Immunocytochemistry of band 3 protein in kidney and other tissues of control and cystic fibrosis patients

Anion exchanger 1 in red blood cells and kidney: Band 3's in a pod

The bicarbonate/chloride exchanger 1 (AE1, Band 3) is abundantly expressed in the red blood cell membrane, where it is involved in gas exchange and functions as a major site of cytoskeletal attachment to the erythrocyte membrane. A truncated kidney isoform (kAE1) is highly expressed in type A intercalated cells of the distal tubules, where it is vital for urinary acidification. Recently, kAE1 has emerged as a novel physiologically significant protein in the kidney glomerulus. This minireview will discuss the known interactions of kAE1 in the podocytes and the possible mechanisms whereby this important multispanning membrane protein may contribute to the function of the glomerular filtration barrier and prevent proteinuria.

Anion exchanger 1 interacts with nephrin in podocytes

The central role of the multifunctional protein nephrin within the macromolecular complex forming the glomerular slit diaphragm is well established, but the mechanisms linking the slit diaphragm to the cytoskeleton and to the signaling pathways involved in maintaining the integrity of the glomerular filter remain incompletely understood. Here, we report that nephrin interacts with the bicarbonate/chloride transporter kidney anion exchanger 1 (kAE1), detected by yeast two-hybrid assay and confirmed by immunoprecipitation and co-localization studies. We confirmed low-level glomerular expression of kAE1 in human and mouse kidneys by immunoblotting and immunofluorescence microscopy. We observed less kAE1 in human glomeruli homozygous for the NPHS1(FinMaj) nephrin mutation, whereas kAE1 expression remained unchanged in the collecting duct. We could not detect endogenous kAE1 expression in NPHS1(FinMaj) podocytes in primary culture, but heterologous re-introduction of wild-type nephrin into these podocytes rescued kAE1 expression. In kidneys of Ae1(-/-) mice, nephrin abundance was normal but its distribution was altered along with the reported kAE1-binding protein integrin-linked kinase (ILK). Ae1(-/-) mice had increased albuminuria with glomerular enlargement, mesangial expansion, mesangiosclerosis, and expansion of the glomerular basement membrane. Glomeruli with ILK-deficient podocytes also demonstrated altered AE1 and nephrin expression, further supporting the functional interdependence of these proteins. These data suggest that the podocyte protein kAE1 interacts with nephrin and ILK to maintain the structure and function of the glomerular basement membrane.

Molecular mapping of human band 3 aging antigenic sites and active amino acids using synthetic peptides

An aging antigen, senescent cell antigen appears on old cells and marks them for death by initiating the binding of IgG autoantibody and subsequent removal by phagocytes. This antigen is derived from the major anion transport protein, protein band 3, that is involved in respiration and acid base balance. We use synthetic peptides from the transmembrane, anion transport segment of band 3 to "walk" band 3 to identify potential aging antigenic sites. A competitive inhibition assay with affinity purified IgG autoantibody from senescent red cells was used. Results indicate that: aging antigenic sites reside on human band 3 residues 538-554, 593-601, and 812-830; and that the smallest residues which act as aging antigenic sites are 593-601 and 813-818. The contribution of lysine and/or arginine to antigenicity is examined by synthesizing peptide analogs in which glycines or arginines are substituted for lysines or arginines. Substitution of neutral glycine for the positively charged amino acids arginine or lysine or both arginine and lysine did not result in a significant difference in antigenicity between the analog and the native band 3 peptide. Substitution of the positively charged arginine for the positively charged lysine resulted in a significant reduction in antigenicity. The chicken sequence of band 3 peptides 538-554 and 812-827 differs from that of the human peptides at several sites. Antigenicity of these chicken "analogs" were tested and compared to the human peptides. The data suggest that the three-dimensional configuration of band 3 segments plays a dominant role in defining the antigenic determinants reactive with senescent cell IgG autoantibodies.

Abnormal fucosylation of-ileal mucus in cystic fibrosis: II. A histochemical study using monoclonal antibodies to fucosyl oligosaccharides

Abnormal fucosylation of cystic fibrosis mucin was previously shown using peroxidase conjugated lectins on ileal tissue sections. These abnormally fucosylated glycoproteins were investigated further using monoclonal antibodies to fucosyl oligosaccharides based on type 1 and type 2 blood group precursor chains. The results of this study, using monoclonal antibodies to blood group glycoproteins in cystic fibrosis, were negative, yet abnormal fucosylation had been found using lectin histochemistry. Using monoclonal antibodies, lectins, and appropriate enzymes, such as glycosyl hydrolases, it should be possible to delineate further the abnormality found in glycoproteins in cystic fibrosis on appropriately fixed ileal sections, obtained from infants at term presenting with meconium ileus.

Monoclonal antibodies to the membrane domain of the human erythrocyte anion transport protein. Localization of the C-terminus of the protein to the cytoplasmic side of the red cell membrane and distribution of the protein in some human tissues

(1) We have prepared murine monoclonal antibodies to the membrane domain of the human erythrocyte anion transport protein (band 3). (2) All of these antibodies react with regions of the protein located at the cytoplasmic surface of the red cell. (3) One of the antibodies reacts with an epitope present on a cytoplasmic loop of the protein located between the C-terminus and a point 168 amino acids from the C-terminus. The other antibodies recognize different epitopes on the C-terminal tail of the protein and the sequences likely to be involved in these epitopes are defined. (4) Our results show that the C-terminus of the red-cell anion transport protein is located on the cytoplasmic side of the red-cell membrane. (5) None of the antibodies inhibited sulphate exchange transport when introduced into resealed red-cell membranes; however, the bivalent form of one of the antibodies reduced the inhibitory potency of 4-acetamido-4'-isothiocyanatostilbene disulphonate on sulphate exchange transport in resealed erythrocyte membranes. (6) Immunostaining of human kidney sections with the antibodies showed strong staining of the basolateral membrane of some but not all of the epithelial cells of distal tubules and the initial connecting segment of collecting tubules. With human liver, only the haematopoeitic cells of fetal liver reacted with all the antibodies.

Intercalated cells as a probable source for the development of renal oncocytoma

Renal oncocytoma is a distinct type of epithelial tumor said to arise from the collecting duct system. Here we show that in nine of ten oncocytomas the tumor cells expressed an analog of the erythrocyte anion exchanger band 3. In the normal kidney band 3 is confined to the basolateral surface of the majority of intercalated cells which comprise up to 50% of the cortical collecting duct epithelium. Carbonic anhydrase c is another protein abundant in intercalated cells, and this was also expressed in six of the ten oncocytomas investigated. Immunoreactivity specific for band 3 and carbonic anhydrase c was not detected in any of the 20 renal cell carcinomas examined. At favourable section planes direct transitions between normal collecting ducts and oncocytic tubules were observed. These findings suggest that oncocytomas may develop from intercalated cells of the collecting duct epithelium.