Opposing effects of sirtuins on neuronal survival: SIRT1-mediated neuroprotection is independent of its deacetylase activity

Expression patterns and potential roles of SIRT1 in human medulloblastoma cells in vivo and in vitro

Medulloblastoma is a primitive neuroectodermal tumor, which originates in the cerebellum, presumably due to the alterations of some neurogenetic elements. Sirtuin 1 (SIRT1), a class III histone deacetylase (HDAC), regulates differentiation of neuronal stem cells but its status in medulloblastomas remains largely unknown. The current study aimed to address this issue by checking SIRT1 expression in noncancerous cerebellar tissues, medulloblastoma tissues and established cell lines. The roles of SIRT1 in proliferation and survival of UW228-3 medulloblastoma cells were analyzed by SIRT1 small interfering RNA (siRNA) transfection and SIRT1 inhibitor nicotinamide treatment. The results revealed that the frequency of SIRT1 expression in medulloblastoma tissues was 64.17% (77/120), while only one out of seven tumor-surrounding noncancerous cerebellar tissues showed restricted SIRT1 expression in the cells within the granule layer. Of the three morphological subtypes, the rates of SIRT1 detection in the large cell/anaplastic cell (79.07%; 34/43) and the classic medulloblastomas (60.29%; 41/68) are higher than that (22.22%; 2/9) in nodular/desmoplastic medulloblastomas (P<0.01 and P<0.05, respectively). Heterogeneous SIRT1 expression was commonly observed in classic medulloblastoma. Inhibition of SIRT1 expression by siRNA arrested 64.96% of UW228-3 medulloblastoma cells in the gap 1 (G1) phase and induced 14.53% of cells to apoptosis at the 48-h time point. Similarly, inhibition of SIRT1 enzymatic activity with nicotinamide brought about G1 arrest and apoptosis in a dose-related fashion. Our data thus indicate: (i) that SIRT1 may act as a G1-phase promoter and a survival factor in medulloblastoma cells; and (ii) that SIRT1 expression is correlated with the formation and prognosis of human medulloblastomas. In this context, SIRT1 would be a potential therapeutic target of medulloblastomas.

Resveratrol Targeting of Carcinogen-Induced Brain Endothelial Cell Inflammation Biomarkers MMP-9 and COX-2 is Sirt1-Independent

The occurrence of a functional relationship between the release of metalloproteinases (MMPs) and the expression of cyclooxygenase (COX)-2, two inducible pro-inflammatory biomarkers with important pro-angiogenic effects, has recently been inferred. While brain endothelial cells play an essential role as structural and functional components of the blood-brain barrier (BBB), increased BBB breakdown is thought to be linked to neuroinflammation. Chemopreventive mechanisms targeting both MMPs and COX-2 however remain poorly investigated. In this study, we evaluated the pharmacological targeting of Sirt1 by the diet-derived and antiinflammatory polyphenol resveratrol. Total RNA, cell lysates, and conditioned culture media from human brain microvascular endothelial cells (HBMEC) were analyzed using qRT-PCR, immunoblotting, and zymography respectively. Tissue scan microarray analysis of grade I-IV brain tumours cDNA revealed increased gene expression of Sirt-1 from grade I-III but surprisingly not in grade IV brain tumours. HBMEC were treated with a combination of resveratrol and phorbol 12-myristate 13-acetate (PMA), a carcinogen known to increase MMP-9 and COX-2 through NF-κB. We found that resveratrol efficiently reversed the PMA-induced MMP-9 secretion and COX-2 expression. Gene silencing of Sirt1, a critical modulator of angiogenesis and putative target of resveratrol, did not lead to significant reversal of MMP-9 and COX-2 inhibition. Decreased resveratrol inhibitory potential of carcinogen-induced IκB phosphorylation in siSirt1-transfected HBMEC was however observed. Our results suggest that resveratrol may prevent BBB disruption during neuroinflammation by inhibiting MMP-9 and COX-2 and act as a pharmacological NF-κB signal transduction inhibitor independent of Sirt1.

The NAMPT inhibitor FK866 reverts the damage in spinal cord injury

Background

Emerging data implicate nicotinamide phosphoribosyl transferase (NAMPT) in the pathogenesis of cancer and inflammation. NAMPT inhibitors have proven beneficial in inflammatory animal models of arthritis and endotoxic shock as well as in autoimmune encephalitis. Given the role of inflammatory responses in spinal cord injury (SCI), the effect of NAMPT inhibitors was examined in this setting.

Methods

We investigated the effects of the NAMPT inhibitor FK866 in an experimental compression model of SCI.

Results

Twenty-four hr following induction of SCI, a significant functional deficit accompanied widespread edema, demyelination, neuron loss and a substantial increase in TNF-α, IL-1β, PAR, NAMPT, Bax, MPO activity, NF-κB activation, astrogliosis and microglial activation was observed. Meanwhile, the expression of neurotrophins BDNF, GDNF, NT3 and anti-apoptotic Bcl-2 decreased significantly. Treatment with FK866 (10 mg/kg), the best known and characterized NAMPT inhibitor, at 1 h and 6 h after SCI rescued motor function, preserved perilesional gray and white matter, restored anti-apoptotic and neurotrophic factors, prevented the activation of neutrophils, microglia and astrocytes and inhibited the elevation of NAMPT, PAR, TNF-α, IL-1β, Bax expression and NF-κB activity.

We show for the first time that FK866, a specific inhibitor of NAMPT, administered after SCI, is capable of reducing the secondary inflammatory injury and partly reduce permanent damage. We also show that NAMPT protein levels are increased upon SCI in the perilesional area which can be corrected by administration of FK866.

Conclusions

Our findings suggest that the inflammatory component associated to SCI is the primary target of these inhibitors.

Peptide switch is essential for Sirt1 deacetylase activity

In mammals, the Sirtuins are composed of seven Sir2 orthologs (Sirt1-7) with a conserved deacetylase core that utilizes NAD(+) as a cofactor. Interestingly, the deacetylase core of Sirt1 by itself has no catalytic activity. We found within the C-terminal domain a 25 aa sequence that is essential for Sirt1 activity (ESA). Our results indicate that the ESA region interacts with and functions as an "on switch" for the deacetylase core. The endogenous Sirt1 inhibitor DBC1, which also binds to the deacetylase core, competes with and inhibits the ESA region from interacting with the deacetylase core. We discovered an ESA mutant peptide that can bind to the deacetylase core and inhibit Sirt1 in trans. By using this mutant peptide, we were able to inhibit Sirt1 activity and to increase the chemosensitivity of androgen-refractory prostate cancer cells. Therefore, the ESA region is a potential target for development of therapies to regulate Sirt1.

