Characterization of calcium-triggered secretion in permeabilized rat basophilic leukemia cells. Possible role of vectorially acting G proteins

Sorting soluble tumor necrosis factor (TNF) receptor for storage and regulated secretion in hematopoietic cells

Hematopoietic cells contain secretory lysosomes that degranulate at sites of inflammation. We envisage that secretory granules can act as vehicles for targeting inflammatory sites, including malignancies, and thereafter, locally release therapeutically active agents to these sites. Exogenous proteins, such as the soluble tumor necrosis factor receptor 1 (sTNFR1), have been shown previously to be targeted to secretory lysosomes [1]. In this work, we asked whether exogenous, secretory lysosome-targeted proteins were subject to regulated secretion. sTNFR1-transmembrane (tm)-cytosol-sorting signal (Y) and sTNFR1-tm-Y-enhanced green fluorescent protein (egfp) were expressed in rat basophilic leukemia cell clones having different secretory capacities. sTNFR1-tm-Y was targeted directly from the Golgi to secretory lysosomes, followed by generation of membrane-free sTNFR1, whose secretion could be triggered by a Ca2+ ionophore or immunoglobulin E receptor activation. In contrast, sTNFR1-tm-Y-egfp was targeted to the plasma membrane and then subjected to endocytosis and presumably, secretory lysosome targeting, as judged by results from antibody ligation and cell-surface biotinylation. Activation of protein kinase C with phorbol ester promoted ectodomain shedding at the cell surface, resulting in sTNFR1 release from sTNFR1-tm-Y-egfp. These results support a concept for using the storage organelles of hematopoietic cells as vehicles for targeting sites of inflammation with therapeutically active agents.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Secretory granule exocytosis

Regulated exocytosis of secretory granules or dense-core granules has been examined in many well-characterized cell types including neurons, neuroendocrine, endocrine, exocrine, and hemopoietic cells and also in other less well-studied cell types. Secretory granule exocytosis occurs through mechanisms with many aspects in common with synaptic vesicle exocytosis and most likely uses the same basic protein components. Despite the widespread expression and conservation of a core exocytotic machinery, many variations occur in the control of secretory granule exocytosis that are related to the specialized physiological role of particular cell types. In this review we describe the wide range of cell types in which regulated secretory granule exocytosis occurs and assess the evidence for the expression of the conserved fusion machinery in these cells. The signals that trigger and regulate exocytosis are reviewed. Aspects of the control of exocytosis that are specific for secretory granules compared with synaptic vesicles or for particular cell types are described and compared to define the range of accessory control mechanisms that exert their effects on the core exocytotic machinery.

Robust, high-resolution, whole cell patch-clamp capacitance measurements using square wave stimulation

High-resolution, whole cell capacitance measurements are usually performed using sine wave stimulation using a single frequency or a sum of two frequencies. We present here a high-resolution technique for whole-cell capacitance measurements based on square-wave stimulation. The square wave represents a sum of sinusoidal frequencies at odd harmonics of the base frequency, the amplitude of which is highest for the base frequency and decreases as the frequency increases. The resulting currents can be analyzed by fitting the current relaxations with exponentials, or by a phase-sensitive detector technique. This method provides a resolution undistinguishable from that of single-frequency sine wave stimulation, and allows for clear separation of changes in capacitance, membrane conductance, and access resistance. In addition, it allows for the analysis of more complex equivalent circuits as associated with the presence of narrow fusion pores during degranulation, tracking many equivalent circuit parameters simultaneously. The method is insensitive to changes in the reversal potential, pipette capacitance, or widely varying cell circuit parameters. It thus provides important advantages in terms of robustness for measuring cell capacitances, and allows analysis of complicated changes of the equivalent circuits.

