Rat mast cells permeabilised with streptolysin O secrete histamine in response to Ca2+ at concentrations buffered in the micromolar range

Calcium-dependent subquantal peptide release from single docked lawn-resident vesicles of pituitary lactotrophs

Regulated exocytosis consists of the fusion between vesicles and the plasma membranes, leading to the formation of a narrow fusion pore through which secretions exit the vesicle lumen into the extracellular space. An increase in the cytosolic concentration of free Ca²⁺ ([Ca²⁺]_i) is considered the stimulus of this process. However, whether this mechanism can be preserved in a simplified system of membrane lawns with docked secretory vesicles, devoid of cellular components, is poorly understood. Here, we studied peptide discharge from individual secretory vesicles docked at the plasma membrane, prepared from primary endocrine pituitary cells (the lactotrophs), releasing hormone prolactin. To label secretory vesicles, we transfected lactotrophs to express the fluorescent atrial natriuretic peptide (ANP.emd), previously shown to be expressed in and released from prolactin-containing vesicles. We used stimulating solutions containing different [Ca²⁺] to evoke vesicle peptide discharge, which appeared similar in membrane lawns and in intact stimulated lactotrophs. All vesicles examined discharged peptides in a subquantal manner, either exhibiting a unitary or sequential time course. In the membrane lawns, the unitary vesicle peptide discharge was predominant and slightly slower than that recorded in intact cells, but with a shorter delay with respect to the stimulation onset. This study revealed directly that Ca²⁺ triggers peptide discharge from docked single vesicles in the membrane lawns with a half-maximal response of ∼8 µM [Ca²⁺], consistent with previous whole-cell patch-clamp studies in endocrine cells where the rapid component of exocytosis, interpreted to represent docked vesicles, was fully activated at <10 µM [Ca²⁺]. Interestingly, the sequential subquantal peptide vesicle discharge indicates that fluctuations between constricted and dilated fusion pore states are preserved in membrane lawns and that fusion pore regulation appears to be an autonomously controlled process.

Sodium Sulfite-Induced Mast Cell Pyroptosis and Degranulation

Sodium sulfite, a common food additive, has been proved to cause allergic reaction. Pyroptosis is an inflammatory form of programmed cell death with plasma membrane lysis. In this study, we found that sodium sulfite triggered pyroptosis, which depended on reactive oxygen species (ROS)/NOD-like receptor protein 3 (NLRP3) in RBL-2H3 mast cells. Sodium sulfite increased the generation of ROS and the expression of NLRP3, caspase-1, gasdermin D N-terminal (GSDMD-N), interleukin-1β (IL-1β), and interleukin-18 (IL-18). The ROS scavenger N-acetyl-L-carnosine (NAC) and the NLRP3 inhibitor MCC950 reversed these effects. Furthermore, using a lactate dehydrogenase kit, propidium iodide staining, scanning electron microscopy, colocalization of GSDMD-N with histamine, and neutral red staining, we found that sodium sulfite notably induced cell membrane rupture. Because β-Hexosaminidase and histamine play a key role in allergic reactions, we detected the release of β-Hexosaminidase and histamine. The data showed that the release of β-Hexosaminidase and histamine induced by sodium sulfite was increased with dose independence, which were inhibited after treatment with NAC or MCC950. Overall, evidence suggested that pyroptosis induced by sodium sulfite may rupture the cell membrane and result in degranulation of mast cells. Our study may provide new insights for the mechanism by which sodium sulfite induces mast cell death and sensitization.

Molecular Mechanisms of Mast Cell Activation by Cholesterol-Dependent Cytolysins

Mast cells are potent immune sensors of the tissue microenvironment. Within seconds of activation, they release various preformed biologically active products and initiate the process of de novo synthesis of cytokines, chemokines, and other inflammatory mediators. This process is regulated at multiple levels. Besides the extensively studied IgE and IgG receptors, toll-like receptors, MRGPR, and other protein receptor signaling pathways, there is a critical activation pathway based on cholesterol-dependent, pore-forming cytolytic exotoxins produced by Gram-positive bacterial pathogens. This pathway is initiated by binding the exotoxins to the cholesterol-rich membrane, followed by their dimerization, multimerization, pre-pore formation, and pore formation. At low sublytic concentrations, the exotoxins induce mast cell activation, including degranulation, intracellular calcium concentration changes, and transcriptional activation, resulting in production of cytokines and other inflammatory mediators. Higher toxin concentrations lead to cell death. Similar activation events are observed when mast cells are exposed to sublytic concentrations of saponins or some other compounds interfering with the membrane integrity. We review the molecular mechanisms of mast cell activation by pore-forming bacterial exotoxins, and other compounds inducing cholesterol-dependent plasma membrane perturbations. We discuss the importance of these signaling pathways in innate and acquired immunity.

Unraveling the mechanisms of calcium-dependent secretion

Ca2+-dependent secretion is a process by which important signaling molecules that are produced within a cell-including proteins and neurotransmitters-are expelled to the extracellular environment. The cellular mechanism that underlies secretion is referred to as exocytosis. Many years of work have revealed that exocytosis in neurons and neuroendocrine cells is tightly coupled to Ca2+ and orchestrated by a series of protein-protein/protein-lipid interactions. Here, we highlight landmark discoveries that have informed our current understanding of the process. We focus principally on reductionist studies performed using powerful model secretory systems and cell-free reconstitution assays. In recent years, molecular cloning and genetics have implicated the involvement of a sizeable number of proteins in exocytosis. We expect reductionist approaches will be central to attempts to resolve their roles. The Journal of General Physiology will continue to be an outlet for much of this work, befitting its tradition of publishing strongly mechanistic, basic research.

