Store-Operated Calcium Channels

Orai1 is an essential pore subunit of the CRAC channel

Stimulation of immune cells causes depletion of Ca2+ from endoplasmic reticulum (ER) stores, thereby triggering sustained Ca2+ entry through store-operated Ca2+ release-activated Ca2+ (CRAC) channels, an essential signal for lymphocyte activation and proliferation. Recent evidence indicates that activation of CRAC current is initiated by STIM proteins, which sense ER Ca2+ levels through an EF-hand located in the ER lumen and relocalize upon store depletion into puncta closely associated with the plasma membrane. We and others recently identified Drosophila Orai and human Orai1 (also called TMEM142A) as critical components of store-operated Ca2+ entry downstream of STIM. Combined overexpression of Orai and Stim in Drosophila cells, or Orai1 and STIM1 in mammalian cells, leads to a marked increase in CRAC current. However, these experiments did not establish whether Orai is an essential intracellular link between STIM and the CRAC channel, an accessory protein in the plasma membrane, or an actual pore subunit. Here we show that Orai1 is a plasma membrane protein, and that CRAC channel function is sensitive to mutation of two conserved acidic residues in the transmembrane segments. E106D and E190Q substitutions in transmembrane helices 1 and 3, respectively, diminish Ca2+ influx, increase current carried by monovalent cations, and render the channel permeable to Cs+. These changes in ion selectivity provide strong evidence that Orai1 is a pore subunit of the CRAC channel.

Ginsenoside-Rd from panax notoginseng blocks Ca2+ influx through receptor- and store-operated Ca2+ channels in vascular smooth muscle cells

Previously, it was found that total saponins from panax notoginseng inhibited Ca2+ influx coupling to activation of alpha1-adrenoceptor. This study was designed to investigate the effects of ginsenoside-Rd from total saponins of panax notoginseng on receptor-operated (ROCC) and store-operated (SOCC) Ca2+ channels in vascular smooth muscle cells using fura-2 fluorescence, whole cell patch clamp ion channel recording, radio-ligand-receptor binding, 45Ca2+ radio-trace and organ bath techniques. It was found that ginsenoside-Rd reduced phenylephrine-induced contractile responses and Ca2+ influx in normal media without significant effect on these responses in Ca2+ -free media. Ginsenoside-Rd also decreased phenylephrine- and thapsigargin-induced inward Ca2+ currents, and attenuated thapsigargin- and 1-oleoy-2-acetyl-sn-glycerol (OAG)-induced cation entries that are coupled to ROCC and SOCC respectively. Ginsenoside-Rd failed to inhibit KCl-induced contraction of rat aortal rings and Ca2+ influx, and did not alter voltage-dependent inward Ca2+ current (VDCC) which was blocked by nifedipine. Also, ginsenoside-Rd did not change binding site and affinity of [3H]-prazosin for alpha1-adrenoceptor in the vascular plasma membrane. These results suggest that ginsenoside-Rd, as an inhibitor, remarkably inhibits Ca2+ entry through ROCC and SOCC without effects on VDCC and Ca2+ release in vascular smooth muscle cells.

STIM1 carboxyl-terminus activates native SOC, I(crac) and TRPC1 channels

Receptor-evoked Ca2+ signalling involves Ca2+ release from the endoplasmic reticulum, followed by Ca2+ influx across the plasma membrane. Ca2+ influx is essential for many cellular functions, from secretion to transcription, and is mediated by Ca2+-release activated Ca2+ (I(crac)) channels and store-operated calcium entry (SOC) channels. Although the molecular identity and regulation of I(crac) and SOC channels have not been precisely determined, notable recent findings are the identification of STIM1, which has been indicated to regulate SOC and I(crac) channels by functioning as an endoplasmic reticulum Ca2+ sensor, and ORAI1 (ref. 7) or CRACM1 (ref. 8)--both of which may function as I(crac) channels or as an I(crac) subunit. How STIM1 activates the Ca2+ influx channels and whether STIM1 contributes to the channel pore remains unknown. Here, we identify the structural features that are essential for STIM1-dependent activation of SOC and I(crac) channels, and demonstrate that they are identical to those involved in the binding and activation of TRPC1. Notably, the cytosolic carboxyl terminus of STIM1 is sufficient to activate SOC, I(crac) and TRPC1 channels even when native STIM1 is depleted by small interfering RNA. Activity of STIM1 requires an ERM domain, which mediates the selective binding of STIM1 to TRPC1, 2 and 4, but not to TRPC3, 6 or 7, and a cationic lysine-rich region, which is essential for gating of TRPC1. Deletion of either region in the constitutively active STIM1(D76A) yields dominant-negative mutants that block native SOC channels, expressed TRPC1 in HEK293 cells and I(crac) in Jurkat cells. These observations implicate STIM1 as a key regulator of activity rather than a channel component, and reveal similar regulation of SOC, I(crac) and TRPC channel activation by STIM1.

Local Ca2+ rise near store operated Ca2+ channels inhibits cell coupling during capacitative Ca2+ influx

Using a new fluorescence imaging technique, LAMP, we recently reported that Ca(2+) influx through store operated Ca(2+) channels (SOCs) strongly inhibits cell coupling in primary human fibroblasts (HF) expressing Cx43. To understand the mechanism of inhibition, we studied the involvement of cytosolic pH (pH(i)) and Ca(2+)([Ca(2+)](i)) in the process by using fluorescence imaging and ion clamping techniques. During the capacitative Ca(2+) influx, there was a modest decline of pH(i) measured by BCECF. Decreasing pH(i) below neutral using thioacetate had little effect by itself on cell coupling, and concomitant pH(i) drop with thioacetate and bulk [Ca(2+)(i) rise with ionomycin was much less effective in inhibiting cell coupling than Ca(2+) influx. Moreover, clamping pH(i) with a weak acid and a weak base during Ca(2+) influx largely suppressed bulk pH(i) drop, yet the inhibition of cell coupling was not affected. In contrast, buffering [Ca(2+)(i) with BAPTA, but not EGTA, efficiently prevented cell uncoupling by Ca(2+) influx. We concluded that local Ca(2+) elevation subjacent to the plasma membrane is the primary cause for closing Cx43 channels during capacitative Ca(2+) influx. To assess how Ca(2+) influx affects junctional coupling mediated by other types of connexins, we applied the LAMP assay to Hela cells expressing Cx26. Capacitative Ca(2+) influx also caused a strong reduction of cell coupling, suggesting that the inhibitory effect by Ca(2+) influx may be a more general phenomenon.

Retrograde regulation of store-operated calcium channels by the ryanodine receptor-associated protein triadin 95 in rat skeletal myotubes

The 95kDa triadin (or T95), the main skeletal muscle triadin isoform, negatively regulates the mechanism of excitation-contraction coupling. T95 is a ryanodine receptor (RyR)-interacting protein but it also possesses a calsequestrin-interacting domain. RyR and calsequestrin are involved in Ca2+ signalling and, for instance, influence the activity of store-dependent Ca2+ channels (SOC). This work was undertaken to determine whether T95 was able to modulate the entry of Ca2+ through SOC. The experiments were carried out on differentiated rat myotubes over-expressing T95 or DsRed (control cells) by means of an adenovirus infection. Intracellular Ca2+ signals were analyzed using the Ca2+ indicator Fluo-4. The sarco-endoplasmic reticulum Ca2+-ATPase inhibitor thapsigargin was used to deplete intracellular Ca2+ stores. When applied in the presence of a Ca2+-free medium, thapsigargin elicited transient but long-lasting Fluo-4 responses by elevating the cytoplasmic concentration of Ca2+ ([Ca2+]i). The over-expression of T95 reduced the thapsigargin-dependent [Ca2+]i increase, with respect to control myotubes. Addition of extracellular Ca2+after the depletion of this Ca2+ pool was accompanied by a [Ca2+]i increase that was sensitive to the SOC blockers 2-APB, SKF-96365 and La3+. The over-expression of T95 reduced this Ca2+ influx, without changing its pharmacological properties, showing that T95 over-expression did not alter the properties of the SOC. In conclusion, the RyR-interacting molecule T95, recently shown to inhibit the excitation-contraction coupling, has also the ability to interfere with the skeletal muscle Ca2+ signalling by depressing thapsigargin-dependent Ca2+ release and influx.

