Tau In Alzheimer's Disease Brain Research Articles

125I]INFT: Synthesis and Evaluation of a New Imaging Agent for Tau Protein in Post-Mortem Human Alzheimer's Disease Brain.

Aggregation of Tau protein into paired helical filaments causing neurofibrillary tangles (NFT) is a neuropathological feature in Alzheimer's disease (AD). This study aimed to develop and evaluate the effectiveness of a novel radioiodinated tracer, 4-[125I]iodo-3-(1H-pyrrolo[2,3-c]pyridine-1-yl)pyridine ([125I]INFT), for binding to Tau protein in postmortem human AD brain. Radiosynthesis of [125I]INFT was carried out using electrophilic destannylation by iodine-125 and purified chromatographically. Computational modeling of INFT binding on Tau fibril was compared with IPPI. In vitro, autoradiography studies were conducted with [125I]INFT for Tau in AD and cognitively normal (CN) brains. [125I]INFT was produced in >95% purity. Molecular modeling of INFT revealed comparable binding energies to IPPI at site-1 of the Tau fibril with an affinity of IC50 = 7.3 × 10-8 M. Binding of [125I]INFT correlated with the presence of Tau in the AD brain, confirmed by anti-Tau immunohistochemistry. The ratio of average grey matter (GM) [125I]INFT in AD versus CN was found to be 5.9, and AD GM/white matter (WM) = 2.5. Specifically bound [125I]INFT to Tau in AD brains was displaced by IPPI (>90%). Monoamine oxidase inhibitor deprenyl had no effect and clorgyline had little effect on [125I]INFT binding. [125I]INFT is a less lipophilic imaging agent for Tau in AD.

The Sensitivity of Tau Tracers for the Discrimination of Alzheimer's Disease Patients and Healthy Controls by PET.

Hyperphosphorylated tau aggregates, also known as neurofibrillary tangles, are a hallmark neuropathological feature of Alzheimer's disease (AD). Molecular imaging of tau by positron emission tomography (PET) began with the development of [18F]FDDNP, an amyloid β tracer with off-target binding to tau, which obtained regional specificity through the differing distributions of amyloid β and tau in AD brains. A concerted search for more selective and affine tau PET tracers yielded compounds belonging to at least eight structural categories; 18F-flortaucipir, known variously as [18F]-T807, AV-1451, and Tauvid®, emerged as the first tau tracer approved by the American Food and Drug Administration. The various tau tracers differ concerning their selectivity over amyloid β, off-target binding at sites such as monoamine oxidase and neuromelanin, and degree of uptake in white matter. While there have been many reviews of molecular imaging of tau in AD and other conditions, there has been no systematic comparison of the fitness of the various tracers for discriminating between AD patient and healthy control (HC) groups. In this narrative review, we endeavored to compare the binding properties of the various tau tracers in vitro and the effect size (Cohen's d) for the contrast by PET between AD patients and age-matched HC groups. The available tracers all gave good discrimination, with Cohen's d generally in the range of two-three in culprit brain regions. Overall, Cohen's d was higher for AD patient groups with more severe illness. Second-generation tracers, while superior concerning off-target binding, do not have conspicuously higher sensitivity for the discrimination of AD and HC groups. We suppose that available pharmacophores may have converged on a maximal affinity for tau fibrils, which may limit the specific signal imparted in PET studies.

Effect of KIBRA on tau-mediated synaptic and memory deficits.