SIRT1 activates MAO-A in the brain to mediate anxiety and exploratory drive

SIRT1 is a NAD(+)-dependent deacetylase that governs a number of genetic programs to cope with changes in the nutritional status of cells and organisms. Behavioral responses to food abundance are important for the survival of higher animals. Here we used mice with increased or decreased brain SIRT1 to show that this sirtuin regulates anxiety and exploratory drive by activating transcription of the gene encoding the monoamine oxidase A (MAO-A) to reduce serotonin levels in the brain. Indeed, treating animals with MAO-A inhibitors or selective serotonin reuptake inhibitors (SSRIs) normalized anxiety differences between wild-type and mutant animals. SIRT1 deacetylates the brain-specific helix-loop-helix transcription factor NHLH2 on lysine 49 to increase its activation of the MAO-A promoter. Both common and rare variations in the SIRT1 gene were shown to be associated with risk of anxiety in human population samples. Together these data indicate that SIRT1 mediates levels of anxiety, and this regulation may be adaptive in a changing environment of food availability.

Sirtuin 1 in immune regulation and autoimmunity

The NAD-dependent histone deacetylase sirtuin (Sirt)1 is implicated in a wide variety of physiological processes, ranging from tumorigenesis to mitochondrial biogenesis to neuronal development. Recent studies indicate that Sirt1 is a critical regulator of both the innate and adaptive immune response in mice and its altered functions are likely involved in autoimmune diseases. Small molecules that modulate Sirt1 functions are potential therapeutic reagents for autoimmune inflammatory diseases. In this review, we highlight the functions of Sirt1 in the immune system focusing on the underlying molecular mechanisms, and the potential of Sirt1 as a therapeutic target for autoimmune diseases.

Misfolded proteins inhibit proliferation and promote stress-induced death in SV40-transformed mammalian cells

Protein misfolding is implicated in neurodegenerative diseases and occurs in aging. However, the contribution of the misfolded ensembles to toxicity remains largely unknown. Here we introduce 2 primate cell models of destabilized proteins devoid of specific cellular functions and interactors, as bona fide misfolded proteins, allowing us to isolate the gain-of-function of non-native structures. Both GFP-degron and a mutant chloramphenicol-acetyltransferase fused to GFP (GFP-Δ9CAT) form perinuclear aggregates, are degraded by the proteasome, and colocalize with and induce the chaperone Hsp70 (HSPA1A/B) in COS-7 cells. We find that misfolded proteins neither significantly compromise chaperone-mediated folding capacity nor induce cell death. However, they do induce growth arrest in cells that are unable to degrade them and promote stress-induced death upon proteasome inhibition by MG-132 and heat shock. Finally, we show that overexpression of all heat-shock factor-1 (HSF1) and Hsp70 proteins, as well as wild-type and deacetylase-deficient (H363Y) SIRT1, rescue survival upon stress, implying a noncatalytic action of SIRT1 in response to protein misfolding. Our study establishes a novel model and extends our knowledge on the mechanism of the function-independent proteotoxicity of misfolded proteins in dividing cells.

Tumor necrosis factor α-mediated cleavage and inactivation of SirT1 in human osteoarthritic chondrocytes

OBJECTIVE

The protein deacetylase SirT1 positively regulates cartilage-specific gene expression, while the proinflammatory cytokine tumor necrosis factor α (TNFα) negatively regulates these same genes. This study was undertaken to test the hypothesis that SirT1 is adversely affected by TNFα, resulting in altered gene expression.

METHODS

Cartilage-specific gene expression, SirT1 activity, and results of chromatin immunoprecipitation analysis at the α2(I) collagen enhancer site were determined in RNA, protein extracts, and nuclei of human osteoarthritic chondrocytes left untreated or treated with TNFα. Protein extracts from human chondrocytes transfected with epitope-tagged SirT1 that had been left untreated or had been treated with TNFα were analyzed by immunoblotting with SirT1 and epitope-specific antibodies. The 75-kd SirT1-reactive protein present in TNFα-treated extracts was identified by mass spectroscopy, and its amino-terminal cleavage site was identified via Edman sequencing. SirT1 activity was assayed following an in vitro cathepsin B cleavage reaction. Cathepsin B small interfering RNA (siRNA) was transfected into chondrocytes left untreated or treated with TNFα.

RESULTS

TNFα-treated chondrocytes had impaired SirT1 enzymatic activity and displayed 2 forms of the enzyme: a full-length 110-kd protein and a smaller 75-kd fragment. The 75-kd SirT1 fragment was found to lack the carboxy-terminus. Cathepsin B was identified as the TNFα-responsive protease that cleaves SirT1 at residue 533. Reducing cathepsin B levels via siRNA following TNFα exposure blocked the generation of the 75-kd SirT1 fragment.

CONCLUSION

These data indicate that TNFα, a cytokine that mediates joint inflammation in arthritis, induces cathepsin B-mediated cleavage of SirT1, resulting in reduced SirT1 activity. This reduced SirT1 activity correlates with the reduced cartilage-specific gene expression evident in these TNFα-treated cells.

SirT1 catalytic activity is required for male fertility and metabolic homeostasis in mice

The protein encoded by the sirt1 gene is an enzyme, SirT1, that couples the hydrolysis of NAD(+) to the deacetylation of acetyl-lysine residues in substrate proteins. Mutations of the sirt1 gene that fail to encode protein have been introduced into the mouse germ line, and the animals homozygous for these null mutations have various physiological abnormalities. To determine which of the characteristics of these sirt1(-/-) mice are a consequence of the absence of the catalytic activity of the SirT1 protein, we created a mouse strain carrying a point mutation (H355Y) that ablates the catalytic activity but does not affect the amount of the SirT1 protein. Mice carrying point mutations in both sirt1 genes, sirt1(Y/Y), have a phenotype that is overlapping but not identical to that of the sirt1-null animals. The sirt1(Y/Y) phenotype is significantly milder than that seen in the sirt1(-/-) animals. For example, female sirt1(Y/Y) animals are fertile, while sirt1(-/-) females are sterile. On the other hand, both sirt1(-/-) and sirt1(Y/Y) male mice are sterile and hypermetabolic. We report that sirt1(Y/Y) mice respond aberrantly to caloric restriction, although the effects are more subtle than seen in sirt1(-/-) mice. Thus, the SirT1 protein has functions that can be attributed to the catalytic activity of the protein, as well as other functions that are conferred by the protein itself.

