Robert Aarhus

Activation and inactivation of Ca2+ release by NAADP+

Nicotinic acid adenine dinucleotide phosphate (NAADP) is a recently identified metabolite of NADP that is as potent as inositol trisphosphate (IP) and cyclic ADP-ribose (cADPR) in mobilizing intracellular Ca2 in sea urchin eggs and microsomes (Clapper, D. L., Walseth, T. F., Dargie, P. J., and Lee, H. C.(1987) J. Biol. Chem. 262, 9561-9568; Lee, H. C., and Aarhus, R.(1995) J. Biol. Chem. 270, 2152-2157). The mechanism of Ca release activated by NAADP and the Ca stores it acts on are different from those of IP and cADPR. In this study we show that photolyzing caged NAADP in intact sea urchin eggs elicits long term Ca oscillations. On the other hand, uncaging threshold amounts of NAADP produces desensitization. In microsomes, this self-inactivation mechanism exhibits concentration and time dependence. Binding studies show that the NAADP receptor is distinct from that of cADPR, and at subthreshold concentrations, NAADP can fully inactivate subsequent binding to the receptor in a time-dependent manner. Thus, the NAADP-sensitive Ca release process has novel regulatory characteristics, which are distinguishable from Ca release mediated by either IP or cADPR. This battery of release mechanisms may provide the necessary versatility for cells to respond to diverse signals that lead to Ca mobilization.

Wide distribution of an enzyme that catalyzes the hydrolysis of cyclic ADP-ribose

Cyclic ADP-ribose (cADPR) is a metabolite of NAD+ that is as effective as inositol trisphosphate in mobilizing intracellular-Ca2+ stores. Its synthesizing enzyme, ADP-ribosyl cyclase, has been shown to be present in mammalian and invertebrate tissues. In this study we identify another widely-distributed enzyme that can hydrolyze cADPR to ADP-ribose. Incubation of cADPR with brain extracts resulted in progressive decrease in its Ca2+ mobilizing activity. The degradation of cADPR was catalyzed by a heat-labile protein factor in the brain extracts. Analysis by HPLC indicated a single degradation product was produced in equal molar quantity and that it has identical elution time as ADP-ribose. Proton NMR confirmed that the product was ADP-ribose. The degradation enzyme had a Michaelis constant of 0.16 mM and a broad pH maximum around neutrality. Centrifugation studies of the total brain extracts showed that the degradation activity was membrane-bound. Survey of tissues from various animals established that both the degradation and the synthesizing enzyme of cADPR were widely distributed from mammals to invertebrates. Since the degradation enzyme hydrolyzes an unique linkage between the adenine group and the terminal ribosyl moiety of cADPR, we propose to call it cyclic ADP-ribose hydrolase.

Determination of endogenous levels of cyclic ADP-ribose in rat tissues.

Cyclic ADP-ribose (cADPR) is a potent mediator of calcium mobilization in sea urchin eggs. The cADPR synthesizing enzyme is present not only in the eggs but also in various mammalian tissue extracts. The purpose of this study was to ascertain whether cADPR is a naturally occurring nucleotide in mammalian tissues. Rat tissues were frozen and powdered in liquid N2, followed by extraction with perchloric acid at -10 degrees C. [32P]cADPR was prepared and used as a tracer. The acid extracts were chromatographed on a Mono-Q column and cADPR in the fractions were determined by its ability to release Ca2+ from egg homogenates. That the release was mediated by cADPR and not inositol trisphosphate (IP3) in the extracts was shown by the fact that the homogenates, subsequent to Ca2+ release induced by active fractions, were desensitized to authentic cADPR but not to IP3. Furthermore, the Ca2+ release activity was shown to co-elute with [32P]cADPR. The endogenous level of cADPR determined in rat liver is 3.37 +/- 0.64 pmol/mg, in heart is 1.04 +/- 0.08 pmol/mg and in brain is 2.75 +/- 0.35 pmol/mg. These results indicate cADPR is a naturally occurring nucleotide and suggest that it may be a general second messenger for mobilizing intracellular Ca2+.

