Tau Truncation Research Articles

Homocysteine Increases Tau Phosphorylation, Truncation and Oligomerization

Increased plasma homocysteinemia is considered a risk factor of dementia, including Alzheimer’s disease (AD) and vascular dementia. However, the reason elevated plasma homocysteinemia increases the risk of dementia remains unknown. A pathological hallmark of AD is neurofibrillary tangles (NFTs) that consist of pathologically phosphorylated tau proteins. The effect of homocysteine (Hcy) on tau aggregation was explored using human neuroblastoma M1C cells that constitutively express human wild-type tau (4R0N) under the control of a tetracycline off system, primary mouse cultured neurons, and by inducing hyperhomocysteinemia in a mouse model of tauopathy (HHCy mice). A wide range of Hcy concentrations (10–1000 µM) increased total tau and phosphorylated tau protein levels. Hcy activated glycogen synthase kinase 3, and cyclin dependent kinase 5, major tau phosphokinases, and inactivated protein phosphatase 2A, a main tau phosphatase. Hcy exhibited cytotoxic effects associated with enhanced activation of caspase. Truncation of tau in the C-terminus, the cleavage site of caspase 3 (i.e., D421, detected by the TauC3 antibody) was also increased. Total tau, phosphorylated tau, as well as C-terminal cleaved tau were increased in the sarkosyl insoluble tau fraction. Hcy also increased the level of tau oligomers, as indicated by the tau oligomer complex 1 (TOC1) antibody that specifically identifies oligomeric tau species, in the tris insoluble, sarkosyl soluble fraction. The levels of TOC1-positive oligomeric tau were increased in brain lysates from HHCy mice, and treating HHCy mice with S-adenosylmethionine, an intermediate of Hcy, reduced the levels of oligomeric tau to control levels. These observations suggest that Hcy increases the levels of phosphorylated tau as well as truncated tau species via caspase 3 activation, and enhanced tau oligomerization and aggregation.

Relevance of Phosphorylation and Truncation of Tau to the Etiopathogenesis of Alzheimer's Disease.

Microtubule (MT) associated protein tau is abnormally hyperphosphorylated and aggregated into paired helical filaments (PHFs), which manifest as neurofibrillary tangles (NFTs) in the brains of individuals with Alzheimer’s disease (AD) and related tauopathies. Hyperphosphorylation and truncation of tau have been linked to the progression of the disease. However, the nature of phosphorylation and truncation of tau in AD brain are not very clear. In the present study we investigated the association of phosphorylation and truncation with high-molecular weight oligomers of tau (HMW-tau) in post-mortem AD brain by western blots. We found that tau from AD brain appears as a smear from low molecular weight (LMW) to HMW tau species in western blots developed with pan-tau antibodies. Similar level of LMW-tau was found in AD and control brains, whereas HMW-tau was found in AD brain only. HMW-tau was hyperphosphorylated at multiple sites and not unphosphorylated at Ser46 or Ser198/199/202. HMW-tau was weakly labeled by tau antibodies 43D against a.a. 6–18 and HT7 against a.a. 159–163 of tau, whereas, the C-terminal antibodies, tau46 and tau46.1, strongly labeled HMW-tau. The ratio of HMW-tau/LMW-tau detected by tau antibodies increased as the epitope of the tau antibodies ranges from N-terminal to C-terminal. The level of tau truncated at Asp421 was increased in AD brain, but was poorly associated with the HMW-tau. These findings suggest that tau pathogenesis involves both hyperphosphorylation and dominantly N-terminal truncation of tau in AD.

A Simple Method to Avoid Nonspecific Signal When Using Monoclonal Anti-Tau Antibodies in Western Blotting of Mouse Brain Proteins.

