β2-glycoprotein-I (apolipoprotein H) and β2-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 β2-glycoprotein-I/apolipoprotein H as a plasma component binding to the renal epithelial endocytic receptor megalin. A calcium-dependent megalin-mediated β2-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 β2- glycoprotein-I in urine of mice with disrupted megalin gene established that megalin is the renal clearance receptor for β2-glycoprotein-I. A significant increase in functional affinity for purified megalin was observed when β2-glycoprotein-I was bound to the acidic phospholipids, phosphatidylserine and cardiolipin. The binding of β2-glycoprotein-I and β2-glycoprotein-I-phospholipid complexes to megalin was completely blocked by receptor-associated protein. In conclusion, we have demonstrated a novel receptor recognition feature of β2-glycoprotein-I. In addition to explaining the high urinary excretion of β2-glycoprotein-I in patients with renal tubule failure, the data provide molecular evidence for the suggested function of β2-glycoprotein-I as a linking molecule mediating cellular recognition of phosphatidylserine-exposing particles.