Ubiquitin is associated with early truncation of tau protein at aspartic acid(421) during the maturation of neurofibrillary tangles in Alzheimer's disease

Tau truncation in the pathogenesis of Alzheimer's disease: a narrative review

ABSTRACT

Alzheimer's disease is characterized by two major neuropathological hallmarks-the extracellular β-amyloid plaques and intracellular neurofibrillary tangles consisting of aggregated and hyperphosphorylated Tau protein. Recent studies suggest that dysregulation of the microtubule-associated protein Tau, especially specific proteolysis, could be a driving force for Alzheimer's disease neurodegeneration. Tau physiologically promotes the assembly and stabilization of microtubules, whereas specific truncated fragments are sufficient to induce abnormal hyperphosphorylation and aggregate into toxic oligomers, resulting in them gaining prion-like characteristics. In addition, Tau truncations cause extensive impairments to neural and glial cell functions and animal cognition and behavior in a fragment-dependent manner. This review summarizes over 60 proteolytic cleavage sites and their corresponding truncated fragments, investigates the role of specific truncations in physiological and pathological states of Alzheimer's disease, and summarizes the latest applications of strategies targeting Tau fragments in the diagnosis and treatment of Alzheimer's disease.

Post-Translational Modifications in Tau and Their Roles in Alzheimer's Pathology

Microtubule-Associated Protein Tau (also known as tau) has been shown to accumulate into paired helical filaments and neurofibrillary tangles, which are known hallmarks of Alzheimer's disease (AD) pathology. Decades of research have shown that tau protein undergoes extensive post-translational modifications (PTMs), which can alter the protein's structure, function, and dynamics and impact the various properties such as solubility, aggregation, localization, and homeostasis. There is a vast amount of information describing the impact and role of different PTMs in AD pathology and neuroprotection. However, the complex interplay between these PTMs remains elusive. Therefore, in this review, we aim to comprehend the key post-translational modifications occurring in tau and summarize potential connections to clarify their impact on the physiology and pathophysiology of tau. Further, we describe how different computational modeling methods have helped in understanding the impact of PTMs on the structure and functions of the tau protein. Finally, we highlight the tau PTM-related therapeutics strategies that are explored for the development of AD therapy.

Reversible protein assemblies in the proteostasis network in health and disease

While proteins populating their native conformations constitute the functional entities of cells, protein aggregates are traditionally associated with cellular dysfunction, stress and disease. During recent years, it has become clear that large aggregate-like protein condensates formed via liquid-liquid phase separation age into more solid aggregate-like particles that harbor misfolded proteins and are decorated by protein quality control factors. The constituent proteins of the condensates/aggregates are disentangled by protein disaggregation systems mainly based on Hsp70 and AAA ATPase Hsp100 chaperones prior to their handover to refolding and degradation systems. Here, we discuss the functional roles that condensate formation/aggregation and disaggregation play in protein quality control to maintain proteostasis and why it matters for understanding health and disease.

A Potential Mechanism for Targeting Aggregates With Proteasomes and Disaggregases in Liquid Droplets

Insoluble protein deposits are hallmarks of neurodegenerative disorders and common forms of dementia. The aberrant aggregation of misfolded proteins involves a complex cascade of events that occur over time, from the cellular to the clinical phase of neurodegeneration. Declining neuronal health through increased cell stress and loss of protein homeostasis (proteostasis) functions correlate with the accumulation of aggregates. On the cellular level, increasing evidence supports that misfolded proteins may undergo liquid-liquid phase separation (LLPS), which is emerging as an important process to drive protein aggregation. Studying, the reverse process of aggregate disassembly and degradation has only recently gained momentum, following reports of enzymes with distinct aggregate-disassembly activities. In this review, we will discuss how the ubiquitin-proteasome system and disaggregation machineries such as VCP/p97 and HSP70 system may disassemble and/or degrade protein aggregates. In addition to their canonically associated functions, these enzymes appear to share a common feature: reversibly assembling into liquid droplets in an LLPS-driven manner. We review the role of LLPS in enhancing the disassembly of aggregates through locally increasing the concentration of these enzymes and their co-proteins together within droplet structures. We propose that such activity may be achieved through the concerted actions of disaggregase machineries, the ubiquitin-proteasome system and their co-proteins, all of which are condensed within transient aggregate-associated droplets (TAADs), ultimately resulting in aggregate clearance. We further speculate that sustained engagement of these enzymatic activities within TAADs will be detrimental to normal cellular functions, where these activities are required. The possibility of facilitating endogenous disaggregation and degradation activities within TAADs potentially represents a novel target for therapeutic intervention to restore protein homeostasis at the early stages of neurodegeneration.

