stim2 protein mediates distinct store-dependent...
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The FASEB Journal Research Communication
STIM2 protein mediates distinct store-dependentand store-independent modes of CRACchannel activation
Suhel Parvez,*,1 Andreas Beck,*,1 Christine Peinelt,* Jonathan Soboloff, Annette Lis,*Mahealani Monteilh-Zoller,* Donald. L. Gill, Andrea Fleig,*and Reinhold Penner,*,2
*Center for Biomedical Research at The Queens Medical Center and John A. Burns School ofMedicine at the University of Hawaii, Honolulu, Hawaii, USA; and Department of Biochemistry,School of Medicine, Temple University, Philadelphia, Pennsylvania, USA
ABSTRACT STIM1 and CRACM1 (or Orai1) areessential molecular components mediating store-oper-ated Ca2 entry (SOCE) and Ca2 release-activatedCa2 (CRAC) currents. Although STIM1 acts as aluminal Ca2 sensor in the endoplasmic reticulum(ER), the function of STIM2 remains unclear. Here wereveal that STIM2 has two distinct modes of activatingCRAC channels: a store-operated mode that is activatedthrough depletion of ER Ca2 stores by inositol 1,4,5-trisphosphate (InsP3) and store-independent activationthat is mediated by cell dialysis during whole-cell per-fusion. Both modes are regulated by calmodulin(CaM). The store-operated mode is transient in intactcells, possibly reflecting recruitment of CaM, whereasloss of CaM in perfused cells accounts for the persis-tence of the store-independent mode. The inhibition byCaM can be reversed by 2-aminoethoxydiphenyl borate(2-APB), resulting in rapid, store-independent activa-tion of CRAC channels. The aminoglycoside antibioticG418 is a highly specific and potent inhibitor of STIM2-dependent CRAC channel activation. The results reveala novel bimodal control of CRAC channels by STIM2,the store dependence and CaM regulation, which indi-cates that the STIM2/CRACM1 complex may be underthe control of both luminal and cytoplasmic Ca2
levels.Parvez, S., Beck, A., Peinelt, C., Soboloff, J.,Lis, A., Monteilh-Zoller, M., Gill, D. L., Fleig, A.,Penner, R. STIM2 protein mediates distinct store-dependent and store-independent modes of CRACchannel activation. FASEB J. 22, 000000 (2008)
Key Words: STIM1 calmodulin store-operated calcium entry
Changes in intracellular free calcium concentra-tion ([Ca2]i) represent one of the most widespreadand important signaling events, regulating a plethoraof cellular responses. Many cell types employ store-operated Ca2 entry (SOCE) as their principal pathwayfor Ca2 influx (13). This mechanism is engaged after
Ca2 release from stores, where the depleted storeslead to activation of Ca2 release-activated Ca2
(CRAC) channels (46). Recent work (711) has iden-tified STIM1 and CRACM1 (or Orai1) as essentialcomponents for functional SOCE. When overexpressedjointly, but not individually, STIM1 and CRACM1 re-constitute large CRAC currents (1114). In mammals,there exist several homologs of these proteins: STIM1and STIM2 in the endoplasmic reticulum (ER) andCRACM1, CRACM2, and CRACM3 in the plasma mem-brane. On Ca2 depletion of stores, STIM1 translocatesinto junctional structures close to the plasma mem-brane (8, 1517), where it may bind to and activateCRACM1 (18, 19). Mutational analysis revealed thatseveral key amino acids in CRACM1 determine theselectivity of CRAC currents, demonstrating thatCRACM1 forms the ion-selective pore of the CRACchannel (1820).
All three CRACM homologs represent functionalstore-operated channels with differential properties(12, 21, 22). The role of STIM2 in SOCE appears to becomplex and remains incompletely understood. Whenoverexpressed alone, STIM2 inhibits SOCE (16), yetcoexpressed with CRACM1, STIM2 causes constitutiverather than store-dependent Ca2 entry that is stronglyenhanced by 2-aminoethoxydiphenyl borate (2-APB;ref. 14). When expressed in HEK293 cells, this proteinresides exclusively in the ER, but unlike STIM1, it doesnot redistribute into junctions after store depletion, un-less STIM1 is also overexpressed (16). To gain insight intothe functional properties of STIM2, we performed acareful analysis of how it regulates CRAC channels.
1 These authors contributed equally to this work.2 Correspondence: Center for Biomedical Research, The
Queens Medical Center, 1301 Punchbowl St., UHT 8, Hono-lulu, HI 96813, USA. E-mail: [email protected]
The FASEB Journal article fj.07-9449com. Published online September 28, 2007.
MATERIALS AND METHODS
Subcloning and overexpression of CRACM and STIM
Full-length human CRACM1, CRACM2, and CRACM3 weresubcloned as described previously (18, 22). For electrophysi-ological analysis, CRACM proteins were overexpressed inHEK293 cells stably expressing STIM1 and STIM2 (16) usinglipofectamine 2000 (Invitrogen, Carlsbad, CA, USA). Exper-iments were performed 2448 h post-transfection. For theexperiments with HEK293 cells stably expressing STIM2, cellswere grown for several days or weeks in the absence orpresence of G418 (500 g/ml). Green cells were analyzed forthe effects of CRACM overexpression.
