Overexpression of SIRT1 in mouse forebrain impairs lipid/glucose metabolism and motor function

PD-L1 signaling in reactive astrocytes counteracts neuroinflammation and ameliorates neuronal damage after traumatic brain injury

BACKGROUND

Tissue damage and cellular destruction are the major events in traumatic brain injury (TBI), which trigger sterile neuroimmune and neuroinflammatory responses in the brain. While appropriate acute and transient neuroimmune and neuroinflammatory responses facilitate the repair and adaptation of injured brain tissues, prolonged and excessive neuroimmune and neuroinflammatory responses exacerbate brain damage. The mechanisms that control the intensity and duration of neuroimmune and neuroinflammatory responses in TBI largely remain elusive.

METHODS

We used the controlled cortical impact (CCI) model of TBI to study the role of immune checkpoints (ICPs), key regulators of immune homeostasis, in the regulation of neuroimmune and neuroinflammatory responses in the brain in vivo.

RESULTS

We found that de novo expression of PD-L1, a potent inhibitory ICP, was robustly and transiently induced in reactive astrocytes, but not in microglia, neurons, or oligodendrocyte progenitor cells (OPCs). These PD-L1⁺ reactive astrocytes were highly enriched to form a dense zone around the TBI lesion. Blockade of PD-L1 signaling enlarged brain tissue cavity size, increased infiltration of inflammatory Ly-6C^High monocytes/macrophages (M/Mɸ) but not tissue-repairing Ly-6C^LowF4/80⁺ M/Mɸ, and worsened TBI outcomes in mice. PD-L1 gene knockout enhanced production of CCL2 that is best known for its ability to interact with its cognate receptor CCR2 on Ly-6C^High M/Mϕ to chemotactically recruit these cells into inflammatory sites. Mechanically, PD-L1 signaling in astrocytes likely exhibits dual inhibitory activities for the prevention of excessive neuroimmune and neuroinflammatory responses to TBI through (1) the PD-1/PD-L1 axis to suppress the activity of brain-infiltrating PD-1⁺ immune cells, such as PD-1⁺ T cells, and (2) PD-L1 intrinsic signaling to regulate the timing and intensity of astrocyte reactions to TBI.

CONCLUSIONS

PD-L1⁺ astrocytes act as a gatekeeper to the brain to control TBI-related neuroimmune and neuroinflammatory responses, thereby opening a novel avenue to study the role of ICP-neuroimmune axes in the pathophysiology of TBI and other neurological disorders.

Protective effect of liquiritin on coronary heart disease through regulating the proliferation of human vascular smooth muscle cells via upregulation of sirtuin1

This study aimed to explore whether liquiritin affects the development of coronary heart disease by regulating the proliferation and migration of human vascular smooth muscle cells (hVSMCs). A 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2 H-tetrazolium bromide (MTT) assay and lactate dehydrogenase (LDH) release detection were performed to measure the toxic effects of liquiritin on hVSMCs. An in vitro atherosclerosis model in hVSMCs was established using oxidized low-density lipoprotein (ox-LDL), and cell proliferation and apoptosis were detected using an MTT assay and flow cytometry analysis. Western blotting and reverse transcriptase quantitative polymerase chain reaction (RT-qPCR) were used to detect protein and mRNA expressions, respectively. Caspase3 activity and cell migration were measured using an activity detection kit and Transwell assay, respectively. The results indicated that liquiritin at doses <160 μM had no significant effect on cell viability and LDH release in hVSMCs. Ox-LDL significantly induced cell proliferation and migration, and inhibited hVSMCs apoptosis. Liquiritin significantly inhibited cell proliferation and migration, and enhanced cell apoptosis in ox-LDL induced hVSMCs. Sirtuin1 (SIRT1) was lowly expressed in atherosclerotic plaque tissues in coronary heart disease patients and in ox-LDL-induced hVSMCs. Liquiritin improved SIRT1 expression in ox-LDL-induced hVSMCs, whereas the improvement was inhibited by Selisistat (EX 527, an effective SIRT1 inhibitor) treatment. EX 527 reversed the effects of liquiritin on cell proliferation, migration, and apoptosis in ox-LDL-induced hVSMCs In conclusion, liquiritin plays a protective role in coronary heart disease by regulating the proliferation and migration of hVSMCs by increasing SIRT1 expression.

