Renal epithelial cells rapidly bind and internalize calcium oxalate monohydrate crystals

Antioxidant Activity of Auricularia auricula Polysaccharides with Different Molecular Weights and Cytotoxicity Difference of Polysaccharides Regulated CaOx to HK-2 Cells

Objective

This study aimed to investigate the growth of calcium oxalate (CaOx) crystals regulated by Auricularia auricular polysaccharides (AAPs) with different viscosity-average molecular weights (Mv), the toxicity of AAP-regulated CaOx crystals toward HK-2 cells, and the prevention and treatment capabilities of AAPs for CaOx stones.

Methods

The scavenging capability and reducing capacity of four kinds of AAPs (Mv of 31.52, 11.82, 5.86, and 3.34 kDa) on hydroxyl, ABTS, and DPPH free radicals and their capability to chelate divalent iron ions were detected. AAP-regulated CaOx crystals were evaluated by using zeta potential, thermogravimetric analysis, X-ray diffraction, and scanning electron microscopy. The cytotoxicity of AAP-regulated crystals was evaluated through examination of cell viability, cell death, malondialdehyde (MDA) content, and cell surface hyaluronic acid (HA) expression.

Results

The in vitro antioxidant activities of the four AAPs were observed in the following order: AAP0 < AAP1 < AAP2 < AAP3. Thus, AAP3, which had the smallest Mv, had the strongest antioxidant activity. AAPs can inhibit the growth of CaOx monohydrate (COM), induce the formation of CaOx dihydrate (COD), and reduce the degree of crystal aggregation, with AAP3 exhibiting the strongest capability. Cell experiments showed the lowest cytotoxicity in AAP3-regulated CaOx crystals, along with the lowest MDA content, HA expression, and cell mortality. In addition, COD presented less cytotoxicity than COM. Meanwhile, the cytotoxicity of blunt crystals was less than that of sharp crystals.

Conclusion

AAPs, particularly AAP3, showed an excellent antioxidative capability in vitro, and AAP3-regulated CaOx crystals presented minimal cytotoxicity.

The upregulation of lamin A/C as a compensatory mechanism during tight junction disruption in renal tubular cells mediated by calcium oxalate crystals

Calcium oxalate monohydrate (COM), the most important crystal causing kidney stone disease, upregulates lamin A/C but downregulates zonula occludens-1 (ZO-1) in renal tubular cells. While roles for F-actin and α-tubulin and their association with ZO-1 are known to regulate COM-mediated tight junction (TJ) disruption, roles of lamin A/C and its interplay with ZO-1 in COM kidney stone model remain unclear and are thus the objectives of this study. Lamin A/C was knocked down in MDCK cells by silencing RNA specific for LMNA (siLMNA). Both wild-type (WT) and siLMNA cells were treated with COM for 48-h compared with the untreated (control) cells. Western blotting and immunofluorescence staining revealed upregulated lamin A/C and downregulated ZO-1 in the COM-treated WT cells. siLMNA successfully reduced lamin A/C expression in both control and COM-treated cells. Nonetheless, siLMNA did not reverse the effect of COM on the decreases in ZO-1 and transepithelial resistance, but further reduced their levels in both control and COM-treated cells. Protein-protein interaction analysis demonstrated that two cytoskeletal proteins (actin and tubulin) served as the linkers to connect lamin A/C with ZO-1 and occludin (both of which are the TJ proteins). Altogether, these data implicate that lamin A/C and ZO-1 are indirectly associated to control TJ function, and ZO-1 expression is regulated by lamin A/C. Moreover, COM-induced upregulation of lamin A/C most likely serves as a compensatory mechanism to cope with the downregulation of ZO-1 during COM-mediated TJ disruption.

Corn Silk Polysaccharides with Different Carboxyl Contents Reduce the Oxidative Damage of Renal Epithelial Cells by Inhibiting Endocytosis of Nano-calcium Oxalate Crystals

OBJECTIVE

Renal epithelial cell injury and cell-crystal interaction are closely related to kidney stone formation.

METHODS

This study aims to explore the inhibition of endocytosis of nano-sized calcium oxalate monohydrate (nano-COM) crystals and the cell protection of corn silk polysaccharides (CCSPs) with different carboxyl contents (3.92, 7.75, 12.90, and 16.38%). The nano-COM crystals protected or unprotected by CCSPs were co-cultured with human renal proximal tubular epithelial cells (HK-2), and then the changes in the endocytosis of nano-COM and cell biochemical indicators were detected.

RESULTS

CCSPs could inhibit the endocytosis of nano-COM by HK-2 cells and reduce the accumulation of nano-COM in the cells. Under the protection of CCSPs, cell morphology is restored, intracellular superoxide dismutase levels are increased, lipid peroxidation product malondialdehyde release is decreased, and mitochondrial membrane potential and lysosomal integrity are increased. The release of Ca²⁺ ions in the cell, the level of cell autophagy, and the rate of cell apoptosis and necrosis are also reduced. CCSPs with higher carboxyl content have better cell protection abilities.

CONCLUSION

CCSPs could inhibit the endocytosis of nano-COM crystals and reduce cell oxidative damage. CCSP3, with the highest carboxyl content, shows the best biological activity.

