Novel targets for antithrombotic drug discovery

PEAR1 polymorphisms as a prognostic factor in hemostasis and cardiovascular diseases

Platelet Endothelial Aggregation Receptor (PEAR1), as a platelet receptor, plays a vital role in hemostasis. This receptor, by its extracellular part, causes platelet adhesion and consequently initiates platelet aggregation. Dysfunction of PEAR1 can disrupt platelet aggregation in patients with cardiovascular diseases (CVDs). The content used in this paper has been taken from English language articles (2005-2020) retrieved from Pubmed database and Google scholar search engine using "Cardiovascular Disease", "PEAR1", "Polymorphism", and "Platelet Aggregation" keywords. Some PEAR1 polymorphisms can disrupt homeostasis and interfere with the function mechanism of cardiac drugs. Since polymorphisms in this gene affect platelet function and the platelet aggregation process, PEAR1 could be further studied in the future as an essential factor in controlling the treatment process of patients with cardiovascular diseases. PEAR1 polymorphisms through disruption of the platelet aggregation process can be a risk factor in patients with CVDs. Therefore, controlling patients through genetic testing and the evaluation of PEAR1 polymorphisms can help improve the treatment process of patients. According to the studies on the PEAR1 gene and the effect of different polymorphisms on some crucial issues in CVDs patients (changes in platelet activity), it is clear that if there is a significant relationship between polymorphisms and CVDs, they can be used as prognostic and diagnostic markers. This study aims to evaluate the prognosis and drug treatment of the PEAR1 gene in CVDs patients.

CD84 cell surface signaling molecule: An emerging biomarker and target for cancer and autoimmune disorders

CD84 (SLAMF5) is a member of the SLAM family of cell-surface immunoreceptors. Broadly expressed on most immune cell subsets, CD84 functions as a homophilic adhesion molecule, whose signaling can activate or inhibit leukocyte function depending on the cell type and its stage of activation or differentiation. CD84-mediated signaling regulates diverse immunological processes, including T cell cytokine secretion, natural killer cell cytotoxicity, monocyte activation, autophagy, cognate T:B interactions, and B cell tolerance at the germinal center checkpoint. Recently, alterations in CD84 have been related to autoimmune and lymphoproliferative disorders. Specific allelic variations in CD84 are associated with autoimmune diseases such as systemic lupus erythematosus and rheumatoid arthritis. In chronic lymphocytic leukemia, CD84 mediates intrinsic and stroma-induced survival of malignant cells. In this review, we describe our current understanding of the structure and function of CD84 and its potential role as a therapeutic target and biomarker in inflammatory autoimmune disorders and cancer.

Effect of PEAR1 Genetic Variants on 1-Year Outcomes in Chinese Patients with Acute Myocardial Infarction After Percutaneous Coronary Intervention

AIMS

Platelet endothelial aggregation receptor-1 (PEAR1) is a platelet transmembrane protein that plays an important role on platelet aggregation. The aim of this study was to investigate whether PEAR1 genetic variations are associated with 1-year outcomes in Chinese patients with acute myocardial infarction after percutaneous coronary intervention.

METHODS

A total of 647 consecutive Chinese patients with acute myocardial infarction that underwent percutaneous coronary intervention and that were exposed to standard dual antiplatelet therapy with aspirin and clopidogrel were enrolled in this study. Six single nucleotide polymorphisms of PEAR1 were detected using the ligase detection reaction method. The follow-up period was 12 months.

RESULTS

Overall, 66 (10.2%) adverse ischemic events occurred. Multivariate Cox regression analysis showed that carriage of the PEAR1 rs56260937 minor allele was an independent predictor of revascularization events (OR=2.15, 95% CI 1.12-4.15, p=0.022) after adjusting for confounding factors.

CONCLUSIONS

Genetic testing for PEAR1 variants can be helpful in predicting adverse ischemic events in Chinese patients with acute myocardial infarction after percutaneous coronary intervention.

Association of PEAR1 rs12041331 polymorphism and pharmacodynamics of ticagrelor in healthy Chinese volunteers

