Imaging the endothelial glycocalyx in vitro by rapid freezing/freeze substitution transmission electron microscopy

Endothelial barrier function is differentially regulated by CEACAM1-mediated signaling

Carcinoembryonic antigen-related cell adhesion molecule-1 (CEACAM1) is known to be crucial to vasculogenesis and angiogenesis. Recently, CEACAM1 deficiency was shown to result in the formation of aortic plaque-like lesions, indicating a role for CEACAM1 in adult vessels as well. The underlying mechanisms remained largely elusive. Therefore, we aimed to elucidate the role of CEACAM1 in endothelial homeostasis. Here, we show that CEACAM1 deficiency causes subcellular eNOS redistribution in endothelial cells ( i.e., by eNOS depalmitoylation) and alters endothelial glycocalyx that confers antiadhesive properties to the endothelium ( i.e., by repression of glycocalyx-degrading enzymes). Accordingly, our analysis revealed an increased leukocyte-endothelial interaction in CEACAM1-deficient endothelium. In addition, CEACAM1 age dependently modulated basal and TNF-α-mediated endothelial barrier (EB) leakiness. In younger mice, CEACAM1 was protective for EB, whereas in aged mice it promoted EB leakiness. EB function depends on interendothelial adherence junctions formed by β-catenin/vascular endothelial-cadherin complexes. We show here that CEACAM1 influenced basal and TNF-α-mediated phosphorylation of β-catenin and caveolin-1, which are essential players in EB modulation. Both increased adhesiveness to leukocytes and EB modulation due to CEACAM1 deficiency may facilitate inflammatory cell transmigration into the vascular wall and subsequent plaque formation. Collectively, these results identify a crucial role for CEACAM1 in endothelial homeostasis of adult blood vessels.-Ghavampour, S., Kleefeldt, F., Bömmel, H., Volland, J., Paus, A., Horst, A., Pfeiffer, V., Hübner, S., Wagner, N., Rueckschloss, U., Ergün, S. Endothelial barrier function is differentially regulated by CEACAM1-mediated signaling.

SPARC preserves endothelial glycocalyx integrity, and protects against adverse cardiac inflammation and injury during viral myocarditis

Myocardial damage as a consequence of cardiotropic viruses leads to a broad variety of clinical presentations and is still a complicated condition to diagnose and treat. Whereas the extracellular matrix protein Secreted Protein Acidic and Rich in Cysteine or SPARC has been implicated in hypertensive and ischemic heart disease by modulating collagen production and cross-linking, its role in cardiac inflammation and endothelial function is yet unknown. Absence of SPARC in mice resulted in increased cardiac inflammation and mortality, and reduced cardiac systolic function upon coxsackievirus-B3 induced myocarditis. Intra-vital microscopic imaging of the microvasculature of the cremaster muscle combined with electron microscopic imaging of the microvasculature of the cardiac muscle uncovered the significance of SPARC in maintaining endothelial glycocalyx integrity and subsequent barrier properties to stop inflammation. Moreover, systemic administration of recombinant SPARC restored the endothelial glycocalyx and consequently reversed the increase in inflammation and mortality observed in SPARC KO mice in response to viral exposure. Reducing the glycocalyx in vivo by systemic administration of hyaluronidase, an enzyme that degrades the endothelial glycocalyx, mimicked the barrier defects found in SPARC KO mice, which could be restored by subsequent administration of recombinant SPARC. In conclusion, the secreted glycoprotein SPARC protects against adverse cardiac inflammation and mortality by improving the glycocalyx function and resulting endothelial barrier function during viral myocarditis.

The Role of Plasma Transfusion in Massive Bleeding: Protecting the Endothelial Glycocalyx?

Massive hemorrhage is a leading cause of death worldwide. During the last decade several retrospective and some prospective clinical studies have suggested a beneficial effect of early plasma-based resuscitation on survival in trauma patients. The underlying mechanisms are unknown but appear to involve the ability of plasma to preserve the endothelial glycocalyx. In this mini-review, we summarize current knowledge on glycocalyx structure and function, and present data describing the impact of hemorrhagic shock and resuscitation fluids on glycocalyx. Animal studies show that hemorrhagic shock leads to glycocalyx shedding, endothelial inflammatory changes, and vascular hyper-permeability. In these animal models, plasma administration preserves glycocalyx integrity and functions better than resuscitation with crystalloids or colloids. In addition, we briefly present data on the possible plasma components responsible for these effects. The endothelial glycocalyx is increasingly recognized as a critical component for the physiological vasculo-endothelial function, which is destroyed in hemorrhagic shock. Interventions for preserving an intact glycocalyx shall improve survival of trauma patients.

Implications Enzymatic Degradation of the Endothelial Glycocalyx on the Microvascular Hemodynamics and the Arteriolar Red Cell Free Layer of the Rat Cremaster Muscle

The endothelial glycocalyx is a complex network of glycoproteins, proteoglycans, and glycosaminoglycans; it lines the vascular endothelial cells facing the lumen of blood vessels forming the endothelial glycocalyx layer (EGL). This study aims to investigate the microvascular hemodynamics implications of the EGL by quantifying changes in blood flow hydrodynamics post-enzymatic degradation of the glycocalyx layer. High-speed intravital microscopy videos of small arteries (around 35 μm) of the rat cremaster muscle were recorded at various time points after enzymatic degradation of the EGL. The thickness of the cell free layer (CFL), blood flow velocity profiles, and volumetric flow rates were quantified. Hydrodynamic effects of the presence of the EGL were observed in the differences between the thickness of CFL in microvessels with an intact EGL and glass tubes of similar diameters. Maximal changes in the thickness of CFL were observed 40 min post-enzymatic degradation of the EGL. Analysis of the frequency distribution of the thickness of CFL allows for estimation of the thickness of the endothelial surface layer (ESL), the plasma layer, and the glycocalyx. Peak flow, maximum velocity, and mean velocity were found to statistically increase by 24, 27, and 25%, respectively, after enzymatic degradation of the glycocalyx. The change in peak-to-peak maximum velocity and mean velocity were found to statistically increase by 39 and 32%, respectively, after 40 min post-enzymatic degradation of the EGL. The bluntness of blood flow velocity profiles was found to be reduced post-degradation of the EGL, as the exclusion volume occupied by the EGL increased the effective volume impermeable to RBCs in microvessels. This study presents the effects of the EGL on microvascular hemodynamics. Enzymatic degradation of the EGL resulted in a decrease in the thickness of CFL, an increase in blood velocity, blood flow, and decrease of the bluntness of the blood flow velocity profile in small arterioles. In summary, the EGL functions as a molecular sieve to solute transport and as a lubrication layer to protect the endothelium from red blood cell (RBC) motion near the vessel wall, determining wall shear stress.

