The extracellular redox state modulates mitochondrial function, gluconeogenesis, and glycogen synthesis in murine hepatocytes

Circulating redox state changes, determined by the ratio of reduced/oxidized pairs of different metabolites, have been associated with metabolic diseases. However, the pathogenic contribution of these changes and whether they modulate normal tissue function is unclear. As alterations in hepatic gluconeogenesis and glycogen metabolism are hallmarks that characterize insulin resistance and type 2 diabetes, we tested whether imposed changes in the extracellular redox state could modulate these processes. Thus, primary hepatocytes were treated with different ratios of the following physiological extracellular redox couples: β-hydroxybutyrate (βOHB)/acetoacetate (Acoc), reduced glutathione (GSH)/oxidized glutathione (GSSG), and cysteine/cystine. Exposure to a more oxidized ratio via extracellular βOHB/Acoc, GSH/GSSG, and cysteine/cystine in hepatocytes from fed mice increased intracellular hydrogen peroxide without causing oxidative damage. On the other hand, addition of more reduced ratios of extracellular βOHB/Acoc led to increased NAD(P)H and maximal mitochondrial respiratory capacity in hepatocytes. Greater βOHB/Acoc ratios were also associated with decreased β-oxidation, as expected with enhanced lipogenesis. In hepatocytes from fasted mice, a more extracellular reduced state of βOHB/Acoc led to increased alanine-stimulated gluconeogenesis and enhanced glycogen synthesis capacity from added glucose. Thus, we demonstrated for the first time that the extracellular redox state regulates the major metabolic functions of the liver and involves changes in intracellular NADH, hydrogen peroxide, and mitochondrial respiration. Because redox state in the blood can be communicated to all metabolically sensitive tissues, this work confirms the hypothesis that circulating redox state may be an important regulator of whole body metabolism and contribute to alterations associated with metabolic diseases.

Metabolic profiling of alternative NAD biosynthetic routes in mouse tissues

NAD plays essential redox and non-redox roles in cell biology. In mammals, its de novo and recycling biosynthetic pathways encompass two independent branches, the "amidated" and "deamidated" routes. Here we focused on the indispensable enzymes gating these two routes, i.e. nicotinamide mononucleotide adenylyltransferase (NMNAT), which in mammals comprises three distinct isozymes, and NAD synthetase (NADS). First, we measured the in vitro activity of the enzymes, and the levels of all their substrates and products in a number of tissues from the C57BL/6 mouse. Second, from these data, we derived in vivo estimates of enzymes'rates and quantitative contributions to NAD homeostasis. The NMNAT activity, mainly represented by nuclear NMNAT1, appears to be high and nonrate-limiting in all examined tissues, except in blood. The NADS activity, however, appears rate-limiting in lung and skeletal muscle, where its undetectable levels parallel a relative accumulation of the enzyme's substrate NaAD (nicotinic acid adenine dinucleotide). In all tissues, the amidated NAD route was predominant, displaying highest rates in liver and kidney, and lowest in blood. In contrast, the minor deamidated route showed higher relative proportions in blood and small intestine, and higher absolute values in liver and small intestine. Such results provide the first comprehensive picture of the balance of the two alternative NAD biosynthetic routes in different mammalian tissues under physiological conditions. This fills a gap in the current knowledge of NAD biosynthesis, and provides a crucial information for the study of NAD metabolism and its role in disease.

Mitochondrial glutathione depletion reveals a novel role for the pyruvate dehydrogenase complex as a key H2O2-emitting source under conditions of nutrient overload

Once regarded as a "by-product" of aerobic metabolism, the production of superoxide/H2O2 is now understood to be a highly specialized and extensively regulated process responsible for exerting control over a vast number of thiol-containing proteins, collectively referred to as the redox-sensitive proteome. Although disruptions within this process, secondary to elevated peroxide exposure, have been linked to disease, the sources and mechanisms regulating increased peroxide burden remain poorly defined and as such are difficult to target using pharmacotherapy. Here we identify the pyruvate dehydrogenase complex (PDC) as a key source of H2O2 within skeletal muscle mitochondria under conditions of depressed glutathione redox buffering integrity. Treatment of permeabilized myofibers with varying concentrations of the glutathione-depleting agent 1-chloro-2,4-dinitrobenzene led to a dose-dependent increase in pyruvate-supported JH2O2 emission (the flux of H2O2 diffusing out of the mitochondrial matrix into the surrounding assay medium), with emission rates eventually rising to exceed those of all substrate combinations tested. This striking sensitivity to glutathione depletion was observed in permeabilized fibers prepared from multiple species and was specific to PDC. Physiological oxidation of the cellular glutathione pool after high-fat feeding in rodents was found to elevate PDC JH2O2 emission, as well as increasing the sensitivity of the complex to GSH depletion. These findings reveal PDC as a potential major site of H2O2 production that is extremely sensitive to mitochondrial glutathione redox status.

