Urinary macromolecular inhibition of crystal adhesion to renal epithelial cells is impaired in male stone formers

Defining physicochemical properties of urinary proteins that determine their inhibitory activities against calcium oxalate kidney stone formation

We have recently reported a set of urinary proteins that inhibited calcium oxalate (CaOx) stone development. However, physicochemical properties that determine their inhibitory activities remained unknown. Herein, human urinary proteins were chromatographically fractionated into 15 fractions and subjected to various CaOx crystal assays and identification by nanoLC-ESI-Qq-TOF MS/MS. Their physicochemical properties and crystal inhibitory activities were subjected to Pearson correlation analysis. The data showed that almost all urinary protein fractions had crystal inhibitory activities. Up to 128 proteins were identified from each fraction. Crystallization inhibitory activity correlated with percentages of Ca2+-binding proteins, stable proteins, polar amino acids, alpha helix, beta turn, and random coil, but inversely correlated with number of Ox2--binding motifs/protein and percentage of unstable proteins. Crystal aggregation inhibitory activity correlated with percentage of stable proteins but inversely correlated with percentage of unstable proteins. Crystal adhesion inhibitory activity correlated with percentage of stable proteins and GRAVY, but inversely correlated with pI, instability index and percentages of unstable proteins and positively charged amino acids. However, there was no correlation between crystal growth inhibitory activity and any physicochemical properties. In summary, some physicochemical properties of urinary proteins can determine and may be able to predict their CaOx stone inhibitory activities.

Large-scale identification of calcium oxalate stone inhibitory proteins in normal human urine

Recent evidence has shown that proteins in normal human urine can inhibit calcium oxalate (CaOx) kidney stone formation. Herein, we performed fast protein liquid chromatography (FPLC) to fractionate normal human urinary proteins using anion-exchange (DEAE) and size-exclusion (Superdex 200) materials. FPLC fractions (F1-F15) were examined by CaOx crystallization, growth, aggregation and crystal-cell adhesion assays. The fractions with potent inhibitory activities against CaOx crystals were then subjected to mass spectrometric protein identification. The data revealed that 13 of 15 fractions showed inhibitory activities in at least one crystal assay. Integrating CaOx inhibitory scores demonstrated that F6, F7 and F8 had the most potent inhibitory activities. NanoLC-ESI-Qq-TOF MS/MS identified 105, 93 and 53 proteins in F6, F7 and F8, respectively. Among them, 60 were found in at least two fractions and/or listed among known inhibitors with solid experimental evidence in the StoneMod database (https://www.stonemod.org). Interestingly, 10 of these 60 potential inhibitors have been reported with lower urinary levels in CaOx stone formers compared with healthy (non-stone) individuals, strengthening their roles as potent CaOx stone inhibitors. Our study provides the largest dataset of potential CaOx stone inhibitory proteins that will be useful for further elucidations of stone-forming mechanisms and ultimately for therapeutic/preventive applications.

Kidney Stone Prevention

Kidney stone disease (KSD) (alternatively nephrolithiasis or urolithiasis) is a global health care problem that affects almost people in developed and developing countries. Its prevalence has been continuously increasing with a high recurrence rate after stone removal. Although effective therapeutic modalities are available, preventive strategies for both new and recurrent stones are required to reduce physical and financial burdens of KSD. To prevent kidney stone formation, its etiology and risk factors should be first considered. Low urine output and dehydration are the common risks of all stone types, whereas hypercalciuria, hyperoxaluria, and hypocitraturia are the major risks of calcium stones. In this article, up-to-date knowledge on strategies (nutrition-based mainly) to prevent KSD is provided. Important roles of fluid intake (2.5-3.0 L/d), diuresis (>2.0-2.5 L/d), lifestyle and habit modifications (for example, maintain normal body mass index, fluid compensation for working in high-temperature environment, and avoid cigarette smoking), and dietary management [for example, sufficient calcium at 1000-1200 mg/d, limit sodium at 2 or 3-5 g/d of sodium chloride (NaCl), limit oxalate-rich foods, avoid vitamin C and vitamin D supplements, limit animal proteins to 0.8-1.0 g/kg body weight/d but increase plant proteins in patients with calcium and uric acid stone and those with hyperuricosuria, increase proportion of citrus fruits, and consider lime powder supplementation] are summarized. Moreover, uses of natural bioactive products (for example, caffeine, epigallocatechin gallate, and diosmin), medications (for example, thiazides, alkaline citrate, other alkalinizing agents, and allopurinol), bacterial eradication, and probiotics are also discussed. Adv Nutr 2023;x:xx-xx.

Gallic acid ameliorates calcium oxalate crystal-induced renal injury via upregulation of Nrf2/HO-1 in the mouse model of stone formation

BACKGROUND

High prevalence and reoccurrence rate of nephrolithiasis bring about serious socioeconomic and healthcare burden, necessitating the need of effective therapeutic agents. Previous study revealed that gallic acid (GAL) alters the nucleation pathway of calcium oxalate (CaOx). On the other hand, it appears protective role against oxidative injury. Whether GAL could protect against crystal-induced lesion in vivo, and its underlying mechanism is yet unsolved.

PURPOSE

This study aims to investigate the protective effects of GAL on the crystal-induced renal injury and its underlying mechanism in the mouse model of stone formation induced by glyoxylic acid.

STUDY DESIGN AND METHODS

The mouse model of stone formation was established via successive intraperitoneal injection of glyoxylate. Proximal tubular epithelial cell line HK-2 treated with calcium oxalate monohydrate (COM) was used as in vitro model. The protective role of GAL on nephrolithiasis was tested by determination of tubular injury, crystal deposition and adhesion, levels of inflammatory cytokines, macrophage infiltration and the redox status of kidney. In vitro, effect of GAL on the ROS level and oxidative tubular injury induced by COM were detected, as well as major antioxidant pathway Nrf2/HO-1.

