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

The "jaws" of the tau-microtubule interaction

Tau is the major microtubule-associated protein in neuronal axons. It aggregates into "neurofibrillary tangles" during the course of Alzheimer disease. Binding to microtubules and microtubule assembly requires the "repeat domain" in the C-terminal half of Tau, as well as the two regions flanking the repeats. Here we report the NMR characterization of a 198-residue Tau fragment composed of the four tandem repeats and the flanking domains and containing the full microtubule binding and assembly activity of Tau. NMR secondary chemical shifts and dipolar couplings detect the highest propensity for beta-structure within the four-repeat region, whereas the flanking domains are largely random coil, with an increased rigidity in the proline-rich region. Chemical shift perturbation experiments identify two motifs in the upstream flanking domain, (225)KVAVVRT(231) and (243)LQTA(246), and one downstream of the repeats, (370)KIETHKTFREN(380), which strongly contribute to the binding to the acidic outside of microtubules, as well as to the binding of other polyanions such as heparin. This is consistent with the "jaws" model of Tau-microtubule interactions and highlights the importance of the regions flanking the repeats for both microtubule binding and pathological Tau aggregation.

Effect of DNA on Filament Formation of Tau Microtubule-Binding Domain: Structural Dependence of DNA

To examine whether or not DNA accelerates the paired helical filament (PHF) formation of tau, the effect of various types of DNAs on filament formations of three-repeated and four-repeated microtubule-binding domains (3RMBD and 4RMBD, respectively) of tau protein was investigated by monitoring the change of thioflavin S fluorescence intensity, that is parallel to the filament formation. Consequently, the followings were clarified: 1) the structurally rigid double-stranded DNA such as poly(dG-dC) or calf thymus DNA has the high potency of promoting the filament formations of 3RMBD and 4RMBD, 2) the filament formation of 3RMBD was more promoted than that of 4RMBD, due to the intermolecular dimer formation of 3RMBD, 3) the DNA-promoted filament formations of these MBDs were temperature-dependent, and the single-stranded DNA such as poly(dA) or poly(dT) reversely protected 4RMBD from the molecular assembly at 20 degrees C. These are the first report on the function of DNA for the PHF formation of tau protein.

Alzheimer’s-disease-associated conformation of intrinsically disordered tau protein studied by intrinsically disordered protein liquid-phase competitive enzyme-linked immunosorbent assay

Tau protein, the major constituent of paired helical filaments in Alzheimer's disease, belongs to the intrinsically disordered proteins (IDPs). IDPs are an emerging group in the protein kingdom characterized by the absence of a rigid three-dimensional structure. Disordered proteins usually acquire a "functional fold" upon binding to their interaction partner(s). This property of IDPs implies the need for innovative approaches to measure their binding affinity. We have mapped and measured the Alzheimer's-disease-associated epitope on intrinsically disordered tau protein with a novel two-step sandwich competitive enzyme-linked immunosorbent assay (ELISA). This approach allowed us to determine the binding affinity of disordered tau protein in liquid phase without any disturbance to the competitive equilibrium and without any need for covalent or noncovalent modification of tau protein. Furthermore, the global fitting method, used for the reconstruction of tau binding curves, significantly improved the assay readout. The proposed novel competitive ELISA allowed us to determine the changes in the standard Gibbs energy of binding, thus enabling measurement of tau protein conformation in the core of paired helical filaments. IDP competitive ELISA results showed, for the first time, that the tau protein C terminus of the Alzheimer's-disease-derived paired helical filaments core subunit adopts beta-turn type I' fold and is accessible from solution.

Role of ApoE in conformation-prone diseases and atherosclerosis

Three isoforms of human plasma apolipoprotein E (apoE) are ligands to lipoprotein receptors and influence in different manner the synthesis and catabolism of pro-atherogenic triglyceride-rich lipoproteins. Among three isoforms, the apoE4 isoform is associated with increased frequency of atherosclerosis and Alzheimer's disease (AD). The conformational transitions of beta-amyloid (Abeta) influenced by apoE and serum amyloid P (SAP) component are key events in AD development, the accumulation of intermediate diffusible and soluble oligomers of Abeta being of particular significance. SAP and apoE, in a different manner for the three isoforms, serve as "pathological" chaperones during the aggregation of Abeta considered as a conformation-prone process. In turn, apoE consisting of two domains self-associates in solution and intermediate structures differently populated for the three isoforms exist. The different structures of the three isoforms determine their different distribution among various plasma lipoproteins. The structural and metabolic consideration of the common apoE pathway(s) in two pathologies assumes four molecular targets for AD correction: (i) inhibition of the accumulation of diffusible soluble Abeta oligomers; (ii) inhibition of apoE synthesis and secretion by astrocytes, in particular, under lipid-lowering therapy; (iii) inhibition of the binding of apoE and/or SAP to Abeta; (iv) stimulation of the expression of cholesterol transporter ABCA1.

