Increase in tau tyrosine phosphorylation correlates with the formation of tau aggregates

Conformational changes and cleavage; are these responsible for the tau aggregation in Alzheimer’s disease?

In the past, post-translational modifications of tau protein, such as phosphorylation, cleavage and conformational changes, have long been implicated in the pathogenesis of Alzheimer’s disease. Unfortunately, the accurate role and relationship between these pathological modifications during tau aggregation remains under extensive study. We had proposed a chronological model of tau pathological processing during Alzheimer´s disease, in which phosphorylation and cleavage could lead to conformational changes causing aggregation and therefore, cell toxicity. We discuss this issue and review in vitro and in situ evidence that supports the relevance of tau modifications that cause its pathological conformations and toxic aggregation. Thus, we offer a brief discussion regarding conformational change and cleavage as future clinical targets.

Charge-pairing mechanism of phosphorylation effect upon amyloid fibrillation of human tau core peptide

Phosphorylation of a fibrillogenic protein, human tau, is believed to play crucial roles in the pathogenesis of Alzheimer's disease. For elucidating molecular mechanisms of the phosphorylation effect on tau fibrillation, we synthesized a peptide, VQIVY 310K (PHF6) and its phosphorylated derivative (PHF6pY). PHF6 is a partial peptide surrounding a plausible in vivo phosphorylation site Tyr310 and forms amyloid-type fibrils similar to those generated by full-length tau. Fibrillation of PHF6 and PHF6pY were studied by spectroscopic and microscopic methods, and the critical concentration of the fibrillation was determined for comparing the fibril stability. The results showed that the phosphorylation strongly influenced the fibrillation propensity of PHF6 by changing its dependency on pH and ionic strength. On the basis of the observations, we suggested that charged sites on the phosphate group and its electrostatic pairing with the neighboring charged residues were physical origins of the phosphorylation effect. To verify this charge-pairing mechanism, we conducted experiments using a series of PHF6 derivatives with non-native charge distributions. The electrostatic interaction in an intermolecular mode was also demonstrated by the system composed of two different peptide species, which found that fibrillation of nonphosphorylated PHF6 was drastically enhanced when a trace amount of phosphorylated PHF6 molecules coexisted. A simulation analysis utilizing crystal coordinates of the PHF6 fibril was also performed for interpreting the experimental results in a molecular level. The present study using the model peptide system gave us a microscopically insightful view on the roles of tau phosphorylation in amyloid-related diseases.

Detailed immunohistochemical characterization of temporal and spatial progression of Alzheimer's disease-related pathologies in male triple-transgenic mice

Background

Several transgenic animal models genetically predisposed to develop Alzheimer's disease (AD)-like pathology have been engineered to facilitate the study of disease pathophysiology and the vetting of potential disease-modifying therapeutics. The triple transgenic mouse model of AD (3xTg-AD) harbors three AD-related genetic loci: human PS1^M146V, human APP^swe, and human tau^P301L. These mice develop both amyloid plaques and neurofibrillary tangle-like pathology in a progressive and age-dependent manner, while these pathological hallmarks are predominantly restricted to the hippocampus, amygdala, and the cerebral cortex the main foci of AD neuropathology in humans. This model represents, at present, one of the most advanced preclinical tools available and is being employed ever increasingly in the study of mechanisms underlying AD, yet a detailed regional and temporal assessment of the subtleties of disease-related pathologies has not been reported.

Methods and results

In this study, we immunohistochemically documented the evolution of AD-related transgene expression, amyloid deposition, tau phosphorylation, astrogliosis, and microglial activation throughout the hippocampus, entorhinal cortex, primary motor cortex, and amygdala over a 26-month period in male 3xTg-AD mice. Intracellular amyloid-beta accumulation is detectable the earliest of AD-related pathologies, followed temporally by phospho-tau, extracellular amyloid-beta, and finally paired helical filament pathology. Pathology appears to be most severe in medial and caudal hippocampus. While astrocytic staining remains relatively constant at all ages and regions assessed, microglial activation appears to progressively increase temporally, especially within the hippocampal formation.

Conclusion

These data fulfill an unmet need in the ever-widening community of investigators studying 3xTg-AD mice and provide a foundation upon which to design future experiments that seek to examine stage-specific disease mechanisms and/or novel therapeutic interventions for AD.

