Discriminating between capacitative and arachidonate-activated Ca(2+) entry pathways in HEK293 cells

Inhibition of adenylyl cyclase 8 prevents the upregulation of Orai1 channel, which improves cardiac function after myocardial infarction

The upregulation of Orai1 and subsequent store-operated Ca2+ entry (SOCE) has been associated with adverse cardiac remodeling and heart failure (HF). However, the mechanism underlying Orai1 upregulation and its role in myocardial infarction remains unclear. Our study investigated the role of Orai1 in activating adenylyl cyclase 8 (AC8) and cyclic AMP (cAMP) response element-binding protein (CREB), as well as its contribution to cardiac dysfunction induced by ischemia and reperfusion (I/R). We found that I/R evoked an increase in the expression of Orai1 and AC8 in rats' hearts, resulting in a substantial rise in diastolic Ca2+ concentration ([Ca2+]i), and reduced ventricular contractions. The expression of Orai1 and AC8 was also increased in ventricular biopsies of post-ischemic HF patients. Mechanistically, we demonstrate that I/R activation of Orai1 stimulated AC8, which produced cAMP and phosphorylated CREB. Subsequently, p-CREB activated the ORAI1 promoter, resulting in Orai1 upregulation and SOCE exacerbation. Intramyocardial administration of AAV9 carrying AC8 short hairpin RNA decreased the expression of AC8, Orai1 and CREB, which restored diastolic [Ca2+]i and improved cardiac contraction. Therefore, our data suggests that the axis composed by Orai1/AC8/CREB plays a critical role in I/R-induced cardiac dysfunction, representing a potential new therapeutic target to limit the progression of the disease toward HF.

Cross-Talk Between the Adenylyl Cyclase/cAMP Pathway and Ca2+ Homeostasis

Cyclic AMP and Ca²⁺ are the first second or intracellular messengers identified, unveiling the cellular mechanisms activated by a plethora of extracellular signals, including hormones. Cyclic AMP generation is catalyzed by adenylyl cyclases (ACs), which convert ATP into cAMP and pyrophosphate. By the way, Ca²⁺, as energy, can neither be created nor be destroyed; Ca²⁺ can only be transported, from one compartment to another, or chelated by a variety of Ca²⁺-binding molecules. The fine regulation of cytosolic concentrations of cAMP and free Ca²⁺ is crucial in cell function and there is an intimate cross-talk between both messengers to fine-tune the cellular responses. Cancer is a multifactorial disease resulting from a combination of genetic and environmental factors. Frequent cases of cAMP and/or Ca²⁺ homeostasis remodeling have been described in cancer cells. In those tumoral cells, cAMP and Ca²⁺ signaling plays a crucial role in the development of hallmarks of cancer, including enhanced proliferation and migration, invasion, apoptosis resistance, or angiogenesis. This review summarizes the cross-talk between the ACs/cAMP and Ca²⁺ intracellular pathways with special attention to the functional and reciprocal regulation between Orai1 and AC8 in normal and cancer cells.

The Orai1-AC8 Interplay: How Breast Cancer Cells Escape from Orai1 Channel Inactivation

The interplay between the Ca2+-sensitive adenylyl cyclase 8 (AC8) and Orai1 channels plays an important role both in the activation of the cAMP/PKA signaling and the modulation of Orai1-dependent Ca2+ signals. AC8 interacts with a N-terminal region that is exclusive to the Orai1 long variant, Orai1α. The interaction between both proteins allows the Ca2+ that enters the cell through Orai1α to activate the generation of cAMP by AC8. Subsequent PKA activation results in Orai1α inactivation by phosphorylation at serine-34, thus shaping Orai1-mediated cellular functions. In breast cancer cells, AC8 plays a relevant role supporting a variety of cancer hallmarks, including proliferation and migration. Breast cancer cells overexpress AC8, which shifts the AC8-Orai1 stoichiometry in favor of the former and leads to the impairment of PKA-dependent Orai1α inactivation. This mechanism contributes to the enhanced SOCE observed in triple-negative breast cancer cells. This review summarizes the functional interaction between AC8 and Orai1α in normal and breast cancer cells and its relevance for different cancer features.

Endoplasmic Reticulum-Plasma Membrane Contact Sites as an Organizing Principle for Compartmentalized Calcium and cAMP Signaling

In eukaryotic cells, ultimate specificity in activation and action-for example, by means of second messengers-of the myriad of signaling cascades is primordial. In fact, versatile and ubiquitous second messengers, such as calcium (Ca²⁺) and cyclic adenosine monophosphate (cAMP), regulate multiple-sometimes opposite-cellular functions in a specific spatiotemporal manner. Cells achieve this through segregation of the initiators and modulators to specific plasma membrane (PM) subdomains, such as lipid rafts and caveolae, as well as by dynamic close contacts between the endoplasmic reticulum (ER) membrane and other intracellular organelles, including the PM. Especially, these membrane contact sites (MCSs) are currently receiving a lot of attention as their large influence on cell signaling regulation and cell physiology is increasingly appreciated. Depletion of ER Ca²⁺ stores activates ER membrane STIM proteins, which activate PM-residing Orai and TRPC Ca²⁺ channels at ER-PM contact sites. Within the MCS, Ca²⁺ fluxes relay to cAMP signaling through highly interconnected networks. However, the precise mechanisms of MCS formation and the influence of their dynamic lipid environment on their functional maintenance are not completely understood. The current review aims to provide an overview of our current understanding and to identify open questions of the field.

Signaling through Ca2+ Microdomains from Store-Operated CRAC Channels

Calcium (Ca²⁺) ion microdomains are subcellular regions of high Ca²⁺ concentration that develop rapidly near open Ca²⁺ channels in the plasma membrane or internal stores and generate local regions of high Ca²⁺ concentration. These microdomains are remarkably versatile in that they activate a range of responses that differ enormously in both their temporal and spatial profile. In this review, we describe how Ca²⁺ microdomains generated by store-operated calcium channels, a widespread and conserved Ca²⁺ entry pathway, stimulate different signaling pathways, and how the spatial extent of a Ca²⁺ microdomain can be influenced by Ca²⁺ ATPase pumps.

Expression Profiling of Calcium Channels and Calcium-Activated Potassium Channels in Colorectal Cancer

Background: Colorectal cancer (CRC) is a highly devastating cancer. Ca²⁺-dependent channels are now considered key regulators of tumor progression. In this study, we aimed to investigate the association of non-voltage gated Ca²⁺ channels and Ca²⁺-dependent potassium channels (KCa) with CRC using the transcriptional profile of their genes. Methods: We selected a total of 35 genes covering KCa channels KCNN1–4, KCNMA1 and their subunits KCNMB1–4, endoplasmic reticulum (ER) calcium sensors STIM1 and STIM2, Ca²⁺ channels ORAI1–3 and the family of cation channels TRP (TRPC1–7, TRPA1, TRPV1/2,4–6 and TRPM1–8). We analyzed their expression in two public CRC datasets from The Cancer Genome Atlas (TCGA) and GSE39582. Results: KCNN4 and TRPM2 were induced while KCNMA1 and TRPM6 were downregulated in tumor tissues comparing to normal tissues. In proximal tumors, STIM2 and KCNN2 were upregulated while ORAI2 and TRPM6 were downregulated. ORAI1 decreased in lymph node metastatic tumors. TRPC1 and ORAI3 predicted poor prognosis in CRC patients. Moreover, we found that ORAI3/ORAI1 ratio is increased in CRC progression and predicted poor prognosis. Conclusions: KCa and Ca²⁺ channels could be important contributors to CRC initiation and progression. Our results provide new insights on KCa and Ca²⁺ channels remodeling in CRC.

Artificial Rod and Cone Photoreceptors with Human-Like Spectral Sensitivities

Photosensitive materials contain biologically engineered elements and are constructed using delicate techniques, with special attention devoted to efficiency, stability, and biocompatibility. However, to date, no photosensitive material has been developed to replace damaged visual-systems to detect light and transmit the signal to a neuron in the human body. In the current study, artificial nanovesicle-based photosensitive materials are observed to possess the characteristics of photoreceptors similar to the human eye. The materials exhibit considerably effective spectral characteristics according to each pigment. Four photoreceptors originating from the human eye with color-distinguishability are produced in human embryonic kidney (HEK)-293 cells and partially purified in the form of nanovesicles. Under various wavelengths of visible light, electrochemical measurements are performed to analyze the physiological behavior and kinetics of the photoreceptors, with graphene, performing as an electrode, playing an important role in the lipid bilayer deposition and oxygen reduction processes. Four nanovesicles with different photoreceptors, namely, rhodopsin (Rho), short-, medium-, and longwave sensitive opsin 1 (1SW, 1MW, 1LW), show remarkable color-dependent characteristics, consistent with those of natural human retina. With four different light-emitting diodes for functional verification, the photoreceptors embedded in nanovesicles show remarkably specific color sensitivity. This study demonstrates the potential applications of light-activated platforms in biological optoelectronic industries.

