Sirtuins: double players in Huntington's disease

SIRTUINS MODULATORS COUNTERACT MITOCHONDRIAL DYSFUNCTION IN CELLULAR MODELS OF HYPOXIA: RELEVANCE TO SCHIZOPHRENIA

Schizophrenia (SZ) is a neurodevelopmental-associated disorder strongly related to environmental factors, such as hypoxia. Because there is no cure for SZ or any pharmacological approach that could revert hypoxia-induced cellular damages, we evaluated whether modulators of sirtuins could abrogate hypoxia-induced mitochondrial deregulation as a neuroprotective strategy. Firstly, astrocytes from control (Wistar) and Spontaneously Hypertensive Rats (SHR), a model of both SZ and neonatal hypoxia, were submitted to chemical hypoxia. Then, cells were exposed to different concentrations of Nicotinamide (NAM), Resveratrol (Resv), and Sirtinol (Sir) for 48hrs. Our data indicate that sirtuins modulation reduces cell death increasing the acetylation of histone 3. This outcome is related to the rescue of loss of mitochondrial membrane potential, changes in mitochondrial calcium buffering capacity, decreased O₂^-• levels and increased expression of metabolic regulators (Nrf-1 and Nfe2l2) and mitochondrial content. Such findings are relevant not only for hypoxia-associated conditions, named pre-eclampsia but also for SZ since prenatal hypoxia is a relevant environmental factor related to this burdensome neuropsychiatric disorder.

Sirtuins and redox signaling interplay in neurogenesis, neurodegenerative diseases, and neural cell reprogramming

Since the discovery of Neural Stem Cells (NSCs) there are still mechanism to be clarified, such as the role of mitochondrial metabolism in the regulation of endogenous adult neurogenesis and its implication in neurodegeneration. Although stem cells require glycolysis to maintain their stemness, they can perform oxidative phosphorylation and it is becoming more and more evident that mitochondria are central players, not only for ATP production but also for neuronal differentiation's steps regulation, through their ability to handle cellular redox state, intracellular signaling, epigenetic state of the cell, as well as the gut microbiota-brain axis, upon dietary influences. In this scenario, the 8-oxoguanine DNA glycosylase (OGG1) repair system would link mitochondrial DNA integrity to the modulation of neural differentiation. On the other side, there is an increasing interest in NSCs generation, from induced pluripotent stem cells, as a clinical model for neurodegenerative diseases (NDs), although this methodology still presents several drawbacks, mainly related to the reprogramming process. Indeed, high levels of reactive oxygen species (ROS), associated with telomere shortening, genomic instability, and defective mitochondrial dynamics, lead to pluripotency limitation and reprogramming efficiency's reduction. Moreover, while a physiological or moderate ROS increase serves as a signaling mechanism, to activate differentiation and suppress self-renewal, excessive oxidative stress is a common feature of NDs and aging. This ROS-dependent regulatory effect might be modulated by newly identified ROS suppressors, including the NAD⁺-dependent deacetylase enzymes family called Sirtuins (SIRTs). Recently, the importance of subcellular localization of NAD synthesis has been coupled to different roles for NAD in chromatin stability, DNA repair, circadian rhythms, and longevity. SIRTs have been described as involved in the control of both telomere's chromatin state and expression of nuclear gene involved in the regulation of mitochondrial gene expression, as well as in several NDs and aging. SIRTs are ubiquitously expressed in the mammalian brain, where they play important roles. In this review we summarize the current knowledge on how SIRTs-dependent modulation of mitochondrial metabolism could impact on neurogenesis and neurodegeneration, focusing mainly on ROS function and their role in SIRTs-mediated cell reprogramming and telomere protection.

Resveratrol-Mediated Regulation of Mitochondria Biogenesis-associated Pathways in Neurodegenerative Diseases: Molecular Insights and Potential Therapeutic Applications

Neurodegenerative disorders include different neurological conditions that affect nerve cells, causing the progressive loss of their functions and ultimately leading to loss of mobility, coordination, and mental functioning. The molecular mechanisms underpinning neurodegenerative disease pathogenesis are still unclear. Nonetheless, there is experimental evidence to demonstrate that the perturbation of mitochondrial function and dynamics play an essential role. In this context, mitochondrial biogenesis, the growth, and division of preexisting mitochondria, by controlling mitochondria number, plays a vital role in maintaining proper mitochondrial mass and function, thus ensuring efficient synaptic activity and brain function. Mitochondrial biogenesis is tightly associated with the control of cell division and variations in energy demand in response to extracellular stimuli; therefore, it may represent a promising therapeutic target for developing new curative approaches to prevent or counteract neurodegenerative disorders. Accordingly, several inducers of mitochondrial biogenesis have been proposed as pharmacological targets for treating diverse central nervous system conditions. The naturally occurring polyphenol resveratrol has been shown to promote mitochondrial biogenesis in various tissues, including the nervous tissue, and an ever-growing number of studies highlight its neurotherapeutic potential. Besides preventing cognitive impairment and neurodegeneration through its antioxidant and anti-inflammatory properties, resveratrol has been shown to be able to enhance mitochondria biogenesis by acting on its main effectors, including PGC-1α, SIRT1, AMPK, ERRs, TERT, TFAM, NRF-1 and NRF-2. This review aims to present and discuss the current findings concerning the impact of resveratrol on the machinery and main effectors modulating mitochondrial biogenesis in the context of neurodegenerative diseases.

