Proline-directed and non-proline-directed phosphorylation of PHF-tau

Hyperphosphorylation induces self-assembly of tau into tangles of paired helical filaments/straight filaments

The microtubule-associated protein tau is a family of six isoforms that becomes abnormally hyperphosphorylated and accumulates in the form of paired helical filaments (PHF) in the brains of patients with Alzheimer's disease (AD) and patients with several other tauopathies. Here, we show that the abnormally hyperphosphorylated tau from AD brain cytosol (AD P-tau) self-aggregates into PHF-like structures on incubation at pH 6.9 under reducing conditions at 35 degrees C during 90 min. In vitro dephosphorylation, but not deglycosylation, of AD P-tau inhibits its self-association into PHF. Furthermore, hyperphosphorylation induces self-assembly of each of the six tau isoforms into tangles of PHF and straight filaments, and the microtubule binding domains/repeats region in the absence of the rest of the molecule can also self-assemble into PHF. Thus, it appears that tau self-assembles by association of the microtubule binding domains/repeats and that the abnormal hyperphosphorylation promotes the self-assembly of tau into tangles of PHF and straight filaments by neutralizing the inhibitory basic charges of the flanking regions.

The kinase DYRK phosphorylates protein-synthesis initiation factor eIF2Bepsilon at Ser539 and the microtubule-associated protein tau at Thr212: potential role for DYRK as a glycogen synthase kinase 3-priming kinase

The substrate specificity of glycogen synthase kinase 3 (GSK3) is unusual in that efficient phosphorylation only occurs if another phosphoserine or phosphothreonine residue is already present four residues C-terminal to the site of GSK3 phosphorylation. One such substrate is the epsilon-subunit of rat eukaryotic protein-synthesis initiation factor 2B (eIF2Bepsilon), which is inhibited by the GSK3-catalysed phosphorylation of Ser(535). There is evidence that GSK3 is only able to phosphorylate eIF2Bepsilon at Ser(535) if Ser(539) is already phosphorylated by another protein kinase. However, no protein kinases capable of phosphorylating Ser(539) have so far been identified. Here we show that Ser(539) of eIF2Bepsilon, which is followed by proline, is phosphorylated specifically by two isoforms of dual-specificity tyrosine phosphorylated and regulated kinase (DYRK2 and DYRK1A), but only weakly or not at all by other 'proline-directed' protein kinases tested. We also establish that phosphorylation of Ser(539) permits GSK3 to phosphorylate Ser(535) in vitro and that eIF2Bepsilon is highly phosphorylated at Ser(539) in vivo. The DYRK isoforms also phosphorylate human microtubule-associated protein tau at Thr(212) in vitro, a residue that is phosphorylated in foetal tau and hyperphosphorylated in filamentous tau from Alzheimer's-disease brain. Phosphorylation of Thr(212) primes tau for phosphorylation by GSK3 at Ser(208) in vitro, suggesting a more general role for DYRK isoforms in priming phosphorylation of GSK3 substrates.

PP2A mRNA expression is quantitatively decreased in Alzheimer's disease hippocampus

Since abnormal tau phosphorylation may play a role in neurofibrillary tangle (NFT) formation in aging and Alzheimer's disease (AD), we probed the distribution and abundance of protein phosphatase 2A (PP2A) catalytic (Calpha) and regulatory (PR55alpha and gamma, PR61varepsilon and delta) subunit mRNA in control and AD hippocampus using in situ hybridization. Quantitation of grain density per neuron area of PP2A subunits and beta-actin was determined for the CA3 region of hippocampus and cerebellum, while a qualitative assessment was performed for CA1, CA4, and dentate gyrus. All subunits are expressed in neurons, while PR55gamma and PR55alpha mRNA are also evident in glia. The expression levels of Calpha, all PP2A regulatory subunits studied, and beta-actin were similar in control and AD cerebellum. beta-Actin mRNA was, however, reduced in AD hippocampus. In addition to the generalized reduction of mRNA, as indicated by decreased beta-actin signal, there was a significant loss of Calpha, PR55gamma, and PR61epsilon mRNA in the CA3 hippocampus of AD. This study delineates the distribution of critical PP2A mRNAs and reveals a neuron- and subunit-specific reduction in PP2A catalytic and regulatory mRNA in AD hippocampus. This could result in decreased protein expression and phosphatase activity, leading to the hyperphosphorylation of tau and the formation of NFTs, as well as neuron degeneration in AD.

Phosphorylation of tau is regulated by PKN

For the phosphorylation state of microtubule-associated protein, tau plays a pivotal role in regulating microtubule networks in neurons. Tau promotes the assembly and stabilization of microtubules. The potential for tau to bind to microtubules is down-regulated after local phosphorylation. When we investigated the effects of PKN activation on tau phosphorylation, we found that PKN triggers disruption of the microtubule array both in vitro and in vivo and predominantly phosphorylates tau in microtubule binding domains (MBDs). PKN has a catalytic domain highly homologous to protein kinase C (PKC), a kinase that phosphorylates Ser-313 (= Ser-324, the number used in this study) in MBDs. Thus, we identified the phosphorylation sites of PKN and PKC subtypes (PKC-alpha, -betaI, -betaII, -gamma, -delta, -epsilon, -zeta, and -lambda) in MBDs. PKN phosphorylates Ser-258, Ser-320, and Ser-352, although all PKC subtypes phosphorylate Ser-258, Ser-293, Ser-324, and Ser-352. There is a PKN-specific phosphorylation site, Ser-320, in MBDs. HIA3, a novel phosphorylation-dependent antibody recognizing phosphorylated tau at Ser-320, showed immunoreactivity in Chinese hamster ovary cells expressing tau and the active form of PKN, but not in Chinese hamster ovary cells expressing tau and the inactive form of PKN. The immunoreactivity for phosphorylated tau at Ser-320 increased in the presence of a phosphatase inhibitor, FK506 treatment, which means that calcineurin (protein phosphatase 2B) may be involved in dephosphorylating tau at Ser-320 site. We also noted that PKN reduces the phosphorylation recognized by the phosphorylation-dependent antibodies AT8, AT180, and AT270 in vivo. Thus PKN serves as a regulator of microtubules by specific phosphorylation of tau, which leads to disruption of tubulin assembly.

Site-specific phosphorylation of tau accompanied by activation of mitogen-activated protein kinase (MAPK) in brains of Niemann-Pick type C mice

Niemann-Pick type C (NPC) disease is characterized by an accumulation of cholesterol in most tissues and progressive neurodegeneration with the formation of neurofibrillary tangles. Neurofibrillary tangles are composed of paired helical filaments (PHF), a major component of which is the hyperphosphorylated tau. In this study we used NPC heterozygous and NPC homozygous mouse brains to investigate the molecular mechanism responsible for tauopathy in NPC. Immunoblot analysis using anti-tau antibodies (Tau-1, PHF-1, AT-180, and AT-100) revealed site-specific phosphorylation of tau at Ser-396 and Ser-404 in the brains of NPC homozygous mice. Mitogen-activated protein kinase, a potential serine kinase known to phosphorylate tau, was activated, whereas other serine kinases such as glycogen synthase kinase-3beta and cyclin-dependent kinase 5 were inactive. Morphological examination demonstrated that a number of neurons, the perikarya of which strongly immunostained with PHF-1, exhibited polymorphorous cytoplasmic inclusion bodies and multi-concentric lamellar-like bodies. Importantly, the accumulation of intracellular cholesterol in NPC mouse brains was determined to be a function of age. From these results we conclude that abnormal cholesterol metabolism due to the genetic mutation in NPC1 may be responsible for activation of the mitogen-activated protein kinase-signaling pathway and site-specific phosphorylation of tau in vivo, leading to tauopathy in NPC.

