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

CD38 promotes hematopoietic stem cell dormancy

A subpopulation of deeply quiescent, so-called dormant hematopoietic stem cells (dHSCs) resides at the top of the hematopoietic hierarchy and serves as a reserve pool for HSCs. The state of dormancy protects the HSC pool from exhaustion throughout life; however, excessive dormancy may prevent an efficient response to hematological stresses. Despite the significance of dHSCs, the mechanisms maintaining their dormancy remain elusive. Here, we identify CD38 as a novel and broadly applicable surface marker for the enrichment of murine dHSCs. We demonstrate that cyclic adenosine diphosphate ribose (cADPR), the product of CD38 cyclase activity, regulates the expression of the transcription factor c-Fos by increasing the release of Ca2+ from the endoplasmic reticulum (ER). Subsequently, we uncover that c-Fos induces the expression of the cell cycle inhibitor p57Kip2 to drive HSC dormancy. Moreover, we found that CD38 ecto-enzymatic activity at the neighboring CD38-positive cells can promote human HSC quiescence. Together, CD38/cADPR/Ca2+/c-Fos/p57Kip2 axis maintains HSC dormancy. Pharmacological manipulations of this pathway can provide new strategies to improve the success of stem cell transplantation and blood regeneration after injury or disease.

PTEN participates in airway remodeling of asthma by regulating CD38/Ca2+/CREB signaling

Both phosphatase and tensin homologue deleted on chromosome ten (PTEN) and cluster of differentiation 38 (CD38) have been suggested to be key regulators of the pathogenesis of asthma. However, the precise role and molecular mechanisms by which PTEN and CD38 are involved in airway remodeling throughout asthma pathogenesis remains poorly understood. This study aimed to elucidate the role of PTEN and CD38 in airway remodeling of asthma. Exposure to tumor necrosis factor-α (TNF-α) in airway smooth muscle (ASM) cells markedly decreased PTEN expression, and increased expression of CD38. Overexpression of PTEN suppressed the expression of CD38 and downregulated proliferation and migration induced by TNF-α stimulation, which was partially reversed by CD38 overexpression. PTEN/CD38 axis regulated Ca2+ levels and cyclic AMP response-element binding protein (CREB) phosphorylation in TNF-α-stimulated ASM cells. The in vitro knockdown of CD38 or overexpression of PTEN remarkably restricted airway remodeling and decreased Ca2+ concentrations and CREB phosphorylation in asthmatic mice. CD38 overexpression abolished the inhibitory effects of PTEN overexpression on airway remodeling. These findings demonstrate that PTEN inhibits airway remodeling of asthma through the downregulation of CD38-mediated Ca2+/CREB signaling, highlighting a key role of PTEN/CD38/Ca2+/CREB signaling in the molecular pathogenesis of asthma.

NAADP Receptors

Of the established Ca²⁺-mobilizing messengers, NAADP is arguably the most tantalizing. It is the most potent, often efficacious at low nanomolar concentrations, and its receptors undergo dramatic desensitization. Recent studies have identified a new class of calcium-release channel, the two-pore channels (TPCs), as the likely targets for NAADP regulation, even though the effect may be indirect. These channels localized at endolysosomes, where they mediate local Ca²⁺ release, and have highlighted a new role of acidic organelles as targets for messenger-evoked Ca²⁺ mobilization. Three distinct roles of TPCs have been identified. The first is to effect local Ca²⁺ release that may play a role in endolysosomal function including vesicular fusion and trafficking. The second is to trigger global calcium release by recruiting Ca²⁺-induced Ca²⁺-release (CICR) channels at lysosomal-endoplasmic reticulum (ER) junctions. The third is to regulate plasma membrane excitability by the targeting of Ca²⁺ release from appropriately positioned subplasma membrane stores to regulate plasma membrane Ca²⁺-activated channels. In this review, I discuss the role of nicotinic acid adenine nucleotide diphosphate (NAADP)-mediated Ca²⁺ release from endolysosomal stores as a widespread trigger for intracellular calcium signaling mechanisms, and how studies of TPCs are beginning to enhance our understanding of the central role of lysosomes in Ca²⁺ signaling.

Roles for the Endoplasmic Reticulum in Regulation of Neuronal Calcium Homeostasis

By influencing Ca²⁺ homeostasis in spatially and architecturally distinct neuronal compartments, the endoplasmic reticulum (ER) illustrates the notion that form and function are intimately related. The contribution of ER to neuronal Ca²⁺ homeostasis is attributed to the organelle being the largest reservoir of intracellular Ca²⁺ and having a high density of Ca²⁺ channels and transporters. As such, ER Ca²⁺ has incontrovertible roles in the regulation of axodendritic growth and morphology, synaptic vesicle release, and neural activity dependent gene expression, synaptic plasticity, and mitochondrial bioenergetics. Not surprisingly, many neurological diseases arise from ER Ca²⁺ dyshomeostasis, either directly due to alterations in ER resident proteins, or indirectly via processes that are coupled to the regulators of ER Ca²⁺ dynamics. In this review, we describe the mechanisms involved in the establishment of ER Ca²⁺ homeostasis in neurons. We elaborate upon how changes in the spatiotemporal dynamics of Ca²⁺ exchange between the ER and other organelles sculpt neuronal function and provide examples that demonstrate the involvement of ER Ca²⁺ dyshomeostasis in a range of neurological and neurodegenerative diseases.

