The microtubule binding of Tau and high molecular weight Tau in apoptotic PC12 cells is impaired because of altered phosphorylation

Targeting the Cation-Chloride Co-Transporter NKCC1 to Re-Establish GABAergic Inhibition and an Appropriate Excitatory/Inhibitory Balance in Selective Neuronal Circuits: A Novel Approach for the Treatment of Alzheimer's Disease

GABA, the main inhibitory neurotransmitter in the adult brain, depolarizes and excites immature neurons because of an initially higher intracellular chloride concentration [Cl-]i due to the delayed expression of the chloride exporter KCC2 at birth. Depolarization-induced calcium rise via NMDA receptors and voltage-dependent calcium channels is instrumental in shaping neuronal circuits and in controlling the excitatory (E)/inhibitory (I) balance in selective brain areas. An E/I imbalance accounts for cognitive impairment observed in several neuropsychiatric disorders. The aim of this review is to summarize recent data on the mechanisms by which alterations of GABAergic signaling alter the E/I balance in cortical and hippocampal neurons in Alzheimer's disease (AD) and the role of cation-chloride co-transporters in this process. In particular, we discuss the NGF and AD relationship and how mice engineered to express recombinant neutralizing anti-NGF antibodies (AD11 mice), which develop a neurodegenerative pathology reminiscent of that observed in AD patients, exhibit a depolarizing action of GABA due to KCC2 impairment. Treating AD and other forms of dementia with bumetanide, a selective KCC2 antagonist, contributes to re-establishing a proper E/I balance in selective brain areas, leading to amelioration of AD symptoms and the slowing down of disease progression.

Phosphorylation of SET mediates apoptosis via P53 hyperactivation and NM23-H1 nuclear import

Apoptosis plays an important role in neuron loss in Alzheimer's disease (AD). SET, an endogenous inhibitor of protein phosphatase-2A, is phosphorylated in AD brains and positively correlates with cell apoptosis. However, the mechanism underlying phosphorylated SET association with apoptosis remains unknown. Here, we show that mimetic phosphorylation of SET (S9E) induced apoptosis of primary cultured neurons. To investigate its mechanism, we overexpressed SET (S9E) in HEK293/tau cells and observed apoptosis accompanied with a marked increase of cleaved caspase-3 and cytoplasmic SET (S9E) retention with enhanced protein phosphatase-2A inhibition, which subsequently caused p53 hyperphosphorylation and activation. In addition, it caused the release of nucleoside diphosphate kinase A isoform a, a positive regulator of p53 with a DNase activity from SET/nucleoside diphosphate kinase A isoform a complex, and migration into the nucleus, resulting in DNA damage. Besides, it reduced nuclear tau accumulation leading to DNA protection deficiency. These findings suggest that SET phosphorylation is involved in the neuronal apoptotic pathway in AD and provide a new insight into the mechanism of this pathology.

New evidence on α-synuclein and Tau binding to conformation and sequence specific GC* rich DNA: Relevance to neurological disorders

Background:

Deoxyribonucleic acid (DNA) topology plays a critical role in maintaining the integrity of the genome and cellular functions. Although changes in DNA conformation and structural dynamics in the brain have been associated with various neurological disorders, its precise role in the pathogenesis is still unclear. Previous studies from our laboratory have shown that there is a conformational change in the genomic DNA of Parkinson's disease (PD) (B to altered B-DNA) and Alzheimer's disease brain (B to Z-DNA). However, there is limited information on the mechanism on DNA dynamics changes in brain.

Objective:

In the present study, we have investigated the DNA conformation and sequence specific binding ability of α-Synuclein and Tau with reference to B-DNA and Z-DNA using oligonucleotide (CGCGCGCG)₂ as a novel model DNA system. This sequence is predominantly present in the promoter region of the genes of biological relevance.

Materials and Methods:

Natively, (CGCGCGCG)₂ sequence exists in B-DNA conformation, but in the presence of high sodium concentration (4 M NaCl), the oligo converts into Z-DNA form. We used circular dichroism, melting temperature and fluorescence studies to understand protein-DNA interactions.

Results:

CD studies indicated that both α-Synuclein and Tau bind to B-DNA conformation of (CGCGCGCG)₂ and induce altered B-form. Further, these proteins increased the melting temperature and decreased the number of EtBr molecules bound per base pair of DNA in B-form indicating that DNA stability is favored to alter B-DNA conformation, which could be an intermediate form favoring Z-DNA conformation. Moreover, both α-Synuclein and Tau also bound to disease-linked Z-DNA conformation of (CGCGCGCG)₂ and further stabilized the Z-conformation.

Conclusions:

The present study provides vital mechanistic information on Synuclein and Tau binding to DNA in a conformation-specific manner causing conformational transition. Furthermore, both the proteins stabilize Z-DNA conformation. These have altered minor and major groove patterns and thus may have significant biological implications in relevance to gene expression pattern in neurodegeneration. We discuss the implications of α-Synuclein/Tau binding to DNA and stabilizing the altered conformations of DNA in neuronal cell dysfunction.

