Distinct roles for Rap1b protein in platelet secretion and integrin αIIbβ3 outside-in signaling

Roles of G proteins and their GTPase-activating proteins in platelets

Platelets are small anucleate blood cells supporting vascular function. They circulate in a quiescent state monitoring the vasculature for injuries. Platelets adhere to injury sites and can be rapidly activated to secrete granules and to form platelet/platelet aggregates. These responses are controlled by signalling networks that include G proteins and their regulatory guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). Recent proteomics studies have revealed the complete spectrum of G proteins, GEFs, and GAPs present in platelets. Some of these proteins are specific for platelets and very few have been characterised in detail. GEFs and GAPs play a major role in setting local levels of active GTP-bound G proteins in response to activating and inhibitory signals encountered by platelets. Thus, GEFs and GAPs are highly regulated themselves and appear to integrate G protein regulation with other cellular processes. This review focuses on GAPs of small G proteins of the Arf, Rab, Ras, and Rho families, as well as of heterotrimeric G proteins found in platelets.

Platelet C3G: a key player in vesicle exocytosis, spreading and clot retraction

C3G is a Rap1 GEF that plays a pivotal role in platelet-mediated processes such as angiogenesis, tumor growth, and metastasis by modulating the platelet secretome. Here, we explore the mechanisms through which C3G governs platelet secretion. For this, we utilized animal models featuring either overexpression or deletion of C3G in platelets, as well as PC12 cell clones expressing C3G mutants. We found that C3G specifically regulates α-granule secretion via PKCδ, but it does not affect δ-granules or lysosomes. C3G activated RalA through a GEF-dependent mechanism, facilitating vesicle docking, while interfering with the formation of the trans-SNARE complex, thereby restricting vesicle fusion. Furthermore, C3G promotes the formation of lamellipodia during platelet spreading on specific substrates by enhancing actin polymerization via Src and Rac1-Arp2/3 pathways, but not Rap1. Consequently, C3G deletion in platelets favored kiss-and-run exocytosis. C3G also controlled granule secretion in PC12 cells, including pore formation. Additionally, C3G-deficient platelets exhibited reduced phosphatidylserine exposure, resulting in decreased thrombin generation, which along with defective actin polymerization and spreading, led to impaired clot retraction. In summary, platelet C3G plays a dual role by facilitating platelet spreading and clot retraction through the promotion of outside-in signaling while concurrently downregulating α-granule secretion by restricting granule fusion.

Atorvastatin-mediated inhibition of prenylation of Rab27b and Rap1a in platelets attenuates their prothrombotic capacity and modulates clot structure

Statins inhibit the mevalonate pathway by impairing protein prenylation via depletion of lipid geranylgeranyl diphosphate (GGPP). Rab27b and Rap1a are small GTPase proteins involved in dense granule secretion, platelet activation, and regulation. We analyzed the impact of statins on prenylation of Rab27b and Rap1a in platelets and the downstream effects on fibrin clot properties. Whole blood thromboelastography revealed that atorvastatin (ATV) delayed clot formation time (P < .005) and attenuated clot firmness (P < .005). ATV pre-treatment inhibited platelet aggregation and clot retraction. Binding of fibrinogen and P-selectin exposure on stimulated platelets was significantly lower following pre-treatment with ATV (P < .05). Confocal microscopy revealed that ATV significantly altered the structure of platelet-rich plasma clots, consistent with the reduced fibrinogen binding. ATV enhanced lysis of Chandler model thrombi 1.4-fold versus control (P < .05). Western blotting revealed that ATV induced a dose-dependent accumulation of unprenylated Rab27b and Rap1a in the platelet membrane. ATV dose-dependently inhibited ADP release from activated platelets. Exogenous GGPP rescued the prenylation of Rab27b and Rap1a, and partially restored the ADP release defect, suggesting these changes arise from reduced prenylation of Rab27b. These data demonstrate that statins attenuate platelet aggregation, degranulation, and binding of fibrinogen thereby having a significant impact on clot contraction and structure.

Altered Cortical Palmitoylation Induces Widespread Molecular Disturbances in Parkinson's Disease

The relationship between Parkinson's disease (PD), the second-most common neurodegenerative disease after Alzheimer's disease, and palmitoylation, a post-translational lipid modification, is not well understood. In this study, to better understand the role of protein palmitoylation in PD and the pathways altered in this disease, we analyzed the differential palmitoyl proteome (palmitome) in the cerebral cortex of PD patients compared to controls (n = 4 per group). Data-mining of the cortical palmitome from PD patients and controls allowed us to: (i) detect a set of 150 proteins with altered palmitoylation in PD subjects in comparison with controls; (ii) describe the biological pathways and targets predicted to be altered by these palmitoylation changes; and (iii) depict the overlap between the differential palmitome identified in our study with protein interactomes of the PD-linked proteins α-synuclein, LRRK2, DJ-1, PINK1, GBA and UCHL1. In summary, we partially characterized the altered palmitome in the cortex of PD patients, which is predicted to impact cytoskeleton, mitochondrial and fibrinogen functions, as well as cell survival. Our study suggests that protein palmitoylation could have a role in the pathophysiology of PD, and that comprehensive palmitoyl-proteomics offers a powerful approach for elucidating novel cellular pathways modulated in this neurodegenerative disease.

Reversible Platelet Integrin αIIbβ3 Activation and Thrombus Instability

Integrin αIIbβ3 activation is essential for platelet aggregation and, accordingly, for hemostasis and arterial thrombosis. The αIIbβ3 integrin is highly expressed on platelets and requires an activation step for binding to fibrinogen, fibrin or von Willebrand factor (VWF). A current model assumes that the process of integrin activation relies on actomyosin force-dependent molecular changes from a bent-closed and extended-closed to an extended-open conformation. In this paper we review the pathways that point to a functional reversibility of platelet αIIbβ3 activation and transient aggregation. Furthermore, we refer to mouse models indicating that genetic defects that lead to reversible platelet aggregation can also cause instable thrombus formation. We discuss the platelet agonists and signaling pathways that lead to a transient binding of ligands to integrin αIIbβ3. Our analysis points to the (autocrine) ADP P2Y₁ and P2Y₁₂ receptor signaling via phosphoinositide 3-kinases and Akt as principal pathways linked to reversible integrin activation. Downstream signaling events by protein kinase C, CalDAG-GEFI and Rap1b have not been linked to transient integrin activation. Insight into the functional reversibility of integrin activation pathways will help to better understand the effects of antiplatelet agents.

Platelets inhibit development of atherosclerosis in atherosclerotic mice

Platelets can protect from lipopolysaccharide-induced septic shock by inhibiting inflammation, but it is unknown whether platelets have an anti-atherosclerotic effect. The aim of this study was to investigate the effect of platelet transfusion on atherosclerosis (AS) in a mouse model of AS. Apolipoprotein E deficiency (ApoE^-/-) mice were fed with a high-fat diet (HFD) for 8 weeks to establish a mouse model of AS. Mice weekly underwent bi-weekly injection with or without platelets during AS induction (HFD+platelet). Hematoxylin-eosin (H&E), Oil Red O, and Sudan IV stainings were used to assess pathological and morphological changes in the aortic tissue. Lipid levels, and liver and kidney function were examined using an automatic biochemical analyzer. Immune histochemical assays were used to detect the infiltration and distribution of inflammatory cytokines, including tumor necrosis factor-α (TNF-α), interleukin 1β (IL-1β), IL-6, and monocyte chemotactic protein-1 (MCP-1) in the aortic arch. Western blot and enzyme-linked immunosorbent assay (ELISA) were used to examine the expression levels of TNF-α, IL-1β, IL-6, and MCP-1 in the aorta or the peripheral blood, respectively. Compared with the HFD group, AS pathological lesions from the aortic arch in the HFD+platelet group were significantly smaller and alterations in the lipid metabolism were also less pronounced. Furthermore, TNF-α, IL-1β, IL-6, and MCP-1 levels were all significantly reduced in mice that received platelet injection. Platelets transfusion can effectively ameliorate lipid metabolism, suppress the inflammatory response in the vascular wall, and inhibit the development of AS in mice.