Caged Nicotinic Acid Adenine Dinucleotide Phosphate SYNTHESIS AND USE

Nicotinic acid adenine dinucleotide phosphate (NAADP) is a metabolite of NADP with Ca2+ mobilizing activity. The Ca2+ release mechanism activated by NAADP as well as the Ca2+ stores that it acts on are different from those activated by either cyclic ADP-ribose or inositol 1,4,5-trisphosphate (IP3) (Lee, H. C., and Aarhus, R. (1995) J. Biol. Chem. 270, 2152-2157). In order to demonstrate unambiguously that NAADP can mobilize Ca2+ stores in live cells, a caged analog was synthesized by reacting NAADP with 1-(2-nitrophenyl)diazoethane. Anion exchange high pressure liquid chromatography (HPLC) was used to purify one particular caged form from the mixture of products. Phosphate analyses following specific enzymatic cleavage indicate that the caging group is on the 2′-phosphate. This is confirmed by 31P NMR spectroscopy, showing that the 2′-phosphate of the caged compound exhibits an altered chemical shift of −2.6 ppm as compared with 2.3 ppm determined for the 2′-phosphate of NAADP. Caged NAADP had no Ca2+ releasing activity at a concentration as high as 1 μM when tested on sea urchin egg microsomes. After photolysis, it released Ca2+, was effective in nanomolar range, and was indistinguishable from authentic NAADP. The regeneration of NAADP after photolysis was also confirmed by HPLC analyses. The analog is particularly susceptible to UV and can be efficiently photolyzed using a spectrofluorimeter. To demonstrate its utility in live cells, caged NAADP was microinjected into sea urchin eggs. Photolysis effectively regenerated NAADP and activated Ca2+ oscillations in the eggs. Removal of external Ca2+ did not prevent the Ca2+ oscillations but only delayed the second Ca2+ peak by about 45 s, indicating that the oscillations are due to release from internal stores and not caused by Ca2+ influx. A mechanism based on sensitization of the Ca2+ release by Ca2+ loading is proposed to account for the Ca2+ oscillation observed.

Characterization of the active site of ADP-ribosyl cyclase

ADP-ribosyl cyclase synthesizes two Ca2+ messengers by cyclizing NAD to produce cyclic ADP-ribose and exchanging nicotinic acid with the nicotinamide group of NADP to produce nicotinic acid adenine dinucleotide phosphate. Recombinant Aplysia cyclase was expressed in yeast and co-crystallized with a substrate, nicotinamide. x-ray crystallography showed that the nicotinamide was bound in a pocket formed in part by a conserved segment and was near the central cleft of the cyclase. Glu98, Asn107 and Trp140 were within 3.5 Å of the bound nicotinamide and appeared to coordinate it. Substituting Glu98 with either Gln, Gly, Leu, or Asn reduced the cyclase activity by 16–222-fold, depending on the substitution. The mutant N107G exhibited only a 2-fold decrease in activity, while the activity of W140G was essentially eliminated. The base exchange activity of all mutants followed a similar pattern of reduction, suggesting that both reactions occur at the same active site. In addition to NAD, the wild-type cyclase also cyclizes nicotinamide guanine dinucleotide to cyclic GDP-ribose. All mutant enzymes had at least half of the GDP-ribosyl cyclase activity of the wild type, some even 2–3-fold higher, indicating that the three coordinating amino acids are responsible for positioning of the substrate but not absolutely critical for catalysis. To search for the catalytic residues, other amino acids in the binding pocket were mutagenized. E179G was totally devoid of GDP-ribosyl cyclase activity, and both its ADP-ribosyl cyclase and the base exchange activities were reduced by 10,000- and 18,000-fold, respectively. Substituting Glu179 with either Asn, Leu, Asp, or Gln produced similar inactive enzymes, and so was the conversion of Trp77 to Gly. However, both E179G and the double mutant E179G/W77G retained NAD-binding ability as shown by photoaffinity labeling with [32P]8-azido-NAD. These results indicate that both Glu179 and Trp77 are crucial for catalysis and that Glu179 may indeed be the catalytic residue.

A single residue at the active site of CD38 determines its NAD cyclizing and hydrolyzing activities.