Store-operated Ca²⁺-entry and adenylyl cyclase

One of the longest-standing effects of SOCE is in its selective regulation of Ca(2+)-sensitive adenylyl cyclase (AC) activity in non-excitable cells. Remarkably it was this source of Ca(2+) (SOCE) rather than the apparent magnitude of the Ca(2+)-rise that conferred AC responsiveness. The molecular basis for this dependence is now resolved in the case of adenylyl cyclase 8 (AC8). Sensors for Ca(2+) and cAMP targeted to ACs have been particularly useful in dissecting the influences upon and composition of what turn out to be signalling microdomains centred on ACs. A number of physiological processes depend on the regulation by SOCE of ACs, but the issue is under-studied. Here I will expand on these topics and point to some immediate unresolved questions.

TRPC1 contributes to the Ca2+-dependent regulation of adenylate cyclases

SOCE (store-operated Ca2+ entry) is mediated via specific plasma membrane channels in response to ER (endoplasmic reticulum) Ca2+ store depletion. This route of Ca2+ entry is central to the dynamic interplay between Ca2+ and cAMP signalling in regulating the activity of Ca2+-sensitive adenylate cyclase isoforms (AC1, AC5, AC6 and AC8). Two proteins have been identified as key components of SOCE: STIM1 (stromal interaction molecule 1), which senses ER Ca2+ store content and translocates to the plasma membrane upon store depletion, where it then activates Orai1, the pore-forming component of the CRAC (Ca2+ release-activated Ca2+) channel. Previous studies reported that co-expression of STIM1 and Orai1 in HEK-293 (human embryonic kidney 293) cells enhances Ca2+-stimulated AC8 activity and that AC8 and Orai1 directly interact to enhance this regulation. Nonetheless, the additional involvement of TRPC (transient receptor potential canonical) channels in SOCE has also been proposed. In the present study, we evaluate the contribution of TRPC1 to SOCE-mediated regulation of Ca2+-sensitive ACs in HEK-293 cells stably expressing AC8 (HEK-AC8) and HSG (human submandibular gland) cells expressing an endogenous Ca2+-inhibited AC6. We demonstrate a role for TRPC1 as an integral component of SOCE, alongside STIM1 and Orai1, in regulating Ca2+ fluxes within AC microdomains and influencing cAMP production.

Rapid recycling of Ca2+ between IP3-sensitive stores and lysosomes

Inositol 1,4,5-trisphosphate (IP₃) evokes release of Ca²⁺ from the endoplasmic reticulum (ER), but the resulting Ca²⁺ signals are shaped by interactions with additional intracellular organelles. Bafilomycin A₁, which prevents lysosomal Ca²⁺ uptake by inhibiting H⁺ pumping into lysosomes, increased the amplitude of the initial Ca²⁺ signals evoked by carbachol in human embryonic kidney (HEK) cells. Carbachol alone and carbachol in combination with parathyroid hormone (PTH) evoke Ca²⁺ release from distinct IP₃-sensitive Ca²⁺ stores in HEK cells stably expressing human type 1 PTH receptors. Bafilomycin A₁ similarly exaggerated the Ca²⁺ signals evoked by carbachol or carbachol with PTH, indicating that Ca²⁺ released from distinct IP₃-sensitive Ca²⁺ stores is sequestered by lysosomes. The Ca²⁺ signals resulting from store-operated Ca²⁺ entry, whether evoked by thapsigargin or carbachol, were unaffected by bafilomycin A₁. Using Gd³⁺ (1 mM) to inhibit both Ca²⁺ entry and Ca²⁺ extrusion, HEK cells were repetitively stimulated with carbachol to assess the effectiveness of Ca²⁺ recycling to the ER after IP₃-evoked Ca²⁺ release. Blocking lysosomal Ca²⁺ uptake with bafilomycin A₁ increased the amplitude of each carbachol-evoked Ca²⁺ signal without affecting the rate of Ca²⁺ recycling to the ER. This suggests that Ca²⁺ accumulated by lysosomes is rapidly returned to the ER. We conclude that lysosomes rapidly, reversibly and selectively accumulate the Ca²⁺ released by IP₃ receptors residing within distinct Ca²⁺ stores, but not the Ca²⁺ entering cells via receptor-regulated, store-operated Ca²⁺ entry pathways.

Activation of ERK1/2 by store-operated calcium entry in rat parotid acinar cells

The regulation of intracellular Ca²⁺ concentration ([Ca²⁺]_i) plays a critical role in a variety of cellular processes, including transcription, protein activation, vesicle trafficking, and ion movement across epithelial cells. In many cells, the activation of phospholipase C-coupled receptors hydrolyzes membrane phosphoinositides and produces the depletion of endoplasmic reticulum Ca²⁺ stores, followed by the sustained elevation of [Ca²⁺]_i from Ca²⁺ entry across the plasma membrane via store-operated Ca²⁺ entry (SOCE). Ca²⁺ entry is also increased in a store-independent manner by arachidonate-regulated Ca²⁺ (ARC) channels. Using rat parotid salivary gland cells, we examined multiple pathways of Ca²⁺ entry/elevation to determine if they activated cell signaling proteins and whether this occurred in a pathway-dependent manner. We observed that SOCE activates extracellular signal-related kinases 1 and 2 (ERK1/2) to ∼3-times basal levels via a receptor-independent mechanism when SOCE was initiated by depleting Ca²⁺ stores using the endoplasmic reticulum Ca²⁺-ATPase inhibitor thapsigargin (TG). TG-initiated ERK1/2 phosphorylation increased as rapidly as that initiated by the muscarinic receptor agonist carbachol, which promoted an increase to ∼5-times basal levels. Notably, ERK1/2 phosphorylation was not increased by the global elevation of [Ca²⁺]_i by Ca²⁺ ionophore or by Ca²⁺ entry via ARC channels in native cells, although ERK1/2 phosphorylation was increased by Ca²⁺ ionophore in Par-C10 and HSY salivary cell lines. Agents and conditions that blocked SOCE in native cells, including 2-aminoethyldiphenyl borate (2-APB), SKF96363, and removal of extracellular Ca²⁺, also reduced TG- and carbachol-stimulated ERK1/2 phosphorylation. TG-promoted ERK1/2 phosphorylation was blocked when SRC and Protein Kinases C (PKC) were inhibited, and it was blocked in cells pretreated with β-adrenergic agonist isoproterenol. These observations demonstrate that ERK1/2 is activated by a selective mechanism of Ca²⁺ entry (SOCE) in these cells, and suggest that ERK1/2 may contribute to events downstream of SOCE.

Organization of cAMP signalling microdomains for optimal regulation by Ca2+ entry

Cross-talk between cAMP and Ca2+ signalling pathways plays a critical role in cellular homoeostasis. Several AC (adenylate cyclase) isoforms, catalysing the production of cAMP from ATP, display sensitivity to submicromolar changes in intracellular Ca2+ and, as a consequence, are key sites for Ca2+ and cAMP interplay. Interestingly, these Ca2+-regulated ACs are not equally responsive to equivalent Ca2+ rises within the cell, but display a remarkable selectivity for regulation by SOCE (store-operated Ca2+ entry). Over the years, considerable efforts at investigating this phenomenon have provided indirect evidence of an intimate association between Ca2+-sensitive AC isoforms and sites of SOCE. Now, recent identification of the molecular components of SOCE [namely STIM1 (stromal interaction molecule 1) and Orai1], coupled with significant advances in the generation of high-resolution targeted biosensors for Ca2+ and cAMP, have provided the first detailed insight into the organization of the cellular microdomains associated with Ca2+-regulated ACs. In the present review, I summarize the findings that have helped to provide our most definitive understanding of the selective regulation of cAMP signalling by SOCE.

