Distinct pathways regulate Syk protein activation downstream of immune tyrosine activation motif (ITAM) and hemITAM receptors in platelets

Equine β-defensin 1 regulates cytokine expression and phagocytosis in S. aureus-infected mouse monocyte macrophages via the Paxillin-FAK-PI3K pathway

β-defensin-1 (BD-1) is a rich source of disulfide bonds and antibacterial peptides that exhibit direct bactericidal function. The expression of BD-1 is primarily induced by external stimulation and is known to correlate with TLR-mediated inflammation, suggesting its association with innate immune responses. Equine β-defensin-1 (eBD-1) belongs to the BD-1 family. Our previous study demonstrated that eBD-1 enhances cytokine expression and promotes macrophage phagocytosis of S. aureus, although the underlying mechanism remains unknown. In this study, we utilized a PI-3K inhibitor (PKI-402) to treat eBD-1 -treated S. aureus-infected macrophages in vitro. Our results revealed that PKI-402 decreased the expression of eBD-1-promoted TNF-α, IL-6, CXCL10, CD40, RANTES, and p65 mRNA. To further investigate the relationship between eBD-1 and phagocytosis, we examined the expression of paxillin and FcγRIII (CD16 receptor) using western blot and immunofluorescence techniques. Our findings demonstrated that eBD-1 enhanced CD16 and paxillin expression in S. aureus -infected macrophages. Considering the correlation between paxillin expression and focal adhesion kinase (FAK), we transfected FAK siRNA into macrophages and evaluated paxillin expression using western blot analysis. Additionally, we quantified the number of S. aureus phagocytosed by macrophages. The results indicated a reduction in both paxillin expression and the number of S. aureus phagocytosed by macrophages upon FAK siRNA treatment. Our study showed the eBD-1 promotes cytokine mRNA expression in S. aureus-infected macrophages regulated by PI-3K-NF-κB pathway, and it increases macrophage phagocytosis of S. aureus associated with the FAK-paxillin signaling pathway.

Phosphorylation of spleen tyrosine kinase at Y346 negatively regulates ITAM-mediated signaling and function in platelets

Spleen tyrosine kinase (Syk) is expressed in a variety of hemopoietic cells. Upon phosphorylation of the platelet immunoreceptor-based activation motif of the glycoprotein VI (GPVI)/Fc receptor gamma chain collagen receptor, both the tyrosine phosphorylation and activity of Syk are increased leading to downstream signaling events. Although it has been established that the activity of Syk is regulated by tyrosine phosphorylation, the specific roles of individual phosphorylation sites remain to be elucidated. We observed that Syk Y346 in mouse platelets was still phosphorylated when GPVI-induced Syk activity was inhibited. We then generated Syk Y346F mice and analyzed the effect this mutation exerts on platelet responses. Syk Y346F mice bred normally, and their blood cell count was unaltered. We did observe potentiation of GPVI-induced platelet aggregation and ATP secretion as well as increased phosphorylation of other tyrosines on Syk in the Syk Y346F mouse platelets when compared to WT littermates. This phenotype was specific for GPVI-dependent activation, since it was not seen when AYPGKF, a PAR4 agonist, or 2-MeSADP, a purinergic receptor agonist, was used to activate platelets. Despite a clear effect of Syk Y346F on GPVI-mediated signaling and cellular responses, there was no effect of this mutation on hemostasis as measured by tail-bleeding times, although the time to thrombus formation determined using the ferric chloride injury model was reduced. Thus, our results indicate a significant effect of Syk Y346F on platelet activation and responses in vitro and reveal its complex nature manifesting itself by the diversified translation of platelet activation into physiological responses.

Identification of redundancy between human FcεRIβ and MS4A6A proteins points toward additional complex mechanisms for FcεRI trafficking and signaling

BACKGROUND

Allergic diseases are triggered by signaling through the high-affinity IgE receptor, FcεRI. In both mast cells (MCs) and basophils, FcεRI is a tetrameric receptor complex comprising a ligand-binding α subunit (FcεRIα), a tetraspan β subunit (FcεRIβ, MS4A2) responsible for trafficking and signal amplification, and a signal transducing dimer of single transmembrane γ subunits (FcεRIγ). However, FcεRI also exists as presumed trimeric complexes that lack FcεRIβ and are expressed on several cell types outside the MC and basophil lineages. Despite known differences between humans and mice in the presence of the trimeric FcεRI complex, questions remain as to how it traffics and whether it signals in the absence of FcεRIβ. We have previously reported that targeting FcεRIβ with exon-skipping oligonucleotides eliminates IgE-mediated degranulation in mouse MCs, but equivalent targeting in human MCs was not effective at reducing degranulation.

RESULTS

Here, we report that the FcεRIβ-like protein MS4A6A exists in human MCs and compensates for FcεRIβ in FcεRI trafficking and signaling. Human MS4A6A promotes surface expression of FcεRI complexes and facilitates degranulation. MS4A6A and FcεRIβ are encoded by highly related genes within the MS4A gene family that cluster within the human gene loci 11q12-q13, a region linked to allergy and asthma susceptibility.

CONCLUSIONS

Our data suggest the presence of either FcεRIβ or MS4A6A is sufficient for degranulation, indicating that MS4A6A could be an elusive FcεRIβ-like protein in human MCs that performs compensatory functions in allergic disease.

Recent progress in ITP treatment

In this review, the recently approved drugs avatrombopag and fostamatinib, which were not extensively covered within 2019 international recommendations for ITP, will be discussed in some detail. Avatrombopag appears more convenient than eltrombopag as it does not require dietary restrictions or subcutaneous administration like romiplostim. However, data on quality of life (QoL) are lacking and the rate of thromboembolic events in exposed patients is not negligible. Efficacy of fostamatinib, an inhibitor of macrophagic activity, is supported by placebo-controlled trials in patients refractory to several therapies, including TPO-RA. While hypertension and diarrhea have been reported, only one minor thrombotic event occurred in 146 exposed patients. In addition, several new treatment combinations and new agents entered clinical investigation in recent years. In a UK trial, combining mycophenolate mofetil with corticosteroids as first line therapy was more effective than corticosteroids alone, but at the cost of worse QoL. No combination, including oseltamivir or all-trans retinoic acid or danazol, resulted in convincing evidence of superior efficacy and safety when used in first or later lines of treatment. Agents targeting specific mechanisms are also discussed: sutimlimab (complement inhibitor); rilzabrutinib (BTK inhibitor) and efgartigimod (modified Fc fragment inhibiting FcRn). Only efgartigimod has completed phase 3 investigation.

Filamin A in platelets: Bridging the (signaling) gap between the plasma membrane and the actin cytoskeleton

Platelets are anucleate cells that are essential for hemostasis and wound healing. Upon activation of the cell surface receptors by their corresponding extracellular ligands, platelets undergo rapid shape change driven by the actin cytoskeleton; this shape change reaction is modulated by a diverse array of actin-binding proteins. One actin-binding protein, filamin A (FLNA), cross-links and stabilizes subcortical actin filaments thus providing stability to the cell membrane. In addition, FLNA binds the intracellular portion of multiple cell surface receptors and acts as a critical intracellular signaling scaffold that integrates signals between the platelet's plasma membrane and the actin cytoskeleton. This mini-review summarizes how FLNA transduces critical cell signals to the platelet cytoskeleton.

