Reciprocal regulation between nicotinamide adenine dinucleotide metabolism and abscisic acid and stress response pathways in Arabidopsis

Nicotinamide adenine dinucleotide (NAD) is an essential coenzyme that has emerged as a central hub linking redox equilibrium and signal transduction in living organisms. The homeostasis of NAD is required for plant growth, development, and adaption to environmental cues. In this study, we isolated a chilling hypersensitive Arabidopsis thaliana mutant named qs-2 and identified the causal mutation in the gene encoding quinolinate synthase (QS) critical for NAD biosynthesis. The qs-2 mutant is also hypersensitive to salt stress and abscisic acid (ABA) but resistant to drought stress. The qs-2 mutant accumulates a reduced level of NAD and over-accumulates reactive oxygen species (ROS). The ABA-hypersensitivity of qs-2 can be rescued by supplementation of NAD precursors and by mutations in the ABA signaling components SnRK2s or RBOHF. Furthermore, ABA-induced over-accumulation of ROS in the qs-2 mutant is dependent on the SnRK2s and RBOHF. The expression of QS gene is repressed directly by ABI4, a transcription factor in the ABA response pathway. Together, our findings reveal an unexpected interplay between NAD biosynthesis and ABA and stress signaling, which is critical for our understanding of the regulation of plant growth and stress responses.

N-terminal protein acetylation by NatB modulates the levels of Nmnats, the NAD+ biosynthetic enzymes in Saccharomyces cerevisiae

NAD+ is an essential metabolite participating in cellular biochemical processes and signaling. The regulation and interconnection among multiple NAD+ biosynthesis pathways are incompletely understood. Yeast (Saccharomyces cerevisiae) cells lacking the N-terminal (Nt) protein acetyltransferase complex NatB exhibit an approximate 50% reduction in NAD+ levels and aberrant metabolism of NAD+ precursors, changes that are associated with a decrease in nicotinamide mononucleotide adenylyltransferase (Nmnat) protein levels. Here, we show that this decrease in NAD+ and Nmnat protein levels is specifically due to the absence of Nt-acetylation of Nmnat (Nma1 and Nma2) proteins and not of other NatB substrates. Nt-acetylation critically regulates protein degradation by the N-end rule pathways, suggesting that the absence of Nt-acetylation may alter Nmnat protein stability. Interestingly, the rate of protein turnover (t½) of non-Nt-acetylated Nmnats did not significantly differ from those of Nt-acetylated Nmnats. Accordingly, deletion or depletion of the N-end rule pathway ubiquitin E3 ligases in NatB mutants did not restore NAD+ levels. Next, we examined whether the status of Nt-acetylation would affect the translation of Nmnats, finding that the absence of Nt-acetylation does not significantly alter the polysome formation rate on Nmnat mRNAs. However, we observed that NatB mutants have significantly reduced Nmnat protein maturation. Our findings indicate that the reduced Nmnat levels in NatB mutants are mainly due to inefficient protein maturation. Nmnat activities are essential for all NAD+ biosynthesis routes, and understanding the regulation of Nmnat protein homeostasis may improve our understanding of the molecular basis and regulation of NAD+ metabolism.

Quinolinate as a Marker for Kynurenine Metabolite Formation and the Unresolved Question of NAD+ Synthesis During Inflammation and Infection

