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Originally published In Press as doi:10.1074/jbc.M311088200 on December 2, 2003

J. Biol. Chem., Vol. 279, Issue 8, 7234-7240, February 20, 2004
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Organelle Selection Determines Agonist-specific Ca2+ Signals in Pancreatic Acinar and {beta} Cells*

Michiko Yamasaki{ddagger}§, Roser Masgrau{ddagger}§, Anthony J. Morgan{ddagger}, Grant C. Churchill{ddagger}, Sandip Patel||, Stephen J. H. Ashcroft**, and Antony Galione{ddagger}

From the {ddagger}Department of Pharmacology, University of Oxford, Mansfield Road, Oxford OX1 3QT, the ||Department of Physiology, University College London, Gower Street, London WC1E 6BT, and **Oxford Centre for Diabetes, Endocrinology and Metabolism, Churchill Hospital, Oxford OX3 7LJ, United Kingdom

Received for publication, October 8, 2003 , and in revised form, November 19, 2003.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 EXPERIMENTAL PROCEDURES
 RESULTS
 DISCUSSION
 REFERENCES
 
How different extracellular stimuli can evoke different spatiotemporal Ca2+ signals is uncertain. We have elucidated a novel paradigm whereby different agonists use different Ca2+-storing organelles ("organelle selection") to evoke unique responses. Some agonists select the endoplasmic reticulum (ER), and others select lysosome-related (acidic) organelles, evoking spatial Ca2+ responses that mirror the organellar distribution. In pancreatic acinar cells, acetylcholine and bombesin exclusively select the ER Ca2+ store, whereas cholecystokinin additionally recruits a lysosome-related organelle. Similarly, in a pancreatic {beta} cell line MIN6, acetylcholine selects only the ER, whereas glucose mobilizes Ca2+ from a lysosome-related organelle. We also show that the key to organelle selection is the agonist-specific coupling messenger(s) such that the ER is selected by recruitment of inositol 1,4,5-trisphosphate (or cADP-ribose), whereas lysosome-related organelles are selected by NAADP.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
EXPERIMENTAL PROCEDURES
 RESULTS
 DISCUSSION
 REFERENCES
 
Increases in the Ca2+ concentration ([Ca2+]i)1 represent a ubiquitous transduction mechanism in response to stimuli as diverse as fertilization, neurotransmitters, hormones, and blood glucose (1). In turn, Ca2+-binding proteins responsible for decoding these Ca2+ signals promote an array of cellular responses from cell division to cell death, notably including secretion (1). However, what remains perplexing is that not all stimuli that increase Ca2+ elicit the same physiological response, implying differences in their [Ca2+]i handling. Some early indications arose from single cell and subcellular [Ca2+]i studies which revealed that each stimulus evoked its own unique Ca2+ response in time and space (exemplified by agonist-specific "Ca2+ signatures") (2). But the mechanism(s) underlying stimulus-specific Ca2+ signaling is poorly understood.

It has become clear that a primary factor governing agonist specificity is the release of Ca2+ from intracellular stores, a process that encompasses multiple channel families regulated by the second messengers inositol 1,4,5-trisphosphate (IP3), cyclic ADP-ribose (cADPR), and nicotinic acid adenine dinucleotide phosphate (NAADP). By differential recruitment of second messenger complements, different agonists could, in principle, elicit different Ca2+ signals, and several cellular systems have indeed been shown to exploit messenger diversity, e.g. pancreatic acinar cells (3).

However, the use of different messengers per se is not sufficient to evoke different signals; only if these systems are non-equivalent and differ in their properties, regulation, or spatial distribution will this hold. In spatial terms, IP3 and cADPR mobilize the endoplasmic reticular (ER) Ca2+ store in nearly all cell types (1, 4), although an outstanding issue has been the identity and location of the NAADP-sensitive store. Only recently has this been characterized in sea urchin eggs as a lysosome-related organelle (5), with its mammalian counterpart remaining elusive beyond an isolated report that secretory vesicles support NAADP-induced Ca2+ release in permeabilized cells (6).

