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

Brain energy metabolism: A roadmap for future research

Although we have learned much about how the brain fuels its functions over the last decades, there remains much still to discover in an organ that is so complex. This article lays out major gaps in our knowledge of interrelationships between brain metabolism and brain function, including biochemical, cellular, and subcellular aspects of functional metabolism and its imaging in adult brain, as well as during development, aging, and disease. The focus is on unknowns in metabolism of major brain substrates and associated transporters, the roles of insulin and of lipid droplets, the emerging role of metabolism in microglia, mysteries about the major brain cofactor and signaling molecule NAD+, as well as unsolved problems underlying brain metabolism in pathologies such as traumatic brain injury, epilepsy, and metabolic downregulation during hibernation. It describes our current level of understanding of these facets of brain energy metabolism as well as a roadmap for future research.

Large scale microfluidic CRISPR screening for increased amylase secretion in yeast

Key to our ability to increase recombinant protein production through secretion is a better understanding of the pathways that interact to translate, process and export mature proteins to the surrounding environment, including the supporting cellular machinery that supplies necessary energy and building blocks. By combining droplet microfluidic screening with large-scale CRISPR libraries that perturb the expression of the majority of coding and non-coding genes in S. cerevisiae, we identified 345 genes for which an increase or decrease in gene expression resulted in increased secretion of α-amylase. Our results show that modulating the expression of genes involved in the trafficking of vesicles, endosome to Golgi transport, the phagophore assembly site, the cell cycle and energy supply improve α-amylase secretion. Besides protein-coding genes, we also find multiple long non-coding RNAs enriched in the vicinity of genes associated with endosomal, Golgi and vacuolar processes. We validated our results by overexpressing or deleting selected genes, which resulted in significant improvements in α-amylase secretion. The advantages, in terms of precision and speed, inherent to CRISPR based perturbations, enables iterative testing of new strains for increased protein secretion.

Neurodegenerative disease-associated inclusion bodies are cleared by selective autophagy in budding yeast

Protein misfolding, aggregation, and accumulation cause neurodegenerative disorders. One such disorder, Huntington's disease, is caused by an increased number of glutamine-encoding trinucleotide repeats CAG in the first exon of the huntingtin (HTT) gene. Mutant proteins of Htt exon 1 with polyglutamine expansion are prone to aggregation and form pathological inclusion bodies in neurons. Extensive studies have shown that misfolded proteins are cleared by the ubiquitin-proteasome system or autophagy to alleviate their cytotoxicity. Misfolded proteins can form small soluble aggregates or large insoluble inclusion bodies. Previous works have elucidated the role of autophagy in the clearance of misfolded protein aggregates, but autophagic clearance of inclusion bodies remains poorly characterized. Here we use mutant Htt exon 1 with 103 polyglutamine (Htt103QP) as a model substrate to study the autophagic clearance of inclusion bodies in budding yeast. We found that the core autophagy-related proteins were required for Htt103QP inclusion body autophagy. Moreover, our evidence indicates that the autophagy of Htt103QP inclusion bodies is selective. Interestingly, Cue5/Tollip, a known autophagy receptor for aggrephagy, is dispensable for this inclusion body autophagy. From the known selective autophagy receptors in budding yeast, we identified three that are essential for inclusion body autophagy. Amyloid beta peptide (Aβ42) is a major component of amyloid plaques found in Alzheimer's disease brains. Interestingly, a similar selective autophagy pathway contributes to the clearance of Aβ42 inclusion bodies in budding yeast. Therefore, our results reveal a novel autophagic pathway specific for inclusion bodies associated with neurodegenerative diseases, which we have termed IBophagy.

Cellular and Mitochondrial NAD Homeostasis in Health and Disease

The mitochondrion has a unique position among other cellular organelles due to its dynamic properties and symbiotic nature, which is reflected in an active exchange of metabolites and cofactors between the rest of the intracellular compartments. The mitochondrial energy metabolism is greatly dependent on nicotinamide adenine dinucleotide (NAD) as a cofactor that is essential for both the activity of respiratory and TCA cycle enzymes. The NAD level is determined by the rate of NAD synthesis, the activity of NAD-consuming enzymes, and the exchange rate between the individual subcellular compartments. In this review, we discuss the NAD synthesis pathways, the NAD degradation enzymes, and NAD subcellular localization, as well as NAD transport mechanisms with a focus on mitochondria. Finally, the effect of the pathologic depletion of mitochondrial NAD pools on mitochondrial proteins' post-translational modifications and its role in neurodegeneration will be reviewed. Understanding the physiological constraints and mechanisms of NAD maintenance and the exchange between subcellular compartments is critical given NAD's broad effects and roles in health and disease.

Activation of cardiac Nmnat/NAD+/SIR2 pathways mediates endurance exercise resistance to lipotoxic cardiomyopathy in aging Drosophila

Endurance exercise is an important way to resist and treat high-fat diet (HFD)-induced lipotoxic cardiomyopathy, but the underlying molecular mechanisms are poorly understood. Here, we used Drosophila to identify whether cardiac Nmnat/NAD+/SIR2 pathway activation mediates endurance exercise-induced resistance to lipotoxic cardiomyopathy. The results showed that endurance exercise activated the cardiac Nmnat/NAD+/SIR2/FOXO pathway and the Nmnat/NAD+/SIR2/PGC-1α pathway, including up-regulating cardiac Nmnat, SIR2, FOXO and PGC-1α expression, superoxide dismutase (SOD) activity and NAD+ levels, and it prevented HFD-induced or cardiac Nmnat knockdown-induced cardiac lipid accumulation, malondialdehyde (MDA) content and fibrillation increase, and fractional shortening decrease. Cardiac Nmnat overexpression also activated heart Nmnat/NAD+/SIR2 pathways and resisted HFD-induced cardiac malfunction, but it could not protect against HFD-induced lifespan reduction and locomotor impairment. Exercise improved lifespan and mobility in cardiac Nmnat knockdown flies. Therefore, the current results confirm that cardiac Nmnat/NAD+/SIR2 pathways are important antagonists of HFD-induced lipotoxic cardiomyopathy. Cardiac Nmnat/NAD+/SIR2 pathway activation is an important underlying molecular mechanism by which endurance exercise and cardiac Nmnat overexpression give protection against lipotoxic cardiomyopathy in Drosophila.