Mechanotransmission in endothelial cells subjected to oscillatory and multi-directional shear flow

Local haemodynamics are linked to the non-uniform distribution of atherosclerosic lesions in arteries. Low and oscillatory (reversing in the axial flow direction) wall shear stress (WSS) induce inflammatory responses in endothelial cells (ECs) mediating disease localization. The objective of this study is to investigate computationally how the flow direction (reflected in WSS variation on the EC surface over time) influences the forces experienced by structural components of ECs that are believed to play important roles in mechanotransduction. A three-dimensional, multi-scale, multi-component, viscoelastic model of focally adhered ECs is developed, in which oscillatory WSS (reversing or non-reversing) parallel to the principal flow direction, or multi-directional oscillatory WSS with reversing axial and transverse components are applied over the EC surface. The computational model includes the glycocalyx layer, actin cortical layer, nucleus, cytoskeleton, focal adhesions (FAs), stress fibres and adherens junctions (ADJs). We show the distinct effects of atherogenic flow profiles (reversing unidirectional flow and reversing multi-directional flow) on subcellular structures relative to non-atherogenic flow (non-reversing flow). Reversing flow lowers stresses and strains due to viscoelastic effects, and multi-directional flow alters stress on the ADJs perpendicular to the axial flow direction. The simulations predict forces on integrins, ADJ filaments and other substructures in the range that activate mechanotransduction.

Tumor Metastasis in the Microcirculation

Tumor cell metastasis through blood circulation is a complex process and is one of the great challenges in cancer research as metastatic spread is responsible for ∼90% of cancer-related mortality. Tumor cell intravasation into, arrest and adhesion at, and extravasation from the microvessel walls are critical steps in metastatic spread. Understanding these steps 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 endothelial cells is the principal pathway for water and solute transport through the microvessel wall in health. Recently, this cleft has been found to be the location for tumor cell adhesion and extravasation. The blood-flow-induced hydrodynamic factors such as shear rates and stresses, shear rate and stress gradients, as well as vorticities, especially at the branches and turns of microvasculatures, also play important roles in tumor cell arrest and adhesion. This chapter therefore reports the current advances from in vivo animal studies and in vitro culture cell studies to demonstrate how the endothelial integrity or microvascular permeability, hydrodynamic factors, microvascular geometry, cell adhesion molecules, and surrounding extracellular matrix affect critical steps of tumor metastasis in the microcirculation.

The Role of Endothelial Surface Glycocalyx in Mechanosensing and Transduction

The endothelial cells (ECs) forming the inner wall of every blood vessel are constantly exposed to the mechanical forces generated by blood flow. The EC responses to these hemodynamic forces play a critical role in the homeostasis of the circulatory system. A variety of mechanosensors and transducers, locating on the EC surface, intra- and trans-EC membrane, and within the EC cytoskeleton, have thus been identified to ensure proper functions of ECs. Among them, the most recent candidate is the endothelial surface glycocalyx (ESG), which is a matrix-like thin layer covering the luminal surface of the EC. It consists of various proteoglycans, glycosaminoglycans, and plasma proteins and is close to other prominent EC mechanosensors and transducers. This chapter summarizes the ESG composition, thickness, and structure observed by different labeling and visualization techniques and in different types of vessels. It also presents the literature in determining the ESG mechanical properties by atomic force microscopy and optical tweezers. The molecular mechanisms by which the ESG plays the role in EC mechanosensing and transduction are described as well as the ESG remodeling by shear stress, the actin cytoskeleton, the membrane rafts, the angiogenic factors, and the sphingosine-1-phosphate.

Endothelial Glycocalyx as Biomarker for Cardiovascular Diseases: Mechanistic and Clinical Implications

INTRODUCTION

The endothelial surface layer is covered with abundant proteoglycans, of which syndecans and glycosaminoglycans are major constituents.

RECENT FINDINGS

Among the endothelial glycocalyx (eGC) constituents, syndecan-1 (sdc1) is a main component, and an elevated serum level of sdc1 may indicate the degradation of eGC. In patients with ischemic heart disease or heart failure, elevation of serum sdc1 has been associated with worsening cardiac and renal function; however, the causal relationship between degradation of eGC and clinical outcomes is unclear. Herein, we review the previous literature on eGC in cardiovascular and noncardiovascular diseases and their clinical implications.

More than a biomarker: the systemic consequences of heparan sulfate fragments released during endothelial surface layer degradation (2017 Grover Conference Series)

Advances in tissue fixation and imaging techniques have yielded increasing appreciation for the glycosaminoglycan-rich endothelial glycocalyx and its in vivo manifestation, the endothelial surface layer (ESL). Pathological loss of the ESL during critical illness promotes local endothelial dysfunction and, consequently, organ injury. Glycosaminoglycan fragments, such as heparan sulfate, are released into the plasma of animals and humans after ESL degradation and have thus served as a biomarker of endothelial injury. The development of state-of-the-art glycomic techniques, however, has revealed that these circulating heparan sulfate fragments are capable of influencing growth factor and other signaling pathways distant to the site of ESL injury. This review summarizes the current state of knowledge concerning the local (i.e. endothelial injury) and systemic (i.e. para- or endocrine) consequences of ESL degradation and identifies opportunities for future, novel investigations.

Glycocalyx in Atherosclerosis-Relevant Endothelium Function and as a Therapeutic Target

Purpose of Review

The cell surface-attached extracellular glycocalyx (GCX) layer is a major contributor to endothelial cell (EC) function and EC-dependent vascular health and is a first line of defense against vascular diseases including atherosclerosis. Here, we highlight our findings regarding three GCX-dependent EC functions, which are altered when GCX is shed and in atherosclerosis. We discuss why the GCX is a viable option for the prevention and treatment of atherosclerosis.

Recent Findings

GCX regulated EC activities such as barrier and filtration function, active cell-to-cell communication, and vascular tone mediation contribute to function of the entire vascular wall. Atheroprone vessel regions, including bifurcation sites, exhibit breakdown in GCX. This GCX degradation allows increased lipid flux and thereby promotes lipid deposition in the vessel walls, a hallmark of atherosclerosis. GCX degradation also alters EC-to-EC communication while increasing EC-to-inflammatory cell interactions that enable inflammatory cells to migrate into the vessel wall. Inflammatory macrophages and foam cells, to be specific, appear in early stages of atherosclerosis. Furthermore, GCX degradation deregulates vascular tone, by causing ECs to reduce their expression of endothelial nitric oxide synthase (eNOS) which produces the vasodilator, nitric oxide. Loss of vasodilation supports vasoconstriction, which promotes the progression of atherosclerosis.

Summary

Common medicinal atherosclerosis therapies include lipid lowering and anti-platelet therapies. None of these treatments specifically target the endothelial GCX, although the GCX is at the front-line in atherosclerosis combat. This review demonstrates the viability of targeting the GCX therapeutically, to support proper EC functionality and prevent and/or treat atherosclerosis.

