NAD Synthase Research Articles

Cardiomyocyte-specific deletion of the mitochondrial transporter Abcb10 causes cardiac dysfunction via lysosomal-mediated ferroptosis.

Heart function is highly dependent on mitochondria, which not only produce energy but also regulate many cellular functions. Therefore, mitochondria are important therapeutic targets in heart failure. Abcb10 is a member of the ABC transporter superfamily located in the inner mitochondrial membrane and plays an important role in haemoglobin synthesis, biliverdin transport, antioxidant stress, and stabilization of the iron transporter mitoferrin-1. However, the mechanisms underlying the impairment of mitochondrial transporters in the heart remain poorly understood. Here, we generated mice with cardiomyocyte-specific loss of Abcb10. The Abcb10 knockouts exhibited progressive worsening of cardiac fibrosis, increased cardiovascular risk markers and mitochondrial structural abnormalities, suggesting that the pathology of heart failure is related to mitochondrial dysfunction. As the mitochondrial dysfunction was observed early but mildly, other factors were considered. We then observed increased Hif1α expression, decreased NAD synthase expression, and reduced NAD+ levels, leading to lysosomal dysfunction. Analysis of ABCB10 knockdown HeLa cells revealed accumulation of Fe2+ and lipid peroxides in lysosomes, leading to ferroptosis. Lipid peroxidation was suppressed by treatment with iron chelators, suggesting that lysosomal iron accumulation is involved in ferroptosis. We also observed that Abcb10 knockout cardiomyocytes exhibited increased ROS production, iron accumulation, and lysosomal hypertrophy. Our findings suggest that Abcb10 is required for the maintenance of cardiac function and reveal a novel pathophysiology of chronic heart failure related to lysosomal function and ferroptosis.

Genetically encoded probiotic EcN 1917 alleviates alcohol-induced acute liver injury and restore gut microbiota homeostasis

Alcohol abuse increases the risk of acute liver injury. Escherichia coli Nissle 1917(EcN) has been comprehensively demonstrated as a safe oral probiotic. In this study, we genetically modified EcN expressing alcohol dehydrogenase gene, acetaldehyde dehydrogenase gene, NAD synthase gene, and NADH oxidase gene. Applied strains in the mouse experiment showed a 54.44% reduction in serum ethanol. EcN-treated mice also exhibited lower liver AST, ALT, MDA, TG, T-CHO, TNF-α, IL-1β and increased SOD and GSH levels versus untreated ones, indicating restored liver function alongside depressed oxidative stress, lipid peroxidation, and inflammation. Consistently, aberrant gene expression correlating with alcohol metabolism and injured liver induced by alcoholic food were restored following EcN inoculation. The efficacy EcN conferred was accompanied by the restoration of gut microbiota homeostasis. In conclusion, the engineered EcN has been shown to detoxify ethanol metabolites in liver disease models and which may be attributed to intestinal recovery.

Mechanistic Basis for Client Recognition and Amyloid Inhibition of NMNAT

Imbalance of proteostasis leads to abnormal protein aggregations as a common pathological hallmark in a variety of devastating human neurodegenerative diseases such as AD and PD. Chaperones play essential roles in maintaining proteostasis and preventing protein from abnormal aggregation. NMNAT, a NAD synthase firstly identified as a key protector in Wallerian axon degeneration, was recently found to possess chaperone activity and exhibits protective effects in cells and animal models of neurodegeneration. However, it remains enigmatic how an enzyme evolves chaperone activity and prevents pathological amyloid aggregation. Here, we found that NMNAT recognizes a full spectrum of client amyloid proteins (e.g. α-syn, Tau, Aβ and IAPP), and prevents their aggregation and cytotoxicity. By using multiple biophysical approaches, we identified the client binding pocket of NMNAT which is partially overlapped with its catalytic pocket, providing the structural basis of how NMNAT exhibits both chaperone and enzyme activity. We found that both electrostatic and hydrophobic interactions are crucial for the client-chaperone recognition. In addition to binding various amyloid client proteins, NMNAT shows high binding affinity and selectively to phosphorylated Tau, which is closely related to diseases. Moreover, NMNAT can inhibit both liquid-liquid phase separation and amyloid aggregation of phosphorylated Tau. Our work helps to explain how a chaperone halt its client proteins from aggregation and highlight the importance of NMNAT as a maintenance factor in neurodegenerative diseases. Reference:

