Regulation of expression, phosphorylation and biological activity of tau during differentiation in SY5Y cells

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.

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

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

Tibolone modulates neuronal plasticity through regulating Tau, GSK3β/Akt/PI3K pathway and CDK5 p35/p25 complexes in the hippocampus of aged male mice

Aging is a key risk factor for cognitive decline and age-related neurodegenerative disorders. Also, an age-related decrease in sex steroid hormones may have a negative impact on the formation of neurofibrillary tangles (NFTs); these hormones can regulate Tau phosphorylation and the principal kinase GSK3β involved in this process. Hormone replacement therapy decreases NFTs, but it increases the risk of some types of cancer. However, other synthetic hormones such as tibolone (TIB) have been used for hormone replacement therapy. The aim of this work was to evaluate the long-term effects of TIB (0.01 mg/kg and 1 mg/kg, intragastrically for 12 weeks) on the content of total and hyperphosphorylated Tau (PHF-1) proteins and the regulation of GSK3β/Akt/PI3K pathway and CDK5/p35/p25 complexes in the hippocampus of aged male mice. We observed that the content of PHF-1 decreased with TIB administration. In contrast, no changes were observed in the active form of GSK3β or PI3K. TIB decreased the expression of the total and phosphorylated form of Akt while increased that of p110 and p85. The content of CDK5 was differentially modified with TIB: it was increased at low doses and decreased at high doses. When we analyzed the content of CDK5 activators, an increase was found on p35; however, the content of p25 decreased with administration of low dose of TIB. Our results suggest a possible mechanism of action of TIB in the hippocampus of aged male mice. Through the regulation of Tau and GSK3β/Akt/PI3K pathway, and CDK5/p35/p25 complexes, TIB may modulate neuronal plasticity and regulate learning and memory processes.

Altered Machinery of Protein Synthesis in Alzheimer's: From the Nucleolus to the Ribosome

Ribosomes and protein synthesis have been reported to be altered in the cerebral cortex at advanced stages of Alzheimer's disease (AD). Modifications in the hippocampus with disease progression have not been assessed. Sixty-seven cases including middle-aged (MA) and AD stages I-VI were analyzed. Nucleolar chaperones nucleolin, nucleophosmin and nucleoplasmin 3, and upstream binding transcription factor RNA polymerase I gene (UBTF) mRNAs are abnormally regulated and their protein levels reduced in AD. Histone modifications dimethylated histone H3K9 (H3K9me2) and acetylated histone H3K12 (H3K12ac) are decreased in CA1. Nuclear tau declines in CA1 and dentate gyrus (DG), and practically disappears in neurons with neurofibrillary tangles. Subunit 28 ribosomal RNA (28S rRNA) expression is altered in CA1 and DG in AD. Several genes encoding ribosomal proteins are abnormally regulated and protein levels of translation initiation factors eIF2α, eIF3η and eIF5, and elongation factor eEF2, are altered in the CA1 region in AD. These findings show alterations in the protein synthesis machinery in AD involving the nucleolus, nucleus and ribosomes in the hippocampus in AD some of them starting at first stages (I-II) preceding neuron loss. These changes may lie behind reduced numbers of dendritic branches and reduced synapses of CA1 and DG neurons which cause hippocampal atrophy.

mTor mediates tau localization and secretion: Implication for Alzheimer's disease

Abnormally hyperphosphorylated tau aggregates form paired helical filaments (PHFs) in neurofibrillary tangles, a key hallmark of Alzheimer's disease (AD) and other tauopathies. The cerebrospinal fluid (CSF) levels of soluble total tau and phospho-tau from clinically diagnosed AD patients are significantly higher compared with controls. Data from both in vitro and in vivo AD models have implied that an aberrant increase of mammalian target of rapamycin (mTor) signaling may be a causative factor for the formation of abnormally hyperphosphorylated tau. In the present study, we showed that in post-mortem human AD brain, tau was localized within different organelles (autophagic vacuoles, endoplasmic reticulum, Golgi complexes, and mitochondria). In human SH-SY5Y neuroblastoma cells stably carrying different genetic variants of mTor, we found a common link between the synthesis and distribution of intracellular tau. mTor overexpression or the lack of its expression was responsible for the altered balance of phosphorylated (p-)/-non phosphorylated (Np-) tau in the cytoplasm and different cellular compartments, which might facilitate tau deposition. Up-regulated mTor activity resulted in a significant increase in the amount of cytosolic tau as well as its re-localization to exocytotic vesicles that were not associated with exosomes. These results have implicated that mTor is involved in regulating tau distribution in subcellular organelles and in the initiation of tau secretion from cells to extracellular space.