Quinolinate (Quin) is a classic example of a biochemical double-edged sword, acting as both essential metabolite and potent neurotoxin. Quin is an important metabolite in the kynurenine pathway of tryptophan catabolism leading to the de novo synthesis of nicotinamide adenine dinucleotide (NAD+). As a precursor for NAD+, Quin can direct a portion of tryptophan catabolism toward replenishing cellular NAD+ levels in response to inflammation and infection. Intracellular Quin levels increase dramatically in response to immune stimulation [e.g., lipopolysaccharide (LPS) or pokeweed mitogen (PWM)] in macrophages, microglia, dendritic cells, and other cells of the immune system. NAD+ serves numerous functions including energy production, the poly ADP ribose polymerization (PARP) reaction involved in DNA repair, and the activity of various enzymes such as the NAD+-dependent deacetylases known as sirtuins. We used highly specific antibodies to protein-coupled Quin to delineate cells that accumulate Quin as a key aspect of the response to immune stimulation and infection. Here, we describe Quin staining in the brain, spleen, and liver after LPS administration to the brain or systemic PWM administration. Quin expression was strong in immune cells in the periphery after both treatments, whereas very limited Quin expression was observed in the brain even after direct LPS injection. Immunoreactive cells exhibited diverse morphology ranging from foam cells to cells with membrane extensions related to cell motility. We also examined protein expression changes in the spleen after kynurenine administration. Acute (8 h) and prolonged (48 h) kynurenine administration led to significant changes in protein expression in the spleen, including multiple changes involved with cytoskeletal rearrangements associated with cell motility. Kynurenine administration resulted in several expression level changes in proteins associated with heat shock protein 90 (HSP90), a chaperone for the aryl-hydrocarbon receptor (AHR), which is the primary kynurenine metabolite receptor. We propose that cells with high levels of Quin are those that are currently releasing kynurenine pathway metabolites as well as accumulating Quin for sustained NAD+ synthesis from tryptophan. Further, we propose that the kynurenine pathway may be linked to the regulation of cell motility in immune and cancer cells.

The NAD+-mitophagy axis in healthy longevity and in artificial intelligence-based clinical applications

Nicotinamide adenine dinucleotide (NAD+) is an important natural molecule involved in fundamental biological processes, including the TCA cycle, OXPHOS, β-oxidation, and is a co-factor for proteins promoting healthy longevity. NAD+ depletion is associated with the hallmarks of ageing and may contribute to a wide range of age-related diseases including metabolic disorders, cancer, and neurodegenerative diseases. One of the central pathways by which NAD+ promotes healthy ageing is through regulation of mitochondrial homeostasis via mitochondrial biogenesis and the clearance of damaged mitochondria via mitophagy. Here, we highlight the contribution of the NAD+-mitophagy axis to ageing and age-related diseases, and evaluate how boosting NAD+ levels may emerge as a promising therapeutic strategy to counter ageing as well as neurodegenerative diseases including Alzheimer's disease. The potential use of artificial intelligence to understand the roles and molecular mechanisms of the NAD+-mitophagy axis in ageing is discussed, including possible applications in drug target identification and validation, compound screening and lead compound discovery, biomarker development, as well as efficacy and safety assessment. Advances in our understanding of the molecular and cellular roles of NAD+ in mitophagy will lead to novel approaches for facilitating healthy mitochondrial homoeostasis that may serve as a promising therapeutic strategy to counter ageing-associated pathologies and/or accelerated ageing.

Mitochondrial oxidative capacity and NAD+ biosynthesis are reduced in human sarcopenia across ethnicities

The causes of impaired skeletal muscle mass and strength during aging are well-studied in healthy populations. Less is known on pathological age-related muscle wasting and weakness termed sarcopenia, which directly impacts physical autonomy and survival. Here, we compare genome-wide transcriptional changes of sarcopenia versus age-matched controls in muscle biopsies from 119 older men from Singapore, Hertfordshire UK and Jamaica. Individuals with sarcopenia reproducibly demonstrate a prominent transcriptional signature of mitochondrial bioenergetic dysfunction in skeletal muscle, with low PGC-1α/ERRα signalling, and downregulation of oxidative phosphorylation and mitochondrial proteostasis genes. These changes translate functionally into fewer mitochondria, reduced mitochondrial respiratory complex expression and activity, and low NAD+ levels through perturbed NAD+ biosynthesis and salvage in sarcopenic muscle. We provide an integrated molecular profile of human sarcopenia across ethnicities, demonstrating a fundamental role of altered mitochondrial metabolism in the pathological loss of skeletal muscle mass and function in older people.

