Increased Tau Phosphorylation Research Articles

DHA Selectively Protects SAMP-8-Associated Cognitive Deficits Through Inhibition of JNK.

A potential role of marine n-3 polyunsaturated fatty acids (ω-3 PUFAs) has been suggested in memory, learning, and cognitive processes. Therefore, ω-3 PUFAs might be a promising treatment option, albeit controversial, for Alzheimer's disease (AD). Among the different mechanisms that have been proposed as responsible for the beneficial effects of ω-3 PUFAs, inhibition of JNK stands as a particularly interesting candidate. In the present work, it has been studied whether the administration of two different PUFAs (docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA)) and a DHA-derived specialized pro-resolving lipid mediator (MaR1) is able to reverse cognitive deficits in the senescence-accelerated mouse prone 8 (SAMP8) mouse model of sporadic AD. The novel object recognition test (NORT) test showed that recognition memory was significantly impaired in SAMP8 mice, as shown by a significantly decreased discrimination index that was reversed by MaR1 and DHA. In the retention phase of the Morris water maze (MWM) task, SAMP8 mice showed memory deficit that only DHA treatment was able to reverse. pJNK levels were significantly increased in the hippocampus of SAMP8 mice compared to SAMR1 mice, and only DHA treatment was able to significantly reverse these increased pJNK levels. Similar results were found when measuring c-Jun, the main JNK substrate. Consequently to the increases in tau phosphorylation after increased pJNK, it was checked that tau phosphorylation (PHF-1) was increased in SAMP mice, and this effect was reversed after DHA treatment. Altogether, DHA could represent a new approach for the treatment of AD through JNK inhibition.

Phosphorylation of Tau protein correlates with changes in hippocampal theta oscillations and reduces hippocampal excitability in Alzheimer's model

Tau hyperphosphorylation at several sites, including those close to the microtubule domain region (MDr), is considered a key pathological event in the development of Alzheimer's disease (AD). Recent studies indicate that at the very early stage of this disease, increased phosphorylation in Tau's MDr domain correlates with reduced levels of neuronal excitability. Mechanistically, we show that pyramidal neurons and some parvalbumin-positive interneurons in 1-month-old triple-transgenic AD mice accumulate hyperphosphorylated Tau protein and that this accumulation correlates with changes in theta oscillations in hippocampal neurons. Pyramidal neurons from young triple-transgenic AD mice exhibited less spike accommodation and power increase in subthreshold membrane oscillations. Furthermore, triple-transgenic AD mice challenged with the potassium channel blocker 4-aminopyridine had reduced theta amplitude compared with 4-aminopyridine-treated control mice and, unlike these controls, displayed no seizure-like activity after this challenge. Collectively, our results provide new insights into AD pathogenesis and suggest that increases in Tau phosphorylation at the initial stages of the disease represent neuronal responses that compensate for brain circuit overexcitation.

The influence of anesthetics on substantia nigra tyrosine hydroxylase expression and tau phosphorylation in the hypoxic-ischemic near-term lamb.

BackgroundGeneral anesthetics could protect key neurotransmitter systems, such as the dopaminergic system, from hypoxic-ischemic encephalopathy (HIE) by limiting excessive glutamatergic neurotransmission. However, anesthetics may adversely affect inflammation and tau phosphorylation.MethodsA near-term sheep model of HIE by umbilical cord occlusion (UCO) under anesthesia was used. The effect of propofol and isoflurane on the dopaminergic neurotransmitter phenotype in the substantia nigra (SN) was studied using tyrosine hydroxylase immunohistochemistry. The overall microglial response and tau phosphorylation were also measured in the SN, surrounding the midbrain gray matter structures and the hippocampal white matter.ResultsThe isoflurane-treated UCO group had fewer tyrosine hydroxylase-expressing neurons in the SN at 8 h after the insult than the propofol-treated UCO or sham-operated groups (P<0.05). The microglial response was unchanged in the SN region. In the thalamus and the hippocampal stratum moleculare layer, the propofol-treated UCO group had a lower microglial response than the corresponding sham-operated group. Both UCO and the use of anesthetics additively increased tau phosphorylation in the SN region, thalamus, and hippocampus.ConclusionThe choice of anesthetics is important for an emergency C-section. Propofol could potentially protect the dopaminergic neurotransmitter phenotype within the SN at the cost of a widespread increase in tau phosphorylation.

