SIRT2 controls the pentose phosphate switch

SIRT2 and ALDH1A1 as critical enzymes for astrocytic GABA production in Alzheimer's disease

BACKGROUND

Alzheimer's Disease (AD) is a neurodegenerative disease with drastically altered astrocytic metabolism. Astrocytic GABA and H2O2 are associated with memory impairment in AD and synthesized through the Monoamine Oxidase B (MAOB)-mediated multi-step degradation of putrescine. However, the enzymes downstream to MAOB in this pathway remain unidentified.

METHODS

Using transcriptomics analysis, we identified two candidate enzymes, Aldehyde Dehydrogenase 1 family member A1 (ALDH1A1) and Sirtuin 2 (SIRT2) for the steps following MAOB in the astrocytic GABA production pathway. We used immunostaining, metabolite analysis and electrophysiology, both in vitro and in vivo, to confirm the participation of these enzymes in astrocytic GABA production. We checked for the presence of SIRT2 in human AD patients as well as the mouse model APP/PS1 and finally, we selectively ablated SIRT2 in the astrocytes of APP/PS1 mice to observe its effects on pathology.

RESULTS

Immunostaining, metabolite analysis, and electrophysiology recapitulated the participation of ALDH1A1 and SIRT2 in GABA production. Inhibition of SIRT2 reduced the production of astrocytic GABA but not H2O2, a key molecule in neurodegeneration. Elevated expression of these enzymes was found in hippocampal astrocytes of AD patients and APP/PS1 mice. Astrocyte-specific gene-silencing of SIRT2 in APP/PS1 mice restored GABA production and partially improved memory function.

CONCLUSIONS

Our study is the first to identify the specific role of SIRT2 in reactive astrogliosis and determine the specific pathway and metabolic step catalyzed by the enzyme. We determine the partial, yet significant role of ALDH1A1 in this process, thereby highlighting 2 new players the astrocytic GABA production pathway. Our findings therefore, offer SIRT2 as a new tool to segregate GABA from H2O2 production, aiding future research in neurodegenerative diseases.

Metabolic mechanisms orchestrated by Sirtuin family to modulate inflammatory responses

Maintaining metabolic homeostasis is crucial for cellular and organismal health throughout their lifespans. The intricate link between metabolism and inflammation through immunometabolism is pivotal in maintaining overall health and disease progression. The multifactorial nature of metabolic and inflammatory processes makes study of the relationship between them challenging. Homologs of Saccharomyces cerevisiae silent information regulator 2 protein, known as Sirtuins (SIRTs), have been demonstrated to promote longevity in various organisms. As nicotinamide adenine dinucleotide-dependent deacetylases, members of the Sirtuin family (SIRT1-7) regulate energy metabolism and inflammation. In this review, we provide an extensive analysis of SIRTs involved in regulating key metabolic pathways, including glucose, lipid, and amino acid metabolism. Furthermore, we systematically describe how the SIRTs influence inflammatory responses by modulating metabolic pathways, as well as inflammatory cells, mediators, and pathways. Current research findings on the preferential roles of different SIRTs in metabolic disorders and inflammation underscore the potential of SIRTs as viable pharmacological and therapeutic targets. Future research should focus on the development of promising compounds that target SIRTs, with the aim of enhancing their anti-inflammatory activity by influencing metabolic pathways within inflammatory cells.

NMN partially rescues cuproptosis by upregulating sirt2 to increase intracellular NADPH

Cuproptosis is characterized by lipoylated protein aggregation and loss of iron-sulfur (Fe-S) proteins, which are crucial for a wide range of important cellular functions, including DNA replication and damage repair. Sirt2 and sirt4 are lipoamidases that remove the lipoyl moiety from lipoylated proteins using nicotinamide adenine dinucleotide (NAD+) as a cofactor. However, to date, it is not clear whether nicotinamide mononucleotide (NMN), a precursor of NAD+, affects cellular sensitivity to cuproptosis. Therefore, in the current study, cuproptosis was induced by the copper (Cu) ionophore elesclomol (Es) in HeLa cells. It was also found that Es/Cu treatment increased cellular DNA damage level. On the other hand, NMN treatment partially rescued cuproptosis in a dose-dependent manner, as well as reduced cellular DNA damage level. In addition, NMN upregulated the expression of Fe-S protein POLD1, without affecting the aggregation of lipoylated proteins. Mechanistic study revealed that NMN increased the expression of sirt2 and cellular reduced nicotinamide adenine dinucleotide phosphate (NADPH) level. Overexpression of sirt2 and sirt4 did not change the aggregation of lipoylated proteins, however, sirt2, but not sirt4, increased cellular NADPH levels and partially rescued cuproptosis. Inhibition of NAD+ kinase (NADK), which is responsible for generating NADPH, abolished the rescuing function of NMN and sirt2 for Es/Cu induced cell death. Taken together, our results suggested that DNA damage is a characteristic feature of cuproptosis. NMN can partially rescue cuproptosis by upregulating sirt2, increase intracellular NADPH content and maintain the level of Fe-S proteins, independent of the lipoamidase activity of sirt2.

