Malabendu Jana

Fibrillar Amyloid-β Peptides Activate Microglia via TLR2: Implications for Alzheimer’s Disease

Microglial activation is an important pathological component in brains of patients with Alzheimer’s disease (AD), and fibrillar amyloid-β (Aβ) peptides play an important role in microglial activation in AD. However, mechanisms by which Aβ peptides induce the activation of microglia are poorly understood. The present study underlines the importance of TLR2 in mediating Aβ peptide-induced activation of microglia. Fibrillar Aβ1–42 peptides induced the expression of inducible NO synthase, proinflammatory cytokines (TNF-α, IL-1β, and IL-6), and integrin markers (CD11b, CD11c, and CD68) in mouse primary microglia and BV-2 microglial cells. However, either antisense knockdown of TLR2 or functional blocking Abs against TLR2 suppressed Aβ1–42-induced expression of proinflammatory molecules and integrin markers in microglia. Aβ1–42 peptides were also unable to induce the expression of proinflammatory molecules and increase the expression of CD11b in microglia isolated from TLR2−/− mice. Finally, the inability of Aβ1–42 peptides to induce the expression of inducible NO synthase and to stimulate the expression of CD11b in vivo in the cortex of TLR2−/− mice highlights the importance of TLR2 in Aβ-induced microglial activation. In addition, ligation of TLR2 alone was also sufficient to induce microglial activation. Consistent to the importance of MyD88 in mediating the function of various TLRs, antisense knockdown of MyD88 also inhibited Aβ1–42 peptide-induced expression of proinflammatory molecules. Taken together, these studies delineate a novel role of TLR2 signaling pathway in mediating fibrillar Aβ peptide-induced activation of microglia.

Human immunodeficiency virus type 1 (HIV-1) tat induces nitric-oxide synthase in human astroglia

Human immunodeficiency virus type 1 (HIV-1) infection is known to cause neuronal injury and dementia in a significant proportion of patients. However, the mechanism by which HIV-1 mediates its deleterious effects in the brain is poorly defined. The present study was undertaken to investigate the effect of the HIV-1tat gene on the expression of inducible nitric-oxide synthase (iNOS) in human U373MG astroglial cells and primary astroglia. Expression of the tat gene as RSV-tat but not that of the CAT gene as RSV-CAT in U373MG astroglial cells led to the induction of NO production and the expression of iNOS protein and mRNA. Induction of NO production by recombinant HIV-1 Tat protein and inhibition of RSV-tat-induced NO production by anti-Tat antibodies suggest that RSV-tat-induced production of NO is dependent on Tat and that Tat is secreted from RSV-tat-transfected astroglia. Similar to U373MG astroglial cells, RSV-tat also induced the production of NO in human primary astroglia. The induction of human iNOS promoter-derived luciferase activity by the expression of RSV-tat suggests that RSV-tat induces the transcription of iNOS. To understand the mechanism of induction of iNOS, we investigated the role of NF-κB and C/EBPβ, transcription factors responsible for the induction of iNOS. Activation of NF-κB as well as C/EBPβ by RSV-tat, stimulation of RSV-tat-induced production of NO by the wild type of p65 and C/EBPβ, and inhibition of RSV-tat-induced production of NO by Δp65, a dominant-negative mutant of p65, and ΔC/EBPβ, a dominant-negative mutant of C/EBPβ, suggest that RSV-tat induces iNOS through the activation of NF-κB and C/EBPβ. In addition, we show that extracellular signal-regulated kinase (ERK) but not that p38 mitogen-activated protein kinase (MAPK) is involved in RSV-tat induced production of NO. Interestingly, PD98059, an inhibitor of the ERK pathway, and ΔERK2, a dominant-negative mutant of ERK2, inhibited RSV-tat-induced production of NO through the inhibition of C/EBPβ but not that of NF-κB. This study illustrates a novel role for HIV-1 tat in inducing the expression of iNOS in human astrocytes that may participate in the pathogenesis of HIV-associated dementia.