Cell Protection and Crystal Endocytosis Inhibition by Sulfated Laminaria Polysaccharides Against Nano-COM-Induced Oxidative Damage in Renal Epithelial Cells

Background: The damage to renal tubular epithelial cells is closely related to the formation of kidney stones. At present, research on drugs that can protect cells from damage remains limited. Methods: This study aims to explore the protective effects of four different sulfate groups (-OSO₃ ^-) of Laminaria polysaccharides (SLPs) on human kidney proximal tubular epithelial (HK-2) cells and determine the difference in the endocytosis of nano-sized calcium oxalate monohydrate (COM) crystals before and after protection. COM with a size of 230 ± 80 nm was used to damage HK-2 cells to establish a damage model. The protection capability of SLPs (LP0, SLP1, SLP2, and SLP3) with -OSO₃ ^- contents of 0.73, 15, 23, and 31%, respectively, against COM crystal damage and the effect of SLPs on the endocytosis of COM crystals were studied. Results: Compared with that of the SLP-unprotected COM-injured group, the cell viability of the SLP-protected group was improved, healing capability was enhanced, cell morphology was restored, production of reactive oxygen species was reduced, mitochondrial membrane potential and lysosome integrity were increased, intracellular Ca²⁺ level and autophagy were decreased, cell mortality was reduced, and internalized COM crystals were lessened. The capability of SLPs to protect cells from damage and inhibit the endocytosis of crystals in cells enhanced with an increase in the -OSO₃ ^- content of SLPs. Conclusions: SLPs with a high -OSO₃ ^- content may become a potential green drug for preventing the formation of kidney stones.

Carboxymethylation of Corn Silk Polysaccharide and Its Inhibition on Adhesion of Nanocalcium Oxalate Crystals to Damaged Renal Epithelial Cells

The purpose of this study was to explore the repair effect of carboxymethyl-modified corn silk polysaccharide (CSP) on oxidatively damaged renal epithelial cells and the difference in adhesion between cells and calcium oxalate crystals. The CSP was degraded and modified through carboxymethylation. An oxidatively damaged cell model was constructed by oxalate damage to human kidney proximal tubular epithelial (HK-2) cells. Then, the damaged cells were repaired by modified polysaccharides, and the changes in biochemical indexes and adhesion ability between cells and crystals before and after repair were detected. Four modified polysaccharides with carboxyl group (-COOH) contents of 3.92% (CSP0), 7.75% (CCSP1), 12.90% (CCSP2), and 16.38% (CCSP3) were obtained. Compared with CSP0, CCSPs had stronger antioxidant activity, could repair damaged HK-2 cells, and could reduce phosphorylated serine eversion on the cell membrane, the expression of osteopontin (OPN) and Annexin A1, and crystal adhesion. However, its effect on the expression of hyaluronic acid synthase was not substantial. The carboxymethyl modification of the CSP can improve its ability to repair cells and inhibit crystal adhesion and aggregation. A high carboxymethylation degree results in strong polysaccharide activity. CCSPs are expected to reduce the risk of kidney stone formation and recurrence.

Degraded Porphyra yezoensis polysaccharide protects HK-2 cells and reduces nano-COM crystal toxicity, adhesion and endocytosis

We studied the protection of degraded Porphyra yezoensis polysaccharide (PYP) on human proximal tubular epithelial cells (HK-2) from cytotoxicity of nano-calcium oxalate monohydrate (COM) crystal, and the regulation of adhesion and endocytosis of the COM crystal. Four degraded fractions, namely, PYP1, PYP2, PYP3, and PYP4, were successfully obtained, with molecular weights (Mws) of 576.2, 49.5, 12.6, and 4.02 kDa, respectively. PYP protection reduced the crystal toxicity, prevented the destruction of cell morphology and cytoskeleton, inhibited the production of reactive oxygen species and the decline of lysosomal integrity, and reduced the expression of osteopontin and transmembrane protein (CD44). PYPi inhibited the adhesion and endocytosis of HK-2 cells by nano-COM. Endocytic COM crystals were accumulated in the lysosomes. With decreasing molecular weight, the ability of PYP to reduce cell damage and inhibit cell adhesion and endocytosis increased. PYP4, which has the smallest molecular weight, weaker intramolecular hydrogen bonds and more reducing groups, showed the best biological activity. PYPi can reduce the oxidative damage of the crystal to the cell, inhibit the adhesion and endocytosis of the crystal, and reduce the risk of kidney stone formation. Therefore, PYP, especially PYP4, has potential for use as a green drug to inhibit the formation and recurrence of calcium oxalate stones.

Induction of retinopathy by fibrillar oxalate assemblies

The formation of metabolite fibrillar assemblies represents a paradigm shift in the study of human metabolic disorders. Yet, direct clinical relevance has been attributed only to metabolite crystals. A notable example for metabolite crystallization is calcium oxalate crystals observed in various diseases, including primary hyperoxaluria. We unexpectedly observed retinal damage among young hyperoxaluria patients in the absence of crystals. Exploring the possible formation of alternative supramolecular organizations and their biological role, here we show that oxalate can form ordered fibrils with no associated calcium. These fibrils inflict intense retinal cytotoxicity in cultured cells. A rat model injected with oxalate fibrils recaptures patterns of retinal dysfunction observed in patients. Antibodies purified from hyperoxaluria patient sera recognize oxalate fibrils regardless of the presence of calcium. These findings highlight a new molecular basis for oxalate-associated disease, and to our knowledge provide the first direct clinical indication for the pathogenic role of metabolite fibrillar assemblies.

