| Osteopontin (OPN) is reported to have two important functions in kidney stone disease: inhibition of calcium oxalate monohydrate (COM) crystal aggregation and promotion of inflammation. However, many of these studies supporting these functions were carried out in vitro or in animal models of acute renal injury; thus the role of OPN in chronic renal disease is not well-defined. 2,8-dihydroxyadenine (2,8-DHA) kidney stones and inflammation are prominent features of adenine phosphoribosyltransferase (APRT) deficiency in humans and mice. We therefore created Aprt−/− Opn−/− double knockout (DKO) mice to dissect the role of OPN in vivo. We hypothesized that if OPN interacted with 2,8-DHA crystals in the same manner as COM crystals, there would be increased 2,8-DHA crystal burden in Aprt−/−Opn −/− DKO mice compared with Aprt−/− Opn+/+ mice. Furthermore, DKO mice would have increased inflammation and fibrosis due to the increased crystal burden, unless OPN was the sole initiator of inflammation. We characterized the phenotypes of DKO and Aprt−/−Opn+/+ male and female mice at ages 6 and 12 weeks of age. We observed a less severe phenotype in female mice at any age compared to males. In DKO mice versus Aprt knockout mice at 6 weeks of age, we observed no significant differences in renal pathology or in urine purine content. In DKO mice versus Aprt knockout mice at 12 weeks of age, we observed increased macrophage infiltration, collagen deposition, and general pathology. Urine and renal crystal quantification data showed that there were more stones in male DKO mice compared with male Aprt−/−Opn+/+ mice at 12 weeks of age. Increased inflammation in male DKO mice is likely due to the increased stone burden, and possibly to a compensatory upregulation of certain pro-inflammatory cytokines, including Small inducible cytokine A7. These studies suggest that OPN is a major inhibitor of the aggregation of crystals into stones in vivo. They also suggest that OPN is one of many, but not the primary, inducer of inflammation. We aim to verify these findings a cystinuria knockout mouse model of kidney stone disease, which is under development in our laboratory. |
