Tau in Health and Neurodegenerative Diseases

*Dandan Chu and Fei Liu*

## **Abstract**

Tau, one of the major microtubule-associated proteins, modulates the dynamic properties of microtubules in the mammalian nervous system. Tau is abundantly expressed in the brain, particularly in the hippocampus. Insoluble and filamentous inclusions of tau in neurons or glia are discovered in neurodegenerative diseases termed 'tauopathies', including Alzheimer's disease (AD), argyrophilic grain disease (AGD), corticobasal degeneration (CBD), frontotemporal dementia (FTD), Pick's disease (PiD) and progressive supranuclear palsy (PSP). Accumulation of intracellular neurofibrillary tangles (NFTs), which are composed of hyperphosphorylated tau, is directly correlated with the degree of Alzheimer's dementia. This chapter reviews the role of tau protein in physiological conditions and the pathological changes of tau related to neurodegenerative diseases. The applications of tau as a therapeutic target are also discussed.

**Keywords:** tau, neurodegenerative diseases, pathology, Alzheimer's disease, therapy

### **1. Introduction**

The human hippocampus is critically involved in spatial navigation, the forming, storing and retrieval of episodic memory and the processing of many other types of memory [1]. In the normal human brains, the hippopotamus volume remains relatively stable before the age of 60, and then gradually atrophies. In Alzheimer's disease (AD) patients, the pattern of brain atrophy follows a stereotypical pathway that initiates from the entorhinal cortex and the hippocampus, and then spreads to the medial parietal, lateral temporal and frontal regions, eventually to the neocortex. As the connections between the hippocampus and its neighboring cortical structures are selectively vulnerable to neurodegeneration in AD, hippocampal volume loss is considered an important indicator of AD neuropathology [2]. In addition, since neurogenesis in adult brain only occurs in the dentate gyrus of the hippocampus and the olfactory bulb, hippocampus atrophy in AD also alters the production of newborn neurons [3].

Tau, initially isolated as a microtubule-associated protein from the porcine brain in 1975, is predominantly expressed in the hippocampus [4, 5]. In the previous year, neurofibrillary tangles (NFTs) and a paired helical filament (PHF) protein had been identified from the brains of patients with Alzheimer's disease (AD) [6], but it was not until 1986 that tau was discovered to be a major component of PHF [7]. Subsequently, tau was implicated in the pathogenesis of over 25 human neurological disorders (termed "tauopathies"), including Alzheimer's disease (AD), argyrophilic grain disease (AGD), corticobasal degeneration (CBD), frontotemporal dementia

(FTD), globular glial tauopathy (GGT), primary age-related tauopathy (PART), Pick's disease (PiD) and progressive supranuclear palsy (PSP) [8–10].

As the most common neurodegenerative disease, AD is characterized by early impairments in learning and memory, followed by progressive loss of complex attention, executive function, language, orientation and self-care ability, changes in mood, loss of motivation, and impairments in thinking, behavior and/or social comportment [11]. The two major neuropathological hallmarks of AD are the extracellular deposition of β-amyloid (Aβ) plaques and the intracellular neurofibrillary tangles (NFTs) consisting of aggregated hyperphosphorylated tau [12]. NFTs pathology in AD is initiated in the locus coeruleus and transentorhinal cortex, from where it spreads to the limbic system (e.g., entorhinal cortex and hippocampus) and further to the neocortex, leading to six Braak stages [13]. The progression of cognitive decline in AD correlates with the accumulation of NFTs and loss of hippocampal volume, but not deposition of Aβ plaques [14, 15]. Since most therapies targeting Aβ failed in late-stage clinical trials for AD in the past decades, increasing research revealing the roles of tau in disease has inspired tau-targeting approaches in the treatment of AD and related tauopathies [16, 17].

This chapter reviews the expression and functions of tau in physiological conditions, the pathological changes of tau in diseases, such as genetic variants, posttranslational modifications (PTMs) and prion-like seeding and propagation. Recent advances in the development of tau-based therapies for AD and other neurodegenerative diseases are also discussed.