Homer 1 mediates store- and inositol 1,4,5-trisphosphate receptor-dependent translocation and retrieval of TRPC3 to the plasma membrane

Store-operated Ca(2+) channels (SOCs) mediate receptor-stimulated Ca(2+) influx. Accumulating evidence indicates that members of the transient receptor potential (TRP) channel family are components of SOCs in mammalian cells. Agonist stimulation activates SOCs and TRP channels directly and by inducing translocation of channels in intracellular vesicles to the plasma membrane (PM). The mechanism of TRP channel translocation in response to store depletion and agonist stimulation is not known. Here we use TRPC3 as a model to show that IP(3) and the scaffold Homer 1 (H1) regulate the rate of translocation and retrieval of TRPC3 from the PM. In resting cells, TRPC3 exists in TRPC3-H1b/c-IP(3)Rs complexes that are located in part at the PM and in part in intracellular vesicles. Binding of IP(3) to the IP(3)Rs dissociates the interaction between IP(3)Rs and H1 but not between H1 and TRPC3 to form IP(3)Rs-TRPC3-H1b/c. TIRFM and biotinylation assays show robust receptor- and store-dependent translocation of the TRPC3 to the PM and their retrieval upon termination of cell stimulation. The translocation requires depletion of stored Ca(2+) and is prevented by inhibition of the IP(3)Rs. In HEK293, dissociating the H1b/c-IP(3)R complex with H1a results in TRPC3 translocation to the PM, where it is spontaneously active. The TRPC3-H1b/c-IP(3)Rs complex is reconstituted by infusing H1c into these cells. Reconstitution is inhibited by IP(3). Deletion of H1 in mice markedly reduces the rates of translocation and retrieval of TRPC3. Conversely, infusion of H1c into H1(-/-) cells eliminates spontaneous channel activity and increases the rate of channel activation by agonist stimulation. The effects of H1c are inhibited by IP(3). These findings together with our earlier studies demonstrating gating of TRPC3 by IP(3)Rs were used to develop a model in which assembly of the TRPC3-H1b/c-IP(3)Rs complexes by H1b/c mediates both the translocation of TRPC3-containing vesicles to the PM and gating of TRPC3 by IP(3)Rs.

Dual effect of calmodulin on store-operated Ca2+ -permeable cation channels in rabbit portal vein myocytes

We have previously described a Ca(2+)-permeable non-selective cation channel in freshly dispersed rabbit ear artery myocytes, which is activated by agents that deplete internal Ca(2+) stores and also by protein kinase C (PKC). In the present study, we investigated the effect of calmodulin (CaM) on store-operated channels (SOCs) with electrophysiological techniques. Bath application of the CaM inhibitor calmidazolium (CMZ) to quiescent cells produced transient activation of SOC activity in cell-attached patches. CMZ produced a dual effect on cyclopiazonic acid (CPA)-evoked SOCs by initially inducing an increase in mean open probability (NP(o)) and subsequently producing a marked inhibition of SOC activity. In contrast, SOCs activated by the cell-permeable Ca(2+) chelator 1,2-bis (2-aminophenoxy)ethane-N-N,N',N'-tetraacetic acid (BAPTA-AM) were inhibited by CMZ. In inside-out patches where SOCs were activated by CPA or the PKC activator phorbol-12,13-dibutyrate (PDBu), bath application of CaM induced an initial inhibition followed by an increase in SOC activity. In contrast, CaM only enhanced BAPTA-AM-evoked SOC activity in inside-out patches. Bath application of CaM to the cytoplasmic surface of quiescent inside-out patches evoked single channel currents with biophysical properties similar to SOCs. The inhibitory action of CaM on PDBu-induced SOC activity was inhibited by the calmodulin-dependent kinase II (CaM kinase II) inhibitor autocamtide-related inhibitory peptide (AIP) but not by inhibitors of calcineurin or myosin light chain kinase (MLCK). In addition, CaM-evoked channel currents were inhibited by coapplication of purified CaM kinase II but not by inhibitors of CaM kinase II, calcineurin or MLCK. With whole-cell and cell-attached recording, bath application of the CaM kinase II inhibitors KN93 and AIP evoked SOCs in unstimulated myocytes. These results indicate that CaM has pronounced dual inhibitory and excitatory actions on SOCs with the inhibitory effect of CaM mediated by CaM kinase II. Moreover, the present work provides strong evidence that CaM has an important role in activating SOCs, possibly through a direct action on channel/associated proteins.

Voltage- and Ca2+-activated potassium channels in Ca2+ store control Ca2+ release

Ca2+ release from Ca2+ stores is a 'quantal' process; it terminates after a rapid release of stored Ca2+. To explain the quantal nature, it has been supposed that a decrease in luminal Ca2+ acts as a 'brake' on store release. However, the mechanism for the attenuation of Ca2+ efflux remains unknown. We show that Ca2+ release is controlled by voltage- and Ca2+-activated potassium channels in the Ca2+ store. The potassium channel was identified as the big or maxi-K (BK)-type, and was activated by positive shifts in luminal potential and luminal Ca2+ increases, as revealed by patch-clamp recordings from an exposed nuclear envelope. The blockage or closure of the store BK channel due to Ca2+ efflux developed lumen-negative potentials, as revealed with an organelle-specific voltage-sensitive dye [DiOC5(3); 3,3'-dipentyloxacarbocyanine iodide], and suppressed Ca2+ release. The store BK channels are reactivated by Ca2+ uptake by Ca2+ pumps regeneratively with K+ entry to allow repetitive Ca2+ release. Indeed, the luminal potential oscillated bistably by approximately 45 mV in amplitude. Our study suggests that Ca2+ efflux-induced store BK channel closures attenuate Ca2+ release with decreases in counter-influx of K+.

Ca2+ entry through plasma membrane IP3 receptors

Inositol 1,4,5-trisphosphate receptors (IP3Rs) release calcium ions, Ca2+, from intracellular stores, but their roles in mediating Ca2+ entry are unclear. IP3 stimulated opening of very few (1.9 +/- 0.2 per cell) Ca2+-permeable channels in whole-cell patch-clamp recording of DT40 chicken or mouse B cells. Activation of the B cell receptor (BCR) in perforated-patch recordings evoked the same response. IP3 failed to stimulate intracellular or plasma membrane (PM) channels in cells lacking IP3R. Expression of IP3R restored both responses. Mutations within the pore affected the conductances of IP3-activated PM and intracellular channels similarly. An impermeant pore mutant abolished BCR-evoked Ca2+ signals, and PM IP3Rs were undetectable. After introduction of an alpha-bungarotoxin binding site near the pore, PM IP3Rs were modulated by extracellular alpha-bungarotoxin. IP(3)Rs are unusual among endoplasmic reticulum proteins in being also functionally expressed at the PM, where very few IP3Rs contribute substantially to the Ca2+ entry evoked by the BCR.

A store-operated current (SOC) mediates oxytocin autocontrol in the developing rat hypothalamus

Oxytocin (OT) and vasopressin (VP) autocontrol their secreting neurons in the supraoptic nucleus (SON) by modulating action potential firing through activation of specific metabotropic receptors. However, the mechanisms linking receptor activation to firing remain unknown. In almost all cell types, activation of plasma membrane metabotropic receptors triggers signalling cascades that induce mobilization of calcium from intracellular stores. In turn, emptying the calcium stores may evoke calcium influx through store-operated channels (SOCs), the functions of which remain largely unknown in neurons. In this study, we show that these channels play a key role in the SON, at least in the response to OT. In isolated rat SON neurons, store depletion by thapsigargin induced an influx of calcium, demonstrating the presence of SOCs in these neurons. This calcium influx was specifically inhibited by 0.2 mM 1-(2-trifluoromethylphenyl-)imidazole (TRIM). At 2 mM, this compound affected neither the resting electrophysiological properties nor the voltage-dependant inward currents. In fresh slices, TRIM (2 mM) did not affect the resting potential of SON neurons, action potential characteristics, spontaneous action potential firing or synaptic activity; this compound thus appears to be a specific blocker of SOCs in SON neurons. TRIM (0.2 mM) specifically reduced the increase in action potential firing triggered by OT but did not affect the VP-induced response. These observations demonstrate that store operated channels exist in hypothalamic neurons and specifically mediate the response to OT in the SON.