In Alzheimer's disease and other tauopathies, tau displays several abnormal post-translation modifications such as hyperphosphorylation, truncation, conformation, and oligomerization. Mouse monoclonal antibodies have been raised against such tau modifications for research, diagnostic, and therapeutic purposes. However, many of these primary antibodies are at risk of giving nonspecific signals in common Western blotting procedures. Not because they are unspecific, but because the secondary antibodies used to detect them will also detect the heavy chain of endogenous mouse immunoglobulins (Igs), and give a nonspecific signal at the same molecular weight than tau protein (around 50 kDa). Here, we propose the use of anti-light chain secondary antibodies as a simple and efficient technique to prevent nonspecific Igs signals at around 50 kDa. We demonstrate the efficacy of this method by removing artifactual signals when using monoclonal antibodies directed at tau phosphorylation (AT100, 12E8, AT270), tau truncation (TauC3), tau oligomerization (TOMA), or tau abnormal conformation (Alz50), in wild-type, 3×Tg-AD, and tau knockout mice.

Ser422 phosphorylation blocks human Tau cleavage by caspase-3: Biochemical implications to Alzheimer’s Disease

Proteolytic truncation of microtubule associated human (h) Tau protein by caspase-3 at the carboxy (C) terminus has been linked to the pathogenesis of Alzheimer’s Disease (AD). This cleavage likely occurs between Asp421↓Ser422 leading to the formation of 421-mer truncated Tau protein which has been found to be present as aggregate in high level after phosphorylation in mortal AD brain tissue compared to normal. At least 50 phosphorylation sites involving Ser, Thr and Tyr residues have been identified or proposed in hTau and a selected number of them have been implicated in hTau aggregation following latter’s proteolytic truncation. Interestingly, it is further noted that Ser422 residue present in the P1′ position of hTau caspase-3 cleavage region is a potential phosphorylation site. So we became interested to examine in vitro the effect of phospho-Ser422 residue on hTau cleavage by caspase-3 which is a crucial upstream event associated with hTau self-assembly leading to AD pathogenesis. The goal of this project is to study in vitro the caspase-3 cleavage site of hTau protein and to examine the kinetics of this cleavage following Ser422 phosphorylation and treatment with caspase-3 inhibitors. This is achieved by designing peptides from the sequence of hTau protein containing the proposed caspase-3 cleavage region. Peptides were designed from 441-mer major human Tau protein sequence that encompasses the proposed caspase-3 cleavage site [Asp421↓Ser422]. Corresponding phospho-, dextro-Ser422 and dextro-Asp421 analogs were also designed. Peptides were synthesized by solid phase chemistry, purified and fully characterized by mass spectrometry. These were then incubated with recombinant caspase-3 enzyme under identical condition for digestion and analyzed for cleavage by mass spectrometry and RP-HPLC chromatograms. Our results indicated that while the control peptide is efficiently cleaved by caspase-3 at Asp421↓Ser422 site producing the expected N- and C-terminal fragment peptides, the corresponding phospho-Ser422 peptide remained completely resistant to the cleavage. Substitution of Asp421 by its dextro isoform also blocks peptide cleavage by caspase-3. However substitution of Ser422 by its dextro isoform in the peptide did not affect the cleavage significantly. The above results were further confirmed by caspase-3 digestion experiment in the presence of varying amounts of caspase-3 inhibitor (Ac-DQVD-aldehyde) which was found to block this cleavage in a highly effective manner. Our results highlighted the crucial significance of Ser422 phosphorylation and suggest that the kinase associated with this Ser-phosphorylation may protect Tau from aggregation. Thus specific promoters/activators of this kinase may find useful therapeutic benefits in arresting Tau truncation by caspase-3 and the progression of AD. In addition our data demonstrated that Tau-peptides where Ser422 or Asp421 are substituted by their respective dextro isomers, exhibit different cleavage kinetics by caspase-3 and this may have important implications in therapeutic intervention of Tau aggregation and associated AD.

Subacute Changes in Cleavage Processing of Amyloid Precursor Protein and Tau following Penetrating Traumatic Brain Injury.

