Tau protein

Tau is a microtubule-associated phosphoprotein (MAP) that is abundantly expressed in both the central and peripheral nervous system. By promoting microtubule (MT) assembly and stability, tau plays a fundamental role in maintaining neuronal integrity and axoplasmic transport (Refs Reference Cleveland, Hwo and Kirschner23, Reference Weingarten24). The tau gene (MAPT) is located on chromosome 17q21 and has two major haplotypes, H1 and H2, which are defined by a set of single nucleotide polymorphisms and a 238 base pair deletion in intron 9 (Ref. Reference Baker25). The H2 haplotype is also associated with an inversion of a ~900 kb region that includes MAPT (Ref. Reference Stefansson26). MAPT contains 16 exons, 11 of which encode the major tau protein isoforms (Ref. Reference Andreadis, Brown and Kosik27). Through alternative mRNA splicing of exons 2, 3 and 10, a set of six isoforms, ranging from 352 to 441 amino acids, are generated in the adult human brain. Exons 9 to 12 encode four microtubule-binding motifs, which are imperfect repeats of 31 or 32 amino acids, in the C-terminal half of the tau molecule. It is these binding domains that mediate the interaction between tau and MTs. Alternative splicing of exon 10 generates tau isoforms with either three (exon 10 missing) or four (exon 10 present) repeat domains, known as 3R and 4R tau, respectively (Fig. 1a) (Refs Reference Goedert and Spillantini28, Reference Goedert29). In Alzheimer disease (AD), abnormally hyperphosphorylated tau is the major component of the neurofibrillary lesions (known as neurofibrillary tangles, neuropil threads and dystrophic neurites), whereas β-amyloid peptide is the major component of senile plaques. Abnormal intracellular accumulation of tau is also characteristic of a number of other neurodegenerative disorders, collectively known as ‘tauopathies’ (Refs Reference Tolnay and Probst30, Reference Lee, Goedert and Trojanowski31). Comparison of the tau aggregates in these disorders reveals differences in both phosphorylation and the content of different tau isoforms. Thus, tauopathies might be subdivided into disorders with inclusions made predominantly of 3R or 4R tau or an admixture of both (Ref. Reference Delacourte32). A number of these tauopathies may be associated with clinical FTD (FTLD-tau), including Pick disease (PiD), progressive supranuclear palsy (PSP), corticobasal degeneration (CBD), argyrophilic grain disease (AGD) and hereditary frontotemporal dementia and parkinsonism linked to chromosome 17, as a result of MAPT mutations (FTDP-17T) (Ref. Reference Cairns21).

Figure 1.

Tau isoforms and tau gene mutations. (a) Six isoforms of the tau protein found in the adult human brain result from alternative splicing of exons 2 (red), 3 (green) and 10 (yellow). Exons 9–12 encode imperfectly repeated microtubule-binding repeats (shown in black). (b) Mutations in the tau gene in frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17T). Thirty-six coding region mutations in exons (E) 1 and 9–13 and eight intronic mutations flanking exon 10 are shown.

FTLD-tau (PiD)

PiD is the clinicopathological prototype of FTD (Refs Reference Onari and Spatz33, Reference Alzheimer34, Reference Pick35). The most distinctive gross pathological feature is marked atrophy of the frontal and anterior temporal lobes, also known as ‘knife-edge’ lobar atrophy (Fig. 2a). The main histological features consist of severe neuronal loss, ballooned neurons (Pick cells) and pathognomonic spherical argyrophilic neuronal inclusions, called Pick bodies (PBs) (Fig. 2b). PBs are found mainly in neocortical neurons of layers II–IV, the granule cells of the dentate gyrus, pyramidal cells of the hippocampus, subiculum and entorhinal cortex, and subcortical nerve cell populations. PBs are composed predominantly of 3R tau isoforms (Ref. Reference Delacourte32), whereas 4R tau deposits might be present in glia in a subset of cases (Refs Reference Zhukareva36, Reference Mott37).

