Sex differences in angiotensin II-induced hypertension and kidney injury: role of AT1a receptors in the proximal tubule of the kidney

In the present study, we tested the hypothesis that there are significant sex differences in angiotensin II (Ang II)-induced hypertension and kidney injury using male and female wildtype (WT) and proximal tubule-specific AT1a receptor knockout mice (PT-Agtr1a-/-). Twelve groups (n=8-12 per group) of adult male and female WT and PT-Agtr1a-/- mice were infused with a pressor dose of Ang II via osmotic minipump for 2 weeks (1.5 mg/kg/day, i.p.) and simultaneously treated with or without losartan (20 mg/kg/day, p.o.) to determine the respective roles of AT1a receptors in the proximal tubules versus systemic tissues. Basal systolic, diastolic, and mean arterial pressure were approximately 13 ± 3 mmHg lower (P<0.01), while basal 24-h urinary Na+, K+, and Cl- excretion were significantly higher in both male and female PT-Agtr1a-/- mice than WT controls (P<0.01) without significant sex differences between different strains. Both male and female WT and PT-Agtr1a-/- mice developed hypertension (P<0.01), and the magnitudes of the pressor responses to Ang II were similar between male and female WT and PT-Agtr1a-/- mice (n.s.). Likewise, Ang II-induced hypertension was significantly attenuated in both male and female PT-Agtr1a-/- mice (P<0.01). Furthermore, losartan attenuated the hypertensive responses to Ang II to similar extents in both male and female WT and PT-Agtr1a-/- mice. Finally, Ang II-induced kidney injury was attenuated in PT-Agtr1a-/- mice (P<0.01). In conclusion, the present study demonstrates that deletion of AT1a receptors in the proximal tubules of the kidney attenuates Ang II-induced hypertension and kidney injury without revealing significant sex differences.

Involvement of necroptosis in contrast-induced nephropathy in a rat CKD model

BACKGROUND

The risk of contrast-induced nephropathy (CIN) is high in patients with chronic kidney disease (CKD). However, the mechanism of CIN in CKD is not fully understood. Here, we prepared a clinically relevant model of CIN and examined the role of necroptosis, which potentially cross-talks with autophagy, in CIN.

METHODS

In Sprague-Dawley rats, CKD was induced by subtotal nephrectomy (SNx, 5/6 nephrectomy) 4 weeks before induction of CIN. CIN was induced by administration of a contrast medium (CM), iohexol, following administration of indomethacin and N-omega-Nitro-L-arginine methyl ester. Renal function and tissue injuries were assessed 48 h after CM injection.

RESULTS

Serum creatinine (s-Cre) and BUN were increased from 0.28 ± 0.01 to 0.52 ± 0.02 mg/dl and from 15.1 ± 0.7 to 29.2 ± 1.2 mg/dl, respectively, after SNx alone. CM further increased s-Cre and BUN to 0.69 ± 0.03 and 37.2 ± 2.1, respectively. In the renal tissue after CM injection, protein levels of receptor-interacting serine/threonine-protein kinase (RIP) 1, RIP3, cleaved caspase 3, and caspase 8 were increased by 64 ~ 212%, while there was reduction in LC3-II and accumulation of p62. Necrostatin-1, an RIP1 inhibitor, administered before and 24 h after CM injection significantly suppressed elevation of s-Cre, BUN and urinary albumin levels, kidney injury molecule-1 expression and infiltration of CD68-positive macrophages in renal tissues after CM injection.

CONCLUSION

The results suggest that necroptosis of proximal tubular cells contributes to CIN in CKD and that suppression of protective autophagy by pro-necroptotic signaling may also be involved.

Cyst formation in proximal renal tubules caused by dysfunction of the microtubule minus-end regulator CAMSAP3

Epithelial cells organize an ordered array of non-centrosomal microtubules, the minus ends of which are regulated by CAMSAP3. The role of these microtubules in epithelial functions, however, is poorly understood. Here, we show that the kidneys of mice in which Camsap3 is mutated develop cysts at the proximal convoluted tubules (PCTs). PCTs were severely dilated in the mutant kidneys, and they also exhibited enhanced cell proliferation. In these PCTs, epithelial cells became flattened along with perturbation of microtubule arrays as well as of certain subcellular structures such as interdigitating basal processes. Furthermore, YAP and PIEZO1, which are known as mechanosensitive regulators for cell shaping and proliferation, were activated in these mutant PCT cells. These observations suggest that CAMSAP3-mediated microtubule networks are important for maintaining the proper mechanical properties of PCT cells, and its loss triggers cell deformation and proliferation via activation of mechanosensors, resulting in the dilation of PCTs.

SIRT1 attenuates sepsis-induced acute kidney injury via Beclin1 deacetylation-mediated autophagy activation

Our previous studies showed that silent mating-type information regulation 2 homologue-1 (SIRT1, a deacetylase) upregulation could attenuate sepsis-induced acute kidney injury (SAKI). Upregulated SIRT1 can deacetylate certain autophagy-related proteins (Beclin1, Atg5, Atg7 and LC3) in vitro. However, it remains unclear whether the beneficial effect of SIRT1 is related to autophagy induction and the underlying mechanism of this effect is also unknown. In the present study, caecal ligation and puncture (CLP)-induced mice, and an LPS-challenged HK-2 cell line were established to mimic a SAKI animal model and a SAKI cell model, respectively. Our results demonstrated that SIRT1 activation promoted autophagy and attenuated SAKI. SIRT1 deacetylated only Beclin1 but not the other autophagy-related proteins in SAKI. SIRT1-induced autophagy and its protective effect against SAKI were mediated by the deacetylation of Beclin1 at K430 and K437. Moreover, two SIRT1 activators, resveratrol and polydatin, attenuated SAKI in CLP-induced septic mice. Our study was the first to demonstrate the important role of SIRT1-induced Beclin1 deacetylation in autophagy and its protective effect against SAKI. These findings suggest that pharmacologic induction of autophagy via SIRT1-mediated Beclin1 deacetylation may be a promising therapeutic approach for future SAKI treatment.

Characterization of ferroptosis in kidney tubular cell death under diabetic conditions

Kidney tubular cell death induced by transforming growth factor-β1 (TGF-β1) is known to contribute to diabetic nephropathy, a major complication of diabetes. Caspase-3-dependent apoptosis and caspase-1-dependent pyroptosis are also involved in tubular cell death under diabetic conditions. Recently, ferroptosis, an atypical form of iron-dependent cell death, was reported to cause kidney disease, including acute kidney injury. Ferroptosis is primed by lipid peroxide accumulation through the cystine/glutamate antiporter system Xc- (xCT) and glutathione peroxidase 4 (GPX4)-dependent mechanisms. The aim of this study was to evaluate the role of ferroptosis in diabetes-induced tubular injury. TGF-β1-stimulated proximal tubular epithelial cells and diabetic mice models were used for in vitro and in vivo experiments, respectively. xCT and GPX4 expression, cell viability, glutathione concentration, and lipid peroxidation were quantified to indicate ferroptosis. The effect of ferroptosis inhibition was also assessed. In kidney biopsy samples from diabetic patients, xCT and GPX4 mRNA expression was decreased compared to nondiabetic samples. In TGF-β1-stimulated tubular cells, intracellular glutathione concentration was reduced and lipid peroxidation was enhanced, both of which are related to ferroptosis-related cell death. Ferrostatin-1 (Fer-1), a ferroptosis inhibitor, alleviated TGF-β1-induced ferroptosis. In diabetic mice, kidney mRNA and protein expressions of xCT and GPX4 were reduced compared to control. Kidney glutathione concentration was decreased, while lipid peroxidation was increased in these mice, and these changes were alleviated by Fer-1 treatment. Ferroptosis is involved in kidney tubular cell death under diabetic conditions. Ferroptosis inhibition could be a therapeutic option for diabetic nephropathy.