Given the uncertainties surrounding the role and properties of different potential Ca2+ stores, we therefore show for the first time that different agonists generate their specific signals by signaling through Ca2+ mobilization from different intracellular stores (organelle selection). This is determined by their second messenger complements, with NAADP-linked agonists coupling to lysosome-related organelles in mammalian cells and those linked to IP3/cADPR coupling to ER pools.


   EXPERIMENTAL PROCEDURES
TOP
 ABSTRACT
 INTRODUCTION
 EXPERIMENTAL PROCEDURES
RESULTS
 DISCUSSION
 REFERENCES
 
Cell Preparation—To obtain pancreatic acinar cells, pancreata were excised from male CD1 mice 8-10 weeks old, and small clusters of pancreatic acinar cells were prepared by collagenase digestion as described previously (7). MIN6 cells were cultured in Dulbecco's modified Eagle's medium (25 mM glucose) supplemented with 10% fetal calf serum, 2 mM glutamine, 100 units/ml penicillin, 100 µg/ml streptomycin, and 50 µM {beta}-mercaptoethanol equilibrated with 5% CO2 and 95% air at 37 °C. Twenty hours before each experiment, cells were placed in low glucose Dulbecco's modified Eagle's medium (5 mM glucose).

Ca2+ Imaging—Both acinar and MIN6 cells were seeded onto polylysine-coated number 1 glass coverslips and loaded with 1-5 µM fura-2 acetoxymethyl ester (fura-2/AM) for 60 min at room temperature. Acinar cells and MIN6 cells were maintained in buffer of the following compositions: for acinar cells (in mM), 140 NaCl, 4.7 KCl, 1.1 MgCl2, 1 CaCl2, 10 Hepes, 10 glucose, pH 7.2; for MIN6 cells (in mM), 119 NaCl, 4.75 KCl, 5 NaHCO3, 1.2 MgSO4, 1.18 KH2PO4, 20 Hepes, 2.54 CaCl2, and 2.8 glucose, pH 7.4. After the loading period, cells were washed and imaged immediately. Coverslips were mounted in a static chamber (Harvard Apparatus), on an inverted Zeiss 35 Axiovert microscope, and imaged with a conventional epifluorescence system, using Metafluor software (Universal Imaging). Cells were excited alternately with 340 and 380 nm light (emission 510 nm), and ratio images of clusters were recorded every 4-5 s, using a 12-bit CCD camera (MicroMax, Princeton Instruments). All experiments were conducted at room temperature for acinar cells and at 37 °C for MIN6 cells.

Imaging Lysosomes—Acidic organelles in both cell types were labeled by incubating cells with 50 nM Lysotracker Red for 20 min at room temperature. Labeling was visualized after 20-40 min of removing excess dye using a Leica TCS NT laser scanning confocal microscope (excitation 568 nm, emission >590 nm).

Flash Photolysis—Acinar cells and MIN6 cells were pressure-microinjected (Femtojet, Eppendorf) with Oregon Green BAPTA-1 dextran (OGBD, final concentration of 20 and 5 µM, respectively) with caged compounds. In acinar cells the Ca2+-sensitive dye was imaged (excitation 490 nm, emission 530 nm) as mentioned, and the caged compounds were photolysed with an XF-10 arc lamp (HI-TECH Scientific, the ultraviolet flash efficiency was 0.5-1%). On the other hand, MIN6 cells were imaged by laser-scanning confocal microscopy (Leica TCS NT), and caged compounds were photolysed with an ultraviolet laser (efficiency of uncaging of ~50%). Images were processed using Metamorph software (Universal Imaging). Ca2+ concentration is given as the ratio F/Fo where Fo is the fluorescence before stimulation, and F the fluorescence at a given time. Changes in Ca2+ concentration are given as increases in the mentioned ratio ({Delta}F/Fo).

