Platelet Integrin Research Articles

The relative importance of platelet integrins in hemostasis, thrombosis and beyond.

Integrins are heterodimeric transmembrane receptors composed of α and β chains, with an N-terminal extracellular domain forming a globular head corresponding to the ligand binding site. Integrins regulate various cellular functions including adhesion, migration, proliferation, spreading and apoptosis. On platelets, integrins play a central role in adhesion and aggregation on subendothelial matrix proteins of the vascular wall, thereby ensuring hemostasis. Platelet integrins belong either to the β1 family (α2β1, α5β1 and α6β1) or to the β3 family (αIIbβ3 and αvβ3). On resting platelets, integrins can engage their ligands when the latter are immobilized but not in their soluble form. The effects of various agonists promote an inside-out signal in platelets, increasing the affinity of integrins for their ligands and conveying a modest signal reinforcing platelet activation, called outside-in signaling. This outside-in signal ensures platelet adhesion, shape change, granule secretion and aggregation. In this review, we examine the role of each platelet integrin in hemostatic plug formation, hemostasis and arterial thrombosis and also beyond these classical functions, notably in tumor metastasis and sepsis.

Abstract A004: The role of calcium in metastatic progression

Abstract Calcium is essential to the structural stability of integrin proteins implicated in platelet–cancer cell interactions, namely integrins GPIIb/IIIa and avb3, which share a common beta subunit (Pelletier et al., J Biol Chem, 1996), (Zhang and Chen, Cell Adh Migr, 2012). Additionally, calcium is essential for ligand binding to these integrins (e.g., fibrinogen) that bridge platelets and cancer cells (Raborn et al., Biochemistry, 2011) and for the downstream signaling following mechanical and chemical stimulation in the integrin microenvironment (Michelson, Platelets, 2013). Platelet-cancer cell interactions are key players in cancer progression as platelets support metastatic dissemination (Morris et al., Biochim. Biophys. Acta BBA - Rev. Cancer, 2022). Despite previous studies recognizing calcium’s role in integrin stability and function, calcium’s importance for platelet support in cancer progression is not well understood. Therefore, we aim to (1) characterize the receptor expression and ligand binding ability of GPIIb/IIIa and avb3 on respective cells via anti-CD41a (for platelets and breast cancer MDA-MB-231 cells) and anti-avb3 antibody (for lung cancer A549 cells), (2) characterize platelet function by assessing platelet aggregation using light transmission aggregometry and platelet adhesion on a thrombogenic surface, (3) characterize platelet-cancer cell interaction via flow cytometry, and (4) assess cancer cell invasion in the presence of platelets via transwell assay; all in the presence of an environment with calcium chelation and hypercalcemic conditions. Our results demonstrate that platelet GPIIb/IIIa, cancer cell GPIIb/IIIa (MDA-MB-231) and cancer cell integrin avb3 (A549) are modulated with calcium chelation. Additionally, platelet function, specifically aggregation is significantly decreased with calcium chelation and significantly increased with hypercalcemia. We also observed that the interaction between platelets and cancer cells is reduced with calcium chelation, platelet-MDA-MB-231 interaction specifically being significantly reduced in the presence of sodium citrate. Our overall hypothesis is that the observed effect of calcium levels of platelet and cancer cell integrins could have a significant influence on platelet-cancer cell interactions and cancer cell migration in hypercalcemia, often seen in patients with malignancy. A greater understanding of these interactions could have impact for the design of specific inhibitors with anti-metastatic effects and potentially lead to novel targeted therapies to combat metastatic dissemination. Citation Format: Kenise Morris, Anne-Laure Papa. The role of calcium in metastatic progression [abstract]. In: Proceedings of the AACR Special Conference: Cancer Metastasis; 2022 Nov 14-17; Portland, OR. Philadelphia (PA): AACR; Cancer Res 2022;83(2 Suppl_2):Abstract nr A004.

Fatty acid oxidation fuels agonist-induced platelet activation and thrombus formation: Targeting β-oxidation of fatty acids as an effective anti-platelet strategy.

Platelet mitochondria possess remarkable plasticity for oxidation of energy substrates, where metabolic dependency on glucose or fatty acids is higher than glutamine. Since platelets metabolize nearly the entire pool of glucose to lactate rather than fluxing through mitochondrial tricarboxylic acid cycle, we posit that majority of mitochondrial ATP, which is essential for platelet granule secretion and thrombus formation, is sourced from oxidation of fatty acids. We performed a comprehensive analysis of bioenergetics and function of stimulated platelets in the presence of etomoxir, trimetazidine and oxfenicine, three pharmacologically distinct inhibitors of β-oxidation. Each of them significantly impaired oxidative phosphorylation in unstimulated as well as thrombin-stimulated platelets leading to a small but consistent drop in ATP level in activated cells due to a lack of compensation from glycolytic ATP. Trimetazidine and oxfenicine attenuated platelet aggregation, P-selectin externalization and integrin αIIb β3 activation. Both etomoxir and trimetazidine impeded agonist-induced dense granule release and platelet thrombus formation on collagen under arterial shear. The effect of inhibitors on platelet aggregation and dense granule release was dose- and incubation time- dependent with significant inhibition at higher doses and prolonged incubation times. Neither of the inhibitors could protect mice from collagen-epinephrine-induced pulmonary embolism or prolong mouse tail bleeding times. However, mice pre-administered with etomoxir, trimetazidine and oxfenicine were protected from ferric chloride-induced mesenteric thrombosis. In conclusion, β-oxidation of fatty acids sustains ATP level in stimulated platelets and is therefore essential for energy-intensive agonist-induced platelet responses. Thus, fatty acid oxidation may constitute an attractive therapeutic target for novel antiplatelet agents.