A novel calcium-binding protein is associated with tau proteins in tauopathy

Tauopathies are a group of neurological disorders characterized by the presence of intraneuronal hyperphosphorylated and filamentous tau. Mutations in the tau gene have been found in kindred with tauopathy. The expression of the human tau mutant in transgenic mice induced neurodegeneration, indicating that tau plays a central pathological role. However, the molecular mechanism leading to tau-mediated neurodegeneration is poorly understood. To gain insights into the role that tau plays in neurodegeneration, human tau proteins were immunoprecipitated from brain lysates of the tauopathy mouse model JNPL3, which develops neurodegeneration in age-dependent manner. In the present work, a novel EF-hand domain-containing protein was found associated with tau proteins in brain lysate of 12-month-old JNPL3 mice. The association between tau proteins and the novel identified protein appears to be induced by the neurodegeneration process as these two proteins were not found associated in young JNPL3 mice. Consistently, the novel protein co-purified with the pathological sarkosyl insoluble tau in terminally ill JNPL3 mice. Calcium-binding assays demonstrated that this protein binds calcium effectively. Finally, the association between tau and the novel calcium-binding protein is conserved in human and enriched in Alzheimer's disease brain. Taken together, the identification of a novel calcium-binding protein associated with tau protein in terminally ill tauopathy mouse model and its confirmation in human brain lysate suggests that this association may play an important physiological and/or pathological role.

Phosphorylation regulates tau interactions with Src homology 3 domains of phosphatidylinositol 3-kinase, phospholipase Cgamma1, Grb2, and Src family kinases

The microtubule-associated protein tau can associate with various other proteins in addition to tubulin, including the SH3 domains of Src family tyrosine kinases. Tau is well known to aggregate to form hyperphosphorylated filamentous deposits in several neurodegenerative diseases (tauopathies) including Alzheimer disease. We now report that tau can bind to SH3 domains derived from the p85alpha subunit of phosphatidylinositol 3-kinase, phospholipase Cgamma1, and the N-terminal (but not the C-terminal) SH3 of Grb2 as well as to the kinases Fyn, cSrc, and Fgr. However, the short inserts found in neuron-specific isoforms of Src prevented the binding of tau. The experimentally determined binding of tau peptides is well accounted for when modeled into the peptide binding cleft in the SH3 domain of Fyn. After phosphorylation in vitro or in transfected cells, tau showed reduced binding to SH3 domains; no binding was detected with hyperphosphorylated tau isolated from Alzheimer brain, but SH3 binding was restored by phosphatase treatment. Tau mutants with serines and threonines replaced by glutamate, to mimic phosphorylation, showed reduced SH3 binding. These results strongly suggest that tau has a potential role in cell signaling in addition to its accepted role in cytoskeletal assembly, with regulation by phosphorylation that may be disrupted in the tauopathies including Alzheimer disease.

Microtubule-associated protein tau in development, degeneration and protection of neurons

As a principal neuronal microtubule-associated protein, tau has been recognized to play major roles in promoting microtubule assembly and stabilizing the microtubules and to maintain the normal morphology of the neurons. Recent studies suggest that tau, upon alternative mRNA splicing and multiple posttranslational modifications, may participate in the regulations of intracellular signal transduction, development and viability of the neurons. Furthermore, tau gene mutations, aberrant mRNA splicing and abnormal posttranslational modifications, such as hyperphosphorylation, have also been found in a number of neurodegenerative disorders, collectively known as tauopathies. Therefore, changes in expression of the tau gene, alternative splicing of its mRNA and its posttranslational modification can modulate the normal architecture and functions of neurons as well as in a situation of tauopathies, such as Alzheimer's disease. The primary aim of this review is to summarize the latest developments and perspectives in our understanding about the roles of tau, especially hyperphosphorylation, in the development, degeneration and protection of neurons.