Tau-tubulin kinase phosphorylates tau at Ser-208 and Ser-210, sites found in paired helical filament-tau

Hyperphosphorylated tau protein is known to be a major component of the paired helical filaments (PHFs) that accumulate in the brain of Alzheimer's patients. The kinase that phosphorylated Ser-208 and Ser-210 in PHF-tau had remained unknown. We used anti-pS208 and anti-pS210 antibodies and Western blots to confirm that the tau-tubulin kinase (TTK) phosphorylates tau at Ser-208 and at Ser-210. Using partial amino acid sequences of purified bovine brain TTK, a mouse cDNA of TTK was isolated and the sequence was determined. Its 963 bp coding region is composed of 320 amino acids and encodes a 36 kDa protein indistinguishable in size from authentic bovine brain TTK. Our immunoblot analysis demonstrated that TTK is ubiquitously distributed in the rat tissues, and that it is developmentally regulated in the rat brain.

From genetics to pathology: tau and alpha-synuclein assemblies in neurodegenerative diseases

The most common degenerative diseases of the human brain are characterized by the presence of abnormal filamentous inclusions in affected nerve cells and glial cells. These diseases can be grouped into two classes, based on the identity of the major proteinaceous components of the filamentous assemblies. The filaments are made of either the microtubule-associated protein tau or the protein alpha-synuclein. Importantly, the discovery of mutations in the tau gene in familial forms of frontotemporal dementia and of mutations in the alpha-synuclein gene in familial forms of Parkinson's disease has established that dysfunction of tau protein and alpha-synuclein can cause neurodegeneration.

Molecular analysis of mutant and wild-type tau deposited in the brain affected by the FTDP-17 R406W mutation

Frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17) is a familial neurological disorder, characterized genetically by autosomal dominant inheritance, clinically by behavioral abnormalities and parkinsonism, and neuropathologically by tauopathy. Linkage analyses of affected families have led to identification of several exonic and intronic mutations in the tau gene. In this study, we analyzed molecular species of tau in the soluble and insoluble fractions of brain affected by the FTDP-17 R406W mutation. Protein chemical analysis and Western blotting using site-specific antibodies indicated that almost equal amounts of wild-type and mutant tau were present in the Sarkosyl-insoluble fraction of the R406W brain. Consistent with this, wild-type and mutant tau colocalized in neurofibrillary tangles in the frontal cortex and hippocampus of the R406W brain. In contrast to soluble R406W tau, which was less phosphorylated than soluble wild-type tau, the Sarkosyl-insoluble mutant tau was highly phosphorylated as well as the insoluble wild-type tau.

Calpain-mediated degradation of p35 to p25 in postmortem human and rat brains

Tau in Alzheimer neurofibrillary tangles has been shown to be hyperphosphorylated and CDK5, GSK3, MAP kinase and SAP kinases are the candidate kinases for the phosphorylation of tau. Recently, it was reported that the conversion of p35, the activator of CDK5, to p25 was upregulated in Alzheimer's disease (AD) brains, and that p35 is cleaved to yield p25 by calpain. Here we show that p35 is rapidly cleaved to p25 in rat and human brains within a short postmortem delay and that the conversion of p35 to p25 is partially dependent on calpain activity. Immunoblot analysis of brains prepared from patients with AD or age-matched control individuals with a short postmortem delay revealed no specific increase in the levels of p25 in AD brains, whereas the levels of active form of calpain were increased in AD brains compared to the those in controls. These observations suggest that the conversion of p35 to p25 is a postmortem degradation event and may not be upregulated in AD brains.

Glycogen synthase kinase-3beta phosphorylates protein tau and rescues the axonopathy in the central nervous system of human four-repeat tau transgenic mice

Protein tau filaments in brain of patients suffering from Alzheimer's disease, frontotemporal dementia, and other tauopathies consist of protein tau that is hyperphosphorylated. The responsible kinases operating in vivo in neurons still need to be identified. Here we demonstrate that glycogen synthase kinase-3beta (GSK-3beta) is an effective kinase for protein tau in cerebral neurons in vivo in adult GSK-3beta and GSK-3beta x human tau40 transgenic mice. Phosphorylated protein tau migrates slower during electrophoretic separation and is revealed by phosphorylation-dependent anti-tau antibodies in Western blot analysis. In addition, its capacity to bind to re-assembled paclitaxel (Taxol((R)))-stabilized microtubules is reduced, compared with protein tau isolated from mice not overexpressing GSK-3beta. Co-expression of GSK-3beta reduces the number of axonal dilations and alleviates the motoric impairment that was typical for single htau40 transgenic animals (Spittaels, K., Van den Haute, C., Van Dorpe, J., Bruynseels, K., Vandezande, K., Laenen, I., Geerts, H., Mercken, M., Sciot, R., Van Lommel, A., Loos, R., and Van Leuven, F. (1999) Am. J. Pathol. 155, 2153-2165). Although more hyperphosphorylated protein tau is available, neither an increase in insoluble protein tau aggregates nor the presence of paired helical filaments or tangles was observed. These findings could have therapeutic implications in the field of neurodegeneration, as discussed.

Phosphorylated, but not native, tau protein assembles following reaction with the lipid peroxidation product, 4-hydroxy-2-nonenal

A correlation between hyperphosphorylation of tau protein and its aberrant assembly into paired helical filaments has lead to suggestions that phosphorylation controls assembly, but lacked a mechanistic basic. In this work, we have found that phosphorylated, but not native, tau protein is able to form polymers after the reaction with 4-hydroxy-2-nonenal, a highly toxic product of lipid peroxidation. Phosphorylation of tau by both proline or non-proline directed kinases, was able to assemble it into polymers.

Tau protein is hyperphosphorylated in a site-specific manner in apoptotic neuronal PC12 cells

Alterations in the status of microtubules contribute to the cytoskeletal rearrangements that occur during apoptosis. The microtubule-associated protein tau regulates microtubule dynamics and thus is likely to play an important role in the cytoskeletal changes that occur in apoptotic cells. Previously, we demonstrated that the phosphorylation of tau at the Tau-1 epitope was increased during neuronal PC12 cell apoptosis, and further that the microtubule binding of tau from apoptotic cells was significantly impaired because of altered phosphorylation. The fact that the microtubule-binding capacity of tau from apoptotic cells was reduced to approximately 30% of control values indicated that sites in addition to those within the Tau-1 epitope were hyperphosphorylated during apoptosis. In this study using a combination of immunological and biochemical approaches, numerous sites were found to be hyperphosphorylated on tau isolated from apoptotic cells. Further, during apoptosis, the activities of cell division control protein kinase (cdc2) and cyclin-dependent kinase 5 (cdk5) were selectively and significantly increased. The association of these two protein kinases with tau was also increased during apoptosis. These findings are intriguing because many of the sites found to be hyperphosphorylated on tau during apoptosis are also hyperphosphorylated on tau from Alzheimer's disease brain. Likewise, there are data indicating that in Alzheimer's disease the activities of cdc2 and cdk5 are also increased.

