Isolation of anionic sialoproteins from the rat glomerulus

Interaction of polycations with cell-surface negative charges of epithelial cells

The interaction between various polycations and cultured glomerular epithelial cells was studied by cell electrophoresis. It was shown that the glomerular epithelial cell presents a negatively charged surface which imparts a zeta potential of -29.0 +/- 1.5 mV at the peripheral layer of the plasma membrane. The pH at which the GEC charge became 50% reduced (pKa) was determined to be 3.0. A variety of polycations of various sizes and fixed and flexible geometries were tested for their capacity to neutralize the cell charge. All the polycations except cytochrome c and lysozyme were capable of completely neutralizing the cell. Cytochrome c could maximally neutralize only 50% of charge and lysozyme only 72% of charge. However, reduced and 'relaxed' molecules of cytochrome c and lysozyme efficiently neutralized the cell surface, as did larger sized 'flexible' polylysines. On the basis of these findings, we conclude that all polycations are not equal in their capacity to neutralize the cell surface. Flexible molecules in contrast to molecules with rigid structures were more effective in neutralizing the cell. This may likely be due to the exposure and availability of more cationic groups in a flexible molecule which results in stabilization of interaction with cells.

Identification of molecules in the kidney utilizing immunocytochemistry

Immunocytochemistry of the kidney is a unique method to integrate physiology, biochemistry, and molecular biology with morphology. Both mono- and polyclonal antibody reagents are useful and each has advantages and disadvantages. Specificity with low background is of the greatest importance. Tissue preparation techniques depend on the antigen being studied as well as the methodology to be used. Pre- and postembedding techniques combined with visualization with peroxidase reaction products or with particulate markers such as ferritin and gold must be chosen for each individual circumstance to be studied. Important applications in the kidney have included studies of glomerular antigens, specific transport proteins, and segment-specific antigens of unknown specificity. Future utilization of this technique with new molecular probes will greatly enhance our knowledge of the biology of the kidney.

Monoclonal antibodies to human glomerular antigens

Using cultured human fetal kidney cortical cells as antigen, two monoclonal antibodies (moAbs) against human glomeruli were produced. One of these moAbs, H-4, recognized the cell surface of glomerular epithelial cells, and the other, H-13, recognized the extracellular matrix present in the mesangial area. Both also reacted with liver, H-4 recognizing antigen present on the hepatocyte, and H-13 recognizing antigen distributed along the sinusoid. Species specificity for these moAbs was examined using mouse, rat, guinea pig and rabbit glomeruli, which revealed that H-4 reacted with rat glomerular epithelial cells and H-13 stained guinea pig glomerular mesangium. In the human fetal kidney, H-13 reacted with the mesangium, glomerular and tubular basement membrane and Bowman's capsule, and H-4 with the glomerular and tubular epithelial cells. Dot immunobinding assay of fibronectin purified from glomerular culture supernatant and plasma revealed that H-13 recognized both plasma and cellular fibronectin. Immunoblot analysis of 2.0 M guanidine HCl extract after dissociation in sodium dodecyl sulfate and electrophoresis demonstrated binding of H-4 to a 125 kd polypeptide. Immunoblot analysis of thermolysin-digested fibronectin exhibited binding of H-13 to 145 kd and 110 kd fragments, but not to 38 kd-29 kd fragments. In renal biopsy specimens from patients with membranous nephropathy. H-13 stained the glomerular basement membrane (GBM), but not the mesangium, whereas anti-fibronectin antisera stained both the GBM and the mesangium. In those from patients with minimal change nephrotic syndrome (MCNS), IgA glomerulonephritis (IgAGN) and membranoproliferative glomerulonephritis (MPGN), the staining pattern with H-13 was similar to that with polyclonal anti-fibronectin antisera. These results indicate that H-4 recognizes a 125 kd polypeptide constituent of the glomerular epithelial cell membrane and that H-13 recognizes the cell binding domain of fibronectin as well as revealing structural alterations in the mesangium and GBM.