Orai channel-dependent activation of phospholipase C-δ: a novel mechanism for the effects of calcium entry on calcium oscillations

The frequency of oscillatory Ca(2+) signals is a major determinant in the selective activation of discrete downstream responses in non-excitable cells. An important modulator of this oscillation frequency is known to be the rate of agonist-activated Ca(2+) entry. However precisely how this is achieved and the respective roles of store-operated versus store-independent Ca(2+) entry pathways in achieving this are unclear. Here, we examine the possibility that a direct stimulation of a phospholipase C (PLC) by the entering Ca(2+) can induce a modulation of Ca(2+) oscillation frequency, and examine the roles of the endogenous store-operated and store-independent Orai channels (CRAC and ARC channels, respectively) in such a mechanism. Using the decline in the magnitude of currents through expressed PIP(2)-dependent Kir2.1 channels as a sensitive assay for PLC activity, we show that simple global increases in Ca(2+) concentrations over the physiological range do not significantly affect PLC activity. Similarly, maximal activation of endogenous CRAC channels also fails to affect PLC activity. In contrast, equivalent activation of endogenous ARC channels resulted in a 10-fold increase in the measured rate of PIP(2) depletion. Further experiments show that this effect is strictly dependent on the Ca(2+) entering via these channels, rather than the gating of the channels or the arachidonic acid used to activate them, and that it reflects the activation of a PLCδ by local Ca(2+) concentrations immediately adjacent to the active channels. Finally, based on the effects of expression of either a dominant-negative mutant Orai3 that is an essential component of the ARC channel, or a catalytically compromised mutant PLCδ, it was shown that this specific action of the store-independent ARC channel-mediated Ca(2+) entry on PLCδ has a significant impact on the oscillation frequency of the Ca(2+) signals activated by low concentrations of agonist.

Regulation by Ca2+-signaling pathways of adenylyl cyclases

Interplay between the signaling pathways of the intracellular second messengers, cAMP and Ca(2+), has vital consequences for numerous essential physiological processes. Although cAMP can impact on Ca(2+)-homeostasis at many levels, Ca(2+) either directly, or indirectly (via calmodulin [CaM], CaM-binding proteins, protein kinase C [PKC] or Gβγ subunits) may also regulate cAMP synthesis. Here, we have evaluated the evidence for regulation of adenylyl cyclases (ACs) by Ca(2+)-signaling pathways, with an emphasis on verification of this regulation in a physiological context. The effects of compartmentalization and protein signaling complexes on the regulation of AC activity by Ca(2+)-signaling pathways are also addressed. Major gaps are apparent in the interactions that have been assumed, revealing a need to comprehensively clarify the effects of Ca(2+) signaling on individual ACs, so that the important ramifications of this critical interplay between Ca(2+) and cAMP are fully appreciated.

The mechanism of agonist induced Ca2+ signalling in intact endothelial cells studied confocally in in situ arteries

In endothelial cells there remain uncertainties in the details of how Ca²⁺ signals are generated and maintained, especially in intact preparations. In particular the role of the sarco-endoplasmic reticulum Ca²⁺-ATPase (SERCA), in contributing to the components of agonist-induced signals is unclear.

The aim of this work was to increase understanding of the detailed mechanism of Ca²⁺ signalling in endothelial cells using real time confocal imaging of Fluo-4 loaded intact rat tail arteries in response to muscarinic stimulation. In particular we have focused on the role of SERCA, and its interplay with capacitative Ca²⁺ entry (CCE) and ER Ca²⁺ release and uptake. We have determined its contribution to the Ca²⁺ signal and how it varies with different physiological stimuli, including single and repeated carbachol applications and brief and prolonged exposures.

In agreement with previous work, carbachol stimulated a rise in intracellular Ca²⁺ in the endothelial cells, consisting of a rapid initial phase, then a plateau upon which oscillations of Ca²⁺ were superimposed, followed by a decline to basal Ca²⁺ levels upon carbachol removal. Our data support the following conclusions: (i) the size (amplitude and duration) of the Ca²⁺ spike and early oscillations are limited by SERCA activity, thus both are increased if SERCA is inhibited. (ii) SERCA activity is such that brief applications of carbachol do not trigger CCE, presumably because the fall in luminal Ca²⁺ is not sufficient to trigger it. However, longer applications sufficient to deplete the ER or even partial SERCA inhibition stimulate CCE. (iii) Ca²⁺ entry occurs via STIM-mediated CCE and SERCA contributes to the cessation of CCE. In conclusion our data show how SERCA function is crucial to shaping endothelial cell Ca signals and its dynamic interplay with both CCE and ER Ca releases.

Biochemical properties of heterologously expressed and native adenylyl cyclases from the honeybee brain (Apis mellifera L.)

Cyclic AMP is an important intracellular signaling molecule participating e.g. in sensory signal transduction, cardiac myocyte regulation, learning and memory. The formation of cAMP is catalyzed by adenylyl cyclases. A variety of factors can modulate the properties of these enzymes and lead to dynamic changes of the intracellular cAMP concentration. Here we determined the tissue distribution of a recently cloned adenylyl cyclase (AmAC3) in honeybee brain. The protein is present in all neuropils. Intensive immunoreactivity was found in parts of the proto- and deutocerebrum and in the suboesophageal ganglion. Biochemical and pharmacological properties of AmAC3 and of native adenylyl cyclases in subregions of the honeybee brain were examined. Values for half-maximal activation with NKH477 were in the low micromolar range with 10.2 μM for AmAC3 and 3.6-8.1 μM for native enzymes. Biosynthesis of cAMP was specifically blocked by P-site inhibitors. Adenylyl cyclases in antennal lobes and AmAC3 share the inhibitory profile with 2',5'dd3'ATP>3'AMP>2'deoxyadenosine. In addition to P-site inhibitors AmAC3 activity was impaired by Ca(2+)/calmodulin. The results suggest that AmAC3 is a likely candidate to fulfill an integrative role in sensory, motor and higher-order information processing in the honeybee brain.

Direct demonstration of discrete Ca2+ microdomains associated with different isoforms of adenylyl cyclase

Ca(2+)-sensitive adenylyl cyclases (ACs) orchestrate dynamic interplay between Ca(2+) and cAMP that is a crucial feature of cellular homeostasis. Significantly, these ACs are highly selective for capacitative Ca(2+) entry (CCE) over other modes of Ca(2+) increase. To directly address the possibility that these ACs reside in discrete Ca(2+) microdomains, we tethered a Ca(2+) sensor, GCaMP2, to the N-terminus of Ca(2+)-stimulated AC8. GCaMP2-AC8 measurements were compared with global, plasma membrane (PM)-targeted or Ca(2+)-insensitive AC2-targeted GCaMP2. In intact cells, GCaMP2-AC8 responded rapidly to CCE, but was largely unresponsive to other types of Ca(2+) rise. The global GCaMP2, PM-targeted GCaMP2 and GCaMP2-AC2 sensors reported large Ca(2+) fluxes during Ca(2+) mobilization and non-specific Ca(2+) entry, but were less responsive to CCE than GCaMP2-AC8. Our data reveal that different AC isoforms localize to distinct Ca(2+)-microdomains within the plasma membrane. AC2, which is regulated via protein kinase C, resides in a microdomain that is exposed to a range of widespread Ca(2+) signals seen throughout the cytosol. By contrast, a unique Ca(2+) microdomain surrounds AC8 that promotes selectivity for Ca(2+) signals arising from CCE, and optimizes CCE-mediated cAMP synthesis. This direct demonstration of discrete compartmentalized Ca(2+) signals associated with specific signalling proteins provides a remarkable insight into the functional organization of signalling microdomains.

Electrophysiological characterization of store-operated and agonist-induced Ca2+ entry pathways in endothelial cells

In endothelial cells, agonist-induced Ca(2+) entry takes place via the store-operated Ca(2+) entry pathway and/or via channel(s) gated by second messengers. As cell stimulation leads to both a partial Ca(2+) store depletion as well as the production of second messengers, these two pathways are problematic to distinguish. We showed that passive endoplasmic reticulum (ER) depletion by thapsigargin or cell stimulation by histamine activated a similar Ca(2+)-release-activated Ca(2+) current (CRAC)-like current when 10 mM Ba(2+)/2 mM Ca(2+) was present in the extracellular solution. Importantly, during voltage clamp recordings, histamine stimulation largely depleted the ER Ca(2+) store, explaining the activation of a CRAC-like current (due to store depletion) upon histamine in Ba(2+) medium. On the contrary, in the presence of 10 mM Ca(2+), the ER Ca(2+) content remained elevated, and histamine induced an outward rectifying current that was inhibited by Ni(2+) and KB-R7943, two blockers of the Na(+)/Ca(2+) exchanger (NCX). Both blockers also reduced histamine-induced cytosolic Ca(2+) elevation. In addition, removing extracellular Na(+) increased the current amplitude which is in line with a current supported by the NCX. These data are consistent with the involvement of the NCX working in reverse mode (Na(+) out/Ca(2+) in) during agonist-induced Ca(2+) entry in endothelial cells.

