Neuronal SIRT1 Activation as a Novel Mechanism Underlying the Prevention of Alzheimer Disease Amyloid Neuropathology by Calorie Restriction

Crystal structures of human SIRT3 displaying substrate-induced conformational changes

SIRT3 is a major mitochondrial NAD(+)-dependent protein deacetylase playing important roles in regulating mitochondrial metabolism and energy production and has been linked to the beneficial effects of exercise and caloric restriction. SIRT3 is emerging as a potential therapeutic target to treat metabolic and neurological diseases. We report the first sets of crystal structures of human SIRT3, an apo-structure with no substrate, a structure with a peptide containing acetyl lysine of its natural substrate acetyl-CoA synthetase 2, a reaction intermediate structure trapped by a thioacetyl peptide, and a structure with the dethioacetylated peptide bound. These structures provide insights into the conformational changes induced by the two substrates required for the reaction, the acetylated substrate peptide and NAD(+). In addition, the binding study by isothermal titration calorimetry suggests that the acetylated peptide is the first substrate to bind to SIRT3, before NAD(+). These structures and biophysical studies provide key insight into the structural and functional relationship of the SIRT3 deacetylation activity.

Some highlights of research on aging with invertebrates, 2009

This annual review focuses on invertebrate model organisms, which shed light on new mechanisms in aging and provide excellent systems for both genome-wide and in-depth analysis. This year, protein interaction networks have been used in a new bioinformatic approach to identify novel genes that extend replicative lifespan in yeast. In an extended approach, using a new, human protein interaction network, information from the invertebrates was used to identify new, candidate genes for lifespan extension and their orthologues were validated in the nematode Caenorhabditis elegans. Chemosensation of diffusible substances from bacteria has been shown to limit lifespan in C. elegans, while a systematic study of the different methods used to implement dietary restriction in the worm has shown that they involve mechanisms that are partially distinct and partially overlapping, providing important clarification for addressing whether or not they are conserved in other organisms. A new theoretical model for the evolution of rejuvenating cell division has shown that asymmetrical division for either cell size or for damaged cell constituents results in increased fitness for most realistic levels of cellular protein damage. Work on aging-related disease has both refined our understanding of the mechanisms underlying one route to the development of Parkinson's disease and has revealed that in worms, as in mice, dietary restriction is protective against cellular proteotoxicity. Two systematic studies genetically manipulating the superoxide dismutases of C. elegans support the idea that damage from superoxide plays little or no role in aging in this organism, and have prompted discussion of other kinds of damage and other kinds of mechanisms for producing aging-related decline in function.

Rho kinase inhibitor Y-27632 down-regulates norepinephrine synthesis and release in PC12 cells

Rho kinase inhibition is beneficial for neurite outgrowth and nerve disorders, and the Rho kinase inhibitors have been regarded as promising agents to treat neural diseases. The main aim of the study was to elucidate how Rho kinase inhibitor Y-27632 regulates neurotransmitter norepinephrine synthesis and release in PC12 cells when neurite outgrowth was induced. PC12 cells were treated with Y-27632 for 6 days. The amount of norepinephrine synthesized in PC12 cells and the amount released evoked by acetylcholine or by KCl were determined by norepinephrine enzyme-linked immunosorbent assay kits. The results showed that the amount of norepinephrine both synthesized and released was down-regulated with a concentration-dependent relationship. Further results of Western blotting found that the protein expression of tyrosine hydroxylase and synapsin I (especially its active form, synapsin I phosphoSer603) was also down-regulated, which were directly related to synthesis and release of norepinephrine, respectively. All the results suggest that Y-27632 is able to down-regulate norepinephrine synthesis and release, the direct mechanism of which may be associated with down-regulation on expression of some proteins, including tyrosine hydroxylase and synapsin I.

Biochemical characterization, localization, and tissue distribution of the longer form of mouse SIRT3

SIRT3 is a key mitochondrial protein deacetylase proposed to play key roles in regulating mitochondrial metabolism but there has been considerable debate about its actual size, the sequences required for activity, and its subcellular localization. A previously cloned mouse SIRT3 has high sequence similarity with the C-terminus of human SIRT3 but lacks an N-terminal mitochondrial targeting sequence and has no detectable deacetylation activity in vitro. Using 5' rapid amplification of cDNA ends, we cloned the entire sequence of mouse SIRT3, as well as rat and rabbit SIRT3. Importantly, we find that full-length SIRT3 protein localizes exclusively to the mitochondria, in contrast to reports of SIRT3 localization to the nucleus. We demonstrate that SIRT3 has no deacetylation activity in vitro unless the protein is truncated, consistent with human SIRT3. In addition, we determined the inhibition constants and mechanism of action for nicotinamide and a small molecule SIRT3 inhibitor against active mouse SIRT3 and show that the mechanisms are different for the two compounds with respect to peptide substrate and NAD(+). Thus, identification and characterization of the actual SIRT3 sequence should help resolve the debate about the nature of mouse SIRT3 and identify new mechanisms to modulate enzymatic activity.

Sirtuin inhibitors

BACKGROUND

The sirtuin family of deacetylase enzymes comprises seven proteins (SIRT1-7) that are dependent on NAD(+) for their activity. Three proteins are located in the nucleus, three in the mitochondria and only one is predominantly cytoplasmic. Caloric restriction and oxidative stress generally up-regulate their expression. SIRT1, the orthologue of yeast Sir2, is the mammalian sirtuin that has been most extensively studied to date. Among other targets, SIRT1 down-regulates the activity of the nuclear transcription factor p53, being this related with an increase in lifespan and cell survival associated to stress resistance.

