Nicotinamide 2-fluoroadenine dinucleotide unmasks the NAD+ glycohydrolase activity of Aplysia californica adenosine 5'-diphosphate ribosyl cyclase

Different substrate specificities of the two ADPR binding sites in TRPM2 channels of Nematostella vectensis and the role of IDPR

NvTRPM2 (Nematostella vectensis Transient Receptor Potential Melastatin 2), the species variant of the human apoptosis-related cation channel hTRPM2, is gated by ADP-ribose (ADPR) independently of the C-terminal NUDT9H domain that mediates ADPR-directed gating in hTRPM2. The decisive binding site in NvTRPM2 is likely to be identical with the N-terminal ADPR binding pocket in zebra fish DrTRPM2. Our aim was a characterization of this binding site in NvTRPM2 with respect to its substrate specificity, in comparison to the classical ADPR interaction site within NUDT9H that is highly homologous in hTRPM2 and NvTRPM2, although only in NvTRPM2, catalytic (ADPRase) activity is conserved. With various ADPR analogues, key differences of the two sites were identified. Particularly, two reported antagonists on hTRPM2 were agonists on NvTRPM2. Moreover, IDP-ribose (IDPR) induced currents both in hTRPM2 and NvTRPM2 but not in NvTRPM2 mutants in which NUDT9H was absent. Thus, IDPR acts on NUDT9H rather than N-terminally, revealing a regulatory function of NUDT9H in NvTRPM2 opposed to that in hTRPM2. We propose that IDPR competitively inhibits the ADPRase function of NUDT9H and evokes ADPR accumulation. The findings provide important insights into the structure-function relationship of NvTRPM2 and will allow further characterization of the novel ADPR interaction site.

Role of Nicotinamide Adenine Dinucleotide and Related Precursors as Therapeutic Targets for Age-Related Degenerative Diseases: Rationale, Biochemistry, Pharmacokinetics, and Outcomes

Significance: Nicotinamide adenine dinucleotide (NAD+) is an essential pyridine nucleotide that serves as an essential cofactor and substrate for a number of critical cellular processes involved in oxidative phosphorylation and ATP production, DNA repair, epigenetically modulated gene expression, intracellular calcium signaling, and immunological functions. NAD+ depletion may occur in response to either excessive DNA damage due to free radical or ultraviolet attack, resulting in significant poly(ADP-ribose) polymerase (PARP) activation and a high turnover and subsequent depletion of NAD+, and/or chronic immune activation and inflammatory cytokine production resulting in accelerated CD38 activity and decline in NAD+ levels. Recent studies have shown that enhancing NAD+ levels can profoundly reduce oxidative cell damage in catabolic tissue, including the brain. Therefore, promotion of intracellular NAD+ anabolism represents a promising therapeutic strategy for age-associated degenerative diseases in general, and is essential to the effective realization of multiple benefits of healthy sirtuin activity. The kynurenine pathway represents the de novo NAD+ synthesis pathway in mammalian cells. NAD+ can also be produced by the NAD+ salvage pathway. Recent Advances: In this review, we describe and discuss recent insights regarding the efficacy and benefits of the NAD+ precursors, nicotinamide (NAM), nicotinic acid (NA), nicotinamide riboside (NR), and nicotinamide mononucleotide (NMN), in attenuating NAD+ decline in degenerative disease states and physiological aging. Critical Issues: Results obtained in recent years have shown that NAD+ precursors can play important protective roles in several diseases. However, in some cases, these precursors may vary in their ability to enhance NAD+ synthesis via their location in the NAD+ anabolic pathway. Increased synthesis of NAD+ promotes protective cell responses, further demonstrating that NAD+ is a regulatory molecule associated with several biochemical pathways. Future Directions: In the next few years, the refinement of personalized therapy for the use of NAD+ precursors and improved detection methodologies allowing the administration of specific NAD+ precursors in the context of patients' NAD+ levels will lead to a better understanding of the therapeutic role of NAD+ precursors in human diseases.

