Expression of chloride channel, ClC-5, and its role in receptor-mediated endocytosis of albumin in OK cells

Functionally Essential Tubular Proteins Are Lost to Urine-Excreted, Large Extracellular Vesicles during Chronic Renal Insufficiency

Background

The 5/6 nephrectomy (5/6Nx) rat model recapitulates many elements of human CKD. Within weeks of surgery, 5/6Nx rats spontaneously exhibit proximal tubular damage, including the production of very large extracellular vesicles and brush border shedding. We hypothesized that production and elimination of these structures, termed large renal tubular extracellular vesicles (LRT-EVs), into the urine represents a pathologic mechanism by which essential tubule proteins are lost.

Methods

LRT-EVs were isolated from 5/6Nx rat urine 10 weeks after surgery. LRT-EV diameters were measured. LRT-EV proteomic analysis was performed by tandem mass spectrometry. Data are available via the ProteomeXchange Consortium with identifier PXD019207. Kidney tissue pathology was evaluated by trichrome staining, TUNEL staining, and immunohistochemistry.

Results

LRT-EV size and a lack of TUNEL staining in 5/6Nx rats suggest LRT-EVs to be distinct from exosomes, microvesicles, and apoptotic bodies. LRT-EVs contained many proximal tubule proteins that, upon disruption, are known to contribute to CKD pathologic hallmarks. Select proteins included aquaporin 1, 16 members of the solute carrier family, basolateral Na⁺/K⁺-ATPase subunit ATP1A1, megalin, cubilin, and sodium-glucose cotransporters (SLC5A1 and SLC5A2). Histologic analysis confirmed the presence of apical membrane proteins in LRT-EVs and brush border loss in 5/6Nx rats.

Conclusions

This study provides comprehensive proteomic analysis of a previously unreported category of extracellular vesicles associated with chronic renal stress. Because LRT-EVs contain proteins responsible for essential renal functions known to be compromised in CKD, their formation and excretion may represent an underappreciated pathogenic mechanism.

CLC Chloride Channels and Transporters: Structure, Function, Physiology, and Disease

CLC anion transporters are found in all phyla and form a gene family of eight members in mammals. Two CLC proteins, each of which completely contains an ion translocation parthway, assemble to homo- or heteromeric dimers that sometimes require accessory β-subunits for function. CLC proteins come in two flavors: anion channels and anion/proton exchangers. Structures of these two CLC protein classes are surprisingly similar. Extensive structure-function analysis identified residues involved in ion permeation, anion-proton coupling and gating and led to attractive biophysical models. In mammals, ClC-1, -2, -Ka/-Kb are plasma membrane Cl−channels, whereas ClC-3 through ClC-7 are 2Cl−/H+-exchangers in endolysosomal membranes. Biological roles of CLCs were mostly studied in mammals, but also in plants and model organisms like yeast and Caenorhabditis elegans. CLC Cl−channels have roles in the control of electrical excitability, extra- and intracellular ion homeostasis, and transepithelial transport, whereas anion/proton exchangers influence vesicular ion composition and impinge on endocytosis and lysosomal function. The surprisingly diverse roles of CLCs are highlighted by human and mouse disorders elicited by mutations in their genes. These pathologies include neurodegeneration, leukodystrophy, mental retardation, deafness, blindness, myotonia, hyperaldosteronism, renal salt loss, proteinuria, kidney stones, male infertility, and osteopetrosis. In this review, emphasis is laid on biophysical structure-function analysis and on the cell biological and organismal roles of mammalian CLCs and their role in disease.

