Loss-of-function mutations of the ClC-5 chloride channel lead to Dent's disease, a syndrome characterized by low molecular weight proteinuria, hypercalciuria, and kidney stones. We show that ClC-5 is expressed in renal proximal tubule cells, which normally endocytose proteins passing the glomerular filter. Expression is highest below the brush border in a region densely packed with endocytotic vesicles, where ClC-5 colocalizes with the H ؉ -ATPase and with internalized proteins early after uptake. In intercalated cells of the collecting duct it again localizes to apical intracellular vesicles and colocalizes with the proton pump in ␣-intercalated cells. In transfected cells, ClC-5 colocalizes with endocytosed ␣ 2 -macroglobulin. Cotransfection with a GTPasedeficient rab5 mutant leads to enlarged early endosomes that stain for ClC-5. We suggest that ClC-5 may be essential for proximal tubular endocytosis by providing an electrical shunt necessary for the efficient acidification of vesicles in the endocytotic pathway, explaining the proteinuria observed in Dent's disease.

ClC-5 is the Cl- channel that is mutated in Dent's disease, an X-chromosome-linked disease characterized by low molecular weight proteinuria, hypercalciuria, and kidney stones. It is predominantly expressed in endocytically active renal proximal cells. We investigated whether this Cl- channel could also be expressed in intestinal tissues that have endocytotic machinery. ClC-5 mRNA was detected in the rat duodenum, jejunum, ileum, and colon. Western blot analyses revealed the presence of the 83-kDa ClC-5 protein in these tissues. Indirect immunofluorescence studies showed that ClC-5 was mainly concentrated in the cytoplasm above the nuclei of enterocytes and colon cells. ClC-5 partially colocalized with the transcytosed polymeric immunoglobulin receptor but was not detectable together with the brush-border-anchored sucrase isomaltase. A subfractionation of vesicles obtained by differential centrifugation showed that ClC-5 is associated with the vacuolar 70-kDa H+-ATPase and the small GTPases rab4 and rab5a, two markers of early endosomes. Thus these results indicate that ClC-5 is present in the small intestine and colon of rats and suggest that it plays a role in the endocytotic pathways of intestinal cells.

The effects of ammonium hydroxide treatment of complexes consisting of biotinylated nucleic acids and immobilized streptavidin were investigated. It was found that incubation of such complexes with ammonium hydroxide at room temperature leads to denaturation of double-stranded DNA molecules, liberating only the complementary nonbiotinylated strand, whereas incubation at elevated temperatures leads to an efficient dissociation of biotin-streptavidin complexes. The introduced procedure is especially suitable as a purification and conditioning format prior to matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometric analysis of DNA from complex enzymatic reactions. This is demonstrated by analysis of polymerase chain reaction (PCR) and sequencing products.

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