The microtubule-associated protein tau is phosphorylated by Syk

Aberrant phosphorylation of tau protein on serine and threonine residues has been shown to be critical in neurodegenerative disorders called tauopathies. An increasing amount of data suggest that tyrosine phosphorylation of tau might play an equally important role in pathology, with at least three putative tyrosine kinases of tau identified to date. It was recently shown that the tyrosine kinase Syk could efficiently phosphorylate α-synuclein, the aggregated protein found in Parkinson's disease and other synucleinopathies. We report herein that Syk is also a tau kinase, phosphorylating tau in vitro and in CHO cells when both proteins are expressed exogenously. In CHO cells, we have also demonstrated by co-immunoprecipitation that Syk binds to tau. Finally, by site-directed mutagenesis substituting the tyrosine residues of tau with phenylalanine, we established that tyrosine 18 was the primary residue in tau phosphorylated by Syk. The identification of Syk as a common tyrosine kinase of both tau and α-synuclein may be of potential significance in neurodegenerative disorders and also in neuronal physiology. These results bring another clue to the intriguing overlaps between tauopathies and synucleinopathies and provide new insights into the role of Syk in neuronal physiology.

Early axonopathy preceding neurofibrillary tangles in mutant tau transgenic mice

Neurodegenerative diseases characterized by brain and spinal cord involvement often show widespread accumulations of tau aggregates. We have generated a transgenic mouse line (Tg30tau) expressing in the forebrain and the spinal cord a human tau protein bearing two pathogenic mutations (P301S and G272V). These mice developed age-dependent brain and hippocampal atrophy, central and peripheral axonopathy, progressive motor impairment with neurogenic muscle atrophy, and neurofibrillary tangles and had decreased survival. Axonal spheroids and axonal atrophy developed early before neurofibrillary tangles. Neurofibrillary inclusions developed in neurons at 3 months and were of two types, suggestive of a selective vulnerability of neurons to form different types of fibrillary aggregates. A first type of tau-positive neurofibrillary tangles, more abundant in the forebrain, were composed of ribbon-like 19-nm-wide filaments and twisted paired helical filaments. A second type of tau and neurofilament-positive neurofibrillary tangles, more abundant in the spinal cord and the brainstem, were composed of 10-nm-wide neurofilaments and straight 19-nm filaments. Unbiased stereological analysis indicated that total number of pyramidal neurons and density of neurons in the lumbar spinal cord were not reduced up to 12 months in Tg30tau mice. This Tg30tau model thus provides evidence that axonopathy precedes tangle formation and that both lesions can be dissociated from overt neuronal loss in selected brain areas but not from neuronal dysfunction.

A decade of tau transgenic animal models and beyond

The first tau transgenic mouse model was established more than a decade ago. Since then, much has been learned about the role of tau in Alzheimer's disease and related disorders. Animal models, both in vertebrates and invertebrates, were significantly improved and refined as a result of the identification of pathogenic mutations in Tau in human cases of frontotemporal dementia. They have been instrumental for dissecting the cross-talk between tau and the second hallmark lesion of Alzheimer's disease, the Abeta peptide-containing amyloid plaque. We discuss how the tau models have been used to unravel the pathophysiology of Alzheimer's disease, to search for disease modifiers and to develop novel treatment strategies. While tau has received less attention than Abeta, it is rapidly acquiring a more prominent position and the emerging view is one of a synergistic action of Abeta and tau in Alzheimer's disease. Moreover, the existence of a number of neurodegenerative diseases with tau pathology in the absence of extracellular deposits underscores the relevance of research on tau.

Regulation of c-Src activity in glutamate-induced neurodegeneration

c-Src is heavily expressed in the brain and in human neural tissues. Our pursuit for characterization of the neuroprotective mechanisms of tocotrienols led to the first evidence demonstrating that rapid c-Src activation plays a central role in executing glutamate-induced neurodegeneration. It is now known that Src deficiency or blockade of Src activity in mice provides cerebral protection following stroke. Here, we sought to examine the mechanisms that regulate inducible c-Src activity in glutamate-challenged HT4 neural cells and primary cortical neurons. Knockdown of c-Src protected cells against glutamate-induced loss of viability. Consistently, microinjection of siRNA against c-Src protected cells against glutamate. Using overexpression and knockdown approaches, we noted that SHP-1 may be implicated in glutamate-induced c-Src activation. Following such activation, Cbp and caveolin-1 were phosphorylated and associated with Csk. Csk was translocated to the membrane where it down-regulated glutamate-induced c-Src activity by catalyzing the inhibitory phosphorylation of a tyrosine residue in c-Src. Findings of this study present a new paradigm that addresses the regulation of c-Src under neurodegenerative conditions.

