Cyclic ADP ribose as a calcium-mobilizing messenger

Identification of a small chemical as a lysosomal calcium mobilizer and characterization of its ability to inhibit autophagy and viral infection

We previously identified glyceraldehyde 3-phosphate dehydrogenase (GAPDH) as one of the cyclic adenosine diphosphoribose (cADPR)'s binding proteins and found that GAPDH participates in cADPR-mediated Ca2+ release from endoplasmic reticulum via ryanodine receptors (RyRs). Here, we aimed to chemically synthesise and pharmacologically characterise novel cADPR analogues. Based on the simulated cADPR-GAPDH complex structure, we performed the structure-based drug screening, identified several small chemicals with high docking scores to cADPR's binding pocket in GAPDH and showed that two of these compounds, C244 and C346, are potential cADPR antagonists. We further synthesised several analogues of C346 and found that its analogue, G42, also mobilised Ca2+ release from lysosomes. G42 alkalised lysosomal pH and inhibited autophagosome-lysosome fusion. Moreover, G42 markedly inhibited Zika virus (ZIKV, a flavivirus) or murine hepatitis virus (MHV, a β-coronavirus) infections of host cells. These results suggest that G42 inhibits virus infection, likely by triggering lysosomal Ca2+ mobilisation and inhibiting autophagy.

The First Line of Defense: Receptor-like Protein Kinase-Mediated Stomatal Immunity

Stomata regulate gas and water exchange between the plant and external atmosphere, which are vital for photosynthesis and transpiration. Stomata are also the natural entrance for pathogens invading into the apoplast. Therefore, stomata play an important role in plants against pathogens. The pattern recognition receptors (PRRs) locate in guard cells to perceive pathogen/microbe-associated molecular patterns (PAMPs) and trigger a series of plant innate immune responses, including rapid closure of stomata to limit bacterial invasion, which is termed stomatal immunity. Many PRRs involved in stomatal immunity are plasma membrane-located receptor-like protein kinases (RLKs). This review focuses on the current research progress of RLK-mediated signaling pathways involved in stomatal immunity, and discusses questions that need to be addressed in future research.

A lectin receptor kinase as a potential sensor for extracellular nicotinamide adenine dinucleotide in Arabidopsis thaliana

Nicotinamide adenine dinucleotide (NAD+) participates in intracellular and extracellular signaling events unrelated to metabolism. In animals, purinergic receptors are required for extracellular NAD+ (eNAD+) to evoke biological responses, indicating that eNAD+ may be sensed by cell-surface receptors. However, the identity of eNAD+-binding receptors still remains elusive. Here, we identify a lectin receptor kinase (LecRK), LecRK-I.8, as a potential eNAD+ receptor in Arabidopsis. The extracellular lectin domain of LecRK-I.8 binds NAD+ with a dissociation constant of 436.5 ± 104.8 nM, although much higher concentrations are needed to trigger in vivo responses. Mutations in LecRK-I.8 inhibit NAD+-induced immune responses, whereas overexpression of LecRK-I.8 enhances the Arabidopsis response to NAD+. Furthermore, LecRK-I.8 is required for basal resistance against bacterial pathogens, substantiating a role for eNAD+ in plant immunity. Our results demonstrate that lectin receptors can potentially function as eNAD+-binding receptors and provide direct evidence for eNAD+ being an endogenous signaling molecule in plants.

CD38 mediates angiotensin II-induced intracellular Ca(2+) release in rat pulmonary arterial smooth muscle cells