Intermittent streptozotocin administration induces behavioral and pathological features relevant to Alzheimer's disease and vascular dementia

RationaleDiabetes mellitus (DM) is a major risk factor for Alzheimer's disease and vascular dementia. Few animal models exist that focus on the metabolic contributions to dementia onset and progression. Thus, there is strong scientific rationale to explore the effects of streptozotocin (STZ), a diabetogenic compound, on vascular and inflammatory changes within the brain. Objective and methodsThe present study was designed to evaluate the effect of staggered, low-dose administration of STZ on behavioral and cognitive deficits, neuroinflammation, tau pathology, and histopathological alterations related to dementia. ResultsStaggered administration (Days 1, 2, 3, 14, 15) of streptozotocin (40 mg/kg/mL) induced a diabetic-like state in mice, resulting in sustained hyperglycemia. STZ-treated animals displayed memory deficits in the novel object recognition task as well as increased tau phosphorylation and increased neuroinflammation. Additionally, STZ led to altered insulin signaling, exhibited by decreased plasma insulin and decreased levels of insulin degrading enzyme and pAKT within the hippocampus. ConclusionsSTZ-treated animals exhibit cognitive deficits and histopathological changes seen in dementia. This model of dementia warrants continued investigation to better understand the role that DM plays in dementia-related alterations.

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.

Neuroprotective effects of N-acetyl cysteine on primary hippocampus neurons against hydrogen peroxide-induced injury are mediated via inhibition of mitogen-activated protein kinases signal transduction and antioxidative action.

N-acetyl cysteine (NAC) has been extensively reported to exert neuroprotective effects on the central nervous system. Oxidative stress may contribute to the underlying mechanisms causing Alzheimer's disease (AD). The effect of NAC against oxidative stress injury was investigated in a cellular model of AD in the present study and the underlying mechanisms were revealed. The neuroprotective action of NAC (1, 10, 100 and 1,000µmol/l) on a cellular model of AD [hydrogen peroxide (H2O2)‑induced (3, 30 and 300µmol/l) toxicity in primary rat hippocampus neurons] demonstrated the underlying mechanisms. Cytotoxicity was measured using the MTT assay, and light microscopy and the dichloro-dihydro-fluorescein diacetate method were used to detect the reactive oxygen species (ROS) levels. Furthermore, the levels of mitogen-activated protein kinases (MAPKs) signal transduction and tau protein phosphorylation were measured via western blotting. NAC (100µmol/l) protected hippocampus neurons against H2O2‑mediated toxicity, as evidenced by enhanced cell viability. Using MTT assay and light microscopy for the observation of cell death, NAC ameliorated cell viability, which was induced by H2O2 injury (P<0.05). NAC was found to mitigate the excessive production of ROS (P<0.05). Another mechanism involved in the neuroprotective action of NAC may be its ability to inhibit MAPK signal transduction following H2O2 exposure. In addition, NAC may protect cells against H2O2‑induced toxicity by attenuating increased tau phosphorylation. Thus, the protective ability of NAC is hypothesized to result from inhibition of oxidative stress and downregulation of MAPK signal transduction and tau phosphorylation.

Impaired insulin signaling and spatial learning in middle-aged rats: The role of PTP1B

The insulin and Brain-Derived Neurotrophic Factor (BDNF) signaling in the hippocampus promotes synaptic plasticity and memory formation. On the other hand, aging is related to the cognitive decline and is the main risk factor for Alzheimer's Disease (AD). The Protein-Tyrosine Phosphatase 1B (PTP1B) is related to several deleterious processes in neurons and emerges as a promising target for new therapies. In this context, our study aims to investigate the age-related changes in PTP1B content, insulin signaling, β-amyloid content, and Tau phosphorylation in the hippocampus of middle-aged rats. Young (3 months) and middle-aged (17 months) Wistar rats were submitted to Morris-water maze (MWM) test, insulin tolerance test, and molecular analysis in the hippocampus. Aging resulted in increased body weight, and insulin resistance and decreases learning process in MWM. Interestingly, the middle-aged rats have higher levels of PTP-1B, lower phosphorylation of IRS-1, Akt, GSK3β, mTOR, and TrkB. Also, the aging process increased Tau phosphorylation and β-amyloid content in the hippocampus region. In summary, this study provides new evidence that aging-related PTP1B increasing, contributing to insulin resistance and the onset of the AD.

