Focal and regional variations in the composition of the glycocalyx of large vessel endothelium

Glycocalyx Disruption Triggers Human Monocyte Activation in Acute Heart Failure Syndromes

PURPOSE

Acute heart failure (AHF) syndromes manifest increased inflammation and vascular dysfunction; however, mechanisms that integrate the two in AHF remain largely unknown. The glycocalyx (GAC) is a sugar-based shell that envelops all mammalian cells. Much GAC research has focused on its role in vascular responses, with comparatively little known about how the GAC regulates immune cell function.

METHODS

In this study, we sought to determine if GAC degradation products are elevated in AHF patients, how these degradation products relate to circulating inflammatory mediators, and whether the monocyte GAC (mGAC) itself modulates monocyte activation. Inflammatory markers and GAC degradation products were profiled using ELISAs. Flow cytometry was used to assess the mGAC and RNA-seq was employed to understand the role of the mGAC in regulating inflammatory activation programs.

RESULTS

In a cohort of hospitalized AHF patients (n = 17), we found that (1) the GAC degradation product heparan sulfate (HS) was elevated compared with age-matched controls (4396 and 2903 ng/mL; p = 0.01) and that (2) HS and soluble CD14 (a marker of monocyte activation) levels were closely related (Pearson's r = 0.65; p = 0.002). Mechanistically, Toll-like receptor (TLR) activation of human monocytes results in GAC remodeling and a decrease in the mGAC (71% compared with no treatment; p = 0.0007). Additionally, we found that ex vivo enzymatic removal of HS and disruption of the mGAC triggers human monocyte activation and amplifies monocyte inflammatory responses. Specifically, using RNA-seq, we found that enzymatic degradation of the mGAC increases transcription of inflammatory (IL6, CCL3) and vascular (tissue factor/F3) mediators.

CONCLUSION

These studies indicate that the mGAC is dynamically remodeled during monocyte activation and that mGAC remodeling itself may contribute to the heightened inflammation associated with AHF.

Effects of different dehydration methods on the preservation of aortic and renal glycocalyx structures in mice

Glycocalyx is located outside the vascular endothelial cells playing an important role in vascular homeostasis. However, lacking efficient detection methods is one of the biggest obstacles to study the glycocalyx. In this study, three dehydration methods were used to compare the preservation of HUVEC, aorta and kidney glycocalyx by transmission electron microscope. The chemical pre-fixation was performed by lanthanum nitrate staining, and the mice aorta and renal glycocalyx were prepared by different dehydration methods such as ethanol gradient, acetone gradient and low temperature dehydration. HUVEC glycocalyx was prepared by acetone gradient and low temperature dehydration. Low temperature dehydration method preserves HUVEC and mice aortic glycocalyx completely, which had a certain thickness and presented a needle-like structure. But for mice kidney, the acetone gradient dehydration preparation method could better preserve the glycocalyx integrity than other two methods. In conclusion, low temperature dehydration method is suitable for HUVEC and aortic glycocalyx preservation, acetone gradient dehydration method is more suitable for kidney glycocalyx preservation.

Pharmacokinetics and pharmacodynamics in the treatment of cutaneous leishmaniasis - challenges and opportunities

Pharmacological efficacy is obtained when adequate concentrations of a potent drug reach the target site. In cutaneous leishmaniasis, a heterogeneous disease characterised by a variety of skin manifestations from simple nodules, skin discoloration, plaques to extensive disseminated forms, the parasites are found in the dermal layers of the skin. Treatment thus involves the release of the active compound from the formulation (administered either topically or systemically), it's permeation into the skin, accumulation by the local macrophages and further transport into the phagolysosome of the macrophage. The pharmacodynamic activity of a drug against the parasite is relatively straight forward to evaluate both in vivo and in vitro. The pharmacokinetic processes taking place inside the skin are more complex to elucidate due to the multi-lamellar structure of the skin, heterogeneous distribution of drugs within the tissue, the difficulty of accessing the site of infection complicating sampling and the lack of surrogate markers reflecting the activity of a drug in the skin. This review will discuss the difficulties encountered when investigating drug distribution, PK PD relationships and efficacy in the skin with a focus on cutaneous leishmaniasis treatment.

Glycocalyx sialic acids regulate Nrf2-mediated signaling by fluid shear stress in human endothelial cells

Activation of the nuclear factor erythroid 2-related factor 2 (Nrf2) pathway is critical for vascular endothelial redox homeostasis in regions of high, unidirectional shear stress (USS), however the underlying mechanosensitive mediators are not fully understood. The endothelial glycocalyx is disrupted in arterial areas exposed to disturbed blood flow that also exhibit enhanced oxidative stress leading to atherogenesis. We investigated the contribution of glycocalyx sialic acids (SIA) to Nrf2 signaling in human endothelial cells (EC) exposed to atheroprotective USS or atherogenic low oscillatory shear stress (OSS). Cells exposed to USS exhibited a thicker glycocalyx and enhanced turnover of SIA which was reduced in cells cultured under OSS. Physiological USS, but not disturbed OSS, enhanced Nrf2-mediated expression of antioxidant enzymes, which was attenuated following SIA cleavage with exogenous neuraminidase. SIA removal disrupted kinase signaling involved in the nuclear accumulation of Nrf2 elicited by USS and promoted mitochondrial reactive oxygen species accumulation. Notably, knockdown of the endogenous sialidase NEU1 potentiated Nrf2 target gene expression, directly implicating SIA in regulation of Nrf2 signaling by USS. In the absence of SIA, deficits in Nrf2 responses to physiological flow were also associated with a pro-inflammatory EC phenotype. This study demonstrates that the glycocalyx modulates endothelial redox state in response to shear stress and provides the first evidence of an atheroprotective synergism between SIA and Nrf2 antioxidant signaling. The endothelial glycocalyx therefore represents a potential therapeutic target against EC dysfunction in cardiovascular disease and redox dyshomeostasis in ageing.

Three-dimensional electron microscopy for endothelial glycocalyx observation using Alcian blue with silver enhancement

Glycocalyx (GCX) is a thin layer of negatively charged glycoproteins that covers the vascular endothelial surface and regulates various biological processes. Because of the delicate and fragile properties of this structure, it is difficult to detect GCX morphologically. We established a simple method for a three-dimensional visualization of endothelial GCX using low-vacuum scanning electron microscopy (LVSEM) on formalin-fixed paraffin-embedded (FFPE) sections. Mouse kidney tissue was fixed with 10% buffered formalin containing 1% Alcian blue (ALB) via perfusion and immersion. FFPE sections were observed by light microscopy (LM) and LVSEM, and formalin-fixed epoxy resin-embedded ultrathin sections were observed by transmission electron microscopy (TEM). The endothelial GCX from various levels of kidney blood vessels was stained blue in LM and confirmed as a thin osmiophilic layer in TEM. In LVSEM, the sections stained by periodic acid methenamine silver (PAM) revealed the endothelial GCX as a layer of dense silver-enhanced particles, in both the samples fixed via perfusion and immersion. Correlative light and electron microscopy (CLEM) revealed the fine visible structure of endothelial GCX. This simple method using FFPE samples with ALB will enable the three-dimensional evaluation of endothelial GCX alterations in various human diseases associated with endothelial injury in future studies.

