SEARCH

SEARCH BY CITATION

Keywords:

  • aggregates;
  • Alzheimer’s disease;
  • oligomers;
  • paired helical filament;
  • Tau

Abstract

  1. Top of page
  2. Abstract
  3. Tau protein
  4. Tau oligomers
  5. Inductors of Tau aggregation
  6. Post-translational modifications associated with Tau aggregation
  7. Possible events in the intracellular aggregation of Tau
  8. Toxicity of Tau aggregates
  9. Metabolic dysfunction in Alzheimer’s disease
  10. Tau aggregation inhibitors
  11. Concluding remarks
  12. Acknowledgements
  13. References

J. Neurochem. (2010) 112, 1353–1367.

Abstract

We are analyzing the physiological function of Tau protein and its abnormal pathological behavior when this protein is self-assemble into pathological filaments. These aggregates of Tau protein are the main components in many diseases such as Alzheimer’s disease (AD). Recent studies suggest that Tau acquires complex oligomeric conformations which may be toxic. In this review, we emphasized the possible phenomena implicated in the formation of these oligomers. Studies with chemical inductors indicates that the microtubule-binding domain is the most important region involved in Tau aggregation and showed the requirement of a pre-arrange Tau in abnormal conformation to promote self-assembly. Transgenic animal models and AD neuropathology studies showed that post-translational modifications are also implicated in Tau aggregation and neural cell death during AD development. Therefore, we analyzed some events that could be present during Tau aggregation. Finally, we included a brief discussion of the possible relation between glucose metabolism dysfunction in AD, and data of Tau aggregation by using aggregation inhibitors. In conclusion, the process Tau aggregation deserves further investigations to design possible therapeutic targets to inhibit the toxicity of these aggregates and it is possible that could be extended to other diseases with similar etiology.


Abbreviations used:
AD

Alzheimer’s disease

Cdk5

cyclin-dependent kinase 5

CHIP

C-terminus of the Hsc70-interacting protein

MAP

microtubules associated proteins

MT

microtubule

MTBD

MT-binding domain

NFT

neurofibrillary tangle

PHF

paired helical filament

Tau protein

  1. Top of page
  2. Abstract
  3. Tau protein
  4. Tau oligomers
  5. Inductors of Tau aggregation
  6. Post-translational modifications associated with Tau aggregation
  7. Possible events in the intracellular aggregation of Tau
  8. Toxicity of Tau aggregates
  9. Metabolic dysfunction in Alzheimer’s disease
  10. Tau aggregation inhibitors
  11. Concluding remarks
  12. Acknowledgements
  13. References

The cytoskeleton is a cellular structure that provides neuronal morphology and whose essential components are the microtubules (MTs). The cytoskeleton is important in the formation of axon and dendrites, which are involved both in transport and neurotransmission. The stability and dynamics for the assembly of MTs is promoted by proteins which are referred to as ‘microtubules associated proteins’ (MAPs). Among these MAPs, Tau is a major protein that participates in the association-dissociation of the MTs, conferring dynamics to the system (Weingarten et al. 1975). The human Tau gene is located on chromosome 17q21 and occupies over 100 kb, containing 16 exons. CNS, expressed six isoforms of Tau proteins which are originated by alternative splicing with 2, 3, and 10 exons, ranging proteins with 352–441 amino acids. Tau protein also undergoes different post-translational modifications such as phosphorylation, glycosylation, ubiquitinylation, deamidation, oxidation, tyrosine nitration, cross-linking, and glycation (Ávila et al. 2004; Hernández and Ávila 2007; Wang et al. 2007). Characteristically, Tau protein is divided into two domains: a projection domain and a MT-binding domain (MTBD) (Fig. 1). The projection domain has an acidic region (negative charge) and contains a proline-rich region (positive charge). This domain is responsible for the interaction with other cytoskeletal and plasma proteins. The proline-rich region regulates the interaction of Tau with MTs trough phosphorylation. The phosphorylation sites are located in serine/threonine amino acids. The SH3 domains which are associated to these amino acids regulate the interaction of Tau protein with some plasma membrane proteins, such as Fyn (Lee et al. 1998). Tau protein has been reported also in the nucleus of neuronal and non-neuronal cells, and can interact with DNA and RNA, and could participate in the conformation of the nucleolar structure and/or heterochromatin organization of ribosomal genes (Sjöberg et al. 2006). The MTBD contains two regions: the MTBD per se and an acidic region, at the end portion of the protein. The MTBD is composed by three or four repeats (3MTBD or 4MTBD, respectively), similar but no identical, composed of 31 or 32 amino acids. This region binds to MTs to stabilize them. This binding is possible because tubulin has a negative charge in its C-terminal region and by electrostatic binding interacts with the MTBD of Tau protein, which containing a positive charge (Buée et al. 2000; Rosenberg et al. 2008). Tau protein is rich in polar amino acids with a basic character, and there are only five types of amino acids that compose the half of protein, these are G, K, P, S, and T. As a consequence, Tau protein is a highly soluble molecule with a poor secondary structure. However, Tau contains two motifs which have a regional trend to form β-sheet structures flanked by the second (275–280 amino acids) and the third (306–311 amino acids) repeats of MTBD. When this structure is altered by modifications such as truncation or hyperphosphorylation, such as the case for Alzheimer’s disease (AD), the protein loses the affinity to MTs and begins to self-assemble, resulting in the formation of paired helical filaments (PHFs) and straight filaments (Mandelkow et al. 2007). These Tau polymers, the PHFs and straight filaments are present in a variety of dementia processes including AD, dementia with parkinsonism linked to the chromosome 17, progressive supranuclear palsy, cortico-basal ganglionic degeneration, Guam parkinsonism dementia complex, Niemann–Pick type C, pugilistic dementia, and Pick’s diseases (Sánchez et al. 2001; Binder et al. 2005; Guillozet-Bongaarts et al. 2007; Hernández and Ávila 2007; Iqbal and Grundke-Iqbal 2008). All these disorders also are known as tauopathies (see Table 1) because they are characterized by the presence of abnormal Tau (Ávila et al. 2004).

image

Figure 1.  Tau domains, net charge, and post-translational modifications. All isoforms of Tau protein are divided in a projection domain localized on N-terminal region, and microtubule-binding domain region (MTBD) in C-terminal charge in region. Tau has a positive charged in MTBD and part of the proline-rich region and, on the other hand, the rest of the molecule has negative charged. Either in the projection domain or MTBD Tau is a substrate of phosphorylation (P), glycosylation or glycation (G), nitration (N), and truncation (inline image). Tau is able to interact with members of the Src family, non-associated receptor tyrosin-kinase Fyn, through a motif in the proline-rich region.

Download figure to PowerPoint

Table 1.   Tau aggregation-associated diseases
Neurological diseaseDescriptionAbnormal TauReferences
  1. AP, amyloid plaques; NFT, neurofibrillary tangles; AD, Alzheimer’s disease; PHFs, paired helical filaments; MTBD, MT-binding domain.

Alzheimer’s diseaseIs the major type of dementia histopathologically characterized by abnormal protein aggregation in the brain which is known as AP and NFT.The NFT in early steps of AD are intracellular deposits; however, these progress to extracellular deposits at the end of disease; NFT are composed of PHFs where the major component is Tau protein.Rojo et al. (2006) and Iqbal and Grundke-Iqbal (2008)
Frontotemporal dementia with parkinsonism linked to chromosome 17The patients display frontotemporal atrophy that is the main feature, neuronal loss, gliosis, and cortical spongiform changes in the lobes.This disorder is the consequence of many mutations in Tau gene with an extensive variety of clinical symptoms.Ávila et al. (2004) and Ludolph et al. (2009)
Progressive supranuclear palsyThe patients present prominent postural instability and dementia in later stages.The neuropathological characteristics include midbrain atrophy as consequence of Tau inclusions in neurons (such as NFT), astrocytes, and oligodendrocytes; patients with familial origin have mutations in Tau gene.Dickson et al. (2007) and Williams and Lees (2009)
Cortico-basal ganglionic degenerationIs a disease that includes clinical manifestations such as dementia, movement disorder, cortical sensory loss, oculomotor dysfunction, and dysphagia.The principal lesions of this disease are cortical and nigral atrophy as a consequence of Tau inclusions (only isoforms of 4MTBD) in astrocytic plaques as an exclusive feature of this disorder.Ludolph et al. (2009)
Guam parkinsonism dementia complexThis dementia is characterized by a progressive decline of intellectual faculties, disorientation and behavioral changes, also show rigidity and postural deformities, hyperreflexia, and spinal muscular atrophy.This disorder is characterized by cortical atrophy, neuronal loss and extensive NFTs in neocortex and hippocampus.Murakami (1999) and Winton et al. (2006)
Niemann–Pick type CIs a dementia with a sphingolipid storage disorder as a result from the word was divided inherited deficiencies of intracellular lipid trafficking proteins, which are an accumulation of cholesterol and glycosphingolipids in late endosomes and lysosomes.The neuropathological features include numerous Tau-positive NFTs and neuropil threads in the midbrain and cerebral cortex.Pacheco and Lieberman (2008) and Ludolph et al. (2009)
Dementia pugilisticaThis disorder involves pathological processes which affect parts of the brainstem, the cerebellum and the cerebral hemispheres.These processes include degeneration of the substantia nigra, cerebellar scarring, and partial disintegration of the septum pellucidum and the widespread presence of NFT without neuritic plaques, particularly in the medial temporal cortex.Adams and Bruton (1989)
PickIs a dementia that produces disturbances in language and behavior and is associated with frontal lobe atrophy.This disease is characterized by the presence of cytoplasmic hyperphosphorylated Tau (only isoforms of 3MTBD) inclusions in neurons of the frontal lobe, known as Pick bodies.Ávila et al. (2004) and Robert and Mathuranath (2007)

Tau oligomers

  1. Top of page
  2. Abstract
  3. Tau protein
  4. Tau oligomers
  5. Inductors of Tau aggregation
  6. Post-translational modifications associated with Tau aggregation
  7. Possible events in the intracellular aggregation of Tau
  8. Toxicity of Tau aggregates
  9. Metabolic dysfunction in Alzheimer’s disease
  10. Tau aggregation inhibitors
  11. Concluding remarks
  12. Acknowledgements
  13. References