Wnt signaling loss accelerates the appearance of neuropathological hallmarks of Alzheimer's disease in J20-APP transgenic and wild-type mice.

Alzheimer's disease (AD) is a neurodegenerative pathology characterized by aggregates of amyloid-β (Aβ) and phosphorylated tau protein, synaptic dysfunction, and spatial memory impairment. The Wnt signaling pathway has several key functions in the adult brain and has been associated with AD, mainly as a neuroprotective factor against Aβ toxicity and tau phosphorylation. However, dysfunction of Wnt/β-catenin signaling might also play a role in the onset and development of the disease. J20 APPswInd transgenic (Tg) mouse model of AD was treated i.p. with various Wnt signaling inhibitors for 10weeks during pre-symptomatic stages. Then, cognitive, biochemical and histochemical analyses were performed. Wnt signaling inhibitors induced severe changes in the hippocampus, including alterations in Wnt pathway components and loss of Wnt signaling function, severe cognitive deficits, increased tau phosphorylation and Aβ1-42 peptide levels, decreased Aβ42/Aβ40 ratio and Aβ1-42 concentration in the cerebral spinal fluid, and high levels of soluble Aβ species and synaptotoxic oligomers in the hippocampus, together with changes in the amount and size of senile plaques. More important, we also observed severe alterations in treated wild-type (WT) mice, including behavioral impairment, tau phosphorylation, increased Aβ1-42 in the hippocampus, decreased Aβ1-42 in the cerebral spinal fluid, and hippocampal dysfunction. Wnt inhibition accelerated the development of the pathology in a Tg AD mouse model and contributed to the development of Alzheimer's-like changes in WT mice. These results indicate that Wnt signaling plays important roles in the structure and function of the adult hippocampus and suggest that inhibition of the Wnt signaling pathway is an important factor in the pathogenesis of AD. Read the Editorial Highlight for this article on page 356.

Effect of tibolone pretreatment on kinases and phosphatases that regulate the expression and phosphorylation of Tau in the hippocampus of rats exposed to ozone.

Oxidative stress (OS) is a key process in the development of many neurodegenerative diseases, memory disorders, and other pathological processes related to aging. Tibolone (TIB), a synthetic hormone used as a treatment for menopausal symptoms, decreases lipoperoxidation levels, prevents memory impairment and learning disability caused by ozone (O3) exposure. However, it is not clear if TIB could prevent the increase in phosphorylation induced by oxidative stress of the microtubule-associated protein Tau. In this study, the effects of TIB at different times of administration on the phosphorylation of Tau, the activation of glycogen synthase kinase-3β (GSK3β), and the inactivation of Akt and phosphatases PP2A and PTEN induced by O3 exposure were assessed in adult male Wistar rats. Rats were divided into 10 groups: control group (ozone-free air plus vehicle [C]), control + TIB group (ozone-free air plus TIB 1 mg/kg [C + TIB]); 7, 15, 30, and 60 days of ozone exposure groups [O3] and 7, 15, 30, and 60 days of TIB 1 mg/kg before ozone exposure groups [O3 + TIB]. The effects of O3 exposure and TIB administration were assessed by western blot analysis of total and phosphorylated Tau, GSK3β, Akt, PP2A, and PTEN proteins and oxidative stress marker nitrotyrosine, and superoxide dismutase activity and lipid peroxidation of malondialdehyde by two different spectrophotometric methods (Marklund and TBARS, respectively). We observed that O3 exposure increases Tau phosphorylation, which is correlated with decreased PP2A and PTEN protein levels, diminished Akt protein levels, and increased GSK3β protein levels in the hippocampus of adult male rats. The effects of O3 exposure were prevented by the long-term treatment (over 15 days) with TIB. Malondialdehyde and nitrotyrosine levels increased from 15 to 60 days of exposure to O3 in comparison to C group, and superoxide dismutase activity decreased. Furthermore, TIB administration limited the changes induced by O3 exposure. Our results suggest a beneficial use of hormone replacement therapy with TIB to prevent neurodegeneration caused by O3 exposure in rats.