Increased MMP activity in curved geometries disrupts the endothelial cell glycocalyx creating a proinflammatory environment

Wall shear stress gradients (WSSGs) induce an inflammatory phenotype in endothelial cells (ECs) which is hypothesized to be mediated by mechanotransduction through the EC glycocalyx (GCX). We used a three-dimensional in vitro cell culture model with a 180^o curved geometry to investigate if WSSGs created by curvature can cause EC inflammation and disruption of the GCX. The hydrodynamics of the model elicited a morphological response in ECs as well as a pattern of leukocyte adhesion towards the inner wall of curvature that was attenuated with enzymatic removal of GCX components. GCX degradation was also observed in regions of curvature which corresponded to increased activity of MMPs. Together, these results support the hypothesis that the EC GCX is involved in mechanotransduction of WSSGs and that components of the GCX are regulated by MMP activity in regions of curvature.

Role of the Glycocalyx as a Barrier to Leukocyte-Endothelium Adhesion

Leukocyte (WBC) to endothelial cell (EC) adhesion is a receptor-mediated process governed by the avidity and affinity of selectins, which modulate adhesive forces during WBC rolling, and integrins, which determine the strength of firm adhesion. Adhesion receptors on the EC surface lie below an endothelial surface layer (ESL) comprised of the EC glycocalyx and adsorbed proteins which, in vivo, have a thickness on the order 500 nm. The glycocalyx consists of a matrix of the glycosaminoglycans heparan sulfate and chondroitin sulfate, bound to proteoglycans and encased in hyaluronan. Together, these carbohydrates form a layer that varies in glycan content along the length of post-capillary venules where WBC-EC adhesion occurs. Thickness and porosity of the glycocalyx can vary dramatically during the inflammatory response as observed by increased infiltration and diffusion of macromolecules within the layer following activation of the EC by cytokines and chemoattractants. In models of inflammation in the living animal, the shedding of glycans and diminished thickness of the glycocalyx rapidly occur to facilitate penetration by the WBCs and adhesion to the EC. The primary effectors of glycan shedding appear to be metalloproteases and heparanase released by the EC. Retardation of glycan shedding and WBC-EC adhesion has been demonstrated in vivo using MMP inhibitors and low-molecular-weight heparin (LMWH), where the latter competitively binds to heparanase liberated by the EC. Together, these agents may serve to stabilize the ESL and provide a useful strategy for treatment of inflammatory disorders.

Optimal occlusion uniformly partitions red blood cells fluxes within a microvascular network

In animals, gas exchange between blood and tissues occurs in narrow vessels, whose diameter is comparable to that of a red blood cell. Red blood cells must deform to squeeze through these narrow vessels, transiently blocking or occluding the vessels they pass through. Although the dynamics of vessel occlusion have been studied extensively, it remains an open question why microvessels need to be so narrow. We study occlusive dynamics within a model microvascular network: the embryonic zebrafish trunk. We show that pressure feedbacks created when red blood cells enter the finest vessels of the trunk act together to uniformly partition red blood cells through the microvasculature. Using mathematical models as well as direct observation, we show that these occlusive feedbacks are tuned throughout the trunk network to prevent the vessels closest to the heart from short-circuiting the network. Thus occlusion is linked with another open question of microvascular function: how are red blood cells delivered at the same rate to each micro-vessel? Our analysis shows that tuning of occlusive feedbacks increase the total dissipation within the network by a factor of 11, showing that uniformity of flows rather than minimization of transport costs may be prioritized by the microvascular network.

Arterial trees shuttle red blood cells from the heart to billions of capillaries distributed throughout the body. These trees have long been thought to be organized to minimize transport costs. Yet red blood cells are tightly squeezed within the finest vessels, meaning that these vessels account for as much as half of the total transport costs within the arterial network. It is unclear why vessel diameters and red blood cell diameters are so closely matched in a network that is presumed to optimize transport. Here, we use mathematical modeling and direct observations of red blood cell movements in embryonic zebrafish to show that occlusive feedbacks—the pressure feedbacks that alter the flows into a vessel when it is nearly blocked by a red blood cell—can optimally distribute red blood cells through microvessels. In addition to revealing an adaptive function for the matching of vessel and red blood cell diameters, this work shows that uniformity of red blood cell fluxes can be a unifying principle for understanding the elegant hydraulic organization of microvascular networks.

Hemorrhagic Shock and the Microvasculature

The microvasculature plays a central role in the pathophysiology of hemorrhagic shock and is also involved in arguably all therapeutic attempts to reverse or minimize the adverse consequences of shock. Microvascular studies specific to hemorrhagic shock were reviewed and broadly grouped depending on whether data were obtained on animal or human subjects. Dedicated sections were assigned to microcirculatory changes in specific organs, and major categories of pathophysiological alterations and mechanisms such as oxygen distribution, ischemia, inflammation, glycocalyx changes, vasomotion, endothelial dysfunction, and coagulopathy as well as biomarkers and some therapeutic strategies. Innovative experimental methods were also reviewed for quantitative microcirculatory assessment as it pertains to changes during hemorrhagic shock. The text and figures include representative quantitative microvascular data obtained in various organs and tissues such as skin, muscle, lung, liver, brain, heart, kidney, pancreas, intestines, and mesentery from various species including mice, rats, hamsters, sheep, swine, bats, and humans. Based on reviewed findings, a new integrative conceptual model is presented that includes about 100 systemic and local factors linked to microvessels in hemorrhagic shock. The combination of systemic measures with the understanding of these processes at the microvascular level is fundamental to further develop targeted and personalized interventions that will reduce tissue injury, organ dysfunction, and ultimately mortality due to hemorrhagic shock. Published 2018. Compr Physiol 8:61-101, 2018.

Structural alteration of the endothelial glycocalyx: contribution of the actin cytoskeleton

The endothelial glycocalyx is a carbohydrate–protein layer that lines the luminal surface of the endothelium. It anchors to the cell membrane via its core proteins that share extended link to the actin cytoskeleton. It is widely accepted that those protein domains and the attached carbohydrates are susceptible to pathological changes. It is unclear, however, to what extent the actin cytoskeleton contributes to the glycocalyx stability. In this study, we investigate the role of the actin cytoskeleton in the maintenance of the glycocalyx under static and laminar flow conditions in vitro. Our results show that in the static culture medium neither rapid actin depolymerisation nor prolonged actin disturbance leads to glycocalyx disruption from the apical surface of human umbilical vein endothelial cells. However, when endothelial cells are exposed to laminar flow for 24 h, the glycocalyx is seen to shift to the downstream peripheral region of the cell surface. The mean fluorescence intensity decreases to \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$91.9 \pm 2.5\%$$\end{document}91.9±2.5% of the control. When actin depolymerisation is introduced, the intensity decreases significantly to \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$54.7 \pm 1.3\%$$\end{document}54.7±1.3%, indicating a severe disruption of the glycocalyx. Similar changes are observed in human aortic endothelial cells, where the intensity of the glycocalyx is reduced to \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$72.8 \pm 1.6\%$$\end{document}72.8±1.6% of the control. Collectively, we demonstrate that the actin cytoskeleton contributes to structural stability of the glycocalyx under shear stress. Our results can be used to develop new strategies to prevent shedding of the glycocalyx in cardiovascular diseases.

