Impairment of phosphatase 2A contributes to the prolonged MAP kinase phosphorylation in Alzheimer's disease fibroblasts

Homocysteine in Neurology: A Possible Contributing Factor to Small Vessel Disease

Homocysteine (Hcy) is a sulfur-containing amino acid generated during methionine metabolism, accumulation of which may be caused by genetic defects or the deficit of vitamin B12 and folate. A serum level greater than 15 micro-mols/L is defined as hyperhomocysteinemia (HHcy). Hcy has many roles, the most important being the active participation in the transmethylation reactions, fundamental for the brain. Many studies focused on the role of homocysteine accumulation in vascular or degenerative neurological diseases, but the results are still undefined. More is known in cardiovascular disease. HHcy is a determinant for the development and progression of inflammation, atherosclerotic plaque formation, endothelium, arteriolar damage, smooth muscle cell proliferation, and altered-oxidative stress response. Conversely, few studies focused on the relationship between HHcy and small vessel disease (SVD), despite the evidence that mice with HHcy showed a significant end-feet disruption of astrocytes with a diffuse SVD. A severe reduction of vascular aquaporin-4-water channels, lower levels of high-functioning potassium channels, and higher metalloproteinases are also observed. HHcy modulates the N-homocysteinylation process, promoting a pro-coagulative state and damage of the cellular protein integrity. This altered process could be directly involved in the altered endothelium activation, typical of SVD and protein quality, inhibiting the ubiquitin-proteasome system control. HHcy also promotes a constant enhancement of microglia activation, inducing the sustained pro-inflammatory status observed in SVD. This review article addresses the possible role of HHcy in small-vessel disease and understands its pathogenic impact.

The Controversial Role of Homocysteine in Neurology: From Labs to Clinical Practice

Homocysteine (Hcy) is a sulfur-containing amino acid that is generated during methionine metabolism. Physiologic Hcy levels are determined primarily by dietary intake and vitamin status. Elevated plasma levels of Hcy can be caused by deficiency of either vitamin B12 or folate. Hyperhomocysteinemia (HHcy) can be responsible of different systemic and neurological disease. Actually, HHcy has been considered as a risk factor for systemic atherosclerosis and cardiovascular disease (CVD) and HHcy has been reported in many neurologic disorders including cognitive impairment and stroke, independent of long-recognized factors such as hyperlipidemia, hypertension, diabetes mellitus, and smoking. HHcy is typically defined as levels >15 micromol/L. Treatment of hyperhomocysteinemia with folic acid and B vitamins seems to be effective in the prevention of the development of atherosclerosis, CVD, and strokes. However, data from literature show controversial results regarding the significance of homocysteine as a risk factor for CVD and stroke and whether patients should be routinely screened for homocysteine. HHcy-induced oxidative stress, endothelial dysfunction, inflammation, smooth muscle cell proliferation, and endoplasmic reticulum (ER) stress have been considered to play an important role in the pathogenesis of several diseases including atherosclerosis and stroke. The aim of our research is to review the possible role of HHcy in neurodegenerative disease and stroke and to understand its pathogenesis.

Fusarochromanone-induced reactive oxygen species results in activation of JNK cascade and cell death by inhibiting protein phosphatases 2A and 5

Recent studies have shown that fusarochromanone (FC101), a mycotoxin, is cytotoxic in a variety of cell lines. However, the molecular mechanism underlying its cytotoxicity remains elusive. Here we found that FC101 induced cell death in COS7 and HEK293 cells in part by activating JNK pathway. This is evidenced by the findings that inhibition of JNK with SP600125 or expression of dominant negative c-Jun partially prevented FC101-induced cell death. Furthermore, we observed that FC101-activated JNK pathway was attributed to induction of reactive oxygen species (ROS). Pretreatment with N-acetyl-L-cysteine (NAC), a ROS scavenger and antioxidant, suppressed FC101-induced activation of JNK and cell death. Moreover, we noticed that FC101 inhibited the serine/threonine protein phosphatases 2A (PP2A) and 5 (PP5) in the cells, which was abrogated by NAC. Overexpression of PP2A or PP5 partially prevented FC101-induced activation of JNK and cell death. The results indicate that FC101-induced ROS inhibits PP2A and PP5, leading to activation of JNK pathway and consequently resulting in cell death.