Impaired Platelet Function and Thrombus Formation in PDE5A-Deficient Mice.

Intracellular cyclic GMP (cGMP) inhibits platelet function. Platelet cGMP levels are controlled by phosphodiesterase 5A (PDE5A)-mediated degradation. However, the exact role of PDE5A in platelet function and thrombus formation remains poorly understood. In this study, we characterized the role of PDE5A in platelet activation and function. Platelets were isolated from wild type or PDE5A-/- mice to measure platelet aggregation, activation, phosphatidylserine exposure (annexin-V binding), reactive oxygen species (ROS) generation, platelet spreading as well as clot retraction. Cytosolic calcium mobilization was measured using Fluo-4 AM by a microplate reader. Western blot was used to measure the phosphorylation of VASP, ERK1/2, p38, JNK, and AKT. FeCl3-induced arterial thrombosis and venous thrombosis were assessed to evaluate the in vivo hemostatic function and thrombus formation. Additionally, in vitro thrombus formation was assessed in a microfluidic whole-blood perfusion assay. PDE5A-deficient mice presented significantly prolonged tail bleeding time and delayed arterial and venous thrombus formation. PDE5A deficiency significantly inhibited platelet aggregation, ATP release, P-selectin expression, and integrin aIIbb3 activation. In addition, an impaired spreading on collagen or fibrinogen and clot retraction was observed in PDE5A-deficient platelets. Moreover, PDE5A deficiency reduced phosphatidylserine exposure, calcium mobilization, ROS production, and increased intracellular cGMP level along with elevated VASP phosphorylation and reduced phosphorylation of ERK1/2, p38, JNK, and AKT. In conclusion, PDE5A modulates platelet activation and function and thrombus formation, indicating that therapeutically targeting it might be beneficial for the treatment of thrombotic diseases.

Integrin αIIbβ3 intermediates: From molecular dynamics to adhesion assembly

The platelet integrin αIIbβ3 undergoes long-range conformational transitions associated with its functional conversion from inactive (low-affinity) to active (high-affinity) during hemostasis. Although new conformations that are intermediate between the well-characterized bent and extended states have been identified, their molecular dynamic properties and functions in the assembly of adhesions remain largely unexplored. In this study, we evaluated the properties of intermediate conformations of integrin αIIbβ3 and characterized their effects on the assembly of adhesions by combining all-atom simulations, principal component analysis, and mesoscale modeling. Our results show that in the low-affinity, bent conformation, the integrin ectodomain tends to pivot around the legs; in intermediate conformations, the headpiece becomes partially extended, away from the lower legs. In the fully open, active state, αIIbβ3 is flexible, and the motions between headpiece and lower legs are accompanied by fluctuations of the transmembrane helices. At the mesoscale, bent integrins form only unstable adhesions, but intermediate or open conformations stabilize the adhesions. These studies reveal a mechanism by which small variations in ligand binding affinity and enhancement of the ligand-bound lifetime in the presence of actin retrograde flow stabilize αIIbβ3 integrin adhesions.

Platelet CD36 links overweight and a prothrombotic phenotype in patients with non-valvular atrial fibrillation.

The pathophysiological mechanisms linking the overweight and prothrombotic state of non-valvular atrial fibrillation (NVAF) are incompletely understood. Our objective was to evaluate the effect of platelet CD36 on the risk of stroke associated with overweight in NVAF patients. A cross-sectional study enrolled 182 subjects with NVAF in two groups: normal weight (18.5 < body mass index(BMI) < 25.0 kg/m2) and overweight (BMI ≥ 25.0 kg/m2). Clinical data, medical history, vital signs, transthoracic echocardiography parameters, and medication were recorded. Biochemical characteristics including blood glucose and serum lipid were analyzed in the Laboratory. The expression of platelet CD36 and integrin αIIbβ3 was detected by flow cytometry. Among the 182 patients with NVAF, 68 (37.36%) were classified as normal weight, 114 (62.64%) as overweight. With an increase in BMI, waist-hip ratio, cholesterol, triglycerides, left atrium diameters, and the ratio of mitral inflow E velocity to myocardial e' velocity in the mitral annulus (E/e') increased significantly (P < 0.05). The mean fluorescent intensity of platelet CD36 increased significantly in overweight patients (P < 0.01), in line with platelet activation biomarkers (platelet integrin αIIbβ3). Platelet CD36 was positively correlated with BMI and platelet integrin αIIbβ3, respectively (P < 0.05). Additionally, platelet CD36 and BMI were independent risk factors for platelet activation in patients with NVAF. Platelet CD36 is speculated to mediate the complex crosstalk between overweight and platelet hyperactivity, leading to the prothrombotic state in overweight patients with NVAF. Platelet CD36 could be a potential target for preventing the prothrombotic state in overweight patients with NVAF.

