Tau pathology in Alzheimer disease and other tauopathies

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

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

Retinoic acid attenuates beta-amyloid deposition and rescues memory deficits in an Alzheimer's disease transgenic mouse model

Recent studies have revealed that disruption of vitamin A signaling observed in Alzheimer's disease (AD) leads to beta-amyloid (Abeta) accumulation and memory deficits in rodents. The aim of the present study was to evaluate the therapeutic effect of all-trans retinoic acid (ATRA), an active metabolite of vitamin A, on the neuropathology and deficits of spatial learning and memory in amyloid precursor protein (APP) and presenilin 1 (PS1) double-transgenic mice, a well established AD mouse model. Here we report a robust decrease in brain Abeta deposition and tau phosphorylation in the blinded study of APP/PS1 transgenic mice treated intraperitoneally for 8 weeks with ATRA (20 mg/kg, three times weekly, initiated when the mice were 5 months old). This was accompanied by a significant decrease in the APP phosphorylation and processing. The activity of cyclin-dependent kinase 5, a major kinase involved in both APP and tau phosphorylation, was markedly downregulated by ATRA treatment. The ATRA-treated APP/PS1 mice showed decreased activation of microglia and astrocytes, attenuated neuronal degeneration, and improved spatial learning and memory compared with the vehicle-treated APP/PS1 mice. These results support ATRA as an effective therapeutic agent for the prevention and treatment of AD.

The effects of NOS2 gene deletion on mice expressing mutated human AbetaPP

Nitric oxide synthase 2 (NOS2) and its gene product, inducible NOS (iNOS) play an important role in neuroinflammation by generating nitric oxide (NO), a critical signaling and redox factor in the brain. Although NO is associated with tissue damage, it can also promote cell survival. We hypothesize that during long-term exposure to amyloid-beta (Abeta) in Alzheimer's disease (AD), NO levels fall in the brain to a threshold at which the protective effects of NO cannot be sustained, promoting Abeta mediated damage. Two new mouse models of AD have been developed that utilize this concept of NO's action. These mice express human amyloid-beta protein precursor (AbetaPP) mutations that generate Abeta peptides on a mouse NOS2 knockout background. The APP/NOS2(-/-) bigenic mice progress from Abeta production and amyloid deposition to hyperphosphorylated normal mouse tau at AD-associated epitopes, aggregation and redistribution of tau to somatodendritic regions of neurons and significant neuronal loss including loss of interneurons. This AD-like pathology is accompanied by robust behavioral changes. As APP/NOS2(-/-) bigenic mice more fully model the human AD disease pathology, they may serve as a tool to better understand disease progression in AD and the role of NO in altering chronic neurological disease processes.

Pharmacotherapeutic targets in Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder which is characterized by an increasing impairment in normal memory and cognitive processes that significantly diminishes a person's daily functioning. Despite decades of research and advances in our understanding of disease aetiology and pathogenesis, there are still no effective disease-modifying drugs available for the treatment of AD. However, numerous compounds are currently undergoing pre-clinical and clinical evaluations. These candidate pharma-cotherapeutics are aimed at various aspects of the disease, such as the microtubule-associated τ-protein, the amyloid-β (Aβ) peptide and metal ion dyshomeostasis – all of which are involved in the development and progression of AD. We will review the way these pharmacological strategies target the biochemical and clinical features of the disease and the investigational drugs for each category.

No more brain tangles with DeltaNp73

In Alzheimer's disease (AD), neurons suffer dysfunction and death associated with aberrant tau phosphorylation and subsequent neurofibrillary tangles. A new study reveals a surprising neuroprotective role for a truncated p73 isoform (DeltaNp73). Aged mice with reduced DeltaNp73 levels exhibit tau pathology and cognitive deficits, and DeltaNp73 reduction in mice with amyloid pathology causes extensive tangle formation and neuron death. These findings provide a novel animal model of AD and a potential therapeutic role for DeltaNp73 inducers.

Phosphorylation of tau at Ser214 mediates its interaction with 14-3-3 protein: implications for the mechanism of tau aggregation

The microtubule associated protein tau is a major component of neurofibrillary tangles in Alzheimer disease brain, however the neuropathological processes behind the formation of neurofibrillary tangles are still unclear. Previously, 14-3-3 proteins were reported to bind with tau. 14-3-3 Proteins usually bind their targets through specific serine/threonine -phosphorylated motifs. Therefore, the interaction of tau with 14-3-3 mediated by phosphorylation was investigated. In this study, we show that the phosphorylation of tau by either protein kinase A (PKA) or protein kinase B (PKB) enhances the binding of tau with 14-3-3 in vitro. The affinity between tau and 14-3-3 is increased 12- to 14-fold by phosphorylation as determined by real time surface plasmon resonance studies. Mutational analyses revealed that Ser214 is critical for the phosphorylation-mediated interaction of tau with 14-3-3. Finally, in vitro aggregation assays demonstrated that phosphorylation by PKA/PKB inhibits the formation of aggregates/filaments of tau induced by 14-3-3. As the phosphorylation at Ser214 is up-regulated in fetal brain, tau's interaction with 14-3-3 may have a significant role in the organization of the microtubule cytoskeleton in development. Also as the phosphorylation at Ser214 is up-regulated in Alzheimer's disease brain, tau's interaction with 14-3-3 might be involved in the pathology of this disease.

Neurofibrillary degeneration in Alzheimer's disease: from molecular mechanisms to identification of drug targets

PURPOSE OF REVIEW

Great progress has been made in understanding the pathogenesis of neurofibrillary degeneration in Alzheimer's disease brains in the last two decades. In this review we summarize how neurons are degenerated in Alzheimer's disease brains and highlight the evidence of using kinases such as glycogen synthase kinase 3 and p70 S6 kinase and phosphatases such as protein phosphatase 2A as drug targets to prohibit the formation of neurofibrillary degeneration of Alzheimer's disease.