Ca2+ refilling of the endoplasmic reticulum is largely preserved albeit reduced Ca2+ entry in endothelial cells

In this study the relationship between the efficiency of endoplasmic reticulum (ER) Ca2+ refilling and the extent of Ca2+ entry was investigated in endothelial cells. ER and mitochondrial Ca2+ concentration were measured using genetically encoded Ca2+ sensors, while the amount of entering Ca2+ was controlled by varying either the extracellular Ca2+ or the electrical driving force for Ca2+ by changing the plasma membrane potential. In the absence of an agonist, ER Ca2+ replenishment was fully accomplished even if the Ca2+ concentration applied was reduced from 2 to 0.5mM. A similar strong efficiency of ER Ca2+ refilling was obtained under condition of plasma membrane depolarization. However, in the presence of histamine, ER Ca2+ refilling depended on mitochondrial Ca2+ transport and was more susceptible to membrane depolarization. Store-operated Ca2+ entry (SOCE), was strongly reduced under low Ca2+ and depolarizing conditions but increased if ER Ca2+ uptake was blocked or if ER Ca2+ was released continuously by IP(3). A correlation of the kinetics of ER Ca2+refilling with cytosolic Ca2+ signals revealed that termination of SOCE is a rapid event that is not delayed compared to ER refilling. Our data indicate that ER refilling occurs in priority to, and independently from the cytosolic Ca2+ elevation upon Ca2+ entry and that this important process is widely achieved even under conditions of diminished Ca2+entry.

Calcium ions and integration in neural circuits

Integration in the nervous system is achieved by signal processing within dynamic functional ensembles formed by highly complex neuronal-glial cellular circuits. The interactions between electrically excitable neuronal networks and electrically non-excitable glial syncytium occur through either chemical transmission, which involves the release of transmitters from presynaptic terminals or from astroglial cells, or via direct intercellular contacts, gap junctions. Calcium ions act as a universal intracellular signalling system, which controls many aspects of neuronal-glial communications. In neurones, calcium signalling events regulate the exocytosis of neurotransmitters and establish the link between excitation of postsynaptic cells and integrative intracellular events, which control synaptic strength, expression of genes and memory function. In glial cells metabotropic receptor mediated release of calcium ions from the intracellular endoplasmic reticulum calcium store provide specific form of glial excitability. Glial calcium signals ultimately result in vesicular secretion of "glio" transmitters, which affect neuronal networks thus closing the glial-neuronal circuits. Cellular signalling through calcium ions therefore can be regarded as a molecular mechanism of integration in the nervous system.

STIM2 Is an Inhibitor of STIM1-Mediated Store-Operated Ca2+ Entry

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.

Amplification of CRAC current by STIM1 and CRACM1 (Orai1)

Depletion of intracellular calcium stores activates store-operated calcium entry across the plasma membrane in many cells. STIM1, the putative calcium sensor in the endoplasmic reticulum, and the calcium release-activated calcium (CRAC) modulator CRACM1 (also known as Orai1) in the plasma membrane have recently been shown to be essential for controlling the store-operated CRAC current (I(CRAC)). However, individual overexpression of either protein fails to significantly amplify I(CRAC). Here, we show that STIM1 and CRACM1 interact functionally. Overexpression of both proteins greatly potentiates I(CRAC), suggesting that STIM1 and CRACM1 mutually limit store-operated currents and that CRACM1 may be the long-sought CRAC channel.

Selective Inhibition of Spontaneous But Not Ca2+-Dependent Release Machinery by Presynaptic Group II mGluRs in Rat Cerebellar Slices

Two main forms of neurotransmitter release are known: action potential-evoked and spontaneous release. Action potential-evoked release depends on Ca2+entry through voltage-gated Ca2+channels, whereas spontaneous release is thought to be Ca2+-independent. Generally, spontaneous and action potential-evoked release are believed to use the same release machinery to release neurotransmitter. This study shows, using the whole cell patch-clamp technique in rat cerebellar slices, that at the interneuron- Purkinje cell synapse activation of presynaptic group II metabotropic glutamate receptors suppresses spontaneous GABA release through a mechanism independent of voltage-gated Ca2+channels, store-operated Ca2+channels, and Ca2+release from intracellular Ca2+stores, suggesting that the metabotropic receptors target the release machinery directly. Voltage gated Ca2+channel-independent release following increased presynaptic cAMP production is similarly inhibited by these metabotropic receptors. In contrast, both voltage-gated Ca2+channel-dependent and presynaptic N-methyl-d-aspartate receptor-dependent GABA release were unaffected by activation of group II metabotropic glutamate receptors. Hence, the mechanisms underlying spontaneous and Ca2+-dependent GABA release are distinct in that only the former is blocked by group II metabotropic glutamate receptors. Thus the same neurotransmitter, glutamate, can activate or inhibit neurotransmitter release by selecting different receptors that target different release machineries.

Store-operated calcium channels and pro-inflammatory signals

In non-excitable cells such as T lymphocytes, hepatocytes, mast cells, endothelia and epithelia, the major pathway for calcium [Ca2+] entry is through store-operated Ca2+ channels in the plasma membrane. These channels are activated by the emptying of intracellular Ca2+ stores, however, neither the gating mechanism nor the downstream targets of these channels has been clear established. Here, I review some of the proposed gating mechanisms of store-operated Ca2+ channels and the functional implications in regulating pro-inflammatory signals.

The gap junction cellular internet: connexin hemichannels enter the signalling limelight

Cxs (connexins), the protein subunits forming gap junction intercellular communication channels, are transported to the plasma membrane after oligomerizing into hexameric assemblies called connexin hemichannels (CxHcs) or connexons, which dock head-to-head with partner hexameric channels positioned on neighbouring cells. The double membrane channel or gap junction generated directly couples the cytoplasms of interacting cells and underpins the integration and co-ordination of cellular metabolism, signalling and functions, such as secretion or contraction in cell assemblies. In contrast, CxHcs prior to forming gap junctions provide a pathway for the release from cells of ATP, glutamate, NAD+ and prostaglandin E2, which act as paracrine messengers. ATP activates purinergic receptors on neighbouring cells and forms the basis of intercellular Ca2+ signal propagation, complementing that occuring more directly via gap junctions. CxHcs open in response to various types of external changes, including mechanical, shear, ionic and ischaemic stress. In addition, CxHcs are influenced by intracellular signals, such as membrane potential, phosphorylation and redox status, which translate external stresses to CxHc responses. Also, recent studies demonstrate that cytoplasmic Ca2+ changes in the physiological range act to trigger CxHc opening, indicating their involvement under normal non-pathological conditions. CxHcs not only respond to cytoplasmic Ca2+, but also determine cytoplasmic Ca2+, as they are large conductance channels, suggesting a prominent role in cellular Ca2+ homoeostasis and signalling. The functions of gap-junction channels and CxHcs have been difficult to separate, but synthetic peptides that mimic short sequences in the Cx subunit are emerging as promising tools to determine the role of CxHcs in physiology and pathology.

A TRPC-like non-selective cation current activated by α1-adrenoceptors in rat mesenteric artery smooth muscle cells

The TRPC family of non-selective cation channels has been suggested to play a key role in the responses to alpha1-adrenoceptor stimulation of vascular smooth muscle. However, there are still very few reports of non-selective cation currents activated by alpha1-AR in resistance arteries. Here, we examine the expression of TRPC channels and the currents activated by alpha1-adrenoceptors in rat mesenteric resistance artery smooth muscle. Messenger RNA and protein for TRPC1, TRPC3 and TRPC6 were detected within the arteries by RT-PCR and immunoblotting. Endothelial and adventitial layers were found to express the TRPC1, TRPC3 and TRPC6 proteins whereas only TRPC1 and TRPC6 were detected in the arterial smooth muscle by confocal immunofluorescence microscopy. In whole-cell patch-clamp recordings from isolated mesenteric arterial myocytes, an outwardly rectifying non-selective cation current was activated by both the alpha1-adrenoceptor agonist, phenylephrine (10 microM), and the diacylglycerol analogue, 1-oleoyl-2-acetyl-sn-glycerol (100 microM). Responses to 1-oleoyl-2-acetyl-sn-glycerol were not blocked, but increased, following inhibition of protein-kinase-C with either bisindolylmaleimide-I (1 microM) or chelerythrine (1 microM). The currents activated by both phenylephrine and 1-oleoyl-2-acetyl-sn-glycerol were inhibited by Gd3+ (100 microM) but potentiated by flufenamic acid (100 microM). Collectively, these findings demonstrate for the first time the expression of TRPC1 and TRPC6 in rat mesenteric artery smooth muscle and the existence in rat isolated mesenteric arterial myocytes of a TRPC-like non-selective cation current activated by alpha1-adrenoceptor stimulation and 1-oleoyl-2-acetyl-sn-glycerol.

