Abnormal phosphorylation of tau and the mechanism of Alzheimer neurofibrillary degeneration: sequestration of microtubule-associated proteins 1 and 2 and the disassembly of microtubules by the abnormal tau

Cytoskeletal Transport in the Aging Brain: Focus on the Cholinergic System

There is now compelling evidence for the aging-related breakdown of cytoskeletal support in neurons. Similarly affected are the principal components of the intracellular microtubule system, the transport units involved in active shuttle of organelles and molecules in an antero- and retrograde manner, and the proteins stabilizing the cytoskeleton and providing trophic support. Here, we review the basic organization of the cytoskeleton, and describe its elements and their interactions. We then critically assess the role of these cytoskeletal proteins in physiological aging and aging-related malfunction. Our focus is on the microtubule-associated protein tau, for which comprehensive investigations suggest a critical role in neurodegenerative diseases, for instance tauopathies. These diseases frequently lead to cognitive decline and are often paralleled by reductions in cholinergic neurotransmission. We propose this reduction to be due to destabilization of the cytoskeleton and protein transport mechanisms in these neurons. Therefore, maintenance of the neuronal cytoskeleton during aging may prevent or delay neurodegeneration as well as cognitive decline during physiological aging.

[The search for new autoantigens in Sjögren's syndrome]

OBJECTIVE

To identify new autoantigens related to Sjögren's syndrome and to determine their prevalence in patients and healthy individuals.

MATERIAL AND METHODS

Serological sampling was performed in a patient with Sjögren's syndrome through the use of a human brain expression genotec (SEREX technique) to determine expression of known autoantigens and previously undescribed proteins. The presence of a previously unknown protein was found. Several proteins were obtained and two were selected to be studied (a human protein called Tau and an unknown protein described by our group and named hlscA). Both Tau and hIscA cDNA were transformed into an expression plasmid to obtain their recombinant proteins.

RESULTS

Using a Western-blot technique we investigated the presence of anti-Tau and anti-hlscA autoantibodies in the sera of 19 patients with Sjögren's syndrome and in the sera of 20 controls. No statistically significant differences were found in the expression of anti-Tau antibodies between patients with Sjögren's syndrome and controls but values of anti-hlscA autoantibodies were significantly lower in patients with Sjögren's syndrome.

CONCLUSION

We identified Tau and hIscA proteins as new autoantigens in Sjögren's syndrome. Anti-hlscA antibody values were significantly lower in patients with Sjögren's syndrome than in healthy controls. Although no statistically significant differences in values of anti-Tau antibodies were found between Sjögren's syndrome patients and controls, this is the first time antibodies against this protein have been detected in Sjögren's syndrome.

DNA sequence variations in the prolyl isomerase Pin1 gene and Alzheimer's disease

Senile plaques and neurofibrillary tangles (NFT) are the prominent lesions in the brain of Alzheimer's disease (AD) patients. NFT are mainly composed of an abnormally phosphorylated form of tau protein, which has lost its function to bind microtubules and promote their assembly. Tau hyperphosphorylation critically decreases tau function and precedes neurodegeneration. The majority of tau phosphorylation sites are Ser/Thr-Pro motifs, which are known to exist in two distinct cis and trans conformations. The prolyl isomerase Pin1 catalyses the conversion of those conformations. Pin1 binds to tau specifically at the Thr231-Pro site and restores tau function, either by inducing conformational changes or facilitating dephosphorylation. It has been shown that Pin1 expression levels inversely correlate with the predicted vulnerability of different brain areas to neurodegeneration and soluble Pin1 is depleted in neurons from AD brains; furthermore, Pin1 knock-out mice develop signs and symptoms of tau-related pathologies late in life. It seems that Pin1 plays an important role in maintaining tau function, thereby preserving neuronal homeostasis and preventing age-dependent neurodegeneration. DNA sequence variations in Pin1 gene may affect its expression level or function and influence the individual risk for developing AD. We screened by denaturing high performance liquid chromatography the genomic DNA of 120 AD subjects and 134 age-matched controls and we found very few and rare sequence variations in the promoter region and in exons 2 and 3. We conclude that Pin1 is a very well conserved gene, whose rare nucleotide variations have no effect on the individual genetic risk for AD.

PP2B isolated from human brain preferentially dephosphorylates Ser-262 and Ser-396 of the Alzheimer disease abnormally hyperphosphorylated tau

PP2B is one of the major serine/threonine phosphatases in the brain. We quantitated the dephosphorylation of various sites of Alzheimer disease abnormally hyperphosphorylated tau by PP2B purified from six (three Alzheimer and three control) autopsied human brains. The purified PP2B was essentially homogenous holoenzyme as determined by SDS-PAGE, Western blot analyses and biochemical characterization. Purified PP2B from all six brains efficiently dephosphorylated (32)P-tau with specific activities ranging from 684-1286 pmol (32)Pi/mg/min. Estimated by dot-blot analyses, the purified PP2B (on average from six brains) dephosphorylated Alzheimer tau at pS199, pT217, pS262, pS396 and pS422 by 38%, 32%, 63%, 78%, and 32%, respectively. Dephosphorylation of tau at pT181, pS202, pT205, pT212, pS214, and pS404 by PP2B was undetectable. The preferential dephosphorylation of Ser262 and Ser396 by PP2B suggests a possible involvement of this phosphatase in Alzheimer neurofibrillary degeneration.

Phosphothreonine-212 of Alzheimer abnormally hyperphosphorylated tau is a preferred substrate of protein phosphatase-1

We isolated and characterized several phosphoseryl/phosphothreonyl phosphatase activities (P1-P11) from frontal lobe of six autopsied human brains. Of these, PP1 (P3) was a major tau phosphatase. The enzyme required metal ions and was maximally activated by Mn2+. Western blots with antibodies to known protein phosphatases showed PP1 and PP2B immunoreactivity. However, the removal of PP2B by immunoabsorption or its inhibition with EGTA did not result in appreciable loss of P3 activity. These observations suggest that P3 was an enriched PPI. Dephosphorylation of Alzheimer disease hyperphosphorylated tau (AD P-tau) by PP1 was site-specific. PPI preferentially dephosphorylated pT212 (40%), pT217 (26%), pS262 (33%), pS396 (42%) and pS422 (31%) of AD P-tau. Dephosphorylation of tau at pT181, pS199, pS202, pT205, pS214, and pS404, was undetectable. Of the sites dephosphorylated, pT212 was only a substrate for PP1, as purified/enriched PP2A and PP2B from the same brains did not dephosphorylate this site.

Up-regulation of inhibitors of protein phosphatase-2A in Alzheimer's disease

The activity of protein phosphatase-2A (PP2A) is compromised and is believed to be a cause of the abnormal hyperphosphorylation of tau in Alzheimer's disease (AD) brain. We investigated in AD the role of the two known endogenous PP2A inhibitors, called I1(PP2A) and I2(PP2A), which regulate the intracellular activity of PP2A in mammalian tissues. We found a significant increase in the neocortical levels of I1(PP2A) and I2(PP2A) in AD as compared to control cases by in situ hybridization. The immunohistochemical studies revealed that I2(PP2A) was translocated from neuronal nuclei to cytoplasm in AD. The 39-kd full-length I2(PP2A) was selectively cleaved into an approximately 20-kd fragment in AD brain cytosol. Digestion of the recombinant human I2(PP2A) with AD brain extract showed an increase in the generation of the approximately 20 kd and other fragments of the inhibitor as compared to control brain extract. Double-immunohistochemical studies revealed co-localization of PP2A with PP2A inhibitors in neuronal cytoplasm and co-localization of the inhibitors with abnormally hyperphosphorylated tau. These studies suggest the possible involvement of I1(PP2A) and I2(PP2A) in the abnormal hyperphosphorylation of tau in AD.

