Amphipathic helical behavior of the third repeat fragment in the tau microtubule-binding domain, studied by (1)H NMR spectroscopy

NMR Studies of Tau Protein in Tauopathies

Tauopathies, including Alzheimer's disease (AD), are the most troublesome of all age-related chronic conditions, as there are no well-established disease-modifying therapies for their prevention and treatment. Spatio-temporal distribution of tau protein pathology correlates with cognitive decline and severity of the disease, therefore, tau protein has become an appealing target for therapy. Current knowledge of the pathological effects and significance of specific species in the tau aggregation pathway is incomplete although more and more structural and mechanistic insights are being gained using biophysical techniques. Here, we review the application of NMR to structural studies of various tau forms that appear in its aggregation process, focusing on results obtained from solid-state NMR. Furthermore, we discuss implications from these studies and their prospective contribution to the development of new tauopathy therapies.

Copper (Cu2+) ion-induced misfolding of tau protein R3 peptide revealed by enhanced molecular dynamics simulation

Tau misfolding plays a significant role in some neurodegenerative diseases such as Alzheimer's disease (AD). It is intrinsically disordered and highly soluble under normal physiological conditions. While the protein will aggregate to form paired helical filaments (PHFs) under copper homeostasis at pathological conditions, which is the main substance of neurofibrillary tangles (NFTs) in the brain of AD patients. However, the molecular mechanism underlying the copper (Cu2+) ion-induced tau misfolding is not fully understood. In this study, using the 1/2 third repeat fragment (R3 peptide) of tau protein (residues 318-335: VTSKCGSLGNIHHKPGGG) as a model, a Gaussian accelerated molecular dynamics (GaMD) method followed by efficient trajectory analysis was carried out to investigate the influences of Cu2+ on the tau about the protein fold and the free energy landscape along the simulation. The two-dimensional potential of mean force (PMF) profiles obtained from reweighting of the GaMD simulations as well as clustering analysis revealed the Cu2+ ion induced α-helix fold R3 peptide located at the low-energy wells of free energy map, which is in agreement with the reported experimental result. In contrast, there is no α-helix fold of R3 peptide that appeared during the GaMD simulation without Cu2+ ion existing. Furthermore, the definition of secondary structure of protein (DSSP) analysis indicated that the R3 peptide with Cu2+ ion forms a stable structure of the helix (Lys321-His330 interval of the peptide) at between 400 and 500 ns. Therefore, the structures and free energy profiles from GaMD simulations proposed that Cu2+ triggers the aggregation of R3 peptide into toxic PHFs through a stable α-helix fold form.

Phenothiazine-Tacrine Heterodimers: Pursuing Multitarget Directed Approach in Alzheimer's Disease

Since 2002, no clinical candidate against Alzheimer's disease has reached the market; hence, an effective therapy is urgently needed. We followed the so-called "multitarget directed ligand" approach and designed 36 novel tacrine-phenothiazine heterodimers which were in vitro evaluated for their anticholinesterase properties. The assessment of the structure-activity relationships of such derivatives highlighted compound 1dC as a potent and selective acetylcholinesterase inhibitor with IC₅₀ = 8 nM and 1aA as a potent butyrylcholinesterase inhibitor with IC₅₀ = 15 nM. Selected hybrids, namely, 1aC, 1bC, 1cC, 1dC, and 2dC, showed a significant inhibitory activity toward τ_(306-336) peptide aggregation with percent inhibition ranging from 50.5 to 62.1%. Likewise, 1dC and 2dC exerted a remarkable ability to inhibit self-induced Aβ_1-42 aggregation. Notwithstanding, in vitro studies displayed cytotoxicity toward HepG2 cells and cerebellar granule neurons; no pathophysiological abnormality was observed when 1dC was administered to mice at 14 mg/kg (i.p.). 1dC was also able to permeate to the CNS as shown by in vitro and in vivo models. The maximum brain concentration was close to the IC₅₀ value for acetylcholinesterase inhibition with a relatively slow elimination half-time. 1dC showed an acceptable safety and good pharmacokinetic properties and a multifunctional biological profile.