A cell-physiological description of GE, a GTP-binding protein that mediates exocytosis

Introduction of GTP gamma S or other non-metabolic analogues of GTP into permeabilized myeloid granulocytes (mast cells, eosinophils, neutrophils) constitutes a sufficient stimulus to induce exocytosis. We concentrate on mast cells. Exocytosis from cells permeabilized in isotonic glutamate solution proceeds in the absence of ATP and at exceedingly low levels (< 10(-9) M) of Ca2+. Mg2+ strongly promotes GTP gamma S-induced exocytosis but this requirement can be spared and then obliterated by lifting Ca2+ through 10(-7) to 10(-6) M. GTP provides only a modest support to exocytosis but becomes almost equipotent with GTP gamma S when Mg2+ is excluded. Ca2+ alone is unable to induce exocytosis. We envisage that the terminal stage of exocytosis (membrane fusion) requires activation of GE, a putative GTPase so far undefined as a molecular entity. Ca2+, presumed to act through a Ca(2+)-binding protein (CE, also undefined) supports exocytosis by promoting the exchange of guanine nucleotides on GE. In the absence of Mg2+ the onset of exocytosis is characterized by delays that have concentration-dependent (binding) and independent components. The latter are sensitive to the identity of the stimulating nucleotide (GTP < GTP gamma S < Gpp [NH]p) and may reflect activation of GE. The activation by Ca2+ and Mg2+ and the delays preceding onset of GTP gamma S-triggered exocytosis are reminiscent of the action of glucagon and Mg2+ in the activation of adenylate cyclase in hepatocyte membranes. The cell-physiological description predicts GE to be an alpha beta gamma heterotrimeric GTP-binding protein with functional similarity to GS.

Cholesterol-dependent actin remodeling via RhoA and Rac1 activation by the Streptococcus pneumoniae toxin pneumolysin

The Streptococcus pneumoniae toxin pneumolysin belongs to the group of cholesterol-dependent cytolysins. It produces rapid cell lysis at higher concentrations or apoptosis at lower concentrations. In cell membranes, it forms prepores and pores. Here, we show that sublytic concentrations of pneumolysin produce rapid activation of Rho and Rac GTPases and formation of actin stress fibers, filopodia, and lamellipodia. That Rac1-specific and Rho-associated kinase (ROCK)-specific inhibitors reverted the formation of lamellipodia and stress fibers, respectively, identifies RhoA and Rac1 as key toxin effectors. Live imaging excluded macropore formation (as judged by membrane impermeability toward calcein) but indicated very early membrane depolarization [as judged by bis-(1,3-dibutylbarbituric acid)trimethine oxanol staining], indicative of formation of micropores with ion channel properties. That Rac1-dependent lamellipodia formation was reverted by the voltage-gated calcium channel inhibitor SKF96365 and by toxin exposure in calcium-free medium suggests a role for calcium influx via endogenous calcium channels in the Rac1 activation. Cellular cholesterol depletion by methyl-beta-cyclodextrin or incubation of the toxin with cholesterol before cell treatment eliminated its membrane binding and the subsequent GTPase activation. Thus, that our experiments show small GTPase activation by a cholesterol-dependent cytolysin suggests a membrane cholesterol-dependent activation mechanism.

Inhibition of phospholipase C-independent exocytotic responses in rat peritoneal mast cells by U73122

The aminosteroid U73122 has been established as potent, selective, and cell-permeable inhibitor C-type phosphatidylinositol-specific phospholipases (PI-PLCs), and has been used to define a contribution of PI-PLCs as part of exocytotic signalling pathways in rat peritoneal mast cells (RPMCs). However, doubts have been raised regarding its PI-PLC selectivity of action. Therefore, in the present study, U73122 was tested in RPMCs under experimental conditions allowing to elicit exocytosis PI-PLC independently (streptolysin O [SLO]-permeabilised cells; stimulated by GTPgammaS; in the presence of low concentrations of free Ca2+). The release of [3H]5-hydroxytryptamine ([3H]5-HT) from [3H]5-HT-loaded RPMCs served as measure of secretion. U73122 potently inhibited the exocytotic response induced by 10 microM GTPgammaS (Ca2+: 10(-6) M) in permeabilised cells (IC50: 0.6 microM, n=5) in an insurmountable manner. In intact RPMCs, with a nearly equal potency (IC50: 4 microM, n=4), U73122 also inhibited the PI-PLC-dependent exocytotic response induced by concomitant application of nerve growth factor and lyso-phosphatidylserine (NGF/lyso-PS).

CONCLUSION

U73122 exerts potent PI-PLC-independent secretostatic effects, limiting its use to define PI-PLC function within exocytotic processes.

Ceramide kinase is a mediator of calcium-dependent degranulation in mast cells

Ceramide kinase (CERK) catalyzes the conversion of ceramide to ceramide 1-phosphate (C1P) and is known to be activated by calcium. Although several groups have examined the functions of CERK and its product C1P, the functions of C1P and CERK are not understood. We studied the RBL-2H3 cell line, a widely used model for mast cells, and found that CERK and C1P are required for activation of the degranulation process in mast cells. We found that C1P formation was enhanced during activation induced by IgE/antigen or by Ca(2+) ionophore A23187. The formation of C1P required the intracellular elevation of Ca(2+). We generated RBL-2H3 cells that stably express CERK, and when these cells were treated with A23187, a concomitant C1P formation was observed and degranulation increased 4-fold, compared with mock transfectants. The cell-permeable N-acetylsphingosine (C(2)-ceramide), a poor substrate of CERK, inhibited both the formation of C1P and degranulation, indicating that C1P formation was necessary for degranulation. Exogenous introduction of CERK into permeabilized RBL-2H3 cells caused degranulation. We identified a cytosolic localization of CERK that provides exposure to cytosolic Ca(2+). Taken together, these results indicate that C1P formation is a necessary step in the degranulation pathway in RBL-2H3 cells.

