Major contribution of the P2Y₁receptor in purinergic regulation of TNFα-induced vascular inflammation

Renal ischemia and reperfusion impact the purinergic signaling in a vascular bed distant from the injured site

AIMS

Acute kidney injury (AKI) is a public health problem and represents a risk factor for cardiovascular diseases (CVD) and vascular damage. This study aimed to investigate the impact of AKI on purinergic components in mice aorta.

MAIN METHODS

The kidney ischemia was achieved by the occlusion of the left kidney pedicle for 60 min, followed by reperfusion for 8 (IR8) and 15 (IR15) days. Renal function was assessed through biochemical assays, while gene expression levels were evaluated by RT-qPCR.

KEY FINDINGS

Analyses of renal parameters showed renal remodeling through mass loss in the left kidney and hypertrophy of the right kidney in the IR15 group. Furthermore, after 15 days, local inflammation was evidenced in the aorta. Moreover, the aorta purinergic components were significantly impacted by the renal ischemia and reperfusion model, with increases in gene expression of the pro-inflammatory purinoceptors P2Y1, P2Y2, P2Y6, and P2X4, potentially contributing to the vessel inflammation. The expression of NTPDase2 and ecto-5'-nucleotidase were also significantly increased in the aorta of the same group. In addition, both ATP and AMP hydrolysis were significantly increased in the aorta from IR15 animals, driving the entire purinergic cascade to the production of the anti-inflammatory adenosine.

SIGNIFICANCE

In short, this is the first time that inflammation of the aorta due to AKI was shown to have an impact on purinergic signaling components, with emphasis on the adenosinergic pathway. This seems to be closely implicated in the establishment of vascular inflammation in this model of AKI and deserves to be further investigated.

Quantifying Adhesion of Inflammatory Cells to the Endothelium In Vitro

We present a simple and quantitative assay system that accurately models human endothelium by use of primary human umbilical vein endothelial cells (HUVECs) in cell culture plates coated with gelatin, a matrix that mimics basal lamina, the matrix that is tightly associated with the vascular endothelium and is critical for its proper function. We describe using this system to quantitatively measure adhesion of the inflammatory cells - monocytic THP-1 cell line to the HUVEC monolayer. The THP-1 cells are fluorescently labeled which allows to quantify the number of the fluorescent THP-1 cells adhering to the endothelium under a microscope and the level of florescence - a quantitative measure of the number of adhering fluorescent THP-1 cells using a fluorescent plate reader. After optimization, we were able to detect increased adhesion of the THP-1 cells to the endothelium in response to the inflammatory cytokine TNFα in a dose-dependent manner like what has been observed in vivo.

Purine and purinergic receptors in health and disease

Purines and purinergic receptors are widely distributed throughout the human body. Purine molecules within cells play crucial roles in regulating energy metabolism and other cellular processes, while extracellular purines transmit signals through specific purinergic receptors. The ubiquitous purinergic signaling maintains normal neural excitability, digestion and absorption, respiratory movement, and other complex physiological activities, and participates in cell proliferation, differentiation, migration, and death. Pathological dysregulation of purinergic signaling can result in the development of various diseases, including neurodegeneration, inflammatory reactions, and malignant tumors. The dysregulation or dysfunction of purines and purinergic receptors has been demonstrated to be closely associated with tumor progression. Compared with other subtypes of purinergic receptors, the P2X7 receptor (P2X7R) exhibits distinct characteristics (i.e., a low affinity for ATP, dual functionality upon activation, the mediation of ion channels, and nonselective pores formation) and is considered a promising target for antitumor therapy, particularly in patients with poor response to immunotherapy This review summarizes the physiological and pathological significance of purinergic signaling and purinergic receptors, analyzes their complex relationship with tumors, and proposes potential antitumor immunotherapy strategies from tumor P2X7R inhibition, tumor P2X7R overactivation, and host P2X7R activation. This review provides a reference for clinical immunotherapy and mechanism investigation.

Purinergic signaling: A gatekeeper of blood-brain barrier permeation

This review outlined evidence that purinergic signaling is involved in the modulation of blood-brain barrier (BBB) permeability. The functional and structural integrity of the BBB is critical for maintaining the homeostasis of the brain microenvironment. BBB integrity is maintained primarily by endothelial cells and basement membrane but also be regulated by pericytes, neurons, astrocytes, microglia and oligodendrocytes. In this review, we summarized the purinergic receptors and nucleotidases expressed on BBB cells and focused on the regulation of BBB permeability by purinergic signaling. The permeability of BBB is regulated by a series of purinergic receptors classified as P2Y₁, P2Y₄, P2Y₁₂, P2X4, P2X7, A₁, A_2A, A_2B, and A₃, which serve as targets for endogenous ATP, ADP, or adenosine. P2Y₁ and P2Y₄ antagonists could attenuate BBB damage. In contrast, P2Y₁₂-mediated chemotaxis of microglial cell processes is necessary for rapid closure of the BBB after BBB breakdown. Antagonists of P2X4 and P2X7 inhibit the activation of these receptors, reduce the release of interleukin-1 beta (IL-1β), and promote the function of BBB closure. In addition, the CD39/CD73 nucleotidase axis participates in extracellular adenosine metabolism and promotes BBB permeability through A₁ and A_2A on BBB cells. Furthermore, A_2B and A₃ receptor agonists protect BBB integrity. Thus, the regulation of the BBB by purinergic signaling is complex and affects the opening and closing of the BBB through different pathways. Appropriate selective agonists/antagonists of purinergic receptors and corresponding enzyme inhibitors could modulate the permeability of the BBB, effectively delivering therapeutic drugs/cells to the central nervous system (CNS) or limiting the entry of inflammatory immune cells into the brain and re-establishing CNS homeostasis.

Purinergic signaling during Marek's disease in chickens

Purinergic receptors (PRs) have been reported as potential therapeutic targets for many viral infections including herpesviruses, which urges the investigation into their role in Marek's disease (MD), a herpesvirus induced cancer in chickens that is an important pathogen for the poultry industry. MD is caused by MD virus (MDV) that has a similar viral life cycle as human varicella zoster virus in that it is shed from infected epithelial skin cells and enters the host through the respiratory route. In this report, PR responses during natural MDV infection and disease progression was examined in MD-resistant white Leghorns (WL) and MD-susceptible Pure Columbian (PC) chickens during natural infection. Whole lung lavage cells (WLLC) and liver tissue samples were collected from chickens infected but showing no clinical signs of MD (Infected) or presenting with clinical disease (Diseased). RNA was extracted followed by RT-qPCR analysis with gene specific primers against members of the P1, P2X, and P2Y PR families. Differential expression (p < 0.05) was observed in breed and disease conditions. Some PRs showed tissue specific expression (P1A1, P2X1, and P2X6 in WLLC) whereas others responded to MDV infection only in MD-susceptible (PC) chickens (P1A2A, P2X1, P2X5, P2X7). P2Y PRs had differential expression in both chicken lines in response to MDV infection and MD progression. This study is the first to our knowledge to examine PR responses during MDV infection and disease progression. These results suggest PR signaling may an important area of research for MDV replication and MD.

