Formation Of Paired Helical Filaments Research Articles

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 α-helical structure in aqueous solutions at physiological pH after the addition of 1 mol equiv. of Cu 2+. Since α-helix structure is responsible for the formation of paired helical filaments (PHFs) which aggregate into NFTs, it is hypothesized that Cu 2+ induces R2 peptide to self-assemble into a PHFs-like structure. Hence, it is postulated that Cu 2+ 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.

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.

Sites of Tau Important for Aggregation Populate β-Structure and Bind to Microtubules and Polyanions

The aggregation of the microtubule-associated tau protein and formation of "neurofibrillary tangles" is one of the hallmarks of Alzheimer disease. The mechanisms underlying the structural transition of innocuous, natively unfolded tau to neurotoxic forms and the detailed mechanisms of binding to microtubules are largely unknown. Here we report the high-resolution characterization of the repeat domain of soluble tau using multidimensional NMR spectroscopy. NMR secondary chemical shifts detect residual beta-structure for 8-10 residues at the beginning of repeats R2-R4. These regions correspond to sequence motifs known to form the core of the cross-beta-structure of tau-paired helical filaments. Chemical shift perturbation studies show that polyanions, which promote paired helical filament aggregation, as well as microtubules interact with tau through positive charges near the ends of the repeats and through the beta-forming motifs at the beginning of repeats 2 and 3. The high degree of similarity between the binding of polyanions and microtubules supports the hypothesis that stable microtubules prevent paired helical filament formation by blocking the tau-polyanion interaction sites, which are crucial for paired helical filament formation.

Structural insights into Alzheimer filament assembly pathways based on site‐directed mutagenesis and S‐glutathionylation of three‐repeat neuronal Tau protein

Although Tau and MAP2 readily assemble into straight filaments (SFs), Tau's unique ability to form paired-helical filaments (PHFs) may offer clues as to why Tau's microtubule-binding region (MTBR) is the exclusive building block of the neurofibrillary tangles that accumulate during Alzheimer's disease. To learn more about the factors permitting Tau to form both SFs and PHFs, we investigated the microtubule binding, thiol oxidation, and polymerization reactions of the monomer and dimer forms of Tau and MAP2 MTBRs. This review focuses on electron microscopic evidence (1) that facilitated the identification of amino acid residues within 3-repeat Tau that promote PHF formation; and (2) provided experimental evidence for the polymerization of S-glutathionylated three-repeat Tau, a reaction that unambiguously demonstrates that disulfide-linked Tau-S-S-Tau dimer formation is not a compulsory step in filament assembly. We also consider these findings within the context of current views on the genetic and biochemical basis of Tau fibrillogenesis.

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.

Screening for Inhibitors of Tau Polymerization

The histopathological diagnosis of Alzheimer's disease relies on two kinds of proteinaceous aggregates: the extracellular plaques built from filaments of the Abeta-peptide and the intracellular tangles consisting of tau polymerized into Paired Helical Filaments (PHFs). The order of aggregation events is still under debate, but it is well accepted that tau-related changes have an important impact on the viability of neurons. In neurons, early morphological changes are seen in axons which begin to loose and retract synapses. This process is accompanied by an increase of aggregated tau protein. Thus the prevention of tau aggregation seems to be a valuable target for therapy of Alzheimer's disease. Here we present a screening procedure by which we identified inhibitors of tau polymerization. In the primary screen we used a thioflavin-S based assay which detects PHF formation in solution. These initial hits were further analyzed for their capacity to depolymerize preformed PHFs. These results were confirmed by several secondary assays (tryptophan fluorescence, pelleting, filter trapping and electron microscopy). By this approach it is possible to identify small molecule compounds which prevent or reverse the aggregation of tau and thereby might improve the viability of neurons in a therapeutic approach.