Specific cyclic ADP-ribose phosphohydrolase obtained by mutagenic engineering of Mn2+-dependent ADP-ribose/CDP-alcohol diphosphatase

Cyclic ADP-ribose (cADPR) is a messenger for Ca²⁺ mobilization. Its turnover is believed to occur by glycohydrolysis to ADP-ribose. However, ADP-ribose/CDP-alcohol diphosphatase (ADPRibase-Mn) acts as cADPR phosphohydrolase with much lower efficiency than on its major substrates. Recently, we showed that mutagenesis of human ADPRibase-Mn at Phe³⁷, Leu¹⁹⁶ and Cys²⁵³ alters its specificity: the best substrate of the mutant F37A + L196F + C253A is cADPR by a short difference, Cys²⁵³ mutation being essential for cADPR preference. Its proximity to the 'northern' ribose of cADPR in docking models indicates Cys²⁵³ is a steric constraint for cADPR positioning. Aiming to obtain a specific cADPR phosphohydrolase, new mutations were tested at Asp²⁵⁰, Val²⁵², Cys²⁵³ and Thr²⁷⁹, all near the 'northern' ribose. First, the mutant F37A + L196F + C253G, with a smaller residue 253 (Ala > Gly), showed increased cADPR specificity. Then, the mutant F37A + L196F + V252A + C253G, with another residue made smaller (Val > Ala), displayed the desired specificity, with cADPR k_cat/K_M ≈20-200-fold larger than for any other substrate. When tested in nucleotide mixtures, cADPR was exhausted while others remained unaltered. We suggest that the specific cADPR phosphohydrolase, by cell or organism transgenesis, or the designed mutations, by genome editing, provide opportunities to study the effect of cADPR depletion on the many systems where it intervenes.

Characterization of CD38 in the major cell types of the heart: endothelial cells highly express CD38 with activation by hypoxia-reoxygenation triggering NAD(P)H depletion

The NAD(P)⁺-hydrolyzing enzyme CD38 is activated in the heart during the process of ischemia and reperfusion, triggering NAD(P)(H) depletion. However, the presence and role of CD38 in the major cell types of the heart are unknown. Therefore, we characterize the presence and function of CD38 in cardiac myocytes, endothelial cells, and fibroblasts. To comprehensively evaluate CD38 in these cells, we measured gene transcription via mRNA, as well as protein expression and enzymatic activity. Endothelial cells strongly expressed CD38, while only low expression was present in cardiac myocytes with intermediate levels in fibroblasts. In view of this high level expression in endothelial cells and the proposed role of CD38 in the pathogenesis of endothelial dysfunction, endothelial cells were subjected to hypoxia-reoxygenation to characterize the effect of this stress on CD38 expression and activity. An activity-based CD38 imaging method and CD38 activity assays were used to characterize CD38 activity in normoxic and hypoxic-reoxygenated endothelial cells, with marked CD38 activation seen following hypoxia-reoxygenation. To test the impact of hypoxia-reoxygenation-induced CD38 activation on endothelial cells, NAD(P)(H) levels and endothelial nitric oxide synthase (eNOS)-derived NO production were measured. Marked NADP(H) depletion with loss of NO and increase in superoxide production occurred following hypoxia-reoxygenation that was prevented by CD38 inhibition or knockdown. Thus, endothelial cells have high expression of CD38 which is activated by hypoxia-reoxygenation triggering CD38-mediated NADP(H) depletion with loss of eNOS-mediated NO generation and increased eNOS uncoupling. This demonstrates the importance of CD38 in the endothelium and explains the basis by which CD38 triggers post-ischemic endothelial dysfunction.

Activation of the EIF2α/ATF4 and ATF6 Pathways in DU-145 Cells by Boric Acid at the Concentration Reported in Men at the US Mean Boron Intake

Fruits, nuts, legumes, and vegetables are rich sources of boron (B), an essential plant nutrient with chemopreventive properties. Blood boric acid (BA) levels reflect recent B intake, and men at the US mean intake have a reported non-fasting level of 10 μM. Treatment of DU-145 prostate cancer cells with physiological concentrations of BA inhibits cell proliferation without causing apoptosis and activates eukaryotic initiation factor 2 (eIF2α). EIF2α induces cell differentiation and protects cells by redirecting gene expression to manage endoplasmic reticulum stress. Our objective was to determine the temporal expression of endoplasmic reticulum (ER) stress-activated genes in DU-145 prostate cells treated with 10 μM BA. Immunoblots showed post-treatment increases in eIF2α protein at 30 min and ATF4 and ATF6 proteins at 1 h and 30 min, respectively. The increase in ATF4 was accompanied by an increase in the expression of its downstream genes growth arrest and DNA damage-induced protein 34 (GADD34) and homocysteine-induced ER protein (Herp), but a decrease in GADD153/CCAAT/enhancer-binding protein homologous protein (CHOP), a pro-apoptotic gene. The increase in ATF6 was accompanied by an increase in expression of its downstream genes GRP78/BiP, calreticulin, Grp94, and EDEM. BA did not activate IRE1 or induce cleavage of XBP1 mRNA, a target of IRE1. Low boron status has been associated with increased cancer risk, low bone mineralization, and retinal degeneration. ATF4 and BiP/GRP78 function in osteogenesis and bone remodeling, calreticulin is required for tumor suppressor p53 function and mineralization of teeth, and BiP/GRP78 and EDEM prevent the aggregation of misfolded opsins which leads to retinal degeneration. The identification of BA-activated genes that regulate its phenotypic effects provides a molecular underpinning for boron nutrition and biology.