GC/MS and LC/MS-based Tissue Metabolomic Analysis Detected Increased Levels of Antioxidant Metabolites in Colorectal Cancer

Late-stage colorectal cancer is resistant to current treatments. Understanding the biological processes responsible for the development and progression of colorectal cancer could aid the development of new diagnostic and treatment approaches. We used gas chromatography/mass spectrometry and liquid chromatography/mass spectrometry-based metabolomic analysis to measure metabolite levels in pairs of colorectal cancer tissue samples and samples of the adjacent macroscopically normal mucosal tissue from 10 colon cancer patients. Regarding nucleotide metabolomic intermediates, the colorectal cancer tissue contained lower levels of ribulose 5-phosphate and higher levels of xanthine, adenine, and hypoxanthine than the normal tissue. The levels of antioxidant metabolites, such as sulfur-containing amino acids, were also significantly higher in the colorectal cancer tissue. The level of tryptophan was decreased, and the levels of molecules downstream of the tryptophan pathway, such as kynurenine and quinolinic acid, which protect colorectal cancer against the host's immune system and function in de novo nicotinamide adenine dinucleotide synthesis, were increased in the colorectal cancer tissue. The colorectal cancer tissue samples also contained higher levels of lysophospholipids and fatty acids, especially stearic acid and polyunsaturated fatty acids, including arachidonic acid and docosahexaenoic acid. Thus, understanding these cancer-specific alterations could make it possible to detect colorectal cancer early and aid the development of additional treatments for the disease, leading to improvements in colorectal cancer patients' quality of life.

Developing a single strain for in vitro salvage synthesis of NAD+ at high temperatures and its potential for bioconversion

BACKGROUND

Thermostable enzymes have several advantages over their mesophilic counterparts for industrial applications. However, trade-offs such as thermal instability of enzyme substrates or co-factors exist. Nicotinamide adenine dinucleotide (NAD+) is an important co-factor in many enzyme-catalyzed oxidation-reduction reactions. This compound spontaneously decomposes at elevated temperatures and basic pH, a property that limits catalysis of NAD+/NADH-dependent bioconversions using thermostable enzymes to short timeframes. To address this issue, an "in vitro metabolic pathway" for salvage synthesis of NAD+ using six thermophilic enzymes was constructed to resynthesize NAD+ from its thermal decomposition products at high temperatures.

RESULTS

An integrated strain, E. coli DH5α (pBR-CI857, pGETS118-NAD+), that codes for six thermophilic enzymes in a single operon was constructed. Gene-expression levels of these enzymes in the strain were modulated by their sequential order in the operon. An enzyme solution containing these enzymes was prepared by the heat purification from the cell lysate of the integrated strain, and used as an enzyme cocktail for salvage synthesis of NAD+. The salvage activity for synthesis of NAD+ from its thermal decomposition products was found to be 0.137 ± 0.006 µmol min-1 g-1 wet cells. More than 50% of this initial activity remained after 24 h at 60 °C. The enzyme cocktail could maintain a NAD+ concentration of 1 mM for 12 h at 60 °C. Furthermore, this enzyme cocktail supported continuous NAD+/NADH-dependent redox reactions using only NAD+/NADH derived from host cells, without the need for addition of external NAD+.

CONCLUSIONS

The integrated strain allows preparation of an enzyme cocktail that can solve the problem of NAD+ instability at high temperatures. The strain simplifies preparation of the enzyme cocktail, and thus expands the applicability of the in vitro metabolic engineering method using thermostable enzymes. Further optimization of gene expressions in the integrated strain can be achieved by using various types of ribosome binding sites as well as promoters.

Quantitative analysis of the effects of nicotinamide phosphoribosyltransferase induction on the rates of NAD+ synthesis and breakdown in mammalian cells using stable isotope-labeling combined with mass spectrometry

