Ca2(+)-induced secretion by electropermeabilized human neutrophils. The roles of Ca2+, nucleotides and protein kinase C

Mutation of tyrosine 145 of lymphocyte cytosolic protein 2 protects mice from anaphylaxis and arthritis

BACKGROUND

Lymphocyte cytosolic protein 2, also known as Src homology 2 domain-containing leukocyte phosphoprotein of 76 kilodaltons (SLP-76), is an essential adaptor molecule in myeloid cells, where it regulates FcepsilonRI-induced mast cell (MC) and FcgammaR- and integrin-induced neutrophil (polymorphonuclear leukocyte [PMN]) functions. SLP-76 contains 3 N-terminal tyrosines at residues 112, 128, and 145 that together are critical for its function.

OBJECTIVE

We sought to explore the relative importance of tyrosines 112, 128, and 145 of SLP-76 during MC and PMN activation.

METHODS

We examined in vitro MC and PMN functions using cells isolated from knock-in mice harboring phenylalanine substitution mutations at tyrosines 112 and 128 (Y112/128F) or 145 (Y145F). We also examined the effects of these mutations on in vivo MC and PMN activation using models of anaphylaxis, dermal inflammation, and serum-induced arthritis.

RESULTS

Mutations at Y112/Y128 and Y145 both interfered with SLP-76 activity; however, Y145F had a greater effect than Y112/128F on most in vitro FcR-induced functions. In vitro functional defects were recapitulated in vivo, where mice expressing Y145F exhibited greater attenuation of MC-dependent passive systemic anaphylaxis and PMN-mediated inflammatory responses. Notably, the Y145F mutation completely protected mice against development of joint-specific inflammation in the MC and PMN-dependent K/B x N model of arthritis.

CONCLUSION

Our data indicate that Y145 is the most critical tyrosine supporting SLP-76 function in myeloid cells. Future efforts to dissect how Y145 mediates SLP-76-dependent signaling in MCs and PMNs will increase our understanding of these lineages and provide insights into the treatment of allergy and inflammation.

A peptide against the N-terminus of myristoylated alanine-rich C kinase substrate inhibits degranulation of human leukocytes in vitro

Leukocytes synthesize a variety of inflammatory mediators that are packaged and stored in the cytoplasm within membrane-bound granules. Upon stimulation, the cells secrete the granule contents via an exocytotic process whereby the granules translocate to the cell periphery, the granule membranes fuse with the plasma membrane, and the granule contents are released extracellularly. We have reported previously that another exocytotic process, release of mucin by secretory cells of the airway epithelium, is regulated by the myristoylated alanine-rich C kinase substrate (MARCKS) (Li Y, Martin LD, Spizz G, Adler KB. MARCKS protein is a key molecule regulating mucin secretion by human airway epithelial cells in vitro. J Biol Chem 2001;276:40982-40990; Singer M, Martin LD, Vargaftig BB, Park J, Gruber AD, Li Y, Adler KB. A MARCKS-related peptide blocks mucus hypersecretion in a mouse model of asthma. Nat Med 2004;10:193-196). In those studies, mucin secretion in vitro and in vivo was attenuated by a synthetic peptide identical to the N-terminus of MARCKS, named the MANS peptide (Li and colleagues, 2001). In this study, we used the MANS peptide to investigate possible involvement of MARCKS in secretion of leukocyte granule proteins. In neutrophils isolated from human blood, phorbol 12-myristate 13-acetate-induced myeloperoxidase release was attenuated in a concentration-dependent manner by MANS but not by equal concentrations of a missense control peptide. In additional studies using human leukocyte cell lines, secretion of eosinophil peroxidase from the eosinophil-like cell line HL-60 clone 15, lysozyme from the monocytic leukemia cell line U937, and granzyme from the lymphocyte natural killer cell line NK-92 were attenuated by preincubation of the cells with MANS but not with the missense control peptide. The results indicate that MARCKS protein may play an important role in the secretion of membrane-bound granules from different leukocytes. MARCKS may be an important component of secretory pathways associated with release of granules by different cell types.

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.