CD38 is a multifunctional enzyme that catalyzes the formation of the endogenous Ca(2+)-mobilizing messengers cyclic ADP-ribose (cADPR) and nicotinic acid adenosine dinucleotide phosphate (NAADP) for the activation of ryanodine receptors (RyRs) of sarcoplasmic reticulum and NAADP-sensitive Ca(2+) release channels in endolysosomes, respectively. It plays important roles in systemic vascular functions, but there is little information on CD38 in pulmonary arterial smooth muscle cells (PASMCs). Earlier studies suggested a redox-sensing role of CD38 in hypoxic pulmonary vasoconstriction. This study sought to characterize its roles in angiotensin II (Ang II)-induced Ca(2+) release (AICR) in PASMCs. Examination of CD38 expression in various rat arteries found high levels of CD38 mRNA and protein in pulmonary arteries. The Ang II-elicited Ca(2+) response consisted of extracellular Ca(2+) influx and intracellular Ca(2+) release in PASMCs. AICR activated in the absence of extracellular Ca(2+) was reduced by pharmacological or siRNA inhibition of CD38, by the cADPR antagonist 8-bromo-cADPR or ryanodine, and by the NAADP antagonist Ned-19 or disruption of endolysosomal Ca(2+) stores with the vacuolar H(+)-ATPase inhibitor bafilomycin A1. Suppression of AICR by the inhibitions of cADPR- and NAADP-dependent pathways were nonadditive, indicating interdependence of RyR- and NAADP-gated Ca(2+) release. Furthermore, AICR was inhibited by the protein kinase C inhibitor staurosporine, the nonspecific NADPH oxidase (NOX) inhibitors apocynin and diphenyleneiodonium, the NOX2-specific inhibitor gp91ds-tat, and the scavenger of reactive oxygen species (ROS) tempol. These results provide the first evidence that Ang II activates CD38-dependent Ca(2+) release via the NOX2-ROS pathway in PASMCs.

Exploring the genetic signature of body size in Yucatan miniature pig

Since being domesticated about 10,000-12,000 years ago, domestic pigs (Sus scrofa domesticus) have been selected for traits of economic importance, in particular large body size. However, Yucatan miniature pigs have been selected for small body size to withstand high temperature environment and for laboratory use. This renders the Yucatan miniature pig a valuable model for understanding the evolution of body size. We investigate the genetic signature for selection of body size in the Yucatan miniature pig. Phylogenetic distance of Yucatan miniature pig was compared to other large swine breeds (Yorkshire, Landrace, Duroc and wild boar). By estimating the XP-EHH statistic using re-sequencing data derived from 70 pigs, we were able to unravel the signatures of selection of body size. We found that both selections at the level of organism, and at the cellular level have occurred. Selection at the higher levels include feed intake, regulation of body weight and increase in mass while selection at the molecular level includes cell cycle and cell proliferation. Positively selected genes probed by XP-EHH may provide insight into the docile character and innate immunity as well as body size of Yucatan miniature pig.

Glycoproteins identified from heart failure and treatment models

Conduction abnormalities can lead to dyssynchronous contraction, which significantly worsens morbidity and mortality of heart failure. Cardiac resynchronization therapy (CRT) can reverse ventricular remodeling and improve cardiac function. Although the underlying molecular changes are unknown, the use of a canine model of dyssynchronous heart failure (DHF) and CRT has shown that there are global changes across the cardiac proteome. This study determines changes in serum glycoprotein concentration from DHF and CRT compared to normal. We hypothesize that CRT invokes protective or advantageous pathways that can be reflected in the circulating proteome. Two prong discovery approaches were carried out on pooled normal, DHF, and CRT samples composed of individual canine serum to determine the overall protein concentration and the N-linked glycosites of circulating glycoproteins. The level of the glycoproteins was altered in DHF and CRT compared to control sera, with 63 glycopeptides substantially increased in DHF and/or CRT. Among the 32 elevated glycosite-containing peptides in DHF, 13 glycopeptides were reverted to normal level after CRT therapy. We further verify the changes of glycopeptides using label-free LC-MS from individual canine serum. Circulating glycoproteins such as alpha-fetoprotein, alpha-2-macroglobulin, galectin-3-binding protein, and collectin-10 show association to failing heart and CRT treatment model.

Roles and mechanisms of the CD38/cyclic adenosine diphosphate ribose/Ca2+ signaling pathway

Mobilization of intracellular Ca²⁺ stores is involved in many diverse cell functions, including: cell proliferation; differentiation; fertilization; muscle contraction; secretion of neurotransmitters, hormones and enzymes; and lymphocyte activation and proliferation. Cyclic adenosine diphosphate ribose (cADPR) is an endogenous Ca²⁺ mobilizing nucleotide present in many cell types and species, from plants to animals. cADPR is formed by ADP-ribosyl cyclases from nicotinamide adenine dinucleotide. The main ADP-ribosyl cyclase in mammals is CD38, a multi-functional enzyme and a type II membrane protein. It has been shown that many extracellular stimuli can induce cADPR production that leads to calcium release or influx, establishing cADPR as a second messenger. cADPR has been linked to a wide variety of cellular processes, but the molecular mechanisms regarding cADPR signaling remain elusive. The aim of this review is to summarize the CD38/cADPR/Ca²⁺ signaling pathway, focusing on the recent advances involving the mechanism and physiological functions of cADPR-mediated Ca²⁺ mobilization.