MAPK-interacting kinase 1 regulates platelet production, activation, and thrombosis

The MAPK-interacting kinase (Mnk) family includes Mnk1 and Mnk2, which are phosphorylated and activated in response to extracellular stimuli. Mnk1 contributes to cellular responses by regulating messenger RNA (mRNA) translation, and mRNA translation influences platelet production and function. However, the role of Mnk1 in megakaryocytes and platelets has not previously been studied. The present study investigated Mnk1 in megakaryocytes and platelets using both pharmacological and genetic approaches. We demonstrate that Mnk1, but not Mnk2, is expressed and active in human and murine megakaryocytes and platelets. Stimulating human and murine megakaryocytes and platelets induced Mnk1 activation and phosphorylation of eIF4E, a downstream target of activated Mnk1 that triggers mRNA translation. Mnk1 inhibition or deletion significantly diminished protein synthesis in megakaryocytes as measured by polysome profiling and [35S]-methionine incorporation assays. Depletion of Mnk1 also reduced megakaryocyte ploidy and proplatelet forming megakaryocytes in vitro and resulted in thrombocytopenia. However, Mnk1 deletion did not affect the half-life of circulating platelets. Platelets from Mnk1 knockout mice exhibited reduced platelet aggregation, α granule secretion, and integrin αIIbβ3 activation. Ribosomal footprint sequencing indicated that Mnk1 regulates the translation of Pla2g4a mRNA (which encodes cPLA2) in megakaryocytes. Consistent with this, Mnk1 ablation reduced cPLA2 activity and thromboxane generation in platelets and megakaryocytes. In vivo, Mnk1 ablation protected against platelet-dependent thromboembolism. These results provide previously unrecognized evidence that Mnk1 regulates mRNA translation and cellular activation in platelets and megakaryocytes, endomitosis and thrombopoiesis, and thrombosis.

TULA-Family Regulators of Platelet Activation

The two members of the UBASH3/TULA/STS-protein family have been shown to critically regulate cellular processes in multiple biological systems. The regulatory function of TULA-2 (also known as UBASH3B or STS-1) in platelets is one of the best examples of the involvement of UBASH3/TULA/STS proteins in cellular regulation. TULA-2 negatively regulates platelet signaling mediated by ITAM- and hemITAM-containing membrane receptors that are dependent on the protein tyrosine kinase Syk, which currently represents the best-known dephosphorylation target of TULA-2. The biological responses of platelets to collagen and other physiological agonists are significantly downregulated as a result. The protein structure, enzymatic activity and regulatory functions of UBASH3/TULA/STS proteins in the context of platelet responses and their regulation are discussed in this review.

Platelets and tyrosine kinase inhibitors: clinical features, mechanisms of action, and effects on physiology

Tyrosine kinase inhibitors (TKIs) have emerged as a promising class of target-directed, small molecule inhibitors used to treat hematologic malignancies, inflammatory diseases, and autoimmune disorders. Recently, TKIs have also gained interest as potential antiplatelet-directed therapeutics that could be leveraged to reduce pathologic thrombus formation and atherothrombotic complications, while minimally affecting platelet hemostatic function. This review provides a mechanistic overview and summarizes the known effects of tyrosine kinase inhibitors on platelet signaling and function, detailing prominent platelet signaling pathways downstream of the glycoprotein VI (GPVI) receptor, integrin αIIbβ3, and G protein-coupled receptors (GPCRs). This review focuses on mechanistic as well as clinically relevant and emerging TKIs targeting major families of tyrosine kinases including but not limited to Bruton's tyrosine kinase (BTK), spleen tyrosine kinase (Syk), Src family kinases (SFKs), Janus kinases (JAK), and signal transducers and activators of transcription (STAT) and evaluates their effects on platelet aggregation and adhesion, granule secretion, receptor expression and activation, and protein phosphorylation events. In summation, this review highlights current advances and knowledge on the effects of select TKIs on platelet biology and furthers insight on signaling pathways that may represent novel druggable targets coupled to specific platelet functional responses.

Phosphorylation on Syk Y342 is important for both ITAM and hemITAM signaling in platelets

Immune cells express receptors bearing an immune tyrosine activation motif (ITAM) containing two YXXL motifs or hemITAMs containing only one YXXL motif. Phosphorylation of the ITAM/hemITAM is mediated by Src family kinases allowing for the binding and activation of spleen tyrosine kinase (Syk). It is believed that Syk must be phosphorylated on tyrosine residues for activation, and Tyr342, а conserved tyrosine in the interdomain B region, has been shown to be critical for regulating Syk in FcεR1-activated mast cells. Syk is a key mediator of signaling pathways downstream of several platelet pathways including the ITAM bearing glycoprotein VI (GPVI)/Fc receptor gamma chain collagen receptor and the hemITAM containing C-type lectin-like receptor-2 (CLEC-2). Since platelet activation is a crucial step in both hemostasis and thrombosis, we evaluated the importance of Syk Y342 in these processes by producing an Syk Y342F knock-in mouse. When using a CLEC-2 antibody as an agonist, reduced aggregation and secretion were observed in Syk Y342F mouse platelets when compared with control mouse platelets. Platelet reactivity was also reduced in response to the GPVI agonist collagen-related peptide. Signaling initiated by either GPVI or CLEC-2 was also greatly inhibited, including Syk Y519/520 phosphorylation. Hemostasis, as measured by tail bleeding time, was not altered in Syk Y342F mice, but thrombus formation in response to FeCl₃ injury was prolonged in Syk Y342F mice. These data demonstrate that phosphorylation of Y342 on Syk following stimulation of either GPVI or CLEC-2 receptors is important for the ability of Syk to transduce a signal.

Theaflavin: a natural candidate to restrain thrombosis

Many clinical studies have demonstrated the beneficial effects of black tea on cardiovascular diseases. However, the antiplatelet and antithrombotic activities of theaflavin (TF-1) remain unknown. In this study, we aimed to investigate the beneficial effects of TF-1 on platelet activation and thrombosis formation both in vitro and in vivo. Firstly, the in vitro antiplatelet activity of TF-1 was analyzed using platelets isolated from human blood via aggregometry, flow cytometry, the ELISA kit, western blot and fluorescence microscopy. Subsequently, the in vivo analysis of the hemostatic state and thrombosis formation was carried out in C57BL/6 mice based on the tail bleeding time and an FeCl₃-induced arterial thrombus model. The results showed that TF-1 could prominently inhibit platelet aggregation in a dose-dependent manner, and attenuate P-selectin expression, fibrinogen binding, spreading and thromboxane A2 (TxA2) formation. Western blot analysis showed that TF-1 potently inhibited spleen tyrosine kinase (Syk) and Akt (ser473/474) phosphorylation. The in vivo data further confirmed the inhibition of platelet activation by TF-1 with a prolonged arterial occlusion time (from 15.0 ± 1.1 minutes to 40.0 ± 5.4 minutes). All the results indicated that TF-1 is a powerful inhibitor of platelet activation and thrombosis formation in C57BL/6 mice, and could be developed as a novel food-based inhibitor of thrombotic disorders.

PD-L1 Regulates Platelet Activation and Thrombosis via Caspase-3/GSDME Pathway

Platelets play a central role in hemostasis and thrombosis, regulating the occurrence and development of thrombotic diseases, including ischemic stroke. Programmed death ligand 1 (PD-L1) has recently been detected in platelet, while the function of PD-L1 in platelets remain elusive. Our data reveal a novel mechanism for the role of PD-L1 on platelet activation and arterial thrombosis. PD-L1 knockout does not affect platelet morphology, count, and mean volume under homeostasis and without risk of bleeding, which inhibits platelet activation by suppressing outside-in-activation of integrin by downregulating the Caspase-3/GSDME pathway. Platelet adoptive transfer experiments demonstrate that PD-L1 knockout inhibits thrombosis. And the absence of PD-L1 improves ischemic stroke severity and increases mice survival. Immunohistochemical staining of the internal structure of the thrombus proves that PD-L1 enhances the seriousness of the thrombus by inhibiting platelet activation. This work reveals a regulatory role of PD-L1 on platelet activation and thrombosis while providing novel platelet intervention strategies to prevent thrombosis.