Protein kinase C and guanosine triphosphate combine to potentiate calcium-dependent membrane fusion driven by annexin 7

Exocytotic secretion is promoted by the concerted action of calcium, guanine nucleotide, and protein kinase C. We now show that the calcium-dependent membrane fusion activity of annexin 7 in vitro is further potentiated by the combined addition of guanine nucleotide and protein kinase C. The observed increment involves the simultaneous activation of annexin 7 by these two effectors. Guanosine triphosphate (GTP) and its non-hydrolyzable analogues optimally enhance the phosphorylation of annexin 7 by protein kinase C in vitro. Reciprocally, phosphorylation by protein kinase C significantly potentiates the binding and hydrolysis of GTP by annexin 7. Only protein kinase C-dependent phosphorylation has a significant positive effect on annexin 7 GTPase, although other protein kinases, including cAMP-dependent protein kinase, cGMP-dependent protein kinase, and pp60(c-)(src), have been shown to label the protein with high efficiency. In vivo, the ratio of bound GDP/GTP and phosphorylation of annexin 7 change in direct proportion to the extent of catecholamine release from chromaffin cells in response to stimulation by carbachol, or to inhibition by various protein kinase C inhibitors. These results thus lead us to hypothesize that annexin 7 may serve as a common site of action for calcium, guanine nucleotide, and protein kinase C in the exocytotic membrane fusion process in chromaffin cells.

Phosphorylation of cysteine string protein by protein kinase A. Implications for the modulation of exocytosis

Cyclic AMP-dependent protein kinase (PKA) enhances regulated exocytosis in neurons and most other secretory cells. To explore the molecular basis of this effect, known exocytotic proteins were screened for PKA substrates. Both cysteine string protein (CSP) and soluble NSF attachment protein-alpha (alpha-SNAP) were phosphorylated by PKA in vitro, but immunoprecipitation of cellular alpha-SNAP failed to detect (32)P incorporation. In contrast, endogenous CSP was phosphorylated in synaptosomes, PC12 cells, and chromaffin cells. In-gel kinase assays confirmed PKA to be a cellular CSP kinase, with phosphorylation occurring on Ser(10). PKA phosphorylation of CSP reduced its binding to syntaxin by 10-fold but had little effect on its interaction with HSC70 or G-protein subunits. Furthermore, an in vivo role for Ser(10) phosphorylation at a late stage of exocytosis is suggested by analysis of chromaffin cells transfected with wild type or non-phosphorylatable mutant CSP. We propose that PKA phosphorylation of CSP could modulate the exocytotic machinery, by selectively altering its availability for protein-protein interactions.

GTP-dependent permeabilized neutrophil secretion requires a freely diffusible cytosolic protein

Guanosine triphosphate (GTP) has been implicated in the regulation of Ca(2+)-mediated secretion from neutrophils. We further examined the role of GTP in neutrophil secretion using streptolysin O permeabilized cells. We found that, in the presence of GTP, 1.0 microM free Ca(2+) causes maximum secretion-equivalent to that achieved with 100 microM free Ca(2+)-whereas GTPgammaS inhibits Ca(2+)-stimulated secretion. Interestingly, GTP by itself stimulates secretion. These results indicate the existence of a GTP-regulated mechanism of secretion in neutrophils that requires GTP hydrolysis to stimulate secretion in the presence and absence of Ca(2+). The stimulatory effect of GTP is only observed when GTP is present during permeabilization. Addition of GTP after permeabilization, when the cytosolic contents have leaked out from cells, gives no stimulatory response, implying that the GTP-dependent secretory apparatus requires at least one cytosolic protein. GTP-dependent secretion can be reconstituted with crude HL-60 and bovine liver cytosol. The reconstituting activity binds to GTP-agarose, suggesting that the cytosolic factor is a GTP-binding protein or forms a complex with a GTP-binding protein. However, it is not a member of the rho or rac families of GTPases. By gel filtration chromatography, the secretion-reconstituting activity eluted at 870 and 200 kDa, but in the presence of GTP, eluted at 120 kDa, indicating that it is part of a high-molecular-weight complex that dissociates in the presence of GTP. Retention of adenosine diphosphate-ribosylation factor (ARF) in permeabilized cells and insensitivity of the cytosolic reconstituting activity to brefeldin A led to our speculation that ARF6 may be the GTPase involved in GTP-dependent secretion, and that activity from a BFA-insensitive ARF6 guanine nucleotide exchange factor reconstitutes secretion.