Antiphospholipid antibodies internalised by human syncytiotrophoblast cause aberrant cell death and the release of necrotic trophoblast debris

Antiphospholipid antibodies (aPL) are the strongest maternal risk factor for pre-eclampsia, a hypertensive disease of human pregnancy. Pre-eclampsia is triggered by a toxic factor released from the placenta that activates the maternal endothelium. Antiphospholipid antibodies cause the release of necrotic trophoblast debris from the placental syncytiotrophoblast and this debris can activate endothelial cells. In this study, we investigated how aPL affects syncytiotrophoblast death and production of necrotic trophoblast debris by examining the interaction between aPL and human first trimester placental explants. Human polyclonal and murine monoclonal aPL, but not control antibodies, were rapidly internalised by the syncytiotrophoblast. Inhibitors of endocytosis or the low-density lipoprotein receptor (LDLR) family, but not toll-like receptors, decreased the internalisation of aPL and prevented the release of necrotic trophoblast debris from the syncytiotrophoblast. Once internalised, aPL increased inner mitochondrial membrane leak and Cytochrome c release while depressing oxidative flux through Complex IV of the electron transport system in syncytiotrophoblast mitochondria. These data suggest that the human syncytiotrophoblast internalises aPL by antigen-dependent endocytosis involving LDLR family members. Once internalised by the syncytiotrophoblast, aPL affects the death-regulating mitochondria, causing extrusion of necrotic trophoblast debris which can activate maternal endothelial cells thereby contributing to the pathogenesis of pre-eclampsia.

Endocytic uptake of FITC-albumin by human alveolar epithelial cell line A549

The uptake mechanism of FITC-labeled albumin (FITC-albumin) was examined in human alveolar epithelial cell line A549. FITC-albumin uptake by A549 cells was time- and temperature-dependent, and was markedly suppressed at 4°C compared with that at 37°C. The uptake was saturable, and was mediated by a high-affinity, low-capacity system and by a low-affinity, high-capacity system. In the following experiments, we focused on the low-affinity system. FITC-albumin uptake was markedly inhibited by metabolic inhibitors and by a vacuolar H⁺-ATPase, bafilomycin A₁. The uptake was inhibited by clathrin-mediated endocytosis inhibitors (phenylarsine oxide and chlorpromazine). Potassium depletion and hypertonicity that inhibit clathrin-mediated endocytosis also decreased FITC-albumin uptake. On the other hand, caveolae-mediated endocytosis inhibitors (indomethacin and nystatin) did not affect FITC-albumin uptake. In addition, FITC-albumin uptake was inhibited by macropinocytosis inhibitors such as 5-(N-ethyl-N-isopropyl) amiloride. These results suggest that the low-affinity system of FITC-albumin uptake is mediated by endocytosis in A549 cells, predominantly via a clathrin-mediated pathway. Macropinocytosis, but not caveolae-mediated endocytosis, may also be involved. Considering our previous findings, albumin may be transported by a similar mechanism and/or pathway in rat and human alveolar epithelial cells.

Megalin/cubilin-mediated uptake of FITC-labeled IgG by OK kidney epithelial cells

In this paper, we characterize the uptake mechanism of fluorescein isothiocyanate-labeled human immunoglobulin G (FITC-hIgG) in opossum kidney (OK) epithelial cells, which have been shown to express megalin and cubilin. Confocal immunofluorescence microscopy showed the punctate expression of the neonatal Fc receptor FcRn in the cytoplasm, but not on the cell surface membrane. Temperature- and energy-dependent uptake of FITC-hIgG was observed at pH 7.4 but not at pH 6.0, indicating that the internalization of FITC-hIgG might not be due to FcRn, which has a binding affinity for IgG under acidic conditions. Under physiological pH conditions, human and bovine serum γ-globulin decreased FITC-hIgG uptake in a concentration-dependent manner. In addition, FITC-hIgG uptake was inhibited by various megalin and/or cubilin ligands including albumin, cytochrome c, transferrin and gentamicin. Endosomal acidification inhibitors (bafilomycin A(1) and chloroquine) significantly decreased the uptake of FITC-hIgG. Clathrin-dependent endocytosis inhibitors (phenylarsine oxide and chlorpromazine) decreased FITC-hIgG uptake. Potassium depletion and hypertonicity, conditions known to inhibit clathrin-dependent endocytosis, also decreased FITC-hIgG uptake. In contrast, caveolin-dependent endocytosis inhibitors (nystatin and methyl-β-cyclodextrin) did not decrease, but rather increased the uptake of FITC-hIgG. These observations suggest that the internalization of FITC-hIgG in OK cells might be, at least in part, due to megalin/cubilin-mediated, clathrin-dependent endocytosis.

