Ca(2+)-ATPase inhibitor, cyclopiazonic acid, releases Ca2+ from intracellular stores in RBL-2H3 mast cells and activates a Ca2+ influx pathway that is permeable to sodium and manganese

The Role of TRP Proteins in Mast Cells

Transient receptor potential (TRP) proteins form cation channels that are regulated through strikingly diverse mechanisms including multiple cell surface receptors, changes in temperature, in pH and osmolarity, in cytosolic free Ca(2+) concentration ([Ca(2+)](i)), and by phosphoinositides which makes them polymodal sensors for fine tuning of many cellular and systemic processes in the body. The 28 TRP proteins identified in mammals are classified into six subfamilies: TRPC, TRPV, TRPM, TRPA, TRPML, and TRPP. When activated, they contribute to cell depolarization and Ca(2+) entry. In mast cells, the increase of [Ca(2+)](i) is fundamental for their biological activity, and several entry pathways for Ca(2+) and other cations were described including Ca(2+) release activated Ca(2+) (CRAC) channels. Like in other non-excitable cells, TRP channels could directly contribute to Ca(2+) influx via the plasma membrane as constituents of Ca(2+) conducting channel complexes or indirectly by shifting the membrane potential and regulation of the driving force for Ca(2+) entry through independent Ca(2+) entry channels. Here, we summarize the current knowledge about the expression of individual Trp genes with the majority of the 28 members being yet identified in different mast cell models, and we highlight mechanisms how they can regulate mast cell functions. Since specific agonists or blockers are still lacking for most members of the TRP family, studies to unravel their function and activation mode still rely on experiments using genetic approaches and transgenic animals. RNAi approaches suggest a functional role for TRPC1, TRPC5, and TRPM7 in mast cell derived cell lines or primary mast cells, and studies using Trp gene knock-out mice reveal a critical role for TRPM4 in mast cell activation and for mast cell mediated cutaneous anaphylaxis, whereas a direct role of cold- and menthol-activated TRPM8 channels seems to be unlikely for the development of cold urticaria at least in mice.

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.

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.

Inhibition by calyculin A and okadaic acid of the Ca(2+) release-activated Ca(2+) entry pathway in rat basophilic leukemia cells: evidence for regulation by type 1/2A serine/threonine phosphatase activity

Using a combination of fluorescence measurements of intracellular Ca(2+) ion concentration ([Ca(2+)](i)) and membrane potential we have investigated the sensitivity to serine/threonine phosphatase inhibition of Ca(2+) entry stimulated by activation of the Ca(2+) release-activated Ca(2+) (CRAC) entry pathway in rat basophilic leukemia cells. In both suspension and adherent cells, addition of the type 1/2A phosphatase inhibitor calyculin A, during activation of CRAC uptake, resulted in a fall in [Ca(2+)](i) to near preactivation levels. Pre-treatment with calyculin A abolished the component of the Ca(2+) rise associated with activation of CRAC uptake and inhibited Mn(2+) entry, consistent with a requirement of phosphatase activity for activation of the pathway. Depletion of intracellular Ca(2+) stores is accompanied by a large depolarisation which is absolutely dependent upon Ca(2+) entry via the CRAC uptake pathway. Application of calyculin A or okadaic acid, a structurally unrelated phosphatase antagonist inhibits this depolarisation. Taken in concert, these data demonstrate a marked sensitivity of the CRAC entry pathway to inhibition by calyculin A and okadaic acid.

Na(+)-dependent Ca(2+) transport modulates the secretory response to the Fcepsilon receptor stimulus of mast cells

