Evidence that the purinergic receptor P2Y12 potentiates platelet shape change by a Rho kinase-dependent mechanism

Thrombus remodelling by reversible and irreversible P2Y12 inhibitors

Pharmacological inhibition of the platelet ADP-receptor P2Y₁₂ is a cornerstone in the prevention of atherothrombotic events in adult patients with acute coronary syndrome (ACS). Thienopyridines such as clopidogrel and prasugrel exert their antithrombotic effect by means of active metabolites that irreversibly inhibit P2Y₁₂. Due to the short half-life of these metabolites, a subpopulation of ADP-responsive platelets will form in between dosing. With increased platelet turnover rate or poor patient compliance, the fraction of ADP-responsive platelets will increase, potentially increasing the risk for new thrombotic events. In contrast, the reversible P2Y₁₂ inhibition produced by direct-acting ADP blockers such as ticagrelor and cangrelor inhibit the entire platelet population. In this study, we evaluated the impact of these pharmacological differences on thrombus formation in an ex vivo flow chamber model. A customized image analysis pipeline was used for automatized, large-scale identification and tracking of single platelets incorporated into the thrombus, enabling quantitative analysis of the relative contribution of inhibited and uninhibited platelets to thrombus growth and consolidation. Comparative experiments were conducted using the irreversible and reversible P2Y₁₂ inhibitors prasugrel active metabolite (PAM) and ticagrelor, respectively. Our results show that PAM inhibited thrombus platelet recruitment more gradually than ticagrelor, with a slower onset of inhibition. Further, we show that the presence of a small fraction (<10%) of uninhibited platelets did not abrogate the antithrombotic effect of PAM to any significant extent. Finally, we demonstrate a gradual enrichment of inhibited platelets in the thrombus shell due to selective recruitment of inhibited platelets to the thrombus periphery.

Role of P2Y12 Receptor in Thrombosis

P2Y₁₂ receptor is a 342 amino acid Gi-coupled receptor predominantly expressed on platelets. P2Y₁₂ receptor is physiologically activated by ADP and inhibits adenyl cyclase (AC) to decrease cyclic AMP (cAMP) level, resulting in platelet aggregation. It also activates PI3 kinase (PI3K) pathway leading to fibrinogen receptor activation, and may protect platelets from apoptosis. Abnormalities of P2Y₁₂ receptor include congenital deficiencies or high activity in diseases like diabetes mellitus (DM) and chronic kidney disease (CKD), exposing such patients to a prothrombotic condition. A series of clinical antiplatelet drugs, such as clopidogrel and ticagrelor, are designed as indirect or direct antagonists of P2Y₁₂ receptor to reduce incidence of thrombosis mainly for patients of acute coronary syndrome (ACS) who are at high risk of thrombotic events. Studies on novel dual-/multi-target antiplatelet agents consider P2Y₁₂ receptor as a promising part in combined targets. However, the clinical practical phenomena, such as "clopidogrel resistance" due to gene variations of cytochrome P450 or P2Y₁₂ receptor constitutive activation, call for better antiplatelet agents. Researches also showed inverse agonist of P2Y₁₂ receptor could play a better role over neutral antagonists. Personalized antiplatelet therapy is the most ideal destination for antiplatelet therapy in ACS patients with or without other underlying diseases like DM or CKD, however, there is still a long way to go.

P2Y12 and P2Y13 receptors involved in ADPβs induced the release of IL-1β, IL-6 and TNF-α from cultured dorsal horn microglia

Objective

P2 receptors have been implicated in the release of neurotransmitter and pro-inflammatory cytokines due to their response to neuroexcitatory substances in the microglia. Dorsal horn P2Y₁₂ and P2Y₁₃ receptors are involved in the development of pain behavior induced by peripheral nerve injury. However, it is not known whether P2Y₁₂ and P2Y₁₃ receptors activation is associated with the expression and the release of interleukin-1B (IL-1β), interleukin-6 (IL-6), tumor necrosis factor-α (TNF-α) in cultured dorsal spinal cord microglia. For this reason, we examined the effects of ADPβs (ADP analog) on the expression and the release of IL-1β, IL-6, and TNF-α.

