Tau Pathology Generated by Overexpression of Tau

Abstract

Neurofibrillary changes of abnormally hyperphosphorylated tau are the key lesion in Alzheimer's disease (AD) and a number of other tauopathies. Recent developments in the field of autosomal dominantly inherited dementias, in particular the frontotemporal dementias and Parkinsonism linked to chromosome 17 (FTDP-17) group, have shown that abnormalities in the tau gene result in neurofibrillary degeneration and cell death. Clinically this disorder presents with behavioral abnormalities, which are followed by dementia and, depending on the affected areas, by motor dysfunction. The location of the lesions does not seem to depend so much on the type of mutation as on the individual's genetic background and may vary even in the same family with the identical mutation. For instance, in one family with a P 301 S mutation in exon 10 of tau, the father presented with frontotemporal dementia, whereas the son had corticobasal degeneration.1Bugiani Murrell JR Giaccone G Hasegawa M Ghigo G Tabaton M Morbin M Primavera A Carella F Solaro C Grisoli M Savoiardo M Spillantini MG Tagliavini F Goedert M Ghetti B Frontotemporal dementia and corticobasal degeneration in a family with a P301S mutation in tau.J Neuropathol Exp Neurol. 1999; 58: 667-677Crossref PubMed Scopus (343) Google ScholarFTDP-17 is associated with both exonic and intronic mutations of the tau gene. The microtubule-associated protein (MAP) tau is a family of six proteins derived by alternative mRNA splicing2Himmler A Structure of the bovine tau gene: alternatively spliced transcripts generate a protein family.Mol Cell Biol. 1989; 4: 1389-1396Google Scholar, 3Goedert M Spillantini MG Potier MC Ulrich J Crowther RA Cloning and sequencing of the cDNA encoding an isoform of microtubule-associated protein tau containing four tandem repeats: differential expression of tau protein mRNAs in human brain.EMBO J. 1989; 2: 393-399Google Scholar from a single gene located on chromosome 17. These molecular isoforms of tau differ in whether they contain three or four tubulin binding domains/repeats of 31 or 32 amino acids each near the C-terminal end and no, one, or two inserts of 29 amino acids each at the N-terminal end of the molecule. There are nine missense mutations on tau exons 9 to 13; all but three are on exon 10.4Poorkaj P Bird TD Wijsman E Nemens E Garruto RM Anderson L Andreadis A Wiederholt WC Raskind M Schellenberg GD Tau is a candidate gene for chromosome 17 frontotemporal dementia.Ann Neurol. 1998; 43: 815-825Crossref PubMed Scopus (1214) Google Scholar, 5Hutton M 5′ splice site mutations in tau associated with the inherited dementia FTDP-17 affect a stem-loop structure that regulates alternative splicing of exon 10.J Biol Chem. 1999; 274: 15134-15143Crossref PubMed Scopus (242) Google Scholar, 6Spillantini MG Murrell JR Goedert M Farlows M Klug A Ghetti B Mutation in the tau gene in familial multiple system tauopathy with presenile dementia.Proc Natl Acad Sci USA. 1998; 95: 7737-7741Crossref PubMed Scopus (1297) Google Scholar, 7Mirra SS Murrell JR Gearing M Spillantini MG Goedert M Crowther RA Levey AL Jones R Green J Shoffner JM Wainer BH Schmidt ML Trojanowski JQ Ghetti B Tau pathology in a family with dementia and a P301L mutation in tau.J Neuropathol Exp Neurol. 1999; 58: 335-345Crossref PubMed Scopus (168) Google Scholar Exon 10 codes for the additional insert of the three 4-repeat tau isoforms. The resulting mutated taus possess an altered conformation8Jicha GA Rockwood JM Berenfeld B Hutton M Davies P Altered conformation of recombinant frontotemporal dementia-17 mutant tau proteins.Neurosci Lett. 1999; 260: 153-156Crossref PubMed Scopus (37) Google Scholar and a somewhat reduced ability to bind to and assemble microtubules.9Hong M Zhukareva V Vogelsberg-Ragaglia V Wszokek Z Reed L Miller BI Geschwind DH Bird TD McKeel D Goate A Morris JC Wilhelmsen KC Schellenberg GD Trojanowski JQ Lee VM-Y Mutation-specific functional impairments in distinct tau isoforms of hereditary FTDP-17.Science. 