Skeletal Muscle Gene Expression Profile in Response to Caloric Restriction and Aging: A Role for SirT1

SirT1 plays a crucial role in the regulation of some of the caloric restriction (CR) responsive biological pathways. Aging suppresses SirT1 gene expression in skeletal muscle, suggesting that aging may affect the role of CR in muscle. To determine the role of SirT1 in the regulation of CR regulated pathways in skeletal muscle, we performed high-throughput RNA sequencing using total RNA isolated from the skeletal muscles of young and aged wild-type (WT), SirT1 knockout (SirT1-KO), and SirT1 overexpression (SirT1-OE) mice fed to 20 wk ad libitum (AL) or 40% CR diet. Our data show that aging repressed the global gene expression profile, which was restored by CR via upregulating transcriptional and translational process-related pathways. CR inhibits pathways linked to the extracellular matrix and cytoskeletal proteins regardless of aging. Mitochondrial function and muscle contraction-related pathways are upregulated in aged SirT1 KO mice following CR. SirT1 OE did not affect whole-body energy expenditure or augment skeletal muscle insulin sensitivity associated pathways, regardless of aging or diet. Overall, our RNA-seq data showed that SirT1 and CR have different functions and activation of SirT1 by its activator or exercise may enhance SirT1 activity that, along with CR, likely have a better functional role in aging muscle.

Sirtuins-Mediated System-Level Regulation of Mammalian Tissues at the Interface between Metabolism and Cell Cycle: A Systematic Review

Sirtuins are a family of highly conserved NAD+-dependent proteins and this dependency links Sirtuins directly to metabolism. Sirtuins' activity has been shown to extend the lifespan of several organisms and mainly through the post-translational modification of their many target proteins, with deacetylation being the most common modification. The seven mammalian Sirtuins, SIRT1 through SIRT7, have been implicated in regulating physiological responses to metabolism and stress by acting as nutrient sensors, linking environmental and nutrient signals to mammalian metabolic homeostasis. Furthermore, mammalian Sirtuins have been implicated in playing major roles in mammalian pathophysiological conditions such as inflammation, obesity and cancer. Mammalian Sirtuins are expressed heterogeneously among different organs and tissues, and the same holds true for their substrates. Thus, the function of mammalian Sirtuins together with their substrates is expected to vary among tissues. Any therapy depending on Sirtuins could therefore have different local as well as systemic effects. Here, an introduction to processes relevant for the actions of Sirtuins, such as metabolism and cell cycle, will be followed by reasoning on the system-level function of Sirtuins and their substrates in different mammalian tissues. Their involvement in the healthy metabolism and metabolic disorders will be reviewed and critically discussed.

From Hybrids to New Scaffolds: The Latest Medicinal Chemistry Goals in Multi-target Directed Ligands for Alzheimer's Disease

Alzheimer's disease (AD) is a chronic, progressive, and fatal neurodegenerative disorder affecting cognition, behavior, and function, being one of the most common causes of mental deterioration in elderly people. Once thought as being just developed because of β amyloid depositions or neurofibrillary Tau tangles, during the last decades, numerous AD-related targets have been established, the multifactorial nature of AD became evident. In this context, the one drug-one target paradigm has resulted in being inefficient in facing AD and other disorders with complex etiology, opening the field for the emergence of the multitarget approach. In this review, we highlight the recent advances within this area, emphasizing in hybridization tools of well-known chemical scaffolds endowed with pharmacological properties concerning AD, such as curcumin-, resveratrol-, chromone- and indole-. We focus mainly on well established and incipient AD therapeutic targets, AChE, BuChE, MAOs, β-amyloid deposition, 5-HT4 and Serotonin transporter, with the aim to shed light about new insights in the AD multitarget therapy.

Spatiotemporal gating of SIRT1 functions by O-GlcNAcylation is essential for liver metabolic switching and prevents hyperglycemia

Inefficient physiological transitions are known to cause metabolic disorders. Therefore, investigating mechanisms that constitute molecular switches in a central metabolic organ like the liver becomes crucial. Specifically, upstream mechanisms that control temporal engagement of transcription factors, which are essential to mediate physiological fed-fast-refed transitions are less understood. SIRT1, a NAD⁺-dependent deacetylase, is pivotal in regulating hepatic gene expression and has emerged as a key therapeutic target. Despite this, if/how nutrient inputs regulate SIRT1 interactions, stability, and therefore downstream functions are still unknown. Here, we establish nutrient-dependent O-GlcNAcylation of SIRT1, within its N-terminal domain, as a crucial determinant of hepatic functions. Our findings demonstrate that during a fasted-to-refed transition, glycosylation of SIRT1 modulates its interactions with various transcription factors and a nodal cytosolic kinase involved in insulin signaling. Moreover, sustained glycosylation in the fed state causes nuclear exclusion and cytosolic ubiquitin-mediated degradation of SIRT1. This mechanism exerts spatiotemporal control over SIRT1 functions by constituting a previously unknown molecular relay. Of note, loss of SIRT1 glycosylation discomposed these interactions resulting in aberrant gene expression, mitochondrial dysfunctions, and enhanced hepatic gluconeogenesis. Expression of nonglycosylatable SIRT1 in the liver abrogated metabolic flexibility, resulting in systemic insulin resistance, hyperglycemia, and hepatic inflammation, highlighting the physiological costs associated with its overactivation. Conversely, our study also reveals that hyperglycosylation of SIRT1 is associated with aging and high-fat-induced obesity. Thus, we establish that nutrient-dependent glycosylation of SIRT1 is essential to gate its functions and maintain physiological fitness.