Comparison of the adhesion and endocytosis of calcium oxalate dihydrate to HK-2 cells before and after repair by Astragalus polysaccharide

This work investigated the effects of repairing injured renal proximal tubular epithelial (HK-2) cells by using three Astragalus polysaccharides (APS) with different molecular weights and the adhesion and endocytosis of HK-2 cells to the calcium oxalate dihydrate (COD) nanocrystals before and after repair to develop new products that can protect against kidney stones. HK-2 cells cultured in vitro were injured with 2.6 mmol/L oxalic acid to establish a damaged cell model. Three kinds of APS (APS0, APS1, and APS2 with molecular weights of 11.03, 4.72, and 2.60 kDa, respectively) were used to repair the damaged cells. The changes in the adhesion and endocytosis of 100 nm COD crystals to cells before and after the repair were detected. After the repair of HK-2 cells by the APS, the speed of wound healing of the damaged HK-2 cells increased, and the amount of phosphatidylserine (PS) ectropion decreased. In addition, the proportion of cells with adhered COD crystals decreased, whereas the proportion of cells with internalized crystals increased. As a result of the repair activity, APS can inhibit the adhesion and promote the endocytosis of COD nanocrystals to damaged cells. APS1, which had a moderate molecular weight, displayed the strongest abilities to repair the cells, inhibit adhesion, and promote endocytosis. Thus, APS, particularly APS1, may serve as potential green drugs for preventing kidney stones.

The Long Pentraxin PTX3 Is an Endogenous Inhibitor of Hyperoxaluria-Related Nephrocalcinosis and Chronic Kidney Disease

The long pentraxin 3 (PTX3) exerts a variety of regulatory functions in acute and chronic tissue inflammation. In particular, PTX3 acts as an opsonin for a variety of pathogens and endogenous particles. We hypothesized that PTX3 would exhibit opsonin-like functions toward calcium oxalate crystals, too, and inhibit crystal growth. This process is fundamental in kidney stone disease as well as in hyperoxaluria-related nephrocalcinosis, the paradigmatic cause of chronic kidney disease (CKD) in children with primary hyperoxaluria type I due to genetic defects in oxalate metabolism. Direct effects of PTX3 on calcium oxalate crystals were investigated in chemico by adding recombinant PTX3 to supersaturated calcium and oxalate solutions. PTX3, but not isomolar concentrations of albumin, dose-dependently inhibited crystal growth. In vivo, the PTX3 protein was undetectable in tubular epithelial cells and urine of wild-type mice under physiological conditions. However, its levels increased within 3 weeks of feeding an oxalate-rich diet, an exposure inducing hyperoxaluria-related nephrocalcinosis and CKD in selected mouse strains (male and female C57BL/6N and male Balb/c mice) but not in others (male and female 129SV and CD-1, male and female Balb/c mice). Genetic ablation of ptx3 in nephrocalcinosis un-susceptible B6;129 mice was sufficient to raise the oxalate nephropathy phenotype observed in susceptible strains. We conclude that PTX3 is an endogenous inhibitor of calcium oxalate crystal growth. This mechanism limits hyperoxaluria-related nephrocalcinosis, e.g., in primary or secondary hyperoxaluria, and potentially also in the more prevalent kidney stone disease.

Kidney Stone Disease: An Update on Current Concepts

Kidney stone disease is a crystal concretion formed usually within the kidneys. It is an increasing urological disorder of human health, affecting about 12% of the world population. It has been associated with an increased risk of end-stage renal failure. The etiology of kidney stone is multifactorial. The most common type of kidney stone is calcium oxalate formed at Randall's plaque on the renal papillary surfaces. The mechanism of stone formation is a complex process which results from several physicochemical events including supersaturation, nucleation, growth, aggregation, and retention of urinary stone constituents within tubular cells. These steps are modulated by an imbalance between factors that promote or inhibit urinary crystallization. It is also noted that cellular injury promotes retention of particles on renal papillary surfaces. The exposure of renal epithelial cells to oxalate causes a signaling cascade which leads to apoptosis by p38 mitogen-activated protein kinase pathways. Currently, there is no satisfactory drug to cure and/or prevent kidney stone recurrences. Thus, further understanding of the pathophysiology of kidney stone formation is a research area to manage urolithiasis using new drugs. Therefore, this review has intended to provide a compiled up-to-date information on kidney stone etiology, pathogenesis, and prevention approaches.

Systematic evaluation for effects of urine pH on calcium oxalate crystallization, crystal-cell adhesion and internalization into renal tubular cells

Urine pH has been thought to be an important factor that can modulate kidney stone formation. Nevertheless, there was no systematic evaluation of such pH effect. Our present study thus addressed effects of differential urine pH (4.0–8.0) on calcium oxalate (CaOx) crystallization, crystal-cell adhesion, crystal internalization into renal tubular cells, and binding of apical membrane proteins to the crystals. Microscopic examination revealed that CaOx monohydrate (COM), the pathogenic form, was crystallized with greatest size, number and total mass at pH 4.0 and least crystallized at pH 8.0, whereas COD was crystallized with the vice versa order. Fourier-transform infrared (FT-IR) spectroscopy confirmed such morphological study. Crystal-cell adhesion assay showed the greatest degree of crystal-cell adhesion at the most acidic pH and least at the most basic pH. Crystal internalization assay using fluorescein isothiocyanate (FITC)-labelled crystals and flow cytometry demonstrated that crystal internalization into renal tubular cells was maximal at the neutral pH (7.0). Finally, there were no significant differences in binding capacity of the crystals to apical membrane proteins at different pH. We concluded that the acidic urine pH may promote CaOx kidney stone formation, whereas the basic urine pH (i.e. by alkalinization) may help to prevent CaOx kidney stone disease.

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.

