Alternatively spliced isoforms of fibronectin in immune-mediated glomerulosclerosis: the role of TGF? and IL-4

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Abstract
Fibronectin (FN) is the main extracellular matrix component in glomerulosclerotic lesions. There are different FN isoforms that result from alternative splicing at the EDA and EDB regions of FN mRNA. Increased inclusion of EDA and EDB, which can be elicited by TGFβ, may be conducive to the development of glomerulosclerosis (GS). TGFβ and IL‐4 have previously been shown to play a role in the development of GS. In this study, the mRNA splicing patterns for EDA+ and EDB+ fibronectin were investigated in vivo in various experimental sclerotic glomerulopathies, in vitro in rat mesangial cells (MC) that were stimulated by TGFβ or transfected with IL‐4, and in human kidney biopsies with GS from patients with various kidney diseases. Analysis of glomerular FN mRNA demonstrated inclusion of both ED regions in rats with anti‐Thy1 nephritis or chronic serum sickness and in mice with anti‐GBM glomerulonephritis. Inclusion of both the EDA and EDB regions was associated with glomerular TGFβ expression. In contrast, in mice with Th2‐mediated graft‐versus‐host disease, a model for lupus nephritis, the FN transcripts included neither the EDA nor the EDB region, and renal TGFβ expression was absent. Compared to normal MCs in culture, MCs transfected with IL‐4 produced lower amounts of FN and demonstrated less EDA inclusion, while MC that had been treated with TGFβ showed increased production of FN and more EDA inclusion. Renal biopsies from patients with renal diseases, except those taken from patients with lupus nephritis, showed higher TGFβ levels, higher FN levels, and more EDA inclusion than controls. TGFβ may be a key player in the development of GS by inducing local FN production and alternative splicing of FN mRNA. In lupus glomerulonephritis, in which the involvement of TGFβ in GS is less prominent, Th2 cytokines such as IL‐4 probably account for increased intrarenal collagen synthesis and subsequent FN accumulation from the circulation. In conclusion, neither alternative FN splicing, nor a high transcription level of TGFβ, appears to be a general prerequisite for the development of GS. Copyright © 2004 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.