Structural Determination of a Cyclic Metabolite of NAD+ with Intracellular Ca2+-mobilizing Activity

Diazirine-AIOC-NAADP, a Clickable-Photoactive NAADP Analog for Sea Urchin NAADP Binding Proteins

Calcium ions (Ca2+) play a vital role as intracellular messengers, regulating essential cellular processes. Nicotinic acid adenine dinucleotide phosphate (NAADP) serves as a potent second messenger, responsible for releasing Ca2+ in both mammals and echinoderms. Despite identification of two human NAADP receptor proteins, their counterparts in sea urchins remain elusive. Sea urchin NAADP binding proteins are important due to their unique identities and NAADP binding properties which may illuminate new signaling modalities in other species. Consequently, the development of new photoactive and clickable NAADP analogs with specificity for binding targets in sea urchin egg homogenates is a priority. We designed and synthesized diazirine-AIOC-NAADP, a photoactive and "clickable" NAADP analog, to specifically label and identify sea urchin NAADP receptors. This analog, synthesized using a chemo-enzymatic approach, induced Ca2+ release from sea urchin egg homogenates at low-micromolar concentrations. The ability of diazirine-AIOC-NAADP to mobilize Ca2+ in cultured human cells was investigated by microinjection of the probe into U2OS cells. Microinjected NAADP elicited a robust Ca2+ release, but even 6000-fold higher concentrations of diazirine-AIOC-NAADP were unable to release Ca2+. Our results indicate that our new probe is specifically recognized at low concentration by sea urchin egg NAADP receptors but not by the NAADP receptors in a human cultured cell line.

The Role of NAD+ in Metabolic Regulation of Adipose Tissue: Implications for Obesity-Induced Insulin Resistance

Obesity-induced insulin resistance is among the key factors in the development of type 2 diabetes, atherogenic dyslipidemia and cardiovascular disease. Adipose tissue plays a key role in the regulation of whole-body metabolism and insulin sensitivity. In obesity, adipose tissue becomes inflamed and dysfunctional, exhibiting a modified biochemical signature and adipokine secretion pattern that promotes insulin resistance in peripheral tissues. An important hallmark of dysfunctional obese adipose tissue is impaired NAD+/sirtuin signaling. In this chapter, we summarize the evidence for impairment of the NAD+/sirtuin pathway in obesity, not only in white adipose tissue but also in brown adipose tissue and during the process of beiging, together with correlative evidence from human studies. We also describe the role of PARPs and CD38 as important NAD+ consumers and discuss findings from experimental studies that investigated potential NAD+ boosting strategies and their efficacy in restoring impaired NAD+ metabolism in dysfunctional obese adipose tissue. In sum, these studies suggest a critical role of NAD+ metabolism in adipose biology and provide a basis for the potential development of strategies to restore metabolic health in obesity.

NAADP-Evoked Ca2+ Signaling Leads to Mutant Huntingtin Aggregation and Autophagy Impairment in Murine Astrocytes

Huntington’s disease (HD) is a progressive neurodegenerative disease characterized by mutations in the huntingtin gene (mHtt), causing an unstable repeat of the CAG trinucleotide, leading to abnormal long repeats of polyglutamine (poly-Q) in the N-terminal region of the huntingtin, which form abnormal conformations and aggregates. Alterations in Ca2+ signaling are involved in HD models and the accumulation of mutated huntingtin interferes with Ca2+ homeostasis. Lysosomes are intracellular Ca2+ storages that participate in endocytic and lysosomal degradation processes, including autophagy. Nicotinic acid adenine dinucleotide phosphate (NAADP) is an intracellular second messenger that promotes Ca2+ release from the endo-lysosomal system via Two-Pore Channels (TPCs) activation. Herein, we show the impact of lysosomal Ca2+ signals on mHtt aggregation and autophagy blockade in murine astrocytes overexpressing mHtt-Q74. We observed that mHtt-Q74 overexpression causes an increase in NAADP-evoked Ca2+ signals and mHtt aggregation, which was inhibited in the presence of Ned-19, a TPC antagonist, or BAPTA-AM, a Ca2+ chelator. Additionally, TPC2 silencing revert the mHtt aggregation. Furthermore, mHtt has been shown co-localized with TPC2 which may contribute to its effects on lysosomal homeostasis. Moreover, NAADP-mediated autophagy was also blocked since its function is dependent on lysosomal functionality. Taken together, our data show that increased levels of cytosolic Ca2+ mediated by NAADP causes mHtt aggregation. Additionally, mHtt co-localizes with the lysosomes, where it possibly affects organelle functions and impairs autophagy.

Enzymology of Ca2+-Mobilizing Second Messengers Derived from NAD: From NAD Glycohydrolases to (Dual) NADPH Oxidases

Nicotinamide adenine dinucleotide (NAD) and its 2'-phosphorylated cousin NADP are precursors for the enzymatic formation of the Ca²⁺-mobilizing second messengers adenosine diphosphoribose (ADPR), 2'-deoxy-ADPR, cyclic ADPR, and nicotinic acid adenine dinucleotide phosphate (NAADP). The enzymes involved are either NAD glycohydrolases CD38 or sterile alpha toll/interleukin receptor motif containing-1 (SARM1), or (dual) NADPH oxidases (NOX/DUOX). Enzymatic function(s) are reviewed and physiological role(s) in selected cell systems are discussed.

A conformation-specific nanobody targeting the nicotinamide mononucleotide-activated state of SARM1

Sterile alpha (SAM) and Toll/interleukin-1 receptor (TIR) motif containing 1 (SARM1) is an autoinhibitory NAD-consuming enzyme that is activated by the accumulation of nicotinamide mononucleotide (NMN) during axonal injury. Its activation mechanism is not fully understood. Here, we generate a nanobody, Nb-C6, that specifically recognizes NMN-activated SARM1. Nb-C6 stains only the activated SARM1 in cells stimulated with CZ-48, a permeant mimetic of NMN, and partially activates SARM1 in vitro and in cells. Cryo-EM of NMN/SARM1/Nb-C6 complex shows an octameric structure with ARM domains bending significantly inward and swinging out together with TIR domains. Nb-C6 binds to SAM domain of the activated SARM1 and stabilized its ARM domain. Mass spectrometry analyses indicate that the activated SARM1 in solution is highly dynamic and that the neighboring TIRs form transient dimers via the surface close to one BB loop. We show that Nb-C6 is a valuable tool for studies of SARM1 activation.

