Tau pathology in Alzheimer disease and other tauopathies

Cdk5: multitasking between physiological and pathological conditions

Cyclin-dependent kinase 5 (Cdk5) is a peculiar proline-directed serine/threonine kinase. Unlike the other members of the Cdk family, Cdk5 is not directly involved in cell cycle regulation, being normally associated with neuronal processes such as migration, cortical layering and synaptic plasticity. This kinase is present mainly in post-mitotic neurons and its activity is tightly regulated by the interaction with the specific activators, p35 and p39. Despite its pivotal role in CNS development, Cdk5 dysregulation has been implicated in different pathologies, such as Alzheimer's disease (AD), amyotrophic lateral sclerosis (ALS), Parkinson's disease (PD) and, most recently, prion-related encephalopathies (PRE). In these neurodegenerative conditions, Cdk5 overactivation and relocalization occurs upon association with p25, a truncated form of the normal activator p35. This activator switching will cause a shift in the phosphorylative pattern of Cdk5, with an alteration both in targets and activity, ultimately leading to neuronal demise. In AD and PRE, two disorders that share clinical and neuropathological features, Cdk5 dysregulation is a linking event between the major neuropathological markers: amyloid plaques, tau hyperphosphorylation and synaptic and neuronal loss. Moreover, this kinase was shown to be involved in abortive cell cycle re-entry, a feature recently proposed as a possible step in the neuronal apoptosis mechanism of several neurological diseases. This review focuses on the role of Cdk5 in neurons, namely in the regulation of cytoskeletal dynamics, synaptic function and cell survival, both in physiological and in pathological conditions, highlighting the relevance of Cdk5 in the main mechanisms of neurodegeneration in Alzheimer's disease and other brain pathologies.

Melatonin reduces the impairment of axonal transport and axonopathy induced by calyculin A

Previous studies have reported that calyculin A (CA), a selective inhibitor of protein phosphatase (PP)-2A and PP-1, impairs axonal transport in neuroblastoma N2a cells. Melatonin prevents Alzheimer-like hyperphosphorylation of cytoskeletal proteins and the impairment of spatial memory retention induced by CA. In this study, we tested the effects of melatonin on the impairment of axonal transport induced by CA in neuroblastoma N2a cells. We found that melatonin protected the cells from CA-induced toxicity in metabolism and viability as well as hyperphosphorylation of tau and neurofilaments. Furthermore, melatonin partially reversed the CA-induced phosphorylation of the catalytic subunit of PP-2A at tyrosine 307, a crucial site that negatively regulates the activity of PP-2A, and reduced the levels of malondialdehyde and the activity of superoxide dismutase, which are markers of oxidative stress. Melatonin also significantly reversed the CA-induced impairment of axonal transport. These results suggest that melatonin may have a role in protecting against the CA-induced impairment of axonal transport by modulating the activity of PP-2A and oxidative stress.

Increased CDK5 expression in HIV encephalitis contributes to neurodegeneration via tau phosphorylation and is reversed with Roscovitine

Recent treatments with highly active antiretroviral therapy (HAART) regimens have been shown to improve general clinical status in patients with human immunodeficiency virus (HIV) infection; however, the prevalence of cognitive alterations and neurodegeneration has remained the same or has increased. These deficits are more pronounced in the subset of HIV patients with the inflammatory condition known as HIV encephalitis (HIVE). Activation of signaling pathways such as GSK3β and CDK5 has been implicated in the mechanisms of HIV neurotoxicity; however, the downstream mediators of these effects are unclear. The present study investigated the involvement of CDK5 and tau phosphorylation in the mechanisms of neurodegeneration in HIVE. In the frontal cortex of patients with HIVE, increased levels of CDK5 and p35 expression were associated with abnormal tau phosphorylation. Similarly, transgenic mice engineered to express the HIV protein gp120 exhibited increased brain levels of CDK5 and p35, alterations in tau phosphorylation, and dendritic degeneration. In contrast, genetic knockdown of CDK5 or treatment with the CDK5 inhibitor roscovitine improved behavioral performance in the water maze test and reduced neurodegeneration, abnormal tau phosphorylation, and astrogliosis in gp120 transgenic mice. These findings indicate that abnormal CDK5 activation contributes to the neurodegenerative process in HIVE via abnormal tau phosphorylation; thus, reducing CDK5 might ameliorate the cognitive impairments associated with HIVE.

Splicing factor SC35 promotes tau expression through stabilization of its mRNA

Altered alternative splicing and accumulation of brain microtubule-associated protein tau are found in several tauopathies and are believed to lead to these neurodegenerative diseases. We found that in addition to promoting tau exon 10 inclusion, splicing factor SC35 also promoted tau expression in HEK-293T cells. The activity of SC35 in promotion of tau expression was limited to exon 10 containing tau isoforms. SC35 did not affect tau transcription, but stabilized tau mRNA by binding to the SC35-like element of exon 10. These results provide novel insight into the regulation of tau expression and a molecular mechanism of tauopathies.

Inhibition of β-amyloid1-42 internalization attenuates neuronal death by stabilizing the endosomal-lysosomal system in rat cortical cultured neurons

A number of recent studies have indicated that accumulation of β amyloid (Aβ) peptides within neurons is an early event which may trigger degeneration of neurons and subsequent development of Alzheimer's disease (AD) pathology. However, very little is known about the internalization and/or subcellular sites involved in trafficking of Aβ peptides into the neurons that are vulnerable in AD pathology. To address this issue we evaluated internalization of fluoroscein conjugated Aβ1-42 (FAβ1-42) and subsequent alteration of endosomal-lysosomal (EL) markers such as cathepsin D, Rab5 and Rab7 in rat cortical cultured neurons. It is evident from our results that internalization of FAβ1-42, which occurred in a dose- and time-dependent manner, triggered degeneration of neurons along with increased levels and/or altered distribution of cathepsin D, Rab5 and Rab7. Our results further revealed that FAβ1-42 internalization was attenuated by phenylarsine oxide (a general inhibitor of endocytosis) and sucrose (an inhibitor of clathrin-mediated endocytosis) but not by antagonists of N-methyl-d-aspartate (NMDA) glutamate receptors. Additionally, inhibition of FAβ1-42 endocytosis not only protected neurons against toxicity but also reversed the altered levels/distributions of EL markers. These results, taken together, suggest that internalization of exogenous Aβ1-42, which is partly mediated via a clathrin-dependent process, can lead to degeneration of neurons, possibly by activating the EL system. Inhibition of FAβ endocytosis attenuated toxicity, thus suggesting a potential strategy for preventing loss of neurons in AD pathology.