RESULTS

Administration of GAL alleviates the renal deposition and adhesion of CaOx stone. Meanwhile, GAL ameliorates the inflammation and renal tubular injury. Level of intracellular ROS, osteopontin and CD44 are reduced, either in the mouse model of stone formation or in the COM-treated HK-2 cells after treatment of GAL. Mechanistically, GAL activates Nrf2/HO-1 pathway in HK-2 cells. Silencing Nrf2 abrogates the protective effect of GAL on the oxidative injury and adhesion of COM in HK-2 cells.

CONCLUSION

Taken together, our study demonstrates the protective effect of GAL on the deposition of kidney stone and consequent tubular injury. Induction of the antioxidant pathway Nrf2/HO-1 was found to decrease the level of ROS and oxidative injury, thus implying that GAL could be a potential therapeutic agent for the treatment of nephrolithiasis.

Oxidized forms of uromodulin promote calcium oxalate crystallization and growth, but not aggregation

Roles of an abundant human urinary protein, uromodulin (UMOD), in kidney stone disease were previously controversial. Recently, we have demonstrated that oxidative modification reverses overall modulatory activity of whole urinary proteins, from inhibition to promotion of calcium oxalate (CaOx) stone-forming processes. We thus hypothesized that oxidation is one of the factors causing those previously controversial UMOD data on stone modulation. Herein, we addressed effects of performic-induced oxidation on CaOx crystal modulatory activity of UMOD. Sequence analyses revealed two EGF-like calcium-binding domains (65th-107th and 108th-149th), two other calcium-binding motifs (65th-92nd and 108th-135th), and three oxalate-binding motifs (199th-207th, 361st-368th and 601st-609th) in UMOD molecule. Analysis of tandem mass spectrometric dataset of whole urinary proteins confirmed marked increases in oxidation, dioxidation and trioxidation of UMOD in the performic-modified urine samples. UMOD was then purified from the normal urine and underwent performic-induced oxidative modification, which was confirmed by Oxyblotting. The oxidized UMOD significantly promoted CaOx crystallization and crystal growth, whereas the unmodified native UMOD inhibited CaOx crystal growth. However, the oxidized UMOD did not affect CaOx crystal aggregation. Therefore, our data indicate that oxidized forms of UMOD promote CaOx crystallization and crystal growth, which are the important processes for CaOx kidney stone formation.

Oxidative Modifications Switch Modulatory Activities of Urinary Proteins From Inhibiting to Promoting Calcium Oxalate Crystallization, Growth, and Aggregation

The incidence/prevalence of kidney stone disease has been increasing around the globe, but its pathogenic mechanisms remained unclear. We evaluated effects of oxidative modifications of urinary proteins on calcium oxalate (CaOx) stone formation processes. Urinary proteins derived from 20 healthy individuals were modified by performic oxidation, and the presence of oxidatively modified urinary proteins was verified, quantified, and characterized by Oxyblot assay and tandem MS (nanoLC-electrospray ionization-linear trap quadrupole-Orbitrap-MS/MS). Subsequently, activities of oxidatively modified urinary proteins on CaOx stone formation processes were examined. Oxyblot assay confirmed the marked increase in protein oxidation level in the modified urine. NanoLC-electrospray ionization-linear trap quadrupole-Orbitrap-MS/MS identified a total of 193 and 220 urinary proteins in nonmodified and modified urine samples, respectively. Among these, there were 1121 and 5297 unambiguous oxidatively modified peptides representing 42 and 136 oxidatively modified proteins in the nonmodified and modified urine samples, respectively. Crystal assays revealed that oxidatively modified urinary proteins significantly promoted CaOx crystallization, crystal growth, and aggregation. By contrast, the nonmodified urinary proteins had inhibitory activities. This is the first direct evidence demonstrating that oxidative modifications of urinary proteins increase the risk of kidney stone disease by switching their modulatory activities from inhibiting to promoting CaOx crystallization, crystal growth, and aggregation.

Regulation of endoplasmic reticulum stress on the damage and apoptosis of renal tubular epithelial cells induced by calcium oxalate crystals

This study aimed to observe whether calcium oxalate (CaOx) crystals can induce the activation of endoplasmic reticulum (ER) stress in human renal cortex proximal tubule epithelial (HK-2) cells and to explore the regulatory of ER stress on the damage and apoptosis of HK-2 cells induced by CaOx crystals. We detected the optimal CaOx crystal concentration and intervention time by Western blot. ER stress modifiers tunicamycin (TM) and 4-phenylbutyric acid (4-PBA) were used to regulate the ER stress of HK-2 cells. The activities of ER stress marker proteins GRP78 and CHOP were evaluated by Western blot and immunohistochemistry. Western blot and TUNEL staining were used to detect cell apoptosis. We observed cell-crystal adhesion with an optical microscope. Lactate dehydrogenase (LDH) test kit and IL-1β enzyme-linked immunosorbent assay kit were used to detect and evaluate HK-2 cell damage. We found that the expression of ER stress marker proteins GRP78 and CHOP gradually increased with the increase in CaOx crystal concentration and intervention time and reached the maximum at 2.0 mmol/L and 24 h. The use of ER stress modifiers TM and 4-PBA can effectively regulate the ER stress level induced by CaOx crystals, and the level of apoptosis is positively correlated with the level of ER stress. 4-PBA pretreatment remarkably reduced cell-crystal adhesion and the secretions of IL-1β and LDH, whereas the results of TM pretreatment were the opposite. In summary, the damage and apoptosis of HK-2 cells induced by CaOx crystals are closely related to the level of ER stress. Inhibiting the ER stress of HK-2 cells can substantially reduce the cell damage and apoptosis induced by CaOx crystals.

iTRAQ-Based Comparative Proteomics Analysis of Urolithiasis Rats Induced by Ethylene Glycol

Nephrolithiasis is a frequent chronic urological condition with a high prevalence and recurrence rate. Proteomics studies on urolithiasis rat models are highly important in characterizing the pathophysiology of kidney stones and identifying potential approaches for preventing and treating kidney stones. The isobaric tags for relative and absolute quantification (iTRAQ) were performed to identify differentially expressed proteins (DEPs) in the kidney between urolithiasis rats and control rats. The results showed that 127 DEPs (85 upregulated and 42 downregulated) were identified in urolithiasis and control rats. The functions of DEPs were predicted by Gene Ontology (GO) analysis, Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis, and protein-protein interaction (PPI) network analysis. The expression of four upregulated proteins (Tagln, Akr1c9, Spp1, and Fbn1) and four downregulated proteins (Hbb, Epb42, Hmgcs2, and Ca1) were validated by parallel reaction monitoring (PRM). Proteomics studies of ethylene glycol-induced urolithiasis rat models using iTRAQ and PRM helped to elucidate the molecular mechanism governing nephrolithiasis and to identify candidate proteins for the treatment of kidney stones.

