Characterization of sugar binding by osteoclast inhibitory lectin

Abstract

Osteoclast inhibitory lectin (OCIL) is a membrane-bound C-type lectin that blocks osteoclast differentiation and, via binding to its cognate receptor NKRP1D, inhibits natural killer cell-mediated cytotoxicity. OCIL is a member of the natural killer cell receptor C-type lectin group that includes CD69 and NKRP1D. We investigated carbohydrate binding of soluble recombinant human and mouse OCIL in enzyme-linked immunosorbent assay-based assays. OCIL bound immobilized high molecular weight sulfated glycosaminoglycans, including fucoidan, λ-carrageenan, and dextran sulfate, but not unsulfated dextran or sialated hyaluronic acid. Carbohydrate binding was Ca2+- independent. Binding of immobilized low molecular weight glycosaminoglycans, including chondroitin sulfate (A, B, and C forms) and heparin, was not observed. However, the soluble forms of these low molecular weight glycosaminoglycans competed for OCIL binding of immobilized fucoidan (as did soluble fucoidan, dextran sulfate, and λ-carrageenan), indicating that OCIL does recognize these carbohydrates. Inhibition constants for chondroitin sulfate A and heparin binding were 380 and 5 nM, respectively. Immobilized and soluble monosaccharides did not bind OCIL. The presence of saturating levels of fucoidan, dextran sulfate, and λ-carrageenan did not affect OCIL inhibition of osteoclast formation. The fucoidan-binding lectins Ulex eurepaeus agglutinin I and Anguilla anguilla agglutinin did not block osteoclast formation or affect the inhibitory action of OCIL. Although the osteoclast inhibitory action of OCIL is independent of sugar recognition, we have found that OCIL, a lectin widely distributed, but notably localized in bone, skin, and other connective tissues, binds a range of physiologically important glycosaminoglycans, and this property may modulate OCIL actions upon other cells.