Renal epithelial cells rapidly bind and internalize calcium oxalate monohydrate crystals

Renal tubular fluid is supersaturated with calcium and oxalate ions, which can nucleate to form crystals of calcium oxalate monohydrate (COM), the most abundant constituent of kidney stones. However, the mechanisms by which nascent crystals are retained in the nephron and then grow into kidney stones are unclear. An interaction of COM crystals with the surface of renal epithelial cells could be a critical initiating event in nephrolithiasis. To investigate this possibility we used cultures of monkey kidney epithelial cells (BSC-1 line) as a model system and found that [14C]COM crystals bound to the cell surface within seconds. Scanning electron microscopy revealed that crystals bind first to apical microvilli, which subsequently migrate over the crystalline surface. When visualized by transmission electron microscopy, intracellular crystals were located within vesicles. Cytoskeletal responses to crystal uptake were sought by immunofluorescence microscopy, which revealed concentration of F-actin at sites of crystal contact as well as a generalized reorganization of the intermediate filament network containing cytokeratin 8. Uptake of COM crystals did not adversely affect renal epithelial cell growth, and internalized crystals were apparently distributed to daughter cells during division. Rapid adherence of COM crystals to the apical surface of tubular epithelial cells could promote crystal retention in the kidney. Elucidation of factors that regulate this process may provide insight into the pathogenesis of nephrolithiasis.