Embryonic fibronectin isoforms are synthesized in crescents in experimental autoimmune glomerulonephritis

Crescents are a severe and stereotyped glomerular response to injury that occur in several forms of glomerulonephritis that progress to renal failure. The key pathogenetic step that leads to glomerular scarring is unknown, but fibronectin (FN), the clotting system, macrophages, and proliferating parietal epithelial cells are known to participate. This study was designed to determine whether FN is synthesized locally, and in what molecular isoform, and whether cytokines known to promote FN synthesis are present in the crescent. Rats immunized with bovine glomerular basement membrane develop cellular crescents by 14 days and fibrous crescents and glomerulosclerosis by 35 days. In situ hybridization was performed with oligonucleotides specific for sequences common to all FN isoforms (total FN) or sequences specific for the alternatively spliced segments (EIIIA, EIIIB, and V). Throughout the time period (14, 21, and 35 days) all crescents and glomerular tufts contained cells with strong ISH signals for total and V⁺ mRNA, with the strongest signals present in large cellular crescents at day 21. In contrast, EIIIA⁺ and EIIIB⁺ mRNAs showed maximal abundance within sclerosing crescents at 35 days. Protein deposition of EIIIA⁺, EIIIB⁺, and V⁺ FN isoforms was confirmed by immunofluorescence with segment-specific FN antibodies. Transforming growth factor-β and interleukin-1β, both known to promote FN synthesis, were found in cellular crescents (days 14 and 21) and were still present, but greatly diminished, in the sclerotic phase (day 35). In summary, EIIIA^-, EIIIB^-, and V⁺ FN mRNA plasma isoforms predominate in cellular crescents, whereas in the fibrosing stage, mainly the oncofetal EIIIA⁺, EIIIB⁺, and V⁺ isoforms are synthesized and accumulate.