Glomerular clusterin is associated with PKC-alpha/beta regulation and good outcome of membranous glomerulonephritis in humans

Mechanisms for human membranous glomerulonephritis (MGN) remain elusive. Most up-to-date concepts still rely on the rat model of Passive Heymann Nephritis that derives from an autoimmune response to glomerular megalin, with complement activation and membrane attack complex assembly. Clusterin has been reported as a megalin ligand in immunodeposits, although its role has not been clarified. We studied renal biopsies of 60 MGN patients by immunohistochemistry utilizing antibodies against clusterin, C5b-9, and phosphorylated-protien kinase C (PKC) isoforms (pPKC). In vitro experiments were performed to investigate the role of clusterin during podocyte damage by MGN serum and define clusterin binding to human podocytes, where megalin is known to be absent. Clusterin, C5b-9, and pPKC-alpha/beta showed highly variable glomerular staining, where high clusterin profiles were inversely correlated to C5b-9 and PKC-alpha/beta expression (P=0.029), and co-localized with the low-density lipoprotein receptor (LDL-R). Glomerular clusterin emerged as the single factor influencing proteinuria at multivariate analysis and was associated with a reduction of proteinuria after a follow-up of 1.5 years (-88.1%, P=0.027). Incubation of podocytes with MGN sera determined strong upregulation of pPKC-alpha/beta that was reverted by pre-incubation with clusterin, serum de-complementation, or protein-A treatment. Preliminary in vitro experiments showed podocyte binding of biotinilated clusterin, co-localization with LDL-R and specific binding inhibition with anti-LDL-R antibodies and with specific ligands. These data suggest a central role for glomerular clusterin in MGN as a modulator of inflammation that potentially influences the clinical outcome. Binding of clusterin to the LDL-R might offer an interpretative key for the pathogenesis of MGN in humans.

[Etiopathogenesis of membranous nephropathy: is there a correlation between experimental and human pathology?]

The Authors discuss the etiologic, pathogenetic and immunopathologic aspects of Heymann nephritis, in order to compare the numerous acquisitions concerning this nephropathy with the scanty knowledge of human membranous nephropathy, of which it represents the experimental counterpart. This rat disease can be obtained by inoculation of tubular brush border preparations (active form) or of the relevant antibodies (passive form); after an initial hypothesis of glomerular deposition of circulating immune complexes, studies on its pathogenetic mechanisms, instead demonstrated that in situ immunoaggregates, caused by an interaction between circulating antibodies and fixed glomerular antigens, are formed. Recent investigations have led to the identification of a major nephritogenic antigen (gp330), which is a tubular brush border glycoprotein expressed by coated pits located at the glomerular epithelial cell surface. Studies on antigen-antibody interactions at this level have demonstrated that there is a quick redistribution and accumulation of the so-formed immune complexes, and when polyclonal antibodies were utilized, growth of subepithelial electron dense deposits was observed. Although other tubulo-glomerular antigens, which can also be expressed by endothelial cells, play an uncertain role, they seem to favour transmembrane passing of anti-gp330 antibodies. Immune complex formation gives rise to the onset of proteinuria through complement system activation, without leukocyte involvement: in particular a MAC and C9 fraction lytic effect was demonstrated on cultured epithelial cells. In conclusion, studies on Heymann nephritis contribute to our understanding of the etiopathogenetic mechanisms regarding human membranous nephropathy, and emphasize a possible role played by tubular antigens and in situ formed immune complexes.