Glomerular epithelial and mesangial cells differentially modulate the binding specificities of VLA-1 and VLA-2

Maintenance of glomerular cell interaction with the complex basement membrane is crucial for the normal functioning of the kidney. Because little is known about the receptors utilized by glomerular cells, we examined the attachment of cultured glomerular cells to extracellular matrix proteins.

We produced monoclonal antibodies that inhibited the function of rat VLA-1 and VLA-2 and used these antibodies alone and in combination to explore the attachment of glomerular epithelial cells (GEC) and mesangial cells to extracellular matrix proteins in vitro.

Cultured GEC utilize only VLA-2 for attachment to collagen but use it together with VLA-1 for adhesion to laminin. In contrast, mesangial cells use both receptors for their attachment to collagen but utilize only VLA-1 in their interaction with laminin. The use of VLA-1 by GEC and of VLA-2 by mesangial cells was unexpected because the expression of these receptors was barely detectable in an enzyme-linked immunosorbent assay and by immunocytochemistry.

VLA-1, VLA-2, and VLA-3 are integrin receptors with overlapping specificities in that all have the potential to interact with collagen and laminin. Our studies demonstrate that cultured GEC use VLA-1 and VLA-2 almost exclusively for their adhesion to these ligands, and thus VLA-3 appears to play a negligible role in such attachment. Interestingly, GEC and mesangial cells differentially modulate the ligand binding specificities of VLA-1 and VLA-2. In situ, VLA-1 has been localized within the mesangium, whereas VLA-2 has not been detected within the glomerulus leading to the conclusion that GEC do not use VLA-1 or VLA-2 and that mesangial cells fail to utilize VLA-2. However, our studies have shown that, even when such receptors are barely detectable on the surface of cultured cells by sensitive techniques, they can play a functional role. These results suggest either that the levels of expression in situ are too low for the relatively insensitive immunohistochemical techniques employed, and thus the importance of these receptors to glomerular cell attachment in vivo is under appreciated or that such receptors are the result of de novo expression by glomerular cells when they are subjected to in vitro culture conditions. Because it is known that such conditions may mimic pathologic stress, we are presently examining the expression of these receptors by glomerular cells in various disease models.