Hyperoxaluria Requires TNF Receptors to Initiate Crystal Adhesion and Kidney Stone Disease

Intrarenal crystals trigger inflammation and renal cell necroptosis, processes that involve TNF receptor (TNFR) signaling. Here, we tested the hypothesis that TNFRs also have a direct role in tubular crystal deposition and progression of hyperoxaluria-related CKD. Immunohistochemical analysis revealed upregulated tubular expression of TNFR1 and TNFR2 in human and murine kidneys with calcium oxalate (CaOx) nephrocalcinosis-related CKD compared with controls. Western blot and mRNA expression analyses in mice yielded consistent data. When fed an oxalate-rich diet, wild-type mice developed progressive CKD, whereas Tnfr1-, Tnfr2-, and Tnfr1/2-deficient mice did not. Despite identical levels of hyperoxaluria, Tnfr1-, Tnfr2-, and Tnfr1/2-deficient mice also lacked the intrarenal CaOx deposition and tubular damage observed in wild-type mice. Inhibition of TNFR signaling prevented the induced expression of the crystal adhesion molecules, CD44 and annexin II, in tubular epithelial cells in vitro and in vivo, and treatment with the small molecule TNFR inhibitor R-7050 partially protected hyperoxaluric mice from nephrocalcinosis and CKD. We conclude that TNFR signaling is essential for CaOx crystal adhesion to the luminal membrane of renal tubules as a fundamental initiating mechanism of oxalate nephropathy. Furthermore, therapeutic blockade of TNFR might delay progressive forms of nephrocalcinosis in oxalate nephropathy, such as primary hyperoxaluria.

Calcium oxalate monohydrate crystals internalized into renal tubular cells are degraded and dissolved by endolysosomes

Interaction between calcium oxalate crystals and renal tubular cells has been recognized as one of the key mechanisms for kidney stone formation. While crystal adhesion and internalization have been extensively investigated, subsequent phenomena (i.e. crystal degradation and dissolution) remained poorly understood. To explore these mechanisms, we used fluorescein isothiocyanate (FITC)-labelled calcium oxalate monohydrate (COM) crystals (1000 μg/ml of crystals/culture medium) to confirm crystal internalization into MDCK (Type II) renal tubular cells after exposure to the crystals for 1 h and to trace the internalized crystals. Crystal size, intracellular and extracellular fluorescence levels were measured using a spectrofluorometer for up to 48 h after crystal internalization. Moreover, markers for early endosome (Rab5), late endosome (Rab7) and lysosome (LAMP-2) were examined by laser-scanning confocal microscopy. Fluorescence imaging and flow cytometry confirmed that FITC-labelled COM crystals were internalized into MDCK cells (14.83 ± 0.85%). The data also revealed a reduction of crystal size in a time-dependent manner. In concordance, intracellular and extracellular fluorescence levels were decreased and increased, respectively, indicating crystal degradation/dissolution inside the cells and the degraded products were eliminated extracellularly. Moreover, Rab5 and Rab7 were both up-regulated and were also associated with the up-regulated LAMP-2 to form large endolysosomes in the COM-treated cells at 16-h after crystal internalization. We demonstrate herein, for the first time, that COM crystals could be degraded/dissolved by endolysosomes inside renal tubular cells. These findings will be helpful to better understand the crystal fate and protective mechanism against kidney stone formation.

Genetic modulation of nephrocalcinosis in mouse models of ectopic mineralization: the Abcc6(tm1Jfk) and Enpp1(asj) mutant mice

Ectopic mineralization of renal tissues in nephrocalcinosis is a complex, multifactorial process. The purpose of this study was to examine the role of genetic modulation and the role of diet in nephrocalcinosis using two established mouse models of ectopic mineralization, Abcc6(tm1Jfk) and Enpp1(asj) mice, which serve as models for pseudoxanthoma elasticum and generalized arterial calcification of infancy, two heritable disorders, respectively. These mutant mice, when on standard rodent diet, develop nephrocalcinosis only at a very late age. In contrast, when placed on an 'acceleration diet' composed of increased phosphate and reduced magnesium content, they showed extensive mineralization of the kidneys affecting primarily the medullary tubules as well as arcuate and renal arteries, as examined by histopathology and quantitated by chemical assay for calcium. Mineralization could also be detected noninvasively by micro computed tomography. Whereas the heterozygous mice did not develop nephrocalcinosis, compound heterozygous mice carrying both mutant alleles, Abcc6(tm1Jfk/+) and Enpp1(+/asj), developed ectopic mineralization similar to that noted in homozygous mice for either gene, indicating that deletion of one Abcc6 allele along with Enpp1 haploinsufficiency resulted in renal mineralization. Thus, synergistic genetic defects in the complex mineralization/antimineralization network can profoundly modulate the degree of ectopic mineralization in nephrocalcinosis.

Update on oxalate crystal disease

Oxalate arthropathy is a rare cause of arthritis characterized by deposition of calcium oxalate crystals within synovial fluid. This condition typically occurs in patients with underlying primary or secondary hyperoxaluria. Primary hyperoxaluria constitutes a group of genetic disorders resulting in endogenous overproduction of oxalate, whereas secondary hyperoxaluria results from gastrointestinal disorders associated with fat malabsorption and increased absorption of dietary oxalate. In both conditions, oxalate crystals can deposit in the kidney leading to renal failure. Since oxalate is primarily renally eliminated, it accumulates throughout the body in renal failure, a state termed oxalosis. Affected organs can include bones, joints, heart, eyes, and skin. Since patients can present with renal failure and oxalosis before the underlying diagnosis of hyperoxaluria has been made, it is important to consider hyperoxaluria in patients who present with unexplained soft tissue crystal deposition. The best treatment of oxalosis is prevention. If patients present with advanced disease, treatment of oxalate arthritis consists of symptom management and control of the underlying disease process.