OBJECTIVE

Because sirtuin modulation could have beneficial effects on several human diseases, there is a growing interest in the discovery and development of small molecules that modify its activity. This review will be focused on sirtuin inhibitors.

CONCLUSIONS

Several specific inhibitors of SIRT1 have been described. These compounds could be mainly useful for the treatment of cancers by increasing p53 activity that stops the formation of tumours and induces apoptosis. A p53-independent massive induction of apoptosis has been also described for one inhibitor. In addition, a potent and selective SIRT2 inhibitor that ameliorates the alpha-synuclein fibril formation in Parkinson disease has been proposed to treat this kind of neurodegenerative disease.

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.

SirT1-null mice develop tumors at normal rates but are poorly protected by resveratrol

The function of the class III histone deacetylase, Sir2, in promoting lifespan extension is well established in small model organisms. By analogy, SirT1, the mammalian orthologue of Sir2, is a candidate gene to slow down aging and forestall the onset of age-associated diseases. We have used SirT1-null mice to study the function of SirT1 in susceptibility to tumorigenesis. The number of intestinal polyps induced in mice carrying the Apc(min) mutation was unaffected by the SirT1 genotype although the average polyp size was slightly smaller in the SirT1-null animals. Similarly, the presence or absence of SirT1 had no effect on incidence and tumor load of skin papillomas induced by the classical two-stage carcinogenesis protocol. We found that resveratrol topically applied to the skin profoundly reduced tumorigenesis. This chemoprotective effect was significantly reduced but not ablated in SirT1-null mice, suggesting that part of the protection afforded by resveratrol requires the SirT1-encoded protein. Thus, our results suggest that SirT1 does not behave like a classical tumor-suppressor gene but the antitumor activity of resveratrol is mediated at least in part by SirT1.

Metabolism control by the circadian clock and vice versa

Circadian rhythms govern a wide variety of physiological and metabolic functions in most organisms. At the heart of these regulatory pathways in mammals is the clock machinery, a remarkably coordinated transcription-translation system that relies on dynamic changes in chromatin states. Recent findings indicate that regulation also goes the other way, as specific elements of the clock can sense changes in cellular metabolism. Understanding in full detail the intimate links between cellular metabolism and the circadian clock machinery will provide not only crucial insights into system physiology but also new avenues toward pharmacological intervention of metabolic disorders.

CADASIL management or what to do when there is little one can do

Cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL) is a rare disease that leads to migraine, mood disorders, recurrent lacunar strokes and early vascular dementia. This autosomal-dominant condition is caused by mutations in the NOTCH3 gene and is characterized by degeneration of vascular smooth muscle cells. At present, no evidence-based treatment for CADASIL is available and only relief of symptoms can be offered to patients. This review focuses on an update of CADASIL management, based on the recent clinical and basic evidence, and discusses possible new treatment targets for CADASIL.

PGC-1alpha expression decreases in the Alzheimer disease brain as a function of dementia

OBJECTIVES

To explore mechanisms through which altered peroxisome proliferator-activated receptor gamma coactivator 1alpha (PGC-1alpha) expression may influence Alzheimer disease (AD) amyloid neuropathology and to test the hypothesis that promotion of PGC-1alpha expression in neurons might be developed as a novel therapeutic strategy in AD.

DESIGN

Case-control. Patients Human postmortem brain (hippocampal formation) samples from AD cases and age-matched non-AD cases.

RESULTS

Using genome-wide complementary DNA microarray analysis, we found that PGC-1alpha messenger RNA expression was significantly decreased as a function of progression of clinical dementia in the AD brain. Following confirmatory real-time polymerase chain reaction assay, we continued to explore the role of PGC-1alpha in clinical dementia and found that PGC-1alpha protein content was negatively associated with both AD-type neuritic plaque pathology and beta-amyloid (Abeta)(X-42) contents. Moreover, we found that the predicted elevation of amyloidogenic Abeta(1-42) and Abeta(1-40) peptide accumulation in embryonic cortico-hippocampal neurons derived from Tg2576 AD mice under hyperglycemic conditions (glucose level, 182-273 mg/dL) coincided with a dose-dependent attenuation in PGC-1alpha expression. Most importantly, we found that the reconstitution of exogenous PGC-1alpha expression in Tg2576 neurons attenuated the hyperglycemic-mediated beta-amyloidogenesis through mechanisms involving the promotion of the "nonamyloidogenic" alpha-secretase processing of amyloid precursor protein through the attenuation of the forkheadlike transcription factor 1 (FoxO3a) expression.

CONCLUSION

Therapeutic preservation of neuronal PGC-1alpha expression promotes the nonamyloidogenic processing of amyloid precursor protein precluding the generation of amyloidogenic Abeta peptides.

Sirtuin 1 reduction parallels the accumulation of tau in Alzheimer disease

Aging and metabolism-related disorders are risk factors for Alzheimer disease (AD). Because sirtuins may increase the life span through regulation of cellular metabolism, we compared the concentration of sirtuin 1 (SIRT1) in the brains of AD patients (n = 19) and controls (n = 22) using Western immunoblots and in situ hybridization. We report a significant reduction of SIRT1 (messenger RNA [mRNA], -29%; protein, -45%) in the parietal cortex of AD patients, but not in the cerebellum. Further analyses in a second cohort of 36 subjects confirmed that cortical SIRT1 was decreased in AD but not in individuals with mild cognitive impairment. SIRT1 mRNA and its translated protein correlated negatively with the duration of symptoms (mRNA, r2 = -0.367; protein, r2 = -0.326) and the accumulation of paired helical filament tau (mRNA, r2 = -0.230; protein, r2 = -0.119), but weakly with insoluble amyloid-beta 42 (mRNA, r2= -0.090; protein, r2 = -0.072). A significant relationship between SIRT1 levels and global cognition scores proximate to death was also found (r2= +0.09, p = 0.049). In contrast, cortical SIRT1 levels remained unchanged in a triple-transgenic animal model of AD. Collectively, our results indicate that loss of SIRT1 is closely associated with the accumulation of amyloid-beta and tau in the cerebral cortex of persons with AD.

