Activation and regulation of store-operated calcium entry

The process of store-operated Ca²⁺ entry (SOCE), whereby Ca²⁺ influx across the plasma membrane is activated in response to depletion of intracellular Ca²⁺ stores in the endoplasmic reticulum (ER), has been under investigation for greater than 25 years; however, only in the past 5 years have we come to understand this mechanism at the molecular level. A surge of recent experimentation indicates that STIM molecules function as Ca²⁺ sensors within the ER that, upon Ca²⁺ store depletion, rearrange to sites very near to the plasma membrane. At these plasma membrane-ER junctions, STIM interacts with and activates SOCE channels of the Orai family. The molecular and biophysical data that have led to these findings are discussed in this review, as are several controversies within this rapidly expanding field.

TRPC channels function independently of STIM1 and Orai1

Recent studies have defined roles for STIM1 and Orai1 as calcium sensor and calcium channel, respectively, for Ca(2+)-release activated Ca(2+) (CRAC) channels, channels underlying store-operated Ca(2+) entry (SOCE). In addition, these proteins have been suggested to function in signalling and constructing other channels with biophysical properties distinct from the CRAC channels. Using the human kidney cell line, HEK293, we examined the hypothesis that STIM1 can interact with and regulate members of a family of non-selective cation channels (TRPC) which have been suggested to also function in SOCE pathways under certain conditions. Our data reveal no role for either STIM1 or Orai1 in signalling of TRPC channels. Specifically, Ca(2+) entry seen after carbachol treatment in cells transiently expressing TRPC1, TRPC3, TRPC5 or TRPC6 was not enhanced by the co-expression of STIM1. Further, knockdown of STIM1 in cells expressing TRPC5 did not reduce TRPC5 activity, in contrast to one published report. We previously reported in stable TRPC7 cells a Ca(2+) entry which was dependent on TRPC7 and appeared store-operated. However, we show here that this TRPC7-mediated entry was also not dependent on either STIM1 or Orai1, as determined by RNA interference (RNAi) and expression of a constitutively active mutant of STIM1. Further, we determined that this entry was not actually store-operated, but instead TRPC7 activity which appears to be regulated by SERCA. Importantly, endogenous TRPC activity was also not regulated by STIM1. In vascular smooth muscle cells, arginine-vasopressin (AVP) activated non-selective cation currents associated with TRPC6 activity were not affected by RNAi knockdown of STIM1, while SOCE was largely inhibited. Finally, disruption of lipid rafts significantly attenuated TRPC3 activity, while having no effect on STIM1 localization or the development of I(CRAC). Also, STIM1 punctae were found to localize in regions distinct from lipid rafts. This suggests that TRPC signalling and STIM1/Orai1 signalling occur in distinct plasma membrane domains. Thus, TRPC channels appear to be activated by mechanisms dependent on phospholipase C which do not involve the Ca(2+) sensor, STIM1.

Glutathione depletion and disruption of intracellular ionic homeostasis regulate lymphoid cell apoptosis

Intracellular glutathione (GSH) depletion is an important hallmark of apoptosis. We have recently shown that GSH depletion by its extrusion regulates apoptosis independently of excessive reactive oxygen species accumulation. However, the mechanisms by which GSH depletion regulates apoptosis are still unclear. Because disruption of intracellular ionic homeostasis, associated with apoptotic volume decrease (AVD), is necessary for the progression of apoptotic cell death, we sought to evaluate the relationship between GSH transport and ionic homeostasis during Fas ligand (FasL)-induced apoptosis in Jurkat cells. GSH depletion in response to FasL was paralleled by distinct degrees of AVD identified by differences in cellular forward scatter and electronic impedance analysis. Inhibition of GSH efflux prevented AVD, K+ loss, and the activation of two distinct ionic conductances, mediated by Kv1.3 and outward rectifying Cl- channels. Reciprocally, stimulation of GSH loss accelerated the loss of K+, AVD, and consequently the progression of the execution phase of apoptosis. Although high extracellular K+ inhibited FasL-induced apoptosis, GSH depletion was largely independent of K+ loss. These results suggest that deregulation of GSH and ionic homeostasis converge in the regulation of apoptosis in lymphoid cells.

