Renal osteopontin expression in experimental urolithiasis

Renal macrophages monitor and remove particles from urine to prevent tubule obstruction

When the filtrate of the glomerulus flows through the renal tubular system, various microscopic sediment particles, including mineral crystals, are generated. Dislodging these particles is critical to ensuring the free flow of filtrate, whereas failure to remove them will result in kidney stone formation and obstruction. However, the underlying mechanism for the clearance is unclear. Here, using high-resolution microscopy, we found that the juxtatubular macrophages in the renal medulla constitutively formed transepithelial protrusions and "sampled" urine contents. They efficiently sequestered and phagocytosed intraluminal sediment particles and occasionally transmigrated to the tubule lumen to escort the excretion of urine particles. Mice with decreased renal macrophage numbers were prone to developing various intratubular sediments, including kidney stones. Mechanistically, the transepithelial behaviors of medulla macrophages required integrin β1-mediated ligation to the tubular epithelium. These findings indicate that medulla macrophages sample urine content and remove intratubular particles to keep the tubular system unobstructed.

Osteopontin phosphopeptide mitigates calcium oxalate stone formation in a Drosophila melanogaster model

Kidney stone disease affects nearly one in ten individuals and places a significant economic strain on global healthcare systems. Despite the high frequency of stones within the population, effective preventative strategies are lacking and disease prevalence continues to rise. Osteopontin (OPN) is a urinary protein that can inhibit the formation of renal calculi in vitro. However, the efficacy of OPN in vivo has yet to be determined. Using an established Drosophila melanogaster model of calcium oxalate urolithiasis, we demonstrated that a 16-residue synthetic OPN phosphopeptide effectively reduced stone burden in vivo. Oral supplementation with this peptide altered crystal morphology of calcium oxalate monohydrate (COM) in a similar manner to previous in vitro studies, and the presence of the OPN phosphopeptide during COM formation and adhesion significantly reduced crystal attachment to mammalian kidney cells. Altogether, this study is the first to show that an OPN phosphopeptide can directly mitigate calcium oxalate urolithiasis formation in vivo by modulating crystal morphology. These findings suggest that OPN supplementation is a promising therapeutic approach and may be clinically useful in the management of urolithiasis in humans.

Association of osteopontin with kidney function and kidney failure in chronic kidney disease patients: the GCKD study

BACKGROUND

Osteopontin (OPN), synthesized in the thick ascending limb of Henle's loop and in the distal tubule, is involved in the pathogenesis of kidney fibrosis, a hallmark of kidney failure (KF). In a cohort of chronic kidney disease (CKD) patients, we evaluated OPN's association with kidney markers and KF.

METHODS

OPN was measured from baseline serum samples of German Chronic Kidney Disease study participants. Cross-sectional regression models for estimated glomerular filtration rate (eGFR) and urinary albumin-to-creatinine ratio (UACR) as well as Cox regression models for all-cause mortality and KF were evaluated to estimate the OPN effect. Additionally, predictive ability, of OPN and time-dependent population-attributable fraction were evaluated.

RESULTS

Over a median follow-up of 6.5 years, 471 KF events and 629 deaths occurred among 4,950 CKD patients. One-unit higher log(OPN) was associated with 5.5 mL/min/1.73m2 lower eGFR (95%CI: [-6.4,-4.6]) and 1% change in OPN with 0.7% higher UACR (estimated effect 0.7, 95%CI: [0.6,0.8]). Moreover, higher OPN levels were associated with a higher risk of KF (hazard ratio [HR] 1.4, 95%CI: [1.2,1.7]) and all-cause mortality (HR 1.5, 95%CI: [1.3,1.8]). After 6 years, 31% of the KF events could be attributed to higher OPN levels (95%CI: [3%,56%]).

CONCLUSIONS

In this study, higher OPN levels were associated with kidney function markers worsening, and a higher risk for adverse outcomes. A larger proportion of KF could be attributed to higher OPN levels warranting further research on OPN with regards to its role in CKD progression and possible treatment options.

