Crucial role of the cryptic epitope SLAYGLR within osteopontin in renal crystal formation of mice

Current Status of Protein Biomarkers in Urolithiasis-A Review of the Recent Literature

Urolithiasis is an increasingly common clinical problem worldwide. The formation of stones is a combination of metabolic status, environmental factors, family history and many other aspects. It is important to find new ways to quickly detect and assess urolithiasis because it causes sudden, severe pain and often comes back. One way to do this is by exploring new biomarkers. Current advances in proteomic studies provide a great opportunity for breakthroughs in this field. This study focuses on protein biomarkers and their connection to kidney damage and inflammation during urolithiasis.

Linking 24-h urines to clinical phenotypes: what alternatives does the future bring?

PURPOSE OF REVIEW

The 24-h urine test is recommended as part of the metabolic evaluation for patients with nephrolithiasis to guide preventive interventions. However, this test may be challenging to interpret and has limits in its predictive ability. In this review, we summarize and discuss the most recent research on the opportunities and challenges for utilizing urinary biomarkers for kidney stone prevention.

RECENT FINDINGS

Contemporary studies utilizing the 24-h urine test have improved our understanding of how to better administer testing and interpret test results. Beyond the standard panel of 24-h urine parameters, recent applications of proteomics and metabolomics have identified protein and metabolic profiles of stone formers. These profiles can be assayed in future studies as potential biomarkers for risk stratification and prediction. Broad collaborative efforts to create large datasets and biobanks from kidney stone formers will be invaluable for kidney stone research.

SUMMARY

Recent advances in our understanding of kidney stone risk have opened opportunities to improve metabolic testing for kidney stone formers. These strategies do not appear to be mutually exclusive of 24-h urine testing but instead complementary in their approach. Finally, large clinical datasets hold promise to be leveraged to identify new avenues for stone prevention.

Proteomic analysis of a urinary stone with two layers composed of calcium oxalate monohydrate and uric acid

We examined the mechanism of urinary stone formation by analyzing the matrix proteins in a urinary stone with two layers composed of different crystals. Micro-area X-ray spectrometry and infrared spectroscopy revealed calcium oxalate monohydrate in the outside and uric acid in the inside. We also examined the interface. After the outside, inside, and interface parts were separated, proteomic analysis identified 48, 7, and 4 matrix proteins, respectively. Urinary stones with two layers are considered to have grown under different conditions. The matrix proteins in each part differed among the crystal components and may reveal the stone-generating process. The proteins in the interface likely function to enlarge the stone via the addition of different crystals.

Can urine osteopontin levels, which may be correlated with nutrition intake and body composition, be used as a new biomarker in the diagnosis of nephrolithiasis?

BACKGROUND AND AIM

The nephrolithiasis has a multifactorial etiology resulting from the interaction of metabolic, genetic and environmental factors. Parameters such as nutrition and urinary osteopontin (OPN) level may affect kidney stone formation. The purpose of this study is to evaluate the correlation between urinary OPN level and kidney stone formation and effect of nutrition on OPN level in nephrolithiasis.

MATERIALS AND METHODS

This study was conducted on 88 volunteers including 44 healthy individuals and 44 patients diagnosed with nephrolithiasis and aging between 20 and 65 years. Some serum parameters and urinary OPN levels of the individuals were analyzed. Several anthropometric measurements of the individuals were taken and calculated their body mass index. Additionally, 24-hour dietary recall and water intakes were recorded and the participants completed food-frequency questionnaire for the evaluation of their nutritional status.

RESULTS

Urinary OPN (ng/mL) levels of patients were lower than that of control group (p<0.05). Dietary energy, carbohydrate, poly-unsaturated fatty acid (PUFA) and n-6 fatty acids intakes and urinary OPN levels of male patients were positively correlated (p<0.05). Additionally, there was a negative correlation between their urinary OPN (ng/mL) and serum creatinine (mg/dL) levels of female patients (p<0.05). Body weight, waist circumference, hip circumference and body muscle mass values of healthy males were positively correlated with their urinary OPN levels (p<0.05).

CONCLUSIONS

Results of the study showed that low urinary OPN levels were correlated with increased kidney stone risk, and dietary habits can affect urinary OPN level.

