Discrimination of intracellular calcium store subcompartments using TRPV1 (transient receptor potential channel, vanilloid subfamily member 1) release channel activity

Ca(2+) channels on the move

The versatility of Ca(2+) as an intracellular messenger derives largely from the spatial organization of cytosolic Ca(2+) signals, most of which are generated by regulated openings of Ca(2+)-permeable channels. Most Ca(2+) channels are expressed in the plasma membrane (PM). Others, including the almost ubiquitous inositol 1,4,5-trisphosphate receptors (IP(3)R) and their relatives, the ryanodine receptors (RyR), are predominantly expressed in membranes of the sarcoplasmic or endoplasmic reticulum (ER). Targeting of these channels to appropriate destinations underpins their ability to generate spatially organized Ca(2+) signals. All Ca(2+) channels begin life in the cytosol, and the vast majority are then functionally assembled in the ER, where they may either remain or be dispatched to other membranes. Here, by means of selective examples, we review two issues related to this trafficking of Ca(2+) channels via the ER. How do cells avoid wayward activity of Ca(2+) channels in transit as they pass from the ER via other membranes to their final destination? How and why do some cells express small numbers of the archetypal intracellular Ca(2+) channels, IP(3)R and RyR, in the PM?

Capacitative calcium entry: from concept to molecules

Rapid to moderately rapid changes in intracellular Ca2+ concentration, or Ca2+ signals, control a variety of critical cellular functions in the immune system. These signals are comprised of Ca2+ release from intracellular stores coordinated with Ca2+ influx across the plasma membrane. The most common mechanisms by which these two modes of signaling occur is through inositol 1,4,5-trisphosphate (IP3)-induced release of Ca2+ from the endoplasmic reticulum (ER) and store-operated Ca2+ entry across the plasma membrane. The latter process was postulated over 20 years ago, and in just the past few years, the key molecular players have been discovered: STIM proteins serve as sensors of Ca2+ within the ER which communicate with and activate plasma membrane store-operated channels composed of Orai subunits. The process of store-operated Ca2+ entry provides support for oscillating Ca2+ signals from the ER and also provides direct activator Ca2+ that signals to a variety of downstream effectors.

The endoplasmic reticulum of dorsal root ganglion neurons contains functional TRPV1 channels

Transient receptor potential vanilloid type 1 (TRPV1) is a plasma membrane Ca(2+) channel involved in transduction of painful stimuli. Dorsal root ganglion (DRG) neurons express ectopic but functional TRPV1 channels in the endoplasmic reticulum (ER) (TRPV1(ER)). We have studied the properties of TRPV1(ER) in DRG neurons and HEK293T cells expressing TRPV1. Activation of TRPV1(ER) with capsaicin or other vanilloids produced an increase of cytosolic Ca(2+) due to Ca(2+) release from the ER. The decrease of [Ca(2+)](ER) was directly revealed by an ER-targeted aequorin Ca(2+) probe, expressed in DRG neurons using a herpes amplicon virus. The sensitivity of TRPV1(ER) to capsaicin was smaller than the sensitivity of the plasma membrane TRPV1 channels. The low affinity of TRPV1(ER) was not related to protein kinase A- or C-mediated phosphorylations, but it was due to inactivation by cytosolic Ca(2+) because the sensitivity to capsaicin was increased by loading the cells with the Ca(2+) chelator BAPTA. Decreasing [Ca(2+)](ER) did not affect the sensitivity of TRPV1(ER) to capsaicin. Disruption of the TRPV1 calmodulin-binding domains at either the C terminus (Delta35AA) or the N terminus (K155A) increased 10-fold the affinity of TRPV1(ER) for capsaicin, suggesting that calmodulin is involved in the inactivation. The lack of TRPV1 sensitizers, such as phosphatidylinositol 4,5-bisphosphate, in the ER could contribute to decrease the affinity for capsaicin. The low sensitivity of TRPV1(ER) to agonists may be critical for neuron health, because otherwise Ca(2+) depletion of ER could lead to ER stress, unfolding protein response, and cell death.

Increased expression of TRPV1 in squamous cell carcinoma of the human tongue

OBJECTIVES

Recent reports have unambiguously identified the presence and the growth-modulatory role of transient receptor potential vanilloid-1 (TRPV1), a central integrator of pain sensation, on numerous non-neuronal cell types and, of great importance, in certain malignancies. In this study, we have investigated the molecular expression of TRPV1 in the human tongue and its high-incidence malignant (squamous cell carcinoma, SCC) and premalignant (leukoplakia) conditions.

METHODS

Immunohistochemistry, Western blotting and quantitative 'real-time' Q-PCR were performed to define the expression of TRPV1.

RESULTS

A weak and sparse TRPV1-specific immunoreactivity was identified in the basal layers of the healthy human tongue epithelium. By contrast, we observed a dramatically elevated TRPV1-immunoreactivity in all layers of the epithelium both in precancerous and malignant samples. Furthermore, statistical analysis revealed that the marked overexpression of TRPV1 found in all grades of SCC showed no correlation with the degree of malignancy of the tumours. Finally, the molecular expression of TRPV1 was also identified in an SCC-derived cell line and was shown to be increased in parallel with the accelerated growth of the cells.

CONCLUSION

Collectively, our findings identify TRPV1 as a novel, promising target molecule in the supportive treatment and diagnosis of human tongue SCC.

A small component of the endoplasmic reticulum is required for store-operated Ca2+ channel activation in liver cells: evidence from studies using TRPV1 and taurodeoxycholic acid

The question of whether the activation of SOCs (store-operated Ca(2+) channels) requires the whole or part of the ER (endoplasmic reticulum) has not been fully resolved. The role of a putative sub-compartment of the ER in SOC activation in liver cells was investigated using ectopically expressed TRPV1 (transient receptor potential vanilloid 1), a non-selective cation channel, and TDCA (taurodeoxycholic acid), an activator of SOCs, to release Ca(2+) from different regions of the ER. TRPV1 was expressed in the ER and in the plasma membrane. The amount of Ca(2+) released from the ER by a TRPV1 agonist, measured using fura-2, was the same as that released by a SERCA (sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase) inhibitor, indicating that TRPV1 agonist-sensitive stores substantially overlap with SERCA inhibitor-sensitive stores. In contrast with SERCA inhibitors, TRPV1 agonists did not activate store-operated Ca(2+) entry. These findings were confirmed by patch-clamp recording. Using FFP-18, it was shown that SERCA inhibitors release Ca(2+) from the ER located closer to the plasma membrane than the region from which TRPV1 agonists release Ca(2+). In contrast with SERCA inhibitors, TRPV1 agonists did not induce a redistribution of STIM1 (stromal interaction molecule 1). TDCA caused the release of Ca(2+) from the ER, which was detected by FFP-18 but not by fura-2, and a redistribution of STIM1 to puncta similar to that caused by SERCA inhibitors. It is concluded that in liver cells, Ca(2+) release from a small component of the ER located near the plasma membrane is required to induce STIM1 redistribution and SOC activation.

