Generation of Nicotinic Acid Adenine Dinucleotide Phosphate and Cyclic ADP-Ribose by Glucagon-Like Peptide-1 Evokes Ca2+ Signal That Is Essential for Insulin Secretion in Mouse Pancreatic Islets

  1. Uh-Hyun Kim1,4
  1. 1Department of Biochemistry, Chonbuk National University Medical School, Jeonju, Republic of Korea
  2. 2Department of Internal Medicine, Chonbuk National University Medical School, Jeonju, Republic of Korea
  3. 3Department of Advanced Biological Sciences for Regeneration, Tohoku University Graduate School of Medicine, Sendai, Japan
  4. 4Institute of Cardiovascular Research, Chonbuk National University Medical School, Jeonju, Republic of Korea
  1. Address correspondence and reprint requests to Uh-Hyun Kim, MD, PhD, Department of Biochemistry, Chonbuk National University Medical School, Keum-am dong, Jeonju, 561-182, Republic of Korea. E-mail: uhkim{at}chonbuk.ac.kr

Abstract

OBJECTIVE—Glucagon-like peptide-1 (GLP-1) increases intracellular Ca2+ concentrations ([Ca2+]i), resulting in insulin secretion from pancreatic β-cells. The molecular mechanism(s) of the GLP-1–mediated regulation of [Ca2+]i was investigated.

RESEARCH DESIGN AND METHODS—GLP-1–induced changes in [Ca2+]i were measured in β-cells isolated from Cd38+/+ and Cd38−/− mice. Calcium-mobilizing second messengers were identified by measuring levels of nicotinic acid adenine dinucleotide phosphate (NAADP) and cyclic ADP-ribose (ADPR), using a cyclic enzymatic assay. To locate NAADP- and cyclic ADPR–producing enzyme(s), cellular organelles were separated using the sucrose gradient method.

RESULTS—A GLP-1–induced [Ca2+]i increase showed a cooperative Ca2+ signal, i.e., an initial [Ca2+]i rise mediated by the action of NAADP that was produced in acidic organelles and a subsequent long-lasting increase of [Ca2+]i by the action of cyclic ADPR that was produced in plasma membranes and secretory granules. GLP-1 sequentially stimulated production of NAADP and cyclic ADPR in the organelles through protein kinase A and cAMP-regulated guanine nucleotide exchange factor II. Furthermore, the results showed that NAADP production from acidic organelles governed overall Ca2+ signals, including insulin secretion by GLP-1, and that in addition to CD38, enzymes capable of synthesizing NAADP and/or cyclic ADPR were present in β-cells. These observations were supported by the study with Cd38−/− β-cells, demonstrating production of NAADP, cyclic ADPR, and Ca2+ signal with normal insulin secretion stimulated by GLP-1.

CONCLUSIONS—Our findings demonstrate that the GLP-1–mediated Ca2+ signal for insulin secretion in pancreatic β-cells is a cooperative action of NAADP and cyclic ADPR spatiotemporally formed by multiple enzymes.

An increase of intracellular Ca2+ concentration ([Ca2+]i) through release from intracellular Ca2+ stores and/or extracellular Ca2+ entry induces insulin secretion from pancreatic β-cells (14). An elevation of blood glucose levels stimulates insulin secretion through a specialized pathway that requires mitochondrial ATP synthesis, which leads to the closure of ATP-sensitive K+ channels, cell depolarization, and Ca2+ influx (5). In addition, glucose-mediated elevation of [Ca2+]i is also achieved through two Ca2+-releasing receptors in the endoplasmic reticulum (ER): receptor for inositol 1,4,5-trisphosphate (IP3) stimulated by IP3/phospholipase C activation and ryanodine receptor activated by cyclic ADP-ribose (ADPR) (6,7). Recent studies (8,9) have indicated that cyclic ADPR also induces Ca2+ entry. An additional pathway for intracellular Ca2+ release channel is the nicotinic acid adenine dinucleotide phosphate (NAADP)-sensitive receptor channel reportedly present in acidic lysosome–related granules (10). The production of cyclic ADPR and NAADP is catalyzed by ADPR cyclases, including CD38 (11,12). Glucose-stimulated Ca2+ mobilization and insulin secretion are elevated by CD38 overexpression (13) and reduced by knockout (14). Low levels of CD38 expression have been observed in diabetic β-cells such as ob/ob mouse islets and RINm5F insulinoma cells with poor glucose-stimulated insulin production/release (15). A recent study (16) has indicated that NAADP initiates and propagates Ca2+ signals in response to insulin and is involved in insulin synthesis. Along the same lines, NAADP-sensitive Ca2+ store–controlled Ca2+ signaling and the production of NAADP by glucose stimulus in β-cells have also been demonstrated (17,18).

