Mesangial cells organize the glomerular capillaries by adhering to the G domain of laminin alpha5 in the glomerular basement membrane

Mesangial cells organize the glomerular capillaries by adhering to the G domain of laminin alpha5 in the glomerular basement membrane. n developing glomeruli, laminin ␣ 5 replaces laminin ␣ 1 in the glomerular basement membrane (GBM) at the capillary loop stage, a transition required for glomerulogenesis. To investigate domain-specific functions of laminin ␣ 5 during glomerulogenesis, we produced transgenic mice that express a chimeric laminin composed of laminin ␣ 5 domains VI through I fused to the human laminin ␣ 1 globular (G) domain, designated Mr51. Transgene-derived protein accumulated in many basement membranes, including the developing GBM. When bred onto the Lama5 Ϫ / Ϫ background, Mr51 supported GBM formation, preventing the breakdown that normally occurs in Lama5 Ϫ / Ϫ glomeruli. In addition, podocytes exhibited their typical arrangement in a single cell layer epithelium adjacent to the GBM, but convolution I of glomerular capillaries did not occur. Instead, capillaries were distended and exhibited a ballooned appearance, a phenotype similar to that observed in the total absence of mesangial cells. However, here the phenotype could be attributed to the lack of mesangial cell adhesion to the GBM, suggesting that the G domain of laminin ␣ 5 is essential for this adhesion. Analysis of an additional chimeric trans-gene allowed us to narrow the region of the ␣ 5 G domain essential for mesangial cell adhesion to ␣ 5LG3-5. Finally, in vitro studies showed that integrin ␣ 3 ␤ 1 and the Lutheran glycoprotein mediate adhesion of mesangial cells to laminin ␣ 5. Our results elucidate a mechanism whereby mesangial cells organize the glomerular capillaries by adhering to the G domain of laminin ␣ 5 in the GBM.