Patch-clamp experiments were performed in the tight-sealwhole-cell configuration at 2125C. High-resolution currentrecordings were acquired using the EPC-9 (HEKA). Voltageramps of 50 ms duration spanning a range of 100 to 100mV were delivered from a holding potential of 0 mV at a rateof 0.5 Hz over a period of 100700 s. All voltages werecorrected for a liquid junction potential of 10 mV (3 mV withCl as main internal anion). Currents were filtered at 2.9 kHzand digitized at 100 s intervals. Capacitive currents weredetermined and corrected before each voltage ramp. Extract-ing the current amplitude at 80 mV from individual rampcurrent records assessed the low-resolution temporal develop-ment of currents. Where applicable, statistical errors ofaveraged data are mean se with n determinations. Standardexternal solutions were as follows (in mM): 120 NaCl, 10 CsCl,2.8 KCl, 2 MgCl2, 10 CaCl2, 10 TEA-Cl, 10 HEPES, and 10glucose, pH 7.2 with NaOH, 300 mosM. In some experiments,2, 5, or 50 M 2-APB or 10 M G418 was added to thestandard external solution and applied through wide-tippedpuffer pipettes. Standard internal solutions were as follows(in mM): 120 Cs-glutamate, 8 NaCl, 20 CsBAPTA, 3 MgCl2,10 HEPES, and 0.02 inositol 1,4,5-trisphosphate (InsP3), pH7.2 with CsOH, 300 mosM. As indicated in the figure legends,for some experiments [Ca2]i was buffered to 150 or 100 nMby 20 mM CsBAPTA and 8 mM CaCl2 or 10 mM CsEGTA and3.6 mM CaCl2, respectively. For passive-depletion experi-ments, the internal solution was supplemented withCsBAPTA in the absence of InsP3 and Ca
2. In someexperiments, Cs-glutamate and CsBAPTA were equimolarlyreplaced by KCl and KBAPTA. In others, G418, NaATP andNaGTP, or calmodulin (CaM) were added to intracellularsolutions as specified in the figure legends. All chemicals werepurchased from Sigma-Aldrich (St. Louis, MO, USA).
Cells grown on coverslips were placed in external solutioncontaining (in mM): 107 NaCl, 7.2 KCl, 1 CaCl2, 1.2 MgCl2,11.5 glucose, and 10 HEPES, pH 7.2 with NaOH, and loadedwith fura-2 acetoxymethylester (2 M) for 30 min at 20C.Cells were washed, and dye was allowed to de-esterify for aminimum of 30 min at 20C. Approximately 95% of the dyewas confined to the cytoplasm as determined by the signalremaining after saponin permeabilization. Cells on coverslipswere placed in external solution in the absence or presence of1 mM CaCl2. Ca
2 measurements were made using an InCytdual-wavelength fluorescence imaging system (IntracellularImaging Inc., Cincinnati, OH, USA). Fluorescence emissionat 505 nm was monitored with excitation at 340 and 380 nm;intracellular Ca2 measurements are shown as 340/380 nmratios obtained from groups (3545) of single cells. The
details of these Ca2 measurements were described previously(23). All measurements shown represent minimum of threeindependent experiments.
Initial experiments, in which we compared CRAC cur-rents in HEK293 cells stably overexpressing STIM1 orSTIM2, confirmed that both proteins slightly modifyendogenous ICRAC. In response to Ca
2 store depletionby InsP3, STIM2-expressing cells exhibited reducedICRAC (0.2 pA/pF) and STIM1-expressing cells hadslightly increased ICRAC (0.7 pA/pF) compared towild-type cells (0.5 pA/pF; see Fig. 1A; ref. 13). Toinvestigate the functional interaction of STIM proteinswith CRACM1, HEK293 cells stably overexpressingSTIM1 or STIM2 were transiently transfected withCRACM1 and ICRAC was measured in response to InsP3.Consistent with our previous observations (13, 14),STIM1 cells generated large CRAC currents, whereasSTIM2 cells did not (Fig. 1B). Although we did notobserve readily discernable constitutive CRAC channelactivity corresponding to the previously described Ca2
entry (14), a small basal CRAC current of 1 pA/pF atbreak-in could have gone unnoticed. However, a smallCRAC current developed very slowly, on averageamounting to approximately 1 pA/pF at 80 mV.Under identical experimental conditions, despite thelack of InsP3-induced CRACM1 currents, external ap-plication of 2-APB (2 M) induced large CRACM1currents with the typical inwardly rectifying current-voltage (I-V) relationships (Fig. 1B, C). 2-APB is acompound that has facilitatory effects on CRAC cur-rents at low doses (5 M) but inhibits them at highdoses (10 M; refs. 2326). In cells expressing STIM2and CRACM1, 2-APB applied at 50 M caused a rapid,large, and transient activation of CRAC current (Fig.1D). In these experiments, we omitted InsP3 from thepatch pipette and also buffered [Ca2]i to 150 nM tokeep stores repleted, so the 2-APB effect observed heredid not require store depletion.
To better resolve the kinetics of this response, weperformed high-resolution recordings of the 2-APBeffect at a fixed membrane potential of 80 mV. Figure1E shows that the large 2-APB-induced current peakedwithin 8 s and disappeared to almost zero within afurther 10 s. This 2-APB-induced effect was specific tocells overexpressing STIM2 and CRACM1, since it wasnot observed in HEK293 cells that overexpressed eitherSTIM2 or CRACM1 alone or a combination of STIM1and CRACM1 (Fig. 1D). The 2-APB-induced activationseems to be too rapid to be mediated by store-depen-dent translocation of STIM2 to couple to CRACM1. Wetherefore surmise that a significant portion of CRACM1channels must somehow be in a readily activated state,possibly precoupled in a complex with STIM2. Immu-nofluorescence analysis reveals that STIM2 is distrib-uted in ER structures throughout the cell but is alsofound close to the plasma membrane, where it partially
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overlaps with CRACM1 localization (Fig. 2B). Storedepletion had no effect on this distribution of STIM2.This is in contrast to STIM1, which redistributes intoCRACM1-associated junctions after store depletion(Fig. 2A). Interestingly, coimmunoprecipitation exper-iments revealed that not only STIM1 but also STIM2can physically interact with CRACM1 (Fig. 2C), whichindicates that both STIM proteins interact withCRACM1. If STIM2 and CRACM1 exist coupled with-out store depletion and can generate large CRACcurrents in response to 2-APB, what prevents theCRACM1 channel from being open? As described here,it appears that an endogenous inhibitor prevents acti-vation of the channel and that 2-APB reverses theaction of this inhibitor.