Change in tau phosphorylation associated with neurodegeneration in the ME7 model of prion disease

Hyperphosphorylation of the microtubule-associated protein tau is a significant determinant in AD (Alzheimer's disease), where it is associated with disrupted axonal transport and probably causes synaptic dysfunction. Although less well studied, hyperphosphorylation has been observed in prion disease. We have investigated the expression of hyperphosphorylated tau in the hippocampus of mice infected with the ME7 prion agent. In ME7-infected animals, there is a selective loss of CA1 synapse, first discernable at 13 weeks of disease. There is a potential that dysfunctional axonal transport contributes to this synaptopathy. Thus investigating hyperphosphorylated tau that is dysfunctional in AD could illuminate whether and how they are significant in prion disease. We observed no differences in the levels of phosphorylated tau (using MC1, PHF-1 and CP13 antibodies) in detergent-soluble and detergent-insoluble fractions extracted from ME7- and NBH- (normal brain homogenate) treated animals across disease. In contrast, we observed an increase in phospho-tau staining for several epitopes using immunohistochemistry in ME7-infected hippocampal sections. Although the changes were not of the magnitude seen in AD tissue, clear differences for several phospho-tau species were seen in the CA1 and CA3 of ME7-treated animals (pSer(199-202)>pSer(214)>PHF-1 antibody). Temporally, these changes were restricted to animals at 20 weeks and none of the disease-related staining was associated with the axons or dendrites that hold CA1 synapses. These findings suggest that phosphorylation of tau at the epitopes examined does not underpin the early synaptic dysfunction. These data suggest that the changes in tau phosphorylation recorded here and observed by others relate to end-stage prion pathology when early dysfunctions have progressed to overt neuronal loss.

New evidences on Tau-DNA interactions and relevance to neurodegeneration

Tau is mainly distributed in cytoplasm and also found to be localized in the nucleus. There is limited data on DNA binding potential of Tau. We provide novel evidence on nicking of DNA by Tau. Tau nicks the supercoiled DNA leading to open circular and linear forms. The metal ion magnesium (a co-factor for endonuclease) enhanced the Tau DNA nicking ability, while an endonuclease specific inhibitor, aurinetricarboxylic acid (ATA) inhibited the Tau DNA nicking ability. Further, we also evidenced that Tau induces B-C-A mixed conformational transition in DNA and also changes DNA stability. Tau-scDNA complex is more sensitive to DNAse I digestion indicating stability changes in DNA caused by Tau. These findings indicate that Tau alters DNA helicity and integrity and also nicks the DNA. The relevance of these novel intriguing findings regarding the role Tau in neuronal dysfunction is discussed.

p63α and γ Induce TAU Phosphorylation in Cultured Mammalian Cells

Here we show by western blotting that transcriptionally active isoforms of p63 (p63α and p63γ) induce the phosphorylation of human 2N4R tau at the tau-1/AT8 epitope in HEK293a cells; a phospho-epitope increased in Alzheimer's disease. Confocal microscopy shows that tau and p63α are spatially separated intracellularly. Tau was found in the cytoskeletal compartment, whilst p63α was located in the nucleus, indicating that the effects of p63 on tau phosphorylation are indirectly mediated. Tau phosphorylation occurred independently of the known tau kinases, protein kinase C delta (PKCδ), c-Jun N-terminal kinase (JNK), extracellular-signal-regulated kinase (ERK), p38 mitogen-activated protein kinase (p38), glycogen synthase kinase 3 (GSK3), v-akt murine thymoma viral oncogene homolog (AKT) and cyclin-dependent kinase 5 (Cdk5) and the tau protein phosphatases (PP), PP1 and PP2A-Aα/β. Considering that p63 and tau are both associated with developmental processes, these findings have ramifications for neuronal development and synaptic plasticity and also neurodegenerative diseases such as Alzheimer's disease and other tauopathies.

Endogenous Aβ causes cell death via early tau hyperphosphorylation

Alzheimer's disease (AD) is characterized by Aβ overproduction and tau hyperphosphorylation. We report that an early, transient and site-specific AD-like tau hyperphosphorylation at Ser262 and Thr231 epitopes is temporally and causally related with an activation of the endogenous amyloidogenic pathway that we previously reported in hippocampal neurons undergoing cell death upon NGF withdrawal [Matrone, C., Ciotti, M.T., Mercanti, D., Marolda, R., Calissano, P., 2008b. NGF and BDNF signaling control amyloidogenic route and Ab production in hippocampal neurons. Proc. Natl. Acad. Sci. 105, 13138-13143]. Such tau hyperphosphorylation, as well as apoptotic death, is (i) blocked by 4G8 and 6E10 Aβ antibodies or by specific β and/or γ-secretases inhibitors; (ii) temporally precedes tau cleavage mediated by a delayed (6-12h after NGF withdrawal) activation of caspase-3 and calpain-I; (iii) under control of Akt-GSK3β-mediated signaling. Finally, we show that such site-specific tau hyperphosphorylation causes tau detachment from microtubules and an impairment of mitochondrial trafficking. These results depict, for the first time, a rapid interplay between endogenous Aβ and tau post-translational modifications which act co-ordinately to compromise neuronal functions in the same neuronal system, under physiological conditions as seen in AD brain.

β-Amyloid impairs axonal BDNF retrograde trafficking

The neurotrophin, brain-derived neurotrophic factor (BDNF), is essential for synaptic function, plasticity and neuronal survival. At the axon terminal, when BDNF binds to its receptor, tropomyosin-related kinase B (TrkB), the signal is propagated along the axon to the cell body, via retrograde transport, regulating gene expression and neuronal function. Alzheimer disease (AD) is characterized by early impairments in synaptic function that may result in part from neurotrophin signaling deficits. Growing evidence suggests that soluble β-amyloid (Aβ) assemblies cause synaptic dysfunction by disrupting both neurotransmitter and neurotrophin signaling. Utilizing a novel microfluidic culture chamber, we demonstrate a BDNF retrograde signaling deficit in AD transgenic mouse neurons (Tg2576) that can be reversed by γ-secretase inhibitors. Using BDNF-GFP, we show that BDNF-mediated TrkB retrograde trafficking is impaired in Tg2576 axons. Furthermore, Aβ oligomers alone impair BDNF retrograde transport. Thus, Aβ reduces BDNF signaling by impairing axonal transport and this may underlie the synaptic dysfunction observed in AD.