Link between DYRK1A overexpression and several-fold enhancement of neurofibrillary degeneration with 3-repeat tau protein in Down syndrome

Triplication of chromosome 21 in Down syndrome (DS) results in overexpression of the minibrain kinase/dual-specificity tyrosine phosphorylated and regulated kinase 1A gene (DYRK1A). DYRK1A phosphorylates cytoplasmic tau protein and appears in intraneuronal neurofibrillary tangles (NFTs). We have previously shown significantly more DYRK1A-positive NFTs in DS brains than in sporadic Alzheimer disease (AD) brains. This study demonstrates a gene dosage-proportional increase in the level of DYRK1A in DS in the cytoplasm and the cell nucleus, and enhanced cytoplasmic and nuclear immunoreactivity of DYRK1A in DS. The results suggest that overexpressed DYRK1A may alter both phosphorylation of tau and alternative splicing factor (ASF). Two-dimensional electrophoresis revealed modification of ASF phosphorylation in DS/AD and AD in comparison to controls. Altered phosphorylation of ASF by overexpressed nuclear DYRK1A may contribute to the alternative splicing of the tau gene and an increase by 2.68 × of the 3R/4R ratio in DS/AD, and a several-fold increase in the number of 3R tau-positive NFTs in DS/AD subjects compared with that in sporadic AD subjects. These data support the hypothesis that phosphorylation of ASF by overexpressed DYRK1A may contribute to alternative splicing of exon 10, increased expression of 3R tau, and early onset of neurofibrillary degeneration in DS.

Modeling human diseases in Caenorhabditis elegans

Genes linked to human diseases often function in evolutionarily conserved pathways, which can be readily dissected in simple model organisms. Because of its short lifespan and well-known biology, coupled with a completely sequenced genome that shares extensive homology with that of mammals, Caenorhabditis elegans is one of the most versatile and powerful model organisms. Research in C. elegans has been instrumental for the elucidation of molecular pathways implicated in many human diseases. In this review, we introduce C. elegans as a model organism for biomedical research and we survey recent relevant findings that shed light on the basic molecular determinants of human disease pathophysiology. The nematode holds promise of providing clear leads towards the identification of potential targets for the development of new therapeutic interventions against human diseases.

Chronic mild cerebrovascular dysfunction as a cause for Alzheimer's disease?

Alzheimer's disease (AD) is a progressive chronic disorder and is characterized by β-amyloid plaques and angiopathy, tau pathology, neuronal cell death, and inflammatory responses. The reasons for this disease are not known. This review proposes the hypothesis that a chronic mild longlasting cerebrovascular dysfunction could initiate a cascade of events leading to AD. It is suggested that (vascular) risk factors (e.g. hypercholesterolemia, type 2 diabetes, hyperhomocysteinemia) causes either damage of the cerebrovascular system including silent strokes or causes dysregulation of beta-amyloid clearance at the blood-brain barrier resulting in increased brain beta-amyloid. A cascade of subsequent downstream events may lead to disturbed metabolic changes, and neuroinflammation and tau pathology. The role of NGF on the cell death of cholinergic neurons is discussed. Additional risk factors (e.g. acidosis, metals) contribute to plaque development.

Alzheimer's disease neurofibrillary degeneration: pivotal and multifactorial

Independent of the aetiology, AD (Alzheimer's disease) neurofibrillary degeneration of abnormally hyperphosphorylated tau, a hallmark of AD and related tauopathies, is apparently required for the clinical expression of the disease and hence is a major therapeutic target for drug development. However, AD is multifactorial and heterogeneous and probably involves several different aetiopathogenic mechanisms. On the basis of CSF (cerebrospinal fluid) levels of Abeta(1-42) (where Abeta is amyloid beta-peptide), tau and ubiquitin, five different subgroups, each with its own clinical profile, have been identified. A successful development of rational therapeutic disease-modifying drugs for AD will require understanding of the different aetiopathogenic mechanisms involved and stratification of AD patients by different disease subgroups in clinical trials. We have identified a novel aetiopathogenic mechanism of AD which is initiated by the cleavage of SET, also known as inhibitor-2 (I(2)(PP2A)) of PP2A (protein phosphatase 2A) at Asn(175) into N-terminal (I(2NTF)) and C-terminal (I(2CTF)) halves and their translocation from the neuronal nucleus to the cytoplasm. AAV1 (adeno-associated virus 1)-induced expression of I(2CTF) in rat brain induces inhibition of PP2A activity, abnormal hyperphosphorylation of tau, neurodegeneration and cognitive impairment in rats. Restoration of PP2A activity by inhibition of the cleavage of I(2)(PP2A)/SET offers a promising therapeutic opportunity in AD with this aetiopathogenic mechanism.

Lead dysregulates serine/threonine protein phosphatases in human neurons

It is well established that lead (Pb) exposure in humans leads to learning and memory impairment. However, the biological and molecular mechanisms are still not clearly understood. When over activated, serine/threonine protein phosphatases are known to function as a constraint on learning and memory. Activation of these phosphatases can also result in cytoskeletal changes that will adversely affect learning and memory. We investigated the effects of Pb exposure on these phosphatases in primary cultures of human neurons. Neurons were exposed to physiologically relevant concentrations of Pb (5, 10, 20 and 40 μg/dL) and total phosphatase and PP2A activities were determined in neuronal lysate using para-nitrophenyl phosphate (pNPP), and a PP2A-specific phosphopeptide as substrates. Expression of various serine/threonine phosphatases, tau and its phosphorylation state were determined by Western blot (WB) and immunocytochemistry (ICC). We found that the total phosphatase activity in the neuronal lysate was increased by 30–50% by all the concentrations of Pb tested. PP2A activity was increased by 5 μg/dL Pb only. PP1 expression was increased (ranging from 25–50%) by 10, 20 and 40 μg/dL of Pb. PP2B expression was increased substantially (up to 2.5-fold) by 10 μg/dL Pb, whereas, higher concentrations did not show any effect. On the other hand, Pb (at all concentrations used) decreased expression of PP2A and PP5. Pb exposure induced substantial hyperphosphorylation of tau at serine 199/202 by 5 and 10 μg/dL Pb, and Threonine 231 at higher doses. Expression of total tau was mostly unaffected by lead. Immunocytochemistry data confirmed the WB results of expression of PP1, PP2A, tau protein and the phosphorylation of tau. These results support our hypothesis that Pb exposure up regulates some of the serine/threonine phosphatases (PP1 and PP2B) that are known to impair memory formation, and suggest a novel mechanism of Pb neurotoxicity.