RECENT FINDINGS

In general there are two types of neuronal degeneration in Alzheimer's disease brains: neurofibrillary formation and apoptosis. The microtubule-associated protein tau that stabilizes neuronal microtubules under normal physiological conditions is abnormally hyperphosphorylated in Alzheimer's disease brains, resulting in the generation of aberrant aggregates that are toxic to neurons. The processes of tau hyperphosphorylation and the formation of neurofibrillary tangles are caused by the imbalance of the activities of protein kinases and protein phosphatases in Alzheimer's disease brains. Recent findings from our and other groups have suggested glycogen synthase kinase 3 and p70 S6 kinase as main tau kinases and protein phosphatase 2A as the main tau phosphatase involved in the formation of these processes. Activities of these targets are implicated by Abeta peptide, the major component of another hallmark in Alzheimer's disease brains, senile plaques.

SUMMARY

To prevent the clinical progression of neurodegeneration, a combination strategy is suggested to target both senile plaques with immunization and neurofibrillary tangles with drugs to prevent the synthesis and phosphorylation of tau.

Co-localization of hyperphosphorylated tau and caspases in the brainstem of Alzheimer's disease patients

Hyperphosphorylation of microtubule associated protein tau had limited studies in Alzheimer's disease (AD) brainstem. We compared the distribution and number of neurons with hyperphosphorylated tau in two age groups of AD brainstems with mean ages of 65.4 +/- 5.7 and 91.1 +/- 6.4 years. The degree of co-localization of hyperphosphorylated tau positive cells with either cleaved caspase-3 or cleaved caspase-6 was also quantified. Results showed hyperphosphorylated tau mainly occurred in hypoglossal, dorsal motor vagal, trigeminal sensory/motor nuclei as well as in dorsal raphe, locus coeruleus and substantia nigra. Older AD brainstem consistently had higher density of hyperphosphorylated tau cells. Up to 70% of tau positive cells also displayed either cleaved caspase-3 or caspase-6, and the number of co-localized tau cells in each caspase subfamily group was always higher in older aged group. Some hyperphosphorylated tau cells with cleaved caspases had TUNEL positive nuclei. These findings suggest that these latter cells went through the apoptotic process or DNA fragmentation.

Relevance of transgenic mouse models to human Alzheimer disease

During the past 2 decades, the elucidation of susceptibility and causative genes for Alzheimer disease as well as proteins involved in the pathogenic process has greatly facilitated the development of genetically altered mouse models. These models have played a major role in defining critical disease-related mechanisms and in evaluating novel therapeutic approaches, with many treatments currently in clinical trial owing their origins to studies initially performed in mice. This review discusses the utility of transgenic mice as a research tool and their contributions to our understanding of Alzheimer disease.

Increased dosage of Dyrk1A alters alternative splicing factor (ASF)-regulated alternative splicing of tau in Down syndrome

Two groups of tau, 3R- and 4R-tau, are generated by alternative splicing of tau exon 10. Normal adult human brain expresses equal levels of them. Disruption of the physiological balance is a common feature of several tauopathies. Very early in their life, individuals with Down syndrome (DS) develop Alzheimer-type tau pathology, the molecular basis for which is not fully understood. Here, we demonstrate that Dyrk1A, a kinase encoded by a gene in the DS critical region, phosphorylates alternative splicing factor (ASF) at Ser-227, Ser-234, and Ser-238, driving it into nuclear speckles and preventing it from facilitating tau exon 10 inclusion. The increased dosage of Dyrk1A in DS brain due to trisomy of chromosome 21 correlates to an increase in 3R-tau level, which on abnormal hyperphosphorylation and aggregation of tau results in neurofibrillary degeneration. Imbalance of 3R- and 4R-tau in DS brain by Dyrk1A-induced dysregulation of alternative splicing factor-mediated alternative splicing of tau exon 10 represents a novel mechanism of neurofibrillary degeneration and may help explain early onset tauopathy in individuals with DS.

Language performance in Alzheimer's disease and mild cognitive impairment: a comparative review

Mild cognitive impairment (MCI) manifests as memory impairment in the absence of dementia and progresses to Alzheimer's disease (AD) at a rate of around 15% per annum, versus 1-2% in the general population. It thus constitutes a primary target for investigation of early markers of AD. Language deficits occur early in AD, and performance on verbal tasks is an important diagnostic criterion for both AD and MCI. We review language performance in MCI, compare these findings to those seen in AD, and identify the primary issues in understanding language performance in MCI and selecting tasks with diagnostic and prognostic value.

Role of Dickkopf-1, an antagonist of the Wnt/beta-catenin signaling pathway, in estrogen-induced neuroprotection and attenuation of tau phosphorylation

17beta-Estradiol (E2) has been implicated to be neuroprotective in a variety of neurodegenerative disorders, although the mechanism remains poorly understood. The current study sheds light on this issue by demonstrating that low physiological levels of E2 protects the hippocampus CA1 against global cerebral ischemia by preventing elevation of dickkopf-1 (Dkk1), an antagonist of the Wnt/beta-catenin signaling pathway, which is a principal mediator of neurodegeneration in cerebral ischemia and Alzheimer's disease. E2 inhibition of Dkk1 elevation correlated with a reduction of phospho-beta-catenin and elevation of nuclear beta-catenin levels, as well as enhancement of Wnt-3, suggesting E2 activation of the Wnt/beta-catenin signaling pathway. In agreement, the beta-catenin downstream prosurvival factor, survivin, was induced by E2 at 24 and 48 h after cerebral ischemia, an effect observed only in surviving neurons because degenerating neurons lacked survivin expression. E2 suppression of Dkk1 elevation was found to be caused by attenuation of upstream c-Jun N-terminal protein kinase (JNK)/c-Jun signaling, as E2 attenuation of JNK/c-Jun activation and a JNK inhibitor significantly blocked Dkk1 induction. Tau hyperphosphorylation has been implicated to have a prodeath role in Alzheimer's disease and cerebral ischemia, and E2 attenuates tau hyperphosphorylation. Our study demonstrates that tau hyperphosphorylation is strongly induced after global cerebral ischemia, and that E2 inhibits tau hyperphosphorylation by suppressing activation of the JNK/c-Jun/Dkk1 signaling pathway. Finally, exogenous Dkk1 replacement via intracerebroventricular administration completely reversed E2-induced neuroprotection, nuclear beta-catenin induction, and phospho-tau attenuation, further suggesting that E2 inhibition of Dkk1 is a critical mechanism underlying its neuroprotective and phospho-tau regulatory effects after cerebral ischemia.

Biomarkers for early diagnosis of Alzheimer disease: 'ALZheimer ASsociated gene'--a new blood biomarker?