Statistical Analysis—Data are presented as means ± S.E. Statistical significance were evaluated by paired Student's t test and, for multiple comparisons, analysis of variance followed by Fisher's Least Significant Difference test (Statview, Abacus Concepts).

Materials—Caged IP3 was from Calbiochem. Caged cADPR, Fura-2/AM, OGBD, and Lysotracker Red were from Molecular Probes, and collagenase was from Worthington. Caged NAADP was synthesized essentially as described previously (8). All other reagents were from Sigma.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 EXPERIMENTAL PROCEDURES
 RESULTS
DISCUSSION
 REFERENCES
 
Of all the mammalian cell models used for investigating agonist-specific Ca2+ signals, one of the most extensively studied non-excitable cells is the pancreatic acinar cell. The gastrointestinal peptides cholecystokinin and bombesin, and the neurotransmitter acetylcholine, evoke subtly different Ca2+ signatures resulting in differential control of fluid secretion, exocytosis, and trophic effects (3). We first addressed whether these agonists differentially recruited lysosome-related or endoplasmic reticular Ca2+ stores by using inhibitors to selectively abrogate Ca2+ storage in each organelle; bafilomycin A1 inhibits the vacuolar H+-ATPase responsible for the proton gradient that drives lysosomal Ca2+ uptake (9), whereas thapsigargin directly blocks the ER Ca2+-ATPase (SERCA) (10).

Whether or not bafilomycin A1 had an effect upon Ca2+ oscillations depended upon the agonist used. Cholecystokinin-induced Ca2+ oscillations were profoundly inhibited, whether bafilomycin A1 was added before (Fig. 1a) or after (Fig. 1b) the agonist. By contrast, bafilomycin A1 had very little effect upon acetylcholine- or bombesin-stimulated oscillations (Fig. 1, c-f). To confirm that bafilomycin A1 was indeed acting at a lysosome-related organelle, we also eliminated such stores with glycyl-phenylalanine 2-naphthylamide (GPN), a substrate of lysosomal cathepsin C whose cleavage results in osmotic lysis (5, 11). Essentially identical results were obtained with GPN, notably the selective block of the response to cholecystokinin over acetylcholine or bombesin (Fig. 1, g-l). Moreover, in cells where cholecystokinin-induced oscillations were blocked by GPN, subsequent addition of acetylcholine could rescue Ca2+ spiking, highlighting the specificity of the response (Fig. 1m). The data show that, of the agonists tested, cholecystokinin is unique in recruiting a lysosome-related organelle that can function as a Ca2+ store.



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  		FIG. 1.
Effect of eliminating Ca2+ stores on agonist-dependent Ca2+ signals in pancreatic acinar cells. In pancreatic acinar cells, 3 µM bafilomycin A1 differentially inhibited Ca2+ oscillations (a-f). Responses to 0.5-2.5 pM cholecystokinin were profoundly inhibited by pre- (a, 70/98 cells) or postincubation (b, 72/117 cells), whereas neither 25-50 nM acetylcholine- (c, 34/36; d, 35/47 cells) nor 1.0-2.5 pM bombesin (e, 29/29; f, 52/67 cells)-induced responses were significantly affected. Similarly, the effect of 50 µM GPN (g-m) was agonist-dependent; cholecystokinin-induced oscillations were blocked (g, 70/77; h, 52/78 cells), and there was little effect upon acetylcholine (i, 56/60; j, 21/24 cells) or bombesin responses (k, 43/43; l, 29/46 cells). In cells where cholecystokinin-induced oscillations were blocked, further addition of acetylcholine restored spiking (m, 21/24 cells). Preincubation with 1 µM thapsigargin completely blocked the response to 2 pM cholecystokinin (n, 23/23 cells).