Glucagon-like peptide-1 (GLP-1), a peptide hormone released from gut l-cells, is a physiologically important potentiator of glucose-induced insulin secretion (19,20). The peptide elevates intracellular cAMP concentrations and causes activation of protein kinase A (PKA) and cAMP-regulated guanine nucleotide exchange factor II (Epac) (21,22). Although these cAMP-binding proteins have been shown to play a role in GLP-1–mediated transient and sustained increase of [Ca2+]i (23), it remains to be clarified whether the increase of [Ca2+]i is mediated through activation of Ca2+ channels directly or through generation of Ca2+-mobilizing second messengers by the molecules PKA and Epac.

In this study, we examined the possibility that GLP-1 signaling activates ADPR cyclases/CD38 in the context of NAADP- and/or cyclic ADPR–mediated regulation of Ca2+ signals and insulin secretion. Our results demonstrate that the GLP-1–induced Ca2+ signal is mediated in a spatiotemporally different mode by both NAADP and cyclic ADPR, which are produced by as yet unidentified enzyme(s), and CD38. In particular, we demonstrate that the GLP-1–induced Ca2+ signal is regulated in a concerted and sequential way by the second messengers NAADP and cyclic ADPR.

RESEARCH DESIGN AND METHODS

Drugs.

Anti-mouse CD38 monoclonal antibody was obtained from eBioscience (San Diego, CA,). Dulbecco’s modified Eagle’s medium containing low glucose and antibiotics was from Life Technologies (Grand Island, NY). GLP-1 (7-36) amide was purchased from American Peptide Company (Vista, CA). A 125I-insulin radioimmunoassay kit was from Linco Research (Charles, MO). Biolog Life Science (Bremen, Germany) provided 8-pCPT-2′-O-Me-cAMP and N6-benzoyl-cAMP. Recombinant nicotinic acid mononucleotide adenylyltransferase was a gift from Dr. Se Won Suh (Department of Chemistry, Seoul National University, Seoul, Korea) (24). All other reagents were obtained from Sigma.

Animals.

Cd38−/− mice with genetic background ICR were inbred in the animal facility of Chonbuk National University Medical School. The generation and characterization of Cd38−/− mice have been described previously (14). All experimental animals used were under a protocol approved by the institutional animal care and use committee of the Chonbuk National University Medical School. Standard guidelines for laboratory animal care were followed (25).

Preparation of islets.

Pancreatic islets were isolated from Cd38+/+ and Cd38−/− mice weighing 25–30 g using a collagenase method (14,26), with the exception of changing Krebs-Ringer buffer to Krebs-Ringer bicarbonate buffer (KRBB) (in millimoles per liter: 2 CaCl2, 2.8 glucose, 145 NaCl, 1.19 KCl, 2.54 MgCl2, 1.19 KH2PO4, 5 NaHCO3, and 20 HEPES, pH 7.3). Briefly, mouse pancreata were distended by infusion of KRBB containing 0.15 mg/ml type V collagenase through the bile duct. Islets were isolated, washed with KRBB, and stabilized by culturing in a humidified incubator (95% air, 5% CO2) overnight at 37°C in low glucose (5 mmol/l), Dulbecco’s modified Eagle’s medium supplemented with 10% (vol/vol) fetal bovine serum, 100 units/ml penicillin G, and 100 μg/ml streptomycin (culture media).

Complete methods, including a description of the procedures for measurement of [Ca2+]i, measurement of intracellular cyclic ADPR and NAADP concentration, separation of organelles, reconstitution study, analysis of CD38 in organelles, and measurement of insulin, are provided in the online appendix (available at http://dx.doi.org/10.2337/db07-0443).

Statistical analysis.

Data represent means ± SEM of at least three separate experiments. Statistical analysis was performed using Student’s t test. A value of P < 0.05 was considered significant.

RESULTS

GLP-1 induces Ca2+ signal by releasing Ca2+ from ER and non-ER Ca2+ stores in β-cells.

Dependency of glucose concentrations on GLP-1–mediated regulation of [Ca2+]i in β-cells was first examined. At 2.8 mmol/l glucose, GLP-1 was not able to elevate [Ca2+]i (Fig. 1A). In contrast, the addition of 12 mmol/l glucose increased [Ca2+]i and [Ca2+]i slowly decreased, but not to the basal level, and sustained (Fig. 1B). At the sustained Ca2+ levels, application of GLP-1 induced a rapid and large rise of [Ca2+]i that was also sustained (Fig. 1B). It should be noted here that our preliminary studies showed that at 2.8 mmol/l glucose, none of the molecules used in this study generated Ca2+ signals (data not shown).

FIG. 1.