NAD+ Metabolism, Metabolic Stress, and Infection

Nicotinamide adenine dinucleotide (NAD⁺) is an essential metabolite with wide-ranging and significant roles in the cell. Defects in NAD⁺ metabolism have been associated with many human disorders; it is therefore an emerging therapeutic target. Moreover, NAD⁺ metabolism is perturbed during colonization by a variety of pathogens, either due to the molecular mechanisms employed by these infectious agents or by the host immune response they trigger. Three main biosynthetic pathways, including the de novo and salvage pathways, contribute to the production of NAD⁺ with a high degree of conservation from bacteria to humans. De novo biosynthesis, which begins with l-tryptophan in eukaryotes, is also known as the kynurenine pathway. Intermediates of this pathway have various beneficial and deleterious effects on cellular health in different contexts. For example, dysregulation of this pathway is linked to neurotoxicity and oxidative stress. Activation of the de novo pathway is also implicated in various infections and inflammatory signaling. Given the dynamic flexibility and multiple roles of NAD⁺ intermediates, it is important to understand the interconnections and cross-regulations of NAD⁺ precursors and associated signaling pathways to understand how cells regulate NAD⁺ homeostasis in response to various growth conditions. Although regulation of NAD⁺ homeostasis remains incompletely understood, studies in the genetically tractable budding yeast Saccharomyces cerevisiae may help provide some molecular basis for how NAD⁺ homeostasis factors contribute to the maintenance and regulation of cellular function and how they are regulated by various nutritional and stress signals. Here we present a brief overview of recent insights and discoveries made with respect to the relationship between NAD⁺ metabolism and selected human disorders and infections, with a particular focus on the de novo pathway. We also discuss how studies in budding yeast may help elucidate the regulation of NAD⁺ homeostasis.

Salvage NAD+ biosynthetic pathway enzymes moonlight as molecular chaperones to protect against proteotoxicity

Human neurodegenerative proteinopathies are disorders associated with abnormal protein depositions in brain neurons. They include polyglutamine (polyQ) conditions such as Huntington's disease (HD) and α-synucleinopathies such as Parkinson's disease (PD). Overexpression of NMNAT/Nma1, an enzyme in the NAD+ biosynthetic salvage pathway, acts as an efficient suppressor of proteotoxicities in yeast, fly and mouse models. Screens in yeast models of HD and PD allowed us to identify three additional enzymes of the same pathway that achieve similar protection against proteotoxic stress: Npt1, Pnc1 and Qns1. The mechanism by which these proteins maintain proteostasis has not been identified. Here, we report that their ability to maintain proteostasis in yeast models of HD and PD is independent of their catalytic activity and does not require cellular protein quality control systems such as the proteasome or autophagy. Furthermore, we show that, under proteotoxic stress, the four proteins are recruited as molecular chaperones with holdase and foldase activities. The NAD+ salvage proteins act by preventing misfolding and, together with the Hsp90 chaperone, promoting the refolding of extended polyQ domains and α-synuclein (α-Syn). Our results illustrate the existence of an evolutionarily conserved strategy of repurposing or moonlighting housekeeping enzymes under stress conditions to maintain proteostasis. We conclude that the entire salvage NAD+ biosynthetic pathway links NAD+ metabolism and proteostasis and emerges as a target for therapeutics to combat age-associated neurodegenerative proteotoxicities.

Nicotinamide adenine dinucleotide treatment alleviates the symptoms of experimental autoimmune encephalomyelitis by activating autophagy and inhibiting the NLRP3 inflammasome

Nicotinamide adenine dinucleotide (NAD + ) is an essential cofactor in numerous metabolic pathways, and so may support protective and reparative processes against central nervous system diseases such as multiple sclerosis (MS). Here, we investigated the therapeutic potential of NAD + administration in the experimental autoimmune encephalomyelitis (EAE) mouse model of MS and the contributions of autophagic regulation and NLRP3 inflammasome activity. EAE was induced in female C57BL/6 mice by immunization with myelin oligodendrocyte glycoprotein (MOG) p35-55 and disease severity analyzed by neurological function score and histological scores of spinal cord sections stained with hematoxylin-eosin or luxol fast blue. Outcomes were compared among control mice and EAE groups receiving daily post-immunization vehicle injections, NAD + injections, injection of the autophagy inhibitor 3-methyladenine (3-MA), or co-injection of NAD + and 3-MA. Expression levels of autophagy-related proteins (Beclin1, LC3-II/I, and p62/SQSTM1) were assessed by Western blotting, the activated microglial cells were evaluated by immunohistochemistry, while mRNA expression levels of NOD-like receptor family pyrin domain containing 3 (NLRP3), interleukin (IL)-1β, IL-2, IL-17, IL-18, interferon-γ (IFN-γ) and IL-10 were detected by real-time PCR. The proportions of Th1 and Th17 cells in spleen were evaluated using flow cytometry. Treatment with NAD + alleviated demyelination, nerve injury, microglial activation and motor function abnormalities of EAE mice. In addition, NAD + increased the expressions of the autophagy-related proteins LC3-II/I and Beclin 1, and reduced the expression of p62. Treatment with NAD + also inhibited the expressions of NLRP3 and modulated the differentiation of Th1 and Th17 cells, reduced the expressions of the pro-inflammatory factors IL-1β, IL-2, IL-18, IFN-γ and IL-17, and increased the expression of anti-inflammatory IL-10. Conversely, 3-MA aggravated spinal cord inflammation and demyelination, and delayed spontaneous remission from EAE. Furthermore, the beneficial effects of NAD + were abolished by 3-MA cotreatment. Our results indicate that NAD + suppresses the NLRP3 inflammasome at least in part through the activation of autophagy to relieve the symptoms of EAE. Therefore, regulation of autophagy by NAD + treatment may be an effective therapeutic strategy for MS.

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.

NAD+ Metabolism and Regulation: Lessons From Yeast

Nicotinamide adenine dinucleotide (NAD+) is an essential metabolite involved in various cellular processes. The cellular NAD+ pool is maintained by three biosynthesis pathways, which are largely conserved from bacteria to human. NAD+ metabolism is an emerging therapeutic target for several human disorders including diabetes, cancer, and neuron degeneration. Factors regulating NAD+ homeostasis have remained incompletely understood due to the dynamic nature and complexity of NAD+ metabolism. Recent studies using the genetically tractable budding yeast Saccharomyces cerevisiae have identified novel NAD+ homeostasis factors. These findings help provide a molecular basis for how may NAD+ and NAD+ homeostasis factors contribute to the maintenance and regulation of cellular function. Here we summarize major NAD+ biosynthesis pathways, selected cellular processes that closely connect with and contribute to NAD+ homeostasis, and regulation of NAD+ metabolism by nutrient-sensing signaling pathways. We also extend the discussions to include possible implications of NAD+ homeostasis factors in human disorders. Understanding the cross-regulation and interconnections of NAD+ precursors and associated cellular pathways will help elucidate the mechanisms of the complex regulation of NAD+ homeostasis. These studies may also contribute to the development of effective NAD+-based therapeutic strategies specific for different types of NAD+ deficiency related disorders.

