The endoplasmic reticulum (ER) and acidic organelles (endo-lysosomes) act as separate Ca2+ stores that release Ca2+ in response to the second messengers IP3 and cADPR (ER) or NAADP (acidic organelles). Typically, trigger Ca2+ released from acidic organelles by NAADP subsequently recruits IP3 or ryanodine receptors on the ER, an anterograde signal important for amplification and Ca2+ oscillations/waves. We therefore investigated whether the ER can signal back to acidic organelles, using organelle pH as a reporter of NAADP action. We show that Ca2+ released from the ER can activate the NAADP pathway in two ways: first, by stimulating Ca2+-dependent NAADP synthesis; second, by activating NAADP-regulated channels. Moreover, the differential effects of EGTA and BAPTA (slow and fast Ca2+ chelators, respectively) suggest that the acidic organelles are preferentially activated by local microdomains of high Ca2+ at junctions between the ER and acidic organelles. Bidirectional organelle communication may have wider implications for endo-lysosomal function as well as the generation of Ca2+ oscillations and waves.
A universal signal transduction mechanism for extracellular stimuli is the release of Ca2+ from intracellular stores (Berridge et al., 2003) with stimulus-specific Ca2+ patterns fine-tuned by appropriate combinations of three Ca2+-mobilizing intracellular messengers: D-myo-inositol 1,4,5-trisphosphate (IP3), cyclic ADP-ribose (cADPR), and nicotinic acid adenine dinucleotide phosphate (NAADP; Cancela et al., 2002; Morgan and Galione, 2008). Thus, multiple messengers entrain Ca2+ oscillations and waves, e.g., at fertilization (Churchill and Galione, 2001; Santella et al., 2004; Moccia et al., 2006; Whitaker, 2006; Davis et al., 2008), the activation of T cells (Steen et al., 2007; Davis et al., 2012) or pancreatic acinar cells (Cancela et al., 2002; Yamasaki et al., 2004).
While IP3 and cADPR target their cognate receptors on the neutral sarcoplasmic or endoplasmic reticulum (SR/ER), NAADP evokes Ca2+ release from acidic Ca2+ stores (Churchill et al., 2002; Morgan et al., 2011), probably by activating complexes of the two-pore channel (TPC) family (E. Brailoiu et al., 2009, 2010a; Calcraft et al., 2009; Zong et al., 2009; Ruas et al., 2010; Morgan et al., 2011; Davis et al., 2012).
To date, the cross talk (“channel chatter”; Patel et al., 2001) between the NAADP and IP3/cADPR pathways has centered upon “anterograde” signaling from the acidic stores to the ER in the “trigger” hypothesis (or two-pool model; Churchill and Galione, 2001): NAADP activates TPCs on acidic stores to provide the critical “pacemaker” trigger Ca2+ that is subsequently amplified by IP3 receptors (IP3Rs) and/or ryanodine receptors (RyRs) on the neutral ER/SR (Morgan et al., 2011), either via Ca2+-induced Ca2+ release (CICR; Patel et al., 2001; Kinnear et al., 2004; Zong et al., 2009; Brailoiu et al., 2010b; Ruas et al., 2010; Davis et al., 2012) or by luminal priming (Churchill and Galione, 2001; Macgregor et al., 2007).
It is unknown whether Ca2+ signals travel in the reverse direction from ER to the acidic Ca2+ stores to make channel chatter a two-way conversation that might be important for regenerative cycles of Ca2+ oscillations and waves. Consequently, we have investigated whether ER (IP3/cADPR) signals communicate with acidic Ca2+ stores (NAADP) by using a novel single-cell approach for monitoring acidic store activation. The activation of acidic Ca2+ stores is difficult to extract from cytosolic Ca2+ recordings that are the net result of multiple processes. This issue can be offset by monitoring the organelle lumen itself using optical reporters, e.g., targeted to ER, mitochondria, and secretory granules (Arnaudeau et al., 2001; Pinton et al., 2007; Santodomingo et al., 2008). By analogy, we have monitored the luminal pH (pHL) of acidic Ca2+ stores as a readout of activation because a prompt alkalinization accompanies NAADP-induced Ca2+ release in sea urchin eggs (Morgan and Galione, 2007a,b; Morgan, 2011), pancreatic acinar cells (Cosker et al., 2010), and atrial myocytes (Collins et al., 2011).
Hence, we have investigated sea urchin eggs where the sperm stimulus couples to NAADP, cADPR, and IP3 (Morgan, 2011). Given that, of these three, NAADP is unique in changing acidic store pHL in egg homogenate and that these stores are well distributed throughout the sea urchin egg (Lee and Epel, 1983; Churchill et al., 2002; Morgan and Galione, 2007a,b; Ramos et al., 2010), we have imaged Ca2+ store alkalinization to “map” where and when acidic stores are activated during a physiological stimulus. Our results suggest that the ER and acidic vesicles are in close apposition and that Ca2+ released from the ER by IP3/cADPR stimulates the NAADP pathway in an unexpected retrograde manner, thereby amplifying acidic Ca2+ store signaling. This may have profound implications for Ca2+ oscillations and waves in all systems.
Ca2+ release evoked by NAADP or fertilization is accompanied by a rapid increase in the pHL of acidic Ca2+ stores (Morgan and Galione, 2007a,b). In the intact sea urchin egg, this predominantly cortical pHL response was termed a pHLash (pronounced “flash”) that was independent of cytosolic pH changes and exocytosis (Morgan and Galione, 2007a). Because NAADP is unique in evoking this pHL change (Morgan and Galione, 2007b), we exploited this as a “reporter” of acidic store activation (i.e., NAADP-induced Ca2+ release), first testing its sensitivity and spatiotemporal fidelity.