Tau Protein Is Cross-Linked by Transglutaminase in P301L Tau Transgenic Mice

The microtubule-associated protein tau is highly soluble under physiological conditions. However, in tauopathies, tau protein aggregates into insoluble filaments and neurofibrillary tangles (NFTs). The mechanisms underlying the formation of tau filaments and NFTs in tauopathies remain unclear. Several lines of evidence suggest that transglutaminase may cross-link tau into stable, insoluble aggregates, leading to the formation of NFTs in Alzheimer's disease and progressive supranuclear palsy. To further determine the contribution of transglutaminase in the formation of NFTs, we compared the levels of cross-linked tau protein from P301L tau transgenic mice that develop NFTs to four-repeat wild-type (4RWT) tau transgenic and nontransgenic mice that do not develop NFT pathology. Immunoprecipitation and immunoblotting experiments show that transglutaminase cross-links phosphorylated tau in the hindbrain of P301L tau transgenic mice but not in mice overexpressing 4RWT tau and nontransgenic mice. Cross-linked, phosphorylated tau from P301L tau transgenic mice runs as high-molecular mass aggregates on Western blots, similar to cross-linked tau from paired helical filaments of Alzheimer's disease. We also used double-label immunofluorescence to demonstrate colocalization of PHF-1-immunoreactive tau and the transglutaminase-catalyzed cross-link in the hindbrain, spinal cord, and cortex of P301L tau transgenic mice. In the spinal cord, 87% of PHF-1-labeled cells colocalize with the transglutaminase-catalyzed cross-link. Additionally, transglutaminase enzymatic activity is significantly elevated in the spinal cord of P301L tau transgenic mice. These studies further implicate transglutaminase in the formation and/or stabilization of NFT and paired helical filaments and provide a model system to investigate the therapeutic potential of transglutaminase inhibitors in tauopathies.

Anthraquinones Inhibit Tau Aggregation and Dissolve Alzheimer's Paired Helical Filaments in Vitro and in Cells

The abnormal aggregation of tau protein into paired helical filaments (PHFs) is one of the hallmarks of Alzheimer's disease. Aggregation takes place in the cytoplasm and could therefore be cytotoxic for neurons. To find inhibitors of PHF aggregation we screened a library of 200,000 compounds. The hits found in the PHF inhibition assay were also tested for their ability to dissolve preformed PHFs. The results were obtained using a thioflavin S fluorescence assay for the detection and quantification of tau aggregation in solution, a tryptophan fluorescence assay using tryptophan-containing mutants of tau, and confirmed by a pelleting assay and electron microscopy of the products. Here we demonstrate the feasibility of the approach with several compounds from the family of anthraquinones, including emodin, daunorubicin, adriamycin, and others. They were able to inhibit PHF formation with IC50 values of 1-5 microm and to disassemble preformed PHFs at DC50 values of 2-4 microm. The compounds had a similar activity for PHFs made from different tau isoforms and constructs. The compounds did not interfere with the stabilization of microtubules by tau. Tau-inducible neuroblastoma cells showed the formation of tau aggregates and concomitant cytotoxicity, which could be prevented by inhibitors. Thus, small molecule inhibitors could provide a basis for the development of tools for the treatment of tau pathology in AD and other tauopathies.

Induction of hyperphosphorylated tau in primary rat cortical neuron cultures mediated by oxidative stress and glycogen synthase kinase-3

Neurofibrillary tangles (NFT) containing paired helical filaments (PHF) composed of abnormally phosphorylated tau are one of the hallmark lesions of the Alzheimer's disease (AD) brain. Although phosphorylation of tau is thought to precede the formation of PHF, the kinases/phosphatases involved remain poorly understood. Here we report that treatment of primary rat cortical neuron cultures with cuprizone, a copper chelator, in combination with oxidative stress (Fe(2+)/H(2)O(2)), significantly increased aberrant tau phosphorylation identified by TG3 immunochemistry. To determine the potential contribution of glycogen synthase kinase-3 (GSK-3) to the phosphorylation of tau in this model, activity of GSK-3 was determined. Cultures treated with cuprizone/Fe(2+)/H(2)O(2) showed significantly increased GSK-3 activity compared with control cultures or cultures treated with cuprizone, or Fe(2+)/H(2)O(2) alone. Concomitant treatment of cultures with lithium, a GSK-3 inhibitor, significantly decreased GSK-3 activity and reduced TG3 staining. Together these data suggest a culture model of hyperphosphorylated tau that implicates increased GSK-3 activity.

Serum or cerebrospinal fluid levels of glyceraldehyde-derived advanced glycation end products (AGEs) may be a promising biomarker for early detection of Alzheimer’s disease

