Tau-proximity ligation assay detects pathology ahead of tau neurofibrillary tangles in Alzheimer's disease
Nora Bengoa-Vergniory, Ana Maria Silva, Connor Scott, Richard Wade-Martins and Javier Alegre-Abarrategui
OPDC, Dept of Physiology, Anatomy and Genetics, Oxford, UK
Alzheimer's disease is a neurodegenerative disorder affecting thousands of patients worldwide. While the main neuropathological features remain amyloid plaques and neurofibrillary tangles, it is still debated whether these are toxic or helpful for neuronal health and survival. Visualizing early aggregates will help us answer this question and will shed light on disease progression and pathology. Therefore, we have developed a new technique, the tau proximity ligation assay (tau-PLA) that allows us to visualize early pathology in the form of fibrillar tau aggregates. We have validated this technique in vitro using bimolecular fluorescent complementation assays, and electron microscopy in order to prove that tau-PLA detects only aggregates of tau, and that the nature of these aggregates is fibrillar. Unsurprisingly, in situ tau-PLA detects neurofibrillary tangles and neuritic plaques, but in addition, tau-PLA was also able to detect striking accumulations of fibrillar tau diffusely in the neuropil across hippocampal regions, particularly extensive in the prominent in white matter tracts such as at the alveus / stratum oriens. This diffuse stain appeared extensively and abruptly from the very early stages of the AD pathological process (Braak stages I, II, and III) while it seemed to decrease as disease progressed into later stages (Braak stages IV, V and VI). Importantly, these aggregates were invisible to classical immunohistochemistry such as AT8 and only detected by tau-PLA, which was able to detect pathology earlier than traditional methods. These results highlight a new methodology capable of detecting early pathology in situ, and suggest that the early fibrils recognized by tau-PLA initiate disease in axonal tracts, and then progress into classical lesions. These exciting new findings will allow future research to determine whether early treatment tackling these tau molecular species could be a new therapeutic avenue to explore for the treatment of AD.