Regulation of protein sorting at the TGN by plasma membrane receptor activation

We show that in the rat basophilic leukemia cell line RBL, the physiological stimulation of the IgE receptor or direct activation of PKC leads to the missorting of proteins to the plasma membrane, diverting them from their normal intracellular destination. This is demonstrated for two classes of proteins that are normally targeted to the secretory lysosomes via completely different mechanisms, i.e. proteoglycans and the aspartic protease cathepsin D. In the latter case, normal processing of the enzyme is also affected, leading to secretion of the immature form of cathepsin. The present study shows how completely different sorting mechanisms, such as those for delivering proteoglycans and cathepsin D to secretory lysosomes, might share common regulatory signals and are similarly affected when the levels of these signals are perturbed. Finally, protein kinase C appears to be a major player in the signal transduction pathways, leading to proteoglycan and cathepsin D missorting.

Characterization of chemical inhibitors of brefeldin A-activated mono-ADP-ribosylation

Brefeldin A, a toxin inhibitor of vesicular traffic, induces the selective mono-ADP-ribosylation of two cytosolic proteins, glyceraldehyde-3-phosphate dehydrogenase and the novel GTP-binding protein BARS-50. Here, we have used a new quantitative assay for the characterization of this reaction and the development of specific pharmacological inhibitors. Mono-ADP-ribosylation is activated by brefeldin A with an EC50 of 17.0 +/- 3.1 microg/ml, but not by biologically inactive analogs including a brefeldin A stereoisomer. Brefeldin A acts by increasing the Vmax of the reaction, whereas it does not influence the Km of the enzyme for NAD+ (154 +/- 13 microM). The enzyme is an integral membrane protein present in most tissues and is modulated by Zn2+, Cu2+, ATP (but not by other nucleotides), pH, temperature, and ionic strength. To identify inhibitors of the reaction, a large number of drugs previously tested as blockers of bacterial ADP-ribosyltransferases were screened. Two classes of molecules, one belonging to the coumarin group (dicumarol, coumermycin A1, and novobiocin) and the other to the quinone group (ilimaquinone, benzoquinone, and naphthoquinone), rather potently and specifically inhibited brefeldin A-dependent mono-ADP-ribosylation. When tested in living cells, these molecules antagonized the tubular reticular redistribution of the Golgi complex caused by brefeldin A at concentrations similar to those active in the mono-ADP-ribosylation assay in vitro, suggesting a role for mono-ADP-ribosylation in the cellular actions of brefeldin A.

Measuring serotonin distribution in live cells with three-photon excitation

Tryptophan and serotonin were imaged with infrared illumination by three-photon excitation (3PE) of their native ultraviolet (UV) fluorescence. This technique, established by 3PE cross section measurements of tryptophan and the monoamines serotonin and dopamine, circumvents the limitations imposed by photodamage, scattering, and indiscriminate background encountered in other UV microscopies. Three-dimensionally resolved images are presented along with measurements of the serotonin concentration ( approximately 50 mM) and content (up to approximately 5 x 10(8) molecules) of individual secretory granules.

Regulation of constitutive exocytic transport by membrane receptors. A biochemical and morphometric study

Biochemical and morphometric approaches were combined to examine whether constitutive secretory transport might be controlled by plasma membrane receptors, as this possibility would have significant physiological implications. Indeed, IgE receptor stimulation in rat basophilic leukemia cells potently increased the rate of transport of soluble pulse-labeled 35S-sulfated glycosaminoglycans from distal Golgi compartments to the cell surface. This effect was largely protein kinase C (PKC)-dependent. Direct activation of PKC also stimulated constitutive transport of glycosaminoglycans, as indicated by the use of agonistic and antagonistic PKC ligands. PKC ligands also had potent, but different, effects on the exocytic transport from distal Golgi compartments to the plasma membrane of a membrane-bound protein (vesicular stomatitis virus glycoprotein), which was slightly stimulated by activators and profoundly suppressed by inhibitors of PKC. Morphological analysis showed impressive changes of the organelles of the secretory pathway in response to IgE receptor stimulation and to direct PKC activation (enhanced number of buds and vesicles originating from the endoplasmic reticulum and Golgi and increase in surface and volume of Golgi compartments), suggestive of an overall activation of exocytic movements. These results show that rapid and large changes in constitutive transport fluxes and in the morphology of the exocytic apparatus can be induced by membrane receptors (as well as by direct PKC stimulation).