Calcium signaling in physiology and pathophysiology

Calcium ions are the most ubiquitous and pluripotent cellular signaling molecules that control a wide variety of cellular processes. The calcium signaling system is represented by a relatively limited number of highly conserved transporters and channels, which execute Ca2+ movements across biological membranes and by many thousands of Ca2+-sensitive effectors. Molecular cascades, responsible for the generation of calcium signals, are tightly controlled by Ca2+ ions themselves and by genetic factors, which tune the expression of different Ca2+-handling molecules according to adaptational requirements. Ca2+ ions determine normal physiological reactions and the development of many pathological processes.

Induced TRPC1 expression sensitizes intestinal epithelial cells to apoptosis by inhibiting NF-kappaB activation through Ca2+ influx

Apoptosis occurs within crypts and at the intestinal luminal surface and plays a critical role in mucosal homoeostasis. NF-kappaB (nuclear factor-kappaB) is the central regulator of the transcription of genes involved in apoptosis, and its activity is highly regulated in the intestinal mucosa. We have recently demonstrated that TRPC1 (transient receptor potential canonical-1) is expressed in IECs (intestinal epithelial cells) and functions as a Ca2+ permeable channel activated by Ca2+ store depletion. The present study tests the hypothesis that TRPC1 channels are implicated in the regulation of apoptosis by inhibiting NF-kappaB through the induction of TRPC1-mediated Ca2+ influx in the IEC-6 line. The expression of TRPC1 induced by stable transfection of IEC-6 cells with the wild-type TRPC1 gene (IEC-TRPC1 cells) increased Ca2+ influx after Ca2+ store depletion and repressed NF-kappaB transactivation, which was associated with an increase in susceptibility to apoptosis induced by exposure to TNFalpha (tumour necrosis factor-alpha) plus CHX (cycloheximide) (TNF-alpha/CHX), or STS (staurosporine). By contrast, the induction of endogenous NF-kappaB activity, by the depletion of cellular polyamines, promoted resistance to apoptosis, which was prevented by the ectopic expression of the IkappaBalpha super-repressor. Furthermore, inhibition of TRPC1 expression by transfection with siRNA (small interfering RNA) targeting TRPC1 (siTRPC1) decreased Ca2+ influx, increased NF-kappaB transactivation, and prevented the increased susceptibility of IEC-TRPC1 cells to apoptosis. Decreasing Ca2+ influx by exposure to a Ca2+-free medium also induced NF-kappaB activity and blocked the increased susceptibility to apoptosis of stable IEC-TRPC1 cells. These results indicate that induced TRPC1 expression sensitizes IECs to apoptosis by inhibiting NF-kappaB activity as a result of the stimulation of Ca2+ influx.

Large store-operated calcium selective currents due to co-expression of Orai1 or Orai2 with the intracellular calcium sensor, Stim1

The molecular nature of store-operated Ca(2+)-selective channels has remained an enigma, due largely to the continued inability to convincingly demonstrate Ca(2+)-selective store-operated currents resulting from exogenous expression of known genes. Recent findings have implicated two proteins, Stim1 and Orai1, as having essential roles in store-operated Ca(2+) entry across the plasma membrane. However, transient overexpression of these proteins on their own results in little or no increase in store-operated entry. Here we demonstrate dramatic synergism between these two mediators; co-transfection of HEK293 cells with Stim1 and Orai1 results in an approximate 20-fold increase in store-operated Ca(2+) entry and Ca(2+)-selective current. This demonstrates that these two proteins are limiting for both the signaling and permeation mechanisms for Ca(2+)-selective store-operated Ca(2+) entry. There are three mammalian homologs of Orai1, and in expression experiments they all produced or augmented store-operated Ca(2+) entry with efficacies in the order Orai1 > Orai2 > Orai3. Stim1 apparently initiates the signaling process by acting as a Ca(2+) sensor in the endoplasmic reticulum. This results in rearrangement of Stim1 within the cell and migration toward the plasma membrane to regulate in some manner Orai1 located in the plasma membrane. However, we demonstrate that Stim1 does not incorporate in the surface membrane, and thus likely regulates or interacts with Orai1 at sites of close apposition between the plasma membrane and an intracellular Stim1-containing organelle.

On the activation mechanism of store-operated calcium channels

The development of the patch clamp technique has revolutionised our understanding of the life sciences. One area in which it has made an enormous contribution is cellular signalling. In many cell types, calcium influx across the plasma membrane is essential for the regulation of a wide range of critical physiological responses including secretion, gene transcription and cell growth. For many years the calcium influx pathways in non-excitable cells remained unknown, despite their importance in physiological and pathophysiological states. Very careful and insightful work by James Putney led to the formulation of the capacitative calcium entry (store-operated calcium influx) model, in which the process of emptying intracellular calcium stores resulted in the activation of calcium entry channels. Unequivocal evidence for this revolutionary model was provided by patch clamp studies carried out by Markus Hoth and Reinhold Penner, who demonstrated that store depletion activated a novel class of calcium channel called the CRAC channel. This review provides a historical perspective on the development of store-operated calcium influx and how patch clamping resolved a long-standing controversy in cell physiology. The review also discusses current ideas relating to how store emptying opens channels in the plasma membrane.

Orai1 and STIM reconstitute store-operated calcium channel function

The two membrane proteins, STIM1 and Orai1, have each been shown to be essential for the activation of store-operated channels (SOC). Yet, how these proteins functionally interact is not known. Here, we reveal that STIM1 and Orai1 expressed together reconstitute functional SOCs. Expressed alone, Orai1 strongly reduces store-operated Ca(2+) entry (SOCE) in human embryonic kidney 293 cells and the Ca(2+) release-activated Ca(2+) current (I(CRAC)) in rat basophilic leukemia cells. However, expressed along with the store-sensing STIM1 protein, Orai1 causes a massive increase in SOCE, enhancing the rate of Ca(2+)entry by up to 103-fold. This entry is entirely store-dependent since the same coexpression causes no measurable store-independent Ca(2+) entry. The entry is completely blocked by the SOC blocker, 2-aminoethoxydiphenylborate. Orai1 and STIM1 coexpression also caused a large gain in CRAC channel function in rat basophilic leukemia cells. The close STIM1 homologue, STIM2, inhibited SOCE when expressed alone but coexpressed with Orai1 caused substantial constitutive (store-independent) Ca(2+) entry. STIM proteins are known to mediate Ca(2+) store-sensing and endoplasmic reticulum-plasma membrane coupling with no intrinsic channel properties. Our results revealing a powerful gain in SOC function dependent on the presence of both Orai1 and STIM1 strongly suggest that Orai1 contributes the PM channel component responsible for Ca(2+) entry. The suppression of SOC function by Orai1 overexpression likely reflects a required stoichiometry between STIM1 and Orai1.