Regulation of blood vascular permeability in the skin

Regulation of blood vessel permeability is essential for the homeostasis of peripheral tissues. This regulation controls the trafficking of plasma contents, including water, vitamins, ions, hormones, cytokines, amyloids, lipoproteins, carrier proteins, and immunoglobulins. The properties of blood vessels vary among tissues based on their structural differences: continuous, fenestrated, or sinusoidal. These three types of blood vessels have different charge and size barrier properties. The anionic luminal glycocalyx layer on endothelial cells establishes the "charge barrier" that repels the attachment of negatively charged blood cells and plasma molecules. In contrast, the "size barrier" of blood vessels largely relies on the interendothelial junctions (IEJs) between endothelial cells, which define the paracellular permeability. As in most peripheral tissues, blood capillaries in the skin are composed of continuous and/or fenestrated blood vessels that have relatively tighter IEJs compared to those in the internal organs. Small vesicles in the capillary endothelium were discovered in the 1950s, and studies have since confirmed that blood endothelial cells transport the plasma contents by endocytosis and subsequent transcytosis and exocytosis-this process is called transcellular permeability. The permeability of blood vessels is highly variable as a result of intrinsic and extrinsic factors. It is significantly elevated upon tissue inflammations as a result of disabled IEJs and increased paracellular permeability due to inflammatory mediators. An increase in transcellular permeability during inflammation has also been postulated. Here, we provide an overview of the general properties of vascular permeability based on our recent observations of murine skin inflammation models, and we discuss its physiological significance in peripheral homeostasis.

Stenosis Hemodynamics Disrupt the Endothelial Cell Glycocalyx by MMP Activity Creating a Proinflammatory Environment

Hemodynamic forces are known to be able to induce an inflammatory phenotype in endothelial cells (ECs). The EC glycocalyx (GCX) is a dynamic structure which is regulated in response to different stimuli and hypothesized as an important contributor to the mechanotransduction of wall shear stresses (WSS). In this work, we used a three dimensional in vitro EC culture model with a 50% asymmetric stenosis to investigate degradation of the GCX by increased matrix metalloproteinase (MMP) activity in regions of WSS gradients and how this degradation might create a proinflammatory environment. Experiments showed GCX degradation was observed in regions of WSSGs created by a 50% asymmetric stenosis. Furthermore, inhibition of MMP activity abolished this regional degradation. The integrity of the GCX altered EC morphological elongation to flow and leukocyte adhesion patterns. These results help strengthen the hypothesis that the EC GCX is involved in the mechanotransduction of hemodynamic forces and that the GCX is regulated by MMP activity in regions of WSSGs.

Inhibition of inflammation induced shedding of the endothelial glycocalyx with low molecular weight heparin

The endothelial surface layer (ESL) consists of the endothelial cell (EC) glycocalyx and adsorbed proteins, and forms a barrier between blood and the EC. Enzymatic shedding of the ESL in response to cytokines may expose receptors for leukocyte (WBC) adhesion and increase vascular permeability. Thus, intravital microscopy was used to explore stabilization of the ESL with low molecular weight heparin (LMWH) to mitigate structural changes with inflammation. Following bolus infusions (i.v.) of LMWH (0.12-1.6mg/kg), shedding of glycans in response to 10^-7M fMLP was measured by loss of fluorescently labeled lectins bound to the EC and WBC-EC adhesion was monitored in post-capillary venules of rat mesentery. During a 30min exposure to fMLP, a 50% reduction in fluorescence (indicative of glycan shedding) occurred at the lowest dose of LMWH whereas a 50% increase occurred (indicative of ESL compaction) at the highest dose. Shedding was reduced by LMWH in a dose dependent manner with an EC50 of 0.6mg/kg. Concomitant WBC-EC adhesion increased over 3-fold for all doses of LMWH. However, at a dose of 1.6mg/kg, WBC-EC adhesion did not rise significantly during the initial 10min exposure to fMLP. Correlation of WBC adhesion with intensity of the lectin stain for all measurements revealed a significant 40% reduction in adhesion as intensity increased 50%. This relationship was attributed to LMWH inhibition of heparanase and/or binding to components of the glycocalyx that resulted in mitigation of glycan shedding, compaction of the lectin stain and stabilization of the glycocalyx.

Novel regulators of endothelial barrier function

Endothelial barrier function is an essential and tightly regulated process that ensures proper compartmentalization of the vascular and interstitial space, while allowing for the diffusive exchange of small molecules and the controlled trafficking of macromolecules and immune cells. Failure to control endothelial barrier integrity results in excessive leakage of fluid and proteins from the vasculature that can rapidly become fatal in scenarios such as sepsis or the acute respiratory distress syndrome. Here, we highlight recent advances in our understanding on the regulation of endothelial permeability, with a specific focus on the endothelial glycocalyx and endothelial scaffolds, regulatory intracellular signaling cascades, as well as triggers and mediators that either disrupt or enhance endothelial barrier integrity, and provide our perspective as to areas of seeming controversy and knowledge gaps, respectively.

Rubbing salt into wounded endothelium: sodium potentiates proatherogenic effects of TNF-α under non-uniform shear stress

Increased consumption of sodium is a risk factor for hypertension and cardiovascular diseases. In vivo studies indicated that high dietary sodium may have a direct negative influence on endothelium. We investigated the effects of high sodium on the endothelial activation during early steps of atherogenesis. Endothelial cells (HUVECs) grown in a model of arterial bifurcations were exposed to shear stress in the presence of normal or high (+ 30 mmol/l) sodium. Adherent THP-1 cells, and the adhesion molecule expression were quantified. Sodium channel blockers, pathways' inhibitors, and siRNA against tonicity-responsive enhancer binding protein (TonEBP) were used to identify the mechanisms of sodium effects on endothelium. ApoE-deficient mice on low-fat diet received water containing normal or high salt (8% w/v) for four weeks, and the influence of dietary salt on inflammatory cell adhesion in the common carotid artery and carotid bifurcation was measured by intravital microscopy. In vitro, high sodium dramatically increased the endothelial responsiveness to tumour necrosis factor-α under non-uniform shear stress. Sodium-induced increase in monocytic cell adhesion was mediated by reactive oxygen species and the endothelial nitric oxygen synthase, and was sensitive to the knockdown of TonEBP. The results were subsequently confirmed in the ApoE-deficient mice. As compared with normal-salt group, high-salt intake significantly enhanced the adhesion of circulating CD11b+ cells to carotid bifurcations, but not to the straight segment of common carotid artery. In conclusion, elevated sodium has a direct effect on endothelial activation under atherogenic shear stress in vitro and in vivo, and promotes the endothelial-leukocyte interactions even in the absence of increased lipid concentrations.

Nanomechanics of the endothelial glycocalyx in experimental sepsis

The endothelial glycocalyx (eGC), a carbohydrate-rich layer lining the luminal side of the endothelium, regulates vascular adhesiveness and permeability. Although central to the pathophysiology of vascular barrier dysfunction in sepsis, glycocalyx damage has been generally understudied, in part because of the aberrancy of in vitro preparations and its degradation during tissue handling. The aim of this study was to analyze inflammation-induced damage of the eGC on living endothelial cells by atomic-force microscopy (AFM) nanoindentation technique. AFM revealed the existence of a mature eGC on the luminal endothelial surface of freshly isolated rodent aorta preparations ex vivo, as well as on cultured human pulmonary microvascular endothelial cells (HPMEC) in vitro. AFM detected a marked reduction in glycocalyx thickness (266 ± 12 vs. 137 ± 17 nm, P<0.0001) and stiffness (0.34 ± 0.03 vs. 0.21 ± 0.01 pN/mn, P<0.0001) in septic mice (1 mg E. coli lipopolysaccharides (LPS)/kg BW i.p.) compared to controls. Corresponding in vitro experiments revealed that sepsis-associated mediators, such as thrombin, LPS or Tumor Necrosis Factor-α alone were sufficient to rapidly decrease eGC thickness (-50%, all P<0.0001) and stiffness (-20% P<0.0001) on HPMEC. In summary, AFM nanoindentation is a promising novel approach to uncover mechanisms involved in deterioration and refurbishment of the eGC in sepsis.