 
The ER, on the other hand, is the established Ca2+ reservoir for many G protein-coupled receptors, including those for acetylcholine and bombesin. In pancreatic acinar cells, these stimuli are well documented to release Ca2+ from the ER using both IP3 and ryanodine receptors (12, 13). Furthermore, cholecystokinin-induced Ca2+ signals were also confirmed to derive from the ER as evidenced by the marked inhibition by thapsigargin (Fig. 1n). Together, the evidence supports cholecystokinin recruiting both lysosomes and ER, whereas acetylcholine and bombesin only target the ER in order to generate [Ca2+]i signals.

An obvious issue is whether this differential organellar recruitment is specific to acinar cells or a universal blueprint for other mammalian cell types and stimuli. In choosing another model, the pancreatic {beta} cell, we opted for a system with very different properties from the acinar cell, the new one being an excitable cell in which Ca2+ signals are elicited by nutrients as well as by G protein-coupled agonists (14, 15). In the {beta} cell line, MIN6 (16), we compared three different stimuli, glucose, acetylcholine, and K+, with regard to their relative sensitivities to bafilomycin A1 or thapsigargin. Mechanistically, glucose metabolism generates intracellular signals that culminate in a complex interplay between Ca2+ influx and Ca2+ release from intracellular stores (17); muscarinic acetylcholine receptors are G protein-coupled to phospholipase C (15), whereas high K+ depolarizes the plasma membrane to induce voltage-operated Ca2+ entry (18, 19)

In MIN6 cells, the sensitivity of Ca2+ responses to bafilomycin A1 was, like acinar cells, highly dependent upon the stimulus. Remarkably glucose responses were profoundly inhibited by a preincubation with bafilomycin A1 (Fig. 2, a and b), whereas neither acetylcholine nor K+ stimulation was affected (Fig. 2, d and e and g and h). That the acidic stores of the glucose response were lysosome-related was confirmed by the inhibition by GPN (data not shown). Moreover, these results confirm the specificity of bafilomycin and GPN because neither interacts with the IP3-calcium release pathway (acetylcholine) nor calcium influx (K+). Remarkably, the effects of interfering with ER stores with thapsigargin were almost the mirror of those with bafilomycin A1. Glucose-induced Ca2+ signals were not inhibited by ER depletion (Fig. 2c) but rather appeared to be potentiated because thapsigargin greatly reduced the lag phase and eliminated the initial fall in basal [Ca2+]i. Similarly, responses induced by K+ were also slightly potentiated (Fig. 2i), attesting to the role of the ER as a Ca2+ sink in {beta} cells (20). On the other hand, the ER appeared to play a major role during acetylcholine-induced Ca2+ mobilization because responses were completely eliminated by thapsigargin (Fig. 2f). Taken together, the data in this cell type support a model of the reciprocal recruitment of different organelles where metabolic activation is heavily reliant upon lysosome-related Ca2+ stores, contrasting with an exclusive ER role in response to neurotransmitter.



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  		FIG. 2.
Effect of eliminating Ca2+ stores on agonist-dependent Ca2+ signals in pancreatic {beta} cells. In MIN6 cells, 20 mM glucose induced a sustained plateau with a mean increase in the fluorescence ratio (340/380) of 25.7 ± 1.5% in controls cells (n = 140) (a). This plateau response was profoundly inhibited by 2 µM bafilomycin A1 (b, 6.0 ± 1.0, n = 90, p < 0.001) but not affected by 1 µM thapsigargin (c, 27.8 ± 2.8%, n = 90, p = 0.4). By contrast, the 100 µM acetylcholine-induced increase of the fluorescence ratio (d, 24.5 ± 1.6%, n = 159) was unaffected by bafilomycin A1 (e, 25.7 ± 1.8%, n = 71, p = 0.6) but eliminated by thapsigargin (f, 0.8 ± 0.3, n = 70, p < 0.001). Finally, 40 mM KCl induced a peak response in control cells of 62.8 ± 2.6% (g, n = 157) that was unaffected by bafilomycin A1 (h, 62.8 ± 2.2, n = 70, p = 0.9) but was increased by thapsigargin (i, 86.0 ± 5.7, n = 60, p < 0.001).

