Extracellular matrix modifiers

Atherosclerotic lesions are thought to arise from transport and retention of plasma LDL through the endothelial cell layer into the extracellular matrix of the subendothelial space. Once in the artery wall, LDL is chemically modified through oxidation and nonenzymatic glycation. Mildly oxidized LDL then recruits monocytes into the artery wall. These monocytes then become transformed into macrophages that accelerate LDL oxidation. 

Dietary sugars are also modified for synthesis of glycoproteins and proteoglycans, especially for serum proteins and extracellular matrix structural proteins. 

Figure 8-17 Working model of the protein-protein interactions in focal adhesions determined by in vitro binding experiments and immunolocalization. In addition, several interactions are of relatively low affinity in solution but may be enhanced at the membrane surface. Abbreviations are ECM, extracellular matrix PM, plasma membrane p-Tyr- , unknown phosphoty-rosine-containing protein R/E/M, member of the radixin/ezrin/moesin family VASP, vasodilator-stimulated phosphopro-tein. Diagram is modified from Simon et al.285...

Figure 1. The muscle dystrophin-glycoprotein complex. The dystrophin-glycoprotein complex normally spans the plasma membrane of the skeletal muscle cell and may stabilize the sarcolemma and cytoskeleton to allow force transduction between the intracellular cytoskeleton (F-actin filaments) and the extracellular matrix. The molecules indicated are core components of the dystrophin-glycoprotein complex. Laminin 2 is the predominant laminin isoform in skeletal muscle basement membranes. Modified from McNeil and Steinhardt (2003)...

There are several self-assembling macromolecules that are of interest to us in this text. They include (1) collagen, the primary structural material found in the extracellular matrix (2) actin, a component of the cell cytoskeleton that is involved in cell locomotion and in formation of the thin filaments of muscle (3) microtubules, which are involved in cell mitosis, movement, and organelle movement and finally (4) fibrinogen, which forms fibrin networks that minimize bleeding from cut vessels. Self-assembly is important in these systems because the function of these macromolecules can be modified via processes that increase the molecular axial ratio and hence decrease the solubility. 

Of the several other protein components of the extracellular matrix, the best understood is fibronectin. This glycoprotein is a heterodimer composed of two very similar, but not identical, disulfide-bonded polypeptide chains (Mr — 220,000). It is found in several forms as a bound complex on the surface of cells such as fibroblasts as large aggregates in the extracellular space and in a modified form as the so-called cold-insoluble globulin (i.e., it readily precipitates at 0°C) in plasma. 

Levels ofVWF in the subendotelium are variable and depend on the origin of endothelial cells and their growth conditions. It is likely that the activation status of endothelial cells is wdiat determines the amoimt of VWF secreted towards the subendothelium. Studies on cultured endothelial cells have shown that activation induced by phorbol yristate or thrombin modify the reactivity of die extracellular matrix towards platelets [33]. Although in these in vitro studies, endothelial cell activation is followed by the release of VWF to the supernatant, it may be speculated that, in vivo, chronic endothelial activation could result in an increase in subendothelial VWF which, together with rapid de-endoflielization could result in thrombotic episodes [34]. 

The development of parallel-plate perfusion chambers [67,68] made possible the study of platelet interaction with the extracellular matrix (ECM) generated by cells in culture or with isolated subendothelial components under defined experimental conditions. The use of the ECM produced by human umbilical vein endothelial cells (HUVEC) in culture as adhesive substrate has Su tated the understanding of the mechanisms involved in primary hemostasis [68]. HUVECs are immature and not subjected to flow conditions during their culture, two Ikctors which may influence the reactivity of their ECM towards platelets [69]. Interestingly, the properties and reactivity of the underlying ECM can be modified by exposure of HUVECs to different stimuli, an experimental approach which has fevored the investigation of basic mechanisms of thrombosis [33]. 

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