Antineuroinflammatory Effect of NF-κB Essential Modifier-Binding Domain Peptides in the Adoptive Transfer Model of Experimental Allergic Encephalomyelitis

It has been shown that peptides corresponding to the NF-κB essential modifier-binding domain (NBD) of IκB kinase α or IκB kinase β specifically inhibit the induction of NF-κB activation without inhibiting the basal NF-κB activity. The present study demonstrates the effectiveness of NBD peptides in inhibiting the disease process in adoptively transferred experimental allergic encephalomyelitis (EAE), an animal model of multiple sclerosis. Clinical symptoms of EAE were much lower in mice receiving wild-type (wt)NBD peptides compared with those receiving mutated (m)NBD peptides. Histological and immunocytochemical analysis showed that wtNBD peptides inhibited EAE-induced spinal cord mononuclear cell invasion and normalized p65 (the RelA subunit of NF-κB) expression within the spinal cord. Analysis of lymph node cells isolated from donor and recipient mice showed that wtNBD peptides but not mNBD peptides were able to shift the immune response from a Th1 to a Th2 profile. Consistently, wtNBD peptides but not mNBD peptides inhibited the encephalitogenicity of myelin basic protein-specific T cells. Furthermore, i.p. injection of wtNBD peptides but not mNBD peptides was also able to reduce LPS- and IFN-γ-induced expression of inducible NO synthase, IL-1β, and TNF-α in vivo in the cerebellum. Taken together, our results support the conclusion that NBD peptides are antineuroinflammatory, and that NBD peptides may have therapeutic effect in neuroinflammatory disorders such as multiple sclerosis.

MAPK p38 Regulates Transcriptional Activity of NF-κB in Primary Human Astrocytes via Acetylation of p65

MAPK-p38 plays an important role in inflammation. Several studies have shown that blocking p38 activity attenuates the transcriptional activity of the proinflammatory transcription factor NF-κB without altering its DNA-binding activity. We have also observed that blocking p38 in human primary astrocytes suppresses the transcriptional but not the DNA-binding activity of NF-κB and down-regulates the expression of an NF-κB-dependent gene, inducible NO synthase. However, the molecular mechanism of p38-mediated regulation of NF-κB remains largely unknown. In this study, we delineate that p38 controls the transcriptional activity of NF-κB by regulating acetylation of p65, but not its phosphorylation. The combination of IL-1β and IFN-γ, previously shown to strongly induce inducible NO synthase in human primary astrocytes, induced p38-dependent phosphorylation of acetyltransferase coactivator p300, but not p65, and subsequent association of p300 with p65. Furthermore, immunocomplex-histone acetyltransferase assays demonstrated that cytokine-induced association of p65 with biologically active immunocomplex-histone acetyltransferase assay was dependent on p38. It has been previously reported that acetylation of p65 at K310 residue is important for transcriptional activity of NF-κB. Accordingly, we found that cytokine-induced association of p65 with p300 led to acetylation of p65 at K310. Because p38 regulated the association between p65 and p300, blocking p38 activity also led to attenuation of p65-K310 acetylation in cytokine-stimulated astrocytes. Taken together, this study illuminates a novel regulatory role of p38 during neuroinflammation where this MAP kinase controls acetylation of NF-κB p65 by regulating acetyltransferase activity of coactivator p300.

Induction of tumor necrosis factor‐α (TNF‐α) by interleukin‐12 p40 monomer and homodimer in microglia and macrophages

The present study was undertaken to explore the role of interleukin‐12 (IL‐12) p40 in the expression of TNF‐α in microglia. Interestingly, we have found that IL‐12 p70, p402 (the p40 homodimer) and p40 (the p40 monomer) dose‐dependently induced the production of TNF‐α and the expression of TNF‐α mRNA in BV‐2 microglial cells. In addition to BV‐2 microglial cells, p70, p402 and p40 also induced the production of TNF‐α in mouse primary microglia and peritoneal macrophages. As the activation of both NF‐κB and CCAAT/enhancer binding protein β (C/EBPβ) is important for the expression of TNF‐α in microglial cells, we investigated the effect of p40 on the activation of NF‐κB as well as C/EBPβ. Activation of NF‐κB as well as C/EBPβ by p40 and inhibition of p40‐induced expression of TNF‐α by Δp65, a dominant‐negative mutant of p65, and ΔC/EBPβ, a dominant‐negative mutant of C/EBPβ, suggests that p40 induces the expression of TNF‐α through the activation of NF‐κB and C/EBPβ. In addition, we show that p40 induced the activation of both extracellular signal‐regulated kinase (ERK) and p38 mitogen‐activated protein kinase (MAPK). Interestingly, PD98059, an inhibitor of ERK, inhibited p40‐induced expression of TNF‐α through the inhibition of C/EBPβ, but not that of NF‐κB, whereas SB203580, an inhibitor of p38 MAPK, inhibited p40‐induced expression of TNF‐α through the inhibition of both NF‐κB and C/EBPβ. This study delineates a novel biological function of p40 in inducing TNF‐α in microglia and macrophages.

