Calcium release-activated calcium current (ICRAC) is a direct target for sphingosine

Increased S1P induces S1PR2 internalization to blunt the sensitivity of colorectal cancer to 5-fluorouracil via promoting intracellular uracil generation

Sphingosine-1-phosphate (S1P), the backbone of most sphingolipids, activating S1P receptors (S1PRs) and the downstream G protein signaling has been implicated in chemoresistance. In this study we investigated the role of S1PR2 internalization in 5-fluorouracil (5-FU) resistance in human colorectal cancer (CRC). Clinical data of randomly selected 60 CRC specimens showed the correlation between S1PR2 internalization and increased intracellular uracil (P < 0.001). Then we explored the regulatory mechanisms in CRC model of villin-S1PR2-/- mice and CRC cell lines. We showed that co-administration of S1P promoted S1PR2 internalization from plasma membrane (PM) to endoplasmic reticulum (ER), thus blunted 5-FU efficacy against colorectal tumors in WT mice, compared to that in S1PR2-/- mice. In HCT116 and HT-29 cells, application of S1P (10 μM) empowered S1PR2 to internalize from PM to ER, thus inducing 5-FU resistance, whereas the specific S1PR2 inhibitor JTE-013 (10 μM) effectively inhibited S1P-induced S1PR2 internalization. Using Mag-Fluo-AM-labeling [Ca2+]ER and LC-ESI-MS/MS, we revealed that internalized S1PR2 triggered elevating [Ca2+]ER levels to activate PERK-eLF2α-ATF4 signaling in HCT116 cells. The activated ATF4 upregulated RNASET2-mediated uracil generation, which impaired exogenous 5-FU uptake to blunt 5-FU therapy. Overall, this study reveals a previously unrecognized mechanism of 5-FU resistance resulted from S1PR2 internalization-upregulated uracil generation in colorectal cancer, and provides the novel insight into the significance of S1PR2 localization in predicting the benefit of CRC patients from 5-FU-based chemotherapy.

Sphingolipids and their enigmatic role in asthma

Asthma is defined as a chronic inflammatory condition in the lung and is characterized by episodic shortness of breath with expiratory wheezing and cough. Asthma is a serious public health concern globally with an estimated incidence over 300 million. Asthma is a complex disease in that it manifests as disease of gene and environmental interactions. Sphingolipids are a unique class of lipids involved in a host of biological functions ranging from serving as key cellular membrane lipids to acting as critical signaling molecules. To date sphingolipids have been studied across various human conditions ranging from neurological disorders to cancer to infection to autoimmunity. This review will focus on the role of sphingolipids in asthma development and pathology with particular focus on the role of mast cell sphingolipid biology.

Store-Operated Calcium Channels

Store-operated calcium channels (SOCs) are a major pathway for calcium signaling in virtually all metozoan cells and serve a wide variety of functions ranging from gene expression, motility, and secretion to tissue and organ development and the immune response. SOCs are activated by the depletion of Ca(2+) from the endoplasmic reticulum (ER), triggered physiologically through stimulation of a diverse set of surface receptors. Over 15 years after the first characterization of SOCs through electrophysiology, the identification of the STIM proteins as ER Ca(2+) sensors and the Orai proteins as store-operated channels has enabled rapid progress in understanding the unique mechanism of store-operate calcium entry (SOCE). Depletion of Ca(2+) from the ER causes STIM to accumulate at ER-plasma membrane (PM) junctions where it traps and activates Orai channels diffusing in the closely apposed PM. Mutagenesis studies combined with recent structural insights about STIM and Orai proteins are now beginning to reveal the molecular underpinnings of these choreographic events. This review describes the major experimental advances underlying our current understanding of how ER Ca(2+) depletion is coupled to the activation of SOCs. Particular emphasis is placed on the molecular mechanisms of STIM and Orai activation, Orai channel properties, modulation of STIM and Orai function, pharmacological inhibitors of SOCE, and the functions of STIM and Orai in physiology and disease.

Sphingosine derivatives inhibit cell signaling by electrostatically neutralizing polyphosphoinositides at the plasma membrane

Mast cell stimulation via IgE receptors causes activation of multiple processes, including Ca(2+) mobilization, granule exocytosis, and outward trafficking of recycling endosomes to the plasma membrane. We used fluorescein-conjugated cholera toxin B (FITC-CTxB) to label GM(1) in recycling endsomes and to monitor antigen-stimulated trafficking to the plasma membrane in both fluorimeter and imaging-based assays. We find that the sphingosine derivatives D-sphingosine and N,N'-dimethylsphingosine effectively inhibit this outward trafficking response, whereas a quarternary ammonium derivative, N,N',N″-trimethylsphingosine, does not inhibit. This pattern of inhibition is also found for Ca(2+) mobilization and secretory lysosomal exocytosis, indicating a general effect on Ca(2+)-dependent signaling processes. This inhibition correlates with the capacity of sphingosine derivatives to flip to the inner leaflet of the plasma membrane that is manifested as changes in plasma membrane-associated FITC-CTxB fluorescence and cytoplasmic pH. Using a fluorescently labeled MARCKS effector domain to monitor plasma membrane-associated polyphosphoinositides, we find that these sphingosine derivatives displace the electrostatic binding of this MARCKS effector domain to the plasma membrane in parallel with their capacity to inhibit Ca(2+)-dependent signaling. Our results support roles for plasma membrane polyphosphoinositides in Ca(2+) signaling and stimulated exocytosis, and they illuminate a mechanism by which D-sphingosine regulates signaling responses in mammalian cells.

Diacylglycerol kinase γ regulates antigen-induced mast cell degranulation by mediating Ca(2+) influxes

Diacylglycerol (DAG) is an important lipid that acts as a signaling messenger during mast cell degranulation after allergen cross-linking of immunoglobulin (Ig) E-bound FcεRI receptors. In this study, we determined the role of diacylglycerol kinase (DGK), which negatively regulates DAG-dependent signaling by converting DAG to phosphatidic acid (PA), in the regulation of mast cell degranulation. Treating RBL (rat basophilic leukemia)-2H3 mast cells with a type I DGK inhibitor significantly reduced antigen-induced degranulation and PA production. Among type I DGK isoforms, we observed that DGKα and DGKγ mRNAs were expressed in RBL-2H3 mast cells using reverse transcription polymerase chain reaction. DGKγ knockdown, but not DGKα, by isoform-specific short hairpin RNAs reduced mast cell degranulation and Ca(2+) influxes from the extracellular environment. These results suggest that DGKγ regulates mast cell degranulation after FcεRI cross-linking through mobilization of intracellular Ca(2+) through Ca(2+) influxes.

