Prolonged inhibition of protein kinase A results in metalloproteinase-dependent platelet GPIbalpha shedding

Actin polymerization regulates glycoprotein Ibα shedding

Glycoprotein (GP) Ibα shedding mediated by ADAM17 (a disintegrin and metalloproteinase 17) plays an important role in negatively regulating platelet function and thrombus formation. However, the mechanism of GPIbα shedding remains elusive. Here, we show that jasplakinolide (an actin-polymerizing peptide)-induced actin polymerization results in GPIbα shedding and impairs platelet function. Thrombin and A23187-induced GPIbα shedding is increased by jasplakinolide; in contrast, GPIbα shedding is reduced by a depolymerization regent (cytochalasin B). We find that actin polymerization activates calpain leading to filamin A hydrolyzation. We further demonstrate that the interaction of filamin A with the cytoplasmic domain of GPIbα plays a critical role in regulating actin polymerization-induced GPIbα shedding. Taken together, these data demonstrate that actin polymerization regulates ADAM17-mediated GPIbα shedding, suggesting a novel strategy to negatively regulate platelet function.

Platelet Membrane Receptor Proteolysis: Implications for Platelet Function

The activities of adhesion and signaling receptors in platelets are controlled by several mechanisms. An important way of regulation is provided by proteolytic cleavage of several of these receptors, leading to either a gain or a loss of platelet function. The proteases involved are of different origins and types: (i) present as precursor in plasma, (ii) secreted into the plasma by activated platelets or other blood cells, or (iii) intracellularly activated and cleaving cytosolic receptor domains. We provide a comprehensive overview of the proteases acting on the platelet membrane. We describe how these are activated, which are their target proteins, and how their proteolytic activity modulates platelet functions. The review focuses on coagulation-related proteases, plasmin, matrix metalloproteinases, ADAM(TS) isoforms, cathepsins, caspases, and calpains. We also describe how the proteolytic activities are determined by different platelet populations in a thrombus and conversely how proteolysis contributes to the formation of such populations.

Cyclic nucleotide-dependent inhibitory signaling interweaves with activating pathways to determine platelet responses

Platelets are regulated by extracellular cues that impact on intracellular signaling. The endothelium releases prostacyclin and nitric oxide which stimulate the synthesis of cyclic nucleotides cAMP and cGMP leading to platelet inhibition. Other inhibitory mechanisms involve immunoreceptor tyrosine-based inhibition motif-containing receptors, intracellular receptors and receptor desensitization. Inhibitory cyclic nucleotide pathways are traditionally thought to represent a passive background system keeping platelets in a quiescent state. In contrast, cyclic nucleotides are increasingly seen to be dynamically involved in most aspects of platelet regulation. This review focuses on crosstalk between activating and cyclic nucleotide-mediated inhibitory pathways highlighting emerging new hub structures and signaling mechanisms. In particular, interactions of plasma membrane receptors like P2Y12 and GPIb/IX/V with the cyclic nucleotide system are described. Furthermore, differential regulation of the RGS18 complex, second messengers, protein kinases, and phosphatases are presented, and control over small G-proteins by guanine-nucleotide exchange factors and GTPase-activating proteins are outlined. Possible clinical implications of signaling crosstalk are discussed.

The regulation of Sema4D exodomain shedding by protein kinase A in platelets

We have previously shown that Sema4D expressed on the platelet plasma membrane can be cleaved by the metalloprotease ADAM17, producing a 120-kDa exodomain fragment that retains biological activity and remnant fragments of 24-28 kDa that remain associated with the platelet membrane. This process is modulated by calmodulin. Here we investigated the potential role of protein kinase A (PKA) in these events. Using a pharmacological approach, we now show that inhibition of PKA by H89 is sufficient to induce Sema4D exodomain shedding, while activation of PKA inhibits agonist-initiated shedding. Studies on the regulatory mechanism show that the shedding induced by PKA inhibition is mediated by ADAM17, but, unlike agonist-induced shedding, does not involve the dissociation of calmodulin from the Sema4D cytoplasmic domain. In attempt to identify the cleavage sites for shedding, we found that ADAM17 mediates variable cleavages in the juxtamembrane region. Therefore, our data reveal a potential regulatory mechanism for the shedding of Sema4D in platelets.