The multiple functions and mechanisms of osteopontin

Osteopontin (OPN) is a highly phosphorylated glycophosphoprotein having acidic characteristics and rich in aspartic acid. OPN, a multifunctional protein, has important functions on cardiovascular diseases, cancer, diabetes and kidney stone diseases and in the process of inflammation, biomineralization, cell viability and wound healing. Osteopontin acts on organisms by playing a key role in secretion levels of interleukin-10 (IL-10), interleukin-12 (IL-12), interleukin-3 (IL-3), interferon-γ (IFN-γ), integrin αvB3, nuclear factor kappa B (NF-kB), macrophage and T cells, regulating the osteoclast function and affecting CD44 receptors. The aim of the present review is to address majority of different functions of OPN protein which are known, suspected or suggested through the data obtained about this protein yet.

The potential role of Osteopontin in the maintenance of commensal bacteria homeostasis in the intestine

Osteopontin (Opn), a multifunctional extracellular matrix protein, is implicated in the pathogenesis of various inflammatory disorders. Under physiologic conditions, its expression is restricted to certain tissues including bone and kidney tubule. However, cellular activation during disease development induces Opn expression in various immune cells. In this study, using Opn-EGFP knock-in (KI) mice we found that CD8α⁺ T cells in the intestinal tissues, including Peyer’s patch, lamina propria and epithelium, express Opn under steady state conditions. Therefore, we examined the role of Opn-expressing CD8α⁺ T cells in intestinal homeostasis. Interestingly, Opn knockout (KO) mice had altered fecal microflora concordant with a reduction of TCRγδ⁺ intraepithelial lymphocytes (IELs). Consistent with this result, both treatment with anti-Opn blocking antibody and deficiency of Opn resulted in decreased survival of TCRγδ⁺ and TCRαβ⁺ IELs. This data suggests that a possibility that Opn may function as a survival factor for IELs in the intestinal tissue. Collectively, these data suggest the possibility that Opn might regulate the homeostasis of intestinal microflora through maintenance of TCRγδ⁺ IELs, possibly by support of IEL survival.

Impaired urinary osteopontin excretion in Npt2a-/- mice

Mutations in the renal sodium-dependent phosphate cotransporters NPT2a and NPT2c have been reported in patients with renal stone disease and nephrocalcinosis. Oral phosphate supplementation is currently thought to reduce risk by reversing the hypercalciuria, but the exact mechanism remains unclear and the relative contribution of modifiers of mineralization such as osteopontin (Opn) to the formation of renal mineral deposits in renal phosphate wasting disorders has not been studied. We observed a marked decrease of renal gene expression and urinary excretion of Opn in Npt2a^-/- mice, a mouse model of these disorders, at baseline. Following supplementation with phosphate Opn gene expression was restored to wild-type levels in Npt2a^-/- mice; however, urine excretion of the protein remained low. To further investigate the role of Opn, we used a double-knockout strategy, which provides evidence that loss of Opn worsens the nephrocalcinosis and nephrolithiasis observed in these mice on a high-phosphate diet. These studies suggest that impaired Opn gene expression and urinary excretion in Npt2a^-/- mice may be an additional risk factor for nephrolithiasis, and normalizing urine Opn levels may improve the therapy of phosphaturic disorders.

Inhibition of Cellular Adhesion by Immunological Targeting of Osteopontin Neoepitopes Generated through Matrix Metalloproteinase and Thrombin Cleavage

Osteopontin (OPN), a secreted protein involved in inflammatory processes and cancer, induces cell adhesion, migration, and activation of inflammatory pathways in various cell types. Cells bind OPN via integrins at a canonical RGD region in the full length form as well as to a contiguous cryptic site that some have shown is unmasked upon thrombin or matrix metalloproteinase cleavage. Thus, the adhesive capacity of osteopontin is enhanced by proteolytic cleavage that may occur in inflammatory conditions such as obesity, atherosclerosis, rheumatoid arthritis, tumor growth and metastasis. Our aim was to inhibit cellular adhesion to recombinant truncated proteins that correspond to the N-terminal cleavage products of thrombin- or matrix metalloproteinase-cleaved OPN in vitro. We specifically targeted the cryptic integrin binding site with monoclonal antibodies and antisera induced by peptide immunization of mice. HEK 293 cells adhered markedly stronger to truncated OPN proteins than to full length OPN. Without affecting cell binding to the full length form, the raised monoclonal antibodies specifically impeded cellular adhesion to the OPN fragments. Moreover, we show that the peptides used for immunization were able to induce antisera, which impeded adhesion either to all OPN forms, including the full-length form, or selectively to the corresponding truncated recombinant proteins. In conclusion, we developed immunological tools to selectively target functional properties of protease-cleaved OPN forms, which could find applications in treatment and prevention of various inflammatory diseases and cancers.