Immunological stimulation of rat mucosal-type mast cells (RBL-2H3 line) by clustering of their Fcepsilon receptors (FcepsilonRI) causes a rapid and transient increase in free cytoplasmic Ca(2+) ion concentration ([Ca(2+)](i)) because of its release from intracellular stores. This is followed by a sustained elevated [Ca(2+)](i), which is attained by Ca(2+) influx. Because an FcepsilonRI-induced increase in the membrane permeability for Na(+) ions has also been observed, and secretion is at least partially inhibited by lowering of extracellular sodium ion concentrations ([Na(+)](o)), the operation of a Na(+)/Ca(2+) exchanger has been considered. We found significant coupling between the Ca(2+) and Na(+) ion gradients across plasma membranes of RBL-2H3 cells, which we investigated employing (23)Na-NMR, (45)Ca(2+), (85)Sr(2+), and the Ca(2+)-sensitive fluorescent probe indo-1. The reduction in extracellular Ca(2+) concentrations ([Ca(2+)](o)) provoked a [Na(+)](i) increase, and a decrease in [Na(+)](o) results in a Ca(2+) influx as well as an increase in [Ca(2+)](i). Mediator secretion assays, monitoring the released beta-hexosaminidase activity, showed in the presence of extracellular sodium a sigmoidal dependence on [Ca(2+)](o). However, the secretion was not affected by varying [Ca(2+)](o) as [Na(+)](o) was lowered to 0.4 mM, while it was almost completely inhibited at [Na(+)](o) = 136 mM and [Ca(2+)](o) < 0.05 mM. Increasing [Na(+)](o) caused the secretion to reach a minimum at [Na(+)](o) = 20 mM, followed by a steady increase to its maximum value at 136 mM. A parallel [Na(+)](o) dependence of the Ca(2+) fluxes was observed: Antigen stimulation at [Na(+)](o) = 136 mM caused a pronounced Ca(2+) influx. At [Na(+)](o) = 17 mM only a slight Ca(2+) efflux was detected, whereas at [Na(+)](o) = 0.4 mM no Ca(2+) transport across the cell membrane could be observed. Our results clearly indicate that the [Na(+)](o) dependence of the secretory response to FcepsilonRI stimulation is due to its influence on the [Ca(2+)](i), which is mediated by a Na(+)-dependent Ca(2+) transport.

Manganese transport through human erythrocyte membranes. An EPR study

Manganese uptake by human erythrocytes was investigated in the concentration range 0.5-20 mM in the suspending solution, by using the EPR technique. S shaped dependencies of manganese influx on manganese doping solution concentration for both fresh and vanadate treated erythrocytes were found, with maximum influx values of 4.1 +/- 1.9 x 10(-10) mol/m2 x s and 2.1 +/- 0.3 x 10(-9) mol/m2 x s, respectively. At low manganese concentrations (< 2 mM) the manganese permeability coefficient increases with increasing the doping concentration, the ions cooperate for achieving a transport event. For high manganese concentration (> 5 mM) the permeability coefficient decreases with increasing the doping concentration, the ions competing for the limited amount of transport system. A similar increase in manganese uptake as in vanadate treated erythrocytes was measured for 'in vitro' aged erythrocytes. These results might suggest that human erythrocytes possess an active transport mechanism by which, they oppose to manganese influx. This hypothesis is also supported by the 10-15 min time lag between the moment of doping and the start of the manganese influx into the fresh erythrocytes. The manganese uptake inhibition by nifedipine, a calcium channel blocker, for the case of vanadate treated erythrocytes, suggests that, at least partially, manganese uptake by the cells occurs via the 'calcium channels'.

On the inhibition mechanism of sarcoplasmic or endoplasmic reticulum Ca2+-ATPases by cyclopiazonic acid

Ca2+-ATPase inhibition by stoichiometric and substoichiometric concentrations of cyclopiazonic acid was studied in sarcoplasmic reticulum preparations from rabbit fast-twitch muscle. The apparent affinity of the nonphosphorylated enzyme for ATP showed a Kd of approximately 3 microM in the absence of cyclopiazonic acid and approximately 28 microM in the presence of the drug. Fractional saturation of the enzyme by cyclopiazonic acid was accompanied by the appearance of two ATP-binding populations (enzyme with and without drug) and a progressive increase in the half-maximal concentration for saturating the ATP-binding sites. Enzyme turnover in the presence of stoichiometric concentrations of cyclopiazonic acid displayed lower apparent affinity for ATP and lower maximal hydrolytic activity than in the absence of the drug. When cyclopiazonic acid is in the substoichiometric range, the observed kinetic parameters will correspond to the simultaneous contribution of two different reaction cycles sustained by the enzyme with and without drug. The inhibition could be elicited by adding ATP to allow the enzyme turnover when cyclopiazonic acid was preincubated with the enzyme in the presence of Ca2+. The onset of inhibition during enzyme cycling was observed over a period of seconds, revealing the existence of a low inhibition rate constant. It is concluded that cyclopiazonic acid decreases enzyme affinity for ATP in non-turnover conditions by approximately one order of magnitude. This allows enzyme cycling after drug binding, provided that a high ATP concentration is used. Cyclopiazonic acid and ATP do not compete for the same binding site.