Alzheimer's disease (AD) is the most common cause of dementia in developed countries. AD is characterized pathologically by the presence of senile plaques and neurofibrillary tangles (NFTs), the major constituents of which are the amyloid beta protein (Abeta) and tau protein, respectively. Several epidemiological studies have reported moderately increased risks of AD in diabetic patients compared with general population. In diabetes mellitus, the formation and accumulation of advanced glycation end products (AGEs) progress. Recent understandings of this process have confirmed that AGEs - their receptor (RAGE) interactions may play a role in the pathogenesis of diabetic vascular complications and neurodegenerative disorders including AD. Indeed, it has been demonstrated that AGEs can be identified immunohistochemically to be present in both senile plaques and NFTs from patients with AD. Glycation of Abeta markedly enhances its aggregation in vitro, and the glycation of tau, in addition to hyperphosphorylation, appears to enhance the formation of paired helical filaments. Further, RAGE has been found a specific cell surface receptor for Abeta peptite, thus eliciting neuronal cell perturbation. The active participation of RAGE in the pathogenesis of AD has also been confirmed in RAGE-overexpressed transgenic mice. Moreover, we have recently found that glyceraldehyde-derived AGEs, one of the representative ligands for RAGE, exerted cytopathic effects on cultured neuronal cells and that neurotoxic effect of diabetic serum was completely blocked by neutralizing antibodies against glyceraldehydes-derived AGEs. These observations led us to hypothesize that serum or cerebrospinal fluid (CSF) levels of glyceraldehyde-derived AGEs could become a promising biomarker for early detection of AD. We also would like to propose the possible ways of testing our hypothesis. Are the concentrations of glyceraldehyde-derived AGEs in serum or CSF elevated early in the course of dementia? Are these levels correlated with disease severity and progression, especially in patients with diabetes? These clinical studies clarify whether use of serum or CSF levels of glyceraldehyde-derived AGEs as a biomarker for AD might enable more effective diagnosis and treatment of patients with this devastating disorder.

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 CαH 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 α-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.

Residual Structure in the Repeat Domain of Tau: Echoes of Microtubule Binding and Paired Helical Filament Formation

The microtubule-associated protein tau is found aggregated into paired helical filaments in the intraneuronal neurofibrillary tangle deposits of victims of Alzheimer's disease (AD) and other related dementias. Tau contains a repeat domain consisting of three or four 31-32-residue imperfect repeats that forms the core of tau filaments and is capable of self-assembling into filaments in vitro. We have used high-resolution NMR spectroscopy to characterize the structural properties of the three-repeat domain of tau at the level of individual residues. We find that three distinct regions of the polypeptide corresponding to previously mapped microtubule interaction sites exhibit a preference for helical conformations, suggesting that these sites adopt a helical structure when bound to microtubules. In addition, we directly observe a marked preference for extended or beta-strand-like conformations in a stretch of residues between two of the helical regions, which corresponds closely to a region previously implicated as an early site of beta-strand structure formation and intermolecular interactions leading to paired helical filament (PHF) formation. This observation supports the idea that this region of the protein plays a crucial role in the formation of tau aggregates. We further show that disulfide-bond-mediated dimer formation does not affect and is not responsible for the observed structural preferences of the protein. Our results provide the first high-resolution view of the structural properties of the protein tau, are consistent with an important role for beta structure in PHF formation, and may also help explain recent reports that tau filaments contain helical structure.

A Cdk5 inhibitory peptide reduces tau hyperphosphorylation and apoptosis in neurons

The extracellular aggregation of amyloid beta (Abeta) peptides and the intracellular hyperphosphorylation of tau at specific epitopes are pathological hallmarks of neurodegenerative diseases such as Alzheimer's disease (AD). Cdk5 phosphorylates tau at AD-specific phospho-epitopes when it associates with p25. p25 is a truncated activator, which is produced from the physiological Cdk5 activator p35 upon exposure to Abeta peptides. We show that neuronal infections with Cdk5 inhibitory peptide (CIP) selectively inhibit p25/Cdk5 activity and suppress the aberrant tau phosphorylation in cortical neurons. Furthermore, Abeta(1-42)-induced apoptosis of these cortical neurons was also reduced by coinfection with CIP. Of particular importance is our finding that CIP did not inhibit endogenous or transfected p35/Cdk5 activity, nor did it inhibit the other cyclin-dependent kinases such as Cdc2, Cdk2, Cdk4 and Cdk6. These results, therefore, provide a strategy to address, and possibly ameliorate, the pathology of neurodegenerative diseases that may be a consequence of aberrant p25 activation of Cdk5, without affecting 'normal' Cdk5 activity.

