Adenosine and blood platelets

High-Resolution Proteomics Unveils Salivary Gland Disruption and Saliva-Hemolymph Protein Exchange in Plasmodium-Infected Mosquitoes

Plasmodium sporozoites, the stage that initiates a malaria infection, must invade the mosquito salivary glands (SGs) before transmitting to a vertebrate host. However, the effects of sporozoite invasion on salivary gland physiology and saliva composition remain largely unexplored. We examined the impact of Plasmodium infection on Anopheles gambiae salivary glands using high-resolution proteomics, gene expression, and morphological analysis. The data revealed differential expression of various proteins, including the enrichment of humoral proteins in infected salivary glands originating from the hemolymph. These proteins diffused into the SGs due to structural damage caused by the sporozoites during invasion. Conversely, saliva proteins diffused out into the circulation of infected mosquitoes. Moreover, infection altered saliva protein composition, as shown by proteomes from saliva collected from mosquitoes infected by P. berghei or P. falciparum, revealing a significant reduction of immune proteins compared to uninfected mosquitoes. This reduction is likely due to the association of these proteins with the surface of sporozoites within the mosquito salivary secretory cavities. The saliva protein profiles from mosquitoes infected with both Plasmodium species were remarkably similar, suggesting a conserved interaction between sporozoites and salivary glands. Our results provide a foundation for understanding the molecular interactions between Plasmodium sporozoites and mosquito salivary glands.

The Effects of Caffeine on Blood Platelets and the Cardiovascular System through Adenosine Receptors

Caffeine is the most popular and widely consumed behaviourally active substance in the world. This review describes the influence of caffeine on the cardiovascular system, with a special focus on blood platelets. For many years, caffeine was thought to have a negative effect on the cardiovascular system mainly due to increasing blood pressure. However, more recent data suggest that habitual caffeine consumption may reduce the risk of cardiovascular disease and hypertension. This could be a significant finding as cardiovascular disease is the leading cause of death worldwide. Caffeine is known to inhibit A1 adenosine receptors, through which it is believed to modulate inter alia coronary blood flow, total peripheral resistance, diuresis, and heart rate. It has been shown that coffee possesses antiplatelet activity, but depending on the dose and the term of its use, caffeine may stimulate or inhibit platelet reactivity. Also, chronic exposure to caffeine may sensitize or upregulate the adenosine receptors in platelets causing increased cAMP accumulation and anti-aggregatory effects and decrease calcium levels elicited by AR agonists. The search for new, selective, and safe AR agonists is one of the new strategies for improving antiplatelet therapy involving targeting multiple pathways of platelet activation. Therefore, this review examines the AR-dependent impact of caffeine on blood platelets in the presence of adenosine receptor agonists.

Platelet-Mediated Transfer of Cardioprotection by Remote Ischemic Conditioning and Its Abrogation by Aspirin But Not by Ticagrelor

PURPOSE

The role of platelets during myocardial ischemia/reperfusion (I/R) is ambivalent. They contribute to injury but also to cardioprotection. Repeated blood flow restriction and reperfusion in a tissue/organ remote from the heart (remote ischemic conditioning, RIC) reduce myocardial I/R injury and attenuate platelet activation. Whether or not platelets mediate RIC's cardioprotective signal is currently unclear.

METHODS AND RESULTS

Venous blood from healthy volunteers (without or with pretreatment of 500/1000 mg aspirin or 180 mg ticagrelor orally, 2-3 h before the study, n = 18 each) was collected before and after RIC (3 × 5 min blood pressure cuff inflation at 200 mmHg on the left upper arm/5 min deflation). Washed platelets were isolated. Platelet-poor plasma was used to prepare plasma-dialysates. Platelets (25 × 103/µL) or plasma-dialysates (1:10) prepared before and after RIC from untreated versus aspirin- or ticagrelor-pretreated volunteers, respectively, were infused into isolated buffer-perfused rat hearts. Hearts were subjected to global 30 min/120 min I/R. Infarct size was stained. Infarct size was less with infusion of platelets/plasma-dialysate after RIC (18 ± 7%/23 ± 9% of ventricular mass) than with platelets/plasma-dialysate before RIC (34 ± 7%/33 ± 8%). Aspirin pretreatment abrogated the transfer of RIC's cardioprotection by platelets (after/before RIC, 34 ± 7%/33 ± 7%) but only attenuated that by plasma-dialysate (after/before RIC, 26 ± 8%/32 ± 5%). Ticagrelor pretreatment induced an in vivo formation of cardioprotective factor(s) per se (platelets/plasma-dialysate before RIC, 26 ± 7%/26 ± 7%) but did not impact on RIC's cardioprotection by platelets/plasma-dialysate (20 ± 7%/21 ± 5%).

CONCLUSION

Platelets serve as carriers for RIC's cardioprotective signal through an aspirin-sensitive and thus cyclooxygenase-dependent mechanism. The P2Y12 inhibitor ticagrelor per se induces a humoral cardioprotective signal.

Association of ADP-Induced Whole-Blood Platelet Aggregation with Serum Low-Density Lipoprotein Cholesterol in Patients with Coronary Artery Disease When Receiving Maintenance Ticagrelor-Based Dual Antiplatelet Therapy

The degree of platelet inhibition in patients undergoing dual antiplatelet therapy (DAPT) affects cardiovascular outcomes after acute coronary syndromes (ACS) and/or percutaneous coronary intervention. Our aim was to search for correlates of residual ex vivo platelet reactivity and circulating soluble P-selectin (sP-selectin), an index of in vivo platelet activation, in patients being treated by DAPT with ticagrelor. Adenosine diphosphate (ADP)-induced platelet aggregability (by multiple electrode aggregometry) and plasma sP-selectin were estimated in 62 stable post-ACS subjects (46 men and 16 women; mean age: 64 ± 10 years; 30 with type 2 diabetes (T2DM)) undergoing maintenance DAPT with ticagrelor and aspirin. These patients did not exhibit heart failure or other relevant coexistent diseases except for properly controlled T2DM, mild renal insufficiency, and hypertension. We also assessed this in 64 subjects on clopidogrel-based DAPT matched for age, sex, and T2DM status. ADP-induced platelet aggregation was below the optimal levels (190-460 arbitrary units (AU) * min) in most patients receiving ticagrelor-based DAPT, especially in those with below-median (<1.9 mmol/L) serum concentrations of low-density lipoprotein cholesterol (LDL-c) (128 ± 61 vs. 167 ± 73 AU * min for below-median and above-median LDL-c, respectively, p = 0.025). In contrast, platelet reactivity did not differ by LDL-c on clopidogrel-based DAPT (246 ± 101 vs. 268 ± 108 AU * min for below-median and above-median LDL-c, respectively, p > 0.4). Plasma sP-selectin was found to be unrelated to serum LDL-c when receiving DAPT with ticagrelor (p > 0.4) or clopidogrel (p > 0.8). In conclusion, our preliminary observational study suggests the association of lower residual ex vivo platelet aggregability with better LDL-c control in patients undergoing ticagrelor-based maintenance DAPT, which does not appear to be reflected by plasma sP-selectin. Whether the serum LDL-c level should be considered among the factors affecting the degree of platelet inhibition for those treated with ticagrelor-based DAPT needs to be investigated in larger studies.