Fc epsilon RI-stimulated Ca(2+)-dependent secretion from rat basophilic leukemia (RBL-2H3) cells permeabilized with Staphylococcal alpha-toxin: Fc epsilon RI-operated signals are not mimicked by the actions of GTP gamma S

1. RBL-2H3 cells permeabilized with alpha-toxin responded to dinitrophenol (30-40 mol/mol)-conjugated human serum albumin, as antigen, to secrete [14C]serotonin in the micromolar range of free Ca2+. 2. Calcium ion alone did not cause substantial secretion. 3. Guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) (100 microM) in combination with Ca2+ produced only negligible [14C]serotonin secretion. 4. GTP gamma S, in the presence of cytochalasin D, caused optimal secretion of [14C]serotonin in a Ca(2+)-dependent manner.

Receptor and protein kinase C-mediated regulation of ARF binding to the Golgi complex

The formation of constitutive transport vesicles involves the association of non-clathrin coat proteins to transport organelles. Here we report that IgE receptors and protein kinase C (PKC) regulate the GTP-dependent binding of the two coat proteins ADP-ribosylation factor (ARF) and beta-COP to Golgi membranes in rat basophilic leukaemia cells. Activation of IgE receptors and PKC prevented the ARF and beta-COP dissociation from Golgi membranes that occurs in permeabilized cells in the absence of GTP and potentiated the association-promoting effects of GTP and the G protein activator fluoroaluminate. In contrast, PKC downregulation and PKC inhibition abolished the activity of GTP and fluoroaluminae in promoting ARF binding to the Golgi complex. Studies of ARF binding to isolated Golgi membranes gave similar results. These findings suggest that coat assembly on Golgi membranes, and thus possibly constitutive secretory traffic, is modulated by membrane receptors and second messengers.

Inhibition of pituitary hormone exocytosis by a synthetic peptide related to the rab effector domain

GTP-binding proteins of the rab family are believed to function at several steps in intracellular vesicular transport. We examined the effects of a rab-related peptide in permeabilized pituitary cells, in which exocytosis can be triggered by distinct Ca(2+)-dependent or Ca(2+)-independent pathways. We report that a synthetic peptide of 18 amino acids related to the rab effector domain, rab3AL (30-47) inhibited luteinizing hormone (LH) and growth hormone (GH) exocytosis triggered by either pathway. Ca(2+)-stimulated LH and GH release were inhibited by more than 80% and 50%, respectively, by 100 microM peptide. The peptide (100 microM) also inhibited LH and GH exocytosis stimulated by phorbol myristate acetate plus cAMP by more than 45% and 80%, respectively. The effect was sequence-specific since a second peptide, lacking the first 3 amino acids but otherwise identical failed to inhibit exocytosis. These results suggest that a protein of the rab family is involved in regulated pituitary hormone exocytosis, and they identify 3 amino acids of the putative rab effector domain which may be functionally important in exocytosis.

GTP gamma S stimulates exocytosis in patch-clamped rat melanotrophs

We investigated the molecular mechanisms regulating exocytosis in patch-clamped melanotrophs by measuring the membrane capacitance. Ca(2+)-dependent exocytosis could be induced by membrane depolarization or by including solutions containing 2 microM free Ca2+ in the patch pipette. Ca(2+)-dependent exocytosis was inhibited by GDP beta S, suggesting involvement of a GTP-binding protein. The hydrolysis-resistant GTP analogs, GTP gamma S and GppNHp, were able to stimulate exocytosis at low free Ca2+ concentrations. The stimulatory response to GTP gamma S was abolished by both GDP beta S and GTP. The latter suggests that a sustained activation of a GTP-binding protein is necessary for exocytosis. This behavior is similar to the stimulation of exocytosis by guanine nucleotides in mast cells and other nonexcitable cells and suggests a common regulatory mechanism.

