Systematic characterizations of a urinary protein, prosaposin, as a novel inhibitor of calcium oxalate stone formation.

The development of calcium oxalate (CaOx) stones in the kidney comprises multiple processes involving sophisticated interactions between stone minerals and other molecules, especially urinary proteins. Prosaposin (PSAP) has recently been identified in the urine from healthy (non-stone) subjects, not in the urine from CaOx stone subjects. Consequently, PSAP may serve as a CaOx stone inhibitor, but without experimental evidence. This study thus aimed to systematically investigate the stone-modulatory effects of PSAP on four critical CaOx stone-forming processes. Recombinant human PSAP protein was successfully produced and purified in a soluble form and then subjected to all crystal assays at final concentrations of 0.001, 0.01, 0.1, 1 and 10 μg/ml. The results showed that PSAP concentration-dependently inhibited CaOx crystallization, crystal growth and crystal aggregation. Also, PSAP at all concentrations reduced crystal adhesion on the surfaces of tubular cells to the same degree. To address mechanisms underlying its crystal-inhibitory effects, we further investigated the binding abilities of PSAP with Ca2+ and Ox2- ions, and CaOx crystal surfaces. Ca2+- and Ox2--consumption assays revealed that PSAP concentration-dependently bound with both Ca2+ and Ox2- ions. Immunofluorescence study revealed that PSAP effectively bound to the crystal surfaces. These findings indicate that PSAP serves as a novel inhibitor of CaOx stone formation at crystallization, crystal growth, aggregation and crystal-cell interactions phases via its binding abilities with Ca2+ and Ox2- ions and CaOx crystal surfaces.