Structure-activity relationship of adenosine 5'-diphosphoribose at the transient receptor potential melastatin 2 (TRPM2) channel: rational design of antagonists

Adenosine 5′-diphosphoribose (ADPR) activates TRPM2, a Ca²⁺, Na⁺, and K⁺ permeable cation channel. Activation is induced by ADPR binding to the cytosolic C-terminal NudT9-homology domain. To generate the first structure–activity relationship, systematically modified ADPR analogues were designed, synthesized, and evaluated as antagonists using patch-clamp experiments in HEK293 cells overexpressing human TRPM2. Compounds with a purine C8 substituent show antagonist activity, and an 8-phenyl substitution (8-Ph-ADPR, 5) is very effective. Modification of the terminal ribose results in a weak antagonist, whereas its removal abolishes activity. An antagonist based upon a hybrid structure, 8-phenyl-2′-deoxy-ADPR (86, IC₅₀ = 3 μM), is more potent than 8-Ph-ADPR (5). Initial bioisosteric replacement of the pyrophosphate linkage abolishes activity, but replacement of the pyrophosphate and the terminal ribose by a sulfamate-based group leads to a weak antagonist, a lead to more drug-like analogues. 8-Ph-ADPR (5) inhibits Ca²⁺ signalling and chemotaxis in human neutrophils, illustrating the potential for pharmacological intervention at TRPM2.

DNA-N-glycosylases process novel O-glycosidic sites in DNA

After the hydrolysis of the N-glycosyl bond between a damaged base and C1' of a deoxyribosyl moiety of DNA, human alkyladenine DNA glycosylase (AAG) and Escherichia coli 3-methyladenine DNA glycosylase II (AlkA) bind tightly to their abasic DNA products, potentially protecting these reactive species. Here we show that both AAG and AlkA catalyze reactions between bound abasic DNA and small, primary alcohols to form novel DNA-O-glycosides. The synthesis reactions are reversible, as the DNA-O-glycosides are converted back into abasic DNA upon being incubated with AAG or AlkA in the absence of alcohol. AAG and AlkA are therefore able to hydrolyze O-glycosidic bonds in addition to N-glycosyl bonds. The newly discovered DNA-O-glycosidase activities of both enzymes compare favorably with their known DNA-N-glycosylase activities: AAG removes both methanol and 1,N(6)-ethenoadenine (εA) from DNA with single-turnover rate constants that are 2.9 × 10(5)-fold greater than the corresponding uncatalyzed rates, whereas the rate enhancement of 3.7 × 10(7) for removal of methanol from DNA by AlkA is 300-fold greater than its rate enhancement for removal of εA from DNA. Although the biological significance of the DNA-O-glycosidase reactions is not known, the evolution of new DNA repair pathways may be aided by enzymes that practice catalytic promiscuity, such as these two unrelated DNA glycosylases.

Aberrant cyclization affords a C-6 modified cyclic adenosine 5'-diphosphoribose analogue with biological activity in Jurkat T cells

Two nicotinamide adenine dinucleotide (NAD(+)) analogues modified at the 6 position of the purine ring were synthesized, and their substrate properties toward Aplysia californica ADP-ribosyl cyclase were investigated. 6-N-Methyl NAD(+) (6-N-methyl nicotinamide adenosine 5'-dinucleotide 10) hydrolyzes to give the linear 6-N-methyl ADPR (adenosine 5'-diphosphoribose, 11), whereas 6-thio NHD(+) (nicotinamide 6-mercaptopurine 5'-dinucleotide, 17) generates a cyclic dinucleotide. Surprisingly, NMR correlation spectra confirm this compound to be the N1 cyclic product 6-thio N1-cIDPR (6-thio cyclic inosine 5'-diphosphoribose, 3), although the corresponding 6-oxo analogue is well-known to cyclize at N7. In Jurkat T cells, unlike the parent cyclic inosine 5'-diphosphoribose N1-cIDPR 2, 6-thio N1-cIDPR antagonizes both cADPR- and N1-cIDPR-induced Ca(2+) release but possesses weak agonist activity at higher concentration. 3 is thus identified as the first C-6 modified cADPR (cyclic adenosine 5'-diphosphoribose) analogue antagonist; it represents the first example of a fluorescent N1-cyclized cADPR analogue and is a new pharmacological tool for intervention in the cADPR pathway of cellular signaling.