Association of α-1-Antichymotrypsin Expression with the Development of Conformational Changes of Tau Protein in Alzheimer's Disease Brain

In Alzheimer's disease (AD), two mutually exclusive amino-terminal-dependent conformations have been reported to occur during the aggregation of Tau protein into neurofibrillary tangles (NFTs). An early conformation of full-length Tau, involving the bending of the amino terminus over the third repeated domain, is recognized by the Alz-50 antibody, followed by a second conformation recognized by Tau-66 antibody that depends on the folding of the proline-rich region over the third repeated domain in a molecule partially truncated at the amino- and carboxyl-termini. α-1-antichymotrypsin (ACT) is an acute phase serum glycoprotein that accumulates abnormally in the brain of AD patients, and since it is considered to promote the in vitro and in vivo aggregation of amyloid-β, we here seek further evidence that ACT may also contribute to the abnormal aggregation of Tau in AD. By analyzing brain samples from a population of AD cases under immunofluorescence and high-resolution confocal microscopy, we demonstrate here the abundant expression of ACT in hippocampal neurons, visualized as a granular diffuse accumulation, frequently reaching the nuclear compartment. In a significant number of these neurons, intracellular NFTs composed of abnormally phosphorylated and truncated Tau at Asp⁴²¹ were also observed to coexist in separated regions of the cytoplasm. However, we found strong colocalization between ACT and diffuse aggregates of Tau-66-positive granules, which was not observed with Alz-50 antibody. These results suggest that ACT may play a role during the development of Tau conformational changes facilitating its aggregation during the formation of the neurofibrillary pathology in AD.

Implications of Valosin-containing Protein in Promoting Autophagy to Prevent Tau Aggregation

Chaperones and cellular degradative mechanisms modulate Tau aggregation. During aging and neurodegenerative disorders, the cellular proteostasis is disturbed due to impaired protective mechanisms. This results in accumulation of aberrant Tau aggregates in the neuron that leads to microtubule destabilization and neuronal degeneration. The intricate mechanisms to prevent Tau aggregation involve chaperones, autophagy, and proteasomal system have gained main focus about concerning to therapeutic intervention. However, the thorough understanding of other key proteins, such as Valosin-containing protein (VCP), is limited. In various neurodegenerative diseases, the chaperone-like activity of VCP is involved in preventing protein aggregation and mediating the degradation of aberrant proteins by proteasome and autophagy. In the case of Tau aggregation associated with Alzheimer's disease, the importance of VCP is poorly understood. VCP is known to co-localize with Tau, and alterations in VCP cause aberrant accumulation of Tau. Nevertheless, the direct mechanism of VCP in altering Tau aggregation is not known. Hence, we speculate that VCP might be one of the key modulators in preventing Tau aggregation and can disintegrate Tau aggregates by directing its clearance by autophagy.