Identification of transglutaminase reactive residues in human osteopontin and their role in polymerization

Osteopontin (OPN) is a highly posttranslationally modified protein present in several tissues where it is implicated in numerous physiological processes. OPN primarily exerts its functions through interaction with integrins via the Arg-Gly-Asp and Ser-Val-Val-Tyr-Gly-Leu-Arg sequences located in the N-terminal part of the protein. OPN can be polymerized by the cross-linking enzyme transglutaminase 2 (TG2), and polymerization has been shown to enhance the biological activity of OPN. However, little is known about the reactivity and location of the glutamine and lysine residues involved in the TG2-mediated modification of OPN. Here we show that TG2 catalyses the incorporation of 5-(Biotinamido)pentylamine at glutamines in both the N- and C-terminal parts of OPN, whereas TG2 primarily incorporated the glutamine-donor peptide biotinyl-TVQQEL-OH into the C-terminal part of OPN. By mass spectrometric analyses we identified Gln34, Gln42, Gln193 and Gln248 as the major TG2 reactive glutamines in OPN. The distribution of reactive Gln and Lys residues in OPN proved to be important, as the full-length protein but not the physiologically highly active integrin-binding N-terminal part of OPN were able to polymerize in a TG2-mediated reaction. Collectively, these data provide important new molecular knowledge about the mechanism of OPN polymerization.

Biomolecular mechanism of urinary stone formation involving osteopontin

Urinary stones consist of two phases-an inorganic (mineral) phase and an organic (matrix) phase. Studies on the organic components of kidney stones have been undertaken later than those on the inorganic components. After osteopontin was identified as one of the matrix components, the biomolecular mechanism of urinary stone formation became clearer. It also triggered the development of new preventive treatments. Osteopontin expression is sporadically observed in normal distal tubular cells and is markedly increased in stone-forming kidneys. Calcium oxalate crystals adhering to renal tubular cells are incorporated into cells by the involvement of osteopontin. Stimulation of crystal-cell adhesion impairs the opening of mitochondrial permeability transition pores (mPTP) in tubular cells and produces oxidative stress, apoptosis, and osteopontin expression. Macrophages phagocytose and digest a small amount of crystals, but many crystals aggregate into a mass containing osteopontin and epithelial cell debris and are excreted into the renal tubular lumen, becoming nuclei of urinary stones. This biomolecular mechanism is similar to atherosclerotic calcification. Based on these findings, new preventive treatments have been developed. Dietary control such as low-cholesterol intake and the ingestion of antioxidative foods and vegetables have successfully reduced the 5-year recurrence rate. Osteopontin antibodies and cyclosporine A, which blocks the opening of mPTP, have markedly inhibited the expression of osteopontin and urinary stone formation in animal models.

Post-translational modification of osteopontin: effects on in vitro hydroxyapatite formation and growth

The manuscript tests the hypothesis that posttranslational modification of the SIBLING family of proteins in general and osteopontin in particular modify the abilities of these proteins to regulate in vitro hydroxyapatite (HA) formation. Osteopontin has diverse effects on hydroxyapatite (HA) mineral crystallite formation and growth depending on the extent of phosphorylation. We hypothesized that different regions of full-length OPN would also have distinct effects on the mineralization process. Thrombin fragmentation of milk OPN (mOPN) was used to test this hypothesis. Three fragments were tested in a de novo HA formation assay; an N-terminal fragment (aa 1-147), a central fragment (aa 148-204) denoted SKK-fragment and a C-terminal fragment (aa 205-262). Compared to intact mOPN the C- and N-terminal fragments behaved comparably, promoting HA formation and growth, but the central SKK-fragment acted as a mineralization inhibitor. In a seeded growth experiment all fragments inhibited mineral proliferation, but the SKK-fragment was the most effective inhibitor. These effects, seen in HA-formation and seeded growth assays in a gelatin gel system and in a pH-stat experiment were lost when the protein or fragments were dephosphorylated. Effects of the fully phosphorylated protein and fragments were also altered in the presence of fibrillar collagen. The diverse effects can be explained in terms of the intrinsically disordered nature of OPN and its fragments which enable them to interact with their multiple partners.