Seminars in Cell & Developmental Biology 15 (2004) 4549

Tau protein and neurodegenerationMichel Goedert

Medical Research Council Laboratory of Molecular Biology, Hills Road, Cambridge CB2 2QH, UK

Abstract

Tau protein is the major component of the intracellular filamentous deposits that define a number of neurodegenerative diseases. Theyinclude the largely sporadic Alzheimers disease, progressive supranuclear palsy (PSP), corticobasal degeneration (CBD), Picks disease(PiD), argyrophilic grain disease, as well as the inherited frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17).The identification of mutations in Tau as the cause of FTDP-17 established that dysfunction or misregulation of tau protein is sufficient tocause neurodegeneration and dementia. At an experimental level, the new understanding is leading to the development of good transgenicanimal models of the tauopathies. 2004 Elsevier Ltd. All rights reserved.

Keywords: Alternative mRNA splicing; Amyloid; Frontotemporal dementia; Microtubule assembly; Neurodegeneration; Tau protein

1. Introduction

Recent progress in our understanding of some of the mostcommon neurodegenerative diseases was made possible bythe coming together of two independent lines of research.First, the biochemical study of the neuropathological lesionsthat define these diseases led to the identification of theirmolecular components. Second, the study of familial formsof disease led to the identification of gene defects that causethe inherited variants of the different diseases. Remarkably,in most cases, the defective genes were found to encode orincrease the expression of the main components of the neu-ropathological lesions. It has therefore been established thatthe basis of the familial forms of these diseases is a toxicproperty conferred by mutations in the proteins that makeup the filamentous lesions. A corollary of this insight is thata similar toxic property might also underlie the sporadicdisease forms.

Alzheimers disease (AD) is the most common neurode-generative disease. Neuropathologically, it is defined by thepresence of abundant extracellular neuritic plaques madeof the -amyloid peptide and intraneuronal neurofibrillarylesions made of the microtubule-associated protein tau [1].Similar tau lesions, in the absence of extracellular deposits,are also the defining characteristic of a number of otherneurodegenerative diseases, the best known of which areprogressive supranuclear palsy (PSP), corticobasal degen-

Tel.: +44-1223-402036; fax: +44-1223-402197.E-mail address: mg@mrc-lmb.cam.ac.uk (M. Goedert).

eration (CBD) and Picks disease (PiD) [2]. Until recently,there was no genetic evidence implicating tau protein in theneurodegenerative process. This changed with the discoveryof tau gene mutations in a familial form of frontotempo-ral dementia and parkinsonism [35]. Here I review theevidence implicating tau protein in this and other neurode-generative diseases.

2. Tau isoforms in human brain and theirinteractions with microtubules

Tau is a microtubule-binding protein that is believed tobe important for the assembly and stabilization of micro-tubules. In nerve cells, tau is normally found in axons, butin the tauopathies it is redistributed to the cell body anddendrites. In normal adult human brain, there are six iso-forms of tau, produced from a single gene by alternativemRNA splicing [6]. They differ from one another by thepresence or absence of a 29- or 58-amino acid insert in theamino-terminal half of the protein and by the inclusion, ornot, of a 31-amino acid repeat, encoded by exon 10 of Tau,in the carboxy-terminal half of the protein. The exclusionof exon 10 leads to the production of three isoforms, eachcontaining three repeats, and its inclusion leads to a furtherthree isoforms, each containing four repeats. The repeatsconstitute the microtubule-binding region of tau protein. Innormal adult human cerebral cortex, there are similar levelsof three-repeat and four-repeat isoforms. In developing hu-man brain, only the shortest isoform (three repeats and noamino-terminal inserts) is expressed.