FTLD-tau (PSP)

Also known as Steele-Richardson-Olszewski syndrome (Ref. Reference Steele, Richardson and Olszewski38), PSP is the second most-common parkinsonian disorder, after classical Parkinson disease. Clinical features typically include early postural instability, axial dystonia, supranuclear vertical gaze palsy, pseudobulbar palsy and parkinsonism that is resistant to levodopa therapy. Although cognitive dysfunction is often mild, some cases of PSP present with dementia and only mild motor features (Ref. Reference Sha39). The pathology of PSP is characterised by gross atrophy of the midbrain (especially the pretectal region), superior cerebellar peduncles, subthalamic nucleus and substantia nigra. Fibrillar tau pathology occurs in both neurons and glia. Glial inclusions include tufted astrocytes (Fig. 2c), which are fairly specific for PSP, as well as thorn-shaped astrocytes, coiled bodies in oligodendrocytes and neuropil threads, all of which are also found in other tauopathies. Neurofibrillary tangles typically have a round or globose appearance (Fig. 2d). Many subcortical regions are affected, including the striatum, pallidum, subthalamic nucleus, substantia nigra, oculomotor complex, periaqueductal grey, superior colliculi, basis pontis and dentate nucleus. Cases of PSP with dementia are also found to have significant tau pathology in the cerebral cortex (Ref. Reference Bigio, Brown and White40). Biochemically, the inclusions in PSP consist predominantly of 4R tau isoforms (Ref. Reference Dickson, Rademakers and Hutton41). Inheritance of the H1 tau gene haplotype predisposes to the sporadic 4R tauopathies, PSP and CBD (Refs Reference Pittman42, Reference Di Maria43).

FTLD-tau (CBD)

CBD is an increasingly recognised neurodegenerative disease with both motor and cognitive dysfunction (Ref. Reference Rebeiz, Kolodny and Richardson44). The initial motor symptoms are akinesia, rigidity and apraxia. There is often parkinsonism, which is resistant to dopaminergic drugs. Recent studies have found that FTD is also a common clinical presentation of CBD (Refs Reference Sha39, Reference Grimes, Lang and Bergeron45). Gross examination typically shows depigmentation of the substantia nigra and frontoparietal atrophy that is most severe in the pre- and post-central areas and often asymmetrical. Swollen, achromatic neurons were initially thought to be the most characteristic histological change (Ref. Reference Rebeiz, Kolodny and Richardson44); however, it is now recognised that the affected cortex and subcortical regions have abundant glial and neuronal intracytoplasmic tau pathology. In addition to various neuronal inclusions, abundant thread pathology, coiled bodies and thorn-shaped astrocytes, a particularly characteristic feature of CBD is the presence of circular rings of short tau-positive cell processes, referred to as ‘astrocytic plaques’ (Fig. 2e) (Ref. Reference Feany and Dickson46). The biochemical profile of the pathological tau in CBD is similar to that in PSP, and consists predominantly of 4R tau isoforms (Refs Reference Tolnay and Probst30, Reference Lee, Goedert and Trojanowski31).

FTLD-tau (AGD)

AGD is a sporadic late-onset dementia that accounts for approximately 5% of all cases of dementia (Refs Reference Tolnay and Probst47, Reference Braak and Braak48). The clinical features are still poorly understood, and it can be difficult to distinguish from AD (Refs Reference Steuerwald49, Reference Togo50). However, a subset of AGD patients present with clinical features of FTD (Refs Reference Ishihara51, Reference Tsuchiya52). The hallmark lesions are abundant argyrophilic grains (AGs) in neuronal processes (Fig. 2f) and coiled bodies in oligodendrocytes. AGs are abundant in various limbic structures, including the CA1 sector of the hippocampus, layers II and III of the entorhinal and transentorhinal cortices, the amygdala and the hypothalamic lateral tuberal nuclei (Ref. Reference Tolnay and Probst47). Recent biochemical studies have revealed that AGD is also a 4R tauopathy (Refs Reference Zhukareva53, Reference Togo54, Reference Tolnay55). At present, there is controversy over whether the H1 tau haplotype is also a risk factor for AGD (Ref. Reference Tolnay and Probst47).

