Progress in hereditary tauopathies: a mutation in the Tau gene (G389R) causes a Pick disease-like syndrome

Biochemical approaches to assess the impact of post-translational modifications on pathogenic tau conformations using recombinant protein

Tau protein is associated with many neurodegenerative disorders known as tauopathies. Aggregates of tau are thought of as a main contributor to neurodegeneration in these diseases. Increasingly, evidence points to earlier, soluble conformations of abnormally modified monomers and multimeric tau as toxic forms of tau. The biological processes driving tau from physiological species to pathogenic conformations remain poorly understood, but certain avenues are currently under investigation including the functional consequences of various pathological tau changes (e.g. mutations, post-translational modifications (PTMs), and protein-protein interactions). PTMs can regulate several aspects of tau biology such as proteasomal and autophagic clearance, solubility, and aggregation. Moreover, PTMs can contribute to the transition of tau from normal to pathogenic conformations. However, our understating of how PTMs specifically regulate the transition of tau into pathogenic conformations is partly impeded by the relative lack of structured frameworks to assess and quantify these conformations. In this review, we describe a set of approaches that includes several in vitro assays to determine the contribution of PTMs to tau's transition into known pathogenic conformations. The approaches begin with different methods to create recombinant tau proteins carrying specific PTMs followed by validation of the PTMs status. Then, we describe a set of biochemical and biophysical assays that assess the contribution of a given PTM to different tau conformations, including aggregation, oligomerization, exposure of the phosphatase-activating domain, and seeding. Together, these approaches can facilitate the advancement of our understanding of the relationships between PTMs and tau conformations.

Isoform-specific patterns of tau burden and neuronal degeneration in MAPT-associated frontotemporal lobar degeneration

Frontotemporal lobar degeneration with MAPT pathogenic variants (FTLD-MAPT) has heterogeneous tau pathological inclusions postmortem, consisting of three-repeat (3R) or four-repeat (4R) tau isoforms, or a combination (3R + 4R). Here, we studied grey matter tau burden, its relation to neuronal degeneration, and regional patterns of pathology in different isoform groups of FTLD-MAPT. We included 38 FTLD-MAPT autopsy cases with 10 different MAPT pathogenic variants, grouped based on predominant tau isoform(s). In up to eleven regions (ten cortical and one striatal), we quantified grey matter tau burden using digital histopathological analysis and assigned semi-quantitative ratings for neuronal degeneration (i.e. 0-4) and separate burden of glial and neuronal tau inclusions (i.e. 0-3). We used mixed modelling to compare pathology measures (1) across the entire cohort and (2) within isoform groups. In the total cohort, tau burden and neuronal degeneration were positively associated and most severe in the anterior temporal, anterior cingulate and transentorhinal cortices. Isoform groups showed distinctive features of tau burden and neuronal degeneration. Across all regions, the 3R isoform group had lower tau burden compared to the 4R group (p = 0.008), while at the same time showing more severe neuronal degeneration than the 4R group (p = 0.002). The 3R + 4R group had an intermediate profile with relatively high tau burden along with relatively severe neuronal degeneration. Neuronal tau inclusions were most frequent in the 4R group (p < 0.001 vs. 3R), while cortical glial tau inclusions were most frequent in the 3R + 4R and 4R groups (p ≤ 0.009 vs. 3R). Regionally, neuronal degeneration was consistently most severe in the anterior temporal cortex within each isoform group. In contrast, the regions with the highest tau burden differed in isoform groups (3R: striatum; 3R + 4R: striatum, inferior parietal lobule, middle frontal cortex, anterior cingulate cortex; 4R: transentorhinal cortex, anterior temporal cortex, fusiform gyrus). We conclude that FTLD-MAPT isoform groups show distinctive features of overall neuronal degeneration and regional tau burden, but all share pronounced anterior temporal neuronal degeneration. These data suggest that distinct isoform-related mechanisms of genetic tauopathies, with slightly divergent tau distribution, may share similar regional vulnerability to neurodegeneration within the frontotemporal paralimbic networks.