GLP-1 induces a rapid and sustained increase in [Ca2+]i in β-cells at high concentrations of glucose, and GLP-1–induced Ca2+ increase involves release of Ca2+ from ER and non-ER Ca2+ stores. A: The effect of 10 nmol/l GLP-1 on the Ca2+ signal at 2.8 mmol/l glucose (n = 13). B: The effect of GLP-1 on the Ca2+ signal at 12 mmol/l glucose (n = 12). C: Comparison of the 10 nmol/l GLP-1–induced signal with 6 μmol/l forskolin-induced Ca2+ signal at 12 mmol/l glucose (GLP-1, n = 10 and forskolin, n = 16). D: Activator of PKA (100 μmol/l N6-benzoyl-cAMP) and Epac (100 μmol/l 8-pCPT-2′-O-Me-cAMP) induces a rapid and sustained increases in [Ca2+]i (PKA activator; n = 16 and Epac activator; n = 30). E: Depletion of ER Ca2+ store with 1 μmol/l thapsigargin blocks the late phase of the Ca2+ signal induced by GLP-1 (n = 11). F: Blocking of ryanodine receptor with 20 μmol/l ryanodine inhibits the sustained Ca2+ signal induced by GLP-1 (n = 10). G: A cyclic ADPR antagonist, 100 μmol/l 8-bromo-cyclic ADPR, inhibits the sustained Ca2+ signal induced by GLP-1 (n = 10). H: Blocking of IP3 receptor with 2 μmol/l xestospongin C does not affect the GLP-1–induced Ca2+ signals (n = 9). I: A direct comparison of mean [Ca2+]i during increases of [Ca2+]i. The data shown were analyzed at 130 (□), 200 ( Graphic), and 500 (▪) s. *P < 0.05 vs. control; #P < 0.05 vs. 10 nmol/l GLP-1.

To examine whether the GLP-1–induced Ca2+ signal is mediated by cAMP, we utilized forskolin, an activator of adenylyl cyclase. Treatment of β-cells with forskolin produced an increase in the Ca2+ signal similar to that observed with GLP-1 (Fig. 1C). Next, we asked whether PKA and Epac mediate the GLP-1–induced Ca2+ signal. As shown in Fig. 1D, activation of cAMP-dependent PKA or Epac also induced a rapid and sustained Ca2+ signal and the GLP-1–induced Ca2+ signaling or insulin secretion was only partially blocked by PKA inhibitors (10 μmol/l H89 or 100 μmol/l Rp-cAMP; online appendix Fig. 1). These results indicate that these two cAMP-sensitive molecules play a role in GLP-1–induced Ca2+ signal.

To evaluate whether ADPR cyclase/CD38 is involved in the GLP-1–induced Ca2+ signal and to identify a Ca2+ store, we first examined ER Ca2+ store. Thapsigargin, an ER Ca2+ ATPase inhibitor, completely blocked only the late phase of Ca2+ signals, while maintaining an initial sharp Ca2+ rise (Fig. 1E). Pretreatment of the cells with high concentrations of ryanodine, which is known to inhibit ryanodine receptor or a cyclic ADPR antagonistic analog 8-Br–cyclic ADPR, resulted in a significant inhibition of only the GLP-1–induced sustained Ca2+ signal (Figs. 1F and G, respectively). Xestospongin C, an IP3 receptor inhibitor, did not show any effects on the GLP-1–induced Ca2+ signal (Fig. 1H). However, acetylcholine-induced Ca2+ signaling, which was proved to be due to IP3 production (27), was blocked by the reagent (online appendix Fig. 2). The effects of various inhibitors on the initial and sustained Ca2+ increases are summarized in Fig. 1I. Together, these data suggest that GLP-1 induces a rapid and sustained Ca2+ signal by releasing Ca2+ from ER and non-ER Ca2+ stores in β-cells through generation of cAMP and that ADPR cyclase/CD38 is involved in the GLP-1–mediated sustained rise of [Ca2+]i via generation of cyclic ADPR.

NAADP induces a rapid and sustained increase of [Ca2+]i by releasing Ca2+ from ER and non-ER Ca2+ stores.