Other sporadic tauopathies

Some additional rare forms or variants of tauopathies may be associated with clinical FTD and/or demonstrate a frontotemporal lobar distribution of tau pathology. These include the frontal variant of AD (fvAD) (Fig. 2g) (Refs Reference Johnson56, Reference Taylor57), neurofibrillary tangle dementia (reviewed in Ref. Reference Jellinger and Attems58), sporadic multiple system tauopathy with dementia (MSTD) (Ref. Reference Bigio59), and white matter tauopathy with globular inclusions (Ref. Reference Kovacs60). Although insoluble tau in fvAD and neurofibrillary tangle dementia contains both 3R and 4R tau isoforms, sporadic MSTD and white matter tauopathy with globular glial inclusions are 4R tauopathies. Hippocampal sclerosis (HS) is a descriptive term for near-complete, selective loss of pyramidal neurons in the CA1 sector of the hippocampus and subiculum. HS can be the primary pathology in some cases of late-onset dementia (Ref. Reference Dickson61), but it more frequently occurs in association with tauopathies (Fig. 2h) (Ref. Reference Beach62) or FTLD-TDP (see below) (reviewed in Ref. Reference Probst, Taylor and Tolnay63).

Figure 2.

Pathological features in FTLD-tau. (a) Severe atrophy of the frontal and temporal lobes and (b) Pick bodies in the temporal cortex of a patient with Pick disease. (c) Tufted astrocyte and (d) a globose tangle in a case with progressive supranuclear palsy. (e) Astrocytic plaque as a hallmark lesion of corticobasal degeneration. (f) Abundant argyrophilic grains in the hippocampus of a patient with argyrophilic grain disease. (g) Neurofibrillary tangles and neuropil threads in the frontal cortex of a case with frontal variant Alzheimer disease. Arrow indicates a neuritic plaque. (h) Tau pathology in sector CA2 of the hippocampus in hippocampal sclerosis dementia. (i) Neuronal and glial tau pathology in the frontal cortex of a patient with an intron 10 +3 splice-site mutation in the tau gene. b, d, e, f, h, tau immunohistochemistry; c, g, i, Gallyas-Braak silver stain. Scale bars: 50 µm.

FTDP-17T

In 1994, a familial form of FTD and parkinsonism was linked to chromosome 17q21-22 (Ref. Reference Wilhelmsen64). Over the following years, other families with autosomal dominant FTD were identified with linkage to the same region and, based on similarities in the clinical, neuropathological, and genetic findings, these were grouped under the umbrella of FTD and parkinsonism linked to chromosome 17 (FTDP-17) (Ref. Reference Foster65). About half of the original FTDP-17 families were found to have MAPT mutations (FTDP-17T) (Refs Reference Hutton11, Reference Poorkaj12, Reference Spillantini13), whereas the genetic cause in the others remained unknown until recently (see section on FTLD-TDP). At present, 44 MAPT mutations have been described in 131 families (Fig. 1b) (AD&FTD Mutation database: http://www.molgen.ua.ac.be/FTDMutations). Clinically, FTDP-17T is typically characterised by personality changes, motor symptoms and cognitive decline. The earliest symptoms are usually disinhibition, loss of initiative, obsessive-compulsive behaviour, and/or psychosis, followed by cognitive decline leading to profound dementia (Ref. Reference van Swieten and Spillantini66). However, some MAPT mutations result in a clinical phenotype that more closely resembles PiD, CBD, PSP, AGD or AD. In all cases of FTDP-17T, the neuropathology includes various types of fibrillar tau pathology. Some mutations are associated with inclusions in both neurons and glia (Fig. 2i), whereas others lead to a predominance of neuronal tau pathology. Depending on the mutation site, tau aggregates can be composed predominantly of 3R or 4R tau isoforms, or a mixture of both. MAPT mutations are located in the coding region or in the intron flanking the alternatively spliced exon 10 (Fig. 1b) and fall into two main functional categories: those that affect the alternative splicing of tau pre-mRNA, and those whose primary effect is at the protein level. The mechanisms by which the various MAPT mutations impair tau functions and generate filamentous tau inclusions are beyond the scope of this review, but they have recently been reviewed elsewhere (Refs Reference Goedert and Spillantini28, Reference van Swieten and Spillantini66, Reference Goedert67).