Nephrolithiasis: molecular mechanism of renal stone formation and the critical role played by modulators

Urinary stone disease is an ailment that has afflicted human kind for many centuries. Nephrolithiasis is a significant clinical problem in everyday practice with a subsequent burden for the health system. Nephrolithiasis remains a chronic disease and our fundamental understanding of the pathogenesis of stones as well as their prevention and cure still remains rudimentary. Regardless of the fact that supersaturation of stone-forming salts in urine is essential, abundance of these salts by itself will not always result in stone formation. The pathogenesis of calcium oxalate stone formation is a multistep process and essentially includes nucleation, crystal growth, crystal aggregation, and crystal retention. Various substances in the body have an effect on one or more of the above stone-forming processes, thereby influencing a person's ability to promote or prevent stone formation. Promoters facilitate the stone formation while inhibitors prevent it. Besides low urine volume and low urine pH, high calcium, sodium, oxalate and urate are also known to promote calcium oxalate stone formation. Many inorganic (citrate, magnesium) and organic substances (nephrocalcin, urinary prothrombin fragment-1, osteopontin) are known to inhibit stone formation. This review presents a comprehensive account of the mechanism of renal stone formation and the role of inhibitors/promoters in calcium oxalate crystallisation.

Pathomechanisms of nephrolithiasis

Lithiasis continues to be an important factor in chronic renal disease, since it leads to chronic tubulo-interstitial nephritis, which is estimated to be involved in 15- 30% of cases of end stage chronic renal insufficiency. It is believed that in order for a stone to be formed, a solid phase needs to be first produced from microcrystals (the nucleus), which are formed from salts (and other substances) that are found dissolved in the urine (nucleosis of crystals). Afterwards, the crystals that constitute the core increase in size and link up with each other (incorporation). The main physiochemical factors that participate in the creation of the nucleus are the hypersaturation of urine, the lack of inhibitors of nucleosis and probably the organic substrate. In order for the increase in size and the incorporation of crystals to take place, hypersaturation, the lack of inhibitors, the organic substrate and the epitaxis, during which crystals of a substance are attached to the surface of other crystals of a different chemical structure (e.g. crystals of oxalic calcium onto crystals of uric acid) are needed. Various molecules have been found in urine, which modify to an important degree the adherence of crystals to the surface of epithelial cells. It also seems very likely that certain reactions of renal epithelial cells that follow the uptake of calcium oxalate monohydrate (COM) crystals are due to oxalate ions, which are released during the process of deconstruction of the intracellular crystals. From here, the crystals migrate in the median tissue, where an inflammatory reaction takes place and finally the crystals are destroyed. Macrophages gather in the crystals of the median tissue. The osteopontin which is related to the crystals acts as a chemotactic factor for the macrophages and therefore is perhaps involved in this process too. The uptake of crystals appears to be subjected to regulating mechanisms, as molecules which regulate the endocytosis of COM crystals, a process that is related to changes in the special components of the cytoskeleton, have been observed. In conclusion, the processes of adherence and of endocytosis promote the detention of crystals in the nephron, whilst intracellular deconstruction is an important factor of defence against the deposition of calcium in the kidney.

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.

Oral Reference Dose for ethylene glycol based on oxalate crystal-induced renal tubule degeneration as the critical effect

Several risk assessments have been conducted for ethylene glycol (EG). These assessments identified the kidney as the primary target organ for chronic effects. None of these assessments have incorporated the robust database of species-specific toxicokinetic and toxicodynamic studies with EG and its metabolites in defining uncertainty factors used in reference value derivation. Pertinent in vitro and in vivo studies related to one of these metabolites, calcium oxalate, and its role in crystal-induced nephropathy are summarized, and the weight of evidence to establish the mode of action for renal toxicity is reviewed. Previous risk assessments were based on chronic rat studies using a strain of rat that was later determined to be less sensitive to the toxic effects of EG. A recently published 12-month rat study using the more sensitive strain (Wistar) was selected to determine the point of departure for a new risk assessment. This approach incorporated toxicokinetic and toxicodynamic data and used Benchmark Dose methods to calculate a Human Equivalent Dose. Uncertainty factors were chosen, depending on the quality of the studies available, the extent of the database, and scientific judgment. The Reference Dose for long-term repeat oral exposure to EG was determined to be 15 mg/kg bw/d.

Aluminum citrate prevents renal injury from calcium oxalate crystal deposition

Calcium oxalate monohydrate crystals are responsible for the kidney injury associated with exposure to ethylene glycol or severe hyperoxaluria. Current treatment strategies target the formation of calcium oxalate but not its interaction with kidney tissue. Because aluminum citrate blocks calcium oxalate binding and toxicity in human kidney cells, it may provide a different therapeutic approach to calcium oxalate-induced injury. Here, we tested the effects of aluminum citrate and sodium citrate in a Wistar rat model of acute high-dose ethylene glycol exposure. Aluminum citrate, but not sodium citrate, attenuated increases in urea nitrogen, creatinine, and the ratio of kidney to body weight in ethylene glycol-treated rats. Compared with ethylene glycol alone, the addition of aluminum citrate significantly increased the urinary excretion of both crystalline calcium and crystalline oxalate and decreased the deposition of crystals in renal tissue. In vitro, aluminum citrate interacted directly with oxalate crystals to inhibit their uptake by proximal tubule cells. These results suggest that treating with aluminum citrate attenuates renal injury in rats with severe ethylene glycol toxicity, apparently by inhibiting calcium oxalate's interaction with, and retention by, the kidney epithelium.