Antenatal betamethasone has a sex-dependent effect on the in vivo response to endothelin in adult sheep

Antenatal steroid administration is associated with multiple cardiometabolic alterations, including hypertension; however, the mechanisms underlying this phenomenon are unclear. The aim of the present study was to ascertain, in vivo, the contribution of the endothelin system to the development of hypertension in the adult offspring and the signaling pathway involved. Pregnant sheep were treated with two doses of betamethasone (n = 23) or vehicle (n = 22) at 80 days (~0.55) gestation and allowed to deliver at term. Adult sheep were chronically instrumented under general anesthesia to place vascular catheters and a femoral artery flow probe. Blood pressure and flow were recorded continuously, and femoral artery vascular resistance was calculated before and during administration of endothelin 1 (ET-1). Selective blockers (dantrolene, BQ123, niacinamide) or saline were administered simultaneously. Betamethasone-exposed animals exhibited a significant elevation in mean blood pressure (female: 98 ± 1.8 vs. 92 ± 2.1; males: 97 ± 3.4 vs. 90 ± 2.3; mmHg; P < 0.05). ET-1 elicited a significant increase in blood pressure (F = 56.4; P < 0.001) and in vascular resistance (F = 44.3; P < 0.001) in all groups. A betamethasone effect in the vascular resistance response to ET-1 (F = 25.7; P < 0.001) was present in females only, and the effect was partially blunted by niacinamide (F = 6.6; P < 0.01). Combined administration of niacinamide and BQ123, as well as of dantrolene abolished the betamethasone effect on vascular resistance. No significant differences in mRNA expression of ET(A) or ET(B) in endothelial or smooth muscle cells of resistance-size arteries were observed. We conclude that the betamethasone effect on vascular resistance is mediated by an enhanced response to ET-1 through ET(A) receptor via the cyclic ADPR/ryanodine pathway.

Inhibition of cardiomyocytes differentiation of mouse embryonic stem cells by CD38/cADPR/Ca2+ signaling pathway

Cyclic adenosine diphosphoribose (cADPR) is an endogenous Ca(2+) mobilizing messenger that is formed by ADP-ribosyl cyclases from nicotinamide adenine dinucleotide (NAD). The main ADP-ribosyl cyclase in mammals is CD38, a multi-functional enzyme and a type II membrane protein. Here we explored the role of CD38-cADPR-Ca(2+) in the cardiomyogenesis of mouse embryonic stem (ES) cells. We found that the mouse ES cells are responsive to cADPR and possess the key components of the cADPR signaling pathway. In vitro cardiomyocyte (CM) differentiation of mouse ES cells was initiated by embryoid body (EB) formation. Interestingly, beating cells appeared earlier and were more abundant in CD38 knockdown EBs than in control EBs. Real-time RT-PCR and Western blot analyses further showed that the expression of several cardiac markers, including GATA4, MEF2C, NKX2.5, and α-MLC, were increased markedly in CD38 knockdown EBs than those in control EBs. Similarly, FACS analysis showed that more cardiac Troponin T-positive CMs existed in CD38 knockdown or 8-Br-cADPR, a cADPR antagonist, treated EBs compared with that in control EBs. On the other hand, overexpression of CD38 in mouse ES cells significantly inhibited CM differentiation. Moreover, CD38 knockdown ES cell-derived CMs possess the functional properties characteristic of normal ES cell-derived CMs. Last, we showed that the CD38-cADPR pathway negatively modulated the FGF4-Erks1/2 cascade during CM differentiation of ES cells, and transiently inhibition of Erk1/2 blocked the enhanced effects of CD38 knockdown on the differentiation of CM from ES cells. Taken together, our data indicate that the CD38-cADPR-Ca(2+) signaling pathway antagonizes the CM differentiation of mouse ES cells.