FTLD-tau due to other gene abnormalities

In addition to FTDP-17T, there remain many familial FTLD-tau cases for which no MAPT mutation has been discovered. Recently, an interesting association has been described between familial FTLD-tau and presenilin 1 (PS-1). Mutations in the genes encoding PS-1 and PS-2 (PSEN1 and PSEN2) account for the majority of early-onset familial AD (Ref. Reference Roberson and Mucke68). However, PSEN1 mutations have also been identified in patients with an FTD presentation (Ref. Reference van der Zee, Sleegers and Van Broeckhoven69). In one patient with familial FTD caused by the G183V PSEN1 mutation, the neuropathology was found to be consistent with PiD in the absence of β-amyloid deposits (Ref. Reference Dermaut70), whereas another FTD family with the M146L mutation had both PBs and AD pathology (Ref. Reference Halliday71). However, some FTD patients with PSEN1 mutations have turned out to have typical AD pathology (Refs Reference Zekanowski72, Reference Rippon73). Finally, a family with FTD that was originally attributed to the PS-1 insR352 change was recently shown to have a GRN mutation and characteristic FTLD-TDP pathology (Refs Reference Boeve74, Reference Pickering-Brown75). Currently, the mechanisms by which PSEN1 mutations contribute to neurodegeneration in a subset of FTD subjects are still elusive (Refs Reference Kumar-Singh and Van Broeckhoven76, Reference Hutton77).

Normal function of TDP-43

TDP-43 is a protein comprising 414 amino acids, which is encoded by the TARDBP gene on chromosome 1 (Fig. 3a). It was first cloned as a human protein capable of binding to the transactive response DNA of human immunodeficiency virus Type 1 (Ref. Reference Ou90), and later identified as part of a complex involved in splicing of the cystic fibrosis transmembrane conductance regulator gene (Ref. Reference Buratti91) and the apolipoprotein-A-II gene (Ref. Reference Mercado92). TDP-43 is highly conserved, ubiquitously expressed and predominantly localised to the nucleus. It consists of two RNA-recognition motifs and a glycine-rich C-terminal region (Fig. 3a). Recent studies have shown that TDP-43 continuously shuttles between the nucleus and cytoplasm – a process partially regulated by nuclear localisation signal (NLS) and nuclear export signal (NES) motifs (Refs Reference Winton93, Reference Ayala94). In addition to its well-characterised role in regulation of transcription and splicing, more recent studies suggest that TDP-43 is involved in other cellular processes such as micro RNA biogenesis, apoptosis, cell division, mRNA stabilisation and regulation of neuronal plasticity by acting as neuronal activity response factor (Refs Reference Strong95, Reference Wang96). The exon skipping and splicing inhibitory activity requires the C-terminal region of TDP-43, which interacts with other members of the heterogeneous nuclear ribonucleoprotein (hnRNP) family (Ref. Reference Buratti97). Finally, TDP-43 can also act as scaffold for nuclear bodies through interaction with survival motor neuron protein (Ref. Reference Wang, Reddy and Shen98).