(Endo)cannabinoids mediate different Ca2+ entry mechanisms in human bronchial epithelial cells

In human bronchial epithelial (16HBE14o(-)) cells, CB(1) and CB(2) cannabinoid receptors are present, and their activation by the endocannabinoid virodhamine and the synthetic non-selective receptor agonist CP55,940 inhibits adenylyl cyclase and cellular interleukin-8 release. Here, we analyzed changes in intracellular calcium ([Ca2+](i)) evoked by Delta(9)-tetrahydrocannabinol (Delta(9)-THC), CP55,940, and virodhamine in 16HBE14o(-) cells. Delta(9)-THC induced [Ca2+](i) increase and a large transient [Ca2+](i) mobilization, the latter probably reflecting store-depletion-driven capacitative Ca2+ entry (CCE). In contrast, CP55,940 induced a rather moderate Ca2+ influx and a sustained [Ca2+](i) mobilization. CP55,940-induced Ca2+ influx was inhibited by Ni2+, indicating CCE, possibly mediated by transient receptor potential channel TRPC1, the mRNA of which is expressed in 16HBE14o(-) cells. CP55,940-induced calcium alterations were mimicked by virodhamine concentrations below 30 microM. Interestingly, higher virodhamine induced an additional Ca2+ entry, insensitive to Ni2+, but sensitive to the TRPV1 antagonist capsazepine, the TRPV1-TRPV4 inhibitor ruthenium red, and the non-CCE (NCCE) inhibitors La3+ and Gd3+. Such pharmacological profile is supported by the presence of TRPV1, TRPV4, and TRPC6 mRNAs as well as TRPV1 and TRPC6 proteins in 16HBE14o(-) cells. Cannabinoid receptor antagonists increased virodhamine-induced Ca2+ entry. Virodhamine also enhanced arachidonic acid release, which was insensitive to cannabinoid receptor antagonism, but sensitive to the phospholipase A(2) inhibitor quinacrine, and to capsazepine. Arachidonic acid induced [Ca2+](i) increase similar to virodhamine. Collectively, these observations suggest that [Ca2+](i) alterations induced by Delta(9)-THC, CP55,940 and by low concentrations of virodhamine involve mobilization and subsequent CCE mechanisms, whereas such responses by high virodhamine concentrations involve NCCE pathways.

Capacitative Ca2+ entry via Orai1 and stromal interacting molecule 1 (STIM1) regulates adenylyl cyclase type 8

Capacitative Ca(2+) entry (CCE), which occurs through the plasma membrane as a result of Ca(2+) store depletion, is mediated by stromal interacting molecule 1 (STIM1), a sensor of intracellular Ca(2+) store content, and the pore-forming component Orai1. However, additional factors, such as C-type transient receptor potential (TRPC) channels, may also participate in the CCE apparatus. To explore whether the store-dependent Ca(2+) entry reconstituted by coexpression of Orai1 and STIM1 has the functional properties of CCE, we used the Ca(2+)-calmodulin stimulated adenylyl cyclase type 8 (AC8), which responds selectively to CCE, whereas other modes of Ca(2+) entry, including those activated by arachidonate and the ionophore ionomycin, are ineffective. In addition, the Ca(2+) entry mediated by previous CCE candidates, diacylglycerol-activated TRPC channels, does not activate AC8. Here, we expressed Orai1 and STIM1 in HEK293 cells and saw a robust increment in CCE, and a proportional increase in CCE-stimulated AC8 activity. Inhibitors of the CCE assembly process ablated the effects on cyclase activity in both AC8-overexpressing HEK293 cells and insulin-secreting MIN6 cells endogenously expressing Ca(2+)-sensitive AC isoforms. AC8 is believed to be closely associated with the source of CCE; indeed, not only were AC8, Orai1, and STIM1 colocalized at the plasma membrane but also all three proteins occurred in lipid rafts. Together, our data indicate that Orai1 and STIM1 can be integral components of the cAMP and CCE microdomain associated with adenylyl cyclase type 8.

Insights into the residence in lipid rafts of adenylyl cyclase AC8 and its regulation by capacitative calcium entry

Adenylyl cyclases (ACs) are a family of critically important signaling molecules that are regulated by multiple pathways. Adenylyl cyclase 8 (AC8) is a Ca(2+) stimulated isoform that displays a selective regulation by capacitative Ca(2+) entry (CCE), the process whereby the entry of Ca(2+) into cells is triggered by the emptying of intracellular stores. This selectivity was believed to be achieved through the localization of AC8 in lipid raft microdomains, along with components of the CCE apparatus. In the present study, we show that an intact leucine zipper motif is required for the efficient N-linked glycosylation of AC8, and that this N-linked glycosylation is important to target AC8 into lipid rafts. Disruption of the leucine zipper by site-directed mutagenesis results in the elimination of N-glycosylated forms and their exclusion from lipid rafts. Mutants of AC8 that cannot be N-glycosylated are not demonstrably associated with rafts, although they can still be regulated by CCE; however, raft integrity is required for the regulation of these mutants. These findings suggest that raft localized proteins in addition to AC8 are needed to mediate its regulation by CCE.

Distinct mechanisms of regulation by Ca2+/calmodulin of type 1 and 8 adenylyl cyclases support their different physiological roles

Nine membrane-bound mammalian adenylyl cyclases (ACs) have been identified. Type 1 and 8 ACs (AC1 and AC8), which are both expressed in the brain and are stimulated by Ca(2+)/calmodulin (CaM), have discrete neuronal functions. Although the Ca(2+) sensitivity of AC1 is higher than that of AC8, precisely how these two ACs are regulated by Ca(2+)/CaM remains elusive, and the basis for their diverse physiological roles is quite unknown. Distinct localization of the CaM binding domains within the two enzymes may be essential to differential regulation of the ACs by Ca(2+)/CaM. In this study we compare in detail the regulation of AC1 and AC8 by Ca(2+)/CaM both in vivo and in vitro and explore the different role of each Ca(2+)-binding lobe of CaM in regulating the two enzymes. We also assess the relative dependence of AC1 and AC8 on capacitative Ca(2+) entry. Finally, in real-time fluorescence resonance energy transfer-based imaging experiments, we examine the effects of dynamic Ca(2+) events on the production of cAMP in cells expressing AC1 and AC8. Our data demonstrate distinct patterns of regulation and Ca(2+) dependence of AC1 and AC8, which seems to emanate from their mode of regulation by CaM. Such distinctive properties may contribute significantly to the divergent physiological roles in which these ACs have been implicated.

The regulation of cell motility and chemotaxis by phospholipid signaling

Phosphoinositide 3-kinase (PI3K), PTEN and localized phosphatidylinositol (3,4,5)-trisphosphate [PtdIns(3,4,5)P3] play key roles in chemotaxis, regulating cell motility by controlling the actin cytoskeleton in Dictyostelium and mammalian cells. PtdIns(3,4,5)P3, produced by PI3K, acts via diverse downstream signaling components, including the GTPase Rac, Arf-GTPases and the kinase Akt (PKB). It has become increasingly apparent, however, that chemotaxis results from an interplay between the PI3K-PTEN pathway and other parallel pathways in Dictyostelium and mammalian cells. In Dictyostelium, the phospholipase PLA2 acts in concert with PI3K to regulate chemotaxis, whereas phospholipase C (PLC) plays a supporting role in modulating PI3K activity. In adenocarcinoma cells, PLC and the actin regulator cofilin seem to provide the direction-sensing machinery, whereas PI3K might regulate motility.

Ca2+ microdomains near plasma membrane Ca2+ channels: impact on cell function

In eukaryotic cells, a rise in cytoplasmic Ca(2+) can activate a plethora of responses that operate on time scales ranging from milliseconds to days. Inherent to the use of a promiscuous signal like Ca(2+) is the problem of specificity: how can Ca(2+) activate some responses but not others? We now know that the spatial profile of the Ca(2+) signal is important Ca(2+) does not simply rise uniformly throughout the cytoplasm upon stimulation but can reach very high levels locally, creating spatial gradients. The most fundamental local Ca(2+) signal is the Ca(2+) microdomain that develops rapidly near open plasmalemmal Ca(2+) channels like voltage-gated L-type (Cav1.2) and store-operated CRAC channels. Recent work has revealed that Ca(2+) microdomains arising from these channels are remarkably versatile in triggering a range of responses that differ enormously in both temporal and spatial profile. Here, I delineate basic features of Ca(2+) microdomains and then describe how these highly local signals are used by Ca(2+)-permeable channels to drive cellular responses.