Overexpression of Sirtuin 1 protein in neurons prevents and reverses experimental diabetic neuropathy

In diabetic neuropathy, there is activation of axonal and sensory neuronal degeneration pathways leading to distal axonopathy. The nicotinamide-adenine dinucleotide (NAD+)-dependent deacetylase enzyme, Sirtuin 1 (SIRT1), can prevent activation of these pathways and promote axonal regeneration. In this study, we tested whether increased expression of SIRT1 protein in sensory neurons prevents and reverses experimental diabetic neuropathy induced by a high fat diet (HFD). We generated a transgenic mouse that is inducible and overexpresses SIRT1 protein in neurons (nSIRT1OE Tg). Higher levels of SIRT1 protein were localized to cortical and hippocampal neuronal nuclei in the brain and in nuclei and cytoplasm of small to medium sized neurons in dorsal root ganglia. Wild-type and nSIRT1OE Tg mice were fed with either control diet (6.2% fat) or a HFD (36% fat) for 2 months. HFD-fed wild-type mice developed neuropathy as determined by abnormal motor and sensory nerve conduction velocity, mechanical allodynia, and loss of intraepidermal nerve fibres. In contrast, nSIRT1OE prevented a HFD-induced neuropathy despite the animals remaining hyperglycaemic. To test if nSIRT1OE would reverse HFD-induced neuropathy, nSIRT1OE was activated after mice developed peripheral neuropathy on a HFD. Two months after nSIRT1OE, we observed reversal of neuropathy and an increase in intraepidermal nerve fibre. Cultured adult dorsal root ganglion neurons from nSIRT1OE mice, maintained at high (30 mM) total glucose, showed higher basal and maximal respiratory capacity when compared to adult dorsal root ganglion neurons from wild-type mice. In dorsal root ganglion protein extracts from nSIRT1OE mice, the NAD+-consuming enzyme PARP1 was deactivated and the major deacetylated protein was identified to be an E3 protein ligase, NEDD4-1, a protein required for axonal growth, regeneration and proteostasis in neurodegenerative diseases. Our results indicate that nSIRT1OE prevents and reverses neuropathy. Increased mitochondrial respiratory capacity and NEDD4 activation was associated with increased axonal growth driven by neuronal overexpression of SIRT1. Therapies that regulate NAD+ and thereby target sirtuins may be beneficial in human diabetic sensory polyneuropathy.

Neuronal SIRT1 Regulates Metabolic and Reproductive Function and the Response to Caloric Restriction

Sirt1 is an NAD-dependent, class III deacetylase that functions as a cellular energy sensor. In addition to its well-characterized effects in peripheral tissues, emerging evidence suggests that neuronal Sirt1 activity plays a role in the central regulation of energy balance and glucose metabolism. In this study, we generated mice expressing an enzymatically inactive form (N-MUT) or wild-type (WT) SIRT1 (N-OX) in mature neurons. N-OX male and female mice had impaired glucose tolerance, and N-MUT female, but not male, mice had improved glucose tolerance compared with that of WT littermates. Furthermore, glucose tolerance was improved in all mice with caloric restriction (CR) but was greater in the N-OX mice, who had better glucose tolerance than their littermates. At the reproductive level, N-OX females had impaired estrous cycles, with increased cycle length and more time in estrus. LH and progesterone surges were absent on the evening of proestrus in the N-OX mice, suggesting a defect in spontaneous ovulation, which was confirmed by the ovarian histology revealing fewer corpora lutea. Despite this defect, the mice were still fertile when mated to WT mice on the day of proestrus, indicating that the mice could respond to normal pheromonal or environmental cues. When subjected to CR, the N-OX mice went into diestrus arrest earlier than their littermates. Together, these results suggested that the overexpression of SIRT1 rendered the mice more sensitive to the metabolic improvements and suppression of reproductive cycles by CR, which was independent of circadian rhythms.

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.

Protection against High-Fat-Diet-Induced Obesity in MDM2C305F Mice Due to Reduced p53 Activity and Enhanced Energy Expenditure

The RPL11-MDM2 interaction constitutes a p53 signaling pathway activated by deregulated ribosomal biosynthesis in response to stress. Mice bearing an MDM2^C305F mutation that disrupts RPL11-MDM2 binding were analyzed on a high-fat diet (HFD). The Mdm2^C305F/C305F mice, although phenotypically indistinguishable from wild-type (WT) mice when fed normal chow, demonstrated decreased fat accumulation along with improved insulin sensitivity and glucose tolerance after prolonged HFD feeding. We found that HFD increases expression of c-MYC and RPL11 in both WT and Mdm2^C305F/C305F mice; however, p53 was induced in WT but not in Mdm2^C305F/C305F mice. Reduced p53 activity in HFD-fed Mdm2^C305F/C305F mice resulted in higher levels of p53 downregulated targets GLUT4 and SIRT1, leading to increased biosynthesis of NAD⁺, and increased energy expenditure. Our study reveals a role for the RPL11-MDM2-p53 pathway in fat storage during nutrient excess and suggests that targeting this pathway may be a potential treatment for obesity.