L-selectin signaling of neutrophil adhesion and degranulation involves p38 mitogen-activated protein kinase

The adhesion molecules known as selectins mediate the capture of neutrophils from the bloodstream. We have previously reported that ligation and cross-linking of L-selectin on the neutrophil surface enhances the adhesive function of beta(2)-integrins in a synergistic manner with chemotactic agonists. In this work, we examined degranulation and adhesion of neutrophils in response to cross-linking of L-selectin and addition of interleukin-8. Cross-linking of L-selectin induced priming of degranulation that was similar to that observed with the alkaloid cytochalasin B. Activation mediated by L-selectin of neutrophil shape change and adhesion through CD11b/CD18 were strongly blocked by Merck C, an imidazole-based inhibitor of p38 mitogen-activated protein kinase (MAPK), but not by a structurally similar non-binding regioisomer. Priming by L-selectin of the release of secondary, tertiary, and secretory, but not primary, granules was blocked by inhibition of p38 MAPK. Peak phosphorylation of p38 MAPK was observed within 1 min of cross-linking L-selectin, whereas phosphorylation of ERK1/2 was highest at 10 min. Phosphorylation of p38 MAPK, but not ERK1/2, was inhibited by Merck C. These data suggest that signal transduction as a result of clustering L-selectin utilizes p38 MAPK to effect neutrophil shape change, integrin activation, and the release of secondary, tertiary, and secretory granules.

Alpha-granule secretion from alpha-toxin permeabilized, MgATP-exposed platelets is induced independently by H+ and Ca2+

In order to better understand granule release from platelets, we developed an alpha-toxin permeabilized platelet model to study alpha-granule secretion. Secretion of alpha-granules was analyzed by flow cytometry using P-selectin as a marker for alpha-granule release. P-selectin surface expression occurred when platelets were permeabilized in the presence of Ca2+. Responsiveness to Ca2+ was lost 30 min after permeabilization but could be reconstituted with MgATP. Alpha-toxin-permeabilized, MgATP-exposed platelets also degranulated within a pH range of 5.4-5.9 without exposure to and independent of Ca2+. ATP, GTP, CTP, UTP, and ITP supported Ca2+-induced alpha-granule secretion, while H+-induced alpha-granule secretion occurred only with ATP and GTP. Both Ca2+- and H+-induced alpha-granule secretion required ATP hydrolysis. Kinase inhibitors blocked both Ca2+- and H+-induced secretion. These data suggest that alpha-granule secretion in this permeabilized platelet system shares many characteristics with granule secretion studied in other permeabilized cell models. Furthermore, these results show that H+ can trigger alpha-granule release independent of Ca2+.

High micromolar Ca2+ beneath the plasma membrane in stimulated neutrophils

Ca2+ near the inner face of the plasma membrane, as reported by the membrane associated fluorescent Ca2+ probe FFP-18, was higher than the bulk cytosolic free Ca2+ concentration both in resting neutrophils and in response to f-met-leu-phe. Influx caused Ca2+ close to the plasma membrane to rise more rapidly than the bulk cytosolic free Ca2+ and to reach a peak concentration of at least 30 microM. This zone of high Ca2+ was localised to just beneath the plasma membrane and did not extend more than 0.1 micron into the cell, as it was undetected by the bulk cytosolic free Ca2+ probes magfura2 and fura2. From these data, reconstruction of the distribution of Ca2+ within the neutrophil showed that the high Ca2+ signal at the cell cortex rapidly subsided to give a uniform free Ca2+ across the cell.

Ca2+-induced exocytosis in individual human neutrophils: high- and low-affinity granule populations and submaximal responses

We have investigated Ca2+-induced exocytosis from human neutrophils using the whole cell patch-clamp capacitance technique. Microperfusion of Ca2+ buffer solutions (<30 nM to 5 mM free Ca2+) through the patch-clamp pipette revealed a biphasic activation of exocytosis by Ca2+. The first phase was characterized by high affinity (1.5-5 microM) and low apparent cooperativity (<=2) for Ca2+, and the second phase by low affinity (approximately 100 microM) and high cooperativity (>6). Only the second phase was accompanied by loss of myeloperoxidase, suggesting that the low-affinity exocytosis reflected release of peroxidase-positive (primary) granules, while the high-affinity exocytosis reflected release of peroxidase-negative (secondary and tertiary) granules. At submaximal Ca2+ concentrations, only a fraction of a given granule population was released. This submaximal release cannot simply be explained by Ca2+ modulation of the rate of exocytosis, and it suggests that the secretory response of individual cells is adjusted to the strength of the stimulus. The Ca2+ dependence of the high- and low-affinity phases of neutrophil exocytosis bears a resemblance to endocrine and neuronal exocytosis, respectively. The occurrence of such high- and low-affinity exocytosis in the same cell is novel, and suggests that the Ca2+ sensitivity of secretion is granule-, rather than cell-specific.