Endothelial Cell Dysfunction and Increased Cardiovascular Risk in Patients With Chronic Kidney Disease

The endothelium is considered to be the gatekeeper of the vessel wall, maintaining and regulating vascular integrity. In patients with chronic kidney disease, protective endothelial cell functions are impaired due to the proinflammatory, prothrombotic and uremic environment caused by the decline in kidney function, adding to the increase in cardiovascular complications in this vulnerable patient population. In this review, we discuss endothelial cell functioning in healthy conditions and the contribution of endothelial cell dysfunction to cardiovascular disease. Further, we summarize the phenotypic changes of the endothelium in chronic kidney disease patients and the relation of endothelial cell dysfunction to cardiovascular risk in chronic kidney disease. We also review the mechanisms that underlie endothelial changes in chronic kidney disease and consider potential pharmacological interventions that can ameliorate endothelial health.

Effects of uridine and nucleotides on hemostasis parameters

Several purinergic receptors have been identified on platelets which are involved in hemostatic and thrombotic processes. The aim of the present study was to investigate the effects of uridine and its nucleotides on platelet aggregation and hemostasis in platelet-rich plasma (PRP) and whole blood. The effects of uridine, UMP, UDP, and UTP at different final concentrations (1 to 1000 µM) on platelet aggregation were studied using an aggregometer. In PRP samples, platelet aggregation was induced by ADP, collagen and epinephrine 3 min after addition of uridine, UMP, UDP, UTP and saline (as a control). All thromboelastogram experiments were performed at 1000 µM final concentrations of uridine and its nucleotides in whole blood. UDP and UTP were also tested in thromboelastogram with PRP. Our results showed that UDP, and especially UTP, inhibited ADP- and collagen-induced aggregation in a concentration-dependent manner. In whole blood thromboelastogram experiments, UDP stimulated clot formation while UTP suppressed clot formation. When thromboelastogram experiments were repeated with PRP, UTP's inhibitory effect on platelets was confirmed, while UDP's stimulated clot forming effect disappeared. Collectively, our data showed that UTP inhibited platelet aggregation in a concentration-dependent manner and suppressed clot formation. On the other hand, UDP exhibited distinct effects on whole blood or PRP in thromboelastogram. These data suggest that the difference on effects of UTP and UDP might have arisen from the different receptors that they stimulate and warrant further investigation with regard to their in vivo actions on platelet aggregation and hemostasis.

Mechanistic considerations for adenosine-lidocaine-magnesium (ALM) in controlling coagulopathy

Adenosine-lidocaine-magnesium (ALM) mixture is a cardioplegic agent that improves survivability in rodent, but not swine, models of noncompressible torso hemorrhage (NCTH). However, despite protection from comorbid coagulopathy being the one common effect reported in both NCTH models, the underlying prothrombotic mechanism for ALM has not been elucidated in either. Here, we undertook a component-based approach focusing on individual drugs in the mixture to elaborate on the protective mechanism against coagulopathy within the frames of adenosine signaling and metabolic pathways. Additionally, the translational potential of small and large animal models of NCTH for human survival is critically appraised, owing to substantial quantitative/qualitative differences between humans and rodents, particularly regarding the genetics of G protein-coupled receptors (GPCRs) interacting with ALM's constituents.

Adenosine 2A Receptor Activation Amplifies Ibrutinib Antiplatelet Effect; Implications in Chronic Lymphocytic Leukemia

Chronic lymphocytic leukemia patients have an increased bleeding risk with the introduction of Bruton tyrosine kinase (BTK) inhibitors. BTK is a signaling effector downstream of the platelet GPVI receptor. Innate platelet dysfunction in CLL patients and the contribution of the leukemia microenvironment to the anti-platelet effect of BTK inhibitors are still not well defined. Herein, we investigated platelet function in stable, untreated CLL patients in comparison to age-matched healthy subjects as control. Secondly, we proposed a novel mechanism of platelet dysfunction via the adenosinergic pathway during BTK inhibitor therapy. Our data indicate that the nucleotidase that produces adenosine, CD73, was expressed on one-third of B-cells in CLL patients. Inhibition of CD73 improved platelet response to ADP in the blood of CLL patients ex vivo. Using healthy platelets, we show that adenosine 2A (A2A) receptor activation amplifies the anti-platelet effect of ibrutinib (10 nM). Ibrutinib plus an A2A agonist-but not ibrutinib as a single agent-significantly inhibited collagen (10 µg/mL)-induced platelet aggregation. Mechanistically, A2A activation attenuated collagen-mediated inhibition of p-VASP and synergized with ibrutinib to inhibit the phosphorylation of AKT, ERK and SYK kinases. This manuscript highlights the potential role of adenosine generated by the microenvironment in ibrutinib-associated bleeding in CLL patients.

Endothelial biomarkers and platelet reactivity on ticagrelor versus clopidogrel in patients after acute coronary syndrome with and without concomitant type 2 diabetes: a preliminary observational study

BACKGROUND

Pleiotropic effects have been implicated in clinical benefits of ticagrelor compared to thienopyridine P2Y₁₂ antagonists. There are conflicting data regarding effects of ticagrelor vs. thienopyridine P2Y₁₂ blockers on endothelial function. Our aim was to compare endothelial biomarkers and their relations with platelet reactivity in real-world patients after acute coronary syndrome (ACS) on maintenance dual antiplatelet therapy (DAPT) with ticagrelor or clopidogrel stratified by diabetes status.

METHODS

Biochemical indices of endothelial dysfunction/activation and platelet reactivity by multiple electrode aggregometry were compared in 126 stable post-ACS subjects (mean age: 65 ± 10 years, 92 men and 34 women), including patients with (n = 61) or without (n = 65) coexistent type 2 diabetes (T2DM) on uneventful maintenance DAPT with either ticagrelor (90 mg b.d.) or clopidogrel (75 mg o.d.) in addition to low-dose aspirin. Exclusion criteria included a complicated in-hospital course, symptomatic heart failure, left ventricular ejection fraction < 40% and relevant coexistent diseases except for well-controlled diabetes, mild renal insufficiency or hypertension.

RESULTS

Clinical characteristics were similar in patients on ticagrelor (n = 62) and clopidogrel (n = 64). The adenosine diphosphate-induced platelet aggregation and circulating soluble P-selectin (sP-selectin) were decreased in ticagrelor users irrespective of T2DM status (p < 0.001 and p < 0.01 for platelet reactivity and sP-selectin, respectively). Plasma levels of soluble vascular cell adhesion molecule-1 (sVCAM-1) were lower in T2DM subjects on ticagrelor vs. clopidogrel (758 ± 162 vs. 913 ± 217 µg/L, p < 0.01). In contrast, plasma sVCAM-1 was similar in non-diabetic patients on ticagrelor and clopidogrel (872 ± 203 vs. 821 ± 210 µg/L, p > 0.7). The concentrations of sE-selectin, monocyte chemoattractant protein-1 and asymmetric dimethylarginine did not differ according to the type of P2Y₁₂ antagonist regardless of T2DM status. Platelet reactivity was unrelated to any endothelial biomarker in subjects with or without T2DM.

CONCLUSIONS

Our preliminary findings may suggest an association of ticagrelor-based maintenance DAPT with favorable endothelial effects compared to clopidogrel users in stable post-ACS patients with T2DM. If proven, this could contribute to more pronounced clinical benefits of ticagrelor in diabetic subjects.

Assessment of myocardial salvage in patients with STEMI undergoing thrombolysis: ticagrelor versus clopidogrel

Background

In the setting of ST-segment elevation myocardial infarction (STEMI), the faster and stronger antiplatelet action of ticagrelor compared to clopidogrel, as well as its pleiotropic effects, could result in a greater degree of cardioprotection and final infarct size (FIS) limitation. The aim of our study was to comparatively evaluate the effect of ticagrelor and clopidogrel on myocardial salvage index (MSI) in STEMI patients undergoing thrombolysis.

