N-acetyl-L-leucine for Niemann-Pick type C: a multinational double-blind randomized placebo-controlled crossover study

BACKGROUND

Niemann-Pick disease type C (NPC) is a rare autosomal recessive neurodegenerative lysosomal disease characterized by multiple symptoms such as progressive cerebellar ataxia and cognitive decline. The modified amino acid N-acetyl-leucine has been associated with positive symptomatic and neuroprotective, disease-modifying effects in various studies, including animal models of NPC, observational clinical case studies, and a multinational, rater-blinded phase IIb clinical trial. Here, we describe the development of a study protocol (Sponsor Code "IB1001-301") for the chronic treatment of symptoms in adult and pediatric patients with NPC.

METHODS

This multinational double-blind randomized placebo-controlled crossover phase III study will enroll patients with a genetically confirmed diagnosis of NPC patients aged 4 years and older across 16 trial sites. Patients are assessed during a baseline period and then randomized (1:1) to one of two treatment sequences: IB1001 followed by placebo or vice versa. Each sequence consists of a 12-week treatment period. The primary efficacy endpoint is based on the Scale for the Assessment and Rating of Ataxia, and secondary outcomes include cerebellar functional rating scales, clinical global impression, and quality of life assessments.

DISCUSSION

Pre-clinical as well as observational and phase IIb clinical trials have previously demonstrated that IB1001 rapidly improved symptoms, functioning, and quality of life for pediatric and adult NPC patients and is safe and well tolerated. In this placebo-controlled cross-over trial, the risk/benefit profile of IB1001 for NPC will be evaluated. It will also give information about the applicability of IB1001 as a therapeutic paradigm for other rare and common neurological disorders.

TRIAL REGISTRATIONS

The trial (IB1001-301) has been registered at www.

CLINICALTRIALS

gov (NCT05163288) and www.clinicaltrialsregister.eu (EudraCT: 2021-005356-10). Registered on 20 December 2021.

A master protocol to investigate a novel therapy acetyl-L-leucine for three ultra-rare neurodegenerative diseases: Niemann-Pick type C, the GM2 gangliosidoses, and ataxia telangiectasia

BACKGROUND

The lack of approved treatments for the majority of rare diseases is reflective of the unique challenges of orphan drug development. Novel methodologies, including new functionally relevant endpoints, are needed to render the development process more feasible and appropriate for these rare populations and thereby expedite the approval of promising treatments to address patients' high unmet medical need. Here, we describe the development of an innovative master protocol and primary outcome assessment to investigate the modified amino acid N-acetyl-L-leucine (Sponsor Code: IB1001) in three separate, multinational, phase II trials for three ultra-rare, autosomal-recessive, neurodegenerative disorders: Niemann-Pick disease type C (NPC), GM2 gangliosidoses (Tay-Sachs and Sandhoff disease; "GM2"), and ataxia telangiectasia (A-T).

METHODS/DESIGN

The innovative IB1001 master protocol and novel CI-CS primary endpoints were developed through a close collaboration between the Industry Sponsor, Key Opinion Leaders, representatives of the Patient Communities, and National Regulatory Authorities. As a result, the open-label, rater-blinded study design is considerate of the practical limitations of recruitment and retention of subjects in these ultra-orphan populations. The novel primary endpoint, the Clinical Impression of Change in Severity© (CI-CS), accommodates the heterogenous clinical presentation of NPC, GM2, and A-T: at screening, the principal investigator appoints for each patient a primary anchor test (either the 8-m walk test (8MWT) or 9-hole peg test of the dominant hand (9HPT-D)) based on his/her unique clinical symptoms. The anchor tests are videoed in a standardized manner at each visit to capture all aspects related to the patient's functional performance. The CI-CS assessment is ultimately performed by independent, blinded raters who compare videos of the primary anchor test from three periods: baseline, the end of treatment, and the end of a post-treatment washout. Blinded to the time point of each video, the raters make an objective comparison scored on a 7-point Likert scale of the change in the severity of the patient's neurological signs and symptoms from video A to video B. To investigate both the symptomatic and disease-modifying effects of treatment, N-acetyl-L-leucine is assessed during two treatment sequences: a 6-week parent study and 1-year extension phase.

DISCUSSION

The novel CI-CS assessment, developed through a collaboration of all stakeholders, is advantageous in that it better ensures the primary endpoint is functionally relevant for each patient, is able to capture small but meaningful clinical changes critical to the patients' quality of life (fine-motor skills; gait), and blinds the primary outcome assessment. The results of these three trials will inform whether N-acetyl-L-leucine is an effective treatment for NPC, GM2, and A-T and can also serve as a new therapeutic paradigm for the development of future treatments for other orphan diseases.

TRIAL REGISTRATION

The three trials (IB1001-201 for Niemann-Pick disease type C (NPC), IB1001-202 for GM2 gangliosidoses (Tay-Sachs and Sandhoff), IB1001-203 for ataxia telangiectasia (A-T)) have been registered at www.clinicaltrials.gov (NCT03759639; NCT03759665; NCT03759678), www.clinicaltrialsregister.eu (EudraCT: 2018-004331-71; 2018-004406-25; 2018-004407-39), and https://www.germanctr.de (DR KS-ID: DRKS00016567; DRKS00017539; DRKS00020511).

The role of calcium and nicotinic acid adenine dinucleotide phosphate (NAADP) in human osteoclast formation and resorption

Osteoclasts are specialised bone resorbing cells which form by fusion of circulating mononuclear phagocyte precursors. Bone resorption results in the release of large amounts of calcium into the extracellular fluid (ECF), but it is not certain whether changes in extracellular calcium concentration [Ca(2+)]e influence osteoclast formation and resorption. In this study, we sought to determine the effect of [Ca(2+)]e and NAADP, a potent calcium mobilising messenger that induces calcium uptake, on human osteoclast formation and resorption. CD14+ human monocytes were cultured with M-CSF and RANKL in the presence of different concentrations of calcium and NAADP and the effect on osteoclast formation and resorption evaluated. We found that the number of TRAP+ multinucleated cells and the extent of lacunar resorption were reduced when there was an increase in extracellular calcium and NAADP. This was associated with a decrease in RANK mRNA expression by CD14+ cells. At high concentrations (20 mM) of [Ca(2+)]e mature osteoclast resorption activity remained unaltered relative to control cultures. Our findings indicate that osteoclast formation is inhibited by a rise in [Ca(2+)]e and that RANK expression by mononuclear phagocyte osteoclast precursors is also [Ca(2+)]e dependent. Changes in NAADP also influence osteoclast formation, suggesting a role for this molecule in calcium handling. Osteoclasts remained capable of lacunar resorption, even at high ECF [Ca(2+)]e, in keeping with their role in physiological and pathological bone resorption.

NAADP, a new intracellular messenger that mobilizes Ca2+ from acidic stores

NAADP (nicotinic acid-adenine dinucleotide phosphate) is a recently described Ca2+-mobilizing molecule. First characterized in the sea urchin egg, it has been shown to mobilize Ca2+ from intracellular stores in a wide range of cells from different organisms. It is a remarkably potent molecule, and recent reports show that its cellular levels change in response to a variety of agonists, confirming its role as a Ca2+-mobilizing messenger. In many cases, NAADP appears to interact with other Ca2+-mobilizing messengers such as IP3 (inositol 1,4,5-trisphosphate) and cADP-ribose in shaping cytosolic Ca2+ signals. What is not clear is the molecular nature of the NAADP-sensitive Ca2+ release mechanism and its subcellular localization. This review focuses on the recent progress made in sea urchin eggs, which indicates that NAADP activates a novel Ca2+ release channel distinct from the relatively well-characterized IP3 and ryanodine receptors. Furthermore, in the sea urchin egg, the NAADP-sensitive store appears to be separate from the endoplasmic reticulum and is most likely an acidic store. These findings have also been reinforced by similar findings in mammalian cells, and a unified model for NAADP-induced Ca2+ signalling is presented.