Alzheimer's disease (AD) is characterized by an accumulation of tau in the brain and progressive memory decline. Previous work showed that aberrant acetylation of K274 and K281 on tau in postmortem human AD brains is linked to dementia. Transgenic mice expressing human tau with mutations to mimic acetylation at K274 and K281 (tauKQ) have impaired long-term potentiation (LTP) and exhibited hippocampal-dependent memory deficits. Postsynaptic protein levels of KIdney/BRAin (KIBRA) are reduced in tauKQ mice and lower KIBRA protein levels are associated with dementia in human AD brains. KIBRA is a scaffolding protein containing several protein binding domains involved in multiple signaling pathways that regulate neuronal function. Our research aims to uncover how dysregulation of KIBRA signaling contributes to the loss of synaptic plasticity in AD and whether KIBRA signaling can be targeted for therapeutic intervention to prevent memory loss. We generated an N-terminal fragment of KIBRA (N-term KIBRA) and C-terminal fragment of KIBRA (C-term KIBRA) that were expressed either in dissociated rat hippocampal cultures or in vivo using lentivirus-mediated expression in tauKQ mice. We performed immunostaining, electrophysiology, and mouse behavior experiments to examine the effects of either N-term KIBRA or C-term KIBRA on neurons expressing pathogenic tau. We found that C-term KIBRA restores AMPAR trafficking during LTP in neurons expressing pathogenic tau. N-term KIBRA failed to restore AMPAR trafficking required for LTP. We injected lentivirus expressing C-term KIBRA in the hippocampus of tauKQ mice and found that C-term KIBRA restored LTP and hippocampal-dependent memory deficits in tauKQ mice. C-term KIBRA interacts with the brain-specific kinase protein kinase Mζ (PKMζ), and PKMζ is required for C-term KIBRA rescue of AMPAR trafficking during LTP in neurons expressing pathogenic tau. We have shown that the C-terminus region of KIBRA can restore LTP and memory impairments in mice with pathogenic tau that mimics the accumulation of acetylated tau in AD brain. Our work suggests C-term KIBRA enhances synaptic plasticity through a direct interaction with PKMζ to modulate AMPAR trafficking.

G protein-coupled receptor kinases are associated with Alzheimer's disease pathology.

Alzheimer's disease (AD) is characterised by extracellular deposition of amyloid-β (Aβ) in amyloid plaques and intracellular aggregation and accumulation of hyperphosphorylated tau in neurofibrillary tangles (NFTs). Although several kinases have been identified to contribute to the pathological phosphorylation of tau, kinase-targeted therapies for AD have not been successful in clinical trials. Critically, the kinases responsible for numerous identified tau phosphorylation sites remain unknown. G protein-coupled receptor (GPCR) kinases (GRKs) have recently been implicated in phosphorylation of non-GPCR substrates, for example, tubulin and α-synuclein, and in neurological disorders, including schizophrenia and Parkinson's disease. Accordingly, we investigated the involvement of GRKs in the pathophysiology of AD. We performed a comprehensive immunohistochemical and biochemical analysis of the ubiquitously expressed GRKs, namely, GRK2, 3, 5 and 6, in postmortem human brain tissue of control subjects and AD patients. GRKs display unique cell-type-specific expression patterns in neurons, astrocytes and microglia. Levels of GRKs 2, 5 and 6 are specifically decreased in the CA1 region of the AD hippocampus. Biochemical evidence indicates that the GRKs differentially associate with total, soluble and insoluble pools of tau in the AD brain. Complementary immunohistochemical studies indicate that the GRKs differentially colocalise with total tau, phosphorylated tau and NFTs. Notably, GRKs 3 and 5 also colocalise with amyloid plaques. These studies establish a link between GRKs and the pathological phosphorylation and accumulation of tau and amyloid pathology in AD brains and suggest a novel role for these kinases in regulation of the pathological hallmarks of AD.

Thiamme2-G, a Novel O-GlcNAcase Inhibitor, Reduces Tau Hyperphosphorylation and Rescues Cognitive Impairment in Mice.