Tau binds to lipid membrane surfaces via short amphipathic helices located in its microtubule-binding repeats

Tau is a microtubule-associated protein that is genetically linked to dementia and linked to Alzheimer's disease via its presence in intraneuronal neurofibrillary tangle deposits, where it takes the form of aggregated paired helical and straight filaments. Although the precise mechanisms by which tau contributes to neurodegeneration remain unclear, tau aggregation is commonly considered to be a critical component of tau-mediated pathogenicity. Nevertheless, the context in which tau aggregation begins in vivo is unknown. Tau is enriched in membrane-rich neuronal structures such as axons and growth cones, and can interact with membranes both via intermediary proteins and directly via its microtubule-binding domain (MBD). Membranes efficiently facilitate tau aggregation in vitro, and may therefore provide a physiologically relevant context for nucleating tau aggregation in vivo. Furthermore, tau-membrane interactions may potentially play a role in tau's poorly understood normal physiological functions. Despite the potential importance of direct tau-membrane interactions for tau pathology and physiology, the structural mechanisms that underlie such interactions remain to be elucidated. Here, we employ electron spin resonance spectroscopy to investigate the secondary and long-range structural properties of the MBD of three-repeat tau isoforms when bound to lipid vesicles and membrane mimetics. We show that the membrane interactions of the tau MBD are mediated by short amphipathic helices formed within each of the MBD repeats in the membrane-bound state. To our knowledge, this is the first detailed elucidation of helical tau structure in the context of intact lipid bilayers. We further show, for the first time (to our knowledge), that these individual helical regions behave as independent membrane-binding sites linked by flexible connecting regions. These results represent the first (to our knowledge) detailed structural view of membrane-bound tau and provide insights into potential mechanisms for membrane-mediated tau aggregation. Furthermore, the results may have implications for the structural basis of tau-microtubule interactions and microtubule-mediated tau aggregation.

Structural transitions in tau k18 on micelle binding suggest a hierarchy in the efficacy of individual microtubule-binding repeats in filament nucleation

The protein tau is found in an aggregated filamentous state in the intraneuronal paired helical filament deposits characteristic of Alzheimer's disease and other related dementias and mutations in tau protein and mRNA cause frontotemproal dementia. Tau isoforms include a microtubule-binding domain containing either three or four imperfect tandem microtubule binding repeats that also form the core of tau filaments and contain hexapaptide motifs that are critical for tau aggregation. The tau microtubule-binding domain can also engage in direct interactions with detergents, fatty acids, or membranes, which can greatly facilitate tau aggregation and may also mediate some tau functions. Here, we show that the alternatively spliced second microtubule-binding repeat exhibits significantly different structural characteristics compared with the other three repeats in the context of the intact repeat domain. Most notably, the PHF6* hexapeptide motif located at the N-terminus of repeat 2 has a lower propensity to form strand-like structure than the corresponding PHF6 motif in repeat 3, and unlike PHF6 converts to partially helical structure in the micelle-bound state. Interestingly, the behavior of the Module-B motif, located at the beginning of repeat 4, resembles that of PHF6* rather than PHF6. Our observations, combined with previous results showing that PHF6* and Module-B are both less effective than PHF6 in nucleating tau aggregation, suggest a hierarchy in the efficacy of these motifs in nucleating tau aggregation that originates in differences in their intrinsic propensities for extended strand-like structure and the resistance of these propensities to changes in tau's environment.

[Overview of structural study on conformations and intermolecular interactions of biomolecules]

Information on the conformational feature and specific intermolecular interaction of biomolecules is important to understand the biological function and to develop device for treating disorder caused by the abnormal function. Thus the 3D structures of the biologically active molecules and the specific interactions with their target molecules at the atomic level have been investigated by various physicochemical approaches. Herein, the following five subjects are reviewed: (1) function-linked conformations of biomolecules including natural annular products, opioid peptides and neuropeptides; (2) π-π stacking interactions of tryptophan derivatives with coenzymes and nucleic acid bases; (3) mRNA cap recognition of eukaryotic initiation factor 4E and its regulation by 4E-binding protein; (4) conformational feature of histamine H2 receptor antagonists and design of cathepsin B inhibitors; (5) self-aggregation mechanism of tau protein and its inhibition.