The role of mitochondria-derived reactive oxygen species in hyperthermia-induced platelet apoptosis

A combination of hyperthermia with radiotherapy and chemotherapy for various solid tumors has been practiced clinically. However, hyperthermic therapy has side effects, such as thrombocytopenia. Up to now, the pathogenesis of hyperthermia-induced thrombocytopenia remains unclear. Previous studies have shown that hyperthermia induces platelet apoptosis. However, the signaling pathways and molecular mechanisms involved in hyperthermia-induced platelet apoptosis have not been determined. Here we show that hyperthermia induced intracellular reactive oxygen species (ROS) production and mitochondrial ROS generation in a time-dependent manner in platelets. The mitochondria-targeted ROS scavenger Mito-TEMPO blocked intracellular ROS and mitochondrial ROS generation. By contrast, inhibitors of reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, nitric oxide synthase, cyclooxygenase and lipoxygenase did not. Furthermore, Mito-TEMPO inhibited hyperthermia-induced malonyldialdehyde production and cardiolipin peroxidation. We also showed that hyperthermia-triggered platelet apoptosis was inhibited by Mito-TEMPO. Furthermore, Mito-TEMPO ameliorated hyperthermia-impaired platelet aggregation and adhesion function. Lastly, hyperthermia decreased platelet manganese superoxide dismutase (MnSOD) protein levels and enzyme activity. These data indicate that mitochondrial ROS play a pivotal role in hyperthermia-induced platelet apoptosis, and decreased of MnSOD activity might, at least partially account for the enhanced ROS levels in hyperthermia-treated platelets. Therefore, determining the role of mitochondrial ROS as contributory factors in platelet apoptosis, is critical in providing a rational design of novel drugs aimed at targeting mitochondrial ROS. Such therapeutic approaches would have potential clinical utility in platelet-associated disorders involving oxidative damage.

Mechanism of platelet functional changes and effects of anti-platelet agents on in vivo hemostasis under different gravity conditions

Serious thrombotic and hemorrhagic problems or even fatalities evoked by either microgravity or hypergravity occur commonly in the world. We recently reported that platelet functions are inhibited in microgravity environments and activated under high-G conditions, which reveals the pathogenesis for gravity change-related hemorrhagic and thrombotic diseases. However, the mechanisms of platelet functional variations under different gravity conditions remain unclear. In this study we show that the amount of filamin A coimmunoprecipitated with GPIbalpha was enhanced in platelets exposed to modeled microgravity and, in contrast, was reduced in 8 G-exposed platelets. Hypergravity induced actin filament formation and redistribution, whereas actin filaments were reduced in platelets treated with modeled microgravity. Furthermore, intracellular Ca2+ levels were elevated by hypergravity. Pretreatment of platelets with the cell-permeable Ca2+ chelator BAPTA-AM had no effect on cytoskeleton reorganization induced by hypergravity but significantly reduced platelet aggregation induced by ristocetin/hypergravity. Two anti-platelet agents, aspirin and tirofiban, effectively reversed the shortened tail bleeding time and reduced the death rate of mice exposed to hypergravity. Furthermore, the increased P-selectin surface expression was obviously reduced in platelets from mice treated with aspirin/hypergravity compared with those from mice treated with hypergravity alone. These data suggest that the actin cytoskeleton reorganization and intracellular Ca2+ level play key roles in the regulation of platelet functions in different gravitational environments. The results with anti-platelet agents not only further confirm the activation of platelets in vivo but also suggest a therapeutic potential for hypergravity-induced thrombotic diseases.

The glycoprotein Ibalpha-von Willebrand factor interaction induces platelet apoptosis

BACKGROUND

The interaction of glycoprotein (GP) Ibalpha with von Willebrand factor (VWF) initiates platelet adhesion, and simultaneously triggers intracellular signaling cascades leading to platelet aggregation and thrombus formation. Some of the signaling events are similar to those occurring during apoptosis, however, it is still unclear whether platelet apoptosis is induced by the GPIbalpha-VWF interaction.

OBJECTIVES

To investigate whether the GPIbalpha-VWF interaction induces platelet apoptosis and the role of 14-3-3zeta in apoptotic signaling.

