Alzheimer's disease-like phosphorylation of the microtubule-associated protein tau by glycogen synthase kinase-3 in transfected mammalian cells.

BACKGROUND: Paired helical filaments (PHFs) are a characteristic pathological feature of Alzheimer's disease; their principal component is the microtubule-associated protein tau. The tau in PHFs (PHF-tau) is hyperphosphorylated, but the cellular mechanisms responsible for this hyperphosphorylation have yet to be elucidated. A number of kinases, including mitogen-activated protein (MAP) kinase, glycogen synthase kinase (GSK)-3 alpha, GSK-3 beta and cyclin-dependent kinase-5, phosphorylate recombinant tau in vitro so that it resembles PHF-tau as judged by its reactivity with a panel of antibodies capable of discriminating between normal tau and PHF-tau, and by a reduced electrophoretic mobility that is characteristic of PHF-tau. To determine whether MAP kinase, GSK-3 alpha and GSK-3 beta can also induce Alzheimer's disease-like phosphorylation of tau in mammalian cells, we studied the phosphorylation status of tau in primary neuronal cultures and transfected COS cells following changes in the activities of MAP kinase and GSK-3. RESULTS: Activating MAP kinase in cultures of primary neurons or transfected COS cells expressing tau isoforms did not increase the level of phosphorylation for any PHF-tau epitope investigated. But elevating GSK-3 activity in the COS cells by co-transfection with GSK-3 alpha or GSK-3 beta decreased the electrophoretic mobility of tau so that it resembled that of PHF-tau, and induced reactivity with eight PHF-tau-selective monoclonal antibodies. CONCLUSIONS: Our data indicate that GSK-3 alpha and/or GSK-3 beta, but not MAP kinase, are good candidates for generating PHF-type phosphorylation of tau in Alzheimer's disease. The involvement of other kinases in the generation of PHFs cannot, however, be eliminated. Our results suggest that aberrant regulation of GSK-3 may be a pathogenic mechanism in Alzheimer's disease.

Chicken synucleins: cloning and expression in the developing embryo.

Synucleins comprise a family of small intracellular proteins that have recently attracted considerable attention because of their involvement in human diseases. Mutations of alpha-synuclein has been found in several families with hereditary early-onset Parkinson's disease and accumulation of this protein in characteristic cytoplasmic inclusions is a pathohistological hallmark of several neurodegenerative diseases that have been recently classified as 'alpha;-synucleinopathies' (reviewed in Brain Res. Bull. 50 (1999) 465; J. Neurosci. Res. 58 (1999) 120; Philos. Trans. R. Soc. Lond. Biol. Sci. 354 (1999) 1101; Brain Pathol. 9 (1999) 733). Aggregates of beta-synuclein and persyn (gamma-synuclein) also have been found in dystrophic neurites associated with Parkinson's and other neurodegenerative diseases (Proc. Natl. Acad. Sci. USA 96 (1999) 13450; and our unpublished observations). Moreover, persyn has been implicated in malignization of breast tumours (Cancer Res. 57 (1997) 759; Cancer Res. 59 (1999) 742; Hum. Mol. Genet. 7 (1998) 1417). All synucleins have distinct, although overlapping, patterns of expression in the embryonic, postnatal and adult mammalian nervous systems, suggesting important, although still not clear, biological functions in neuronal developing. Chicken embryo is a unique object for developmental studies that allows in vivo manipulations not always possible for mammalian embryos. Studies of synucleins expression in this model system could shed light on their functions in the developing nervous system. We cloned three chicken synucleins from the embryonic neural cDNA libraries and studied their expression in normal chicken embryonic tissues by Northern and in situ hybridization with specific probes. Our results demonstrate that primary structures and expression patterns of synucleins are similar in birds and mammals, suggesting that conserved function of synucleins is important for embryonic development of vertebrates.

Quantitative analysis of tau isoform transcripts in sporadic tauopathies.

