Altered proteins in MDCK renal tubular cells in response to calcium oxalate dihydrate crystal adhesion: a proteomics approach

The interaction between crystals and renal tubular cells has been proposed to be a crucial event that elicits subsequent cellular responses, leading to kidney stone formation. Nevertheless, the molecular mechanisms of these cellular responses remain poorly understood. We performed a gel-based differential proteomics study to examine cellular responses (as determined by altered protein expression) in Madin-Darby canine kidney (MDCK) cells, which were derived from dog kidney and exhibited distal renal tubule phenotype, during calcium oxalate dihydrate (COD) crystal adhesion. MDCK cells were grown in a medium without or with COD crystals (100 microg/ml) for 48 h. Crystal adhesion was illustrated by phase-contrast and scanning electron microscopy. Flow cytometry using annexin V/propidium iodide double staining showed that the percentage of cell death did not significantly differ between cells with and without COD crystal adhesion. Cellular proteins were then extracted, resolved with two-dimensional gel electrophoresis (2-DE), and visualized by SYPRO Ruby staining ( n = 5 gels per group). Quantitative intensity analysis revealed 11 significantly altered proteins, 10 of which were successfully identified by quadrupole time-of-flight peptide mass fingerprinting (MS) and/or tandem MS (MS/MS), including metabolic enzymes, cellular structural protein, calcium-binding protein, adhesion molecule, protein involved in RNA metabolism, and chaperone. An increase in annexin II was confirmed by 2-D Western blot analysis. These data may lead to better understanding of the cellular responses in distal renal tubular cells during COD crystal adhesion.     

Response of renal tubular cells to differential types and doses of calcium oxalate crystals: Integrative proteome network analysis and functional investigations

We have previously identified changes in the cellular proteome of renal tubular cells induced by low‐dose (100 μg/mL) and high‐dose (1000 μg/mL) calcium oxalate monohydrate (COM) and dihydrate (COD) crystals. However, the functional significance of such expression data remained unclear. In this study, we performed comparative analyses and functional investigations of four proteomic datasets to define potential mechanisms by which renal tubular cells responded to differential crystal types and doses. The data showed that high‐dose induced greater changes than low‐dose, whereas COM induced more changes than COD. Luciferin–luciferase ATP assay revealed increased intracellular ATP level by high‐dose of both COM and COD. OxyBlot assay and Western blotting showed accumulated intracellular oxidized proteins but decreased ubiquitinated proteins by high‐dose of both crystals. Flow cytometric analysis of cell death showed that high‐dose of both crystals, particularly COM, significantly increased cell death. Also, crystal adhesion assay showed higher degree of cell–crystal adhesion in high‐dose and COM when compared to low‐dose and COD, respectively. Finally, pretreatment of epigallocatechin‐3‐gallate revealed a protective effect on COM/COD crystals‐induced oxidative stress and cell‐crystal adhesion. Collectively, these data may provide a better understanding of cellular responses of renal tubular cells to COM/COD crystals in kidney stone disease.     

Characterization of hyaluronic acid interaction with calcium oxalate crystals: implication of crystals faces, pH and citrate

Interaction between hyaluronic acid (HA) present at the surface of tubular epithelial cells and calcium oxalate monohydrate (COM) crystals is thought to play an important role in kidney stone formation. AFM-based force spectroscopy, where HA is covalently attached to AFM-probes, was used to quantify the interaction between HA and the surfaces of COM crystals. The work of adhesion of the HA-probe as well as the rupture force of single HA molecules were quantified in order to understand the molecular regulation of HA binding to COM crystals. Our results reveal that HA adsorbs to the crystal surface in physiological conditions. We also observed increased adhesion when the pH is lowered to a value that increases the risk of kidney stone formation. HA adhesion to the COM crystal surface can be suppressed by citrate, a physiological inhibitor of stone retention currently used in the treatment and prevention of kidney stone formation. Interestingly, we also observed preferential binding of HA onto the [100] face versus the [010] face, suggesting a major contribution of the [100] faces in the crystal retention process at the surface of tubular epithelial cells and the promotion of stone formation. Our results clearly establish a direct role for the glycosaminoglycan HA present at the surface of kidney tubular epithelium in the process of COM crystal retention.     