The effect of doxycycline on shedding of the glycocalyx due to reactive oxygen species

The structure and composition of the endothelial cell (EC) glycocalyx reflect a balance of the biosynthesis of glycans and their shear dependent removal. Shedding of glycans from the EC surface has been shown to occur in response to reactive oxygen species (ROS) and inflammatory mediators. Using sub-antimicrobial doses of doxycycline, a broad spectrum matrix metalloprotease (MMP) inhibitor, inhibition of chemoattractant induced glycan shedding has suggested that MMPs may be a major effector of the loss of glycans. However, it has also been reported that doxycycline is a scavenger of ROS that may also activate MMPs. To clarify the basis for doxycycline as an inhibitor of glycan shedding, the present studies were undertaken to determine its effect on ROS induced shedding in post-capillary venules of the exteriorized mesentery of the rat. To this end, hypoxanthine (HX) and xanthine oxidase (XO) were rapidly mixed on the mesenteric surface for a 2min period to generate superoxide anion (O2(-)·) and the time course of glycan shedding was monitored in post-capillary venules over a 30min period. Glycan shedding was quantitated by loss of adherent fluorescently labeled lectin coated microspheres (FLMs, 0.1μm diameter) that were systemically infused. It was found that HX/XO caused FLM adhesion to decrease 45% within 30min. This effect could be inhibited in a dose dependent manner by the addition of superoxide dismutase to the superfusion solution, thus confirming the role of O2(-)·. In contrast, 0.5μM doxycycline had no effect on FLM shedding in response to HX/XO, contrary to its ability to attenuate shedding in response to the chemoattractant fMLP. Thus it is suggested that the efficacy of doxycycline as an inhibitor of glycan shedding during inflammation arises from its ability to inhibit MMP activation.

The structural stability of the endothelial glycocalyx after enzymatic removal of glycosaminoglycans

Rationale

It is widely believed that glycosaminoglycans (GAGs) and bound plasma proteins form an interconnected gel-like structure on the surface of endothelial cells (the endothelial glycocalyx layer–EGL) that is stabilized by the interaction of its components. However, the structural organization of GAGs and proteins and the contribution of individual components to the stability of the EGL are largely unknown.

Objective

To evaluate the hypothesis that the interconnected gel-like glycocalyx would collapse when individual GAG components were almost completely removed by a specific enzyme.

Methods and Results

Using confocal microscopy, we observed that the coverage and thickness of heparan sulfate (HS), chondroitin sulfate (CS), hyaluronic acid (HA), and adsorbed albumin were similar, and that the thicknesses of individual GAGs were spatially nonuniform. The individual GAGs were degraded by specific enzymes in a dose-dependent manner, and decreased much more in coverage than in thickness. Removal of HS or HA did not result in cleavage or collapse of any of the remaining components. Simultaneous removal of CS and HA by chondroitinase did not affect HS, but did reduce adsorbed albumin, although the effect was not large.

Conclusion

All GAGs and adsorbed proteins are well inter-mixed within the structure of the EGL, but the GAG components do not interact with one another. The GAG components do provide binding sites for albumin. Our results provide a new view of the organization of the endothelial glycocalyx layer and provide the first demonstration of the interaction between individual GAG components.

The glycocalyx and cardiovascular disease in diabetes: should we judge the endothelium by its cover?

Patients with diabetes mellitus are characterized by an extraordinary vascular vulnerability. Traditionally, glucose-induced damage to the vascular endothelium is believed to be one of the first steps in the development of vascular damage. However, in the healthy vessel the endothelium is protected by a matrix layer of highly glycosylated proteins that form a physical barrier between the endothelium and the blood flowing past. Although its presence has been known for half a century, this so-called glycocalyx earned little attention from researchers in the past because of an underestimation of its size. In the last decade it has become clear that its full thickness actually exceeds that of the vascular endothelium. Accumulating research into the functional relevance of the endothelial glycocalyx suggests an important role for this layer in the development of cardiovascular disease in diabetes mellitus. Here we will present an overview of the biochemistry of the intact glycocalyx, current methods to assess the glycocalyx, and its possible role in the pathophysiology of cardiovascular disease in diabetes.

Quantification of the endothelial surface glycocalyx on rat and mouse blood vessels

The glycocalyx on the surface of endothelium lining blood vessel walls modulates vascular barrier function, cell adhesion and also serves as a mechano-sensor for blood flow. Reduction of glycocalyx has been reported in many diseases including atherosclerosis, inflammation, myocardial edema, and diabetes. The surface glycocalyx layer (SGL) is composed of proteoglycans and glycosaminoglycans, of which heparan sulfate is one of the most abundant. To quantify the SGL thickness on the microvessels of rat mesentery and mouse cremaster muscle in situ, we applied a single vessel cannulation and perfusion technique to directly inject FITC-anti-heparan sulfate into a group of microvessels for immuno-labeling the SGL. We also used anti-heparan sulfate for immuno-labeling the SGL on rat and mouse aortas ex vivo. High resolution confocal microscopy revealed that the thickness of the SGL on rat mesenteric capillaries and post-capillary venules is 0.9±0.1 μm and 1.2±0.3 μm, respectively; while the thickness of the SGL on mouse cremaster muscle capillaries and post-capillary venules is 1.5±0.1 μm and 1.5±0.2 μm, respectively. Surprisingly, there was no detectable SGL in either rat mesenteric or mouse cremaster muscle arterioles. The SGL thickness is 2.5±0.1 μm and 2.1±0.2 μm respectively, on rat and mouse aorta. In addition, we observed that the SGL is continuously and evenly distributed on the aorta wall but not on the microvessel wall.

The endothelial glycocalyx as a barrier to leukocyte adhesion and its mediation by extracellular proteases

The endothelial cell (EC) surface is coated with a layer of polysaccharides linked to membrane-bound and trans-membrane proteoglycans that comprise the glycocalyx, which is augmented by adsorbed proteins derived from the blood stream. This surface layer has been shown to affect hemodynamics in small blood vessels of the microcirculation, the resistance to flow, and leukocyte (WBC) to EC adhesion. Parallel studies of WBC-EC adhesion in response to chemoattractants and cytokines, and shedding of constituents of the glycocalyx, have suggested a role for activation of extracellular proteases in mediating the dynamics of WBC adhesion in response to inflammatory and ischemic stimuli. Likely candidates among the many proteases present are the matrix metalloproteases (MMPs). Inhibition of MMP activation with sub-antimicrobial doses of doxycycline, or zinc chelators, has also inhibited WBC adhesion and shedding of glycans from the EC surface in response to the chemoattractant fMLP. Taken together, these studies suggest that shedding of the EC glycocalyx exposes adhesion receptors and thus enhances WBC-EC adhesion. Future therapeutic strategies for treating pathologies such as the low flow state and inflammation may benefit by further exploration of the mechanics of the glycocalyx in light of protease activation and shear-dependent effects.