Two pathologically confirmed cases of novel mutations in the MAPT gene causing frontotemporal dementia

MAPT mutations were the first discovered genetic cause of frontotemporal dementia (FTD) in 1998. Since that time, over 60 MAPT mutations have been identified, usually causing behavioral variant FTD and/or parkinsonism clinically. We describe 2 novel MAPT mutations, D252V and G389_I392del, each presenting in a patient with behavioral variant FTD and associated language and cognitive deficits. Neuroimaging revealed asymmetrical left greater than right temporal lobe atrophy in the first case, and bifrontal atrophy in the second case. Disease duration was 8 years and 5 years, respectively. Postmortem examination in both patients revealed a 3-repeat predominant tauopathy, similar in appearance to Pick's disease. These 2 mutations add to the literature on genetic FTD, both presenting with similar clinical and imaging features to previously described cases, and pathologically showing a primary tauopathy similar to a number of other MAPT mutations.

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Over 60 MAPT mutations are currently described—we describe 2 novel mutations: p.D252V and p.G389_I392del.

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Both novel mutations are associated with a frontotemporal dementia clinical syndrome similar to previously described MAPT mutations.

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Both the novel mutations are associated with a primary 3-repeat tauopathy at postmortem.

MAPT mutations, tauopathy, and mechanisms of neurodegeneration

In multiple neurodegenerative diseases, including Alzheimer's disease (AD), a prominent pathological feature is the aberrant aggregation and inclusion formation of the microtubule-associated protein tau. Because of the pathological association, these disorders are often referred to as tauopathies. Mutations in the MAPT gene that encodes tau can cause frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17), providing the clearest evidence that tauopathy plays a causal role in neurodegeneration. However, large gaps in our knowledge remain regarding how various FTDP-17-linked tau mutations promote tau aggregation and neurodegeneration, and, more generally, how the tauopathy is linked to neurodegeneration. Herein, we review what is known about how FTDP-17-linked pathogenic MAPT mutations cause disease, with a major focus on the prion-like properties of wild-type and mutant tau proteins. The hypothesized mechanisms by which mutations in the MAPT gene promote tauopathy are quite varied and may not provide definitive insights into how tauopathy arises in the absence of mutation. Further, differences in the ability of tau and mutant tau proteins to support prion-like propagation in various model systems raise questions about the generalizability of this mechanism in various tauopathies. Notably, understanding the mechanisms of tauopathy induction and spread and tau-induced neurodegeneration has important implications for tau-targeting therapeutics.

Retiring the term FTDP-17 as MAPT mutations are genetic forms of sporadic frontotemporal tauopathies

See Josephs (doi:10.1093/brain/awx367) for a scientific commentary on this article.In many neurodegenerative disorders, familial forms have provided important insights into the pathogenesis of their corresponding sporadic forms. The first mutations associated with frontotemporal lobar degeneration (FTLD) were found in the microtubule-associated protein tau (MAPT) gene on chromosome 17 in families with frontotemporal degeneration and parkinsonism (FTDP-17). However, it was soon discovered that 50% of these families had a nearby mutation in progranulin. Regardless, the original FTDP-17 nomenclature has been retained for patients with MAPT mutations, with such patients currently classified independently from the different sporadic forms of FTLD with tau-immunoreactive inclusions (FTLD-tau). The separate classification of familial FTLD with MAPT mutations implies that familial forms cannot inform on the pathogenesis of the different sporadic forms of FTLD-tau. To test this assumption, this study pathologically assessed all FTLD-tau cases with a known MAPT mutation held by the Sydney and Cambridge Brain Banks, and compared them to four cases of four subtypes of sporadic FTLD-tau, in addition to published case reports. Ten FTLD-tau cases with a MAPT mutation (K257T, S305S, P301L, IVS10+16, R406W) were screened for the core differentiating neuropathological features used to diagnose the different sporadic FTLD-tau subtypes to determine whether the categorical separation of MAPT mutations from sporadic FTLD-tau is valid. Compared with sporadic cases, FTLD-tau cases with MAPT mutations had similar mean disease duration but were younger at age of symptom onset (55 ± 4 years versus 70 ± 6 years). Interestingly, FTLD-tau cases with MAPT mutations had similar patterns and severity of neuropathological features to sporadic FTLD-tau subtypes and could be classified into: Pick's disease (K257T), corticobasal degeneration (S305S, IVS10‰+‰16, R406W), progressive supranuclear palsy (S305S) or globular glial tauopathy (P301L, IVS10‰+‰16). The finding that the S305S mutation could be classified into two tauopathies suggests additional modifying factors. Assessment of our cases and previous reports suggests that distinct MAPT mutations result in particular FTLD-tau subtypes, supporting the concept that they are likely to inform on the varied cellular mechanisms involved in distinctive forms of sporadic FTLD-tau. As such, FTLD-tau cases with MAPT mutations should be considered familial forms of FTLD-tau subtypes rather than a separate FTDP-17 category, and continued research on the effects of different mutations more focused on modelling their impact to produce the very different sporadic FTLD-tau pathologies in animal and cellular models.

