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

Investigation of the Structure of Full-Length Tau Proteins with Coarse-Grained and All-Atom Molecular Dynamics Simulations

Tau proteins not only have many important biological functions but also are associated with several neurodegenerative diseases, such as Parkinson's disease and Alzheimer's disease (AD). However, it is still a challenge to identify the atomic structure of full-length tau proteins due to their lengthy and disordered characteristics and the factor that there are no crystal structures of full-length tau proteins available. We performed multi- and large-scale molecular dynamics simulations of the full-length tau monomer (the 2N4R isoform and 441 residues) in aqueous solution under biological conditions with coarse-grained and all-atom force fields. The obtained atomic structures produced radii of gyration and chemical shifts that are in excellent agreement with those of experiment. The generated monomer structure ensemble would be very useful for further studying the oligomerization mechanism and discovering tau oligomerization inhibitors, which are important events in AD drug development.

Interaction kinetics reveal distinct properties of conformational ensembles of three-repeat and four-repeat tau proteins

Tau protein is an intrinsically disordered protein. Its physiological state is best described as a conformational ensemble (CE) of metastable structures interconverting on the local and molecular scale. The monoclonal antibody DC39C recognizes a linear C-terminal tau epitope, and as the tau interaction partner, its binding parameters report about tau CE. Association kinetics of DC39C binding, together with crosslinking mass spectrometry, show differences in the accessibility of the C-terminus in CEs of tau isoforms. Furthermore, removal of the C-terminus accelerated the aggregation kinetics of three-repeat tau proteins. Our results suggest a novel mechanism of splicing-driven regulation of the tau C-terminal domain with consequences on the specific roles of tau isoforms in microtubule assembly and pathological aggregation.

Tau and Membranes: Interactions That Promote Folding and Condensation

Tau misfolding and assembly is linked to a number of neurodegenerative diseases collectively described as tauopathies, including Alzheimer’s disease (AD) and Parkinson’s disease. Anionic cellular membranes, such as the cytosolic leaflet of the plasma membrane, are sites that concentrate and neutralize tau, primarily due to electrostatic interactions with tau’s microtubule binding repeat domain (RD). In addition to electrostatic interactions with lipids, tau also has interactions with membrane proteins, which are important for tau’s cellular functions. Tau also interacts with lipid tails to facilitate direct translocation across the membrane and can form stable protein-lipid complexes involved in cell-to-cell transport. Concentrated tau monomers at the membrane surface can form reversible condensates, change secondary structures, and induce oligomers, which may eventually undergo irreversible crosslinking and fibril formation. These β-sheet rich tau structures are capable of disrupting membrane organization and are toxic in cell-based assays. Given the evidence for relevant membrane-based tau assembly, we review the emerging hypothesis that polyanionic membranes may serve as a site for phase-separated tau condensation. Membrane-mediated phase separation may have important implications for regulating tau folding/misfolding, and may be a powerful mechanism to spatially direct tau for native membrane-mediated functions.

Pharmacological Modulators of Tau Aggregation and Spreading

Tauopathies are neurodegenerative disorders characterized by the deposition of aggregates composed of abnormal tau protein in the brain. Additionally, misfolded forms of tau can propagate from cell to cell and throughout the brain. This process is thought to lead to the templated misfolding of the native forms of tau, and thereby, to the formation of newer toxic aggregates, thereby propagating the disease. Therefore, modulation of the processes that lead to tau aggregation and spreading is of utmost importance in the fight against tauopathies. In recent years, several molecules have been developed for the modulation of tau aggregation and spreading. In this review, we discuss the processes of tau aggregation and spreading and highlight selected chemicals developed for the modulation of these processes, their usefulness, and putative mechanisms of action. Ultimately, a stronger understanding of the molecular mechanisms involved, and the properties of the substances developed to modulate them, will lead to the development of safer and better strategies for the treatment of tauopathies.

The Distance between N and C Termini of Tau and of FTDP-17 Mutants Is Modulated by Microtubule Interactions in Living Cells

The microtubule (MT)-associated protein Tau is a natively unfolded protein, involved in a number of neurodegenerative disorders, collectively called tauopathies, aggregating in neurofibrillary tangles (NFT). It is an open question how the conversion from a MT bound molecule to an aggregation-prone Tau species occurs and, also, if and how tauopathy-related mutations affect its behavior in the cell. To address these points, we exploited a genetically encoded FRET sensor based on the full length Tau protein, to monitor in real time Tau conformational changes in different conditions in live cells. By studying the FRET signal we found that soluble Tau molecules, detached from MTs, display an unfolded structure. On the contrary, we observed an increased FRET signal generated by Tau monomers bound to MT, indicating that the association with MTs induced a folding of Tau protein, decreasing the distance between its N and C termini. We exploited the FRET sensor to investigate the impact of FTDP-17 mutations and of phosphorylation-site mutations on Tau folding and mobility in live cells. We demonstrated that the FTDP-17 Tau mutations weaken the interaction of Tau with cellular MTs, shifting the equilibrium towards the soluble pool while, conversely, phosphorylation site mutations shift the equilibrium of Tau towards the MT-bound state and a more closed conformation.