Key role of an ADP − ribose - dependent transcriptional regulator of NAD metabolism for fitness and virulence of Pseudomonas aeruginosa

NAD is an essential co-factor of redox reactions and metabolic conversions of NAD-dependent enzymes. NAD biosynthesis in the opportunistic pathogen Pseudomonas aeruginosa has yet not been experimentally explored. The in silico search for orthologs in the P. aeruginosa PAO1 genome identified the operon pncA − pncB1–nadE (PA4918–PA4920) to encode the nicotinamidase, nicotinate phosporibosyltransferase and Nad synthase of salvage pathway I. The functional role of the preceding genes PA4917 and PA4916 was resolved by the characterization of recombinant protein. PA4917 turned out to encode the nicotinate mononucleotide adenylyltransferase NadD2 and PA4916 was determined to encode the transcriptional repressor NrtR that binds to an intergenic sequence between nadD2 and pncA. Complex formation between the catalytically inactive Nudix protein NrtR and its DNA binding site was suppressed by the antirepressor ADP-ribose. NrtR plasposon mutagenesis abrogated virulence of P. aeruginosa TBCF10839 in a murine acute airway infection model and constrained its metabolite profile. When grown together with other isogenic plasposon mutants, the nrtR knock-out was most compromised in competitive fitness to persist in nutrient-rich medium in vitro or murine airways in vivo. This example demonstrates how tightly metabolism and virulence can be intertwined by key elements of metabolic control.

Increased Rate of NAD Metabolism Shortens Plant Longevity by Accelerating Developmental Senescence in Arabidopsis.

NAD is a well-known co-enzyme that mediates hundreds of redox reactions and is the basis of various processes regulating cell responses to different environmental and developmental cues. The regulatory mechanism that determines the amount of cellular NAD and the rate of NAD metabolism remains unclear. We created Arabidopsis thaliana plants overexpressing the NAD synthase (NADS) gene that participates in the final step of NAD biosynthesis. NADS overexpression enhanced the activity of NAD biosynthesis but not the amounts of NAD+, NADH, NADP+ or NADPH. However, the amounts of some intermediates were elevated, suggesting that NAD metabolism increased. The NAD redox state was greatly facilitated by an imbalance between NAD generation and degradation in response to bolting. Metabolite profiling and transcriptional analysis revealed that the drastic modulation of NAD redox homeostasis increased tricarboxylic acid flux, causing the ectopic generation of reactive oxygen species. Vascular bundles suffered from oxidative stress, leading to a malfunction in amino acid and organic acid transportation that caused early wilting of the flower stalk and shortened plant longevity, probably due to malnutrition. We concluded that the mechanism regulating the balance between NAD synthesis and degradation is important in the systemic plant response to developmental cues during the growth-phase transition.

Two for the Price of One: A Neuroprotective Chaperone Kit within NAD Synthase Protein NMNAT2.

One of the most fascinating properties of the brain is the ability to function smoothly across decades of a lifespan. Neurons are nondividing mature cells specialized in fast electrical and chemical communication at synapses. Often, neurons and synapses operate at high levels of activity through sophisticated arborizations of long axons and dendrites that nevertheless stay healthy throughout years. On the other hand, aging and activity-dependent stress strike onto the protein machineries turning proteins unfolded and prone to form pathological aggregates associated with neurodegeneration. How do neurons protect from those insults and remain healthy for their whole life? Ali and colleagues now present a molecular mechanism by which the enzyme nicotinamide mononucleotide adenylyltransferase 2 (NMNAT2) acts not only as a NAD synthase involved in axonal maintenance but as a molecular chaperone helping neurons to overcome protein unfolding and protein aggregation.

