Linkage-dependent contribution of repeat peptides to self-aggregation of three- or four-repeat microtubule-binding domains in tau protein

Although one of the priorities in Alzheimer's research is to clarify the filament formation mechanism for the tau protein, it is still unclear how it is transformed from a normal structure in a neuron. To examine the linkage-dependent contribution of each repeat peptide (R1-R4) to filament formation of the three- or four-repeat microtubule-binding domain (MBD) in the tau protein, four two-repeat peptides (R12, R13, R23 and R34) and two three-repeat peptides (R123 and R234) were prepared, and their in vitro self-aggregation was investigated by thioflavin S fluorescence and circular dichroism measurements, and by electron microscopy in neutral buffer (pH 7.6). Comparison of these aggregation behaviors with previous results for single-repeat peptides and wild-type 3RMBD (R134) and 4RMBD (R1234) indicated that (a) the two-repeat R23, not the R2 or R3 single repeat, forms the core structure in self-aggregation of 4RMBD, whereas that of 3RMBD comprises the R3 single repeat, (b) co-existence of R1 and R4 repeats is necessary for the aggregation behavior inherent in 3RMBD and 4RMBD, whereas the R1 or R4 repeat alone functions as a repressor or modifier of the filament formation, (c) 4RMBD aggregation is accompanied by R1-driven transition from random and alpha-helix structures to a beta-sheet structure, whereas 3RMBD aggregation involves three-repeat R134-specific transition from a random structure to an alpha-helix structure without the participation of a beta-sheet structure, and (d) the peptides that include the R1 repeat form a long filament irrespective of the absence or presence of the R4 repeat, whereas those that include the R4 repeat, but not the R1 repeat, form a relatively short filament. To the best of our knowledge, a systematic study of the linkage-dependent contribution of each repeat peptide to the paired helical filament formation of tau MBD has not been carried out previously, and thus the present information is useful for understanding the essence of the filament formation of tau MBD.     

Possible role of each repeat structure of the microtubule-binding domain of the tau protein in in vitro aggregation

Although one of the priorities in Alzheimer's research is to clarify the filament formation mechanism of the tau protein, it is currently unclear how it is transformed from a normal structure in a neuron. To examine which part and what structural change in the tau protein are involved in its transformation into a pathological entity, the initial in vitro self-aggregation features of each repeat peptide (R1-R4) constituting a three- or four-repeat microtubule-binding domain (3RMBD or 4RMBD) in the tau protein was investigated by measuring both the fluorescence and light scattering (LS) spectra on the same instrument, because these MBD domains constitute the core moiety of the tau paired helical filament (PHF) structure. The conformational features of the R1 and R4 peptides in trifluoroethanol were also investigated by (1)H-NMR and molecular modeling analyses and compared with those of the R2 and R3 peptides. The analyses of the LS spectra clarified (i) the self-aggregation rates of R1-R4, 3RMBD and 4RMBD at a fixed concentration (15 mM), (ii) their minimum concentrations for starting filament extension, and (iii) the concentration dependence of their self-aggregations. The fluorescence analyses showed that the R2 and R3 peptides have high self-aggregation abilities at the extension and nucleation steps, respectively, in their filament formation processes. It was shown that the R2 repeat exhibits a positive synergistic effect on the aggregation of 4RMBD. The R1 and R4 repeats, despite their weak self-aggregation abilities, are necessary for the intact PHF formation of tau MBD, whereas they exerted a negative effect on the R3-driven aggregation of 3RMBD. The conformational analyses showed the importance of the amphipathic conformational features of the R1 to R4 peptides, and the intermolecular disulfide bonding abilities of the R2 and R3 peptides for the PHF formation. On the basis of the present spectral and conformational results, the possible role of each repeat structure in the dimeric formation of MBD at the initial in vitro aggregation stage is discussed.     