The role of transcriptional regulators in central control of appetite and body weight

Individuals who live in industrialized countries often eat a calorie-rich diet and perform little physical activity. These habits are thought to be critical contributors to the rapidly rising incidence of obesity, a condition that affects hundreds of millions of people worldwide. High-calorie intake alters metabolic-sensing pathways in central nervous system neurons, and these changes have pathogenic roles in the development of obesity. This review aims to summarize our current knowledge about the neuronal populations (the central melanocortin system in particular) and transcriptional regulators, including STAT3 and FOXO1, that are involved in the maintenance of normal body weight. We describe the interactions between these transcriptional factors and their target genes, which encode the main appetite-regulating neuropeptides (agouti-related peptide and alpha-melanocyte-stimulating hormone). We discuss the transcriptional co-activator PGC-1-alpha and the supposed metabolic-sensor protein SIRT1, and their potential roles as targets for novel antiobesity medications.

Inhibition of Rho kinases enhances the degradation of mutant huntingtin

Huntington disease (HD) is a fatal hereditary neurodegenerative disease caused by an expansion of the polyglutamine (polyQ) stretch in huntingtin (htt). Whereas the pathological significance of the expanded polyQ has been clearly established and a tremendous effort to develop therapeutic tools for HD has been exerted, there is yet no effective cure. Whereas many molecules able to reduce the polyQ accumulation and aggregation have been identified, including several Rho kinase (ROCK) inhibitors, it remains very important to determine the mechanism of action of the potential drugs. ROCK inhibitors, including Y-27632 were reported to decrease aggregation of htt and androgen receptor (AR) through ROCK1 and protein kinase C-related protein kinase-2 (PRK-2). A downstream effector of ROCK1, actin-binding factor profilin, was shown to inhibit the mutant htt aggregation but not AR by direct interaction. We found that the anti-aggregation effect of ROCK inhibitors was not limited to the mutant htt and AR and that Y-27632 was also able to reduce the aggregation of ataxin-3 and atrophin-1 with expanded polyQ. These results suggested that in addition to the mechanism reported for htt and AR, there might also be other common mediators involved in the reduced aggregation of different polyQ proteins. In this study, we show that Y-27632 not only reduced the mutant htt aggregation by enhancing its degradation, but surprisingly was able to activate the main cellular degradation pathways, proteasome, and macroautophagy. We also show that this unique effect was mediated by ROCK1 and ROCK2.

AMPK and the biochemistry of exercise: implications for human health and disease

AMPK (AMP-activated protein kinase) is a phylogenetically conserved fuel-sensing enzyme that is present in all mammalian cells. During exercise, it is activated in skeletal muscle in humans, and at least in rodents, also in adipose tissue, liver and perhaps other organs by events that increase the AMP/ATP ratio. When activated, AMPK stimulates energy-generating processes such as glucose uptake and fatty acid oxidation and decreases energy-consuming processes such as protein and lipid synthesis. Exercise is perhaps the most powerful physiological activator of AMPK and a unique model for studying its many physiological roles. In addition, it improves the metabolic status of rodents with a metabolic syndrome phenotype, as does treatment with AMPK-activating agents; it is therefore tempting to attribute the therapeutic benefits of regular physical activity to activation of AMPK. Here we review the acute and chronic effects of exercise on AMPK activity in skeletal muscle and other tissues. We also discuss the potential role of AMPK activation in mediating the prevention and treatment by exercise of specific disorders associated with the metabolic syndrome, including Type 2 diabetes and Alzheimer's disease.

Dietary activators of Sirt1

Calorie restriction (CR) is a non-genetic manipulation that reliably results in extended lifespan of several species ranging from yeast to dogs. The lifespan extension effect of CR has been strongly associated with an increased level and activation of the silent information regulator 2 (Sir2) histone deacetylase and its mammalian ortholog Sirt1. This association led to the search for potential Sirt1-activating, life-extending molecules. This review briefly outlines the experimental findings on resveratrol and other dietary activators of Sirt1.

Zyxin is a novel interacting partner for SIRT1

BACKGROUND

SIRT1 is a mammalian homologue of NAD+-dependent deacetylase sirtuin family. It regulates longevity in several model organisms and is involved with cell survival, differentiation, metabolism among other processes in mammalian cells. SIRT1 modulates functions of various key targets via deacetylation. Recent studies have revealed SIRT1 protects neurons from axonal degeneration or neurodegeneration. Further, SIRT1 null mice exhibit growth retardation and developmental defects, suggesting its critical roles in neurons and development.

RESULTS

To identify novel binding partners for SIRT1 in the central nervous system, we performed yeast two-hybrid screening on human fetal brain cDNA library and found that zyxin is a possible binding partner. SIRT1 and zyxin transcript were both preferentially expressed in developmental mouse brain. Zyxin accumulates in the nucleus where it is co-localized with SIRT1 after treatment with leptomycin B in COS-7 cells. Furthermore, SIRT1 deacetylates zyxin, suggesting SIRT1 could interact with nuclear-accumulated zyxin and modulate its function through deacetylation.