Calcium-induced secretion from permeabilized rat mast cells: requirements for guanine nucleotides

Cells used in this work were permeabilized by streptolysin-O and then washed to remove freely soluble components. The secretory responsiveness of these cells to various combinations of calcium, MgATP and guanine nucleotide was characterized and in most respects was found to be similar to that of the metabolically inhibited (unwashed) cells. The content of adenosine and guanine nucleotides, which remain within the permeabilized cells after washing, was estimated as 0.83 and 0.12 mM (extrapolated to intact cells), which constitutes 18 and 25%, respectively, of the total nucleotide content of mast cells. High (> mM) concentrations of MgATP, required for the calcium-induced secretion, were reduced to microM levels by suboptimal concentrations of GTP, which also markedly increased both the rate and extent of the response. Similarly, microM concentrations of MgATP reduced the requirements of the calcium-dependent secretion for GTP. The synergy of the GTP and ATP effects suggests that, together, the two nucleotides can maintain a pool of free GTP, presumably as a result of transphosphorylation from ATP to GDP. Thus, MgATP may work by transphosphorylating the endogenous GDP. However, neither GTP nor GTP-gamma-S were effective as substitutes for MgATP in the calcium-induced secretion, particularly that from metabolically inhibited cells. This indicates that MgATP does not act simply by providing GTP but is needed to maintain a phosphorylated state of the system. The synergistic effects of ATP and GTP were observed only in the presence of calcium. To test whether calcium/MgATP-induced secretion requires an activated G protein, the effects of G-protein inactivators were studied. GDP, deoxy GDP and GDP-beta-S exerted differing degrees of inhibition on secretory responses induced by various combinations of effectors. The response to calcium/MgATP was less sensitive to these inhibitors than that to GTP-gamma-S (with or without calcium). However, all three 'inhibitors' were also capable of stimulating calcium/MgATP-dependent secretion, indicating a transphosphorylation, producing GTP, dGTP and GTP-beta-S. Thus, in the absence of any specific inhibitors for either G proteins or the transphosphorylation reaction, the degree of dependence of the calcium-induced secretion on a G protein remains unclear.

Cell-to-cell spread of calcium signals mediated by ATP receptors in mast cells

Rat basophilic leukaemia cells, like mast cells from which they are derived, have surface Fc epsilon receptors that trigger secretion of inflammatory mediators when crosslinked. Both GTP-binding proteins and a rise in cytosolic calcium concentration ([Ca2+]i) are implicated in the secretory mechanism. Here we use a video-imaging technique to report that transient rises in [Ca2+]i initiated in an individual cell can spread from cell to cell in a wave-like pattern by means of a secreted intermediate, in the absence of gap-junctional communication. We find that the leukaemia cells, peritoneal mast cells and mucosal mast cells have cell-surface P2-type purinergic receptors that can trigger similar [Ca2+]i transients. We provide evidence that ATP is rapidly released, and that it can amplify [Ca2+]i signals and initial secretory responses during antigen-stimulation of rat basophilic leukaemia cells.

Synthetic peptides of the effector-binding domain of rab enhance secretion from digitonin-permeabilized chromaffin cells

There is evidence that the rab class of low molecular weight GTP-binding proteins is involved in vesicular transfer from endoplasmic reticulum to Golgi and between Golgi cisternae. To determine whether similar proteins play a role in regulated exocytosis, the effects of synthetic peptides derived from low molecular weight GTP-binding proteins on catecholamine secretion from digitonin-permeabilized chromaffin cells were investigated. The synthetic peptides represent the putative effector-binding domains of the rab, ras and ral classes of low molecular weight GTP-binding proteins and correspond to ras(33-48). Two rab peptides but neither a ras nor a ral peptide enhanced Ca(2+)-dependent secretion by approximately 30%. Maximal secretion in response to Ca2+ was increased. The enhancement was not blocked by the pseudosubstrate inhibitor of protein kinase C, PKC(19-31), thus indicating that activation of protein kinase C was not responsible for the enhancement of secretion. Similarly a rab peptide but neither a ras nor a ral peptide enhanced GppNHp-induced secretion 30-70%. The peptides had little or no effect in the absence of Ca2+ or GppNHp. The data are consistent with a protein of the rab class playing a role in regulated exocytosis.

