Glomerular endothelial cells and podocytes jointly synthesize laminin-1 and-11

Background. The glomerular basement membrane (GBM) originates in development from fusion of subendothelial and subepithelial matrices. Subsequently, newly synthesized subepithelial matrix is added as glomerular capillary loops expand. During GBM assembly, the laminin-1 heterotrimer (alpha1, beta1, and gamma1 chains), initially expressed in vascular clefts of comma- and S-shaped bodies, is eventually replaced by laminin-11 (alpha5, beta2, and gamma1 chains), which persists into maturation. The cellular source(s) of these laminins is not known and prompted this study. Methods. To determine which cells synthesize the various laminin chains, postfixation immunoelectron microscopy of developing mouse kidney was performed using monoclonal and polyclonal antibodies that specifically recognized laminin alpha1, beta1, alpha5, or beta2 chains. Results. Intracellular labeling for laminin alpha1, beta1 (laminin-1), and alpha5 and beta2 (laminin-11) chains was observed in developing glomerular endothelial cells and podocytes of comma- and S-shaped nephric figures. Laminin-1 was also seen in unfused GBMs at this stage, whereas laminin-11 was only found intracellularly. In capillary loop stage GBMs, laminin alpha1 chain was completely absent, whereas labeling for laminin alpha5 was intense, indicating rapid substitution between alpha chains. In contrast, laminin beta1 chain labeling remained strong both intracellularly and in GBMs of capillary loop stage glomeruli, and beta2 was up-regulated as well. In maturing stage glomeruli, beta1 labeling declined, and alpha5 and beta2 remained at high levels intracellularly in both endothelial cells and podocytes and in GBMs. Conclusions. Our results show that both endothelial cells and podocytes synthesize laminin-1 and -11 chains throughout glomerular development. The sustained and comparatively high level of laminin synthesis by endothelial cells was unexpected, suggesting that the endothelium may be an important source of GBM proteins in glomerular disease.