Mathematical model of mitochondrial ionic homeostasis: three modes of Ca2+ transport

Mitochondria play an important role in regulation of Ca2+ homeostasis in a cell. Here we present a mathematical model of mitochondrial ion transport and use this model to analyse different modes of Ca2+ uptake by mitochondria. The model includes transport of H+, Ca2+, K+, inorganic phosphate and oxidative substrates across the inner mitochondrial membrane harboring permeability transition pore (PTP). The detailed description of ion fluxes is based on the experimental ion kinetics in isolated mitochondria. Using the model we show that the kinetics of Ca2+ uptake by mitochondria is regulated by the total amount of Ca2+ in the system and the rate of Ca2+ infusion. Varying these parameters we find three different modes of ion transport. When the total amount of Ca2+ is below 140 nmol Ca2+/mg protein, all available Ca2+ is accumulated in the matrix without activation of the PTP. Between 140 and 160 nmol Ca2+/mg protein, accumulation of Ca2+ generates periodic opening and closure of the PTP and oscillations of ion fluxes. Higher levels of Ca2+ (> 160 nmol Ca2+/mg protein) result in a permanently open PTP, membrane depolarization and loss of small ions from the matrix. We show that in the intermediate range of Ca2+ concentrations the rate of Ca2+ infusion regulates the PTP state, so that slow Ca2+ infusion does not lead to PTP opening, while fast Ca2+ infusion results in an oscillatory state.

A road map for TR(I)Ps

The development of our knowledge on the structure, molecular regulation, and cell function on transient receptor potential (TRP) channels has been growing dramatically during the last few years. Many meetings in the past and upcoming events are now focused on TRP channels as general sensor molecules in cell physiology. However, most of the scientists in the field still feel that we are just beginning to understand these truly remarkable proteins, called TRPs, and there is still a long way to go from structure via molecular regulation to cell and organ function. This generally accepted but exciting view about the long road to the understanding of TRPs dominated all presentations given at the 2006 Minerva-Gentner Symposium on TRP channels and calcium signalling, which was held in Eilat, Israel, and was excellently organized by Baruch Minke (Jerusalem, Israel) and supported by Veit Flockerzi (Homburg, Germany).

Genome-wide RNAi screen of Ca(2+) influx identifies genes that regulate Ca(2+) release-activated Ca(2+) channel activity

Recent studies by our group and others demonstrated a required and conserved role of Stim in store-operated Ca(2+) influx and Ca(2+) release-activated Ca(2+) (CRAC) channel activity. By using an unbiased genome-wide RNA interference screen in Drosophila S2 cells, we now identify 75 hits that strongly inhibited Ca(2+) influx upon store emptying by thapsigargin. Among these hits are 11 predicted transmembrane proteins, including Stim, and one, olf186-F, that upon RNA interference-mediated knockdown exhibited a profound reduction of thapsigargin-evoked Ca(2+) entry and CRAC current, and upon overexpression a 3-fold augmentation of CRAC current. CRAC currents were further increased to 8-fold higher than control and developed more rapidly when olf186-F was cotransfected with Stim. olf186-F is a member of a highly conserved family of four-transmembrane spanning proteins with homologs from Caenorhabditis elegans to human. The endoplasmic reticulum (ER) Ca(2+) pump sarco-/ER calcium ATPase (SERCA) and the single transmembrane-soluble N-ethylmaleimide-sensitive (NSF) attachment receptor (SNARE) protein Syntaxin5 also were required for CRAC channel activity, consistent with a signaling pathway in which Stim senses Ca(2+) depletion within the ER, translocates to the plasma membrane, and interacts with olf186-F to trigger CRAC channel activity.

Tyrosine kinase modulation of protein kinase C activity regulates G protein-linked Ca2+ signaling in leukemic hematopoietic cells

We have used a recombinant mouse pre-B cell line (TonB210.1, expressing Bcr/Abl under the control of an inducible promoter) and several human leukemia cell lines to study the effect of high tyrosine kinase activity on G protein-coupled receptor (GPCR) agonist-stimulated cellular Ca(2+) release and store-operated Ca(2+) entry (SOCE). After induction of Bcr/Abl expression, GPCR-linked SOCE increased. The effect was reverted in the presence of the specific Abl inhibitor imatinib (1microM) and the Src inhibitor PP2 (10microM). In leukemic cell lines constitutively expressing high tyrosine kinase activity, Ca(2+) transients were reduced by imatinib and/or PP2. Ca(2+) transients were enhanced by specific inhibitors of PKC subtypes and this effect was amplified by tyrosine kinase inhibition in Bcr/Abl expressing TonB210.1 and K562 cells. Under all conditions Ca(2+) transients were essentially blocked by the PKC activator PMA. In Bcr/Abl expressing (but not in native) TonB210.1 cells, tyrosine kinase inhibitors enhanced PKCalpha catalytic activity and PKCalpha co-immunoprecipitated with Bcr/Abl. Unlike native TonB210.1 cells, Bcr/Abl expressing cells showed a high rate of cell death if Ca(2+) influx was reduced by complexing extracellular Ca(2+) with BAPTA. Our data suggest that tonic inhibition of PKC represents a mechanism by which high tyrosine kinase activity can enhance cellular Ca(2+) transients and thus exert profound effects on the proliferation, apoptosis and chemotaxis of leukemic cells.

mGluR1/5 subtype-specific calcium signalling and induction of long-term potentiation in rat hippocampal oriens/alveus interneurones

Hippocampal inhibitory interneurones demonstrate pathway- and synapse-specific rules of transmission and plasticity, which are key determinants of their role in controlling pyramidal cell excitability. Mechanisms underlying long-term changes at interneurone excitatory synapses, despite their importance, remain largely unknown. We use two-photon calcium imaging and whole-cell recordings to determine the Ca2+ signalling mechanisms linked specifically to group I metabotropic glutamate receptors (mGluR1alpha and mGluR5) and their role in hebbian long-term potentiation (LTP) in oriens/alveus (O/A) interneurones. We demonstrate that mGluR1alpha activation elicits dendritic Ca2+ signals resulting from Ca2+ influx via transient receptor potential (TRP) channels and Ca2+ release from intracellular stores. By contrast, mGluR5 activation produces dendritic Ca2+ transients mediated exclusively by intracellular Ca2+ release. Using Western blot analysis and immunocytochemistry, we show mGluR1alpha-specific extracellular signal-regulated kinase (ERK1/2) activation via Src in CA1 hippocampus and, in particular, in O/A interneurones. Moreover, we find that mGluR1alpha/TRP Ca2+ signals in interneurone dendrites are dependent on activation of the Src/ERK cascade. Finally, this mGluR1alpha-specific Ca2+ signalling controls LTP at interneurone synapses since blocking either TRP channels or Src/ERK and intracellular Ca2+ release prevents LTP induction. Thus, our findings uncover a novel molecular mechanism of interneurone-specific Ca2+ signalling, critical in regulating synaptic excitability in hippocampal networks.

Effects of 2-methyl-1,4-naphtoquinone (menadione) on cellular signaling in RBL-2H3 cells

Reactive oxygen species (ROS) are produced in various cells and affect many biologic processes. In this study, we examined the effects of 2-methyl-1,4-naphtoquinone (menadione; vitamin K3) on signal transduction in mast cells. Several lines of evidence suggest that H2O2 affects the antigen-induced responses in mast cells but its mechanism is not clearly understood. Unlike H2O2, menadione produces ROS only inside cells. Thus, it is possible to investigate the effects of ROS produced intracellularly. Pretreatment of mast cells (RBL-2H3) with menadione inhibited exocytotic secretion (degranulation) induced by antigen stimulation dose dependently. Menadione also inhibited the intracellular Ca2+ increase induced by antigen stimulation. Menadione did not inhibit the Ca2+ increase due to Ca2+ release from the intracellular calcium store in the absence of extracellular Ca2+, but inhibited the Ca2+ influx from the extracellular medium. These results suggest that reactive oxygen generated inside RBL cells by menadione inhibited degranulation by decreasing Ca2+ influx through the store operated Ca2+ channel on the plasma membrane.

Consequences of depleted SERCA2-gated calcium stores in the skin

Sarco(endo)plasmic reticulum Ca2+-ATPase isoform 2 (SERCA2) pumps belong to the family of Ca2+-ATPases responsible for the maintenance of calcium in the endoplasmic reticulum. In epidermal keratinocytes, SERCA2-controlled calcium stores are involved in cell cycle exit and onset of terminal differentiation. Hence, their dysfunction was thought to provoke impaired keratinocyte cohesion and hampered terminal differentiation. Here, we assessed cultured keratinocytes and skin biopsies from a canine family with an inherited skin blistering disorder. Cells from lesional and phenotypically normal areas of one of these dogs revealed affected calcium homeostasis due to depleted SERCA2-gated stores. In phenotypically normal patient cells, this defect compromised upregulation of p21(WAF1) and delayed the exit from the cell cycle. Despite this abnormality it failed to impede the terminal differentiation process in the long term but instead coincided with enhanced apoptosis and appearance of chronic wounds, suggestive of secondary mutations. Collectively, these findings provide the first survey on phenotypic consequences of depleted SERCA-gated stores for epidermal homeostasis that explain how depleted SERCA2 calcium stores provoke focal lesions rather than generalized dermatoses, a phenotype highly reminiscent of the human genodermatosis Darier disease.

