The NAD+ precursor NMN activates dSarm to trigger axon degeneration in Drosophila.

Axon degeneration contributes to the disruption of neuronal circuit function in diseased and injured nervous systems. Severed axons degenerate following the activation of an evolutionarily conserved signaling pathway, which culminates in the activation of SARM1 in mammals to execute the pathological depletion of the metabolite NAD+. SARM1 NADase activity is activated by the NAD+ precursor nicotinamide mononucleotide (NMN). In mammals, keeping NMN levels low potently preserves axons after injury. However, it remains unclear whether NMN is also a key mediator of axon degeneration and dSarm activation in flies. Here, we demonstrate that lowering NMN levels in Drosophila through the expression of a newly generated prokaryotic NMN-Deamidase (NMN-D) preserves severed axons for months and keeps them circuit-integrated for weeks. NMN-D alters the NAD+ metabolic flux by lowering NMN, while NAD+ remains unchanged in vivo. Increased NMN synthesis by the expression of mouse nicotinamide phosphoribosyltransferase (mNAMPT) leads to faster axon degeneration after injury. We also show that NMN-induced activation of dSarm mediates axon degeneration in vivo. Finally, NMN-D delays neurodegeneration caused by loss of the sole NMN-consuming and NAD+-synthesizing enzyme dNmnat. Our results reveal a critical role for NMN in neurodegeneration in the fly, which extends beyond axonal injury. The potent neuroprotection by reducing NMN levels is similar to the interference with other essential mediators of axon degeneration in Drosophila.

Structural Basis of Human Dimeric α-Amino-ß-Carboxymuconate-ε-Semialdehyde Decarboxylase Inhibition With TES-1025.

Human α-amino-ß-carboxymuconate-ε-semialdehyde decarboxylase (ACMSD) stands at a branch point of the de novo NAD+ synthesis pathway and plays an important role in maintaining NAD+ homeostasis. It has been recently identified as a novel therapeutic target for a wide range of diseases, including inflammatory, metabolic disorders, and aging. So far, in absence of potent and selective enzyme inhibitors, only a crystal structure of the complex of human dimeric ACMSD with pseudo-substrate dipicolinic acid has been resolved. In this study, we report the crystal structure of the complex of human dimeric ACMSD with TES-1025, the first nanomolar inhibitor of this target, which shows a binding conformation different from the previously published predicted binding mode obtained by docking experiments. The inhibitor has a K i value of 0.85 ± 0.22 nM and binds in the catalytic site, interacting with the Zn2+ metal ion and with residues belonging to both chains of the dimer. The results provide new structural information about the mechanism of inhibition exerted by a novel class of compounds on the ACMSD enzyme, a novel therapeutic target for liver and kidney diseases.

De novo NAD+ synthesis enhances mitochondrial function and improves health.

Nicotinamide adenine dinucleotide (NAD+) is a co-substrate for several enzymes, including the sirtuin family of NAD+-dependent protein deacylases. Beneficial effects of increased NAD+ levels and sirtuin activation on mitochondrial homeostasis, organismal metabolism and lifespan have been established across species. Here we show that α-amino-ß-carboxymuconate-ε-semialdehyde decarboxylase (ACMSD), the enzyme that limits spontaneous cyclization of α-amino-ß-carboxymuconate-ε-semialdehyde in the de novo NAD+ synthesis pathway, controls cellular NAD+ levels via an evolutionarily conserved mechanism in Caenorhabditis elegans and mouse. Genetic and pharmacological inhibition of ACMSD boosts de novo NAD+ synthesis and sirtuin 1 activity, ultimately enhancing mitochondrial function. We also characterize two potent and selective inhibitors of ACMSD. Because expression of ACMSD is largely restricted to kidney and liver, these inhibitors may have therapeutic potential for protection of these tissues from injury. In summary, we identify ACMSD as a key modulator of cellular NAD+ levels, sirtuin activity and mitochondrial homeostasis in kidney and liver.

α-Amino-ß-carboxymuconate-ε-semialdehyde Decarboxylase (ACMSD) Inhibitors as Novel Modulators of De Novo Nicotinamide Adenine Dinucleotide (NAD+) Biosynthesis.

NAD+ has a central function in linking cellular metabolism to major cell-signaling and gene-regulation pathways. Defects in NAD+ homeostasis underpin a wide range of diseases, including cancer, metabolic disorders, and aging. Although the beneficial effects of boosting NAD+ on mitochondrial fitness, metabolism, and lifespan are well established, to date, no therapeutic enhancers of de novo NAD+ biosynthesis have been reported. Herein we report the discovery of 3-[[[5-cyano-1,6-dihydro-6-oxo-4-(2-thienyl)-2-pyrimidinyl]thio]methyl]phenylacetic acid (TES-1025, 22), the first potent and selective inhibitor of human ACMSD (IC50 = 0.013 µM) that increases NAD+ levels in cellular systems. The results of physicochemical-property, ADME, and safety profiling, coupled with in vivo target-engagement studies, support the hypothesis that ACMSD inhibition increases de novo NAD+ biosynthesis and position 22 as a first-class molecule for the evaluation of the therapeutic potential of ACMSD inhibition in treating disorders with perturbed NAD+ supply or homeostasis.