More than a drug target: Purinergic signalling as a source for diagnostic tools in epilepsy

Epilepsy is one of the most common and disabling chronic neurological diseases affecting people of all ages. Major challenges of epilepsy management include the persistently high percentage of drug-refractoriness among patients, the absence of disease-modifying treatments, and its diagnosis and prognosis. To date, long-term video-electroencephalogram (EEG) recordings remain the gold standard for an epilepsy diagnosis. However, this is very costly, has low throughput, and in some instances has very limited availability. Therefore, much effort is put into the search for non-invasive diagnostic tests. Purinergic signalling, via extracellularly released adenosine triphosphate (ATP), is gaining increasing traction as a therapeutic strategy for epilepsy treatment which is supported by evidence from both experimental models and patients. This includes in particular the ionotropic P2X7 receptor. Besides that, other components from the ATPergic signalling cascade such as the metabotropic P2Y receptors (e.g., P2Y₁ receptor) and ATP-release channels (e.g., pannexin-1), have also been shown to contribute to seizures and epilepsy. In addition to the therapeutic potential of purinergic signalling, emerging evidence has also shown its potential as a diagnostic tool. Following seizures and epilepsy, the concentration of purines in the blood and the expression of different compounds of the purinergic signalling cascade are significantly altered. Herein, this review will provide a detailed discussion of recent findings on the diagnostic potential of purinergic signalling for epilepsy management and the prospect of translating it for clinical application. This article is part of the Special Issue on 'Purinergic Signaling: 50 years'.

Hypoxia induces purinergic receptor signaling to disrupt endothelial barrier function

Blood-brain-barrier permeability is regulated by endothelial junctional proteins and is vital in limiting access to and from the blood to the CNS. When stressed, several cells, including endothelial cells, can release nucleotides like ATP and ADP that signal through purinergic receptors on these cells to disrupt BBB permeability. While this process is primarily protective, unrestricted, uncontrolled barrier disruption during injury or inflammation can lead to serious neurological consequences. Purinergic receptors are broadly classified into two families: the P1 adenosine and P2 nucleotide receptors. The P2 receptors are further sub-classified into the P2XR ion channels and the P2YR GPCRs. While ATP mainly activates P2XRs, P2YRs have a broader range of ligand selectivity. The P2Y1R, essential for platelet function, is reportedly ubiquitous in its expression. Prior studies using gene knockout and specific antagonists have shown that these approaches have neuroprotective effects following occlusive stroke. Here we investigated the expression of P2Y1R in primary cultured brain endothelial cells and its relation to the maintenance of BBB function. Results show that following in vitro hypoxia and reoxygenation, P2Y1R expression is upregulated in both control and diabetic cells. At the same time, endothelial junctional markers, ZO-1 and VE-cadherin, were downregulated, and endothelial permeability increased. siRNA knockdown of P2Y1R and MRS 2500 effectively blocked this response. Thus, we show that P2Y1R signaling in endothelial cells leads to the downregulation of endothelial barrier function.

Pannexin1 channels-a potential therapeutic target in inflammation

An exaggerated inflammatory response is the hallmark of a plethora of disorders. ATP is a central signaling molecule that orchestrates the initiation and resolution of the inflammatory response by enhancing activation of the inflammasome, leukocyte recruitment and activation of T cells. ATP can be released from cells through pannexin (Panx) channels, a family of glycoproteins consisting of three members, Panx1, Panx2, and Panx3. Panx1 is ubiquitously expressed and forms heptameric channels in the plasma membrane mediating paracrine and autocrine signaling. Besides their involvement in the inflammatory response, Panx1 channels have been shown to contribute to different modes of cell death (i.e., pyroptosis, necrosis and apoptosis). Both genetic ablation and pharmacological inhibition of Panx1 channels decrease inflammation in vivo and contribute to a better outcome in several animal models of inflammatory disease involving various organs, including the brain, lung, kidney and heart. Up to date, several molecules have been identified to inhibit Panx1 channels, for instance probenecid (Pbn), mefloquine (Mfq), flufenamic acid (FFA), carbenoxolone (Cbx) or mimetic peptides like 10Panx1. Unfortunately, the vast majority of these compounds lack specificity and/or serum stability, which limits their application. The recent availability of detailed structural information on the Panx1 channel from cryo-electron microscopy studies may open up innovative approaches to acquire new classes of synthetic Panx1 channel blockers with high target specificity. Selective inhibition of Panx1 channels may not only limit acute inflammatory responses but may also prove useful in chronic inflammatory diseases, thereby improving human health. Here, we reviewed the current knowledge on the role of Panx1 in the initiation and resolution of the inflammatory response, we summarized the effects of Panx1 inhibition in inflammatory pathologies and recapitulate current Panx1 channel pharmacology with an outlook towards future approaches.

Physiologic roles of P2 receptors in leukocytes

Since their discovery in the 1970s, purinergic receptors have been shown to play key roles in a wide variety of biologic systems and cell types. In the immune system, purinergic receptors participate in innate immunity and in the modulation of the adaptive immune response. In particular, P2 receptors, which respond to extracellular nucleotides, are widely expressed on leukocytes, causing the release of cytokines and chemokines and the formation of inflammatory mediators, and inducing phagocytosis, degranulation, and cell death. The activity of these receptors is regulated by ectonucleotidases-expressed in these same cell types-which regulate the availability of nucleotides in the extracellular environment. In this article, we review the characteristics of the main purinergic receptor subtypes present in the immune system, focusing on the P2 family. In addition, we describe the physiologic roles of the P2 receptors already identified in leukocytes and how they can positively or negatively modulate the development of infectious diseases, inflammation, and pain.

The Interplay of Endothelial P2Y Receptors in Cardiovascular Health: From Vascular Physiology to Pathology

The endothelium plays a key role in blood vessel health. At the interface of the blood, it releases several mediators that regulate local processes that protect against the development of cardiovascular disease. In this interplay, there is increasing evidence for a role of extracellular nucleotides and endothelial purinergic P2Y receptors (P2Y-R) in vascular protection. Recent advances have revealed that endothelial P2Y₁-R and P2Y₂-R mediate nitric oxide-dependent vasorelaxation as well as endothelial cell proliferation and migration, which are processes involved in the regeneration of damaged endothelium. However, endothelial P2Y₂-R, and possibly P2Y₁-R, have also been reported to promote vascular inflammation and atheroma development in mouse models, with endothelial P2Y₂-R also being described as promoting vascular remodeling and neointimal hyperplasia. Interestingly, at the interface with lipid metabolism, P2Y₁₂-R has been found to trigger HDL transcytosis through endothelial cells, a process known to be protective against lipid deposition in the vascular wall. Better characterization of the role of purinergic P2Y-R and downstream signaling pathways in determination of the endothelial cell phenotype in healthy and pathological environments has clinical potential for the prevention and treatment of cardiovascular diseases.

Purinergic signaling in systemic sclerosis

Systemic sclerosis (SSc) is a chronic autoimmune rheumatic disease that involves numerous organs and presents major management challenges. The histopathologic hallmarks of SSc include vasculopathy, fibrosis and autoimmune phenomena involving both innate and adaptive immune systems. Purinergic signalling is a pathway that may be implicated in the pathophysiology of several of these disease manifestations. Extracellular purines are potent signalling mediators, which have been shown to be dysregulated in SSc. As examples, purines can exacerbate vasculopathy and provoke platelet dysfunction; as well as contributing to immune dysregulation. Elements of purinergic signalling further promote organ and tissue fibrosis in several disease models. Here, we provide an overview of extracellular purine metabolism in purinergic signalling and link disorders of these to the molecular pathology of SSc. We also discuss targeting the purinergic signalling and explore the translational applications for new therapeutic options in SSc.

G protein-coupled purinergic P2Y receptor oligomerization: Pharmacological changes and dynamic regulation

P2Y receptors (P2YRs) are a δ group of rhodopsin-like G protein-coupled receptors (GPCRs) with many essential functions in physiology and pathology, such as platelet aggregation, immune responses, neuroprotective effects, inflammation, and cellular proliferation. Thus, they are among the most researched therapeutic targets used for the clinical treatment of diseases (e.g., the antithrombotic drug clopidogrel and the dry eye treatment drug diquafosol). GPCRs transmit signals as dimers to increase the diversity of signalling pathways and pharmacological activities. Many studies have frequently confirmed dimerization between P2YRs and other GPCRs due to their functions in cardiovascular and cerebrovascular processes in vivo and in vitro. Recently, some P2YR dimers that dynamically balance physiological functions in the body were shown to be involved in effective signal transduction and exert pathological responses. In this review, we summarize the types, pharmacological changes, and active regulators of P2YR-related dimerization, and delineate new functions and pharmacological activities of P2YR-related dimers, which may be a novel direction to improve the effectiveness of medications.