TRPM3

Like most other members of the TRP family, the Trpm3 gene encodes proteins that form cation-permeable ion channels on the plasma membrane. However, TRPM3 proteins have several unique features that set them apart from the other members of this diverse family. The Trpm3 gene encodes for a surprisingly large number of isoforms generated mainly by alternative splicing. Only for two of the (at least) eight sites at which sequence diversity is generated the functional consequences have been elucidated, one leading to nonfunctional channels, the other one profoundly affecting the ionic selectivity. In the Trpm3 gene an intronic microRNA (miR-204) is co-transcribed with Trpm3. By regulating the expression of a multitude of genes, miR-204 increases the functional complexity of the Trpm3 locus. Over the past years, important progress has been made in discovering pharmacological tools to manipulate TRPM3 channel activity. These substances have facilitated the identification of endogenously expressed functional TRPM3 channels in nociceptive neurons, pancreatic beta cells, and vascular smooth muscle cells, among others. TRPM3 channels, which themselves are temperature sensitive, thus have been implicated in sensing noxious heat, in modulating insulin release, and in secretion of inflammatory cytokines. However, in many tissues where TRPM3 proteins are known to be expressed, no functional role has been identified for these channels so far. Because of the availability of adequate pharmacological and genetic tools, it is expected that future investigations on TRPM3 channels will unravel important new aspects and functions of these channels.

TRPM7

The channel kinases TRPM6 and TRPM7 are fusion proteins with an ion transport domain and an enzymatically active kinase domain. TRPM7 has been found in every mammalian tissue investigated to date. The two-in-one protein structure, the ubiquitous expression profile, and the protein's unique biophysical characteristics that enable divalent ion transport involve TRPM7 in a plethora of (patho)physiological processes. With its prominent role in cellular and systemic magnesium homeostasis, TRPM7 emerges as a key player in embryonic development, global ischemia, cardiovascular disease, and cancer.

Vesicle size determines unitary exocytic properties and their sensitivity to sphingosine

Neuroendocrine cells contain small and large vesicles, but the functional significance of vesicle diameter is unclear. We studied unitary exocytic events of prolactin-containing vesicles in lactotrophs by monitoring discrete steps in membrane capacitance. In the presence of sphingosine, which recruits VAMP2 for SNARE complex formation, the frequency of transient and full fusion events increased. Vesicles with larger diameters proceeded to full fusion, but smaller vesicles remained entrapped in transient exocytosis. The diameter of vesicle dense cores released by full fusion exocytosis into the extracellular space was larger than the diameter of the remaining intracellular vesicles beneath the plasma membrane. Labeling with prolactin- and VAMP2-antibodies revealed a correlation between the diameters of colocalized prolactin- and VAMP2-positive structures. It is proposed that sphingosine-mediated facilitation of regulated exocytosis is not only related to the number of SNARE complexes per vesicle but also depends on the vesicle size, which may determine the transition between transient and full fusion exocytosis.

Sphingosine and FTY720 are potent inhibitors of the transient receptor potential melastatin 7 (TRPM7) channels

BACKGROUND AND PURPOSE

Transient receptor potential melastatin 7 (TRPM7) is a unique channel kinase which is crucial for various physiological functions. However, the mechanism by which TRPM7 is gated and modulated is not fully understood. To better understand how modulation of TRPM7 may impact biological processes, we investigated if TRPM7 can be regulated by the phospholipids sphingosine (SPH) and sphingosine-1-phosphate (S1P), two potent bioactive sphingolipids that mediate a variety of physiological functions. Moreover, we also tested the effects of the structural analogues of SPH, N,N-dimethyl-D-erythro-sphingosine (DMS), ceramides and FTY720 on TRPM7.

EXPERIMENTAL APPROACH

HEK293 cells stably expressing TRPM7 were used for whole-cell, single-channel and macropatch current recordings. Cardiac fibroblasts were used for native TRPM7 current recording.

KEY RESULTS

SPH potently inhibited TRPM7 in a concentration-dependent manner, whereas S1P and other ceramides did not produce noticeable effects. DMS also markedly inhibited TRPM7. Moreover, FTY720, an immunosuppressant and the first oral drug for treatment of multiple sclerosis, inhibited TRPM7 with a similar potency to that of SPH. In contrast, FTY720-P has no effect on TRPM7. It appears that SPH and FTY720 inhibit TRPM7 by reducing channel open probability. Furthermore, endogenous TRPM7 in cardiac fibroblasts was markedly inhibited by SPH, DMS and FTY720.

CONCLUSIONS AND IMPLICATIONS

This is the first study demonstrating that SPH and FTY720 are potent inhibitors of TRPM7. Our results not only provide a new modulation mechanism of TRPM7, but also suggest that TRPM7 may serve as a direct target of SPH and FTY720, thereby mediating S1P-independent physiological/pathological functions of SPH and FTY720.

Sphingosine and the transient receptor potential channel kinase(s)

Transient receptor potential melastatin 7 (TRPM7) is a multifunctional ion channel playing crucial roles during development. TRPM7 is a member of the highly diverse TRP ion channel family. Most TRP channels are regulated by membrane phospholipids, especially phosphoinositides. In this issue of the British Journal of Pharmacology, Qin et al. describes the regulation of TRPM7 and its close homologue TRPM6 by a different kind of membrane lipid: sphingosine. The study finds that sphingosine is a potent and specific inhibitor of TRPM7 and TRPM6 channels. This commentary briefly summarizes the findings of the study, their potential significance and discusses open question and future directions.

Roles, regulation and inhibitors of sphingosine kinase 2

The bioactive sphingolipids ceramide, sphingosine and sphingosine-1-phosphate (S1P) are important signalling molecules that regulate a diverse array of cellular processes. Most notably, the balance of the levels of these three sphingolipids in cells, termed the 'sphingolipid rheostat', can dictate cell fate, where ceramide and sphingosine enhance apoptosis and S1P promotes cell survival and proliferation. The sphingosine kinases (SKs) catalyse the production of S1P from sphingosine and are therefore central regulators of the sphingolipid rheostat and attractive targets for cancer therapy. Two SKs exist in humans: SK1 and SK2. SK1 has been extensively studied and there is a large body of evidence to demonstrate its role in promoting cell survival, proliferation and neoplastic transformation. SK1 is also elevated in many human cancers which appears to contribute to carcinogenesis, chemotherapeutic resistance and poor patient outcome. SK2, however, has not been as well characterized, and there are contradictions in the key physiological functions that have been proposed for this isoform. Despite this, many studies are now emerging that implicate SK2 in key roles in a variety of diseases, including the development of a range of solid tumours. Here, we review the literature examining SK2, its physiological and pathophysiological functions, the current knowledge of its regulation, and recent developments in targeting this complex enzyme.