The Neuronal Cytoskeleton as a Potential Therapeutical Target in Neurodegenerative Diseases and Schizophrenia

The cytoskeleton plays a key role in maintaining the highly asymmetrical shape and structural polarity of neurons that are essential for neuronal physiology. Cytoskeletal reorganization plays a key role in neuritogenesis. In neurodegenerative diseases, the cytoskeleton is abnormally assembled and impairment of neurotransmission occurs. In Alzheimer's disease, abundant amyloid plaques and neurofibrillary tangles constitute the two major neuropathologic alterations present in the brain. Neurofibrillary tangles are formed of paired helical filaments consisting nearly entirely of the microtubule-associated protein tau. Under normal conditions tau binds to microtubules, stabilizing neuron structure and integrity. Hyperphosphorylation of tau is assumed to be the cause of formation of paired helical filaments. Another example of cytoskeletal abnormalities present in neurodegenerative diseases are the Lewy bodies considered as cytopathologic markers of Parkinson's disease. Lewy bodies are constituted of tubulin, MAP1, and MAP2. Neuronal shape, loss of dendrites and spines, as well as irregular distribution of neuronal elongations occur in specific brain areas of schizophrenic patients. Increase in non-phosphorylated MAP2 and MAP1B at hippocampus has been suggested as responsible for somatodendritic and cytoarchitectural abnormalities found in schizophrenia. In addition, neurofibrillary tangles are more frequent among schizophrenic patients who received pharmacologic antipsychotic treatment. Cumulative evidence suggests that neurodegenerative diseases and psychiatric illnesses are associated with cytoskeletal alterations in neurons that, in turn, loose synaptic connectivity and the ability to transmit incoming axonal information to the somatodendritic domain. We will review evidence supporting that the neuronal cytoskeleton is disrupted in neurodegenerative and some psychiatric diseases, and therefore could be a target for drug therapy. In addition, current data indicating that melatonin, a hormone secreted by the pineal gland, promotes neuritogenesis through cytoskeletal rearrangements and in addition to the potential therapeutic use of melatonin in neurodegenerative diseases will be discussed.

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.

Neurodegeneration and plasticity

Neurofibrillary degeneration, associated with the formation of paired helical filaments (PHF), is one of the critical neuropathological hallmarks of Alzheimer's disease (AD). Although the microtubule-associated protein tau in a hyperphosphorylated form has been established as primary PHF constituent, the process of tau phosphorylation and its potential link to degeneration is not very well understood, mostly because of the lack of a physiological in vivo model of PHF-like tau phosphorylation. PHF formation in AD follows a hierarchial pattern of development throughout different cortical areas, which closely matches the pattern of neuronal plasticity in the adult brain. Those brain areas are most early and most severely affected which are involved in the regulation of memory, learning, perception, self-awareness, consciousness, and higher brain functions that require a life-long re-fitting of connectivity, a process based on a particularly high degree of plasticity. Failures of synaptic plasticity are, thus, assumed to represent early events in the course of AD that eventually lead to alteration of tau phosphorylation. Recently, we have used the hibernation cycle, a physiological model of adaptation associated with an extraordinary high degree of structural neuronal plasticity, to analyze the potential link between synaptic plasticity, synaptic detachment and the regulation of tau phosphorylation. During torpor, a natural state of hypothermia, synaptic contacts between mossy fibers and hippocampal pyramidal neurons undergo dramatic regressive changes that are fully reversible very rapidly during euthermy. This rapid, reversible, and repeated regression of synaptic and dendritic components on CA3 neurons is associated with a reversible PHF-like phosphorylation of tau at a similar time course. The repeated formation and degradation of PHF-tau might, thus, represent a physiological mechanism not necessarily associated with pathological effects. These findings implicate an essential link between neuronal plasticity and PHF-like phosphorylation of tau, potentially involved in neurofibrillary degeneration.

Conformational transition state is responsible for assembly of microtubule-binding domain of tau protein

In the brains of Alzheimer’s disease patients, the tau protein dissociates from the axonal microtubule and abnormally aggregates to form a paired helical filament (PHF). One of the priorities in Alzheimer research is to clarify the mechanism of PHF formation. Although several reports on the regulation of tau assembly have been published, it is not yet clear whether in vivo PHFs are composed of β-structures or α-helices. Since the four-repeat microtubule-binding domain (4RMBD) of the tau protein has been considered to play an essential role in PHF formation, its heparin-induced assembly propensity was investigated by the thioflavin fluorescence method to clarify what conformation is most preferred for the assembly. We analyzed the assembly propensity of 4RMBD in Tris–HCl buffer with different trifluoroethanol (TFE) contents, because TFE reversibly induces the transition of the random structure to the α-helical structure in an aqueous solution. Consequently, it was observed that the 4RMBD assembly is most significantly favored to proceed in the 10–30% TFE solution, the concentration of which corresponds to the activated transition state of 4RMBD from a random structure to an α-helical structure, as determined from the circular dichroism (CD) spectral changes. Since such an assembly does not occur in a buffer containing TFE of <10% or >40%, the intermediate conformation between the random and α-helical structures could be most responsible for the PHF formation of 4RMBD. This is the first report to clarify that the non-native α-helical intermediate in transition from random coil is directly associated with filament formation at the start of PHF formation.

