The long and arduous road to CRAC

Syntaxin11 Deficiency Inhibits CRAC Channel Priming To Suppress Cytotoxicity And Gene Expression In FHLH4 Patient T Lymphocytes

CRAC channels enable calcium entry from the extracellular space in response to a variety of stimuli and are crucial for gene expression and granule exocytosis in lymphocytes. Here we find that Syntaxin11, a Q-SNARE, associated with FHLH4 disease in human patients, directly binds Orai1, the pore forming subunit of CRAC channels. Syntaxin11 depletion strongly inhibited SOCE, CRAC currents, IL-2 expression and cytotoxicity in cell lines and FHLH4 patient T lymphocytes. Constitutively active H134 Orai1 mutant completely reconstituted calcium entry in Syntaxin11 depleted cells and the defects of granule exocytosis as well as gene expression could be bypassed by ionomycin induced calcium influx in FHLH4 T lymphocytes. Our data reveal a Syntaxin11 induced pre-activation state of Orai which is necessary for its subsequent coupling and gating by the endoplasmic reticulum resident Stim protein. We propose that ion channel regulation by specific SNAREs is a primary and conserved function which may have preceded their role in vesicle fusion.

High affinity associations with α-SNAP enable calcium entry via Orai1 channels

Molecular steps that activate store-operated calcium entry (SOCE) via Orai channel supramolecular complex remain incompletely defined. We have earlier shown that α-SNAP regulates the on-site functional assembly and calcium selectivity of Orai1 channels. Here we investigate the molecular basis of its association with Orai, Stim and find that the affinity of α-SNAP for Orai and Stim is substantially higher than previously reported affinities between Stim and Orai sub-domains. α-SNAP binds the coiled-coil 3 (CC3) sub-domain of Stim1. Mutations of Tryptophan 430 in Stim1-CC3 disrupted α-SNAP association and SOCE, demonstrating a novel α-SNAP dependent function for this crucial subdomain. Further, α-SNAP binds the hinge region near the C-terminus of Orai1 and an additional broad region near the N-terminus and Valine 262 and Leucine 74 were necessary for these respective interactions, but not Orai, Stim co-clustering. Thus, high affinity interactions with α-SNAP are necessary for imparting functionality to Stim, Orai clusters and induction of SOCE.

Huntingtin-Associated Protein 1A Regulates Store-Operated Calcium Entry in Medium Spiny Neurons From Transgenic YAC128 Mice, a Model of Huntington's Disease

Huntington's disease (HD) is a hereditary neurodegenerative disease that is caused by polyglutamine expansion within the huntingtin (HTT) gene. One of the cellular activities that is dysregulated in HD is store-operated calcium entry (SOCE), a process by which Ca2+ release from the endoplasmic reticulum (ER) induces Ca2+ influx from the extracellular space. HTT-associated protein-1 (HAP1) is a binding partner of HTT. The aim of the present study was to examine the role of HAP1A protein in regulating SOCE in YAC128 mice, a transgenic model of HD. After Ca2+ depletion from the ER by the activation of inositol-(1,4,5)triphosphate receptor type 1 (IP3R1), we detected an increase in the activity of SOC channels when HAP1 protein isoform HAP1A was overexpressed in medium spiny neurons (MSNs) from YAC128 mice. A decrease in the activity of SOC channels in YAC128 MSNs was observed when HAP1 protein was silenced. In YAC128 MSNs that overexpressed HAP1A, an increase in activity of IP3R1 was detected while the ionomycin-sensitive ER Ca2+ pool decreased. 6-Bromo-N-(2-phenylethyl)-2,3,4,9-tetrahydro-1H-carbazol-1-amine hydrochloride (C20H22BrClN2), identified in our previous studies as a SOCE inhibitor, restored the elevation of SOCE in YAC128 MSN cultures that overexpressed HAP1A. The IP3 sponge also restored the elevation of SOCE and increased the release of Ca2+ from the ER in YAC128 MSN cultures that overexpressed HAP1A. The overexpression of HAP1A in the human neuroblastoma cell line SK-N-SH (i.e., a cellular model of HD (SK-N-SH HTT138Q)) led to the appearance of a pool of constitutively active SOC channels and an increase in the expression of STIM2 protein. Our results showed that HAP1A causes the activation of SOC channels in HD models by affecting IP3R1 activity.

Patient-Specific iPSC-Based Models of Huntington's Disease as a Tool to Study Store-Operated Calcium Entry Drug Targeting

Neurodegenerative pathologies are among the most serious and socially significant problems of modern medicine, along with cardiovascular and oncological diseases. Several attempts have been made to prevent neuronal death using novel drugs targeted to the cell calcium signaling machinery, but the lack of adequate models for screening markedly impairs the development of relevant drugs. A potential breakthrough in this field is offered by the models of hereditary neurodegenerative pathologies based on endogenous expression of mutant proteins in neurons differentiated from patient-specific induced pluripotent stem cells (iPSCs). Here, we study specific features of store-operated calcium entry (SOCE) using an iPSCs-based model of Huntington's disease (HD) and analyze the pharmacological effects of a specific drug targeted to the calcium channels. We show that SOCE in gamma aminobutyric acid-ergic striatal medium spiny neurons (GABA MSNs) was mediated by currents through at least two different channel groups, ICRAC and ISOC. Both of these groups were upregulated in HD neurons compared with the wild-type neurons. Thapsigargin-induced intracellular calcium store depletion in GABA MSNs resulted in predominant activation of either ICRAC or ISOC. The potential anti-HD drug EVP4593, which was previously shown to have neuroprotective activity in different HD models, affected both ICRAC and ISOC.

