Correction: SIRT2 Ablation Has No Effect on Tubulin Acetylation in Brain, Cholesterol Biosynthesis or the Progression of Huntington's Disease Phenotypes In Vivo.

[This corrects the article DOI: 10.1371/journal.pone.0034805.].

Mitochondrial Biology and Neurological Diseases.

Mitochondria are extremely active organelles that perform a variety of roles in the cell including energy production, regulation of calcium homeostasis, apoptosis, and population maintenance through fission and fusion. Mitochondrial dysfunction in the form of oxidative stress and mutations can contribute to the pathogenesis of various neurodegenerative diseases such as Parkinson's (PD), Alzheimer's (AD), and Huntington's diseases (HD). Abnormalities of Complex I function in the electron transport chain have been implicated in some neurodegenerative diseases, inhibiting ATP production and generating reactive oxygen species that can cause major damage to mitochondria. Mutations in both nuclear and mitochondrial DNA can contribute to neurodegenerative disease, although the pathogenesis of these conditions tends to focus on nuclear mutations. In PD, nuclear genome mutations in the PINK1 and parkin genes have been implicated in neurodegeneration [1], while mutations in APP, PSEN1 and PSEN2 have been implicated in a variety of clinical symptoms of AD [5]. Mutant htt protein is known to cause HD [2]. Much progress has been made to determine some causes of these neurodegenerative diseases, though permanent treatments have yet to be developed. In this review, we discuss the roles of mitochondrial dysfunction in the pathogenesis of these diseases.

SIRT3 attenuates MPTP-induced nigrostriatal degeneration via enhancing mitochondrial antioxidant capacity.

Parkinson's disease (PD) is one of the most common neurodegenerative diseases, which is characterized by progressive degeneration of nigrostriatal dopaminergic neurons. There is a growing consensus that mitochondrial dysfunction and oxidative stress play a crucial role in PD pathogenesis. Sirtuin3 (SIRT3) is the major mitochondria NAD(+)-dependent deacetylase that acts as a regulator of mitochondrial protein function; it is essential for maintaining mitochondrial integrity. Although SIRT3 was reported to have anti-oxidative stress activity in an in vitro study, there is no explicit in vivo evidence for the involvement of SIRT3 in the etiology of PD. The present study shows that SIRT3 null mice do not exhibit motor and non-motor deficits compared with wild-type controls. However, SIRT3 deficiency dramatically exacerbated the degeneration of nigrostriatal dopaminergic neurons in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced PD mice. SIRT3 null mice exposed to MPTP also exhibited decreased superoxide dismutase 2, a specific mitochondrial antioxidant enzyme, and reduced glutathione peroxidase expression compared with wild-type controls. Taken together, these findings strongly support that SIRT3 has a possible role in MPTP-induced neurodegeneration via preserving free radical scavenging capacity in mitochondria.

Protective role of SIRT5 against motor deficit and dopaminergic degeneration in MPTP-induced mice model of Parkinson's disease.

Parkinson's disease (PD) is characterized by progressive loss of nigrostriatal dopaminergic neurons that results in motor deficits including resting tremor, rigidity, bradykinesia, and postural instability. Despite decades of intensive study, the underlying molecular mechanisms are not fully understood. Multiple lines of evidence indicate that mitochondrial dysfunction and oxidative stress contribute to neuronal death, which is the key feature of neurodegeneration. Mitochondria are pivotal organelles that host essential functions in neuronal viability including energy production, oxidative phosphorylation, calcium buffering, redox homeostasis and apoptosis. SIRT5, which localizes in the mitochondrial matrix, is nicotinamide adenine dinucleotide (NAD(+))-dependent histone deacetylase. The physiological and pathophysiological functions of SIRT5 in vivo remain elusive although it is known to be an important energy sensor. Here, we investigated the role of SIRT5 in the pathogenesis of PD mice induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). We present evidence that SIRT5 deficiency, by itself, does not affect motor and non-motor functions; however, lack of SIRT5 exacerbates MPTP-induced motor deficits. Consistently, MPTP-exposed SIRT5 knockout mice exhibited more severe nigrostriatal dopaminergic degeneration than that observed in wild-type controls. Furthermore, deletion of SIRT5 leads to a larger decrease, relative to control, in the expression level of manganese superoxide dismutase (SOD2), a mitochondria-specific antioxidant enzyme, after MPTP induction. These findings indicate that SIRT5 ameliorates MPTP-induced nigrostriatal dopaminergic degeneration via preserving mitochondrial antioxidant capacity.