Unveiling the Potential of Purinergic Signaling in Schistosomiasis Treatment

Schistosomiasis is a neglected tropical disease. It is related to long-lasting granulomatous fibrosis and inflammation of target organs, and current sub-optimal pharmacological treatment creates global public health concerns. Intravascular worms and eggs release antigens and extracellular vesicles that target host endothelial cells, modulate the immune system, and stimulate the release of damageassociated molecular patterns (DAMPs). ATP, one of the most studied DAMPs, triggers a cascade of autocrine and paracrine actions through purinergic P2X and P2Y receptors, which are shaped by ectonucleotidases (CD39). Both P2 receptor families, and in particular P2Y₁, P2Y₂, P2Y₁₂, and P2X7 receptors, have been attracting increasing interest in several inflammatory diseases and drug development. Current data obtained from the murine model unveiled a CD39-ADP-P2Y₁/P2Y₁₂ receptors signaling pathway linked to the liver and mesenteric exacerbations of schistosomal inflammation. Therefore, we proposed that members of this purinergic signaling could be putative pharmacological targets to reduce schistosomal morbidity.

Purinergic Signaling in Controlling Macrophage and T Cell Functions During Atherosclerosis Development

Atherosclerosis is a hardening and narrowing of arteries causing a reduction of blood flow. It is a leading cause of death in industrialized countries as it causes heart attacks, strokes, and peripheral vascular disease. Pathogenesis of the atherosclerotic lesion (atheroma) relies on the accumulation of cholesterol-containing low-density lipoproteins (LDL) and on changes of artery endothelium that becomes adhesive for monocytes and lymphocytes. Immunomediated inflammatory response stimulated by lipoprotein oxidation, cytokine secretion and release of pro-inflammatory mediators, worsens the pathological context by amplifying tissue damage to the arterial lining and increasing flow-limiting stenosis. Formation of thrombi upon rupture of the endothelium and the fibrous cup may also occur, triggering thrombosis often threatening the patient’s life. Purinergic signaling, i.e., cell responses induced by stimulation of P2 and P1 membrane receptors for the extracellular nucleotides (ATP, ADP, UTP, and UDP) and nucleosides (adenosine), has been implicated in modulating the immunological response in atherosclerotic cardiovascular disease. In this review we will describe advancements in the understanding of purinergic modulation of the two main immune cells involved in atherogenesis, i.e., monocytes/macrophages and T lymphocytes, highlighting modulation of pro- and anti-atherosclerotic mediated responses of purinergic signaling in these cells and providing new insights to point out their potential clinical significance.

Purinergic Regulation of Endothelial Barrier Function

Increased vascular permeability is a hallmark of several cardiovascular anomalies, including ischaemia/reperfusion injury and inflammation. During both ischaemia/reperfusion and inflammation, massive amounts of various nucleotides, particularly adenosine 5'-triphosphate (ATP) and adenosine, are released that can induce a plethora of signalling pathways via activation of several purinergic receptors and may affect endothelial barrier properties. The nature of the effects on endothelial barrier function may depend on the prevalence and type of purinergic receptors activated in a particular tissue. In this review, we discuss the influence of the activation of various purinergic receptors and downstream signalling pathways on vascular permeability during pathological conditions.

Targeting Platelet in Atherosclerosis Plaque Formation: Current Knowledge and Future Perspectives

Besides their role in hemostasis and thrombosis, it has become increasingly clear that platelets are also involved in many other pathological processes of the vascular system, such as atherosclerotic plaque formation. Atherosclerosis is a chronic vascular inflammatory disease, which preferentially develops at sites under disturbed blood flow with low speeds and chaotic directions. Hyperglycemia, hyperlipidemia, and hypertension are all risk factors for atherosclerosis. When the vascular microenvironment changes, platelets can respond quickly to interact with endothelial cells and leukocytes, participating in atherosclerosis. This review discusses the important roles of platelets in the plaque formation under pro-atherogenic factors. Specifically, we discussed the platelet behaviors under disturbed flow, hyperglycemia, and hyperlipidemia conditions. We also summarized the molecular mechanisms involved in vascular inflammation during atherogenesis based on platelet receptors and secretion of inflammatory factors. Finally, we highlighted the studies of platelet migration in atherogenesis. In general, we elaborated an atherogenic role of platelets and the aspects that should be further studied in the future.

Inflammation and thrombosis in COVID-19 pathophysiology: proteinase-activated and purinergic receptors as drivers and candidate therapeutic targets

Evolving information has identified disease mechanisms and dysregulation of host biology that might be targeted therapeutically in coronavirus disease 2019 (COVID-19). Thrombosis and coagulopathy, associated with pulmonary injury and inflammation, are emerging clinical features of COVID-19. We present a framework for mechanisms of thrombosis in COVID-19 that initially derive from interaction of SARS-CoV-2 with ACE2, resulting in dysregulation of angiotensin signaling and subsequent inflammation and tissue injury. These responses result in increased signaling by thrombin (proteinase-activated) and purinergic receptors, which promote platelet activation and exert pathological effects on other cell types (e.g., endothelial cells, epithelial cells, and fibroblasts), further enhancing inflammation and injury. Inhibitors of thrombin and purinergic receptors may, thus, have therapeutic effects by blunting platelet-mediated thromboinflammation and dysfunction in other cell types. Such inhibitors include agents (e.g., anti-platelet drugs) approved for other indications, and that could be repurposed to treat, and potentially improve the outcome of, COVID-19 patients. COVID-19, caused by the SARS-CoV-2 virus, drives dysregulation of angiotensin signaling, which, in turn, increases thrombin-mediated and purinergic-mediated activation of platelets and increase in inflammation. This thromboinflammation impacts the lungs and can also have systemic effects. Inhibitors of receptors that drive platelet activation or inhibitors of the coagulation cascade provide opportunities to treat COVID-19 thromboinflammation.

Activated platelets promote an osteogenic programme and the progression of calcific aortic valve stenosis

AIMS

Calcific aortic valve stenosis (CAVS) is characterized by a fibrocalcific process. Studies have shown an association between CAVS and the activation of platelets. It is believed that shear stress associated with CAVS promotes the activation of platelets. However, whether platelets actively participate to the mineralization of the aortic valve (AV) and the progression of CAVS is presently unknown. To identify the role of platelets into the pathobiology of CAVS.

METHODS AND RESULTS

Explanted control non-mineralized and mineralized AVs were examined by scanning electron microscope (SEM) for the presence of activated platelets. In-depth functional assays were carried out with isolated human valve interstitial cells (VICs) and platelets as well as in LDLR-/- apoB100/100 IGFII (IGFII) mice. Scanning electron microscope and immunogold markings for glycoprotein IIb/IIIa (GPIIb/IIIa) revealed the presence of platelet aggregates with fibrin in endothelium-denuded areas of CAVS. In isolated VICs, collagen-activated platelets induced an osteogenic programme. Platelet-derived adenosine diphosphate induced the release of autotaxin (ATX) by VICs. The binding of ATX to GPIIb/IIIa of platelets generated lysophosphatidic acid (LysoPA) with pro-osteogenic properties. In IGFII mice with CAVS, platelet aggregates were found at the surface of AVs. Administration of activated platelets to IGFII mice accelerated the development of CAVS by 2.1-fold, whereas a treatment with Ki16425, an antagonist of LysoPA receptors, prevented platelet-induced mineralization of the AV and the progression of CAVS.

CONCLUSIONS

These findings suggest a novel role for platelets in the progression of CAVS.