The regulatory role of mitochondria in capacitative calcium entry

Capacitative regulation of calcium entry is a major mechanism of Ca2+ influx into electrically non-excitable cells, but it also operates in some excitable ones. It participates in the refilling of intracellular calcium stores and in the generation of Ca2+ signals in excited cells. The mechanism which couples depletion of intracellular calcium stores located in the endoplasmic reticulum with opening of store-operated calcium channels in the plasma membrane is not clearly understood. Mitochondria located in close proximity to Ca2+ channels are exposed to high Ca2+ concentration, and therefore, they are able to accumulate this cation effectively. This decreases local Ca2+ concentration and thereby affects calcium-dependent processes, such as depletion and refilling of the intracellular calcium stores and opening of the store-operated channels. Finally, mitochondria modulate the intensity and the duration of calcium signals induced by extracellular stimuli. Ca2+ uptake by mitochondria requires these organelles to be in the energized state. On the other hand, Ca2+ flux into mitochondria stimulates energy metabolism. To sum up, mitochondria couple cellular metabolism with calcium homeostasis and signaling.

Cyclic GMP modulates store-operated calcium entry inducing phosphatidylserine translocation at the surface of megakaryocytic cells

When subjected to stimulation, cells from the vascular compartment show a spontaneous collapse of the plasma membrane phospholipid asymmetry and phosphatidylserine is exposed at the external leaflet. Thus, phosphatidylserine externalization is essential for normal hemostasis and phagocytosis. The mechanism governing the migration of phosphatidylserine to the exoplasmic leaflet is not yet fully understood. We have proposed that store-operated calcium entry (SOCE) constitutes a key step of this process. Here, interaction of [Ca(2+)](i), cAMP and cGMP pathways and phosphatidylserine exposure was examined in human megakaryocytic cells. The membrane permeable cAMP and cGMP analogues, pCPT-cAMP and pCPT-cGMP, enhanced the Ca(2+) signal induced by ionophore and SOCE. Responses to pCPT-cAMP and pCPT-cGMP were independent of protein kinase A, protein kinase G (PKG) or ERK pathways. Inhibition of small G-proteins reduced or abolished the increase of [Ca(2+)](i) induced by pCPT-cAMP or pCPT-cGMP, respectively. pCPT-cGMP but not pCPT-cAMP enhanced the ability of cells to expose phosphatidylserine. This effect was not prevented by the inhibition of PKG or small G-proteins. These results show the differential role of cyclic nucleotides in the Ca(2+)-dependent membrane remodeling. Hence, pCPT-cGMP is another regulatory element for the completion of SOCE-induced phosphatidylserine transmembrane redistribution in HEL cells through a mechanism implicating small G-proteins.

Effect of cyclosporin and tacrolimus (FK506) on the antigen-induced mediator release, membrane potential and 86Rb+/K+ and Ca2+ fluxes in the RBL-2H3 cell line

The immunosuppressants cyclosporin A (CsA) and tacrolimus (FK506) inhibit the activation by antigen of T-lymphocytes as well as mast cells. The mechanism of their action on mast cells has yet to be elucidated. We, therefore, assessed their effect on antigen-induced histamine and beta-hexosaminidase release, membrane potential changes (bis-oxonol fluorescent probe), 86RB+ (marker for K+)-efflux, the intracellular free calcium concentration ([Ca2+]i in single cells) and 45Ca2+ uptake (CsA only) in RBL-2H3 cells, a mucosal-type mast cell line, passively sensitized with monoclonal mouse IgE antibody. Antigen addition induced depolarization within 1-2 min, followed by slower repolarization, reaching a steady state (approximately 90% repolarization) after 7-9 min. CsA and FK506 each dose-dependently inhibited antigen-induced histamine and beta-hexosaminidase secretion and the membrane repolarization phase, with similar IC50s for both actions, approximately 20 nM for CsA and approximately 2 nM for FK506. Antigen-induced 86Rb+-efflux was also significantly inhibited. Antigen-evoked increase in [Ca2+]i (area under the curve, AUC) was reduced by 35% and 52% in the presence of CsA or FK506 (1 microM each), respectively. However, 45Ca2+-uptake was not inhibited by CsA. These results suggest that both CsA and FK506 may inhibit mediator release from mast cells via blocking two interrelated processes, which are involved in the secretory process: 1. Membrane repolarization phase, which is essential for optimal mediator secretion and is mediated by a Ca2+-sensitive K+-efflux, yet to be further characterized, and (2) Increase in [Ca2+]i, probably via reduction of Ca(+2)-release from intracellular stores, [Ca2+]s.

Pharmacological and electrophysiological characterization of store-operated currents and capacitative Ca(2+) entry in vascular smooth muscle cells

Capacitative Ca(2+) entry (CCE) in vascular smooth muscle cells contributes to vasoconstrictor and mitogenic effects of vasoactive hormones. In A7r5 rat aortic smooth muscle cells, measurements of cytosolic free Ca(2+) concentration ([Ca(2+)](i)) have demonstrated that depletion of intracellular Ca(2+) stores activates CCE. However, there is disagreement in published studies regarding the regulation of this mechanism by the vasoconstrictor hormone [Arg(8)]-vasopressin (AVP). We have employed electrophysiological methods to characterize the membrane currents activated by store depletion [store-operated current (I(SOC))]. Because of different recording conditions, it has not been previously determined whether I(SOC) corresponds to CCE measured using fura-2; nor has the channel protein responsible for CCE been identified. In the present study, the pharmacological characteristics of I(SOC), including its sensitivity to blockade by 2-aminoethoxydiphenylborane, diethylstilbestrol, or micromolar Gd(3+), were found to parallel the effects of these drugs on thapsigargin- or AVP-activated CCE measured under identical external ionic conditions using fura-2. Thapsigargin-stimulated I(SOC) was also measured in freshly isolated rat mesenteric artery smooth muscle cells (MASMC). Members of the transient receptor potential (TRP) family of nonselective cation channels, TRPC1, TRPC4, and TRPC6, were detected by reverse transcription-polymerase chain reaction and Western blot in both A7r5 cells and MASMC. TRPC1 expression was reduced in a stable A7r5 cell line expressing a small interfering RNA (siRNA) or by infection of A7r5 cells with an adenovirus expressing a TRPC1 antisense nucleotide sequence. Thapsigargin-stimulated I(SOC) was reduced in both the TRPC1 siRNA- and TRPC1 antisense-expressing cells, suggesting that the TRPC1 channel contributes to the I(SOC)/CCE pathway.

Mitochondrial dynamics and Ca2+ signaling

Recent data shed light on two novel aspects of the mitochondria-Ca2+ liaison. First, it was extensively investigated how Ca2+ handling is controlled by mitochondrial shape, and positioning; a playground also of cell death and survival regulation. On the other hand, significant progress has been made to explore how intra- and near-mitochondrial Ca2+ signals modify mitochondrial morphology and cellular distribution. Here, we shortly summarize these advances and provide a model of Ca2+-mitochondria interactions.

