Introduction 66 86 4 74 51 52 63 83 101 83 17 51 129 125 67 28 Composition 62 86 Visualization techniques 1 Fig. 1 Left Right GAG-chain ec-SOD AT III  Below, state-of-the-art knowledge on the various components of the endothelial glycocalyx will be provided. Although many molecules have been identified as being part of the glycocalyx, information on their distribution is still scarce; if present, such knowledge was mostly obtained indirectly and nonquantitatively. Proteoglycans 1 n n 12 25 44 50 Table 1 Characteristics of proteoglycan core proteins in the vascular endothelial glycocalyx Core protein group Core protein size (kDa) Number of subtypes Number of GAG-chains linked Type of GAG-chains linked Structural relation to cell membrane Syndecan 19–35 4 5 HS/CS Membrane-spanning Glypican 57–69 6 3 HS/CS GPI-anchor Perlecan 400 1 3 HS/CS Secreted Versican 370 1 10–30 CS/DS Secreted Decorin 40 1 1 CS/DS Secreted Biglycan 40 1 2 CS/DS Secreted Mimecan 35 1 2–3 KS Secreted GAG HS CS DS KS GPI 89 70 2 22 27 57 58 114 Table 2 Composition of the disaccharides of various glycosaminoglycan chains   Heparan sulfate Chondroitin sulfate a Hyaluronan Keratan sulfate Uronic acid GlcA(2S) IdoA(2S) GlcA GlcA IdoA(2S) GlcA Gal(6S) Disaccharide link 1β4 1β3 1β3 1β3 1β4 Hexosamine GlcNAc(NS)(3S)(6S) a a GalNAc(4S)(6S) GlcNAc GlcNAc(6S) Polymerization link 1β4 1β4 1β4 1β4 1β3 N \documentclass[12pt]{minimal}
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\begin{document}$${\left( {{\text{GlcNH}}^{ + }_{3} } \right)},$$\end{document} GlcA IdoA Gal GlcNAc N GalNAc N 2S O 3S O 4S O 6S O NS N a O N O N 43 86 119 70 90 4 72 135 57 31 15 23 58 cis N O cis trans 3 4 131 Glycoproteins Besides the proteoglycans with their long linear side chains, certain glycoproteins are also regarded as “backbone” molecules, connecting the glycocalyx to the endothelial cell membrane. This group of endothelial glycoproteins, characterized by relatively small (2–15 sugar residues) and branched carbohydrate side chains, comprises a number of molecules that all have been studied intensively; major classes that will be discussed in more detail below are the endothelial cell adhesion molecules and components of the coagulation and fibrinolysis system. It is beyond the scope of this review to categorically discuss all glycoproteins that can be expressed by endothelial cells. Furthermore, one should realize that the level of glycoprotein expression on the endothelial cell membrane varies considerably with cell activation or stimulation. The endothelial cell adhesion molecules are well-defined glycoproteins that play a major role in cell recruitment from the bloodstream and in cell signaling. The three families of cell adhesion molecules present in the endothelial glycocalyx are the selectin family, the integrin family, and the immunoglobulin superfamily. 29 48 108 19 53 109 47 78 139 142 V 3 11 2 1 5 1 6 1 98 71 113 41 65 99 7 115 141 Soluble components 42 54 88 145 1 103 104 117 Functional importance The endothelial glycocalyx as the endothelial gatekeeper 35 130 125 124 121 126 112 137 68 40 144 79 14 91 45 129 128 13 13 36 124 125 67 111 81 13 36 70 128 102 33 106 138 46 78 62 84 85 The endothelial glycocalyx as mechanotransducer 16 18 96 24 69 69 30 123 138 116 The endothelial glycocalyx as control center for the microenvironment 3 Table 3 Molecules dependent on interaction with the endothelial glycocalyx for proper functioning Interacting molecule Primary function in vasculature Reference number Antithrombin III Potent inactivator of pro-coagulant proteases such as thrombin, factor Xa and factor IXa; activity enhanced by heparin or heparan sulfate 107 Heparin cofactor II Inactivator of the procoagulant protease thrombin; activated by dermatan sulfate in the endothelial glycocalyx 120 TFPI Anticoagulant protein blocking activated factor VII and X 38 49 LPL Enzyme involved in breakdown of low density lipoproteins 133 LDL Transports cholesterol and triglycerides through the circulation 140 VEGF Potent stimulator of angiogenesis, production of which is triggered by hypoxia 92 TGFβ1/2 Growth factor known to mediate in a lot of signaling pathways, including smooth muscle cell differentiation and vascular tone and reactivity 64 FGF(r) Growth factor (receptor) involved in endothelial cell proliferation and angiogenesis 3 26 Ec-SOD Extracellular quencher of reactive oxygen species 11 IL 2, 3, 4, 5, 7, 8, 12, RANTES Chemotaxis of leukocytes to the subendothelium; involved in arrest and diapedesis 2 39 100 110 143 TFPI LPL LDL VEGF TGFβ1/2 FGF(r) ec-SOD IL RANTES 3 26 133 140 61 82 87 107 80 120 136 49 38 21 9 10 59 The endothelial glycocalyx in pathophysiology In healthy vessels, the endothelial glycocalyx determines vascular permeability, attenuates blood cell–vessel wall interactions, mediates shear stress sensing, enables balanced signaling, and fulfills a vasculoprotective role. But when it is disrupted or modified, these properties are lost, as has been shown through direct targeting of the glycocalyx in experimental settings. In the last few years, evidence is emerging that (damage to) the glycocalyx plays a pivotal role in several vascular pathologies. Here, we will discuss its suspected roles in diabetes, ischemia/reperfusion, and atherosclerosis. Diabetes 73 1 20 76 75 118 Ischemia/reperfusion 32 105 77 56 8 132 55 70 97 70 6 122 134 Atherosclerosis 34 60 93 128 123 74 4 Visualization techniques 66 121 51 124 2 37 94 95 Fig. 2 a Bar 124 b left pane right pane bar in the left pane 129 c WGA green blue arrows X Y Z 3 67  129 2 33 106 138 129 35 36 130 13 97 2 5 24 70 24 70 5 70 127 −18 67 2 127 Conclusions Overlying the vascular endothelium, the glycocalyx is a membrane-bound mesh in which plasma-derived molecules integrate. It exerts a variety of functions, important in normal vascular physiology and also in vascular disease. Although data from experiments in microcirculation, and more recently, in macrocirculation strongly suggests a vasculoprotective role for the glycocalyx, research on this subject is hampered by lack of a good visualization technique. Two-photon laser scanning microscopy may prove to be a successful tool in achieving direct visualization of the glycocalyx in larger arteries in rodents, both ex vivo and in vivo, with the possibility to analyze focal variations in the composition or integrity of this layer.