1. Introduction
Proteins are essential macromolecules that play important roles in almost any cellular process. They usually do not function alone but rather as complexes with other molecules, mainly with proteins. Protein-protein interactions (PPIs) are elementary for many processes and it is proposed that their dysfunction or deregulation is located upstream, leading to various pathological conditions
[1]. In Alzheimer’s disease and other tauopathies, tau protein undergoes pathological modifications that lead to the formation of paired helical filaments (PHF) and neurofibrillary tangles (NT) which belong to the main hallmarks of these diseases. The conversion of physiological tau into its pathological forms and their participation in disease etiology have not been fully understood yet and are still under investigation. In physiological conditions, tau interacts with many protein partners which maintain their proper structure and function. Under pathological conditions, proteins interacting with tau can also participate in its non-physiological modifications leading to the development of neurodegenerative diseases. Interaction partners of tau protein and involved molecular pathways can either initiate and drive the tau pathology or can have neuroprotective roles, by reducing pathological tau changes or inflammation.
Tau protein belongs to the family of microtubule-associated proteins (MAPs)
[2][3] and can influence axonal transport and growth
[4], neuronal polarization
[5], and thus the normal function of neurons and the brain (
Figure 1)
[6][7].
Figure 1. Schematic representation of physiological tau protein functions in neurons. The axonal tau (A) stabilizes microtubules (MTs), and it can also bind actin filaments thus facilitating cytoskeleton networking. Furthermore, tau regulates MT dynamics by interacting with end-binding (EB) proteins. The EB proteins promote and regulate MT nucleation and elongation. Tau inhibits the EB protein binding to MTs and this inhibition is reversed by tau phosphorylation. Tau also competitively inhibits the interaction of dynein and kinesin to MTs and thus influences the intraneuronal transport and cargo release. In the synapses (B), tau protein can be directly translated, and during neuronal activity, it is released into the synaptic cleft. Through linking the MTs and actin filaments, tau can influence synaptic plasticity. Moreover, tau is a known interacting partner of the BAR domain-containing proteins such as BAIAP2, PACSIN1, and BIN1, that ensure the curvature and shaping of the neuronal membrane. Tau may play a role in the removal of developmental NMDARs and their replacement for mature NMDARs in dendrites (dashed arrows) as it is a known substrate of the protein kinase C (PKC), which is activated by the G protein-coupled receptors (GPCR). The PACSIN1 recruits clathrin and dynamin endocytic machinery to the developmental NMDARs and thus mediates their removal. The developmental/mature NMDARs exchange is important for the formation of new synaptic connections. ER—endoplasmic reticulum. In the nucleus (C), tau interacts with DNA and RNA, maintains their integrity, and protects them from oxidative damage. Furthermore, tau may be involved in rRNA-coding DNA transcription and rRNA processing.
In humans, tau protein is expressed mainly in neurons
[8], and in lower amounts in oligodendrocytes and astrocytes
[9][10][11][12]. Besides the central nervous system (CNS), tau is expressed by peripheral neurons
[13], and more recently, tau immunoreactivity was also found in the human submandibular gland and sigmoid colon tissues
[14]. Human tau protein is encoded by
MAPT gene localized on chromosome 17q21 and consists of 16 exons. In the adult human brain, six tau isoforms are expressed ranging from 352 to 441 amino acids (see
Figure 2). Three decades ago, it was proposed that individual tau isoforms may have different functions since they are differently expressed in the fetal and developed brain
[15][16][17]. Now, it is known that alternative splicing even varies across neuronal cell types and during neuronal maturation
[18][19].
Figure 2. Schematic representation of six human tau protein isoforms and their domains. The N-terminal part is more acidic due to N1 and N2 inserts. The microtubule-binding domain (MTBD) is composed of repeats R1–R4. The number of repeats and N-terminal inserts varies according to the type of tau protein isoform.
Tau protein is distinctly divided into the N-terminal part, proline-rich region (PRR), microtubule-binding domain (MTBD), and C-terminus. The N-terminal domain length is dependent on alternative splicing of exons 2 and 3 which encode acidic amino acids. In MTBD, the splicing-dependent manner of exon 10 results in either 3 (3R) or 4 (4R) microtubule-binding repeats which are essential for binding of tau to individual tubulin heterodimers, and through this interaction tau stabilizes microtubules (MT)
[20].
Despite the MT-stabilizing function of tau, its removal or modification had no significant impact on microtubule stability, cellular function, or cognition in mouse models
[21][22][23][24][25]. It was shown that the MT-stabilizing function of tau protein is replaceable by microtubule-associated proteins MAP1a
[22][26] or MAP1b
[5]. However, experiments performed by several other groups on tau-knockout mice revealed that tau can influence the regulation of neuronal activity
[27], synaptic plasticity
[28], neurogenesis
[29], iron export from neurons
[30], and long-term depression of synapses
[31][32]. Along with other animal disease models where the tau levels were reduced, it was found that depletion of tau protein had a protective effect on neurons against amyloid-beta (Aβ) induced excitotoxicity or by other excitotoxins in mice over-expressing amyloid precursor protein (APP) and presenilin 1
[33][34][35][36][37]. In chemically induced seizure models, hyperexcitability of neurons was reduced in mice with attenuated tau expression
[38]. This suggests that endogenous tau is integral for regulating, or rather, upregulating neuronal hyperexcitability in diseased animals. In addition, in mice with impaired function of voltage-dependent sodium and potassium channels, the depletion of tau protein had a protective effect on neurons
[39]. These studies suggest that tau protein may play a role in the regulation of neuronal network activity in both pathological and physiological conditions. Moreover, this is also supported by the evidence that tau protein has several additional roles in neurons and the brain
[6].
In pathological conditions, the accumulation of insoluble tau aggregates occurs inside neurons, in extracellular space
[40][41], and other brain cells such as astrocytes and oligodendrocytes
[42][43]. The formation of this stable material is the consequence of abnormally modified and truncated tau proteins which self-aggregate and gradually mature to paired helical filaments (PHF) and neurofibrillary tangles (NFT) which are common hallmarks of several neurodegenerative diseases
[44]. The formation of PHFs and NFTs is associated with the engulfment of the cytosol of neurons, failure in intracellular trafficking, and gradual disruption of basic physiological processes that end in apoptosis and neuronal death
[45][46][47]. The formation of these aggregates is accompanied by inflammation which on one hand could help in their clearance, but on the other, can exacerbate the pathological processes
[48][49][50]. Tau pathology is the main cause of dementia in Alzheimer’s disease and other neurodegenerative diseases, including frontotemporal dementia
[51], argyrophilic grain disease
[52], corticobasal degeneration
[53][54], progressive supranuclear palsy
[55], and several other diseases
[44]. These disorders, where the accumulation of abnormal tau protein in the brain occurs, are referred to as tauopathies.