Inhibitors of protein phosphatase-2A from human brain structures, immunocytological localization and activities towards dephosphorylation of the Alzheimer type hyperphosphorylated tau

Protein phosphatase (PP)-2A, which regulates the phosphorylation of tau, is regulated by two endogenous inhibitor proteins, I(1)(PP2A) and I(2)(PP2A), in mammalian tissues. Here, we report the cloning of I(1)(PP2A) and I(2)(PP2A) from human brain, and show that in PC12 cells and in I(1)(PP2A)-GFP or I(2)(PP2A)-GFP transfected NIH3T3 and human neural progenitor cells, I(1)(PP2A) is localized mostly in the cell cytoplasm and I(2)(PP2A) mostly in the nucleus. The recombinant I(1)(PP-2A) and I(2)(PP-2A) inhibit PP-2A activity towards hyperphosphorylated tau in vitro; the dephosphorylation of the hyperphosphorylated tau at specific sites is selectively inhibited. Overexpression of I(1)(PP2A) as well as I(2)(PP2A) results in tau hyperphosphorylation and degeneration of PC 12 cells.

Pathways of tau fibrillization

New methods for analyzing tau fibrillization have yielded insights into the biochemical transitions involved in the process. Here we review the parallels between the sequential progression of tau fibrillization observed macroscopically in Alzheimer's disease (AD) lesions and the pathway of tau aggregation observed in vitro with purified tau preparations. In addition, pharmacological agents for further dissection of fibrillization mechanism and lesion formation are discussed.

Tau pathology in Alzheimer disease and other tauopathies

Just as neuronal activity is essential to normal brain function, microtubule-associated protein tau appears to be critical to normal neuronal activity in the mammalian brain, especially in the evolutionary most advanced species, the homo sapiens. While the loss of functional tau can be compensated by the other two neuronal microtubule-associated proteins, MAP1A/MAP1B and MAP2, it is the dysfunctional, i.e., the toxic tau, which forces an affected neuron in a long and losing battle resulting in a slow but progressive retrograde neurodegeneration. It is this pathology which is characteristic of Alzheimer disease (AD) and other tauopathies. To date, the most established and the most compelling cause of dysfunctional tau in AD and other tauopathies is the abnormal hyperphosphorylation of tau. The abnormal hyperphosphorylation not only results in the loss of tau function of promoting assembly and stabilizing microtubules but also in a gain of a toxic function whereby the pathological tau sequesters normal tau, MAP1A/MAP1B and MAP2, and causes inhibition and disruption of microtubules. This toxic gain of function of the pathological tau appears to be solely due to its abnormal hyperphosphorylation because dephosphorylation converts it functionally into a normal-like state. The affected neurons battle the toxic tau both by continually synthesizing new normal tau and as well as by packaging the abnormally hyperphosphorylated tau into inert polymers, i.e., neurofibrillary tangles of paired helical filaments, twisted ribbons and straight filaments. Slowly but progressively, the affected neurons undergo a retrograde degeneration. The hyperphosphorylation of tau results both from an imbalance between the activities of tau kinases and tau phosphatases and as well as changes in tau's conformation which affect its interaction with these enzymes. A decrease in the activity of protein phosphatase-2A (PP-2A) in AD brain and certain missense mutations seen in frontotemporal dementia promotes the abnormal hyperphosphorylation of tau. Inhibition of this tau abnormality is one of the most promising therapeutic approaches to AD and other tauopathies.

Tau alteration and neuronal degeneration in tauopathies: mechanisms and models

Tau becomes characteristically altered both functionally and structurally in several neurodegenerative diseases now collectively called tauopathies. Although increasing evidence supports that alterations of tau may directly cause neuronal degeneration and cell death, the mechanisms, which render tau to become a toxic agent are still unclear. In addition, it is obscure, whether neurodegeneration in tauopathies occurs via a common mechanism or specific differences exist. The aim of this review is to provide an overview about the different experimental models that currently exist, how they are used to determine the role of tau during degeneration and what has been learnt from them concerning the mechanistic role of tau in the disease process. The review begins with a discussion about similarities and differences in tau alteration in paradigmatic tauopathies such as frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17) and Alzheimer's disease (AD). The second part concentrates on major experimental models that have been used to address the mechanistic role of tau during degeneration. This will include a discussion of cell-free assays, culture models using cell lines or dissociated neurons, and animal models. How these models aid to understand (i) alterations in the function of tau as a microtubule-associated protein (MAP), (ii) direct cytotoxicity of altered tau protein, and (iii) the potential role of tau aggregation in neurodegenerative processes will be the central theme of this part. The review ends with concluding remarks about a general mechanistic model of the role of tau alteration and neuronal degeneration in tauopathies and future perspectives.

Metabolic/signal transduction hypothesis of Alzheimer's disease and other tauopathies

Alzheimer's disease (AD), the major cause of dementia in middle- to old-aged individuals, is multifactorial. Independent of the etiology, whether genetic or non-genetic, this disease is characterized by extracellular beta-amyloid plaques and intraneuronal neurofibrillary tangles of abnormally hyperphosphorylated tau. However, the molecular mechanisms of neither AD nor other tauopathies are completely understood. To date, the most popular hypothesis of AD is the "Amyloid cascade hypothesis", according to which beta-amyloid, the cleavage product of beta-amyloid precursor protein (APP), is neurotoxic and causes neurodegeneration and dementia. However, this hypothesis is inconsistent with the presence in normal aged human brain of the beta-amyloid plaque burden similar to that in AD, and the absence of neurofibrillary pathology and neurodegeneration in mutated APP, presenilin-1 and presenilin-2 transgenic mice that show extensive beta-amyloid plaque pathology. Here we propose an alternate hypothesis, the "Metabolic/signal transduction hypothesis", which is consistent both with the pathology seen in AD and other tauopathies and as well as all experimental animal conditions. In this hypothesis, with increasing age, the fluidity of neuronal membranes is progressively reduced, which makes it less resistant to environmental/metabolic insults affecting one or more signal transduction pathways, which lead to a protein phosphorylation/dephosphorylation imbalance and abnormal hyperphosphorylation of tau. The hyperphosphorylated tau sequesters normal tau, MAP1 and MAP2, which results in breakdown of the microtubule network and, consequently, a progressive retrograde degeneration of the affected neurons and, ultimately, dementia.

Pseudophosphorylation and Glycation of Tau Protein Enhance but Do Not Trigger Fibrillization in Vitro

Alzheimer's disease is defined in part by the intraneuronal aggregation of tau protein into filamentous lesions. The pathway is accompanied by posttranslational modifications including phosphorylation and glycation, each of which has been shown to promote tau fibrillization in vitro when present at high stoichiometry. To clarify the site-specific impact of posttranslational modification on tau fibrillization, the ability of recombinant full-length four repeat tau protein (htau40) and 11 pseudophosphorylation mutants to fibrillize in the presence of anionic inducer was assayed in vitro using transmission electron microscopy and laser light scattering assays. Tau glycated with d-glucose was examined as well. Both glycated tau and pseudophosphorylation mutants S199E, T212E, S214E, double mutant T212E/S214E, and triple mutant S199E/S202E/T205E yielded increased filament mass at equilibrium relative to wild-type tau. Increases in filament mass correlated strongly with decreases in critical concentration, indicating that both pseudophosphorylation and glycation promoted fibrillization by shifting equilibrium toward the fibrillized state. Analysis of reaction time courses further revealed that increases in filament mass were not associated with reduced lag times, indicating that these posttranslational modifications did not promote filament nucleation. The results suggest that site-specific posttranslational modifications can stabilize filaments once they nucleate, and thereby support their accumulation at low intracellular tau concentrations.