Considering the possibility that additional Ca2+-mobilizing messengers targeting intracellular Ca2+ stores beside ER could be involved in GLP-1–induced Ca2+ signaling, we thought NAADP would be an interesting candidate since it is one of the most potent Ca2+-releasing messengers found thus far (28,29). Therefore, we initially tested whether NAADP can be transported into β-cells when applied extracellularly because NAADP is known to be transported in certain cell types (30). Figure 2A shows that NAADP was transported in a time-dependent manner and that the NAADP transport was Ca2+ and glucose dependent and partially blocked by dipyridamole (Fig. 2B). Interestingly, cyclic ADPR externally applied to β-cells was also similarly transported, except in a Ca2+-independent manner (online appendix Fig. 3). To rule out any possibility of the effect on purinoreceptors by extracellularly added NAADP, we also tested the effects of equimolar concentration of NAD and NADP under the same condition. However, we could not find any significant signals with even high concentration of the nucleotides (online appendix Fig. 4). Treatment of β-cells with exogenous NAADP elicited Ca2+ signals in a bell-shaped concentration response, and the Ca2+ signal peaked at a 50 nmol/l concentration of NAADP (Fig. 2C). However, the NAADP-induced Ca2+ signal was not observed in the presence of 2.8 mmol/l glucose or in the Ca2+-free condition (online appendix Fig. 5), where NAADP was not transported into the cells (Fig. 2B). As shown in Fig. 2D, NAADP (50 nmol/l) induced a rapid and sustained [Ca2+]i increase in a manner similar to that observed with GLP-1 or forskolin (see Fig. 1C). An important property of NAADP signaling in mammalian cells is a self-desensitization mechanism induced by its high concentrations (31). Similarly, treatment of β-cells with high concentrations of NAADP (10 μmol/l) blocked the low NAADP concentration (50 nmol/l)–mediated Ca2+ signals (Fig. 2E). Analyses of characteristics of NAADP-mediated Ca2+ signals revealed that NAADP-mediated Ca2+ signals were very similar to those observed with GLP-1. Thus, pretreatment of β-cells with thapsigargin blocked NAADP-mediated late-phase Ca2+ signal but not the initial spiky Ca2+ rise (Fig. 2F). Ryanodine and 8-Br–cyclic ADPR also inhibited the NAADP-mediated sustained Ca2+ signal (Figs. 2G and H). The effects of various inhibitors on NAADP-induced Ca2+ increases are summarized in Fig. 2I.

FIG. 2.

Treatment of β-cells with exogenous NAADP induces rapid and sustained [Ca2+]i, and NAADP-mediated Ca2+ rise induces the release of Ca2+ from the ER Ca2+ store. A and B: NAADP transport experiments were executed by a rapid oil-stop procedure (50), and transported NAADP was measured using a cyclic enzymatic assay as described in the manuscript. Islets preincubated with high glucose concentrations were treated with 100 μmol/l NAADP for the indicated time. NAADP was transported in a time-dependent manner (A), and the NAADP transport was Ca2+ and glucose dependent and partially blocked by dipyridamole (B). C: Amplitude of [Ca2+]i rise induced by various concentrations of NAADP. Peak amplitudes are given as the means ± SE from 3 to 5 independent experiments. D: A total of 50 nmol/l NAADP induces a rapid and sustained increase in [Ca2+]i (n = 14). E: Blocking of NAADP receptor with 10 μmol/l NAADP abolishes completely the Ca2+ signal mediated by 50 nmol/l NAADP (n = 12). F: Thapsigargin blocks the late phase of Ca2+ signal induced by NAADP (n = 10). G: Inhibition of ryanodine receptor results in inhibition of the sustained Ca2+signal induced by NAADP (n = 18). H: 8-Bromo-cyclic ADPR inhibits the NAADP-induced sustained Ca2+ signal (n = 4). I: A direct comparison of mean [Ca2+]i during increases of [Ca2+]i. The data shown were analyzed at 150 (□), 220 ( Graphic), and 500 (▪) s. *P < 0.05 vs. control; #P < 0.05 vs. 50 nmol/l NAADP.

NAADP-induced Ca2+ mobilization from acidic organelles governs overall GLP-1–mediated Ca2+ signal.

Numerous studies (18,3234) have indicated that NAADP receptors exist in acidic organelles in a variety of cell types, including β-cells. We hypothesized that the initial Ca2+ rise, which was not blocked by thapsigargin, was due to its release from acidic organelles. Treatment of β-cells with bafilomycin A1, an inhibitor of vacuolar H+-ATPase, transiently increased [Ca2+]i and, following treatment with NAADP, failed to induce an increase of [Ca2+]I; nevertheless, thapsigargin-induced elevation of [Ca2+]i was observed (Fig. 3A). A similar result was also observed by treatment of the cells with glycylphenylalanine 2-naphthylamide (GPN), which selectively disrupts these organelles via osmotic lysis (data not shown). After treatment of β-cells with bafilomycin A1 (Fig. 3B) or GPN (Fig. 3C), the increase of [Ca2+]i mediated by GLP-1 or forskolin could not be found. High concentrations of NAADP prevented the induction of Ca2+ signals mediated by GLP-1 or forskolin (Fig. 3D). In contrast, cyclic ADPR–mediated Ca2+ increase was not blocked by the treatment of 10 μmol/l NAADP (Fig. 3E). The cyclic ADPR–mediated Ca2+ increase was also not affected by the treatment with either bafilomycin A1 or GPN (online appendix Fig. 6). In addition, bafilomycin A1 and GPN blocked generation of Epac- or PKA-induced Ca2+ signals but not depolarization-evoked Ca2+ influx (Figs. 3F and G). High concentrations of NAADP also completely abolished the Epac- or PKA-induced Ca2+ signals (Fig. 3H). Together, these results show that NAADP acts as the initiator and propagator of the GLP-1–induced Ca2+ signal. The effects of inhibitors on various stimulator-induced Ca2+ increases are summarized in Fig. 3I.