Differential effects of apolipoprotein E isoforms on phosphorylation at specific sites on tau by glycogen synthase kinase-3 beta identified by nano-electrospray mass spectrometry

Previously published data have shown an allele-specific variation in the in vitro binding of apolipoprotein E (apoE) to tau, which prompted the hypothesis that apoE binding may protect tau from phosphorylation, apoE3 being more efficient than apoE4. We have, therefore, investigated the effects of apoE on tau phosphorylation in vitro by the proline-directed kinase, glycogen synthase kinase (GSK)-3 beta. The phosphopeptide maps of tau alone, of tau with apoE3 and of tau with apoE4 were very similar. When apoE2 was present a further four spots were evident. Additionally, of the 15 peptides phosphorylated in the presence or absence of apoE, subtle differences, some isoform-specific, in the relative amounts of phosphorylation were observed.

The natural osmolyte trimethylamine N-oxide (TMAO) restores the ability of mutant tau to promote microtubule assembly

Coding region and intronic mutations in the gene for microtubule-associated protein tau cause frontotemporal dementia and Parkinsonism linked to chromosome 17 (FTDP-17). Most coding region mutations effect a reduced ability of tau protein to interact with microtubules and lead to the formation of a filamentous pathology made of hyperphosphorylated tau. Here we show that trimethylamine N-oxide (TMAO) restores the ability of tau with FTDP-17 mutations to promote microtubule assembly. To mimic phosphorylation, serine and threonine residues in tau were singly or multiply mutated to glutamic acid, resulting in a reduced ability of tau to promote microtubule assembly. With the exception of the most heavily substituted protein (27 glutamic acid residues), TMAO increased the ability of mutant tau to promote microtubule assembly. However, it had no significant effect on heparin-induced assembly of tau into filaments.

Microtubule-affinity regulating kinase (MARK) is tightly associated with neurofibrillary tangles in Alzheimer brain: a fluorescence resonance energy transfer study

Paired helical filaments, the main structural components of the neurofibrillary tangles in Alzheimer disease, consist of phosphorylated tau protein. Because the levels and degree of phosphorylation are significantly higher in paired helical filament (PHF)-derived tau than in normal adult tau, and because phosphorylation of tau severely disrupts microtubule stability, it is postulated that tau phosphorylation is an important step in PHF formation. The kinases and/or phosphatases that act in vivo to help induce such a pathological state of tau, however, are not yet known. In this study we implicate the non-proline directed kinase MARK in PHF-tau phosphorylation, by virtue of its close intermolecular association with the phosphorylated Ser262 epitope on PHF-tau as assessed by fluorescence resonance energy transfer. Moreover, because this tight enzyme-substrate association is observed in neurofibrillary tangles in Alzheimer tissue, we suggest that PHF-tau phosphorylation may occur to some extent on assembled PHF filaments.

Structural and functional implications of tau hyperphosphorylation: information from phosphorylation-mimicking mutated tau proteins

Abnormal tau-immunoreactive filaments are a hallmark of tauopathies, including Alzheimer's disease (AD). A higher phosphorylation ("hyperphosphorylation") state of tau protein may represent a critical event. To determine the potential role of tau hyperphosphorylation in these disorders, mutated tau proteins were produced where serine/threonine residues known to be highly phosphorylated in tau filaments isolated from AD patients were substituted for glutamate to simulate a paired helical filament (PHF)-like tau hyperphosphorylation. We demonstrate that, like hyperphosphorylation, glutamate substitutions induce compact structure elements and SDS-resistant conformational domains in tau protein. Hyperphosphorylation-mimicking glutamate-mutated tau proteins display a complete functional loss in its ability to promote microtubule nucleation which can partially be overcome by addition of the osmolyte trimethylamine N-oxide (TMAO), which is similar to phosphorylated tau. In addition, glutamate-mutated tau proteins fail to interact with the dominant brain protein phosphatase 2A isoform ABalphaC, and exhibit a reduced ability to assemble into filaments. Interestingly, wild-type tau and phosphorylation-mimicking tau similarly bind to microtubules when added alone, but the mutated tau is almost completely displaced from the microtubule surface by equimolar concentrations of wild-type tau. The data indicate that glutamate-mutated tau proteins provide a useful model for analyzing the functional consequences of tau hyperphosphorylation. They suggest that several mechanisms contribute to the abnormal tau accumulation observed during tauopathies, in particular a selective displacement of hyperphosphorylated tau from microtubules, a functional loss in promoting microtubule nucleation, and a failure to interact with phosphatases.

Significance of tau phosphorylation and protein kinase regulation in the pathogenesis of Alzheimer disease

The role of the phosphatidylinositol-3 kinase pathway in the hyperphosphorylation of tau protein was investigated in cultured cells. Human kidney 293T-cells were cotransfected with tau and glycogen synthase kinase-3 (GSK-3) genes or tau and protein kinase B genes. The phosphorylation of tau protein was increased by cotransfection with GSK-3; however, it was decreased by cotransfection with protein kinase B. Human neuroblastoma SY5Y cells were treated with wortmannin, an inhibitor of phosphatidylinositol-3 kinase, and only transient (after 1 hour) activation of GSK-3 and hyperphosphorylation of tau protein were observed. However, continuous inactivation of protein kinase B was observed, suggesting the involvement of protein kinases other than protein kinase B in the phosphorylation and inactivation of GSK-3 after 3 hours. In cells treated with wortmannin, protein kinase C delta fragments were observed, and the protein kinase C activity increased after 3 hours, whereas treatment of cells with z-DEVD-fmk, an inhibitor of caspase-3, inhibited fragmentation of protein kinase C delta and induced continuous activation of GSK-3. It is suggested that fragmentation of protein kinase C delta during the process of apoptosis results in the phosphorylation and the inactivation of GSK-3. Those data suggest that, in Alzheimer disease, more complicated mechanisms are involved in the process of phosphorylation of tau protein predominantly regulated by P13K pathway.

Synthesis of a triply phosphorylated pentapeptide from human tau-protein

Two different strategies for the synthesis of a triply phosphorylated pentapeptide are described. In both cases a monophosphorylated selectively N-deprotected tripeptide is employed as C-terminal fragment. Coupling of this building block with a C-terminally unmasked bis-phosphorylated seryl-dipeptide unexpectedly failed due to decomposition of this peptide upon activation with different coupling reagents. Instead stepwise N-terminal elongation of the peptide chain with serine derivatives and subsequent O-phosphorylation of the serine OH-groups was successful. These results indicate that assembly of multiply phosphorylated peptides from preformed multiply phosphorylated phosphopeptide building blocks in general may be problematic and that a stepwise elongation of the amino acid chain may be preferable.