Up-regulation of casein kinase 1\u03b5 is involved in tau pathogenesis in Alzheimer\u2019s disease

Hyperphosphorylation of tau and imbalanced expression of 3R-tau and 4R-tau as a result of dysregulation of tau exon 10 splicing are believed to be pivotal to the pathogenesis of tau pathology, but the molecular mechanism leading to the pathologic tau formation in Alzheimer’s disease (AD) brain is not fully understood. In the present study, we found that casein kinase 1ε (CK1ε) was increased significantly in AD brains. Overexpression of CK1ε in cultured cells led to increased tau phosphorylation at many sites. Moreover, we found that CK1ε suppressed tau exon 10 inclusion. Levels of CK1ε were positively correlated to tau phosphorylation, 3R-tau expression and tau pathology, and negatively correlated to 4R-tau in AD brains. Overexpression of CK1ε in the mouse hippocampus increased tau phosphorylation and impaired spontaneous alternation behavior. These data suggest that CK1ε is involved in the regulation of tau phosphorylation, the alternative splicing of tau exon 10, and cognitive performance. Up-regulation of CK1ε might contribute to tau pathology by hyperphosphorylating tau and by dysregulating the alternative splicing of tau exon 10 in AD.

Nimodipine attenuates tau phosphorylation at Ser396 via miR-132/GSK-3β pathway in chronic cerebral hypoperfusion rats

Chronic cerebral hypofusion (CCH) promotes hyperphosphorylation of tau proteins, a key neuropathological trait that reflects neurodegenerative disorders. Nimodipine, an L-type calcium channel antagonist, has been reported to show neuroprotective effects. In this study, we investigated the potential mechanism of nimodipine in tauopathies induced by CCH. MiR-132 is downregulated in tauopathies such as AD and directly targets tau mRNA to regulate its expression. Here, we report that CCH induced miR-132 deficiency and increased tau phosphorylation at Ser396 while tau expression was not influenced. Nimodipine treatment attenuated CCH induced tau phosphorylation by up-regulating expression of miR-132. Furthermore, nimodipine inhibited activation of GSK-3β and neuronal apoptosis induced by CCH. Interestingly, GSK-3βmRNA level negatively correlated with the expression of miR-132. These findings support a role for nimodipine inhibiting tau phosphorylation at Ser396 via miR-132/GSK-3β. Therefore, nimodipine may be a candidate for the treatment of tauopathy present in CCH.

A\u03b2 accumulation causes MVB enlargement and is modelled by dominant negative VPS4A

BackgroundAlzheimer’s disease (AD)-linked β-amyloid (Aβ) accumulates in multivesicular bodies (MVBs) with the onset of AD pathogenesis. Alterations in endosomes are among the earliest changes associated with AD but the mechanism(s) that cause endosome enlargement and the effects of MVB dysfunction on Aβ accumulation and tau pathology are incompletely understood.MethodsMVB size and Aβ fibrils in primary neurons were visualized by electron microscopy and confocal fluorescent microscopy. MVB-dysfunction, modelled by expression of dominant negative VPS4A (dnVPS4A), was analysed by biochemical methods and exosome isolation.ResultsHere we show that AD transgenic neurons have enlarged MVBs compared to wild type neurons. Uptake of exogenous Aβ also leads to enlarged MVBs in wild type neurons and generates fibril-like structures in endocytic vesicles. With time fibrillar oligomers/fibrils can extend out of the endocytic vesicles and are eventually detectable extracellularly. Further, endosomal sorting complexes required for transport (ESCRT) components were found associated with amyloid plaques in AD transgenic mice. The phenotypes previously reported in AD transgenic neurons, with net increased intracellular levels and reduced secretion of Aβ, were mimicked by blocking recycling of ESCRT-III by dnVPS4A. DnVPS4A further resembled AD pathology by increasing tau phosphorylation at serine 396 and increasing markers of autophagy.ConclusionsWe demonstrate that Aβ leads to MVB enlargement and that amyloid fibres can form within the endocytic pathway of neurons. These results are consistent with the scenario of the endosome-lysosome system representing the site of initiation of Aβ aggregation. In turn, a dominant negative form of the CHMP2B-interacting protein VPS4A, which alters MVBs, leads to accumulation and aggregation of Aβ as well as tau phosphorylation, mimicking the cellular changes in AD.

