Guanine nucleotides and Ca2(+)-dependent lysosomal secretion in electropermeabilised human platelets

Molecular mechanisms of platelet exocytosis: role of SNAP-23 and syntaxin 2 and 4 in lysosome release

On stimulation by strong agonists, platelets release the contents of 3 storage compartments in 2 apparent waves of exocytosis. The first wave is the release of α- and dense core granule contents and the second is the release of lysosomal contents. Using a streptolysin O-permeabilized platelet exocytosis assay, we show that hexosaminidase release is stimulated by either Ca++ or by GTP-γ-S. This release step retains the same temporal separation from serotonin release as seen in intact platelets. This assay system was also used to dissect the molecular mechanisms of lysosome exocytosis. Lysosome release requires adenosine triphosphate and the general membrane fusion protein, N-ethylmaleimide sensitive factor. Uniquely, 2 syntaxin t-SNAREs, syntaxin 2 and 4, which localize to granules and open canalicular membranes, together with the general target membrane SNAP receptor (t-SNARE) protein SNAP-23 appear to make up the heterodimeric t-SNAREs required for lysosome exocytosis. These studies further show that regardless of stimuli (Ca++or GTP-γ-S) serotonin and hexosaminidase release requires the same membrane fusion machinery.

Molecular mechanisms of platelet exocytosis: role of SNAP-23 and syntaxin 2 and 4 in lysosome release

On stimulation by strong agonists, platelets release the contents of 3 storage compartments in 2 apparent waves of exocytosis. The first wave is the release of α- and dense core granule contents and the second is the release of lysosomal contents. Using a streptolysin O-permeabilized platelet exocytosis assay, we show that hexosaminidase release is stimulated by either Ca++ or by GTP-γ-S. This release step retains the same temporal separation from serotonin release as seen in intact platelets. This assay system was also used to dissect the molecular mechanisms of lysosome exocytosis. Lysosome release requires adenosine triphosphate and the general membrane fusion protein, N-ethylmaleimide sensitive factor. Uniquely, 2 syntaxin t-SNAREs, syntaxin 2 and 4, which localize to granules and open canalicular membranes, together with the general target membrane SNAP receptor (t-SNARE) protein SNAP-23 appear to make up the heterodimeric t-SNAREs required for lysosome exocytosis. These studies further show that regardless of stimuli (Ca++or GTP-γ-S) serotonin and hexosaminidase release requires the same membrane fusion machinery.

Molecular mechanisms of platelet exocytosis: requirements for alpha-granule release

Platelets function by secreting components necessary for primary clot formation. This report describes an in vitro assay that measures alpha-granule secretion. Using permeabilized platelets, it is possible to recreate Ca(2+)-stimulated release of platelet factor 4 (PF4) that is ATP- and temperature-dependent. Though other divalent cations can replace Ca(2+) (i.e., Sr(2+), Mn(2+), Zn(2+)), there is no effect of Ba(2+). Analysis by electron microscopy indicates that the in vitro assay also mimics the cytoskeletal rearrangements and granule centralization that occurs upon platelet activation in vivo. Antibody inhibition studies show that PF4 release requires the general membrane fusion protein N-ethylmaleimide-sensitive factor (NSF) and well as the target membrane SNAP receptors (t-SNAREs), syntaxin 2, 4, and SNAP-23. As shown by electron microscopy, the anti-t-SNARE antibodies block granule to target membrane fusion. This finding is unique in that it is the first report of a role for two syntaxins in the same exocytosis event.

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.