Regeneration of glycocalyx by heparan sulfate and sphingosine 1-phosphate restores inter-endothelial communication

Vasculoprotective endothelium glycocalyx (GCX) shedding plays a critical role in vascular disease. Previous work demonstrated that GCX degradation disrupts endothelial cell (EC) gap junction connexin (Cx) proteins, likely blocking interendothelial molecular transport that maintains EC and vascular tissue homeostasis to resist disease. Here, we focused on GCX regeneration and tested the hypothesis that vasculoprotective EC function can be stimulated via replacement of GCX when it is shed. We used EC with [i] intact heparan sulfate (HS), the most abundant GCX component; [ii] degraded HS; or [iii] HS that was restored after enzyme degradation, by cellular self-recovery or artificially. Artificial HS restoration was achieved via treatment with exogenous HS, with or without the GCX regenerator and protector sphingosine 1- phosphate (S1P). In these cells we immunocytochemically examined expression of Cx isotype 43 (Cx43) at EC borders and characterized Cx-containing gap junction activity by measuring interendothelial spread of gap junction permeable Lucifer Yellow dye. With intact HS, 60% of EC borders expressed Cx43 and dye spread to 2.88 ± 0.09 neighboring cells. HS degradation decreased Cx43 expression to 30% and reduced dye spread to 1.87± 0.06 cells. Cellular self-recovery of HS restored baseline levels of Cx43 and dye transfer. Artificial HS recovery with exogenous HS partially restored Cx43 expression to 46% and yielded dye spread to only 1.03 ± 0.07 cells. Treatment with both HS and S1P, recovered HS and restored Cx43 to 56% with significant dye transfer to 3.96 ± 0.23 cells. This is the first evidence of GCX regeneration in a manner that effectively restores vasculoprotective EC communication.

Structural Behavior of the Endothelial Glycocalyx Is Associated With Pathophysiologic Status in Septic Mice: An Integrated Approach to Analyzing the Behavior and Function of the Glycocalyx Using Both Electron and Fluorescence Intravital Microscopy

BACKGROUND

The endothelial surface layer (ESL) regulates vascular permeability to maintain fluid homeostasis. The glycocalyx (GCX), which has a complex and fragile ultrastructure, is an important component of the ESL. Abnormalities of the GCX have been hypothesized to trigger pathological hyperpermeability. Here, we report an integrated in vivo analysis of the morphological and functional properties of the GCX in a vital organ.

METHODS

We examined the behavior of the ESL and GCX, using both electron microscopy (EM) and intravital microscopy (IVM). We also compared morphological changes in the ESL of mouse skin in a glycosidase-treated and control group. Combined approaches were also used to examine both morphology and function in a lipopolysaccharide-induced septic model and the pathophysiological features of leukocyte-endothelial interactions and in vivo vascular permeability.

RESULTS

Using IVM, we identified an illuminated part of the ESL as the GCX and confirmed our observation using morphological and biochemical means. In septic mice, we found that the GCX was thinner than in nonseptic controls in both an EM image analysis (0.98 ± 2.08 nm vs 70.68 ± 36.36 nm, P< .001) and an IVM image analysis (0.36 ± 0.15 μm vs 1.07 ± 0.39 μm, P< .001). Under septic conditions, syndecan-1, a representative core protein of the GCX, was released into the blood serum at a higher rate in septic animals (7.33 ± 3.46 ng/mL) when compared with controls (below the limit of detection, P< .001). Significant increases in leukocyte-endothelial interactions, defined as the numbers of rolling or firm-sticking leukocytes, and molecular hyperpermeability to the interstitium were also observed after GCX shedding in vivo.

CONCLUSIONS

Using IVM, we visualized an illuminated part of the ESL layer that was subsequently confirmed as the GCX using EM. Severe sepsis induced morphological degradation of the GCX, accompanied by shedding of the syndecan-1 core protein and an increase in leukocyte-endothelial interactions affecting the vascular permeability. Our in vivo model describes a new approach to deciphering the relationship between structural and functional behaviors of the GCX.

Endothelial Glycocalyx-Mediated Nitric Oxide Production in Response to Selective AFM Pulling

Nitric oxide (NO) is a regulatory molecule in the vascular system and its inhibition due to endothelial injury contributes to cardiovascular disease. The glycocalyx is a thin layer of glycolipids, glycoproteins, and proteoglycans on the surface of mammalian epithelial cells. Extracellular forces are transmitted through the glycocalyx to initiate intracellular signaling pathways. In endothelial cells (ECs), previous studies have shown the glycocalyx to be a significant mediator of NO production; degradation of the endothelial glycocalyx layer (EGL) drastically reduces EC production of NO in response to fluid shear stress. However, the specific EGL components involved in this process are not well established. Recent work using short-hairpin RNA approaches in vitro suggest that the proteoglycan glypican-1, not syndecan-1, is the dominant core protein mediating shear-induced NO production. We utilized atomic force microscopy (AFM) to apply force selectively to components of the EGL of confluent rat fat pad ECs (RFPECs), including proteoglycans and glycosaminoglycans, to observe how each component individually contributes to force-induced production of NO. 4,5-diaminofluorescein diacetate, a cell-permeable fluorescent molecule, was used to detect changes in intracellular NO production. Antibody-coated AFM probes exhibited strong surface binding to RFPEC monolayers, with 100-300 pN mean adhesion forces. AFM pulling on glypican-1 and heparan sulfate for 10 min caused significantly increased NO production, whereas pulling on syndecan-1, CD44, hyaluronic acid, and with control probes did not. We conclude that AFM pulling can be used to activate EGL-mediated NO production and that the heparan sulfate proteoglycan glypican-1 is a primary mechanosensor for shear-induced NO production.

The Biomechanical Effects of Resuscitation Colloids on the Compromised Lung Endothelial Glycocalyx

BACKGROUND

The endothelial glycocalyx is an important component of the vascular permeability barrier, forming a scaffold that allows serum proteins to create a gel-like layer on the endothelial surface and transmitting mechanosensing and mechanotransduction information that influences permeability. During acute inflammation, the glycocalyx is degraded, changing how it interacts with serum proteins and colloids used during resuscitation and altering its barrier properties and biomechanical characteristics. We quantified changes in the biomechanical properties of lung endothelial glycocalyx during control conditions and after degradation by hyaluronidase using biophysical techniques that can probe mechanics at (1) the aqueous/glycocalyx interface and (2) inside the glycocalyx. Our goal was to discern the location-specific effects of albumin and hydroxyethyl starch (HES) on glycocalyx function.

METHODS

The effects of albumin and HES on the mechanical properties of bovine lung endothelial glycocalyx were studied using a combination of atomic force microscopy and reflectance interference contrast microscopy. Logistic regression was used to determine the odds ratios for comparing the effects of varying concentrations of albumin and HES on the glycocalyx with and without hyaluronidase.