Methods

Forty-two STEMI patients treated with thrombolysis were randomized to receive clopidogrel (n = 21) or ticagrelor (n = 21), along with aspirin. Myocardial area at risk (AAR) was calculated according to the BARI and the APPROACH jeopardy scores. FIS was quantified by cardiac magnetic resonance imaging (CMR) performed 5–6 months post-randomization. MSI was calculated as (AAR-FIS)/AAR × 100%. Primary endpoint of our study was MSI. Secondary endpoints were FIS and CMR-derived left ventricular ejection fraction (LVEF) at 5 –6 months post-randomization.

Results

By using the BARI score for AAR calculation, mean MSI was 52.25 ± 30.5 for the clopidogrel group and 54.29 ± 31.08 for the ticagrelor group (p = 0.83), while mean MSI using the APPROACH score was calculated at 51.94 ± 30 and 53.09 ± 32.39 (p = 0.9), respectively. Median CMR-derived FIS—as a percentage of LV—was 10.7% ± 8.25 in the clopidogrel group and 12.09% ± 8.72 in the ticagrelor group (p = 0.6). Mean LVEF at 5–6 months post-randomization did not differ significantly between randomization groups.

Conclusions

Our results suggest that the administration of ticagrelor in STEMI patients undergoing thrombolysis offer a similar degree of myocardial salvage, compared to clopidogrel.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12872-022-02735-1.

Curcumin by activation of adenosine A2A receptor stimulates protein kinase a and potentiates inhibitory effect of cangrelor on platelets

Curcumin is a natural polyphenol derived from the turmeric plant (Curcuma longa) which exhibits numerous beneficial effects on different cell types. Inhibition of platelet activation by curcumin is well known, however molecular mechanisms of its action on platelets are not fully defined. In this study, we used laser diffraction method for analysis of platelet aggregation and Western blot for analysis of intracellular signaling mechanisms of curcumin effects on platelets. We identified two new molecular mechanisms involved in the inhibitory effects of curcumin on platelet activation. Firstly, curcumin by activation of adenosine A_2A receptor stimulated protein kinase A activation and phosphorylation of Vasodilator-stimulated phosphoprotein. Secondly, we demonstrated that curcumin even at low doses, which did not inhibit platelet aggregation, potentiated inhibitory effect of ADP receptor P2Y₁₂ antagonist cangrelor which partly could be explained by activation of adenosine A_2A receptor.

Effects of Platelet Agonists and Priming on the Formation of Platelet Populations

Platelets from healthy donors display heterogeneity in responsiveness to agonists. The response thresholds of platelets are controlled by multiple bioactive molecules, acting as negatively or positively priming substances. Higher circulating levels of priming substances adenosine and succinate, as well as the occurrence of hypercoagulability, have been described for patients with ischaemic heart disease. Here, we present an improved methodology of flow cytometric analyses of platelet activation and the characterisation of platelet populations following activation and priming by automated clustering analysis.Platelets were treated with adenosine, succinate, or coagulated plasma before stimulation with CRP-XL, 2-MeSADP, or TRAP6 and labelled for activated integrin α_IIbβ₃ (PAC1), CD62P, TLT1, CD63, and GPIX. The Super-Enhanced Dmax subtraction algorithm and 2% marker (quadrant) setting were applied to identify populations, which were further defined by state-of-the-art clustering techniques (tSNE, FlowSOM).Following activation, five platelet populations were identified: resting, aggregating (PAC1 + ), secreting (α- and dense-granules; CD62P + , TLT1 + , CD63 + ), aggregating plus α-granule secreting (PAC1 + , CD62P + , TLT1 + ), and fully active platelet populations. The type of agonist determined the distribution of platelet populations. Adenosine in a dose-dependent way suppressed the fraction of fully activated platelets (TRAP6 > 2-MeSADP > CRP-XL), whereas succinate and coagulated plasma increased this fraction (CRP-XL > TRAP6 > 2-MeSADP). Interestingly, a subset of platelets showed a constant response (aggregating, secreting, or aggregating plus α-granule secreting), which was hardly affected by the stimulus strength or priming substances.

Thrombo-Inflammation: A Focus on NTPDase1/CD39

There is increasing evidence for a link between inflammation and thrombosis. Following tissue injury, vascular endothelium becomes activated, losing its antithrombotic properties whereas inflammatory mediators build up a prothrombotic environment. Platelets are the first elements to be activated following endothelial damage; they participate in physiological haemostasis, but also in inflammatory and thrombotic events occurring in an injured tissue. While physiological haemostasis develops rapidly to prevent excessive blood loss in the endothelium activated by inflammation, hypoxia or by altered blood flow, thrombosis develops slowly. Activated platelets release the content of their granules, including ATP and ADP released from their dense granules. Ectonucleoside triphosphate diphosphohydrolase-1 (NTPDase1)/CD39 dephosphorylates ATP to ADP and to AMP, which in turn, is hydrolysed to adenosine by ecto-5'-nucleotidase (CD73). NTPDase1/CD39 has emerged has an important molecule in the vasculature and on platelet surfaces; it limits thrombotic events and contributes to maintain the antithrombotic properties of endothelium. The aim of the present review is to provide an overview of platelets as cellular elements interfacing haemostasis and inflammation, with a particular focus on the emerging role of NTPDase1/CD39 in controlling both processes.

Ganoderma lucidum (Reishi) an edible mushroom; a comprehensive and critical review of its nutritional, cosmeceutical, mycochemical, pharmacological, clinical, and toxicological properties

Reishi owes an exceptional value in nutritional, cosmeceutical, and medical treatments; however, none of the studies has provided its future-driven critical assessment. This study documents an up-to-date review (2015-2020, wherever applicable) and provide valuable insights (preclinical and clinical evidence-based) with comprehensive and critical assessments. Various databases 'Google scholar', 'Web of Science', 'ScienceDirect', 'PubMed', 'Springer Link', books, theses, and library resources were used. The taxonomic chaos of G. lucidum and its related species was discussed in detail with solution-oriented emphasis. Reishi contains polysaccharides (α/β-D-glucans), alkaloids, triterpenoids (ganoderic acids, ganoderenic acids, ganoderol, ganoderiol, lucidenic acids), sterols/ergosterol, proteins (LZ-8, LZ-9), nucleosides (adenosine, inosine, uridine), and nucleotides (guanine, adenine). Some active drugs are explored at an optimum level to make them potential drug candidates. The pharmacological potential was observed in diabetes, inflammation, epilepsy, neurodegeneration, cancer, anxiety, sedation, cardiac diseases, depression, hepatic diseases, and immune disorders; however, most of the studies are preclinical with a number of drawbacks. In particular, quality clinical data are intensely needed to support pharmacological activities for human use. The presence of numerous micro-, macro, and trace elements imparts an essential nutritional and cosmeceutical value to Reishi, and various marketed products are available already, but the clinical studies regarding safety and efficacy, interactions with foods/drinks, chronic use, teratogenicity, mutagenicity, and genotoxicity are missing for Reishi. Reishi possesses many valuable pharmacological activities, and the number of patents and clinical trials is increasing for Reishi. Yet, a gap in research exists for Reishi, which is discussed in detail in the forthcoming sections.