The cellular distribution and Ser262 phosphorylation of tau protein are regulated by BDNF in vitro

The brain-enriched microtubule-associated protein tau, a critical regulator of cytoskeletal dynamics, forms insoluble aggregates in a number of neurodegenerative diseases termed tauopathies, including Alzheimer's disease (AD). Hyperphosphorylation of tau protein is an important mechanism for aggregation, so many studies on the pathogenesis of AD and other tauopathies have focused on regulation of tau phosphorylation by kinases and phosphatases. Less studied are mechanisms of tau transcriptional and post-transcriptional regulation by extracellular signals such as BDNF and how such changes alter neuronal function. Previously, we reported that tau is required for morphological plasticity induced by BDNF. Here, we further explore tau modification during BDNF-induced changes in neuronal cell morphology. In undifferentiated SH-SY5Y cells lacking neurites, tau formed a sphere within the soma as revealed by immunocytochemistry. In contrast, tau was enriched in the neurites and sparse in the soma of SH-SY5Y cells induced to differentiate by retinoic acid (RA). Treatment with RA also increased total tau protein levels but decreased expression of tau phosphorylated at Ser262 as determined by Western blot. Both effects were further enhanced by subsequent BDNF treatment. Upregulation of tau protein and downregulation of p-Ser262 tau were correlated with total neurite length (R = .94 and R = −.98, respectively). When primary E18 hippocampal neurons were treated with nocodazole, a blocker of microtubule polymerization, nascent neurites were lost and tau shifted to the soma. This process of retrograde tau movement away from neurites was reversed by BDNF. These results indicate that tau is redistributed to neurites and dephosphorylated during RA- and BDNF-mediated differentiation.

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.

The Fox and the Rabbits-Environmental Variables and Population Genetics (1) Replication Problems in Association Studies and the Untapped Power of GWAS (2) Vitamin A Deficiency, Herpes Simplex Reactivation and Other Causes of Alzheimer's Disease

Classical population genetics shows that varying permutations of genes and risk factors permit or disallow the effects of causative agents, depending on circumstance. For example, genes and environment determine whether a fox kills black or white rabbits on snow or black ash covered islands. Risk promoting effects are different on each island, but obscured by meta-analysis or GWAS data from both islands, unless partitioned by different contributory factors. In Alzheimer's disease, the foxes appear to be herpes, borrelia or chlamydial infection, hypercholesterolemia, hyperhomocysteinaemia, diabetes, cerebral hypoperfusion, oestrogen depletion, or vitamin A deficiency, all of which promote beta-amyloid deposition in animal models—without the aid of gene variants. All relate to risk factors and subsets of susceptibility genes, which condition their effects. All are less prevalent in convents, where nuns appear less susceptible to the ravages of ageing. Antagonism of the antimicrobial properties of beta-amyloid by Abeta autoantibodies in the ageing population, likely generated by antibodies raised to beta-amyloid/pathogen protein homologues, may play a role in this scenario. These agents are treatable by diet and drugs, vitamin supplementation, pathogen detection and elimination, and autoantibody removal, although again, the beneficial effects of individual treatments may be tempered by genes and environment.

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.