RESULTS

Atomic force microscopy measurements demonstrated that both 0.1% and 4% albumin increased the thickness and reduced the stiffness of glycocalyx when compared with 1% albumin. The effect of HES on glycocalyx thickness was similar to albumin, with thickness increasing significantly between 0.1% and 1% HES and a trend toward a softer glycocalyx at 4% HES. Reflectance interference contrast microscopy revealed a concentration-dependent softening of the glycocalyx in the presence of albumin, but a concentration-dependent increase in stiffness with HES. After glycocalyx degradation with hyaluronidase, stiffness was increased only at 4% albumin and 1% HES.

CONCLUSIONS

Albumin and HES induced markedly different effects on glycocalyx mechanics and had notably different effects after glycocalyx degradation by hyaluronidase. We conclude that HES is not comparable with albumin for studies of vascular permeability and glycocalyx-dependent signaling. Characterizing the molecular and biomechanical effects of resuscitation colloids on the glycocalyx should clarify their indicated uses and permit a better understanding of how HES and albumin affect vascular function.

Glycocalyx bending by an electric field increases cell motility

The application of physiological strength electric fields may produce a wide range of effects on cells. The mechanisms by which cells detect the presence of these fields, however, are not fully understood. Previous experiments have shown that directionality of cells in the field is governed by an electromechanical mechanism in which the field exerts a torque on the negatively charged, inner glycocalyx that is then transmitted as a force on the cytoskeleton. This mechanism is similar to that by which cells detect fluid shear forces. Several authors, however, have reported that cell directionality and motility behave differently in an electric field. We propose here a second electromechanical mechanism in which the field bends the negatively charged, outer glycocalyx in proximity to the substrate, increasing cell adhesion and, thus, cell motility. The increase in motility depends not only on the field strength, but also on the adhesion of the cell to the substrate prior to application of the field. We show that these mechanisms are common to both human cells and amoebae and, hence, are evolutionarily conserved. Furthermore, the mechanism for detection of electric fields is simply an extension of the mechanism for detecting fluid shears. Bioelectromagnetics. 38:482-493, 2017. © 2017 Wiley Periodicals, Inc.

Electron microscopy approach for the visualization of the epithelial and endothelial glycocalyx

This study presents a methodological approach for the visualization of the glycocalyx by electron microscopy. The glycocalyx is a three dimensional network mainly composed of glycolipids, glycoproteins and proteoglycans associated with the plasma membrane. Since less than a decade, the epithelial and endothelial glycocalyx proved to play an important role in physiology and pathology, increasing its research interest especially in vascular functions. Therefore, visualization of the glycocalyx requires reliable techniques and its preservation remains challenging due to its fragile and dynamic organization, which is highly sensitive to the different process steps for electron microscopy sampling. In this study, chemical fixation was performed by perfusion as a good alternative to conventional fixation. Additional lanthanum nitrate in the fixative enhances staining of the glycocalyx in transmission electron microscopy bright field and improves its visualization by detecting the elastic scattered electrons, thus providing a chemical contrast.

Microvasculature on a chip: study of the Endothelial Surface Layer and the flow structure of Red Blood Cells

Microvasculatures-on-a-chip, i.e. in vitro models that mimic important features of microvessel networks, have gained increasing interest in recent years. Such devices have allowed investigating pathophysiological situations involving abnormal biophysical interactions between blood cells and vessel walls. Still, a central question remains regarding the presence, in such biomimetic systems, of the endothelial glycocalyx. The latter is a glycosaminoglycans-rich surface layer exposed to blood flow, which plays a crucial role in regulating the interactions between circulating cells and the endothelium. Here, we use confocal microscopy to characterize the layer expressed by endothelial cells cultured in microfluidic channels. We show that, under our culture conditions, endothelial cells form a confluent layer on all the walls of the circuit and display a glycocalyx that fully lines the lumen of the microchannels. Moreover, the thickness of this surface layer is found to be on the order of 600 nm, which compares well with measurements performed ex or in vivo on microcapillaries. Furthermore, we investigate how the presence of endothelial cells in the microchannels affects their hydrodynamic resistance and the near-wall motion of red blood cells. Our study thus provides an important insight into the physiological relevance of in vitro microvasculatures.

Eukaryotic protein glycosylation: a primer for histochemists and cell biologists

Proteins undergo co- and posttranslational modifications, and their glycosylation is the most frequent and structurally variegated type. Histochemically, the detection of glycan presence has first been performed by stains. The availability of carbohydrate-specific tools (lectins, monoclonal antibodies) has revolutionized glycophenotyping, allowing monitoring of distinct structures. The different types of protein glycosylation in Eukaryotes are described. Following this educational survey, examples where known biological function is related to the glycan structures carried by proteins are given. In particular, mucins and their glycosylation patterns are considered as instructive proof-of-principle case. The tissue and cellular location of glycoprotein biosynthesis and metabolism is reviewed, with attention to new findings in goblet cells. Finally, protein glycosylation in disease is documented, with selected examples, where aberrant glycan expression impacts on normal function to let disease pathology become manifest. The histological applications adopted in these studies are emphasized throughout the text.

The glycocalyx regulates the uptake of nanoparticles by human endothelial cells in vitro

AIM

To assess the role of the endothelial glycocalyx (eGCX) for the uptake of nanoparticles by endothelial cells.

METHODS

The expression of the eGCX on cultured human umbilical vein endothelial cells was determined by immunostaining of heparan sulfate. Enzymatic degradation of the eGCX was achieved by incubating the cells with eGCX-shedding enzymes. The uptake of 50-nm polystyrene nanospheres was quantified by confocal microscopy.

RESULTS

Human umbilical vein endothelial cells expressed a robust eGCX when cultured for 10 days. The uptake of both carboxylated and aminated polystyrene nanospheres was significantly increased in cells in which the glycocalyx was enzymatically degraded, while it remained at a low level in cells with an intact glycocalyx.

CONCLUSION

The eGCX constitutes a barrier against the internalization of blood-borne nanoparticles by endothelial cells.

THE GLYCOCALYX AND TRAUMA: A REVIEW

In the United States trauma is the leading cause of mortality among those under the age of 45, claiming approximately 192,000 lives each year. Significant personal disability, lost productivity, and long-term healthcare needs are common and contribute 580 billion dollars in economic impact each year. Improving resuscitation strategies and the early acute care of trauma patients has the potential to reduce the pathological sequelae of combined exuberant inflammation and immune suppression that can co-exist, or occur temporally, and adversely affect outcomes. The endothelial and epithelial glycocalyx has emerged as an important participant in both inflammation and immunomodulation. Constituents of the glycocalyx have been used as biomarkers of injury severity and have the potential to be target(s) for therapeutic interventions aimed at immune modulation. In this review, we provide a contemporary understanding of the physiologic structure and function of the glycocalyx and its role in traumatic injury with a particular emphasis on lung injury.