Phosphoproteomic analysis reveals site-specific changes in GFAP and NDRG2 phosphorylation in frontotemporal lobar degeneration

Frontotemporal lobar degeneration (FTLD) is a progressive neurodegenerative disease characterized by behavioral abnormalities, personality changes, language dysfunction, and can co-occur with the development of motor neuron disease. One major pathological form of FTLD is characterized by intracellular deposition of ubiquitinated and phosphorylated TAR DNA binding protein-43 (TDP-43), suggesting that dysregulation in phosphorylation events may contribute to disease progression. However, to date systematic analysis of the phosphoproteome in FTLD brains has not been reported. In this study, we employed immobilized metal affinity chromatography (IMAC) followed by liquid chromatography-tandem mass spectrometry (LC-MS/MS) to identify phosphopeptides from FTLD and age-matched control post-mortem human brain tissue. Using this approach, we identified 786 phosphopeptides in frontal cortex (control and FTLD), in which the population of phosphopeptides represented approximately 50% of the total peptides analyzed. Label-free quantification using spectral counts revealed six proteins with significant changes in the FTLD phosphoproteome. N-myc-Downstream regulated gene 2 (NDRG2) and glial fibrillary acidic protein (GFAP) had an increased number of phosphospectra in FTLD, whereas microtubule associated protein 1A (MAP1A), reticulon 4 (RTN4; also referred to as neurite outgrowth inhibitor (Nogo)), protein kinase C gamma (PRKCG), and heat shock protein 90 kDa alpha, class A member 1(HSP90AA1) had significantly fewer phosphospectra compared to control brain. To validate these differences, we examined NDRG2 phosphorylation in FTLD brain by immunoblot analyses, and using a phosphoserine-13 (pSer13) GFAP monoclonal antibody we show an increase in pSer13 GFAP levels by immunoblot concomitant with increased overall GFAP levels in FTLD cases. These data highlight the utility of combining proteomic and phosphoproteomic strategies to characterize post-mortem human brain tissue.

Silencing of CDK5 reduces neurofibrillary tangles in transgenic alzheimer's mice

Alzheimer's disease is a major cause of dementia for which treatments remain unsatisfactory. Cyclin-dependent kinase 5 (CDK5) is a relevant kinase that has been hypothesized to contribute to the tau pathology. Several classes of chemical inhibitors for CDK5 have been developed, but they generally lack the specificity to distinguish among various ATP-dependent kinases. Therefore, the efficacy of these compounds when tested in animal models cannot definitively be attributed to an effect on CDK5. However, RNA interference (RNAi) targeting of CDK5 is specific and can be used to validate CDK5 as a possible treatment target. We delivered a CDK5 RNAi by lentiviral or adenoassociated viral vectors and analyzed the results in vitro and in vivo. Silencing of CDK5 reduces the phosphorylation of tau in primary neuronal cultures and in the brain of wild-type C57BL/6 mice. Furthermore, the knockdown of CDK5 strongly decreased the number of neurofibrillary tangles in the hippocampi of triple-transgenic mice (3×Tg-AD mice). Our data suggest that this downregulation may be attributable to the reduction of the CDK5 availability in the tissue, without affecting the CDK5 kinase activity. In summary, our findings validate CDK5 as a reasonable therapeutic target for ameliorating tau pathology.

Basis for MAP4 dephosphorylation-related microtubule network densification in pressure overload cardiac hypertrophy

Increased activity of Ser/Thr protein phosphatases types 1 (PP1) and 2A (PP2A) during maladaptive cardiac hypertrophy contributes to cardiac dysfunction and eventual failure, partly through effects on calcium metabolism. A second maladaptive feature of pressure overload cardiac hypertrophy that instead leads to heart failure by interfering with cardiac contraction and intracellular transport is a dense microtubule network stabilized by decoration with microtubule-associated protein 4 (MAP4). In an earlier study we showed that the major determinant of MAP4-microtubule affinity, and thus microtubule network density and stability, is site-specific MAP4 dephosphorylation at Ser-924 and to a lesser extent at Ser-1056; this was found to be prominent in hypertrophied myocardium. Therefore, in seeking the etiology of this MAP4 dephosphorylation, we looked here at PP2A and PP1, as well as the upstream p21-activated kinase 1, in maladaptive pressure overload cardiac hypertrophy. The activity of each was increased persistently during maladaptive hypertrophy, and overexpression of PP2A or PP1 in normal hearts reproduced both the microtubule network phenotype and the dephosphorylation of MAP4 Ser-924 and Ser-1056 seen in hypertrophy. Given the major microtubule-based abnormalities of contractile and transport function in maladaptive hypertrophy, these findings constitute a second important mechanism for phosphatase-dependent pathology in the hypertrophied and failing heart.

Growth hormone/insulin-like growth factor I axis in neurodegenerative diseases

Neurodegenerative diseases (ND) are a group of heterogeneous disorders characterized by unknown etiology, subtle onset, and progressive involvement of neuronal systems leading to degeneration and dysfunction. They represent a challenge for basic science and clinical medicine because of increasing prevalence, social cost, complex biochemistry and pathology, and lack of mechanism-based treatments. Endocrine modifications may accompany the progression of ND, due to the intimate connections between central nervous and endocrine systems. Reported data on endocrine changes in different ND have often been non-conclusive or conflicting. GH/IGF-I axis is involved in the regulation of brain growth, development, and metabolism. Dysfunctions in GH/IGF-I axis in most of ND are therefore reviewed. Whether GH deficiency, when present, may act as a contributory factor in the pathogenesis of these diseases, or might represent a consequence of it is presently unknown. A thorough effort in investigating every possible involvement of GH/IGF-I axis is warranted, in the light of future possible therapeutic strategies.