Metabolomic Profile in Venous Thromboembolism (VTE)

Venous thromboembolism (VTE) is a condition comprising deep venous thrombosis (DVT) and pulmonary embolism (PE). The prevalence of this disease is constantly increasing and it is also a chief reason for morbidity. Therefore, the primary prevention of VTE remains a highly important public health issue. At present, its diagnosis generally relies on subjective clinical examination and ultrasound imaging. D-dimer is also used as a biomarker, but it is considered to be poorly specific and only moderately sensitive. There are also no reliable methods that could accurately guide the type of treatment and potentially identify patients who may benefit from more aggressive therapies without the risk of bleeding. The application of metabolomics profiling in the area of vascular diseases may become a turning point in early diagnosis and patient management. Among the most described metabolites possibly related to VTE are carnitine species, glucose, phenylalanine, 3-hydroxybutarate, lactic acid, tryptophan and some monounsaturated and polyunsaturated fatty acids. The cell response to acute PE was suggested to involve the uncoupling between glycolysis and oxidative phosphorylation. Despite technological advancement in the identification of metabolites and their alteration in thrombosis, we still do not understand the mechanisms and pathways responsible for the occurrence of observed alterations.

Adenosine Receptor Agonist HE-NECA Enhances Antithrombotic Activities of Cangrelor and Prasugrel in vivo by Decreasing of Fibrinogen Density in Thrombus

Blood platelets' adenosine receptors (AR) are considered to be a new target for the anti-platelet therapy. This idea is based on in vitro studies which show that signaling mediated by these receptors leads to a decreased platelet response to activating stimuli. In vivo evidence for the antithrombotic activity of AR agonists published to date were limited, however, to the usage of relatively high doses given in bolus. The present study was aimed at verifying if these substances used in lower doses in combination with inhibitors of P2Y12 could serve as components of dual anti-platelet therapy. We have found that a selective A2A agonist 2-hexynyl-5'-N-ethylcarboxamidoadenosine (HE-NECA) improved the anti-thrombotic properties of either cangrelor or prasugrel in the model of ferric chloride-induced experimental thrombosis in mice. Importantly, HE-NECA was effective not only when applied in bolus as other AR agonists in the up-to-date published studies, but also when given chronically. In vitro thrombus formation under flow conditions revealed that HE-NECA enhanced the ability of P2Y12 inhibitors to decrease fibrinogen content in thrombi, possibly resulting in their lower stability. Adenosine receptor agonists possess a certain hypotensive effect and an ability to increase the blood-brain barrier permeability. Therefore, the effects of anti-thrombotic doses of HE-NECA on blood pressure and the blood-brain barrier permeability in mice were tested. HE-NECA applied in bolus caused a significant hypotension in mice, but the effect was much lower when the substance was given in doses corresponding to that obtained by chronic administration. At the same time, no significant effect of HE-NECA was observed on the blood-brain barrier. We conclude that chronic administration of the A2A agonist can be considered a potential component of a dual antithrombotic therapy. However, due to the hypotensive effect of the substances, dosage and administration must be elaborated to minimize the side-effects. The total number of animals used in the experiments was 146.

Possible Role of Adenosine in COVID-19 Pathogenesis and Therapeutic Opportunities

The outbreak of the novel coronavirus disease 2019 (COVID-19) caused by Severe Acute Respiratory Syndrome CoronaVirus-2 (SARS-CoV-2) requires urgent clinical interventions. Crucial clinical needs are: 1) prevention of infection and spread of the virus within lung epithelia and between people, 2) attenuation of excessive lung injury in Advanced Respiratory Distress Syndrome, which develops during the end stage of the disease, and 3) prevention of thrombosis associated with SARS-CoV-2 infection. Adenosine and the key adenosine regulators adenosine deaminase (ADA), adenosine kinase (ADK), and equilibrative nucleoside transporter 1 may play a role in COVID-19 pathogenesis. Here, we highlight 1) the non-enzymatic role of ADA by which it might out-compete the virus (SARS-CoV-2) for binding to the CD26 receptor, 2) the enzymatic roles of ADK and ADA to increase adenosine levels and ameliorate Advanced Respiratory Distress Syndrome, and 3) inhibition of adenosine transporters to reduce platelet activation, thrombosis and improve COVID-19 outcomes. Depending on the stage of exposure to and infection by SARS-CoV-2, enhancing adenosine levels by targeting key adenosine regulators such as ADA, ADK and equilibrative nucleoside transporter 1 might find therapeutic use against COVID-19 and warrants further investigation.

Ticagrelor: a cardiometabolic drug targeting erythrocyte-mediated purinergic signaling?

Cardiometabolic diseases lead to vascular complications, which cause increasing morbidity and mortality worldwide. The underlying mechanisms are multifactorial and complex but may involve altered purinergic signaling that significantly contributes to cardiovascular dysfunction. Ticagrelor is a successful purinergic drug directly targeting ADP-mediated P2Y₁₂R signaling for platelet aggregation and is widely used in patients with acute coronary syndrome. In addition, ticagrelor can target red blood cells (RBCs) to release ATP and inhibit adenosine uptake by RBCs, which subsequently activate purinergic signaling. This involvement in purinergic signaling may allow ticagrelor to mediate pleiotropic effects and contribute to the beneficial cardiovascular outcomes observed in clinical studies. Recent studies have established a novel function of RBCs, which is that RBCs act as disease mediators for the development of cardiovascular complications in type 2 diabetes (T2D). RBC-released ATP is defective in T2D, which has implications for the induction of vascular dysfunction by dysregulating purinergic signaling. Ticagrelor might target RBCs and restore the bioavailability of ATP and adenosine, thereby attenuating cardiovascular complications. The present perspective discusses the pleiotropic effect of ticagrelor, with a focus on the possibility of ticagrelor for the treatment of cardiometabolic complications by targeting RBCs and initiating purinergic activation. A better understanding of the proposed cardiometabolic effects could support novel clinical indications for ticagrelor application.

Cilostazol increases adenosine plasma concentration in patients with acute coronary syndrome

WHAT IS KNOWN AND OBJECTIVE

Cilostazol is a specific and strong inhibitor of phosphodiesterase (PDE) type III which can suppress the platelet aggregation by increasing cyclic adenosine monophosphate (cAMP) levels. The clinical benefit of cilostazol in ACS patients suggested that the drug may have non-platelet-directed properties. Some in vitro and animal studies also indicated that the 'pleiotropic' properties of cilostazol might be related to the interaction with adenosine metabolism. Adenosine is an important regulatory metabolite and an inhibitor of platelet activation. However, no human study has been conducted to determine whether cilostazol could increase the adenosine plasma concentration in vivo. As a result, this study aimed to investigate the impact of cilostazol on adenosine plasma concentration (APC) in acute coronary syndrome (ACS) patients.

METHODS

We prospectively analysed 149 ACS patients undergoing percutaneous coronary intervention (PCI) with drug-eluting stents. The included patients were divided into two groups according to the presence (cilostazol group, n = 64) or absence (aspirin group, n = 85) of aspirin intolerance. The inhibition of platelet aggregation (IPA), APC and cAMP concentration was measured. Patient characteristics, medications and 30-day clinical outcomes were examined.

RESULTS

Patients receiving cilostazol had a significantly higher adenosine and cAMP plasma concentration than patients receiving aspirin (3.00 ± 0.67 vs 2.56 ± 0.74 mol/L, P < .001; 28.10 ± 14.74 vs 20.48 ± 11.35 pmol/mL, P = .0014). Cilostazol was associated with a higher inhibition rate of ADP induced platelet aggregation than aspirin (63.35 ± 26.71 vs 52.2 ± 28.35, P = .036). The plasma levels of adenosine and cAMP showed a positive correlation with ADP induced platelet aggregation.