Heterogeneous Tau-Tubulin Complexes Accelerate Microtubule Polymerization

Tau is an intrinsically disordered protein with a central role in the pathology of a number of neurodegenerative diseases. Tau normally functions to stabilize neuronal microtubules, although the mechanism underlying this function is not well understood. Of note is that the interaction between tau and soluble tubulin, which has implications both in understanding tau function as well as its role in disease, is underexplored. Here we investigate the relationship between heterogeneity in tau-tubulin complexes and tau function. Specifically, we created a series of truncated and scrambled tau constructs and characterized the size and heterogeneity of the tau-tubulin complexes formed under nonpolymerizing conditions. Function of the constructs was verified by tubulin polymerization assays. We find that, surprisingly, the pseudo-repeat region of tau, which flanks the core microtubule-binding domain of tau, contributes largely to the formation of large, heterogeneous tau tubulin complexes; additional independent tubulin binding sites exist in repeats two and three of the microtubule binding domain. Of particular interest is that we find positive correlation between the size and heterogeneity of the complexes and rate of tau-promoted microtubule polymerization. We propose that tau-tubulin can be described as a "fuzzy" complex, and our results demonstrate the importance of heterogeneous complex formation in tau function. This work provides fundamental insights into the functional mechanism of tau, and more broadly underscores the relevance of heterogeneous and dynamic complexes in the functions of intrinsically disordered proteins.

FTDP-17 with Pick body-like inclusions associated with a novel tau mutation, p.E372G

Mutations in microtubule-associated protein tau gene (MAPT) cause frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17). Here, we describe a patient with FTDP-17 and a novel missense mutation in exon 13 of MAPT, p.E372G. We compare clinicopathologic features of this patient to two previously unreported patients with another exon 13 mutation, p.G389R. The patient with the p.E372G mutation was a 40-year-old man with behavioral variant frontotemporal dementia (bvFTD), who subsequently developed agrammatic speech and parkinsonism. One of the FTDP-17 patients with p.G389R mutation presented at age 24 with agrammatic variant of primary progressive aphasia, and subsequently behavioral dysfunction. The other presented at age 53 with bvFTD, followed by agrammatic speech and corticobasal syndrome. Neuropathologic features of FTDP-17 due to p.E372G were similar to those of p.G389R, including tau-immunoreactive Pick body-like neuronal inclusions and swollen, tapering thread-like processes in white matter immunoreactive for 3-repeat and 4-repeat tau. Biochemical analysis of insoluble tau showed similar isoform compositions in p.E372G and p.G389R. Functional studies of the p.E372G mutation showed marked increase in tau filament formation and its reduced ability to promote microtubule assembly. Together these findings indicate that p.E372G is a pathogenic MAPT mutation that causes FTDP-17 similar to p.G389R.

Presence of a carboxy-terminal pseudorepeat and disease-like pseudohyperphosphorylation critically influence tau's interaction with microtubules in axon-like processes

A refined FDAP approach is used to analyze tau’s behavior in axon-like processes. A conserved C-terminal pseudorepeat and disease-like pseudohyperphosphorylation critically influence tau’s microtubule interaction. The results contribute to an understanding of pathological processes that lead to tau’s redistribution during disease.