Function and expression of ATP-binding cassette transporters in cultured human Y79 retinoblastoma cells

The aim of this study was to reveal the expression and function of P-glycoprotein and multidrug resistance-associated proteins (MRP), members of the ATP-binding cassette (ABC) superfamily of drug transporters, in cultured human Y79 retinoblastoma cells. ABC transporter mRNA expression was evaluated by conventional reverse transcription-polymerase chain reaction (RT-PCR) and real-time PCR analyses. Cellular accumulation of rhodamine 123 (P-glycoprotein substrate), calcein (MRP substrate), and doxorubicin (P-glycoprotein/MRP substrate) was analyzed by fluorometry. Conventional RT-PCR analysis showed the expression of multidrug resistance 1 (MDR1), MRP1, MRP2 and lung resistance-related protein (LRP) mRNAs. Real-time RT-PCR analysis revealed that the expression levels of the MDR1 and MRP2 genes in Y79 cells were much lower than those in human intestinal cell line Caco-2, while the expression level of MRP1 was higher than that in Caco-2 cells. The accumulation of rhodamine 123 was not enhanced by verapamil or reversin 205, inhibitors of P-glycoprotein, indicating no function of P-glycoprotein in Y79 cells. The accumulation of calcein was significantly increased by various MRP inhibitors including probenecid, indicating that MRP functions in Y79 cells. The accumulation of doxorubicin was increased in the presence of metabolic inhibitors (10 mM 2-deoxyglucose and 5 mM sodium azide). However, most MRP inhibitors such as probenecid and indomethacin did not affect doxorubicin accumulation, while cyclosporin A and taclorimus significantly increased doxorubicin accumulation. These results suggest that MRP, but not P-glycoprotein, functions in Y79 cells, and that the efflux of doxorubicin from Y79 cells may be due to an ATP-dependent transporter, which has not been identified yet.

Effects of endocytosis inhibitors on internalization of human IgG by Caco-2 human intestinal epithelial cells

AIMS

The purpose of this study was to characterize the internalization mechanism of human IgG into the epithelial cells of human small intestine, employing human intestinal epithelial cell line Caco-2 as an in vitro model system.

MAIN METHODS

Real-time PCR analysis and uptake studies of fluorescein isothiocyanate-labeled IgG (FITC-IgG) from human serum were performed using Caco-2 cells.

KEY FINDINGS

Real-time PCR analysis showed that mRNA level of the neonatal Fc receptor (FcRn) was increased during the differentiation process in Caco-2 cells. The binding of FITC-labeled human IgG to the membrane surface of Caco-2 cells increased with a decrease in pH of incubation buffer. The uptake of FITC-IgG was also stimulated at acidic pH and was time-dependent. The binding and uptake of FITC-IgG at pH 6.0 was partially, but significantly, decreased by human gamma-globulin in a concentration-dependent manner. A mixture of metabolic inhibitors (sodium azide and 2-deoxyglucose) significantly inhibited the uptake, but not the binding, of FITC-IgG. In addition, endosomal acidification inhibitors such as bafilomycin A(1) and chloroquine significantly increased the accumulation of FITC-IgG. Clathrin-dependent endocytosis inhibitors (phenylarsine oxide and chlorpromazine) and caveolin-dependent endocytosis inhibitors (nystatin and indomethacin) did not decrease the uptake of FITC-IgG at pH 6.0. In contrast, macropinocytosis inhibitors such as cytochalasin B and 5-(N-ethyl-N-isopropyl) amiloride significantly decreased the uptake of FITC-IgG at pH 6.0.

SIGNIFICANCE

The internalization of human IgG in human intestine might be, at least in part, due to FcRn-mediated endocytosis, which could occur by a process other than clathrin- and caveolin-dependent mechanisms.