Mast cell chemotaxis - chemoattractants and signaling pathways

Migration of mast cells is essential for their recruitment within target tissues where they play an important role in innate and adaptive immune responses. These processes rely on the ability of mast cells to recognize appropriate chemotactic stimuli and react to them by a chemotactic response. Another level of intercellular communication is attained by production of chemoattractants by activated mast cells, which results in accumulation of mast cells and other hematopoietic cells at the sites of inflammation. Mast cells express numerous surface receptors for various ligands with properties of potent chemoattractants. They include the stem cell factor (SCF) recognized by c-Kit, antigen, which binds to immunoglobulin E (IgE) anchored to the high affinity IgE receptor (FcεRI), highly cytokinergic (HC) IgE recognized by FcεRI, lipid mediator sphingosine-1-phosphate (S1P), which binds to G protein-coupled receptors (GPCRs). Other large groups of chemoattractants are eicosanoids [prostaglandin E₂ and D₂, leukotriene (LT) B₄, LTD₄, and LTC₄, and others] and chemokines (CC, CXC, C, and CX3C), which also bind to various GPCRs. Further noteworthy chemoattractants are isoforms of transforming growth factor (TGF) β1–3, which are sensitively recognized by TGF-β serine/threonine type I and II β receptors, adenosine, C1q, C3a, and C5a components of the complement, 5-hydroxytryptamine, neuroendocrine peptide catestatin, tumor necrosis factor-α, and others. Here we discuss the major types of chemoattractants recognized by mast cells, their target receptors, as well as signaling pathways they utilize. We also briefly deal with methods used for studies of mast cell chemotaxis and with ways of how these studies profited from the results obtained in other cellular systems.

Sphingosine 1-phosphate, sphingosine kinase and autocrine calcium signalling in thyroid cells

In many cell types metabolites of sphingomyelin have a profound role in cellular signalling. One particular field where these derivatives have obtained a crucial role is calcium signalling. This is an interesting aspect on how lipids may wield their physiological role, as calcium is probably one of the most versatile signalling molecules in the cell, and modulation of calcium signalling may have profound effects on cellular physiology. In this review we discuss a novel aspect of sphingolipid signalling, i.e. the autocrine role of sphingosine 1-phosphate (S1P) in regulating calcium entry in thyroid cells. Although many investigations have highlighted the importance of S1P as a regulator of both calcium release from the endoplasmic reticulum and calcium entry through plasma membrane channels, the autocrine mechanism presented here introduces a new aspect of S1P signalling in thyroid cells. This mechanism may be physiologically relevant in many other cell types, including cancer cells.

Regulators of Ca(2+) signaling in mast cells: potential targets for treatment of mast cell-related diseases?

A calcium signal is essential for degranulation, generation of eicosanoids and optimal production of cytokines in mast cells in response to antigen and other stimulants. The signal is initiated by phospholipase C-mediated production of inositol1,4,5-trisphosphate resulting in release of stored Ca(2+) from the endoplasmic reticulum (ER) and Golgi. Depletion of these stores activates influx of extracellular Ca(2+), usually referred to as store-operated calcium entry (SOCE), through the interaction of the Ca(2+)-sensor, stromal interacting molecule-1 (STIM1 ), in ER with Orai1(CRACM1) and transient receptor potential canonical (TRPC) channel proteins in the plasma membrane (PM). This interaction is enabled by microtubular-directed reorganization of ER to form ER/PM contact points or "punctae" in which STIM1 and channel proteins colocalize. The ensuing influx of Ca(2+) replenishes Ca(2+) stores and sustains elevated levels of cytosolic Ca(2+) ions-the obligatory signal for mast-cell activation. In addition, the signal can acquire spatial and dynamic characteristics (e.g., calcium puffs, waves, oscillations) that encode signals for specific functional outputs. This is achieved by coordinated regulation of Ca(2+) fluxes through ATP-dependent Ca(2+)-pumps and ion exchangers in mitochondria, ER and PM. As discussed in this chapter, studies in mast cells revealed much about the mechanisms described above but little about allergic and autoimmune diseases although studies in other types of cells have exposed genetic defects that lead to aberrant calcium signaling in immune diseases. Pharmacologic agents that inhibit or activate the regulatory components of calcium signaling in mast cells are also discussed along with the prospects for development of novel SOCE inhibitors that may prove beneficial in the treatment inflammatory mast-cell related diseases.

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.

Physiological function and molecular basis of STIM1-mediated calcium entry in immune cells

Calcium signals in immune cells regulate a variety of physiological responses such as cell activation, differentiation, gene transcription, and effector functions. Surface receptor stimulation induces an increase in the concentration of cytosolic calcium ions (Ca2+), which are derived mainly from two sources, intracellular endoplasmic reticulum (ER) Ca2+ stores and the extracellular space. The major cascade for Ca2+ entry in immune cells is through store-operated Ca2+ entry (SOCE) and Ca2+ release-activated Ca2+ (CRAC) channels. Activation of SOCE is triggered by depletion of intracellular ER Ca2+ stores, but the molecular mechanism was a long-standing issue. With the recent molecular identification of the ER Ca2+ sensor [stromal interacting molecule-1 (STIM1)] and a pore-forming subunit of the CRAC channel (Orai1), our understanding of the SOCE activation pathway has increased dramatically. These advances have now made it possible to shed some light on important questions: what is the physiological significance of SOCE, and what is its molecular basis? This review focuses on the recent progress in the field and the exciting opportunities for understanding how SOCE influences diverse immune functions.

Sphingosine kinase as a regulator of calcium entry through autocrine sphingosine 1-phosphate signaling in thyroid FRTL-5 cells

Calcium entry is one of the main regulators of intracellular signaling. Here, we have described the importance of sphingosine, sphingosine kinase 1 (SK1), and sphingosine 1-phosphate (S1P) in regulating calcium entry in thyroid FRTL-5 cells. In cells incubated with the phosphatase inhibitor calyculin A, which evokes calcium entry without mobilizing sequestered intracellular calcium, sphingosine inhibited calcium entry in a concentration-dependent manner. Furthermore, inhibiting SK1 or the ATP-binding cassette ABCC1 multidrug transporter attenuated calcium entry. The addition of exogenous S1P restored calcium entry. Neither sphingosine nor inhibition of SK1 attenuated thapsigargin-evoked calcium entry. Blocking S1P receptor 2 or phospholipase C attenuated calcium entry, whereas blocking S1P receptor 3 did not. Overexpression of wild-type SK1, but not SK2, enhanced calyculin-evoked calcium entry compared with mock-transfected cells, whereas calcium entry was decreased in cells transfected with the dominant-negative G82D SK1 mutant. Exogenous S1P restored calcium entry in G82D cells. Our results suggest that the calcium entry pathway is blocked by sphingosine and that activation of SK1 and the production of S1P, through an autocrine mechanism, facilitate calcium entry through activation of S1P receptor 2. This is a novel mechanism by which the sphingosine-S1P rheostat regulates cellular calcium homeostasis.