PARPs and ADP-ribosylation in RNA biology: from RNA expression and processing to protein translation and proteostasis

In this review, Kim et al. discuss the importance of PARP family members and ADPRylation in gene regulation, mRNA processing, and protein abundance.

ADP-ribosylation (ADPRylation) is a posttranslational modification of proteins discovered nearly six decades ago, but many important questions remain regarding its molecular functions and biological roles, as well as the activity of the ADP-ribose (ADPR) transferase enzymes (PARP family members) that catalyze it. Growing evidence indicates that PARP-mediated ADPRylation events are key regulators of the protein biosynthetic pathway, leading from rDNA transcription and ribosome biogenesis to mRNA synthesis, processing, and translation. In this review we describe the role of PARP proteins and ADPRylation in all facets of this pathway. PARP-1 and its enzymatic activity are key regulators of rDNA transcription, which is a critical step in ribosome biogenesis. An emerging role of PARPs in alternative splicing of mRNAs, as well as direct ADPRylation of mRNAs, highlight the role of PARP members in RNA processing. Furthermore, PARP activity, stimulated by cellular stresses, such as viral infections and ER stress, leads to the regulation of mRNA stability and protein synthesis through posttranscriptional mechanisms. Dysregulation of PARP activity in these processes can promote disease states. Collectively, these results highlight the importance of PARP family members and ADPRylation in gene regulation, mRNA processing, and protein abundance. Future studies in these areas will yield new insights into the fundamental mechanisms and a broader utility for PARP-targeted therapeutic agents.

Nicotinamide Riboside Enhances Mitochondrial Proteostasis and Adult Neurogenesis through Activation of Mitochondrial Unfolded Protein Response Signaling in the Brain of ALS SOD1G93A Mice

Amyotrophic lateral sclerosis (ALS) is caused by the progressive degeneration of motor neurons in the spinal cord, the brain stem, and the motor cortex. So far, there is still a lack of effective drugs. Nicotinamide adenine dinucleotide (NAD+) takes part in redox reactions and the NAD-dependent signaling pathway. The NAD+ decline is related with many neurological diseases, leading to the accumulation of neurotoxic protein in the central nervous system. Moreover, the NAD+ supplementation is shown to promote neural stem cells/neuronal precursor cells (NSCs/NPCs) pool maintenance. Regulatory mechanisms and functions of NAD+ metabolism in ALS are still unknown. Thus, we hypothesized the aggregation of human SOD1 toxic protein and the fate of NSCs/NPCs in the ALS disease could be improved by the administration of nicotinamide riboside (NR), an NAD+ precursor. In this study, we treated SOD1G93A transgenic and wild-type mice by the oral administration of 20 mg/ml NR starting at 50 days of age. Effects of NR on the body weight, the motor function, the onset and the survival were assessed during the experiment. The expression of mutant hSOD1 protein, mitochondrial unfolded protein response (UPRmt) related protein, mitophagy markers and NAD+ metabolism related protein were detected by immunoblotting. Effects of NR on the NSCs/NPCs in neurogenic niches of brain were identified by the immunofluorescence staining. Our investigation elucidated that the NR treatment exhibited better hanging wire endurance but did not postpone the onset or extend the life span of SOD1G93A mice. Besides, we observed that the NR repletion promoted the clearance of mitochondrial hSOD1 neurotoxic protein. Meanwhile, the mitochondrial function pathway was disrupted in the brain of SOD1G93A mice. What's more, we demonstrated that the inadequate function of NAD+ salvage synthesis pathway was the primary explanation behind the decline of NAD+, and the NR treatment enhanced the proliferation and migration of NSCs/NPCs in the brain of SOD1G93A mice. At last, we found that levels of UPRmt related protein were significantly increased in the brain of SOD1G93A mice after the NR treatment. In summary, these findings reveal that the administration of NR activates UPRmt signaling, modulates mitochondrial proteostasis and improves the adult neurogenesis in the brain of SOD1G93A mice.

Nicotinamide mononucleotide adenylyltransferase uses its NAD+ substrate-binding site to chaperone phosphorylated Tau

Tau hyper-phosphorylation and deposition into neurofibrillary tangles have been found in brains of patients with Alzheimer's disease (AD) and other tauopathies. Molecular chaperones are involved in regulating the pathological aggregation of phosphorylated Tau (pTau) and modulating disease progression. Here, we report that nicotinamide mononucleotide adenylyltransferase (NMNAT), a well-known NAD⁺ synthase, serves as a chaperone of pTau to prevent its amyloid aggregation in vitro as well as mitigate its pathology in a fly tauopathy model. By combining NMR spectroscopy, crystallography, single-molecule and computational approaches, we revealed that NMNAT adopts its enzymatic pocket to specifically bind the phosphorylated sites of pTau, which can be competitively disrupted by the enzymatic substrates of NMNAT. Moreover, we found that NMNAT serves as a co-chaperone of Hsp90 for the specific recognition of pTau over Tau. Our work uncovers a dedicated chaperone of pTau and suggests NMNAT as a key node between NAD⁺ metabolism and Tau homeostasis in aging and neurodegeneration.

Why is NMNAT Protective against Neuronal Cell Death and Axon Degeneration, but Inhibitory of Axon Regeneration?

Nicotinamide mononucleotide adenylyltransferase (NMNAT), a key enzyme for NAD⁺ synthesis, is well known for its activity in neuronal survival and attenuation of Wallerian degeneration. Recent investigations in invertebrate models have, however, revealed that NMNAT activity negatively impacts upon axon regeneration. Overexpression of Nmnat in laser-severed Drosophila sensory neurons reduced axon regeneration, while axon regeneration was enhanced in injured mechanosensory axons in C. elegansnmat-2 null mutants. These diametrically opposite effects of NMNAT orthologues on neuroprotection and axon regeneration appear counterintuitive as there are many examples of neuroprotective factors that also promote neurite outgrowth, and enhanced neuronal survival would logically facilitate regeneration. We suggest here that while NMNAT activity and NAD⁺ production activate neuroprotective mechanisms such as SIRT1-mediated deacetylation, the same mechanisms may also activate a key axonal regeneration inhibitor, namely phosphatase and tensin homolog (PTEN). SIRT1 is known to deacetylate and activate PTEN which could, in turn, suppress PI3 kinase–mTORC1-mediated induction of localized axonal protein translation, an important process that determines successful regeneration. Strategic tuning of Nmnat activity and NAD⁺ production in axotomized neurons may thus be necessary to promote initial survival without inhibiting subsequent regeneration.