FIG. 3.

NAADP induces Ca2+ mobilization from acidic organelles and governs overall GLP-1–mediated Ca2+ signal in β-cells. A: Treatment of β-cells with 300 nmol/l bafilomycin A1 hampers the NAADP-induced Ca2+ signal (n = 10). B: Bafilomycin A1 obstructs GLP-1 –and forskolin-induced Ca2+ signals (GLP-1, n = 14 and forskolin, n = 25). C: A total of 50 μmol/l GPN obstructs GLP-1 –and forskolin-induced Ca2+ signals (GLP-1, n = 10 and forskolin, n = 6). D: The effect of blocking NAADP receptor with 10 μmol/l NAADP on GLP-1–and forskolin-induced Ca2+ signals (GLP-1, n = 11 and forskolin, n = 17). E: Blocking of NAADP receptor with NAADP has no effect on cyclic ADPR-mediated Ca2+ signal (n = 9). F: Bafilomycin A1 obstructs Epac- and PKA-mediated Ca2+ signals (Epac, n = 8 and PKA, n = 20). G: GPN obstructs Epac- and PKA-mediated Ca2+ signals (Epac, n = 17 and PKA, n = 18). H: Blocking of NAADP receptor with NAADP results in the absence of PKA- and Epac-induced Ca2+ signals (Epac, n = 8 and PKA, n = 11). I: A direct comparison of mean [Ca2+]i during increases of [Ca2+]i.

Acidic organelles play an essential role in insulin secretion mediated by GLP-1.

We evaluated the effects of acidic organelles disturbing agents on the insulin secretion induced by 12 mmol/l glucose or GLP-1 signaling molecules. Bafilomycin A1 and concanamycin A, inhibitors of vacuolar H+-ATPase, have been reported to inhibit glucose-induced insulin secretion by mouse islets (35). The glucose-induced insulin secretions were not affected by GPN or bafilomycin A1 (Fig. 4A) or by concanamycin A (online appendix Fig. 7). In contrast, however, pretreatment of the cells with GPN or bafilomycin A1 completely blocked GPL-1–, PKA-, or Epac-induced insulin secretion (Fig. 4A). Indeed, treatment with NAADP resulted in enhancement of insulin secretion at high glucose concentrations only, which was abolished by GPN or bafilomycin A1 (Fig. 4B). These results indicate that acidic organelles, which involve NAADP-mediated signaling and probably Ca2+ release, play critical roles in GLP-1–induced insulin secretion but not in glucose-induced insulin secretion.

FIG. 4.

cAMP-potentiated insulin secretion in response to 12 mmol/l glucose is essential to produce NAADP from acidic granule. A: Islets were stimulated with either 12 mmol/l glucose or 12 mmol/l glucose plus 10 nmol/l GLP-1, 100 μmol/l PKA, or 100 μmol/l Epac activator or a combination of PKA and Epac activator for 30 min. Preincubation with 50 μmol/l GPN or 300 nmol/l bafilomycin A1 did not affect 12 mmol/l glucose-induced insulin secretion but completely inhibited insulin secretion mediated by 12 mmol/l glucose plus GLP-1, PKA or Epac. *P < 0.05 vs. 2.8 mmol/l glucose; **P < 0.005 vs. 12 mmol/l glucose; #P < 0.005 vs. 12 mmol/l glucose plus 10 nmol/l GLP-1. B: Islets were stimulated with 50 nmol/l NAADP containing 12 mmol/l glucose. NAADP-stimulated insulin secretion is completely blocked by 50 μmol/l GPN or 300 nmol/l bafilomycin A1. *P < 0.01 vs. 12 mmol/l glucose; #P < 0.01 vs. 50 nmol/l NAADP.

GLP-1 stimulates generation of NAADP and cyclic ADPR sequentially in the presence of high glucose and Ca2+ concentrations.