BETASCAN: probable beta-amyloids identified by pairwise probabilistic analysis

Amyloids and prion proteins are clinically and biologically important β-structures, whose supersecondary structures are difficult to determine by standard experimental or computational means. In addition, significant conformational heterogeneity is known or suspected to exist in many amyloid fibrils. Recent work has indicated the utility of pairwise probabilistic statistics in β-structure prediction. We develop here a new strategy for β-structure prediction, emphasizing the determination of β-strands and pairs of β-strands as fundamental units of β-structure. Our program, BETASCAN, calculates likelihood scores for potential β-strands and strand-pairs based on correlations observed in parallel β-sheets. The program then determines the strands and pairs with the greatest local likelihood for all of the sequence's potential β-structures. BETASCAN suggests multiple alternate folding patterns and assigns relative a priori probabilities based solely on amino acid sequence, probability tables, and pre-chosen parameters. The algorithm compares favorably with the results of previous algorithms (BETAPRO, PASTA, SALSA, TANGO, and Zyggregator) in β-structure prediction and amyloid propensity prediction. Accurate prediction is demonstrated for experimentally determined amyloid β-structures, for a set of known β-aggregates, and for the parallel β-strands of β-helices, amyloid-like globular proteins. BETASCAN is able both to detect β-strands with higher sensitivity and to detect the edges of β-strands in a richly β-like sequence. For two proteins (Aβ and Het-s), there exist multiple sets of experimental data implying contradictory structures; BETASCAN is able to detect each competing structure as a potential structure variant. The ability to correlate multiple alternate β-structures to experiment opens the possibility of computational investigation of prion strains and structural heterogeneity of amyloid. BETASCAN is publicly accessible on the Web at http://betascan.csail.mit.edu.

Amyloid is a highly ordered form of protein aggregation that a wide variety of proteins can form. While the earliest discovered amyloids were associated with systemic and neurodegenerative diseases, recent findings indicate amyloids may have myriad roles and functions ranging from learning and memory, to yeast epigenetics, to biofilm and melanin production. In this study, we expand the range and flexibility of our ability to understand how amyloid properties arise from their polypeptide sequence. By taking advantage of the intrinsic properties of a characteristic amyloid structure—parallel β-strands—and data from available protein structures, we construct and test an algorithm to predict the probability that particular portions of a protein will form amyloid. Our method has the advantage of more accurate detection of the edges of such zones, as well as the ability to consider and evaluate the likelihood of multiple folding patterns.

Spectroscopic Approaches to the Conformation of Tau Protein in Solution and in Paired Helical Filaments

The abnormal aggregation of the microtubule-associated protein tau into paired helical filaments is one the hallmarks of Alzheimer's disease. This aggregation is based in the partial formation of beta-structure. In contrast, the soluble protein shows a mostly random coil structure, as judged by circular dichroism, Fourier transform infrared, X-ray scattering and biochemical assays. Here, we review the basis of the natively unstructured character of tau, as well as recent studies of residual structure and long-range interactions between different domains of the protein. Analysis of the primary structure reveals a very low content of hydrophobic amino acids and a high content of charged residues, both of which tend to counteract a well-folded globular state of proteins. In the case of tau, the low overall hydrophobicity is sufficient to explain the lack of folding. This is in contrast to other proteins which also carry an excess charge at physiological pH. By tryptophan scanning mutagenesis and fluorimetry we found that most of the sequence is solvent exposed. Analysis of the hydrodynamic radii confirms a mostly random coil structure of various tau isoforms and tau domains. The proteins can be further expanded by denaturation with GdHCl which indicates some global folding. This was substantiated by a FRET-based approach where the distances between different domains of tau were determined. The combined data show that tau is mostly disordered and flexible but tends to assume a hairpin-like overall fold which may be important in the transition to a pathological aggregate.