Transcriptional and Metabolic Profiling Reveals Alpha-Ketoglutarate As a Novel Mediator of Aberrant Platelet Activity and Inflammation in Myeloproliferative Neoplasms

Prior studies by our group and others have demonstrated that platelets from patients with myeloproliferative neoplasms (MPNs) exhibit a hyperreactive phenotype. However, a complete understanding of platelet alterations in MPNs remains lacking, and the mechanisms by which platelets contribute to MPN-related thrombosis, as well as other MPN disease features, are incompletely understood. In this study, utilizing multiomic approaches to interrogate platelet phenotypes in patient samples, in conjunction with relevant MPN animal models (Figure 1A), we aim to investigate mechanisms of dysregulated platelet activity in MPNs, and explore how these findings may be leveraged to uncover novel therapeutic strategies. We initially studied platelet activation in peripheral blood from patients with essential thromobocythemia (ET) and compared to age-, sex-matched healthy controls. We found significantly increased P-selectin exposure in conjunction with increased platelet-leukocyte aggregates indicating activation of platelets from ET patients. To investigate the transcriptional signature of PBMCs and platelets at the single cell level, we performed single cell RNA-seq (scRNA-seq) in PBMCs from ET patients and healthy controls. Monocytes were increased in ET patients and displayed the highest inflammation index, implicating monocytes as the primary source of inflammation in ET. GSEA analysis revealed enrichment of platelet activation and oxidative phosphorylation (OXPHOS) genes in platelets from ET patients. Liquid chromatography-mass spectrometry (LC-MS) metabolomics analysis showed distinct metabolic phenotypes consisting of elevated tricarboxylic acid (TCA) cycle components, ATP generation and lower alpha-ketoglutarate (α-KG), in platelets from ET patients, all consistent with increased OXPHOS. α-KG is a key TCA cycle intermediate, which inhibits ATP generation via the suppression of ATP synthase. Extracellular flux analysis by Seahorse analysis confirmed bioenergetic alterations in MPN patient platelets. Enhanced mitochondrial respiration at both baseline and after ex vivo stimulation with the platelet agonist thrombin receptor activator peptide (TRAP6) was also observed from MPN patient platelets. Notably, α-KG supplementation drastically reduced oxygen consumption and ATP generation in platelets from MPN patients. We further investigated the effects of α-KG on MPN platelet activation. Ex vivo incubation of platelets from both MPN patients and Jak2 V617F knock-in mice with α-KG significantly reduced platelet surface P-selectin and integrin a2bb3 activation. Additionally, α-KG inhibited the spreading and adhesion of platelets from Jak2 V617F knock-in mice to fibrinogen-coated surfaces. Platelet phosphoblots demonstrated significant downregulation of p-STAT3, p-AKT and p-ERK after treatment with α-KG, suggesting these signaling pathways may be responsible for the inhibitory effects of α-KG on platelet activation. Thus, α-KG inhibited platelet activities in both human and mouse MPN samples. To test the therapeutic impact of α-KG on MPN disease features, we treated Jak2 V617F knock-in mice with α-KG for 6 weeks. Oral α-KG supplementation decreased splenomegaly and reduced elevated platelets and hematocrit. Additionally, monocytes were significantly decreased as early as 2 weeks after α-KG treatment in Jak2 V617F knock-in mice. In ex vivo studies with MPN patient CD34+ cells, α-KG treatment for 10 days led to a decrease in CD41+ CD61+ cells, suggesting decreased megakaryocyte commitment. We further observed that α-KG incubation significantly decreased the secretion of proinflammatory cytokines from sorted CD14+ human monocytes. Mass cytometry analysis of whole blood from MPN patients demonstrated inhibition of MAPK pathway signaling after α-KG treatment. Taken together, these results suggest that α-KG supplementation may exert therapeutic effects through both direct inhibition of MPN platelet activity and via quenching of monocyte hyper-inflammation (Figure 1B). In summary, these studies reveal a previously unrecognized metabolic disorder in platelets from MPN patients and highlight a prominent role for α-KG in aberrant MPN platelet activity and monocyte-driven inflammation. These findings have potential relevance for novel therapeutic approaches for MPN patients. Figure 1View largeDownload PPTFigure 1View largeDownload PPT Close modal

Actin Bundling Protein L-Plastin Regulates Megakaryocyte Membrane Rigidity and Platelet Spreading