Purinergic Dysfunction in Pulmonary Arterial Hypertension

Pulmonary arterial hypertension (PAH) is a life‐threatening disease characterized by increased pulmonary arterial pressure and pulmonary vascular resistance, which result in an increase in afterload imposed onto the right ventricle, leading to right heart failure. Current therapies are incapable of reversing the disease progression. Thus, the identification of novel and potential therapeutic targets is urgently needed. An alteration of nucleotide‐ and nucleoside‐activated purinergic signaling has been proposed as a potential contributor in the pathogenesis of PAH. Adenosine‐mediated purinergic 1 receptor activation, particularly A_2AR activation, reduces pulmonary vascular resistance and attenuates pulmonary vascular remodeling and right ventricle hypertrophy, thereby exerting a protective effect. Conversely, A_2BR activation induces pulmonary vascular remodeling, and is therefore deleterious. ATP‐mediated P2X₇R activation and ADP‐mediated activation of P2Y₁R and P2Y₁₂R play a role in pulmonary vascular tone, vascular remodeling, and inflammation in PAH. Recent studies have revealed a role of ectonucleotidase nucleoside triphosphate diphosphohydrolase, that degrades ATP/ADP, in regulation of pulmonary vascular remodeling. Interestingly, existing evidence that adenosine activates erythrocyte A_2BR signaling, counteracting hypoxia‐induced pulmonary injury, and that ATP release is impaired in erythrocyte in PAH implies erythrocyte dysfunction as an important trigger to affect purinergic signaling for pathogenesis of PAH. The present review focuses on current knowledge on alteration of nucleot(s)ide‐mediated purinergic signaling as a potential disease mechanism underlying the development of PAH.

More purinergic receptors deserve attention as therapeutic targets for the treatment of cardiovascular disease

Cardiovascular disease is a major cause of morbidity and mortality worldwide. Innovative new treatment options for this cardiovascular pandemic are urgently needed. Activation of purinergic receptors (PRs) is critically involved in the development and progression of cardiovascular disease including atherosclerosis, ischemic heart disease, hypertension, and diabetes. PRs have been targeted for the treatment of several cardiovascular diseases in a clinical setting. The P2Y₁₂R antagonists such as clopidogrel, ticagrelor, and others are the most successful class of purinergic drugs targeting platelets for the treatment of acute coronary syndrome. In addition to targeting platelets, ticagrelor may exert P2Y₁₂R-independent effect by targeting erythrocyte-mediated purinergic activation. The partial A₁R agonist neladenoson and the A_2AR agonist regadenoson have been applied in cardiovascular medicine. In experimental studies, many other PRs have been shown to play a significant role in the development and progression of cardiovascular diseases, and targeting these receptors have resulted in promising outcomes. Therefore, many of these PRs including A_2BR, A₃R, P2X₃R, P2X₄R, P2X₇R, P2Y₁R, P2Y₄R, P2Y₆R, and P2Y₁₁R can be considered as therapeutic targets. However, the multitude of PR subtypes expressed in different cells of the cardiovascular system may constitute a challenge whether single or multiple receptors should be targeted at the same time for the best efficacy. The present review discusses the promising purinergic drugs used in clinical studies for the treatment of cardiovascular disease. We also update experimental evidence for many other PRs that can be considered as therapeutic targets for future drug development.

Update of P2Y receptor pharmacology: IUPHAR Review 27

Eight G protein-coupled P2Y receptor subtypes respond to extracellular adenine and uracil mononucleotides and dinucleotides. P2Y receptors belong to the δ group of rhodopsin-like GPCRs and contain two structurally distinct subfamilies: P2Y₁ , P2Y₂ , P2Y₄ , P2Y₆ , and P2Y₁₁ (principally G_q protein-coupled P2Y₁ -like) and P2Y_12-14 (principally G_i protein-coupled P2Y₁₂ -like) receptors. Brain P2Y receptors occur in neurons, glial cells, and vasculature. Endothelial P2Y₁ , P2Y₂ , P2Y₄ , and P2Y₆ receptors induce vasodilation, while smooth muscle P2Y₂ , P2Y₄ , and P2Y₆ receptor activation leads to vasoconstriction. Pancreatic P2Y₁ and P2Y₆ receptors stimulate while P2Y₁₃ receptors inhibits insulin secretion. Antagonists of P2Y₁₂ receptors, and potentially P2Y₁ receptors, are anti-thrombotic agents, and a P2Y₂ /P2Y₄ receptor agonist treats dry eye syndrome in Asia. P2Y receptor agonists are generally pro-inflammatory, and antagonists may eventually treat inflammatory conditions. This article reviews recent developments in P2Y receptor pharmacology (using synthetic agonists and antagonists), structure and biophysical properties (using X-ray crystallography, mutagenesis and modelling), physiological and pathophysiological roles, and present and potentially future therapeutic targeting.

P2 Receptors as Therapeutic Targets in the Salivary Gland: From Physiology to Dysfunction

Although often overlooked in our daily lives, saliva performs a host of necessary physiological functions, including lubricating and protecting the oral cavity, facilitating taste sensation and digestion and maintaining tooth enamel. Therefore, salivary gland dysfunction and hyposalivation, often resulting from pathogenesis of the autoimmune disease Sjögren's syndrome or from radiotherapy of the head and neck region during cancer treatment, severely reduce the quality of life of afflicted patients and can lead to dental caries, periodontitis, digestive disorders, loss of taste and difficulty speaking. Since their initial discovery in the 1970s, P2 purinergic receptors for extracellular nucleotides, including ATP-gated ion channel P2X and G protein-coupled P2Y receptors, have been shown to mediate physiological processes in numerous tissues, including the salivary glands where P2 receptors represent a link between canonical and non-canonical saliva secretion. Additionally, extracellular nucleotides released during periods of cellular stress and inflammation act as a tissue alarmin to coordinate immunological and tissue repair responses through P2 receptor activation. Accordingly, P2 receptors have gained widespread clinical interest with agonists and antagonists either currently undergoing clinical trials or already approved for human use. Here, we review the contributions of P2 receptors to salivary gland function and describe their role in salivary gland dysfunction. We further consider their potential as therapeutic targets to promote physiological saliva flow, prevent salivary gland inflammation and enhance tissue regeneration.

Pharmacological strategies for targeting platelet activation in asthma

The activation of platelets during host defence and inflammatory disorders has become increasingly documented. Clinical studies of patients with asthma reveal heightened platelet activation and accumulation into lung tissue. Accompanying studies in animal models of allergic lung inflammation, using protocols of experimentally induced thrombocytopenia proclaim an important role for platelets during the leukocyte recruitment cascade, tissue integrity, and lung function. The functions of platelets during these inflammatory events are clearly distinct to platelet functions during haemostasis and clot formation, and have led to the concept that a dichotomy (or polytomy, depending on what else platelets do) in platelet activation exists. The platelet, therefore, presents us with novel opportunities for modulating these inflammatory responses. This review discusses the rationale and effectiveness of current anti-platelet drugs in their use to supress inflammation with regard to asthma, and the need to consider novel possibilities for pharmacological modulation of platelet function associated with inflammation that are pharmacologically distinct to current anti-platelet therapies.

Autocrine stimulation of P2Y1 receptors is part of the purinergic signaling mechanism that regulates T cell activation

Previous studies have shown that T cell receptor (TCR) and CD28 coreceptor stimulation involves rapid ATP release, autocrine purinergic feedback via P2X receptors, and mitochondrial ATP synthesis that promote T cell activation. Here, we show that ADP formation and autocrine stimulation of P2Y1 receptors are also involved in these purinergic signaling mechanisms. Primary human CD4 T cells and the human Jurkat CD4 T cell line express P2Y1 receptors. The expression of this receptor increases following T cell stimulation. Inhibition of P2Y1 receptors impairs the activation of mitochondria, as assessed by mitochondrial Ca²⁺ uptake, and reduces cytosolic Ca²⁺ signaling in response to TCR/CD28 stimulation. We found that the addition of exogenous ADP or overexpression of P2Y1 receptors significantly increased IL-2 mRNA transcription in response to TCR/CD28 stimulation. Conversely, antagonists or silencing of P2Y1 receptors reduced IL-2 mRNA transcription and attenuated T cell functions. We conclude that P2Y1 and P2X receptors have non-redundant, synergistic functions in the regulation of T cell activation. P2Y1 receptors may represent potential therapeutic targets to modulate T cell function in inflammation and host defense.