The NAADP receptor: commentary on Billington et al

NAADP is a recently described calcium-mobilizing messenger. First discovered as a potent calcium-releasing molecule in sea urchin eggs, its actions have now been reported in several mammalian cell types. In the sea urchin egg, NAADP-sensitive calcium release channels appear distinct from inositol trisphosphate or ryanodine receptors, and are mainly localized to acidic compartments. In this study, Billington et al. extend the pharmacology of the putative NAADP receptor utilizing molecules unrelated to NAADP itself. This work may provide an important step in developing selective NAADP receptor modulators that will help define the role of NAADP in cell signalling.

Interactions between calcium release pathways: multiple messengers and multiple stores

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

Ca2+ release induced by cyclic ADP ribose in mice lacking type 3 ryanodine receptor

The action of cyclic-ADP-ribose was studied on calcium release from sarcoplasmic reticulum of skeletal muscles of neonatal and adult wild-type and RyR3-deficient mice. cADPR increased calcium efflux from microsomes, enhanced caffeine-induced calcium release, and, in 20% of the tests, triggered calcium release in single muscle fibers. These responses occurred only in the diaphragm of adult RyR3-deficient mice. cADPR action was abolished by ryanodine, ruthenium red, and 8-brome-cADPR. These results strongly favor a specific action of cADPR on RyR1. The responsiveness of RyR1 appears in adult muscles when RyR3 is lacking.

Pharmacological characterization of the putative cADP-ribose receptor

cADP-ribose (cADPR), a naturally occurring metabolite of NAD(+), has been shown to be an important regulator of intracellular Ca(2+) release. Considerable evidence suggests that cADPR is the endogenous modulator of the ryanodine receptor (RyR), which mediates Ca(2+)-induced Ca(2+) release (CICR). Indeed, cADPR-mediated Ca(2+) release is subject to functional regulation by other modulators of CICR, including Ca(2+), caffeine and calmodulin. However, the underlying basis behind the effect of such agents on cADPR activity (in particular whether they regulate cADPR binding), as well as the precise nature of the cADPR receptor remains unclear. In the present study, use of (32)P-radiolabelled cADPR has enabled a detailed pharmacological characterization of cADPR-binding sites in sea urchin egg homogenates. We report that cADPR binds specifically to a single class of high affinity receptor. Retainment of binding to membranes after a high-salt wash suggests the involvement of either an integral membrane protein (possibly the RyR itself) or a peripheral protein tightly associated to the membrane. Insensitivity of [(32)P]cADPR binding to either FK506 or rapamycin suggests that this does not concern the FK506-binding protein. Significantly, binding is highly robust, being relatively insensitive to both endogenous and pharmacological modulators of RyR-mediated CICR. In turn, this suggests that such agents modulate cADPR-mediated Ca(2+) release primarily by tuning the 'gain' of the CICR system, upon which cADPR acts, rather than influencing the interaction of cADPR with its target receptor. The exception to this is calmodulin, for which our results indicate an additional role in facilitating cADPR binding.

Distinct pharmacology of 2-hydroxycarbazole-induced Ca2+ release in the sea urchin egg

2-hydroxycarbazole, a compound structurally related to the Ca2+-mobilizing marine toxin 9-methyl-7-bromoeudistomin, has recently been proposed to activate both type 1 and type 2 ryanodine receptors in skeletal and cardiac muscle, respectively. This study was undertaken to evaluate the activity of this compound in the sea urchin egg homogenate, a model system used to characterize intracellular Ca2+ mobilization mechanisms. 2-Hydroxycarbazole was found to potently release Ca2+ in a concentration-dependent manner via a specific mechanism displaying apparent desensitization. Use of selective inhibitors of the Ca2+-mobilizing messengers inositol 1,4,5-trisphosphate, cyclic adenosine diphosphate ribose, and nicotinic acid adenine dinucleotide phosphate, as well as desensitization of homogenates to each of these molecules, failed to inhibit the response to 2-hydroxycarbazole. However, the response to 2-hydroxycarbazole was competitively antagonized by caffeine. Investigation of the Ca2+ stores accessed by 2-hydroxycarbazole revealed Ca2+ release from a thapsigargin-insensitive pool. Finally, 2-hydroxycarbazole failed to enhance [3H]ryanodine binding, suggesting the operation of a nonryanodine receptor mechanism. These results demonstrate that 2-hydroxycarbazole is acting to modulate a Ca2+ release mechanism with distinct pharmacological properties to those previously reported in the sea urchin egg.

Coordination of Ca2+ signalling by NAADP

Nicotinic acid adenine dinucleotide phosphate (NAADP) mobilizes intracellular Ca2+ stores in several cell types. Ample evidence suggests that NAADP activates intracellular Ca2+ channels distinct from those that are sensitive to inositol trisphosphate and ryanodine/cyclic ADP-ribose. Recent studies in intact cells have demonstrated functional coupling ('channel chatter') between Ca2+ release pathways mediated by NAADP, inositol trisphosphate and cyclic ADP-ribose. Thus, NAADP is probably an important determinant in shaping cytosolic Ca2+ signals.

NAADP induces Ca2+ oscillations via a two-pool mechanism by priming IP3- and cADPR-sensitive Ca2+ stores

In sea urchin eggs, Ca2+ mobilization by nicotinic acid adenine dinucleotide phosphate (NAADP) potently self-inactivates but paradoxically induces long-term Ca2+ oscillations. We investigated whether NAADP-induced Ca2+ oscillations arise from the recruitment of other Ca2+ release pathways. NAADP, inositol trisphosphate (IP3) and cyclic ADP-ribose (cADPR) all mobilized Ca2+ from internal stores but only NAADP consistently induced Ca2+ oscillations. NAADP-induced Ca2+ oscillations were partially inhibited by heparin or 8-amino-cADPR alone, but eliminated by the presence of both, indicating a requirement for both IP3- and cADPR-dependent Ca2+ release. Thapsigargin completely blocked IP3 and cADPR responses as well as NAADP-induced Ca2+ oscillations, but only reduced the NAADP-mediated Ca2+ transient. Following NAADP-mediated release from this Ca2+ pool, the amount of Ca2+ in the Ca2+-induced Ca2+ release stores was increased. These results support a mechanism in which Ca2+ oscillations are initiated by Ca2+ release from NAADP-sensitive Ca2+ stores (pool 1) and perpetuated through cycles of Ca2+ uptake into and release from Ca2+-induced Ca2+ release stores (pool 2). These results provide the first direct evidence in support of a two-pool model for Ca2+ oscillations.

