Endothelial glycocalyx: permeability barrier and mechanosensor

Abstract

Endothelial cells are covered with a polysaccharide rich layer more than 400 nm thick, mechanical properties of which limit access of circulating plasma components to endothelial cell membranes. The barrier properties of this endothelial surface layer are deduced from the rate of tracer penetration into the layer and the mechanics of red and white cell movement through capillary microvessels. This review compares the mechanosensor and permeability properties of an inner layer (100-150 nm, close to the endothelial membrane) characterized as a quasi-periodic structure which accounts for key aspects of transvascular exchange and vascular permeability with those of the whole endothelial surface layers. We conclude that many of the barrier properties of the whole surface layer are not representative of the primary fiber matrix forming the molecular filter determining transvascular exchange. The differences between the properties of the whole layer and the inner glycocalyx structures likely reflect dynamic aspects of the endothelial surface layer including tracer binding to specific components, synthesis and degradation of key components, activation of signaling pathways in the endothelial cells when components of the surface layer are lost or degraded, and the spatial distribution of adhesion proteins in microdomains of the endothelial cell membrane.

FIGURE 1

Methods to estimate the endothelial glycocalyx thickness. (a) Alcian blue labeling and transmission electron microscopy of thin sectioned rat myocardial capillary reveals a layer up to 500 nm thick. (b) The width of the column of red blood cells (left) or the width of a column of fluorescently labeled dextran (right) is less than the anatomic width of the capillary in hamster cremaster (glycocalyx thickness estimated to be 400–500 nm). (c) Rapid freezing and freeze substitution of cultured endothelium reveals several micron depth of glycocalyx structures. Model of glycocalyx structure derived from autocorrelation functions and Fourier transforms of representative areas of electron micrograph images (from endothelial cells in frog mesentery and usually extending shorter distances (100–200 nm) from the endothelial cell membrane) showing quasi-periodic structure perpendicular (d) and parallel (e) to the endothelial surface. (a) and (b) Reprinted with permission of Circulation Research, (c) Reprinted with permission of Arteriosclerosis Thrombosis and Vascular Biology, and (d) & (e) Reprinted with permission from Elsevier.

FIGURE 2

Models used for transport barrier and mechanosensor functions of the glycocalyx. (a) Flow through surface glycocalyx-junction break model of the endothelium. The resistance of the glycocalyx to water and solute flows is described using hydrodynamic models. Fiber diameters range from small (1–2 nm, representative of GAG side chains) to larger fibers (10 nm and adsorbed plasma proteins). Flows through the glycocalyx are funneled into infrequent breaks in the tight junction strands. Various combinations of fiber size and arrangement have been investigated which describe water and solute flows when the size and frequency of the breaks in the junction strand are measured (see text). (b) A specific form of the surface glycocalyx-junction break model based on the quasi periodic structures in Fig 1(d) and (e) is shown. The glycocalyx is modeled as branching clusters anchored to the peripheral actin band in the endothelial cell. The structures form the primary molecular filter on the luminal side of the intercellular junctions. When the fibers extend about 150 nm from the surface and breaks in the junction stand, up to 400 nm long and 20 nm wide, are present every 2–4 microns, the model describes the permeability properties of rat venular microvessels. Shear stress on the edge of these clusters transmits displacements to their anchoring sites in the endothelial actin cytoskeleton of the order of 10 nm. (a) Reprinted with permission from Elsevier and (b) Copyright 2003 National Academy of Sciences, USA, used with permission.

FIGURE 3

(a) The multistep process of leucocyte recruitment requires interaction of membrane-bound macromolecules near the surface of the leucocyte and the endothelium. The adhesion molecules extend only tens of nm from the cell surface. The common cartoon of this cascade does not take into account the presence of a thick endothelial surface layer which limits access to the cell surface receptors. (b) Leucocytes (arrows) in frog mesentery vessels preferentially localize to endothelial cell borders as revealed by silver staining. (a) Courtesy of Professor Scott I. Simon and (b) used with permission of the American Physiological Society.