Simple, non-invasive tests for an early detection of degenerative dementia by use of biomarkers are urgently required. However, up to the present, no validated extracerebral diagnostic markers (plasma/serum, platelets, urine, connective tissue) for the early diagnosis of Alzheimer disease (AD) are available. In disease stages with evident cognitive disturbances, the clinical diagnosis of probable AD is made with around 90% accuracy using modern clinical, neuropsychological and imaging methods. Diagnostic sensitivity and specificity even in early disease stages are improved by CSF markers, in particular combined tau and amyloid beta peptides (Abeta) and plasma markers (eg, Abeta-42/Abeta-40 ratio). Recently, a novel gene/protein--ALZAS (Alzheimer Associated Protein)--with a 79 amino acid sequence, containing the amyloid beta-42 fragment (Abeta-42), the amyloid precursor protein (APP) transmembrane signal and a 12 amino acid C-terminal, not present in any other known APP alleles, has been discovered on chromosome 21 within the APP region. Reverse transcriptase-PCR revealed the expression of the transcript of this protein in the cortex and hippocampal regions as well as in lymphocytes of human AD patients. The expression of ALZAS is mirrored by a specific autoimmune response in AD patients, directed against the ct-12 end of the ALZAS-peptide but not against the Abeta-sequence. ELISA studies of plasma detected highest titers of ALZAS in patients with mild cognitive impairment (presymptomatic AD), but only moderately increased titers in autopsy-confirmed AD, whereas low or undetectable ct-12 titers were found in cognitively intact age-matched subjects and young controls. The antigen, ALZAS protein, was detected in plasma in later clinical stages of AD. It is suggested that ALZAS represents an indicator in a dynamic equilibrium between both peripheral and brain degenerative changes in AD and may become a useful "non-invasive" diagnostic marker via a simple blood test.

Pharmacogenomics and treatment for dementia induced by Alzheimer’s disease

Pharmacogenomics is the study of interindividual genetic variability, which plays a significant role in defining drug response and toxicity. As research has graduated from studying single candidate genes to whole-genome scans, pharmacogenomics is beginning to make its impact on the therapeutics of complex CNS disorders, such as schizophrenia, Parkinson’s disease and Alzheimer’s disease. Alzheimer’s disease is a progressive complex disorder, where genetic predisposition interacts with environmental factors. With conventional therapeutics only providing symptomatic treatment, the current focus of the pharmaceutical industry is on novel strategies with an etiopathogenic orientation. In this review, we have summarized the current knowledge of pathogenetic mechanisms of Alzheimer’s disease, with a focus on the known relevant molecules and the potential of pharmacogenomics in translating this knowledge of human genome variability into efficacious and safer therapeutics.

Phosphoproteomic analysis of human brain by calcium phosphate precipitation and mass spectrometry

Alzheimer's disease (AD), the most common form of dementia, is manifested in the brain by the aggregation of amyloid plaques and neurofibrillary tangles. The tangles are primarily composed of microtubule-associated protein tau that is aberrantly hyperphosphorylated, suggesting that deregulated phosphorylation may contribute to AD pathogenesis. However, systematic analysis of the phosphoproteome in AD brain tissues has not been reported. We used calcium phosphate precipitation to analyze an AD postmortem brain, followed by liquid chromatography-tandem mass spectrometry. The protein sample was first resolved by one-dimensional polyacrylamide gel electrophoresis and subjected to gel excision and in-gel digestion. Phosphopeptides in the resulting peptide mixtures were enriched in a single step of calcium phosphate precipitation, and then analyzed by the LC-MS/MS approach. After database search, stringent filtering, and manual validation of neutral loss in the MS/MS spectra, a total of 466 phosphorylation sites on 185 proteins including tau were identified. A majority of sites were not described previously. This study demonstrates the feasibility of combining calcium phosphate precipitation with mass spectrometry for phosphoproteome analysis of postmortem human brain tissue.

Biomarkers of Mild Cognitive Impairment and Alzheimer’s Disease

Alzheimer’s disease (AD) is currently diagnosed only via clinical assessments and confirmed by postmortem brain pathology. Biochemical and neuroimaging markers could facilitate diagnosis, predict AD progression from a pre-AD state of mild cognitive impairment (MCI), and be used to monitor efficacies of disease-modifying therapies. It is now clear that cerebrospinal fluid (CSF) levels of Aβ40, Aβ42, total tau and phosphorylated tau have diagnostic values in AD. Measurements of the above CSF markers in combination are useful in predicting the risk of progression from MCI to AD. Recent advances further support a notion that plasma Aβ levels, expressed as an Aβ42/Aβ40 ratio, could also be of value. New potential biomarkers are emerging, and CSF or plasma marker profiles may eventually become part of the clinician’s toolkit for accurate AD diagnosis and management. These biomarkers, along with clinical assessment, neuropsychological testing and neuroimaging could achieve a much higher diagnostic accuracy for AD and related disorders in the future. Key words: Alzheimer’s disease, b-amyloid (Ab), Biomarkers, Mild cognitive impairment (MCI), Tau

A novel transgenic mouse expressing double mutant tau driven by its natural promoter exhibits tauopathy characteristics

The neurofibrillary-tangles (NTFs), characteristic of tauopathies including Alzheimer's-disease (AD), are the pathological features which correlate best with dementia. The objective of our study was to generate an authentic transgenic (tg) animal model for NFT pathology in tauopathy/AD. Previous NFT-tg mice were driven by non-related/non-homologous promoters. Our strategy was to use the natural tau promoter for expressing the human-tau (htau) gene with two mutations K257T/P301S (double mutant, DM) associated with severe phenotypes of frontotemporal-dementia in humans. Cellular, biochemical, behavioral and electrophysiological studies were subsequently conducted. The tg mice showed a tolerated physiological level of the DM-htau protein, mostly in cortex and hippocampus. The mice demonstrated tauopathy-like characteristics, which increased with age, that included NFT-related pathology, astrogliosis, argyrophilic plaque-like (amyloid-free) structures in brain, with memory deficits and signs of anxiety. Moreover, the tg mice showed a robust synaptic plasticity deficit selectively expressed in a severe impairment in their ability to maintain hippocampal long-term-potentiation (LTP) in response to stimulation of the perforant path, providing evidence that "tau-pathology only" is sufficient to cause this memory and learning-associated deficit. This is a unique mutant-htau-tg model which presents a wide spectrum of features characteristic of tauopathy/AD, which does not show unrelated motor deficits described in other models of tauopathy. In addition, expressing the DM-htau in a neuronal cell model resulted in tau-aggregation, as well as impaired microtubule arrangement. Both animal and cell models, which were regulated under the natural tau promoter (of rat origin), provide authentic and reliable models for tauopathy, and offer valuable tools for understanding the molecular events underlying tauopathies including AD.