Acetylation State of Lysine 14 of Histone H3.3 Affects Mutant Huntingtin Induced Pathogenesis

Huntington's Disease (HD) is a fatal neurodegenerative disorder caused by the expansion of a polyglutamine-coding CAG repeat in the Huntingtin gene. One of the main causes of neurodegeneration in HD is transcriptional dysregulation that, in part, is caused by the inhibition of histone acetyltransferase (HAT) enzymes. HD pathology can be alleviated by increasing the activity of specific HATs or by inhibiting histone deacetylase (HDAC) enzymes. To determine which histone's post-translational modifications (PTMs) might play crucial roles in HD pathology, we investigated the phenotype-modifying effects of PTM mimetic mutations of variant histone H3.3 in a Drosophila model of HD. Specifically, we studied the mutations (K→Q: acetylated; K→R: non-modified; and K→M: methylated) of lysine residues K9, K14, and K27 of transgenic H3.3. In the case of H3.3K14Q modification, we observed the amelioration of all tested phenotypes (viability, longevity, neurodegeneration, motor activity, and circadian rhythm defects), while H3.3K14R had the opposite effect. H3.3K14Q expression prevented the negative effects of reduced Gcn5 (a HAT acting on H3K14) on HD pathology, while it only partially hindered the positive effects of heterozygous Sirt1 (an HDAC acting on H3K14). Thus, we conclude that the Gcn5-dependent acetylation of H3.3K14 might be an important epigenetic contributor to HD pathology.

Role of Sirtuins in Physiology and Diseases of the Central Nervous System

Silent information regulators, sirtuins (SIRTs), are a family of enzymes which take part in major posttranslational modifications of proteins and contribute to multiple cellular processes, including metabolic and energetic transformations, as well as regulation of the cell cycle. Recently, SIRTs have gained increased attention as the object of research because of their multidirectional activity and possible role in the complex pathomechanisms underlying human diseases. The aim of this study was to review a current literature evidence of SIRTs' role in the physiology and pathology of the central nervous system (CNS). SIRTs have been demonstrated to be crucial players in the crosstalk between neuroinflammation, neurodegeneration, and metabolic alterations. The elucidation of SIRTs' role in the background of various CNS diseases offers a chance to define relevant markers of their progression and promising candidates for novel therapeutic targets. Possible diagnostic and therapeutic implications from SIRTs-related investigations are discussed, as well as their future directions and associated challenges.

Versatile role of sirtuins in metabolic disorders: From modulation of mitochondrial function to therapeutic interventions

Sirtuins (SIRT1-7) are distinct histone deacetylases (HDACs) whose activity is determined by cellular metabolic status andnicotinamide adenine dinucleotide (NAD⁺ ) levels. HDACs of class III are the members of the SIRT's protein family. SIRTs are the enzymes that modulate mitochondrial activity and energy metabolism. SIRTs have been linked to a number of clinical and physiological operations, such as energy responses to low-calorie availability, aging, stress resistance, inflammation, and apoptosis. Mammalian SIRT2 orthologs have been identified as SIRT1-7 that are found in several subcellular sections, including the cytoplasm (SIRT1, 2), mitochondrial matrix (SIRT3, 4, 5), and the core (SIRT1, 2, 6, 7). For their deacetylase or ADP-ribosyl transferase action, all SIRTs require NAD⁺ and are linked to cellular energy levels. Evolutionarily, SIRT1 is related to yeast's SIRT2 as well as received primary attention in the circulatory system. An endogenous protein, SIRT1 is involved in the development of heart failure and plays a key role in cell death and survival. SIRT2 downregulation protects against ischemic-reperfusion damage. Increase in human longevity is caused by an increase in SIRT3 expression. Cardiomyocytes are also protected by SIRT3 from oxidative damage and aging, as well as suppressing cardiac hypertrophy. SIRT4 and SIRT5 perform their roles in the heart. SIRT6 has also been linked to a reduction in heart hypertrophy. SIRT7 is known to be involved in the regulation of stress responses and apoptosis in the heart.

Multifaced role of protein deacetylase sirtuins in neurodegenerative disease

Sirtuins, a class III histone/protein deacetylase, is a central regulator of metabolic function and cellular stress response. This plays a pivotal role in the pathogenesis and progression of diseases such as cancer, neurodegeneration, metabolic syndromes, and cardiovascular disease. Sirtuins regulate biological and cellular processes, for instance, mitochondrial biogenesis, lipid and fatty acid oxidation, oxidative stress, gene transcriptional activity, apoptosis, inflammatory response, DNA repair mechanism, and autophagic cell degradation, which are known components for the progression of the neurodegenerative diseases (NDDs). Emerging evidence suggests that sirtuins are the useful molecular targets against NDDs like, Alzheimer's Disease (AD), Parkinson's Disease (PD), Huntington's Disease (HD), and Amyotrophic Lateral Sclerosis (ALS). However, the exact mechanism of neuroprotection mediated through sirtuins remains unsettled. The manipulation of sirtuins activity with its modulators, calorie restriction (CR), and micro RNAs (miR) is a novel therapeutic approach for the treatment of NDDs. Herein, we reviewed the current putative therapeutic role of sirtuins in regulating synaptic plasticity and cognitive functions, which are mediated through the different molecular phenomenon to prevent neurodegeneration. We also explained the implications of sirtuin modulators, and miR based therapies for the treatment of life-threatening NDDs.