WHAT IS NEW AND CONCLUSION

Cilostazol increases adenosine concentration compared with aspirin. Its potent antiplatelet effect in ACS patients may be partly mediated by adenosine.

Biased signaling in platelet G-protein coupled receptors

Platelets are small megakaryocyte-derived, anucleate, disk-like structures that play an outsized role in human health and disease. Both a decrease in the number of platelets and a variety of platelet function disorders result in petechiae or bleeding that can be life threatening. Conversely, the inappropriate activation of platelets, within diseased blood vessels, remains the leading cause of death and morbidity by affecting heart attacks and stroke. The fine balance of the platelet state in healthy individuals is controlled by a number of receptor-mediated signaling pathways that allow the platelet to rapidly respond and maintain haemostasis. G-protein coupled receptors (GPCRs) are particularly important regulators of platelet function. Here we focus on the major platelet-expressed GPCRs and discuss the roles of downstream signaling pathways (e.g., different G-protein subtypes or β-arrestin) in regulating the different phases of the platelet activation. Further, we consider the potential for selectively targeting signaling pathways that may contribute to platelet responses in disease through development of biased agonists. Such selective targeting of GPCR-mediated signaling pathways by drugs, often referred to as biased signaling, holds promise in delivering therapeutic interventions that do not present significant side effects, especially in finely balanced physiological systems such as platelet activation in haemostasis.

Adenosine Receptor Agonists Increase the Inhibition of Platelet Function by P2Y12 Antagonists in a cAMP- and Calcium-Dependent Manner

We have shown previously that platelet activity can be lowered through the simultaneous inhibition of P2Y₁₂ receptor and activation of adenosine receptors (AR). This work explores this concept by testing the antiplatelet potential of nine AR agonists in combination with P2Y₁₂ receptor antagonists—cangrelor and prasugrel metabolite. A panel of in vitro methods was used to assess platelet viability, P-selectin expression, GPIIb-IIIa activation, fibrinogen binding, calcium ion mobilization, VASP-P level, and cAMP formation, utilizing whole blood or isolated platelets from healthy volunteers. The AR agonists demonstrated anti-platelet effects, but stimulated signaling pathways to varying degrees. AR agonists and P2Y₁₂ antagonists reduced expression of both P-selectin and the activated form of GPIIb-IIIa on platelets; however, the combined systems (AR agonist + P2Y₁₂ antagonist) demonstrated stronger effects. The antiplatelet effects of AR when combined with P2Y₁₂ were more pronounced with regard to exogenous fibrinogen binding and calcium mobilization. The cAMP levels in both resting and ADPactivated platelets were increased by AR agonist treatment, and more so when combined with P2Y₁₂ inhibitor. In conclusion, as AR agonists are fast-acting compounds, the methods detecting early activation events are more suitable for assessing their antiplatelet action. The exogenous fibrinogen binding, calcium mobilisation and cAMP level turned out to be sensitive markers for detecting the inhibition caused by AR agonists alone or in combination with P2Y₁₂ receptor antagonists.

Flow Augmentation in the Myocardium by Ultrasound Cavitation of Microbubbles: Role of Shear-Mediated Purinergic Signaling

BACKGROUND

Ultrasound-mediated cavitation of microbubble contrast agents produces high intravascular shear. We hypothesized that microbubble cavitation increases myocardial microvascular perfusion through shear-dependent purinergic pathways downstream from ATP release that is immediate and sustained through cellular ATP channels such as Pannexin-1.

METHODS

Quantitative myocardial contrast echocardiography perfusion imaging and in vivo optical imaging of ATP was performed in wild-type and Pannexin-1-deficient (Panx1^-/-) mice before and 5 and 30 minutes after 10 minutes of ultrasound-mediated (1.3 MHz, mechanical index 1.3) myocardial microbubble cavitation. Flow augmentation in a preclinical model closer to humans was evaluated in rhesus macaques undergoing myocardial contrast echocardiography perfusion imaging after high-power cavitation in the apical four-chamber plane for 10 minutes.

RESULTS

Microbubble cavitation in wild-type mice (n = 7) increased myocardial perfusion by 64% ± 25% at 5 minutes and 95% ± 55% at 30 minutes compared with baseline (P < .05). In Panx1^-/- mice (n = 5), perfusion increased by 28% ± 26% at 5 minutes (P = .04) but returned to baseline at 30 minutes. Myocardial ATP signal in wild-type (n = 7) mice undergoing cavitation compared with sham-treated controls (n = 3) was 450-fold higher at 5 minutes and 90-fold higher at 30 minutes after cavitation (P < .001). The ATP signal in Panx1^-/- mice (n = 4) was consistently 10-fold lower than that in wild-type mice and was similar to sham controls at 30 minutes. In macaques (n = 8), myocardial perfusion increased twofold in the cavitation-exposed four-chamber plane, similar in degree to that produced by adenosine, but did not increase in the control two-chamber plane.

CONCLUSIONS

Cavitation of microbubbles in the myocardial microcirculation produces an immediate release of ATP, likely from cell microporation, as well as sustained release, which is channel dependent and responsible for persistent flow augmentation. These findings provide mechanistic insight by which cavitation improves perfusion and reduces infarct size in patients with myocardial infarction.

Nucleotide ecto-enzyme metabolic pattern and spatial distribution in calcific aortic valve disease; its relation to pathological changes and clinical presentation

BACKGROUND

Extracellular nucleotide metabolism contributes to chronic inflammation, cell differentiation, and tissue mineralization by controlling nucleotide and adenosine concentrations and hence its purinergic effects. This study investigated location-specific changes of extracellular nucleotide metabolism in aortic valves of patients with calcific aortic valve disease (CAVD). Individual ecto-enzymes and adenosine receptors involved were analyzed together with correlation with CAVD severity and risk factors.

RESULTS

Nucleotide and adenosine degradation rates were adversely modified on the aortic surface of stenotic valve as compared to ventricular side, including decreased ATP removal (1.25 ± 0.35 vs. 2.24 ± 0.61 nmol/min/cm2) and adenosine production (1.32 ± 0.12 vs. 2.49 ± 0.28 nmol/min/cm2) as well as increased adenosine deamination (1.28 ± 0.31 vs. 0.67 ± 0.11 nmol/min/cm2). The rates of nucleotide to adenosine conversions were lower, while adenosine deamination was higher on the aortic sides of stenotic vs. non-stenotic valve. There were no differences in extracellular nucleotide metabolism between aortic and ventricular sides of non-stenotic valves. Furthermore, nucleotide degradation rates, measured on aortic side in CAVD (n = 62), negatively correlated with echocardiographic and biochemical parameters of disease severity (aortic jet velocity vs. ATP hydrolysis: r = - 0.30, p < 0.05; vs. AMP hydrolysis: r = - 0.44, p < 0.001; valvular phosphate concentration vs. ATP hydrolysis: r = - 0.26, p < 0.05; vs. AMP hydrolysis: r = - 0.25, p = 0.05) while adenosine deamination showed positive correlation trend with valvular phosphate deposits (r = 0.23, p = 0.07). Nucleotide and adenosine conversion rates also correlated with CAVD risk factors, including hyperlipidemia (AMP hydrolysis vs. serum LDL cholesterol: r = - 0.28, p = 0.05; adenosine deamination vs. total cholesterol: r = 0.25, p = 0.05; LDL cholesterol: r = 0.28, p < 0.05; triglycerides: r = 0.32, p < 0.05), hypertension (adenosine deamination vs. systolic blood pressure: r = 0.28, p < 0.05) and thrombosis (ATP hydrolysis vs. prothrombin time: r = - 0.35, p < 0.01). Functional assays as well as histological and immunofluorescence, flow cytometry and RT-PCR studies identified all major ecto-enzymes engaged in nucleotide metabolism in aortic valves that included ecto-nucleotidases, alkaline phosphatase, and ecto-adenosine deaminase. We have shown that changes in nucleotide-converting ecto-enzymes were derived from their altered activities on valve cells and immune cell infiltrate. We have also demonstrated a presence of A1, A2a and A2b adenosine receptors with diminished expression of A2a and A2b in stenotic vs. non-stenotic valves. Finally, we revealed that augmenting adenosine effects by blocking adenosine deamination with deoxycoformycin decreased aortic valve thickness and reduced markers of calcification via adenosine-dependent pathways in a mouse model of CAVD.