1084-9521/$ see front matter 2004 Elsevier Ltd. All rights reserved.doi:10.1016/j.semcdb.2003.12.015

46 M. Goedert / Seminars in Cell & Developmental Biology 15 (2004) 4549

The tau molecule can be subdivided into an amino-terminaldomain that projects from the microtubule surface andthe carboxy-terminal microtubule-binding domain. Recentstructural work using gold-labelled tau co-assembled withmicrotubules has begun to shed light on the way thattau protein interacts with microtubules. When bound totaxol-stabilized microtubules, tau was found to localizealong the outer ridges of the microtubule protofilaments [7].A different conclusion was reached when tau was boundto microtubules in the absence of taxol. The tau repeatswere now found to bind to the inner surface of the mi-crotubule, close to the taxol-binding site on -tubulin [8].Taxol binds to a site on -tubulin, where -tubulin has aconserved extra loop of eight amino acids. Interestingly,this extended loop has significant sequence homology withthe tau repeats. Since tubulin cannot have evolved to bindtaxol, this work may answer the question of what type ofnatural substrate binds to this pocket in -tubulin. In thismodel, part of the proline-rich region of tau must providethe link between the amino-terminal projection domain onthe outside of the microtubule and the repeat motifs onthe inside surface. It could thread through one of the holesbetween protofilaments.

3. Tau filaments as nerve cell amyloid

In human diseases with tau pathology, the normally sol-uble tau protein is present in an abnormal filamentous form(Fig. 1). It is also hyperphosphorylated. In AD, tau filamentsconsist principally of paired helical filaments (PHFs), withstraight filaments (SFs) being a minority species. Electronmicrographs of negatively stained isolated filaments showimages in which the width of the filament varies betweenabout 8 and 20 nm with a spacing between cross-overs ofabout 80 nm [1]. Although the filament morphologies and

Fig. 1. Electron micrograph of a negatively stained preparation of taufilaments of the Alzheimer-type. Examples of the characteristic PHF (P)and SF (S) are indicated. Scale bar, 100 nm.

their tau isoform compositions vary between diseases [2],it is the repeat region of tau that forms the core of the fila-ment, with the amino- and carboxy-terminal regions forminga fuzzy coat around the filament [9].

The discovery that incubation of bacterially expressedhuman tau with sulphated glycosaminoglycans leads tobulk assembly of tau filaments [10,11] made it possibleto obtain structural information [12]. Filaments assembledfrom either three- or four-repeat tau showed cross- struc-ture by selected area electron diffraction, X-ray diffractionfrom macroscopic fibres and Fourier transform infraredspectroscopy. This work was extended to PHFs and SFs ex-tracted from diseased human brain. There had been contro-versy in the literature with regard to the internal molecularfine structure of these filaments. The difficulty had been toproduce from human brain pure preparations of filamentsfor analysis. This problem was circumvented by using se-lected area diffraction from small groups of filaments ofdefined morphology. Using this approach, PHFs and SFshad a clear cross- structure, which is the defining featureof amyloid fibres. They share this structure with the extra-cellular deposits present in the systemic and organ-specificamyloid diseases. It is therefore appropriate to consider thetauopathies a form of brain amyloidosis.

4. Mutations causing tauopathy

Frontotemporal dementias occur as familial forms and,more commonly, as sporadic diseases. They are character-ized by a remarkably circumscribed atrophy of the frontaland temporal lobes of the cerebral cortex, often withadditional, subcortical changes. In 1994, an autosomal-dominantly inherited form of frontotemporal dementia withparkinsonism was linked to chromosome 17q21.2 [13].Subsequently, other familial forms of frontotemporal de-mentia were found to be linked to this region, resulting inthe denomination frontotemporal dementia and parkinson-ism linked to chromosome 17 (FTDP-17) for this class ofdisease. All cases of FTDP-17 have so far shown a filamen-tous pathology made of hyperphosphorylated tau protein. InJune 1998, the first mutations in Tau in FTDP-17 patientswere reported [35]. Currently, 31 different mutations havebeen described in over 80 families with FTDP-17.

Tau mutations are either missense, deletion or silent mu-tations in the coding region, or intronic mutations locatedclose to the splice-donor site of the intron following the al-ternatively spliced exon 10 (Fig. 2). Functionally, they fallinto two largely non-overlapping categoriesthose whoseprimary effect is at the protein level and those that influ-ence the alternative splicing of tau pre-mRNA. Most mis-sense mutations reduce the ability of tau protein to interactwith microtubules, as reflected by a reduction in the abilityof mutant tau to promote microtubule assembly [14,15]. Anumber of mutations in Tau may cause FTDP-17, at least inpart, by promoting the aggregation of tau protein [16,17].