CSF phosporylated TAU protein levels correlate with cerebral glucose metabolism assessed with PET in Alzheimer's disease

One major goal of drug development would be the establishment of biomarkers as objective indicators of normal biological and pathogenetic processes, or pharmacological response to a therapeutic intervention. A potential approach is to investigate proteins in CSF linked to key neuropathological features of Alzheimer's disease (AD). Recently CSF phosphorylated-Tau (p-Tau) levels have been reported to reflect neurofibrillary changes within the brain of patients with AD, however the use of serial CSF investigations in order to monitor the disease progression is not applicable. PET with FDG reveals characteristic patterns in AD patients, however so far no correlation between in vivo metabolic information and pathological features has been reported. In the present study, we tested whether CSF Tau levels correlate with metabolic rate for glucose consumption in a cohort of 28 AD patients. We found a statistically significative correlation between both CSF total and p-TAU protein and relative metabolic indexes obtained from 18FDG-PET scans in parietal, temporal and occipital lobes bilaterally. These results indicate the existence of a correlation between impairment of cerebral metabolism, estimated throughout FDG-PET, and CSF Tau protein levels.

The ubiquitin-proteasome system in Alzheimer's disease

Accumulation of proteins is a recurring event in many neurodegenerative diseases, including Alzheimer's disease (AD).Evidence has suggested that protein accumulation may result from a dysfunction in the ubiquitin proteasome system (UPS). Indeed, there is clear genetic and biochemical evidence of an involvement of the ubiquitin proteasome system in AD. This review summarizes the data supporting an involvement of the UPS in the pathogenesis of AD, focusing on the data showing the relationship between Aβ and tau, the two hallmark lesions of AD, and the UPS.

I1PP2A affects tau phosphorylation via association with the catalytic subunit of protein phosphatase 2A

In Alzheimer disease (AD) brain, the level of I (1)(PP2A), a 249-amino acid long endogenous inhibitor of protein phosphatase 2A (PP2A), is increased, the activity of the phosphatase is decreased, and the microtubule-associated protein Tau is abnormally hyperphosphorylated. However, little is known about the detailed regulatory mechanism by which PP2A activity is inhibited by I (1)(PP2A) and the consequent events in mammalian cells. In this study, we found that both I (1)(PP2A) and its N-terminal half I (1)(PP2A(1-120)), but neither I (1)(PP2A(1-163)) nor I (1)(PP2A(164-249)), inhibited PP2A activity in vitro, suggesting an autoinhibition by amino acid residues 121-163 and its neutralization by the C-terminal region. Furthermore, transfection of NIH3T3 cells produced a dose-dependent inhibition of PP2A activity by I (1)(PP2A)(1). I (PP2A) and PP2A were found to colocalize in PC12 cells. I (1)(PP2A) could only interact with the catalytic subunit of PP2A (PP2Ac) and had no interaction with the regulatory subunits of PP2A (PP2A-A or PP2A-B) using a glutathione S-transferase-pulldown assay. The interaction was further confirmed by coimmunoprecipitation of I (1)(PP2A) and PP2Ac from lysates of transiently transfected NIH3T3 cells. The N-terminal isotype specific region of I (1)(PP2A) was required for its association with PP2Ac as well as PP2A inhibition. In addition, the phosphorylation of Tau was significantly increased in PC12/Tau441 cells transiently transfected with full-length I (1)(PP2A) and with PP2Ac-interacting I (1)(PP2A) deletion mutant 1-120 (I (1)(PP2A)DeltaC2). Double immunofluorescence staining showed that I (1)(PP2A) and I (1)(PP2A)DeltaC2 increased Tau phosphorylation and impaired the microtubule network and neurite outgrowth in PC12 cells treated with nerve growth factor.

Alzheimer neurofibrillary degeneration: significance, etiopathogenesis, therapeutics and prevention

Alzheimer disease (AD) is multi-factorial and heterogeneous. Independent of the aetiology, this disease is characterized clinically by chronic and progressive dementia and histopathologically by neurofibrillary degeneration of abnormally hyperphosphorylated tau seen as intraneuronal neurofibrillary tangles, neuropil threads and dystrophic neurites, and by neuritic (senile) plaques of beta-amyloid. The neurofibrillary degeneration is apparently required for the clinical expression of AD, and in related tauopathies it leads to dementia in the absence of amyloid plaques. While normal tau promotes assembly and stabilizes microtubules, the abnormally hyperphosphorylated tau sequesters normal tau, MAP1 and MAP2, and disrupts microtubules. The abnormal hyperphosphorylation of tau also promotes its self-assembly into tangles of paired helical and or straight filaments. Tau is phosphorylated by a number of protein kinases. Glycogen synthase kinase-3 (GSK-3) and cyclin dependent protein kinase 5 (cdk5) are among the kinases most implicated in the abnormal hyperphosphorylation of tau. Among the phosphatases which regulate the phosphorylation of tau, protein phosphatase-2A (PP-2A), the activity of which is down-regulated in AD brain, is by far the major enzyme. The inhibition of abnormal hyperphosphorylation of tau is one of the most promising therapeutic targets for the development of disease modifying drugs. A great advantage of inhibiting neurofibrillary degeneration is that it can be monitored by evaluating the levels of total tau and tau phosphorylated at various known abnormally hyperphosphorylated sites in the cerebrospinal fluid of patients, obtained by lumbar puncture. There are at least five subgroups of AD, each is probably caused by a different etiopathogenic mechanism. The AD subgroup identification of patients can help increase the success of clinical trials and the development of specific and potent disease modifying drugs.

Neuroprotective actions of 2,4-dinitrophenol: Friend or foe?

