Molecular cloning and characterization of SCaMPER, a sphingolipid Ca2+ release-mediating protein from endoplasmic reticulum

Sphingosylphosphorylcholine induces itch via activation of TRPM3 and TRPA1 in mice

Itch is a prevalent symptom in atopic dermatitis (AD), often leading to a strong urge to scratch. Elevated levels of sphingosylphosphorylcholine (SPC) are found in the stratum corneum of AD patients, and while SPC is known to induce itch, its molecular targets are not well understood. This study aims to identify the signaling pathway of SPC-induced itch under AD conditions. We demonstrate that SPC specifically activates the Transient Receptor Potential Melastatin 3 (TRPM3) channel in sensory neurons. In HEK293T cells expressing TRPM3, SPC treatment caused a significant increase in intracellular calcium, which was inhibited by TRPM3 antagonists. Among various TRP channels tested, TRPM3 exhibited the highest reactivity to SPC, followed by TRPA1. Molecular docking analysis also supported interactions between SPC and both TRPM3 and TRPA1. In an AD mouse model, SPC-induced responses were dependent on TRPM3 and TRPA1, and the expression of these channels increased in dorsal root ganglion neurons. SPC-induced scratching behaviors were significantly reduced by TRPM3 and TRPA1 antagonists, with TRPM3 playing a critical role in spontaneous scratching. This study identifies TRPM3 and TRPA1 as key mediators of SPC-induced itch, providing potential therapeutic targets for treating itch in AD patients.

Cellular and mitochondrial calcium communication in obstructive lung disorders

Calcium (Ca²⁺) signalling is well known to dictate cellular functioning and fate. In recent years, the accumulation of Ca²⁺ in the mitochondria has emerged as an important factor in Chronic Respiratory Diseases (CRD) such as Asthma and Chronic Obstructive Pulmonary Disease (COPD). Various reports underline an aberrant increase in the intracellular Ca²⁺, leading to mitochondrial ROS generation, and further activation of the apoptotic pathway in these diseases. Mitochondria contribute to Ca²⁺ buffering which in turn regulates mitochondrial metabolism and ATP production. Disruption of this Ca²⁺ balance leads to impaired cellular processes like apoptosis or necrosis and thus contributes to the pathophysiology of airway diseases. This review highlights the key role of cytoplasmic and mitochondrial Ca²⁺ signalling in regulating CRD, such as asthma and COPD. A better understanding of the dysregulation of mitochondrial Ca²⁺ homeostasis in these diseases could provide cues for the development of advanced therapeutic interventions in these diseases.

NAADP Receptors

Of the established Ca²⁺-mobilizing messengers, NAADP is arguably the most tantalizing. It is the most potent, often efficacious at low nanomolar concentrations, and its receptors undergo dramatic desensitization. Recent studies have identified a new class of calcium-release channel, the two-pore channels (TPCs), as the likely targets for NAADP regulation, even though the effect may be indirect. These channels localized at endolysosomes, where they mediate local Ca²⁺ release, and have highlighted a new role of acidic organelles as targets for messenger-evoked Ca²⁺ mobilization. Three distinct roles of TPCs have been identified. The first is to effect local Ca²⁺ release that may play a role in endolysosomal function including vesicular fusion and trafficking. The second is to trigger global calcium release by recruiting Ca²⁺-induced Ca²⁺-release (CICR) channels at lysosomal-endoplasmic reticulum (ER) junctions. The third is to regulate plasma membrane excitability by the targeting of Ca²⁺ release from appropriately positioned subplasma membrane stores to regulate plasma membrane Ca²⁺-activated channels. In this review, I discuss the role of nicotinic acid adenine nucleotide diphosphate (NAADP)-mediated Ca²⁺ release from endolysosomal stores as a widespread trigger for intracellular calcium signaling mechanisms, and how studies of TPCs are beginning to enhance our understanding of the central role of lysosomes in Ca²⁺ signaling.

Extracellular and intracellular sphingosine-1-phosphate distinctly regulates exocytosis in chromaffin cells

Sphingosine-1-phosphate (S1P) is an essential bioactive sphingosine lipid involved in many neurological disorders. Sphingosine kinase 1 (SphK1), a key enzyme for S1P production, is concentrated in presynaptic terminals. However, the role of S1P/SphK1 signaling in exocytosis remains elusive. By detecting catecholamine release from single vesicles in chromaffin cells, we show that a dominant negative SphK1 (SphK1^DN ) reduces the number of amperometric spikes and increases the duration of foot, which reflects release through a fusion pore, implying critical roles for S1P in regulating the rate of exocytosis and fusion pore expansion. Similar phenotypes were observed in chromaffin cells obtained from SphK1 knockout mice compared to those from wild-type mice. In addition, extracellular S1P treatment increased the number of amperometric spikes, and this increase, in turn, was inhibited by a selective S1P3 receptor blocker, suggesting extracellular S1P may regulate the rate of exocytosis via activation of S1P3. Furthermore, intracellular S1P application induced a decrease in foot duration of amperometric spikes in control cells, indicating intracellular S1P may regulate fusion pore expansion during exocytosis. Taken together, our study represents the first demonstration that S1P regulates exocytosis through distinct mechanisms: extracellular S1P may modulate the rate of exocytosis via activation of S1P receptors while intracellular S1P may directly control fusion pore expansion during exocytosis. OPEN SCIENCE BADGES: This article has received a badge for *Open Materials* because it provided all relevant information to reproduce the study in the manuscript. The complete Open Science Disclosure form for this article can be found at the end of the article. More information about the Open Practices badges can be found at https://cos.io/our-services/open-science-badges/.

Intracellular Ca(2+) channels - a growing community

The Ca(2+) signals that control almost every cellular activity are generated by regulating Ca(2+) transport, usually via Ca(2+)-permeable channels, across the plasma membrane or the membranes of intracellular organelles. The most widespread and best understood of the intracellular Ca(2+) channels are inositol trisphosphate receptors (IP(3)R) and ryanodine receptors, most of which are expressed in the endoplasmic or sarcoplasmic reticulum. However, accumulating evidence suggests physiological roles for many additional Ca(2+) channels in both ER and other intracellular organelles. Interactions between these channels, whether mediated by Ca(2+) itself or interactions between proteins, is a recurrent feature of the Ca(2+) signals evoked by physiological stimuli. We focus on two specific examples, clustering of IP(3)Rs and NAADP (nicotinic acid dinucleotide phosphate)-evoked Ca(2+) release from endo-lysosomes, to illustrate the diversity of Ca(2+) channels and the interplay between them.