Interestingly, the distinct effect of STIM2 could beselectively modified by the aminoglycoside antibioticG418. G418 is routinely present at 500 g/ml (720 M)in the growth medium to maintain selection pressureon cells stably transfected with STIM1 or STIM2.HEK293 cells grown for several days in the absence ofG418 exhibited enhanced responses to 2-APB andstrikingly different responses to store depletion, yetthere was no significant decrease in protein levels (Fig.2D). As shown in Fig. 1E, these cells responded witheven faster kinetics and with larger current amplitudesto 50 M 2-APB. To appreciate the speed of thisresponse, Fig. 1F superimposes the fastest possiblestore-operated activation of CRACM1 through STIM1using InsP3 as a stimulus and the 2-APB-mediatedactivation of CRACM1 through STIM2, demonstratingthat full activation of CRACM1 in STIM2-expressing
cells is complete before the activation of STIM1-medi-ated CRAC currents even begins. These results suggestthat the aminoglycoside inhibits store-operated gatingof CRAC channels through STIM2 and that 2-APB canovercome this inhibition. This effect of G418 wasspecific to STIM2, since STIM1-expressing cells, whichwere also routinely grown in G418, produced largeCRACM1-mediated currents (see Fig. 1B) with similarproperties as those produced by transient coexpressionof STIM1 and CRACM1 in HEK293 cells that remainedunexposed to G418 (13, 18).
The removal of G418 provided an important condi-tion under which to analyze STIM2 activation in re-sponse to store depletion. Thus, in cells not exposed toG418, InsP3 perfusion now generated amplified CRACcurrents (Fig. 3A). These CRAC currents were charac-terized by a biphasic activation pattern consisting of aninitial fast activation phase followed by a slower phasethat set in after 100 s and rarely reached steady statewithin 300 s. The relative proportion of the two phaseswas somewhat variable across experimental days andmay reflect the different coupling mechanisms ofSTIM2 populations (see below). Typical current ampli-tudes at 80 mV obtained 300 s into the experimentwere approximately 15 pA/pF and had the signatureI/V curves typical of CRAC currents (Fig. 3B).
Since STIM1 can activate all three CRACM homologs(22), we examined whether STIM2 could also couple toCRACM2 and CRACM3. Figure 3A demonstrates thatthis is indeed the case. Coexpression of STIM2 with anyof the three homologs produced biphasic currents,although the secondary phase of activation was less
Figure 1. Store-independent activation of CRACM1 bySTIM2 through 2-APB. In all experiments (except blacktrace in A and green traces in E and F), data are fromHEK293 cells grown in the presence of 500 g/mlG418. A) Average CRAC current densities at 80 mV inwild-type HEK293 cells (black, n14) and in cells stablyoverexpressing STIM1 (blue, n14) or STIM2 (red,n8) in response to 20 M InsP3. [Ca
2]i was clampedto near zero with 20 mM BAPTA. B) Average CRAC
current densities induced by 20 M InsP3 20 mM BAPTA in STIM1CRACM1 cells (black, n15), STIM2CRACM1cells (blue, n10), and STIM2CRACM1 cells stimulated by 2 M 2-APB (red, n9). C) Representative current-voltage(I/V) relationships of CRAC currents in STIM1CRACM1-expressing cells or 2 M 2-APB-induced currents inSTIM2CRACM1-expressing cells shown in B. D) Average CRAC current densities in HEK293 cells expressing STIM2alone (purple, n5; trace is flat around 0 pA/pF and masked by blue trace), CRACM1 alone (black, n3, trace is flataround 0 pA/pF and masked by blue trace), STIM1CRACM1 (blue, n8), and STIM2CRACM1 (red, n9). In allcells, [Ca2]i was buffered to 150 nM using 20 mM BAPTA and 8 mM CaCl2. E) High-resolution average CRAC currentsat 80 mV in STIM2CRACM1 cells grown in the presence (red, n7) or absence (green, n5) of G418 induced by 50M 2-APB as in D. F) Comparison of activation kinetics of InsP3- and 2-APB-induced gating of CRAC channels. Data setsare taken from B and E and plotted on same time scale to illustrate speed of STIM2-dependent and 2-APB-inducedactivation relative to STIM1-dependent and store-operated activation of CRACM1.
3STIM2 PROTEIN AND CRAC CHANNEL ACTIVATION
pronounced in CRACM2- and CRACM3-expressingcells. The I/V relationships of these currents were verysimilar and exhibited typical CRAC inward rectification(Fig. 3B), suggesting that both STIM homologs cancouple to any of the CRACM homologs to activatestore-operated currents.