Synthesis and biological evaluation of novel photo-clickable adenosine and cyclic ADP-ribose analogs: 8-N3-2'-O-propargyladenosine and 8-N3-2'-O-propargyl-cADPR

A photo-clickable analog of adenosine was devised and synthesized in which the photoactive functional group (8-azidoadenosine) and the click moiety (2'-O-propargyl-ether) were compactly combined within the structure of the adenosine nucleoside itself. We synthesized 8-N3-2'-O-propargyl adenosine in four steps starting from adenosine. This photo-clickable adenosine was 5'-phosphorylated and coupled to nicotinamide mononucleotide to form the NAD analog 8-N3-2'-O-propargyl-NAD. This NAD analog was recognized by Aplysia californica ADP-ribosyl cyclase and enzymatically cyclized producing 8-N3-2'-O-propargyl cyclic ADP-ribose. Photo-clickable cyclic-ADP-ribose analog was envisioned as a probe to label cyclic ADP-ribose binding proteins. The monofunctional 8-N3-cADPR has previously been shown to be an antagonist of cADPR-induced calcium release [T.F. Walseth et. al., J. Biol. Chem (1993) 268, 26686-26691]. 2'-O-propargyl-cADPR was recognized as an agonist which elicited Ca2+ release when added at low concentration to sea urchin egg homogenates. The bifunctional 8-N3-2'-O-propargyl cyclic ADP-ribose did not elicit Ca2+ release at low concentration or impact cyclic ADP-ribose mediated Ca2+ release either when added to sea urchin egg homogenates or when microinjected into cultured human U2OS cells. The photo-clickable adenosine will none-the-less be a useful scaffold for synthesizing photo-clickable probes for identifying proteins that interact with a variety of adenosine nucleotides.

Anti-CD38 antibody therapy for patients with relapsed/refractory multiple myeloma: differential mechanisms of action and recent clinical trial outcomes

CD38 is a transmembrane glycoprotein that functions both as a receptor and an ectoenzyme, playing key roles in the regulation of calcium signaling and migration of immune cells to tumor microenvironments. High expression on multiple myeloma (MM) cells and limited expression on normal cells makes CD38 an ideal target for the treatment of MM patients. Two monoclonal antibodies directed at CD38, isatuximab and daratumumab, are available for use in patients with relapsed and/or refractory MM (RRMM); daratumumab is also approved in newly diagnosed MM and light-chain amyloidosis. Clinical experience has shown that anti-CD38 antibody therapy is transforming treatment of MM owing to its anti-myeloma efficacy and manageable safety profile. Isatuximab and daratumumab possess similarities and differences in their mechanisms of action, likely imparted by their binding to distinct, non-overlapping epitopes on the CD38 molecule. In this review, we present the mechanistic properties of these two antibodies and outline available evidence on their abilities to induce adaptive immune responses and modulate the bone marrow niche in MM. Further, we discuss differences in regulatory labeling between these two agents and analyze recent key clinical trial results, including evidence in patients with underlying renal impairment and other poor prognostic factors. Finally, we describe the limited existing evidence for the use of isatuximab or daratumumab after disease progression on prior anti-CD38 mono- or combination therapy, highlighting the need for additional clinical evaluations to define optimal anti-CD38 antibody therapy selection and sequencing in RRMM.

Paracrine ADP Ribosyl Cyclase-Mediated Regulation of Biological Processes

ADP-ribosyl cyclases (ADPRCs) catalyze the synthesis of the Ca2+-active second messengers Cyclic ADP-ribose (cADPR) and ADP-ribose (ADPR) from NAD+ as well as nicotinic acid adenine dinucleotide phosphate (NAADP+) from NADP+. The best characterized ADPRC in mammals is CD38, a single-pass transmembrane protein with two opposite membrane orientations. The first identified form, type II CD38, is a glycosylated ectoenzyme, while type III CD38 has its active site in the cytosol. The ectoenzymatic nature of type II CD38 raised long ago the question of a topological paradox concerning the access of the intracellular NAD+ substrate to the extracellular active site and of extracellular cADPR product to its intracellular receptors, ryanodine (RyR) channels. Two different transporters, equilibrative connexin 43 (Cx43) hemichannels for NAD+ and concentrative nucleoside transporters (CNTs) for cADPR, proved to mediate cell-autonomous trafficking of both nucleotides. Here, we discussed how type II CD38, Cx43 and CNTs also play a role in mediating several paracrine processes where an ADPRC+ cell supplies a neighboring CNT-and RyR-expressing cell with cADPR. Recently, type II CD38 was shown to start an ectoenzymatic sequence of reactions from NAD+/ADPR to the strong immunosuppressant adenosine; this paracrine effect represents a major mechanism of acquired resistance of several tumors to immune checkpoint therapy.

Radiolabeling and cytotoxicity of monoclonal antibody Isatuximab functionalized silver nanoparticles on the growth of multiple myeloma

The goal of this article was to develop a new therapeutic strategy based on nanotechnology for multiple myeloma (MM) treatment which shows a synergism of different mechanisms. In this concern, 12.9 nm-sized silver nanoparticles (AgNPs) were prepared and functionalized with Isatuximab, anti-MM monoclonal antibody (mAb). Furthermore, the synthesized nanocomposite was radiolabelled with iodine-131 radionuclide and yielded 95.5 ± 1.5%. Then, the synergistic MM-proliferation inhibition efficacy of the radionanocomposite (¹³¹I-Isatuximab/AgNPs) was explored in-vitro in comparison to each single agent. The MTT investigation showed that the antiproliferation effect of ¹³¹I-Isatuximab/AgNPs increased by more than 1.5 fold if compared with Isatuximab, AgNPs, Isatuximab/AgNPs or ¹³¹I-Isatuximab. Additionally, ¹³¹I-Isatuximab/AgNPs exhibited an apoptotic effect on MM cells which was more than that of Isatuximab, AgNPs, Isatuximab/AgNPs or ¹³¹I-Isatuximab by 2, 1.8, 1.7 and 1.5 folds, respectively. In conclusion, the results expressed ¹³¹I-Isatuximab/AgNPs as a potential new anti-MM agent.

Sarm1 activation produces cADPR to increase intra-axonal Ca++ and promote axon degeneration in PIPN

Cancer patients frequently develop chemotherapy-induced peripheral neuropathy (CIPN), a painful and long-lasting disorder with profound somatosensory deficits. There are no effective therapies to prevent or treat this disorder. Pathologically, CIPN is characterized by a "dying-back" axonopathy that begins at intra-epidermal nerve terminals of sensory neurons and progresses in a retrograde fashion. Calcium dysregulation constitutes a critical event in CIPN, but it is not known how chemotherapies such as paclitaxel alter intra-axonal calcium and cause degeneration. Here, we demonstrate that paclitaxel triggers Sarm1-dependent cADPR production in distal axons, promoting intra-axonal calcium flux from both intracellular and extracellular calcium stores. Genetic or pharmacologic antagonists of cADPR signaling prevent paclitaxel-induced axon degeneration and allodynia symptoms, without mitigating the anti-neoplastic efficacy of paclitaxel. Our data demonstrate that cADPR is a calcium-modulating factor that promotes paclitaxel-induced axon degeneration and suggest that targeting cADPR signaling provides a potential therapeutic approach for treating paclitaxel-induced peripheral neuropathy (PIPN).