A possible role for the chloroplast pyruvate dehydrogenase complex in plant glycolate and glyoxylate metabolism

Glyoxylate is a peroxisomal intermediate of photorespiration, the recycling pathway for 2-phosphoglycolate (2-PG) produced by the oxygenase activity of Rubisco. Under hot and dry growth conditions, photorespiratory intermediates can accumulate and must be detoxified by alternative pathways, including plastidal reactions. Moreover, there is evidence that chloroplasts are capable of actively producing glyoxylate from glycolate. Further metabolic steps are unknown, but probably include a CO2 release step. Here, we report that CO2 production from glycolate and glyoxylate in isolated tobacco chloroplasts can be inhibited by pyruvate, but not related compounds. We isolated a protein fraction that was enriched for the chloroplast pyruvate dehydrogenase complex (PDC). The fraction contained a protein complex of several MDa in size that included all predicted subunits of the chloroplast PDC and a so far unidentified HSP93-V/ClpC1 heat shock protein. Glyoxylate competitively inhibited NADH formation from pyruvate in this fraction. The Km for pyruvate and the Ki for glyoxylate were 330 and 270 μM, respectively. Glyoxylate decarboxylation was also enriched in this fraction and could be in turn inhibited by pyruvate. Based on these data, we suggest that the chloroplast PDC might be part of a pathway for glycolate and/or glyoxylate oxidation in chloroplasts.

Regulation of SIRT1 by microRNAs

Sirtuin 1 (SIRT1) is an NAD(+)-dependent deacetylase that connects cellular energy levels to homeostatic responses by deacetylating and modulating the activities of many transcriptional regulators. Discovered as a longevity protein in yeast, the mammalian SIRT1 has been intensively studied because of its great potential as a therapeutic target to benefit human health by preventing and improving many age-related diseases. There has been, therefore, substantial interest in developing agents that upregulate SIRT1 expression and activity. SIRT1 is regulated at multiple levels, including post-transcriptionally by microRNAs (miRs), powerful regulators of diverse biological pathways. Here we discuss how expression and activity of SIRT1 and other sirtuins are inhibited by miRs and further discuss the therapeutic potential of targeting miRs for age-related diseases that involve SIRT1 dysfunction, focusing on obesityrelated diseases.

NAD+ salvage pathway proteins suppress proteotoxicity in yeast models of neurodegeneration by promoting the clearance of misfolded/oligomerized proteins

Increased levels of nicotinamide/nicotinic acid mononucleotide adenylyltransferase (NMNAT) act as a powerful suppressor of Wallerian degeneration and ataxin- and tau-induced neurodegeneration in flies and mice. However, the nature of the suppression mechanism/s remains controversial. Here, we show that in yeast models of proteinopathies, overexpression of the NMNAT yeast homologs, NMA1 and NMA2, suppresses polyglutamine (PolyQ) and α-synuclein-induced cytotoxicities. Unexpectedly, overexpression of other genes in the salvage pathway for NAD(+) biosynthesis, including QNS1, NPT1 and PNC1 also protected against proteotoxicity. Our data revealed that in all cases, this mechanism involves extensive clearance of the non-native protein. Importantly, we demonstrate that suppression by NMA1 does not require the presence of a functional salvage pathway for NAD(+) biosynthesis, SIR2 or an active mitochondrial oxidative phosphorylation (OXPHOS) system. Our results imply the existence of histone deacetylase- and OXPHOS-independent crosstalk between the proteins in the salvage pathway for NAD(+) biosynthesis and the proteasome that can be manipulated to achieve cellular protection against proteotoxic stress.