Plaques, tangles, and memory loss in mouse models of neurodegeneration

Within the past decade, our understanding of the pathogenic mechanisms in Alzheimer's disease (AD) has dramatically advanced because of the development of transgenic mouse models that recapitulate the key pathological and behavioral phenotypes of the disease. These mouse models have allowed investigators to test detailed questions about how pathology develops and to evaluate potential therapeutic approaches that could slow down the development of this disease. In this review, we discuss the status of transgenic mouse models and review the complex relationship between pathology and behavior in the development of neuropathological syndromes in AD.

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.

SRp54 (SFRS11), a regulator for tau exon 10 alternative splicing identified by an expression cloning strategy

The tau gene encodes a microtubule-associated protein that is critical for neuronal survival and function. Splicing defects in the human tau gene lead to frontotemporal dementia with Parkinsonism linked to chromosome 17 (FTDP-17), an autosomal dominant neurodegenerative disorder. Genetic mutations associated with FTDP-17 often affect tau exon 10 alternative splicing. To investigate mechanisms regulating tau exon 10 alternative splicing, we have developed a green fluorescent protein reporter for tau exon 10 skipping and an expression cloning strategy to identify splicing regulators. A role for SRp54 (also named SFRS11) as a tau exon 10 splicing repressor has been uncovered using this strategy. The overexpression of SRp54 suppresses tau exon 10 inclusion. RNA interference-mediated knock-down of SRp54 increases exon 10 inclusion. SRp54 interacts with a purine-rich element in exon 10 and antagonizes Tra2beta, an SR-domain-containing protein that enhances exon 10 inclusion. Deletion of this exonic element eliminates the activity of SRp54 in suppressing exon 10 inclusion. Our data support a role of SRp54 in regulating tau exon 10 splicing. These experiments also establish a generally useful approach for identifying trans-acting regulators of alternative splicing by expression cloning.

Removal of pattern-breaking sequences in microtubule binding repeats produces instantaneous tau aggregation and toxicity

Aggregated and highly phosphorylated tau protein is a pathological hallmark of Alzheimer's disease (AD) and other tauopathies. We identified motifs of alternating polar and apolar amino acids within the microtubule-binding repeats of tau which were interrupted by small breaking stretches. Minimal mutation of these breaking sequences yielded a unique instantly aggregating tau mutant containing longer stretches of polar/apolar amino acids without losing its microtubule-binding capacity. These modifications produced rapid aggregation and cytotoxicity with accompanying occurrence of pathologic tau phosphoepitopes (AT8, AT180, AT270, AT100, Ser(422), and PHF-1) and conformational epitopes (MC-1 and Alz50) in cells. Similar to pathological tau in the pretangle state, toxicity appeared to occur early without the requirement for extensive fibril formation. Thus, our mutant protein provides a novel platform for the investigation of the molecular mechanisms for toxicity and cellular behavior of pathologically aggregated tau proteins and the identification of its interaction partners.

Jupiter, a new Drosophila protein associated with microtubules

In this study we describe a novel Drosophila protein Jupiter, which shares properties with several structural microtubule-associated proteins (MAPs) including TAU, MAP2, MAP4. Jupiter is a soluble unfolded molecule with the high net positive charge, rich in Glycine. It possesses two degenerated repeats around the sequence PPGG, separated by a Serine-rich region. Jupiter associates with microtubules in vitro and, fused with the green fluorescent protein (GFP), is an excellent marker to follow microtubule dynamics in vivo. In a jupiter transgenic Drosophila strain generated by the "protein-trap" technique, Jupiter:GFP fusion protein localizes to the microtubule network through the cell cycle at the different stages of development. We found particularly high Jupiter:GFP concentrations in the young embryo, larval nervous system, precursors of eye photoreceptors and adult ovary. Moreover, from jupiter:gfp embryos we have established two permanent cell lines presenting strongly fluorescent microtubules during the whole cell cycle. In these cells, the distribution of the Jupiter:GFP fusion protein reproduces microtubule behavior upon treatment by the drugs colchicine and taxol. The jupiter cell lines and fly strain should be of wide interest for biologists interested in in vivo analysis of microtubule dynamics.