1. Genetic polymorphisms in platelet endothelial aggregation receptor 1 (PEAR1) were associated with responsiveness to aspirin and P2Y12 receptor antagonists. This study aimed to investigate whether PEAR1 polymorphism is associated with ticagrelor pharmacodynamics in healthy Chinese subjects. 2. The in vitro inhibition of platelet aggregation (IPA) was evaluated before and after ticagrelor incubated with platelet-rich plasma from 196 healthy Chinese male subjects. Eight polymorphisms at PEAR1 locus were genotyped. Eighteen volunteers (six in each rs12041331 genotype group) were randomly selected. After a single oral 180 mg dose of ticagrelor, plasma levels of ticagrelor and the active metabolite AR-C124910XX were measured and pharmacodynamics parameters including IPA and VASP-platelet reactivity index (PRI) were assessed. 3. No significant difference in ticagrelor pharmacokinetics among rs12041331 genotype was observed. As compared with rs12041331 G allele carriers, AA homozygotes exhibited increased IPA after 15 μM ticagrelor incubation (p < 0.01), increased area under the time-effect curve of IPA and lower PRI at 2 h after ticagrelor administration (p < 0.05, respectively). Rs4661012 GG homozygotes showed increased IPA after 50 μM ticagrelor incubation as compared to T allele carriers (p < 0.01). 4. PEAR1 polymorphism may influence ticagrelor pharmacodynamics in healthy Chinese subjects.

PEAR1 gene polymorphism in a Chinese pedigree with pulmonary thromboembolism

To explore the correlation between platelet endothelial aggregation receptor-1 (PEAR1) genetic polymorphism and pulmonary thromboembolism (PTE).Variant loci of the PEAR1 gene were screened in a PTE pedigree, followed by verification using Sanger sequencing. These polymorphic loci were validated in 101 PTE patients and 132 matched normal patients using MassARRAY single nucleotide polymorphism (SNP) genotyping methods. The frequency differences between the allele and genotypes were compared using the Hardy-Weinberg equilibrium test and Chi-square test. The correlation between the PEAR1 gene SNP and PTE was analyzed by comparing the between-group variance differences using the χ test.Three SNPs were identified in the PTE pedigree. There was a heterozygous transition of T>C in rs1952294, and a transition of C>T in rs778026543 in 2 members in the pedigree; however, the rs778026543 was not identified in the 101 PTE patients and 132 healthy controls. The genotype and allele frequencies of rs822442 did not differ significantly between PTE patients and healthy controls (P > 0.05). The variance difference at rs778026543 between pedigree members and healthy controls was significant (P < 0.001), supporting its potential heredity.The PEAR1 polymorphism, rs778026543, but not rs1952294 and rs822442, may be a susceptibility SNP for PTE.

Association of PEAR1 genetic variants with platelet reactivity in response to dual antiplatelet therapy with aspirin and clopidogrel in the Chinese patient population after percutaneous coronary intervention

INTRODUCTION

Platelet Endothelial Aggregation Receptor-1 (PEAR1) is a recently reported platelet transmembrane protein which plays an important role in platelet aggregation. The aim of this study was to investigate whether PEAR1 genetic variations were associated with platelet reactivity as assessed by adenosine diphosphate(ADP)-induced platelet aggregation in Chinese patients treated with aspirin and clopidogrel.

METHODS

Patients with coronary heart disease (CHD) who underwent percutaneous coronary intervention (PCI) were enrolled in the study. All patients were on dual antiplatelet therapy with aspirin and clopidogrel. ADP-induced platelet aggregation was measured by thromboelastography and defined as percent inhibition of platelet aggregation (IPA). Patients (n=204) with IPA <30% were identified as high on-treatment platelet reactivity (HPR). Patients (n=201) with IPA >70% were identified as low on-treatment platelet reactivity (LPR). Sixteen single nucleotide polymorphisms (SNPs) of PEAR1 were determined by a method of improved multiple ligase detection reaction.

RESULTS

Among the 16 SNPs examined by univariate analysis, 5 SNPs were significantly associated with ADP-induced platelet aggregation. Minor allele C at rs11264580 (p=0.033), minor allele G at rs2644592 (p=0.048), minor allele T at rs3737224 (p=0.033) and minor allele T at rs41273215 (p=0.025) were strongly associated with HPR, whereas homozygous TT genotype at rs57731889 (p=0.009) was associated with LPR. Multivariate logistic regression analysis further revealed that the minor allele T at rs41273215 (p=0.038) was an independent predictor of HPR and the homozygous TT genotype at rs57731889 (p=0.003) was an independent predictor of LPR.

CONCLUSIONS

PEAR1 genetic variations were strongly associated with ADP-induced platelet aggregation in Chinese patients with CHD treated with aspirin and clopidogrel. These genetic variations may contribute to the variability in platelet function. The utility of PEAR1 genetic variants in the assessment and prediction of cardiovascular risk warrants further investigation.

Mice lacking the SLAM family member CD84 display unaltered platelet function in hemostasis and thrombosis

Background

Platelets are anuclear cell fragments derived from bone marrow megakaryocytes that safeguard vascular integrity by forming thrombi at sites of vascular injury. Although the early events of thrombus formation—platelet adhesion and aggregation—have been intensively studied, less is known about the mechanisms and receptors that stabilize platelet-platelet interactions once a thrombus has formed. One receptor that has been implicated in this process is the signaling lymphocyte activation molecule (SLAM) family member CD84, which can undergo homophilic interactions and becomes phosphorylated upon platelet aggregation.