The role of protein oligomeric aggregation intermediates has recently received a considerable attention in several neurodegenerative diseases because of their link to toxicity (Berger et al. 2007). In AD neuropathology, as a possible consequence of the transition of an unfolded conformation to an amyloidogenic state, it is be possible that the formation of oligomers play a crucial role in the formation of intracellular aggregates of the Tau protein. Some data produced by in vitro assays suggested that oligomerization of Tau is required for stabilization of MTs through neuronal development (Makrides et al. 2003). In addition, it has been reported that these oligomers are able to act as an electrostatic zipper during MT stabilization (Rosenberg et al. 2008). In AD, the majority of studies are focused to understand the mechanism of the formation of the insoluble PHFs in which the presence of oligomers may play a key role. These Tau oligomers can cause neurodegeneration and memory impairment in the absence of amyloid-β (Santacruz et al. 2005; Mocanu et al. 2008; Kayed and Jackon 2009). Regarding the nature of the early Tau protein oligomers, possible monomer or dimeric subunits appears to be constituted by highly phosphorylated or truncated Tau (Fig. 2) (Zilka et al. 2006). In the oligomers, Tau protein also seems to be structurally modified. Tau oligomers are rich in β-sheet structures and it can promote the formation of fibrils once the oligomers reaches a size of 20 nm (Maeda et al. 2007). Oligomeric Tau protein may contain three or four repeat of the MTBD isoforms (Sugino et al. 2009). Using an in vitro Tau aggregation system, granular-shaped protofilaments of Tau has been identified (Maeda et al. 2006). These granular Tau oligomers appear to be composed of nearly 40 Tau molecules and are able to become filaments in a dose-dependent manner (Maeda et al. 2006). Surprisingly, in brain tissue derived from AD patients which was analyzed by atomic force microscopy, these granular oligomers also was observed The presence of these oligomers were detected early in Braak 0 stage brain samples, but were significantly increased in Braak 1 stage, suggesting that these oligomers increases in parallel to the pre-symptomatic stages of the disease, suggesting that the granular Tau oligomers may be present before the neurofibrillary tangles (NFTs) are detectable (Braak and Braak 1991; Maeda et al. 2006). These results are in agreement with the evidences that similar oligomers have been identified in a time-dependent manner in some rodent models of tauopathies such as the Tg4510 (Berger et al. 2007) and the Tau P301L transgenic mice (JNPL3) (Sahara et al. 2007a). Oligomeric structures also have been described in cellular cultures (Zhang et al. 2005). Taken together, all this information strongly support that oligomerization is an essential step for Tau aggregation leading to the formation of abnormal filaments. In this regard, it is possible that these oligomers appear in the cellular environment where their presence is somewhat permissible for their further assembly to higher-order oligomers to eventually become overt filaments (Sahara et al. 2007a). Barghorn and Mandelkow (2002) reported that the sequence of MTBD itself is capable to form dimers and oligomers. These authors also suggested that Tau protein needed to change from a mostly random-coiled structure to a conformation where part of the repeated domain adopts an extended conformation ready to enter to a β-sheet structure with interactions with other molecules. Accordingly, two molecules of Tau could be aligned each other to form a dimer. Further association of many other monomers and dimers could lead to the formation of a ‘nucleation’ or aggregation center, and from here to grow up longer to become a filament (Barghorn and Mandelkow 2002). Another important event that might be important in the mechanism of the formation of oligomers and from there to filaments is the nitration. Nitration could inhibit Tau MT-binding activity. This process could allow that nitrated Tau molecules may sequester normal Tau, implying a role of Tau nitration in disruption of the MT system, as seen in AD pathology (Zhang et al. 2005). Other possible important mechanism for the oligomers formation is the interaction of Tau molecules with so-called chaperon molecules. It has been reported that levels of granular Tau oligomers are inversely proportional to those of heat-shock proteins, suggesting that molecular chaperone–Tau protein complexes may likely trigger the Tau aggregation processing (Sahara et al. 2007b). On the other hand, it has been suggested that autophagic-lysosomal perturbations may promote the accumulation of these oligomers composed by C-terminal truncated Tau as well as full-length Tau cross-linked (Hamano et al. 2008). In addition, phosphorylated Tau is able not only to sequestrate normal Tau, also is able to capture another high molecular weight MAPs such as MAP2 and MAP1 (Alonso et al. 1997), indicating the importance of these proteins in Tau oligomerization. Likewise, it is possible that the origin of Tau oligomers might be because of the interaction of the Tau–Tau monomers could be stabilized by anionic factors such as arachidonic acid (Barghorn and Mandelkow 2002) and heparin (Maeda et al. 2007), through the disulfide bounds between cysteine residues which these compounds elicit (Sahara et al. 2007a). Isolated monomers of the repeats of the MTBD could also play a differential role in the process of oligomerization of Tau protein (Sahara et al. 2007a). Regarding to this, Petterson and coworkers suggested that hexapeptides motifs VQIINK280 and VQIVYK311 (Fig. 3) act as mediators of intermolecular interactions between isolated the Tau molecules monomers which were stabilized by the anionic factor heparin (Petterson et al. 2008). Furthermore, it has been proved that β-sheet structures play an important role in the formation of Tau oligomers and the aggregation process (von Bergen et al. 2000). Recent confocal microscopy studies have demonstrated that Tau aggregation process is accompanied by conformational changes resulting from a putative cascade of phosphorylation occurring at the N-part of the molecule and truncations at its the C end (Luna-Muñoz et al. 2007; Basurto-Islas et al. 2008). These molecular events may favor the elongation of Tau aggregates to become filaments. Likewise, using an all-atom minimalistic model and structure analysis it was possible to determine that a little fragment of Tau molecule is able not only to promote the formation of stable oligomers but capable of stabilizing these structures, this because such a fragments are characteristically rich in β-sheet structures (Li et al. 2008).

image

Figure 2.  Immunolabeling with TauC3 and dye with Thiazine red to detect Tau truncation in Asp 421 and Tau aggregates, respectively. The image shows a positive signal with TauC3 antibody for Tau oligomers which are negative with dye of Thiazine red.

Download figure to PowerPoint

image

Figure 3.  Important regions implicated in Tau aggregation. Sequences in MTBD are important in the promotion of Tau oligo-merization and aggregation like PHF core (I297–E391) and inner motifs VQNIIK280 and VQIYNK311. The N-terminal region is important in Tau–Tau assembly inhibition.

Download figure to PowerPoint

Inductors of Tau aggregation

  1. Top of page
  2. Abstract
  3. Tau protein
  4. Tau oligomers
  5. Inductors of Tau aggregation
  6. Post-translational modifications associated with Tau aggregation
  7. Possible events in the intracellular aggregation of Tau
  8. Toxicity of Tau aggregates
  9. Metabolic dysfunction in Alzheimer’s disease
  10. Tau aggregation inhibitors
  11. Concluding remarks
  12. Acknowledgements
  13. References

In the presence of cofactors as reducing agents, including polyanions, anionic micelles, and planar anionic aromatic dyes, full-length Tau protein and its MTBD are highly soluble and aggregates in vitro. This strong interaction accelerates and increases Tau aggregation by the stabilization of the protein assembly. These cofactors also reduce the repulsion between cationic molecules like Tau, this property enhances the local concentration of this protein allowing its aggregation (Alonso et al. 2001; Konno et al. 2004; Chirita et al. 2005; Kuret et al. 2005). Tau monomers maintained in urea solution (also a reducing agent) are able to polymerize in a fashion very much alike to PHFs (Montejo de Garcini et al. 1988). There are other inductors of aggregating, such as polyanions include RNA, DNA, sulfate glycosaminoglycans, and taurine (Goedert et al. 1996; Kampers et al. 1996; Hikosou et al. 2007; Santa-María et al. 2007). Tau filaments formed with RNA depends of intermolecular disulfide bridges (C322) in the third repeat of Tau; the interaction with the polyanion opens up the folded conformation in the MTBD leading to the next step of antiparallel dimerization of Tau (early intermediary). These polymers are morphologically similar to the PHFs and the six isoforms of Tau can be aggregated in the presence of this polyanion (Kampers et al. 1996). In the case of the DNA–Tau interaction, several studies support that this interaction is important because Tau stabilizes and protects the structure of DNA from radical attack, however, recently it has been observed that a single or double strand DNA (such as poly dA or poly dT and poly dG-dC, respectively), also promote a filament formation of three and four repeats MTBD of Tau (Hikosou et al. 2007). In general, these data demonstrated the formation of intermolecular dimers via disulfide bond on C17 in the third repeat of Tau of 3MTBD, but in the case of 4MTBD tends to form an intramolecular crossbridge between two C in the second and third repeat. Based upon this information, it is possible to suggest that these isoforms offer a resistance to the process of filament formation (Hua and He 2003; Hikosou et al. 2007; Wei et al. 2008). On the other hand, the assays with the RNA demonstrated that sulfated glycosaminoglycans, such as heparin or heparin sulfate could enhance Tau aggregation of the three repeats isoforms in vitro, creating PHFs. It has also been found that heparin sulfate and hyperphosphorylated Tau coexist in AD at earliest stages of dementia, suggesting that heparin sulfate could be a key factor in the formation of NFTs. The region Tau protein which allows this interaction includes one part of the MTBD, i.e. 306–311 or 317–335 amino acids corresponding to a segment of the third repeat (Goedert et al. 1996; von Bergen et al. 2000; Pérez et al. 2001). Recently, it was discovered that Taurine is a polyanion able to induce Tau aggregation in vitro (Santa-María et al. 2007). On the other hand, anionic micelles such as arachidonic acid, also promote the fast aggregation of Tau monomers with a nucleation phase (in seconds), followed by a slower elongation phase (hours) to finish in the formation of PHFs ultra structurally similar to the PHFs found in AD brains. It appears that in the process of Tau aggregation using polyanions the presence of 314–320 amino acid (this moiety includes the first, second, and the small portion of third repeat)-containing region is not mandatory (King et al. 1999; Abraha et al. 2000; Ksiezak-Reding and Wall 2005).