Reduced lifespan of erythrocytes in Dahl/Salt sensitive rats is the cause of the renal proximal tubule damage

We studied the mechanisms of anemia and the influence of anemia on renal pathology in Dahl/Salt Sensitive (Dahl/SS) rat, a model of cardio-renal-anemia syndrome. Erythrocyte lifespan was shortened and associated with decreased hemoglobin level in the Dahl/SS rats given high-salt diet. Serum haptoglobin decreased, reticulocytes increased, and erythropoiesis in the bone marrow and extramedullary hematopoiesis in the spleen was markedly stimulated by increased serum erythropoietin in them. As a mechanism of hemolysis, we investigated the incidence of eryptosis, suicidal death of erythrocytes. Eryptosis was increased, and red blood cell-derived microparticles, small particle which are generated in hemolytic disease, were also increased in Dahl/SS rats fed with high-salt diet. Deposition of hemosiderin and mitochondrial morphologic abnormality, a sign of ferroptosis, in proximal renal tubules was associated with intravascular hemolysis. Treatment with deferasirox, an oral iron chelator, reduced the renal proximal tubular injury and the glomerular sclerosis in Dahl/SS rats fed with high-salt diet. In conclusion, reduced half-life of erythrocytes induced by hemolysis is the major cause of anemia in Dahl/SS rat. Iron accumulation induced by hemolysis causes renal proximal tubule injury and accelerates renal damage in this model.

SARS-CoV-2 causes a specific dysfunction of the kidney proximal tubule

Coronavirus disease 2019 (COVID-19) is commonly associated with kidney damage, and the angiotensin converting enzyme 2 (ACE2) receptor for SARS-CoV-2 is highly expressed in the proximal tubule cells. Whether patients with COVID-19 present specific manifestations of proximal tubule dysfunction remains unknown. To test this, we examined a cohort of 49 patients requiring hospitalization in a large academic hospital in Brussels, Belgium. There was evidence of proximal tubule dysfunction in a subset of patients with COVID-19, as attested by low-molecular-weight proteinuria (70-80%), neutral aminoaciduria (46%), and defective handling of uric acid (46%) or phosphate (19%). None of the patients had normoglycemic glucosuria. Proximal tubule dysfunction was independent of pre-existing comorbidities, glomerular proteinuria, nephrotoxic medications or viral load. At the structural level, kidneys from patients with COVID-19 showed prominent tubular injury, including in the initial part of the proximal tubule, with brush border loss, acute tubular necrosis, intraluminal debris, and a marked decrease in the expression of megalin in the brush border. Transmission electron microscopy identified particles resembling coronaviruses in vacuoles or cisternae of the endoplasmic reticulum in proximal tubule cells. Among features of proximal tubule dysfunction, hypouricemia with inappropriate uricosuria was independently associated with disease severity and with a significant increase in the risk of respiratory failure requiring invasive mechanical ventilation using Cox (adjusted hazard ratio 6.2, 95% CI 1.9-20.1) or competing risks (adjusted sub-distribution hazard ratio 12.1, 95% CI 2.7-55.4) survival models. Thus, our data establish that SARS-CoV-2 causes specific manifestations of proximal tubule dysfunction and provide novel insights into COVID-19 severity and outcome.

Autophagy Deficiency in Renal Proximal Tubular Cells Leads to an Increase in Cellular Injury and Apoptosis under Normal Fed Conditions

Renal proximal tubular epithelial cells are significantly damaged during acute kidney injury. Renal proximal tubular cell-specific autophagy-deficient mice show increased sensitivity against renal injury, while showing few pathological defects under normal fed conditions. Considering that autophagy protects the proximal tubular cells from acute renal injury, it is reasonable to assume that autophagy contributes to the maintenance of renal tubular cells under normal fed conditions. To clarify this possibility, we generated a knock out mouse model which lacks Atg7, a key autophagosome forming enzyme, in renal proximal tubular cells (Atg7^flox/flox;KAP-Cre⁺). Analysis of renal tissue from two months old Atg7^flox/flox;KAP-Cre⁺ mouse revealed an accumulation of LC3, binding protein p62/sequestosome 1 (a selective substrate for autophagy), and more interestingly, Kim-1, a biomarker for early kidney injury, in the renal proximal tubular cells under normal fed conditions. TUNEL (TdT-mediated dUTP Nick End Labeling)-positive cells were also detected in the autophagy-deficient renal tubular cells. Analysis of renal tissue from Atg7^flox/flox;KAP-Cre⁺ mice at different age points showed that tubular cells positive for p62 and Kim-1 continually increase in number in an age-dependent manner. Ultrastructural analysis of tubular cells from Atg7^flox/flox;KAP-Cre⁺ revealed the presence of intracellular inclusions and abnormal structures. These results indicated that autophagy-deficiency in the renal proximal epithelial tubular cells leads to an increase in injured cells in the kidney even under normal fed conditions.

Exocytosis-Mediated Urinary Full-Length Megalin Excretion Is Linked With the Pathogenesis of Diabetic Nephropathy

Efficient biomarkers for diabetic nephropathy (DN) have not been established. Using ELISA, we found previously that urinary levels of full-length megalin (C-megalin), a multiligand endocytic receptor in proximal tubules, was positively correlated with DN progression in patients with type 2 diabetes mellitus (T2DM). Here, we found that urinary extracellular vesicle (UEV) excretion and C-megalin content in UEVs or in their exosomal fraction increased along with the progression of the albuminuric stages in patients with T2DM. Cultured immortalized rat proximal tubule cells (IRPTCs) treated with fatty acid-free BSA or advanced glycation end product-modified BSA (AGE-BSA), endocytic ligands of megalin, increased EV excretion, and their C-megalin content. C-megalin excretion from IRPTCs via extracellular vesicles was significantly blocked by an exosome-specific inhibitor, GW4869, indicating that this excretion is mainly exocytosis-mediated. AGE-BSA treatment of IRPTCs caused apparent lysosomal dysfunction, which stimulated multivesicular body formation, resulting in increased exosomal C-megalin excretion. In a high-fat diet-induced, megalin-mediated kidney injury model in mice, urinary C-megalin excretion also increased via UEVs. Collectively, exocytosis-mediated urinary C-megalin excretion is associated with the development and progression of DN in patients with T2DM, particularly due to megalin-mediated lysosomal dysfunction in proximal tubules, and hence it could be a candidate biomarker linked with DN pathogenesis.