Creation and characterization of an immortalized canine myoblast cell line: Myok9

The availability of an in vitro canine cell line would reduce the need for dogs for primary in vitro cell culture and reduce overall cost in pre-clinical studies. An immortalized canine muscle cell line, named Myok9, from primary myoblasts of a normal dog has been developed by the authors. Immortalization was performed by SV40 viral transfection of the large T antigen into the primary muscle cells. Proliferation assays, growth curves, quantitative PCR, western blotting, mass spectrometry, and light microscopy were performed to characterize the MyoK9 cell line at different stages of growth and differentiation. The expression of muscle-related genes was determined to assess myogenic origin. Myok9 cells expressed dystrophin and other muscle-specific proteins during differentiation, as detected with mass spectrometry and western blotting. Using the Myok9 cell line, new therapies before moving to pre-clinical studies to enhance the number and speed of analyses and reduce the cost of early experimentation can be tested now. This cell line will be made available to the research community to further evaluate potential therapeutics.

The role of osteopontin in kidney diseases

BACKGROUND

Osteopontin (OPN) is a pleiotropic glycoprotein expressed in various cell types in animals and in humans, including bone, immune, smooth muscle, epithelial and endothelial cells. Moreover, OPN is found in kidneys (in the thick ascending limbs of the loop of Henle and in distal nephrons) and urine. The protein plays an important role in mineralization and bone resorption. In addition, OPN is involved in the regulation of immunity and inflammation, angiogenesis and apoptosis. It was demonstrated that OPN and some OPN gene polymorphic variants are associated with the pathogenesis and progression of multiple disorders, such as cancer, autoimmune, neurodegenerative and cardiovascular diseases. Moreover, recent studies suggested that OPN is associated with the pathogenesis of renal failure.

METHODS

In this review, I briefly discussed the role of OPN and its gene polymorphisms in kidney physiology, as well as in various kidney diseases.

FINDINGS AND CONCLUSION

Most studies reported that OPN expression is elevated in urolithiasis, and also in acute and chronic kidney diseases, and in renal allograft dysfunction. Moreover, it was demonstrated that polymorphic variants of the OPN gene may be associated with renal failure. However, some reports suggested that OPN is essential for tubulogenesis, and that it inhibits calcium oxalate crystal formation and retention, nitric oxide synthesis, cell apoptosis and promotes cell regeneration. Thus, further studies are required to fully understand the role of OPN in kidney physiology and pathology. Eventually, these studies may result in the identification of OPN as a valuable marker for renal dysfunction prognosis and treatment.

Association between polymorphisms in osteopontin gene (SPP1) and first episode calcium oxalate urolithiasis

We examined whether single nucleotide polymorphisms (SNPs) in SPP1 gene are associated with risk of calcium oxalate urolithiasis (COU). We genotyped nine known SNPs in SPP1 gene (rs11739060, rs28357094, rs2728127, rs11730582, rs1126772, rs9138, rs2853744, rs4754=p.Asp80Asp, and rs1126616=p.Ala236Ala). Genomic DNA from 1,026 individuals (n = 342 patients with first episode COU, and n = 684 healthy unrelated controls) was analyzed for nine SPP1 SNPs using polymerase chain reaction and melting curve analysis by means of a pair of fluorescence resonance energy transfer probes. Serum and urine osteopontin (OPN) levels were also measured using enzyme-linked immunosorbent assay kits. rs9138 AA genotype was protective (OR 0.62, 95 % CI 0.47-0.81; P = 0.004). rs28357094 TT genotype (OR 2.52, 95 % CI 1.74-3.79; P = 0.021), rs2728127 GG genotype (OR 2.64, 95 % CI 1.42-4.81; P = 0.002), and rs2853744 GG genotype (OR 1.68, 95 % CI 1.22-3.87; P = 0.003) were predisposing. None of the other examined SPP1 SNPs was associated with COU susceptibility. Subjects with protective and predisposing polymorphisms had increased and decreased serum levels of OPN, respectively. Urinary calcium/OPN ratios were higher and lower in subjects with predisposing and protective SNPs of SPP1 gene, respectively. Of 28 constructed haplotypes, 6 demonstrated significant association with COU risk. There was no sex difference in the obtained results. The SPP1 gene polymorphisms are associated with the COU susceptibility.