Ca2+ waves initiate antigen-stimulated Ca2+ responses in mast cells

Ca(2+) mobilization is central to many cellular processes, including stimulated exocytosis and cytokine production in mast cells. Using single cell stimulation by IgE-specific Ag and high-speed imaging of conventional or genetically encoded Ca(2+) sensors in rat basophilic leukemia and bone marrow-derived rat mast cells, we observe Ca(2+) waves that originate most frequently from the tips of extended cell protrusions, as well as Ca(2+) oscillations throughout the cell that usually follow the initiating Ca(2+) wave. In contrast, Ag conjugated to the tip of a micropipette stimulates local, repetitive Ca(2+) puffs at the region of cell contact. Initiating Ca(2+) waves are observed in most rat basophilic leukemia cells stimulated with soluble Ag and are sensitive to inhibitors of Ca(2+) release from endoplasmic reticulum stores and to extracellular Ca(2+), but they do not depend on store-operated Ca(2+) entry. Knockdown of transient receptor potential channel (TRPC)1 and TRPC3 channel proteins by short hairpin RNA reduces the sensitivity of these cells to Ag and shifts the wave initiation site from protrusions to the cell body. Our results reveal spatially encoded Ca(2+) signaling in response to immunoreceptor activation that utilizes TRPC channels to specify the initiation site of the Ca(2+) response.

Regulation of Ca2+ signaling with particular focus on mast cells

Calcium signals mediate diverse cellular functions in immunological cells. Early studies with mast cells, then a preeminent model for studying Ca2+-dependent exocytosis, revealed several basic features of calcium signaling in non-electrically excitable cells. Subsequent studies in these and other cells further defined the basic processes such as inositol 1,4,5-trisphosphate-mediated release of Ca2+ from Ca2+ stores in the endoplasmic reticulum (ER); coupling of ER store depletion to influx of external Ca2+ through a calcium-release activated calcium (CRAC) channel now attributed to the interaction of the ER Ca2+ sensor, stromal interacting molecule-1 (STIM1), with a unique Ca2+-channel protein, Orai1/CRACM1, and subsequent uptake of excess Ca2+ into ER and mitochondria through ATP-dependent Ca2+ pumps. In addition, transient receptor potential channels and ion exchangers also contribute to the generation of calcium signals that may be global or have dynamic (e.g., waves and oscillations) and spatial resolution for specific functional readouts. This review discusses past and recent developments in this field of research, the pharmacologic agents that have assisted in these endeavors, and the mast cell as an exemplar for sorting out how calcium signals may regulate multiple outputs in a single cell.

Cross-linking of FcepsilonRI causes Ca2+ mobilization via a sphingosine kinase pathway in a clathrin-dependent manner

Clathrin-coated pits are now recognized to be involved in cell signaling in addition to receptor down-regulation. Here we tried to identify signaling pathways that might be dependent on clathrin. Our initial data with pharmacological inhibitors of formation of clathrin-coated pits or lipid-rafts indicated that Ca(2+) response evoked by cross-linking of the high affinity receptors for IgE (FcepsilonRI) was dependent on clathrin. To confirm this finding, we created clathrin-knockdown cells by transfecting the mast cell line RBL-2H3 with a shRNA-clathrin heavy chain construct. In these cells, the FcepsilonRI-mediated Ca(2+) response was almost completely abolished, which was accompanied by the inhibition of sphingosine 1-phosphate (S1P) production with no changes in inositol 1,4,5-trisphosphate (IP(3)) production. This suggests that the Ca(2+) signaling pathway via a sphingosine kinase (SK) is dependent on clathrin. Furthermore, antigen-induced tyrosine phosphorylation of p85 and p110 subunits of PI3K was almost completely inhibited in clathrin-knockdown cells. In contrast, antigen-induced tyrosine phosphorylation of phospholipase Cgamma was not affected by clathrin-knockdown and tyrosine phosphorylation of Syk and degranulation were partially inhibited in clathrin-knockdown cells. The present study identifies the SK/Ca(2+) pathway to be dependent on clathrin.

Molecular clustering of STIM1 with Orai1/CRACM1 at the plasma membrane depends dynamically on depletion of Ca2+ stores and on electrostatic interactions

Activation of store operated Ca(2+) entry involves stromal interaction molecule 1 (STIM1), localized to the endoplasmic reticulum (ER), and calcium channel subunit (Orai1/CRACM1), localized to the plasma membrane. Confocal microscopy shows that thapsigargin-mediated depletion of ER Ca(2+) stores in RBL mast cells causes a redistribution of STIM1, labeled with monomeric red fluorescent protein (mRFP), to micrometer-scale ER-plasma membrane junctions that contain Orai1/CRACM1, labeled with monomeric Aequorea coerulescens green fluorescent protein (AcGFP). Using fluorescence resonance energy transfer (FRET), we determine that this visualized coredistribution is accompanied by nanoscale interaction of STIM1-mRFP and AcGFP-Orai1/CRACM1. We find that antigen stimulation of immunoglobulin E receptors causes much less Orai1/CRACM1 and STIM1 association, but strong interaction is observed under conditions that prevent refilling of ER stores. Stimulated association monitored by FRET is inhibited by sphingosine derivatives in parallel with inhibition of Ca(2+) influx. Similar structural and functional effects are caused by mutation of acidic residues in the cytoplasmic segment of Orai1/CRACM1, suggesting a role for electrostatic interactions via these residues in the coupling of Orai1/CRACM1 to STIM1. Our results reveal dynamic molecular interactions between STIM1 and Orai1/CRACM1 that depend quantitatively on electrostatic interactions and on the extent of store depletion.

N,N-Dimethyl-D-erythro-sphingosine inhibits store-operated Ca2+ entry in U937 monocytes

Calcium is a ubiquitous second messenger that controls a broad range of cellular functions, and store-operated calcium entry (SOCE) is the primary mechanism of regulated Ca(2+) entry in non-excitable immunocytes. In this study, we found that N,N-dimethyl-D-erythro-sphingosine (DMS) inhibited SOCE. In U937 cells, treatment with DMS for 2 h inhibited thapsigargin-induced SOCE by about 70%. DMS inhibited SOCE in a concentration-dependent manner when it was added to the cells after SOCE reached a plateau. DMS-induced SOCE inhibition was also confirmed by the Mn(2+)-quenching method, which monitors only Ca(2+) influx. Because sphingosine kinase inhibitors or protein kinase C (PKC) inhibitors could not mimic the SOCE inhibition, sphingosine kinase and PKC could be excluded as targets of DMS-induced inhibition of SOCE. Furthermore, disruption of lipid rafts with methyl-beta-cyclodextrin and bacterial sphingomyelinase did not influence DMS-induced inhibition of SOCE. DMS-induced inhibition of SOCE in U937 human monocytes is a unique observation and could serve as a basis to study modulation of intracellular Ca(2+) concentration by sphingolipids, although the precise mechanism should be elucidated in the future.