Uromodulin in mineral metabolism

PURPOSE OF REVIEW

Uromodulin (UMOD), also known as Tamm-Horsfall protein, is the most abundant protein in human urine. UMOD has multiple functions such as protection against urinary tract infections and nephrolithiasis. This review outlines recent progress made in UMOD's role in renal physiology, tubular transport, and mineral metabolism.

RECENT FINDINGS

UMOD is mostly secreted in the thick ascending limb (TAL) and to a lesser degree in the distal convoluted tubule (DCT). UMOD secretion is regulated by the calcium-sensing receptor. UMOD upregulates ion channels [e.g., renal outer medullary potassium channel, transient receptor potential cation channel subfamily V member 5, and transient receptor potential melastatin 6 (TRPM6)] and cotransporters [e.g., Na,K,2Cl cotransporter (NKCC2) and sodium-chloride cotransporter (NCC)] in the TAL and DCT. Higher serum UMOD concentrations have been associated with higher renal function and preserved renal reserve. Higher serum UMOD has also been linked to a lower risk of cardiovascular disease and diabetes mellitus.

SUMMARY

With better serum UMOD detection assays the extent of different functions for UMOD is still expanding. Urinary UMOD regulates different tubular ion channels and cotransporters. Variations of urinary UMOD secretion can so contribute to common disorders such as hypertension or nephrolithiasis.

The role of the gasotransmitter hydrogen sulfide in pathological calcification

Calcification is the deposition of minerals, mainly hydroxyapatite, inside the cell or in the extracellular matrix. Physiological calcification is central for many aspects of development including skeletal and tooth growth; conversely, pathological mineralization occurs in soft tissues and is significantly associated with malfunction and impairment of the tissue where it is located. Various mechanisms have been proposed to explain calcification. However, this research area lacks a more integrative, systemic, and global perspective that could explain both physiological and pathological processes. In this review, we propose such an integrated explanation. Hydrogen sulfide (H₂ S) is a newly recognized multifunctional gasotransmitters and tis actions have been studied in different physiological and pathological contexts, but little is known about its potential role on calcification. Interestingly, we found that H₂ S promotes calcification under physiological conditions and has an inhibitory effect on pathological processes. This makes H₂ S a potential therapy for diseases related to pathological calcification. LINKED ARTICLES: This article is part of a themed section on Hydrogen Sulfide in Biology & Medicine. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v177.4/issuetoc.

Genetic, pathophysiological, and clinical aspects of nephrocalcinosis

Nephrocalcinosis describes the ectopic deposition of calcium salts in the kidney parenchyma. Nephrocalcinosis can result from a number of acquired causes but also an even greater number of genetic diseases, predominantly renal but also extrarenal. Here we provide a review of the genetic causes of nephrocalcinosis, along with putative mechanisms, illustrated by human and animal data.

Histological aspects of the "fixed-particle" model of stone formation: animal studies

Crystallization by itself is not harmful as long as the crystals are not retained in the kidneys and are allowed to pass freely down the renal tubules to be excreted in the urine. A number of theories have been proposed, and studies performed, to determine the mechanisms involved in crystal retention within the kidneys. It has been suggested that urinary transit through the nephron is too fast for crystals to grow large enough to be retained. Thus, free particle mechanism alone cannot lead to stone formation, and there must be a mechanism for crystal fixation within the kidneys. Animal model studies suggest that crystal retention is possible through both the free- and fixed-particle mechanisms. Crystal-cell interaction leads to pathological changes which promote crystal attachment to either epithelial cells or their basement membrane. Alternatively, crystals aggregate and produce large enough particles to block the tubules particularly at sites, where urinary flow is affected because of changes in the luminal diameter of the tubule. Crystal deposits plugging the openings of the ducts of Bellini may be the result of such a phenomenon. Intratubular crystals translocating to renal interstitium may produce osteogenic changes in the epithelial or endothelial cells resulting in the formation of the Randall's plaques. Thus, fixation appears to be either through the formation of Randall's plugs, crystal plugs clogging the openings of the ducts of Bellini or sub-epithelial crystal deposits, and the Randall's plaques.

Protective Effects of Degraded Soybean Polysaccharides on Renal Epithelial Cells Exposed to Oxidative Damage

This study aimed to investigate the protective effects of degraded soybean polysaccharides (DSP) on oxidatively damaged African green monkey kidney epithelial (Vero) cells. Low DSP concentration (10 μg/mL) elicited an evident protective effect on H₂O₂-induced cell injury (0.3 mmol/L). The cell viabilities of the H₂O₂-treated group and the DSP-protected group were 57.3 and 93.1%, respectively. The cell viability decreased to 88.3% when the dosage was increased to 100 μg/mL. DSP protected Vero cells from H₂O₂-mediated oxidative damage by enhancing cellular superoxide dismutase activity and total antioxidant capacity and by decreasing malonaldehyde content and lactate dehydrogenase release. The H₂O₂-treated cells stimulated the aggregation of calcium oxalate monohydrate crystals. DSP could also reduce the crystal size, decrease the attached crystal content, and prevent the cell aggregation by alleviating oxidative injury and lipid peroxidation, enhancing antioxidant capacity, and decreasing hyaluronan expression on cellular surfaces. The internalization ability of the injured cells was improved after these cells were exposed to DSP solution. The regulation ability of DSP-repaired cells on calcium oxalate dihydrate formation, crystal attachment, aggregation, and internalization was lower than that of normal cells but was higher than that of the injured cells. DSP may be a potential green drug to prevent calcium oxalate (CaOx) stone formation because DSP could protect cells from oxidative damage and inhibit CaOx crystal formation.