IP(3) receptor subtype-dependent activation of store-operated calcium entry through I(CRAC)

The store-operated, calcium release-activated calcium current I(CRAC) is activated by the depletion of inositol 1,4,5-trisphosphate (IP(3))-sensitive stores. The significantly different dose-response relationships of IP(3)-mediated Ca(2+) release and CRAC channel activation indicate that I(CRAC) is activated by a functionally, and possibly physically, distinct sub-compartment of the endoplasmic reticulum (ER), the so-called CRAC store. Vertebrate genomes contain three IP(3) receptor (IP(3)R) genes and most cells express at least two subtypes, but the functional relevance of various IP(3)R subtypes with respect to store-operated Ca(2+) entry is completely unknown. We here demonstrate in avian B cells (chicken DT40) that IP(3)R type II and type III participate in IP(3)-induced activation of I(CRAC), but IP(3)R type I does not. This suggests that the expression pattern of IP(3)R contributes to the formation of specialized CRAC stores in B cells.

Secretogranin III directs secretory vesicle biogenesis in mast cells in a manner dependent upon interaction with chromogranin A

Mast cells are granular immunocytes that reside in the body's barrier tissues. These cells orchestrate inflammatory responses. Proinflammatory mediators are stored in granular structures within the mast cell cytosol. Control of mast cell granule exocytosis is a major therapeutic goal for allergic and inflammatory diseases. However, the proteins that control granule biogenesis and abundance in mast cells have not been elucidated. In neuroendocrine cells, whose dense core granules are strikingly similar to mast cell granules, granin proteins regulate granulogenesis. Our studies suggest that the Secretogranin III (SgIII) protein is involved in secretory granule biogenesis in mast cells. SgIII is abundant in mast cells, and is organized into vesicular structures. Our results show that over-expression of SgIII in mast cells is sufficient to cause an expansion of a granular compartment in these cells. These novel granules store inflammatory mediators that are released in response to physiological stimuli, indicating that they function as bona fide secretory vesicles. In mast cells, as in neuroendocrine cells, we show that SgIII is complexed with Chromogranin A (CgA). CgA is granulogenic when complexed with SgIII. Our data show that a novel non-granulogenic truncation mutant of SgIII (1-210) lacks the ability to interact with CgA. Thus, in mast cells, a CgA-SgIII complex may play a key role in secretory granule biogenesis. SgIII function in mast cells is unlikely to be limited to its partnership with CgA, as our interaction trap analysis suggests that SgIII has multiple binding partners, including the mast cell ion channel TRPA1.

The effect of rottlerin in calcium regulation in HMC-1(560) cells is mediated by a PKC-delta independent effect

The human mast cell line (HMC-1(560)) is a good model for Ca(2+) signaling studies, because intracellular alkalinization is the mainly histamine release stimulus without changes in the intracellular Ca(2+) levels. This fact allows us to study Ca(2+) changes without degranulation, since this process can affected cellular viability. Ionomycin and thapsigargin have been fully used for induced Ca(2+) influx across SOC channels. When HMC-1(560) cells are incubated with rottlerin, 5 microM, for 5 min a strong inhibition of ionomycin-induced Ca(2+) influx is observed. However, when thapsigargin stimulates Ca(2+) influx, rottlerin did not show any effect on Ca(2+) levels. This fact point two possibilities, ionomycin and thapsigargin might activate different SOC channels or that these drugs might activate the same channel but in a different way in HMC-1(560) cells. The rottlerin inhibition of ionomycin-induced Ca(2+) influx is PKC-delta independent and this effect is not related with the store depletion, since rottlerin has the same effect when it is added before or after the stores are empty. FCCP, a know uncoupler of oxidative phosphorylation in mitochondria, induces the same inhibition in ionomycin Ca(2+) influx than rottlerin which point to the mitochondria as a cellular target to rottlerin.

Structure-activity relationships of 1,4-dihydropyridines that act as enhancers of the vanilloid receptor 1 (TRPV1)

Vanilloid agonists such as capsaicin activate ion flux through the TRPV1 channel, a heat- and ligand-gated cation channel that transduces painful chemical or thermal stimuli applied to peripheral nerve endings in skin or deep tissues. We have probed the SAR of a variety of 1,4-dihydropyridine (DHP) derivatives as novel 'enhancers' of TRPV1 activity by examining changes in capsaicin-induced elevations in (45)Ca(2+)-uptake in either cells ectopically expressing TRPV1 or in cultured dorsal root ganglion (DRG) neurons. The enhancers increased the maximal capsaicin effect on (45)Ca(2+)-uptake by typically 2- to 3-fold without producing an action when used alone. The DHP enhancers contained 6-aryl substitution and small alkyl groups at the 1 and 4 positions, and a 3-phenylalkylthioester was tolerated. Levels of free intracellular Ca(2+), as measured by calcium imaging, were also increased in DRG neurons when exposed to the combination of capsaicin and the most efficacious enhancer 23 compared to capsaicin alone. Thus, DHPs can modulate TRPV1 channels in a positive fashion.

TRP channels: an overview

The TRP ("transient receptor potential") family of ion channels now comprises more than 30 cation channels, most of which are permeable for Ca2+, and some also for Mg2+. On the basis of sequence homology, the TRP family can be divided in seven main subfamilies: the TRPC ('Canonical') family, the TRPV ('Vanilloid') family, the TRPM ('Melastatin') family, the TRPP ('Polycystin') family, the TRPML ('Mucolipin') family, the TRPA ('Ankyrin') family, and the TRPN ('NOMPC') family. The cloning and characterization of members of this cation channel family has exploded during recent years, leading to a plethora of data on the roles of TRPs in a variety of tissues and species, including mammals, insects, and yeast. The present review summarizes the most pertinent recent evidence regarding the structural and functional properties of TRP channels, focusing on the regulation and physiology of mammalian TRPs.

