Oxalate-induced redistribution of phosphatidylserine in renal epithelial cells: implications for kidney stone disease

Meta-data analysis of kidney stone disease highlights ATP1A1 involvement in renal crystal formation

Nephrolithiasis is a complicated disease affected by various environmental and genetic factors. Crystal-cell adhesion is a critical initiation process during kidney stone formation. However, genes regulated by environmental and genetic factors in this process remain unclear. In the present study, we integrated the gene expression profile data and the whole-exome sequencing data of patients with calcium stones, and found that ATP1A1 might be a key susceptibility gene involved in calcium stone formation. The study showed that the T-allele of rs11540947 in the 5'-untranslated region of ATP1A1 was associated with a higher risk of nephrolithiasis and lower activity of a promoter of ATP1A1. Calcium oxalate crystal deposition decreased ATP1A1 expression in vitro and in vivo and was accompanied by the activation of the ATP1A1/Src/ROS/p38/JNK/NF-κB signaling pathway. However, the overexpression of ATP1A1 or treatment with pNaKtide, a specific inhibitor of the ATP1A1/Src complex, inhibited the ATP1A1/Src signal system and alleviated oxidative stress, inflammatory responses, apoptosis, crystal-cell adhesion, and stone formation. Moreover, the DNA methyltransferase inhibitor 5-aza-2'-deoxycytidine reversed ATP1A1 down-regulation induced by crystal deposition. In conclusion, this is the first study to show that ATP1A1, a gene modulated by environmental factors and genetic variations, plays an important role in renal crystal formation, suggesting that ATP1A1 may be a potential therapeutic target for treating calcium stones.

Nephro-protective effect of Daphnetin in hyperoxaluria-induced rat renal injury via alterations of the gut microbiota

It is well proved that hyperoxaluria induces the renal injury and finally causes the end stage kidney disease. Daphnetin (coumarin derivative) already confirmed renal protective effect in renal model, but hyperoxaluria protective effect still unexplore. The objective of this research was to scrutinize the renal protective effect of daphnetin against ethylene glycol (GC)-induced hyperoxaluria via altering the gut microbiota. GC (1% v/v) was used for the induction of hyperoxaluria in the rats and the rats were received the oral administration of daphnetin (5, 10 and 15 mg/kg). The body and renal weight were assessed. Urine, renal, inflammatory cytokines, antioxidant, inflammatory parameters, and gut microbiota were appraised. Daphnetin effectually improved the body weight and reduced the renal weight. Its also remarkably boosted the magnesium, calcium, citrate level and suppressed the level of uric acid and oxalate formation. Daphnetin significantly (p < .001) ameliorate the level of urinary kidney injury molecule 1 (KIM-1), blood urea nitrogen (BUN), urea, serum creatinine (Scr), neutrophil gelatinase-associated lipocalin (NGAL) and uric acid along with inflammatory cytokines and inflammatory mediators. Daphnetin considerably repressed the malonaldehyde (MDA) level, protein carbonyl and improved the level of glutathione reductase (GR), superoxide dismutase (SOD), glutathione (GSH) and catalase (CAT). Daphnetin treatment considerably altered the microbial composition of different bacteria at phylum, genus and family level. Daphnetin significantly suppressed the Firmicutes relative abundance and boosted the Bacteroidetes relative abundance. Our result clearly indicated that daphnetin remarkably ameliorates the GC induced hyperoxaluria in rats via altering the oxidative stress, inflammatory reaction and gut microbiota. PRACTICAL APPLICATION: Nephrotoxicity is a serious health disease worldwide. We induce the renal toxicity in the experimental rats using the ethylene glycol and scrutinized the renal protective effect of daphnetin. Daphnetin considerably suppress the renal, urine parameters. For estimation the underlying mechanism, we estimated the gut microbiota in all group rats. Daphnetin remarkably altered the level of gut microbiota and suggesting the renal protective effect.

Two independent modes of kidney stone suppression achieved by AIM/CD5L and KIM-1

The prevalence of kidney stones is increasing and its recurrence rate within the first 5 years is over 50%. No treatments that prevent the occurrence/recurrence of stones have reached the clinic. Here, we show that AIM (also called CD5L) suppresses stone development and improves stone-associated physical damages. The N-terminal domain of AIM associates with calcium oxalate crystals via charge-based interaction to impede the development of stones, whereas the 2nd and C-terminal domains capture the inflammatory DAMPs to promote their phagocytic removal. Accordingly, when stones were induced by glyoxylate in mice, recombinant AIM (rAIM) injection dramatically reduced stone development. Expression of injury molecules and inflammatory cytokines in the kidney and overall renal dysfunction were abrogated by rAIM. Among various negatively charged substances, rAIM was most effective in stone prevention due to its high binding affinity to crystals. Furthermore, only AIM was effective in improving the physical complaints including bodyweight-loss through its DAMPs removal effect. We also found that tubular KIM-1 may remove developed stones. Our results could be the basis for the development of a comprehensive therapy against kidney stone disease.

The circulating protein apoptosis inhibitor of macrophage (AIM) reduces kidney stone development and prevents build up, providing the basis for kidney stone disease therapy.