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.

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.

Structure of core domain of fibril-forming PHF/Tau fragments

Short peptide sequences within the microtubule binding domain of the protein Tau are proposed to be core nucleation sites for formation of amyloid fibrils displaying the paired helical filament (PHF) morphology characteristic of neurofibrillary tangles. To study the structure of these proposed nucleation sites, we analyzed the x-ray diffraction patterns from the assemblies formed by a variety of PHF/tau-related peptide constructs containing the motifs VQIINK (PHF6*) in the second repeat and VQIVYK (PHF6) in the third repeat of tau. Peptides included: tripeptide acetyl-VYK-amide (AcVYK), tetrapeptide acetyl-IVYK-amide (AcPHF4), hexapeptide acetyl-VQIVYK-amide (AcPHF6), and acetyl-GKVQIINKLDLSNVQKDNIKHGSVQIVYKPVDLSKVT-amide (AcTR4). All diffraction patterns showed reflections at spacings of 4.7 A, 3.8 A, and approximately 8-10 A, which are characteristic of an orthogonal unit cell of beta-sheets having dimensions a=9.4 A, b=6.6 A, and c=approximately 8-10 A (where a, b, and c are the lattice constants in the H-bonding, chain, and intersheet directions). The sharp 4.7 A reflections indicate that the beta-crystallites are likely to be elongated along the H-bonding direction and in a cross-beta conformation. The assembly of the AcTR4 peptide, which contains both the PHF6 and PHF6* motifs, consisted of twisted sheets, as indicated by a unique fanning of the diffuse equatorial scattering and meridional accentuation of the (210) reflection at 3.8 A spacing. The diffraction data for AcVYK, AcPHF4, and AcPHF6 all were consistent with approximately 50 A-wide tubular assemblies having double-walls, where beta-strands constitute the walls. In this structure, the peptides are H-bonded together in the fiber direction, and the intersheet direction is radial. The positive-charged lysine residues face the aqueous medium, and tyrosine-tyrosine aromatic interactions stabilize the intersheet (double-wall) layers. This particular contact, which may be involved in PHF fibril formation, is proposed here as a possible aromatic target for anti-tauopathy drugs.

Increased levels of granular tau oligomers: an early sign of brain aging and Alzheimer's disease

Development of neurofibrillary tangles (NFTs) is a pathological hallmark in various neurodegenerative disorders including Alzheimer's disease (AD). Recently, we identified a granular tau oligomer having a pre-filamentous structure. To determine the role of this oligomer in NFT formation, we quantified the amount of granular tau oligomer in 21 frontal cortex samples, each displaying varying degrees of Braak-staged NFT pathology. Here we report that granular tau oligomer levels in frontal cortex were significantly increased, even in brains displaying Braak-stage I neuropathology, a stage at which clinical symptoms of AD and NFTs in frontal cortex are believed to be absent. This suggests that increases in granular tau oligomer levels occur before NFTs form and before individuals manifest clinical symptoms of AD. Increased granular tau oligomer levels, therefore, may lead to NFT formation in frontal cortex, eventually leading to the development of AD. Thus, increases in granular tau oligomer levels may represent a very early sign of NFT formation and AD.

Paired helical filaments contain small amounts of cholesterol, phosphatidylcholine and sphingolipids

By using qualitative and quantitative high-performance thin layer chromatography (hpTLC) we found lipids associated with purified Alzheimer's (AD) paired helical filaments (PHF) in an amount of 1.4+/-0.2% of the total anhydrous mass. Compared to normal brain tissue these lipids have an unusual lipid class composition. The most prominent lipid classes were phosphatidylcholine (PC), cholesterol (CH), galactocerebrosides (GC) and sphingomyelin (SM). In addition, the use of micro high-performance liquid chromatography (HPLC) in combination with matrix-assisted laser desorption and ionisation time-of-flight mass spectrometry (MALDI-TOF-MS) allowed the determination of the molecular species of the polar membrane lipid classes present in PHF. The lipid pattern of intracellular PHF shows many characteristics of the conserved lipid pattern previously described for extracellular amyloid fibrils, suggesting similarities in their pathway of formation.