Calcium plays a central role in the sensitization of TRPV3 channel to repetitive stimulations

Transient receptor potential channels are involved in sensing chemical and physical changes inside and outside of cells. TRPV3 is highly expressed in skin keratinocytes, where it forms a nonselective cation channel activated by hot temperatures in the innocuous and noxious range. The channel has also been implicated in flavor sensation in oral and nasal cavities as well as being a molecular target of some allergens and skin sensitizers. TRPV3 is unique in that its activity is sensitized upon repetitive stimulations. Here we investigated the role of calcium ions in the sensitization of TRPV3 to repetitive stimulations. We show that the sensitization is accompanied by a decrease of Ca(2+)-dependent channel inhibition mediated by calmodulin acting at an N-terminal site (amino acids 108-130) and by an acidic residue (Asp(641)) at the pore loop of TRPV3. These sites also contribute to the voltage dependence of TRPV3. During sensitization, the channel displayed a gradual shift of the voltage dependence to more negative potentials as well as uncoupling from voltage sensing. The initial response to ligand stimulation was increased and sensitization to repetitive stimulations was decreased by increasing the intracellular Ca(2+)-buffering strength, inhibiting calmodulin, or disrupting the calmodulin-binding site. Mutation of Asp(641) to Asn abolished the high affinity extracellular Ca(2+)-mediated inhibition and greatly facilitated the activation of TRPV3. We conclude that Ca(2+) inhibits TRPV3 from both the extracellular and intracellular sides. The inhibition is sequentially reduced, appearing as sensitization to repetitive stimulations.

Feedback mechanisms in the regulation of intracellular calcium ([Ca2+]i) in the peripheral nociceptive system: role of TRPV-1 and pain related receptors

Multimodal stimuli like heat, cold, bacterial or mechanical events are able to elicit pain, which is necessary to guarantee survival. However, the control of pain is of major clinical importance. The perception and transduction of pain is differentially modulated in the peripheral and central nervous system (CNS): while peripheral structures modulate these signals, the perception of pain occurs in the CNS. In recent years major advances have been made in the understanding of the processes which are involved in pain sensation. For the peripheral pain reception, the importance of specific pain receptors of the transition receptor pore (TRP)-family (e.g. the TRPV-1 receptor) has been analyzed. These receptors/channels are localized at the cell membrane of nociceptive neurones as well as in membranes of intracellular calcium stores like the endoplasmic reticulum. While the associated channel conducts different ions, a major proportion is calcium. Therefore, this review focuses on (1) the modulations of intracellular calcium ([Ca2+]i) initiated by the activation of pain receptors and (2) the consequences of [Ca2+]i changes for the processing of pain signals at the peripheral side. The possible interference of TRPV-1 induced [Ca2+]i modulations to the function of other membrane receptors and channels, like voltage gated calcium, sodium or potassium channels, or co-expressed CB1-receptors will be discussed. The latter interactions are of specific interest since the analgetic properties of endo- and exo-cannabinoids are mediated by CB1 receptors and their activation significantly modulates the calcium induced release of pain related transmitters. Furthermore, multiple cross links between different pain modulating intracellular pathways and their dependence on [Ca2+]i modulations will be illuminated. Overall, this review will summarize new insights resulting in the understanding of the prominent influence of [Ca2+]i for processes which are involved in pain sensation.

New molecular players in capacitative Ca2+ entry

Capacitative Ca2+ entry links the emptying of intracellular Ca2+ stores to the activation of store-operated Ca2+ channels in the plasma membrane. In the twenty years since the inception of the concept of capacitative Ca2+ entry, a number of activation mechanisms have been proposed, and there has been considerable interest in the possibility that TRP channels function as store-operated channels. However, in the past two years, two major players in both the signaling and permeation mechanisms for store-operated channels have been discovered: Stim1 and the Orai proteins. Stim1 is an endoplasmic reticulum Ca2+ sensor. It appears to act by redistributing within a small component of the endoplasmic reticulum, approaching the plasma membrane, but does not seem to translocate into the plasma membrane. Stim1 signals to plasma membrane Orai proteins, which constitute pore-forming subunits of store-operated channels.

Analysis of calcium concentration fluctuations in hepatopancreatic R cells of Marsupenaeus japonicus during the molting cycle

In this study we examined the fluctuations of the intracellular calcium concentration in isolated hepatopancreatic R cells during the four molting stages of the prawn Marsupenaeus japonicus. In addition, we used the Fura-2-AM fluorescence technique to investigate the release of calcium from mitochondria and ATP-sensitive calcium stores (endoplasmic reticulum (ER), Golgi, and nucleus) into cytoplasm during the molting cycle. Results demonstrate that both the cytosolic free calcium concentration and the total cell calcium (free, bound to calcium-binding proteins, and stored in amorphous form) in the R cells strictly depend upon the molting cycle. Interestingly, the total cell calcium was higher (approximately 10 mmol l(-1)) in postmolt than in premolt (approximately 1 mmol l(-1)) and intermolt (approximately 0.3 mmol l(-1)). The calcium released from mitochondria was higher during premolt than during postmolt and intermolt, but the amount of calcium released from ATP-sensitive calcium stores was similar during all four stages. All together, our results suggest that the mitochondria-ATP-sensitive calcium stores system does not play a key role in calcium storage during the molting cycle but that it is involved in transcellular calcium flux. We hypothesize that lysosome or membrane-clad concretion vacuoles could represent the main site of calcium storage in hepatopancreatic R cells.

Recent breakthroughs in the molecular mechanism of capacitative calcium entry (with thoughts on how we got here)

Activation of phospholipase C by G-protein-coupled receptors results in release of intracellular Ca(2+) and activation of Ca(2+) channels in the plasma membrane. The intracellular release of Ca(2+) is signaled by the second messenger, inositol 1,4,5-trisphosphate. Ca(2+) entry involves signaling from depleted intracellular stores to plasma membrane Ca(2+) channels, a process referred to as capacitative calcium entry or store-operated calcium entry. The electrophysiological current associated with capacitative calcium entry is the calcium-release-activated calcium current, or I(crac). In the 20 years since the inception of the concept of capacitative calcium entry, a variety of activation mechanisms have been proposed, and there has been considerable interest in the possibility of transient receptor potential channels functioning as store-operated channels. However, in the past 2 years, two major players in both the signaling and permeation mechanisms for store-operated channels have been discovered: Stim1 (and possibly Stim2) and the Orai proteins. Activation of store-operated channels involves an endoplasmic reticulum Ca(2+) sensor called Stim1. Stim1 acts by redistributing within a small component of the endoplasmic reticulum, approaching the plasma membrane, but does not appear to translocate into the plasma membrane. Stim1, either directly or indirectly, signals to plasma membrane Orai proteins which constitute pore-forming subunits of store-operated channels.

Cell biology of polycystin-2

Naturally occurring mutations in two separate, but interacting loci, pkd1 and pkd2 are responsible for almost all cases of autosomal dominant polycystic kidney disease (ADPKD). ADPKD is one of the most common genetic diseases resulting primarily in the formation of large kidney, liver, and pancreatic cysts. Homozygous deletion of either pkd1 or pkd2 results in embryonic lethality in mice due to kidney and heart defects illustrating their indispensable roles in mammalian development. However, the mechanism by which mutations in these genes cause ADPKD and other developmental defects are unknown. Research in the past several years has revealed that PKD2 has multiple functions depending on its subcellular localization. It forms a receptor-operated, non-selective cation channel in the plasma membrane, a novel intracellular Ca2+ release channel in the endoplasmic reticulum (ER), and a mechanosensitive channel in the primary cilium. This review focuses on the functional compartmentalization of PKD2, its modes of activation, and PKD2-mediated signal transduction.