2,4-dinitrophenol (DNP) has long been known to be toxic at high concentrations, an effect related to uncoupling of mitochondrial oxidative phosphorylation. Five years ago, however, we reported that low concentrations of DNP protect neurons against the toxicity of the amyloid-β peptide. Since then, a number of other studies have provided evidence of beneficial actions of DNP (at low concentrations), including neuroprotection against different types of insult, blockade of amyloid aggregation, stimulation of neurite outgrowth and neuronal differentiation, and even extension of lifespan in certain organisms. Some of these effects appear due to mild mitochondrial uncoupling and prevention of oxidative stress, whereas other actions are related to activation of additional intracellular signaling pathways. This study discusses the evidence supporting beneficial neuroprotective actions of DNP. DNP and other compounds with similar biological activities may be of interest in the development of novel therapeutic approaches for neurodegenerative diseases and other neurological disorders.

Site-specific effects of tau phosphorylation on its microtubule assembly activity and self-aggregation

Microtubule-associated protein tau is abnormally hyperphosphorylated and aggregated into neurofibrillary tangles in brains with Alzheimer's disease. The phosphorylation sites of tau are mainly localized in the proline-rich (residues 172-251) and C-terminal tail (residues 368-441) regions, which flank the microtubule-binding repeats. Here, we investigated the effects of tau phosphorylation at these distinct sites/regions on its activity of stimulating microtubule assembly and its self-aggregation. We found that tau phosphorylation at the proline-rich region by dual-specificity tyrosine-phosphorylated and -regulated kinase 1A inhibited its microtubule assembly activity moderately and promoted its self-aggregation slightly. Tau phosphorylation at the C-terminal tail region by glycogen synthase kinase-3beta increased its activity and promoted its self-aggregation markedly. Tau phosphorylation at both regions plus the microtubule-binding region by cAMP-dependent protein kinase diminished its activity (approximately 70% inhibition) and disrupted microtubules. These studies reveal the differential regulation of tau's biological activity and self-aggregation by phosphorylation at various sites/regions.

Efficacy of small-molecule glycogen synthase kinase-3 inhibitors in the postnatal rat model of tau hyperphosphorylation

BACKGROUND AND PURPOSE

Glycogen synthase kinase-3 (GSK-3) affects neuropathological events associated with Alzheimeŕs disease (AD) such as hyperphosphorylation of the protein, tau. GSK-3beta expression, enzyme activity and tau phosphorylated at AD-relevant epitopes are elevated in juvenile rodent brains. Here, we assess five GSK-3beta inhibitors and lithium in lowering phosphorylated tau (p-tau) and GSK-3beta enzyme activity levels in 12-day old postnatal rats.

EXPERIMENTAL APPROACH

Brain levels of inhibitors following treatment in vivo were optimized based on pharmacokinetic data. At optimal doses, p-tau (Ser(396)) levels in brain tissue was measured by immunoblotting and correlated with GSK-3beta enzyme activities in the same tissues. Effects of GSK inhibitors on p-tau, GSK-3beta activities and cell death were measured in a human neuronal cell line (LUHMES).

KEY RESULTS

Lithium and CHIR98014 reduced tau phosphorylation (Ser(396)) in the cortex and hippocampus of postnatal rats, while Alsterpaullone and SB216763 were effective only in hippocampus. AR-A014418 and Indirubin-3'-monoxime were ineffective in either brain region. Inhibition of p-tau in brain required several-fold higher levels of GSK inhibitors than the IC(50) values obtained in recombinant or cell-based GSK-3beta enzyme activity assays. The inhibitory effect on GSK-3beta activity ex vivo correlated with protection against cell death and decrease of p-tau- in LUHMES cells, using low microM inhibitor concentrations.

CONCLUSIONS AND IMPLICATIONS

Selective small-molecule inhibitors of GSK-3 reduce tau phosphorylation in vivo. These findings corroborate earlier suggestions that GSK-3beta may be an attractive target for disease-modification in AD and related conditions where tau phosphorylation is believed to contribute to disease pathogenesis.

Developing pharmacological therapies for Alzheimer disease

Alzheimer disease (AD), while chronic and progressive with an average progression of 7 - 10 years, is both multifactorial and heterogeneous. Thus, AD offers a large window of opportunity and a large number of therapeutic targets to inhibit it. The selection of a therapeutic target, however, is one of the biggest challenges in developing a pharmacological treatment of this multifactorial disease. Inhibition of a pivotal downstream event is likely to benefit more patients than inhibition of an upstream event in AD pathogenesis. Neurofibrillary degeneration of abnormally hyperphosphorylated tau offers such a pivotal therapeutic target. Abnormal hyperphosphorylation of tau and not its aggregation into filaments appears to be the most deleterious step in neurofibrillary degeneration. Tau can be abnormally hyperphosphorylated by downregulation of protein phosphatase-2A activity or by upregulation of more than one tau kinase. Restoration of the phosphatase activity which is downregulated in AD brain or inhibition of GSK-3beta and cdk5, which are required for AD-type abnormal hyperphosphorylation of tau, are among the most promising therapeutic strategies.

14-3-3 proteins and protein phosphatases are not reduced in tau-deficient mice

Tau is an axonal microtubule-associated protein, whose dysfunction causes neurodegenerative diseases such as Alzheimer's disease and other tauopathies. Earlier studies have shown the interactions of tau with glycogen synthase kinase-3beta, 14-3-3zeta, protein phosphatase 1 and protein phosphatase 2A. In this study, we compared the amounts of these tau-interacting proteins in brain microtubule-enriched fractions from wild-type and tau-deficient mice. Contrary to our expectation, we detected no difference in the amount of these proteins between wild-type and tau-deficient mice. Our findings indicate that only a small portion of tau-interacting proteins are bound to tau in vivo, and suggest the existence of other scaffolding proteins. We propose that tau-deficient mice are an ideal system for confirming the function of tau-interacting proteins.

