Influence of Fibrillar Collagen Structure on the Mechanisms of Platelet Thrombus Formation Under Flow

Abstract

We have used real-time video microscopy to study the mechanisms of platelet adhesion to type I collagen fibrils of distinct structure exposed to flowing blood. Electron microscopy analysis by surface replication demonstrated morphological differences between acid-insoluble fibrils, displaying a regularly repeating striated pattern (banded collagen), and acid-soluble fibrils generated by pepsin treatment of insoluble collagen, smaller in size with a helical configuration (nonbanded collagen). These structural differences proved to be related to the role of platelet integrin 2β1 in stabilizing adhesion to collagen under a variety of flow conditions. Blocking 2β1 function with a monoclonal antibody had no effect on platelet adhesion to insoluble type I collagen coated at high density on a glass surface, whereas there was an absolute dependence of 2β1 function for the initial permanent arrest of platelets and subsequent thrombus formation on pepsin-solubilized type I collagen under the same conditions. In contrast, reconstituted, banded fibrils prepared from pepsin-solubilized type I collagen supported platelet adhesion and thrombus development even when platelet 2β1 function was blocked, a process that was greatly accelerated by pre-exposure of this substrate to autologous plasma under flow. These results implicate a collagen receptor(s) on platelets other than 2β1 that can selectively engage domains in banded, but not nonbanded type I collagen when 2β1 function is blocked. In addition, collagen structure may regulate the extent and affinity of the binding under flow of plasma components such as von Willebrand factor and/or other IIbβ3 ligands.