Mitochondrial disorders and the eye

The clinical significance of disturbed mitochondrial function in the eye has emerged since mitochondrial DNA (mtDNA) mutation was described in Leber's hereditary optic neuropathy. The spectrum of mitochondrial dysfunction has become apparent through increased understanding of the contribution of nuclear and somatic mtDNA mutations to mitochondrial dynamics and function. Common ophthalmic manifestations of mitochondrial dysfunction include optic atrophy, pigmentary retinopathy, and ophthalmoplegia. The majority of patients with ocular manifestations of mitochondrial disease also have variable central and peripheral nervous system involvement. Mitochondrial dysfunction has recently been associated with age-related retinal disease including macular degeneration and glaucoma. Therefore, therapeutic targets directed at promoting mitochondrial biogenesis and function offer a potential to both preserve retinal function and attenuate neurodegenerative processes.

Calorie restriction and stroke

Stroke, a major cause of disability and mortality in the elderly, occurs when a cerebral blood vessel is occluded or ruptured, resulting in ischemic damage and death of brain cells. The injury mechanism involves metabolic and oxidative stress, excitotoxicity, apoptosis and inflammatory processes, including activation of glial cells and infiltration of leukocytes. In animal models, dietary energy restriction, by daily calorie reduction (CR) or intermittent fasting (IF), extends lifespan and decreases the development of age-related diseases. Dietary energy restriction may also benefit neurons, as suggested by experimental evidence showing that CR and IF protect neurons against degeneration in animal models. Recent findings by our group and others suggest the possibility that dietary energy restriction may protect against stroke induced brain injury, in part by inducing the expression of neurotrophic factors, such as brain-derived neurotrophic factor (BDNF) and basic fibroblast growth factor (bFGF); protein chaperones, including heat shock protein 70 (Hsp70) and glucose regulated protein 78 (GRP78); antioxidant enzymes, such as superoxide dismutases (SOD) and heme oxygenase-1 (HO-1), silent information regulator T1 (SIRT1), uncoupling proteins and anti-inflammatory cytokines. This article discusses the protective mechanisms activated by dietary energy restriction in ischemic stroke.

Sirt1 overexpression in neurons promotes neurite outgrowth and cell survival through inhibition of the mTOR signaling

The mammalian nicotinamide-adenine dinucleotide (NAD)-dependent deacetylase Sirt1 impacts different processes involved in the maintenance of brain integrity and in the pathogenic pathways associated with several neurodegenerative disorders, including Alzheimer's disease. Here we used human Sirt1 transgenic mice to demonstrate that neuron-specific Sirt1 overexpression promoted neurite outgrowth and improved cell viability under normal and nutrient-limiting conditions in primary culture systems and that Sirt1-overexpressing neurons exhibited higher tolerance to cell death or degeneration induced by amyloid-β1-42 oligomers. Coincidentally, we found that enhanced Sirt1 expression in neurons downregulated the mammalian target of rapamycin (mTOR) protein levels and its phosphorylation without changes in its mRNA levels, which was accompanied by concomitant inhibition of the mTOR downstream signaling activity as revealed by decreased p70S6 kinase (p70S6K) phosphorylation at Thr389. Consistently with this, using a Sirt1 siRNA transfection approach, we observed that reduction of endogenous mouse Sirt1 led to increased levels of mTOR and phosphorylation of itself and p70S6K as well as impaired cell survival and neurite outgrowth in wild-type mouse primary neurons, corroborating a suppressing effect of mTOR by Sirt1. Correspondingly, the mTOR inhibitor rapamycin markedly improved neuronal cell survival in response to nutrient deprivation and significantly enhanced neurite outgrowth in wild-type mouse neurons. The protective effect of rapamycin was extended to neurons even with Sirt1 siRNA knockdown that displayed developmental abnormalities compared with siRNA control-treated cells. Collectively, our findings suggest that Sirt1 may act to promote growth and survival of neurons in the central nervous system via its negative modulation of mTOR signaling.

Closer association of mitochondria with lipid droplets in hepatocytes and activation of Kupffer cells in resveratrol-treated senescence-accelerated mice

Resveratrol has been extensively investigated because of its beneficial effects in delaying age-related diseases, thus extending the lifespan, possibly by mimicking calorie restriction. For this study, cell biological techniques were used to examine how resveratrol influenced hepatocytes in a senescence-accelerated mouse P10 (SAMP10), treated from 35 to 55 weeks of age, with special emphasis on the relationship between mitochondria and lipid droplets. Survival ratio, body weight and food intake of SAMP10 did not differ significantly between the control and resveratrol-treated groups. Compared with the control, the treated livers were altered significantly, as follows. Lipid droplets were reduced and mitochondria were increased in number in hepatocytes. Phosphorylation of acetyl-CoA carboxylase and the expression of both the mitochondrial ATP synthase β subunit and Mn superoxide dismutase (SOD2) were increased. Mitochondria, expressing more SOD2, were more tightly associated with lipid droplets, suggesting the enhancement of lipolysis through the activation of mitochondrial functions. Cathepsin D expression was less in hepatocytes but enhanced in Kupffer cells, which were increased in number and size with more numerous lysosome-related profiles. Together, resveratrol may activate mitochondria resulting in consuming lipids, and may also activate Kupffer cells by which a beneficial milieu for hepatocytes may be created. Both might be related to improvement in the functioning of the liver, which is the organ that is central to metabolic regulation.