### **2. Tau gene**

Human tau protein is encoded by the microtubule-associated protein tau (MAPT) gene, which locates on chromosome 17q21.31 and consists of 16 exons. In the central nervous system (CNS), the alternative splicing of exons 2, 3, and 10 gives rise to six tau isoforms with zero (0 N), one (1 N) or two (2 N) N-terminal inserts and three (3R-tau) or four (4R-tau) microtubule-binding repeats (**Figure 1**) [18]. The longest isoform of human brain tau consists of 441 amino acids (2N4R, tau441) with an apparent molecular weight (MW) of 46 kDa. Exons 4a and 6 are predominantly expressed in the peripheral nervous system (PNS), producing proteins of apparent MW of 110 kDa, named big tau [19].

### **2.1 Alternative splicing of tau pre-mRNA**

The expression of tau isoforms is developmentally and pathologically regulated. 3R-tau isoforms are expressed throughout life, including in the fetal brain, whereas 4R-tau isoforms are specifically expressed in adults, resulting in approximately equal levels of 3R-tau and 4R-tau in the adult human brain [18]. Rodent tau shares about 90% homology with human tau. Unlike humans, rodents express 3R-tau only in fetus and infant, and mainly 4R-tau in adulthood [20].

Tau pre-mRNA contains multiple cis-elements that allow the interaction of trans-acting factors like the serine and arginine-rich (SR) proteins and heterogeneous nuclear ribonucleoproteins (hnRNPs), including an SC35-like enhancer, a polypurine enhancer (PPE) and an A/C-rich enhancer (ACE) at the 5′ terminus, and an exonic splicing silencer (ESS) and an exonic splicing enhancer (ESE) at the 3′ end of tau exon 10, and an intronic splicing silencer (ISS) and an intronic splicing modulator (ISM) at 5′ end of intron 10 [21]. Binding of trans-acting factors to these cis-elements either suppresses (SRSF3, SRSF4, SRSF7, SRSF11, U2AF, PTB

*Tau in Health and Neurodegenerative Diseases DOI: http://dx.doi.org/10.5772/intechopen.101299*

### **Figure 1.**

*Gene structure and expression of human MAPT gene. MAPT gene is localized to chromosome 17 and consists of 16 exons. Exons 4a, 6, and 8 are expressed in PNS. Alternative splicing of tau exon 2 (green, encodes N1) or exon 3 (yellow, encodes N2) produces isoforms with zero (0 N), one (1 N) or two (2 N) N-terminal inserts. Exons 9, 11 and 12 encode the microtubule-binding repeats R1, R3, and R4 (blue). Alternative splicing of exon 10 (red, encodes R2) generates isoforms containing four (4R-tau) or three (3R-tau) microtubule-binding repeats.*

and hnRNP G) or promotes (hTRA2-beta1, CELF3, CELF4, SRSF1, SRSF2, SRSR6, SRSF9, RNA helicase p68, RNA binding motif protein 4 and Tar DNA-binding protein 43) the inclusion of exon 10 [22].

Mutations in the MAPT gene or dysregulation of the splicing factors that alter the expression of tau exon 10 are involved in the pathogenesis of neurodegenerative diseases [21]. PiD is a prototypical 3R-tauopathy with spherical tau inclusions (Pick bodies), whereas PSP, CBD, AGD and GGT are primary 4R-tauopathies. AD, FTD and chronic traumatic encephalopathy (CTE), progressive neurodegeneration associated with repetitive mild traumatic injury, comprise both 3R and 4R-tau pathologies [23]. Studies on the amount of the six tau transcripts in AD brain have been contradictory [24, 25]. However, 4R- and 3R-tau proteins appear equally in the early-stage and some late-stage AD cases [26, 27]. In some cases of late-stage AD, 3R-tau protein is predominantly expressed in the subiculum, entorhinal cortex and area CA1 of the hippocampus [27], implying that dysregulation of tau exon 10 splicings could be related to AD progression rather than initiation.