Platelet protects angiotensin II-driven abdominal aortic aneurysm formation through inhibition of inflammation

BACKGROUND

Inflammation is the primary cause of abdominal aortic aneurysm (AAA) formation and development. It has been reported that platelets protect against septic shock by inhibiting inflammation. However, it is unclear whether platelets protect AAA progress via suppressing inflammation.

METHODS

A mouse model of AAA was established by a daily administration of angiotensin II (Ang II, 1000 ng/kg/min) for 28-day. The AAA mice received 1 × 10⁹ platelets transfusion in normal saline every 3rd day for 1 month. Hematoxylin and eosin, Masson's trichrome, and elastic van Gieson staining techniques were used to analyze the morphology of the abdominal aorta. Immunohistochemistry was used to detect any infiltration of inflammatory cells, inflammatory factors, and matrix metalloproteins (MMPs) in the aortic tissue. Western blot and enzyme-linked immunosorbent assay (ELISA) were used to examine the inflammatory factor proteins levels in the aortic wall and peripheral blood, respectively.

RESULTS

Platelets infusion significantly suppressed the Ang II-driven elevation of aortic diameter, AAA formation (52.38% decrease, P < 0.05), aortic expansion, elastic lamina destruction, and inflammatory response. In addition, MMP-2 and MMP-9 production were also reduced by platelets transfusion.

CONCLUSIONS

For the first time, our study reported the beneficial effect of platelet transfusion in suppressing the Ang II-driven AAA progress in mice through the inhibition of inflammation.

Roxadustat Does Not Affect Platelet Production, Activation, and Thrombosis Formation

Objective: Roxadustat is a new medication for the treatment of renal anemia. EPO (erythropoietin)-the current treatment standard-has been reported to enhance platelet activation and production. However, to date, the effect of roxadustat on platelets is unclear. To address this deficiency, herein, we have evaluated the effect of roxadustat on platelet production and function. Approach and Results: We performed several mouse platelet functional assays in the presence/absence of in vitro and in vivo roxadustat treatment. Both healthy and 5/6 nephrectomized mice were utilized. The effect of roxadustat on platelet function of healthy volunteers and chronic kidney disease patients was also evaluated. For platelet production, megakaryocyte maturation and proplatelet formation were assayed in vitro. Peripheral platelet and bone marrow megakaryocyte counts were also determined. We found that roxadustat could not stimulate washed platelets directly, and platelet aggregation, spreading, clot retraction, and P-selectin/JON/A exposure were similar with or without in vitro or in vivo roxadustat treatment among both healthy and 5/6 nephrectomized mice. In vivo mouse thrombosis models were additionally performed, and no differences were detected between the vehicle and roxadustat treatment groups. EPO, which was considered a positive control in the present study, promoted platelet function and production as reported previously. Megakaryocyte maturation and proplatelet formation were also not significantly different between control mice and those treated with roxadustat. After receiving roxadustat for 14 days, no difference in the peripheral platelet count was observed in the mice. Conclusions: Administration of roxadustat has no significant impact on platelet production and function.

MiR-181a Reduces Platelet Activation via the Inhibition of Endogenous RAP1B

Aim

Since RAP1B is critical for platelet functions, including hemostasis, this study was conducted to identify RAP1B regulating microRNAs (miRNAs) in ex vivo stored platelets.

Background

Previous studies with platelets identified factors affecting RAP1B activity but regulatory miRNAs that affect RAP1B protein expression have not been reported.

Objective

To understand the functional significance of miRNA mediated regulation of RAP1B in stored platelets, using microRNA, miR-181a as an example.

Methods

A Tagged RNA Affinity approach (MS2-TRAP) was employed to identify miRNAs that bound to the 3` untranslated region (3`UTR) of the RAP1B mRNA in HeLa cells as an assay system. And subsequently, the mRNA 3’UTR:miRNA interactions were verified in platelets through the ectopic expression of miR-181a mimic and appropriate controls. The interaction of such miRNAs with RAP1B mRNA was also validated by qRT-PCR and Western analysis.

Results

Sixty-two miRNAs from MS2 assay were then compared with already known 171 platelet abundant miRNAs to identify a common set of miRNAs. This analysis yielded six miRNAs (miR-30e, miR-155, miR-181a, miR-206, miR-208a and miR-454), which are also predicted to target RAP1B mRNA. From this pool, miR-181a was selected for further study since RAP1B harbors two binding sites for miR-181a in its 3′UTR. Ectopic expression of miR-181a mimic in platelets resulted in lowering the endogenous RAP1B at both mRNA and protein levels. Further, miR-181a ectopic expression reduced the surface expression of the platelet activation marker, P-selectin.

Conclusion

MicroRNA-181a can target RAP1B and this interaction has the potential to regulate platelet activation during storage.

Comparative transcriptome reveals the response of oriental river prawn (Macrobrachium nipponense) to sulfide toxicity at molecular level

Aquatic environmental pollutants have various impacts on aquaculture. Specifically, sulfide has been established as being toxic to aquatic animals including the oriental river prawn Macrobrachium nipponense. In response, the hepatopancreas has been broadly studied, as it plays a pivotal role in arthropod nutrient digestion and absorption, energy supply, and organ development as well as in crustacean immunity. However, the underlying molecular mechanisms of hepatopancreas's response to sulfide toxicity are still poorly understand. Herein, we used Nova-seq 6000 platform to conduct a comparative transcriptome analysis of gene expression profiles in the hepatopancreas of M. nipponense, while it was under the influence of a semi-lethal sulfide concentration (3.20 mg/L at 48 h). A total of 139 million raw reads were obtained, in which 67,602 transcripts were clustered into 37,041 unigenes for further analysis. After constant sulfide exposure for 48 h, 235 differentially expressed genes, i.e., DEGs (151 up-regulated and 84 down-regulated) were identified in the sulfide treatment group (TGHP) compared with the control group (CGHP). We used GO and KEGG databases to annotate all the DEGs into 1180 functions and 123 pathways, respectively. The metabolic pathways included proximal tubule bicarbonate reclamation, sulfur metabolism, glycolysis and gluconeogenesis, and the TCA cycle; while immune-related pathways contained Ras, Rap1, focal adhesion and platelet activation. Additionally, apoptosis-involved pathways e.g., lysosome, also exhibited remarkable alteration in the presence of sulfide stress. Notably, responses to external stimuli and detoxification genes- such as GSKIP, CRT2, APOD, TRET1, CYP4C3 and HR39- were significantly altered by the sulfide stress, indicating that significant toxicity was transferred through energy metabolism, growth, osmoregulatory processes and immunity. Finally, we demonstrated that in the present of sulfide stress, M. nipponense altered the expression of detoxification- and extracellular stimulation-related genes, and displayed positive resistance via tight junction activation and lysosome pathways. The results of these novel experiments shed light on the hepatopancreas's molecular response to sulfide stress resistance and the corresponding adaptation mechanism; and enable us to identify several potential biomarkers for further studies.