It is important to ascertain that the InsP3-inducedcurrents observed in Fig. 3A developed as a conse-quence of store depletion. Therefore, we performedadditional experiments in which store depletion wasinduced independently of InsP3. We first examinedwhether CRAC currents were activated when prevent-ing store refilling with 20 mM BAPTA in the pipette.This also produced large CRAC-like currents but with asignificant delay (Fig. 3C, blue trace), which would beconsistent with the time needed to passively deplete
stores through leak pathways. We then performed thesame experiments but used the Ca2 ionophore iono-mycin to rapidly release Ca2 from intracellular stores(Fig. 3C, black trace). With this protocol, we observedbiphasic activation of CRACM1 currents that weresimilar in both amplitude and kinetics to those acti-vated by InsP3. If both phases of CRAC current werestore-dependent, we should have been able to suppresscurrent activation entirely by perfusing cells with anInsP3-free solution and buffering [Ca
2]i to 150 nM toavoid store depletion. As can be seen in Fig. 3D (greentrace), these conditions indeed prevent store-depen-dent activation of CRAC currents in STIM1- andCRACM1-expressing cells. Surprisingly, however, thesame experimental conditions still generated the slowsecondary phase in STIM2- and CRACM1-expressing
Figure 2. Immunolocalization of STIM and CRACM proteins. Images were obtained by confocal microscopy, while colocalization(yellow) was demonstrated by merging the images using ImageJ software (NIH). A) Cells stably expressing CFP-CRACM1(green) were transfected with STIM1 (red), treated with 2 M thapsigargin for 10 min, and then fixed, permeabilized, andblocked. Cells were sequentially double-stained with sheep STIM1-specific and rabbit anti-GFP antibodies (Invitrogen), followedby corresponding 2 antibodies. B) Cells stably expressing STIM2 (red) were transfected with CFP-CRACM1 (green), treatedwith 2 M thapsigargin for 10 min, and then fixed, permeabilized, and blocked. Cells were sequentially double-stained withsheep STIM2-specific and rabbit anti-GFP antibodies (Invitrogen), followed by corresponding 2 antibodies. C) Binding of STIMproteins to CRACM1 was assessed after transient transfection of STIM1 or STIM2 into HEK293 cells stably expressing CRACM1.Approximate molecular masses of proteins were 85 kDa for STIM1, 105 kDa for STIM2, and 55 kDa for CFP-CRACM1, theglycosylated form of which is 70 kDa. After transfection, cells were lysed and the proteins were quantified. For immunopre-cipitations, 200 g of protein/sample were incubated with protein G beads coated with rabbit anti-GFP antibodies (Invitrogen)for 2 h followed by separation with SDS-PAGE and transfer to PVDF membranes. Protein expression was confirmed by loading5 g of untransfected lysate. STIM proteins were detected by Western blot using an antibody cross-reacting with both STIM1 andSTIM2 (BD Biosciences, San Jose, CA, USA), while pulldown and expression of CFP-CRACM1 was demonstrated by strippingand reprobing a goat-anti-GFP antibody (Abcam). D) STIM2-expressing cells were incubated for 3 wk in the presence or absenceof G418. Cells were lysed and the proteins were quantified. Proteins (5 g/sample) were separated by SDS-PAGE and transferredto PVDF membranes. Levels of STIM expression were demonstrated by Western blot with an antibody cross-reacting to bothSTIM1 and STIM2 (BD Biosciences).
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cells (Fig. 3D, red trace). These results indicate that thefirst phase of the CRAC current is indeed caused bystore depletion, but the second phase develops regard-less of the filling state of intracellular stores and is storeindependent. As we reveal below, this secondary, store-independent phase of current activation appears to becaused by the diffusional escape of Ca2-CaM into thepatch pipette during perfusion of cells in patch-clamprecordings.
Although the above experiments suggest that storedepletion via InsP3 or ionomycin results in activation ofCRAC channels, the store dependence of STIM2 couldhave resulted from its comigration with STIM1 intojunctions. We therefore examined whether STIM2 canserve as a Ca2 sensor and activate CRAC channels in astore-dependent manner when STIM1 expression wassuppressed by small interfering RNA (siRNA). Weconfirmed the efficacy of siRNA treatment in wild-type(wt) HEK293 cells, where endogenous CRAC currentswere almost completely suppressed after siRNA treat-ment (Fig. 3E). However, in parallel experiments per-formed in STIM2- and CRACM1-expressing cells,STIM1 knockdown did not significantly affect the bi-phasic CRAC currents (Fig. 3F), suggesting that STIM2has the intrinsic ability to respond to store depletion.We surmised that the Ca2-sensing function of STIM2might be mediated by an EF-hand motif located in theN terminus of the protein facing the lumen of the ER,since such a motif is also found in STIM1 and has beenshown to serve this function (8, 17, 27). To test thisidea, we introduced a point mutation into the EF-handmotif of the STIM2 protein (D80A) that would be
predicted to disrupt Ca2 binding. The equivalentmutation in STIM1 (D76A) has been shown to abolishits Ca2-sensing function and results in constitutiveactivation of SOCE (8, 17, 27). As illustrated in Fig. 3F,the D80A mutant of STIM2 failed to support the rapid,store-operated activation phase of CRAC channels,whereas the delayed, store-independent activationphase remained unaffected. Together, these resultssuggest that STIM2 can mediate both store-operatedand store-independent activation of CRAC channelsindependent of STIM1 and that the Ca2-sensing func-tion of STIM2 resides within its luminal EF-hand motif.
Since the cells grown in the absence of G418 re-sponded to InsP3-induced store depletion, we were ableto assess the inhibitory effect of this antibiotic onSTIM2-mediated activation of CRACM1 by perfusingcells with defined concentrations of G418. Figure 4Aillustrates the dose-dependent inhibition of bothphases of the InsP3-induced CRACM1 currents by in-creasing concentrations of G418, resulting in a half-maximal inhibitory concentration of 0.6 M (Fig.4B). Under identical experimental conditions, where10 M G418 was perfused intracellularly, it did notmodify InsP3-induced CRAC currents in STIM1- andCRACM1-coexpressing cells (Fig. 4C). Although themost potent effect of G418 appears to be mediatedfrom the intracellular space and specifically affectsSTIM2-expressing cells, we also observed some smallinhibition of the aminoglycoside when it was applied at10 M from the extracellular side. This effect, however,did not seem to be specific for STIM2, since it wassimilar in both STIM1- and STIM2-expressing cells (Fig.
