Biomedical subjects

Cheng-Xin Gong

Publications and source records attributed to Cheng-Xin Gong.

At least 19 recordsLinked to original sources

PubMed2006

PKA modulates GSK-3beta- and cdk5-catalyzed phosphorylation of tau in site- and kinase-specific manners.

Phosphorylation of tau protein is regulated by several kinases, especially glycogen synthase kinase 3beta (GSK-3beta), cyclin-dependent protein kinase 5 (cdk5) and cAMP-dependent protein kinase (PKA). Phosphorylation of tau by PKA primes it for phosphorylation by GSK-3beta, but the site-specific modulation of GSK-3beta-catalyzed tau phosphorylation by the prephosphorylation has not been well investigated. Here, we found that prephosphorylation by PKA promotes GSK-3beta-catalyzed tau phosphorylation at Thr181, Ser199, Ser202, Thr205, Thr217, Thr231, Ser396 and Ser422, but inhibits its phosphorylation at Thr212 and Ser404. In contrast, the prephosphorylation had no significant effect on its subsequent phosphorylation by cdk5 at Thr181, Ser199, Thr205, Thr231 and Ser422; inhibited it at Ser202, Thr212, Thr217 and Ser404; and slightly promoted it at Ser396. These studies reveal the nature of the inter-regulation of tau phosphorylation by the three major tau kinases.

Amino Acids↗

PubMed2006

Concurrent alterations of O-GlcNAcylation and phosphorylation of tau in mouse brains during fasting.

Impaired brain glucose uptake/metabolism precedes the symptoms of Alzheimer disease (AD) and is likely to play a role in the development of the disease, but the mechanism by which it contributes to AD is not understood. Because glucose uptake/metabolism regulates protein O-GlcNAcylation, and the latter modulates phosphorylation of tau inversely, we investigated, in fasting Kunming mice, whether impaired brain glucose uptake/metabolism causes abnormal hyperphosphorylation of tau and, consequently, facilitates the neurofibrillary degeneration of AD via down-regulation of tau O-GlcNAcylation. We found that fasting caused decreased tau O-GlcNAcylation and concurrent hyperphosphorylation of tau at most of the phosphorylation sites studied. The hippocampus was found more vulnerable to the tau alterations than the cerebral cortex, which is consistent with the fact that it is the hippocampus that is first affected in AD. Furthermore, hyperphosphorylation of tau induced by fasting was reversible in the brain after re-feeding. These findings provide a novel mechanism explaining how impaired brain glucose uptake/metabolism contributes to AD and suggest that it may be feasible to treat AD by reversing the abnormal hyperphosphorylation of tau at early stages of the disease.

Acylation↗

PubMed2006

Dysregulation of protein phosphorylation/dephosphorylation in Alzheimer's disease: a therapeutic target.

Studies during the last two decades have provided new insights into the molecular mechanism of Alzheimer's disease (AD). One of the milestone findings in AD research was the demonstration that neurofibrillary degeneration characterized by tau pathology is central to the pathogenesis of AD and other tauopathies and that abnormal hyperphosphorylation of tau is pivotal to neurofibrillary degeneration. This article reviews the recent research advances in tau pathology and the underlying dysregulation of the protein phosphorylation/dephosphorylation system. An updated model of the mechanism of neurofibrillary degeneration is also presented, and a promising therapeutic target to treat AD by correcting dysregulation of protein phosphorylation/dephosphorylation is discussed.

Journal Article↗

PubMed2006

Impaired brain glucose metabolism leads to Alzheimer neurofibrillary degeneration through a decrease in tau O-GlcNAcylation.

Neurofibrillary degeneration characterized by abnormal hyperphosphorylation and aggregation of tau in affected neurons is directly associated with dementia symptoms and plays a pivotal role in the pathogenesis of Alzheimer disease (AD) and related tauopathies. It is well established that brain glucose uptake/metabolism is impaired in AD, but how this impairment contributes to the disease is unknown. We recently found that tau in human brain is also modified by O-GlcNAcylation in addition to phosphorylation and that the former negatively regulates the latter. On the basis of these findings, we propose a novel hypothesis that the impaired glucose uptake/metabolism contributes to AD by facilitating abnormal hyperphosphorylation of tau. Further studies of this mechanism are likely to offer a novel therapeutic target for preventing and treating AD.

Acetylglucosaminidase↗

PubMed2005

Truncation and activation of calcineurin A by calpain I in Alzheimer disease brain.

