Staging the pathological assembly of truncated tau protein into paired helical filaments in Alzheimer's disease.
Tau protein, which is incorporated into the core of paired helical filaments (PHFs) in Alzheimer's disease (AD), can be characterised immunochemically by C-terminal truncation at Glu-391 recognised by monoclonal antibody (mAb) 423, and acid-reversible occlusion of a generic tau epitope in the tandem repeat region recognised by mAb 7.51. PHFs are also characterised by the presence of binding sites for a fluorescent dye (thiazin red) which can be used to differentiate between amorphous and fibrillar states of tau and beta-amyloid proteins in AD. We have used double labelling confocal microscopy to investigate that state of aggregation of the tau antigens associated with the core structure of the PHF at early stages of neurofibrillary pathology. We report that the early abnormal tau deposits in cells vulnerable to neurofibrillary degeneration are characterised by C-terminal truncation at Glu-391, acid-reversible occlusion of the mAb 7.51 epitope, and the absence of binding sites for thiazin red, consistent with the amorphous non-fibrillar structure demonstrated by immunoelectron microscopy. Transition to the fibrillar state in the PHF is associated with acid-reversible occlusion of both mAb 7.51 and 423 epitopes, and acquisition of binding sites for thiazin red. In neurites, the transition between the two states of aggregation shows distal to proximal polarity, with the fibrillar state found nearest the cell body. These findings demonstrate that the assembly of tau protein into the PHF occurs in at least two stages, an amorphous stage characterised by C-terminal truncation and occlusion of sites within the tandem repeat region, and a fibrillar stage characterised by acid-reversible occlusion of both epitopes via addition of intact tau molecules in the fuzzy coat of the PHF.