DeepSIRT: A deep neural network for identification of sirtuin targets and their subcellular localizations

Sirtuins are a family of proteins that play a key role in regulating a wide range of cellular processes including DNA regulation, metabolism, aging/longevity, cell survival, apoptosis, and stress resistance. Sirtuins are protein deacetylases and include in the class III family of histone deacetylase enzymes (HDACs). The class III HDACs contains seven members of the sirtuin family from SIRT1 to SIRT7. The seven members of the sirtuin family have various substrates and are present in nearly all subcellular localizations including the nucleus, cytoplasm, and mitochondria. In this study, a deep neural network approach using one-dimensional Convolutional Neural Networks (CNN) was proposed to build a prediction model that can accurately identify the outcome of the sirtuin protein by targeting their subcellular localizations. Therefore, the function and localization of sirtuin targets were analyzed and annotated to compartmentalize into distinct subcellular localizations. We further reduced the sequence similarity between protein sequences and three feature extraction methods were applied in datasets. Finally, the proposed method has been tested and compared with various machine-learning algorithms. The proposed method is validated on two independent datasets and showed an average of up to 85.77 % sensitivity, 97.32 % specificity, and 0.82 MCC for seven members of the sirtuin family of proteins.

Sirtuin Acetylation and Deacetylation: a Complex Paradigm in Neurodegenerative Disease

Sirtuins are the class III of histone deacetylases that depend on nicotinamide adenine dinucleotide for their activity. Sirtuins can influence the progression of neurodegenerative disorders by switching between deacetylation and acetylation processes. Histone acetylation occurs when acetyl groups are added to lysine residues on the N-terminal part of histone proteins. Deacetylation, on the other hand, results in the removal of acetyl groups. Pharmacological modulation of sirtuin activity has been shown to influence various neurodegenerative disorders including Alzheimer's disease, Parkinson's disease, Huntington's disease, stroke, and amyotrophic lateral sclerosis. In this review, mechanistic perspective of sirtuins has been discussed in anti-inflammatory, antiapoptotic, and neuroprotective effects in various disorders. We have discussed the structure, neurobiology, and physiology of sirtuins in neurodegenerative disease. Recent preclinical and clinical studies and their outcome have also been elucidated. The aim of this review is to fill in the gaps in our understanding of sirtuins' role in histone acetylation and deacetylation in all neurodegenerative diseases. Here, we emphasized on reviewing all the studies carried out in various labs depicting the role of sirtuin modulators in neuroprotection and highlighted the ideas that can be considered for future perspectives. Taken together, sirtuins may serve as a promising therapeutic target for the treatment of neurodegenerative disorders.

Potential roles of natural products in the targeting of proteinopathic neurodegenerative diseases

Defective proteostasis is associated with the gradual accumulations of misfolded proteins and is a hallmark of many age-associated neurodegenerative diseases. In the aged brain, maintenance of the proteostasis network presents a substantial challenge, and its loss contributes to the onset and progression of neurological diseases associated with cognitive decline due to the generation of toxic protein aggregates, a process termed 'proteinopathy'. Emerging evidence suggests that reversing proteinopathies by boosting proteostasis might provide an effective means of preventing neurodegeneration. From this perspective, phytochemicals may play significant roles as potent modulators of the proteostasis network, as previous reports have suggested they can interact with various network components to modify pathologies and confer neuroprotection. This review focuses on some potent phytochemicals that directly or indirectly modulate the proteostasis network and on their possible molecular targets. In addition, we propose strategies for the natural product-based modulation of proteostasis machinery that target proteinopathies.

Autophagy regulation by acetylation-implications for neurodegenerative diseases

Posttranslational modifications of proteins, such as acetylation, are essential for the regulation of diverse physiological processes, including metabolism, development and aging. Autophagy is an evolutionarily conserved catabolic process that involves the highly regulated sequestration of intracytoplasmic contents in double-membrane vesicles called autophagosomes, which are subsequently degraded after fusing with lysosomes. The roles and mechanisms of acetylation in autophagy control have emerged only in the last few years. In this review, we describe key molecular mechanisms by which previously identified acetyltransferases and deacetylases regulate autophagy. We highlight how p300 acetyltransferase controls mTORC1 activity to regulate autophagy under starvation and refeeding conditions in many cell types. Finally, we discuss how altered acetylation may impact various neurodegenerative diseases in which many of the causative proteins are autophagy substrates. These studies highlight some of the complexities that may need to be considered by anyone aiming to perturb acetylation under these conditions.

SIRT1 and SIRT2 Activity Control in Neurodegenerative Diseases

Sirtuins are NAD+ dependent histone deacetylases (HDAC) that play a pivotal role in neuroprotection and cellular senescence. SIRT1-7 are different homologs from sirtuins. They play a prominent role in many aspects of physiology and regulate crucial proteins. Modulation of sirtuins can thus be utilized as a therapeutic target for metabolic disorders. Neurological diseases have distinct clinical manifestations but are mainly age-associated and due to loss of protein homeostasis. Sirtuins mediate several life extension pathways and brain functions that may allow therapeutic intervention for age-related diseases. There is compelling evidence to support the fact that SIRT1 and SIRT2 are shuttled between the nucleus and cytoplasm and perform context-dependent functions in neurodegenerative diseases including Alzheimer's disease (AD), Parkinson's disease (PD), and Huntington's disease (HD). In this review, we highlight the regulation of SIRT1 and SIRT2 in various neurological diseases. This study explores the various modulators that regulate the activity of SIRT1 and SIRT2, which may further assist in the treatment of neurodegenerative disease. Moreover, we analyze the structure and function of various small molecules that have potential significance in modulating sirtuins, as well as the technologies that advance the targeted therapy of neurodegenerative disease.

