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

From protein uptake to Dent disease: An overview of the CLCN5 gene

Proteinuria is a well-known risk factor, not only for renal disorders, but also for several other problems such as cardiovascular diseases and overall mortality. In the kidney, the chloride channel Cl^-/H⁺ exchanger ClC-5 encoded by the CLCN5 gene is actively involved in preventing protein loss. This action becomes evident in patients suffering from the rare proximal tubulopathy Dent disease because they carry a defective ClC-5 due to CLCN5 mutations. In fact, proteinuria is the distinctive clinical sign of Dent disease, and mainly involves the loss of low-molecular-weight proteins. The identification of CLCN5 disease-causing mutations has greatly improved our understanding of ClC-5 function and of the ClC-5-related physiological processes in the kidney. This review outlines current knowledge regarding the CLCN5 gene and its protein product, providing an update on ClC-5 function in tubular and glomerular cells, and focusing on its relationship with proteinuria and Dent disease.

Dent disease: A window into calcium and phosphate transport

This review examines calcium and phosphate transport in the kidney through the lens of the rare X-linked genetic disorder Dent disease. Dent disease type 1 (DD1) is caused by mutations in the CLCN5 gene encoding ClC-5, a Cl^- /H⁺ antiporter localized to early endosomes of the proximal tubule (PT). Phenotypic features commonly include low molecular weight proteinuria (LMWP), hypercalciuria, focal global sclerosis and chronic kidney disease; calcium nephrolithiasis, nephrocalcinosis and hypophosphatemic rickets are less commonly observed. Although it is not surprising that abnormal endosomal function and recycling in the PT could result in LMWP, it is less clear how ClC-5 dysfunction disturbs calcium and phosphate metabolism. It is known that the majority of calcium and phosphate transport occurs in PT cells, and PT endocytosis is essential for calcium and phosphorus reabsorption in this nephron segment. Evidence from ClC-5 KO models suggests that ClC-5 mediates parathormone endocytosis from tubular fluid. In addition, ClC-5 dysfunction alters expression of the sodium/proton exchanger NHE3 on the PT apical surface thus altering transcellular sodium movement and hence paracellular calcium reabsorption. A potential role for NHE3 dysfunction in the DD1 phenotype has never been investigated, either in DD models or in patients with DD1, even though patients with DD1 exhibit renal sodium and potassium wasting, especially when exposed to even a low dose of thiazide diuretic. Thus, insights from the rare disease DD1 may inform possible underlying mechanisms for the phenotype of hypercalciuria and idiopathic calcium stones.

Barttin Regulates the Subcellular Localization and Posttranslational Modification of Human Cl-/H+ Antiporter ClC-5

Dent disease 1 (DD1) is a renal salt-wasting tubulopathy associated with mutations in the Cl-/H+ antiporter ClC-5. The disease typically manifests with proteinuria, hypercalciuria, nephrocalcinosis, and nephrolithiasis but is characterized by large phenotypic variability of no clear origin. Several DD1 cases have been reported lately with additional atypical hypokalemic metabolic alkalosis and hyperaldosteronism, symptoms usually associated with another renal disease termed Bartter syndrome (BS). Expression of the Bartter-like DD1 mutant ClC-5 G261E in HEK293T cells showed that it is retained in the ER and lacks the complex glycosylation typical for ClC-5 WT. Accordingly, the mutant abolished CLC ionic transport. Such phenotype is not unusual and is often observed also in DD1 ClC-5 mutants not associated with Bartter like phenotype. We noticed, therefore, that one type of BS is associated with mutations in the protein barttin that serves as an accessory subunit regulating the function and subcellular localization of ClC-K channels. The overlapping symptomatology of DD1 and BS, together with the homology between the proteins of the CLC family, led us to investigate whether barttin might also regulate ClC-5 transport. In HEK293T cells, we found that barttin cotransfection impairs the complex glycosylation and arrests ClC-5 in the endoplasmic reticulum. As barttin and ClC-5 are both expressed in the thin and thick ascending limbs of the Henle's loop and the collecting duct, interactions between the two proteins could potentially contribute to the phenotypic variability of DD1. Pathologic barttin mutants differentially regulated trafficking and processing of ClC-5, suggesting that the interaction between the two proteins might be relevant also for the pathophysiology of BS. Our findings show that barttin regulates the subcellular localization not only of kidney ClC-K channels but also of the ClC-5 transporter, and suggest that ClC-5 might potentially play a role not only in kidney proximal tubules but also in tubular kidney segments expressing barttin. In addition, they demonstrate that the spectrum of clinical, genetic and molecular pathophysiology investigation of DD1 should be extended.