Amyloid-β and islet amyloid pathologies link Alzheimer's disease and type 2 diabetes in a transgenic model.

Alzheimer's disease (AD) and type 2 diabetes (T2D) present a significant risk to each other. AD and T2D are characterized by deposition of cerebral amyloid-β (Aβ) and pancreatic human islet amyloid polypeptide (hIAPP), respectively. We investigated the role of amyloidogenic proteins in the interplay between these diseases. A novel double transgenic mouse model combining T2D and AD was generated and characterized. AD-related amyloid transgenic mice coexpressing hIAPP displayed peripheral insulin resistance, hyperglycemia, and glucose intolerance. Aβ and IAPP amyloid co-deposition increased tau phosphorylation, and a reduction in pancreatic β-cell mass was detected in islets. Increased brain Aβ deposition and tau phosphorylation and reduced insulin levels and signaling were accompanied by extensive synaptic loss and decreased neuronal counts. Aβ immunization rescued the peripheral insulin resistance and hyperglycemia, suggesting a role for Aβ in T2D pathogenesis for individuals predisposed to AD. These findings demonstrate that Aβ and IAPP are key factors in the overlapping pathologies of AD and T2D.-Wijesekara, N., Ahrens, R., Sabale, M., Wu, L., Ha, K., Verdile, G., Fraser, P. E. Amyloid-β and islet amyloid pathologies link Alzheimer's disease and type 2 diabetes in a transgenic model.

Aβ42-mediated proteasome inhibition and associated tau pathology in hippocampus are governed by a lysosomal response involving cathepsin B: Evidence for protective crosstalk between protein clearance pathways.

Impaired protein clearance likely increases the risk of protein accumulation disorders including Alzheimer’s disease (AD). Protein degradation through the proteasome pathway decreases with age and in AD brains, and the Aβ42 peptide has been shown to impair proteasome function in cultured cells and in a cell-free model. Here, Aβ42 was studied in brain tissue to measure changes in protein clearance pathways and related secondary pathology. Oligomerized Aβ42 (0.5–1.5 μM) reduced proteasome activity by 62% in hippocampal slice cultures over a 4-6-day period, corresponding with increased tau phosphorylation and reduced synaptophysin levels. Interestingly, the decrease in proteasome activity was associated with a delayed inverse effect, >2-fold increase, regarding lysosomal cathepsin B (CatB) activity. The CatB enhancement did not correspond with the Aβ42-mediated phospho-tau alterations since the latter occurred prior to the CatB response. Hippocampal slices treated with the proteasome inhibitor lactacystin also exhibited an inverse effect on CatB activity with respect to diminished proteasome function. Lactacystin caused earlier CatB enhancement than Aβ42, and no correspondence was evident between up-regulated CatB levels and the delayed synaptic pathology indicated by the loss of pre- and postsynaptic markers. Contrasting the inverse effects on the proteasomal and lysosomal pathways by Aβ42 and lactacystin, such were not found when CatB activity was up-regulated two-fold with Z-Phe-Ala-diazomethylketone (PADK). Instead of an inverse decline, proteasome function was increased marginally in PADK-treated hippocampal slices. Unexpectedly, the proteasomal augmentation was significantly pronounced in Aβ42-compromised slices, while absent in lactacystin-treated tissue, resulting in >2-fold improvement for nearly complete recovery of proteasome function by the CatB-enhancing compound. The PADK treatment also reduced Aβ42-mediated tau phosphorylation and synaptic marker declines, corresponding with the positive modulation of both proteasome activity and the lysosomal CatB enzyme. These findings indicate that proteasomal stress contributes to AD-type pathogenesis and that governing such pathology occurs through crosstalk between the two protein clearance pathways.