NAD+ is mainly synthesized from nicotinamide (Nam) by the rate-limiting enzyme Nam phosphoribosyltransferase (Nampt) and degraded to Nam by NAD+-degrading enzymes in mammals. Numerous studies report that tissue NAD+ levels decrease during aging and age-related diseases and suggest that NAD+ replenishment promotes healthy aging. Although increased expression of Nampt might be a promising intervention for healthy aging, forced expression of Nampt gene, inducing more than 10-fold increases in the enzyme protein level, has been reported to elevate NAD+ levels only 40-60% in mammalian cells. Mechanisms underlying the limited increases in NAD+ levels remain to be determined. Here we show that Nampt is inhibited in cells and that enhanced expression of Nampt activates NAD+ breakdown. Combined with the measurement of each cell's volume, we determined absolute values (μM/h) of the rates of NAD+ synthesis (RS) and breakdown (RB) using a flux assay with a 2H (D)-labeled Nam, together with the absolute NAD+ concentrations in various mammalian cells including primary cultured cardiomyocytes under the physiological conditions and investigated the relations among total cellular Nampt activity, RS, RB, and the NAD+ concentration. NAD+ concentration was maintained within a narrow range (400-700 μM) in the cells. RS was much smaller than the total Nampt activity, indicating that NAD+ synthesis from Nam in the cells is suppressed. Forced expression of Nampt leading to 6-fold increase in total Nampt activity induced only a 1.6-fold increase in cellular NAD+ concentration. Under the conditions, RS increased by 2-fold, while 2-fold increase in RB was also observed. The small increase in cellular NAD+ concentration is likely due to both inhibited increase in the NAD+ synthesis and the activation of its breakdown. Our findings suggest that cellular NAD+ concentrations do not vary dramatically by the physiological fluctuation of Nampt expression and show the tight link between the NAD+ synthesis and its breakdown.

Dual control of NAD+ synthesis by purine metabolites in yeast

Metabolism is a highly integrated process resulting in energy and biomass production. While individual metabolic routes are well characterized, the mechanisms ensuring crosstalk between pathways are poorly described, although they are crucial for homeostasis. Here, we establish a co-regulation of purine and pyridine metabolism in response to external adenine through two separable mechanisms. First, adenine depletion promotes transcriptional upregulation of the de novo NAD⁺ biosynthesis genes by a mechanism requiring the key-purine intermediates ZMP/SZMP and the Bas1/Pho2 transcription factors. Second, adenine supplementation favors the pyridine salvage route resulting in an ATP-dependent increase of intracellular NAD⁺. This control operates at the level of the nicotinic acid mononucleotide adenylyl-transferase Nma1 and can be bypassed by overexpressing this enzyme. Therefore, in yeast, pyridine metabolism is under the dual control of ZMP/SZMP and ATP, revealing a much wider regulatory role for these intermediate metabolites in an integrated biosynthesis network.

The E3 ligase Highwire promotes synaptic transmission by targeting the NAD-synthesizing enzyme dNmnat

The ubiquitin ligase Highwire restrains synaptic growth and promotes evoked neurotransmission at NMJ synapses in Drosophila Highwire regulates synaptic morphology by downregulating the MAP3K Wallenda, but excess Wallenda signaling does not account for the decreased presynaptic release observed in highwire mutants. Hence, Highwire likely has a second substrate that inhibits neurotransmission. Highwire targets the NAD+ biosynthetic and axoprotective enzyme dNmnat to regulate axonal injury responses. dNmnat localizes to synapses and interacts with the active zone protein Bruchpilot, leading us to hypothesize that Highwire promotes evoked release by downregulating dNmnat. Here, we show that excess dNmnat is necessary in highwire mutants and sufficient in wild-type larvae to reduce quantal content, likely via disruption of active zone ultrastructure. Catalytically active dNmnat is required to drive defects in evoked release, and depletion of a second NAD+ synthesizing enzyme is sufficient to suppress these defects in highwire mutants, suggesting that excess NAD+ biosynthesis is the mechanism inhibiting neurotransmission. Thus, Highwire downregulates dNmnat to promote evoked synaptic release, suggesting that Highwire balances the axoprotective and synapse-inhibitory functions of dNmnat.