Indicator displacement assays: from concept to recent developments

Overcoming the synthetic burden related to covalently connected receptors with appropriate indicators for sensing various analytes via an indicator spacer receptor (ISR) approach, the indicator displacement assay (IDA) seems to be a very sophisticated and versatile supramolecular sensing paradigm, and it has taken the phenomenon of molecular recognition to the next level in the realm of host-guest chemistry. Due to the unavailability of a comprehensive report on what has been done in the last decade in relation to IDAs, we decided to set down this account illustrating diverse indicator displacement assays (IDAs) in detail from the concept stage to recent developments relating to the detection of cationic, anionic, and neutral analytes. The authors conclude this account with future perspectives and highlight the limitations and challenges relating to IDAs which need to be overcome in order to realize the full potential of this popular sensing phenomenon. While we were finalizing our account for publication, a tutorial review by the research groups of Anslyn, Sessler, and Sun was published, which focuses mainly on diverse aspects of the chemistry related to IDAs. As can be seen, our review, besides discussing various basic IDA concepts, has a vast collection of information published in the past decade and hence, hopefully, will be very informative for the supramolecular community. We believe that this work will offer new insights for the construction of novel sensors operating through the IDA approach.

Evaluations of the curative efficacy of G. fruticosus solvent extracts in experimentally induced nephrolithiatic Wistar male rats

BACKGROUND

In Ethiopian folk medicine, there is a claim that medicinal plants can treat urolithiasis although there is insufficient scientific evidence. The objective of this study was to evaluate the curative efficacy of Gomphocarpus fruticosus extracts in experimentally induced nephrolithiatic rats.

METHODS

Urolithiasis was induced in male Wistar rats by feeding ethylene glycol in drinking water for 28 days. The curative effects were evaluated after oral administrations of 200 mg/kg of the extracts from 15 to 28 days. Urine samples were collected 1 day before sacrificing the rats. Blood, liver and kidney samples were gathered under anaesthetic condition at day 28. Crystals in the urine were also analyzed by light microscopy.

RESULTS

G. fruticosus EtOAc extract reduced significantly the level of sodium (P < 0.001), whereas it was significantly elevated the levels of magnesium and citrate (P < 0.01) compared to lithiatic control. G. fruticosus BuOH extract lowered the levels of potassium (P < 0.01), calcium and phosphate in urolithiatic rats. It was also observed that G. fruticosus EtOAc extract decreased the level of oxalate in the urine (P < 0.001), whereas it was increased the levels of magnesium (P < 0.05) and citrate (P < 0.01) in serum analysis after exposure to BuOH extract. In the kidneys, CaOx crystal deposits were reduced significantly by G. fruticosus EtOAc extract (P < 0.01).

CONCLUSION

It has been noted that G. fruticosus EtOAc extract was potent in treating urolithiasis. However, further study is required to assess the efficacy of the active compounds against urolithiasis.

Calcium-sensing receptor promotes calcium oxalate crystal adhesion and renal injury in Wistar rats by promoting ROS production and subsequent regulation of PS ectropion, OPN, KIM-1, and ERK expression

OBJECTIVES

To explore the mechanism of calcium-sensing receptors (CaSRs) during the development of nephrolithiasis.

MATERIALS AND METHODS

Wistar rats were treated with ethylene glycol to induce calcium oxalate crystallization, and gadolinium chloride (GdCl₃, an agonist of CaSR) and NPS 2390 (an antagonist of CaSR) were added. Oxidative stress (OS) and calcium oxalate crystals in the kidney were observed. CaSR expression and the expression of extracellular signal-regulated protein kinase (ERK), OPN, and KIM-1 were determined by western blotting. In addition, renal tubular epithelial cells were isolated from the kidney to observe phosphatidylserine (PS) ectropion using flow cytometric analysis. Various biochemical parameters were assessed in serum and urine at the end of the experiment.

RESULTS

Calcium oxalate increased OS, crystal adhesion, PS ectropion, and the expression of CaSR and ERK, OPN, and KIM-1 in vivo. In addition, lower levels of urine citrate as well as increased serum creatinine and urea levels were observed after treatment with calcium oxalate (p < .05). Compared with calcium oxalate treatment alone, the above deleterious changes were further significantly confirmed by GdCl₃ but were reversed by NPS-2390. However, urine calcium excretion was decreased after ethylene glycol treatment but was significantly reduced by NPS 2390 and increased by GdCl₃ (p < .05).

CONCLUSIONS

The results suggest that CaSR might play significant roles in the induction of nephrolithiasis in rats by regulating reactive oxygen species (ROS) and PS ectropion and the composition of urine, OPN, KIM-1, and ERK expression.

Effects of Selenized Astragalus Polysaccharide on the Adhesion and Endocytosis of Nanocalcium Oxalate Dihydrate after the Repair of Damaged HK-2 Cells

An oxidative damage model of human proximal renal epithelial cells (HK-2) was established using oxalate damage. The repair effects of Astragalus polysaccharide (APS) and selenized APS (Se-APS) on damaged HK-2 cells were investigated. Differences in the adhesion and endocytosis of HK-2 cells to calcium oxalate dihydrate crystals with a size of approximately 100 nm before and after APS and Se-APS repair were also explored. The results showed that after being repaired by APS and Se-APS, HK-2 cells exhibited increased cell viability, restored cell morphology, reduced reactive oxygen species level, increased mitochondrial membrane potential, reduced phosphatidylserine eversion, and osteopontin expression. Moreover, the amount of adherent crystals on the cell surface decreased, but the amount of endocytic crystals increased. At the same concentration, Se-APS exhibited better repair effects on the damaged HK-2 cells than APS. All these findings revealed that Se-APS may be a potential drug candidate for inhibiting the formation of kidney stones.