Sphingosine kinase/sphingosine 1-phosphate signalling in central nervous system

Sphingolipids were once regarded as inert structural components of cell membranes. Now these metabolites are generally believed to be important bioactive molecules that control a wide repertoire of cellular processes such as proliferation and survival of cells. Along with these ubiquitous cell functions observed in many peripheral tissues sphingolipid metabolites, especially sphingosine 1-phosphate, exert important neuron-specific functions such as regulation of neurotransmitter release. This review summarizes physiological and pathological roles of sphingolipid metabolites emphasizing the role of sphingosine 1-phosphate in the central nervous system.

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.

Differential signaling of sphingosine derivatives in U937 human monocytes depends on the degree of N-methylation

Previously, we studied N,N-dimethyl-D-erythro-sphingosine (DMS)-induced cell death and signaling in U937 human monocytes; we found that DMS-induced sphingosine kinase- and PKC-independent apoptosis. In the present study, we studied apoptotic responses by three N-methyl derivatives of sphingosine: N-monomethyl-D-erythro-sphingosine (MMS), N,N,N-trimethyl-D-erythro-sphingosine (TMS), and D-erythro-sphingosine (SPH). The potency order in the apoptotic response was DMS>or=MMS>TMS>SPH. We compared cellular responses to the derivatives in terms of activities of MAPK signaling molecules, mitochondrial membrane potential (DeltaPsi(m)), and reactive oxygen species (ROS) generation. Our results suggest that the degree of N-methylation affects the apoptosis-inducing capacity and other related responses including MAPK modulation, DeltaPsi(m), and ROS generation. Dimethylation and monomethylation on the C2 amine of sphingosine enhance the apoptotic response; however, trimethylation induces differential modulation of signaling molecules and less cytotoxicity. Our investigation will be useful for understanding the actions of sphingolipids in apoptosis and for developing chemotherapeutics based on DMS structure.

N, N-dimethyl-D-erythro-sphingosine increases intracellular Ca2+ concentration via Na+-Ca2+-exchanger in HCT116 human colon cancer cells

N,N-dimethyl-D-erythro-sphingosine (DMS), an N-methyl derivative of sphingosine, is an inhibitor of protein kinase C (PKC) and sphingosine kinase (SK). In previous reports, DMS-induced intracellular Ca2+ increase concentration ([Ca2+]i) was studied in T lymphocytes, monocytes, astrocytes and neuronal cells. In the present study, we studied DMS-induced increase of [Ca2+]i in HCT116 human colon cancer cells. We found that the DMS-induced increase of [Ca2+]i in colon cancer cells is composed of Ca2+ release from intracellular Ca2+ stores and subsequent Ca2+ influx. The Ca2+ release is not related to modulation of inositol 1,4,5-trisphosphate (IP3) receptor or ryanodine receptor. On the other hand, the Ca2+ influx is mediated largely through Ca2+ channels sensitive to verapamil, nifedipine, Ga3+, and La3+. Furthermore, we found that the response is inhibited by bepridil and Ni2+, specific inhibitors of Na+-Ca2+-exchanger, suggesting involvement of Na+-Ca2+ exchanger in the DMS-induced [Ca2+]i increase in colon cancer cells. This inhibition was also observed in U937 monocytes, but not in 1321N1 astrocytes.

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.

Essential function for the calcium sensor STIM1 in mast cell activation and anaphylactic responses

Mast cells have key functions as effectors of immunoglobulin E-mediated allergic inflammatory diseases. Allergen stimulation induces Ca2+ influx and elicits the secretion of inflammatory mediators from mast cells. Here we show that the Ca2+-binding endoplasmic reticulum protein STIM1 is critical to mast cell function. STIM1-deficient fetal liver-derived mast cells had impaired Ca2+ influx mediated by the high-affinity immunoglobulin E receptor FcepsilonRI and activation of the transcription factors NF-kappaB and NFAT. Mast cells lacking STIM1 also had much less degranulation and cytokine production after FcepsilonRI stimulation. In addition, alterations in STIM1 expression affected the sensitivity of immunoglobulin E-mediated immediate-phase anaphylactic responses in vivo. Thus, STIM1 is key in promoting the Ca2+ influx that is essential for FcepsilonRI-mediated mast cell activation and anaphylaxis.

New developments in FcepsilonRI regulation, function and inhibition

The high-affinity Fc receptor for IgE (FcepsilonRI), a multimeric immune receptor, is a crucial structure for IgE-mediated allergic reactions. In recent years, advances have been made in several important areas of the study of FcepsilonRI. The first area relates to FcepsilonRI-mediated biological responses that are antigen independent. The second area encompasses the biological relevance of the distinct signalling pathways that are activated by FcepsilonRI; and the third area relates to the accumulated evidence for the tight control of FcepsilonRI signalling through a broad array of inhibitory mechanisms, which are being developed into promising therapeutic approaches.

The sphingosine kinase-sphingosine-1-phosphate axis is a determinant of mast cell function and anaphylaxis

Sphingosine-1-phosphate, a key mediator in immune cell trafficking, is elevated in the lungs of asthmatic patients and regulates pulmonary epithelium permeability. Stimulation of mast cells by allergens induces two mammalian sphingosine kinases (Sphk1 and Sphk2) to produce sphingosine-1-phosphate (S1P). Little is known about the individual role of these kinases in regulating immune cell function. Here we show that in mast cells, Sphk2 is required for production of S1P, for calcium influx, for activation of protein kinase C, and for cytokine production and degranulation. However, susceptibility to in vivo anaphylaxis is determined both by S1P within the mast cell compartment and by circulating S1P generated by Sphk1 predominantly from a non-mast cell source(s). Thus, sphingosine kinases are determinants of mast cell responsiveness, demonstrating a previously unrecognized relationship with anaphylaxis.

Normal acute and chronic inflammatory responses in sphingosine kinase 1 knockout mice

Sphingosine-1-phosophate, generated from the phosphorylation of sphingosine by sphingosine kinase enzymes, is suggested to function as an intracellular second messenger for inflammatory mediators, including formyl peptide, C5a, and Fc. More recently, a role for sphingosine kinases during inflammation has also been proposed. Here we show that sphingosine kinase 1 knockout mice exhibit normal inflammatory cell recruitment during thioglycollate-induced peritonitis and that sphingosine kinase 1-null neutrophils respond normally to formyl peptide. In the collagen-induced arthritis model of rheumatoid arthritis, sphingosine kinase 1 knockout mice developed arthritis with normal incidence and severity. Our findings show that sphingosine kinase 1 is dispensable for inflammatory responses and support the need for more extensive studies of sphingosine kinases in inflammation.