Traumatic brain injury (TBI) is an established risk factor for the development of Alzheimer’s disease (AD). Here the effects of severe penetrating TBI on APP and tau cleavage processing were investigated in a rodent model of penetrating ballistic-like brain injury (PBBI). PBBI was induced by stereotactically inserting a perforated steel probe through the right frontal cortex of the anesthetized rat and rapidly inflating/deflating the probe’s elastic tubing into an elliptical shaped balloon to 10% of total rat brain volume causing temporary cavitation injury. Separate animals underwent probe injury (PrI) alone without balloon inflation. Shams underwent craniectomy. Brain tissue was collected acutely (4h, 24h, 3d) and subacutely (7d) post-injury and analyzed by immunoblot for full length APP (APP-FL) and APP beta c-terminal fragments (βCTFs), full length tau (tau-FL) and tau truncation fragments and at 7d for cytotoxic Beta amyloid (Aβ) peptides Aβ40 and Aβ42 analysis. APP-FL was significantly decreased at 3d and 7d following PBBI whereas APP βCTFs were significantly elevated by 4h post-injury and remained elevated through 7d post-injury. Effects on βCTFs were mirrored with PrI, albeit to a lesser extent. Aβ40 and Aβ42 were significantly elevated at 7d following PBBI and PrI. Tau-FL decreased substantially 3d and 7d post-PBBI and PrI. Importantly, a 22 kDa tau fragment (tau22), similar to that found in AD, was significantly elevated by 4h and remained elevated through 7d post-injury. Thus both APP and tau cleavage was dramatically altered in the acute and subacute periods post-injury. As cleavage of these proteins has also been implicated in AD, TBI pathology shown here may set the stage for the later development of AD or other tauopathies.

PHF-tau propagation in the presence of Amyloid ß pathology

PHF-tau propagation in the presence of Amyloid ß pathology

Neuroinflammatory Gene Regulation, Mitochondrial Function, Oxidative Stress, and Brain Lipid Modifications With Disease Progression in Tau P301S Transgenic Mice as a Model of Frontotemporal Lobar Degeneration-Tau.

Tau P301S transgenic mice (PS19 line) are used as a model of frontotemporal lobar degeneration (FTLD)-tau. Behavioral alterations in these mice begin at approximately 4 months of age. We analyzed molecular changes related to disease progression in these mice. Hyperphosphorylated 4Rtau increased in neurons from 1 month of age in entorhinal and piriform cortices to the neocortex and other regions. A small percentage of neurons developed an abnormal tau conformation, tau truncation, and ubiquitination only at 9/10 months of age. Astrocytosis, microgliosis, and increased inflammatory cytokine and immune mediator expression also occurred at this late stage; hippocampi were the most markedly affected. Altered mitochondrial function, increased reactive oxygen species production, and limited protein oxidative damage were observed in advanced disease. Tau oligomers were only present in P301S mice, they were found in somatosensory cortex and hippocampi at the age of 3 months, and they increased across time in the somatosensory cortex and were higher and sustained in hippocampi. Age-related modifications in lipid composition occurred in both P301S and wild-type mice with regional and phenotypic differences; however, changes of total lipids did not seem to have pathogenic implications. Apoptosis only occurred in restricted regions in late disease. The complex tau pathology, mitochondrial alterations, oxidative stress damage, glial reactions, neuroinflammation, and cell death in P301S mice likely parallel those in FTLD-tau. Thus, therapies should focus first on abnormal tau rather than secondary events that appear late in the course of FTLD-tau.

CXCL1 Triggers Caspase-3 Dependent Tau Cleavage in Long-Term Neuronal Cultures and in the Hippocampus of Aged Mice: Implications in Alzheimer's Disease.