CD38 promotes angiotensin II-induced cardiac hypertrophy

Cardiac hypertrophy is an early hallmark during the clinical course of heart failure and regulated by various signalling pathways. Recently, we observed that mouse embryonic fibroblasts from CD38 knockout mice were significantly resistant to oxidative stress such as H₂O₂‐induced injury and hypoxia/reoxygenation‐induced injury. In addition, we also found that CD38 knockout mice protected heart from ischaemia reperfusion injury through activating SIRT1/FOXOs‐mediated antioxidative stress pathway. However, the role of CD38 in cardiac hypertrophy is not explored. Here, we investigated the roles and mechanisms of CD38 in angiotensin II (Ang‐II)‐induced cardiac hypertrophy. Following 14 days of Ang‐II infusion with osmotic mini‐pumps, a comparable hypertension was generated in both of CD38 knockout and wild‐type mice. However, the cardiac hypertrophy and fibrosis were much more severe in wild‐type mice compared with CD38 knockout mice. Consistently, RNAi‐induced knockdown of CD38 decreased the gene expressions of atrial natriuretic factor (ANF) and brain natriuretic peptide (BNP) and reactive oxygen species generation in Ang‐II‐stimulated H9c2 cells. In addition, the expression of SIRT3 was elevated in CD38 knockdown H9c2 cells, in which SIRT3 may further activate the FOXO3 antioxidant pathway. The intracellular Ca²⁺ release induced by Ang‐II markedly decreased in CD38 knockdown H9c2 cells, which might be associated with the decrease of nuclear translocation of NFATc4 and inhibition of ERK/AKT phosphorylation. We concluded that CD38 plays an essential role in cardiac hypertrophy probably via inhibition of SIRT3 expression and activation of Ca²⁺‐NFAT signalling pathway. Thus, CD38 may be a novel target for treating cardiac hypertrophy.

Implication of CD38 gene in autophagic degradation of collagen I in mouse coronary arterial myocytes

Collagen deposition is a hallmark of atherosclerosis. Although compromised collagen I degradation has been implied in the pathogenesis of atherosclerosis, the molecular mechanisms are still unclear. Thus, we determined the role of CD38, an enzyme involved in cellular calcium modulation and autophagic flux, in the regulation of collagen I degradation in coronary arterial myocytes (CAMs).In primary cultured CAMs from CD38^-/- mice, collagen I protein accumulation but not mRNA abundance was significantly increased compared with cells from CD38^+/+ mice either under control or upon TGF-Beta stimulation. Pharmacological inhibition of the formation of autophagosomes with 3-methyladenine or of autophagolysosomes with a lysosomal functional blocker, bafilomycin A1, induced a similar increase in collagen protein levels, while inhibition of the proteasome by MG132 had no effects on collagen I accumulation. In addition, CD38-deficiency did not change the protein expression of matrix metalloprotein-9 (MMP-9) or tissue inhibitor of metalloproteinase-1 (TIMP-1) in CAMs. Confocal microscopy showed that collagen I deposition was mainly lied within lysosomes or autophagosomes in CD38^-/- or TGF-Beta treated CAMs. Collagen I deposition increased when CAMs lack CD38 expression or if autophagy was blocked, which is associated with impaired autophagic degradation of collagen I. This CD38 regulation of autophagic flux may represent a novel mechanism for extracellular matrix (ECM) plasticity of coronary arteries upon atherogenic stimulation.

Requirement of IP3 receptor 3 (IP3R3) in nitric oxide induced cardiomyocyte differentiation of mouse embryonic stem cells

Nitric oxide (NO) markedly induces cardiomyocyte (CM) differentiation of embryonic stem (ES) cells. Here we examined the role of the Ca(2+) signaling in the NO-induced CM differentiation of mouse ES cells. We found that NO induced intracellular Ca(2+) increases in ES cells in a dose-dependent manner, and application of IP3 pathway antagonists not only significantly inhibited this induced Ca(2+) increase but also abolished NO-induced CM differentiation of ES cells. Subsequently, all 3 types of inositol 1, 4, 5-trisphosphate (IP3) receptors (IP3Rs) in mouse ES cells were individually or triply knocked down. Interestingly, only knockdown of type 3 IP3R (IP3R3) or triple-knockdown of three types of IP3Rs significantly inhibited the NO-induced Ca(2+) increases. Consistently, IP3R3 knockdown blocked the NO-induced CM differentiation of ES cells. CMs derived from IP3R3 knockdown ES cells also showed both structural and functional defects. In summary, our results indicate that the IP3R3-Ca(2+) pathway is required for NO-induced CM differentiation of ES cells.