From Yeast to Humans: Leveraging New Approaches in Yeast to Accelerate Discovery of Therapeutic Targets for Synucleinopathies

Neurodegenerative diseases (ND) represent a growing, global health crisis, one that lacks any disease-modifying therapeutic strategy. This critical need for new therapies must be met with an exhaustive approach to exploit all tools available. A yeast (Saccharomyces cerevisiae) model of α-synuclein toxicity-the protein causally linked to Parkinson's disease and other synucleinopathies-offers a powerful approach that takes advantage of the unique offerings of this system: tractable genetics, robust high-throughput screening strategies, unparalleled data repositories, powerful computational tools, and extensive evolutionary conservation of fundamental biological pathways. These attributes have enabled genetic and small molecule screens that have revealed toxic phenotypes and drug targets that translate directly to patient-derived iPSC neurons. Extending these insights, recent advances in genetic network analyses have generated the first "humanized" α-synuclein network, which has identified druggable proteins and led to validation of the toxic phenotypes in patient-derived cells. Unbiased phenotypic small molecule screens can identify compounds targeting critical proteins within α-synuclein networks. While identification of direct drug targets for phenotypic screen hits represents a bottleneck, high-throughput chemical genetic methods provide a means to uncover cellular targets and pathways for large numbers of compounds in parallel. Taken together, the yeast α-synuclein model and associated tools can reveal insights into underlying cellular pathologies, lead molecules and their cognate targets, and strategies to translate mechanisms of toxicity and cytoprotection into complex neuronal systems.

NAD-biosynthetic enzyme NMNAT1 reduces early behavioral impairment in the htau mouse model of tauopathy

NAD metabolism and the NAD biosynthetic enzymes nicotinamide nucleotide adenylyltransferases (NMNATs) are thought to play a key neuroprotective role in tauopathies, including Alzheimer's disease. Here, we investigated whether modulating the expression of the NMNAT nuclear isoform NMNAT1, which is important for neuronal maintenance, influences the development of behavioral and neuropathological abnormalities in htau mice, which express non-mutant human tau isoforms and represent a model of tauopathy relevant to Alzheimer's disease. Prior to the development of cognitive symptoms, htau mice exhibit tau hyperphosphorylation associated with a selective deficit in food burrowing, a behavior reminiscent to activities of daily living which are impaired early in Alzheimer's disease. We crossed htau mice with Nmnat1 transgenic and knockout mice and tested the resulting offspring until the age of 6 months. We show that overexpression of NMNAT1 ameliorates the early deficit in food burrowing characteristic of htau mice. At 6 months of age, htau mice did not show neurodegenerative changes in both the cortex and hippocampus, and these were not induced by downregulating NMNAT1 levels. Modulating NMNAT1 levels produced a corresponding effect on NMNAT enzymatic activity but did not alter NAD levels in htau mice. Although changes in local NAD levels and subsequent modulation of NAD-dependent enzymes cannot be ruled out, this suggests that the effects seen on behavior may be due to changes in tau phosphorylation. Our results suggest that increasing NMNAT1 levels can slow the progression of symptoms and neuropathological features of tauopathy, but the underlying mechanisms remain to be established.

Quercetin enhances stress resistance in Saccharomyces cerevisiae tel1 mutant cells to different stressors

The Saccharomyces cerevisiae TEL1 gene is an ortholog of the human ATM (Ataxia telangiectasia mutated) gene. S. cerevisiae tel1 mutant (tel1∆) lacking Tel1p, share some of the cellular defects with ATM mutation that includes prevention of oxidative damage repair, premature aging and apoptosis. In the present study, we investigated the protective effects of quercetin on the sensitivity of yeast S. cerevisiae tel1∆ cells exposed to oxidative, apoptotic and DNA damaging stress and viability of tel1∆ cells during chronological aging. Quercetin improved the stress resistance of tel1∆ cells when challenged with oxidants such as hydrogen peroxide (H₂O₂), menadine bisulphite (MBS) and tertiary butyl hydroperoxide (t-BHP) by scavenging reactive oxygen species (ROS). Quercetin protected the tel1∆ cells from acetic acid-induced apoptotic cell death and sensitivity against hydroxyurea. We found that quercetin attenuated ROS accumulation and apoptotic markers in tel1∆ cells and therefore an increase in cell viability during chronological aging. Our results from the S. cerevisiae model, suggest that use of quercetin as a food supplement might alleviate oxidative stress mediated DNA damage, apoptosis and age related damaging effects in AT patients and also improve health beneficial effects in humans.

Mechanisms of α-Synuclein Induced Synaptopathy in Parkinson's Disease

Parkinson's disease (PD) is characterized by intracellular inclusions of aggregated and misfolded α-Synuclein (α-Syn), and the loss of dopaminergic (DA) neurons in the brain. The resulting motor abnormalities mark the progression of PD, while non-motor symptoms can already be identified during early, prodromal stages of disease. Recent studies provide evidence that during this early prodromal phase, synaptic and axonal abnormalities occur before the degenerative loss of neuronal cell bodies. These early phenotypes can be attributed to synaptic accumulation of toxic α-Syn. Under physiological conditions, α-Syn functions in its native conformation as a soluble monomer. However, PD patient brains are characterized by intracellular inclusions of insoluble fibrils. Yet, oligomers and protofibrils of α-Syn have been identified to be the most toxic species, with their accumulation at presynaptic terminals affecting several steps of neurotransmitter release. First, high levels of α-Syn alter the size of synaptic vesicle pools and impair their trafficking. Second, α-Syn overexpression can either misregulate or redistribute proteins of the presynaptic SNARE complex. This leads to deficient tethering, docking, priming and fusion of synaptic vesicles at the active zone (AZ). Third, α-Syn inclusions are found within the presynaptic AZ, accompanied by a decrease in AZ protein levels. Furthermore, α-Syn overexpression reduces the endocytic retrieval of synaptic vesicle membranes during vesicle recycling. These presynaptic alterations mediated by accumulation of α-Syn, together impair neurotransmitter exocytosis and neuronal communication. Although α-Syn is expressed throughout the brain and enriched at presynaptic terminals, DA neurons are the most vulnerable in PD, likely because α-Syn directly regulates dopamine levels. Indeed, evidence suggests that α-Syn is a negative modulator of dopamine by inhibiting enzymes responsible for its synthesis. In addition, α-Syn is able to interact with and reduce the activity of VMAT2 and DAT. The resulting dysregulation of dopamine levels directly contributes to the formation of toxic α-Syn oligomers. Together these data suggest a vicious cycle of accumulating α-Syn and deregulated dopamine that triggers synaptic dysfunction and impaired neuronal communication, ultimately causing synaptopathy and progressive neurodegeneration in Parkinson's disease.