The streptococcal exotoxin streptolysin O activates mast cells to produce tumor necrosis factor alpha by p38 mitogen-activated protein kinase- and protein kinase C-dependent pathways

Streptolysin O (SLO), a major virulence factor of pyogenic streptococci, binds to cholesterol in the membranes of eukaryotic cells and oligomerizes to form large transmembrane pores. While high toxin doses are rapidly cytocidal, low doses are tolerated because a limited number of lesions can be resealed. Here, we report that at sublethal doses, SLO activates primary murine bone marrow-derived mast cells to degranulate and to rapidly induce or enhance the production of several cytokine mRNAs, including tumor necrosis factor alpha (TNF-alpha). Mast cell-derived TNF-alpha plays an important protective role in murine models of acute inflammation, and the production of this cytokine was analyzed in more detail. Release of biologically active TNF-alpha peaked approximately 4 h after stimulation with SLO. Production of TNF-alpha was blunted upon depletion of protein kinase C by pretreatment of the cells with phorbol-12 myristate-13 acetate. Transient permeabilization of mast cells with SLO also led to the activation of the stress-activated protein kinases p38 mitogen-activated protein (MAP) kinase and c-jun N-terminal kinase (JNK), and inhibition of p38 MAP kinase markedly reduced production of TNF-alpha. In contrast, secretion of preformed granule constituents triggered by membrane permeabilization was not dependent on p38 MAP kinase or on protein kinase C. Thus, transcriptional activation of mast cells following transient permeabilization might contribute to host defense against infections via the beneficial effects of TNF-alpha. However, hyperstimulation of mast cells might also lead to overproduction of TNF-alpha, which would then promote the development of toxic streptococcal syndromes.

Latrunculin B or ATP depletion induces cofilin-dependent translocation of actin into nuclei of mast cells

Increasing cellular G-actin, using latrunculin B, in either intact or permeabilized rat peritoneal mast cells, caused translocation of both actin and an actin regulatory protein, cofilin, into the nuclei. The effect was not associated with an increase in the proportion of apoptotic cells. The major part of the nuclear actin was not stained by rhodamine-phalloidin but could be visualized with an actin antibody, indicating its monomeric or a conformationally distinct state, e.g. cofilin-decorated filaments. Introduction of anti-cofilin into permeabilized cells inhibited nuclear actin accumulation, implying that an active, cofilin-dependent, import exists in this system. Nuclear actin was localized outside the ethidium bromide-stained region, in the extrachromosomal nuclear domain. In permeabilized cells, the appearance of nuclear actin and cofilin was not significantly affected by increasing [Ca(2+)] and/or adding guanosine 5'-O-(3-thiotriphosphate), but was greatly promoted when ATP was withdrawn. Similarly, ATP depletion in intact cells also induced nuclear actin accumulation. In contrast to the effects of latrunculin B, ATP depletion was associated with an increase in cortical F-actin. Our results suggest that the presence of actin in the nucleus may be required for certain stress-induced responses and that cofilin is essential for the nuclear import of actin.

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.

Phosphatidylinositol transfer proteins and protein kinase C make separate but non-interacting contributions to the phosphorylation state necessary for secretory competence in rat mast cells

Mast cells permeabilized by streptolysin O undergo exocytosis when stimulated with Ca(2+) and guanosine 5'-[gamma-thio]triphosphate but become progressively refractory to this stimulus if it is delayed. This run-down of responsiveness occurs over a period of 20-30 min, during which the cells leak soluble and tethered proteins. We show here that withdrawal of ATP during the process of run-down is strongly inhibitory but that as little as 25 microM ATP can extend responsiveness significantly; this effect is maximal at 50 microM. When phosphatidylinositol transfer proteins (PITPs) are provided to cells at the time of permeabilization, run-down is retarded. We conclude that in the presence of ATP they convey substrates for phosphorylation that are essential for exocytosis and thus interact with the regulatory machinery. Furthermore, we show that PITPalpha and PITPbeta have additive effects in this mechanism, suggesting that they are not functionally redundant. Alternatively, secretion from run-down cells can be inhibited by the aminoglycoside antibiotic neomycin, which is understood to bind to phosphoinositide headgroups, and by a PH (pleckstrin homology) domain polypeptide that binds phosphoinositides. The apparent displacement of neomycin by exogenous PITPs suggests that these proteins screen essential lipids. Secretion from run-down cells is also inhibited by 1-O-hexadecyl-2-O-methyl-rac-glycerol (AMG-C(16)), an inhibitor of protein kinase C. The lack of synergy between neomycin and AMG-C(16) suggests that protein kinase C independently provides a second essential component through protein phosphorylation and that there are two independent phosphorylation pathways necessary for secretion competence.

Influence of salmeterol and benzalkonium chloride on G-protein-mediated exocytotic responses of rat peritoneal mast cells

The long-acting beta(2)-adrenoceptor agonist salmeterol and the invert soap benzalkonium chloride share physicochemically important structures, namely a polar head group and a long aliphatic chain. Low concentrations of benzalkonium chloride have been shown to inhibit exocytotic responses in rat peritoneal mast cells by selectively interacting with heterotrimeric G-proteins of the G(i)-type. The present study investigates whether salmeterol inhibits, independently of beta-adrenoceptors, exocytotic responses of rat peritoneal mast cells induced by the direct agonists at G-proteins mastoparan or guanosine 5'-O-(3-thiotriphosphate) (++GTP gamma S++). Exocytosis was studied by secretion assays ([3H]5-hydroxytryptamine ([3H]5-HT)-release) using intact, streptolysin O-permeabilised or metabolically inhibited (antimycin, deoxyglucose) rat peritoneal mast cells. Both amphiphilics, salmeterol, and benzalkonium chloride, dose-dependently exerted biphasic effects on mastoparan-induced [3H]5-HT release in intact mast cells. In contrast to benzalkonium chloride, the dose-response curves for secretostatic and celltoxic effects of salmeterol markedly overlapped. Similar to benzalkonium chloride, salmeterol in non-cytotoxic concentrations (1-25 microg/ml) dose-dependently inhibited exocytosis induced by mastoparan (intact cells) or ++GTP gamma S (permeabilised cells). These findings indicate a direct, adrenoceptor-independent affection of G proteins by salmeterol in mast cells.