What is nephrocalcinosis?

The available publications on nephrocalcinosis are wide-ranging and have documented multiple causes and associations of macroscopic or radiological nephrocalcinosis, most often located in the renal medulla, with various metabolic and genetic disorders; in fact, so many and various are these that it is difficult to define a common underlying mechanism. We have reviewed nephrocalcinosis in relation to its definition, genetic associations, animal models, and putative mechanisms. We have concluded, and hypothesized, that nephrocalcinosis is primarily a renal interstitial process, resembling metastatic calcification, and that it may have some features in common with, and pathogenic links to, vascular calcification.

Interstitial calcinosis in renal papillae of genetically engineered mouse models: relation to Randall's plaques

Genetically engineered mouse models (GEMMs) have been highly instrumental in elucidating gene functions and molecular pathogenesis of human diseases, although their use in studying kidney stone formation or nephrolithiasis remains relatively limited. This review intends to provide an overview of several knockout mouse models that develop interstitial calcinosis in the renal papillae. Included herein are mice deficient for Tamm-Horsfall protein (THP; also named uromodulin), osteopontin (OPN), both THP and OPN, Na(+)-phosphate cotransporter Type II (Npt2a) and Na(+)/H(+) exchanger regulatory factor (NHERF-1). The baseline information of each protein is summarized, along with key morphological features of the interstitial calcium deposits in mice lacking these proteins. Attempts are made to correlate the papillary interstitial deposits found in GEMMs with Randall's plaques, the latter considered precursors of idiopathic calcium stones in patients. The pathophysiology that underlies the renal calcinosis in the knockout mice is also discussed wherever information is available. Not all the knockout models are allocated equal space because some are more extensively characterized than others. Despite the inroads already made, the exact physiological underpinning, origin, evolution and fate of the papillary interstitial calcinosis in the GEMMs remain incompletely defined. Greater investigative efforts are warranted to pin down the precise role of the papillary interstitial calcinosis in nephrolithiasis using the existing models. Additionally, more sophisticated, second-generation GEMMs that allow gene inactivation in a time-controlled manner and "compound mice" that bear several genetic alterations are urgently needed, in light of mounting evidence that nephrolithiasis is a multifactorial, multi-stage and polygenic disease.

Osteopontin knockdown in the kidneys of hyperoxaluric rats leads to reduction in renal calcium oxalate crystal deposition

Osteopontin (OPN) expression is increased in kidneys of rats with ethylene glycol (EG) induced hyperoxaluria and calcium oxalate (CaOx) nephrolithiasis. The aim of this study is to clarify the effect of OPN knockdown by in vivo transfection of OPN siRNA on deposition of CaOx crystals in the kidneys. Hyperoxaluria was induced in 6-week-old male Sprague-Dawley rats by administering 1.5% EG in drinking water for 2 weeks. Four groups of six rats each were studied: Group A, untreated animals (tap water); Group B, administering 1.5% EG; Group C, 1.5% EG with in vivo transfection of OPN siRNA; Group D, 1.5% EG with in vivo transfection of negative control siRNA. OPN siRNA transfections were performed on day 1 and 8 by renal sub-capsular injection. Rats were killed at day 15 and kidneys were removed. Extent of crystal deposition was determined by measuring renal calcium concentrations and counting renal crystal deposits. OPN siRNA transfection resulted in significant reduction in expression of OPN mRNA as well as protein in group C compared to group B. Reduction in OPN expression was associated with significant decrease in crystal deposition in group C compared to group B. Specific suppression of OPN mRNA expression in kidneys of hyperoxaluric rats leads to a decrease in OPN production and simultaneously inhibits renal crystal deposition.

Catechin prevents the calcium oxalate monohydrate induced renal calcium crystallization in NRK-52E cells and the ethylene glycol induced renal stone formation in rat

BACKGROUND

Reactive oxygen species play important roles in renal calcium crystallization. In this study, we examined the effects of catechin, which have been shown to have antioxidant properties on the renal calcium crystallization.

METHODS

In the vitro experiment, the changes of the mitochondrial membrane potential, expression of superoxide dismutase (SOD), 4-hydroxynonenal (4-HNE), cytochrome c, and cleaved caspase 3 were measured to show the effects of catechin treatment on the NRK-52E cells induced by calcium oxalate monohydrate (COM). In the vivo study, Sprague-Dawley rats were administered 1% ethylene glycol (EG) to generate a rat kidney stone model and then treated with catechin (2.5 and 10 mg/kg/day) for 14 days. The urine and serum variables were dected on 7 and 14 days after EG administration. The expression of cytochrome c, cleaved caspase 3, SOD, osteopontin (OPN), malondialdehyde (MDA), 8-hydroxy-2'-deoxyguanosine (8-OHdG) in kidney were measured. Furthermore, the mitochondrial microstructure in the kidney was also examined by transmission electron microscopy.

RESULTS

Catechin treatment could prevent the changes in mitochondrial membrane potential and expression of SOD, 4-HNE, cytochrome c, and cleaved caspase 3 in NRK-52E cells induced by the COM. For the in vivo experiments, the EG administration induced renal calcium crystallization was also prevented by the catechin. The expression of SOD, OPN, MDA, OPN and 8-OHdG, were increased after EG administration and this increase was diminished by catechin. Moreover, catechin also prevented EG induced mitochondrial collapse in rat.

CONCLUSIONS

Catechin has preventive effects on renal calcium crystallization both in vivo and in vitro, and provide a potential therapeutic treatment for this disease.