The Inhibitory Effects of Purple Sweet Potato Color on Hepatic Inflammation Is Associated with Restoration of NAD⁺ Levels and Attenuation of NLRP3 Inflammasome Activation in High-Fat-Diet-Treated Mice

Purple sweet potato color (PSPC), a class of naturally occurring anthocyanins, exhibits beneficial effects on metabolic syndrome. Sustained inflammation plays a crucial role in the pathogenesis of metabolic syndrome. Here we explored the effects of PSPC on high-fat diet (HFD)-induced hepatic inflammation and the mechanisms underlying these effects. Mice were divided into four groups: Control group, HFD group, HFD + PSPC group, and PSPC group. PSPC was administered by daily oral gavage at doses of 700 mg/kg/day for 20 weeks. Nicotinamide riboside (NR) was used to increase NAD⁺ levels. Our results showed that PSPC effectively ameliorated obesity and liver injuries in HFD-fed mice. Moreover, PSPC notably blocked hepatic oxidative stress in HFD-treated mice. Furthermore, PSPC dramatically restored NAD⁺ level to abate endoplasmic reticulum stress (ER stress) in HFD-treated mouse livers, which was confirmed by NR treatment. Consequently, PSPC remarkably suppressed the nuclear factor-κB (NF-κB) p65 nuclear translocation and nucleotide oligomerization domain protein1/2 (NOD1/2) signaling in HFD-treated mouse livers. Thereby, PSPC markedly diminished the NLR family, pyrin domain containing 3 (NLRP3) inflammasome activation, ultimately lowering the expressions of inflammation-related genes in HFD-treated mouse livers. In summary, PSPC protected against HFD-induced hepatic inflammation by boosting NAD⁺ level to inhibit NLRP3 inflammasome activation.

NMNAT: It's an NAD+ synthase… It's a chaperone… It's a neuroprotector

Nicotinamide mononucleotide adenylyl transferases (NMNATs) are a family of highly conserved proteins indispensable for cellular homeostasis. NMNATs are classically known for their enzymatic function of catalyzing NAD⁺ synthesis, but also have gained a reputation as essential neuronal maintenance factors. NMNAT deficiency has been associated with various human diseases with pronounced consequences on neural tissues, underscoring the importance of the neuronal maintenance and protective roles of these proteins. New mechanistic studies have challenged the role of NMNAT-catalyzed NAD⁺ production in delaying Wallerian degeneration and have specified new mechanisms of NMNAT's chaperone function critical for neuronal health. Progress in understanding the regulation of NMNAT has uncovered a neuronal stress response with great therapeutic promise for treating various neurodegenerative conditions.

Exploring the power of yeast to model aging and age-related neurodegenerative disorders

Aging is a multifactorial process determined by molecular, cellular and systemic factors and it is well established that advancing age is a leading risk factor for several neurodegenerative diseases. In fact, the close association of aging and neurodegenerative disorders has placed aging as the greatest social and economic challenge of the 21st century, and age-related diseases have also become a key priority for countries worldwide. The growing need to better understand both aging and neurodegenerative processes has led to the development of simple eukaryotic models amenable for mechanistic studies. Saccharomyces cerevisiae has proven to be an unprecedented experimental model to study the fundamental aspects of aging and to decipher the intricacies of neurodegenerative disorders greatly because the molecular mechanisms underlying these processes are evolutionarily conserved from yeast to human. Moreover, yeast offers several methodological advantages allowing a rapid and relatively easy way of establishing gene-protein-function associations. Here we review different aging theories, common cellular pathways driving aging and neurodegenerative diseases and discuss the major contributions of yeast to the state-of-art knowledge in both research fields.

Aspartate facilitates mitochondrial function, growth arrest and survival during doxorubicin exposure

Genomic screens of doxorubicin toxicity in S. cerevisiae have identified numerous mutants in amino acid and carbon metabolism which express increased doxorubicin sensitivity. This work examines the effect of amino acid metabolism on doxorubicin toxicity. S. cerevisiae were treated with doxorubicin in combination with a variety of amino acid supplements. Strains of S. cerevisiae with mutations in pathways utilizing aspartate and other metabolites were examined for sensitivity to doxorubicin. S. cerevisiae cultures exposed to doxorubicin in minimal media showed significantly more toxicity than cultures exposed in rich media. Supplementing minimal media with aspartate, glutamate or alanine reduced doxorubicin toxicity. Cell cycle response was assessed by examining the budding pattern of treated cells. Cultures exposed to doxorubicin in minimal media arrested growth with no apparent cell cycle progression. Aspartate supplementation allowed cultures exposed to doxorubicin in minimal media to arrest after one division with a budding pattern and survival comparable to cultures exposed in rich media. Aspartate provides less protection from doxorubicin in cells mutant in either mitochondrial citrate synthase (CIT1) or NADH oxidase (NDI1), suggesting aspartate reduces doxorubicin toxicity by facilitating mitochondrial function. These data suggest glycolysis becomes less active and mitochondrial respiration more active following doxorubicin exposure.

Nicotinamide Suppresses the DNA Damage Sensitivity of Saccharomyces cerevisiae Independently of Sirtuin Deacetylases

Nicotinamide is both a reaction product and an inhibitor of the conserved sirtuin family of deacetylases, which have been implicated in a broad range of cellular functions in eukaryotes from yeast to humans. Phenotypes observed following treatment with nicotinamide are most often assumed to stem from inhibition of one or more of these enzymes. Here, we used this small molecule to inhibit multiple sirtuins at once during treatment with DNA damaging agents in the Saccharomyces cerevisiae model system. Since sirtuins have been previously implicated in the DNA damage response, we were surprised to observe that nicotinamide actually increased the survival of yeast cells exposed to the DNA damage agent MMS. Remarkably, we found that enhanced resistance to MMS in the presence of nicotinamide was independent of all five yeast sirtuins. Enhanced resistance was also independent of the nicotinamide salvage pathway, which uses nicotinamide as a substrate to generate NAD+, and of a DNA damage-induced increase in the salvage enzyme Pnc1 Our data suggest a novel and unexpected function for nicotinamide that has broad implications for its use in the study of sirtuin biology across model systems.

