A specific amyloid-b protein assembly in the brain impairs memory

Abstract

Memory function often declines with age, and is believed to deteriorate initially because of changes in synaptic function rather than loss of neurons. Some individuals then go on to develop Alzheimer’s disease with neurodegeneration. Here we use Tg2576 mice, which express a human amyloid-b precursor protein (APP) variant linked to Alzheimer’s disease, to investigate the cause of memory decline in the absence of neurodegeneration or amyloid-b protein amyloidosis. Young Tg2576 mice (<6 months old) have normal memory and lack neuropathology, middle-aged mice (6–14 months old) develop memory deficits without neuronal loss, and old mice (>14 months old) form abundant neuritic plaques containing amyloid-b (refs 3–6). We found that memory deficits inmiddle-agedTg2576mice are caused by the extracellular accumulation of a 56-kDa soluble amyloid-b assembly, which we term Ab*56 (Ab star 56). Ab*56 purified from the brains of impaired Tg2576 mice disrupts memory when administered to young rats. We propose that Ab*56 impairs memory independently of plaques or neuronal loss, andmay contribute to cognitive deficits associated with Alzheimer’s disease. Poor memory function can predict Alzheimer’s disease up to 15 years before diagnosis, and non-demented individuals at risk genetically for Alzheimer’s disease show abnormalities in functional brain imaging tests. These and other studies imply that Alzheimer’s disease has an insidious onset, which blurs the boundary between age-associated memory impairment and Alzheimer’s disease. Tg(APPSWE)2576Kahs mice (hereafter Tg2576 mice) express a human amyloid-b precursor protein (APP) variant linked to Alzheimer’s disease, and develop many neuropathological features of Alzheimer’s, including amyloid plaques, dystrophic neurites and inflammatory changes. However, Tg2576 mice lack neurofibrillary tangles, significant neuronal loss and gross atrophy. They may therefore be a good model to study pre-clinical stages of Alzheimer’s disease, before the diagnosis of dementia or the onset of neuronal loss. In Tg2576 mice, as in other APP transgenic mice, there is strong evidence that amyloid-b (Ab) is responsible for age-related memory decline. However, there are several paradoxical findings about the relationship between Ab and cognitive decline that suggest a complex role for Ab in cognitive impairment. For example, spatial reference memory in Tg2576 mice declines modestly but significantly at 6 months of age and then remains stable for 7 to 8 months (Fig. 1a, b). However, no candidate Ab species measured to date corresponds with the decline in memory observed at 6 months and the cognitive stability observed thereafter (see Supplementary Table 1). Hence, we are faced with the paradox that a rapidly increasing amount of Ab, the molecule believed to be responsible for memory loss, is associated with no change in memory function. One solution to this conundrum is to posit the existence of soluble Ab assemblies that disrupt memory, which we designated Ab* (Ab star) and sought to identify in Tg2576 mice. A challenge in analysing Ab in the brain lies in reliably separating the specific cellular pools of Ab (for example, extracellular, intracellular, membrane-associated and insoluble). We overcame this obstacle by developing a high-fidelity extraction procedure that separates proteins in known cellular compartments (Supplementary Fig. 1). Our new extraction method allowed us to quantify and compare four independent pools of transgene-derived Ab species. To resolve the problem of a mismatch between Ab levels and memory deficits, we used our extraction procedure to search for Ab* in Tg2576 mice between 4 and 25 months of age. We required candidate Ab* molecules to satisfy two criteria. First, their appearance should coincide with memory loss at 6 months. Second, their levels should remain stable in middle-aged mice (6–14 months old). By immunoblotting immunoglobulin-depleted forebrain extracts, we found a set of apparent assemblies of Ab in the soluble, extracellular-enriched fraction from 6-month-old mice (Fig. 1c). In addition to a faint 4-kDa band corresponding to Ab monomers, 6E10and 4G8-immunoreactive proteins (see Methods) were detected at molecular masses theoretically corresponding to trimeric (14 kDa), hexameric (27 kDa), nonameric (40 kDa) and dodecameric (56 kDa) Ab1–42 assemblies. These species represent multiples of trimeric Ab oligomers, with high-molecular-mass assemblies (.20 kDa) appearing in mice older than 6 months. The detection of similar bands using 6E10 and 4G8 antibodies excludes the possibility that they represent degradation products of soluble APP, which lacks themid-domain Ab epitope (Ab17–24) recognized by 4G8 (Supplementary Fig. 2a). The bands were not recognized by 22C11 or APPC17-Cter antibodies, indicating they were neither APP nor APP cleavage-end products (data not shown). Although this result suggests that ageing induces Ab trimers to associate and form high-molecular-mass assemblies, we also considered the possibility that they might represent Ab oligomers complexed to binding proteins. However, this is unlikely on the basis of their biochemical properties and immunospecificity. First, we examined their properties in urea, a common denaturant of globular proteins. To our surprise, the Ab oligomers were unaltered in SDS–polyacrylamide gel electrophoresis (SDS–PAGE) containing 8M urea (Fig. 2a). However, when exposed to$10% hexafluoroisopropanol (HFIP), a solvent with strong hydrogen-bonding properties, the theoretical hexamers, nonamers and dodecamers depolymerized, with a parallel increase in levels of tetramers, trimers and, to a lesser extent, monomers (Fig. 2b). In.20% HFIP, only the LETTERS

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@inproceedings{Lesn2008ASA, title={A specific amyloid-b protein assembly in the brain impairs memory}, author={Sylvain E Lesn{\'e} and Ming Teng Koh and Linda A. Kotilinek and Rakez Kayed and Charles G. Glabe and Austin J. Yang and Michela Gallagher and Karen Hsiao Ashe}, year={2008} }