Alzheimer's disease (AD) is characterized by large numbers of extracellular amyloid plaques with dense amyloid cores that are associated with dystrophic neurites and neuroinflammatory changes as well as intraneuronal neurofibrillary tangles. Pathological aging (PA) patients also have abundant and widespread amyloid plaques; however, these plaques have typically been described as diffuse in nature. In PA there are fewer cored plaques and there is little or no inflammatory reaction, neuritic pathology or neurofibrillary tangles in the cortex. These patients are reported to be cognitively normal prior to death [1–3]. Based on our current understanding of the progression of AD, PA may represent a prodromal phase of AD (for example, preclinical stage 1 AD, plaque only), a benign form of Aβ accumulation, or inherent individual resistance to the toxic effects of Aβ accumulation [3, 4].
Aβ is the principle component of amyloid deposits in the AD brain. It is a secreted peptide produced through sequential cleavage of the Amyloid-β Protein Precursor (APP) by β- and γ-secretases [5–7]. Aβ peptides have a heterogeneous carboxyl-terminus with the majority (approximately 40% to 70%) composed of 40 amino acids Aβ1-40, while a minor product (approximately 5% to 20%) contains a two amino acid extension Aβ1-42. Additional minor Aβ peptides are also normally produced (for example, 1-34, 1-37, 1-38 and 1-39), although few reports have quantified the levels of these peptides in the brain . Aβ1-42 is more amyloidogenic and has been implicated as the pathogenic form of Aβ . A recent study also suggested that Aβ1-43 could play a critical role in Aβ accumulation . Furthermore, a variety of truncated and modified Aβ peptides have been described (for example, 1-28, 1-29, 1-45, 2-46, 3-44, 3-47, 2-42, 4-42, 5-42, 6-42, 7-45, 8-42, 1-42Met35ox, pE3-42, pE11-42) [11–18]. Of these truncated and modified forms the pyroglutamate modified forms, AβpE3-42 and AβpE11-42, have been highly investigated, as key species possibly involved in initial nucleation or seeding events [19–22].
Once liberated from APP, Aβ can self-associate to form various aggregates. These aggregates include soluble oligomers, protofibrils, and amyloid fibrils [23, 24]. Although there is currently debate within the field regarding which form or forms of Aβ aggregates are the most pathogenic, there is general consensus that the aggregated forms of Aβ are harmful and that Aβ1-42 or possibly Aβ1-43 is required for aggregation in the absence of internal mutations within Aβ [25–30].
Because many different forms of Aβ exist and accumulate in various higher order assemblies, it is possible that the relatively poor correlation between cognitive deficits and plaque load is attributable to either qualitative or quantitative differences between a particular species or assembly of Aβ. This poor correlation could also reflect an inherent difference in vulnerability to 'toxic' effects of different forms of Aβ aggregates. Additionally, given the growing acceptance of the concept of preclinical AD [4, 31, 32], where the initial stage is defined by the presence of Aβ deposits in the absence of other pathologies and no evidence for cognitive impairment, the poor correlation could be attributable to differences in time from initial deposition to frank neurodegeneration and clinical deterioration.
As PA represents the most clear cut example of the dissociation between Aβ accumulation and cognitive impairment, investigation of the type and species of Aβ peptides in PA cohorts could provide novel insights into the poor correlation between Aβ and cognition. Previous studies investigating differences in Aβ1-40 and Aβ1-42 species extracted from PA and AD brains demonstrated that Aβ1-40 levels were as much as approximately 20-fold higher in AD brains compared to PA brains whereas Aβ1-42 levels were only about 2-fold higher . A more recent and extensive study using both ELISAs and western blotting to analyze Aβ levels and oligomeric assemblies failed to detect major differences in PA and AD . Other more anecdotal studies comparing oligomeric assemblies in a single PA brain versus AD brains failed to detect significantly elevated levels of Aβ dimer in the PA brain extracts compared to AD brain extracts .
Although these studies suggest that there may be both quantitative and qualitative differences in Aβ peptides in PA brains as opposed to AD brains, we felt that a more extensive investigation with larger cohorts was warranted. Here we report on our analysis of Aβ peptides sequentially extracted from the pre-frontal cortices of 16 AD patients, eight PA patients, and six non-demented controls (NDC) using a battery of biochemical tests. Our analysis shows that AD and PA brains are clearly distinct from controls, but there is extensive overlap between PA and AD with respect to extractable Aβ levels as measured by ELISA. Using immunoprecipitation mass spectrometry (IP/MS) to profile individual Aβ species in the PA and AD brain extracts we find that there is also extensive overlap in the profiles of accumulated Aβ. However, individual AD brains showed more extensive heterogeneity with an increase of diversity of Aβ species, particularly amino-terminally truncated Aβ species. Assessment of SDS-stable oligomers by western blotting also showed no consistent differences between PA and NDC.