CONCLUSIONS

This work highlights profound changes in extracellular nucleotide and adenosine metabolism in CAVD. Altered extracellular nucleotide hydrolysis and degradation of adenosine in stenotic valves may affect purinergic responses to support a pro-stenotic milieu and valve calcification. This emphasizes a potential mechanism and target for prevention and therapy. .

Effect of remote ischaemic conditioning on coagulation function as measured by whole blood impedance aggregometry and rotational thromboelastometry in off-pump coronary artery bypass surgery: A randomised controlled trial

INTRODUCTION

Remote ischaemic conditioning (RIC) has been shown to prevent platelet activation during ablation for atrial fibrillation. RIC has also been associated with more postoperative transfusion in the off-pump coronary artery bypass graft surgery (OPCAB) patients. We evaluated the effects of RIC on coagulation function in OPCAB patients.

METHODS

A total of 58 patients undergoing OPCAB were randomised to the RIC or control group. In the RIC group, four cycles of 5 min of ischaemia and 5 min of reperfusion were applied twice to the upper arm after the induction of anaesthesia (preconditioning), and after the completion of coronary anastomoses (postconditioning). Whole blood impedance aggregometry (Multiplate®) and rotational thromboelastometry (ROTEM®) were performed before the induction of anaesthesia, at the end of surgery, and at postoperative day 1.

RESULTS

The trend towards a decrease in adenosine diphosphate-induced whole blood aggregation at the end of surgery was greater in the RIC group than in the control group, but this effect was not statistically significant (-10.4 [18.1] vs. -5.7 [24.8] U, P = 0.424). In ROTEM® analysis, the EXTEM area under the velocity curve was lower in the RIC group than in the control group at the end of surgery (3567 [1399-5794] vs. 5693 [4718-6179] mm∗100, respectively; P = 0.030). A tendency of larger perioperative blood loss was identified in the RIC group.

CONCLUSIONS

Although some parameters indicated a tendency for hypocoagulation in the RIC group at the end of surgery, most effects were not statistically significant. RIC does not significantly affect perioperative platelet aggregability and coagulation in patients undergoing OPCAB.

Adenosine Receptor Agonists Exhibit Anti-Platelet Effects and the Potential to Overcome Resistance to P2Y12 Receptor Antagonists

Large inter-individual variation in platelet response to endogenous agonists and pharmacological agents, including resistance to antiplatelet therapy, prompts a search for novel platelet inhibitors and development new antithrombotic strategies. The present in vitro study evaluates the beneficial effects of three adenosine receptor (AR) agonists (regadenoson, LUF 5835 and NECA), different in terms of their selectivity for platelet adenosine receptors, when used alone and in combination with P2Y₁₂ inhibitors, such as cangrelor or prasugrel metabolite. The anti-platelet effects of AR agonists were evaluated in healthy subjects (in the whole group and after stratification of individuals into high- and low-responders to P2Y₁₂ inhibitors), using whole blood techniques, under flow (thrombus formation) and static conditions (study of platelet activation and aggregation). Compared to P2Y₁₂ antagonists, AR agonists were much less or not effective under static conditions, but demonstrated similar antiplatelet activity in flow. In most cases, AR agonists significantly enhanced the anti-platelet effect of P2Y₁₂ antagonists, despite possessing different selectivity profiles and antiplatelet activities. Importantly, their inhibitory effects in combination with P2Y₁₂ antagonists were similar in high- and low-responders to P2Y₁₂ inhibitors. In conclusion, a combination of anti-platelet agents acting via the P1 and P2 purinergic receptors represents a promising alternative to existing antithrombotic therapy.

Platelet Oxidative Stress and its Relationship with Cardiovascular Diseases in Type 2 Diabetes Mellitus Patients

Enhanced platelet activation and thrombosis are linked to various cardiovascular diseases (CVD). Among other mechanisms, oxidative stress seems to play a pivotal role in platelet hyperactivity. Indeed, upon stimulation by physiological agonists, human platelets generate and release several types of reactive oxygen species (ROS) such as O2 -, H2O2 or OH-, further amplifying the platelet activation response via various signalling pathways, including, formation of isoprostanes, Ca2+ mobilization and NO inactivation. Furthermore, excessive platelet ROS generation, incorporation of free radicals from environment and/or depletion of antioxidants induce pro-oxidant, pro-inflammatory and platelet hyperaggregability effects, leading to the incidence of cardiovascular events. Here, we review the current knowledge regarding the effect of oxidative stress on platelet signaling pathways and its implication in CVD such as type 2 diabetes mellitus. We also summarize the role of natural antioxidants included in vegetables, fruits and medicinal herbs in reducing platelet function via an oxidative stress-mediated mechanism.

Blood Platelet Adenosine Receptors as Potential Targets for Anti-Platelet Therapy

Adenosine receptors are a subfamily of highly-conserved G-protein coupled receptors. They are found in the membranes of various human cells and play many physiological functions. Blood platelets express two (A_2A and A_2B) of the four known adenosine receptor subtypes (A₁, A_2A, A_2B, and A₃). Agonization of these receptors results in an enhanced intracellular cAMP and the inhibition of platelet activation and aggregation. Therefore, adenosine receptors A_2A and A_2B could be targets for anti-platelet therapy, especially under circumstances when classic therapy based on antagonizing the purinergic receptor P2Y₁₂ is insufficient or problematic. Apart from adenosine, there is a group of synthetic, selective, longer-lasting agonists of A_2A and A_2B receptors reported in the literature. This group includes agonists with good selectivity for A_2A or A_2B receptors, as well as non-selective compounds that activate more than one type of adenosine receptor. Chemically, most A_2A and A_2B adenosine receptor agonists are adenosine analogues, with either adenine or ribose substituted by single or multiple foreign substituents. However, a group of non-adenosine derivative agonists has also been described. This review aims to systematically describe known agonists of A_2A and A_2B receptors and review the available literature data on their effects on platelet function.

Schistosomes can hydrolyze proinflammatory and prothrombotic polyphosphate (polyP) via tegumental alkaline phosphatase, SmAP

Schistosoma mansoni is a long-lived intravascular trematode parasite that can infect humans causing the chronic debilitating disease, schistosomiasis. We hypothesize that the action of host-interactive proteins found at the schistosome surface allows the worms to maintain a safe, anti-thrombotic and anti-inflammatory environment around them in the bloodstream. One such protein is the ˜60 kDa alkaline phosphatase SmAP which is known to be expressed in the outer tegument of all intravascular life stages. We demonstrate in this work that the parasites (schistosomula as well as adult males and females) can hydrolyze polyphosphate (polyP) - an anionic, linear polymer of inorganic phosphates that is produced and released by immune cells as well as by activated platelets and that induce proinflammatory and prothrombotic pathways. Purified recombinant SmAP can likewise cleave polyP and with a Km of 6.9 ± 1 mM. Finally, parasites whose SmAP gene has been suppressed by RNAi are significantly impaired in their ability to hydrolyze polyP. SmAP-mediated cleavage of polyP may contribute to the armamentarium of schistosomes that promotes their survival in the hostile intravascular habitat. This is the first report of any pathogen cleaving this bioactive metabolite.