 
Next we provide a mechanism to couple particular extracellular stimuli to specific intracellular organelles with the appropriate fidelity. Interestingly, our data show an absolute correlation between those stimuli that recruit lysosome-related organelles and those known to utilize NAADP as a Ca2+-mobilizing messenger (21), i.e. cholecystokinin in pancreatic acinar cells (22) and glucose in {beta} cells (16). The agonists that fail to require acidic stores are well known to couple to IP3 and/or ryanodine receptors that mobilize ER Ca2+ stores (13, 23, 24). We therefore tested whether NAADP was the unique link to acidic stores, with IP3 and/or cADPR showing a preference for non-acidic (ER) stores.

First, in pancreatic acinar cells, photorelease of IP3 or cADPR from their caged precursors evoked robust, monotonic Ca2+ transients in control cells (Fig. 3, a and b) comparable in magnitude to the subsequent response to acetylcholine. Elimination of lysosomal Ca2+ storage by preincubation with GPN had no effect upon the magnitude of the [Ca2+]i rise in response to either IP3 or cADPR (or the following acetylcholine responses) (Fig. 3, d and e). By contrast, GPN profoundly inhibited the Ca2+ oscillations following uncaging of NAADP (Fig. 3, c and f). Note that the NAADP-induced Ca2+ spikes are initially small and became progressively amplified by Ca2+-induced Ca2+ release mechanism through the recruitment of IP3 and ryanodine receptors (3, 22, 25-27). In agreement with the effects of GPN, bafilomycin A1 displayed an identical and selective block of NAADP-induced over IP3-induced responses (data not shown, n = 4). The very fact that the Ca2+ responses to second messengers alone are inhibited strongly suggests that bafilomycin A1 and GPN are working downstream of NAADP, and not at an upstream element of the signaling cascade initiated by agonist. We conclude that only NAADP couples to the lysosome-related Ca2+ store in acinar cells, whereas cADPR and IP3 couple to the ER.



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  		FIG. 3.
Effect of eliminating Ca2+ stores upon the response to photolysis of caged second messengers in acinar cells. Pancreatic acinar cells were microinjected with the fluorescent Ca2+ indicator, OGBD, plus the appropriate caged precursor of various Ca2+-releasing agents and photolysis initiated by exposure to UV light where indicated. Control responses to photorelease of IP3 (approximated final intracellular concentration of 300 nM) (a, n = 9) or cADPR (approximated final intracellular concentration of 1 µM) (b, n = 13) were comparable with that for the subsequent 10 µM acetylcholine response, whereas NAADP (approximated final intracellular concentration of 100 nM) responses often showed complex Ca2+ oscillations (c, n = 8). 20-30 min of preincubation with 50 µM GPN had little effect upon either IP3 (d and g, n = 17) or cADPR (e and h, n = 15), whereas NAADP responses were abolished (f and i, n = 6).

 
Our hypothesis is also supported in experiments with the {beta} cell system; in these cells NAADP-dependent Ca2+ release occurs via specific binding sites (16). Just as glucose and acetylcholine manifest a reciprocal dependence upon lysosomes and ER, so NAADP and IP3 displayed this mutually exclusive pattern. NAADP photorelease stimulated a Ca2+ increase that was inhibited by bafilomycin A1 but not by thapsigargin (Fig. 4, a, c and e), whereas the converse occurred when photoreleasing IP3 (Fig. 4, b, d, and f). Thapsigargin-induced depletion of stores profoundly inhibited IP3 transients, which were otherwise insensitive to bafilomycin A1. Once again, NAADP selects lysosome-related stores, whereas IP3 predominantly selects the ER. Hence, the data suggest that lysosome-related stores couple via NAADP to particular extracellular stimuli (16), cholecystokinin and glucose, respectively. Furthermore, by recruiting NAADP, agonists select a novel Ca2+ store with distinct properties, distribution, and ramifications from the ER (primarily the domain of IP3 and cADPR).