Interaction between submicron COD crystals and renal epithelial cells

Objectives

This study aims to investigate the adhesion characteristics between submicron calcium oxalate dihydrate (COD) with a size of 150 ± 50 nm and African green monkey kidney epithelial cells (Vero cells) before and after damage, and to discuss the mechanism of kidney stone formation.

Methods

Vero cells were oxidatively injured by hydrogen peroxide to establish a model of injured cells. Scanning electron microscopy was used to observe Vero–COD adhesion. Inductively coupled plasma emission spectrometry was used to quantitatively measure the amount of adhered COD microcrystals. Nanoparticle size analyzer and laser scanning confocal microscopy were performed to measure the change in the zeta potential on the Vero cell surface and the change in osteopontin expression during the adhesion process, respectively. The level of cell injury was evaluated by measuring the changes in malonaldehyde content, and cell viability during the adhesion process.

Results

The adhesion capacity of Vero cells in the injury group to COD microcrystals was obviously stronger than that of Vero cells in the control group. After adhesion to COD, cell viability dropped, both malonaldehyde content and cell surface zeta potential increased, and the fluorescence intensity of osteopontin decreased because the osteopontin molecules were successfully covered by COD. Submicron COD further damaged the cells during the adhesion process, especially for Vero cells in the control group, leading to an elevated amount of attached microcrystals.

Conclusion

Submicron COD can further damage injured Vero cells during the adhesion process. The amount of attached microcrystals is proportional to the degree of cell damage. The increased amount of microcrystals that adhered to the injured epithelial cells plays an important role in the formation of early-stage kidney stones.

The effect of intracrystalline and surface-bound osteopontin on the degradation and dissolution of calcium oxalate dihydrate crystals in MDCKII cells

In vivo, urinary crystals are associated with proteins located within the mineral bulk as well as upon their surfaces. Proteins incarcerated within the mineral phase of retained crystals could act as a defence against urolithiasis by rendering them more vulnerable to destruction by intracellular and interstitial proteases. The aim of this study was to examine the effects of intracrystalline and surface-bound osteopontin (OPN) on the degradation and dissolution of urinary calcium oxalate dihydrate (COD) crystals in cultured Madin Darby canine kidney (MDCK) cells. [(14)C]-oxalate-labelled COD crystals with intracrystalline (IC), surface-bound (SB) and IC + SB OPN, were generated from ultrafiltered (UF) urine containing 0, 1 and 5 mg/L human milk OPN and incubated with MDCKII cells, using UF urine as the binding medium. Crystal size and degradation were assessed using field emission scanning electron microscopy (FESEM) and dissolution was quantified by the release of radioactivity into the culture medium. Crystal size decreased directly with OPN concentration. FESEM examination indicated that crystals covered with SB OPN were more resistant to cellular degradation than those containing IC OPN, whose degree of disruption appeared to be related to OPN concentration. Whether bound to the crystal surface or incarcerated within the mineral interior, OPN inhibited crystal dissolution in direct proportion to its concentration. Under physiological conditions OPN may routinely protect against stone formation by inhibiting the growth of COD crystals, which would encourage their excretion in urine and thereby perhaps partly explain why, compared with calcium oxalate monohydrate crystals, COD crystals are more prevalent in urine, but less common in kidney stones.

Recurrent urinary tract infections in an infant with antenatal Bartter syndrome

BACKGROUND

antenatal variant of Bartter syndrome is characterized by a history of polyhydramnios, premature birth, metabolic alkalosis, hypokalemia, polyuria and renal salt wasting. In this report we present a premature female baby with antenatal Barter syndrome who had three episodes of urinary tract infection (UTI), without evidence for congenital anomaly of the kidneys or urinary tract.

METHODS

antenatal Bartter syndrome was diagnosed according to the standard criteria. Ultrasound scan and voiding cystourethrography were performed to exclude congenital anomaly of the kidneys and urinary tract.

RESULTS

the baby presented with early hyperkalemia and acidosis. The typical biochemical features of the Bartter syndrome were observed in the second month. Despite appropriate treatment she had persistent hypercalciuria. The clinical course was complicated with recurrent episodes of febrile UTIs. Urinary tract system imaging did not demonstrate congenital anomalies. She finally died of severe dehydration, acidosis and renal failure.

CONCLUSION

since no congenital anomaly of the kidneys or urinary tract was demonstrated in our patient, we believe that severe, persistent hypercalciuria is the most important risk factor for development of recurrent UTIs.