Glycocalyx Degradation Induces a Proinflammatory Phenotype and Increased Leukocyte Adhesion in Cultured Endothelial Cells under Flow

Leukocyte adhesion to the endothelium is an early step in the pathogenesis of atherosclerosis. Effective adhesion requires the binding of leukocytes to their cognate receptors on the surface of endothelial cells. The glycocalyx covers the surface of endothelial cells and is important in the mechanotransduction of shear stress. This study aimed to identify the molecular mechanisms underlying the role of the glycocalyx in leukocyte adhesion under flow. We performed experiments using 3-D cell culture models, exposing human abdominal aortic endothelial cells to steady laminar shear stress (10 dynes/cm2 for 24 hours). We found that with the enzymatic degradation of the glycocalyx, endothelial cells developed a proinflammatory phenotype when exposed to uniform steady shear stress leading to an increase in leukocyte adhesion. Our results show an up-regulation of ICAM-1 with degradation compared to non-degraded controls (3-fold increase, p<0.05) and we attribute this effect to a de-regulation in NF-κB activity in response to flow. These results suggest that the glycocalyx is not solely a physical barrier to adhesion but rather plays an important role in governing the phenotype of endothelial cells, a key determinant in leukocyte adhesion. We provide evidence for how the destabilization of this structure may be an early and defining feature in the initiation of atherosclerosis.

Glycocalyx and its involvement in clinical pathophysiologies

Vascular hyperpermeability is a frequent intractable feature involved in a wide range of diseases in the intensive care unit. The glycocalyx (GCX) seemingly plays a key role to control vascular permeability. The GCX has attracted the attention of clinicians working on vascular permeability involving angiopathies, and several clinical approaches to examine the involvement of the GCX have been attempted. The GCX is a major constituent of the endothelial surface layer (ESL), which covers most of the surface of the endothelial cells and reduces the access of cellular and macromolecular components of the blood to the surface of the endothelium. It has become evident that this structure is not just a barrier for vascular permeability but contributes to various functions including signal sensing and transmission to the endothelium. Because GCX is a highly fragile and unstable layer, the image had been only obtained by conventional transmission electron microscopy. Recently, advanced microscopy techniques have enabled direct visualization of the GCX in vivo, most of which use fluorescent-labeled lectins that bind to specific disaccharide moieties of glycosaminoglycan (GAG) chains. Fluorescent-labeled solutes also enabled to demonstrate vascular leakage under the in vivo microscope. Thus, functional analysis of GCX is advancing. A biomarker of GCX degradation has been clinically applied as a marker of vascular damage caused by surgery. Fragments of the GCX, such as syndecan-1 and/or hyaluronan (HA), have been examined, and their validity is now being examined. It is expected that GCX fragments can be a reliable diagnostic or prognostic indicator in various pathological conditions. Since GCX degradation is strongly correlated with disease progression, pharmacological intervention to prevent GCX degradation has been widely considered. HA and other GAGs are candidates to repair GCX; further studies are needed to establish pharmacological intervention. Recent advancement of GCX research has demonstrated that vascular permeability is not regulated by simple Starling’s law. Biological regulation of vascular permeability by GCX opens the way to develop medical intervention to control vascular permeability in critical care patients.

Transport of Gold Nanoparticles by Vascular Endothelium from Different Human Tissues

The selective entry of nanoparticles into target tissues is the key factor which determines their tissue distribution. Entry is primarily controlled by microvascular endothelial cells, which have tissue-specific properties. This study investigated the cellular properties involved in selective transport of gold nanoparticles (<5 nm) coated with PEG-amine/galactose in two different human vascular endothelia. Kidney endothelium (ciGENC) showed higher uptake of these nanoparticles than brain endothelium (hCMEC/D3), reflecting their biodistribution in vivo. Nanoparticle uptake and subcellular localisation was quantified by transmission electron microscopy. The rate of internalisation was approximately 4x higher in kidney endothelium than brain endothelium. Vesicular endocytosis was approximately 4x greater than cytosolic uptake in both cell types, and endocytosis was blocked by metabolic inhibition, whereas cytosolic uptake was energy-independent. The cellular basis for the different rates of internalisation was investigated. Morphologically, both endothelia had similar profiles of vesicles and cell volumes. However, the rate of endocytosis was higher in kidney endothelium. Moreover, the glycocalyces of the endothelia differed, as determined by lectin-binding, and partial removal of the glycocalyx reduced nanoparticle uptake by kidney endothelium, but not brain endothelium. This study identifies tissue-specific properties of vascular endothelium that affects their interaction with nanoparticles and rate of transport.

Endothelial glycocalyx conditions influence nanoparticle uptake for passive targeting

Cardiovascular diseases are facilitated by endothelial cell (EC) dysfunction and coincide with EC glycocalyx coat shedding. These diseases may be prevented by delivering medications to affected vascular regions using circulating nanoparticle (NP) drug carriers. The objective of the present study was to observe how the delivery of 10 nm polyethylene glycol-coated gold NPs (PEG-AuNP) to ECs is impacted by glycocalyx structure on the EC surface. Rat fat pad endothelial cells were chosen for their robust glycocalyx, verified by fluorescent immunolabeling of adsorbed albumin and integrated heparan sulfate (HS) chains. Confocal fluorescent imaging revealed a ~3 µm thick glycocalyx layer, covering 75% of the ECs and containing abundant HS. This healthy glycocalyx hindered the uptake of PEG-AuNP as expected because glycocalyx pores are typically 7 nm wide. Additional glycocalyx models tested included: a collapsed glycocalyx obtained by culturing cells in reduced protein media, a degraded glycocalyx obtained by applying heparinase III enzyme to specifically cleave HS, and a recovered glycocalyx obtained by supplementing exogenous HS into the media after enzyme degradation. The collapsed glycocalyx waŝ2 µm thick with unchanged EC coverage and sustained HS content. The degraded glycocalyx showed similar changes in EC thickness and coverage but its HS thickness was reduced to 0.7 µm and spanned only 10% of the original EC surface. Both dysfunctional models retained six- to sevenfold more PEG-AuNP compared to the healthy glycocalyx. The collapsed glycocalyx permitted NPs to cross the glycocalyx into intracellular spaces, whereas the degraded glycocalyx trapped the PEG-AuNP within the glycocalyx. The repaired glycocalyx model partially restored HS thickness to 1.2 µm and 44% coverage of the ECs, but it was able to reverse the NP uptake back to baseline levels. In summary, this study showed that the glycocalyx structure is critical for NP uptake by ECs and may serve as a passive pathway for delivering NPs to dysfunctional ECs.