Adp-ribosyl cyclase and cyclic ADP-ribose hydrolase act as a redox sensor. a primary role for cyclic ADP-ribose in hypoxic pulmonary vasoconstriction

Hypoxic pulmonary vasoconstriction is unique to pulmonary arteries and serves to match lung perfusion to ventilation. However, in disease states this process can promote hypoxic pulmonary hypertension. Hypoxic pulmonary vasoconstriction is associated with increased NADH levels in pulmonary artery smooth muscle and with intracellular Ca(2+) release from ryanodine-sensitive stores. Because cyclic ADP-ribose (cADPR) regulates ryanodine receptors and is synthesized from beta-NAD(+), we investigated the regulation by beta-NADH of cADPR synthesis and metabolism and the role of cADPR in hypoxic pulmonary vasoconstriction. Significantly higher rates of cADPR synthesis occurred in smooth muscle homogenates of pulmonary arteries, compared with homogenates of systemic arteries. When the beta-NAD(+):beta-NADH ratio was reduced, the net amount of cADPR accumulated increased. This was due, at least in part, to the inhibition of cADPR hydrolase by beta-NADH. Furthermore, hypoxia induced a 10-fold increase in cADPR levels in pulmonary artery smooth muscle, and a membrane-permeant cADPR antagonist, 8-bromo-cADPR, abolished hypoxic pulmonary vasoconstriction in pulmonary artery rings. We propose that the cellular redox state may be coupled via an increase in beta-NADH levels to enhanced cADPR synthesis, activation of ryanodine receptors, and sarcoplasmic reticulum Ca(2+) release. This redox-sensing pathway may offer new therapeutic targets for hypoxic pulmonary hypertension.

Prolonged inactivation of nicotinic acid adenine dinucleotide phosphate-induced Ca2+ release mediates a spatiotemporal Ca2+ memory

Although numerous extracellular stimuli are coupled to increases in intracellular Ca(2+), different stimuli are thought to achieve specificity by eliciting different spatiotemporal Ca(2+) increases. We investigated the effect of nicotinic acid adenine dinucleotide phosphate (NAADP) inactivation on spatiotemporal Ca(2+) signals in intact sea urchin eggs. The photorelease of NAADP but not inositol 1,4,5-trisphosphate or cyclic ADP-ribose resulted in self-inactivation. When NAADP was released first locally and subsequently globally, the spatial pattern of the first response shaped that of the second. Specifically, the local release of NAADP created a Ca(2+) gradient that was reversed during the subsequent global release of NAADP. Neither cyclic ADP-ribose nor inositol 1,4,5-trisphosphate showed a similar effect. In contrast to homogenates, NAADP inactivation was reversible in intact eggs with resensitization occurring in approximately 20 min. Because initial NAADP responses affect later responses, NAADP can serve as a mechanism for a Ca(2+) memory that has both spatial and temporal components. This NAADP-mediated Ca(2+) memory provides a novel mechanism for cells to control spatiotemporal Ca(2+) increases.

Unique kinetics of nicotinic acid-adenine dinucleotide phosphate (NAADP) binding enhance the sensitivity of NAADP receptors for their ligand

Nicotinic acid-adenine dinucleotide phosphate (NAADP) is a novel and potent Ca(2+)-mobilizing agent in sea urchin eggs and other cell types. Little is known, however, concerning the properties of the putative intracellular NAADP receptor. In the present study we have characterized NAADP binding sites in sea urchin egg homogenates. [(32)P]NAADP bound to a single class of high-affinity sites that were reversibly inhibited by NaCl but insensitive to pH and Ca(2+). Binding of [(32)P]NAADP was lost in preparations that did not mobilize Ca(2+) in response to NAADP, indicating that [(32)P]NAADP probably binds to a receptor mediating Ca(2+) mobilization. Addition of excess unlabelled NAADP, at various times after initiation of [(32)P]NAADP binding, did not result in displacement of bound [(32)P]NAADP. These data show that NAADP becomes irreversibly bound to its receptor immediately upon association. Accordingly, incubation of homogenates with low concentrations of NAADP resulted in maximal labelling of NAADP binding sites. This unique property renders NAADP receptors exquisitely sensitive to their ligand, thereby allowing detection of minute changes in NAADP levels.

Spatial control of Ca2+ signaling by nicotinic acid adenine dinucleotide phosphate diffusion and gradients

Intracellular Ca(2+) is able to control numerous cellular responses through complex spatiotemporal organization. Ca(2+) waves mediated by inositol trisphosphate or ryanodine receptors propagate by Ca(2+)-induced Ca(2+) release and therefore do not have an absolute requirement for a gradient in either inositol trisphosphate or cyclic ADP-ribose, respectively. In contrast, we report that although Ca(2+) increases induced by nicotinic acid adenine dinucleotide phosphate (NAADP) are amplified by Ca(2+)-induced Ca(2+) release locally, Ca(2+) waves mediated by NAADP have an absolute requirement for an NAADP gradient. If NAADP is increased such that its concentration is spatially uniform in one region of an egg, the Ca(2+) increase occurs simultaneously throughout this area, and only where there is diffusion out of this area to establish an NAADP gradient is there a Ca(2+) wave. A local increase in NAADP results in a Ca(2+) increase that spreads by NAADP diffusion. NAADP diffusion is restricted at low but not high concentrations of NAADP, indicating that NAADP diffusion is strongly influenced by binding to immobile and saturable sites, probably the NAADP receptor itself. Thus, the range of action of NAADP can be tuned by its concentration from that of a local messenger, like Ca(2+), to that of a global messenger, like IP(3) or cyclic ADP-ribose.

Widespread distribution of binding sites for the novel Ca2+-mobilizing messenger, nicotinic acid adenine dinucleotide phosphate, in the brain

Nicotinic acid adenine dinucleotide phosphate (NAADP) is a potent Ca(2+)-mobilizing agent in invertebrate eggs that has recently been shown to be active in certain mammalian and plant systems. Little, however, is known concerning the properties of putative NAADP receptors. Here, for the first time, we report binding sites for NAADP in brain. In contrast to sea urchin egg homogenates, [(32)P]NAADP bound reversibly to multiple sites in brain membranes. The rank order of potency of NAADP, 2',3'-cyclic NAADP and 3'-NAADP in displacing [(32)P]NAADP was, however, the same in the two systems and in agreement with their ability to mobilize Ca(2+) from homogenates. These data indicate that [(32)P]NAADP likely binds to receptors mediating Ca(2+) mobilization. Autoradiography revealed striking heterogeneity in the distribution of [(32)P]NAADP binding sites throughout the brain. Our data strongly support a role for NAADP-induced Ca(2+) signaling in the brain.

Two neuropeptides recruit different messenger pathways to evoke Ca2+ signals in the same cell

Bombesin and cholecystokinin (CCK) peptides act as signalling molecules in both the central nervous system and gastrointestinal tract [1-4]. It was reported recently that nicotinic acid adenine dinucleotide phosphate (NAADP) releases Ca2+ from mammalian brain microsomes [5] and triggers Ca2+ signals in pancreatic acinar cells, where it is proposed to mediate CCK-evoked Ca2+ signals [6]. Here, for the first time, we have finely resolved bombesin-induced cytosolic Ca2+ oscillations in single pancreatic acinar cells by whole-cell patch-clamp monitoring of Ca2+-dependent ionic currents [6-8]. Picomolar concentrations of bombesin and CCK evoked similar patterns of cytosolic Ca2+ oscillations, but high, desensitising, NAADP concentrations selectively inhibited CCK, but not bombesin-evoked signals. Inhibiting inositol trisphosphate (IP3) receptors with a high concentration of caffeine blocked both types of oscillations. We further tested whether NAADP is involved in Ca2+ signals triggered by activation of the low-affinity CCK receptor sites. Nanomolar concentrations of CCK evoked non-oscillatory Ca2+ signals, which were not affected by desensitising NAADP receptors. Our results suggest that Ca2+-release channels gated by the novel Ca2+-mobilising molecule NAADP are only essential in specific Ca2+-mobilising pathways, whereas the IP3 receptors are generally required for Ca2+ signals. Thus, the same cell may use different combinations of intracellular Ca2+-releasing messengers to encode different external messages.