Pharmacology of P2Y receptors

P2Y receptors are G-protein-coupled receptors (GPCRs) for extracellular nucleotides. There are eight mammalian P2Y receptor subtypes divided into two subgroups (P2Y₁, P2Y₂, P2Y₄, P2Y₆, and P2Y₁₁) and (P2Y₁₂, P2Y₁₃, and P2Y₁₄). The P2Y receptors are expressed in various cell types and play important roles in physiology and pathophysiology including inflammatory responses and neuropathic pain. The antagonism of P2Y₁₂ receptors is used in pharmacotherapy for the prevention and therapy of cardiovascular events. The nucleoside analogue ticagrelor and active metabolites of the thienopyridine compounds ticlopidine, clopidogrel and prasugrel inhibit platelet P2Y₁₂ receptors and reduce thereby platelet aggregation. The P2Y₂ receptor agonist diquafosol is used for the treatment of the dry eye syndrome. The P2Y receptor subtypes differ in their amino acid sequences, their pharmacological profiles and their signaling transduction pathways. Recently, selective receptor ligands have been developed for all subtypes. The published crystal structures of the human P2Y₁ and P2Y₁₂ receptors as well as receptor models will facilitate the development of novel drugs for pharmacotherapy.

Mechanisms of ATP Release by Inflammatory Cells

Extracellular nucleotides (e.g., ATP, ADP, UTP, UDP) released by inflammatory cells interact with specific purinergic P2 type receptors to modulate their recruitment and activation. The focus of this review is on stimuli and mechanisms of extracellular nucleotide release and its consequences during inflammation. Necrosis leads to non-specific release of nucleotides, whereas specific release mechanisms include vesicular exocytosis and channel-mediated release via connexin or pannexin hemichannels. These release mechanisms allow stimulated inflammatory cells such as macrophages, neutrophils, and endothelial cells to fine-tune autocrine/paracrine responses during acute and chronic inflammation. Key effector functions of inflammatory cells are therefore regulated by purinergic signaling in acute and chronic diseases, making extracellular nucleotide release a promising target for the development of new therapies.

Pin1 induces the ADP-induced migration of human dental pulp cells through P2Y1 stabilization

PIN1, which belongs to a family of prolyl isomerases, specifically binds to phosphorylated Ser/Thr-pro motifs to catalytically regulate the post-phosphorylation conformation of its substrates. This study aimed to investigate the importance of Pin1 expression in human dental pulp cells (hDPCs) to understand the involvement of Pin1 in the regulation of P2Y1 and the activation of ADP-mediated P2Y1 signaling. This study found that the protein levels of P2Y1 gradually decreased after the onset of cell recovery following heat stress. Interestedly, hDPC migration significantly decreased during the recovery period. An in vitro study revealed that the silencing of PIN1 by siRNA or the pharmacologic inhibition of its activity decreased the migration of P2Y1 and P2Y1 expression in these cells. In addition, we found that Pin1 directly interacts with S252 of P2Y1 and that its binding stabilizes the P2Y1 protein to increase migration activity. These results strongly suggest that Pin1 mediates cell migration by stabilizing P2Y1 and that the Pin1/P2Y1 signaling pathways might serve as a novel mechanism of cell migration progression in hDPCs.

VPO1 mediates oxidation of LDL and formation of foam cells

Deposition of oxidized-LDL in vascular walls is essential in the initiation of atherosclerosis. Oxidation of LDL has been attributed to myeloperoxidase as its generation of potent oxidants. However, the exact mechanism of LDL oxidation and foam cell formation in atherosclerosis remains to be elucidated. Vascular peroxidase-1 (VPO1), a newly-identified heme-containing peroxidase, is primarily expressed in cardiovascular systems, and secreted into the circulation. The present study evaluates VPO1-mediated LDL oxidation and its role in atherosclerosis. VPO1 was first demonstrated binding to LDL. VPO1-mediated oxidation of proteins and lipids in LDL was verified by a variety of methods including immunoblot analysis, free tryptophan assay, UV absorbance, and thiobarbituric acid assay. VPO1-oxidized LDL caused accumulation of LDL in monocyte-like cells and promoted formation of foam cells. Administration of inflammation factors, LPS or TNF-α, induced increasing expression of VPO1 in aorta and secretion to plasma. TNF-α also promoted formation and retention of VPO1-oxidized LDL in aortic walls. Our data suggest that VPO1 contributes to oxidation and retention of LDL in vessel walls, and formation foam cells, indicating VPO1 as a novel potential mediator of atherosclerosis.

Ligand binding to a G protein-coupled receptor captured in a mass spectrometer

G protein (heterotrimeric guanine nucleotide-binding protein)-coupled receptors belong to the largest family of membrane-embedded cell surface proteins and are involved in a diverse array of physiological processes. Despite progress in the mass spectrometry of membrane protein complexes, G protein-coupled receptors have remained intractable because of their low yield and instability after extraction from cell membranes. We established conditions in the mass spectrometer that preserve noncovalent ligand binding to the human purinergic receptor P2Y1. Results established differing affinities for nucleotides and the drug MRS2500 and link antagonist binding with the absence of receptor phosphorylation. Overall, therefore, our results are consistent with drug binding, preventing the conformational changes that facilitate downstream signaling. More generally, we highlight opportunities for mass spectrometry to probe effects of ligand binding on G protein-coupled receptors.

The P2Y1 receptor-mediated leukocyte adhesion to endothelial cells is inhibited by melatonin

Extracellular ATP (released by endothelial and immune cells) and its metabolite ADP are important pro-inflammatory mediators via the activation of purinergic P2 receptors (P2Y and P2X), which represent potential new targets for anti-inflammatory therapy. Endothelial P2Y₁ receptor (P2Y₁R) induces endothelial cell activation triggering leukocyte adhesion. A number of data have implicated melatonin as a modulator of immunity, inflammation, and endothelial cell function, but to date no studies have investigated whether melatonin modulates endothelial P2YR signaling. Here, we evaluated the putative effect of melatonin on P2Y₁R-mediated leukocyte adhesion to endothelial cells and TNF-α production, using mesenteric endothelial cells and fresh peripheral blood mononuclear cells isolated from rats. Endothelial cells were treated with the P2Y₁R agonist 2MeSATP, alone or in combination with melatonin, and then exposed to mononuclear cells. 2MeSATP increased leukocyte adhesion to endothelial cells and TNF-α production in vitro, and melatonin inhibited both effects without altering P2Y₁R protein expression. In addition, assays with the Ca²⁺ chelator BAPTA-AM indicate that the effect of melatonin on 2MeSATP-stimulated leukocyte adhesion depends on intracellular Ca²⁺ modulation. P2Y₁R is considered a potential target to control chronic inflammation. Therefore, our data unveiled a new endothelial cell modulator of purinergic P2Y₁ receptor signaling.