Transient receptor potential cation channels in disease

The transient receptor potential (TRP) superfamily consists of a large number of cation channels that are mostly permeable to both monovalent and divalent cations. The 28 mammalian TRP channels can be subdivided into six main subfamilies: the TRPC (canonical), TRPV (vanilloid), TRPM (melastatin), TRPP (polycystin), TRPML (mucolipin), and the TRPA (ankyrin) groups. TRP channels are expressed in almost every tissue and cell type and play an important role in the regulation of various cell functions. Currently, significant scientific effort is being devoted to understanding the physiology of TRP channels and their relationship to human diseases. At this point, only a few channelopathies in which defects in TRP genes are the direct cause of cellular dysfunction have been identified. In addition, mapping of TRP genes to susceptible chromosome regions (e.g., translocations, breakpoint intervals, increased frequency of polymorphisms) has been considered suggestive of the involvement of these channels in hereditary diseases. Moreover, strong indications of the involvement of TRP channels in several diseases come from correlations between levels of channel expression and disease symptoms. Finally, TRP channels are involved in some systemic diseases due to their role as targets for irritants, inflammation products, and xenobiotic toxins. The analysis of transgenic models allows further extrapolations of TRP channel deficiency to human physiology and disease. In this review, we provide an overview of the impact of TRP channels on the pathogenesis of several diseases and identify several TRPs for which a causal pathogenic role might be anticipated.

TRPM8-independent Menthol-induced Ca2+ Release from Endoplasmic Reticulum and Golgi

Menthol, a secondary alcohol produced by the peppermint herb, Mentha piperita, is widely used in the food and pharmaceutical industries as a cooling/soothing compound and odorant. It induces Ca2+ influx in a subset of sensory neurons from dorsal root and trigeminal ganglia, due to activation of TRPM8, a Ca2+-permeable, cold-activated member of the TRP superfamily of cation channels. Menthol also induces Ca2+ release from intracellular stores in several TRPM8-expressing cell types, which has led to the suggestion that TRPM8 can function as an intracellular Ca2+-release channel. Here we show that menthol induces Ca2+ release from intracellular stores in four widely used cell lines (HEK293, lymph node carcinoma of the prostate (LNCaP), Chinese hamster ovary (CHO), and COS), and provide several lines of evidence indicating that this release pathway is TRPM8-independent: 1) menthol-induced Ca2+ release was potentiated at higher temperatures, which contrasts to the cold activation of TRPM8; 2) overexpression of TRPM8 did not enhance the menthol-induced Ca2+) release; 3) menthol-induced Ca2+ release was mimicked by geraniol and linalool, which are structurally related to menthol, but not by the more potent TRPM8 agonists icilin or eucalyptol; and 4) TRPM8 expression in HEK293 cells was undetectable at the protein and mRNA levels. Moreover, using a novel TRPM8-specific antibody we demonstrate that both heterologously expressed TRPM8 (in HEK293 cells) and endogenous TRPM8 (in LNCaP cells) are mainly localized in the plasma membrane, which contrast to previous localization studies using commercial anti-TRPM8 antibodies. Finally, aequorin-based measurements demonstrate that the TRPM8-independent menthol-induced Ca2+ release originates from both endoplasmic reticulum and Golgi compartments.

Nicotinic acid adenine dinucleotide phosphate and cyclic ADP-ribose regulate TRPM2 channels in T lymphocytes

TRPM2 (previously designated TRPC7 or LTRPC2) is a Ca2+-permeable nonselective cation channel that contains a C-terminal enzymatic domain with pyrophosphatase activity, which specifically binds ADP-ribose. Cyclic ADP-ribose (cADPR) and hydrogen peroxide (H2O2) can facilitate ADPR-mediated activation of heterologously expressed TRPM2. Here, we show that the two Ca2+-mobilizing second messengers cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP) strongly activate natively expressed TRPM2 channels in Jurkat T cells. TRPM2 activation by both agonists can be partially suppressed by the ADPR antagonist adenosine monophosphate (AMP), which suggests that cADPR and NAADP lead to mobilization of endogenous ADPR presumably via metabolic conversion. The remaining channel activity is due to direct gating of TRPM2 by the two agonists and can be completely suppressed by 8-Br-cADPR, which suggests that cADPR and NAADP share a common binding site on TRPM2 that can regulate TRPM2 activity in synergy with ADPR. We conclude that cADPR and NAADP, in combination with ADPR, represent physiological co-activators of TRPM2 that contribute to Ca2+ influx in T lymphocytes and presumably other cell types that express this channel.

The role of canonical transient receptor potential 7 in B-cell receptor-activated channels

Phospholipase C signaling stimulates Ca2+ entry across the plasma membrane through multiple mechanisms. Ca2+ store depletion stimulates store-operated Ca2+-selective channels, or alternatively, other phospholipase C-dependent events activate Ca2+-permeable non-selective cation channels. Transient receptor potential 7 (TRPC7) is a non-selective cation channel that can be activated by both mechanisms when ectopically expressed, but the regulation of native TRPC7 channels is not known. We knocked out TRPC7 in DT40 B-cells, which expresses both forms of Ca2+ entry. No difference in the store-operated current I(crac) was detected between TRPC7-/- and wild-type cells. Wild-type cells demonstrated nonstore-operated cation entry and currents in response to activation of the B-cell receptor or protease-activated receptor 2, intracellular dialysis with GTPgammaS, or application of the synthetic diacylglycerol oleyl-acetyl-glycerol. These responses were absent in TRPC7-/- cells but could be restored by transfection with human TRPC7. In conclusion, in B-lymphocytes, TRPC7 appeared to participate in the formation of ion channels that could be activated by phospholipase C-linked receptors. This represents the first demonstration of a physiological function for endogenous TRPC7 channels.

Anandamide acts as an intracellular messenger amplifying Ca2+ influx via TRPV1 channels

The endocannabinoid anandamide is able to interact with the transient receptor potential vanilloid 1 (TRPV1) channels at a molecular level. As yet, endogenously produced anandamide has not been shown to activate TRPV1, but this is of importance to understand the physiological function of this interaction. Here, we show that intracellular Ca2+ mobilization via the purinergic receptor agonist ATP, the muscarinic receptor agonist carbachol or the Ca(2+)-ATPase inhibitor thapsigargin leads to formation of anandamide, and subsequent TRPV1-dependent Ca2+ influx in transfected cells and sensory neurons of rat dorsal root ganglia (DRG). Anandamide metabolism and efflux from the cell tonically limit TRPV1-mediated Ca2+ entry. In DRG neurons, this mechanism was found to lead to TRPV1-mediated currents that were enhanced by selective blockade of anandamide cellular efflux. Thus, endogenous anandamide is formed on stimulation of metabotropic receptors coupled to the phospholipase C/inositol 1,4,5-triphosphate pathway and then signals to TRPV1 channels. This novel intracellular function of anandamide may precede its action at cannabinoid receptors, and might be relevant to its control over neurotransmitter release.

