Human cerebrospinal fluid apolipoprotein E isoforms are apparently inefficient at complexing with synthetic Alzheimer's amyloid-β peptide (Aβ^1-40) in vitro

Background: Variation at the apolipoprotein E locus on chromosome 19 plays a role in more cases of Alzheimer's disease than does any other identified genetic determinant. We have previously reported the isoform-specific interaction of native human apolipoprotein E (APOE, gene; apoE, protein) ε3 with the amyloid-β peptide, Aβ^1-40, the major component of the cerebral amyloid deposits that appear to cause Alzheimer's disease. Materials and Methods: In order to investigate the apoE: Aβ interaction further, a modified assay was developed based on co-immunoprecipitation of the complex using an anti-apoE antibody (anti-apoE IP assay). Results: Application of this assay demonstrated that the interaction of Aβ^1-40 and apoE can be distinguished into two types: sodium dodecyl sulfate (SDS) -resistant and SDS-releasable. The SDS-resistant interaction between ε3 and Aβ^1-40 is apparently maximal at an Aβ^1-40 concentration of ∼75 μM, and an Aβ^1-40/ε3 molar ratio of about 250:1. The major apoE-isoform-specific difference in interaction with Aβ^1-40 is the ability of Aβ^1-40 to form SDS-resistant complexes with ε3 but not with ε4. Using the anti-apoE co-IP assay, we found that human cerebrospinal fluid (CSF) ε3 can also form an SDS-resistant complex with Aβ^1-40 but human CSF ε4 cannot. However, when compared with apoE ε3 collected from the conditioned medium of APOE ε3-transfected cells, the competence of equal concentrations of CSF apoE ε3 to form SDS-resistant complexes with Aβ^1-40 is apparently diminished. A 1:1 mixture of CSF plus apoE ε3-containing conditioned medium is associated with diminished Aβ^1-40/ε3 complex formation to a greater extent than that observed when an identical volume of phosphate-buffered saline is added to apoE ε3 medium. Conclusions: These results suggest the presence in CSF of factors that interfere with the formation of complexes between synthetic Aβ^1-40 and apoE ε3.