Masking of anionic sites by deposits in lamina rara externa in immune complex nephritis in rats

Glomerular heparan sulfate alterations: mechanisms and relevance for proteinuria

Heparan sulfate (HS) is the anionic polysaccharide side chain of HS proteoglycans (HSPGs) present in basement membranes, in extracellular matrix, and on cell surfaces. Recently, agrin was identified as a major HSPG present in the glomerular basement membrane (GBM). An increased permeability of the GBM for proteins after digestion of HS by heparitinase or after antibody binding to HS demonstrated the importance of HS for the permselective properties of the GBM. With recently developed antibodies directed against the GBM HSPG (agrin) core protein and the HS side chain, we demonstrated a decrease in HS staining in the GBM in different human proteinuric glomerulopathies, such as systemic lupus erythematosus (SLE), minimal change disease, membranous glomerulonephritis, and diabetic nephropathy, whereas the staining of the agrin core protein remained unaltered. This suggested changes in the HS side chains of HSPG in proteinuric glomerular diseases. To gain more insight into the mechanisms responsible for this observation, we studied GBM HS(PG) expression in experimental models of proteinuria. Similar HS changes were found in murine lupus nephritis, adriamycin nephropathy, and active Heymann nephritis. In these models, an inverse correlation was found between HS staining in the GBM and proteinuria. From these investigations, four new and different mechanisms have emerged. First, in lupus nephritis, HS was found to be masked by nucleosomes complexed to antinuclear autoantibodies. This masking was due to the binding of cationic moieties on the N-terminal parts of the core histones to anionic determinants in HS. Second, in adriamycin nephropathy, glomerular HS was depolymerized by reactive oxygen species (ROS), mainly hydroxyl radicals, which could be prevented by scavengers both in vitro (exposure of HS to ROS) and in vivo. Third, in vivo renal perfusion of purified elastase led to a decrease of HS in the GBM caused by proteolytic cleavage of the agrin core protein near the attachment sites of HS by the HS-bound enzyme. Fourth, in streptozotocin-induced diabetic nephropathy and during culture of glomerular cells under high glucose conditions, evidence was obtained that hyperglycemia led to a down-regulation of HS synthesis, accompanied by a reduction in the degree of HS sulfation.

Reduction in glomerular heparan sulfate correlates with complement deposition and albuminuria in active Heymann nephritis

In a time-study of active Heymann nephritis, the expression of agrin, the main heparan sulfate proteoglycan in the glomerular basement membrane, was analyzed in relation to deposition of IgG and complement in the glomerular capillary wall and the development of albuminuria. Binding of IgG autoantibodies to the glomerular capillary wall could be detected from 2 wk onward, followed by activation of complement after 6 wk. Progressive albuminuria developed from 6 wk onward to a level of 274+/-68 mg/18 h at week 12. The staining intensity for the agrin core protein decreased slightly, and the staining intensity for the heparan sulfate stubs that were still attached to the core protein after heparitinase digestion remained normal. From week 6 onward, however, a progressive decrease was seen in the staining of two monoclonal antibodies (mAb) directed against different epitopes on the heparan sulfate polysaccharide side chain of agrin (to 35 and 30% of the control level, respectively, at week 12, both mAb P = 0.016). Moreover, albuminuria was inversely correlated with heparan sulfate staining as revealed by these antibodies (r(s) = -0.82 and r(s) = -0.75, respectively, both mAb P < 0.0001). This decrease in heparan sulfate staining was due to a progressive reduction of glomerular heparan sulfate content to 46 and 32% of control level at week 10 and week 12 of the disease, respectively, as measured biochemically. It is speculated that the observed decrease in glomerular heparan sulfate in active Heymann nephritis is due to complement-mediated cleavage of heparan sulfate, resulting in an increased permeability of the glomerular basement membrane to macromolecules.