Store-operated Ca2+ influx and stimulation of exocytosis in HL-60 granulocytes

This study addresses the role of store-operated Ca2+ influx in the regulation of exocytosis in inflammatory cells. In HL-60 granulocytes, which do not possess voltage-operated Ca2+ channels, the chemotactic peptide fMet-Leu-Phe (fMLP) was able to stimulate store-operated Ca2+ influx and to trigger exocytosis of primary granules. An efficient triggering of exocytosis by fMLP required the presence of extracellular Ca2+ and was inhibited by blockers of store-operated Ca2+ influx. However, receptor-independent activation of store-operated Ca2+ influx through thapsigargin did not trigger exocytosis. fMLP was unable to stimulate exocytosis in the absence of cytosolic free Ca2+ concentration [Ca2+]c elevations. However, a second signal generated by fMLP synergized with store-operated Ca2+ influx to trigger exocytosis and led to a left shift of the exocytosis/[Ca2+]c relationship in ionomycin-stimulated cells. The synergistic fMLP-generated signaling cascade was long-lasting, involved a pertussis toxin-sensitive G protein and a phosphatidylinositol 3-kinase. In summary, store-operated Ca2+ influx is crucial for the efficient triggering of exocytosis in HL-60 granulocytes, but, as opposed to Ca2+ influx through voltage-operated Ca2+ channels in neurons, it is not a sufficient stimulus by itself and requires synergistic receptor-generated signals.

Characterization of peptide diffusion into electropermeabilized neutrophils

The superoxide (O2-)-generating NADPH oxidase of human neutrophils consists of membrane-bound and cytosolic proteins that assemble in the plasma membrane of activated cells. To date, most of our understanding of the assembly of the NADPH oxidase has been obtained through the use of a cell-free assay, and a number of peptides that mimic regions of NADPH oxidase proteins have been shown to block oxidase assembly using this assay. However, the cell-free assay provides an incomplete representation of the assembly and regulation of the NADPH oxidase in vivo, and it has become necessary to develop methods for introducing biomolecules, such as peptides, into intact neutrophils where their effects can be investigated. One such method is electropermeabilization. Although this method has been used previously with human neutrophils, it has not been well characterized. We report here a detailed characterization of the electropermeabilized neutrophil assay system, including optimal conditions for membrane electropermeabilization with maximal retention of functional capacity, optimal conditions for analyzing the effects of experimental peptides, quantification of internalized peptide concentration, and molecular size limits for diffusion of molecules into these cells. Our results demonstrate that optimal neutrophil permeabilization (98-100%) can be achieved using significantly lower electrical fields than previously reported, resulting in the retention of higher levels of O2(-)-generating activity. We also found that biomolecules as large as 2.3 kDa readily diffuse into permeabilized cells. Analysis of flavocytochrome b peptides that were shown previously to inhibit NADPH oxidase activity in a cell-free assay demonstrated that these peptides also blocked O2- production in electropermeabilized human neutrophils; although at higher effective concentrations than in the cell-free system. Thus, electropermeabilized neutrophils provide a model system for evaluating the effects of peptides and other pharmacological agents in intact cells which closely mimic neutrophils in vivo.

Role of protein phosphorylation in the degranulation of electropermeabilized human neutrophils

We have studied the role of protein phosphorylation in the degranulation response of human neutrophils by measuring the effect of ATP depletion and the addition of the protein kinase inhibitor staurosporine in electropermeabilized human neutrophils, activated with Ca2+ and/or GTP-gamma-S. Our studies were carried out in the presence of cytochalasin B to prevent inhibitory effects of actin polymerization on the degranulation response. It was found that protein phosphorylation plays an important role in the degranulation response in cells stimulated with the single stimuli Ca2+ or GTP-gamma-S. However, in neutrophils stimulated with the combination of these activators degranulation can occur without the apparent need for protein phosphorylation, albeit with a slower rate than with protein phosphorylation.