Shedding of the endothelial glycocalyx in arterioles, capillaries, and venules and its effect on capillary hemodynamics during inflammation

The endothelial glycocalyx has been identified as a barrier to transvascular exchange of fluid, macromolecules, and leukocyte-endothelium [endothelial cell (EC)] adhesion during the inflammatory process. Shedding of glycans and structural changes of the glycocalyx have been shown to occur in response to several agonists. To elucidate the effects of glycan shedding on microvascular hemodynamics and capillary resistance to flow, glycan shedding in microvessels in mesentery (rat) was induced by superfusion with 10(-7) M fMLP. Shedding was quantified by reductions of fluorescently labeled lectin (BS-1) bound to the EC and reductions in thickness of the barrier to infiltration of 70-kDa dextran on the EC surface. Red cell velocities (two-slit technique), pressure drops (dual servo-null method), and capillary hematocrit (direct cell counting) were measured in parallel experiments. The results indicate that fMLP caused shedding of glycans in all microvessels with reductions in thickness of the barrier to 70-kDa dextran of 110, 80, and 123 nm, in arterioles, capillaries, and venules, respectively. Intravascular volumetric flows fell proportionately in all three divisions in response to rapid obstruction of venules by white blood cell (WBC)-EC adhesion, and capillary resistance to flow rose 18% due to diminished deformability of activated WBCs. Capillary resistance fell significantly 26% over a 30-min period, as glycans were shed from the EC surface to increase effective capillary diameter, whereas capillary hematocrit and anatomic diameter remained invariant. This decrease in capillary resistance mitigates the increase in resistance due to diminished WBC deformability, and hence these concurrent rheological events may be of equal importance in affecting capillary flow during the inflammatory process.

Compositional or charge density modification of the endothelial glycocalyx accelerates flow-dependent concentration polarization of low-density lipoproteins

We hypothesized that diminished endothelial glycocalyx (GCX) at atherosclerotic lesion-prone sites accelerates flow-dependent concentration polarization of low-density lipoproteins (LDLs) at the luminal surface, and in turn contributes to vulnerability of these sites to atherosclerosis. A parallel plate flow chamber was applied to expose cultured endothelial monolayers to three different levels of shear stress (3, 12, 20 dyn/cm2). Heparinase III (Hep.III) was employed to degrade heparan sulfate proteoglycans selectively and 3-(N-morpholino) propanesulfonic acid-buffered physiological salt solutions (MOPS-PSS) were used at either normal ionic strength (Normal-MOPS), low ionic strength (LO-MOPS) or high ionic strength (HI-MOPS) to modify the effective charge density of the endothelial GCX. Water filtration velocity ( V w) across the endothelial monolayer, the luminal concentration of LDLs ( C w) and the uptake of LDLs by endothelial cells were measured and compared among the following five groups of cells: (1) Control; (2) Hep.III treatment; (3) LO-MOPS; (4) Normal-MOPS; and (5) HI-MOPS. The results obtained substantiated the aforementioned hypothesis and demonstrated that compositional or charge density modification of the endothelial GCX facilitated water filtration across the endothelium, enhanced the accumulation of LDLs on the luminal surface and increased the uptake of LDLs by endothelial cells, therefore contributing to atherogenesis.

Stiffness and heterogeneity of the pulmonary endothelial glycocalyx measured by atomic force microscopy

The mechanical properties of endothelial glycocalyx were studied using atomic force microscopy with a silica bead (diameter ∼18 μm) serving as an indenter. Even at indentations of several hundred nanometers, the bead exerted very low compressive pressures on the bovine lung microvascular endothelial cell (BLMVEC) glycocalyx and allowed for an averaging of stiffness in the bead-cell contact area. The elastic modulus of BLMVEC glycocalyx was determined as a pointwise function of the indentation depth before and after enzymatic degradation of specific glycocalyx components. The modulus-indentation depth profiles showed the cells becoming progressively stiffer with increased indentation. Three different enzymes were used: heparinases III and I and hyaluronidase. The main effects of heparinase III and hyaluronidase enzymes were that the elastic modulus in the cell junction regions increased more rapidly with the indentation than in BLMVEC controls, and that the effective thickness of glycocalyx was reduced. Cytochalasin D abolished the modulus increase with the indentation. The confocal profiling of heparan sulfate and hyaluronan with atomic force microscopy indentation data demonstrated marked heterogeneity of the glycocalyx composition between cell junctions and nuclear regions.

Protease Activity and the Role of the Endothelial Glycocalyx in Inflammation

A new paradigm for governance of leukocyte-endothelium (WBC-EC) adhesion during the inflammatory process is presented in which shedding of the endothelial glycocalyx exposes adhesion molecules on the EC surface, which promotes WBC-EC adhesion. It is postulated that the effector of this shedding is the activation of extracellular proteases, one of which may be a member of the matrix metalloproteinase (MMP) family of zinc dependent endopetidases. This model for the role of the glycocalyx as a barrier to WBC-EC adhesion includes the additional participation of normally active extracellular proteolytic enzymes, i.e. sheddases, which may cleave proteoglycans or activate lyases that cleave GAG chains in the glycocalyx. In support of this hypothesis, studies are examined which have established the concurrent activation of MMP proenzymes on the EC surface, shedding of the glycocalyx, and enhanced WBC-EC adhesion.

Myeloperoxidase attracts neutrophils by physical forces

Recruitment of polymorphonuclear neutrophils (PMNs) remains a paramount prerequisite in innate immune defense and a critical cofounder in inflammatory vascular disease. Neutrophil recruitment comprises a cascade of concerted events allowing for capture, adhesion and extravasation of the leukocyte. Whereas PMN rolling, binding, and diapedesis are well characterized, receptor-mediated processes, mechanisms attenuating the electrostatic repulsion between the negatively charged glycocalyx of leukocyte and endothelium remain poorly understood. We provide evidence for myeloperoxidase (MPO), an abundant PMN-derived heme protein, facilitating PMN recruitment by its positive surface charge. In vitro, MPO evoked highly directed PMN motility, which was solely dependent on electrostatic interactions with the leukocyte's surface. In vivo, PMN recruitment was shown to be MPO-dependent in a model of hepatic ischemia and reperfusion, upon intraportal delivery of MPO and in the cremaster muscle exposed to local inflammation or to intraarterial MPO application. Given MPO's affinity to both the endothelial and the leukocyte's surface, MPO evolves as a mediator of PMN recruitment because of its positive surface charge. This electrostatic MPO effect not only displays a so far unrecognized, catalysis-independent function of the enzyme, but also highlights a principal mechanism of PMN attraction driven by physical forces.

Composition of the endothelial glycocalyx and its relation to its thickness and diffusion of small solutes

The endothelial glycocalyx is well endowed with the glycosaminoglycans (GAGs) heparan sulfate, chondroitin sulfate and hyaluronan. The current studies aimed to assess the relative contributions of each of these GAGs to the thickness and permeability of the glycocalyx layer by direct enzymatic removal of each using micropipettes to infuse heparinase, chondroitinase and hyaluronidase into post-capillary venules of the intestinal mesentery of the rat. The relative losses of GAGs due to enzymatic removal were compared with stimulated shedding of glycans induced by superfusing the mesentery with 10(-)(7)M fMLP. Thickness of the glycocalyx was assessed by infiltration of the glycocalyx with circulating FITC labeled 70kDa dextran (Dx70) and measuring the distance from the dye front to the surface of the endothelium (EC), which averaged 463nm under control conditions. Reductions in thickness were 43.3%, 34.1% and 26.1% following heparinase, chondroitinase and hyaluronidase, respectively, and 89.7% with a mixture of all three enzymes. Diffusion coefficients of FITC in the glycocalyx were determined using a 1-D diffusion model. By comparison of measured transients in radial intensity of a bolus of FITC with that of a computational model a diffusion coefficient D was obtained. Values of D were obtained corresponding to the thickness of the layer demarcated by Dx70 (D(Dx70)), and a smaller sublayer 173nm above the EC surface (D(173)), prior to and following enzyme infusion and superfusion with fMLP. The magnitude of D(Dx70) was twice that of D(173) suggesting that the glycocalyx is more compact near the EC surface. Chondroitinase and hyaluronidase significantly increased both D(Dx70) and D(173). However, heparinase decreased D(Dx70), and did not induce any significant change for the D(173). These observations suggest that the three GAGs are not evenly distributed throughout the glycocalyx and that they each contribute to permeability of the glycocalyx to a differing extent. The fMLP-induced shedding caused a reduction in glycocalyx thickness (which may increase permeability) and as with heparinase, decreased the diffusion coefficient of solutes (which may decrease permeability). This behavior suggests that the removal of heparan sulfate may cause a collapse of the glycocalyx which counters decreases in thickness by compacting the layer to maintain a constant resistance to filtration.

