The platelet as a Ca(2+)-driven cell: mechanisms which may modulate Ca(2+)-driven responses

Human platelets exhibit chemotaxis using functional N-formyl peptide receptors

OBJECTIVE

Activated platelets participate in inflammatory and microbicidal processes by upregulation of surface selectins, shedding of CD40 ligand, and release of platelet microbicidal proteins and microparticles. Given their myeloid lineage, we hypothesized that platelets express functional N-formyl peptide receptors and respond to the bacterially derived chemotactic peptide N-formyl peptide with gradient-driven chemotaxis.

METHODS AND RESULTS

Here we show specific binding of N-formyl peptides to the surface of activated platelets. Platelet expression and function of the formyl peptide receptor, FPR, was verified by RT-PCR of the differentiated megakaryocyte MEG-01 cell line, immunoblotting of platelet proteins, and calcium mobilization in platelets with formyl peptide binding. Furthermore, we demonstrate gradient-driven chemotaxis of platelets by video microscopy and transwell migration toward formyl peptides. We also show that endogenous formyl peptides, released by eukaryotic mitochondria from necrotic cells, induce chemotaxis using formyl peptide receptors expressed by thrombin-activated platelets. Conversely, supernatants from cells undergoing apoptotic cell death do not induce platelet chemotaxis. Platelet chemotaxis to formyl peptides was blocked with FPR-specific antibody as well as by pertussis toxin inhibition of the formyl peptide G-coupled receptor.

CONCLUSION

These data establish a new role for platelets in host defense and suggest reexamination of their active function in microbicidal and other host defense activities.

Role of Ca(2+) and protein kinase C in the serotonin (5-HT) transport in human platelets

Accumulation of serotonin (5-HT) into human platelets was not affected by the presence of the extra-cellular calcium chelator EGTA, while decreased by platelet incubation with the membrane permeant chelator BAPTA-AM. Serotonin uptake also diminished upon platelet exposure to EGTA/thapsigargin or EGTA/ionomycin which increased the cytosolic [Ca(2+)] to levels lower than those inducing secretion of dense granules. The latter inhibition depended in part on changes of intra-granular pH, since the accumulation of acridine orange, which is driven into the dense granules by the intra-granular acid pH gradient, was slightly decreased in the presence of EGTA/thapsigargin. These compounds also inhibited the 5-HT uptake in platelets pre-incubated with reserpine and bafilomycin that prevent 5-HT from entering into the dense granules. Inhibitors of protease, protein phosphatase, Na(+)/H(+) exchanger or ciclo-oxygenase activities did not modify the serotonin accumulation. Addition of EGTA/thapsigargin to reserpine-treated, [(14)C]5-HT-loaded, platelets caused an imipramine-insensitive release of labelled serotonin. This release was reduced by both BAPTA-AM or protein kinase C inhibitor bisindoylmaleimide (GF). The latter compound, either alone or together with EGTA/thapsigargin, inhibited the 5-HT accumulation in reserpine-treated platelets. It is concluded that both cytosolic [Ca(2+)] and protein kinase C are involved in the regulation of the plasma membrane 5-HT transport.

Inhibition of human platelet function by sulfatides

Sulfatides are glycolipid constituents of human platelet cell membranes and have been shown to interact with platelet-binding proteins involved in hemostasis. Because little is known about the physiological role of sulfatides in platelet function, the effect of sulfatide on platelet adhesion, aggregation, release, and ristocetin-induced platelet agglutination (RIPA) was studied. These processes are inhibited when exogenous sulfatide is present in vitro. Inhibition of aggregation induced by collagen, thrombin, and ristocetin by sulfatide was dose dependent. Adenosine diphosphate-mediated adhesion and aggregation were not significantly affected by sulfatide, nor was serotonin- and epinephrine-mediated aggregation. Collagen mediate release of serotonin was reduced sulfatide. RIPA demonstrated dose-dependent inhibition in response to sulfatide. These results suggest that sulfatide may play a role in modulating platelet activation.

Marked Temperature Dependence of the Platelet Calcium Signal Induced by Human von Willebrand Factor

Interaction of von Willebrand factor (vWF) with the platelet is essential to hemostasis when vascular injury occurs. This interaction elevates the intracellular free calcium concentration ([Ca2+]i) and promotes platelet activation. The present study investigated the temperature dependence of vWF-induced [Ca2+]i signaling in human platelets. The influence of temperature can provide invaluable insight into the underlying mechanism. Platelet [Ca2+]i was monitored with Fura-PE3. Ristocetin-mediated binding of vWF induced a transient platelet [Ca2+]i increase at 37°C, but no response at lower temperatures (20°C to 25°C). This temperature dependence could not be attributed to a reduction in vWF binding, as ristocetin-mediated platelet aggregation and agglutination were essentially unaffected by temperature. Most other platelet agonists (U-46619, -thrombin, and adenosine 5′-diphosphate [ADP]) induced a [Ca2+]isignal whose amplitude did not diminish at lower temperatures. The [Ca2+]i signal in response to arachidonic acid, however, showed similar temperature dependence to that seen with vWF. Assessment of thromboxane A2 production showed a strong temperature dependence for metabolism of arachidonic acid by the cyclo-oxygenase pathway. vWF induced thromboxane A2production in the platelet. Aspirin treatment abolished the vWF-induced [Ca2+]i signal. These observations suggest that release of arachidonic acid and its conversion to thromboxane A2 play a central role in vWF-mediated [Ca2+]i signaling in the platelet at physiological temperatures.