Since the above observations suggest that NAADP and cyclic ADPR differentially regulate Ca2+ signals mediated by GLP-1 and that NAADP initiates and propagates the GLP-1–induced Ca2+ signal, we examined kinetics of the production of these two Ca2+-mobilizing messengers in response to GLP-1. Treatment of islets with GLP-1 generated NAADP first and then cyclic ADPR, with a delay of ∼10 s (Fig. 5A), suggesting that the production of cyclic ADPR depends on the NAADP-induced initial Ca2+ signal (Fig. 2D and F). Moreover, treatment of islets with NAADP increased the formation of cyclic ADPR, which was dependent on the presence of extracellular Ca2+ (Fig. 5B). We tested whether the cyclic ADPR production would be stimulated by a general Ca2+ signal. Ionomycin-induced Ca2+ signals did not affect cyclic ADPR production (online appendix Fig. 8) as in lymphokine-activated killer cells (8), indicating that NAADP-induced Ca2+ signaling specifically produces cyclic ADPR, as in sea urchin eggs (36). We next evaluated dependency of glucose and Ca2+ on the GLP-1–stimulated production of NAADP and cyclic ADPR. As shown in Fig. 5C, high glucose concentrations increased the levels of NAADP and cyclic ADPR compared with low glucose concentrations. Addition of GLP-1 in the presence of high glucose concentrations further increased the production of NAADP and cyclic ADPR by ∼3- and 2.5-fold, respectively. GLP-1–mediated production of NAADP and cyclic ADPR was significantly lower in the presence of 2.8 mmol/l glucose or absence of extracellular Ca2+ than that in the presence of high glucose and extracellular Ca2+ concentrations. These results indicate that high glucose and extracellular Ca2+ concentrations are prerequisite for the GLP-1–induced production of the Ca2+-mobilizing second messengers. Bafilomycin A1 completely blocked the GLP-1–induced formation of NAADP and cyclic ADPR (Fig. 5D), suggesting that NAADP is formed in acidic organelles. These results also indicate that the formation of cyclic ADPR may depend on NAADP signaling.

FIG. 5.

GLP-1 signaling stimulates production of NAADP and cyclic ADPR. A: The effect of 10 nmol/l GLP-1 on the production of NAADP and cyclic ADPR. Islets were preincubated for 40 min with KRBB containing 12 mmol/l glucose and were stimulated for different periods of time with GLP-1. *P < 0.01 vs. 0 time. B: A total of 50 nmol/l NAADP induces the production of cyclic ADPR in the presence of 2 mmol/l extracellular Ca2+. *P < 0.001 vs. without extracellular Ca2+ condition. C: GLP-1–stimulated NAADP and cyclic ADPR requires extracellular Ca2+ and high concentration of glucose. *P < 0.05 vs. 2.8 mmol/l glucose; **P < 0.05 vs. 12 mmol/l glucose; #P < 0.05 vs. 12 mmol/l glucose plus GLP-1. D: GLP-1–stimulated NAADP and cyclic ADPR production is inhibited by disrupting acidic organelles with 300 nmol/l bafilomycin A1. *P < 0.002 vs. vehicle; #P < 0.01 vs. vehicle plus GLP-1. Data represent means + SE from three independent experiments. C and D: ▪, NAADP; □, cADPR.

NAADP and cyclic ADPR are produced in different organelles via CD38 and non-CD38 ADPR cyclase.

CD38 (transmembrane glycoprotein) is known to catalyze the synthesis of NAADP and cyclic ADPR (7,11,12). However, it has not yet been clarified which organelles in the pancreatic β-cells are involved in the production of NAADP and cyclic ADPR. To address the question, we resolved cellular organelles into at least three parts: plasma membranes, lysosomes, and secretory granules (Fig. 6A). NADase activity was detected in all three organelles; however, the lysosomes-containing fraction contained the highest NADase activity among the three fractions (Fig. 6B and C). To determine the presence of enzyme(s) other than CD38 in these compartments, CD38 in each fraction was immunoprecipitated with a CD38-specific antibody, and NADase activity in the precipitates and supernatants was further determined. CD38 activity was found mainly in the lysosome fraction and less in the plasma membrane fraction. However, in the supernatants from all fractions, significant levels of NADase activity were also observed consistently (Figs. 6B and C), suggesting an existence of non-CD38 enzyme(s) in β-cells.

FIG. 6.

Localization of CD38/ADPR cyclase in the cellular organelles. A: Distribution of the organelles was assessed in each fraction by the measurements of Na+/K+ ATPase (Mem, plasma membranes), β-d-galactosidase (Lys, lysosomes), and insulin (SG, secretory granules). B and C: NADase activity in each fraction (•, Origin). Localization of CD38 was assessed by measuring NADase activity of CD38 immunoprecipitate of each fraction (▵, CD38 Abs). Control immunoprecipitates using anti-mouse IgG were measured for NADase activity (○, Control Abs). Unbound NADase activity was determined by measurement of NADase activity in the supernatant (▪). D: Production of NAADP and cyclic ADPR in organelles. Each fraction was reconstituted with the lysate prepared from islets stimulated with 10 nmol/l GLP-1. NAADP and cyclic ADPR levels were determined. E: Epac- and PKA-mediated NAADP production in orgarnelles. F: Epac- and PKA-mediated cyclic ADPR in organelles. Each fraction was reconstituted with equal amount of the lysate prepared from islets that were stimulated with Epac or PKA activator. The production of NAADP and cyclic ADPR was measured as described above. Data represent means ± SE from three independent experiments. ▵, Epac-specific activator; ○, PKA-specific activator.

