**6. Truncation of tau protein at Asp-421**

tau and mAb 423 immunoreactivity of NFTs, have a close clinical-pathological relationship; the density of NFTs immunolabelled with mAb 423 is correlated with the progression of neurofibrilary pathology, as determined by Braak staging criteria (Fig. 2). Most significantly, there is a correlation between mAb 423 immunoreactivity and both clinical severity and pro‐ gression to dementia [3]. On the other hand, over-expression of PHF-core tau, in cell culture, favors a programmed cell death or apoptosis, which shows that it is highly toxic[59]. Re‐ combinant tau protein truncated at Glu-391 also shows increased rates of polymerization compared with recombinant full-length tau. From confocal microscopy studies, it has been shown that this fragment of tau is hidden within the PHF-core and exposed by formic acid treatment [57]. In the cytoplasm of susceptible neurons, this truncated tau triggers an auto‐ catalytic process in which the fragment has a high affinity for full length tau and, once bound, leads to cycles of proteolysis and further tau binding to form a proto-filament [60]. In this scenario, the initiating tau species that gave rise to the filament is hidden within a large number of covering tau molecules, some of which become hyperphosphorylated. This would correspond to the early aggregation of tau protein associated with PHF in small NFT. Tau molecules of the NFT would become exposed on death of the neuron to reveal the ex‐ tracellular NFT, or "ghost tangle" (Fig. 1D, small arrow) which shares the properties of being stable, insoluble and immunoreactive with mAb 423 [57, 61]. The proteases responsible for

**Figure 6.** Location of tau phosphorylation sites and epitopes for tau antibodies. Multiple amino acids are phosphory‐ lated with some those observed in AD brain [5], normal brain (green) and both normal and AD brains (blue). Putative phosphorylation sites that have not yet been demonstrated *in vitro* or *in vivo* (black). Localization of antibody epitopes

are indicated arrows. Residues are numbered according to the longest tau isoform.

truncation at Glu-391 are not known.

96 Understanding Alzheimer's Disease

In 2003, a second truncation of tau protein was found to be associated with PHFs [62-65]. This truncation is found at position Asp-421 in the C-terminus of the tau molecule and its presence can be detected specifically by using mAb TauC-3[25, 63]. Unlike truncation at Glu-391, for which the protease responsible is unknown, caspase-3 (an enzyme involved in the apoptotic pathway) is responsible for the truncation at Asp-421 *in vitro* [59, 62, 63]. This suggests that cleavage of the carboxyl terminus of tau protein, could result as a neuronal re‐ sponse to prevent or control the polymerization of tau in PHF [58]. In 2005, Binder and col‐ leagues discovered a truncation at the amino terminus of the tau protein associated with PHFs. This cut corresponds to Asp-13, which is produced by caspase-6, another enzyme in‐ volved in the apoptotic pathway [66]. An antibody to detect this cleavage site of the amino terminus of tau protein has not yet been generated.

Tau-C3 has an affinity for NFTs, NDs and neuropil threads in AD brains. Immunohisto‐ chemical studies indicated that truncation at Asp-421 occurs after conformational change; the antibody binds with greatest affinity when the amino terminus of tau molecule contacts the third microtubule binding repeat (MTBR), as recognized by mAb Alz-50 [26]. However, other studies have shown Tau-C3 immunoreactivity in pre-tangle cells before they become Alz-50 immunoreactive and in the absence of PHFs [64, 67].