A number of neurodegenerative diseases, including Alzheimer's disease (AD), are characterized by intraneuronal accumulation of the tau protein. Some forms of FTDP-17 are caused by mutations in the tau gene affecting exon 10 splicing. Therefore, dysregulation of tau pre-mRNA splicing may be a contributing factor to sporadic tauopathies. To address this question, we devised a real-time RT-PCR strategy based on the use of a single fluorogenic probe to evaluate the ratio between tau isoforms containing or lacking exon 10 (4R/3R ratio) in post-mortem brain samples. We found a two- to six-fold increase in the 4R/3R ratio in cases of FTDP-17 linked to a splice site mutation, hence confirming the validity of the strategy. The difference in the 4R/3R ratio in the superior temporal and superior frontal gyri between AD and control brains was not statistically significant. Similarly, there was no significant difference in the 4R/3R ratio between Pick's disease cases and controls, indicating that the predominance of tau3R protein in PiD reflects post-translational modifications of specific isoforms. This study indicates that post-translational events are likely to be the main factors controlling tau isoform composition in sporadic tauopathies and highlights the benefit of quantitative RT-PCR in the assessment of splicing abnormalities in tauopathies.

Neurodegenerative changes including altered tau phosphorylation and neurofilament immunoreactivity in mice transgenic for the serine/threonine kinase Mos.

Transgenic mice expressing the oncogenic protein-serine/threonine kinase Mos at high levels in the brain display progressive neuronal degeneration and gliosis. Gliosis developed in parallel with the onset of postnatal transgene expression and led to a dramatic increase in the number of astrocytes positive for GFAP, vimentin, and possibly tau. Interestingly, vimentin is normally expressed only in immature or neoplastic astrocytes, but appears to be induced to high levels in Mos-transgenic, mature astrocytes. Mos can activate mitogen activated protein kinase (MAPK) and MAPK has been implicated in Alzheimer-type tau phosphorylation. In the Mos-transgenic brain we found increased levels of phosphorylation at one epitope on tau containing serines 199 and 202 (numbering according to human tau), a pattern similar but not identical to that found in Alzheimer's disease. In addition, Mos-transgenic mice express a novel neurofilament-related protein that might be a proteolytic neurofilament heavy chain degradation product. These results suggest that activation of protein phosphorylation in neurons can result in changes in cytoskeletal proteins that might contribute to neuronal degeneration.

Modulation of PHF-like tau phosphorylation in cultured neurones and transfected cells.

Two cellular systems have been used to investigate the modulation of tau hyperphosphorylation. In the first system, the effects of the excitatory amino acid glutamate, the microtubule destabilising agent colchicine, and beta 25-35-amyloid peptide on tau phosphorylation were studied in rat cortical neurones in primary culture. Using immunocytochemistry and western blot analysis, we demonstrated that tau in these cultures is normally highly phosphorylated, but a proportion becomes rapidly dephosphorylated following treatment of the cultures with glutamate or colchicine. These changes in tau phosphorylation occurred prior to cell death. In the second system, the ability of p42 MAP and p44 MAP kinases, glycogen synthase kinases 3 alpha and 3 beta (GSK-3 alpha and GSK-3 beta) to phosphorylate tau in transfected COS cells was investigated. Both GSK-3 alpha and GSK-3 beta phosphorylated tau to produce a PHF-like state of phosphorylation but the MAP kinases failed to induce such a transformation in tau. These results suggest that aberrant regulation of GSK-3 alpha/beta may be a pathogenic mechanism in Alzheimer's disease.

PHF-tau from Alzheimer's brain comprises four species on SDS-PAGE which can be mimicked by in vitro phosphorylation of human brain tau by glycogen synthase kinase-3 beta.

Extensive in vitro phosphorylation of a purified preparation of control human brain tau consistently produces four rather than, as previously believed, three tau species on SDS-PAGE. The species thus generated are shifted on SDS-PAGE to positions that match those of PHF-tau isolated from Alzheimer's disease brain. A mixture of recombinant human tau isoforms phosphorylated by GSK-3 beta gave similar results to those obtained with control human brain tau. In vitro phosphorylation of the individual recombinant isoforms by GSK-3 beta showed that the four bands of PHF-tau are likely to consist of isoforms 3R,0 alone; 4R,0 with 3R,29; 4R,29 with 3R,58 and 4R,58 alone.

