Preferential interactions between ApoE-containing lipoproteins and Aß revealed by a detection method that combines size exclusion chromatography with non-reducing gel-shift.

ABSTRACT

The association between apolipoprotein E (apoE) and amyloid-ß peptide (Aß) may significantly impact the function of both proteins, thus affecting the etiology of Alzheimer's disease (AD). However, apoE/Aß interactions remain fundamentally defined by the stringency of the detection method. Here we use size exclusion chromatography (SEC) as a non-stringent approach to the detection of apoE/Aß interactions in solution, specifically apoE and both endogenous and exogenous Aß from plasma, CSF and astrocyte conditioned media. By SEC analysis, Aß association with plasma and CNS lipoproteins is apoE-dependent. While endogenous Aß elutes to specific human plasma lipoproteins distinct from those containing apoE, it is the apoE-containing lipoproteins that absorb excess amounts of exogenous Aß40. In human CSF, apoE, endogenous Aß and phospholipid elute in an almost identical profile, as do apoE, exogenous Aß and phospholipid from astrocyte conditioned media. Combining SEC fractionation with subsequent analysis for SDS-stable apoE/Aß complex reveals that apoE-containing astrocyte lipoproteins exhibit the most robust interactions with Aß. Thus, standardization of the methods for detecting apoE/Aß complex is necessary to determine its functional significance in the neuropathology characteristic of AD. Importantly, a systematic understanding of the role of apoE-containing plasma and CNS lipoproteins in Aß homeostasis could potentially contribute to identifying a plasma biomarker currently over-looked because it has multiple components.