Mechanosensing at the vascular interface

Abstract

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.

Keywords: endothelium; glycocalyx; glycoprotein; proteoglycan; red cell; shear stress; sphingosine-1 phosphate; vascular disease; vascular permeability; white cell.

Figure 1

The glycocalyx on the surface of a capillary in the rat mesentery labeled with antiheparan sulfate antibody and imaged by laser-scanning confocal microscopy. Images of the glycocalyx (a) in a longitudinal series of cross sections, (b) in a centerline longitudinal section, and (c) in a three-dimensional reconstruction. (Figure adapted with permission from .)

Figure 2

Schematic diagram showing the components and spatial organization of the endothelial glycocalyx. In the diagram, a denotes the region where the cytoplasmic tail of syndecan links to the actin cytoskeleton, b indicates oligomerization of syndecans, and c labels caveolin protein within a caveola. All other components are labeled in the diagram. (Figure adapted with permission from .)

Figure 3

TEM of GCX-covered BAECs (a) preserved conventionally, labeled with ruthenium red and osmium tetroxide, and alcohol dehydrated and (b) preserved by cryo-EM and osmium tetroxide stained. (c) High-magnification image of a conventionally preserved BAEC GCX. Arrows indicate extended strands of GCX. (d) High-magnification image of a cryo-EM BAEC GCX, showing (from left to right) locations near the cell membrane, farther away from the cell membrane, in the center region of the GCX, and at the most apical surface of the GCX. Abbreviations: BAECs, bovine aortic endothelial cells; EM, electron microscopy; GCX, glycocalyx; TEM, transmission electron microscopy. (Figure adapted with permission from .)

Figure 4

Redistribution of heparan sulfate (HS) under shear stress. (a) Representative immunofluorescent images of an HS-labeled rat fat pad endothelial cell (RFPEC) under static and shear stress conditions. Respective changes in the (b) mean fluorescence intensity (MFI), (c) coverage, and (d) average radial profile of HS. The zero-radius represents the center of the cell. Significant difference: ^*P < 0.05; ^**P < 0.01. (Figure adapted with permission from .)

Figure 5

The GCX mediates NO production in response to shear stress. Bovine aortic ECs were exposed to a step increase in shear stress from static to 20 dyn/cm² at time 0 in a parallel-plate flow chamber. The HS GAG component was either partially removed by a heparinase enzyme treatment or left intact, and NO released into the media was monitored over time. Shear-induced NO production was completely blocked by the enzyme treatment. Abbreviations: GAG, glycosaminoglycan; GCX, glycocalyx; HS, heparan sulfate; NO, nitric oxide. (Figure adapted with permission from .)

Figure 6

S1P-activated pathways that regulate (a) the stability of the glycocalyx by regulating MMP release (top) and (b) the interendothelial junctions and adhesion to the basement membrane (bottom). For both panels, signaling involves ligation of S1P to S1P1 to stimulate Rac1 activation. In the top panel, serum albumin and HDL carry S1P that activates S1P1. The activation of S1P1 inhibits the activity of MMPs. The S1P1 agonist activity can be blocked by the receptor antagonist W146. After the bound S1P on the cell surface is cleared by removal of albumin (or S1P), the inhibition of MMP activity (MMP-9 and -13) is attenuated, and this induces the shedding of syndecan-1 by cleaving its ectodomain. The glycocalyx is protected when MMP activity is inhibited. The specific pathways leading control of MMP release in the top panel have not been investigated in as much detail as those that regulate the interendothelial junctions shown in the bottom panel (see 29). In the bottom panel, activation of Rac1 induces AJ and TJ assembly, cytoskeletal reorganization, and formation of focal adhesions that combine to enhance vascular barrier function. Other S1P-dependent mechanisms include an increase in intracellular Ca²⁺ concentration and transactivation of S1P1 signaling by other barrier-enhancing agents. Abbreviations: AJ, adherens junction; APC, activated protein C signaling through the thrombin receptor; CS, chondroitin sulfate; ECM, extracellular matrix; FAK, focal adhesion kinase; HDL, high-density lipoprotein; HS, heparan sulfate; MLCK, endothelial myosin light chain kinase; MMPs, matrix metalloproteinases; P, phosphorylated focal adhesion kinase; PI3K, phosphoinositide 3-kinase; PLC, phospholipase C; Rac1, a Rho family GTPase; S1P, sphingosine-1-phosphate; S1P1, sphingosine-1-phosphate receptor 1; Tiam1, T-cell lymphoma invasion and metastasis gene 1; TJ, tight junction; VE-cad, vascular endothelial cadherin; ZO-1, zona occludens-1. (See text for more details; figure adapted with permission from and .)