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.

Memantine inhibits and reverses the Alzheimer type abnormal hyperphosphorylation of tau and associated neurodegeneration

Memantine, an N-methyl-D-aspartate (NMDA) receptor antagonist, reduces the clinical deterioration in moderate-to-severe Alzheimer disease (AD) for which other treatments are not available. The activity of protein phosphatase (PP)-2A is compromised in AD brain and is believed to be a cause of the abnormal hyperphosphorylation of tau and the consequent neurofibrillary degeneration. Here we show that memantine inhibits and reverses the PP-2A inhibition-induced abnormal hyperphosphorylation and accumulation of tau in organotypic culture of rat hippocampal slices. Such restorative effects of memantine were not detected either with 5,7-dichlorokynurenic acid or with D(-)-2-amino-5-phosphopentanoic acid, NMDA receptor antagonists active at the glycine binding site and at the glutamate binding site, respectively. These findings show (1) that memantine inhibits and reverses the PP-2A inhibition-induced abnormal hyperphosphorylation of tau/neurofibrillary degeneration and (2) that this drug might be useful for the treatment of AD and related tauopathies.

The retinoic acid and brain-derived neurotrophic factor differentiated SH-SY5Y cell line as a model for Alzheimer’s disease-like tau phosphorylation

The paired helical filaments of highly phosphorylated tau protein are the main components of neurofibrillary tangles (NFT) in Alzheimer's disease (AD). Protein kinases including glycogen synthase kinase 3 beta (GSK3beta), cyclin-dependent kinase 5 (Cdk5), and c-Jun N-terminal kinase (JNK) have been implicated in NFT formation making the use of selective kinase inhibitors an attractive treatment possibility in AD. When sequentially treated with retinoic acid (RA) and brain-derived neurotrophic factor (BDNF), the human neuroblastoma SH-SY5Y differentiates to neuron-like cells. We found that coincident with morphologically evident neurite outgrowth, both the content and phosphorylation state of tau increased in RA-BDNF differentiated SH-SY5Y cells. Tau phosphorylation increased at all the examined sites ser-199, ser-202, thr-205, ser-396, and ser-404, all of which are hyperphosphorylated in AD brain. We also investigated whether GSK3beta, Cdk5 or JNK was involved in tau phosphorylation in the differentiated SH-SY5Y cells. We found that GSK3beta contributed most and that Cdk5 made a minor contribution. JNK was not involved in tau phosphorylation in this system. The GSK3beta-inhibitor, lithium, inhibited tau phosphorylation in a concentration-dependent manner and with good reproducibility, which enables ranking of substances in this cell model. RA-BDNF differentiated SH-SY5Y cells could serve as a suitable model for studying the mechanisms of tau phosphorylation and for screening potential GSK3beta inhibitors.

Promotion of hyperphosphorylation by frontotemporal dementia tau mutations

Mutations in the tau gene are known to cosegregate with the disease in frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17). However, the molecular mechanism by which these mutations might lead to the disease is not understood. Here, we show that four of the FTDP-17 tau mutations, R406W, V337M, G272V, and P301L, result in tau proteins that are more favorable substrates for phosphorylation by brain protein kinases than the wild-type, largest four-repeat protein tau4L and tau4L more than tau3L. In general, at all the sites studied, mutant tau proteins were phosphorylated faster and to a higher extent than tau4L and tau4L > tau3L. The most dramatic difference found was in the rate and level of phosphorylation of tau4L(R406W) at positions Ser-396, Ser-400, Thr-403, and Ser-404. Phosphorylation of this mutant tau was 12 times faster and 400% greater at Ser-396 and less than 30% at Ser-400, Thr-403, and Ser-404 than phosphorylation of tau4L. The mutated tau proteins polymerized into filaments when 4-6 mol of phosphate per mol of tau were incorporated, whereas wild-type tau required approximately 10 mol of phosphate per mol of protein to self-assemble. Mutated and wild-type tau proteins were able to sequester normal tau upon incorporation of approximately 4 mol of phosphate per mol of protein, which was achieved at as early as 30 min of phosphorylation in the case of mutant tau proteins. These findings taken together suggest that the mutations in tau might cause neurodegeneration by making the protein a more favorable substrate for hyperphosphorylation.

Role of tau protein in both physiological and pathological conditions

The morphology of a neuron is determined by its cytoskeletal scaffolding. Thus proteins that associate with the principal cytoskeletal components such as the microtubules have a strong influence on both the morphology and physiology of neurons. Tau is a microtubule-associated protein that stabilizes neuronal microtubules under normal physiological conditions. However, in certain pathological situations, tau protein may undergo modifications, mainly through phosphorylation, that can result in the generation of aberrant aggregates that are toxic to neurons. This process occurs in a number of neurological disorders collectively known as tauopathies, the most commonly recognized of which is Alzheimer's disease. The purpose of this review is to define the role of tau protein under normal physiological conditions and to highlight the role of the protein in different tauopathies.

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.

Overactivation of glycogen synthase kinase-3 by inhibition of phosphoinositol-3 kinase and protein kinase C leads to hyperphosphorylation of tau and impairment of spatial memory

Neurofibrillary tangles (NFTs) consisting of the hyperphosphorylated microtubule-associated protein tau are a defining pathological characteristic of Alzheimer's disease (AD). Hyperphosphorylation of tau is hypothesized to impair the microtubule stabilizing function of tau, leading to the formation of paired helical filaments and neuronal death. Glycogen synthase kinase-3 (GSK-3) has been shown to be one of several kinases that mediate tau hyperphosphorylation in vitro. However, molecular mechanisms underlying overactivation of GSK-3 and its potential linkage to AD-like pathologies in vivo remain unclear. Here, we demonstrate that injection of wortmannin (a specific inhibitor of phosphoinositol-3 kinase) or GF-109203X (a specific inhibitor of protein kinase C) into the left ventricle of rat brains leads to overactivation of GSK-3, hyperphosphorylation of tau at Ser 396/404/199/202 and, most significantly, impaired spatial memory. The effects of wortmannin and GF-109203X are additive. Significantly, specific inhibition of GSK-3 activity by LiCl prevents hyperphosphorylation of tau, and spatial memory impairment resulting from PI3K and PKC inhibition. These results indicate that in vivo inhibition of phosphoinositol-3 kinase and protein kinase C results in overactivation of GSK-3 and tau hyperphosphorylation and support a direct role of GSK-3 in the formation of AD-like cognitive deficits.

Role of protein kinase B in Alzheimer's neurofibrillary pathology

Protein kinase B (PKB) is an important intermediate in the phosphatidylinositol-3 kinase signaling cascade that acts to phosphorylate glycogen synthase kinase-3 (GSK-3) at its serine 9 residue, thereby inactivating it. Activated GSK-3 has been previously shown to be preferentially associated with neurofibrillary tangles (NFTs) in Alzheimer's disease (AD) brain. In the present study, we performed immunohistochemistry with an antibody to the active form of PKB in brains with different stages of neurofibrillary degeneration. We found that the amount of activated PKB (p-Thr308) increased in correlation to the progressive sequence of AT8 immunoreactivity and neurofibrillary changes assessed according to Braak's criteria. By confocal microscopy, activated PKB (p-Thr308) was found to appear in particular in neurons that are known to later develop NFTs in AD. Western blotting showed that activated PKB was increased by more than 50% in the 16,000- g supernatants of AD brains as compared with normal aged and Huntington's disease controls. This increase in PKB levels corresponded with a several-fold increase in the levels of total tau and abnormally hyperphosphorylated tau at the Tau-1 site. These studies suggest the involvement of PKB/GSK-3 signaling in Alzheimer neurofibrillary degeneration.