Proteostasis of tau. Tau overexpression results in its secretion via membrane vesicles

Increasing amounts of tau protein were expressed in non-neuronal cells. When intracellular amounts reached a threshold level, tau protein was released to the extracellular culture medium in association with membrane vesicles. Hence, we propose that tau might be secreted through membrane vesicles as a cellular mechanism to eliminate the excess of tau protein, thereby avoiding its toxicity.

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.

Probing interactions of tubulin with small molecules, peptides, and protein fragments by solution nuclear magnetic resonance

The description of the molecular mechanisms of interaction between tubulin or microtubules and partners at atomic scale is expected to have critical impacts on the understanding of basic physiological processes. This information will also help the design of future drug candidates that may be used to fight various pathologies such as cancer or neurological diseases. For these reasons, this aspect of tubulin research has been tackled since the seventies using many different methods and at different scales. NMR appears as a unique approach to provide, with atomic resolution, the solution structure and dynamical properties of tubulin/microtubule partners in free and bound states. Though tubulin is not directly amenable to solution NMR, the NMR ligand-based experiments allow one to obtain valuable data on the molecular mechanisms that sustain structure-function relationship, in particular atomic details on the partner binding site. We will first describe herein some basic principles of solution NMR spectroscopy that should not be missed for a comprehensive reading of NMR reports. A series of results will then be presented to illustrate the wealth and variety of NMR experiments and how this approach enlightens tubulin/microtubules interaction with partners.

Structural studies of specific intermolecular interactions and self-aggregation of biomolecules and their application to drug design

Information on the structural basis of intermolecular recognition or self-aggregation of biomolecules at the atomic level is important to understand biological functions and to develop devices for treating disorders caused by abnormal functions. Thus structural analysis of specific intermolecular or intramolecular interactions of biomolecules has been performed using various physicochemical approaches. Herein, the following three subjects are reviewed: (1) structural analyses of mRNA cap structure recognition by eukaryotic initiation factor 4E and its functional regulation by endogenous 4E-binding protein; (2) structural studies of self-aggregation mechanism of microtubule-binding domain in tau protein and aggregation inhibitor; and (3) molecular design of cathepsin B-specific inhibitor.

The effect of a DeltaK280 mutation on the unfolded state of a microtubule-binding repeat in Tau

Tau is a natively unfolded protein that forms intracellular aggregates in the brains of patients with Alzheimer's disease. To decipher the mechanism underlying the formation of tau aggregates, we developed a novel approach for constructing models of natively unfolded proteins. The method, energy-minima mapping and weighting (EMW), samples local energy minima of subsequences within a natively unfolded protein and then constructs ensembles from these energetically favorable conformations that are consistent with a given set of experimental data. A unique feature of the method is that it does not strive to generate a single ensemble that represents the unfolded state. Instead we construct a number of candidate ensembles, each of which agrees with a given set of experimental constraints, and focus our analysis on local structural features that are present in all of the independently generated ensembles. Using EMW we generated ensembles that are consistent with chemical shift measurements obtained on tau constructs. Thirty models were constructed for the second microtubule binding repeat (MTBR2) in wild-type (WT) tau and a DeltaK280 mutant, which is found in some forms of frontotemporal dementia. By focusing on structural features that are preserved across all ensembles, we find that the aggregation-initiating sequence, PHF6*, prefers an extended conformation in both the WT and DeltaK280 sequences. In addition, we find that residue K280 can adopt a loop/turn conformation in WT MTBR2 and that deletion of this residue, which can adopt nonextended states, leads to an increase in locally extended conformations near the C-terminus of PHF6*. As an increased preference for extended states near the C-terminus of PHF6* may facilitate the propagation of beta-structure downstream from PHF6*, these results explain how a deletion at position 280 can promote the formation of tau aggregates.

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.