A novel fluorescent cell membrane-permeable caged cyclic ADP-ribose analogue

Cyclic adenosine diphosphate ribose is an endogenous Ca(2+) mobilizer involved in diverse cellular processes. A cell membrane-permeable cyclic adenosine diphosphate ribose analogue, cyclic inosine diphosphoribose ether (cIDPRE), can induce Ca(2+) increase in intact human Jurkat T-lymphocytes. Here we synthesized a coumarin-caged analogue of cIDPRE (Co-i-cIDPRE), aiming to have a precisely temporal and spatial control of bioactive cIDPRE release inside the cell using UV uncaging. We showed that Co-i-cIDPRE accumulated inside Jurkat cells quickly and efficiently. Uncaging of Co-i-cIDPRE evoked Ca(2+) release from endoplasmic reticulum, with concomitant Ca(2+) influx in Jurkat cells. Ca(2+) release evoked by uncaged Co-i-cIDPRE was blocked by knockdown of ryanodine receptors (RyRs) 2 and 3 in Jurkat cells. The associated Ca(2+) influx, on the other hand, was abolished by double knockdown of Stim1 and TRPM2 in Jurkat cells. Furthermore, Ca(2+) release or influx evoked by uncaged Co-i-cIDPRE was recapitulated in HEK293 cells that overexpress RyRs or TRPM2, respectively, but not in wild-type cells lacking these channels. In summary, our results indicate that uncaging of Co-i-cIDPRE incites Ca(2+) release from endoplasmic reticulum via RyRs and triggers Ca(2+) influx via TRPM2.

Intracellular organelles in the saga of Ca2+ homeostasis: different molecules for different purposes?

An increase in the concentration of cytosolic free Ca(2+) is a key component regulating different cellular processes ranging from egg fertilization, active secretion and movement, to cell differentiation and death. The multitude of phenomena modulated by Ca(2+), however, do not simply rely on increases/decreases in its concentration, but also on specific timing, shape and sub-cellular localization of its signals that, combined together, provide a huge versatility in Ca(2+) signaling. Intracellular organelles and their Ca(2+) handling machineries exert key roles in this complex and precise mechanism, and this review will try to depict a map of Ca(2+) routes inside cells, highlighting the uniqueness of the different Ca(2+) toolkit components and the complexity of the interactions between them.

Synthesis and use of cell-permeant cyclic ADP-ribose

Cyclic ADP-ribose (cADPR) is a second messenger that acts on ryanodine receptors to mobilize Ca(2+). cADPR has a net negative charge at physiological pH making it not passively membrane permeant thereby requiring it to be injected, electroporated or loaded via liposomes. Such membrane impermeance of other charged intracellular messengers (including cyclic AMP, inositol 1,4,5-trisphosphate and nicotinic acid adenine dinucleotide phosphate) and fluorescent dyes (including fura-2 and fluorescein) has been overcome by synthesizing masked analogs (prodrugs), which are passively permeant and hydrolyzed to the parent compound inside cells. We now report the synthesis and biological activity of acetoxymethyl (AM) and butoxymethyl (BM) analogs of cADPR. Extracellular addition of cADPR-AM or cADPR-BM to neuronal cells in primary culture or PC12 neuroblastoma cells induced increases in cytosolic Ca(2+). Pre-incubation of PC12 cells with thapsigargin, ryanodine or caffeine eliminated the response to cADPR-AM, whereas the response still occurred in the absence of extracellular Ca(2+). Combined, these data demonstrate that masked cADPR analogs are cell-permeant and biologically active. We hope these cell-permeant tools will facilitate cADPR research and reveal its diverse physiological functions.

Pyridine nucleotide metabolites and calcium release from intracellular stores

Ca(2+) signals are probably the most common intracellular signaling elements, controlling an extensive range of responses in virtually all cells. Many cellular stimuli, often acting at cell surface receptors, evoke Ca(2+) signals by mobilizing Ca(2+) from intracellular stores. Inositol trisphosphate (IP₃) was the first messenger shown to link events at the plasma membrane to release of Ca(2+) from the endoplasmic reticulum (ER), through activation of IP₃-gated Ca(2+) release channels (IP₃ receptors). Subsequently, two additional Ca(2+) mobilizing messengers were discovered, cADPR and NAADP. Both are metabolites of pyridine nucleotides, and may be produced by the same class of enzymes, ADP-ribosyl cyclases, such as CD38. Whilst cADPR mobilizes Ca(2+) from the ER by activation of ryanodine receptors (RyRs), NAADP releases Ca(2+) from acidic stores by a mechanism involving the activation of two pore channels (TPCs).