Several experiments using dyes have been designed to support the linkage theory (by which the dye concentration would induce Tau monomer to self-association reaction). These dyes which are planar anionic aromatic compounds such as Thiazine red, Congo red, and thioflavin-S are capable to stimulate the aggregation of full-length Tau monomers and the mechanism which has been proposed for this is based on the fact such compounds increase the rates of filament nucleation of Tau through conformational change to β-sheet structure (Chirita et al. 2005). Congo red at 10 μM, is so far, the most potent dye able to induce Tau aggregation, followed by thioflavin-S at 10-fold lower and then by Thiazine red, yielding few filaments which are longer in length. All these compounds accelerate the process of Tau aggregation by the increment of the rates of filament nucleation of Tau (Chirita et al. 2005). It is important to emphasize that the MTBD is the most important region involved in Tau aggregation and the conformational features of this domain is crucial, and is the responsible of promote the polymerization of this protein (Fig. 1). On the other hand Congo Red, for example, was used to induce Tau aggregation within human embryonic kidney 293 cells expressing full-length Tau and the results show the formation of detergent-insoluble aggregates after 7 days of treatment. In addition, Tau aggregation was related with cell death (Bandyopadhyay et al. 2007). Oxidative stress has also been involved in the Tau aggregation. It was found that SHSY5Y differentiated cells, treated with dopamine oxidation products such as p-benzoquinone in the presence of iron, showed that Tau polymerization is induced, resulting in amorphous aggregates, however, dopaminergic neurons show a low content of Tau and for that reason rarely have Tau aggregation (Santa-María et al. 2005). On the other hand, the methanol intake by adulterated alcoholic drinks have been associated with toxicity in the CNS, because its metabolites formic acid and formaldehyde induce progressively neuronal chronic damage, this acquires great interest because formaldehyde have a potential effect on protein misfolding (Cullen and Halliday 1995a). This may explain in part the presence NFTs which are found in brain of chronic alcoholics (Cullen and Halliday 1995b). This background was very useful to try to design potential pharmacological approaches in vivo cell models. Likewise, some recent reports have shown that formaldehyde at low concentrations (0.0005%) promoted the formation of amyloid-like aggregates of Tau in three types of cell culture: human embryonic kidney 293 cells transfected with full-length Tau, SHSY5Y cell line, and hippocampal cells. These aggregates were detected by thioflavin-S assay and sodium dodecyl sulfate–polyacrylamide gel electrophoresis and found only in the latter two cell lines. In this same system, Tau induces apoptosis probably because the interaction with formaldehyde is linked to the ‘denatured-like’ structure, in which the ε-amino groups of K and thiol groups of C of Tau protein becomes exposed to the exterior and consequently formaldehyde can bind to these groups (Nie et al. 2007a,b). The main conclusion of these set of experiments using in vivo cell models is that although Tau isoforms have a rate-limiting barrier to aggregate, by the fact that Tau protein is a highly soluble molecule with a poor secondary structure, this barrier could be reduced by several inducers. This reduction may promote the formation of intermediates which show a pre-arrange of an abnormal conformation that diminishes the thermodynamic barrier for polymerization, thus, inducing the formation of Tau oligomers and its subsequent polymerization (Gamblin et al. 2003; Chirita et al. 2005; Carlson et al. 2007).

Post-translational modifications associated with Tau aggregation

  1. Top of page
  2. Abstract
  3. Tau protein
  4. Tau oligomers
  5. Inductors of Tau aggregation
  6. Post-translational modifications associated with Tau aggregation
  7. Possible events in the intracellular aggregation of Tau
  8. Toxicity of Tau aggregates
  9. Metabolic dysfunction in Alzheimer’s disease
  10. Tau aggregation inhibitors
  11. Concluding remarks
  12. Acknowledgements
  13. References

In the majority of all tauopathies, several post-translational modifications have been involved, including hyperphosphorylation, truncation, glycosylation, glycation, and ubiquitination. While the former abnormal process appears to play a protective role in Tau aggregation (Fig. 2) the latter processes may contribute to the actual aggregation of the molecule. In this review, we mainly focused such modifications involved in Tau abnormal polymerization.

Abnormal phosphorylation is, so far, the most studied post-translational modifications of Tau in human neurodegenerative processes. Tau belongs to a family of a phospho-proteins and its interaction with other molecules can be regulated for the specific site of phosphorylation. These principal sites of phosphorylation are on T and S residues, principally located in the projection domain of Tau and in the MTBD, which lead to regulate MT interaction and consequentially cytoskeleton stability (Lin et al. 2007). It has been reported that Y phosphorylation especially regulates plasma membrane interaction (Lee et al. 1998) and perhaps could be pathologically important (Cripps et al. 2005). Many kinases which have been implicated in Tau phosphorylation maybe or not associated with MTs. Among the kinases responsible for Tau phosphorylation are glycogen synthase kinase 3β (Lin et al. 2007), cyclin-dependent kinase 5 (Cdk5) (Liu et al. 2002), MT-affinity regulatory kinase, cAMP-dependent protein kinase (Johnson and Stoothoff 2004), dual-specificity tyrosine-phosphorylated and regulated kinase 1A (Liu et al. 2007), Tau–tubulin kinase 1 (Sato et al. 2006), and calmodulin-dependent protein kinase 2 (Wang et al. 2007). Phosphorylation of Tau at T231, S262, and S356 in the MTBD inhibits its activity to stimulate MT assembly (Johnson and Stoothoff 2004; Liu et al. 2007) and phosphorylation at proline-rich region reduce dramatically Tau capacity for de novo MT assembly (Brandt et al. 2005). These phosphorylated sites are regulated during physiological conditions by phosphatases like protein phosphatase 2A (PP2-A) (Wang et al. 2007). When the dynamic equilibrium between phosphorylation and dephosphorylation fails, hyperphosphorylation processing appears to be triggered leading to Tau aggregation both in vitro (Wang et al. 2007) and in vivo assays (Braak et al. 2006). As per the neurofibrillary degeneration, abnormal hyperphosphorylation of Tau appears to be the key event for it to be formed. Both in situ and in vitro systems have been suggested that the cytosolic abnormally hyperphosphorylated Tau and not its polymerization into PHF are apparently involved in the breakdown of the neuronal MT network (Wang et al. 2007). Several proteins such as the 14-3-3ζ regulatory protein have been implicated in certain manner in this abnormal phosphorylation or even in the aggregation pathway. The 14-3-3ζ regulatory protein has been implicated in fibrils formation when S214 is phosphorylated possibly by cAMP-dependent protein kinase (Hernández et al. 2004; Sadik et al. 2009). Another protein that could play an important role in Tau aggregation in a phospho-dependent manner is the peptidyl-prolyl cis-trans isomerase, Pin1, which binds Tau when it is phosphorylated at T231 (Holzter et al. 2002). Some reports suggest that the bundles PHF–Tau in AD neurons sequesters Pin1 into the insoluble NFT, thereby depleting the soluble pool of Pin1 and contributing to cell death (Lu et al. 1999). Although some reports suggest that Tau prolyl isomerization may provide to cells with a compensating mechanism to the adverse effects of Tau hyperphosphorylation, especially by reducing the free intracellular Tau pool (Takahashi et al. 2008). In view of this evidence, it might be argued that the appearance of Pin1-positive granules within the neuronal cells prone to fibrillar degeneration, may indicate either that this event occurs at early stage of the pathological alterations which precede the formation of PHF–Tau or reflect an alternative pathway for dealing with an aberrant molecular mechanism, which protects against neurofibrillary degeneration in AD patients (Holzter et al. 2002). Tyrosines 18, 197, and 394 have been shown to be phosphorylated in the brain of patients with AD whereas Y394 is the only residue that has been described to date that is phosphorylated in physiological conditions. According to recent data, phosphorylation in Tyrosine residues might be an early event in the pathophysiology of AD and that Fyn and c-Abl are critical in the neurodegenerative process which occurs in tauopathies (Hanger et al. 2009; Lebouvier et al. 2009). Tau abnormal phosphorylation at specific sites are strongly associated to other post-translational modifications such endogenous proteolysis or truncation. This abnormal process is defined as a protein cutting that could also promote aberrant aggregation (Ávila et al. 2004). In the case of Tau molecule, the lost of any of its C- and N-extremes leads to a change of properties of the molecule. The lost of the N-terminal has been proposed as an early event in Tau aggregation process because of the capability of this extreme of inhibit Tau oligomerization in vitro (Ghoshal et al. 2002; Horowitz et al. 2006). On the other hand, C-terminal proteolysis is strongly correlated with neuropathological lesions and cognitive impairment (García-Sierra et al. 2001). By in vitro assays, several enzymes were identified that may contribute to truncated Tau forms in vivo, including caspase 6 (Horowitz et al. 2004), thrombin (Arai et al. 2005), chymotrypsin (Mandelkow et al. 2007), and puromycin-sensitive aminopeptidase (Sengupta et al. 2006) that acts in the projection domain of the molecule. Similarly, in the MTBD region the enzymes able to process Tau are calpain (Yang and Ksiezak-Reding 1995), caspase 3, and a non-identified protease that is able to truncate Tau at the position E391 of the C-terminal region (Novak et al. 1993; Basurto-Islas et al. 2008). Taken all these data together, it was proposed that conformational changes, maybe induced by specific phosphorylation sites (Mondragón-Rodríguez et al. 2008), are strongly associated to the generation of truncated forms of Tau (Binder et al. 2005). Based upon to in vitro assays (Novak et al. 1993) and cellular models (Lira-De León et al. 2009), it has been also suggested that a minimal resistant fragment, namely, the PHF core (Wischik et al. 1988a,b; Skrabana et al. 2004, 2006) is able to promote an the formation of aggregating sites of dimers composed of truncated Tau/full-length Tau molecules within the neuronal milieu. According to Wischik et al. (1996) hypothesis such increasing aggregates may eventually lead Tau polymerization into PHF in an antiparallel assembly of these structures. This hypothesis has been supported with cellular models in which the expression of truncated forms lead to apoptosis (Fasulo et al. 2000) and with a rodent model that expresses a truncated Tau, masking of the PHF core by clustering of full-length Tau molecule abnormally phosphorylated. This cascade of events including the presence of the PHF core and its consequent occlusion within the assembling polymers would promote the NFT formation. As, it has been well established, densities of NFT in specific brain areas correlate with the cognitive impairment in AD (Zilka et al. 2006; Hrnkova et al. 2007; Koson et al. 2008). All these results strongly suggest that the mechanisms involved in the truncating Tau processing of Tau play a crucial role in the abnormal aggregation and eventual polymerization into insoluble PHF.