Exploring histone deacetylases in type 2 diabetes mellitus: pathophysiological insights and therapeutic avenues

Diabetes mellitus is a chronic disease that impairs metabolism, and its prevalence has reached an epidemic proportion globally. Most people affected are with type 2 diabetes mellitus (T2DM), which is caused by a decline in the numbers or functioning of pancreatic endocrine islet cells, specifically the β-cells that release insulin in sufficient quantity to overcome any insulin resistance of the metabolic tissues. Genetic and epigenetic factors have been implicated as the main contributors to the T2DM. Epigenetic modifiers, histone deacetylases (HDACs), are enzymes that remove acetyl groups from histones and play an important role in a variety of molecular processes, including pancreatic cell destiny, insulin release, insulin production, insulin signalling, and glucose metabolism. HDACs also govern other regulatory processes related to diabetes, such as oxidative stress, inflammation, apoptosis, and fibrosis, revealed by network and functional analysis. This review explains the current understanding of the function of HDACs in diabetic pathophysiology, the inhibitory role of various HDAC inhibitors (HDACi), and their functional importance as biomarkers and possible therapeutic targets for T2DM. While their role in T2DM is still emerging, a better understanding of the role of HDACi may be relevant in improving insulin sensitivity, protecting β-cells and reducing T2DM-associated complications, among others.

Metabolomics analysis to interpret changes in physiological and metabolic responses to chronic heat stress in Pekin ducks

Heat stress (HS) is a major environmental threat that affects duck production in subtropical and tropical regions, especially in summer. This study aimed to evaluate the physiological and metabolic responses of Pekin ducks to chronic HS conditions via liquid chromatography-mass spectrometry (LC-MS) using a paired-fed (PF) experimental design. On the basis of equivalent feed intake (HS vs. PF), HS significantly reduced growth performance and the percentage of leg and breast muscles, however, markedly increased the percentage of abdominal fat and breast skin fat. Serum metabolomics results revealed that heat-stressed ducks showed enhanced glycolysis and pentose phosphate pathways, as demonstrated by higher glucose 6-phosphate and 6-phogluconic acid levels in the PF vs. HS comparison. HS decreased hepatic mRNA levels of mitochondrial fatty acid β-oxidation-related genes (MCAD and SCAD) compared to the PF group, resulting in acetylcarnitine accumulation in serum. Moreover, HS elevated the concentrations of serum amino acids and mRNA levels of ubiquitination-related genes (MuRF1 and MAFbx) in the skeletal muscle and amino acid transporter-related genes (SLC1A1 and SLC7A1) and gluconeogenesis-related genes (PCK1 and PCase) in the liver compared to the PF group. When compared to the normal control group (NC), HS further decreased growth performance, but it elevated the abdominal fat rate. However, increased mRNA levels of ubiquitination-related genes and serum amino acid accumulation were not observed in the HS group compared to the NC group, implying that reduced feed intake masked the effect of HS on skeletal muscle breakdown and is a form of protection for the organism. These results suggest that chronic HS induces protein degradation in the skeletal muscle to provide amino acids for hepatic gluconeogenesis to provide sufficient energy, as Pekin ducks under HS conditions failed to efficiently oxidise fatty acids and ketones in the mitochondria, leading to poor growth performance and slaughter characteristics.

