Openings Through Endothelial Cells Associated with Increased Microvascular Permeability

Biomechanics of Schlemm's canal endothelium and intraocular pressure reduction

Ocular hypertension in glaucoma develops due to age-related cellular dysfunction in the conventional outflow tract, resulting in increased resistance to aqueous humor outflow. Two cell types, trabecular meshwork (TM) and Schlemm's canal (SC) endothelia, interact in the juxtacanalicular tissue (JCT) region of the conventional outflow tract to regulate outflow resistance. Unlike endothelial cells lining the systemic vasculature, endothelial cells lining the inner wall of SC support a transcellular pressure gradient in the basal to apical direction, thus acting to push the cells off their basal lamina. The resulting biomechanical strain in SC cells is quite large and is likely to be an important determinant of endothelial barrier function, outflow resistance and intraocular pressure. This review summarizes recent work demonstrating how biomechanical properties of SC cells impact glaucoma. SC cells are highly contractile, and such contraction greatly increases cell stiffness. Elevated cell stiffness in glaucoma may reduce the strain experienced by SC cells, decrease the propensity of SC cells to form pores, and thus impair the egress of aqueous humor from the eye. Furthermore, SC cells are sensitive to the stiffness of their local mechanical microenvironment, altering their own cell stiffness and modulating gene expression in response. Significantly, glaucomatous SC cells appear to be hyper-responsive to substrate stiffness. Thus, evidence suggests that targeting the material properties of SC cells will have therapeutic benefits for lowering intraocular pressure in glaucoma.

The role of vascular endothelial growth factor in the pathogenesis, diagnosis and treatment of malignant pleural effusion

Malignant pleural effusions (MPEs) are a significant source of cancer-related morbidity. Over 150,000 patients in the United States suffer from breathlessness and diminished quality of life due to MPE each year. Current management strategies are of mostly palliative value and focus on symptom control; they do not address the pathobiology of the effusion, nor do they improve survival. Further elucidation of the pathophysiological mechanisms, coupled with the development of novel treatments such as intrapleural chemotherapeutics targeting this process, has the potential to greatly improve the efficacy of our current management options. Vascular endothelial growth factor-A (VEGF-A) has been implicated as a critical cytokine in the formation of malignant pleural effusions. Elevated levels of VEGF produced by tumor cells, mesothelial cells, and infiltrating immune cells result in increased vascular permeability, cancer cell transmigration, and angiogenesis. Therefore antiangiogenic therapies such as Bevacizumab, a monoclonal antibody targeting VEGF-A, may have a potential role in the management of malignant pleural effusions. Herein we review the pathogenesis and potential treatment strategies of malignant pleural effusions, with a focus on angiogenesis and antiangiogenic therapeutics.

Pneumococcal immune evasion: ZmpC inhibits neutrophil influx

Neutrophil recruitment is essential in clearing pneumococcal infections. The first step in neutrophil extravasation involves the interaction between P-selectin on activated endothelium and P-Selectin Glycoprotein 1 (PSGL-1) on neutrophils. Here, we identify pneumococcal Zinc metalloproteinase C as a potent inhibitor of PSGL-1. ZmpC degrades the N-terminal domain of PSGL-1, thereby disrupting the initial rolling of neutrophils on activated human umbilical vein endothelial cells. Furthermore, mice infected with wild-type strain in the model of pneumococcal pneumonia showed lower lungs neutrophil infiltration compare to animals infected with ZmpC mutant. In addition, we confirmed the association of zmpC with serotype 8 and 11A and found it to be associated with serotype 33F as well. In conclusion, wereport PSGL-1 as a novel target for ZmpC and show that ZmpC inhibits neutrophil extravasation during pneumococcal pneumonia.