Expression of Tau40 induces activation of cultured rat microglial cells

Accumulation of microtubule-associated protein tau has been observed in the brain of aging and tauopathies. Tau was observed in microglia, but its role is not illustrated. By immunofluorescence staining and the fractal dimension value assay in the present study, we observed that microglia were activated in the brains of rats and mice during aging, simultaneously, the immunoreactivities of total tau and the phosphorylated tau were significantly enhanced in the activated microglia. Furtherly by transient transfection of tau40 (human 2N/4R tau) into the cultured rat microglia, we demonstrated that expression of tau40 increased the level of Iba1, indicating activation of microglia. Moreover, expression of tau40 significantly enhanced the membranous localization of the phosphorylated tau at Ser396 in microglia possibly by a mechanism involving protein phosphatase 2A, extracellular signal-regulated kinase and glycogen synthase kinase-3β. It was also found that expression of tau40 promoted microglial migration and phagocytosis, but not proliferation. And we observed increased secretion of several cytokines, including interleukin (IL)-1β, IL-6, IL-10, tumor necrosis factor-α and nitric oxide after the expression of tau40. These data suggest a novel role of human 2N/4R tau in microglial activation.

Early postnatal lead exposure induces tau phosphorylation in the brain of young rats

Cognitive impairment is a common feature of both lead exposure and hyperphosphorylation of tau. We, therefore, investigated whether lead exposure would induce tau hyperphosphorylation. Wistar rat pups were exposed to 0.2% lead acetate via their dams' drinking water from postnatal day 1 to 21. Lead in blood and brain were measured by atomic absorption spectrophotometry and the expression of tau, phosphorylated tau and various serine/threonine protein phosphatases (PP1, PP2A, PP2B and PP5) in the brain was analyzed by Western blot. Lead exposure significantly impaired learning and resulted in a significant reduction in the expression of tau but increased the phosphorylation of tau at Ser199/202, Thr212/Ser214 and Thr231. PP2A expression decreased, whereas, PP1 and PP5 expression increased in lead-exposed rats. These results demonstrate that early postnatal exposure to lead decrease PP2A expression and induce tau hyperphosphorylation at several serine and threonine residues. Hyperphosphorylation of tau may be a mechanism of Pb-induced deficits in learning and memory.

Curcumin inhibits protein phosphatases 2A and 5, leading to activation of mitogen-activated protein kinases and death in tumor cells

Curcumin can induce p53-independent apoptosis. However, the underlying mechanism remains to be defined. Here, we show that curcumin-induced apoptosis in a panel of tumor cells with mutant p53. Curcumin rapidly induced activation of the mitogen-activated protein kinases (MAPKs) including extracellular signal-regulated kinase 1/2 (Erk1/2) and c-Jun N-terminal kinase (JNK). Inhibition of JNK (with SP600125) or Erk1/2 (with U0126) partially prevented curcumin-induced cell death in the cells. Similarly, expression of dominant negative c-Jun or downregulation of Erk1/2 in part attenuated curcumin-induced cell death. It appears that curcumin-induced activation of MAPKs and apoptosis was due to induction of reactive oxygen species (ROS), as pretreatment with N-acetyl-L-cysteine, a ROS scavenger, blocked these events. Furthermore, we found that curcumin-induced activation of MAPK pathways was related to inhibition of the serine/threonine protein phosphatases 2A (PP2A) and 5 (PP5). Overexpression of PP2A or PP5 partially prevented curcumin-induced activation of JNK and Erk1/2 phosphorylation as well as cell death. The results suggest that curcumin induction of ROS activates MAPKs, at least partially by inhibiting PP2A and PP5, thereby leading to p53-independent apoptosis in tumor cells.