Photolysis of caged sphingosine-1-phosphate induces barrier enhancement and intracellular activation of lung endothelial cell signaling pathways

Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid that mediates cellular functions by ligation via G protein-coupled S1P receptors. In addition to its extracellular action, S1P also has intracellular effects; however, the signaling pathways modulated by intracellular S1P remain poorly defined. We have previously demonstrated a novel pathway of intracellular S1P generation in human lung endothelial cells (ECs). In the present study, we examined the role of intracellular S1P generated by photolysis of caged S1P on EC barrier regulation and signal transduction. Intracellular S1P released from caged S1P caused mobilization of intracellular calcium, induced activation of MAPKs, redistributed cortactin, vascular endothelial cadherin, and β-catenin to cell periphery, and tightened endothelial barrier in human pulmonary artery ECs. Treatment of cells with pertussis toxin (PTx) had no effect on caged S1P-mediated effects on Ca(2+) mobilization, reorganization of cytoskeleton, cell adherens junction proteins, and barrier enhancement; however, extracellular S1P effects were significantly attenuated by PTx. Additionally, intracellular S1P also activated small GTPase Rac1 and its effector Ras GTPase-activating-like protein IQGAP1, suggesting involvement of these proteins in the S1P-mediated changes in cell-to-cell adhesion contacts. Downregulation of sphingosine kinase 1 (SphK1), but not SphK2, with siRNA or inhibition of SphK activity with an inhibitor 2-(p-hydroxyanilino)-4-(p-chlorophenyl) thiazole (CII) attenuated exogenously administrated S1P-induced EC permeability. Furthermore, S1P1 receptor inhibitor SB649164 abolished exogenous S1P-induced transendothelial resistance changes but had no effect on intracellular S1P generated by photolysis of caged S1P. These results provide evidence that intracellular S1P modulates signal transduction in lung ECs via signaling pathway(s) independent of S1P receptors.

Regulation of ryanodine receptors by sphingosylphosphorylcholine: involvement of both calmodulin-dependent and -independent mechanisms

Sphingosylphosphorylcholine (SPC), a lipid mediator with putative second messenger functions, has been reported to regulate ryanodine receptors (RyRs), Ca2+ channels of the sarco/endoplasmic reticulum. RyRs are also regulated by the ubiquitous Ca2+ sensor calmodulin (CaM), and we have previously shown that SPC disrupts the complex of CaM and the peptide corresponding to the CaM-binding domain of the skeletal muscle Ca2+ release channel (RyR1). Here we report that SPC also displaces Ca2+-bound CaM from the intact RyR1, which we hypothesized might lead to channel activation by relieving the negative feedback Ca2+CaM exerts on the channel. We could not demonstrate such channel activation as we have found that SPC has a direct, CaM-independent inhibitory effect on channel activity, confirmed by both single channel measurements and [3H]ryanodine binding assays. In the presence of Ca2+CaM, however, the addition of SPC did not reduce [3H]ryanodine binding, which we could explain by assuming that the direct inhibitory action of the sphingolipid was negated by the simultaneous displacement of inhibitory Ca2+CaM. Additional experiments revealed that RyRs are unlikely to be responsible for SPC-elicited Ca2+ release from brain microsomes, and that SPC does not exert detergent-like effects on sarcoplasmic reticulum vesicles. We conclude that regulation of RyRs by SPC involves both CaM-dependent and -independent mechanisms, thus, the sphingolipid might play a physiological role in RyR regulation, but channel activation previously attributed to SPC is unlikely.

Dissociation of calmodulin-target peptide complexes by the lipid mediator sphingosylphosphorylcholine: implications in calcium signaling

Previously we have identified the lipid mediator sphingosylphosphorylcholine (SPC) as the first potentially endogenous inhibitor of the ubiquitous Ca2+ sensor calmodulin (CaM) (Kovacs, E., and Liliom, K. (2008) Biochem. J. 410, 427-437). Here we give mechanistic insight into CaM inhibition by SPC, based on fluorescence stopped-flow studies with the model CaM-binding domain melittin. We demonstrate that both the peptide and SPC micelles bind to CaM in a rapid and reversible manner with comparable affinities. Furthermore, we present kinetic evidence that both species compete for the same target site on CaM, and thus SPC can be considered as a competitive inhibitor of CaM-target peptide interactions. We also show that SPC disrupts the complex of CaM and the CaM-binding domain of ryanodine receptor type 1, inositol 1,4,5-trisphosphate receptor type 1, and the plasma membrane Ca2+ pump. By interfering with these interactions, thus inhibiting the negative feedback that CaM has on Ca2+ signaling, we hypothesize that SPC could lead to Ca2+ mobilization in vivo. Hence, we suggest that the action of the sphingolipid on CaM might explain the previously recognized phenomenon that SPC liberates Ca2+ from intracellular stores. Moreover, we demonstrate that unlike traditional synthetic CaM inhibitors, SPC disrupts the complex between not only the Ca2+-saturated but also the apo form of the protein and the target peptide, suggesting a completely novel regulation for target proteins that constitutively bind CaM, such as ryanodine receptors.

Sphingosine kinase regulates oxidized low density lipoprotein-mediated calcium oscillations and macrophage survival

We recently reported that oxidized LDL (oxLDL) induces an oscillatory increase in intracellular calcium ([Ca(2+)](i)) levels in macrophages. Furthermore, we have shown that these [Ca(2+)](i) oscillations mediate oxLDL's ability to inhibit macrophage apoptosis in response to growth factor deprivation. However, the signal transduction pathways by which oxLDL induces [Ca(2+)](i) oscillations have not been elucidated. In this study, we show that these oscillations are mediated in part by intracellular mechanisms, as depleting extracellular Ca(2+) did not completely abolish the effect. Inhibiting sarco-endoplasmic reticulum ATPase (SERCA) completely blocked [Ca(2+)](i) oscillations, suggesting a role for Ca(2+) reuptake by the ER. The addition of oxLDL resulted in an almost immediate activation of sphingosine kinase (SK), which can increase sphingosine-1-phosphate (S1P) levels by phosphorylating sphingosine. Moreover, S1P was shown to be as effective as oxLDL in blocking macrophage apoptosis and producing [Ca(2+)](i) oscillations. This suggests that the mechanism in which oxLDL generates [Ca(2+)](i) oscillations may be 1) activation of SK, 2) SK-mediated increase in S1P levels, 3) S1P-mediated Ca(2+) release from intracellular stores, and 4) SERCA-mediated Ca(2+) reuptake back into the ER.

Ca(2+) transfer from the ER to mitochondria: when, how and why

The heterogenous subcellular distribution of a wide array of channels, pumps and exchangers allows extracellular stimuli to induce increases in cytoplasmic Ca(2+) concentration ([Ca(2+)]c) with highly defined spatial and temporal patterns, that in turn induce specific cellular responses (e.g. contraction, secretion, proliferation or cell death). In this extreme complexity, the role of mitochondria was considered marginal, till the direct measurement with targeted indicators allowed to appreciate that rapid and large increases of the [Ca(2+)] in the mitochondrial matrix ([Ca(2+)]m) invariably follow the cytosolic rises. Given the low affinity of the mitochondrial Ca(2+) transporters, the close proximity to the endoplasmic reticulum (ER) Ca(2+)-releasing channels was shown to be responsible for the prompt responsiveness of mitochondria. In this review, we will summarize the current knowledge of: i) the mitochondrial and ER Ca(2+) channels mediating the ion transfer, ii) the structural and molecular foundations of the signaling contacts between the two organelles, iii) the functional consequences of the [Ca(2+)]m increases, and iv) the effects of oncogene-mediated signals on mitochondrial Ca(2+) homeostasis. Despite the rapid progress carried out in the latest years, a deeper molecular understanding is still needed to unlock the secrets of Ca(2+) signaling machinery.