Electron tomography of vesicles

In this issue of Microcirculation, Wagner, Modla, Hossler and Czmmek [25] describe the use of electron tomography to visualize the three-dimensional arrangement of small endothelial vesicles and caveolae of muscle capillaries. Their images show the well-known clusters of fused vesicles communicating with caveolae at the luminal and abluminal surfaces. The advantages of electron tomography are shown by well resolved images of single cytoplasmic vesicles separate from fused vesicle clusters and also by occasional chains of fused vesicles forming trans-endothelial channels. Twenty five to thirty years ago the existence of both trans-endothelial channels and single unattached vesicles was disputed. Also, since some single vesicles and all of the trans-endothelial channels are labeled with a lanthanide tracer present in the perfusate at the time of fixation, this evidence once again raises the question of whether vesicles have a role in vascular permeability to macromolecules. This brief review describes the origin of the vesicle controversy, some of the more recent evidence for and against the participation of vesicles in macromolecular transport and considers some criticisms of ultra-structural evidence for vesicular transport that still require answers.

Microvascular transport and tumor cell adhesion in the microcirculation

One critical step in tumor metastasis is tumor cell adhesion to the endothelium forming the microvessel wall. Understanding this step may lead to new therapeutic concepts for tumor metastasis. Vascular endothelium forming the microvessel wall and the glycocalyx layer at its surface are the principal barriers to, and regulators of the material exchange between circulating blood and body tissues. The cleft between adjacent ECs (interendothelial cleft) is the principal pathway for water and solutes transport through the microvessel wall in health. It is also suggested to be the pathway for high molecular weight plasma proteins, leukocytes and tumor cells across microvessel walls in disease. Thus the first part of the review introduced the mathematical models for water and solutes transport through the interendothelial cleft. These models, combined with the experimental results from in vivo animal studies and electron microscopic observations, are used to evaluate the role of the endothelial surface glycocalyx, the junction strand geometry in the interendothelial cleft, and the surrounding extracellular matrix and tissue cells, as the determinants of microvascular transport. The second part of the review demonstrated how the microvascular permeability, hydrodynamic factors, microvascular geometry and cell adhesion molecules affect tumor cell adhesion in the microcirculation.

Effects of preconditioning with sevoflurane on TNF-α-induced permeability and activation of p38 MAPK in rat pulmonary microvascular endothelial cells

Preconditioning with sevoflurane (SPC) diminishes effusion of rat alveolar membrane during inflammation. It is not clear whether this preconditioning directly inhibits permeability of pulmonary microvascular endothelial cell (PMVEC) monolayer. In this article, we evaluated effects of SPC on permeability of PMVEC monolayer and identified signaling pathways involved in these effects. PMVEC monolayer was exposed to different conditions (5-hydroxydecanoate (5-HD), TNF-α, SPC, SPC with subsequent exposure to TNF-α and 5-HD, and SPC with subsequent exposure to TNF-α alone), and the permeability of PMVEC monolayer was assessed using FITC-bovine serum albumin (ELISA). Expression of ICAM-1 (Western blot and RT-PCR) and activation of p38 MAPK (Western blot) were also assessed. Compared to the TNF-α group, permeability of PMVEC monolayer in the SPC + TNF-α group was significantly lower. Activation of p38 MAPK was also diminished in the TNF-α group. Pre-treatment with 5-HD reverted beneficial effects of SPC. Expression of ICAM-1 was not modulated by any of the tested experimental exposures. The results of this study demonstrate that SPC is capable of diminishing the TNF-α-induced increase of permeability of PMVEC monolayer, and that this beneficial effect is partly reversed by 5-HD. Further, SPC suppresses activation of p38 MAPK.

CRM197-induced blood-brain barrier permeability increase is mediated by upregulation of caveolin-1 protein