Kinetic study of NTPDase immobilization and its effect of haemocompatibility on polyethylene terephthalate

Poor haemocompatibility of material surfaces is a serious limitation that can lead to failure of blood-contacting devices and implants. In this work, we have improved the haemocompatibility of polyethylene terephthalate (PET) surfaces by immobilizing apyrase/ecto-nucleoside triphosphate diphosphohydrolase (NTPDase) on to the carboxylated PET. NTPDase immobilized PET surfaces scavenge the ADP released by activated platelets, which prevents further platelet activation and aggregation. The surface properties of the modified PET were characterized by scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDAX), and contact angle measurement. The enzyme attachment and stability on the modified PET surfaces were evaluated. The kinetics of free enzyme and immobilized enzyme were studied and fitted using the Michaelis-Menten kinetic model. Both free and immobilized NTPDase followed Michaelis-Menten kinetics with similar Michaelis-Menten constants (K_m). This suggests that the activity of NTPDase was unchanged upon immobilization. Protein adsorption and %hemolysis was significantly reduced for carboxylated PET and NTPDase immobilized PET surfaces compared to unmodified PET. Lactate dehydrogenase assay showed that the number of adhered platelets reduced by more than an order of magnitude for the NTPDase immobilized PET surface compared to unmodified PET. These results clearly indicate that NTPDase immobilization significantly enhances the haemocompatibility of PET surfaces.

Adrenoceptor α2A signalling countervails the taming effects of synchronous cyclic nucleotide-elevation on thrombin-induced human platelet activation and aggregation

The healthy vascular endothelium constantly releases autacoids which cause an increase of intracellular cyclic nucleotides to tame platelets from inappropriate activation. Elevating cGMP and cAMP, in line with previous reports, cooperated in the inhibition of isolated human platelet intracellular calcium-mobilization, dense granules secretion, and aggregation provoked by thrombin. Further, platelet alpha granules secretion and, most relevant, integrin α_IIaβ₃ activation in response to thrombin are shown to be prominently affected by the combined elevation of cGMP and cAMP. Since stress-related sympathetic nervous activity is associated with an increase in thrombotic events, we investigated the impact of epinephrine in this setting. We found that the assessed signalling events and functional consequences were to various extents restored by epinephrine, resulting in full and sustained aggregation of isolated platelets. The restoring effects of epinephrine were abolished by either interfering with intracellular calcium-elevation or with PI3-K signalling. Finally, we show that in our experimental setting epinephrine likewise reconstitutes platelet aggregation in heparinized whole blood, which may indicate that this mechanism could also apply in vivo.

Platelet Reactivity And Circulating Platelet-Derived Microvesicles Are Differently Affected By P2Y12 Receptor Antagonists

Background: Platelet-derived microvesicles (PMVs), shed from platelet surface membranes, constitute the majority of circulating microvesicles and have been implicated in procoagulant, pro-inflammatory and pro-atherosclerotic effects. Our aim was to compare plasma PMVs numbers in relation to platelet reactivity during dual antiplatelet therapy (DAPT) with various P2Y₁₂ adenosine diphosphate (ADP) receptor antagonists. Methods: In pre-discharge men treated with DAPT for an acute coronary syndrome, plasma PMVs were quantified by flow cytometry on the basis of CD62P (P-selectin) and CD42 (glycoprotein Ib) positivity, putative indices of PMVs release from activated and all platelets, respectively. ADP-induced platelet aggregation was measured by multiple-electrode aggregometry. Results: Clinical characteristics were similar in patients on clopidogrel (n=16), prasugrel (n=10) and ticagrelor (n=12). Platelet reactivity was comparably reduced on ticagrelor or prasugrel versus clopidogrel (p<0.01). Compared to clopidogrel-treated patients, CD42⁺/CD62P⁺ PMVs counts were 3-4-fold lower in subjects receiving ticagrelor (p=0.001) or prasugrel (p<0.05), while CD42⁺ PMVs were significantly reduced on ticagrelor (by about 6-fold, p<0.001), but not prasugrel (p=0.3). CD42⁺/CD62P⁺ PMVs numbers correlated positively to the ADP-induced aggregation on clopidogrel (p<0.01) or prasugrel (p<0.05), which was absent in ticagrelor users (p=0.8). CD42⁺ PMVs counts were unrelated to platelet reactivity (p>0.5). Conclusions: Higher antiplatelet potency of prasugrel and ticagrelor versus clopidogrel is associated with decreased plasma CD42⁺/CD62P⁺ PMVs numbers. However, in contrast to thienopyridines, the association of reduced CD42⁺/CD62P⁺ PMVs counts with ticagrelor use appears independent of its anti-aggregatory effect. Despite similar platelet-inhibitory activity of ticagrelor and prasugrel, only the treatment with ticagrelor seems associated with lower total PMVs release. Our preliminary findings may suggest a novel pleiotropic effect of ticagrelor extending beyond pure anti-aggregatory properties of the drug.

Adenosine receptor agonists deepen the inhibition of platelet aggregation by P2Y12 antagonists

Several adenosine receptor (AR) agonists have been shown in the past to possess anti-platelet potential; however, the adjunctive role of AR agonists in anti-platelet therapy with the use of P2Y₁₂ receptor inhibitors has not been elucidated so far. This in vitro aggregation-based study investigates whether the inhibition of platelet function mediated by cangrelor or prasugrel metabolite can be potentiated by AR agonists. It evaluates the effect of non-selective (2-chloroadenosine), A_2A-selective (UK 432097, MRE 0094, PSB 0777) and A_2B-selective AR agonists (BAY 60-6583) on platelet function in relation to their toxicity, specificity towards adenosine receptor subtypes, structure and solubility. UK 432097, 2-chloroadenosine, MRE 0094 and PSB 0777 were found to be more or less potent inhibitors of ADP-induced platelet aggregation when acting alone, and that they remained non-cytotoxic to the cells. These AR agonists were also effective in the potentiation of the effects exerted by P2Y₁₂ antagonists. Considering the estimated IC₅₀ value, UK 432097, showing a relatively high binding affinity to the A_2A adenosine receptor, has been identified as the most potent anti-aggregatory agent. This compound diminished platelet aggregation at nanomolar concentrations and further augmented platelet inhibition by P2Y₁₂ antagonists by approx. 60% (P < .01). Our results indicate the importance of adenosine receptors as therapeutic targets and point out challenges and potential benefits of therapeutic use of a combined therapy of P2Y₁₂ antagonist and AR agonist in cardioprotection. Our comparative analysis of the effects of AR agonists on platelet response in plasma and whole blood may indirectly suggest that other blood morphology elements contribute little to the inhibition of platelet function by AR agonists.