Synthesis of cyclic adenosine 5'-diphosphate ribose analogues: a C2'endo/syn "southern" ribose conformation underlies activity at the sea urchin cADPR receptor

Novel 8-substituted base and sugar-modified analogues of the Ca(2+) mobilizing second messenger cyclic adenosine 5'-diphosphate ribose (cADPR) were synthesized using a chemoenzymatic approach and evaluated for activity in sea urchin egg homogenate (SUH) and in Jurkat T-lymphocytes; conformational analysis investigated by (1)H NMR spectroscopy revealed that a C2'endo/syn conformation of the "southern" ribose is crucial for agonist or antagonist activity at the SUH-, but not at the T cell-cADPR receptor.

An NAADP-gated two-pore channel targeted to the plasma membrane uncouples triggering from amplifying Ca2+ signals

Nicotinic acid adenine dinucleotide phosphate (NAADP) is a ubiquitous messenger proposed to stimulate Ca(2+) release from acidic organelles via two-pore channels (TPCs). It has been difficult to resolve this trigger event from its amplification via endoplasmic reticulum Ca(2+) stores, fuelling speculation that archetypal intracellular Ca(2+) channels are the primary targets of NAADP. Here, we redirect TPC2 from lysosomes to the plasma membrane and show that NAADP evokes Ca(2+) influx independent of ryanodine receptors and that it activates a Ca(2+)-permeable channel whose conductance is reduced by mutation of a residue within a putative pore. We therefore uncouple TPC2 from amplification pathways and prove that it is a pore-forming subunit of an NAADP-gated Ca(2+) channel.

CD38/cADPR/Ca2+ pathway promotes cell proliferation and delays nerve growth factor-induced differentiation in PC12 cells

Intracellular Ca(2+) mobilization plays an important role in a wide variety of cellular processes, and multiple second messengers are responsible for mediating intracellular Ca(2+) changes. Here we explored the role of one endogenous Ca(2+)-mobilizing nucleotide, cyclic adenosine diphosphoribose (cADPR), in the proliferation and differentiation of neurosecretory PC12 cells. We found that cADPR induced Ca(2+) release in PC12 cells and that CD38 is the main ADP-ribosyl cyclase responsible for the acetylcholine (ACh)-induced cADPR production in PC12 cells. In addition, the CD38/cADPR signaling pathway is shown to be required for the ACh-induced Ca(2+) increase and cell proliferation. Inhibition of the pathway, on the other hand, accelerated nerve growth factor (NGF)-induced neuronal differentiation in PC12 cells. Conversely, overexpression of CD38 increased cell proliferation but delayed NGF-induced differentiation. Our data indicate that cADPR plays a dichotomic role in regulating proliferation and neuronal differentiation of PC12 cells.

Extracellular pyridine nucleotides induce PR gene expression and disease resistance in Arabidopsis

Although it is well known that the pyridine nucleotides NAD and NADP function inside the cell to regulate intracellular signaling processes, recent evidence from animal studies suggests that NAD(P) also functions in the extracellular compartment (ECC). Extracellular NAD(P) [eNAD(P)] can either directly bind to plasma membrane receptors or be metabolized by ecto-enzymes to produce cyclic ADP-ribose and nicotinic acid adenine dinucleotide phosphate, and/or may ADP-ribosylate cell-surface receptors, resulting in activation of transmembrane signaling. In this study, we report that, in plants, exogenous NAD(P) induces the expression of pathogenesis-related (PR) genes and resistance to the bacterial pathogen Pseudomonas syringae pv. maculicola ES4326. Chelation of Ca(2+) by EGTA significantly inhibits the induction of PR genes by exogenous NAD(P), suggesting that exogenous NAD(P) may induce PR genes through a pathway that involves Ca(2+) signaling. We show that exogenous application of NAD(P) causes accumulation of the defense signal molecule salicylic acid (SA), and induces both SA/NPR1-dependent and -independent PR gene expression and disease resistance. Furthermore, we demonstrate that NAD(P) leaks into the plant ECC after mechanical wounding and pathogen infection, and that the amount of NAD(P) leaking into the ECC after P. syringae pv. tobacco DC3000/avrRpt2 infection is sufficient for induction of both PR gene expression and disease resistance. We propose that NAD(P) leakage from cells losing membrane integrity upon environmental stress may function as an elicitor to activate plant defense responses. Our data provide evidence that eNAD(P) functions in plant signaling, and illustrate the potential importance of eNAD(P) in plant innate immunity.