Human Tau Isoforms and Proteolysis for Production of Toxic Tau Fragments in Neurodegeneration

The human tau protein is implicated in a wide range of neurodegenerative “tauopathy” diseases, consisting of Alzheimer’s disease (AD) and frontotemporal lobar degeneration which includes progressive supranuclear palsy, corticobasal degeneration, Pick’s disease, and FTLD-tau (frontotemporal dementia with parkinsonism caused by MAPT mutations). Tau gene transcripts in the human brain undergo alternative splicing to yield 6 different tau protein isoforms that are expressed in different ratios in neurodegeneration which result in tau pathology of paired-helical filaments, neurofibrillary tangles, and tau fibrillar aggregates with detrimental microtubule destabilization. Protease-mediated tau truncation is an important post-translational modification (PTM) which drives neurodegeneration in a tau fragment-dependent manner. While numerous tau fragments have been identified, knowledge of the proteolytic steps that convert each parent tau isoform into specific truncated tau fragments has not yet been fully defined. An improved understanding of the relationships between tau isoforms and their proteolytic processing to generate neurotoxic tau fragments is important to the field. This review evaluates tau isoform expression patterns including PTMs and mutations that influence proteolysis of tau to generate toxic fragments that drive cognitive deficits in AD and other tauopathy models. This assessment identifies the gap in the field on understanding the details of proteolytic steps used to convert each tau isoform into fragments. Knowledge of the processing mechanisms of tau isoforms can lead to new protease targeted drug strategies to prevent the formation of toxic tau fragments in tauopathy neurodegenerative diseases.

To target Tau pathologies, we must embrace and reconstruct their complexities

The accumulation of hyperphosphorylated fibrillar Tau aggregates in the brain is one of the defining hallmarks of Tauopathy diseases, including Alzheimer's disease. However, the primary events or molecules responsible for initiation of the pathological Tau aggregation and spreading remain unknown. The discovery of heparin as an effective inducer of Tau aggregation in vitro was instrumental to enabling different lines of research into the role of Tau aggregation in the pathogenesis of Tauopathies. However, recent proteomics and cryogenic electron microscopy (cryo-EM) studies have revealed that heparin-induced Tau fibrils generated in vitro do not reproduce the biochemical and ultrastructural properties of disease-associated brain-derived Tau fibrils. These observations demand that we reassess our current approaches for investigating the mechanisms underpinning Tau aggregation and pathology formation. Our review article presents an up-to-date survey and analyses of 1) the evolution of our understanding of the interactions between Tau and heparin, 2) the various structural and mechanistic models of the heparin-induced Tau aggregation, 3) the similarities and differences between brain-derived and heparin-induced Tau fibrils; and 4) emerging concepts on the biochemical and structural determinants underpinning Tau pathological heterogeneity in Tauopathies. Our analyses identify specific knowledge gaps and call for 1) embracing the complexities of Tau pathologies; 2) reassessment of current approaches to investigate, model and reproduce pathological Tau aggregation as it occurs in the brain; 3) more research towards a better understanding of the naturally-occurring cofactor molecules that are associated with Tau brain pathology initiation and propagation; and 4) developing improved approaches for in vitro production of the Tau aggregates and fibrils that recapitulate and/or amplify the biochemical and structural complexity and diversity of pathological Tau in Tauopathies. This will result in better and more relevant tools, assays, and mechanistic models, which could significantly improve translational research and the development of drugs and antibodies that have higher chances for success in the clinic.

Tau Post-translational Modifications: Dynamic Transformers of Tau Function, Degradation, and Aggregation