Progress in the development of new drugs in Alzheimer's disease

Alzheimer's disease (AD) is an age-related neurodegenerative disease with a global prevalence estimated at 26.55 million in 2006. During the past decades, several agents have been approved that enhance cognition of AD patients. However, the effectiveness of these treatments are limited or controversial and they do not modify disease progression. Recent advances in understanding AD pathogenesis have led to the development of numerous compounds that might modify the disease process. AD is mainly characterized neuropathologically by the presence of two kinds of protein aggregates: extracellular plaques of Abeta-peptide and intracellular neurofibrillary tangles. Abeta and tau could interfere in an original way contributing to a cascade of events leading to neuronal death and transmitter deficits. Investigation for novel therapeutic approaches targeting the presumed underlying pathogenic mechanisms is major focus of research. Antiamyloid agents targeting production, accumulation, clearance, or toxicity associated with Abeta peptide, are some approaches under investigation to limit extracellular plaques of Abeta-peptide accumulation. We can state as an example: Abeta passive and active immunization, secretases modulation, Abeta degradation enhancement, or antiaggregation and antifibrillization agents. Tau-related therapies are also under clinical investigation but few compounds are available. Another alternative approach under development is neuroprotective agents such as antioxidants, anti-inflammatory drugs, compounds acting against glutamate mediated neurotoxicity. Neurorestorative approaches through neurotrophin or cell therapy also represent a minor avenue in AD research. Finally, statins, receptor for advanced glycation end products inhibitors, thiazolidinediones, insulin, and hormonal therapies are some other ways of research for a therapeutic approach of Alzheimer's disease. Taking into account AD complexity, it becomes clear that polypharmacology with drugs targeting different sites could be the future treatment approach and a majority of the recent drugs under evaluation seems to act on multiple targets. This article exposes general classes of disease-modifying therapies under investigation.

The excitotoxin quinolinic acid induces tau phosphorylation in human neurons

Some of the tryptophan catabolites produced through the kynurenine pathway (KP), and more particularly the excitotoxin quinolinic acid (QA), are likely to play a role in the pathogenesis of Alzheimer's disease (AD). We have previously shown that the KP is over activated in AD brain and that QA accumulates in amyloid plaques and within dystrophic neurons. We hypothesized that QA in pathophysiological concentrations affects tau phosphorylation. Using immunohistochemistry, we found that QA is co-localized with hyperphosphorylated tau (HPT) within cortical neurons in AD brain. We then investigated in vitro the effects of QA at various pathophysiological concentrations on tau phosphorylation in primary cultures of human neurons. Using western blot, we found that QA treatment increased the phosphorylation of tau at serine 199/202, threonine 231 and serine 396/404 in a dose dependent manner. Increased accumulation of phosphorylated tau was also confirmed by immunocytochemistry. This increase in tau phosphorylation was paralleled by a substantial decrease in the total protein phosphatase activity. A substantial decrease in PP2A expression and modest decrease in PP1 expression were observed in neuronal cultures treated with QA. These data clearly demonstrate that QA can induce tau phosphorylation at residues present in the PHF in the AD brain. To induce tau phosphorylation, QA appears to act through NMDA receptor activation similar to other agonists, glutamate and NMDA. The QA effect was abrogated by the NMDA receptor antagonist memantine. Using PCR arrays, we found that QA significantly induces 10 genes in human neurons all known to be associated with AD pathology. Of these 10 genes, 6 belong to pathways involved in tau phosphorylation and 4 of them in neuroprotection. Altogether these results indicate a likely role of QA in the AD pathology through promotion of tau phosphorylation. Understanding the mechanism of the neurotoxic effects of QA is essential in developing novel therapeutic strategies for AD.