TRP channels: a TR(I)P through a world of multifunctional cation channels

The "transient receptor potential" (TRP) family of ion channels comprises more than 50 cation-permeable channels expressed from yeast to man. On the basis of structural homology, the TRP family can be subdivided in to seven main subfamilies: the TRPC ('Canonical') group, the TRPV ('Vanilloid') group, the TRPM ('Melastatin') group, the TRPP ('Polycystin'), the TRPML ('Mucolipin'), the TRPA ('Ankyrin') and the TRPN ('NOMP') family. The cloning and characterization of members of this cation channel family has exploded during recent years, leading to a plethora of data concerning TRPs in a variety of cell types, tissues and species. This paper briefly reviews the TRP superfamily and the basic properties of its many members as a reader's guide in this Special Issue. Hopefully, a better understanding of TRP channel physiology will provide important insight into the relationship between TRP channel dysfunction and human diseases.

Suramin and disulfonated stilbene derivatives stimulate the Ca2+-induced Ca2+ -release mechanism in A7r5 cells

We have described previously a novel Ca2+-induced Ca2+-release (CICR) mechanism in permeabilized A7r5 cells (embryonic rat aorta) and 16HBE14o-cells (human bronchial mucosa) cells (J Biol Chem 278:27548-27555, 2003). This CICR mechanism was activated upon the elevation of the free cytosolic calcium concentration [Ca2+]c and was not inhibited by pharmacological inhibitors of the inositol-1,4,5-trisphosphate (IP3) receptor nor of the ryanodine receptor. This CICR mechanism was inhibited by calmodulin (CaM)1234, a Ca2+-insensitive CaM mutant, and by different members of the superfamily of CaM-like Ca2+-binding proteins. Here, we present evidence that the CICR mechanism that is expressed in A7r5 and 16HBE14o-cells is strongly activated by suramin and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS). We found several indications that both activation mechanisms are indeed two different modes of the same release system. Suramin/DIDS-induced Ca2+ release was only detected in cells that displayed the CICR mechanism, and cell types that do not express this type of CICR mechanism did not exhibit suramin/DIDS-induced Ca2+ release. Furthermore, we show that the suramin-stimulated Ca2+ release is regulated by Ca2+ and CaM in a similar way as the previously described CICR mechanism. The pharmacological characterization of the suramin/DIDS-induced Ca2+ release further confirms its properties as a novel CaM-regulated Ca2+-release mechanism. We also investigated the effects of disulfonated stilbene derivatives on IP3-induced Ca2+ release and found, in contrast to the effect on CICR, a strong inhibition by DIDS and 4'-acetoamido-4'-isothiocyanostilbene-2',2'-disulfonic acid.

Store-Operated Calcium Channels

In electrically nonexcitable cells, Ca2+influx is essential for regulating a host of kinetically distinct processes involving exocytosis, enzyme control, gene regulation, cell growth and proliferation, and apoptosis. The major Ca2+entry pathway in these cells is the store-operated one, in which the emptying of intracellular Ca2+stores activates Ca2+influx (store-operated Ca2+entry, or capacitative Ca2+entry). Several biophysically distinct store-operated currents have been reported, but the best characterized is the Ca2+release-activated Ca2+current, ICRAC. Although it was initially considered to function only in nonexcitable cells, growing evidence now points towards a central role for ICRAC-like currents in excitable cells too. In spite of intense research, the signal that relays the store Ca2+content to CRAC channels in the plasma membrane, as well as the molecular identity of the Ca2+sensor within the stores, remains elusive. Resolution of these issues would be greatly helped by the identification of the CRAC channel gene. In some systems, evidence suggests that store-operated channels might be related to TRP homologs, although no consensus has yet been reached. Better understood are mechanisms that inactivate store-operated entry and hence control the overall duration of Ca2+entry. Recent work has revealed a central role for mitochondria in the regulation of ICRAC, and this is particularly prominent under physiological conditions. ICRACtherefore represents a dynamic interplay between endoplasmic reticulum, mitochondria, and plasma membrane. In this review, we describe the key electrophysiological features of ICRACand other store-operated Ca2+currents and how they are regulated, and we consider recent advances that have shed insight into the molecular mechanisms involved in this ubiquitous and vital Ca2+entry pathway.

Mast cell exocytosis can be triggered by ammonium chloride with just a cytosolic alkalinization and no calcium increase

A human mast cell line (HMC-1) has been used to study the effect of cytosolic alkaline pH in exocytosis. Compound 48/80, concanavalin A, and thapsigargin do not induce histamine release in HMC-1 cells. Although thapsigargin does not activate histamine release, it does show a large increase in cytosolic Ca(2+), and no change in cytosolic pH. However, when HMC-1 cells were activated with ionomycin, a significant histamine release takes place, and this effect is higher in the presence of thapsigargin. Both drugs show an additive effect on cytosolic Ca(2+) levels. Ammonium chloride (NH(4)Cl) does activate cytosolic alkalinization and histamine release, with no increase in cytosolic Ca(2+). NH(4)Cl does block the release of internal Ca(2+) by thapsigargin, not by ionomycin, and decreases Ca(2+) influx stimulated by these drugs. Under conditions in which the alkalinization induced by NH(4)Cl is blocked by acidification with sodium propionate, histamine release is inhibited. The release of histamine is also observed when NH(4)Cl is added after propionate addition, regardless of the final pH value attained. Our results show that a shift in pH alkaline values, even with final pH below 7.2 is enough to activate histamine release. A shift to less acidic values is a sufficient signal to activate the cells.