Nitric oxide reacts with intracellular glutathione and activates the hexose monophosphate shunt in human neutrophils: evidence for S-nitrosoglutathione as a bioactive intermediary

We performed experiments to determine whether nitric oxide promoted the formation of intracellular S-nitrosothiol adducts in human neutrophils. At concentrations sufficient to inhibit chemoattractant-induced superoxide anion production, nitric oxide caused a depletion of measurable intracellular glutathione as determined by both the monobromobimane HPLC method and the glutathione reductase recycling assay. The depletion of glutathione could be shown to be due to the formation of intracellular S-nitrosoglutathione as indicated by the ability of sodium borohydride treatment of cytosol to result in the complete recovery of measurable glutathione. The formation of intracellular S-nitrosylated compounds was confirmed by the capacity of cytosol derived from nitric oxide-treated cells to ADP-ribosylate glyceraldehyde-3-phosphate dehydrogenase. Depletion of intracellular glutathione was accompanied by a rapid and concomitant activation of the hexose monophosphate shunt (HMPS) following exposure to nitric oxide. Kinetic studies demonstrated that nitric oxide-dependent activation of the HMPS was reversible and paralleled nitric oxide-induced glutathione depletion. Synthetic preparations of S-nitrosoglutathione shared with nitric oxide the capacity to inhibit superoxide anion production and activate the HMPS. These data suggest that nitric oxide may regulate cellular functions via the formation of intracellular S-nitrosothiol adducts and the activation of the HMPS.

Immunoelectron microscopy reveals significant granule fusion upon stimulation of electropermeabilized human neutrophils

Although electropermeabilization has become an important tool for studying the signal requirements of exocytosis, relatively little is known about the morphological changes accompanying this response in electropermeabilized cells. In this study, we determined that electropermeabilization of human neutrophils by itself caused only minor changes in the morphology as determined by transmission electron microscopy. The structure of the plasma membrane did not show detectable changes, whereas the cytoplasm was more electron lucent as compared to intact cells. Activation of intact neutrophils with formyl-methionyl-leucyl-phenylalanine (FMLP), in the presence of cytochalasin-B, caused the development of invaginations of the plasma membrane. In contrast, activation of electropermeabilized cells with 1 microM Ca2+ and/or 50 microM GTP-gamma-S caused the development of vacuoles that did not seem to be in contact (or had previously been in contact) with the extracellular environment. However, fusion of azurophilic and specific granules with these vacuoles clearly had taken place. The response characteristics of this fusion induced by Ca2+ and GTP-gamma-S were quite similar to those of the direct fusion of granules with the plasma membrane. We conclude that in electropermeabilized human neutrophils, two processes involving granule fusion can be distinguished. First, a direct fusion of granules with the plasma membrane. Secondly, the fusion of granules leading to the formation of vacuoles, not in contact with the extracellular space.

GTP[S] stimulates migration of electropermeabilized neutrophils via a pertussis toxin-sensitive G-protein

Electropermeabilized neutrophils were used to study the role of G-proteins in neutrophil migration. Rabbit neutrophils, under specific conditions, retained their ability to migrate after electropermeabilization. Introduction of guanosine-5'-[3-thio] triphosphate (GTP[S]) into the cell interior stimulated random migration and enhanced migration activated by a suboptimal concentration of formyl-methionyl-leucyl-phenylalanine (fMet-Leu-Phe) (10(-11) M). GTP[S] had no effect on random migration by intact cells, or on migration of intact cells activated with a suboptimal concentration of fMet-Leu-Phe, indicating that the effect of GTP[S] was intracellular. The effects of GTP[S] were inhibited by pertussis toxin and by guanosine-5'-[2-thio] diphosphate (GDP beta S) indicating that a pertussis toxin-sensitive G-protein was involved. GTP stimulated random migration to the same extent as GTP[S], but had only a small effect on migration activated by a suboptimal concentration of fMet-Leu-Phe (10(-11) M). Several other nucleotides tested had no effect on random migration or migration activated with 10(-11) M fMet-Leu-Phe. The results show that neutrophil migration can be potentiated by direct activation of a pertussis toxin-sensitive G-protein, and the results obtained with GTP suggest that possibly more than one G-protein is involved in this process.