CONCLUSION

Zyxin could be a novel interacting partner of SIRT1. Zyxin is an adaptor protein at focal adhesion plaque, regulating cytoskeletal dynamics and signal transduction to convey signal from the ECM (extracellular matrix) to the nucleus. Our results raise the possibility that SIRT1 regulates signal transmission from ECM to the nucleus by modulating the functions of zyxin via deacetylation.

Regulation of forkhead transcription factor FoxO3a contributes to calorie restriction-induced prevention of Alzheimer's disease-type amyloid neuropathology and spatial memory deterioration

Forkhead transcription factor FoxO3a, also known as DAF-16 in Caenorhabditis elegans, is a key regulator of the insulin receptor (IR)/insulin-like growth factor-I signaling pathway mediated extension of life span in worms and yeast. In this study, we report that calorie restriction (CR)-mediated activation of the IR signaling pathway leads to hyperphosphorylation of FoxO3a transcription factor and, consequently, its exclusion from the nucleus. This inactivation of FoxO3a activity is correlated with attenuation of Alzheimer's disease (AD)-type amyloid neuropathology and with preservation of spatial reference memory in the Tg2576 mouse model of AD. Further, in vitro studies reveal that exogenous expression of viral, triple-mutant, constitutively active FoxO3a resulting in increased nuclear FoxO3a activity in primary neuron cultures derived from Tg2576 mouse embryos, causally promotes AD amyloid-beta peptide (Abeta) levels by inhibiting nonamyloidogenic alpha-secretase activity, indicating the existence of an inverse correlation between FoxO3a activity and cerebral Abeta amyloidosis. Moreover, we report for the first time that the exclusion of the FoxO3a transcription factor from the nucleus in combination with inhibition of nuclear FoxO3a activity by SIRT1-mediated deacetylation in response to CR is a mechanism resulting in the repression of Rho-associated protein kinase-1 gene expression, thereby activating nonamyloidogenic alpha-secretase processing of the amyloid precursor protein and lowering Abeta generation. This study provides a novel metabolic pathway for prevention and/or treatment of AD.

Mitochondrial approaches for neuroprotection

A large body of evidence from postmortem brain tissue and genetic analysis in humans and biochemical and pathological studies in animal models (transgenic and toxin) of neurodegeneration suggest that mitochondrial dysfunction is a common pathological mechanism. Mitochondrial dysfunction from oxidative stress, mitochondrial DNA deletions, pathological mutations, altered mitochondrial morphology, and interaction of pathogenic proteins with mitochondria leads to neuronal demise. Therefore, therapeutic approaches targeting mitochondrial dysfunction and oxidative damage hold great promise in neurodegenerative diseases. This review discusses the potential therapeutic efficacy of creatine, coenzyme Q10, idebenone, synthetic triterpenoids, and mitochondrial targeted antioxidants (MitoQ) and peptides (SS-31) in in vitro studies and in animal models of Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis, and Alzheimer's disease. We have also reviewed the current status of clinical trials of creatine, coenzyme Q10, idebenone, and MitoQ in neurodegenerative disorders. Further, we discuss newly identified therapeutic targets, including peroxisome proliferator-activated receptor-gamma-coactivator and sirtuins, which provide promise for future therapeutic developments in neurodegenerative disorders.

Therapeutic potential of resveratrol in Alzheimer's disease

Several epidemiological studies indicate that moderate consumption of red wine is associated with a lower incidence of dementia and Alzheimer's disease. Red wine is enriched in antioxidant polyphenols with potential neuroprotective activities. Despite scepticism concerning the bioavailability of these polyphenols, in vivo data have clearly demonstrated the neuroprotective properties of the naturally occurring polyphenol resveratrol in rodent models for stress and diseases. Furthermore, recent work in cell cultures and animal models has shed light on the molecular mechanisms potentially involved in the beneficial effects of resveratrol intake against the neurodegenerative process in Alzheimer's disease.

Single sample extraction protocol for the quantification of NAD and NADH redox states in Saccharomyces cerevisiae

A robust redox extraction protocol for quantitative and reproducible metabolite isolation and recovery has been developed for simultaneous measurement of nicotinamide adenine dinucleotide (NAD) and its reduced form, NADH, from Saccharomyces cerevisiae. Following culture in liquid media, yeast cells were harvested by centrifugation and then lysed under nonoxidizing conditions by bead blasting in ice-cold, nitrogen-saturated 50 mM ammonium acetate. To enable protein denaturation, ice cold nitrogen-saturated CH(3)CN/50 mM ammonium acetate (3:1 v/v) was added to the cell lysates. Chloroform extractions were performed on supernatants to remove organic solvent. Samples were lyophilized and resuspended in 50 mM ammonium acetate. NAD and NADH were separated by HPLC and quantified using UV-Vis absorbance detection. NAD and NADH levels were evaluated in yeast grown under normal (2% glucose) and calorie restricted (0.5% glucose) conditions. Results demonstrate that it is possible to perform a single preparation to reliably and robustly quantitate both NAD and NADH contents in the same sample. Robustness of the protocol suggests it will be (i) applicable to quantification of these metabolites in other cell cultures; and (ii) amenable to isotope labeling strategies to determine the relative contribution of specific metabolic pathways to total NAD and NADH levels in cell cultures.