Phagocytosis: a repertoire of receptors and Ca(2+) as a key second messenger

Receptor-mediated phagocytosis is a complex process that mediates the internalization, by a cell, of other cells and large particles; this is an important physiological event not only in mammals, but in a wide diversity of organisms. Of simple unicellular organisms that use phagocytosis to extract nutrients, to complex metazoans in which phagocytosis is essential for the innate defence system, as a first line of defence against invading pathogens, as well as for the clearance of damaged, dying or dead cells. Evolution has armed multicellular organisms with a range of receptors expressed on many cells that serve as the molecular basis to bring about phagocytosis, regardless of the organism or the specific physiological event concerned. Key to all phagocytic processes is the finely controlled rearrangement of the actin cytoskeleton, in which Ca(2+) signals play a major role. Ca(2+) is involved in cytoskeletal changes by affecting the actions of a number of contractile proteins, as well as being a cofactor for the activation of a number of intracellular signalling molecules, which are known to play important roles during the initiation, progression and resolution of the phagocytic process. In mammals, the requirement of Ca(2+) for the initial steps in phagocytosis, and the subsequent phagosome maturation, can be quite different depending on the type of cell and on the type of receptor that is driving phagocytosis. In this review we discuss the different receptors that mediate professional and non-professional phagocytosis, and discuss the role of Ca(2+) in the different steps of this complex process.

Sphingosylphosphocholine effects on cultured astrocytes reveal mechanisms potentially involved in neurotoxicity in Niemann-Pick type A disease

Niemann-Pick type A is a disease characterized by the absence of a functional SMPD1 (acidic sphingomyelinase) gene and the abnormal accumulation of sphingomyelin. Under these conditions, also sphingosylphosphocholine (SPC, a sphingomyelin metabolite) accumulates in various tissues, including the brain, where it might act as a toxic stimulus, contributing to the appearance of the neurological symptoms. We studied the effects of SPC on astrocytic and neuronal cultures from rat. In particular, we investigated the possibility that SPC acts on astrocytes and that this represents the first step leading to neurodegeneration. Our results show that acute administration of SPC to astrocytes in culture promotes Ca2+ responses and a release of glutamate that, in turn, leads to cytosolic [Ca2+] elevation in neurons. We also show that chronic stimulation by SPC leads astrocytes to proliferate, but can also change their phenotype towards an activated state that might contribute to the inflammatory responses. Interestingly, upon acute SPC stimulation, activated astrocytes release more glutamate. In conclusion, we show that both chronic and acute exposure to SPC can constitute harmful signals that may have a role in the sequence of events leading to neurodegeneration.

Sphingosine-1-phosphate-induced intracellular Ca2+ mobilization in human endothelial cells

The authors have studied the effect of sphingosine-1-phosphate (S1P) on Ca2+ release from intracellular stores in cultured human umbilical vein endothelial cells (HUVECs). In the presence of extracellular Ca2+, S1P increased intracellular Ca2+ concentration ([Ca2+]i) and this increase was partially inhibited by La3+ (1 microM), indicating that S1P induces Ca2+ influx from extracellular pool and Ca2+ release from intracellular stores. S1P increased [Ca2+]i concentration dependently in Ca2+-free extracellular solution. The Hill coefficient (1.7) and EC50 (420 nM) was obtained from the concentration-response relationship. When caffeine depleted Ca2+ store in the presence of ryanodine, S1P did not induce intracellular Ca2+ release. Furthermore, the Ca2+-induced Ca2+ release inhibitors ruthenium red or dantrolene completely inhibited S1P-induced intracellular Ca2+ release. S1P-induced intracellular Ca2+ release was inhibited by the phospholipase C (PLC) inhibitors neomycin and U73312, or the inositol 1,4,5-triphosphate (IP3)-gated Ca2+ channel blocker aminoethoxybiphenyl borane (2-APB). In contrast, S1P-induced intracellular Ca2+ release was not inhibited by the mitochondrial Ca2+ uptake inhibitor CCCP or the mitochondrial Ca2+ release inhibitor cyclosporin A. These results show that S1P mobilizes Ca2+ from intracellular stores primarily via Ca2+-induced and IP3-induced Ca2+ release and this Ca2+ mobilization is independent of mitochondrial Ca2+ stores.

Microdomains of intracellular Ca2+: molecular determinants and functional consequences

Calcium ions are ubiquitous and versatile signaling molecules, capable of decoding a variety of extracellular stimuli (hormones, neurotransmitters, growth factors, etc.) into markedly different intracellular actions, ranging from contraction to secretion, from proliferation to cell death. The key to this pleiotropic role is the complex spatiotemporal organization of the [Ca(2+)] rise evoked by extracellular agonists, which allows selected effectors to be recruited and specific actions to be initiated. In this review, we discuss the structural and functional bases that generate the subcellular heterogeneity in cellular Ca(2+) levels at rest and under stimulation. This complex choreography requires the concerted action of many different players; the central role is, of course, that of the calcium ion, with the main supporting characters being all the entities responsible for moving Ca(2+) between different compartments, while the cellular architecture provides a determining framework within which all the players have their exits and their entrances. In particular, we concentrate on the molecular mechanisms that lead to the generation of cytoplasmic Ca(2+) microdomains, focusing on their different subcellular location, mechanism of generation, and functional role.

Sphingosine releases Ca2+ from intracellular stores via the ryanodine receptor in sea urchin egg homogenates

Various reports have demonstrated that the sphingolipids sphingosine and sphingosine-1-phosphate are able to induce Ca2+ release from intracellular stores in a similar way to second messengers. Here, we have used the sea urchin egg homogenate, a model system for the study of intracellular Ca2+ release mechanisms, to investigate the effect of these sphingolipids. While ceramide and sphingosine-1-phosphate did not display the ability to release Ca2+, sphingosine stimulated transient Ca2+ release from thapsigargin-sensitive intracellular stores. This release was inhibited by ryanodine receptor blockers (high concentrations of ryanodine, Mg2+, and procaine) but not by pre-treatment of homogenates with cADPR, 8-bromo-cADPR or blockers of other intracellular Ca2+ channels. However, sphingosine rendered the ryanodine receptor refractory to cADPR. We propose that, in the sea urchin egg, sphingosine is able to activate the ryanodine receptor via a mechanism distinct from that used by cADPR.

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.

Lysophospholipid receptors: signaling and biology

Lysophospholipids (LPs), such as lysophosphatidic acid and sphingosine 1-phosphate, are membrane-derived bioactive lipid mediators. LPs can affect fundamental cellular functions, which include proliferation, differentiation, survival, migration, adhesion, invasion, and morphogenesis. These functions influence many biological processes that include neurogenesis, angiogenesis, wound healing, immunity, and carcinogenesis. In recent years, identification of multiple cognate G protein-coupled receptors has provided a mechanistic framework for understanding how LPs play such diverse roles. Generation of LP receptor-null animals has allowed rigorous examination of receptor-mediated physiological functions in vivo and has identified new functions for LP receptor signaling. Efforts to develop LP receptor subtype-specific agonists/antagonists are in progress and raise expectations for a growing collection of chemical tools and potential therapeutic compounds. The rapidly expanding literature on the LP receptors is herein reviewed.