Binding of copper (II) ion to an Alzheimer's tau peptide as revealed by MALDI-TOF MS, CD, and NMR

The tau protein plays an important role in some neurodegenerative diseases including Alzheimer's disease (AD). Neurofibrillary tangles (NFTs), a biological marker for AD, are aggregates of bundles of paired helical filaments (PHFs). In general, the alpha-sheet structure favors aberrant protein aggregates. However, some reports have shown that the alpha-helix structure is capable of triggering the formation of aberrant tau protein aggregates and PHFs have a high alpha-helix content. In addition, the third repeat fragment in the four-repeat microtubule-binding domain of the tau protein (residues 306-336: VQIVYKPVDLSKVTSKCGSLGNIHHKPGGGQ, according to the longest tau protein) adopts a helical structure in trifluoroethanol (TFE) and may be a self-assembly model in the tau protein. In the human brain, there is a very small quantity of copper, which performs an important function. In our study, by means of matrix assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF MS), circular dichroism (CD), and nuclear magnetic resonance (NMR) spectroscopy, the binding properties of copper (II) ion to the R3 peptide derived from the third repeat fragment (residues 318-335: VTSKCGSLGNIHHKPGGG) have been investigated. The results show that copper ions bind to the R3 peptide. CD spectra, ultraviolet (UV)-visible absorption spectra, and MALDI-TOF MS show pH dependence and stoichiometry of Cu2+ binding. Furthermore, CD spectra and NMR spectroscopy elucidate the copper binding sites located in the R3 peptide. Finally, CD spectra reveal that the R3 peptide adopts a mixture structure of random structures, alpha-helices, and beta-turns in aqueous solutions at physiological pH. At pH 7.5, the addition of 0.25 mol eq of Cu2+ induces the conformational change from the mixture mentioned above to a monomeric helical structure, and a beta-sheet structure forms in the presence of 1 mol eq of Cu2+. As alpha-helix and beta-sheet structures are responsible for the formation of PHFs, it is hypothesized that Cu2+ is an inducer of self-assembly of the R3 peptide and makes the R3 peptide form a structure like PHF. Hence, it is postulated that Cu2+ plays an important role in the aggregation of the R3 peptide and tau protein and that copper (II) binding may be another possible involvement in AD.     

Rapid tau aggregation and delayed hippocampal neuronal death induced by persistent thrombin signaling

Tau hyperphosphorylation, leading to self-aggregation, is widely held to underlie the neurofibrillary degeneration found in Alzheimer's disease (AD) and other tauopathies. However, it is unclear exactly what environmental factors may trigger this pathogenetic tau hyperphosphorylation. From several perspectives, the coagulation serine protease, thrombin, has been implicated in AD and activates several different protein kinase pathways but has not previously been shown how it may contribute to AD pathogenesis. Here we report that nanomolar thrombin induced rapid tau hyperphosphorylation and aggregation in murine hippocampal neurons via protease-activated receptors, which was followed by delayed synaptophysin reduction and apoptotic neuronal death. Mechanistic study revealed that a persistent thrombin signaling via protease-activated receptor 4 and prolonged downstream p44/42 mitogenactivated protein kinase activation are at least in part responsible. These results pathogenetically linked thrombin to subpopulations of AD and other tauopathies associated with cerebrovascular damage. Such knowledge may be instrumental in transforming therapeutic paradigms.     

The solution structure of the C-terminal segment of tau protein.

Pathological changes in the microtubule associated protein tau, leading to tau-containing filamentous lesions, are a major hallmark common to many types of human neurodegenerative diseases, including Alzheimer's disease (AD). No structural data are available which could rationalize the extensive conformational changes that occur when tau protein is converted to Alzheimer's paired helical filaments (PHF). The C-terminal portion of tau plays a crucial role in the aggregation of tau into PHF and in the truncation process that generates cytotoxic segments of tau. Therefore, we investigated the solution structure of the hydrophobic C-terminal segment 423-441 of tau protein (PQLATLADEVSASLAKQGL) by 1H 2D NMR spectroscopy. The peptide displays the typical NMR evidence consistent with a alpha-helix geometry with a stabilizing C-capping motif. The reported data represent the first piece of structural information on an important portion of the molecule and can have implications towards the understanding of its pathophysiology.     

