Morphological Conversion of Calcium Oxalate Crystals Into Stones Is Regulated by Osteopontin in Mouse Kidney

Abstract

An important process in kidney stone formation is the conversion of retentive crystals in renal tubules to concrete stones. Osteopontin (OPN) is the major component of the kidney calcium-containing stone matrix. In this study, we estimated OPN function in early morphological changes of calcium oxalate crystals using OPN knockout mice: 100 mg/kg glyoxylate was intra-abdominally injected into wildtype mice (WT) and OPN knockout mice (KO) for a week, and 24-h urine oxalate excretion showed no significant difference between WT and KO. Kidney crystal depositions were clearly detected by Pizzolato staining but not by von Kossa staining in both genotypes, and the number of crystals in KO was significantly fewer than in WT. Morphological observation by polarized light optical microphotography and scanning electron microphotography (SEM) showed large flower-shaped crystals growing in renal tubules in WT and small and uniform crystals in KO. X-ray diffraction detected the crystal components as calcium oxalate monohydrate in both genotypes. Immunohistochemical staining of OPN showed that the WT crystals contained OPN protein but not KO crystals. We concluded that OPN plays a crucial role in the morphological conversion of calcium oxalate crystals to stones in mouse kidneys.

Renal macrophage migration and crystal phagocytosis via inflammatory-related gene expression during kidney stone formation and elimination in mice: Detection by association analysis of stone-related gene expression and microstructural observation

“…Spp1 encodes OPN, which is a major constituent of calcium-containing kidney stones. (40) We have reported evidence of OPN as a promoter of kidney stone formation (5)(6)(7)(8)(9) ; however, opposing opinions suggest that OPN is an inhibitor. (10)(11)(12) This controversy has not been resolved, but knowledge of stone elimination (13)(14)(15) indicates two possible functions of OPN during kidney stone formation-as a promoter of early crystal conversion to stones and as an inhibitor of kidney stone formation, acting as an opsonin-like molecule involved in crystal digestion by macrophages.…”

“…The generated crystals had morphologic features of a rosette shape and specific birefringence, as reported previously. (8) Quantification of the number of crystals by NIH imaging software indicated a significant increase in crystals on day 6 compared with day 0 and a decrease on day 15 compared with day 6 (Fig. 1B).…”

“…OPN expression was detected on the apical side of tubular cells and seemed to be contained in crystal depositions, and the same findings were described previously. (8) FN was distributed in tubular cells surrounding crystal depositions and along widened …”

“…(3)(4)(5) Since this discovery, a new perspective on kidney stones as ''genetic products'' as well as ''mineral matter'' has been developed, and biomolecular approaches have been initiated. Recently, using two experimental systems, OPN antisense expressing renal tubular cells (6,7) and OPN knockout mice, (8) we found that OPN could play a crucial role in CaOx crystal attachment to the renal tubular cell surface and crystal growth and kidney stone formation. Moreover, using two types of transgenic mice with conversion of the Arg-Gly-Asp (RGD) sequence, the cell-attachment domain of OPN, to the Arg-Gly-Glu (RGE) sequence and lack of two calcium-binding sites consisting of an aspartate chain, we found that each domain could affect the number of kidney CaOx crystals and crystal morphology, respectively, and confirmed OPN function in kidney stone formation as a promoter.…”

The Isolation and Characterization of Glycosylated Phosphoproteins from Herring Fish Bones

“…This is due to observations and experimental evidence that they are involved in a number of biological events such as regulation of biomineralization and cellular activity/ behavior in normally mineralizing tissues including bone (1-7), cartilage (8), and dentin (9). ECM phosphoproteins have been also found in the pathologically mineralizing tissues such as atherosclerotic plaque (10), kidney stones (11), dental calculus (12), and breast tumors (13). Two most abundant and well known are bone sialoprotein (BSP) and osteopontin (OPN), which are synthesized by osteoblasts during bone formation (1-7, 14, 15).…”