NAD+ accumulation during pollen maturation in Arabidopsis regulating onset of germination

Although the nicotinamide nucleotides NAD(H) and NADP(H) are essential for various metabolic reactions that play major roles in maintenance of cellular homeostasis, the significance of NAD biosynthesis is not well understood. Here, we investigated the dynamics of pollen nicotinamide nucleotides in response to imbibition, a representative germination cue. Metabolic analysis with capillary electrophoresis electrospray ionization mass spectrometry revealed that excess amount of NAD+ is accumulated in freshly harvested dry pollen, whereas it dramatically decreased immediately after contact with water. Importantly, excess of NAD+ impaired pollen tube growth. Moreover, NAD+ accumulation was retained after pollen was imbibed in the presence of NAD+-consuming reaction inhibitors and pollen germination was greatly retarded. Pollen deficient in the nicotinate/nicotinamide mononucleotide adenyltransferase (NMNAT) gene, encoding a key enzyme in NAD biosynthesis, and a lack of NAD+ accumulation in the gametophyte, showed precocious pollen tube germination inside the anther locule and vigorous tube growth under high-humidity conditions. Hence, the accumulation of excess NAD+ is not essential for pollen germination, but instead participates in regulating the timing of germination onset. These results indicate that NAD+ accumulation acts to negatively regulate germination and a decrease in NAD+ plays an important role in metabolic state transition.

The NAD metabolome--a key determinant of cancer cell biology

NAD is a vital molecule in all organisms. It is a key component of both energy and signal transduction--processes that undergo crucial changes in cancer cells. NAD(+)-dependent signalling pathways are many and varied, and they regulate fundamental events such as transcription, DNA repair, cell cycle progression, apoptosis and metabolism. Many of these processes have been linked to cancer development. Given that NAD(+)-dependent signalling reactions involve the degradation of the molecule, permanent nucleotide resynthesis through different biosynthetic pathways is crucial for incessant cancer cell proliferation. This necessity supports the targeting of NAD metabolism as a new therapeutic concept for cancer treatment.

Glucose is a pH-dependent motor for sperm beat frequency during early activation

To reach the egg in the ampulla, sperm have to travel along the female genital tract, thereby being dependent on external energy sources and substances to maintain and raise the flagellar beat. The vaginal fluid is rich in lactate, whereas in the uterine fluid glucose is the predominant substrate. This evokes changes in the lactate content of sperm as well as in the intracellular pH (pH(i)) since sperm possess lactate/proton co-transporters. It is well documented that glycolysis yields ATP and that HCO(3)- is a potent factor in the increase of beat frequency. We here show for the first time a pathway that connects both parts. We demonstrate a doubling of beat frequency in the mere presence of glucose. This effect can reversibly be blocked by 2-deoxy-D-glucose, dichloroacetate and aminooxyacetate, strongly suggesting that it requires both glycolysis and mitochondrial oxidation of glycolytic end products. We show that the glucose-mediated acceleration of flagellar beat and ATP production are hastened by a pH(i) ≥7.1, whereas a pH(i) ≤7.1 leaves both parameters unchanged. Since we observed a diminished rise in beat frequency in the presence of specific inhibitors against carbonic anhydrases, soluble adenylyl cyclase and protein kinase, we suggest that the glucose-mediated effect is linked to CO(2) hydration and thus the production of HCO(3)- by intracellular CA isoforms. In summary, we propose that, in sperm, glycolysis is an additional pH(i)-dependent way to produce HCO(3)-, thus enhancing sperm beat frequency and contributing to fertility.

Influence of anaerobiosis and low temperature on Bacillus cereus growth, metabolism, and membrane properties

The impact of simultaneous anaerobiosis and low temperature on growth parameters, metabolism, and membrane properties of Bacillus cereus ATCC 14579 was studied. No growth was observed under anaerobiosis at 12°C. In bioreactors, growth rates and biomass production were drastically reduced by simultaneous anaerobiosis and low temperature (15°C). The two conditions had a synergistic effect on biomass reduction. In anaerobic cultures, fermentative metabolism was modified by low temperature, with a marked reduction in ethanol production leading to a lower ability to produce NAD(+). Anaerobiosis reduced unsaturated fatty acids at both low optimal temperatures. In addition, simultaneous anaerobiosis and low temperatures markedly reduced levels of branched-chain fatty acids compared to all other conditions (accounting for 33% of total fatty acids against more 71% for low-temperature aerobiosis, optimal-temperature aerobiosis, and optimal-temperature anaerobiosis). This corresponded to high-melting-temperature lipids and to low-fluidity membranes, as indicated by differential scanning calorimetry, 1,6-diphenyl-1,3,5-hexatriene (DPH) fluorescence anisotropy, and infrared spectroscopy. This is in contrast to requirements for cold adaptation. A link between modification in the synthesis of metabolites of fermentative metabolism and the reduction of branched-chain fatty acids at low temperature under anaerobiosis, through a modification of the oxidizing capacity, is assumed. This link may partly explain the impact of low temperature and anaerobiosis on membrane properties and growth performance.