TDP-43 is the major pathological protein in FTLD-TDP

As demonstrated in the initial reports and rapidly confirmed by numerous subsequent studies, antibodies against TDP-43 have proven to be the most sensitive and specific tool to detect all of the different types of the ub-ir pathology found in most cases of FTLD-U, including the NCIs, DNs and neuronal intranuclear inclusions (NIIs) (Fig. 3b, d) (Refs Reference Neumann88, Reference Arai89, Reference Davidson99, Reference Cairns100). Moreover, the pathological forms of TDP-43 demonstrate evidence of abnormal processing, being hyperphosphorylated, ubiquitinated and N-terminally truncated (Ref. Reference Neumann88) (Fig. 3c). Of potential functional importance, the formation of TDP-43-positive inclusion bodies (either NCIs or NIIs) is consistently associated with a dramatic reduction of the normally diffuse nuclear staining (Ref. Reference Neumann88) (Fig. 3b), raising the possibility that essential nuclear functions of TDP-43 might be lost in FTLD-TDP.

Figure 3.

TDP-43 and pathological spectrum of FTLD-TDP. (a) Schematic diagram of TDP-43 with its characteristic functional domains. (b) TDP-43-positive cytoplasmic inclusions in the dentate granule cells. Note the absence of normal nuclear staining in cells with cytoplasmic inclusions (arrows). (c) Immunoblot analysis of urea fractions isolated from FTLD-TDP brain shows the highly characteristic biochemical signature with pathological bands at 25 kDa (*), 45 kDa (**) and a high molecular mass smear (***), in addition to the normal TDP-43 band. (d) Based on morphological parameters, four FTLD-TDP subtypes can be delineated, each of which correlates with specific clinical phenotype and genetic abnormalities. Scale bars: 20 µm (b, d). Abbreviations: RRM, RNA recognition motif; NLS, nuclear localisation sequence; NES, nuclear export sequence.

The use of TDP-43 immunohistochemistry has clarified the spectrum of pathology in this FTLD subtype, by demonstrating previously unrecognised ubiquitin-negative pathology, including glial cytoplasmic inclusions (GCIs) in cells of presumed oligodendroglial lineage (Ref. Reference Neumann101), more diffuse neuronal cytoplasmic ‘preinclusions’ (Refs Reference Brandmeir102, Reference Geser103) and delicate neurites in the CA1 region in a subset of FTLD-TDP (Ref. Reference Hatanpaa104). Antibodies raised against phosphorylated epitopes of TDP-43 (serine residues 379, 403, 404, 409 and 410) have further facilitated the neuropathological assessment, because they label only abnormal TDP-43 in inclusions, not the physiological nuclear TDP-43 (Refs Reference Hasegawa105, Reference Inukai106, Reference Neumann107).

Pathological subtypes of FTLD-TDP

FTLD-TDP pathology is heterogeneous with respect to the morphology and laminar distribution of pathological inclusions, leading to the description of four distinct pathological subtypes (Fig. 3d) (Refs Reference Mackenzie86, Reference Sampathu87, Reference Cairns100). Using the classification scheme originally described by Sampathu and colleagues, FTLD-TDP Type 1 is characterised by an abundance of long neuritic profiles, predominantly in superficial cortical laminae, with few or no NCIs, NIIs or GCIs. Type 2 shows a predominance of NCIs in both superficial and deep cortical layers and frequent ‘preinclusions’ and GCIs. Neurites may be present but NIIs are rare or absent. NCIs, which are typical of MND, are often present in lower motor neurons. Type 3 cases have abundant small neuritic profiles and NCIs, predominantly in the superficial cortical layers. Moderate numbers of lentiform NIIs can be found in affected cortical regions, especially in cases with a positive family history. The characteristic neuropathological feature of FTLD-TDP Type 4 pathology is an abundance of NIIs and DNs with only rare NCIs in cortical regions, and the absence of inclusions in the hippocampal dentate granule cells (Refs Reference Neumann108, Reference Forman109). The relevance of this heterogeneity is supported by relatively specific correlations between the various patterns of pathology, the clinical phenotypes and the known genetic causes (Table 1, Fig. 3d, see below) (Refs Reference Mackenzie86, Reference Davidson99).