Paclitaxel resistance in untransformed human mammary epithelial cells is associated with an aneuploidy-prone phenotype

Despite its increasing clinical use, almost no data are currently available about paclitaxel effects on non-cancerous mammary epithelial cells. We have previously established paclitaxel-resistant sub-cell lines (paclitaxel-surviving populations, PSPs; n=20), and sensitive controls (control clones, CCs; n=10), from the untransformed human mammary epithelial cell line HME1. In this study, we aimed to establish whether paclitaxel resistance was associated with a modified sensitivity to paclitaxel-induced aneuploidy. For this purpose, we analysed basal and paclitaxel-induced chromosome missegregation, apoptosis and aberrant spindle multipolarisation as well as microtubular network composition for each subline. PSP sublines showed higher basal and paclitaxel-induced chromosome missegregation than the CC sublines. This phenomenon was associated with resistance to paclitaxel-induced apoptosis. No significant difference in paclitaxel-induced spindle pole abnormalities between CC and PSP sublines was found. Besides, we showed that a majority of PSPs display a constitutively disrupted microtubular network composition due to aberrant tubulin expression and post-translational modifications. These results clearly indicate that paclitaxel resistance in untransformed human mammary epithelial cells is related to an increased susceptibility to acquire aneuploidy in response to this agent. The consequences of these paclitaxel-associated alterations could be deleterious as they can potentially trigger tumorigenesis.

Untangling tau hyperphosphorylation in drug design for neurodegenerative diseases

Aggregation of hyperphosphorylated tau is one of the characteristic neuropathological lesions of Alzheimer's disease and other neurodegenerative disorders. Pharmacological modulation of tau hyperphosphorylation might represent a valid and feasible therapeutic strategy for such disorders. Here, we consider recent evidence supporting the validity of the three most relevant kinases affecting tau hyperphosphorylation - GSK3beta, CDK5 and ERK2 - as drug targets and describe progress in the design of inhibitors for these kinases.

p53 is upregulated in Alzheimer's disease and induces tau phosphorylation in HEK293a cells

p53 and tau are both associated with neurodegenerative disorders. Here, we show by Western blotting that p53 is upregulated approximately 2-fold in the superior temporal gyrus of Alzheimer's patients compared to healthy elderly control subjects. Moreover, p53 was found to induce phosphorylation of human 2N4R tau at the tau-1/AT8 epitope in HEK293a cells. Confocal microscopy revealed that tau and p53 were spatially separated intracellularly. Tau was found in the cytoskeletal compartment, whilst p53 was located in the nucleus, indicating that the effects of p53 on tau phosphorylation are indirect. Collectively, these findings have ramifications for neuronal death associated with Alzheimer's disease and other tauopathies.

TAp73alpha induces tau phosphorylation in HEK293a cells via a transcription-dependent mechanism

p73 and tau both play roles in neurodevelopment and neurodegeneration. In this pilot study we show by Western blotting that TAp73alpha induces phosphorylation of human 2N4R tau at threonine-205 and at the PHF-1 epitope (serine366/serine404) in HEK293a cells. Neither the dominant negative isoform, DeltaNp73, nor a transcriptionally inactive mutant TAp73alpha(R292H) altered tau phosphorylation indicating that tau phosphorylation is dependent on the transcriptional activity of TAp73alpha. Consistent with this, confocal microscopy revealed that tau and TAp73alpha were spatially separated within the cell; tau being located in the cytoskeletal compartment whilst TAp73alpha was found in the nucleus. These findings have ramifications for microtubule dynamics associated with axonal growth during development and for neuronal death associated with Alzheimer's disease and other tauopathies.

Myotonic dystrophy expanded CUG repeats disturb the expression and phosphorylation of τ in PC12 cells

Mental retardation is a main feature of the congenital form of myotonic dystrophy (DM1), however, the molecular mechanisms underlying the central nervous system symptoms of DM1 are poorly understood. We have established a PC12 cell line-based model expressing the DM1 expanded CUG repeats (CTG90 cells) to analyze the effects of this mutation on neuronal functions. Previously, we have reported that CTG90 cells displayed impaired NGF-induced neuronal differentiation. Because disruption of normal expression of the microtubule associated protein tau and neuronal aggregates of hyperphosphorylated tau have been associated with DM1, this study analyzes the behavior of tau in the CTG90 cells. Several alterations of tau were observed in the PC12 cells that express expanded CUG repeats, including a subtle but reproducible reduction in the expression of the tau mRNA splicing isoform containing exon 10, decreased expression of tau and hyperphosphorylation of both tau and high molecular weight tau as well as abnormal nuclear localization of tau phosphorylated at Ser396/404. Interestingly, phosphorylation regulates negatively the activity of tau as microtubule-associated protein. In addition, impaired activity of the Akt/GSK3beta pathway, which phosphorylates tau, was also identified in the CTG90 cells. Besides tau phosphorylation, the Akt/GSK3beta signaling pathway regulates other key processes of PC12 cells, such as apoptosis and neuronal differentiation. Our results indicate that defective neuronal differentiation exhibited by the PC12 cells expressing expanded CUG repeats could be the result of combinatory effects derived from the altered behavior of tau and the impaired activation of the Akt/GSK3beta signaling pathway.

Human protein tau represses DNA replication in vitro

Here, in the experiments of both PCR and real-time PCR, a repression of DNA amplification was observed in the presence of protein tau. Furthermore, a strong repression appeared when an in vitro DNA replication assay was performed at the physiological temperature (37 degrees C). The incorporation of dNTP was markedly decreased to approximately 12% of control by the presence of tau23 and to approximately 15% by tau40. In the competitive experiments, the PCR product could be restored when the competitor DNA was added, indicating that the association of tau with the template gave rise to the repression. However, tau did not repress the yield of RNA in transcription, suggesting that tau was replaced or ejected from the template by the elongating T7 RNA polymerase.