Depletion of NADP(H) due to CD38 activation triggers endothelial dysfunction in the postischemic heart

In the postischemic heart, coronary vasodilation is impaired due to loss of endothelial nitric oxide synthase (eNOS) function. Although the eNOS cofactor tetrahydrobiopterin (BH4) is depleted, its repletion only partially restores eNOS-mediated coronary vasodilation, indicating that other critical factors trigger endothelial dysfunction. Therefore, studies were performed to characterize the unidentified factor(s) that trigger endothelial dysfunction in the postischemic heart. We observed that depletion of the eNOS substrate NADPH occurs in the postischemic heart with near total depletion from the endothelium, triggering impaired eNOS function and limiting BH4 rescue through NADPH-dependent salvage pathways. In isolated rat hearts subjected to 30 min of ischemia and reperfusion (I/R), depletion of the NADP(H) pool occurred and was most marked in the endothelium, with >85% depletion. Repletion of NADPH after I/R increased NOS-dependent coronary flow well above that with BH4 alone. With combined NADPH and BH4 repletion, full restoration of NOS-dependent coronary flow occurred. Profound endothelial NADPH depletion was identified to be due to marked activation of the NAD(P)ase-activity of CD38 and could be prevented by inhibition or specific knockdown of this protein. Depletion of the NADPH precursor, NADP(+), coincided with formation of 2'-phospho-ADP ribose, a CD38-derived signaling molecule. Inhibition of CD38 prevented NADP(H) depletion and preserved endothelium-dependent relaxation and NO generation with increased recovery of contractile function and decreased infarction in the postischemic heart. Thus, CD38 activation is an important cause of postischemic endothelial dysfunction and presents a novel therapeutic target for prevention of this dysfunction in unstable coronary syndromes.

Cyclic adenosine 5'-diphosphoribose (cADPR) mimics used as molecular probes in cell signaling

Cyclic adenosine 5'-diphosphate ribose (cADPR) is a second messenger in the Ca(2+) signaling pathway. To elucidate its molecular mechanism in calcium release, a series of cADPR analogues with modification on ribose, nucleobase, and pyrophosphate have been investigated. Among them, the analogue with the modification of the northern ribose by ether linkage substitution (cIDPRE) exhibits membrane-permeate Ca(2+) agonistic activity in intact HeLa cells, human T cells, mouse cardiac myocytes and neurosecretory PC12 cell lines; thus, cIDPRE and coumarin-caged cIDPRE are valuable probes to investigate the cADPR-mediated Ca(2+) signal pathway.

Determinants of the membrane orientation of a calcium signaling enzyme CD38

CD38 catalyzes the synthesis of two structurally distinct messengers for Ca²⁺-mobilization, cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP), from cytosolic substrates, NAD and NADP, respectively. CD38 is generally thought of as a type II membrane protein with its catalytic site facing outside. We recently showed that CD38 exists, instead, in two opposite membrane orientations. The determinant for the membrane topology is unknown. Here, specific antibodies against type III CD38 were designed and produced. We show that mutating the positively charged residues in the N-terminal tail of CD38 converted its orientation to type III, with the catalytic domain facing the cytosol and it was fully active in producing intracellular cADPR. Changing the serine residues to aspartate, which is functionally equivalent to phosphorylation, had a similar effect. The mutated CD38 was expressed intracellularly and was un-glycosylated. The membrane topology could also be modulated by changing the highly conserved di-cysteine. The results indicate that the net charge of the N-terminal segment is important in determining the membrane topology of CD38 and that the type III orientation can be a functional form of CD38 for Ca²⁺-signaling. This article is part of a Special Issue entitled: 13th European Symposium on Calcium.

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.

Two pore channel 2 (TPC2) inhibits autophagosomal-lysosomal fusion by alkalinizing lysosomal pH

Autophagy is an evolutionarily conserved lysosomal degradation pathway, yet the underlying mechanisms remain poorly understood. Nicotinic acid adenine dinucleotide phosphate (NAADP), one of the most potent Ca²⁺ mobilizing messengers, elicits Ca²⁺ release from lysosomes via the two pore channel 2 (TPC2) in many cell types. Here we found that overexpression of TPC2 in HeLa or mouse embryonic stem cells inhibited autophagosomal-lysosomal fusion, thereby resulting in the accumulation of autophagosomes. Treatment of TPC2 expressing cells with a cell permeant-NAADP agonist, NAADP-AM, further induced autophagosome accumulation. On the other hand, TPC2 knockdown or treatment of cells with Ned-19, a NAADP antagonist, markedly decreased the accumulation of autophagosomes. TPC2-induced accumulation of autophagosomes was also markedly blocked by ATG5 knockdown. Interestingly, inhibiting mTOR activity failed to increase TPC2-induced autophagosome accumulation. Instead, we found that overexpression of TPC2 alkalinized lysosomal pH, and lysosomal re-acidification abolished TPC2-induced autophagosome accumulation. In addition, TPC2 overexpression had no effect on general endosomal-lysosomal degradation but prevented the recruitment of Rab-7 to autophagosomes. Taken together, our data demonstrate that TPC2/NAADP/Ca²⁺ signaling alkalinizes lysosomal pH to specifically inhibit the later stage of basal autophagy progression.

