Collagen-mediated platelet aggregation. Evidence for multivalent interactions of intermediate specificity between collagen and platelets.

Abstract

A B S T R A C T We have shown previously that periodate oxidation ofcollagen carbohydrate does not affect its ability to aggregate platelets. We now describe an additional characterization of periodate-modified collagen which demonstrates that collagen devoid of intact carbohydrate is fully capable of fibril formation, and we confirm its capacity to initiate platelet aggregation. Furthermore, we demonstrate that the platelet aggregating abilities of Types I, II, and III fibrillar collagen are quite similar despite differences in carbohydrate content and amino acid sequence. We also demonstrate that monomeric, pepsin-solubilized Type I human collagen is ineffective in inhibiting aggregation by preformed fibrils derived from the same molecule, thus establishing that the affinity of platelets for collagen depends upon prior polymerization of collagen. We interpret these and other findings to demonstrate that the hydroxylysyl glycoside regions of collagen are not highly specific sites involved in platelet-collagen interactions leading to "physiological" aggregation, and that the possibility must be considered that multiple interactions involving collagen sites of comparatively low structural specificity may be the initiating events in release of platelet ADP and the ensuing aggregation.

Fibronectin and the multiple interaction model for platelet-collagen adhesion.

“…Santoro and Cunningham (8) have recently proposed that adhesion of platelets to collagen and their subsequent aggregation result from-the interaction of multiple binding sites on the platelet surface, which may exhibit only modest affinity in the monovalent state for some'structural feature of the collagen molecule also multiply present in the collagen fibril. This concept is consistent with the similar capacities of the genetically distinct collagen types (8,12,13), including the (aI)3 trimer (14), to initiate aggregation provided they are in a fibrillar form. This occurs despite the existence of regions of variation in amino acid sequence and different polypeptide chain compositions and may suggest that completely identical platelet binding sites are'not present on the surface of these different collagen fibers.…”

“…This occurs despite the existence of regions of variation in amino acid sequence and different polypeptide chain compositions and may suggest that completely identical platelet binding sites are'not present on the surface of these different collagen fibers. The inability of monomeric collagen to inhibit aggregation induced by the identical collagen in fibrillar form (8) further supports this model. Fibronectin, a collagen-binding protein with molecular weight of 440,000 composed of two 220,000-dalton polypeptide chains (15), is found in plasma and on the surface of various cells and has been implicated in the adhesion of fibroblasts to collagenous substrates (16,17).…”

“…Despite early support (4)(5)(6), studies in this (7,8) and other laboratories (9,10) indicate that this mechanism is unlikely.…”

“…The adhesion assay has been used to examine the role of platelet surface fibronectin in collagen-platelet adhesion as recently proposed by Bensusan et al (18) in light of our proposal concerning the role of multivalent interactions (8). Treatment of platelets with purified antibody to fibronectin results in only 25% inhibition of adhesion.…”

Evidence that fibronectin is the collagen receptor on platelet membranes.

“…(8) have shown that only denatured collagen can act as a substrate for the platelet glucosyl transferase, whereas only native, fibrous collagen serves as a platelet substrate in the collagen-platelet interaction (9,10). In addition, others have shown that the carbohydrate residues of collagen can be completely destroyed by periodate oxidation without affecting the ability of the fibrous form of the oxidized collagen to interact normally with platelets (10,11).…”