Large-scale identification of calcium oxalate monohydrate crystal-binding proteins on apical membrane of distal renal tubular epithelial cells

Adhesion of calcium oxalate monohydrate (COM) crystals onto apical surface of renal tubular epithelial cells is a crucial mechanism for crystal retention, leading to kidney stone formation. Various proteins on apical membrane may bind to COM crystals; however, these crystal-binding proteins remained unidentified. The present study therefore aimed to identify COM crystal-binding proteins on apical membrane of distal renal tubular epithelial cells. Madin-Darby Canine Kidney (MDCK) cells were cultivated to be polarized epithelial cells and apical membrane was isolated from these cells using a peeling method established recently. Enrichment and purity of isolated apical membrane were confirmed by Western blot analysis for specific markers of apical (gp135) and basolateral (Na(+)/K(+)-ATPase) membranes. Proteins derived from the isolated apical membrane were then resuspended in artificial urine and incubated with COM crystals. The bound proteins were eluted, resolved by SDS-PAGE, and analyzed by Q-TOF MS and MS/MS, which identified 96 proteins. Among these, expression and localization of annexin II on apical surface of MDCK cells were confirmed by Western blot analysis, immunofluorescence staining, and laser-scanning confocal microscopic examination. Finally, the function of annexin II as the COM crystal-binding protein was successfully validated by COM crystal-binding assay. This large data set offers many opportunities for further investigations of kidney stone disease and may lead to the development of new therapeutic targets.     

Proteomic analysis of calcium oxalate monohydrate crystal-induced cytotoxicity in distal renal tubular cells

Calcium oxalate monohydrate (COM) is the major crystalline component found in kidney stones and its adhesion to renal tubular cells provokes tubular injury, which in turn enhances COM crystal adhesion. However, COM-induced toxic effects in these tubular cells remain largely unknown. We performed a proteomics study to characterize changes in the cellular proteome in MDCK distal renal tubular cells after an exposure to high-dose (1000 microg/mL) COM crystals for 48 h, at which percentage of cell death was significantly increased. Proteins were extracted from MDCK cells cultured with COM-containing or COM-free medium ( n = 5 individual flasks per group), resolved in individual 2-D gels, and stained with SYPRO Ruby fluorescence dye. Quantitative and statistical analyses revealed 53 proteins whose abundance levels were altered (25 were increased, whereas other 28 were decreased) by COM-induced toxicity. Among these, 50 were successfully identified by quadrupole time-of-flight (Q-TOF) mass spectrometry (MS) and/or tandem MS (MS/MS) analyses. The proteomic data were clearly confirmed by 2-D Western blot analysis. While three chaperones (GRP78, Orp150 and Hsp60) were increased, other proteins involved in protein biosynthesis, ATP synthesis, cell cycle regulator, cellular structure, and signal transduction were decreased. These data provide some novel mechanistic insights into the molecular mechanisms of COM crystal-induced tubular toxicity.     

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

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

EGCG decreases binding of calcium oxalate monohydrate crystals onto renal tubular cells via decreased surface expression of alpha-enolase

Crystal retention on tubular cell surface inside renal tubules is considered as the earliest and crucial step for kidney stone formation. Therapeutics targeting this step would cease the development of kidney stone. This study thus aimed to investigate the potential role of epigallocatechin-3-gallate (EGCG), a major antioxidant found in green tea leaves, in the reduction of calcium oxalate monohydrate (COM) crystal binding onto renal tubular cells. Pretreatment of the cells with EGCG for up to 6 h significantly diminished crystal-binding capability in a dose-dependent manner. Indirect immunofluorescence assay without and with cell permeabilization followed by laser-scanning confocal microscopy revealed that EGCG significantly reduced surface expression of alpha-enolase, whereas its intracellular level was increased. Western blot analysis confirmed such contradictory changes in membrane and cytosolic fractions of EGCG-treated cells, whereas the total level in whole cell lysate remained unchanged. Moreover, overexpression of surface alpha-enolase and enhancement of cell–crystal adhesion induced by 10 mM sodium oxalate were completely abolished by EGCG. Taken together, these data indicate that EGCG decreases binding of COM crystals onto renal tubular cells by decreasing the surface expression of alpha-enolase via re-localization or inhibition of alpha-enolase shuttling from the cytoplasm to the plasma membrane. These findings may also explain the effects of EGCG in reducing COM crystal deposition in previous animal models of kidney stone disease. Thus, EGCG may be useful for the prevention of new or recurrent stone formation.     

