Tau-proximity ligation assay reveals extensive previously undetected pathology prior to neurofibrillary tangles in preclinical Alzheimer's disease

Abstract

Tauopathies, with Alzheimer’s disease (AD) the most common neurodegenerative disorder, are caused by the abnormal folding and aggregation of proteins that seed themselves and spread throughout the brain in a prion-like manner causing irreversible neuronal cell death and progressive neurological symptoms. The intracellular aggregates of the hyperphosphorylated tau in neurofibrillary tangles (NFTs), together with the extracellular deposits of amyloid beta, are the fundamental neuropathological characteristics of AD. It is unclear which tau species cause the toxicity and have seeding capacity, responsible for the prion-like spread of tau pathology. While neuronal loss correlates with the number of NFTs in AD, it is disproportionally higher, suggesting oligomeric tau species rather than NFTs being the toxic species. This also suggests that a proportion of tau pathology is not being detected by standard histopathological markers (which focus on the detection of phosphorylated tau), particularly since tau fibrillisation can occur independently of tau post-translational modifications. We have now developed a new assay, tau proximity ligation assay (tau-PLA), for the histopathological visualisation of a range of tau pathology, including previously invisible early tau multimers, in situ with anatomical and subcellular detail. Tau-PLA detected self-interacting tau in vitro and in situ in animal and post-mortem AD human brain with high specificity, recognising different patterns of tau pathology. In contrast with standard histopathological markers, tau-PLA detected a previously unreported early type of tau pathology revealing an extensive diffuse tau distribution in non-pathological brain regions of the brain along the Braak tau pathway. It also revealed that tau multimerization is one of the earliest detectable molecular events of AD tau pathology preceding tau hyperphosphorylation and misfolding. Tau Real-Time Quaking-Induced Conversion showed that the early tau multimers have high seeding activity and might be the toxic species contributing to the disease progress. Therefore, our findings open a new window to the study of early tau pathology, with potential implications in early diagnosis and the design of therapeutic strategies.