Uriniferous tubule: structural and functional organization

The uriniferous tubule is divided into the proximal tubule, the intermediate (thin) tubule, the distal tubule and the collecting duct. The present chapter is based on the chapters by Maunsbach and Christensen on the proximal tubule, and by Kaissling and Kriz on the distal tubule and collecting duct in the 1992 edition of the Handbook of Physiology, Renal Physiology. It describes the fine structure (light and electron microscopy) of the entire mammalian uriniferous tubule, mainly in rats, mice, and rabbits. The structural data are complemented by recent data on the location of the major transport- and transport-regulating proteins, revealed by morphological means(immunohistochemistry, immunofluorescence, and/or mRNA in situ hybridization). The structural differences along the uriniferous tubule strictly coincide with the distribution of the major luminal and basolateral transport proteins and receptors and both together provide the basis for the subdivision of the uriniferous tubule into functional subunits. Data on structural adaptation to defined functional changes in vivo and to genetical alterations of specified proteins involved in transepithelial transport importantly deepen our comprehension of the correlation of structure and function in the kidney, of the role of each segment or cell type in the overall renal function,and our understanding of renal pathophysiology.

Dent's disease

Dent's disease is a renal tubular disorder characterized by manifestations of proximal tubule dysfunction, including low-molecular-weight proteinuria, hypercalciuria, nephrolithiasis, nephrocalcinosis, and progressive renal failure. These features are generally found in males only, and may be present in early childhood, whereas female carriers may show a milder phenotype. Prevalence is unknown; the disorder has been reported in around 250 families to date. Complications such as rickets or osteomalacia may occur. The disease is caused by mutations in either the CLCN5 (Dent disease 1) or OCRL1 (Dent disease 2) genes that are located on chromosome Xp11.22 and Xq25, respectively. CLCN5 encodes the electrogenic Cl⁻/H(+) exchanger ClC-5, which belongs to the CLC family of Cl⁻ channels/transporters. OCRL1 encodes a phosphatidylinositol bisphosphate (PIP₂) 5-phosphatase and mutations are also associated with Lowe Syndrome. The phenotype of Dent's disease is explained by the predominant expression of ClC-5 in the proximal tubule segments of the kidney. No genotype-phenotype correlation has been described thus far, and there is considerable intra-familial variability in disease severity. A few patients with Dent's disease do not harbour mutations in CLCN5 and OCRL1, pointing to the involvement of other genes. Diagnosis is based on the presence of all three of the following criteria: low-molecular-weight proteinuria, hypercalciuria and at least one of the following: nephrocalcinosis, kidney stones, hematuria, hypophosphatemia or renal insufficiency. Molecular genetic testing confirms the diagnosis. The differential diagnosis includes other causes of generalized dysfunction of the proximal tubules (renal Fanconi syndrome), hereditary, acquired, or caused by exogenous substances. Antenatal diagnosis and pre-implantation genetic testing is not advised. The care of patients with Dent's disease is supportive, focusing on the treatment of hypercalciuria and the prevention of nephrolithiasis. The vital prognosis is good in the majority of patients. Progression to end-stage renal failure occurs between the 3rd and 5th decades of life in 30-80% of affected males.