Glycosylation and glycation are other important modifications which have been associated with Tau aggregation (Gendron and Petrucelli 2009). Both these modifications may participate in the formation of disulfite bridges and thus, favoring the formation of Tau aggregates (Kuhla et al. 2007). Glycosylation is characterized by the covalent attachment of oligosaccharides to protein side chains. Glycosidic bonds are classified either as N-linked or O-linked (Gendron and Petrucelli 2009). In vitro assays have been shown that glycosylation facilitates the subsequent phosphorylation which is catalyzed either by Cdk5 or glycogen synthase kinase 3β at several abnormal hyperphosphorylation sites, in a site-specific manner (Liu et al. 2002). N-glycosylation is enhanced in PHF–Tau compared with normal Tau; moreover, a decreased level of O-glycosylation has been detected in AD brains (Wang and Liu 2008), probably because this molecular event can slowdown the process of aggregation at least in vitro (Yu et al. 2008). Glycation refers to non-enzymatic linkage of sugars to the amino side chain of polypeptide (Wang and Liu 2008) and in AD brains has been reported that PHFs present this modification (Ledesma et al. 1994). This modification is exclusively associated to AD tissue (Wang and Liu 2008) and it was suggested that the enrichment of small glycans in the PHF probably represents a more advanced stage of the processing of glycans in these structures (Sato et al. 2001). In addition, because of the relationship with phosphorylation (Ledesma et al. 1994), glycation might facilitate the formation of Tau aggregates.

Nitration has also been suggested as a modification which could participate in Tau aggregation. Tau nitration may represent oxidative damage of the brain and the accumulation of oxidants may play an important role in Tau nitration in AD brains. Abnormally nitrated Tau has been found in NFTs in AD brains: the levels of 3-nitrotyrosine and dityrosine were elevated consistently in the hippocampus, neocortical regions, and CSF. In vitro assays denote that peroxynitrite treatments lead to a Tau nitration, an inhibition of MT stabilization, and a consequent oligomerization of the molecules (Zhang et al. 2005). Select nitration at Y29 and Y197 increases the average filament length without changing the steady state polymer mass. In contrast, nitration at residues Y18 and Y394 decreases the average filament length and/or number relative to wild-type (Reynolds et al. 2005). According with studies in AD patient’s tissue, nitration at Y29 is a disease-related event that may alter the biochemical state of Tau in the neurodegenerative tauopathies (Reynolds et al. 2006). Tau-nY18 recognizes nitrated soluble Tau isolated from age-matched controls (Braak stages I–III), as well as nitrated soluble Tau from severely affected AD brains (Braak stages V and VI). In situ, Tau-nY18 sparsely labels the neuronal pathological hallmarks of the disease, including the NFT and neuritic plaques. In contrast to its intermittent reactivity with the neuronal pathology, Tau-nY18 robustly labels nitrated Tau within activated, glial fibrillary acidic protein-positive astrocytes from both AD brains and age-matched controls. In most instances, Tau-nY18-labeled astrocytes are associated with or in proximity to AP, suggesting that nitration of Tau at Y18 occurs in soluble Tau from normal controls (Braak stage I–III), as well as soluble and insoluble PHF–Tau preparations from severely affected AD brains (Braak stages V and VI), an event that appears to be associated with astrocyte activation and/or amyloid deposition (Reyes et al. 2008). According to previous reports, while some nitrations may appear to exert disparate effects in polymerization in vitro, it is possible that their accumulative effect in vivo would promote self-association and NFT formation (Reynolds et al. 2007).

Ubiquitination has also been involved in the mechanism of Tau aggregation. Ubiquitin is a highly conserved protein of 76 amino acids and plays a regulatory role in proteolytical systems for damaged or abnormal proteins. Polyubiquitinated proteins are targeted for degradation by an ubiquitin-proteosome system (Cripps et al. 2005). This protein has been found closely associated with PHFs (Manetto et al. 1988) at sites K6, K11, K48, K254, K311, and K353 in association with phosphorylation (Cripps et al. 2005). The association of Ubiquitin with PHF is such that is the only protein which remains bound to PHF after their treatment with Pronase (Goedert et al. 1988). Recent studies have suggested that a proteosome deficit may also contribute to the accumulation of Tau proteins in selective neurons in AD (Liu et al. 2008a). In addition, it was suggested that the ubiquitin system might become saturated by an excess of altered cytoskeletal proteins, which would aggregate in highly ubiquitinated, morphologically abnormal filaments in AD and even to participate in the mechanism of formation of Lewy’s bodies (Manetto et al. 1988). As well, it was observed in a rodent model lacking the C-terminus of the 70 kD heat shock cognate protein (Hsc70)-interacting protein (CHIP), and expressing P301L mutant form of Tau, that CHIP is critical for degradation of phosphorylated Tau species and may play a role in the formation of neurofibrillary pathology. Thus, ubiquitination of phospho-Tau species by CHIP is critical to prevent their accumulation in aggregates. According to this, if the delicate balance of phospho-Tau turnover is altered by impaired proteosome function, Tau over-expression, or enhanced Tau phosphorylation, aggregation of these hyperphosphorylated Tau species into NFTs may be a CHIP/chaperone-mediated protective mechanism rather than a direct route to neuronal cell death (Dickey et al. 2006).

Possible events in the intracellular aggregation of Tau

  1. Top of page
  2. Abstract
  3. Tau protein
  4. Tau oligomers
  5. Inductors of Tau aggregation
  6. Post-translational modifications associated with Tau aggregation
  7. Possible events in the intracellular aggregation of Tau
  8. Toxicity of Tau aggregates
  9. Metabolic dysfunction in Alzheimer’s disease
  10. Tau aggregation inhibitors
  11. Concluding remarks
  12. Acknowledgements
  13. References

According to many investigations, the elucidation of the mechanisms by which Tau polymerizes may provide clues to the initiating modifications that precede neurodegeneration (Kayed and Jackon 2009). In Fig. 4, we summarize some of the events that could be present during Tau aggregation. For this aggregation process, first, Tau must be dissociated from MTs so that cytosolic concentration can exceed the minimal Tau concentration necessary to support aggregation (Golde 2006). This dissociation could be mediated by a site-specific phosphorylation (Liu et al. 2007), probably truncation (Binder et al. 2005) or aggregation inducers (Chirita et al. 2005) could lead to a conformational change and promote the adoption for an aggregation-competent conformation (von Bergen et al. 2000). This change in Tau nature would impact directly in the presence of β-sheet structures, and leads to an electrostatic modification in the molecule that make it able to form a side chain/side chain interactions ending in the eventual formation of the first Tau–Tau dimers (Andronesi et al. 2008). These early dimmer may span the core of the filament, which adopts parallel, in register β-sheet conformation. These dimers could be also able to be promoted by post-translational modifications. Once these dimers are formed and adopt a stable structure, they can begin a process of nucleation with the help of a Tau-membrane interaction (Gray et al. 1987; Kuret et al. 2005; Lira-De León et al. 2009). Later, once the critical nucleus cluster size is reached, subsequent additions to the nascent filament end are energetically favorable, and elongation proceeds efficiently, forming oligomers in a dose–time-dependent manner. Subsequently, oligomers are capable to continue aggregation process and begin forming subunits of filaments, termed protomers, which adopt the parallel, in register cross β-sheet structure typical of amyloid aggregates. On the basis of morphology and mass-per unit length measurements, mature Tau filaments, termed PHFs, consist of two protofilaments around each other (Congdon et al. 2008). A posterior event would imply the formation of NFTs. Recently, a cell culture model has been proposed, in which extracellular Tau aggregates can transmit a misfolded state from the outside to the inside of cell through a process of endocytosis. This data allow a better understanding of how misfolded protein aggregates to spreads to other areas of the brain (Frost et al. 2009).

image

Figure 4.  Schematic representation of a possible pathway of Tau aggregation and NFT formation in AD. See text for details.

Download figure to PowerPoint

Toxicity of Tau aggregates

  1. Top of page
  2. Abstract
  3. Tau protein
  4. Tau oligomers
  5. Inductors of Tau aggregation
  6. Post-translational modifications associated with Tau aggregation
  7. Possible events in the intracellular aggregation of Tau
  8. Toxicity of Tau aggregates
  9. Metabolic dysfunction in Alzheimer’s disease
  10. Tau aggregation inhibitors
  11. Concluding remarks
  12. Acknowledgements
  13. References

The toxicity of Tau aggregates originates the loss of physiological functions and the gain of pathological functions (Lovestone and McLoughlin 2002; Ding and Johnson 2008; Gendron and Petrucelli 2009). In the first case, Tau aggregates are unable to bind and stabilize the MTs, causing an increase of MT instability; this would also increase the expression of Tau protein as a compensatory mechanism. It has been found alterations in the Tau isoforms expression with an excess of Tau with 4MTBD. These isoforms bind tightly to MTs and its imbalance could favor an impaired axonal disruption (Deshpande et al. 2008). In addition, studies with over-expression of full-length Tau in Chinese hamster ovary (CHO) cells and N2a cells showed that Tau alters the distribution of various organelles (mitochondria and endoplasmic reticulum) by inhibiting kinesin-dependent traffic and disturbs the stability in the growing axonal neurites (Ebneth et al. 1998; Dubey et al. 2008). An in vivo model with formaldehyde treatment showed that Tau aggregates could be inducing apoptosis after the removal of the promoter of Tau polymerization (Nie et al. 2007a). The same toxicity was observed in a cell model that used N2a cell transfected with a fragment of mutant K18ΔK280 Tau (S258–I360) alone or together with mutant Tau (ΔK280) causes cytotoxicity, and the cells were positives to thioflavin-S staining for Tau aggregates (Wang et al. 2007).

Metabolic dysfunction in Alzheimer’s disease

  1. Top of page
  2. Abstract
  3. Tau protein
  4. Tau oligomers
  5. Inductors of Tau aggregation
  6. Post-translational modifications associated with Tau aggregation
  7. Possible events in the intracellular aggregation of Tau
  8. Toxicity of Tau aggregates
  9. Metabolic dysfunction in Alzheimer’s disease
  10. Tau aggregation inhibitors
  11. Concluding remarks
  12. Acknowledgements
  13. References

Metabolism of glucose in AD patients is impaired and precedes the clinical symptoms, nevertheless the molecular mechanism involved is unknown (Liu et al. 2009). Recently, the characterization of gene expression in vulnerable anatomical regions such as entorhinal cortex, hippocampus, and posterior cingulate cortex in normal and AD aged brains provides a clue to elucidate the possible mechanism implicated in the glucose system dysfunction. These results showed in AD brains a lower expression of nuclear genes encoding subunits of the mitochondrial electron transport chain in all regions mentioned compared with controls. Moreover, the metabolic rate for glucose also was decreased in AD brains, opening the possibility to relate this abnormality with mitochondrial dysfunction (Liang et al. 2007, 2008). Afterward, the analysis with fluorodeoxy glucose–positron emission tomography of subjects with a clinical dementia rating of 0.5, allowed establishing a baseline for glucose metabolism and comparing with early AD cases. The data obtained showed that AD patients have a significant decline in glucose metabolism principally in right cingulated, left inferior parietal, and left temporal gyrus compared with controls. Alterations were also observed in the brain structures of AD that can also be related with diminish in glucose metabolism, mainly in temporal parietal lobes associated with an inadequate episodic memory and deteriorate hippocampus (Kuczynski et al. 2008; Ishii et al. 2009). Furthermore, other investigation have shown that the process of O-glycosylation regulate by glucose metabolism correlated negatively with a hyperphosphorylation of Tau protein, presented a reduction of fourfold O-Glycosylation compared with a non-hyperphosphorylated Tau (Liu et al. 2009). This finding allows suggesting a connection between glucose metabolic changes and abnormal Tau modifications. On the other hand, the study of hippocampal metabolite levels in normal elderly controls, subjects with mild cognitive impairment, and AD showed a decrease of glutamate in the latter two groups may be as a consequence of the synapses destruction (Rupsingh et al. 2009). However, these results disagree with studies using the SHSY5Y cells, in which it was observed an increment of Tau phosphorylation after the treatment with glutamate, probably caused by an increase of Cdk5 activity (Jämsäet al. 2006). A similar patron was observed in a model of chronic stress in old rats, in which a decreased in the enzyme activity of glutamate decarboxylase and increased of hyperphosphorylated Tau was found in the hippocampus (El-faramawy et al. 2009). These findings correlate well with the possible mechanism related with post-translational modification in protein Tau which has been associated with the loss of the MT network and also promoted the self-assembly of Tau to form aggregates.