A comparative study of experimental rat models of renal calcium oxalate stone formation

PURPOSE

The aim of this study was to design a simple and convenient rat model with significant calcium oxalate crystal deposition in the kidney.

MATERIALS AND METHODS

Sprague-Dawley rats were divided into seven groups of 30 rats each. One group of rats was untreated to serve as controls. Two of the groups of rats were provided with drinking water supplemented with 1% ethylene glycol (EG), and of these two groups, one group was also administered vitamin D. Two groups of rats received intraperitoneal injections of EG, and of these two groups, one group also received vitamin D. Two groups of rats received intraperitoneal injections of glyoxylate, and of these two groups, one group also received vitamin D. We analyzed 24-hour urine samples for urinary constituents for all experimental groups, including calcium, oxalate, citrate, uric acid, pH, and urine volume. The kidneys were examined for crystal deposition using histologic examination and for osteopontin expression using immunohistochemical staining.

RESULTS

Calcium-oxalate crystals were found in all rats injected with glyoxylate after 1 week. The degree of crystal deposition in rats injected with glyoxylate for 1 week was significantly increased compared with rats fed EG for 4 weeks.

CONCLUSIONS

Intraperitoneal injection of glyoxylate is a faster, more exact, and more reliable method to produce calcium oxalate crystal deposition in the kidney than previous urolithiasis animal models.

Involvement of urinary proteins in the rat strain difference in sensitivity to ethylene glycol-induced renal toxicity

Ethylene glycol (EG) exposure is a common model for kidney stones, because animals accumulate calcium oxalate monohydrate (COM) in kidneys. Wistar rats are more sensitive to EG than Fischer 344 (F344) rats, with greater COM deposition in kidneys. The mechanisms by which COM accumulates differently among strains are poorly understood. Urinary proteins inhibit COM adhesion to renal cells, which could alter COM deposition in kidneys. We hypothesize that COM accumulates more in Wistar rat kidneys because of lower levels of inhibitory proteins in urine. Wistar and F344 rats were treated with 0.75% EG in drinking water for 8 wk. Twenty-four-hour urine was collected every 2 wk for analysis of urinary proteins. Similar studies were conducted for 2 wk using 2% hydroxyproline (HP) as an alternative oxalate source. Total urinary protein was higher in F344 than Wistar rats at all times. Tamm-Horsfall protein was not different between strains. Osteopontin (OPN) levels in Wistar urine and kidney tissue were higher and were further increased by EG treatment. This increase in OPN occurred before renal COM accumulation. Untreated F344 rats showed greater CD45 and ED-1 staining in kidneys than untreated Wistars; in contrast, EG treatment increased CD45 and ED-1 staining in Wistars more than in F344 rats, indicating macrophage infiltration. This increase occurred in parallel with the increase in OPN and before COM accumulation. Like EG, HP induced markedly greater oxalate concentrations in the plasma and urine of Wistar rats compared with F344 rats. These results suggest that OPN upregulation and macrophage infiltration do not completely protect against COM accumulation and may be a response to crystal retention. Because the two oxalate precursors, EG and HP, produced similar elevations of oxalate, the strain difference in COM accumulation may result more so from metabolic differences between strains than from differences in urinary proteins or inflammatory responses.

Osteopontin: an effector and an effect of tumor metastasis

Osteopontin (OPN) is a matricellular protein that is produced by multiple tissues in our body and is most abundant in bone. It is also produced by cancer cells and plays a determinative role in the growth, progression and metastasis of cancer. Clinically, OPN has been reported to be upregulated in tumor cells per se; this is also reflected by increased levels of OPN in the circulation. Thus, increased OPN levels the plasma are an effect of tumor growth and progression. Functionally, high OPN levels are determinative of higher incidence of bone metastases in mouse models and are clinically correlated with metastatic bone disease and bone resorption in advanced breast cancer patients. Several research efforts have been made to therapeutically target and inhibit the activities of OPN. In this article we have reviewed OPN in its role as an effector of critical steps in tumor progression and metastasis, with a particular emphasis on its role in facilitating bone metastasis of breast cancer. We have also addressed the role of the host-derived OPN in influencing the malignant behavior of the tumor cells.

