Ultrastructure of glomerular basement membrane in active Heymann nephritisrats revealed by tissue-negative staining method

Recently, we have developed a tissue-negative staining method, and successf ully visualized fine meshwork structure of the glomerular basement membrane (GBM). To clarify the mechanism of proteinuria in active Heymann nephritis , we performed tissue-negative staining and investigated the ultrastructura l alterations of the GEM. Active Heymann nephritis, the animal model of hum an membranous nephropathy, was induced in Lewis rats by the injection of pr oximal tubular brush border antigen, i.e. Fx1A. Urinary protein excretion w as measured and histological studies were performed over 15 weeks following the Fx1A injection. Proteinuria developed at 10 weeks after injection (38. 2 +/-7.4 mg/day) and progressively increased (160.2 +/- 20.6 mg/day at 15 w eeks). Capillary fine deposits of IgG and C3 were seen by immunofluorescenc e, and subepithelial electron dense deposits (EDD) by transmission electron microscopy (TEM). Using the tissue-negative staining method, regular meshw ork structure consisted of fine fibrils and pores (2.5 +/- 0.7 nm in short dimension) was observed in the GEM of control rats. At 10 and 15 weeks afte r injection, the GEM, directly facing the endothelial side of EDD, containe d enlarged pores and nephrotic tunnels. Mean values of the short dimension of enlarged pores were 2.9 +/- 0.5 nm at 10 weeks and 3.1 +/-0.4 nm at 15 w eeks, which were significantly larger than that of control rats (p < 0.01). The rest area of the GEM, including newly produced GEM covering the epithe lial side of EDD, had no significant difference in size of the pores from c ontrol GEM and no tunnels. Although there was no significant difference in the size of enlarged pores between 10 and 15 weeks, the percentage area of GBM with impaired size barrier increased at 15 weeks (51.4<plus/minus>8.1%) compared with 10 weeks (24.0 +/-8.3%) and related to severity of proteinur ia. The density of the tunnels also increased at 15 weeks. In conclusion, i mmune deposits may affect the GBM biosynthesis and induce the defect of siz e barrier of the GEM, which is responsible for proteinuria in active Heyman n nephritis. Copyright (C) 2001 S. Karger AG, Basel.