Nicotinamide mononucleotide adenylyltransferase maintains active zone structure by stabilizing Bruchpilot

Active zones are specialized presynaptic structures critical for neurotransmission. We show that a neuronal maintenance factor, nicotinamide mononucleotide adenylyltransferase (NMNAT), is required for maintaining active zone structural integrity in Drosophila by interacting with the active zone protein, Bruchpilot (BRP), and shielding it from activity-induced ubiquitin-proteasome-mediated degradation. NMNAT localizes to the peri-active zone and interacts biochemically with BRP in an activity-dependent manner. Loss of NMNAT results in ubiquitination, mislocalization and aggregation of BRP, and subsequent active zone degeneration. We propose that, as a neuronal maintenance factor, NMNAT specifically maintains active zone structure by direct protein-protein interaction.

The role of autophagy in Nmnat-mediated protection against hypoxia-induced dendrite degeneration

The selective degeneration of dendrites precedes neuronal cell death in hypoxia-ischemia (HI) and is a neuropathological hallmark of stroke. While it is clear that a number of different molecular pathways likely contribute to neuronal cell death in HI, the mechanisms that govern HI-induced dendrite degeneration are largely unknown. Here, we show that the NAD synthase nicotinamide mononucleotide adenylyltransferase (Nmnat) functions endogenously to protect Drosophila class IV dendritic arborization (da) sensory neurons against hypoxia-induced dendritic damage. Whereas dendrites of wild-type class IV neurons are largely resistant to morphological changes during prolonged periods of hypoxia (<1.0% O(2)), class IV neurons of nmnat heterozygous mutants exhibit significant dendrite loss and extensive fragmentation of the dendritic arbor under the same hypoxic conditions. Although basal levels of autophagy are required for neuronal survival, we demonstrate that autophagy is dispensable for maintaining the dendritic integrity of class IV neurons. However, we find that genetically blocking autophagy can suppress hypoxia-induced dendrite degeneration of nmnat heterozygous mutants in a cell-autonomous manner, suggestive of a self-destructive role for autophagy in this context. We further show that inducing autophagy by overexpression of the autophagy-specific kinase Atg1 is sufficient to cause dendrite degeneration of class IV neurons under hypoxia and that overexpression of Nmnat fails to protect class IV dendrites from the effects of Atg1 overexpression. Our studies reveal an essential neuroprotective role for endogenous Nmnat in hypoxia and demonstrate that Nmnat functions upstream of autophagy to mitigate the damage incurred by dendrites in neurons under hypoxic stress.

Nmnat exerts neuroprotective effects in dendrites and axons

Dendrites can be maintained for extended periods of time after they initially establish coverage of their receptive field. The long-term maintenance of dendrites underlies synaptic connectivity, but how neurons establish and then maintain their dendritic arborization patterns throughout development is not well understood. Here, we show that the NAD synthase Nicotinamide mononucleotide adenylyltransferase (Nmnat) is cell-autonomously required for maintaining type-specific dendritic coverage of Drosophila dendritic arborization (da) sensory neurons. In nmnat heterozygous mutants, dendritic arborization patterns of class IV da neurons are properly established before increased retraction and decreased growth of terminal branches lead to progressive defects in dendritic coverage during later stages of development. Although sensory axons are largely intact in nmnat heterozygotes, complete loss of nmnat function causes severe axonal degeneration, demonstrating differential requirements for nmnat dosage in the maintenance of dendritic arborization patterns and axonal integrity. Overexpression of Nmnat suppresses dendrite maintenance defects associated with loss of the tumor suppressor kinase Warts (Wts), providing evidence that Nmnat, in addition to its neuroprotective role in axons, can function as a protective factor against progressive dendritic loss. Moreover, motor neurons deficient for nmnat show progressive defects in both dendrites and axons. Our studies reveal an essential role for endogenous Nmnat function in the maintenance of both axonal and dendritic integrity and present evidence of a broad neuroprotective role for Nmnat in the central nervous system.