Cross-reacting material 197 (CRM197), a non-toxin mutant of diphtheria toxin, could act as a diphtheria toxin receptor-specific carrier protein for the targeted delivery of macromolecular substances across the blood-brain barrier (BBB) in vitro. This study was performed to investigate the effects and mechanisms of CRM197 on the permeability of BBB in guinea pigs. Data from the Evans blue extravasation showed that the BBB permeability significantly increased after CRM197 injection in a dose-dependent manner. Transmission electron microscopy indicated CRM197 could induce increased pinocytotic vesicles and vacuoles in brain microvascular endothelial cells. Immunohistochemistry and western blot assay revealed that CRM197 enhanced caveolin-1 protein expression in brain microvessels. The caveolin-1 protein in the membrane fraction of microvessels began to upregulate at 5 min and reached the peak at 10 min after CRM197 treatment, associated by diminished expression of several tight junction-associated proteins ZO-1, occludin, and claudin-5. Thus, our results indicate that the in vivo targeting CRM197 leads to increased BBB permeability via upregulation of caveolin-1 protein, increased pinocytotic vesicles, and redistribution of tight junction-associated proteins in brain microvessels. CRM197 may have a potential application for targeted drug delivery across the BBB.

Neutrophils cross the BBB primarily on transcellular pathways: an in vitro study

The cerebral microcapillary endothelium forms a highly important barrier between the blood and the interstitial fluid of the brain (blood-brain barrier) that controls the passage of molecules and cells in and out of the CNS. Several CNS diseases include leukocyte extravasation through the endothelium via two mechanistically distinct routes, the paracellular and the transcellular pathway. We established a new in vitro model of the inflamed blood-brain barrier consisting of primary cultured porcine brain capillary endothelial cells which express a tight endothelial barrier even under inflammatory conditions. By means of this specialized blood-brain barrier model we extensively studied the transmigration of neutrophils. Electron and scanning force microscopy as well as immunofluorescence imaging captured the penetrating neutrophil on the endothelial cellular body in between the junctions clearly suggesting a transcellular migration pathway. Electric cell-substrate impedance sensing and transendothelial electrical resistance measurements in combination with expression analysis of tight junction proteins demonstrate that the neutrophil-endothelial interaction does not disrupt the barrier. In conclusion, this study, based on an in vitro model of the blood-brain barrier under inflammatory conditions, evidently implicates that neutrophils preferentially migrate across the BBB via the transcellular route without impairing endothelial barrier function whereas paracellular transmigration plays only a minor role if the barrier is strongly expressed.

Excess neutrophil infiltration during cytomegalovirus brain infection of interleukin-10-deficient mice

Wild-type mice control murine cytomegalovirus (MCMV) brain infection, but identical infection is lethal to animals deficient in interleukin (IL)-10. Here, we report that MCMV-infected IL-10 knockout (KO) mice displayed a marked increase in neutrophil infiltration into the infected, IL-10-deficient brain when compared to wild-type animals. Enhanced microglial cell activation, determined by MHC class II up-regulation, overexpression of CXCL2, and elevated P-selectin mRNA levels were observed. In vivo blocking of CXCL2 attenuated neutrophil infiltration and significantly improved the outcome of infection. Collectively, these data indicate that the absence of IL-10 results in pathologic neutrophil infiltration into MCMV-infected brains.

Does telomerase reverse transcriptase induce functional de-differentiation of human endothelial cells?

By counteracting the shortening of chromosome telomeres, telomerase reverse transcriptase (hTERT) prevents senescence and age-related cell death. Embryonic cells display a high telomerase activity that declines rapidly with cell differentiation. Conversely, de-differentiated tumor cells tend to re-express telomerase. In view of the controversial data on the reciprocal correlation between cell proliferation and differentiation, we questioned whether telomerase overexpression and the resulting immortalization would affect the functional phenotype of human endothelial cells. Our comparative analysis addressed (1) distinct cell adhesion to different ECM-proteins analyzed on miniaturized multisubstrate arrays (MSA), (2) protein expression of diverse markers, (3) the uptake of DiI-Ac-LDL, (4) the inflammatory response based on upregulation of ICAM-1, (5) tube formation, and (6) the barrier properties of cell monolayers in transfilter cultures. Our results, based on some 40 data sets, demonstrate that immortalization of primary endothelial cells by hTERT maintains the typical endothelial characteristics without any sign of functional de-differentiation.