Alteration of anionic sites in renal glomerular basement membrane of pigs

Ultrastructural alteration of anionic sites (ASs) in the glomerular basement membrane (GBM) was studied in 10 cases of swine mesangial proliferative glomerulonephritis using a cationic ultrastructural tracer, 0.5% polyethyleneimine (M.W. = 1,800). Glomerular ASs were seen as discrete electron-dense particles in the GBM, mesangial matrix and epithelial cell surfaces by electron microscopy. In the lamina rara externa (LRE) of the normal GBM, ASs were distributed regularly in a single layer. In those areas of the LRE that contained electron dense deposits or clusters of spherical microparticles (SMPs), however, a distinct reduction or loss of ASs was observed in all the pigs. Quantitative assessment of ASs in the LRE over 1,000 nm of the GBM revealed a significant reduction in ASs in one case with diffuse global thickening of the GBM as compared with the remaining nine pigs without GBM thickening (P < 0.001, Mann-Whitney's U-test). There were no ASs in the lamina densa (LD) of the normal GBM, but an irregular distribution of ASs was seen within the LD of the pig showing diffuse global thickening of the GBM. These results suggest that a disturbance of the charge-selective barrier in the GBM may be induced by electron-dense deposits or SMPs, in the LRE as well as thickening of the GBM in swine glomerulonephritis.

Glomerular extracellular matrices and anionic sites in aging ddY mice: a morphometric study

A morphometric study was undertaken to examine age-related changes in glomerular ultrastructure and anionic sites in ddY male mice at various ages. A progressive increase in glomerular extracellular matrices, including thickening of the glomerular basement membrane (GBM), formation of GBM nodules, and mesangial matrix increase, was found to be the primary age-related ultrastructural change in aging mice; there were also electron-dense deposits in mesangial and subepithelial regions. The extent of GBM thickening in mice was less than was reported in rats. Rather, the GBM nodules, which had the same electron density as the lamina densa (LD) and protruded on the subepithelial side of the GBM, were more striking. Quantitative evaluation showed that GBM thickness, number and size of GBM nodules, and the area of the mesangial matrix were significantly correlated with the age of the mice. The distribution of anionic sites in the glomeruli of aging animals was described for the first time. No statistically significant differences were noted between the number of glomerular anionic sites in the different age groups. These results indicate that the increase in glomerular extracellular matrices reported in aged rats was also present in aged mice, although the extent of various changes was different. The results also indicate that this increase in glomerular extracellular matrices with age was not accompanied by significant alteration in glomerular anionic sites.

Distribution of GBM heparan sulfate proteoglycan core protein and side chains in human glomerular diseases

Using monoclonal antibodies (mAbs) recognizing either the core protein or the heparan sulfate (HS) side chain of human GBM heparan sulfate proteoglycan (HSPG), we investigated their glomerular distribution on cryostat sections of human kidney tissues. The study involved 95 biopsies comprising twelve different glomerulopathies. Four normal kidney specimens served as controls. A homogenous to linear staining of the GBM was observed in the normal kidney with anti-HSPG-core mAb (JM-72) and anti-HS mAb (JM-403). In human glomerulopathies the major alteration was a segmental or total absence of GBM staining with anti-HS mAb JM-403, which is most pronounced in lupus nephritis, membranous glomerulonephritis (GN), minimal change disease and diabetic nephropathy, whereas the HSPG-core staining by mAb JM-72 was unaltered. In addition we found HSPG-core protein in the mesangial matrix when this was increased in membranoproliferative GN Type I, Schönlein-Henoch GN, IgA nephropathy, lupus nephritis, diabetic nephropathy and in focal glomerulosclerosis. Also staining with the anti-HS mAb JM-403 became positive within the mesangium, although to a lesser extent. Furthermore, amyloid deposits in AL and AA amyloidosis clearly stained with anti-HSPG-core mAb JM-72, and to a lesser degree with anti-HS mAb JM-403. Finally, in membranous GN (stage II and III), the GBM staining with anti-HSPG-core mAb JM-72 became irregular or granular, probably related to the formation of spikes. In conclusion, major alterations were observed in the glomerular distribution of HS and HSPG-core in various human glomerulopathies. The mAbs can be useful to further delineate the significance of HSPG and HS for glomerular diseases.