Leukocyte Membrane Enzymes Play the Cell Adhesion Game

For a long time, proteins with enzymatic activity have not been usually considered to carry out other functions different from catalyzing chemical reactions within or outside the cell. Nevertheless, in the last few years several reports have uncovered the participation of numerous enzymes in other processes, placing them in the category of moonlighting proteins. Some moonlighting enzymes have been shown to participate in complex processes such as cell adhesion. Cell adhesion plays a physiological role in multiple processes: it enables cells to establish close contact with one another, allowing communication; it is a key step during cell migration; it is also involved in tightly binding neighboring cells in tissues, etc. Importantly, cell adhesion is also of great importance in pathophysiological scenarios like migration and metastasis establishment of cancer cells. Cell adhesion is strictly regulated through numerous switches: proteins, glycoproteins and other components of the cell membrane. Recently, several cell membrane enzymes have been reported to participate in distinct steps of the cell adhesion process. Here, we review a variety of examples of membrane bound enzymes participating in adhesion of immune cells.

The calcium signaling enzyme CD38 - a paradigm for membrane topology defining distinct protein functions

CD38 is a single-pass transmembrane enzyme catalyzing the synthesis of two nucleotide second messengers, cyclic ADP-ribose (cADPR) from NAD and nicotinic acid adenine dinucleotide phosphate (NAADP) from NADP. The former mediates the mobilization of the endoplasmic Ca²⁺-stores in response to a wide range of stimuli, while NAADP targets the endo-lysosomal stores. CD38 not only possesses multiple enzymatic activities, it also exists in two opposite membrane orientations. Type III CD38 has the catalytic domain facing the cytosol and is responsible for producing cellular cADPR. The type II CD38 has an opposite orientation and is serving as a surface receptor mediating extracellular functions such as cell adhesion and lymphocyte activation. Its ecto-NADase activity also contributes to the recycling of external NAD released by apoptosis. Endocytosis can deliver surface type II CD38 to endo-lysosomes, which acidic environment favors the production of NAADP. This article reviews the rationale and evidence that have led to CD38 as a paradigm for membrane topology defining distinct functions of proteins. Also described is the recent discovery of a hitherto unknown cADPR-synthesizing enzyme, SARM1, ushering in a new frontier in cADPR-mediated Ca²⁺-signaling.

NAADP: From Discovery to Mechanism

Nicotinic acid adenine dinucleotide 2'-phosphate (NAADP) is a naturally occurring nucleotide that has been shown to be involved in the release of Ca²⁺ from intracellular stores in a wide variety of cell types, tissues and organisms. Current evidence suggests that NAADP may function as a trigger to initiate a Ca²⁺ signal that is then amplified by other Ca²⁺ release mechanisms. A fundamental question that remains unanswered is the identity of the NAADP receptor. Our recent studies have identified HN1L/JPT2 as a high affinity NAADP binding protein that is essential for the modulation of Ca²⁺ channels.

Precursor comparisons for the upregulation of nicotinamide adenine dinucleotide. Novel approaches for better aging

Nicotinamide adenine dinucleotide (NAD) is a coenzyme found in every human cell and regulates a number of systems across multiple cellular compartments and tissue types via an endogenous and exogenous influence. NAD levels are demonstrated to decline with age and therefore measures to counteract the waning of NAD have been devised. A number of NAD precursor candidates such as nicotinamide riboside (NR), nicotinamide mononucleotide (NMN), the reduced form of nicotinamide mononucleotide (NMNH), nicotinic acid (NA) nicotinamide (NAM), and dihydronicotinamide riboside (DNR) increase NAD levels in vitro and in vivo. This discussion will focus on the precursors NR, NMN, NMNH, and DNR in the upregulation of NAD. There are many publications on NAD precursors as it has become popular for human consumption in recent years due to its vital importance to the general consumer. However, there is no consensus between researchers and this was the aim of this review, to determine and discuss their areas of agreement versus disagreement, to highlight the gaps in research, and to give recommendations for future work. Bioavailability and potency of NR, NMNH, NMN, and DNR is also examined on the light of the most recent literature.

Isatuximab for the treatment of relapsed/refractory multiple myeloma

INTRODUCTION

Although new drug classes have significantly extended survival of patients with multiple myeloma, they continue to experience multiple relapses and/or become refractory to treatment. Therefore, novel therapies and treatment combinations with different mechanisms of action are needed to improve the outcomes of patients with relapsed/refractory multiple myeloma.

AREAS COVERED

Here, the authors review the published data regarding the development and clinical investigation of isatuximab, a CD38 monoclonal antibody, for treatment of patients with relapsed/refractory multiple myeloma. The mechanisms of action, clinical efficacy, and safety of isatuximab treatment are summarized.

EXPERT OPINION

Isatuximab is approved in combination with pomalidomide/dexamethasone for the treatment of adults with relapsed/refractory multiple myeloma who have received at least two prior therapies, including lenalidomide and a proteasome inhibitor. Isatuximab displays a manageable safety profile, with infusion reactions being the most common adverse events. Isatuximab is currently being further evaluated in combination with other backbone regimens in relapsed/refractory and newly diagnosed multiple myeloma.

Ca2+ Signaling as the Untact Mode during Signaling in Metastatic Breast Cancer

Metastatic features of breast cancer in the brain are considered a common pathology in female patients with late-stage breast cancer. Ca2+ signaling and the overexpression pattern of Ca2+ channels have been regarded as oncogenic markers of breast cancer. In other words, breast tumor development can be mediated by inhibiting Ca2+ channels. Although the therapeutic potential of inhibiting Ca2+ channels against breast cancer has been demonstrated, the relationship between breast cancer metastasis and Ca2+ channels is not yet understood. Thus, we focused on the metastatic features of breast cancer and summarized the basic mechanisms of Ca2+-related proteins and channels during the stages of metastatic breast cancer by evaluating Ca2+ signaling. In particular, we highlighted the metastasis of breast tumors to the brain. Thus, modulating Ca2+ channels with Ca2+ channel inhibitors and combined applications will advance treatment strategies for breast cancer metastasis to the brain.