Therapeutic strategies targeting the endothelial glycocalyx: acute deficits, but great potential

Damage of the endothelial glycocalyx, which ranges from 200 to 2000 nm in thickness, decreases vascular barrier function and leads to protein extravasation and tissue oedema, loss of nutritional blood flow, and an increase in platelet and leucocyte adhesion. Thus, its protection or the restoration of an already damaged glycocalyx seems to be a promising therapeutic target both in an acute critical care setting and in the treatment of chronic vascular disease. Drugs that can specifically increase the synthesis of glycocalyx components, refurbish it, or selectively prevent its enzymatic degradation do not seem to be available. Pharmacological blockers of radical production may be useful to diminish the oxygen radical stress on the glycocalyx. Tenable options are the application of hydrocortisone (inhibiting mast-cell degranulation), use of antithrombin III (lowering susceptibility to enzymatic attack), direct inhibition of the cytokine tumour necrosis factor-alpha, and avoidance of the liberation of natriuretic peptides (as in volume loading and heart surgery). Infusion of human plasma albumin (to maintain mechanical and chemical stability of the endothelial surface layer) seems the easiest treatment to implement.

Dextran sulfate modulates MAP kinase signaling and reduces endothelial injury in a rat aortic clamping model

OBJECTIVE

Mitogen-activated protein kinases (MAPKs), including JNK, p38, and ERK1/2, noticeably influence ischemia/reperfusion injury (IRI). The complement inhibitor dextran sulfate (DXS) associates with damaged endothelium denudated of its heparan sulfate proteoglycan (HSPG) layer. Other glycosaminoglycan analogs are known to influence MAPK signaling. Hypothetically therefore, targeted intravascular cytoprotection by DXS may function in part through influencing MAPK activation to reduce IRI-induced damage of the vasculature.

METHODS

IRI of the infrarenal aorta of male Wistar rats was induced by 90 minutes clamping followed by 120 minutes reperfusion. DXS (5 mg/mL) or physiologic saline (NaCl controls) was infused locally into the ischemic aortic segment immediately prior to reperfusion. Ninety minutes ischemia-only and heparinase infusion (maximal damage) experiments, as well as native rat aorta, served as controls. Aortas were excised following termination of the experiments for further analysis.

RESULTS

DXS significantly inhibited IRI-induced JNK and ERK1/2 activation (P = .043; P =.005) without influencing the p38 pathway (P =.110). Reduced aortic injury, with significant inhibition of apoptosis (P = .032 for DXS vs NaCl), correlated with decreased nuclear factor kappaB translocation within the aortic wall. DXS treatment clearly reduced C1q, C4b/c, C3b/c, and C9 complement deposition, whilst preserving endothelial cell integrity and reducing reperfusion-induced HSPG shedding. Protection was associated with binding of fluorescein labeled DXS to ischemically damaged tissue.

CONCLUSIONS

Local application of DXS into ischemic vasculature immediately prior to reperfusion reduces complement deposition and preserves endothelial integrity, partially through modulating activation of MAPKs and may offer a new approach to tackle IRI in vascular surgical procedures.

CLINICAL RELEVANCE

The purpose of the present study was to determine the role of dextran sulfate (DXS), a glycosaminoglycan analog and complement inhibitor, in modulating intracellular MAPK signaling pathways, reducing complement activation and ultimately attenuating ischemia/reperfusion injury (IRI) in a rat aortic-clamping model, in part a surrogate model to study the microvasculature. The study shows a role for DXS in ameliorating endothelial injury by reducing IRI-mediated damage and intravascular, local inflammation in the affected aortic segment. DXS may be envisaged as an endothelial protectant in vascular injury, such as occurs during vascular surgical procedures.

The endothelial glycocalyx is hydrodynamically relevant in arterioles throughout the cardiac cycle

The existence of a hydrodynamically relevant endothelial glycocalyx of approximately 0.5 microm in thickness is well established in capillaries and venules in vivo. Since the glycocalyx is likely to have implications for broad areas of vascular physiology and pathophysiology, including endothelial-cell mechanotransduction, vascular permeability, and atherosclerosis, it is necessary to determine the extent to which the glycocalyx is present on arteriolar endothelium. We applied microviscometric analysis to data obtained using microparticle image velocimetry in cremaster-muscle arterioles of wild-type mice. Due to the pulsatile nature of the flow regimes in arterioles, data acquisition was triggered with the electrocardiogram at specific time points in the cardiac cycle. Results show the existence of a hydrodynamically relevant glycocalyx having a mean thickness of 0.38 microm in arterioles approximately 20-70 microm in diameter (n = 20), which is approximately 0.13 microm thinner (p = 0.03) than that found previously in venules having a similar diameter range and under similar hemodynamic conditions. Results from data obtained at multiple time points in the cardiac cycle show that the glycocalyx remains hydrodynamically relevant in arterioles with statistically insignificant changes in mean thickness throughout the cardiac cycle, despite the inherent unsteadiness of the flow regimes in these microvessels. These results provide direct in vivo confirmation of the existence of a hydrodynamically relevant surface glycocalyx that essentially eliminates fluid shear stress on arteriolar endothelium throughout the entire cardiac cycle.

Properties of the Glomerular Barrier and Mechanisms of Proteinuria

This review focuses on the intricate properties of the glomerular barrier. Other reviews have focused on podocyte biology, mesangial cells, and the glomerular basement membrane (GBM). However, since all components of the glomerular membrane are important for its function, proteinuria will occur regardless of which layer is affected by disease. We review the properties of endothelial cells and their surface layer, the GBM, and podocytes, discuss various methods of studying glomerular permeability, and analyze data concerning the restriction of solutes by size, charge, and shape. We also review the physical principles of transport across biological or artificial membranes and various theoretical models used to predict the fluxes of solutes and water. The glomerular barrier is highly size and charge selective, in qualitative agreement with the classical studies performed 30 years ago. The small amounts of albumin filtered will be reabsorbed by the megalin-cubulin complex and degraded by the proximal tubular cells. At present, there is no unequivocal evidence for reuptake of intact albumin from urine. The cellular components are the key players in restricting solute transport, while the GBM is responsible for most of the resistance to water flow across the glomerular barrier.

Glomerular endothelial glycocalyx constitutes a barrier to protein permeability

Glycocalyx, composed of glycoproteins including proteoglycans, coats the luminal surface of the glomerular capillaries. Human heparanase degrades heparan sulphate glycosaminoglycans and is up-regulated in proteinuric states. In this study, we analyze the structure of the human glomerular endothelial cell glycocalyx in vitro and examine its functional relevance, especially after treatment with human heparanase. Electron microscopy of conditionally immortalized glomerular endothelial cells revealed a 200-nm thick glycocalyx over the plasma membrane, which was also demonstrated by confocal microscopy. Neuraminidase treatment removed the majority of glycocalyx, reduced trans-endothelial electrical resistance by 59%, and increased albumin flux by 207%. Heparinase III and human heparanase specifically cleaved heparan sulphate: this caused no change in trans-endothelial electrical resistance, but increased the albumin passage across the monolayers by 40% and 39%, respectively. Therefore, we have characterized the glomerular endothelial cell glycocalyx and have shown that it contributes to the barrier to flux of albumin across the cell layer. These results suggest an important role for this glycocalyx in the restriction of glomerular protein passage in vivo and suggest ways in which human heparanase levels may be linked to proteinuria in clinical disease.