The Sirtuin 2 microtubule deacetylase is an abundant neuronal protein that accumulates in the aging CNS

Sirtuin 2 (SIRT2) is one of seven known mammalian protein deacetylases homologous to the yeast master lifespan regulator Sir2. In recent years, the sirtuin protein deacetylases have emerged as candidate therapeutic targets for many human diseases, including metabolic and age-dependent neurological disorders. In non-neuronal cells, SIRT2 has been shown to function as a tubulin deacetylase and a key regulator of cell division and differentiation. However, the distribution and function of the SIRT2 microtubule (MT) deacetylase in differentiated, postmitotic neurons remain largely unknown. Here, we show abundant and preferential expression of specific isoforms of SIRT2 in the mammalian central nervous system and find that a previously uncharacterized form, SIRT2.3, exhibits age-dependent accumulation in the mouse brain and spinal cord. Further, our studies reveal that focal areas of endogenous SIRT2 expression correlate with reduced α-tubulin acetylation in primary mouse cortical neurons and suggest that the brain-enriched species of SIRT2 may function as the predominant MT deacetylases in mature neurons. Recent reports have demonstrated an association between impaired tubulin acetyltransferase activity and neurodegenerative disease; viewed in this light, our results showing age-dependent accumulation of the SIRT2 neuronal MT deacetylase in wild-type mice suggest a functional link between tubulin acetylation patterns and the aging brain.

The controversial links among calorie restriction, SIRT1, and resveratrol

It has been widely known that slow metabolism induced by calorie restriction (CR) can extend the life span of model organisms though the underlying mechanism remains poorly understood. Accumulated evidence suggests that SIRT1 may be actively involved in CR-induced signaling pathways. As a putative activator of SIRT1, resveratrol, known for the French paradox, can partially mimic the physiological effects of CR. While the deacetylase activity of SIRT1 is important for the beneficial effects of resveratrol, resveratrol-induced SIRT1 activation has recently been challenged by the observations that resveratrol could not induce SIRT1-mediated deacetylation of native substrates in vitro. To resolve the discrepancy of resveratrol-induced activation of SIRT1 deacetylase activity between the in vitro and in vivo assays, a model of indirect SIRT1 activation by resveratrol is proposed. In this review, we will discuss the emerging roles of SIRT1 and resveratrol in CR and focus on debate over the links between SIRT1 and resveratrol.

[Genetic profiles of longevity and healthy cognitive aging in nonagenarians from the Basque Country]

INTRODUCTION

Currently there are notable differences in the aging of individuals in modern populations. While some of them enjoy a long healthy aging, others develop neurodegenerative diseases, such as Alzheimer's disease (AD). Environmental factors are critical, but genetics could explain the differences observed. It has recently been postulated that longevity genes might also be neuroprotective.

OBJECTIVES

To assess whether certain genetic variants associated with longevity might have a neuroprotective effect.

METHODS

The subjects of this study are people older than 90 years. We will collect sociodemographic and clinical data and multiple assessments, cognitive, functional, anthropometric, nutritional, sensory and physical each participant. In addition, 64 SNPs loci distributed in 13 candidate genes FOXO3, SIRT1, TOMM40, APOE, PICALM, COMT, CETP, CLU, CR1, IL-6, PCK-1, ZNF224 and ACE will be analysed by Taqman array.

RESULTS

It is hoped to gain more knowledge about under/over-represented alleles in nonagenarians. Furthermore, comparison of the genetic characteristics of nonagenarians with AD with those free of disease will enable links to be seen between certain alleles with protection or the risk of AD. Associated information on the participants will create subgroups showing the interactions between environment and genetic variation in relation to healthy aging and AD.

CONCLUSION

The study of the genetic variability of nonagenarians can give us information on the alleles associated with longevity and neuroprotection.

SIRT3 and cancer: tumor promoter or suppressor?

Sirtuins (SIRT1-7), the mammalian homologues of the Sir2 gene in yeast, have emerging roles in age-related diseases, such as cardiac hypertrophy, diabetes, obesity, and cancer. However, the role of several sirtuin family members, including SIRT1 and SIRT3, in cancer has been controversial. The aim of this review is to explore and discuss the seemingly dichotomous role of SIRT3 in cancer biology with particular emphasis on its potential role as a tumor promoter and tumor suppressor. This review will also discuss the potential role of SIRT3 as a novel therapeutic target to treat cancer.

Selective toxicity by HDAC3 in neurons: regulation by Akt and GSK3beta

Although it is well established that pharmacological inhibitors of classical histone deacetylases (HDACs) are protective in various in vivo models of neurodegenerative disease, the identity of the neurotoxic HDAC(s) that these inhibitors target to exert their protective effects has not been resolved. We find that HDAC3 is a protein with strong neurotoxic activity. Forced expression of HDAC3 induces death of otherwise healthy rat cerebellar granule neurons, whereas shRNA-mediated suppression of its expression protects against low-potassium-induced neuronal death. Forced expression of HDAC3 also promotes the death of rat cortical neurons and hippocampally derived HT22 cells, but has no effect on the viability of primary kidney fibroblasts or the HEK293 and HeLa cell lines. This suggests that the toxic effect of HDAC3 is cell selective and that neurons are sensitive to it. Neurotoxicity by HDAC3 is inhibited by treatment with IGF-1 as well as by the expression of a constitutively active form of Akt, an essential mediator of IGF-1 signaling. Protection against HDAC3-induced neurotoxicity is also achieved by the inhibition of GSK3β, a kinase inhibited by Akt that is widely implicated in the promotion of neurodegeneration in experimental models and in human pathologies. HDAC3 is directly phosphorylated by GSK3β, suggesting that the neuronal death-promoting action of GSK3β could be mediated through HDAC3 phosphorylation. In addition to demonstrating that HDAC3 has neurotoxic effects, our study identifies it as a downstream target of GSK3β.

Neuronal Sirt3 protects against excitotoxic injury in mouse cortical neuron culture

Background

Sirtuins (Sirt), a family of nicotinamide adenine nucleotide (NAD) dependent deacetylases, are implicated in energy metabolism and life span. Among the known Sirt isoforms (Sirt1-7), Sirt3 was identified as a stress responsive deacetylase recently shown to play a role in protecting cells under stress conditions. Here, we demonstrated the presence of Sirt3 in neurons, and characterized the role of Sirt3 in neuron survival under NMDA-induced excitotoxicity.