A current challenge of cell biology is to investigate molecular interactions in subcellular compartments of living cells to overcome the artificial character of in vitro studies. To dissect the interaction of the neuronal microtubule (MT)-associated protein tau with MTs in axon-like processes, we used a refined fluorescence decay after photoactivation approach and single-molecule tracking. We found that isoform variation had only a minor influence on the tau–MT interaction, whereas the presence of a C-terminal pseudorepeat region (PRR) greatly increased MT binding by a greater-than-sixfold reduction of the dissociation rate. Bioinformatic analysis revealed that the PRR contained a highly conserved motif of 18 amino acids. Disease-associated tau mutations in the PRR (K369I, G389R) did not influence apparent MT binding but increased its dynamicity. Simulation of disease-like tau hyperphosphorylation dramatically diminished the tau–MT interaction by a greater-than-fivefold decrease of the association rate with no major change in the dissociation rate. Apparent binding of tau to MTs was similar in axons and dendrites but more sensitive to increased phosphorylation in axons. Our data indicate that under the conditions of high MT density that prevail in the axon, tau’s MT binding and localization are crucially affected by the presence of the PRR and tau hyperphosphorylation.

Mitotic epitopes are incorporated into age-dependent neurofibrillary tangles in Niemann-Pick disease type C

The mechanism underlying neurofibrillary tangles (NFTs) in Alzheimer's disease (AD) and other neurodegenerative disorders remains elusive. Niemann-Pick disease type C (NPC) is a kind of genetic neurovisceral disorder in which the intracellular sequestration of cholesterol and other lipids in neurons, NFT formation and neuronal degeneration in brain are the neuropathology hallmarks. The age of onset and progression of the disease vary dramatically. We have analyzed the hippocampus from 17 NPC cases, aged from 7 months to 55 years, to depict the temporal characteristics of NFT formation. Unexpectedly, classic NFT was observed in about 4-year-old NPC brain, suggesting that NFT is not aging dependent, and that juvenile brain neurons satisfy the requirements for NFT formation. NFT in the hippocampus of NPC was significantly increased in number with the advance of age. More importantly, multiple mitotic phase markers, which are not usually found in normal mature neurons, were abundant in the affected neurons and incorporated into NFT. The unusual activation of cdc2/cyclin B kinase and downstream mitotic indices are closely associated with the age-dependent NFT formation, signifying the contribution of abortive cell cycle to neurodegeneration. The cdc2 inhibitors may be therapeutically used for early intervention of neurodegeneration and NFT formation in NPC.

Progranulin and frontotemporal lobar degeneration

Frontotemporal lobar degeneration is the term used to describe the non-Alzheimer clinical syndromes of frontotemporal dementia, semantic dementia and progressive non-fluent aphasia, regardless of the underlying neuropathological features. Considerable progress has been made in recent years in our understanding of the aetiology of this disorder, notably the identification of mutations in tau and progranulin genes, both on chromosome 17q21. Mutations in tau appear to affect the ability of tau to bind microtubules and/or increase this protein's ability to form fibrils. In contrast, progranulin mutations cause haploinsufficiency leading to TDP-43 accumulation. These genes collectively account for 10-20% of FTLD. However, it is clear that much remains to be discovered before our knowledge of this heterogeneous condition is complete.

The complex aetiology of frontotemporal lobar degeneration

Frontotemporal lobar degeneration (FTLD) is now a widely recognised form of dementia. This heterogeneous disease has been of particular interest to geneticists due to its high rate of heritability with up to 40% of patients reporting a family history of the disease in at least one extra family member. There have been several chromosome loci linked to this disorder and three genes have already been identified. Remarkably, it has been recently demonstrated that 2 of these are only 1.7 Mb from one another on chromosome 17q21, these being tau and progranulin. The identification of these genes has contributed greatly to our understanding of the differing neuropathologies associated with FTLD. Furthermore, the discovery that TDP-43 is a component of the neuronal inclusions seen in the most common neuropathological subtype has also helped expand the biochemical pathways that are the focus of much FTLD research. Nevertheless, other genes causing FTLD remain to be identified and their biology elucidated before we have a complete understanding of the complex aetiology of this disease.

