Nicotinic acid adenine dinucleotide phosphate (NAADP+)

NAADP-Mediated Ca2+ Signalling

The discovery of NAADP-evoked Ca²⁺ release in sea urchin eggs and then as a ubiquitous Ca²⁺ mobilizing messenger has introduced several novel paradigms to our understanding of Ca²⁺ signalling, not least in providing a link between cell stimulation and Ca²⁺ release from lysosomes and other acidic Ca²⁺ storage organelles. In addition, the hallmark concentration-response relationship of NAADP-mediated Ca²⁺ release, shaped by striking activation/desensitization mechanisms, influences its actions as an intracellular messenger. There has been recent progress in our understanding of the molecular mechanisms underlying NAADP-evoked Ca²⁺ release, such as the identification of the endo-lysosomal two-pore channel family of cation channels (TPCs) as their principal target and the identity of NAADP-binding proteins that complex with them. The NAADP/TPC signalling axis has gained recent prominence in pathophysiology for their roles in such disease processes as neurodegeneration, tumorigenesis and cellular viral entry.

The synthesis and characterization of a clickable-photoactive NAADP analog active in human cells

Nicotinic acid adenine dinucleotide phosphate (NAADP) is a potent Ca²⁺ mobilizing second messenger which triggers Ca²⁺ release in both sea urchin egg homogenates and in mammalian cells. The NAADP binding protein has not been identified and the regulation of NAADP mediated Ca²⁺ release remains controversial. To address this issue, we have synthesized an NAADP analog in which 3-azido-5-azidomethylbenzoic acid is attached to the amino group of 5-(3-aminopropyl)-NAADP to produce an NAADP analog which is both a photoaffinity label and clickable. This 'all-in-one-clickable' NAADP (AIOC-NAADP) elicited Ca²⁺ release when microinjected into cultured human SKBR3 cells at low concentrations. In contrast, it displayed little activity in sea urchin egg homogenates where very high concentrations were required to elicit Ca²⁺ release. In mammalian cell homogenates, incubation with low concentrations of [³²P]AIOC-NAADP followed by irradiation with UV light resulted in labeling 23 kDa protein(s). Competition between [³²P]AIOC-NAADP and increasing concentrations of NAADP demonstrated that the labeling was selective. We show that this label recognizes and selectively photodervatizes the 23 kDa NAADP binding protein(s) in cultured human cells identified in previous studies using [³²P]5-N₃-NAADP.

Nicotinic acid adenine dinucleotide phosphate analogues containing substituted nicotinic acid: effect of modification on Ca(2+) release

Analogues of nicotinic acid adenine dinucleotide phosphate (NAADP) with substitution at either the 4- or the 5-position position of the nicotinic acid moiety have been synthesized from NADP enzymatically using Aplysia californica ADP-ribosyl cyclase or mammalian NAD glycohydrolase. Substitution at the 4-position of the nicotinic acid resulted in the loss of agonist potency for release of Ca(2+)-ions from sea urchin egg homogenates and in potency for competition ligand binding assays using [(32)P]NAADP. In contrast, several 5-substituted NAADP derivatives showed high potency for binding and full agonist activity for Ca(2+) release. 5-Azido-NAADP was shown to release calcium from sea urchin egg homogenates at low concentration and to compete with [(32)P]NAADP in a competition ligand binding assay with an IC(50) of 18 nM, indicating that this compound might be a potential photoprobe useful for specific labeling and identification of the NAADP receptor.

Structural characterization of novel adenine dinucleotide phosphate conjugates of imatinib in incubations with rat and human liver microsomes

Imatinib, a potent and selective protein tyrosine kinase inhibitor, has been approved for the treatment of chronic myelogenous leukemia and metastatic and unresectable malignant gastrointestinal stromal tumors. In vitro metabolism of imatinib was investigated in rat and human liver microsomes. Besides several oxidative metabolites and an N-desmethyl metabolite, as previous reported, a novel metabolite with a mass addition of 621 atomic mass units to the parent was detected as the major metabolite in the incubations with rat liver microsomes, using NADPH as a cofactor. The analysis of MS(2) and MS(n) data revealed that this metabolite corresponded to adenine dinucleotide phosphate (ADP+) conjugate of imatinib. The ADP+ adduct was scaled up from rat liver microsomal incubations and isolated for NMR analysis. NMR data confirmed and conclusively showed the conjugation had occurred between the pyridine nitrogen of imatinib to the ribose ring of ADP+ moiety. The formation of this adduct was enzymatic and required NADP+ as a reactant. In addition, ADP+ adducts of imatinib N-oxide and desmethyl imatinib were also detected as minor metabolites in the incubations with rat liver microsomes. In contrast, only trace levels of ADP+ adducts of imatinib and desmethyl imatinib were detected in the incubations with human liver microsomes. Imatinib-ADP+ adducts have been observed only in in vitro studies to date. The physiological role of these adducts is not clear, nor is their in vivo relevance.