Molecular profiling reveals diversity of stress signal transduction cascades in highly penetrant Alzheimer's disease human skin fibroblasts

The serious and growing impact of the neurodegenerative disorder Alzheimer's disease (AD) as an individual and societal burden raises a number of key questions: Can a blanket test for Alzheimer's disease be devised forecasting long-term risk for acquiring this disorder? Can a unified therapy be devised to forestall the development of AD as well as improve the lot of present sufferers? Inflammatory and oxidative stresses are associated with enhanced risk for AD. Can an AD molecular signature be identified in signaling pathways for communication within and among cells during inflammatory and oxidative stress, suggesting possible biomarkers and therapeutic avenues? We postulated a unique molecular signature of dysfunctional activity profiles in AD-relevant signaling pathways in peripheral tissues, based on a gain of function in G-protein-coupled bradykinin B2 receptor (BKB2R) inflammatory stress signaling in skin fibroblasts from AD patients that results in tau protein Ser hyperphosphorylation. Such a signaling profile, routed through both phosphorylation and proteolytic cascades activated by inflammatory and oxidative stresses in highly penetrant familial monogenic forms of AD, could be informative for pathogenesis of the complex multigenic sporadic form of AD. Comparing stimulus-specific cascades of signal transduction revealed a striking diversity of molecular signaling profiles in AD human skin fibroblasts that express endogenous levels of mutant presenilins PS-1 or PS-2 or the Trisomy 21 proteome. AD fibroblasts bearing the PS-1 M146L mutation associated with highly aggressive AD displayed persistent BKB2R signaling plus decreased ERK activation by BK, correctible by gamma-secretase inhibitor Compound E. Lack of these effects in the homologous PS-2 mutant cells indicates specificity of presenilin gamma-secretase catalytic components in BK signaling biology directed toward MAPK activation. Oxidative stress revealed a JNK-dependent survival pathway in normal fibroblasts lost in PS-1 M146L fibroblasts. Complex molecular profiles of signaling dysfunction in the most putatively straightforward human cellular models of AD suggest that risk ascertainment and therapeutic interventions in AD as a whole will likely demand complex solutions.

A cellular model of Alzheimer's disease therapeutic efficacy: PKC activation reverses Abeta-induced biomarker abnormality on cultured fibroblasts

PKC signaling is critical for the non-toxic degradation of amyloid precursor protein (APP) and inhibition of GSK3beta, which controls phosphorylation of tau protein in Alzheimer's disease (AD). Thus the misregulation of PKC signaling could contribute to the origins of AD. Bryostatin, a potent PKC modulator, has the potential to ameliorate both the neurodegeneration and the recent memory loss associated with AD. As reported herein bryostatin and a potent synthetic analog (picolog) are found to cause stimulation of non-amyloidogenic pathways by increasing alpha-secretase activity and thus lowering the amount of toxic Abeta produced. Both bryostatin and picolog increased the secretion of the alpha-secretase product (s-APP-alpha) of APP at sub-nanomolar to nanomolar concentrations. A peripheral AD-Biomarker has previously been autopsy-validated. This Biomarker, based on bradykinin-induced differential phosphorylation of Erk1 and Erk2, has been used here to test the therapeutic efficacy both for bryostatin and picolog. Both of these PKC activators are then shown to convert the AD Erk1/2 phenotype of fibroblasts into the phenotype of "normal" control skin fibroblasts. This conversion occurred for both the abnormal Erk1/2 phenotype induced by application of Abeta(1-42) to the fibroblasts or the phenotype observed for fibroblasts of AD patients. The Abeta(1-42)-induction, and PKC modulator reversal of the AD Erk1/2 biomarker phenotype demonstrate the AD-Biomarker's potential to monitor both disease progression and treatment response. Additionally, this first demonstration of the therapeutic potential in AD of a synthetically accessible bryostatin analog warrants further preclinical advancement.