An adequate number of normally functioning platelets is required for normal hemostasis, while thrombocytopenia and platelet dysfunction are common causes of pathologic bleeding. Platelets are released into the blood from proplatelets (PPs) that extend from megakaryocyte (MK) intracellular invaginated membrane system (IMS) that is continuous with the plasma membrane. Tubulin provides the motor for PP extension, and actin reorganization is believed to be critical for MK migration and PP branching. However, the role for actin in PP formation (PPF) is less well understood. The initiation of PPF requires podosomes - actin-based structures with an F-actin core surrounded by a ring of adhesion molecules (e.g., integrins) and many actin-binding proteins. We have been intrigued by how podosomes form and how the IMS is able to penetrate the dense cortical actin cytoskeleton to form PPs. We recently showed that human MKs and platelets express a novel actin bundling protein, L-plastin. L-plastin levels were inversely correlated with platelet number, and its expression was down-regulated during MK maturation. L-plastin knockdown in cultured human CD34+ derived MKs caused increased PP, IMS, and podosome formation. These data indicate L-plastin is a negative regulator of MK platelet production. To assess the in vivo effects of L-plastin, we have investigated platelet production and function in vivo using L-plastin knockout (KO) mice (Lcp1-/-). Compared to wildtype (WT) littermate controls, Lcp1 KO mice showed both significantly increased platelet counts and increased PPF from cultured murine bone marrow MKs. Elegant work in both HUVECs and epithelial cells has shown high membrane proximal F-actin (MPA) prohibits membrane protrusions, and lowering MPA density is required to sensitize areas for membrane protrusion (Bisaria, Science 2020). We hypothesize that normal MK levels of L-plastin inhibits PPF in less mature MKs by F-actin bundling, which raises the MPA. Conversely, lower L-plastin levels result in lower MPA and enabling PP protrusion. Enhanced L-plastin-mediated F-actin bundling is expected to increase membrane stiffness, so we measured MK membrane stiffness at single cell level using a state-of-the-art Biomembrane Force Probe (BFP). MKs from KO mice showed 68% reduction in membrane stiffness as compared to WT controls (p<0.0001) (Fig 1). Ultrastructure of MKs also revealed increased IMS in Lcp1-/- MKs. Preliminary data suggests that L-plastin deficiency also associated with increased MK podosome formation in vitro, which is consistent with our findings in human CD34+ derived MKs. Although L-plastin levels decrease during terminal MK differentiation, they persist at moderate levels in platelets. Platelet integrin activation, granule release and clot retraction also depend on actin reorganization, but L-plastin function in platelets has not been studied. We observed no difference between WT and Lcp1 KO platelets for measures of inside-out integrin signaling (activation-induced JON/A binding or P-selectin expression). However, when plated on fibrinogen and stimulated with thrombin, platelets from KO mice showed significantly increased spreading compared to WT mice (Fig 2). A similar trend was observed for platelet clot retraction. Taken together these results show that L-plastin regulates MK membrane stiffness, which likely inhibits PPF. L-plastin also appears to participate in platelet outside-in, but not inside-out, integrin signaling. On-going studies are determining whether integrin beta3 is the molecular link between these two findings. Figure 1View largeDownload PPTFigure 1View largeDownload PPT Close modal

Asebogenin suppresses thrombus formation via inhibition of Syk phosphorylation.

Thrombosis is a major cause of morbidity and mortality worldwide. Platelet activation by exposed collagen through glycoprotein VI (GPVI) and formation of neutrophil extracellular traps (NETs) are critical pathogenic factors for arterial and venous thrombosis. Both events are regulated by spleen tyrosine kinase (Syk)-mediated signalling events. Asebogenin is a dihydrochalcone whose pharmacological effects remain largely unknown. This study aims to investigate the antithrombotic effects of asebogenin and the underlying molecular mechanisms. Platelet aggregation was assessed using an aggregometer. Platelet P-selectin exposure, integrin activation and calcium mobilization were determined by flow cytometry. NETs formation was assessed by SYTOX Green staining and immunohistochemistry. Quantitative phosphoproteomics, microscale thermophoresis, in vitro kinase assay and molecular docking combined with dynamics simulation were performed to characterize the targets of asebogenin. The in vivo effects of asebogenin on arterial thrombosis were investigated using FeCl3 -induced and laser-induced injury models, whereas those of venous thrombosis were induced by stenosis of the inferior vena cava. Asebogenin inhibited a series of GPVI-induced platelet responses and suppressed NETs formation induced by proinflammatory stimuli. Mechanistically, asebogenin directly interfered with the phosphorylation of Syk at Tyr525/526, which is important for its activation. Further, asebogenin suppressed arterial thrombosis demonstrated by decreased platelet accumulation and fibrin generation and attenuated venous thrombosis determined by reduced neutrophil accumulation and NETs formation, without increasing bleeding risk. Asebogenin exhibits potent antithrombotic effects by targeting Syk and is a potential lead compound for the development of efficient and safe antithrombotic agents.

Pro-Inflammatory Chemokines CCL5, CXCL12, and CX3CL1 Bind to and Activate Platelet Integrin αIIbβ3 in an Allosteric Manner.

