INTERACTION OF APOLIPOPROTEIN J-AMYLOID BETA-PEPTIDE COMPLEX WITH LOW-DENSITY-LIPOPROTEIN RECEPTOR-RELATED PROTEIN-2 MEGALIN - A MECHANISM TO PREVENT PATHOLOGICAL ACCUMULATION OF AMYLOID BETA-PEPTIDE

Apolipoprotein J (apoJ) has been shown to be the predominant amyloid b eta-peptide (A beta)-binding protein in cerebrospinal fluid. We have p reviously demonstrated that the endocytic receptor low density lipopro tein receptor-related protein-2/megalin (LRP-2), which is expressed by choroid plexus epithelium and ependymal cells lining the brain ventri cles and neural tube, binds and mediates cellular uptake of apoJ (Koun nas, M. Z., Loukinova, E. B., Stefansson, S., Harmony, J. A., Brewer, B., Strickland, D. K., and Argraves, W. S. (1995) J. Biol. Chem. 270, 13070-13075). In the present study, we evaluated the ability of apoJ t o mediate binding of A beta(1-40)-apoJ complex to LRP-2 in vitro. Immu noblot analysis showed that incubation of apoJ with A beta(1-40) resul ted in the formation of A beta(1-40)-apoJ complex and the inhibition o f the formation of A beta(1-40) aggregates. Using an enzyme-linked imm unosorbent assay, an estimated dissociation constant (K-d) of 4.8 nM w as derived for the interaction between A beta(1-40) and apoJ. Enzyme-l inked immunosorbent assay was also used to study the interaction of th e A beta(1-40)-apoJ complex with LRP-2. The results showed that A beta alone did not bind directly to LRP-2; however, when A beta(1-40) was combined with apoJ to form a complex, binding to LRP-2 took place. The binding interaction could be blocked by inclusion of the receptor-ass ociated protein, an antagonist of apoJ binding to LRP-2. When LRP-2-ex pressing cells were given I-125-A beta(1-40), cellular uptake of the r adiolabeled peptide was promoted by co-incubation with apoJ. When the cells were provided purified I-125-A beta(1-40)-apoJ complex, the comp lex was internalized and degraded, and both processes were inhibited w ith polyclonal LRP-2 antibodies. Furthermore, chloroquine treatment in hibited the cellular degradation of the complex. The data indicate tha t apoJ facilitates A beta(1-40) binding to LRP-2 and that the receptor mediates cellular clearance of A beta(1-40)-apoJ complex leading to l ysosomal degradation of A beta(1-40). The findings support the possibi lity that LRP-2 can act in vivo to mediate clearance of the complex fr om biological fluids such as cerebrospinal fluid and thereby play a ro le in the regulation of A beta accumulation.