Phosphorylation modulates the alpha-helical structure and polymerization of a peptide from the third tau microtubule-binding repeat

Paired helical filaments (PHFs) isolated from patients with Alzheimer's disease (AD) mainly consist of the microtubule-associated protein tau in a hyperphosphorylated form. It has been found that PHFs are the first example of pathological protein aggregation associated with formation of alpha-helices [Biochemistry (2002) 41, 7150-5]. In an effort to investigate the interplay between phosphorylation and the putative role of short regions of alpha-helix in the polymerization of tau, we have focused on the region of tau encompassing residues 317 to 335. This region is able to form protein fibrils in vitro and has two serines that are often found phosphorylated in PHFs. Using trifluoroethanol as an indicator of the alpha-helix, we find that the stability of the alpha-helix conformation is enhanced by phosphorylation. Circular dichroism data show that the phosphorylated peptide in water presents a content in alpha-helix similar to the unphosphorylated peptide at 40% of trifluoroethanol. Phosphorylation also stimulates the effect of juglone in promoting the in vitro polymerization. Furthermore, Fourier transformed infrared spectroscopy of samples of phosphorylated peptide polymerized with juglone renders a spectrum with maxima at approximately 1665 and approximately 1675 cm(-1), which are suggestive of a mixture of turns and alpha-helix conformations. Our results provide a direct mechanistic connection between phosphorylation and polymerization in tau. The connection between phosphorylation and polymerization appears to involve formation of alpha-helix structure.     

Copper binding properties of a tau peptide associated with Alzheimer's disease studied by CD, NMR, and MALDI-TOF MS

We have previously reported the copper binding properties of R3 peptide (residues 318-335: VTSKCGSLGNIHHKPGGG, according to the longest tau protein) derived from the third repeat microtubule-binding domain of water-soluble tau protein. In this work, we have investigated copper binding properties of R2 peptide (residues 287-304: VQSKCGSKDNIKHVPGGG) derived from the second repeat region of tau protein. Similar to R3 peptide, R2 peptide also plays an important role in the formation of neurofibrillary tangles (NFTs) which is one of the two main biological characteristics of Alzheimer's disease (AD). Based on the copper binding properties of R2 peptide, the possible influences of the binding on the formation of NFTs were investigated. Results from circular dichroism (CD) spectra, nuclear magnetic resonance (NMR) spectroscopy, and matrix assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF MS) suggest that the binding is pH-dependent and stoichiometry-determined. In addition, these results also reveal that R2 peptide adopts a monomeric alpha-helical structure in aqueous solutions at physiological pH after the addition of 1 mol equiv. of Cu2+. Since alpha-helix structure is responsible for the formation of paired helical filaments (PHFs) which aggregate into NFTs, it is hypothesized that Cu2+ induces R2 peptide to self-assemble into a PHFs-like structure. Hence, it is postulated that Cu2+ plays an important role in the aggregation of R2 peptide and tau protein and that copper binding to R2 peptide may be another possible involvement in AD.     

Mercury(II) promotes the in vitro aggregation of tau fragment corresponding to the second repeat of microtubule‐binding domain: Coordination and conformational transition

The loss of metal homeostasis and the toxic effect of metal ion are important events in neurodegenerative and age‐related diseases, such as Alzheimer's disease (AD). For the first time, we investigated the impacts of mercury(II) ions on the folding and aggregation of Alzheimer's tau fragment R2 (residues 275‐305: VQIIN KKLDL SNVQS KCGSK DNIKH VPGGGS), corresponding to the second repeat unit of the microtubule‐binding domain, which was believed to be pivotal to the biochemical properties of full tau protein. By ThS fluorescence assay and electron microscopy, we found that mercury(II) dramatically promoted heparin‐induced aggregation of R2 at an optimum molar ratio of 1: 2 (metal: protein), and the resulting R2 filaments became smaller. Isothermal titration calorimetry (ITC) experiment revealed that the strong coordination of mercury(II) with R2 was an enthalpy‐controlled, entropy‐decreased thermodynamic process. The exceptionally large magnitude of heat release (ΔH₁ = ?34.8 Kcal mol^?1) suggested that the most possible coordinating site on the R2 peptide chain was the thiol group of cysteine residue (Cys291), and this was further confirmed by a control experiment using Cys291 mutated R2. Circular dichroism spectrum demonstrated that this peptide underwent a significant conformational change from random coil to β‐turn structure upon its binding to mercury(II) ion. This study was undertaken to better understand the mechanism of tau aggregation, and evaluate the possible role of mercury(II) in the pathogenesis of AD. © 2010 Wiley Periodicals, Inc. Biopolymers 93: 1100–1107, 2010. This article was originally published online as an accepted preprint. The “Published Online” date corresponds to the preprint version. You can request a copy of the preprint by emailing the Biopolymers editorial office at biopolymers@wiley.com     