Inclusion-body myositis: muscle-fiber molecular pathology and possible pathogenic significance of its similarity to Alzheimer's and Parkinson's disease brains

Sporadic inclusion-body myositis (s-IBM), the most common muscle disease of older persons, is of unknown cause and lacks successful treatment. Here we summarize diagnostic criteria and discuss our current understanding of the steps in the pathogenic cascade. While it is agreed that both degeneration and mononuclear-cell inflammation are components of the s-IBM pathology, how each relates to the pathogenesis remains unsettled. We suggest that the intra-muscle-fiber degenerative component plays the primary role, leading to muscle-fiber destruction and clinical weakness, since anti-inflammatory treatments are not of sustained benefit. We discuss possible treatment strategies aimed toward ameliorating a degenerative component, for example, lithium and resveratrol. Also discussed are the intriguing phenotypic similarities between s-IBM muscle fibers and the brains of Alzheimer and Parkinson's diseases, the most common neurodegenerative diseases associated with aging. Similarities include, in the respective tissues, cellular aging, mitochondrial abnormalities, oxidative and endoplasmic-reticulum stresses, proteasome inhibition and multiprotein aggregates.

Evaluation of potential pro-survival pathways regulated by melatonin in a murine senescence model

We examined the effect of melatonin on pro-survival processes in three groups of mice. Untreated senescence-accelerated mice (SAMP8), melatonin-treated SAMP8 and untreated senescence-accelerated resistant mice (SAMR1) of 10 months old were studied. Melatonin (10 mg/kg) or vehicle (ethanol at 0.066%) was supplied in the drinking water from the end of the first month until the end of the ninth month of life. Differences in the Akt/Erk1-2 pathway and downstream targets were examined and no significant changes were observed, except for beta-catenin. However, sirtuin 1 expression was significantly lower in SAMP8 than in SAMR1. In addition, acetylated p53 and NFkappaB expression were lower in SAMP8 than in SAMR1. These changes were prevented by melatonin. Moreover, the concentration/expression of alpha-secretase was lower and that of amyloid beta aggregates (Abeta) was higher in untreated SAMP8 than in SAMR1. Likewise, the levels of Bid were higher, whereas Bcl-2(XL) levels were lower in SAMP8 than in SAMR1. Melatonin reduced all these changes. We conclude that melatonin improves pro-survival signals and reduces pro-death signals in age-related impairments of neural processes.

Caloric restriction and brain function

PURPOSE OF REVIEW

In addition to extending lifespan, animal research shows that specific diets benefit brain functioning. Indeed, it has been proven that caloric restriction prevents age-related neuronal damage. What are those mechanisms involved in the effects of caloric restriction on brain functioning? Could caloric restriction be proposed in the future to prevent or treat neurodegenerative disorders such as Alzheimer's disease? Is there a future for caloric restriction interventions in adults?

RECENT FINDINGS

Hypotheses linking caloric restriction to cognitive capability include anti-inflammatory mechanisms, reduction of neural oxidative stress, promotion of synaptic plasticity, induction of various stress and neurotrophic/neuroprotective factors. Caloric restriction may also prevent beta-amyloid neuropathology in Alzheimer transgenic models. Finally, both exercise and caloric restriction enhance neurogenesis via different mechanisms suggesting that their combination may decrease the risk of neurodegenerative disease.

SUMMARY

It is now well established that caloric restriction could be used to promote successful brain aging. Data from randomized controlled trials in humans are limited. No positive effect on cognitive impairment was found probably due to methodological limitations. The long-term effects of caloric restriction in adults must be clarified before engaging in such preventive strategy. Additional animal studies must be conducted in the future to test the effects of 'multidomain' interventions (caloric restriction plus regular exercise) on age-related cognitive decline.

Emerging cues mediating astroglia lineage restriction of progenitor cells in the injured/diseased adult CNS

Other than specific neurogenic regions, the adult central nervous system (CNS) is not conducive for neuronal regeneration and neurogenesis, particularly at sites of injury or neurodegeneration. Engraftment of neural stem/progenitor cells into non-neurogenic regions or sites of injury/disease invariably results mainly in astroglia differentiation. The reasons for such a lineage restriction have not been well defined. Recent findings have brought to light some underlying novel mechanistic basis for this preferential differentiation into astroglia. The more oxidized state of pathological brain tissue leads to upregulation of the protein deacetylase sirtuin 1 (Sirt1). Sirt1 appears to stabilize a co-repressor complex of Hairy/enhancer of split (Hes)1, thereby suppressing expression of the proneuronal transcription factor Mash1, and directs progenitor cell differentiation towards the glia lineage. Sirt1 upregulated by CNS inflammation may also inhibit neuronal differentiation. Myelin-associated inhibitors such as Nogo, acting through the Nogo-66 receptor (NgR), also appear to promote neural stem/progenitor cell differentiation into astrocytes. Understanding the molecular basis of glia lineage restriction of neural progenitors in the injured or diseased CNS would provide handles to improving the success of stem cell-based transplantation therapy.

Sirtuins--novel therapeutic targets to treat age-associated diseases

Sirtuins post-translationally modulate the function of many cellular proteins that undergo reversible acetylation-deacetylation cycles, affecting physiological responses that have implications for treating diseases of ageing. Potent small-molecule modulators of sirtuins have shown efficacy in preclinical models of metabolic, neurodegenerative and inflammatory diseases, and so hold promise for drug discovery efforts in multiple therapeutic areas. Here, we discuss current knowledge and data that strengthens sirtuins as a druggable set of enzymes for the treatment of age-associated diseases, including activation of SIRT1 in type 2 diabetes.

