Glomerular heparan sulfate alterations: Mechanisms and relevance for proteinuria

Heparan sulfate (HS) is the anionic polysaccharide side chain of HS proteog lycans (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 G BM for proteins after digestion of HS by heparitinase or after antibody bin ding to HS demonstrated the importance of HS for the permselective properti es 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, membra nous 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 insig ht into the mechanisms responsible for this observation, we studied GBM HS( PG) expression in experimental models of proteinuria. Similar HS changes we re found in murine lupus nephritis, adriamycin nephropathy, and active Heym ann nephritis. In these models, an inverse correlation was found between HS staining in the GBM and proteinuria. From these investigations, four new a nd different mechanisms have emerged. First, in lupus nephritis, HS was fou nd to be masked by nucleosomes complexed to antinuclear autoantibodies. Thi s masking was due to the binding of cationic moieties on the N-terminal par ts of the core histones to anionic determinants in HS. Second, in adriamyci n 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 perfusi on of purified elastase led to a decrease of HS in the GBM caused by proteo lytic cleavage of the agrin core protein near the attachment sites of HS by the MS-bound enzyme. Fourth, in streptozotocin-induced diabetic nephropath y and during culture of glomerular cells under high glucose conditions, evi dence was obtained that hyperglycemia led to a down-regulation of HS synthe sis, accompanied by a reduction in the degree of HS sulfation.