Methodology/Principal Findings

To induce excitotoxic injury, we exposed primary cultured mouse cortical neurons to NMDA (30 µM). NMDA induced a rapid decrease of cytoplasmic NAD (but not mitochondrial NAD) in neurons through poly (ADP-ribose) polymerase-1 (PARP-1) activation. Mitochondrial Sirt3 was increased following PARP-1 mediated NAD depletion, which was reversed by either inhibition of PARP-1 or exogenous NAD. We found that massive reactive oxygen species (ROS) produced under this NAD depleted condition mediated the increase in mitochondrial Sirt3. By transfecting primary neurons with a Sirt3 overexpressing plasmid or Sirt3 siRNA, we showed that Sirt3 is required for neuroprotection against excitotoxicity.

Conclusions

This study demonstrated for the first time that mitochondrial Sirt3 acts as a prosurvival factor playing an essential role to protect neurons under excitotoxic injury.

Characterization of nuclear sirtuins: molecular mechanisms and physiological relevance

Sirtuins are protein deacetylases/mono-ADP-ribosyltransferases found in organisms ranging from bacteria to humans. This group of enzymes relies on nicotinamide adenine dinucleotide (NAD(+)) as a cofactor linking their activity to the cellular metabolic status. Originally found in yeast, Sir2 was discovered as a silencing factor and has been shown to mediate the effects of calorie restriction on lifespan extension. In mammals seven homologs (SIRT1-7) exist which evolved to have specific biological outcomes depending on the particular cellular context, their interacting proteins, and the genomic loci to where they are actively targeted. Sirtuins biological roles are highlighted in the early lethal phenotypes observed in the deficient murine models. In this chapter, we summarize current concepts on non-metabolic functions for sirtuins, depicting this broad family from yeast to mammals.

Beyond histone and deacetylase: an overview of cytoplasmic histone deacetylases and their nonhistone substrates

Acetylation of lysines is a prominent form of modification in mammalian proteins. Deacetylation of proteins is catalyzed by histone deacetylases, traditionally named after their role in histone deacetylation, transcriptional modulation, and epigenetic regulation. Despite the link between histone deacetylases and chromatin structure, some of the histone deacetylases reside in various compartments in the cytoplasm. Here, we review how these cytoplasmic histone deacetylases are regulated, the identification of nonhistone substrates, and the functional implications of their nondeacetylase enzymatic activities.

Cellular stress responses, the hormesis paradigm, and vitagenes: novel targets for therapeutic intervention in neurodegenerative disorders

Despite the capacity of chaperones and other homeostatic components to restore folding equilibrium, cells appear poorly adapted for chronic oxidative stress that increases in cancer and in metabolic and neurodegenerative diseases. Modulation of endogenous cellular defense mechanisms represents an innovative approach to therapeutic intervention in diseases causing chronic tissue damage, such as in neurodegeneration. This article introduces the concept of hormesis and its applications to the field of neuroprotection. It is argued that the hormetic dose response provides the central underpinning of neuroprotective responses, providing a framework for explaining the common quantitative features of their dose-response relationships, their mechanistic foundations, and their relationship to the concept of biological plasticity, as well as providing a key insight for improving the accuracy of the therapeutic dose of pharmaceutical agents within the highly heterogeneous human population. This article describes in mechanistic detail how hormetic dose responses are mediated for endogenous cellular defense pathways, including sirtuin and Nrf2 and related pathways that integrate adaptive stress responses in the prevention of neurodegenerative diseases. Particular attention is given to the emerging role of nitric oxide, carbon monoxide, and hydrogen sulfide gases in hormetic-based neuroprotection and their relationship to membrane radical dynamics and mitochondrial redox signaling.

Neuroprotection by histone deacetylase-7 (HDAC7) occurs by inhibition of c-jun expression through a deacetylase-independent mechanism

Histone deacetylase (HDAC) 7 is a member of the HDAC family of deacetylases. Although some of the HDAC proteins have been shown to regulate neuronal survival and death, whether HDAC7 has a similar role is not known. In this study, we show that HDAC7 protects neurons from apoptosis. In cerebellar granule neurons (CGNs) primed to undergo apoptosis by low potassium treatment, expression of HDAC7 protein is reduced. Reduced expression is also observed in CGNs induced to die by pharmacological inhibition of the proteasome, in cortical neurons treated with homocysteic acid, and in the striatum of R6/2 transgenic mice, a commonly used genetic model of Huntington disease. Forced expression of HDAC7 in cultured CGNs blocks low potassium-induced death, and shRNA-mediated suppression of its expression induces death in otherwise healthy neurons. HDAC7-mediated neuroprotection does not require its catalytic domain and cannot be inhibited by chemical inhibitors of HDACs. Moreover, pharmacological inhibitors of the PI3K-Akt or Raf-MEK-ERK signaling pathways or that of PKA, PKC, and Ca(2+)/calmodulin-dependent protein kinase fail to reduce neuroprotection by HDAC7. We show that stimulation of c-jun expression, an essential feature of neuronal death, is prevented by HDAC7. shRNA-mediated suppression of HDAC7 expression leads to an increase in c-jun expression. Inhibition of c-jun expression by HDAC7 is mediated at the transcriptional level by its direct association with the c-jun gene promoter. Taken together, our results indicate that HDAC7 is a neuroprotective protein acting by a mechanism that is independent of its deacetylase activity but involving the inhibition of c-jun expression.

SIRT1 undergoes alternative splicing in a novel auto-regulatory loop with p53

BACKGROUND

The NAD-dependent deacetylase SIRT1 is a nutrient-sensitive coordinator of stress-tolerance, multiple homeostatic processes and healthspan, while p53 is a stress-responsive transcription factor and our paramount tumour suppressor. Thus, SIRT1-mediated inhibition of p53 has been identified as a key node in the common biology of cancer, metabolism, development and ageing. However, precisely how SIRT1 integrates such diverse processes remains to be elucidated.

