Effects of impaired functional domains of osteopontin on renal crystal formation: Analyses of OPN transgenic and OPN knockout mice.

Osteopontin (OPN) has been described as playing a nonredundant role in renal crystal formation. Here we investigated the effects of impaired domains of OPN, namely, the Arg-Gly-Asp (RGD) sequence and two calcium-binding sites on crystal formation. We used wild-type mice (WT group), OPN knockout mice (KO group), and OPN knockout mice carrying either a transgene in which the RGD sequence had been modified to Arg-Gly-Glu (RGE group) or whose two calcium-binding sites had been deleted (CaX group). Following intraperitoneal injection of glyoxylate for 9 days, the changes occurring in three parameters of crystal formation--localization, number, and microstructure--were analyzed. In the WT group, crystal deposits increased gradually at the renal corticomedullary junction in an orderly fashion, whereas those in the KO group were observed sporadically in the renal cortex. In both the CaX and RGE groups, deposits were localized near the corticomedullary junction. Crystal deposition was greatest in the WT group and least in the KO group. The number of deposits in the RGE group was nearly equal to that in the KO group. Microscopic observations revealed that the crystal nuclei in the CaX group were stratified and occurred in a disordered pattern; this pattern was dissimilar to that in the WT group, in which a rosette petal-like radial pattern was observed. In the RGE group, the nuclei exhibited a radial pattern similar to that in the WT group. The results indicated the possibility that each domain contributes to the mechanism by which OPN stimulates crystal formation.