### **2.2 Tau mutations**

So far, 112 mutations have been identified in human the MAPT gene, of which 13 were located in an intron (https://www.alzforum.org/mutations). Not all the mutations are pathogenic, and at least 27 benign mutations are not responsible for significant clinical symptoms. The largest number of MAPT mutations (55) is found in FTD. Second, 15 mutations are identified in AD, most of which are benign mutations causing no significant neuropathology. The rest mutations are correlated to PSP (10 mutations), Parkinson's disease (PD) (8), PiD (7), CBD (4), AGD (1) and other tauopathies (**Table 1**). As genetic risk factors for neurodegenerative diseases, pathogenic tau mutations alter the protein sequence or/and the balance between 4R- and 3R-tau by changing alternative splicing [28].


**Table 1.**

*MAPT mutations found in main neurodegenerative diseases.*

Two major extended haplotypes cover the MAPT gene: H1 and H2 [29]. The frequency of the haplotypes differs between population groups; the H2 haplotype is found primarily in Caucasian and southwest Asians, but barely reported in the Chinese Han population [29, 30]. H1 haplotype was shown overrepresented in Caucasian patients with PSP [31]. Nevertheless, the relationship between MAPT haplotype and AD is still contradictory [32].

### **3. Tau protein**

### **3.1 Protein structure of tau**

Tau is a highly water-soluble and basic protein with little secondary structure. As an intrinsically disordered protein, tau consists of a large number of serine/ threonine and arginine/lysine/histidine residues, which makes the protein easy to be hyperphosphorylated. Full-length human tau (tau441) is composed of four domains: (i) an acidic amino-terminal projection domain that projects away from the surface of the microtubule, (ii) a proline-rich region required for the interaction with SH3-domain-containing proteins like tau kinases, (iii) a microtubule-binding domain involved in mediating tubulin assembly and tau aggregation, and (iv) a C-terminus (**Figure 1**) [33].

### **3.2 Localization of tau**

Human tau is highly expressed in the frontal and temporal cortices and is decreased to approximately a quarter in the cerebellum. In rodent brains, the highest levels of tau are detected in the hippocampus and entorhinal cortex. The cerebellum and olfactory bulb showed the lowest total tau level, being about 2/3 of that in

### *Tau in Health and Neurodegenerative Diseases DOI: http://dx.doi.org/10.5772/intechopen.101299*

the frontal cortex [5]. The expression of 3R-tau is comparable throughout different regions in the adult rat brains, such as the hippocampus, entorhinal cortex, frontal cortex, occipital-temporal cortex, parietal–temporal cortex, striatum, thalamus, olfactory bulb and cerebellum. However, the distribution of 4R-tau showed significant regional differences, with the highest levels in the entorhinal and frontal cortices and the lowest in the cerebellum. The uneven expression of tau protein in brain regions may contribute to distinct vulnerability/resistance to tau pathology [5].

In the human brain, tau is primarily expressed in neurons and also expressed at lower levels in oligodendrocytes and astrocytes [34]. In physiological conditions, tau was believed to be predominantly localized to axons, but limited in the soma and dendrites of neurons. Tau levels are comparable in gray matter and white matter in normal elderly brains, but higher in gray matter than whiter matter in AD brains [35]. Normally, tau monomer is difficult to be immunostained. Only aggregated tau or microtubule-binding tau can be detected by immunostaining. Although high levels in axons, a pre-synaptic abundance of tau is low. The mechanism for polarized neuronal distribution of tau could be (i) relocalization of tau from axon to soma may be blocked by the axon initial segment (AIS), (ii) Annexin A2 in the distal part of the axon interacts with tau and provides a sink for the redistribution of tau [16]. Under pathological conditions, endogenous tau translocates from axon to the soma and dendrite, and into the post-synapse. Early studies in AD and FTD have revealed that NFTs composed of aggregated hyperphosphorylated tau are localized in the soma and dendrites [36]. Translocation of tau depends on its interaction with microtubules [37] and are modulated through multiple mechanisms involving pathological posttranslational modification (e.g., hyperphosphorylation) and/or non-physiological overexpression of tau protein [38], imbalanced expression of tau isoforms (e.g., 2 N tau) [39] and dysregulation of extracellular signals [16]. Furthermore, tau messenger RNA (mRNA) might be recruited to the dendrite and post-synapses, resulting in local somatodendritic translation of tau protein [40, 41]. Lastly, tau is also detected in the nucleus, where it is likely to protect DNA integrity from stress [42].