De novo NAD synthesis is required for intracellular replication of Coxiella burnetii, the causative agent of the neglected zoonotic disease Q fever

Coxiella burnetii is an intracellular Gram-negative bacterium responsible for the important zoonotic disease Q fever. Improved genetic tools and the ability to grow this bacterium in host cell-free media has advanced the study of C. burnetii pathogenesis, but the mechanisms that allow it to survive inside the hostile phagolysosome remain incompletely understood. Previous screening of a transposon mutant library for replication within HeLa cells has suggested that nadB, encoding a putative l-aspartate oxidase required for de novo NAD synthesis, is needed for intracellular replication. Here, using genetic complementation of two independent nadB mutants and intracellular replication assays, we confirmed this finding. Untargeted metabolite analyses demonstrated key changes in metabolites in the NAD biosynthetic pathway in the nadB mutant compared with the WT, confirming the involvement of NadB in de novo NAD synthesis. Bioinformatic analysis revealed the presence of a functionally conserved arginine residue at position 275. Using site-directed mutagenesis to substitute this residue with leucine, which abolishes the activity of Escherichia coli NadB, and expression of WT and R275L GST-NadB fusion proteins in E. coli JM109, we found that purified recombinant WT GST-NadB has l-aspartate oxidase activity and that the R275L NadB variant is inactive. Complementation of the C. burnetii nadB mutant with a plasmid expressing this inactive R275L NadB failed to restore replication to WT levels, confirming the link between de novo NAD synthesis and intracellular replication of C. burnetii This suggests that targeting this prokaryotic-specific pathway could advance the development of therapeutics to combat C. burnetii infections.

New approaches to NAD(P)H regeneration in the biosynthesis systems

Nicotinamide adenine dinucleotide (NADH) and nicotinamide adenine dinucleotide phosphate (NADPH), as two kinds of well-known cofactor, are widely used in the most of enzymatic redox reactions, playing an important role in industrial catalysis. In general, supply of NAD(P)H is a major challenged factor in redox fermentation systems due to its high cost and low stability, which have stimulated the development of NADH regeneration systems in recent years. Until now, a series of NAD(P)H regeneration systems have been developed. This review focuses primarily on new approaches of NAD(P)H cofactor regeneration in the biosynthesis systems, such as single cell in vivo NADH regeneration system, double cell coupling NADH regeneration system, in vitro enzyme-coupled NADH regeneration system, microbial cell surface display NADH regeneration system. Finally, the prospect and tendency of NADH regeneration are discussed.

Characterization of l-aspartate oxidase from Arabidopsis thaliana

The flavoprotein l-aspartate oxidase (LASPO) is the first enzyme of the de novo biosynthetic pathway of NAD⁺ in plants. Although LASPO is considered pivotal to maintain NAD⁺ homeostasis, it has not been hitherto characterized in plants. Here, the cDNA encoding the LASPO from the model plant Arabidopsis thaliana (AtLASPO, At5g14760) has been cloned and expressed in Escherichia coli for subsequent enzyme characterization. The purified AtLASPO enzyme displayed a K_m of 0.79 mM for l-aspartate and a k_cat of 0.25 s^-1. We could further detect an l-aspartate: fumarate oxidoreductase activity of the recombinant plant enzyme. In addition, results indicated that NADP⁺ but not NAD⁺, and even more strongly NADH, inhibited AtLASPO at physiological concentrations by competing with the flavin for binding to the apoprotein. LASPO optimal pH and temperature, as well as plastidial pyridine nucleotide concentrations may contribute to an increased NAD⁺ production in planta. Moreover, in Arabidopsis thaliana AtLASPO gene expression exhibited a clear correlation between LASPO activity and NAD⁺ levels, thus demonstrating that plant LASPO catalyzes a key metabolic step of NAD⁺ synthesis.

Quantitative Analysis of NAD Synthesis-Breakdown Fluxes

The redox cofactor nicotinamide adenine dinucleotide (NAD) plays a central role in metabolism and is a substrate for signaling enzymes including poly-ADP-ribose-polymerases (PARPs) and sirtuins. NAD concentration falls during aging, which has triggered intense interest in strategies to boost NAD levels. A limitation in understanding NAD metabolism has been reliance on concentration measurements. Here, we present isotope-tracer methods for NAD flux quantitation. In cell lines, NAD was made from nicotinamide and consumed largely by PARPs and sirtuins. In vivo, NAD was made from tryptophan selectively in the liver, which then excreted nicotinamide. NAD fluxes varied widely across tissues, with high flux in the small intestine and spleen and low flux in the skeletal muscle. Intravenous administration of nicotinamide riboside or mononucleotide delivered intact molecules to multiple tissues, but the same agents given orally were metabolized to nicotinamide in the liver. Thus, flux analysis can reveal tissue-specific NAD metabolism.