Cerebellar Predominant Increase in mRNA Expression Levels of Sirt1 and Sirt3 Isoforms in a Transgenic Mouse Model of Huntington's Disease

The potential role of Sirt1 and Sirt2 subtypes of Sirtuins (class III NAD⁺-dependent deacetylases) in the pathogenesis of Huntington’s disease (HD) has been extensively studied yielding some controversial results. However, data regarding the involvement of Sirt3 and their variants in HD are considerably limited. The aim of this study was to assess the expression pattern of Sirt1 and three Sirt3 mRNA isoforms (Sirt3-M1/2/3) in the striatum, cortex and cerebellum in respect of the effect of gender, age and the presence of the transgene using the N171-82Q transgenic mouse model of HD. Striatal, cortical and cerebellar Sirt1-Fl and Sirt3-M1/2/3 mRNA levels were measured in 8, 12 and 16 weeks old N171-82Q transgenic mice and in their wild-type littermates. Regarding the striatum and cortex, the presence of the transgene resulted in a significant increase in Sirt3-M3 and Sirt1 mRNA levels, respectively, whereas in case of the cerebellum the transgene resulted in increased expression of all the assessed subtypes and isoforms. Aging exerted minor influence on Sirt mRNA expression levels, both in transgene carriers and in their wild-type littermates, and there was no interaction between the presence of the transgene and aging. Furthermore, there was no difference between genders. The unequivocal cerebellar Sirtuin activation with presumed compensatory role suggests that the cerebellum might be another key player in HD in addition to the most severely affected striatum. The mitochondrially acting Sirt3 may serve as an interesting novel therapeutic target in this deleterious condition.

Analysis of the Circadian Regulation of Cancer Hallmarks by a Cross-Platform Study of Colorectal Cancer Time-Series Data Reveals an Association with Genes Involved in Huntington's Disease

Accumulating evidence points to a link between circadian clock dysfunction and the molecular events that drive tumorigenesis. Here, we investigated the connection between the circadian clock and the hallmarks of cancer in an in vitro model of colorectal cancer (CRC). We used a cross-platform data normalization method to concatenate and compare available microarray and RNA-sequencing time series data of CRC cell lines derived from the same patient at different disease stages. Our data analysis suggests differential regulation of molecular pathways between the CRC cells and identifies several of the circadian and likely clock-controlled genes (CCGs) as cancer hallmarks and circadian drug targets. Notably, we found links of the CCGs to Huntington’s disease (HD) in the metastasis-derived cells. We then investigated the impact of perturbations of our candidate genes in a cohort of 439 patients with colon adenocarcinoma retrieved from the Cancer Genome Atlas (TCGA). The analysis revealed a correlation of the differential expression levels of the candidate genes with the survival of patients. Thus, our study provides a bioinformatics workflow that allows for a comprehensive analysis of circadian properties at different stages of colorectal cancer, and identifies a new association between cancer and HD.

Increased plasma melatonin in presymptomatic Huntington disease sheep (Ovis aries): Compensatory neuroprotection in a neurodegenerative disease?

Melatonin is a pleiotrophic hormone, synthesised primarily by the pineal gland under the control of the suprachiasmatic nuclei (SCN). It not only provides a hormonal signal of darkness but also has neuroprotective properties. Huntington's disease (HD) is a progressive neurodegenerative disorder characterised by abnormal motor, cognitive and psychiatric symptoms. There is growing evidence, particularly from animal models, that circadian rhythms may also be disturbed in HD. We measured two circadian-regulated hormones, melatonin and cortisol, in plasma samples collected around-the-clock from normal and presymptomatic transgenic HD sheep (Ovis aries) at 5 and 7 years of age, to assess SCN-driven rhythms and the effect of genotype, sex and age. Melatonin-related precursors and metabolites (tryptophan, serotonin, kynurenine) were also measured by liquid chromatography (LC)-mass spectrometry (MS). At 5 years of age in both rams and ewes, plasma melatonin levels were significantly elevated in HD sheep. In ewes measured 2 years later, there was still a significant elevation of nocturnal melatonin. Furthermore, the daytime baseline levels of melatonin were significantly higher in HD sheep. Since increased melatonin could have global beneficial effects on brain function, we suggest that the increased melatonin measured in presymptomatic HD sheep is part of an autoprotective response to mutant huntingtin toxicity that may account, at least in part, for the late onset of disease that characterises HD.

PPAR-γ promotes p38 MAP kinase-mediated endothelial cell permeability through activating Sirt3

Background

Ischemia-reperfusion (I/R)-induced vascular dysfunction is the main factor to acute ischemic stroke. Sirt3 is one of the sirtuin family members, which plays an important role in the development of neurological diseases.

Methods

In this study, we constructed I/R injury model on HBMEC cells and induced the overexpression of Sirt3 in model cells. Meanwhile, the p38 activator U-46619 was used to examine the connection between Sirt3 and p38. We also examined the level of endothelial associated proteins, including occluding, ZO-1 and claudin-4 by using qRT-PCR and western blot.

Results

Our findings indicated that overexpression of Sirt3 decreased the permeability of model cells and promoted in the growth of endothelial cells. However, the activation of p38 could antagonize the function of Sirt3 in HBMEC cells. Moreover, Our results indicated a positive correlation between Sirt3 and inter-endothelial junction proteins. Importantly, PPAR-γ agonist and inhibitor were utilized to investigate the role of PPAR-γ in Sirt3 mediated cell function. Sirt3 was targeted by PPAR-γ in model cells.