METHODOLOGY/PRINCIPAL FINDINGS

Here we report that SIRT1 is alternatively spliced in mammals, generating a novel SIRT1 isoform: SIRT1-ΔExon8. We show that SIRT1-ΔExon8 is expressed widely throughout normal human and mouse tissues, suggesting evolutionary conservation and critical function. Further studies demonstrate that the SIRT1-ΔExon8 isoform retains minimal deacetylase activity and exhibits distinct stress sensitivity, RNA/protein stability, and protein-protein interactions compared to classical SIRT1-Full-Length (SIRT1-FL). We also identify an auto-regulatory loop whereby SIRT1-ΔExon8 can regulate p53, while in reciprocal p53 can influence SIRT1 splice variation.

CONCLUSIONS/SIGNIFICANCE

We characterize the first alternative isoform of SIRT1 and demonstrate its evolutionary conservation in mammalian tissues. The results also reveal a new level of inter-dependency between p53 and SIRT1, two master regulators of multiple phenomena. Thus, previously-attributed SIRT1 functions may in fact be distributed between SIRT1 isoforms, with important implications for SIRT1 functional studies and the current search for SIRT1-activating therapeutics to combat age-related decline.

Mitochondrial dysfunction in glaucoma and emerging bioenergetic therapies

The similarities between glaucoma and mitochondrial optic neuropathies have driven a growing interest in exploring mitochondrial function in glaucoma. The specific loss of retinal ganglion cells is a common feature of mitochondrial diseases - not only the classic mitochondrial optic neuropathies of Leber's Hereditary Optic Neuropathy and Autosomal Dominant Optic Atrophy - but also occurring together with more severe central nervous system involvement in many other syndromic mitochondrial diseases. The retinal ganglion cell, due to peculiar structural and energetic constraints, appears acutely susceptible to mitochondrial dysfunction. Mitochondrial function is also well known to decline with aging in post-mitotic tissues including neurons. Because age is a risk factor for glaucoma this adds another impetus to investigating mitochondria in this common and heterogeneous neurodegenerative disease. Mitochondrial function may be impaired by either nuclear gene or mitochondrial DNA genetic risk factors, by mechanical stress or chronic hypoperfusion consequent to the commonly raised intraocular pressure in glaucomatous eyes, or by toxic xenobiotic or even light-induced oxidative stress. If primary or secondary mitochondrial dysfunction is further established as contributing to glaucoma pathogenesis, emerging therapies aimed at optimizing mitochondrial function represent potentially exciting new clinical treatments that may slow retinal ganglion cell and vision loss in glaucoma.

Mammalian sirtuins: biological insights and disease relevance

Aging is accompanied by a decline in the healthy function of multiple organ systems, leading to increased incidence and mortality from diseases such as type II diabetes mellitus, neurodegenerative diseases, cancer, and cardiovascular disease. Historically, researchers have focused on investigating individual pathways in isolated organs as a strategy to identify the root cause of a disease, with hopes of designing better drugs. Studies of aging in yeast led to the discovery of a family of conserved enzymes known as the sirtuins, which affect multiple pathways that increase the life span and the overall health of organisms. Since the discovery of the first known mammalian sirtuin, SIRT1, 10 years ago, there have been major advances in our understanding of the enzymology of sirtuins, their regulation, and their ability to broadly improve mammalian physiology and health span. This review summarizes and discusses the advances of the past decade and the challenges that will confront the field in the coming years.

trans-Resveratrol as a neuroprotectant

Epidemiological evidence indicates that nutritionally-derived polyphenols such as resveratrol (RES) have neuroprotective properties. Administration of RES to culture media protects a wide variety of neuronal cell types from stress-induced death. Dietary supplementation of RES can ameliorate neuronal damage and death resulting from both acute and chronic stresses in rodents. The specific molecular mechanisms by which RES acts at the cellular level remain incompletely understood. However, many experimental data indicate that RES reduces or prevents the occurrence of oxidative damage. Here we discuss possible mechanisms by which RES might exert protection against oxidative damage and cell death. Evidence suggesting that RES’s chemical antioxidant potential is not sufficient explanation for its effects is discussed. Putative biological activities, including interactions with estrogen receptors and sirtuins are critically discussed. We provide a synthesis of how RES’s phytoestrogenic properties might mediate the neuronal stress resistance underlying its observed neuroprotective properties.

HDAC inhibitors and neurodegeneration: at the edge between protection and damage

The use of histone deacetylase inhibitors (HDACIs) as a therapeutic tool for neurodegenerative disorders has been examined with great interest in the last decade. The functional response to treatment with broad-spectrum inhibitors however, has been heterogeneous: protective in some cases and detrimental in others. In this review we discuss potential underlying causes for these apparently contradictory results. Because HDACs are part of repressive complexes, the functional outcome has been characteristically attributed to enhanced gene expression due to increased acetylation of lysine residues on nucleosomal histones. However, it is important to take into consideration that the up-regulation of diverse sets of genes (i.e. pro-apoptotic and anti-apoptotic) may orchestrate different responses in diverse cell types. An alternative possibility is that broad-spectrum pharmacological inhibition may target nuclear or cytosolic HDAC isoforms, with distinct non-histone substrates (i.e. transcription factors; cytoskeletal proteins). Thus, for any given neurological disorder, it is important to take into account the effect of HDACIs on neuronal, glial and inflammatory cells and define the relative contribution of distinct HDAC isoforms to the pathological process. This review article addresses how opposing effects on distinct cell types may profoundly influence the overall therapeutic potential of HDAC inhibitors when investigating treatments for neurodegenerative disorders.

Epigenetic regulation in the pathophysiology of Alzheimer's disease

With the aging of the population, the growing incidence and prevalence of Alzheimer's disease (AD) increases the burden on individuals and society as a whole. To date, the pathophysiology of AD is not yet fully understood. Recent studies have suggested that epigenetic mechanisms may play a pivotal role in its course and development. The most frequently studied epigenetic mechanisms are DNA methylation and histone modifications, and investigations relevant to aging and AD are presented in this review. Various studies on human postmortem brain samples and peripheral leukocytes, as well as transgenic animal models and cell culture studies relevant to AD will be discussed. From those, it is clear that aging and AD are associated with epigenetic dysregulation at various levels. Moreover, data on e.g. twin studies in AD support the notion that epigenetic mechanisms mediate the risk for AD. Conversely, it is still not fully clear whether the observed epigenetic changes actually represent a cause or a consequence of the disease. This is mainly due to the fact that most clinical investigations on epigenetics in AD are conducted in samples of patients already in an advanced stage of the disease. Evidently, more research is needed in order to clarify the exact role of epigenetic regulation in the course and development of AD. Research on earlier stages of the disease could provide more insight into its underlying pathophysiology, possibly contributing to the establishment of early diagnosis and the development of more effective treatment strategies.