Catalytic properties of the isolated diaphorase fragment of the NAD-reducing [NiFe]-hydrogenase from Ralstonia eutropha

The NAD⁺-reducing soluble hydrogenase (SH) from Ralstonia eutropha H16 catalyzes the H₂-driven reduction of NAD⁺, as well as reverse electron transfer from NADH to H⁺, in the presence of O₂. It comprises six subunits, HoxHYFUI₂, and incorporates a [NiFe] H⁺/H₂ cycling catalytic centre, two non-covalently bound flavin mononucleotide (FMN) groups and an iron-sulfur cluster relay for electron transfer. This study provides the first characterization of the diaphorase sub-complex made up of HoxF and HoxU. Sequence comparisons with the closely related peripheral subunits of Complex I in combination with UV/Vis spectroscopy and the quantification of the metal and FMN content revealed that HoxFU accommodates a [2Fe2S] cluster, FMN and a series of [4Fe4S] clusters. Protein film electrochemistry (PFE) experiments show clear electrocatalytic activity for both NAD⁺ reduction and NADH oxidation with minimal overpotential relative to the potential of the NAD⁺/NADH couple. Michaelis-Menten constants of 56 µM and 197 µM were determined for NADH and NAD⁺, respectively. Catalysis in both directions is product inhibited with K_I values of around 0.2 mM. In PFE experiments, the electrocatalytic current was unaffected by O₂, however in aerobic solution assays, a moderate superoxide production rate of 54 nmol per mg of protein was observed, meaning that the formation of reactive oxygen species (ROS) observed for the native SH can be attributed mainly to HoxFU. The results are discussed in terms of their implications for aerobic functioning of the SH and possible control mechanism for the direction of catalysis.

Mitochondria-localized NAD biosynthesis by nicotinamide mononucleotide adenylyltransferase in Jerusalem artichoke (Helianthus tuberosus L.) heterotrophic tissues

Current studies in plants suggest that the content of the coenzyme NAD is variable and potentially important in determining cell fate. In cases that implicate NAD consumption, re-synthesis must occur to maintain dinucleotide pools. Despite information on the pathways involved in NAD synthesis in plants, the existence of a mitochondrial nicotinamide mononucleotide adenylyltransferase (NMNAT) activity which catalyses NAD synthesis from nicotinamide mononucleotide (NMN) and ATP has not been reported. To verify the latter assumed pathway, experiments with purified and bioenergetically active mitochondria prepared from tubers of Jerusalem artichoke (Helianthus tuberosus L.) were performed. To determine whether NAD biosynthesis might occur, NMN was added to Jerusalem artichoke mitochondria (JAM) and NAD biosynthesis was tested by means of HPLC and spectroscopically. Our results indicate that JAM contain a specific NMNAT inhibited by Na-pyrophosphate, AMP and ADP-ribose. The dependence of NAD synthesis rate on NMN concentration shows saturation kinetics with K (m) and V (max) values of 82 ± 1.05 μM and 4.20 ± 0.20 nmol min(-1) mg(-1) protein, respectively. The enzyme's pH and temperature dependence were also investigated. Fractionation studies revealed that mitochondrial NMNAT activity was present in the soluble matrix fraction. The NAD pool needed constant replenishment that might be modulated by environmental inputs. Thus, the mitochondrion in heterotrophic plant tissues ensures NAD biosynthesis by NMNAT activity and helps to orchestrate NAD metabolic network in implementing the survival strategy of cells.