Methods and results

In this study, we observed the effect of P2Y receptor agonist ADPβs on the expression and release of IL-1β, IL-6 and TNF-α by using real-time fluorescence quantitative polymerase chain reaction (PCR) and enzyme-linked immunosorbent assay (ELISA). ADPβs induced the increased expression of Iba-1, IL-1β, IL-6 and TNF-α at the level of messenger RNA (mRNA). ADPβs-evoked increase in Iba-1, IL-1β, IL-6 and TNF-α mRNA expression was inhibited only partially by P2Y₁₂ receptor antagonist MRS2395 or P2Y₁₃ receptor antagonist MRS2211, respectively. Similarly, ADPβs-evoked release of IL-1β, IL-6 and TNF-α was inhibited only partially by MRS2395 or MRS2211. Furthermore, ADPβs-evoked increased expression of Iba-1, IL-1β, IL-6 and TNF-α mRNA, and release of IL-1β, IL-6 and TNF-α were nearly all blocked after co-administration of MRS2395 plus MRS2179. Further evidence indicated that P2Y₁₂ and P2Y₁₃ receptor-evoked increased gene expression of IL-1β, IL-6 and TNF-α were inhibited by Y-27632 (ROCK inhibitor), SB203580 (P38MAPK inhibitor) and PDTC (NF-κb inhibitor), respectively. Subsequently, P2Y₁₂ and P2Y₁₃ receptor-evoked release of IL-1β, IL-6 and TNF-α, were also inhibited by Y-27632, SB203580 and PDTC, respectively.

Conclusion

These observations suggest that P2Y₁₂ and P2Y₁₃ receptor-evoked gene expression and release of IL-1β, IL-6 and TNF-α are associated with ROCK/P38MAPK/NF-κb signaling pathway.

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.

Blood cells: an historical account of the roles of purinergic signalling

The involvement of purinergic signalling in the physiology of erythrocytes, platelets and leukocytes was recognised early. The release of ATP and the expression of purinoceptors and ectonucleotidases on erythrocytes in health and disease are reviewed. The release of ATP and ADP from platelets and the expression and roles of P1, P2Y(1), P2Y(12) and P2X1 receptors on platelets are described. P2Y(1) and P2X(1) receptors mediate changes in platelet shape, while P2Y(12) receptors mediate platelet aggregation. The changes in the role of purinergic signalling in a variety of disease conditions are considered. The successful use of P2Y(12) receptor antagonists, such as clopidogrel and ticagrelor, for the treatment of thrombosis, myocardial infarction and stroke is discussed.

RhoA/ROCK pathway mediates p38 MAPK activation and morphological changes downstream of P2Y12/13 receptors in spinal microglia in neuropathic pain

Recent studies have indicated an important role of ATP receptors in spinal microglia, such as P2Y12 or P2Y13, in the development of chronic pain. However, intracellular signaling cascade of these receptors have not been clearly elucidated. We found that intrathecal injection of 2-(methylthio)adenosine 5'-diphosphate (2Me-SADP) induced mechanical hypersensitivity and p38 mitogen-activated protein kinase (MAPK) phosphorylation in the spinal cord. Intrathecal administration of P2Y12/P2Y13 antagonists and Rho-associated coiled-coil-containing protein kinase (ROCK) inhibitor H1152 suppressed not only p38 MAPK phosphorylation, but also mechanical hypersensitivity induced by 2Me-SADP. In the rat peripheral nerve injury model, intrathecal administration of antagonists for the P2Y12/P2Y13 receptor suppressed activation of p38 MAPK in the spinal cord. In addition, subarachnoidal injection of H1152 also attenuated nerve injury-induced spinal p38 MAPK phosphorylation and neuropathic pain behavior, suggesting an essential role of ROCK in nerve injury-induced p38 MAPK activation. We also found that the antagonists of the P2Y12/P2Y13 receptor and H1152 had inhibitory effects on the morphological changes of microglia such as retraction of processes in both 2Me-SADP and nerve injured rats. In contrast these treatments had no effect on the number of Iba1-positive cells in the nerve injury model. Collectively, our results have demonstrated roles of ROCK in the spinal microglia that is involved in p38 MAPK activation and the morphological changes. Inhibition of ROCK signaling may offer a novel target for the development of a neuropathic pain treatment.

Molecular mechanisms of platelet P2Y(12) receptor regulation

Platelets are critical for haemostasis, however inappropriate activation can lead to the development of arterial thrombosis, which can result in heart attack and stroke. ADP is a key platelet agonist that exerts its actions via stimulation of two surface GPCRs (G-protein-coupled receptors), P2Y(1) and P2Y(12). Similar to most GPCRs, P2Y receptor activity is tightly regulated by a number of complex mechanisms including receptor desensitization, internalization and recycling. In the present article, we review the molecular mechanisms that underlie P2Y(1) and P2Y(12) receptor regulation, with particular emphasis on the structural motifs within the P2Y(12) receptor, which are required to maintain regulatory protein interaction. The implications of these findings for platelet responsiveness are also discussed.