Lysophospholipid receptor-dependent and -independent calcium signaling

Changes in cellular Ca(2+) concentrations form a ubiquitous signal regulating numerous processes such as fertilization, differentiation, proliferation, contraction, and secretion. The Ca(2+) signal, highly organized in space and time, is generated by the cellular Ca(2+) signaling toolkit. Lysophospholipids, such as sphingosine-1-phosphate (S1P), sphingosylphosphorylcholine (SPC), or lysophosphatidic acid (LPA) use this toolkit in a specific manner to initiate their cellular responses. Acting as agonists at G protein-coupled receptors, S1P, SPC, and LPA increase the intracellular free Ca(2+) concentration ([Ca(2+)](i)) by using the classical, phospholipase C (PLC)-dependent pathway as well as PLC-independent pathways such as sphingosine kinase (SphK)/S1P. The S1P(1) receptor, via protein kinase C, inhibits the [Ca(2+)](i) transients caused by other receptors. Both S1P and SPC also act intracellularly to regulate [Ca(2+)](i). Intracellular S1P mobilizes Ca(2+) in intact cells independently of G protein-coupled S1P receptors, and Ca(2+) signaling by many agonists requires SphK-mediated S1P production. As shown for the FcepsilonRI receptor, PLC and SphK may contribute specific components to the overall [Ca(2+)](i) transient. Of the many open questions, identification of the intracellular S1P target site(s) appears to be of particular importance.

Sphingosine-1-phosphate is a high-affinity ligand for the G protein-coupled receptor GPR6 from mouse and induces intracellular Ca2+ release by activating the sphingosine-kinase pathway

We identified and cloned the mouse orthologue of human GPR6 as a new member of the lysophospholipid-receptor family. Sphingosine-1-phosphate (S1P) activated GPR6, transiently expressed in frog oocytes or in Chinese hamster ovary (CHO) cells, with high specificity and nanomolar affinity. The GPR6 gene was found to be located on chromosome 10B1 and a single exon coded for the entire open-reading frame. Signal transduction of S1P was inhibited by pertussis toxin, suggesting a coupling of GPR6 to an inhibitory G protein. In CHO cells transfected with GPR6, the sphingosine-kinase pathway mediated Ca(2+) mobilization from internal stores. Apoptotic cell death was induced by serum deprivation or H(2)O(2) treatment and was prevented by S1P in GPR6-, but not in vector-transfected CHO cells. The antiapoptotic effect of S1P required activation of sphingosine kinase and was accompanied by an increase in MAP-kinase phosphorylation.

Extra- and intracellular sphingosylphosphorylcholine promote spontaneous transmitter release from frog motor nerve endings

Similar to phosphatidylinositol bisphosphate, sphingomyelin breakdown generates several lipids, including sphingosylphosphorylcholine (SPC), that are putative signaling molecules. The present study was undertaken to evaluate the involvement of SPC in transmitter release process. Intracellular recordings were made from isolated frog sciatic-sartorius nerve-muscle preparations, and the effects of SPC on neurosecretion in the form of miniature endplate potentials (MEPPs) were assessed. Extracellular application of SPC mixture (D,L-SPC) at 1, 10, and 25 microM increased the MEPP frequency by 68, 96, and 127%, respectively. D-erythro-SPC (dissolved in dimethyl sulfoxide but not coupled to bovine serum albumin), but not L-threo-SPC, was active extracellular; the former (at 10 microM) increased the MEPP frequency by 143%. D-erythro-SPC treatment did not significantly change the median amplitude or frequency-distribution of MEPPs. Intracellular delivery via liposomes, in which 10, 100, or 1000 microM SPC mixture was entrapped in liposomal aqueous phase, induced a concentration-dependent increase in MEPP frequency of 45, 91, and 100%, respectively. D-erythro-SPC and L-threo-SPC at the concentration of 100 microM increased the MEPP frequency by 117 and 67%, respectively, or 91 and 61%, respectively, when coupled to bovine serum albumin. Pretreatment with thapsigargin significantly reduced but did not abolish the effects of extracellular D-erythro-SPC (10 microM) or liposomes containing 100 microM D-erythro-SPC. Liposomes filled with 100 microM D-myo-inositol 1,4,5-trisphosphate (IP3) enhanced the MEPP frequency to the same magnitude as 100 microM D-erythro-SPC entrapped in liposomes. However, administration of 100 microM D-erythro-SPC and IP3 entrapped in the same liposomes enhanced the MEPP frequency by 70%, which was less than that produced by these two compounds alone. The result provides the first electrophysiological evidence that SPC can modulate transmitter release by an extra- or intracellular action at the frog motor nerve ending.

Translational control of Scamper expression via a cell-specific internal ribosome entry site

The mRNA of Scamper, a putative intracellular calcium channel activated by sphingosylphosphocholine, contains a long 5' transcript leader with several upstream AUGs. In this work we have investigated the role this sequence plays in the translational control of Scamper expression. The cytosolic transcription machinery of a T7 RNA polymerase recombinant vaccinia virus was used to avoid artifacts arising from cryptic promoters or mRNA processing. Based on transient transfection experiments of dicistronic and bi-monocistronic plasmids expressing reporter genes, we present evidence that the 5' transcript leader of Scamper contains a functional internal ribosome entry site (IRES). Our data indicate that Scamper translation in Madin-Darby canine kidney cells is driven by a cap-independent mechanism supported by the IRES activity of its mRNA. Finally, the Scamper IRES appears to be the first IRES with specificity for kidney epithelial cells.

Expression and functional characterization of SCaMPER: a sphingolipid-modulated calcium channel of cardiomyocytes

Calcium channels are important in a variety of cellular events including muscle contraction, signaling, proliferation, and apoptosis. Sphingolipids have been recognized as mediators of intracellular calcium release through their actions on a calcium channel, sphingolipid calcium release-mediating protein of the endoplasmic reticulum (SCaMPER). The current study investigates the expression and function of SCaMPER in cardiomyocytes. Northern analyses and RT-PCR cloning and sequencing revealed SCaMPER expression in both human and rat cardiac tissue. Immunofluorescence and Western blot analyses demonstrated that SCaMPER is abundant in cardiac tissue and is localized to the sarcotubular junction. This was confirmed by the colocalization of SCaMPER with dihydropyridine and ryanodine receptors by confocal microscopy. Purified T tubules were shown to contain SCaMPER and immunoelectron micrographs suggested that SCaMPER is located to the junctional T tubules, but a junctional SR localization cannot be ruled out. The sphingolipid ligand for SCaMPER, sphingosylphosphorylcholine (SPC), initiated calcium release from the cardiomyocyte SR. Importantly, antisense knockdown of SCaMPER mRNA produced a substantial reduction of sphingolipid-induced calcium release, suggesting that SCaMPER is a potentially important calcium channel of cardiomyocytes.