The glycocalyx and its significance in human medicine

Cells are covered by a surface layer of glycans that is referred to as the 'glycocalyx'. In this review, we focus on the role of the glycocalyx in vascular diseases (atherosclerosis, stroke, hypertension, kidney disease and sepsis) and cancer. The glycocalyx and its principal glycosaminoglycans [heparan sulphate (HS) and hyaluronic acid (HA)] and core proteins (syndecans and glypicans) are degraded in vascular diseases, leading to a breakdown of the vascular permeability barrier, enhanced access of leucocytes to the arterial intima that propagate inflammation and alteration of endothelial mechanotransduction mechanisms that protect against disease. By contrast, the glycocalyx on cancer cells is generally robust, promoting integrin clustering and growth factor signalling, and mechanotransduction of interstitial flow shear stress that is elevated in tumours to upregulate matrix metalloproteinase release which enhances cell motility and metastasis. HS and HA are consistently elevated on cancer cells and are associated with tumour growth and metastasis. Later, we will review the agents that might be used to enhance or protect the glycocalyx to combat vascular disease, as well as a different set of compounds that can degrade the cancer cell glycocalyx to suppress cell growth and metastasis. It is clear that what is beneficial for either vascular disease or cancer will not be so for the other. The overarching conclusions are that (i) the importance of the glycocalyx in human medicine is only beginning to be recognized, and (ii) more detailed studies of glycocalyx involvement in vascular diseases and cancer will lead to novel treatment modalities.

Adhesion Molecules: Master Controllers of the Circulatory System

This manuscript will review our current understanding of cellular adhesion molecules (CAMs) relevant to the circulatory system, their physiological role in control of vascular homeostasis, innate and adaptive immune responses, and their importance in pathophysiological (disease) processes such as acute lung injury, atherosclerosis, and pulmonary hypertension. This is a complex and rapidly changing area of research that is incompletely understood. By design, we will begin with a brief overview of the structure and classification of the major groups of adhesion molecules and their physiological functions including cellular adhesion and signaling. The role of specific CAMs in the process of platelet aggregation and hemostasis and leukocyte adhesion and transendothelial migration will be reviewed as examples of the complex and cooperative interplay between CAMs during physiological and pathophysiological processes. The role of the endothelial glycocalyx and the glycobiology of this complex system related to inflammatory states such as sepsis will be reviewed. We will then focus on the role of adhesion molecules in the pathogenesis of specific disease processes involving the lungs and cardiovascular system. The potential of targeting adhesion molecules in the treatment of immune and inflammatory diseases will be highlighted in the relevant sections throughout the manuscript.

Endothelial and Microcirculatory Function and Dysfunction in Sepsis

The microcirculation is a series of arterioles, capillaries, and venules that performs essential functions of oxygen and nutrient delivery, customized to the unique physiologic needs of the supplied organ. The homeostatic microcirculatory response to infection can become harmful if overactive and/or dysregulated. Pathologic microcirculatory dysfunction can be directly visualized by intravital microscopy or indirectly measured via detection of circulating biomarkers. Although several treatments have been shown to protect the microcirculation during sepsis, they have not improved patient outcomes when applied indiscriminately. Future outcomes-oriented studies are needed to test sepsis therapeutics when personalized to a patient's microcirculatory dysfunction.

Friction and morphology of pleural mesothelia

To verify the hypothesis that by enmeshing lubricants, microvilli reduce the coefficient of kinetic friction (μ) of pleural mesothelium, μ was measured during reciprocating sliding of rabbit's visceral against parietal pleura before and after addition of hyaluronan, and related to the morphological features of the microvillar network. Because no relation was found between μ or μ changes after hyaluronan and microvillar characteristics, the latter are not determinants of the frictional forces which oppose sliding of normal mesothelial surfaces under physiological conditions, nor of the effects of hyaluronan. Addition of hyaluronan increased μ slightly but significantly in normal specimens, probably by altering the physiological mix of lubricants, but decreased μ of damaged mesothelia, suggesting protective, anti-abrasion properties. Indeed, while sliding of an injured against a normal pleura heavily damaged the latter and increased μ when Ringer was interposed between the surfaces, both effects were limited or prevented when hyaluronan was interposed between the injured and normal pleura before onset of sliding.

Sphingosine 1-phosphate induced synthesis of glycocalyx on endothelial cells

Sphingosine 1-phosphate (S1P) protects glycocalyx against shedding, playing important roles in endothelial functions. We previously found that glycocalyx on endothelial cells (ECs) was shed after plasma protein depletion. In the present study, we investigated the role of S1P on the recovery of glycocalyx, and tested whether it is mediated by phosphoinositide 3-kinase (PI3K) pathway. After depletion of plasma protein, ECs were treated with S1P for another 6h. And then, the major components of glycocalyx including syndecan-1 with attached heparan sulfate (HS) and chondroitin sulfate (CS) on endothelial cells were detected using confocal fluorescence microscopy. Role of PI3K in the S1P-induced synthesis of glycocalyx was confirmed by using the PI3K inhibitor (LY294002). Syndecan-1 with attached HS and CS were degraded with duration of plasma protein depletion. S1P induced recovery of syndecan-1 with attached HS and CS. The PI3K inhibitor LY294002 abolished the effect of S1P on recovery of glycocalyx. Thus, S1P induced synthesis of glycocalyx on endothelial cells and it is mediated by PI3K pathway.

Role of the endothelial surface layer in neutrophil recruitment

Neutrophil recruitment in most tissues is limited to postcapillary venules, where E- and P-selectins are inducibly expressed by venular endothelial cells. These molecules support neutrophil rolling via binding of PSGL-1 and other ligands on neutrophils. Selectins extend ≤ 38 nm above the endothelial plasma membrane, and PSGL-1 extends to 50 nm above the neutrophil plasma membrane. However, endothelial cells are covered with an ESL composed of glycosaminoglycans that is ≥ 500 nm thick and has measurable resistance against compression. The neutrophil surface is also covered with a surface layer. These surface layers would be expected to completely shield adhesion molecules; thus, neutrophils should not be able to roll and adhere. However, in the cremaster muscle and in many other models investigated using intravital microscopy, neutrophils clearly roll, and their rolling is easily and quickly induced. This conundrum was thought to be resolved by the observation that the induction of selectins is accompanied by ESL shedding; however, ESL shedding only partially reduces the ESL thickness (to 200 nm) and thus is insufficient to expose adhesion molecules. In addition to its antiadhesive functions, the ESL also presents neutrophil arrest-inducing chemokines. ESL heparan sulfate can also bind L-selectin expressed by the neutrophils, which contributes to rolling and arrest. We conclude that ESL has both proadhesive and antiadhesive functions. However, most previous studies considered either only the proadhesive or only the antiadhesive effects of the ESL. An integrated model for the role of the ESL in neutrophil rolling, arrest, and transmigration is needed.