Traffic at the intersection of neurotrophic factor signaling and neurodegeneration

Advances in understanding the biology of neurotrophic factors and their signaling pathways have provided important insights into the normal growth, differentiation and maintenance of neurons. Stimulated by neuropathological observations and genetic discoveries, studies in cell and animal models of neurodegenerative disorders have begun to clarify pathogenetic mechanisms. We examine the intersection of these research themes and identify several potential mechanisms for linking failed neurotrophic factor signaling to neurodegeneration. Studies of nerve growth factor signaling in a mouse model of Down syndrome encourage the views that neuronal dysfunction and atrophy might be linked to failed neurotrophic support and that additional studies focused on this possibility would enhance our understanding of the mechanisms of neurodegenerative disorders and their treatment.

Pharmacological targets to inhibit Alzheimer neurofibrillary degeneration

Neurofibrillary degeneration appears to be required for the clinical expression of Alzheimer disease (AD) and related tauopathies. Given the polyetiology of these diseases and the pivotal involvement of neurofibrillary degeneration in their pathogenesis, inhibition of this lesion offers a promising therapeutic target. Studies from our laboratories have shown that there is a protein phosphorylation/dephosphorylation imbalance and that the microtubule associated protein tau is abnormally hyperphosphorylated in the brain of patients with AD and in this form it is the major protein subunit of paired helical filaments/neurofibrillary tangles (PHF/NFT). The abnormal tau which is polymerized into PHF/NFT neither promotes or inhibits in vitro microtubule assembly. In contrast the cytosolic abnormally hyperphosphorylated tau from AD brain, the AD P-tau neither associates with tubulin nor promotes in vitro microtubule assembly but instead it sequesters normal tau, MAP1 and MAP2 and inhibits microtubule assembly. The AD P-tau readily self-assembles in vitro into tangles of PHF/straight filaments under physiological conditions of protein concentration, pH, ionic strength and reducing conditions and this self assembly requires the abnormal hyperphosphorylation of this protein. The activity of phosphoseryl/phosphothreonyl protein phosphatase (PP)-2A which regulates the phosphorylation of tau, is compromised in AD brain. Thus, modulation of the activities of protein phosphatase-2A and tau kinases and inhibition of the sequestration of normal MAPs by AD P-tau offer promising therapeutic opportunities to inhibit neurofibrillary degeneration and the diseases characterized by this lesion.

Tau assembly in inducible transfectants expressing wild-type or FTDP-17 tau

Conditional expression systems for 4-repeat wild-type (WT) tau or the corresponding mutants V337M and R406W were established in human neuroglioma H4 cells to study the effect of tau mutations on the physicochemical properties of tau, and to develop a cellular model for the formation of filamentous tau characteristic of frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17) and Alzheimer's disease. Upon induction tau expression increased, reaching maximal levels at 5 to 7 days. WT tau was phosphorylated at amino acids T181, S202/T205, T231, and S396/S404. The R406W mutation decreased tau phosphorylation at each of these sites as did the V337M mutation except for S396/S404 sites that increased. Most tau in postnuclear cell lysates was recovered in the supernatant fraction after centrifugation at 200,000 x g. The amount of tau in the pellet fraction increased more in mutant transfectants compared to WT when the induction was extended beyond 5 days. This particulate tau could be partially extracted with salt, Triton X-100, or sarkosyl. Of the transfectants, R406W had the highest proportion of sarkosyl-insoluble tau by day 7. This insoluble fraction was thioflavin S-positive and contained 15- to 5-nm-wide filaments with tau immunoreactivities. The R406W filaments were more abundant than those detected in similar preparations from WT or V337M transfectants. At the light microscopy level, most tau was found with microtubules, or diffusely distributed in the cytoplasm, but none of this appeared thioflavin S-positive. The results suggest that conditional tau transfectants are in a pretangle stage making them an attractive model system for studying intracellular tangle accumulation and for testing potential therapeutic agents as inhibitors for tau aggregation.

Involvement of aberrant glycosylation in phosphorylation of tau by cdk5 and GSK-3beta

Microtubule-associated protein tau is abnormally hyperphosphorylated, glycosylated, and aggregated in affected neurons in the brains of individuals with Alzheimer's disease (AD). We recently found that the glycosylation might precede hyperphosphorylation of tau in AD. In this study, we investigated the effect of glycosylation on phosphorylation of tau catalyzed by cyclin-dependent kinase 5 (cdk5) and glycogen synthase kinase-3beta (GSK-3beta). The phosphorylation of the longest isoform of recombinant human brain tau, tau(441), at various sites was detected by Western blots and by radioimmuno-dot-blot assay with phosphorylation-dependent and site-specific tau antibodies. We found that cdk5 phosphorylated tau(441) at Thr-181, Ser-199, Ser-202, Thr-205, Thr-212, Ser-214, Thr-217, Thr-231, Ser-235, Ser-396, and Ser-404, but not at Ser-262, Ser-400, Thr-403, Ser-409, Ser-413, or Ser-422. GSK-3beta phosphorylated all the cdk5-catalyzed sites above except Ser-235. Deglycosylation by glycosidases depressed the subsequent phosphorylation of AD-tau (i) with cdk5 at Thr-181, Ser-199, Ser-202, Thr-205, and Ser-404, but not at Thr-212; and (ii) with GSK-3beta at Thr-181, Ser-202, Thr-205, Ser-217, and Ser-404, but not at Ser-199, Thr-212, Thr-231, or Ser-396. These data suggest that aberrant glycosylation of tau in AD might be involved in neurofibrillary degeneration by promoting abnormal hyperphosphorylation by cdk5 and GSK-3beta.

Significance and mechanism of Alzheimer neurofibrillary degeneration and therapeutic targets to inhibit this lesion

Abnormally hyperphosphorylated tau which is the major protein subunit of paired helical filaments (PHF)/neurofibrillary tangles is the pivotal lesion in Alzheimer disease (AD) and related tauopathies. The cosegregation of tau mutations with disease in inherited cases of frontotemporal dementia has confirmed that abnormalities in this protein can be a primary cause of neurodegeneration. Unlike normal tau that promotes assembly and maintains the structure of microtubules, the abnormally hyperphosphorylated protein sequesters normal tau, MAP1 and MAP2 and consequently disassembles microtubules. The abnormal hyperphosphorylation also promotes the self assembly of tau into tangles of PHF. The hyperphosphorylation of tau in AD is probably due to a protein phosphorylation/dephosphorylation imbalance produced by a decrease in the activity of protein phosphatase (PP)-2A and increase in the activities of tau kinases which are directly or indirectly regulated by PP-2A. Two of the most promising pharmacologic therapeutic approaches to AD are (1) the development of drugs that can inhibit the sequestration of normal MAPs by the abnormally hyperphosphorylated tau, and (2) the development of drugs that can reverse the abnormal hyperphosphorylation of tau by correcting the protein phosphorylation/dephosphorylation imbalance.

Tau function and dysfunction in neurons: its role in neurodegenerative disorders

Alzheimer's disease (AD) is the most usual neurodegenerative disorder leading to dementia in the aged human population. It is characterized by the presence of two main brain pathological hallmarks: senile plaques and neurofibrillary tangles (NFTs). NFTs are composed of fibrillar polymers of the abnormally phosphorylated cytoskeletal protein tau.