Liquid chromatography/time-of-flight mass spectrometry for the analysis of plant samples: a method for simultaneous screening of common cofactors or nucleotides and application to an engineered plant line

Intense efforts are currently devoted to improve plant metabolomic analyses so as to describe more accurately the whole picture of metabolic pathways. Analyses based on liquid chromatography/time-of-flight mass spectrometry (LC-TOF) are now widely distributed among plant science laboratories. However, the use of reliable, sensitive LC-TOF methods to identify and quantify micromolar or inframicromolar key metabolites is often impeded by the sensitivity of the technique to sample preparation or chromatographic conditions. Typically, the sample matrix has a substantial influence on ionization efficiency and therefore, on the detectability of such compounds. Here, we describe a new method to analyze simultaneously 23 nucleotides and cofactors from plant extracts, taking advantage of solid-phase extraction (SPE) prior to injection. The influence of common m/z fragments in several metabolites and adducts is considered. We applied this method to characterise metabolic intermediates of NAD biosynthesis in Arabidopsis thaliana, using a wild-type and an engineered transgenic plant line that produces bacterial quinolinate phosphoribosyl transferase (nadc). We show that sample pre-purification with SPE is strictly required not only for compound quantification and identification but also to allow ionization of matrix-sensitive compounds (e.g. nicotinamide) or alleviate fragmentation of others (e.g. NAD). When exogenous substrate quinolinate was infiltrated into Arabidopsis leaves to increase the natural content in downstream metabolites, a clear correlation between intermediates of NAD biosynthesis was seen, showing the accuracy of our method for quantification in biological samples. Nadc plants only showed very modest changes in NAD-related metabolites and furthermore, they were associated with slightly lower photosynthetic performance and ATP production.

Genomic characterization of Haemophilus parasuis SH0165, a highly virulent strain of serovar 5 prevalent in China

Haemophilus parasuis can be either a commensal bacterium of the porcine respiratory tract or an opportunistic pathogen causing Glässer's disease, a severe systemic disease that has led to significant economical losses in the pig industry worldwide. We determined the complete genomic sequence of H. parasuis SH0165, a highly virulent strain of serovar 5, which was isolated from a hog pen in North China. The single circular chromosome was 2,269,156 base pairs in length and contained 2,031 protein-coding genes. Together with the full spectrum of genes detected by the analysis of metabolic pathways, we confirmed that H. parasuis generates ATP via both fermentation and respiration, and possesses an intact TCA cycle for anabolism. In addition to possessing the complete pathway essential for the biosynthesis of heme, this pathogen was also found to be well-equipped with different iron acquisition systems, such as the TonB system and ABC-type transport complexes, to overcome iron limitation during infection and persistence. We identified a number of genes encoding potential virulence factors, such as type IV fimbriae and surface polysaccharides. Analysis of the genome confirmed that H. parasuis is naturally competent, as genes related to DNA uptake are present. A nine-mer DNA uptake signal sequence (ACAAGCGGT), identical to that found in Actinobacillus pleuropneumoniae and Mannheimia haemolytica, followed by similar downstream motifs, was identified in the SH0165 genome. Genomic and phylogenetic comparisons with other Pasteurellaceae species further indicated that H. parasuis was closely related to another swine pathogenic bacteria A. pleuropneumoniae. The comprehensive genetic analysis presented here provides a foundation for future research on the metabolism, natural competence and virulence of H. parasuis.

Synthesizing and salvaging NAD: lessons learned from Chlamydomonas reinhardtii

The essential coenzyme nicotinamide adenine dinucleotide (NAD⁺) plays important roles in metabolic reactions and cell regulation in all organisms. Bacteria, fungi, plants, and animals use different pathways to synthesize NAD⁺. Our molecular and genetic data demonstrate that in the unicellular green alga Chlamydomonas NAD⁺ is synthesized from aspartate (de novo synthesis), as in plants, or nicotinamide, as in mammals (salvage synthesis). The de novo pathway requires five different enzymes: L-aspartate oxidase (ASO), quinolinate synthetase (QS), quinolate phosphoribosyltransferase (QPT), nicotinate/nicotinamide mononucleotide adenylyltransferase (NMNAT), and NAD⁺ synthetase (NS). Sequence similarity searches, gene isolation and sequencing of mutant loci indicate that mutations in each enzyme result in a nicotinamide-requiring mutant phenotype in the previously isolated nic mutants. We rescued the mutant phenotype by the introduction of BAC DNA (nic2-1 and nic13-1) or plasmids with cloned genes (nic1-1 and nic15-1) into the mutants. NMNAT, which is also in the de novo pathway, and nicotinamide phosphoribosyltransferase (NAMPT) constitute the nicotinamide-dependent salvage pathway. A mutation in NAMPT (npt1-1) has no obvious growth defect and is not nicotinamide-dependent. However, double mutant strains with the npt1-1 mutation and any of the nic mutations are inviable. When the de novo pathway is inactive, the salvage pathway is essential to Chlamydomonas for the synthesis of NAD⁺. A homolog of the human SIRT6-like gene, SRT2, is upregulated in the NS mutant, which shows a longer vegetative life span than wild-type cells. Our results suggest that Chlamydomonas is an excellent model system to study NAD⁺ metabolism and cell longevity.