Calcium release from the endoplasmic reticulum of higher plants elicited by the NADP metabolite nicotinic acid adenine dinucleotide phosphate

Higher plants share with animals a responsiveness to the Ca(2+) mobilizing agents inositol 1,4,5-trisphosphate (InsP(3)) and cyclic ADP-ribose (cADPR). In this study, by using a vesicular (45)Ca(2+) flux assay, we demonstrate that microsomal vesicles from red beet and cauliflower also respond to nicotinic acid adenine dinucleotide phosphate (NAADP), a Ca(2+)-releasing molecule recently described in marine invertebrates. NAADP potently mobilizes Ca(2+) with a K(1/2) = 96 nM from microsomes of nonvacuolar origin in red beet. Analysis of sucrose gradient-separated cauliflower microsomes revealed that the NAADP-sensitive Ca(2+) pool was derived from the endoplasmic reticulum. This exclusively nonvacuolar location of the NAADP-sensitive Ca(2+) pathway distinguishes it from the InsP(3)- and cADPR-gated pathways. Desensitization experiments revealed that homogenates derived from cauliflower tissue contained low levels of NAADP (125 pmol/mg) and were competent in NAADP synthesis when provided with the substrates NADP and nicotinic acid. NAADP-induced Ca(2+) release is insensitive to heparin and 8-NH(2)-cADPR, specific inhibitors of the InsP(3)- and cADPR-controlled mechanisms, respectively. However, NAADP-induced Ca(2+) release could be blocked by pretreatment with a subthreshold dose of NAADP, as previously observed in sea urchin eggs. Furthermore, the NAADP-gated Ca(2+) release pathway is independent of cytosolic free Ca(2+) and therefore incapable of operating Ca(2+)-induced Ca(2+) release. In contrast to the sea urchin system, the NAADP-gated Ca(2+) release pathway in plants is not blocked by L-type channel antagonists. The existence of multiple Ca(2+) mobilization pathways and Ca(2+) release sites might contribute to the generation of stimulus-specific Ca(2+) signals in plant cells.

Cyclic ADP ribose as a calcium-mobilizing messenger

This Perspective by Galione and Churchill is one in a series on intracellular calcium release mechanisms. The authors review the evidence for cyclic adenosine diphosphate ribose (cADPR) being a second messenger involved in regulating intracellular calcium. In addition, the physiological stimuli and responses mediated by cADPR are discussed. The Perspective is accompanied by a movie showing a calcium wave triggered by cADPR.

NAADP-induced calcium release in sea urchin eggs

Nicotinic acid adenine dinucleotide phosphate (NAADP) is the most potent activator of Ca2+ release from intracellular stores described. It acts on a mechanism distinct from inositol trisphosphate and ryanodine receptors, the two major Ca2+ release channels characterised. NAADP-gated Ca2+ release channels do not appear to be regulated by Ca2+ and may be better suited for triggering Ca2+ signals rather than propagating them. They exhibit a remarkable pharmacology for a putative intracellular Ca2+ release channel in that they are selectively blocked by potassium and L-type Ca2+ channel antagonists. Furthermore, in contrast to microsomal Ca2+ stores expressing IP3Rs and RyRs, those sensitive to NAADP are thapsigargin-insensitive, suggesting that they may be expressed on a different part of the endoplasmic reticulum. Perhaps the most unusual feature of the NAADP-gated Ca2+ release mechanisms is its inactivation properties. Unlike the mechanisms regulated by IP3 and cADPR in sea urchin eggs which after induction of Ca2+ release appear to become refractory to subsequent activation, very low concentrations of NAADP are able to inactivate NAADP-induced Ca2+ release fully at concentrations well below those required to activate Ca2+ release. The mechanism and physiological significance of this most unusual desensitisation phenomenon are unclear. More recently, NAADP has been shown to mobilise Ca2+ in ascidian oocytes, brain microsomes and pancreatic acinar cells suggesting a more widespread role in Ca2+ signalling. A possible role for this novel Ca2+ release mechanism in sea urchin egg fertilisation is discussed.

Microinjection of cyclic ADP-ribose triggers a regenerative wave of Ca2+ release and exocytosis of cortical alveoli in medaka eggs

Medaka (Oryzias latipes) eggs microinjected with the Ca(2+)-mobilising messenger cyclic adenosine diphosphate ribose (cADPR) underwent a wave of exocytosis of cortical alveoli and were thus activated. The number of eggs activated was sharply dependent on the concentration of cADPR in the pipette, the threshold concentration was approximately 60 nM. After injection, a pronounced latency preceded the onset of cortical alveoli exocytosis; this latency was independent of the concentration of cADPR but decreased markedly with increasing temperature. Heat-treated cADPR, which yields the inert non-cyclised product ADP-ribose, was ineffective in activating eggs. When cADPR was injected into aequorin-loaded eggs, a wave of luminescence arose at the site of cADPR injection and then swept out across the egg with a mean velocity of approximately 13 microns/s; the velocity was independent of the concentration of injected cADPR. In such a large cell (diameter of around 1 mm), this is considerably faster than that possible by simple diffusion of cADPR, which unambiguously demonstrates that cADPR must activate a regenerative process. cADPR has been demonstrated to modulate Ca(2+)-induced Ca2+ release (CICR) via ryanodine receptors (RyRs) in many cell types, and consistent with this was the finding that microinjection of the pharmacological RyR modulator, ryanodine, also activated medaka eggs. These results suggest that a cADPR-sensitive Ca2+ release mechanism is present in the medaka egg, that cADPR is the most potent activator of medaka eggs described to date, and that it activates eggs by triggering a wave of CICR from internal stores that in turn stimulates a wave of exocytosis.

Effects of photoreleased cADP-ribose on calcium transients and calcium sparks in myocytes isolated from guinea-pig and rat ventricle

Actions of photoreleased cADP-ribose (cADPR), a novel regulator of calcium-induced calcium release (CICR) from ryanodine-sensitive stores, were investigated in cardiac myocytes. Photoreleased cADPR caused an increase in the magnitude of whole-cell calcium transients studied in mammalian cardiac ventricular myocytes (both guinea-pig and rat) using confocal microscopy). Approx. 15 s was required following photorelease of cADPR for the development of its maximal effect. Photoreleased cADPR also increased the frequency of calcium 'sparks', which are thought to be elementary events which make up the whole-cell calcium transient, and were studied in rat myocytes, but had little or no effect on spark characteristics (amplitude, rise time, decay time and distance to half amplitude). The potentiating effects of photoreleased cADPR on both whole-cell transients and the frequency of calcium sparks were prevented by cytosolic application of the antagonist 8-amino-cADPR (5 microM). These experiments, therefore, provide the first evidence in any cell type for an effect of cADPR on calcium sparks, and are the first to show the actions of photoreleased cADPR on whole-cell calcium transients in mammalian cells. The observations are consistent with the effects of cADPR in enhancing the calcium sensitivity of CICR from the sarcoplasmic reticulum in cardiac ventricular myocytes, leading to an increase in the probability of occurrence of calcium sparks and to an increase in whole-cell calcium transients. The slow time-course for development of the full effect on whole-cell calcium transients might be taken to indicate that the influence of cADPR on CICR may involve complex molecular interactions rather than a simple direct action of cADPR on the ryanodine-receptor channels.