Kinetic properties of Ca2+/calmodulin-dependent phosphodiesterase isoforms dictate intracellular cAMP dynamics in response to elevation of cytosolic Ca2+

Multiply regulated adenylyl cyclases (AC) and phosphodiesterases (PDE) can yield complex intracellular cAMP signals. Ca2+-sensitive ACs have received far greater attention than the Ca2+/calmodulin-dependent PDE (PDE1) family in governing intracellular cAMP dynamics in response to changes in the cytosolic Ca2+ concentration ([Ca2+]i). Here, we have stably expressed two isoforms of PDE1, PDE1A2 and PDE1C4, in HEK-293 cells to determine whether they exert different impacts on cellular cAMP. Fractionation and imaging showed that both PDEs occurred mainly in the cytosol. However, PDE1A2 and PDE1C4 differed considerably in their ability to hydrolyze cAMP and in their susceptibility to inhibition by the non-selective PDE inhibitor, IBMX and the PDE1-selective inhibitor, MMX. PDE1A2 had an approximately 30-fold greater Km for cAMP than PDE1C4 and yet was more susceptible to inhibition by IBMX and MMX than was PDE1C4. These differences were mirrored in intact cells when thapsigargin-induced capacitative Ca2+ entry (CCE) activated the PDEs. Mirroring their kinetic properties, PDE1C4 was active at near basal cAMP levels, whereas PDE1A2 required agonist-triggered levels of cAMP, produced in response to stimulation of ACs. The effectiveness of IBMX and MMX to inhibit PDE1A2 and PDE1C4 in functional studies was inversely related to their respective affinities for cAMP. To assess the impact of the two isoforms on cAMP dynamics, real-time cAMP measurements were performed in single cells expressing the two PDE isoforms and a fluorescent Epac-1 cAMP biosensor, in response to CCE. These measurements showed that prostaglandin E1-mediated cAMP production was markedly attenuated in PDE1C4-expressing cells upon induction of CCE and cAMP hydrolysis occurred at a faster rate than in cells expressing PDE1A2 under similar conditions. These results prove that the kinetic properties of PDE isoforms play a major role in determining intracellular cAMP signals in response to physiological elevation of [Ca2+]i and thereby provide a rationale for the utility of diverse PDE1 species.

Discrete influx events refill depleted Ca2+ stores in a chick retinal neuron

The depletion of ER Ca2+ stores, following the release of Ca2+ during intracellular signalling, triggers the Ca2+ entry across the plasma membrane known as store-operated calcium entry (SOCE). We show here that brief, local [Ca2+]i increases (motes) in the thin dendrites of cultured retinal amacrine cells derived from chick embryos represent the Ca2+ entry events of SOCE and are initiated by sphingosine-1-phosphate (S1P), a sphingolipid with multiple cellular signalling roles. Externally applied S1P elicits motes but not through a G protein-coupled membrane receptor. The endogenous precursor to S1P, sphingosine, also elicits motes but its action is suppressed by dimethylsphingosine (DMS), an inhibitor of sphingosine phosphorylation. DMS also suppresses motes induced by store depletion and retards the refilling of depleted stores. These effects are reversed by exogenously applied S1P. In these neurons formation of S1P is a step in the SOCE pathway that promotes Ca2+ entry in the form of motes.

Functional consequences of activating store-operated CRAC channels

Store-operated CRAC channels, which are activated by the emptying of the endoplasmic reticulum Ca(2+) stores, are an important and widespread route for triggering rises in cytoplasmic Ca(2+). The cellular responses that are activated in response to Ca(2+) entry through CRAC channels are being dissected out, and recent evidence has established that CRAC channels can induce both short-term (safeguarding the Ca(2+) content of the endoplasmic reticulum, maintenance of cytoplasmic Ca(2+) oscillations, enzyme activation, secretion) and long-term (gene expression) changes in cells. CRAC channel activation is therefore capable of evoking a range of temporally distinct responses, highlighting the versatility of this ubiquitous Ca(2+) entry pathway.

Organization and Ca2+Regulation of Adenylyl Cyclases in cAMP Microdomains

The adenylyl cyclases are variously regulated by G protein subunits, a number of serine/threonine and tyrosine protein kinases, and Ca2+. In some physiological situations, this regulation can be readily incorporated into a hormonal cascade, controlling processes such as cardiac contractility or neurotransmitter release. However, the significance of some modes of regulation is obscure and is likely only to be apparent in explicit cellular contexts (or stages of the cell cycle). The regulation of many of the ACs by the ubiquitous second messenger Ca2+provides an overarching mechanism for integrating the activities of these two major signaling systems. Elaborate devices have been evolved to ensure that this interaction occurs, to guarantee the fidelity of the interaction, and to insulate the microenvironment in which it occurs. Subcellular targeting, as well as a variety of scaffolding devices, is used to promote interaction of the ACs with specific signaling proteins and regulatory factors to generate privileged domains for cAMP signaling. A direct consequence of this organization is that cAMP will exhibit distinct kinetics in discrete cellular domains. A variety of means are now available to study cAMP in these domains and to dissect their components in real time in live cells. These topics are explored within the present review.

Regulation of store-operated calcium entry by calcium-independent phospholipase A2 in rat cerebellar astrocytes

We have studied store-operated Ca2+ entry (SOCE) in Bergmann glia and granule cell layer astrocytes in acute brain slices of the rat cerebellum, using the Ca2+-sensitive fluorescent dye Fluo-4 and confocal laser scanning microscopy. Astrocytes were identified by their morphology, location, and their Ca2+ response in K+-free solution. Depletion of Ca2+ stores by cyclopiazonic acid (CPA) (20 microM) induced SOCE in both types of astrocyte. A similar Ca2+ influx was elicited by the calmodulin antagonist calmidazolium (CMZ) (1 microM). The SOCE channel blocker 2-aminoethoxy-diphenylborate (2-APB) (100 microM) and the Ca2+ release-activated channel blocker 3,5-bistrifluoromethyl pyrazole derivative (BTP2) (20 microM) suppressed the CPA- and the CMZ-induced Ca2+ influx. Pretreatment of acute slices with the specific Ca2+-independent phospholipase A2 (iPLA2) inhibitor bromoenol lactone (BEL) (25 microM) blocked the CPA- and the CMZ-induced Ca2+ influx. The lysophospholipid products of iPLA2, lysophosphatidylcholine (250 nM) and lysophosphatidylinositol (250 nM), but not lysophosphatidic acid (250 nM), induced a BTP2- and 2-APB-sensitive, but BEL-insensitive, Ca2+ influx. CPA or CMZ enhanced the BEL-sensitive enzymatic activity of iPLA2 in cerebellar astrocyte culture. Inhibition of iPLA2 expression by specific antisense oligodeoxynucleotide of iPLA2 reduced the SOCE and the Ca2+ store refilling in cultured astrocytes. Spontaneous Ca2+ oscillations in astrocytes in situ were reduced after inhibiting SOCE channels or iPLA2 activity. The results suggest that the depletion of Ca2+ stores activates iPLA2 to open Ca2+ channels in the plasma membrane by the formation of lysophospholipids in astrocytes, presumably to refill the stores and allow normal Ca2+ signaling.

Evidence against Calcium as a Mediator of Mitochondrial Dysfunction during Apoptosis Induced by Arachidonic Acid and Other Free Fatty Acids

Apoptosis is often accompanied by activation of phospholipase A(2), causing release of free fatty acids (FFAs), which in turn are thought to contribute to the loss of mitochondrial transmembrane potential (Deltapsi(m)). In these experiments, we asked whether calcium plays a role as an intermediate in this process. A total of 14 FFAs were compared for their ability to cause loss of Deltapsi(m) and for their ability to affect levels of intracellular calcium. Among the FFAs, unsaturated FFAs tended to induce apoptosis while saturated FFAs did not. Arachidonic acid (AA) was most damaging, causing loss of Deltapsi(m) and cell death in 8-10 h while linoleic acid, gamma-linolenic acid, and docosapentaenoic also strongly induced apoptosis. Effects of the FFAs on levels of intracellular calcium were very different. Many caused strong calcium responses; however, the ability to induce a strong calcium response was not predictive of ability to induce apoptosis, and overall, we did not find a correlation between apoptosis and calcium induction. Also, verapamil and TMB-8 were able to block the calcium response, but these inhibitors did not prevent loss of Deltapsi(m), indicating that the calcium response is not necessary for FFA-induced loss of Deltapsi(m). In contrast, we found that cyclosporine A could inhibit the AA-induced loss of Deltapsi(m) with both whole cells and isolated mitochondria, confirming that the antimitochondrial effects of FFA can stem from direct effects on the mitochondrial permeability transition pore. Finally, we show that the strong apoptosis-inducing activity of AA may stem from its ability to selectively induce its own release.