Abnormal hyperphosphorylation of microtubule-associated protein tau plays a pivotal role in Alzheimer's disease (AD). We previously found that O-GlcNAcylation inversely correlates to hyperphosphorylation of tau in AD brain, and downregulation of brain O-GlcNAcylation promotes tau hyperphosphorylation and AD-like neurodegeneration in mice. Herein we investigated the effect of increasing O-GlcNAcylation by using intermittent dosing with low doses of a potent novel O-GlcNAcase (OGA) inhibitor on AD-like brain changes and cognitive function in a mouse model of sporadic AD (sAD) induced by intracerebroventricular (ICV) injection of streptozotocin (STZ). STZ was injected into the lateral ventricle of C57BL/6J mice. From the second day, Thiamme2-G (TM2G) or saline, as a vehicle control, was orally administered to the ICV-STZ mice three times per week for five weeks. A separate group of ICV-saline mice treated with saline was used as a baseline control. Behavioral tests, including open field and novel object recognition, were conducted three weeks after the first dose of the TM2G or saline. Protein O-GlcNAcylation, tau hyperphosphorylation, synaptic proteins, and neuroinflammation in the mouse brain were assessed by western blotting. ICV-STZ caused decreased protein O-GlcNAcylation. Enhancement of O-GlcNAcylation to moderate levels by using low-dose OGA inhibitor in ICV-STZ mice prevented STZ-induced body weight loss, rescued cognitive impairments, and restored AD-like pathologies, including hyperphosphorylation of tau and abnormalities in synaptic proteins and neuroinflammation. These findings suggest that moderately increasing protein O-GlcNAcylation by using low doses of OGA inhibitor may be a suitable therapeutic strategy for sAD.

Development and evaluation of [125 I]IPPI for Tau imaging in postmortem human Alzheimer's disease brain.

Alzheimer's disease (AD) is a neurodegenerative disease characterized by aggregation of Tau protein into paired helical filaments causing neurofibrillary tangles (NFT) in the brain. The aim of this study was to develop and evaluate the effectiveness of a novel radioiodinated tracer, 6-[125 I]iodo-3-(1H-pyrrolo[2,3-c]pyridine-1-yl)isoquinoline ([125 I]IPPI), for binding to Tau protein (Ki=0.75nM) in postmortem human brain (AD and cognitively normal (CN). Radiosynthesis of [125 I]IPPI was carried out by radioiododestannylation and purified chromatographically. Computational modeling studies of IPPI and MK-6240 binding on Tau fibril were evaluated. In vitro autoradiography studies were carried out with [3 H]PIB for Aβ plaques and [125 I]IPPI for Tau in AD and CN brains and evaluate drug effects. [125 I]IPPI was produced in >95% purity. Molecular modeling of IPPI revealed binding energies of IPPI (-7.8, -8.1, -8.2, -7.5 Kcal/mol) at the four sites were comparable to MK-6240 (-8.7, -8.5, -8.3, -7.5 Kcal/mol). Ratio of average grey matter (GM) [125 I]IPPI in AD versus CN was found to be 7.31 (p=.07) and AD GM/ white matter (WM) = 4.35 (p=.09). Ratio of average GM/WM [125 I]IPPI in CN was 1.21. Binding of [125 I]IPPI correlated with the presence of Tau, confirmed by anti-Tau Dako A0024. Specifically bound [125 I]IPPI to Tau in AD brains was displaced by MK-6240 and IPPI (>90%). Monoamine oxidase inhibitors (MAO) inhibitors deprenyl and clorgyline effected [125 I]IPPI binding at >1µM concentrations. [125 I]IPPI exhibited high binding in human AD frontal cortex and anterior cingulate and is a suitable radioiodinated ligand for Tau imaging.

Tau's Three-Repeat Domain and EFhd2 Co-incubation Leads to Increased Thioflavin Signal.

Aggregation of the protein tau is a pathological hallmark of Alzheimer’s disease (AD) and related disorders. However, the molecular mechanisms that lead to tau protein aggregation are still unclear. Previously, we showed that EFhd2 protein is associated with pathological aggregated forms of tau in AD brain. Further, immuno-gold analyses of purified tau aggregates showed that EFhd2 co-localized with filamentous tau structures. We demonstrated that EFhd2’s coiled-coil domain is required for its association with tau proteins. However, it is unknown the role that EFhd2 plays in tau aggregation. Here, we show that incubation of K19-tau with substoichiometric amount of EFhd2 promote the formation of amyloid structures in vitro. The result suggests that EFhd2 may play a role in the biogenesis of aggregated tau.