Two pore channel 2 differentially modulates neural differentiation of mouse embryonic stem cells

Nicotinic acid adenine dinucleotide phosphate (NAADP) is an endogenous Ca²⁺ mobilizing nucleotide presented in various species. NAADP mobilizes Ca²⁺ from acidic organelles through two pore channel 2 (TPC2) in many cell types and it has been previously shown that NAADP can potently induce neuronal differentiation in PC12 cells. Here we examined the role of TPC2 signaling in the neural differentiation of mouse embryonic stem (ES) cells. We found that the expression of TPC2 was markedly decreased during the initial ES cell entry into neural progenitors, and the levels of TPC2 gradually rebounded during the late stages of neurogenesis. Correspondingly, TPC2 knockdown accelerated mouse ES cell differentiation into neural progenitors but inhibited these neural progenitors from committing to neurons. Overexpression of TPC2, on the other hand, inhibited mouse ES cell from entering the early neural lineage. Interestingly, TPC2 knockdown had no effect on the differentiation of astrocytes and oligodendrocytes of mouse ES cells. Taken together, our data indicate that TPC2 signaling plays a temporal and differential role in modulating the neural lineage entry of mouse ES cells, in that TPC2 signaling inhibits ES cell entry to early neural progenitors, but is required for late neuronal differentiation.

A cell permeable NPE caged ADP-ribose for studying TRPM2

Transient potential receptor melastatin-2 (TRPM2) is a non-selective Ca²⁺-permeable cation channel of the TRPM channel subfamily and is mainly activated by intracellular adenosine diphosphate ribose (ADPR). Here we synthesized a 1-(2-nitrophenyl)ethyl caged ADPR (NPE-ADPR) and found that uncaging of NPE-ADPR efficiently stimulated Ca²⁺, Mg²⁺, and Zn²⁺ influx in a concentration-dependent manner in intact human Jurkat T-lymphocytes. The cation influx was inhibited by inhibitors or knockdown of TRPM2. Likewise, uncaging of NPE-ADPR markedly induced cation entry in HEK 293 cells that overexpress TRPM2. As expected, high temperature increased the ability of the photolyzed NPE-ADPR to induce cation entry, whereas acidic pH inhibited. Moreover, the absence of extracellular Ca²⁺ significantly inhibited Mg²⁺ and Zn²⁺ influx after uncaging NPE-ADPR. On the other hand, the absence of extracellular Na⁺ or Mg²⁺ had no effect on photolyzed NPE-ADPR induced Ca²⁺ entry. Taken together, our results indicated that NPE-ADPR is a cell permeable ADPR analogue that is useful for studying TRPM2-mediated cation entry in intact cells.

Platelet-derived growth factor CC-mediated neuroprotection against HIV Tat involves TRPC-mediated inactivation of GSK 3beta

Platelet-derived growth factor-CC (PDGF-CC) is the third member of the PDGF family, and has been implicated both in embryogenesis and development of the CNS. The biological function of this isoform however, remains largely unexplored in the context of HIV-associated dementia (HAD). In the present study, we demonstrate that exposure of human neuroblastoma cells SH-SY5Y to HIV transactivator protein Tat resulted in decreased intrinsic expression of PDGF-CC as evidenced by RT-PCR and western blot assays. Reciprocally, pretreatment of SH-SY5Y cells with PDGF-CC abrogated Tat-mediated neurotoxicity by mitigating apoptosis and neurite & MAP-2 loss. Using pharmacological and loss of function approaches we identified the role of phosphatidylinositol 3-kinase (PI3K)/Akt signaling in PDGF-CC-mediated neuroprotection. We report herein a novel role about the involvement of transient receptor potential canonical (TRPC) channel 1 in modulation of calcium transients in PDGF-CC-mediated neuroprotection. Furthermore we also demonstrated PDGF-CC-mediated inactivation of the downstream mediator - glycogen synthase kinase 3β (GSK3β) evidenced by its phosphorylation at Ser-9. This was further validated by gain and loss of function studies using cells transfected with either the wild type or mutant GSK3β constructs. Intriguingly, pretreatment of cells with either the PI3K inhibitor or TRPC blocker resulted in failure of PDGF-CC to inactivate GSK3β, thereby suggesting the intersection of PI3K and TRPC signaling at GSK3β. Taken together our findings lead to the suggestion that PDGF-CC could be developed as a therapeutic target to reverse Tat-mediated neurotoxicity with implications for HAD.

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

Cyclic adenosine diphosphoribose (cADPR) is an endogenous Ca²⁺ 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-Ca²⁺ 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-Ca²⁺ signaling pathway antagonizes the CM differentiation of mouse ES cells.

Cyclic ADP-ribose and nicotinic acid adenine dinucleotide phosphate (NAADP) as messengers for calcium mobilization

Cyclic ADP-ribose and nicotinic acid adenine dinucleotide phosphate were discovered >2 decades ago. That they are second messengers for mobilizing Ca(2+) stores has since been firmly established. Separate stores and distinct Ca(2+) channels are targeted, with cyclic ADP-ribose acting on the ryanodine receptors in the endoplasmic reticulum, whereas nicotinic acid adenine dinucleotide phosphate mobilizes the endolysosomes via the two-pore channels. Despite the structural and functional differences, both messengers are synthesized by a ubiquitous enzyme, CD38, whose crystal structure and catalytic mechanism have now been well elucidated. How this novel signaling enzyme is regulated remains largely unknown and is the focus of this minireview.