Proteome changes in human monocytes upon interaction with calcium oxalate monohydrate crystals

Monocytic infiltration in renal interstitium is commonly found surrounding the site of calcium oxalate (CaOx) crystal deposition in the kidney. Monocytes are supposed to eliminate the deposited crystals. However, effects of CaOx crystals on the infiltrating monocytes remain unknown. Therefore, this study investigated the altered cellular proteome of human monocytes in response to interaction with CaOx monohydrate (COM) crystals. After 24-h culture with or without 100 microg/mL COM crystals, U937 cells were harvested and subjected to 2-DE analysis with Deep Purple fluorescence staining (n = 5 gels/group; each was derived from independent culture). Spot matching, quantitative intensity analysis, and statistics revealed 22 differentially expressed proteins (9 up-regulated and 13 down-regulated proteins), which were successfully identified by Q-TOF MS and MS/MS analyses, including those involved in cell cycle, cellular structure, carbohydrate metabolism, lipid metabolism, mRNA processing, and protein synthesis, stabilization, and degradation. Randomly selected changes [up-regulated ALG-2 interacting protein 1 (Alix), elongation factor-2 (EF-2), and down-regulated beta-actin] were confirmed by Western blot analysis. Our data may help to understand how monocytes interact with COM crystals. These processes are proposed to cause subsequent inflammatory response in kidney stone disease through oxidative stress pathway(s).

Hemolysis of human erythrocytes induced by melamine-cyanurate complex

Melamine is a widely-used chemical in industries. In recent years, melamine has been found to be involved in outbreaks of renal injury in infants and animals. Pathological studies indicated that the melamine-induced acute renal failure was related to the concurrence of melamine and other triazine analogs such as cyanuric acid. In the present study, human erythrocytes were used as an in vitro model to explore the cytotoxicity of melamine and its complex with cyanuric acid. The results demonstrated that mixing melamine and cyanuric acid resulted in the formation of insoluble particles and that the insoluble melamine-cyanurate complex induced membrane damages of human erythrocytes. The membrane damages included hemolysis, K(+) leakage, alterations in cell shape and membrane fragility, and inhibition of enzymatic activity. By contrast, either melamine or cyanuric acid alone had no effect on erythrocyte membranes. The results of this study may provide a fresh insight into the melamine toxicology.

The tubular epithelium in the initiation and course of intratubular nephrocalcinosis

Intratubular nephrocalcinosis is defined as the histological observation of calcium oxalate and/or calcium phosphate deposits retained within the lumen of the renal tubules. As the tubular epithelium is the primary interaction partner of crystals formed in the tubular fluid, the role of the epithelial cells in nephrocalcinosis has been investigated intensively. This review summarizes our current understanding on how the tubular epithelium mechanistically appears to be involved both in the initiation and in the course of nephrocalcinosis, with emphasis on in vivo observations.

The initial and subsequent inflammatory events during calcium oxalate lithiasis

BACKGROUND

Crystallization is believed to be the initiation step of urolithiasis, even though it is unknown where inside the nephron the first crystal nucleation occurs.

METHODS

Direct nucleation of calcium oxalate and subsequent events including crystal retention, cellular damage, endocytosis, and hyaluronan (HA) expression, were tested in a two-compartment culture system with intact human proximal tubular HK-2 cell monolayer.

RESULTS

Calcium oxalate dihydrate (COD) was nucleated and bound onto the apical surface of the HK-2 cells under hypercalciuric and hyperoxaluric conditions. These cells displayed mild cellular damage and internalized some of the adhered crystals within 18h post-COD-exposure, as revealed by electron microscopy. Prolonged incubation in complete medium caused significant damage to disrupt the monolayer integrity. Furthermore, hyaluronan disaccharides were detected in the harvested media, and were associated with HAS-3 mRNA expression.

CONCLUSION

Human proximal cells were able to internalize COD crystals which nucleated directly onto the apical surface, subsequently triggering cellular damage and HAS-3 specific hyaluronan synthesis as an inflammatory response. The proximal tubule cells here demonstrate that it plays an important role in facilitating urolithiasis via endocytosis and creating an inflammatory environment whereby free hyaluronan in tubular fluid can act as crystal-binding molecule at the later segments of distal and collecting tubules.

Renal toxicity of ethylene glycol results from internalization of calcium oxalate crystals by proximal tubule cells

Ethylene glycol exposure can lead to the development of renal failure due to the metabolic formation of calcium oxalate monohydrate (COM) crystals. The renal damage is closely linked to the degree of COM accumulation in the kidney and most likely results from a COM-induced injury to proximal tubule (PT) cells. The present studies have measured the binding and internalization of COM by primary cultures of normal PT cells from humans and from Wistar and Fischer-344 rats in order to examine the roles of these uptake processes in the resulting cytotoxicity. Internalization was determined by incubation of cells with [(14)C]-COM at 37 degrees C, removal of bound COM with an EDTA incubation, followed by solubilization of cells, as well as by transmission electron microscopy of COM-exposed cells. COM crystals were internalized by PT cells in time- and concentration-dependent manners. COM crystals were bound to and internalized by rat cells about five times more than by human cells. Binding and internalization values were similar between PT cells from Wistar and Fischer-344 rats, indicating that a differential uptake of COM does not explain the known strain difference in sensitivity to ethylene glycol renal toxicity. Internalization of COM correlated highly with the degree of cell death, which is greater in rat cells than in human cells. Thus, surface binding and internalization of COM by cells play critical roles in cytotoxicity and explain why rat cells are more sensitive to COM crystals. At the same level of COM accumulation after ethylene glycol exposure or hyperoxaluria in vivo, rats would be more susceptible than humans to COM-induced damage.

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.

Renal tubular epithelial cell injury and oxidative stress induce calcium oxalate crystal formation in mouse kidney

OBJECTIVES

To clarify the role of renal tubular cell (RTC) injury and oxidative stress in the early stage of renal calcium oxalate crystal formation in a mouse model.