Effects of three different Ca(2+)-ATPase inhibitors on Ca2+ response and leukotriene release in RBL-2H3 cells

The effects of three Ca(2+)-ATPase inhibitors, thapsigargin (TG), cyclopiazonic acid (CPA), and 2,5-di(tert-butyl)-1,4-hydroquinone (DTBHQ), on the Ca2+ response, degranulation, and leukotriene C4 (LTC4) release in RBL-2H3 cells were investigated. All three compounds elevated the intracellular free Ca2+ concentration ([Ca2+]i), and caused degranulation in the presence of 12-O-tetradecanoylphorbol-13-acetate (TPA), a protein kinase C activator. The dose-dependency of each compound in the Ca2+ response was in good agreement with that in degranulation. TG and CPA also caused the release of LTC4 in a dose-dependent manner, and this effect was unaffected by TPA or calphostin C, a selective PKC inhibitor. DTBHQ, however, did not induce LTC4 release, and rather inhibited the antigen-induced release of LTC4. These results suggest [1] that both degranulation and LTC4 release caused by these compounds are dependent on their [Ca2+]i increasing effect, [2] that degranulation and LTC4 release are mediated via independent pathways following the Ca2+ response, and [3] that DTBHQ additionally prevents the synthesis of LTC4 possibly by inhibition of 5-lipoxygenase.

Ionic dependency of membrane potential and autorhythmicity in the atrium of the whelk Busycon canaliculatum

1. Calcium-free media usually caused a cessation of all electrical and mechanical activity of the Busycon atrium. Where any electrical activity survived, the action potential consisted of a pre- and plateau-like potential devoid of the usual terminal spike. 2. High Ca salines induced tonic force, membrane depolarization and reduction in generation of spontaneous action potentials. The Ca ionophore A23187 enhanced contractions and the SR CaATPase inhibitor cyclopiazonic acid induced slight depolarization, tonic contractures and increased action potential firing. 3. The inorganic Ca antagonist Co2+ was without effect on the preparations, although the lanthanide Gd3+ inhibited contractions and spontaneous action potentials as well as inducing membrane potential depolarization. 4. The organic Ca entry-blocker nifedipine enhanced both spontaneous action potential amplitude and the phasic contractions they generated. 5. High K salines considerably depolarized atrial preparations with accompanying large tonic contractures and suppression of action potentials. The K channel-blocker 4AP enhanced action potential amplitude with slight increase in contractions, and TEA depolarized the atrium, and enhanced action potentials and rhythmic contractions. 6. Sodium-free salines strongly hyperpolarized atrial preparations and abolished spontaneous action potentials and, on washout, the membrane potential became temporarily unstable. In 2 preparations, low chloride and chloride-free media induced significant membrane potential hyperpolarization. 7. It is concluded that, in the atrium, the resting membrane potential is largely determined by the transmembrane K gradient, but with significant conductances to Na and Cl though probably not Ca. The action potential spike appears to be a Ca-dependent event and the plateau-like phase may be a Na-dependent event.

Differential effects of the protein kinase C activator phorbol 12-myristate 13-acetate on calcium responses and secretion in adherent and suspended RBL-2H3 mucosal mast cells

Adhesion of RBL-2H3 mucosal mast cells to fibronectin-coated surfaces has been linked to changes in secretion and tyrosine kinase activity. We now show that adhesion affects the sensitivity of RBL cells to the protein kinase C activator phorbol 12-myristate 13-acetate (PMA). In suspended cells, PMA inhibited antigen-induced calcium influx (as measured by manganese influx) and changes in intracellular free calcium and had complex effects on antigen-stimulated secretion. However, in adherent cells PMA had little effect on these responses. Suspended cells only secreted in response to thapsigargin if they were co-treated with PMA, while adherent cells secreted in response to thapsigargin alone. The thapsigargin-induced secretion in adherent cells was inhibited by protein kinase C down-regulation and by the protein kinase C inhibitor GF 109203X, but not by calphostin C. We suggest that protein kinase C is constitutively activated in adherent cells, possibly due to modification of the regulatory domain of the enzyme.