Truncation of tau protein is considered an early event in Alzheimer's disease (AD) and is believed to play a major pathogenic role in sporadic AD. However, causative factors that trigger tau truncation in AD remain poorly understood. In the present study, we demonstrate that CXCL1 (C-X-C motif ligand 1), a specific ligand for the chemokine receptor CXCR2, induced cleavage of tau at ASP421 in a caspase-3-dependent manner in long-term but not short-term cultured neurons. The cleaved tau formed varicosities or bead-like structures along the neurites, an abnormal distribution of tau induced by CXCL1 that has not been observed previously. CXCL1-induced activation of GSK3β and the subsequent phosphorylation of tau preceded and were required for caspase-3 activation and tau cleavage. Moreover, intrahippocampal microinjection of lentiviral CXCL1 induced tau cleavage in hippocampal neurons in aged (15-18 months of age) but not adult mice (5-10 months of age). Our data highlight a new role of CXCR2 in tau cleavage and suggest that targeting CXCR2 may offer therapeutic benefits to patients with AD and potentially other tauopathies.

NH2-truncated human tau induces deregulated mitophagy in neurons by aberrant recruitment of Parkin and UCHL-1: implications in Alzheimer's disease.

Disarrangement in functions and quality control of mitochondria at synapses are early events in Alzheimer's disease (AD) pathobiology. We reported that a 20-22 kDa NH2-tau fragment mapping between 26 and 230 amino acids of the longest human tau isoform (aka NH2htau): (i) is detectable in cellular and animal AD models, as well in synaptic mitochondria and cerebrospinal fluids (CSF) from human AD subjects; (ii) is neurotoxic in primary hippocampal neurons; (iii) compromises the mitochondrial biology both directly, by inhibiting the ANT-1-dependent ADP/ATP exchange, and indirectly, by impairing their selective autophagic clearance (mitophagy). Here, we show that the extensive Parkin-dependent turnover of mitochondria occurring in NH2htau-expressing post-mitotic neurons plays a pro-death role and that UCHL-1, the cytosolic Ubiquitin-C-terminal hydrolase L1 which directs the physiological remodeling of synapses by controlling ubiquitin homeostasis, critically contributes to mitochondrial and synaptic failure in this in vitro AD model. Pharmacological or genetic suppression of improper mitophagy, either by inhibition of mitochondrial targeting to autophagosomes or by shRNA-mediated silencing of Parkin or UCHL-1 gene expression, restores synaptic and mitochondrial content providing partial but significant protection against the NH2htau-induced neuronal death. Moreover, in mitochondria from human AD synapses, the endogenous NH2htau is stably associated with Parkin and with UCHL-1. Taken together, our studies show a causative link between the excessive mitochondrial turnover and the NH2htau-induced in vitro neuronal death, suggesting that pathogenetic tau truncation may contribute to synaptic deterioration in AD by aberrant recruitment of Parkin and UCHL-1 to mitochondria making them more prone to detrimental autophagic clearance.

The relationship between truncation and phosphorylation at the C-terminus of tau protein in the paired helical filaments of Alzheimer's disease.

We previously demonstrated that, in the early stages of tau processing in Alzheimer's disease, the N-terminal part of the molecule undergoes a characteristic cascade of phosphorylation and progressive misfolding of the proteins resulting in a structural conformation detected by Alz-50. In this immunohistochemical study of AD brain tissue, we have found that C-terminal truncation of tau at Asp-421 was an early event in tau aggregation and analyzed the relationship between phospho-dependent tau epitopes located at the C-terminus with truncation at Glu-391. The aim of this study was to determine whether C-terminal truncation may trigger events leading to the assembly of insoluble PHFs from soluble tau aggregates present in pre-tangle cells. Our findings suggest that there is a complex interaction between phosphorylated and truncated tau species. A model is presented here in which truncated tau protein represents an early neurotoxic species while phosphorylated tau species may provide a neuroprotective role in Alzheimer's disease.

N-terminal truncation of microtubule associated protein tau dysregulates its cellular localization.