Familial FTLD-TDP

Molecular mechanisms underlying FTLD-TDP

The mechanistic aspects of abnormal TDP-43 processing and accumulation, the functional consequences of these abnormalities (loss-of-function versus toxic gain-of-function) as well as the links with GRN and VCP mutations are currently not well understood. The dramatic change in subcellular distribution of TDP-43 from the nucleus to the cytoplasm in inclusion-bearing cells suggests that loss of pivotal nuclear TDP-43 functions in transcription and mRNA processing might have a pathogenic role. Consistent with this hypothesis, knockdown of TDP-43 in cell lines has been shown to have important consequences on essential metabolic processes, such as regulation of nuclear shape, cell cycle and apoptosis (Ref. Reference Ayala, Misteli and Baralle121). Moreover, TDP-43-knockout mice are embryonic lethal (M. Neumann, unpublished). Alternatively, the generation and sequestration of abnormal TDP-43 species might have a toxic effect. Cortical inclusions in FTLD-TDP are selectively enriched for hyperphosphorylated C-terminal fragments (CTFs), compared with spinal cord inclusions, which contain more full-length TDP-43 (Refs Reference Neumann107, Reference Igaz122). This suggests that TDP-43 is differentially processed in different neuroanatomical regions and that TDP-43 CTFs might act as seeds for inclusion formation and aggregation, particularly in cortical neurons. Recently, one specific cleavage site (Arg208) was identified in the pathological TDP-43 CTFs purified from FTLD-TDP brains (Ref. Reference Igaz123). Overexpression of this CTF in cultured cells recapitulated some key features of FTLD-TDP, such as the formation of ubiquitinated and phosphorylated cytoplasmic aggregates; however, no clear effect on cell viability was observed (Ref. Reference Igaz123). An interesting link between TDP-43 and PGRN was recently reported by Zhang and co-workers who demonstrated caspase-dependent cleavage of TDP-43 upon PGRN knockdown in cultured cells (Ref. Reference Zhang124). Although it is widely accepted that TDP-43 can be cleaved by caspases, the influence of PGRN levels on this process was not replicated by another group (Ref. Reference Shankaran125). Finally, an in vitro study showed that restricting TDP-43 from entering the nucleus by changing the NLS motif leads to the formation of cytoplasmic TDP-43 aggregates, with subsequent sequestration of endogenous TDP-43, resulting in a depletion of nuclear TDP-43 (Ref. Reference Winton93). Thus, perturbation of the normal shuttling of TDP-43 between the nucleus and cytoplasm might predispose to both the formation of cytoplasmic inclusions and a loss of nuclear TDP-43.

Although initial studies failed to identify mutations in the TARDBP gene in FTLD-TDP (Refs Reference Gijselinck126, Reference Schumacher127, Reference Rollinson128), mutations were identified in familial and sporadic ALS, in early 2008 (Refs Reference Kabashi129, Reference Sreedharan130). At least 30 different TARDBP mutations have now been identified, mostly in familial and sporadic ALS (Refs Reference Rutherford131, Reference Gitcho132, Reference Van Deerlin133, Reference Kuhnlein134, Reference Corrado135, Reference Daoud136, Reference Yokoseki137, Reference Pamphlett138, Reference Lemmens139, Reference Del Bo140). However, very recently, a TARDBP mutation (G295S) was reported in two unrelated patients with FTD and MND (Ref. Reference Benajiba141). Except for the Y374X truncation mutation, all others are missense mutations, mostly in exon 6, which effect highly conserved amino residues in the C-terminal region of TDP-43. Elucidating the functional consequences of ALS-causing TARDBP mutations is likely to also provide important insights into the mechanism of neurodegeneration in FTLD-TDP. One current hypothesis is that TARDBP mutations might interfere with protein–protein interaction, thereby affecting intracellular transport or crucial transcriptional and splicing activities. Alternatively, the mutations might increase the generation of CTFs, promote the aggregation of TDP-43 and/or result in abnormal phosphorylation (Refs Reference Kabashi129, Reference Rutherford131, Reference Corrado135).