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

Cyclic adenosine diphosphate ribose is an endogenous Ca²⁺ mobilizer involved in diverse cellular processes. A cell membrane-permeable cyclic adenosine diphosphate ribose analogue, cyclic inosine diphosphoribose ether (cIDPRE), can induce Ca²⁺ 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 Ca²⁺ release from endoplasmic reticulum, with concomitant Ca²⁺ influx in Jurkat cells. Ca²⁺ release evoked by uncaged Co-i-cIDPRE was blocked by knockdown of ryanodine receptors (RyRs) 2 and 3 in Jurkat cells. The associated Ca²⁺ influx, on the other hand, was abolished by double knockdown of Stim1 and TRPM2 in Jurkat cells. Furthermore, Ca²⁺ 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 Ca²⁺ release from endoplasmic reticulum via RyRs and triggers Ca²⁺ influx via TRPM2.

Cyclic ADP ribose is a novel regulator of intracellular Ca2+ oscillations in human bone marrow mesenchymal stem cells

Bone marrow mesenchymal stem cells (MSCs) are a promising cell source for regenerative medicine. However, the cellular biology of these cells is not fully understood. The present study characterizes the cyclic ADP-ribose (cADPR)-mediated Ca(2+) signals in human MSCs and finds that externally applied cADPR can increase the frequency of spontaneous intracellular Ca(2+) (Ca(2+) (i) ) oscillations. The increase was abrogated by a specific cADPR antagonist or an inositol trisphosphate receptor (IP3R) inhibitor, but not by ryanodine. In addition, the cADPR-induced increase of Ca(2+) (i) oscillation frequency was prevented by inhibitors of nucleoside transporter or by inhibitors of the transient receptor potential cation melastatin-2 (TRPM2) channel. RT-PCR revealed mRNAs for the nucleoside transporters, concentrative nucleoside transporters 1/2 and equilibrative nucleoside transporters 1/3, IP3R1/2/3 and the TRPM2 channel, but not those for ryanodine receptors and CD38 in human MSCs. Knockdown of the TRPM2 channel by specific short interference RNA abolished the effect of cADPR on the Ca(2+) (i) oscillation frequency, and prevented the stimulation of proliferation by cADPR. Moreover, cADPR remarkably increased phosphorylated extracellular-signal-regulated kinases 1/2 (ERK1/2), but not Akt or p38 mitogen-activated protein kinase (MAPK). However, cADPR had no effect on adipogenesis or osteogenesis in human MSCs. Our results indicate that cADPR is a novel regulator of Ca(2+) (i) oscillations in human MSCs. It permeates the cell membrane through the nucleoside transporters and increases Ca(2+) oscillation via activation of the TRPM2 channel, resulting in enhanced phosphorylation of ERK1/2 and, thereby, stimulation of human MSC proliferation. This study delineates an alternate signalling pathway of cADPR that is distinct from its well-established role of serving as a Ca(2+) messenger for mobilizing the internal Ca(2+) stores. Whether cADPR can be used clinically for stimulating marrow function in patients with marrow disorders remains to be further studied.

Intracellular alkalinization induces cytosolic Ca2+ increases by inhibiting sarco/endoplasmic reticulum Ca2+-ATPase (SERCA)

Intracellular pH (pHi) and Ca(2+) regulate essentially all aspects of cellular activities. Their inter-relationship has not been mechanistically explored. In this study, we used bases and acetic acid to manipulate the pHi. We found that transient pHi rise induced by both organic and inorganic bases, but not acidification induced by acid, produced elevation of cytosolic Ca(2+). The sources of the Ca(2+) increase are from the endoplasmic reticulum (ER) Ca(2+) pools as well as from Ca(2+) influx. The store-mobilization component of the Ca(2+) increase induced by the pHi rise was not sensitive to antagonists for either IP(3)-receptors or ryanodine receptors, but was due to inhibition of the sarco/endoplasmic reticulum Ca(2+)-ATPase (SERCA), leading to depletion of the ER Ca(2+) store. We further showed that the physiological consequence of depletion of the ER Ca(2+) store by pHi rise is the activation of store-operated channels (SOCs) of Orai1 and Stim1, leading to increased Ca(2+) influx. Taken together, our results indicate that intracellular alkalinization inhibits SERCA activity, similar to thapsigargin, thereby resulting in Ca(2+) leak from ER pools followed by Ca(2+) influx via SOCs.

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.

Cu(ii)- and disulfide bonds-induced stabilization during the guanidine hydrochloride- and thermal-induced denaturation of NAD-glycohydrolase from the venom of Agkistrodon acutus

NAD-glycohydrolase (AA-NADase) from Agkistrodon acutus venom is a unique multicatalytic enzyme with both NADase and AT(D)Pase-like activities. Among all identified NADases, only AA-NADase is a disulfide-linked dimer and contains Cu(2+). Cu(2+) and disulfide bonds are essential for its multicatalytic activity. In this study, the effects of Cu(2+) and disulfide-bonds on guanidine hydrochloride (GdnHCl)- and thermal-induced unfolding of AA-NADase have been investigated by fluorescence, circular dichroism (CD) and differential scanning calorimetry (DSC). Cu(2+) and disulfide bonds not only increase the free energy change during the GdnHCl-induced unfolding as determined by fluorescence, but also increase the overall enthalpy change and the transition temperature during the thermal-induced unfolding as determined by CD and DSC. The slope of the GdnHCl-induced unfolding curve at its midpoint and the heat capacity of thermal-induced unfolding are slightly affected by Cu(2+) but significantly decrease after reduction of three disulfide-bonds. This work suggests that Cu(2+) stabilizes the folded state by increasing the enthalpy of unfolding, while disulfide-bonds stabilize the folded state by increasing the enthalpy of unfolding and stabilizing the packing of hydrophobic residues. Thus both Cu(2+) and disulfide bonds play a structural role in its multicatalytic activity.

