|10. Differential Effects of 3 and 4 Repeat Tau
According to the Microtubule Model for Excitability proposed in Chapter 9, neuronal excitability can be regulated by the interaction between Ankyrin-G and microtubules at the axon initial segment. The microtubule associated protein Tau may interfere with this interaction, thereby modulating neuronal excitability. In the Tau protein, the repeat region is the microtubule binding domain. Thus, the 4-repeat (4R) Tau binds to the microtubule much stronger than the 3-repeat (3R) Tau (see Chapter 5), resulting in differential effects on excitability.
In a healthy adult human brain, the levels of 4R and 3R Tau proteins are approximately equal. Distortion of the balance toward 4R Tau will lead to neurodegeneration, as observed in Alzheimer's disease (Yasojima et al., 1999; Ginsberg et al., 2006; Glatz et al., 2006), Huntington's disease (Fernández-Nogales et al., 2014) and FTDP-17 (see last section).
Modulation of Excitability by Tau
A microtubule may associate with the plasma membrane through interaction with Ankyrin-G (Figure 9-1). Higher Ankyrin-G level will provide more "anchor points", which restrict microtubule bending away from the membrane, thereby reducing the open probability of Nav1.6 and excitability. The end binding protein, EB1 or EB3, may facilitate the binding between microtubules and Ankyrin-G (Leterrier et al., 2011). Tau is also known to interact with EB proteins (Sayas et al., 2015). Hence, Tau can increase excitability by interfering with the microtubule association with the membrane. This explains why in animal models Tau reduction or knockout attenuates hyperexcitability (Holth et al., 2013; DeVos et al., 2013; Li et al., 2014).
The 4R Tau binds on the microtubule stronger than the 3R Tau. Therefore, 4R Tau should have greater impact on enhancing excitability than the 3R Tau. It seems that for adult humans the equal level of 4R and 3R Tau can produce optimal excitability. Higher level of 4R Tau will lead to hyperexcitability, whereas elevated 3R Tau may reduce neuronal activity.
Modulation of Excitability by mTOR
The mechanistic target of rapamycin (mTOR) plays crucial roles in diseases and aging (Appendix D). It is now well-established that mTOR activation can enhance excitability (see Epilepsy and mTOR). While hyperexcitability could arise from alteration at synapses, the hyperactive mTOR does not necessarily lead to changes in synaptic transmission (Lasarge and Danzer, 2014; Wang et al., 2015). Furthermore, the mTOR-induced hyperexcitability is likely to originate from persistent Na+ current, INaP (Manuel and Heckman, 2011; Carunchio et al., 2010). The major source of INaP is the ionic current through non-inactivating Na+ channel, Nav1.6, which is regulated by Ankyrin-G (Shirahata et al., 2006).
A principal function of mTOR is the synthesis of proteins, including Tau (Caccamo et al., 2013; Tang et al., 2013; Tang et al., 2015). Thus, the above results provide strong evidence that hyperexcitability can indeed be induced by over-production of Tau proteins, which promote opening of the Nav1.6 channel.
Tauopathies and Seizure
Tauopathies are characterized by the deposition of abnormal (usually hyperphosphorylated) Tau proteins in the brain (Kovacs, 2015). As explained above, elevated 4R Tau may give rise to hyperexcitability via activation of the Nav1.6 channel which plays a critical role in epilepsy (seizure) (O'Brien and Meisler, 2013). The 4R tauopathies are indeed associated with seizure.
FTDP-17: Testing the Model
The frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17) is caused by mutations of the Tau gene MAPT, which is located on chromosome 17. In most cases, the mutation may either increase the 4R Tau level or reduce the Tau-microtubule binding (Hyman et al., 2005). According to the present model, the two biochemical alterations should have opposite effects on neuronal excitability: increase in 4R Tau will enhance, whereas reduction in microtubule binding will attenuate, neuronal excitability.