Sphingosine-1-phosphate via activation of a G-protein-coupled receptor(s) enhances the excitability of rat sensory neurons

Sphingosine-1-phosphate (S1P) is released by immune cells and is thought to play a key role in chemotaxis and the onset of the inflammatory response. The question remains whether this lipid mediator also contributes to the enhanced sensitivity of nociceptive neurons that is associated with inflammation. Therefore we examined whether S1P alters the excitability of small diameter, capsaicin-sensitive sensory neurons by measuring action potential (AP) firing and two of the membrane currents critical in regulating the properties of the AP. External application of S1P augments the number of APs evoked by a depolarizing current ramp. The enhanced firing is associated with a decrease in the rheobase and an increase in the resistance at firing threshold although neither the firing threshold nor the resting membrane potential are changed. Treatment with S1P enhanced the tetrodotoxin-resistant sodium current and decreased the total outward potassium current (IK). When sensory neurons were internally perfused with GDP-beta-S, a blocker of G protein activation, the S1P-induced increase in APs was completely blocked and suggests the excitatory actions of S1P are mediated through G-protein-coupled receptors called endothelial differentiation gene or S1PR. In contrast, internal perfusion with GDP-beta-S and S1P increased the number of APs evoked by the current ramp. These results and our finding that the mRNAs for S1PRs are expressed in both the intact dorsal root ganglion and cultures of adult sensory neurons supports the notion that S1P acts on S1PRs linked to G proteins. Together these findings demonstrate that S1P can regulate the excitability of small diameter sensory neurons by acting as an external paracrine-type ligand through activation of G-protein-coupled receptors and thus may contribute to the hypersensitivity during inflammation.

Death or survival: Membrane ceramide controls the fate and activation of antigen-specific T-cells depending on signal strength and duration

Sphingomyelinase (SMase)-mediated release of ceramide in the plasma membrane of T-lymphocytes induced by different stimuli such as ligation of Fas/CD95, irradiation, stress, inflammation or anticancer drugs primarily involves mitochondrial apoptosis signaling, but under specific conditions non-apoptotic Fas-signaling was also reported. Here we investigated, using a quantitative simulation model with exogenous C2-ceramide (and SMase), the dependence of activation and fate of T-cells on the strength and duration of ceramide accumulation. A murine, influenza virus hemagglutinin-specific T-helper cell (IP12-7) alone or together with interacting antigen presenting B-cells (APC) was used. C2-ceramide induced apoptosis of TH cells above a 'threshold' stimulus (>25 microM in 'strength' or >30 min in duration), while below the threshold C2-ceramide was non-apoptotic, as confirmed by early and late apoptotic markers (PS-translocation, mitochondrial depolarization, caspase-3 activation, DNA-fragmentation). The modest ceramide stimuli strongly suppressed the calcium response and inhibited several downstream signal events (e.g. ERK1/2-, JNK-phosphorylation, CD69 expression or IL-2 production) in TH cells during both anti-CD3 induced and APC-triggered activation. Ceramide moderately affected the Ca2+ -release from internal stores upon antigen-specific engagement of TCR in immunological synapses, while the influx phase was remarkably reduced in both amplitude and rate, suggesting that the major target(s) of ceramide-effects are membrane-proximal. Ceramide inhibited Kv1.3 potassium channels, store operated Ca2+ -entry (SOC) and depolarized the plasma membrane to which contribution of spontaneously formed ceramide channels is possible. The impaired function of these transporters may be coupled to the quantitative, membrane raft-remodeling effect of ceramide and responsible, in a concerted action, for the suppressed activation. Our results suggest that non-apoptotic Fas stimuli, received from previously activated, FasL+ interacting lymphocytes in the lymph nodes, may negatively regulate subsequent antigen-specific T-cell activation and thus modulate the antigen-specific T-cell response.

Antigen-induced Ca2+ mobilization in RBL-2H3 cells: Role of I(1,4,5)P3 and S1P and necessity of I(1,4,5)P3 production

Inositol 1,4,5-trisphosphate (IP3) has long been recognized as a second messenger for intracellular Ca2+ mobilization. Recently, sphingosine 1-phosphate (S1P) has been shown to be involved in Ca2+ release from the endoplasmic reticulum (ER). Here, we investigated the role of S1P and IP3 in antigen (Ag)-induced intracellular Ca2+ mobilization in RBL-2H3 mast cells. Antigen-induced intracellular Ca2+ mobilization was only partially inhibited by the sphingosine kinase inhibitor dl-threo-dihydrosphingosine (DHS) or the IP3 receptor inhibitor 2-aminoethoxydiphenyl borate (2-APB), whereas preincubation with both inhibitors led to complete inhibition. In contrast, stimulation of A3 adenosine receptors with N5-ethylcarboxamidoadenosine (NECA) caused intracellular Ca2+ mobilization that was completely abolished by 2-APB but not by DHS, suggesting that NECA required only the IP3 pathway, while antigen used both the IP3 and S1P pathways. Interestingly, however, inhibition of IP3 production with the phospholipase C inhibitor U73122 completely abolished Ca2+ release from the ER induced by either stimulant. This suggested that S1P alone, without concomitant production of IP3, would not cause intracellular Ca2+ mobilization. This was further demonstrated in some clones of RBL-2H3 cells excessively overexpressing a beta isoform of Class II phosphatidylinositol 3-kinase (PI3KC2beta). In such clones including clone 5A4C, PI3KC2beta was overexpressed throughout the cell, although endogenous PI3KC2beta was normally expressed only in the ER. Overexpression of PI3KC2beta in the cytosol and the PM led to depletion of phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2), resulting in a marked reduction in IP3 production. This could explain the abolishment of intracellular Ca2+ mobilization in clone 5A4C. Supporting this hypothesis, the Ca2+ mobilization was reconstituted by the addition of exogenous PI(4,5)P2 in these cells. Our results suggest that both IP3 and S1P contribute to FcvarepsilonRI-induced Ca2+ release from the ER and production of IP3 is necessary for S1P to cause Ca2+ mobilization from the ER.