Isoaspartate formation and neurodegeneration in Alzheimer's disease

We reviewed here that protein isomerization is enhanced in amyloid-beta peptides (Abeta) and paired helical filaments (PHFs) purified from Alzheimer's disease (AD) brains. Biochemical analyses revealed that Abeta purified from senile plaques and vascular amyloid are isomerized at Asp-1 and Asp-7. A specific antibody recognizing isoAsp-23 of Abeta further suggested the isomerization of Abeta at Asp-23 in vascular amyloid as well as in the core of senile plaques. Biochemical analyses of purified PHFs also revealed that heterogeneous molecular weight tau contains L-isoaspartate at Asp-193, Asn-381, and Asp-387, indicating a modification, other than phosphorylation, that differentiates between normal tau and PHF tau. Since protein isomerization as L-isoaspartate causes structural changes and functional inactivation, or enhances the aggregation process, this modification is proposed as one of the progression factors in AD. Protein L-isoaspartyl methyltransferase (PIMT) is suggested to play a role in the repair of isomerized proteins containing L-isoaspartate. We show here that PIMT is upregulated in neurodegenerative neurons and colocalizes in neurofibrillary tangles (NFTs) in AD. Taken together with the enhanced protein isomerization in AD brains, it is implicated that the upregulated PIMT may associate with increased protein isomerization in AD. We also reviewed studies on PIMT-deficient mice that confirmed that PIMT plays a physiological role in the repair of isomerized proteins containing L-isoaspartate. The knockout study also suggested that the brain of PIMT-deficient mice manifested neurodegenerative changes concomitant with accumulation of L-isoaspartate. We discuss the pathological implications of protein isomerization in the neurodegeneration found in model mice and AD.

14-3-3zeta is an effector of tau protein phosphorylation

Neurofibrillary tangles associated with Alzheimer's disease are composed mainly of paired helical filaments that are formed by the aggregation of abnormally phosphorylated microtubule-associated protein tau. 14-3-3, a highly conserved protein family that exists as seven isoforms and regulates diverse cellular processes is present in neurofibrillary tangles (Layfield, R., Fergusson, J., Aitken, A., Lowe, J., Landon, M., Mayer, R. J. (1996) Neurosci. Lett. 209, 57-60). The role of 14-3-3 in Alzheimer's disease pathogenesis is not known. In this study, we found that the 14-3-3zeta isoform is associated with tau in brain extract and profoundly stimulates cAMP-dependent protein kinase catalyzed in vitro phosphorylation on Ser(262)/Ser(356) located within the microtubule-binding region of tau. 14-3-3zeta binds to both phosphorylated and nonphosphorylated tau, and the binding site is located within the microtubule-binding region of tau. From brain extract, 14-3-3zeta co-purifies with microtubules, and tubulin blocks 14-3-3zeta-tau binding. Among four 14-3-3 isoforms tested, beta and zeta but not gamma and epsilon associate with tau. Our data suggest that 14-3-3zeta is a tau protein effector and may be involved in the abnormal tau phosphorylation occurring during Alzheimer's disease ontogeny.

Tau protein isoforms, phosphorylation and role in neurodegenerative disorders

Tau proteins belong to the family of microtubule-associated proteins. They are mainly expressed in neurons where they play an important role in the assembly of tubulin monomers into microtubules to constitute the neuronal microtubules network. Microtubules are involved in maintaining the cell shape and serve as tracks for axonal transport. Tau proteins also establish some links between microtubules and other cytoskeletal elements or proteins. Tau proteins are translated from a single gene located on chromosome 17. Their expression is developmentally regulated by an alternative splicing mechanism and six different isoforms exist in the human adult brain. Tau proteins are the major constituents of intraneuronal and glial fibrillar lesions described in Alzheimer's disease and numerous neurodegenerative disorders referred to as 'tauopathies'. Molecular analysis has revealed that an abnormal phosphorylation might be one of the important events in the process leading to their aggregation. Moreover, a specific set of pathological tau proteins exhibiting a typical biochemical pattern, and a different regional and laminar distribution could characterize each of these disorders. Finally, a direct correlation has been established between the progressive involvement of the neocortical areas and the increasing severity of dementia, suggesting that pathological tau proteins are reliable marker of the neurodegenerative process. The recent discovery of tau gene mutations in frontotemporal dementia with parkinsonism linked to chromosome 17 has reinforced the predominant role attributed to tau proteins in the pathogenesis of neurodegenerative disorders, and underlined the fact that distinct sets of tau isoforms expressed in different neuronal populations could lead to different pathologies.

Structure of tau protein and assembly into paired helical filaments

Over the past few years the systematic investigation of paired helical filament assembly from tau protein in vitro has become feasible. We review our current understanding of the structure and conformations of tau protein and how this affects tau's assembly into the pathological paired helical filaments in Alzheimer's disease.

Age-related amyloid beta deposition in transgenic mice overexpressing both Alzheimer mutant presenilin 1 and amyloid beta precursor protein Swedish mutant is not associated with global neuronal loss

To analyze the relationship between the deposition of amyloid beta peptides (Abeta) and neuronal loss in transgenic models of Alzheimer's disease (AD), we examined the frontal neocortex (Fc) and CA1 portion of hippocampus (CA1) in PSAPP mice doubly expressing AD-associated mutant presenilin 1 (PS1) and Swedish-type mutant beta amyloid precursor protein (APPsw) by morphometry of Abeta burden and neuronal counts. Deposition of Abeta was detected as early as 3 months of age in the Fc and CA1 of PSAPP mice and progressed to cover 28.3% of the superior frontal cortex and 18.4% of CA1 at 12 months: approximately 20- (Fc) and approximately 40- (CA1) fold greater deposition than in APPsw mice. There was no significant difference in neuronal counts in either CA1 or the frontal cortex between nontransgenic (non-tg), PS1 transgenic, APPsw, and PSAPP mice at 3 to 12 months of age. In the PSAPP mice, there was disorganization of the neuronal architecture by compact amyloid plaques, and the average number of neurons was 8 to 10% fewer than the other groups (NS, P > 0.10) in CA1 and 2 to 20% fewer in frontal cortex (NS, P = 0.31). There was no loss of total synaptophysin immunoreactivity in the Fc or dentate gyrus molecular layer of the 12-month-old PSAPP mice. Thus, although co-expression of mutant PS1 with Swedish mutant betaAPP leads to marked cortical and limbic Abeta deposition in an age-dependent manner, it does not result in the dramatic neuronal loss in hippocampus and association cortex characteristic of AD.

Tau aggregation into fibrillar polymers: taupathies

Different neurological disorders, known as taupathies have been recently described. In these disorders it has been suggested that modifications in the microtubule-associated protein tau could cause neural degeneration in specific regions. Although these regions are different in the different taupathies, some common features appear to occur in all of them: abnormal hyperphosphorylation of tau and aberrant tau aggregation. These two features are commented upon in this review.

