Calcium oxalate monohydrate aggregation induced by aggregation of desialylated Tamm-Horsfall protein
Tamm-Horsfall protein (THP) is thought to protect against calcium oxalate monohydrate (COM) stone formation by inhibiting COM aggregation. Several studies reported that stone formers produce THP with reduced levels of glycosylation, particularly sialic acid levels, which leads to reduced negative charge. In this study, normal THP was treated with neuraminidase to remove sialic acid residues, confirmed by an isoelectric point shift to higher pH. COM aggregation assays revealed that desialylated THP (ds-THP) promoted COM aggregation, while normal THP inhibited aggregation. The appearance of protein aggregates in solutions at ds-THP concentrations ≥1 μg/mL in 150 mM NaCl correlated with COM aggregation promotion, implying that ds-THP aggregation induced COM aggregation. The aggregation-promoting effect of the ds-THP was independent of pH above its isoelectric point, but was substantially reduced at low ionic strength, where protein aggregation was much reduced. COM aggregation promotion was maximized at a ds-THP to COM mass ratio of ~0.025, which can be explained by a model wherein partial COM surface coverage by ds-THP aggregates promotes crystal aggregation by bridging opposing COM surfaces, whereas higher surface coverage leads to repulsion between adsorbed ds-THP aggregates. Thus, desialylation of THP apparently abrogates a normal defensive action of THP by inducing protein aggregation, and subsequently COM aggregation, a condition that favors kidney stone formation.
KeywordsSialic acid THP Crystal aggregation Protein aggregation Calcium oxalate Kidney stone
The authors thank Dr. Brian K. Olmsted for helpful discussions and Dr. William J. Zachowicz for performing the electrophoretic mobility (zeta potential) measurements reported in this study. This study was supported by the Veterans Affairs Merit Review Program (9305), the National Institutes of Health (NIDDK R01-DK068551), the Jacob Lemann Jr. Endowment Grant from the Medical College of Wisconsin, and the NYU Molecular Design Institute.
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