All-trans-retinoic acid inhibits collapsin response mediator protein-2 transcriptional activity during SH-SY5Y neuroblastoma cell differentiation

Neurons are highly polarized cells composed of two structurally and functionally distinct parts, the axon and the dendrite. The establishment of this asymmetric structure is a tightly regulated process. In fact, alterations in the proteins involved in the configuration of the microtubule lattice are frequent in neuro-oncologic diseases. One of these cytoplasmic mediators is the protein known as collapsin response mediator protein-2, which interacts with and promotes tubulin polymerization. In this study, we investigated collapsin response mediator protein-2 transcriptional regulation during all-trans-retinoic acid-induced differentiation of SH-SY5Y neuroblastoma cells. All-trans-retinoic acid is considered to be a potential preventive and therapeutic agent, and has been extensively used to differentiate neuroblastoma cells in vitro. Therefore, we first demonstrated that collapsin response mediator protein-2 mRNA levels are downregulated during the differentiation process. After completion of deletion construct analysis and mutagenesis and mobility shift assays, we concluded that collapsin response mediator protein-2 basal promoter activity is regulated by the transcription factors AP-2 and Pax-3, whereas E2F, Sp1 and NeuroD1 seem not to participate in its regulation. Furthermore, we finally established that reduced expression of collapsin response mediator protein-2 after all-trans-retinoic acid exposure is associated with impaired Pax-3 and AP-2 binding to their consensus sequences in the collapsin response mediator protein-2 promoter. Decreased attachment of AP-2 is a consequence of its accumulation in the cytoplasm. On the other hand, Pax-3 shows lower binding due to all-trans-retinoic acid-mediated transcriptional repression. Unraveling the molecular mechanisms behind the action of all-trans-retinoic acid on neuroblastoma cells may well offer new perspectives for its clinical application.

Pin1 allows for differential Tau dephosphorylation in neuronal cells

Neurofibrillary degeneration is likely to be related to abnormal Tau phosphorylation and aggregation. Among abnormal Tau phosphorylation sites, pThr231 is of particular interest since it is associated with early stages of Alzheimer's disease and is a binding site of Pin1, a peptidyl-prolyl cis/trans isomerase mainly involved in cell cycle regulation. In the present work, Pin1 level was found strongly increased during neuronal differentiation and tightly correlated with Tau dephosphorylation at Thr231. Likewise, we showed in cellular model that Pin1 allowed for specific Tau dephosphorylation at Thr231, whereas other phosphorylation sites were unchanged. Moreover, cells displaying Tau phosphorylation at Thr231 did not show any Pin1 nuclear depletion. Altogether, these data indicate that Pin1 has key function(s) in neuron and is at least involved in the regulation of Tau phosphorylation at relevant sites. Hence, Pin1 dysfunction, unlikely by nuclear depletion, may have critical consequences on Tau pathological aggregation and neuronal death.

A Simple, Flexible, Nonfluorescent System for the Automated Screening of Neurite Outgrowth

Measurement of neurite outgrowth is a common assay of neurotrophic activity. However, currently available techniques for measuring neurite outgrowth are either time or resource intensive. The authors established a system in which chronic treatment of a subcloned SH-SY5Y cell line with aphidicolin and various concentrations of nerve growth factor (NGF) induced discernable alterations in proliferation and differentiation. Cells were fixed, labeled with a nonfluorescent dye, and evaluated both manually and with an automated analysis system. NGF increased multiple parameters of differentiation, including neurite length, the proportion of cells extending neurites, and branching, as well as promoting cellular survival/proliferation. Interestingly, although NGF treatment increased the total number of branches, it actually decreased the proportion of branches per neurite length. The authors observed no differences in results obtained using the manual and automated systems, but the automated system was orders of magnitude faster. To demonstrate the flexibility of the system, the authors also show that they could measure changes in differentiation induced by a small-molecule Rho kinase inhibitor, as well as by retinoic acid cotreatment with brain-derived neurotrophic factor. In addition to this flexibility, this system does not require specialized equipment or fluorescent antibodies for analysis and therefore provides a less resource-intensive alternative to fluorescence-based systems.

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.