METHODS

Daily intra-abdominal injections of glyoxylate (1.35 mmol/kg/day) into 8-week-old mice were carried out over 6 days. Kidneys were extracted before and at 6, 12 and 24 h and 3 and 6 days after glyoxylate injection. Crystal formation was detected using Pizzolato staining and polarized light optical microscopy. Immunohistochemical staining and western blotting of superoxide dismutase, and 4-hydroxynonenal and malondialdehyde were carried out in order to observe oxidative stress and lipid peroxidation, respectively. RTC microstructural damage and crystal nuclei formation were observed using transmission electron microscopy. To ameliorate RTC injury, mice were treated with green tea 1 week before and 1 week after glyoxylate administration. The number of crystals and RTC damage were observed and comparisons were made between glyoxylate-treated mice with and without green tea administration.

RESULTS

Oxidative stress and lipid peroxidation were observed after 6 h. Crystal nuclei containing collapsed mitochondria and fallen microvilli appeared in the renal distal tubular lumen after 24 h. Crystals occupying the tubular lumen were detected on day 3. The number of crystals in mice receiving green tea was significantly lower than in those receiving glyoxylate alone.

CONCLUSIONS

RTC injury, especially mitochondrial damage, and oxidative stress induce the early stage of calcium oxalate crystal formation in mice.

A novel calcium oxalate crystal growth inhibitory protein from the seeds of Dolichos biflorus (L.)

Recurrence and persistent side effects of present day treatment for urolithiasis restrict their use, so an alternate, using phytotherapy is being sought. Dolichos biflorus seeds, which are used as dietary food in India, possess antilithiatic properties. In the present study, a novel dimeric antilithiatic protein (98 kDa) from its seeds was purified based on its ability to inhibit calcium oxalate crystallization in vitro. Amino acid analysis of Dolichos biflorus antilithiatic protein showed abundant acidic amino acids. The mascot search engine presented sequence similarity with a calcium binding protein, calnexin of Pisum sativum from the m/z data obtained by MALDI TOF mass spectrometer. Above results demonstrate the anticalcifying/antilithiatic nature of a novel protein from the seeds of Dolichos biflorus and thus open new vistas for using plant proteins as therapeutic agents to treat urolithiasis.

Calcium oxalate crystals in fetal bovine serum: implications for cell culture, phagocytosis and biomineralization studies in vitro

Cell culture methods and models are key investigative tools for cell and molecular biology studies. Fetal bovine serum (FBS) is commonly used as an additive during cell culture since its constituents promote cell survival, proliferation and differentiation. Here we report that commercially available FBS from different major suppliers consistently contain precipitated, calcium oxalate crystals-either in the monohydrate (COM) or dihydrate (COD) form. Mineral structure and phase identification of the crystals were determined by X-ray diffraction, chemical composition by energy-dispersive X-ray microanalysis, and imaging and measurement of crystal growth steps by atomic force microscopy-all identified and confirmed crystallographic parameters for COM and COD. Proteins binding to the crystals were identified by immunoblotting, revealing the presence of osteopontin and fetuin-A (alpha(2)HS-glycoprotein)--known regulators of crystal growth found in serum. Macrophage cell cultures exposed to calcium oxalate crystals showed internalization of the crystals by phagocytosis in a process that induced disruption of cell-cell adhesion, release of reactive oxygen species and membrane damage, events that may be linked to the release of inflammatory cytokines by these cells into the culture media. In conclusion, calcium oxalate crystals found in commercially available FBS are toxic to cells, and their presence may confound results from in vitro studies where, amongst others, phagocytosis, biomineralization, renal cell and molecular biology, and drug and biomaterial testing are being examined.

Intracrystalline urinary proteins facilitate degradation and dissolution of calcium oxalate crystals in cultured renal cells

We have previously proposed that intracrystalline proteins would increase intracellular proteolytic disruption and dissolution of calcium oxalate (CaOx) crystals. Chauvet MC, Ryall RL. J Struct Biol 151: 12-17, 2005; Fleming DE, van Riessen A, Chauvet MC, Grover PK, Hunter B, van Bronswijk W, Ryall RL. J Bone Miner Res 18: 1282-1291, 2003; Ryall RL, Fleming DE, Doyle IR, Evans NA, Dean CJ, Marshall VR. J Struct Biol 134: 5-14, 2001. The aim of this investigation was to determine the effect of increasing concentrations of intracrystalline protein on the rate of CaOx crystal dissolution in Madin-Darby canine kidney (MDCKII) cells. Crystal matrix extract (CME) was isolated from urinary CaOx monohydrate (COM) crystals. Cold and [14C]oxalate-labeled COM crystals were precipitated from ultrafiltered urine containing 0-5 mg/l CME. Crystal surface area was estimated from scanning electron micrographs, and synchrotron X-ray diffraction was used to determine nonuniform strain and crystallite size. Radiolabeled crystals were added to MDCKII cells and crystal dissolution, expressed as radioactive label released into the medium, was measured. Increasing CME content did not significantly alter crystal surface area. However, nonuniform strain increased and crystallite size decreased in a dose-response manner, both reaching saturation at a CME concentration of 3 mg/ and demonstrating unequivocally the inclusion of increasing quantities of proteins in the crystals. This was confirmed by Western blotting. Crystal dissolution also followed saturation kinetics, increasing proportionally with final CME concentration and reaching a plateau at a concentration of approximately 2 mg/l. These findings were complemented by field emission scanning electron microscopy, which showed that crystal degradation also increased relative to CME concentration. Intracrystalline proteins enhance degradation and dissolution of CaOx crystals and thus may constitute a natural defense against urolithiasis. The findings have significant ramifications in biomineral metabolism and pathogenesis of renal stones.