Attenuation of polyglutamine-induced toxicity by enhancement of mitochondrial OXPHOS in yeast and fly models of aging

Defects in mitochondrial biogenesis and function are common in many neurodegenerative disorders, including Huntington's disease (HD). We have previously shown that in yeast models of HD, enhancement of mitochondrial biogenesis through overexpression of Hap4, the catalytic subunit of the transcriptional complex that regulates mitochondrial gene expression, alleviates the growth arrest induced by expanded polyglutamine (polyQ) tract peptides in rapidly dividing cells. However, the mechanism through which HAP4 overexpression exerts this protection remains unclear. Furthermore, it remains unexplored whether HAP4 overexpression and increased respiratory function during growth can also protect against polyQ-induced toxicity during yeast chronological lifespan. Here, we show that in yeast, mitochondrial respiration and oxidative phosphorylation (OXPHOS) are essential for protection against the polyQ-induced growth defect by HAP4 overexpression. In addition, we show that not only increased HAP4 levels, but also alternative interventions, including calorie restriction, that result in enhanced mitochondrial biogenesis confer protection against polyQ toxicity during stationary phase. The data obtained in yeast models guided experiments in a fly model of HD, where we show that enhancement of mitochondrial biogenesis can also protect against neurodegeneration and behavioral deficits. Our results suggest that therapeutic interventions aiming at the enhancement of mitochondrial respiration and OXPHOS could reduce polyQ toxicity and delay disease onset.

Nmnat1 protects neuronal function without altering phospho-tau pathology in a mouse model of tauopathy

Objective

The nicotinamide‐nucleotide adenylyltransferase protein Nmnat1 is a potent inhibitor of axonal degeneration in models of acute axonal injury. Hyperphosphorylation and aggregation of the microtubule‐associated protein Tau are associated with neurodegeneration in Alzheimer's Disease and other disorders. Previous studies have demonstrated that other Nmnat isoforms can act both as axonoprotective agents and have protein chaperone function, exerting protective effects in drosophila and mouse models of tauopathy. Nmnat1 targeted to the cytoplasm (cytNmnat1) is neuroprotective in a mouse model of neonatal hypoxia‐ischemia, but the effect of cytNmnat1 on tauopathy remains unknown.

Methods

We examined the impact of overexpression of cytNmnat1 on tau pathology, neurodegeneration, and brain functional connectivity in the P301S mouse model of chronic tauopathy.

Results

Overexpression of cytNmnat1 preserved cortical neuron functional connectivity in P301S mice in vivo. However, whereas Nmnat1 overexpression decreased the accumulation of detergent‐insoluble tau aggregates in the cerebral cortex, it exerted no effect on immunohistochemical evidence of pathologic tau phosphorylation and misfolding, hippocampal atrophy, or inflammatory markers in P301S mice.

Interpretation

Our results demonstrate that cytNmnat1 partially preserves neuronal function and decreases biochemically insoluble tau in a mouse model of chronic tauopathy without preventing tau phosphorylation, formation of soluble aggregates, or tau‐induced inflammation and atrophy. Nmnat1 might thus represent a therapeutic target for tauopathies.

Alzheimer's as a Systems-Level Disease Involving the Interplay of Multiple Cellular Networks

Alzheimer's disease (AD), and many neurodegenerative disorders, are multifactorial in nature. They involve a combination of genomic, epigenomic, interactomic and environmental factors. Progress is being made, and these complex diseases are beginning to be understood as having their origin in altered states of biological networks at the cellular level. In the case of AD, genomic susceptibility and mechanisms leading to (or accompanying) the impairment of the central Amyloid Precursor Protein (APP) processing and tau networks are widely accepted as major contributors to the diseased state. The derangement of these networks may result in both the gain and loss of functions, increased generation of toxic species (e.g., toxic soluble oligomers and aggregates) and imbalances, whose effects can propagate to supra-cellular levels. Although well sustained by empirical data and widely accepted, this global perspective often overlooks the essential roles played by the main counteracting homeostatic networks (e.g., protein quality control/proteostasis, unfolded protein response, protein folding chaperone networks, disaggregases, ER-associated degradation/ubiquitin proteasome system, endolysosomal network, autophagy, and other stress-protective and clearance networks), whose relevance to AD is just beginning to be fully realized. In this chapter, an integrative perspective is presented. Alzheimer's disease is characterized to be a result of: (a) intrinsic genomic/epigenomic susceptibility and, (b) a continued dynamic interplay between the deranged networks and the central homeostatic networks of nerve cells. This interplay of networks will underlie both the onset and rate of progression of the disease in each individual. Integrative Systems Biology approaches are required to effect its elucidation. Comprehensive Systems Biology experiments at different 'omics levels in simple model organisms, engineered to recapitulate the basic features of AD may illuminate the onset and sequence of events underlying AD. Indeed, studies of models of AD in simple organisms, differentiated cells in culture and rodents are beginning to offer hope that the onset and progression of AD, if detected at an early stage, may be stopped, delayed, or even reversed, by activating or modulating networks involved in proteostasis and the clearance of toxic species. In practice, the incorporation of next-generation neuroimaging, high-throughput and computational approaches are opening the way towards early diagnosis well before irreversible cell death. Thus, the presence or co-occurrence of: (a) accumulation of toxic Aβ oligomers and tau species; (b) altered splicing and transcriptome patterns; (c) impaired redox, proteostatic, and metabolic networks together with, (d) compromised homeostatic capacities may constitute relevant 'AD hallmarks at the cellular level' towards reliable and early diagnosis. From here, preventive lifestyle changes and tailored therapies may be investigated, such as combined strategies aimed at both lowering the production of toxic species and potentiating homeostatic responses, in order to prevent or delay the onset, and arrest, alleviate, or even reverse the progression of the disease.

Alternative splicing of Drosophila Nmnat functions as a switch to enhance neuroprotection under stress

Nicotinamide mononucleotide adenylyltransferase (NMNAT) is a conserved enzyme in the NAD synthetic pathway. It has also been identified as an effective and versatile neuroprotective factor. However, it remains unclear how healthy neurons regulate the dual functions of NMNAT and achieve self-protection under stress. Here we show that Drosophila Nmnat (DmNmnat) is alternatively spliced into two mRNA variants, RA and RB, which translate to protein isoforms with divergent neuroprotective capacities against spinocerebellar ataxia 1-induced neurodegeneration. Isoform PA/PC translated from RA is nuclear-localized with minimal neuroprotective ability, and isoform PB/PD translated from RB is cytoplasmic and has robust neuroprotective capacity. Under stress, RB is preferably spliced in neurons to produce the neuroprotective PB/PD isoforms. Our results indicate that alternative splicing functions as a switch that regulates the expression of functionally distinct DmNmnat variants. Neurons respond to stress by driving the splicing switch to produce the neuroprotective variant and therefore achieve self-protection.

Nicotinamide mononucleotide adenylyltransferase (NMNAT) acts in the NAD biosynthesis pathway and has neuroprotective activity. Ruan et al. show that the neuroprotective activity of NMNAT is restricted to a splice variant of the enzyme, and that this variant is preferentially spliced in response to stress.