Low-affinity transport of FITC-albumin in alveolar type II epithelial cell line RLE-6TN

FITC-albumin uptake by cultured alveolar type II epithelial cells, RLE-6TN, is mediated by high- and low-affinity transport systems. In this study, characteristics of the low-affinity transport system were evaluated. The uptake of FITC-albumin was time and temperature dependent and was inhibited by metabolic inhibitors and bafilomycin A1. Confocal laser scanning microscopic analysis showed punctate localization of the fluorescence in the cells, which was partly localized in lysosomes. FITC-albumin taken up by the cells gradually degraded over time, as shown by fluoroimage analyzer after SDS-PAGE. The uptake of FITC-albumin by RLE-6TN cells was not inhibited by caveolae-mediated endocytosis inhibitors such as nystatin, but was inhibited by clathrin-mediated endocytosis inhibitors such as phenylarsine oxide. The uptake was also inhibited by potassium depletion and hypertonicity, conditions known to inhibit clathrin-mediated endocytosis. In addition, macropinocytosis inhibitors such as 5-(N-ethyl-N-isopropyl) amiloride inhibited the uptake. These results indicate that the low-affinity transport of FITC-albumin in RLE-6TN cells is at least in part mediated by clathrin-mediated endocytosis, but not by caveolae-mediated endocytosis. Possible involvement of macropinocytosis was also suggested.

Comparison of Albumin Uptake in Rat Alveolar Type II and Type I-like Epithelial Cells in Primary Culture

PURPOSE

To elucidate and compare the activity and mechanism of albumin uptake in primary cultured alveolar type II and type I-like epithelial cells.

MATERIALS AND METHODS

Type II epithelial cells isolated from rat lungs were cultured for 2 days at 5 x 10(6) cells/35-mm dish or for 6 days at 2 x 10(6) cells/35-mm dish. The mRNA expression of marker genes and FITC-albumin uptake were examined.

RESULTS

The cells cultured for 2 days exhibited cuboidal type II epithelial morphology with lamellar bodies inside the cells, while the cells cultured for 6 days exhibited squamous type I epithelial morphology. These morphological characteristics were consistent with the changes in mRNA expression pattern of marker genes. FITC-albumin uptake in both cells was temperature-dependent and was inhibited by metabolic inhibitors and bafilomycin A1. The rate of uptake was much higher in type II cells than type I-like cells. In both cells, FITC-albumin uptake was inhibited by clathrin mediated-endocytosis inhibitors, but not by caveolae mediated-endocytosis inhibitors.

CONCLUSIONS

These findings indicate that albumin in alveolar lining fluid is internalized into type II and type I epithelial cells via clathrin-mediated endocytosis, and the rate of albumin uptake is higher in type II cells than type I cells.

Evidence for a Clathrin-Mediated Recycling of Albumin in Human Term Placenta1

During human pregnancy, the trophoblast layer is in direct contact with maternal albumin. In contrast to immunoglobulins, albumin does not cross the placental barrier. However, albumin affects the trophoblast placental lactogen and chorionic gonadotroph secretion. The present study investigated the interaction between albumin and syncytiotrophoblast using human term placental explants. Bovine serum albumin, labeled with either 125I or fluorescein isothio-cyanate, was taken up rapidly by placental explants. This process was temperature-sensitive. The internalized labeled BSA quickly outflowed from the tissue at the maternal side, largely without any major modification in molecular weight. Colchicine (1 mM), which disrupts the microtubule network, or cytochalasin B (40 microM), which disassembles filamentous actin, did not interfere with the placental transmembrane movements of labeled BSA. Megalin, clathrin, and caveolin 1 are three membrane proteins associated with albumin endocytosis in other tissues, but only megalin and clathrin were detected in the syncytiotrophoblast layer by immunohistochemistry. The uptake of labeled BSA into placental explants was not modified by 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (1 mM) or 5-nitro-2-(3-phenylpropylamino)benzoic acid (100 microM), two pharmacological tools known to disturb megalin-mediated albumin endocytosis. By contrast, methyl-beta-cyclodextrin (10 mM) and chlorpromazine (1.4 mM), both of which disrupt the clathrin-mediated endocytotic system, significantly reduced the uptake of labeled BSA. These data suggest, to our knowledge for the first time, that maternal albumin is actively internalized into the human trophoblast according to an apical recycling pathway. This temperature-sensitive process does not depend on an intact cytoskeleton, but it is associated with a clathrin-mediated endocytotic system.