Ceramide 1-phosphate increases intracellular free calcium concentrations in thyroid FRTL-5 cells: evidence for an effect mediated by inositol 1,4,5-trisphosphate and intracellular sphingosine 1-phosphate

Sphingolipid (SP) derivatives have diverse effects on the regulation of intracellular free calcium concentrations ([Ca2+]i) in a multitude of non-excitable cells. In the present investigation, the effect of C2-ceramide 1-phosphate (C1P) on [Ca2+]i was investigated in thyroid FRTL-5 cells. C1P evoked a concentration-dependent increase in [Ca2+]i, both in a calcium-containing and a calcium-free buffer. A substantial part of the C1P-evoked increase in [Ca2+]i was due to calcium entry. The effect of C1P was attenuated by overnight pretreatment of the cells with pertussis toxin. Similar results were obtained with C8-ceramide 1-phosphate, although the magnitude of the responses was smaller than with C1P. The phospholipase C inhibitor U73122 attenuated the effect of C1P. C1P invoked a small, but significant, increase in inositol 1,4,5-trisphosphate (IP3). However, the effect of C1P on [Ca2+]i was inhibited by neither Xestospongin C, 2-aminoethoxydiphenylborate nor neomycin. C1P mobilized calcium from an IP3-sensitive calcium store, as C1P did not increase [Ca2+]i in cells pretreated with thapsigargin. The effect of C1P on [Ca2+]i was potently attenuated by dihydrosphingosine and dimethylsphingosine, two inhibitors of sphingosine kinase, but not by the inactive SP-derivative N -acetyl sphingosine. Stimulating the cells with C1P evoked an increase in the production of intracellular sphingosine 1-phosphate. C1P did not modulate DNA synthesis or the forskolin-evoked production of cAMP. The results indicate that C1P may be an important SP participating in cellular signalling.

Sphingosine kinase: a point of convergence in the action of diverse neutrophil priming agents

Neutrophils are a vital component of the early acute inflammatory response, but can cause profound tissue damage when activated to excess or prevented from undergoing apoptosis. However, much remains unknown about the intracellular signaling pathways regulating neutrophil activity. The structurally diverse neutrophil-priming agents platelet-activating factor, TNF-alpha, and the substance P analog [D-Arg(6), D-Trp(7,9),N(me)Phe(8)]-substance P(6-11) (SP-G) stimulated a rapid increase in sphingosine kinase activity in freshly isolated human neutrophils. This activity was blocked by preincubation with the sphingosine kinase inhibitor N,N-dimethylsphingosine (DMS). DMS also inhibited the increase in intracellular calcium concentration stimulated by platelet-activating factor, fMLP, and SP-G. This suggests that the increase in intracellular calcium concentration by these agents is dependent on sphingosine kinase activation and the generation of sphingosine-1-phosphate. Changes in cell polarization and the augmentation of the fMLP-induced superoxide anion generation, by all priming agents were also inhibited by DMS, while only the superoxide anion release was blocked by the phosphatidylinositol 3-kinase inhibitor LY294002. Moreover, SP-G and GM-CSF inhibited constitutive neutrophil apoptosis which was completely blocked by DMS. These results suggest a novel role for sphingosine kinase in the regulation of neutrophil priming.

The endoplasmic reticulum: a multifunctional signaling organelle

The endoplasmic reticulum (ER) is a multifunctional signaling organelle that controls a wide range of cellular processes such as the entry and release of Ca(2+), sterol biosynthesis, apoptosis and the release of arachidonic acid (AA). One of its primary functions is as a source of the Ca(2+) signals that are released through either inositol 1,4,5-trisphosphate (InsP(3)) or ryanodine receptors (RYRs). Since these receptors are Ca(2+)-sensitive, the ER functions as an excitable system capable of spreading signals throughout the cell through a process of Ca(2+)-induced Ca(2+) release (CICR). This regenerative capacity is particularly important in the control of muscle cells and neurons. Its role as an internal reservoir of Ca(2+) must be accommodated with its other major role in protein synthesis where a constant luminal level of Ca(2+) is essential for protein folding. The ER has a number of stress signaling pathways that activate various transcriptional cascades that regulate the luminal content of the Ca(2+)-dependent chaperones responsible for the folding and packaging of secretory proteins.Another emerging function of the ER is to regulate apoptosis by operating in tandem with mitochondria. Anti-apoptotic regulators of apoptosis such as Bcl-2 may act by reducing the ebb and flow of Ca(2+) through the ER/mitochondrial couple. Conversely, the presenilins that appear to increase the Ca(2+) content of the ER lumen make cells more susceptible to apoptosis.

Sphingosine 1-phosphate: a Ca2+ release mediator in the balance

Sphingosine 1-phosphate (S1P) is a lipid signalling molecule with Ca(2+) mobilising properties. Importantly for a role as a Ca(2+) release messenger, intracellular levels of S1P can be regulated by a variety of extracellular stimuli, via the enzyme sphingosine kinase. However, neither the mechanism underlying S1P generation, nor its actions at the endoplasmic reticulum are clear. Thus, the role of S1P as an intracellular mediator of Ca(2+) release remains in the balance.

Sphingosine 1-phosphate induces Ca2+ transients and cytoskeletal rearrangement in C2C12 myoblastic cells

In many cell systems, sphingosine 1-phosphate (SPP) increases cytosolic Ca2+ concentration ([Ca2+]i) by acting as intracellular mediator and extracellular ligand. We recently demonstrated (Meacci E, Cencetti F, Formigli L, Squecco R, Donati C, Tiribilli B, Quercioli F, Zecchi-Orlandini S, Francini F, and Bruni P. Biochem J 362: 349-357, 2002) involvement of endothelial differentiation gene (Edg) receptors (Rs) specific for SPP in agonist-mediated Ca2+ response of a mouse skeletal myoblastic (C2C12) cell line. Here, we investigated the Ca2+ sources of SPP-mediated Ca2+ transients in C2C12 cells and the possible correlation of ion response to cytoskeletal rearrangement. Confocal fluorescence imaging of C2C12 cells preloaded with Ca2+ dye fluo 3 revealed that SPP elicited a transient Ca2+ increase propagating as a wave throughout the cell. This response required extracellular and intracellular Ca2+ pool mobilization. Indeed, it was significantly reduced by removal of external Ca2+, pretreatment with nifedipine (blocker of L-type plasma membrane Ca2+ channels), and inositol 1,4,5-trisphosphate [Ins(1,4,5)P3]-mediated Ca2+ pathway inhibitors. Involvement of EdgRs was tested with suramin (specific inhibitor of Edg-3). Fluorescence associated with Ins(1,4,5)P3Rs and L-type Ca2+ channels was evident in C2C12 cells. SPP also induced C2C12 cell contraction. This event, however, was unrelated to [Ca2+]i increase, because the two phenomena were temporally shifted. We propose that SPP may promote C2C12 cell contraction through Ca2+-independent mechanisms.

Dichotomy of Ca2+ signals triggered by different phospholipid pathways in antigen stimulation of human mast cells

Mast cell activation triggers Ca(2+) signals and the release of enzyme-containing granules, events that play a major role in allergic/hypersensitivity reactions. However, the precise molecular mechanisms that regulate antigen-triggered degranulation and Ca(2+) fluxes in human mast cells are still poorly understood. Here we show, for the first time, that a receptor can trigger Ca(2+) via two separate molecular mechanisms. Using an antisense approach, we show that IgE-antigen stimulation of human bone marrow-derived mast cells triggers a sphingosine kinase (SPHK) 1-mediated fast and transient Ca(2+) release from intracellular stores. However, phospholipase C (PLC) gamma1 triggers a second (slower) wave of calcium release from intracellular stores, and it is this PLCgamma1-generated signal that is responsible for Ca(2+) entry. Surprisingly, FcepsilonRI (a high affinity receptor for IgE)-triggered mast cell degranulation depends on the first, sphingosine kinase-mediated Ca(2+) signal. These two pathways act independently because antisense knock down of either enzyme does not interfere with the activity of the other enzyme. Of interest, similar to PLCgamma1, SPHK1 translocates rapidly to the membrane after FcepsilonRI cross-linking. Here we also show that SPHK1 activity depends on phospholipase D1 and that FcepsilonRI-triggered mast cell degranulation depends primarily on the activation of both phospholipase D1 and SPHK1.