Some flavonoids and DHEA-S prevent the cis-effect of expanded CTG repeats in a stable PC12 cell transformant

Expanded CUG triplet repeats carrying mRNA seem to be responsible for myotonic dystrophy type 1 (DM1). To study the pathogenesis of DM1, we constructed a DM1 cell culture model using a PC12 neuronal cell line and screened flavonoids that ameliorate this mRNA gain of function. The expanded 250 CTG repeat was subcloned into the 3'-untranslated region of the luciferase gene yielding a stable transformant of PC12 (CTG-250). The cytotoxicity of CTG-250 was evaluated by intracellular LDH activity, and the cis-effect by luciferase activity. To find agents that alter CTG-250 toxic effects, 235 bioflavonoids were screened. An increased cis-effect and cytotoxicity were found when CTG-250 was treated with nerve growth factor to induce differentiation. Western blotting with anti-caspase-3 antibody suggested that cell death was caused by apoptosis. Screening analysis confirmed that a flavone (toringin), an isoflavones (genistein and formononetin), a flavanone (isosakuranetin), and DHEA-S prevent both the cytotoxicity and cis-effect of CTG-250 and that a flavanone (naringenin), isoflavone (ononin), and xanthylatin strongly inhibit the cis-effect of CTG repeats. In conclusion, we found that this neuronal cell line, which expresses the CUG repeat-bearing mRNA, showed cis-effects through the reporter gene and neuronal death after cell differentiation in vitro. However, some flavonoids and DHEA-S inhibit both the cis-effect and cytotoxicity, indicating that their chemical structures work to ameliorate both these toxic effects. This system makes it easy to evaluate the toxic effects of expanded CTG repeats and therefore should be useful for screening other DM1 treatments for their efficacies.

The Role of Caspase Cleavage of Tau in Alzheimer Disease Neuropathology

Alzheimer disease (AD) is characterized by the accumulation of amyloid plaques and neurofibrillary tangles within selective brain regions. In addition, cell death pathways become active leading to neurodegeneration. Caspase activation, a key step in the programmed cell death pathway known as apoptosis, occurs in AD and leads to the proteolytic cleavage of several neuronal proteins. Previously, it was hypothesized that the development of the classical hallmarks of AD, amyloid plaques and neurofibrillary tangles, occur independently and do not involve the activation of caspases. However, recent studies suggest that plaques, tangles, and caspase activation share a common pathway. Beta-amyloid, the main component of amyloid plaques, activates caspases. Activated caspases can in turn cleave tau, the main component of neurofibrillary tangles. Caspase-cleaved tau (deltatau) may initiate or accelerate the development of tangle pathology. Tau, when cleaved by caspases at Asp421, "seeds" filamentous aggregates in vitro. Caspase-cleaved tau also adopts the MC1 conformation, one of the earliest pathologic events in tangle formation. Importantly, deltatau occurs early in the development of tangle pathology within AD brains and in a transgenic mouse model of AD. This review summarizes recent evidence suggesting that caspase cleavage of tau plays an important role in the development of neurofibrillary tangle pathology. In addition, a model is presented whereby caspase cleavage of tau provides a mechanistic link between the development of amyloid and tangle pathologies.

Pinning down phosphorylated tau and tauopathies

Neurofibrillary tangles (NFTs) are prominent neuronal lesions in a large subset of neurodegenerative diseases, including Alzheimer's disease (AD). NFTs are mainly composed of insoluble Tau that is hyperphosphorylated on many serine or threonine residues preceding proline (pSer/Thr-Pro). Tau hyperphosphorylation abolishes its biological function to bind microtubules and promotes microtubule assembly and precedes neurodegeneration. Not much is known about how tau is further regulated following phosphorylation. Notably, we have recently shown that phosphorylated Ser/Thr-Pro motifs exist in two distinct conformations. The conversion between two conformations in some proteins is catalyzed by the prolyl isomerase Pin1. Pin1 binds to tau phosphorylated specifically on the Thr231-Pro site and probably catalyzes cis/trans isomerization of pSer/Thr-Pro motif(s), thereby inducing conformational changes in tau. Such conformational changes can directly restore the ability of phosphorylated Tau to bind microtubules and promote microtubule assembly and/or facilitate tau dephosphorylation by its phosphatase PP2A, as PP2A activity is conformation-specific. Furthermore, Pin1 expression inversely correlates with the predicted neuronal vulnerability in normally aged brain and also with actual neurofibrillary degeneration in AD brain. Moreover, deletion of the gene encoding Pin1 in mice causes progressive age-dependent neuropathy characterized by motor and behavioral deficits, tau hyperphosphorylation, tau filament formation and neuronal degeneration. Distinct from all other mouse models where transgenic overexpression of specific proteins elicits tau-related pathologies, Pin1 is the first protein whose depletion causes age-dependent neurodegeneration and tau pathologies. Thus, Pin1 is pivotal in maintaining normal neuronal function and preventing age-dependent neurodegeneration. This could represent a promising interventive target to prevent neurodegenerative diseases.

Glycogen synthase kinase 3 beta induces caspase-cleaved tau aggregation in situ

Tau is a substrate of caspases, and caspase-cleaved tau has been detected in Alzheimer's disease brain but not in control brain. Furthermore, in vitro studies have revealed that caspase-cleaved tau is more fibrillogenic than full-length tau. Considering these previous findings, the purpose of this study was to determine how the caspase cleavage of tau affected tau function and aggregation in a cell model system. The effects of glycogen synthase kinase 3 beta (GSK3 beta), a well established tau kinase, on these processes also were examined. Tau or tau that had been truncated at Asp-421 to mimic caspase cleavage (Tau-D421) was transfected into cells with or without GSK3 beta, and phosphorylation, microtubule binding, and tau aggregation were examined. Tau-D421 was not as efficiently phosphorylated by GSK3 beta as full-length tau. Tau-D421 efficiently bound microtubules, and in contrast to the full-length tau, co-expression with GSK3 beta did not result in a reduction in the ability of Tau-D421 to bind microtubules. In the absence of GSK3 beta, neither Tau-D421 nor full-length tau formed Sarkosyl-insoluble inclusions. However, in the presence of GSK3 beta, Tau-D421, but not full-length tau, was present in the Sarkosyl-insoluble fraction and formed thioflavin-S-positive inclusions in the cell. Nonetheless, co-expression of GSK3 beta and Tau-D421 did not result in an enhancement of cell death. These data suggest that a combination of phosphorylation events and caspase activation contribute to the tau oligomerization process in Alzheimer's disease, with GSK3 beta-mediated tau phosphorylation preceding caspase cleavage.