Regulation of tau phosphorylation and protection against beta-amyloid-induced neurodegeneration by lithium. Possible implications for Alzheimer's disease

Alzheimer's disease is a neurodegenerative disorder characterized by the accumulation of the beta-amyloid peptide and the hyperphosphorylation of the tau protein, among other features. The most widely accepted hypothesis on the etiopathogenesis of this disease proposes that the aggregates of the beta-amyloid peptide are the main triggers of tau hyperphosphorylation and the subsequent degeneration of affected neurons. In support of this view, fibrillar aggregates of synthetic beta-amyloid peptide induce tau hyperphosphorylation and cell death in cultured neurons. We have previously reported that lithium inhibits tau hyperphosphorylation and also significantly protects cultured neurons from cell death triggered by beta-amyloid peptide. As lithium is a relatively specific inhibitor of glycogen synthase kinase-3 (in comparison with other protein kinases), and other studies also point to a relevant role of this enzyme, we favor the view that glycogen synthase kinase-3 is a crucial element in the pathogenesis of Alzheimer's disease. In our opinion, the possibility of using lithium, or other inhibitors of glycogen synthase kinase-3, in experimental trials aimed to ameliorate neurodegeneration in Alzheimer's disease should be considered.

Protein phosphatase 2A methylation: a link between elevated plasma homocysteine and Alzheimer's Disease

Tau hyperphosphorylation is a central event in the development of Alzheimer's Disease (AD). Protein phosphatase 2A (PP2A) heterotrimer formation is necessary for efficient dephosphorylation of the tau protein. S-Adenosylmethionine-dependent carboxyl methylation is essential for the assembly of PP2A heterotrimers. Epidemiological evidence indicates that elevated plasma homocysteine is an independent risk factor for AD. Homocysteine is a key intermediate in the methyl cycle and elevated plasma homocysteine results in a global decrease in cellular methylation. We propose that the PP2A methylation system is the link relating elevated plasma homocysteine to AD.

A pool of beta-tubulin is hyperphosphorylated at serine residues in Alzheimer disease brain

In Alzheimer disease (AD) brain, activities of protein phosphatase (PP)-2A/PP-1 which are known to be associated with microtubules are compromised and are probably a cause of neurofibrillary degeneration through hyperphosphorylation of microtubule proteins. In the present study, an increase of approximately 11 pmol phosphate/microg protein in 100,000 x g pellet from AD compared with age-matched control brains was found. Tau protein, which is hyperphosphorylated in AD can only account for approximately 4 pmol phosphate/microg protein, suggesting the presence of non-tau hyperphosphorylated proteins in the diseased brain. Western blot analysis with phosphoserine antibodies revealed a approximately 54 kDa non-tau protein to be significantly hyperphosphorylated in AD compared with age-matched control cases in the particulate fraction. The approximately 54 kDa protein was purified by preparative sodium dodecyl sulfate-polyacrylamide gel electrophoresis and identified as beta-tubulin by immunolabeling with specific antibodies, mass spectrometry analysis and by N-terminal amino acid sequencing. The purified protein was hyperphosphorylated at serine residues in AD.

Interaction of tau isoforms with Alzheimer's disease abnormally hyperphosphorylated tau and in vitro phosphorylation into the disease-like protein

The microtubule-associated protein tau is a family of six isoforms that becomes abnormally hyperphosphorylated and accumulates in neurons undergoing neurodegeneration in the brains of patients with Alzheimer disease (AD). We investigated the isoform-specific interaction of normal tau with AD hyperphosphorylated tau (AD P-tau). We found that the binding of AD P-tau to normal human recombinant tau was tau4L > tau4S > tau4 and tau3L > tau3S > tau3, and that its binding to tau4L was greater than to tau3L. AD P-tau also inhibited the assembly of microtubules promoted by each tau isoform and caused disassembly when added to preassembled microtubules. This inhibition and depolymerization of microtubules by the AD P-tau corresponded directly to the degree of its interaction with the different tau isoforms. In vitro hyperphosphorylation of recombinant tau (P-tau) conferred AD P-tau-like characteristics. Like AD P-tau, P-tau interacted with and sequestered normal tau and inhibited microtubule assembly. These studies suggest that the AD P-tau interacts preferentially with the tau isoforms that have the amino-terminal inserts and four microtubule binding domain repeats and that hyperphosphorylation of tau appears to be sufficient to acquire AD P-tau characteristics. Thus, lack of amino-terminal inserts and extra microtubule binding domain repeat in fetal human brain might be protective from Alzheimer's neurofibrillary degeneration.

Functional differences of tau isoforms containing 3 or 4 C-terminal repeat regions and the influence of oxidative stress

We report functional differences between tau isoforms with 3 or 4 C-terminal repeats and a difference in susceptibility to oxidative conditions, with respect to the regulation of microtubule dynamics in vitro and tau-microtubule binding in cultured cells. In the presence of dithiothreitol in vitro, a 3-repeat tau isoform promotes microtubule nucleation, reduces the tubulin critical concentration for microtubule assembly, and suppresses dynamic instability. Under non-reducing conditions, threshold concentrations of 3-repeat tau and tubulin exist below which this isoform still promotes microtubule nucleation and assembly but fails to reduce the tubulin critical concentration or suppress dynamic instability; above these threshold concentrations, amorphous aggregates of 3-repeat tau and tubulin can be produced at the expense of microtubule formation. A 4-repeat tau isoform is less sensitive to the oxidative potential of the environment, behaving under oxidative conditions similarly to the 3-repeat isoform under reducing conditions. Under conditions of oxidative stress, in Chinese hamster ovary cells stably expressing either 3- or 4-repeat tau, 3-repeat tau disassociates from microtubules more readily than the 4-repeat isoform, and tau-containing high molecular weight aggregates are preferentially observed in lysates from the Chinese hamster ovary cells expressing 3-repeat tau, indicating greater susceptibility of 3-repeat tau to oxidative conditions, compared with 4-repeat tau in vivo.

Mechanism of neurofibrillary degeneration and pharmacologic therapeutic approach

Neurofibrillary degeneration is a key histopathological brain lesion of Alzheimer disease (AD) and related neurodegenerative disorders such as frontotemporal dementia and Parkinsonism linked to chromosome 17 (FTDP-17), commonly referred to as tauopathies. Microtubule associated protein (MAP) tau, which is a major MAP of a normal mature neuron is abnormally hyperphosphorylated in tauopathies and is the major protein subunit of paired helical filaments (PHF)/straight filaments (SF) which accumulate in the soma (as neurofibrillary tangles) and dystrophic neurites (as neuropil threads and as dystrophic neurites surrounding the beta-amyloid core in neuritic plaques in AD) of the affected neurons. Unlike normal tau which stimulates assembly and stabilizes microtubules, the abnormally hyperphosphorylated tau inhibits assembly and disrupts microtubules. The abnormally hyperphosphorylated tau competes with tubulin/microtubules in associating with normal tau, MAP1 and MAP2. This sequestration of normal MAPs by the abnormal tau results in the breakdown of the microtubules. The association of the abnormal tau with normal tau and not with MAP1 or MAP2 results in the formation of tangles of tau filaments. All these toxic properties of the abnormally hyperphosphorylated tau are eliminated by its enzymatic dephosphorylation. Activities of phosphoseryl/phosphothreonyl protein phosphatases (PP)-2A and PP-1 which can dephosphorylate the abnormal tau to a normal-like state are compromised in AD brain. Dephosphorylation by PP-2A and PP-2B and to a lesser extent by PP-1 restores the normal microtubule assembly promoting activity in AD P-tau in vitro. Neurofibrillary tangles of PHF isolated from AD brain are also dissociated on in vitro dephosphorylation with PP-2A, and the tau released by this treatment can stimulate microtubule assembly. Thus, it appears that the abnormal hyperphosphorylation of tau leads to neurodegeneration through breakdown of the microtubule network and that the abnormal tau on association with normal tau forms neurofibrillary tangles of tau filaments i.e. PHF/SF. Increase in tau phosphatase activity is a promising approach to inhibit neurofibrillary degeneration and thereby the diseases characterized by this lesion.