[Biomarkers for early diagnosis of Alzheimer's disease: current update and future directions]

INTRODUCTION

The increased prevalence of the sporadic form of Alzheimer's disease (AD) has become a significant health issue in the elderly population. The need for early diagnosis is imperative because this, along with the development of novel therapeutic treatments, would permit the rapid and perhaps more efficient treatment of these debilitating disorders early on.

BACKGROUND

Over the last decade, the potential use of certain biomarkers in the cerebrospinal fluid (CSF), and more recently, in the plasma has been investigated. Among the candidates studied includes the neurotoxic amyloid beta peptide and the Tau protein. However, although these two proteins have been clearly shown to be directly related to the pathophysiology of this disorder, it has proven difficult to establish a clear relationship between plasma or CSF levels of Abeta and Tau and the incidence and severity of AD in patients. This is due in part to differences in methodologies related to the detection sensitivity, as well as the variations in the biological data and consequent interpretation of the biochemical and biological data. Peripheral cells, in particular platelets and skin fibroblasts, could be an alternative solution as peripheral biological markers for the early diagnosis of AD. These cells are easily accessible from patients. Furthermore, they would provide a means not only to validate potential therapeutic strategies, but also to study the mechanisms involved in the development of AD, including APP processing.

PERSPECTIVES

A combined strategy using both a fundamental mechanistic and an analytical approach of patient peripheral cells will allow the identification of new biological markers for AD, and hence permit immediate therapeutic strategies to be implemented.

Early diagnostic accuracy and pathophysiologic relevance of an autopsy-confirmed Alzheimer's disease peripheral biomarker

Treatment of Alzheimer's disease (AD) and the discovery of promising drug candidates depend on early diagnosis. Few currently available diagnostic tests have significantly improved this early uncertainty, while the "gold standard" diagnosis continues to require clinical dementia in life and the presence of pathologic brain lesions of amyloid plaques and neurofibrillary tangles in the brain at autopsy. Here, the inflammatory agonist bradykinin, a small nano-peptide, that induces PKC-mediated phosphorylation of Erk1 and Erk2 in fibroblasts, was applied to punch-biopsy-obtained human skin fibroblasts. Quantitative imaging of the phosphorylated Erk1 and Erk2 bands was then used in a ratio that is mathematically configured into an AD-Biomarker Index (AD-Index). In the population described here (N=264), there were 64 autopsy examinations. Demented individuals were clinically diagnosed as AD with an overall accuracy of 78%. Among the 42 autopsy-confirmed cases for which there were also AD-Biomarker measurements, the overall accuracy of the AD-Biomarker was 98%. Among both the autopsy-confirmed and the clinically diagnosed patients, the AD-Index values were inversely correlated with the duration of disease, i.e., the time from the onset of dementia symptoms. Among the autopsy-confirmed cases, the AD-Biomarker diagnosis showed remarkably high sensitivity (97%) and specificity (100%) compared to clinical diagnosis (sensitivity: 78% and specificity: 20%). Using autopsy validation, the clinical diagnosis was only accurate at 52% level vs. the AD-Biomarker accuracy of 100% for cases with dementia not larger than 4 years of duration. Finally, application of soluble Abeta(1-42) to the fibroblasts of normal controls induced the abnormal AD-Biomarker phenotype, suggesting the pathophysiologic relevance of this AD-Biomarker measurement. In summary, the AD-Biomarker, as confirmed by autopsy validation, showed significantly higher sensitivity and specificity than did clinical diagnosis, particularly at early stages of disease, and pathophysiological relevance was demonstrated for the mechanistic basis of the AD-Biomarker measurements.