Less is more: Nutrient limitation induces cross-talk of nutrient sensing pathways with NAD+ homeostasis and contributes to longevity

Nutrient sensing pathways and their regulation grant cells control over their metabolism and growth in response to changing nutrients. Factors that regulate nutrient sensing can also modulate longevity. Reduced activity of nutrient sensing pathways such as glucose-sensing PKA, nitrogen-sensing TOR and S6 kinase homolog Sch9 have been linked to increased life span in the yeast, Saccharomyces cerevisiae, and higher eukaryotes. Recently, reduced activity of amino acid sensing SPS pathway was also shown to increase yeast life span. Life span extension by reduced SPS activity requires enhanced NAD⁺ (nicotinamide adenine dinucleotide, oxidized form) and nicotinamide riboside (NR, a NAD⁺ precursor) homeostasis. Maintaining adequate NAD⁺ pools has been shown to play key roles in life span extension, but factors regulating NAD⁺ metabolism and homeostasis are not completely understood. Recently, NAD⁺ metabolism was also linked to the phosphate (Pi)-sensing PHO pathway in yeast. Canonical PHO activation requires Pi-starvation. Interestingly, NAD⁺ depletion without Pi-starvation was sufficient to induce PHO activation, increasing NR production and mobilization. Moreover, SPS signaling appears to function in parallel with PHO signaling components to regulate NR/NAD⁺ homeostasis. These studies suggest that NAD⁺ metabolism is likely controlled by and/or coordinated with multiple nutrient sensing pathways. Indeed, cross-regulation of PHO, PKA, TOR and Sch9 pathways was reported to potentially affect NAD⁺ metabolism; though detailed mechanisms remain unclear. This review discusses yeast longevity-related nutrient sensing pathways and possible mechanisms of life span extension, regulation of NAD⁺ homeostasis, and cross-talk among nutrient sensing pathways and NAD⁺ homeostasis.

Dietary restriction, mitochondrial function and aging: from yeast to humans

Dietary restriction (DR) attenuates many detrimental effects of aging and consequently promotes health and increases longevity across organisms. While over the last 15 years extensive research has been devoted towards understanding the biology of aging, the precise mechanistic aspects of DR are yet to be settled. Abundant experimental evidence indicates that the DR effect on stimulating health impinges several metabolic and stress-resistance pathways. Downstream effects of these pathways include a reduction in cellular damage induced by oxidative stress, enhanced efficiency of mitochondrial functions and maintenance of mitochondrial dynamics and quality control, thereby attenuating age-related declines in mitochondrial function. However, the literature also accumulates conflicting evidence regarding how DR ameliorates mitochondrial performance and whether that is enough to slow age-dependent cellular and organismal deterioration. Here, we will summarize the current knowledge about how and to which extent the influence of different DR regimes on mitochondrial biogenesis and function contribute to postpone the detrimental effects of aging on health-span and lifespan. This article is part of a Special Issue entitled: Mitochondrial Dysfunction in Aging.

Overexpression of alpha-synuclein at non-toxic levels increases dopaminergic cell death induced by copper exposure via modulation of protein degradation pathways

Gene multiplications or point mutations in alpha (α)-synuclein are associated with familial and sporadic Parkinson's disease (PD). An increase in copper (Cu) levels has been reported in the cerebrospinal fluid and blood of PD patients, while occupational exposure to Cu has been suggested to augment the risk to develop PD. We aimed to elucidate the mechanisms by which α-synuclein and Cu regulate dopaminergic cell death. Short-term overexpression of wild type (WT) or mutant A53T α-synuclein had no toxic effect in human dopaminergic cells and primary midbrain cultures, but it exerted a synergistic effect on Cu-induced cell death. Cell death induced by Cu was potentiated by overexpression of the Cu transporter protein 1 (Ctr1) and depletion of intracellular glutathione (GSH) indicating that the toxic effects of Cu are linked to alterations in its intracellular homeostasis. Using the redox sensor roGFP, we demonstrated that Cu-induced oxidative stress was primarily localized in the cytosol and not in the mitochondria. However, α-synuclein overexpression had no effect on Cu-induced oxidative stress. WT or A53T α-synuclein overexpression exacerbated Cu toxicity in dopaminergic and yeast cells in the absence of α-synuclein aggregation. Cu increased autophagic flux and protein ubiquitination. Impairment of autophagy by overexpression of a dominant negative Atg5 form or inhibition of the ubiquitin/proteasome system (UPS) with MG132 enhanced Cu-induced cell death. However, only inhibition of the UPS stimulated the synergistic toxic effects of Cu and α-synuclein overexpression. Our results demonstrate that α-synuclein stimulates Cu toxicity in dopaminergic cells independent from its aggregation via modulation of protein degradation pathways.

Quantitative proteomics of the yeast Hsp70/Hsp90 interactomes during DNA damage reveal chaperone-dependent regulation of ribonucleotide reductase

The highly conserved molecular chaperones Hsp90 and Hsp70 are indispensible for folding and maturation of a significant fraction of the proteome, including many proteins involved in signal transduction and stress response. To examine the dynamics of chaperone-client interactions after DNA damage, we applied quantitative affinity-purification mass spectrometry (AP-MS) proteomics to characterize interactomes of the yeast Hsp70 isoform Ssa1 and Hsp90 isoform Hsp82 before and after exposure to methyl methanesulfonate. Of 256 proteins identified and quantified via (16)O(/18)O labeling and LC-MS/MS, 142 are novel Hsp70/90 interactors. Nearly all interactions remained unchanged or decreased after DNA damage, but 5 proteins increased interactions with Ssa1 and/or Hsp82, including the ribonucleotide reductase (RNR) subunit Rnr4. Inhibiting Hsp70 or 90 chaperone activity destabilized Rnr4 in yeast and its vertebrate homolog hRMM2 in breast cancer cells. In turn, pre-treatment of cancer cells with chaperone inhibitors sensitized cells to the RNR inhibitor gemcitabine, suggesting a novel chemotherapy strategy. All MS data have been deposited in the ProteomeXchange with identifier PXD001284.

BIOLOGICAL SIGNIFICANCE

This study provides the dynamic interactome of the yeast Hsp70 and Hsp90 under DNA damage which suggest key roles for the chaperones in a variety of signaling cascades. Importantly, the cancer drug target ribonucleotide reductase was shown to be a client of Hsp70 and Hsp90 in both yeast and breast cancer cells. As such, this study highlights the potential of a novel cancer therapeutic strategy that exploits the synergy of chaperone and ribonucleotide reductase inhibitors.