Figure 3. Store-operated activation of CRACM1 throughSTIM2. AF) STIM2-expressing HEK293 cells were grownwithout G418. A) Average CRAC current densities inducedby 20 M InsP3 in STIM2-expressing HEK293 cells trans-fected with CRACM1 (black, n34), CRACM2 (blue,n5), and CRACM3 (red, n5) with [Ca2]i clamped tonear zero with 20 mM BAPTA. B) Average current-voltage(I/V) relationships of CRAC currents extracted from
representative STIM2CRACM1, 2, and 3 cells shown in A at 300 s into experiment. Data represent leak-subtractedcurrents evoked by 50 ms voltage ramps from 100 to 100 mV, normalized to cell capacitance (pA/pF). C) AverageCRAC current densities in STIM2-expressing HEK293 cells transfected with CRACM1, where [Ca2]i was clamped to nearzero with 20 mM BAPTA to induce passive store depletion (blue, n7). In other cells (black, n6), active store depletionwas induced by brief (2 s) application of 2 M ionomycin (indicated by the arrow). D) Average CRAC current densitiesin STIM1- (green, n6) or STIM2-expressing HEK293 cells (red, n7) transfected with CRACM1, where store depletionwas prevented by omission of InsP3 and buffering [Ca2]i to 150 nM with 20 mM BAPTA 8 mM CaCl2. E) Average CRACcurrent densities at 80 mV in wt HEK293 cells (black, n14) or cells treated with STIM1-specific siRNA (red, n6) inresponse to 20 M InsP3. [Ca
2]i was clamped to near zero with 20 mM BAPTA. Data demonstrate that STIM1 siRNAtreatment was effective and suppressed native CRAC currents. F) Average CRAC current densities in STIM2CRACM1cells that were cotransfected with siRNA against STIM1 (black, n5) in parallel and exactly as in E. Average CRAC currentdensities in CRACM1-expressing cells that were cotransfected with the D80A EF-hand mutant of STIM2 (red, n5). Inboth experimental sets, store depletion was induced by 20 M InsP3 and [Ca
2]i buffered to near zero by 20 mM BAPTA.
5STIM2 PROTEIN AND CRAC CHANNEL ACTIVATION
4C, D), suggesting that it might be a pharmacologicaleffect on CRACM1. The specific and potent inhibitionof STIM2-mediated CRAC currents caused by intracel-lular G418 (and possibly other aminoglycoside antibi-otics) thus may provide a powerful pharmacologicaltool to assess STIM2 function in native cell systems.
Figure 3 shows CRAC currents that developed duringactive store depletion by InsP3, which caused the typicalbiphasic activation of CRACM1 currents, with the firstphase being store-operated, whereas the second phasedeveloped independent of the filling state of stores.Hence, the secondary phase is either activated by someingredient of our pipette filling solution or it is causedby the loss of some cytosolic factor that constitutivelysuppresses CRAC channels. We tested for these possi-bilities by exchanging the major ingredients of thestandard pipette filling solution. However, the substitu-tion of the primary intracellular salt Cs-glutamate withKCl (n5, data not shown) and the replacement ofBAPTA with EGTA (see Fig. 5B) were both ineffectivein suppressing the secondary current component.Thus, we conclude that the secondary CRAC currentphase is likely to result from the washout of a cellularfactor. To substantiate this idea, we performed experi-ments in which we varied pipette tip diameters andanalyzed biphasic currents. Smaller pipette tips (withhigher series resistance) would reduce the rate ofdiffusional escape of the cytosolic inhibitor, and Fig. 5Ademonstrates that this indeed delayed the developmentof the secondary phase. The cytosolic factor does notappear to be either of the two major cytosolic nucleo-tides, since adding physiological concentrations of ATP(3 mM) and GTP (0.3 mM) failed to suppress thesecondary phase (n5; data not shown).
We next considered CaM as a possible factor, sinceCaM can regulate numerous ion channels (28), includ-ing CRAC channels (29, 30). We performed experi-ments in which [Ca2]i was buffered to near restinglevels of 100 nM using EGTA as chelator and CaM waseither omitted or included in the pipette filling solu-
tion. In the absence of CaM, with [Ca2]i buffered toresting levels, the CRAC current activated after a delay(Fig. 5B, black trace), similar to the experiments de-scribed in Fig. 3D (red trace), where BAPTA was used aschelator. However, additional inclusion of CaM (100M) essentially prevented the store-independent acti-vation of CRAC currents (Fig. 5B, red and blue traces),suggesting that CaM may indeed represent the soughtcytosolic inhibitor. Subsequent exposure of cells toionomycin (Fig. 5B, red trace) to deplete stores wasineffective in activating CRAC currents, but channelsremained activable by 5 M 2-APB (Fig. 5B, blue trace).The inhibitory effect of CaM required some [Ca2]i,since the same concentration of CaM failed to suppressCRAC current activation when [Ca2]i was clamped tonear zero with 10 mM EGTA and no added Ca2 (Fig.5B, green trace). Thus, STIM2-mediated CRAC cur-rents are inhibited by Ca2-bound CaM but not byresting [Ca2]i alone or Ca
2-free apo-CaM. We furtherconfirmed that CaM can inhibit store-operated CRACchannel activation by perfusing cells with both InsP3and CaM and [Ca2]i buffered to 100 nM (Fig. 5C, bluetrace). This resulted in a transient CRAC current,presumably due to fast InsP3-mediated store depletionand STIM2-mediated activation of CRACM1, which wasthen curtailed by inhibition through CaM as it accumu-lated intracellularly. CaM also prevented the secondary,store-independent CRAC current phase, normally seenin the absence of CaM (Fig. 5C, black trace). Neverthe-less, the CRAC channels remained available for activa-tion by 5 M 2-APB and were suppressed again by CaMon washout of 2-APB.