Post-translational modifications (PTMs) on tau have long been recognized as affecting protein function and contributing to neurodegeneration. The explosion of information on potential and observed PTMs on tau provides an opportunity to better understand these modifications in the context of tau homeostasis, which becomes perturbed with aging and disease. Prevailing views regard tau as a protein that undergoes abnormal phosphorylation prior to its accumulation into the toxic aggregates implicated in Alzheimer's disease (AD) and other tauopathies. However, the phosphorylation of tau may, in fact, represent part of the normal but interrupted function and catabolism of the protein. In addition to phosphorylation, tau undergoes another forms of post-translational modification including (but not limited to), acetylation, ubiquitination, glycation, glycosylation, SUMOylation, methylation, oxidation, and nitration. A holistic appreciation of how these PTMs regulate tau during health and are potentially hijacked in disease remains elusive. Recent studies have reinforced the idea that PTMs play a critical role in tau localization, protein-protein interactions, maintenance of levels, and modifying aggregate structure. These studies also provide tantalizing clues into the possibility that neurons actively choose how tau is post-translationally modified, in potentially competitive and combinatorial ways, to achieve broad, cellular programs commensurate with the distinctive environmental conditions found during development, aging, stress, and disease. Here, we review tau PTMs and describe what is currently known about their functional impacts. In addition, we classify these PTMs from the perspectives of protein localization, electrostatics, and stability, which all contribute to normal tau function and homeostasis. Finally, we assess the potential impact of tau PTMs on tau solubility and aggregation. Tau occupies an undoubtedly important position in the biology of neurodegenerative diseases. This review aims to provide an integrated perspective of how post-translational modifications actively, purposefully, and dynamically remodel tau function, clearance, and aggregation. In doing so, we hope to enable a more comprehensive understanding of tau PTMs that will positively impact future studies.

Early but not late conformational changes of tau in association with ubiquitination of neurofibrillary pathology in Alzheimer's disease brains

In Alzheimer's disease, tau protein undergoes post-translational modifications including hyperphosphorylation and truncation, which promotes two major conformational changes associated with progressive N-terminal folding. Along with the development of the disease, tau ubiquitination was previously shown to emerge in the early and intermediate stages of the disease, which is closely associated with early tau truncation at aspartic acid 421, but not with a subsequently truncated tau molecule at glutamic acid 391. In the same group of cases, using multiple immunolabeling and confocal microscopy, a possible relationship between the ubiquitin-targeting of tau and the progression of conformational changes adopted by the N-terminus of this molecule was further studied. A comparable number of neurofibrillary tangles was found displaying ubiquitin, an early conformation recognized by the Alz-50 antibody, and a phosphorylation. However, a more reduced number of neurofibrillary tangles were immunoreactive to Tau-66 antibody, a late tau conformational change marker. When double-labeling profiles of neurofibrillary tangles were assessed, ubiquitination was clearly demonstrated in tau molecules undergoing early N-terminal folding, but was barely observed in late conformational changes of the N-terminus adopted by tau. The same pattern of colocalization was visualized in neuritic pathology. Overall, these results indicate that a more intact conformation of the N-terminus of tau may facilitate tau ubiquitination, but this modification may not occur in a late truncated and more compressed folding of the N-terminus of the tau molecule.

Evaluation of the Common Molecular Basis in Alzheimer's and Parkinson's Diseases

Alzheimer's disease (AD) and Parkinson's disease (PD) are the most common neurodegenerative disorders related to aging. Though several risk factors are shared between these two diseases, the exact relationship between them is still unknown. In this paper, we analyzed how these two diseases relate to each other from the genomic, epigenomic, and transcriptomic viewpoints. Using an extensive literature mining, we first accumulated the list of genes from major genome-wide association (GWAS) studies. Based on these GWAS studies, we observed that only one gene (HLA-DRB5) was shared between AD and PD. A subsequent literature search identified a few other genes involved in these two diseases, among which SIRT1 seemed to be the most prominent one. While we listed all the miRNAs that have been previously reported for AD and PD separately, we found only 15 different miRNAs that were reported in both diseases. In order to get better insights, we predicted the gene co-expression network for both AD and PD using network analysis algorithms applied to two GEO datasets. The network analysis revealed six clusters of genes related to AD and four clusters of genes related to PD; however, there was very low functional similarity between these clusters, pointing to insignificant similarity between AD and PD even at the level of affected biological processes. Finally, we postulated the putative epigenetic regulator modules that are common to AD and PD.