Regulation of exocytosis from rat peritoneal mast cells by G protein beta gamma-subunits

We applied G protein-derived beta gamma-subunits to permeabilized mast cells to test their ability to regulate exocytotic secretion. Mast cells permeabilized with streptolysin-O leak soluble (cytosol) proteins over a period of 5 min and become refractory to stimulation by Ca2+ and GTPgammaS over approximately 20-30 min. beta gamma-Subunits applied to the permeabilized cells retard this loss of sensitivity to stimulation (run-down) and it can be inferred that they interact with the regulatory mechanism for secretion. While alpha-subunits are without effect, beta gamma-subunits at concentrations >10(-8 )M enhance the secretion due to Ca2+ and GTPgammaS. Unlike the small GTPases Rac and Cdc42, beta gamma-subunits cannot induce secretion in the absence of an activating guanine nucleotide, and thus further GTP-binding proteins (likely to be Rho-related GTPases) must be involved. The enhancement due to beta gamma-subunits is mediated largely through interaction with pleckstrin homology (PH) domains. It remains manifest in the face of maximum activation by PMA and inhibition of PKC with the pseudosubstrate inhibitory peptide. Soluble peptides mimicking PH domains inhibit the secretion due to GTPgammaS and block the enhancement due to beta gamma-subunits. Our data suggest that beta gamma-subunits are components of the pathway of activation of secretion due to receptor-mimetic ligands such as mastoparan and compound 48/80.

Partial agonists and antagonists reveal a second permeability state of human lymphocyte P2Z/P2X7 channel

Extracellular ATP is known to trigger apoptosis of thymocytes and lymphocytes through a P2Z receptor at which ATP is a partial agonist, giving only 70% of the maximum response of 3'-O-(4-benzoyl)benzoyl-adenosine 5'-triphosphate (BzATP), a full agonist. This cytolytic receptor and its associated ion channel are Ca2+ (and Ba2+) selective but also pass molecules up to the size of ethidium cation (314 Da). RT-PCR showed identity between lymphocyte P2Z and the hP2X7 gene recently cloned from human monocytes. When human leukemic B lymphocytes were incubated with ATP and 133Ba2+, an immediate influx of isotope occurred. It was augmented by 45% when ATP was added 10 min before isotope. Time-resolved flow cytometry was used to examine kinetics of ethidium uptake in cells incubated with BzATP or the partial agonists ATP, 2-methylthioadenosine 5'-triphosphate, or adenosine 5'-O-(3-thiotriphosphate). Maximally effective concentrations of BzATP (50 microM) induced immediate uptake of ethidium at a rate linear with time. In contrast, a delay was observed (30 s) before ethidium uptake commenced after addition of maximally effective ATP concentrations (500 microM) at 37 degreesC, and the delay was longer at 24 degreesC. ATP addition 2-10 min before ethidium abolished the delay. The delay was longer with other partial agonists and inversely related to maximal flux produced by agonist. A delay was also observed for submaximal BzATP concentrations (10-20 microM). P2Z/P2X7 inhibitors, KN-62 and 5-(N, N-hexamethylene)-amiloride, reduced the rate of agonist-induced ethidium uptake and lengthened the delay. The results support a model in which agonists for P2Z/P2X7 receptor mediate an immediate channel opening allowing passage of small inorganic cations, followed by a slow further permeability increase allowing passage of larger permeant cations like ethidium. The rate of the second step depends on time and temperature and the efficacy and concentration of agonist and is slowed by antagonists, suggesting it depends on the fraction of P2Z/P2X7 channels held in the initial open state.

Large dense-core vesicle exocytosis in PC12 cells

A current major challenge in the study of regulated exocytosis is the identification of essential proteins that mediate the transit of secretory vesicles through trafficking stages such as recruitment, docking, and fusion. Defining the physiological roles and mechanisms of action of these essential proteins is paramount. The reconstitution of stages of regulated exocytosis in cell-free systems provides the opportunity to identify required proteins and establish their stage-specific mechanisms of action. PC12 cells, clonal cell lines of adrenal medullary origin, possess large dense-core vesicles that retain their competence for regulated exocytosis in a variety of permeable cell and isolated membrane preparations. We describe several cell-free systems for studies of regulated exocytosis derived from PC12 cells.

1,25-Dihydroxyvitamin D3 enhances degranulation of mast cells

The mast cell lines rat basophilic leukemia (RBL) and mouse C57 cells respond to IgE/antigen complexes by degranulation. Treatment of these cells with 1,25-dihydroxyvitamin D3 (1,25(OH)2D3), (10-100 nM) for 24-48 h enhanced IgE/antigen-induced exocytosis as monitored by release of hexosaminidase. A short term incubation with the hormone did not affect exocytosis, ruling out a rapid non genomic mechanism. The presence of vitamin D receptors, demonstrated by immunoblotting and the lack of effect of 24,25(OH)2D3 suggest a role for these receptors in the enhancing effect. 1,25(OH)2D3 also enhanced exocytosis induced by the calcium ionophore A23187 in the presence or absence of phorbol ester indicating modulation of events distal to signal transduction. 1,25(OH)2D3 enhanced exocytosis in the presence of cytochalasin D, indicating that the action of the hormone is not due to effects on microfilament structure. The results of this study suggest that 1,25(OH)2D3 may affect the allergic or pro-inflammatory potential of mast cells.