Imeglimin amplifies glucose-stimulated insulin release from diabetic islets via a distinct mechanism of action

Pancreatic islet β-cell dysfunction is characterized by defective glucose-stimulated insulin secretion (GSIS) and is a predominant component of the pathophysiology of diabetes. Imeglimin, a novel first-in-class small molecule tetrahydrotriazine drug candidate, improves glycemia and GSIS in preclinical models and clinical trials in patients with Type 2 diabetes; however, the mechanism by which it restores β-cell function is unknown. Here, we show that imeglimin acutely and directly amplifies GSIS in islets isolated from rodents with Type 2 diabetes via a mode of action that is distinct from other known therapeutic approaches. The underlying mechanism involves increases in the cellular nicotinamide adenine dinucleotide (NAD+) pool-potentially via the salvage pathway and induction of nicotinamide phosphoribosyltransferase (NAMPT) along with augmentation of glucose-induced ATP levels. Further, additional results suggest that NAD+ conversion to a second messenger, cyclic ADP ribose (cADPR), via ADP ribosyl cyclase/cADPR hydrolase (CD38) is required for imeglimin's effects in islets, thus representing a potential link between increased NAD+ and enhanced glucose-induced Ca2+ mobilization which-in turn-is known to drive insulin granule exocytosis. Collectively, these findings implicate a novel mode of action for imeglimin that explains its ability to effectively restore-β-cell function and provides for a new approach to treat patients suffering from Type 2 diabetes.

Nasser S, Younis N, Bhagani H, Al-Dhaheri Y, Pintus G, Eid AA, El-Yazbi AF, Eid AH. Visfatin: A Possible Role in Cardiovasculo-Metabolic Disorders

Visfatin/NAMPT (nicotinamide phosphoribosyltransferase) is an adipocytokine with several intriguing properties. It was first identified as pre-B-cell colony-enhancing factor but turned out to possess enzymatic functions in nicotinamide adenine dinucleotide biosynthesis, with ubiquitous expression in skeletal muscles, liver, cardiomyocytes, and brain cells. Visfatin exists in an intracellular (iNAMPT) and extracellular (eNAMPT) form. Intracellularly, visfatin/iNAMPT plays a regulatory role in NAD⁺ biosynthesis and thereby affects many NAD-dependent proteins such as sirtuins, PARPs, MARTs and CD38/157. Extracellularly, visfatin is associated with many hormone-like signaling pathways and activates some intracellular signaling cascades. Importantly, eNAMPT has been associated with several metabolic disorders including obesity and type 1 and 2 diabetes. In this review, a brief overview about visfatin is presented with special emphasis on its relevance to metabolic diseases. Visfatin/NAMPT appears to be a unique molecule with clinical significance with a prospective promising diagnostic, prognostic, and therapeutic applications in many cardiovasculo-metabolic disorders.

Facts and Challenges in Immunotherapy for T-Cell Acute Lymphoblastic Leukemia

T-cell acute lymphoblastic leukemia (T-ALL), a T-cell malignant disease that mainly affects children, is still a medical challenge, especially for refractory patients for whom therapeutic options are scarce. Recent advances in immunotherapy for B-cell malignancies based on increasingly efficacious monoclonal antibodies (mAbs) and chimeric antigen receptors (CARs) have been encouraging for non-responding or relapsing patients suffering from other aggressive cancers like T-ALL. However, secondary life-threatening T-cell immunodeficiency due to shared expression of targeted antigens by healthy and malignant T cells is a main drawback of mAb—or CAR-based immunotherapies for T-ALL and other T-cell malignancies. This review provides a comprehensive update on the different immunotherapeutic strategies that are being currently applied to T-ALL. We highlight recent progress on the identification of new potential targets showing promising preclinical results and discuss current challenges and opportunities for developing novel safe and efficacious immunotherapies for T-ALL.

25 Years of Collaboration with A Genius: Deciphering Adenine Nucleotide Ca2+ Mobilizing Second Messengers Together with Professor Barry Potter

Ca²⁺-mobilizing adenine nucleotide second messengers cyclic adenosine diphosphoribose, (cADPR), nicotinic acid adenine dinucleotide phosphate (NAADP), adenosine diphosphoribose (ADPR), and 2'deoxy-ADPR were discovered since the late 1980s. They either release Ca²⁺ from endogenous Ca²⁺ stores, e.g., endoplasmic reticulum or acidic organelles, or evoke Ca²⁺ entry by directly activating a Ca²⁺ channel in the plasma membrane. For 25 years, Professor Barry Potter has been one of the major medicinal chemists in this topical area, designing and contributing numerous analogues to develop structure-activity relationships (SAR) as a basis for tool development in biochemistry and cell biology and for lead development in proof-of-concept studies in disease models. With this review, I wish to acknowledge our 25-year-long collaboration on Ca²⁺-mobilizing adenine nucleotide second messengers as a major part of Professor Potter's scientific lifetime achievements on the occasion of his retirement in 2020.

NAD+ Metabolism as an Emerging Therapeutic Target for Cardiovascular Diseases Associated With Sudden Cardiac Death

In addition to its central role in mediating oxidation reduction in fuel metabolism and bioenergetics, nicotinamide adenine dinucleotide (NAD+) has emerged as a vital co-substrate for a number of proteins involved in diverse cellular processes, including sirtuins, poly(ADP-ribose) polymerases and cyclic ADP-ribose synthetases. The connection with aging and age-associated diseases has led to a new wave of research in the cardiovascular field. Here, we review the basics of NAD+ homeostasis, the molecular physiology and new advances in ischemic-reperfusion injury, heart failure, and arrhythmias, all of which are associated with increased risks for sudden cardiac death. Finally, we summarize the progress of NAD+-boosting therapy in human cardiovascular diseases and the challenges for future studies.

Direct Gating of the TRPM2 Channel by cADPR via Specific Interactions with the ADPR Binding Pocket

cADPR is a well-recognized signaling molecule by modulating the RyRs, but considerable debate exists regarding whether cADPR can bind to and gate the TRPM2 channel, which mediates oxidative stress signaling in diverse physiological and pathological processes. Here, we show that purified cADPR evoked TRPM2 channel currents in both whole-cell and cell-free single-channel recordings and specific binding of cADPR to the purified NUDT9-H domain of TRPM2 by surface plasmon resonance. Furthermore, by combining computational modeling with electrophysiological recordings, we show that the TRPM2 channels carrying point mutations at H1346, T1347, L1379, S1391, E1409, and L1484 possess distinct sensitivity profiles for ADPR and cADPR. These results clearly indicate cADPR is a bona fide activator at the TRPM2 channel and clearly delineate the structural basis for cADPR binding, which not only lead to a better understanding in the gating mechanism of TRPM2 channel but also shed light on a cADPR-induced RyRs-independent Ca²⁺ signaling mechanism.

