This double-labelling confocal microscopy study of the neuropathology of
Alzheimer's disease (AD) reports the use of a
fluorescent dye,
thiazin red, which has staining properties similar to
thioflavin-S.
Thiazin red fluorescence can be visualised selectively in the red channel, and we have used this property to compare it with the labelling seen using
monoclonal antibody (mAb) 423, which detects
tau protein C-terminally truncated at Glu-391, and mAb 4G8, which detects
beta-amyloid protein.
Thiazin red is shown to recognized the typical histopathological deposits associated with both
proteins. However, not all deposits containing these
proteins are stained. Specifically, diffuse
beta-amyloid plaques and severely degraded extracellular tangles are unlabelled. Likewise a characteristic mAb 423-reactive granular plaque-like structure, typically present in cases with abundant extracellular tangels, is unlabelled by
thiazin red. Such plaques can be shown to be continuous with the basal dendrites of degraded tangle-bearing pyramidal cells. These findings suggest that paired helical filaments (PHFs) continue to undergo degradation in the extracellular space, which is associated with loss of
thiazin red binding sites, but preservation of mAb 423 immunoreactivity. This
epitope appears to be characteristic of a stable core
element of the PHF which is highly resistant to proteolysis. Compounds such as
thiazin red with high affinity for beta-pleated
protein structures can be used to monitor the state of pathological assembly of
amyloidogenic protein species found in AD.