Protein Network Analysis and Functional Studies of Calcium Oxalate Crystal‐Induced Cytotoxicity in Renal Tubular Epithelial Cells

Our previous expression study has reported a set of proteins with altered levels in renal tubular cells after exposure to calcium oxalate monohydrate (COM) crystals, which are the main composition of kidney stones. However, their functional significance remained largely unknown. In this study, protein network analysis revealed that the significantly altered proteins induced by COM crystals were involved mainly in three main functional networks, including i) cell proliferation and wound healing; ii) oxidative stress and mitochondrial function; and iii) cellular junction complex and integrity. Cell proliferation and wound healing assays showed that the COM‐treated cells had defective proliferation and tissue healing capability, respectively. Oxyblot analysis demonstrated accumulation of the oxidized proteins, whereas intracellular ATP level was significantly increased in the COM‐treated cells. Additionally, level of zonula occludens‐1 (ZO‐1), a tight junction protein, was significantly decreased, consistent with the significant declines in transepithelial resistance (TER) and level of RhoA signaling molecule in the COM‐treated cells. These findings indicate significant perturbations in mitochondrial and oxidative stress axis that cause defective cell proliferation, tissue healing capability, junctional protein complex, and cellular integrity of renal tubular epithelial cells exposed to COM crystals that may play important roles in kidney stone pathogenesis.     

Surface heat shock protein 90 serves as a potential receptor for calcium oxalate crystal on apical membrane of renal tubular epithelial cells

Adhesion of calcium oxalate monohydrate (COM) crystals on renal tubular epithelial cells is a crucial step in kidney stone formation. Finding potential crystal receptors on the apical membrane of the cells may lead to a novel approach to prevent kidney stone disease. Our previous study identified a large number of crystal-binding proteins on the apical membrane of MDCK cells. However, their functional role as potential crystal receptors had not been validated. The present study aimed to address the potential role of heat shock protein 90 (HSP90) as a COM crystal receptor. The apical membrane was isolated from polarized MDCK cells by the peeling method and recovered proteins were incubated with COM crystals. Western blot analysis confirmed the presence of HSP90 in the apical membrane and the crystal-bound fraction. Immunofluorescence staining without permeabilization and laser-scanning confocal microscopy confirmed the surface HSP90 expression on the apical membrane of the intact cells. Crystal adhesion assay showed that blocking surface HSP90 by specific anti-HSP90 antibody and knockdown of HSP90 by small interfering RNA (siRNA) dramatically reduced crystal binding on the apical surface of MDCK cells (by approximately 1/2 and 2/3, respectively). Additionally, crystal internalization assay revealed the presence of HSP90 on the membrane of endocytic vesicle containing the internalized COM crystal. Moreover, pretreatment of MDCK cells with anti-HSP90 antibody significantly reduced crystal internalization (by approximately 1/3). Taken together, our data indicate that HSP90 serves as a potential receptor for COM crystals on the apical membrane of renal tubular epithelial cells and is involved in endocytosis/internalization of the crystals into the cells.     

Renal tubular cell membranes inhibit growth but promote aggregation of calcium oxalate monohydrate crystals

Cell membranes have been proposed to serve as promoters for calcium oxalate monohydrate (COM) kidney stone formation. However, direct evidence to demonstrate the modulatory effects of renal tubular cell membranes on COM crystals does not currently exist. We thus examined the effects of intact MDCK cells and their fragmented membranes on COM crystal growth, aggregation and transformation. COM crystals were generated in the absence (control) or presence of intact MDCK cells or their membrane fragments. Intact MDCK cells and their membrane fragments significantly inhibited COM crystal growth (22.6% and 25.2% decreases in size, respectively) and significantly reduced COM total crystal mass (23.1% and 25.6% decreases, respectively). In contrast, both of them markedly promoted crystal aggregation (1.9-fold and 3.2-fold increases, respectively). Moreover, both intact cells and membrane fragments could transform COM to calcium oxalate dihydrate (COD) crystals. Finally, COM crystal growth inhibitory activities of both membrane forms were successfully confirmed by a spectrophotometric oxalate-depletion assay. Our data provide the first direct evidence to demonstrate the dual modulatory effects of MDCK membranes on COM crystals. Although growth of individual COM crystals was inhibited, their aggregation was promoted. These findings provide additional insights into the mechanisms of COM kidney stone formation.     

