Protein-tyrosine phosphatases: a new frontier in platelet signal transduction

Platelets-related signature based diagnostic model in rheumatoid arthritis using WGCNA and machine learning

Background and aim

Rheumatoid arthritis (RA) is an autoinflammatory disease that may lead to severe disability. The diagnosis of RA is limited due to the need for biomarkers with both reliability and efficiency. Platelets are deeply involved in the pathogenesis of RA. Our study aims to identify the underlying mechanism and screening for related biomarkers.

Methods

We obtained two microarray datasets (GSE93272 and GSE17755) from the GEO database. We performed Weighted correlation network analysis (WGCNA) to analyze the expression modules in differentially expressed genes identified from GSE93272. We used KEGG, GO and GSEA enrichment analysis to elucidate the platelets-relating signatures (PRS). We then used the LASSO algorithm to develop a diagnostic model. We then used GSE17755 as a validation cohort to assess the diagnostic performance by operating Receiver Operating Curve (ROC).

Results

The application of WGCNA resulted in the identification of 11 distinct co-expression modules. Notably, Module 2 exhibited a prominent association with platelets among the differentially expressed genes (DEGs) analyzed. Furthermore, a predictive model consisting of six genes (MAPK3, ACTB, ACTG1, VAV2, PTPN6, and ACTN1) was constructed using LASSO coefficients. The resultant PRS model demonstrated excellent diagnostic accuracy in both cohorts, as evidenced by area under the curve (AUC) values of 0.801 and 0.979.

Conclusion

We elucidated the PRSs occurred in the pathogenesis of RA and developed a diagnostic model with excellent diagnostic potential.

Multicolor flow cytometry in clinical samples for platelet signaling assessment

Background

Availability of multichannel cytometers and specific commercial antibodies makes flow cytometry a new option to simultaneously assess multiple intracellular platelet signaling pathways for clinical purposes, in small volume of blood or low platelet count.

Objectives

To describe a multicolor flow cytometry with fluorescent barcoding technique for screening signaling pathways downstream membrane receptors of major platelet agonists (adenosine diphosphate, thrombin, thromboxane, and collagen).

Methods

By comparison with immunoblotting, we first selected the target phosphoproteins, AKT, P38MAPK, LIMK, and SPL76; the times of stimulation; and phosphoflow barcoding conditions. We then performed a clinical study on whole blood of patients without evidence of blood platelet disorder on standard biological screening, consulting for trivial or occasionally provoked bleeds without familial antecedent (bleeding of unknown origin, n = 23) or type-1 von Willebrand disease (n = 9). In addition, we included a small group of patients with definite platelet disorders (Glanzmann thrombasthenia, δ-storage pool deficiency, and immune glycoprotein VI-related disease with granule secretion defect).

Results

The range, kinetics, and distribution of fluorescence intensity were established for each agonist-target protein combination. Principal component analysis indicates a correlation in response to a target phosphoprotein (AKT and P38MAPK) to different agonists but no correlation in the response of different target phosphoproteins to the same agonist. The heterogeneity of individual responses in the whole population displayed was analyzed using clustering algorithm. Patients with platelet storage pool deficiency were positioned as lowest responders on the heatmap.

Conclusion

In complement of functional tests, this study introduces a new approach for rapid platelet signaling profiling in clinical practice.

Differential Regulation of GPVI-Induced Btk and Syk Activation by PKC, PKA and PP2A in Human Platelets

Bruton's tyrosine kinase (Btk) and spleen tyrosine kinase (Syk) are major signaling proteins in human platelets that are implicated in atherothrombosis and thrombo-inflammation, but the mechanisms controlling their activities are not well understood. Previously, we showed that Syk becomes phosphorylated at S297 in glycoprotein VI (GPVI)-stimulated human platelets, which limits Syk activation. Here, we tested the hypothesis that protein kinases C (PKC) and A (PKA) and protein phosphatase 2A (PP2A) jointly regulate GPVI-induced Btk activation in platelets. The GPVI agonist convulxin caused rapid, transient Btk phosphorylation at S180 (pS180↑), Y223 and Y551, while direct PKC activation strongly increased Btk pS180 and pY551. This increase in Btk pY551 was also Src family kinase (SFK)-dependent, but surprisingly Syk-independent, pointing to an alternative mechanism of Btk phosphorylation and activation. PKC inhibition abolished convulxin-stimulated Btk pS180 and Syk pS297, but markedly increased the tyrosine phosphorylation of Syk, Btk and effector phospholipase Cγ2 (PLCγ2). PKA activation increased convulxin-induced Btk activation at Y551 but strongly suppressed Btk pS180 and Syk pS297. PP2A inhibition by okadaic acid only increased Syk pS297. Both platelet aggregation and PLCγ2 phosphorylation with convulxin stimulation were Btk-dependent, as shown by the selective Btk inhibitor acalabrutinib. Together, these results revealed in GPVI-stimulated platelets a transient Syk, Btk and PLCγ2 phosphorylation at multiple sites, which are differentially regulated by PKC, PKA or PP2A. Our work thereby demonstrated the GPVI-Syk-Btk signalosome as a tightly controlled protein kinase network, in agreement with its role in atherothrombosis.

An optimized herbal medicine containing Scutellaria baicalensis Georgi, Alisma orientale Juzepzuk, and Atractylodes japonica Koidzumi has potent antiplatelet and antithrombotic activities

Background and aim

Platelet-derived thrombosis is important in the pathogenesis of cardiovascular diseases. HTB is an optimized herbal medicine including Scutellaria baicalensis Georgi, Alisma orientale Juzepzuk, and Atractylodes japonica Koidzumi. It is widely used in traditional medicine due to its anti-inflammatory and antioxidant effects. However, its antiplatelet and antithrombotic activities have not been completely validated. The current study aimed to examine the inhibitory effect of the novel herb formula HTB against platelet activation and thrombus formation.