The inhibitory effect of CaM could occur at variouspoints in the signal transduction process of SOCE,including InsP3 receptors, STIM sensors, and/or CRACchannels. We examined this issue by performing exper-iments in STIM1- and CRACM1-expressing cells underidentical experimental conditions as those for STIM2shown in Fig. 5C. With [Ca2]i buffered to near restinglevels of 100 nM and CaM absent from the pipette
Figure 4. Effects of G418 on STIM1- and STIM2-mediated activation of CRACM1. A) Average CRAC current densities inSTIM2CRACM1 cells in the absence of G418 (closed circles, n34) or presence of 300 nM (open circles, n9), 1 M (closedsquares, n11), 3 M (open squares, n8), or 10 M (closed triangles, n8) of G418 in the pipette. B) Dose-responserelationship of average CRAC current densities as a function of G418 concentration extracted at 300 s from the recordingsshown in E. Data were fitted with a dose-response curve, yielding an IC50 value of 640 nM and a Hill coefficient of 2.6. C) AverageCRAC currents in STIM1CRACM1 cells (n7) grown in the presence of G418. Pipette solutions contained InsP3 (20 M) and10 M G418, demonstrating that these cells are largely insensitive to intracellular G418. Extracellular application of 10 M G418caused a small and reversible reduction in CRAC current. D) Effects of extracellularly applied G418 on STIM2CRACM1.Average CRAC currents in STIM2CRACM1 cells grown without G418. Pipette solutions contained InsP3 (20 M) to activatebiphasic CRAC currents. Bar indicates extracellular application of various concentrations of G418. These caused a small andreversible reduction in CRAC current: 300 nM (filled circles, n3), 1 M (open circles, n4), and 10 M (filled squares, n5).
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filling solution, InsP3 induced a sustained CRAC cur-rent with monophasic activation kinetics (Fig. 5D, blacktrace). Addition of 100 M CaM resulted in a similaractivation of CRAC current, but the current was rapidlyinactivated (Fig. 5D, red trace). This transient CRACcurrent response was similar to that observed withSTIM2; however, in contrast to STIM2-expressing cells(see Fig. 5B, green trace), subsequent ionomycin appli-cation in STIM1-expressing cells completely restoredCRAC currents (Fig. 5D, red trace). This result suggeststhat the CaM-induced inhibition of CRAC currentsdoes not occur at the level of either STIM1 or CRACM1but instead appears to be mediated by store refilling.Indeed, CaM has been demonstrated to inhibit InsP3receptors (31, 32), and this mechanism would enablestores to refill even in the presence of InsP3. Together,these data suggest that the CaM-mediated inhibition ofCRAC currents can occur indirectly through inhibitionof InsP3 receptors and more directly through inhibitionof STIM2-mediated CRAC currents. The latter mecha-nism is specific for STIM2, since CRAC currents pro-duced through STIM1 remain unaffected by CaM ifstores are emptied in an InsP3-independent fashion.
The above data suggest that CaM acts as a specificinhibitor of STIM2 coupled to CRAC channels anddialysis of cells in whole-cell recordings leads to wash-out of CaM and disinhibition of CRAC channels. Bysupplementing the pipette solution with CaM, thewashout is counteracted and CRAC channels are sup-
pressed; however, 2-APB can still activate the CaM-inhibited channels. A simple interpretation of thisresult would be that 2-APB might displace the inhibi-tory CaM from the STIM2/CRACM1 complex. Weobtained support for this hypothesis from experimentsin which we probed the effects of 2-APB before andafter the secondary phase had fully developed. Figure5E illustrates that the same concentration of 2-APB (50M) produced almost identical peak amplitudes ofapproximately 10 pA/pF of STIM2-dependent CRACcurrent when applied 10 s after whole-cell establish-ment and after the CRAC current reached a plateau,with the second application timed when a significantamount of the endogenous CaM had washed out of thecell. Thus, the absolute efficacy was essentially the sameas during the first application, whereas the relativeefficacy of the second 2-APB application in facilitatingthe CRAC current was lower after the inhibitor hadwashed out when compared to the first application.This result would be compatible with the hypothesisthat 2-APB interferes with CaM binding to the STIM2/CRACM1 complex and such a mechanism would pre-sumably occur at an intracellular site. However, wecould not obtain direct and conclusive evidence for anintracellular effect of 2-APB. Inclusion of 50 M 2-APBin the patch pipette failed to activate CRAC currents byitself, although InsP3-induced CRAC currents appearedto be delayed and slightly reduced (Fig. 5F). Subse-quent extracellular application of 50 M 2-APB, how-
Figure 5. STIM2 causes transient store-operated activationof CRACM1. A-F) STIM2-expressing HEK293 cells weregrown without G418. A) Average CRAC current densities inSTIM2CRACM1 cells, where store depletion was inducedby 20 M InsP3 20 mM BAPTA. Traces represent binneddata of experiments in which pipette series resistances werewithin the following ranges: 24 M (black, n17), 57M (blue, n7), or 79 M (green, n7). B) AverageCRAC current densities in STIM2CRACM1 cells. CRAC
currents developed normally both in the absence of added CaM with [Ca2]i buffered to 100 nM with 10 mM EGTA 3.6mM CaCl2 (black, n5) or in its presence (100 M) with [Ca
2]i buffered to 0 with 10 mM EGTA and no added Ca2
(green, n9). However, CRAC currents were suppressed by the combined presence of 100 M CaM and 100 nM [Ca2]i(blue, n7; red, n4). Application of 2 M ionomycin for 3 s (red) or 5 M 2-APB for 60 s (blue) is indicated in graph.C) Average CRAC currents in STIM2CRACM1 cells induced by 20 M InsP3 with [Ca
2]i buffered to 100 nM with 10 mMEGTA 3.6 mM CaCl2 in the absence (black, n7) or additional presence of 100 M CaM (blue, n7) in pipette.Application of 5 M 2-APB for 60 s to CaM-treated cells (blue) is indicated in graph. D) Average CRAC currents inSTIM1CRACM1 cells induced by 20 M InsP3 with [Ca
2]i buffered to 100 nM using 10 mM EGTA 3.6 mM CaCl2.CRAC currents developed in the absence of added CaM (black, n9) but were rapidly suppressed by 100 M CaM inpipette (red, n6). Application of 2 M ionomycin for 3 s (red) reversed the CaM-mediated inhibition of CRAC currents.E) Average CRAC current densities in STIM2-expressing HEK293 cells transfected with CRACM1, where [Ca2]i wasclamped to near zero with 20 mM BAPTA. 2-APB (50 M) was applied for 30 s as indicated in graph (n7). F) AverageCRAC current densities at 80 mV in STIM2CRACM1 cells in response to 20 M InsP3 20 mM BAPTA. Pipette solutionadditionally contained 50 M 2-APB. At the indicated time, 50 M 2-APB was applied extracellularly.