1998; 282: 1914-1917Crossref PubMed Scopus (809) Google Scholar, 10Hasegawa M Smith MJ Goedert M Tau proteins with FTDP-17 mutations have a reduced ability to promote microtubule assembly.FEBS Lett. 1998; 437: 207-210Abstract Full Text Full Text PDF PubMed Scopus (414) Google Scholar In addition to the exonic mutations, mutations at several sites have been found in the predicted stem loop structure in the 5′ splice site to exon 10. These intronic mutations and certain mutations in exon 10 that are close to the stem loop, and thus able to disrupt it, lead to two- to sixfold higher proportion of tau mRNA containing exon 10 than in control brains.5Hutton M 5′ splice site mutations in tau associated with the inherited dementia FTDP-17 affect a stem-loop structure that regulates alternative splicing of exon 10.J Biol Chem. 1999; 274: 15134-15143Crossref PubMed Scopus (242) Google Scholar The tau protein resulting from intronic mutations is normal, but the ratio of 4-repeat to 3-repeat isoforms is increased. Presently it is believed that due to the increased proportion of 4-repeat tau in the case of intronic mutations and the compromised biological activity of the tau with missense mutations, excess tau is not bound to the microtubules, which can then be hyperphosphorylated and would lead to neurofibrillary degeneration.In contrast to the FTDP-17 group of diseases, no mutations in the tau gene have been reported in AD at the time of writing this Commentary. In more than 90% of the AD patients the disease occurs sporadically above 60 years of age. In less than 5% of the cases the disease segregates with mutations in the amyloid precursor protein (APP), presenilin-1 (PS-1), or presenilin-2 (PS-2) genes.11Finch C Tanzi RE Genetics of aging.Science. 1997; 278: 407-411Crossref PubMed Scopus (377) Google Scholar Frameshift mutations of APP and ubiquitin at the level of transcription have been reported to be associated with sporadic and familial AD and Down's syndrome.12van Leeuwen FW de Kleijn DP van den Hurk HH Neubauer A Sonnemans MA Sluijs JA Koycu S Ramdjielal RDJ Salehi A Martens GJM Grosveld FG Peter J Burbach H Hol EM Frameshift mutants of β amyloid precursor protein and ubiquitin-B in Alzheimer β and Down patients.Science. 1998; 279: 242-247Crossref PubMed Scopus (482) Google Scholar The occurrence of the apolipoprotein E4 allele13Corder EH Saunders AM Strittmatter WJ Schmechel DE Gaskell PC Small GW Roses AD Haines JL Pericak-Vance MA Gene dose of apolipoprotein E Type 4 allele and the risk of Alzheimer's disease in late onset families.Science. 1993; 261: 921-923Crossref PubMed Scopus (7160) Google Scholar and, most recently, mutations in the β2 macroglobulin gene14Blacker D Wilcox MA Laird NM Rodes L Horvath SM Go RCP Perry R Watson Jr, B Bassett SS McInnis MG Albert MS Hyman BT Tanzi RE Alpha-2 macroglobulin is genetically associated with Alzheimer's disease.Nature Genetics. 