Recent Evidence Regarding a Role for Cdk5 Dysregulation in Alzheimers Disease

Based on a growing literature, cyclin-dependent kinase 5 (Cdk5) has been implicated in the pathological processes that contribute to neurodegeneration in Alzheimer's disease (AD). Cdk5 is ubiquitously expressed, but its activity is largely localized to post-mitotic neurons due to neuron-specific expression of its activators p35 and p39. Sufficient Cdk5 activity is critical to normal central nervous system development, as in its absence, neuronal migration and axonal path finding are deranged. Conversely, excessive and mislocalized Cdk5 activity appears to be detrimental to neuronal function. In fact, the pathological hallmarks of AD, beta-amyloid aggregates and neurofibrillary tangles, have been linked to Cdk5-mediated neuronal death. In this model, beta-amyloid is the toxic stimulus that disrupts intracellular calcium homeostasis, leading to activation of calpains, a family of calcium-dependent proteases. Calpain-mediated cleavage of p35, yields a truncated p25 fragment that possesses a longer half-life, lacks the necessary sequence targeting it to membranes, but retains the capacity to activate Cdk5. The resulting excessive and mislocalized Cdk5 activity targets tau as a substrate for hyperphosphorylation, which is a prerequisite of paired helical filament (PHF) formation. A number of recent reports, utilizing diverse methods, lend further support to this model of AD neurodegeneration, and several strategies for combating Cdk5 dysregulation have even been devised. However, the study of Cdk5 in AD is not without controversy, and questions remain regarding its role in the pathology. Herein, the most recent findings regarding this model are reviewed.

β-Amyloid Induces Paired Helical Filament-like Tau Filaments in Tissue Culture

Paired helical filaments (PHF) are the principal pathologic components of neurofibrillary tangles in Alzheimer's disease (AD). To reproduce the formation of PHF in tissue culture, we stably expressed human tau with and without pathogenic mutations in human SH-SY5Y cells and exposed them for 5 days to aggregated synthetic beta-amyloid peptide (A beta 42). This caused a decreased solubility of tau along with the generation of PHF-like tau-containing filaments. These were 20 nm wide and had periodicities of 130-140 nm in the presence of P301L mutant tau or 150-160 nm in the presence of wild-type tau. Mutagenesis of the phosphoepitope serine 422 of tau prevented both the A beta 42-mediated decrease in solubility and the generation of PHF-like filaments, suggesting a role of serine 422 or its phosphorylation in tau filament formation. Together, our data underscore a role of A beta 42 in the formation of PHF-like filaments. Our culture system will be useful to map phosphoepitopes of tau involved in PHF formation and to identify and characterize modifiers of the tau pathology. Further adaptation of the system may allow the screening and validation of compounds designed to prevent PHF formation.

Reversible Paired Helical Filament-Like Phosphorylation of Tau Is an Adaptive Process Associated with Neuronal Plasticity in Hibernating Animals

Neurofibrillary pathology [paired helical filaments (PHFs)] formed by the microtubule-associated protein tau in a hyperphosphorylated form is a major hallmark of Alzheimer's disease and related disorders. The process of tau phosphorylation, thought to be of critical importance for PHF formation, and its potential link to neurodegeneration, however, is not understood very well, mostly because of the lack of a physiological in vivo model of PHF-like tau phosphorylation. Here we describe the formation of highly phosphorylated tau, containing a number of PHF-like epitopes in torpor during hibernation. PHF-like phosphorylation of tau was not associated with fibril formation and was fully reversible after arousal. Distribution of PHF-like tau followed a consistent pattern, being most intense in the entorhinal cortex, hippocampus, and isocortical areas. Within the hippocampus, a particularly high labeling was seen in CA3 pyramidal cells. Somewhat lesser reactivity was present in CA1 neurons while dentate gyrus granule cells were not reactive. Formation of PHF-like tau in CA3 neurons was paralleled by the regression of synaptic contacts of the mossy fiber system terminating on CA3 apical dendrites. Mossy fiber afferentation was re-established during arousal, concomitantly with the decrease of PHF-like tau in CA3 neurons. These findings implicate an essential link between neuronal plasticity and PHF-like phosphorylation of tau. The repeated formation and degradation of PHF-like tau might, thus, represent a physiological mechanism not necessarily associated with pathological effects. Hibernation will, therefore, be a valuable model to study the regulation of PHF-like tau-phosphorylation and its cell biological sequelae under physiological in vivo conditions.