Homomeric and Heteromeric Aß Species Exist in Human Brain and CSF Regardless of Alzheimer's Disease Status and Risk Genotype.

ABSTRACT

Background: A fundamental question in Alzheimer's disease (AD) is whether amyloid-ß (Aß) peptides and their deposition in the brain signify a direct pathological role or they are mere outcome of the disease pathophysiological events affecting neuronal function. It is therefore important to decipher their physiological role in the brain. So far, the overwhelming focus has been on the potential toxicity of Aß, often studied outside the crucial AD characteristics, i.e. (i) the slow, decades-long disease progression that precedes clinical symptoms; (ii) the link to apolipoprotein-E ε4 allele as major risk factor; (iii) the selective early degeneration of cholinergic neurons. Previous studies, in vitro and cerebrospinal fluid (CSF) only, indicated one possible native function of Aß peptides is the allosteric modulation of acetylcholine homeostasis, via molecular interactions between Aß, apolipoprotein-E, and the acetylcholine-degrading enzymes, cholinesterases, resulting in the formation of acetylcholine-hydrolyzing complexes (BAßACs). Methods: Here, by combining sucrose-density gradient fractionation of post-mortem brains and in-house developed sensitive ELISA assays on the obtained fractions, we investigated the presence, levels and molecular interactions between Aß, apolipoprotein-E and cholinesterases for the first time in brain tissues. We examined three distinct brain regions of Alzheimer and non-demented subjects, plus a large number of Alzheimer CSF samples. Results: We report that both monomeric and oligomeric (homomeric and heteromeric) forms of Aß peptides are present in the brain of Alzheimer and non-demented individuals. Heteromeric Aß was found in stable complexes with apolipoprotein-E and/or cholinesterases, irrespective of APOE genotype or disease status, arguing in favor of a physiological dynamic formation and function for these complexes in the brain. The patterns and molecular sizes of the detected soluble Aß forms were closely matched between CSF and brain samples. This evinces that the detected Aß-apolipoprotein-E complexes and BAßACs in CSF most likely originate from the interstitial fluids of the brain. Conclusions: In conclusion, both light homomeric Aß oligomers and heteromeric Aß-ApoE and BAßACs are present and readily detectable in the brain, regardless of disease status and APOE4 genotype. Deeper knowledge of the physiological function of Aß is crucial for better understanding the early pathological events that decades later lead to manifestation of AD.