Optogenetic toolkit for precise control of calcium signaling

Calcium acts as a second messenger to regulate a myriad of cell functions, ranging from short-term muscle contraction and cell motility to long-term changes in gene expression and metabolism. To study the impact of Ca2+-modulated 'ON' and 'OFF' reactions in mammalian cells, pharmacological tools and 'caged' compounds are commonly used under various experimental conditions. The use of these reagents for precise control of Ca2+ signals, nonetheless, is impeded by lack of reversibility and specificity. The recently developed optogenetic tools, particularly those built upon engineered Ca2+ release-activated Ca2+ (CRAC) channels, provide exciting opportunities to remotely and non-invasively modulate Ca2+ signaling due to their superior spatiotemporal resolution and rapid reversibility. In this review, we briefly summarize the latest advances in the development of optogenetic tools (collectively termed as 'genetically encoded Ca2+ actuators', or GECAs) that are tailored for the interrogation of Ca2+ signaling, as well as their applications in remote neuromodulation and optogenetic immunomodulation. Our goal is to provide a general guide to choosing appropriate GECAs for optical control of Ca2+ signaling in cellulo, and in parallel, to stimulate further thoughts on evolving non-opsin-based optogenetics into a fully fledged technology for the study of Ca2+-dependent activities in vivo.

Na+ influx via Orai1 inhibits intracellular ATP-induced mTORC2 signaling to disrupt CD4 T cell gene expression and differentiation

T cell effector functions require sustained calcium influx. However, the signaling and phenotypic consequences of non-specific sodium permeation via calcium channels remain unknown. α-SNAP is a crucial component of Orai1 channels, and its depletion disrupts the functional assembly of Orai1 multimers. Here we show that α-SNAP hypomorph, hydrocephalus with hopping gait, Napa^hyh/hyh mice harbor significant defects in CD4 T cell gene expression and Foxp3 regulatory T cell (Treg) differentiation. Mechanistically, TCR stimulation induced rapid sodium influx in Napa^hyh/hyh CD4 T cells, which reduced intracellular ATP, [ATP]_i. Depletion of [ATP]_i inhibited mTORC2 dependent NFκB activation in Napa^hyh/hyh cells but ablation of Orai1 restored it. Remarkably, TCR stimulation in the presence of monensin phenocopied the defects in Napa^hyh/hyh signaling and Treg differentiation, but not IL-2 expression. Thus, non-specific sodium influx via bonafide calcium channels disrupts unexpected signaling nodes and may provide mechanistic insights into some divergent phenotypes associated with Orai1 function.

DOI:http://dx.doi.org/10.7554/eLife.25155.001

The STIM-Orai Pathway: Light-Operated Ca2+ Entry Through Engineered CRAC Channels

Ca²⁺ signals regulate a plethora of cellular functions that include muscle contraction, heart beating, hormone secretion, lymphocyte activation, gene expression, and metabolism. To study the impact of Ca²⁺ signals on biological processes, pharmacological tools and caged compounds have been commonly applied to induce fluctuations of intracellular Ca²⁺ concentrations. These conventional approaches, nonetheless, lack rapid reversibility and high spatiotemporal resolution. To overcome these disadvantages, we and others have devised a series of photoactivatable genetically encoded Ca²⁺ actuators (GECAs) by installing light sensitivities into a bona fide highly selective Ca²⁺ channel, the Ca²⁺ release-activated Ca²⁺ (CRAC) channel. Store-operated CRAC channel serves as a major route for Ca²⁺ entry in many cell types. These GECAs enable remote and precise manipulation of Ca²⁺ signaling in both excitable and non-excitable cells. When combined with nanotechnology, it becomes feasible to wirelessly photo-modulate Ca²⁺-dependent activities in vivo. In this chapter, we briefly review most recent advances in engineering CRAC channels to achieve optical control over Ca²⁺ signaling, outline their design principles and kinetic features, and present exemplary applications of GECAs engineered from CRAC channels.

α-SNAP regulates dynamic, on-site assembly and calcium selectivity of Orai1 channels

Ion channel subunits typically assemble in the endoplasmic reticulum. α-SNAP orchestrates a unique assembly and calcium selectivity of Orai1 subunits into functional multimers. Dynamic assembly of Orai1 and its dependence on α-SNAP could enable localization of calcium signals and regulation of rate and amount of calcium influx.

Orai1 forms a highly calcium-selective pore of the calcium release activated channel, and α-SNAP is necessary for its function. Here we show that α-SNAP regulates on-site assembly of Orai1 dimers into calcium-selective multimers. We find that Orai1 is a dimer in resting primary mouse embryonic fibroblasts but displays variable stoichiometry in the plasma membrane of store-depleted cells. Remarkably, α-SNAP depletion induces formation of higher-order Orai1 oligomers, which permeate significant levels of sodium via Orai1 channels. Sodium permeation in α-SNAP–deficient cells cannot be corrected by tethering multiple Stim1 domains to Orai1 C-terminal tail, demonstrating that α-SNAP regulates functional assembly and calcium selectivity of Orai1 multimers independently of Stim1 levels. Fluorescence nanoscopy reveals sustained coassociation of α-SNAP with Stim1 and Orai1, and α-SNAP–depleted cells show faster and less constrained mobility of Orai1 within ER-PM junctions, suggesting Orai1 and Stim1 coentrapment without stable contacts. Furthermore, α-SNAP depletion significantly reduces fluorescence resonance energy transfer between Stim1 and Orai1 N-terminus but not C-terminus. Taken together, these data reveal a unique role of α-SNAP in the on-site functional assembly of Orai1 subunits and suggest that this process may, in part, involve enabling crucial low-affinity interactions between Orai1 N-terminus and Stim1.