GTP gamma S and phorbol ester act synergistically to stimulate both Ca(2+)-independent secretion and phospholipase D activity in permeabilized human platelets. Inhibition by BAPTA and analogues

We have tested the hypothesis that phospholipase D (PLD) is the effector of the unidentified G protein (GE) mediating Ca(2+)-independent exocytosis in platelets. Although GTP gamma S, and to a lesser extent phorbol 12-myristate 13-acetate (PMA), caused some secretion of 5-HT from electropermeabilized human platelets in the effective absence of Ca2+ (pCa > 9), these stimuli had much more potent synergistic effects when added together. In all cases, secretion of 5-HT was closely correlated to the stimulus-induced formation of [3H]phosphatidic acid ([3H]PA) from [3H]arachidonate-labelled phospholipids. Addition of ethanol inhibited both secretion and [3H]PA formation and led to the accumulation of [3H]phosphatidylethanol ([3H]PEt), indicating that [3H]PA was formed largely by activation of PLD. BAPTA and analogues caused dose-dependent inhibitions of both GTP gamma S-induced secretion and PLD activity in the permeabilized platelets. This action of BAPTA did not appear to be mediated by chelation of Ca2+ or by direct inhibition of protein kinase C (PKC). The results suggest that PLD is the target of GE in platelets and that BAPTA can block PLD activation.

Two independently regulated secretory pathways in mast cells

We review here evidence that mast cells independently secrete large and small vesicles. Release of small vesicles is triggered by Ca2+. Combining capacitance measurements and flash photolysis of caged Ca2+, we demonstrate Ca-triggered exocytosis that is kinetically identical to the release of dense-core peptidergic vesicles from pituitary melanotrophs. It is suggested that the secretory pathway used for hormone secretion in neuroendocrine cells is also active in mast cells and eosinophils. The nature of the mediator thus secreted is unknown.

Evidence that activation of phospholipase D can mediate secretion from permeabilized platelets

Studies on electropermeabilized human platelets indicated that any two of three distinct factors must be present for marked secretion of dense or alpha-granule constituents to occur. These factors are Ca2+, activation of protein kinase C (PKC) and activation of an unidentified GTP-binding protein ('GE'). Thus, in the absence of Ca2+, phorbol ester and GTP[S] acted synergistically to promote secretion, whereas in the presence of Ca2+, either activation of PKC or addition of GTP[S] was sufficient. In all cases, secretion correlated with the activation of phospholipase D (PLD), as detected by the formation of [3H]phosphatidic acid (PA) in the absence of ethanol or of [3H]phosphatidylethanol (PEt) in the presence of ethanol. Secretion did not correlate with phospholipase C (PLC) activity or with the accumulation of 1,2-diacylglycerol (DAG), both of which required Ca2+ and were inhibited by phorbol ester. Ethanol partially inhibited secretion in the absence of Ca2+. BAPTA, a known inhibitor of Ca(2+)-independent secretion in permeabilized cells, caused parallel inhibitions of secretion and PLD activity. GTP[S] enhanced PKC activity, as indicated by pleckstrin phosphorylation, apparently by stimulating the formation of PA in the absence of Ca2+, as well as of DAG in the presence of Ca2+. PA and stable analogues, including PEt, stimulated the Ca(2+)-independent phosphorylation of pleckstrin and other proteins in platelet supernatant fraction. The results suggest that PA formed by activation of PLD may mediate secretion from permeabilized platelets by PKC-dependent and independent mechanisms. However, in intact platelets stimulated by thrombin, PLD accounted for only 10-20% of the total PA formed and can only play a major role in secretion if this PA fraction is distinct from that formed by the combined actions of PLC and DAG kinase.

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.

Mastoparan promotes exocytosis and increases intracellular cyclic AMP in human platelets. Evidence for the existence of a Ge-like mechanism of secretion