Tyrosine 302 in RACK1 is essential for insulin-like growth factor-I-mediated competitive binding of PP2A and beta1 integrin and for tumor cell proliferation and migration

Insulin-like growth factor (IGF)-I regulates a mutually exclusive interaction of PP2A and beta1 integrin with the WD repeat scaffolding protein RACK1. This interaction is required for the integration of IGF-I receptor (IGF-IR) and adhesion signaling. Here we investigated the nature of the binding site for PP2A and beta1 integrin in RACK1. A WD7 deletion mutant of RACK1 did not associate with PP2A but retained some interaction with beta1 integrin, whereas a WD6/WD7 mutant lost the ability to bind to both PP2A and beta1 integrin. Using immobilized peptide arrays representing the entire RACK1 protein, we identified a common cluster of amino acids (FAGY) at positions 299-302 within WD7 of RACK1 which were essential for binding of both PP2A and beta1 integrin to RACK1. PP2A showed a higher level of association with a peptide in which Tyr-302 was phosphorylated compared with an unphosphorylated peptide, whereas beta1 integrin binding was not affected by phosphorylation. RACK1 mutants in which either the FAGY cluster or Tyr-302 were mutated to AAAF, or Phe, respectively, did not interact with either PP2A or beta1 integrin. These mutants were unable to rescue the decrease in PP2A activity caused by suppression of RACK1 in MCF-7 cells with small interfering RNA. MCF-7 cells and R+ (IGF-IR-overexpressing fibroblasts) expressing these mutants exhibited decreased proliferation and migration, whereas R- cells (IGF-IR null fibroblasts) were unaffected. Taken together, the data demonstrate that Tyr-302 in RACK1 is required for interaction with PP2A and beta1 integrin, for regulation of PP2A activity, and for IGF-I-mediated cell migration and proliferation.

Proteomic analysis of peripheral leukocytes in Alzheimer's disease patients treated with divalproex sodium

The molecular profiling of peripheral tissues, including circulating leukocytes, may hold promise in the discovery of biomarkers for diagnosing and treating neurodegenerative diseases, including Alzheimer's disease (AD). As a proof-of-concept, we performed a proteomics study on peripheral leukocytes from patients with AD both before and during treatment with divalproex sodium. Using two-dimensional gel electrophoresis and MALDI-TOF mass spectrometry, we identified 10 differentially expressed proteins: two up-regulated proteins, 14-3-3 protein epsilon and peroxiredoxin 2; and eight down-regulated proteins, actin-interacting protein, mitogen activated protein kinase 1, beta actin, annexin A1, glyceraldehyde 3-phosphate dehydrogenase, transforming protein RhoA, acidic leucine-rich nuclear phosphoprotein 32 family member B, and a currently unidentified protein. A subset was validated on both the transcript and protein levels in normal human peripheral blood mononuclear cell cultures treated with valproic acid. These proteins comprise a number of functional classes that may be important to the biology of AD and to the therapeutic action of valproate. These data also suggest the potential of using peripheral leukocytes to monitor pharmaceutical action for neurodegenerative diseases.

Different mechanisms account for extracellular-signal regulated kinase activation in distinct brain regions following global ischemia and reperfusion

Oxidative stress after cerebral ischemia and reperfusion activates extracellular signal-regulated kinases (ERK) in brain. However, the mechanism of this activation has not been elucidated. We have previously reported that in an in vitro model of oxidative stress in immature cortical neuronal cultures, the inhibition of ERK phosphatase activity contributes to ERK1/2 activation and subsequent neuronal toxicity. This study examined whether ERK activation was associated with altered activity of ERK phosphatases in a rat cardiac arrest model. Rats in experimental groups were subjected to asphyxial cardiac arrest for 8 min and then resuscitated for 30 min. Significant ERK activation was detected in both cortex and hippocampus following ischemia/reperfusion by immunoblotting. ERK phosphatase activity was reversibly inhibited in cerebral cortex but not affected in hippocampus following ischemia/reperfusion. MEK1/2 was activated in both cerebral cortex and hippocampus following ischemia/reperfusion. Using a specific inhibitor of protein phosphatase 2A (PP2A), okadaic acid (OA), we have identified PP2A to be the major ERK phosphatase that is responsible for regulating ERK activation in ischemic brain tissues. Orthovanadate inhibited ERK phosphatase activity in brain tissues, suggesting that tyrosine phosphatases and dual specificity phosphatases may also contribute to the ERK phosphatase activity in brain tissues. Together, these data implicate ERK phosphatase in the regulation of ERK activation in distinct brain regions following global ischemia.