Layers of organization of cAMP microdomains in a simple cell

Based on a variety of single-cell measurements, the notion that cAMP microdomains exist in cells is being increasingly embraced. The cellular and molecular underpinnings of this organization are also steadily being revealed. A dependence of Ca(2+)-sensitive ACs (adenylate cyclases) in HEK-293 cells (human embryonic kidney cells) on capacitative Ca(2+) entry is enforced by their presence in lipid rafts and protein-protein interactions. In these cells, many of the participants in the cAMP cascade, including AC, phosphodiesterase 4, cAMP-dependent protein kinase [PKA (protein kinase A)] and protein phosphatase 2A, are now seen to be involved in higher order assemblies. Moreover, the presence of Na(+)/H(+) exchanger 1 in these domains creates a microclimate, protected against global swings in cellular pH. The Ca(2+)-stimulatable AC8, which is targeted to these regions, can sequester calmodulin for its own regulatory purposes. These devices are a sampling of the multiple layers of organization that are in place -- even in a simple cell -- to ensure faithful and economical communication of the cAMP message.

Higher-order organization and regulation of adenylyl cyclases

There is increasing awareness of the compartmentalization of cAMP signalling--the means by which cAMP levels change in discrete domains of the cell with discrete local consequences. Current developments in understanding the organization of adenylyl cyclases in the plasma membrane are illuminating how the earliest part of cAMP compartmentalization could occur. This review focuses on recent findings regarding three levels of adenylyl cyclase organization--oligomerization, positioning to lipid rafts and participation in multiprotein signalling complexes. This organization, coupled with the role of scaffolding proteins in arranging the downstream effectors of cAMP, helps to identify complexes that greatly facilitate the translation of enzyme activation into local consequences.

Capacitative and 1-oleyl-2-acetyl-sn-glycerol-activated Ca(2+) entry distinguished using adenylyl cyclase type 8

Although the molecular identity of capacitative Ca(2+) entry (CCE) channels remains elusive, transient receptor potential channel (TRPC) family members 3, 6, and 7, which are activated by diacylglycerol (DAG), have been put forward as possible candidates. Because human embryonic kidney (HEK) 293 cells endogenously express these TRP subunits, this cell line is suitable for investigating whether DAG-activated TRP subunits form part of the putative multimeric assemblies that mediate CCE. Adenylyl cyclase type 8 (AC8) is activated by CCE in nonexcitable cells but is not responsive to other forms of Ca(2+) entry, such as ionophore- or arachidonate-activated entry through the plasma membrane. In this study, we exploited this unique dependence of AC8 on CCE to determine whether the DAG analog, 1-oleyl-2-acetyl-sn-glycerol (OAG), activates the same subset of Ca(2+) channels as store depletion, which triggers CCE. In populations of HEK 293 cells, OAG evoked a faster and greater influx of Ca(2+) than CCE. Both pathways of Ca(2+) entry could be triggered simultaneously in the same batch of cells, with additive effects. It is striking that OAG-mediated Ca(2+) entry, unlike CCE, did not stimulate AC8 activity in populations of cells. In single cells, OAG evoked a highly heterogeneous response, whereas CCE occurred as a smooth and sustained increase in [Ca(2+)](i). Taken together, our results indicate that, in HEK 293 cells, OAG-activated Ca(2+) entry is distinct from CCE. The inability of the OAG-activated Ca(2+) entry pathway to regulate AC8 further reinforces the absolute dependence of this enzyme on CCE.

Carboxyamidotriazole-induced inhibition of mitochondrial calcium import blocks capacitative calcium entry and cell proliferation in HEK-293 cells

Blocking calcium entry may prevent normal and pathological cell proliferation. There is evidence suggesting that molecules such as carboxyamidotriazole, widely used in anti-cancer therapy based on its ability to block calcium entry in nonexcitable cells, also have antiproliferative properties. We found that carboxyamidotriazole and the capacitative calcium entry blocker 2-aminoethoxydiphenyl borate inhibited proliferation in HEK-293 cells with IC50 values of 1.6 and 50 μM, respectively. Capacitative calcium entry is activated as a result of intracellular calcium store depletion. However, non-capacitative calcium entry pathways exist that are independent of store depletion and are activated by arachidonic acid and diacylglycerol, generated subsequent to G protein coupled receptor stimulation. We found that carboxyamidotriazole completely inhibited the capacitative calcium entry and had no effect on the amplitude of arachidonic-acid-activated non-capacitative calcium entry. However, investigation of the effects of carboxyamidotriazole on mitochondrial calcium dynamics induced by carbachol, capacitative calcium entry and exogenously set calcium loads in intact and digitonin-permeabilized cells revealed that carboxyamidotriazole inhibited both calcium entry and mitochondrial calcium uptake in a time-dependent manner. Mitochondrial inner-membrane potential was altered by carboxyamidotriazole treatment, suggesting that carboxyamidotriazole antagonizes mitochondrial calcium import and thus local calcium clearance, which is crucial for the maintenance of capacitative calcium entry.

Mitochondria play a critical role in shaping the exocytotic response of rat pancreatic acinar cells

We have previously demonstrated [M. Campos-Toimil, T. Bagrij, J.M. Edwardson, P. Thomas, Two modes of secretion in pancreatic acinar cells: involvement of phosphatidylinositol 3-kinase and regulation by capacitative Ca(2+) entry, Curr. Biol. 12 (2002) 211-215] that in rat pancreatic acinar cells, Gd(3+)-sensitive Ca(2+) entry is instrumental in governing which second messenger pathways control secretory activity. However, in those studies, we were unable to demonstrate a significant increase in cytoplasmic [Ca(2+)] during agonist application as a result of this entry pathway. In the present study, we combined pharmacology with ratiometric imaging of fura-2 fluorescence to resolve this issue. We found that 2 microM Gd(3+) significantly inhibits store-mediated Ca(2+) entry. Furthermore, both the protonophore, CCCP (5 microM) and the mitochondrial Ca(2+)-uptake blocker, RU360 (10 microM), led to an enhancement of the plateau phase of the biphasic Ca(2+) response induced by acetylcholine (1 microM). This enhancement was completely abolished by Gd(3+); and as has been previously shown for Gd(3+), RU360 led to a switch to a wortmannin-sensitive form of exocytosis. Using MitoTracker Red staining we found a close association of mitochondria with the lateral plasma membrane. We propose that in rat pancreatic acinar cells, capacitative Ca(2+) entry is targeted directly to mitochondria; and that as a result of Ca(2+) uptake, these mitochondria release "third" messengers which both enhance exocytosis and suppress phosphatidylinositol 3-kinase-dependent secretion.

Direct coupling between arachidonic acid-induced Ca2+ release and Ca2+ entry in HEK293 cells

Arachidonic acid (AA) modulates intracellular Ca2+ signaling via Ca2+ release or/and Ca2+ entry. However, the mechanism underlies either process is unknown; nor is it clear as to whether the two processes are mechanistically linked. By using Fura2/AM, we found that AA induced mobilization of internal Ca2+ store and an increment in Ca2+, Mn2+ and Ba2+ influx in HEK293 cells. The AA-mediated Ca2+ signaling was not due to AA metabolites, and insensitive to capacitative Ca2+ entry inhibitors. Interestingly, isotetrandrine and Gd3+ inhibited both AA-induced Ca2+ release and Ca2+ entry in a concentration-dependent manner without affecting Ca2+ discharge caused by carbachol, caffeine, or thapsigargin. Additionally, similar pattern of inhibition was observed with tetracaine treatment. More importantly, the three compounds exhibited almost equal potent inhibition of AA-initiated Ca2+ release as well as Ca2+ influx. Therefore, this study, for the first time, provides evidence for a direct coupling between AA-mediated Ca2+ release and Ca2+ entry.