Morphological conversion of calcium oxalate crystals into stones is regulated by osteopontin in mouse kidney

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.

Specific adsorption of osteopontin and synthetic polypeptides to calcium oxalate monohydrate crystals

Protein-crystal interactions are known to be important in biomineralization. To study the physicochemical basis of such interactions, we have developed a technique that combines confocal microscopy of crystals with fluorescence imaging of proteins. In this study, osteopontin (OPN), a protein abundant in urine, was labeled with the fluorescent dye AlexaFluor-488 and added to crystals of calcium oxalate monohydrate (COM), the major constituent of kidney stones. In five to seven optical sections along the z axis, scanning confocal microscopy was used to visualize COM crystals and fluorescence imaging to map OPN adsorbed to the crystals. To quantify the relative adsorption to different crystal faces, fluorescence intensity was measured around the perimeter of the crystal in several sections. Using this method, it was shown that OPN adsorbs with high specificity to the edges between {100} and {121} faces of COM and much less so to {100}, {121}, or {010} faces. By contrast, poly-L-aspartic acid adsorbs preferentially to {121} faces, whereas poly-L-glutamic acid adsorbs to all faces approximately equally. Growth of COM in the presence of rat bone OPN results in dumbbell-shaped crystals. We hypothesize that the edge-specific adsorption of OPN may be responsible for the dumbbell morphology of COM crystals found in human urine.

Successful formation of calcium oxalate crystal deposition in mouse kidney by intraabdominal glyoxylate injection

The establishment of an experimental animal model would be useful to study the mechanism of kidney stone formation. A calcium kidney stone model in rats induced by ethylene glycol has been used for research; however, to investigate the genetic basis affecting kidney stone formation, which will contribute to preventive medicine, the establishment of a kidney stone model in mice is essential. This study indicates the optimum conditions for inducing calcium oxalate stones in normal mouse kidney. Various doses of oxalate precursors, ethylene glycol, glycolate and glyoxylate, were administered either by free drinking or intraabdominal injection for 2 months as a preliminary study. Stone formation was detected with light microscopy, polarized light optical microscopy and electron microscopy. Stone components were detected with X-ray diffraction analysis. The expression of osteopontin (OPN), a major stone-related protein, was detected with immunohistochemical staining, in situ hybridization and quantitative reverse transcriptase polymerase chain reaction. Kidney stones were not detected in ethylene glycol- or glycolate-treated groups even at the highest dose of LD(50). Whereas, numerous kidney stones were detected in glyoxylate-treated mice (more than 60 mg/kg) at 3, 6 and 9 days after glyoxylate were administered intraabdominally. However, the number of kidney stones decreased gradually at day 12, and was hardly detected at day 15. The stone component was further analyzed as calcium oxalate monohydrate. A dramatic increase in the expression of OPN was observed by the administration of glyoxylate. We established a mouse kidney stone experimental system in this study. The difficulty of inducing kidney stones suggested that mice have greater intrinsic ability to prevent stone formation with hyperoxaluric stress than rats. The differing response to hyperoxaluric stress between mice and rats possibly contributes to the molecular mechanism of kidney stone formation and will aid preventive medicine in the future.