Characterizing responses to photolysis of caged NAADP
We photo-released NAADP from its microinjected caged precursor using a UV laser. No pHL (or Ca2+) responses to UV light were observed in the absence of caged NAADP (Δratio: 0.03 ± 0.01; n = 10, P > 0.05). While measuring pHL, the uniform uncaging of NAADP (Fig. 1 A) replicated the injection of free NAADP (Morgan and Galione, 2007a), i.e., a prompt increase in the pHL of acidic vesicles with the largest response in the cell periphery and a small but detectable response in the egg center. In contrast, when measuring Ca2+, the uniform uncaging of NAADP evoked a uniform Ca2+ response (Fig. 1 B). However, rapid Ca2+ diffusion and the contribution of ER Ca2+ stores (Churchill and Galione, 2000) render it unsuitable for mapping acidic store activation, so we focused on pHL as a more reliable readout.
By varying the UV laser power, the pHL response of both the periphery and the center increased as a function of the NAADP concentration (Fig. 1 C). Indeed, when the magnitude of the responses was normalized to the maximum response, there was no difference in the sensitivity of the two regions, only in their dynamic range (Fig. 1 D). Consequently, the ratio of the responses in the periphery and center is almost invariant with NAADP concentration (the center being ∼30% of the periphery; Fig. 1 E). Kinetically, the maximal pHLash response (at 70% UV) occurred with a time to peak of 6.4 ± 0.4 s and a lag of 3.2 ± 0.3 s (n = 14). Such response times are congruent with other second messenger reporters (Nikolaev et al., 2004).
To address spatial fidelity, we focally uncaged NAADP at one pole of the egg (Fig. 1, F–H). In the majority of eggs (n = 22), the pHL response precisely overlapped with the site of exposure to UV and remained at the site until it waned (Fig. 1, F–H; Fig. S1). This indicated that neither the diffusion of NAADP nor of the target vesicles themselves were confounding factors over this period. Thus, pHL faithfully mapped experimental increases in cytosolic NAADP and acidic store activation. However, in a minority of eggs (n = 6) the initial, polarized response did eventually propagate to the antipode after remaining stationary at the UV site for 19 ± 3 s (Fig. S1 F). This regenerative phase clearly required positive feedback and probably reflects the secondary Ca2+ oscillations and waves that can be entrained by uncaging NAADP (Lee et al., 1997; Churchill and Galione, 2000, 2001).
Effect of NAADP antagonists on pHL
If pHL is a faithful reporter of the NAADP/TPC pathway, then responses should be inhibited by NAADP antagonists. Therefore, we tested several antagonists with the fertilization-induced pHLash. Note that at fertilization, the pHL of acidic vesicles changes differently in different regions of the egg: the periphery shows a prompt alkalinization (the pHLash) that accompanies the main Ca2+ wave and can be mimicked by NAADP (Morgan and Galione, 2007a), whereas the center shows a slow acidification that is driven by an unrelated mechanism dependent upon extracellular Na+ (Lee and Epel, 1983; Morgan and Galione, 2007a).
We first tested the fluorescent NAADP receptor antagonist, Ned-19 (Fig. S2; Naylor et al., 2009; Barceló-Torns et al., 2011). Preincubation of eggs with Ned-19 resulted in a concentration-dependent increase in Ned-19 loading, as assessed by its intrinsic fluorescence (Fig. 2 C), but which did not affect the resting pHL (% control, 80 µM, 95 ± 2; 160 µM, 105 ± 3; P > 0.05, n = 47–83). In keeping with a role for NAADP, Ned-19 inhibited the sperm-induced pHLash (Fig. 2, A–C). This was not due to a general interference with fertilization because it had little effect upon the slower acidification of the central acidic granules (Fig. 2, A–C; Lee and Epel, 1983; Morgan and Galione, 2007a) or upon fertilization envelope lifting driven by the other remaining fertilization messengers, cADPR and IP3 (Fig. 2 B, inset). Furthermore, we confirmed that Ned-19 inhibited the pHLash evoked by injection of NAADP itself (Fig. 2 D).
Unlike RyRs and IP3Rs, NAADP receptors are inhibited by high concentrations of L-type Ca2+ channel blockers in sea urchin egg and other cell types (Genazzani et al., 1997; Mándi et al., 2006; Zhang and Li, 2007). Conveniently, both the main fertilization-induced Ca2+ wave (Shen and Buck, 1993) and the pHLash itself (Fig. S2 E) are independent of Ca2+ influx. As a selective NAADP antagonist (Fig. S2, A and B), the phenylalkylamine, diltiazem, selectively inhibited the sperm-induced pHLash compared with the central acidification (Fig. 2 F) and, like Ned-19, did not interfere with fertilization envelope lifting (unpublished data). Diltiazem also inhibited the responses to NAADP injection in terms of the pHLash and fertilization envelope lifting (consistent with a block of Ca2+ release, Fig. 2 G).
Similarly, another L-type Ca2+ channel blocker, nifedipine (a dihydropyridine), also inhibited the sperm-induced pHLash (but not the central acidification, Fig. 2 E). This partial inhibition by nifedipine was at its limit of solubility and probably reflects its lower membrane permeability compared with diltiazem (XlogP3 of 2.2 and 3.1, respectively). Finally, the nucleotide mimetic, PPADS, inhibited NAADP responses (Fig. S2; Billington and Genazzani, 2007) and selectively inhibited the pHLash when microinjected into the egg, precluding an extracellular site of action (Fig. 2 E). Together, the data affirm NAADP as the main pHL messenger at fertilization and strengthen pHL as a physiologically relevant reporter of the NAADP pathway.