Cyclic ADP ribose-dependent Ca2+ release by group I metabotropic glutamate receptors in acutely dissociated rat hippocampal neurons

Group I metabotropic glutamate receptors (group I mGluRs; mGluR1 and mGluR5) exert diverse effects on neuronal and synaptic functions, many of which are regulated by intracellular Ca²⁺. In this study, we characterized the cellular mechanisms underlying Ca²⁺ mobilization induced by (RS)-3,5-dihydroxyphenylglycine (DHPG; a specific group I mGluR agonist) in the somata of acutely dissociated rat hippocampal neurons using microfluorometry. We found that DHPG activates mGluR5 to mobilize intracellular Ca²⁺ from ryanodine-sensitive stores via cyclic adenosine diphosphate ribose (cADPR), while the PLC/IP₃ signaling pathway was not involved in Ca²⁺ mobilization. The application of glutamate, which depolarized the membrane potential by 28.5±4.9 mV (n = 4), led to transient Ca²⁺ mobilization by mGluR5 and Ca²⁺ influx through L-type Ca²⁺ channels. We found no evidence that mGluR5-mediated Ca²⁺ release and Ca²⁺ influx through L-type Ca²⁺ channels interact to generate supralinear Ca²⁺ transients. Our study provides novel insights into the mechanisms of intracellular Ca²⁺ mobilization by mGluR5 in the somata of hippocampal neurons.

Cyclic ADP-ribose requires CD38 to regulate the release of ATP in visceral smooth muscle

It is well established that the intracellular second messenger cADP-ribose (cADPR) activates Ca(2+) release from the sarcoplasmic reticulum through ryanodine receptors. CD38 is a multifunctional enzyme involved in the formation of cADPR in mammals. CD38 has also been reported to transport cADPR in several cell lines. Here, we demonstrate a role for extracellular cADPR and CD38 in modulating the spontaneous, but not the electrical field stimulation-evoked, release of ATP in visceral smooth muscle. Using a small-volume superfusion assay and an HPLC technique with fluorescence detection, we measured the spontaneous and evoked release of ATP in bladder detrusor smooth muscles isolated from CD38(+/+) and CD38(-/-) mice. cADPR (1 nM) enhanced the spontaneous overflow of ATP in bladders isolated from CD38(+/+) mice. This effect was abolished by the inhibitor of cADPR receptors on sarcoplasmic reticulum 8-bromo-cADPR (80 μM) and by ryanodine (50 μm), but not by the nonselective P2 purinergic receptor antagonist pyridoxal phosphate 6-azophenyl-2',4'-disulfonate (30 μM). cADPR failed to facilitate the spontaneous ATP overflow in bladders isolated from CD38(-/-) mice, indicating that CD38 is crucial for the enhancing effects of extracellular cADPR on spontaneous ATP release. Contractile responses to ATP were potentiated by cADPR, suggesting that the two adenine nucleotides may work in synergy to maintain the resting tone of the bladder. In conclusion, extracellular cADPR enhances the spontaneous release of ATP in the bladder by influx via CD38 and subsequent activation of intracellular cADPR receptors, probably causing an increase in intracellular Ca(2+) in neuronal cells.

Cytosolic CD38 protein forms intact disulfides and is active in elevating intracellular cyclic ADP-ribose

CD38 catalyzes the synthesis of cyclic ADP-ribose (cADPR), a Ca(2+) messenger responsible for regulating a wide range of physiological functions. It is generally regarded as an ectoenzyme, but its intracellular localization has also been well documented. It is not known if internal CD38 is enzymatically active and contributes to the Ca(2+) signaling function. In this study, we engineered a novel soluble form of CD38 that can be efficiently expressed in the cytosol and use cytosolic NAD as a substrate to produce cADPR intracellularly. The activity of the engineered CD38 could be decreased by mutating the catalytic residue Glu-226 and increased by the double mutation E146A/T221F, which increased its cADPR synthesis activity by >11-fold. Remarkably, the engineered CD38 exhibited the ability to form the critical disulfide linkages required for its enzymatic activity. This was verified by using a monoclonal antibody generated against a critical disulfide, Cys-254-Cys-275. The specificity of the antibody was established by x-ray crystallography and site-directed mutagenesis. The engineered CD38 is thus a novel example challenging the general belief that cytosolic proteins do not possess disulfides. As a further refinement of this approach, the engineered CD38 was placed under the control of tetracycline using an autoregulated construct. This study has set the stage for in vivo manipulation of cADPR metabolism.