Activation of platelet integrin αIIbβ3, a key event for hemostasis and thrombus formation, is known to be mediated exclusively by inside-out signaling. We showed that inflammatory chemokines CX3CL1 and CXCL12 in previous studies, and CCL5 in this study, bound to the allosteric binding site (site 2) of vascular integrin αvβ3, in addition to the classical ligand binding site (site 1), and allosterically activated integrins independent of inside-out signaling. Since αIIbβ3 is exposed to inflammatory chemokines at increased concentrations during inflammation (e.g., cytokine/chemokine storm) and platelet activation, we hypothesized that these chemokines bind to and activate αIIbβ3 in an allosteric activation mechanism. We found that these chemokines bound to αIIbβ3. Notably, they activated soluble αIIbβ3 in 1 mM Ca2+ by binding to site 2. They activated cell-surface αIIbβ3 on CHO cells, which lack machinery for inside-out signaling or chemokine receptors, quickly (<1 min) and at low concentrations (1–10 ng/mL) compared to activation of soluble αIIbβ3, probably because chemokines bind to cell surface proteoglycans. Furthermore, activation of αIIbβ3 by the chemokines was several times more potent than 1 mM Mn2+. We propose that CCL5 and CXCL12 (stored in platelet granules) may allosterically activate αIIbβ3 upon platelet activation and trigger platelet aggregation. Transmembrane CX3CL1 on activated endothelial cells may mediate platelet–endothelial interaction by binding to and activating αIIbβ3. Additionally, these chemokines in circulation over-produced during inflammation may trigger αIIbβ3 activation, which is a possible missing link between inflammation and thrombosis.

Physiologically-based pharmacokinetic modeling of immunoglobulin and antibody coadministration in patients with primary human immunodeficiency.

Intravenous immunoglobulin (IVIG) (2000 mg/kg) increased the clearance of the mouse monoclonal antibody 7E3, directed against platelet integrin IIb/IIIa (alpha IIb beta 3, CD41/CD61) in rodents. We wanted to investigate the effect of IVIG on clearance of monoclonal antibodies in humans as there is extremely limited data regarding this interaction in the literature. Using the tyrosine protein kinase KIT anti‐cluster of differentiation 117 (c‐Kit) humanized monoclonal antibody (JSP191) as a case study, we used physiologically‐based pharmacokinetic (PBPK) modeling to evaluate the pharmacokinetic interaction between monoclonal antibodies and IVIG at doses (300–600 mg/kg) administered to patients with primary human immunodeficiency (PI). We first characterized the interaction between monoclonal antibodies and IVIG in PK‐Sim®/MoBi® using published literature data, including the following: IVIG plus 7E3 in mice and rats and IVIG plus the human anti‐C5 monoclonal antibody tesidolumab in adults with end‐stage renal disease. We next developed a PBPK model using digitized data for JSPI91 alone in older adults with myelodysplastic syndrome and acute myeloid leukemia and in pediatric patients with severe combined immunodeficiency (SCID). Finally, we simulated the impact of IVIG (300–2000 mg/kg) coadministration with JSP191 on the area under the curve of JSP191 in patients with SCID. Model predictions were within 1.5‐fold of observed values for 7E3 plus IVIG and tesidolumab plus IVIG as well as for JSP191 administered alone. Based on our simulations, IVIG doses ≥500 mg exceeded the 80%–125% no‐effect boundaries. IVIG treatment with monoclonal antibodies in patients with PI may result in a clinically significant interaction depending on the IVIG dose administered and the exposure–response relationship for the specific monoclonal antibody.

Contribution of septins to human platelet structure and function

SummaryAlthough septins have been well-studied in nucleated cells, their role in anucleate blood platelets remains obscure. Here, we elucidate the contribution of septins to human platelet structure and functionality. We show that Septin-2 and Septin-9 are predominantly distributed at the periphery of resting platelets and co-localize strongly with microtubules. Activation of platelets by thrombin causes clustering of septins and impairs their association with microtubules. Inhibition of septin dynamics with forchlorfenuron (FCF) reduces thrombin-induced densification of septins and lessens their colocalization with microtubules in resting and activated platelets. Exposure to FCF alters platelet shape, suggesting that septins stabilize platelet cytoskeleton. FCF suppresses platelet integrin αIIbβ3 activation, promotes phosphatidylserine exposure on activated platelets, and induces P-selectin expression on resting platelets, suggesting septin involvement in these processes. Inhibition of septin dynamics substantially reduces platelet contractility and abrogates their spreading on fibrinogen-coated surfaces. Overall, septins strongly contribute to platelet structure, activation and biomechanics.

Hemodynamic Mimic Shear Stress for Platelet Membrane Nanobubbles Preparation and Integrin αIIbβ3 Conformation Regulation.