RELEVANCE OF PHOSPHORYLATION AND TRUNCATION OF TAU TO THE ETIOPATHOGENESIS OF ALZHEIMER’S DISEASE

Alzheimer's disease (AD), the major neurodegenerative disease and cause of dementia, is a slow progressive disorder pathologically characterized by extracellular amyloid-b plaques and intracellular neurofibrillary tangles (NFTs) of abnormally hyperphosphorylated tau. 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 is not very clear. In the present study we investigated the association of 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 tau antibodies ranges from the N- to the C-termini of tau. The level of tau truncated at Asp421 was increased in AD brain, but was poorly associated with the HMW-tau. HMW-tau is hyperphosphorylated at multiple sites, and unphosphorylated tau, such as at Ser46 and at Ser198/199/202, is absent in these aggregates. The N-terminally truncated tau is associated with HMW-tau more than the C-terminal truncated tau. These findings provide a novel insight into the association of the hyperphosphorylation and predominantly N-terminal truncation of tau with tau aggregation into oligomers in AD.

Ca2+/calmodulin-dependent protein kinase II promotes neurodegeneration caused by tau phosphorylated at Ser262/356 in a transgenic Drosophila model of tauopathy.

Abnormal deposition of the microtubule-associated protein tau is a common pathological feature of multiple neurodegenerative diseases, including Alzheimer's disease (AD), and plays critical roles in their pathogenesis. Disruption of calcium homeostasis and the downstream kinase Ca2+/calmodulin-dependent protein kinase II (CaMKII) coincides with pathological phosphorylation of tau in AD brains. However, it remains unclear whether and how dysregulation of CaMKII affects tau toxicity. Using a Drosophila model, we found that CaMKII promotes neurodegeneration caused by tau phosphorylated at the AD-associated sites Ser262/356. Overexpression of CaMKII promoted, while RNA-mediated knockdown of CaMKII and inhibition of CaMKII activity by expression of an inhibitory peptide suppressed, tau-mediated neurodegeneration. Blocking tau phosphorylation at Ser262/356 by alanine substitutions suppressed promotion of tau toxicity by CaMKII, suggesting that tau phosphorylation at these sites is required for this phenomenon. However, neither knockdown nor overexpression of CaMKII affected tau phosphorylation levels at Ser262/356, suggesting that CaMKII is not directly involved in tau phosphorylation at Ser262/356 in this model. These results suggest that a pathological cascade of events, including elevated levels of tau phosphorylated at Ser262/356 and aberrant activation of CaMKII, work in concert to promote tau-mediated neurodegeneration.

Novel Treatment Strategies Using TiO2-Nanowired Delivery of Histaminergic Drugs and Antibodies to Tau With Cerebrolysin for Superior Neuroprotection in the Pathophysiology of Alzheimer's Disease.

More than 5.5 million Americans of all ages are suffering from Alzheimer's disease (AD) till today for which no suitable therapy has been developed so far. Thus, there is an urgent need to explore novel therapeutic measures to contain brain pathology in AD. The hallmark of AD includes amyloid-beta peptide (AβP) deposition and phosphorylation of tau in AD brain. Recent evidences also suggest a marked decrease in neurotrophic factors in AD. Thus, exogenous supplement of neurotrophic factors could be one of the possible ways for AD therapy. Human postmortem brain in AD shows alterations in histamine receptors as well, indicating an involvement of the amine in AD-induced brain pathology. In this review, we focused on role of histamine 3 and 4 receptor-modulating drugs in the pathophysiology of AD. Moreover, antibodies to histamine and tau appear to be also beneficial in reducing brain pathology, blood-brain barrier breakdown, and edema formation in AD. Interestingly, TiO2-nanowired delivery of cerebrolysin-a balanced composition of several neurotrophic factors attenuated AβP deposition and reduced tau phosphorylation in AD brain leading to neuroprotection. Coadministration of cerebrolysin with histamine antibodies or tau antibodies has further enhanced neuroprotection in AD. These novel observations strongly suggest a role of nanomedicine in AD that requires further investigation.