Effect of sphingosine on Ca2+ entry and mitochondrial potential of Jurkat T cells--interaction with Bcl2

Triggers of Jurkat T cell apoptosis include sphingosine and ceramide. Sphingosine and ceramide further inhibit capacitative Ca2+ entry (ICRAC), an effect leading to inactivation but not death of Jurkat T cells. Mitochondria are key organelles in the machinery leading to apoptosis and on the other hand have been shown to participate in the regulation of Ca2+ entry. The present experiments were performed to explore whether treatment of Jurkat T cells with sphingosine leads to apoptosis and reduced Ca2+ entry and whether those effects are sensitive to expression of the antiapoptotic protein Bcl2, localized in the outer mitochondrial membrane. Exposure of Jurkat T cells to 10 microM spingosine was according to DiOC6 fluorescence followed by mitochondrial depolarization and according to Fura-red/Fluo-3 fluorescence followed by decreased capacitative Ca2+ entry. Mitochondrial depolarization was significantly delayed in cells overexpressing wild type Bcl2 or Bcl2 targeted to the mitochondrial membrane, whereas no significant influence on mitochondrial depolarization was observed in cells expressing Bcl2 lacking the membrane targeting motif or Bcl2 targeted to the endoplasmatic reticulum. In contrast to mitochondrial potential, the blunting of capacitative Ca2+ entry following sphingosine treatment was not sensitive to mitochondrial Bcl2 expression. In conclusion sphingosine exposure leads to both, mitochondrial depolarization and inhibition of capacitative Ca2+ entry. Mitochondrial Bcl2 reverses the effect on mitochondria but not on Ca2+ entry and thus leads to dissociation of those two sequelae of sphingosine treatment.

Sphingosine-1-phosphate activates BKCa channels independently of G protein-coupled receptor in human endothelial cells

The effect of sphingosine-1-phosphate (S1P) on large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels was examined in primary cultured human umbilical vein endothelial cells by measuring intracellular Ca(2+) concentration ([Ca(2+)](i)), whole cell membrane currents, and single-channel activity. In nystatin-perforated current-clamped cells, S1P hyperpolarized the membrane and simultaneously increased [Ca(2+)](i). [Ca(2+)](i) and membrane potentials were strongly correlated. In whole cell clamped cells, BK(Ca) currents were activated by increasing [Ca(2+)](i) via cell dialysis with pipette solution, and the activated BK(Ca) currents were further enhanced by S1P. When [Ca(2+)](i) was buffered at 1 microM, the S1P concentration required to evoke half-maximal activation was 403 +/- 13 nM. In inside-out patches, when S1P was included in the bath solution, S1P enhanced BK(Ca) channel activity in a reversible manner and shifted the relationship between Ca(2+) concentration in the bath solution and the mean open probability to the left. In whole cell clamped cells or inside-out patches loaded with guanosine 5'-O-(2-thiodiphosphate) (GDPbetaS; 1 mM) using a patch pipette, GDPbetaS application or pretreatment of cells with pertussis toxin (100 ng/ml) for 15 h did not affect S1P-induced BK(Ca) current and channel activation. These results suggest that S1P enhances BK(Ca) channel activity by increasing Ca(2+) sensitivity. This channel activation hyperpolarizes the membrane and thereby increases Ca(2+) influx through Ca(2+) entry channels. Inasmuch as S1P activates BK(Ca) channels via a mechanism independent of G protein-coupled receptors, S1P may be a component of the intracellular second messenger that is involved in Ca(2+) mobilization in human endothelial cells.

Sphingosine-1-phosphate: an emerging therapeutic target

Sphingosine-1-phosphate (SPP) is a polar sphingolipid metabolite that has received increasing attention as both an extracellular mediator and an intracellular second messenger. SPP is the ligand of a family of specific cell surface G-protein coupled receptors (GPCR), known as the endothelial differentiation gene-1 (EDG-1) family. These receptors, which include EDG-1, -3, -5, -6 and -8, regulate diverse processes including cell migration, angiogenesis, vascular maturation, heart development, neurite retraction and soma rounding. In addition, abundant evidence indicates that SPP also acts as an intracellular lipid messenger, regulating calcium mobilisation, cell growth and survival. The relative intracellular level of SPP and ceramide, another sphingolipid metabolite associated with cell death and cell growth arrest, is an important factor in determining cell fate. Changes in SPP and ceramide have been implicated in a number of pathological conditions in which apoptosis plays an important role, including cancer and neurodegenerative disorders, as well as in atherosclerosis and allergic responses. This review will examine the biosynthesis, metabolism and potential functions of SPP in diverse diseases in order to illuminate targets for the pharmaceutical and therapeutic manipulation of SPP levels.

Arachidonic acid inhibits the store-operated Ca2+ current in rat liver cells

Vasopressin and other phospholipase-C-coupled hormones induce oscillations (waves) of [Ca2+]cyt (cytoplasmic Ca2+ concentration) in liver cells. Maintenance of these oscillations requires replenishment of Ca2+ in intracellular stores through Ca2+ inflow across the plasma membrane. While this may be achieved by SOCs (store-operated Ca2+ channels), some studies in other cell types indicate that it is dependent on AA (arachidonic acid)-activated Ca2+ channels. We studied the effects of AA on membrane conductance of rat liver cells using whole-cell patch clamping. We found no evidence that concentrations of AA in the physiological range could activate Ca2+-permeable channels in either H4IIE liver cells or rat hepatocytes. However, AA (1-10 microM) did inhibit (IC50=2.4+/-0.1 microM) Ca2+ inflow through SOCs (ISOC) initiated by intracellular application of Ins(1,4,5)P3 in H4IIE cells. Pre-incubation with AA did not inhibit ISOC development, but decreased maximal amplitude of the current. Iso-tetrandrine, widely used to inhibit receptor-activation of phospholipase A2, and therefore AA release, inhibited ISOC directly in H4IIE cells. It is concluded that (i) in rat liver cells, AA does not activate an AA-regulated Ca2+-permeable channel, but does inhibit SOCs, and (ii) iso-tetrandrine and tetrandrine are effective blockers of CRAC (Ca2+-release-activated Ca2+) channel-like SOCs. These results indicate that AA-activated Ca2+-permeable channels do not contribute to hormone-induced increases or oscillations in [Ca2+]cyt in liver cells. However, AA may be a physiological modulator of Ca2+ inflow in these cells.

The mammalian melastatin-related transient receptor potential cation channels: an overview

The mammalian melastatin-related transient receptor potential (TRPM) subfamily contains eight members. TRPM proteins, consisting of six putative transmembrane domains and intracellular N and C termini, form monovalent-permeable cation channels with variable selectivity for Ca(2+), Mg(2+) and other divalent cations. Some functions are linked to their individual cation selectivity: the highly divalent-permeable cation channels TRPM6 and TRPM7 are involved in the control of Mg(2+) influx, whereas the Ca(2+)-impermeable channels TRPM4 and TRPM5 modulate cellular Ca(2+) entry by determining the membrane potential. TRPM2, TRPM3 and TRPM8 mediate a direct influx of Ca(2+) in response to specific stimuli. Electrophysiological properties of the founding member, melastatin (TRPM1), are unexplored. The individual TRPM members are activated by different stimuli, including voltage, Ca(2+), temperature, cell swelling, lipid compounds and other endogenous or exogenous ligands. This review summarizes molecular features, activation mechanisms, biophysical properties and modulators of TRPM channels.