Glycoconjugates in normal human kidney. A histochemical study using 13 biotinylated lectins

The avidin-biotin-peroxidase complex technique was used with 13 lectins to study the glycoconjugates of normal human renal tissue. The evaluated lectins included Triticum vulgaris (WGA), Concanavalin ensiformis (ConA), Phaseolus vulgaris leukoagglutinin and erythroagglutinin (PHA-L and PHA-E), Lens culinaris (LCA), Pisum sativum (PSA), Dolichos biflorus (DBA), Glycine max (SBA), Arachis hypogaea (PNA), Sophora japonica (SJA), Bandeiraea simplicifolia I (BSL-I), Ulex europaeus I (UEA-I) and Ricinus communis I (RCA-I). Characteristic and reproducible staining patterns were observed. WGA and ConA stained all tubules; PHA-L, PHA-E, LCA, PSA stained predominantly proximal tubules; DBA, SBA, PNA, SJA and BSL-I stained predominantly distal portions of nephrons. In glomeruli, WGA and PHA-L stained predominantly visceral epithelial cells; ConA stained predominantly basement membranes and UEA-I stained exclusively endothelial cells. UEA-I also stained endothelial cells of other blood vessels and medullary collecting ducts. Sialidase treatment before staining caused marked changes of the binding patterns of several lectins including a focal loss of glomerular and tubular staining by WGA; an acquired staining of endothelium by PNA and SBA; and of glomeruli by PNA, SBA, PHA-E, LCA, PSA and RCA-I. The known saccharide specificities and binding patterns of the lectins employed in this study allowed some conclusions about the nature and the distribution of the sugar residues in the oligosaccharide chains of renal glycoconjugates. The technique used in this report may be applicable to other studies such as evaluation of normal renal maturation, classification of renal cysts and pathogenesis of nephrotic syndrome. The observations herein reported may serve as a reference for these studies.

Antibody, antigen, and glomerular capillary wall charge interactions: influence of antigen location on in situ immune complex formation

These studies examined the charge interactions between the glomerular capillary wall, antibody and antigen at different sites in the glomerulus. Sheep IgG was separated into differently charged subclasses and immunologically placed in one of two glomerular locations (subepithelial or subendothelial) to serve as planted antigen. Single kidneys with planted antigen were transplanted into uninephrectomized recipients that received affinity-purified, cationic and anionic rat anti-sheep IgG labelled with 125I and 131I, respectively. Glomerular bound antibody was determined and corrected for antibody delivery. Specificity of antibody binding was confirmed by comparison of kidneys with or without planted antigen. The results indicate that the influence of charge on glomerular antibody binding depends on the site of the antigen. When antigen was planted in the subepithelial space, significantly more (15 to 25%) cationic than anionic antibody bound despite the fact that the antigen was cationic. Conversely, when the antigen was planted subendothelially, significantly more anionic (13 to 22%) antibody bound when the antigen was cationic, and significantly more cationic (7 to 16%) antibody bound when the antigen was anionic. Thus, the negatively-charged glomerular filtration barrier retards the permeation of anionic antibodies that complex with antigens located in the subepithelial space, but antigen-antibody charge interactions appear to predominate when the antigen is more proximally located.

[Molecular pathology of the glomerular sialoglycoprotein podocalyxin, a major component of the glomerular polyanion in experimental and human minimal glomerular change]

Visceral glomerular epithelial cells (or podocytes) are endowed with a highly polyanionic glycokalyx which is rich in sialic acid. We have identified a single sialoprotein - MW 140 kD - which we have called "Podocalyxin", and which is the major carrier of glomerular sialic acid. We have shown that podocalyxin of normal rat glomeruli contains 20 sialic acid residues per molecule, whereas in aminonucleoside nephrotic glomeruli only 4-5 sialic acid per molecule are present. We conclude that the loss of histochemical staining for the "glomerular polyanion" is due both to the reduction of sialic acid per podocalyxin molecule, and to the reduction of the surface of the podocyte's plasmalemma caused by spreading of footprocesses. We furtheron report that also in human glomeruli there is a sialoprotein which is distinct from rat podocalyxin in its electrophoretic mobility and immunological structure, but otherwise shows all features which are characteristic for rat podocalyxin.