Induction of hippocampal LTD requires nitric-oxide-stimulated PKG activity and Ca2+ release from cyclic ADP-ribose-sensitive stores

Long-term depression (LTD) of synaptic transmission can be induced by several mechanisms, one thought to involve Ca2+-dependent activation of postsynaptic nitric oxide (NO) synthase and subsequent diffusion of NO to the presynaptic terminal. We used the stable NO donor S-nitroso-N-acetylpenicillamine (SNAP) to study the NO-dependent form of LTD at Schaffer collateral-CA1 synapses in vitro. SNAP (100 microM) enhanced the induction of LTD via a cascade that was blocked by the N-methyl-D-aspartate receptor antagonist D-2-amino-5-phosphonopentanoic acid (50 microM), NO guanylyl cyclase inhibitor 1H-[1,2,4] oxadiazolo [4,3-a] quinoxalin-1-one (10 microM), and the PKG inhibitor KT5823 (1 microM). We further show that LTD induced by low-frequency stimulation in the absence of SNAP also is blocked by KT5823 or Rp-8-(4-chlorophenylthio)-guanosine 3',5'-cyclic monophosphorothioate (10 microM), cyclic guanosine 3',5' monophosphate-dependent protein kinase (PKG) inhibitors with different mechanisms of action. Furthermore SNAP-facilitated LTD was blocked when release from intracellular calcium stores was inhibited by ryanodine (10 microM). Finally, two cell-permeant antagonists of the cyclic ADP-ribose binding site on ryanodine receptors also were able to block the induction of LTD. These results support a cascade for induction of homosynaptic, NO-dependent LTD involving activation of guanylyl cyclase, production of guanosine 3',5' cyclic monophosphate and subsequent PKG activation. This process has an additional requirement for release of Ca2+ from ryanodine-sensitive stores, perhaps dependent on the second-messenger cyclic ADP ribose.

Effects of glyburide-cyclosporin A interaction on interleukin-2 production in rats

The effects of simultaneous administrations of Cyclosporin A (CsA) and Glyburide on the immune system of rats has been evaluated in terms of Interleukin-2 (IL-2) production by Concanavalin A (ConA) stimulated splenocytes and exogenous IL-2 binding capacity. The inhibitory effect of Cyclosporin A on IL-2 production of lymphoid cells is well known. Spleen cells from rats receiving CsA had reduced levels of IL-2 when compared to untreated controls or rats receiving Glyburide only. Splenocytes from rats receiving both drugs had reduced levels of IL-2 when they were sacrificed 24 hours after one or three CsA administrations; instead when the animals were sacrificed 6 days after three CsA administrations, their ability of producing IL-2 is increased as well as increasing exogenous IL-2 binding capacity. These findings let us hypothesize that when there are lower concentrations of CsA in lymphocytes there is an increase of cellular metabolism induced by Glyburide that leads to an increase in IL-2 secretion and in IL-2 receptor expression on cellular surface restoring these levels to normal or slightly above normal levels.

Potentiation of cADPR-induced Ca(2+)-release by methylxanthine analogues

Caffeine and other methylxanthines are known to induce Ca(2+)-release from intracellular stores via the ryanodine receptor. In the present work, a range of caffeine analogues, in which methyl groups at the 1 and 7 positions were replaced with alkyl chains containing different functional groups (oxo, hydroxyl, propargyl, ester, and acids), were synthesized. These compounds were then screened for their ability to potentiate Ca(2+)-release induced by cADPR (an endogenous modulator of ryanodine receptors) in sea urchin egg homogenates. Two of the synthesized methylxanthines, 1, 3-dimethyl-7-(7-hydroxyoctyl)xanthine (37) and 3-methyl-7-(7-oxooctyl)-1-propargylxanthine (66), were shown to be more potent than caffeine in potentiating cADPR-induced Ca(2+)-release, while 1,3-dimethyl-7-(5-ethylcarboxypentyl)xanthine (14) was shown to be more efficacious. The development of new methylxanthine analogues may lead to a better understanding of ryanodine receptor function and could possibly provide novel therapeutic agents.

Nicotinic acid adenine dinucleotide phosphate triggers Ca2+ release from brain microsomes

Mobilization of Ca2+ from intracellular stores is an important mechanism for generating cytoplasmic Ca2+ signals [1]. Two families of intracellular Ca(2+)-release channels - the inositol-1,4, 5-trisphosphate (IP3) receptors and the ryanodine receptors (RyRs) - have been described in mammalian tissues [2]. Recently, nicotinic acid adenine dinucleotide phosphate (NAADP), a molecule derived from NADP+, has been shown to trigger Ca2+ release from intracellular stores in invertebrate eggs [3] [4] [5] [6] and pancreatic acinar cells [7]. The nature of NAADP-induced Ca2+ release is unknown but it is clearly distinct from the IP3- and cyclic ADP ribose (cADPR)-sensitive mechanisms in eggs (reviewed in [8] [9]). Furthermore, mammalian cells can synthesize and degrade NAADP, suggesting that NAADP-induced Ca2+ release may be widespread and thus contribute to the complexity of Ca2+ signalling [10] [11]. Here, we show for the first time that NAADP evokes Ca2+ release from rat brain microsomes by a mechanism that is distinct from those sensitive to IP3 or cADPR, and has a remarkably similar pharmacology to the action of NAADP in sea urchin eggs [12]. Membranes prepared from the same rat brain tissues are able to support the synthesis and degradation of NAADP. We therefore suggest that NAADP-mediated Ca2+ signalling could play an important role in neuronal Ca2+ signalling.

The relative order of IP3 sensitivity of types 1 and 3 IP3 receptors is pH dependent

The type-3 inositol 1,4,5-trisphosphate (IP3) receptor, in contrast to the type-1 IP3 receptor (IP3R), is not stimulated by sulfhydryl oxidation and is less sensitive to adenosine 5'-triphosphate. In the present study we compared the effect of pH on the Ca2+ release induced by IP3 and cytosolic Ca2+ between IP3R3-expressing 16HBE14o- cells and IP3R1-expressing A7r5 cells. Changing pH from 6.8 to 7.5 decreased the IP3 concentration required for half-maximal stimulation of IP3R3 (EC50) 10.7-fold (from 2.14 to 0.20 microM). Similar alkalinization decreased the IP3 concentration (EC50) for stimulation of IP3R1 only 2.5-fold (from 0.87 to 0.35 microM). IP3R1 is therefore the more sensitive isoform at pH 6.8, while IP3R3 is more sensitive at pH 7.5. Stimulation and inhibition of IP3R1 and -3 by low and high cytosolic [Ca2+] respectively was observed at both pH 6.8 and 7.5. Increasing [H+] shifted the Ca2+-activation curve of IP3R1 towards higher [Ca2+] but did not affect the Ca2+ dependence of IP3R3. We conclude that IP3R1 and -3 differ markedly in their response to protons.

An antagonist of cADP-ribose inhibits arrhythmogenic oscillations of intracellular Ca2+ in heart cells

Oscillations of Ca2+ in heart cells are a major underlying cause of important cardiac arrhythmias, and it is known that Ca2+-induced release of Ca2+ from intracellular stores (the sarcoplasmic reticulum) is fundamental to the generation of such oscillations. There is now evidence that cADP-ribose may be an endogenous regulator of the Ca2+ release channel of the sarcoplasmic reticulum (the ryanodine receptor), raising the possibility that cADP-ribose may influence arrhythmogenic mechanisms in the heart. 8-Amino-cADP-ribose, an antagonist of cADP-ribose, suppressed oscillatory activity associated with overloading of intracellular Ca2+ stores in cardiac myocytes exposed to high doses of the beta-adrenoreceptor agonist isoproterenol or the Na+/K+-ATPase inhibitor ouabain. The oscillations suppressed by 8-amino-cADP-ribose included intracellular Ca2+ waves, spontaneous action potentials, after-depolarizations, and transient inward currents. Another antagonist of cADP-ribose, 8-bromo-cADP-ribose, was also effective in suppressing isoproterenol-induced oscillatory activity. Furthermore, in the presence of ouabain under conditions in which there was no arrhythmogenesis, exogenous cADP-ribose was found to be capable of triggering spontaneous contractile and electrical activity. Because enzymatic machinery for regulating the cytosolic cADP-ribose concentration is present within the cell, we propose that 8-amino-cADP-ribose and 8-bromo-cADP-ribose suppress cytosolic Ca2+ oscillations by antagonism of endogenous cADP-ribose, which sensitizes the Ca2+ release channels of the sarcoplasmic reticulum to Ca2+.