Sirtuin-3 Promotes Adipogenesis, Osteoclastogenesis, and Bone Loss in Aging Male Mice

Sirtuin-3 (Sirt3) is an essential metabolic regulatory enzyme that plays an important role in mitochondrial metabolism, but its role in bone marrow and skeletal homeostasis remains largely unknown. In this study, we hypothesize that increased expression of Sirt3 plays a role in skeletal aging. Using mice that overexpress Sirt3 [i.e., Sirt3 transgenic (Sirt3Tg)], we show that Sirt3 is a positive regulator of adipogenesis and osteoclastogenesis and a negative regulator of skeletal homeostasis. Sirt3Tg mice exhibited more adipocytes in the tibia compared with control mice. Bone marrow stromal cells (BMSCs) from Sirt3Tg mice displayed an enhanced ability to differentiate into adipocytes compared with control BMSCs. We found a 2.5-fold increase in the number of osteoclasts on the bone surface in Sirt3Tg mice compared with control mice (P < 0.03), and increased osteoclastogenesis in vitro. Importantly, Sirt3 activates the mechanistic target of rapamycin (mTOR) pathway to regulate osteoclastogenesis. Sirt3Tg male mice exhibited a significant reduction in cortical thickness at the tibiofibular junction (P < 0.05). In summary, Sirt3 activity in bone marrow cells is associated with increased adipogenesis, increased osteoclastogenesis through activation of mTOR signaling, and reduced bone mass. Interestingly, Sirt3 expression in bone marrow cells increases during aging, suggesting that Sirt3 promotes age-related adipogenesis and osteoclastogenesis associated with bone loss. These findings identify Sirt3 as an important regulator of adipogenesis and skeletal homeostasis in vivo and identify Sirt3 as a potential target for the treatment of osteoporosis.

The brain, sirtuins, and ageing

In mammals, recent studies have demonstrated that the brain, the hypothalamus in particular, is a key bidirectional integrator of humoral and neural information from peripheral tissues, thus influencing ageing both in the brain and at the 'systemic' level. CNS decline drives the progressive impairment of cognitive, social and physical abilities, and the mechanisms underlying CNS regulation of the ageing process, such as microglia-neuron networks and the activities of sirtuins, a class of NAD⁺-dependent deacylases, are beginning to be understood. Such mechanisms are potential targets for the prevention or treatment of age-associated dysfunction and for the extension of a healthy lifespan.

AAV-mediated Sirt1 overexpression in skeletal muscle activates oxidative capacity but does not prevent insulin resistance

Type 2 diabetes is characterized by triglyceride accumulation and reduced lipid oxidation capacity in skeletal muscle. SIRT1 is a key protein in the regulation of lipid oxidation and its expression is reduced in the skeletal muscle of insulin resistant mice. In this tissue, Sirt1 up-regulates the expression of genes involved in oxidative metabolism and improves mitochondrial function mainly through PPARGC1 deacetylation. Here we examined whether Sirt1 overexpression mediated by adeno-associated viral vectors of serotype 1 (AAV1) specifically in skeletal muscle can counteract the development of insulin resistance induced by a high fat diet in mice. AAV1-Sirt1-treated mice showed up-regulated expression of key genes related to β-oxidation together with increased levels of phosphorylated AMP protein kinase. Moreover, SIRT1 overexpression in skeletal muscle also increased basal phosphorylated levels of AKT. However, AAV1-Sirt1 treatment was not enough to prevent high fat diet-induced obesity and insulin resistance. Although Sirt1 gene transfer to skeletal muscle induced changes at the muscular level related with lipid and glucose homeostasis, our data indicate that overexpression of SIRT1 in skeletal muscle is not enough to improve whole-body insulin resistance and that suggests that SIRT1 has to be increased in other metabolic tissues to prevent insulin resistance.

MEK1/2 inhibitors activate macrophage ABCG1 expression and reverse cholesterol transport-An anti-atherogenic function of ERK1/2 inhibition

Expression of ATP-binding cassette transporter G1 (ABCG1), a molecule facilitating cholesterol efflux to HDL, is activated by liver X receptor (LXR). In this study, we investigated if inhibition of ERK1/2 can activate macrophage ABCG1 expression and functions. MEK1/2 inhibitors, PD98059 and U0126, increased ABCG1 mRNA and protein expression, and activated the natural ABCG1 promoter but not the promoter with the LXR responsive element (LXRE) deletion. Inhibition of ABCG1 expression by ABCG1 siRNA did enhance the formation of macrophage/foam cells and it attenuated the inhibitory effect of MEK1/2 inhibitors on foam cell formation. MEK1/2 inhibitors activated macrophage cholesterol efflux to HDL in vitro, and they enhanced reverse cholesterol transport (RCT) in vivo. ApoE deficient (apoE(-/-)) mice receiving U0126 treatment had reduced sinus lesions in the aortic root which was associated with activated macrophage ABCG1 expression in the lesion areas. MEK1/2 inhibitors coordinated the RXR agonist, but not the LXR agonist, to induce ABCG1 expression. Furthermore, induction of ABCG1 expression by MEK1/2 inhibitors was associated with activation of SIRT1, a positive regulator of LXR activity, and inactivation of SULT2B1 and RIP140, two negative regulators of LXR activity. Taken together, our study suggests that MEK1/2 inhibitors activate macrophage ABCG1 expression/RCT, and inhibit foam cell formation and lesion development by multiple mechanisms, supporting the concept that ERK1/2 inhibition is anti-atherogenic.