Specific populations of urinary extracellular vesicles and proteins differentiate type 1 primary hyperoxaluria patients without and with nephrocalcinosis or kidney stones

BACKGROUND

Primary hyperoxaluria type 1 (PH1) is associated with nephrocalcinosis (NC) and calcium oxalate (CaOx) kidney stones (KS). Populations of urinary extracellular vesicles (EVs) can reflect kidney pathology. The aim of this study was to determine whether urinary EVs carrying specific biomarkers and proteins differ among PH1 patients with NC, KS or with neither disease process.

METHODS

Mayo Clinic Rare Kidney Stone Consortium bio-banked cell-free urine from male and female PH1 patients without (n = 10) and with NC (n = 6) or KS (n = 9) and an eGFR > 40 mL/min/1.73 m2 were studied. Urinary EVs were quantified by digital flow cytometer and results expressed as EVs/ mg creatinine. Expressions of urinary proteins were measured by customized antibody array and results expressed as relative intensity. Data were analyzed by ANCOVA adjusting for sex, and biomarkers differences were considered statistically significant among groups at a false discovery rate threshold of Q < 0.20.

RESULTS

Total EVs and EVs from different types of glomerular and renal tubular cells (11/13 markers) were significantly (Q < 0.20) altered among PH1 patients without NC and KS, patients with NC or patients with KS alone. Three cellular adhesion/inflammatory (ICAM-1, MCP-1, and tissue factor) markers carrying EVs were statistically (Q < 0.20) different between PH1 patients groups. Three renal injury (β2-microglobulin, laminin α5, and NGAL) marker-positive urinary EVs out of 5 marker assayed were statistically (Q < 0.20) different among PH1 patients without and with NC or KS. The number of immune/inflammatory cell-derived (8 different cell markers positive) EVs were statistically (Q < 0.20) different between PH1 patients groups. EV generation markers (ANO4 and HIP1) and renal calcium/phosphate regulation or calcifying matrixvesicles markers (klotho, PiT1/2) were also statistically (Q < 0.20) different between PH1 patients groups. Only 13 (CD14, CD40, CFVII, CRP, E-cadherin, EGFR, endoglin, fetuin A, MCP-1, neprilysin, OPN, OPGN, and PDGFRβ) out of 40 proteins were significantly (Q < 0.20) different between PH1 patients without and with NC or KS.

CONCLUSIONS

These results imply activation of distinct renal tubular and interstitial cell populations and processes associated with KS and NC, and suggest specific populations of urinary EVs and proteins are potential biomarkers to assess the pathogenic mechanisms between KS versus NC among PH1 patients.

Urinary extracellular vesicle-associated MCP-1 and NGAL derived from specific nephron segments differ between calcium oxalate stone formers and controls

Randall's plaque (RP; subepithelial calcification) appears to be an important precursor of kidney stone disease. However, RP cannot be noninvasively detected. The present study investigated candidate biomarkers associated with extracellular vesicles (EVs) in the urine of calcium stone formers (CSFs) with low (<5% papillary surface area) and high (≥5% papillary surface area) percentages of RP and a group of nonstone formers. RPs were quantitated via videotaping and image processing in consecutive CSFs undergoing percutaneous surgery for stone removal. Urinary EVs derived from cells of different nephron segments of CSFs (n = 64) and nonstone formers (n = 40) were quantified in biobanked cell-free urine by standardized and validated digital flow cytometer using fluorophore-conjugated antibodies. Overall, the number of EVs carrying surface monocyte chemoattractant protein (MCP)-1 and neutrophil gelatinase-associated lipocalin (NGAL) were significantly lower in CSFs compared with nonstone former controls (P < 0.05) but did not differ statistically between CSFs with low and high RPs. The number of EVs associated with osteopontin did not differ between any groups. Thus, EVs carrying MCP-1 and NGAL may directly or indirectly contribute to stone pathogenesis as evidenced by the lower of these populations of EVs in stone formers compared with nonstone formers. Validation of EV-associated MCP-1 and NGAL as noninvasive biomarkers of kidney stone pathogenesis in larger populations is warranted.

UPLC/MS-Based Metabolomics Investigation of the Protective Effect of Hydrogen Gas Inhalation on Mice with Calcium Oxalate-Induced Renal Injury

Hydrogen has a significant protective effect on calcium oxalate-induced renal injury, but its effect on metabolic profiles is unknown. This study showed the effects of hydrogen on serum and urine metabolites in a renal injury model. Ultra-HPLC quadrupole time-of-flight-MS-based metabolomics was used to characterise metabolic variations. Twenty-five serum metabolites and 14 urine metabolites showed differences in the the nitrogen and oxygen inhalation (NO), nitrogen and oxygen inhalation combined with calcium oxalate induction (CaOx), and hydrogen inhalation combined with calcium oxalate induction (HO+CaOx) groups. Nineteen serum metabolites and 7 urine metabolites showed significant restoration to normal levels after hydrogen gas (H₂) treatment. These metabolites are primarily related to amino acid metabolism, fatty acid metabolism, and phospholipid metabolism. This study showed that a comprehensive metabolomics approach is an effective strategy to elucidate the mechanisms underlying the effects of hydrogen treatment on calcium oxalate-induced renal injury.