Talin-1 is the principal platelet Rap1 effector of integrin activation

Ras-related protein 1 (Rap1) is a major convergence point of the platelet-signaling pathways that result in talin-1 binding to the integrin β cytoplasmic domain and consequent integrin activation, platelet aggregation, and effective hemostasis. The nature of the connection between Rap1 and talin-1 in integrin activation is an important remaining gap in our understanding of this process. Previous work identified a low-affinity Rap1-binding site in the talin-1 F0 domain that makes a small contribution to integrin activation in platelets. We recently identified an additional Rap1-binding site in the talin-1 F1 domain that makes a greater contribution than F0 in model systems. Here we generated mice bearing point mutations, which block Rap1 binding without affecting talin-1 expression, in either the talin-1 F1 domain (R118E) alone, which were viable, or in both the F0 and F1 domains (R35E,R118E), which were embryonic lethal. Loss of the Rap1-talin-1 F1 interaction in platelets markedly decreases talin-1-mediated activation of platelet β1- and β3-integrins. Integrin activation and platelet aggregation in mice whose platelets express only talin-1(R35E, R118E) are even more impaired, resembling the defect seen in platelets lacking both Rap1a and Rap1b. Although Rap1 is important in thrombopoiesis, platelet secretion, and surface exposure of phosphatidylserine, loss of the Rap1-talin-1 interaction in talin-1(R35E, R118E) platelets had little effect on these processes. These findings show that talin-1 is the principal direct effector of Rap1 GTPases that regulates platelet integrin activation in hemostasis.

C3G contributes to platelet activation and aggregation by regulating major signaling pathways

C3G is a GEF (guanine nucleotide exchange factor) for Rap GTPases, among which the isoform Rap1b is an essential protein in platelet biology. Using transgenic mouse models with platelet-specific overexpression of C3G or mutant C3GΔCat, we have unveiled a new function of C3G in regulating the hemostatic function of platelets through its participation in the thrombin-PKC-Rap1b pathway. C3G also plays important roles in angiogenesis, tumor growth, and metastasis through its regulation of the platelet secretome. In addition, C3G contributes to megakaryopoiesis and thrombopoiesis. Here, we used a platelet-specific C3G-KO mouse model to further support the role of C3G in hemostasis. C3G-KO platelets showed a significant delay in platelet activation and aggregation as a consequence of the defective activation of Rap1, which resulted in decreased thrombus formation in vivo. Additionally, we explored the contribution of C3G-Rap1b to platelet signaling pathways triggered by thrombin, PMA or ADP, in the referenced transgenic mouse model, through the use of a battery of specific inhibitors. We found that platelet C3G is phosphorylated at Tyr504 by a mechanism involving PKC-Src. This phosphorylation was shown to be positively regulated by ERKs through their inhibition of the tyrosine phosphatase Shp2. Moreover, C3G participates in the ADP-P2Y12-PI3K-Rap1b pathway and is a mediator of thrombin-TXA2 activities. However, it inhibits the synthesis of TXA2 through cPLA2 regulation. Taken together, our data reveal the critical role of C3G in the main pathways leading to platelet activation and aggregation through the regulation of Rap1b.

Hsp47 promotes cancer metastasis by enhancing collagen-dependent cancer cell-platelet interaction

Increased expression of extracellular matrix (ECM) proteins in circulating tumor cells (CTCs) suggests potential function of cancer cell-produced ECM in initiation of cancer cell colonization. Here, we showed that collagen and heat shock protein 47 (Hsp47), a chaperone facilitating collagen secretion and deposition, were highly expressed during the epithelial-mesenchymal transition (EMT) and in CTCs. Hsp47 expression induced mesenchymal phenotypes in mammary epithelial cells (MECs), enhanced platelet recruitment, and promoted lung retention and colonization of cancer cells. Platelet depletion in vivo abolished Hsp47-induced cancer cell retention in the lung, suggesting that Hsp47 promotes cancer cell colonization by enhancing cancer cell-platelet interaction. Using rescue experiments and functional blocking antibodies, we identified type I collagen as the key mediator of Hsp47-induced cancer cell-platelet interaction. We also found that Hsp47-dependent collagen deposition and platelet recruitment facilitated cancer cell clustering and extravasation in vitro. By analyzing DNA/RNA sequencing data generated from human breast cancer tissues, we showed that gene amplification and increased expression of Hsp47 were associated with cancer metastasis. These results suggest that targeting the Hsp47/collagen axis is a promising strategy to block cancer cell-platelet interaction and cancer colonization in secondary organs.

Stimulation of mTORC2 by integrin αIIbβ3 is required for PI3Kβ-dependent activation of Akt but is dispensable for platelet spreading on fibrinogen

Phosphatidylinositol 3 kinase (PI3K) is a major player in platelet activation and regulates thrombus formation and stabilization. The β isoform of PI3K is implicated in integrin αIIbβ3 outside-in signaling, is required for the phosphorylation of Akt, and controls efficient platelet spreading upon adhesion to fibrinogen. In this study we found that during integrin αIIbβ3 outside-in signaling PI3Kβ-dependent phosphorylation of Akt on Serine473 is mediated by the mammalian target of rapamycin complex 2 (mTORC2). The activity of mTORC2 is stimulated upon platelet adhesion to fibrinogen, as documented by increased autophosphorylation. However, mTORC2 activation downstream of integrin αIIbβ3 is PI3Kβ-independent. Inhibition of mTORC2, but not mTORC1, also prevents Akt phosphorylation of Threonine308 and affects Akt activity, resulting in the inhibition of GSK3α/β phosphorylation. Nevertheless, mTORC2 or Akt inhibition does not alter PI3Kβ-dependent platelet spreading on fibrinogen. The activation of the small GTPase Rap1b downstream of integrin αIIbβ3 is regulated by PI3Kβ but is not affected upon inhibition of either mTORC2 or Akt. Altogether, these results demonstrate for the first time the activation of mTORC2 and its involvement in Akt phosphorylation and stimulation during integrin αIIbβ3 outside-in signaling. Moreover, the results demonstrate that the mTORC2/Akt pathway is dispensable for PI3Kβ-regulated platelet spreading on fibrinogen.

Focal Adhesion Kinases in Platelet Function and Thrombosis

The focal adhesion kinase family includes 2 homolog members, FAK and Pyk2 (proline-rich tyrosine kinase 2), primarily known for their roles in nucleated cells as regulators of cytoskeletal dynamics and cell adhesion. FAK and Pyk2 are also expressed in megakaryocytes and platelets and are activated by soluble agonists and on adhesion to the subendothelial matrix. Despite high sequence homology and similar molecular organization, FAK and Pyk2 play different roles in platelet function. Whereas FAK serves mostly as a traditional focal adhesion kinase activated downstream of integrins, Pyk2 coordinates multiple signals from different receptors. FAK, but not Pyk2, is involved in megakaryocyte maturation and platelet production. In circulating platelets, FAK is recruited by integrin αIIbβ3 to regulate hemostasis, whereas it plays minimal roles in thrombosis. By contrast, Pyk2 is implicated in platelet activation and is an important regulator of thrombosis. The direct activation of Pyk2 by calcium ions provides a connection between GPCRs (G-protein coupled receptors) and Src family kinases. In this review, we provide the comprehensive overview of >20 years of investigations on the role and regulation of focal adhesion kinases in blood platelets, highlighting common and distinctive features of FAK and Pyk2 in hemostasis and thrombosis.

Critical roles for the phosphatidylinositide 3-kinase isoforms p110β and p110γ in thrombopoietin-mediated priming of platelet function

Thrombopoietin (TPO) enhances platelet activation through activation of the tyrosine kinase; JAK2 and the lipid kinase phosphatidylinositide 3-kinase (PI3K). The aim of our study was to identify the PI3K isoforms involved in mediating the effect of TPO on platelet function and elucidate the underlying mechanism. We found that p110β plays an essential role in TPO-mediated (i) priming of protease-activated receptor (PAR)-mediated integrin α_IIbβ₃ activation and α-granule secretion, (ii) synergistic enhancement of PAR-mediated activation of the small GTPase RAP1, a regulator of integrin activation and (iii) phosphorylation of the PI3K effector Akt. More importantly, the synergistic effect of TPO on phosphorylation of extracellular-regulated kinase (ERK1/2) and thromboxane (TxA₂) synthesis was dependent on both p110β and p110γ. p110β inhibition/deletion, or inhibition of p110γ, resulted in a partial reduction, whereas inhibiting both p110β and p110γ completely prevented the synergistic effect of TPO on ERK1/2 phosphorylation and TxA₂ synthesis. The latter was ablated by inhibition of MEK, but not p38, confirming a role for ERK1/2 in regulating TPO-mediated increases in TxA₂ synthesis. In conclusion, the synergistic effect of TPO on RAP1 and integrin activation is largely mediated by p110β, whereas p110β and p110γ contribute to the effect of TPO on ERK1/2 phosphorylation and TxA₂ formation.