Methods for studying store-operated calcium entry

Activation of surface membrane receptors coupled to phospholipase C results in the generation of cytoplasmic Ca2+ signals comprised of both intracellular Ca2+ release, and enhanced entry of Ca2+ across the plasma membrane. A primary mechanism for this Ca2+ entry process is attributed to store-operated Ca2+ entry, a process that is activated by depletion of Ca2+ ions from an intracellular store by inositol 1,4,5-trisphosphate. Our understanding of the mechanisms underlying both Ca2+ release and store-operated Ca2+ entry have evolved from experimental approaches that include the use of fluorescent Ca2+ indicators and electrophysiological techniques. Pharmacological manipulation of this Ca2+ signaling process has been somewhat limited; but recent identification of key molecular players, STIM and Orai family proteins, has provided new approaches. Here we describe practical methods involving fluorescent Ca2+ indicators and electrophysiological approaches for dissecting the observed intracellular Ca2+ signal to reveal characteristics of store-operated Ca2+ entry, highlighting the advantages, and limitations, of these approaches.

Complex functions of phosphatidylinositol 4,5-bisphosphate in regulation of TRPC5 cation channels

The canonical transient receptor potential (TRPC) proteins have been recognized as key players in calcium entry pathways activated through phospholipase-C-coupled receptors. While it is clearly demonstrated that members of the TRPC3/6/7 subfamily are activated by diacylglycerol, the mechanism by which phospholipase C activates members of the TRPC1/4/5 subfamily remains a mystery. In this paper, we provide evidence for both negative and positive modulatory roles for membrane polyphosphoinositides in the regulation of TRPC5 channels. Depletion of polyphosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate (PIP2) through inhibition of phosphatidylinositol 4-kinase activates calcium entry and membrane currents in TRPC5-expressing but not in TRPC3- or TRPC7-expressing cells. Inclusion of polyphosphatidylinositol 4-phosphate or PIP2, but not phosphatidylinositol 3,4,5-trisphosphate, in the patch pipette inhibited TRPC5 currents. Paradoxically, depletion of PIP2 with a directed 5-phosphatase strategy inhibited TRPC5. Furthermore, when the activity of single TRPC5 channels was examined in excised patches, the channels were robustly activated by PIP2. These findings indicate complex functions for regulation of TRPC5 by PIP2, and we propose that membrane polyphosphoinositides may have at least two distinct functions in regulating TRPC5 channel activity.

Complex actions of 2-aminoethyldiphenyl borate on store-operated calcium entry

Store-operated Ca2+ entry (SOCE) is likely the most common mode of regulated influx of Ca2+ into cells. However, only a limited number of pharmacological agents have been shown to modulate this process. 2-Aminoethyldiphenyl borate (2-APB) is a widely used experimental tool that activates and then inhibits SOCE and the underlying calcium release-activated Ca2+ current (I CRAC). The mechanism by which depleted stores activates SOCE involves complex cellular movements of an endoplasmic reticulum Ca2+ sensor, STIM1, which redistributes to puncta near the plasma membrane and, in some manner, activates plasma membrane channels comprising Orai1, -2, and -3 subunits. We show here that 2-APB blocks puncta formation of fluorescently tagged STIM1 in HEK293 cells. Accordingly, 2-APB also inhibited SOCE and I(CRAC)-like currents in cells co-expressing STIM1 with the CRAC channel subunit, Orai1, with similar potency. However, 2-APB inhibited STIM1 puncta formation less well in cells co-expressing Orai1, indicating that the inhibitory effects of 2-APB are not solely dependent upon STIM1 reversal. Further, 2-APB only partially inhibited SOCE and current in cells co-expressing STIM1 and Orai2 and activated sustained currents in HEK293 cells expressing Orai3 and STIM1. Interestingly, the Orai3-dependent currents activated by 2-APB showed large outward currents at potentials greater than +50 mV. Finally, Orai3, and to a lesser extent Orai1, could be directly activated by 2-APB, independently of internal Ca2+ stores and STIM1. These data reveal novel and complex actions of 2-APB effects on SOCE that can be attributed to effects on both STIM1 as well as Orai channel subunits.

Role of the microtubule cytoskeleton in the function of the store-operated Ca2+ channel activator STIM1

We examined the role of the microtubule cytoskeleton in the localization and store-operated Ca(2+) entry (SOCE) function of the endoplasmic reticulum (ER) Ca(2+) sensor stromal interaction molecule 1 (STIM1) in HEK 293 cells. STIM1 tagged with an enhanced yellow fluorescent protein (EYFP-STIM1) exhibited a fibrillar localization that colocalized with endogenous alpha-tubulin. Depolymerization of microtubules with nocodazole caused a change from a fibrillar EYFP-STIM1 localization to one that was similar to that of the ER. Treatment of HEK 293 cells with nocodazole had a detrimental impact on SOCE and the associated Ca(2+) release-activated Ca(2+) current (I(CRAC)). This inhibition was significantly reversed in cells overexpressing EYFP-STIM1, implying that the primary inhibitory effect of nocodazole is related to STIM1 function. Surprisingly, nocodazole treatment alone induced significant SOCE and I(CRAC) in cells expressing EYFP-STIM1, and this was accompanied by an increase in EYFP-STIM1 fluorescence near the plasma membrane. We conclude that microtubules play a facilitative role in the SOCE signaling pathway by optimizing the localization of STIM1.