Comparison of extent of tau pathology in patients with frontotemporal dementia with Parkinsonism linked to chromosome 17 (FTDP-17), frontotemporal lobar degeneration with Pick bodies and early onset Alzheimer's disease

In order to gain insight into the pathogenesis of frontotemporal lobar degeneration (FTLD), the mean tau load in frontal cortex was compared in 34 patients with frontotemporal dementia linked to chromosome 17 (FTDP-17) with 12 different mutations in the tau gene (MAPT), 11 patients with sporadic FTLD with Pick bodies and 25 patients with early onset Alzheimer's disease (EOAD). Tau load was determined, as percentage of tissue occupied by stained product, by image analysis of immunohistochemically stained sections using the phospho-dependent antibodies AT8, AT100 and AT180. With AT8 and AT180 antibodies, the amount of tau was significantly (P < 0.001 in each instance) less than that in EOAD for both FTDP-17 (8.5% and 10.0% respectively) and sporadic FTLD with Pick bodies (16.1% and 10.0% respectively). With AT100, the amount of tau detected in FTDP-17 was 54% (P < 0.001) of that detected in EOAD, but no tau was detected in sporadic FTLD with Pick bodies using this particular antibody. The amount of insoluble tau deposited within the brain in FTDP-17 did not depend in any systematic way upon where the MAPT mutation was topographically located within the gene, or on the physiological or structural change generated by the mutation, regardless of which anti-tau antibody was used. Not only does the amount of tau deposited in the brain differ between the three disorders, but the pattern of phosphorylation of tau also varies according to disease. These findings raise important questions relating to the role of aggregated tau in neurodegeneration - whether this represents an adaptive response which promotes the survival of neurones, or whether it is a detrimental change that directly, or indirectly, brings about the demize of the affected cell.

An immunohistochemical study of cases of sporadic and inherited frontotemporal lobar degeneration using 3R- and 4R-specific tau monoclonal antibodies

The pathological distinctions between the various clinical and pathological manifestations of frontotemporal lobar degeneration (FTLD) remain unclear. Using monoclonal antibodies specific for 3- and 4-repeat isoforms of the microtubule associated protein, tau (3R- and 4R-tau), we have performed an immunohistochemical study of the tau pathology present in 14 cases of sporadic forms of FTLD, 12 cases with Pick bodies and two cases without and in 27 cases of familial FTLD associated with 12 different mutations in the tau gene (MAPT), five cases with Pick bodies and 22 cases without. In all 12 cases of sporadic FTLD where Pick bodies were present, these contained only 3R-tau isoforms. Clinically, ten of these cases had frontotemporal dementia and two had progressive apraxia. Only 3R-tau isoforms were present in Pick bodies in those patients with familial FTLD associated with L266V, Q336R, E342V, K369I or G389R MAPT mutations. Patients with familial FTLD associated with exon 10 N279K, N296H or +16 splice site mutations showed tau pathology characterised by neuronal neurofibrillary tangles (NFT) and glial cell tangles that contained only 4R-tau isoforms, as did the NFT in P301L MAPT mutation. With the R406W mutation, NFT contained both 3R- and 4R-tau isoforms. We also observed two patients with sporadic FTLD, but without Pick bodies, in whom the tau pathology comprised only of 4R-tau isoforms. We have therefore shown by immunohistochemistry that different specific tau isoform compositions underlie the various kinds of tau pathology present in sporadic and familial FTLD. The use of such tau isoform specific antibodies may refine pathological criteria underpinning FTLD.

Caspase-cleaved tau accumulation in neurodegenerative diseases associated with tau and alpha-synuclein pathology

Alzheimer's disease (AD), Pick's disease (PiD), progressive supranuclear palsy (PSP), corticobasal degeneration (CBD) and dementia with Lewy bodies (DLB) are diseases associated with the accumulation of tau or alpha-synuclein. In AD, beta-amyloid (Abeta)-associated caspase activation and cleavage of tau at Asp421 (DeltaTau) may be an early step in neurofibrillary tangle (NFT) formation. To examine whether DeltaTau accumulates in other diseases not characterized by extracellular Abeta accumulation, we examined PiD, PSP, and CBD cases in comparison to those without extensive tau accumulation including frontotemporal lobar degeneration without Pick bodies (FTLD) and control cases. Additionally, we studied DeltaTau accumulation in DLB cases associated with intracellular alpha-synuclein. DeltaTau was observed in all disease cases except non-PiD FTLD and controls. These results demonstrate that the accumulation of DeltaTau may represent a common pathway associated with abnormal accumulation of intracellular tau or alpha-synuclein and may be relatively less dependent on the extracellular accumulation of Abeta in non-AD dementias.