A Novel Role of CD38 and Oxytocin as Tandem Molecular Moderators of Human Social Behavior

Oxytocin is an important modulator of human affiliative behaviors, including social skills, human pair bonding, and friendship. CD38 will be discussed as an immune marker and then in more detail the mechanisms of CD38 on releasing brain oxytocin. Mention is made of the paralogue of oxytocin, vasopressin, that has often overlapping and complementary functions with oxytocin on social behavior. Curiously, vasopressin does not require CD38 to be released from the brain. This review discusses the social salience hypothesis of oxytocin action, a novel view of how this molecule influences much of human social behaviors often in contradictory ways. The oxytocinergic-vasopressinergic systems are crucial modulators of broad aspects of human personality. Of special interest are studies of these two hormones in trust related behavior observed using behavioral economic games. This review also covers the role of oxytocin in parenting and parental attachment. In conclusion, the effects of oxytocin on human behavior depend on the individual's social context and importantly as well, the individual's cultural milieu, viz. East and West. ACRONYMS: ACC = Anterior Cingulate ADP = Adenosine diphosphate AQ = Autism Quotient cADPR = Cyclic ADP-ribose CNS = Central nervous system DA = Dopamine eQTLC = Expression Quantitative Trait Loci LC-NE = Locus Coeruleus-Norepinephrine MRI = Magnetic Resonance Imaging OFC = Orbitofrontal cortices OXT = Oxytocin RAGE = Receptor for advanced glycation end-products SARM1 = Sterile Alpha and toll/interleukin-1 receptor motif-containing 1 TRPM2= Transient Receptor Potential Cation Channel Subfamily M Member 2 AVP = Vasopressin.

Resolving the topological enigma in Ca2+ signaling by cyclic ADP-ribose and NAADP

Cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP) are two structurally distinct messengers that mobilize the endoplasmic and endolysosomal Ca2+ stores, respectively. Both are synthesized by the CD38 molecule (CD38), which has long been thought to be a type II membrane protein whose catalytic domain, intriguingly, faces to the outside of the cell. Accordingly, for more than 20 years, it has remained unresolved how CD38 can use cytosolic substrates such as NAD and NADP to produce messengers that target intracellular Ca2+ stores. The discovery of type III CD38, whose catalytic domain faces the cytosol, has now begun to clarify this topological conundrum. This article reviews the ideas and clues leading to the discovery of the type III CD38; highlights an innovative approach for uncovering its natural existence; and discusses the regulators of its activity, folding, and degradation. We also review the compartmentalization of cADPR and NAADP biogenesis. We further discuss the possible mechanisms that promote type III CD38 expression and appraise a proposal of a Ca2+-signaling mechanism based on substrate limitation and product translocation. The surprising finding of another enzyme that produces cADPR and NAADP, sterile α and TIR motif-containing 1 (SARM1), is described. SARM1 regulates axonal degeneration and has no sequence similarity with CD38 but can catalyze the same set of multireactions and has the same cytosolic orientation as the type III CD38. The intriguing finding that SARM1 is activated by nicotinamide mononucleotide to produce cADPR and NAADP suggests that it may function as a regulated Ca2+-signaling enzyme like CD38.

Therapeutic Opportunities with Pharmacological Inhibition of CD38 with Isatuximab

CD38 is a transmembrane glycoprotein with ectoenzymatic activity involved in regulation of migration, signal transduction, and receptor-mediated adhesion. CD38 is highly expressed on various malignant cells, including multiple myeloma (MM), and at relatively low levels in other tissues, making it a suitable target for therapeutic antibodies. Several anti-CD38 therapies have been, or are being, developed for the treatment of MM, including daratumumab and isatuximab (SAR650984), respectively. Studies have shown that anti-CD38 therapies are effective in the treatment of relapsed/refractory MM and are well tolerated, with infusion reactions being the most common side effects. They can be used as monotherapy or in combination with immunomodulatory agents, such as pomalidomide, or proteasome inhibitors to potentiate their activity. Here we examine isatuximab and several anti-CD38 agents in development that were generated using new antibody engineering techniques and that may lead to more effective CD38 targeting. We also summarize trials assessing these antibodies in MM, other malignancies, and solid organ transplantation. Finally, we propose that further research on the mechanisms of resistance to anti-CD38 therapy and the development of biomarkers and new backbone regimens with CD38 antibodies will be important steps in building more personalized treatment for patients with MM.

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.

Pyridine Nucleotide Metabolites and Calcium Release from Intracellular Stores

Ca²⁺ signals are probably the most common intracellular signaling cellular events, controlling an extensive range of responses in virtually all cells. Many cellular stimuli, often acting at cell surface receptors, evoke Ca²⁺ signals by mobilizing Ca²⁺ from intracellular stores. Inositol trisphosphate (IP₃) was the first messenger shown to link events at the plasma membrane to release Ca²⁺ from the endoplasmic reticulum (ER), through the activation of IP₃-gated Ca²⁺ release channels (IP₃ receptors). Subsequently, two additional Ca²⁺ 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 Ca²⁺ from the ER by activation of ryanodine receptors (RyRs), NAADP releases Ca²⁺ from acidic stores by a mechanism involving the activation of two pore channels (TPCs). In addition, other pyridine nucleotides have emerged as intracellular messengers. ADP-ribose and 2'-deoxy-ADPR both activate TRPM2 channels which are expressed at the plasma membrane and in lysosomes.

The transferrin receptor CD71 regulates type II CD38, revealing tight topological compartmentalization of intracellular cyclic ADP-ribose production

The CD38 molecule (CD38) catalyzes biogenesis of the calcium-mobilizing messenger cyclic ADP-ribose (cADPR). CD38 has dual membrane orientations, and type III CD38, with its catalytic domain facing the cytosol, has low abundance but is efficient in cyclizing cytosolic NAD to produce cADPR. The role of cell surface type II CD38 in cellular cADPR production is unknown. Here we modulated type II CD38 expression and assessed the effects of this modulation on cADPR levels. We developed a photoactivatable cross-linking probe based on a CD38 nanobody, and, combining it with MS analysis, we discovered that cell surface CD38 interacts with CD71. CD71 knockdown increased CD38 levels, and CD38 knockout reciprocally increased CD71, and both could be cocapped and coimmunoprecipitated. We constructed a chimera comprising the N-terminal segment of CD71 and a CD38 nanobody to mimic CD71's ligand property. Overexpression of this chimera induced a dramatically large decrease in CD38 via lysosomes. Remarkably, cellular cADPR levels did not decrease correspondingly. Bafilomycin-mediated blockade of lysosomal degradation greatly elevated active type II CD38 by trapping it in the lysosomes but also did not increase cADPR levels. Retention of type II CD38 in the endoplasmic reticulum (ER) by expressing an ER construct that prevented its transport to the cell surface likewise did not change cADPR levels. These results provide first and direct evidence that cADPR biogenesis occurs in the cytosol and is catalyzed mainly by type III CD38 and that type II CD38, compartmentalized in the ER or lysosomes or on the cell surface, contributes only minimally to cADPR biogenesis.