Direct Rap1/Talin1 interaction regulates platelet and neutrophil integrin activity in mice

Targeting Talin1 to the plasma membrane is a crucial step in integrin activation, which in leukocytes is mediated by a Rap1/RIAM/Talin1 pathway, whereas in platelets, it is RIAM independent. Recent structural, biochemical, and cell biological studies have suggested direct Rap1/Talin1 interaction as an alternative mechanism to recruit Talin1 to the membrane and induce integrin activation. To test whether this pathway is of relevance in vivo, we generated Rap1 binding-deficient Talin1 knockin (Tln13mut) mice. Although Tln13mut mice showed no obvious abnormalities, their platelets exhibited reduced integrin activation, aggregation, adhesion, and spreading, resulting in prolonged tail-bleeding times and delayed thrombus formation and vessel occlusion in vivo. Surprisingly, neutrophil adhesion to different integrin ligands and β2 integrin-dependent phagocytosis were also significantly impaired, which caused profound leukocyte adhesion and extravasation defects in Tln13mut mice. In contrast, macrophages exhibited no defect in adhesion or spreading despite reduced integrin activation. Taken together, our findings suggest that direct Rap1/Talin1 interaction is of particular importance in regulating the activity of different integrin classes expressed on platelets and neutrophils, which both depend on fast and dynamic integrin-mediated responses.

An Antithrombotic Strategy by Targeting Phospholipase D in Human Platelets

Phospholipase D (PLD) is involved in many biological processes. PLD1 plays a crucial role in regulating the platelet activity of mice; however, the role of PLD in the platelet activation of humans remains unclear. Therefore, we investigated whether PLD is involved in the platelet activation of humans. Our data revealed that inhibition of PLD1 or PLD2 using pharmacological inhibitors effectively inhibits platelet aggregation in humans. However, previous studies have showed that PLD1 or PLD2 deletion did not affect mouse platelet aggregation in vitro, whereas only PLD1 deletion inhibited thrombus formation in vivo. Intriguingly, our data also showed that the pharmacological inhibition of PLD1 or PLD2 does not affect mouse platelet aggregation in vitro, whereas the inhibition of only PLD1 delayed thrombus formation in vivo. These findings indicate that PLD may play differential roles in humans and mice. In humans, PLD inhibition attenuates platelet activation, adhesion, spreading, and clot retraction. For the first time, we demonstrated that PLD1 and PLD2 are essential for platelet activation in humans, and PLD plays different roles in platelet function in humans and mice. Our findings also indicate that targeting PLD may provide a safe and alternative therapeutic approach for preventing thromboembolic disorders.

Small GTPases in platelet membrane trafficking

Our understanding of fundamental biological processes within platelets is continually evolving. A critical feature of platelet biology relates to the intricate uptake, packaging and release of bioactive cargo from storage vesicles, essential in mediating a range of classical (haemostasis/thrombosis) and non-classical (regeneration/inflammation/metastasis) roles platelets assume. Pivotal to the molecular control of these vesicle trafficking events are the small GTPases of the Ras superfamily, which function as spatially distinct, molecular switches controlling essential cellular processes. Herein, we specifically focus on members of the Rab, Arf and Ras subfamilies, which comprise over 130 members and platelet proteomic datasets suggest that more than half of these are expressed in human platelets. We provide an update of current literature relating to trafficking roles for these GTPases in platelets, particularly regarding endocytic and exocytic events, but also vesicle biogenesis and provide speculative argument for roles that other related GTPases and regulatory proteins may adopt in platelets. Advances in our understanding of small GTPase function in the anucleate platelet has been hampered by the lack of specific molecular tools, but it is anticipated that this will be greatly accelerated in the years ahead and will be crucial to the identification of novel therapeutic targets controlling different platelet processes.

Potentially critical roles of TNPO1, RAP1B, ZDHHC17, and PPM1B in the progression of coronary atherosclerosis through microarray data analysis

OBJECTIVE

This study aimed to identify more potentially critical genes associated with atherosclerotic coronary artery disease (CAD).

MATERIALS AND METHODS

Gene expression profile of GSE12288 was downloaded from the Gene Expression Omnibus (GEO) database. Differentially expressed genes (DEGs) were screened in atherosclerotic CAD samples compared with controls, followed by weighed gene coexpression network analysis (WGCNA) by which the most significant module was identified. Gene coexpression network was constructed based on genes in the most significant module, and functional annotation was also performed. In addition, microRNAs (miRNAs) that were directly associated with CAD were screened from the literature, and the miRNA-target regulatory network was constructed based on genes in the most significant module, followed by Gene Ontology (GO) and pathway enrichment analysis. Furthermore, we used another data set of GSE42148 from the GEO database to perform data validation.

RESULTS

WGCNA analysis showed that the turquoise module may have the most important role in atherosclerotic CAD. Genes in this module were involved in translational elongation and intracellular signal transduction. Besides, we identified five confirmed CAD-related miRNAs. TNPO1, RAP1B, and ZDHHC17 could be targeted by four of these miRNAs. Genes such as PPM1B could be regulated by three miRNAs. Moreover, TNPO1 and ZDHHC17 were involved in the GO terms associated with protein localization and transport and the immune system; RAP1B and PPM1B were linked with intracellular signal transduction-related pathways. In addition, PPM1B and ZDHHC17 had accordantly significant expression changes in another data set GSE42148.

CONCLUSION

TNPO1, RAP1B, ZDHHC17, and PPM1B may play essential roles in the progression of coronary atherosclerosis.

Functional redundancy between RAP1 isoforms in murine platelet production and function

RAP GTPases, important regulators of cellular adhesion, are abundant signaling molecules in the platelet/megakaryocytic lineage. However, mice lacking the predominant isoform, RAP1B, display a partial platelet integrin activation defect and have a normal platelet count, suggesting the existence of a RAP1-independent pathway to integrin activation in platelets and a negligible role for RAP GTPases in megakaryocyte biology. To determine the importance of individual RAP isoforms on platelet production and on platelet activation at sites of mechanical injury or vascular leakage, we generated mice with megakaryocyte-specific deletion (mKO) of Rap1a and/or Rap1b Interestingly, Rap1a/b-mKO mice displayed a marked macrothrombocytopenia due to impaired proplatelet formation by megakaryocytes. In platelets, RAP isoforms had redundant and isoform-specific functions. Deletion of RAP1B, but not RAP1A, significantly reduced α-granule secretion and activation of the cytoskeleton regulator RAC1. Both isoforms significantly contributed to thromboxane A₂ generation and the inside-out activation of platelet integrins. Combined deficiency of RAP1A and RAP1B markedly impaired platelet aggregation, spreading, and clot retraction. Consistently, thrombus formation in physiological flow conditions was abolished in Rap1a/b-mKO, but not Rap1a-mKO or Rap1b-mKO, platelets. Rap1a/b-mKO mice were strongly protected from experimental thrombosis and exhibited a severe defect in hemostasis after mechanical injury. Surprisingly, Rap1a/b-mKO platelets were indistinguishable from controls in their ability to prevent blood-lymphatic mixing during development and hemorrhage at sites of inflammation. In summary, our studies demonstrate an essential role for RAP1 signaling in platelet integrin activation and a critical role in platelet production. Although important for hemostatic/thrombotic plug formation, platelet RAP1 signaling is dispensable for vascular integrity during development and inflammation.