NAMPT and NAPRT1: novel polymorphisms and distribution of variants between normal tissues and tumor samples

Nicotinamide phosphoribosyltransferase (NAMPT) and nicotinate phosphoribosyltransferase domain containing 1 (NAPRT1) are the main human NAD salvage enzymes. NAD regulates energy metabolism and cell signaling, and the enzymes that control NAD availability are linked to pathologies such as cancer and neurodegeneration. Here, we have screened normal and tumor samples from different tissues and populations of origin for mutations in human NAMPT and NAPRT1, and evaluated their potential pathogenicity. We have identified several novel polymorphisms and showed that NAPRT1 has a greater genetic diversity than NAMPT, where any alteration can have a greater functional impact. Some variants presented different frequencies between normal and tumor samples that were most likely related to their population of origin. The novel mutations described that affect protein structure or expression levels can be functionally relevant and should be considered in a disease context. Particularly, mutations that decrease NAPRT1 expression can predict the usefulness of Nicotinic Acid in tumor treatments with NAMPT inhibitors.

Regulation of NAD+ metabolism, signaling and compartmentalization in the yeast Saccharomyces cerevisiae

Pyridine nucleotides are essential coenzymes in many cellular redox reactions in all living systems. In addition to functioning as a redox carrier, NAD(+) is also a required co-substrate for the conserved sirtuin deacetylases. Sirtuins regulate transcription, genome maintenance and metabolism and function as molecular links between cells and their environment. Maintaining NAD(+) homeostasis is essential for proper cellular function and aberrant NAD(+) metabolism has been implicated in a number of metabolic- and age-associated diseases. Recently, NAD(+) metabolism has been linked to the phosphate-responsive signaling pathway (PHO pathway) in the budding yeast Saccharomyces cerevisiae. Activation of the PHO pathway is associated with the production and mobilization of the NAD(+) metabolite nicotinamide riboside (NR), which is mediated in part by PHO-regulated nucleotidases. Cross-regulation between NAD(+) metabolism and the PHO pathway has also been reported; however, detailed mechanisms remain to be elucidated. The PHO pathway also appears to modulate the activities of common downstream effectors of multiple nutrient-sensing pathways (Ras-PKA, TOR, Sch9/AKT). These signaling pathways were suggested to play a role in calorie restriction-mediated beneficial effects, which have also been linked to Sir2 function and NAD(+) metabolism. Here, we discuss the interactions of these pathways and their potential roles in regulating NAD(+) metabolism. In eukaryotic cells, intracellular compartmentalization facilitates the regulation of enzymatic functions and also concentrates or sequesters specific metabolites. Various NAD(+)-mediated cellular functions such as mitochondrial oxidative phosphorylation are compartmentalized. Therefore, we also discuss several key players functioning in mitochondrial, cytosolic and vacuolar compartmentalization of NAD(+) intermediates, and their potential roles in NAD(+) homeostasis. To date, it remains unclear how NAD(+) and NAD(+) intermediates shuttle between different cellular compartments. Together, these studies provide a molecular basis for how NAD(+) homeostasis factors and the interacting signaling pathways confer metabolic flexibility and contribute to maintaining cell fitness and genome stability.

YCL047C/POF1 is a novel nicotinamide mononucleotide adenylyltransferase (NMNAT) in Saccharomyces cerevisiae

NAD(+) is an essential metabolic cofactor involved in various cellular biochemical processes. Nicotinamide riboside (NR) is an endogenously produced key pyridine metabolite that plays important roles in the maintenance of NAD(+) pool. Using a NR-specific cell-based screen, we identified mutants that exhibit altered NR release phenotype. Yeast cells lacking the ORF YCL047C/POF1 release considerably more NR compared with wild type, suggesting that POF1 plays an important role in NR/NAD(+) metabolism. The amino acid sequence of Pof1 indicates that it is a putative nicotinamide mononucleotide adenylyltransferase (NMNAT). Unlike other yeast NMNATs, Pof1 exhibits NMN-specific adenylyltransferase activity. Deletion of POF1 significantly lowers NAD(+) levels and decreases the efficiency of NR utilization, resistance to oxidative stress, and NR-induced life span extension. We also show that NR is constantly produced by multiple nucleotidases and that the intracellular NR pools are likely to be compartmentalized, which contributes to the regulation of NAD(+) homeostasis. Our findings may contribute to the understanding of the molecular basis and regulation of NAD(+) metabolism in higher eukaryotes.

NMNATs, evolutionarily conserved neuronal maintenance factors

Proper brain function requires neuronal homeostasis over a range of environmental challenges. Neuronal activity, injury, and aging stress the nervous system, and lead to neuronal dysfunction and degeneration. Nevertheless, most organisms maintain healthy neurons throughout life, implying the existence of active maintenance mechanisms. Recent studies have revealed a key neuronal maintenance and protective function for nicotinamide mononucleotide adenylyl transferases (NMNATs). We review evidence that NMNATs protect neurons through multiple mechanisms in different contexts, and highlight functions that either require or are independent of NMNAT catalytic activity. We then summarize data supporting a role for NMNATs in neuronal maintenance and raise intriguing questions on how NMNATs preserve neuronal integrity and facilitate proper neural function throughout life.

Mislocalization of neuronal mitochondria reveals regulation of Wallerian degeneration and NMNAT/WLD(S)-mediated axon protection independent of axonal mitochondria

Axon degeneration is a common and often early feature of neurodegeneration that correlates with the clinical manifestations and progression of neurological disease. Nicotinamide mononucleotide adenylytransferase (NMNAT) is a neuroprotective factor that delays axon degeneration following injury and in models of neurodegenerative diseases suggesting a converging molecular pathway of axon self-destruction. The underlying mechanisms have been under intense investigation and recent reports suggest a central role for axonal mitochondria in both degeneration and NMNAT/WLD(S) (Wallerian degeneration slow)-mediated protection. We used dorsal root ganglia (DRG) explants and Drosophila larval motor neurons (MNs) as models to address the role of mitochondria in Wallerian degeneration (WD). We find that expression of Drosophila NMNAT delays WD in human DRG neurons demonstrating evolutionary conservation of NMNAT function. Morphological comparison of mitochondria from WLD(S)-protected axons demonstrates that mitochondria shrink post-axotomy, though analysis of complex IV activity suggests that they retain their functional capacity despite this morphological change. To determine whether mitochondria are a critical site of regulation for WD, we genetically ablated mitochondria from Drosophila MN axons via the mitochondria trafficking protein milton. Milton loss-of-function did not induce axon degeneration in Drosophila larval MNs, and when axotomized WD proceeded stereotypically in milton distal axons although with a mild, but significant delay. Remarkably, the protective effects of NMNAT/WLD(S) were also maintained in axons devoid of mitochondria. These experiments unveil an axon self-destruction cascade governing WD that is not initiated by axonal mitochondria and for the first time illuminate a mitochondria-independent mechanism(s) regulating WD and NMNAT/WLD(S)-mediated axon protection.