Normal endothelium

In recent decades, it has become evident that the endothelium is by no means a passive inner lining of blood vessels. This 'organ' with a large surface (approximately 350 m2) and a comparatively small total mass (approximately 110 g) is actively involved in vital functions of the cardiovascular system, including regulation of perfusion, fluid and solute exchange, haemostasis and coagulation, inflammatory responses, vasculogenesis and angiogenesis. The present chapter focusses on two central aspects of endothelial structure and function: (1) the heterogeneity in endothelial properties between species, organs, vessel classes and even within individual vessels and (2) the composition and role of the molecular layer on the luminal surface of endothelial cells. The endothelial lining of blood vessels in different organs differs with respect to morphology and permeability and is classified as 'continuous', 'fenestrated' or 'discontinuous'. Furthermore, the mediator release, antigen presentation or stress responses of endothelial cells vary between species, different organs and vessel classes. Finally there are relevant differences even between adjacent endothelial cells, with some cells exhibiting specific functional properties, e.g. as pacemaker cells for intercellular calcium signals. Organ-specific structural and functional properties of the endothelium are marked in the vascular beds of the lung and the brain. Pulmonary endothelium exhibits a high constitutive expression of adhesion molecules which may contribute to the margination of the large intravascular pool of leucocytes in the lung. Furthermore, the pulmonary microcirculation is less permeable to protein and water flux as compared to large pulmonary vessels. Endothelial cells of the blood-brain barrier exhibit a specialised phenotype with no fenestrations, extensive tight junctions and sparse pinocytotic vesicular transport. This barrier allows a strict control of exchange of solutes and circulating cells between the plasma and the interstitial space. It was observed that average haematocrit levels in muscle capillaries are much lower as compared to systemic haematocrit, and that flow resistance of microvascular beds is higher than expected from in vitro studies of blood rheology. This evidence stimulated the concept of a substantial layer on the luminal endothelial surface (endothelial surface layer, ESL) with a thickness in the range of 0.5-1 microm. In comparison, the typical thickness of the glycocalyx directly anchored in the endothelial plasma membrane, as seen in electron micrographs, amounts to only about 50-100 microm. Therefore it is assumed that additional components, e.g. adsorbed plasma proteins or hyaluronan, are essential in constituting the ESL. Functional consequences of the ESL presence are not yet sufficiently understood and acknowledged. However, it is evident that the thick endothelial surface layer significantly impacts haemodynamic conditions, mechanical stresses acting on red cells in microvessels, oxygen transport, vascular control, coagulation, inflammation and atherosclerosis.

Change in Properties of the Glycocalyx Affects the Shear Rate and Stress Distribution on Endothelial Cells

The endothelial glycocalyx mediates interactions between the blood flow and the endothelium. This study aims to evaluate, quantitatively, effects of structural change of the glycocalyx on stress distribution and shear rate on endothelial cells. In the study, the endothelial glycocalyx is modeled as a surface layer of fiber matrix and when exposed to laminar shear flow, the matrix deforms. Fluid velocity and stress distribution inside the matrix and on cell membranes are studied based on a binary mixture theory. Parameters, such as the height and porosity of the matrix and the drag coefficient between fluid and matrix fibrils, are based on available data and estimation from experiments. Simple theoretical solutions are achieved for fluid velocity and stress distribution in the surface matrix. Degradation of the matrix, e.g., by enzyme digestion, is represented by reductions in the volume fraction of fibrils, height, and drag coefficient. From a force balance, total stress on endothelial surface remains constant regardless of structural alteration of the glycocalyx. However, the stress that is transmitted to endothelial cells by direct “pulling” of fiber branches of the glycocalyx is reduced significantly. Fluid shear rate at the cell membrane, on the other hand, increases. The study gives quantitative insight into the effect of the structural change of the glycocalyx on the shear rate and pulling stress on the endothelium. Results can be used to interpret experiments on effects of the glycocalyx in shear induced endothelial responses.

Change in shear stress (Deltatau)/hydraulic conductivity (Lp) relationship after pronase treatment of individual capillaries in situ

A complex glycoprotein meshwork covers the inner wall of blood vessels and is implicated in mechanotransduction of fluid shear stress (tau). A relationship between Deltatau and capillary Lp has been established. The purpose of this study was to evaluate Lp in response to Deltatau after exposing the capillary lumen to a mild, non-specific protease selected to disrupt its inner matrix. We hypothesized that Lp would not correlate with Deltatau after enzyme treatment. Frogs (Rana pipiens, n=69) were pithed and the mesentery was exteriorized. Lp was assessed at 30 cm H2O using the modified Landis technique after an abrupt, square wave Deltatau produced by a physiologically relevant increase in pressure. Perfusate solutions were 10 mg ml-1 BSA/frog Ringer's (Control) or 0.1 mg ml-1 pronase in BSA/Ringer's (1 min) then BSA/Ringer's alone (Test). Mean (+/-SE) control Lp following Deltatau was 2.2+/-0.2 x 10(-7) cm s-1 cm H2O-1 and individual values correlated positively with Deltatau (r=0.85, P<0.0001, n=41). After pronase, mean Test Lp (17.6+/-2.5 x 10(-7) cm s-1 cm H2O-1) was higher compared to control and Deltatau/Lp plots revealed two subsets of capillaries. Lp correlated strongly with Deltatau in capillaries with diameters<or=15 microm (r=0.91, P=0.0006, n=14) and also in a second subset of capillaries with diameters >15 microm (r=0.96, P=0.0001, n=8). Slopes were 3.9- and 8.7-fold higher, respectively, compared to control. These data suggest a protective role for luminal constituents of intact capillaries. Mechanisms involved in capillary responses to flow-induced, mechanical stimuli may be located in the cellular structures that form capillaries.

Vasculoprotective properties of the endothelial glycocalyx: effects of fluid shear stress

The endothelial glycocalyx exerts a wide array of vasculoprotective effects via inhibition of coagulation and leucocyte adhesion, by contributing to the vascular permeability barrier and by mediating shear stress-induced NO release. In this review, we will focus on the relationship between fluid shear stress and the endothelial glycocalyx. We will address the hypothesis that modulation of glycocalyx synthesis by fluid shear stress may contribute to thinner glycocalyces, and therefore more vulnerable endothelium, at lesion-prone sites of arterial bifurcations. Finally, we will discuss the effects of known atherogenic stimuli such as hyperglycaemia on whole body glycocalyx volume in humans and its effect on endothelial function.