Experimental procedure

The antiplatelet activities of HTB via platelet aggregation, granule secretion, reactive oxygen species generation, and intracellular calcium mobilization were evaluated. Moreover, the antithrombotic effect of HTB via FeCl₃-induced arterial thrombus formation in vivo in mice was assessed. The inhibitory effect of HTB against primary hemostasis was investigated based on transection tail bleeding time.

Results and conclusion

HTB treatment significantly inhibited glycoprotein VI-mediated platelet aggregation, granule secretion, reactive oxygen species generation, and intracellular calcium mobilization. Biochemical studies revealed that HTB inhibited glycoprotein VI-mediated platelet signal transduction during cell activation. Further, its antioxidant effect might be derived by reducing the phosphorylation of the p47^phox/Hic5 axis signalosome. Oral HTB treatment was effective in decreasing FeCl₃-induced arterial thrombus formation without prolonging the tail bleeding time. HTB can be an effective therapeutic agent against thrombotic diseases.

A phenotype driven integrative framework uncovers molecular mechanisms of a rare hereditary thrombophilia

Antithrombin resistance is a rare subtype of hereditary thrombophilia caused by prothrombin gene variants, leading to thrombotic disorders. Recently, the Prothrombin Belgrade variant has been reported as a specific variant that leads to antithrombin resistance in two Serbian families with thrombosis. However, due to clinical data scarcity and the inapplicability of traditional genome-wide association studies (GWAS), a broader perspective on molecular and phenotypic mechanisms associated with the Prothrombin Belgrade variant is yet to be uncovered. Here, we propose an integrative framework to address the lack of genomic samples and support the genomic signal from the full genome sequences of five heterozygous subjects by integrating it with subjects' phenotypes and the genes' molecular interactions. Our goal is to identify candidate thrombophilia-related genes for which our subjects possess germline variants by focusing on the resulting gene clusters of our integrative framework. We applied a Non-negative Matrix Tri-Factorization-based method to simultaneously integrate different data sources, taking into account the observed phenotypes. In other words, our data-integration framework reveals gene clusters involved with this rare disease by fusing different datasets. Our results are in concordance with the current literature about antithrombin resistance. We also found candidate disease-related genes that need to be further investigated. CD320, RTEL1, UCP2, APOA5 and PROZ participate in healthy-specific or disease-specific subnetworks involving thrombophilia-annotated genes and are related to general thrombophilia mechanisms according to the literature. Moreover, the ADRA2A and TBXA2R subnetworks analysis suggested that their variants may have a protective effect due to their connection with decreased platelet activation. The results show that our method can give insights into antithrombin resistance even if a small amount of genetic data is available. Our framework is also customizable, meaning that it applies to any other rare disease.

Protein tyrosine phosphatase PTPN22 negatively modulates platelet function and thrombus formation

Protein tyrosine phosphatase nonreceptor type 22 (PTPN22) is a protein tyrosine phosphatase that negatively regulates T-cell signaling. However, whether it is expressed and functions in platelets remains unknown. Here we investigated the expression and role of PTPN22 in platelet function. We reported PTPN22 expression in both human and mouse platelets. Using PTPN22-/- mice, we showed that PTPN22 deficiency significantly shortened tail-bleeding time and accelerated arterial thrombus formation without affecting venous thrombosis and the coagulation factors VIII and IX. Consistently, PTPN22-deficient platelets exhibited enhanced platelet aggregation, granule secretion, calcium mobilization, lamellipodia formation, spreading, and clot retraction. Quantitative phosphoproteomic analysis revealed the significant difference of phosphodiesterase 5A (PDE5A) phosphorylation in PTPN22-deficient platelets compared with wild-type platelets after collagen-related peptide stimulation, which was confirmed by increased PDE5A phosphorylation (Ser92) in collagen-related peptide-treated PTPN22-deficient platelets, concomitant with reduced level and vasodilator-stimulated phosphoprotein phosphorylation (Ser157/239). In addition, PTPN22 interacted with phosphorylated PDE5A (Ser92) and dephosphorylated it in activated platelets. Moreover, purified PTPN22 but not the mutant form (C227S) possesses intrinsic serine phosphatase activity. Furthermore, inhibition of PTPN22 enhanced human platelet aggregation, spreading, clot retraction, and increased PDE5A phosphorylation (Ser92). In conclusion, our study shows a novel role of PTPN22 in platelet function and arterial thrombosis, identifying new potential targets for future prevention of thrombotic or cardiovascular diseases.

A proteomic surrogate for cardiovascular outcomes that is sensitive to multiple mechanisms of change in risk

A reliable, individualized, and dynamic surrogate of cardiovascular risk, synoptic for key biologic mechanisms, could shorten the path for drug development, enhance drug cost-effectiveness and improve patient outcomes. We used highly multiplexed proteomics to address these objectives, measuring about 5000 proteins in each of 32,130 archived plasma samples from 22,849 participants in nine clinical studies. We used machine learning to derive a 27-protein model predicting 4-year likelihood of myocardial infarction, stroke, heart failure, or death. The 27 proteins encompassed 10 biologic systems, and 12 were associated with relevant causal genetic traits. We independently validated results in 11,609 participants. Compared to a clinical model, the ratio of observed events in quintile 5 to quintile 1 was 6.7 for proteins versus 2.9 for the clinical model, AUCs (95% CI) were 0.73 (0.72 to 0.74) versus 0.64 (0.62 to 0.65), c-statistics were 0.71 (0.69 to 0.72) versus 0.62 (0.60 to 0.63), and the net reclassification index was +0.43. Adding the clinical model to the proteins only improved discrimination metrics by 0.01 to 0.02. Event rates in four predefined protein risk categories were 5.6, 11.2, 20.0, and 43.4% within 4 years; median time to event was 1.71 years. Protein predictions were directionally concordant with changed outcomes. Adverse risks were predicted for aging, approaching an event, anthracycline chemotherapy, diabetes, smoking, rheumatoid arthritis, cancer history, cardiovascular disease, high systolic blood pressure, and lipids. Reduced risks were predicted for weight loss and exenatide. The 27-protein model has potential as a "universal" surrogate end point for cardiovascular risk.