CD38 is a multifunctional enzyme involved in metabolizing two Ca2+ messengers, cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP). When incubated with NAD, CD38 predominantly hydrolyzes it to ADP-ribose (NAD glycohydrolase), but a trace amount of cADPR is also produced through cyclization of the substrate. Site-directed mutagenesis was used to investigate the amino acid important for controlling the hydrolysis and cyclization reactions. CD38 and its mutants were produced in yeast, purified, and characterized by immunoblot. Glu-146 is a conserved residue present in the active site of CD38. Its replacement with Phe greatly enhanced the cyclization activity to a level similar to that of the NAD hydrolysis activity. A series of additional replacements was made at the Glu-146 position including Ala, Asn, Gly, Asp, and Leu. All the mutants exhibited enhanced cyclase activity to various degrees, whereas the hydrolysis activity was inhibited greatly. E146A showed the highest cyclase activity, which was more than 3-fold higher than its hydrolysis activity. All mutants also cyclized nicotinamide guanine dinucleotide to produce cyclic GDP. This activity was enhanced likewise, with E146A showing more than 9-fold higher activity than the wild type. In addition to NAD, CD38 also hydrolyzed cADPR effectively, and this activity was correspondingly depressed in the mutants. When all the mutants were considered, the two cyclase activities and the two hydrolase activities were correlated linearly. The Glu-146 replacements, however, only minimally affected the base-exchange activity that is responsible for synthesizing NAADP. Homology modeling was used to assess possible structural changes at the active site of E146A. These results are consistent with Glu-146 being crucial in controlling specifically and selectively the cyclase and hydrolase activities of CD38.

Cyclic 3-deaza-adenosine diphosphoribose: a potent and stable analog of cyclic ADP-ribose.

Cyclic 3-deaza-adenosine diphosphoribose (3-deaza-cADPR), an analog of cyclic adenosine diphosphoribose (cADPR) was synthesized. 3-deaza-cADPR differs from cADPR by only the substitution of carbon for nitrogen at the 3-position of the purine ring. Similar to cADPR, the analog has potent calcium releasing activity in sea urchin egg homogenates and was able to induce calcium release at concentrations as low as 0.3 nM. The EC(50) value for 3-deaza-cADPR-induced calcium release was 1 nM, which is about 70 times more potent than cADPR. The properties of calcium release induced by 3-deaza-cADPR in all other respects were similar to those of cADPR. Thus, 3-deaza-cADPR and cADPR were capable of cross-desensitizing each other and their calcium releasing activities were potentiated by Sr(2+) as well as caffeine. 8-amino-cADPR, a selective antagonist of cADPR, was also able to inhibit 3-deaza-cADPR induced calcium release. Taken together, these data suggest that 3-deaza-cADPR releases calcium through the same mechanism as cADPR. 3-deaza-cADPR was found to be resistant to both heat and enzymatic hydrolysis. Only 15% of 3-deaza-cADPR was destroyed after boiling this compound for 2 h. No loss of 3-deaza-cADPR was observed when treated with CD38 under conditions where cADPR was completely hydrolyzed. Thus, 3-deaza-cADPR is a potent and stable analog of cADPR. These properties should make 3-deaza-cADPR a useful probe in studies focused on the mechanism of cADPR action.

Preparation of cyclic ADP-ribose antagonists and caged cyclic ADP- ribose

Publisher Summary Cyclic ADP-ribose (cADPR) is a naturally occurring metabolite of NAD + that is capable of mobilizing calcium from intracellular stores in a variety of biological systems. This chapter presents the procedures for the synthesis of three 8-substituted (8-amino-, 8-azido-, and 8-bromo-) analogs and a caged analog of cyclic ADP-ribose. The use of these analogs demonstrates that the occupation of the cADPR receptor site does not necessarily lead to Ca 2+ release; appropriate interactions between the 8-position of the ligand and receptor are also required. All three compounds are competitive antagonists that bind to the same site as cADPR. The 8-azido-cADPR, with its photoactive azido group, is particularly useful as a photoaffinity probe for the identification and characterization of cADPR-binding proteins. The synthesis of high specific activity 8-azido-[ 32 P]cADPR for use in photoaffinity labeling experiments is also described in the chapter. The use of photoactivable “caged” compounds represents a powerful tool for controlling the release of biologically active molecules in intact cells. Generally, the caging group incorporated into an active molecule is designed to render the molecule inactive. Removal of the caging group by photolysis results in liberation of the active substance, which can be controlled both spatially and temporally.