Ca2+ and protein kinase C activation of mucin granule exocytosis in permeabilized SPOC1 cells

Mucin secretion by airway goblet cells is under the control of apical P2Y2, phospholipase C-coupled purinergic receptors. In SPOC1 cells, the mobilization of intracellular Ca2+ by ionomycin or the activation of protein kinase C (PKC) by phorbol 12-myristate 13-acetate (PMA) stimulates mucin secretion in a fully additive fashion [L. H. Abdullah, J. D. Conway, J. A. Cohn, and C. W. Davis. Am. J. Physiol. 273 (Lung Cell. Mol. Physiol. 17): L201-L210, 1997]. This apparent independence between PKC and Ca2+ in the stimulation of mucin secretion was tested in streptolysin O-permeabilized SPOC1 cells. These cells were fully competent to secrete mucin when Ca2+ was elevated from 100 nM to 3.1 microM for 2 min following permeabilization; the Ca2+ EC50 was 2.29 +/- 0.07 microM. Permeabilized SPOC1 cells were exposed to PMA or 4alpha-phorbol at Ca2+ activities ranging from 10 nM to 10 microM. PMA, but not 4alpha-phorbol, increased mucin release at all Ca2+ activities tested: at 10 nM Ca2+ mucin release was 2.1-fold greater than control and at 4.7 microM Ca2+ mucin release was maximal (3.6-fold increase). PMA stimulated 27% more mucin release at 4.7 microM than at 10 nM Ca2+. Hence, SPOC1 cells possess Ca2+-insensitive, PKC-dependent, and Ca2+-dependent PKC-potentiated pathways for mucin granule exocytosis.

Induction of exocytosis from permeabilized mast cells by the guanosine triphosphatases Rac and Cdc42

We applied recombinant forms of the Rho-related small guanosine triphosphatases (GTPases) Rac2 and Cdc42/G25K to permeabilized mast cells to test their ability to regulate exocytotic secretion. Mast cells permeabilized with streptolysin-O leak soluble (cytosol) proteins over a period of 5 min and become refractory to stimulation by Ca2+ and guanosine triphosphate (GTP)gammaS over about 20-30 min. This loss of sensitivity is likely to be due to loss of key regulatory proteins that are normally tethered at intracellular locations. Exogenous proteins that retard this loss of sensitivity to stimulation may be similar, if not identical, to those secretory regulators that are lost. Recombinant Rac and Cdc42/G25K, preactivated by binding GTPgammaS, retard the loss of sensitivity (run-down) and, more importantly, enable secretion to be stimulated by Ca2+ alone. Investigation of the concentration dependence of each of these two GTPases applied individually to the permeabilized cells, and of Cdc42/G25K applied in the presence of an optimal concentration of Rac2, has provided evidence for a shared effector pathway and also a second effector pathway activated by Cdc42/G25K alone. Dominant negative mutant (N17) forms of Rac2 and Cdc42/G25K inhibit secretion induced by Ca2+ and GTPgammaS. Our data suggest that Rac2 and Cdc42 should be considered as candidates for GE, GTPases that mediate exocytosis in cells of hematopoeitic origin.

Anti-allergic activity of cyclosporin-A metabolites and their interaction with the parent compound and FK 506

The ability of cyclosporin-A (CSA) and four of its metabolites M1, M17, M18 and M21, to inhibit antigen-stimulated release of beta-hexoseaminidase from IgE-sensitized rat basophilic leukemia cells (RBL-2H3), as an in vitro correlate of anti-allergic effect, was studied Metabolites M17, M1 and M21 were effective in inhibiting enzyme release, though less potent than the parent compound. The concentrations achieving 50% inhibition (IC50 values) were 53.3, 315.5 and 875.7 ng/ml for CSA, M17 and M1, respectively. M21 had approximately same IC50 as M1 while M18 was essentially inactive. At the highest concentration tested (1000 ng/ml) the mean maximum percentage inhibitions were 98.6, 79.5, 53.9, 48.6 and 12.2 for CSA, M17, M1, M21 and M18, respectively. The relative anti-allergic potency of the metabolites was similar to their reported relative immunosuppressive potency. Combinations of low concentrations of CSA and its metabolites were synergistic in inhibiting enzyme release whereas at higher concentrations interactions were either additive or antagonistic. Even the concentrations of the metabolites that have little or no activity when used alone also potentiated the effect of CSA. The immunosuppressor FK 506 was found to be about three times more potent than CSA in this system and the interactions between FK 506 (3, 10 and 30 ng/ml) and CSA (10, 30 and 100 ng/ml) or M17 (20, 100 and 500 ng/ml) were synergistic at all combinations. Both CSA and M17 synergized more strongly with FK 506 than they did between themselves. These results show that some metabolites of CSA, like the parent compound, possess anti-allergic effects and that at concentrations that are obtainable in transplant patients, synergistic interaction occurs between CSA and its metabolites, and this may be of some therapeutic significance.

Purification and identification of FOAD-II, a cytosolic protein that regulates secretion in streptolysin-O permeabilized mast cells, as a rac/rhoGDI complex

Mast cells permeabilized by treatment with streptolysin-O in the presence of Ca2+ and GTP-gamma-S can secrete almost 100% of their contained N-acetyl-beta-D-glucosaminidase. If these stimuli are provided to the permeabilized cells after a delay, the response is diminished and the ability of the cells to undergo secretion runs down progressively over a period of about 30 min. This is thought to be due to the loss of key proteins involved in the exocytotic mechanism. Using this effect as the basis of a biological assay, we have isolated a protein from bovine brain cytosol that retards the loss of responsiveness to stimulation by Ca2+ and GTP-gamma-S. Purification of this protein and peptide sequencing have enabled us to identify it as the small GTP-binding protein rac complexed to the guanine nucleotide exchange inhibitor rhoGDI. Both proteins are required to retard the loss of the secretory response, while purified rhoGDI applied alone accelerates the rundown.