Tau protein is a member of microtubule-associated protein family. Under pathological conditions, tau undergoes multiple modifications that lead to the formation of insoluble deposits in neurons, resulting in neuronal dysfunction in several neurodegenerative disorders collectively called tauopathies, with Alzheimer's disease being the most frequent example. This typical cytosolic protein has been shown to translocate into the nucleus and participate in DNA protection upon stress conditions. In our study, we demonstrate that truncated Tau151-391/4R changes its usual behavior and gains constitutive access into the nucleus of both primary rat neurons and human neuroblastoma cells. Our results show that partial/dysregulated nuclear localization of tau results from the removal of the N-terminal (1-150) residues of the protein. Data obtained by cell fractionation data were supported by confocal microscopy analysis of GFP-fused tau proteins. Furthermore, neither addition of the fusion protein, nor increased tau phosphorylation had any effect on the intracellular distribution of truncated tau. Our data further suggest that differential tau phospho-status between cytosolic and nuclear fractions is rather a consequence than a cause of truncated tau nuclear localization. Finally, truncated tau in the nucleus is engaged in interactions with subnuclear structure(s), since it exhibits reduced mobility. We conclude that N-terminal truncation of tau proteins leads to their nonphysiological subcellular distribution as a result of modified tau conformation.

Overexpression of heme oxygenase 1 causes cognitive decline and affects pathways for tauopathy in mice.

The stress protein heme oxygenase-1 (HO-1) is upregulated and co-localizes to pathological features, including tauopathies in the brains of individuals with Alzheimer's disease. However, the relationship between HO-1 and Alzheimer's disease remains unclear. In our previous research, the long-term overexpression of HO-1 was shown to promote tau aggregation by inducing tau phosphorylation in the mouse brain. In this study, we found that the long-term overexpression of HO-1 led to cognitive decline in transgenic mice, as determined by the water maze test, and that HO-1 can affect two pathways for tauopathy. Through one pathway, HO-1 promotes the expression of CDK5 by accumulating reactive oxygen species, which are produced by HO-1 downstream products of iron in neuro2a cell lines and mouse brain. Through the second pathway, HO-1 induces tau truncation at D421 in vivo and in vitro. Clearly, there is a HO-1-dependent mechanism responsible for tau protein phosphorylation and tau truncation in vivo and in vitro. Taken together, our results suggest that HO-1 plays an important role in the disease process of tauopathies in AD.

Asparagine endopeptidase cleaves tau and promotes neurodegeneration.

Truncation of tau contributes to the formation of neurofibrillary tangles in Alzheimer's disease. A new study finds a direct role for a lysosomal cysteine protease, asparagine endopeptidase, in cleaving tau into neurotoxic fragments.

BRI2 ectodomain affects Aβ42 fibrillation and tau truncation in human neuroblastoma cells.

Alzheimer's disease (AD) is pathologically characterized by the presence of misfolded proteins such as amyloid beta (Aβ) in senile plaques, and hyperphosphorylated tau and truncated tau in neurofibrillary tangles (NFT). The BRI2 protein inhibits Aβ aggregation via its BRICHOS domain and regulates critical proteins involved in initiating the amyloid cascade, which has been hypothesized to be central in AD pathogenesis. We recently detected the deposition of BRI2 ectodomain associated with Aβ plaques and concomitant changes in its processing enzymes in early stages of AD. Here, we aimed to investigate the effects of recombinant BRI2 ectodomain (rBRI276-266) on Aβ aggregation and on important molecular pathways involved in early stages of AD, including the unfolded protein response (UPR), phosphorylation and truncation of tau, as well as apoptosis. We found that rBRI276-266 delays Aβ fibril formation, although less efficiently than the BRI2 BRICHOS domain (BRI2 residues 113-231). In human neuroblastoma SH-SY5Y cells, rBRI276-266 slightly decreased cell viability and increased up to two-fold the Bax/Bcl-2 ratio and the subsequent activity of caspases 3 and 9, indicating activation of apoptosis. rBRI276-266 upregulated the chaperone BiP but did not modify the mRNA expression of other UPR markers (CHOP and Xbp-1). Strikingly, rBRI276-266 induced the activation of GSK3β but not the phosphorylation of tau. However, exposure to rBRI276-266 significantly induced the truncation of tau, indicating that BRI2 ectodomain can contribute to NFT formation. Since BRI2 can also regulate the metabolism of Aβ, the current data suggests that BRI2 ectodomain is a potential nexus between Aβ, tau pathology and neurodegeneration.