Neurotrophin effects on intracellular Ca2+ and force in airway smooth muscle

Neurotrophins [e.g., brain-derived neurotrophic factor (BDNF), neurotrophin 4 (NT4)], known to affect neuronal structure and function, are expressed in nonneuronal tissues including the airway. However, their function is unclear. We examined the effect of acute vs. prolonged neurotrophin exposure on regulation of airway smooth muscle (ASM) intracellular Ca(2+) concentration ([Ca(2+)](i)): sarcoplasmic reticulum (SR) Ca(2+) release and Ca(2+) influx (specifically store-operated Ca(2+) entry, SOCE). Human ASM cells were incubated for 30 min in medium (control) or 1 or 10 nM BDNF, NT3, or NT4 (acute exposure) or overnight in 1 nM BDNF, NT3, or NT4 (prolonged exposure) and imaged after loading with the Ca(2+) indicator fura-2 AM. [Ca(2+)](i) responses to ACh, histamine, bradykinin, and caffeine and SOCE following SR Ca(2+) depletion were compared across cell groups. Force measurements were performed in human bronchial strips exposed to neurotrophins. Basal [Ca(2+)](i), peak responses to all agonists, SOCE, and force responses to ACh and histamine were all significantly enhanced by both acute and prolonged BDNF exposure (smaller effect of NT4) but decreased by NT3. Inhibition of the BDNF/NT4 receptor trkB by K252a prevented enhancement of [Ca(2+)](i) responses. ASM cells showed positive immunostaining for BDNF, NT3, NT4, trkB, and trkC (NT3 receptor). These novel data demonstrate that neurotrophins influence ASM [Ca(2+)](i) and force regulation and suggest a potential role for neurotrophins in airway diseases.

Cyclic ADP-ribose requires FK506-binding protein to regulate intracellular Ca2+ dynamics and catecholamine release in acetylcholine-stimulated bovine adrenal chromaffin cells

The present study was undertaken to elucidate whether cyclic ADP-ribose (cADPR) mediates the amplification of Ca2+ signaling and catecholamine release via the involvement of FK506-binding proteins (FKBPs)/ryanodine receptor (RyR) in bovine adrenal chromaffin cells. cADPR induced Ca2+ release in digitonin-permeabilized chromaffin cells and this was blocked by FK506 and rapamycin, ligands for FKBPs; 8Br-cADPR, a competitive antagonist for cADPR; and antibody for FKBP12/12.6, while it was enhanced by cyclosporin A. Ryanodine-induced Ca2+ release was not affected by 8Br-cADPR and was remarkably enhanced by FK506, rapamycin, cyclosporin A, and cADPR. FK506 binds to FKBP12.6 and removes it from RyRs, but cADPR did not affect the binding between FKBP12.6 and RyR. In intact chromaffin cells, 8Br-cADPR, FK506, and rapamycin, but not cyclosporin A attenuated the sustained intracellular free Ca2+ concentration ([Ca2+]i) rise induced by acetylcholine (ACh). 8Br-cADPR, FK506, and SK&F 96365 reduced the Mn2+ entry stimulated with ACh only when Ca2+ was present in the extracellular medium. 8Br-cADPR, FK506, and rapamycin concentration-dependently inhibited the ACh-induced catecholamine (CA) release. Here, we present evidence that FKBP12.6 associated with RyR may be required for Ca2+ release induced by cADPR in bovine adrenal chromaffin cells. cADPR-mediated Ca2+ release from endoplasmic reticulum in ACh-stimulated chromaffin cells is coupled with Ca2+ influx through the plasma membrane which is essential for ACh-stimulated CA release.

CRACM1 is a plasma membrane protein essential for store-operated Ca2+ entry

Store-operated Ca2+ entry is mediated by Ca2+ release-activated Ca2+ (CRAC) channels following Ca2+ release from intracellular stores. We performed a genome-wide RNA interference (RNAi) screen in Drosophila cells to identify proteins that inhibit store-operated Ca2+ influx. A secondary patch-clamp screen identified CRACM1 and CRACM2 (CRAC modulators 1 and 2) as modulators of Drosophila CRAC currents. We characterized the human ortholog of CRACM1, a plasma membrane-resident protein encoded by gene FLJ14466. Although overexpression of CRACM1 did not affect CRAC currents, RNAi-mediated knockdown disrupted its activation. CRACM1 could be the CRAC channel itself, a subunit of it, or a component of the CRAC signaling machinery.

Modeling the impact of store-operated Ca2+ entry on intracellular Ca2+ oscillations

Calcium (Ca2+) oscillations play fundamental roles in various cell signaling processes and have been the subject of numerous modeling studies. Here we have implemented a general mathematical model to simulate the impact of store-operated Ca2+ entry on intracellular Ca2+ oscillations. In addition, we have compared two different models of the inositol 1,4,5-trisphosphate (IP3) receptor (IP3R) and their influences on intracellular Ca2+ oscillations. Store-operated Ca2+ entry following Ca2+ depletion of endoplasmic reticulum (ER) is an important component of Ca2+ signaling. We have developed a phenomenological model of store-operated Ca2+ entry via store-operated Ca2+ (SOC) channels, which are activated upon ER Ca2+ depletion. The depletion evokes a bi-phasic Ca2+ signal, which is also produced in our mathematical model. The IP3R is an important regulator of intracellular Ca2+ signals. This IP3 sensitive Ca2+ channel is also regulated by Ca2+. We apply two IP3R models, the Mak-McBride-Foskett model and the De Young and Keizer model, with significantly different channel characteristics. Our results show that the two separate IP3R models evoke intracellular Ca2+ oscillations with different frequencies and amplitudes. Store-operated Ca2+ entry affects the oscillatory behavior of these intracellular Ca2+ oscillations. The IP3 threshold is altered when store-operated Ca2+ entry is excluded from the model. Frequencies and amplitudes of intracellular Ca2+ oscillations are also altered without store-operated Ca2+ entry. Under certain conditions, when intracellular Ca2+ oscillations are absent, excluding store-operated Ca2+ entry induces an oscillatory response. These findings increase knowledge concerning store-operated Ca2+ entry and its impact on intracellular Ca2+ oscillations.

Potential Roles of Electrogenic Ion Transport and Plasma Membrane Depolarization in Apoptosis

Apoptosis is characterized by the programmed activation of specific biochemical pathways leading to the organized demise of cells. To date, aspects of the intracellular signaling machinery involved in this phenomenon have been extensively dissected and characterized. However, recent studies have elucidated a novel role for changes in the intracellular milieu of the cells as important modulators of the cell death program. Specially, intracellular ionic homeostasis has been reported to be a determinant in both the activation and progression of the apoptotic cascade. Several apoptotic insults trigger specific changes in ionic gradients across the plasma membrane leading to depolarization of the plasma membrane potential (PMP). These changes lead to ionic imbalance early during apoptosis. Several studies have also suggested the activation and/or modulation of specific ionic transport mechanisms including ion channels, transporters and ATPases, as mediators of altered intracellular ionic homeostasis leading to PMP depolarization during apoptosis. However, the role of PMP depolarization and of the changes in ionic homeostasis during the progression of apoptosis are still unclear. This review summarizes the current knowledge regarding the causes and consequences of PMP depolarization during apoptosis. We also review the potential electrogenic ion transport mechanisms associated with this event, including the net influx/efflux of cations and anions. An understanding of these mechamisms could lead to the generation of new therapeutic approaches for a variety of diseases involving apoptosis.

Cell and tissue responses of a range of Urotensin II analogs at cloned and native urotensin II receptors. Evidence for coupling promiscuity

Urotensin II (U-II) is the peptide ligand for the G-protein-coupled U-II receptor (UT). U-II has been dubbed "the most potent vasoconstrictor identified to date". However, in vivo studies with this system are hampered by the paucity of available ligands. Here, we characterise Chinese hamster ovary (CHO) cells expressing the human UT receptor in the following assays; (1) [(125)I]U-II binding, (2) GTPgamma[(35)S] binding, (3) cAMP formation, and (4) intracellular Ca(2+). We assess activity of 9 U-II analogues using these paradigms and examine their ability to contract isolated rat aorta. CHO(hUT) cells bound [(125)I]U-II with a B (max) and K (d) of 1,110+/-70 fmol/mg protein and 742 pM, respectively. hU-II stimulated GTPgamma[(35)S] binding (pEC(50) 8.38), optimal at low (0.1 muM) GDP concentrations. The hU-II GTPgamma[(35)S] response was partially PTx sensitive and there was a potent (pEC(50) 9.23) low efficacy ( approximately 20% inhibition) coupling to adenylyl cyclase. In CHO(hUT) cells hU-II stimulates calcium release from intracellular stores (pEC(50) 8.80) and calcium influx in a PTx-insensitive manner. In our structure-activity relationship study most ligands acted as full agonists. However, urantide behaved as a partial agonist (pEC(50) 7.67/pK(B) 7.55) in GTPgamma[(35)S] binding, a full agonist (pEC(50) 8.11) for increases in intracellular Ca(2+) and a competitive antagonist in the rat aorta bioassay (pK(B) 8.59). Collectively, these data show promiscuity at high expression and indicate the need for careful multi-assay evaluation of novel U-II analogues. Further modification of urantide, in order to eliminate residual agonist activity and to identify novel ligands for in vivo cardiovascular studies are clearly warranted.

