Modulation of apolipoprotein E receptor-2 by ApoE4, amyloid β-peptide, reelin, and secreted amyloid precursor protein: a common point of impact in Alzheimer's disease pathogenesis.

INTRODUCTION: Apolipoprotein E (ApoE), reelin, and several other proteins bind ApoE-receptor 2 (apoER2), distinguished from other members of its receptor family by signal transduction which enhances the activity of N-methyl D-aspartate (NMDA) receptors. Evidence indicates that this signal transduction depends upon apoER2 forming dimers or other high-order clusters. It seems noteworthy therefore that protein products of major APOE gene variants differ in their numbers of cysteines capable of forming disulfide dimers, with the allele (ε4) associated with highest rates of Alzheimer's disease (AD) possessing none. Thus, lower AD risk may be associated with the ability of ApoE to dimerize and thereby promote apoER2 dimerization and signaling. METHODS: We examined calcium fluxes via the NMDA receptor in neurons derived from the NTera2 cell line in response to conditioned medium from human astrocytes differing in APOE genotype, recombinant ApoE proteins, reelin, amyloid β-peptide (Aβ) preparations differing in their aggregation states, and secreted amyloid precursor protein (sAPP). Signaling through apoER2 was inhibited by receptor-associated protein (RAP) or siRNA directed against apoER2. RESULTS: Reelin, fibrillar Aβ, ApoE3, and conditioned medium from APOE ε3 astrocytes elevated calcium fluxes, and this phenomenon required apoER2. By contrast, ApoE4 and oligomeric Aβ antagonized activation. sAPP showed high-affinity binding to apoER2 and enhanced responses to reelin. DISCUSSION: These findings suggest a comprehensive hypothesis for the pathogenesis of AD whereby the common factor in development of disease is antagonism of apoER2, likely to include agents that cannot promote the receptor's dimerization yet competitively inhibit those ligands that can cause dimerization.