Evidence of a role for cyclic ADP-ribose in long-term synaptic depression in hippocampus

Ca2+ released from presynaptic and postsynaptic intracellular stores plays important roles in activity-dependent synaptic plasticity, including long-term depression (LTD) of synaptic strength. At Schaffer collateral-CA1 synapses in the hippocampus, presynaptic ryanodine receptor-gated stores appear to mobilize some of the Ca2+ necessary to induce LTD. Cyclic ADP-ribose (cADPR) has recently been proposed as an endogenous activator of ryanodine receptors in sea urchin eggs and several mammalian cell types. Here, we provide evidence that cADPR-mediated signaling pathways play a key role in inducing LTD. We show that biochemical production of cGMP increases cADPR concentration in hippocampal slices in vitro, and that blockade of cGMP-dependent protein kinase, cADPR receptors, or ryanodine-sensitive Ca2+ stores each prevent the induction of LTD at Schaffer collateral-CA1 synapses. A lack of effect of postsynaptic infusion of either cADPR antagonist indicates a probable presynaptic site of action.

Coordination of agonist-induced Ca2+-signalling patterns by NAADP in pancreatic acinar cells

Many hormones and neurotransmitters evoke Ca2+ release from intracellular stores, often triggering agonist-specific signatures of intracellular Ca2+ concentration. Inositol trisphosphate (InsP3) and cyclic adenosine 5'-diphosphate-ribose (cADPR) are established Ca2+-mobilizing messengers that activate Ca2+ release through intracellular InsP3 and ryanodine receptors, respectively. However, in pancreatic acinar cells, neither messenger can explain the complex pattern of Ca2+ signals triggered by the secretory hormone cholecystokinin (CCK). We show here that the Ca2+-mobilizing molecule nicotinic acid adenine dinucleotide phosphate (NAADP), an endogenous metabolite of beta-NADP, triggers a Ca2+ response that varies from short-lasting Ca2+ spikes to a complex mixture of short-lasting (1-2s) and long-lasting (0.2-1 min) Ca2+ spikes. Cells were significantly more sensitive to NAADP than to either cADPR or InsP3, whereas higher concentrations of NAADP selectively inactivated CCK-evoked Ca2+ signals in pancreatic acinar cells, indicating that NAADP may function as an intracellular messenger in mammalian cells.

ADP-ribose gates the fertilization channel in ascidian oocytes

We report an ion channel in the plasma membrane of unfertilized oocytes of the ascidian Ciona intestinalis that is directly gated by the second messenger ADP-ribose. The ion channel is permeable to Ca2+ and Na+ and is characterized by a reversal potential between 0 and +20 mV and a unitary conductance of 140 pS. Preinjection of the Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) or antagonists of intracellular Ca2+ release channels into oocytes did not inhibit the ADP-ribose current, demonstrating that the channel is activated in a Ca2+-independent manner. Both the fertilization current and the current induced by the injection of nicotinamide nucleotides are blocked by nicotinamide, suggesting that the ADP-ribose channel is activated at fertilization in a nicotinamide-sensitive manner. These data suggest that ascidian sperm trigger the hydrolysis of nicotinamide nucleotides in the oocyte to ADP-ribose and that this mechanism is responsible for the production of the fertilization current.

Mechanisms of calcium release and sequestration in eggs of Chaetopterus pergamentaceus

Increases in the intracellular free calcium concentration are of great importance to the initiation of development in deuterostomes. Their involvement has not yet been clearly defined in protostomes. We used endogenous ligands (IP3, cADPR, ryanodine and NAADP) and pharmacological agents (thapsigargin [Tg], thimerosal, caffeine and heparin) to study smooth endoplasmic reticulum Ca2+ pump and release mechanisms in eggs of an annelid, Chaetopterus. Oocyte homogenates effectively sequestered Ca2+ and released it in response to IP3 in a concentration-dependent manner. Repeated additions of IP3 were unable to cause further release. Heparin inhibited Ca2+ release in response to IP3. The homogenates also released Ca2+ in response to thimerosal, and this release was sensitive to heparin. Two antibodies to IP3 receptors recognized an appropriate band in Chaetopterus egg lysates. These results indicate that the oocytes possess type-1 IP3-gated Ca2+ channels. Neither calcium itself, nor strontium, cADPR, ryanodine, caffeine nor NAADP released appreciable Ca2+. At low concentrations, Tg caused a slow release of Ca2+; at higher concentrations, it elicited a rapid release. Release of Ca2+ by Tg activated development. Since one theory of fertilization invokes the introduction of a Ca2+ releasing soluble protein into the egg upon sperm-egg fusion, we also tested whether soluble extracts of Chaetopterus sperm could stimulate Ca2+ release in Chaetopterus egg homogenates. There was no Ca2+ release when the sperm extract was added to the homogenate; however, homogenates exposed to sperm extract became refractory to IP3. Thus, Ca2+ release at fertilization in these oocytes occurs through IP3-gated channels.

Differential regulation of nicotinic acid-adenine dinucleotide phosphate and cADP-ribose production by cAMP and cGMP

The sea urchin egg has been used as a system to study calcium-release mechanisms induced by inositol 1,4,5-trisphosphate (IP3), cADP-ribose (cADPR), and more recently, nicotinic acid-adenine dinucleotide phosphate (NAADP). In order that cADPR and NAADP may be established as endogenous messengers for calcium release, the existence of intracellular enzymes capable of metabolizing these molecules must be demonstrated. In addition, intracellular levels of cADPR and NAADP should be under the control of extracellular stimuli. It has been shown that cGMP stimulates the synthesis of cADPR in the sea urchin egg. The present study shows that the sea urchin egg is capable of synthesizing and degrading NAADP. cADPR and NAADP synthetic activities appear to be separate, with different cellular localizations, pH and temperature optima. We suggest that in the sea urchin egg, cADPR and NAADP production may be differentially regulated by receptor-coupled second messengers, with cADPR production being regulated by cGMP and NAADP production modulated by cAMP.

Cyclic ADP-ribose and the regulation of calcium-induced calcium release in eggs and cardiac myocytes

Cyclic ADP-ribose (cADPR) is a cyclic metabolite of NAD+ synthesised in cells and tissues expressing ADP-ribosyl cyclases. Although it was first discovered in sea-urchin egg extracts as a potent calcium mobilizing agent, subsequent studies have indicated that it may have a widespread action in the activation of calcium-release channels in such diverse systems as mammalian neurones, myocytes, blood cells, eggs, and plant microsomes. In this review we focus on recent work suggesting that cADPR enhances the sensitivity of ryanodine-sensitive calcium-release channels (RyRs) to activation by calcium, a phenomenon termed calcium-induced calcium release (CICR). Two roles for cADPR in calcium signaling are discussed. The first is as a classical second messenger where its levels are controlled by extracellular stimuli, and the second mode of cellular regulation is that the levels of intracellular cADPR may set the sensitivity of RyRs to activation by an influx of calcium in excitable cells. These two possible actions of cADPR are illustrated by considering the signal transduction events during the fertilization of the sea-urchin egg and the modulation of CICR during excitation-coupling in isolated guinea-pig ventricular myocytes, respectively.