The role of metabotropic glutamate receptors for the generation of calcium oscillations in rat hippocampal astrocytes in situ

Ca2+ oscillations are part of the intra- and intercellular signalling in many cell types. We have studied Ca2+ oscillations in astrocytes in acute brain slices of the hippocampus of juvenile rats (postnatal 8-14 days old), using confocal laser scanning microscopy and bulk-loading of the Ca2+ -sensitive dye Fluo-4. Astrocytes were identified morphologically in the stratum radiatum, and by their Ca2+ response in the absence of external K+. Thirty-five per cent of astrocytes (43 slices) showed spontaneous Ca2+ oscillations, with a frequency of 1.26 +/- 0.11 transients/min (n = 366). These Ca2+ signals were unaffected by tetrodotoxin (0.5 microM) and Ni2+ (2 mM), but were sensitive to interference with the phospholipase C-mediated Ca2+ release from intracellular stores. Spontaneous Ca2+ oscillations were reduced or suppressed by antagonists of metabotropic glutamate receptors (mGluRs) of groups I and II, but not affected by antagonists of group III. Glutamate (1-100 microM) and specific agonists of mGluR groups I and II evoked concentration-dependent Ca2+ signals, which were oscillatory at intermediate concentrations (e.g. at 10 microM glutamate). Our results indicate that mGluRs of both groups I and II are involved in mediating Ca2+ oscillations in astrocytes, which might be glial responses to micromolar changes of glutamate in the extracellular spaces.

Ca2+-calmodulin-dependent phosphodiesterase (PDE1): Current perspectives

Ca2+-calmodulin-dependent phosphodiesterases (PDE1), like Ca2+-sensitive adenylyl cyclases (AC), are key enzymes that play a pivotal role in mediating the cross-talk between cAMP and Ca2+ signalling. Our understanding of how ACs respond to Ca2+ has advanced greatly, with significant breakthroughs at both the molecular and functional level. By contrast, little is known of the mechanisms that might underlie the regulation of PDE1 by Ca2+ in the intact cell. In living cells, Ca2+ signals are complex and diverse, exhibiting different spatial and temporal properties. The potential therefore exists for dynamic changes in the subcellular distribution and activation of PDE1 in relation to intracellular Ca2+ dynamics. PDE1s are a large family of multiply-spliced gene products. Therefore, it is possible that a cell-type specific response to elevation in [Ca2+]i can occur, depending on the isoform of PDE1 expressed. In this article, we summarize current knowledge on Ca2+ regulation of PDE1 in the intact cell and discuss approaches that might be undertaken to delineate the responses of this important group of enzymes to changes in [Ca2+]i.

Regulation of fluid and electrolyte secretion in salivary gland acinar cells

The secretion of fluid and electrolytes by salivary gland acinar cells requires the coordinated regulation of multiple water and ion transporter and channel proteins. Notably, all the key transporter and channel proteins in this process appear to be activated, or are up-regulated, by an increase in the intracellular Ca2+ concentration ([Ca2+]i). Consequently, salivation occurs in response to agonists that generate an increase in [Ca2+]i. The mechanisms that act to modulate these increases in [Ca2+]i obviously influence the secretion of salivary fluid. Such modulation may involve effects on mechanisms of both Ca2+ release and Ca2+ entry and the resulting spatial and temporal aspects of the [Ca2+]i signal, as well as interactions with other signaling pathways in the cells. The molecular cloning of many of the transporter and regulatory molecules involved in fluid and electrolyte secretion has yielded a better understanding of this process at the cellular level. The subsequent characterization of mice with null mutations in many of these genes has demonstrated the physiological roles of individual proteins. This review focuses on recent developments in determining the molecular identification of the proteins that regulate the fluid secretion process.

The blocking of capacitative calcium entry by 2-aminoethyl diphenylborate (2-APB) and carboxyamidotriazole (CAI) inhibits proliferation in Hep G2 and Huh-7 human hepatoma cells

Calcium entry is a component of the processes regulating the proliferative phenotype of some types of cancer. In non-excitable cells, capacitative calcium entry (CCE) and non-capacitative calcium entry (NCCE) are thought to be the main pathways of Ca2+ influx into cells. Thus, blocking calcium entry may prevent normal and pathological cell proliferation and there is evidence to suggest that molecules blocking calcium entry also have antiproliferative properties. Carboxyamidotriazole (CAI), a novel inhibitor of the non-voltage-dependent calcium entry has been shown to have such properties in model systems in vitro and in vivo. We used Hep G2 and Huh-7 human hepatoma cells to investigate the effects of calcium entry blockers on cell proliferation. CAI (10 microM) and 2-APB (20 microM) completely blocked CCE in thapsigargin-treated Huh-7, and CAI and 2-APB inhibited cell proliferation with IC50 of 4.5 and 43 microM, respectively. The plateau phase of the [Ca2+]i increases triggered by 10% FCS were abolished in the absence of external Ca2+ and in the presence of CAI or 2-APB. We, therefore, suggest that CCE is the main pathway involved in regulation of the processes leading to cell proliferation.

Long lasting inhibition of adenylyl cyclase selectively mediated by inositol 1,4,5-trisphosphate-evoked calcium release

In A7r5 smooth muscle cells, vasopressin stimulates release of Ca2+ from intracellular stores and Ca2+ entry, and it inhibits adenylyl cyclase (AC) activity. Inhibition of AC is prevented by inhibition of phospholipase C or when the increase in cytosolic [Ca2+] is prevented by the Ca2+ buffer, BAPTA. It is unaffected by pertussis toxin, inhibition of protein kinase C, or L-type Ca2+ channels or by removal of extracellular Ca2+. The independence of extracellular Ca2+ occurs despite inhibition of AC by vasopressin persisting for at least 15 min, whereas the cytosolic [Ca2+] returns to its basal level within 1-2 min in Ca2+-free medium. Although capacitative Ca2+ entry (CCE), activated by emptying stores with thapsigargin, inhibits AC, Ca2+ entry via CCE or L-type Ca2+ channels activated by vasopressin is ineffective. Temporally separating vasopressin-evoked Ca2+ release from the assessment of AC activity revealed that the transient Ca2+ signal resulting from Ca2+ mobilization causes a long lasting inhibition of AC. By contrast, inhibition of AC by thapsigargin-evoked CCE reverses rapidly after removal of extracellular Ca2+. Inhibition of AC by vasopressin is prevented by inhibition of Ca2+-calmodulin-dependent protein kinase II. We conclude that persistent inhibition of AC (probably AC-3) by vasopressin is mediated by inositol trisphosphate-evoked Ca2+ release causing activation of Ca2+-calmodulin-dependent protein kinase II. Our results establish that an important interaction between two ubiquitous signaling pathways is tuned selectively to Ca2+ release via inositol trisphosphate receptors and that the interaction transduces a transient Ca2+ signal into a long lasting inhibition of AC.

Calmidazolium and arachidonate activate a calcium entry pathway that is distinct from store-operated calcium influx in HeLa cells

Agonists that deplete intracellular Ca2+ stores also activate Ca2+ entry, although the mechanism by which store release and Ca2+ influx are linked is unclear. A potential mechanism involves 'store-operated channels' that respond to depletion of the intracellular Ca2+ pool. Although SOCE (store-operated Ca2+ entry) has been considered to be the principal route for Ca2+ entry during hormonal stimulation of non-electrically excitable cells, recent evidence has suggested that alternative pathways activated by metabolites such as arachidonic acid are responsible for physiological Ca2+ influx. It is not clear whether such messenger-activated pathways exist in all cells, whether they are truly distinct from SOCE and which metabolites are involved. In the present study, we demonstrate that HeLa cells express two pharmacologically and mechanistically distinct Ca2+ entry pathways. One is the ubiquitous SOCE route and the other is an arachidonate-sensitive non-SOCE. We show that both these Ca2+ entry pathways can provide long-lasting Ca2+ elevations, but that the channels are not the same, based on their differential sensitivity to 2-aminoethoxydiphenyl borate, LOE-908 [(R,S)-(3,4-dihydro-6,7-dimethoxy-isochinolin-1-yl)-2-phenyl-N,N-di[2-(2,3,4-trimethoxyphenyl)ethyl]acetamid mesylate] and gadolinium. In addition, non-SOCE and not SOCE was permeable to strontium. Furthermore, unlike SOCE, the non-SOCE pathway did not require store depletion and was not sensitive to displacement of the endoplasmic reticulum from the plasma membrane using jasplakinolide or ionomycin pretreatment. These pathways did not conduct Ca2+ simultaneously due to the dominant effect of arachidonate, which rapidly curtails SOCE and promotes Ca2+ influx via non-SOCE. Although non-SOCE could be activated by exogenous application of arachidonate, the most robust method for stimulation of this pathway was application of the widely used calmodulin antagonist calmidazolium, due to its ability to activate phospholipase A2.