Objective

The role of CD84 in platelet physiology and thrombus formation was investigated in CD84-deficient mice.

Methods and Results

We generated CD84-deficient mice and analyzed their platelets in vitro and in vivo. Cd84^−/− platelets exhibited normal activation and aggregation responses to classical platelet agonists. Furthermore, CD84 deficiency did not affect integrin-mediated clot retraction and spreading of activated platelets on fibrinogen. Notably, also the formation of stable three-dimensional thrombi on collagen-coated surfaces under flow ex vivo was unaltered in the blood of Cd84^−/− mice. In vivo, Cd84^−/− mice exhibited unaltered hemostatic function and arterial thrombus formation.

Conclusion

These results show that CD84 is dispensable for thrombus formation and stabilization, indicating that its deficiency may be functionally compensated by other receptors or that it may be important for platelet functions different from platelet-platelet interactions.

The SLAM family member CD84 is regulated by ADAM10 and calpain in platelets

BACKGROUND AND OBJECTIVE

Ectodomain shedding is a major mechanism to modulate platelet receptor signaling and to downregulate platelet reactivity. Proteins of the a disintegrin and metalloproteinase (ADAM) family are implicated in the shedding of various platelet receptors. The signaling lymphocyte activation molecule (SLAM) family receptor CD84 is highly expressed in platelets and immune cells, but its role in platelet physiology is not well explored. Because of its ability to form homodimers, CD84 has been suggested to mediate contact-dependent signaling and contribute to thrombus stability. However, nothing is known about the cellular regulation of CD84.

METHODS

We studied the regulation of CD84 in murine platelets by biochemical approaches and use of three different genetically modified mouse lines. Regulation of CD84 in human platelets was studied using inhibitors and biochemical approaches.

RESULTS

We show that CD84 is cleaved from the surface of human and murine platelets in response to different shedding inducing agents and platelet receptor agonists. CD84 downregulation occurs through ectodomain-shedding and intracellular cleavage. Studies in transgenic mice identified ADAM10 as the principal sheddase responsible for CD84 cleavage, whereas ADAM17 was dispensable. Western blot analyses revealed calpain-mediated intracellular cleavage of the CD84 C-terminus, occurring simultaneously with, but independently of, ectodomain shedding. Furthermore, analysis of plasma and serum samples from transgenic mice demonstrated that CD84 is constitutively shed from the platelet surface by ADAM10 in vivo.

CONCLUSIONS

These results reveal a dual regulation mechanism for platelet CD84 by simultaneous extra- and intracellular cleavage that may modulate platelet-platelet and platelet-immune cell interactions.

Platelet receptors and signaling in the dynamics of thrombus formation

Hemostasis and pathological thrombus formation are dynamic processes that require a co-ordinated series of events involving platelet membrane receptors, bidirectional intracellular signals, and release of platelet proteins and inflammatory substances. This review aims to summarize current knowledge in the key steps in the dynamics of thrombus formation, with special emphasis on the crucial participation of platelet receptors and signaling in this process. Initial tethering and firm adhesion of platelets to the exposed subendothelium is mediated by glycoprotein (GP) Ib/IX/V complex and collagen receptors, GP VI and alpha(2)beta(1) integrin, in the platelet surface, and by VWF and fibrillar collagen in the vascular site. Interactions between these elements are largely influenced by flow and trigger signaling events that reinforce adhesion and promote platelet activation. Thereafter, soluble agonists, ADP, thrombin, TxA(2), produced/released at the site of vascular injury act in autocrine and paracrine mode to amplify platelet activation and to recruit circulating platelets to the developing thrombus. Specific interactions of these agonists with their G-protein coupled receptors generate inside-out signaling leading to conformational activation of integrins, in particular alpha(IIb)beta(3), increasing their ligand affinity. Binding of alpha(IIb)beta(3) to its ligands, mainly fibrinogen, supports processes such as clot retraction and platelet aggregation. Stabilization of thrombi is supported by the late wave of signaling events promoted by close contact between aggregated platelets. The best known contact-dependent signaling is outside-in signaling through alphaIb beta(3), but new ones are being clarified such as those mediated by interaction of Eph receptors with ephrins, or by Sema 4D and Gas-6 binding to their receptors. Finally, newly identified mechanisms appear to control thrombus growth, including back-shifting of activated integrins and actuation of compensatory molecules such as ESAM or PECAM-1. The expanding knowledge of thrombotic disease is expected to translate into the development of new drugs to help management and prevention of thrombosis.