Regulation between O-GlcNAcylation and phosphorylation of neurofilament-M and their dysregulation in Alzheimer disease

The medium subunit of neurofilament (NF-M) is extensively modified by phosphate and O-linked beta-N-acetylglucosamine (O-GlcNAc). Phosphorylation of NF-M plays a critical role in regulating its translocation, filament formation, and function. However, the regulation of NF-M phosphorylation and the role of NF-M O-GlcNAcylation (a modification by which GlcNAc is attached to the serine/threonine residues of a protein via an O-linked glycosidic bond) are largely unknown. Here, we demonstrate that O-GlcNAcylation and phosphorylation of NF-M regulate each other reciprocally in cultured neuroblastoma cells and in metabolically active rat brain slices. In animal models of fasting rats, which mimicked the decreased glucose uptake/metabolism observed in brains of individuals with Alzheimer disease (AD), we found a decrease in O-GlcNAcylation and increase in phosphorylation of NF-M. We also observed decreased O-GlcNAcylation and an increased phosphorylation of NF-M in AD brain. These results suggest that O-GlcNAcylation and phosphorylation of NF-M are regulated reciprocally and that the hyperphosphorylation and accumulation of NF-M in AD brain might be caused by impaired brain glucose uptake/metabolism via down-regulation of NF-M O-GlcNAcylation.

A comparison of the neuronal dysfunction caused by Drosophila tau and human tau in a Drosophila model of tauopathies

Hyperphosphorylation and aggregation of tau into tangles is a feature of disorders such as Alzheimer's disease and other Tauopathies. To model these disorders in Drosophila melanogaster, human tau has been over-expressed and a variety of phenotypes have been observed including neurotoxicity, disrupted neuronal and synaptic function and locomotor impairments. Neuronal dysfunction has been seen prior to neuronal death and in the absence of tangle formation. The Drosophila tau protein shares a large degree of homology with human tau but differs in the crucial microtubule binding domains. Although like human tau Drosophila tau can induce neurotoxicity, little is known about its ability to disrupt neuronal function. In this study we demonstrate that like human tau, over-expression of Drosophila tau results in disrupted axonal transport, altered neuromuscular junction morphology and locomotor impairments. This indicates that like human tau, over-expression of Drosophila tau compromises neuronal function despite significant differences in microtubule binding regions.

The modulation of metal bio-availability as a therapeutic strategy for the treatment of Alzheimer's disease

The postmortem Alzheimer's disease brain is characterized histochemically by the presence of extracellular amyloid plaques and neurofibrillary tangles. Also consistent with the disease is evidence for chronic oxidative damage within the brain. Considerable research data indicates that these three critical aspects of Alzheimer's disease are interdependent, raising the possibility that they share some commonality with respect to the ever elusive initial factor(s) that triggers the development of Alzheimer's disease. Here, we discuss reports that show a loss of metal homeostasis is also an important event in Alzheimer's disease, and we identify how metal dyshomeostasis may contribute to development of the amyloid-beta, tau and oxidative stress biology of Alzheimer's disease. We propose that therapeutic agents designed to modulate metal bio-availability have the potential to ameliorate several of the dysfunctional events characteristic of Alzheimer's disease. Metal-based therapeutics have already provided promising results for the treatment of Alzheimer's disease, and new generations of pharmaceuticals are being developed. In this review, we focus on copper dyshomeostasis in Alzheimer's disease, but we also discuss zinc and iron.

A Caenorhabditis elegans model of tau hyperphosphorylation: induction of developmental defects by transgenic overexpression of Alzheimer's disease-like modified tau

The microtubule-associated tau proteins become functionally and structurally altered in Alzheimer's disease (AD). To analyze tau modification and its role in a non-vertebrate animal model, we produced transgenic Caenorhabditis elegans strains with a panneuronal expression of human tau and a pseudohyperphosphorylated (PHP) tau construct that mimics AD-relevant tau modification. We show that human tau in C. elegans becomes highly phosphorylated and exhibits conformational changes similar to PHP tau and human PHF tau. Both, wt tau and PHP tau induced a progressive age-dependent development of a phenotype of uncoordinated locomotion (unc) in the absence of neuronal degeneration. However, only PHP tau induced a defective pattern of motor neuron development as indicated by the presence of gaps in the dorsal cord, commissures on the wrong side and local broadening of axons. The data indicate that C. elegans is capable of highly phosphorylating human tau to an AD-like state whereas only stable disease-like tau modification induce developmental defects suggesting a specific interference of pathologic tau with intracellular mechanisms of axonal outgrowth and pathfinding.

Mitochondrial oxidative stress causes hyperphosphorylation of tau

Age-related neurodegenerative disease has been mechanistically linked with mitochondrial dysfunction via damage from reactive oxygen species produced within the cell. We determined whether increased mitochondrial oxidative stress could modulate or regulate two of the key neurochemical hallmarks of Alzheimer's disease (AD): tau phosphorylation, and beta-amyloid deposition. Mice lacking superoxide dismutase 2 (SOD2) die within the first week of life, and develop a complex heterogeneous phenotype arising from mitochondrial dysfunction and oxidative stress. Treatment of these mice with catalytic antioxidants increases their lifespan and rescues the peripheral phenotypes, while uncovering central nervous system pathology. We examined sod2 null mice differentially treated with high and low doses of a catalytic antioxidant and observed striking elevations in the levels of tau phosphorylation (at Ser-396 and other phospho-epitopes of tau) in the low-dose antioxidant treated mice at AD-associated residues. This hyperphosphorylation of tau was prevented with an increased dose of the antioxidant, previously reported to be sufficient to prevent neuropathology. We then genetically combined a well-characterized mouse model of AD (Tg2576) with heterozygous sod2 knockout mice to study the interactions between mitochondrial oxidative stress and cerebral Ass load. We found that mitochondrial SOD2 deficiency exacerbates amyloid burden and significantly reduces metal levels in the brain, while increasing levels of Ser-396 phosphorylated tau. These findings mechanistically link mitochondrial oxidative stress with the pathological features of AD.

Stepwise proteolysis liberates tau fragments that nucleate the Alzheimer-like aggregation of full-length tau in a neuronal cell model

Tau is a highly soluble protein, yet it aggregates abnormally in Alzheimer's disease. Here, we address the question of proteolytic processing of tau and the nucleation of aggregates by tau fragments. We show in neuronal cell models that fragments of the repeat domain of tau containing mutations of FTDP17 (frontotemporal dementia with parkinsonism linked to chromosome 17), produced by endogenous proteases, can induce the aggregation of full-length tau. Fragments are generated by successive cleavages, first N-terminally between K257 and S258, then C-terminally around residues 353-364; conversely, when the N-terminal cleavage is inhibited, no fragmentation and aggregation takes place. The C-terminal truncation and the coaggregation of fragments with full-length tau depends on the propensity for beta-structure. The aggregation is modulated by phosphorylation but does not depend on it. Aggregation but not fragmentation as such is toxic to cells; conversely, toxicity can be prevented by inhibiting either aggregation or proteolysis. The results reveal a novel pathway of abnormal tau aggregation in neuronal cells.