A Cell-Permeant Mimetic of NMN Activates SARM1 to Produce Cyclic ADP-Ribose and Induce Non-apoptotic Cell Death

SARM1, an NAD-utilizing enzyme, regulates axonal degeneration. We show that CZ-48, a cell-permeant mimetic of NMN, activated SARM1 in vitro and in cellulo to cyclize NAD and produce a Ca2+ messenger, cADPR, with similar efficiency as NMN. Knockout of NMN-adenylyltransferase elevated cellular NMN and activated SARM1 to produce cADPR, confirming NMN was its endogenous activator. Determinants for the activating effects and cell permeability of CZ-48 were identified. CZ-48 activated SARM1 via a conformational change of the auto-inhibitory domain and dimerization of its catalytic domain. SARM1 catalysis was similar to CD38, despite having no sequence similarity. Both catalyzed similar set of reactions, but SARM1 had much higher NAD-cyclizing activity, making it more efficient in elevating cADPR. CZ-48 acted selectively, activating SARM1 but inhibiting CD38. In SARM1-overexpressing cells, CZ-48 elevated cADPR, depleted NAD and ATP, and induced non-apoptotic death. CZ-48 is a specific modulator of SARM1 functions in cells.

Niacin in the Central Nervous System: An Update of Biological Aspects and Clinical Applications

Niacin (also known as "vitamin B₃" or "vitamin PP") includes two vitamers (nicotinic acid and nicotinamide) giving rise to the coenzymatic forms nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP). The two coenzymes are required for oxidative reactions crucial for energy production, but they are also substrates for enzymes involved in non-redox signaling pathways, thus regulating biological functions, including gene expression, cell cycle progression, DNA repair and cell death. In the central nervous system, vitamin B₃ has long been recognized as a key mediator of neuronal development and survival. Here, we will overview available literature data on the neuroprotective role of niacin and its derivatives, especially focusing especially on its involvement in neurodegenerative diseases (Alzheimer's, Parkinson's, and Huntington's diseases), as well as in other neuropathological conditions (ischemic and traumatic injuries, headache and psychiatric disorders).

Probing Ca2+ release mechanisms using sea urchin egg homogenates

Sea urchin eggs have been extensively used to study Ca²⁺ release through intracellular Ca²⁺-permeable channels. Their amenability to homogenization yields a robust, cell-free preparation that was central to establishing the Ca²⁺ mobilizing actions of cyclic ADP-ribose and NAADP. Egg homogenates have continued to provide insight into the basic properties and pharmacology of intracellular Ca²⁺ release channels. In this chapter, we describe methods for the preparation of egg homogenates and monitoring Ca²⁺ release using fluorimetry and radiotracer flux.

Synthesis and Biological Evaluation of a New Structural Simplified Analogue of cADPR, a Calcium-Mobilizing Secondary Messenger Firstly Isolated from Sea Urchin Eggs

Herein, we reported on the synthesis of cpIPP, which is a new structurally-reduced analogue of cyclic ADP-ribose (cADPR), a potent Ca2+-releasing secondary messenger that was firstly isolated from sea urchin eggs extracts. To obtain cpIPP the "northern" ribose of cADPR was replaced by a pentyl chain and the pyrophosphate moiety by a phophono-phosphate anhydride. The effect of the presence of the new phosphono-phosphate bridge on the intracellular Ca2+ release induced by cpIPP was assessed in PC12 neuronal cells in comparison with the effect of the pyrophosphate bridge of the structurally related cyclic N1-butylinosine diphosphate analogue (cbIDP), which was previously synthesized in our laboratories, and with that of the linear precursor of cpIPP, which, unexpectedly, revealed to be the only one provided with Ca2+ release properties.

NAD+ Intermediates: The Biology and Therapeutic Potential of NMN and NR

Research on the biology of NAD+ has been gaining momentum, providing many critical insights into the pathogenesis of age-associated functional decline and diseases. In particular, two key NAD+ intermediates, nicotinamide riboside (NR) and nicotinamide mononucleotide (NMN), have been extensively studied over the past several years. Supplementing these NAD+ intermediates has shown preventive and therapeutic effects, ameliorating age-associated pathophysiologies and disease conditions. Although the pharmacokinetics and metabolic fates of NMN and NR are still under intensive investigation, these NAD+ intermediates can exhibit distinct behavior, and their fates appear to depend on the tissue distribution and expression levels of NAD+ biosynthetic enzymes, nucleotidases, and presumptive transporters for each. A comprehensive concept that connects NAD+ metabolism to the control of aging and longevity in mammals has been proposed, and the stage is now set to test whether these exciting preclinical results can be translated to improve human health.

From insulin synthesis to secretion: Alternative splicing of type 2 ryanodine receptor gene is essential for insulin secretion in pancreatic β cells

Increases in the intracellular Ca²⁺ concentration in pancreatic islets, resulting from the Ca²⁺ mobilization from the intracellular source through the ryanodine receptor, are essential for insulin secretion by glucose. Cyclic ADP-ribose, a potent Ca²⁺ mobilizing second messenger synthesized from NAD⁺ by CD38, regulates the opening of ryanodine receptor. A novel ryanodine receptor mRNA (the islet-type ryanodine receptor) was found to be generated from the type 2 ryanodine receptor gene by the alternative splicing of exons 4 and 75. The islet-type ryanodine receptor mRNA is expressed in a variety of tissues such as pancreatic islets, cerebrum, cerebellum, and other neuro-endocrine cells, whereas the authentic type 2 ryanodine receptor mRNA (the heart-type ryanodine receptor) was found to be generated using GG/AG splicing of intron 75 and is expressed in the heart and the blood vessel. The islet-type ryanodine receptor caused a greater increase in the Ca²⁺ release by caffeine when expressed in HEK293 cells pre-treated with cyclic ADP-ribose, suggesting that the novel ryanodine receptor is an intracellular target for the CD38-cyclic ADP-ribose signal system in mammalian cells and that the tissue-specific alternative splicing of type 2 ryanodine receptor mRNA plays an important role in the functioning of the cyclic ADP-ribose-sensitive Ca²⁺ release.