SIRT2 transgenic over-expression does not impact lifespan in mice

The NAD+ -dependent deacylase family of sirtuin enzymes have been implicated in biological ageing, late-life health and overall lifespan, though of these members, a role for sirtuin-2 (SIRT2) is less clear. Transgenic overexpression of SIRT2 in the BubR1 hypomorph model of progeria can rescue many aspects of health and increase overall lifespan, due to a specific interaction between SIRT2 and BubR1 that improves the stability of this protein. It is less clear whether SIRT2 is relevant to biological ageing outside of a model where BubR1 is under-expressed. Here, we sought to test whether SIRT2 over-expression would impact the overall health and lifespan of mice on a nonprogeroid, wild-type background. While we previously found that SIRT2 transgenic overexpression prolonged female fertility, here, we did not observe any additional impact on health or lifespan, which was measured in both male and female mice on standard chow diets, and in males challenged with a high-fat diet. At the biochemical level, NMR studies revealed an increase in total levels of a number of metabolites in the brain of SIRT2-Tg animals, pointing to a potential impact in cell composition; however, this did not translate into functional differences. Overall, we conclude that strategies to enhance SIRT2 protein levels may not lead to increased longevity.

Variation in pentose phosphate pathway-associated metabolism dictates cytotoxicity outcomes determined by tetrazolium reduction assays

Tetrazolium reduction and resazurin assays are the mainstay of routine in vitro toxicity batteries. However, potentially erroneous characterization of cytotoxicity and cell proliferation can arise if verification of baseline interaction of test article with method employed is neglected. The current investigation aimed to demonstrate how interpretation of results from several standard cytotoxicity and proliferation assays vary in dependence on contributions from the pentose phosphate pathway (PPP). Non-tumorigenic Beas-2B cells were treated with graded concentrations of benzo[a]pyrene (B[a]P) for 24 and 48 h prior to cytotoxicity and proliferation assessment with commonly used MTT, MTS, WST1, and Alamar Blue assays. B[a]P caused enhanced metabolism of each dye assessed despite reductions in mitochondrial membrane potential and was reversed by 6-aminonicotinamide (6AN)-a glucose-6-phosphate dehydrogenase inhibitor. These results demonstrate differential sensitivity of standard cytotoxicity assessments on the PPP, thus (1) decoupling "mitochondrial activity" as an interpretation of cellular formazan and Alamar Blue metabolism, and (2) demonstrating the implicit requirement for investigators to sufficiently verify interaction of these methods in routine cytotoxicity and proliferation characterization. The nuances of method-specific extramitochondrial metabolism must be scrutinized to properly qualify specific endpoints employed, particularly under the circumstances of metabolic reprogramming.

Metabolic Regulation of Hematopoietic Stem Cells

Cellular metabolism is a key regulator of hematopoietic stem cell (HSC) maintenance. HSCs rely on anaerobic glycolysis for energy production to minimize the production of reactive oxygen species and shift toward mitochondrial oxidative phosphorylation upon differentiation. However, increasing evidence has shown that HSCs still maintain a certain level of mitochondrial activity in quiescence, and exhibit high mitochondrial membrane potential, which both support proper HSC function. Since glycolysis and the tricarboxylic acid (TCA) cycle are not directly connected in HSCs, other nutrient pathways, such as amino acid and fatty acid metabolism, generate acetyl-CoA and provide it to the TCA cycle. In this review, we discuss recent insights into the regulatory roles of cellular metabolism in HSCs. Understanding the metabolic requirements of healthy HSCs is of critical importance to the development of new therapies for hematological disorders.

Mitochondria Turnover and Lysosomal Function in Hematopoietic Stem Cell Metabolism

Hematopoietic stem cells (HSCs) reside in a hypoxic microenvironment that enables glycolysis-fueled metabolism and reduces oxidative stress. Nonetheless, metabolic regulation in organelles such as the mitochondria and lysosomes as well as autophagic processes have been implicated as essential for the determination of HSC cell fate. This review encompasses the current understanding of anaerobic metabolism in HSCs as well as the emerging roles of mitochondrial metabolism and lysosomal regulation for hematopoietic homeostasis.