Iron (III) induces aggregation of hyperphosphorylated tau and its reduction to iron (II) reverses the aggregation: implications in the formation of neurofibrillary tangles of Alzheimer's disease

Iron as well as aluminum is reported to accumulate in neurons with neurofibrillary tangles (NFTs) of Alzheimer's disease (AD) brain. Previously we demonstrated that aluminum (III) shows phosphate-dependent binding with hyperphosphorylated tau (PHFtau), the major constituent of NFTs, thereby inducing aggregation of PHFtau. Herein we report that iron (III) can also induce aggregation of soluble PHFtau. Importantly, for the aggregation of PHFtau to occur, iron in the oxidized state (III) is essential since iron in the reduced state (II) lacks such ability. Furthermore, iron (III)-induced aggregation is reversed by reducing iron (III) to iron (II). Thus the iron-participating aggregation is mediated not only by tau phosphorylation but also by the transition of iron between reduced (II) and oxidized (III) states. Further incubation of insoluble PHFtau aggregates isolated from AD brain with reducing agents produced liberation of solubilized PHFtau and iron (II), indicating that PHFtau in association with iron (III) constitutes the insoluble pool of PHFtau. These results indicate that iron might play a role in the aggregation of PHFtau leading to the formation of NFTs in AD brain.     

Cooperative folding of tau peptide by coordination of group IIB metal cations during heparin-induced aggregation

The group IIB elements, especially Cd(II) and Hg(II), are increasingly considered as potential environmental neurotoxins. This study demonstrates that the Alzheimer’s tau fragment R2, corresponding to the second repeat of the microtubule-binding domain, can bind to Zn(II), Cd(II) and Hg(II). Isothermal titration calorimetry experiments suggest that the most likely coordination site is the thiol group of Cys291, and this is further confirmed by a control experiment using a C291A mutant peptide. Circular dichroism spectrum reveals that the coordination of group IIB cations, especially Hg(II), can induce pronounced conformational conversions in natively unfolded R2, from random coil to other ordered structures. ThS fluorescence assays and electron microscopy indicate that the group IIB cations promote heparin-induced aggregation of R2, giving relatively small R2 filaments. The efficiency in promoting aggregation, as well as inducing conformational conversion, varies strongly with the cation’s polarizability. Based on these results, a model is proposed in which the cooperative folding of R2 through cross-bridging of group IIB cations is suggested to be a key factor in promoting aggregation, in addition to the effective neutralization of coulombic charge–charge repulsion by heparin, the poly-anion inducer. Our results provide clues to understanding the potential pathogenic role of group IIB metals in the development of neurofibrillary tangles, a typical hallmark of Alzheimer’s disease.     

TMAO promotes fibrillization and microtubule assembly activity in the C-terminal repeat region of tau

Alzheimer's disease most closely correlates with the appearance of the neurofibrillary tangles (NFTs), intracellular fibrous aggregates of the microtubule-associated protein, tau. Under native conditions, tau is an unstructured protein, and its physical characterization has revealed no clues about the three-dimensional structural determinants essential for aggregation or microtubule binding. We have found that the natural osmolyte trimethylamine N-oxide (TMAO) induces secondary structure in a C-terminal fragment of tau (tau(187)) and greatly promotes both self-aggregation and microtubule (MT) assembly activity. These processes could be distinguished, however, by a single-amino acid substitution (Tyr(310) --> Ala), which severely inhibited aggregation but had no effect on MT assembly activity. The inability of this mutant to aggregate could be completely reversed by TMAO. We propose a model in which TMAO induces partial order in tau(187), resulting in conformers that may correspond to on-pathway intermediates of either aggregation or tau-dependent MT assembly or both. These studies set the stage for future high-resolution structural characterization of these intermediates and the basis by which Tyr(310) may direct pathologic versus normal tau function.     