Brain SIRT1: anatomical distribution and regulation by energy availability

SIRT1 is a nicotinamide adenosine dinucleotide-dependent deacetylase that orchestrates key metabolic adaptations to nutrient deprivation in peripheral tissues. SIRT1 is induced also in the brain by reduced energy intake. However, very little is known about SIRT1 distribution and the biochemical phenotypes of SIRT1-expressing cells in the neuraxis. Unknown are also the brain sites in which SIRT1 is regulated by energy availability and whether these regulations are altered in a genetic model of obesity. To address these issues, we performed in situ hybridization histochemistry analyses and found that Sirt1 mRNA is highly expressed in metabolically relevant sites. These include, but are not limited to, the hypothalamic arcuate, ventromedial, dorsomedial, and paraventricular nuclei and the area postrema and the nucleus of the solitary tract in the hindbrain. Of note, our single-cell reverse transcription-PCR analyses revealed that Sirt1 mRNA is expressed in pro-opiomelanocortin neurons that are critical for normal body weight and glucose homeostasis. We also found that SIRT1 protein levels are restrictedly increased in the hypothalamus in the fasted brain. Of note, we found that this hypothalamic-specific, fasting-induced SIRT1 regulation is altered in leptin-deficient, obese mice. Collectively, our findings establish the distribution of Sirt1 mRNA throughout the neuraxis and suggest a previously unrecognized role of brain SIRT1 in regulating energy homeostasis.

Nicotinic acid, nicotinamide, and nicotinamide riboside: a molecular evaluation of NAD+ precursor vitamins in human nutrition

Although baseline requirements for nicotinamide adenine dinucleotide (NAD+) synthesis can be met either with dietary tryptophan or with less than 20 mg of daily niacin, which consists of nicotinic acid and/or nicotinamide, there is growing evidence that substantially greater rates of NAD+ synthesis may be beneficial to protect against neurological degeneration, Candida glabrata infection, and possibly to enhance reverse cholesterol transport. The distinct and tissue-specific biosynthetic and/or ligand activities of tryptophan, nicotinic acid, nicotinamide, and the newly identified NAD+ precursor, nicotinamide riboside, reviewed herein, are responsible for vitamin-specific effects and side effects. Because current data suggest that nicotinamide riboside may be the only vitamin precursor that supports neuronal NAD+ synthesis, we present prospects for human nicotinamide riboside supplementation and propose areas for future research.

The neuroprotective properties of calorie restriction, the ketogenic diet, and ketone bodies

Both calorie restriction and the ketogenic diet possess broad therapeutic potential in various clinical settings and in various animal models of neurological disease. Following calorie restriction or consumption of a ketogenic diet, there is notable improvement in mitochondrial function, a decrease in the expression of apoptotic and inflammatory mediators and an increase in the activity of neurotrophic factors. However, despite these intriguing observations, it is not yet clear which of these mechanisms account for the observed neuroprotective effects. Furthermore, limited compliance and concern for adverse effects hamper efforts at broader clinical application. Recent research aimed at identifying compounds that can reproduce, at least partially, the neuroprotective effects of the diets with less demanding changes to food intake suggests that ketone bodies might represent an appropriate candidate. Ketone bodies protect neurons against multiple types of neuronal injury and are associated with mitochondrial effects similar to those described during calorie restriction or ketogenic diet treatment. The present review summarizes the neuroprotective effects of calorie restriction, of the ketogenic diet and of ketone bodies, and compares their putative mechanisms of action.

The ups and downs of SIRT1

Reversible acetylation has emerged as a key post-translational modification of proteins. Although the number of acetylated proteins is rapidly growing, the ways in which protein acetyltransferases and deacetylases connect with extracellular stimuli remain unclear. Recently, a regulatory network has emerged that controls the expression and activity of SIRT1, a mammalian class-III protein deacetylase. SIRT1 is an important regulator of metabolism, senescence, cancer and, possibly, longevity and is connected with crucial stress-responsive signal-transduction pathways. These connections provide important clues about how protein acetylation and deacetylation mediate cellular adaptations to extrinsic stress.

The role of calorie restriction and SIRT1 in prion-mediated neurodegeneration

A central focus of aging research is to determine how calorie restriction (CR) extends lifespan and delays diseases of aging. SIRT1, the mammalian ortholog of Sir2 in yeast, is a longevity factor which mediates dietary restriction in diverse species. In addition, SIRT1 plays a protective role in several models of neurodegenerative disease. We tested the role of SIRT1 in mediating the effects of CR in a mouse model of prion disease. Prion diseases are protein misfolding disorders of the central nervous system with many similarities to other neurodegenerative diseases, including deposition of aggregated protein, gliosis, and loss of synapses and neurons. We report that the onset of prion disease is delayed by CR and in the SIRT1 KO mice fed ad libitum. CR exerts no further effect on the SIRT1 KO strain, suggesting the effects of CR and SIRT1 deletion are mechanistically coupled. In conjunction, SIRT1 is downregulated in certain brain regions of CR mice. The expression of PrP mRNA and protein is reduced in the brains of CR mice and in SIRT1 knockout mice, suggesting a possible mechanism for the delayed onset of disease, as PrP levels are a critical determinant of how quickly mice succumb to prion disease. Surprisingly, CR greatly shortens the duration of clinical symptoms of prion disease and ultimately shortens lifespan of prion-inoculated mice in a manner that is independent of SIRT1. Taken together, our results suggest a more complex interplay between CR, SIRT1, and neurodegenerative diseases than previously appreciated.