Conclusions

Taken together, this research not only demonstrated PPAR-γ might benefit to the growth of endothelial cell though activating Sirt3 but also indicated its potential value in the treatment for ischemic stroke.

Resveratrol attenuates myocardial hypoxia/reoxygenation-induced cell apoptosis through DJ-1-mediated SIRT1-p53 pathway

Resveratrol, a multi-functional phytoalexin, has been well indicated to exert cardioprotective effects by weakening ischemia/reperfusion (I/R) injury, and cell apoptosis is a vital way in I/R injury. SIRT1-p53 pathway has strong significance in regulating cell apoptosis. DJ-1 can directly bind to SIRT1 and stimulate the activity of SIRT1-p53. Therefore, the current study was determined whether Resveratrol attenuates hypoxia/reoxygenation (H/R)-induced cell apoptosis, and whether DJ-1-mediated SIRT1 activation involves in the cardioprotective effects of Resveratrol. The results showed that remarkable decrease in the number of apoptotic cells along with reduction of lactate dehydrogenase release and restoration of cell viability emerged when Resveratrol was applied in the H9c2 cells exposed to H/R. Moreover, Resveratrol increased DJ-1 expression and promoted the interaction of DJ-1 with SIRT1, which further contributed to subsequent restoration of SIRT1 activity and decrease of acetylation level of p53. However, above cardioprotective effects of Resveratrol were abrogated by DJ-1 siRNA and SIRT1 specific inhibitor Sirtinol. In conclusion, the current study demonstrated that Resveratrol suppressed H/R-induced cell apoptosis, which may be conducted by up-regulating DJ-1, and later activating SIRT1 activity and subsequently inhibiting p53 acetylation level in the H9c2 cells.

Catalytic-independent neuroprotection by SIRT1 is mediated through interaction with HDAC1

SIRT1, a NAD+-dependent deacetylase, protects neurons in a variety of in vitro and in vivo models of neurodegenerative disease. We have previously described a neuroprotective effect by SIRT1 independent of its catalytic activity. To confirm this conclusion we tested a panel of SIRT1 deletion mutant constructs, designated Δ1–Δ10, in cerebellar granule neurons induced to undergo apoptosis by low potassium treatment. We find that deletions of its N-terminal, those lacking portions of the catalytic domain, as well as one that lacks the ESA (Essential for SIRT1 Activity) motif, are as protective as wild-type SIRT1. In contrast, deletion of the region spanning residues 542–609, construct Δ8, substantially reduced the neuroprotective activity of SIRT1. As observed with LK-induced apoptosis, all SIRT1 constructs except Δ8 protect neurons against mutant huntingtin toxicity. Although its own catalytic activity is not required, neuroprotection by SIRT1 is abolished by inhibitors of Class I HDACs as well as by knockdown of endogenous HDAC1. We find that SIRT1 interacts with HDAC1 and this interaction is greatly increased by deleting regions of SIRT1 necessary for its catalytic activity. However, SIRT1-mediated protection is not dependent on HDAC1 deacetylase activity. Although other studies have described that catalytic activity of SIRT1 mediates is neuroprotective effect, our study suggests that in cerebellar granule neurons its deacetylase activity is not important and that HDAC1 contributes to the neuroprotective effect of SIRT1.

Investigating the role of Sirtuins in cell reprogramming

Cell reprogramming has been considered a powerful technique in the regenerative medicine field. In addition to diverse its strengths, cell reprogramming technology also has several drawbacks generated during the process of reprogramming. Telomere shortening caused by the cell reprogramming process impedes the efficiency of cell reprogramming. Transcription factors used for reprogramming alter genomic contents and result in genetic mutations. Additionally, defective mitochondria functioning such as excessive mitochondrial fission leads to the limitation of pluripotency and ultimately reduces the efficiency of reprogramming. These problems including genomic instability and impaired mitochondrial dynamics should be resolved to apply cell reprograming in clinical research and to address efficiency and safety concerns. Sirtuin (NAD+-dependent histone deacetylase) has been known to control the chromatin state of the telomere and influence mitochondria function in cells. Recently, several studies reported that Sirtuins could control for genomic instability in cell reprogramming. Here, we review recent findings regarding the role of Sirtuins in cell reprogramming. And we propose that the manipulation of Sirtuins may improve defects that result from the steps of cell reprogramming.

New developments in investigational HDAC inhibitors for the potential multimodal treatment of cachexia

INTRODUCTION

Cachexia is a frequent feature of chronic diseases. This syndrome includes loss of body weight, depletion of skeletal muscle mass and altered metabolic homeostasis. Acceleration of protein and energy metabolism, impaired myogenesis, and systemic inflammation contribute to cachexia. Its occurrence impinges on treatment tolerance and on the quality of life of the patient, however, no effective therapy is available yet.

AREAS COVERED

This review focuses on the use of histone deacetylase inhibitors as pharmacological tools to prevent or delay cachexia, with reference to muscle wasting.

EXPERT OPINION

Novel histone deacetylase inhibitors could be considered as exercise mimetics and this supports their use as a treatment for muscle-wasting associated diseases, such as cachexia. The ability of some of these inhibitors to modulate the release of extracellular vesicles from tumor cells is a potential tool for restricting the development of cancer-induced muscle protein depletion. There are few clinical trials that are testing histone deacetylase inhibitors as a treatment for cachexia; this reflects the lack of robust experimental evidence of effectiveness. The determination of the pathogenic mechanisms of muscle wasting and the identification of suitable histone deacetylase inhibitors that target such mechanisms are necessary.