Identification and isolation of a hyperphosphorylated, conformationally changed intermediate of human protein tau expressed in yeast

Hyperphosphorylation and aggregation of protein tau are typical for neurodegenerative tauopathies, including Alzheimer's disease (AD). We demonstrate here that human tau expressed in yeast acquired pathological phosphoepitopes, assumed a pathological conformation, and formed aggregates. These processes were modulated by yeast kinases Mds1 and Pho85, orthologues of GSK-3beta and cdk5, respectively. Surprisingly, inactivation of Pho85 increased phosphorylation of tau-4R, concomitant with increased conformational change defined by antibody MC1 and a 40-fold increase in aggregation. Soluble protein tau, purified from yeast lacking PHO85, spontaneously and rapidly formed tau filaments in vitro. Further fractionation of tau by anion-exchange chromatography yielded a hyperphosphorylated monomeric subfraction, termed hP-tau/MC1, with slow electrophoretic mobility and enriched with all major epitopes, including MC1. Isolated hP-tau/MC1 vastly accelerated in vitro aggregation of wild-type tau-4R, demonstrating its functional capacity to initiate aggregation, as well as its structural stability. Combined, this novel yeast model recapitulates hyperphosphorylation, conformation, and aggregation of protein tau, provides insight in molecular changes crucial in tauopathies, offers a source for isolation of modified protein tau, and has potential for identification of modulating compounds and genes.     

Heparin-induced conformational change in microtubule-associated protein Tau as detected by chemical cross-linking and phosphopeptide mapping

In Alzheimer's disease, microtubule-associated protein tau becomes abnormally phosphorylated and aggregates into paired helical filaments. Sulfated glycosaminoglycans such as heparin and heparan sulfate were shown to accumulate in pretangle neurons, stimulate in vitro tau phosphorylation, and cause tau aggregation into paired helical filament-like filaments. The sulfated glycosaminoglycan-tau interaction was suggested to be the central event in the development of neuropathology in Alzheimer's disease brain (Goedert, M., Jakes, R., Spillantini, M. G., Hasegawa, M., Smith, M. J., and Crowther, R. A. (1996) Nature 383, 550-553). The biochemical mechanism by which sulfated glycosaminoglycans stimulate tau phosphorylation and cause tau aggregation remains unclear. In this study, disuccinimidyl suberate (DSS), a bifunctional chemical cross-linker, cross-linked tau dimers, tetramers, high molecular size aggregates, and two tau species of sizes 72 and 83 kDa in the presence of heparin. In the absence of heparin only dimeric tau was cross-linked by DSS. Fast protein liquid chromatography gel filtration revealed that 72- and 83-kDa species were formed by intramolecular cross-linking of tau by DSS. These observations indicate that heparin, in addition to causing aggregation, also induces a conformational change in tau in which reactive groups are unmasked or move closer leading to the DSS cross-linking of 72- and 83-kDa species. Heparin-induced structural changes in tau molecule depended on time of heparin exposure. Dimerization and tetramerization peaked at 48 h, whereas conformational change was completed within 30 min of heparin exposure. Heparin exposure beyond 48 h caused an abrupt aggregation of tau into high molecular size species. Heparin stimulated tau phosphorylation by neuronal cdc2-like kinase (NCLK) and cAMP-dependent protein kinase. Phosphopeptide mapping and phosphopeptide sequencing revealed that tau is phosphorylated by NCLK on Thr212 and Thr231 and by cAMP-dependent protein kinase on Ser262 only in the presence of heparin. Heparin stimulation of tau phosphorylation by NCLK showed dependence on time of heparin exposure and correlated with the heparin-induced conformational change of tau. Our data suggest that heparin-induced conformational change exposes new sites for phosphorylation within tau molecule.     