Mammary Gland Secretory Concretions Contain Non-Collagenous Bone Matrix Proteins

Secretory concretions in mammary gland alveoli are commonly of microscopical size. However, some concretions reach clinically palpable dimensions and may occlude teat canals and obstruct milk flow. We studied secretory concretions in sheep, goat and cow mammary glands, using routine histological staining methods, conventional histochemistry and electron microscopy. As concretions frequently mineralize, immunostaining for keratan sulphate and calcium-binding non-collagenous bone matrix proteins (bone sialoprotein, osteocalcin, osteonectin and osteopontin) was performed. Concretions consisted of organic matrix (condensed secretions) with calcium precipitates. Mineralized deposits mostly show concentric organization, bound haematoxylin, and were readily identified in H&E-stained sections. Mineral components of concretions reacted for calcium carbonate and phosphate, organic matrix was found to contain sialoglycan material. Immunohistochemistry revealed bone sialoprotein, osteonectin and keratan sulphate in cow and goat concretions. Osteocalcin was detected in sheep, cow and goat concretions, whilst osteopontin was not identified in any of the specimens studied. Our results indicate the presence of non-collagenous bone matrix proteins (except osteopontin) in mammary gland concretions. These glycoproteins are commonly thought to govern mineralization of organic matrix and are assumed also to promote mineral deposition in mammary gland secretory concretions. Besides caseins, these particular glycoproteins have to be considered as calcium-binding milk proteins.

Renal prothrombin mRNA is significantly decreased in a hyperoxaluric rat model of nephrolithiasis

Although urinary prothrombin fragment 1 (UPTF1) possesses several hallmarks expected of a regulatory protein in urolithiasis, its precise role remains unknown. To determine the relationship between renal prothrombin (PT), the parent molecule of UPTF1, and lithogenesis, this study quantified and compared levels of renal PT mRNA in healthy rats (n = 10) and rats rendered lithogenic (n = 10) by ingestion of 0.75% ethylene glycol for 8 weeks. Studies included morphological and histological examination of the kidneys with scanning electron microscopy of the urinary filtrates of control and experimental animals. Haematuria and calcium oxalate (CaOx) crystals occurred in the urine of all experimental rats, but not in those of controls. Histological examination showed birefringent nephroliths and associated damage in kidneys of lithogenic rats, which were not seen in the control group. The amounts of total RNA extracted from both groups of rats were similar, but the median ratio of PT to beta-actin transcript of 11.14 x 10(-4) (10.65 x 10(-4) +/- 2.24 x 10(-4)) in the control rats was significantly (p < or = 0.001) reduced to 6.47 x 10(-4) (6.57 x 10(-4) +/- 2.72 x 10(-4)) in the lithogenic group. These results demonstrate that renal PT mRNA is reduced by approximately 42% in lithogenic rats and confirm the existence of a direct association between renal PT synthesis and calculogenesis. Attempts to compare renal PT and urinary levels of PTF1 were unsuccessful because of interference from hepatic PT circulating in the blood, haematuria, and the presence of urinary CaOx crystals. This is the first report of a significant reduction in the renal expression of a urinary protein well documented to inhibit CaOx crystal growth and aggregation in undiluted human urine in vitro.

Calcium oxalate crystal localization and osteopontin immunostaining in genetic hypercalciuric stone-forming rats

BACKGROUND

The inbred genetic hypercalciuric stone-forming (GHS) rats develop calcium phosphate (apatite) stones when fed a normal 1.2% calcium diet. The addition of 1% hydroxyproline to this diet does not alter the type of stone formed, while rats fed this diet with 3% hydroxyproline form mixed apatite and calcium oxalate stones and those with 5% hydroxyproline added form only calcium oxalate stones. The present study was designed to determine the localization of stone formation and if this solid phase resulted in pathologic changes to the kidneys.

METHODS

GHS rats were fed 15 g of the standard diet or the diet supplemented with 1%, 3%, or 5% hydroxyproline for 18 weeks. A separate group of Sprague-Dawley rats (the parental strain of the GHS rats), fed the standard diet for a similar duration, served as an additional control. At 18 weeks, all kidneys were perfusion-fixed for structural analysis, detection of crystal deposits using the Yasue silver substitution method, and osteopontin immunostaining.

RESULTS

There were no crystal deposits found in the kidneys of Sprague-Dawley rats. Crystal deposits were found in the kidneys of all GHS rats and this Yasue-stained material was detected only in the urinary space. No crystal deposits were noted within the cortical or medullary segments of the nephron and there was no evidence for tubular damage in any group. The only pathologic changes occurred in 3% and 5% hydroxyproline groups with the 5% group showing the most severe changes. In these rats, which form only calcium oxalate stones, focal sites along the urothelial lining of the papilla and fornix of the urinary space demonstrated a proliferative response characterized by increased density of urothelial cells that surrounded the crystal deposits. At the fornix, some crystals were lodged within the interstitium, deep to the proliferative urothelium. There was increased osteopontin immunostaining in the proliferating urothelium.