Interacting Effects of Sea Louse (Lepeophtheirus salmonis) Infection and Formalin-Killed Aeromonas salmonicida on Atlantic Salmon Skin Transcriptome

Lepeophtheirus salmonis (sea lice) and bacterial co-infection threatens wild and farmed Atlantic salmon performance and welfare. In the present study, pre-adult L. salmonis-infected and non-infected salmon were intraperitoneally injected with either formalin-killed Aeromonas salmonicida bacterin (ASAL) or phosphate-buffered saline (PBS). Dorsal skin samples from each injection/infection group (PBS/no lice, PBS/lice, ASAL/no lice, and ASAL/lice) were collected at 24 h post-injection and used for transcriptome profiling using a 44K salmonid microarray platform. Microarray results showed no clear inflammation gene expression signatures and revealed extensive gene repression effects by pre-adult lice (2,189 down and 345 up-regulated probes) in the PBS-injected salmon (PBS/lice vs. PBS/no lice), which involved basic cellular (e.g., RNA and protein metabolism) processes. Lice repressive effects were not observed within the group of ASAL-injected salmon (ASAL/lice vs. ASAL/no lice); on the contrary, the observed skin transcriptome changes –albeit of lesser magnitude (82 up and 1 down-regulated probes)– suggested the activation in key immune and wound healing processes (e.g., neutrophil degranulation, keratinocyte differentiation). The molecular skin response to ASAL was more intense in the lice-infected (ASAL/lice vs. PBS/lice; 272 up and 11 down-regulated probes) than in the non-infected fish (ASAL/no lice vs. PBS/no lice; 27 up-regulated probes). Regardless of lice infection, the skin’s response to ASAL was characterized by the putative activation of both antibacterial and wound healing pathways. The transcriptomic changes prompted by ASAL+lice co-stimulation (ASAL/lice vs. PBS/no lice; 1878 up and 3120 down-regulated probes) confirmed partial mitigation of lice repressive effects on fundamental cellular processes and the activation of pathways involved in innate (e.g., neutrophil degranulation) and adaptive immunity (e.g., antibody formation), as well as endothelial cell migration. The qPCR analyses evidenced immune-relevant genes co-stimulated by ASAL and lice in an additive (e.g., mbl2b, bcl6) and synergistic (e.g., hampa, il4r) manner. These results provided insight on the physiological response of the skin of L. salmonis-infected salmon 24 h after ASAL stimulation, which revealed immunostimulatory properties by the bacterin with potential applications in anti-lice treatments for aquaculture. As a simulated co-infection model, the present study also serves as a source of candidate gene biomarkers for sea lice and bacterial co-infection.

RASopathies and hemostatic abnormalities: key role of platelet dysfunction

Background

Bleeding anomalies have been reported in patients affected by Noonan syndrome. No study has been performed in patients with molecularly confirmed RASopathy. We aimed to characterize the frequency and types of bleeding disorders in patients with RASopathies and evaluate any significant association with laboratory findings.

Patients and methods

Forty-nine individuals (PTPN11, n = 27; SOS1, n = 7; RIT1, n = 3; SPRED1, n = 1; LZTR1, N = 3; RAF1, n = 2; BRAF, n = 4; MEK1, n = 1; MEK2, n = 1), and 49 age- and sex-matched controls were enrolled. The “Paediatric Bleeding Questionnaire Scoring Key” was administered to patients and families. Laboratory screening tests including clotting factors dosing, platelet count, Prothrombin Time and Partial Thromboplastin Time, were employed both in patients and controls to characterize the bleeding diathesis. A subgroup of 29/49 patients and 29/49 controls was also tested for platelet function.

Results

Regardless of the gene involved, pathological paediatric bleeding scores were recorded in 14/49 (28.5%) patients. Indeed, 7 were mutated in PTPN11, 3 in SOS1, 2 in RIT1, 1 in BRAF, and 1 in MEK1. Compared to patients with normal bleeding scores, those with pathologic bleeding score showed higher prevalence of splenomegaly (p = 0.006), prolonged aPTT (p = 0.04), lower levels of coagulation factor V (FV, p = 0.001), FVII (p = 0.003), FX (p = 0.0008) and FXIII (p = 0.002), higher vWAg (p = 0.04), and lower platelet sensitivity to Ristocetin (p = 0.001), arachidonic acid (AA) (p = 0.009) and collagen (p = 0.01). The presence of hematomas inversely correlated with factor V (p = 0.002), factor VII (p = 0.003), factor X (p = 0.002) and factor XIII (p = 0.004) levels, and directly correlated with platelet response to collagen (p = 0.02) and AA (p = 0.01). The presence of splenomegaly directly correlated with the presence of hematoma (p = 0.006), platelet response to Ristocetin (p = 0.04) and AA (p = 0.04), and inversely correlated with factor V levels (p = 0.03).

Conclusions

Patients with RASopathies and a bleeding tendency exhibit multiple laboratory abnormalities, including platelet-related disorders. Splenomegaly is frequently detected and might be a suggestive sign for qualitative platelet dysfunction. A comprehensive clinical assessment should be carried out at diagnosis, during the follow-up and before any surgical procedures. Since there is currently no consensus on management of bleeding complications, it is important that physicians closely monitor these patients.