Inositol trisphosphate analogues selective for types I and II inositol trisphosphate receptors exert differential effects on vasopressin-stimulated Ca2+ inflow and Ca2+ release from intracellular stores in rat hepatocytes

Previous studies have shown that adenophostin A is a potent initiator of the activation of SOCs (store-operated Ca2+ channels) in rat hepatocytes, and have suggested that, of the two subtypes of Ins(1,4,5)P3 receptor predominantly present in rat hepatocytes [Ins(1,4,5)P3R1 (type I receptor) and Ins(1,4,5)P3R2 (type II receptor)], Ins(1,4,5)P3R1s are required for SOC activation. We compared the abilities of Ins(1,4,6)P3 [with higher apparent affinity for Ins(1,4,5)P3R1] and Ins(1,3,6)P3 and Ins(1,2,4,5)P4 [with higher apparent affinities for Ins(1,4,5)P3R2] to activate SOCs. The Ins(1,4,5)P3 analogues were microinjected into single cells together with fura 2, and dose-response curves for the activation of Ca2+ inflow and Ca2+ release from intracellular stores obtained for each analogue. The concentration of Ins(1,4,6)P3 which gave half-maximal stimulation of Ca2+ inflow was substantially lower than that which gave half-maximal stimulation of Ca2+ release. By contrast, for Ins(1,3,6)P3 and Ins(1,2,4,5)P3, the concentration which gave half-maximal stimulation of Ca2+ inflow was substantially higher than that which gave half-maximal stimulation of Ca2+ release. The distribution of Ins(1,4,5)P3R1 and Ins(1,4,5)P3R2 in rat hepatocytes cultured under the same conditions as those employed for the measurement of Ca2+ inflow and release was determined by immunofluorescence. Ins(1,4,5)-P3R1s were found predominantly at the cell periphery, whereas Ins(1,4,5)P3R2s were found at the cell periphery, the cell interior and nucleus. It is concluded that the idea that a small region of the endoplasmic reticulum enriched in Ins(1,4,5)P3R1 is required for the activation of SOCs is consistent with the present results for hepatocytes.

TRPV channels and modulation by hepatocyte growth factor/scatter factor in human hepatoblastoma (HepG2) cells

Using patch clamp and Ca(2+) imaging techniques, we have studied Ca(2+) entry pathways in human hepatoblastoma (HepG2) cells. These cells express the mRNA of TRPV1, TRPV2, TRPV3 and TRPV4 channels, but not those of TRPV5 and TRPV6. Functional assessment showed that capsaicin (10 microM), 4alpha-phorbol-12,13-didecanoate (4alphaPDD, 1 microM), arachidonic acid (10 microM), hypotonic stress, and heat all stimulated increases in [Ca(2+)](i) within minutes. The increase in [Ca(2+)](i) depended on extracellular Ca(2+) and on the transmembrane potential, which indicated that both driving forces affected Ca(2+) entry. Capsaicin also stimulated an increase in [Ca(2+)](i) in nominally Ca(2+)-free solutions, which was compatible with the receptor functioning as a Ca(2+) release channel. Hepatocyte growth factor/scatter factor (HGF/SF) modulated Ca(2+) entry. Ca(2+) influx was greater in HepG2 cells incubated with HGF/SF (20 ng/ml for 20 h) compared with non-stimulated cells, but this occurred only in those cells with a migrating phenotype as determined by presence of a lamellipodium and trailing footplate. The effect of capsaicin on [Ca(2+)](i) was greater in migrating HGF/SF-treated cells, and this was inhibited by capsazepine. The difference between control and HGF/SF-treated cells was not found in Ca(2+)-free solutions. 4alphaPDD also had no greater effect on HGF/SF-treated cells. We conclude that TRPV1 and TRPV4 channels provide Ca(2+) entry pathways in HepG2 cells. HGF/SF increases Ca(2+) entry via TRPV1, but not via TRPV4. This rise in [Ca(2+)](i) may constitute an early response of a signalling cascade that gives rise to cell locomotion and the migratory phenotype.

Activation of protein kinase C reverses capsaicin-induced calcium-dependent desensitization of TRPV1 ion channels

Ca2+ selective ion channels of vanilloid receptor subtype-1 (TRPV1) in capsaicin-sensitive dorsal root ganglion (DRG) neurons and TRPV1 transfected Chinese hamster ovarian (CHO) cells are desensitized following calcium-dependent tachyphylaxis induced by successive applications of 100 nM capsaicin. Tachyphylaxis of TRPV1 to 100 nM capsaicin stimuli was not observed in the absence of extracellular calcium. Capsaicin sensitivity of desensitized TRPV1 ion channels recovered on application of phorbol-12-myristate-13-acetate (PMA). PMA-induced recovery of desensitized TRPV1 was primarily due to influx of extracellular calcium observed during re-application of capsaicin following desensitization. Capsazepine blocked the re-sensitization to capsaicin by PMA. Protein kinase C (PKC) inhibitory peptide PKC fragment 19-36 also inhibited re-sensitization to capsaicin by PMA. Reversal of capsaicin-induced desensitization by PMA was prevented by a mutation of TRPV1 where phosphorylation sites serine502 and serine800 were replaced with alanine. This study provides evidence for a role of PKC in reversing capsaicin-induced calcium-dependent desensitization of TRPV1 ion channels.

A TRPV2-PKA signaling module for transduction of physical stimuli in mast cells

Cutaneous mast cell responses to physical (thermal, mechanical, or osmotic) stimuli underlie the pathology of physical urticarias. In vitro experiments suggest that mast cells respond directly to these stimuli, implying that a signaling mechanism couples functional responses to physical inputs in mast cells. We asked whether transient receptor potential (vanilloid) (TRPV) cation channels were present and functionally coupled to signaling pathways in mast cells, since expression of this channel subfamily confers sensitivity to thermal, osmotic, and pressure inputs. Transcripts for a range of TRPVs were detected in mast cells, and we report the expression, surface localization, and oligomerization of TRPV2 protein subunits in these cells. We describe the functional coupling of TRPV2 protein to calcium fluxes and proinflammatory degranulation events in mast cells. In addition, we describe a novel protein kinase A (PKA)–dependent signaling module, containing PKA and a putative A kinase adapter protein, Acyl CoA binding domain protein (ACBD)3, that interacts with TRPV2 in mast cells. We propose that regulated phosphorylation by PKA may be a common pathway for TRPV modulation.

An alternative splicing product of the murine trpv1 gene dominant negatively modulates the activity of TRPV1 channels

Transient receptor potential vanilloid 1 (TRPV1), or vanilloid receptor 1, is the founding member of the vanilloid type of TRP superfamily of nonselective cation channels. TRPV1 is activated by noxious heat, acid, and alkaloid irritants as well as several endogenous ligands and is sensitized by inflammatory factors, thereby serving important functions in detecting noxious stimuli in the sensory system and pathological states in different parts of the body. Whereas numerous studies have been carried out using the rat and human TRPV1 cDNA, the mouse TRPV1 cDNA has not been characterized. Here, we report molecular cloning of two TRPV1 cDNA variants from dorsal root ganglia of C57BL/6 mice. The deduced proteins are designated TRPV1alpha and TRPV1beta and contain 839 and 829 amino acids, respectively. TRPV1beta arises from an alternative intron recognition signal within exon 7 of the trpv1 gene. We found a predominant expression of TRPV1alpha in many tissues and significant expression of TRPV1beta in dorsal root ganglia, skin, stomach, and tongue. When expressed in HEK 293 cells or Xenopus oocytes, TRPV1alpha formed a Ca(2+)-permeable channel activated by ligands known to stimulate TRPV1. TRPV1beta was not functional by itself but its co-expression inhibited the function of TRPV1alpha. Furthermore, although both isoforms were synthesized at a similar rate, less TRPV1beta than TRPV1alpha protein was found in cells and on the cell surface, indicating that the beta isoform is highly unstable. Our data suggest that TRPV1beta is a naturally occurring dominant-negative regulator of the responses of sensory neurons to noxious stimuli.