Novel phosphorylation sites in tau from Alzheimer brain support a role for casein kinase 1 in disease pathogenesis

Tau in Alzheimer disease brain is highly phosphorylated and aggregated into paired helical filaments comprising characteristic neurofibrillary tangles. Here we have analyzed insoluble Tau (PHF-tau) extracted from Alzheimer brain by mass spectrometry and identified 11 novel phosphorylation sites, 10 of which were assigned unambiguously to specific amino acid residues. This brings the number of directly identified sites in PHF-tau to 39, with an additional six sites indicated by reactivity with phosphospecific antibodies to Tau. We also identified five new phosphorylation sites in soluble Tau from control adult human brain, bringing the total number of reported sites to nine. To assess which kinases might be responsible for Tau phosphorylation, we used mass spectrometry to determine which sites were phosphorylated in vitro by several kinases. Casein kinase 1delta and glycogen synthase kinase-3beta were each found to phosphorylate numerous sites, and each kinase phosphorylated at least 15 sites that are also phosphorylated in PHF-tau from Alzheimer brain. A combination of casein kinase 1delta and glycogen synthase kinase-3beta activities could account for over three-quarters of the serine/threonine phosphorylation sites identified in PHF-tau, indicating that casein kinase 1delta may have a role, together with glycogen synthase kinase-3beta, in the pathogenesis of Alzheimer disease.

Tau-tubulin kinase 1 (TTBK1), a neuron-specific tau kinase candidate, is involved in tau phosphorylation and aggregation

Neurofibrillary tangles, which are major pathological hallmarks of Alzheimer's disease (AD), are composed of paired helical filaments (PHFs) containing hyperphosphorylated tau. Specific kinases regulate tau phosphorylation and are closely linked to the pathogenesis of AD. We have characterized a human tau-tubulin kinase 1 (TTBK1) gene located on chromosome 6p21.1. TTBK1 is a serine/threonine/tyrosine kinase that is conserved among species and belongs to the casein kinase 1 superfamily. It is specifically expressed in the brain, especially in the cytoplasm of cortical and hippocampal neurons. TTBK1 phosphorylates tau proteins in both a Mg2+- and a Mn2+-dependent manner. Phosphopeptide mapping and immunoblotting analysis confirmed a direct tau phosphorylation by TTBK1 at Ser198, Ser199, Ser202 and Ser422, which are also phosphorylated in PHFs. TTBK1 also induces tau aggregation in human neuronal cells in a dose-dependent manner. We conclude that TTBK1 is a neuron-specific dual kinase involved in tau phosphorylation at AD-related sites and is also associated with tau aggregation.

Alzheimer disease-specific conformation of hyperphosphorylated paired helical filament-Tau is polyubiquitinated through Lys-48, Lys-11, and Lys-6 ubiquitin conjugation

One of the key pathological hallmarks of Alzheimer disease (AD) is the accumulation of paired helical filaments (PHFs) of hyperphosphorylated microtubule-associated protein Tau. Tandem mass spectrometry was employed to examine PHF-Tau post-translational modifications, in particular protein phosphorylation and ubiquitination, to shed light on their role in the early stages of Alzheimer disease. PHF-Tau from Alzheimer disease brain was affinity-purified by MC1 monoclonal antibody to isolate a soluble fraction of PHF-Tau in a conformation unique to human AD brain. A large number of phosphorylation sites were identified by employing a data-dependent neutral loss algorithm to trigger MS3 scans of phosphopeptides. It was found that soluble PHF-Tau is ubiquitinated at its microtubule-binding domain at residues Lys-254, Lys-311, and Lys-353, suggesting that ubiquitination of PHF-Tau may be an earlier pathological event than previously thought and that ubiquitination could play a regulatory role in modulating the integrity of microtubules during the course of AD. Tandem mass spectrometry data for ubiquitin itself indicate that PHF-Tau is modified by three polyubiquitin linkages, at Lys-6, Lys-11, and Lys-48. Relative quantitative analysis indicates that Lys-48-linked polyubiquitination is the primary form of polyubiquitination with a minor portion of ubiquitin linked at Lys-6 and Lys-11. Because modification by Lys-48-linked polyubiquitin chains is known to serve as the essential means of targeting proteins for degradation by the ubiquitin-proteasome system, and it has been reported that modification at Lys-6 inhibits ubiquitin-dependent protein degradation, a failure of the ubiquitin-proteasome system could play a role in initiating the formation of degradation-resistant PHF tangles.