Brian Savage

Initiation of Platelet Adhesion by Arrest onto Fibrinogen or Translocation on von Willebrand Factor

We have identified two distinct mechanisms initiating the adhesion of flowing platelets to thrombogenic surfaces. The intergrin alpha IIb beta 3 promotes immediate arrest onto fibrinogen but is fully efficient only at wall shear rates below 600-900 s-1, perhaps because of a relatively slow rate of bond formation or low resistance to tensile stress. In contrast, glycoprotein Ib alpha binding to immobilized von Willebrand factor (vWF) appears to have fast association and dissociation rates as well as high resistance to tensile stress, supporting slow movement of platelets in continuous contact with the surface even at shear rates in excess of 6000 s-1. This eventually allows activated alpha IIb beta 3 to arrest platelets onto vWF under conditions not permissive of direct binding to fibrinogen. The coupling of these different functions may be crucial for thrombogenesis.

Specific Synergy of Multiple Substrate–Receptor Interactions in Platelet Thrombus Formation under Flow

We have used confocal videomicroscopy in real time to delineate the adhesive interactions supporting platelet thrombus formation on biologically relevant surfaces. Type I collagen fibrils exposed to flowing blood adsorb von Willebrand factor (vWF), to which platelets become initially tethered with continuous surface translocation mediated by the membrane glycoprotein Ib alpha. This step is essential at high wall shear rates to allow subsequent irreversible adhesion and thrombus growth mediated by the integrins alpha2beta1 and alpha(IIb)beta3. On subendothelial matrix, endogenous vWF and adsorbed plasma vWF synergistically initiate platelet recruitment, and alpha2beta1 remains key along with alpha(IIb)beta3 for normal thrombus development at all but low shear rates. Thus, hemodynamic forces and substrate characteristics define the platelet adhesion pathways leading to thrombogenesis.

Mechanisms of platelet aggregation.

Platelet aggregation is initiated by receptor activation coupled to intracellular signaling leading to activation of integrin alphaIIbbeta3. Recent advances in the study of platelet receptors for collagen, von Willebrand factor, thrombin, and adenosine diphosphate are providing new insights into the mechanisms of platelet aggregation.

Interaction of Integrin αIIbβ3 with Multiple Fibrinogen Domains during Platelet Adhesion

We have investigated how modulation of integrin αIIbβ3 function influences the mechanisms that initiate platelet thrombus formation onto surface-bound fibrinogen and isolated fibrinogen domains. Under stationary conditions and with full activation of platelets blocked by prostaglandin E1, the carboxyl-terminal γ400-411 sequence is necessary for establishing initial contact with the immobilized substrate. Molecules containing a single copy of this sequence, like the plasmin-generated fibrinogen fragment D, support platelet spreading, but the resulting attachment to the surface is loose and disrupted by minimal peeling force. In contrast, platelets adhere firmly to intact fibrinogen under the same conditions, suggesting that recognition of contact sites outside a single D domain can secure the firm interaction not supported by a single γ400-411 sequence. If platelets are activated, the γ400-411 sequence is no longer necessary to initiate the adhesion process but becomes sufficient, even as a single copy, to mediate stable surface attachment in the absence of shear stress. Under conditions of flow, however, intact fibrinogen but not fragment D can support adhesion, regardless of whether platelets have the potential to become activated or not. These results indicate the functional relevance of multiple fibrinogen domains during the initial stages of the platelet adhesion process.

CHAPTER 18 – Platelet Thrombus Formation in Flowing Blood

The inner lining of normal blood vessels is composed of endothelial cells that form a surface resistant to the adhesion of circulating platelets. In areas where the endothelium is altered, however, or at sites of vascular damage where subendothelial or other extracellular matrices are exposed, firm platelet attachment rapidly occurs. This is a critical initial step in hemostasis and thrombosis, as well as in infl ammatory and immunopathogenic responses,1 requiring the concerted interaction of matrix proteins with platelet receptors ultimately leading to platelet activation and aggregation. The hemostatic response to vascular injury is contingent on the extent of damage, the specific matrix proteins exposed, and flow conditions. In this chapter, we focus first on the mechanisms responsible for the initial interaction of platelets with thrombogenic surfaces associated with vascular and tissue trauma. We then discuss the secondary events that, pursuant to the initial adhesion of platelets, lead to thrombus propagation and stabilization by mechanisms that mostly depend on the formation of interplatelet contacts.