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  		FIG. 4.
Effect of eliminating Ca2+ stores upon the response to photolysis of caged second messengers in pancreatic {beta} cells. In OGBD-injected MIN6 {beta} cells, control responses to flash photolysis of caged NAADP (approximated final intracellular concentration of 100 nM) (a, n = 58) or caged IP3 (approximated final intracellular concentration of 1 µM) (b, n = 38) were similar. 1 µM thapsigargin had no effect upon NAADP responses (c and g, n = 18) but eliminated IP3 responses (d and h, n = 24). Conversely, 2 µM bafilomycin A1 inhibited NAADP (e and g, n = 31) but had no effect upon IP3 (f and h, n = 24).

 
To confirm that the distribution of lysosomal Ca2+ stores indeed has a bearing upon the spatial profile of the Ca2+ response, we compared NAADP-mediated Ca2+ release with the distribution of the organelle in live cells. Lysotracker Red labeling was markedly polarized and confined to the apical region of pancreatic acinar cells reminiscent of secretory vesicle staining (Fig. 5, a and b) (28), which are highly related if not overlapping organelles (29). The observed pattern with Lysotracker Red faithfully reflected lysosomal staining as confirmed by the elimination of the punctate fluorescence by treatment with either GPN (Fig. 5a) or bafilomycin A1 (Fig. 5b). Interestingly, the organelle distribution coincided with the ensuing NAADP-evoked small Ca2+ oscillations, which do not fully recruit Ca2+-induced Ca2+ release (22, 30) and were confined to the apical pole in acinar cells (Fig. 5c). In contrast, in {beta} cells Lysotracker Red comprised staining of bright punctate bodies, albeit superimposed on a diffuse fluorescent background. This punctate staining was uniformly dispersed throughout the cytoplasm but excluded from the nucleus (Fig. 6, a-c). As in acinar cells, the Lysotracker Red granular staining was eliminated by GPN (Fig. 6a) or bafilomycin A1 (Fig. 6b) but not by thapsigargin (Fig. 6c). Moreover, in MIN6 cells, the response to NAADP in {beta} cells was essentially global (Fig. 6d). Specifically, the close spatial correlation of Ca2+ release and the acidic store distribution strongly imply that there is a substantive rationale for using different Ca2+-storing organelles.



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  		FIG. 5.
Comparison of lysosome-related store distribution and NAADP-induced Ca2+ signals in acinar cells. Micrographs showing vital fluorescence staining of pancreatic acinar cells labeled with 50 nM Lysotracker Red (a and b, left side) plus corresponding bright field images of pancreatic acinar cells (a and b, left side, lower panels). The predominantly apical fluorescence was dramatically reduced by pretreatment with 50 µM GPN (a, right-hand upper panel, n = 4) or 3 µM bafilomycin A1 (b, right-hand upper panel, n = 3). Cells were arbitrarily treated for 25-40 min but fluorescence began to fall immediately upon their application. The local Ca2+ response to photolysis of caged NAADP in a single acinar cell is also confined to the apical pole (c, blue trace), but not to the basal pole (c, red trace) (n = 8).

 



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  		FIG. 6.
Comparison of lysosome-related store distribution and NAADP-induced Ca2+ signals in {beta} cells. The distribution of 50 nM Lysotracker Red fluorescence in pancreatic {beta} cells was detected throughout the cells (except nucleus) (a-c, left panels) and eliminated by 50 µM GPN (a, n = 6) and 2 µM bafilomycin (b, n = 4) but not by thapsigargin (c, n = 4). Photolysis of caged NAADP in {beta} cells induced a global Ca2+ response, and identical localized responses are obtained all over the cell (d, n = 58).