Molecular bases of caloric restriction regulation of neuronal synaptic plasticity

Aging is associated with the decline of cognitive properties. This situation is magnified when neurodegenerative processes associated with aging appear in human patients. Neuronal synaptic plasticity events underlie cognitive properties in the central nervous system. Caloric restriction (CR; either a decrease in food intake or an intermittent fasting diet) can extend life span and increase disease resistance. Recent studies have shown that CR can have profound effects on brain function and vulnerability to injury and disease. Moreover, CR can stimulate the production of new neurons from stem cells (neurogenesis) and can enhance synaptic plasticity, which modulate pain sensation, enhance cognitive function, and may increase the ability of the brain to resist aging. The beneficial effects of CR appear to be the result of a cellular stress response stimulating the production of proteins that enhance neuronal plasticity and resistance to oxidative and metabolic insults; they include neurotrophic factors, neurotransmitter receptors, protein chaperones, and mitochondrial biosynthesis regulators. In this review, we will present and discuss the effect of CR in synaptic processes underlying analgesia and cognitive improvement in healthy, sick, and aging animals. We will also discuss the possible role of mitochondrial biogenesis induced by CR in regulation of neuronal synaptic plasticity.

Metabolic syndrome and the role of dietary lifestyles in Alzheimer's disease

Since Alzheimer's disease (AD) has no cure or preventive treatment, an urgent need exists to find a means of preventing, delaying the onset, or reversing the course of the disease. Clinical and epidemiological evidence suggests that lifestyle factors, especially nutrition, may be crucial in controlling AD. Unhealthy lifestyle choices lead to an increasing incidence of obesity, dyslipidemia and hypertension--components of the metabolic syndrome. These disorders can also be linked to AD. Recent research supports the hypothesis that calorie intake, among other non-genetic factors, can influence the risk of clinical dementia. In animal studies, high calorie intake in the form of saturated fat promoted AD-type amyloidosis, while calorie restriction via reduced carbohydrate intake prevented it. Pending further study, it is prudent to recommend to those at risk for AD--e.g. with a family history or features of metabolic syndrome, such as obesity, insulin insensitivity, etc.--to avoid foods and beverages with added sugars; to eat whole, unrefined foods with natural fats, especially fish, nuts and seeds, olives and olive oil; and to minimize foods that disrupt insulin and blood sugar balance.

Mitochondrial biogenesis and healthy aging

Aging is associated with an overall loss of function at the level of the whole organism that has origins in cellular deterioration. Most cellular components, including mitochondria, require continuous recycling and regeneration throughout the lifespan. Mitochondria are particularly susceptive to damage over time as they are the major bioenergetic machinery and source of oxidative stress in cells. Effective control of mitochondrial biogenesis and turnover, therefore, becomes critical for the maintenance of energy production, the prevention of endogenous oxidative stress and the promotion of healthy aging. Multiple endogenous and exogenous factors regulate mitochondrial biogenesis through the peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1alpha). Activators of PGC-1alpha include nitric oxide, CREB and AMPK. Calorie restriction (CR) and resveratrol, a proposed CR mimetic, also increase mitochondrial biogenesis through activation of PGC-1alpha. Moderate exercise also mimics CR by inducing mitochondrial biogenesis. Negative regulators of PGC-1alpha such as RIP140 and 160MBP suppress mitochondrial biogenesis. Another mechanism involved in mitochondrial maintenance is mitochondrial fission/fusion and this process also involves an increasing number of regulatory proteins. Dysfunction of either biogenesis or fission/fusion of mitochondria is associated with diseases of the neuromuscular system and aging, and a greater understanding of the regulation of these processes should help us to ultimately control the aging process.

Sirt1 protects against high-fat diet-induced metabolic damage

The identification of new pharmacological approaches to effectively prevent, treat, and cure the metabolic syndrome is of crucial importance. Excessive exposure to dietary lipids causes inflammatory responses, deranges the homeostasis of cellular metabolism, and is believed to constitute a key initiator of the metabolic syndrome. Mammalian Sirt1 is a protein deacetylase that has been involved in resveratrol-mediated protection from high-fat diet-induced metabolic damage, but direct proof for the implication of Sirt1 has remained elusive. Here, we report that mice with moderate overexpression of Sirt1 under the control of its natural promoter exhibit fat mass gain similar to wild-type controls when exposed to a high-fat diet. Higher energy expenditure appears to be compensated by a parallel increase in food intake. Interestingly, transgenic Sirt1 mice under a high-fat diet show lower lipid-induced inflammation along with better glucose tolerance, and are almost entirely protected from hepatic steatosis. We present data indicating that such beneficial effects of Sirt1 are due to at least two mechanisms: induction of antioxidant proteins MnSOD and Nrf1, possibly via stimulation of PGC1alpha, and lower activation of proinflammatory cytokines, such as TNFalpha and IL-6, via down-modulation of NFkappaB activity. Together, these results provide direct proof of the protective potential of Sirt1 against the metabolic consequences of chronic exposure to a high-fat diet.

Mitochondrial medicine for aging and neurodegenerative diseases

Mitochondria are key cytoplasmic organelles, responsible for generating cellular energy, regulating intracellular calcium levels, altering the reduction-oxidation potential of cells, and regulating cell death. Increasing evidence suggests that mitochondria play a central role in aging and in neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis, and Freidriech ataxia. Further, several lines of evidence suggest that mitochondrial dysfunction is an early event in most late-onset neurodegenerative diseases. Biochemical and animal model studies of inherited neurodegenerative diseases have revealed that mutant proteins of these diseases are associated with mitochondria. Mutant proteins are reported to block the transport of nuclear-encoded mitochondrial proteins to mitochondria, interact with mitochondrial proteins and disrupt the electron transport chain, induce free radicals, cause mitochondrial dysfunction, and, ultimately, damage neurons. This article discusses critical issues of mitochondria causing dysfunction in aging and neurodegenerative diseases, and discusses the potential of developing mitochondrial medicine, particularly mitochondrially targeted antioxidants, to treat aging and neurodegenerative diseases.