Impacts of Cd(II) on the conformation and self-aggregation of Alzheimer's tau fragment corresponding to the third repeat of microtubule-binding domain

Environmental exposure to some heavy metals such as cadmium appears to be a risk factor for Alzheimer's disease (AD), however, definite mechanism of their toxicity in AD remains to be elucidated. Previous studies largely focused on the metal ions binding to beta-amyloid, however, very few papers concerned the interaction between tau and metal ions. For the first time, we investigated the impacts of Cd(II) on the conformation and self-aggregation of Alzheimer's tau peptide R3, corresponding to the third repeat of microtubule-binding domain. The initial state of R3 was proven to be dimeric linked by intermolecular disulfide bond, in the non-reducing buffer (Tris-HCl buffer pH7.5, containing no reducing reagent). In this paper, we show that Cd(II) can accelerate heparin-induced aggregation of R3 or independently induce the aggregation of R3, as monitored by ThS fluorescence. In the presence of Cd(II), the resulting R3 filaments became much smaller, as revealed by electron microscopy. Binding to the Cd(II) ion, the dimeric R3 partially lost its random coil, and converted to alpha-helix structure, as revealed by CD and Raman spectrum. Stoichiometric analysis of CD signal against the ratio of [Cd(II)]/[R3] suggested that the coordination intermediate consisted of two R3 dimers binding to a central cadmium ion. As the seed, the coordination intermediate could extensively accelerate the self-aggregation of R3 via promoting the nucleation step. On the other hand, gain in alpha-helix structure on the peptide chain, by coordinating with Cd(II), could be a critical role to promote self-aggregation, as revealed by Raman spectrum. These results provide a further insight into the mechanism of tau filament formation and emphasize the possible involvement of Cd(II) in the pathogenesis of AD.

Folding of the repeat domain of tau upon binding to lipid surfaces

The microtubule-associated protein tau is impacted in neurodegeneration and dementia through its deposition in the form of paired helical filaments in Alzheimer's disease neurofibrillary tangles and through mutations linking it to the autosomal dominant disorder frontotemporal dementia with Parkinsonism. When isolated in solution tau is intrinsically unstructured and does not fold, while the conformation of the protein in the microtubule-bound state remains uncharacterized. Here we show that the repeat region of tau, which has been reported both to mediate tau microtubule interactions and to constitute the proteolysis-resistant core of disease-associated tau aggregates, associates with lipid micelles and vesicles and folds into an ordered structure upon doing so. In addition to providing the first structural insights into a folded state of tau, our results support a role for lipid membranes in mediating tau function and tau pathology.

Intrinsically Disordered Proteins in the Neurodegenerative Processes: Formation of Tau Protein Paired Helical Filaments and Their Analysis

1. Several intrinsically disordered proteins (IDPs) play principal role in the neurodegenerative processes of various types. Among them, alpha-synuclein is involved in Parkinson's disease, prion protein in transmissible spongiform encephalopathies, and tau protein in Alzheimer's disease (AD) and related tauopathies. Neuronal damage in AD is accompanied by the presence of tau protein fibrils composed of paired helical filaments (PHF). 2. Tau protein represents a typical IDP. IDPs do not exhibit any stable secondary structure in the free form, but they are able to fold after binding to targets and contain regions with large propensity to adopt a defined type of secondary structure. Binding-folding event at tau protein leading to PHF generation is believed to happen in the course of tauopathies. 3. Detailed molecular topology of PHF formation is unknown. There are evidences about the cross-beta structure in PHF core; however the precise arrangement of the tau polypeptide chain is unclear. In this review we summarize current attempts at in vitro PHF reconstruction and the development of methods for PHF structure determination. The emphasis is put on the monoclonal antibodies used as structural molecular probes for research on the role of IDPs in pathogenesis of neurodegenerative diseases.

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.

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.

Importance of local structures of second and third repeat fragments of microtubule-binding domain for tau filament formation

To investigate the importance of the seventh residue of the second and third repeat fragments (R2 and R3 peptides) of the microtubule-binding domain (MBD) for tau filamentous assembly, the residues Lys and Pro were substituted (R2-K7P and R3-P7K). The filament formations of the R2 and R3 peptides were almost lost due to their substitutions despite their overall conformational similarities. The NOE analyses showed the importance of the conformational flexibility for the R2 peptide and the coupled extended and helical conformations for the R3 peptide in their limited N-terminal regions around their seventh residues. The result shows that the filament formation of MBD is initiated from a short fragment region containing the minimal conformational or functional motif.