Proinflammatory effects of copper deficiency on neutrophils and lung endothelial cells

Dietary copper deficiency increases the accumulation of circulating neutrophils in the rat lung microcirculation. This process includes neutrophil adhesion to, migration along, and emigration though the vascular endothelium. The current study was designed to examine the role of copper in each of these steps. Neutrophils were isolated from rats fed either a copper-adequate (CuA, 6.1 microg Cu/g diet) or copper-deficient diet (CuD, 0.3 microg Cu/g diet) for 4 weeks. First, transient and firm adhesion of neutrophils to P-selectin in a flow chamber showed there were more adhered CuD neutrophils than CuA ones. This effect is probably caused by the increased expression of CD11b that was observed in the current study. Second, the evaluation of neutrophil migration under agarose showed that the CuD neutrophils moved farther than the CuA group in response to IL-8 but not fMLP; this suggests an increased sensitivity to a CD11/CD18-independent signalling pathway. Third, the contractile mechanism of endothelial cells was studied. Elevated F-actin formation in Cu-chelated lung microvascular endothelial cells suggests that neutrophil emigration may be promoted by enhanced cytoskeletal reorganization of the endothelium during copper deficiency. Combined, these results support the theory that dietary copper deficiency has proinflammatory effects on both neutrophils and the microvascular endothelium that promote neutrophil-endothelial interactions.

Vacuole formation in the endothelium of rat extremity vessels depends on fixation techniques and vessel type

Applying immersion fixation for electron microscopy, huge clear endothelial membrane-bound vacuoles of 0.1-3 microm diameter were noted in the extremity veins of Sprague-Dawley rats. Histological and electron microscopic histochemical methods were applied to determine whether they were the product of programmed cell death or any other kind of cell damage. Image analyzer was used to measure the total area of the vacuoles in the endothelium cells. Neither lipid content nor acidic phosphatase activity could be identified in the vacuoles. In saphenous and brachial veins, the vacuoles occupied 20.6 +/- 2.21% and 18 +/- 2.45% of the endothelium, respectively. Venous endothelium of two different strains of rat also contained the vacuoles. No such structures appeared in extremity arteries. Long-term tilting did not influence vacuolization. Using in vivo whole body fixation, only pinocytotic and dense microvesicles, but no vacuoles were noted. In conclusion, the clear vacuolar structures represent neither lipid inclusions nor secondary lysosomes. The method of tissue fixation is critical when venous endothelial vesicles are investigated. It is presumed that the vacuoles originated from intra- or intercellular microstructures, and that in case of the collapsible vein segments, their size is increased under the pathological-hypoxic and low-pressure-conditions of in vitro fixation.

Role of Rho kinase and actin filament in the increased vascular permeability of skin venules in rats after scalding

OBJECTIVE

To investigate the role of the Small GTPase Rho and endothelial cytoskeleton in the increased vascular permeability of rat skin after scalding.

METHODS

Rats were subjected to scalding local ventral skin and a venule was isolated from scalded skin and cannulated by micropipette. The venular permeability was measured with a fluorescence ratio technique and expressed with the permeability coefficient to albumin (P(a)). The venular F-actin filaments were observed by staining with rhodamine phalloidin and laser confocal scanning microscopy. A specific Rho kinase inhibitor Y-27632 was added into vessel bathing solution or preincubated with vessels to evaluate the role of Rho kinase in regulating of vascular barrier function.

RESULTS

Scalding increased P(a) value of skin venule about threefold compared to normal skin venules (P<0.01) and was maintained for 120 min. Inhibition of Rho kinase with Y-27632 (30 micromol/l in low-concentration group; 60 micromol/l in high-concentration group) significantly attenuated the hyperpermeability responses to scalding in a dose dependent fashion. A prominent peripheral actin rim (PAR) existed at the outer area of endothelial cells and apparently delineated the cell-to-cell borders. In the control group, the PARs were arranged smoothly and fairly continuously. However, occasionally PARs did show focal interruption with focal fluorescein isothiocyanate (FITC)-albumin leakage. In the burned group, PARs were less organized and accompanied by a large amount of FITC-albumin leakage. Inhibition of Rho kinase with Y-27632 dramatically reduced P(a) value with recovery of actin filament arrangement in venule after scalding.