Visualizing Pathology Deposits in the Living Brain of Patients with Alzheimer's Disease

One of the major neuropathological changes characteristic of Alzheimer's disease (AD) are deposits of beta-amyloid plaques and neurofibrillary tangles in neocortical and subcortical regions of the AD brain. The histochemical detection of these lesions in postmortem brain tissue is necessary for definitive diagnosis of AD. Methods for their in vivo detection would greatly aid the diagnosis of AD in early stages when neuronal loss and related functional impairment are still limited and also open the opportunity for effective therapeutic interventions. Positron emission tomography (PET) using an appropriate radiolabeled imaging probe with high binding affinity for these lesions is one of such techniques. We have developed 2-(1-{6-[(2-[F-18]fluoroethyl)(methyl)amino]-2-naphthyl}ethylidene)malononitrile ([F-18]FDDNP), a naphthalene-based radiofluorinated PET imaging probe with binding affinity for amyloid and amyloid-like structures, and applied it for in vivo brain imaging of patients with Alzheimer's disease and cognitively normal controls with PET. Analysis of in vivo [F-18]FDDNP imaging data using Logan plot graphical analysis with the cerebellum as a reference region was performed, and the binding levels in several areas of neocortex were determined. We observed increased levels of [F-18]FDDNP binding in patients in several neocortical regions in Alzheimer's disease compared with the cerebellum. In contrast, control subjects have uniformly low levels of [F-18]FDDNP binding in all areas, which is comparable to that of cerebellum.

RNA-binding protein is involved in aggregation of light neurofilament protein and is implicated in the pathogenesis of motor neuron degeneration

Abnormal protein aggregation is emerging as a common theme in the pathogenesis of neurodegenerative disease. Our previous studies have shown that overexpression of untranslated light neurofilament (NF-L) RNA causes motor neuron degeneration in transgenic mice, leads to accumulation of ubiquitinated aggregates in degenerating cultured motor neurons and triggers aggregation of NF-L protein and co-aggregation of mutant SOD1 protein in neuronal cells. Here, we report that p190RhoGEF, an RNA-binding protein that binds to a destabilizing element in NF-L mRNA, is involved in aggregation of NF-L protein and is implicated in the pathogenesis of motor neuron degeneration. We show that p190RhoGEF co-aggregates with unassembled NF-L protein and that co-aggregation is associated with down-regulation of parent NF-L mRNA in neuronal cells. Co-expression of NF-M increases NF assembly and reduces RNA-triggered aggregation as well as loss of solubility of NF-L protein. siRNA-induced down-regulation of p190RhoGEF not only reduces aggregation and promotes assembly of NF-L and NF-M, but also causes reversal of aggregation and recovery of NF assembly in transfected cells. Examination of transgenic models of motor neuron disease shows that prominent aggregates of p190RhoGEF and NF-L and down-regulation of NF-L expression occur in degenerating motor neurons of mice expressing untranslated NF-L RNA or a G93A mutant SOD1 transgene. Moreover, aggregates of p190RhoGEF and NF-L appear as early pathological changes in presymptomatic G93A mutant SOD1 transgenic mice. Together, the findings indicate that p190RhoGEF is involved in aggregation of NF-L protein and support a working hypothesis that aggregation of p190RhoGEF and NF-L is an upstream event triggering neurotoxicity in motor neuron disease.

Characterization of paired helical filaments by scanning transmission electron microscopy

Paired helical filaments (PHFs) are abnormal twisted filaments composed of hyperphosphorylated tau protein. They are found in Alzheimer's disease and other neurodegenerative disorders designated as tauopathies. They are a major component of intracellular inclusions known as neurofibrillary tangles (NFTs). The objective of this review is to summarize various structural studies of PHFs in which using scanning transmission electron microscopy (STEM) has been particularly informative. STEM provides shape and mass per unit length measurements important for studying ultrastructural aspects of filaments. These include quantitative comparisons between dispersed and aggregated populations of PHFs as well as comparative studies of PHFs in Alzheimer's disease and other neurodegenerative disorders. Other approaches are also discussed if relevant or complementary to studies using STEM, e.g., application of a novel staining reagent, Nanovan. Our understanding of the PHF structure and the development of PHFs into NFTs is presented from a historical perspective. Others goals are to describe the biochemical and ultrastructural complexity of authentic PHFs, to assess similarities between authentic and synthetic PHFs, and to discuss recent advances in PHF modeling.