1998; 19: 357-360Crossref PubMed Scopus (579) Google Scholar have been reported to be risk factors for the development of the late onset, sporadic AD. AD has two prominent neuropathological lesions, the extracellular deposits of the amyloid β peptide (Aβ) as plaques and the intraneuronal paired helical filaments (PHF) of abnormally hyperphosphorylated tau, which accumulate in the neuronal cell body as neurofibrillary tangles, in the neuropil (the so-called neuropil threads),15Braak H Braak E Grundke-Iqbal I Iqbal K Occurrence of neuropil threads in the senile human brain and in Alzheimer's disease: a third location of paired helical filaments outside of neurofibrillary tangles and neuritic plaques.Neurosci Lett. 1986; 65: 351-355Crossref PubMed Scopus (365) Google Scholar and in the dystrophic neurites surrounding the neuritic plaques. The direct relationship, if any, between the tangles and β-amyloid is not yet understood. At the one extreme, in the normal aged brain there is the significant β-amyloid accumulation and minimal neurofibrillary degeneration, whereas in the early onset familial AD with mutations in the β-APP or presenilin gene, massive β-amyloid deposits, tau hyperphosphorylation, and tangle formation are always seen. The other extreme situation is represented by extensive neurofibrillary degeneration and minimal β-amyloidosis. These include tauopathies like the tangle-predominant form of senile dementia (tangle-only dementia), Guam Parkinsonism dementia complex, dementia with argyrophilic grains, Nieman Pick's disease type C, subacute sclerosing panencephalitis, Pick's disease, and the dementia group with mutations in the tau gene.16Tolnay M Probst A Tau protein pathology in Alzheimer's disease and related disorders.Neuropathol Appl Neurobiol. 1999; 25: 171-187Crossref PubMed Scopus (193) Google Scholar Although at present the exact role of PHF and β-amyloid in the pathogenesis of AD is not established, there is growing evidence from a number of laboratories that the intellectual deterioration in AD patients is associated with neurofibrillary degeneration.17Tomlinson BE Blessed G Roth M Observations on the brains of demented old people.J Neurol Sci. 1970; 11: 205-242Abstract Full Text PDF PubMed Scopus (1361) Google Scholar, 18Alafuzoff I Iqbal K Friden H Adolfsson R Winblad B Histopathological criteria for progressive dementia disorders: clinical-pathological correlation and classification by multivariate data analysis.Acta Neuropathol (Berl). 1987; 74: 209-225Crossref PubMed Scopus (244) Google Scholar, 19Arigada PA Growdon JH Hedley-White ET Hyman BT Neurofibrillary tangles but not senile plaques parallel duration and severity of Alzheimer's disease.Neurology. 1992; 42: 631-639Crossref PubMed Google Scholar, 20Dickson DW Crystal HA Mattiace LA Masur DM Blau AD Davies P Yen S-H Aronson M Identification of normal and pathological aging in prospectively studied non-demented elderly humans.Neurobiol Aging. 1991; 13: 179-189Abstract Full Text PDF Scopus (527) Google Scholar A third and well characterized phenomenon is synaptic loss and cell death exceeding 50% in certain areas of the brain.21Gomez-Isla T Hollister R West H Mui S Growdon JH Petersen RC Parisi JE Hyman BT Neuronal loss correlates with but exceeds neurofibrillary tangles in Alzheimer's disease.Ann Neurol. 1997; 41: 17-24Crossref PubMed Scopus (1100) Google Scholar, 22Mann DMA Yates PO Marcynuik B Some morphometric observations on the cerebral cortex and hippocampus in presenile Alzheimer's disease, senile dementia of Alzheimer type and Down's syndrome in middle age.J Neurol Sci. 1985; 69: 139-159Abstract Full Text PDF PubMed Scopus (196) Google ScholarRole of Abnormal Hyperphosphorylation of Tau in Neurofibrillary DegenerationThe MAP tau in abnormally hyperphosphorylated form is the major protein subunit of the paired helical filaments (PHF).23Grundke-Iqbal I Iqbal K Quinlan M Tung Y-C Zaidi MS Wisniewski HM Microtubule-associated protein tau: a component of Alzheimer paired helical filaments.J Biol Chem. 1986; 261: 6084-6089Abstract Full Text PDF PubMed Google Scholar, 24Grundke-Iqbal I Iqbal K Tung Y-C Quinlan M Wisniewski HM Binder LI Abnormal phosphorylation of the microtubule associated protein τ (tau) in Alzheimer cytoskeletal pathology.Proc Natl Acad Sci USA. 1986; 83: 4913-4917Crossref PubMed Scopus (2814) Google Scholar, 25Iqbal K Grundke-Iqbal I Alzheimer abnormally phosphorylated tau is more hyperphosphorylated than the fetal tau and causes the disruption of microtubules.Neurobiol Aging. 