Shear-induced force transmission in a multicomponent, multicell model of the endothelium

Haemodynamic forces applied at the apical surface of vascular endothelial cells (ECs) provide the mechanical signals at intracellular organelles and through the inter-connected cellular network. The objective of this study is to quantify the intracellular and intercellular stresses in a confluent vascular EC monolayer. A novel three-dimensional, multiscale and multicomponent model of focally adhered ECs is developed to account for the role of potential mechanosensors (glycocalyx layer, actin cortical layer, nucleus, cytoskeleton, focal adhesions (FAs) and adherens junctions (ADJs)) in mechanotransmission and EC deformation. The overriding issue addressed is the stress amplification in these regions, which may play a role in subcellular localization of mechanotransmission. The model predicts that the stresses are amplified 250-600-fold over apical values at ADJs and 175-200-fold at FAs for ECs exposed to a mean shear stress of 10 dyne cm(-2). Estimates of forces per molecule in the cell attachment points to the external cellular matrix and cell-cell adhesion points are of the order of 8 pN at FAs and as high as 3 pN at ADJs, suggesting that direct force-induced mechanotransmission by single molecules is possible in both. The maximum deformation of an EC in the monolayer is calculated as 400 nm in response to a mean shear stress of 1 Pa applied over the EC surface which is in accord with measurements. The model also predicts that the magnitude of the cell-cell junction inclination angle is independent of the cytoskeleton and glycocalyx. The inclination angle of the cell-cell junction is calculated to be 6.6° in an EC monolayer, which is somewhat below the measured value (9.9°) reported previously for ECs subjected to 1.6 Pa shear stress for 30 min. The present model is able, for the first time, to cross the boundaries between different length scales in order to provide a global view of potential locations of mechanotransmission.

Endothelial Cell Senescence Increases Traction Forces due to Age-Associated Changes in the Glycocalyx and SIRT1

Endothelial cell (EC) aging and senescence are key events in atherogenesis and cardiovascular disease development. Age-associated changes in the local mechanical environment of blood vessels have also been linked to atherosclerosis. However, the extent to which cell senescence affects mechanical forces generated by the cell is unclear. In this study, we sought to determine whether EC senescence increases traction forces through age-associated changes in the glycocalyx and antioxidant regulator deacetylase Sirtuin1 (SIRT1), which is downregulated during aging. Traction forces were higher in cells that had undergone more population doublings and changes in traction force were associated with altered actin localization. Older cells also had increased actin filament thickness. Depletion of heparan sulfate in young ECs elevated traction forces and actin filament thickness, while addition of heparan sulfate to the surface of aged ECs by treatment with angiopoietin-1 had the opposite effect. While inhibition of SIRT1 had no significant effect on traction forces or actin organization for young cells, activation of SIRT1 did reduce traction forces and increase peripheral actin in aged ECs. These results show that EC senescence increases traction forces and alters actin localization through changes to SIRT1 and the glycocalyx.

Mechanosensing at the vascular interface

Mammals are endowed with a complex set of mechanisms that sense mechanical forces imparted by blood flow to endothelial cells (ECs), smooth muscle cells, and circulating blood cells to elicit biochemical responses through a process referred to as mechanotransduction. These biochemical responses are critical for a host of other responses, including regulation of blood pressure, control of vascular permeability for maintaining adequate perfusion of tissues, and control of leukocyte recruitment during immunosurveillance and inflammation. This review focuses on the role of the endothelial surface proteoglycan/glycoprotein layer-the glycocalyx (GCX)-that lines all blood vessel walls and is an agent in mechanotransduction and the modulation of blood cell interactions with the EC surface. We first discuss the biochemical composition and ultrastructure of the GCX, highlighting recent developments that reveal gaps in our understanding of the relationship between composition and spatial organization. We then consider the roles of the GCX in mechanotransduction and in vascular permeability control and review the prominent interaction of plasma-borne sphingosine-1 phosphate (S1P), which has been shown to regulate both the composition of the GCX and the endothelial junctions. Finally, we consider the association of GCX degradation with inflammation and vascular disease and end with a final section on future research directions.

Modulation of endothelial glycocalyx structure under inflammatory conditions

The glycocalyx of the endothelium is an intravascular compartment that creates a barrier between circulating blood and the vessel wall. The glycocalyx is suggested to play an important role in numerous physiological processes including the regulation of vascular permeability, the prevention of the margination of blood cells to the vessel wall, and the transmission of shear stress. Various theoretical models and experimental approaches provide data about changes to the structure and functions of the glycocalyx under various types of inflammatory conditions. These alterations are suggested to promote inflammatory processes in vessels and contribute to the pathogenesis of number of diseases. In this review we summarize current knowledge about the modulation of the glycocalyx under inflammatory conditions and the consequences for the course of inflammation in vessels. The structure and functions of endothelial glycocalyx are briefly discussed in the context of methodological approaches regarding the determination of endothelial glycocalyx and the uncertainty and challenges involved in glycocalyx structure determination. In addition, the modulation of glycocalyx structure under inflammatory conditions and the possible consequences for pathogenesis of selected diseases and medical conditions (in particular, diabetes, atherosclerosis, ischemia/reperfusion, and sepsis) are summarized. Finally, therapeutic strategies to ameliorate glycocalyx dysfunction suggested by various authors are discussed.

Shear-induced endothelial NOS activation and remodeling via heparan sulfate, glypican-1, and syndecan-1

Mammalian epithelial cells are coated with a multifunctional surface glycocalyx (GCX). On vascular endothelial cells (EC), intact GCX is atheroprotective. It is degraded in many vascular diseases. GCX heparan sulfate (HS) is essential for healthy flow-induced EC nitric oxide (NO) release, elongation, and alignment. The HS core protein mechanisms involved in these processes are unknown. We hypothesized that the glypican-1 (GPC1) HS core protein mediates flow-induced EC NO synthase (eNOS) activation because GPC1 is anchored to caveolae where eNOS resides. We also hypothesized that the HS core protein syndecan-1 (SDC1) mediates flow-induced EC elongation and alignment because SDC1 is linked to the cytoskeleton which impacts cell shape. We tested our hypotheses by exposing EC monolayers treated with HS degrading heparinase III (HepIII), and monolayers with RNA-silenced GPC1, or SDC1, to 3 to 24 hours of physiological shear stress. Shear-conditioned EC with intact GCX exhibited characteristic eNOS activation in short-term flow conditions. After long-term exposure, EC with intact GCX were elongated and aligned in the direction of flow. HS removal and GPC1 inhibition, not SDC1 reduction, blocked shear-induced eNOS activation. EC remodeling in response to flow was attenuated by HS degradation and in the absence of SDC1, but preserved with GPC1 knockdown. These findings clearly demonstrate that HS is involved in both centralized and decentralized GCX-mediated mechanotransduction mechanisms, with GPC1 acting as a centralized mechanotransmission agent and SDC1 functioning in decentralized mechanotransmission. This foundational work demonstrates how EC can transform fluid shear forces into diverse biomolecular and biomechanical responses.