Molecular features underlying differential SHP1/SHP2 binding of immune checkpoint receptors

A large number of inhibitory receptors recruit SHP1 and/or SHP2, tandem-SH2-containing phosphatases through phosphotyrosine-based motifs immunoreceptor tyrosine-based inhibitory motif (ITIM) and immunoreceptor tyrosine-based switch motif (ITSM). Despite the similarity, these receptors exhibit differential effector binding specificities, as exemplified by the immune checkpoint receptors PD-1 and BTLA, which preferentially recruit SHP2 and SHP1, respectively. The molecular basis by which structurally similar receptors discriminate SHP1 and SHP2 is unclear. Here, we provide evidence that human PD-1 and BTLA optimally bind to SHP1 and SHP2 via a bivalent, parallel mode that involves both SH2 domains of SHP1 or SHP2. PD-1 mainly uses its ITSM to prefer SHP2 over SHP1 via their C-terminal SH2 domains (cSH2): swapping SHP1-cSH2 with SHP2-cSH2 enabled PD-1:SHP1 association in T cells. In contrast, BTLA primarily utilizes its ITIM to prefer SHP1 over SHP2 via their N-terminal SH2 domains (nSH2). The ITIM of PD-1, however, appeared to be de-emphasized due to a glycine at pY+1 position. Substitution of this glycine with alanine, a residue conserved in BTLA and several SHP1-recruiting receptors, was sufficient to induce PD-1:SHP1 interaction in T cells. Finally, structural simulation and mutagenesis screening showed that SHP1 recruitment activity exhibits a bell-shaped dependence on the molecular volume of the pY+1 residue of ITIM. Collectively, we provide a molecular interpretation of the SHP1/SHP2-binding specificities of PD-1 and BTLA, with implications for the mechanisms of a large family of therapeutically relevant receptors.

SON inhibits megakaryocytic differentiation via repressing RUNX1 and the megakaryocytic gene expression program in acute megakaryoblastic leukemia

A high incidence of acute megakaryoblastic leukemia (AMKL) in Down syndrome patients implies that chromosome 21 genes have a pivotal role in AMKL development, but the functional contribution of individual genes remains elusive. Here, we report that SON, a chromosome 21-encoded DNA- and RNA-binding protein, inhibits megakaryocytic differentiation by suppressing RUNX1 and the megakaryocytic gene expression program. As megakaryocytic progenitors differentiate, SON expression is drastically reduced, with mature megakaryocytes having the lowest levels. In contrast, AMKL cells express an aberrantly high level of SON, and knockdown of SON induced the onset of megakaryocytic differentiation in AMKL cell lines. Genome-wide transcriptome analyses revealed that SON knockdown turns on the expression of pro-megakaryocytic genes while reducing erythroid gene expression. Mechanistically, SON represses RUNX1 expression by directly binding to the proximal promoter and two enhancer regions, the known +23 kb enhancer and the novel +139 kb enhancer, at the RUNX1 locus to suppress H3K4 methylation. In addition, SON represses the expression of the AP-1 complex subunits JUN, JUNB, and FOSB which are required for late megakaryocytic gene expression. Our findings define SON as a negative regulator of RUNX1 and megakaryocytic differentiation, implicating SON overexpression in impaired differentiation during AMKL development.

Fine-Tuning of Platelet Responses by Serine/Threonine Protein Kinases and Phosphatases-Just the Beginning

Comprehensive proteomic analyses of human and murine platelets established an extraordinary intracellular repertoire of signaling components, which control crucial functions. The spectrum of platelet serine/threonine protein kinases (more than 100) includes the AGC family (protein kinase A, G, C [PKA, PKG, PKC]), the mitogen-activated protein kinases (MAPKs), and others. PKA and PKG have multiple significantly overlapping substrates in human platelets, which possibly affect functions with clear "signaling nodes" of regulation by multiple protein kinases/phosphatases. Signaling nodes are intracellular Ca²⁺ stores, the contractile system (myosin light chains), and other signaling components such as G-proteins, protein kinases, and protein phosphatases. An example for this fine-tuning is the tyrosine kinase Syk, a crucial component of platelet activation, which is controlled by several serine/threonine and tyrosine protein kinases as well as phosphatases. Other protein kinases including PKA/PKG modulate protein phosphatase 2A, which may be a master regulator of MAPK signaling in human platelets. Protein kinases and in particular MAPKs are targeted by an increasing number of clinically used inhibitors. However, the precise regulation and fine-tuning of these protein kinases and their effects on other signaling components in platelets are only superficially understood-just the beginning. However, promising future approaches are in sight.

Platelet Src family kinases: A tale of reversible phosphorylation

Sarcoma (Src) family kinases (SFKs) have occupied a central place in platelet research for over 40 years. Discovered by virologists and oncologists as the proto proto-oncogene, Src tyrosine kinase spurred a phenomenal burst of research on reversible tyrosine phosphorylation and signal transduction. For a time, platelets were adopted as the model of choice for studying the biological functions of Src, owing to their ease of isolation, high Src expression, and lack of a nucleus, only to be abandoned due to challenges of culturing and manipulating using common molecular biology-based techniques. For platelet biologists, SFKs have remained an important area of investigation, initiating and amplifying signals from all major adhesion, activation, and inhibitory receptors, including the integrin αIIbβ3, the collagen receptor complex glycoprotein VI-Fc receptor γ-chain, the G protein-coupled ADP receptor P2Y₁₂ and the inhibitory receptors platelet endothelial cell adhesion molecule-1 and G6b-B. The vital roles of SFKs in platelets is highlighted by the severe phenotypes of null and gain-of-function mutations in SFKs in mice and humans, and effects of pharmacologic inhibitors on platelet activation, thrombosis, and hemostasis. The recent description of critical regulators of SFKs in platelets, namely, C-terminal Src kinase (Csk), Csk homologous kinase (Chk), the receptor-type protein-tyrosine phosphatase receptor type J (PTPRJ) helps explain some of the bleeding side effects of tyrosine kinase inhibitors and are novel therapeutic targets for regulating the thrombotic and hemostatic capacity of platelets. Recent findings from Chk, Csk, and PTPRJ knockout mouse models highlighted that SFKs are able to autoinhibit by phosphorylating their C-terminal tyrosine residues, providing fundamental insights into SFK autoregulation.