Atherogenic region and diet diminish glycocalyx dimension and increase intima-to-media ratios at murine carotid artery bifurcation

It was hypothesized that endothelial glycocalyx perturbation contributes to increased vulnerability of the arterial wall exposed to atherogenic risk factors. Glycocalyx and intima-to-media ratios (IMR) were studied at a low- and a high-risk region within the murine carotid artery (common region) and internal carotid branch (sinus region) in control C57BL/6J (C57BL6) and age-matched C57BL/6J/apoE*3-Leiden (apoE*3; on an atherogenic diet) mice. Electron micrographs revealed significantly thinner glycocalyces [73 (SD 36) vs. 399 (SD 174) nm, P < 0.05] and greater IMR [0.096 (SD 0.045) vs. 0.044 (SD 0.023), P < 0.05] at the sinus region of C57BL6 mice than in the common region. Thinner glycocalyces [100 (SD 27) vs. 399 (SD 174) nm, P < 0.05] and greater IMR [0.071 (SD 0.024) vs. 0.044 (SD 0.023), P < 0.05] were also observed in the common region of age-matched apoE*3 mice on an atherogenic diet for 6 wk vs. C57BL6 mice on a normal diet. Greater IMR were due to greater intima layers, without significant changes in media layer dimension. In addition, atherogenic diet resulted in increased endothelial cell thickness at the sinus region [0.85 (SD 0.49) vs. 0.53 (SD 0.28) μm, P < 0.05] but not at the common region [0.66 (SD 0.37) vs. 0.62 (SD 0.32) μm]. It is concluded that both regional and diet-induced increases in atherogenic risk are associated with smaller glycocalyx dimensions and greater IMR and that vascular sites with diminished glycocalyx are more vulnerable to proinflammatory and atherosclerotic sequelae.

Signaling Mechanisms Regulating Endothelial Permeability

The microvascular endothelial cell monolayer localized at the critical interface between the blood and vessel wall has the vital functions of regulating tissue fluid balance and supplying the essential nutrients needed for the survival of the organism. The endothelial cell is an exquisite “sensor” that responds to diverse signals generated in the blood, subendothelium, and interacting cells. The endothelial cell is able to dynamically regulate its paracellular and transcellular pathways for transport of plasma proteins, solutes, and liquid. The semipermeable characteristic of the endothelium (which distinguishes it from the epithelium) is crucial for establishing the transendothelial protein gradient (the colloid osmotic gradient) required for tissue fluid homeostasis. Interendothelial junctions comprise a complex array of proteins in series with the extracellular matrix constituents and serve to limit the transport of albumin and other plasma proteins by the paracellular pathway. This pathway is highly regulated by the activation of specific extrinsic and intrinsic signaling pathways. Recent evidence has also highlighted the importance of the heretofore enigmatic transcellular pathway in mediating albumin transport via transcytosis. Caveolae, the vesicular carriers filled with receptor-bound and unbound free solutes, have been shown to shuttle between the vascular and extravascular spaces depositing their contents outside the cell. This review summarizes and analyzes the recent data from genetic, physiological, cellular, and morphological studies that have addressed the signaling mechanisms involved in the regulation of both the paracellular and transcellular transport pathways.

Fluid shear stress stimulates incorporation of hyaluronan into endothelial cell glycocalyx

Vascular endothelial cells are shielded from direct exposure to flowing blood by the endothelial glycocalyx, a highly hydrated mesh of glycoproteins, sulfated proteoglycans, and associated glycosaminoglycans (GAGs). Recent data indicate that the incorporation of the unsulfated GAG hyaluronan into the endothelial glycocalyx is essential to maintain its permeability barrier properties, and we hypothesized that fluid shear stress is an important stimulus for endothelial hyaluronan synthesis. To evaluate the effect of shear stress on glycocalyx synthesis and the shedding of its GAGs into the supernatant, cultured human umbilical vein endothelial cells (i.e., the stable cell line EC-RF24) were exposed to 10 dyn/cm2 nonpulsatile shear stress for 24 h, and the incorporation of [3H]glucosamine and Na2[35S]O4 into GAGs was determined. Furthermore, the amount of hyaluronan in the glycocalyx and in the supernatant was determined by ELISA. Shear stress did not affect the incorporation of 35S but significantly increased the amount of glucosamine-containing GAGs incorporated in the endothelial glycocalyx [168 (SD 17)% of static levels, P < 0.01] and shedded into the supernatant [231 (SD 41)% of static levels, P < 0.01]. Correspondingly with this finding, shear stress increased the amount of hyaluronan in the glycocalyx [from 26 (SD 24) × 10−4 to 46 (SD 29) × 10−4 ng/cell, static vs. shear stress, P < 0.05] and in the supernatant [from 28 (SD 11) × 10−4 to 55 (SD 16) × 10−4 ng·cell−1·h−1, static vs. shear stress, P < 0.05]. The increase in the amount of hyaluronan incorporated in the glycocalyx was confirmed by a threefold higher level of hyaluronan binding protein within the glycocalyx of shear stress-stimulated endothelial cells. In conclusion, fluid shear stress stimulates incorporation of hyaluronan in the glycocalyx, which may contribute to its vasculoprotective effects against proinflammatory and pro-atherosclerotic stimuli.

The endothelial glycocalyx: a potential barrier between health and vascular disease

PURPOSE OF REVIEW

Although cardiovascular prevention has improved substantially, we still face the challenge of finding new targets to reduce the sequelae of atherosclerosis further. In this regard, optimizing the vasculoprotective effects of the vessel wall itself warrants intensive research. In particular, the endothelial glycocalyx, consisting of proteoglycans, glycoproteins and adsorbed plasma proteins, may play an essential role in protecting the vessel wall from atherosclerosis.

RECENT DEVELOPMENTS

In this review, we will discuss the different vasculoprotective effects exerted by the endothelial glycocalyx, the factors that damage it, and the first preliminary data on the glycocalyx dimension in humans. Whereas most glycocalyx research has traditionally focused on the microvasculature, more recent data have underscored the importance of the glycocalyx in protecting the macrovasculature against pro-atherogenic insults. It has been shown that glycocalyx loss is accompanied by a wide array of unfavourable changes in both small and larger vessels. Pro-atherogenic stimuli increase the shedding of glycocalyx constituents into the circulation, contributing to the progressive loss of the vasculoprotective properties of the vessel wall. Novel techniques have facilitated reproducible measurements of systemic glycocalyx volume in humans. Consistent with experimental data, the volume of the human glycocalyx is also severely perturbed by exposure to atherogenic risk factors.

SUMMARY

Cumulating evidence suggests that an intact glycocalyx protects the vessel wall, whereas disruption of the glycocalyx upon atherogenic stimuli increases vascular vulnerability for atherogenesis.

Primary human glomerular endothelial cells produce proteoglycans, and puromycin affects their posttranslational modification

This article describes the possible role of the endothelial cell-surface coat, containing proteoglycans (PGs) with connected glycosaminoglycans (GAGs), in maintaining glomerular permselectivity. Primary human glomerular endothelial cells (HGEC) in culture were treated with the nephrosis-inducing agent puromycin aminonucleoside (PAN). Analysis was made by TaqMan real-time PCR, Western blot analysis, and by metabolic labeling with [35S]sulfate. The HGECs express several PGs: syndecan, versican, glypican, perlecan, decorin, and biglycan, which may contribute to the glomerular charge barrier. PAN treatment downregulated both the protein expression (by 25%) and the mRNA expression (by 37 ± 6%, P < 0.001, n = 8) of versican compared with control. Transferases important for chondroitin and heparan sulfate biosynthesis were also significantly downregulated by PAN, resulting in less sulfate groups, shorter GAG chains, and reduced PG net-negative charge. Moreover, analysis of the cell media after PAN treatment revealed a reduced content of [35S]sulfate-labeled PGs (40% of control). We conclude that PAN may cause proteinuria by affecting the endothelial cell-surface layer and not only by disrupting the foot process arrangement of the podocytes. Thus the endothelium may be a more important component of the glomerular barrier than hitherto acknowledged.