BINDING OF INJECTED LAMININ TO DEVELOPING KIDNEY GLOMERULAR MESANGIALMATRICES AND BASEMENT-MEMBRANES IN-VIVO

During glomerular development, subendothelial and -epithelial basement membrane layers fuse to produce the glomerular basement membrane (GEM ) shared by endothelial cells and epithelial podocytes. As glomeruli m ature, additional basement membrane derived from podocytes is spliced into the fused GEM and loose mesangial matrices condense. The mechanis ms for GBM fusion, splicing, and mesangial matrix condensation are not known but might involve intermolecular bond formation between matrix molecules. To test for laminin binding sites, we intravenously injecte d mouse laminin containing alpha 1-, beta 1-, and gamma 1-chains into 2-day-old rats. Kidneys were immunolabeled for fluorescence and electr on microscopy with domain-specific rat anti-mouse laminin monoclonal a ntibodies (MAbs), which recognized only mouse and not endogenous rat l aminin. Intense labeling for injected laminin was found in mesangial m atrices and weaker labeling was seen in GBMs of maturing glomeruli. Th ese patterns persisted for at least 2 weeks after injection. In contro l newborns receiving sheep IgG, no binding of injected protein was obs erved and laminin did not bind adult rat glomeruli. To assess which mo lecular domains might mediate binding to immature glomeruli, three pro teolytic laminin fragments were affinity-isolated by MAbs and injected into newborns. These failed to bind glomeruli, presumably owing to en zymatic digestion of binding domains. Alternatively, stable incorporat ion may require multivalent laminin binding. We conclude that laminin binding sites are transiently present in developing glomeruli and may be functionally important for GEM assembly and mesangial matrix conden sation.