Regulation of microtubule-associated proteins, protein kinases and protein phosphatases during differentiation of SY5Y cells

Regulation of expression and function of microtubule-associated proteins (MAPs) is critical for neurons to maintain normal cytoskeletal architecture and functions. We have shown previously that in differentiated human neuroblastoma SY5Y cells, the expression of tau, a major neuronal MAP, is dramatically increased, and tau phosphorylation is differentially regulated. In the present study, we investigated the expression, the subcellular distribution and the microtubule-binding activities of several MAPs in SY5Y cells upon differentiation. We also studied the activities of protein kinases and phosphatases that are involved in regulation of tau phosphorylation during cell differentiation. We found that the expression of MAP1b in addition to tau was upregulated upon differentiation. Tau, MAP1a, MAP1b and MAP2 had distinct immunocytochemical staining patterns in differentiated SY5Y cells, suggesting differential biological functions. The microtubule-binding activity of tau increased after cell differentiation, whereas the activities of MAP1a and MAP2 decreased. Upon differentiation, the phosphorylation of tau at Ser198/Ser199/Ser202 and Ser396/Ser404 was increased, but that at Ser262/Ser356 was decreased. These changes in tau phosphorylation were accompanied by an upregulation of activities of several protein kinases (cdk5, MAPK, PKC and CK-1) as well as protein phosphatases PP-1 and PP-2A. These results suggest that the expression, post-translational modifications and biological activities of various MAPs are differentially regulated to meet the biological needs during cell differentiation.

Role of tau phosphorylation by glycogen synthase kinase-3beta in the regulation of organelle transport

Anterograde organelle transport is known to be inhibited by overexpression of the microtubule-associated protein tau in cultured cells. However, the molecular mechanism regulating this function of tau protein has not previously been understood. We found that in PC12 cells treated with NGF or fibroblast growth factor-2, glycogen synthase kinase-3beta and tau were upregulated simultaneously from around day 2 of differentiation, with increasing glycogen synthase kinase-3-mediated tau phosphorylation. This phosphorylation did not alter tau's ability to bind to microtubules but appeared to be required for the maintenance of the anterograde organelle transport in differentiated cells. Lithium, alsterpaullone or valproate, three independent glycogen synthase kinase-3 inhibitors, but not butyrolactone 1, an inhibitor of cyclin-dependent protein kinases, induced mitochondrial clustering in association with tau dephosphorylation. In CHO cells transfected with human tau(441), mitochondrial clustering was found in cells in which tau was unphosphorylated. These findings raise the possibility that the phosphorylation of tau by glycogen synthase kinase-3 might be involved in the regulation of organelle transport.

beta-Amyloid induces paired helical filament-like tau filaments in tissue culture

Paired helical filaments (PHF) are the principal pathologic components of neurofibrillary tangles in Alzheimer's disease (AD). To reproduce the formation of PHF in tissue culture, we stably expressed human tau with and without pathogenic mutations in human SH-SY5Y cells and exposed them for 5 days to aggregated synthetic beta-amyloid peptide (A beta 42). This caused a decreased solubility of tau along with the generation of PHF-like tau-containing filaments. These were 20 nm wide and had periodicities of 130-140 nm in the presence of P301L mutant tau or 150-160 nm in the presence of wild-type tau. Mutagenesis of the phosphoepitope serine 422 of tau prevented both the A beta 42-mediated decrease in solubility and the generation of PHF-like filaments, suggesting a role of serine 422 or its phosphorylation in tau filament formation. Together, our data underscore a role of A beta 42 in the formation of PHF-like filaments. Our culture system will be useful to map phosphoepitopes of tau involved in PHF formation and to identify and characterize modifiers of the tau pathology. Further adaptation of the system may allow the screening and validation of compounds designed to prevent PHF formation.

Mitotic-like tau phosphorylation by p25-Cdk5 kinase complex

Among tau phosphorylation sites, some phosphoepitopes referred to as abnormal ones are exclusively found on tau aggregated into filaments in Alzheimer's disease. Recent data suggested that molecular mechanisms similar to those encountered during mitosis may play a role in abnormal tau phosphorylation. In particular, TG-3 phosphoepitope is associated with early stages of neurofibrillary tangles (NFTs). In this study, we reported a suitable cell model consisting of SH-SY5Y cells stably transfected with an inducible p25 expression vector. It allows investigation of tau phosphorylation by p25-Cdk5 kinase complex in a neuronal context and avoiding p25-induced cytotoxicity. Immunoblotting analyses showed that p25-Cdk5 strongly phosphorylates tau protein not only at the AT8 epitope but also at the AT180 epitope and at the Alzheimer's mitotic epitope TG-3. Further biochemical analyses showed that abnormal phosphorylated tau accumulated in cytosol as a microtubule-free form, suggesting its impact on tau biological activity. Since tau abnormal phosphorylation occurred in dividing cells, TG-3 immunoreactivity was also investigated in differentiated neuronal ones, and both TG-3-immunoreactive tau and nucleolin, another early marker for NFT, were also generated. These data suggest that p25-Cdk5 is responsible for the mitotic-like phosphoepitopes present in NFT and argue for a critical role of Cdk5 in neurodegenerative mechanisms.