SIRT1 is increased in affected brain regions and hypothalamic metabolic pathways are altered in Huntington disease

AIMS

Metabolic dysfunction is involved in modulating the disease process in Huntington disease (HD) but the underlying mechanisms are not known. The aim of this study was to investigate if the metabolic regulators sirtuins are affected in HD.

METHODS

Quantitative real-time polymerase chain reactions were used to assess levels of SIRT1-3 and downstream targets in post mortem brain tissue from HD patients and control cases as well as after selective hypothalamic expression of mutant huntingtin (HTT) using recombinant adeno-associated viral vectors in mice.

RESULTS

We show that mRNA levels of the metabolic regulator SIRT1 are increased in the striatum and the cerebral cortex but not in the less affected cerebellum in post mortem HD brains. Levels of SIRT2 are only increased in the striatum and SIRT3 is not affected in HD. Interestingly, mRNA levels of SIRT1 are selectively increased in the lateral hypothalamic area (LHA) and ventromedial hypothalamus (VMH) in HD. Further analyses of the LHA and VMH confirmed pathological changes in these regions including effects on SIRT1 downstream targets and reduced mRNA levels of orexin (hypocretin), prodynorphin and melanin-concentrating hormone (MCH) in the LHA and of brain-derived neurotrophic factor (BDNF) in the VMH. Analyses after selective hypothalamic expression of mutant HTT suggest that effects on BDNF, orexin, dynorphin and MCH are early and direct, whereas changes in SIRT1 require more widespread expression of mutant HTT.

CONCLUSIONS

We show that SIRT1 expression is increased in HD-affected brain regions and that metabolic pathways are altered in the HD hypothalamus.

Molecular Imaging of Sirtuin1 Expression-Activity in Rat Brain Using Positron-Emission Tomography-Magnetic-Resonance Imaging with [18F]-2-Fluorobenzoylaminohexanoicanilide

Sirtuin 1 (SIRT1) is a class III histone deacetylase that plays significant roles in the regulation of lifespan, metabolism, memory, and circadian rhythms and in the mechanisms of many diseases. However, methods of monitoring the pharmacodynamics of SIRT1-targeted drugs are limited to blood sampling because of the invasive nature of biopsies. For the noninvasive monitoring of the spatial and temporal dynamics of SIRT1 expression-activity in vivo by PET-CT-MRI, we developed a novel substrate-type radiotracer, [¹⁸F]-2-fluorobenzoylaminohexanoicanilide (2-[¹⁸F]BzAHA). PET-CT-MRI studies in rats demonstrated increased accumulation of 2-[¹⁸F]BzAHA-derived radioactivity in the hypothalamus, hippocampus, nucleus accumbens, and locus coeruleus, consistent with autoradiographic and immunofluorescent (IMF) analyses of brain-tissue sections. Pretreatment with the SIRT1 specific inhibitor, EX-527 (5 mg/kg, ip), resulted in about a 20% reduction of 2-[¹⁸F]BzAHA-derived-radioactivity accumulation in these structures. In vivo imaging of SIRT1 expression-activity should facilitate studies that improve the understanding of SIRT1-mediated regulation in the brain and aid in the development and clinical translation of SIRT1-targeted therapies.

Sirt3 deficiency impairs neurovascular recovery in ischemic stroke

AIMS

Sirt3 is one member of the NAD⁺ -dependent protein deacetylase family and plays crucial roles in diverse aspects of mammalian biological function. Then the role of Sirt3 on ischemia stroke is unknown.

METHODS

To examine the effect of Sirt3 on ischemic stroke, we performed transient middle cerebral artery occlusion (tMCAO) in adult male Sirt3 knockout (KO) and wild-type (WT) mice.

RESULTS

The level of Sirt3 in infarct region is decreased after ischemic stroke. In addition, we found that Sirt3 KO mice showed worse neurobehavioral outcome compared with WT mice, accompanied by decreased neurogenesis and angiogenesis as shown by the reduction in number of DCX⁺ /BrdU⁺ cells, NeuN⁺ /BrdU⁺ cells, and CD31⁺ /BrdU⁺ cells in the perifocal region during recovery phase after ischemic stroke. Furthermore, Sirt3 deficiency reduced the activation of vascular endothelial growth factor (VEGF), AKT, and extracellular signal-regulated kinases (ERK) signaling pathways.

CONCLUSION

Our results indicated that Sirt3 is beneficial to neurovascular and functional recovery following chronic ischemic stroke.

Perturbations in the p53/miR-34a/SIRT1 pathway in the R6/2 Huntington's disease model

The three factors, p53, the microRNA-34 family and Sirtuin 1 (SIRT1), interact in a positive feedback loop involved in cell cycle progression, cellular senescence and apoptosis. Each factor in this triad has roles in metabolic regulation, maintenance of mitochondrial function, and regulation of brain-derived neurotrophic factor (BDNF). Thus, this regulatory network holds potential importance for the pathophysiology of Huntington's disease (HD), an inherited neurodegenerative disorder in which both mitochondrial dysfunction and impaired neurotrophic signalling are observed. We investigated expression of the three members of this regulatory triad in the R6/2 HD mouse model. Compared to wild-type littermates, we found decreased levels of miR-34a-5p, increased SIRT1 mRNA and protein levels, and increased levels of p53 protein in brain tissue from R6/2 mice. The upregulation of SIRT1 did not appear to lead to an increased activity of the enzyme, as based on measures of p53 acetylation. In other words, the observed changes did not reflect the known interactions between these factors, indicating a general perturbation of the p53, miR-34a and SIRT1 pathway in HD. This is the first study investigating the entire triad during disease progression in an HD model. Given the importance of these three factors alone and within the triad, our results indicate that outside factors are regulating - or dysregulating - this pathway in HD.