Oxygen nano-bubble water reduces calcium oxalate deposits and tubular cell injury in ethylene glycol-treated rat kidney

Renal tubular cell injury induced by oxalate plays an important role in kidney stone formation. Water containing oxygen nano-bubbles (nanometer-sized bubbles generated from oxygen micro-bubbles; ONB) has anti-inflammatory effects. Therefore, we investigated the inhibitory effects of ONB water on kidney stone formation in ethylene glycol (EG)-treated rats. We divided 60 rats, aged 4 weeks, into 5 groups: control, the water-fed group; 100 % ONB, the 100 % ONB water-fed group; EG, the EG treated water-fed group; EG + 50 % ONB and EG + 100 % ONB, water containing EG and 50 % or 100 % ONB, respectively. Renal calcium oxalate (CaOx) deposition, urinary excretion of N-acetyl-β-D-glucosaminidase (NAG), and renal expression of inflammation-related proteins, oxidative stress biomarkers, and the crystal-binding molecule hyaluronic acid were compared among the 5 groups. In the control and 100 % ONB groups, no renal CaOx deposits were detected. In the EG + 50 % ONB and EG + 100 % ONB groups, ONB water significantly decreased renal CaOx deposits, urinary NAG excretion, and renal monocyte chemoattractant protein-1, osteopontin, and hyaluronic acid expression and increased renal superoxide dismutase-1 expression compared with the EG group. ONB water substantially affected kidney stone formation in the rat kidney by reducing renal tubular cell injury. ONB water is a potential prophylactic agent for kidney stones.

Uromodulin upregulates TRPV5 by impairing caveolin-mediated endocytosis

Uromodulin (UMOD) is synthesized in the thick ascending limb and secreted into urine as the most abundant protein. Association studies in humans suggest protective effects of UMOD against calcium-containing kidney stones. Mice carrying mutations of Umod found in human uromodulin-associated kidney disease (UAKD) and Umod deficient mice exhibit hypercalciuria. The mechanism for UMOD regulation of urinary Ca²⁺ excretion is incompletely understood. We examined if UMOD regulates TRPV5 and TRPV6, channels critical for renal transcellular Ca²⁺ reabsorption. Coexpression with UMOD increased whole-cell TRPV5 current density in HEK293 cells. In biotinylation studies UMOD increased TRPV5 cell-surface abundance. Extracellular application of purified UMOD upregulated TRPV5 current density within physiological relevant concentration ranges. UMOD exerted a similar effect on TRPV6. TRPV5 undergoes constitutive caveolin-mediated endocytosis. UMOD had no effect on TRPV5 in a caveolin-1 deficient cell line. Expression of recombinant caveolin-1 in these cells restored the ability of UMOD to upregulate TRPV5. Secretion of UAKD-mutant UMOD was markedly reduced and coexpression of mutant UMOD with TRPV5 failed to increase its current. Immunofluorescent studies demonstrated lower TRPV5 expression in Umod^−/− mice compared to wild-type. UMOD upregulates TRPV5 by acting from extracellular and by decreasing endocytosis of TRPV5. The stimulation of Ca²⁺ reabsorption via TRPV5 by UMOD may contribute to protection against kidney stone formation.

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.

Mitochondrial permeability transition pore opening induces the initial process of renal calcium crystallization

Renal tubular cell injury induced by oxidative stress via mitochondrial collapse is thought to be the initial process of renal calcium crystallization. Mitochondrial collapse is generally caused by mitochondrial permeability transition pore (mPTP) opening, which can be blocked by cyclosporine A (CsA). Definitive evidence for the involvement of mPTP opening in the initial process of renal calcium crystallization, however, is lacking. In this study, we examined the physiological role of mPTP opening in renal calcium crystallization in vitro and in vivo. In the in vitro study, cultured renal tubular cells were exposed to calcium oxalate monohydrate (COM) crystals and treated with CsA (2 μM). COM crystals induced depolarization of the mitochondrial membrane potential and generated oxidative stress as evaluated by Cu-Zn SOD and 4-HNE. Furthermore, the expression of cytochrome c and cleaved caspase 3 was increased and these effects were prevented by CsA. In the in vivo study, Sprague-Dawley rats were administered 1% ethylene glycol (EG) to generate a rat kidney stone model and then treated with CsA (2.5, 5.0, and 10.0 mg/kg/day) for 14 days. EG administration induced renal calcium crystallization, which was prevented by CsA. Mitochondrial collapse was demonstrated by transmission electron microscopy, and oxidative stress was evaluated by measuring Cu-Zn SOD, MDA, and 8-OHdG generated by EG administration, all of which were prevented by CsA. Collectively, our results provide compelling evidence for a role of mPTP opening and its associated mitochondrial collapse, oxidative stress, and activation of the apoptotic pathway in the initial process of renal calcium crystallization.

Effect of Cystone® on urinary composition and stone formation over a one year period

Kidney stones are a common problem for which inadequate prevention exists. We recruited ten recurrent kidney stone formers with documented calcium oxalate stones into a two phased study to assess safety and effectiveness of Cystone(®), an herbal treatment for prevention of kidney stones. The first phase was a randomized double-blinded 12 week cross over study assessing the effect of Cystone(®) vs. placebo on urinary supersaturation. The second phase was an open label one year study of Cystone(®) to determine if renal stone burden decreased, as assessed by quantitative and subjective assessment of CT. Results revealed no statistically significant effect of Cystone(®) on urinary composition short (6 weeks) or long (52 weeks) term. Average renal stone burden increased rather than decreased on Cystone(®). Therefore, this study does not support the efficacy of Cystone(®) to treat calcium oxalate stone formers. Future studies will be needed to assess effects on stone passage, or on other stone types.

Are calcium oxalate crystals involved in the mechanism of acute renal failure in ethylene glycol poisoning?

INTRODUCTION

Ethylene glycol (EG) poisoning often results in acute renal failure, particularly if treatment with fomepizole or ethanol is delayed because of late presentation or diagnosis. The mechanism has not been established but is thought to result from the production of a toxic metabolite.