Rit promotes MEK-independent neurite branching in human neuroblastoma cells

Rit, by sequence homology, is a member of the Ras subfamily of small guanine triphosphatases (GTPases). In PC6 cells, Rit signals through pathways both common to and different from those activated by Ras to promote cell survival and neurite outgrowth. However, the specific morphological changes induced by Rit in human cells are not known. Here, we show in a human neuronal model that Rit increases neurite outgrowth and branching through MEK-dependent and MEK-independent signaling mechanisms, respectively. Adenoviral expression of wild-type or constitutively active Rit increased neurite initiation, elongation and branching on endogenous matrix or a purified laminin-1 substratum of SH-SY5Y cells as assessed using image analysis. This outgrowth was morphologically distinct from that promoted by constitutively active Ras or Raf (evidenced by increased branching and elongation). Constitutively active Rit increased phosphorylation of ERK 1/2, but not Akt, and the MEK inhibitor PD 098059 blocked constitutively active Rit-induced neurite initiation but not elongation or branching. These results suggest that Rit plays a key role in human neuronal development and regeneration through activating both known and as yet undefined signaling pathways.

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.

Paradoxical phosphorylation of the serine 199 on tau proteins from young individuals

The microtubule-associated tau proteins are abnormally aggregated in many tauopathies. Phosphorylation modulates the functions of tau. The serine 199 residue of tau is abnormally phosphorylated at early and late stages of Alzheimer's disease. The presence of the phosphorylated Ser199 was investigated in autopsy-derived and biopsy-derived brain tissue samples from non-demented individuals. A paradoxical expression was found in the hippocampus of the youngest ones, in granule cells of the dentate gyrus and in pyramidal cells of the Ammon's horn, which are particularly prone to neurodegeneration in several tauopathies. The rate of positive cells decreased with age. These data emphasize the importance of the phosphorylation of the Ser199 residue of tau in ageing and susceptibility to neurodegeneration.

Effect of human neuronal tau on denaturation and reactivation of rabbit muscle D-glyceraldehyde-3-phosphate dehydrogenase

Human neuronal tau-40 (htau-40) has been used to study denaturation and renaturation of rabbit muscle D-glyceraldehyde-3-phosphate dehydrogenase (GAPDH, EC 1.2.1.12). Inactivation of GAPDH incubated with tau was more distinguishably detected than that of control GAPDH during thermal and guanidine hydrochloride (GdnHCl) denaturation. However, tau did not influence the activity of GAPDH at room temperature or in solution without GdnHCl. A marked change in both the emission intensity and emission maximum of the intrinsic fluorescence at 335 nm of GAPDH with tau was observed when GdnHCl concentration was 0.8 M, but that of the control without tau occurred in 1.2 M GdnHCl. The first-order rate of the decrease in the fluorescence intensity of the enzyme with tau was approximately twice as great as that of GAPDH without tau. Kinetics of inactivation of GAPDH with tau in 0.2 M GdnHCl was a monophasic procedure, instead of the biphasic procedure followed by the control, as described before [He, Zhao, Yan and Li (1993) Biochim. Biophys. Acta 1163, 315-320]. Similar results were obtained when the enzyme was thermally denatured at 45 degrees C. It revealed that tau bound to the denatured GAPDH but not the native molecule. On the other hand, tau suppressed refolding and reactivation of GAPDH when this enzyme was reactivated by dilution of GdnHCl solution. Furthermore, tau improved the aggregation of the non-native GAPDH in solutions. It suggested that tau acted in an anti-chaperone-like manner towards GAPDH in vitro. However, tau lost that function when it was aggregated or phosphorylated by neuronal cdc2-like protein kinase. It showed that tau's anti-chaperone-like function depended on its native conformation.