Linking alterations in tau phosphorylation and cleavage during neuronal apoptosis

Neurofibrillary tangles (NFTs) are classic lesions of Alzheimer's disease. NFTs are bundles of abnormally phosphorylated tau, the paired helical filaments. The initiating mechanisms of NFTs and their role in neuronal loss are still unknown. Accumulating evidence supports a role for the activation of proteolytic enzymes, caspases, in neuronal death observed in brains of patients with Alzheimer's disease. Alterations in tau phosphorylation and tau cleavage by caspases have been previously reported in neuronal apoptosis. However, the links between the alterations in tau phosphorylation and its proteolytic cleavage have not yet been documented. Here, we show that, during staurosporine-induced neuronal apoptosis, tau first undergoes transient hyperphosphorylation, which is followed by dephosphorylation and cleavage. This cleavage generated a 10-kDa fragment in addition to the 17- and 50-kDa tau fragments previously reported. Prior tau dephosphorylation by a glycogen synthase kinase-3beta inhibitor, lithium, enhanced tau cleavage and sensitized neurons to staurosporine-induced apoptosis. Caspase inhibition prevented tau cleavage without reversing changes in tau phosphorylation linked to apoptosis. Furthermore, the microtubule depolymerizing agent, colchicine, induced tau dephosphorylation and caspase-independent tau cleavage and degradation. Both phenomena were blocked by inhibiting protein phosphatase 2A (PP2A) by okadaic acid. These experiments indicate that tau dephosphorylation precedes and is required for its cleavage and degradation. We propose that the absence of cleavage and degradation of hyperphosphorylated tau (due to PP2A inhibition) may lead to its accumulation in degenerating neurons. This mechanism may contribute to the aggregation of hyperphosphorylated tau into paired helical filaments in Alzheimer's disease where reduced PP2A activity has been reported.

Overexpression of tau leads to the stimulation of neurite outgrowth, the activation of caspase 3 activity, and accumulation and phosphorylation of tau in neuroblastoma cells on cAMP treatment

To explore changes to the tau molecule in Alzheimer's disease, we studied the effect of tau expression in stably transfected neuroblastoma x glioma hybrid NG108-15 cells (tau cells). Tau cells had a similar shape to, but more neurites than, wild type NG108-15 cells (wild type cells). When treated with cAMP, tau cells began to form neurites within 2h. After that, these neurites became longer and thicker than those of wild type cells. An accumulation and increased phosphorylation of tau were observed after 8 h and caspase 3 activity was increased after 4 h in tau cells, but not in wild type cells, upon treatment with cAMP. Caspase 3 activity was activated after the initiation of morphological change, and before the accumulation of tau in tau cells. Under these conditions, apoptotic cell death was not observed and tau was colocalized with tubulin. However, the accumulated tau molecules did not associate with tubulin and were dislocated around and in the nuclei of tau cells. These observations have implications for the cellular causes of Alzheimer's disease where the accumulation and mislocation of tau occur concomitant with neuronal degeneration.

In vitro cultured neurons for molecular studies correlating apoptosis with events related to Alzheimer disease

This short review analyses the possible molecular events linking a general program of death such as apoptosis to highly specific intracellular pathways involving the function and degradation of two proteins--tau and amyloid precursor protein--which in their aggregated state constitute the hallmark of Alzheimer disease. By surveying the recent studies carried out in 'in vitro' neuronal cultures--with special emphasis to cerebellar granule neurons--the apparent correlation between onset of apoptosis, tau cleavage with formation of potential toxic fragments, and activation of an amyloidogenic route are discussed. Within this framework, proteasomes seem to play a crucial role upstream of the proteolytic cascade involving calpain(s) and caspase(s) by contributing to tau and amyloid precursor protein-altered breakdown and consequent tendency to aggregation of their degradation fragments. Thus, apoptotic death due to altered supply of anti apoptotic agents, neurotrophic factors, deafferentiation or other causes, may constitute a major trigger of the onset of Alzheimer disease.

Primed phosphorylation of tau at Thr231 by glycogen synthase kinase 3beta (GSK3beta) plays a critical role in regulating tau's ability to bind and stabilize microtubules

Site-specific phosphorylation of tau negatively regulates its ability to bind and stabilize microtubule structure. Although tau is a substrate of glycogen synthase kinase 3beta (GSK3beta), the exact sites on tau that are phosphorylated by this kinase in situ have not yet been established, and the effect of these phosphorylation events on tau-microtubule interactions have not been fully elucidated. GSK3beta phosphorylates both primed and unprimed sites on tau, but only primed phosphorylation events significantly decrease the ability of tau to bind microtubules. The focus of the present study is on determining the importance of the GSK3beta-mediated phosphorylation of a specific primed site, Thr231, in regulating tau's function. Pre-phosphorylation of Ser235 primes tau for phosphorylation by GSK3beta at Thr231. Phosphorylation by GSK3beta of wild-type tau or tau with Ser235 mutated to Ala decreases tau-microtubule interactions. However, when Thr231 alone or Thr231 and Ser235 in tau were mutated to Ala, phosphorylation by GSK3beta did not decrease the association of tau with the cytoskeleton. Further, T231A tau was still able to efficiently bind microtubules after phosphorylation by GSK3beta. Expression of each tau construct alone increased tubulin acetylation, a marker of microtubule stability. However, when cells were cotransfected with wild-type tau and GSK3beta, the level of tubulin acetylation was decreased to vector-transfected levels. In contrast, coexpression of GSK3beta with mutated tau (T231A/S235A) did not significantly decrease the levels of acetylated tubulin. These results strongly indicate that phosphorylation of Thr231 in tau by GSK3beta plays a critical role in regulating tau's ability to bind and stabilize microtubules.