Calcium signalling in early embryos

The onset of development in most species studied is triggered by one of the largest and longest calcium transients known to us. It is the most studied and best understood aspect of the calcium signals that accompany and control development. Its properties and mechanisms demonstrate what embryos are capable of and thus how the less-understood calcium signals later in development may be generated. The downstream targets of the fertilization calcium signal have also been identified, providing some pointers to the probable targets of calcium signals further on in the process of development. In one species or another, the fertilization calcium signal involves all the known calcium-releasing second messengers and many of the known calcium-signalling mechanisms. These calcium signals also usually take the form of a propagating calcium wave or waves. Fertilization causes the cell cycle to resume, and therefore fertilization signals are cell-cycle signals. In some early embryonic cell cycles, calcium signals also control the progress through each cell cycle, controlling mitosis. Studies of these early embryonic calcium-signalling mechanisms provide a background to the calcium-signalling events discussed in the articles in this issue.

Regulation of poly(ADP-ribose) polymerase 1 activity by the phosphorylation state of the nuclear NAD biosynthetic enzyme NMN adenylyl transferase 1

Nuclear NAD(+) metabolism constitutes a major component of signaling pathways. It includes NAD(+)-dependent protein deacetylation by members of the Sir2 family and protein modification by poly(ADP-ribose) polymerase 1 (PARP-1). PARP-1 has emerged as an important mediator of processes involving DNA rearrangements. High-affinity binding to breaks in DNA activates PARP-1, which attaches poly(ADP-ribose) (PAR) to target proteins. NMN adenylyl transferases (NMNATs) catalyze the final step of NAD(+) biosynthesis. We report here that the nuclear isoform NMNAT-1 stimulates PARP-1 activity and binds to PAR. Its overexpression in HeLa cells promotes the relocation of apoptosis-inducing factor from the mitochondria to the nucleus, a process known to depend on poly(ADP-ribosyl)ation. Moreover, NMNAT-1 is subject to phosphorylation by protein kinase C, resulting in reduced binding to PAR. Mimicking phosphorylation, substitution of the target serine residue by aspartate precludes PAR binding and stimulation of PARP-1. We conclude that, depending on its state of phosphorylation, NMNAT-1 binds to activated, automodifying PARP-1 and thereby amplifies poly(ADP-ribosyl)ation.

Nicotinic acid adenine dinucleotide phosphate and cyclic ADP-ribose regulate TRPM2 channels in T lymphocytes

TRPM2 (previously designated TRPC7 or LTRPC2) is a Ca2+-permeable nonselective cation channel that contains a C-terminal enzymatic domain with pyrophosphatase activity, which specifically binds ADP-ribose. Cyclic ADP-ribose (cADPR) and hydrogen peroxide (H2O2) can facilitate ADPR-mediated activation of heterologously expressed TRPM2. Here, we show that the two Ca2+-mobilizing second messengers cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP) strongly activate natively expressed TRPM2 channels in Jurkat T cells. TRPM2 activation by both agonists can be partially suppressed by the ADPR antagonist adenosine monophosphate (AMP), which suggests that cADPR and NAADP lead to mobilization of endogenous ADPR presumably via metabolic conversion. The remaining channel activity is due to direct gating of TRPM2 by the two agonists and can be completely suppressed by 8-Br-cADPR, which suggests that cADPR and NAADP share a common binding site on TRPM2 that can regulate TRPM2 activity in synergy with ADPR. We conclude that cADPR and NAADP, in combination with ADPR, represent physiological co-activators of TRPM2 that contribute to Ca2+ influx in T lymphocytes and presumably other cell types that express this channel.