Ca2+ drives NAADP synthesis and pHL responses
With a means of mapping acidic store activation, we asked whether Ca2+ released from the ER can signal to acidic vesicles. First, we elevated Ca2+ independently of sperm using ionomycin, a Ca2+ ionophore that mobilizes Ca2+ from neutral (and not acidic) Ca2+ stores (Fasolato et al., 1991; see Fig. 5 A) and which acts only weakly at the plasma membrane (Morgan and Jacob, 1994). Ionomycin evoked a prompt Ca2+ rise in eggs (Fig. 3, Eii), as well as a pHLash (Fig. 3, Ei). However, the pHL response was slower, mimicking the sperm response in its magnitude (Fig. 3, A–C) and kinetics (time to peak (s) ionomycin: 69 ± 2; sperm: 60 ± 2; n = 31–39). Because ionomycin has no direct effect upon the pHL of sea urchin egg acidic vesicles (Morgan and Galione, 2007b) it follows that it is the released Ca2+ that affects acidic vesicles.
Their relative kinetics imply that the pHLash is downstream of Ca2+ and this is consistent with the one mirroring the other in space: the pHL (Fig. 3, Di) and Ca2+ (Fig. 3, Dii) responses to sperm each occurred as waves (that propagate away from the sperm entry point; Morgan and Galione, 2007a), whereas ionomycin induced a synchronous cortical elevation of either pHL or Ca2+ (Fig. 3, Ei and ii). Taken together, the kinetic and spatial interrelationship of the two parameters is consistent with Ca2+ driving pHL changes.
How could the Ca2+ released by ionomycin be stimulating acidic vesicles? As argued previously for sperm (Morgan and Galione, 2007a), the pHLash is not limited by the time of the acidic stores to respond to NAADP (an ∼10-times faster process), so the kinetics may reflect the time taken to generate NAADP. We therefore tested whether ionomycin could elevate NAADP levels, as measured in populations of eggs with a radioreceptor assay (Lewis et al., 2007). As observed previously, fertilization increased NAADP in two phases: the first phase represents an increase in NAADP in the sperm as they contact the egg jelly, the second phase is due to de novo synthesis of NAADP inside the egg (Fig. 3 F; Churchill et al., 2003). Remarkably, ionomycin increased NAADP in eggs with kinetics that not only overlapped with the second “egg” phase but also that mirrored the slow time to peak of the pHL response (each peaked at ∼70 s; Fig. 3 F). The maximum magnitude of the ionomycin-evoked NAADP increase was variable between preparations, being 30 ± 18% (n = 4) of that evoked by sperm.
If ionomycin increases NAADP, then its pHLash should be sensitive to NAADP inhibitors. We also used SKF96365 to block NAADP (Moccia et al., 2004; Bezin et al., 2008) after first confirming its >50-fold selectivity for NAADP over cADPR and IP3 (Fig. S2, C and D). Although SKF96365 could not be used with sperm because it blocks sperm chemotaxis and the acrosome reaction (Hirohashi and Vacquier, 2003; Yoshida et al., 2003; Treviño et al., 2006), it was an effective inhibitor of the ionomycin-induced pHLash (Fig. 3 I). Similarly, Ned-19 and diltiazem also inhibited the ionomycin pHLash (Fig. 3, G and H) with a similar potency (and selectivity over the slow central acidification) to that seen against the sperm-induced response (Fig. 2, C and F). The data suggest that Ca2+ release from the ER by ionomycin can stimulate a pHLash via NAADP generation and action.
ER channel activation evokes a pHLash
We next investigated if more physiological routes of Ca2+ release from the ER could support acidic store activation. Sea urchin eggs are sensitive to both ER-targeting messengers, IP3 and cADPR (Churchill and Galione, 2001; Morgan and Galione, 2007b), and we first confirmed that microinjection of these messengers elicited robust Ca2+ responses similar to those with NAADP itself (Fig. 4, A and B). When subsequently measuring pHL changes, not only did NAADP evoke a pHLash but so did cADPR and IP3 (Fig. 4, C and D). Although the examples shown are among the best responses seen with cADPR and IP3, overall they were weaker stimuli of the pHLash than was NAADP (Fig. 4 E), with IP3 being the least efficacious. This differential messenger profile for the pHLash contrasted with the similar global Ca2+ responses (Fig. 4, A and B). The fact that IP3 and cADPR do not directly affect pHL in egg homogenate (Morgan and Galione, 2007b) implies that these receptors do not reside on acidic vesicles themselves, and that the mechanism that normally couples Ca2+ to pHL is lost upon homogenization.
Differential coupling of Ca2+ signals to the pHLash
Such differential coupling of second messengers to the pHLash might be explained by local Ca2+ domains; i.e., a high local Ca2+ required for a pHLash might be readily attained by NAADP but less so by cADPR and IP3. We therefore tested whether other Ca2+ signals differentially couple to the pHLash. First, release of Ca2+ from the ER was evoked by inhibiting SERCA (sarco-endoplasmic reticulum Ca2+ ATPase) with cyclopiazonic acid (CPA). In most cells, CPA stimulated a small, slow Ca2+ release (Fig. 5, A–C) that failed to evoke a pHLash (Fig. 5, D–F), even though a subsequent addition of ionomycin was successful (Fig. 5, D–F). This is consistent with the CPA-induced Ca2+ rise failing to reach a local Ca2+ threshold. Nonetheless, in ∼28% of cells, CPA produced a secondary peak of Ca2+ release after a long delay (285 ± 24 s; Fig. 5, A–C) that translated into a pHLash after 361 ± 15 s. Overall, CPA coupled weakly to a pHLash.
In contrast to CPA, photolysis of caged Ca2+ elicited a very rapid increase in Ca2+ whose amplitude was indistinguishable (P > 0.05) from that produced by sperm (Fig. 5, G–I). However, in spite of the globally similar Ca2+ signals, the photolysis of caged Ca2+ failed to produce a pHLash in 90% of eggs (only two eggs gave a pHLash; Fig. 5, J–L). This weak coupling provides strong evidence that it is the local and not global Ca2+ that is an important determinant of the coupling efficiency.