Interaction of neuronal Cdc2-like protein kinase with microtubule-associated protein tau

Neuronal Cdc2-like protein kinase (NCLK), a approximately 58-kDa heterodimer, was isolated from neuronal microtubules (Ishiguro, K., Takamatsu, M., Tomizawa, K., Omori, A., Takahashi, M., Arioka, M., Uchida, T. and Imahori, K. (1992) J. Biol. Chem. 267, 10897-10901). The biochemical nature of NCLK-microtubule association is not known. In this study we found that NCLK is released from microtubules upon microtubule disassembly as a 450-kDa species. The 450-kDa species is an NCLK.tau complex, and NCLK-bound tau is in a nonphosphorylated state. Tau phosphorylation causes NCLK.tau complex dissociation, and phosphorylated tau does not bind to NCLK. In vitro, the Cdk5 subunit of NCLK binds to the microtubule-binding region of tau and NCLK associates with microtubules only in the presence of tau. Our data indicate that in brain extract NCLK is complexed with tau in a tau phosphorylation-dependent manner and that tau anchors NCLK to microtubules. Recently NCLK has been suggested to be aberrantly activated and to hyperphosphorylate tau in Alzheimer's disease brain (Patrick, G. N., Zukerberg, L., Nikolic, M., de la Monte, S., Dikkes, P, and Tsai, L.-H. (1999) Nature 402, 615-622). Our findings may explain why in Alzheimer's disease NCLK specifically hyperphosphorylates tau, although this kinase has a number of protein substrates in the brain.

Binding specificity of monoclonal antibody AD2: influence of the phosphorylation state of tau

Using recombinant human tau protein phosphorylated by a brain extract and the glycogen synthase kinase-3beta in the absence or the presence of heparin, we showed that phosphorylation-dependent antibody AD2 recognition only requires phosphorylated Ser-396. By the Spot multiple peptide synthesis method, we showed that Tyr-394, Ser(P)-396 and Pro-397 are critical for AD2 binding. A decrease in the binding of AD2 was observed with increasing phosphorylation of residues in the vicinity of Ser(P)-396.

Interaction of tau with the neural membrane cortex is regulated by phosphorylation at sites that are modified in paired helical filaments

The axonal microtubule-associated phosphoprotein tau interacts with neural plasma membrane (PM) components during neuronal development (Brandt, R., Léger, J., and Lee, G. (1995) J. Cell Biol. 131, 1327-1340). To analyze the mechanism and potential regulation of tau's PM association, a method was developed to isolate PM-associated tau using microsphere separation of surface-biotinylated cells. We show that tau's PM association requires an intact membrane cortex and that PM-associated tau and cytosolic tau are differentially phosphorylated at sites detected by several Alzheimer's disease (AD) diagnostic antibodies (Ser(199)/Ser(202), Thr(231), and Ser(396)/Ser(404)). In polar neurons, the association of endogenous tau phosphoisoforms with the membrane cortex correlates with an enrichment in the axonal compartment. To test for a direct effect of AD-specific tau modifications in determining tau's interactions, a phosphomutant that simulates an AD-like hyperphosphorylation of tau was produced by site-directed mutagenesis of Ser/Thr residues to negatively charged amino acids (Glu). These mutations completely abolish tau's association with the membrane cortex; however, the construct retains its capability to bind to microtubules. The data suggest that a loss of tau's association with the membrane cortex as a result of phosphorylation at sites that are modified during disease contributes to somatodendritic tau accumulation, axonal microtubule disintegration, and neuronal death characteristic for AD.

Phosphorylation sites on tau identified by nanoelectrospray mass spectrometry: differences in vitro between the mitogen-activated protein kinases ERK2, c-Jun N-terminal kinase and P38, and glycogen synthase kinase-3beta

The stress-activated kinases c-Jun N-terminal kinase (JNK) and p38 are members of the mitogen-activated protein (MAP) kinase family and take part in signalling cascades initiated by various forms of stress. Their targets include the microtubule-associated protein tau, which becomes hyperphosphorylated in Alzheimer's disease. It is necessary, as a forerunner for in vivo studies, to identify the protein kinases and phosphatases that are responsible for phosphate turnover at individual sites. Using nanoelectrospray mass spectrometry, we have undertaken an extensive comparison of phosphorylation in vitro by several candidate tau kinases, namely, JNK, p38, ERK2, and glycogen synthase kinase 3beta (GSK3beta). Between 10 and 15 sites were identified for each kinase. The three MAP kinases phosphorylated Ser202 and Thr205 but not detectably Ser199, whereas conversely GSK3beta phosphorylated Ser199 but not detectably Ser202 or Thr205. Phosphorylated Ser404 was found with all of these kinases except JNK. The MAP kinases may not be strictly proline specific: p38 phosphorylated the nonproline sites Ser185, Thr245, Ser305, and Ser356, whereas ERK2 was the most strict. All of the sites detected except Thr245 and Ser305 are known or suspected phosphorylation sites in paired helical filament-tau extracted from Alzheimer brains. Thus, the three MAP kinases and GSK3beta are importantly all strong candidates as tau kinases that may be involved in the pathogenic hyperphosphorylation of tau in Alzheimer's disease.

The formation of PHF-1 and SMI-31 positive dystrophic neurites in rat hippocampus following acute injection of okadaic acid

Within neurofibrillary tangles and dystrophic neurites of Alzheimer's disease (AD), tau protein is hyperphosphorylated. In the present study, we provide evidence that acute injection of okadaic acid (1 mM, 0.5 microliter) into the dorsal hippocampus induces the formation of paired helical filament (PHF)-1, sternberger monoclonals incorporated (SMI)-31, and amyloid precursor protein (APP) positive dystrophic neurites in the lacunosum-molecular layer of CA1 and molecular layer of dentate gyrus. Okadaic acid evoked a marked loss of microtubule associated protein (MAP)-2 immunoreactivity. PHF-1 immunoreactive terminals were fine, and SMI-31 immunoreactive terminals appeared at granular terminals and at the ring-like or elongated dystrophic neurites. APP positive dystrophic neurites exhibited large bulb-like globular terminals. Interestingly, APP dystrophic neurites were co-localized with SMI-31 immunoreactivity in the core. APP immunoreactivity became stronger over 24 h even in vehicle injected area. These results may provide the morphological evidence for the animal model to study dystrophic neurites formation of AD.

Inactivation of glycogen synthase kinase-3 by protein kinase C delta: implications for regulation of tau phosphorylation

The role of the phosphatidylinositol 3-kinase (PI3K) pathway in the hyperphosphorylation of tau was investigated in SY5Y human neuroblastoma cells. Wortmannin, an inhibitor of PI3K, induced transient (after 1 h) activation of glycogen synthase kinase-3 (GSK-3), hyperphosphorylation of tau and dose-dependent cytotoxicity. However, continuous inactivation of protein kinase (PK) B was observed from 1 to 24 h, suggesting the involvement of protein kinase(s) other than PKB in the phosphorylation and inactivation of GSK-3 after 3 h. In cells treated with wortmannin, PKC delta fragments were observed, and the PKC activity increased after 3 h, whereas treatment of cells with z-DEVD-fmk, an inhibitor of caspase 3, also inhibited fragmentation of PKC delta and induced continuous activation of GSK-3. It is suggested that fragmentation of PKC delta during the process of apoptosis results in the phosphorylation and inactivation of GSK-3 and consequently inhibition of the phosphorylation of tau.