The GPIbα intracellular tail - role in transducing VWF- and collagen/GPVI-mediated signaling

The GPIbT-VWF A1 domain interaction is essential for platelet tethering under high shear. Synergy between GPIbα and GPVI signaling machineries has been suggested previously, however its molecular mechanism remains unclear. We generated a novel GPIbα transgenic mouse (GpIbαΔsig/Δsig) by CRISPR-Cas9 technology to delete the last 24 residues of the GPIbα intracellular tail that harbors the 14-3-3 and phosphoinositide-3 kinase binding sites. GPIbαΔsig/Δsig platelets bound VWF normally under flow. However, they formed fewer filopodia on VWF/botrocetin in the presence of a oIIbI3 blocker, demonstrating that despite normal ligand binding, VWF-dependent signaling is diminished. Activation of GpIbαΔsig/Δsig platelets with ADP and thrombin was normal, but GpIbαΔsig/Δsig platelets stimulated with collagen-related-peptide (CRP) exhibited markedly decreased P-selectin exposure and eIIbI3 activation, suggesting a role for the GpIbaaintracellular tail in GPVI-mediated signaling. Consistent with this, while haemostasis was normal in GPIbαΔsig/Δsig mice, diminished tyrosine-phosphorylation, (particularly pSYK) was detected in CRP-stimulated GpIbαΔsig/Δsig platelets as well as reduced platelet spreading on CRP. Platelet responses to rhodocytin were also affected in GpIbαΔsig/Δsig platelets but to a lesser extent than those with CRP. GpIbαΔsig/Δsig platelets formed smaller aggregates than wild-type platelets on collagen-coated microchannels at low, medium and high shear. In response to both VWF and collagen binding, flow assays performed with plasma-free blood or in the presence of bIIbI3- or GPVI-blockers suggested reduced bIIbI3 activation contributes to the phenotype of the GpIbαΔsig/Δsig platelets. Together, these results reveal a new role for the intracellular tail of GPIbiiin transducing both VWF-GPIbGGand collagen-GPVI signaling events in platelets.

Clustering extent-dependent differential signaling by CLEC-2 receptors in platelets

Background

C-type lectin receptor family members play a role in many cells including platelets, where they are crucial in the separation of lymphatic and blood vessels during development. The C-type lectin-like receptor 2 (CLEC-2) receptor contains the canonical intracellular hemITAM motif through which it signals to activate Syk.

Objectives

One proposed hypothesis for signaling cascade is that Syk bridges two receptors through phosphorylated hemITAM motifs. We demonstrated that the phosphorylated hemITAM stimulates PI3 kinase/Btk pathways to activate Syk. To address this controversy, we used a CLEC-2 selective agonist and studied the role of Btk in platelet activation.

Results and Conclusions

Platelet activation and downstream signaling were abolished in murine and human platelets in the presence of the Btk inhibitors ibrutinib or acalabrutinib when a low concentration of a CLEC-2 antibody was used to crosslink CLEC-2 receptors. This inhibition was overcome by increasing concentrations of the CLEC-2 antibody. Similar results were obtained in X-linked immunodeficient mouse platelets, with an inactivating mutation in Btk or in Lyn null platelets. We conclude that at low crosslinking conditions of CLEC-2, Btk plays an important role in the activation of Syk, but at higher crosslinking conditions their role becomes less important and other mechanisms take over to activate Syk.

Megakaryocyte-specific knockout of the Mir-99b/let7e/125a cluster lowers platelet count without altering platelet function

The purpose of this research was to assess the effects of a microRNA (miRNA) cluster on platelet production. Human chromosome 19q13.41 harbors an evolutionarily conserved cluster of three miRNA genes (MIR99B, MIRLET7E, MIR125A) within 727 base-pairs. We now report that levels of miR-99b-5p, miR-let7e-5p and miR-125a-5p are strongly correlated in human platelets, and all are positively associated with platelet count, but not white blood count or hemoglobin level. Although the cluster regulates hematopoietic stem cell proliferation, the function of this genomic locus in megakaryocyte (MK) differentiation and platelet production is unknown. Furthermore, studies of individual miRNAs do not represent broader effects in the context of a cluster. To address this possibility, MK/platelet lineage-specific Mir-99b/let7e/125a knockout mice were generated. Compared to wild type littermates, cluster knockout mice had significantly lower platelet counts and reduced MK proplatelet formation, but no differences in MK numbers, ploidy, maturation or ultra-structural morphology, and no differences in platelet function. Compared to wild type littermates, knockout mice showed similar survival after pulmonary embolism. The major conclusions are that the effect of the Mir-99b/let7e/125a cluster is confined to a late stage of thrombopoiesis, and this effect on platelet number is uncoupled from platelet function.

Molecular Proteomics and Signalling of Human Platelets in Health and Disease

Platelets are small anucleate blood cells that play vital roles in haemostasis and thrombosis, besides other physiological and pathophysiological processes. These roles are tightly regulated by a complex network of signalling pathways. Mass spectrometry-based proteomic techniques are contributing not only to the identification and quantification of new platelet proteins, but also reveal post-translational modifications of these molecules, such as acetylation, glycosylation and phosphorylation. Moreover, target proteomic analysis of platelets can provide molecular biomarkers for genetic aberrations with established or non-established links to platelet dysfunctions. In this report, we review 67 reports regarding platelet proteomic analysis and signalling on a molecular base. Collectively, these provide detailed insight into the: (i) technical developments and limitations of the assessment of platelet (sub)proteomes; (ii) molecular protein changes upon ageing of platelets; (iii) complexity of platelet signalling pathways and functions in response to collagen, rhodocytin, thrombin, thromboxane A₂ and ADP; (iv) proteomic effects of endothelial-derived mediators such as prostacyclin and the anti-platelet drug aspirin; and (v) molecular protein changes in platelets from patients with congenital disorders or cardiovascular disease. However, sample sizes are still low and the roles of differentially expressed proteins are often unknown. Based on the practical and technical possibilities and limitations, we provide a perspective for further improvements of the platelet proteomic field.

Multifaceted Immunomodulatory Effects of the BTK Inhibitors Ibrutinib and Acalabrutinib on Different Immune Cell Subsets - Beyond B Lymphocytes

The clinical success of the two BTK inhibitors, ibrutinib and acalabrutinib, represents a major breakthrough in the treatment of chronic lymphocytic leukemia (CLL) and has also revolutionized the treatment options for other B cell malignancies. Increasing evidence indicates that in addition to their direct effects on B lymphocytes, both BTK inhibitors also directly impact the homeostasis, phenotype and function of many other cell subsets of the immune system, which contribute to their high efficacy as well as adverse effects observed in CLL patients. In this review, we attempt to provide an overview on the overlapping and differential effects of ibrutinib and acalabrutinib on specific receptor signaling pathways in different immune cell subsets other than B cells, including T cells, NK cells, monocytes, macrophages, granulocytes, myeloid-derived suppressor cells, dendritic cells, osteoclasts, mast cells and platelets. The shared and distinct effects of ibrutinib versus acalabrutinib are mediated through BTK-dependent and BTK-independent mechanisms, respectively. Such immunomodulatory effects of the two drugs have fueled myriad explorations of their repurposing opportunities for the treatment of a wide variety of other human diseases involving immune dysregulation.

The structure of CLEC-2: mechanisms of dimerization and higher-order clustering

The platelet C-type lectin-like receptor CLEC-2 drives inflammation-driven venous thrombosis in mouse models of thrombo-inflammatory disease with a minimal effect on hemostasis identifying it as a target for a new class of antiplatelet agent. Here, we discuss how the protein structure and dynamic arrangement of CLEC-2 on the platelet membrane helps the receptor, which has a single YxxL motif (known as a hemITAM), to trigger intracellular signaling. CLEC-2 exists as a monomer and homo-dimer within resting platelets and forms higher-order oligomers following ligand activation, a process that is mediated by the multivalent nature of its ligands and the binding of the tandem SH2 domains of Syk to the phosphorylated hemITAM and concomitantly to PIP2 or PIP3 to localize it to the membrane. We propose that a low level of active Syk is present at the membrane in resting platelets due to phosphorylation by Src family kinases and that clustering of receptors disturbs the equilibrium between kinases and phosphatases, triggering phosphorylation of the CLEC-2 hemITAM and recruitment of Syk. Knowledge of the structure of CLEC-2 and the mechanism of platelet activation has important implications for development of therapeutics.