METHODS

A literature review utilizing PubMed identified papers dealing with renal toxicity and EG or oxalate. The list of papers was culled to those relevant to the mechanism and treatment of the renal toxicity associated with either compound. ROLE OF METABOLITES: Although the "aldehyde" metabolites of EG, glycolaldehyde, and glyoxalate, have been suggested as the metabolites responsible, recent studies have shown definitively that the accumulation of calcium oxalate monohydrate (COM) crystals in kidney tissue produces renal tubular necrosis that leads to kidney failure. In vivo studies in EG-dosed rats have correlated the severity of renal damage with the total accumulation of COM crystals in kidney tissue. Studies in cultured kidney cells, including human proximal tubule (HPT) cells, have demonstrated that only COM crystals, not the oxalate ion, glycolaldehyde, or glyoxylate, produce a necrotic cell death at toxicologically relevant concentrations. COM CRYSTAL ACCUMULATION: In EG poisoning, COM crystals accumulate to high concentrations in the kidney through a process involving adherence to tubular cell membranes, followed by internalization of the crystals. MECHANISM OF TOXICITY: COM crystals have been shown to alter membrane structure and function, to increase reactive oxygen species and to produce mitochondrial dysfunction. These processes are likely to be involved in the mechanism of cell death.

CONCLUSIONS

Accumulation of COM crystals in the kidney is responsible for producing the renal toxicity associated with EG poisoning. The development of a pharmacological approach to reduce COM crystal adherence to tubular cells and its cellular interactions would be valuable as this would decrease the renal toxicity not only in late treated cases of EG poisoning, but also in other hyperoxaluric diseases such as primary hyperoxaluria and kidney stone formation.

Surface aggregation of urinary proteins and aspartic Acid-rich peptides on the faces of calcium oxalate monohydrate investigated by in situ force microscopy

The growth of calcium oxalate monohydrate in the presence of Tamm-Horsfall protein (THP), osteopontin, and the 27-residue synthetic peptides (DDDS)(6)DDD and (DDDG)(6)DDD (D = aspartic acid, S = serine, and G = glycine) was investigated via in situ atomic force microscopy. The results show that these four growth modulators create extensive deposits on the crystal faces. Depending on the modulator and crystal face, these deposits can occur as discrete aggregates, filamentary structures, or uniform coatings. These proteinaceous films can lead to either the inhibition of or an increase in the step speeds (with respect to the impurity-free system), depending on a range of factors that include peptide or protein concentration, supersaturation, and ionic strength. While THP and the linear peptides act, respectively, to exclusively increase and inhibit growth on the (101) face, both exhibit dual functionality on the (010) face, inhibiting growth at low supersaturation or high modulator concentration and accelerating growth at high supersaturation or low modulator concentration. Based on analyses of growth morphologies and dependencies of step speeds on supersaturation and protein or peptide concentration, we propose a picture of growth modulation that accounts for the observations in terms of the strength of binding to the surfaces and steps and the interplay of electrostatic and solvent-induced forces at the crystal surface.

An active renal crystal clearance mechanism in rat and man

The kidney has several defense mechanisms to avert nephrocalcinosis by preventing intratubular crystal formation and adherence. Little is known about the fate of luminally adhered crystals. In order to study post-crystal adhesion defense mechanisms we quantified the number and morphology of crystal-containing tubules in rats at various time points following ethylene glycol administration as well as in renal biopsies of patients diagnosed with nephrocalcinosis of different etiology. In rats, nephrocalcinosis was completely cleared by epithelial overgrowth of adherent crystals, which were then translocated to the interstitium and subsequently disintegrated. These processes correlated with a low to moderate infiltration of inflammatory cells. Patients with nephrocalcinosis due either to acute phosphate nephropathy, primary hyperoxaluria, preterm birth, or transplantation also showed epithelial crystal overgrowth independent of the underlying disorder or the nature of the crystals. Our study found a quantitative association between changes in tubular and crystalline morphology and crystal clearance, demonstrating the presence of an important and active nephrocalcinosis-clearing mechanism in both rat and man.

Mass spectroscopic characteristics of low molecular weight proteins extracted from calcium oxalate stones: preliminary study

It is believed that boundary compositions of matrix proteins might play a role in stone formation; however, few proteomic studies concerning matrix proteins in urinary stones have been conducted. In this study, we extracted low molecular weight proteins from calcium oxalate stones and measured their characteristic patterns by mass spectroscopy. A total of 10 stones were surgically removed from patients with urolithiasis. Proteins were extracted from the stones and identified by one-dimensional electrophoresis (sodium dodecyl sulfate buffer [SDS]-polyacrylamide gel electrophoresis [SDS-PAGE]). After in-gel digest, samples were analyzed by the surface-enhanced laser desorption ionization-time of flight (SELDI-TOF) technique. The peptide sequences were analyzed from the data of mass spectroscopy. Proteins were identified from Database Search (SwissProt Protein Database; Swiss Institute of Bioinformatics; http://www.expasy.org/sprot) on a MASCOT server (Matrix Science Ltd.; http://www.matrixscience.com). A total of three bands of proteins (27, 18, and 14 kDa) were identified from SDS-PAGE in each stone sample. A database search (SwissProt) on a MASCOT server revealed that the most frequently seen proteins from band 1 (27 kDa) were leukocyte elastase precursor, cathepsin G precursor, azurocidin precursor, and myeloblastin precursor (EC 3.4.21.76) (leukocyte proteinase 3); band 2 (18 kDa) comprised calgranulin B, eosinophil cationic protein precursor, and lysozyme C precursor; band 3 (14 kDa) showed neutrophil defensin 3 precursor, calgranulin A, calgranulin C, and histone H4. The modifications and deamidations found from the mass pattern of these proteins may provide information for the study of matrix proteins. Various lower molecular weight proteins can be extracted from calcium oxalate stones. The characteristic patterns and their functions of those proteins should be further tested to investigate their roles in stone formation.

