The assembly of nonadhesive fibrinogen matrices depends on the αC regions of the fibrinogen molecule.

Adsorption of fibrinogen on fibrin clots and other surfaces strongly reduces integrin-mediated adhesion of platelets and leukocytes with implications for the surface-mediated control of thrombus growth and blood compatibility of biomaterials. The underlying mechanism of this process is surface-induced aggregation of fibrinogen, resulting in the assembly of a nanoscale multilayered matrix. The matrix is extensible, which makes it incapable of transducing strong mechanical forces via cellular integrins, resulting in insufficient intracellular signaling and weak cell adhesion. To determine the mechanism of the multilayer formation, the physical and adhesive properties of fibrinogen matrices prepared from human plasma fibrinogen (hFg), recombinant normal (rFg), and fibrinogen with the truncated αC regions (FgAα251) were compared. Using atomic force microscopy and force spectroscopy, we show that whereas hFg and rFg generated the matrices with a thickness of ∼8 nm consisting of 7-8 molecular layers, the deposition of FgAα251 was terminated at two layers, indicating that the αC regions are essential for the multilayer formation. The extensibility of the matrix prepared from FgAα251 was 2-fold lower than that formed from hFg and rFg. In agreement with previous findings that cell adhesion inversely correlates with the extensibility of the fibrinogen matrix, the less extensible FgAα251 matrix and matrices generated from human fibrinogen variants lacking the αC regions supported sustained adhesion of leukocytes and platelets. The persistent adhesiveness of matrices formed from fibrinogen derivatives without the αC regions may have implications for conditions in which elevated levels of these molecules are found, including vascular pathologies, diabetes, thrombolytic therapy, and dysfibrinogenemia.

Origin of the nonadhesive properties of fibrinogen matrices probed by force spectroscopy.

The deposition of a multilayered fibrinogen matrix on various surfaces results in a dramatic reduction of integrin-mediated cell adhesion and outside-in signaling in platelets and leukocytes. The conversion of a highly adhesive, low-density fibrinogen substrate to the nonadhesive high-density fibrinogen matrix occurs within a very narrow range of fibrinogen coating concentrations. The molecular events responsible for this transition are not well understood. Herein, single-cell and molecular force spectroscopy were used to determine the early steps in the formation of nonadhesive fibrinogen substrates. We show that the adsorption of fibrinogen in the form of a molecular bilayer coincides with a several-fold reduction in the adhesion forces generated between the AFM tip and the substrate as well as between a cell and the substrate. The subsequent deposition of new layers at higher coating concentrations of fibrinogen results in a small additional decrease in adhesion forces. The poorly adhesive fibrinogen bilayer is more extensible under an applied tensile force than is the surface-bound fibrinogen monolayer. Following chemical cross-linking, the stabilized bilayer displays the mechanical and adhesive properties characteristic of a more adhesive fibrinogen monolayer. We propose that a greater compliance of the bi- and multilayer fibrinogen matrices has its origin in the interaction between the molecules forming the adjacent layers. Understanding the mechanical properties of nonadhesive fibrinogen matrices should be of importance in the therapeutic control of pathological thrombosis and in biomaterials science.