The role of sirtuins in aging and age-related diseases

Sirtuins, initially described as histone deacetylases and gene silencers in yeast, are now known to have much more functions and to be much more abundant in living organisms. Sirtuins gained much attention when they were first acknowledged to be responsible for some beneficial and longevity-promoting effects of calorie restriction in many species of animals - from fruit flies to mammals. In this paper, we discuss some detailed molecular mechanisms of inducing these effects, and wonder if they could be possibly mimicked without actually applying calorie restriction, through induction of sirtuin activity. It is known now that sirtuins, when adjusting the pattern of cellular metabolism to nutrient availability, can regulate many metabolic functions significant from the standpoint of aging research - including DNA repair, genome stability, inflammatory response, apoptosis, cell cycle, and mitochondrial functions. While carrying out these regulations, sirtuins cooperate with many transcription factors, including PGC-1a, NFKB, p53 and FoxO. This paper contains some considerations about possible use of facilitating activity of the sirtuins in prevention of aging, metabolic syndrome, chronic inflammation, and other diseases.

Role of microRNAs in Alcohol-Induced Multi-Organ Injury

Alcohol consumption and its abuse is a major health problem resulting in significant healthcare cost in the United States. Chronic alcoholism results in damage to most of the vital organs in the human body. Among the alcohol-induced injuries, alcoholic liver disease is one of the most prevalent in the United States. Remarkably, ethanol alters expression of a wide variety of microRNAs that can regulate alcohol-induced complications or dysfunctions. In this review, we will discuss the role of microRNAs in alcoholic pancreatitis, alcohol-induced liver damage, intestinal epithelial barrier dysfunction, and brain damage including altered hippocampus structure and function, and neuronal loss, alcoholic cardiomyopathy, and muscle damage. Further, we have reviewed the role of altered microRNAs in the circulation, teratogenic effects of alcohol, and during maternal or paternal alcohol consumption.

Personalized epigenetic management of diabetes

Epigenetic regulation of gene expression allows the organism to respond/adapt to environmental conditions without changing the gene coding sequence. Epigenetic modifications have also been found to control gene expression in various diseases, including diabetes. Epigenetic changes induced by hyperglycemia in multiple target organs contribute to metabolic memory of diabetic complications. The long-lasting development of diabetic complications even after achieving glucose control has been partly attributed to epigenetic changes in target cells. Specific epigenetic drugs might rescue chromatin conformation associated to hyperglycemia possibly slowing down the onset of diabetes-related complications. The current review will describe the updated epigenetics in diabetes that can be used to personalize a more focused treatment.

SIRT1 in the brain-connections with aging-associated disorders and lifespan

The silent mating type information regulation 2 proteins (sirtuins) 1 of class III histone deacetylases (HDACs) have been associated with health span and longevity. SIRT1, the best studied member of the mammalian sirtuins, has a myriad of roles in multiple tissues and organs. However, a significant part of SIRT1's role that impinges on aging and lifespan may lie in its activities in the central nervous system (CNS) neurons. Systemically, SIRT1 influences energy metabolism and circadian rhythm through its activity in the hypothalamic nuclei. From a cell biological perspective, SIRT1 is a crucial component of multiple interconnected regulatory networks that modulate dendritic and axonal growth, as well as survival against stress. This neuronal cell autonomous activity of SIRT1 is also important for neuronal plasticity, cognitive functions, as well as protection against aging-associated neuronal degeneration and cognitive decline. We discuss recent findings that have shed light on the various activities of SIRT1 in the brain, which collectively impinge on aging-associated disorders and lifespan.

The promise and perils of HDAC inhibitors in neurodegeneration

Histone deacetylases (HDACs) represent emerging therapeutic targets in the context of neurodegeneration. Indeed, pharmacologic inhibition of HDACs activity in the nervous system has shown beneficial effects in several preclinical models of neurological disorders. However, the translation of such therapeutic approach to clinics has been only marginally successful, mainly due to our still limited knowledge about HDACs physiological role particularly in neurons. Here, we review the potential benefits along with the risks of targeting HDACs in light of what we currently know about HDAC activity in the brain.