The internal calcium concentration of human platelets increases during chilling

Human platelets must be stored at 22 degreesC in blood banks, because of the well-known phenomenon of cold-induced activation. When human platelets are chilled below room temperature, they undergo shape change and vesicle secretion that resembles physiological agonist-mediated activation. The trigger for the cascade of events leading to platelet activation at hypothermic temperatures is not known, although an increase in the internal calcium concentration ([Ca]i) due to passage of the platelet membranes through their thermotropic phase transition has been proposed. We report here that the fluorescent calcium-sensitive probe, Indo-1, has been used to estimate the internal calcium concentration of human platelets during a reduction in temperature from 20 degreesC to 5 degreesC at a rate of 0.5 degreesC/min. An increase on the order of 100 nM was recorded. Almost all of the increase in [Ca2+]i occurs during the chilling process, as incubation of platelets for 1 h at low temperature did not lead to a continued calcium concentration increase. The increase in [Ca2+]i during chilling is likely to be due to more than a single mechanism, but might include some release of the calcium stores from the dense tubule system. Loading platelets with the calcium chelator BAPTA (1, 2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid) dramatically reduced the increase in [Ca2+]i seen during chilling. Antifreeze glycoproteins (AFGPs) isolated from the blood serum of Antarctic fishes, which are known to protect platelets from cold-induced activation, did not eliminate the rise in [Ca2+]i during chilling, suggesting that signaling mechanisms are likely to be involved in cold-induced activation.

Synergistic inhibition of thrombin-induced platelet aggregation by the novel nitric oxide-donor GEA 3175 and adenosine

1. The influence of the novel nitric oxide-donor GEA 3175 on thrombin- and ionomycin-stimulated human platelets was investigated. The effect of GEA 3175 was compared with that of adenosine, an activator of platelet adenylyl cyclase. 2. GEA 3175 inhibited thrombin-induced secretion of ATP but did not affect aggregation; similar results were obtained with adenosine. 3. Thrombin-stimulated rises in the cytosolic free Ca2+ concentration, [Ca2+]i, were dose-dependently inhibited by GEA 3175 and adenosine. GEA 3175 and adenosine maximally reduced the initial rise in [Ca2+]i by 41% and 35%, respectively. 4. Simultaneous exposure to GEA 3175 and adenosine nearly abolished both the functional responses (i.e. aggregation and degranulation) and the rises in [Ca2+]i in thrombin-stimulated platelets. 5. Aggregation and increases in [Ca2+]i triggered in platelets by the Ca(2+)-ionophore ionomycin were only marginally affected by a combination of GEA 3175 and adenosine. 6. GEA 3175 potently increased the guanosine 3':5'-cyclic monophosphate (cyclic GMP) content in platelets but did not affect adenosine 3':5'-cyclic monophosphate (cyclic AMP) levels. Adenosine did not increase either the cyclic AMP or the cyclic GMP levels in platelets. However, adenosine and GEA 3175 combined significantly elevated the platelet cyclic AMP content. 7. The results show that simultaneous exposure to GEA 3175 and adenosine promotes potent anti-aggregatory properties in platelets in vitro. The findings suggest that blockage of the cytosolic Ca(2+)-signal, which is probably mediated by an amplified cyclic nucleotide response, is an important event during the synergistic inhibition of thrombin-induced aggregation.

Ca2+ concentration during binding determines the manner in which annexin V binds to membranes

Annexins are a family of calcium-binding proteins that have been implicated in a wide range of intracellular processes. We have previously reported that stimulation of platelets with thrombin can induce the association of intracellular annexin V with membranes in two distinct ways. First, in such a way that it can be eluted from the membrane with EGTA and secondly in a manner such that it is tightly bound to the membrane and requires the non-ionic detergent Triton X-100 for its solubilization. We report that exposure of platelets to the calcium ionophore A23187 mimics the relocation induced by stimulation with thrombin. In separate experiments we demonstrate that a calcium ion concentration [Ca2+] of 0.8 microM is sufficient for maximum binding of the EGTA-resistant form to membranes. In contrast a higher [Ca2+] was required to induce maximal binding of the annexin V which could be extracted with EGTA. We demonstrate that following temperature-induced phase separation in Triton X-114, the membrane-associated annexin V partitions predominantly into the aqueous phase. We also show that the isoelectric point of annexin V does not change following membrane association. These observations suggest that a covalent modification, of annexin V itself, is not responsible for its association with the membrane. Millimolar [Ca2+] is required for maximal binding of purified annexin V to phospholipid vesicles. We show that binding to phospholipids can be reversed entirely by subsequent treatment with EGTA. This suggests that the EGTA-resistant form of annexin V is binding to a membrane component other than phosphatidylserine. Annexin V has previously been shown to bind to protein kinase C. Relocation of annexin V to membranes paralleled that of protein kinase C in thrombin-stimulated cells but not in cells treated with A23187, suggesting that these proteins are not functionally linked in platelet activation. Using bifunctional cross-linking reagents we have identified an 85 kDa complex containing annexin V. This may represent an association between annexin V and an annexin V-binding protein with a molecular mass of approximately 50 kDa.