Protease-activated receptor 4 causes Akt phosphorylation independently of PI3 kinase pathways

PI-3 Kinase plays an important role in platelet activation mainly through regulation of RASA3. Akt phosphorylation is an indicator for the activity of PI3 kinase. The aim of this study is to characterize the pathways leading to Akt phosphorylation in platelets. We performed concentration response curves of LY294002, a pan-PI3 kinase inhibitor, on platelet aggregation and Akt phosphorylation, in washed human and mouse platelets. At concentrations as low as 3.12 µM, LY294002 abolished Akt phosphorylation induced by 2MeSADP and SFLLRN, but not by AYPGKF. It required much higher concentrations of LY294002 (12.5-25 µM) to abolish AYPGKF-induced Akt phosphorylation, both in wild type and P2Y12 null mouse platelets. We propose that 3.12 µM LY294002 is sufficient to inhibit PI3 kinase isoforms in platelets and higher concentrations might inhibit other pathways regulating Akt phosphorylation by AYPGKF. We conclude that Protease-activated receptor 4 (PAR4) might cause Akt phosphorylation through pathways distinctly different from those of Protease-activated receptor 1 (PAR1).

Assessment of thrombotic risk during long-term treatment of immune thrombocytopenia with fostamatinib

Background:

Patients with immune thrombocytopenia (ITP) are at risk of bleeding and, paradoxically, thromboembolic events (TEEs), irrespective of thrombocytopenia. The risk of thrombosis is increased by advanced age, obesity, and prothrombotic comorbidities: cancer, hyperlipidemia, diabetes, hypertension, coronary artery disease, and chronic kidney disease, among others. Certain ITP treatments further increase the risk of TEE, especially splenectomy and thrombopoietin receptor agonists. Spleen tyrosine kinase (SYK) is a key signaling molecule common to thromboembolic and hemostatic (in addition to inflammatory) pathways. Fostamatinib is an orally administered SYK inhibitor approved in the USA and Europe for treatment of chronic ITP in adults.

Methods:

The phase III and extension studies included heavily pretreated patients with long-standing ITP, many of whom had risk factors for thrombosis prior to initiating fostamatinib. This report describes long-term safety and efficacy of fostamatinib in 146 patients with up to 5 years of treatment, a total of 229 patient-years, and assesses the incidence of thromboembolic events (by standardized MedDRA query).

Results:

Platelet counts ⩾50,000/µL were achieved in 54% of patients and the safety profile was as described in the phase III clinical studies with no new toxicities observed over the 5 years of follow-up. The only TEE occurred in one patient (0.7%, or 0.44/100 patient-years), who experienced a mild transient ischemic attack. This is a much lower rate than might be expected in ITP patients.

Conclusion:

This report demonstrates durable efficacy and a very low incidence of TEE in patients receiving long-term treatment of ITP with the SYK inhibitor fostamatinib.

ClinicalTrials.gov identifiers:

NCT02076399, NCT02076412, and NCT02077192.

Assessment of the effects of Syk and BTK inhibitors on GPVI-mediated platelet signaling and function

Spleen tyrosine kinase (Syk) and Bruton's tyrosine kinase (BTK) play critical roles in platelet physiology, facilitating intracellular immunoreceptor tyrosine-based activation motif (ITAM)-mediated signaling downstream of platelet glycoprotein VI (GPVI) and GPIIb/IIIa receptors. Small molecule tyrosine kinase inhibitors (TKIs) targeting Syk and BTK have been developed as antineoplastic and anti-inflammatory therapeutics and have also gained interest as antiplatelet agents. Here, we investigate the effects of 12 different Syk and BTK inhibitors on GPVI-mediated platelet signaling and function. These inhibitors include four Syk inhibitors, Bay 61-3606, R406 (fostamatinib), entospletinib, TAK-659; four irreversible BTK inhibitors, ibrutinib, acalabrutinib, ONO-4059 (tirabrutinib), AVL-292 (spebrutinib); and four reversible BTK inhibitors, CG-806, BMS-935177, BMS-986195, and fenebrutinib. In vitro, TKIs targeting Syk or BTK reduced platelet adhesion to collagen, dense granule secretion, and alpha granule secretion in response to the GPVI agonist cross-linked collagen-related peptide (CRP-XL). Similarly, these TKIs reduced the percentage of activated integrin α_IIbβ₃ on the platelet surface in response to CRP-XL, as determined by PAC-1 binding. Although all TKIs tested inhibited phospholipase C γ2 (PLCγ2) phosphorylation following GPVI-mediated activation, other downstream signaling events proximal to phosphoinositide 3-kinase (PI3K) and PKC were differentially affected. In addition, reversible BTK inhibitors had less pronounced effects on GPIIb/IIIa-mediated platelet spreading on fibrinogen and differentially altered the organization of PI3K around microtubules during platelets spreading on fibrinogen. Select TKIs also inhibited platelet aggregate formation on collagen under physiological flow conditions. Together, our results suggest that TKIs targeting Syk or BTK inhibit central platelet functional responses but may differentially affect protein activities and organization in critical systems downstream of Syk and BTK in platelets.

Low-dose Btk inhibitors selectively block platelet activation by CLEC-2

Inhibitors of the tyrosine kinase Btk have been proposed as novel antiplatelet agents. In this study we show that low concentrations of the Btk inhibitor ibrutinib block CLEC-2-mediated activation and tyrosine phosphorylation including Syk and PLCγ2 in human platelets. Activation is also blocked in patients with X-linked agammaglobulinemia (XLA) caused by a deficiency or absence of Btk. In contrast, the response to GPVI is delayed in the presence of low concentrations of ibrutinib or in patients with XLA, and tyrosine phosphorylation of Syk is preserved. A similar set of results is seen with the second-generation inhibitor, acalabrutinib. The differential effect of Btk inhibition in CLEC-2 relative to GPVI signalling is explained by the positive feedback role involving Btk itself, as well as ADP and thromboxane A2 mediated activation of P2Y12 and TP receptors, respectively. This feedback role is not seen in mouse platelets and, consistent with this, CLEC-2-mediated activation is blocked by high but not by low concentrations of ibrutinib. Nevertheless, thrombosis was absent in 8 out of 13 mice treated with ibrutinib. These results show that Btk inhibitors selectively block activation of human platelets by CLEC-2 relative to GPVI suggesting that they can be used at 'low dose' in patients to target CLEC-2 in thrombo-inflammatory disease.

Molecular Drivers of Platelet Activation: Unraveling Novel Targets for Anti-Thrombotic and Anti-Thrombo-Inflammatory Therapy

Cardiovascular diseases (CVDs) are the leading cause of death globally-partly a consequence of increased population size and ageing-and are major contributors to reduced quality of life. Platelets play a major role in hemostasis and thrombosis. While platelet activation and aggregation are essential for hemostasis at sites of vascular injury, uncontrolled platelet activation leads to pathological thrombus formation and provokes thrombosis leading to myocardial infarction or stroke. Platelet activation and thrombus formation is a multistage process with different signaling pathways involved to trigger platelet shape change, integrin activation, stable platelet adhesion, aggregation, and degranulation. Apart from thrombotic events, thrombo-inflammation contributes to organ damage and dysfunction in CVDs and is mediated by platelets and inflammatory cells. Therefore, in the past, many efforts have been made to investigate specific signaling pathways in platelets to identify innovative and promising approaches for novel antithrombotic and anti-thrombo-inflammatory strategies that do not interfere with hemostasis. In this review, we focus on some of the most recent data reported on different platelet receptors, including GPIb-vWF interactions, GPVI activation, platelet chemokine receptors, regulation of integrin signaling, and channel homeostasis of NMDAR and PANX1.