Distribution of tau protein kinase I/glycogen synthase kinase-3beta, phosphatases 2A and 2B, and phosphorylated tau in the developing rat brain

When trying to elucidate the role played by tau protein kinase I/glycogen synthase kinase-3beta (TPKI/GSK-3beta) in tau phosphorylation, it is important to consider the balance that exists between the various kinases and phosphatases that are involved in vivo. We studied developmental changes in the expressions of TPKI/GSK-3beta and phosphatases 2A and 2B in rat brains using immunoblot analysis. The expression of the kinase peaked postnatally at days 8-11 and returned then to low level after 5 weeks. Phosphatase 2A showed a similar pattern, increasing postnatally until day 14 and decreasing thereafter. On the other hand, phosphatase 2B was undetectable at the juvenile stage, but later its presence increased rapidly to peak at 5 weeks after birth, after which it was maintained at high levels throughout the adult stage. Immunohistochemical studies using the PAP method revealed details of the distribution of TPKI/GSK-3beta. At postnatal days 3-21 both gray and white matter were immunoreactive. Later, after 5 weeks, the immunoreactivity became more restricted to the gray matter. The staining of tau phosphorylated at Ser 199, Ser 396, and Ser 413 followed mostly the pattern of the kinase distribution throughout all stages of development. These data, therefore, confirm that TPKI/GSK-3beta is expressed primarily in neurons and especially in neurites until postnatal day 21, whereafter the distribution is concentrated mostly in the cell soma and the proximal neurite region.

Conformation of paired helical filaments blocks dephosphorylation of epitopes shared with fetal tau except Ser199/202 and Ser202/Thr205

To determine if the high phosphate content of paired helical filaments (PHFs) in Alzheimer's disease (AD) is a result of limited access to filament phosphorylation sites, we studied in vitro dephosphorylation of intact PHFs, PHFs with filamentous structure abolished by formic acid treatment (PHF(FA)) and fetal human tau protein. Samples were treated with alkaline phosphatase for up to 24 h at 37 degrees C and then immunoblotted with eight well characterized tau antibodies, that recognize two phosphorylation-insensitive sites and six phosphorylation-sensitive epitopes at Thr181, Ser199/202, Ser202/Thr205, Thr231, Ser262/356 and Ser396/404. Intact PHFs were effectively dephosphorylated only at the two N-terminal epitopes Ser199/202 and Ser202/Thr205, with little change in electrophoretic mobility. In contrast, PHF(FA) were dephosphorylated at all epitopes, with particular effectiveness at those in the C-terminus and with significant increase in electrophoretic mobility. The fetal tau epitopes were effectively dephosphorylated except at Thr181 and Thr231 with marked increase in mobility. The extent of dephosphorylation of PHF(FA) was equal or more effective than in fetal tau, except for Thr181 that was minimally dephosphorylated in both proteins. The results indicate that intact PHFs, but not PHF(FA) or fetal tau display differential dephosphorylation of the N- and C-terminal epitopes. The results confirm that the filamentous conformation may significantly contribute to hyperphosphorylation of PHFs in the C-terminus. The filamentous conformation, however, does not limit access to two N-terminal epitopes Ser199/202 and Ser202/Thr205. The access to these sites in AD may be limited by other factors, e.g., inhibition of phosphatase binding.

Tau dephosphorylation at tau-1 site correlates with its association to cell membrane

It has been considered that tau protein is mainly a cytoplasmic protein since it is a microtubule associated protein. However, it has also been suggested that tau could be located in the cell nucleus and membrane. In our work, the cellular distribution of tau has been studied by immunofluorescence and western blot analysis, after subcellular fractionation in neuroblastoma cells and in tau-transfected non neural cells using, mainly, two types of tau antibodies; antibody 7.51 (that recognizes tau independent of its phosphorylation level); and antibody Tau-1 (that recognizes tau only in its dephosphorylated form). Also, tau was expressed in COS-1 cells to test for the features involved in the sorting of tau to different cell localizations. Our results show that tau associated to cell membrane has a lower phosphorylation level in its proline-rich region. Additionally, in differentiated neuroblastoma cells, tau phosphorylation, at that region, decreases and the amount of tau associated to cell membrane increases.

The microtubule binding of Tau and high molecular weight Tau in apoptotic PC12 cells is impaired because of altered phosphorylation

Although the importance of the microtubule network throughout cell life is well established, the dynamics of microtubules during apoptosis, a regulated cell death process, is unclear. In a previous study (Davis, P. K., and Johnson, G. V. (1999) Biochem. J. 340, 51-58) we demonstrated that the phosphorylation of the microtubule-associated protein tau was increased during neuronal PC12 cell apoptosis. The purpose of this study was to determine whether the increased tau phosphorylation that occurred during apoptosis impaired the microtubule binding capacity of tau. This study is the first demonstration that microtubule-binding by tau and high molecular weight tau is significantly impaired as a result of altered phosphorylation during a naturally occurring process, apoptosis. Furthermore, co-immunofluorescence studies reveal for the first time that tau populations within an apoptotic neuronal PC12 cell exhibit differential phosphorylation. In control PC12 cells, Tau-1 staining (Tau-1 recognizes an unphosphorylated epitope) is evident throughout the entire cell body. In contrast, Tau-1 immunoreactivity in apoptotic PC12 cells is retained in the nuclear/perinuclear region but is significantly decreased in the cytoplasm up to the plasma membrane. The selective distribution of phosphorylated tau in apoptotic PC12 cells indicates that tau likely plays a significant role in the cytoskeletal changes that occur during apoptosis.

Starvation induces tau hyperphosphorylation in mouse brain: implications for Alzheimer's disease

Hyperphosphorylated tau is the major component of paired helical filaments in neurofibrillary tangles found in Alzheimer's disease brains, and tau hyperphosphorylation is thought to be a critical event in the pathogenesis of this disease. The objective of this study was to reproduce tau hyperphosphorylation in an animal model by inducing hypoglycemia. Food deprivation of mice for 1 to 3 days progressively enhanced tau hyperphosphorylation in the hippocampus, to a lesser extent in the cerebral cortex, but the effect was least in the cerebellum, in correspondence with the regional selectivity of tauopathy in Alzheimer's disease. This hyperphosphorylation was reversible by refeeding for 1 day. We discuss possible mechanisms of this phenomenon, and propose the starved mouse as a simple model to study in vivo tau phosphorylation and dephosphorylation which are altered in Alzheimer's disease.

Phosphorylation of MAP2c and MAP4 by MARK kinases leads to the destabilization of microtubules in cells

Microtubules serve as transport tracks in molecular mechanisms governing cellular shape and polarity. Rapid transitions between stable and dynamic microtubules are regulated by several factors, including microtubule-associated proteins (MAPs). We have shown that MAP/microtubule affinity regulating kinases (MARK) can phosphorylate the microtubule-associated-proteins MAP4, MAP2c, and tau on their microtubule-binding domain in vitro. This leads to their detachment from microtubules (MT) and an increased dynamic instability of MT. Here we show that MARK protein kinases phosphorylate MAP2 and MAP4 on their microtubule-binding domain in transfected CHO cells. In CHO cells expressing MARK1 or MARK2 under control of an inducible promoter, MARK2 phosphorylates an endogenous MAP4-related protein. Prolonged expression of MARK2 results in microtubule-disruption, detachment of cells from the substratum, and cell death. Concomitant with microtubule disruption, we also observed a breakdown of the vimentin network, whereas actin fibers remained unaffected. Thus, MARK seems to play an important role in controlling cytoskeletal dynamics.