Purinergic P2Y6 receptors: A new therapeutic target of age-dependent hypertension

Aging has a remarkable effect on cardiovascular homeostasis and it is known as the major non-modifiable risk factor in the development of hypertension. Medications targeting sympathetic nerve system and/or renin-angiotensin-aldosterone system are widely accepted as a powerful therapeutic strategy to improve hypertension, although the control rates remain unsatisfactory especially in the elder patients with hypertension. Purinergic receptors, activated by adenine, uridine nucleotides and nucleotide sugars, play pivotal roles in many biological processes, including platelet aggregation, neurotransmission and hormone release, and regulation of cardiovascular contractility. Since clopidogrel, a selective inhibitor of G protein-coupled purinergic P2Y₁₂ receptor (P2Y₁₂R), achieved clinical success as an anti-platelet drug, P2YRs has been attracted more attention as new therapeutic targets of cardiovascular diseases. We have revealed that UDP-responsive P2Y₆R promoted angiotensin type 1 receptor (AT1R)-stimulated vascular remodeling in mice, in an age-dependent manner. Moreover, the age-related formation of heterodimer between AT1R and P2Y₆R was disrupted by MRS2578, a P2Y₆R-selective inhibitor. These findings suggest that P2Y₆R is a therapeutic target to prevent age-related hypertension.

In silico Approach for Anti-Thrombosis Drug Discovery: P2Y1R Structure-Based TCMs Screening

Cardiovascular diseases (CVDs), including thrombosis, which is induced by platelet aggregation, are the leading cause of mortality worldwide. The P2Y1 receptor (P2Y1R) facilitates platelet aggregation and is thus an important potential anti-thrombotic drug target. The P2Y1R protein structure contains a binding site for receptor antagonist MRS2500 within its seven-transmembrane bundle, which also provides suitable pockets for numerous other ligands to act as nucleotide antagonists of P2Y1R. The Traditional Chinese Medicine Systems Pharmacology Database and Analysis Platform (TCMSP) comprises 499 Chinese Pharmacopoeia-registered herbs and the structure information for 29,384 ingredients. In silico docking of these compounds into the P2Y1R protein structure within the MRS2500 pocket can identify potential antithrombotic drugs from natural medicinal plants. Docking studies were performed and scored to evaluate ligand-binding affinities. In this study, a total of 8987 compounds from Traditional Chinese Medicine (TCM) were filtered by Lipinski's rule of five, and their ideal oral-intake properties were evaluated. Of these, 1656 compounds distributed in 443 herbs docked into the P2Y1R-MRS2500 structure in 16,317 poses. A total of 38 compounds were ranked with a DockScore above 70, and these may have significant potential for development into anti-thrombosis drugs. These computational results suggested that licorice (Glycyrrhiza uralensis Fisch), cimicifugae (Cimicifuga foetida L.), and ganoderma (Ganoderma lucidum Karst) and their chemical constituents, which have not previously been widely used for anti-thrombosis, may have unexpected effects on platelet aggregation. Moreover, two types of triterpene scaffolds summarized from 10 compounds were distributed in these three herbs and also docked into P2Y1R. These scaffold structures may be utilized for the development of drugs to inhibit platelet aggregation.

Lung injury during LPS-induced inflammation occurs independently of the receptor P2Y1

Disruption of the lung endothelial and epithelial barriers during acute inflammation leads to excessive neutrophil migration. It is likely that activated platelets promote pulmonary recruitment of neutrophils during inflammation, and previous studies have found that anti-platelet therapy and depletion of circulating platelets have lung-protective effects in different models of inflammation. Because ADP signaling is important for platelet activation, I investigated the role of the ADP-receptor P2Y₁, a G protein-coupled receptor expressed on the surface of circulating platelets, during lipopolysaccharide (LPS)-induced inflammation and lung injury in P2Y₁-null and wild-type mice. Systemic inflammation was induced by a single intraperitoneal dose of LPS (3 mg/kg), and the mice were analyzed 24 h posttreatment. The data show that the LPS-induced inflammation levels were comparable in the P2Y₁-null and wild-type mice. Specifically, splenomegaly, counts of circulating platelets and white blood cells (lymphocytes and neutrophils), and assessments of lung injury (tissue architecture and cell infiltration) were similar in the P2Y₁-null and wild-type mice. Based on my results, I conclude that lung injury during LPS-induced inflammation in mice is independent of P2Y₁ signaling. I propose that if a blockade of purinergic signaling in platelets is a potential lung-protective strategy in the treatment of acute inflammation, then it is more likely to be a result of the disruption of the signaling pathway mediated by P2Y₁₂, another G protein-coupled receptor that mediates platelet responses to ADP.

Purinergic signaling in schistosomal infection

Human schistosomiasis is a chronic inflammatory disease caused by blood fluke worms belonging to the genus Schistosoma. Health metrics indicate that the disease is related to an elevated number of years lost-to-disability and years lost-to-life. Schistosomiasis is an intravascular disease that is related to a Th1 and Th2 immune response polarization, and the degree of polarization affects the outcome of the disease. The purinergic system is composed of adenosine and nucleotides acting as key messenger molecules. Moreover, nucleotide-transforming enzymes and cell-surface purinergic receptors are obligatory partners of this purinergic signaling. In mammalian cells, purinergic signaling modulates innate immune responses and inflammation among other functions; conversely purinergic signaling may also be modulated by inflammatory mediators. Moreover, schistosomes also express some enzymes of the purinergic system, and it is possible that worms modulate host purinergic signaling. Current data obtained in murine models of schistosomiasis support the notion that the host purinergic system is altered by the disease. The dysfunction of adenosine receptors, metabotropic P2Y and ionotropic P2X₇ receptors, and NTPDases likely contributes to disease morbidity.

The Role of Purine Metabolites as DAMPs in Acute Graft-versus-Host Disease

Acute graft-versus-host disease (GvHD) causes high mortality in patients undergoing allogeneic hematopoietic cell transplantation. An early event in the classical pathogenesis of acute GvHD is tissue damage caused by the conditioning treatment or infection that consecutively leads to translocation of bacterial products [pathogen-associated molecular patterns (PAMPs)] into blood or lymphoid tissue, as well as danger-associated molecular patterns (DAMPs), mostly intracellular components that act as pro-inflammatory agents, once they are released into the extracellular space. A subtype of DAMPs is nucleotides, such as adenosine triphosphate released from dying cells that can activate the innate and adaptive immune system by binding to purinergic receptors. Binding to certain purinergic receptors leads to a pro-inflammatory microenvironment and promotes allogeneic T cell priming. After priming, T cells migrate to the acute GvHD target organs, mainly skin, liver, and the gastrointestinal tract and induce cell damage that further amplifies the release of intracellular components. This review summarizes the role of different purinergic receptors in particular P2X7 and P2Y2 as well as nucleotides in the pathogenesis of GvHD.

BPTU, an allosteric antagonist of P2Y1 receptor, blocks nerve mediated inhibitory neuromuscular responses in the gastrointestinal tract of rodents

P2Y1 receptors mediate nerve mediated purinergic inhibitory junction potentials (IJP) and relaxations in the gastrointestinal (GI) tract in a wide range of species including rodents and humans. A new P2Y1 antagonist, with a non-nucleotide structure, BPTU, has recently been described using X-ray crystallography as the first allosteric G-protein-coupled receptor antagonist located entirely outside of the helical bundle. In this study, we tested its effect on purinergic responses in the gastrointestinal tract of rodents using electrophysiological and myographic techniques. BPTU concentration dependently inhibited purinergic inhibitory junction potentials and inhibition of spontaneous motility induced by electrical field stimulation in the colon of rats (EC50 = 0.3 μM) and mice (EC50 = 0.06 μM). Mechanical inhibitory responses were also concentration-dependently blocked in the stomach of both species. Compared to MRS2500, BPTU displays a lower potency. In the rat colon nicotine induced relaxation was also blocked by BPTU. BPTU also blocked the cessation of spontaneous contractility elicited by ADPβS and the P2Y1 agonist MRS2365. We conclude that BPTU is a novel antagonist with different structural and functional properties than nucleotidic antagonists that is able to block the P2Y1 receptor located at the neuromuscular junction of the GI tract.