[Current antiplatelet agents, new inhibitors and therapeutic targets]

Cardiovascular diseases are the leading cause of deaths in the world. Platelets play a major role in the occurrence of these diseases and the development of antiplatelet drugs is a priority in the fight against cardiovascular diseases-associated mortality. Aspirin and thienopyridine-based P2Y12 inhibitors are the main drugs currently used. These molecules target the initiation of platelets activation and are responsible for a moderate inhibitory action. Other antiplatelet agents, as glycoprotein (GP) IIb/IIIa antagonists, inhibit platelet aggregation independently of initial activation-associated pathways, but are responsible for increased hemorrhagic events. Regarding each antiplatelet agent's specific characteristics, the prescription of these drugs must take into account the type of cardiovascular event, the age of the patient, the past medical history, and the potential hemorrhagic adverse events. Thus, there is a need for the development of new molecules with a more targeted effect, maintaining optimal efficiency but with a reduction of the hemorrhagic risk, which is the principal limitation of these treatments.

Platelet CLEC-2 and lung development

In this article, the State of the Art lecture "Platelet CLEC-2 and Lung Development" presented at the ISTH congress 2019 is reviewed. During embryonic development, blood cells are often considered as porters of nutrition and oxygen but not as active influencers of cell differentiation. However, recent studies revealed that platelets actively facilitate cell differentiation by releasing biological substances during development. C-type lectin-like receptor 2 (CLEC-2) has been identified as a receptor for the platelet-activating snake venom rhodocytin. An endogenous ligand of CLEC-2 is the membrane protein podoplanin (PDPN), which is expressed on the surface of certain types of tumor cells and lymphatic endothelial cells (LECs). Deletion of CLEC-2 from platelets in mice results in death just after birth due to lung malformation and blood/lymphatic vessel separation. During development, lymphatic vessels are derived from cardinal veins. At this stage, platelets are activated by binding of CLEC-2 to LEC PDPN and release trandforming growth factor-β (TGF-β). This cytokine inhibits LEC migration and proliferation, facilitating blood/lymphatic vessel separation. TGF-β released upon platelet-expressed CLEC-2/LEC PDPN also facilitates differentiation of lung mesothelial cells into alveolar duct myofibroblasts (adMYFs) in the developing lung. AdMYFs generate elastic fibers inside the lung, so that the lung can be properly inflated. Thus, platelets act as an ultimate natural drug delivery system that enables biological substances to be specifically delivered to the target at high concentrations by receptor/ligand interactions during development.

Control of Platelet CLEC-2-Mediated Activation by Receptor Clustering and Tyrosine Kinase Signaling

Platelets are blood cells responsible for vascular integrity preservation. The activation of platelet receptor C-type lectin-like receptor II-type (CLEC-2) could partially mediate the latter function. Although this receptor is considered to be of importance for hemostasis, the rate-limiting steps of CLEC-2-induced platelet activation are not clear. Here, we aimed to investigate CLEC-2-induced platelet signal transduction using computational modeling in combination with experimental approaches. We developed a stochastic multicompartmental computational model of CLEC-2 signaling. The model described platelet activation beginning with CLEC-2 receptor clustering, followed by Syk and Src family kinase phosphorylation, determined by the cluster size. Active Syk mediated linker adaptor for T cell protein phosphorylation and membrane signalosome formation, which resulted in the activation of Bruton's tyrosine kinase, phospholipase and phosphoinositide-3-kinase, calcium, and phosphoinositide signaling. The model parameters were assessed from published experimental data. Flow cytometry, total internal reflection fluorescence and confocal microscopy, and western blotting quantification of the protein phosphorylation were used for the assessment of the experimental dynamics of CLEC-2-induced platelet activation. Analysis of the model revealed that the CLEC-2 receptor clustering leading to the membrane-based signalosome formation is a critical element required for the accurate description of the experimental data. Both receptor clustering and signalosome formation are among the rate-limiting steps of CLEC-2-mediated platelet activation. In agreement with these predictions, the CLEC-2-induced platelet activation, but not activation mediated by G-protein-coupled receptors, was strongly dependent on temperature conditions and cholesterol depletion. Besides, the model predicted that CLEC-2-induced platelet activation results in cytosolic calcium spiking, which was confirmed by single-platelet total internal reflection fluorescence microscopy imaging. Our results suggest a refined picture of the platelet signal transduction network associated with CLEC-2. We show that tyrosine kinase activation is not the only rate-limiting step in CLEC-2-induced activation of platelets. Translocation of receptor-agonist complexes to the signaling region and linker adaptor for T cell signalosome formation in this region are limiting CLEC-2-induced activation as well.

ELMO1 deficiency enhances platelet function

Phosphatidylinositol 3-kinase is an important signaling molecule that, once activated, leads to the generation of phosphatidylinositol (3,4,5)-trisphosphate (PIP₃). We performed a proteomic screen to identify PIP₃-interacting proteins in human platelets_. Among these proteins, we found engulfment and cell motility 1 (ELMO1), a scaffold protein with no catalytic activity. ELMO1 is expressed in platelets and interacts with active RhoG. However, the function of ELMO1 in platelets is not known. The focus of this study was to determine the function of ELMO1 in platelets utilizing ELMO1^-/- mice. Platelet aggregation, granule secretion, integrin αIIbβ3 activation, and thromboxane generation were enhanced in ELMO1^-/- platelets in response to glycoprotein VI (GPVI) agonists but unaltered when a protease-activated receptor 4 agonist was used. The kinetics of spreading on immobilized fibrinogen was enhanced in ELMO1^-/- platelets compared with wild-type (WT) littermate controls. This suggests that ELMO1 plays a role downstream of the GPVI and integrin αIIbβ3 pathway. Furthermore, whole blood from ELMO1^-/- mice perfused over collagen exhibited enhanced thrombus formation compared with WT littermate controls. ELMO1^-/- mice showed reduced survival compared with control following pulmonary embolism. ELMO1^-/- mice also exhibited a shorter time to occlusion using the ferric-chloride injury model and reduced bleeding times compared with WT littermate controls. These results indicate that ELMO1 plays an important role in hemostasis and thrombosis in vivo. RhoG activity was enhanced in ELMO1^-/- murine platelets compared with WT littermate controls in response to GPVI agonist. Together, these data suggest that ELMO1 negatively regulates GPVI-mediated thrombus formation via RhoG.

Pathway Analysis Integrating Genome-Wide and Functional Data Identifies PLCG2 as a Candidate Gene for Age-Related Macular Degeneration

Purpose

Age-related macular degeneration (AMD) is the worldwide leading cause of blindness among the elderly. Although genome-wide association studies (GWAS) have identified AMD risk variants, their roles in disease etiology are not well-characterized, and they only explain a portion of AMD heritability.

Methods

We performed pathway analyses using summary statistics from the International AMD Genomics Consortium's 2016 GWAS and multiple pathway databases to identify biological pathways wherein genetic association signals for AMD may be aggregating. We determined which genes contributed most to significant pathway signals across the databases. We characterized these genes by constructing protein-protein interaction networks and performing motif analysis.

Results

We determined that eight genes (C2, C3, LIPC, MICA, NOTCH4, PLCG2, PPARA, and RAD51B) "drive" the statistical signals observed across pathways curated in the Kyoto Encyclopedia of Genes and Genomes (KEGG), Reactome, and Gene Ontology (GO) databases. We further refined our definition of statistical driver gene to identify PLCG2 as a candidate gene for AMD due to its significant gene-level signals (P < 0.0001) across KEGG, Reactome, GO, and NetPath pathways.