Kidins220 Correlates with Tau inAlzheimer's Disease Brain andCerebrospinal Fluid.

Identification of neurodegeneration-monitoring biomarkers would be of great clinical value for Alzheimer's disease (AD) diagnosis. Using N- or C-terminal antibodies, we studied the pro-survival synaptic effector, Kidins220, in the brain and cerebrospinal fluid (CSF) of controls and AD patients. Only the N-terminal antibody showed apositive correlation between Kidins220 and phosphorylated tau in AD brains. Using this antibody, Kidins220 was detected in CSF from AD patients where it positively correlated with CSF phosphorylated tau and tau. This study highlights the potential of Kidins220 as aCSF biomarker in AD.

Truncation and Activation of Dual Specificity Tyrosine Phosphorylation-regulated Kinase 1A by Calpain I: A MOLECULAR MECHANISM LINKED TO TAU PATHOLOGY IN ALZHEIMER DISEASE

Hyperphosphorylation and dysregulation of exon 10 splicing of Tau are pivotally involved in pathogenesis of Alzheimer disease (AD) and/or other tauopathies. Alternative splicing of Tau exon 10, which encodes the second microtubule-binding repeat, generates Tau isoforms containing three and four microtubule-binding repeats, termed 3R-Taus and 4R-Taus, respectively. Dual specificity tyrosine-phosphorylation-regulated kinase 1A (Dyrk1A) lies at the Down syndrome critical region of chromosome 21. Overexpression of this kinase may contribute to the early Tau pathology in Down syndrome via phosphorylation of Tau and dysregulation of Tau exon 10. Here, we report that Dyrk1A was truncated at the C terminus and was associated with overactivation of calpain I in AD brain. Calpain I proteolyzed Dyrk1A in vitro first at the C terminus and further at the N terminus and enhanced its kinase activity toward Tau via increased Vmax but not Km. C-terminal truncation of Dyrk1A resulted in stronger activity than its full-length protein in promotion of exon 10 exclusion and phosphorylation of Tau. Dyrk1A was truncated in kainic acid-induced excitotoxic mouse brains and coincided with an increase in 3R-Tau expression and phosphorylation of Tau via calpain activation. Moreover, truncation of Dyrk1A was correlated with an increase in the ratio of 3R-Tau/4R-Tau and Tau hyperphosphorylation in AD brain. Collectively, these findings suggest that truncation/activation of Dyrk1A by Ca(2+)/calpain I might contribute to Tau pathology via promotion of exon 10 exclusion and hyperphosphorylation of Tau in AD brain.

Passive immunotherapy of tauopathy targeting pSer413-tau: a pilot study in mice.

ObjectiveCellular inclusions of hyperphosphorylated tau are a hallmark of tauopathies, which are neurodegenerative disorders that include Alzheimer's disease (AD). Active and passive immunization against hyperphosphorylated tau has been shown to attenuate phenotypes in model mice. We developed new monoclonal antibodies to hyperphosphorylated tau and sought high therapeutic efficacy for future clinical use.MethodsUsing more than 20 antibodies, we investigated which sites on tau are phosphorylated early and highly in the tauopathy mouse models tau609 and tau784. These mice display tau hyperphosphorylation, synapse loss, memory impairment at 6 months, and tangle formation and neuronal loss at 15 months. We generated mouse monoclonal antibodies to selected epitopes and examined their effects on memory and tau pathology in aged tau609 and tau784 mice by the Morris water maze and by histological and biochemical analyses.ResultsImmunohistochemical screening revealed that pSer413 is expressed early and highly. Monoclonal antibodies to pSer413 and to pSer396 (control) were generated. These antibodies specifically recognized pathological tau in AD brains but not normal tau in control brains according to Western blots. Representative anti-pSer413 and anti-pSer396 antibodies were injected intraperitoneally into 10–11- or 14-month-old mice once a week at 0.1 or 1 mg/shot 5 times. The anti-pSer413 antibody significantly improved memory, whereas the anti-pSer396 antibodies showed less effect. The cognitive improvement paralleled a reduction in the levels of tau hyperphosphorylation, tau oligomer accumulation, synapse loss, tangle formation, and neuronal loss.InterpretationThese results indicate that pSer413 is a promising target in the treatment of tauopathy.