METHODS

Apoptotic events were assessed in platelets or Chinese hamster ovary (CHO) cells expressing wild-type (1b9) or mutant GPIb-IX interacting with VWF by flow cytometry or western blotting.

RESULTS

Ristocetin-induced GPIbalpha-VWF interaction elicited apoptotic events in platelets, including phosphatidylserine exposure, elevations of Bax and Bak, gelsolin cleavage, and depolarization of mitochondrial inner transmembrane potential. Apoptotic events were also elicited in platelets exposed to pathologic shear stresses in the presence of VWF; however, the shear-induced apoptosis was eliminated by the anti-GPIbalpha antibody AK2. Furthermore, apoptotic events occurred in 1b9 cells stimulated with VWF and ristocetin, but were significantly diminished in two CHO cell lines expressing mutant GPIb-IX with GPIbalpha truncated at residue 551 or a serine-to-alanine mutation at the 14-3-3zeta-binding site in GPIbalpha.

CONCLUSIONS

This study demonstrates that the GPIbalpha-VWF interaction induces apoptotic events in platelets, and that the association of 14-3-3zeta with the cytoplasmic domain of GPIbalpha is essential for apoptotic signaling. This finding may suggest a novel mechanism for platelet clearance or some thrombocytopenic diseases.

The role of calpain in the regulation of ADAM17-dependent GPIbalpha ectodomain shedding

There are evidence that both a disintegrin and metalloproteinase 17 (ADAM17) and calpain are involved in platelet glycoprotein (GP)Ibalpha ectodomain cleavage. However, the relationship between the two enzymes in the shedding process remains unclear. Here we show that calcium ionophore A23187- and alpha-thrombin-induced GPIbalpha shedding is completely inhibited by the metalloproteinase inhibitor GM6001, whereas it is only partially inhibited by calpain inhibitors. Calpain activator dibucaine-induced GPIbalpha shedding was completely inhibited by both metalloproteinase and calpain inhibitors. On the other hand, calpain inhibitors did not inhibit GPIbalpha shedding induced by the reagents that specifically activate ADAM17. Furthermore, A23187-induced GPIbalpha shedding in Chinese hamster ovary cells expressing wild-type or mutant GPIb-IX was also partially inhibited by calpain inhibitors and almost completely inhibited by GM6001. Therefore, these data indicate that calpain plays an important role in the regulation of ADAM17-dependent GPIbalpha ectodomain shedding in both platelets and nucleated cells.

Shear-induced interaction of platelets with von Willebrand factor results in glycoprotein Ibα shedding

Shear-induced platelet adhesion through the interaction of glycoprotein (GP) Ibα with von Willebrand factor (VWF) exposed at the injured vessel wall or atherosclerotic plaque rupture is a prerequisite for the physiological hemostatic process or pathological thrombus formation in stenosed arteries. Here we show that shear-induced interaction of platelets with immobilized VWF results in GPIbα ectodomain shedding. Washed platelets were exposed to VWF-coated glass capillary or cone-and-plate viscometer at different shear rates, and GPIbα ectodomain was shed from platelets, while a small mass of GPIbα COOH-terminal peptide, ∼17 kDa, was increased correspondingly. The extent of GPIbα shedding was enhanced with the concentration of immobilized VWF and the time duration of constant shear stress, whereas it was obviously reduced with the decreased number of adherent platelets. Pretreatment of platelets with membrane-permeable calpain inhibitors and metalloproteinase inhibitor abolished shear-induced GPIbα shedding. Furthermore, GPIbα shedding was obviously diminished by anti-integrin-αIIbβ3monoclonal antibody SZ21, phosphatidylinositol 3-kinase inhibitor wortmannin, and cell-permeable calcium chelator 1,2-bis( o-aminophenoxy)ethane- N, N, N′, N′-tetraacetic acid. These results indicate that shear-induced platelet-VWF interaction results in calpain and metalloproteinase-dependent GPIbα ectodomain shedding. These findings not only have a physiological implication in understanding the presence of glycocalicin in normal circulation, but also suggest a novel mechanism for the negative regulation of platelet function and the limitation of platelet thrombus infinite formation under pathophysiological flow conditions.