The complex relationship between soluble and insoluble tau in tauopathies revealed by efficient dephosphorylation and specific antibodies.

Phosphorylated tau is deposited as insoluble inclusion bodies in the tauopathies. We have used a new efficient method to dephosphorylate tau extracted from control and tauopathy brain. In some tauopathies, including Alzheimer's disease and progressive supranuclear palsy, the pattern of insoluble tau isoforms reflected that of soluble tau. In contrast, in corticobasal degeneration, Pick's disease, and some forms of fronto-temporal dementia, specific tau isoforms were selectively sequestered into insoluble inclusion-forming tau. Therefore the overall expression of individual tau isoforms does not predict which tau isoforms are deposited in all tauopathies and different mechanisms must operate that result in the deposition of specific tau isoforms.

Sites of phosphorylation in tau and factors affecting their regulation.

The microtubule-associated protein, tau, is the principal component of paired helical filaments (PHFs) in Alzheimer's disease. PHF-tau is highly phosphorylated and a total of 25 sites of phosphorylation have so far been identified. Many of these sites are serine or threonine residues that are immediately followed in the sequence by proline residues, and hence are candidate phosphorylation sites for proline-directed kinases. In vitro, glycogen synthase kinase-3 (GSK-3), extracellular signal-related kinase-1 and -2, and mitogen-activated protein kinases, p38 kinase and c-jun N-terminal kinase, all phosphorylate many of these sites, although with different efficiencies for particular sites. Phosphorylation studies in transfected cells and neurons show that GSK-3 phosphorylates tau more extensively than do these other proline-directed kinases. Mutations in tau have been shown to affect in vitro phosphorylation of tau by GSK-3. The Arg406-->Trp (R406W) tau mutation also affects tau phosphorylation in cells.

Autopsy-confirmed familial early-onset Alzheimer disease caused by the l153V presenilin 1 mutation.

BACKGROUND: Three affected individuals are described from a small English kindred with early-onset autosomal dominant familial Alzheimer disease (FAD) caused by a leucine-to-valine change at codon 153 (L153V) of the presenilin 1 (PSEN1) gene. METHODS: Clinical information on the pedigree was collected directly from family members and from hospital records. Samples of DNA were screened by means of direct sequencing of all coding exons of PSEN1. One patient underwent neuropathological examination. RESULTS: Mean age at onset of symptoms was 35.3 years (95% confidence interval [CI], 34.6-36.0 years); at death, 44.0 years (95% CI, 39.1-48.9 years). Mean duration of illness was 8.3 years (95% CI, 4.7-11.9 years). Myoclonus was a late feature in 1 patient; seizures were not reported in any subjects. Spastic paraparesis and extrapyramidal signs were absent. The neuropsychometric profile of 1 patient showed relatively preserved naming skills in the setting of global cognitive deficits. Results of neuropathological examination demonstrated the signature lesions of Alzheimer disease and the presence of occasional cortical Lewy bodies. CONCLUSIONS: The PSEN1 L153V mutation lies in the main mutation cluster of PSEN1 in the second transmembrane domain. It causes early-onset FAD with clinical features similar to those of other reported FAD pedigrees.

Differential involvement and heterogeneous phosphorylation of tau isoforms in progressive supranuclear palsy.

We found previously that aggregated insoluble tau protein in progressive supranuclear palsy (PSP) brains exhibits a heterogeneous pattern that is not segregated by the type of clinical presentation. Here we have investigated tau isoform composition from 20 PSP cases and found marked variation between different brains. Cases were classified into three groups, each comprising essentially of (1) 1N4R; (2) 1N4R and 1N3R; or (3) 1N4R, 1N3R and 0N4R tau isoforms. There was also an absence of a simple relationship between isoform composition and the pattern of insoluble tau before dephosphorylation. We conclude that there is distinct molecular heterogeneity in the involvement of tau isoforms in the tau pathology in PSP.