We next performed reconstitution studies to determine the production of NAADP and cyclic ADPR in the organelles isolated. The reconstitution study revealed that cyclic ADPR was produced in both plasma membrane–and secretory granule–containing fractions (Fig. 6D). Interestingly, CD38 was not detected in the fractions of secretory granules. On the other hand, the production of NAADP was observed only in fractions containing lysosomes (Fig. 6D). Moreover, the effect of PKA or Epac activator on the production of NAADP and cyclic ADPR showed specificity for the organelles; both PKA and Epac stimulated the generation of NAADP in the lysosomes only (Fig. 6E). Epac activated the cyclic ADPR production in both plasma membranes and secretory granules, and PKA stimulated the cyclic ADPR formation only in the plasma membranes (Fig. 6F). These data indicate that activation of CD38/ADPR cyclase(s) by GLP-1 depends on their intracellular localization, where assembly of cognate signaling proteins such as PKA and Epac may be different.

Production of NAADP and cyclic ADPR in Cd38−/− islets is reduced in response to GLP-1 without alteration of insulin secretion.

To further confirm the above observations that enzymes producing NAADP and/or cyclic ADPR exist in β-cells, β-cells isolated from CD38 knockout mice (Cd38−/−) and the littermates (Cd38+/+) were used. In the presence of 12 mmol/l glucose, GLP-1–mediated Ca2+ signals in these β-cells were somewhat different. The early Ca2+ increases were similar to each other; however, the late phase of Ca2+ signals in Cd38−/− β-cells was gradually decreased while the Ca2+ signals in Cd38+/+ β-cells were sustained (Fig. 7A). When the production of NAADP and cyclic ADPR was determined, no alterations in the levels of NAADP and cyclic ADPR were observed in the presence of 12 mmol/l glucose only. However, GLP-1–stimulated production of NAADP and cyclic ADPR in Cd38−/− islets was substantially higher than that with glucose alone and significantly reduced compared with that in Cd38+/+ islets (Fig. 7B). Insulin secretion of Cd38−/− islets stimulated by glucose or GLP-1 was similar to that of Cd38+/+ islets (Fig. 7C). These findings clearly demonstrate that ADPR cyclase(s)/NAADP-producing enzyme(s) other than CD38 exists in β-cells.

FIG. 7.

GLP-1 signaling in Cd38+/+ and Cd38−/− β-cells is not impaired. A: GLP-1–induced increase in [Ca2+]i. (Cd38+/+, n = 8 and Cd38−/−, n = 12). B: Production of NAADP and cyclic ADPR in response to GLP-1 (10 nmol/l) in the presence of 12 mmol/l glucose. *P < 0.01 vs. 12 mmol/l glucose; #P < 0.05 vs. NAADP concentration of Cd38+/+ induced by GLP-1; §P < 0.01 vs. cyclic ADPR concentration of Cd38+/+ induced by GLP-1. ▪, NAADP; □, cADPR. C: Insulin secretion from Cd38+/+ and Cd38−/− islets in response to 10 nmol/l GLP-1 in the presence of 12 mmol/l glucose. *P < 0.01 vs. 12 mmol/l glucose. Data represent means ± SE from three independent experiments.

DISCUSSION

In this study, we showed that GLP-1 elevates Ca2+ via stimulation of NAADP and cyclic ADPR production and that NAADP-induced Ca2+ mobilization from acidic stores contributes to the GLP-1–stimulated Ca2+ signal. The action of GLP-1 is complex, with multiple kinetic components shaping the Ca2+ response. Our results firmly established a role of thapsigargin- and ryanodine-sensitive stores and also provide new data implicating a source of Ca2+ in acidic organelles.

Many equivocal results have been reported (9,13,14) on the possible role of CD38 and/or cyclic ADPR in glucose signaling in β-cells. Our results showed that high glucose concentrations alone could elevate cyclic ADPR and NAADP levels (Fig. 5C). Consistent with our observations, Masgrau et al. (17) showed that high glucose levels (20 mmol/l) increase NAADP levels in MIN6 cells, a clonal pancreatic β-cell line. However, the glucose-induced production of NAADP and cyclic ADPR was not altered in Cd38−/− islets compared with that in Cd38+/+ islets (Fig. 7B). These findings together with immunoprecipitation (Figs. 6B and C) and reconstitution studies (Fig. 6D) suggest that additional ADPR cyclase(s) and/or NAADP-producing enzyme(s) besides CD38 exist in islets. Recently, an existence of enzymes capable of generating NAADP and/or cyclic ADPR beside CD38 has been reported in various tissues (37).