The multifaceted role of PIP2 in leukocyte biology

Phosphatidylinositol 4,5-bisphosphate (PIP2) represents about 1 % of plasma membrane phospholipids and behaves as a pleiotropic regulator of a striking number of fundamental cellular processes. In recent years, an increasing body of literature has highlighted an essential role of PIP2 in multiple aspects of leukocyte biology. In this emerging picture, PIP2 is envisaged as a signalling intermediate itself and as a membrane-bound regulator and a scaffold of proteins with specific PIP2 binding domains. Indeed PIP2 plays a key role in several functions. These include directional migration in neutrophils, integrin-dependent adhesion in T lymphocytes, phagocytosis in macrophages, lysosomes secretion and trafficking at immune synapse in cytolytic effectors and secretory cells, calcium signals and gene transcription in B lymphocytes, natural killer cells and mast cells. The coordination of these different aspects relies on the spatio-temporal organisation of distinct PIP2 pools, generated by the main PIP2 generating enzyme, phosphatidylinositol 4-phosphate 5-kinase (PIP5K). Three different isoforms of PIP5K, named α, β and γ, and different splice variants have been described in leukocyte populations. The isoform-specific coupling of specific isoforms of PIP5K to different families of activating receptors, including integrins, Fc receptors, toll-like receptors and chemokine receptors, is starting to be reported. Furthermore, PIP2 is turned over by multiple metabolising enzymes including phospholipase C (PLC) γ and phosphatidylinositol 3-kinase (PI3K) which, along with Rho family small G proteins, is widely involved in strategic functions within the immune system. The interplay between PIP2, lipid-modifying enzymes and small G protein-regulated signals is also discussed.

Calcium channels in Fc receptor signaling

The calcium ion (Ca(2+)) is the main common second messenger involved in signaling transduction subsequent to immunoreceptor activation. Its rapid intracellular elevation induces multiple cellular responses, such as secretion, proliferation, mobility, and gene transcription. Intracellular levels of Ca(2+) need to reach a specific threshold to efficiently transduce the signal to activate transcription factors through the recruitment of Ca(2+)-binding molecules. However, since Ca(2+) cannot be metabolized, its intracellular concentration is tightly regulated to avoid the induction of programmed cell death. This highly controlled regulation of Ca(2+) homeostasis has recently been clarified by the uncovering of new ion channels. The regulation of these channels allows the role of Ca(2+) in Fc receptor transduction pathways to be more precisely defined.

Calcium signaling in lacrimal glands

Lacrimal glands provide the important function of lubricating and protecting the ocular surface. Failure of proper lacrimal gland function results in a number of debilitating dry eye diseases. Lacrimal glands secrete lipids, mucins, proteins, salts and water and these secretions are at least partially regulated by neurotransmitter-mediated cell signaling. The predominant signaling mechanism for lacrimal secretion involves activation of phospholipase C, generation of the Ca(2+)-mobilizing messenger, IP3, and release of Ca(2+) stored in the endoplasmic reticulum. The loss of Ca(2+) from the endoplasmic reticulum then triggers a process known as store-operated Ca(2+) entry, involving a Ca(2+) sensor in the endoplasmic reticulum, STIM1, which activates plasma membrane store-operated channels comprised of Orai subunits. Recent studies with deletions of the channel subunit, Orai1, confirm the important role of SOCE in both fluid and protein secretion in lacrimal glands, both in vivo and in vitro.

New insights into store-independent Ca(2+) entry: secretory pathway calcium ATPase 2 in normal physiology and cancer

Recent studies in secretory pathway calcium ATPases (SPCA) revealed novel functions of SPCA2 in interacting with store-operated Ca(2+) channel Orai1 and inducing Ca(2+) influx at the cell surface. Importantly, SPCA2-mediated Ca(2+) signaling is uncoupled from its conventional role of Ca(2+)-ATPase and independent of store-operated Ca(2+) signaling pathway. SPCA2-induced store-independent Ca(2+) entry (SICE) plays essential roles in many important physiological processes, while unbalanced SICE leads to enhanced cell proliferation and tumorigenesis. Finally, we have summarized the clinical implication of SICE in oral cancer prognosis and treatment. Inhibition of SICE may be a new target for the development of cancer therapeutics.

CRAC inhibitors: identification and potential

BACKGROUND

Ca(2+) release-activated Ca(2+) (CRAC) channels, a subfamily of store-operated channels, play an essential role in various diseases such as immune disorders and allergic responses.

OBJECTIVE

The successful treatment of these diseases requires the identification of specific inhibitors. So far, a variety of chemical compounds blocking CRAC have been identified; however, they have all turned out to be less specific. Recently two proteins, STIM1 and ORAI1, have been identified as the essential components that fully reconstitute CRAC currents with a similar biophysical fingerprint.