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 EXPERIMENTAL PROCEDURES
 RESULTS
 DISCUSSION
REFERENCES
 
In this present report we provide evidence for a novel mechanism that explains how agonists evoke their own characteristic Ca2+ response, that of organelle selection (Fig. 7). According to this model, different agonists (even within the same cell) mobilize Ca2+ in different ways in time and space by coupling to (selecting) different Ca2+-storing organelles with their own unique properties and distribution. Moreover, we reveal that a given organelle couples via a particular second messenger, such that an agonist selects organelles by recruiting the appropriate messenger complement. Although our recent results in sea urchin egg (5) provided a framework (that different messengers mobilize Ca2+ from different organelles), the differential recruitment of these organelles by different agonists has never been shown.



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  		FIG. 7.
A minimal model for achieving stimulus specificity through organelle selection. Different external stimuli are coupled to different organelles via the stimulus-specific recruitment of non-promiscuous second messenger complements. In pancreatic acinar cells, cholecystokinin (CCK) selects both lysosome-related organelles and endoplasmic reticulum by recruiting NAADP, cADPR, and IP3 respectively. By contrast, acetylcholine (ACh) and bombesin only select the ER using IP3/cADPR. In {beta} cells, glucose stimulates NAADP synthesis to release Ca2+ from lysosome-related organelles. Although our data do not favor a major role for the endoplasmic reticulum via IP3 (or cADPR), we cannot formally exclude their involvement (31). Acetylcholine in {beta} cells uses only the ER/IP3 pathway.

 
In essence, irrespective of cell type, agonists can be divided into those that recruit lysosome-related organelles (cholecystokinin and glucose) and those that do not (acetylcholine and bombesin). Such a conclusion is drawn from pharmacological studies using two mechanistically and chemically distinct inhibitors of lysosomal function, bafilomycin A1 and GPN. Whether added before or during Ca2+ oscillations, these agents selectively inhibited responses to the former pair of agonists, while having little or no effect upon the latter. We are confident that these agents act specifically upon acidic Ca2+ stores because of the following: (a) they did not indiscriminately inhibit all agonists, as evidenced by their lack of effect upon the ER-coupled acetylcholine and bombesin; (b) their site of action is likely downstream of second messengers themselves as indicated by photolysis studies (see below) and therefore not an upstream signal; (c) they do not block depolarization-induced Ca2+ entry; and (d) they eliminate Lysotracker Red staining.

We have proceeded to show that cell surface receptors couple to an intracellular store type by virtue of a characteristic selecting messenger, i.e. NAADP was unique in coupling to lysosome-related organelles, whereas IP3/cADPR coupled to the ER. Not only does the published messenger profiles of the agonists support our hypothesis (3, 15), but the sensitivity of the second messengers themselves to various store inhibitors showed an absolute agreement. Therefore, sea urchin eggs are not anomalous in having acidic stores sensitive to NAADP but rather are vindicated as an excellent model system to study mammalian Ca2+ signaling. Moreover, our data are of interest in the light of a previous study (6) in permeabilized MIN6 cells suggesting that NAADP releases Ca2+ from secretory vesicles, themselves an acidic organelle. It should be noted that our data differ from that by Mitchell et al. (6) because (a) we have used intact cells; (b) we show agonist (glucose) coupling to acidic stores via NAADP; and (c) in our hands NAADP predominantly releases Ca2+ from a bafilomycin A1-sensitive and lysosomal-related store.

At first sight, it might appear contradictory that selective elimination of acidic Ca2+ stores has such a marked effect upon cholecystokinin when clearly there is an additional ER component (Fig. 1) (22). More surprisingly, glucose-stimulated Ca2+ signals in {beta} cells also manifest a profound sensitivity to acidic store blockade when there ought to be a substantial residual Ca2+ entry component (17, 18) (and perhaps an ER component) (31). Although there is currently no complete mechanistic explanation for this absolute dependence, it has been empirically determined that desensitization of the NAADP receptor by its own ligand ablates both the cholecystokinin- as well as the glucose-induced Ca2+ signals (16, 22). Therefore, the effects of bafilomycin A1 and GPN remain entirely consistent with the blockade of the NAADP store. For the acinar cells, it has been suggested that the ER is essential to amplify NAADP-induced Ca2+ release via Ca2+-induced Ca2+ release at the IP3 or ryanodine receptors (3, 25). It is currently less clear how NAADP might affect Ca2+ entry in {beta} cells, but in sea urchin eggs a link between NAADP signaling and voltage-gated Ca2+ channels has been suggested (32).