Ca2+ release from the ER stimulates NAADP receptors
We then asked what mechanism couples IP3/cADPR to a pHLash, with potential pathways depicted in Fig. 6 A. We first tested whether ER Ca2+ recruits the NAADP pathway by using three NAADP receptor antagonists. At concentrations that block NAADP itself, Ned-19, diltiazem, and SKF96365 also inhibit the pHLash response to both cADPR and IP3 (Fig. 6, B and C). This places NAADP action downstream of ER Ca2+ because the inhibitors do not affect Ca2+ release evoked by IP3 or cADPR (Fig. S2; Genazzani et al., 1997; Naylor et al., 2009), as confirmed by the weak effect of the inhibitors upon Ca2+-dependent exocytosis (fertilization envelope lifting; Fig. 6 D). We therefore exclude pathway 2 (Fig. 6 A).
Because IP3Rs and RyRs are on the ER, Ca2+ must diffuse from the ER to target sites that affect acidic stores, and this should be blocked by EGTA. Accordingly, EGTA ablated the pHLash response to either cADPR or IP3 (Fig. 6, B and C). This contrasted with the robust pHL response to NAADP in the presence of EGTA (Fig. 6, B and C; Morgan and Galione, 2007b). The data suggest that IP3 and cADPR stimulate acidic stores after Ca2+ diffusion from the ER to target domains (Fig. 6 A, pathway 1 or 3).
Interestingly, EGTA did slightly modify the response to NAADP, reducing the amplitude (Fig. 5, B and C) and slowing the upstroke kinetics (time to peak [s]: Ctrl, 6 ± 0; EGTA, 28 ± 3; P < 0.001). The fact that EGTA (a slow Ca2+ buffer) strongly inhibited the IP3/cADPR pHLash but was weaker toward the NAADP pHLash was further evidence of local Ca2+ domains around acidic vesicles facilitating the pHLash. As a final confirmation, we tested the fast Ca2+ buffer, BAPTA, which is able to dissipate Ca2+ gradients in microdomains (Kidd et al., 1999; G. Brailoiu et al., 2009). The NAADP-induced pHLash was almost completely abolished by BAPTA (Fig. 6, B and C). We conclude that Ca2+ release by NAADP evokes locally high Ca2+ concentrations around the acidic vesicles that are required for acidic vesicle activation (alkalinization). Together the data support the main highlighted pathway 3 in Fig. 6 A in which IP3- or cADPR-induced Ca2+ release from the ER activates acidic vesicles via the Ca2+-sensitive NAADP pathway.
To complement NAADP inhibition, we examined the effect of NAADP receptor desensitization. Microinjection of NAADP into the egg demonstrably desensitized both the Ca2+ and pHLash responses to a second NAADP injection (Fig. 7, Ai and Ci); crucially, NAADP desensitization inhibited the IP3-induced pHLash (Fig. 7, Ciii) without affecting IP3-induced Ca2+ release (Fig. 7, Aiii). Thus, desensitization uncoupled IP3-induced Ca2+ release from the pHLash and mimicked the action of NAADP antagonism.
Conversely, if IP3 recruits NAADP receptors, then injection of IP3 first should cross-desensitize them. We found that both the Ca2+ (Fig. 7, Aiv) and pHLash (Fig. 7, Civ) responses to NAADP were reduced by prior IP3 injection, consistent with heterologous desensitization of the NAADP receptors.
Unfortunately, the analogous cross-desensitization of the cADPR pHLash response was not technically possible because when NAADP was injected first, the subsequent cADPR-induced Ca2+ release was profoundly cross-desensitized (Fig. S3), probably reflecting the secondary recruitment of the cADPR pathway by NAADP (Churchill and Galione, 2000) and its consequent, persistent desensitization (Thomas et al., 2002). Nonetheless, the data overall support our model of bidirectional communication.
Fertilization uses bidirectional Ca2+ signaling
The corollary of such cross talk is that ER Ca2+ is important physiologically for facilitating acidic store activation via NAADP. We first tested whether cytosolic Ca2+ was important for the pHLash by microinjecting eggs with EGTA. After verifying that EGTA blocked the Ca2+ response at fertilization (Fig. 8 A), we showed that it did indeed inhibit the pHLash (when measured throughout the entire periphery; Fig. 8 B). However, when analyzed more closely, some eggs injected with EGTA showed a highly localized, nonpropagating pHL increase (Fig. 8, C and D). Moreover, this “hot spot” coincided with the point of sperm contact (Fig. 8 C, inset) and may correspond to the bolus of NAADP delivered to the egg by sperm (see Discussion; Churchill et al., 2003). An additional hot spot of pHL was observed in another part of the egg periphery (presumably due to polyspermy when fertilization envelope lifting is inhibited by EGTA). This suggests that Ca2+ is physiologically important for propagating acidic store activation, apparently by amplifying the initial sperm-induced trigger.
We then tested whether this facilitating Ca2+ was released from the ER by IP3/cADPR. Consequently, fertilization was effected in eggs that had been microinjected with a cocktail of IP3 and cADPR antagonists (heparin and 8-NH2-cADPR, respectively; Churchill and Galione, 2000) or with an injection marker alone. In the absence of inhibitors, the pHLash proceeded as a wave away from the point of sperm entry (Morgan and Galione, 2007a) to be subsequently mirrored in the antipode (Fig. 3, Di; Morgan and Galione, 2007a). One would predict that the effect of blocking ER Ca2+ release would be similar to microinjecting EGTA and, qualitatively, this is what we observed: a nonpropagating, local pHL response remained at one pole of the cell (Fig. 8, E and F) that was smaller in amplitude (Fig. 8 G) and slower (Fig. 8 H) than the initiation site response in control eggs. These data are consistent with ER Ca2+ release amplifying acidic store activation during fertilization.