Conclusions

We performed pathway analyses on the largest available collection of advanced AMD cases and controls in the world. Eight genes strongly contributed to significant pathways from the three larger databases, and one gene (PLCG2) was central to significant pathways from all four databases. This is, to our knowledge, the first study to identify PLCG2 as a candidate gene for AMD based solely on genetic burden. Our findings reinforce the utility of integrating in silico genetic and biological pathway data to investigate the genetic architecture of AMD.

[CLEC-2 induced signalling in blood platelets]

Platelet activating receptor CLEC-2 has been identified on platelet surface a decade ago. The only confirmed endogenous CLEC-2 agonist is podoplanin. Podoplanin is a transmembrane protein expressed by lymphatic endothelial cells, reticular fibroblastic cells in lymph nodes, kidney podocytes and by cells of certain tumors. CLEC-2 and podoplanin are involved in the processes of embryonic development (blood-lymph vessel separation and angiogenesis), maintaining of vascular integrity of small vessels during inflammation and prevention of blood-lymphatic mixing in high endothelial venules. However, CLEC-2 and podoplanin are contributing to tumor methastasis progression, Salmonella sepsis, deep-vein thrombosis. CLEC-2 signalling cascade includes tyrosine-kinases (Syk, SFK, Btk) as well as adapter LAT and phospholipase Cg2, which induces calcium signalling. CLEC-2, podoplanin and proteins, participating in CLEC-2 signalling cascade, are perspective targets for antithrombotic therapy.

Establishment and maintenance of blood-lymph separation

Hippocratic Corpus, a collection of Greek medical literature, described the functional anatomy of the lymphatic system in the fifth century B.C. Subsequent studies in cadavers and surgical patients firmly established that lymphatic vessels drain extravasated interstitial fluid, also known as lymph, into the venous system at the bilateral lymphovenous junctions. Recent advances revealed that lymphovenous valves and platelet-mediated hemostasis at the lymphovenous junctions maintain life-long separation of the blood and lymphatic vascular systems. Here, we review murine models that exhibit failure of blood-lymph separation to highlight the novel mechanisms and molecular targets for the modulation of lymphatic disorders. Specifically, we focus on the transcription factors, cofactors, and signaling pathways that regulate lymphovenous valve development and platelet-mediated lymphovenous hemostasis, which cooperate to maintain blood-lymph separation.

miR-15a-5p regulates expression of multiple proteins in the megakaryocyte GPVI signaling pathway

Essentials The action of microRNAs (miRs) in human megakaryocyte signaling is largely unknown. Cord blood-derived human megakaryocytes (MKs) were used to test the function of candidate miRs. miR-15a-5p negatively regulated MK GPVI-mediated αIIbβ3 activation and α-granule release. miR-15a-5p acts as a potential "master-miR" regulating genes in the MK GPVI signaling pathway. SUMMARY: Background Megakaryocytes (MKs) invest their progeny platelets with proteins and RNAs. MicroRNAs (miRs), which inhibit mRNA translation into protein, are abundantly expressed in MKs and platelets. Although platelet miRs have been associated with platelet reactivity and disease, there is a paucity of information on the function of miRs in human MKs. Objective To identify MK miRs that regulate the GPVI signaling pathway in the MK-platelet lineage. Methods Candidate miRs associated with GPVI-mediated platelet aggregation were tested for functionality in cultured MKs derived from cord blood. Results An unbiased, transcriptome-wide screen in 154 healthy donors identified platelet miR-15a-5p as significantly negatively associated with CRP-induced platelet aggregation. Platelet agonist dose-response curves demonstrated activation of αIIbβ3 in suspensions of cord blood-derived cultured MKs. Overexpression and knockdown of miR-15a-5p in these MKs reduced and enhanced, respectively, CRP-induced αIIbβ3 activation but did not alter thrombin or ADP stimulation. FYN, SRGN, FCER1G, MYLK. and PRKCQ, genes involved in GPVI signaling, were identified as miR-15a-5p targets and were inhibited or de-repressed in MKs with miR-15a-5p overexpression or inhibition, respectively. Lentiviral overexpression of miR-15a-5p also inhibited GPVI-FcRγ-mediated phosphorylation of Syk and PLCγ2, GPVI downstream signaling molecules, but effects of miR-15a-5p on αIIbβ3 activation did not extend to other ITAM-signaling receptors (FcγRIIa and CLEC-2). Conclusion Cord blood-derived MKs are a useful human system for studying the functional effects of candidate platelet genes. miR-15a-5p is a potential "master-miR" for specifically regulating GPVI-mediated MK-platelet signaling. Targeting miR-15a-5p may have therapeutic potential in hemostasis and thrombosis.

Physiological and pathophysiological aspects of blood platelet activation through CLEC-2 receptor

Platelet activating receptor CLEC-2 has been identified on platelet surface a decade ago. The only confirmed endogenous CLEC-2 agonist is podoplanin. Podoplanin is a transmembrane protein expressed by lymphatic endothelial cells, reticular fibroblastic cells in lymph nodes, kidney podocytes and by cells of certain tumors. Association of CLEC-2 with podoplanin is involved in processes of embryonic development (blood-lymph vessel separation and angiogenesis), maintaining of vascular integrity of small vessels during inflammation and prevention of blood-lymphatic mixing in high endothelial venules. However, CLEC-2 and podoplanin are contributing to tumor metastasis progression, Salmonella sepsis and deep-vein thrombosis. This makes CLEC-2 and podoplanin a perspective target for pharmacological treatment. Aspirin and Ibrutinib are considered to be perspective for abrogation of podoplanin-induced platelet activation via CLEC-2. The present review discusses already known pathological and physiological roles of CLEC-2 and possibilities of a targeted therapy for CLEC-2 associated diseases.

TULA-family proteins: Jacks of many trades and then some

UBASH3/STS/TULA is a novel two-member family, which exerts several key regulatory effects in multiple cell types. UBASH3B/STS-1/TULA-2 is a highly active protein tyrosine phosphatase; its major target appears to be a specific regulatory site of protein tyrosine kinases of the Syk family, dephosphorylation of which inhibits Syk and Zap-70 kinases and suppresses receptor signaling mediated by these kinases. UBASH3A/STS-2/TULA exhibits substantial homology to UBASH3B/STS-1/TULA-2, but possesses only a small fraction of phosphatase activity of UBASH3B/STS-1/TULA-2, and thus, its regulatory effect may be based also on the phosphatase-independent mechanisms. Critical physiologic effects of these proteins have been demonstrated in T lymphocytes, platelets, stem cells, and other important cell types. These proteins have also been shown to play a key role in such pathologic conditions as autoimmunity, cancer, and thrombosis. The review focuses on the recent studies of this important family of cellular regulators.

PDK1 governs thromboxane generation and thrombosis in platelets by regulating activation of Raf1 in the MAPK pathway

Essentials Phosphoinositide 3-kinase and MAPK pathways crosstalk via PDK1. PDK1 is required for adenosine diphosphate-induced platelet activation and thromboxane generation. PDK1 regulates RAF proto-oncogene Ser/Thr kinase (Raf1) activation in the MAPK pathway. Genetic ablation of PDK1 protects against platelet-dependent thrombosis in vivo.