Involvement of Phosphatidylinositol 3-Kinase-Mediated Up-Regulation of IκBα in Anti-Inflammatory Effect of Gemfibrozil in Microglia

The present study underlines the importance of PI3K in mediating the anti-inflammatory effect of gemfibrozil, a prescribed lipid-lowering drug for humans, in mouse microglia. Gemfibrozil inhibited LPS-induced expression of inducible NO synthase (iNOS) and proinflammatory cytokines in mouse BV-2 microglial cells and primary microglia. By overexpressing wild-type and dominant-negative constructs of peroxisome proliferator-activated receptor-α (PPAR-α) in microglial cells and isolating primary microglia from PPAR-α−/− mice, we have demonstrated that gemfibrozil inhibits the activation of microglia independent of PPAR-α. Interestingly, gemfibrozil induced the activation of p85α-associated PI3K (p110β but not p110α) and inhibition of that PI3K by either chemical inhibitors or dominant-negative mutants abrogated the inhibitory effect of gemfibrozil. Conversely, overexpression of the constitutively active mutant of p110 enhanced the inhibitory effect of gemfibrozil on LPS-induced expression of proinflammatory molecules. Similarly, gemfibrozil also inhibited fibrillar amyloid β (Aβ)-, prion peptide (PrP)-, dsRNA (poly IC)-, HIV-1 Tat-, and 1-methyl-4-phenylpyridinium (MPP+)-, but not IFN-γ-, induced microglial expression of iNOS. Inhibition of PI3K also abolished the inhibitory effect of gemfibrozil on Aβ-, PrP-, poly IC-, Tat-, and MPP+-induced microglial expression of iNOS. Involvement of NF-κB activation in LPS-, Aβ-, PrP-, poly IC-, Tat-, and MPP+-, but not IFN-γ-, induced microglial expression of iNOS and stimulation of IκBα expression and inhibition of NF-κB activation by gemfibrozil via the PI3K pathway suggests that gemfibrozil inhibits the activation of NF-κB and the expression of proinflammatory molecules in microglia via PI3K-mediated up-regulation of IκBα.

Gemfibrozil Ameliorates Relapsing-Remitting Experimental Autoimmune Encephalomyelitis Independent of Peroxisome Proliferator-Activated Receptor-α

The present study underlines the importance of gemfibrozil, a lipid-lowering drug and an activator of peroxisome proliferator-activated receptor-α (PPAR-α), in inhibiting the disease process of adoptively transferred experimental allergic encephalomyelitis (EAE), an animal model of relapsing-remitting multiple sclerosis. Clinical symptoms of EAE, infiltration of mononuclear cells, and demyelination were significantly lower in SJL/J female mice receiving gemfibrozil through food chow than those without gemfibrozil. It is noteworthy that the drug was equally effective in treating EAE in PPAR-α wild-type as well as knockout mice. Gemfibrozil also inhibited the encephalitogenicity of MBP-primed T cells and switched the immune response from a Th1 to a Th2 profile independent of PPAR-α. Gemfibrozil consistently inhibited the expression and DNA-binding activity of T-bet, a key regulator of interferon-γ (IFN-γ) expression and stimulated the expression and DNA-binding activity of GATA3, a key regulator of IL-4. Gemfibrozil treatment decreased the number of T-bet–positive T cells and increased the number of GATA3-positive T cells in spleen of donor mice. The histological and immunohistochemical analyses also demonstrate the inhibitory effect of gemfibrozil on the invasion of T-bet–positive T cells into the spinal cord of EAE mice. Furthermore, we demonstrate that the differential effect of gemfibrozil on the expression of T-bet and GATA3 was due to its inhibitory effect on NO production. Although excess NO favored the expression of T-bet, scavenging of NO stimulated the expression of GATA-3. Taken together, our results suggest gemfibrozil, an approved drug for hyperlipidemia in humans, may find further therapeutic use in multiple sclerosis.