NMNAT Expression and its Relation to NAD Metabolism

Nicotinamide mononucleotide adenylyltransferease (NMNAT), a rate-limiting enzyme present in all organisms, reversibly catalyzes the important step in the biosynthesis of NAD from ATP and NMN. NAD and NADP are used reversibly in anabolic and catabolic reactions. NAD is necessary for cell survival in oxidative stress and DNA damage. Based on their localization, three different NMNAT's have been recognized, NMNAT-1 (homohexamer) in the nucleus (chromosome 1 p32-35), NMNAT-2 (homodimer) in the cytoplasm (chromosome 1q25) and NMNAT-3 (homotetramer) in the mitochondria. NMNAT also catalyzes the metabolic conversion of potent antitumor prodrugs like tiazofurin and benzamide riboside to their active forms which are analogs of NAD. NAD synthase-NMNAT acts as a chaperone to protect against neurodegeneration, injury-induced axonal degeneration and also correlates with DNA synthesis during cell cycle. Since its activity is rather low in tumor cells it can be exploited as a source for therapeutic targeting. Steps involved in NAD synthesis are being utilized as targets for chemoprevention, radiosensitization and therapy of wide range of diseases, such as cancer, multiple sclerosis, neurodegeneration and Huntington's disease.

Identification of MOAG-4/SERF as a Regulator of Age-Related Proteotoxicity

Fibrillar protein aggregates are the major pathological hallmark of several incurable, age-related, neurodegenerative disorders. These aggregates typically contain aggregation-prone pathogenic proteins, such as amyloid-beta in Alzheimer's disease and alpha-synuclein in Parkinson's disease. It is, however, poorly understood how these aggregates are formed during cellular aging. Here we identify an evolutionarily highly conserved modifier of aggregation, MOAG-4, as a positive regulator of aggregate formation in C. elegans models for polyglutamine diseases. Inactivation of MOAG-4 suppresses the formation of compact polyglutamine aggregation intermediates that are required for aggregate formation. The role of MOAG-4 in driving aggregation extends to amyloid-beta and alpha-synuclein and is evolutionarily conserved in its human orthologs SERF1A and SERF2. MOAG-4/SERF appears to act independently from HSF-1-induced molecular chaperones, proteasomal degradation, and autophagy. Our results suggest that MOAG-4/SERF regulates age-related proteotoxicity through a previously unexplored pathway, which will open up new avenues for research on age-related, neurodegenerative diseases.

NAD synthase NMNAT acts as a chaperone to protect against neurodegeneration

Neurodegeneration can be triggered by genetic or environmental factors. Although the precise cause is often unknown, many neurodegenerative diseases share common features such as protein aggregation and age dependence. Recent studies in Drosophila have uncovered protective effects of NAD synthase nicotinamide mononucleotide adenylyltransferase (NMNAT) against activity-induced neurodegeneration and injury-induced axonal degeneration. Here we show that NMNAT overexpression can also protect against spinocerebellar ataxia 1 (SCA1)-induced neurodegeneration, suggesting a general neuroprotective function of NMNAT. It protects against neurodegeneration partly through a proteasome-mediated pathway in a manner similar to heat-shock protein 70 (Hsp70). NMNAT displays chaperone function both in biochemical assays and cultured cells, and it shares significant structural similarity with known chaperones. Furthermore, it is upregulated in the brain upon overexpression of poly-glutamine expanded protein and recruited with the chaperone Hsp70 into protein aggregates. Our results implicate NMNAT as a stress-response protein that acts as a chaperone for neuronal maintenance and protection. Our studies provide an entry point for understanding how normal neurons maintain activity, and offer clues for the common mechanisms underlying different neurodegenerative conditions.