Cognitive Impairment and Brain and Peripheral Alterations in a Murine Model of Intraventricular Hemorrhage in the Preterm Newborn.

Germinal matrix hemorrhage-intraventricular hemorrhage (GMH-IVH) remains a serious complication in the preterm newborn. The significant increase of survival rates in extremelye preterm newborns has also contributed to increase the absolute number of patients developing GMH-IVH. However, there are relatively few available animal models to understand the underlying mechanisms and peripheral markers or prognostic tools. In order to further characterize central complications and evolution of GMH-IVH, we injected collagenase intraventricularly to P7 CD1 mice and assessed them in the short (P14) and the long term (P70). Early complications at P14 included ventricle enlargement, increased bleeding, and inflammation. These alterations were maintained at P70, when increased tau phosphorylation and decreased neurogenesis were also observed, resulting in impaired learning and memory in these early adult mice. We additionally analyzed peripheral blood biomarkers in both our mouse model and preterm newborns with GMH-IVH. While MMP9 levels were not significantly altered in mice or newborns, reduced gelsolin levels and increased ubiquitin carboxy-terminal hydrolase L1 and tau levels were detected in GMH-IVH patients at birth. A similar profile was observed in our mouse model after hemorrhage. Interestingly, early changes in gelsolin and carboxy-terminal hydrolase L1 levels significantly correlated with the hemorrhage grade in newborns. Altogether, our data support the utility of this animal model to reproduce the central complications and peripheral changes observed in the clinic, and support the consideration of gelsolin, carboxy-terminal hydrolase L1, and tau as feasible biomarkers to predict the development of GMH-IVH.

ON THE MECHANISM OF THE TAU R406W MUTATION

The tau-R406W mutation is observed to have a causative effective in neurodegeneration in human tauopathies, and is widely used to induce neurodegeneration in mouse models of Alzheimer's disease. Despite its clinical importance and wide use in research, the mechanism by which the R406W mutation leads to tau aggregation and neurodegeneration is not understood. We tested a series of hypotheses for potential mechanisms of functional effects of tau-R406W mutation: (1) increased hydrophobicity; (2) increased tau phosphorylation via increased activity by the kinases cdk5, GSK-3b, and/or PKA; (3) increased tau phosphorylation through decreased phosphorylation by the phosphatases PP2A or PP2B; (4) conformational change via increased cis amide bond formation due to a cis-proline-aromatic interaction. A series of peptides derived from the tau C-terminal domain was synthesized, in non-phosphorylated form and with phosphorylation at Ser396, Ser400, Thr403, Ser404, and/or Ser409. In addition, peptides were synthesized with R406 changed to Trp or other aromatic amino acids. Peptides were characterized by NMR spectroscopy. The R406W mutation did not increase phosphorylation by any kinase tested. Surprisingly, R406W led to increased dephosphorylation by PP2A, contrary to expectations. In contrast, the R406W mutation led to a substantial increase in cis amide bond at Ser404-Pro405, via a cis-proline-aromatic C-H/Π interaction that significantly stabilizes the cis proline amide bond. A significant increase in cis amide bond was also observed upon Ser404 phosphorylation, with the effects further modulated by phosphorylation at other sites. Additional structural changes dependent on phosphorylation and/or tauR406W were observed and characterized by NMR. The phosphorylation-dependent proline isomerism Pin1 has been strongly implicated in tau aggregation and Alzheimer's disease. These data suggest that tau phosphorylation is not directly increased by the R406W mutation. Instead, the data suggest induction of proline-405 cis amide bond formation and subsequent structural changes as a primary mechanism for the functional effects of the R406W mutation.

Insulin: To the periphery and beyond.

Increased tau phosphorylation resulting from insulin depletion is reversed by insulin treatment in a murine model.

High glucose induces formation of tau hyperphosphorylation via Cav-1-mTOR pathway: A potential molecular mechanism for diabetes-induced cognitive dysfunction.