Sequencing of peptides phosphorylated on serines and threonines by post-source decay in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

In the era of complete genome sequences, biochemical and medical research will focus more on the dynamic proteome of a cell. Regulation of proteins by post-translational modifications, which are not determined by the gene sequence, are already intensively studied. One example is phosphorylation of serines and threonines, probably the single most common cellular regulatory mechanism. In this paper we describe the sequencing of mono- and bisphosphorylated peptides, including identification of the phosphorylation sites, by post-source decay (PSD) in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. In addition to dephosphorylation of the parent ions, we studied the influence of the phosphate group on the fragmentation of peptides. Generally, peptides phosphorylated on serine and threonine residues displayed no difference in their fragmentation patterns. The intensities of the resulting fragment ion signals depend only on the peptide sequence and not on either the phosphorylated amino acid or its position in the peptide chain. Phosphorylation increased the bond cleavage C-terminal to the phosphorylation site more than 10-fold, resulting in abundant signals, which typically dominated the PSD spectra. The produced C-terminally phosphorylated b-type fragment ions showed characteristic dephosphorylated fragment ions b(n) -H(3)PO(4) (-98 Da) and b(n) -HPO(3) (-80 Da) of higher abundances than the phosphorylated fragment ion. As a second layer to identify the phosphorylation site, all internally phosphorylated fragment ions were accompanied by minor, but always detectable, signals of the dephosphorylated fragment ions. Interpretation of PSD spectra of phosphopeptides was not more complicated than for unphosphorylated peptides, despite the increased number of obtained fragment ion signals.

Comparative biochemistry of tau in progressive supranuclear palsy, corticobasal degeneration, FTDP-17 and Pick's disease

Neurodegenerative disorders referred to as tauopathies have cellular hyperphosphorylated tau protein aggregates in the absence of amyloid deposits. Comparative biochemistry of tau aggregates shows that they differ in both phosphorylation and content of tau isoforms. The six tau isoforms found in human brain contain either three (3R) or four microtubule-binding domains (4R). In Alzheimer's disease, all six tau isoforms are abnormally phosphorylated and aggregate into paired helical filaments. They are detected by immunoblotting as a major tau triplet (tau55, 64 and 69). In corticobasal degeneration and progressive supranuclear palsy, only 4R-tau isoforms aggregate into twisted and straight filaments respectively. They appear as a major tau doublet (tau64 and 69). Finally, in Pick's disease, only 3R-tau isoforms aggregate into random coiled filaments. They are characterized by another major tau doublet (tau55 and 64). These differences in tau isoforms may be related to either the degeneration of particular cell populations in a given disorder or aberrant cell trafficking of particular tau isoforms. Finally, recent findings provide a direct link between a genetic defect in tau and its abnormal aggregation into filaments in fronto-temporal dementia with Parkinsonism linked to chromosome 17, demonstrating that tau aggregation is sufficient for nerve cell degeneration. Thus, tau mutations and polymorphisms may also be instrumental in many neurodegenerative disorders.

Molecular interactions among protein phosphatase 2A, tau, and microtubules. Implications for the regulation of tau phosphorylation and the development of tauopathies

Hyperphosphorylated forms of the neuronal microtubule (MT)-associated protein tau are major components of Alzheimer's disease paired helical filaments. Previously, we reported that ABalphaC, the dominant brain isoform of protein phosphatase 2A (PP2A), is localized on MTs, binds directly to tau, and is a major tau phosphatase in cells. We now describe direct interactions among tau, PP2A, and MTs at the submolecular level. Using tau deletion mutants, we found that ABalphaC binds a domain on tau that is indistinguishable from its MT-binding domain. ABalphaC binds directly to MTs through a site that encompasses its catalytic subunit and is distinct from its binding site for tau, and ABalphaC and tau bind to different domains on MTs. Specific PP2A isoforms bind to MTs with distinct affinities in vitro, and these interactions differentially inhibit the ability of PP2A to dephosphorylate various substrates, including tau and tubulin. Finally, tubulin assembly decreases PP2A activity in vitro, suggesting that PP2A activity can be modulated by MT dynamics in vivo. Taken together, these findings indicate how structural interactions among ABalphaC, tau, and MTs might control the phosphorylation state of tau. Disruption of these normal interactions could contribute significantly to development of tauopathies such as Alzheimer's disease.

Interaction of aluminum with PHFtau in Alzheimer's disease neurofibrillary degeneration evidenced by desferrioxamine-assisted chelating autoclave method

To demonstrate that aluminum III (Al) interacts with PHFtau in neurofibrillary degeneration (NFD) of Alzheimer's disease (AD) brain, we developed a "chelating autoclave method" that allows Al chelation by using trivalent-cationic chelator desferrioxamine. Its application to AD brain sections before Morin histochemistry for Al attenuated the positive fluorescence of neurofibrillary tangles, indicating Al removal from them. This method, applied for immunostaining with phosphorylation-dependent anti-tau antibodies, significantly enhanced the PHFtau immunoreactivity of the NFD. These results suggest that each of the phosphorylated epitopes in PHFtau are partially masked by Al binding. Incubation of AD sections with AlCl(3) before Morin staining revealed Al accumulation with association to neurofibrillary tangles. Such incubation before immunostaining with the phosphorylation-dependent anti-tau antibodies abolished the immunolabeling of the NFD and this abolition was reversed by the Al chelation. These findings indicate cumulative Al binding to and thereby antigenic masking of the phosphorylated epitopes of PHFtau. Al binding was further documented for electrophoretically-resolved PHFtau on immunoblots, indicating direct Al binding to PHFtau. In vitro aggregation by AlCl(3) was observed for PHFtau but was lost on dephosphorylation of PHFtau. Taken together, phosphorylation-dependent and direct PHFtau-Al interaction occurs in the NFD of the AD brain.

Sequence of neurodegeneration and accumulation of phosphorylated tau in cultured neurons after okadaic acid treatment

Within neurofibrillary tangles and dystrophic neurites of Alzheimer's disease (AD), the cytoskeletal protein tau is abnormally hyperphosphorylated. In the present study, we examined the effect of okadaic acid (OA), a protein phosphatase inhibitor, in rat cultured neurons. Low concentrations of OA induce degeneration of neurites, rounding of cell bodies, detachment from the substratum, and eventual neuronal death. During OA-induced degeneration, SMI-31 immunoreactivity became punctate in neurites at 6 h after OA treatment, and over time, accumulated in cell bodies and dystrophic neurites. Hyperphosphorylation of tau and marked loss of MAP-2-positive dendrites occurred after 6 h of treatment with OA. Thereafter, AT-8 and PHF-1 immunoreactivity accumulated in cell bodies and subsequently appeared in distal axon-like neurites. These results demonstrate that OA treatment induced hyperphosphorylation of tau and preferential dendritic damage, with subsequent accumulation of phosphorylated tau in cell bodies and dystrophic axon-like neurites. OA-induced neurodegeneration may provide a useful model to study AD.