Transcriptome profiling in neurodegenerative disease

Changes in gene expression and splicing patterns (that occur prior to the onset and during the progression of complex diseases) have become a major focus of neurodegenerative disease research. These signature patterns of gene expression provide clues about the mechanisms involved in the molecular pathogenesis of neurodegenerative disease and may facilitate the discovery of novel therapeutic drugs. With the development of array technologies and the very recent RNA-seq technique, our understanding of the pathogenesis of neurodegenerative disease is expanding exponentially. Here, we review the technologies involved in gene expression and splicing analysis and the related literature on three common neurodegenerative diseases: Alzheimer's disease, Parkinson's disease and Huntington's disease.

Using topological indices to predict anti-Alzheimer and anti-parasitic GSK-3 inhibitors by multi-target QSAR in silico screening

Plasmodium falciparum, Leishmania, Trypanosomes, are the causers of diseases such as malaria, leishmaniasis and African trypanosomiasis that nowadays are the most serious parasitic health problems worldwide. The great number of deaths and the few drugs available against these parasites, make necessary the search for new drugs. Some of these antiparasitic drugs also are GSK-3 inhibitors. GSKI-3 are candidates to develop drugs for the treatment of Alzheimer's disease. In this work topological descriptors for a large series of 3,370 active/non-active compounds were initially calculated with the ModesLab software. Linear Discriminant Analysis was used to fit the classification function and it predicts heterogeneous series of compounds like paullones, indirubins, meridians, etc. This study thus provided a general evaluation of these types of molecules.

Spatially pathogenic forms of tau detected in Alzheimer's disease brain tissue by fluorescence lifetime-based Förster resonance energy transfer

In tauopathies including Alzheimer's disease (AD) tau molecules have lost their normal spatial distance to each other and appear in oligomeric or aggregated forms. Conventional immunostaining methods allow detection of abnormally phosphorylated or conformationally altered aggregated tau proteins, but fail to visualize oligomeric forms of tau. Here we show that tau molecules that lost their normal spatial localization can be detected on a subcellular level in postmortem central nervous system (CNS) tissue sections of AD patients by fluorescence lifetime-based Förster resonance energy transfer (FRET). Paraffin sections were co-immunostained with two tau-specific monoclonal antibodies recognizing the same epitope, but labeled with distinct fluorescence dyes suitable for spatial resolution at a nanometer scale by lifetime-based FRET. A FRET signal was detected in neuritic plaques and neurofibrillary tangles of CNS tissue sections of AD patients, showing associated tau proteins typically reflecting either fibrillary, oligomeric or aggregated tau. The 'pretangle-like' structures within the neuronal perikarya did not contain spatially pathogenic forms of tau accordingly to this method. Data demonstrate that fluorescence lifetime-based FRET can be applied to human brain tissue sections to detect pathogenic forms of tau molecules that lost their normal spatial distance.

Genome wide profiling of altered gene expression in the neocortex of Alzheimer's disease

Alzheimer's disease (AD) is characterized by a complex neurodegenerative process affecting multiple genes and proteins in the neocortex, many of which have not been well-studied. In this study, we investigated genome-wide gene alterations in the temporal cortex of a well-characterized cohort of AD patients using a recently developed microarray platform, and compared some of the transcript changes with immunoblotting. Of the 5485 genes found to be significantly altered in AD, there were consistent patterns of changes which show that the AD transcriptome in neocortex is characterized by changes indicative of synaptic dysfunction, perturbed neurotransmission and activation of neuroinflammation. We also highlighted several genes of potential pathogenic significance which have not been well studied in AD. The current study aims to add to the growing body of knowledge relating to gene changes in AD and provide further insights into pathogenic mechanisms and potential targets of pharmacotherapy.

Tau proteins in the cerebrospinal fluid of patients with subacute sclerosing panencephalitis

Neurodegenerative diseases characterized by cytoskeletal deformation and neurofibrillary tangles are associated with altered levels of tau and related proteins in cerebrospinal fluid (CSF). Neuronal or glial fibrillary tangles have been shown in 20% of subacute sclerosing panencephalitis (SSPE) patients. We therefore investigated CSF samples from 60 newly diagnosed SSPE and 31 neurological control patients for total tau (t-tau), phosphorylated tau (p-tau), and S100-B levels by ELISA. There was no difference between patient and control groups in t-tau and S100-B levels. p-Tau was lower in the SSPE group (p=0.009). Past history of measles infection, measles immunization status, latent period between measles and onset of SSPE, duration of symptoms, frequency of myoclonia, neurological deficit index, stage and progression rate of the disease, CSF glucose levels and cell counts, CSF and serum measles IgG titer, distribution of lesions on brain magnetic resonance imaging were not related to t-tau, p-tau and S100-B levels. Mental status and age were negatively correlated with t-tau, and male gender and EEG abnormalities were associated with higher t-tau levels. The levels of tau proteins in our patients suggest there is no, or only scarce and immature, neurofibrillary tangle formation in SSPE. Autopsy studies showing neurofibrillary tangles might have examined older patients with longer disease and more parenchymal involvement.