SUMMARY

Background Platelets are dynamic effector cells with functions that span hemostatic, thrombotic and inflammatory continua. Phosphoinositide-dependent protein kinase 1 (PDK1) regulates protease-activated receptor 4-induced platelet activation and thrombus formation through glycogen synthase kinase3β. However, whether PDK1 also signals through the ADP receptor and its functional importance in vivo remain unknown. Objective To establish the mechanism of PDK1 in ADP-induced platelet activation and thrombosis. Methods We assessed the role of PDK1 on 2MeSADP-induced platelet activation by measuring aggregation, thromboxane generation and phosphorylation events in the presence of BX-795, which inhibits PDK1, or by using platelet-specific PDK1 knockout mice and performing western blot analysis. PDK1 function in thrombus formation was assessed with an in vivo pulmonary embolism model. Results PDK1 inhibition with BX-795 reduced 2-methylthio-ADP (2MeSADP)-induced aggregation of human and murine platelets by abolishing thromboxane generation. Similar results were observed in pdk1-/- mice. PDK1 was also necessary for the phosphorylation of mitogen-activated protein kinase kinase 1/2 (MEK1/2), extracellular signal-regulated kinase 1/2, and cytosolic phospholipase A2, indicating that PDK1 regulates an upstream kinase in the mitogen-activated protein kinase (MAPK) pathway. We next determined that this upstream kinase is Raf-1, a serine/threonine kinase that is necessary for the phosphorylation of MEK1/2, as pharmacological inhibition and genetic ablation of PDK1 were sufficient to prevent Raf1 phosphorylation. Furthermore, in vivo inhibition or genetic ablation of PDK1 protected mice from collagen/epinephrine-induced pulmonary embolism. Conclusion PDK1 governs thromboxane generation and thrombosis in platelets that are stimulated with 2MeSADP by regulating activation of the MAPK pathway.

HemITAM: A single tyrosine motif that packs a punch

Innate immune cells sense danger through a plethora of germline-encoded receptors that recognize pathogen-associated molecular patterns (PAMPs) or cellular molecules that are exposed only by stressed, infected, malignant, or dead cells. Many of these danger-sensing receptors belong to the C-type lectin-like superfamily (CLSF) and therefore are called C-type lectin-like receptors (CTLRs). Certain activating CTLRs, namely, CLEC-2, Dectin-1, DNGR-1, NKp80, and NKp65, which are encoded by genes that are clustered together in a subregion of the mammalian natural killer gene complex (NKC), use a single copy tyrosine signaling module termed the hemi-immunoreceptor tyrosine-based activation motif (hemITAM). These hemITAM-bearing CTLRs are present on myeloid cells and innate lymphocytes and stimulate various functions, such as phagocytosis, cytokine production, and cytotoxicity. Proximal signaling mechanisms involve the tyrosine phosphorylation of the hemITAM and the subsequent activation of the kinase Syk. Signaling and Syk recruitment by the hemITAM appear to be tuned by variable amino acids within or near the hemITAM, which give rise to differences in downstream signaling events and diverging functional outcomes among hemITAM-bearing receptors.

Platelets and vascular integrity: how platelets prevent bleeding in inflammation

Platelets play a central role in primary hemostasis by forming aggregates that plug holes in injured vessels. Half a century ago, detailed studies of the microvasculature by electron microscopy revealed that under inflammatory conditions that do not induce major disruption to vascular structure, individual platelets are mobilized to the vessel wall, where they interact with leukocytes and appear to seal gaps that arise between endothelial cells. Recent developments in genetic engineering and intravital microscopy have allowed further molecular and temporal characterization of these events. Surprisingly, it turns out that platelets support the recruitment of leukocytes to sites of inflammation. In parallel, however, they exercise their hemostatic function by securing the integrity of inflamed blood vessels to prevent bleeding from sites of leukocyte infiltration. It thus appears that platelets not only serve in concert as building blocks of the hemostatic plug but also act individually as gatekeepers of the vascular wall to help preserve vascular integrity while coordinating host defense. Variants of this recently appreciated hemostatic function of platelets that we refer to as "inflammation-associated hemostasis" are engaged in different contexts in which the endothelium is challenged or dysfunctional. Although the distinguishing characteristics of these variants and the underlying mechanisms of inflammation-associated hemostasis remain to be fully elucidated, they can differ notably from those supporting thrombosis, thus presenting therapeutic opportunities.

Mouse podoplanin supports adhesion and aggregation of platelets under arterial shear: A novel mechanism of haemostasis

The podoplanin-CLEC-2 axis is critical in mice for prevention of hemorrhage in the cerebral vasculature during mid-gestation. This raises the question as to how platelets are captured by podoplanin on neuroepithelial cells in a high shear environment. In this study, we demonstrate that mouse platelets form stable aggregates on mouse podoplanin at arterial shear through a CLEC-2 and Src kinase-dependent pathway. Adhesion and aggregation are also dependent on the platelet glycoprotein (GP) receptors, integrin αIIbβ3 and GPIb, and the feedback agonists ADP and thromboxane A₂ (TxA₂). CLEC-2 does not bind to von Willebrand factor (VWF) suggesting that the interaction with podoplanin is sufficient to both tether and activate platelets. Consistent with this, the surface plasmon resonance measurements reveal that mouse CLEC-2 binds to mouse podoplanin with nanomolar affinity. The present findings demonstrate a novel pathway of hemostasis in which podoplanin supports platelet capture and activation at arteriolar rates of shear.

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.

Comparison of Acalabrutinib, A Selective Bruton Tyrosine Kinase Inhibitor, with Ibrutinib in Chronic Lymphocytic Leukemia Cells

Purpose: Ibrutinib inhibits Bruton tyrosine kinase (BTK) by irreversibly binding to the Cys-481 residue in the enzyme. However, ibrutinib also inhibits several other enzymes that contain cysteine residues homologous to Cys-481 in BTK. Patients with relapsed/refractory or previously untreated chronic lymphocytic leukemia (CLL) demonstrate a high overall response rate to ibrutinib with prolonged survival. Acalabrutinib, a selective BTK inhibitor developed to minimize off-target activity, has shown promising overall response rates in patients with relapsed/refractory CLL. A head-to-head comparison of ibrutinib and acalabrutinib in CLL cell cultures and healthy T cells is needed to understand preclinical biologic and molecular effects.Experimental Design: Using samples from patients with CLL, we compared the effects of both BTK inhibitors on biologic activity, chemokine production, cell migration, BTK phosphorylation, and downstream signaling in primary CLL lymphocytes and on normal T-cell signaling to determine the effects on other kinases.Results: Both BTK inhibitors induced modest cell death accompanied by cleavage of PARP and caspase-3. Production of CCL3 and CCL4 chemokines and pseudoemperipolesis were inhibited by both drugs to a similar degree. These drugs also showed similar inhibitory effects on the phosphorylation of BTK and downstream S6 and ERK kinases. In contrast, off-target effects on SRC-family kinases were more pronounced with ibrutinib than acalabrutinib in healthy T lymphocytes.Conclusions: Both BTK inhibitors show similar biological and molecular profile in primary CLL cells but appear different on their effect on normal T cells. Clin Cancer Res; 23(14); 3734-43. ©2016 AACR.

Gq pathway regulates proximal C-type lectin-like receptor-2 (CLEC-2) signaling in platelets

Platelets play a key role in the physiological hemostasis or pathological process of thrombosis. Rhodocytin, an agonist of the C-type lectin-like receptor-2 (CLEC-2), elicits powerful platelet activation signals in conjunction with Src family kinases (SFKs), spleen tyrosine kinase (Syk), and phospholipase γ2 (PLCγ2). Previous reports have shown that rhodocytin-induced platelet aggregation depends on secondary mediators such as thromboxane A2 (TxA2) and ADP, which are agonists for G-protein-coupled receptors (GPCRs) on platelets. How the secondary mediators regulate CLEC-2-mediated platelet activation in terms of signaling is not clearly defined. In this study, we report that CLEC-2-induced Syk and PLCγ2 phosphorylation is potentiated by TxA2 and that TxA2 plays a critical role in the most proximal event of CLEC-2 signaling, i.e. the CLEC-2 receptor tyrosine phosphorylation. We show that the activation of other GPCRs, such as the ADP receptors and protease-activated receptors, can also potentiate CLEC-2 signaling. By using the specific G_q inhibitor, UBO-QIC, or G_q knock-out murine platelets, we demonstrate that G_q signaling, but not other G-proteins, is essential for GPCR-induced potentiation of Syk phosphorylation downstream of CLEC-2. We further elucidated the signaling downstream of G_q and identified an important role for the PLCβ-PKCα pathway, possibly regulating activation of SFKs, which are crucial for initiation of CLEC-2 signaling. Together, these results provide evidence for novel G_q-PLCβ-PKCα-mediated regulation of proximal CLEC-2 signaling by G_q-coupled receptors.