MRP-1 and BCRP Promote the Externalization of Phosphatidylserine in Oxalate-treated Renal Epithelial Cells: Implications for Calcium Oxalate Urolithiasis

OBJECTIVE

To investigate the possible involvement of multidrug resistance-associated protein 1 (MRP-1) and breast cancer resistance protein (BCRP) in the oxalate-induced redistribution of phosphatidylserine (PS) in renal epithelial cell membranes.

METHODS

A western blot analysis was used to examine the MRP-1 and BCRP expression levels. Surface-expressed PS was detected by the annexin V-binding assay. The cell-permeable fluorogenic probe 2,7-dichlorofluorescein diacetate was used to measure the intracellular reactive oxygen species (ROS) level. A rat model of hyperoxaluria was obtained using 0.5% ethylene glycol and 1.0% ammonium chloride. In addition, certain animals received verapamil (50 mg/kg body weight), which is a common inhibitor of MRP-1 and BCRP. The degree of nephrolithiasis was assessed histomorphometrically using sections stained by Pizzolato method and by measuring the calcium oxalate crystal content in the renal tissue.

RESULTS

Oxalate produced a concentration-dependent increase in the synthesis of MRP-1 and BCRP. Treatment with MK571 and Ko143 (MRP-1- and BCRP-specific inhibitors, respectively) significantly attenuated the oxalate-induced PS externalization. Adding the antioxidant N-acetyl-l-cysteine significantly reduced MRP-1 and BCRP expression. In vivo, markedly decreased nephrocalcinosis was observed compared with that in the rat model of hyperoxaluria without verapamil treatment.

CONCLUSION

Oxalate induces the upregulation of MRP-1 and BCRP, which act as phospholipid floppases causing PS externalization in the renal epithelial cell membrane. The process is mediated by intracellular ROS production. The ROS-mediated increase in the synthesis of MRP-1 and BCRP can play an important role in hyperoxaluria-promoted calcium oxalate urolithiasis by facilitating phosphatidylserine redistribution in renal epithelial cells.

Time-dependent subcellular structure injuries induced by nano-/micron-sized calcium oxalate monohydrate and dihydrate crystals

Comparative studies were conducted to investigate the time effect of cell injury induced by nano-sized (50nm) and micron-sized (10μm) calcium oxalate monohydrate (COM) and dihydrate (COD) crystals in African green monkey renal epithelial (Vero) cells. The effects of nano-/micron-sized COM and COD exposure on Vero cells were investigated by detecting the cell viability, cell morphology, LDH release, reactive oxygen species, mitochondrial membrane potential, cell cycle, and cell apoptosis, as well as the intracellular and extracellular crystal distribution. Nano-/micron-sized COM and COD exposure lead to subcellular organelle injury in varying degrees, but the injury sequence of various organelles differed. The time sequence of organelle injury presenting significant variation was described as follows: cell membrane injury (1h)<mitochondrial membrane potential decrease (3h to 6h)≈cell-cycle arrest (3h to 6h)<cell apoptosis (12h). Nano-sized crystals lead organelle injury faster than micron-sized crystals, and COM crystals showed more obvious time-dependent effects than the same-sized COD crystals. This study may provide insights into the damage to renal epithelial cells induced by urinary crystals and the formation mechanism of kidney stones.

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.

Calcium oxalate toxicity in renal epithelial cells: the mediation of crystal size on cell death mode

The cytotoxicity of calcium oxalate (CaOx) in renal epithelial cells has been studied extensively, but the cell death mode induced by CaOx with different physical properties, such as crystal size and crystal phase, has not been studied in detail. In this study, we comparatively investigated the differences of cell death mode induced by nano-sized (50 nm) and micron-sized (10 μm) calcium oxalate monohydrate (COM) and calcium oxalate dihydrate (COD) to explore the cell death mechanism. The effect of the exposure of nano-/micron-sized COM and COD crystals toward the African green monkey renal epithelial (Vero) cells were investigated by detecting cell cytoskeleton changes, lysosomal integrity, mitochondrial membrane potential (Δψm), apoptosis and/or necrosis, osteopontin (OPN) expression, and malondialdehyde (MDA) release. Nano-/micron-sized COM and COD crystals could cause apoptosis and necrosis simultaneously. Nano-sized crystals primarily caused apoptotic cell death, leading to cell shrinkage, phosphatidylserine ectropion, and nuclear shrinkage, whereas micron-sized crystals primarily caused necrotic cell death, leading to cell swelling and cell membrane and lysosome rupture. Nano-sized COM and COD crystals induced much greater cell death (sum of apoptosis and necrosis) than micron-sized crystals, and COM crystals showed higher cytotoxicity than the same-sized COD crystals. Both apoptosis and necrosis could lead to mitochondria depolarization and elevate the expression of OPN and the generation of lipid peroxidation product MDA. The amount of expressed OPN and generated MDA was positively related to cell injury degree. The physicochemical properties of crystals could affect the cell death mode. The results of this study may provide a basis for future studies on cell death mechanisms.

Recent advances in the construction of fluorescent metallocycles and metallocages via coordination-driven self-assembly

During the last few years, the construction of fluorescent metallocycles and metallocages has attracted considerable attention because of their wide applications in fluorescence detection of metal ions, anions, or small molecules, mimicking complicated natural photo-processes, and preparing photoelectric devices, etc. This perspective focuses on the recent advances in the construction of a variety of fluorescent metallocycles and metallocages via coordination-driven self-assembly. In addition, the fluorescence properties and the applications of these organometallic architectures have also been discussed.