Sodium Phenylacetate Inhibits Adoptive Transfer of Experimental Allergic Encephalomyelitis in SJL/J Mice at Multiple Steps

Experimental allergic encephalomyelitis (EAE) is the animal model for multiple sclerosis. The present study underlines the importance of sodium phenylacetate (NaPA), a drug approved for urea cycle disorders, in inhibiting the disease process of adoptively transferred EAE in female SJL/J mice at multiple steps. Myelin basic protein (MBP)-primed T cells alone induced the expression of NO synthase (iNOS) and the activation of NF-κB in mouse microglial cells through cell-cell contact. However, pretreatment of MBP-primed T cells with NaPA markedly inhibited its ability to induce microglial expression of iNOS and activation of NF-κB. Consistently, adoptive transfer of MBP-primed T cells, but not that of NaPA-pretreated MBP-primed T cells, induced the clinical symptoms of EAE in female SJL/J mice. Furthermore, MBP-primed T cells isolated from NaPA-treated donor mice were also less efficient than MBP-primed T cells isolated from normal donor mice in inducing iNOS in microglial cells and transferring EAE to recipient mice. Interestingly, clinical symptoms of EAE were much less in mice receiving NaPA through drinking water than those without NaPA. Similar to NaPA, sodium phenylbutyrate, a chemically synthesized precursor of NaPA, also inhibited the disease process of EAE. Histological and immunocytochemical analysis showed that NaPA inhibited EAE-induced spinal cord mononuclear cell invasion and normalized iNOS, nitrotyrosine, and p65 (the RelA subunit of NF-κB) expression within the spinal cord. Taken together, our results raise the possibility that NaPA or sodium phenylbutyrate taken through drinking water or milk may reduce the observed neuroinflammation and disease process in multiple sclerosis patients.

HMG-CoA Reductase Inhibitors Bind to PPARα to Upregulate Neurotrophin Expression in the Brain and Improve Memory in Mice

Neurotrophins are important for neuronal health and function. Here, statins, inhibitors of HMG-CoA reductase and cholesterol lowering drugs, were found to stimulate expression of neurotrophins in brain cells independent of the mevalonate pathway. Time-resolved fluorescence resonance energy transfer (FRET) analyses, computer-derived simulation, site-directed mutagenesis, thermal shift assay, and de novo binding followed by electrospray ionization tandem mass spectrometry (ESI-MS) demonstrates that statins serve as ligands of PPARα and that Leu331 and Tyr 334 residues of PPARα are important for statin binding. Upon binding, statins upregulate neurotrophins via PPARα-mediated transcriptional activation of cAMP-response element binding protein (CREB). Accordingly, simvastatin increases CREB and brain-derived neurotrophic factor (BDNF) in the hippocampus of Ppara null mice receiving full-length lentiviral PPARα, but not L331M/Y334D statin-binding domain-mutated lentiviral PPARα. This study identifies statins as ligands of PPARα, describes neurotrophic function of statins via the PPARα-CREB pathway, and analyzes the importance of PPARα in the therapeutic success of simvastatin in an animal model of Alzheimers disease.

Regulation of Cyclic AMP Response Element Binding and Hippocampal Plasticity-Related Genes by Peroxisome Proliferator-Activated Receptor α

Peroxisome proliferator-activated receptor α (PPARα) is a transcription factor that regulates genes involved in fatty acid catabolism. Here, we provide evidence that PPARα is constitutively expressed in nuclei of hippocampal neurons and, surprisingly, controls calcium influx and the expression of various plasticity-related genes via direct transcriptional regulation of cyclic AMP response element binding (CREB). Accordingly, Pparα-null, but not Pparβ-null, mice are deficient in CREB and memory-associated proteins and have decreased spatial learning and memory. Small hairpin RNA knockdown of PPARα in the hippocampus suppressed CREB and NR2A, rendering wild-type animals markedly poor in consolidating spatial memory, whereas introduction of PPARα to the hippocampus of Pparα-null mice increased hippocampal CREB and NR2A and improved spatial learning and memory. Through detailed analyses of CREB and NR2A activity, as well as spatial learning and memory in bone marrow chimeric animals lacking PPARα in the CNS, we uncover a mechanism for transcriptional control of Creb and associated plasticity genes by PPARα.