Deferasirox-induced iron depletion promotes BclxL downregulation and death of proximal tubular cells

Iron deficiency has been associated with kidney injury. Deferasirox is an oral iron chelator used to treat blood transfusion-related iron overload. Nephrotoxicity is the most serious and common adverse effect of deferasirox and may present as an acute or chronic kidney disease. However, scarce data are available on the molecular mechanisms of nephrotoxicity. We explored the therapeutic modulation of deferasirox-induced proximal tubular cell death in culture. Deferasirox induced dose-dependent tubular cell death and AnexxinV/7AAD staining showed features of apoptosis and necrosis. However, despite inhibiting caspase-3 activation, the pan-caspase inhibitor zVAD-fmk failed to prevent deferasirox-induced cell death. Moreover, zVAD increased deferasirox-induced cell death, a feature sometimes found in necroptosis. Electron microscopy identified mitochondrial injury and features of necrosis. However, neither necrostatin-1 nor RIP3 knockdown prevented deferasirox-induced cell death. Deferasirox caused BclxL depletion and BclxL overexpression was protective. Preventing iron depletion protected from BclxL downregulation and deferasirox cytotoxicity. In conclusion, deferasirox promoted iron depletion-dependent cell death characterized by BclxL downregulation. BclxL overexpression was protective, suggesting a role for BclxL downregulation in iron depletion-induced cell death. This information may be used to develop novel nephroprotective strategies. Furthermore, it supports the concept that monitoring kidney tissue iron depletion may decrease the risk of deferasirox nephrotoxicity.

Puerarin protects against cadmium-induced proximal tubular cell apoptosis by restoring mitochondrial function

Puerarin (PU) is a potent free radical scavenger with a protective effect in nephrotoxin-mediated oxidative damage. Here, we show a novel molecular mechanism by which PU exerts its anti-apoptotic effects in cadmium (Cd)-exposed primary rat proximal tubular (rPT) cells. Morphological assessment and flow cytometric analysis revealed that PU significantly decreased Cd-induced apoptotic cell death of rPT cells. Administration of PU protected cells against Cd-induced depletion of mitochondrial membrane potential (ΔΨm) and lipid peroxidation. Cd-mediated mitochondrial permeability transition pore (MPTP) opening, disruption of mitochondrial ultrastructure, mitochondrial cytochrome c (cyt-c) release, caspase-3 activation and subsequently poly ADP-ribose polymerase (PARP) cleavage could be effectively blocked by the addition of PU. Moreover, up-regulation of Bcl-2 and down-regulation of Bax and hence increased Bcl-2/Bax ratio were observed with the PU administration. In addition, PU reversed Cd-induced ATP depletion by restoring ΔΨm to affect ATP production and by regulating expression levels of ANT-1 and ANT-2 to improve ATP transport. In summary, PU inhibited Cd-induced apoptosis in rPT cells by ameliorating the mitochondrial dysfunction.

A Comprehensive Study of Easter Lily Poisoning in Cats

This study was conducted with 3 objectives in mind: first, to identify the toxic fraction (aqueous or organic) in leaves and flowers; second, to identify diagnostic marker(s) of toxicosis in cats; and, third, to evaluate the morphologic effects of intoxication. The study was conducted in 2 phases. Phase 1 was to identify which extract, organic or aqueous, was nephrotoxic and also to determine the appropriate dose for use in the phase 2 studies. Results indicated that only the aqueous extracts of leaves and flowers were nephrotoxic and pancreotoxic. To identify the proximate toxic compound, cats in the phase 2 study were orally exposed to subfractions of the aqueous flower extract, 1 subfraction per cat. Results confirmed vomiting, depression, polyuria, polydipsia, azotemia, glucosuria, proteinuria, and isosthenuria as toxic effects of the Easter lily plant. Another significant finding in serum was elevated creatinine kinase. Significant histologic kidney changes included acute necrosis of proximal convoluted tubules and degeneration of pancreatic acinar cells. Renal ultrastructural changes included swollen mitochondria, megamitochondria, edema, and lipidosis. Subfraction IIa3 of the aqueous floral extract contained most of the toxic compound(s). These studies reproduced the clinical disease, identified the most toxic fraction of the Easter lily, and helped characterize the clinical pathology, histopathology, and ultrastructural pathology associated with the disease.

Myosin 1b Regulates Amino Acid Transport by Associating Transporters with the Apical Plasma Membrane of Kidney Cells

Amino acid transporters (AATers) in the brush border of the apical plasma membrane (APM) of renal proximal tubule (PT) cells mediate amino acid transport (AAT). We found that the membrane-associated class I myosin myosin 1b (Myo1b) localized at the apical brush border membrane of PTs. In opossum kidney (OK) 3B/2 epithelial cells, which are derived from PTs, expressed rat Myo1b-GFP colocalized in patched microvilli with expressed mouse V5-tagged SIT1 (SIT1-V5), which mediates neutral amino acid transport in OK cells. Lentivirus-mediated delivery of opossum Myo1b-specific shRNA resulted in knockdown (kd) of Myo1b expression, less SIT1-V5 at the APM as determined by localization studies, and a decrease in neutral AAT as determined by radioactive uptake assays. Myo1b kd had no effect on Pi transport or noticeable change in microvilli structure as determined by rhodamine phalloidin staining. The studies are the first to define a physiological role for Myo1b, that of regulating renal AAT by modulating the association of AATers with the APM.

Early lesions of the nephron in paraproteinemia

Electron microscopy of renal biopsies in eight patients with paraproteinemia (six multiple myeloma, one Waldenström's disease, one idiopathic paraproteinemia) and without impairment of renal function demonstrates an increase of mesangial matrix and thickening of the lamina rara interna of the basement membrane in the glomerulus by fibrillar material without periodicity. These lesions are thought to exhibit precursors of amyloid deposits. The proximal tubules show signs of increased cellular activity probably because of handling an increased protein load. In the basement membranes there are proteins with a fibrillar structure and definite periodicity. These are suggested to be consistent with large aggregates of immunoglobulin fragments.

Cadherin expression, vectorial active transport, and metallothionein isoform 3 mediated EMT/MET responses in cultured primary and immortalized human proximal tubule cells

BACKGROUND

Cultures of human proximal tubule cells have been widely utilized to study the role of EMT in renal disease. The goal of this study was to define the role of growth media composition on classic EMT responses, define the expression of E- and N-cadherin, and define the functional epitope of MT-3 that mediates MET in HK-2 cells.

METHODS

Immunohistochemistry, microdissection, real-time PCR, western blotting, and ELISA were used to define the expression of E- and N-cadherin mRNA and protein in HK-2 and HPT cell cultures. Site-directed mutagenesis, stable transfection, measurement of transepithelial resistance and dome formation were used to define the unique amino acid sequence of MT-3 associated with MET in HK-2 cells.