Evidence for the existence of a specific g protein-coupled receptor activated by guanosine

Guanosine, released extracellularly from neurons and glial cells, plays important roles in the central nervous system, including neuroprotection. The innovative DELFIA Eu-GTP binding assay was optimized for characterization of the putative guanosine receptor binding site at rat brain membranes by using a series of novel and known guanosine derivatives. These nucleosides were prepared by modifying the purine and sugar moieties of guanosine at the 6- and 5'-positions, respectively. Results of these experiments prove that guanosine, 6-thioguanosine, and their derivatives activate a G protein-coupled receptor that is different from the well-characterized adenosine receptors.

A single residue in a novel ADP-ribosyl cyclase controls production of the calcium-mobilizing messengers cyclic ADP-ribose and nicotinic acid adenine dinucleotide phosphate

Cyclic ADP-ribose and nicotinic acid adenine dinucleotide phosphate are ubiquitous calcium-mobilizing messengers produced by the same family of multifunctional enzymes, the ADP-ribosyl cyclases. Not all ADP-ribosyl cyclases have been identified, and how production of different messengers is achieved is incompletely understood. Here, we report the cloning and characterization of a novel ADP-ribosyl cyclase (SpARC4) from the sea urchin, a key model organism for the study of calcium-signaling pathways. Like several other members of the ADP-ribosyl cyclase superfamily, SpARC4 is a glycoprotein targeted to the plasma membrane via a glycosylphosphatidylinositol anchor. However, unlike most other members, SpARC4 shows a remarkable preference for producing cyclic ADP-ribose over nicotinic acid adenine dinucleotide phosphate. Mutation of a single residue (tyrosine 142) within a noncanonical active site reversed this striking preference. Our data highlight further diversification of this unusual enzyme family, provide mechanistic insight into multifunctionality, and suggest that different ADP-ribosyl cyclases are fine-tuned to produce specific calcium-mobilizing messengers.

Structural basis for enzymatic evolution from a dedicated ADP-ribosyl cyclase to a multifunctional NAD hydrolase

Cyclic ADP-ribose (cADPR) is a universal calcium messenger molecule that regulates many physiological processes. The production and degradation of cADPR are catalyzed by a family of related enzymes, including the ADP-ribosyl cyclase from Aplysia california (ADPRAC) and CD38 from human. Although ADPRC and CD38 share a common evolutionary ancestor, their enzymatic functions toward NAD and cADPR homeostasis have evolved divergently. Thus, ADPRC can only generate cADPR from NAD (cyclase), whereas CD38, in contrast, has multiple activities, i.e. in cADPR production and degradation, as well as NAD hydrolysis (NADase). In this study, we determined a number of ADPRC and CD38 structures bound with various nucleotides. From these complexes, we elucidated the structural features required for the cyclization (cyclase) reaction of ADPRC and the NADase reaction of CD38. Using the structural approach in combination with site-directed mutagenesis, we identified Phe-174 in ADPRC as a critical residue in directing the folding of the substrate during the cyclization reaction. Thus, a point mutation of Phe-174 to glycine can turn ADPRC from a cyclase toward an NADase. The equivalent residue in CD38, Thr-221, is shown to disfavor the cyclizing folding of the substrate, resulting in NADase being the dominant activity. The comprehensive structural comparison of CD38 and APDRC presented in this study thus provides insights into the structural determinants for the functional evolution from a cyclase to a hydrolase.

Emerging roles of NAD+ and its metabolites in cell signaling

Nicotinamide adenine dinucleotide (NAD(+)) is the universal currency of energy metabolism and electron transfer. Recent studies indicate that apart from its role as a coenzyme, NAD(+) and its metabolites also function in cell signaling pathways; for example, they are substrates for nucleotide-metabolizing enzymes and ligands for extra- and intracellular receptors and ion channels. Moreover, the NAD(+) and NAD(+) phosphate metabolites adenosine 5'-diphosphoribose (ADP-ribose), cyclic ADP-ribose, and nicotinic acid adenine dinucleotide phosphate (NAADP) have emerged as key second messengers in Ca(2+) signaling. A symposium in Hamburg, Germany, brought together 120 researchers from various fields, who were all engaged in the molecular characterization of the key players of NAD(+) signaling (www.NAD2008.de).