M. Goedert / Seminars in Cell & Developmental Biology 15 (2004) 4549 47

Fig. 2. Mutations in the tau gene in frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17). (a) Schematic diagram of the sixtau isoforms (352441 amino acids) that are expressed in adult human brain, with mutations in the coding region indicated using the numbering of the441 amino acid isoform. Nineteen missense mutations, two deletion mutations and three silent mutations are shown. The six tau isoforms are producedby alternative mRNA splicing from a single gene. They differ by the presence or absence of three inserts, shown in red (encoded by exon 2), green(encoded by exon 3) and yellow (encoded by exon 10), respectively. (b) Stem-loop structure in the pre-mRNA at the boundary between exon 10 andthe intron that follows it. Nine mutations are shown, two of which (S305N and S305S) are located in exon 10. They destabilize the stem-loop structure,resulting in increased inclusion of exon 10 and the relative overproduction of four-repeat tau. Exon sequences are boxed and shown in capital, withintron sequences being shown in lower-case letters. The two-dimensional representation of the tau exon 10 splicing regulatory element RNA is based onthe three-dimensional structure determined in [34].

Most of these mutations lead to the formation of nerve cellinclusions that consist of filaments made of all six tau iso-forms. Mutations P301L and P301S in exon 10 are excep-tions, since they lead to the formation of neuronal and glialinclusions that are made of tau isoforms with four-repeats.

The intronic mutations and most coding region mutationsin exon 10 increase the splicing of exon 10, thus changing theratio between three- and four-repeat tau isoforms, resultingin the overproduction of soluble four-repeat tau [4,5]. Thisleads to the formation of filaments made of four-repeat tauin both nerve cells and glial cells. Approximately half of theknown Tau mutations have their primary effect at the RNAlevel. They affect splicing enhancer or silencer sequencesin exon 10 or destabilise a stem-loop structure located atthe boundary between exon 10 and the intron that follows it(Fig. 2b). Thus, to a significant degree, FTDP-17 is a diseaseof the alternative mRNA splicing of exon 10 of the tau gene.It follows that a correct ratio of three-repeat to four-repeattau isoforms is essential for preventing neurodegenerationand dementia in mid-life.

5. Relevance for the sporadic tauopathies

The study of FTDP-17 has established that dysfunctionor misregulation of tau protein can cause neurodegeneration

and dementia. It follows that tau protein is most probablyalso of central importance in the pathogenesis of diseases,such as AD, PSP, CBD, and PiD. This is further underlinedby the fact that the aforementioned diseases are partially orcompletely phenocopied by cases of FTDP-17 [2].

Eight missense mutations in Tau have been shown to giverise to a clinical and neuropathological phenotype that isclosely related to PiD. The finding that overproduction offour-repeat tau causes disease and leads to the assembly oftau isoforms with only four repeats in nerve cells and glialcells may shed light on the pathogenesis of PSP and CBD.Both diseases are characterized by a neuronal and glial taupathology, with the filaments comprising only four-repeattau [2]. An association between progressive supranuclearpalsy and a dinucleotide repeat polymorphism in the intronbetween exons 9 and 10 of Tau has been described [18].The alleles at this locus carry 1115 repeats. The A0 allele,with 11 repeats, has a frequency of over 90% in patientswith PSP and about 70% in controls. Subsequently, twocommon tau haplotypes that differ at the nucleotide level,but not at the level of the protein coding sequence, havebeen reported [19]. Homozygosity of the more common al-lele H1 predisposes to PSP and CBD, but not to AD or PiD[1922]. This work strongly suggests that dysfunction oftau protein is of central importance in PSP and CBD, as it isin PiD.

