beta2-glycoprotein-I (apolipoprotein H) and beta2-glycoprotein-I-phospholipid complex harbor a recognition site for the endocytic receptor megalin.

Abstract

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

Megalin Knockout Mice as an Animal Model of Low Molecular Weight Proteinuria

“…Binding of ligands to megalin was quantified by BIAcore analysis (Biosensor, Sweden) as described previously. 5 A continuous flow of HBS buffer (10 mmol/L HEPES, 3.4 mmol/L EDTA, 150 mmol/L NaCl, 0.005% surfactant P20, pH 7.4) passing over the sensor surface was maintained at 5 l/minute. The carboxylated dextran matrix of the sensor chip flow cell was activated by injection of 60 l of a solution containing 0.2 mol/L N-ethyl-NЈ-(3-dimethylaminopropyl) carbodiimide and 0.05 mol/L N-hydroxysuccimide in H 2 O.…”

“…Endosomes and lysosomes contain proteins absorbed from the glomerular filtrate. 5 The physiological importance of tubular uptake processes is underscored by the pathological changes observed in patients with a resorption deficiency of the proximal tubules. This defect is referred to as the Fanconi syndrome (Lignac-de Toni-Debré-Fanconi syndrome).…”

“…4 This receptor binds several proteins known to be reabsorbed in the proximal tubules (eg, insulin, ␤ 2 -glycoprotein, retinol-binding protein). 5,11,12 Therefore, it was postulated that megalin might be involved in the clearance of filtered plasma proteins. Alternatively, a role for the receptor in tubular uptake of calcium has been suggested.…”

“…Fortunately, the severity of forebrain malformations in megalin knockout mice is variable and approximately 2% of the animals survive to adulthood. 5 These mice enabled us to study the consequence of receptor deficiency for renal function.…”

Inflammatory response and the endothelium

“…Megalin-an endocytic receptor expressed on placenta, kidney and other tissues-was reported as a possible receptor for h2GPI [21]. However, Cao et al [22] showed that megalin-mRNA was not expressed in both human vein and arterial umbilical EC.…”