Loss of SIRT4 decreases GLT-1-dependent glutamate uptake and increases sensitivity to kainic acid.

Glutamate transport is a critical process in the brain that maintains low extracellular levels of glutamate to allow for efficient neurotransmission and prevent excitotoxicity. Loss of glutamate transport function is implicated in epilepsy, traumatic brain injury, and amyotrophic lateral sclerosis. It remains unclear whether or not glutamate transport can be modulated in these disease conditions to improve outcome. Here, we show that sirtuin (SIRT)4, a mitochondrial sirtuin, is up-regulated in response to treatment with the potent excitotoxin kainic acid. Loss of SIRT4 leads to a more severe reaction to kainic acid and decreased glutamate transporter expression and function in the brain. Together, these results indicate a critical and novel stress response role for SIRT4 in promoting proper glutamate transport capacity and protecting against excitotoxicity.

SIRT2 enhances 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced nigrostriatal damage via apoptotic pathway.

Sirtuins are NAD-dependent protein deacetylases that were shown to have protective effects against different age-related diseases. SIRT2 is a strong deacetylase that is highly expressed in brain. It has been associated with neurodegenerative diseases. MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) is a dopaminergic neurotoxin that displays clinical features of Parkinson's Disease (PD). MPTP leads to the degeneration of nigrostriatal dopaminergic pathway after its systemic administration. Chronic administration of MPTP induces lesion via apoptosis. We show here that SIRT2 deacetylates Foxo3a, increases RNA and protein levels of Bim, and as a result enhances apoptosis in the MPTP model of PD. We also show that neurodegeneration induced by chronic MPTP regimen is prevented by genetic deletion of SIRT2 in mouse. Deletion of SIRT2 leads to the reduction of apoptosis due to an increase in acetylation of Foxo3a and a decrease in Bim levels. We demonstrate that SIRT2 deacetylates Foxo3a, activates Bim, and induces apoptosis only in MPP(+)-treated cells. Therefore, designing SIRT2 inhibitors might be helpful in developing effective treatments for PD.

Mitochondrial sirtuins as therapeutic targets for age-related disorders.

Sirtuins are a class of histone deacetylases that have a wide range of regulatory roles in the cell. Three sirtuins, SIRT3 to SIRT5, localize to and function within the mitochondria. Mitochondrial dysfunction is thought to be the underlying mechanism of several age-related diseases, such as metabolic syndrome, cancer, and neurodegeneration. This review examines current evidence that mitochondrial sirtuins are involved in regulating mitochondrial function and pathogenesis.

The role of sirtuins in Alzheimer's disease.

Sirtuins are highly conserved NAD(+)-dependent enzymes that were shown to have beneficial effects against age-related diseases. Alzheimer's Disease (AD) is the most common neurodegenerative disorder associated with aging and the effects of sirtuins on AD have been investigated using different mouse and cell culture models. In most of these studies, it has been found that the overexpression of SIRT1 has protective effects against the AD phenotype. Therefore, designing therapeutics based on SIRT1 activity might be important to investigate treatment methods for this disease. In this review, we summarize the recent research regarding the functions of sirtuins and their potential roles in designing therapeutics for AD.

Sirtuins as possible targets in neurodegenerative diseases.