Cell-permeant small-molecule modulators of NAADP-mediated Ca2+ release

Nicotinic acid adenine dinucleotide phosphate (NAADP, 1) is the most potent intracellular Ca2+ mobilizing agent in important mammalian cells and tissues, yet the identity of the NAADP receptor is elusive. Significantly, the coenzyme NADP is completely inactive in this respect. Current studies are restricted by the paucity of any chemical probes beyond NAADP itself, and importantly, none is cell permeant. We report simple nicotinic acid-derived pyridinium analogs as low molecular weight compounds that (1) inhibit Ca2+ release via the NAADP receptor (IC50 approximately 15 microM - 1 mM), (2) compete with NAADP binding, (3) cross the cell membrane of sea urchin eggs to inhibit NAADP-evoked Ca2+ release, and (4) selectively ablate NAADP-dependent Ca2+ oscillations induced by the external gastric peptide hormone agonist cholecystokinin (CCK) in murine pancreatic acinar cells.

Nicotinic acid adenine dinucleotide phosphate: a new intracellular second messenger?

Nicotinic acid adenine dinucleotide phosphate (NAADP) is one of the most potent stimulators of intracellular Ca2+ release known to date. The role of the NAADP system in physiological processes is being extensively investigated at the present time. Exciting new discoveries in the last 5 years suggest that the NAADP-regulated system may have a significant role in intracellular Ca2+ signaling. The NAADP receptor and its associated Ca2+ pool have been hypothesized to be important in several physiological processes including fertilization, T cell activation, and pancreatic secretion. However, whether NAADP is a new second messenger or a tool for the discovery of a new Ca2+ channel is still an unanswered question.

Nicotinic acid adenine dinucleotide phosphate: a new Ca2+ releasing agent in kidney

Nicotinic acid adenine dinucleotide phosphate (NAADP), a molecule derived from beta-NADP, has been shown to trigger Ca2+ release from intracellular stores of invertebrate eggs and mammalian cell microsomes. NAADP-induced Ca2+ release occurs through a mechanism distinct from that of inositol-1,4,5-trisphosphate- or cyclic ADP-ribose-elicited Ca2+ release. This study investigated whether NAADP can be synthesized in rat kidney. Extracts from glomeruli, mesangial cells, and papilla have high NAADP synthetic capacities. Conversely, synthesis of NAADP in kidney cortex was almost undetectable. Furthermore, 9-cis-retinoic acid significantly up-regulated NAADP synthesis in mesangial cells. Authenticity of NAADP biosynthesis in glomeruli was affirmed by HPLC analysis. NAADP stimulated Ca2+ release from mesangial cell microsomes through a pathway distinct from that of inositol-1,4,5-trisphosphate or cyclic ADP-ribose. NAADP-triggered Ca2+ release may play an important role in regulation of renal function.

Enzymatic synthesis of caged NADP cofactors: aqueous NADP photorelease and optical properties

The synthesis of caged NADP analogues 18, 19, and 20 has been accomplished by utilizing the transglycosidase activity of solubilized NAD glycohydrolase (porcine brain) to incorporate caged nicotinamides 2, 3, and 4 into NADP. The synthesis of several nicotinamides modified at the carboxamide with o-nitrobenzyl photolabile groups is demonstrated as well as their potential for enzymatic transglycosidation. These results further demonstrate the feasibility of direct enzymatic transglycosidation of sterically hindered substrates into NAD(P), although high nicotinamide analogue water solubility was found to be a necessary trait for yield enhancement with certain analogues. Caged analogues were surveyed under aqueous conditions for net NADP photorelease, while the UV and fluorescent properties of both analogues and their photobyproducts were assessed for compatibility with systems that rely on optical monitoring of enzyme activity. A highly water-soluble alpha-methyl-o-nitrobenzyl group 8 was developed for the synthesis of 20 in order to enhance net NADP photorelease. Compound 20 demonstrated a high 75% net NADP photoreleased without substantial UV optical blackening or fluorescent byproducts. Analogues 18 and 19 were shown by ESI/MALDI-MS to photogenerate primarily adducts of NADP with deleterious UV and fluorescent properties. Our work stresses the superior release properties conferred by alpha-methyl substitution on aqueous carboxamide photorelease from o-nitrobenzyl compounds.