Loss of syndecan-1 induces a pro-inflammatory phenotype in endothelial cells with a dysregulated response to atheroprotective flow

Fluid shear stresses are potent regulators of vascular homeostasis and powerful determinants of vascular disease progression. The glycocalyx is a layer of glycoaminoglycans, proteoglycans, and glycoproteins that lines the luminal surface of arteries. The glycocalyx interacts directly with hemodynamic forces from blood flow and, consequently, is a prime candidate for the mechanosensing of fluidic shear stresses. Here, we investigated the role of the glycocalyx component syndecan-1 (sdc-1) in controlling the shear stress-induced signaling and flow-mediated phenotypic modulation in endothelial cells. We found that knock-out of sdc-1 abolished several key early signaling events of endothelial cells in response to shear stress including the phosphorylation of Akt, the formation of a spatial gradient in paxillin phosphorylation, and the activation of RhoA. After exposure to atheroprotective flow, we found that sdc-1 knock-out endothelial cells had a phenotypic shift to an inflammatory/pro-atherosclerotic phenotype in contrast to the atheroprotective phenotype of wild type cells. Consistent with these findings, we found increased leukocyte adhesion to sdc-1 knock-out endothelial cells in vitro that was reduced by re-expression of sdc-1. In vivo, we found increased leukocyte recruitment and vascular permeability/inflammation in sdc-1 knock-out mice. Taken together, our studies support a key role for sdc-1 in endothelial mechanosensing and regulation of endothelial phenotype.

The adaptive remodeling of endothelial glycocalyx in response to fluid shear stress

The endothelial glycocalyx is vital for mechanotransduction and endothelial barrier integrity. We previously demonstrated the early changes in glycocalyx organization during the initial 30 min of shear exposure. In the present study, we tested the hypothesis that long-term shear stress induces further remodeling of the glycocalyx resulting in a robust layer, and explored the responses of membrane rafts and the actin cytoskeleton. After exposure to shear stress for 24 h, the glycocalyx components heparan sulfate, chondroitin sulfate, glypican-1 and syndecan-1, were enhanced on the apical surface, with nearly uniform spatial distributions close to baseline levels that differed greatly from the 30 min distributions. Heparan sulfate and glypican-1 still clustered near the cell boundaries after 24 h of shear, but caveolin-1/caveolae and actin were enhanced and concentrated across the apical aspects of the cell. Our findings also suggest the GM₁-labelled membrane rafts were associated with caveolae and glypican-1/heparan sulfate and varied in concert with these components. We conclude that remodeling of the glycocalyx to long-term shear stress is associated with the changes in membrane rafts and the actin cytoskeleton. This study reveals a space- and time- dependent reorganization of the glycocalyx that may underlie alterations in mechanotransduction mechanisms over the time course of shear exposure.

The role of the glomerular endothelium in albumin handling

The unique permeability characteristics of the glomerular capillary wall depend on its three-layer structure, consisting of endothelial cells, the basement membrane and podocytes. These components form the glomerular filtration barrier (GFB). That albuminuria may occur in the absence of changes in podocyte foot processes suggests that GFB components other than podocytes have essential roles in albumin handling. The endothelium forms the first part of the GFB and is characterized by fenestrations-transcellular holes that are filled with endothelial glycocalyx, a hydrated mesh principally comprised of proteoglycans. The glycocalyx and adsorbed plasma constituents form the endothelial surface layer (ESL). Human and animal studies have shown that the glomerular ESL restricts macromolecule passage and ensures that plasma albumin is largely excluded from the GFB. The glomerular endothelium is also likely to indirectly influence glomerular albumin handling by modifying podocyte behaviour. These modifications may occur physiologically through soluble mediators and/or pathologically through increased exposure of podocytes to plasma components as a consequence of ESL dysfunction. The importance of the glomerular endothelium and ESL in albumin handling also sheds light on the relationship between albuminuria and vascular disease. The therapeutic potential that this relationship offers will become evident with better understanding of the structure, composition and regulation of the glycocalyx.

Sphingosine-1-phosphate protects endothelial glycocalyx by inhibiting syndecan-1 shedding

Endothelial cells (ECs) are covered by a surface glycocalyx layer that forms part of the barrier and mechanosensing functions of the blood-tissue interface. Removal of albumin in bathing media induces collapse or shedding of the glycocalyx. The electrostatic interaction between arginine residues on albumin, and negatively charged glycosaminoglycans (GAGs) in the glycocalyx have been hypothesized to stabilize the glycocalyx structure. Because albumin is one of the primary carriers of the phospholipid sphingosine-1-phosphate (S1P), we evaluated the alternate hypothesis that S1P, acting via S1P1 receptors, plays the primary role in stabilizing the endothelial glycocalyx. Using confocal microscopy on rat fat-pad ECs, we demonstrated that heparan sulfate (HS), chondroitin sulfate (CS), and ectodomain of syndecan-1 were shed from the endothelial cell surface after removal of plasma protein but were retained in the presence of S1P at concentrations of >100 nM. S1P1 receptor antagonism abolished the protection of the glycocalyx by S1P and plasma proteins. S1P reduced GAGs released after removal of plasma protein. The mechanism of protection from loss of glycocalyx components by S1P-dependent pathways was shown to be suppression of metalloproteinase (MMP) activity. General inhibition of MMPs protected against loss of CS and syndecan-1. Specific inhibition of MMP-9 and MMP-13 protected against CS loss. We conclude that S1P plays a critical role in protecting the glycocalyx via S1P1 and inhibits the protease activity-dependent shedding of CS, HS, and the syndecan-1 ectodomain. Our results provide new insight into the role for S1P in protecting the glycocalyx and maintaining vascular homeostasis.

Heparan Sulfate Regrowth Profiles Under Laminar Shear Flow Following Enzymatic Degradation

The local hemodynamic shear stress waveforms present in an artery dictate the endothelial cell phenotype. The observed decrease of the apical glycocalyx layer on the endothelium in atheroprone regions of the circulation suggests that the glycocalyx may have a central role in determining atherosclerotic plaque formation. However, the kinetics for the cells' ability to adapt its glycocalyx to the environment have not been quantitatively resolved. Here we report that the heparan sulfate component of the glycocalyx of HUVECs increases by 1.4-fold following the onset of high shear stress, compared to static cultured cells, with a time constant of 19 h. Cell morphology experiments show that 12 h are required for the cells to elongate, but only after 36 h have the cells reached maximal alignment to the flow vector. Our findings demonstrate that following enzymatic degradation, heparan sulfate is restored to the cell surface within 12 h under flow whereas the time required is 20 h under static conditions. We also propose a model describing the contribution of endocytosis and exocytosis to apical heparan sulfate expression. The change in HS regrowth kinetics from static to high-shear EC phenotype implies a differential in the rate of endocytic and exocytic membrane turnover.