Age-related diseases pose as an enormous problem on aging populations of the world. Despite the fact that many advances have been made on understanding of the neurodegeneration, there is still no cure available for the age-related brain disorders. Sirtuins are NAD-dependent protein deacetylases that were shown to have beneficial effects against age-related diseases. SIRT1 and SIRT2 have been studied mostly in terms of neurodegenerative diseases and seem to have opposite effects. According to the recent findings, activators of SIRT1 and inhibitors of SIRT2 would benefit the brain from neurodegeneration. Despite the enormous amount of research that has been conducted so far, there is still no cure or treatment for almost all of the neurodegenerative disorders. In addition, the mechanisms underlying brain aging and also the link between aging and neurodegeneration are not understood. This review focuses on the role of sirtuins as possible drug targets for neurodegenerative diseases such as Alzheimer's, Parkinson's and Huntington's Diseases.

SIRT1 and SIRT2: emerging targets in neurodegeneration.

Sirtuins are NAD-dependent protein deacetylases known to have protective effects against age-related diseases such as cancer, diabetes, cardiovascular and neurodegenerative diseases. In mammals, there are seven sirtuins (SIRT1-7), which display diversity in subcellular localization and function. While SIRT1 has been extensively investigated due to its initial connection with lifespan extension and involvement in calorie restriction, important biological and therapeutic roles of other sirtuins have only recently been recognized. Here, we review the potential roles and effects of SIRT1 and SIRT2 in neurodegenerative diseases. We discuss different functions and targets of SIRT1 and SIRT2 in a variety of neurodegenerative diseases including Alzheimer's disease (AD), Parkinson's disease (PD) and Huntington's Disease (HD). We also cover the role of SIRT1 in neuronal differentiation due to the possible implications in neurodegenerative conditions, and conclude with an outlook on the potential therapeutic value of SIRT1 and SIRT2 in these disorders.

The Effects of SIRT1 on Alzheimer's Disease Models.

Sirtuins are highly conserved NAD(+)-dependent enzymes that were shown to have beneficial effects against age-related diseases. Aging is the major risk factor for all neurodegenerative disorders including Alzheimer's Disease (AD). Sirtuins have been widely studied in the context of AD using different mouse models. In most of these studies, overexpression of SIRT1 has been shown to have protective effects against AD. Therefore, designing therapeutics based on increasing SIRT1 activity might be important for investigating the ways of treatment for this disease. This paper summarizes the recent research on the effect of SIRT1 in AD animal models and also the potential of SIRT1 being a therapeutical target for AD.

Sirtuin 2 (SIRT2) enhances 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced nigrostriatal damage via deacetylating forkhead box O3a (Foxo3a) and activating Bim protein.

Sirtuins are NAD-dependent protein deacetylases that were shown to have beneficial effects against age-related diseases. SIRT2 is a strong deacetylase that is highly expressed in brain. It has been associated with neurodegenerative diseases. 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is a dopaminergic neurotoxin that replicates most of the clinical features of Parkinson disease (PD) and produces a reliable and reproducible lesion of the nigrostriatal dopaminergic pathway and neurodegeneration after its systemic administration. Chronic administration of MPTP induces lesion via apoptosis. We show here that SIRT2 deacetylates Foxo3a, increases RNA and protein levels of Bim, and as a result, enhances apoptosis in the MPTP model of PD. We also show that neurodegeneration induced by chronic MPTP regimen is prevented by genetic deletion of SIRT2 in mouse. Deletion of SIRT2 leads to the reduction of apoptosis due to an increase in acetylation of Foxo3a and a decrease in Bim levels. We demonstrate that SIRT2 deacetylates Foxo3a, activates Bim, and induces apoptosis only in 1-methyl-4-phenylpyridinium-treated cells. Therefore, designing SIRT2 inhibitors might be helpful to develop effective treatments for PD.

The neurobiology of sirtuins and their role in neurodegeneration.