Tau aggregation inhibitors

  1. Top of page
  2. Abstract
  3. Tau protein
  4. Tau oligomers
  5. Inductors of Tau aggregation
  6. Post-translational modifications associated with Tau aggregation
  7. Possible events in the intracellular aggregation of Tau
  8. Toxicity of Tau aggregates
  9. Metabolic dysfunction in Alzheimer’s disease
  10. Tau aggregation inhibitors
  11. Concluding remarks
  12. Acknowledgements
  13. References

It is known that the pathological aggregation of Tau correlates closely with the progression of AD, therefore, is important to look for therapies for this disorder. To date, available therapies are based on the use of cholinesterase inhibitors and NMDA receptor antagonist, and newer approaches focus on inhibition of Tau phosphorylation (Liu et al. 2008b) and aminopeptidase activation. Protein aggregation is closely associated with cellular malfunction and cytotoxicity (Congdon et al. 2007). The development of Tau aggregation inhibitors that would also be able to disaggregate filaments could provide an alternative to existing pharmaceutics strategies (Bulic et al. 2007). In principle, the inhibitory effect of those potential compounds could take place on different levels, for example, interference with a particular Tau conformation, association of dimers or oligomers, elongation of filaments, and so on. In particular, this compound could interfere with the initial generation of nuclei or with the further elongation of fibrils. This could be achieved by tight binding of the compound to the protein monomer or oligomer. Other therapeutic includes the steric obstruction of the protein–protein interaction by the compound, or interference with the polyanion inducers of aggregation (Pickhart et al. 2007). For Tau protein, in vitro assays for N774 (Congdon et al. 2007), rhodamines (Bulic et al. 2007), phenylthiazolylhydrazide (Pickhart et al. 2007), and phenothiazines (Wischik et al. 1996; Taniguchi et al. 2005) (Table 2), encourage the search for improved potencies in aggregation inhibition, with particular emphasis on disaggregation ability, permeability, and cytotoxicity. The design of the cell-permeable compounds would be a key step in the future development of new generations of aggregation inhibitors. Especially the negative or positive charges shared by most aggregation inhibitors impede membrane permeability (Bulic et al. 2009). In general, useful drugs can be developed on the basis of a combination of biological activity and drug-like properties. Thus, the compounds identified so far will have to be optimized with regard to oral bioavailability, aqueous solubility, and metabolic clearance (Pickhart et al. 2007). Together this data suggest that a possible alternative to AD treatment and other tauopathies is the inhibition of the formation of Tau oligomers, its nucleation and aggregation.

Table 2.   Various inhibitors employed in Tau aggregation assays
Compound employedAssays characteristicsReferences
Anthraquinones: emodin, daunorubicin, adriamycin, PHF016, PHF005K19 fragment was induced to aggregation with heparin and N2a cells with expression of K18ΔK280. Measurements of inhibition with thioflavin-S and tryptophan fluorescence spectroscopy; filaments observations in Electronic Microscopy (EM).Pickhart et al. (2005)
N-phenylamines: B4D3, B1C11, B4A1N2a cells expressed K18 or K18ΔK280 constructions with an inducible promoter. Quantification of aggregation with thioflavin-S assays. Observations PHF-like with EM.Khlistunova et al. (2005)
Phenotiazines: methylene blue, azure A, azure B, quinancrine mustard. Polyphenols: myricetin, epicatechin 3-gallate, gossypetin, 2,3,4,2′,4′-pentahydroxyphenone Porphyrins: ferric dehydroporphyrin IXAggregation induction of recombinant Tau with heparin and also assembly of Aβ was investigated in parallel. Evaluation of inhibition with thioflavin-S assays, EM, and Sarkosyl-insoluble Tau. Compounds that inhibited Tau filament assembly were also found to inhibit the formation of Aβ fibrils. Possibly inhibits Tau oligomerization.Taniguchi et al. (2005)
Rhodanine-derived: 5, 14, 17, 19, 20, 21, 30, 31, 1Quantification of aggregation with thioflavin-S assays. Observations of filaments with EM in N2a cells expressing K18ΔK280 Tau.Bulic et al. (2007)
Phenylthiazolyl-hydrazide: BSc3094Induction of aggregation with heparin in N2a cells with inducible expression of K18ΔK280 construction. Analysis by saturation transfer difference NMR and surface plasmon resonance.Pickhart et al. (2007)
Cyanine dyes: thiacarbocyanine dye N744Protein was incubated in assembly buffer with or without fibrillization inducers C18H37SO4Na or carboxylate-modified microspheres. Measures were performed with absorbance spectroscopy. Shows limited action according to dosage.Congdon et al. (2007)
Cyanine dyes: 3,3′-diethyl-9-methyl-thiacarbocyanine iodide C11An organotypic slice culture model derived from Tau transgenic mice and split Green fluorescent protein in HEK cultures expressing Tau24 were used for inhibitor evaluation by sarkosyl-insoluble fractions, quantitative immunoblot analysis and filament quantification.Congdon et al. (2009)

Concluding remarks

  1. Top of page
  2. Abstract
  3. Tau protein
  4. Tau oligomers
  5. Inductors of Tau aggregation
  6. Post-translational modifications associated with Tau aggregation
  7. Possible events in the intracellular aggregation of Tau
  8. Toxicity of Tau aggregates
  9. Metabolic dysfunction in Alzheimer’s disease
  10. Tau aggregation inhibitors
  11. Concluding remarks
  12. Acknowledgements
  13. References

The field of Tau oligomers and aggregates has becoming an extremely important issue which should be addressed because these structures by becoming PHF and NFTs will be causing the neurological and irreversible damage because of neuronal death in AD. The proposed mechanisms that regulate the process of polymerization of Tau deserves further investigations to allow us to improve the knowledge at this respect and design possible therapeutic targets to inhibit the toxicity of these abnormal polymers of Tau.