Interestingly, at low concentrations of glucose (2.8 mmol/l), none of the following molecules generate Ca2+ signals in β-cells: GLP-1, forskolin, activators of PKA and Epac, and NAADP. These molecules generate Ca2+ signals only under the condition of high glucose–induced elevated basal Ca2+ level (Fig. 1B). In agreement with our observations, several studies have also reported that GLP-1 has no significant effects on β-cells at low concentrations of glucose (38,39). However, the reasons for the insensitivity of GLP-1 and other downstream signaling molecules at low concentrations of glucose remain completely unknown. Intriguingly, KCl-induced Ca2+ signals could not substitute for the effect of high glucose concentrations in triggering GLP-1–induced Ca2+ signals (online appendix Fig. 9). Glucose-induced Ca2+ signals as well as metabolites of glucose may coordinate the cognate signaling molecules and/or organelles for the effective responses to subsequent stimuli in β-cells.

In the present study, exogenous NAADP could be a useful tool for analyzing NAADP-mediated Ca2+ signaling by virtue of the property of NAADP transport into β-cells. The characteristics of NAADP transport were very similar to those observed in RBL-2H3 cells, which were originally observed (30). However, the exact mechanisms involved in the transport of NAADP in pancreatic islets remain to be clarified. Dependencies of the NAADP transport on high glucose and external Ca2+ are the important issues to be solved in conjunction with the Ca2+ signals in β-cells.

Our data (Fig. 1E) show that thapsigargin greatly reduced the overall Ca2+ response, while leaving Ca2+ transient. Previously, Holz et al. (40) demonstrated that GLP-1, as well as cAMP (through Epac), mobilized Ca2+ from intracellular Ca2+ stores, which was abolished by thapsigargin. Our data suggest that Ca2+ signaling induced by GLP-1 consists of transient and sustained components and that the former is thapsigargin insensitive and the latter is sensitive to thapsigargin or ryanodine. Furthermore, we could differentiate the effects of bafilomycin or GPN on the transient and sustained Ca2+ by GLP-1; bafilomycin or GPN pretreatment had no effects on cyclic ADPR–induced Ca2+, which represents sustained Ca2+ (online appendix Fig. 6), while completely blocking the NAADP-induced Ca2+, which represents transient Ca2+ (Fig. 3A).

Mitchell et al. (34) showed that NAADP decreased free Ca2+ concentrations of dense-core secretory vesicles in permeabilized MIN6 β-cells. Duman et al. (41) reported also that dense-core secretory granules comprise a GPN-sensitive acidic Ca2+ store. Our results also showed a similar result in that 50 μmol/l GPN released Ca2+ from secretory granule–containing fractions and lysosome-containing fractions (online appendix Fig. 10). Therefore, further studies are needed to clarify these NAADP-responsive Ca2+ stores in β-cells.

Johnson and Misler (16) suggested that NAADP initiates subsequent oscillatory Ca2+ signaling in insulin signaling in human β-cells. However, their results are quite different from ours in two aspects: 1) Ca2+-free solutions did not affect initial Ca2+ rise in the insulin-mediated Ca2+ signals, whereas GLP-induced Ca2+ signals were completely blocked in the Ca2+-free solutions (data not shown), and 2) thapsigargin resulted in a complete abolishment of the insulin-mediated Ca2+ signals, while the agent inhibited the late phase of GLP-1–induced Ca2+ signals but not the initial Ca2+ spike (Fig. 1E). Indeed, stimulation of mouse β-cells with 200 nmol/l insulin under the same conditions used by Johnson and Misler did not affect NAADP levels (online appendix Fig. 11).

In conclusion, our results revealed for the first time that GLP-1 elevates [Ca2+]i via stimulation of NAADP and cyclic ADPR production and that NAADP acts as an initiator for GLP-1–mediated Ca2+ signals and effectively increases insulin secretion as much as GLP-1 does. Our results also indicated that the downstream modulators for GLP-1 consist of CD38 and hitherto unidentified ADPR cyclase(s)/NAADP–producing enzyme(s), which produces NAADP and cyclic ADPR spatiotemporally or differentially. Finally, consistent with previous findings (42,43) of GLP-1–evoked interrelationships between cAMP and Ca2+, our results indicate that both NAADP and cyclic ADPR may act as key modulators interdependent of [cAMP]i and [Ca2+]i oscillation.

Acknowledgments

M.-J.I. and B.-J.K. are the recipients of the BK21 Program of the Ministry of Education of Korea. This work was supported by Korea Research Foundation Grant KRF-2004-005-E00108, a fund of International Collaboration Study from Chonbuk National University (U.-H.K.), and a grant from the National R&D Program for Cancer Control, Ministry of Health and Welfare of Korea (0620220-1) (C.-Y.Y.)

We thank Sang-Hee Yu for the excellent technical support.

Footnotes

  • Published ahead of print at http://diabetes.diabetesjournals.org on 9 January 2008. DOI: 10.2337/db07-0443.

    Additional information for this article can be found in an online appendix at http://dx.doi.org/10.2337/db07-0443.

    The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

  • Received April 2, 2007.
  • Accepted January 2, 2008.

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