Fluorescent analogs of NAADP with calcium mobilizing activity

Nicotinic acid adenine dinucleotide phosphate (NAADP) mobilizes Ca2+ through a mechanism totally independent of cyclic ADP-ribose or inositol trisphosphate. Fluorescent analogs of NAADP were synthesized in this study to facilitate further characterization of this novel Ca2+ release mechanism. The base-exchange reaction catalyzed by ADP-ribosyl cyclase was utilized to convert nicotinamide 1,N6-ethenoadenine dinucleotide phosphate to a fluorescent product, nicotinic acid 1,N6-ethenoadenine dinucleotide phosphate (etheno-NAADP). The excitation spectrum of the product showed two maxima at 275 nm and 300 nm and an emission maximum at 410 nm. An aza derivative of etheno-NAADP was also synthesized by sequential treatments with NaOH and nitrite. The product, nicotinic acid 1,N6-etheno-2-aza-adenine dinucleotide phosphate (etheno-aza-NAADP) had excitation maxima at 280 nm and 360 nm and an emission maximum at 470 nm. The fluorescence of both analogs was sensitive to polarity and exhibited a 3-4-fold enhancement going from an aqueous buffer to an organic solvent. Proton-NMR measurements confirmed the presence of the etheno ring in both analogs. In the aza derivative the proton at the 2-position of the adenine ring was absent, consistent with the conversion of the 2-carbon to a nitrogen. Both analogs could activate Ca2+ release from sea urchin egg homogenates and the half-maximal concentrations for etheno-aza-NAADP and etheno-NAADP were at about 2.5 microM and 5 microM, respectively. At sub-threshold concentrations, both analogs could also function as antagonists, inactivating the NAADP-sensitive Ca2+ release with a half-maximal concentration of 60-80 nM. Microinjection of etheno-aza-NAADP into live eggs activated Ca2+ increase and triggered a cortical exocytotic reaction confirming its effectiveness in vivo. These fluorescent analogs are potentially useful for visualizing the novel Ca2+ stores that are sensitive to NAADP in live cells.

Thio-NADP is not an antagonist of NAADP

Nicotinic acid adenine dinucleotide phosphate (NAADP) is a metabolite of NADP, which can release Ca2+ from stores that are distinct from those activated by either cyclic ADP-ribose or inositol 1,4,5-trisphosphate (IP3). It has previously been suggested that thio-NADP is a specific antagonist of NAADP (Chini et al. [1995]J. Biol. Chem.270, 3216–3223). Its effects in sea-urchin egg homogenates were investigated. At 50 μM, thio-NADP activates partial Ca2+ release and totally inhibits subsequent challenge with a saturating concentration of NAADP. Purification by HPLC eliminates the Ca2+ releasing activity of 50 μM thio-NADP and reduces the subsequent inhibition by 73.7±1.3%. The residual inhibitory effect is no more than that exerted by 50 μM of either NADP itself or nicotinic acid adenine dinucleotide (NAAD). These results are confirmed by32P-NAADP binding studies. Unpurified thio-NADP inhibits the specific32P-NAADP binding to egg microsomes with an IC50 of 40 μM. After HPLC purification, only 20% inhibition is seen at a concentration as high as 50 μM, similar to the extent of inhibition effected by 40 μM NADP. These results indicate the inhibitory substance in thio-NADP is a contaminant. The partial Ca2+ release activity of unpurified thio-NADP suggests the contaminant is NAADP itself. This is supported by the fact that pretreatment with a subthreshold concentration of only 2 nM NAADP totally desensitizes the egg homogenates such that no Ca2+ response is seen with saturating NAADP. Estimation from the binding studies shows that a contamination of 0.012% of NAADP in the unpurified thio-NADP samples is sufficient to account for the inhibitory effects. These results indicate thio-NADP is not an antagonist of NAADP.