7STIM2 PROTEIN AND CRAC CHANNEL ACTIVATION
ever, remained fully effective in first facilitating andthen blocking CRAC currents. Hence, the exact mech-anism and site of action of 2-APB remains to beinvestigated.
Unlike the situation in whole-cell recordings, theendogenous CaM cannot escape from intact cells, and,therefore, one would expect the initial store-operatedactivation of CRAC channels by STIM2 to be followedby inhibition, resulting in a transient SOCE phase. Wetested this hypothesis in intact cells loaded with theCa2 indicator Fura-2 and stimulated store-operatedCa2 influx through muscarinic receptors using carba-chol to rapidly deplete stores and thapsigargin toprevent store refilling. This experiment was performedboth in the presence and absence of extracellular Ca2
and in stable STIM2-expressing HEK293 cells that weretransiently transfected with an empty vector or withCRACM1. For comparison, we also included STIM1-and CRACM1-expressing cells. As illustrated in Fig.6AC, all cell populations produced a similar transientincrease in [Ca2]i when Ca
2 was absent, due toInsP3-mediated release of Ca
2 from intracellularstores. As expected, the presence of extracellular Ca2
produced a plateau phase of elevated [Ca2]i that waslikely due to SOCE in all cell populations. However, theCRACM1-expressing cells exhibited a more pro-nounced shoulder after the release transient due tostore-operated activation by STIM1 or STIM2. Tobetter appreciate the differences in Ca2 entry be-tween the two populations, we subtracted the signalsobtained in the absence of Ca2 from those in thepresence of Ca2 to arrive at the net influx compo-nents in the cell populations. As can be seen in Fig.6D, the STIM1-expressing cells produced a sustainedincrease in [Ca2]i, whereas STIM2-expressing cellsproduced a transient increase in SOCE that decayed.This transient increase in SOCE reflects the SOCEinduced by STIM2 coupling to CRACM1 channelsafter store depletion and presumably the subsequentinhibition by CaM. The slightly increased basal[Ca2]i of the STIM2- and CRACM1-expressing cellsmay result from a low level of constitutively openCRAC channels.
Taken together, the above results provide compellingevidence for both store-dependent and store-indepen-dent activation of CRAC channels by STIM2. Our datasuggest that in stably transfected HEK293 cells, STIM2exists in at least two functional states, with most of theSTIM2 molecules coupling to CRAC channels in astore-independent manner (constitutively coupled)and a smaller population of STIM2 molecules remain-ing available to activate CRAC channels after storedepletion (store coupled). Store-dependent activationcan be demonstrated both in patch-clamp experimentswhen cells are perfused with InsP3 (Fig. 3A) or whenstore depletion is caused by ionomycin (Fig. 3C), whichboth result in rapid activation of CRAC currents. SOCEis also evident in [Ca2]i imaging experiments in intactcells when cells are stimulated with an InsP3-mobilizingagonist and blocking store refilling with thapsigargin(Fig. 6). The main difference between CRAC in patch-clamp experiments and SOCE in intact cells is that thelatter manifests itself as a transient increase in [Ca2]i(Fig. 6), whereas whole-cell recordings exhibit a morepersistent CRAC current that is followed by an evenlarger, secondary activation of CRAC channel activity(Fig. 3A, C, F). The transient nature of STIM2-mediatedSOCE in intact cells may also explain why the store-operated mechanism remained undetected in typicalassays of SOCE (14, 16), where thapsigargin-inducedstore depletion in zero extracellular Ca2 is followed byCa2 readmission after several hundreds of seconds toprobe SOCE. During the thapsigargin/zero Ca2 expo-sure, any STIM2-mediated SOCE would remain unde-tectable and channel activity would have subsided bythe time Ca2 is readmitted.
The second population of STIM2 molecules is notcoupled to store depletion (14). It is observed inpatch-clamp experiments as a large CRAC current thattypically activates after 100200 s under a variety ofexperimental conditions, regardless of the filling stateof intracellular stores. Although the store-operatedpopulation can be suppressed experimentally in main-taining the stores filled with Ca2, the store-indepen-dent population can only be delayed by reducing the
Figure 6. STIM2 causes transient SOCE in intact cells. A) Changes in [Ca2]i measured as ratios of fura-2 fluorescence excitedat 340 and 380 nm in STIM2 cells transfected with empty vector in the absence (black) or presence (blue) of 1 mM extracellularCa2. Carbachol (100 M) and thapsigargin (2 M) were applied as indicated by arrow and maintained for duration ofexperiment. B) Identical experimental conditions as in A, but for STIM1CRACM1 cells. C) Identical experimental conditionsas in A, but for STIM2CRACM1 cells. D) These traces represent subtracted traces from AC, where [Ca2]i signal in Ca
2-freesolution was subtracted from that obtained with Ca2 to yield net Ca2 entry.