A disturbance of calcium homeostasis is believed to play an important role in the neurodegeneration of the brains of Alzheimer disease (AD) patients, but the molecular pathways by which it contributes to the disease are not well understood. Here we studied the activation of two major Ca(2+)-regulated brain proteins, calpain and calcineurin, in AD brain. We found that calpain I is activated, which in turn cleaves and activates calcineurin in AD brain. Mass spectrometric analysis indicated that the cleavage of calcineurin by calpain I is at lysine 501, a position C-terminal to the autoinhibitory domain, which produces a 57-kDa truncated form. The 57-kDa calcineurin maintains its Ca(2+)/calmodulin dependence of the phosphatase activity, but the phosphatase activity is remarkably activated upon truncation. The cleavage and activation of calcineurin correlate to the number of neurofibrillary tangles in human brains. These findings suggest that the overactivation of calpain I and calcineurin may mediate the role of calcium homeostatic disturbance in the neurodegeneration of AD.

Aged↗

PubMed2005

NF-kappaB precursor, p105, and NF-kappaB inhibitor, IkappaBgamma, are both elevated in Alzheimer disease brain.

The nuclear factor-kappaB (NF-kappaB) signal transduction pathway regulates several vital cellular processes. During our studies of the glycoproteins involved in Alzheimer disease (AD), we found a significant increase of a 45-kDa protein band that was stained by lectin Maackia amurensis agglutinin (MAA). Mass spectrometry and Western blot analyses indicated that this 45-kDa MMA-positive protein was an inhibitor of NF-kappaB, IkappaBgamma. By Western blot analysis, the levels of both IkappaBgamma and NF-kappaB precursor, p105, were found to be elevated in AD brain as compared to age-matched controls. Our findings suggest that the NF-kappaB pathway might be involved in the molecular mechanism of AD.

Aged↗

PubMed2005

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.

Alzheimer Disease↗

PubMed2005

Bilateral injection of isoproterenol into hippocampus induces Alzheimer-like hyperphosphorylation of tau and spatial memory deficit in rat.

The abnormal hyperphosphorylation of tau protein is one of the hallmarks of Alzheimer disease and other tauopathies; as yet the exact role of various tau kinases in this pathology is not fully understood. Here, we show that injection of isoproterenol, an activator of cAMP-dependent kinase (PKA), into rat hippocampus bilaterally results in the activation of PKA, calcium/calmodulin-dependent kinase II and cyclin-dependent kinase-5, inhibition of protein phosphatase-2A, hyperphosphorylation of tau at several Alzheimer-like epitopes and a disturbance of spatial memory retention 48 h after the drug injection. These findings suggest the involvement of PKA and PKA-mediated signaling pathway in the Alzheimer-like tau hyperphosphorylation and memory impairment.

Alzheimer Disease↗

PubMed2005

Contributions of protein phosphatases PP1, PP2A, PP2B and PP5 to the regulation of tau phosphorylation.

Abnormal hyperphosphorylation of tau is believed to lead to neurofibrillary degeneration in Alzheimer's disease (AD) and other tauopathies. Recent studies have shown that protein phosphatases (PPs) PP1, PP2A, PP2B and PP5 dephosphorylate tau in vitro, but the exact role of each of these phosphatases in the regulation of site-specific phosphorylation of tau in the human brain was unknown. Hence, we investigated the contributions of these PPs to the regulation of tau phosphorylation quantitatively. We found that these four phosphatases all dephosphorylated tau at Ser199, Ser202, Thr205, Thr212, Ser214, Ser235, Ser262, Ser396, Ser404 and Ser409, but with different efficiencies toward different sites. The K(m) values of tau dephosphorylation catalysed by PP1, PP2A and PP5 were 8-12 microm, similar to the intraneuronal tau concentration of human brain, whereas the K(m) of PP2B was fivefold higher. PP2A, PP1, PP5 and PP2B accounted for approximately 71%, approximately 11%, approximately 10% and approximately 7%, respectively, of the total tau phosphatase activity of human brain. The total phosphatase activity and the activities of PP2A and PP5 toward tau were significantly decreased, whereas that of PP2B was increased in AD brain. PP2A activity negatively correlated to the level of tau phosphorylation at the most phosphorylation sites in human brains. Our findings indicate that PP2A is the major tau phosphatase that regulates its phosphorylation at multiple sites in human brain. The abnormal hyperphosphorylation of tau is partially due to a downregulation of PP2A activity in AD brain.

Aged, 80 and over↗

PubMed2004

Dephosphorylation of tau by protein phosphatase 5: impairment in Alzheimer's disease.