SIRT1 overexpression ameliorates a mouse model of SOD1-linked amyotrophic lateral sclerosis via HSF1/HSP70i chaperone system

Background

Dominant mutations in superoxide dismutase 1 (SOD1) cause degeneration of motor neurons in a subset of inherited amyotrophic lateral sclerosis (ALS). The pathogenetic process mediated by misfolded and/or aggregated mutant SOD1 polypeptides is hypothesized to be suppressed by protein refolding. This genetic study is aimed to test whether mutant SOD1-mediated ALS pathology recapitulated in mice could be alleviated by overexpressing a longevity-related deacetylase SIRT1 whose substrates include a transcription factor heat shock factor 1 (HSF1), the master regulator of the chaperone system.

Results

We established a line of transgenic mice that chronically overexpress SIRT1 in the brain and spinal cord. While inducible HSP70 (HSP70i) was upregulated in the spinal cord of SIRT1 transgenic mice (PrP-Sirt1), no neurological and behavioral alterations were detected. To test hypothetical benefits of SIRT1 overexpression, we crossbred PrP-Sirt1 mice with two lines of ALS model mice: A high expression line that exhibits a severe phenotype (SOD1^G93A-H) or a low expression line with a milder phenotype (SOD1^G93A-L). The Sirt1 transgene conferred longer lifespan without altering the time of symptomatic onset in SOD1^G93A-L. Biochemical analysis of the spinal cord revealed that SIRT1 induced HSP70i expression through deacetylation of HSF1 and that SOD1^G93A-L/PrP-Sirt1 double transgenic mice contained less insoluble SOD1 than SOD1^G93A-L mice. Parallel experiments showed that Sirt1 transgene could not rescue a more severe phenotype of SOD1^G93A-H transgenic mice partly because their HSP70i level had peaked out.

Conclusions

The genetic supplementation of SIRT1 can ameliorate a mutant SOD1-linked ALS mouse model partly through the activation of the HSF1/HSP70i chaperone system. Future studies shall include testing potential benefits of pharmacological enhancement of the deacetylation activity of SIRT1 after the onset of the symptom.

Systemic regulation of mammalian ageing and longevity by brain sirtuins

Sirtuins regulate numerous important biological processes in mammals, including various age-associated pathophysiologies. However, whether sirtuins are critical to control ageing and longevity in mammals has been controversial. Recent studies have demonstrated critical roles of sirtuins in the brain, especially the hypothalamus, in governing multiple physiological functions. These data provide strong evidence that brain sirtuins regulate mammalian ageing and longevity at the organismal level.

Expression of Nampt in hippocampal and cortical excitatory neurons is critical for cognitive function

Nicotinamide adenine dinucleotide (NAD(+)) is an enzyme cofactor or cosubstrate in many essential biological pathways. To date, the primary source of neuronal NAD(+) has been unclear. NAD(+) can be synthesized from several different precursors, among which nicotinamide is the substrate predominantly used in mammals. The rate-limiting step in the NAD(+) biosynthetic pathway from nicotinamide is performed by nicotinamide phosphoribosyltransferase (Nampt). Here, we tested the hypothesis that neurons use intracellular Nampt-mediated NAD(+) biosynthesis by generating and evaluating mice lacking Nampt in forebrain excitatory neurons (CaMKIIαNampt(-/-) mice). CaMKIIαNampt(-/-) mice showed hippocampal and cortical atrophy, astrogliosis, microgliosis, and abnormal CA1 dendritic morphology by 2-3 months of age. Importantly, these histological changes occurred with altered intrahippocampal connectivity and abnormal behavior; including hyperactivity, some defects in motor skills, memory impairment, and reduced anxiety, but in the absence of impaired sensory processes or long-term potentiation of the Schaffer collateral pathway. These results clearly demonstrate that forebrain excitatory neurons mainly use intracellular Nampt-mediated NAD(+) biosynthesis to mediate their survival and function. Studying this particular NAD(+) biosynthetic pathway in these neurons provides critical insight into their vulnerability to pathophysiological stimuli and the development of therapeutic and preventive interventions for their preservation.

The sirtuin class of histone deacetylases: regulation and roles in lipid metabolism

After the completion of the human genome sequence and that from many other organisms, last decade has witnessed a spectacular gain of knowledge on gene functions. These studies provided new insights on the roles of genes in physiology and disease. Nonetheless, the availability of genetically modified models and of "omics" technologies such as next generation sequencing unveiled clear evidences on epigenetic regulation of many cellular functions. At this regard, sirtuins, belonging to class III histone deacetylase family, have emerged as regulators of metabolism as well as other cellular processes and seem ideally suited as targets of future therapeutical interventions. This review deals on general aspects of the biology of sirtuins and focuses on their relevance in lipid metabolism in different tissues, pointing to their exploitation as potential pharmacological targets of compounds that could be used as new therapeutic alternatives in several disorders ranging from type 2 diabetes and obesity to age-related cardiovascular and neurodegenerative diseases.