Cytotoxicity of alkylating agents in isolated rat kidney proximal tubular and distal tubular cells

Patterns of chemical-induced cytotoxicity in different regions of the nephron were studied with freshly isolated proximal tubular and distal tubular cells from rat kidney. Three model alkylating agents, methyl vinyl ketone, allyl alcohol, and N-dimethylnitrosamine, were used as test chemicals. Methyl vinyl ketone and a metabolite of allyl alcohol, acrolein, are Michael acceptors that bind to cellular protein sulfhydryl groups and GSH. N-Dimethylnitrosamine binds to cellular protein and DNA. Lactate dehydrogenase leakage was used to assess irreversible cellular injury. Distal tubular cells were more susceptible than proximal tubular cells to injury produced by methyl vinyl ketone or allyl alcohol while the two cell populations were equally susceptible to injury produced by N-dimethylnitrosamine. Preincubation of both proximal tubular and distal tubular cells with GSH protected them from methyl vinyl ketone- and allyl alcohol-induced cytotoxicity but had no effect on N-dimethylnitrosamine-induced cytotoxicity. Similarly, incubation of cells with methyl vinyl ketone or allyl alcohol, but not N-dimethylnitrosamine, altered cellular GSH status. As with GSH status, incubation of cells with methyl vinyl ketone or allyl alcohol, but not N-dimethylnitrosamine, caused pronounced inhibitory effects on mitochondrial function, as evidenced by ATP depletion and inhibition of cellular oxygen consumption. These results demonstrate that alkylating agents are cytotoxic to both proximal tubular and distal tubular cells, and that interaction with cellular GSH is a factor determining nephron cell type specificity of injury.     

Macropinocytosis is the Major Mechanism for Endocytosis of Calcium Oxalate Crystals into Renal Tubular Cells

During an initial phase of kidney stone formation, the internalization of calcium oxalate (CaOx) crystals by renal tubular cells has been thought to occur via endocytosis. However, the precise mechanism of CaOx crystal endocytosis remained unclear. In the present study, MDCK renal tubular cells were pretreated with inhibitors specific to individual endocytic pathways, including nystatin (lipid raft/caveolae-mediated), cytochalasin D (actin-dependent or macropinocytosis), and chlorpromazine (CPZ; clathrin-mediated) before exposure to plain (non-labeled), or fluorescence-labeled CaOx monohydrate (COM) crystals. Quantitative analysis by flow cytometry revealed that pretreatment with nystatin and CPZ slightly decreased the crystal internalization, whereas the cytochalasin D pretreatment caused a marked decrease in crystal uptake. Immunofluorescence study and laser-scanning confocal microscopic examination confirmed that the cytochalasin D-pretreated cells had dramatic decrease of the internalized crystals, whereas the total number of crystals interacted with the cells was unchanged (crystals could adhere but were not internalized). These data have demonstrated for the first time that renal tubular cells endocytose COM crystals mainly via macropinocytosis. These novel findings will be useful for further tracking the endocytosed crystals inside the cells during the course of kidney stone formation.     

Ceftriaxone crystallization and its potential role in kidney stone formation

Drug-induced nephrolithiasis contributes to 1–2% of the incidence of renal calculi. We examined whether ceftriaxone at therapeutic doses could be crystallized in the urine and also explored its role in kidney stone formation. Crystallization was induced by mixing ceftriaxone sodium at therapeutic urinary excretion levels (0.5–4.0 mg/ml) to calcium chloride at physiologic urinary concentration (5 mM) in deionized (dI) water or artificial urine (AU). The results showed that ceftriaxone was crystallized with free calcium in dose- and time-dependent manner. These ceftriaxone/calcium crystals showed birefringence property under polarized microscope. Individual crystals had needle-shape (5–100 μm in length), whereas the aggregated form had star-burst and irregular-plate shape (40–200 μm in diameter) (note that the crystal sizes were much larger than renal tubular lumens). Calcium-depletion assay revealed that crystallization required free calcium as a substrate. In AU, crystallization remained although it was partially inhibited when compared to that in dI water. Finally, these crystals could tightly adhere onto renal tubular cell surface. Our data demonstrated that ceftriaxone at therapeutic levels could be crystallized with free calcium in the urine under physiologic condition. We hypothesize that tubular occlusion and crystal–cell adhesion may play important role in pathogenic mechanisms of ceftriaxone-induced nephrolithiasis.     