Increase of intracellular Ca(2+) during ischemia/reperfusion injury of heart is mediated by cyclic ADP-ribose

While the molecular mechanisms by which oxidants cause cytotoxicity are still poorly understood, disruption of Ca(2+) homeostasis appears to be one of the critical alterations during the oxidant-induced cytotoxic process. Here, we examined the possibility that oxidative stress may alter the metabolism of cyclic ADP-ribose (cADPR), a potent Ca(2+)-mobilizing second messenger in the heart. Isolated heart perfused by Langendorff technique was subjected to ischemia/reperfusion injury and endogenous cADPR level was determined using a specific radioimmunoassay. Following ischemia/reperfusion injury, a significant increase in intracellular cADPR level was observed. The elevation of cADPR content was closely correlated with the increase in ADP-ribosyl cyclase activity. Inclusion of oxygen free radical scavengers, 2,2,6,6-tetramethyl-1-piperidinyloxy and mannitol, in the reperfusate prevented the ischemia/reperfusion-induced increases in cADPR level and the ADP-ribosyl cyclase activity. Exposure of isolated cardiomyocytes to t-butyl hydroperoxide increased the ADP-ribosyl cyclase activity, cADPR level, and intracellular Ca(2+) concentration ([Ca(2+)](i)) and consequently resulting in cell lethal damage. The oxidant-induced elevation of [Ca(2+)](i) as well as cell lethal damage was blocked by a cADPR antagonist, 8-bromo-cADPR. These results provide evidence for involvement of cADPR and its producing enzyme in alteration of Ca(2+) homeostasis during the ischemia/reperfusion injury of the heart.

Interactions between calcium release pathways: multiple messengers and multiple stores

The discovery of cyclic adenosine diphosphate ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP) as Ca(2+) releasing messengers has provided additional insight into how complex Ca(2+) signalling patterns are generated. There is mounting evidence that these molecules along with the more established messenger, myo-inositol 1,4,5-trisphosphate (IP(3)), have a widespread messenger role in shaping Ca(2+) signals in many cell types. These molecules have distinct structures and act on specific Ca(2+) release mechanisms. Emerging principles are that cADPR enhances the Ca(2+) sensitivity of ryanodine receptors (RYRs) to produce prolonged Ca(2+) signals through Ca(2+)-induced Ca(2+) release (CICR), while NAADP acts on a novel Ca(2+) release mechanism to produce a local trigger Ca(2+) signal which can be amplified by CICR by recruiting other Ca(2+) release mechanisms. Whilst IP(3) and cADPR mobilise Ca(2+) from the endoplasmic reticulum (ER), recent evidence from the sea urchin egg suggests that the major NAADP-sensitive Ca(2+) stores are reserve granules, acidic lysosomal-related organelles. In this review we summarise the role of multiple Ca(2+) mobilising messengers, Ca(2+) release channels and Ca(2+) stores, and the interplay between them, in the generation of specific Ca(2+) signals. Focusing upon cADPR and NAADP, we discuss how cellular stimuli may draw upon different combinations of these messengers to produce distinct Ca(2+) signalling signatures.

Calcium-based signalling systems in guard cells

Calcium is a ubiquitous intracellular signal responsible for controlling numerous cellular processes in both plants and animals. As an example, Ca²⁺ has been shown to be a second messenger in the signal transduction pathways by which stomatal guard cells respond to external stimuli. Regulated increases in the cytosolic concentration of free calcium ions ([Ca²⁺ ]_cyt ) in guard cells have been observed to be a common intermediate in many of the pathways leading to either opening or closing of the stomatal pore. This observation has prompted investigations into how specificity is encoded in the Ca²⁺ signal. It has been suggested that the key to generating stimulus-specific calcium signatures lies in the ability to access differentially the cellular machinery controlling calcium influx and release from intracellular stores. Several important components of the calcium-based signalling pathways have been identified in guard cells including cADPR, phospholipase C-InsP₃ , InsP₆ and H₂ O₂ . These data suggest that the pathways for intracellular mobilization of Ca²⁺ are evolutionarily conserved between plants and animals. ABBREVIATIONS: ABA, abscisic acid; [Ca²⁺ ]_cyt , cytosolic free calcium concentration; [Ca²⁺ ]_ext , external calcium concentration; I_K,in ; inward-rectifying K⁺ currents; InsP₃ , inositol-1,4,5-trisphosphate; InsP₆ , inositol hexakisphosphate; PLC, phospholipase C; PLD, phospholipase D; PA, phosphatidic acid; H₂ O₂ , hydrogen peroxide; AAPK, ABA-activated serine-threonine protein kinase; cADPR, cyclic adenosine 5'-diphosphoribose; U73122, 1-(6-{[17â-3-methoxyestra-1,3,5(10)-trien-17-yl]amino}hexyl)-1H-pyrrole-2, 5-dione; RyR; ryanodine receptor; CICR; calcium-induced calcium-release; I_Ca , inward calcium current.