Biochemical pharmacology of the vanilloid receptor TRPV1. An update

There is mounting evidence that the vanilloid (capsaicin) receptor; transient receptor potential channel, vanilloid subfamily member 1 (TRPV1), is subjected to multiple interacting levels of control. The first level is by reversible phosphorylation catalyzed by intrinsic kinases (e.g. protein kinase A and C) and phosphatases (e.g. calcineurin), which plays a pivotal role in receptor sensitization vs. tachyphylaxis. In addition, this mechanism links TRPV1 to intracellular signaling by various important endogenous as well as exogenous substances such as bradykinin, ethanol, nicotin and insulin. It is not clear, however, whether phosphorylation per se is sufficient to liberate TRPV1 under the inhibitory control of phosphatydylinositol-4,5-bisphosphate. The second level of control is by forming TRPV1 heteromers and their association with putative regulatory proteins. The next level of regulation is by subcellular compartmentalization. The membrane form of TRPV1 functions as a nonselective cation channel. On the endoplasmic reticulum, TRPV1 is present in two differentially regulated forms, one of which is inositol triphosphate-dependent whereas the other is not. These three TRPV1 compartments provide a versatile regulation of intracellular Ca(2+) levels. Last, there is a complex and poorly understood regulation of TRPV1 activity via control of gene expression. Factors that downregulate TRPV1 expression include vanilloid treatment and growth factor (notably, nerve growth factor) deprivation. By contrast, TRPV1 appears to be upregulated during inflammatory conditions. Interestingly, following experimental nerve injury and in animal models of diabetic neuropathy TRPV1 is present on neurons that do not normally express TRPV1. Combined, these findings imply an important role for aberrant TRPV1 expression in the development of neuropathic pain and hyperalgesia. In humans, disease-related changes in TRPV1 expression have already been described (e.g. inflammatory bowel disease and irritable bowel syndrome). The mechanisms that regulate TRPV1 gene expression under pathological conditions are unknown but a better understanding of these pathways has obvious implications for rational drug development.

Evaluation, using targeted aequorins, of the roles of the endoplasmic reticulum and its (Ca2++Mg2+)ATP-ases in the activation of store-operated Ca2+ channels in liver cells

The process by which store-operated Ca2+ channels (SOCs) deliver Ca2+ to the endoplasmic reticulum (ER) and the role of (Ca2++Mg2+)ATP-ases of the ER in the activation of SOCs in H4-IIE liver cells were investigated using cell lines stably transfected with apo-aequorin targeted to the cytoplasmic space or the ER. In order to measure the concentration of Ca2+ in the ER ([Ca2+]er), cells were pre-treated with 2,5-di-tert-butylhydroquinone (DBHQ) to deplete Ca2+ in the ER before reconstitution of holo-aequorin. The addition of extracellular Ca2+ (Cao2+) to Ca2+-depleted cells induced refilling of the ER, which was complete within 5 min. This was associated with a sharp transient increase in the cytoplasmic Ca2+ concentration ([Ca2+]cyt) of about 15 s duration (a Cao2+-induced [Ca2+]cyt spike) after which [Ca2+]cyt remained elevated slightly above the basal value for a period of about 2 min (low [Ca2+]cyt plateau). The Cao2+-induced [Ca2+]cyt spike was inhibited by Gd3+, not affected by tetrakis-(2-pyridymethyl) ethylenediamine (TPEN), and broadened by ionomycin and the intracellular Ca2+ chelators BAPTA and EGTA. Refilling of the ER was inhibited by caffeine. Neither thapsigargin nor DBHQ caused a detectable inhibition or change in shape of the Cao2+-induced [Ca2+]cyt spike or the low [Ca2+]cyt plateau whereas each inhibited the inflow of Ca2+ to the ER by about 80%. Experiments conducted with carbonyl cyanide m-chlorophenyl-hydrazone (CCCP) indicated that thapsigargin did not alter the amount of Ca2+ accumulated in mitochondria. The changes in [Ca2+]cyt reported by aequorin were compared with those reported by fura-2. It is concluded that (i) there are significant quantitative differences between the manner in which aequorin and fura-2 sense changes in [Ca2+]cyt and (ii) thapsigargin and DBHQ inhibit the uptake of Ca2+ to the bulk of the ER but this is not associated with inhibition of the activation of SOCs. The possible involvement of a small sub-region of the ER (or another intracellular Ca2+ store), which contains thapsigargin-insensitive (Ca2++Mg2+)ATP-ases, in the activation of SOCs is briefly discussed.

Vanilloid receptor 1 regulates multiple calcium compartments and contributes to Ca2+-induced Ca2+ release in sensory neurons

Vanilloid receptor 1 belongs to the transient receptor potential ion channel family and transduces sensations of noxious heat and inflammatory hyperalgesia in nociceptive neurons. These neurons contain two vanilloid receptor pools, one in the plasma membrane and the other in the endoplasmic reticulum. The present experiments characterize these two pools and their functional significance using calcium imaging and 45Ca uptake in stably transfected cells or dorsal root ganglion neurons. The plasma membrane localized receptor is directly activated by vanilloids. The endoplasmic reticulum pool was demonstrated to be independently activated with 20 microm capsaicin or 1.6 microm resiniferatoxin using a bathing solution containing 10 microm Ruthenium Red (to selectively block plasma membrane-localized receptors) and 100 microm EGTA. We also demonstrate an overlap between the endoplasmic reticulum-localized vanilloid receptor regulated stores and thapsigargin-sensitive stores. Direct depletion of calcium via activation of endoplasmic reticulum-localized vanilloid receptor 1 triggered store operated calcium entry. Furthermore, we found that, in the presence of low extracellular calcium (10(-5) m), either 2 microm capsaicin or 0.1 nm-1.6 microm resiniferatoxin caused a pronounced calcium-induced calcium release in either vanilloid receptor-expressing neurons or heterologous expression systems. This phenomenon may allow new insight into how nociceptive neuron function in response to a variety of nociceptive stimuli both acutely and during prolonged nociceptive signaling.