Acidic vesicle/ER junctions
These functional data imply a close physical apposition of acidic Ca2+ stores and the ER that was borne out by examining the cellular architecture. In the cortex where the pHLash was observed, acidic vesicles and ER were densely packed and, irrespective of the slice depth (1–22 µm), were consistently closely apposed (Fig. 9). Indeed, at this spatial resolution, the vast majority of vesicles were juxtaposed to ER cisternae (Fig. 9, inset), possibly reflecting vesicle tethering in order to maintain acidic vesicle–ER junctions (Kilpatrick et al., 2012; Patel and Brailoiu, 2012).
The current anterograde model of NAADP signaling describes acidic Ca2+ stores as the providers of local, trigger Ca2+, which is amplified by Ca2+-sensitive ER Ca2+ channels, IP3Rs, or RyRs (channel chatter; Patel et al., 2001; G. Brailoiu et al., 2009; Morgan et al., 2011). Indeed, in the sea urchin egg, NAADP couples to either channel family (Churchill and Galione, 2000, 2001), hinting at a close apposition of all three channels that is made possible by the structural intimacy of the ER and acidic vesicles observed by either light (Davis et al., 2008; Morgan, 2011) or electron microscopy (Sardet, 1984; Poenie and Epel, 1987; Henson et al., 1989; Fishkind et al., 1990; McPherson et al., 1992). With such a functional triad, we wondered whether channel chatter is a two-way conversation with the ER signaling in a retrograde manner back to the acidic vesicle.
Therefore, we used pHL as a marker of acidic store activation by NAADP. By uncaging NAADP, we verified that pHL responds over an appropriate, quasi-linear concentration range and is rapid, spatially sensitive and is not distorted by diffusion. Importantly, NAADP is unique in directly activating acidic vesicles because the alkalinization in egg homogenate cannot be recapitulated by ER Ca2+ release agents (IP3, cADPR, ionomycin, or SERCA inhibition; Morgan and Galione, 2007b). Finally, we verify that fertilization-induced pHL changes are primarily dependent upon the NAADP pathway (Morgan and Galione, 2007a) judging by the effect of four NAADP inhibitors. pHL therefore bears all the requisite hallmarks of an NAADP/acidic store reporter.
Ca2+ release from the ER recruits the NAADP pathway
Our primary hypothesis was that Ca2+ release from the ER resulted in acidic vesicle activation, and it was supported by several lines of evidence. First, the Ca2+ and pHL responses mirror each other in space (see the focal photolysis of caged NAADP, sperm-induced waves, or uniform ionomycin-induced responses). Second, Ca2+ precedes pHL with sperm (Morgan and Galione, 2007a) or ionomycin. Third, releasing Ca2+ from the ER (with ionomycin, CPA, IP3, or cADPR) can drive a pHLash. Fourth, inhibiting a rise of Ca2+ (with Ca2+ buffers or intracellular channel antagonists) abrogates the pHLash.
In terms of the underlying pathway, we conclude that NAADP is an obligate component because the pHLash was blocked by NAADP antagonism or desensitization, irrespective of the ER stimulus (ionophore, second messenger, fertilization). We are confident that the antagonist pharmacology is reliable because: (1) five structurally unrelated inhibitors inhibit the pHLash; (2) these inhibitors are selective for the pHLash over the NAADP-independent central acidification; (3) they are weak inhibitors of Ca2+ release evoked by IP3 or cADPR (Genazzani et al., 1997; Mándi et al., 2006; Zhang and Li, 2007; Naylor et al., 2009) or Ca2+-dependent exocytosis of the fertilization envelope; (4) NAADP desensitization mimicked NAADP antagonism; and (5) pHLash blockade is independent of effects upon plasma membrane Ca2+ channels or basal pHL. But how does ER Ca2+ activate acidic stores? We propose that there are two Ca2+-dependent processes that can contribute to the pHLash—one drives NAADP synthesis, the second facilitates NAADP receptor (TPC) activation—and we shall discuss the evidence.
Ca2+-dependent NAADP synthesis
The most direct evidence for Ca2+-dependent NAADP synthesis was that ionomycin stimulated NAADP levels in egg populations and, correspondingly, the ionomycin-evoked pHLash was kinetically sluggish (consistent with the time to generate NAADP) and sensitive to NAADP antagonism. Unfortunately, we are prevented from directly testing the importance of Ca2+ during fertilization because L. pictus eggs possess low esterase activity, which precludes EGTA/AM-loading in the NAADP radioreceptor assay (Morgan and Galione, 2008). Ca2+-dependent messenger production has precedents in IP3 (Thaler et al., 2004), cADPR (Kim et al., 2008; Shawl et al., 2009), and cAMP (Cooper, 2003) and is an emerging theme for NAADP too, having been postulated in sperm (Vasudevan et al., 2010). At present we cannot say whether Ca2+ is the sole stimulus of NAADP synthesis at fertilization, but it suggests that Ca2+ can be a component.
Ca2+ feedback at NAADP-sensitive channels
Nevertheless, NAADP synthesis cannot readily explain all aspects of the Ca2+-dependent activation of acidic vesicles. For instance, the slow kinetics (≤70 s) of the ionomycin-induced pHLash are credibly consistent with synthesis, but the more rapid effects of IP3 or cADPR (≤4 s) are more difficult to rationalize in such terms (unless exquisitely coupled). We therefore invoke a direct action of Ca2+ upon the acidic stores themselves.
First, for acidic vesicle activation, NAADP exhibited a co-requirement for Ca2+ because the pHLash was sensitive to Ca2+ chelators, EGTA and BAPTA. Moreover, the degree of inhibition was commensurate with the kinetics of Ca2+ buffering (slow EGTA was weaker than was fast BAPTA). With ∼2.5 mM chelator in the cytosol, one can calculate (Stern, 1992; Dargan and Parker, 2003 [assuming a 200 µm2/s Ca2+ diffusion coefficient appropriate for high concentrations of Ca2+ around a channel; Allbritton et al., 1992; Naraghi and Neher, 1997]) that the maximum range of Ca2+ action after release from a channel would be ∼300 nm in the presence of EGTA but ∼10 nm in the presence of BAPTA. In other words, the Ca2+ released by NAADP must be acting locally in microdomains 10–300 nm from the TPC to facilitate the pHLash (compare microdomains in other systems; G. Brailoiu et al., 2009). Given that Ca2+ channel complexes span 15–28 nm in diameter (Samsó and Wagenknecht, 1998; Wolf et al., 2003; Taylor et al., 2004), we propose that Ca2+ feedback reinforces TPC activation at the intra- as well as intermolecular level.