References

  1. Top of page
  2. Abstract
  3. Tau protein
  4. Tau oligomers
  5. Inductors of Tau aggregation
  6. Post-translational modifications associated with Tau aggregation
  7. Possible events in the intracellular aggregation of Tau
  8. Toxicity of Tau aggregates
  9. Metabolic dysfunction in Alzheimer’s disease
  10. Tau aggregation inhibitors
  11. Concluding remarks
  12. Acknowledgements
  13. References
  • Abraha A., Ghoshal N., Gamblin T. C., Cryns V., Berry R. W., Kuret J. and Binder L. I. (2000) C-terminal inhibition of tau assembly in vitro and in Alzheimer’s disease. J. Cell Sci. 113, 37373745.
  • Adams C. W. M. and Bruton C. J. (1989) The cerebral vasculature in dementia pugilistica. J. Neurol. Neurosurg. Psychiatry 52, 600604.
  • Alonso A. D., Grundke-Iqbal I., Barra H. S. and Iqbal K. (1997) Abnormal phosphorylation of tau and the mechanism of Alzheimer neurofibrillary degeneration: sequestration of microtubule-associated proteins 1 and 2 and the disassembly of microtubules by the abnormal tau. Proc. Natl Acad. Sci. USA 94, 298303.
  • Alonso A. C., Zaidi T., Novak M., Grundke-Iqbal I. and Iqbal K. (2001) Hyperphosphorylation induces self-assembly of t into tangles of paired helical filaments/straight filaments. Proc. Natl Acad. Sci. USA 98, 69236928.
  • Andronesi O. C., Von Bergen M., Biernat J., Seidel K., Griesinger C., Mandelkow E. and Baldus M. (2008) Characterization of Alzheimer’s-like paired helical filaments from the core domain of tau protein using solid-state NMR spectroscopy. J. Am. Chem. Soc. 130, 59225928.
  • Arai T., Guao J. P. and McGeer P. L. (2005) Proteolysis of non-phosphorylated and phosphorylated Tau by Thrombin. J. Cell Biol. 280, 51455153.
  • Ávila J., Lucas J. J., Pérez M. and Hernández F. (2004) Role of tau protein in both physiological and pathological conditions. Physiol. Rev. 84, 361384.
  • Bandyopadhyay B., Li G., Yin H. and Kuret J. (2007) Tau aggregation and toxicity in a cell culture model of tauopathy. J. Biol. Chem. 282, 1645416464.
  • Barghorn S. and Mandelkow E. (2002) Toward a unified scheme for the aggregation of tau into Alzheimer paired helical filaments. Biochemistry 41, 1488514896.
  • Basurto-Islas G., Luna-Muñoz J., Guillozet-Bongaart A., Binder L., Mena R. and García-Sierra F. (2008) Accumulation of aspartic acid 421-and glutamic acid 391-cleaved Tau in neurofibrillary tangles correlates with progression in Alzheimer disease. J. Neuropathol. Exp. Neurol. 67, 114.
  • Von Bergen M., Friedhoff P., Biernat J., Heberle J., Mandelkow E. M. and Mandelkow E. (2000) Assembly of t protein into Alzheimer paired helical filaments depends on a local sequence motif (306VQIVYK311) forming β structure. Proc. Natl Acad. Sci. USA 97, 51295134.
  • Berger Z., Roder H., Hanna A. et al. (2007) Accumulation of pathological tau species and memory loss in a conditional model of tauophaty. J. Neurosci. 27, 36503662.
  • Binder L. I., Guillozet-Bongaartsa A. L., Garcia-Sierra F. and Berry R. W. (2005) Tau, tangles, and Alzheimer’s disease. Biochim. Biophys. Acta 1739, 216223.
  • Braak H. and Braak E. (1991) Neuropathological staging of Alzheimer-related changes. Acta Neuropathol. 82, 239259.
  • Braak H., Alafuzoff I., Arzberger T., Kretzshmar H. and Del Tredici K. (2006) Staging of Alzheimer disease-associated neurofbrillary pathology using paraffin sections and immunocytochemistry. Acta Neuropathol. 112, 389404.
  • Brandt R., Hundetl M. and Shahani N. (2005) Tau alteration and neuronal degeneration in Tauopathies: mechanisms and models. Biochim. Biophys. Acta 1739, 331354.
  • Buée L., Bussière T., Buée-Scherrer V., Delacourte A. and Hof P. R. (2000) Tau protein isoforms, phosphorylation and role in neurodegenerative disorders. Brain Res. Rev. 33, 95130.
  • Bulic B., Pickhart M., Khlistunova I., Biernat J., Mandelkow E.-M., Mandelkow E. and Waldmann H. (2007) Rhodanine-based tau aggregation inhibitors in cell models of tauopathy Angew. Chem. Int. Ed. 46, 16.
  • Bulic B., Pickhart M., Schmidt B., Mandelow E.-M., Waldmann H. and Mandelkow E. (2009) Development of Tau aggregation inhibitors for Alzheimer Disease Angrew. Chem. Int. Ed. 48, 17401752.
  • Carlson S. W., Branden M., Voss K., Sun Q., Rankin C. A. and Gamblin T. C. (2007) A complex mechanism for inducer mediated tau polymerization. Biochemistry. 46, 88388849.
  • Chirita C. N., Congdon E. E., Yin H. and Kuret J. (2005) Triggers of full-length tau aggregation: a role for partially folded intermediates. Biochemistry. 44, 58625872.
  • Congdon E. E., Necula M., Blackstone R. D. and Kuret J. (2007) Potency of a Tau fibrillization inhibitor is influenced by its aggregation state. Arch. Biochem. Biophys. 465, 127135.
  • Congdon E. E., Kim S., Bonchak J., Songrug T., Matzavinos A. and Kuret J. (2008) Nucleation-dependent tau filament formation, the importance of dimerization and an estimation of elementary rat constants. J. Biol. Chem. 283, 1380613816.
  • Congdon E. E., Figueroa Y., Wang L., Toneva G., Chang E., Kuret J., Conrad C. and Duff K. (2009) Inhibition of Tau polymerization with a cyanine dye in two distinct model systems. J. Biol. Chem. 284(31), 2083020839.
  • Cripps D., Thomas S. N., Jeng Y., Yang F., Davies P. and Yang A. J. (2005) Alzheimer disease-specific conformation of hyperphophorylated paired helical filament-tau is polyubiquitinated through Lys-48, Lys-11, and Lys-6 ubiquitin conjugation. J. Biol. Chem. 281, 1082510838.
  • Cullen K. M. and Halliday G. M. (1995a) Mechanisms of cell death in cholinergic basal forebrain neurons in chronic alcoholics. Metab. Brain Dis. 10, 8191.
  • Cullen K. M. and Halliday G. M. (1995b) Neurofibrillary tangles in chronic alcoholics. Neuropathol. Appl. Neurobiol. 21, 312318.
  • Deshpande A., Win K. M. and Buschiglio J. (2008) Tau isoform expression and regulation in human cortical neurons. FASEB J. 22, 23572367.
  • Dickey C. A., Yue M., Lin W. L. et al. (2006) Deletion of the ubiquitin ligase CHIP leads to the accumulation, but not the aggregation, of both endogenous phospho- and caspase-3-cleaved Tau species. J. Neurosci. 26, 69856996.
  • Dickson D. W., Rademarkers R. and Hutton M. L. (2007) Progressive supranuclear palsy: pathology and genetics. Brain Pathol. 17, 7482.
  • Ding H. and Johnson V. W. (2008) The last tangle of tau. J. Alzheimers Dis. 14, 441447.
  • Dubey M., Chaudhury P., Kabiru H. and Shea T. B. (2008) Tau inhibits anterograde axonal transport and perturbs stability in growing axonal neurites in part by displacing kinesin cargo: neurofilaments attenuate tau-mediated neurite instability. Cell Motil. Cytoskeleton 65, 8999.
  • Ebneth A., Godemann R., Stamer K., Illenberger S., Trinczek B., Mandelkow E. M. and Mandelkow E. (1998) Overexpression of tau protein inhibits kinesin-dependent trafficking of vesicles, mitochondria, and endoplasmic reticulum: implications for Alzheimer’s disease. J. Cell Biol. 143, 777794.
  • El-faramawy Y. A., El-banouby M. H., Sergeev P., Mortagy A. K., Amer M. S. and Abdel-tawab A. M. (2009) Changes in glutamate decarboxylase enzyme activity and tau-protein phosphorylation in the hippocampus of old rats exposed to chronic mild stress: reversal with the neuronal nitric oxide synthase inhibitor 7-nitroindazole. Pharmacol. Biochem. Behav. 91, 339344.
  • Fasulo L., Ugolini G., Visitin M., Bradbury A., Brancolini C., Verzilo V., Novak M. and Cattaneo A. (2000) The neuronal microtubule-associated protein Tau is a substrate for caspase-3 and effector of apoptosis. J. Neurochem. 75, 624633.
  • Frost B., Jacks R. L. and Diamond M. I. (2009) Propagation of tau misfolding from the outside to the inside of a cell. J. Biol. Chem. 284, 1284512852.
  • Gamblin T. C., Berry R. W. and Binder L. I. (2003) Tau polymerization: role of the amino terminus. Biochemistry 42, 22522257.
  • García-Sierra F., Wishick C. M., Harrington C. R., Luna-Muñoz J. and Mena R. (2001) Accumulation of C-terminally truncated Tau protein associated with vulnerability of the perforant pathway in early stages of neurofibrillary pathology in Alzheimer’s disease. J. Chem. Neuroanat. 22, 6577.
  • Gendron T. F. and Petrucelli L. (2009) The role of tau in neurodegeneration. Mol. Neurodegener. 4, 13 (1–19).
  • Ghoshal N., García-Sierra F., Wuu J., Leurgans S., Bennett D. A., Berry R. W. and Binder L. I. (2002) Tau conformational changes correspond to impairments of episodic memory in mild cognitive impairment and Alzheimer’s disease. Exp. Neurol. 177, 475493.
  • Goedert M., Wischik C. M., Crowther R. A., Walker J. E. and Klug A. (1988) Cloning and sequencing of the cDNA encoding a core protein of the paired helical filament of Alzheimer disease: identification as the microtubule-associated protein tau. Proc. Natl Acad. Sci. USA 85(11), 40514055.
  • Goedert M., Jakes R., Spillantini M. G., Hasegawa M., Smith M. J. and Crowther R. A. (1996) Assembly of microtubule-associated protein tau into Alzheimer-like filaments induced by sulphated glycosaminoglycans. Nature 383, 550553.
  • Golde T. E. (2006) Disease modifying therapy for AD? J. Neurochem. 99, 689707.
  • Gray E. G., Paula-Barbosa M. and Roher A. (1987) Alzheimer’s disease: paired helical filaments and cytomembranes. Neuropathol. Appl. Neurobiol. 13, 91110.
  • Guillozet-Bongaarts A. L., Glajch K. E., Libson E. G., Cahill M. E., Bigio E., Berry R. W. and Binder L. I. (2007) Phosphorylation and cleavage of tau in non-AD tauopathies. Acta Neuropathol. 113, 513520.
  • Hamano T., Gendron T. F., Causevic E., Yen S.-H., Lin W.-L., Isidoro C., DeTure M. and Ko L.-W. (2008) Autophagic-lysosomal perturbation enhances tau aggregation in transfectants with induced wild-type tau expression. Eur. J. Neuorsci. 27, 11191130.