Strontium and barium induce exocytosis in electropermeabilized neutrophils

Calcium, strontium and barium induced an exocytotic response in electropermeabilized rabbit neutrophils while magnesium was without any effect. The extent of enzyme release was found to depend upon the concentration of these cations. For all cations, an optimum concentration was found with the same maximum enzyme release. At concentrations higher than optimum a decrease in lysozyme release was observed. Efficiency to induce enzyme release was in the order: Ca2+ > Sr2+ > Ba2+. Enzyme release was significantly enhanced by guanosine-5'-[gamma-thio]triphosphate (GTP gamma S) resulting in a shift to the left of the dose/response curve. The enhancement by GTP gamma S was strongest with Ca2+, was less with Sr2+, and was very little with Ba2+. The time course of lysozyme release was the same for Ca2+, Sr2+, and Ba2+ in the presence and absence of GTP gamma S when suboptimal cation concentrations were used. A decrease in responsiveness to the effectors after electropermeabilization was observed with Ca2+, Sr2+ and Ba2+ in the presence and absence of GTP gamma S. The lysozyme release induced by the different cations was not inhibited by the protein kinase C inhibitor staurosporine and was slightly affected by pertussis toxin. Ca2+ and Sr2+, but not Ba2+, potentiated formyl-methionyl-leucyl-phenylalanine (fMet-Leu-Phe) induced enzyme release in intact neutrophils. The divalent cation ionophore A23187 induced enzyme release in the presence of Ca2+ and Sr2+ but not in the presence of Ba2+. The results obtained with electropermeabilized neutrophils indicate that Sr2+ and Ba2+ can act as substitutes for Ca2+ in activating exocytosis, and that permeabilized neutrophils provide the best tool to investigate the effects of alkaline earth ions in exocytosis.

Exocytosis in electropermeabilized neutrophils. Responsiveness to calcium and guanosine 5'-[gamma-thio]triphosphate

Electropermeabilized neutrophils were used to study the exocytotic response in rabbit neutrophils. Enzyme release from electropermeabilized neutrophils could be induced by elevating the Ca2+ concentration. Ca(2+)-induced secretion was significantly enhanced by guanosine 5'-[gamma-thio]triphosphate (GTP[S]) in a concentration-dependent manner. The effect of GTP[S] could be blocked by guanosine 5'-[beta-thio]diphosphate (GDP[S]) and was not affected by pertussis toxin. GTP[S] did not induce enzyme release in the absence of Ca2+. Induction of an exocytotic response did not require addition of ATP. However, neutrophils permeabilized in the absence of ATP became refractory to stimulation due to a reduction in their affinity for Ca2+. Responsiveness to the effectors Ca2+ or Ca2+ + GTP[S] could be prolonged or restored by ATP. ATP was not the only agent that prolonged responsiveness; other nucleotides and inorganic phosphates were also effective. The protein kinase C inhibitors staurosporine and 1-O-hexadecyl-2-methyl-sn-glycerol did not inhibit exocytosis and had only a small effect on the prolongation and restoration of responsiveness by ATP. A hypothesis is presented suggesting that the loss of responsiveness is caused by dephosphorylation and that the restoration or prolongation of responsiveness is not mediated by protein kinase C. It is possible that an as yet unidentified Ca(2+)-binding protein is dephosphorylated, resulting in a decrease in Ca2+ affinity.

Differential up-regulation of specific and azurophilic granule membrane markers in electropermeabilized neutrophils

We have developed an alternative method to study the degranulation in electropermeabilized human neutrophils by measuring the up-regulation of the specific membrane markers CD63 (residing in the azurophilic granules of resting neutrophils) and CD67 (present in specific granules). The expression of these marker proteins was measured by the binding of specific antibodies to paraformaldehyde-fixed cells and subsequent flow cytometry. We first investigated whether the changes in CD63 and CD67 expression after stimulation of intact cells were comparable with earlier measurements of neutrophil degranulation, in which the release of soluble marker proteins was measured. These experiments indicated that this new method compares favourably with earlier studies, both with respect to kinetics and stimulus dependency. Subsequently, we applied this method (which does not include centrifugation of the cells) to study degranulation in electropermeabilized neutrophils. In permeabilized neutrophils, a clear up-regulation of the specific granule marker CD67 was observed upon incubation with a free Ca2+ concentration of 1 microM, a value of the cytosolic free Ca2+ concentration occurring in formylmethionyl-leucyl-phenylalanine (FMLP)-activated neutrophils. The azurophilic granule marker CD63 required GTP-gamma-S besides 1 microM Ca2+ for a significant up-regulation. Hence, our study indicates a different requirement for intracellular signals of the two main types of granules in human neutrophils.

