The Interaction of Cross-Linked Fibrin Fragment D-Dimer with αIIbβ3 Requires Receptor Activation, Occurs at Sites Other Than the γ-404-411 Sequence, and Contributes to Clot Retraction

Background: The interaction between the fibrinogen (fbg) γ-chain sequence 404-411 and the ‘RGD pocket’ formed by αIIb and β3 plays a major role in platelet (P) aggregation. It requires activation of αIIbβ3 when fbg is in solution, but not when fbg is immobilized. Less is known about the interaction of Ps with cross-linked fibrin, which presumably is the dominant form of fibrin(ogen) in human thrombi and the form that participates in clot retraction. αIIbβ3 is required for clot retraction since it is decreased or absent in Glanzmann thrombasthenia patients who lack a functional receptor. Paradoxically, clot retraction is essentially normal with fbg lacking γ-408-411 or in the presence EDTA, which eliminates fbg binding to the RGD pocket. We recently studied the interaction of αIIbβ3 with fbg fragments D100 (which has γ-404-411) and 'D98' (which essentially lacks γ-406-411) to identify fbg binding sites on αIIbβ3 other than the RGD pocket. We found that: 1. activated, but not unactivated, Ps adhere well to immobilized 'D98;' 2. cells expressing normal αIIbβ3 could bind to: a) fbg in the absence of EDTA, but not in the presence of EDTA, b) D100 in the absence or presence of EDTA, c) 'D98' in the presence, but not the absence, of EDTA; 3. cells expressing constitutively active αIIbβ3 mutants, but not cells expressing normal αIIbβ3, adhere well to immobilized 'D98.' These data support there being two separate mechanisms of binding, one involving the interaction of γ-404-411 (in fbg and D100) with the αIIbβ3 RGD binding pocket, and another involving a site on D100 and 'D98' (but cryptic in fbg) that interacts with a site on αIIbβ3 exposed by receptor activation or induced by EDTA.