RAP GTPases and platelet integrin signaling

Platelets are highly specialized cells that continuously patrol the vasculature to ensure its integrity (hemostasis). At sites of vascular injury, they are able to respond to trace amounts of agonists and to rapidly transition from an anti-adhesive/patrolling to an adhesive state (integrin inside-out activation) required for hemostatic plug formation. Pathological conditions that disturb the balance in the underlying signaling processes can lead to unwanted platelet activation (thrombosis) or to an increased bleeding risk. The small GTPases of the RAP subfamily, highly expressed in platelets, are critical regulators of cell adhesion, cytoskeleton remodeling, and MAP kinase signaling. Studies by our group and others demonstrate that RAP GTPases, in particular RAP1A and RAP1B, are the key molecular switches that turn on platelet activation/adhesiveness at sites of injury. In this review, we will summarize major findings on the role of RAP GTPases in platelet biology with a focus on the signaling pathways leading to the conversion of integrins to a high-affinity state.

GAS6/TAM Pathway Signaling in Hemostasis and Thrombosis

The GAS6/TYRO3-AXL-MERTK (TAM) signaling pathway is essential for full and sustained platelet activation, as well as thrombus stabilization. Inhibition of this pathway decreases platelet aggregation, shape change, clot retraction, aggregate formation under flow conditions, and surface expression of activation markers. Transgenic mice deficient in GAS6, or any of the TAM family of receptors that engage this ligand, exhibit in vivo protection against arterial and venous thrombosis but do not demonstrate either spontaneous or prolonged bleeding compared to their wild-type counterparts. Comparable results are observed in wild-type mice treated with pharmacological inhibitors of the GAS6-TAM pathway. Thus, GAS6/TAM inhibition offers an attractive novel therapeutic option that may allow for a moderate reduction in platelet activation and decreased thrombosis while still permitting the primary hemostatic function of platelet plug formation.

The small-molecule MERTK inhibitor UNC2025 decreases platelet activation and prevents thrombosis

Essentials Signaling by Gas6 through Tyro3/Axl/Mer receptors is essential for stable platelet aggregation. UNC2025 is a small molecule inhibitor of the Mer tyrosine kinase. UNC2025 decreases platelet activation in vitro and thrombus formation in vivo. UNC2025's anti-platelet effect is synergistic with inhibition of the ADP receptor, P2Y12 .

SUMMARY

Background Growth arrest-specific protein 6 signals through the TAM (TYRO-3-AXL-MERTK) receptor family, mediating platelet activation and thrombus formation via activation of the aggregate-stabilizing αIIb β3 integrin. Objective To describe the antithrombotic effects mediated by UNC2025, a small-molecule MERTK tyrosine kinase inhibitor. Methods MERTK phosphorylation and downstream signaling were assessed by immunoblotting. Light transmission aggregometry, flow cytometry and microfluidic analysis were used to evaluate the impact of MERTK inhibition on platelet activation and stability of aggregates in vitro. The effects of MERTK inhibition on arterial and venous thrombosis, platelet accumulation at microvascular injury sites and tail bleeding times were determined with murine models. The effects of combined treatment with ADP-P2Y1&12 pathway antagonists and UNC2025 were also evaluated. Results and Conclusions Treatment with UNC2025 inhibited MERTK phosphorylation and downstream activation of AKT and SRC, decreased platelet activation, and protected animals from pulmonary embolism and arterial thrombosis without increasing bleeding times. The antiplatelet effect of UNC2025 was enhanced in combination with ADP-P2Y1&12 pathway antagonists, and a greater than additive effect was observed when these two agents with different mechanisms of inhibition were coadministered. TAM kinase signaling represents a potential therapeutic target, as inhibition of this axis, especially in combination with ADP-P2Y pathway antagonism, mediates decreased platelet activation, aggregate stability, and thrombus formation, with less hemorrhagic potential than current treatment strategies. The data presented here also demonstrate antithrombotic activity mediated by UNC2025, a novel translational agent, and support the development of TAM kinase inhibitors for clinical applications.

Platelets at the Vascular Interface

In this brief review paper, we will summarize the State-of-the-Art on how platelet reactivity is regulated in circulation and at sites of vascular injury. Our review discusses recent and ongoing work, presented at this year's International Society on Thrombosis and Haemostasis (ISTH) meeting, on the role of platelets in (1) classical hemostasis at sites of mechanical injury, and (2) the maintenance of vascular integrity at sites of inflammation.

Platelet Integrins in Tumor Metastasis: Do They Represent a Therapeutic Target?

Platelets are small anucleated cell fragments that ensure the arrest of bleeding after a vessel wall injury. They are also involved in non-hemostatic function such as development, immunity, inflammation, and in the hematogeneous phase of metastasis. While the role of platelets in tumor metastasis has been recognized for 60 years, the molecular mechanism underlying this process remains largely unclear. Platelets physically and functionally interact with various tumor cells through surface receptors including integrins. Platelets express five integrins at their surface, namely α2β1, α5β1, α6β1, αvβ3, and αIIbβ3, which bind preferentially to collagen, fibronectin, laminin, vitronectin, and fibrinogen, respectively. The main role of platelet integrins is to ensure platelet adhesion and aggregation at sites of vascular injury. Two of these, α6β1 and αIIbβ3, were proposed to participate in platelet–tumor cell interaction and in tumor metastasis. It has also been reported that pharmacological agents targeting both integrins efficiently reduce experimental metastasis, suggesting that platelet integrins may represent new anti-metastatic targets. This review focuses on the role of platelet integrins in tumor metastasis and discusses whether these receptors may represent new potential targets for novel anti-metastatic approaches.

New Concepts and Mechanisms of Platelet Activation Signaling

Upon blood vessel injury, platelets are exposed to adhesive proteins in the vascular wall and soluble agonists, which initiate platelet activation, leading to formation of hemostatic thrombi. Pathological activation of platelets can induce occlusive thrombosis, resulting in ischemic events such as heart attack and stroke, which are leading causes of death globally. Platelet activation requires intracellular signal transduction initiated by platelet receptors for adhesion proteins and soluble agonists. Whereas many platelet activation signaling pathways have been established for many years, significant recent progress reveals much more complex and sophisticated signaling and amplification networks. With the discovery of new receptor signaling pathways and regulatory networks, some of the long-standing concepts of platelet signaling have been challenged. This review provides an overview of the new developments and concepts in platelet activation signaling.

The Phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P3) Binder Rasa3 Regulates Phosphoinositide 3-kinase (PI3K)-dependent Integrin αIIbβ3 Outside-in Signaling

The class I PI3K family of lipid kinases plays an important role in integrin α_IIbβ₃ function, thereby supporting thrombus growth and consolidation. Here, we identify Ras/Rap1GAP Rasa3 (GAP1^IP4BP) as a major phosphatidylinositol 3,4,5-trisphosphate-binding protein in human platelets and a key regulator of integrin α_IIbβ₃ outside-in signaling. We demonstrate that cytosolic Rasa3 translocates to the plasma membrane in a PI3K-dependent manner upon activation of human platelets. Expression of wild-type Rasa3 in integrin α_IIbβ₃-expressing CHO cells blocked Rap1 activity and integrin α_IIbβ₃-mediated spreading on fibrinogen. In contrast, Rap1GAP-deficient (P489V) and Ras/Rap1GAP-deficient (R371Q) Rasa3 had no effect. We furthermore show that two Rasa3 mutants (H794L and G125V), which are expressed in different mouse models of thrombocytopenia, lack both Ras and Rap1GAP activity and do not affect integrin α_IIbβ₃-mediated spreading of CHO cells on fibrinogen. Platelets from thrombocytopenic mice expressing GAP-deficient Rasa3 (H794L) show increased spreading on fibrinogen, which in contrast to wild-type platelets is insensitive to PI3K inhibitors. Together, these results support an important role for Rasa3 in PI3K-dependent integrin α_IIbβ₃-mediated outside-in signaling and cell spreading.