Chloride channels and endocytosis: new insights from Dent's disease and ClC-5 knockout mice

Dent's disease is a hereditary renal tubular disorder characterized by low-molecular weight (LMW) proteinuria, hypercalciuria and nephrolithiasis. The disease is due to mutations of ClC-5, a member of the family of voltage-gated CLC chloride channels. ClC-5 is expressed in part in cells lining the proximal tubule (PT) of the kidney, where it colocalizes with albumin-containing endocytic vesicles belonging to the receptor-mediated endocytic pathway that ensures efficient reabsorption of ultrafiltrated LMW proteins. Since progression along the endocytic apparatus requires endosomal acidification, it has been suggested that dysfunction of ClC-5 in endosomes may lead to inefficient reabsorption of LMW proteins and dysfunction of PT cells. Analysis of a ClC-5 knockout (KO) mouse model, displaying all the characteristic renal tubular defects of Dent's disease, showed evidence of a severe LMW proteinuria. Cytochemical studies with the endocytic tracer, peroxidase, showed poor transfer into early endocytic vesicles, suggesting that impairment of receptor-mediated endocytosis in PT cells is the basis for the defective uptake of LMW proteins in patients with Dent's disease. Endocytosis and processing of LMW proteins involve the multiligand tandem receptors, megalin and cubilin, that are abundantly expressed at the brush border of PT cells. Characterization of the endocytic defect in ClC-5 KO mice revealed that ligands of both megalin and cubilin were affected. The total kidney content of megalin and especially cubilin at the protein level was decreased but, more importantly, using analytical subcellular fractionation and quantitative immunogold labelling we demonstrated a selective disappearance of megalin and cubilin at the brush border of PT cells. These observations allowed us to conclude that defective protein endocytosis linked to ClC-5 inactivation is due at least in part to a major and selective loss of megalin and cubilin at the brush border, reflecting a trafficking defect in renal PT cells. These results improve our understanding of Dent's disease, taken as a paradigm for renal Fanconi syndrome and nephrolithiasis, and demonstrate multiple roles for ClC-5 in the kidney. These studies also provided insights into important functions such as apical endocytosis, handling of proteins by renal tubular cells, calcium metabolism, and urinary acidification.

Endocytosis of the somatostatin analogue, octreotide, by the proximal tubule-derived opossum kidney (OK) cell line

BACKGROUND

Nephrotoxicity of cancer therapy using radiolabeled somatostatin analogues such as octreotide is due to ultrafiltration and reuptake by proximal tubular cells (PTCs). The mechanism of uptake is unknown. It could occur either by receptor-mediated endocytosis via a somatostatin receptor or, alternatively, the multiligand megalin/cubilin tandem receptor, or by fluid-phase endocytosis. To define the mechanisms of internalization and to identify potential receptors, we have studied the uptake and processing of octreotide by the PTC-derived opossum kidney (OK) cell line.

METHODS

We compared the kinetics of uptake and fate of (111)In-diethylenetriamine pentaacetic acid (DTPA)-D-Phe(1)-octreotide and (125)I-human serum albumin ((125)I-HSA). To determine the contribution of receptor-mediated endocytosis, we tested competition for uptake by octreotide and somatostatin and by various megalin/cubilin ligands [receptor-associated protein (RAP), albumin, transferrin, insulin, polymixin B] or basic amino acids. The subcellular localization of fluorescein isothiocyanate (FITC)-D-Phe(1)-octreotide was studied by confocal microscopy.

RESULTS

Kinetics of uptake of (111)In-DTPA-D-Phe(1)-octreotide and (125)I-HSA by OK cells were comparable, but only the somatostatin analogue was significantly retained intact. All megalin/cubilin ligands and basic amino acids strongly inhibited (125)I-HSA uptake, but these could not compete for >50% of (111)In-DTPA-D-Phe(1)-octreotide uptake. The same was found for somatostatin and octreotide. The noncompetable uptake of (111)In-DTPA-D-Phe(1)-octreotide was comparable to the clearance of Lucifer Yellow, a marker of fluid-phase endocytosis. By confocal microscopy, FITC-D-Phe(1)-octreotide colocalized with transferrin in endosomes, then accumulated in lysosomes.

CONCLUSION

Receptor-mediated endocytosis via megalin/cubilin and fluid-phase endocytosis contribute about equally to the uptake of radiolabeled somatostatin analogues by OK cells.