Carbohydrate heterogeneity of fibronectins. Synovial fluid fibronectin resembles the form secreted by cultured synoviocytes but differs from the plasma form

Abstract

Large quantities of fibronectin (Fn) are present in inflammatory synovial fluid. Inflammatory synovial fluid Fn, while indistinguishable from plasma Fn on the basis of reactivity to polyclonal antibodies, displays alterations in molecular size and charge. Since biochemical differences between plasma and synovial fluid fibronectins might be in part due to differences in glycosylation we have compared the carbohydrate composition of plasma Fn, synovial fluid Fn, and Fn from synoviocyte conditioned medium by biochemical assay, glycopeptide analysis, and binding to a series of lectins. Synovial fluid Fn has a greater carbohydrate content but contains less sialic acid when compared with plasma Fn. Glycopeptides formed from synovial fluid Fn are smaller than plasma Fn glycopeptides. These data suggest the presence of an additional N-linked oligosaccharide chain on synovial fluid Fn. In addition, synovial fluid Fn contains N-acetyl galactosamine indicating the presence of O-linked oligosaccharides. Synovial fluid Fn and Fn isolated from rheumatoid synoviocyte-conditioned medium display strong reactivity with the lectins wheat germ agglutinin (WGA) and peanut agglutinin (PNA), whereas normal and rheumatoid plasma Fn react weakly. The PNA reactivity of synovial fluid Fn is mediated by terminal beta-galactose residues on the gelatin-binding domain, whereas the enhanced WGA reactivity of synovial Fn is mediated by a sialic acid containing oligosaccharide located on a 27-kD C-terminal fragment. These data demonstrate domain-specific biochemical differences between plasma and synovial fluid fibronectins. These differences suggest a local origin for synovial fluid Fn and may contribute to functional differences between these forms of the protein.