Role of protein phosphatase-2A and -1 in the regulation of GSK-3, cdk5 and cdc2 and the phosphorylation of tau in rat forebrain

In Alzheimer disease brain the activities of protein phosphatase (PP)-2A and PP-1 are decreased and the microtubule-associated protein tau is abnormally hyperphosphorylated at several sites at serine/threonine. Employing rat forebrain slices kept metabolically active in oxygenated artificial CSF as a model system, we investigated the role of PP-2A/PP-1 in the regulation of some of the major abnormally hyperphosphorylated sites of tau and the protein kinases involved. Treatment of the brain slices with 1.0 microM okadaic acid inhibited approximately 65% of PP-2A and produced hyperphosphorylation of tau at Ser 198/199/202, Ser 396/404 and Ser 422. No significant changes in the activities of glycogen synthase kinase-3 (GSK-3) and cyclin dependent protein kinases cdk5 and cdc2 were observed. Calyculin A (0.1 microM) inhibited approximately 50% PP-1, approximately 20% PP-2A, 50% GSK-3 and approximately 30% cdk5 but neither inhibited the activity of cyclin AMP dependent protein kinase A (PKA) nor resulted in the hyperphosphorylation of tau at any of the above sites. Treatment of brain slices with 1 microM okadaic acid plus 0.1 microM calyculin A inhibited approximately 100% of both PP-2A and PP-1, approximately 80% of GSK-3, approximately 50% of cdk5 and approximately 30% of cdc2 but neither inhibited PKA nor resulted in the hyperphosphorylation of tau at any of the above sites. These studies suggest (i) that PP-1 upregulates the phosphorylation of tau at Ser 198/199/202 and Ser 396/404 indirectly by regulating the activities of GSK-3, cdk5 and cdc2 whereas PP-2A regulates the phosphorylation of tau directly by dephosphorylation at the above sites, and (ii) that a decrease in the PP-2A activity leads to abnormal hyperphosphorylation of tau at Ser 198/199/202, Ser 396/404 and Ser 422.

Phosphorylation of tau alters its association with the plasma membrane

1. The potential functions of the microtubule-associated protein tau have been expanded by the recent demonstration of its interaction with the plasma membrane. Since the association of tau with microtubules is regulated by phosphorylation, herein we examine whether or not the association of tau with the plasma membrane is also regulated by phosphorylation. 2. A range of tau isoforms migrating from 46 to 64 kDa was associated with crude particulate fractions derived from SH-SY-5Y human neuroblastoma cells, and were retained during the initial stages of plasma membrane purification. During the extensive washing utilized in purification of the plasma membrane, portions of each of these isoforms were depleted from the resultant purified membrane. Immunoblot analysis with phospho-dependent and -independent antibodies revealed selective depletion of phospho isoforms during membrane washing. This effect was more pronounced for the slowest-migrating (64-kDa) tau isoform. 3. This putative influence of phosphorylation on the association of tau with the plasma membrane was further probed by transfection of SH-SY-5Y human neuroblastoma cells with a tau construct that could associate with the plasma membrane but not with microtubules. Treatment with phorbol ester or calcium ionophore, both of which increased phospho-tau levels within the cytosol and plasma membrane, was accompanied by the dissociation of this tau construct from the membrane. 4. These data indicate that phosphorylation regulates the association with the plasma membrane. Dissociation from the membrane by phosphorylation may place tau at risk for hyperphosphorylation and ultimate PHF formation in a manner previously considered for tau dissociated from microtubules.

Wnt signaling function in Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disease with progressive dementia accompanied by three main structural changes in the brain: diffuse loss of neurons; intracellular protein deposits termed neurofibrillary tangles (NFT) and extracellular protein deposits termed amyloid or senile plaques, surrounded by dystrophic neurites. Two major hypotheses have been proposed in order to explain the molecular hallmarks of the disease: The 'amyloid cascade' hypothesis and the 'neuronal cytoskeletal degeneration' hypothesis. While the former is supported by genetic studies of the early-onset familial forms of AD (FAD), the latter revolves around the observation in vivo that cytoskeletal changes - including the abnormal phosphorylation state of the microtubule associated protein tau - may precede the deposition of senile plaques. Recent studies have suggested that the trafficking process of membrane associated proteins is modulated by the FAD-linked presenilin (PS) proteins, and that amyloid beta-peptide deposition may be initiated intracellularly, through the secretory pathway. Current hypotheses concerning presenilin function are based upon its cellular localization and its putative interaction as macromolecular complexes with the cell-adhesion/signaling beta-catenin molecule and the glycogen synthase kinase 3beta (GSK-3beta) enzyme. Developmental studies have shown that PS proteins function as components in the Notch signal transduction cascade and that beta-catenin and GSK-3beta are transducers of the Wnt signaling pathway. Both pathways are thought to have an important role in brain development, and they have been connected through Dishevelled (Dvl) protein, a known transducer of the Wnt pathway. In addition to a review of the current state of research on the subject, we present a cell signaling model in which a sustained loss of function of Wnt signaling components would trigger a series of misrecognition events, determining the onset and development of AD.

Tau aggregation into fibrillar polymers: taupathies

Different neurological disorders, known as taupathies have been recently described. In these disorders it has been suggested that modifications in the microtubule-associated protein tau could cause neural degeneration in specific regions. Although these regions are different in the different taupathies, some common features appear to occur in all of them: abnormal hyperphosphorylation of tau and aberrant tau aggregation. These two features are commented upon in this review.

Beta-amyloid-induced calcium influx induces apoptosis in culture by oxidative stress rather than tau phosphorylation

Beta-amyloid (betaA) toxicity in culture is accompanied by multiple events culminating in apoptosis. Calcium influx may represent the initial event, since calcium chelation prevents all subsequent events, while subsequent events include increased generation of reactive oxygen species (ROS) and hyperphosphorylation of tau. In the present study, we undertook to determine whether ROS generation or tau hyperphosphorylation mediate betaA-induced apoptosis. The anti-oxidant vitamin E or the kinase inhibitor N-(6-aminohexyl)-5-chloro-1-naphthalenslfonamide (W7) was added following brief treatment of differentiated SH-SY-5Y human neuroblastoma cells with 22 microM betaA. Under these conditions, vitamin E prevented ROS generation and apoptosis, but did not prevent intracellular calcium accumulation or tau phosphorylation. W7 prevented tau phosphorylation but did not block betaA-induced calcium influx, ROS generation or apoptosis. While these studies do not address the long-term consequences of PHF formation, they indicate that ROS generation, rather than tau hyperphosphorylation, leads to apoptosis following betaA-induced calcium influx into cultured cells.

beta-Amyloid(1-42) binds to alpha7 nicotinic acetylcholine receptor with high affinity. Implications for Alzheimer's disease pathology

Alzheimer's disease pathology is characterized by the presence of neuritic plaques and the loss of cholinergic neurons in the brain. The underlying mechanisms leading to these events are unclear, but the 42-amino acid beta-amyloid peptide (Abeta(1-42)) is involved. Immunohistochemical studies on human sporadic Alzheimer's disease brains demonstrate that Abeta(1-42) and a neuronal pentameric cation channel, the alpha7 nicotinic acetylcholine receptor (alpha7nAChR), are both present in neuritic plaques and co-localize in individual cortical neurons. Using human brain tissues and cells that overexpress either alpha7nAChR or amyloid precursor protein as the starting material, Abeta(1-42) and alpha7nAChR can be co-immunoprecipitated by the respective specific antibodies, suggesting that they are tightly associated. The formation of the alpha7nAChR.Abeta(1-42) complex can be efficiently suppressed by Abeta(12-28), implying that this Abeta sequence region contains the binding epitope. Receptor binding experiments show that Abeta(1-42) and alpha7nAChR bind with high affinity, and this interaction can be inhibited by alpha7nAChR ligands. Human neuroblastoma cells overexpressing alpha7nAChR are readily killed by Abeta(1-42), whereas alpha7nAChR agonists such as nicotine and epibatidine offered protection. Because Abeta(1-42) inhibits alpha7nAChR-dependent calcium activation and acetylcholine release, two processes critically involved in memory and cognitive functions, and the distribution of alpha7nAChR correlates with neuritic plaques in Alzheimer's disease brains, we propose that interaction of the alpha7nAChR and Abeta(1-42) is a pivotal mechanism involved in the pathophysiology of Alzheimer's disease.