Sirtuin 6 protects the brain from cerebral ischemia/reperfusion injury through NRF2 activation

Sirtuin 6 (SIRT6), a member of the sirtuin family of NAD(+)-dependent deacetylases, has been shown to produce beneficial effects in myocardial ischemia/reperfusion (I/R). However, the role of SIRT6 in cerebral I/R is largely unclear. In this study, we investigated the effects of SIRT6 overexpression in regulating I/R injury in a mouse cerebral I/R model in vivo and in oxygen-glucose-deprivation/reoxygenation (OGD/R)-stimulated neuro-2a neuroblastoma cells in vitro. We found that cerebral I/R (1 h/24 h) resulted in decreased SIRT6 expression in the cerebral cortex (P < 0.01). SIRT6 overexpression in the brain by in vivo gene transfer enhanced the antioxidant NRF2 signaling (P < 0.05), reduced oxidative stress (P < 0.05), and attenuated cerebral I/R-induced brain tissue damage and neurological deficits (P < 0.05). These neuroprotective effects of SIRT6 overexpression were abolished in NRF2 knockout mice. In neuro-2A neuroblastoma cells, SIRT6 overexpression increased total and nuclear NRF2 levels (P < 0.05), reduced oxidative stress (P < 0.05), and attenuated OGD/R-induced cell death (P < 0.05); these protective effects were blocked by NRF2 knockdown (P < 0.05). Moreover, in OGD/R-stimulated neuro-2A cells, SIRT6 overexpression produced similar protective effects to those induced by the antioxidant NAC, but no added benefits were detected when SIRT6 overexpression was used in combination with NAC (P > 0.05). These findings provide evidence that SIRT6 can protect the brain from cerebral I/R injury by suppressing oxidative stress via NRF2 activation. Thus, SIRT6 may serve as a potential therapeutic target for ischemic stroke.

Huntington's Disease-Update on Treatments

Huntington's disease (HD) is an autosomal dominantly inherited neurodegenerative disease characterized by progressive motor, behavioral, and cognitive decline, ending in death. Despite the discovery of the underlying genetic mutation more than 20 years ago, treatment remains focused on symptomatic management. Chorea, the most recognizable symptom, responds to medication that reduces dopaminergic neurotransmission. Psychiatric symptoms such as depression and anxiety may also respond well to symptomatic therapies. Unfortunately, many other symptoms do not respond to current treatments. Furthermore, high-quality evidence for treatment of HD in general remains limited. To date, there has been minimal success with identifying a disease-modifying therapy based upon molecular models. However, one of the emerging gene silencing techniques may provide a breakthrough in treating this devastating disease.

Reversible modulation of SIRT1 activity in a mouse strain

The SIRT1 protein deacetylase is reported to have a remarkably wide spectrum of biological functions affecting such varied processes as aging, cancer, metabolism, neurodegeneration and immunity. However, the SIRT1 literature is also full of contradictions. To help establish the role(s) of SIRT1 in these and other biological processes, we set out to create a mouse in which the SIRT1 activity could be toggled between on and off states by fusing the estrogen receptor ligand-binding domain (ER) to the C terminus of the SIRT1 protein. We found that the catalytic activity of the SIRT1-ER fusion protein increased 4–5 fold in cells treated with its ligand, 4-hydroxy-tamoxifen (4OHT). The 4OHT-induced activation of SIRT1-ER was due in large part to a 2 to 4-fold increase in abundance of the SIRT1-ER protein in cells in culture and in tissues in vivo. This increase is reversible and is a consequence of 4OHT-induced stabilization of the SIRT1-ER protein. Since changes in SIRT1 level or activity of 2–4 fold are frequently reported to be sufficient to affect its biological functions, this mouse should be helpful in establishing the causal relationships between SIRT1 and the diseases and processes it affects.

Function of the SIRT3 mitochondrial deacetylase in cellular physiology, cancer, and neurodegenerative disease

In mammals, seven members of the sirtuin protein family known as class III histone deacetylase have been identified for their characteristic features. These distinguished characteristics include the tissues where they are distributed or located, enzymatic activities, molecular functions, and involvement in diseases. Among the sirtuin members, SIRT3 has received much attention for its role in cancer genetics, aging, neurodegenerative disease, and stress resistance. SIRT3 controls energy demand during stress conditions such as fasting and exercise as well as metabolism through the deacetylation and acetylation of mitochondrial enzymes. SIRT3 is well known for its ability to eliminate reactive oxygen species and to prevent the development of cancerous cells or apoptosis. This review article provides a comprehensive review on numerous (noteworthy) molecular functions of SIRT3 and its effect on cancer cells and various diseases including Huntington's disease, amyotrophic lateral sclerosis, and Alzheimer's disease.