Cytosolic interaction of type III human CD38 with CIB1 modulates cellular cyclic ADP-ribose levels

CD38 catalyzes the synthesis of the Ca²⁺ messenger, cyclic ADP-ribose (cADPR). It is generally considered to be a type II protein with the catalytic domain facing outside. How it can catalyze the synthesis of intracellular cADPR that targets the endoplasmic Ca²⁺ stores has not been resolved. We have proposed that CD38 can also exist in an opposite type III orientation with its catalytic domain facing the cytosol. Here, we developed a method using specific nanobodies to immunotarget two different epitopes simultaneously on the catalytic domain of the type III CD38 and firmly established that it is naturally occurring in human multiple myeloma cells. Because type III CD38 is topologically amenable to cytosolic regulation, we used yeast-two-hybrid screening to identify cytosolic Ca²⁺ and integrin-binding protein 1 (CIB1), as its interacting partner. The results from immunoprecipitation, ELISA, and bimolecular fluorescence complementation confirmed that CIB1 binds specifically to the catalytic domain of CD38, in vivo and in vitro. Mutational studies established that the N terminus of CIB1 is the interacting domain. Using shRNA to knock down and Cas9/guide RNA to knock out CIB1, a direct correlation between the cellular cADPR and CIB1 levels was demonstrated. The results indicate that the type III CD38 is functionally active in producing cellular cADPR and that the activity is specifically modulated through interaction with cytosolic CIB1.

The Plant Hormone Abscisic Acid Is a Prosurvival Factor in Human and Murine Megakaryocytes

Abscisic acid (ABA) is a phytohormone involved in pivotal physiological functions in higher plants. Recently, ABA has been proven to be also secreted and active in mammals, where it stimulates the activity of innate immune cells, mesenchymal and hematopoietic stem cells, and insulin-releasing pancreatic β cells through a signaling pathway involving the second messenger cyclic ADP-ribose (cADPR). In addition to behaving like an animal hormone, ABA also holds promise as a nutraceutical plant-derived compound in humans. Many biological functions of ABA in mammals are mediated by its binding to the LANCL-2 receptor protein. A putative binding of ABA to GRP78, a key regulator of endoplasmic reticulum stress, has also been proposed. Here we investigated the role of exogenous ABA in modulating thrombopoiesis, the process of platelet generation. Our results demonstrate that expression of both LANCL-2 and GRP78 is up-regulated during hematopoietic stem cell differentiation into mature megakaryocytes (Mks). Functional ABA receptors exist in mature Mks because ABA induces an intracellular Ca²⁺ increase ([Ca²⁺] _i ) through PKA activation and subsequent cADPR generation. In vitro exposure of human or murine hematopoietic progenitor cells to 10 μm ABA does not increase recombinant thrombopoietin (rTpo)-dependent Mk differentiation or platelet release. However, under conditions of cell stress induced by rTpo and serum deprivation, ABA stimulates, in a PKA- and cADPR-dependent fashion, the mitogen-activated kinase ERK 1/2, resulting in the modulation of lymphoma 2 (Bcl-2) family members, increased Mk survival, and higher rates of platelet production. In conclusion, we demonstrate that ABA is a prosurvival factor for Mks in a Tpo-independent manner.

Development of Ca2+-release mechanisms during oocyte maturation of the starfish Asterina pectinifera

An important step for successful fertilization and further development is the increase in intracellular Ca2+ in the activated oocyte. It has been known that starfish oocytes become increasingly sensitive to inositol 1,4,5-trisphosphate (IP3) during meiotic maturation to exhibit highly efficient IP3-induced Ca2+ release (IICR) by the time of germinal vesicle breakdown (GVBD). However, we noted that the peak level of intracellular Ca2+ increase after insemination is already high in the maturing oocytes before GVBD. Using maturing oocytes before GVBD, we investigated Ca2+ release mechanisms other than IICR. We report here that Ca2+-release mechanisms dependent on nicotinic acid adenine dinucleotide phosphate (NAADP) and nicotinamide adenine dinucleotide (NADP), the precursor of NAADP, became functional prior to the development of IICR mechanisms. As with IP3, but unlike NAADP, the Ca2+ stores responsive to NADP are sensitized during the meiotic maturation induced by 1-methyladenine (1-MA). This suggests that the process may represent a physiological response to the maturation hormone. NADP-dependent Ca2+ release in immature oocytes, however, did not induce oocyte maturation by itself, but was enhanced by the conditions mimicking the increases of intracellular Ca2+ and pH that take place in the maturing oocytes of starfish.

Metabotropic glutamate receptor-mediated cyclic ADP ribose signalling

Group I metabotropic glutamate receptors (I-mGluRs) modulate numerous cellular functions such as specific membrane currents and neurotransmitter release linked to their ability to mobilize calcium from intracellular calcium stores. As such, most I-mGluR research to date has focused on the coupling of these receptors to phospholipase C (PLC)-dependent and inositol (1,4,5) trisphosphate (IP3)-mediated calcium release via activation of IP3 receptors located upon the sarco/endoplasmic reticulum. However, there are now numerous examples of PLC- and IP3-independent I-mGluR-evoked signals, which may instead be mediated by activation of ryanodine receptors (RyRs). A prime candidate for mediating this coupling between I-mGluR activation and RyR opening is cyclic ADP ribose (cADPR) and, indeed, several of these PLC-/IP3-independent I-mGluR-evoked calcium signals have now been shown to be mediated wholly or partly by cADPR-evoked activation of RyRs. The contribution of cADPR signalling to I-mGluR-mediated responses is relatively complex, dependent as it is on factors such as cell type, excitation state of the cell and location of I-mGluRs on the cell. However, these factors notwithstanding, I-mGluR-mediated cADPR signalling remains poorly characterized, with several key aspects yet to be fully elucidated such as (1) the range of stimuli which evoke cADPR production, (2) the specific molecular mechanism(s) coupling cADPR to RyR activation and (3) the contribution of cADPR-mediated responses to downstream outputs such as synaptic plasticity. Furthermore, it is possible that the cADPR pathway may play a role in diseases underpinned by dysregulated calcium homoeostasis such as Alzheimer's disease (AD).

The CD38 genotype (rs1800561 (4693C>T): R140W) is associated with an increased risk of admission to the neonatal intensive care unit

BACKGROUNDS

Preterm birth (PTB)/admission to the neonatal intensive care unit (NICU) is a complex disorder associated with significant neonatal mortality and morbidity and long-term adverse health consequences. Multiple lines of evidence suggest that genetic factors play an important role in its etiology.