8 Vol. 22 March 2008 PARVEZ ET AL.The FASEB Journal
rate of diffusion through the pipette and the cytosol(Fig. 5A), which effectively slows down the washout andloss of a cytosolic factor that normally inhibits theconstitutive activity of STIM2 and CRACM1. We pro-pose that this cytosolic factor is CaM, since inclusion ofCaM in the pipette filling solution completely sup-presses activation of the slow CRAC current phase (Fig.5B). This inhibition requires at least resting levels of[Ca2]i approximately 100 nM, as apo-CaM is ineffec-tive when [Ca2]i is buffered to near zero (Fig. 5B). Inthe case of store-operated activation of CRACM1 bySTIM2, we surmise that the CaM is initially absent, soactivation can take place. Subsequently, possibly medi-ated by the increase in [Ca2]ii, Ca
2-CaM is recruitedto the STIM2/CRACM1 complex and inhibits channelactivity to cause transient SOCE. In addition, CaMappears to also have a more general effect on SOCEthat is mediated through inhibition of InsP3 receptoractivity (31, 32). This effect allows stores to refill even inthe presence of high levels of InsP3 and results insuppression of CRAC channel activity independently ofwhether CRAC channels are activated through STIM1or STIM2. Sidestepping this InsP3-dependent regula-tion by enforcing store depletion through ionomycinreveals that the inhibitory effect of CaM at the STIM/CRAC complex is specific to STIM2, as it is not ob-served with STIM1 (Fig. 5C, D).
Similar to CaM, G418 appears to be an effective andpotent inhibitor of STIM2-mediated CRAC channelactivation. In cells grown in the presence of G418, bothpopulations of STIM2 molecules are blocked by theantibiotic at the intracellular side (Fig. 4A, B). Anattractive hypothesis that remains to be substantiatedwould be that G418 acts as a CaM mimetic and occupiesthe same inhibitory binding site as CaM on the STIM2/CRACM1 complexes. Consistent with this notion is thefinding that regardless of whether the STIM2/CRACM1 complexes are suppressed by G418 or theendogenous CaM, they can be rapidly activated by2-APB. This activation is so rapid that it cannot possiblyresult from store depletion. The fastest store-depen-dent activation of CRAC currents by InsP3 is typicallycharacterized by 46 s delay and then proceeds over60 s to reach full activation. However, the 2-APB-mediated activation occurs with a half-time of 2 s andis virtually complete within 5 s (Fig. 1E, F), i.e., at a timewhen store-dependent activation just begins. Thisrapid, store-independent gating of CRAC channelsappears to be mediated by some close interaction ofSTIM2 and CRACM1, as it is not observed in cellsoverexpressing CRACM1 alone or in combination withSTIM1. It is tempting to speculate that agonist-medi-ated signals might recruit these precoupled STIM2/CRAC complexes to activate CRAC channels even with-out store depletion and with very fast kinetics. In thiscontext, we note that the carbachol-induced activationof Ca2 influx illustrated in Fig. 6 develops consider-ably more quickly in STIM2-expressing cells than inSTIM1 expressors (see Fig. 6D).
At this point, it remains unclear how 2-APB canachieve such fast gating of CRAC channels and evenovercome the apparent block by G418 and CaM. If weaccept that STIM2 activates CRACM1 via direct bindingin a conformational coupling manner, the simplestinterpretation would be that 2-APB simply displacesG418 or CaM from its binding site on the STIM2/CRACM1 complex. This would imply an intracellularaction 2-APB; however, this issue remains unresolved,since both facilitatory and inhibitory effects of 2-APBare only seen when the compound is applied extracel-lularly (26). We confirmed that intracellularly applied2-APB (50 M) is indeed ineffective in activating orsuppressing CRAC currents, whereas subsequent extra-cellular application of 50 M 2-APB first activates andthen inhibits CRAC currents (Fig. 5F). However, li-pophilic compounds are often ineffective when appliedintracellularly in patch-clamp experiments and extra-cellularly applied 2-APB has been demonstrated to bemembrane permeable and suppress InsP3 receptorfunction (33), suggesting that, in principle, the com-pound might gain access to the cytosol when appliedextracellularly.
Taken together, our data provide compelling evi-dence for a bimodal functional coupling betweenSTIM2 to CRACM channels, with both store-dependentand -independent components. Since both STIM1 andSTIM2 are widely expressed (34), both may servedistinct, but important, roles in SOCE. Relative differ-ences in ER Ca2 sensing by the two proteins mayprovide a broader sensitivity to store emptying. STIM2is additionally activable in a store-independent manner,with the rapid action of 2-APB revealing that a substan-tial component of STIM2 appears to be in a precoupledconfiguration. Identifying physiological mechanismsthat regulate this process would be an important futurearea of investigation. Indeed, the revelation that bothcomponents of STIM2-mediated channel activation aremodified by CaM provides an important clue to the roleof a potentially powerful cytosolic Ca2-mediated con-trol process in the activation of CRAC channels, com-plementing the clear role of luminal Ca2 in activatingthe channels.
We thank M. Bellinger and A. Love for help with cellculture and transfections. Supported in part by NationalInstitutes of Health (NIH) grants R01-AI050200 and R01-NS040927 (R.P.) and AI058173 (D.L.G.). C.P. was supportedby a fellowship from the Deutsche Forschungsgemeinschaft(PE-1478/11).
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Received for publication July 22, 2007.Accepted for publication August 30, 2007.
10 Vol. 22 March 2008 PARVEZ ET AL.The FASEB Journal