Phosphorylated tau can promote tubulin assembly

Phosphorylation can affect the function of microtubule-associated protein tau. Here, the human brain tau with 441 amino acids was phosphorylated by cyclic-AMP-dependent protein kinase (PKA) or glycogen synthase kinase-3beta. PKA-phosphorylated tau (2.7 mol phosphates/mol) does not promote tubulin assembly as judged by spectrophotometric and atomic force microscopy measurements, unless trimethylamine N-oxide (TMAO), a natural occurring osmolyte, is included in these assays. TMAO is also found to promote tubulin assembly of glycogen synthase kinase-3beta-phosphorylated tau (1.6 mol phosphates/mol). TMAO does not act by causing a chemical dephosphorylation of phosphorylated tau, but it acts to overcome the functional deficit caused by phosphorylation. PKA-phosphorylated tau binds to tubulin in the presence of TMAO and lowers the critical concentration of tubulin needed for assembly. From these data, we conclude that PKA-phosphorylated tau retains the ability to bind tubulin and promote tubulin assembly. TMAO is required, however, to sensitize the reaction. Possible uses of TMAO in relation to studies of tubulin assembly in vitro, in intact cells, and in relation to Alzheimer's disease are presented in this report.

Regulation of expression, phosphorylation and biological activity of tau during differentiation in SY5Y cells

Tau, one of the best characterized microtubule-associated proteins (MAPs), is a phosphoprotein, the biological activity of which is regulated by its degree of phosphorylation. The objective of the present study was to evaluate the regulation, phosphorylation and the biological activity of tau during differentiation. On differentiation, the tau/tubulin ratio increased about 3-fold regardless whether cells were optimally differentiated with retinoic acid and aphidicolin or with retinoic acid alone which does not inhibit proliferation. The phosphorylation at the Tau-1 (Ser-195/Ser-198/Ser-199/Ser-202) and PHF-1 (Ser-396/Ser-404) sites was increased, mostly in the retinoic acid treated cells, whereas phosphorylation of tau at the 12E8 (Ser-262/Ser-356) epitope was decreased in both groups by approximately 60%. Phosphorylation at the 12E8 site is thought to be one of the most prominent factors affecting the biological activity of tau. However, the microtubule binding activity of tau increased only slightly upon differentiation. Furthermore, a large part of the tau that bound to taxol-stabilized microtubules was phosphorylated at all three sites indicating that these sites are not major sites determining the biological activity of tau. These data show that differentiation of SY5Y cells results in increased tau levels rather than dephosphorylation of tau to meet the additional need in tau's biological activity.

Transient increases in intracellular calcium result in prolonged site-selective increases in Tau phosphorylation through a glycogen synthase kinase 3beta-dependent pathway

Calcium is a universal intracellular signaling molecule. Through variations in both the amplitude and frequency of intracellular calcium increases, the same calcium ion can elicit different responses. In this report, we investigated the effect of a calcium transient, lasting 2-5 min, on alterations in the phosphorylation state of the cytoskeletal protein, tau. Transient increases in calcium result in a prolonged (1-4 h) approximately 60% increase in tau phosphorylation at the Tau-1 epitope. These increases in tau phosphorylation appear to be more dependent upon the duration of the increase in intracellular calcium and less on the amplitude. The calcium-induced increases in tau phosphorylation are not dependent upon protein synthesis, nor are protein kinase C or calcium/calmodulin-dependent protein kinase II involved in the response. However, the calcium-induced increase in tau phosphorylation was inhibited by lithium, a noncompetitive inhibitor of glycogen synthase kinase-3beta (GSK-3beta), and by the tyrosine kinase inhibitor, genistein. Furthermore, transient increases in calcium resulted in a prolonged increase in GSK-3beta tyrosine phosphorylation concomitant with the increase in tau phosphorylation. Therefore, this study is the first to indicate that transient increases in intracellular calcium result in increased tyrosine phosphorylation and activation of GSK-3beta which subsequently results in a sustained increase in the phosphorylation state of tau.

Phosphorylation of tau protein by recombinant GSK-3beta: pronounced phosphorylation at select Ser/Thr-Pro motifs but no phosphorylation at Ser262 in the repeat domain

Glycogen synthase kinase-3beta (GSK-3beta) has been described as a proline-directed kinase which phosphorylates tau protein at several sites that are elevated in Alzheimer paired helical filaments. However, it has been claimed that GSK-3beta can also phosphorylate the non-proline-directed KXGS motifs in the presence of heparin, including Ser262 in the repeat domain of tau, which could induce the detachment of tau from microtubules. We have analyzed the activity of recombinant GSK-3beta and of GSK-3beta preparations purified from tissue, using two-dimensional phosphopeptide mapping, immunoblotting with phosphorylation-sensitive antibodies, and phosphopeptide sequencing. The most prominent phosphorylation sites on tau are Ser396 and Ser404 (PHF-1 epitope), Ser46 and Thr50 in the first insert, followed by a less efficient phosphorylation of other Alzheimer phosphoepitopes (antibodies AT-8, AT-270, etc). We also show that the non-proline-directed activity at KXGS motifs is not due to GSK-3beta itself, but to kinase contaminations in common GSK-3beta preparations from tissues which are activated upon addition of heparin.

Filamentous nerve cell inclusions in neurodegenerative diseases: tauopathies and alpha-synucleinopathies

Alzheimer's disease and Parkinson's disease are the most common neurodegenerative diseases. They are characterized by the degeneration of selected populations of nerve cells that develop filamentous inclusions before degeneration. The neuronal inclusions of Alzheimer's disease are made of the microtubule-associated protein tau, in a hyperphosphorylated state. Recent work has shown that the filamentous inclusions of Parkinson's disease are made of the protein alpha-synuclein and that rare, familial forms of Parkinson's disease are caused by missense mutations in the alpha-synuclein gene. Besides Parkinson's disease, the filamentous inclusions of two additional neurodegenerative diseases, namely dementia with Lewy bodies and multiple system atrophy, have also been found to be made of alpha-synuclein. Abundant filamentous tau inclusions are not limited to Alzheimer's disease. They are the defining neuropathological characteristic of frontotemporal dementias such as Pick's disease, and of progressive supranuclear palsy and corticobasal degeneration. The recent discovery of mutations in the tau gene in familial forms of frontotemporal dementia has provided a direct link between tau dysfunction and dementing disease. The new work has established that tauopathies and alpha-synucleinopathies account for most late-onset neurodegenerative diseases in man. The formation of intracellular filamentous inclusions might be the gain of toxic function that leads to the demise of affected brain cells.

Stable expression in Chinese hamster ovary cells of mutated tau genes causing frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17)

Extensive neuronal loss and aggregation of tau as cytoplasmic inclusions in neurons and glial cells in selected cortical and subcortical regions is the most striking characteristic of frontotemporal dementia and parkinsonism linked to chromosome 17, which is caused by exonic or intronic mutations in the tau gene. Here, we examined the effects of four exonic mutations in four-repeat tau using stably transfected Chinese hamster ovary cells. The proportion of polymerized tubulin was the largest in the P301L transfectant. G272V and P301L transfectants showed greater instability of microtubules in the presence of Colcemid than wild-type tau, V337M, or R406W transfectants. Thus no distinct phenotypes were shared by the mutant tau transfectants with regard to microtubule assembly and stability. Unexpectedly, R406W showed low and negligible levels of phosphorylation at Thr 231 and Ser 396, respectively, in the transfectant. This presents a sharp contrast to the observation that tau aggregates in R406W-affected brains are heavily phosphorylated at these two sites. This result suggests that hyperphosphorylation at these sites cannot occur in the tau R406W bound to microtubules, and thus that the hyperphosphorylated species of tau may be generated only after disruption of microtubules.