Reducing endogenous tau ameliorates amyloid beta-induced deficits in an Alzheimer's disease mouse model

Many potential treatments for Alzheimer's disease target amyloid-beta peptides (Abeta), which are widely presumed to cause the disease. The microtubule-associated protein tau is also involved in the disease, but it is unclear whether treatments aimed at tau could block Abeta-induced cognitive impairments. Here, we found that reducing endogenous tau levels prevented behavioral deficits in transgenic mice expressing human amyloid precursor protein, without altering their high Abeta levels. Tau reduction also protected both transgenic and nontransgenic mice against excitotoxicity. Thus, tau reduction can block Abeta- and excitotoxin-induced neuronal dysfunction and may represent an effective strategy for treating Alzheimer's disease and related conditions.

Drosophila lacking the Cdk5 activator, p35, display defective axon guidance, age-dependent behavioral deficits and reduced lifespan

The cyclin-dependent kinase Cdk5 has attracted a great deal of attention both because of its roles in cell migration and axon patterning, and the extensive data implicating it in adult-onset neurodegeneration in mammals. Both the kinase activity and the biological effects of Cdk5 are absolutely dependent on association with an activating subunit, called p35. We show here that Drosophila lacking the Cdk5 activator, D-p35, display a wide range of defects in embryonic axon patterning. We further show that, while viable and fertile, p35 mutant adults display progressive, age-dependent loss of motor function and have a significantly shortened lifespan.

Disruption of microtubule network by Alzheimer abnormally hyperphosphorylated tau

Hyperphosphorylated tau has long been proposed as the key molecule disrupting normal neuronal microtubule dynamics and leading to neurofibrillary degeneration in Alzheimer disease. Here we provide a direct evidence of hyperphosphorylated tau-induced disruption of microtubule network. Using Nocodozole-treated and detergent-extracted cells, we created a neuronal environment in mouse embryonic fibroblasts, 3T3 cells, by replacing their cytoplasm with adult rat brain cytosol. By recreating neuronal microtubule network in these cells, we were able to follow the effects of hyperphosphorylated tau on microtubule dynamics in real time. Whereas recombinant human brain tau promoted assembly and bundling of microtubules, abnormally hyperphosphorylated tau isolated from Alzheimer disease brain cytosol (AD P-tau) inhibited the assembly and disrupted preformed microtubule network by sequestering normal brain tau and MAP2. This breakdown of the microtubule network was reversed by treatment of the extracted cells with protein phosphatase-2A. This study, for the first time, provides direct mechanistic insights into the molecular basis of both axonal and dendritic neurodegeneration seen in Alzheimer disease.

Modulation of hippocampal calcium signalling and plasticity by serine/threonine protein phosphatases

Kinases and phosphatases act antagonistically to maintain physiological phosphorylation/dephosphorylation at numerous intracellular sites critical for neuronal signalling. In this study, it was found that inhibition of serine/threonine phosphatases by exposure of hippocampal slices to okadaic acid (OA) or cantharidin (CA; 100 nmol/L) for 2 h resulted in reduced basal synaptic transmission and blocked the induction of synaptic plasticity in the form of long-term potentiation as determined by electrophysiological analysis. Fura-2 Ca(2+) imaging revealed a bidirectional modulation of N-methyl-D-aspartate (NMDA) -mediated Ca(2+) responses and reduced KCl-mediated Ca(2+) responses in neonatal cultured hippocampal neurons after phosphatase inhibition. While OA inhibited NMDA-induced Ca(2+) influx both acutely and after incubation, CA-enhanced receptor-mediated Ca(2+) signalling at low concentrations (1 nmol/L) but reduced NMDA and KCl-mediated Ca(2+) responses at higher concentrations (100 nmol/L). Changes in Ca(2+) signalling were accompanied by increased phosphorylation of cytoskeletal proteins tau and neurofilament and the NMDA receptor subunit NR1 in selective treatments. Incubation with OA (100 nmol/L) also led to the disruption of the microtubule network. This study highlights novel signalling effects of prolonged inhibition of protein phosphatases and suggests reduced post-synaptic signalling as a major mechanism for basal synaptic transmission and long-term potentiation impairments.

Kinases and phosphatases and tau sites involved in Alzheimer neurofibrillary degeneration

Microtubule associated protein (MAP) tau is abnormally hyperphosphorylated in Alzheimer's disease (AD) and related tauopathies; in this form it is the major protein subunit of paired helical filaments (PHF)/neurofibrillary tangles. However, the nature of protein kinases and phosphatases and tau sites involved in this lesion has been elusive. We investigated self-assembly and microtubule assembly promoting activities of hyperphosphorylated tau isolated from Alzheimer disease brain cytosol, the AD abnormally hyperphosphorylated tau (AD P-tau) before and after dephosphorylation by phosphoseryl/phosphothreonyl protein phosphatase-2A (PP-2A), and then rephosphorylation by cyclic AMP-dependent protein kinase (PKA), calcium, calmodulin-dependent protein kinase II (CaMKII), glycogen synthase kinase-3beta (GSK-3beta) and cyclin-dependent protein kinase 5 (cdk5) in different kinase combinations. We found that (i) dephosphorylation of AD P-tau by PP-2A inhibits its polymerization into PHF/straight filaments (SF) and restores its binding and ability to promote assembly of tubulin into microtubules; (ii) rephosphorylation of PP-2A-dephosphorylated AD P-tau by sequential phosphorylation by PKA, CaMKII and GSK-3beta or cdk5, and as well as by cdk5 and GSK-3beta, promotes its self-assembly into tangles of PHF similar to those seen in Alzheimer brain, and (iii) phosphorylation of tau sites required for this pathology are Thr231 and Ser262, along with several sites flanking the microtubule binding repeat region. Phosphorylation of recombinant human brain tau(441) yielded similar results as the PP-2A dephosphorylated AD P-tau, except that mostly SF were formed. The conditions for the abnormal hyperphosphorylation of tau that promoted its self-assembly also induced the microtubule assembly inhibitory activity. These findings suggest that activation of PP-2A or inhibition of either both GSK-3beta and cdk5 or one of these two kinases plus PKA or CaMKII might be required to inhibit Alzheimer neurofibrillary degeneration.

