Glomerular heparan sulfate alterations: Mechanisms and relevance for proteinuria

Abstract

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.

Podocyte Changes after Induction of Acute Albuminuria in Mice by Anti-Aminopeptidase A mAb

“…Our study provided no arguments for one of the first three possibilities. The expression of JM403, an antibody against heparan sulfate used to characterize GBM-negative charge [15, 16], remained normal, thus suggesting that the acute albuminuria is not related to a defect or reduction of GBM-negative charge. We also did not observe reduced expression of α3-integrin or dystroglycan, arguing against changes in podocyte-GBM adhesion.…”

Isolation, Identification, and Quantitation of Urinary Glycosaminoglycans

“…Recently, Raats et al [4]elegantly reviewed the mechanisms underlying alterations of glomerular heparan sulfate. They proposed several mechanisms leading to reduced heparan sulfate function: masking by nucleosome-containing immune complexes [5], depolymerization by reactive oxygen species [6], neutral serine proteinases such as elastase and/or cathepsin, and hyperglycemia-induced reduction of heparan sulfate synthesis and sulfation.…”

“…The main heparan sulfate proteoglycans in the basement membrane are perlecan, agrin, and collagen XVIII. Perlecan and collagen XVIII are most abundant in the mesangial matrix, whereas agrin is a major heparan sulfate proteoglycan in the glomerular basement membrane [4]. Two classes of proteinases have been identified in mesangial cells in vivo and in vitro [3]: matrix metalloproteinases and serine proteases (plasminogen activators), together with their inhibitors.…”

Genetic kidney diseases disclose the pathogenesis of proteinuria

“…The molecular mechanims leading to proteinuria have been mostly unknown. Based on animal models and some human data, it was believed for decades that defects in the GBM were responsible for proteinuria (10,33,34). Loss of negative charge of the GBM was thought to lead to the lack of electrical repulsion between the GBM and negatively charged plasma proteins, and in this way enable the leakage of proteins through the capillary wall.…”

“…Loss of negative charge of the GBM was thought to lead to the lack of electrical repulsion between the GBM and negatively charged plasma proteins, and in this way enable the leakage of proteins through the capillary wall. As proteoglycans are mainly responsible for the anionic charge of the GBM, defects were suspected especially in the HSPGs (10,(34)(35)(36)(37). Despite these data, the role of the GBM in proteinuria remained controversial.…”