An Ru(ii)–Fe(iii) bimetallic complex as a multifunctional device for detecting, signal amplifying, and degrading oxalate

A multifunctional device that can monitor the level of pollutants, magnify weak signals, and subsequently degrade pollutants is highly desirable. A new Ru(ii)–Fe(iii) complex—[Ru^II(^tBubpy)(CN)₄]₂–[Fe^III(H₂O)₃Cl]₂·8H₂O (1,^tBubpy = 4,4′-di-tert-butyl-2,2′-bipyridine)—was synthesized and characterized.

NADPH oxidase as a therapeutic target for oxalate induced injury in kidneys

A major role of the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase family of enzymes is to catalyze the production of superoxides and other reactive oxygen species (ROS). These ROS, in turn, play a key role as messengers in cell signal transduction and cell cycling, but when they are produced in excess they can lead to oxidative stress (OS). Oxidative stress in the kidneys is now considered a major cause of renal injury and inflammation, giving rise to a variety of pathological disorders. In this review, we discuss the putative role of oxalate in producing oxidative stress via the production of reactive oxygen species by isoforms of NADPH oxidases expressed in different cellular locations of the kidneys. Most renal cells produce ROS, and recent data indicate a direct correlation between upregulated gene expressions of NADPH oxidase, ROS, and inflammation. Renal tissue expression of multiple NADPH oxidase isoforms most likely will impact the future use of different antioxidants and NADPH oxidase inhibitors to minimize OS and renal tissue injury in hyperoxaluria-induced kidney stone disease.

Aluminum citrate blocks toxicity of calcium oxalate crystals by preventing binding with cell membrane phospholipids

BACKGROUND/AIMS

Renal damage from ethylene glycol and primary hyperoxaluria is linked to accumulation of calcium oxalate monohydrate (COM) crystals in the renal proximal tubule (PT). In vitro studies have shown that aluminum citrate (AC), uniquely among citrate salts, blocks COM cytotoxicity to tubular cells. These studies were designed to evaluate the interaction of COM with membrane phospholipids and the ability of AC to reduce COM toxicity by interfering with this interaction.

METHODS

Interaction of COM with phospholipids was assessed using differential scanning calorimetric analysis of structural changes in specific liposomes. Interaction of COM with cell membranes was studied by measuring binding of radiolabeled crystals by human PT (HPT) cells.

RESULTS

Analysis of liposomes prepared from phosphatidylserine (PS) or phosphatidylcholine (PC) showed that COM interfered with the gel-liquid transition of PS liposomes, but not that of PC liposomes. AC reversed the COM-induced changes in liposomal structure. AC inhibited the binding of [(14)C]-COM by HPT cells in a concentration-dependent manner. AC blocked COM binding by interacting with the crystal surface and not the cell membrane.

CONCLUSION

These results indicate that AC blocks the binding of COM by PT cells, and consequently its cytotoxicity, by attaching to the surface of the crystal. Thus, AC, or a related compound that works by the same mechanism, could be a useful adjunct therapy to reduce the renal damage produced by severe hyperoxaluria.

Oxalate impairs aminophospholipid translocase activity in renal epithelial cells via oxidative stress: implications for calcium oxalate urolithiasis

PURPOSE

We evaluated the possible involvement of phospholipid transporters and reactive oxygen species in the oxalate induced redistribution of renal epithelial cell phosphatidylserine.

MATERIALS AND METHODS

Madin-Darby canine kidney cells were labeled with the fluorescent phospholipid NBD-PS in the inner or outer leaflet of the plasma membrane and then exposed to oxalate in the presence or absence of antioxidant. This probe was tracked using a fluorescent quenching assay to assess the bidirectional transmembrane movement of phosphatidylserine. Surface expressed phosphatidylserine was detected by annexin V binding assay. The cell permeable fluorogenic probe DCFH-DA was used to measure the intracellular reactive oxygen species level.

RESULTS

Oxalate produced a time and concentration dependent increase in phosphatidylserine, which may have resulted from impaired aminophospholipid translocase mediated, inward directed phosphatidylserine transport and from enhanced phosphatidylserine outward transport. Adding the antioxidant N-acetyl-L-cysteine significantly attenuated phosphatidylserine externalization by effectively rescuing aminophospholipid translocase activity.

CONCLUSIONS

To our knowledge our findings are the first to show that oxalate induced increased reactive oxygen species generation impairs aminophospholipid translocase activity and decreased aminophospholipid translocase activity has a role in hyperoxaluria promoted calcium oxalate urolithiasis by facilitating phosphatidylserine redistribution in renal epithelial cells.

Morphology, particle size distribution, aggregation, and crystal phase of nanocrystallites in the urine of healthy persons and lithogenic patients

BACKGROUND

The first step in urinary stone formation is the nucleation of urinary mineral from supersaturated urine. The formed nuclei then grow or/and aggregate to a pathological size. Thus, the nanocrystallites in urine may be related to the formation of urinary stones.

METHODS

Nanocrystallites with a size of less than 1000 nm in the urine samples of 85 healthy persons and 65 lithogenic patients were comparatively investigated using laser scattering spectroscopy, TEM, and X-ray diffraction.

RESULTS

Most of the nanocrystallites in healthy urine samples were spheroidal, less aggregated, well-dispersed, and with a narrow particle size distribution from about 20 to 350 nm. In contrast, most of the particles in lithogenic urines had sharply angled edges and tips, were aggregated, and had a broad particle size distribution from 1.1 to 1000 nm. More calcium oxalate dihydrate (COD) nanocrystallites were found in healthy urine; however, more calcium oxalate monohydrate (COM) nanocrystallites were found in lithogenic urine.