Protective Cellular Mechanism of Estrogen Against Kidney Stone Formation: A Proteomics Approach and Functional Validation

Females have less incidence/prevalence of kidney stone disease than males. Estrogen thus may serve as the protective factor but with unclear mechanism. This study explores cellular mechanism underlying such stone preventive mechanism of estrogen. Madin darby canine kidney (MDCK) renal tubular cells are incubated with or without 20 nm 17β‐estradiol for 7 days. Comparative proteomics reveals 58 differentially expressed proteins in estrogen‐treated versus control cells that are successfully identified by nanoLC–ESI–Q‐TOF‐MS/MS. Interestingly, these altered proteins are involved mainly in “binding and receptor,” “metabolic process,” and “migration and healing” networks. Functional investigations demonstrate reduction of calcium oxalate (CaOx) crystal‐binding capability of the estrogen‐treated cells consistent with the decreased levels of annexin A1 and α‐enolase (the known CaOx crystal‐binding receptors) on the cell surface. High‐calcium and high‐oxalate challenge initially enhances surface expression of annexin A1 and α‐enolase, respectively, both of which return to their basal levels by estrogen. Additionally, estrogen reduces intracellular ATP level and promotes cell migration and tissue healing. Taken together, estrogen causes changes in cellular proteome of renal tubular cells that lead to decreased surface expression of CaOx crystal receptors, reduced intracellular metabolism, and enhanced cell proliferation and tissue healing, all of which may contribute, at least in part, to stone prevention.     

Cloning and preliminary characterization of a calcium-binding protein closely related to nucleolin on the apical surface of inner medullary collecting duct cells.

Calcium stone crystal attachment to the urinary epithelium plays an essential role in the development of kidney stones by allowing small crystals to be retained in the kidney until they become macroscopic. We among others have described attachment of stone crystals to cultured renal epithelia (Wiessner, J. H., Kleinman, J. G., Blumenthal, S. S., Garancis, J. C., and Mandel, G. S. (1987) J. Urol. 138, 640-643). To isolate protein(s) that may participate in crystal attachment, apical membranes of cultured renal inner medullary collecting duct were biotinylated, the cells were lysed with detergent, the lysate was subjected to hydroxyapatite chromatography, and fractions were incubated with calcium oxalate monohydrate. Electrophoresis of material solubilized from the crystals showed several selectively adsorbed protein bands. A 110-kDa band stained positively for biotin and for glycosides and bound (45)Ca. The amino acid sequence of this band was determined to be that of a protein closely related to rat nucleolin (nucleolin-related protein; NRP). NRP was cloned and sequenced and was 83% homologous with the previously sequenced nucleolar protein nucleolin. Using temperature-induced phase partitioning with Triton X-114, NRP was associated with both the insoluble membrane skeleton pellet and the soluble aqueous phase but not the soluble detergent phase. This association with the membrane skeleton was increased in the presence of calcium. Thus, NRP is associated with the apical membranes of cultured renal tubular cells and is bound to membrane skeletal elements in a calcium-dependent fashion. The physiological role of NRP remains to be determined; however, a pathophysiological role may be that of mediating the attachment to the renal tubular epithelium of calcium stone crystals.     

Epithelial structural changes in the urinary tubules of the kidney of the white rat in the early period of corrosive sublimate nephrosis

Examination of serial semithin (0.5 micron) methacrylate and paraffin (8 micron) sections of the rat kidney 6, 12, 24 and 48 hours after subcutaneous injection of sublimate in a dose of 0.6 mg/100 g bw has demonstrated that the damage to the different parts of the tubular component of nephron is heterogeneous in nature. Both complete and partial necrosis of nephrocyte cytoplasm can be seen in the proximal part of nephron. The distal parts of nephron and collective tubules are characterized by partial necrosis of the apical areas of the cytoplasm. During the period between 12 and 24 hours after sublimate injection, one can observe the onset of destructive processes together with intracellular recovery processes in partially damaged but still viable nephrocytes, which is confirmed by the enlargement of the nucleolar size. The regeneration of the tubular epithelium at the expense of cellular renewal was unmarked 24 hours after sublimate injection.     