Ibrutinib-associated bleeding: pathogenesis, management and risk reduction strategies

Ibrutinib is an irreversible inhibitor of Bruton's tyrosine kinase (Btk) that has proven to be an effective therapeutic agent for multiple B-cell-mediated lymphoproliferative disorders. Ibrutinib, however, carries an increased bleeding risk compared with standard chemotherapy. Bleeding events range from minor mucocutaneous bleeding to life-threatening hemorrhage, due in large part to the effects of ibrutinib on several distinct platelet signaling pathways. There is currently a minimal amount of data to guide clinicians regarding the use of ibrutinib in patients at high risk of bleeding or on anticoagulant or antiplatelet therapy. In addition, the potential cardiovascular protective effects of ibrutinib monotherapy in patients at risk of vascular disease are unknown. Patients should be cautioned against using non-steroidal anti-inflammatory drugs, fish oils, vitamin E and aspirin-containing products, and consider replacing ibrutinib with a different agent if dual antiplatelet therapy is indicated. Patients should not take vitamin K antagonists concurrently with ibrutinib; direct oral anticoagulants should be used if extended anticoagulation is strongly indicated. In this review, we describe the pathophysiology of ibrutinib-mediated bleeding and suggest risk reduction strategies for common clinical scenarios associated with ibrutinib.

Targeting of C-type lectin-like receptor 2 or P2Y12 for the prevention of platelet activation by immunotherapeutic CpG oligodeoxynucleotides

Essentials CpG oligodeoxynucleotide (ODN) immuotherapeutics cause undesired platelet activating effects. It is crucial to understand the mechanisms of these effects to identify protective strategies. CpG ODN-induced platelet activation depends on C-type lectin-like receptor 2 (CLEC-2) and P2Y12. Targeting CLEC-2 or P2Y12 fully prevents CpG ODN-induced platelet activation and thrombosis.

SUMMARY

Background Synthetic phosphorothioate-modified CpG oligodeoxynucleotides (ODNs) show potent immunostimulatory properties that are widely exploited in clinical trials of anticancer treatment. Unexpectedly, a recent study indicated that CpG ODNs activate human platelets via the immunoreceptor tyrosine-based activation motif (ITAM)-coupled receptor glycoprotein VI. Objective To further analyze the mechanisms of CpG ODN-induced platelet activation and identify potential inhibitory strategies. Methods In vitro analyses were performed on human and mouse platelets, and on cell lines expressing platelet ITAM receptors. CpG ODN platelet-activating effects were evaluated in a mouse model of thrombosis. Results We demonstrated platelet uptake of CpG ODNs, resulting in platelet activation and aggregation. C-type lectin-like receptor 2 (CLEC-2) expressed in DT40 cells bound CpG ODNs. CpG ODN uptake did not occur in CLEC-2-deficient mouse platelets. Inhibition of human CLEC-2 with a blocking antibody inhibited CpG ODN-induced platelet aggregation. CpG ODNs caused CLEC-2 dimerization, and provoked its internalization. They induced dense granule release before the onset of aggregation. Accordingly, pretreating platelets with apyrase, or inhibiting P2Y12 with cangrelor or clopidogrel, prevented CpG ODN platelet-activating effect. In vivo, intravenously injected CpG ODN interacted with platelets adhered to mouse injured endothelium, and promoted thrombus growth, which was inhibited by CLEC-2 deficiency or by clopidogrel. Conclusions CLEC-2 and P2Y12 are required for CpG ODN-induced platelet activation and thrombosis, and might be targeted to prevent adverse events in patients at risk.

Myc enhances B-cell receptor signaling in precancerous B cells and confers resistance to Btk inhibition

Dysregulation of the oncogenic transcription factor MYC induces B cell transformation and is a driver for B cell non-Hodgkin lymphoma (B-NHL). MYC overexpression in B-NHL is associated with more aggressive phenotypes and poor prognosis. Although genomic studies suggest a link between MYC overexpression and B cell receptor (BCR) signaling molecules in B-NHL, signaling pathways essential to Myc-mediated B-cell transformation have not been fully elucidated. We utilized intracellular phospho-flow cytometry to investigate the relationship between Myc and BCR signaling in pre-malignant B cells. Utilizing the Eμ-myc mouse model, where Myc is overexpressed specifically in B cells, both basal and stimulated BCR signaling were increased in precancerous B lymphocytes from Eμ-myc mice compared to wild-type littermates. B cells overexpressing Myc displayed constitutively higher levels of activated CD79α, Btk, Plcγ2, and Erk1/2. Notably, Myc overexpressing B cells maintained elevated BCR signaling despite treatment with ibrutinib, a Bruton’s tyrosine kinase inhibitor. Furthermore, PI3K/Akt pathway signaling was also increased in Eμ-myc B cells, and this increase was partially suppressed with ibrutinib. Additionally, experiments with Btk-null B cells revealed off-target effects of ibrutinib on BCR signaling. Our data show that in pre-malignant B cells, Myc overexpression is sufficient to activate BCR and PI3K/Akt signaling pathways and further enhances signaling following BCR ligation. Therefore, our results indicate precancerous B cells have already acquired enhanced survival and growth capabilities prior to transformation, and that elevated MYC levels confer resistance to pharmacologic inhibitors of BCR signaling, which has significant implications for B-NHL treatment.

The Activating C-type Lectin-like Receptor NKp65 Signals through a Hemi-immunoreceptor Tyrosine-based Activation Motif (hemITAM) and Spleen Tyrosine Kinase (Syk)

NKp65 is an activating human C-type lectin-like receptor (CTLR) triggering cellular cytotoxicity and cytokine secretion upon high-affinity interaction with the cognate CTLR keratinocyte-associated C-type lectin (KACL) selectively expressed by human keratinocytes. Previously, we demonstrated that NKp65-mediated cellular cytotoxicity depends on tyrosine 7, located in a cytoplasmic sequence motif of NKp65 resembling a hemi-immunoreceptor tyrosine-based activation motif (hemITAM). HemITAMs have been reported for a few activating myeloid-specific CTLRs, including Dectin-1 and CLEC-2, and consist of a single tyrosine signaling unit preceded by a triacidic motif. Upon receptor engagement, the hemITAM undergoes phosphotyrosinylation and specifically recruits spleen tyrosine kinase (Syk), initiating cellular activation. In this study, we addressed the functionality of the putative hemITAM of NKp65. We show that NKp65 forms homodimers and is phosphorylated at the hemITAM-embedded tyrosine 7 upon engagement by antibodies or KACL homodimers. HemITAM phosphotyrosinylation initiates a signaling pathway involving and depending on Syk, leading to cellular activation and natural killer (NK) cell degranulation. However, although NKp65 utilizes Syk for NK cell activation, a physical association of Syk with the NKp65 hemITAM could not be detected, unlike shown previously for the hemITAM of myeloid CTLR. Failure of NKp65 to recruit Syk is not due to an alteration of the triacidic motif, which rather affects the efficiency of hemITAM phosphotyrosinylation. In summary, NKp65 utilizes a hemITAM-like motif for cellular activation that requires Syk, although Syk appears not to be recruited to NKp65.