CONCLUSIONS

The morphology, particle size, aggregation, and crystal phase of nanocrystallites in the urine of lithogenic patients are pronouncedly different from those of healthy persons. The results suggest, in ascending order of importance, that making nanocrystallites rounded, diminishing their size differentiation, and decreasing their aggregation in urine by physical and chemical methods maybe the means to prevent urinary stone formation. The most crucial among the four differences is the crystal phase differential of calcium oxalate (CaOxa). That is, the formation of COD nanocrystallites in urine can be considered as being relatively more favorable in preventing stone formation than the formation of COM nanocrystallites, which are in accord with those found for larger crystallites.

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.

Crystalluric and tubular epithelial parameters during the onset of intratubular nephrocalcinosis: illustration of the 'fixed particle' theory in vivo

BACKGROUND

The 'fixed particle' theory states that, besides crystal formation in the tubular fluid, crystal adhesion to the tubular epithelium is a prerequisite for the development of intratubular nephrocalcinosis. It has been hypothesized that the tubular epithelium, in order to bind crystals, needs to be phenotypically altered. Whereas most evidence hereto is provided by in vitro experiments, we set out to illustrate this theory in vivo.

METHODS

We simultaneously investigated the temporal changes of nephrocalcinosis-associated parameters during and shortly after a 4-day ethylene glycol (EG)-administration period in rats. We measured oxaluria, crystal formation, crystalluria, apoptosis, epithelial injury/ regeneration and luminal membrane expression of several crystal-binding molecules [hyaluronan (HA), osteopontin (OPN) and for the first time in vivo, annexin-2 (ANX2) and nucleolin-related-protein (NRP) and one of their receptors (CD44, HA/OPN-receptor]. Clinically, renal biopsies of preterm infants, transplant patients and acute phosphate nephropathy patients were stained for ANX2, NRP, HA and OPN.

RESULTS

In the presence of a rather constant and persistent intratubular crystal formation, crystal retention gradually increased during EG-administration and markedly increased after arrest thereof, indicating that the development of crystal adhesion requires more than just the presence of crystals in the tubular fluid. All luminal membrane markers and a regenerating/dedifferentiated epithelium, unlike apoptosis, to various extents were upregulated concurrently and in association with crystal adhesion. However, both in humans and rats, expression of luminal molecules was not confined to crystal-containing tubules.

CONCLUSIONS

Altogether, these findings allow better insight into the mechanisms underlying the 'fixed particle' theory in vivo and indicate that an altered epithelial phenotype with crystal-binding properties precedes crystal adhesion, thereby corroborating the requirement of tubular epithelial phenotypical changes in the development of intratubular nephrocalcinosis.

Mechanisms of star fruit-induced acute renal failure

We have previously discovered that star fruit can induce oliguric acute renal failure. To investigate the mechanisms of star fruit-associated acute oxalate nephropathy, the nephrotoxic effect of star fruit was examined in both cellular experiments and animal models. We evaluated renal function, pathological changes in kidney tissues and apoptotic effects using terminal deoxynucleotidyl transferase nick-end labeling (TUNEL) assay in four groups of rats -- a control group (CG), fed with tap water (1); a star fruit group (SG), fed with star fruit juice naturally containing 0.2M oxalate (2); and oxalate groups (OxG), fed with 0.2M (3) or 0.4M (4) oxalate solution. The effects of both star fruit juice and oxalate on MDCK cells were also analyzed by flow cytometry. We found that the mean creatinine clearance was significantly lower in the SG, 0.2M OxG and 0.4M OxG. Dose-dependent apoptotic effects were evident from the TUNEL assay, and flow cytometry analysis of treated MDCK cells showed dose- and time-dependent effects. Our findings suggest that star fruit juice produces acute renal injury, not only through the obstructive effect of calcium oxalate crystals, but also by inducing apoptosis of renal epithelial cells, which may be caused by the levels of oxalate in the fruit.

Intracellular events in the initiation of calcium oxalate stones

This review summarizes our current understanding of intracellular events in the initiation of kidney stone formation, focusing on results from studies using renal epithelial cells in vitro. Such studies have shown that oxalate - either in crystalline or in soluble form - triggers a spectrum of responses in renal cells that favor stone formation, including alterations in membrane surface properties that promote crystal attachment and alterations in cell viability that provide debris for crystal nucleation. Activation of cytosolic PLA2 appears to play an important role in oxalate actions, triggering a signaling cascade that generates several lipid mediators (arachidonic acid, AA; lysophosphatidylcholine, Lyso-PC; ceramide) that act on key intracellular targets (mitochondria, nucleus). The net effect is increased production of reactive oxygen molecules (that in turn affect other cellular processes), an increase in cell death and an induction of a number of genes in surviving cells, some of which may promote proliferation for replacement of damaged cells, or may promote secretion of urinary macromolecules that serve to modulate crystal formation. A scheme is provided that explains how such oxalate-induced alterations could initiate stone formation in vivo.