Practical considerations regarding the use of streptolysin-O as a permeabilising agent for cells in the investigation of exocytosis

Streptolysin-O is widely used in cell biological investigations in order to make large (>12 nm) pores in the plasma membrane and so to render the cytosol directly accessible to experimental manipulation. We have compared the effect of streptolysin-O commercially formulated (Murex Diagnostics) as a diagnostic reagent in pathology with two pure reagents (a conventional purified protein, and a recombinant protein generated in E.coli) on exocytotic secretion from mast cells. For mast cells permeabilised by streptolysin obtained from the commercial source, exocytosis (of beta-D-N-acetylglucosaminidase) is dependent on provision of both Ca(2)+ and a guanine nucleotide. In contrast, for cells permeabilised by either of the two pure proteins, a substantial extent of Ca(2)+-independent exocytosis can be elicited. When the Murex material is subject to dialysis or ultrafiltration, some secretion can be induced in the absence of Ca(2)+, indicating a modulatory function of the low mol wt additives of formulation, mainly phosphate and cysteine. However, Ca(2)+-independent exocytosis is still manifest when the pure proteins are reconstituted with ultrafiltrates from the Murex material. These observations indicate that reagents used to permeabilise cells should be characterised thoroughly and used with great care. Confirmation that the cytolytic activity of the Murex material derives from a cholesterol directed factor was demonstrated by inhibition of exocytosis when red blood cell derived (and hence cholesterol containing) sonicated liposomes were provided.

Inhibition by cromoglycate and some flavonoids of nucleoside diphosphate kinase and of exocytosis from permeabilized mast cells

1. The anti-allergic compound, cromoglycate, is reported to possess affinity for, and to suppress the autophosphorylation of a 72kDa protein having the sequence of nucleoside diphosphate kinase (NDPK). 2. We have tested the ability of cromoglycate, and a panel of ten structurally related flavonoids of plant origin, to inhibit the NDPK reaction and the exocytotic process of permeabilized mast cells. The conditions of permeabilization (use of an isotonic medium based on sodium glutamate) were selected to ensure that NDPK activity would be an essential component in the induction of Ca(2+)-induced exocytosis in which ATP is required for generation of GTP. For comparison, we also measured the inhibition of exocytosis induced by GTP-gamma-S; this proceeds in the absence of ATP and bypasses the need for NDPK activity. 3. We found that cromoglycate does not discriminate between Ca2+ and GTP-gamma-S-induced exocytosis and is a poor inhibitor of NDPK activity. Concentrations in the millimolar range are required for inhibition of all these functions. By comparison, many of the flavonoids are effective at concentrations in the micromolar range. 4. While we were unable to discern any systematic relationships between their ability to inhibit the three functions, two compounds, quercetin and genistein, inhibit Ca(2+)-induced, but not GTP-gamma-S-induced exocytosis. Inhibition of the late stages of the stimulus-response pathway in mast cells by these compounds is therefore likely to be due to inhibition of NDPK and the consequent failure to generate GTP.

Secretion from permeabilised mast cells is enhanced by addition of gelsolin: contrasting effects of endogenous gelsolin

Permeabilised rat mast cells were exposed to gelsolin and its N-terminal half (S1-3), proteins that sever actin filaments in a calcium-dependent and independent manner, respectively. Gelsolin and S1-3 induced a decrease in cellular F-actin content and an increase in the extent of the secretory response. The calcium sensitivities of both these effects were consistent with the differential calcium requirements of the two proteins. Segment 1 (S1), which binds G-actin and caps filaments but does not sever them, did not show these effects. Thus, secretion of mast cells is promoted as a consequence of the severing activity of exogenous gelsolin or S1-3. Most of the endogenous gelsolin remained within permeabilised, washed mast cells and its distribution in resting state was predominantly cortical. Addition of calcium in the absence of MgATP did not reduce the F-actin content; by contrast, calcium with MgATP induced F-actin loss that was unaffected by the presence of anti-gelsolin. Because this antibody inhibits the severing activity of gelsolin, these results indicate that in permeabilised mast cells the severing activity of the remaining endogenous gelsolin is not involved in cortical actin filaments disassembly. Upon exposure to GTP-gamma-S in the absence of calcium, the content of cortical gelsolin was reduced. This parallels our previous observation of a GTP-gamma-S induced reduction of cortical actin filaments followed by their relocation to the cell's interior (Norman et al. (1994) J. Cell Biol. 126, 1005–1015) and suggests that actin redistribution may be a consequence of dissociation of gelsolin caps brought about by activation of a GTP-binding protein.

Permeabilization of MDCK cells with cholesterol binding agents: dependence on substratum and confluency

We studied systematically the susceptibility of Madin-Darby canine kidney (MDCK) cells to permeabilization by two cholesterol binding agents, digitonin and streptolysin-O (SLO), under different culture conditions. Monolayers grown on polycarbonate filter chambers (Transwells) required twice the concentration of digitonin effective on monolayers grown on glass or plastic (80 vs. 40 micrograms/ml) to allow antibody penetration or the release of 90% of the cytosolic protein lactate dehydrogenase (LDH). Neither the apical nor the basolateral surface showed preferential susceptibility to digitonin. Confluent MDCK cells, cultured either on filters or on impermeable substrates, showed poor antibody permeability after addition of commercial SLOs, even when used at concentrations 100 times higher (20 U/ml) than those effective on nonepithelial Chinese hamster ovary cells. Surprisingly, culture conditions that prevent tight junction formation and the acquisition of a polarized phenotype (< 10 microM Ca2+) increased dramatically the susceptibility to permeabilization by SLO. On restoration of normal Ca2+ levels, susceptibility to SLO quickly decreased. Thus conditions that lead to the full establishment of polarity result in decreased sensitivity to disruption by digitonin and SLO.