Site-specific microtubule-associated protein 4 dephosphorylation causes microtubule network densification in pressure overload cardiac hypertrophy

In severe pressure overload-induced cardiac hypertrophy, a dense, stabilized microtubule network forms that interferes with cardiocyte contraction and microtubule-based transport. This is associated with persistent transcriptional up-regulation of cardiac alpha- and beta-tubulin and microtubule-stabilizing microtubule-associated protein 4 (MAP4). There is also extensive microtubule decoration by MAP4, suggesting greater MAP4 affinity for microtubules. Because the major determinant of this affinity is site-specific MAP4 dephosphorylation, we characterized this in hypertrophied myocardium and then assessed the functional significance of each dephosphorylation site found by mimicking it in normal cardiocytes. We first isolated MAP4 from normal and pressure overload-hypertrophied feline myocardium; volume-overloaded myocardium, which has an equal degree and duration of hypertrophy but normal functional and cytoskeletal properties, served as a control for any nonspecific growth-related effects. After cloning cDNA-encoding feline MAP4 and obtaining its deduced amino acid sequence, we characterized by mass spectrometry any site-specific MAP4 dephosphorylation. Solely in pressure overload-hypertrophied myocardium, we identified striking MAP4 dephosphorylation at Ser-472 in the MAP4 N-terminal projection domain and at Ser-924 and Ser-1056 in the assembly-promoting region of the C-terminal microtubule-binding domain. Site-directed mutagenesis of MAP4 cDNA was then used to switch each serine to non-phosphorylatable alanine. Wild-type and mutated cDNAs were used to construct adenoviruses; microtubule network density, stability, and MAP4 decoration were assessed in normal cardiocytes following an equivalent level of MAP4 expression. The Ser-924 --> Ala MAP4 mutant produced a microtubule phenotype indistinguishable from that seen in pressure overload hypertrophy, such that Ser-924 MAP4 dephosphorylation during pressure overload hypertrophy may be central to this cytoskeletal abnormality.

Cerebrospinal fluid and plasma biomarkers in Alzheimer disease

Intense multidisciplinary research has provided detailed knowledge of the molecular pathogenesis of Alzheimer disease (AD). This knowledge has been translated into new therapeutic strategies with putative disease-modifying effects. Several of the most promising approaches, such as amyloid-beta immunotherapy and secretase inhibition, are now being tested in clinical trials. Disease-modifying treatments might be at their most effective when initiated very early in the course of AD, before amyloid plaques and neurodegeneration become too widespread. Thus, biomarkers are needed that can detect AD in the predementia phase or, ideally, in presymptomatic individuals. In this Review, we present the rationales behind and the diagnostic performances of the core cerebrospinal fluid (CSF) biomarkers for AD, namely total tau, phosphorylated tau and the 42 amino acid form of amyloid-beta. These biomarkers reflect AD pathology, and are candidate markers for predicting future cognitive decline in healthy individuals and the progression to dementia in patients who are cognitively impaired. We also discuss emerging plasma and CSF biomarkers, and explore new proteomics-based strategies for identifying additional CSF markers. Furthermore, we outline the roles of CSF biomarkers in drug discovery and clinical trials, and provide perspectives on AD biomarker discovery and the validation of such markers for use in the clinic.

Accumulation of tumor-suppressor PTEN in Alzheimer neurofibrillary tangles

The phosphatase and tensin homologue deleted on chromosome 10 (PTEN) negatively regulates intracellular levels of PIP3 and antagonizes the PI3K signaling pathway important for cell survival. The present study determined whether altered distribution of PTEN occurs in Alzheimer's disease (AD) brains. We investigated a possible role for PTEN in postmortem brain tissues from elderly controls and patients with AD using immunoblotting and microscopic analyses. Intense immunolabeling was found in the large neurons such as pyramidal cells. In normal neurons, PTEN was located in the nucleus, the cytoplasm of cell bodies and the proximal portion of apical dendrites. Reduced expression and redistribution of PTEN was seen in the remaining neurons in AD. In addition, PTEN was redistributed in damaged neurons from the nucleus and cytoplasm to neuritic pathology such as intracellular neurofibrillary tangles (NFTs), neuropil threads and dystrophic neurites within senile plaques in AD hippocampus, subiculum, entorhinal cortex and angular gyrus. Furthermore, double immunofluorescence staining showed dual labeling of intracellular NFTs for PTEN and tau, labeling of some axons for PTEN and phosphorylated neurofilament, and weak labeling of a few reactive astrocytes around senile plaques for PTEN and GFAP. Double labeling of NFTs was observed in a subset of tangle-bearing neurons either for PTEN and GSK3beta or for PTEN and MEK. Thus our results suggest that PTEN delocalized from the nucleus to the cytoplasm and to intracellular NFTs may cause a deregulation of PI3K pathway in the cytoplasm and may induce the nuclear dysfunction of PTEN in AD degenerating neurons.

Mouse models of neurodegenerative diseases: criteria and general methodology

The major symptom of Alzheimer's disease is rapidly progressing dementia, coinciding with the formation of amyloid and tau deposits in the central nervous system, and neuronal death. At present familial cases of dementias provide the most promising foundation for modelling neurodegeneration. We describe the mnemonic and other major behavioral symptoms of tauopathies, briefly outline the genetics underlying familiar cases and discuss the arising implications for modelling the disease in mostly transgenic mouse lines. We then depict to what degree the most recent mouse models replicate pathological and cognitive characteristics observed in patients.There is no universally valid behavioral test battery to evaluate mouse models. The selection of individual tests depends on the behavioral and/or memory system in focus, the type of a model and how well it replicates the pathology of a disease and the amount of control over the genetic background of the mouse model. However it is possible to provide guidelines and criteria for modelling the neurodegeneration, setting up the experiments and choosing relevant tests. One should not adopt a "one (trans)gene, one disease" interpretation, but should try to understand how the mouse genome copes with the protein expression of the transgene in question. Further, it is not possible to recommend some mouse models over others since each model is valuable within its own constraints, and the way experiments are performed often reflects the idiosyncratic reality of specific laboratories. Our purpose is to improve bridging molecular and behavioural approaches in translational research.

Protein carboxyl methylation and the biochemistry of memory

Bacterial chemotaxis is mediated by two reversible protein modification chemistries: phosphorylation and carboxyl methylation. Attractants bind to membrane chemoreceptors that control the activity of a protein kinase which acts in turn to control flagellar motor activity. Coordinate changes in receptor carboxyl methylation provide a negative feedback mechanism that serves a memory function. Protein carboxyl methylation might play an analogous role in the nervous system. Two protein carboxyl methyltransferases serve to regulate signal transduction pathways in eukaryotic cells. One is highly expressed in the Purkinje layer of the cerebellum where it methyl esterifies prenylated cysteine residues at the carboxyl-termini of Ras-related and heterotrimeric G-proteins. The other is abundant throughout the brain where it methylates the carboxyl-terminus of protein phosphatase 2A. The phosphatase methyltransferase and the protein methylesterase that reverses phosphatase methylation are structurally related to the corresponding bacterial chemotaxis methylating and demethylating enzymes. Recent results indicate that deficiencies in phosphatase methylation play an important role in the etiology of Alzheimer's disease.