Dual effects of guanosine 5'-[gamma-thio]triphosphate on secretion by electroporated human neutrophils

It is generally believed that G-proteins play stimulatory roles on cell activation. In contrast, we found that guanosine 5'-[gamma-thio]triphosphate (GTP[S]) was a potent inhibitor of Ca(2+)-induced secretion from specific granules (as monitored by vitamin B-12-binding protein). GTP[S] inhibition of specific-granule release occurred in the presence or absence of adenine nucleotides, required Mg2+ (1-3 mM), and was half-maximal at 30 microM-GTP[S]. The dual stimulatory and inhibitory effects of GTP[S] could be readily observed and differentiated when degranulation was monitored over a range of Ca2+ concentrations. Inhibition of specific-granule release by GTP[S] was observed at low Ca2+ concentrations and resulted from shifting the Ca2+ dose-response curves to the right. In contrast, GTP[S] promoted azurophil-granule secretion at relatively high concentrations of Ca2+ and appeared to be due to a general enhancement at all Ca2+ concentrations. A series of hydrolysable and non-hydrolysable nucleotides did not mimic GTP[S] or block its action. Inhibition by GTP[S] occurred in cells which were sensitized with a protein kinase C agonist, suggesting that inhibition of secretion took place distal to this enzyme. However, the inhibitory effects of GTP[S] on specific-granule secretion were reversed by cytochalasin D, which prevents new microfilament formation; this compound also enhanced the stimulation of azurophil-granule release by GTP[S]. We also found that GTP[S] greatly increased the F-actin content of permeabilized neutrophils, whereas Ca2+ (to a lesser extent) decreased F-actin. These data are consistent with the hypothesis that at least two G-proteins are involved in regulating secretion: one which has been previously described as stimulating Ca(2+)-induced secretion (particularly from azurophil granules) and a second, possibly involved in promoting microfilament assembly, which inhibits the discharge of specific granules.

Guanine-nucleotide- and adenine-nucleotide-dependent regulation of phospholipase D in electropermeabilized HL-60 granulocytes

We have characterized the regulation of phospholipase D (PLD) in electropermeabilized HL-60 granulocytes in which endogenous phospholipids were pre-labelled with [3H]oleic acid. Treatment of these permeabilized cells with the non-hydrolysable GTP analogues guanosine 5'-[gamma-thio]triphosphate (GTP[S]) and guanosine 5'-[beta gamma-imido]triphosphate induced a sustained (near-linear for up to 60 min) accumulation of phosphatidic acid (PA). In the presence of ethanol a sustained production of phosphatidylethanol (PEt) was also observed. With increasing concentrations of ethanol, PEt formation increased, whereas PA formation declined; this indicated involvement of a PLD-type effector enzyme. The ability of GTP[S] to stimulate this PLD activity was Mg(2+)-dependent and was inhibited by GDP and its non-hydrolysable beta-thio analogue. Ca2+, at concentrations less than or equal to nM, had no effect on the GTP[S]-dependent PLD activity. However, higher concentrations of Ca2+ produced a significant potentiation of this activity. Inclusion of MgATP (greater than or equal to 0.1 mM), but not other nucleoside triphosphates, also induced a large potentiation of GTP[S]-dependent PLD activation. In the absence of guanine nucleotides, MgATP elicited no significant activation of PLD. Significantly, this effect of ATP was not mimicked by adenosine 5'-[beta gamma-methylene]triphosphate, a non-hydrolysable ATP analogue. Rather, this analogue inhibited both basal and ATP-potentiated GTP[S]-dependent PLD activity. This suggests that the ability of ATP to potentiate GTP[S]-dependent PLD activity involves phosphotransferase action rather than simple allosteric effects induced by adenine nucleotide binding. The absolute magnitude of the GTP[S]-dependent PLD activity which could be potentiated by MgATP was decreased by 90% when the permeabilized cells were preincubated for various times before addition of these stimulatory agents. This time-dependent loss of MgATP-induced potentiation was prevented when the permeabilized cells were preincubated in the presence of GTP[S]. These results demonstrate that electropermeabilized HL-60 granulocytes can be used to discriminate synergistic roles for a GTP-binding protein(s) and an ATP-dependent process (kinase?) in the regulation of phospholipase D activity.