Adhesion of hydroxyapatite crystals to anionic sites on the surface of renal epithelial cells

Adhesion of microcrystals that nucleate in tubular fluid to the apical surface of renal tubular cells could be a critical step in the formation of kidney stones, 20% of which contain hydroxyapatite (HA). HA crystals bound rapidly to monolayer cultures of monkey kidney epithelial cells (BSC-1 line), used to model the surface of the nephron, in a concentration-dependent manner. Adhesion was blocked by diverse polyanions including heparin, pentosan polysulfate, polyaspartate, and polyglutamate, as well as many found in tubular fluid such as chondroitin sulfates A and B, heparan sulfate, citrate, nephrocalcin, and osteopontin. The polycations cetylpyridinium chloride and cationized ferritin, as well as the cationic dyes alcian blue, polyethylenimine, and brilliant blue R, also inhibited adhesion of HA crystals, as did specific lectins including Triticum vulgaris (wheat germ agglutinin). Anions that inhibited adhesion of crystals appeared to act on the crystal surface, whereas cations and lectins exerted their effect on the cell. Treatment of cells with neuraminidase inhibited binding of crystals, suggesting that anionic cell surface sialic acid residues function as HA crystal receptor sites that can be blocked by specific cations or lectins. Adherence of HA crystals to cells of another renal line (MDCK) and to 3T3 fibroblasts was also inhibited by heparin, polyaspartate, alcian blue, and T. vulgaris lectin, suggesting that these crystals bind to analogous molecules on the surface of different types of cells. These results suggest that the structure, quantity, and/or function of soluble anions in tubular fluid, as well as those anchored to the cell surface, could be critical determinants of HA crystal retention in the nephron and the subsequent formation of a renal stone.