Effects of intracellular pH and Ca2+ on the activity of stretch-sensitive cation channels in leech neurons

The effects of intracellular pH and calcium on the activity of the leech mechanosensitive cation channels have been studied. These channels exhibited two activity modes denoted as spike-like (SL) and multiconductance (MC). In the absence of mechanical stimulation, acidification of the intracellular side of membrane patches from 7.2 to 6.2 reversibly increased the mean channel open time as well as the opening frequency in the SL mode. Channels in MC mode were activated by a pH(i) reduction from 7.2 to 6.2, but were inhibited at pH(i) 5.5. Unlike MC mode, SL mode was strongly activated by intracellular Ca(2+). Fura-2 imaging experiments showed that intracellular calcium was induced to increase by hypotonic cell swelling. The major component of this response did not require extracellular calcium. A component of the swelling-induced calcium response was sensitive to blockers of stretch-sensitive cation channels. The results indicate that the two activity modes of mechanosensitive channels of leech neurons respond differently to changes of intracellular pH and calcium. The sensitivity of the channel to micromolar concentrations of internal free calcium, along with its permeability to this ion, is consistent with a role in the amplification of mechanically induced Ca(2+) signals in leech neurons.

A mutation in Orai1 causes immune deficiency by abrogating CRAC channel function

Antigen stimulation of immune cells triggers Ca2+ entry through Ca2+ release-activated Ca2+ (CRAC) channels, promoting the immune response to pathogens by activating the transcription factor NFAT. We have previously shown that cells from patients with one form of hereditary severe combined immune deficiency (SCID) syndrome are defective in store-operated Ca2+ entry and CRAC channel function. Here we identify the genetic defect in these patients, using a combination of two unbiased genome-wide approaches: a modified linkage analysis with single-nucleotide polymorphism arrays, and a Drosophila RNA interference screen designed to identify regulators of store-operated Ca2+ entry and NFAT nuclear import. Both approaches converged on a novel protein that we call Orai1, which contains four putative transmembrane segments. The SCID patients are homozygous for a single missense mutation in ORAI1, and expression of wild-type Orai1 in SCID T cells restores store-operated Ca2+ influx and the CRAC current (I(CRAC)). We propose that Orai1 is an essential component or regulator of the CRAC channel complex.

Inhibition of glucose-stimulated activation of extracellular signal-regulated protein kinases 1 and 2 by epinephrine in pancreatic beta-cells

Glucose sensing is essential for the ability of pancreatic beta-cells to produce insulin in sufficient quantities to maintain blood glucose within the normal range. Stress causes the release of adrenergic hormones that increase circulating glucose by promoting glucose production and inhibiting insulin release. We have shown that extracellular signal-regulated kinases 1 and 2 (ERK1/2) are responsive to glucose in pancreatic beta-cells and that glucose activates ERK1/2 by mechanisms independent of insulin. Here we show that glucose-induced activation of ERK1/2 is inhibited by epinephrine through the alpha2-adrenergic receptor. Epinephrine and the selective alpha2-adrenergic agonist UK14304 reduced insulin secretion and glucose-stimulated ERK1/2 activation in a pertussis toxin-sensitive manner, implicating the alpha subunit of a Gi family member. Alpha2-adrenergic agonists also reduced stimulation of ERK1/2 by glucagon-like peptide 1 and KCl, but not by phorbol ester or nerve growth factor. Our findings suggest that alpha2-adrenergic agonists act via a Gi family member on early steps in ERK1/2 activation, supporting the idea that ERK1/2 are regulated in a manner that reflects insulin demand.

Norepinephrine activates store-operated Ca2+entry coupled to large-conductance Ca2+-activated K+channels in rat pinealocytes

Norepinephrine (NE) is one of the major neurotransmitters that determine melatonin production in the pineal gland. Although a substantial amount of Ca2+influx is triggered by NE, the Ca2+entry pathway and its physiological relevance have not been elucidated adequately. Herein we report that the Ca2+influx triggered by NE significantly regulates the protein level of serotonin N-acetyltransferase, or arylalkylamine N-acetyltransferase (AANAT), a critical enzyme in melatonin production, and is responsible for maintaining the Ca2+response after repetitive stimulation. Ca2+entry evoked by NE was dependent on PLC activation. NE evoked a substantial amount of Ca2+entry even after cells were treated with 1-oleoyl-2-acetyl- sn-glycerol (OAG), an analog of diacylglycerol. To the contrary, further OAG treatment after cells had been exposed to OAG did not evoke additional Ca2+entry. Moreover, NE failed to induce further Ca2+entry after the development of Ca2+entry induced by thapsigargin (Tg), suggesting that the pathway of Ca2+entry induced by NE might be identical to that of Tg. Interestingly, Ca2+entry evoked by NE or Tg induced membrane hyperpolarization that was reversed by iberiotoxin (IBTX), a specific inhibitor of large-conductance Ca2+-activated K+(BK) channels. Moreover, IBTX-sensitive BK current was observed during application of NE, suggesting that activation of the BK channels was responsible for the hyperpolarization. Furthermore, the activation of BK channels triggered by NE contributed to regulation of the protein level of AANAT. Collectively, these results suggest that NE triggers Ca2+entry coupled to BK channels and that NE-induced Ca2+entry is important in the regulation of AANAT.

A flow cytometric comparison of Indo-1 to fluo-3 and Fura Red excited with low power lasers for detecting Ca2+ flux

Indo-1 and high-power water-cooled lasers have been the standard for flow cytometric based Ca(2+) flux measurements. With advances in technology and the availability of low-power air-cooled lasers, there is interest in alternative protocols. Here, we have compared Indo-1 with the combination of fluo-3 and Fura Red calcium indicator dyes using low-power air-cooled lasers as the excitation source. The reagents were examined in parallel to detect Ca(2+) flux in peripheral blood T lymphocytes and in a T lymphoblastoid cell line. Ca(2+) flux was detected with a FACSVantage SE equipped with an Omnichrome Series 74 Helium-Cadmium, or a Spectra Physics 177-G1202 Argon ion air-cooled laser. Following determination of optimal loading conditions, Ca(2+) flux was examined in response to membrane receptor stimulation or intracellular Ca(2+) mobilization. Dose dependent Ca(2+) flux to anti-CD3 and thapsigargin was detected with either Indo-1 or with fluo-3 and Fura Red. The profile of the Ca(2+) flux detected by Indo-1 or with fluo-3 and Fura Red appeared similar, with the combination of fluo-3 and Fura Red more sensitive under the particular test conditions. The results clearly demonstrated that Indo-1 could be usefully excited with a low-power air-cooled laser. The alternative use of fluo-3 and Fura Red does not require the availability of a UV capable laser and produced equivalent data.

A role for voltage gated T-type calcium channels in mediating “capacitative” calcium entry?

Calcium entry through plasma membrane calcium channels is one of the most important cell signaling mechanism involved in such diverse functions as secretion, contraction and cell growth by regulating gene expression, proliferation and apoptosis. The identity of plasma membrane calcium channels, the main regulators of calcium entry, involved in cell proliferation has been thus extensively sought. Among these, a calcium entry pathway called capacitative calcium entry (CCE), activated by calcium store depletion, is particularly important in non-excitable cells. Though this capacitative calcium entry is generally supposed to occur through TRP channels there is some evidence that voltage-dependent T-type calcium channels may contribute to calcium entry after store depletion. Here we show that though mibefradil, a T-type calcium channel blocker, is able to reduce capacitative calcium entry induced by either thapsigargin or ATP, this was not mimicked by any other T-type calcium channel inhibitors even in cells overexpressing alpha(1H) T-type calcium channels, leading us to conclude that T-type calcium channels are not responsible for the capacitative calcium entry observed in different cancer cell lines. On the contrary, we show that the action of mibefradil on capacitative calcium entry is due to an action on store-operated calcium channels.