A cytosolic sperm protein factor mobilizes Ca2+ from intracellular stores by activating multiple Ca2+ release mechanisms independently of low molecular weight messengers

Ca2+ oscillations can be induced in mammalian eggs and somatic cells by microinjection of a cytosolic sperm protein factor. The nature of the sperm factor-induced Ca2+ signaling was investigated by adding sperm protein extracts to homogenates of sea urchin eggs, which contain multiple classes of Ca2+ release mechanisms. We show that the sperm factor mobilizes Ca2+ from non-mitochondrial Ca2+ stores in egg homogenates after a distinct latency. This latency is abolished by preincubation of sperm extracts with egg cytosol. The preincubation step is highly temperature-dependent and generates a high molecular weight, protein-based Ca2+-releasing agent that can also mobilize Ca2+ from purified egg microsomes. This Ca2+ release appears to be mediated via both inositol 1,4,5-trisphosphate and ryanodine receptors, since homologous desensitization of these two release mechanisms by their respective agonists inhibits further release by the sperm factor. However, sperm factor-induced Ca2+ release by these channels is independent of inositol 1,4, 5-trisphosphate or cADPR since antagonists of either of these two messengers did not block the Ca2+ release effected by the sperm factor. The sperm protein factor may cause Ca2+ release via an enzymatic step that generates a protein-based Ca2+-releasing agent.

Actions of cADP-ribose and its antagonists on contraction in guinea pig isolated ventricular myocytes. Influence of temperature

Although it is becoming widely accepted that cADP-ribose (cADPR) can regulate calcium release from the endoplasmic reticulum in sea urchin eggs and in a variety of mammalian cell types, it remains controversial whether this substance might influence calcium release during excitation-contraction coupling in cardiac muscle. We have investigated possible actions of cADPR in intact cells isolated from guinea pig ventricle, paying particular attention to the possible influence of temperature. At 36 degrees C, myocyte contraction was influenced by cytosolic application of cADPR in a concentration-dependent manner (showing an approximately 30% increase in contraction with 5 mumol/L cADPR applied via a patch pipette in myocytes stimulated to fire action potentials at 1 Hz). Calcium transients measured with fura 2 were also increased by 5 mumol/L cADPR. Antagonists of cADPR reduced contraction at 36 degrees C (by approximately 35% with either 50 mumol/L 8-Br-cADPR or 5 mumol/L 8-amino-cADPR applied via the patch pipette). At room temperature (approximately 20 degrees C to 24 degrees C), no significant effects on contraction were detected with either cADPR or its antagonists. At 36 degrees C, treatment of the cells with a mixture of 2 mumol/L ryanodine and 1 mumol/L thapsigargin to suppress function of the sarcoplasmic reticulum stores of calcium prevented the action of 5 mumol/L cADPR applied via a patch pipette. These observations are consistent with an action of cytosolic cADPR to enhance calcium-induced calcium release from the sarcoplasmic reticulum in guinea pig ventricular myocytes at 36 degrees C. The observed influence of temperature under the conditions of our experiments is one factor that might help to account for failure to detect actions of cADPR and its analogues in some previous studies.

Nitric oxide gates fertilization channels in ascidian oocytes through nicotinamide nucleotide metabolism

In this paper we use the nitric oxide (NO) donor sodium nitroprusside to examine the response of the unfertilised oocyte of the ascidian Ciona intestinalis to nitric oxide. We show that the release of NO triggers an inward current that displays similar properties to the ascidian fertilisation current. Furthermore, the production of NO causes the release of intracellular calcium through a ruthenium-red sensitive mechanism. Our data suggest that these effects are due to the stimulation of nicotinamide nucleotide metabolism, but the active second messenger is not cyclic adenosine diphosphate ribose (cADPr). Finally, we show that NO production increases at fertilisation. The results suggest that ascidian sperm trigger the release of NO and this second messenger causes the breakdown of nicotinamide nucleotides leading to the production of a second messenger which induces the fertilisation current and may assist in the production of the increase in calcium.

Cyclic ADP-ribose enhances coupling between voltage-gated Ca2+ entry and intracellular Ca2+ release

Ca2+ release from intracellular stores can be activated in neurons by influx of Ca2+ through voltage-gated Ca2+ channels. This process, called Ca2+-induced Ca2+ release, relies on the properties of the ryanodine receptor and represents a mechanism by which Ca2+ influx during neuronal activity can be amplified into large intracellular Ca2+ signals. In a differentiated neuroblastoma cell line, we show that caffeine, a pharmacological activator of the ryanodine receptor, released Ca2+ from intracellular stores in a Ca2+-dependent and ryanodine-sensitive manner. The pyridine nucleotide, cyclic ADP-ribose, thought to be an endogenous modulator of ryanodine receptors also amplified Ca2+-induced Ca2+ release in these neurons. Cyclic ADP-ribose enhanced the total cytoplasmic Ca2+ levels during controlled Ca2+ influx through voltage gated channels, in a concentration-dependent and ryanodine-sensitive manner and also increased the sensitivity with which a small amount of Ca2+ influx could trigger additional release from the ryanodine-sensitive intracellular Ca2+ stores. Single cell imaging showed that following the Ca2+ influx, cyclic ADP-ribose enhanced the spatial spread of the Ca2+ signal from the edge of the cell into its center. These powerful actions suggest a role for cyclic ADP-ribose in the functional coupling of neuronal depolarization, Ca2+ entry, and global intracellular Ca2+ signaling.

Roles for adenosine ribose hydroxyl groups in cyclic adenosine 5'-diphosphate ribose-mediated Ca2+ release

Cyclic adenosine diphosphate ribose (cADPR) is a naturally occurring and potent Ca2+-mobilizing agent. Structural analogues are currently required as pharmacological tools for the investigation of this topical molecule, but modifications to date have concentrated primarily upon the purine ring. Two novel dehydroxylated analogues of cADPR have now been prepared from chemically synthesized nicotinamide adenine dinucleotide (NAD+) precursors modified in the ribose moiety linked to adenine. ADP-ribosyl cyclase of Aplysia californica catalyzed the conversion of 2'A-deoxy-NAD+ and 3'A-deoxy-NAD+ into the corresponding 2'A-deoxy-cADPR and 3'A-deoxy-cADPR analogues, respectively. These analogues were used to assess the effect of 2'- and 3'-hydroxyl group deletion in the adenosine ribose moiety of cADPR on the Ca2+-releasing potential of cADPR. These compounds were found to have comparatively markedly different activities as agonists for Ca2+ mobilization in sea urchin egg homogenate. 2'A-Deoxy-cADPR is similar to cADPR, whereas 3'A-deoxy-cADPR is at least 100-fold less potent, indicating that the 3'A-hydroxyl group, but not the 2'A-hydroxyl group, is essential for calcium releasing activity. EC50 values recorded were 32 nM, 58 nM, and 5 microM for cADPR, 2'A-deoxy-cADPR, and 3'A-deoxy-cADPR, respectively. Moreover, 200 nM 2'A-deoxy-cADPR was required to desensitize the cADPR-sensitive Ca2+ channel to a subsequent addition of 100 nM cADPR, but 20 microM 3'A-deoxy-cADPR was required to produce the same desensitizing effect. This is in accordance with the 100-fold lower potency exhibited by the latter analogue. To further investigate the importance of the 3'-hydroxyl group, we have also synthesized 3'A-O-methyl-cADPR, in which the 3'-hydroxyl group of adenosine has been methylated and its ability potentially to donate a hydrogen atom in a hydrogen bond has been removed. Although inactive in releasing Ca2+, 3'A-O-methyl-cADPR inhibited cADPR-induced Ca2+ release in a dose-dependent manner with an approximate IC50 value of 5 microM, whereas 3'-O-methyladenosine had no effect. This further supports the requirement of a 3'-OH group for Ca2+ releasing activity. In addition, however, it suggests that this group may not be crucial for ligand-receptor recognition. Thus, replacement of the hydrogen atom of the hydroxyl with a methyl group effects a change of activity from an agonist to an antagonist of cADPR-induced Ca2+ release. Two other analogues with modifications in the 2' and/or 3' positions, 3'-cADPR phosphate and 2',3'-cyclic-cADPR phosphate, were synthesized and tested for their Ca2+-mobilizing activity in sea urchin egg homogenates. Both analogues were inactive with respect to both agonistic and antagonistic activities on the cADPR-sensitive Ca2+ release mechanism. These are the first steps toward a wider structure-activity relationship for cADPR, and this is the first study to implicate a crucial role for the adenosine ribose hydroxyl groups of cADPR in the biological activity of this cyclic nucleotide. Additionally, this is the first report of a cADPR receptor antagonist that is not modified at the 8-position of the purine ring.