Mutations at Introns
In a gene, the sections that encode a protein sequence are called exons, and the non-coding sections are referred to as introns. Thus, mutations in introns should not affect protein sequences. For the Tau gene, mutations in the introns flanking exon 10 may affect alternative splicing, resulting in 4R:3R Tau imbalance. Distortion toward 4R Tau will enhance excitability. As discussed in Chapter 12, hyperexcitability may cause calcium overload to activate calpain, leading to Tau hyperphosphorylation. The hyperphosphorylated Tau proteins may aggregate to form inclusion bodies such as neurofibrillary tangles or Pick bodies (King et al., 2001). On the other hand, distortion toward 3R Tau will reduce neuronal activity. No Tau hyperphosphorylation or inclusion is expected. These predictions agree with the following observations.
Neuronal activity may stimulate BDNF signaling, leading to mTOR activation (Chapter 11, Figure 11-3 ). In addition to protein synthesis, mTOR also plays important roles in cell survival (Appendix D). Activation of mTOR inhibits cell apoptosis. Therefore, reduced neuronal activity can enhance apoptosis.
Mutations at Microtubule Binding Domains
In FTDP-17 brains, mutations at microtubule binding domains all lead to Tau hyperphosphorylation (Hyman et al., 2005; de Silva et al., 2006). It was proposed that the mutations may induce a conformational change in the Tau proteins that make them more favorable for phosphorylation (Alonso Adel et al., 2004). However, mutant Tau proteins were not phosphorylated more than wild-type Tau either in transfected cultured cells or in vitro (Sakaue et al., 2005). The present model predicts that reduced Tau-microtubule binding should attenuate neuronal excitability, which does not promote phosphorylation. Therefore, Tau hyperphosphorylation in FTDP-17 brains must arise from other mechanisms. The most likely one is the activation of mTOR.
Misfolding may Activate mTOR
mTOR can sense a wide range of signals, including misfolded proteins (Qian et al., 2010). This capability allows the activated mTOR to trigger synthesis of chaperones (e.g. heat shock proteins HSP70, HSP90, etc.), which are responsible for correct protein folding (Conn and Qian, 2011). Chaperones also play an important role in Tau-microtubule binding (Dou et al., 2003). Therefore, Tau mutations at microtubule binding domains may alter conformation (Jicha et al., 1999; Frost et al., 2009), resulting in mTOR activation, and leading to hyperexcitability. In support of this mechanism, rapamycin (a potent mTOR inhibitor) has been shown to attenuate the progression of tau pathology in P301S tau transgenic mice (Ozcelik et al., 2013).
Future Testing: Tau Knockin
In the past, the vast majority of animal models employed overexpression of mutant Tau. Since the Tau level becomes higher than the endogenous level, as expected, they all led to hyperexcitability (García-Cabrero et al., 2013) and hyperphosphorylation even overexpressing 3R Tau at high level (Rockenstein et al., 2015). By contrast, the knockin animal model replaces the original gene with an inserted gene (e.g. a mutant human Tau gene). This approach does not artificially overexpress Tau proteins.
In "P301L" Tau knockin mice, Tau phosphorylation was reduced and Tau inclusion was not observed. The mice displayed increased spontaneous locomotor activity in old age (Gilley et al., 2012) and loss of axonal mitochondria (Rodríguez-Martín et al., 2015). These results parallel the effects of mutant SOD1 in a mouse model of amyotrophic lateral sclerosis, where misfolding of mutant SOD1 may activate mTOR, resulting in neuronal hyperexcitability and mitochondrial dysfunction (Appendix B).
In "P301L" Tau knockin mice, as in SOD1-mutated transgenic mice, the mTOR-induced hyperexcitability may not be sufficient to activate calpain to induce hyperphosphorylation within the lifetime of a mouse. However, in FTDP-17 patients, the constant activation of mTOR by Tau misfolding could eventually lead to hyperphosphorylation over the course of several decades. It should be interesting to see if mTOR inhibition can suppress spontaneous locomotor activity and rescue mitochondrial loss in "P301L" Tau knockin mice. If confirmed, mTOR inhibitors could be a promising therapeutic for FTDP-17.
Author: Frank Lee