Association of osteopontin gene haplotypes with nephrolithiasis

Osteopontin (OPN) is one of the glycosylated phosphoproteins produced in the kidney that can modulate nephrolithiasis. We had previously found a modest association between OPN gene polymorphisms and the risk for urinary stone formation. In order to determine if sequence variants within the OPN gene could be linked to the risk of nephrolithiasis; we sequenced the entire OPN gene of 45 stone forming patients and 54 control patients of Japanese ancestry. We identified 61 polymorphisms and of these evaluated four haplotype-tagging single nucleotide polymorphisms in a total of 126 kidney stone cases and 214 healthy individuals; all of Japanese ancestry. There was a significant association of two of these haplotypes located in the OPN promoter with the relative probability of nephrolithiasis; one of increased and one of reduced risk. Our findings provide potential support for significant increased and decreased associations between OPN gene haplotypes and the relative potential of stone formation in the Japanese population. We suggest that such genetic findings may help to clarify the function of OPN in nephrolithiasis.

Inhibitors of calcification in blood and urine

In bone and teeth formation, coordinated calcification is a highly desirable biological process. However, heterotopic calcification at unwanted tissue sites leads to dysfunction, disease and, potentially, to death and therefore requires prevention and treatment. With the recent discovery of calcification inhibitors we now know that biological calcification is not passive but a complex, active and highly regulated process. Calcification at vascular sites is the most threatening localization and manifests as part of atherosclerosis or arteriosclerosis. Atherosclerosis is often accompanied by intimal plaque calcification, whereas arteriosclerosis is characterized by calcification of the media. The severity of calcification of cerebral or coronary atherosclerotic plaques is associated with an increased incidence of events such as stroke or myocardial infarction. Medial calcification is the major cause of arterial stiffness, which contributes to left ventricular dysfunction and heart failure. Patients with chronic kidney disease are at especially increased risk for both intimal and medial calcification. In this context, it is currently thought that calcium-regulatory factors including fetuin-A, matrix Gla protein, osteoprotegerin, and pyrophosphates act in a local or systemic manner to prevent calcifications of the vasculature, and that dys-regulations of such calcification inhibitors may contribute to progressive calcifications. Nephrolithiasis represents another process of unwanted calcification responsible for significant morbidity. More than 80% of renal stones contain calcium. Urinary factors inhibiting calcification are citrate, glycosaminoglycans, Tamm-Horsfall protein, and osteopontin. This review summarizes current experimental and clinical data underlining the biological importance of these calcification inhibitors.

Cell biology of pathologic renal calcification: contribution of crystal transcytosis, cell-mediated calcification, and nanoparticles

INTRODUCTION

The earliest lesion in the kidneys of idiopathic calcium oxalate stone formers is deposition of calcium phosphate in the interstitium, termed a Randall's plaque. Yet the cellular and molecular factors leading to their formation are unknown.

METHODS

The influence of urinary proteins on adhesion of preformed calcium oxalate crystals to rat continuous inner medullary collecting duct (cIMCD) cells was studied in vitro, and cIMCD cells were also exposed to calcifying media containing beta-glycerophosphate for up to 28 days. Renal tissue was obtained from a stone-forming and non-stone-forming individual at the time of nephrectomy. These nanoparticles, isolated from renal stones obtained at the time of surgical resection, were analyzed and propagated in standard cell culture medium.

RESULTS

Urinary proteins influence crystal adhesion to renal epithelial cells, and this activity is abnormal in the urine of stone-forming patients. cIMCD cells assumed an osteoblastic phenotype when exposed to the calcifying medium, expressing two bone matrix proteins (osteopontin and bone sialoprotein) that were also identified in the kidney of the stone-forming patient and associated with crystal deposition. Nanoparticles were propagated from the majority of renal stones. Isolates were susceptible to selected metabolic inhibitors and antibiotics and contained conserved bacterial proteins and deoxyribonucleic acid (DNA).

CONCLUSIONS

These results suggest new paradigms for Randall's plaque formation and idiopathic calcium oxalate stone disease. It seems unlikely that these events are driven solely by physical chemistry; rather, they are critically influenced by specific proteins and cellular responses, and understanding these events will provide clues toward novel therapeutic targets.

The importance of a clean face: the effect of different washing procedures on the association of Tamm-Horsfall glycoprotein and other urinary proteins with calcium oxalate crystals

This study was undertaken to determine whether the use of different washing procedures could explain dissident findings in published studies examining the role of urinary macromolecules in urolithiasis. Calcium oxalate monohydrate (COM) crystals were deposited from or added to the same sieved urine, washed with copious or limited amounts of distilled water, or with methanol, and examined by field emission scanning electron microscopy (FESEM). Demineralized extracts were analysed by SDS-PAGE and Western blotting for Tamm-Horsfall glycoprotein (THG), human serum albumin (HSA), osteopontin (OPN) and prothrombin fragment 1 (PTF1). Synchrotron X-ray diffraction (SXRD) with Rietveld whole-pattern peak fitting and profile analysis was used to determine non-uniform crystal strain and crystallite size in crystals generated from inorganic solutions in the presence of increasing concentrations of THG and prothrombin (PT). HSA and PTF1 were present in all demineralized crystal extracts, confirming their inclusion within COM. OPN was present in all extracts except those derived from pure inorganic COM crystals, because of its occlusion within small numbers of calcium oxalate dihydrate (COD) crystals contaminating the COM population. THG was absent from the demineralized extracts of all crystals washed copiously with water, but present in those washed with methanol or limited amounts of water. FESEM showed extraneous organic material associated only with crystals whose extracts contained THG, confirming that the protein does not bind permanently to the COM crystal surface and is not occluded within the mineral bulk. This was confirmed by SXRD, which showed that non-uniform strain and crystallite size remained unaltered in crystals grown in the presence of increasing THG concentrations. However, non-uniform strain increased and crystallite size decreased with increasing PT concentrations, demonstrating unambiguously that PT is included in COM crystals. It was concluded that scrupulous care must be taken to ensure the complete removal of extraneous THG adventitiously associated with CaOx crystals in order to avoid inaccurate analysis of crystal matrix protein content and possible misinterpretation of experimental data.