Demonstration of sialic acid groups in the glomerular basement membrane of the rat with phosphotungstic acid at low pH

This paper reports an unrecognized aspect of phosphotungstic acid staining at low pH. It provides an on-section staining method in which sialic acid-containing molecules can be demonstrated in the laminae rarae of the rat glomerular basement membrane. The staining in the basement membrane became negative after perfusion with the following cations: protamine sulphate, hexadimethrine, Alcian Blue, Ruthenium Red and Toluidine Blue. Blocking was not achieved with Alcian Blue at about pH 1. The staining was also abolished after mild methylation and demethylation restored the contrast. This is suggestive of the involvement of carboxyl groups. Prior digestion with pronase, trypsin and neuraminidase rendered the laminae rarae negative, whereas hyaluronidase, chondroitinase ABC and crude heparinase were without effect. This indicates that sialic acid groups are detected by this method and that heparan sulphate does not interfere. The staining of the epithelial plasma membrane, also carrying sialic acid groups, remained positive after neuraminidase treatment. It is presumed that this method can be applied successfully for detecting changes in the sialic acid content of the laminae rarae in rat glomerular basement membranes under normal and pathological conditions.

Localization and distribution of anionic charges in the glomerular mesangium of normal and nephrotic rats

Sulfated glycosaminoglycans and sialoglycoproteins are thought to play a pivotal role in the glomerular capillary wall barrier to filtration since these anionic charged elements are important in the maintenance of capillary wall integrity and constitute a charge-selective filter. The development of proteinuria in puromycin aminonucleoside (PAN) nephrosis is associated with polyanion loss from the glomerular capillary wall structures. Since in PAN nephrosis the permeability of the mesangial area to plasma proteins and tracer substances has also been shown to be increased, the purpose of this study was to analyse the localization and distribution of anionic charges in the glomerular mesangium in this experimental model. Glycosaminoglycans were labeled by perfusion of the kidneys with ruthenium red solution (RR). Electron microscopic examination revealed the presence of distinct small RR granules ("anionic sites") in the mesangial intercellular matrix substance and in the laminae rarae of the glomerular basement membrane (GBM). The center-to-center spacing of the granules was measured and a frequency distribution of intervals in different interval classes was constructed. In normal glomeruli the anionic sites in the mesangial matrix showed a distribution pattern identical to the GBM with a maximal interval incidence at the 31-40 nm class. In nephrotic rats anionic site distributions in matrix and GBM did not change significantly. Sialoglycoproteins were labeled with colloidal iron (CI). In PAN nephrosis a decrease of CI binding was observed at the epithelial-basement membrane junction of the glomerular capillary wall. However, CI labeling of the mesangial matrix and mesangial cell membranes did not differ from that of normal glomeruli.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification and characterization of podocalyxin--the major sialoprotein of the renal glomerular epithelial cell

The glomerular epithelial polyanion is a specialized cell surface component found on renal glomerular epithelial cells (podocytes) that is rich in sialoprotein(s), as detected by staining with cationic dyes (colloidal iron, alcian blue) and wheat germ agglutinin (WGA). We have isolated rat glomeruli and analyzed their protein composition by SDS PAGE in 5-10% gradient gels. When the gels were stained with alcian blue or "Stains All," a single band with an apparent Mr of 140,000 was detected that also stained very prominently with silver, but not with Coomassie Blue. This band predominated in fluorograms of gels of isolated glomeruli that had been labeled in their sialic acid residues by periodate-[3H]borohydride. In lectin overlays, the 140-kilodalton (kd) band was virtually the only one that bound [125I]wheat germ agglutinin, and this binding could be prevented by predigestion with neuraminidase. [125I]Peanut lectin bound exclusively to the 140-kd band after neuraminidase treatment. An antibody was prepared that specifically recognizes only the 140-kd band by immunoprecipitation and immuneoverlay. By immunoperoxidase and immunogold techniques, it was localized to the surface coat of the glomerular epithelium and, less extensively, to that of endothelial cells. When analyzed (after electroelution from preparative SDS gels), the 140-kd band was found to contain approximately 20% hexose and approximately 4.5% sialic acid. These findings indicate that the 140-kd protein is the major sialoprotein of the glomerulus, and it is the only component of glomerular lysates with an affinity for cationic dyes and lectins identical to that defined histochemically for the epithelial polyanion in situ. Since this molecule is a major component of the cell coat or glycocalyx of the podocytes, we have called it "podocalyxin."