RESULTS

It was shown that both E- and N-cadherin mRNA and protein are expressed in the human renal proximal tubule. It was shown, based on the pattern of cadherin expression, connexin expression, vectorial active transport, and transepithelial resistance, that the HK-2 cell line has already undergone many of the early features associated with EMT. It was shown that the unique, six amino acid, C-terminal sequence of MT-3 is required for MT-3 to induce MET in HK-2 cells.

CONCLUSIONS

The results show that the HK-2 cell line can be an effective model to study later stages in the conversion of the renal epithelial cell to a mesenchymal cell. The HK-2 cell line, transfected with MT-3, may be an effective model to study the process of MET. The study implicates the unique C-terminal sequence of MT-3 in the conversion of HK-2 cells to display an enhanced epithelial phenotype.

Cellular localization of uranium in the renal proximal tubules during acute renal uranium toxicity

Renal toxicity is a hallmark of uranium exposure, with uranium accumulating specifically in the S3 segment of the proximal tubules causing tubular damage. As the distribution, concentration and dynamics of accumulated uranium at the cellular level is not well understood, here, we report on high-resolution quantitative in situ measurements by high-energy synchrotron radiation X-ray fluorescence analysis in renal sections from a rat model of uranium-induced acute renal toxicity. One day after subcutaneous administration of uranium acetate to male Wistar rats at a dose of 0.5 mg uranium kg(-1) body weight, uranium concentration in the S3 segment of the proximal tubules was 64.9 ± 18.2 µg g(-1) , sevenfold higher than the mean renal uranium concentration (9.7 ± 2.4 µg g(-1) ). Uranium distributed into the epithelium of the S3 segment of the proximal tubules and highly concentrated uranium (50-fold above mean renal concentration) in micro-regions was found near the nuclei. These uranium levels were maintained up to 8 days post-administration, despite more rapid reductions in mean renal concentration. Two weeks after uranium administration, damaged areas were filled with regenerating tubules and morphological signs of tissue recovery, but areas of high uranium concentration (100-fold above mean renal concentration) were still found in the epithelium of regenerating tubules. These data indicate that site-specific accumulation of uranium in micro-regions of the S3 segment of the proximal tubules and retention of uranium in concentrated areas during recovery are characteristics of uranium behavior in the kidney.

[On the issue of non-mutagenic non-targeted effects in low renewable tissues. Analysis of low dose radiation effects on the rat renal tubule epithelium]

Irradiation of rats with γ-quanta at relatively low doses induces a sustainable dose-independent increase in the occurrence of lethal cytoplasmic disorders in the renal tubules epithelium together with sustainable and as well dose-independent subcelluar compensation and restorative processes. Over the period of research (6 months) these processes led to no population recovery. The detected alterations are referred to the category of non-targeted non-mutagenic effects and they are of interest because they address the issue of the sensitivity of low renewable tissues to radiation.

Novel effect of the inhibitor of mitochondrial cyclophilin D activation, N-methyl-4-isoleucine cyclosporin, on renal calcium crystallization

OBJECTIVES

To experimentally evaluate the clinical application of N-methyl-4-isoleucine cyclosporin, a novel selective inhibitor of cyclophilin D activation.

METHODS

In vitro, cultured renal tubular cells were exposed to calcium oxalate monohydrate crystals and treated with N-methyl-4-isoleucine cyclosporin. The mitochondrial membrane was stained with tetramethylrhodamine ethyl ester perchlorate and observed. In vivo, Sprague-Dawley rats were divided into four groups: a control group, an ethylene glycol group (administration of ethylene glycol to induce renal calcium crystallization), a N-methyl-4-isoleucine cyclosporin group (administration of N-methyl-4-isoleucine cyclosporin) and an ethylene glycol + N-methyl-4-isoleucine cyclosporin group (administration of ethylene glycol and N-methyl-4-isoleucine cyclosporin). Renal calcium crystallization was evaluated using Pizzolato staining. Oxidative stress was evaluated using superoxide dismutase and 8-hydroxy-deoxyguanosine. Mitochondria within renal tubular cells were observed by transmission electron microscopy. Cell apoptosis was evaluated using cleaved caspase-3.

RESULTS

In vitro, calcium oxalate monohydrate crystals induced depolarization of the mitochondrial membrane potential, which was remarkably prevented by N-methyl-4-isoleucine cyclosporin. In vivo, ethylene glycol administration induced renal calcium crystallization, oxidative stress, mitochondrial collapse and cell apoptosis in rats, which were significantly prevented by N-methyl-4-isoleucine cyclosporin.

CONCLUSIONS

Herein we first report a new treatment agent determining renal calcium crystallization through cyclophilin D activation.

Impairment of renal structure and function following heterogeneous chemical mixture exposure in rats

Renal structural and functional alterations following an exposure to a heterogeneous chemical mixture (HCM) of phthalic acid di butyl ester, 1, 2-dichlorobenzene, cadmium chloride and chromium trioxide, administered through oral gavage in low doses (1/100 and 1/1000 of LD50 value of individual chemical) for 60 days, followed by withdrawal till 120 days resulted in significant rise in kidney lipid peroxidation and fall in the activities of enzymatic antioxidants. However, withdrawal of HCM treatment restored most of these altered parameters. Degenerative changes in the kidney included proximal convoluted tubules devoid of brush boarder with cytoplasmic blebbing, dissolution and sloughing of nuclei. Cortical glomeruli were also affected with epithelial disintegration, pyknosis of podocyte nuclei and mesengial cell hyperplasia. The morphological alterations recovered fully in the low dose compared to the high dose treatment group.

Proximal nephron

The kidney plays a fundamental role in maintaining body salt and fluid balance and blood pressure homeostasis through the actions of its proximal and distal tubular segments of nephrons. However, proximal tubules are well recognized to exert a more prominent role than distal counterparts. Proximal tubules are responsible for reabsorbing approximately 65% of filtered load and most, if not all, of filtered amino acids, glucose, solutes, and low molecular weight proteins. Proximal tubules also play a key role in regulating acid-base balance by reabsorbing approximately 80% of filtered bicarbonate. The purpose of this review article is to provide a comprehensive overview of new insights and perspectives into current understanding of proximal tubules of nephrons, with an emphasis on the ultrastructure, molecular biology, cellular and integrative physiology, and the underlying signaling transduction mechanisms. The review is divided into three closely related sections. The first section focuses on the classification of nephrons and recent perspectives on the potential role of nephron numbers in human health and diseases. The second section reviews recent research on the structural and biochemical basis of proximal tubular function. The final section provides a comprehensive overview of new insights and perspectives in the physiological regulation of proximal tubular transport by vasoactive hormones. In the latter section, attention is particularly paid to new insights and perspectives learnt from recent cloning of transporters, development of transgenic animals with knockout or knockin of a particular gene of interest, and mapping of signaling pathways using microarrays and/or physiological proteomic approaches.