Cleavage of tau by asparagine endopeptidase mediates the neurofibrillary pathology in Alzheimer's disease.

Neurofibrillary tangles (NFTs), composed of truncated and hyperphosphorylated tau, are a common feature of numerous aging-related neurodegenerative diseases including Alzheimer’s disease (AD). However, the molecular mechanisms mediating tau truncation and aggregation during aging remain elusive. Here we show that asparagine endopeptidase (AEP), a lysosomal cysteine proteinase, is activated during aging and proteolytically degrades tau, abolishes its microtubule assembly function, induces tau aggregation, and triggers neurodegeneration. AEP is upregulated and active during aging, and is activated in tau P301S transgenic mice and human AD brain, leading to tau truncation in NFTs. Deletion of AEP from tau P301S transgenic mice substantially reduces tau hyperphosphorylation, alleviates the synapse loss and rescues impaired hippocampal synaptic function and the cognitive deficits. Infection of uncleavable tau N255AN368A mutant rescues tau P301S-induced pathological and behavioral defects. Together, these observations indicate that AEP acts as a crucial mediator of tau-related clinical and neuropathological changes in neurodegenerative diseases. Inhibition of AEP may be therapeutically useful for treating tau-mediated neurodegenerative diseases.

Glial and Neuronal Tau Pathology in Tauopathies

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.

A synergic role of caspase-6 and caspase-3 in Tau truncation at D421 induced by H2O 2.

Tau truncation is widely detected in Alzheimer's disease brain. Caspases activation is suggested to play a significant role in tau truncation at Aspartate 421 (D421) according to their ability to cleave recombinant tau in vitro. Ample evidence has shown that caspase-6 is involved in cognitive impairment and expressed in AD brain. Reactive oxygen species (ROS) can lead to caspase-6 activation and correlate with AD. Here, we transfected human embryonic kidney 293 (HEK 293) cells with Tau 441 plasmid and investigated the role of caspase-6 and caspase-3 in ROS-mediated tau truncation. Our data demonstrated that H2O2 induced oxidative stress and increased tau truncation. Caspase-6 and caspase-3 activity also increased in a dose-dependent manner in HEK 293/Tau cells during H2O2 insult. When cells were treated with an ROS inhibitor N-acetyl-L-cysteine, tau truncation was significantly suppressed. Compared with H2O2 (100 μM)/non-inhibitor group or single-inhibitor groups (z-VEID-fmk, caspase-6 inhibitor or z-DEVD-fmk, and caspase-3 inhibitor), tau truncation induced by H2O2 was effectively reduced in the combinative inhibitors group. Similar results were shown when cells were transfected with specific caspase-3 and caspase-6 siRNA. Inhibition of caspase-6 led to decline of caspase-3 activation. Taken together, our results suggest that the combination of caspase-6 and caspase-3 aggravates tau truncation at D421 induced by H2O2. Caspase-6 may play an important part in activating caspase-3. Further investigation of how the synergic role of caspase-6 and caspase-3 affects tau truncation may provide new visions for potential AD therapies.

Proteolytic Cleavage of Polymeric Tau Protein by Caspase-3: Implications for Alzheimer Disease