Assessing THK523 selectivity for tau deposits in Alzheimer’s disease and non–Alzheimer’s disease tauopathies

IntroductionThe introduction of tau imaging agents such as 18F-THK523 offers new hope for the in vivo assessment of tau deposition in tauopathies such as Alzheimer’s disease (AD), where preliminary 18F-THK523-PET studies have demonstrated significantly higher cortical retention of 18F-THK523 in AD compared to age-matched healthy individuals. In addition to AD, tau imaging with PET may also be of value in assessing non-AD tauopathies, such as corticobasal degeneration (CBD), progressive supranuclear palsy (PSP) and Pick’s disease (PiD).MethodsTo further investigate the ability of THK523 to recognize tau lesions, we undertook immunohistochemical and fluorescence studies in serial brain sections taken from individuals with AD (n = 3), CBD (n = 2), PSP (n = 1), PiD (n = 2) and Parkinson’s disease (PD; n = 2). In addition to the neuropathological analysis, one PSP patient had undergone a 18F-THK523 PET scan 5 months before death.ResultsAlthough THK523 labelled tau-containing lesions such as neurofibrillary tangles and neuropil threads in the hippocampus and frontal regions of AD brains, it failed to label tau-containing lesions in non-AD tauopathies. Furthermore, though THK523 faintly labelled dense-cored amyloid-β plaques in the AD frontal cortex, it failed to label α-synuclein-containing Lewy bodies in PD brain sections.ConclusionThe results of this study suggest that 18F-THK523 selectively binds to paired helical filament tau in AD brains but does not bind to tau lesions in non-AD tauopathies, or to α-synuclein in PD brains.

Interaction of 14-3-3ζ with Microtubule-Associated Protein Tau within Alzheimer’s Disease Neurofibrillary Tangles

Alzheimer's disease (AD) is characterized by the presence of abnormal, straight filaments and paired helical filaments (PHFs) that are coated with amorphous aggregates. When PHFs are treated with alkali, they untwist and form filaments with a ribbonlike morphology. Tau protein is the major component of all of these ultrastructures. 14-3-3ζ is present in NFTs and is significantly upregulated in AD brain. The molecular basis of the association of 14-3-3ζ within NFTs and the pathological significance of its association are not known. In this study, we have found that 14-3-3ζ is copurified and co-immunoprecipitates with tau from NFTs of AD brain extract. In vitro, tau binds to both phosphorylated and nonphosphorylated tau. When incubated with 14-3-3ζ, tau forms amorphous aggregates, single-stranded, straight filaments, ribbonlike filaments, and PHF-like filaments, all of which resemble the corresponding ultrastructures found in AD brain. Immuno-electron microscopy determined that both tau and 14-3-3ζ are present in these ultrastructures and that they are formed in an incubation time-dependent manner. Amorphous aggregates are formed first. As the incubation time increases, the size of amorphous aggregates increases and they are incorporated into single-stranded filaments. Single-stranded filaments laterally associate to form double-stranded, ribbonlike, and PHF-like filaments. Both tau and phosphorylated tau aggregate in a similar manner when they are incubated with 14-3-3ζ. Our data suggest that 14-3-3ζ has a role in the fibrillization of tau in AD brain, and that tau phosphorylation does not affect 14-3-3ζ-induced tau aggregation.

