Platelet adhesion to purified surface-immobilized fibronectin under flow conditions was investigated. Fibronectin was found to support attachment and spreading of platelets. The extent of platelet spreading depended on the amount of immobilized fibronectin. An antiglycoprotein (anti-GP) IIb/IIIa antibody and an Arg-Gly-Asp (RGD)-containing peptide inhibited adhesion almost completely, whereas antibodies directed against platelet GP Ic/IIa (very late antigen 5) inhibited by 50%. Similar results with the antibodies and the peptide were found in a static system. A comparison of different anticoagulants showed no difference in adhesion using citrate or hirudin. However, unfractionated heparin (UFH) or low-molecular-weight heparin (LMWH) as the only anticoagulant or in combination with citrate maximally inhibited adhesion by 80% and 60%, respectively. Preincubation of the immobilized fibronectin with UFH resulted in a maximal inhibition of 90%, whereas preincubation with LMWH had no effect. When we preincubated the surface with heparins of different size, we observed 40% inhibition of adhesion with heparins with an average MW of up to 18 kD, whereas a heparin with an average MW of 21 kD almost completely blocked adhesion. These results indicate that platelet adhesion to fibronectin in flow involves several receptors, is highly RGD-mediated, does not require physiologic levels of divalent cations, and can be inhibited by direct binding of heparin to the fibronectin surface.

We describe glycoprotein (GP) Ib as a mediator of adhesion to fibronectin, specifically in flow. A monoclonal antibody (MoAb) directed to the von Willebrand factor (vWF)-binding site on this receptor or the absence of this receptor on the platelet membrane, in the case of a patient with the Bernard-Soulier syndrome, reduced platelet coverage to fibronectin to approximately 30% of the control value. A MoAb directed to the GP Ib-binding site on vWF showed a similar effect. With washed platelets in the absence of plasma vWF, the inhibitory effect of the anti-GP Ib antibody was the same as with whole blood. No inhibition with the anti-GP Ib antibody was observed when we used blood from patients with severe von Willebrand disease (vWD) or from a patient with vWD type I (platelet low). Addition of vWF to vWD blood resulted in restoration of adhesion. Immunoelectron microscopy on platelets adhering to fibronectin showed that GP Ib was homogeneously distributed over the entire surface of the platelet. vWF was present at the central zone and the edges of the platelet and at the basal interface between the platelet and the fibronectin surface. No direct binding of vWF to fibronectin could be demonstrated. These data indicate that GP Ib-mediated adhesion to fibronectin fully depends on vWF and that normal levels of plasma or platelet vWF are sufficient for optimal adhesion to fibronectin. The data suggest that the presence of platelets during perfusion is a prerequisite for vWF to support platelet adhesion to fibronectin.

Abstract. Species-specific binding between the flagellar surfaces of mating types plus and minus (mt ÷ and mt-) gametes of Chlamydomonas eugametos is mediated by mating type-specific agglutinins. Their interaction triggers several mating responses that are necessary for cell fusion, such as flagellar twitching, flagellar tip activation, redistribution of agglutinin molecules to the flagellar tip (tipping), and mating structure activation. Earlier, we reported that a monoclonal antibody (mAb 66. 3) 107:177-189). Here we report that the lectin wheat germ agglutinin (WGA), which does not bind to the agglutinins, can also invoke all these mating reactions. We show, by immunofluorescence studies using anti-WGA and an agglutinin-specific monoclonal antibody (mAb 66.3), that WGA induces the redistribution of agglutinin to the flagellar tips of mt-gametes. Vice versa, when agglutinin tipping is induced by mAb 66.3, the WGA-binding glycoproteins are also tipped. Under the same conditions, the major flagellar glycoproteins are not redistributed, indicating that membrane transport is limited to a few components. We conclude that each agglutinin is associated with a WGA-binding glycoprotein. When cells lacking agglutinin or cells possessing inactive agglutinins are treated with WGA, mating responses are again elicited.The data suggest that clustering of agglutinincontaining complexes results in the production of intracellular signals, such as cAMP, and the coupling of the complex to a force generating system. In nature, the complexes are clustered via the agglutinins, but artificially they can be clustered by lectins or antibodies directed against other proteins in the complex.

Abstract-We investigated the involvement of different domains of fibronectin in mediating platelet adhesion to fibronectin in the extracellular matrix (ECM) of cultured endothelial cells under flow conditions. Polyclonal anti-fibronectin antibodies were absorbed with Sepharose to which no protein, intact fibronectin, or different fibronectin fragments had been coupled to obtain supernatants (Sups) (Sup 0 , Sup FN , and Sup name of the fragment , respectively) from which a specific part of the antibodies had been removed. Treatment of the ECM before perfusion with Sup 0 resulted in a 36% decrease in platelet coverage, whereas treatment with Sup FN resulted in maximal adhesion. Treatment of the ECM with supernatants from which antibodies directed against the gelatin-or heparin-binding domain had been removed showed the same inhibition as treatment with Sup 0 . Removal of antibodies directed to the 120-kDa cell-binding domain resulted in a level of adhesion equal to the level found when the ECM was treated with Sup FN . Further analysis of this central region showed that only treatment with supernatants from which antibodies directed to the ninth type III repeat (III-9) of fibronectin had been removed resulted in a significantly higher adhesion than treatment with Sup 0 . Studies of adhesion to the fragments themselves showed that only fragments containing III-10 were able to support adhesion. Mutation of the Arg-Gly-Asp (RGD) sequence into Arg-Gly-Glu (RGE) in one of those fragments resulted in a complete loss of adhesive capacity. These data suggest that platelet adhesion to fibronectin in the ECM depends on III-9, whereas III-10 does not seem to be required. For platelet adhesion to isolated fibronectin, an intact RGD sequence seems to be crucial. Key Words: platelet adhesion Ⅲ fibronectin Ⅲ endothelial cell matrix T he extracellular matrix (ECM) protein fibronectin is involved in a variety of biological processes by mediating cell adhesion and migration. 1 As a constituent of the subendothelium of the vessel wall, it is recognized by blood platelets. In this way, fibronectin contributes to the process of hemostasis, which follows after the vessel has been damaged and the integrity of the endothelial cell layer has been lost.Fibronectin is composed of 3 types of homologous repeats, designated as types I, II, and III. 2 Proteolysis yields proteaseresistant functional domains that interact with heparin, collagen, fibrin, and cells. The cell-binding domain, which occupies the central region in the molecule, consists of type III repeats, each Ϸ90 amino acids in length. 3 The first sequence in fibronectin found to possess celladhesive properties was the arginine-glycine-aspartic acid (RGD) sequence, which is located in the 10th type III repeat (III-10) of the cell-binding domain. 4 On platelets, this sequence in fibronectin is recognized by 2 receptors, glycoprotein (GP) IIb/IIIa 5,6 and very late antigen 5 (VLA-5), 7 corresponding to GP Ic/IIa on the platelet. 8 -10 Both receptors are members of the superfamily o...