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.

Activation of exocytosis by GTP analogues in adrenal chromaffin cells revealed by patch-clamp capacitance measurement

The role of GTP-binding proteins in exocytosis in bovine adrenal chromaffin cells was examined using patch-clamp capacitance measurement. Internal dialysis with the non-hydrolysable GTP analogue guanosine 5'-[beta gamma-imido]triphosphate and xanthosine triphosphate (XTP) activated a capacitance increase. Exocytosis triggered by XTP was blocked by guanosine 5'-[beta-thio]diphosphate (GDP beta S) but Ca(2+)-induced exocytosis was unaffected. The capacitance increase due to XTP could not be explained by Ca2+ mobilisation since Ins(1,4,5)P3 and caffeine did not mimic the response. Chromaffin cells appear to possess a Ca(2+)-independent pathway for exocytosis that involves GTP-binding proteins. The magnitude of the response to XTP suggested that GTP analogues stimulate both exocytosis and recruitment of secretory granules.

Degranulation of individual mast cells in response to Ca2+ and guanine nucleotides: an all-or-none event

Widespread experience indicates that application of suboptimal concentrations of stimulating ligands (secretagogues) to secretory cells elicits submaximal extents of secretion. Similarly, for permeabilized secretory cells, the extent of secretion is related to the concentration of applied intracellular effectors. We investigated the relationship between the extent of secretion from mast cells (assessed as the release of hexosaminidase) and the degranulation (exocytosis) responses of individual cells. For permeabilized mast cells stimulated by the effector combination Ca2+ plus GTP-gamma-S and for intact cells stimulated by the Ca2+ ionophore ionomycin, we found that exocytosis has the characteristics of an all-or-none process at the level of the individual cells. With a suboptimal stimulus, the population comprised only totally degranulated cells and fully replete cells. In contrast, a suboptimal concentration of compound 48/80 applied to intact cells induced a partial degree of degranulation. This was determined by observing the morphological changes accompanying degranulation by light and electron microscopy and also as a reduction in the intensity of light scattered at 90 degrees, indicative of a change in the cell-refractive index. These results may be explained by the existence of a threshold sensitivity to the combined effectors that is set at the level of individual cells and not at the granule level. We used flow cytometry to establish the relationship between the extent of degranulation in individual rat peritoneal mast cells and the extent of secretion in the population (measured as the percentage release of total hexosaminidase). For comparison, secretion was also elicited by applying the Ca2+ ionophore ionomycin or compound 48/80 to intact cells. For permeabilized cells and also for intact cells stimulated with the ionophore, levels of stimulation that generate partial secretion gave rise to bimodal frequency distributions of 90 degrees light scatter. In contrast, a partial stimulus to secretion by compound 48/80 resulted in a single population of partially degranulated cells, the degree of degranulation varying across the cell population. The difference between the all-or-none responses of the permeabilized or ionophore-treated cells and the graded responses of cells activated by compound 48/80 is likely to stem from differences in the effective calcium stimulus. Whereas cell stimulated with receptor-directed agonists can undergo transient and localized Ca2+ changes, a homogeneous and persistent stimulus is sensed at every potential exocytotic site in the permeabilized cells.

Regulated exocytosis

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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.

Calcium and GTP-gamma-S as single effectors of secretion from permeabilized rat mast cells: requirements for ATP

Treatment with metabolic inhibitors and addition of exogenous MgATP exerted different effects on secretion from streptolysin-O-permeabilized mast cells, responding to calcium and GTP-gamma-S as single effectors (i.e., independently of each other). These effects were also strongly dependent on the experimental conditions. Thus cells, triggered by Ca2+ at the time of permeabilization, did not require MgATP, but after metabolic inhibition rapidly became absolutely dependent on its provision, requiring high (> mM) concentrations. AMP-PNP was not effective. After longer treatment with metabolic inhibitors, the absolute dependence on MgATP was also exhibited by cells responding to dual effectors (i.e., Ca2+ and GTP-gamma-S applied together). In contrast, calcium independent secretion due to GTP-gamma-S was more resistant to metabolic inhibition, exhibiting no absolute requirements for MgATP. Once the responsiveness to GTP-gamma-S had been lost, it could not be restored by addition of MgATP. MgATP, in fact, inhibited the response of permeabilized cells to GTP-gamma-S. This effect could be mimicked by AMP-PNP. When permeabilized cells were washed before triggering, MgATP (0.1-1 mM concentration range) was no longer inhibitory but stimulatory. These differences between Ca(2+)- and GTP-gamma-S-induced responses indicate that ATP utilization is essential to the calcium, but not to the guanine nucleotide, pathway to secretion. The rate of the response to calcium/MgATP was much slower in the absence than in the presence of GTP-gamma-S. The onset of secretion occurred after an initial delay. This lag phase was abolished by addition of GTP-gamma-S, suggesting that a GTP-binding protein may control a reaction which constitutes a rate-limiting step in the secretory process.

Inositol-lipid-specific phospholipase C isoenzymes and their differential regulation by receptors

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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.