Truncated tau protein at Asp(421) is associated with neurofibrillary pathology in Alzheimer disease (AD); however, little is known about its presence in the form of nonfibrillary aggregates. Here, we report immunohistochemical staining of the Tau-C3 antibody, which recognizes Asp(421)-truncated tau, in a group of AD cases with different extents of cognitive impairment. In the hippocampus, we found distinct nonfibrillary aggregates of Asp(421)-truncated tau. Unlike Asp(421)-composed neurofibrillary tangles, however, these nonfibrillary pathologies did not increase significantly with respect to the Braak staging and, therefore, make no significant contribution to cognitive impairment. On the other hand, despite in vitro evidence that caspase-3 cleaves monomeric tau at Asp(421), to date, this truncation has not been demonstrated to be executed by this protease in polymeric tau entities. We determined that Asp(421) truncation can be produced by caspase-3 in oligomeric and multimeric complexes of recombinant full-length tau in isolated native tau filaments in vitro and in situ in neurofibrillary tangles analyzed in fresh brain slices from AD cases. Our data suggest that generation of this pathologic Asp(421) truncation of tau in long-lasting fibrillary structures may produce further permanent toxicity for neurons in the brains of patients with AD.

Structural Determinants of Tau Aggregation Inhibitor Potency

Small-molecule Tau aggregation inhibitors are under investigation as potential therapeutic agents against Alzheimer disease. Many such inhibitors have been identified in vitro, but their potency-driving features, and their molecular targets in the Tau aggregation pathway, have resisted identification. Previously we proposed ligand polarizability, a measure of electron delocalization, as a candidate descriptor of inhibitor potency. Here we tested this hypothesis by correlating the ground state polarizabilities of cyanine, phenothiazine, and arylmethine derivatives calculated using ab initio quantum methods with inhibitory potency values determined in the presence of octadecyl sulfate inducer under reducing conditions. A series of rhodanine analogs was analyzed as well using potency values disclosed in the literature. Results showed that polarizability and inhibitory potency directly correlated within all four series. To identify putative binding targets, representative members of the four chemotypes were added to aggregation reactions, where they were found to stabilize soluble, but SDS-resistant Tau species at the expense of filamentous aggregates. Using SDS resistance as a secondary assay, and a library of Tau deletion and missense mutants as targets, interaction with cyanine was localized to the microtubule binding repeat region. Moreover, the SDS-resistant phenotype was completely dependent on the presence of octadecyl sulfate inducer, but not intact PHF6/PH6* hexapeptide motifs, indicating that cyanine interacted with a species in the aggregation pathway prior to nucleus formation. Together the data suggest that flat, highly polarizable ligands inhibit Tau aggregation by interacting with folded species in the aggregation pathway and driving their assembly into soluble but highly stable Tau oligomers.

Increased misfolding and truncation of tau in APP/PS1/tau transgenic mice compared to mutant tau mice

Neurofibrillary degeneration in transgenic models of tauopathies has been observed to be enhanced when these models are crossed with transgenic models developing an Aβ pathology. The mechanisms leading to this enhanced tau pathology are not well understood. We have performed a detailed analysis of tau misprocessing in a new transgenic mouse model combining APP, PS1 and tau mutations (5xFAD×Tg30 mice) by comparison with littermates expressing only a FTD mutant tau (Tg30 mice). These 5xFAD×Tg30 mice showed a more severe deficient motor phenotype than Tg30 mice and developed with age a dramatically accelerated NFT load in the brain compared to Tg30 mice. Insoluble tau in 5xFAD×Tg30 mice compared to insoluble tau in Tg30 mice showed increased phosphorylation, enhanced misfolding and truncation changes mimicking more closely the post-translational changes characteristic of PHF-tau in Alzheimer's disease. Endogenous wild-type mouse tau was recruited at much higher levels in insoluble tau in 5xFAD×Tg30 than in Tg30 mice. Extracellular amyloid load, Aβ40 and Aβ42, β-CTFs and β-CTF phosphorylation levels were lower in 5xFAD×Tg30 mice than in 5xFAD mice. Despite this reduction of Aβ, a significant hippocampal neuronal loss was observed in 5xFAD×Tg30 but not in 5xFAD mice indicating its closer association with increased tau pathology. This 5xFAD×Tg30 model thus mimics more faithfully tau pathology and neuronal loss observed in AD and suggests that additional post-translational changes in tau and self-recruitment of endogenous tau drive the enhanced tau pathology developing in the presence of Aβ pathology.