Re-evaluation of primary structure, topology, and localization of Scamper, a putative intracellular Ca2+ channel activated by sphingosylphosphocholine

Naturally occurring sphingoid molecules control vital functions of the cell through their interaction with specific receptors. Proliferation, differentiation and programmed death result in fact from a fine balance of signals, among which sphingosine and structurally related molecules play fundamental roles, acting as either first or second messengers. The corresponding receptors need to be identified in order that the role of sphingoid molecules can be established. Among them, several G-protein-coupled receptors specific for sphingosine 1-phosphate, sphingosylphosphocholine, or both, have already been investigated. In contrast, the identification of the postulated intracellular receptors has been problematical. In the present study we re-evaluated the molecular characterization of Scamper, the first proposed intracellular receptor for sphingosylphosphocholine [Mao, Kim, Almenoff, Rudner, Kearney and Kindman (1996) Proc. Natl. Acad. Sci. U.S.A. 93, 1993-1996] and commonly believed to be a Ca(2+) channel of the endoplasmic reticulum (the name "SCaMPER" used by Mao et al. being derived from "sphingolipid Ca(2+)-release-mediating protein of the endoplasmic reticulum"). In contrast with what has been believed hitherto, our primary-structure and overexpression experiments indicate that Scamper is a 110-amino-acid protein spanning the membrane once with a Nexo/Ccyt topology [von Heijne and Gavel (1988) Eur. J. Biochem. 174, 671-678]. Overexpression of either wild-type or tagged Scamper induces a specific phenotype characterized by the rapid extension of actin-containing protrusions, followed by cell death.

A role for G protein-coupled lysophospholipid receptors in sphingolipid-induced Ca2+ signaling in MC3T3-E1 osteoblastic cells

Sphingolipids have been proposed to modulate cell function by acting as intracellular second messengers and through binding to plasma membrane receptors. Exposure of MC3T3-E1 osteoblastic cells to sphingosine (SPH), sphingosine-1-phosphate (SPP), or sphingosylphosphorylcholine (SPC) led to the release of Ca2+ from the endoplasmic reticulum (ER) and acute elevations in cytosolic-free Ca2+ ([Ca2+]i). Desensitization studies suggest that SPP and SPC bind plasma membrane endothelial differentiation gene (Edg) receptors for lysophosphatidic acid (LPA). Consistent with the coupling of Edg receptors to G proteins, SPP- and SPC-induced Ca2+ signaling was inhibited by pretreatment of the cells with pertussis toxin (PTx). Of the Edg receptors known to bind SPH derivatives in other cell types, MC3T3-E1 cells were found to express transcripts encoding Edg-1 and Edg-5 but not Edg-3, Edg-6, or Edg-8. In contrast to SPP and SPC, the ability of SPH to elicit [Ca2+]i elevations was affected neither by prior exposure of cells to LPA nor by PTx treatment. However, LPA-induced Ca2+ signaling was blocked in MC3T3-E1 cells previously exposed to SPH. Elevations in [Ca2+]i were not evoked by SPP or SPC in cells treated with 2-aminoethoxydiphenylborate (2-APB), an inhibitor of inositol 1,4,5-trisphosphate (IP3)-gated Ca2+ channels in the ER. No effect of 2-APB was observed on SPH-or LPA-induced [Ca2+]i elevations. The data support a model in which SPP and SPC bind Edg-1 and/or Edg-5 receptors in osteoblasts leading to the release of Ca2+ from the ER through IP3-gated channels.

Calcium signalling in cells of oligodendroglial lineage

Intracellular Ca2+ is the key signal that regulates the efficacy of neurotransmitter release and synaptic plasticity in neurons but is also an important second messenger involved in the signal transduction and modulation of gene expression in both excitable and non-excitable cells. Glial cells, including cells of oligodendroglial (OLG) lineage, are capable of responding to extracellular stimuli via changes in the intracellular Ca2+. This review article focuses on the mechanisms of Ca2+ signalling in cells of OLG lineage with the goal of providing the basis for understanding the relevance of receptor- and non-receptor-mediated signalling to oligodendroglial development, myelination, and demyelination. Conclusions to date indicate that cells of OLG lineage exhibit remarkable plasticity with regard to the expression of ion channels and receptors linked to Ca2+ signalling and that perturbation of [Ca2](i) homeostasis contributes to the pathogenesis of demyelinating diseases.

Intracellular sphingosine 1-phosphate production: a novel pathway for Ca2+ release

Sphingolipids such as sphingosine 1-phosphate (SPP) and sphingosylphosphorylcholine have long been recognized to possess Ca2+ mobilizing activity, yet to date little is known about their mechanism of action, or indeed their significance as Ca2+ mobilizing intracellular messengers. The recent discovery of extracellular receptors for the sphingolipids has further complicated the interpretation of many experiments in this field. This paper reviews the current literature in which molecular and pharmacological approaches have begun to uncover the signalling components associated with intracellular SPP production and Ca2+ mobilization. The functional significance of this novel Ca2+ release pathway is also discussed.

The versatility and universality of calcium signalling

The universality of calcium as an intracellular messenger depends on its enormous versatility. Cells have a calcium signalling toolkit with many components that can be mixed and matched to create a wide range of spatial and temporal signals. This versatility is exploited to control processes as diverse as fertilization, proliferation, development, learning and memory, contraction and secretion, and must be accomplished within the context of calcium being highly toxic. Exceeding its normal spatial and temporal boundaries can result in cell death through both necrosis and apoptosis.

Ruled by waves? Intracellular and intercellular calcium signalling

The field of calcium signalling has evolved rapidly the last 20 years. Physiologists had worked with cytosolic Ca2+ as the coupler of excitation and contraction of muscles and as a secretory signal in exocrine glands and in the synapses of the brain for several decades before the discovery of cellular calcium as a second messenger. Development of powerful techniques for measuring the concentration of cytosolic free calcium ions in cell suspensions and later in single cells and even in different cellular compartments, has resulted in an upsurge in the knowledge of the cellular machinery involved in intracellular calcium signalling. However, the focus on intracellular mechanisms might have led this field of study away from physiology. During the last few years there is an increasing evidence for an important role of calcium also as an intercellular signal. Via gap junctions calcium is able to co-ordinate cell populations and even organs like the liver. Here we will give an overview of the general mechanisms of intracellular calcium signalling, and then review the recent data on intercellular calcium signals. A functional coupling of cells in different tissues and organs by the way of calcium might be an important mechanism for controlling and synchronizing physiological responses

Function- and agonist-specific Ca2+signalling: The requirement for and mechanism of spatial and temporal complexity in Ca2+signals

Calcium signals have been implicated in the regulation of many diverse cellular processes. The problem of how information from extracellular signals is delivered with specificity and fidelity using fluctuations in cytosolic Ca2+concentration remains unresolved. The capacity of cells to generate Ca2+signals of sufficient spatial and temporal complexity is the primary constraint on their ability to effectively encode information through Ca2+. Over the past decade, a large body of literature has dealt with some basic features of Ca2+-handling in cells, as well as the multiplicity and functional diversity of intracellular Ca2+stores and extracellular Ca2+influx pathways. In principle, physiologists now have the necessary information to attack the problem of function- and agonist-specificity in Ca2+signal transduction. This review explores the data indicating that Ca2+release from diverse sources, including many types of intracellular stores, generates Ca2+signals with sufficient complexity to regulate the vast number of cellular functions that have been reported as Ca2+-dependent. Some examples where such complexity may relate to neuroendocrine regulation of hormone secretion/synthesis are discussed. We show that the functional and spatial heterogeneity of Ca2+stores generates Ca2+signals with sufficient spatiotemporal complexity to simultaneously control multiple Ca2+-dependent cellular functions in neuroendocrine systems.Key words: signal coding, IP3receptor, ryanodine receptor, endoplasmic reticulum, Golgi, secretory granules, mitochondria, exocytosis.