Sirtuins are highly conserved NAD(+)-dependent enzymes that have beneficial effects against age-related diseases. Aging is the major unifying risk factor for all neurodegenerative disorders. Sirtuins modulate major biological pathways, such as stress response, protein aggregation, and inflammatory processes, that are involved in age-related neurodegenerative diseases. Therefore, sirtuins have been widely studied in the context of the nervous system and neurodegeneration. They are especially interesting because it is possible to alter the activities of sirtuins using small molecules that could be developed into drugs. Indeed, it has been shown that manipulation of SIRT1 activity genetically or pharmacologically impacts neurodegenerative disease models. This review summarizes recent research in sirtuin neurobiology and neurodegenerative diseases and analyzes the potential of therapeutic applications based on sirtuin research.

SIRT2 ablation has no effect on tubulin acetylation in brain, cholesterol biosynthesis or the progression of Huntington's disease phenotypes in vivo.

Huntington's disease (HD) is a devastating neurodegenerative disorder for which there are no disease-modifying treatments. The molecular pathogenesis of HD is complex and many mechanisms and cellular processes have been proposed as potential sites of therapeutic intervention. However, prior to embarking on drug development initiatives, it is essential that therapeutic targets can be validated in mammalian models of HD. Previous studies in invertebrate and cell culture HD models have suggested that inhibition of SIRT2 could have beneficial consequences on disease progression. SIRT2 is a NAD(+)-dependent deacetylase that has been proposed to deacetylate α-tubulin, histone H4 K16 and to regulate cholesterol biogenesis - a pathway which is dysregulated in HD patients and HD mouse models. We have utilized mice in which SIRT2 has been reduced or ablated to further explore the function of SIRT2 and to assess whether SIRT2 loss has a beneficial impact on disease progression in the R6/2 mouse model of HD. Surprisingly we found that reduction or loss of SIRT2 had no effect on the acetylation of α-tubulin or H4K16 or on cholesterol biosynthesis in the brains of wild type mice. Equally, genetic reduction or ablation of SIRT2 had no effect on HD progression as assessed by a battery of physiological and behavioural tests. Furthermore, we observed no change in aggregate load or levels of soluble mutant huntingtin transprotein. Intriguingly, neither the constitutive genetic loss nor acute pharmacological inhibition of SIRT2 affected the expression of cholesterol biosynthesis enzymes in the context of HD. Therefore, we conclude that SIRT2 inhibition does not modify disease progression in the R6/2 mouse model of HD and SIRT2 inhibition should not be prioritised as a therapeutic option for HD.

SIRT1 protects against α-synuclein aggregation by activating molecular chaperones.

α-Synuclein is a key molecule in the pathogenesis of synucleinopathy including dementia with Lewy bodies, Parkinson's disease, and multiple system atrophy. Sirtuins are NAD(+)-dependent protein deacetylases that are highly conserved and counter aging in lower organisms. We show that the life span of a mouse model with A53T α-synuclein mutation is increased by overexpressing SIRT1 and decreased by knocking out SIRT1 in brain. Furthermore, α-synuclein aggregates are reduced in the brains of mice with SIRT1 overexpression and increased by SIRT1 deletion. We show that SIRT1 deacetylates HSF1 (heat shock factor 1) and increases HSP70 RNA and protein levels, but only in the brains of mice with A53T and SIRT1 expression. Thus, SIRT1 responds to α-synuclein aggregation-induced stress by activating molecular chaperones to protect against disease.

Hepatic overexpression of SIRT1 in mice attenuates endoplasmic reticulum stress and insulin resistance in the liver.