Bioactivity Profiles of Cytoprotective Short-Chain Quinones

Short-chain quinones (SCQs) have been investigated as potential therapeutic candidates against mitochondrial dysfunction, which was largely thought to be associated with the reversible redox characteristics of their active quinone core. We recently reported a library of SCQs, some of which showed potent cytoprotective activity against the mitochondrial complex I inhibitor rotenone in the human hepatocarcinoma cell line HepG2. To better characterize the cytoprotection of SCQs at a molecular level, a bioactivity profile for 103 SCQs with different compound chemistries was generated that included metabolism related markers, redox activity, expression of cytoprotective proteins and oxidative damage. Of all the tested endpoints, a positive correlation with cytoprotection by SCQs in the presence of rotenone was only observed for the NAD(P)H:quinone oxidoreductase 1 (NQO1)-dependent reduction of SCQs, which also correlated with an acute rescue of ATP levels. The results of this study suggest an unexpected mode of action for SCQs that appears to involve a modification of NQO1-dependent signaling rather than a protective effect by the reduced quinone itself. This finding presents a new selection strategy to identify and develop the most promising compounds towards their clinical use.

NAD+ metabolism, stemness, the immune response, and cancer

NAD⁺ was discovered during yeast fermentation, and since its discovery, its important roles in redox metabolism, aging, and longevity, the immune system and DNA repair have been highlighted. A deregulation of the NAD⁺ levels has been associated with metabolic diseases and aging-related diseases, including neurodegeneration, defective immune responses, and cancer. NAD⁺ acts as a cofactor through its interplay with NADH, playing an essential role in many enzymatic reactions of energy metabolism, such as glycolysis, oxidative phosphorylation, fatty acid oxidation, and the TCA cycle. NAD⁺ also plays a role in deacetylation by sirtuins and ADP ribosylation during DNA damage/repair by PARP proteins. Finally, different NAD hydrolase proteins also consume NAD⁺ while converting it into ADP-ribose or its cyclic counterpart. Some of these proteins, such as CD38, seem to be extensively involved in the immune response. Since NAD cannot be taken directly from food, NAD metabolism is essential, and NAMPT is the key enzyme recovering NAD from nicotinamide and generating most of the NAD cellular pools. Because of the complex network of pathways in which NAD⁺ is essential, the important role of NAD⁺ and its key generating enzyme, NAMPT, in cancer is understandable. In the present work, we review the role of NAD⁺ and NAMPT in the ways that they may influence cancer metabolism, the immune system, stemness, aging, and cancer. Finally, we review some ongoing research on therapeutic approaches.

NAD+ in Brain Aging and Neurodegenerative Disorders

NAD⁺ is a pivotal metabolite involved in cellular bioenergetics, genomic stability, mitochondrial homeostasis, adaptive stress responses, and cell survival. Multiple NAD⁺-dependent enzymes are involved in synaptic plasticity and neuronal stress resistance. Here, we review emerging findings that reveal key roles for NAD⁺ and related metabolites in the adaptation of neurons to a wide range of physiological stressors and in counteracting processes in neurodegenerative diseases, such as those occurring in Alzheimer's, Parkinson's, and Huntington diseases, and amyotrophic lateral sclerosis. Advances in understanding the molecular and cellular mechanisms of NAD⁺-based neuronal resilience will lead to novel approaches for facilitating healthy brain aging and for the treatment of a range of neurological disorders.

SIRT2 activates G6PD to enhance NADPH production and promote leukaemia cell proliferation

Like most other types of cancer cells, leukaemia cells undergo metabolic reprogramming to support rapid proliferation through enhancing biosynthetic processes. Pentose phosphate pathway (PPP) plays a pivotal role in meeting the anabolic demands for cancer cells. However, the molecular mechanism by which PPP contributes to leukaemia remains elusive. Here, we report that leukaemia cell proliferation is dependent on the oxidative branch of PPP, in particular the first and rate-limiting enzyme glucose-6-phosphate dehydrogenase (G6PD). Knockdown of G6PD reduces NADPH level in acute myeloid leukaemia (AML) cell lines. Exogenous lipid supplements partially restore the proliferation of G6PD-depleted cells. Deacetylase SIRT2 promotes NADPH production through deacetylating G6PD at lysine 403 (K403). Activation of G6PD by SIRT2 supports the proliferation and clonogenic activity of leukaemia cells. Chemical inhibitors against SIRT2 suppress G6PD activity, leading to reduced cell proliferation of leukaemia cells, but not normal hematopoietic stem and progenitor cells. Importantly, SIRT2 is overexpressed in clinical AML samples, while K403 acetylation is downregulated and G6PD catalytic activity is increased comparing to that of normal control. Together, our study reveals that acetylation regulation of G6PD is involved in the metabolic reprogramming of AML, and SIRT2 serves as a promising target for further therapeutic investigations.