**5. C-terminal truncation of tau protein in AD**

#### **5.1. PHF-core concept**

It is technically difficult to determine the half life of the different tau isoforms and several factors may regulate tau degradation such as, for example, the extent of phosphorylation and acetylation of tau (see below). The half life of tau decreases in rats by neonatal period

Two main mechanisms for tau protein degradation have been documented: 1) the proteaso‐ mal ubiquitin pathway and 2) the lysosomal autophagic pathway. Proteasomal degradation of tau protein has been described by 20S proteasomal processing [51], although there have also been reports suggesting that tau is not normally degraded by the proteasome [52]. Tau, modified by phosphorylation, can be ubiquitinated by the CHIP-hsc70 complex and degrad‐ ed by the proteasome [53]. Furthermore, acetylation of tau can regulate its proteasomal deg‐ radation by modifying those lysine residues needed for ubiquitination. In this way, acetylation of tau inhibits its degradation through a competition between ubiquitination and

On the other hand, tau may get processed through a lysosomal autophagic mechanism. It has been reported that tau can be degraded by lysosomal proteases [55] and, more recently, it was shown that tau fragmentation and clearance can occur by lysosomal processing [56]. Tau protein is a microtubule-associated protein. It's mostly abundant in neurons in the Cen‐ tral Nervous System (CNS). The main function of Tau protein is to interact with tubulin to stabilize microtubules and promote tubulin assembly into microtubules. Tau protein con‐

Normally, the tau protein is very important, as it manages the transport of materials within soma and other cellular regions through the myelin sheaths. Once it spotted something sus‐ picious or irrelevant, it stops the information sending process automatically. However, in Alzheimer's disease, the tau proteins started to perform uncommon reaction, where it trans‐

Once the above problem happening, it causes the brain overloading with information and might lead to inflammation, clumps or tangles, which kill most of the brain cells (Fig. 5).

Protein phosphorylation is the addition of a phosphate group, by esterification, to one of three different amino acids: serine, threonine and tyrosine. Phosphorylation is the most common post-translational modification of tau described and increased tau phosphorylation reduces its affinity for microtubules leading to cytoskeletal destabilization. Eighty-five puta‐ tive phosphorylation sites on tau protein have been described in AD brain tissue (Fig. 6). The formation of fibrillar aggregates of post-translationally modified tau protein in the brain are characteristic of AD and other tauopathies. The phosphorylation of tau protein affects its solubility, localization, function, interaction with partners and susceptibility to other posttranslational modifications. However, the role of specific sites of tau phosphorylation in ear‐ ly neurodegenerative mechanisms is unknown. The molecular mechanisms of aggregation

trols microtubule stability in two different ways : isoforms and phosphorylation.

mitting the information to the brain simultaneously, regardless of its validity.

**4. Phosphporylation of tau protein**

P20 and there is less demand for tau in non-dividing, mature neurons [50].

acetylation [54].

94 Understanding Alzheimer's Disease

In 1988, Wischik et al, [7, 22] identified tau protein as the major constituent of Pronase-resist‐ ant PHFs and tau was characterized by a specific C-terminal truncation of the protein at Glu-391. This truncation is identified by the monoclonal antibody (mAb) 423 [23, 31], and the acid-reversible occlusion of the intact core tau domain. PHFs are labeled by the fluores‐ cent dye, thiazin-red, a dye which can be used to differentiate between amorphous and fi‐ brillar states of tau and amyloid proteins in AD. The minimum tau fragment in the PHF [20, 24] corresponds to the tandem repeat region in the C-terminal domain of tau protein, a spe‐ cies having a molecular weight of 12.5 kDa. Characteristically, this fragment is highly stable to proteolysis, insoluble and toxic and is referred to as PHF-core tau [22, 57, 58]. PHF-core 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 truncation at Glu-391 are not known.

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

terminus of tau protein has not yet been generated.

**of tau protein in AD**

Alz-50 immunoreactive and in the absence of PHFs [64, 67].

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

Phosphorylation of Tau Protein Associated as a Protective Mechanism in the Presence of Toxic…

http://dx.doi.org/10.5772/54228

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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

**7. Impact of phosphorylation and truncation on the abnormal processing**

**7.1. A neuroprotective mechanism for the phosphorylation of tau protein in the AD brain** During neurodegeneration in AD, tau protein is abnormally phosphorylated in the prolinerich region at Ser and Thr residues [68], and such phosphorylation sites can be identified us‐ ing highly specific antibodies such as: AT8 (Ser-202/Thr-205) AT100 (Ser-212 and Ser-214), TG3 (Thr-231/Ser-235) and PHF-1 (Ser-396/Ser-404), among others (Fig. 6). However, NFTs are found in viable neurons at late stages of the disease, and they persist in neuronal cells for decades with a significant number of NFTs being found in the cognitively intact elderly [69, 70]. Such NFT-bearing neurons contain normal content and structure of microtubules [68]. The findings from studies in transgenic mice and human data, suggest that tau accu‐ mulation in the somatodendritic compartment may represent the manifestation of a protec‐ tive mechanism or a cellular adaptation that arises with advancing age. An increase in tau phosphorylation in AD brain has been associated with a protective mechanism against oxi‐ dative stress [71]. In another study, intact microtubules were found in NFT-bearing neurons [8], calling into question whether accumulation of phosphorylated tau and destabilization of microtubules are necessarily linked. Although microtubules are depolymerized in neurons with fibrillary degeneration, one study found evidence that the reduction of microtubules in AD is marked and specifically limited to vulnerable pyramidal neurons, and that even these alterations were observed in the absence of PHF [72]. This finding is also consistent with

**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.