1995; 16: 375-379Abstract Full Text PDF Scopus (23) Google Scholar These findings on the hyperphosphorylation of tau have been confirmed by a number of laboratories.26Flament S Delacourte A Abnormal tau species are produced during Alzheimer's disease neurodegenerating process.FEBS Lett. 1989; 247: 213-216Crossref PubMed Scopus (49) Google Scholar, 27Sternberger LA Zhang H Herman MM Rubenstein LJ Binder LI Sternberger NH Multiple proteins are abnormally processed in Alzheimer disease.Alzheimer Dis Assoc Disord. 1988; 2: 193Crossref Google Scholar, 28Lee VM-Y Balin BJ Otvos Jr, L Trojanowski JQ A68: a major subunit of paired helical filaments and derivatized forms of normal tau.Science. 1991; 251: 675-678Crossref PubMed Scopus (1247) Google Scholar, 29Brion JP Hanger DP Bruce M Couck AM Flament-Durant J Anderton BT Tau in Alzheimer neurofibrillary tangles.Biochem J. 1991; 273: 127-133Crossref PubMed Scopus (142) Google Scholar In a normal neuron the biological function is dependent on an intact microtubule network through which much of the axoplasmic transport is supported. Tau is one of the major MAPs and its function is regulated by phosphorylation. In neurons with neurofibrillary tangles the normal cytoskeleton is disrupted and replaced by bundles of PHF.30Dustin P Flament-Durand J Disturbances of axoplasmic transport in Alzheimer's disease.in: Weiss DG Gorio A Axoplasmic transport and pathology. Springer-Verlag, Berlin, Heidelberg1982: 131-136Crossref Google Scholar The disruption of the microtubule network probably compromises the axonal transport and starts retrograde degeneration of the affected neurons. The degeneration takes place apparently over a long period of time and neurons devoid of most of their axonal and dendritic arborizations have been reported in brains of patients who suffered several years of this progressive disease.31Scheibel ME Lindsay RD Tomiyasu U Scheibel AB Progressive dendritic changes in the aging human limbic system.Exp Neurol. 1976; 53: 420-430Crossref PubMed Scopus (127) Google Scholar, 32Morsch R Simon W Coleman PD Neurons may live for decades with neurofibrillary tangles.J Neuropathol Exp Neurol. 1999; 58: 188-197Crossref PubMed Scopus (315) Google Scholar These neurons eventually die, leaving behind the extracellular tombstones, or ghost tangles.Tau in PHF is posttranslationally modified. The earliest known modification seems to be its phosphorylation, which is followed at later stages of tangle formation by ubiquitination.33Bancher C Brunner C Lassmann HM Budka H Jellinger K Wiche G Seitelberger F Grundke-Iqbal I Iqbal K Wisniewski HM Accumulation of abnormally phosphorylated tau precedes the formation of neurofibrillary tangles in Alzheimer's disease.Brain Res. 1989; 477: 90-99Crossref PubMed Scopus (696) Google Scholar The very late stages of neurofibrillary tangles also stain immunocytochemically with antibodies to advanced glycation end products (AGE), suggesting that tau in PHF might be glycated.34Smith MA Taneda S Richey PL Miyata S Yan SD Stern D Sayre LM Monnier VM Perry G Advanced Maillard rection end products are associated with Alzheimer disease pathology.Proc Natl Acad Sci USA. 1994; 91: 5710-5714Crossref PubMed Scopus (726) Google Scholar, 35Ledesma MD Bonay P Colaço C Avila J Analysis of microtubule-associated protein tau glycation in paired helical filaments.J Biol Chem. 1994; 269: 21614-21619Abstract Full Text PDF PubMed Google