Hereditary Pick's disease with the G272V tau mutation shows predominant three-repeat tau pathology

Frontotemporal dementia and parkinsonism linked to chromosome 17 have been associated with mutations in the microtubule associated protein tau (MAPT or tau) gene. This disorder is characterized by a large spectrum of neuronal and glial tau lesions in different brain regions. Pick bodies were found in a family with hereditary Pick's disease with the G272V mutation and in several families with other tau mutations in exons 9 and 11-13. The biochemical composition of Pick bodies varies between these mutations. Until recently, no detailed biochemical characterization of G272V brain material was done owing to unavailability of fresh frozen brain material. We now report a detailed study using the immunohistochemistry, western blots and electron microscopy of two brains with the G272V mutation that recently became available. Both brains showed severe neuronal loss in the temporal cortex, whereas in the frontal cortex the loss was less; and abundant Pick bodies in the dentate gyrus of the hippocampus, and caudate nucleus. The Pick bodies consisted exclusively of three-repeat (3R) isoforms, as was demonstrated by isoform-specific antibodies and supported by western blot analysis of sarkosyl-insoluble tau. These observations confirm that this family diagnosed with hereditary Pick disease meets all the criteria for this condition, including the presence of Pick bodies that are unphosphorylated at Ser262 and contain twisted filaments with long periodicity consisting only of 3R tau.

Tau and α-Synuclein Inclusions in a Case of Familial Frontotemporal Dementia and Progressive Aphasia

Recent studies have shown that neurofibrillary tangles are frequently accompanied by alpha-synuclein inclusions in sporadic and familial Alzheimer disease, in Down syndrome, in progressive supranuclear palsy, and Parkinsonism dementia complex of Guam. Here we report the cases of 2 brothers with familial progressive aphasia who developed features of frontotemporal dementia with predominant tau pathology but also alpha-synuclein pathology. The 2 patients' brains revealed abundant tau pathology in the hippocampus and basal ganglia, whereas tau and alpha-synuclein aggregates coexisted only in the nucleus basalis of Meynert, the only region where alpha-synuclein was present. In this brain region, abundant Lewy bodies, Lewy neurites, and tau inclusions were found; the pathology was more abundant in the older than in the younger brother. Sarkosyl-insoluble tau extracted from brains of the 2 patients showed the presence of tau filaments that contained 3 major tau bands of 60, 64, and 68 kDa on Western blot analysis. These bands contained mainly tau with 3 and 4 repeats and no amino-terminal inserts and tau with 4 repeats and one amino-terminal insert. No mutations were identified in the tau, alpha-synuclein, beta-synuclein, or parkin genes. We think that this is the first report showing a specific colocalization of neurofibrillary tangles and Lewy bodies in a family with progressive aphasia.

Pathological inclusion bodies in tauopathies contain distinct complements of tau with three or four microtubule-binding repeat domains as demonstrated by new specific monoclonal antibodies

Pathological inclusions containing fibrillar aggregates of hyperphosphorylated tau protein are a characteristic feature in the tauopathies, which include Alzheimer's disease, frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17), progressive supranuclear palsy, corticobasal degeneration and Pick's disease. Tau isoform composition and cellular and regional distribution as well as morphology of these inclusions vary in each disorder. Recently, several pathological missense and exon 10 splice-donor site mutations of the tau gene were identified in FTDP-17. Exon 10 codes for the second of four microtubule-binding repeat domains. The splice-site mutations result in increased inclusion of exon 10 which causes a relative increase in tau isoforms containing four microtubule-binding repeat domains over those containing three repeat domains. This could be a central aetiological mechanism in FTDP-17 and, perhaps, other related tauopathies. We have investigated changes in the ratio and distribution of three-repeat and four-repeat tau in the different tauopathies as a basis of the phenotypic range of these disorders and the selective vulnerability of different subsets of neurones. In this study, we have developed two monoclonal antibodies, RD3 and RD4 that effectively distinguish these closely related tau isoforms. These new isoform-specific antibodies are useful tools for analysing tau isoform expression and distribution as well as pathological changes in the human brain.