Expression of a truncated tau protein induces oxidative stress in a rodent model of tauopathy

Truncation of tau protein and oxidative stress have been implicated as important pathogenetic events in tauopathies including Alzheimer's disease (AD). We have generated a transgenic rat model that expresses a human truncated tau protein analogous to a variant form derived from sporadic AD. We employed this model to investigate the relationship between tau protein truncation and oxidative stress. We have found that rat cortical neurons (derived from transgenic animals) that had been cultured in vitro for 16 days showed an increased accumulation of reactive oxygen species (up to 1.4-fold increase; P < 0.01) when compared to neurons derived from nontransgenic control animals. Transgene-expressing neurons treated with inducers of oxidative stress, such as glucose oxidase (GO) and buthionine sulfoximine (BSO), displayed dramatically reduced survival (31.4 +/- 3.3 and 24.9 +/- 3.6%, respectively; both P < 0.001) compared to neurons from control animals (79.9 +/- 7.1%, survival following treatment with GO and to 98.2 +/- 3.8%, survival following treatment with BSO). The number of mitochondria in processes of neurons from transgenic animals was decreased by about one-third from that present in neurons from control animals. The results reveal that expression of a human truncated variant form of tau protein leads to the accumulation of reactive oxygen species and sensitizes rat cortical neurons to cell death induced by oxidative stress. This indicates that truncation of tau may precede oxidative stress in the pathogenesis of neurodegenerative diseases such as AD and other tauopathies. These findings may have implications for therapeutic strategies aiming at prevention of neurofibrillary degeneration and cognitive decline, and identify potential new targets for drug development.

PET and SPECT imaging in psychiatric disorders

OBJECTIVES

To review recent findings from positron emission tomography (PET) and single photon emission computed tomography (SPECT) studies that investigate the pathophysiology and treatment of schizophrenia, depression, and dementia.

METHODS

We carried out a review of the literature.

RESULTS

PET and SPECT studies have provided evidence of dopamine system dysregulation in patients with schizophrenia and variable loss of monoamines in patients with depression. Antipsychotic response has been demonstrated to be associated with blockade of dopamine D2 receptors, and antidepressant response has now been linked to blockade of serotonin transporter receptors. PET and SPECT have been extensively evaluated as diagnostic procedures for dementia. Substantial progress has been made in developing radioligands that bind to amyloid deposits in the brain, which should provide new opportunities for early diagnosis and treatment monitoring in Alzheimer's disease.

CONCLUSION

Advances in PET and SPECT imaging have provided new insights into the biology of major psychiatric disorders and their treatment. In the future, we can expect that these imaging techniques will become more central to the management of psychiatric disorders.

14-3-3ζ Facilitates GSK3β-catalyzed tau phosphorylation in HEK-293 cells by a mechanism that requires phosphorylation of GSK3β on Ser9

Hyperphosphorylated tau is the prominent component of paired helical filaments, which are the major component of neurofibrillary tangles associated with Alzheimer's disease (AD). Glycogen synthase kinase 3beta (GSK3beta) is implicated to phosphorylate tau in normal and AD brain. Previously, we isolated a large multiprotein complex containing tau, Ser9-phosphorylated GSK3beta and 14-3-3zeta from bovine brain microtubules. We showed that within the complex, 14-3-3zeta binds to tau and GSK3beta and mediates GSK3beta-catalyzed tau phosphorylation. A recent report however indicated that 14-3-3zeta does not bind to tau or GSK3beta and does not increase tau phosphorylation by GSK3beta in cell models [T.A. Matthews, G.V.W. Johnson, Neurosci. Lett. 384 (2005) 211-216]. In the current study we have thoroughly analyzed the binding of 14-3-3zeta with tau and GSK3beta and evaluated the effect of 14-3-3zeta on tau phosphorylation by GSK3beta in HEK-293 cells. We found that 14-3-3zeta binds to tau and Ser9-phosphorylated GSK3beta. Nonphosphorylated GSK3beta phosphorylates tau without being influenced by 14-3-3zeta. Ser9-phosphorylated GSK3beta on the other hand phosphorylates tau significantly only in the presence of 14-3-3zeta. Our data demonstrate that 14-3-3zeta mediates tau phosphorylation by Ser9-phosphorylated GSK3beta in HEK-293 cells.

A novel monoclonal antibody DC63 reveals that inhibitor 1 of protein phosphatase 2A is preferentially nuclearly localised in human brain

Abnormal phosphorylation of tau protein represents one of the major candidate pathological mechanisms leading to Alzheimer's disease (AD) and related tauopathies. Altered phosphorylation status of neuronal tau protein may result from upregulation of tau-specific kinases or from inhibition of tau-specific phosphatases. Increased expression of the protein inhibitor 1 of protein phosphatase 2A (I1PP2A) could therefore indirectly regulate the phosphorylation status of tau. As an important step towards elucidation of the role of I1PP2A in the physiology and pathology of tau phosphorylation, we developed a novel monoclonal antibody, DC63, which recognizes I1PP2A. Specificity of the antibody was examined by mass spectrometry and Western blot. This analysis supports the conclusion that the antibody does not recognize any of the other proteins of the 9-member leucine-rich acidic nuclear phosphoprotein family to which I1PP2A belongs. Immunoblot detection revealed that the inhibitor I1PP2A is expressed throughout the brain, including the hippocampus, temporal cortex, parietal cortex, subcortical nuclei and brain stem. The cerebellum displayed significantly higher levels of expression of I1PP2A than was seen elsewhere in the brain. Imunohistochemical analysis of normal human brain showed that I1PP2A is expressed in both neurons and glial cells and that the protein is preferentially localized to the nucleus. We conclude that the novel monoclonal antibody DC63 could be successfully employed as a mass spectrometry-validated molecular probe that may be used for in vitro and in vivo qualitative and quantitative studies of physiological and pathological pathways involving I1PP2A.