Neurobiological pathways to Alzheimer's disease: Amyloid-beta, TAU protein or both?

Alzheimer’s disease (AD) is a neurodegenerative disease characterized by progressive cognitive decline, including memory loss, behavioral and psychological symptoms and personality changes. The neuropathological hallmarks of AD are the presence of neuritic (senile) plaques (NP) and neurofibrillary tangles (NFT), along with neuronal loss, dystrophic neurites, and gliosis. Neuritic plaques are extracellular lesions and their main constituent is the amyloid-β₄₂ peptide (Aβ₄₂). Neurofibrillary tangles are intracellular lesions that are mainly composed of hyperphosphorylated Tau protein. In this article, we review the major hypotheses concerning the physiopathology of AD, focusing on the β-amyloid cascade as primary events (supported by the “βaptists”) and cytoskeletal abnormalities secondary to the hyperphosphorylation of protein Tau (as advocated by the “Tauists”). We further provide an integrative view of the physiopathology of AD.

Is there a neuropathology difference between mild cognitive impairment and dementia?

The number of studies that have investigated the neuropathology of mild cognitive impairment (MCI) is small, but growing. In this paper we have restricted our focus to the consideration of the presence and extent of postmortem findings relevant to the neuropathology of Alzheimer's disease. We have drawn from studies that have investigated the postmortem neurobiology of the brains of persons with cognitive function at the interface between unimpaired normal function and mild but definite dementia. The data derived from these studies suggest that i) the brains of persons with MCI evidence significant neuropathological and neurobiological changes relative to those without cognitive impairment; ii) in general, the neuropathological and neurobiological changes are qualitatively similar to those observed in the brains of persons with frank AD-like dementia; and iii) the neuropathological and neurobiological brain changes associated with MCI are quantitatively less than those of persons who meet criteria for dementia. Thus, the available, albeit limited, data suggests that MCI is associated with the early stages of the neurobiological and neuropathological changes that culminate in the florid lesions of AD; including the accumulation of neuritic plaques, neurofibrillary tangles, synaptic and neurotransmitter associated deficits, and significant neuronal cell death.

Versatile somatic gene transfer for modeling neurodegenerative diseases

A growing variety of technical approaches allow control over the expression of selected genes in living organisms. The ability to deliver functional exogenous genes involved in neurodegenerative diseases has opened pathological processes to experimental analysis and targeted therapeutic development in rodent and primate preclinical models. Biological adaptability, economic animal use, and reduced model development costs complement improved control over spatial and temporal gene expression compared with conventional transgenic models. A review of viral vector studies, typically adeno-associated virus or lentivirus, for expression of three proteins that are central to major neurodegenerative diseases, will illustrate how this approach has powered new advances and opportunities in CNS disease research.

Calcium in the initiation, progression and as an effector of Alzheimer's disease pathology

The cause(s) of sporadic Alzheimer’s disease (sAD) are complex and currently poorly understood. They likely result from a combination of genetic, environmental, proteomic and lipidomic factors that crucially occur only in the aged brain. Age-related changes in calcium levels and dynamics have the potential to increase the production and accumulation of both amyloid-β peptide (Aβ) and τ pathologies in the AD brain, although these two pathologies themselves can induce calcium dyshomeostasis, particularly at synaptic membranes. This review discuses the evidence for a role for calcium dyshomeostasis in the initiation of pathology, as well as the evidence for these pathologies themselves disrupting normal calcium homeostasis, which lead to synaptic and neuronal dysfunction, synaptotoxicity and neuronal loss, underlying the dementia associated with the disease.

Mechanisms of tau-induced neurodegeneration

Alzheimer disease (AD) and related tauopathies are histopathologically characterized by a specific type of slow and progressive neurodegeneration, which involves the abnormal hyperphosphorylation of the microtubule associated protein (MAP) tau. This hallmark, called neurofibrillary degeneration, is seen as neurofibrillary tangles, neuropil threads, and dystrophic neurites and 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 non-fibrillized, abnormally hyperphosphorylated tau sequesters normal tau, MAP1 and MAP2, and disrupts microtubules. The abnormal hyperphosphorylation of tau, which can be generated by catalysis of several different combinations of protein kinases, also promotes its misfolding, decrease in turnover, and self-assembly into tangles of paired helical and or straight filaments. Some of the abnormally hyperphosphorylated tau ends up both amino and C-terminally truncated. Disruption of microtubules by the non-fibrillized abnormally hyperphosphorylated tau as well as its aggregation as neurofibrillary tangles probably impair axoplasmic flow and lead to slow progressive retrograde degeneration and loss of connectivity of the affected neurons. Among the phosphatases, which regulate the phosphorylation of tau, protein phosphatase-2A (PP2A), the activity of which is down-regulated in AD brain, is by far the major enzyme. The two inhibitors of PP-2A, I (1) (PP2A) and I (2) (PP2A) , which are overexpressed in AD, might be responsible for the decreased phosphatase activity. AD is multifactorial and heterogeneous and involves more than one etiopathogenic mechanism.