Tau pathology in a case of familial Alzheimer's disease with a valine to glycine mutation at position 717 in the amyloid precursor protein.

The brain tissue from a case of familial Alzheimer's disease (FAD) caused by a missense (valine to glycine) mutation at codon 717 of the amyloid precursor protein (APP) gene has been examined for the presence of abnormally phosphorylated paired helical filament tau (PHF-tau). There was abundant PHF-tau present, which on Western blots, was indistinguishable from the PHF-tau typical of cases of sporadic Alzheimer's disease and that of another FAD mutation (valine to isoleucine), previously (Neurosci. Lett., 137 (1992) 221-224). This result implies that the cytoskeletal pathology in Alzheimer's disease is biochemically linked to abnormal APP processing and amyloid deposition.

Glycogen synthase kinase-3 induces Alzheimer's disease-like phosphorylation of tau: generation of paired helical filament epitopes and neuronal localisation of the kinase.

Glycogen synthase kinase-3 (GSK-3) reduced the mobility of human tau on SDS-PAGE, prevented binding of the monoclonal antibody (mAb), Tau.1, and induced binding of the mAb 8D8. Recombinant tau phosphorylated by GSK-3 aligned on SDS-PAGE with the abnormally phosphorylated tau (PHF-tau) associated with the paired helical filaments in Alzheimer's disease brain. Phosphorylated serine396 (numbering of the largest human brain tau isoform) was identified as a binding site on tau for mAb 8D8. The localisation of GSK-3 within granular structures in pyramidal cells indicates that GSK-3 alpha and GSK-3 beta may have a role in the production of PHF-tau in Alzheimer's disease.

Familial Alzheimer's disease with the amyloid precursor protein position 717 mutation and sporadic Alzheimer's disease have the same cytoskeletal pathology.

The cytoskeletal pathology of a patient with familial Alzheimer's disease (AD) associated with the probably causal amyloid precursor protein (APP) codon 717 Val----Ile mutation is described. In addition to moderately extensive beta A4 protein deposition within the substance of the brain and in blood vessel walls (congophilic angiopathy), there was abundant cytoskeletal pathology in the form of neurofibrillary tangles, plaque neurites and neuropil threads. Interestingly, plentiful cortical and subcortical Lewy bodies were also seen. In order to compare the cytoskeletal pathology in this case with that seen in sporadic cases of AD we (1) studied the immunohistochemical profile of the amyloid and cytoskeletal pathology with antibodies to beta A4 protein, tau, phosphorylated neurofilament epitopes and ubiquitin and (2) performed a biochemical fractionation and Western blot analysis for the abnormally phosphorylated form of tau (A68) characteristically seen in AD. No substantial difference between the familial case and sporadic cases could be found. We conclude that it is now reasonable to hypothesise that an abnormality in APP metabolism is responsible not only for the deposition of beta A4 protein, but also for the range of cytoskeletal pathology, typical of AD.

Pathological lesions of Alzheimer's disease and dementia with Lewy bodies brains exhibit immunoreactivity to an ATPase that is a regulatory subunit of the 26S proteasome.

MS73 is one of a family of ATPases that act as regulatory subunits of the 26S proteasome. Localisation of this ATPase in histological sections of hippocampus from Alzheimer's disease (AD) and in cingulate gyrus sections of dementia with Lewy bodies (DLB) brains was examined immunohistochemically. In all cases of AD (n = 10) neurofibrillary tangles (NFT), plaque neurites and neuropil threads were immunoreactive for MS73. In seven out of the nine cases of DLB, distinctive MS73-positive structures were detected within cortical Lewy bodies. The association of MS73 with these neuronal abnormalities provides further evidence that proteolytic processing involving the 26S proteasome occurs in lesions of AD and DLB.

Tau protein in the glial cytoplasmic inclusions of multiple system atrophy can be distinguished from abnormal tau in Alzheimer's disease.