Extracellular ATP Induces Vascular Inflammation and Atherosclerosis via Purinergic Receptor Y2 in Mice

OBJECTIVE

A solid body of evidence supports a role of extracellular ATP and its P2 receptors in innate and adaptive immunity. It promotes inflammation as a danger signal in various chronic inflammatory diseases. Thus, we hypothesize contribution of extracellular ATP and its receptor P2Y2 in vascular inflammation and atherosclerosis.

APPROACH AND RESULTS

Extracellular ATP induced leukocyte rolling, adhesion, and migration in vivo as assessed by intravital microscopy and in sterile peritonitis. To test the role of extracellular ATP in atherosclerosis, ATP or saline as control was injected intraperitoneally 3× a week in low-density lipoprotein receptor(-/-) mice consuming high cholesterol diet. Atherosclerosis significantly increased after 16 weeks in ATP-treated mice (n=13; control group, 0.26 mm2; ATP group, 0.33 mm2; P=0.01). To gain into the role of ATP-receptor P2Y2 in ATP-induced leukocyte recruitment, ATP was administered systemically in P2Y2-deficient or P2Y2-competent mice. In P2Y2-deficient mice, the ATP-induced leukocyte adhesion was significantly reduced as assessed by intravital microscopy. P2Y2 expression in atherosclerosis was measured by real-time polymerase chain reaction and immunohistochemistry and demonstrates an increased expression mainly caused by influx of P2Y2-expressing macrophages. To investigate the functional role of P2Y2 in atherogenesis, P2Y2-deficient low-density lipoprotein receptor(-/-) mice consumed high cholesterol diet. After 16 weeks, P2Y2-deficient mice showed significantly reduced atherosclerotic lesions with decreased macrophages compared with P2Y2-competent mice (n=11; aortic arch: control group, 0.25 mm(2); P2Y2-deficient, 0.14 mm2; P=0.04). Mechanistically, atherosclerotic lesions from P2Y2-deficient mice expressed less vascular cell adhesion molecule-1 and intercellular adhesion molecule-1 RNA.

CONCLUSIONS

We show that extracellular ATP induces vascular inflammation and atherosclerosis via activation of P2Y2.

P2Y12 Receptor Modulates Sepsis-Induced Inflammation

OBJECTIVE

Platelets modulate hemostasis and immune responses via interactions with immune cells through secretion of immunemodulators and cell-cell interactions. The P2Y12 receptor mediates ADP-induced aggregation and secretion in platelets.

APPROACH AND RESULTS

Using a mouse model of intra-abdominal sepsis and acute lung injury, we investigated the role of the P2Y12 receptor in neutrophil migration and lung inflammation in P2Y12 null mice and in mice pretreated with the P2Y12 antagonist clopidogrel. Our data show a decrease in circulating white blood cells and a decrease in platelet activation and platelet-leukocyte interactions in treated mice compared with untreated mice. Additionally, lung injury and platelet sequestration were diminished in clopidogrel-treated mice compared with their untreated septic littermates. Similar results were observed in P2Y12 null mice: platelet activation and platelet-leukocyte aggregates were decreased in septic P2Y12 null mice compared with wild-type mice. P2Y12 null mice were refractory to lung injury compared with wild-type mice. Finally, to evaluate P2Y12-independent effects of clopidogrel, we pretreated P2Y12 null mice. Interestingly, the number of circulating neutrophils was reduced in treated septic P2Y12 null mice, suggesting neutrophils as a target for clopidogrel pleiotropic effects. No difference was observed in P2Y1 null mice during sepsis, indicating that the P2Y12 receptor is responsible for the effects.

CONCLUSIONS

P2Y12 null mice are refractory to sepsis-induced lung injury, suggesting a key role for activated platelets and the P2Y12 receptor during sepsis.

Increased expression of NTPDases 2 and 3 in mesenteric endothelial cells during schistosomiasis favors leukocyte adhesion through P2Y1 receptors

Schistosomiasis is caused by an intravascular parasite and linked to phenotypic changes in endothelial cells that favor inflammation. Endothelial cells express P2Y1 receptors (P2Y1R), and their activation by ADP favors leukocyte adhesion to the endothelial monolayer. We aimed to evaluate the influence of schistosomiasis upon endothelial purinergic signaling-mediated leukocyte adhesion. Mesenteric endothelial cells and mononuclear cells from control and Schistosoma mansoni-infected mice were used in co-culture. P2Y1R levels were similar in both groups. Basal leukocyte adhesion was higher in the infected than in the control group; leukocyte adhesion increased after treatment with the P2Y1R agonist 2-MeSATP in both groups, though it only marginally increased in the infected group. Pre-incubation with the selective P2Y1R antagonist MRS2179 (0.3μM) prevented the agonist effect. However, in the infected group it also reduced the basal leukocyte adhesion, suggesting endothelial cell pre-activation. The endothelial expressions of NTPDases 2 and 3 were significantly increased in the infected group, increasing extracellular ATP hydrolysis and ADP formation by endothelial cells. Therefore, mesenteric endothelial cells are primed by schistosomiasis to a pro-inflammatory phenotype characterized by an increased expression of NTPDases 2 and 3, favoring ADP accumulation and mononuclear cell adhesion, possibly contributing to mesenteric inflammation and schistosomiasis morbidity.

Pharmacology and structure of P2Y receptors

P2Y receptors are G-protein-coupled receptors (GPCRs) for extracellular nucleotides. There are eight mammalian P2Y receptor subtypes (P2Y1, P2Y2, P2Y4, P2Y6, P2Y11, P2Y12, P2Y13, and P2Y14). P2Y receptors are widely expressed and play important roles in physiology and pathophysiology. One important example is the ADP-induced platelet aggregation mediated by P2Y1 and P2Y12 receptors. Active metabolites of the thienopyridine compounds ticlopidine, clopidogrel and prasugrel as well as the nucleoside analogue ticagrelor block P2Y12 receptors and thereby platelet aggregation. These drugs are used for the prevention and therapy of cardiovascular events. Moreover, P2Y receptors play important roles in the nervous system. Adenine nucleotides modulate neuronal activity and neuronal fibre outgrowth by activation of P2Y1 receptors and control migration of microglia by P2Y12 receptors. UDP stimulates microglial phagocytosis through activation of P2Y6 receptors. There is evidence for a role for P2Y2 receptors in Alzheimer's disease pathology. The P2Y receptor subtypes are highly diverse in both their amino acid sequences and their pharmacological profiles. Selective receptor ligands have been developed for the pharmacological characterization of the receptor subtypes. The recently published three-dimensional crystal structures of the human P2Y1 and P2Y12 receptors will facilitate the development of therapeutic agents that selectively target P2Y receptors. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

Purinergic Receptors in Thrombosis and Inflammation

Under various pathological conditions, including thrombosis and inflammation, extracellular nucleotide levels may increase because of both active release and passive leakage from damaged or dying cells. Once in the extracellular compartment, nucleotides interact with plasma membrane receptors belonging to the P2 purinergic family, which are expressed by virtually all circulating blood cells and in most blood vessels. In this review, we focus on the specific role of the 3 platelet P2 receptors P2Y1, P2Y12, and P2X1 in hemostasis and arterial thrombosis. Beyond platelets, these 3 receptors, along with the P2Y2, P2Y6, and P2X7 receptors, constitute the main P2 receptors mediating the proinflammatory effects of nucleotides, which play important roles in various functions of circulating blood cells and cells of the vessel wall. Each of these P2 receptor subtypes specifically contributes to chronic or acute vascular inflammation and related diseases, such as atherosclerosis, restenosis, endotoxemia, and sepsis. The potential for therapeutic targeting of these P2 receptor subtypes is also discussed.