DNA repair deficiency sensitizes lung cancer cells to NAD+ biosynthesis blockade

Synthetic lethality is an efficient mechanism-based approach to selectively target DNA repair defects. Excision repair cross-complementation group 1 (ERCC1) deficiency is frequently found in non-small-cell lung cancer (NSCLC), making this DNA repair protein an attractive target for exploiting synthetic lethal approaches in the disease. Using unbiased proteomic and metabolic high-throughput profiling on a unique in-house-generated isogenic model of ERCC1 deficiency, we found marked metabolic rewiring of ERCC1-deficient populations, including decreased levels of the metabolite NAD+ and reduced expression of the rate-limiting NAD+ biosynthetic enzyme nicotinamide phosphoribosyltransferase (NAMPT). We also found reduced NAMPT expression in NSCLC samples with low levels of ERCC1. These metabolic alterations were a primary effect of ERCC1 deficiency, and caused selective exquisite sensitivity to small-molecule NAMPT inhibitors, both in vitro - ERCC1-deficient cells being approximately 1,000 times more sensitive than ERCC1-WT cells - and in vivo. Using transmission electronic microscopy and functional metabolic studies, we found that ERCC1-deficient cells harbor mitochondrial defects. We propose a model where NAD+ acts as a regulator of ERCC1-deficient NSCLC cell fitness. These findings open therapeutic opportunities that exploit a yet-undescribed nuclear-mitochondrial synthetic lethal relationship in NSCLC models, and highlight the potential for targeting DNA repair/metabolic crosstalks for cancer therapy.

NAD Deficiency, Congenital Malformations, and Niacin Supplementation

BACKGROUND

Congenital malformations can be manifested as combinations of phenotypes that co-occur more often than expected by chance. In many such cases, it has proved difficult to identify a genetic cause. We sought the genetic cause of cardiac, vertebral, and renal defects, among others, in unrelated patients.

METHODS

We used genomic sequencing to identify potentially pathogenic gene variants in families in which a person had multiple congenital malformations. We tested the function of the variant by using assays of in vitro enzyme activity and by quantifying metabolites in patient plasma. We engineered mouse models with similar variants using the CRISPR (clustered regularly interspaced short palindromic repeats)-Cas9 system.

RESULTS

Variants were identified in two genes that encode enzymes of the kynurenine pathway, 3-hydroxyanthranilic acid 3,4-dioxygenase (HAAO) and kynureninase (KYNU). Three patients carried homozygous variants predicting loss-of-function changes in the HAAO or KYNU proteins (HAAO p.D162*, HAAO p.W186*, or KYNU p.V57Efs*21). Another patient carried heterozygous KYNU variants (p.Y156* and p.F349Kfs*4). The mutant enzymes had greatly reduced activity in vitro. Nicotinamide adenine dinucleotide (NAD) is synthesized de novo from tryptophan through the kynurenine pathway. The patients had reduced levels of circulating NAD. Defects similar to those in the patients developed in the embryos of Haao-null or Kynu-null mice owing to NAD deficiency. In null mice, the prevention of NAD deficiency during gestation averted defects.

CONCLUSIONS

Disruption of NAD synthesis caused a deficiency of NAD and congenital malformations in humans and mice. Niacin supplementation during gestation prevented the malformations in mice. (Funded by the National Health and Medical Research Council of Australia and others.).