Prostaglandin E(2) inhibits collagen expression and proliferation in patient-derived normal lung fibroblasts via E prostanoid 2 receptor and cAMP signaling

Uncontrolled fibroblast activation is one of the hallmarks of fibrotic lung disease. Prostaglandin E(2) (PGE(2)) has been shown to inhibit fibroblast migration, proliferation, collagen deposition, and myofibroblast differentiation in the lung. Understanding the mechanisms for these effects may provide insight into the pathogenesis of fibrotic lung disease. Previous work has focused on commercially available fibroblast cell lines derived from tissue whose precise origin and histopathology are often unknown. Here, we sought to define the mechanism of PGE(2) inhibition in patient-derived fibroblasts from peripheral lung verified to be histologically normal. Fibroblasts were grown from explants of resected lung, and proliferation and collagen I expression was determined following treatment with PGE(2) or modulators of its receptors and downstream signaling components. PGE(2) inhibited fibroblast proliferation by 33% and collagen I expression by 62%. PGE(2) resulted in a 15-fold increase in intracellular cAMP; other cAMP-elevating agents inhibited collagen I in a manner similar to PGE(2). These effects were reproduced by butaprost, a PGE(2) analog selective for the cAMP-coupled E prostanoid (EP) 2 receptor, but not by selective EP3 or EP4 agonists. Fibroblasts expressed both major cAMP effectors, protein kinase A (PKA) and exchange protein activated by cAMP-1 (Epac-1), but only a selective PKA agonist was able to appreciably inhibit collagen I expression. Treatment with okadaic acid, a phosphatase inhibitor, potentiated the effects of PGE(2). Our data indicate that PGE(2) inhibits fibroblast activation in primary lung fibroblasts via binding of EP2 receptor and production of cAMP; inhibition of collagen I proceeds via activation of PKA.

Mechanisms of homocysteine neurotoxicity in neurodegenerative diseases with special reference to dementia

Mild to moderate hyperhomocysteinemia is a risk factor for neurodegenerative diseases. Human studies suggest that homocysteine (Hcy) plays a role in brain damage, cognitive and memory decline. Numerous studies in recent years investigated the role of Hcy as a cause of brain damage. Hcy itself or folate and vitamin B12 deficiency can cause disturbed methylation and/or redox potentials, thus promoting calcium influx, amyloid and tau protein accumulation, apoptosis, and neuronal death. The Hcy effect may also be mediated by activating the N-methyl-D-aspartate receptor subtype. Numerous neurotoxic effects of Hcy can be blocked by folate, glutamate receptor antagonists, or various antioxidants. This review describes the most important mechanisms of Hcy neurotoxicity and pharmacological agents known to reverse Hcy effects.

α-Synuclein activation of protein phosphatase 2A reduces tyrosine hydroxylase phosphorylation in dopaminergic cells