Edg-6 as a putative sphingosine 1-phosphate receptor coupling to Ca(2+) signaling pathway

The endothelial differentiation gene-6 (Edg-6) was recently identified as an orphan G-protein-coupled receptor. Its predicted amino acid sequence is very close to Edg family of receptor proteins whose ligand is supposed to be lysophosphatidic acid (LPA) or lysosphingolipid such as sphingosine 1-phosphate (S1P) and sphingosylphosphorylcholine (SPC). Transfection of the Edg-6 into Chinese hamster ovary (CHO) cells and K562 cells resulted in the appearance of high-affinity [(3)H]S1P binding activity. Among lipids employed, S1P and, even though less potent, SPC, displaced the [(3)H]S1P binding, but LPA was inactive. In Edg-6-transfected CHO cells, an increase in cytosolic Ca(2+) concentration in response to S1P or SPC was clearly enhanced without change in the LPA-induced action as compared with the vector-transfected cells. The enhancement of the Ca(2+) response was associated with a significant accumulation of inositol phosphate, reflecting activation of phospholipase C. Similar enhancement of Ca(2+) response to S1P or SPC was also observed in Edg-6-expressing K562 cells. These lipid-induced actions in CHO cells and K562 cells expressing Edg-6 were markedly suppressed by pertussis toxin treatment. We conclude that Edg-6 is one of S1P or lysosphingolipid receptors that couple to phospholipase C-Ca(2+) system through pertussis toxin-sensitive G-proteins.

Inhibition of Ca2+ release channel (ryanodine receptor) activity by sphingolipid bases: mechanism of action

Sphingosine inhibits the activity of the skeletal muscle Ca2+ release channel (ryanodine receptor) and is a noncompetitive inhibitor of [3H]ryanodine binding (Needleman et al., Am. J. Physiol. 272, C1465-1474, 1997). To determine the contribution of other sphingolipids to the regulation of ryanodine receptor activity, several sphingolipid bases were assessed for their ability to alter [3H]ryanodine binding to sarcoplasmic reticulum (SR) membranes and to modulate the activity of the Ca2+ release channel. Three lipids, N,N-dimethylsphingosine, dihydrosphingosine, and phytosphingosine, inhibited [3H]ryanodine binding to both skeletal and cardiac SR membranes. However, the potency of these three lipids and sphingosine was lower in rabbit cardiac membranes when compared to rabbit skeletal muscle membranes and when compared to sphingosine. Like sphingosine, the lipids inhibited [3H]ryanodine binding by greatly increasing the rate of dissociation of bound [3H]ryanodine from SR membranes, indicating that these three sphingolipid bases were noncompetitive inhibitors of [3H]ryanodine binding. These bases also decreased the activity of the Ca2+ release channel incorporated into planar lipid bilayers by stabilizing a long closed state. Sphingosine-1-PO4 and C6 to C18 ceramides of sphingosine had no significant effect on [3H]ryanodine binding to cardiac or skeletal muscle SR membranes. Saturation of the double bond at positions 4-5 decreased the ability of the sphingolipid bases to inhibit [3H]ryanodine binding 2-3 fold compared to sphingosine. In summary, our data indicate that other endogenous sphingolipid bases are capable of modulating the activity of the Ca2+ release channel and as a class possess a common mechanism of inhibition.

Effect of sphingosylphosphorylcholine on the single channel gating properties of the cardiac ryanodine receptor

The effects of the lysosphingolipid, sphingosylphosphorylcholine (SPC), on the cardiac ryanodine receptor (RyR) were examined. The open probability of cardiac RyR incorporated in lipid bilayers was decreased by cytoplasmic, but not lumenal side application of micromolar concentrations of SPC. Modification of channel function was characterized by the appearance of a long-lived closed state in addition to the brief channel closings observed in the presence and absence of SPC. Open channel kinetics and ion conduction properties, however, were not altered by this compound. These results suggest that SPC, a putative second messenger derived from sphingomyelin, may regulate Ca(2+) release from the sarcoplasmic reticulum by modifying the gating kinetics of the RyR.

New Ca2+-releasing messengers: are they important in the nervous system?

In the nervous system, Ca2+ signalling is determined primarily by voltage-gated Ca2+-selective channels in the plasma membrane, but there is increasing evidence for involvement of intracellular Ca2+ stores in such signalling. It is generally assumed that neurotransmitter-elicited release of Ca2+ from internal stores is primarily mediated by Ins(1,4,5)P3, as originally discovered in pancreatic acinar cells. The more-recently discovered Ca2+-releasing messenger, cyclic ADP-ribose (cADPR), which activates ryanodine receptors, has so far only been implicated in a few cases, and the possible importance of another Ca2+-releasing molecule, nicotinic acid adenine dinucleotide phosphate (NAADP), has been ignored. Recent investigations of the action of the brain-gut peptide cholecystokinin on pancreatic acinar cells have indicated that NAADP and cADPR receptors are essential for Ca2+ release. Tools are available for testing the possible involvement of NAADP and cADPR in neurotransmitter-elicited intracellular Ca2+ release, and such studies could reveal complex mechanisms that control this release in the nervous system.

Calcium-independent activation of endothelial nitric oxide synthase by ceramide

The endothelial isoform of NO synthase (eNOS) is targeted to sphingolipid-enriched signal-transducing microdomains in the plasma membrane termed caveolae. Among the caveolae-targeted sphingolipids are the ceramides, a class of acylated sphingosine compounds that have been implicated in diverse cellular responses. We have explored the role of ceramide analogues in eNOS signaling in cultured bovine aortic endothelial cells (BAEC). Addition of the ceramide analogue N-acetylsphingosine (C(2)-ceramide; 5 microM) to intact BAEC leads to a significant increase in NO synthase activity (assayed by using the fluorescent indicator 4,5-diaminofluorescein) and translocation of eNOS from the endothelial cell membrane to intracellular sites (measured by using quantitative immunofluorescence techniques); the biologically inactive ceramide N-acetyldihydrosphingosine is entirely without effect. C(2)-ceramide-induced eNOS activation and translocation are unaffected by the intracellular calcium chelator 1, 2-bis-o-aminophenoxyethane-N,N,N',N'-tetraacetic acid (BAPTA). Using the calcium-specific fluorescent indicator fluo-3, we also found that C(2)-ceramide activation of eNOS is unaccompanied by a drug-induced increase in intracellular calcium. These findings stand in sharp contrast to the mechanism by which bradykinin, estradiol, and other mediators acutely activate eNOS, in which a rapid, agonist-promoted increase in intracellular calcium is required. Finally, we show that treatment of BAEC with bradykinin causes a significant increase in cellular ceramide content; the response to bradykinin has an EC(50) of 3 nM and is blocked by the bradykinin B(2)-receptor antagonist HOE140. Bradykinin-induced ceramide generation could represent a mechanism for longer-term regulation of eNOS activity. Our results suggest that ceramide functions independently of Ca(2+)-regulated pathways to promote activation and translocation of eNOS, and that this lipid mediator may represent a physiological regulator of eNOS in vascular endothelial cells.