AIM

Given the role of CD38 in term delivery through oxytocin (OXT) release, we hypothesized that OXT signaling may play a role in the etiology of PTB/admission to the NICU. This study was designed to identify genetic variation in the CD38-oxytocin pathway associated with PTB/admission to the NICU.

METHODS

To identify common genetic variants predisposing individuals to PTB/admission to the NICU, we genotyped two single nucleotide polymorphisms (SNPs) in the CD38-oxytocin pathway in 63 case mothers, 55 control mothers, and 188 female volunteers in Nara Medical University Hospital, Japan.

RESULTS

Maternal genetic effect analysis of the SNP genotype data revealed a significant association between an SNP in CD38 (rs1800561 (4693C>T): R140W), which was reported to be correlated with diabetes and autism, and the risk of NICU admission. On the other hand, an SNP in the oxytocin receptor (OXTR) (rs2254298) showed no correlation with the risk of NICU admission.

CONCLUSION

Our study points to an association between maternal common polymorphisms in the CD38 (rs1800561) gene in Japanese women and susceptibility to PTB/admission to the NICU. Future studies with larger sample sizes are needed to confirm the findings of this study.

The human NAD metabolome: Functions, metabolism and compartmentalization

The metabolism of NAD has emerged as a key regulator of cellular and organismal homeostasis. Being a major component of both bioenergetic and signaling pathways, the molecule is ideally suited to regulate metabolism and major cellular events. In humans, NAD is synthesized from vitamin B3 precursors, most prominently from nicotinamide, which is the degradation product of all NAD-dependent signaling reactions. The scope of NAD-mediated regulatory processes is wide including enzyme regulation, control of gene expression and health span, DNA repair, cell cycle regulation and calcium signaling. In these processes, nicotinamide is cleaved from NAD⁺ and the remaining ADP-ribosyl moiety used to modify proteins (deacetylation by sirtuins or ADP-ribosylation) or to generate calcium-mobilizing agents such as cyclic ADP-ribose. This review will also emphasize the role of the intermediates in the NAD metabolome, their intra- and extra-cellular conversions and potential contributions to subcellular compartmentalization of NAD pools.

Antenatal betamethasone increases vascular reactivity to endothelin-1 by upregulation of CD38/cADPR signaling

Antenatal steroid administration is associated with hypertension in adult life; however, the mechanisms underlying this phenomenon are unclear. The aim of this study was to further characterize the effects of antenatal glucocorticoid exposure on the endothelin (ET-1) system, specifically to ascertain the role of the cyclic adenosine diphosphate ribose (cADPR)/ryanodine receptor pathway in the increased sensitivity to ET-1 observed in the offspring exposed to antenatal glucocorticoids. Pregnant sheep were randomly treated with betamethasone (Beta; 0.17 mg/kg) or vehicle at 80 and 81 days of gestation. In adults, we studied endothelium-denuded arterial segments of the brachial arteries. ET-1-induced vasoconstriction was significantly higher in the arteries from Beta sheep (F=3.5, P<0.05). Inhibition of ADP-ribosyl cyclase with 2-2'-dihydroxy-azobenzene significantly decreased the ET-1-induced contraction in Beta but not in vehicle-treated sheep. Nicotinamide attenuated ET-1 contraction in both, but it was significantly more pronounced in the Beta-treated sheep. No significant differences were observed following KCl-induced (6.25-75 mM) contraction. Nicotinamide (10 mM) significantly attenuated the KCl-induced vasoconstriction in both groups. In KCl (62.5 mM)-constricted arteries, the effect of nicotinamide (NIC) was significantly greater in the vehicle-treated sheep (50% relaxation v. 40% relaxation; t=2.2, P<0.05). In contrast, the sodium nitroprusside (SNP) relaxation was not statistically different. An additive effect was observed when NIC and SNP were used in combination and it was also more pronounced in vehicle-treated sheep. We conclude that the increased response to ET-1 is mediated by activation of the CD38/cADPR signaling pathway. Further studies are required to identify the effectors downstream from cADPR affected by exposure to antenatal steroids.

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.

NAD derived second messengers: Role in spontaneous diastolic Ca(2+) transients in murine cardiac myocytes

Strong β-adrenergic stimulation induced spontaneous diastolic Ca(2+) transients (SCTs) in electrically paced murine cardiac myocytes [28]. To obtain further insights into the underlying mechanism, we developed a method for a simultaneous analysis, in which the free luminal Ca(2+) concentration in the sarcoplasmic reticulum (SR) ([Ca(2+)]SR) and the free cytosolic Ca(2+) concentration ([Ca(2+)]i) were measured in parallel in the same cell. Each spontaneous diastolic Ca(2+) transient was exactly mirrored by a decrease of [Ca(2+)]SR. Since antagonism of the Ca(2+) mobilizing second messenger nicotinic acid adenine dinucleotide phosphate (NAADP) was shown to block SCTs in single cardiac myocytes [28], we analyzed the effect of the novel ADP-ribosyl cyclase inhibitor SAN4825 on both cytosolic and intra-luminal Ca(2+) transients upon strong β-adrenergic stimulation. A strong antagonist effect of SAN4825 on SCTs at low micromolar concentrations was observed. Our results suggest that the underlying mechanism of spontaneous diastolic Ca(2+) transients observed upon strong β-adrenergic stimulation is sensitization of type 2 ryanodine receptor by the Ca(2+) releasing activity of the products of ADP-ribosyl cyclase activity.

Determination of ADP-ribosyl cyclase activity, cyclic ADP-ribose, and nicotinic acid adenine dinucleotide phosphate in tissue extracts

Cyclic ADP-ribose (cADPR) is a novel second messenger that releases calcium from intracellular stores. Although first shown to release calcium in the sea urchin egg, cADPR has been shown since to be active in a variety of cells and tissues, from plant to human. cADPR stimulates calcium release via ryanodine receptors although the mechanism is still not completely understood. cADPR is produced enzymatically from NAD by ADP-ribosyl cyclases; several of these proteins have been identified including one isolated from Aplysia californica, two types found in mammals (CD38 and CD157), and three forms in sea urchin. A cyclase activity has been measured in extracts from Arabidopsis thaliana although the protein is still unidentified. Nicotinic acid adenine dinucleotide phosphate (NAADP) is another novel messenger that releases calcium from internal stores and is produced by these same enzymes by an exchange reaction. NAADP targets lysosomal stores whereas cADPR releases calcium from the endoplasmic reticulum. Due to their importance in cell signaling, cADPR and NAADP have been the focus of numerous investigations over the last 25 years. This chapter describes several assay methods for the measurements of cADPR and NAADP concentration and cyclase activity in extracts from cells.