High resolution helium ion scanning microscopy of the rat kidney

Helium ion scanning microscopy is a novel imaging technology with the potential to provide sub-nanometer resolution images of uncoated biological tissues. So far, however, it has been used mainly in materials science applications. Here, we took advantage of helium ion microscopy to explore the epithelium of the rat kidney with unsurpassed image quality and detail. In addition, we evaluated different tissue preparation methods for their ability to preserve tissue architecture. We found that high contrast, high resolution imaging of the renal tubule surface is possible with a relatively simple processing procedure that consists of transcardial perfusion with aldehyde fixatives, vibratome tissue sectioning, tissue dehydration with graded methanol solutions and careful critical point drying. Coupled with the helium ion system, fine details such as membrane texture and membranous nanoprojections on the glomerular podocytes were visualized, and pores within the filtration slit diaphragm could be seen in much greater detail than in previous scanning EM studies. In the collecting duct, the extensive and striking apical microplicae of the intercalated cells were imaged without the shrunken or distorted appearance that is typical with conventional sample processing and scanning electron microscopy. Membrane depressions visible on principal cells suggest possible endo- or exocytotic events, and central cilia on these cells were imaged with remarkable preservation and clarity. We also demonstrate the use of colloidal gold probes for highlighting specific cell-surface proteins and find that 15 nm gold labels are practical and easily distinguishable, indicating that external labels of various sizes can be used to detect multiple targets in the same tissue. We conclude that this technology represents a technical breakthrough in imaging the topographical ultrastructure of animal tissues. Its use in future studies should allow the study of fine cellular details and provide significant advances in our understanding of cell surface structures and membrane organization.

Carcinogens induce loss of the primary cilium in human renal proximal tubular epithelial cells independently of effects on the cell cycle

The primary cilium is an immotile sensory and signaling organelle found on the majority of mammalian cell types. Of the multitude of roles that the primary cilium performs, perhaps some of the most important include maintenance of differentiation, quiescence, and cellular polarity. Given that the progression of cancer requires disruption of all of these processes, we have investigated the effects of several carcinogens on the primary cilium of the RPTEC/TERT1 human proximal tubular epithelial cell line. Using both scanning electron microscopy and immunofluorescent labeling of the ciliary markers acetylated tubulin and Arl13b, we confirmed that RPTEC/TERT1 cells express primary cilium upon reaching confluence. Treatment with the carcinogens ochratoxin A (OTA) and potassium bromate (KBrO(3)) caused a significant reduction in the number of ciliated cells, while exposure to nifedipine, a noncarcinogenic renal toxin, had no effect on primary cilium expression. Flow cytometric analysis of the effects of all three compounds on the cell cycle revealed that only KBrO(3) resulted in an increase in the proportion of cells entering the cell cycle. Microarray analysis revealed dysregulation of multiple pathways affecting ciliogenesis and ciliary maintenance following OTA and KBrO(3) exposure, which were unaffected by nifedipine exposure. The primary cilium represents a unique physical checkpoint with relevance to carcinogenesis. We have shown that the renal carcinogens OTA and KBrO(3) cause significant deciliation in a model of the proximal tubule. With KBrO(3), this was followed by reentry into the cell cycle; however, deciliation was not found to be associated with reentry into the cell cycle following OTA exposure. Transcriptomic analysis identified dysregulation of Wnt signaling and ciliary trafficking in response to OTA and KBrO(3) exposure.

The appearance of renal cells cytoplasmic degeneration and nuclear destruction might be an indication of GNPs toxicity

BACKGROUND

Advances in nanotechnology have identified promising candidates for many biological and biomedical applications. Since the properties of nanoparticles (NPs) differ from that of their bulk materials, they are being increasingly exploited for medical uses and other industrial applications. The histological and the histochemical alterations in the renal tissues due to gold nanoparticles (GNPs) have not well documented and have not yet been identified. The aim of the present study was to investigate the particle-size effect of GNPs on the renal tissue in an attempt to address their potential toxicity.

METHODS

A total of 70 healthy male Wistar-Kyoto rats were exposed to GNPs received 50 or 100 μl of GNPs infusion of size (10, 20 and 50 nm for 3 or 7 days) to investigate particle-size effect of GNPs on the renal tissue. Animals were randomly divided into groups, 6 GNPs-treated rats groups and one control group. Groups 1, 2 and 3 received infusion of 50 μl GNPs of size 10 nm (3 or 7 days), size 20 nm (3 or 7 days) and 50 nm (3 or 7 days), respectively; while groups 4, 5 and 6 received infusion of 100 μl GNPs of size 10 nm, size 20 nm and 50 nm, respectively.

RESULTS

The histological alterations were mainly seen in the cortex and the proximal renal convoluted tubules were more affected than the distal ones. In comparison with respective control rats, exposure to GNPs doses has produced the following renal tubular alterations: cloudy swelling and renal tubular necrosis. Interstitial alterations included: intertubular blood capillaries dilatation, intertubular hemorrhage and inflammatory cell infiltrations. The glomeruli showed moderate congestion with no hypercelluraity and mesangial proliferation or basement membrane thickening.

CONCLUSIONS

The induced histological alterations might be an indication of injured renal tubules due to GNPs toxicity that become unable to deal with the accumulated residues resulting from metabolic and structural disturbances caused by these NPs. These alterations were size-dependent with smaller ones induced more effects and related with time exposure of GNPs. The produced histological alterations may suggest that GNPs interact with proteins and enzymes of the renal tissue interfering with the antioxidant defense mechanism and leading to reactive oxygen species (ROS) generation which in turn may induce stress in the renal cells to undergo atrophy and necrosis. More histomorphologcal investigations are needed to address the potential threat of GNPs as a therapeutic and diagnostic tool.

A fluorescence method for measurement of glucose transport in kidney cells

BACKGROUND

Diabetes may alter renal glucose reabsorption by sodium (Na(+))-dependent glucose transporters (SGLTs). Radiolabeled substrates are commonly used for in vitro measurements of SGLT activity in kidney cells. We optimized a method to measure glucose uptake using a fluorescent substrate, 2-(N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)-2-deoxyglucose (2-NBDG).

METHODS

Uptake buffers for 2-NBDG were the same as for (14)C-labeled α-methyl-d-glucopyranoside ([(14)C]AMG). Cell lysis buffer was optimized for fluorescence of 2-NBDG and Hoechst DNA stain. Uptake was performed on cultures of primary mouse kidney cells (PMKCs), the LLC-PK(1) proximal tubule cell line, or COS-7 cells transiently overexpressing mouse SGLT1 or SGLT2 by incubating cells at 37°C in buffer containing 50-200 μM 2-NBDG. Microscopy was performed to visualize uptake in intact cells, while a fluorescence microplate reader was used to measure intracellular concentration of 2-NBDG ([2-NBDG](i)) in cell homogenates.

RESULTS

Fluorescent cells were observed in cultures of PMKCs and LLC-PK(1) cells exposed to 2-NBDG in the presence or absence of Na(+). In LLC-PK(1) cells, 2-NBDG transport in the presence of Na(+) had a maximum rate of 0.05 nmol/min/μg of DNA. In these cells, Na(+)-independent uptake of 2-NBDG was blocked with the GLUT inhibitor, cytochalasin B. The Na(+)-dependent uptake of 2-NBDG decreased in response to co-exposure to the SGLT substrate, AMG, and it could be blocked with the SGLT inhibitor, phlorizin. Immunocytochemistry showed overexpression of SGLT1 and SGLT2 in COS-7 cells, in which, in the presence of Na(+), [2-NBDG](i) was fivefold higher than in controls.