METHOD

These two proteins and their activation process represent direct targets for the application of specific CRAC inhibitors.

RESULTS/CONCLUSION

For drug development, fluorescence microscopy adaptable for high-throughput screening will provide a powerful assay to mechanistically identify potential CRAC inhibitors that act on various stages within the STIM1/ORAI1 activation pathway visualized by fluorescent-tagged proteins.

On the role of endothelial TRPC3 channels in endothelial dysfunction and cardiovascular disease

In endothelium, calcium (Ca(2+)) influx through plasma membrane Ca(2+)-permeable channels plays a fundamental role in several physiological functions and in the pathogenesis of cardiovascular disease. Current knowledge on the influence of Ca(2+) influx in signaling events associated to endothelial dysfunction has grown significantly over recent years, particularly after identification of members of the Transient Receptor Potential Canonical (TRPC) family of channel forming proteins as prominent mediators of Ca(2+) entry in endothelial cells. Among TRPC members TRPC3 has been at the center of many of these physiopathological processes. Progress in elucidating the mechanism/s underlying regulation of endothelial TRPC3 and characterization of signaling events downstream TRPC3 activation are of most importance to fully appreciate the role of this peculiar cation channel in cardiovascular disease and its potential use as a therapeutic target. In this updated review we focus on TRPC3 channels, revising and discussing current knowledge on channel expression and regulation in endothelium and the roles of TRPC3 in cardiovascular disease in relation to endothelial dysfunction.

On the role of TRPC1 in control of Ca2+ influx, cell volume, and cell cycle

Ca(+) signaling plays a crucial role in control of cell cycle progression, but the understanding of the dynamics of Ca(2+) influx and release of Ca(2+) from intracellular stores during the cell cycle is far from complete. The aim of the present study was to investigate the role of the free extracellular Ca(2+) concentration ([Ca(2+)](o)) in cell proliferation, the pattern of changes in the free intracellular Ca(2+) concentration ([Ca(2+)](i)) during cell cycle progression, and the role of the transient receptor potential (TRP)C1 in these changes as well as in cell cycle progression and cell volume regulation. In Ehrlich Lettré Ascites (ELA) cells, [Ca(2+)](i) decreased significantly, and the thapsigargin-releasable Ca(2+) pool in the intracellular stores increased in G(1) as compared with G(0). Store-depletion-operated Ca(2+) entry (SOCE) and TRPC1 protein expression level were both higher in G(1) than in G(0) and S phase, in parallel with a more effective volume regulation after swelling [regulatory volume decrease (RVD)] in G(1) as compared with S phase. Furthermore, reduction of [Ca(2+)](o), as well as two unspecific SOCE inhibitors, 2-APB (2-aminoethyldiphenyl borinate) and SKF96365 (1-(β-[3-(4-methoxy-phenyl)propoxyl-4-methoxyphenethyl)1H-imidazole-hydrochloride), inhibited ELA cell proliferation. Finally, Madin-Darby canine kidney cells in which TRPC1 was stably silenced [TRPC1 knockdown (TRPC1-KD) MDCK] exhibited reduced SOCE, slower RVD, and reduced cell proliferation compared with mock controls. In conclusion, in ELA cells, SOCE and TRPC1 both seem to be upregulated in G(1) as compared with S phase, concomitant with an increased rate of RVD. Furthermore, TRPC1-KD MDCK cells exhibit decreased SOCE, decreased RVD, and decreased proliferation, suggesting that, at least in certain cell types, TRPC1 is regulated during cell cycle progression and is involved in SOCE, RVD, and cell proliferation.

An emerging role for the lipid mediator sphingosine-1-phosphate in mast cell effector function and allergic disease

Sphingosine-1-phosphate (S1P) plays important roles regulating functions of diverse biological systems, including the immune system. S1P affects immune cell function mostly by acting through its receptors at the cell membrane but it can also induce S1P receptor-independent responses in the cells where it is generated. S1P produced in allergically-stimulated mast cells mediates degranulation, cytokine and lipid mediator production and migration of mast cells towards antigen by mechanisms that are both S1P receptor-dependent and independent. Even in the absence of an antigen challenge, the differentiation and responsiveness of mast cells can be affected by chronic exposure to elevated S1P from a nonmast cell source, whichmay occur under pathophysiological conditions, potentially leading to the hyper-responsiveness of mast cells. The role of S1P extends beyond the regulation of the function of mast cells to the regulation of the surrounding or distal environment. S1P is exported out of antigen-stimulated mast cells and into the extracellular space and the resulting S1P gradient within the tissue may influence diverse surrounding tissue cells and several aspects of the allergic disease, such as inflammation or tissue remodeling. Furthermore, recent findings indicate that vasoactive mediators released systemically by mast cells induce the production of S1P in nonhematopoietic compartments, where it plays a role in regulating the vascular tone and reducing the hypotension characteristic of the anaphy lactic shock and thus helping the recovery. The dual actions of S1P, promoting the immediate response of mast cells, while controlling the systemic consequences of mast cell activity will be discussed in detail.