SIRT1 and neuronal diseases

SIRT1 is the mammalian homologue of yeast silent information regulator (Sir)-2, a member of the sirtuin family of protein deacetylases which have gained much attention as mediators of lifespan extension in several model organisms. Induction of SIRT1 expression also attenuates neuronal degeneration and death in animal models of Alzheimer's disease and Huntington's disease. SIRT1 induction, either by sirtuin activators such as resveratrol, or metabolic conditioning associated with caloric restriction (CR), could be neuroprotective in several ways. It could promote the non-amyloidogenic cleavage of the amyloid precursor protein, enhance clearance of amyloid beta-peptides, and reduced neuronal damage through potential inhibition of neuroinflammatory signaling pathways. In addition, increased SIRT1 activity could alter neuronal transcription profiles to enhance anti-stress and anti-apoptotic gene activities, and has been proposed to underlie the inhibition of axonal degeneration in the Wallerian degeneration slow (Wld(s)) phenotype. As neuronal degeneration is a major pathophysiological aspect of human aging, understanding the mechanism of SIRT1 neuroprotection promises novel strategies in clinical intervention of neurodegenerative diseases.

Paths of convergence: sirtuins in aging and neurodegeneration

Members of the sirtuin family of protein deacetylases support and promote longevity in diverse organisms and can extend life span when upregulated. Sirtuin pathways also modulate fundamental mechanisms in aging-related neurodegenerative diseases, including protein aggregation, stress responses, mitochondrial homeostasis, and inflammatory processes. In this minireview, we will discuss how progress in understanding the neurobiology of sirtuins is shedding light on the pathogenesis of these devastating conditions. We will also examine the potential and challenges of targeting sirtuin pathways therapeutically.

Neurogenesis directed by Sirt1

Differentiation of neuronal stem cells into astrocytes or neurons is important in maintaining brain function. Oxidative stress and inflammation are now shown to bias differentiation toward astrocytes by modulating activity of the anti-ageing gene Sirt1. These findings link a longevity gene to the activity of neuronal stem cells and their response to stress.

ROCK, PAK, and Toll of synapses in Alzheimer's disease

Alzheimer's disease is a neurodegenerative disorder where the cognitive deficit is the hallmark symptom reflecting the progression of the disease. Synaptic dysfunction is a sensitive parameter of the AD pathology. Rho GTPases and the Rho kinases, ROCK1/2, and PAK1-3, are important regulators of synaptic plasticity, especially in maintaining the actin cytoskeleton of dendritic spines. Recent studies have revealed that beta-amyloid oligomers can inhibit PAK and stimulate ROCK-mediated signaling. Both of these effects enhance the disassembly of synaptic actin filaments and ultimately evoke synaptic loss. Brain tissue in AD recognizes the beta-amyloid peptide oligomers as foreign protein particles and mounts an inflammatory defense via Toll-like receptor (TLR) signaling which causes synaptic impairment. We will review here the dysfunction of ROCK, PAK, and Toll signaling associated with AD pathology. The protection of synapses in AD may provide new therapeutic approaches to combatting the cognitive impairment in AD.

Genetic study between SIRT1, PPARD, PGC-1alpha genes and Alzheimer's disease

Single nucleotide polymorphisms (SNPs) in three diabetes-related genes (SIRT1, PPARD, PGC-1alpha) were investigated with a case-control approach. To examine the genetic association of those genes with Alzheimer's disease (AD) risk, we used the TaqMan technique to genotype five SNP sites for SIRT1, six for PPARD and eight for the PGC-1alpha gene, in 326 Finnish AD cases and 463 controls and conducted a single allele and genotypic distribution comparison as well as estimated haplotype frequencies between cases and controls. No significant differences in AD risk were found in single SNP and haplotype analyses for any of the three genes between 326 cases and 463 controls. However, in a subgroup of women older than 65 years, the frequencies of three SNPs in the SIRT1 gene were significantly different between AD and controls. We conclude that there is no real association with SNPs available in the present study between SIRT1, PPARD or PGC-1alpha genes and AD risk in the Finnish population.

Therapeutic role of sirtuins in neurodegenerative disease

The sirtuins are a family of enzymes which control diverse and vital cellular functions, including metabolism and aging. Manipulations of sirtuin activities cause activation of anti-apoptotic, anti-inflammatory, anti-stress responses, and the modulation of an aggregation of proteins involved in neurodegenerative disorders. Recently, sirtuins were found to be disease-modifiers in various models of neurodegeneration. However, almost in all instances, the exact mechanisms of neuroprotection remain elusive. Nevertheless, the manipulation of sirtuin activities is appealing as a novel therapeutic strategy for the treatment of currently fatal human disorders such as Alzheimer's and Parkinson's diseases. Here, we review current data which support the putative therapeutic roles of sirtuin in aging and in neurodegenerative diseases and the feasibility of the development of sirtuin-based therapies.

Sirt1 protects the heart from aging and stress

The prevalence of heart diseases, such as coronary artery disease and congestive heart failure, increases with age. Optimal therapeutic interventions that antagonize aging may reduce the occurrence and mortality of adult heart diseases. We discuss here how molecular mechanisms mediating life span extension affect aging of the heart and its resistance to pathological insults. In particular, we review our recent findings obtained from transgenic mice with cardiac-specific overexpression of Sirt1, which demonstrated delayed aging and protection against oxidative stress in the heart. We propose that activation of known longevity mechanisms in the heart may represent a novel cardioprotection strategy against aging and certain types of cardiac stress, such as oxidative stress.