One-carbon metabolism in cancer

Cells require one-carbon units for nucleotide synthesis, methylation and reductive metabolism, and these pathways support the high proliferative rate of cancer cells. As such, anti-folates, drugs that target one-carbon metabolism, have long been used in the treatment of cancer. Amino acids, such as serine are a major one-carbon source, and cancer cells are particularly susceptible to deprivation of one-carbon units by serine restriction or inhibition of de novo serine synthesis. Recent work has also begun to decipher the specific pathways and sub-cellular compartments that are important for one-carbon metabolism in cancer cells. In this review we summarise the historical understanding of one-carbon metabolism in cancer, describe the recent findings regarding the generation and usage of one-carbon units and explore possible future therapeutics that could exploit the dependency of cancer cells on one-carbon metabolism.

Nicotinamide adenine dinucleotide biosynthesis promotes liver regeneration

The regenerative capacity of the liver is essential for recovery from surgical resection or injuries induced by trauma or toxins. During liver regeneration, the concentration of nicotinamide adenine dinucleotide (NAD) falls, at least in part due to metabolic competition for precursors. To test whether NAD availability restricts the rate of liver regeneration, we supplied nicotinamide riboside (NR), an NAD precursor, in the drinking water of mice subjected to partial hepatectomy. NR increased DNA synthesis, mitotic index, and mass restoration in the regenerating livers. Intriguingly, NR also ameliorated the steatosis that normally accompanies liver regeneration. To distinguish the role of hepatocyte NAD levels from any systemic effects of NR, we generated mice overexpressing nicotinamide phosphoribosyltransferase, a rate-limiting enzyme for NAD synthesis, specifically in the liver. Nicotinamide phosphoribosyltransferase overexpressing mice were mildly hyperglycemic at baseline and, similar to mice treated with NR, exhibited enhanced liver regeneration and reduced steatosis following partial hepatectomy. Conversely, mice lacking nicotinamide phosphoribosyltransferase in hepatocytes exhibited impaired regenerative capacity that was completely rescued by administering NR.

CONCLUSION

NAD availability is limiting during liver regeneration, and supplementation with precursors such as NR may be therapeutic in settings of acute liver injury. (Hepatology 2017;65:616-630).

Investigate the Metabolic Reprogramming of Saccharomyces cerevisiae for Enhanced Resistance to Mixed Fermentation Inhibitors via 13C Metabolic Flux Analysis

The fermentation inhibitors from the pretreatment of lignocellulosic materials, e.g., acetic acid and furfural, are notorious due to their negative effects on the cell growth and chemical production. However, the metabolic reprogramming of the cells under these stress conditions, especially metabolic response for resistance to mixed inhibitors, has not been systematically investigated and remains mysterious. Therefore, in this study, 13C metabolic flux analysis (13C-MFA), a powerful tool to elucidate the intracellular carbon flux distributions, has been applied to two Saccharomyces cerevisiae strains with different tolerances to the inhibitors under acetic acid, furfural, and mixed (i.e., acetic acid and furfural) stress conditions to unravel the key metabolic responses. By analyzing the intracellular carbon fluxes as well as the energy and cofactor utilization under different conditions, we uncovered varied metabolic responses to different inhibitors. Under acetate stress, ATP and NADH production was slightly impaired, while NADPH tended towards overproduction. Under furfural stress, ATP and cofactors (including both NADH and NADPH) tended to be overproduced. However, under dual-stress condition, production of ATP and cofactors was severely impaired due to synergistic stress caused by the simultaneous addition of two fermentation inhibitors. Such phenomenon indicated the pivotal role of the energy and cofactor utilization in resisting the mixed inhibitors of acetic acid and furfural. Based on the discoveries, valuable insights are provided to improve the tolerance of S. cerevisiae strain and further enhance lignocellulosic fermentation.

SIRT1 and NAD+ precursors: Therapeutic targets in multiple sclerosis a review

Neurodegeneration is an important determinant of disability in multiple sclerosis (MS) but while currently approved treatments reduce inflammation, they have not been shown to reduce neurodegeneration. SIRT1, a NAD dependent protein deacetylase, has been implicated in the pathogenesis of neurodegeneration in neurological diseases including MS. We have studied the role of SIRT1 in experimental autoimmune encephalomyelitis (EAE) and found evidence for a neuroprotective role. In this review we summarize the most recent findings from the use of SIRT1 activators and SIRT1 overexpression in transgenic mice. These data support provide a rational for the use of SIRT1 activators in MS.