Protein phosphatase (PP) 5 is highly expressed in the mammalian brain, but few physiological substrates have yet been identified. Here, we investigated the kinetics of dephosphoryation of phospho-tau by PP5 and found that PP5 had a K(m) of 8-13 microm toward tau, which is similar to that of PP2A, the major known tau phosphatase. This K(m) value is within the range of intraneuronal tau concentration in human brain, suggesting that tau could be a physiological substrate of both PP5 and PP2A. PP5 dephosphorylated tau at all 12 Alzheimer's disease (AD)-associated abnormal phosphorylation sites studied, with different efficiency toward each site. Thr(205), Thr(212), and Ser(409) of tau were the most favorable sites; Ser(199), Ser(202), Ser(214), Ser(396), and Ser(404) were less favorable sites; and Ser(262) was the poorest site for PP5. Overexpression of PP5 in PC12 cells resulted in dephosphorylation of tau at multiple phosphorylation sites. The activity but not the protein level of PP5 was found to be decreased by approximately 20% in AD neocortex. These results suggest that tau is probably a physiological substrate of PP5 and that the abnormal hyperphosphorylation of tau in AD might result in part from the decreased PP5 activity in the diseased brains.

Aged↗

PubMed2004

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.

Antineoplastic Agents, Phytogenic↗

PubMed2004

O-GlcNAcylation regulates phosphorylation of tau: a mechanism involved in Alzheimer's disease.

Microtubule-associated protein tau is abnormally hyperphosphorylated and aggregated into neurofibrillary tangles in brains of individuals with Alzheimer's disease (AD) and other tauopathies. Tau pathology is critical to pathogenesis and correlates to the severity of dementia. However, the mechanisms leading to abnormal hyperphosphorylation are unknown. Here, we demonstrate that human brain tau was modified by O-GlcNAcylation, a type of protein O-glycosylation by which the monosaccharide beta-N-acetylglucosamine (GlcNAc) attaches to serine/threonine residues via an O-linked glycosidic bond. O-GlcNAcylation regulated phosphorylation of tau in a site-specific manner both in vitro and in vivo. At most of the phosphorylation sites, O-GlcNAcylation negatively regulated tau phosphorylation. In an animal model of starved mice, low glucose uptake/metabolism that mimicked those observed in AD brain produced a decrease in O-GlcNAcylation and consequent hyperphosphorylation of tau at the majority of the phosphorylation sites. The O-GlcNAcylation level in AD brain extracts was decreased as compared to that in controls. These results reveal a mechanism of regulation of tau phosphorylation and suggest that abnormal hyperphosphorylation of tau could result from decreased tau O-GlcNAcylation, which probably is induced by deficient brain glucose uptake/metabolism in AD and other tauopathies.

Acetylglucosamine↗

PubMed2004

Elevation of the level and activity of acid ceramidase in Alzheimer's disease brain.

Protein glycosylation modifies the processing of several key proteins involved in the molecular pathogenesis of Alzheimer's disease (AD). Aberrant glycosylation of tau and down-regulation of sialyltransferase in AD brain suggest a possible dysregulation of protein glycosylation that may play a role in AD. We therefore isolated major glycoproteins from AD brain by using lectin-affinity chromatographies and ion-exchange chromatography and further separated them using SDS-polyacylamide gel electrophoresis. Mass spectrometry analysis of 11 isolated glycoproteins led to their identification as: neuronal cell adhesion molecule, beta-globin, IgM heavy chain VH1 region precursor, contactin precursor, dipeptidylpeptidase VI, CD81 partner 3, prenylcysteine lyase, adipocyte plasma-associated protein, acid ceramidase and two novel proteins. We found that the level and activity of acid ceramidase (AC), one of the major identified human brain glycoproteins, were significantly elevated in AD brain. Immunohistochemical staining indicated that AC was located mainly in the cell bodies of neurons and colocalized with neurofibrillary tangles. Our findings suggest that AC might play a role in controlling neuronal apoptosis and that AC-mediated signalling pathways might be involved in the molecular mechanism of AD.

Aged↗

PubMed2004

Dephosphorylation of microtubule-associated protein tau by protein phosphatase 5.