Platelet (PLT) membrane biomimetic nanomaterials have become promising theranostic platforms due to their good biocompatibility and effectiveness. However, in order to achieve precise regulation of cell membrane components, novel controllable construction approaches need to be developed. Inspired by the interaction mechanism among platelet production, activation, and dynamic biomechanical signals in blood circulation, here a platelet nanobubbles (PNBs) with reassembled platelet membrane with ideal echogenicity was fabricated using an adjustable pressure-induced shear stress method. The results demonstrate that the high shear stress during PNBs fabrication led to the enrichment of platelet membrane lipid rafts and proteins, as well as their reassembly on the gas-liquid interface. More importantly, the conformation of platelet integrin αIIbβ3 was transformed into a shear stress-induced intermediate affinity state, which gives PNBs enhanced adhesion ability to the vascular endothelial injury. Taken together, these PNBs have great application potential in the specifically targeted ultrasound diagnosis of vascular endothelial injury.

Bruton's Tyrosine Kinase Inhibitors Impair FcγRIIA-Driven Platelet Responses to Bacteria in Chronic Lymphocytic Leukemia.

Bacterial infections are a major cause of morbidity and mortality in chronic lymphocytic leukemia (CLL), and infection risk increases in patients treated with the Bruton’s tyrosine kinase (Btk) inhibitor, ibrutinib. Btk and related kinases (like Tec) are expressed in non-leukemic hematopoietic cells and can be targeted by ibrutinib. In platelets, ibrutinib therapy is associated with bleeding complications mostly due to off-target effects. But the ability of platelets to respond to bacteria in CLL, and the potential impact of ibrutinib on platelet innate immune functions remain unknown. FcγRIIA is a tyrosine kinase-dependent receptor critical for platelet activation in response to IgG-coated pathogens. Crosslinking of this receptor with monoclonal antibodies causes downstream activation of Btk and Tec in platelets, however, this has not been investigated in response to bacteria. We asked whether ibrutinib impacts on FcγRIIA-mediated activation of platelets derived from CLL patients and healthy donors after exposure to Staphylococcus aureus Newman and Escherichia coli RS218. Platelet aggregation, α-granule secretion and integrin αIIbβ3-dependent scavenging of bacteria were detected in CLL platelets but impaired in platelets from ibrutinib-treated patients and in healthy donor-derived platelets exposed to ibrutinib in vitro. While levels of surface FcγRIIA remained unaffected, CLL platelets had reduced expression of integrin αIIbβ3 and GPVI compared to controls regardless of therapy. In respect of intracellular signaling, bacteria induced Btk and Tec phosphorylation in both CLL and control platelets that was inhibited by ibrutinib. To address if Btk is essential for platelet activation in response to bacteria, platelets derived from X-linked agammaglobulinemia patients (lacking functional Btk) were exposed to S. aureus Newman and E. coli RS218, and FcγRIIA-dependent aggregation was observed. Our data suggest that ibrutinib impairment of FcγRIIA-mediated platelet activation by bacteria results from a combination of Btk and Tec inhibition, although off-target effects on additional kinases cannot be discarded. This is potentially relevant to control infection-risk in CLL patients and, thus, future studies should carefully evaluate the effects of CLL therapies, including Btk inhibitors with higher specificity for Btk, on platelet-mediated immune functions.

Characterization of the Role of Integrin α5β1 in Platelet Function, Hemostasis, and Experimental Thrombosis.

Objective Integrins are key regulators of various platelet functions. The pathophysiological importance of most platelet integrins has been investigated, with the exception of α5β1, a receptor for fibronectin. The aim of this study was to characterize the role of α5β1 in megakaryopoiesis, platelet function, and to determine its importance in hemostasis and arterial thrombosis. Approach and Results We generated a mouse strain deficient for integrin α5β1 on megakaryocytes and platelets (PF4Cre-α5 −/− ). PF4Cre-α5 −/− mice were viable, fertile, and presented no apparent signs of abnormality. Megakaryopoiesis appears unaltered as evidence by a normal megakaryocyte morphology and development, which is in agreement with a normal platelet count. Expression of the main platelet receptors and the response of PF4Cre-α5 −/− platelets to a series of agonists were all completely normal. Adhesion and aggregation of PF4Cre-α5 −/− platelets under shear flow on fibrinogen, laminin, or von Willebrand factor were unimpaired. In contrast, PF4Cre-α5 −/− platelets displayed a marked decrease in adhesion, activation, and aggregation on fibrillar cellular fibronectin and collagen. PF4Cre-α5 −/− mice presented no defect in a tail-bleeding time assay and no increase in inflammatory bleeding in a reverse passive Arthus model and a lipopolysaccharide pulmonary inflammation model. Finally, no defects were observed in three distinct experimental models of arterial thrombosis based on ferric chloride-induced injury of the carotid artery, mechanical injury of the abdominal aorta, or laser-induced injury of mesenteric vessels. Conclusion In summary, this study shows that platelet integrin α5β1 is a key receptor for fibrillar cellular fibronectin but is dispensable in hemostasis and arterial thrombosis.