PGC1α drives NAD biosynthesis linking oxidative metabolism to renal protection

The energetic burden of continuously concentrating solutes against gradients along the tubule may render the kidney especially vulnerable to ischaemia. Acute kidney injury (AKI) affects 3% of all hospitalized patients. Here we show that the mitochondrial biogenesis regulator, PGC1α, is a pivotal determinant of renal recovery from injury by regulating nicotinamide adenine dinucleotide (NAD) biosynthesis. Following renal ischaemia, Pgc1α(-/-) (also known as Ppargc1a(-/-)) mice develop local deficiency of the NAD precursor niacinamide (NAM, also known as nicotinamide), marked fat accumulation, and failure to re-establish normal function. Notably, exogenous NAM improves local NAD levels, fat accumulation, and renal function in post-ischaemic Pgc1α(-/-) mice. Inducible tubular transgenic mice (iNephPGC1α) recapitulate the effects of NAM supplementation, including more local NAD and less fat accumulation with better renal function after ischaemia. PGC1α coordinately upregulates the enzymes that synthesize NAD de novo from amino acids whereas PGC1α deficiency or AKI attenuates the de novo pathway. NAM enhances NAD via the enzyme NAMPT and augments production of the fat breakdown product β-hydroxybutyrate, leading to increased production of prostaglandin PGE2 (ref. 5), a secreted autacoid that maintains renal function. NAM treatment reverses established ischaemic AKI and also prevented AKI in an unrelated toxic model. Inhibition of β-hydroxybutyrate signalling or prostaglandin production similarly abolishes PGC1α-dependent renoprotection. Given the importance of mitochondrial health in ageing and the function of metabolically active organs, the results implicate NAM and NAD as key effectors for achieving PGC1α-dependent stress resistance.

The Warburg Effect: How Does it Benefit Cancer Cells?

Cancer cells rewire their metabolism to promote growth, survival, proliferation, and long-term maintenance. The common feature of this altered metabolism is the increased glucose uptake and fermentation of glucose to lactate. This phenomenon is observed even in the presence of completely functioning mitochondria and, together, is known as the 'Warburg Effect'. The Warburg Effect has been documented for over 90 years and extensively studied over the past 10 years, with thousands of papers reporting to have established either its causes or its functions. Despite this intense interest, the function of the Warburg Effect remains unclear. Here, we analyze several proposed explanations for the function of Warburg Effect, emphasize their rationale, and discuss their controversies.

Mitofusin-2-mediated tethering of mitochondria and endoplasmic reticulum promotes cell cycle arrest of vascular smooth muscle cells in G0/G1 phase

Mitofusin-2 (Mfn-2) is a hyperplasia suppressor. Changes in Mfn-2 expression are thought to reflect mitochondrial remodeling during cell proliferation. However, it is unclear how the participation of Mfn-2 in mitochondrial remodeling prevents cellular proliferation. Here we show that arresting vascular smooth muscle cells (VSMCs) in the G0/G1 phase by serum starvation up-regulates Mfn-2 expression and causes mitochondria to assemble into a tubular network and to attach to the endoplasmic reticulum (ER). In the S phase, short rod-shaped mitochondrial structures that were dissociated from the ER were observed. Levels of glucose, ATP, l-amino acid, and NADP(+) did not vary throughout the cell cycle. However, NAD(+) level was lower and NADH level was higher in the G0/G1 phase than in the S phase. Mitochondrial membrane potential was lower in the S phase than in the G0/G1 phase. Infecting VSMCs with an adenovirus encoding full-length Mfn-2 increased NADH level and reduced NAD(+) level, while infecting the cells with an adenovirus that silences the p21(ras) signature motif produced opposite effects. These results suggest that Mfn-2 up-regulation causes mitochondrial fusion into tubular networks and attachment to the ER, which in turn halts proliferation of VSMCs.