Nicotinamide adenine dinucleotide (NAD⁺) is an essential coenzyme. NAD⁺ is necessary for electron transfer in many metabolic reactions. NAD⁺ functions as a substrate for several enzymes, one of which is sirtuin, an enzyme involved in gene regulation and aging. NAD⁺ can be synthesized either from amino acids (de novo) or metabolites (salvage). Given the importance of NAD⁺, enzymes involved in NAD⁺ synthesis are targets for drug discovery. In the unicellular green alga Chlamydomonas we investigated both the de novo and salvage NAD⁺ biosynthetic pathways. Mutations in the plant-like de novo synthesis pathway lead to a nicotinamide-requiring phenotype. We identified an insertional mutation in the first enzyme in the mammal-like salvage pathway; it has no growth defect in cells with an active de novo synthesis pathway but causes lethality when the de novo synthesis pathway is inactive. Coupled with NAD⁺ biosynthesis, sirtuin is involved in NAD⁺ consumption. Our study links upregulation of a sirtuin gene with extended life span in the nic13-1 mutant strain, which has a defective de novo synthesis pathway and suggests that Chlamydomonas is an excellent genetic model to study NAD⁺ metabolism and cell longevity.

Isonicotinic acid hydrazide conversion to Isonicotinyl-NAD by catalase-peroxidases

Activation of the pro-drug isoniazid (INH) as an anti-tubercular drug in Mycobacterium tuberculosis involves its conversion to isonicotinyl-NAD, a reaction that requires the catalase-peroxidase KatG. This report shows that the reaction proceeds in the absence of KatG at a slow rate in a mixture of INH, NAD(+), Mn(2+), and O(2), and that the inclusion of KatG increases the rate by >7 times. Superoxide, generated by either Mn(2+)- or KatG-catalyzed reduction of O(2), is an essential intermediate in the reaction. Elimination of the peroxidatic process by mutation slows the rate of reaction by 60% revealing that the peroxidatic process enhances, but is not essential for isonicotinyl-NAD formation. The isonicotinyl-NAD(*+) radical is identified as a reaction intermediate, and its reduction by superoxide is proposed. Binding sites for INH and its co-substrate, NAD(+), are identified for the first time in crystal complexes of Burkholderia pseudomallei catalase-peroxidase with INH and NAD(+) grown by co-crystallization. The best defined INH binding sites were identified, one in each subunit, on the opposite side of the protein from the entrance to the heme cavity in a funnel-shaped channel. The NAD(+) binding site is approximately 20 A from the entrance to the heme cavity and involves interactions primarily with the AMP portion of the molecule in agreement with the NMR saturation transfer difference results.

Nicotinate/nicotinamide mononucleotide adenyltransferase-mediated regulation of NAD biosynthesis protects guard cells from reactive oxygen species in ABA-mediated stomatal movement in Arabidopsis

Nicotinamide adenine dinucleotide (NAD) and its derivative nicotinamide adenine dinucleotide phosphate (NADP) are indispensable co-factors in broad-spectrum metabolic events for the maintenance of cellular homeostasis in all living organisms. In this study, the cellular expression levels of NAD biosynthesis genes in Arabidopsis were investigated. A very high expression of nicotinate/nicotinamide mononucleotide adenyltransferase (NMNAT) was observed in the differentiated stomatal guard cells of the leaf surface. Transcriptional analysis confirmed that several genes in the biosynthesis pathway were also highly expressed in stomatal guard cells. In fact, NAD and NADP metabolisms were investigated during stomatal movement. Importantly, the generation of phytohormone ABA-induced reactive oxygen species, which acts as a signal for stomatal closure, was accompanied by markedly decreased levels of NAD. The ABA-induced oxidative stress caused stomatal cell death in the nmnat mutant. Furthermore, stomata partially lost their ability to close leading to drought susceptibility. The stomata were less responsive to opening cues as well. These results indicate that NAD biosynthesis is involved in protecting guard cells from ABA-induced local oxidative stress via the regulation of NMNAT activity. In this study, it is demonstrated that NMNAT is essential for the maintenance of NAD homeostasis enabling sustainable stomatal movement.