Heparan sulfate--rich anionic sites in the human glomerular basement membrane. Decreased concentration in congenital nephrotic syndrome

Recent work suggests that the normal barrier to penetration of the renal glomerular basement membrane by anionic plasma proteins may depend in part on the existence of negatively charged sites within the membrane. We describe an in vitro cytochemical method for the quantitative demonstration of anionic sites in the normal human glomerular basement membrane. In five normal subjects, ranging in age from 10 days to 57 years, the sites were distributed at regular intervals in the lamina rara externa, with a frequency of 23.8 +/- 6.8 sites per 1000-nm length of membrane. A similar distribution was observed in the basement membranes from three normal human fetuses. Ex vivo perfusion of one cadaver kidney revealed a similar distribution of anionic sites. The number of anionic sites in the glomerular basement membranes of five patients with the congenital nephrotic syndrome was reduced to 8.9 +/- 3.7 (P less than 0.001). Prior incubation of sections of normal kidney in purified heparinase resulted in a marked reduction in the number of anionic sites. We conclude that congenital nephrosis results from failure of heparan sulfate--rich anionic sites to develop in the lamina rara externa of the glomerular basement membrane.

Glomerular antigens in glomerulonephritis

Ideas on the immunopathogenesis of glomerulonephritis are evolving to embrace a concept of a dynamic and constantly fluctuating involvement of immune reactants in the production of glomerular inflammation. The glomerulus should be regarded as a template around which the antibody-induced inflammatory events that constitute glomerulonephritis are initiated. Such lesions may be produced by direct antibody attack on glomerular antigens of either intrinsic structural or "planted" type, as discussed in this review, or by the deposition of circulating soluble immune complexes containing extraglomerular antigens. These mechanisms are not mutually exclusive and both may play a role in some situations. Intrinsic glomerular antigens are being increasingly better defined as to site, structure, function, and experimental animal models of spontaneous and induced glomerular injury resulting from direct antibody binding to nonclassic GBM capillary wall antigens are available for study. Similar nonclassic GBM antigens are likely to be found of importance in man. Anti-GBM antibody-induced glomerulonephritis continues to be the best understood example of direct attack on the glomerulus by antibody, and its nephritogenic noncollagenous GBM antigenic constituents are being characterized. The incorporation of extraneous substances as "planted" antigens within glomerular structures is now recognized in experimental animal models, and there is suggestive evidence to support the concept in man. Emphasis needs to be placed on the continuing interplay of free antibody and antigen with deposited reactants which, together with complement components, modulate the quality and quantity of the glomerular immune deposits.

Cell associated glycoproteins synthesized by cultured renal tubular cells

Thin cortical kidney explants from newborn New Zealand rabbits were cultured in Dulbecco's MEM containing 10% fetal bovine serum. Within 24 h the explants formed globular bodies which were completely covered by a monolayered epithelium. The cells show polar differentiation and resemble the renal collecting duct epithelium. By culturing the globular bodies in Dulbecco's MEM with D-valine instead of L-valine additionally a monolayer of renal collecting duct cells was obtained. For the study of glycoprotein synthesis the globular bodies and the collecting duct monolayers were incubated with various labelled carbohydrates, protein and collagen precursors and then fractionated into coarse membrane pellets. The synthesized glycoproteins were regained in 600 x g and 12,000 x g coarse membrane fractions and extracted with Triton X 100 buffer for column chromatography and SDS-polyacrylamide electrophoresis in 6 M urea. In addition to a 85,000 d glycoprotein, a carbohydrate rich collagen like protein (apparent molecular weight in column chromatography 200,000 d, in the SDS-polyacrylamide electrophoresis 150,000 d) was found. The 150,000 d glycoprotein incorporates favorably radioactive proline, sulfate, and smaller amounts of lysine, and leucine. Compared to the 85,000 d glycoprotein a double amount of glucosamine and galactose and four fold amount of fucose was detected. The 85,000 d protein has to be ascribed as a usual glycoprotein, in contrast the 150,000 d protein shows an unusual combination of characteristics and has to be considered as a new type of renal glycoprotein.