Acidic vesicle/ER junctions
It is this Ca2+ sensitivity of the NAADP system that may partly explain the trans-stimulation of acidic stores by Ca2+ from the ER. The NAADP-induced pHLash was robust in the presence of EGTA, indicating that TPC activation remained detectable. Nonetheless, the pHLash evoked by IP3 or cADPR was completely blocked by EGTA pointing to the coupling mechanism being affected. The effect of EGTA is unlikely to be due to simply blocking Ca2+ release at IP3Rs/RyRs because local CICR and ER channel opening persist in the presence of the slow chelator (Dargan and Parker, 2003; Dargan et al., 2004; Shuai and Parker, 2005; Smith and Parker, 2009). We conclude that EGTA prevents a pHLash by buffering the ER Ca2+ within the junction before it reaches target sites on adjacent vesicles.
Our staining of the ER and acidic vesicles revealed a close proximity, and electron micrographs of the sea urchin egg cortex confirm the close (≤300 nm) apposition of acidic vesicles and ER elements (Sardet, 1984; Poenie and Epel, 1987; Henson et al., 1989; Fishkind et al., 1990; McPherson et al., 1992). Nevertheless, our functional data suggest that the resident IP3Rs and RyRs, though close, must be farther than 300 nm from the Ca2+-sensitive targets in order to be EGTA sensitive. This can be rationalized if the channels are slightly along the ER branches and RyR immunogold labeling is consistent with such distances (McPherson et al., 1992), but comparable data for IP3Rs are not available. A preferential closeness of RyRs and TPCs is feasible because RyRs (McPherson et al., 1992) and TPCs (Ruas et al., 2010) are relatively denser in the cortical (i.e., pHLash) region compared with IP3Rs (Parys et al., 1994). In addition, the larger Ca2+ conductance of RyRs compared with IP3Rs (Pitt et al., 2010) might also contribute to preferential coupling. Such communication between ER and acidic vesicles is lost upon homogenization (Morgan and Galione, 2007b), conceivably because it destroys the native apposition of the two organelles.
Given the inhibition by NAADP antagonism/desensitization, ER Ca2+ appears to stimulate TPCs on the acidic vesicles. Considering that cytosolic Ca2+ effects at the NAADP receptor have hitherto been discounted (Chini and Dousa, 1996; Genazzani and Galione, 1996; Bak et al., 1999; Mándi et al., 2006), such a Ca2+ stimulation may instead occur at the luminal face of NAADP-regulated channels (after Ca2+ uptake): high luminal [Ca2+] appears to enhance the conductance of sea urchin NAADP receptors (Morgan et al., 2012), TPC1 (Rybalchenko et al., 2012) and TPC2 (Pitt et al., 2010), analogous to the luminal regulation of IP3Rs and RyRs (Burdakov et al., 2005). Ca2+ transfer from ER to acidic vesicles would be reminiscent of bidirectional communication between other inter-organelle partnerships such as ER/mitochondria (Arnaudeau et al., 2001; Rizzuto and Pozzan, 2006).
Bidirectional signaling during cell stimulation
Is retrograde signaling from ER to acidic vesicle physiological? NAADP antagonism confirmed that the fertilization-induced pHLash is driven primarily by NAADP (Morgan and Galione, 2007a), but the fact that Ca2+ chelation and ER channel blockers also inhibit the pHLash at fertilization points to an additional role of cADPR/IP3. In particular, this blockade of the ER “balkanizes” the pHLash around the sperm entry site and is consistent with the retrograde signal amplifying acidic store activation.
What might be the order of events at fertilization? When a sperm contacts the egg jelly, NAADP first rapidly increases within the sperm head (Churchill et al., 2003) to drive the acrosome reaction (Vasudevan et al., 2010) and, incidentally, to provide a preformed bolus of NAADP that can be delivered to the egg upon fusion (Churchill et al., 2003). In the absence of egg amplification (i.e., plus EGTA or ER channel blockade), the early focal pHL change around the sperm entry point is certainly consistent with a visualization of this NAADP delivery.
Because the volume of the sperm is ∼6 orders of magnitude smaller than the egg’s, dilution of the bolus in the vast egg cytosol necessitates further NAADP production within the egg. We therefore propose that the observed slow second phase of NAADP synthesis (Churchill et al., 2003) can be driven by an increase in Ca2+ that is dependent upon cADPR/IP3; this is analogous to waves of IP3 synthesis stimulated by Ca2+ in sea urchin eggs (Kuroda et al., 2001; Thaler et al., 2004).
If cADPR and IP3 provide ER Ca2+ to facilitate the NAADP pathway, then these ER messengers should precede the second (egg) phase of NAADP synthesis (Churchill et al., 2003); their involvement in the initiation and propagation of the fertilization Ca2+ wave (Galione et al., 1993; Lee et al., 1993; Davis et al., 2008) and the kinetics of cADPR or IP3 production (Kuroda et al., 2001; Leckie et al., 2003; Thaler et al., 2004) favor this idea.
In addition to this novel retrograde signaling mode, conventional anterograde signals from the acidic vesicles to the ER do occur in the egg: NAADP-induced Ca2+ release recruits cADPR and IP3, as expected (Churchill and Galione, 2000). Hence, we envisage mutually supportive Ca2+ feedback between acidic vesicles and ER as the Ca2+ wave front propagates across the egg.