The glial cytoplasmic inclusion (GCI) is a histological hallmark for multiple system atrophy (MSA). These inclusions are in oligodendrocytes, contain microtubular structures of 20-30 nm diameter, and can be labelled immunohistochemically with antibodies to ubiquitin, alphaB-crystallin, alpha- and beta-tubulin, and the microtubule-associated protein tau. GCIs have been compared with neuronal inclusions in other neurodegenerative disorders including the neurofibrillary tangles (NFTs) found in Alzheimer's disease (AD), which also contain tau protein. In order to determine whether the tau protein of GCIs in MSA is similar to that observed in AD we used a panel of antibodies to phosphorylation-independent (SMI51, TP007, TP70), dephosphorylation-dependent (Tau.1), and phosphorylation-dependent antibodies to tau and neurofilaments (AT8, AT180, AT270, SMI31, SMI34, RT97, BF10, 8D8). Immunohistochemistry was performed on paraffin wax-embedded brain tissue of the cerebellum, brainstem, and frontal lobes (Brodmann areas 4/6) of ten clinically and neuropathologically well-characterised cases of MSA, two cases of AD, and two normal controls. The NFTs of the AD cases were labelled with all the phosphorylation-dependent and phosphorylation-independent antibodies and with Tau.1 only after treatment with alkaline phosphatase. In contrast, GCIs were immunolabelled by the phosphorylation-independent antibodies and Tau.1, but not by the phosphorylation-dependent antibodies. These data demonstrate that the tau in GCIs is different from the abnormally phosphorylated tau found in AD and is similar to normal adult tau. The mechanism causing the abnormal accumulation of tau in GCIs remains to be elucidated.

Astrocytes are important mediators of Aß-induced neurotoxicity and tau phosphorylation in primary culture.

Alzheimer's disease (AD) is pathologically characterised by the age-dependent deposition of ß-amyloid (Aß) in senile plaques, intraneuronal accumulation of tau as neurofibrillary tangles, synaptic dysfunction and neuronal death. Neuroinflammation, typified by the accumulation of activated microglia and reactive astrocytes, is believed to modulate the development and/or progression of AD. We have used primary rat neuronal, astrocytic and mixed cortical cultures to investigate the contribution of astrocyte-mediated inflammatory responses during Aß-induced neuronal loss. We report that the presence of small numbers of astrocytes exacerbate Aß-induced neuronal death, caspase-3 activation and the production of caspase-3-cleaved tau. Furthermore, we show that astrocytes are essential for the Aß-induced tau phosphorylation observed in primary neurons. The release of soluble inflammatory factor(s) from astrocytes accompanies these events, and inhibition of astrocyte activation with the anti-inflammatory agent, minocycline, reduces astrocytic inflammatory responses and the associated neuronal loss. Aß-induced increases in caspase-3 activation and the production of caspase-3-truncated tau species in neurons were reduced when the astrocytic response was attenuated with minocycline. Taken together, these results show that astrocytes are important mediators of the neurotoxic events downstream of elevated Aß in models of AD, and suggest that mechanisms underlying pro-inflammatory cytokine release might be an important target for therapy.

Transgenic mouse models of tauopathy in drug discovery.

Tauopathies, including Alzheimer's disease, are neurodegenerative diseases characterized by the deposition of hyperphosphorylated tau protein in the central nervous system, and are the major cause of dementia in later life. Considerable advances have been made in developing mouse models that recapitulate, to varying extents, the development of human tau pathology, and the learning and memory deficits characteristic of some tauopathies. Furthermore, such models have been used to show promising disease-modifying effects in pre-clinical testing of new therapeutics. Various strategies have been utilised to generate mouse models of tauopathies. Some of the most enlightening models developed to date either constitutively or inducibly express pathogenic tau mutations. These animals have been instrumental in defining critical disease-related mechanisms, including the observation that tangles are not the toxic form of tau in disease. Here, we discuss the strengths and weaknesses of well characterised transgenic models that emulate human tauopathy, and include a comprehensive listing of the main phenotypic characteristics of all reported tau transgenic rodents. We summarise the use of tau mice for the development and evaluation of new therapeutic approaches, and their utility in identifying novel drug targets. In addition, we review the parameters to be considered in the development of the next generation of mouse models of tauopathy, aimed at further increasing our understanding of disease aetiology and in evaluating novel treatments.