### **3.3 Function of tau**

Microtubules are more stable in mature neurons than in non-neuronal cells [43], probably due to posttranslational modifications of the tubulin subunits as well as their interaction with specific neuronal microtubule-associated proteins (MAP) such as tau [44]. Tau binds to the interface between tubulin heterodimers through its microtubule-binding repeats [45] and stabilizes microtubules in the test tube and cultured cells [46, 47]. 4R-tau isoforms have a stronger affinity for microtubules than 3R-tau isoforms [48] and are more prone to promote microtubule assembly [49]. Besides, tau can nucleate and bundle microtubules in vitro [50] as well as in axons of mammalian neurons [51]. Recent studies have shown that tau not only acts as a microtubule stabilizer but also positively regulates the elongation of labile domains of microtubules at the plus ends [43]. Additionally, tau is involved in mediating intracellular transport along the axon [52], synaptic structure and function, and signaling pathways in neurons [53].

In AD and other tauopathies, pathological tau would detach from microtubules, leading to decreased microtubule stability, impaired axonal transport and synaptic function [54]. First, tau is more abundant on the labile domains of microtubules to protect them from severing proteins like katanin. Under a pathological condition, hyperphosphorylated tau disassociates with the microtubules and causes rapid and selective shrinkage of microtubule labile domains [55]. Disease-associated tau mutants, like K369I, G389R found in FTD, also showed decreased associations rate with microtubules, resulting in reduced ability in promoting microtubule assembly [56]. Second, the ability of tau to regulate axonal transport alters under pathological conditions. Amino acids (aa) 2–18 of tau protein, termed the phosphatase-activating domain (PAD), activate a signaling cascade involving protein phosphatase 1 (PP1) and glycogen synthase kinase 3β (GSK-3β) that results in disruption of kinesin-1-mediated anterograde fast axonal transport. Y18 (tyrosine 18) phosphorylation of tau, which is stronger in monomers than in filaments, shows reduced inhibition of kinesin-1 and is significantly reduced in disease-associated tau species [57]. Moreover, intracellular tau aggregates have been shown to impair fast axonal transport by increasing the run length, run time and instantaneous velocity of membranous organelles [58]. Third, pathological tau accumulates on both pre- and post-synapses in the AD brain. Presynaptic accumulation of tau induces the depletion of the synaptic vesicle pool, followed by impaired synaptic transmission and plasticity [59]. On the other hand, toxic Aβ oligomers trigger N-methyl-D-aspartate receptor (NMDAR)-mediated excitotoxicity depending on the presence of endogenous tau [16].

Besides, glial tau pathology is also a common feature of many tauopathies and contributes to pathogenesis [34]. Oligodendrocytic tau pathology disrupts the maintenance of myelin sheath [60]. Although found at trace levels in astrocytes [61], tau has been shown to accumulate in astrocytic end feet directly apposed to vascular endothelial cells, and therefore impair the blood–brain barrier (BBB) integrity [62] and cause neuron degeneration without neuronal tau inclusions [63]. Filamentous recombinant tau also activates astrocytes via integrin signaling [64]. The expression and function of tau in microglia remain unclear [34]. However, microglia may regulate the uptake and exosomal secretion of tau, therefore involved in the spreading of tau pathology across the brain [65].