Scholar, 36Yan SD Chen X Schmidt AM Brett J Goodman G Zou YS Scott CW Caputo C Frappier T Smith MA Glycated tau protein in Alzheimer disease: a mechanism for induction of oxidant stress.Proc Natl Acad Sci USA. 1994; 91: 7787-7791Crossref PubMed Scopus (524) Google Scholar In addition, PHF-tau is glycosylated with both O- and N-linked glycans.37Wang JZ Grundke-Iqbal I Iqbal K Restoration of biological activity of Alzheimer abnormally phosphorylated tau by dephosphorylation with protein phosphatase-2A, -2B and -1.Mol Brain Res. 1996; 38: 200-208Crossref PubMed Scopus (155) Google Scholar Apparently these molecules play a supportive role for the paired helical structure of the PHF, which, on digestion of the polysaccharides with endoglycosidase F/N-glycosidase F, untwist and collapse into tightly packed bundles of ∼2.5 nm.Besides being polymerized into PHF, a significant amount of abnormally hyperphosphorylated tau is also present as unpolymerized deposits (AD P-tau) in the neuronal cytoplasm of the AD brain.33Bancher C Brunner C Lassmann HM Budka H Jellinger K Wiche G Seitelberger F Grundke-Iqbal I Iqbal K Wisniewski HM Accumulation of abnormally phosphorylated tau precedes the formation of neurofibrillary tangles in Alzheimer's disease.Brain Res. 1989; 477: 90-99Crossref PubMed Scopus (696) Google Scholar, 38Bancher C Grundke-Iqbal I Iqbal K Fried VA Smith HT Wisniewski HM Abnormal phosphorylation of tau precedes ubiquitination in neurofibrillary pathology of Alzheimer disease.Brain Res. 1991; 539: 11-18Crossref PubMed Scopus (162) Google Scholar Tau in these so-called stage 0 tangles is not ubiquitinated, is soluble under nondenaturing conditions, and can be isolated from AD brain and separated from the accompanying normal tau.39Köpke E Tung Y-C Shaikh S Alonso A del C Iqbal K Grundke-Iqbal I Microtubule associated protein tau: abnormal phosphorylation of a non-paired helical filament pool in Alzheimer disease.J Biol Chem. 1993; 268: 24374-24384Abstract Full Text PDF PubMed Google Scholar Although not polymerized in situ the AD P-tau contains from 5 to 9 moles of phosphate per mole of tau, similar to the phosphorylation level of tau of the mature tangles, making it thus unlikely that the polymerization of tau into PHF might be catalyzed solely by the number of moles of phosphate.A potential role of AD P-tau, and in situ, most probably, at least as important as its involvement in the polymerization of PHF, is its deleterious effect on the integrity of the microtubules. Hyperphosphorylated tau, when polymerized into PHF, is biologically inert whereas AD P-tau is toxic to the system. The AD P-tau competes with tubulin in binding to not only the normal tau but also the high-molecular-weight MAPs, MAP1 and MAP2, and this sequestration of normal MAPs results in inhibition of assembly and disruption of microtubules.40Alonso A del C Zaidi T Grundke-Iqbal I Iqbal K Role of abnormally phosphorylated tau in the breakdown of microtubules in Alzheimer disease.Proc Natl Acad Sci USA. 1994; 91: 5562-5566Crossref PubMed Scopus (588) Google Scholar, 41Alonso A del C Grundke-Iqbal I Iqbal K Alzheimer's disease hyperphosphorylated tau sequesters normal tau into tangles of filaments and disassembles microtubules.Nat Med. 1996; 2: 783-787Crossref PubMed Scopus (681) Google Scholar The association between the abnormal and the normal taus leads to the formation of bundles of ∼2.1-nm tau filaments,42Alonso A del C Grundke-Iqbal I Barra HS Iqbal K Abnormal phosphorylation of tau, and the mechanism of Alzheimer neurofibrillary degeneration: Sequestration of MAP1 and MAP2 and the disassembly of microtubules by the abnormal tau.Proc Natl Acad Sci USA. 1997; 94: 298-303Crossref PubMed Scopus (363) Google Scholar