Beyond the egg
Our current work expands the channel chatter model into a two-way conversation between ER and acidic Ca2+ stores. Reports of NAADP acting downstream of IP3 or cADPR are few and have not detailed a mechanism (e.g., in ascidian oocytes [Albrieux et al., 1998] and T-lymphocytes [Berg et al., 2000]), but the implications may be far reaching: we do not understand how NAADP is choreographed during Ca2+ oscillations and waves in mammalian cells, in spite of its importance (Cancela et al., 1999). Our model provides a reinforcement loop whereby consecutive rounds of acidic vesicle-to-ER and ER-to-acidic vesicle Ca2+ feedback occur during oscillations, e.g., during the pacemaker rise of the next spike or at the Ca2+ wave front (Fig. S4). Local ER/acidic vesicle communication has also been highlighted recently in mammalian cells (Kilpatrick et al., 2012; Sanjurjo et al., 2012). That the ER can signal to acidic vesicles may also have far-reaching implications for endo-lysosomal functions such as resident enzyme activity, autophagy, and the pathology of diseases that affect lysosomal Ca2+ fluxes (Lloyd-Evans et al., 2008; Morgan et al., 2011; Coen et al., 2012).
Materials and methods
Sea urchin eggs from Lytechinus pictus were harvested by intracoelomic injection of 0.5 M KCl and collected in artificial sea water (ASW [mM]: 435 NaCl, 40 MgCl2, 15 MgSO4, 11 CaCl2, 10 KCl, 2.5 NaHCO3, and 20 Tris, pH 8.0), and de-jellied by passage through 100-µm nylon mesh (EMD Millipore). Sperm, on the other hand, were collected “dry” and maintained at 4°C until use.
Confocal laser scanning microscopy
Eggs were maintained at room temperature in ASW and imaged on glass poly-d-lysine–coated coverslips mounted on a confocal laser scanning microscope (LSM 510 Meta; Carl Zeiss); an Axiovert 200M (Carl Zeiss) equipped with Zeiss objectives (10× Neofluar, NA 0.3; 40× Fluar, NA 1.3) was controlled by LSM software (Carl Zeiss). Excitation/emission (nm) wavelengths per channel were 351/>385 (UV), 488/505–530 (green), 543/>560 (red), 633/645–719 (far-red). When inhibitors were tested at fertilization, eggs were preincubated with the inhibitors (or vehicle) but sperm were preactivated in ASW without inhibitors for 20–30 s before their addition to eggs in order to minimize drug effects upon sperm. Images were analyzed using custom-written Magipix software (R. Jacob, King’s College London, London, England, UK).
pHL and Ca2+ measurements
pHL was usually monitored ratiometrically in eggs co-loaded with 10 µM Acridine orange and 1 µM LysoTracker red DND-99 for 15–20 min at room temperature and imaged using green/red channels, respectively, as described previously (Morgan and Galione, 2007a). Data are expressed as the ratio of the Acridine orange/LysoTracker red signals with an increase in the ratio reflecting an increase in pHL.
In pHL experiments where different second messengers were consecutively injected, eggs were labeled with 10 µM Acridine orange only (10–30 min) and its fluorescence monitored simultaneously with that of the injection marker (see following section). Because of the slight change of cell shape elicited by the first injection (due to fertilization envelope lifting), the peripheral region of interest was always redefined to monitor the fluorescence response to the second injection.
Cytosolic Ca2+ was measured in two modes: ratiometric recording involved coinjecting eggs with 10 kD dextran conjugates of fluo-4 (Ca2+-sensitive) and Alexa Fluor 647 (Ca2+-insensitive) at pipette concentrations of 1 mM and 250 µM, respectively (data are expressed as the green/far-red ratio); alternatively, single wavelength recording used either fluo-4 dextran (green channel) or rhod-dextran (high affinity form, red channel) only.
Caged compounds and photolysis
Caged NAADP was synthesized in-house using sequential reactions (Lee et al., 1997; Morgan et al., 2006). In brief, 2-nitroacetophenone hydrazone was synthesized from 2-nitroacetophenone and hydrazine monohydrate under acidic conditions; the chloroform-extracted hydrazone was then converted to 1(2-nitrophenyl)diazoethane using MnO2 and finally incubated with NAADP under acidic conditions to cage the phosphate groups. The caged product was purified by HPLC and stored at −80°C. After treatment with alkaline phosphatase beads (to remove contaminating free NAADP), caged NAADP was then microinjected into eggs (50-µM pipette concentration) together with 1 mM fluo-4 dextran to measure Ca2+ or with 50–200 µM Alexa Fluor 647 dextran as an injection marker for pHL recordings. Photolysis was effected with a Coherent Enterprise UV laser (351 nm) and exposure was either global (70% power, 5 frames at 1 Hz) or focal (10–70% power, 2 iterations) as controlled by an acousto-optical tunable filter.
Caged Ca2+ (NP-EGTA) at 25 mM in 10 mM Hepes (pH 7) was coinjected with either fluo-4 dextran or Alexa Fluor 647 dextran. Focal photolysis was effected by UV laser (351- and 364-nm lines, 20 iterations at 50–75% power).
We exploited the intrinsic fluorescence of Ned-19 to monitor its loading into intact eggs (excitation at 364 nm; emission >385 nm). To minimize photobleaching, images were captured discontinuously after known times of Ned-19 preincubation and all with identical acquisition settings. We also confirmed that the fluorescence of Ned-19 did not interfere with Acridine orange fluorescence in vitro: 10 µM Acridine orange fluorescence was recorded in a fluorimeter (excitation 488 nm, emission 526 nm; model LS-50B, PerkinElmer) in a medium containing (mM): 250 potassium gluconate, 250 N-methylglucamine, 20 Hepes, and 1 MgCl2, pH 5.5. In the presence of 100 and 200 µM Ned-19, the Acridine orange signal was 106 ± 2 and 109 ± 3% of that in the absence of Ned-19, respectively (n = 3, P > 0.1).