N-Terminal Fragments of Tau Inhibit Full-Length Tau Polymerization in Vitro†

The polymerization of the microtubule-associated protein, tau, into insoluble filaments is a common thread in Alzheimer's disease and in a variety of frontotemporal dementias. The conformational change required for tau to transition from an extended monomeric state to a filamentous state with a high beta-sheet content involves the extreme N-terminus coming into contact with distal portions of the molecule; however, these exact interactions are incompletely understood. Here we report that a construct representing amino acids 1-196 (Tau196), which itself does not polymerize, inhibits polymerization of full-length tau (hTau40) in vitro. In addition, we trace the inhibitory effect of Tau196 to amino acids 18-42 of the construct. We also provide evidence that the N-terminal tau fragments require a specific C-terminal region of tau (residues 392-421) to exert their inhibitory effect. The fragments are most effective at inhibiting polymerization when present during the initial 5 min; they remain in the soluble fraction of the polymerization reaction, and they increase the amount of soluble hTau40. The fragments also reduce the number and average length of filaments that are formed. Taken together, these results suggest that the N-terminal tau fragments inhibit hTau40 polymerization by interacting with a specific C-terminal sequence, thereby stabilizing a soluble conformation of tau.

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.

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.

Tau paired helical filaments from Alzheimer's disease brain and assembled in vitro are based on beta-structure in the core domain

Tau protein, a neuronal microtubule-associated protein, forms insoluble fibers ("paired helical filaments") in Alzheimer's disease and other tauopathies. Conflicting views on the structure of the fibers have been proposed recently, ranging from mainly alpha-helical structure to mainly beta-sheet, or a mixture of mostly random coil and beta-sheet. We have addressed this issue by studying tau fibers immunopurified from Alzheimer brain tissue by a conformation-specific antibody and comparing them with fibers reassembled from recombinant tau or tau constructs in vitro, using a combination of electron microscopy and spectroscopic methods. Brain-derived fibers and reassembled fibers both exhibit a typical twisted appearance when examined by electron microscopy. The soluble tau protein is a natively unfolded protein dominated by random coil structure, whereas Alzheimer PHFs and reassembled fibers show a shift toward an increase in the level of beta-structure. The results support a model in which the repeat domain of tau (which lies within the core of PHFs) adopts an increasing level of beta-structure during aggregation, whereas the N- and C-terminal domains projecting away from the PHF core are mostly random coil.

Inhibition of tau polymerization by its carboxy-terminal caspase cleavage fragment

Abnormal aggregation of the microtubule-associated protein, tau, occurs in many neurodegenerative diseases, making it important to understand the mechanisms of tau polymerization. Previous work has indicated that the C-terminal region of tau inhibits polymerization in vitro, and a growing body of evidence implicates caspase cleavage of tau at Asp 421 in the C-terminus as an important inducer of tau polymerization in Alzheimer's disease. In the present study, we provide evidence that the C-terminal peptide fragment produced by caspase cleavage inhibits tau polymerization, suggesting that caspase cleavage of tau enhances its polymerization by removing the inhibitory control element. Moreover, we provide evidence that the peptide assumes an alpha-helical configuration and inhibits tau assembly by interacting with residues 321-375 in the microtubule binding repeat region. These findings indicate that formation of the fibrillar pathologies during the course of Alzheimer's disease may be driven or sustained by apoptotic events leading to caspase activation.

From Alzheimer to Huntington: why is a structural understanding so difficult?

An increasing family of neurodegenerative disorders such as Alzheimer's, Parkinson's and Huntington's diseases, prion encephalopathies and cystic fibrosis is associated with aggregation of misfolded polypeptide chains which are toxic to the cell. Knowledge of the three-dimensional structure of the proteins implicated is essential for understanding why and how endogenous proteins may adopt a non-native fold. Yet, structural work has been hampered by the difficulty of handling proteins insoluble or prone to aggregation, and at the same time that is why it is interesting to study these molecules. In this review, we compare the structural knowledge accumulated for two paradigmatic misfolding disorders, Alzheimer's disease (AD) and the family of poly-glutamine diseases (poly-Q) and discuss some of the hypotheses suggested for explaining aggregate formation. While a common mechanism between these pathologies remains to be proven, a direct comparison may help in designing new strategies for approaching their study.

The neuronal microtubule-associated protein tau is a substrate for caspase-3 and an effector of apoptosis

We have identified a class of tau fragments inducing apoptosis in different cellular contexts, including a human teratocarcinoma-derived cell line (NT2 cells) representing committed human neuronal precursors. We have found a transition point inside the tau molecule beyond which the fragments lose their ability to induce apoptosis. This transition point is located around one of the putative caspase-3 cleavage sites. This is the only site that can be effectively used by caspase-3 in vitro, releasing the C-terminal 19 amino acids of tau. These results establish tau as a substrate for an apoptotic protease that turns tau itself into an effector of apoptosis. Accordingly, tau may be involved in a self-propagating process like what has been predicted for the pathogenesis of different neurodegenerative disorders.