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 beta-peptide (A beta)-binding protein in cerebrospinal fluid. We have previously demonstrated that the endocytic receptor low density lipoprotein receptor-related protein-2/megalin (LRP-2), which is expressed by choroid plexus epithelium and ependymal cells lining the brain ventricles and neural tube, binds and mediates cellular uptake of apoJ (Kounnas, 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 to mediate binding of A beta(1-40)-apoJ complex to LRP-2 in vitro. Immunoblot analysis showed that incubation of apoJ with A beta(1-40) resulted in the formation of A beta(1-40)-apoJ complex and the inhibition of the formation of A beta(1-40) aggregates. Using an enzyme-linked immunosorbent assay, an estimated dissociation constant (K-d) of 4.8 nM was derived for the interaction between A beta(1-40) and apoJ. Enzyme-linked immunosorbent assay was also used to study the interaction of the 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-associated protein, an antagonist of apoJ binding to LRP-2. When LRP-2-expressing cells were given I-125-A beta(1-40), cellular uptake of the radiolabeled peptide was promoted by co-incubation with apoJ. When the cells were provided purified I-125-A beta(1-40)-apoJ complex, the complex was internalized and degraded, and both processes were inhibited with polyclonal LRP-2 antibodies. Furthermore, chloroquine treatment inhibited the cellular degradation of the complex. The data indicate that 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 lysosomal degradation of A beta(1-40). The findings support the possibility that LRP-2 can act in vivo to mediate clearance of the complex from biological fluids such as cerebrospinal fluid and thereby play a role in the regulation of A beta accumulation.