Oxidative Stress Triggers Defective Autophagy in Endothelial Cells: Role in Atherothrombosis Development

Atherothrombosis, a multifactorial and multistep artery disorder, represents one of the main causes of morbidity and mortality worldwide. The development and progression of atherothrombosis is closely associated with age, gender and a complex relationship between unhealthy lifestyle habits and several genetic risk factors. The imbalance between oxidative stress and antioxidant defenses is the main biological event leading to the development of a pro-oxidant phenotype, triggering cellular and molecular mechanisms associated with the atherothrombotic process. The pathogenesis of atherosclerosis and its late thrombotic complications involve multiple cellular events such as inflammation, endothelial dysfunction, proliferation of vascular smooth muscle cells (SMCs), extracellular matrix (ECM) alterations, and platelet activation, contributing to chronic pathological remodeling of the vascular wall, atheromatous plague formation, vascular stenosis, and eventually, thrombus growth and propagation. Emerging studies suggest that clotting activation and endothelial cell (EC) dysfunction play key roles in the pathogenesis of atherothrombosis. Furthermore, a growing body of evidence indicates that defective autophagy is closely linked to the overproduction of reactive oxygen species (ROS) which, in turn, are involved in the development and progression of atherosclerotic disease. This topic represents a large field of study aimed at identifying new potential therapeutic targets. In this review, we focus on the major role played by the autophagic pathway induced by oxidative stress in the modulation of EC dysfunction as a background to understand its potential role in the development of atherothrombosis.

Quercitrin inhibits platelet activation in arterial thrombosis

BACKGROUND

The ingestion of flavonoids has been reported to be associated with reduced cardiovascular disease risk. Quercitrin is a common flavonoid in nature, and it exhibits antioxidant properties. Although the process of thrombogenesis is intimately related to cardiovascular disease risk, it is unclear whether quercitrin plays a role in thrombogenesis.

PURPOSE

The aim of this study was to examine the antiplatelet effect of quercitrin in platelet activation.

METHODS

Platelet aggregation, granule secretion, calcium mobilization, and integrin activation were used to assess the antiplatelet activity of quercitrin. Antithrombotic effect was determined in mouse using ferric chloride (FeCl3)-induced arterial thrombus formation in vivo and thrombus formation on collagen-coated surfaces under arteriolar shear in vitro. Transection tail bleeding time was used to evaluate whether quercitrin inhibited primary hemostasis.

RESULTS

Quercitrin significantly impaired collagen-related peptide-induced platelet aggregation, granule secretion, reactive oxygen species generation, and intracellular calcium mobilization. Outside-in signaling of αIIbβ3 integrin was significantly inhibited by quercitrin in a concentration-dependent manner. The inhibitory effect of quercitrin resulted from inhibition of the glycoprotein VI-mediated platelet signal transduction during cell activation. Further, the antioxidant effect is derived from decreased phosphorylation of components of the TNF receptor-associated factor 4/p47phox/Hic5 axis signalosome. Oral administration of quercitrin efficiently blocked FeCl3-induced arterial thrombus formation in vivo and thrombus formation on collagen-coated surfaces under arteriolar shear in vitro, without prolonging bleeding time. Studies using a mouse model of ischemia/reperfusion-induced stroke indicated that treatment with quercitrin reduced the infarct volume in stroke.

CONCLUSIONS

Our results demonstrated that quercitrin could be an effective therapeutic agent for the treatment of thrombotic diseases.

The role of phospho-tyrosine signaling in platelet biology and hemostasis

Platelets are small enucleated cell fragments specialized in the control of hemostasis, but also playing a role in angiogenesis, inflammation and immunity. This plasticity demands a broad range of physiological processes. Platelet functions are mediated through a variety of receptors, the concerted action of which must be tightly regulated, in order to allow specific and timely responses to different stimuli. Protein phosphorylation is one of the main key regulatory mechanisms by which extracellular signals are conveyed. Despite the importance of platelets in health and disease, the molecular pathways underlying the activation of these cells are still under investigation. Here, we review current literature on signaling platelet biology and in particular emphasize the newly emerging role of phosphatases in these processes.

Total Platelet Transcriptomics and Its Network Analysis by RNA-Seq and miRNA-Seq and PCA Application in Essential Thrombocythaemia

Differentiating the aetiology of thrombocytosis is limited yet crucial in patients with essential thrombocythaemia (ET). MicroRNAs (miRNAs) regulate haematopoiesis and lineage commitment; aberrant expression of miRNAs plays an important role in myeloproliferative neoplasms. However, the miRNA profile has been poorly explored in ET patients compared to patients with reactive thrombocytosis (RT). A total of 9 samples, including 5 ET patient samples, 2 RT patient samples, and 2 healthy control samples, were analysed in this study. We produced 81.43 million reads from transcripts and 59.60 million reads from small RNAs. We generated a comprehensive miRNA-mRNA regulatory network and identified unique 14 miRNA expression patterns associated with ET. Among the 14 miRNAs, miR-1268a was downregulated in ET and showed an inverse correlation with its 8 putative target genes, including genes associated with thrombus formation and platelet activation (CDH6, EHD2, FUT1, KIF26A, LINC00346, PTPRN, SERF1A, and SLC6A9). Principal component analysis (PCA) showed ET and non-ET groups well clustered in space, suggesting each group had a distinctive expression pattern of mRNAs and miRNAs. These results suggest that the significant dysregulation of miR-1268a and its 8 target genes could be a unique expression of platelet mi-RNAs and miRNA/mRNA regulatory network in ET patients.