Design of a High-Affinity Competitive Antagonist of the Vanilloid Receptor Selective for the Calcium Entry-Linked Receptor Population

We describe the synthesis and characterization of N-(4-chlorobenzyl) -N'-(4-hydroxy-3-iodo-5-methoxybenzyl)thiourea (IBTU), a novel antagonist of the vanilloid receptor 1 (TRPV1 or VR1). IBTU competitively inhibited 45Ca2+ uptake into CHO cells heterologously expressing rat TRPV1, whether induced by capsaicin or resiniferatoxin (Ki = 99 +/- 23 and 93 +/- 34 nM, respectively). IBTU was thus somewhat more potent (5-fold) than capsazepine. In contrast to its antagonism of vanilloid-induced calcium uptake, IBTU (30 microM) inhibited [3H]resiniferatoxin binding to TRPV1 by less than 10%. We hypothesize that these dramatically distinct potencies reflect different fractions of TRPV1 in this system: namely, a minor plasma membrane fraction controlling 45Ca2+ uptake, and the predominant intracellular fraction that dominates the [3H]resiniferatoxin binding measurements. Intracellular Ca2+ imaging supports this explanation. IBTU antagonized the elevation in intracellular Ca2+ in response to 50 nM capsaicin with an IC50 of 106 +/- 35 nM. Likewise, 600 nM IBTU was able to antagonize the elevation in intracellular Ca2+ in response to 100 pM resiniferatoxin in the presence of normal (1.8 mM) extracellular Ca2+, where the increase in intracellular calcium reflects calcium influx. In contrast, in the absence of extracellular Ca2+, where in this system resiniferatoxin induces a modest increase in calcium from intracellular stores, IBTU was unable to block the response to resiniferatoxin, although the TRPV1 antagonist 5-iodoresiniferatoxin was able to do so. In summary, IBTU is a novel, potent TRPV1 antagonist with marked selectivity between subpopulations of TRPV1 and may permit the function of these distinct pools to be explored and potentially exploited.

TRPV1 receptors mediate particulate matter-induced apoptosis

Exposure to airborne particulate matter (PM) is a world-wide health problem mainly because it produces adverse cardiovascular and respiratory effects that frequently result in morbidity. Despite many years of epidemiological and basic research, the mechanisms underlying PM toxicity remain largely unknown. To understand some of these mechanisms, we measured PM-induced apoptosis and necrosis in normal human airway epithelial cells and sensory neurons from both wild-type mice and mice lacking TRPV1 receptors using Alexa Fluor 488-conjugated annexin V and propidium iodide labeling, respectively. Exposure of environmental PMs containing residual oil fly ash and ash from Mount St. Helens was found to induce apoptosis, but not necrosis, as a consequence of sustained calcium influx through TRPV1 receptors. Apoptosis was completely prevented by inhibiting TRPV1 receptors with capsazepine or by removing extracellular calcium or in sensory neurons from TRPV1(-/-) mice. Binding of either one of the PMs to the cell membrane induced a capsazepine-sensitive increase in cAMP. PM-induced apoptosis was augmented upon the inhibition of PKA. PKA inhibition on its own also induced apoptosis, thereby suggesting that this pathway may be endogenously protective against apoptosis. In summary, it was found that inhibiting TRPV1 receptors prevents PM-induced apoptosis, thereby providing a potential mechanism to reduce their toxicity.

A novel Ca2+-induced Ca2+ release mechanism in A7r5 cells regulated by calmodulin-like proteins

Intracellular Ca2+ release is involved in setting up Ca2+ signals in all eukaryotic cells. Here we report that an increase in free Ca2+ concentration triggered the release of up to 41 +/- 3% of the intracellular Ca2+ stores in permeabilized A7r5 (embryonic rat aorta) cells with an EC50 of 700 nm. This type of Ca2+-induced Ca2+ release (CICR) was neither mediated by inositol 1,4,5-trisphosphate receptors nor by ryanodine receptors, because it was not blocked by heparin, 2-aminoethoxydiphenyl borate, xestospongin C, ruthenium red, or ryanodine. ATP dose-dependently stimulated the CICR mechanism, whereas 10 mm MgCl2 abolished it. CICR was not affected by exogenously added calmodulin (CaM), but CaM1234, a Ca2+-insensitive CaM mutant, strongly inhibited the CICR mechanism. Other proteins of the CaM-like neuronal Ca2+-sensor protein family such as Ca2+-binding protein 1 and neuronal Ca2+ sensor-1 were equally potent for inhibiting the CICR. Removal of endogenous CaM, using a CaM-binding peptide derived from the ryanodine receptor type-1 (amino acids 3614-3643) prevented subsequent activation of the CICR mechanism. A similar CICR mechanism was also found in 16HBE14o-(human bronchial mucosa) cells. We conclude that A7r5 and 16HBE14o-cells express a novel type of CICR mechanism that is silent in normal resting conditions due to inhibition by CaM but becomes activated by a Ca2+-dependent dissociation of CaM. This CICR mechanism, which may be regulated by members of the family of neuronal Ca2+-sensor proteins, may provide an additional route for Ca2+ release that could allow amplification of small Ca2+ signals.

Activation of vanilloid receptor type I in the endoplasmic reticulum fails to activate store-operated Ca2+ entry

To evaluate interaction of vanilloid receptor type 1 (TRPV1) with endogenous Ca(2+) signalling mechanisms, TRPV1 was expressed in Spodoptera frugiperda (Sf 9) insect cells using recombinant baculovirus. Stimulation of TRPV1-expressing cells, but not control Sf 9 cells, with resiniferatoxin (RTX), capsaicin or anandamide, produced an increase in cytosolic free Ca(2+) concentration ([Ca(2+)](i)), with EC(50) values of 166 pM, 24.5 nM and 3.89 microM respectively. In the absence of extracellular Ca(2+), both capsaicin and RTX caused an increase in [Ca(2+)](i) with EC(50) values of approx. 10 microM and 10 nM respectively. This TRPV1-induced release of Ca(2+) from intracellular stores was not blocked by U73122, suggesting that phospholipase C was not involved. Substantial overlap was found between the thapsigargin- and RTX-sensitive internal Ca(2+) pools, and confocal imaging showed that intracellular TRPV1 immunofluorescence co-localized with the endoplasmic reticulum targeting motif KDEL. To determine if TRPV1-induced mobilization of intracellular Ca(2+) activates endogenous store-operated Ca(2+) entry, the effect of 2-aminoethoxydiphenyl borate (2-APB) on Ba(2+) influx was examined. 2-APB blocked thapsigargin-induced Ba(2+) influx, but not RTX-induced Ba(2+) entry. In the combined presence of thapsigargin and a store-releasing concentration of RTX, the 2-APB-sensitive component was essentially identical with the thapsigargin-induced component. Similar results were obtained in HEK-293 cells stably expressing TRPV1. These results suggest that TRPV1 forms agonist-sensitive channels in the endoplasmic reticulum, which when activated, release Ca(2+) from internal stores, but fail to activate endogenous store-operated Ca(2+) entry. Selective activation of intracellular TRPV1, without concomitant involvement of plasmalemmal Ca(2+) influx mechanisms, could play an important role in Ca(2+) signalling within specific subcellular microdomains.