Fluconazole and testosterone: in vivo and in vitro studies.

Fluconazole (UK-49,858), a novel bis-triazole antifungal agent, was given orally to groups of 10 male volunteers at doses of 25 and 50 mg/day for 28 days. Blood samples for testosterone estimation were taken from these and from a placebo group at several time points on days 1, 14, and 28 of the study, and the assay results demonstrated that the compound had no significant effect on circulating testosterone levels. Similarly, in studies with rat Leydig cells in vitro, fluconazole at concentrations up to 10 micrograms/ml was found to be only a weak inhibitor of testosterone production, whereas ketoconazole caused more than 50% inhibition at 0.1 microgram/ml. It is concluded that fluconazole, in contrast to ketoconazole, has little effect on the biosynthesis of testosterone by mammalian cells.

A68 proteins in Alzheimer's disease are composed of several tau isoforms in a phosphorylated state which affects their electrophoretic mobilities.

The tau-immunoreactive A68 polypeptides found in brains from patients with Alzheimer's disease have been studied by Western blotting using (1) antibodies to synthetic peptides corresponding to sequences that span the complete human tau molecule, and (2) antibodies specific for inserts 1 and 2 found towards the N-terminus of some tau isoforms. The three major A68 polypeptides were labelled by all of the antibodies to sequences common to all tau isoforms, but the faster-migrating A68 polypeptides was not labelled by either of the two antibodies specific for inserts 1 and 2. Treatment with alkaline phosphatase of non-solubilized A68 did not change its electrophoretic mobility on SDS/PAGE under the conditions described here. However, A68 that was solubilized before treating it with alkaline phosphatase was found to move faster on SDS/PAGE than untreated A68, to a position similar to that of normal tau. We also confirmed that A68 preparations contain numerous paired helical filaments (PHF). These PHF were labelled by all anti-tau antibodies, including insert-specific antibodies. Our results further support the notion that PHF contain abnormally phosphorylated tau in an aggregated state, and indicate that these abnormally phosphorylated tau forms are composed of several tau isoforms and that the full length of the tau molecule is present in these polypeptides.

Expression and phosphorylation of a three-repeat isoform of tau in transfected non-neuronal cells.

The neuronal microtubule-associated protein, tau, is expressed as a set of isoforms containing either three or four tandemly repeated 31-amino-acid motifs in the C-terminal half of the molecule that can bind to microtubules. Three-repeat forms are the only ones expressed early in development. A single three-repeat isoform of tau has been stably expressed in non-neuronal cells which do not express endogenous tau. Chinese hamster ovary (CHO) cells were transfected with a full-length cDNA coding for the foetal form of human tau cloned downstream of the simian virus 40 (SV40) promoter, and a cell line constitutively expressing tau, CHO[pSVtau3], was isolated. Double-label immunofluorescence microscopy reveals that tau co-localizes with the microtubular network of normal or taxol-treated CHO[pSVtau3] cells, without inducing any dramatic change in cell morphology. Tau is expressed in CHO[pSVtau3] cells as three bands in SDS/PAGE recognized by antibodies to tau, the slow-migrating tau species being the most abundant. Tau also appears as three bands in a heat-stable fraction from CHO[pSVtau3] cells, but a single band of enhanced immunoreactivity is detected following treatment of this fraction with alkaline phosphatase. This single band co-migrates with the fast-migrating band of untreated fractions or whole-cell extracts. In conclusion, a three-repeat isoform of tau is capable of binding to microtubules in transfected non-neuronal cells; furthermore, in this system, the protein is phosphorylated in at least two different states inducing a reduced electrophoretic mobility.