Mechanisms of action of brain insulin against neurodegenerative diseases

Insulin, a pancreatic hormone, is best known for its peripheral effects on the metabolism of glucose, fats and proteins. There is a growing body of evidence linking insulin action in the brain to neurodegenerative diseases. Insulin present in central nervous system is a regulator of central glucose metabolism nevertheless this glucoregulation is not the main function of insulin in the brain. Brain is known to be specifically vulnerable to oxidative products relative to other organs and altered brain insulin signaling may cause or promote neurodegenerative diseases which invalidates and reduces the quality of life. Insulin located within the brain is mostly of pancreatic origin or is produced in the brain itself crosses the blood-brain barrier and enters the brain via a receptor-mediated active transport system. Brain Insulin, insulin receptor and insulin receptor substrate-mediated signaling pathways play important roles in the regulation of peripheral metabolism, feeding behavior, memory and maintenance of neural functions such as neuronal growth and differentiation, neuromodulation and neuroprotection. In the present review, we would like to summarize the novel biological and pathophysiological roles of neuronal insulin in neurodegenerative diseases and describe the main signaling pathways in use for therapeutic strategies in the use of insulin to the cerebral tissues and their biological applications to neurodegenerative diseases.

Targeting sirtuins for the treatment of diabetes

Sirtuins are a class of NAD⁺-dependent deacetylases, such as deacetylases, that have a wide array of biological functions. Recent studies have suggested that reduced sirtuin action is correlated with Type 2 diabetes. Both overnutrition and aging, which are two major risk factors for diabetes, lead to decreased sirtuin function and result in abnormal glucose and lipid metabolism. Therefore, restoring normal levels of sirtuin action in Type 2 diabetes may be a promising method of treating diabetes. This article reviews the biological functions of three of the seven mammalian sirtuins - SIRT1, SIRT3 and SIRT6 - that have demonstrated prominent metabolic roles and early potential for drug targeting. Clinical trials investigating the use of sirtuin activators for treating diabetes are already underway and show promise as alternatives to current diabetes therapies. Thus, further research into sirtuin activators is warranted and may lead to a new class of safe, effective diabetes treatments.

Seven sirtuins for seven deadly diseases of aging

Sirtuins are a class of NAD(+)-dependent deacetylases having beneficial health effects. This extensive review describes the numerous intracellular actions of the seven mammalian sirtuins, their protein targets, intracellular localization, the pathways they modulate, and their role in common diseases of aging. Selective pharmacological targeting of sirtuins is of current interest in helping to alleviate global disease burden. Since all sirtuins are activated by NAD(+), strategies that boost NAD(+) in cells are of interest. While most is known about SIRT1, the functions of the six other sirtuins are now emerging. Best known is the involvement of sirtuins in helping cells adapt energy output to match energy requirements. SIRT1 and some of the other sirtuins enhance fat metabolism and modulate mitochondrial respiration to optimize energy harvesting. The AMP kinase/SIRT1-PGC-1α-PPAR axis and mitochondrial sirtuins appear pivotal to maintaining mitochondrial function. Downregulation with aging explains much of the pathophysiology that accumulates with aging. Posttranslational modifications of sirtuins and their substrates affect specificity. Although SIRT1 activation seems not to affect life span, activation of some of the other sirtuins might. Since sirtuins are crucial to pathways that counter the decline in health that accompanies aging, pharmacological agents that boost sirtuin activity have clinical potential in treatment of diabetes, cardiovascular disease, dementia, osteoporosis, arthritis, and other conditions. In cancer, however, SIRT1 inhibitors could have therapeutic value. Nutraceuticals such as resveratrol have a multiplicity of actions besides sirtuin activation. Their net health benefit and relative safety may have originated from the ability of animals to survive environmental changes by utilizing these stress resistance chemicals in the diet during evolution. Each sirtuin forms a key hub to the intracellular pathways affected.

Neuronal Sirt1 deficiency increases insulin sensitivity in both brain and peripheral tissues

Sirt1 is a NAD(+)-dependent class III deacetylase that functions as a cellular energy sensor. In addition to its well-characterized effects in peripheral tissues, emerging evidence suggests that neuronal Sirt1 activity plays a role in the central regulation of energy balance and glucose metabolism. To assess this idea, we generated Sirt1 neuron-specific knockout (SINKO) mice. On both standard chow and HFD, SINKO mice were more insulin sensitive than Sirt1(f/f) mice. Thus, SINKO mice had lower fasting insulin levels, improved glucose tolerance and insulin tolerance, and enhanced systemic insulin sensitivity during hyperinsulinemic euglycemic clamp studies. Hypothalamic insulin sensitivity of SINKO mice was also increased over controls, as assessed by hypothalamic activation of PI3K, phosphorylation of Akt and FoxO1 following systemic insulin injection. Intracerebroventricular injection of insulin led to a greater systemic effect to improve glucose tolerance and insulin sensitivity in SINKO mice compared with controls. In line with the in vivo results, insulin-induced AKT and FoxO1 phosphorylation were potentiated by inhibition of Sirt1 in a cultured hypothalamic cell line. Mechanistically, this effect was traced to a reduced effect of Sirt1 to directly deacetylate and repress IRS-1 function. The enhanced central insulin signaling in SINKO mice was accompanied by increased insulin receptor signal transduction in liver, muscle, and adipose tissue. In summary, we conclude that neuronal Sirt1 negatively regulates hypothalamic insulin signaling, leading to systemic insulin resistance. Interventions that reduce neuronal Sirt1 activity have the potential to improve systemic insulin action and limit weight gain on an obesigenic diet.