Effect of urine fractionation on attachment of calcium oxalate crystals to renal epithelial cells: implications for studying renal calculogenesis

Our aim was to determine whether fractionation of human urine affects the attachment of calcium oxalate monohydrate (COM) crystals to renal cells. Urine collected from six healthy subjects was fractionated into sieved (S), centrifuged (C), centrifuged and filtered (CF), or ultrafiltered (UF). Attachment of [(14)C]COM crystals to Madin-Darby canine kidney (MDCK) cells was studied after precoating the crystals or the cells with the urine fractions and by using the same fractions as the binding medium. Protein content of the fractions and precoated crystals was analyzed with SDS-PAGE and Western blotting. All urine fractions inhibited crystal attachment. When fractions from the six urine samples were used to precoat the cells, the median inhibitions of crystal adhesion ( approximately 40%) were not significantly different. Median inhibition after preincubation of crystals was the same for the S, C, and CF fractions ( approximately 40%) but significantly greater than for the UF fraction ( approximately 28%). When fractions were used as the binding medium, median inhibitions decreased from 64% in the S fraction to 47 (C), 42 (CF), and to 29% (UF). SDS-PAGE analysis showed that centrifugation and filtration reduced the amount of Tamm-Horsfall glycoprotein (THG), which was confirmed by Western blotting. Human serum albumin, urinary prothrombin fragment 1, and osteopontin, but not THG, were present in demineralized extracts of the precoated crystals. Fractionation of human urine affects the attachment of COM crystals to MDCK cells. Hence future studies investigating regulation of crystal-cell interactions should be carried out in untreated urine as the binding medium.

Henle loop basement membrane as initial site for Randall plaque formation

Since the early description of Randall plaques in 1937, studies of the pathogenesis of stone formation mainly focused on the chemistry involving salt precipitation and crystallization, rather than tubular and interstitial medullar mechanisms of calcium concentration and supersaturation. In 2003, Bushinsky published a suggestive and inspiring sequence of events aimed to show that the basement membrane of the thin limb of the loop of Henle can be the first site of nucleation, as recently shown by the impressive work by Evan et al. The aim of this minireview is to verify the consistency of the Evan and Bushinsky theory with the current literature in the field.

Sialylation of urinary prothrombin fragment 1 is implicated as a contributory factor in the risk of calcium oxalate kidney stone formation

Urinary glycoproteins are important inhibitors of calcium oxalate crystallization and adhesion of crystals to renal cells, both of which are key mechanisms in kidney stone formation. This has been attributed to glycosylation of the proteins. In South Africa, the black population rarely form stones (incidence < 1%) compared with the white population (incidence 12-15%). A previous study involving urinary prothrombin fragment 1 from both populations demonstrated superior inhibitory activity associated with the protein from the black group. In the present study, we compared N-linked and O-linked oligosaccharides released from urinary prothrombin fragment 1 isolated from the urine of healthy and stone-forming subjects in both populations to elucidate the relationship between glycosylation and calcium oxalate stone pathogenesis. The O-glycans of both control groups and the N-glycans of the black control samples were significantly more sialylated than those of the white stone-formers. This demonstrates a possible association between low-percentage sialylation and kidney stone disease and provides a potential diagnostic method for a predisposition to kidney stones that could lead to the implementation of a preventative regimen. These results indicate that sialylated glycoforms of urinary prothrombin fragment 1 afford protection against calcium oxalate stone formation, possibly by coating the surface of calcium oxalate crystals. This provides a rationale for the established roles of urinary prothrombin fragment 1, namely reducing the potential for crystal aggregation and inhibiting crystal-cell adhesion by masking the interaction of the calcium ions on the crystal surface with the renal cell surface along the nephron.

Protein regulation of intrarenal crystallization

PURPOSE OF REVIEW

In this review we discuss the key role renal proteins appear to play during initiation and growth of renal stones.

RECENT FINDINGS

Specific macromolecules have been identified in urine that can regulate crystal nucleation, growth, aggregation, and adhesion to renal cells. Many are incorporated into the matrix of crystals while they grow, including urinary prothrombin fragment 1, thereby increasing crystal susceptibility to degradation by cellular proteases. None of these macromolecular inhibitors appears to be quantitatively decreased in the urine of stone formers, although functional deficiencies thought due to abnormal glycosylation have been implicated, especially in the case of Tamm Horsfall protein. Increasing information is available on the nature and expression of crystal binding molecules on the renal cell surface, and they appear to be maximally expressed in response to stressful stimuli. Studies that employ atomic force microscopy and knockout mice are now being used to further clarify macromolecule-crystal interactions.

SUMMARY

The exact series of events that transform supersaturation to crystal formation and renal stones are poorly defined. Multiple anchored and soluble renal proteins potentially modulate or even regulate these events. A combination of proteomics and molecular biology seems likely to unravel these critical mediators in the coming years.

Crystal Retention in Renal Stone Disease: A Crucial Role for the Glycosaminoglycan Hyaluronan?

The mechanisms that are involved in renal stone disease are not entirely clear. In this article, the various concepts that have been proposed during the past century are reviewed briefly and integrated into current insights. Much attention is dedicated to hyaluronan (HA), an extremely large glycosaminoglycan that may play a central role in renal stone disease. The precipitation of poorly soluble calcium salts (crystal formation) in the kidney is the inevitable consequence of producing concentrated urine. HA is a major constituent of the extracellular matrix in the renal medullary interstitium and the pericellular matrix of mitogen/stress-activated renal tubular cells. HA is an excellent crystal-binding molecule because of its size, negative ionic charge, and ability to form hydrated gel-like matrices. Crystal binding to HA leads to crystal retention in the renal tubules (nephrocalcinosis) and to the formation of calcified plaques in the renal interstitium (Randall's plaques). It remains to be determined whether one or both forms of renal crystal retention are involved in the development of kidney stones (nephrolithiasis).