Chronic high glucose inhibits albumin reabsorption by lysosomal alkalinization in cultured porcine proximal tubular epithelial cells (LLC-PK1)

Lysosomal acidification is a key step of albumin reabsorption in proximal tubular epithelial cells (PTECs). This study was performed to examine the influence of chronic high glucose on lysosomal acidification in cultured PTECs. Porcine PTECs (LLC-PK(1) cells) were cultured in 16.7 mM (300 mg/dl) glucose (HG) alone or with 0.5 mM phlorizin for 24 weeks and subsequently for 12 weeks in 5.5 mM (100 mg/dl) glucose (NG). Chronic HG inhibited the fluorescein isothiocyanate (FITC)-albumin (A) uptake progressively, while phlorizin reversed the inhibition. NG for 12 weeks after HG normalized the uptake. The time-dependent uptake of FITC-A was inhibited by HG and bafilomycin A(1) (BafA(1)) after 15 min and by 4,4'-diisothiocyanato-2,2'-disulfonic acid (DIDS) and N-ethyl-N-isopropyl-amiloride (EIPA) after 3 min. Cellular ATP was depleted by HG and restored by NG. Lysosomal pH, assessed by an acidotropic fluorescent probe, was alkalinized (pH 4.5-7.8) with 5.5-27.8 mM glucose and normalized by subsequent NG. BafA(1) alkalinized lysosomes, and the concentration required to 50% change for the pH and 50% inhibition of FITC-A uptake was similar. EIPA inhibited FITC-A uptake, but did not influence lysosomal pH. DIDS inhibited FITC-A uptake, and unexpectedly lowered lysosomal pH. Real time PCR showed that HG reduced the mRNA level for vacuolar H(+)-ATPase, but did not alter those of chloride channel-5 and Na(+)-H(+)-exchanger-3. In conclusion, the chronic HG inhibits albumin reabsorption by lysosomal alkalinization in PTECs, probably due to ATP depletion and down-regulation of vacuolar H(+)-ATPase.

Clathrin-mediated endocytosis of FITC-albumin in alveolar type II epithelial cell line RLE-6TN

We examined mechanisms of FITC-albumin uptake by alveolar type II epithelial cells using cultured RLE-6TN cells. Alkaline phosphatase activity and the expression of cytokeratin 19 mRNA, which are characteristic features of alveolar type II epithelial cells, were detected in RLE-6TN cells. The uptake of FITC-albumin by the cells was time and temperature dependent and showed the saturation kinetics of high- and low-affinity transport systems. FITC-albumin uptake was inhibited by native albumin, by chemically modified albumin, and by metabolic inhibitors and bafilomycin A1, an inhibitor of vacuolar H+-ATPase. Confocal laser scanning microscopic analysis after FITC-albumin uptake showed punctate localization of fluorescence in the cells, which was partly localized in lysosomes. FITC-albumin taken up by the cells gradually degraded over time, as shown by fluoroimage analyzer after SDS-PAGE. The uptake of FITC-albumin by RLE-6TN cells was not inhibited by nystatin, indomethacin, or methyl-β-cyclodextrin (inhibitors of caveolae-mediated endocytosis) but was inhibited by phenylarsine oxide and chlorpromazine (inhibitors of clathrin-mediated endocytosis) in a concentration-dependent manner. Uptake was also inhibited by potassium depletion and hypertonicity, conditions known to inhibit clathrin-mediated endocytosis. These results indicate that the uptake of FITC-albumin in cultured alveolar type II epithelial cells, RLE-6TN, is mediated by clathrin-mediated but not by caveolae-mediated endocytosis, and intracellular FITC-albumin is gradually degraded in lysosomes. Possible receptors involved in this endocytic system are discussed.