CONCLUSION

Thus in the GHS rat, the initial stone formation occurred solely in the urinary space. Tubular damage was not observed with either apatite or calcium oxalate stones. The apatite stones do not appear to cause any pathological change while those rats forming calcium oxalate stones have a proliferative response of the urothelium, with increased osteopontin immunostaining, around the crystal deposits in the fornix.

Renal calcification in mice homozygous for the disrupted type IIa Na/Pi cotransporter gene Npt2

Mice homozygous for the disrupted renal type IIa sodium/phosphate (Na/Pi) cotransporter gene (Npt2-/-) exhibit renal Pi wasting, hypophosphatemia, and an adaptive increase in the serum concentration of 1,25-dihydroxyvitamin D with associated hypercalcemia and hypercalciuria. Because hypercalciuria is a risk factor for nephrocalcinosis, we determined whether Npt2-/- mice form renal stones. Analysis of renal sections by von Kossa staining and intact kidneys by microcomputed tomography revealed renal calcification in adult Npt2-/- mice but not in Npt2+/+ littermates. Energy-dispersive spectroscopy and selected-area electron diffraction indicated that the calcifications are comprised of calcium and Pi with an apatitic mineral phase. To determine the age of onset of nephrocalcinosis, we examined renal sections of newborn and weanling mice. At both ages, mutant but not wild-type mice display renal calcification, which is associated with renal Pi wasting and hypercalciuria. Immunohistochemistry revealed that osteopontin co-localizes with the calcifications. Furthermore, renal osteopontin messenger RNA abundance is significantly elevated in Npt2-/- mice compared with Npt2+/+ mice. The onset of renal stones correlated developmentally with the absence of Npt2 expression and the expression of the genes responsible for the renal production (1alpha-hydroxylase) and catabolism (24-hydroxylase) of 1,25-dihydroxyvitamin D. In summary, we show that Npt2 gene ablation is associated with renal calcification and suggest that mutations in the NPT2 gene may contribute to nephrocalcinosis in a subset of patients with familial hypercalciuria.

Expression, roles, receptors, and regulation of osteopontin in the kidney

Osteopontin (OPN) is a secreted glycoprotein in both phosphorylated and non-phosphorylated forms. It contains an Arg-Gly-Asp cell-binding sequence and a thrombin-cleavage site. OPN is mainly present in the loop of Henle and distal nephrons in normal kidneys in animals and humans. After renal damage, OPN expression may be significantly up-regulated in all tubule segments and glomeruli. Studies utilizing OPN gene-deficient mice, antisense-treated or anti-OPN-treated animals have demonstrated that OPN promotes accumulation of macrophages, and may play a role in macrophage-mediated renal injury, but that the effect may be mild and short-lived. On the other hand, OPN has some renoprotective actions in renal injury, such as increasing tolerance to acute ischemia, inhibiting inducible nitric oxide synthase and suppressing nitric oxide synthesis, reducing cell peroxide levels and promoting the survival of cells exposed to hypoxia, decreasing cell apoptosis and participating in the regeneration of cells. In addition, OPN is associated with renal stones, but whether it acts as a promoter or inhibitor of stone formation is controversial. It has been demonstrated that OPN receptors include two families: integrin and CD44. The OPN integrin receptors include alpha(v)beta(3), alpha(v)beta(1), alpha(v)beta(5) and alpha(9)beta(1), and alpha(4)beta(1). In normal human kidneys, standard CD44 is expressed most dominantly. Different OPN functions are mediated via distinct receptors. Parathyroid hormone, vitamin D(3), calcium, phosphate and some cytokines increase OPN expression in vitro or in vivo, whereas female sex hormones and angiotensin-converting enzyme inhibitors or angiotensin II receptor antagonists decrease OPN expression in some renal damage states.