A possibility of nutriceuticals as an anti-aging intervention: activation of sirtuins by promoting mammalian NAD biosynthesis

Aging science has recently drawn much attention, and discussions on the possibility of anti-aging medicine have multiplied. One potential target for the development of anti-aging drugs is the SIR2 (silent information regulator 2) family of NAD-dependent deacetylases/ADP-ribosyltransferases, called "sirtuins." Sirtuins regulate many fundamental biological processes in response to a variety of environmental and nutritional stimuli. In mammals, the mammalian SIR2 ortholog SIRT1 has been most studied, and small molecule SIRT1 activators (STACs), including a plant-derived polyphenolic compound resveratrol, have been developed. On the other hand, sirtuin activity is regulated by NAD biosynthetic pathways, and nicotinamide phosphoribosyltransferase (NAMPT) plays a critical role in the regulation of mammalian sirtuin activity. Recent studies have provided a proof of concept for the idea that nicotinamide mononucleotide (NMN), the NAMPT reaction product, can be used as a nutriceutical to activate SIRT1 activity. Based on these recent findings, the possibility of sirtuin-targeted nutriceutical development will be discussed.

Pyridine salvage and nicotinic acid conjugate synthesis in leaves of mangrove species

The metabolic fate of [carbonyl-14C]nicotinamide was surveyed in leaf disks of seven mangrove species, Bruguiera gymnorrhiza, Rhizophora stylosa, Kandeliaobovata, Sonneratia alba, Pemphis acidula, Lumnitzera racemosa and Avicennia marina, with and without 250 mM NaCl. Uptake of [14C]nicotinamide by leaf disks was stimulated by 250 mM NaCl in K. candel, R. stylosa, A. marina and L. racemosa. [Carbonyl-14C]nicotinamide was converted to nicotinic acid and was utilised for the synthesis of nucleotides and nicotinic acid conjugates. Formation of nicotinic acid by the deaminase reaction was rapid; there was little accumulation of nicotinamide in the disks 3h after administration. Radioactivity from [carbonyl-14C]nicotinamide was incorporated into pyridine nucleotides (mainly NAD and NADP) in all mangrove leaves, and the rates varied from 2% (in L. racemosa) to 15% (S. alba) of the total radioactivity taken up. NaCl generally reduced nicotinic acid salvage for NAD and NADP. In all mangrove leaf disks, the most heavily labelled compounds (up to 70% of total radioactivity) were trigonelline (N-methylnicotinic acid) and/or nicotinic acid N-glucoside. Trigonelline was formed in all mangrove plants, but N-glucoside synthesis was found only in leaves of A. marina and K. obovata. In A. marina, incorporation of radioactivity into N-glucoside (51%) was much greater than incorporation into trigonelline (2%). In general, NaCl stimulates the synthesis of these pyridine conjugates. The rate of decarboxylation of nicotinic acid in roots of A. marina seedlings was much greater than for the corresponding reaction observed in leaves.

Profiles of the biosynthesis and metabolism of pyridine nucleotides in potatoes (Solanum tuberosum L.)

