Alpha8 integrin in glomerular mesangial cells and in experimental glomerulonephritis

Mesangial cell (MC) proliferation and extracellular matrix accumulation are typical responses of renal glomeruli to injury. Extracellular matrix components are known to affect MC behavior, which is mediated primarily via integrin receptors of the beta1 family. In addition to alpha1, alpha3, alpha5, and alpha6 chains of beta1 integrins, recent studies have shown the alpha8 chain to be expressed in glomeruli and renal vasculature. alpha8beta1 can serve as a receptor for fibronectin, which is abundant in the mesangium. We investigated the glomerular expression pattern of the alpha8 chain in renal tissues of mouse, rat, and humans as well as in cultured MCs. In addition, the regulation of alpha8 expression in MCs was studied in culture and in nephritic rats.

The expression of alpha8 protein in kidney tissue and cultured MCs was investigated by immunohistochemistry, immunocytochemistry, and Western blotting. The effects of TGF-beta1 on alpha8 mRNA levels in MCs were studied by Northern blot analysis. In addition, time course studies of glomerular abundance and localization of alpha8 were performed in rats with mesangioproliferative anti-Thy1.1 nephritis.

In tissue sections of normal human, rat, and mouse kidney, we found strong immunohistochemical staining for alpha8 in the mesangium and in the media of renal arterioles. Double staining for alpha8 and Thy1.1, a surface antigen of rat MCs, showed alpha8 to be specifically expressed in MCs but not in glomerular endothelial and epithelial cells. In anti-Thy1.1 nephritis of rats, the glomerular abundance of alpha8 protein was reduced in the early mesangiolytic phase but was increased greatly with subsequent MC proliferation, peaking at day 6 of disease. At later stages of this reversible form of nephritis, the number of MCs and the extent mesangial alpha8 staining declined to control levels. Cell culture experiments revealed that freshly plated MCs organize alpha8 into focal contacts within one hour after attachment to fibronectin and vitronectin substrata, showing colocalization with focal contact proteins vinculin and talin. Stimulation of MCs with transforming growth factor-beta1 led to increases of alpha8 mRNA and protein levels.

These results show that in human, rat, and mouse glomeruli, alpha8 integrin is strongly and exclusively expressed in MCs. Gene expression of alpha8 is regulated in cultured MCs, and alpha8 protein abundance is regulated in vivo and in MC culture. It is currently unclear what functional properties this integrin receptor protein has with regard to MC anchorage to extracellular matrix and modulation of the MC phenotype in normal and diseased glomeruli. However, in view of its abundance in the mesangium, alpha8beta1 integrin could be an important MC receptor of matrix ligands and may play a role in the embryology, physiology, and pathophysiology of the glomerular capillary tuft.