Phosphorylation modulates the local conformation and self-aggregation ability of a peptide from the fourth tau microtubule-binding repeat

Phosphorylation of tau protein modulates both its physiological role and its aggregation into paired helical fragments, as observed in Alzheimer's diseased neurons. It is of fundamental importance to study paired helical fragment formation and its modulation by phosphorylation. This study focused on the fourth microtubule-binding repeat of tau, encompassing an abnormal phosphorylation site, Ser356. The aggregation propensities of this repeat peptide and its corresponding phosphorylated form were investigated using turbidity, thioflavin T fluorescence and electron microscopy. There is evidence for a conformational change in the fourth microtubule-binding repeat of tau peptide upon phosphorylation, as well as changes in aggregation activity. Although both tau peptides have the ability to aggregate, this is weaker in the phosphorylated peptide. This study reveals that both tau peptides are capable of self-aggregation and that phosphorylation at Ser356 can modulate this process.     

Probing copper/tau protein interactions electrochemically

Alzheimer’s disease (AD) is a neurodegenerative disorder that is characterized by peptide and protein misfolding and aggregation, in part due to the presence of excess metal ions such as copper(II) [Cu(II)]. Recently, the brain levels of Cu(II) complexes in vivo were linked to the oxidative stress in neurodegenerative disorders, including AD. Amyloid β-peptide (Aβ), found outside neuronal cells, has been investigated extensively in connection with Cu(II) ion toxicity; however, the effects of metallation on tau are less known. Normal tau protein binds and stabilizes the microtubules in neurons, but in diseased cells tau hyperphosphorylation and aggregation are evident and compromise tau function. There is increasing evidence that the Cu(II) ion may play an important role in tau biochemistry. Here, we present an electrochemical study of the interactions between full-length tau-410 and Cu(II) ions. The coordination of Cu(II) ions to tau immobilized on gold surfaces induces an electrochemical signal at approximately 140 ± 5 mV versus Ag/AgCl due to the Cu(II)/Cu(I) redox couple. Redox potentials and current intensities of Cu(II)-containing nonphosphorylated tau (nTau) and phosphorylated tau (pTau) films were determined at different pH conditions. Greater Cu(II) uptake by pTau over nTau films was observed at low pH. Competitive zinc(II) [Zn(II)] ion binding studies revealed significant Cu(II) ion displacement in pTau films. X-ray photoelectron spectroscopy analysis indicated the presence of Cu 2p and Zn 2p binding energies in protein samples, further supporting metal ion coordination to protein films. The surface-based electrochemical technique requires a minimal protein amount (a few microliters) and allows monitoring the bound Cu(II) ions and the redox activities of the resulting metalloprotein films.     

Fluorescence-coupled CD conformational monitoring of filament formation of tau microtubule-binding repeat domain

To clarify the contribution of the three- or four-repeated peptide moiety in tau microtubule-binding domain (MBD) to paired helical filament (PHF) formation, conformational transition accompanied by heparin-induced filament formation was investigated stepwise for four repeat peptides (R1-R4), one three-repeated R1-R3-R4 peptide (3RMBD), and one four-repeated R1-R2-R3-R4 peptide (4RMBD) using a combination of thioflavin S fluorescence and circular dichroism (CD) measurements in a neutral buffer (pH 7.6). The comparison of the fluorescence profile of each repeat peptide with those of 3RMBD and 4RMBD showed the synergistic contribution of R1-R4 to PHF formation of MBD. The CD spectrum measured as a function of filament formation time indicates that: (i) two conformational transitions occur for the filament formations of R3 (from the random structure to the beta-sheet structure) and 3RMBD (from the random structure to the alpha-helix structure), (ii) the filament formations of R2 and 4RMBD proceed via the synchronized conformational transitions of the alpha-helix and random structures, and (iii) the filament formation of 4RMBD is dependent on the aggregation behavior of R2. These data are useful for elucidating the MBD conformational transition in tau PHF formation.     