Tau could protect DNA double helix structure

The hyperchromic effect has been used to detect the effect of tau on the transition of double-stranded DNA to single-stranded DNA. It was shown that tau increased the melting temperature of calf thymus DNA from 67 to 81 degrees C and that of plasmid from 75 to 85 degrees C. Kinetically, rates of increase in absorbance at 260 nm of DNA incubated with tau were markedly slower than those of DNA and DNA/bovine serum albumin used as controls during thermal denaturation. In contrast, rates of decrease in the DNA absorbance with tau were faster than those of controls when samples were immediately transferred from thermal conditions to room temperature. It revealed that tau prevented DNA from thermal denaturation, and improved renaturation of DNA. Circular dichroic spectra results indicated that there were little detectable conformational changes in DNA double helix when tau was added. Furthermore, tau showed its ability to protect DNA from hydroxyl radical (.OH) attacking in vitro, implying that tau functions as a DNA-protecting molecule to the radical.

Glycogen synthase kinase 3beta phosphorylates tau at both primed and unprimed sites. Differential impact on microtubule binding

Glycogen synthase kinase 3beta (GSK3beta) phosphorylates substrates, including the microtubule-associated protein tau, at both primed and unprimed epitopes. GSK3beta phosphorylation of tau negatively regulates tau-microtubule interactions; however the differential effects of phosphorylation at primed and unprimed epitopes on tau is unknown. To examine the phosphorylation of tau at primed and unprimed epitopes and how this impacts tau function, the R96A mutant of GSK3beta was used, a mutation that prevents phosphorylation of substrates at primed sites. Both GSK3beta and GSK3beta-R96A phosphorylated tau efficiently in situ. However, expression of GSK3beta-R96A resulted in significantly less phosphorylation of tau at primed sites compared with GSK3beta. Conversely, GSK3beta-R96A phosphorylated unprimed tau sites to a significantly greater extent than GSK3beta. Prephosphorylating tau with cdk5/p25 impaired the ability of GSK3beta-R96A to phosphorylate tau, whereas GSK3beta-R96A phosphorylated recombinant tau to a significantly greater extent than GSK3beta. Moreover, the amount of tau associated with microtubules was reduced by overexpression of GSK3beta but only when tau was phosphorylated at primed sites, as phosphorylation of tau by GSK3beta-R96A did not negatively regulate the association of tau with microtubules. These results demonstrate that GSK3beta-mediated phosphorylation of tau at primed sites plays a more significant role in regulating the interaction of tau with microtubules than phosphorylation at unprimed epitopes.

The complex relationship between soluble and insoluble tau in tauopathies revealed by efficient dephosphorylation and specific antibodies

Phosphorylated tau is deposited as insoluble inclusion bodies in the tauopathies. We have used a new efficient method to dephosphorylate tau extracted from control and tauopathy brain. In some tauopathies, including Alzheimer's disease and progressive supranuclear palsy, the pattern of insoluble tau isoforms reflected that of soluble tau. In contrast, in corticobasal degeneration, Pick's disease, and some forms of fronto-temporal dementia, specific tau isoforms were selectively sequestered into insoluble inclusion-forming tau. Therefore the overall expression of individual tau isoforms does not predict which tau isoforms are deposited in all tauopathies and different mechanisms must operate that result in the deposition of specific tau isoforms.

Cdk5 phosphorylates p53 and regulates its activity

Cyclin dependent kinase 5 (Cdk5) is a proline-direct protein kinase that is most active in the CNS, and has been implicated as a contributing factor in certain neurodegenerative diseases. Further, there is evidence to suggest that Cdk5 may facilitate the progression of apoptosis. However, the mechanisms involved have not been elucidated. The tumor suppressor protein p53, a transcription factor that is regulated by phosphorylation, increases the expression of genes that control growth arrest or cell death. To understand how Cdk5 could facilitate apoptosis, the effects of Cdk5 on p53 activity were examined. In the present study it is shown that in apoptotic PC12 cells the levels of p53 and Cdk5 increase concomitantly. Further, Cdk5/p25 effectively phosphorylates recombinant p53 in vitro. Transient transfection of Cdk5/p25 into cells results in an increase in p53 levels, as well as the expression of the p53-responsive genes p21 and Bax. Furthermore, evidence is provided that increased Cdk5 activity increases p53 transcriptional activity significantly, suggesting that p53 is modulated in situ by Cdk5. This is the first demonstration that p53 is a substrate of Cdk5, and that Cdk5 can modulate p53 levels and activity.

Transient expression of fluorescent tau proteins promotes process formation in PC12 cells: contributions of the tau C-terminus to this process

The neuronal microtubule-associated protein tau promotes microtubule assembly and has been implicated in the development of axonal morphology. In this study, PC12 cells were transiently transfected with constructs coding fusion proteins of human tau with green fluorescent protein (GFP). Expression of tau constructs actively stabilized microtubules. Expression of the C-terminus of tau can mimic this effect in living cells, though to a lesser extent because of the absence of the tau N-terminus. However, tau colocalization with microtubules did not require the presence of the tau N-terminus. Transient expression of tau (including tau24, a four-repeat human tau isoform encoded in 383 residues, and tau23, human fetal tau isoform encoded in 352 residues) stimulated process formation in PC12 cells, and this occurred faster with tau24 than with tau23. The residues (residues 154-172 in tau23) that confer microtubule nucleation activity of tau in vitro are not required for tau-directed process formation. However, when tau induces the formation of cellular processes in response to cortical breakdown by cytochalasin B, residues 154-172 must be present. Thus, it appears that tau may serve to promote cellular process outgrowth in cultured neuronal cells and that C-terminus of tau is essential to this process.