Structural evaluation of conformational transition state responsible for self-assembly of tau microtubule-binding domain

In the brains of Alzheimer's disease patients, the tau protein abnormally aggregates to form an insoluble paired helical filament (PHF). Since the third repeat structure (R3) of the tau microtubule-binding domain plays an essential role in PHF formation and self-aggregates most significantly in an aqueous solution of 20-40% trifluoroethanol (TFE), its possible conformation was estimated from the combination of (i) the TFE-dependent deviations of NH and CalphaH proton chemical shifts from those of the random structure in water and (ii) the TFE-dependent NOE effect connectivity diagrams between the neighboring protons. Consequently, it was indicated that the extended structure of the N-terminal VQIVYK moiety and the alpha-helical-like structure of the LSKVTSKC region provide a structural scaffold for initiating the self-assembled filament formation of the R3 structure. To the best of our knowledge, this is the first study that demonstrated the initial structural moiety and its structural feature necessary for starting the tau PHF formation.

Effects of different anti-tau antibodies on tau fibrillogenesis: RTA-1 and RTA-2 counteract tau aggregation

Tau is the major antigenic component of neurofibrillary pathology in tauopathy, including Alzheimer's disease. Although conversion of soluble tau to an insoluble polymerized fibrillar form is a key factor in the pathogenesis of tauopathy, the mechanism of the change is unclear and no inhibitors of fibril formation are available. Monoclonal antibodies against the 1st or 2nd repeat of the microtubule binding domain, but not the C-terminal 16 residues, completely inhibited tau aggregation into PHF. Furthermore, they did not inhibit tau-induced tubulin assembly. Thus, they are useful to investigate tau protein conversion and will be useful therapeutic lead materials.

Evaluating triggers and enhancers of tau fibrillization

Alzheimer's disease is characterized in part by the aggregation of tau protein into filamentous inclusions. Because tau filaments form in brain regions associated with memory retention, and because their appearance correlates well with the degree of dementia, they have emerged as robust markers of disease progression. Yet the discovery that mutations in tau protein can lead directly to filament and tangle formation in humans, and that filament formation is linked to neurodegeneration in model biological systems, suggests that tau aggregation may also contribute directly to degeneration in affected neurons. In this context, the mechanism of tau filament formation and its modulation by mutation and posttranslational modification is of fundamental importance. Here, recent progress on the molecular mechanisms underlying tau aggregation deduced from in vivo and in vitro experimentation is reviewed and a model rationalizing the effect of posttranslational and other structural modifications on assembly kinetics and thermodynamics is presented. We hypothesize that tau aggregation can be described as a heterogeneous nucleation reaction, where exogenous effectors, tau gene mutations, or other modifications that stabilize assembly-competent conformations of tau act to trigger the fibrillization reaction. In contrast, those that modulate postnuclear equilibria can enhance fibrillization by increasing the free energy difference between polymers and unincorporated monomers, resulting in stabilization of filaments at low bulk protein concentrations.

Potential structure/function relationships of predicted secondary structural elements of tau

The microtubule-associated protein tau is believed to be a natively unfolded molecule with virtually no secondary structure. However, this protein self-associates into filamentous forms in various neurodegenerative diseases. Since these filamentous forms show a remarkable degree of higher order due to their regular widths and periodicity, it is widely speculated that tau does contain secondary structures that come together to form tertiary and quaternary structures in the filamentous form. The purpose of this review is to use the primary sequence of tau along with predictive methods in an effort to identify potential secondary structural elements that could be involved in its normal and pathological functions. Although there are few predicted structural elements in the tau molecule, these analyses should lead to a better understanding of the structure/function relationships that regulate the behavior of tau.

Tau aggregation is driven by a transition from random coil to beta sheet structure