Pharmacological properties of the Ca2+-release mechanism sensitive to NAADP in the sea urchin egg

1. The sea urchin egg homogenate is an ideal model to characterize Ca2+-release mechanisms because of its reliability and high signal-to-noise-ratio. Apart from the InsP3- and ryanodine-sensitive Ca2+-release mechanisms, it has been recently demonstrated that this model is responsive to a third independent mechanism, that has the pyridine nucleotide, nicotinic acid adenine dinucleotide phosphate (NAADP), as an endogenous agonist. 2. The sea urchin egg homogenate was used to characterize the pharmacological and biochemical characteristics of the novel Ca2+-releasing agent, NAADP, compared to inositol trisphosphate (InsP3) and cyclic ADP ribose (cyclic ADPR), an endogenous activator of ryanodine receptors. 3. NAADP-induced Ca2+-release was blocked by L-type Ca2+-channel blockers and by Bay K 8644, while InsP3- and cyclic ADPR-induced Ca2+-release were insensitive to these agents. L-type Ca2+-channel blockers did not displace [32P]-NAADP binding, suggesting that their binding site was different. Moreover, stopped-flow kinetic studies revealed that these agents blocked NAADP in a all-or-none fashion. 4. Similarly, a number of K+-channel antagonists blocked NAADP-induced Ca2+-release selectively over InsP3- and cyclic ADPR-induced Ca2+-release. Radioligand studies showed that these agents were not competitive antagonists. 5. As has been shown for InsP3 and ryanodine receptors, NAADP receptors were sensitive to calmodulin antagonists, suggesting that this protein could be a common regulatory feature of intracellular Ca2+-release mechanisms. 6. The presence of K+ was not essential for NAADP-induced Ca2+-release, since substitution of K+ with other monovalent cations in the experimental media did not significantly alter Ca2+ release by NAADP. On the contrary, cyclic ADPR and InsP3-sensitive mechanisms were affected profoundly, although to a different extent depending on the monovalent cation which substituted for K+. Similarly, modifications of the pH in the experimental media from 7.2 to 6.7 or 8.0 only slightly affected NAADP-induced Ca2+-release. While the alkaline condition permitted InsP3 and cyclic ADPR-induced Ca2+-release, the acidic condition completely hampered both Ca2+-release mechanisms. 7. The present results characterize pharmacologically and biochemically the novel Ca2+-release mechanism sensitive to NAADP. Such characterization will help future research aimed at understanding the role of NAADP in mammalian systems.

7-Deaza-8-bromo-cyclic ADP-ribose, the first membrane-permeant, hydrolysis-resistant cyclic ADP-ribose antagonist

Cyclic ADP-ribose (cADPR) is a putative second messenger that has been demonstrated to mobilize Ca2+ in many cell types. Its postulated role as the endogenous regulator of ryanodine-sensitive Ca2+ release channels has been greatly supported by the advent and use of specific cADPR receptor antagonists such as 8-NH2-cADPR (Walseth, T. F., and Lee, H. C. (1993) Biochim. Biophys. Acta 1178, 235-242). However, investigations of the role of cADPR in physiological responses, such as fertilization, stimulus-secretion coupling, and excitation-contraction coupling, have been hindered by the susceptibility of cADPR receptor antagonists to hydrolysis and the need to introduce these molecules into cells by microinjection or patch clamp techniques. We have recently reported on the discovery of a poorly hydrolyzable analogue of cADPR, 7-deaza-cADPR (Bailey, V. C., Sethi, J. K., Fortt, S. M., Galione, A., and Potter, B. V. L. (1997) Chem. Biol. 4, 41-51) but this, like cADPR, is an agonist of ryanodine-sensitive Ca2+ release channels. We therefore explored the possibility of combining antagonistic activity with that of hydrolytic resistance and now report on the biological properties of the first hydrolysis-resistant cADPR receptor antagonist, 7-deaza-8-bromo-cADPR. In addition this compound has the advantage of being membrane-permeable. Together these properties make this hybrid molecule the most powerful tool to date for studying cADPR-mediated Ca2+ signaling in intact cells.

Kinetic properties of nicotinic acid adenine dinucleotide phosphate-induced Ca2+ release

Three endogenous molecules have now been shown to release Ca2+ in the sea urchin egg: inositol trisphosphate (InsP3), cyclic adenosine 5'-diphosphate ribose (cADPR), and nicotinic acid adenine dinucleotide phosphate (NAADP), a derivative of NADP. While the mechanism through which the first two molecules are able to release Ca2+ is established and well characterized with InsP3 and cADPR-activating InsP3 and ryanodine receptors, respectively, the newly described NAADP has been shown to release Ca2+ via an entirely different mechanism. The most striking feature of this novel Ca2+ release mechanism is its inactivation, since subthreshold concentrations of NAADP are able to fully and irreversibly desensitize the channel. In the present study we have investigated the fast kinetics of activation and inactivation of NAADP-induced Ca2+ release. NAADP was found to release Ca2+ in a biphasic manner, and such release was preceded by a pronounced latent period, which was inversely dependent on concentration. Moreover, the kinetic features of NAADP-induced Ca2+ release were not altered by pretreatment with low concentrations of NAADP, although the extent of Ca2+ release was greatly affected. Our data suggest that the inactivation of NAADP-induced Ca2+ release is an all-or-none phenomenon, and while some receptors have been fully inactivated, those that remain sensitive to NAADP do so without any change in kinetic features.

Influence of haemodialysis on lipase activity

Our aim was to determine whether the increase in serum pancreatic lipase values, reported in patients with chronic renal failure maintained on haemodialysis, is the result of haemoconcentration by fluid removal during dialysis, or whether it is due to lipase stimulation by endothelial lipoprotein lipase, induced by the heparin used as an anticoagulant. We therefore compared the increases in serum lipase, when heparin was used, with those observed when this was replaced by the antithrombotic agent, defibrotide, which has no effect on lipoprotein lipase. In addition, in order to determine the effects of haemoconcentration, variations in total protein concentration and haematocrit values were determined on the same samples, both before and after dialysis. The results showed a statistically significant post-dialysis increase in lipase only when heparin was used (p < 0.03). There was also a mean percentage post-dialysis increase of 16.2% in total protein (p < 0.0001) and 15.5% in haematocrit (p < 0.0001), due to fluid removal. No significant correlation in percentage increases was found between lipase vs total protein or haematocrit values. These findings suggest that heparin-induced lipoprotein lipase stimulation is the principal cause of the post-dialysis increase in pancreatic lipase, and that fluid removal during dialysis makes only a minor contribution to this increase.