CONCLUSION

Burn leads to dermal venular permeability increase with endothelial cytoskeleton depolymerization and disruption. Rho signal transduction pathway is involved in these responses.

Contribution of F-actin to barrier properties of the blood-joint pathway

OBJECTIVES

Because fibroblast filamentous actin (F-actin) influences cutaneous interstitial matrix swelling pressure (5), we investigated whether F-actin in fibroblast-derived synoviocytes influences the hydraulic permeability of the trans-synovial interstitial pathway. The study also tested whether F-actin in fenestrated synovial endothelium contributes to the blood-joint barrier in vivo.

METHODS

The clearance of Evans blue-albumin (EVA) from plasma into the knee joint cavity was determined in joint infused with F-actin disrupting cytochalasin D (1-200 microM), latrunculin B (100 microM) or vehicle in anesthetized rabbits. The hydraulic permeability of the lining was determined as the slope relating net trans-synovial flow Q(s) to intra-articular pressure P(j). Synovium was examined histologically after i.v. Monastral blue to assess endothelial leakiness.

RESULTS

EVA permeation in vivo was increased up to 25-fold by cytochalasin (p = 0.0002, ANOVA), with an EC(50) of 23 microM (95% confidence limits 13-43 microM). Washout quickly reversed the increase. Latrunculin had a similar effect. F-actin disruption switched Q(s) from drainage (control) to filtration into the cavity at low P(j) in vivo and raised the conductance Q(s)/dP(j) by 2.13 (p = 0.001, ANOVA). Circulatory arrest abolished these effects. Monastral blue revealed numerous endothelial leaks.

CONCLUSIONS

F-actin is crucial to the barrier function of fenestrated endothelium in situ. No significant effect of synoviocyte F-actin on matrix permeability was detected.

Recognition of fibrinogen by leukocyte integrins

Numerous studies have provided evidence that fibrinogen plays a multifaceted role in the immune and inflammatory response. The ability of fibrinogen to participate in the inflammatory response depends on its specific interaction with leukocyte cell surface adhesion receptors, integrins. Two leukocyte integrins, alpha M beta 2 (CD11b/CD18, Mac-1) and alpha X beta 2 (CD11c/CD18, p150,95), are the main fibrinogen receptors expressed on neutrophils, monocytes, macrophages and several subsets of lymphocytes. The recognition site for alpha M beta 2 has been previously mapped to the carboxyl-terminal globular gamma C domains (gamma 143-411) and two sequences, gamma 190-202 (P1) and gamma 377-395 (P2), were implicated as the putative binding sites. We now demonstrate that a second leukocyte integrin, alpha X beta 2, which is highly homologous to alpha M beta 2, mediates adhesion of the alpha X beta 2-bearing cells to the D fragment and to the recombinant gamma-module, gamma 143-411. Within the gamma C domain, alpha X beta 2 may recognize P1 and P2 sequences since synthetic peptides duplicating these sequences effectively inhibits adhesion of the alpha X beta 2-expressing cells to the D fragment. In addition, neutrophil inhibitory factor, NIF, a potent inhibitor of alpha X beta 2, also inhibited alpha X beta 2-mediated cell adhesion. These data suggest that recognition of the gamma C domain of fibrinogen by alpha M beta 2 and alpha X beta 2 may have common structural requirements.

VPF/VEGF and the angiogenic response

Vasculogenesis, the generation of new blood vessels de novo, and angiogenesis, the formation of new blood vessels from preexisting vessels, are mediated by a number of cytokines and growth factors among which vascular permeability factor/vascular endothelial growth factor (VPF/VEGF) is one of the most important. VPF/VEGF is secreted by many tumor cells, at sites of wound healing and chronic inflammation, and in physiological angiogenesis as in corpus luteum formation. VPF/VEGF is a multifunctional cytokine that interacts with two high affinity tyrosine kinase receptors that are selectively expressed on vascular endothelium. This interaction triggers an angiogenic cascade whose steps, among others, include increased microvascular permeability, leading to deposition of a pro-angiogenic extracellular fibrin matrix, and the formation of mother/daughter vessels.