Ramendra N. Saha

HATs and HDACs in neurodegeneration: a tale of disconcerted acetylation homeostasis

Gradual disclosure of the molecular basis of selective neuronal apoptosis during neurodegenerative diseases reveals active participation of acetylating and deacetylating agents during the process. Several studies have now successfully manipulated neuronal vulnerability by influencing the dose and enzymatic activity of histone acetyltransferases (HATs) and histone deacetylases (HDACs), enzymes regulating acetylation homeostasis within the nucleus, thus focusing on the importance of balanced acetylation status in neuronal vitality. It is now increasingly becoming clear that acetylation balance is greatly impaired during neurodegenerative conditions. Herein, we attempt to illuminate molecular means by which such impairment is manifested and how the compromised acetylation homeostasis is intimately coupled to neurodegeneration. Finally, we discuss the therapeutic potential of reinstating the HAT–HDAC balance to ameliorate neurodegenerative diseases.

Up-regulation of BDNF in Astrocytes by TNF-α: A Case for the Neuroprotective Role of Cytokine

Tumor necrosis factor-alpha (TNF-α) is widely known to be involved in physiological and pathophysiological processes of the brain where this proinflammatory cytokine is implicated with regulation of inflammatory and survival components. We report that TNF-α up-regulates exon-IV-bdnf mRNA and brain-derived neurotrophic factor (BDNF) protein in primary astrocytes. The BDNF protein was detectable both in cellular lysate and in the extracellular medium. Activation of NF-κB by TNF-α and inhibition of TNF-α-induced BDNF expression by Δp65 (a dominant-negative mutant) and NEMO-binding domain peptide (an inhibitor of NF-κB) suggests that TNF-α induces BDNF expression through the activation of NF-κB. Similarly, TNF-α induced the activation of C/EBPβ and the expression of BDNF was sensitive to overexpression of ΔC/EBPβ (a dominant-negative mutant) and ETO (an inhibitor of C/EBPβ). Among three MAP kinases, TNF-α-induced BDNF up-regulation was sensitive only to inhibitors of ERK MAP kinase. However, the ERK MAP kinase pathway was coupled to activation of C/EBPβ but not NF-κB. Taken together, this study identifies a novel property of TNF-α in inducing the expression of BDNF via NF-κB and C/EBPβ in astrocytes that may be responsible for neurotrophic activity of the cytokine.

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.

Tumor necrosis factor‐α at the crossroads of neuronal life and death during HIV‐associated dementia

Human immunodeficiency type‐1 (HIV‐1) infection is known to cause disorders of the CNS, including HIV‐associated dementia (HAD). It is suspected that tumor necrosis factor‐α (TNF‐α) released by infected microglia and macrophages play a role in neuronal injury seen in HAD patients. Accordingly, studies suggest that the level of TNF‐α mRNA increases with increasing severity of dementia in patients, and that inhibitors of TNF‐α release reduces neuronal injury in murine model of HAD. However, the exact role of TNF‐α in relation to neuronal dysfunction is a matter of ongoing debate. One school of thought hails TNF‐α as the inducer and mediator of neurodegeneration and their evidence suggest that TNF‐α kill neurons directly by recruiting caspases or may kill indirectly by various means. In sharp contrast to this, another concept theory envisages a role for TNF‐α in negotiating neuroprotection during HAD. The current compilation examines these contradictory concepts, and evaluates their efficacy in the light of TNF‐α signaling. It also attempts to elaborate the current consensus outlook of TNF‐αs role during HAD.

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.

Signals for the induction of nitric oxide synthase in astrocytes

Nitric oxide (NO), being a double-edged sword depending on its concentration in the microenvironment, is involved in both physiological and pathological processes of many organ systems including brain and spinal cord. It is now well-documented that once inducible nitric oxide synthase (iNOS) is expressed in CNS in a signal-dependent fashion, NO in excess of physiological thresholds is produced and this excess NO then plays a role in the pathogenesis of stroke, demyelination and other neurodegenerative diseases. Therefore, a keen interest has been generated in recent years in comprehending the regulation of this enzyme in brain cells. The present review summarizes our current understanding of signaling mechanisms leading to transcription of the iNOS gene in activated astrocytes. We attempt this comprehension with a hope to identify potential targets to intervene NO-mediated CNS disorders.

Role of protein kinase R in double-stranded RNA-induced expression of nitric oxide synthase in human astroglia

Environmental factor(s), such as viral infection, has been implicated as one of the triggering events leading to neuroinflammation in multiple sclerosis. This study underlines the importance of double‐stranded RNA (dsRNA), the active component of a viral infection, in inducing the expression of inducible nitric oxide synthase (iNOS) in human astroglia. DsRNA in the form of synthetic polyinosinic‐polycytidylic acid (poly IC) induced expression of iNOS and iNOS promoter‐driven luciferase activity through activation of nuclear factor (NF)‐κB and CCAAT/enhancer‐binding proteinβ (C/EBPβ). In addition, we show that inhibitors of protein kinase R attenuated iNOS by suppressing the activation of NF‐κB but not C/EBPβ. In contrast, knock down of p38 mitogen‐activated protein kinase (MAPK) attenuated iNOS by suppressing the activation of C/EBPβ but not NF‐κB. This study delineates a novel role of dsRNA in inducing the expression of iNOS through dsRNA‐activated protein kinase (PKR)‐mediated activation of NF‐κB and p38‐mediated activation of C/EBPβ in human astroglia that may participate in virus‐induced neurological abnormalities.

CNS Cell Signaling

The science of neurobiology is now almost a century older than times when Spanish neuroanatomist and Nobel laureate Santiago Ramón y Cajal had wondered as above. Yet, these ‘mysterious forces’ have only been partially illuminated today and the posed question still remains worth pondering upon in contemporary times. What Cajal identified as ‘forces’ are basically key cellular signals that are transduced preceding growth and ramification. A precipitate of our knowledge today tells us that these ‘forces’ are mostly generated within and amongst members of the central nervous system (CNS). The present chapter is aimed at appreciating cellular signals and their transduction pathways which underlie the functional output of CNS during normal times, diseased conditions, and regeneration.