The agonist-specific recruitment of different organelles also has ramifications in the spatial domain. It has been clearly shown that the apical pole of pancreatic acinar cells has a high density of zymogen granules (33), with only small fingers of ER penetrating into this region, and that the ER is highly concentrated in the basolateral part of the cell (34). The distribution of acidic vesicles may display a more cell-specific pattern; certainly for pancreatic acinar cells, intense Lysotracker Red staining was confined to the apical pole, whereas MIN6 cells appeared to display a more uniform staining. Supporting our model that these are Ca2+ stores, this pattern mirrored the subsequent Ca2+ responses that were evoked upon uncaging NAADP; in pancreatic acinar cells the region of highest NAADP sensitivity was confirmed as the apical pole (30), whereas the MIN6 response was essentially uniform. It should be noted, however, that a previous study in permeabilized cells described the basolateral pole as the region of highest NAADP sensitivity (35). We cannot currently rationalize this apparent discrepancy, but we suggest methodological differences (e.g. permeabilization, uneven distribution of compartmentalized Ca2+ indicator).

In summary, we hypothesize the specific agonist-induced Ca2+ mobilization from lysosomal-like acidic organelles as a universal concept in mammalian cells. The use of distinct, non-contiguous stores would allow the Ca2+ levels therein to be regulated independently, e.g. by altering ATPase (Ca2+ or H+) expression or activity. Indeed, the use of non-ER stores will have other advantages such as avoiding plummeting Ca2+ levels in the ER lumen which affect nascent protein synthesis (36) and protein phosphorylation (37) as well as increasing cell viability by minimizing ER and mitochondria overload and hence apoptosis (38). Conversely, altering the Ca2+ content of acidic stores may affect processes such as secretory vesicle fusion (39), membrane repair (29), and proteolysis (40). Furthermore, if the NAADP-sensitive Ca2+ store is indeed an acidic secretory vesicle (6) or secretory lysosome (29), Ca2+ is delivered precisely where required to evoke exocytosis of zymogens (41), ATP (28), or insulin (6, 42), depending upon cell type, and provides another potential target for treatment of diabetes.


   FOOTNOTES
 
* This work was supported by The Wellcome Trust through a senior fellowship (to A. G.), a Research Career Development fellowship (to S. P.), a Prize Studentship (to M. Y.), and a project grant (to A. G. and S. J. H. A.). 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. Back

§ Both authors contributed equally to this work. Back

To whom correspondence should be addressed: Dept. of Pharmacology, University of Oxford, Mansfield Road, Oxford OX1 3QT, UK. Tel.: 44 01865 271 606; Fax: 44 01865 271 853; E-mail: roser.masgrau-juanola{at}pharmacology.oxford.ac.uk.

1 The abbreviations used are: [Ca2+]i, intracellular Ca2+ concentration; ER, endoplasmic reticulum; IP3, inositol 1,4,5-trisphosphate; cADPR, cyclic ADP-ribose; NAADP, nicotinic acid adenine dinucleotide phosphate; GPN, glycyl-phenylalanine 2-naphthylamide; OGBD, Oregon Green BAPTA-1 dextran. Back


   ACKNOWLEDGMENTS
 
We thank Clive Garnham (University of Oxford) for technical assistance and Justyn M. Thomas (University of Oxford) and Georgina Berridge (University of Oxford) for useful discussions.



   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 EXPERIMENTAL PROCEDURES
 RESULTS
 DISCUSSION
 REFERENCES
 

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