[Show abstract][Hide abstract] ABSTRACT: The coupling mechanism between endoplasmic reticulum (ER) Ca(2+) stores and plasma membrane (PM) store-operated channels (SOCs) remains elusive [1-3]. STIM1 was shown to play a crucial role in this coupling process [4-7]; however, the role of the closely related STIM2 protein remains undetermined. We reveal that STIM2 is a powerful SOC inhibitor when expressed in HEK293, PC12, A7r5, and Jurkat T cells. This contrasts with gain of SOC function in STIM1-expressing cells. While STIM1 is expressed in both the ER and plasma membrane, STIM2 is expressed only intracellularly. Store depletion induces redistribution of STIM1 into distinct "puncta." STIM2 translocates into puncta upon store depletion only when coexpressed with STIM1. Double labeling shows coincidence of STIM1 and STIM2 within puncta, and immunoprecipitation reveals direct interactions between STIM1 and STIM2. Independent of store depletion, STIM2 colocalizes with and blocks the function of a STIM1 EF-hand mutant that preexists in puncta and is constitutively coupled to activate SOCs. Thus, whereas STIM1 is a required mediator of SOC activation, STIM2 is a powerful inhibitor of this process, interfering with STIM1-mediated SOC activation at a point downstream of puncta formation. The opposing functions of STIM1 and STIM2 suggest they may play a coordinated role in controlling SOC-mediated Ca(2+) entry signals.
[Show abstract][Hide abstract] ABSTRACT: Receptor-induced Ca(2+) signals are key to the function of all cells and involve release of Ca(2+) from endoplasmic reticulum (ER) stores, triggering Ca(2+) entry through plasma membrane (PM) "store-operated channels" (SOCs). The identity of SOCs and their coupling to store depletion remain molecular and mechanistic mysteries. The single transmembrane-spanning Ca(2+)-binding protein, STIM1, is necessary in this coupling process and is proposed to function as an ER Ca(2+) sensor to provide the trigger for SOC activation. Here we reveal that, in addition to being an ER Ca(2+) sensor, STIM1 functions within the PM to control operation of the Ca(2+) entry channel itself. Increased expression levels of STIM1 correlate with a gain in function of Ca(2+) release-activated Ca(2+) (CRAC) channel activity. Point mutation of the N-terminal EF hand transforms the CRAC channel current (I(CRAC)) into a constitutively active, Ca(2+) store-independent mode. Mutants in the EF hand and cytoplasmic C terminus of STIM1 alter operational parameters of CRAC channels, including pharmacological profile and inactivation properties. Last, Ab externally applied to the STIM1 N-terminal EF hand blocks both I(CRAC) in hematopoietic cells and SOC-mediated Ca(2+) entry in HEK293 cells, revealing that STIM1 has an important functional presence within the PM. The results reveal that, in addition to being an ER Ca(2+) sensor, STIM1 functions within the PM to exert control over the operation of SOCs. As a cell surface signaling protein, STIM1 represents a key pharmacological target to control fundamental Ca(2+)-regulated processes including secretion, contraction, metabolism, cell division, and apoptosis.
Full-text · Article · Apr 2006 · Proceedings of the National Academy of Sciences
[Show abstract][Hide abstract] ABSTRACT: The ubiquitously expressed canonical transient receptor potential (TRPC) ion channels are considered important in Ca2+ signal generation, but their mechanisms of activation and roles remain elusive. Whereas most studies have examined overexpressed TRPC channels, we used molecular, biochemical, and electrophysiological approaches to assess the expression and function of endogenous TRPC channels in A7r5 smooth muscle cells. Real time PCR and Western analyses reveal TRPC6 as the only member of the diacylglycerol-responsive TRPC3/6/7 subfamily of channels expressed at significant levels in A7r5 cells. TRPC1, TRPC4, and TRPC5 were also abundant. An outwardly rectifying, nonselective cation current was activated by phospholipase C-coupled vasopressin receptor activation or by the diacylglycerol analogue, oleoyl-2-acetyl-sn-glycerol (OAG). Introduction of TRPC6 small interfering RNA sequences into A7r5 cells by electroporation led to 90% reduction of TRPC6 transcript and 80% reduction of TRPC6 protein without any detectable compensatory changes in the expression of other TRPC channels. The OAG-activated nonselective cation current was similarly reduced by TRPC6 RNA interference. Intracellular Ca2+ measurements using fura-2 revealed that thapsigargin-induced store-operated Ca2+ entry was unaffected by TRPC6 knockdown, whereas vasopressin-induced Ca2+ entry was suppressed by more than 50%. In contrast, OAG-induced Ca2+ transients were unaffected by TRPC6 knockdown. Nevertheless, OAG-induced Ca2+ entry bore the hallmarks of TRPC6 function; it was inhibited by protein kinase C and blocked by the Src-kinase inhibitor, 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2). Importantly, OAG-induced Ca2+ entry was blocked by the potent L-type Ca2+ channel inhibitor, *nimodipine. Thus, TRPC6 activation probably results primarily in Na ion entry and depolarization, leading to activation of L-type channels as the mediators of Ca2+ entry. Calculations reveal that even 90% reduction of TRPC6 channels would allow depolarization sufficient to activate L-type channels. This tight coupling between TRPC6 and L-type channels is probably important in mediating smooth muscle cell membrane potential and muscle contraction.