Macrolide antibiotics inhibit mucus secretion and calcium entry in Swine airway submucosal mucous gland cells

Macrolide antibiotics such as erythromycin (EM) and azithromycin (AZM) are beneficial in the treatment of mucus hypersecretion in inflammatory pulmonary diseases. Several indirect and direct mechanisms of action have been proposed. This study investigates the direct effect of macrolides on secretory function of isolated submucosal mucous gland cells (SMGCs). We hypothesize that macrolides inhibit the calcium influx necessary for evoked mucus secretion. To test this, we quantified mucin protein release using enzyme-linked immunosorbent assay, calcium-activated K(+) (K(Ca)), and calcium-activated Cl(-) (Cl(Ca)) currents. We measured nonselective cation current (NSCC) using whole-cell patch-clamp techniques; intracellular calcium concentration ([Ca(2+)](i)) was measured using fura-2 Ca(2+) imaging. We found that both EM and AZM are agonists at muscarinic receptors. EM (10 μM) evoked a small but significant increase in mucin release but inhibited the mucin release induced by subsequent acetylcholine (ACh) treatment. Both EM and AZM (10 μM) evoked K(Ca) and Cl(Ca) whole-cell currents, which were blocked by atropine. EM and AZM also accelerated the decay of inositol trisphosphate-induced K(Ca) and Cl(Ca) currents without changing the peak current amplitudes. Likewise, internal application of AZM (10 μM) enhanced the decay rate of ACh-induced K(Ca) and Cl(Ca) currents. EM (1-10 μM) and AZM (0.1-10 μM) slowly (over 25-30 min) inhibited thapsigargin (TG)-induced Ca(2+) entry when applied during the plateau phase of Ca(2+) entry but blunted TG-induced Ca(2+) entry by 70% after a 5-min pretreatment before initiating calcium entry. EM blocked TG-induced NSCC. We conclude that macrolide antibiotics are partial agonists at muscarinic receptors but inhibit stimulated mucus release by inhibiting calcium entry in SMGCs.

Store-independent activation of Orai1 by SPCA2 in mammary tumors

Ca(2+) is an essential and ubiquitous second messenger. Changes in cytosolic Ca(2+) trigger events critical for tumorigenesis, such as cellular motility, proliferation, and apoptosis. We show that an isoform of Secretory Pathway Ca(2+)-ATPase, SPCA2, is upregulated in breast cancer-derived cells and human breast tumors, and suppression of SPCA2 attenuates basal Ca(2+) levels and tumorigenicity. Contrary to its conventional role in Golgi Ca(2+) sequestration, expression of SPCA2 increased Ca(2+) influx by a mechanism dependent on the store-operated Ca(2+) channel Orai1. Unexpectedly, SPCA2-Orai1 signaling was independent of ER Ca(2+) stores or STIM1 and STIM2 sensors and uncoupled from Ca(2+)-ATPase activity of SPCA2. Binding of the SPCA2 amino terminus to Orai1 enabled access of its carboxyl terminus to Orai1 and activation of Ca(2+) influx. Our findings reveal a signaling pathway in which the Orai1-SPCA2 complex elicits constitutive store-independent Ca(2+) signaling that promotes tumorigenesis.

Independent and cooperative roles of adaptor molecules in proximal signaling during FcepsilonRI-mediated mast cell activation

Activation through FcepsilonRI, a high-affinity IgE-binding receptor, is critical for mast cell function during allergy. The formation of a multimolecular proximal signaling complex nucleated by the adaptor molecules SLP-76 and LAT1 is required for activation through this receptor. Based on previous T-cell studies, current dogma dictates that LAT1 is required for plasma membrane recruitment and function of SLP-76. Unexpectedly, we found that the recruitment and phosphorylation of SLP-76 were preserved in LAT1(-/-) mast cells and that SLP-76(-/-) and LAT1(-/-) mast cells harbored distinct functional and biochemical defects. The LAT1-like molecule LAT2 was responsible for the preserved membrane localization and phosphorylation of SLP-76 in LAT1(-/-) mast cells. Although LAT2 supported SLP-76 phosphorylation and recruitment to the plasma membrane, LAT2 only partially compensated for LAT1-mediated cell signaling due to its decreased ability to stabilize interactions with phospholipase Cgamma (PLCgamma). Comparison of SLP-76(-/-) LAT1(-/-) and SLP-76(-/-) mast cells revealed that some functions of LAT1 could occur independently of SLP-76. We propose that while SLP-76 and LAT1 depend on each other for many of their functions, LAT2/SLP-76 interactions and SLP-76-independent LAT1 functions also mediate a positive signaling pathway downstream of FcepsilonRI in mast cells.

Loss of TRPC1-mediated Ca2+ influx contributes to impaired degranulation in Fyn-deficient mouse bone marrow-derived mast cells

MC degranulation requires the influx of calcium from the extracellular environment. Orai1/STIM1 is essential to MC SOCE, as shown in rat peritoneal MCs, the rat MC lines (RBL-2H3), or in Orai1 null embryo liver-derived, cultured MCs. However, minimal information exists about the role of other calcium channels expressed on these cells. Here, we demonstrate that the nonselective TRPC1 participates in FcεRI-mediated calcium entry in mouse BMMCs. We found that Fyn null MCs, which have an impaired degranulation response, expressed reduced levels of TRPC1, had normal depletion of intracellular calcium stores but an impaired calcium influx, and failed to depolymerize cortical F-actin (a key step for granule-plasma membrane fusion). Partial RNAi silencing of TRPC1 expression in WT MCs (to the level of Fyn null MCs) mimicked the Fyn null defect in calcium influx, cortical F-actin depolymerization, and MC degranulation. Ectopic expression of Fyn or TRPC1 in Fyn null MCs restored calcium responses and cortical F-actin depolymerization and increased MC degranulation. Together with our findings that expression of Orai1 is not altered in Fyn null MCs, our findings suggest that TRPC1 participates in calcium influx and other key events required for MC degranulation. This demonstrates that in addition to a role described previously for Orai1 in promoting MC degranulation, nonselective cation channels participate in promoting the exocytotic response.