SIRT1 suppresses activator protein-1 transcriptional activity and cyclooxygenase-2 expression in macrophages

SIRT1 (Sirtuin type 1), a mammalian orthologue of yeast SIR2 (silent information regulator 2), has been shown to mediate a variety of calorie restriction (CR)-induced physiological events, such as cell fate regulation via deacetylation of the substrate proteins. However, whether SIRT1 deacetylates activator protein-1 (AP-1) to influence its transcriptional activity and target gene expression is still unknown. Here we demonstrate that SIRT1 directly interacts with the basic leucine zipper domains of c-Fos and c-Jun, the major components of AP-1, by which SIRT1 suppressed the transcriptional activity of AP-1. This process requires the deacetylase activity of SIRT1. Notably, SIRT1 reduced the expression of COX-2, a typical AP-1 target gene, and decreased prostaglandin E(2) (PGE(2)) production of peritoneal macrophages (pMPhis). pMPhis with SIRT1 overexpression displayed improved phagocytosis and tumoricidal functions, which are associated with depressed PGE(2). Furthermore, SIRT1 protein level was up-regulated in CR mouse pMPhis, whereas elevated SIRT1 decreased COX-2 expression and improved PGE(2)-related macrophage functions that were reversed following inhibition of SIRT1 deacetylase activity. Thus, our results indicate that SIRT1 may be a mediator of CR-induced macrophage regulation, and its deacetylase activity contributes to the inhibition of AP-1 transcriptional activity and COX-2 expression leading to amelioration of macrophage function.

SIRT2-mediated protein deacetylation: An emerging key regulator in brain physiology and pathology

Protein function is considerably altered by posttranslational modification. In recent years, cycles of acetylation/deacetylation emerged as fundamental regulators adjusting biological activity of many proteins. Particularly, protein deacetylation by Sirtuins, a family of atypical histone deacetylases (HDACs), was demonstrated to regulate fundamental cell biological processes including gene expression, genome stability, mitosis, nutrient metabolism, aging, mitochondrial function and cell motility. Given this wealth of biological functions, perhaps not unexpectedly then, pharmacological compounds targeting Sirtuin activity are now prime therapeutic agents for alleviating severity of major diseases encompassing diabetes, cancer, cardiovascular and neurodegenerative disorders in many organs. In this review, we will focus on the brain and its physiological and pathological processes governed by Sirtuin-mediated deacetylation. Besides discussing Sirtuin function in neurodegenerative diseases, emphasis will be given on the mounting evidence deciphering key developmental brain functions for Sirtuins in neuronal motility, neuroprotection and oligodendrocyte differentiation. In this respect, we will particularly highlight functions of the unconventional family member SIRT2 in post-mitotic neurons and glial cells.

Neuroprotection by radical avoidance: search for suitable agents

Neurodegeneration is frequently associated with damage by free radicals. However, increases in reactive oxygen and nitrogen species, which may ultimately lead to neuronal cell death, do not necessarily reflect its primary cause, but can be a consequence of otherwise induced cellular dysfunction. Detrimental processes which promote free radical formation are initiated, e.g., by disturbances in calcium homeostasis, mitochondrial malfunction, and an age-related decline in the circadian oscillator system. Free radicals generated at high rates under pathophysiological conditions are insufficiently detoxified by scavengers. Interventions at the primary causes of dysfunction, which avoid secondary rises in radical formation, may be more efficient. The aim of such approaches should be to prevent calcium overload, to reduce mitochondrial electron dissipation, to support electron transport capacity, and to avoid circadian perturbations. L-theanine and several amphiphilic nitrones are capable of counteracting excitotoxicity and/or mitochondrial radical formation. Resveratrol seems to promote mitochondrial biogenesis. Mitochondrial effects of leptin include attenuation of electron leakage. Melatonin combines all the requirements mentioned, additionally regulates anti- and pro-oxidant enzymes and is, with few exceptions, very well tolerated. In this review, the perspectives, problems and limits of drugs are compared which may be suitable for reducing the formation of free radicals.

Sirt1's complex roles in neuroprotection

The nicotinamide adenine dinucleotide (NAD)-activated protein deacetylase Sir2p/Sirt1 has been strongly implicated in the modulation of replicative lifespan and promotion of longevity. Part of Sirt1's capacity for lifespan extension in complex organisms may be attributed to its protective activity against neuronal degeneration. Manipulation of Sirt1's activity or levels by pharmacological and genetic means in several models of neurodegenerative diseases demonstrated its neuroprotective credentials. However, recent data have indicated that under certain contexts, Sirt1 inhibition, rather than activation, is neuroprotective. These inconsistencies highlight the complex nature of Sirt1-mediated effects. The enzyme has both histone and nonhistone targets, and could potentially act in both nuclear and cytoplasmic compartments. These activities intertwine in a manner depending on the context of a system under investigation. One needs to be cautious in extrapolating results derived from short-term observations to a longer-term context, and in assessing efficacies of Sirt1-based therapeutic approaches in treating neurodegenerative diseases.

Histone deacetylases as targets for the treatment of human neurodegenerative diseases

Histone deacetylases (HDACs) are a family of proteins that play an important role in regulating transcription as well as the function of a variety of cellular proteins. While these proteins are expressed abundantly in the brain, little is known about their roles in brain function. A growing body of evidence suggests that HDACs regulate neuronal survival. Results from studies conducted in vertebrate and mammalian experimental systems indicate that while some of these proteins are involved in promoting neuronal death, a majority of the HDACs studied thus far protect against neurodegeneration. Here we review the research performed on the role played by individual members of the HDAC family in the regulation of neuronal death. Chemical inhibitors of HDACs have been used in a variety of models of neurodegenerative disorders. We summarize the results from these studies, which indicate that HDAC inhibitors show great promise as therapeutic agents for human neurodegenerative disorders.