CONCLUSION

Glucose transport in cultured kidney cells can be measured with the fluorescence method described in this study.

[Detoxication effect of water-soluble imprinted cross-linked chitosan on depleted uranium induced toxicity to renal cells]

To investigate whether a series of water-soluble cross-linked chitosan derivates synthesized in the guide of imprinting technology could be used as a uranium chelating agent to protect cells exposed to depleted uranium (DU), the imprinted chitosan derivates with high UO2(2+) chelating ability were screened, and cell model of human renal proximal tubule epithelium cells (HK-2) exposed to DU (500 micromol.L-1) was built, chitosan derivates (400 mg.L-1 ) was added to test group and diethylenetriaminepentaacetic acid (DTPA, 50 mg.L-1) was added to positive control group. The results showed that three Cu2+ imprinted chitosan derivates had higher uranium chelating ability (>49 microg.mg-1) than chitosan and non-imprinted chitosan derivates. Compared to the cells exposed to DU only, survival of cells in group added chitosan derivates rose up significantly (increased from 57.3% to 88.7%, and DTPA to 72.6%), and DU intracellular accumulation decreased, membrane damage and DNA damage also eased. Among the imprinted chitosan derivates, Cu2+ imprinted penta dialdehyde cross-linked carboxymethyl chitosan (Cu-P-CMC) was the best, and better than DTPA. From ultrastructure observation, the DU precipitates of test group added Cu-P-CMC were most grouped in a big hairy clusters in a string together outside cells. It is possible that the DU-chitosan derivates precipitates are too big to enter into cells, and from this way, the DU uptake by cells decreased so as to detoxication.

Diagnostic value of brush border enzymes of the proximal renal tubules in rheumatoid arthritis

BACKGROUND

Proximal tubules of the kidney have a dominant function in the excretion of different enzymes in the urine. These enzymes can be used as markers for secondary renal damage under the action of different diseases, medicines, and toxins. The aim of this study was to evaluate the values of alanine aminopeptidase (AAP), gamma-glutamyl transferase (gamma-GT), and beta2 microglobulin (beta2m) in urine of patients with untreated rheumatoid arthritis (RA) and to define the possible association between untreated rheumatoid arthritis and tubular function at the brush border region.

METHODS

We used a kinetic assay for AAP, standard methods by the International Federation for Clinical Chemistry (IFCC) for gamma-GT and Microparticle Enzyme Immunoassay (MEIA), (Abbott A(x)SYM System) for the determination of beta2m in urine of 70 participants (35 untreated RA patients and 35 healthy volunteers (HC)).

RESULTS

From the total of 35 RA patients, AAP enzymuria was found in 24 patients with test sensitivity (68.57%), gamma-GT in 16 patients with test sensitivity (45.71%), while the presence of urinary beta2m was found in a very low percentage of cases. Out of 18 rheumatoid factor (RF) negative patients, 14 patients were AAP and 10 patients were gamma-GT positive, while the presence of beta2m in urine was not detected. Among 17 RF positive RA patients, the presence of AAP and gamma-GT was noticed in 10 and 6 patients, respectively, while the presence of beta2m in urine was not detected.

CONCLUSIONS

In conclusion, AAP had a higher sensitivity than gamma-GT and beta2m in detection of asymptomatic renal lesions in untreated RA.

Microarray analysis of gene expression patterns in human proximal tubule cells over a short and long time course of cadmium exposure

Numerous studies showed that renal proximal tubules cells are the cell type critically affected by chronic exposure to cadmium (Cd(2+)). The aim of the present study was to apply global gene expression technology and a human renal epithelial cell culture model (HPT) to determine whether time of exposure to Cd(2+) exerts a major influence on the resulting pattern of global gene expression. HPT cells were exposed to Cd(2+) for a short, 1-d, period of exposure (9, 27, and 45 μM) versus a longer, 13-d, period (4.5, 9, and 27 μM), with the hypothesis being that the stress response of the cells would be more active during the short time of exposure. The results showed that the differential expression of genes was very extensive for HPT cells exposed to Cd(2+) for 1 d, with more than 1848 genes displaying alterations compared to control and with the major categories of genes being involved in stress responses; cell death; checkpoint arrest, DNA repair, and the cell cycle; inflammatory responses; and cell adhesion, motion and differentiation. In contrast, HPT cells exposed to Cd(2+) for 13 d showed 923 genes to be differentially expressed, with a marked reduction in the number of differentially expressed stress response genes and a significant increase in the number of genes involved in development and differentiation. There were 387 differentially expressed genes common to both times of exposure. Data suggest that unless one is actively seeking to study the acute stress response, global gene expression technology should not be applied within an early time course of toxicant exposure.

Tenofovir-induced kidney disease: an acquired renal tubular mitochondriopathy

Tenofovir, used in combination with other antiretroviral agents, is an effective therapy for HIV infection. Although large clinical studies and post-marketing data support a benign renal profile for tenofovir, numerous cases of kidney injury raise concern for nephrotoxic potential. Early human studies and experimental evidence suggested that tenofovir itself was not associated with mitochondrial toxicity within the kidney. However, recent animal data demonstrate that tenofovir causes mitochondrial DNA depletion and mitochondrial toxicity. Herlitz et al. confirm the nephrotoxicity of tenofovir in humans. They describe its clinical consequences, histopathologic findings, and its mitochondrial toxicity in HIV+ patients.

Role of NHERF and scaffolding proteins in proximal tubule transport

Eukaryotic cells coordinate specific responses to hormones and growth factors by spatial and temporal organization of "signaling components." Through the formation of multiprotein complexes, cells are able to generate "signaling components" that transduce hormone signals through proteins, such as PSD-95/Dlg/ZO-1(PDZ)-containing proteins that associate by stable and dynamic interactions. The PDZ homology domain is a common protein interaction domain in eukaryotes and with greater than 500 PDZ domains identified, it is the most abundant protein interaction domain in eukaryotic cells. The NHERF (sodium hydrogen exchanger regulatory factor) proteins are PDZ domain-containing proteins that play an important role in maintaining and regulating cell function. NHERF-1 was initially identified as a brush border membrane-associated phosphoprotein essential for the cAMP/PKA-induced inhibition of the sodium hydrogen exchanger isoform 3 (NHE3). Mouse, rabbit and human renal proximal tubules also express NHERF-2 (E3KARP), a structurally related protein, which in model cell systems also binds NHE3 and mediates its inhibition by cAMP. PDZK1 (NHERF-3) and IKEPP (NHERF-4) were later identified and found to have similar homology domains, leading to their recent reclassification. Although studies have revealed similar binding partners and overlapping functions for the NHERF proteins, it is clear that there is a significant amount of specificity between them. This review focuses primarily on NHERF-1, as the prototypical PDZ protein and will give a brief summary of its role in phosphate transport and the development of some forms of nephrolithiasis.