Recent studies have shown that mastoparan, an amphiphilic peptide derived from wasp venom, accelerates guanine nucleotide exchange and GTPase activity of purified GTP-binding proteins. In the present study we have examined the functional consequences of exposure of intact human platelets to mastoparan. Mastoparan promoted rapid (less than or equal to 1 min) dose-dependent increases in 5-hydroxy[14C]tryptamine and beta-thromboglobulin release from dense-granule and alpha-granule populations respectively. The exocytotic response did not result from a lytic effect of mastoparan and occurred in the complete absence of platelet shape change and aggregation. Liberation of [3H]arachidonate and increases in cytosolic [Ca2+] (detected with fura 2) were not observed in platelets stimulated with mastoparan. Similarly, in platelets preloaded with [3H]inositol during reversible electroporation, mastoparan did not cause the accumulation of [3H]inositol phosphates. Mastoparan-induced secretion was unaffected by preincubation with either the protein kinase C inhibitor staurosporine (10 nM-10 microM) or prostacyclin (PGI2; 100 ng/ml) and was not accompanied by phosphorylation of the 45 kDa protein kinase C substrate or the 20 kDa protein normally associated with platelet activation. The G-protein inhibitor guanosine 5'-[beta-thio]diphosphate (GDP[S]; 1 mM) attenuated the secretion induced by mastoparan in both intact and saponin-permeabilized platelets. Encapsulation of GDP[S] during reversible permeabilization inhibited mastoparan-induced secretion, providing evidence for an intracellular action of GDP[S]. In all these studies thrombin (0.05-0.2 unit/ml) elicited characteristic responses, and thrombin-induced secretion was inhibited by staurosporine, PGI2 and GDP[S]. Mastoparan also increased intra-platelet cyclic AMP in a dose-dependent manner. Mastoparan and PGI2 increased 32P incorporation into a protein of approx. 24 kDa, whereas phosphorylation of a 50 kDa substrate was only seen in PGI2-stimulated platelets. These results indicate that mastoparan promotes secretion by a mechanism which does not involve stimulation of phospholipase C and suggest that the secretory event may result either from a direct fusogenic action of mastoparan and/or from stimulation of the putative exocytosis-linked G-protein, Ge.

Factors affecting dense and alpha-granule secretion from electropermeabilized human platelets: Ca(2+)-independent actions of phorbol ester and GTP gamma S

Electropermeabilized human platelets containing 5-hydroxy[14C]tryptamine ([14C]5-HT) were suspended in a glutamate medium containing ATP and incubated for 10 min with (in various combinations) Ca2+ buffers, phorbol 12-myristate 13-acetate (PMA), guanine nucleotides, and thrombin. Release of [14C]5-HT and beta-thromboglobulin (beta TG) were used to measure secretion from dense and alpha-granules, respectively. Ca2+ alone induced secretion from both granule types; half-maximal effects were seen at a -log [Ca2+ free] (pCa) of 5.5 and maximal secretion at a pCa of 4.5, when approximately 80% of 5-HT and approximately 50% of beta TG were released. Addition of PMA, guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S), GTP, or thrombin shifted the Ca2+ dose-response curves for secretion of both 5-HT and beta TG to the left and caused small increases in the maximum secretion observed. These results suggested that secretion from alpha-granules, like that from dense granules, is a Ca(2+)-dependent process stimulated by the sequential activation of a G-protein, phospholipase C, and protein kinase C (PKC). However, high concentrations of PMA and GTP gamma S had distinct effects in the absence of Ca2+ (pCa greater than 9); 100 nM PMA released approximately 20% of platelet 5-HT but little beta TG, whereas 100 microM GTP gamma S stimulated secretion of approximately 25% of each. Simultaneous addition of PMA greatly enhanced these effects of GTP gamma S. Phosphorylation of pleckstrin in permeabilized platelets incubated with [gamma-32P]ATP was used as an index of the activation of PKC during secretion. In the absence of Ca2+, 100 nM PMA caused maximal phosphorylation of pleckstrin and 100 microM GTP gamma S was approximately 50% as effective as PMA; neither GTP gamma S nor Ca2+ enhanced the phosphorylation of pleckstrin caused by 100 nM PMA. These results indicate that, although activation of PKC promoted secretion, GTP gamma S exerted additional stimulatory effects on secretion from both dense and alpha-granules that were not mediated by PKC. Measurement of [3H]inositol phosphate formation in permeabilized platelets containing [3H]phosphoinositides showed that GTP gamma S did not stimulate phosphoinositide-specific phospholipase C in the absence of Ca2+. It follows that in permeabilized platelets, GTP gamma S can both stimulate PKC and enhance secretion via G-protein-linked effectors other than this phospholipase.