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

Background. The glomerular basement membrane (GBM) originates in developmen t from fusion of subendothelial and subepithelial matrices. Subsequently, n ewly synthesized subepithelial matrix is added as glomerular capillary loop s expand. During GBM assembly, the laminin-1 heterotrimer (alpha1, beta1, a nd gamma1 chains), initially expressed in vascular clefts of comma- and S-s haped bodies, is eventually replaced by laminin-11 (alpha5, beta2, and gamm a1 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, po stfixation immunoelectron microscopy of developing mouse kidney was perform ed 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. Lam inin-1 was also seen in unfused GBMs at this stage, whereas laminin-11 was only found intracellularly. In capillary loop stage GBMs, laminin alpha1 ch ain was completely absent, whereas labeling for laminin alpha5 was intense, indicating rapid substitution between alpha chains. In contrast, laminin b eta1 chain labeling remained strong both intracellularly and in GBMs of cap illary loop stage glomeruli, and beta2 was up-regulated as well. In maturin g stage glomeruli, beta1 labeling declined, and alpha5 and beta2 remained a t high levels intracellularly in both endothelial cells and podocytes and i n GBMs. Conclusions. Our results show that both endothelial cells and podocytes syn thesize laminin-1 and -11 chains throughout glomerular development. The sus tained and comparatively high level of laminin synthesis by endothelial cel ls was unexpected, suggesting that the endothelium may be an important sour ce of GBM proteins in glomerular disease.