  • Hanger D. P., Anderton B. H. and Noble W. (2009) Tau phosphorylation: the therapeutic challenge for neurodegenerative disease. Trends Mol. Med. 15, 112119.
  • Hernández F. and Ávila J. (2007) Tauopathies. Cell. Mol. Life Sci. 64, 22192233.
  • Hernández F., Cuadros R. and Ávila J. (2004) Zeta 14-3-3 favours the formation of human tau fibrillar polymers. Neurosci. Lett. 357, 143146.
  • Hikosou R., Kurabayashi Y., Doumoto M., Hoshitoku K., Mizushima F., Minoura K., Tomoo K. and Ishida T. (2007) Effect of DNA on filament formation of tau microtubule-binding domain: structural dependence of DNA. Chem. Pharm. Bull. 55, 10301033.
  • Holzter M., Gärtner U., Stöbe A., Härtig W., Gruschka H., Brucknër N. K. and Arendt T. (2002) Inverse association of Pin1 and tau accumulation in Alzheimer’s disease hippocampus. Acta Neuropathol. 104, 471481.
  • Horowitz P. M., Patterson K. R., Guillozet-Bongaarts A. L., Reynolds M. R., Carroll C. A., Weintraub S. T., Bennett D. A., Cryns V. L., Berry R. W. and Binder L. I. (2004) Early N-terminal changes and caspase-6 cleavage of Tau in Alzheimer’s disease. J. Neurosci. 24, 78957902.
  • Horowitz P. M., LaPointe N., Guillozet-Bongaarts A., Berry R. and Binder L. (2006) N-terminal fragments of tau inhibit full-length tau polymerization in vitro. Biochemistry 45, 1285912866.
  • Hrnkova M., Zilka N., Minichova Z., Koson P. and Novak M. (2007) Neurodegeneration caused by expression of human truncated Tau leads to progressive neurobehavioral impairment in transgenic rats. Brain Res. 1130, 206213.
  • Hua Q. and He R. Q. (2003) Tau could protect DNA double helix structure. Biochim. Biophys. Acta 1645, 205211.
  • Iqbal K. and Grundke-Iqbal I. (2008) Alzheimer neurofibrillary degeneration: significance, etiopathogenesis, therapeutics and prevention. J. Cell Mol. Med. 12, 3855.
  • Ishii H., Ishikawa H., Meguro K., Tashiro M. and Yamaguchi S. (2009) Decreased cortical glucose metabolism in converters from CDR 0.5 to Alzheimer’s disease in a community: the Osaki-Tajiri project. Int. Psychogeriatr. 21, 148156.
  • Jämsä A., Bäckström A., Gustafsson E., Dehvari N., Hiller G., Cowburn R. F. and Vasänge M. (2006) Glutamate treatment and p25 transfection increase Cdk5 mediated tau phosphorylation in SH-SY5Y cells. Biochem. Biophys. Res. Commun. 345, 324331.
  • Johnson G. V. W. and Stoothoff W. H. (2004) Tau phosphorylation in neuronal cell function and dysfunction. J. Cell Sci. 117, 57215729.
  • Kampers T., Friedhoff P., Biernat J., Mandelkow E. M. and Mandelkow E. (1996) RNA stimulates aggregation of microtubule-associated protein tau into Alzheimer-like paired helical filaments. FEBS Lett. 399, 344349.
  • Kayed R. and Jackon G. R. (2009) Prefilament tau species as potential targets for immunotherapy for Alzheimer disease and related disorders. Curr. Opin. Inmunol. 21, 359363.
  • Khlistunova I., Biernat J., Wang Y., Pickhart M., Von Berger M., Gazova Z., Mandelkow E. and Mandelkow E.-M. (2005) Inducible expression of Tau repeat domain in cell models of tauopathy Aggregation is toxic to cells but can be reversed by inhibitor drugs. J. Biol. Chem. 281, 12051214.
  • King M. E., Ahuja V., Binder L. I. and Kuret J. (1999) Ligand-dependent tau filament formation: implications for Alzheimer’s disease progression. Biochemistry 38, 1485114859.
  • Konno T., Oiki S., Hasegawa K. and Naiki H. (2004) Anionic contribution for fibrous maturation of protofibrillar assemblies of the human tau repeat domain in a fluoroalcohol Solution. Biochemistry 43, 1361313620.
  • Koson P., Zilka N., Kovac A., Kovacech B., Koreniva M., Filipcik P. and Novak M. (2008) Truncated tau expression levels determine life span of a rat model of tauopathy without causing neuronal loss or correlating with terminal neurofibrillary tangle load. Eur. J. Neurosci. 28, 239246.
  • Ksiezak-Reding H. and Wall J. S. (2005) Characterization of paired helical filaments by scanning transmission electron microscopy. Microsc. Res. Tech. 67, 126140.
  • Kuczynski B., Reed B., Mungas D., Weiner M., Chui H. C. and Jagust W. (2008) Cognitive and Anatomic Contributions of Metabolic Decline in Alzheimer Disease and Cerebrovascular Disease. Arch. Neurol. 65, 650655.
  • Kuhla B., Haase C., Flach K., Lüth H.-J., Arendt T. and Münch G. (2007) Effect of pseudophosphorylation and cross-linking by lipid peroxidation and advanced glycation en d product precursors on tau aggregation and filament formation. J. Biol. Chem. 282, 69846991.
  • Kuret J., Congdon E. E., Li G., Yin H., Yu X. and Zhong Q. I. (2005) Evaluating triggers and enhancers of tau fibrillization. Microsc. Res. Tech. 67, 141155.
  • Lebouvier T., Scales T. M., Williamson R., Noble W., Duyckaerts C., Hanger D. P., Reynolds C. H., Anderton B. H. and Derkinderen P. (2009) The microtubule-associated protein tau is also phosphorylated on tyrosine. J Alzheimers Dis. 18, 19.
  • Ledesma M. D., Bonay P., Colaço C. and Avila J. (1994) Analysis of microtubule-associated protein glycation in paired helical filaments. J. Biol. Chem. 269, 2161421619.
  • Lee G., Newman S. T., Gard D. L., Band H. and Panchamoorthy G. (1998) Tau interacts with src-family non-receptor tyrosine kinases. J. Cell Sci. 111, 31673177.
  • Li D.-W., Mohanty S., Irbäck A. and Huo S. (2008) Formation and growth of oligomers: a Monte Carlo study of an amyloid tau fragment. PLOS Comput. Biol. 4(12), e1000238 (1–12).
  • Liang W. S., Dunckley T., Beach T. G. et al. (2007) Gene expression profiles in anatomically and functionally distinct regions of the normal aged human brain. Physiol. Genomics 28, 311322.
  • Liang W. S., Reiman E. M., Valla J. et al. (2008) Alzheimer’s disease is associated with reduced expression of energy metabolism genes in posterior cingulate neurons. Proc. Natl Acad. Sci. USA 105, 44414446.
  • Lin Y.-T., Cheng J.-T., Liang L.-C., Ko C.-Y., Lo Y.-K. and Lu P.-J. (2007) The binding and phosphorylation of Thr231 is critical for tau’s hyperphosphorylation and functional regulation by glycogen synthase kinase 3β. J. Neurochem. 103, 802813.
  • Lira-De León K. I., De Anda-Hernández M. A., Campos-Peña V. and Meraz-Ríos M. A. (2009) Plasma membrane-associated PHF-core could be the trigger for tau aggregation, in Alzheimer’s Disease Current Hypotheses and Research Milestones in Alzheimer’s Disease (MaccioniR. B. and PerryG., eds.), pp. 93100. Springer, New York.
  • Liu F., Iqbal K., Grundke-iqbak I. and Gong C.-X. (2002) Involvement of aberrant glycosylation in phosphorylation of tau by cdk5 and GSK3β. FEBS Lett. 530, 209214.
  • Liu F., Li B., Tung E.-J., Grundke-Iqbal I., Iqbal K. and Gong C.-X. (2007) Site-specific effects of tau phosphorylation on its microtubule assembly activity and self-aggregation. Eur. J. Neurosci. 26, 34293436.
  • Liu Y.-H., Wei W., Yin J., Lui G.-P., Wang Q., Cao F.-Y. and Wang J.-Z. (2008a) Protesosome inhibition increases tau accumulation independent of phosphorylation. Neurobiol. Aging 30(12), 19491961.
  • Liu M., Choi S., Cuny G. D., Ding K., Dobson B. C., Glicksman M. A., Auserbach K. and Stein R. L. (2008b) Kinetic studies of Cdk5/p25 kinase: phosphorylation of Tau and complex inhibition by two prototype inhibitors. Biochemistry 47, 83678377.
  • Liu F., Shi J., Tanimukai H., Gu J., Gu J., Grundke-Iqbal I., Iqbal K. and Gong C. X. (2009) Reduced O-GlcNAcylation links lower brain glucose metabolism and tau pathology in Alzheimer’s disease. Brain 132, 18201832.
  • Lovestone S. and McLoughlin D. M. (2002) Protein aggregates and dementia: is there a common toxicity? J. Neurol. Neurosurg. Psychiatry 72, 152161.
  • Lu P. J., Wulf G., Zhou X.-Z., Davies P. and Lu K. P. (1999) The prolyl isomerase Pin1 restores the function of Alzheimer-associated phosphorylated tau protein. Nature 399, 784788.
  • Ludolph A. C., Kassubek J., Landwehrmeyer B. G. et al. (2009) Tauopathies with parkinsonism: clinical spectrum, neuropathologic basis, biological markers, and treatment options. Eur J Neurol. 16, 297309.
  • Luna-Muñoz J., Chávez-Macías L., García-Sierra F. and Mena R. (2007) Earliest stages of tau conformational changes are related to the appearance of a sequence of specific phospho-dependent tau epitopes in Alzheimer’s disease. J. Alzheimers Dis. 12, 365375.
  • Maeda S., Sahara N., Saito Y., Muruyama S., Ikai A. and Takashima A. (2006) Increased levels of granular tau oligomers: an early sign of brain aging and Alzheimer’s disease. Neurosci. Res. 54, 197201.
  • Maeda S., Sahara N., Saito Y., Murayama M., Yoshiike Y., Kim H., Miyasaka T., Murayama S., Ikai A. and Takashima A. (2007) Granular Tau oligomers as intermediates of Tau filaments. Biochemistry 46, 38563861.
  • Makrides V., Shen T. E., Bhatia R., Smith B. L., Thimm J., Lal R. and Feinstein S. C. (2003) Microtubule-dependent oligomerization of tau: implications for physiological tau function and tauopathies. J. Biol. Chem. 278, 3329833304.
  • Mandelkow E., Von Bergen M., Biernat J. and Mandelkow E. M. (2007) Structural principles of tau and the paired helical filaments of Alzheimer’s disease. Brain Pathol. 17, 8390.
  • Manetto V., Perry G., Tabaton M., Mulvihill P., Fried V. A., Smith H. T., Gambetti P. and Autilio-Gambetti L. (1988) Ubiquitin is associated with abnormal cytoplasmic filaments characteristic of neurodegenerative diseases. Proc. Natl Acad. Sci. USA 85, 45014505.
  • Mocanu M.-M., Niessen A., Eckermann K. et al. (2008) The potential for β-structure in the repeat domain of tau protein determines aggregation, synaptic decay, neuronal loss, and coassembly with endogenous tau in inducible mouse models of tauopathy. J. Neurosci. 28, 737748.
  • Mondragón-Rodríguez S., Basurto-Islas G., Santa-Maria I., Mena R., Binder L. I., Avila J., Smith M. A., Perry G. and García-Sierra F. (2008) Cleavage and conformational changes of tau protein follow phosphorylation during Alzheimer’s disease. Int. J. Exp. Pathol. 89, 8190.