whereas the association between the abnormal tau and MAP1 and MAP2 does not result in the formation of filaments. The binding of AD P-tau to the MAPs is even stronger than that between tubulin and MAPs because when AD P-tau is added to already formed microtubules, they are disrupted.41Alonso A del C Grundke-Iqbal I Iqbal K Alzheimer's disease hyperphosphorylated tau sequesters normal tau into tangles of filaments and disassembles microtubules.Nat Med. 1996; 2: 783-787Crossref PubMed Scopus (681) Google Scholar, 42Alonso A del C Grundke-Iqbal I Barra HS Iqbal K Abnormal phosphorylation of tau, and the mechanism of Alzheimer neurofibrillary degeneration: Sequestration of MAP1 and MAP2 and the disassembly of microtubules by the abnormal tau.Proc Natl Acad Sci USA. 1997; 94: 298-303Crossref PubMed Scopus (363) Google Scholar The inhibition of the microtubule assembly by AD P-tau, its sequestration of normal MAPs and disruption of microtubules are solely due to its abnormal hyperphosphorylation, because AD P-tau or tau extracted from PHF, when dephosphorylated, lose these characteristics and become fully functional, indistinguishable from normal tau in promoting microtubule assembly.40Alonso A del C Zaidi T Grundke-Iqbal I Iqbal K Role of abnormally phosphorylated tau in the breakdown of microtubules in Alzheimer disease.Proc Natl Acad Sci USA. 1994; 91: 5562-5566Crossref PubMed Scopus (588) Google Scholar, 41Alonso A del C Grundke-Iqbal I Iqbal K Alzheimer's disease hyperphosphorylated tau sequesters normal tau into tangles of filaments and disassembles microtubules.Nat Med. 1996; 2: 783-787Crossref PubMed Scopus (681) Google Scholar, 42Alonso A del C Grundke-Iqbal I Barra HS Iqbal K Abnormal phosphorylation of tau, and the mechanism of Alzheimer neurofibrillary degeneration: Sequestration of MAP1 and MAP2 and the disassembly of microtubules by the abnormal tau.Proc Natl Acad Sci USA. 1997; 94: 298-303Crossref PubMed Scopus (363) Google Scholar, 43Iqbal K Zaidi T Bancher C Grundke-Iqbal I Alzheimer paired helical filaments: restoration of the biological activity by dephosphorylation.FEBS Lett. 1994; 349: 104-108Abstract Full Text PDF PubMed Scopus (145) Google Scholar, 44Wang JZ Gong C-X Zaidi T Grundke-Iqbal I Iqbal K Dephosphorylation of Alzheimer paired helical filaments by protein phosphatase-2A and -2B.J Biol Chem. 1995; 270: 4854-4860Crossref PubMed Scopus (254) Google Scholar, 45Wang JZ Grundke-Iqbal I Iqbal K Glycosylation of microtubule-associated protein tau: an abnormal posttranslational modification in Alzheimer's disease.Nat Med. 1996; 2: 871-875Crossref PubMed Scopus (287) Google Scholar Furthermore, in vitro dephosphorylation of isolated PHF-tangles by protein phosphatases (PP) 2A and 2B disaggregates and disassembles them.45Wang JZ Grundke-Iqbal I Iqbal K Glycosylation of microtubule-associated protein tau: an abnormal posttranslational modification in Alzheimer's disease.Nat Med. 1996; 2: 871-875Crossref PubMed Scopus (287) Google ScholarProtein phosphorylation is one of the major mechanisms for the regulation of cellular function.46Nairn AC Hemmings Jr, HC Greengard P Protein kinases in the brain.Ann Rev Biochem. 1985; 54: 931-976Crossref PubMed Google Scholar The hyperphosphorylation of tau (see above), neurofilaments and MAP1b47Sternberger NH Sternberger LA Ulrich J Aberrant neurofilament phosphorylation in Alzheimer disease.Proc Natl Acad Sci USA. 1985; 82: 4274-4276Crossref PubMed Scopus (330) Google Scholar, 48Hasegawa M Arai T Ihara Y Immunochemical evidence that fragments of phosphorylated MAP5 (MAP1B) are bound to neurofibrillary tangles in Alzheimer's disease.Neuron. 