How to Protect Fly Photoreceptors

When a nerve is injured, axons beyond the site of injury die through a process called Wallerian degeneration. This degeneration is delayed in mice that have a mutation called Wallerian degeneration slow (Wlds); these mice have three copies of a particular stretch of their DNA. Because this piece of DNA includes the gene for nicotinamide mononucleotide adenylyltransferase (NMNAT), which synthesizes a molecule called NAD, there has been a great deal of interest in whether NMNAT or NAD can protect against axonal degeneration. Hugo Bellen and colleagues show that NMNAT can, at least in the fruitfly Drosophila—and that its protective ability is independent of its function as a NAD synthase. Investigations into the putative protective role of NMNAT have produced conflicting results. In vitro evidence supports the idea that NMNAT protects against degeneration, as do studies in which NMNAT was overexpressed in Drosophila. But mice in which NMNAT is overexpressed show no protection. The authors used a screening system that allowed them to look for flies that are homozygous for mutations—meaning that both copies of a gene are mutated—that result in death in cells of the visual system but are heterozygous, with one normal copy and one mutated copy, in the rest of the body. This means that mutations that would normally be lethal can be investigated in living flies. To look for mutations that affect synaptic function or development, the authors screened mutated Drosophila for abnormalities in phototaxis (a test of vision in which the organism moves toward or away from light; normal flies move toward light). They then tested the visual responses of the identified flies and looked for abnormal synapse structure in the eyes. This screening process identified two “nonsense” mutations in the gene for Drosophila NMNAT that prevents the gene from generating the correct protein. When they characterized the protein, the authors found that it was highly homologous to NMNAT proteins in humans and mice. Staining with an antibody against NMNAT showed that the protein is highly enriched in the fly nervous system, mainly in the cell nuclei and nerve terminals. Flies carrying the NMNAT mutation had abnormal photoreceptors that became progressively damaged with age, indicating a degenerative process. Mutant photoreceptors appeared to develop normally but did not survive. NMNAT therefore seems to be required for the maintenance of mature neurons. What is the mechanism of degeneration in nmnat mutant photoreceptors? When mutant flies were raised in the dark, their degeneration was much less severe than when they were raised in the light, indicating that photoreceptor activity contributes to the degenerative process. Flies with double mutations that lacked both NMNAT and functioning photoreceptors also showed reduced neurodegeneration. This led the authors to conclude that the normal function of NMNAT is to protect photoreceptors against light-induced degeneration. They also showed that overexpression of NMNAT could protect photoreceptors against the degeneration caused by excessive activity in flies with mutations that cause continuous activation of photoreceptors. To investigate the importance of NAD synthesis in neuroprotection by NMNAT, the authors generated an enzymatically inactive form of NMNAT. To their surprise, they found that expression of this protein could prevent neurodegeneration in nmnat mutants. However, this inactive NMNAT could not stop a full, homozygous nmnat mutation from being lethal to flies. These results indicate that NMNAT has two functions: its NAD synthase activity is essential for survival, but another activity is responsible for neuroprotection. This study sheds new light on the mechanisms of neural degeneration and the functions of NMNAT. Attention will now turn to the identification of its second, non–NAD-dependent activity, as well as to continued attempts to reconcile the apparently conflicting results of earlier overexpression experiments.

Drosophila NMNAT Maintains Neural Integrity Independent of Its NAD Synthesis Activity

Wallerian degeneration refers to a loss of the distal part of an axon after nerve injury. Wallerian degeneration slow (Wlds) mice overexpress a chimeric protein containing the NAD synthase NMNAT (nicotinamide mononucleotide adenylyltransferase 1) and exhibit a delay in axonal degeneration. Currently, conflicting evidence raises questions as to whether NMNAT is the protecting factor and whether its enzymatic activity is required for such a possible function. Importantly, the link between nmnat and axon degeneration is at present solely based on overexpression studies of enzymatically active protein. Here we use the visual system of Drosophila as a model system to address these issues. We have isolated the first nmnat mutations in a multicellular organism in a forward genetic screen for synapse malfunction in Drosophila. Loss of nmnat causes a rapid and severe neurodegeneration that can be attenuated by blocking neuronal activity. Furthermore, in vivo neuronal expression of mutated nmnat shows that enzymatically inactive NMNAT protein retains strong neuroprotective effects and rescues the degeneration phenotype caused by loss of nmnat. Our data indicate an NAD-independent requirement of NMNAT for maintaining neuronal integrity that can be exploited to protect neurons from neuronal activity-induced degeneration by overexpression of the protein.