Animal Ca2+ release-activated Ca2+ (CRAC) channels appear to be homologous to and derived from the ubiquitous cation diffusion facilitators

BACKGROUND

Antigen stimulation of immune cells triggers Ca2+ entry through Ca2+ release-activated Ca2+ (CRAC) channels, promoting an immune response to pathogens. Defects in a CRAC (Orai) channel in humans gives rise to the hereditary Severe Combined Immune Deficiency (SCID) syndrome. We here report results that define the evolutionary relationship of the CRAC channel proteins of animals, and the ubiquitous Cation Diffusion Facilitator (CDF) carrier proteins.

FINDINGS

CDF antiporters derived from a primordial 2 transmembrane spanner (TMS) hairpin structure by intragenic triplication to yield 6 TMS proteins. Four programs (IC/GAP, GGSEARCH, HMMER and SAM) were evaluated for identifying sequence similarity and establishing homology using statistical means. Overall, the order of sensitivity (similarity detection) was IC/GAP = GGSEARCH > HMMER > SAM, but the use of all four programs was superior to the use of any two or three of them. Members of the CDF family appeared to be homologous to members of the 4 TMS Orai channel proteins.

CONCLUSIONS

CRAC channels derived from CDF carriers by loss of the first two TMSs of the latter. Based on statistical analyses with multiple programs, TMSs 3-6 in CDF carriers are homologous to TMSs 1-4 in CRAC channels, and the former was the precursor of the latter. This is an unusual example of how a functionally and structurally more complex protein may have predated a simpler one.

Negative regulation of Ca(2+) influx during P2Y(2) purinergic receptor activation is mediated by Gbetagamma-subunits

We have previously reported that P2Y(2) purinoceptors and muscarinic M(3) receptors trigger Ca(2+) responses in HT-29 cells that differ in their timecourse, the Ca(2+) response to P2Y(2) receptor activation being marked by a more rapid decline of intracellular Ca(2+) concentration ([Ca(2+)](i)) after the peak response and that this rapid decline of [Ca(2+)](i) was slowed in cells expressing heterologous beta-adrenergic receptor kinase (betaARK). In the present study, we demonstrate that, during P2Y(2) receptor activation, betaARK expression increases the rate of Gd(3+)-sensitive Mn(2+) influx, a measure of the rate of store-operated Ca(2+) entry from the extracellular space, during P2Y(2) activation and that this effect of betaARK is mimicked by exogenous alpha-subunits of G(q), G(11) and G(i2). The effect of betaARK on the rate of Mn(2+) influx is thus attributable to its ability to scavenge G protein betagamma-subunits released during activation of P2Y(2) receptor. We further find that the effect of betaARK on the rate of Mn(2+) influx during P2Y(2) receptor activation can be overcome by arachidonic acid. In addition, the UTP-induced Mn(2+) influx rate was significantly increased by inhibitors of phospholipase A(2) (PLA(2)) and an siRNA directed against PLA(2)beta, but not by an siRNA directed against PLA(2)alpha or by inhibitors of arachidonic acid metabolism. These findings provide evidence for the existence of a P2Y(2) receptor-activated signalling system that acts in parallel with depletion of intracellular Ca(2+) stores to inhibit Ca(2+) influx across the cell membrane. This signalling process is mediated via Gbetagamma and involves PLA(2)beta and arachidonic acid.

GPI-1046 protects dorsal root ganglia from gp120-induced axonal injury by modulating store-operated calcium entry

Human immunodeficiency virus (HIV)-associated sensory neuropathy (HIV-SN) occurs in a large fraction of patients infected with HIV. Viral components, including the coat protein gp120, are thought to exert toxic actions on dorsal root ganglia (DRG) sensory neurons that can be further exacerbated by treatment of HIV infection with some antiretroviral agents. In a tissue culture model of HIV-SN, we found that gp120-induced axonal degeneration in DRG sensory neurons was prevented by treatment with the immunophilin ligand GPI-1046. Gp120 induced a rapid and large release of endoplasmic reticulum (ER) calcium in DRG neurons that was attenuated by treatment with GPI-1046. Further experiments suggested that GPI-1046 reduced the total ER calcium load by attenuating store-operated calcium (SOC) entry. Together, these results suggest that GPI-1046 protects DRG from gp120-induced axonal damage by decreasing the entry of calcium through SOC, thus reducing the total volume of ER calcium that is available to be released by gp120.

Synaptotagmin-2 controls regulated exocytosis but not other secretory responses of mast cells

Mast cell degranulation is a highly regulated, calcium-dependent process, which is important for the acute release of inflammatory mediators during the course of many pathological conditions. We previously found that Synaptotagmin-2, a calcium sensor in neuronal exocytosis, was expressed in a mast cell line. We postulated that this protein may be involved in the control of mast cell-regulated exocytosis, and we generated Synaptotagmin-2 knock-out mice to test our hypothesis. Mast cells from this mutant animal conferred an abnormally decreased passive cutaneous anaphylaxis reaction on mast cell-deficient mice that correlated with a specific defect in mast cell-regulated exocytosis, leaving constitutive exocytosis and nonexocytic mast cell effector responses intact. This defect was not secondary to abnormalities in the development, maturation, migration, morphology, synthesis, and storage of inflammatory mediators, or intracellular calcium transients of the mast cells. Unlike neurons, the lack of Synaptotagmin-2 in mast cells was not associated with increased spontaneous exocytosis.