Catalytic dysregulation of SHP2 leading to Noonan syndromes affects platelet signaling and functions

Src homology 2 domain-containing phosphatase 2 (SHP2), encoded by the PTPN11 gene, is a ubiquitous protein tyrosine phosphatase that is a critical regulator of signal transduction. Germ line mutations in the PTPN11 gene responsible for catalytic gain or loss of function of SHP2 cause 2 disorders with multiple organ defects: Noonan syndrome (NS) and NS with multiple lentigines (NSML), respectively. Bleeding anomalies have been frequently reported in NS, but causes remain unclear. This study investigates platelet activation in patients with NS and NSML and in 2 mouse models carrying PTPN11 mutations responsible for these 2 syndromes. Platelets from NS mice and patients displayed a significant reduction in aggregation induced by low concentrations of GPVI and CLEC-2 agonists and a decrease in thrombus growth on a collagen surface under arterial shear stress. This was associated with deficiencies in GPVI and αIIbβ3 integrin signaling, platelet secretion, and thromboxane A2 generation. Similarly, arterial thrombus formation was significantly reduced in response to a local carotid injury in NS mice, associated with a significant increase in tail bleeding time. In contrast, NSML mouse platelets exhibited increased platelet activation after GPVI and CLEC-2 stimulation and enhanced platelet thrombotic phenotype on collagen matrix under shear stress. Blood samples from NSML patients also showed a shear stress-dependent elevation of platelet responses on collagen matrix. This study brings new insights into the understanding of SHP2 function in platelets, points to new thrombopathies linked to platelet signaling defects, and provides important information for the medical care of patients with NS in situations involving risk of bleeding.

The Cell Cycle Checkpoint System MAST(L)-ENSA/ARPP19-PP2A is Targeted by cAMP/PKA and cGMP/PKG in Anucleate Human Platelets

The cell cycle is controlled by microtubule-associated serine/threonine kinase-like (MASTL), which phosphorylates the cAMP-regulated phosphoproteins 19 (ARPP19) at S62 and 19e/α-endosulfine (ENSA) at S67and converts them into protein phosphatase 2A (PP2A) inhibitors. Based on initial proteomic data, we hypothesized that the MASTL-ENSA/ARPP19-PP2A pathway, unknown until now in platelets, is regulated and functional in these anucleate cells. We detected ENSA, ARPP19 and various PP2A subunits (including seven different PP2A B-subunits) in proteomic studies of human platelets. ENSA-S109/ARPP19–S104 were efficiently phosphorylated in platelets treated with cAMP- (iloprost) and cGMP-elevating (NO donors/riociguat) agents. ENSA-S67/ARPP19-S62 phosphorylations increased following PP2A inhibition by okadaic acid (OA) in intact and lysed platelets indicating the presence of MASTL or a related protein kinase in human platelets. These data were validated with recombinant ENSA/ARPP19 and phospho-mutants using recombinant MASTL, protein kinase A and G. Both ARPP19 phosphorylation sites S62/S104 were dephosphorylated by platelet PP2A, but only S62-phosphorylated ARPP19 acted as PP2A inhibitor. Low-dose OA treatment of platelets caused PP2A inhibition, diminished thrombin-stimulated platelet aggregation and increased phosphorylation of distinct sites of VASP, Akt, p38 and ERK1/2 MAP kinases. In summary, our data establish the entire MASTL(like)–ENSA/ARPP19–PP2A pathway in human platelets and important interactions with the PKA, MAPK and PI3K/Akt systems.

Targeting Tyrosine Phosphatases by 3-Bromopyruvate Overcomes Hyperactivation of Platelets from Gastrointestinal Cancer Patients

Venous thromboembolism (VTE) is one of the most common causes of cancer related mortality. It has been speculated that hypercoagulation in cancer patients is triggered by direct or indirect contact of platelets with tumor cells, however the underlying molecular mechanisms involved are currently unknown. Unraveling these mechanisms may provide potential avenues for preventing platelet-tumor cell aggregation. Here, we investigated the role of protein tyrosine phosphatases in the functionality of platelets in both healthy individuals and patients with gastrointestinal cancer, and determined their use as a target to inhibit platelet hyperactivity. This is the first study to demonstrate that platelet agonists selectively activate low molecular weight protein tyrosine phosphatase (LMWPTP) and PTP1B, resulting in activation of Src, a tyrosine kinase known to contribute to several platelet functions. Furthermore, we demonstrate that these phosphatases are a target for 3-bromopyruvate (3-BP), a lactic acid analog currently investigated for its use in the treatment of various metabolic tumors. Our data indicate that 3-BP reduces Src activity, platelet aggregation, expression of platelet activation makers and platelet-tumor cell interaction. Thus, in addition to its anti-carcinogenic effects, 3-BP may also be effective in preventing platelet-tumor cell aggregationin cancer patients and therefore may reduce cancer mortality by limiting VTE in patients.

Protein tyrosine phosphatases in cardiac physiology and pathophysiology

More than any other organ, the heart is particularly sensitive to gene expression deregulation, often leading in the long run to impaired contractile performances and excessive fibrosis deposition progressing to heart failure. Recent investigations provide evidences that the protein phosphatases (PPs), as their counterpart protein kinases, are important regulators of cardiac physiology and development. Two main groups, the protein serine/threonine phosphatases and the protein tyrosine phosphatases (PTPs), constitute the PPs family. Here, we provide an overview of the role of PTP subfamily in the development of the heart and in cardiac pathophysiology. Based on recent in silico studies, we highlight the importance of PTPs as therapeutic targets for the development of new drugs to restore PTPs signaling in the early and late events of heart failure.