The abnormally hyperphosphorylated tau is thought to be implicated in diabetes-associated cognitive deficits. The role of mammalian target of rapamycin (mTOR) / S6 kinase (S6K) signalling in the formation of tau hyperphosphorylation has been previously studied. Caveolin-1 (Cav-1), the essential structure protein of caveolae, promotes neuronal survival and growth, and inhibits glucose metabolism. In this study, we aimed to investigate the role of Cav-1 in the formation of tau hyperphosphorylation under chronic hyperglycemic condition (HGC). Diabetic rats were induced by streptozotocin (STZ). Primary hippocampal neurons with or without molecular intervention such as the transient over-expression or knock-down were subjected to HGC. The obtained experimental samples were analyzed by real time quantitative RT-PCR, Western blot, immunofluorescence or immunohistochemisty. We found: 1) that a chronic HGC directly decreases Cav-1 expression, increases tau phosphorylation and activates mTOR/S6K signalling in the brain neurons of diabetic rats, 2) that overexpression of Cav-1 attenuates tau hyperphosphorylation induced by chronic HGC in primary hippocampal neurons, whereas down-regulation of Cav-1 using Cav-1 siRNA dramatically worsens tau hyperphosphorylation via mTOR/S6K signalling pathway, and 3) that the down-regulation of Cav-1 induced by HGC is independent of mTOR signalling. Our results suggest that tau hyperphosphorylation and the sustained over-activated mTOR signalling under hyperglycemia may be due to the suppression of Cav-1. Therefore, Cav-1 is a potential therapeutic target for diabetes-induced cognitive dysfunction.

Insulin deficiency results in reversible protein kinase A activation and tau phosphorylation

Alzheimer's disease (AD) is a highly prevalent multifactorial disease for which Diabetes Mellitus (DM) is a risk factor. Abnormal phosphorylation and aggregation of tau is a key hallmark of AD. In animal models, DM induces or exacerbates the phosphorylation of tau, suggesting that DM may influence the risk at AD by directly facilitating tau pathology. Previously we reported that tau phosphorylation induced in response to metabolic stress is reversible. Since identification and understanding of early players in tau pathology is pivotal for therapeutic intervention, we here investigated the mechanism underlying tau phosphorylation in the diabetic brain and its potential for reversibility. To model DM we used streptozotocin-treatment to induce insulin deficiency in rats. Insulin depletion leads to increased tau phosphorylation in the brain and we investigated the activation status of known tau kinases and phosphatases in this model. We identified protein kinase A (PKA) as a tau kinase activated by DM in the brain. The potential relevance of this signaling pathway to AD pathogenesis is indicated by the increased level of active PKA in temporal cortex of early stage AD patients. Our data indicate that activation of PKA and tau phosphorylation are associated with insulin deficiency per se, rather than the downstream energy deprivation. In vitro studies confirm that insulin deficiency results in PKA activation and tau phosphorylation. Strikingly, both active PKA and induced tau phosphorylation are reversed upon insulin treatment in the steptozotocin animal model. Our data identify insulin deficiency as a direct trigger that induces the activity of the tau kinase PKA and results in tau phosphorylation. The reversibility upon insulin treatment underscores the potential of insulin as an early disease-modifying intervention in AD and other tauopathies.

Amyloid precursor protein reduction enhances the formation of neurofibrillary tangles in a mutant tau transgenic mouse model

Alzheimer's disease is characterized by the presence of 2 neuropathological lesions: neurofibrillary tangles, composed of tau proteins which are highly phosphorylated and phosphorylated on uncommon sites, and amyloid plaques, containing the Aß peptides generated from the amyloid precursor protein (APP). Reduction of some APP proteolytic derivatives in Alzheimer's disease such as sAPPα fragment has been reported and sAPPα has been shown to affect tau phosphorylation. To investigate in vivo the effect of absence of APP protein and its fragments on tau phosphorylation and the formation of neurofibrillary tangles, we have generated mice deleted for APP gene and overexpressing a human mutant tau protein and developing neurofibrillary tangles (APPKOTg30 mice). These APPKOTg30 mice showed more severe motor and cognitive deficits, increased tau phosphorylation, increased load of neurofibrillary tangles, and increased p25/35 ratio in the brain, compared with Tg30 mice. These data suggest that APP and/or its proteolytic derivatives interfere with the formation of neurofibrillary tangles in a transgenic mouse model that will be useful for investigating the relationship between APP and tau.