Peroxisome proliferator-activated receptor γ modulates renal crystal retention associated with high oxalate concentration by regulating tubular epithelial cellular transdifferentiation

The differentiated phenotype of renal tubular epithelial cell exerts significant effect on crystal adherence. Peroxisome proliferator-activated receptor γ (PPARγ) has been shown to be critical for the regulation of cell transdifferentiation in many physiological and pathological conditions; however, little is known about its role in kidney stone formation. In the current study, we found that temporarily high oxalate concentration significantly decreased PPARγ expression, induced Madin Darby Canine Kidney cell dedifferentiation, and prompted subsequent calcium oxalate (CaOx) crystal adhesion in vitro. Furthermore, cell redifferentiation after the removal of the high oxalate concentration, along with a decreasing affinity to crystals, was an endogenic PPARγ-dependent process. In addition, the PPARγ antagonist GW9662, which can depress total-PPARγ expression and activity, enhanced cell dedifferentiation induced by high oxalate concentration and inhibited cell redifferentiation after removal of the high oxalate concentration. These effects were partially reversed by the PPARγ agonist 15d-PGJ2. Similar results were observed in animals that suffered from temporary hyperoxaluria followed by a recovery period. The active crystal-clearing process occurs through the transphenotypical morphology of renal tubular epithelial cells, reflecting cell transdifferentiation during the recovery period. However, GW9662 delayed cell redifferentiation and increased the secondary temporary crystalluria-induced crystal retention. This detrimental effect was partially reversed by 15d-PGJ2. Taken together, our results revealed that endogenic PPARγ activity plays a vital regulatory role in crystal clearance, subsequent crystal adherence, and CaOx stone formation via manipulating the transdifferentiation of renal tubular epithelial cells.     

Peeping into Human Renal Calcium Oxalate Stone Matrix: Characterization of Novel Proteins Involved in the Intricate Mechanism of Urolithiasis

Background

The increasing number of patients suffering from urolithiasis represents one of the major challenges which nephrologists face worldwide today. For enhancing therapeutic outcomes of this disease, the pathogenic basis for the formation of renal stones is the need of hour. Proteins are found as major component in human renal stone matrix and are considered to have a potential role in crystal–membrane interaction, crystal growth and stone formation but their role in urolithiasis still remains obscure.

Methods

Proteins were isolated from the matrix of human CaOx containing kidney stones. Proteins having MW>3 kDa were subjected to anion exchange chromatography followed by molecular-sieve chromatography. The effect of these purified proteins was tested against CaOx nucleation and growth and on oxalate injured Madin–Darby Canine Kidney (MDCK) renal epithelial cells for their activity. Proteins were identified by Matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF MS) followed by database search with MASCOT server. In silico molecular interaction studies with CaOx crystals were also investigated.

Results

Five proteins were identified from the matrix of calcium oxalate kidney stones by MALDI-TOF MS followed by database search with MASCOT server with the competence to control the stone formation process. Out of which two proteins were promoters, two were inhibitors and one protein had a dual activity of both inhibition and promotion towards CaOx nucleation and growth. Further molecular modelling calculations revealed the mode of interaction of these proteins with CaOx at the molecular level.

Conclusions

We identified and characterized Ethanolamine-phosphate cytidylyltransferase, Ras GTPase-activating-like protein, UDP-glucose:glycoprotein glucosyltransferase 2, RIMS-binding protein 3A, Macrophage-capping protein as novel proteins from the matrix of human calcium oxalate stone which play a critical role in kidney stone formation. Thus, these proteins having potential to modulate calcium oxalate crystallization will throw light on understanding and controlling urolithiasis in humans.     

Unraveling trafficking of the kidney anion exchanger 1 in polarized MDCK epithelial cells.

Kidney anion exchanger 1 (kAE1) is a membrane glycoprotein expressed at the basolateral membrane of type A intercalated cells in the kidney collecting tubule. Mutations occurring in the gene encoding this protein can give rise to distal renal tubular acidosis (dRTA), a disease characterized by an impaired urine acidification, nephrocalcinosis, and renal failure. Here we review how the study of dRTA mutants in polarized epithelial cells has shed light on the cellular mechanisms resulting in this renal disease.