In vivo activation and nuclear translocation of phosphorylated glycogen synthase kinase-3beta in neuronal apoptosis: links to tau phosphorylation

The roles of glycogen synthase kinase-3beta (GSK-3beta) and tau phosphorylation were examined in seven-day-old rats injected with the NMDA receptor antagonist (MK801) that is known to induce neuronal apoptosis. Immunoblot and immunohistochemical analysis of brain samples demonstrated a site-specific increase in tau phosphorylation associated with the relocalization of the protein to the nuclear/perinuclear region of apoptotic neurons. In addition, a tau 32-kDa fragment was detected, suggesting that tau was a target of intracellular proteolysis in MK801-treated brains. The proteolytically modified form of tau has reduced ability to bind to microtubules. GSK-3beta kinase assay and immunoblottings of active (tyrosine-216) and inactive (serine-9) forms of GSK-3beta revealed a rapid and transient increase in the kinase activity. Lithium chloride, a GSK-3beta inhibitor, prevented tau phosphorylation suggesting that tau phosphorylation is mediated by the activation of GSK-3beta. Confocal microscopy using double labelling of tau and GSK-3beta revealed that the activation of GSK-3beta in neurons was associated with early (2 h) nuclear translocation of tyrosine-216 GSK-3beta. The execution phase of neuronal apoptosis was accompanied by a selective phosphorylation of serine-9 and dephosphorylation of tyrosine-216 GSK-3beta. These findings demonstrate that in vivo, GSK-3beta kinase activation and nuclear translocation are early stress signals of neuronal apoptosis.

Tau phosphorylation during apoptosis of human SH-SY5Y neuroblastoma cells

In Alzheimer's Disease brain, the microtubule-associated protein tau is hyperphosphorylated at specific epitopes and abnormally aggregates into filamentous structures. In addition, there is significant neurodegeneration in Alzheimer's disease brain, and there is data to suggest that apoptotic-like processes may contribute to the neurodegeneration. It has been demonstrated that in PC12 cells undergoing apoptosis due trophic factor removal, tau is hyperphosphorylated prior to chromatin condensation. To establish that increased tau phosphorylation is a generalized outcome of the apoptotic process, and to examine the involvement of the protein kinase in these events, apoptosis was induced in retinoic-acid differentiated human SH-SY5Y neuroblastoma cells using the topoisomerase-1 inhibitor camptothecin. Treatment of the differentiated SH-SY5Y cells with camptothecin resulted in a time and concentration dependent activation of caspase-3 with a concomitant increase in the presence of apoptotic nuclei. Immunoblotting revealed that camptothecin treatment resulted in a significant increase in tau phosphorylation. Addition of a cyclin-dependent kinase inhibitor reduced camptothecin-induced cell death in the differentiated SH-SY5Y cells and decreased the effects of camptothecin on tau phosphorylation. In contrast, a general caspase inhibitor decreased camptothecin-induced cell death, but did not significantly decrease the increases in tau phosphorylation. These results suggest that increased tau phosphorylation is likely a generalized outcome of apoptotic processes in neuron-related cells, and that cyclin-dependent kinases probably play a role in this process.

Tau and HMW tau phosphorylation and compartmentalization in apoptotic neuronal PC12 cells

In the Alzheimer disease brain, the microtubule-associated protein tau is hyperphosphorylated. There is also evidence that apoptotic-like processes may contribute to the neuronal loss in AD. In an apoptotic model that involves replating neuronal PC12 cells without serum and nerve growth factor (NGF), tau was hyperphosphorylated. During replating, however, neurites are removed. Here, differentiated cells were maintained in serum-free media before growth factor removal, thus maintaining neuritic processes during the apoptotic process and allowing for evaluation of neuritic changes. Tau phosphorylation, evaluated by immunoblotting and immunocytochemistry, was compared with various measures of cell death. Compared with control, NGF-deprived cells exhibited gradual and consistent increases of lactate dehydrogenase release over a 5-day period and a peak of caspase-3 activity at Day 2 after NGF removal. Nuclear staining demonstrated chromatin condensation in NGF-deprived cells. Apoptotic cells had thickened, tortuous, and shortened neuritic processes compared with control cells. Immunoblotting showed an increase in both tau and high molecular weight (HMW) tau phosphorylation during the apoptotic process. Immunoreactivity of both tau isoforms shifted from the detergent insoluble cytoskeleton to the detergent soluble compartment in the apoptotic cells. The microtubule binding of both tau isoforms from apoptotic cells also was impaired. Immunoblotting of purified plasma membrane showed preferential association of HMW tau with the plasma membrane during apoptosis. Also, plasma membrane-associated HMW tau was more phosphorylated during apoptosis. Immunocytochemistry demonstrated increased tau phosphorylation in most apoptotic cells, especially in the neurites. Tau was, however, dephosphorylated cells in the last stages of apoptosis.