Vasodilator-Stimulated Phosphoprotein (VASP)-dependent and -independent pathways regulate thrombin-induced activation of Rap1b in platelets

Background

Vasodilator-Stimulated Phosphoprotein (VASP) is involved in the inhibition of agonist-induced platelet aggregation by cyclic nucleotides and the adhesion of platelets to the vascular wall. α_IIbβ₃ is the main integrin responsible for platelet activation and Rap1b plays a key role in integrin signalling. We investigated whether VASP is involved in the regulation of Rap1b in platelets since VASP-null platelets exhibit augmented adhesion to endothelial cells in vivo.

Methods

Washed platelets from wild type and VASP-deficient mice were stimulated with thrombin, the purinergic receptors agonist ADP, or the thromboxane A2 receptor agonist U46619 and Rap1b activation was measured using the GST-RalGDS-RBD binding assay. Interaction of VASP and Crkl was investigated by co-immunoprecipitation, confocal microscopy, and pull-down assays using Crkl domains expressed as GST-fusion proteins.

Results

Surprisingly, we found that activation of Rap1b in response to thrombin, ADP, or U46619 was significantly reduced in platelets from VASP-null mice compared to platelets from wild type mice. However, inhibition of thrombin-induced activation of Rap1b by nitric oxide (NO) was similar in platelets from wild type and VASP-null mice indicating that the NO/cGMP/PKG pathway controls inhibition of Rap1b independently from VASP. To understand how VASP regulated Rap1b, we investigated association between VASP and the Crk-like protein (Crkl), an adapter protein which activates the Rap1b guanine nucleotide exchange factor C3G. We demonstrated the formation of a Crkl/VASP complex by showing that: 1) Crkl co-immunoprecipitated VASP from platelet lysates; 2) Crkl and VASP dynamically co-localized at actin-rich protrusions reminiscent of focal adhesions, filopodia, and lamellipodia upon platelet spreading on fibronectin; 3) recombinant VASP bound directly to the N-terminal SH3 domain of Crkl; 4) Protein Kinase A (PKA) -mediated VASP phosphorylation on Ser157 abrogated the binding of Crkl.

Conclusions

We identified Crkl as a novel protein interacting with VASP in platelets. We propose that the C3G/Crkl/VASP complex plays a role in the regulation of Rap1b and this explains, at least in part, the reduced agonist-induced activation of Rap1b in VASP-null platelets. In addition, the fact that PKA-dependent VASP phosphorylation abrogated its interaction with Crkl may provide, at least in part, a rationale for the PKA-dependent inhibition of Rap1b and platelet aggregation.

Integrin-αIIbβ3-mediated outside-in signalling activates a negative feedback pathway to suppress platelet activation

Integrin-αIIbβ3-mediated outside-in signalling is widely accepted as an amplifier of platelet activation; accumulating evidence suggests that outside-in signalling can, under certain conditions, also function as an inhibitor of platelet activation. The role of integrin-αIIbβ3-mediated outside-in signalling in platelet activation is disputable. We employed flow cytometry, aggregometry, immunoprecipitation, and immunoblotting to investigate the role of integrin-αIIbβ3-mediated outside-in signalling in platelet activation. Integrin αIIbβ3 inhibition enhances agonist-induced platelet ATP secretion. Human platelets lacking expression of αIIbβ3 exhibited more platelet ATP secretion than their wild-type counterparts. Moreover, integrin-αIIbβ3-mediated outside-in signals activate SHIP-1, which in turn mediates p-Akt dephosphorylation, leading to inactivation of PI3K/Akt signalling. Furthermore, 3AC (SHIP-1 inhibitor) inhibits platelet disaggregation, and promotes platelet ATP secretion. Upon ADP stimulation, Talin is recruited to αIIbβ3, and it is dissociated from αIIbβ3 when platelets disaggregate. In addition, treatment with RUC2, an inhibitor of αIIbβ3, which blocks αIIbβ3-mediated outside-in signalling, can markedly prevent the dissociation of talin from integrin. SHIP1 Inhibitor 3AC inhibits the dissociation of talin from integrin-β3. These results suggest that integrin-αIIbβ3-mediated outside-in signalling can serve as a brake to restrict unnecessary platelet activation by activated SHIP-1, which mediated the disassociation of talin from β3, leading to integrin inactivation and blocking of PI3K/Akt signalling to restrict platelet ATP secretion.

Mice Expressing Low Levels of CalDAG-GEFI Exhibit Markedly Impaired Platelet Activation With Minor Impact on Hemostasis

OBJECTIVE

The tight regulation of platelet adhesiveness, mediated by the αIIbβ3 integrin, is critical for hemostasis and prevention of thrombosis. We recently demonstrated that integrin affinity in platelets is controlled by the guanine nucleotide exchange factor, CalDAG-GEFI (CD-GEFI), and its target, RAP1. In this study, we investigated whether low-level expression of CD-GEFI leads to protection from thrombosis without pathological bleeding in mice.

APPROACH AND RESULTS

Cdg1(low) mice were generated by knockin of human CD-GEFI cDNA into the mouse Cdg1 locus. CD-GEFI expression in platelets from Cdg1(low) mice was reduced by ≈90% when compared with controls. Activation of RAP1 and αIIbβ3 was abolished at low agonist concentrations and partially inhibited at high agonist concentrations in Cdg1(low) platelets. Consistently, the aggregation response of Cdg1(low) platelets was weaker than that of wild-type platelets, but more efficient than that observed in Cdg1(-/-) platelets. Importantly, Cdg1(low) mice were strongly protected from arterial and immune complex-mediated thrombosis, with only minimal impact on primary hemostasis.

CONCLUSIONS

Together, our studies suggest the partial inhibition of CD-GEFI function as a powerful new approach to safely prevent thrombotic complications.

RAP1-GTPase signaling and platelet function

Platelets are critical for hemostasis, i.e., the body's ability to prevent blood loss at sites of vascular injury. They patrol the vasculature in a quiescent, non-adhesive state for approximately 10 days, after which they are removed from circulation by phagocytic cells of the reticulo-endothelial system. At sites of vascular injury, they promptly shift to an activated, adhesive state required for the formation of a hemostatic plug. The small GTPase RAP1 is a critical regulator of platelet adhesiveness. Our recent studies demonstrate that the antagonistic balance between the RAP1 regulators, CalDAG-GEFI and RASA3, is critical for the modulation of platelet adhesiveness, both in circulation and at sites of vascular injury. The RAP1 activator CalDAG-GEFI responds to small changes in the cytoplasmic calcium concentration and thus provides sensitivity and speed to the activation response, essential for efficient platelet adhesion under conditions of hemodynamic shear stress. The RAP1 inhibitor RASA3 ensures that circulating platelets remain quiescent by restraining CalDAG-GEFI-dependent RAP1 activation. Upon cellular stimulation, it is turned off by P2Y12 signaling to enable sustained RAP1 activation, required for the formation of a stable hemostatic plug. This review will summarize important studies that elucidated the signaling pathways that control RAP1 activation in platelets.

Characterization of a Novel Integrin Binding Protein, VPS33B, Which Is Important for Platelet Activation and In Vivo Thrombosis and Hemostasis

BACKGROUND

Integrins are heterodimeric (α/β) membrane proteins that play fundamental roles in many biological processes, for example, cell adhesion and spreading, which are important for platelet function and hemostasis. The molecular mechanism that regulates integrin activation is not completely understood.