Intravenous Nanomedicine for Targeted Delivery of Thrombin to Augment Hemostasis

Non-compressible uncontrolled hemorrhage remains a major cause of mortality from traumatic injuries. Additionally, patients with congenital, disease-associated or drug-induced hemostatic dysfunctions, may often be at risk of excessive bleeding. Therefore, treatments that render rapid hemostasis are clinically significant in potentially saving lives. The clinical gold standard for this is the transfusion of whole blood (WB) or blood components (e.g. controlled ratios of platelets, RBCs, and plasma), as evidenced by several clinical studies (e.g. PROPPR, PROMMTT and PAMPer). However, the availability of such blood products is donor-dependent, their shelf-life is limited due to contamination risks, and, their portability and storage is often challenging. While extensive research efforts are currently being focused on addressing these challenges, e.g. using low titer Group O whole blood, cold-storage and freeze-drying of platelets and plasma, in vitro generation of platelets from iPSCs etc., a parallel research focus has emerged in designing biomaterials-based I.V.-administrable technologies (nanoparticles, polymers etc.) that can provide specific functional attributes of hemostasis while allowing donor-independent manufacturing, scale-up, and on-demand availability. Prominent examples of these are ‘synthetic platelet’ (SynthoPlate) nanoparticles that recapitulate platelet's binding interactions with von Willebrand Factor (vWF), collagen and active platelet integrin GPIIb-IIIa, flexible platelet-like particles (PLP) that bind fibrin to recapitulate platelet's biomechanical properties, fibrinogen function-mimicking nanoparticles that amplify the aggregation of active platelets, peptide-modified synthetic polymers (e.g. PolySTAT, HAPPI etc.) that render clot stabilization etc. In this framework, we present the design and evaluation of I.V.-administrable unique platelet-inspired nanoparticles that render injury site-targeted, enzyme-responsive direct delivery of thrombin, to site-specifically augment fibrin generation for hemostasis. Our design is inspired by platelets' crucial hemostatic mechanisms of : (i) rapidly accumulating at the injury site to form a plug and (ii) serving as a coagulation amplifier via presenting anionic phospholipids on the activated platelet surface to render tenase and prothrombinase factor assemblies leading to thrombin (FIIa) burst, which can then site-specifically convert fibrinogen to fibrin. Thrombin delivery to augment hemostasis is clinically well-accepted, as exemplified by products like Tisseel where thrombin and fibrinogen are co-delivered by syringe directly at wound site. Researchers have also studied thrombin-loaded topical dressings and topical administration of thrombin-loaded particles on wounds to mitigate bleeding, but these cannot be used intravenously. A recent interesting study has explored encapsulation of thrombin-loaded nanoparticles inside actual platelets with the idea of the particles being released (analogous to granule secretion) upon platelet activation, but this was only demonstrated in vitro because optimizing this complex strategy for consistent in vivo function may be challenging. Our approach circumvents these challenges by: (i) loading consistent amount of thrombin in I.V.-administrable lipid nanoparticles (LNPs), (ii) directly targeting the thrombin-loaded LNPs (TLNPs) to the injury site via specific binding to vWF and collagen, and (iii) releasing the loaded thrombin via particle destabilization by the action of injury site-specific enzyme phospholipase A2 for in situ fibrin production. We evaluated the TLNPs in vitro in human blood and plasma where hemostatic defects were created by platelet depletion and anticoagulant treatment. Spectrophotometric studies of fibrin generation, rotational thromboelastometry (ROTEM) based studies of clot characteristics and BioFlux microfluidics based real-time imaging of fibrin generation under simulated vascular flow conditions, confirmed the ability of TLNPs to restore fibrin generation in hemostatic dysfunction settings. Subsequently, the in vivo feasibility of these TLNPs was tested in a mouse tail-clip bleeding model where a combination of platelet depletion plus anticoagulant treatment was used to render significant hemostatic defect. TLNPs were able to effectively reduce tail-bleeding in mice. [Display omitted] DisclosuresSen Gupta: Haima Therapeutics: Other: Co-founder, Patents & Royalties: US 9107845, US 9107963.

The binding of autotaxin to integrins mediates hyperhomocysteinemia-potentiated platelet activation and thrombosis in mice and humans

AbstractHyperhomocysteinemia (HHcy) is associated with an exaggerated platelet thrombotic response at sites of vascular injury. In this study, human medical examination showed that elevated human plasma Hcy levels correlated positively with enhanced blood coagulation and platelet activity, suggesting that humans with HHcy are more prone to thrombus formation at the sites of vascular injury. Accordingly, we observed accelerated platelet activation, primary hemostasis, and thrombus formation in apolipoprotein E-deficient (ApoE−/−) mice with acute or chronic HHcy. Upon homocysteine (Hcy) administration in C57BL/6J mice, platelet aggregation, spreading and clot retraction were markedly induced. More important, Hcy increased the affinity of platelet integrin αIIbβ3 with ligands and enhanced integrin outside-in signaling by promoting membrane phosphatidylserine exposure in vitro. Mechanistically, lipidomics analysis showed that lysophosphatidylcholines were the primary metabolites leading to clustering of HHcy-stimulated platelets. Cytosolic phospholipase A2 (cPLA2) activity and autotaxin (ATX, a secreted lysophospholipase D) secretion were upregulated by Hcy, leading to membrane phospholipid hydrolysis and PS exposure. Moreover, secreted ATX directly interacted with integrin β3. Inhibitors of cPLA2 and ATX activity blocked integrin αIIbβ3 outside-in signaling and thrombosis in HHcy ApoE−/− mice. In this study, we identified a novel mechanism by which HHcy promotes platelet membrane phospholipid catabolism and extracellular ATX secretion to activate integrin outside-in signaling, consequently exacerbating thrombosis and the results revealed an innovative approach to treating HHcy-related thrombotic diseases.