Activation of Asparaginyl Endopeptidase Leads to Tau Hyperphosphorylation in Alzheimer Disease

Neurofibrillary pathology of abnormally hyperphosphorylated Tau is a key lesion of Alzheimer disease and other tauopathies, and its density in the brain directly correlates with dementia. The phosphorylation of Tau is regulated by protein phosphatase 2A, which in turn is regulated by inhibitor 2, I2(PP2A). In acidic conditions such as generated by brain ischemia and hypoxia, especially in association with hyperglycemia as in diabetes, I2(PP2A) is cleaved by asparaginyl endopeptidase at Asn-175 into the N-terminal fragment (I2NTF) and the C-terminal fragment (I2CTF). Both I2NTF and I2CTF are known to bind to the catalytic subunit of protein phosphatase 2A and inhibit its activity. Here we show that the level of activated asparaginyl endopeptidase is significantly increased, and this enzyme and I2(PP2A) translocate, respectively, from neuronal lysosomes and nucleus to the cytoplasm where they interact and are associated with hyperphosphorylated Tau in Alzheimer disease brain. Asparaginyl endopeptidase from Alzheimer disease brain could cleave GST-I2(PP2A), except when I2(PP2A) was mutated at the cleavage site Asn-175 to Gln. Finally, an induction of acidosis by treatment with kainic acid or pH 6.0 medium activated asparaginyl endopeptidase and consequently produced the cleavage of I2(PP2A), inhibition of protein phosphatase 2A, and hyperphosphorylation of Tau, and the knockdown of asparaginyl endopeptidase with siRNA abolished this pathway in SH-SY5Y cells. These findings suggest the involvement of brain acidosis in the etiopathogenesis of Alzheimer disease, and asparaginyl endopeptidase-I2(PP2A)-protein phosphatase 2A-Tau hyperphosphorylation pathway as a therapeutic target.

Activation of Protein Phosphatase 2B and Hyperphosphorylation of Tau in Alzheimer's Disease

Protein phosphatase 2B (PP2B) is one of the major brain phosphatases and can dephosphorylate tau at several phosphorylation sites in vitro. Previous studies that measured PP2B activity in human brain crude extracts showed that PP2B activity was either unchanged or decreased in Alzheimer's disease (AD) brain. These results led to the speculation that PP2B might regulate tau phosphorylation and that a down-regulation of PP2B might contribute to abnormal hyperphosphorylation of tau. In this study, we immunoprecipitated PP2B from brains of six AD subjects and seven postmortem- and age-matched controls and then measured the phosphatase activity. We found a three-fold increase in PP2B activity in AD brain as compared with control brains. The activation was due to the partial cleavage of PP2B by calpain I that was activated in AD brain. The truncation of PP2B appeared to alter its intracellular distribution in the brain. In human brains, PP2B activity correlated positively, rather than negatively, to the levels of tau phosphorylation at several sites that can be dephosphorylated by PP2B in vitro. Truncation of PP2B in the frontal cortex was more than in the temporal cortex, and tau phosphorylation was also more in the frontal cortex. Taken together, these results indicate that truncation of PP2B by calpain I elevates its activity but does not counteract the abnormal hyperphosphorylation tau in AD brain.