Microglial and Neuronal TDP-43 Pathology in Anti-IgLON5-Related Tauopathy

A novel neuronal tauopathy, mainly confined to hypothalamus and brainstem tegmentum, has recently been reported in patients with autoantibodies to the neuronal cell-adhesion molecule IgLON5. We describe a patient with anti-IgLON5 syndrome, who presented with dysautonomia and sleep disorder, followed by subacute dementia. Postmortem brain examination disclosed neuronal tau pathology prevailing in the hippocampus, amygdala, and locus coeruleus, in addition to microglial/neuronal TDP-43 pathology, with overexpression of aberrantly phosphorylated forms and neurotoxic truncated fragments, in basal ganglia, nucleus basalis, thalamus, and midbrain. These findings suggest that neurodegeneration in anti-IgLON5 syndrome might also occur via a microglia-triggered non-cell autonomous pathway.

Lysine-Directed Post-translational Modifications of Tau Protein in Alzheimer's Disease and Related Tauopathies

Tau is a microtubule-associated protein responsible mainly for stabilizing the neuronal microtubule network in the brain. Under normal conditions, tau is highly soluble and adopts an "unfolded" conformation. However, it undergoes conformational changes resulting in a less soluble form with weakened microtubule stabilizing properties. Altered tau forms characteristic pathogenic inclusions in Alzheimer's disease and related tauopathies. Although, tau hyperphosphorylation is widely considered to be the major trigger of tau malfunction, tau undergoes several post-translational modifications at lysine residues including acetylation, methylation, ubiquitylation, SUMOylation, and glycation. We are only beginning to define the site-specific impact of each type of lysine modification on tau biology as well as the possible interplay between them, but, like phosphorylation, these modifications are likely to play critical roles in tau's normal and pathobiology. This review summarizes the latest findings focusing on lysine post-translational modifications that occur at both endogenous tau protein and pathological tau forms in AD and other tauopathies. In addition, it highlights the significance of a site-dependent approach of studying tau post-translational modifications under normal and pathological conditions.

Locus coeruleus at asymptomatic early and middle Braak stages of neurofibrillary tangle pathology

AIMS

The present study analyses molecular characteristics of the locus coeruleus (LC) and projections to the amygdala and hippocampus at asymptomatic early and middle Braak stages of neurofibrillary tangle (NFT) pathology.

METHODS

Immunohistochemistry, whole-transcriptome arrays and RT-qPCR in LC and western blotting in hippocampus and amygdala in a cohort of asymptomatic individuals at stages I-IV of NFT pathology were used.

RESULTS

NFTs in the LC increased in parallel with colocalized expression of tau kinases, increased neuroketal adducts and decreased superoxide dismutase 1 in neurons with hyperphosphorylated tau and decreased voltage-dependent anion channel in neurons containing truncated tau were found. These were accompanied by increased microglia and AIF1, CD68, PTGS2, IL1β, IL6 and TNF-α gene expression. Whole-transcriptome arrays revealed upregulation of genes coding for proteins associated with heat shock protein binding and genes associated with ATP metabolism and downregulation of genes coding for DNA-binding proteins and members of the small nucleolar RNAs family, at stage IV when compared with stage I. Tyrosine hydroxylase (TH) immunoreactivity was preserved in neurons of the LC, but decreased TH and increased α_2A adrenergic receptor protein levels were found in the hippocampus and the amygdala.

CONCLUSIONS

Complex alteration of several metabolic pathways occurs in the LC accompanying NFT formation at early and middle asymptomatic stages of NFT pathology. Dopaminergic/noradrenergic denervation and increased expression of α_2A adrenergic receptor in the hippocampus and amygdala occur at first stage of NFT pathology, suggesting compensatory activation in the face of decreased adrenergic input occurring before clinical evidence of cognitive impairment and depression.