ARC Channels: A Novel Pathway for Receptor-Activated Calcium Entry

In many nonexcitable cells, stimulation with low agonist concentrations specifically activates Ca2+entry via arachidonic acid-regulated, highly Ca2+-selective ARC channels. Only at high agonist concentrations are the more widely studied store-operated channels activated, producing sustained elevated cytosolic Ca2+concentration signals. These signals activate calcineurin, which in turn inhibits the ARC channels, resulting in a “reciprocal regulation” of these two distinct Ca2+-entry pathways that may have important functional implications for the cell.

A new regulation of non-capacitative calcium entry in insect pacemaker neurosecretory neurons. Involvement of arachidonic acid, no-guanylyl cyclase/cGMP, and cAMP

Efferent dorsal unpaired median neurons are pacemaker neurosecretory cells. A Ca(2+) background current contributing to the pacemaker activity of cockroach dorsal unpaired median neurons is up-regulated by neurohormone D (NHD), an octapeptide belonging to the adipokinetic hormone family. This modulation accelerates spiking and increases [Ca(2+)](i). Using patch clamp, calcium imaging, and immunocytochemistry, we investigated the signaling pathway of NHD-induced current modulation. The membrane depolarization produced by NHD was related to the increase in membrane conductance for Ca(2+), Ba(2+), or Sr(2+). This increase was abolished by LOE 908, an inhibitor of noncapacitive Ca(2+) entry (NCCE), and it was strongly attenuated by the phospholipase C inhibitor U37122 and the diacylglycerol lipase inhibitor RHC80267. Arachidonic acid and ETYA mimicked the NHD effect on background current. This was abolished by l-NAME and ODQ, inhibitors of NO synthase and NO-sensitive guanylyl cyclase, respectively, but mimicked by the NO donor sodium nitroprusside and 8-bromo-cGMP. Immunocytochemistry using cGMP antibodies indicated that NHD and ETYA increase cGMP. Inhibition of protein kinase G with KT5823 and R(p)-8-pCPT-cGMPS had no effect, whereas zaprinast, a cGMP-specific phosphodiesterase 5,6,9 inhibitor, mimicked the NHD effect. Furthermore, inhibition of the cGMP-activated phosphodiesterase 2 by EHNA and trequinsin abolished the effect of NHD. We conclude that the final step of the NHD signal transduction is the phosphodiesterase 2-induced down-regulation of the cAMP level. This removes a depression of NCCE directly attributed to cAMP because inhibition of protein kinase A with KT5720, R(p)-cAMPS, and PKI14-22 amide did not mimic the NHD effect. We also demonstrate that any mechanism increasing the cGMP level can induce NCCE.

Sustained entry of Ca2+ is required to activate Ca2+-calmodulin-dependent phosphodiesterase 1A

Regulation of adenylyl cyclases (ACs) by Ca2+ requires capacitative Ca2+ entry (CCE) (Cooper, D. M. F. (2003) Biochem. J. 375, 517-529), but whether Ca2+-sensitive phosphodiesterases (PDEs) are similarly discriminating has never been addressed. In the present study, a variety of conditions were devised to manipulate [Ca2+]i so that we could ask whether PDE1 selectively responds to different modes of elevating [Ca2+]i, viz. Ca2+ released from intracellular stores and various modes of Ca2+ entry. In 1321N1 human astrocytoma cells, the endogenous PDE1 (identified as PDE1A by reverse transcriptase-PCR) was largely insensitive to Ca2+ released from carbachol-sensitive stores but was robustly stimulated by a similar rise in [Ca2+]i due to carbachol-induced Ca2+ influx. Gd3+, which effectively blocked thapsigargin-induced CCE and its effect on PDE1A, also inhibited the activation of PDE1A by carbachol-induced Ca2+ entry. However, non-selective ionomycin-mediated Ca2+ entry also activated PDE1A, so that, unlike Ca2+-sensitive ACs, PDE1A cannot discriminate between the different sources of Ca2+ entry. Fractionation of the cells revealed that the Ca2+-calmodulin-stimulated PDE activity was not present at the plasma membrane but was associated with the cytosol and the organellar compartments of the cell. Therefore, the apparent disparity between PDE1A and ACs is likely to be the consequence of their differential subcellular localization. Nevertheless, in a physiological context, where artificial modes of elevating [Ca2+]i are not available, as with ACs, a dependence on CCE would be evident, and it would be the duration of this influx of Ca2+ that would determine how long PDE1A was activated.

Ca2+ influx is not involved in acute cytotoxicity of arachidonic acid

Arachidonic acid (AA; 20:4, n-6) has been implicated in cell damage in the brain under ischemia-reperfusion and other pathological conditions. In our experiments, PC12 cells exposed to >10 microM AA died within 1-2 hr, as assessed by the LDH release assay. Since AA is known to induce Ca2+/cation-permeable conductance in the plasma membrane, we investigated whether Ca2+ influx plays a role in this acute cell death. We found that extracellular Ca2+ was not required for the toxic effect of AA. In fact, the removal of extracellular Ca2+ dramatically accelerated its development: the half-time of the toxic effect of 40 microM AA decreased from 70.1 +/- 0.3 min in the presence of 5 mM Ca2+ to 7.4 +/- 0.3 min in the Ca-free solution. The extent of cell killing depended only weakly on AA concentration and ion composition, remaining within the 70-95% range. The AA-induced acute death was not affected by inhibitors of AA metabolism (nordihydroguaiaretic acid, indomethacin, proadifen), whereas some antioxidants tested (deferoxamine and ellagic acid), but not all (melatonin), partially suppressed it. Also, it was not affected by changes in the extracellular ionic strength or mimicked by an acetylenic analog of AA 5,8,11,14-eicosatetraynoic acid. We conclude that lethal injuries sustained by cells during short exposures to AA were caused by the fatty acid itself and were not mediated by the AA-induced influx of Ca2+/cations. Moreover, direct physical effects of AA on the plasma membrane (changes in membrane fluidity or detergent-like action) were also excluded.

A novel mechanism for the store-operated calcium influx pathway

Activation of store-operated channels (SOCs) and capacitative calcium influx are triggered by depletion of intracellular calcium stores. However, the exact molecular mechanism of such communication remains unclear. Recently, we demonstrated that native SOC channels can be activated by calcium influx factor (CIF) that is produced upon depletion of calcium stores, and showed that Ca(2+)-independent phospholipase A(2) (iPLA(2)) has an important role in the store-operated calcium influx pathway. Here, we identify the key plasma-membrane-delimited events that result in activation of SOC channels. We also propose a novel molecular mechanism in which CIF displaces inhibitory calmodulin (CaM) from iPLA(2), resulting in activation of iPLA(2) and generation of lysophospholipids that in turn activate soc channels and capacitative calcium influx. Upon refilling of the stores and termination of CIF production, CaM rebinds to iPLA(2), inhibits it, and the activity of SOC channels and capacitative calcium influx is terminated.

Activation of muscarinic cholinergic receptors enhances the volume-sensitive efflux of myo-inositol from SH-SY5Y neuroblastoma cells

A mechanism used by cells to regulate their volume under hypo-osmotic conditions is the release of organic osmolytes, one of which is myo-inositol. The possibility that activation of phospholipase-C-linked receptors can regulate this process has been examined for SH-SY5Y neuroblastoma cells. Incubation of cells with hypo-osmolar buffers (160-250 mOsm) led to a biphasic release of inositol which persisted for up to 4 h and could be inhibited by inclusion of anion channel blockers - results which indicate the involvement of a volume-sensitive organic anion channel. Inclusion of oxotremorine-M, a muscarinic cholinergic agonist, resulted in a marked increase (80-100%) in inositol efflux under hypo-osmotic, but not isotonic, conditions. This enhanced release, which was observed under all conditions of hypo-osmolarity tested, could be prevented by inclusion of atropine. Incubation of the cells with either the calcium ionophore, ionomycin, or the phorbol ester, phorbol 12-myristate 13-acetate, partially mimicked the stimulatory effect of muscarinic receptor activation when added singly, and fully when added together. The ability of oxotremorine-M to facilitate inositol release was inhibited by removal of extracellular calcium, depletion of intracellular calcium or down-regulation of protein kinase C. These results indicate that activation of muscarinic cholinergic receptors can regulate osmolyte release in this cell line.