Canonical Wnt signaling negatively regulates platelet function

Wnts regulate important intracellular signaling events, and dysregulation of the Wnt pathway has been linked to human disease. Here, we uncover numerous Wnt canonical effectors in human platelets where Wnts, their receptors, and downstream signaling components have not been previously described. We demonstrate that the Wnt3a ligand inhibits platelet adhesion, activation, dense granule secretion, and aggregation. Wnt3a also altered platelet shape change and inhibited the activation of the small GTPase RhoA. In addition, we found the Wnt-beta-catenin signaling pathway to be functional in platelets. Finally, disruption of the Wnt Frizzled 6 receptor in the mouse resulted in a hyperactivatory platelet phenotype and a reduced sensitivity to Wnt3a. Taken together our studies reveal a novel functional role for Wnt signaling in regulating anucleate platelet function and may provide a tractable target for future antiplatelet therapy.

The critical role of myosin IIA in platelet internal contraction

BACKGROUND

Shape change and centralization of granules surrounded by a microtubular coil (internal contraction) are among the earliest morphologic changes observed following platelet activation. Myosin IIA contributes to initiation of platelet shape change, but its role in internal contraction has not been defined.

OBJECTIVE

To define the contribution of myosin IIA to platelet internal contraction.

METHODS

Aspirin-treated platelets suspended in calcium-free buffer were activated with a low concentration (25 nm) of the thromboxane A(2) analog U46619 which initiated shape change and internal contraction via a Rho kinase pathway. Shape change and internal contraction were assessed by aggregometry and transmission electron microscopy (TEM), and Rho activation and myosin regulatory light chain (MRLC) phosphorylation were studied concurrently.

RESULTS AND CONCLUSIONS

Low-concentration blebbistatin (10 microm) inhibited internal contraction in the majority of platelets with minimal inhibition of shape change without significant suppression of MRLC phosphorylation. Higher blebbistatin concentrations (25-100 microm) produced concentration-dependent inhibition of aggregation, shape change, Rho activation, and MRLC phosphorylation. These data demonstrate: (i) direct platelet myosin IIA participation in internal contraction; and (ii) inhibition of Rho activation and MRLC phosphorylation by >10 microm blebbistatin.

Shaping immune responses through the activation of dendritic cells' P2 receptors

Dendritic cells (DCs) activate and shape the adaptive immune response by capturing antigens, migrating to peripheral lymphoid organs where naïve T cells reside, expressing high levels of MHC and costimulatory molecules and secreting cytokines and chemokines. DCs are endowed with a high degree of functional plasticity and their functions are tightly regulated. Besides initiating adaptive immune responses, DCs play a key role in maintaining peripheral tolerance toward self-antigens. On the basis of the information gathered from the tissue where they reside, DCs adjust their functional activity to ensure that protective immunity is favoured while unwanted or exaggerated immune responses are prevented. A wide variety of signals from neighbouring cells affecting DC functional activity have been described. Here we will discuss the complex role of extracellular nucleotides in the regulation of DC function and the role of P2 receptors as possible tools to manipulate immune responses.

Vasodilator-stimulated phosphoprotein (VASP) is phosphorylated on Ser157 by protein kinase C-dependent and -independent mechanisms in thrombin-stimulated human platelets

VASP (vasodilator-stimulated phosphoprotein) is an actin- and profilin-binding protein that is expressed in platelets at high levels and plays a major role in negatively regulating secretory and adhesive events in these cells. VASP is a major substrate for cAMP- and cGMP-regulated protein kinases and it has been shown to be directly phosphorylated on Ser157 by PKC (protein kinase C). In the present paper, we show that, in human platelets, VASP is phosphorylated by PKC on Ser157, but not Ser239, in response to phorbol ester stimulation, in a manner blocked by the PKC inhibitor BIM I (bisindolylmaleimide I). In response to thrombin, VASP was also phosphorylated on Ser157, but this response was only partially inhibited by BIM I, indicating PKC-dependent and -independent pathways to VASP phosphorylation by thrombin. Using inhibitors, we have ruled out the possibility that the PKC-independent pathway acts through guanylate cyclase generation of cGMP, or through a phosphoinositide 3-kinase-dependent kinase. Inhibition of Rho kinase, however, substantially reduced Ser157 VASP phosphorylation, and its effects were additive with BIM I. This implicates Rho kinase and PKC as the major kinases that phosphorylate VASP Ser157 in response to thrombin in platelets.