Oxalate toxicity in renal cells

Exposure to oxalate, a constituent of the most common form of kidney stones, generates toxic responses in renal epithelial cells, including altered membrane surface properties and cellular lipids, changes in gene expression, disruption of mitochondrial function, formation of reactive oxygen species and decreased cell viability. Oxalate exposure activates phospholipase A2 (PLA2), which increases two lipid signaling molecules, arachidonic acid and lysophosphatidylcholine (Lyso-PC). PLA2 inhibition blocks, whereas exogenous Lyso-PC or arachidonic acid reproduce many of the effects of oxalate on mitochondrial function, gene expression and cell viability, suggesting that PLA2 activation plays a role in mediating oxalate toxicity. Oxalate exposure also elicits potentially adaptive or protective changes that increase expression of proteins that may prevent crystal formation or attachment. Additional adaptive responses may facilitate removal and replacement of dead or damaged cells. The presence of different inflammatory cells and molecules in the kidneys of rats with hyperoxaluria and in stone patients suggests that inflammatory responses play roles in stone disease. Renal epithelial cells can synthesize a variety of cytokines, chemoattractants and other molecules with the potential to interface with inflammatory cells; moreover, oxalate exposure increases the synthesis of these molecules. The present studies demonstrate that oxalate exposure upregulates cyclooxygenase-2, which catalyzes the rate-limiting step in the synthesis of prostanoids, compounds derived from arachidonic acid that can modify crystal binding and may also influence inflammation. In addition, renal cell oxalate exposure promotes rapid degradation of IkappaBalpha, an endogenous inhibitor of the NF-kappaB transcription factor. A similar response is observed following renal cell exposure to lipopolysaccharide (LPS), a bacterial cell wall component that activates toll-like receptor 4 (TLR4). While TLRs are primarily associated with immune cells, they are also found on many other cell types, including renal epithelial cells, suggesting that TLR signaling could directly impact renal function. Prior exposure of renal epithelial cells to oxalate in vitro produces endotoxin tolerance, i.e. a loss of responsiveness to LPS and conversely, prior exposure to LPS elicits a similar heterologous desensitization to oxalate. Renal cell desensitization to oxalate stimulation may have profound effects on the outcome of renal stone disease by impairing protective responses.

Vitamin E therapy prevents hyperoxaluria-induced calcium oxalate crystal deposition in the kidney by improving renal tissue antioxidant status

OBJECTIVE

To determine whether vitamin E prevents hyperoxaluria-induced stone formation, using a new animal model of calcium oxalate stone disease, as our previous in- vitro and in-vivo studies showed that oxalate and hyperoxaluria induce free-radical generation, which results in peroxidative injury to renal tubular cells.

MATERIALS AND METHODS

Ethylene glycol (EG) was administered at 150 mg/day by gavage for 3 weeks to rats fed on diets with adequate (group 1), excess (group 2) or deficient (group 3) vitamin E. Several indicators of peroxidation, free radicals and enzymatic activity were then assessed.

RESULTS

EG treatment in group 1 lead to increased lipid peroxidation, protein thiol, excretion of urinary enzymes, oxalate and decreases in urinary calcium, antioxidant enzymes and altered glutathione redox balance. Although renal function was not altered, there was increased water intake, urine volume and lowered urinary pH in these rats. These changes were more intense, with extensive calcium-oxalate crystal deposition, in rats in group 3, and prevented in rats in group 2, except for urinary oxalate levels, which remained high. Histopathological examination showed that there was no deposition of calcium oxalate crystals in rats in group 2.

CONCLUSION

This is the first study to demonstrate in-vivo evidence that hyperoxaluria-induced peroxidative injury induces individual calcium oxalate crystal attachment in the renal tubules. In addition, excess vitamin E completely prevented calcium oxalate deposition, by preventing peroxidative injury and restoring renal tissue antioxidants and glutathione redox balance. Therefore, vitamin E therapy might provide protection against the deposition of calcium oxalate stones in the kidney of humans.

Mitochondrial dysfunction is a primary event in renal cell oxalate toxicity

BACKGROUND

In cultured renal epithelial cells, exposure to oxalate, a constituent of many kidney stones, elicits a cascade of responses that often leads to cell death. Oxalate toxicity is mediated via generation of reactive oxygen species (ROS) in a process that depends at least in part upon lipid signaling molecules that are generated through membrane events that culminate in phospholipase A2 (PLA2) activation. The present studies asked whether mitochondria, a major site of ROS production, were targets of oxalate toxicity, and if so, whether mitochondrial responses to oxalate were mediated by PLA2 activation.

METHODS

Effects of oxalate and various lipids on mitochondrial membrane potential (DeltaPsim) were measured in Madin-Darby canine kidney (MDCK) cell monolayers using 5,5',6,6'-tetrachloro 1,1',3,3'-tetraethylbenzimidazolylcarbocyanine iodide (JC-1), a DeltaPsim-sensitive dye. Other studies assayed caspases, serine proteases activated during apoptosis, in response to oxalate or lipid signaling molecules. Additional studies asked whether oxalate or lipids produced by PLA2 activation promoted ROS formation in isolated renal mitochondria.

RESULTS

Oxalate exposure decreased MDCK cell DeltaPsim within 30 minutes, a response attenuated by arachidonyl trifluoromethyl ketone (AACOCF3), an inhibitor of cytosolic PLA2 (cPLA2). Exposure to arachidonic acid or to lysophosphatidylcholine (lyso-PC), lipid products of PLA2 activation, or to ceramide, another lipid signal generated in MDCK cells following oxalate exposure, also depolarized MDCK cell DeltaPsim and increased the number of caspase-positive cells. Isolated renal mitochondria responded to oxalate, arachidonic acid, lyso-PC, and ceramide by increasing their accumulation of ROS, lipid peroxides, and oxidized thiol proteins.