Protective actions of sex steroid hormones in Alzheimer's disease

Risk for Alzheimer's disease (AD) is associated with age-related loss of sex steroid hormones in both women and men. In post-menopausal women, the precipitous depletion of estrogens and progestogens is hypothesized to increase susceptibility to AD pathogenesis, a concept largely supported by epidemiological evidence but refuted by some clinical findings. Experimental evidence suggests that estrogens have numerous neuroprotective actions relevant to prevention of AD, in particular promotion of neuron viability and reduction of beta-amyloid accumulation, a critical factor in the initiation and progression of AD. Recent findings suggest neural responsiveness to estrogen can diminish with age, reducing neuroprotective actions of estrogen and, consequently, potentially limiting the utility of hormone therapies in aged women. In addition, estrogen neuroprotective actions are also modulated by progestogens. Specifically, continuous progestogen exposure is associated with inhibition of estrogen actions whereas cyclic delivery of progestogens may enhance neural benefits of estrogen. In recent years, emerging literature has begun to elucidate a parallel relationship of sex steroid hormones and AD risk in men. Normal age-related testosterone loss in men is associated with increased risk to several diseases including AD. Like estrogen, testosterone has been established as an endogenous neuroprotective factor that not only increases neuronal resilience against AD-related insults, but also reduces beta-amyloid accumulation. Androgen neuroprotective effects are mediated both directly by activation of androgen pathways and indirectly by aromatization to estradiol and initiation of protective estrogen signaling mechanisms. The successful use of hormone therapies in aging men and women to delay, prevent, and or treat AD will require additional research to optimize key parameters of hormone therapy and may benefit from the continuing development of selective estrogen and androgen receptor modulators.

Effect of Pin1 or microtubule binding on dephosphorylation of FTDP-17 mutant Tau

Neurodegenerative tauopathies, including Alzheimer disease, are characterized by abnormal hyperphosphorylation of the microtubule-associated protein Tau. One group of tauopathies, known as frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17), is directly associated with mutations of the gene tau. However, it is unknown why mutant Tau is highly phosphorylated in the patient brain. In contrast to in vivo high phosphorylation, FTDP-17 Tau is phosphorylated less than wild-type Tau in vitro. Because phosphorylation is a balance between kinase and phosphatase activities, we investigated dephosphorylation of mutant Tau proteins, P301L and R406W. Tau phosphorylated by Cdk5-p25 was dephosphorylated by protein phosphatases in rat brain extracts. Compared with wild-type Tau, R406W was dephosphorylated faster and P301L slower. The two-dimensional phosphopeptide map analysis suggested that faster dephosphorylation of R406W was due to a lack of phosphorylation at Ser-404, which is relatively resistant to dephosphorylation. We studied the effect of the peptidyl-prolyl isomerase Pin1 or microtubule binding on dephosphorylation of wild-type Tau, P301L, and R406W in vitro. Pin1 catalyzes the cis/trans isomerization of phospho-Ser/Thr-Pro sequences in a subset of proteins. Dephosphorylation of wild-type Tau was reduced in brain extracts of Pin1-knockout mice, and this reduction was not observed with P301L and R406W. On the other hand, binding to microtubules almost abolished dephosphorylation of wild-type and mutant Tau proteins. These results demonstrate that mutation of Tau and its association with microtubules may change the conformation of Tau, thereby suppressing dephosphorylation and potentially contributing to the etiology of tauopathies.

Sex difference in pathology and memory decline in rTg4510 mouse model of tauopathy

Abnormal phosphorylation of tau protein is a common event in many neurodegenerative disorders, including Alzheimer's disease and other tauopathies. We investigated the relationship between hyperphosphorylated tau in brain extracts and mnemonic functions in rTg4510 mouse model of tauopathy. We report that rTg4510 mice showed rapid deterioration in spatial learning and memory, which paralleled a significant increase of hyperphosphorylated tau in the brain between 3 and 5.5 months of age. At 5.5 months, rTg4510 females showed significantly higher levels of hyperphosphorylated tau than males, with no evidence of differential tau transgene expression between the sexes. The increased levels of hyperphosphorylated tau in females were associated with more severe impairment in spatial learning and memory as compared to transgenic males. We also showed that within studied age range, the decrease in memory performance was accompanied by other behavioral disturbances in the water maze related to search strategy, like thigmotaxic swim and cue response. These findings suggest that the onset of abnormal tau biochemistry and coincident cognitive deficits in the rTg4510 mouse model is sex-dependent with females being affected earlier and more aggressively than males.

A peptidyl-prolyl isomerase, FKBP12, accumulates in Alzheimer neurofibrillary tangles

We investigated a possible role in Alzheimer's disease (AD) for FKBP12, a peptidyl-prolyl cis-trans isomerase known to be important in protein assembly, folding and transportation by using Western blotting and microscopic analyses in postmortem brain tissues from elderly controls and the patients with AD. FKBP12 was enriched and localized to neuronal cell bodies and neurites in control brains. Intense immunoreactivity was found in large neurons such as pyramidal cells. Many FKBP12 positive granules were located in the cytoplasm and the proximal portion of dendrites and axons, and in the nuclei. By contrast, the expression of FKBP12 in AD brains was lower than in control brains. Furthermore, numerous intracellular neurofibrillary tangles (NFTs) were stained for FKBP12 in the hippocampal CA1 subfield, subiculum, entorhinal cortex and angular gyrus. Neuritic pathology such as neuropil threads and dystrophic neurites (DNs) within senile plaques (SPs) and some reactive astrocytes were also immunolabeled for FKBP12 in AD. Double immunofluorescence staining showed dual labeling of intracellular NFTs for FKBP12 and tau. Similar results were obtained in reactive astrocytes for the combination of FKBP12 and glial fibrillary acidic protein (GFAP). Labeling for FKBP12 was dense in axons stained for highly phosphorylated neurofilament protein. Thus our results suggest that FKBP12 may be involved in neuronal or astrocytic cytoskeletal organization and in the abnormal metabolism of tau protein in AD damaged neurons.

GSK-3beta phosphorylation of functionally distinct tau isoforms has differential, but mild effects

Background

Tau protein exists as six different isoforms that differ by the inclusion or exclusion of exons 2, 3 and 10. Exon 10 encodes a microtubule binding repeat, thereby resulting in three isoforms with three microtubule binding repeats (3R) and three isoforms that have four microtubule binding repeats (4R). In normal adult brain, the relative amounts of 3R tau and 4R tau are approximately equal. These relative protein levels are preserved in Alzheimer's disease, although in other neurodegenerative tauopathies such as progressive supranuclear palsy, corticobasal degeneration and Pick's disease, the ratio of 3R:4R is frequently altered. Because tau isoforms are not equally involved in these diseases, it is possible that they either have inherently unique characteristics owing to their primary structures or that post-translational modification, such as phosphorylation, differentially affects their properties.