1990; 4: 909-918Abstract Full Text PDF PubMed Scopus (103) Google Scholar, 49Ulloa L de Garcini EM Gòmez-Ramos P Moràn MA Avila J Microtubule-associated protein MAP1B showing a fetal phosphorylation pattern is present in sites of neurofibrillary degeneration in brains of Alzheimer's disease patients.Mol Brain Res. 1994; 26: 113-122Crossref PubMed Scopus (67) Google Scholar suggest a protein phosphorylation/dephosphorylation imbalance in the AD brain. Although increased kinase activities have not been shown as yet, it has been demonstrated that the activities but not the expression of both PP-1 and PP-2A are significantly (20–30%) reduced in AD neocortex.50Gong C-X Singh TJ Grundke-Iqbal I Iqbal K Phosphoprotein phosphatase activities in Alzheimer disease.J Neurochem. 1993; 61: 921-927Crossref PubMed Scopus (452) Google Scholar, 51Gong C-X Shaikh S Wang J-Z Zaidi T Grundke-Iqbal I Iqbal K Phosphatase activity toward abnormally phosphorylated τ: decrease in Alzheimer disease brain.J Neurochem. 1995; 65: 732-738Crossref PubMed Scopus (398) Google Scholar Furthermore, a reduction of PP-2B activity which correlated with neurofibrillary degeneration was also observed.53 In vitro AD P-tau and PHF-tau are dephosphorylated mostly by PP-2A and PP-2B, to a lesser extent by PP-1 but not by PP-2C.54Gong C-X Singh TJ Grundke-Iqbal I Iqbal K Alzheimer disease abnormally phosphorylated tau is dephosphorylated by protein phosphatase 2B (calcineurin).J Neurochem. 1994; 62: 803-806Crossref PubMed Scopus (130) Google Scholar, 55Gong C-X Grundke-Iqbal I Damuni Z Iqbal K Dephosphorylation of microtubule-associated protein tau by protein phosphatase-1 and -2C and its implication in Alzheimer disease.FEBS Lett. 1994; 341: 94-98Abstract Full Text PDF PubMed Scopus (92) Google Scholar, 56Gong C-X Grundke-Iqbal I Iqbal K Dephosphorylation of Alzheimer disease abnormally phosphorylated tau by protein phosphatase-2A.Neurosci. 1994; 61: 765-772Crossref PubMed Scopus (139) Google Scholar Recombinant tau in vitro [32P] phosphorylated can be dephosphorylated by PP-2A and PP-2B.57Goedert M Cohen ES Jakes R Cohen P P42 map kinase phosphorylation sites in microtubule-associated protein tau are dephosphorylated by protein phosphatase 2A1: implications for Alzheimer's disease.FEBS Lett. 1992; 312: 95-99Abstract Full Text PDF PubMed Scopus (259) Google Scholar, 58Drewes G Mandelkow E-M Baumann K Goris J Merlevede W Mandelkow E Dephosphorylation of tau protein and Alzheimer paired helical filaments by calcineurin and phosphatase-2A.FEBS Lett. 1993; 336: 425-432Abstract Full Text PDF PubMed Scopus (128) Google Scholar Furthermore, treatment of neuroblastoma or primary neuronal cell cultures with the phosphatase inhibitor okadaic acid results in the hyperphosphorylation of tau and inhibition of its turnover.59Vincent I Rosado M Kim E Davies P Increased production of paired helical filament epitopes in a cell culture system reduces the turnover of τ.J Neurochem. 1994; 62: 715-723Crossref PubMed Scopus (38) Google Scholar In the human neuroblastoma cell line SY5Y the inhibition of PP-2A and PP-1 by okadaic acid is accompanied by a transient stimulation of a number of proline-directed protein kinases, hyperphosphorylation of tau at several sites, reduced binding of MAPs to microtubules, and microtubule destabilization.60Tanaka T Zhong J Iqbal K Trenkner E Grundke-Iqbal I The regulation of phosphorylation of τ in SY5Y neuroblastoma cells: the role of protein phosphatases.FEBS Lett. 1998; 426: 248-254Abstract Full Text Full Text PDF PubMed Scopus (116) Google ScholarAnimal Models for TauopathiesMultiple attempts to induce Alzheimer-type neurofibrillary degeneration in animals, mostly rodents, have only made relatively small inroads. Because, according to the amyloid cascade hypothesis61Hardy JA Higgins GA Alzheimer's disease: the amyloid cascade hypothesis.Science. 