Design, synthesis and biological characterization of novel inhibitors of CD38

Human CD38 is a novel multi-functional protein that acts not only as an antigen for B-lymphocyte activation, but also as an enzyme catalyzing the synthesis of a Ca(2+) messenger molecule, cyclic ADP-ribose, from NAD(+). It is well established that this novel Ca(2+) signaling enzyme is responsible for regulating a wide range of physiological functions. Based on the crystal structure of the CD38/NAD(+) complex, we synthesized a series of simplified N-substituted nicotinamide derivatives (Compound 1-14). A number of these compounds exhibited moderate inhibition of the NAD(+) utilizing activity of CD38, with Compound 4 showing the highest potency. The crystal structure of CD38/Compound 4 complex and computer simulation of Compound 7 docking to CD38 show a significant role of the nicotinamide moiety and the distal aromatic group of the compounds for substrate recognition by the active site of CD38. Biologically, we showed that both Compounds 4 and 7 effectively relaxed the agonist-induced contraction of muscle preparations from rats and guinea pigs. This study is a rational design of inhibitors for CD38 that exhibit important physiological effects, and can serve as a model for future drug development.

NAADP receptors

Of the established Ca(2+) mobilizing messengers, NAADP is arguably the most tantalizing. It is the most potent, often efficacious at low nanomolar concentrations. Recent studies have identified a new class of calcium release channel, the two-pore channels (TPCs), as the likely targets for NAADP. These channels are endolysosomal in localization where they mediate local Ca(2+) release, and have highlighted a new role of acidic organelles as targets for messenger-evoked Ca(2+) mobilization. Three distinct roles of TPCs have been identified. The first is to effect local Ca(2+) release that may play a role in endolysosomal function including vesicular fusion and trafficking. The second is to trigger global calcium release by recruiting Ca(2+)-induced Ca(2+) release (CICR) channels at lysosomal-ER junctions. The third is to regulate plasma membrane excitability by the targeting of Ca(2+) release from appropriately positioned subplasma membrane stores to regulate plasma membrane Ca(2+)-activated channels. In this review, I discuss the role of NAADP-mediated Ca(2+) release from endolysosomal stores as a widespread trigger for intracellular calcium signaling mechanisms, and how studies of TPCs are beginning to enhance our understanding of the central role of lysosomes in Ca(2+) signaling.

ATRA-induced HL-60 myeloid leukemia cell differentiation depends on the CD38 cytosolic tail needed for membrane localization, but CD38 enzymatic activity is unnecessary

Leukocyte antigen CD38 expression is an early marker of all-trans retinoic acid (ATRA) stimulated differentiation in the leukemic cell line HL-60. It promotes induced myeloid maturation when overexpressed, whereas knocking it down is inhibitory. It is a type II membrane protein with an extracellular C-terminal enzymatic domain with NADase/NADPase and ADPR cyclase activity and a short cytoplasmic N-terminal tail. Here we determined whether CD38 enzymatic activity or the cytoplasmic tail is required for ATRA-induced differentiation. Neither a specific CD38 ectoenzyme inhibitor nor a point mutation that cripples enzymatic activity (CD38 E226Q) diminishes ATRA-induced differentiation or G1/0 arrest. In contrast a cytosolic deletion mutation (CD38 Δ11-20) prevents membrane expression and inhibits differentiation and G1/0 arrest. These results may be consistent with disrupting the function of critical molecules necessary for membrane-expressed CD38 signal transduction. One candidate molecule is the Src family kinase Fgr, which failed to undergo ATRA-induced upregulation in CD38 Δ11-20 expressing cells. Another is Vav1, which also showed only basal expression after ATRA treatment in CD38 Δ11-20 expressing cells. Therefore, the ability of CD38 to propel ATRA-induced myeloid differentiation and G1/0 arrest is unimpaired by loss of its ectoenzyme activity. However a cytosolic tail deletion mutation disrupted membrane localization and inhibited differentiation. ATRA-induced differentiation thus does not require the CD38 ectoenzyme function, but is dependent on a membrane receptor function.

Metal ions binding to NAD-glycohydrolase from the venom of Agkistrodon acutus: regulation of multicatalytic activity

AA-NADase from Agkistrodon acutus venom is a unique multicatalytic enzyme with both NADase and AT(D)Pase activities. Among all identified NADases, only AA-NADase contains Cu(2+) ions that are essential for its multicatalytic activity. In this study, the interactions between divalent metal ions and AA-NADase and the effects of metal ions on its structure and activity have been investigated by equilibrium dialysis, isothermal titration calorimetry, fluorescence, circular dichroism, dynamic light scattering and HPLC. The results show that AA-NADase has two classes of Cu(2+) binding sites, one activator site with high affinity and approximately six inhibitor sites with low affinity. Cu(2+) ions function as a switch for its NADase activity. In addition, AA-NADase has one Mn(2+) binding site, one Zn(2+) binding site, one strong and two weak Co(2+) binding sites, and two strong and six weak Ni(2+) binding sites. Metal ion binding affinities follow the trend Cu(2+) > Ni(2+) > Mn(2+) > Co(2+) > Zn(2+), which accounts for the existence of one Cu(2+) in the purified AA-NADase. Both NADase and ADPase activities of AA-NADase do not have an absolute requirement for Cu(2+), and all tested metal ions activate its NADase and ADPase activities and the activation capacity follows the trend Zn(2+) > Mn(2+) > Cu(2+) ~Co(2+) > Ni(2+). Metal ions serve as regulators for its multicatalytic activity. Although all tested metal ions have no obvious effects on the global structure of AA-NADase, Cu(2+)- and Zn(2+)-induced conformational changes around some Trp residues have been observed. Interestingly, each tested metal ion has a very similar activation of both NADase and ADPase activities, suggesting that the two different activities probably occur at the same site.