ClC-5: role in endocytosis in the proximal tubule

The proper functioning of the Cl(-) channel, ClC-5, is essential for the uptake of low molecular mass proteins through receptor-mediated endocytosis in the proximal tubule. Dent's disease patients with mutant ClC-5 channels and ClC-5 knockout (KO) mice both have low molecular mass proteinuria. To further understand the function of ClC-5, endocytosis was studied in LLC-PK(1) cells and primary cultures of proximal tubule cells from wild-type (WT) and ClC-5 KO kidneys. Endocytosis in the proximal tubule cells from KO mice was reduced compared with that in WT animals. Endocytosis in WT but not in KO cells was inhibited by bafilomycin A-1 and Cl(-) depletion, whereas endocytosis in both WT and KO cells was inhibited by the NHE3 blocker, S3226. Infection with adenovirus containing WT ClC-5 rescued receptor-mediated endocytosis in KO cells, whereas infection with any of the three disease-causing mutants, myc-W22G-ClC-5, myc-S520P-ClC-5, or myc-R704X-ClC-5, did not. WT and the three mutants all trafficked to the apical surface, as assessed by surface biotinylation. WT-ClC-5 and the W22G mutant were internalized similarly, whereas neither the S520P nor the R704X mutants was. These data indicate that ClC-5 is important for Cl(-) and proton pump-mediated endocytosis. However, not all receptor-mediated endocytosis in the proximal tubule is dependent on ClC-5. There is a significant fraction that can be inhibited by an NHE3 blocker. Our data from the mutants suggest that defective targeting and trafficking of mutant ClC-5 to the endosomes are a major determinant in the lack of normal endocytosis in Dent's disease.

Transport of prostaglandin E1 across the blood-brain barrier in rats

The transport of prostaglandin E(1) (PGE(1)) across the blood-brain barrier (BBB) was characterized using an in-situ rat brain perfusion technique. The uptake of [(3)H]PGE(1) was not affected by shortchain monocarboxylic acids (butyric acid and valeric acid). On the other hand, uptake of [(3)H]PGE(1) was significantly inhibited by medium-chain monocarboxylic acids such as hexanoic acid, enanthic acid and octanoic acid. These medium-chain monocarboxylic acids showed a more potent inhibitory effect on [(3)H]PGE(1) uptake with increasing number of carbon atoms. In contrast, there was no decrease in [(3)H]PGE(1) transport by any dicarboxylic acids with 5-8 carbon atoms. Valproic acid decreased [(3)H]PGE(1) uptake, whereas p-aminohippuric acid, a substrate for the organic anion transporter family, did not inhibit [(3)H]PGE(1) transport. Bromocresol green, an inhibitor of prostaglandin transporter (PGT), strongly decreased [(3)H]PGE(1) transport across the BBB. In addition, digoxin and taurocholate, substrates for organic anion transporting polypeptide subtype 2 (Oatp2), significantly inhibited [(3)H]PGE(1) uptake. RT-PCR analysis revealed that PGT mRNA and Oatp2 mRNA are expressed in a capillary-rich fraction from rat brain. Thus, it is suggested that PGE(1) transport across the BBB is mediated by some specific transport systems, possibly by the members of the Oatp family.

High glucose reduces albumin uptake in cultured proximal tubular cells (LLC-PK1)