Function and regulation of the mitochondrial sirtuin isoform Sirt5 in Mammalia

Sirtuins are a family of protein deacetylases that catalyze the nicotinamide adenine dinucleotide (NAD(+))-dependent removal of acetyl groups from modified lysine side chains in various proteins. Sirtuins act as metabolic sensors and influence metabolic adaptation but also many other processes such as stress response mechanisms, gene expression, and organismal aging. Mammals have seven Sirtuin isoforms, three of them - Sirt3, Sirt4, and Sirt5 - located to mitochondria, our centers of energy metabolism and apoptosis initiation. In this review, we shortly introduce the mammalian Sirtuin family, with a focus on the mitochondrial isoforms. We then discuss in detail the current knowledge on the mitochondrial isoform Sirt5. Its physiological role in metabolic regulation has recently been confirmed, whereas an additional function in apoptosis regulation remains speculative. We will discuss the biochemical properties of Sirt5 and how they might contribute to its physiological function. Furthermore, we discuss the potential use of Sirt5 as a drug target, structural features of Sirt5 and of an Sirt5/inhibitor complex as well as their differences to other Sirtuins and the current status of modulating Sirt5 activity with pharmacological compounds.

Transcriptional corepressor SMILE recruits SIRT1 to inhibit nuclear receptor estrogen receptor-related receptor gamma transactivation

SMILE (small heterodimer partner interacting leucine zipper protein) has been identified as a corepressor of the glucocorticoid receptor, constitutive androstane receptor, and hepatocyte nuclear factor 4alpha. Here we show that SMILE also represses estrogen receptor-related receptor gamma (ERRgamma) transactivation. Knockdown of SMILE gene expression increases ERRgamma activity. SMILE directly interacts with ERRgamma in vitro and in vivo. Domain mapping analysis showed that SMILE binds to the AF2 domain of ERRgamma. SMILE represses ERRgamma transactivation partially through competition with coactivators PGC-1alpha, PGC-1beta, and GRIP1. Interestingly, the repression of SMILE on ERRgamma is released by SIRT1 inhibitors, a catalytically inactive SIRT1 mutant, and SIRT1 small interfering RNA but not by histone protein deacetylase inhibitor. In vivo glutathione S-transferase pulldown and coimmunoprecipitation assays validated that SMILE physically interacts with SIRT1. Furthermore, the ERRgamma inverse agonist GSK5182 enhances the interaction of SMILE with ERRgamma and SMILE-mediated repression. Knockdown of SMILE or SIRT1 blocks the repressive effect of GSK5182. Moreover, chromatin immunoprecipitation assays revealed that GSK5182 augments the association of SMILE and SIRT1 on the promoter of the ERRgamma target PDK4. GSK5182 and adenoviral overexpression of SMILE cooperate to repress ERRgamma-induced PDK4 gene expression, and this repression is released by overexpression of a catalytically defective SIRT1 mutant. Finally, we demonstrated that ERRgamma regulates SMILE gene expression, which in turn inhibits ERRgamma. Overall, these findings implicate SMILE as a novel corepressor of ERRgamma and recruitment of SIRT1 as a novel repressive mechanism for SMILE and ERRgamma inverse agonist.

The SIRT1 activator resveratrol protects SK-N-BE cells from oxidative stress and against toxicity caused by alpha-synuclein or amyloid-beta (1-42) peptide

Human sirtuins are a family of seven conserved proteins (SIRT1-7). The most investigated is the silent mating type information regulation-2 homolog (SIRT1, NM_012238), which was associated with neuroprotection in models of polyglutamine toxicity or Alzheimer's disease (AD) and whose activation by the phytocompound resveratrol (RES) has been described. We have examined the neuroprotective role of RES in a cellular model of oxidative stress, a common feature of neurodegeneration. RES prevented toxicity triggered by hydrogen peroxide or 6-hydroxydopamine (6-OHDA). This action was likely mediated by SIRT1 activation, as the protection was lost in the presence of the SIRT1 inhibitor sirtinol and when SIRT1 expression was down-regulated by siRNA approach. RES was also able to protect SK-N-BE from the toxicity arising from two aggregation-prone proteins, the AD-involved amyloid-beta (1-42) peptide (Abeta42) and the familiar Parkinson's disease linked alpha-synuclein(A30P) [alpha-syn(A30P)]. Alpha-syn(A30P) toxicity was restored by sirtinol addition, while a partial RES protective effect against Abeta42 was found even in presence of sirtinol, thus suggesting a direct RES effect on Abeta42 fibrils. We conclude that SIRT1 activation by RES can prevent in our neuroblastoma model the deleterious effects triggered by oxidative stress or alpha-syn(A30P) aggregation, while RES displayed a SIRT1-independent protective action against Abeta42.

Epigenetics, oxidative stress, and Alzheimer disease

Alzheimer disease (AD) is a progressive neurodegenerative disorder whose clinical manifestations appear in old age. The sporadic nature of 90% of AD cases, the differential susceptibility to and course of the illness, as well as the late age onset of the disease suggest that epigenetic and environmental components play a role in the etiology of late-onset AD. Animal exposure studies demonstrated that AD may begin early in life and may involve an interplay between the environment, epigenetics, and oxidative stress. Early life exposure of rodents and primates to the xenobiotic metal lead (Pb) enhanced the expression of genes associated with AD, repressed the expression of others, and increased the burden of oxidative DNA damage in the aged brain. Epigenetic mechanisms that control gene expression and promote the accumulation of oxidative DNA damage are mediated through alterations in the methylation or oxidation of CpG dinucleotides. We found that environmental influences occurring during brain development inhibit DNA-methyltransferases, thus hypomethylating promoters of genes associated with AD such as the beta-amyloid precursor protein (APP). This early life imprint was sustained and triggered later in life to increase the levels of APP and amyloid-beta (Abeta). Increased Abeta levels promoted the production of reactive oxygen species, which damage DNA and accelerate neurodegenerative events. Whereas AD-associated genes were overexpressed late in life, others were repressed, suggesting that these early life perturbations result in hypomethylation as well as hypermethylation of genes. The hypermethylated genes are rendered susceptible to Abeta-enhanced oxidative DNA damage because methylcytosines restrict repair of adjacent hydroxyguanosines. Although the conditions leading to early life hypo- or hypermethylation of specific genes are not known, these changes can have an impact on gene expression and imprint susceptibility to oxidative DNA damage in the aged brain.