No preview · Article · Jan 2006 · Journal of Biological Chemistry
[Show abstract][Hide abstract] ABSTRACT: The coupling between receptor-mediated Ca2+ store release and the activation of "store-operated" Ca2+ entry channels is an important but so far poorly understood mechanism. The transient receptor potential (TRP) superfamily of channels contains several members that may serve the function of store-operated channels (SOCs). The 3,5-bis(trifluoromethyl)pyrazole derivative, BTP2, is a recently described inhibitor of SOC activity in T-lymphocytes. We compared its action on SOC activation in a number of cell types and evaluated its modification of three specific TRP channels, canonical transient receptor potential 3 (TRPC3), TRPC5, and TRPV6, to throw light on any link between SOC and TRP channel function. Using HEK293 cells, DT40 B cells, and A7r5 smooth muscle cells, BTP2 blocked store-operated Ca2+ entry within 10 min with an IC50 of 0.1-0.3 microM. Store-operated Ca2+ entry induced by Ca2+ pump blockade or in response to muscarinic or B cell receptor activation was similarly sensitive to BTP2. Using the T3-65 clonal HEK293 cell line stably expressing TRPC3 channels, TRPC3-mediated Sr2+ entry activated by muscarinic receptors was also blocked by BTP2 with an IC50 of <0.3 microM. Importantly, direct activation of TRPC3 channels by diacylglycerol was also blocked by BTP2 (IC50 approximately 0.3 microM). BTP2 still blocked TRPC3 in medium with N-methyl-D-glucamine-chloride replacing Na+, indicating BTP2 did not block divalent cation entry by depolarization induced by activating monovalent cation entry channels. Whereas whole-cell carbachol-induced TRPC3 current was blocked by 3 microM BTP2, single TRPC3 channel recordings revealed persistent short openings suggesting BTP2 reduces the open probability of the channel rather than its pore properties. TRPC5 channels transiently expressed in HEK293 cells were blocked by BTP2 in the same range as TRPC3. However, function of the highly Ca(2+)-selective TRPV6 channel, with many channel properties akin to SOCs, was entirely unaffected by BTP2. The results indicate a strong functional link between the operation of expressed TRPC channels and endogenous SOC activity.
Preview · Article · Mar 2005 · Journal of Biological Chemistry
[Show abstract][Hide abstract] ABSTRACT: Ca(2+) signals in response to receptors mediate and control countless cellular functions ranging from short-term responses such as secretion and contraction to longer-term regulation of growth, cell division and apoptosis. The spatial and temporal details of Ca(2+) signals have been resolved with great precision in many cells. Ca(2+) signals activated by phospholipase C-coupled receptors have two components: Ca(2+) release from endoplasmic reticulum (ER) stores mediated by inositol 1,4,5-trisphosphate (InsP(3)) receptors, and Ca(2+) entry from outside the cell. The latter remains largely a molecular and mechanistic mystery. The activation of "store-operated" Ca(2+) channels is believed to account for the entry of Ca(2+). However, debate now focuses on how much of a contribution emptying of stores plays to the activation of Ca(2+) entry in response to physiological activation of receptors. Here we discuss recent information and ideas on the exchange of signals between the plasma membrane (PM) and ER that results in activation of Ca(2+) entry channels following receptor stimulation and/or store emptying.
Full-text · Article · Jan 2005 · Biochimica et Biophysica Acta
[Show abstract][Hide abstract] ABSTRACT: The mechanisms by which Ca(2+)-store-release channels and Ca(2+)-entry channels are coupled to receptor activation are poorly understood. Modification of Ca(2+) signals by 2-aminoethoxydiphenyl borate (2-APB), suggests the agent may target entry channels or the machinery controlling their activation. In DT40 B-cells and Jurkat T-cells, complete Ca(2+) store release was induced by 2-APB (EC(50) 10-20 microM). At 75 microM, 2-APB emptied stores completely in both lymphocyte lines, but had no such effect on other cells. In DT40 cells, 2-APB mimicked B-cell receptor (BCR) cross-linking, but no effect was observed in mutant DT40 lines devoid of inositol 1,4,5-trisphosphate (InsP(3)) receptors (InsP(3)Rs) or phospholipase C-gamma2 (PLC-gamma2). Like the BCR, 2-APB activated transfected TRPC3 (canonical transient receptor potential) channels, which acted as sensors for PLC-gamma2-generated diacylglycerol in DT40 cells. The action of 2-APB on InsP(3)Rs and TRPC3 channels was prevented by PLC-inhibition, and required PLC-gamma2 catalytic activity. However, unlike BCR activation, no increased InsP(3) level could be measured in response to 2-APB. Also, calyculin A-induced cytoskeletal reorganization prevented 2-APB-induced InsP(3)R and TRPC3-channel activation, but not that induced by the BCR. 2-APB still activated TRPC3 channels in DT40 cells with fully depleted Ca(2+) stores, indicating its action was not via Ca(2+) release. Significantly, 2-APB-induced InsP(3)R and TRPC3 activation was prevented in DT40 knockout cells devoid of the BCR- and PLC-gamma2-coupled adaptor/kinases, Syk, Lyn, Btk or BLNK. The results suggest that 2-APB activates Ca(2+) signals in lymphocytes by initiating and enhancing coupling between components of the BCR-PLC-gamma2 complex and both Ca(2+)-entry and Ca(2+)-release channels.
Full-text · Article · Jan 2004 · Biochemical Journal