Phosphorylation of microtubule-associated protein tau is regulated by protein phosphatase 2A in mammalian brain. Implications for neurofibrillary degeneration in Alzheimer's disease

Hyperphosphorylated tau, which is the major protein of the neurofibrillary tangles in Alzheimer's disease brain, is most probably the result of an imbalance of tau kinase and phosphatase activities in the affected neurons. By using metabolically competent rat brain slices as a model, we found that selective inhibition of protein phosphatase 2A by okadaic acid induced an Alzheimer-like hyperphosphorylation and accumulation of tau. The hyperphosphorylated tau had a reduced ability to bind to microtubules and to promote microtubule assembly in vitro. Immunocytochemical staining revealed hyperphosphorylated tau accumulation in pyramidal neurons in cornu ammonis and in neocortical neurons. The topography of these changes recalls the distribution of neurofibrillary tangles in Alzheimer's disease brain. Selective inhibition of protein phosphatase 2B with cyclosporin A did not have any significant effect on tau phosphorylation, accumulation, or function. These studies suggest that protein phosphatase 2A participates in regulation of tau phosphorylation, processing, and function in vivo. A down-regulation of protein phosphatase 2A activity can lead to Alzheimer-like abnormal hyperphosphorylation of tau.

Multiple forms of phosphatase from human brain: isolation and partial characterization of affi-gel blue binding phosphatases

Implication of protein phosphatases in Alzheimer disease led us to a systemic investigation of the identification of these enzyme activities in human brain. Human brain phosphatases eluted from DEAE-Sephacel with 0.22 M NaCl were resolved into two main groups by affi-gel blue chromatography, namely affi-gel blue-binding phosphatases and affi-gel blue-nonbinding phosphatases. Affi-gel blue-binding phosphatases were further separated into four different phosphatases, designated P1, P2, P3, and P4 by calmodulin-Sepharose 4B and poly-(L-lysine)-agarose chromatographies. These four phosphatases exhibited activities towards nonprotein phosphoester and two of them, P1 and P4, could dephosphorylate phosphoproteins. The activities of the four phosphatases differed in pH optimum, divalent metal ion requirements, sensitivities to various inhibitors and substrate affinities. The apparent molecular masses as estimated by gel-filtration for P1, P2, P3, and P4 were 97, 45, 42, and 125 kDa, respectively. P1 is markedly similar to PP2B from bovine brain and rabbit skeletal muscle. P4 was labeled with anti-PP2A antibody and may represent a new subtype of PP2A. P1 and P4 were also effective in dephosphorylating Alzheimer disease abnormally hyperphosphorylated tau (AD P-tau). The resulting dephosphorylated AD P-tau had its activity restored in promoting assembly of microtubules in vitro. These results suggest that P1 and P4 might be involved in the regulation of phosphorylation of tau in human brain, especially in neurodegenerative conditions like Alzheimer's disease which are characterized by the abnormal hyperphosphorylation of this protein.

Inhibition of tau phosphorylating protein kinase cdk5 prevents beta-amyloid-induced neuronal death

The key target of this study was the tau protein kinase II system (TPK II) involving the catalytic subunit cdk5 and the regulatory component p35. TPK II is one of the tau phosphorylating systems in neuronal cells, thus regulating its functions in the cytoskeletal dynamics and the extension of neuronal processes. This research led to demonstration that the treatment of rat hippocampal cells in culture with fibrillary beta-amyloid (Abeta) results in a significant increase of the cdk5 enzymatic activity. Interestingly, the data also showed that the neurotoxic effect of 1-20 microM Abeta on primary cultures markedly diminished with co-incubation of hippocampal cells with the amyloid fibers plus the cdk5 inhibitor butyrolactone I. This inhibitor protected brain cells against Abeta-induced cell death in a concentration dependent fashion. Moreover, death was also prevented by a cdk5 antisense probe, but not by an oligonucleotide with a random sequence. The cdk5 antisense also reduced neuronal expression of cdk5 compared with the random oligonucleotide. The studies indicate that cdk5 plays a major role in the molecular path leading to the neurodegenerative process triggered by the amyloid fibers in primary cultures of rat hippocampal neurons. These findings are of interest in the context of the pathogenesis of Alzheimer's disease.

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

Tau, one of the best characterized microtubule-associated proteins (MAPs), is a phosphoprotein, the biological activity of which is regulated by its degree of phosphorylation. The objective of the present study was to evaluate the regulation, phosphorylation and the biological activity of tau during differentiation. On differentiation, the tau/tubulin ratio increased about 3-fold regardless whether cells were optimally differentiated with retinoic acid and aphidicolin or with retinoic acid alone which does not inhibit proliferation. The phosphorylation at the Tau-1 (Ser-195/Ser-198/Ser-199/Ser-202) and PHF-1 (Ser-396/Ser-404) sites was increased, mostly in the retinoic acid treated cells, whereas phosphorylation of tau at the 12E8 (Ser-262/Ser-356) epitope was decreased in both groups by approximately 60%. Phosphorylation at the 12E8 site is thought to be one of the most prominent factors affecting the biological activity of tau. However, the microtubule binding activity of tau increased only slightly upon differentiation. Furthermore, a large part of the tau that bound to taxol-stabilized microtubules was phosphorylated at all three sites indicating that these sites are not major sites determining the biological activity of tau. These data show that differentiation of SY5Y cells results in increased tau levels rather than dephosphorylation of tau to meet the additional need in tau's biological activity.

Hyperphosphorylated tau in SY5Y cells: similarities and dissimilarities to abnormally hyperphosphorylated tau from Alzheimer disease brain

Unlike normal tau, abnormally hyperphosphorylated tau (AD P-tau) from Alzheimer disease (AD) does not promote but instead inhibits microtubule assembly and disrupts already formed microtubules. Tau in the human neuroblastoma cell line SH-SY5Y is hyperphosphorylated at several of the same sites as AD P-tau, and accumulates in the cell body without any association to the cellular microtubule network. The aim of the present study was to elucidate why the SY5Y tau does not affect the viability of the cells. We found that, like AD P-tau, SY5Y tau because of hyperphosphorylation does not bind to microtubules and inhibits the tau-promoted assembly of microtubules. However, the tau/HMW MAP ratio is about 10 times less in SY5Y cells than in AD brain. These findings suggest that the hyperphosphorylated tau from SY5Y cells has similar biological characteristics as AD P-tau from AD brain, but is not lethal to the SY5Y cells because of its low tau/HMW MAP ratio.