Protein phosphatase 5 (PP5) is a 58-kDa novel phosphoseryl/phosphothreonyl protein phosphatase. It is ubiquitously expressed in all mammalian tissues examined, with a high level in the brain, but little is known about its physiological substrates. We found that this phosphatase dephosphorylated recombinant tau phosphorylated with cAMP-dependent protein kinase and glycogen synthase kinase-3beta, as well as abnormally hyperphosphorylated tau isolated from brains of patients with Alzheimer's disease. The specific activity of PP5 toward tau was comparable to those reported with other protein substrates examined to date. The PP5 activity toward tau was stimulated by arachidonic acid by 30- to 45-fold. Immunostaining demonstrated that PP5 was primarily cytoplasmic in PC12 cells and in neurons of postmortem human brain tissue. A small pool of PP5 associated with microtubules. Expression of active PP5 in PC12 cells resulted in reduced phosphorylation of tau, suggesting that PP5 can also dephosphorylate tau in cells. These results suggest that PP5 plays a role in the dephosphorylation of tau and might be involved in the molecular pathogenesis of Alzheimer's disease.

Animals↗

PubMed2003

Inhibition of protein phosphatase 2A induces phosphorylation and accumulation of neurofilaments in metabolically active rat brain slices.

In Alzheimer disease (AD) brain neurofilaments (NF) in addition to tau are hyperphosphorylated and accumulated and the activity of the phosphoprotein phosphatase 2A (PP2A) is compromised. In this study, employing metabolically competent brain slices from adult rats, we investigated whether such a NF abnormality could be caused by the down-regulation of PP2A activity. After 3 h treatment with 1.0 microM okadaic acid, the PP2A activity was inhibited by approximately 70% in the brain slices. We found that the inhibition of PP2A induced an increase in the phosphorylation of NF-H and NF-M subunits and in the level of NF-H. Immunohistochemical examination revealed that the PP2A inhibition resulted in increased phosphorylation of NF in the axon and a general accumulation of NF throughout the whole neuron. These findings suggest that the hyperphosphorylation and accumulation of NF found in AD brain could have been caused by the down-regulation of PP2A.

Animals↗

PubMed2003

Okadaic-acid-induced inhibition of protein phosphatase 2A produces activation of mitogen-activated protein kinases ERK1/2, MEK1/2, and p70 S6, similar to that in Alzheimer's disease.

In Alzheimer's disease (AD) brain the activity of protein phosphatase (PP)-2A is compromised and that of the extracellular signal-regulated protein kinase (ERK1/2) of the mitogen-activated protein kinase (MAPK) family, which can phosphorylate tau, is up-regulated. We investigated whether a decrease in PP-2A activity could underlie the activation of these kinases and the abnormal hyperphosphorylation of tau. Rat brain slices, 400-microm-thick, kept under metabolically active conditions in oxygenated (95% O(2), 5% CO(2)) artificial CSF were treated with 1.0 micromol/L okadaic acid (OA) for 1 hour at 33 degrees C. Under this condition, PP-2A activity was decreased to approximately 35% of the vehicle-treated control slices, and activities of PP-1 and PP-2B were not affected. In the OA-treated slices, we observed a dramatic increase in the phosphorylation/activation of ERK1/2, MEK1/2, and p70 S6 kinase both immunohistochemically and by Western blots using phosphorylation-dependent antibodies against these kinases. Treatment of 6-microm sections of the OA-treated slices with purified PP-2A reversed the phosphorylation/activation of these kinases. Hyperphosphorylation of tau at several abnormal hyperphosphorylation sites was also observed, as seen in AD brain. These results suggest 1) that PP-2A down-regulates ERK1/2, MEK1/2, and p70 S6 kinase activities through dephosphorylation at the serine/threonine residues of these kinases, and 2) that in AD brain the decrease in PP-2A activity could have caused the activation of ERK1/2, MEK1/2, and p70 S6 kinase, and the abnormal hyperphosphorylation of tau both via an increase in its phosphorylation and a decrease in its dephosphorylation.

Alzheimer Disease↗

PubMed2003

Alzheimer neurofibrillary degeneration: therapeutic targets and high-throughput assays.

Neurofibrillary degeneration has primary and pivotal involvement in the pathogenesis of Alzheimer disease (AD) and other tauopathies. The inhibition of this lesion offers a promising therapeutic approach. The microtubule- associated protein (MAP) 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 that is polymerized into PHF/NFT is apparently inert and has no effect on microtubule assembly in vitro. The cytosolic abnormally hyperphosphorylated tau from AD brain, the AD P-tau, does not promote in vitro microtubule assembly but, instead, 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, and this self-assembly requires the abnormal hyperphosphorylation of this protein. Although, to date, an up-regulation of the activity of a tau kinase has not been established, 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 pp-2A and one or more 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. Development of high-throughput screening assays for potential drugs aimed at these therapeutic targets is currently under way.

Alzheimer Disease↗

PubMed2002

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.

Alzheimer Disease↗