The TRPC class of ion channels: a critical review of their roles in slow, sustained increases in intracellular Ca(2+) concentrations

The realization that there exists a multimembered family of cation channels with structural similarity to Drosophila's Trp channel emerged during the second half of the 1990s. In mammals, depending on the species, the TRP family counts 29 or 30 members which has been subdivided into 6 subfamilies on the basis of sequence similarity. TRP channels are nonselective monovalent cation channels, most of which also allow passage of Ca(2+). Many members of each of these families, but not all, are involved in sensory signal transduction. The C-type (for canonical or classical) subfamily, differs from the other TRP subfamilies in that it fulfills two different types of function: membrane depolarization, resembling sensory transduction TRPs, and mediation of sustained increases in intracellular Ca(2+). The mechanism(s) by which the C-class of TRP channels-the TRPCs-are activated is poorly understood and their role in mediating intracellular Ca(2+) increases is being questioned. Both of these questions-mechanism of activation and participation in Ca(2+) entry-are the topics of this review.

A role for Orai in TRPC-mediated Ca2+ entry suggests that a TRPC:Orai complex may mediate store and receptor operated Ca2+ entry

TRPC and Orai proteins have both been proposed to form Ca(2+)-selective, store-operated calcium entry (SOCE) channels that are activated by store-depletion with Ca(2+) chelators or calcium pump inhibitors. In contrast, only TRPC proteins have been proposed to form nonselective receptor-operated calcium entry (ROCE) cation channels that are activated by Gq/Gi-PLCbeta signaling, which is the physiological stimulus for store depletion. We reported previously that a dominant negative Orai1 mutant, R91W, inhibits Ca(2+) entry through both SOCE and ROCE channels, implicating Orai participation in both channel complexes. However, the argument for Orai participating in ROCE independently of store depletion is tenuous because store depletion is an integral component of the ROCE response, which includes formation of IP3, a store-depleting agent. Here we show that the R91W mutant also blocks diacylglycerol (DAG)-activated Ca(2+) entry into cells that stably, or transiently, express DAG-responsive TRPC proteins. This strongly suggests that Orai and TRPC proteins form complexes that participate in Ca(2+) entry with or without activation of store depletion. To integrate these results with recent data linking SOCE with recruitment of Orai and TRPCs to lipid rafts by STIM, we develop the hypothesis that Orai:TRPC complexes recruited to lipid rafts mediate SOCE, whereas the same complexes mediate ROCE when they are outside of lipid rafts. It remains to be determined whether the molecules forming the permeation pathway are the same when Orai:TRPC complexes mediate ROCE or SOCE.

STIM1- and Orai1-dependent store-operated calcium entry regulates human myoblast differentiation

Our previous work on human myoblasts suggested that a hyperpolarization followed by a rise in [Ca(2+)](in) involving store-operated Ca(2+) entry (SOCE) channels induced myoblast differentiation. Advances in the understanding of the SOCE pathway led us to examine more precisely its role in post-natal human myoblast differentiation. We found that SOCE orchestrated by STIM1, the endoplasmic reticulum Ca(2+) sensor activating Orai Ca(2+) channels, is crucial. Silencing STIM1, Orai1, or Orai3 reduced SOCE amplitude and myoblast differentiation, whereas Orai2 knockdown had no effect. Conversely, overexpression of STIM1 with Orai1 increased SOCE and accelerated myoblast differentiation. STIM1 or Orai1 silencing decreased resting [Ca(2+)](in) and intracellular Ca(2+) store content, but correction of these parameters did not rescue myoblast differentiation. Remarkably, SOCE amplitude correlated linearly with the expression of two early markers of myoblast differentiation, MEF2 and myogenin, regardless of the STIM or Orai isoform that was silenced. Unexpectedly, we found that the hyperpolarization also depends on SOCE, placing SOCE upstream of K(+) channel activation in the signaling cascade that controls myoblast differentiation. These findings indicate that STIM1 and Orai1 are key molecules for the induction of human myoblast differentiation.

Methods for studying store-operated calcium entry

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

Anatomical and neurochemical characterization of dopaminergic interplexiform processes in mouse and rat retinas

Dopaminergic (DA) neurons of mouse and rat retinas are of the interplexiform subtype (DA-IPC), i.e., they send processes distally toward the outer retina, exhibiting numerous varicosities along their course. The primary question we addressed was whether distally located DA-IPC varicosities, identified by tyrosine hydroxylase (TH) immunoreactivity, had the characteristic presynaptic proteins associated with calcium-dependent vesicular release of neurotransmitter. We found that TH immunoreactive varicosities in the outer retina possessed vesicular monoamine transporter 2 and vesicular GABA transporter, but they lacked immunostaining for any of nine subtypes of voltage-dependent calcium channel. Immunoreactivity for other channels that may permit calcium influx such as certain ionotropic glutamate receptors and canonical transient receptor potential channels (TRPCs) was similarly absent, although DA-IPC varicosities did show ryanodine receptor immunoreactivity, indicating the presence of intracellular calcium stores. The synaptic vesicle proteins sv2a and sv2b and certain other proteins associated with the presynaptic membrane were absent from DA-IPC varicosities, but the vesicular SNARE protein, vamp2, was present in a fraction of those varicosities. We identified a presumed second class of IPC that is GABAergic but not dopaminergic. Outer retinal varicosities of this putative GABAergic IPC did colocalize synaptic vesicle protein 2a, suggesting they possessed a conventional vesicular release mechanism.