[Ultrastructural peculiarities of the tissue adaptation of the epithelia in some invertebrates and in man]

Using electron microscopy, the reactive changes of epidermal cells in response to hypotonic conditions were studied in the representatives of three groups of marine invertebrates characterized by different levels of organization--Convoluta convoluta (Turbellaria, Acoela), Amphiporus lactifloreus (Nemertini) and Saccoglossus mereschkowskii (Enteropneusta). On the basis of functional resemblance of absorption and pinocytosis, the peculiarities of absorptive proximal tubule epithelium of human kidney under intensive protein transport were examined at the ultrastructural level. The resemblance of the epithelial tissue reactive changes in response to the damaging was demonstrated factors in invertebrates and in humans.

Mitochondrial superoxide plays a crucial role in the development of mitochondrial dysfunction during high glucose exposure in rat renal proximal tubular cells

Diabetic nephropathy is the leading cause of end-stage renal disease in the United States. Despite several studies indicating a role for mitochondrial oxidative stress and mitochondrial dysfunction in the development of diabetic complications, the precise mechanisms underlying renal mitochondrial dysfunction and renal cell injury remain unclear. The hypothesis of the current study was that high-glucose-mediated generation of mitochondrial superoxide is a key early event that leads to mitochondrial injury in renal proximal tubular cells. To ascertain the role of mitochondrial superoxide we have tested whether overexpression of the primary mitochondrial antioxidant, manganese superoxide dismutase (MnSOD), protects against hyperglycemia-induced renal injury using normal rat renal proximal tubular cells (NRK). NRK cells were exposed to high glucose (25 mM) and the changes in the mitochondrial membrane potential, ATP levels, and superoxide generation and the loss of cell viability were measured at 24 and 48 h after high glucose exposure. Our results indicate that high glucose first induced superoxide generation and hyperpolarization in the mitochondria, followed by a secondary event, which involved a decline in ATP levels, partial Complex III inactivation, and loss of cell viability. These high-glucose-induced changes were completely prevented by overexpression of MnSOD in NRK cells. However, MnSOD activity was not changed after high glucose exposure in vitro or during the early stages of diabetes using the streptozotocin rat model. These findings show for the first time that hyperglycemic induction of superoxide production within the mitochondria initiates specific mitochondrial injury (i.e., Complex III) via a mechanism independent of MnSOD inactivation.

Role of mitochondrial dysfunction in cellular responses to S-(1,2-dichlorovinyl)-L-cysteine in primary cultures of human proximal tubular cells

The nephrotoxic metabolite of the environmental contaminant trichloroethylene, S-(1,2-dichlorovinyl)-l-cysteine (DCVC), is known to elicit cytotoxicity in rat and human proximal tubular (rPT and hPT, respectively) cells that involves inhibition of mitochondrial function. DCVC produces a range of cytotoxic and compensatory responses in hPT cells, depending on dose and exposure time, including necrosis, apoptosis, repair, and enhanced cell proliferation. The present study tested the hypothesis that induction of mitochondrial dysfunction is an obligatory step in the cytotoxicity caused by DCVC in primary cultures of hPT cells. DCVC-induced necrosis was primarily a high concentration (> or =50 microM) and late (> or =24h) response whereas apoptosis and increased proliferation occurred at relatively low concentrations (<50 microM) and early time points (< or =24h). Decreases in cellular DNA content, indicative of cell loss, were observed at DCVC concentrations as low as 1 microM. Involvement of mitochondrial dysfunction in DCVC-induced cytotoxicity was supported by showing that DCVC caused modest depletion of cellular ATP, inhibition of respiration, and activation of caspase-3/7. Cyclosporin A protected cells against DCVC-induced apoptosis and both cyclosporin A and ruthenium red protected cells against DCVC-induced loss of mitochondrial membrane potential. DCVC caused little or no activation of caspase-8 and did not significantly induce expression of Fas receptor, consistent with apoptosis occurring only by the mitochondrial pathway. These results support the conclusion that mitochondrial dysfunction is an early and obligatory step in DCVC-induced cytotoxicity in hPT cells.

[Ultrastructural aspects in the development of the human nephron--electron microscopy study]

We have looked for electron microscopy aspects tissue fragments of human nephrons harvested from fetuses of 7,5-19 weeks to show ultrastructure aspects of the nephron during development.

MATERIAL AND METHODS

We have used 9 fragments from human embryos; Four cases were aged of 7,5, 8, 12 and 17 weeks. The other 5 cases were between 18 and 19 weeks. Tissue fragments were fixed in glutaraldehyde at 4 degrees C and processed by the classic technique.

RESULTS

The electron microscopy study shows the evolutive steps of the human nephron (stages I-IV), stages met for the 12-19 weeks embryo. For the cases under this age the meta-nephrogenic blastema was dominant. In the IVth development stage, the cells in the proximal tubule show a higher development degree than the distal segments.

CONCLUSION

These observations indicate that during early development the proximal segment is better developed and probably more functional than the distal segments.

P-glycoprotein-deficient mice have proximal tubule dysfunction but are protected against ischemic renal injury

The multidrug resistance gene 1 product, P-glycoprotein (P-gp), is expressed in several excretory organs, including the apical membrane of proximal tubules. After inducing acute renal failure, P-gp expression is upregulated and this might be a protective function by pumping out toxicants and harmful products of oxidative stress. We characterized renal function of P-gp knockout mice and studied its consequences in renal ischemic damage. Compared with wild-type mice, knockout mice have a lower glomerular filtration rate and renal plasma flow. An augmented urinary excretion of sodium, numerous amino acids, calcium, glucose, and low molecular weight proteins was observed along with an increased diuresis. A higher lithium plasma clearance in the knockout mice suggested proximal tubular dysfunction. Electron microscopy showed mitochondrial abnormalities in proximal tubular cells that could account for decreased adenosine triphosphate levels in the cortex. After inducing ischemia, wild-type mice showed a decrease in creatinine clearance and severe proximal tubular necrosis. In contrast, knockout mice had no signs of tubular damage. Our data indicate that P-gp knockout mice have impaired renal function but are protected against ischemic renal injury.

Expanding the pathologic spectrum of immunoglobulin light chain proximal tubulopathy

CONTEXT

In plasma cell dyscrasias, involvement of the distal tubules is frequent and well characterized. In contrast, proximal tubules have only rarely been reported to show diagnostic pathology such as intracytoplasmic crystals.

OBJECTIVE

To look for additional morphologic features that might be helpful in the diagnosis of proximal tubulopathy associated with an underlying plasma cell dyscrasia.

DESIGN

We examined patients presenting with nonspecific renal symptoms who were found to have light chain restriction limited to proximal tubular epithelium by immunofluorescence. We correlated these results with light microscopy, electron microscopy, and the clinical findings.