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

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

Negative regulators of platelet activation and adhesion

Platelets are small anucleated cells that constantly patrol the cardiovascular system to preserve its integrity and prevent excessive blood loss where the vessel lining is breached. Their key challenge is to form a hemostatic plug under conditions of high shear forces. To do so, platelets have evolved a molecular machinery that enables them to sense trace amounts of signals at the site of damage and to rapidly shift from a non-adhesive to a pro-adhesive state. However, this highly efficient molecular machinery can also lead to unintended platelet activation and cause clinical complications such as thrombocytopenia and thrombosis. Thus, several checkpoints are in place to tightly control platelet activation and adhesiveness in space and time. In this review, we will discuss select negative regulators of platelet activation, which are critical to maintain patrolling platelets in a quiescent, non-adhesive state and/or to limit platelet adhesion to sites of injury.

Platelet populations and priming in hematological diseases

In healthy subjects and patients with hematological diseases, platelet populations can be distinguished with different response spectra in hemostatic and vascular processes. These populations partly overlap, and are less distinct than those of leukocytes. The platelet heterogeneity is linked to structural properties, and is enforced by inequalities in the environment. Contributing factors are variability between megakaryocytes, platelet ageing, and positive or negative priming of platelets during their time in circulation. Within a hemostatic plug or thrombus, platelet heterogeneity is enhanced by unequal exposure to agonists, with populations of contracted platelets in the thrombus core, discoid platelets at the thrombus surface, patches of ballooned and procoagulant platelets forming thrombin, and coated platelets binding fibrin. Several pathophysiological hematological conditions can positively or negatively prime the responsiveness of platelet populations. As a consequence, in vivo and in vitro markers of platelet activation can differ in thrombotic and hematological disorders.

Platelet signaling: a complex interplay between inhibitory and activatory networks

The role of platelets in hemostasis and thrombosis is dependent on a complex balance of activatory and inhibitory signaling pathways. Inhibitory signals released from the healthy vasculature suppress platelet activation in the absence of platelet receptor agonists. Activatory signals present at a site of injury initiate platelet activation and thrombus formation; subsequently, endogenous negative signaling regulators dampen activatory signals to control thrombus growth. Understanding the complex interplay between activatory and inhibitory signaling networks is an emerging challenge in the study of platelet biology, and necessitates a systematic approach to utilize experimental data effectively. In this review, we will explore the key points of platelet regulation and signaling that maintain platelets in a resting state, mediate activation to elicit thrombus formation, or provide negative feedback. Platelet signaling will be described in terms of key signaling molecules that are common to the pathways activated by platelet agonists and can be described as regulatory nodes for both positive and negative regulators.

Regulation of Early Steps of GPVI Signal Transduction by Phosphatases: A Systems Biology Approach

We present a data-driven mathematical model of a key initiating step in platelet activation, a central process in the prevention of bleeding following Injury. In vascular disease, this process is activated inappropriately and causes thrombosis, heart attacks and stroke. The collagen receptor GPVI is the primary trigger for platelet activation at sites of injury. Understanding the complex molecular mechanisms initiated by this receptor is important for development of more effective antithrombotic medicines. In this work we developed a series of nonlinear ordinary differential equation models that are direct representations of biological hypotheses surrounding the initial steps in GPVI-stimulated signal transduction. At each stage model simulations were compared to our own quantitative, high-temporal experimental data that guides further experimental design, data collection and model refinement. Much is known about the linear forward reactions within platelet signalling pathways but knowledge of the roles of putative reverse reactions are poorly understood. An initial model, that includes a simple constitutively active phosphatase, was unable to explain experimental data. Model revisions, incorporating a complex pathway of interactions (and specifically the phosphatase TULA-2), provided a good description of the experimental data both based on observations of phosphorylation in samples from one donor and in those of a wider population. Our model was used to investigate the levels of proteins involved in regulating the pathway and the effect of low GPVI levels that have been associated with disease. Results indicate a clear separation in healthy and GPVI deficient states in respect of the signalling cascade dynamics associated with Syk tyrosine phosphorylation and activation. Our approach reveals the central importance of this negative feedback pathway that results in the temporal regulation of a specific class of protein tyrosine phosphatases in controlling the rate, and therefore extent, of GPVI-stimulated platelet activation.

Counteracting Protein Kinase Activity in the Heart: The Multiple Roles of Protein Phosphatases

Decades of cardiovascular research have shown that variable and flexible levels of protein phosphorylation are necessary to maintain cardiac function. A delicate balance between phosphorylated and dephosphorylated states of proteins is guaranteed by a complex interplay of protein kinases (PKs) and phosphatases. Serine/threonine phosphatases, in particular members of the protein phosphatase (PP) family govern dephosphorylation of the majority of these cardiac proteins. Recent findings have however shown that PPs do not only dephosphorylate previously phosphorylated proteins as a passive control mechanism but are capable to actively control PK activity via different direct and indirect signaling pathways. These control mechanisms can take place on (epi-)genetic, (post-)transcriptional, and (post-)translational levels. In addition PPs themselves are targets of a plethora of proteinaceous interaction partner regulating their endogenous activity, thus adding another level of complexity and feedback control toward this system. Finally, novel approaches are underway to achieve spatiotemporal pharmacologic control of PPs which in turn can be used to fine-tune misleaded PK activity in heart disease. Taken together, this review comprehensively summarizes the major aspects of PP-mediated PK regulation and discusses the subsequent consequences of deregulated PP activity for cardiovascular diseases in depth.