Metal binding modes of Alzheimer's amyloid beta-peptide in insoluble aggregates and soluble complexes

Aggregation of the amyloid beta-peptide (Abeta) into insoluble fibrils is a key pathological event in Alzheimer's disease. Zn(II) induces the Abeta aggregation at acidic-to-neutral pH, while Cu(II) is an effective inducer only at mildly acidic pH. We have examined Zn(II) and Cu(II) binding modes of Abeta and their pH dependence by Raman spectroscopy. The Raman spectra clearly demonstrate that three histidine residues in the N-terminal hydrophilic region provide primary metal binding sites and the solubility of the metal-Abeta complex is correlated with the metal binding mode. Zn(II) binds to the N(tau) atom of the histidine imidazole ring and the peptide aggregates through intermolecular His(N(tau))-Zn(II)-His(N(tau)) bridges. The N(tau)-metal ligation also occurs in Cu(II)-induced Abeta aggregation at mildly acidic pH. At neutral pH, however, Cu(II) binds to N(pi), the other nitrogen of the histidine imidazole ring, and to deprotonated amide nitrogens of the peptide main chain. The chelation of Cu(II) by histidine and main-chain amide groups results in soluble Cu(II)-Abeta complexes. Under normal physiological conditions, Cu(II) is expected to protect Abeta against Zn(II)-induced aggregation by competing with Zn(II) for histidine residues of Abeta.     

Regulation of tau isoform expression and dementia

In the central nervous system (CNS), aberrant changes in tau mRNA splicing and consequently in protein isoform ratios cause abnormal aggregation of tau and neurodegeneration. Pathological tau causes neuronal loss in Alzheimer's disease (AD) and a diverse group of disorders called the frontotemporal dementias (FTD), which are two of the most common forms of dementia and afflict more than 10% of the elderly population. Autosomal dominant mutations in the tau gene cause frontotemporal dementia with parkinsonism-chromosome 17 type (FTDP-17). Just over half the mutations affect tau protein function and decrease its affinity for microtubules (MTs) or increase self-aggregation. The remaining mutations occur within exon 10 (E10) and intron 10 sequences and alter complex regulation of E10 splicing by multiple mechanisms. FTDP-17 splicing mutations disturb the normally balanced levels of distinct protein isoforms that result in altered biochemical and structural properties of tau. In addition to FTDP-17, altered tau isoform levels are also pathogenically associated with other FTD disorders such as progressive supranuclear palsy (PSP), corticobasal degeneration and Pick's disease; however, the mechanisms remain undefined and mutations in tau have not been detected. FTDP-17 highlights the association between splicing mutations and the pronounced variability in pathology as well as phenotype that is characteristic of inherited disorders.     

Copper (II) modulates in vitro aggregation of a tau peptide

Copper (II) has been implicated in the pathology of Alzheimer's disease (AD) for the impaired homeostatic mechanism found in the brains of AD patients. Here we studied the binding properties of Cu(II) with the first microtubule-binding repeat, encompassing residues 256-273 of the human tau441 sequence. Additionally, the effect of Cu(II) on the assembly of this repeat was also investigated. Our results indicate that Cu(II) can bind to this repeat with His(268) involved and has an inhibiting effect on the in vitro aggregation of this repeat. This work provides new insight into the role of Cu(II) in Alzheimer's disease.     

Synergistic interactions between repeats in tau protein and Aβ amyloids may be responsible for accelerated aggregation via polymorphic states

Amyloid plaques and neurofibrillary tangles simultaneously accumulate in Alzheimer's disease (AD). It is known that Aβ and tau exist together in the mitochondria; however, the interactions between Aβ oligomers and tau are controversial. Moreover, it is still unclear which specific domains in the tau protein can interact with Aβ oligomers and what could be the effect of these interactions. Herein, we examine three different Aβ-tau oligomeric complexes. These complexes present interactions of Aβ with three domains in the tau protein; all contain high β-structure propensity in their R2, R3, and R4 repeats. Our results show that, among these, Aβ oligomers are likely to interact with the R2 domain to form a stable complex with better alignment in the turn region and the β-structure domain. We therefore propose that the R2 domain can interact with soluble Aβ oligomers and consequently promote aggregation. EM and AFM images and dimensions revealed highly polymorphic tau aggregates. We suggest that the polymorphic tau and Aβ-tau aggregates may be largely due to repeat sequences which are prone to variable turn locations along the tau repeats.