Endoplasmic reticulum (ER) stress has been implicated in the pathophysiology of human type 2 diabetes (T2DM). Although SIRT1 has a therapeutic effect on metabolic deterioration in T2DM, the precise mechanisms by which SIRT1 improves insulin resistance remain unclear. Here, we demonstrate that adenovirus-mediated overexpression of SIRT1 in the liver of diet-induced insulin-resistant low-density lipoprotein receptor-deficient mice and of genetically obese ob/ob mice attenuates hepatic steatosis and ameliorates systemic insulin resistance. These beneficial effects were associated with decreased mammalian target of rapamycin complex 1 (mTORC1) activity, inhibited the unfolded protein response (UPR), and enhanced insulin receptor signaling in the liver, leading to decreased hepatic gluconeogenesis and improved glucose tolerance. The tunicamycin-induced splicing of X-box binding protein-1 and expression of GRP78 and CHOP were reduced by resveratrol in cultured cells in a SIRT1-dependent manner. Conversely, SIRT1-deficient mouse embryonic fibroblasts challenged with tunicamycin exhibited markedly increased mTORC1 activity and impaired ER homeostasi and insulin signaling. These effects were abolished by mTORC1 inhibition by rapamycin in human HepG2 cells. These studies indicate that SIRT1 serves as a negative regulator of UPR signaling in T2DM and that SIRT1 attenuates hepatic steatosis, ameliorates insulin resistance, and restores glucose homeostasis, largely through the inhibition of mTORC1 and ER stress.

SIRT1 suppresses beta-amyloid production by activating the alpha-secretase gene ADAM10.

A hallmark of Alzheimer's disease (AD) is the accumulation of plaques of Abeta 1-40 and 1-42 peptides, which result from the sequential cleavage of APP by the beta and gamma-secretases. The production of Abeta peptides is avoided by alternate cleavage of APP by the alpha and gamma-secretases. Here we show that production of beta-amyloid and plaques in a mouse model of AD are reduced by overexpressing the NAD-dependent deacetylase SIRT1 in brain, and are increased by knocking out SIRT1 in brain. SIRT1 directly activates the transcription of the gene encoding the alpha-secretase, ADAM10. SIRT1 deacetylates and coactivates the retinoic acid receptor beta, a known regulator of ADAM10 transcription. ADAM10 activation by SIRT1 also induces the Notch pathway, which is known to repair neuronal damage in the brain. Our findings indicate SIRT1 activation is a viable strategy to combat AD and perhaps other neurodegenerative diseases.

Aging and disease: connections to sirtuins.

The sirtuins are highly conserved NAD-dependent deacetylases that were shown to regulate lifespan in lower organisms and affect diseases of aging in mammals, such as diabetes, cancer, and inflammation. Most relevant to the amelioration of disease, the SIR2 ortholog SIRT1 has been shown to deacetylate many important transcription factors to exert an overarching influence on numerous metabolic pathways. Here we discuss several diseases of aging for which SIRT1 has been recently shown to confer protection. These findings suggest that manipulating sirtuin activity pharmacologically may be a fruitful area to improve human health.

Neuronal SIRT1 regulates endocrine and behavioral responses to calorie restriction.

Mammalian life span can be extended by both calorie restriction (CR) and mutations that diminish somatotropic signaling. Sirt1 is a mediator of many effects of CR in mammals, but any role in controlling somatotropic signaling has not been shown. Since the somatotropic axis is controlled by the brain, we created mice lacking Sirt1 specifically in the brain and examined the impacts of this manipulation on somatotropic signaling and the CR response. These mutant mice displayed defects in somatotropic signaling when fed ad libitum, and defects in the endocrine and behavioral responses to CR. We conclude that Sirt1 in the brain is a link between somatotropic signaling and CR in mammals.

The 5' end of U2 snRNA is in close proximity to U1 and functional sites of the pre-mRNA in early spliceosomal complexes.

Recognition and pairing of the correct 5' and 3' splice sites (ss) of a pre-mRNA are critical events that occur early during spliceosome assembly. Little is known about the spatial organization in early spliceosomal complexes of the U1 and U2 snRNPs, which together with several non-snRNP proteins, are involved in juxtapositioning the functional sites of the pre-mRNA. To better understand the molecular mechanisms of splice-site recognition/pairing, we have examined the organization of U2 relative to U1 and pre-mRNA in spliceosomal complexes via hydroxyl-radical probing with Fe-BABE-tethered U2 snRNA. These studies reveal that functional sites of the pre-mRNA are located close to the 5' end of U2 both in E and A complexes. U2 is also positioned close to U1 in a defined orientation already in the E complex, and their relative spatial organization remains largely unchanged during the E to A transition.