  • Montejo de Garcini E., Carrascosa J. L., Correas I., Nieto A. and Ávila J. (1988) Tau factor polymers are similar to paired helical filaments of Alzheimer’s disease. FEBS Lett. 236, 150154.
  • Murakami N. (1999) Parkinsonism-dementia complex on Guam – overview of clinical aspects. J. Neurol. 246(Suppl. 2), II16II18.
  • Nie C. L., Wang X. S., Liu Y., Perrett S. and He R. Q. (2007a) Amyloid-like aggregates of neuronal tau induced by formaldehyde promote apoptosis of neuronal cells. BMC Neurosci. 8, 9.
  • Nie C. L., Wei Y., Chen X., Liu Y. Y., Dui W., Liu Y., Davies M. C., Tendler S. J. B. and He R. G. (2007b) Formaldehyde at low concentration induces protein tau into globular amyloid-like aggregates in vitro and in vivo. PLoS ONE 2(7), e629 (1–13).
  • Novak M., Kabat J. and Wischik C. M. (1993) Molecular characterization of the minimal protease resistant Tau unit of the Alzheimer’s disease paired helical filament. EMBO J. 12, 365370.
  • Pacheco C. D. and Lieberman A. P. (2008) The pathogenesis of Niemann–Pick type C disease: a role for autophagy? Expert Rev. Mol. Med. 10, e26 (1–17).
  • Pérez M., Arrasate M., Montejo de Garcini E., Muñoz V. and Ávila J. (2001) In vitro assembly of tau protein: mapping the regions involved in filament formation. Biochemistry 40, 59835991.
  • Petterson D. W., Zhou H., Dahlquist F. W. and Lew J. (2008) A soluble oligomer of tau associated with fiber formation analyzed by NMR. Biochemistry 47, 73937404.
  • Pickhart M., Gazova Z., Von Bergen M., Khlistunova I., Wang Y., Hascher A., Mandelkow E.-M., Biernat J. and Mandelkow E. (2005) Anthraquinones inhibit tau aggregation and dissolve Alzheimer’s paired helical filaments in vitro and in cells. J. Biol. Chem. 280, 36283635.
  • Pickhart M., Larbig G., Khlistunova I., Coksezen A., Meyer B., Mandelkow E.-M., Schmidt B. and Mandelkow E. (2007) Phenylthiazolyl-hydrazide and its derivates are potent inhibitors of Tau aggregation and toxicity in vitro and in cells. Biochemistry 46, 1001610023.
  • Reyes J. F., Reynolds M. R., Horowitz P. M., Fu Y., Guillozet-Bongaarts A., Berry R. and Binder L. (2008) A possible link between astrocyte activation and Tau nitration in Alzheimer’s disease. Neurobiol. Dis. 31, 198208.
  • Reynolds M. R., Berry R. and Binder L. I. (2005) Site-specific nitration differentially influences assembly in vitro. Biochemistry 44, 1399714009.
  • Reynolds M. R., Reyes J. F., Fu W., Bigio E. H., Guillozet-Bongaarts A., Berry R. and Binder L. (2006) Tau nitration occurs at tyrosine 29 in the fibrillar lesions of Alzheimer’s disease and other tauopathies. J. Neurosci. 26, 1063610645.
  • Reynolds M. R., Berry R. and Binder L. I. (2007) Nitration in neurodegeneration: deciphering the Hows ‘nYs’. Biochemistry 46, 73257336.
  • Robert M. and Mathuranath P. S. (2007) Tau and taupathies. Neurol. India 55, 1116.
  • Rojo L., Sjöber M. K., Henández P., Zambrano C. and Maccioni R. B. (2006) Roles of cholesterol and lipids in the etiopathogenesis of Alzheimer’s disease. J. Biomed. Biotech. 2006, 117.
  • Rosenberg K. J., Ross J. L., Feinstein H. E., Feinstein S. C. and Israelachvili J. (2008) Complementary dimerization of microtubule-associated tau protein: implications for microtubule bundling and tau-mediated pathogenesis. Proc. Natl Acad. Sci. USA 105, 74457450.
  • Rupsingh R., Borrie M., Smith M., Wells J. L. and Bartha R. (2009) Reduced hippocampal glutamate in Alzheimer disease. Neurobiol. Aging (In press NBA-7333).
  • Sadik G., Tanaka T., Kato K., Yamamori H., Nessa B. N., Morihara T. and Takeda M. (2009) Phosphorylation of tau at Ser214 mediates its interaction with 14-3-3 protein: implications for the mechanism of tau aggregation. J. Neurochem. 108, 3343.
  • Sahara N., Maeda S., Murayama M., Suzuki T., Dohmae N., Yen S.-H. and Takashima A. (2007a) Assembly of two distinct dimers and higher-order oligomers from full-length tau. Eur. J. Neurosci. 25, 30203029.
  • Sahara N., Maeda S., Yoshiike Y., Mizoroki T., Yamashita S., Murayama M., Park J.-M., Sito Y., Murayama S. and Takashima A. (2007b) Molecular chaperone-mediated tau protein metabolism counteracts the formation of granular tau oligomers in human brain. J. Neurosci. Res. 85, 30983108.
  • Sánchez M. P., Álvarez-Tallada V. and Ávila J. (2001) La proteína tau en enfermedades neurodegenerativas Taupatías. Rev. Neurol. 33, 169177.
  • Santacruz K., Lewis J., Spires T. et al. (2005) Tau suppression in a neurodegenerative mouse model improves memory function. Science 309, 476481.
  • Santa-María I., Hernández F., Smith M. A., Perry G., Ávila J. and Moreno F. J. (2005) Neurotoxic dopamine quinone facilitates the assembly of tau into fibrillar polymers. Mol. Cell. Biochem. 278, 203212.
  • Santa-María I., Hernández F., Moreno F. J. and Ávila J. (2007) Taurine, an inducer for tau polymerization and a weak inhibitor for amyloid-β-peptide aggregation. Neurosc. Lett. 429, 9194.
  • Sato Y., Naito Y., Grundke-Iqbal I., Iqbal K. and Endo T. (2001) Analysis of N-glycans of pathological tau: possible occurrence of aberrant processing of tau in Alzheimer’s disease. FEBS Lett. 496, 152160.
  • Sato S., Cerny R. L., Buescher J. L. and Ikezu T. (2006) Tau-tubulin kinase (TTBK1), a neuron-specific tau kinase candidate, is involved in tau phosphorylation and aggregation. J. Neurochem. 98, 15731584.
  • Sengupta S., Horowitz P. M., Karsten S. L., Jackson G. R., Geschwind D. H., Fu Y., Berry R. W. and Binder L. I. (2006) Degradation of Tau protein by puromycin-sensitive aminopeptidase in vitro. Biochemistry 50, 1511115119.
  • Sjöberg M. K., Shestakova E., Mansuroglu Z., Maccioni R. B. and Bonnefoy E. (2006) Tau protein binds to pericentromeric DNA: a putative role for nuclear tau in nucleolar organization. J. Cell Sci. 119, 20252034.
  • Skrabana R., Kontsek P., Merderlyova A., Iqbal K. and Novak M. (2004) Folding of Alzheimer’s core PHF subunit revealed by monoclonal antibody 423. FEBS Lett. 568, 178182.
  • Skrabana R., Sevcik J. and Novak M. (2006) Intrinsically disordered proteins in the neurodegenerative process: formation of Tau protein paired helical filaments and their analysis. Cell.Mol. Neurobiol. 26(7/8),10851097.
  • Sugino E., Nishiura C., Minoura K., In Y., Sumida M., Taniguchi T., Tomoo K. and Ishida T. (2009) Three-/four-repeat-dependent aggregation profile of tau microtubule-binding domain clarified by dynamic light scattering analysis. Biochem. Biophys. Res. Commun. 385, 236240.
  • Takahashi K., Uchida C., Shin R. W., Shimazaki K. and Uchida T. (2008) Prolyl isomerase, Pin1: new findings of post-translational modifications and physiological substrates in cancer, asthma and Alzheimer’s disease. Cell. Mol. Life Sci. 65, 359375.
  • Taniguchi S., Suzuki N., Masuda M., Hisanaga S.-I., Iwatsubo T., Goedert M. and Hasegawa M. (2005) Inhibition of heparin-induced tau filaments formation by phenothiazines, polyphenols and porphyrins. J. Biol. Chem. 280, 76147623.
  • Wang J. Z. and Liu F. (2008) Microtubule-associated protein tau in development, degeneration and protection of neurons. Prog. Neurobiol. 85, 148175.
  • Wang Y. P., Biernat J., Pickhardt M., Mandelkow E. and Mandelkow E. M. (2007) Stepwise proteolysis liberates tau fragments that nucleate the Alzheimer-like aggregation of full-length tau in a neuronal cell model. Proc. Natl Acad. Sci. USA 104, 1025210257.
  • Wei Y., Qu M. H., Wang X. S., Chen L., Wang D. L., Liu Y., Qian H. and He R. Q. (2008) Binding to the minor groove of the double-strand, tau protein prevents DNA from damage by peroxidation. PLoS ONE 3(7), e2600 (1–10).
  • Weingarten M. D., Lockwood A. H., Hwo S. Y. and Kirschner M. W. (1975) A protein factor essential for microtubule assembly. Proc. Natl Acad. Sci. USA 72, 18581862.
  • Williams D. R. and Lees A. J. (2009) Progressive supranuclear palsy: clinicopathological concepts and diagnostic challenges. Lancet Neurol. 8, 270279.
  • Winton M. J., Joyce S., Zhukareva V., Practico D., Perl D. P., Galasko D., Craig U., Trojanowski J. Q. and Lee V. M. Y. (2006) Characterization of tau pathologies in gray and white matter of Guam parkinsonism-dementia complex. Acta Neuropathol. 111, 401412.
  • Wischik C. M., Novak M., Thøgersen H. C., Edwards P. C., Runswick M. J., Jakes R., Walker J. E., Milstein C., Rotht M. and Klug A. (1988a) Isolation of a fragment of Tau derived from the core of the paired helical filament of Alzheimer disease. Proc. Natl Acad. Sci. USA 85, 45064510.
  • Wischik C. M., Novak M., Edwards P. C., Klug A., Tichelaar W. and Crowther R. A. (1988b) Structural characterization of the core of paired helical filament of Alzheimer disease. Proc. Natl Acad. Sci. USA 85, 48844888.
  • Wischik C. M., Edwards P. C., Lai R. Y. K., Roth M. and Harrington C. R. (1996) Selective inhibition of Alzheimer disease-like tau aggregation by phenothiazines. Proc. Natl Acad. Sci. USA 93, 1121311218.
  • Yang L.-S. and Ksiezak-Reding H. (1995) Calpain-induced proteolysis of normal human Tau and Tau associated with paired helical filaments. Eur. J. Biochem. 233, 917.
  • Yu C.-H., Si T., Wu W.-H., Hu J., Du J.-T., Zhao Y.-F. and Li Y.-M. (2008) O-GlcNacylation modulates the self-aggregation ability of the fourth microtubule-binding repeat of tau. Biochem. Biophys. Res. Commun. 375, 5962.
  • Zhang Y. J., Xu Y. F., Chen X. Q., Wang X. C. and Wang J. Z. (2005) Nitration and oligomerization of Tau induced by peroxynitrite inhibit its microtubule-binding activity. FEBS Lett. 579, 24212427.
  • Zilka N., Filipcik P., Koson P., Fialova L., Skrabana R., Zilkova M., Rolkova G., Kontsekova E. and Novak M. (2006) Truncated Tau from sporadic Alzheimer’s disease suffices to drive neurofibrillary degeneration in vivo. FEBS Lett. 580, 35823588.