Future innovations in anti-platelet therapies

Platelets have long been recognized to be of central importance in haemostasis, but their participation in pathological conditions such as thrombosis, atherosclerosis and inflammation is now also well established. The platelet has therefore become a key target in therapies to combat cardiovascular disease. Anti-platelet therapies are used widely, but current approaches lack efficacy in a proportion of patients, and are associated with side effects including problem bleeding. In the last decade, substantial progress has been made in understanding the regulation of platelet function, including the characterization of new ligands, platelet-specific receptors and cell signalling pathways. It is anticipated this progress will impact positively on the future innovations towards more effective and safer anti-platelet agents. In this review, the mechanisms of platelet regulation and current anti-platelet therapies are introduced, and strong, and some more speculative, potential candidate target molecules for future anti-platelet drug development are discussed.

A novel variant in the platelet endothelial aggregation receptor-1 gene is associated with increased platelet aggregability

OBJECTIVE

Platelet endothelial aggregation receptor-1 (PEAR1) is a recently identified platelet transmembrane protein that becomes activated by platelet contact. We looked for novel genetic variants in PEAR1 and studied their association with agonist-induced native platelet aggregation and with the inhibitory effect of aspirin on platelets.

METHODS AND RESULTS

We genotyped PEAR1 for 10 single nucleotide polymorphisms (SNPs), selected for optimal gene coverage at a density of 4 kb, in 1486 apparently healthy individuals from two generations of families with premature CAD. Subjects had a mean age of 45 years; 62% were white and 38% black. Platelet aggregation to collagen, epinephrine, and ADP was measured in platelet rich plasma, at baseline and after 2 weeks of aspirin (ASA, 81 mg/d), and genotype-phenotype associations were examined separately by ethnicity using multivariable generalized linear models adjusted for covariates. The C allele of SNP rs2768759 [A/C], located in the promoter region of the gene, was common in whites and uncommon in blacks (allele frequency 70.2% versus 17.7%). The C allele was generally associated in both ethnic groups with increased aggregation of native platelets to each agonist. After ASA, the associations were stronger and more consistent and remained significant when post-ASA aggregation was adjusted for baseline aggregation, consistent with a relationship between the C allele and reduced platelet responsiveness to ASA. The PEAR1 SNP explained up to 6.9% of the locus specific genetic variance in blacks and up to 2.5% of the genetic variance in whites after ASA.

CONCLUSIONS

PEAR1 appears to play an important role in agonist-induced platelet aggregation and in the response to ASA in both whites and blacks.

The role of SLAM family receptors in immune cell signaling

The signaling lymphocyte-activating molecule (SLAM) family immunoreceptors are expressed in a wide array of immune cells, including both T and B lymphocytes. By virtue of their ability to transduce tyrosine phosphorylation signals through the so-called ITSM (immunoreceptor tyrosine-based switch motif) sequences, they play an important part in regulating both innate and adaptive immune responses. The critical role of the SLAM immunoreceptors in mediating normal immune reactions was highlighted in recent findings that SAP, a SLAM-associated protein, modulates the activities of various immune cells through interactions with different members of the SLAM family expressed in these cells. Importantly, mutations or deletions of the sap gene in humans result in the X-linked lymphoproliferative syndrome. In this review, we summarize current knowledge and survey the latest developments in signal transduction events triggered by the activation of SLAM family receptors in different cell types.

Platelet adhesion receptors and their ligands in mouse models of thrombosis

Platelet adhesion and aggregation at sites of vascular injury are two key events in hemostasis and thrombosis. Because of exciting advances in genetic engineering, the mouse has become an important and frequently used model to unravel the molecular mechanisms underlying the multistep process leading to the formation of a stable platelet plug. In gene-targeted mice, the crucial importance of platelet adhesion receptors such as glycoprotein Ib alpha or the alphaIIb beta3 integrin has been confirmed and further clarified. Their absence leads to highly impaired thrombus formation, independent of the model used to induce vascular injury. In contrast, the relative contribution of other receptors, such as glycoprotein VI, or of various platelet ligands may be regulated by the severity of injury, the type of vessel injured, and the signaling pathways that are generated. Murine models have also helped improve understanding of the second wave of events that leads to stabilization of the platelet aggregate. Despite the current limitations due to lack of standardization and the virtual absence of thrombosis models in diseased vessels, there is no doubt that the mouse will play a key role in the discovery and characterization of the next generation of antithrombotic agents. This review focuses on key findings about the molecular mechanisms supporting hemostasis and thrombosis that have been obtained with genetically engineered mouse models deficient in various platelet adhesion receptors and ligands. Combination of these models with sophisticated methods allowing direct visualization of platelet-vessel wall interactions after injury greatly contributed to recent advances in the field.