Characterization of bacterial NMN deamidase as a Ser/Lys hydrolase expands diversity of serine amidohydrolases.

NMN deamidase (PncC) is a bacterial enzyme involved in NAD biosynthesis. We have previously demonstrated that PncC is structurally distinct from other known amidohydrolases. Here, we extended PncC characterization by mutating all potential catalytic residues and assessing their individual roles in catalysis through kinetic analyses. Inspection of these residues' spatial arrangement in the active site, allowed us to conclude that PncC is a serine-amidohydrolase, employing a Ser/Lys dyad for catalysis. Analysis of the PncC structure in complex with a modeled NMN substrate supported our conclusion, and enabled us to propose the catalytic mechanism.

Genomics-guided analysis of NAD recycling yields functional elucidation of COG1058 as a new family of pyrophosphatases.

We have recently identified the enzyme NMN deamidase (PncC), which plays a key role in the regeneration of NAD in bacteria by recycling back to the coenzyme the pyridine by-products of its non redox consumption. In several bacterial species, PncC is fused to a COG1058 domain of unknown function, highly conserved and widely distributed in all living organisms. Here, we demonstrate that the PncC-fused domain is endowed with a novel Co(+2)- and K(+)-dependent ADP-ribose pyrophosphatase activity, and discuss the functional connection of such an activity with NAD recycling. An in-depth phylogenetic analysis of the COG1058 domain evidenced that in most bacterial species it is fused to PncC, while in α- and some δ-proteobacteria, as well as in archaea and fungi, it occurs as a stand-alone protein. Notably, in mammals and plants it is fused to FAD synthase. We extended the enzymatic characterization to a representative bacterial single-domain protein, which resulted to be a more versatile ADP-ribose pyrophosphatase, active also towards diadenosine 5'-diphosphate and FAD. Multiple sequence alignment analysis, and superposition of the available three-dimensional structure of an archaeal COG1058 member with the structure of the enzyme MoeA of the molybdenum cofactor biosynthesis, allowed identification of residues likely involved in catalysis. Their role has been confirmed by site-directed mutagenesis.

Simultaneous single-sample determination of NMNAT isozyme activities in mouse tissues.

A novel assay procedure has been developed to allow simultaneous activity discrimination in crude tissue extracts of the three known mammalian nicotinamide mononucleotide adenylyltransferase (NMNAT, EC 2.7.7.1) isozymes. These enzymes catalyse the same key reaction for NAD biosynthesis in different cellular compartments. The present method has been optimized for NMNAT isozymes derived from Mus musculus, a species often used as a model for NAD-biosynthesis-related physiology and disorders, such as peripheral neuropathies. Suitable assay conditions were initially assessed by exploiting the metal-ion dependence of each isozyme recombinantly expressed in bacteria, and further tested after mixing them in vitro. The variable contributions of the three individual isozymes to total NAD synthesis in the complex mixture was calculated by measuring reaction rates under three selected assay conditions, generating three linear simultaneous equations that can be solved by a substitution matrix calculation. Final assay validation was achieved in a tissue extract by comparing the activity and expression levels of individual isozymes, considering their distinctive catalytic efficiencies. Furthermore, considering the key role played by NMNAT activity in preserving axon integrity and physiological function, this assay procedure was applied to both liver and brain extracts from wild-type and Wallerian degeneration slow (Wld(S)) mouse. Wld(S) is a spontaneous mutation causing overexpression of NMNAT1 as a fusion protein, which protects injured axons through a gain-of-function. The results validate our method as a reliable determination of the contributions of the three isozymes to cellular NAD synthesis in different organelles and tissues, and in mutant animals such as Wld(S).

Reducing expression of NAD+ synthesizing enzyme NMNAT1 does not affect the rate of Wallerian degeneration.

NAD(+) synthesizing enzyme NMNAT1 constitutes most of the sequence of neuroprotective protein Wld(S), which delays axon degeneration by 10-fold. NMNAT1 activity is necessary but not sufficient for Wld(S) neuroprotection in mice and 70 amino acids at the N-terminus of Wld(S), derived from polyubiquitination factor Ube4b, enhance axon protection by NMNAT1. NMNAT1 activity can confer neuroprotection when redistributed outside the nucleus or when highly overexpressed in vitro and partially in Drosophila. However, the role of endogenous NMNAT1 in normal axon maintenance and in Wallerian degeneration has not been elucidated yet. To address this question we disrupted the Nmnat1 locus by gene targeting. Homozygous Nmnat1 knockout mice do not survive to birth, indicating that extranuclear NMNAT isoforms cannot compensate for its loss. Heterozygous Nmnat1 knockout mice develop normally and do not show spontaneous neurodegeneration or axon pathology. Wallerian degeneration after sciatic nerve lesion is neither accelerated nor delayed in these mice, consistent with the proposal that other endogenous NMNAT isoforms play a principal role in Wallerian degeneration.