Mechanisms underlying neurodegeneration in Huntington disease: applications to novel disease-modifying therapies

The CAG repeat expansion mutation that causes Huntington Disease (HD) was discovered more than 20 years ago, yet no treatment has yet been developed to stop the relentless course of the disease. Nonetheless, substantial progress has been made in understanding HD pathogenesis. We review insights that have been gleaned from HD genetics, metabolism, and pathology; HD mouse and cell models; the structure, function and post-translational modification of normal and mutant huntingtin (htt) protein; gene expression profiles in HD cells and tissue; the neurotoxicy of mutant htt RNA; and the expression of an antisense transcript from the HD locus. We conclude that rationale therapeutics for HD is within sight, though many questions remain to be answered.

Investigational agents for the management of Huntington's disease

INTRODUCTION

An inherited, chronic progressive, neurodegenerative disorder is Huntington's disease, characterized by motor, cognitive, and psychiatric symptoms. Predictive genetic testing allows earlier diagnosis and identification of gene carriers for Huntington's disease. These individuals are ideal candidates for testing of therapeutic interventions for disease modification. Areas covered: According to queries in Pubmed, Embase and clinical register databases, research and clinical studies emerge on symptomatic and neuroprotective therapies in Huntington's disease. This review discusses novel agents for symptomatic therapy and disease modification. They are currently in phase I and II of drug development Expert opinion: There are promising, safe and well tolerated compounds for amelioration of motor and neuropsychiatric symptoms, but their efficacy still needs to be proven in clinical trials. Deterioration of mutant huntingtin expression, antiapoptotic or cell death inhibition as disease modifying concepts was efficacious in models of Huntington's disease. However, the risk for clinical trial failures is high not only due to ineffectiveness of the tested agent. Negative study outcomes may also result from design misconceptions, underestimation of the heterogeneity of Huntington's disease, too short study durations and too small study cohorts.

Sirtuins and Their Roles in Brain Aging and Neurodegenerative Disorders

Sirtuins (SIRT1-SIRT7) are unique histone deacetylases (HDACs) whose activity depends on NAD⁺ levels and thus on the cellular metabolic status. SIRTs regulate energy metabolism and mitochondrial function. They orchestrate the stress response and damage repair. Through these functions sirtuins modulate the course of aging and affect neurodegenerative diseases. SIRTSs interact with multiple signaling proteins, transcription factors (TFs) and poly(ADP-ribose) polymerases (PARPs) another class of NAD⁺-dependent post-translational protein modifiers. The cross-talk between SIRTs TFs and PARPs is a highly promising research target in a number of brain pathologies. This review describes updated results on sirtuins in brain aging/neurodegeneration. It focuses on SIRT1 but also on the roles of mitochondrial SIRTs (SIRT3, 4, 5) and on SIRT6 and SIRT2 localized in the nucleus and in cytosol, respectively. The involvement of SIRTs in regulation of insulin-like growth factor signaling in the brain during aging and in Alzheimer's disease was also focused. Moreover, we analyze the mechanism(s) and potential significance of interactions between SIRTs and several TFs in the regulation of cell survival and death. A critical view is given on the application of SIRT activators/modulators in therapy of neurodegenerative diseases.

DJ-1 activates SIRT1 through its direct binding to SIRT1

The DJ-1 gene is a ras-dependent oncogene and also a causative gene for a familial form of Parkinson's disease park7. DJ-1 is a multi-functional protein and plays roles in regulation of cell growth, cells death, metabolism and mitochondrial homeostasis against oxidative stress. To explore various functions, DJ-1 associates with a number of proteins localized in the nucleus, cytoplasm and mitochondria. The oxidative status of a cysteine residue at an amino acid number 106 (C106) of DJ-1 determines the active level of DJ-1. Precise molecular mechanism of exploration of DJ-1 function is, however, not resolved. In this study, we identified Sirtuin family proteins (SIRT1, 2, and 4-6) as DJ-1-binding proteins, and DJ-1 associated with SIRT1 in cells. Sirtuins like DJ-1 also regulates growth, death and metabolism of cells and mitochondrial homeostasis. We found that DJ-1 stimulated deacetylase activity of SIRT1 and that SIRT1-suppressed transcriptional activity of SIRT1-target p53 was further decreased by DJ-1. Furthermore, SIRT1 activity was reduced in DJ-1-knockout cells, and this reduced activity was restored by re-introduction of wild-type DJ-1 but not of C106-mutant DJ-1 into DJ-1-knockout cells. It is first report showing direct connection of DJ-1 with SIRT1.

Novel investigational drugs mimicking exercise for the treatment of cachexia

INTRODUCTION

Cachexia is a syndrome characterized by body weight loss, muscle wasting and metabolic abnormalities, that frequently complicates the management of people affected by chronic diseases. No effective therapy is actually available, although several drugs are under clinical evaluation. Altered energy metabolism markedly contributes to the pathogenesis of cachexia; it can be improved by exercise, which is able to both induce anabolism and inhibit catabolism.

AREAS COVERED

This review focuses on exercise mimetics and their potential inclusion in combined protocols to treat cachexia. The authors pay with particular reference to the cancer-associated cachexia.

EXPERT OPINION

Even though exercise improves muscle phenotype, most patients retain sedentary habits which are quite difficult to disrupt. Moreover, they frequently present with chronic fatigue and comorbidities that reduce exercise tolerance. For these reasons, drugs mimicking exercise could be beneficial to those who are unable to comply with the practice of physical activity. Since some exercise mimetics may exert serious side effects, further investigations should focus on treatments which maintain their effectiveness on muscle phenotype while remaining tolerable at the same time.