α-Synuclein is an abundant presynaptic protein implicated in neuronal plasticity and neurodegenerative diseases. Although the function of α-synuclein is not thoroughly elucidated, we found that α-synuclein regulates dopamine synthesis by binding to and inhibiting tyrosine hydroxylase, the rate limiting enzyme in dopamine synthesis. Understanding α-synuclein function in dopaminergic cells should add to our knowledge of this key protein, which is implicated in Parkinson's disease and other disorders. Herein, we report a mechanism by which α-synuclein diminishes tyrosine hydroxylase phosphorylation and activity in stably transfected dopaminergic cells. Short-term regulation of tyrosine hydroxylase depends on the phosphorylation of key seryl residues in the amino-terminal regulatory domain of the protein. Of these, Ser40 contributes significantly to tyrosine hydroxylase activation and dopamine synthesis. We observed that α-synuclein overexpression caused reduced Ser40 phosphorylation in MN9D cells and inducible PC12 cells. Ser40 is phosphorylated chiefly by the cyclic AMP-dependent protein kinase PKA and dephosphorylated almost exclusively by the protein phosphatase, PP2A. Therefore, we measured the impact of α-synuclein overexpression on levels and activity of PKA and PP2A in our cells. PKA was unaffected by α-synuclein. PP2A protein levels also were unchanged, however, the activity of PP2A increased in parallel with α-synuclein expression. Inhibition of PP2A dramatically increased Ser40 phosphorylation only in α-synuclein overexpressors in which α-synuclein was also found to co-immunoprecipitate with PP2A. Together the data reveal a functional interaction between α-synuclein and PP2A that leads to PP2A activation and underscores a key role for α-synuclein in protein phosphorylation.

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.

Downregulation of protein phosphatase 2A carboxyl methylation and methyltransferase may contribute to Alzheimer disease pathogenesis

ABalphaC, a major protein phosphatase 2A (PP2A) heterotrimeric enzyme, binds to and regulates the microtubule cytoskeleton and tau. We have shown that ABalphaC protein expression levels are selectively reduced in Alzheimer disease (AD). Notably, the carboxyl methylation of PP2A catalytic subunit (PP2A(C)) is critically required for ABalphaC holoenzyme assembly, and catalyzed by a specific methyltransferase (PPMT). Here, we provide the first analysis of human PPMT and methylated PP2A(C) in brain regions from AD, non-AD demented, and aged control autopsy cases. Immunoblotting analyses revealed that PPMT protein expression and PP2A(C) methylation levels were quantitatively decreased in AD-affected brain regions. Immunohistochemical studies showed that PPMT was abundant in neurons throughout the cortex in normal control and non-AD demented cases. However, in AD, there was a regional loss of PPMT immunoreactivity that closely paralleled the severity of tau pathology, but not amyloid plaque burden. We propose that the deregulation of PPMT and PP2A methylation/demethylation cycles contributes to AD pathogenesis, by inducing changes in PP2A heteromultimeric composition and substrate specificity. In turn, PP2A dysfunction compromises the mechanisms that control tau, neuronal plasticity, and survival.

Mitogen-activated protein kinases and the evolution of Alzheimer's: a revolutionary neurogenetic axis for therapeutic intervention?

Alzheimer's disease (AD) is a neurogenetic condition that affects the processes via which the brain functions. Major observable hallmarks of AD are accumulated clusters of proteins in the brain. These clusters, termed neurofibrillary tangles (NFT), resemble pairs of threads wound around each other in a helix fashion accumulating within neurons. These tangles consist of a protein called Tau, which binds to tubulin, thus forming microtubules. Unlike NFTs, deposits of amyloid precursor protein (beta-APP) gather in the spaces between nerve cells. The nearby neurons often look swollen and deformed, and the clusters of protein are usually accompanied by reactive inflammatory cells, microglia, which are part of the brain's immune system responsible for degrading and removing damaged neurons or plaques. Since phosphorylation/dephosphorylation mechanisms are crucial in the regulation of Tau and beta-APP, a superfamily of mitogen-activated protein kinases (MAPKs) has recently emerged as key regulators of the formation of plagues, eventually leading to dementia and AD. The complex molecular interactions between MAPKs and proteins (plagues) associated with the evolution of AD form a cornerstone in the knowledge of a still burgeoning field of neurodegenerative diseases and ageing. This review overviews current understanding of the molecular pathways related to MAPKs and their role in the development of AD and, possibly, dementia. MAPKs, therefore, may constitute a neurogenetic, therapeutic target for the diagnosis and evolution of a preventative medical strategy for early detection, and likely treatment, of Alzheimer's.