Distinctive temporal profiles of detergent-soluble and -insoluble tau and Aβ species in human Alzheimer’s disease

Abstract

Alzheimer's disease (AD) pathology relevant proteins tau and beta-amyloid (Aβ) exist as an array of post-translationally modified and conformationally altered species with varying abundance, solubility and toxicity. Insoluble neurofibrillary tau tangles and Aβ plaques are end-stage AD hallmarks, yet may carry less disease significance compared to soluble species. At present, it is unclear how soluble and insoluble tau and Aβ relate to each other as well as to disease progression. Here, detergent soluble and insoluble fractions generated from post-mortem human temporal lobe samples (Brodmann area 21) were probed for tau and Aβ markers in immuno-dot assays. Measures were quantified according to diagnosis (AD cf. Non-AD), neuropathological severity, and correlated with disease progression (Braak stages). All markers were elevated within AD cases cf. non-AD controls (p < 0.05) independent of solubility. However, when considered according to neuropathological severity, phospho-tau (detected via CP13 and AT8 antibodies) was elevated early within the soluble fraction (p < 0.05 intermediate cf. low severity) and emerged only later within the insoluble fraction (p < 0.05 high cf. low severity). In contrast, PHF1 phospho-tau, TOC1 reactive tau oligomers and amyloid markers rose within the two fractions simultaneously. Independent of solubility, cognitive correlations were observed for tau makers and for fibrillary amyloid (OC), however only soluble total Aβ was significantly correlated with intellectual impairment. Following the exclusion of end-stage cases, only soluble total Aβ remained correlated with cognition. The data indicate differential rates of protein aggregation during AD progression and confirm the disease relevance of early emerging soluble Aβ species.

Synaptic Loss, ER Stress and Neuro-Inflammation Emerge Late in the Lateral Temporal Cortex and Associate with Progressive Tau Pathology in Alzheimer’s Disease

“…Samples were subject to standard western blot protocols as described previously [3]. Protein concentration was determined through bicinchoninic acid assay (BCA, Sigma) and samples adjusted to the desired concentration in lithium dodecyl sulphate (LDS, Thermo Fisher, Paisley, UK) and 15 mM dithiothreitol (DTT, Sigma).…”

“…Membranes were then re-probed for total protein in the case of phospho-markers, as above, or stained for total protein using Coomassie total protein stain (see Fig. S1 for example of Coomassie-stained membranes) as previously described [2,3]. Supportive native state dot blots (as per [2,3]) for AQP4 were conducted due to the incompatibility of the antibody with western blotting protocols.…”

“…Alzheimer's disease (AD), the most common cause of dementia, is characterised by the accumulation of extracellular amyloid-β (Aβ) containing plaques and intracellular neurofibrillary tangles (NFTs) composed of hyper-phosphorylated tau. These hallmark features have largely been the focus of AD research; for decades, the 'amyloid cascade hypothesis' [1] remained the foremost pathogenic concept in the field and has since been revised to incorporate a role for earlier occurring soluble species, which appear to hold more disease relevance [2][3][4].…”

“…Furthermore, the separation between Aβ and tau is evident in their correlative strength with disease status and/or cognitive decline. Despite the genetic association of Aβ with AD, it is tau pathology, genetically linked to frontotemporal lobar degeneration (FTLD), which is the more robust indicator of disease progression and cognitive decline [2,3,10]. Alongside the apparent 'hallmark' pathologies, several additional tissue alterations are also apparent and assumed to contribute to either onset or progression of degenerative processes [11].…”

“…Here, we utilise a post-mortem tissue cohort of nondiseased and AD cases, previously characterised for a variety of tau and Aβ species [2,3], to contextualise the activation of the UPR and neuro-inflammation as well as associations with synaptic loss within the lateral temporal cortex. Indicative markers were determined according to diagnosis, pathological severity and neuropathological staging.…”