In this study, we clarify that high glucose inhibits albumin uptake in cultured LLC-PK1 cells. LLC-PK1 cells cultured for 6 days with 5.5-27.8 mM D-glucose were challenged by fluorescein isothiocyanate (FITC)-conjugated human albumin (HA). FITC-HA binding and uptake were inhibited by >5.5mM glucose (5.5 mM > (P < 0.01) 11.0 mM > (P < 0.05) 16.7 mM approximately= 27.8 mM). Analysis of FITC-HA binding and uptake at 5.5 and 16.7 mM D-glucose (high glucose, HG) showed decreased affinity (K(m) for binding: 35.5 mg/l versus 52.6 mg/l, K(m) for uptake; 41.3 mg/l versus 55.6 mg/l) and maximal velocity (B(max)--0.33 microg versus 0.27 microg/30 min/mg protein; U(max)--4.40 microg versus 3.48 microg/60 min/mg protein) at HG. A comparison of the time courses of FITC-HA binding and uptake at 5.5 mM glucose and at HG showed that HG suppressed them beyond 15 min (P < 0.005-0.001). Phlorizin (>0.25 mM) completely reversed the HG-induced inhibition of FITC-HA binding and uptake. High glucose decreased mRNA of GLUT-1 and SGLT-1, but did not influence that of SGLT-2. The simultaneous presence of Vitamin E (10(-6)M), Vitamin C (10(-6)M) and reduced glutathione (0.25 mM) reversed the suppressed FITC-HA binding and uptake by HG, while any one or two of these molecules, and various inhibitors of advanced glycation end products, failed to do so. In conclusion, a high glucose milieu causes inhibition of albumin binding and uptake in proximal tubular cells by increasing metabolic oxidative stress through excessive glucose flux via the sodium glucose transporter.

Hypertonicity increases CLC-5 expression in mouse medullary thick ascending limb cells

Genetic studies indicated that mutations of the chloride channel CLC-5 in the kidney are responsible for a group of clinical disorders, collectively called Dent's disease. In the kidney, CLC-5 was found to be expressed in the proximal tubule, medullary thick ascending limb (mTAL) of loop of Henle, and intercalated cells of the collecting tubule. In proximal tubular cells, CLC-5 was found to play an important role in receptor-mediated endocytosis. However, the functional roles of CLC-5 in mTAL and collecting tubules remain unclear. Because mTAL is normally exposed to a hypertonic environment, we aimed to examine the effect of hypertonicity on CLC-5 expression in this nephron segment. Our studies revealed that exposure to hypertonicity (up to 550 mosM) increased CLC-5 mRNA and protein levels in a murine mTAL cell line (MTAL) but not in an opossum kidney (OK) proximal tubular cell line. A similar effect was also found in mouse kidneys, where CLC-5 expression was enhanced in renal medulla, but not cortex, after 48 h of water deprivation. We also tested the effect of hypertonicity on endocytotic activity and found that exposure to hypertonicity caused a significant decrease in cellular uptake of FITC-labeled albumin in OK but not in MTAL cells. Our results suggest that CLC-5 expression is upregulated by hypertonicity in mTAL cells but not in proximal tubular cells. We speculate that the increased CLC-5 levels in mTAL may serve to maintain the endocytotic activity in a hypertonic environment.

Expression and roles of Cl- channel ClC-5 in cell cycles of myeloid cells

ClC-5 is a chloride channel known to be expressed in the kidney. We previously reported that ClC-5 mRNA was also strongly expressed in immature human myeloid cell line (HL-60), but weakly expressed in mature neutrophils. To clarify the underlying mechanisms, we examined the relationship between ClC-5 expression and cell cycle. Dimethyl sulfoxide treatment of HL-60 that causes differentiation with G0/G1 cell cycle arrest decreased the expression of ClC-5 mRNA. Cell sorting and synchronization experiments revealed that ClC-5 mRNA expression was high in S and G2/M phases and low in G0/G1 phase. ClC-5 antisense oligonucleotide suppressed proliferation of HL-60 cells with a decrease in ClC-5 protein expression, probably due to G2 arrest. These results suggest that cell cycle-dependent expression of ClC-5 has a role in cell cycle progression in myeloid cells.

