Megalin is an endocytic receptor expressed on the luminal surface of the renal proximal tubules. The receptor is believed to play an important role in the tubular uptake of macromolecules filtered through the glomerulus. To elucidate the role of megalin in vivo and to identify its endogenous ligands , we analyzed the proximal tubular function in mice genetically deficient for the receptor. We demonstrate that megalin-deficient mice exhibit a tubular resorption deficiency and excrete low molecular weight plasma proteins in the urine (low molecular weight proteinuria). The proximal tubules in the kidney are responsible for the retrieval of solutes that have been filtered through the glomerulus. This reuptake pathway selectively captures essential low molecular weight metabolites which pass freely through the glomerular membranes and which would otherwise be lost in the urine. Besides water, the main blood constituents taken up in the proximal tubules are ions, glucose, and amino acids. In addition, small plasma proteins (Ͻ70 kd) are also filtered through the glomerulus and reabsorbed in the proximal tubules. These include plasma carrier proteins (eg, retinol-binding protein), peptide hormones (eg, insulin, parathyroid hormone), and lysozyme.

Screening of serum by using a surface plasmon resonance analysis assay identified ␤ 2 -glycoprotein-I/apolipoprotein H as a plasma component binding to the renal epithelial endocytic receptor megalin. A calcium-dependent megalin-mediated ␤ 2 -glycoprotein-I endocytosis was subsequently demonstrated by ligand blotting of rabbit renal cortex and uptake analysis in megalin-expressing cells. Immunohistochemical and immunoelectron microscopic examination of kidneys and the presence of high concentrations of ␤ 2 -glycoprotein-I in urine of mice with disrupted megalin gene established that megalin is the renal clearance receptor for ␤ 2 -glycoprotein-I. A significant increase in functional affinity for purified megalin was observed when ␤ 2 -glycoprotein-I was bound to the acidic phospholipids, phosphatidylserine and cardiolipin. The binding of ␤ 2 -glycoprotein-I and ␤ 2 -glycoprotein-Iphospholipid complexes to megalin was completely blocked by receptor-associated protein.In conclusion, we have demonstrated a novel receptor recognition feature of ␤ 2 -glycoprotein-I. In addition to explaining the high urinary excretion of

In renal extracts, some renin is present as "high molecular weight renin," a heterodimeric complex of renin with the 46-kDa renin-binding protein (RnBP), also known as N-acyl-D-glucosamine 2-epimerase. Because RnBP specifically inhibits renin activity, the protein was proposed to play an important role in the regulation of the renin-angiotensin system (RAS). Using gene targeting, we have generated mice lacking RnBP and tested this hypothesis in vivo. In particular, we analyzed biosynthesis, secretion, and activity of renin and other components of the RAS in mice lacking RnBP. Despite extensive investigations, we were unable to detect any major effects of RnBP deficiency on the plasma and renal RAS or on blood pressure regulation. Contrary to previous hypotheses, we conclude that RnBP does not play a significant role in the regulation of renin activity in plasma or kidney. However, RnBP knockout mice excrete an abnormal pattern of carbohydrates in the urine, indicating a role of the protein in renal carbohydrate metabolism.