Perspective: Tyrosine phosphatases as novel targets for antiplatelet therapy

Arterial thrombosis is the primary cause of most cases of myocardial infarction and stroke, the leading causes of death in the developed world. Platelets, highly specialized cells of the circulatory system, are key contributors to thrombotic events. Antiplatelet drugs, which prevent platelets from aggregating, have been very effective in reducing the mortality and morbidity of these conditions. However, approved antiplatelet therapies have adverse side effects, most notably the increased risk of bleeding. Moreover, there remains a considerable incidence of arterial thrombosis in a subset of patients receiving currently available drugs. Thus, there is a pressing medical need for novel antiplatelet agents with a more favorable safety profile and less patient resistance. The discovery of novel antiplatelet targets is the matter of intense ongoing research. Recent findings demonstrate the potential of targeting key signaling molecules, including kinases and phosphatases, to prevent platelet activation and aggregation. Here, we offer perspectives to targeting members of the protein tyrosine phosphatase (PTP) superfamily, a major class of enzymes in signal transduction. We give an overview of previously identified PTPs in platelet signaling, and discuss their potential as antiplatelet drug targets. We also introduce VHR (DUSP3), a PTP that we recently identified as a major player in platelet biology and thrombosis. We review our data on genetic deletion as well as pharmacological inhibition of VHR, providing proof-of-principle for a novel and potentially safer VHR-based antiplatelet therapy.

Human platelet IgG Fc receptor FcγRIIA in immunity and thrombosis

Beyond their prominent role in hemostasis and thrombosis, platelets are increasingly recognized as having immunologic functions. Supporting this, human platelets express FcγRIIA (CD32a), a low-affinity Fc receptor (FcR) for the constant region of IgG that recognizes immune complexes (ICs) and IgG-opsonized cells with high avidity. In leukocytes, FcγRIIA engagement initiates strong effector functions that are key for immune and inflammatory responses, including cytokine release, antibody-dependent cell-mediated killing of pathogens, and internalization of ICs. However, the physiologic relevance of platelet-expressed FcγRIIA has received little attention in previous reviews on FcRs. This article summarizes and discusses the available information on human platelet FcγRIIA. The importance of this receptor in heparin-induced thrombocytopenia, a prothrombotic adverse drug effect, is well documented. However, studies demonstrating platelet activation by IgG-opsonized bacteria point to the physiologic relevance of platelet FcγRIIA in immunity. In this context, platelet activation and secretion may facilitate both a direct antimicrobial function of platelets and crosstalk with other immune cells. Additionally, a role for platelet FcγRIIA in IgG-independent hemostasis and physiologic thrombosis, by means of amplifying integrin αII b β3 outside-in signaling, has also been proposed. Nonetheless, the thrombotic complications found in some infective and autoimmune diseases may result from unbalanced FcγRIIA-mediated platelet aggregation. Moreover, FcγRIIA is not expressed in mice, and thrombocytopenia and/or thrombotic events found after drug administration can only be recapitulated by the use of human FcγRIIA-transgenic mice. Altogether, the available data support a functional role for platelet FcγRIIA in health and disease, and emphasize the need for further investigation of this receptor.

Hyperreactivity of junctional adhesion molecule A-deficient platelets accelerates atherosclerosis in hyperlipidemic mice

RATIONALE

Besides their essential role in hemostasis, platelets also have functions in inflammation. In platelets, junctional adhesion molecule (JAM)-A was previously identified as an inhibitor of integrin αIIbβ3-mediated outside-in signaling and its genetic knockdown resulted in hyperreactivity.

OBJECTIVE

This gain-of-function was specifically exploited to investigate the role of platelet hyperreactivity in plaque development.

METHODS AND RESULTS

JAM-A-deficient platelets showed increased aggregation and cellular and sarcoma tyrosine-protein kinase activation. On αIIbβ3 ligation, JAM-A was shown to be dephosphorylated, which could be prevented by protein tyrosine phosphatase nonreceptor type 1 inhibition. Mice with or without platelet-specific (tr)JAM-A-deficiency in an apolipoprotein e (apoe(-/-)) background were fed a high-fat diet. After ≤12 weeks of diet, trJAM-A(-/-)apoe-/- mice showed increased aortic plaque formation when compared with trJAM-A(+/+) apoe(-/-) controls, and these differences were most evident at early time points. At 2 weeks, the plaques of the trJAM-A(-/-) apoe(-/-) animals revealed increased macrophage, T cell, and smooth muscle cell content. Interestingly, plasma levels of chemokines CC chemokine ligand 5 and CXC-chemokine ligand 4 were increased in the trJAM-A(-/-) apoe(-/-)mice, and JAM-A-deficient platelets showed increased binding to monocytes and neutrophils. Whole-blood perfusion experiments and intravital microscopy revealed increased recruitment of platelets and monocytes to the inflamed endothelium in blood of trJAM-A(-/-) apoe(-/-)mice. Notably, these proinflammatory effects of JAM-A-deficient platelets could be abolished by the inhibition of αIIbβ3 signaling in vitro.

CONCLUSIONS

Deletion of JAM-A causes a gain-of-function in platelets, with lower activation thresholds and increased inflammatory activities. This leads to an increase of plaque formation, particularly in early stages of the disease.

Src family kinases: at the forefront of platelet activation

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

Systems biology of platelet-vessel wall interactions

Platelets are small, anucleated cells that participate in primary hemostasis by forming a hemostatic plug at the site of a blood vessel's breach, preventing blood loss. However, hemostatic events can lead to excessive thrombosis, resulting in life-threatening strokes, emboli, or infarction. Development of multi-scale models coupling processes at several scales and running predictive model simulations on powerful computer clusters can help interdisciplinary groups of researchers to suggest and test new patient-specific treatment strategies.