Based on the structure of kidney stones, it is likely that they form as aggregations of preformed crystals, mostly calcium oxalate monohydrate (COM). In this study, we examined the ability of a macromolecular mixture isolated from the urine of normal individuals and stone formers to inhibit aggregation of preformed COM seed crystals in a simple ionic solution using measurements of changes in the particle size distribution (PSD) of preformed COM crystal aggregates. We also examined the effect in this assay of a number of synthetic homopolymers, naturally occurring urine macromolecules, and binary mixtures thereof. The macromolecular mixtures from urine of normals and most stone formers reduced the degree of aggregation of the seed crystals, whereas 22% of stone former urine macromolecules either did not disaggregate or actually promoted further aggregation. Stone formers within one family shared this property, but a non-stone forming sibling did not. Polyanions, either synthetic or naturally occurring, induced disaggregation to an extent similar to that exhibited by normal urine macromolecules, while polycations had no effect on the PSD. However, mixing a polyanion, either poly-aspartate or osteopontin, with the polycation poly-arginine, changed their behavior from disaggregation to aggregation promotion. The disaggregating behavior of normal urinary macromolecules provides a defense against aggregation, but a minority of stone forming individuals lacks this defense, which may contribute to stone formation.
Cultured mouse kidney cortical cells secrete osteopontin, a bone matrix protein that is also found in urine. Osteopontin is associated with cell proliferation/tumerogenesis and also inhibits kidney stone mineral crystal growth [1]. Using antibodies raised against osteopontin isolated from the culture medium, we localized osteopontin in normal rat kidney. Fluorescence, confocal and electron microscopy revealed osteopontin primarily in cells of the descending thin limb of the loop of Henle (DTL) and in papillary surface epithelium (PSE) in the area of the calyceal fornix. In situ hybridization with labeled RNA made from a cDNA that contains the entire coding sequence for mouse osteopontin revealed message at the same sites at which protein was demonstrated by immunocytochemistry. Immunogold labeling was localized to a population of dense vesicles distinct from lysosomes and endosomes. To examine the turnover of osteopontin, rats were injected with the protein synthesis inhibitor cyclohexamide, 14 mg/kg, six hours prior to kidney fixation. These kidneys no longer demonstrated osteopontin in DTL and the immunofluorescence in the papillary surface was attenuated. Thus, osteopontin is secreted at two sites in the kidney where urine is highly concentrated in stone mineral constituents. It has a relatively rapid turnover, suggesting that it could be subject to physiological regulation. Osteopontin may be important in the normal endogenous defense against kidney stone formation.
Urine contains proteins that inhibit the growth of calcium oxalate (CaOx) crystals and may prevent the formation of kidney stones. We have identified a potent crystal growth inhibitor in the conditioned media from primary cultures of mouse kidney cortical cells. Conditioned media, incubated with the kidney cells for 6-72 h, was assayed for crystal growth inhibition; inhibitory activity increased 15-fold by 24 h. Inhibitory activity was purified from serum-free media containing proteinase inhibitors using anion-exchange and gel-filtration chromatography. A single band of molecular weight 80,000 daltons was seen after SDS-polyacrylamide gel electrophoresis. The sequence of the N-terminal 21 amino acids of this protein matched that of osteopontin (OP), a phosphoprotein initially isolated from bone matrix. Antisera raised to fusion proteins produced by plasmids containing the N-terminal or C-terminal portions of OP cDNA also cross-reacted with the protein purified from cell culture media on western blots. The effect of the purified protein on the growth of CaOx crystals was measured using a constant composition assay. A 50% inhibition of growth occurred at a protein concentration of 0.85 micrograms/ml, and the dissociation constant of the protein with respect to CaOx crystal was 3.7 x 10(-8) M. The concentration of OP in mouse urine, measured using antibodies raised to the purified protein, was approximately 8 micrograms/ml. We conclude that OP is synthesized by kidney cortical tubule cells and functions as a crystal growth inhibitory protein in urine.
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