Ceramide increase cytoplasmic Ca2+ concentration in Jurkat T cells by liberation of calcium from intracellular stores and activation of a store-operated calcium channel

The effect of ceramide on the cytoplasmic Ca2+ concentration ([Ca2+]i) varies depending on the cell type. We have found that in Jurkat human T cells ceramide increases the [Ca2+]i from a thapsigargin-sensitive calcium pool and the subsequent activation of a capacitative Ca2+ entry. This effect occurs both in the presence and in the absence of extracellular calcium. Addition of ceramine, a non-hydrolysable analogue of ceramide, reproduced its effect on the [Ca2+]i ruling out that this is due to the conversion of ceramide to sphingosine. The effect of ceramide was additive to that obtained by sphingosine, but not to the Jurkat T cells specific antibody OKT3. However, different to the latter, ceramide do not induced an elevation of InsP3. The opening of a store operated Ca2+ channel by ceramide was corroborated by experiments of Fura-2 quenching, using Mn2+ as a surrogate for Ca2+ and confirmed by whole-cell recording patch clamp techniques.

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.

Sphingolipids and the balancing of immune cell function: lessons from the mast cell

Recent studies reveal that metabolites of sphingomyelin are critically important for initiation and maintenance of diverse aspects of immune cell activation and function. The conversion of sphingomyelin to ceramide, sphingosine, or sphingosine-1-phosphate (S1P) provides interconvertible metabolites with distinct biological activities. Whereas ceramide and sphingosine function to induce apoptosis and to dampen mast cell responsiveness, S1P functions as a chemoattractant and can up-regulate some effector responses. Many of the S1P effects are mediated through S1P receptor family members (S1P(1-5)). S1P(1), which is required for thymocyte emigration and lymphocyte recirculation, is also essential for Ag-induced mast cell chemotaxis, whereas S1P(2) is important for mast cell degranulation. S1P is released to the extracellular milieu by Ag-stimulated mast cells, enhancing inflammatory cell functions. Modulation of S1P receptor expression profiles, and of enzymes involved in sphingolipid metabolism, particularly sphingosine kinases, are key in balancing mast cell and immune cell responses. Current efforts are unraveling the complex underlying mechanisms regulating the sphingolipid pathway. Pharmacological intervention of these key processes may hold promise for controlling unwanted immune responses.

Activation of the melastatin-related cation channel TRPM3 by D-erythro-sphingosine [corrected]

TRPM3, a member of the melastatin-like transient receptor potential channel subfamily (TRPM), is predominantly expressed in human kidney and brain. TRPM3 mediates spontaneous Ca2+ entry and nonselective cation currents in transiently transfected human embryonic kidney 293 cells. Using measurements with the Ca2+-sensitive fluorescent dye fura-2 and the whole-cell patch-clamp technique, we found that D-erythro-sphingosine, a metabolite arising during the de novo synthesis of cellular sphingolipids, activated TRPM3. Other transient receptor potential (TRP) channels tested [classic or canonical TRP (TRPC3, TRPC4, TRPC5), vanilloid-like TRP (TRPV4, TRPV5, TRPV6), and melastatin-like TRP (TRPM2)] did not significantly respond to application of sphingosine. Sphingosine-induced TRPM3 activation was not mediated by inhibition of protein kinase C, depletion of intracellular Ca2+ stores, and intracellular conversion of sphingosine to sphingosine-1-phosphate. Although sphingosine-1-phosphate and ceramides had no effect, two structural analogs of sphingosine, dihydro-D-erythro-sphingosine and N,N-dimethyl-D-erythro-sphingosine, also activated TRPM3. Sphingolipids, including sphingosine, are known to have inhibitory effects on a variety of ion channels. Thus, TRPM3 is the first ion channel activated by sphingolipids.

Ceramide 1-phosphate enhances calcium entry through voltage-operated calcium channels by a protein kinase C-dependent mechanism in GH4C1 rat pituitary cells

Sphingomyelin derivatives modulate a multitude of cellular processes, including the regulation of [Ca2+]i (the intracellular free calcium concentration). Previous studies have shown that these metabolites often inhibit calcium entry through VOCCs (voltage-operated calcium channels). In the present study, we show that, in pituitary GH4C1 cells, C1P (C2-ceramide 1-phosphate) enhances calcium entry in a dose-dependent manner. The phospholipase C inhibitor U73122 attenuated the response. C1P invoked a small, but significant, increase in the formation of inositol phosphates. Pre-treatment of the cells with pertussis toxin was without an effect on the C1P-evoked increase in [Ca2+]i. The effect of C1P was critically dependent on extracellular calcium, since no increase in [Ca2+]i was observed when cells in a calcium-free buffer were stimulated with C1P. Furthermore, if the cells were retreated with 300 nM of the VOCC inhibitor nimodipine, the effect of C1P was almost totally abolished. In addition, ceramide C8-1-phosphate evoked an increase in [Ca2+]i, but the onset of the response was slow compared with that of C1P. In cells treated with 1 mM thapsigargin for 15 min, C1P still evoked an increase in [Ca2+]i. In patch-clamp experiments in the whole-cell mode, C1P enhanced calcium entry through the VOCCs compared with vehicle-treated cells. Dialysis of the cells with C1P did not enhance the calcium current. On-cell patch-clamp experiments showed an enhanced probability of the VOCCs being open (P(open)) in the presence of C1P. Inhibition of PKC (protein kinase C) with GF109203X and down-regulation of PKC with PMA attenuated the C1P-evoked increase in [Ca2+]i. Furthermore, down-regulation of PKC abolished the effect of C1P on P(open). This is the first report showing that a sphingomyelin derivative enhances calcium entry through VOCCs.

Sphingolipid metabolites in neural signalling and function

Sphingolipid metabolites, such as ceramide, sphingosine, sphingosine-1-phosphate (S1P) and complex sphingolipids (gangliosides), are recognized as molecules capable of regulating a variety of cellular processes. The role of sphingolipid metabolites has been studied mainly in non-neuronal tissues. These studies have underscored their importance as signals transducers, involved in control of proliferation, survival, differentiation and apoptosis. In this review, we will focus on studies performed over the last years in the nervous system, discussing the recent developments and the current perspectives in sphingolipid metabolism and functions.