Intracellular band 3 immunostaining in type A intercalated cells of rabbit kidney

Immunolocalization of anion exchanger 1 (Band 3) in the renal collecting duct of the common marmoset

The purpose of this study was to determine the expression and distribution of band 3 in the collecting duct and connecting tubules of the kidney of the marmoset monkey (Callithrix jacchus), and to establish whether band 3 is expressed in type A intercalated cells. The intracellular localization of band 3 in the different populations of intercalated cells was determined by double-labeling immunohistochemistry. Immunohistochemical microscopy demonstrated that band 3 is located in the basolateral plasma membranes of all type A intercalated cells in the connecting tubule (CNT), cortical collecting duct (CCD), and outer medullary collecting duct (OMCD) of the marmoset. However, type B intercalated cells and non-A/non-B intercalated cells did not show band 3 labeling. Electron microscopy of the CNT, CCD and OMCD confirmed the light microscopic observation of the basolateral plasma membrane staining for band 3 in a subpopulation of interacted cells. Basolateral staining was seen on the plasma membrane and small coated vesicles in the perinuclear structure, some of which were located in the Golgi region. In addition, there was no labeling of band 3 in the mitochondria of the CNT, CCD and in OMCD cells. The intensity of the immunostaining of the basolateral membrane was less in the CNT than in the CCD and OMCD. In contrast, band 3 immunoreactivity was greater in the intracellular vesicles of the CNT. From these results, we suggest that the basolateral Cl^-/HCO₃^- exchanger in the monkey kidney is in a more active state in the collecting duct than in the CNT.

Signal- and importin-dependent nuclear targeting of the kidney anion exchanger 1-binding protein kanadaptin

Kanadaptin (kidney anion exchanger adaptor protein) has recently been identified as a protein with binding activity to the cytoplasmic domain of the kidney Na(+)-independent Cl(-)/HCO(-)(3) anion exchanger 1 (kAE1). Since it is widely expressed in tissues devoid of kAE1, however, kanadaptin is likely to have additional cellular roles. This is supported by its multidomain structure, and possession of three clusters of basic amino acids exhibiting similarity to known nuclear localization sequences (NLSs). In the present study, we use immunofluorescence and subcellular fractionation approaches to demonstrate that kanadaptin is localized within the nuclei of various epithelial and non-epithelial cultured cell types. The role of the different NLSs is examined in transfection studies using plasmids encoding full-length kanadaptin with or without green fluorescent protein (GFP) as a fusion tag, as well as truncation derivatives thereof. Strong nuclear localization of fusion proteins containing amino acids 140-230 of kanadaptin, which include the sequence AVSRKRKA(193) (NLS1) was observed. Substitution of Arg(191) with a threonine residue resulted in a cytoplasmic location of the expressed protein, while NLS1 proved sufficient to target an otherwise cytoplasmically localized beta-galactosidase-GFP fusion protein to the nucleus. Using a direct binding assay we show that a fusion protein containing kanadaptin amino acids 1-231 (but not the Thr(191) substituted derivative) is recognized with nM affinity by the conventional NLS-binding importin alpha/beta heterodimer. Nuclear import studies on microinjected and permeabilized rat hepatoma cells demonstrated functionality of the NLS in nuclear targeting, with inhibition by antibodies demonstrating the requirement of both importin alpha and beta for nuclear import of kanadaptin. That kanadaptin possesses a functional importin-alpha/beta-recognized NLS explains the nuclear localization of kanadaptin in various cultured cell types, and opens up the possibility that kanadaptin may have a signalling role in the nucleus.

ClC-5, the chloride channel mutated in Dent's disease, colocalizes with the proton pump in endocytotically active kidney cells

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 alpha-intercalated cells. In transfected cells, ClC-5 colocalizes with endocytosed alpha2-macroglobulin. Cotransfection with a GTPase-deficient 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.