Attachment sites for particles in the urinary tract

The adherence of crystals to the surface of renal tubule epithelial cells is one of the initial events in the development of nephrolithiasis. The accumulation of crystalline material in the kidney will sooner or later result in the formation of a stone. Calcium crystals occasionally are present in the urine of even healthy individuals, and mechanisms responsible for the selective attachment of crystals to the tubular epithelium of stone-forming individuals must exist. Although several types of cell surface molecules, including phosphatidylserine (PS) and sialic acid, have been proposed as receptors for crystals in the tubular system, the exact nature of these crystal-binding sites has not yet been revealed. Previously, it was demonstrated that calcium oxalate monohydrate crystals adhere to subconfluent, but not to confluent, Madin-Darby canine kidney-I cultures. This model was used here to investigate whether the surface of cells with affinity for crystals is enriched with one of the proposed crystal-binding molecules. Annexin V was used for the detection of PS at the cell surface, and Sambucus nigra lectin was used to reveal terminal sialic acid in a (alpha2,6) linkage to galactose units. FITC-annexin V binding studies showed that PS was not exposed at the surface of proliferating or growth-inhibited cells, unless they were pretreated with an apoptosis-inducing cytotoxic agent. Sambucus nigra lectin binding, of which the specificity was confirmed by blocking with N-acetylneuraminyl-lactose, demonstrated the abundant presence of (alpha2,6)-linked sialic acid residues at the cell surface of both subconfluent and confluent cultures. While these results seem to rule out a role for PS in the adherence of calcium oxalate monohydrate crystals to the surface of maturating Madin-Darby canine kidney-I cells, they question the role for cell surface-associated sialylated glycoconjugates in this process.