METHODS AND RESULTS

Here, we show that VPS33B, a member of the Sec1/Munc18 family, binds directly to the integrin β subunit. Overexpression of VPS33B in Chinese hamster ovary cells potentiated αIIbβ3 outside-in signaling but not inside-out signaling. Platelets, from megakaryocyte- and platelet-specific VPS33B conditional knockout mice, had normal morphology, yet their spreading on fibrinogen was impaired and they failed to support clot retraction. Platelet aggregation and ATP secretion in response to low-dose agonists were reduced in the VPS33B knockout mice. αIIbβ3-mediated endocytosis of fibrinogen was also defective. Tail bleeding times and times to occlusion in an FeCl3-induced thrombosis model were prolonged in the VPS33B knockout mice. Furthermore, VPS33B acted upstream of the RhoA-ROCK-MLC and Rac1-dependent pathways that lead to clot retraction and cell spreading, respectively.

CONCLUSIONS

Our work demonstrates that vesicular trafficking complexes, containing VPS33B, are a novel class of modifiers of integrin function. Our data also provide insights into the molecular mechanism and treatment of arthrogryposis, renal dysfunction, and cholestasis syndrome.

Platelet-neutrophil interactions under thromboinflammatory conditions

Platelets primarily mediate hemostasis and thrombosis, whereas leukocytes are responsible for immune responses. Since platelets interact with leukocytes at the site of vascular injury, thrombosis and vascular inflammation are closely intertwined and occur consecutively. Recent studies using real-time imaging technology demonstrated that platelet-neutrophil interactions on the activated endothelium are an important determinant of microvascular occlusion during thromboinflammatory disease in which inflammation is coupled to thrombosis. Although the major receptors and counter receptors have been identified, it remains poorly understood how heterotypic platelet-neutrophil interactions are regulated under disease conditions. This review discusses our current understanding of the regulatory mechanisms of platelet-neutrophil interactions in thromboinflammatory disease.

Pull-down assay for analysis of integrin-mediated activation of Rap proteins in adherent platelets

Rap1 GTPases operate as molecular switches by cycling between a GDP-bound inactive state and a GTP-bound active state and regulate several cellular pathways in response to different stimuli. Circulating blood platelets express high levels of Rap1 proteins, mainly Rap1b, which plays a critical role in platelet adhesion and activation. Rap1 is a key element in the inside-out signaling pathway leading to the conversion of integrins into the high-affinity state for their ligands. In platelets, Rap1b regulates inside-out activation of both integrin αIIbβ3 and α2β1. In addition, Rap1b is also involved in integrin outside-in signaling. Integrin-mediated platelet adhesion leads to accumulation of GTP-bound Rap1b, which promotes integrin-mediated processes such as spreading and clot retraction. Rap1b is thus a bidirectional regulator of platelet integrin function. Here we describe a method to analyze Rap1b activation induced by platelet adhesion via integrin α2β1.

Src family kinases: at the forefront of platelet activation

Src family kinases (SFKs) play a central role in mediating the rapid response of platelets to vascular injury. They transmit activation signals from a diverse repertoire of platelet surface receptors, including the integrin αIIbβ3, the immunoreceptor tyrosine-based activation motif-containing collagen receptor complex GPVI-FcR γ-chain, and the von Willebrand factor receptor complex GPIb-IX-V, which are essential for thrombus growth and stability. Ligand-mediated clustering of these receptors triggers an increase in SFK activity and downstream tyrosine phosphorylation of enzymes, adaptors, and cytoskeletal proteins that collectively propagate the signal and coordinate platelet activation. A growing body of evidence has established that SFKs also contribute to Gq- and Gi-coupled receptor signaling that synergizes with primary activation signals to maximally activate platelets and render them prothrombotic. Interestingly, SFKs concomitantly activate inhibitory pathways that limit platelet activation and thrombus size. In this review, we discuss past discoveries that laid the foundation for this fundamental area of platelet signal transduction, recent progress in our understanding of the distinct and overlapping functions of SFKs in platelets, and new avenues of research into mechanisms of SFK regulation. We also highlight the thrombotic and hemostatic consequences of targeting platelet SFKs.

Human CalDAG-GEFI gene (RASGRP2) mutation affects platelet function and causes severe bleeding

First case of a human RASGRP2 mutation affecting Rap1 activation in platelets and causing severe bleeding.

The nature of an inherited platelet disorder was investigated in three siblings affected by severe bleeding. Using whole-exome sequencing, we identified the culprit mutation (cG742T) in the RAS guanyl-releasing protein-2 (RASGRP2) gene coding for calcium- and DAG-regulated guanine exchange factor-1 (CalDAG-GEFI). Platelets from individuals carrying the mutation present a reduced ability to activate Rap1 and to perform proper αIIbβ3 integrin inside-out signaling. Expression of CalDAG-GEFI mutant in HEK293T cells abolished Rap1 activation upon stimulation. Nevertheless, the PKC- and ADP-dependent pathways allow residual platelet activation in the absence of functional CalDAG-GEFI. The mutation impairs the platelet’s ability to form thrombi under flow and spread normally as a consequence of reduced Rac1 GTP-binding. Functional deficiencies were confined to platelets and megakaryocytes with no leukocyte alteration. This contrasts with the phenotype seen in type III leukocyte adhesion deficiency caused by the absence of kindlin-3. Heterozygous did not suffer from bleeding and have normal platelet aggregation; however, their platelets mimicked homozygous ones by failing to undergo normal adhesion under flow and spreading. Rescue experiments on cultured patient megakaryocytes corrected the functional deficiency after transfection with wild-type RASGRP2. Remarkably, the presence of a single normal allele is sufficient to prevent bleeding, making CalDAG-GEFI a novel and potentially safe therapeutic target to prevent thrombosis.

Platelets protect from septic shock by inhibiting macrophage-dependent inflammation via the cyclooxygenase 1 signalling pathway

Although it has long been known that patients with sepsis often have thrombocytopenia and that septic patients with severe thrombocytopenia have a poor prognosis and higher mortality, the role of platelets in the pathogenesis of sepsis is poorly understood. Here we report a protective role of platelets in septic shock. We show that experimental thrombocytopenia induced by intraperitoneal injection of an anti-glycoprotein Ibα monoclonal antibody increases mortality and aggravates organ failure, whereas transfusion of platelets reduces mortality in lipopolysaccharide-induced endotoxemia and a bacterial infusion mouse sepsis model. Plasma concentrations of proinflammatory cytokines TNF-α and IL-6 are elevated by thrombocytopenia and decreased by platelet transfusion in septic mice. Furthermore, we identify that platelets protect from septic shock by inhibiting macrophage-dependent inflammation via the COX1/PGE₂/EP4-dependent pathway. Thus, these findings demonstrate a previously unappreciated role for platelets in septic shock and suggest that platelet transfusion may be effective in treating severely septic patients.

Secrets of platelet exocytosis - what do we really know about platelet secretion mechanisms?

Upon activation by extracellular matrix components or soluble agonists, platelets release in excess of 300 active molecules from intracellular granules. Those factors can both activate further platelets and mediate a range of responses in other cells. The complex microenvironment of a growing thrombus, as well as platelets' roles in both physiological and pathological processes, require platelet secretion to be highly spatially and temporally regulated to ensure appropriate responses to a range of stimuli. However, how this regulation is achieved remains incompletely understood. In this review we outline the importance of regulated secretion in thrombosis as well as in 'novel' scenarios beyond haemostasis and give a detailed summary of what is known about the molecular mechanisms of platelet exocytosis. We also discuss a number of theories of how different cargoes could be released in a tightly orchestrated manner, allowing complex interactions between platelets and their environment.

Platelet ITAM signaling

PURPOSE OF REVIEW

G protein-coupled receptors (GPCRs) like PAR1/4 and P2Y12 have long been known for their critical role in hemostasis. In contrast, deficiency in the immunoreceptor tyrosine-based activation motif (ITAM)-coupled receptors glycoprotein (GP)VI or C-type lectin-like receptor (CLEC)-2 is associated with only a mild bleeding diathesis in humans and mice. This review summarizes recent developments on the physiological importance of platelet ITAM signaling as well as the molecular mechanisms facilitating this signaling pathway.