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.

Efficient mucociliary transport relies on efficient regulation of ciliary beating

The respiratory mucociliary epithelium is a synchronized and highly effective waste-disposal system. It uses mucus as a vehicle, driven by beating cilia, to transport unwanted particles, trapped in the mucus, away from the respiratory system. The ciliary machinery can function in at least two different modes: a low rate of beating that requires only ATP, and a high rate of beating regulated by second messengers. The mucus propelling velocity is linearly dependent on ciliary beat frequency (CBF). The linear dependence implies that a substantial increase in transport efficiency requires an equally substantial rise in CBF. The ability to enhance beating in response to various physiological cues is a hallmark of mucociliary cells. An intricate signaling network controls ciliary activity, which relies on interplay between calcium and cyclic nucleotide pathways.

Unraveling the complexities of sphingosine-1-phosphate function: the mast cell model

Sphingosine-1-phosphate (S1P) is a lipid mediator involved in diverse biological processes, from vascular and neural development to the regulation of lymphocyte trafficking. Many of its functions are regulated by five widely expressed S1P G-protein-coupled receptors (S1P(1-5)). S1P is produced mostly intracellularly, thus, much of its potential as an autocrine and paracrine mediator depends on how, when, and where it is generated or secreted out of the cells. However, S1P can also have intracellular activity independent of its receptors, adding to the complexity of S1P function. The mast cell, a major effector cell during an allergic response, has proven instrumental towards understanding the complex regulation and function of S1P. Antigen (Ag) engagement of the IgE receptor in mast cells stimulates sphingosine kinases, which generate S1P and are involved in the activation of calcium fluxes critical for mast cell responses. In addition, mast cells secrete considerable amounts of S1P upon activation, thus affecting the surrounding tissues and recruiting inflammatory cells. Export of S1P is also involved in the autocrine transactivation of S1P receptors present in mast cells. The in vivo response of mast cells, however, is not strictly dependent on their ability to generate S1P, but they are also affected by changes in S1P in the environment previous to Ag challenge. This review will discuss the recent advances towards understanding the intricacies of S1P generation, secretion and regulation in mast cells. In addition, how S1P receptors are activated and their involvement in mast cell functions will also be covered, including new insights on the role of S1P in the mast cell-mediated allergic response of systemic anaphylaxis.

Functional interactions among Orai1, TRPCs, and STIM1 suggest a STIM-regulated heteromeric Orai/TRPC model for SOCE/Icrac channels

Receptor-operated Ca(2+) entry (ROCE) and store-operated Ca(2+) entry (SOCE) into cells are functions performed by all higher eukaryotic cells, and their impairment is life-threatening. The main molecular components of this pathway appear to be known. However, the molecular make-up of channels mediating ROCE and SOCE is largely unknown. One hypothesis proposes SOCE channels to be formed solely by Orai proteins. Another proposes SOCE channels to be composed of both Orai and C-type transient receptor potential (TRPC) proteins. Both hypotheses propose that the channels are activated by STIM1, a sensor of the filling state of the Ca(2+) stores that activates Ca(2+) entry when stores are depleted. The role of Orai in SOCE has been proven. Here we show the TRPC-dependent reconstitution of Icrac, the electrophysiological correlate to SOCE, by expression of Orai1; we also show that R91W-Orai1 can inhibit SOCE and ROCE and that Orai1 and STIM1 expression leads to functional expression of Gd-resistant ROCE. Because channels that mediate ROCE are accepted to be formed with the participation of TRPCs, our data show functional interaction between ROCE and SOCE components. We propose that SOCE/Icrac channels are composed of heteromeric complexes that include TRPCs and Orai proteins.

New insights on mast cell activation via the high affinity receptor for IgE

Mast cells are innate immune cells that function as regulatory or effector cells and serve to amplify adaptive immunity. In adaptive immunity these cells function primarily through cell surface Fc receptors that bind immunoglobulin antibodies. The dysregulation of their adaptive role makes them central players in allergy and asthma. Upon encountering an allergen (antigen), which is recognized by immunoglobulin E (IgE) antibodies bound to the high affinity IgE receptor (FcepsilonRI) expressed on their cell surface, mast cells secrete both preformed and newly synthesized mediators of the allergic response. Blocking of these responses is an objective in therapeutic intervention of allergic diseases. Thus, understanding the mechanisms by which antigens elicit mast cell activation (via FcepsilonRI) holds promise toward identifying therapeutic targets. Here we review the most recent advances in understanding antigen-dependent mast cell activation. Specifically, we focus on the requirements for FcepsilonRI activation, the regulation of calcium responses, co-stimulatory signals in FcepsilonRI-mediated mast cell activation and function, and how genetics influences mast cell signaling and responses. These recent discoveries open new avenues of investigation with therapeutic potential.