RESULTS

By immunofluorescence, 5 patients had light chain restriction in proximal tubular epithelium. By light microscopy, only 1 patient had focal rhomboid crystals in the proximal tubular epithelium; all other biopsies failed to show any discernible pathology within the proximal tubules or elsewhere in the kidney. By electron microscopy, proximal tubules from 2 patients showed crystals with a latticelike structure, whereas the remaining 3 patients had only prominent phagolysosomes. However, by immunoelectron microscopy, the lysosomal content showed light chain restriction (in 2 cases studied). Post-kidney biopsy, all patients were diagnosed with multiple myeloma or plasma cell dyscrasia. One patient developed renal failure and had recurrence of crystals in the allograft.

CONCLUSIONS

Light chain proximal tubulopathy may be associated with the presence of crystals or with the presence of phagolysosomes with light chain restriction as the sole abnormality. Both kappa and lambda light chains may be involved. The prognosis is variable and the pathology may recur in transplants.

Controversies in nephrology: renal albumin handling, facts, and artifacts!

In this article, we discuss and contradict a recent publication by Russo et al., which suggests that the filtration of large amounts of albumin followed by transtubular transport of intact albumin is a physiological phenomenon.

Caki-1 cells represent an in vitro model system for studying the human proximal tubule epithelium

BACKGROUND/AIMS

The human proximal tubule (PT) epithelium is distinguished from other nephron segments via several unique characteristics. Studies assessing PT epithelium increasingly employ cell lines, bypassing the complexity of primary cell cultures. However, few human model systems exist for studying PT cells in vitro. The current work involves an intensive characterization of Caki-1 cells, a commercially available human renal cell line.

METHODS

Caki-1 cells were validated as a representative model system for PT cell research via morphological, physiological and biochemical investigations including light and transmission electron microscopy, transepithelial electrical resistance (TER) measurements and the detection of PT markers.

RESULTS

Morphologically, these cells form a polarized monolayer with apical located microvilli and multiple mitochondria per cell. Low TER ranging from 2 to 28 Omega cm(2) was determined for Caki-1 cells, characteristic of the 'leaky' PT epithelium in vivo. Expression of the PT markers: NHE3, GGT, DPP IV, APM and AP were present in Caki-1 cells. Two epithelial markers, E-cadherin and Na(+)/K(+)-ATPase, were additionally observed.

CONCLUSION

The current work is a concise summary which confirms that Caki-1 cells represent well-differentiated polarized PT cells in vitro, regardless of its cancerous origin and multiple passaging. They prove to be a significant contribution to the field of PT research.

Changes in expression of fibrotic markers and histopathological alterations in kidneys of mice chronically exposed to low and high Cd doses

The main target organ for cadmium (Cd) is the kidney, and more specifically the proximal tubular cells. Little is known about the effects of a long-term Cd exposure on the ultrastructure of the kidney and the involvement in tubulointerstitial fibrosis. Therefore, mice were exposed to Cd concentrations varying from 10 to 500 mg CdCl(2)/l in the drinking water during 4, 16 and 23 weeks. Ultrastructural changes were studied by means of light- and electron microscopical analyses. Furthermore, the expression of the extracellular matrix (ECM) proteins collagen I and fibronectin, and the myofibroblast/epithelial-to-mesenchymal transition (EMT) marker alfa-smooth muscle actin (alpha-SMA) were studied by means of immunohistochemistry. The histopathological changes caused by Cd varied considerably from one animal to another, and from one individual cell to another. An exposure to Cd concentrations up to 100mg CdCl(2)/l elicited only minor changes that were restricted to increasing amounts of lysosomes and vacuolisation. When higher Cd concentrations were applied, the changes became more pronounced and featured mitochondrial damage, cellular swelling and loss of basal invaginations. An overproduction of the interstitial matrix component fibronectin and the expression of the myofibroblasts/EMT marker alpha-SMA in kidneys of mice exposed to 100mg CdCl(2)/l clearly indicated that an exposure to relatively low Cd doses might lead ultimately to renal fibrosis. Increasing the Cd dose (up to 500 mg CdCl(2)/l) evoked an increased immunoreactivity for fibrotic markers. In conclusion we may state that concentrations up to 100mg CdCl(2)/l evoked minor changes, although the expression of fibrotic markers was increased. Changes became more pronounced when exposing to higher Cd concentrations.

Isolation of renal proximal tubular brush-border membranes

This protocol describes a method for the isolation and purification of renal proximal tubular brush-border membranes in high yield and high purity. Based on a different reactivity of the brush-border membrane compared to other cellular membranes with divalent cations, such as Mg2+, purified membrane vesicles can be obtained after a few differential centrifugation steps (within approximately 3 h) that are suitable for in vitro studies, such as transport experiments or protein and lipid analysis.

Morphological and functional alterations in human proximal tubular cell line induced by low level inorganic arsenic: evidence for targeting of mitochondria and initiated apoptosis

The kidney is a known target organ for arsenic and is critical for both arsenic biotransformation and elimination. Previous studies have demonstrated that at high doses (ppm) inorganic arsenic is toxic to mitochondria primarily by affecting cellular respiration. However, the effect of inorganic arsenic on mitochondria after low level exposures is not known, particularly in the kidney. Thus the functional and morphological effects of low level inorganic arsenic were investigated in a human proximal tubular cell line, HK-2. Mitochondrial function was assessed at subcytotoxic concentrations of arsenite (< or = 10 microm) by examining the alteration of the mitochondrial membrane potential using MitoTracker Red, a mitochondrion selective dye. In a subset of cells, subcytotoxic arsenite led to mitochondrial membrane depolarization, which could subsequently lead to permeability transition and apoptosis. Subcytotoxic arsenite also induced translocation of phosphatidylserine, indicative of early-stage apoptosis. To confirm whether subcytotoxic arsenite induces cellular and/or mitochondrial morphological alterations consistent with initiated apoptosis, HK-2 cells were evaluated with transmission electron microscopy. Classic morphology of apoptosis was not observed with subcytotoxic arsenite exposures; however, evidence of necrotic changes in the cytoplasmic structure and mitochondrial morphology were apparent. Therefore, based on depolarization of mitochondria and the externalization of phosphatidylserine, HK-2 cells appear to initiate apoptosis following subcytotoxic arsenite insult, but morphological changes indicate that HK-2 cells fail to complete apoptosis and ultimately undergo necrosis. Therefore, subcytotoxic arsenite can be sufficiently toxic to mitochondria that they lose their ability to keep the cell on course for apoptotic cell death.

Isolation, propagation and characterization of primary tubule cell culture from human kidney

Proximal tubule cells (PTC) are the major cell type in the cortical tubulointerstitium. Because PTC play a central role in tubulointerstitial pathophysiology, it is essential to prepare pure PTC from kidney tissue to explore the mechanisms of tubulointerstitial pathology. The authors have successfully refined and characterized primary cultures of human PTC using Percoll density gradient centrifugation as a key PTC enrichment step. The cells obtained by this method retain morphological and functional properties of PTC and are minimally contaminated by other renal cells. In particular, the primary isolates have characteristics of epithelial cells with uniform polarized morphology, tight junction and well-formed apical microvilli. Cytokeratin is uniformly and strongly expressed in the isolates. Brush border enzyme activities and PTC transport properties are retained in the isolates. This method therefore provides an excellent in vitro model for the physiologic study of the human proximal tubule.