48 M. Goedert / Seminars in Cell & Developmental Biology 15 (2004) 4549

6. Transgenic animal models of the tauopathies

Animal models of the tauopathies are essential for elu-cidating the mechanisms by which dysfunction of tauprotein leads to neurodegeneration. In addition, they mayprove useful for the development and testing of noveltherapies.

The discovery of mutations in Tau in FTDP-17 is leadingto the production of transgenic mouse lines that expressmutant human tau protein in nerve cells and glial cells.Abundant tau filaments were described in lines expressingfour-repeat tau with either the P301L or the P301S mutation[23,24]. Filamentous tau protein was hyperphosphorylatedin a similar way to the human diseases and hyperphosphory-lation at most sites appeared to precede the assembly of tauinto filaments. However, it remains to be seen whether hy-perphosphorylation of tau is necessary for filament assemblyin brain and spinal cord. Widely differing conclusions havebeen drawn from in vitro studies [10,25,26]. In a recentstudy in transgenic mice expressing human P301L tau, anincrease in the phosphorylation of soluble tau resulted in in-creased filament formation, suggesting that phosphorylationof tau can drive filament assembly in the brain [27]. In thetransgenic mice, filamentous tau deposits contained mostlythe mutant human tau protein, in line with in vitro find-ings showing that the P301L and P301S mutations stronglyenhance the heparin-induced assembly of tau protein intofilaments. In these lines, non-apoptotic nerve cell loss wasdetected in the spinal cord, with signs of a neurogenic mus-cle atrophy. The mice suffered from a severe paraparesis.

In a mouse line expressing human P301L tau in oligo-dendrocytes, co-expression of mutant human -synucleinresulted in the appearance of thioflavin S-positive stainingthat was not observed with the single transgenic lines [28].Moreover, in vitro experiments showed that -synuclein caninduce the formation of tau filaments, giving a possible ex-planation for the co-occurrence of tau and -synuclein in-clusions in some neurodegenerative diseases.

In human P301L tau mouse lines, co-expression of mutanthuman amyloid precursor protein or the intracerebral injec-tion of -amyloid fibrils was reported to increase the numberof tangle-bearing nerve cells [29,30]. It thus appears that ex-tracellular -amyloid deposits can exacerbate the intraneu-ronal pathology caused by the expression of mutant humantau protein. In contrast to these findings, -amyloid depositsfailed to induce the formation of tau filaments in mice ex-pressing wild-type human tau protein. Based on these find-ings, it would appear that -amyloid can promote, but notinduce, tau filament formation.

The existing transgenic mouse models of tauopathiesindicate a connection between the development of tau fil-aments and nerve cell degeneration. This contrasts withCaenorhabditis elegans and Drosophila melanogaster,where overexpression of mutant human tau resulted innerve cell degeneration, in the apparent absence of tau fil-

aments [3133]. It suggests that conformationally altered,non-filamentous human tau protein can be neurotoxic, atleast in an invertebrate context.

The transgenic mouse lines described so far exhibit theessential features of a human tauopathy, including the for-mation of abundant filaments made of hyperphosphorylatedtau protein and neurodegeneration. They will be invaluablefor a better understanding of the molecular mechanisms bywhich the dysfunction of tau protein causes the death ofnerve cells and this may in turn lead to novel therapeuticstrategies. In particular, the administration of specific proteinkinase inhibitors to these mice ought to establish whetherhyperphosphorylation of tau is either necessary or sufficientfor filament formation or neurodegeneration.

Acknowledgements

I thank Dr R.A. Crowther for Figure 1.

References

[1] St George-Hyslop PH, Farrer LA, Goedert M. Alzheimer diseaseand the frontotemporal dementias: diseases with cerebral depositionof fibrillar proteins. In: Scriver CR, Beaudet AL, Sly WS, Valle D,editors. The metabolic & molecular bases of inherited disease. NewYork: McGraw-Hill; 2001. p. 587599.

[2] Lee VMY, Goedert M, Trojanowski JQ. Neurodegenerativetauopathies. Annu Rev Neurosci 2001;24:112159.