The abnormal aggregation of the microtubule associated protein tau into paired helical filaments (PHFs) is one the hallmarks of Alzheimer's disease. The soluble protein is one of the longest natively unfolded proteins, lacking significant amounts of secondary structure over a sequence of 441 amino acids in the longest isoform. Furthermore, the unfolded character is consistent with some notable features of the protein like stability towards heat and acid treatment. It is still unclear how these characteristics support the physiological function of binding to and stabilization of microtubules. We review here some recent studies on how an unfolded protein such as tau can adopt beta-structure, which then leads to the highly ordered morphology of the PHFs. The core sequence for both microtubule binding and PHF formation is the microtubule binding domain containing three or four repeats. This region alone is sufficient for PHF formation and mostly unfolded in the soluble state. A search for sequence motifs within this region crucial for PHF building revealed two hexapeptides in the second and the third repeat. Some of the genetically linked cases of FTDP-17 show missense mutations in or adjacent to these hexapeptide motifs. Proteins containing the P301L and the DeltaK280 mutations exhibit accelerated aggregation. The importance of the two hexapeptides stems from their capacity to undergo a conformational change from a random coil to a beta sheet structure. The increase of beta sheet structure is a typical feature of an amyloidogenic protein and is the basis of other characteristics like a decreased sensitivity towards proteolytic degradation and Congo red binding. PHFs aggregated in vitro and in vivo contain beta-sheet structure, as judged by circular dichroism (CD) spectroscopy, Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction.

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.

Myelin basic protein-diverse conformational states of an intrinsically unstructured protein and its roles in myelin assembly and multiple sclerosis

The 18.5 kDa isoform of myelin basic protein (MBP) is a major component of the myelin sheath in the central nervous system of higher vertebrates, and a member of a larger family of proteins with a multiplicity of forms and post-translational modifications (PTMs). The 18.5 kDa protein is the exemplar of the family, being most abundant in adult myelin, and thus the most-studied. It is peripherally membrane-associated, but has generally been investigated in isolated form. MBP is an 'intrinsically unstructured' protein with a high proportion (approximately 75%) of random coil, but postulated to have core elements of beta-sheet and alpha-helix. We review here the properties of the MBP family, especially of the 18.5 kDa isoform, and discuss how its three-dimensional (3D) structure may be resolved by direct techniques available to us, viz., X-ray and electron crystallography, and solution and solid-state NMR spectrometry. In particular, we emphasise that creating an appropriate environment in which the protein can adopt a physiologically relevant fold is crucial to such endeavours. By solving the 3D structure of 18.5 kDa MBP and the effects of PTMs, we will attain a better understanding of myelin architecture, and of the molecular mechanisms that transpire in demyelinating diseases such as multiple sclerosis.

Role of tau protein in both physiological and pathological conditions

The morphology of a neuron is determined by its cytoskeletal scaffolding. Thus proteins that associate with the principal cytoskeletal components such as the microtubules have a strong influence on both the morphology and physiology of neurons. Tau is a microtubule-associated protein that stabilizes neuronal microtubules under normal physiological conditions. However, in certain pathological situations, tau protein may undergo modifications, mainly through phosphorylation, that can result in the generation of aberrant aggregates that are toxic to neurons. This process occurs in a number of neurological disorders collectively known as tauopathies, the most commonly recognized of which is Alzheimer's disease. The purpose of this review is to define the role of tau protein under normal physiological conditions and to highlight the role of the protein in different tauopathies.

Different associational and conformational behaviors between the second and third repeat fragments in the tau microtubule-binding domain

The third repeat fragment (R3) in the four-repeat microtubule-binding domain of the water-soluble tau protein has been considered to play an essential role in the protein's filamentous assembly. To clarify the associational and conformational features that differentiate R3 from the second repeat, R2, the heparin-induced assembly profiles of these peptide fragments were monitored by the thioflavin fluorescence method and electron microscopy. The trifluoroethanol-induced reversible conformational change from a random structure to an alpha-helical structure, in an aqueous solution, was monitored by CD measurement, and the structure of R2 in trifluoroethanol solution was analyzed by a combination of two-dimensional 1H-NMR measurements and molecular modeling calculations to facilitate comparison with the structure of R3. The speed of R3 assembly was remarkably faster than that of R2, in spite of their similar amino acid sequences. The averaged NMR conformers of R2 exhibited the whole-spanning alpha-helical structure. Similar features observed in R2 and R3 conformers in trifluoroethanol were that the Leu10-Leu20/Lys20 sequence takes a helical structure with the amphipathic-like distribution of the respective side-chains, whereas the C-terminal moieties are both flexible. In contrast, a notable difference was observed at the N-terminal Val1-Lys6 sequence, namely, a helical conformation for R2 and an extended conformation for R3. These conformational behaviors would be associated with the different self-aggregation speeds and seeding reactions between R2 and R3.