RECENT FINDINGS

Genetic experiments identified a critical role for platelet CLEC-2 signaling in the formation of lymphatic vessels during development. Similarly, signaling by both GPVI and CLEC-2, but not GPCRs, is required for the maintenance of vascular integrity at sites of inflammation in the adult. The molecular mechanisms underlying ITAM signaling in platelets continue to be refined.

SUMMARY

Platelet ITAM signaling plays a key role for the maintenance of vascular integrity in development and the adult. This novel form of hemostasis differs from hemostasis at sites of vascular injury in that it does not depend on major platelet adhesion receptors or GPCR signaling.

Platelets as cellular effectors of inflammation in vascular diseases

Platelets are chief effector cells in hemostasis. In addition, they are multifaceted inflammatory cells with functions that span the continuum from innate immune responses to adaptive immunity. Activated platelets have key thromboinflammatory activities in a variety of vascular disorders and vasculopathies. Recently identified inflammatory and immune activities provide insights into the biology of these versatile blood cells that are directly relevant to human vascular diseases.

The proline-rich tyrosine kinase Pyk2 regulates platelet integrin αIIbβ3 outside-in signaling

BACKGROUND

The proline-rich tyrosine kinase Pyk2 is a focal adhesion kinase expressed in blood platelets, and is activated downstream of G-protein coupled receptors as well as integrin α2β1.

OBJECTIVE

In this study we have investigated the involvement of Pyk2 in integrin αIIbβ3 outside-in signaling in human and murine platelets.

METHODS

We analyzed the stimulation of intracellular signaling pathways in platelets from Pyk2 knockout mice adherent to immobilized fibrinogen.

RESULTS

Pyk2 was rapidly phosphorylated and activated in human and murine platelets adherent to fibrinogen through integrin αIIbβ3. Activation of Pyk2 was Src-dependent, but did not require phospholipase Cγ2 activity. Platelets from Pyk2 knockout mice showed a defective ability to adhere and spread on fibrinogen, in association with a dramatic reduction of phosphatidylinositol 3-kinase (PI3K) activation and Akt phosphorylation. Pharmacological and genetic analysis demonstrated that integrin αIIbβ3 engagement selectively stimulated the β-isoform of PI3K (PI3Kβ), and that, as for Pyk2, PI3Kβ activation required Src family kinases activity, but not phospholipase Cγ2. In fibrinogen-adherent platelets, both Pyk2 and PI3Kβ were necessary for stimulation of the small GTPase Rap1b, a regulator of cell adhesion and spreading. Integrin αIIbβ3 engagement triggered the association of the PI3Kβ regulatory subunit p85 with the adaptor protein c-Cbl, which was mediated by the p85 SH3 domain, and was independent of c-Cbl tyrosine phosphorylation. However, p85-associated c-Cbl was tyrosine phosphorylated by activated Pyk2 in fibrinogen adherent platelets.

CONCLUSIONS

These results identify a novel pathway of integrin αIIbβ3 outside-in signaling and recognize the tyrosine kinase Pyk2 as a major regulator of platelet adhesion and spreading on fibrinogen.

The P2Y(12) antagonists, 2MeSAMP and cangrelor, inhibit platelet activation through P2Y(12)/G(i)-dependent mechanism

Background

ADP is an important physiological agonist that induces integrin activation and platelet aggregation through its receptors P2Y₁ (Gα_q-coupled) and P2Y₁₂ (Gα_i-coupled). P2Y₁₂ plays a critical role in platelet activation and thrombosis. Adenosine-based P2Y₁₂ antagonists, 2-methylthioadenosine 5′-monophosphate triethylammonium salt hydrate (2MeSAMP) and Cangrelor (AR-C69931MX) have been widely used to demonstrate the role of P2Y₁₂ in platelet function. Cangrelor is being evaluated in clinical trials of thrombotic diseases. However, a recent study reported that both 2MeSAMP and Cangrelor raise intra-platelet cAMP levels and inhibit platelet aggregation through a P2Y₁₂-independent mechanism.

Methodology/Principal Findings

The present work, using P2Y₁₂ deficient mice, sought to clarify previous conflicting reports and to elucidate the mechanisms by which 2MeSAMP and Cangrelor inhibit platelet activation and thrombosis. 2MeSAMP and Cangrelor inhibited aggregation and ATP release of wild-type but not P2Y₁₂ deficient platelets. 2MeSAMP and Cangrelor neither raised intracellular cAMP concentrations nor induced phosphorylation of vasodilator-stimulated phosphoprotein (VASP) in washed human or mouse platelets. Furthermore, unlike the activators (PGI₂ and forskolin) of the cAMP pathway, 2MeSAMP and Cangrelor failed to inhibit Ca²⁺ mobilization, Akt phosphorylation, and Rap1b activation in P2Y₁₂ deficient platelets. Importantly, while injection of Cangrelor inhibited thrombus formation in a FeCl₃-induced thrombosis model in wild-type mice, it failed to affect thrombus formation in P2Y₁₂ deficient mice.

Conclusions

These data together demonstrate that 2MeSAMP and Cangrelor inhibit platelet function through the P2Y₁₂-dependent mechanism both in vitro and in vivo.

Headpiece domain of dematin regulates calcium mobilization and signaling in platelets

Dematin is a broadly expressed membrane cytoskeletal protein that has been well characterized in erythrocytes and to a lesser extent in non-erythroid cells. However, dematin's function in platelets is not known. Here, we show that dematin is abundantly expressed in both human and mouse platelets. Platelets harvested from the dematin headpiece knock-out (HPKO) mouse model exhibit a striking defect in the mobilization of calcium in response to multiple agonists of platelet activation. The reduced calcium mobilization in HPKO platelets is associated with concomitant inhibition of platelet aggregation and granule secretion. Integrin α(IIb)β(3) activation in response to agonists is attenuated in the HPKO platelets. The mutant platelets show nearly normal spreading on fibrinogen and an unaltered basal cAMP level; however, the clot retraction was compromised in the mutant mice. Immunofluorescence analysis indicated that dematin is present both at the dense tubular system and plasma membrane fractions of platelets. Proteomic analysis of dematin-associated proteins in human platelets identified inositol 1,4,5-trisphosphate 3-kinase isoform B (IP3KB) as a binding partner, which was confirmed by immunoprecipitation analysis. IP3KB, a dense tubular system protein, is a major regulator of calcium homeostasis. Loss of the dematin headpiece resulted in a decrease of IP3KB at the membrane and increased levels of IP3KB in the cytosol. Collectively, these findings unveil dematin as a novel regulator of internal calcium mobilization in platelets affecting multiple signaling and cytoskeletal functions. Implications of a conserved role of dematin in the regulation of calcium homeostasis in other cell types will be discussed.

A novel interplay between Rap1 and PKA regulates induction of angiogenesis in prostate cancer

Angiogenesis inhibition is an important therapeutic strategy for advanced stage prostate cancer. Previous work from our laboratory showed that sustained stimulation of Rap1 by 8-pCPT-2'-O-Me-cAMP (8CPT) via activation of Epac, a Rap1 GEF, or by expression of a constitutively active Rap1 mutant (cRap1) suppresses endothelial cell chemotaxis and subsequent angiogenesis. When we tested this model in the context of a prostate tumor xenograft, we found that 8CPT had no significant effect on prostate tumor growth alone. However, in cells harboring cRap1, 8CPT dramatically inhibited not only prostate tumor growth but also VEGF expression and angiogenesis within the tumor microenvironment. Subsequent analysis of the mechanism revealed that, in prostate tumor epithelial cells, 8CPT acted via stimulation of PKA rather than Epac/Rap1. PKA antagonizes Rap1 and hypoxic induction of 1α protein expression, VEGF production and, ultimately, angiogenesis. Together these findings provide evidence for a novel interplay between Rap1, Epac, and PKA that regulates tumor-stromal induction of angiogenesis.