Tau in Alzheimer Disease and Related Tauopathies

Tau is the major microtubule associated protein (MAP) of a mature neuron. The other two neuronal MAPs are MAP1 and MAP2. An established function of MAPs is their interaction with tubulin and promotion of its assembly into microtubules and stabilization of the microtubule network. The microtubule assembly promoting activity of tau, a phosphoprotein, is regulated by its degree of phosphorylation. Normal adult human brain tau contains 2-3 moles phosphate/mole of tau protein. Hyperphosphorylation of tau depresses this biological activity of tau. In Alzheimer disease (AD) brain tau is ~three to four-fold more hyperphosphorylated than the normal adult brain tau and in this hyperphosphorylated state it is polymerized into paired helical filaments ([PHF) admixed with straight filaments (SF) forming neurofibrillary tangles. Tau is transiently hyperphosphorylated during development and during anesthesia and hypothermia but not to the same state as in AD brain. The abnormally hyperphosphorylated tau in AD brain is distinguished from transiently hyperphosphorylated tau by its ability (1) to sequester normal tau, MAP1 and MAP2 and disrupt microtubules, and (2) to self-assemble into PHF/SF. The cytosolic abnormally hyperphosphorylated tau, because of oligomerization, unlike normal tau, is sedimentable and on self-assembly into PHF/SF, loses its ability to sequester normal MAPs. Some of the tau in AD brain is truncated which also promotes its self-assembly. Tau mutations found in frontotemporal dementia apparently promote its abnormal hyperphosphorylation. Thus, the AD abnormally hyperphosphorylated tau (1) is distinguishable from both normal and transiently hyperphosphorylated taus, and (2) is inhibitory when in a cytosolic/oligomeric state but not when it is self-assembled into PHF/SF. Inhibition of abnormal hyperphosphorylation of tau offers a promising therapeutic target for AD and related tauopathies.

Modulation of Tau Phosphorylation by the Kinase PKR: Implications in Alzheimer's Disease

Double-stranded RNA dependent kinase (PKR) is a pro-apoptotic kinase that controls protein translation. Previous studies revealed that activated PKR is increased in brains with Alzheimer's disease (AD). Glycogen Synthase Kinase Aβ (GSK-3β) is responsible for tau phosphorylation and controls several cellular functions also including apoptosis. The goal of this work was to determine if PKR could concurrently trigger GSK-3β activation, tau phosphorylation and apoptosis. In AD brains, both activated kinases co-localize with phosphorylated tau in neurons. In SH-SY5Y cell cultures, tunicamycin and Aβ(1-42) activate PKR, GSK-3β and induce tau phosphorylation and all these processes are attenuated by PKR inhibitors or PKR siRNA. Our results demonstrate that neuronal PKR co-localizes with GSK-3β and tau in AD brains and is able to modulate GSK-3β activation, tau phosphorylation and apoptosis in neuroblastoma cells exposed to tunicamycin or Aβ. PKR could represent a crucial signaling point relaying stress signals to neuronal pathways leading to cellular degeneration in AD.

The interleukin-6 gene −572C/G promoter polymorphism modifies Alzheimer's risk in APOE ɛ4 carriers

Inflammatory response is involved in the etiopathology of Alzheimer's disease (AD). Interleukin-6 (IL-6), a pleiotropic inflammatory cytokine, was reported to be associated with both increased Aβ aggregation and the appearance of hyperphosphorylated tau in AD brain. To explore the association of genetic variants in the promoter of IL-6 gene with sporadic AD (SAD), a case–control study was conducted in a North Chinese Han population. A systematic screening of IL-6 promoter was performed using direct sequencing and two polymorphisms were obtained including −572C/G (rs1800796) and −384A/T (rs7802308). Definitive genotyping of these markers and apolipoprotein E ( APOE) polymorphism were surveyed in 341 SAD patients and 421 controls. The results revealed no significant differences in the distributions of alleles or genotypes between SAD and control groups. However, there was an interaction between −572C/G and APOE genotypes ( P = 0.016) using logistic analysis. In the subjects with APOE ɛ4, there were significant differences in the allele ( P = 0.004) and genotype ( P = 0.004) distributions of −572C/G polymorphism between SAD and control groups. The −572CC genotype increased risk for AD by 3.301-fold (Wald = 11.093, adjust OR = 3.301, 95% CI = 1.635–6.665, P = 0.001) compared to CG + GG genotype. The present results suggest the −572 polymorphism could modify the risk for SAD in APOE ɛ4 carriers.