Lysophosphatidic acid-mediated Ca2+ mobilization in human SH-SY5Y neuroblastoma cells is independent of phosphoinositide signalling, but dependent on sphingosine kinase activation

Extracellular application of lysophosphatidic acid (LPA) elevated intracellular Ca(2+) concentration ([Ca(2+)](i)) in human SH-SY5Y neuroblastoma cells. The maximal response to LPA occurred between 0. 1 and 1 microM, at which point [Ca(2+)](i) was increased by approx. 500 nM. This increase was of similar magnitude to that caused by the muscarinic acetylcholine receptor agonist methacholine (MCh), although the initial rate of release by LPA was slower. Both LPA and MCh released Ca(2+) from intracellular stores, as assessed by inhibition of their effects by thapsigargin, a blocker of endoplasmic reticular Ca(2+) uptake, and by the persistence of their action in nominally Ca(2+)-free extracellular medium. Similarly, both agonists appeared to stimulate store-refilling Ca(2+) entry. MCh produced a marked elevation in cellular Ins(1,4,5)P(3) and stimulated [(3)H]InsP accumulation in the presence of Li(+). In contrast, LPA failed to stimulate detectable phosphoinositide turnover. Chronic down-regulation of Ins(1,4,5)P(3) receptor (InsP(3)R) proteins with MCh did not affect Ca(2+) responses to LPA. In addition, heparin, a competitive antagonist of InsP(3)Rs, blocked Ca(2+)-mobilization in permeabilized SH-SY5Y cells in response to MCh or exogenously added Ins(1,4,5)P(3), but failed to inhibit Ca(2+)-release induced by LPA. Elevation of [Ca(2+)](i) elicited by LPA was blocked by guanosine 5'-[beta-thio]-diphosphate, indicating that this agonist acts via a G-protein-coupled receptor. However, pertussis toxin was without effect on LPA-evoked [Ca(2+)](i) responses, suggesting that G(i/o)-proteins were not involved. In the absence of extracellular Ca(2+), N,N-dimethylsphingosine (DMS, 30 microM), a competitive inhibitor of sphingosine kinase, blocked LPA-induced Ca(2+) responses by almost 90%. In addition, MCh-induced Ca(2+) responses were also diminished by the addition of DMS, although to a lesser extent than with LPA. We conclude that LPA mobilizes intracellular Ca(2+)-stores in SH-SY5Y cells independently of the generation and action of Ins(1,4,5)P(3). Furthermore, the Ca(2+)-response to LPA appears to be dependent on sphingosine kinase activation and the potential generation of the putative second messenger sphingosine 1-phosphate.

Sphingosylphosphorylcholine induces cytosolic Ca(2+) elevation in endothelial cells in situ and causes endothelium-dependent relaxation through nitric oxide production in bovine coronary artery

Sphingosylphosphorylcholine (SPC) increased intracellular Ca(2+) concentration ([Ca(2+)]i) and nitric oxide (NO) production in endothelial cells in situ on bovine aortic valves, and induced endothelium-dependent relaxation of bovine coronary arteries precontracted with U-46619. The SPC-induced vasorelaxation was inhibited by N(omega)-monomethyl-L-arginine, an inhibitor of both constitutive and inducible NO synthase (NOS), but not by 1-(2-trifluoromethylphenyl) imidazole, an inhibitor of inducible NOS (iNOS). Immunoblotting revealed that endothelial constitutive NOS, but not iNOS, was present in endothelial cells in situ on the bovine aortic valves. We propose that SPC activates [Ca(2+)]i levels and NO production of endothelial cells in situ, thereby causing an endothelium-dependent vasorelaxation.

N-acetyl-sphingenine-1-phosphate is a potent calcium mobilizing agent

Calcium mobilization induced by phosphorylated sphingoid bases was analyzed in calf pulmonary artery endothelial cells by confocal microscopy. A sphingenine-1-phosphate (SeP) analogue, N-acetyl-sphingenine-1-phosphate (N-C2-SeP), exogenously added to these cells, caused a fast and transient intracellular rise in calcium and was as potent as SeP. A minimal concentration of 0.6 nM for N-C2-SeP versus 1 nM for SeP was determined. The N-C2-SeP-induced Ca2+-signaling, like the response to SeP, was due to a release from thapsigargin-sensitive, ryanodine-insensitive, intracellular Ca2+-stores and not to a Ca2+-influx. N-C2-SeP can be considered as a truncated ceramide-phosphate, a lipid already reported to be mitogenic (Gomez-Munoz, A., Duffy, P.A., Martin, A., O'Brien, L., Byun, H.S., Bittman, R. and Brindley, D.N. (1995) Mol. Pharmacol. 47, 833-839), an effect that might be secondary to Ca2+-mobilization.

Sphingosylphosphorylcholine increases calcium concentration in isolated brain nuclei

Sphingosylphosphorylcholine (SPC) caused a rapid increase of Ca2+ concentration in isolated brain nuclei. This effect was prevented by nimodipine, an inhibitor of L-type Ca2+ channels, and by thapsigargin, an inhibitor of Ca(2+)-ATPase. Neither heparin nor U73122 modified this effect, suggesting that phospholipase C activation and inositol 1,4,5-trisphosphate (IP3) production are not involved. Results also indicated that SPC-induced increase in Ca2+ concentration is not protein kinase C-dependent.

Protein-protein interactions in intracellular Ca2+-release channel function

Release of Ca2+ ions from intracellular stores can occur via two classes of Ca2+-release channel (CRC) protein, the inositol 1,4, 5-trisphosphate receptors (InsP3Rs) and the ryanodine receptors (RyRs). Multiple isoforms and subtypes of each CRC class display distinct but overlapping distributions within mammalian tissues. InsP3Rs and RyRs interact with a plethora of accessory proteins which modulate the activity of their intrinsic channels. Although many aspects of CRC structure and function have been reviewed in recent years, the properties of proteins with which they interact has not been comprehensively surveyed, despite extensive current research on the roles of these modulators. The aim of this article is to review the regulation of CRC activity by accessory proteins and, wherever possible, to outline the structural details of such interactions. The CRCs are large transmembrane proteins, with the bulk of their structure located cytoplasmically. Intra- and inter-complex protein-protein interactions between these cytoplasmic domains also regulate CRC function. Some accessory proteins modulate channel activity of all CRC subtypes characterized, whereas other have class- or even isoform-specific effects. Certain accessory proteins exert both direct and indirect forms of regulation on CRCs, occasionally with opposing effects. Others are themselves modulated by changes in Ca2+ concentration, thereby participating in feedback mechanisms acting on InsP3R and RyR activity. CRCs are therefore capable of integrating numerous signalling events within a cell by virtue of such protein-protein interactions. Consequently, the functional properties of InsP3Rs and RyRs within particular cells and subcellular domains are 'customized' by the accessory proteins present.