Pannexin 1 channels regulate leukocyte emigration through the venous endothelium during acute inflammation

Inflammatory cell recruitment to local sites of tissue injury and/or infection is controlled by a plethora of signalling processes influencing cell-to-cell interactions between the vascular endothelial cells (ECs) in post-capillary venules and circulating leukocytes. Recently, ATP-sensitive P₂Y purinergic receptors have emerged as downstream regulators of EC activation in vascular inflammation. However, the mechanism(s) regulating cellular ATP release in this response remains elusive. Here we report that the ATP-release channel Pannexin1 (Panx1) opens downstream of EC activation by TNF-α. This process involves activation of type-1 TNF receptors, recruitment of Src family kinases (SFK) and SFK-dependent phosphorylation of Panx1. Using an inducible, EC-specific Panx1 knockout mouse line, we report a previously unidentified role for Panx1 channels in promoting leukocyte adhesion and emigration through the venous wall during acute systemic inflammation, placing Panx1 channels at the centre of cytokine crosstalk with purinergic signalling in the endothelium.

Endothelial cell activation by inflammation requires extracellular ATP release. Here the authors show that TNF-α induces Src-family kinase-dependent ATP release by Pannexin1 channels in endothelial cells, and that Pannexin1 is required for leukocyte adhesion and emigration into the inflamed tissue.

Two disparate ligand-binding sites in the human P2Y1 receptor

In response to adenosine 5'-diphosphate, the P2Y1 receptor (P2Y1R) facilitates platelet aggregation, and thus serves as an important antithrombotic drug target. Here we report the crystal structures of the human P2Y1R in complex with a nucleotide antagonist MRS2500 at 2.7 Å resolution, and with a non-nucleotide antagonist BPTU at 2.2 Å resolution. The structures reveal two distinct ligand-binding sites, providing atomic details of P2Y1R's unique ligand-binding modes. MRS2500 recognizes a binding site within the seven transmembrane bundle of P2Y1R, which is different in shape and location from the nucleotide binding site in the previously determined structure of P2Y12R, representative of another P2YR subfamily. BPTU binds to an allosteric pocket on the external receptor interface with the lipid bilayer, making it the first structurally characterized selective G-protein-coupled receptor (GPCR) ligand located entirely outside of the helical bundle. These high-resolution insights into P2Y1R should enable discovery of new orthosteric and allosteric antithrombotic drugs with reduced adverse effects.

Pentraxin 3 Induces Vascular Endothelial Dysfunction Through a P-selectin/Matrix Metalloproteinase-1 Pathway

BACKGROUND

Pentraxin 3 (PTX3), the prototype of long pentraxins, has been described to be associated with endothelial dysfunction in different cardiovascular disorders. No study has yet evaluated the possible direct effect of PTX3 on vascular function.

METHODS AND RESULTS

Through in vitro experiments of vascular reactivity and ultrastructural analyses, we demonstrate that PTX3 induces dysfunction and morphological changes in the endothelial layer through a P-selectin/matrix metalloproteinase-1 pathway. The latter hampered the detachment of endothelial nitric oxide synthase from caveolin-1, leading to an impairment of nitric oxide signaling. In vivo studies showed that administering PTX3 to wild-type mice induced endothelial dysfunction and increased blood pressure, an effect absent in P-selectin-deficient mice. In isolated human umbilical vein endothelial cells, PTX3 significantly blunted nitric oxide production through the matrix metalloproteinase-1 pathway. Finally, using ELISA, we found that hypertensive patients (n=31) have higher plasma levels of PTX3 and its mediators P-selectin and matrix metalloproteinase-1 than normotensive subjects (n=21).

CONCLUSIONS

Our data show for the first time a direct role of PTX3 on vascular function and blood pressure homeostasis, identifying the molecular mechanisms involved. The findings in humans suggest that PTX3, P-selectin, and matrix metalloproteinase-1 may be novel biomarkers that predict the onset of vascular dysfunction in hypertensive patients.

Purinergic signaling in atherosclerosis

Cell surface expression of specific receptors and ecto-nucleotidases makes extracellular nucleotides such as ATP, ADP, UTP, and adenosine suitable as signaling molecules for physiological and pathological events, including tissue stress and damage. Recent data have revealed the participation of purinergic signaling in atherosclerosis, depicting a scenario in which, in addition to some exceptions reflecting dual effects of individual receptor subtypes, adenosine and most P1 receptors, as well as ecto-nucleotidases, show a protective, anti-atherosclerotic function. By contrast, P2 receptors promote atherosclerosis. In consideration of these findings, modulation of purinergic signaling would represent an innovative and valuable tool to counteract atherosclerosis. We summarize recent developments on the participation of the purinergic network in atheroma formation and evolution.

The platelet P2 receptors in inflammation

In addition to their well characterized and established role in haemostasis and thrombosis, platelets contribute to the pathogenesis of inflammation. Adenine nucleotides are signalling molecules that regulate the function of virtually every cell in the body, by interacting with P2 receptors. Their important role in inflammation is well established. In the last few years, the pro-inflammatory roles of adenine nucleotides interacting with their platelet P2 receptors has emerged. In particular, it was shown that the platelet P2Y12 receptor for ADP significantly contributed to the pro-inflammatory effects of cysteinyl leukotrienes (CysLT) in experimental models of asthma in mice. More importantly, it was recently shown that P2Y12 variants were associated with lung function in a large family-based asthma cohort and that the P2Y12 antagonist prasugrel tended to decrease bronchial hyper-reactivity to mannitol in patients with allergic bronchial asthma in a randomized, placebo controlled trial.

CONCLUSION

These data strongly suggest that P2Y12 may represent an important pharmacological target for the treatment of patients with allergic bronchial asthma.

The P2X1 receptor is required for neutrophil extravasation during lipopolysaccharide-induced lethal endotoxemia in mice

Extracellular ATP is becoming increasingly recognized as an important regulator of inflammation. However, the known repertoire of P2 receptor subtypes responsible for the proinflammatory effects of ATP is sparse. We looked at whether the P2X1 receptor, an ATP-gated cation channel present on platelets, neutrophils, and macrophages, participates in the acute systemic inflammation provoked by LPS. Compared with wild-type (WT) mice, P2X1(-/-) mice displayed strongly diminished pathological responses, with dampened neutrophil accumulation in the lungs, less tissue damage, reduced activation of coagulation, and resistance to LPS-induced death. P2X1 receptor deficiency also was associated with a marked reduction in plasma levels of the main proinflammatory cytokines and chemokines induced by LPS. Interestingly, macrophages and neutrophils isolated from WT and P2X1(-/-) mice produced similar levels of proinflammatory cytokines when stimulated with LPS in vitro. Intravital microscopy revealed a defect in LPS-induced neutrophil emigration from cremaster venules into the tissues of P2X1(-/-) mice. Using adoptive transfer of immunofluorescently labeled neutrophils from WT and P2X1(-/-) mice into WT mice, we demonstrate that the absence of the P2X1 receptor on neutrophils was responsible for this defect. This study reveals a major role for the P2X1 receptor in LPS-induced lethal endotoxemia through its critical involvement in neutrophil emigration from venules.

Platelets in inflammation and immunity

The paradigm of platelets as mere mediators of hemostasis has long since been replaced by a dual role: hemostasis and inflammation. Now recognized as key players in innate and adaptive immune responses, platelets have the capacity to interact with almost all known immune cells. These platelet-immune cell interactions represent a hallmark of immunity, as they can potently enhance immune cell functions and, in some cases, even constitute a prerequisite for host defense mechanisms such as NETosis. In addition, recent studies have revealed a new role for platelets in immunity: They are ubiquitous sentinels and rapid first-line immune responders, as platelet-pathogen interactions within the vasculature appear to precede all other host defense mechanisms. Here, we discuss recent advances in our understanding of platelets as inflammatory cells, and provide an exemplary review of their role in acute inflammation.