Calcium Inhibition and Calcium Potentiation of Orai1, Orai2, and Orai3 Calcium Release-activated Calcium Channels

The recent discoveries of Stim1 and Orai proteins have shed light on the molecular makeup of both the endoplasmic reticulum Ca(2+) sensor and the calcium release-activated calcium (CRAC) channel, respectively. In this study, we investigated the regulation of CRAC channel function by extracellular Ca(2+) for channels composed primarily of Orai1, Orai2, and Orai3, by co-expressing these proteins together with Stim1, as well as the endogenous channels in HEK293 cells. As reported previously, Orai1 or Orai2 resulted in a substantial increase in CRAC current (I(crac)), but Orai3 failed to produce any detectable Ca(2+)-selective currents. However, sodium currents measured in the Orai3-expressing HEK293 cells were significantly larger in current density than Stim1-expressing cells. Moreover, upon switching to divalent free external solutions, Orai3 currents were considerably more stable than Orai1 or Orai2, indicating that Orai3 channels undergo a lesser degree of depotentiation. Additionally, the difference between depotentiation from Ca(2+) and Ba(2+) or Mg(2+) solutions was significantly less for Orai3 than for Orai1 or -2. Nonetheless, the Na(+) currents through Orai1, Orai2, and Orai3, as well as the endogenous store-operated Na(+) currents in HEK293 cells, were all inhibited by extracellular Ca(2+) with a half-maximal concentration of approximately 20 mum. We conclude that Orai1, -2, and -3 channels are similarly inhibited by extracellular Ca(2+), indicating similar affinities for Ca(2+) within the selectivity filter. Orai3 channels appeared to differ from Orai1 and -2 in being somewhat resistant to the process of Ca(2+) depotentiation.

VPAC Receptor Modulation of Neuroexcitability in Intracardiac Neurons

Pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal polypeptide (VIP) have been found within mammalian intracardiac ganglia, but the cellular effects of these neuropeptides remain poorly understood. Fluorometric calcium imaging and whole cell patch clamp recordings were used to examine the effects of PACAP and VIP on [Ca2+]i and neuroexcitability, respectively, in intracardiac neurons of neonatal rats. PACAP and VIP evoked rapid increases in [Ca2+]i that exhibited both transient and sustained components. Pharmacological experiments using PAC1 and VPAC receptor-selective antagonists demonstrated that the elevations in [Ca2+]i result from the activation of VPAC receptors. The transient increases in [Ca2+]i were shown to be the product of Ca2+ mobilization from caffeine/ryanodine-sensitive intracellular stores and were not due to inositol 1,4,5-trisphosphate-mediated calcium release. In contrast, the sustained [Ca2+]i elevations were dependent on extracellular Ca2+ and were blocked by the transient receptor channel antagonist, 2-aminoethoxydiphenyl borate, which suggests that they are due to Ca2+ entry via store-operated channels. In addition to elevating [Ca2+]i, both PACAP and VIP depolarized intracardiac neurons, and PACAP was further shown to augment action potential firing in these cells. Depolarization of intracardiac neurons by the neuropeptides was dependent on activation of VPAC receptors and the concomitant increases in [Ca2+]i. Although activation of PAC1 receptors alone had no direct effects on neuroexcitability, PAC1 receptor stimulation potentiated the VPAC receptor-induced depolarizations. Furthermore, enhanced action potential firing was only observed upon concurrent stimulation of PAC1 and VPAC receptors, which indicates that these receptors act synergistically to enhance neuroexcitability in intracardiac neurons.

Heterogeneity of pituitary adenylate cyclase-activating polypeptide and vasoactive intestinal polypeptide receptors in rat intrinsic cardiac neurons

The expression of receptors for pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal polypeptide (VIP) was investigated in isolated parasympathetic neurons of neonatal rat intracardiac ganglia using single-cell reverse transcription-polymerase chain reaction. Individual neurons were shown to express multiple isoforms of the PACAP receptor, PAC1, including PAC1-short, -HOP1 and -HOP2 variants, which differ in the region encoding the G protein-binding domain. The PAC1-HOP1 isoform was the predominant species, being expressed at higher levels and in a greater number of cells than other PAC1 variants. In addition to PAC1, intrinsic cardiac neurons express transcripts for the VIP receptors, VPAC1 and VPAC2, with VPAC2 being found in a greater proportion of the neurons. These findings may explain the complex effects of PACAP and VIP on neuroexcitability in mammalian intracardiac ganglia.