Labeling of ER and acidic vesicles was performed as described previously (Terasaki and Jaffe, 1991; Davis et al., 2008). In summary, a saturated solution of DiI (DiIC18(3)) was prepared by vortexing a few grains of DiI in 200 µl of soybean oil and was microinjected into the egg center. The DiI diffused from the central oil droplet into the contiguous membrane system of the ER in 15–30 min, during which time acidic vesicles were labeled by addition of 1 µM LysoTracker green DND-26 (Invitrogen). Confocal 1-µm optical sections were collected using the standard green and red channel settings.
For the NAADP assay, NAADP and [32P]NAADP were enzymatically synthesized in-house using the base-exchange reaction of Aplysia ADP-ribosyl cyclase (ARC; Lee et al., 1997; Morgan et al., 2006; Vasudevan et al., 2010). For NAADP, 13 mM NADP and 100 mM nicotinic acid were incubated at pH 4.5 with ARC for 1 h at room temperature and NAADP purified by HPLC. Two stages were required for [32P]NAADP: first, [32P]NAD was phosphorylated to [32P]NADP using human NAD kinase and 10 mM ATP; second, the product was converted to [32P]NAADP by incubating with 100 mM nicotinic acid and ARC at pH 4.5 for 1 h at room temperature, and purified by HPLC.
The time course of NAADP changes in populations of eggs stimulated with sperm or ionomycin was determined biochemically. For a given experiment, 1 ml of eggs was diluted into 20 ml of ASW. A 2-ml aliquot was taken from this solution at each time point and centrifuged at 9,000 g. Centrifugation was then stopped as swiftly as possible, and the supernatant discarded. 100 µl HClO4 was then added. To disrupt the cells, sonication was performed (Jencons Vibra-Cell at amplitude 60) for three bursts of 5 s. The time point was taken as the start of sonication and samples were then placed on ice. The denatured protein was pelleted by centrifugation at 9,000 g for 10 min and stored at −80°C for later analysis. The supernatant was neutralized with an equal volume of 2 M KHCO3 and vortexed. Centrifugation at 9,000 g for 10 min was again used to remove the KClO4 precipitate. The resulting supernatant was stored at −80°C for NAADP analysis. The protein concentration in the precipitated pellet was determined using the BCA reagent.
As reported in detail previously (Lewis et al., 2007), NAADP levels were determined using the NAADP-binding protein from sea urchin (L. pictus) egg homogenate, which is highly selective for NAADP (Churamani et al., 2004; Lewis et al., 2007). First, we added 25 µl of test sample to each tube and then added 125 µl of 1% (vol/vol) sea urchin egg homogenate in intracellular medium and incubated the reaction for 10 min at 25°C. To each tube we then added 0.2 nM of [32P]NAADP (∼50,000 cpm) diluted in 100 µl of intracellular medium (250 mM N-methyl-d-glucamine, 250 mM potassium gluconate, 1 mM MgCl2, and 20 mM Hepes, pH 7.2) and incubated the reaction for 10 min at 25°C. Bound NAADP was then trapped onto Whatman GF/B filter papers using a Brandel cell harvester. We washed the filters three times with 1 ml of a buffer containing 20 mM Hepes and 500 mM potassium acetate, pH 7.4, and the bound radioactivity was estimated by phosphorimaging. The amount of NAADP in each test sample was determined by comparison with a standard curve containing known amounts of NAADP. Results are normalized to the protein content (pmol NAADP/mg protein).
Fluorescence traces in all figures are from single cells representative of n eggs from ≥3 preparations unless indicated otherwise. Data are expressed as the mean ± SEM. Two datasets were compared using Student’s t test, whereas multiple groups were analyzed using ANOVA and a Tukey-Kramer or Dunnett’s post-test. A nonparametric ANOVA (Kruskal-Wallis and Dunn post-test) was applied when required. Data were paired where appropriate and significance assumed at P < 0.05.
Caged NAADP, free NAADP, and [32P]NAADP were synthesized in-house. [32P]β-NAD+ was obtained from GE Healthcare. IP3 was from LC Laboratories. Cyclic ADP-ribose, NAADP, IP3, PPADS (pyridoxalphosphate-6-azophenyl-2′4′-disulfonic acid), EGTA, BAPTA, heparin (low MW), 8-NH2-cADPR, diltiazem, nifedipine, cyclopiazonic acid, ADP-ribosyl cyclase, and soybean oil were obtained from Sigma-Aldrich. Ned-19 (mixed isomers) was from IBScreen (Moscow, Russia). Acridine orange, LysoTracker red DND-99, rhod-dextran (10 kD, high affinity form), Fluo-4 dextran (10 kD), Alexa Fluor 647 dextran (10 kD), NP-EGTA (potassium salt), and DiIC18(3) were from Invitrogen. Ionomycin and SKF96365 were from EMD Millipore. NAD kinase was a kind gift from M. Ziegler (University of Bergen, Bergen, Norway). All other reagents were of analytical grade.
Online supplemental material
Fig. S1 shows the regenerative propagation of the pHL response away from the focal photolysis region in a minority of eggs and compares such real responses with theoretical models. Fig. S2 compares the effect of NAADP antagonists upon Ca2+ release in egg homogenate evoked by NAADP, cADPR, and IP3; the effect of removing extracellular Ca2+ on the pHLash in intact eggs is also shown. Fig. S3 shows that cADPR-induced Ca2+ release in intact eggs is desensitized by prior injection of NAADP. Fig. S4 is a scheme summarizing how Ca2+ from the ER may stimulate the NAADP pathway in eggs and mammalian cells.
We thank Clive Garnham for technical assistance, Margarida Ruas for invaluable suggestions, and the Wellcome Trust for financial support.
There are no conflicts of interest.
nicotinic acid adenine dinucleotide phosphate
sarcoendoplasmic reticulum Ca2+ ATPase