The Toll-Like Receptor 2 Ligand Pam2CSK4 Activates Platelet Nuclear Factor-κB and Bruton’s Tyrosine Kinase Signaling to Promote Platelet-Endothelial Cell Interactions

Circulating platelets establish a variety of immunological programs and orchestrate inflammatory responses at the endothelium. Platelets express the innate immunity family of Toll-like receptors (TLRs). While TLR2/TLR1 ligands are known to activate platelets, the effects of TLR2/TLR6 ligands on platelet function remain unclear. Here, we aim to determine whether the TLR2/TLR6 agonists Pam2CSK4 and FSL-1 activate human platelets. In addition, human umbilical vein endothelial cells (HUVECs) and platelets were co-cultured to analyze the role of platelet TLR2/TLR6 on inflammation and adhesion to endothelial cells. Pam2CSK4, but not FSL-1, induced platelet granule secretion and integrin αIIbβ3 activation in a concentration-dependent manner. Moreover, Pam2CSK4 promoted platelet aggregation and increased platelet adhesion to collagen-coated surfaces. Mechanistic studies with blocking antibodies and pharmacologic inhibitors demonstrated that the TLR2/Nuclear factor-κB axis, Bruton’s-tyrosine kinase, and a secondary ADP feedback loop are involved in Pam2CSK4-induced platelet functional responses. Interestingly, Pam2CSK4 showed cooperation with immunoreceptor tyrosine-based activation motif (ITAM)-mediated signaling to enhance platelet activation. Finally, the presence of platelets increased inflammatory responses in HUVECs treated with Pam2CSK4, and platelets challenged with Pam2CSK4 showed increased adhesion to HUVECs under static and physiologically relevant flow conditions. Herein, we define a functional role for platelet TLR2-mediated signaling, which may represent a druggable target to dampen excessive platelet activation in thrombo-inflammatory diseases.

Inhibition of Arginine Methylation Impairs Platelet Function.

Protein arginine methyltransferases (PRMTs) catalyze the transfer of methyl groups to arginine residues in proteins. PRMT inhibitors are novel, promising drugs against cancer that are currently in clinical trials, which include oral administration of the drugs. However, off-target activities of systemically available PRMT inhibitors have not yet been investigated. In this work, we study the relevance of arginine methylation in platelets and investigate the effect of PRMT inhibitors on platelet function and on the expression of relevant platelet receptors. We show that (1) key platelet proteins are modified by arginine methylation; (2) incubation of human platelets with PRMT inhibitors for 4 h results in impaired capacity of platelets to aggregate in response to thrombin and collagen, with IC50 values in the μM range; and (3) treatment with PRMT inhibitors leads to decreased membrane expression and reduced activation of the critical platelet integrin αIIbβ3. Our contribution opens new avenues for research on arginine methylation in platelets, including the repurposing of arginine methylation inhibitors as novel antiplatelet drugs. We also recommend that current and future clinical trials with PRMT inhibitors consider any adverse effects associated with platelet inhibition of these emerging anticancer drugs.

The voltage-gated K+ channel Kv1.3 modulates platelet motility and α2β1 integrin-dependent adhesion to collagen

Kv1.3 is a voltage-gated K+-selective channel with roles in immunity, insulin-sensitivity, neuronal excitability and olfaction. Despite being one of the largest ionic conductances of the platelet surface membrane, its contribution to platelet function is poorly understood. Here we show that Kv1.3-deficient platelets display enhanced ADP-evoked platelet aggregation and secretion, and an increased surface expression of platelet integrin αIIb. In contrast, platelet adhesion and thrombus formation in vitro under arterial shear conditions on surfaces coated with collagen were reduced for samples from Kv1.3−/- compared to wild type mice. Use of collagen-mimetic peptides revealed a specific defect in the engagement with α2β1. Kv1.3−/- platelets developed significantly fewer, and shorter, filopodia than wild type platelets during adhesion to collagen fibrils. Kv1.3−/- mice displayed no significant difference in thrombus formation within cremaster muscle arterioles using a laser-induced injury model, thus other pro-thrombotic pathways compensate in vivo for the adhesion defect observed in vitro. This may include the increased platelet counts of Kv1.3−/- mice, due in part to a prolonged lifespan. The ability of Kv1.3 to modulate integrin-dependent platelet adhesion has important implications for understanding its contribution to normal physiological platelet function in addition to its reported roles in auto-immune diseases and thromboinflammatory models of stroke.