Abnormal tau induces cognitive impairment through two different mechanisms: synaptic dysfunction and neuronal loss

The hyperphosphorylated microtubule-associated protein tau is present in several neurodegenerative diseases, although the causal relationship remains elusive. Few mouse models used to study Alzheimer-like dementia target tau phosphorylation. We created an inducible pseudophosphorylated tau (Pathological Human Tau, PH-Tau) mouse model to study the effect of conformationally modified tau in vivo. Leaky expression resulted in two levels of PH-Tau: low basal level and higher upon induction (4% and 14% of the endogenous tau, respectively). Unexpectedly, low PH-Tau resulted in significant cognitive deficits, decrease in the number of synapses (seen by EM in the CA1 region), reduction of synaptic proteins, and localization to the nucleus. Induction of PH-Tau triggered neuronal death (60% in CA3), astrocytosis, and loss of the processes in CA1. These findings suggest, that phosphorylated tau is sufficient to induce neurodegeneration and that two different mechanisms can induce cognitive impairment depending on the levels of PH-Tau expression.

Specific calpain inhibition by calpastatin prevents tauopathy and neurodegeneration and restores normal lifespan in tau P301L mice

Tau pathogenicity in Alzheimer's disease and other tauopathies is thought to involve the generation of hyperphosphorylated, truncated, and oligomeric tau species with enhanced neurotoxicity, although the generative mechanisms and the implications for disease therapy are not well understood. Here, we report a striking rescue from mutant tau toxicity in the JNPL3 mouse model of tauopathy. We show that pathological activation of calpains gives rise to a range of potentially toxic forms of tau, directly, and by activating cdk5. Calpain overactivation in brains of these mice is accelerated as a result of the marked depletion of the endogenous calpain inhibitor, calpastatin. When levels of this inhibitor are restored in neurons of JNPL3 mice by overexpressing calpastatin, tauopathy is prevented, including calpain-mediated breakdown of cytoskeletal proteins, cdk5 activation, tau hyperphosphorylation, formation of potentially neurotoxic tau fragments by either calpain or caspase-3, and tau oligomerization. Calpastatin overexpression also prevents loss of motor axons, delays disease onset, and extends survival of JNPL3 mice by 3 months to within the range of normal lifespan. Our findings support the therapeutic promise of highly specific calpain inhibition in the treatment of tauopathies and other neurodegenerative states.

Glial and neuronal tau pathology in tauopathies: characterization of disease-specific phenotypes and tau pathology progression

Tauopathies are degenerative diseases characterized by the accumulation of phosphorylated tau in neurons and glial cells. With some exceptions, tau deposits in neurons are mainly manifested as pretangles and tangles unrelated to the tauopathy. It is thought that abnormal tau deposition in neurons occurs following specific steps, but little is known about the progression of tau pathology in glial cells in tauopathies. We compared tau pathology in different astrocyte phenotypes and oligodendroglial inclusions with that in neurons in a large series of tauopathies, including progressive supranuclear palsy, corticobasal degeneration, argyrophilic grain disease, Pick disease, frontotemporal lobar degenerations (FTLD) associated with mutations in the tau gene, globular glial tauopathy (GGT), and tauopathy in the elderly. Our findings indicate that disease-specific astroglial phenotypes depend on i) the primary amino acid sequence of tau (mutated tau, 3Rtau, and 4Rtau); ii) phospho-specific sites of tau phosphorylation, tau conformation, tau truncation, and ubiquitination in that order (which parallel tau modifications related to pretangle and tangle stages in neurons); and iii) modifications of the astroglial cytoskeleton. In contrast to astrocytes, coiled bodies in oligodendrocytes have similar characteristics whatever the tauopathy, except glial globular inclusions in GGT, and coiled bodies and globular oligodendroglial inclusions in FTLD-tau/K317M. These observations indicate that tau pathology in glial cells largely parallels, but is not identical to, that in neurons in many tauopathies.