Electroporation of CTL clones: a useful method to investigate signalling pathways leading to the expression of effector functions

Signal transduction mechanisms leading to effector functions in mouse cytolytic T lymphocyte (CTL) clones were studied following the introduction of exogenous molecules by electroporation. Conditions were defined in which the application of an electric pulse permeabilized the CTL without affecting functions such as antigen-dependent or antibody-mediated cytotoxicity. When a non-permeant Ca2+ chelator such as EGTA was added in the external medium during the electric pulse, it inhibited subsequent target cell cytolysis carried out in the presence of external Ca2+, thereby indicating the efficiency of EGTA uptake. Results obtained in this system, using a 13 amino-acid protein kinase C (PKC) pseudo-substrate peptide, indicated that it selectively inhibited cytolysis, whereas a substrate peptide with one amino-acid substitution was not inhibitory. This suggests that the technique could be used to study the signal transduction mechanisms of CTL clones which lead to the expression of effector functions.

Amylase release from streptolysin O-permeabilized pancreatic acinar cells. Effects of Ca2+, guanosine 5'-[gamma-thio]triphosphate, cyclic AMP, tetanus toxin and botulinum A toxin

The molecular requirements for amylase release and the intracellular effects of botulinum A toxin and tetanus toxin on amylase release were investigated using rat pancreatic acinar cells permeabilized with streptolysin O. Micromolar concentrations of free Ca2+ evoked amylase release from these cells. Maximal release was observed in the presence of 30 microM free Ca2+. Ca(2+)-stimulated, but not basal, amylase release was enhanced by guanosine 5'-[gamma-thio]triphosphate (GTP[S]) (3-4 fold) or cyclic AMP (1.5-2 fold). Neither the two-chain forms of botulinum A toxin and tetanus toxin, under reducing conditions, nor the light chains of tetanus toxin, inhibited amylase release triggered by Ca2+, or combinations of Ca2+ + GTP[S] or Ca2+ + cAMP. The lack of inhibition was not due to inactivation of botulinum A toxin or tetanus toxin by pancreatic acinar cell proteolytic enzymes, as toxins previously incubated with permeabilized pancreatic acinar cells inhibited Ca(2+)-stimulated [3H]noradrenaline release from streptolysin O-permeabilized adrenal chromaffin cells. These data imply that clostridial neurotoxins inhibit a Ca(2+)-dependent mechanism which promotes exocytosis in neural and endocrine cells, but not in exocrine cells.

Induction of antigen-specific CD8+ cytolytic T cells by the exogenous bacterial antigen streptolysin O in rhesus monkeys

We have characterized the T cell responses induced by streptolysin O (SLO), a sulfhydryl-activated hemolysin secreted by streptococci, by applying long-term in vitro culture and cloning rhesus monkey (Macaca mulatta) T cells. T cell lines specific for SLO were obtained from three rhesus monkeys. These T cell lines required autologous antigen-presenting cells (APC) to proliferate in response to SLO and did not respond to purified protein derivative. Phenotypic analysis showed that the cells from two of three SLO-specific T cell lines were more than 85% CD3+CD4-CD8+ after prolonged in vitro culture. The rh 1842 CD8+ T cell line proliferative response to SLO was inhibited by the addition of anti-major histocompatibility complex (MHC) class I and anti-CD8 but not of anti-MHC class II and anti-CD4 monoclonal antibody (mAb). This cell line was able to lyse P815 target cells in the presence of anti-CD3 mAb and did not show natural killer activity. Moreover, specific lysis of autologous but not allogeneic non-rosetting E- cell targets pulsed with SLO was observed. Such lysis was inhibited by the addition of anti-MHC class I mAb. In the attempt to identify the restriction elements involved in SLO presentation APC from six unrelated rhesus monkeys and three humans were used. A CD4+ rh 1842 T cell clone responded when SLO was presented by one of six, and a CD8+ rh 1842 T cell clone by four of six rhesus monkeys APC. Both CD4+ and CD8+ T cell clones did not respond when SLO was presented by human APC. However, both clones responded when APC from all donors were used in conjunction with anti-CD3 mb. Furthermore, SLO required active processing to be presented to CD4+ and CD8+ T cell clones as glutaraldehyde fixation of APC before but not after antigen pulsing inhibited T cell proliferation. The SLO-specific CD8+ cytolytic T cells described here could play a role in the regulation of the immune response occurring during streptococcal infections and/or could participate in the pathogenesis of poststreptococcal nonsuppurative sequelae.

Characterization of the tyrosine phosphorylation of calpactin I (annexin II) induced by platelet-derived growth factor

Stimulation in vivo of Swiss 3T3 fibroblasts with platelet-derived growth factor (PDGF) in the presence of orthovanadate induces the tyrosine phosphorylation of a 39 kDa protein, identified as the phosphorylated slow-migrating form of calpactin I (annexin II) heavy chain, p36. In fact, in PDGF-stimulated cells, anti-(calpactin I) antibodies recognize a doublet of bands, p36 and p39, and the latter disappears upon treatment with phosphatase. In many regards phosphorylation of p39 differs from the rapid and transient phosphorylation of the PDGF receptor and of other substrates: (a) it has slower kinetics but is then stable for longer periods of time; (b) it occurs at 37 degrees C but not at 4 degrees C; and (c) whereas most of the tyrosine-phosphorylated proteins are associated with membrane-enriched preparations, membrane association of p39 only occurs in the presence of Ca2+. Moreover, calpactin I leaks out of permeabilized cells at 0.1 microM free Ca2+, whereas it remains associated with the cells at concentrations of Ca2+ greater than or equal to 1 microM. PDGF does not stimulate phosphoinositide turnover (and thus Ca2+ mobilization) at 4 degrees C; thus it can be suggested that the Ca(2+)-dependent translocation of the protein to membrane/cytoskeletal structures is a necessary condition for its phosphorylation. In addition, calpactin I may not be a direct substrate for the PDGF receptor kinase, but rather the substrate of another tyrosine kinase activated by the receptor.