[3] Poorkaj P, Bird TD, Wijsman E, Nemens E, Garruto RM, AndersonL, et al. Tau is a candidate gene for chromosome 17 frontotemporaldementia. Ann Neurol 1998;43:81525.

[4] Hutton M, Lendon CL, Rizzu P, Baker M, Froelich S, Houlden H, etal. Association of missense and 5-splice-site mutations in tau withthe inherited dementia FTDP-17. Nature 1998;393:7026.

[5] Spillantini MG, Murrell JR, Goedert M, Farlow MR, Klug A, GhettiB. Mutation in the tau gene in familial multiple system tauopathywith presenile dementia. Proc Natl Acad Sci USA 1998;5:773741.

[6] Goedert M, Spillantini MG, Jakes R, Rutherford D, Crowther RA.Multiple isoforms of human microtubule-associated protein tau: se-quences and localization in neurofibrillary tangles in Alzheimersdisease. Neuron 1989;3:51926.

[7] Al-Bassam J, Ozer RS, Safer D, Halpain S, Milligan R. MAP2and tau bind longitudinally along the outer ridges of microtubuleprotofilaments. J Cell Biol 2002;157:118796.

[8] Kar S, Fan J, Smith MJ, Goedert M, Amos LA. Repeat motifs of taubind to the insides of microtubules in the absence of taxol. EMBOJ 2003;22:707.

[9] Wischik CM, Novak M, Thogersen HC, Edwards PC, Runswick MJ,Jakes R, et al. Isolation of a fragment of tau derived from the coreof the paired helical filament of Alzheimer disease. Proc Natl AcadSci USA 1988;85:450610.

[10] Goedert M, Jakes R, Spillantini MG, Hasegawa M, Smith MJ,Crowther RA. Assembly of microtubule-associated protein tau intoAlzheimer-like filaments induced by sulphated glycosaminoglycans.Nature 1996;383:5503.

[11] Perez M, Valpuesta JM, Medina M, Montejo de Garcini E, Avila J.Polymerization of tau into filaments in the presence of heparin: theminimal sequence required for tautau interactions. J Neurochem1996;67:118390.

M. Goedert / Seminars in Cell & Developmental Biology 15 (2004) 4549 49

[12] Berriman J, Serpell LC, Oberg KA, Fink AL, Goedert M, CrowtherRA. Tau filaments from human brain and from in vitro assemblyof recombinant protein show cross- structure. Proc Natl Acad SciUSA 2003;100:90348.

[13] Wilhelmsen KC, Lynch T, Pavlou E, Higgins M, Nygaard TG. Local-ization of disinhibition-dementia-Parkinsonism-amyotrophy complexto 17q21-22. Am J Hum Genet 1994;55:115965.

[14] Hasegawa M, Smith MJ, Goedert M. Tau proteins with FTDP-17mutations have a reduced ability to promote microtubule assembly.FEBS Lett 1998;437:20710.

[15] Hong M, Zhukareva V, Vogelsberg-Ragaglia V, Wszolek Z, Reed L,Miller BI, et al. Mutation-specific functional impairments in distincttau isoforms of hereditary FTDP-17. Science 1998;282:19147.

[16] Nacharaju P, Lewis J, Easson C, Yen S, Hackett J, Hutton M, etal. Accelerated filament formation from tau protein with specificFTDP-17 mutations. FEBS Lett 1999;447:1959.

[17] Goedert M, Jakes R, Crowther RA. Effects of frontotemporal de-mentia FTDP-17 mutations on heparin-induced assembly of tau fil-aments. FEBS Lett 1999;450:30611.

[18] Conrad C, Andreadis A, Trojanowski JQ, Dickson DW, Kang D, ChenX, et al. Genetic evidence for the involvement of tau in progressivesupranuclear palsy. Ann Neurol 1997;41:27781.

[19] Baker M, Litvan I, Houlden H, Adamson J, Dickson D, Perez-TurJ, et al. Association of an extended haplotype in the tau gene withprogressive supranuclear palsy. Hum Mol Genet 1999;8:7115.