Global changes in DNA methylation and hydroxymethylation in Alzheimer's disease human brain

DNA methylation (5-methylcytosine [5mC]) is one of several epigenetic markers altered in Alzheimer's disease (AD) brain. More recently, attention has been given to DNA hydroxymethylation (5-hydroxymethylcytosine [5hmC]), the oxidized form of 5mC. Whereas 5mC is generally associated with the inhibition of gene expression, 5hmC has been associated with increased gene expression and is involved in cellular processes such as differentiation, development, and aging. Recent findings point toward a role for 5hmC in the development of diseases including AD, potentially opening new pathways for treating AD through correcting methylation and hydroxymethylation alterations. In the present study, levels of 5mC and 5hmC were investigated in the human middle frontal gyrus (MFG) and middle temporal gyrus (MTG) by immunohistochemistry. Immunoreactivity for 5mC and 5hmC were significantly increased in AD MFG (N = 13) and MTG (N = 29) compared with age-matched controls (MFG, N = 13 and MTG, N = 29). Global levels of 5mC and 5hmC positively correlated with each other and with markers of AD including amyloid beta, tau, and ubiquitin loads. Our results showed a global hypermethylation in the AD brain and revealed that levels of 5hmC were also significantly increased in AD MFG and MTG with no apparent influence of gender, age, postmortem delay, or tissue storage time. Using double-fluorescent immunolabeling, we found that in control and AD brains, levels of 5mC and 5hmC were low in astrocytes and microglia but were elevated in neurons. In addition, our colocalization study showed that within the same nuclei, 5mC and 5hmC mostly do not coexist. The present study clearly demonstrates the involvement of 5mC and 5hmC in AD emphasizing the need for future studies determining the exact time frame of these epigenetic changes during the progression of AD pathology.

Characterization of thorn-shaped astrocytes in white matter of temporal lobe in Alzheimer's disease brains

Thorn-shaped astrocytes (TsA) are mainly localized in the periventricular white matter of the temporal lobe in a subgroup of aged individuals usually in the context of Alzheimer's disease (AD). Immunohistochemistry of TsA shows 4Rtau deposition, tau phosphorylation at different sites recognized with phosphospecific anti-tau antibodies Thr181, Ser202, Ser214, Thr231, Ser396, Ser422, and clones AT8 and PHF-1, and conformational changes revealed with Alz50 and MC-1 antibodies; TsA are also immunostained with antibodies to active tau kinases MAPK/ERK-P, SAPK/JNK-P, p38-P and GSK-3β. These findings are common to neurofibrillary tangles in AD. However, TsA are not stained with 3Rtau antibodies, and they are seldom stained or not at all with phosphospecific tauSer262 and with Tau-C3 antibody, which recognizes the latter tau truncation at aspartic acid 421. Previous studies have shown that tau phosphorylation at Ser262 reduces tau binding to microtubules and increases caspase-3 activity, whereas tau truncation at aspartic acid 421 is associated with tau ubiquitination, and toxic effects of tau. In this line, ubiquitin is not accumulated in TsA, and in situ end-labeling of nuclear DNA fragmentation shows absence of degeneration in TsA. These observations support the concept that tau lesions in neurons differ from those seen in TsA in AD.

Structure and pathology of tau protein in Alzheimer disease

Alzheimer's disease (AD) is the most common type of dementia. In connection with the global trend of prolonging human life and the increasing number of elderly in the population, the AD becomes one of the most serious health and socioeconomic problems of the present. Tau protein promotes assembly and stabilizes microtubules, which contributes to the proper function of neuron. Alterations in the amount or the structure of tau protein can affect its role as a stabilizer of microtubules as well as some of the processes in which it is implicated. The molecular mechanisms governing tau aggregation are mainly represented by several posttranslational modifications that alter its structure and conformational state. Hence, abnormal phosphorylation and truncation of tau protein have gained attention as key mechanisms that become tau protein in a pathological entity. Evidences about the clinicopathological significance of phosphorylated and truncated tau have been documented during the progression of AD as well as their capacity to exert cytotoxicity when expressed in cell and animal models. This paper describes the normal structure and function of tau protein and its major alterations during its pathological aggregation in AD.