Bioorganic chemistry of cyclic ADP-ribose (cADPR)

The objective of this brief review is to present an overview of the bioorganic chemistry of cyclic-ADP-ribose (cADPR) with special emphasis on the methodology used for the synthesis of analogues of cADPR. New structural analogues of cADPR can be prepared using either the biomimetic method or ADP-ribosyl cyclase from Aplysia californica. For the most part, both procedures give similar product profiles, but higher yields are generally obtained with the enzymatic method. These synthetic methodologies have allowed the transformation of a variety of structurally modified analogues of NAD+ into their corresponding cyclic nucleotides. Several of these novel analogues are more potent than cADPR in inducing calcium release and are also more stable towards degradative enzymes. They could serve as valuable affinity probes for the isolation of cADPR-binding proteins.

ADP-ribosyl cyclase and CD38 catalyze the synthesis of a calcium-mobilizing metabolite from NADP

ADP-ribosyl cyclase catalyzes the cyclization of NAD+ to produce cyclic ADP-ribose (cADPR), which is emerging as an endogenous regulator of the Ca(2+)-induced Ca2+ release mechanism in cells. CD38 is a lymphocyte differentiation antigen which has recently been shown to be a bifunctional enzyme that can synthesize cADPR from NAD+ as well as hydrolyze cADPR to ADP-ribose. In this study, we show that both the cyclase and CD38 can also catalyze the exchange of the nicotinamide group of NADP+ with nicotine acid (NA). The product is nicotinic acid adenine dinucleotide phosphate (NAADP+), a metabolite we have previously shown to be potent in Ca2+ mobilization (Lee, H. C., and Aarhus, R. (1995) J. Biol. Chem. 270, 2152-2157). The switch of the catalysis to the exchange reaction requires acidic pH and NA. The half-maximal effective concentration of NA is about 5 mM for both the cyclase and CD38. In the absence of NA or at neutral pH, the cyclase converts NADP+ to another metabolite, which is identified as cyclic ADP-ribose 2'-phosphate. Under the same conditions, CD38 converts NADP+ to ADP-ribose 2'-phosphate instead, which is the hydrolysis product of cyclic ADP-ribose 2'-phosphate. That two different products of ADP-ribosyl cyclase and CD38, cADPR and NAADP+, are both involved in Ca2+ mobilization suggests a crucial role of these enzymes in Ca2+ signaling.

Nicotinate adenine dinucleotide phosphate (NAADP) triggers a specific calcium release system in sea urchin eggs

Transient fluxes of intracellular ionized calcium (Ca2+) from intracellular stores are integral components of regulatory signaling pathways operating in numerous biological regulations, including in early stages of egg fertilization. Therefore, we explored whether NADP, which is rapidly generated by phosphorylation of NAD upon fertilization may, directly or indirectly, exert a regulatory role as a trigger of Ca2+ release from intracellular stores in sea urchin eggs. NADP had no effect, but we found that the deamidated derivative of NADP, nicotinate adenine dinucleotide phosphate (beta-NAADP), is a potent and specific stimulus (ED50 16 nM) for Ca2+ release in sea urchin egg homogenates. NAADP triggers the Ca2+ release via a mechanism which is distinct from the well-known Ca2+ release systems triggered either by inositol-1,4,5-triphosphate (IP3) or by cyclic adenosine diphospho-ribose (cADPR). The NAADP-induced release of Ca2+ is not blocked by heparin, an antagonist of IP3, or by procaine or ruthenium red, antagonists of cADPR. However, it is selectively blocked by thionicotinamide-NADP which does not inhibit the actions of IP3 or cADPR. NAADP produced by heating of NADP in alkaline (pH = 12) medium or synthetized enzymatically by nicotinic acid-NADP reaction catalyzed by NAD glycohydrolase have identical properties. The results presented herein thus describe a novel endocellular Ca(2+)-releasing system controlled by NAADP as a specific stimulus. The NAADP-controlled Ca2+ release system may be an integral component of multiple intracellular regulations occurring in fertilized sea urchin eggs, which are mediated by intracellular Ca2+ release, and may also have similar role(s) in other tissues.