As part of a research program on nucleotide metabolism in potato tubers (Solanum tuberosum L.), profiles of pyridine (nicotinamide) metabolism were examined based on the in situ metabolic fate of radio-labelled precursors and the in vitro activities of enzymes. In potato tubers, [(3)H]quinolinic acid, which is an intermediate of de novo pyridine nucleotide synthesis, and [(14)C]nicotinamide, a catabolite of NAD, were utilised for pyridine nucleotide synthesis. The in situ tracer experiments and in vitro enzyme assays suggest the operation of multiple pyridine nucleotide cycles. In addition to the previously proposed cycle consisting of seven metabolites, we found a new cycle that includes newly discovered nicotinamide riboside deaminase which is also functional in potato tubers. This cycle bypasses nicotinamide and nicotinic acid; it is NAD --> nicotinamide mononucleotide --> nicotinamide riboside --> nicotinic acid riboside --> nicotinic acid mononucleotide --> nicotinic acid adenine dinucleotide --> NAD. Degradation of the pyridine ring was extremely low in potato tubers. Nicotinic acid glucoside is formed from nicotinic acid in potato tubers. Comparative studies of [carboxyl-(14)C]nicotinic acid metabolism indicate that nicotinic acid is converted to nicotinic acid glucoside in all organs of potato plants. Trigonelline synthesis from [carboxyl-(14)C]nicotinic acid was also found. Conversion was greater in green parts of plants, such as leaves and stem, than in underground parts of potato plants. Nicotinic acid utilised for the biosynthesis of these conjugates seems to be derived not only from the pyridine nucleotide cycle, but also from the de novo synthesis of nicotinic acid mononucleotide.

Metabolism and cancer: the circadian clock connection

Circadian rhythms govern a remarkable variety of metabolic and physiological functions. Accumulating epidemiological and genetic evidence indicates that the disruption of circadian rhythms might be directly linked to cancer. Intriguingly, several molecular gears constituting the clock machinery have been found to establish functional interplays with regulators of the cell cycle, and alterations in clock function could lead to aberrant cellular proliferation. In addition, connections between the circadian clock and cellular metabolism have been identified that are regulated by chromatin remodelling. This suggests that abnormal metabolism in cancer could also be a consequence of a disrupted circadian clock. Therefore, a comprehensive understanding of the molecular links that connect the circadian clock to the cell cycle and metabolism could provide therapeutic benefit against certain human neoplasias.

NADH-generating substrates reduce peroxyl radical toxicity in RL-34 cells

There is general agreement that oxidative stress may induce apoptotic and necrotic cell death. Recently it has been shown that NADH can be considered an important antioxidant as it reacts with peroxyl and alkoxyl radicals under in vitro conditions. Therefore, in the present study we hypothesized that an increase in intracellular NADH using specific substrates will protect RL-34 cells against cytotoxicity of 2'-azobis (2-amidinopropane) dihydrochloride (AAPH), which is a peroxyl radical generating compound. Cells treated for 24 hours with 6.0 mM AAPH were severely damaged: mitochondria were vacuolated, and the level of free radicals significantly increased. Both apoptotic and necrotic cells were detected (11.1% and 11.4%, respectively) even after 5 hours of treatment. Pretreatment of the cells with substrates which increase the intracellular level of NADH, such as lactate, beta-hydroxybutyrate, and ethanol, distinctly inhibited AAPH-induced reactive oxygen species (ROS) formation and cell death. On the other hand, acetoacetate (AcA), which decrease the intracellular level of NADH, had opposite effects. Interestingly, NADH-generating substrates augment, while AcA reduced superoxide radical formation induced by AAPH. These results may suggest that although NADH generating substrates may exert some deleterious effects within a cell by inducing reductive stress, they diminish alkoxyl or peroxyl radical cytotoxicity. The protection is associated with a decrease in ROS formation measured by dichlorofluorescein, but with an increase in superoxide radical formation.

Nmnat2 delays axon degeneration in superior cervical ganglia dependent on its NAD synthesis activity

Axon degeneration is an active program of self-destruction observed in many physiological and pathological settings. There are three Nicotinamide mononucleotide adenylyl transferase (Nmnat, EC2.7.7.1) in mammals. Overexpression of Nmnat1 or Nmnat3 can delay axon degeneration, while the role of Nmnat2 in axon degeneration remains largely unknown. Here we found that Nmnat2 was specifically and highly expressed in brain compared with Nmnat1 and Nmnat3. Furthermore, we found brain Nmnat2 was correlated with Alzheimer's disease in APPswe/PS1dE9 transgenic mice. Nmnat2 delayed Wallerian degeneration in cultured superior cervical ganglia (SCGs) from morphological changes, microtubule destruction and neurofilament degradation, mutation of the conserved enzyme activity site in Nmnat2 disrupted its enzyme activity as well as the axon-protective function. Our results demonstrate that the brain-specific Nmnat2 delays injury-induced axon degeneration dependent on its NAD synthesis activity. These findings provide new clues to further study the molecular mechanisms of axon degeneration and the related neurodegenerative diseases.