Results

We have determined the effects of phosphorylation by a kinase widely believed to be involved in neurodegenerative processes, glycogen synthase kinase-3β (GSK-3β), on the microtubule binding and inducer-initiated polymerization of these isoforms in vitro. We have found that each isoform has a unique microtubule binding and polymerization profile that is altered by GSK-3β. GSK-3β phosphorylation had differential effects on the isoforms although there were similarities between isoforms and the effects were generally mild.

Conclusion

These results indicate that tau phosphorylation by a single kinase can have isoform specific outcomes. The mild nature of these changes, however, makes it unlikely that differential effects of GSK-3β phosphorylation on the isoforms are causative in neurodegenerative disease. Instead, the inherent differences in the isoform interactions themselves and local conditions in the diseased cells are likely the major determinant of isoform involvement in various neurodegenerative disorders.

Familial FTDP-17 missense mutations inhibit microtubule assembly-promoting activity of tau by increasing phosphorylation at Ser202 in vitro

In Alzheimer disease (AD), frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17) and other tauopathies, tau accumulates and forms paired helical filaments (PHFs) in the brain. Tau isolated from PHFs is phosphorylated at a number of sites, migrates as approximately 60-, 64-, and 68-kDa bands on SDS-gel, and does not promote microtubule assembly. Upon dephosphorylation, the PHF-tau migrates as approximately 50-60-kDa bands on SDS-gels in a manner similar to tau that is isolated from normal brain and promotes microtubule assembly. The site(s) that inhibits microtubule assembly-promoting activity when phosphorylated in the diseased brain is not known. In this study, when tau was phosphorylated by Cdk5 in vitro, its mobility shifted from approximately 60-kDa bands to approximately 64- and 68-kDa bands in a time-dependent manner. This mobility shift correlated with phosphorylation at Ser(202), and Ser(202) phosphorylation inhibited tau microtubule-assembly promoting activity. When several tau point mutants were analyzed, G272V, P301L, V337M, and R406W mutations associated with FTDP-17, but not nonspecific mutations S214A and S262A, promoted Ser(202) phosphorylation and mobility shift to a approximately 68-kDa band. Furthermore, Ser(202) phosphorylation inhibited the microtubule assembly-promoting activity of FTDP-17 mutants more than of WT. Our data indicate that FTDP-17 missense mutations, by promoting phosphorylation at Ser(202), inhibit the microtubule assembly-promoting activity of tau in vitro, suggesting that Ser(202) phosphorylation plays a major role in the development of NFT pathology in AD and related tauopathies.

Emerging pathways in genetic Parkinson's disease: tangles, Lewy bodies and LRRK2

The last decade has seen clear links emerge between the genetic determinants and neuropathological hallmarks of parkinsonism and dementia, notably with the discovery of mutations in alpha-synuclein and tau. Following the description of mutations in LRRK2 linked to Parkinson's disease, characterized by variable pathology including either alpha-synuclein or tau deposition, it has been suggested that LRRK2 functions as an upstream regulator of Parkinson's disease pathogenesis. This minireview explores this model, in the context of our current understanding of the biochemistry of LRRK2, alpha-synuclein and tau.

Neuronal protein trafficking associated with Alzheimer disease: from APP and BACE1 to glutamate receptors

Aberrant and/or cumulative amyloid-beta (Abeta) production, resulting from proteolytic processing of the amyloid precursor protein (APP) by beta and gamma-secretases, have been postulated to be a main etiological basis of Alzheimer disease (AD). A number of proteins influence the subcellular trafficking itinerary of APP and the beta-site APP-cleaving enzyme (BACE1) between the cell surface, endosomes and the trans-Golgi network (TGN). Available evidence suggests that co-residence of APP and BACE1 in the endosomal compartments promotes amyloidogenesis. Retrograde transport of APP out of the endosome to the TGN reduces Abeta production, while APP routed to and kept at the cell surface enhances its non-amyloidogenic, alpha-secretase-mediated processing. Changes in post-Golgi membrane trafficking in aging neurons that may influence APP processing is particularly relevant to late-onset, idiopathic AD. Dystrophic axons are key features of AD pathology, and impaired axonal transport could play crucial roles in the pathogenesis of idiopathic AD. Recent evidence has also indicated that Abeta-induced synaptic defects and memory impairment could be explained by a loss of both AMPA and NMDA receptors through endocytosis. Detail understanding of factors that influence these neuronal trafficking processes will open up novel therapeutic avenues for preventing or delaying the onset of symptomatic AD.

PPP2R2B CAG repeat length in the Han Chinese in Taiwan: Association analyses in neurological and psychiatric disorders and potential functional implications

PPP2R2B, a protein widely expressed in neurons throughout the brain, regulates the protein phosphatase 2A (PP2A) activity for the microtubule-associated protein tau and other substrates. Altered PP2A activity has been implicated in spinocerebellar ataxia 12, Alzheimer's disease (AD), and other tauopathies. Through a case-control study and a reporter assay, we investigated the association of PPP2R2B CAG repeat polymorphism with Taiwanese AD, essential tremor (ET), Parkinson's disease (PD), and schizophrenia and clarified the functional implication of this polymorphism. The distribution of the alleles was not significantly different between patients and controls, with 68.6-76.1% alleles at lengths of 10, 13, and 16 triplets. No expanded alleles were detected in either group. However, the frequency of the individuals carrying the short 5-, 6-, and 7-triplet alleles was notably higher in patients with AD (5/180 [2.8%], Fisher's exact test, P = 0.003; including 2 homozygotes) and ET (4/132 [3.0%], Fisher's exact test, P < 0.001) than in the controls (1/625 [0.2%]). The PPP2R2B transcriptional activity was significantly lower in the luciferase reporter constructs containing the (CAG)(5-7) allele than in those containing the common 10-, 13-, and 16-triplet alleles in both neuroblastoma and embryonic kidney cells. Therefore, our preliminary results suggest that the PPP2R2B gene CAG repeat polymorphism may be functional and may, in part, play a role in conferring susceptibility to AD and ET in Taiwan.