Identification of 3'UTR region implicated in tau mRNA stabilization in neuronal cells

Tau, a neuronal microtubule-associated protein (MAP) plays an important role in the formation and maintenance of neuronal polarity. Tau mRNA is a stable message and exhibits a relatively long half-life in neuronal cells. The regulation of mRNA stability is a crucial determinant in controlling mRNA steady-state levels in neuronal cells and thereby influences gene expression. The half-lives of specific mRNAs may be dependent on specific sequences located at their 3'untranslated region (UTR), which in turn, may be recognized by tissue-specific proteins. To identify the sequence elements involved in tau mRNA stabilization, selected regions of the 3'UTR were subcloned downstream to c-fos reporter mRNA or to the coding region of the tau mRNA. Using stably transfected neuronal cells, we have demonstrated that a fragment of 240 bp (H fragment) located in the 3'UTR can stabilize c-fos and tau mRNAs. Analysis of stably transfected cells indicated that the transfected tau mRNAs are associated with the microtubules of neuronal cells, suggesting that this association may play a role in tau mRNA stabilization. This step may be a prerequisite in the multistep process leading to the subcellular localization of tau mRNA in neuronal cells.

The Opitz syndrome gene product, MID1, associates with microtubules

Opitz syndrome (OS) is a genetically heterogeneous disorder characterized by defects of the ventral midline, including hypertelorism, cleft lip and palate, heart defects, and mental retardation. We recently identified the gene responsible for X-linked OS. The ubiquitously expressed gene product, MID1, is a member of the RING finger family. These proteins are characterized by an N-terminal tripartite protein-protein interaction domain and a conserved C terminus of unknown function. Unlike other RING finger proteins for which diverse cellular functions have been proposed, the function of MID1 is as yet undefined. By using the green fluorescent protein as a tag, we show here that MID1 is a microtubule-associated protein that influences microtubule dynamics in MID1-overexpressing cells. We confirm this observation by demonstrating a colocalization of MID1 and tubulin in subcellular fractions and the association of endogenous MID1 with microtubules after in vitro assembly. Furthermore, overexpressed MID1 proteins harboring mutations described in OS patients lack the capability to associate with microtubules, forming cytoplasmic clumps instead. These data give an idea of the possible molecular pathomechanism underlying the OS phenotype.

Tau is phosphorylated by GSK-3 at several sites found in Alzheimer disease and its biological activity markedly inhibited only after it is prephosphorylated by A-kinase

Alzheimer disease is characterized by a specific type of neuronal degeneration in which the microtubule associated protein tau is abnormally hyperphosphorylated causing the disruption of the microtubule network. We have found that the phosphorylation of human tau (tau3L) by A-kinase, GSK-3 or CK-1 inhibits its microtubule assembly-promoting and microtubule-binding activities. However, the inhibition of these activities of tau by GSK-3 is significantly increased if tau is prephosphorylated by A-kinase or CK-1. The most potent inhibition is observed by combination phosphorylation of tau with A-kinase and GSK-3. Under these conditions, only very few microtubules are seen by electron microscopy. Sequencing of 32P-labeled trypsin phosphopeptides from tau prephosphorylated by A-kinase (using unlabeled ATP) and further phosphorylated by GSK-3 in the presence of [gamma-32P]ATP revealed that Ser-195, Ser-198, Ser-199, Ser-202, Thr-205, Thr-231, Ser-235, Ser-262, Ser-356 and Ser-404 are phosphorylated, whereas if tau is not prephosphorylated by A-kinase, GSK-3 phosphorylates it at Thr-181, Ser-184, Ser-262, Ser-356 and Ser-400. These data suggest that (i) prephosphorylation of tau by A-kinase makes additional and different sites accessible for phosphorylation by GSK-3; (ii) phosphorylation of tau at these additional sites further inhibits the biological activity of tau in its ability to bind to microtubules and promote microtubule assembly. Thus a combined role of A-kinase and GSK-3 should be considered in Alzheimer neurofibrillary degeneration.

Mechanisms of neurofibrillary degeneration and the formation of neurofibrillary tangles

Alzheimer disease (AD) has polyetiology. Independent of the etiology the disease is characterized histopathologically by the intraneuronal accumulation of paired helical filaments (PHF), forming neurofibrillary tangles, neuropil threads and dystrophic neurites surrounding the extracellular deposits of beta-amyloid in plaques, the second major lesion. The clincal expression of AD correlates with the presence of neurofibrillary degeneration; beta-amyloid alone does not produce the disease clinically. Thus arresting neurofibrillary degeneration offers a promising key target for therapeutic intervention of AD. The major protein subunit of PHF is the microtubule-associated protein tau. Tau in AD brain, especially PHF, is abnormally hyperphosphorylated and glycosylated. With maturation, the tangles are increasingly ubiquitinated. Levels of tau and conjugated ubiquitin are elevated both in AD brain and CSF. The AD abnormally phosphorylated tau (AD P-tau) does not promote microtubule assembly, but on dephosphorylation its microtubule promoting activity is restored to approximately that of the normal tau. The AD P-tau competes with tubulin in binding to normal tau, MAP1 and MAP2 and inhibits their microtubule assembly promoting activities. Furthermore, the AD P-tau sequesters normal MAPs from microtubules. The association of AD P-tau with normal tau but not with MAP1 or MAP2 results in the formation of tangles of 3.3 +/- 0.5 mm filaments. Deglycosylation of Alzheimer neurofibrillary tangles with endoglycosidase F/N-glycosidase F untwists the PHF resulting in tangles of thin filaments similar to those formed by association between the AD P-tau and normal tau. Dephosphorylation or deglycosylation plus dephosphorylation but not deglycosylation alone restores the microtubule assembly promoting activity of tau. In vitro AD P-tau can be dephosphorylated by protein phosphatases PP-2B, PP-2A and PP-1 but not PP-2C and all the three tau phosphatases are present in brain neurons. Tau phosphatase activity is decreased by approximately 30% in AD brain. Inhibition of PP-2A and PP-1 activities in SY5Y neuroblastoma by 10 nM okadaic acid causes breakdown of microtubules and the degeneration of these cells. It is suggested (I) that a defect(s) in the protein phosphorylation/dephosphorylation system(s) leads to a hyperphosphorylation of tau, (ii) that this altered tau causes disassembly of microtubules and consequently a retrograde neuronal degeneration; (iii) a pharmacological approach to AD is to enhance the tau phosphatase activity; and (iv) that CSF tau and conjugated ubiquitin levels are promising markers of AD brain pathology.

The regulation of phosphorylation of tau in SY5Y neuroblastoma cells: the role of protein phosphatases

In Alzheimer disease brain the microtubule associated protein (MAP) tau is abnormally hyperphosphorylated. The role of protein phosphatases (PP) in the regulation of phosphorylation of tau was studied in undifferentiated SY5Y cells. In cells treated with 10 nM okadaic acid (OA), a PP-2A/PP-1 inhibitor, the PP-1 and -2A activities decreased by 60% and 100% respectively and the activities of MAPKs, cdc2 kinase and cdk5, but not of GSK-3, increased. OA increased the phosphorylation of tau at Thr-231/Ser-235 and Ser-3961404, but not at Ser-262/356 or Ser-199/202. An increase in tyrosinated/detyrosinated tubulin ratio, a decrease in the microtubule binding activities of tau, MAP1b and MAP2, and cell death were observed. Treatment with 1 microm taxol partially inhibited the cell death. These data suggest (1) that OA induced hyperphosphorylation of tau is probably the result of activated MAPK and cdks in addition to decreased PP-2A and PP-1 activities and (2) that in SY5Y cells the OA induced cell death is associated with a decrease in stable microtubules.