Involvement of pertussis toxin-sensitive and -insensitive GTP-binding proteins in luteinizing hormone exocytosis distal to second messenger generation

Inhibition of luteinizing hormone (LH) exocytosis by guanosine 5'-[gamma-thio]triphosphate (GTP gamma S) in permeabilized pituitary cells has indicated the involvement of one or more GTP-binding proteins in the exocytotic mechanism distal to second messenger generation. We now report that two inhibitory sites of action of GTP gamma S can be distinguished by their dependence on GTP gamma S concentration and their sensitivity to pertussis toxin. Ca(2+)-stimulated exocytosis was half-maximally inhibited by 6.8 microM GTP gamma S, a six-fold higher concentration than that required for inhibition of exocytosis stimulated by phorbol ester plus cAMP. In addition, GTP gamma S inhibition of Ca(2+)-stimulated exocytosis was insensitive to pertussis toxin, in contrast to the inhibition of exocytosis stimulated by phorbol ester plus cAMP, which was abolished by pretreatment with pertussis toxin. These results indicate that at least two stimulus-specific GTP-binding proteins are involved in regulating LH exocytosis distal to second messenger generation.

Adenosine receptors in rat basophilic leukaemia cells: transductional mechanisms and effects on 5-hydroxytryptamine release

1. The presence of adenosine receptors linked to adenylate cyclase activity and their functional role in calcium-evoked 5-hydroxytryptamine (5-HT) release was investigated in rat basophilic leukaemia (RBL) cells, a widely used model for studying the molecular mechanisms responsible for stimulus-secretion coupling. 2. In [3H]-5-HT-loaded cells triggered to release by the calcium ionophore A23187, a biphasic modulation of 5-HT secretion was induced by adenosine analogues, with inhibition of stimulated release at nM and potentiation at microM concentrations, suggesting the presence of adenosine receptor subtypes mediating opposite effects on calcium-dependent release. This was also confirmed by results obtained with other agents interfering with adenosine pharmacology, such as adenosine deaminase and the non-selective A1/A2 antagonist 8-phenyl-theophylline. 3. Similar biphasic dose-response curves were obtained with a variety of adenosine analogues on basal adenylate cyclase activity in RBL cells, with inhibition and stimulation of adenosine 3':5'-cyclic monophosphate (cyclic AMP) production at nM and microM concentrations, respectively. The rank order of potency of adenosine analogues for inhibition and stimulation of adenylate cyclase activity and the involvement of G-proteins in modulation of cyclic AMP levels suggested the presence of cyclase-linked A1 high-affinity and A2-like low-affinity adenosine receptor subtypes. However, the atypical antagonism profile displayed by adenosine receptor xanthine antagonists on cyclase stimulation suggested that the A2-like receptor expressed by RBL cells might represent a novel cyclase-coupled A2 receptor subtype.4. Micromolar concentrations of adenosine analogues could also increase inositol phospholipid hydrolysis and inositol tris-phosphate formation in both unstimulated cells and in cells triggered to release by the calcium ionophore. The stimulation was constant, small and additive to that exerted by the calcium ionophore.5. It is concluded that RBL cells express both A1 and A2-like adenosine receptors which exert opposite effects on 5-HT release and intracellular cyclic AMP levels. However, besides modulation of cyclic AMP levels, additional transduction pathways, such as modulation of phospholipase C activity, may contribute to the release response evoked by adenosine analogues in this cell-line.