Crosstalk between Oxidative Stress and SIRT1: Impact on the Aging Process

Increased oxidative stress has been associated with the aging process. However, recent studies have revealed that a low-level oxidative stress can even extend the lifespan of organisms. Reactive oxygen species (ROS) are important signaling molecules, e.g., being required for autophagic degradation. SIRT1, a class III protein deacetylase, is a crucial cellular survival protein, which is also involved in combatting oxidative stress. For instance, SIRT1 can stimulate the expression of antioxidants via the FoxO pathways. Moreover, in contrast to ROS, SIRT1 inhibits NF-κB signaling which is a major inducer of inflammatory responses, e.g., with inflammasome pathway. Recent studies have demonstrated that an increased level of ROS can both directly and indirectly control the activity of SIRT1 enzyme. For instance, ROS can inhibit SIRT1 activity by evoking oxidative modifications on its cysteine residues. Decreased activity of SIRT1 enhances the NF-κB signaling, which supports inflammatory responses. This crosstalk between the SIRT1 and ROS signaling provokes in a context-dependent manner a decline in autophagy and a low-grade inflammatory phenotype, both being common hallmarks of ageing. We will review the major mechanisms controlling the signaling balance between the ROS production and SIRT1 activity emphasizing that this crosstalk has a crucial role in the regulation of the aging process.

Insulin induces neurite outgrowth via SIRT1 in SH-SY5Y cells

Insulin plays diverse roles, including learning and memory, in the central nervous system. SIRT1 has been reported to be involved in the processes of normal learning, memory, and synaptic plasticity. However, whether insulin is directly involved in regulating SIRT1 expression in neurons or whether it affects synapses remains largely unknown. Here, we show that insulin promotes neurite outgrowth and increases SIRT1 expression in SH-SY5Y cells. LY294002, a phosphatidylinositol 3-kinase inhibitor, inhibited the expression of insulin-induced increases in SIRT1. Conversely, the downregulation of SIRT1 using a SIRT1 inhibitor and SIRT1-siRNA resulted in a significant reduction in the length of neurite outgrowth. Taken together, these results suggest that the regulation of SIRT1 by insulin is important for the neurite outgrowth of neuroblastoma cells.

From sirtuin biology to human diseases: an update

Originally rising to notoriety for their role in the regulation of aging, sirtuins are a family of NAD(+)-dependent enzymes that have been connected to a steadily growing set of biological processes. In addition to regulating aging, sirtuins play key roles in the maintenance of organismal metabolic homeostasis. These enzymes also have primarily protective functions in the development of many age-related diseases, including cancer, neurodegeneration, and cardiovascular disease. In this minireview, we provide an update on the known roles for each of the seven mammalian sirtuins in these areas.

The circadian clock: a framework linking metabolism, epigenetics and neuronal function

The circadian clock machinery is responsible for biological timekeeping on a systemic level. The central clock system controls peripheral clocks through a number of output cues that synchronize the system as a whole. There is growing evidence that changing cellular metabolic states have important effects on circadian rhythms and can thereby influence neuronal function and disease. Epigenetic control has also been implicated in the modulation of biological timekeeping, and cellular metabolism and epigenetic state seem to be closely linked. We discuss the idea that cellular metabolic state and epigenetic mechanisms might work through the circadian clock to regulate neuronal function and influence disease states.

Sirtuin biology and relevance to diabetes treatment

Sirtuins are a group of NAD(+)-dependent enzymes that post-translationally modify histones and other proteins. Among seven mammalian sirtuins, SIRT1 has been the most extensively studied and has been demonstrated to play a critical role in all major metabolic organs and tissues. SIRT1 regulates glucose and lipid homeostasis in the liver, modulates insulin secretion in pancreatic islets, controls insulin sensitivity and glucose uptake in skeletal muscle, increases adiponectin expression in white adipose tissue and controls food intake and energy expenditure in the brain. Recently, SIRT3 has been demonstrated to modulate insulin sensitivity in skeletal muscle and systemic metabolism, and Sirt3-null mice manifest characteristics of metabolic syndrome on a high-fat diet. Thus, it is reasonable to believe that enhancing the activities of SIRT1 and SIRT3 may be beneficial for Type 2 diabetes. Although it is controversial, the SIRT1 activator SRT1720 has been reported to be effective in improving glucose metabolism and insulin sensitivity in animal models. More research needs to be conducted so that we can better understand the physiological functions and molecular mechanisms of sirtuins in order to therapeutically target these enzymes for diabetes treatment.