Hyperosmotic stress-induced apoptosis and tau phosphorylation in human neuroblastoma cells

A characteristic hallmark of Alzheimer's disease brain is the presence of hyperphosphorylated tau; however, the mechanisms responsible for the aberrant tau phosphorylation are unknown. Recently, it has been shown that apoptotic-like processes may be involved in some of the neuronal loss in Alzheimer's disease. In consideration of these findings, the relationship between tau phosphorylation and apoptosis was examined in human neuroblastoma SH-SY5Y cells that were subjected to hyperosmotic stress. In this model caspase 3 activity, which served as an indicator of apoptosis, was increased by 30 min of osmotic stress and remained elevated through 4 hr. Hyperosmotic stress also resulted in a robust increase in tau phosphorylation at both Ser/Pro and non-Ser/Pro sites. Phosphorylation of Ser262/356 (12E8) and Ser396/404 (PHF-1) increased by 5 min and remained elevated for at least 1 hr. In contrast, phosphorylation within the Tau-1 epitope did not increase (as evidenced by decreased immunoreactivity) until 30 min after treatment but remained elevated for a much greater period of time. Treatment with insulin-like growth factor-1 delayed but did not prevent apoptotic cell death induced by osmotic stress and attenuated the increase in phosphorylation at the Tau-1 epitope. Li(+), an inhibitor of glycogen synthase kinase 3 beta, had no effect on osmotic stress-induced caspase activation, but reduced phosphorylation at the Tau-1 epitope. Complete inhibition of osmotic stress-induced caspase activation with DEVD-CHO had no effect on the increases in tau phosphorylation. The results of these studies demonstrate that tau phosphorylation is increased at the specific epitopes during apoptosis. However, the changes in tau phosphorylation likely do not significantly impact the apoptotic process but rather occur concurrently as a result of inappropriate activation of specific protein kinases. Nonetheless, there is increasing evidence of a dysregulation of protein kinases that occurs in Alzheimer's disease brain that may be part of the events of apoptosis, which could contribute to aberrant increases in tau phosphorylation.

The levels of cdk5 and p35 proteins and tau phosphorylation are reduced during neuronal apoptosis

Cyclin-dependent kinase 5 (cdk5) is believed to be involved in the phosphorylation of tau protein. We studied the expression of the protein levels of cdk5 and the neuron-specific cdk5 activator p35 as well as cdk5 activity and tau phosphorylation during apoptosis in rat hippocampal neuronal cultures. We observed that in cells treated with etoposide, cyclosporin A, 4-hydroxynonenal (HNE), or okadaic acid, there was an early reduction in the protein levels of p35, and later also in cdk5 with all treatments except etoposide. The level of p25, a calpain cleavage product of p35 suggested to have increased ability to activate cdk5, was reduced paralleling the amount of p35. The changes in the p35 and p25 protein levels coincided with decreases in cdk5 activity and tau phosphorylation after treatment with HNE and etoposide. However, the relationship between the p35 and p25 levels and cdk5 activity was complex. We conclude that neuronal apoptosis is accompanied with a decrease in the levels of p35, p25, and cdk5, and tau phosphorylation. These changes may reinforce the neuronal damage.

Hyperphosphorylation of tau is mediated by ERK activation during anticancer drug-induced apoptosis in neuroblastoma cells

Phosphorylated tau protein is the major component of paired helical filaments in Alzheimer disease (AD). We have previously shown that abnormal tau phosphorylation was induced in neuroblastoma SK-N-SH cells by the anticancer drug, paclitaxel, during apoptosis [Guise et al., 1999: Apoptosis 4:47-58]. In the present study, we first demonstrated a shift from fetal tau to hyperphosphorylated tau after incubation with paclitaxel, that showed some similarities with the hyperphosphorylated tau in AD, by using several tau antibodies, N-Term, Tau-1 and AT-8. Tau phosphorylation occurred independently of caspase-3 activation. We next showed that a sustained activation of ERK (extracellular signal-regulated kinase) induced both tau phosphorylation and apoptosis during paclitaxel treatment (1 microM). The inhibition of ERK activation by using the pharmacological MEK1/2 inhibitor, PD98059 (50 microM), or an antisense strategy, reduced tau phosphorylation and neuronal apoptosis (P < 0.001), indicating a link between ERK activation, tau phosphorylation and apoptosis. Doxorubicin (0.2 microM), an anticancer drug whose mechanism of action is independent of microtubules, also induced ERK activation, tau phosphorylation and apoptosis. Moreover, doxorubicin induced some morphological features of neurodegeneration such as loss of neurites and disorganization of the cytoskeleton in apoptotic neuroblastoma cells. Altogether, our results suggest that tau phosphorylation plays a significant role in apoptosis enhancing disruption of microtubules that in turn leads to formation of apoptotic bodies, suggesting that neurodegeneration and apoptosis are related.

Tau protein is hyperphosphorylated in a site-specific manner in apoptotic neuronal PC12 cells

Alterations in the status of microtubules contribute to the cytoskeletal rearrangements that occur during apoptosis. The microtubule-associated protein tau regulates microtubule dynamics and thus is likely to play an important role in the cytoskeletal changes that occur in apoptotic cells. Previously, we demonstrated that the phosphorylation of tau at the Tau-1 epitope was increased during neuronal PC12 cell apoptosis, and further that the microtubule binding of tau from apoptotic cells was significantly impaired because of altered phosphorylation. The fact that the microtubule-binding capacity of tau from apoptotic cells was reduced to approximately 30% of control values indicated that sites in addition to those within the Tau-1 epitope were hyperphosphorylated during apoptosis. In this study using a combination of immunological and biochemical approaches, numerous sites were found to be hyperphosphorylated on tau isolated from apoptotic cells. Further, during apoptosis, the activities of cell division control protein kinase (cdc2) and cyclin-dependent kinase 5 (cdk5) were selectively and significantly increased. The association of these two protein kinases with tau was also increased during apoptosis. These findings are intriguing because many of the sites found to be hyperphosphorylated on tau during apoptosis are also hyperphosphorylated on tau from Alzheimer's disease brain. Likewise, there are data indicating that in Alzheimer's disease the activities of cdc2 and cdk5 are also increased.