|Alzheimer > 6. Tau Pathology: PAD Exposure|
Phosphorylation of Tau protein at multiple sites is a hallmark of Alzheimer's disease (AD), but exactly how the Tau hyperphosphorylation exerts toxic effects on cellular functions remains unclear. Tau is a microtubule-associated protein. Generally, phosphorylation of the Tau protein reduces its binding affinity to microtubules, particularly at sites serine-262 (S262) and threonine-231 (T231), which have strong impact on the Tau-microtubule binding (Sengupta et al., 1998). It was widely believed that Tau detachment from microtubules represents a major mechanism underlying Tau toxicity by causing microtubule disintegration (Figure 6-1).
As pointed out by Kneynsberg et al. (2017), a few animal studies have revealed that Tau knockout (removal of the Tau gene) leads to only mild changes in behavior, cognition, and neuropathology. These findings do not support the notion that Tau is required for microtubule stability. The following sections will show that Tau phosphorylation could have beneficial effects. Its major toxicity is likely to induce hyperactive GSK-3β when the phosphatase-activating domain (PAD) is exposed (Chapter 5).
Normal Functions of PAD Exposure
The normal function of PAD exposure is proposed to regulate the delivery of cargos (proteins or organelles) carried by conventional kinesin along microtubules (Kanaan et al., 2011, Figure 8). In the paperclip conformation, PAD is hidden, allowing continuous transport of cargoes along microtubules. At appropriate locations where Tau is in an extended conformation, the exposed PAD may activates protein phosphatase 1 (PP1), which in tun will activate GSK-3β by dephosphorylating serine-9. Subsequently, the activated GSK-3β may phosphorylate kinesin's light chains, resulting in cargo release. According to this mechanism, aberrant PAD exposure will impede axonal transport and induce abnormal cargo delivery. More importantly, hyperactive GSK-3β may damage cells through various pathways.
Toxicity of Hyperactive GSK-3β
GSK-3β targets a plethora of substrates, including Tau protein, α-synuclein, CRMP2 and potassium channels. It may phosphorylate Tau at multiple sites (Wang et al., 2007). Phosphorylation at S199, S202, and T205 moves the N-terminal domain away from the C-terminal domain while phosphorylation at S396 and S404 moves the C-terminal domain away from the repeat domain, as demonstrated by pseudo-phosphorylation in which these residues are mutated to negatively charged glutamate (E) (Jeganathan et al., 2008). In either case, PAD is exposed. GSK-3β targets all of these sites (Wang et al., 2007). Therefore, hyperactive GSK-3β may cause aberrant PAD exposure. The PAD-exposed Tau proteins can further activate other GSK-3β as they diffuse within the cell or spread to other neurons.
In addition to Tau protein, GSK-3β may phosphorylate α-synuclein, resulting in the loss of functional transportable microtubules which play a crucial role in long-range synchronization of neural oscillations. Phosphorylation of CRMP2 by GSK-3β may impair outgrowth of neurites (axon and dendrites) (see this article) while phosphorylation of potassium channels increases neuronal excitability.
Tau Toxicity Depends on PAD Exposure
Aggregation of Tau proteins was proposed to underlie Tau toxicity. However, it has been shown that Tau oligomers (small Tau aggregates), not neurofibrillary tangles (NFTs), are the true toxic species (Ward et al., 2012; Shafiei et al., 2017). Other studies suggest that the PAD exposure plays a critical role in Tau toxicity: PAD is exposed in early pre-tangle Tau aggregates, but not in late stage NFTs (Combs et al., 2016; Combs and Kanaan, 2017). These findings indicate that the PAD-exposed Tau proteins, whether they are monomeric or in aggregates, can exert toxic effects through the PAD/PP1/GSK-3β pathway.
Intriguingly, phosphorylation at T175 (pT175) facilitates the formation of NFTs (Moszczynski et al., 2015). pT175 appears only in the late stage of the disease (Moszczynski et al., 2017). Therefore, pT175 could have beneficial effects by hiding PAD in NFTs.
Beneficial Effects of PAD Exposure
Arguably, our body is an efficient physiological system where every cellular process should have beneficial purposes. If Tau phosphorylation were always toxic, it would not have existed. As discussed in Chapter 7, elevated total and 4-repeat Tau increases excitability. Hyperexcitability can lead to high Ca2+ concentration, thereby enhancing the activities of GSK-3β and cyclin dependent kinase 5 (Cdk5). Both GSK-3β and Cdk5 can phosphorylate S199/S202/T205 and S396/S404 to expose PAD. Since PAD exposure would exert detrimental effects, why have this happened?
It has been shown that Tau proteins phosphorylated at S202/T205 or S396/S404 are selectively targeted for degradation (Dickey et al., 2006). Therefore, the real purpose of PAD exposure during hyperexcitability is likely to reduce Tau level, as hyperexcitability could arise from elevated Tau level. In this regard, Tau phosphorylation could be a protective process. It is the failure of the degradation system that causes the disease.
Beneficial Effects of pY18
Phosphorylation of tyrosine-18 (pY18) is catalyzed by tyrosine kinase Fyn. pY18 has been demonstrated to prevent PAD from activating the PP1/GSK-3β signaling cascade (Kanaan et al., 2012). Thus, pY18 can exert beneficial effects by suppressing the GSK-3β activity. Notably, PAD exposure occurs prior to pY18 during the progression of AD. However, there is a large fraction of exposed PAD that is not phosphorylated at Y18 in mild and severe AD. These Tau proteins could contribute to the disease.
Beneficial Effects of pS262 and pT231
As mentioned above, phosphorylation of Tau protein generally reduces its binding affinity to microtubules, particularly pS262 (phosphorylation at S262) and pT231. Chapter 9 will show that the detachment of Tau protein from microtubules at the axon initial segment attenuates neuronal excitability, thereby reducing excitotoxicity, including the deleterious effects of hyperactive GSK-3β. This prediction agrees with the finding that at the early stage of AD, increased phosphorylation within or near Tau's microtubule binding domain correlates with reduced levels of neuronal excitability, suggesting that Tau phosphorylation on these sites represents a compensatory mechanism that mediates neuroprotection against hyperexcitability (Mondragón-Rodríguez et al., 2018).
Author: Frank Lee