion molecules on leukocytes and the adhesive properties of platelets. Our previous study conducted in rat venules found that reduction of EC basal NO synthesis caused EC ICAM-1-mediated firm adhesion of leukocytes within 10–30min. This quick response occurred in the absence of alterations of adhesion molecules on leukocytes and also opposes the classical pattern of ICAM-1-mediated leukocyte adhesion that requires protein synthesis and occurs hours after stimulation. The objective of this study is to investigate the underlying mechanisms of reduced basal NO-induced EC-mediated rapid leukocyte adhesion observed in intact microvessels. The relative levels of ICAM-1 at different cell regions and their activation status were determined with cellular fractionation and western blot using cultured human umbilical vein ECs. ICAM-1 adhesiveness was determined by immunoprecipitation in non-denatured proteins to assess the changes in ICAM-1 binding to its inhibitory antibody, mAb1A29, and antibody against total ICAM-1 with and without NO reduction. The adhesion strength of EC ICAM-1 was assessed by atomic force microscopy (AFM) on live cells. Results showed that reduction of EC basal NO caused by the application of caveolin-1 scaffolding domain (AP-CAV) or NOS inhibitor, L-NMMA, for 30 min significantly increased phosphorylated ICAM-1 and its binding to mAb1A29 in the absence of altered ICAM-1 expression and its distribution at subcellular regions. The Src inhibitor, PP1, inhibited NO reduction-induced increases in ICAM-1 phosphorylation and adhesive binding. AFM detected significant increases in the binding force between AP-CAV-treated ECs and mAb1A29-coated probes. These results demonstrated that reduced EC basal NO lead to a rapid increase in ICAM-1 adhesive binding via Src-mediated phosphorylation without de novo protein synthesis and translocation. This study suggests that a NO-dependent conformational change of constitutive EC membrane ICAM-1 might be the mechanism of rapid ICAM-1 dependent leukocyte adhesion observed in vivo. This new mechanistic insight provides a better understanding of EC/leukocyte interaction-mediated vascular inflammation under many disease conditions that encounter reduced basal NO in the circulation system

Spent Culture Medium from Virulent Borrelia burgdorferi Increases Permeability of Individually Perfused Microvessels of Rat Mesentery

Lyme disease is a common vector-borne disease caused by the spirochete Borrelia burgdorferi (Bb), which manifests as systemic and targeted tissue inflammation. Both in vitro and in vivo studies have shown that Bb-induced inflammation is primarily host-mediated, via cytokine or chemokine production that promotes leukocyte adhesion/migration. Whether Bb produces mediators that can directly alter the vascular permeability in vivo has not been investigated. The objective of the present study was to investigate if Bb produces a mediator(s) that can directly activate endothelial cells resulting in increases in permeability in intact microvessels in the absence of blood cells.The effects of cell-free, spent culture medium from virulent (B31-A3) and avirulent (B31-A) B. burgdorferi on microvessel permeability and endothelial calcium concentration, [Ca(2+)](i), were examined in individually perfused rat mesenteric venules. Microvessel permeability was determined by measuring hydraulic conductivity (Lp). Endothelial [Ca(2+)](i), a necessary signal initiating hyperpermeability, was measured in Fura-2 loaded microvessels. B31-A3 spent medium caused a rapid and transient increase in Lp and endothelial [Ca(2+)](i). Within 2-5 min, the mean peak Lp increased to 5.6+/-0.9 times the control, and endothelial [Ca(2+)](i) increased from 113+/-11 nM to a mean peak value of 324+/-35 nM. In contrast, neither endothelial [Ca(2+)](i) nor Lp was altered by B31-A spent medium.A mediator(s) produced by virulent Bb under culture conditions directly activates endothelial cells, resulting in increases in microvessel permeability. Most importantly, the production of this mediator is associated with Bb virulence and is likely produced by one or more of the 8 plasmid(s) missing from strain B31-A

In Vitro Recapitulation of Functional Microvessels for the Study of Endothelial Shear Response, Nitric Oxide and [Ca2+]i

Microfluidic technologies enable in vitro studies to closely simulate in vivo microvessel environment with complexity. Such method overcomes certain constrains of the statically cultured endothelial monolayers and enables the cells grow under physiological range of shear flow with geometry similar to microvessels in vivo. However, there are still existing knowledge gaps and lack of convincing evidence to demonstrate and quantify key biological features of the microfluidic microvessels. In this paper, using advanced micromanufacturing and microfluidic technologies, we presented an engineered microvessel model that mimicked the dimensions and network structures of in vivo microvessels with a long-term and continuous perfusion capability, as well as high-resolution and real-time imaging capability. Through direct comparisons with studies conducted in intact microvessels, our results demonstrated that the cultured microvessels formed under perfused conditions recapitulated certain key features of the microvessels in vivo. In particular, primary human umbilical vein endothelial cells were successfully cultured the entire inner surfaces of the microchannel network with well-developed junctions indicated by VE-cadherin staining. The morphological and proliferative responses of endothelial cells to shear stresses were quantified under different flow conditions which was simulated with three-dimensional shear dependent numerical flow model. Furthermore, we successfully measured agonist-induced changes in intracellular Ca2+ concentration and nitric oxide production at individual endothelial cell levels using fluorescence imaging. The results were comparable to those derived from individually perfused intact venules. With in vivovalidation of its functionalities, our microfluidic model demonstrates a great potential for biological applications and bridges the gaps between in vitro and in vivo microvascular research

Improved measurements of intracellular nitric oxide in intact microvessels using 4,5-diaminofluorescein diacetate

4,5-Diaminofluorescein diacetate (DAF-2 DA) has been widely used for the measurement of nitric oxide (NO) in living cells and tissues. We previously established a method that demonstrated platelet activating factor (PAF)-induced endothelial NO production in intact venules using DAF-2 DA. In previous applications, the loading dye was removed from the extracellular space before NO measurements. However, in high permeability vessels, endothelial cells quickly released the accumulated intracellular DAF-2 after the washout, which compromises the NO measurement. The objective of this study was to investigate if the presence of DAF-2 DA during NO measurements could overcome the dye retention problem and enhance the sensitivity of NO detection. Experiments were conducted in individually perfused rat venules, and endothelial NO was measured using fluorescence imaging under basal and stimulated conditions with continuous perfusion of DAF-2 DA. Continuous dye perfusion was found to promote a relatively constant endothelial dye concentration in both normal and high permeability vessels throughout the experiment. With the use of this method, the basal and stimulated NO was quantified after endothelial DAF-2 concentrations reached a steady state. Our results showed enhanced sensitivity of detecting PAF-stimulated NO compared with a previous method. We also found that the hydrolyzed intracellular DAF-2, the precursor of DAF-2 triazole, contributed significantly to the measured fluorescence and that an appropriate subtraction of non-NO-dependent intracellular DAF-2 fluorescence is critical for the assessment of NO in living tissues. This method overcame the dye leakage problem, enhanced the sensitivity of NO detection, and improved NO quantification, demonstrating significant advantages over existing methodologies using DAF-2