Targeted prevention of renal accumulation and toxicity of gentamicin by aminoglycoside binding receptor antagonists

Receptor-mediated endocytosis plays an important role in accumulation of aminoglycosides in renal proximal tubule. To prevent aminoglycoside-induced nephrotoxicity following concentrated accumulation of gentamicin in the kidney, effect of cationic proteins and their peptide fragments, which could inhibit gentamicin binding to its binding receptor(s), was investigated. Among several substrates for megalin, an endocytic receptor responsible for renal accumulation of aminoglycosides, cytochrome c potently inhibited gentamicin accumulation in renal cortex. Concentration-dependent inhibition by cytochrome c on gentamicin uptake was also observed in OK kidney epithelial cells expressing megalin. In addition, gentamicin-induced increase in urinary excretion of N-acetyl-beta-d-glucosaminidase (NAG), a marker of renal tubular damage, was significantly reduced by cytochrome c. We next attempted to find a peptide fragment with lower molecular size showing inhibitory effect on gentamicin uptake. Cyto79-88 inhibited gentamicin uptake in OK cells, but had little effect on renal accumulation of gentamicin in mice in vivo. On one hand, a peptide fragment of neural Wiskott-Aldrich syndrome protein (N-WASP), which interacts with acidic phospholipids like aminoglycosides, inhibited gentamicin accumulation not only in OK cells but also in mouse kidney. These results show that substrates and/or their peptide fragments for aminoglycoside binding receptor such as megalin might be useful for preventing aminoglycoside-induced nephrotoxicity.

The chloride channel ClC-4 contributes to endosomal acidification and trafficking

Mutations in the gene coding for the chloride channel ClC-5 cause Dent's disease, a disease associated with proteinuria and renal stones. Studies in ClC-5 knockout mice suggest that this phenotype is related to defective endocytosis of low molecular weight proteins and membrane proteins by the renal proximal tubule. In this study, confocal micrographs of proximal tubules and cultured epithelial cells revealed that the related protein ClC-4 is expressed in endosomal membranes suggesting that this channel may also contribute to the function of this organelle. In support of this hypothesis, specific disruption of endogenous ClC-4 expression by transfection of ClC-4 antisense cDNA acidified endosomal pH and altered transferrin trafficking in cultured epithelial cells to the same extent as the specific disruption of ClC-5. Both channels can be co-immunoprecipitated, arguing that they may partially contribute to endosomal function as a channel complex. These studies prompt future investigation of the role of ClC-4 in renal function in health and in Dent's disease. Future studies will assess whether the severity of Dent's disease relates not only to the impact of particular mutations on ClC-5 but also on the consequences of those mutations on the functional expression of ClC-4.

Cisplatin-induced inhibition of receptor-mediated endocytosis of protein in the kidney

BACKGROUND

Administration of cisplatin, cis-diamminedichloroplatinum (II) (CDDP), causes a severe impairment of renal function, including increases in urinary excretion of proteins. We recently found that CDDP inhibits vacuolar H+-ATPase, which plays an important role in receptor-mediated endocytosis in the renal proximal tubules. Therefore, CDDP-induced proteinuria may be due to an inhibition of the receptor-mediated endocytosis in the renal proximal tubules following a decrease in vacuolar H+-ATPase activity by the drug.

METHODS

Effects of CDDP on receptor-mediated endocytosis of albumin in opossum kidney (OK) epithelial cells, and on urinary excretion of albumin and vitamin D binding protein, which are reabsorbed in the renal proximal tubules by endocytosis, in rats were examined.

RESULTS

CDDP inhibited uptake of fluorescein-isothiocyanate (FITC)-albumin, a receptor-mediated endocytosis marker, by OK cells in a time- and concentration-dependent fashion. In contrast, CDDP treatment did not affect the uptake of FITC-inulin, a fluid-phase endocytosis marker. CDDP caused a decrease in the affinity and in the maximal velocity of FITC-albumin uptake. The adenosine 5'-triphosphate (ATP) content in OK cells was not changed by CDDP at concentrations that inhibited FITC-albumin uptake. The endosomal pH in OK cells was increased by CDDP treatment. Administration of CDDP to rats increased the urinary excretion of albumin and vitamin D binding protein.

CONCLUSIONS

These results suggest that CDDP decreases the receptor-mediated endocytosis of protein following the inhibition of vacuolar H+-ATPase in the renal proximal tubules, and the inhibition of receptor-mediated endocytosis would be the mechanisms underlying the proteinuria induced by CDDP.