CONCLUSION

These studies suggest that lipid signaling molecules released after oxalate-induced PLA2 activation trigger marked, rapid changes in mitochondrial function that may mediate toxicity in renal epithelial cells.

Annexin II is present on renal epithelial cells and binds calcium oxalate monohydrate crystals

Attachment of newly formed crystals to renal epithelial cells appears to be a critical step in the development of kidney stones. The current study was undertaken to identify potential calcium oxalate monohydrate (COM) crystal-binding proteins on the surface of renal tubular cells. Apical membranes were prepared from confluent monolayers of renal epithelial cells (MDCKI line), and COM crystal affinity was used to isolate crystal-binding proteins that were then subjected to electrophoresis and electroblotting. Microsequencing of the most prominent COM crystal-binding protein (M(r) of 37 kD) identified it as annexin II (Ax-II). When exposed proteins on the surface of intact monolayers were biotinylated and then isolated using streptavidin agarose beads, Ax-II was detected, suggesting that at least a portion is exposed on the apical cell surface. Ax-II was not completely extracted by 0.1 M Na(2)CO(3), suggesting that at least a portion of cellular Ax-II is an intrinsic membrane-bound protein. Using confocal immunofluorescence microscopy, Ax-II was visualized together with Caveolin-1 (Cav-1) on the apical membrane of intact MDCKI cells. Cells pretreated with a monoclonal anti-Ax-II antibody bound significantly fewer COM crystals, whereas anti-LDL receptor antibody did not decrease COM binding, further suggesting a functional role for Ax-II during adhesion of crystals to intact cells. These results suggest that Ax-II avidly binds COM crystals and is present on the apical surface of MDCKI cells. Therefore, in the intact nephron, Ax-II could mediate adhesion of COM crystals to cells, and altered exposure of Ax-II on the surface of renal tubular cells could promote crystal retention and possibly kidney stone formation.

Presence of lipids in urine, crystals and stones: implications for the formation of kidney stones

BACKGROUND

Cell membranes and their lipids play critical roles in calcification. Specific membrane phospholipids promote the formation of calcium phosphate and become a part of the organic matrix of growing calcification. We propose that membrane lipids also promote the formation of calcium oxalate (CaOx) and calcium phosphate (CaP) containing kidney stones, and become a part of their stone matrix.

METHODS

Human urine, crystals of CaOx and CaP produced in the urine of healthy individuals, and urinary stones containing struvite, uric acid, CaOx and CaP crystals for the presence of membrane lipids were analyzed. Crystallization of CaOx monohydrate at Langmuir monolayers of dipalmitoylphosphatidylglycerol (DPPG), dipalmitoylphosphatidylcholine (DPPC), dipalmitoylphosphatidylserine (DPPS), dioleoylphosphatidylglycerol (DOPG), palmitoyloleoylphosphatidylglycerol (POPG) and dimyristoylphosphatidylglycerol (DMPG) was investigated to directly demonstrate that phospholipid assemblies can catalyze CaOx nucleation.

RESULTS

Urine as well as CaOx and CaP crystals made in the urine and various types of urinary stones investigated contained some lipids. Urine of both CaOx and uric acid stone formers contained significantly more cholesterol, cholesterol ester and triglycerides than urine of healthy subjects. However, urine of CaOx stone formers contained more acidic phospholipids. The organic matrix of calcific stones contained significantly more acidic and complexed phospholipids than uric acid and struvite stones. For each Langmuir monolayer precipitation was heterogeneous and selective with respect to the orientation and morphology of the CaOx crystals. Crystals were predominantly monohydrate, and most often grew singly with the calcium rich (10-1) face toward the monolayer. The number of crystals/mm2 decreased in the order DPPG> DPPC and was inversely proportional to surface pressure and mean molecular area/molecule.

CONCLUSIONS

Stone forming conditions in the kidneys greatly impact their epithelial cells producing significant differences in the urinary lipids between healthy and stone forming individuals. Altered membrane lipids promote face selective nucleation and retention of calcium oxalate crystals, and in the process become a part of the growing crystals and stones.

Crystal-cell interaction in the pathogenesis of kidney stone disease

PURPOSE OF REVIEW

Renal stone formation depends not so much on the formation of crystals, but on their retention in the kidney. Evidence has emerged that crystal retention is caused predominantly by the adherence of crystals to the epithelial cells lining the renal tubules. Understanding the mechanisms involved in crystal retention could lead to new therapeutic approaches for interfering with the renal stone-forming process in patients. Cell-culture studies have been performed to obtain insights into the susceptibility of the cell surface to crystal attachment, and to uncover cell-surface crystal-binding molecules. This review aims to put the relevant publications of the last decade in perspective.

RECENT FINDINGS

Crystal-cell interaction has been investigated by using various renal tubular cell types in culture. Such studies have yielded several candidate crystal-binding molecules, including phosphatidylserine, sialic acid, collagen IV, osteopontin and, recently, hyaluronan.

SUMMARY

Here, the results obtained in crystal-binding studies are recapitulated, compared and evaluated. Arguments are provided in support of the view that many of the proposed crystal-binding molecules could be linked in the series of events resulting in crystal retention. Under pathological conditions, pericellular matrices rich in the polysaccharide hyaluronan are proposed as the key binding substance for crystals at the surface of renal tubular cells.