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

Background: A fundamental question in Alzheimer's disease (AD) is whether amyloid-beta (A beta) 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 A beta 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 epsilon 4 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 A beta peptides is the allosteric modulation of acetylcholine homeostasis, via molecular interactions between A beta, apolipoprotein-E, and the acetylcholine-degrading enzymes, cholinesterases, resulting in the formation of acetylcholine-hydrolyzing complexes (BA beta ACs). 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 A beta, 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 A beta peptides are present in the brain of Alzheimer and non-demented individuals. Heteromeric A beta 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 A beta forms were closely matched between CSF and brain samples. This evinces that the detected A beta-apolipoprotein-E complexes and BA beta ACs in CSF most likely originate from the interstitial fluids of the brain. Conclusions: In conclusion, both light homomeric A beta oligomers and heteromeric A beta-ApoE and BA beta ACs are present and readily detectable in the brain, regardless of disease status and APOE4 genotype. Deeper knowledge of the physiological function of A beta is crucial for better understanding the early pathological events that decades later lead to manifestation of AD.