The Endothelial Glycocalyx: A Possible Therapeutic Target in Cardiovascular Disorders

Abstract

The physiological, anti-inflammatory, and anti-coagulant properties of endothelial cells (ECs) rely on a complex carbohydrate-rich layer covering the luminal surface of ECs, called the glycocalyx. In a range of cardiovascular disorders, glycocalyx shedding causes endothelial dysfunction and inflammation, underscoring the importance of glycocalyx preservation to avoid disease initiation and progression. In this review we discuss the physiological functions of the glycocalyx with particular focus on how loss of endothelial glycocalyx integrity is linked to cardiovascular risk factors, like hypertension, aging, diabetes and obesity, and contributes to the development of thrombo-inflammatory conditions. Finally, we consider the role of glycocalyx components in regulating inflammatory responses and discuss possible therapeutic interventions aiming at preserving or restoring the endothelial glycocalyx and therefore protecting against cardiovascular disease.

Keywords: atherosclerosis; cardiovascular risk factor; endothelial cell (EC); glycocalyx; heparan sulfate (HS); inflammation; ischemia/reperfusion injury; therapeutic target.

Figure 1

Biosynthesis and structure of the endothelial glycocalyx. Schematic representation of the major glycocalyx components covering the luminal surface of microvascular endothelial cells (EC). On the right panel, syndecan (blue) and glypican (brown) are shown as two examples of proteoglycans (PGs, see text chapter 'Biosynthesis and composition of the endothelial glycocalyx'). PGs carry long glycosaminoglycan (GAGs) side chains, while other glycoproteins (shown in yellow) carry shorter, unbranched carbohydrate side chains. The left panel shows the biosynthesis of heparan sulfate (HS), the major GAG expressed on EC. HS biosynthesis takes place in the Golgi apparatus and is mediated by different enzymes. Synthesis is initiated by the enzyme EXTL1-3 which adds the first sugar to the linker region. Chain elongation is performed by EXT1-2 which add GlcNAc and GlcA. Sulfotransferases then initiate HS sulfation, starting with NDST which sulfates GlcNAc at the N-acetyl position. HS2ST sulfates uronic acid, HS3ST and HS6ST finish sulfation by adding sulfate respectively to the 3-O and 6-O position of GlcNAc. Xyl, xylose; Gal, galactose; GlcNAc, N-acetylglucosamine; GlcA, glucuronic acid; IdoA, iduronic acid; NS, N-sulfation; 2S, 2-O sulfation; 3S, 3-O sulfation; 6S, 6-O sulfation; EXTL, exostosin-like glycosyltransferase; NDST, N-deacetylase/N-sulfotransferase; HS2ST, HS 2O-sulfotransferase; HS3ST, HS 3O-sulfotransferase; HS6ST, HS 6O-sulfotransferase.

Figure 2

Physiological functions of the endothelial glycocalyx. The microvascular glycocalyx fulfills both mechanical and biochemical functions under physiological conditions, the five main functions are depicted here. (A) Sensing and transmission of shear stress by syndecans (shown in blue), located near membrane caveolae, increases eNOS activity and NO release. This assures vasodilatory functions. (B) Transmission of shear stress to the cytoskeleton via the cytoplasmic linker region (green) of syndecans leads to rearrangement of actin filaments and cell alignment in the direction of flow. Mechanotransduction also activates intracellular signaling molecules like Rho GTPases which regulate NF_KB and MAPK. (C) The glycocalyx acts as a charge- and size-selective barrier to proteins. HS is important for maintaining intact junctions and vascular integrity. (D) Different molecules can bind to glycocalyx components: regulatory plasma proteins whose activity is potentiated by interaction with the glycocalyx, chemokines which are locally concentrated, and growth factors. (E) The intact glycocalyx shields off selectins and integrins expressed on EC thereby inhibiting leukocyte adhesion.

Figure 3

Alterations of the endothelial glycocalyx during thrombo-inflammatory conditions. Different mechanisms lead to shedding of the micro- and macrovascular glycocalyx in atherosclerosis and during ischemia/reperfusion injury (IRI). (I) Disturbed flow increases hyaluronidase expression leading to shedding of hyaluronan (shown in gray) and increased plasma syndecan-1 (shown in blue) and hyaluronic acid. (II) Hypertension, vascular stiffness and aging, all risk factors of cardiovascular disorders, cause thinning of the glycocalyx, evidenced by reduction of both glypican-1 and HS, as well as changes in the sulfation pattern of HS. (III) ROS (shown in orange), advanced glycation end products and sheddases all directly damage the endothelial glycocalyx and are linked to cardiovascular risk factors such as diabetes and obesity. (IV) Disruption of the glycocalyx is shown to be linked to reduced NO production and eNOS activity as well as activation of coagulation pathways and disturbed vasodilation. (V) Both complement deposition on the EC surface and neutrophil activation are associated with glycocalyx damage. Upon activation, neutrophils release neutrophil extracellular traps (NETs), decondensed chromatin decorated with neutrophil proteins. NETs can directly alter the glycocalyx or, through the release of MMPs and MPO (shown in purple), shed glycocalyx components and degrade junction proteins such as VE-Cadherin, leading to increased vascular leakage.

Figure 4

Soluble glycocalyx components and mechanism of glycocalyx restoration. (A) Effect of soluble glycocalyx components. Released soluble glycocalyx components such as HS (brown), syndecan-1 and −3 (blue) can propagate inflammatory responses by activating peripheral blood mononuclear cells (PBMCs) and dendritic cells (DCs) via TLR4 signaling. This leads to release of pro-inflammatory cytokines and DC maturation. However, soluble syndecan-1 (blue) has also been shown to reduce inflammation by directly inhibiting cytokine and chemokine release and blocking leukocyte adhesion. (B) Protection and restoration of the endothelial glycocalyx. Maintenance of glycocalyx integrity can be achieved by inhibition of sheddases. Doxycycline (shown in red), berberine (shown in yellow) or S1P (shown in blue) have been shown to directly inhibit matrix metalloproteinase 9 (MMP9) and therefore impede glycocalyx shedding. Regeneration of the glycocalyx can be achieved by upregulating the expression and extravasation of glycocalyx components such as syndecans with S1P and angiopoietin-1 or by replacing shed components such as HS with structurally similar agents such as dextran sulfate (shown in orange), a highly branched polysaccharide.