[20] Di Maria E, Tabaton M, Vigo T, Abbruzzese G, Bellone E, Donati C,et al. Corticobasal degeneration shares a common genetic backgroundwith progressive supranuclear palsy. Ann Neurol 2000;47:3747.

[21] Russ C, Powell JF, Zhao J, Baker M, Hutton M, Crawford F, et al. Themicrotubule-associated protein tau gene and Alzheimers diseaseanassociation study and meta-analysis. Neurosci Lett 2001;314:926.

[22] Russ C, Lovestone S, Baker M, Pickering-Brown SM, Ander-sen PM, Furlong R, et al. The extended haplotype of themicrotubule-associated protein tau gene is not associated with Picksdisease. Neurosci Lett 2001;99:1568.

[23] Lewis J, McGowan E, Rockwood J, Melrose H, Nacharaju P, VanSlegtenhorst M, et al. Neurofibrillary tangles, amyotrophy and pro-gressive motor disturbance in mice expressing mutant (P301L) tauprotein. Nat Genet 2000;25:4025.

[24] Allen B, Ingram E, Takao M, Smith MJ, Jakes R, Virdee K, etal. Abundant tau filaments and nonapoptotic neurodegeneration intransgenic mice expressing human P301S tau protein. J Neurosci2002;22:934051.

[25] Schneider A, Biernat J, von Bergen M, Mandelkow E, MandelkowEM. Phosphorylation that detaches tau protein from microtubules(Ser262, Ser214) also protects it against aggregation into Alzheimerpaired helical filaments. Biochemistry 1999;38:354958.

[26] Alonso AC, Zaidi T, Novak M, Grundke-Iqbal I, Iqbal K. Hyper-phosphorylation induces self-assembly of tau into tangles of pairedhelical/straight filaments. Proc Natl Acad Sci USA 2001;98:69238.

[27] Noble W, Olm V, Takata K, Casey EOM, Meyerson J, Gaynor K,et al. Cdk5 is a key factor in tau aggregation and tangle formationin vivo. Neuron 2003;38:55565.

[28] Giasson BI, Forman MS, Higuchi M, Golbe LI, Graves CL,Kotzbauer PT, et al. Initiation and synergistic fibrillization of tauand alpha-synuclein. Science 2003;300:63640.

[29] Lewis J, Dickson DW, Lin WL, Chisholm L, Corral A, Jones G, et al.Enhanced neurofibrillary degeneration in transgenic mice expressingmutant tau and APP. Science 2001;293:148791.

[30] Gtz J, Chen F, van Dorpe J, Nitsch RM. Formation of neurofibril-lary tangles in P301L tau transgenic mice induced by A42 fibrils.Science 2001;293:14915.

[31] Wittmann CW, Wszolek MF, Shulman JM, Salvaterra PM, Lewis J,Hutton M, et al. Tauopathy in Drosophila: neurodegeneration withoutneurofibrillary tangles. Science 2001;293:7114.

[32] Jackson GR, Wiedau-Pazos M, Sang TK, Wagle S, Brown CA, Mas-sachi S, et al. Human wild-type tau interacts with wingless pathwaycomponents and produces neurofibrillary pathology in Drosophila.Neuron 2002;34:50919.

[33] Kraemer BC, Zhang B, Leverenz JB, Thomas JH, Trojanowski JQ,Schellenberg GD. Neurodegeneration and defective neurotransmis-sion in a Caenorhabditis elegans model of tauopathy. Proc Natl AcadSci USA 2003;100:99805.

[34] Varani L, Hasegawa M, Spillantini MG, Smith MJ, Murrell JR,Ghetti B, et al. Structure of tau exon 10 splicing regulatory elementRNA and destabilization by mutations of frontotemporal dementiaand parkinsonism linked to chromosome 17. Proc Natl Acad SciUSA 1999;96:822934.

Tau protein and neurodegenerationIntroductionTau isoforms in human brain and their interactions with microtubulesTau filaments as nerve cell amyloidMutations causing tauopathyRelevance for the sporadic tauopathiesTransgenic animal models of the tauopathiesAcknowledgementsReferences