1992; 256: 184-185Crossref PubMed Scopus (4906) Google Scholar tau pathology may be secondary and the result of amyloid disposition, this avenue has been primarily explored. Generation of transgenic mice expressing either the normal human amyloid precursor protein β-APP 751,62Higgins LS Rodems JM Catalano R Quon D Cordell B Early Alzheimer disease-like histopathology increased in frequency with age in mice transgenic for β-APP751.Proc Natl Acad Sci USA. 1995; 92: 4402-4406Crossref PubMed Scopus (87) Google Scholar the carboxy terminal 100 amino acids of the amyloid precursor protein, APP-C100,63Neve RL Boyce FM McPhie DL Greenan J Oster-Granite M Transgenic mice expressing APP-C100 in the brain.Neurobiol Aging. 1996; 17: 191-203Abstract Full Text PDF PubMed Scopus (40) Google Scholar or amyloid precursor proteins with mutations found in the familial forms of AD64Sturchler-Pierrat C Abramowski D Duke M Wiederhold K-H Mistl C Rothacher S Ledermann B Bürki Frey P Paganetti PA Waridel C Calhoun ME Jucker M Probst A Staufenbiel M Sommer B Two amyloid precursor protein transgenic mouse models with Alzheimer disease like pathology.Proc Natl Acad Sci. 1997; 94: 13287-13292Crossref PubMed Scopus (1237) Google Scholar, 65Moechars D Dewachter I Lorent K Reverse D Baekelandt V Naidu A Tesseur I Spittaels K Van Den Haute C Checler F Godaux E Cordell B Van Leuven F Early phenotypic changes in transgenic mice that overexpress different mutants of amyloid precursor protein in brain.EMBO J. 1998; 274: 6483-6492Google Scholar in all cases resulted in the deposition of amyloid, in some cases neurotoxicity63Neve RL Boyce FM McPhie DL Greenan J Oster-Granite M Transgenic mice expressing APP-C100 in the brain.Neurobiol Aging. 1996; 17: 191-203Abstract Full Text PDF PubMed Scopus (40) Google Scholar but at the most only modest staining of the neuropil surrounding the plaques with antibodies to phosphorylated tau. The most convincing indication for a role of amyloid in neurodegeneration is the study of Geula et al66Geula C Wu C-K Saroff D Lorenzo A Yuan M Yankner BA Aging renders the brain vulnerable to amyloid-protein neurotoxicity.Nat Med. 1998; 4: 827-831Crossref PubMed Scopus (500) Google Scholar in which the injection into the brain of aged rhesus and marmoset monkeys of polymerized synthetic Aβ peptide fibrils resulted in neurotoxicity and the appearance of phosphorylated tau in neurons and neurites distal to the area with neuronal loss. The more direct approach to induce tauopathy in an animal model is the generation of transgenic mice expressing human tau. The first study in which the longest human tau isoform (two N-terminal inserts and four repeats) was expressed in mice under the control of human Thy-1 promoter was published in 1995.67Götz J Probst A Spillantini MG Schäfer T Jakes R Bürki K Goedert M Somatodendritic localization and hyperphosphorylation of tau protein in transgenic mice expressing the longest human brain tau isoform.EMBO J. 1995; 14: 1304-1313Crossref PubMed Scopus (362) Google Scholar In this study human tau was expressed in most brain regions, but the number of neurons immunolabeled with tau antibodies was relatively small. Moderate immunostaining of some neurons with mAb AT8 to phosphorylated tau was also visible. In contrast to the wild-type mice, tau was not only stained in the axon but also present in the somatodendritic compartment of the cells. Somatodendritic staining of tau with the AT8 antibody is one of the earliest changes in selected neurons of the entorhinal cortex where neurofibrillary degeneration can be first observed.15Braak H Braak E Grundke-Iqbal I Iqbal K Occurrence of neuropil threads in the senile human brain and