Adenosine signaling and adenosine deaminase regulation of immune responses: impact on the immunopathogenesis of HIV infection

Infection by human immunodeficiency virus (HIV) causes the acquired immune deficiency syndrome (AIDS), which has devastating effects on the host immune system. HIV entry into host cells and subsequent viral replication induce a proinflammatory response, hyperactivating immune cells and leading them to death, disfunction, and exhaustion. Adenosine is an immunomodulatory molecule that suppresses immune cell function to protect tissue integrity. The anti-inflammatory properties of adenosine modulate the chronic inflammation and immune activation caused by HIV. Lack of adenosine contributes to pathogenic events in HIV infection. However, immunosuppression by adenosine has its shortcomings, such as impairing the immune response, hindering the elimination of the virus and control of viral replication. By attempting to control inflammation, adenosine feeds a pathogenic cycle affecting immune cells. Deamination of adenosine by ADA (adenosine deaminase) counteracts the negative effects of adenosine in immune cells, boosting the immune response. This review comprises the connection between adenosinergic system and HIV immunopathogenesis, exploring defects in immune cell function and the role of ADA in protecting these cells against damage.

Intravascular Schistosoma mansoni Cleave the Host Immune and Hemostatic Signaling Molecule Sphingosine-1-Phosphate via Tegumental Alkaline Phosphatase

Schistosomes are parasitic flatworms that infect the vasculature of >200 million people around the world. These long-lived parasites do not appear to provoke blood clot formation or obvious inflammation around them in vivo. Proteins expressed at the host-parasite interface (such as Schistosoma mansoni alkaline phosphatase, SmAP) are likely key to these abilities. SmAP is a glycoprotein that hydrolyses the artificial substrate p-nitrophenyl phosphate in a reaction that requires Mg2+ and at an optimal pH of 9. SmAP additionally cleaves the nucleoside monophosphates AMP, CMP, GMP, and TMP, all with a similar Km (~600-650 μM). Living adult worms, incubated in murine plasma for 1 h, alter the plasma metabolome; a decrease in sphingosine-1-phosphate (S1P) is accompanied by an increase in the levels of its component parts-sphingosine and phosphate. To test the hypothesis that schistosomes can hydrolyze S1P (and not merely recruit or activate a host plasma enzyme with this function), living intravascular life-stage parasites were incubated with commercially obtained S1P and cleavage of S1P was detected. Parasites whose SmAP gene was suppressed using RNAi were impaired in their ability to cleave S1P compared to controls. In addition, recombinant SmAP hydrolyzed S1P. Since extracellular S1P plays key roles in controlling inflammation and platelet aggregation, we hypothesize that schistosome SmAP, by degrading S1P, can regulate the level of this bioactive lipid in the environment of the parasites to control these processes in the worm's local environment. This is the first report of any parasite being able to cleave S1P.

Modulatory effect of eugenol on arginase, nucleotidase, and adenosine deaminase activities of platelets in a carrageenan-induced arthritis rat model: A possible anti-arthritic mechanism of eugenol

This study investigated the effect of eugenol on arginase, nucleotidase and adenosine deaminase activities in platelets of carrageenan-induced arthritic rat model to explain a possible anti-arthritic mechanism of eugenol. Fifty adult female rats (140-250 g) were divided into ten (10) groups (n = 5). Group I received oral administration of corn oil, group II received 2.50 mg/kg of eugenol, group III and IV rats received oral administration of 5.0 and 10.0 mg/kg of eugenol respectively, group V received 0.20 mg/kg of dexamethasone orally, group VI rats was injected with 1% carrageenan (arthritic rats) and received saline solution orally (arthritic control rat group), group VII, VIII and IX: arthritic rats received 2.50, 5.0 or 10 mg/kg of eugenol orally respectively, group X: arthritic rats was administered with 0.20 mg/kg of dexamethasone orally. The animals were treated for 21 days, thereafter, tibiofemoral histological examination, thiobabituric acid reactive substances level, arginase, nucleoside triphosphate diphosphohydrolase, 5´-nucleotidase and adenosine deaminase activities were assessed. Tibiofemoral histological examination result showed that infiltration of inflammatory cells was significantly decreased with an increase in eugenol dose. Activities of arginase, adenosine triphosphate and adenosine monophosphate hydrolyses were significantly decreased while adenosine diphosphate hydrolysis and adenosine deaminase activities were significantly increased in arthritic rat groups administered with different doses of eugenol. Therefore, eugenol might be a natural complement and alternative promising anti-arthritic agent. These possible anti-arthritic mechanisms may be partly through the modulation of arginase and adenosine nucleotides hydrolyzing enzyme activities as well as the antioxidative action of eugenol.

Adenosine A2A receptor ligand recognition and signaling is blocked by A2B receptors

The adenosine receptor (AR) subtypes A_2A and A_2B are rhodopsin-like G_s protein-coupled receptors whose expression is highly regulated under pathological, e.g. hypoxic, ischemic and inflammatory conditions. Both receptors play important roles in inflammatory and neurodegenerative diseases, are blocked by caffeine, and have now become major drug targets in immuno-oncology. By Förster resonance energy transfer (FRET), bioluminescence resonance energy transfer (BRET), bimolecular fluorescence complementation (BiFC) and proximity ligation assays (PLA) we demonstrated A_2A-A_2BAR heteromeric complex formation. Moreover we observed a dramatically altered pharmacology of the A_2AAR when co-expressed with the A_2BAR (A_2B ≥ A_2A) in recombinant as well as in native cells. In the presence of A_2BARs, A_2A-selective ligands lost high affinity binding to A_2AARs and displayed strongly reduced potency in cAMP accumulation and dynamic mass redistribution (DMR) assays. These results have major implications for the use of A_2AAR ligands as drugs as they will fail to modulate the receptor in an A_2A-A_2B heteromer context. Accordingly, A_2A-A_2BAR heteromers represent novel pharmacological targets.

Unfolding Role of a Danger Molecule Adenosine Signaling in Modulation of Microbial Infection and Host Cell Response

Ectonucleotidases CD39 and CD73, specific nucleotide metabolizing enzymes located on the surface of the host, can convert a pro-inflammatory environment driven by a danger molecule extracellular-ATP to an adenosine-mediated anti-inflammatory milieu. Accordingly, CD39/CD73 signaling have has strongly implicated in modulating the intensity, duration, and composition of purinergic danger signals delivered to host. Recent studies have eluted potential roles for CD39 and CD73 in selective triggering of a variety of host immune cells and molecules in the presence of pathogenic microorganisms or microbial virulence molecules. Growing evidence also suggests that CD39 and CD73 present complimentary, but likely differential, actions against pathogens to shape the course and severity of microbial infection as well as the associated immune response. Similarly, adenosine receptors A2A and A2B have been proposed to be major immunomodulators of adenosine signaling during chronic inflammatory conditions induced by opportunistic pathogens, such as oral colonizer Porphyromonas gingivalis. Therefore, we here review the recent studies that demonstrate how complex network of molecules in the extracellular adenosine signaling machinery and their interactions can reshape immune responses and may also be targeted by opportunistic pathogens to establish successful colonization in human mucosal tissues and modulate the host immune response.

Adenosine: Synthetic Methods of Its Derivatives and Antitumor Activity

Since 1929, several researchers have conducted studies in relation to the nucleoside of adenosine (1) mainly distribution identifying, characterizing their biological importance and synthetic chemistry to which this type of molecule has been subjected to obtain multiple of its derivatives. The receptors that interact with adenosine and its derivatives, called purinergic receptors, are classified as A1, A2A, A2B and A3. In the presence of agonists and antagonists, these receptors are involved in various physiological processes and diseases. This review describes and compares some of the synthetic methods that have been developed over the last 30 years for obtaining some adenosine derivatives, classified according to substitution processes, complexation, mating and conjugation. Finally, we mention that although the concentrations of these nucleosides are low in normal tissues, they can increase rapidly in pathophysiological conditions such as hypoxia, ischemia, inflammation, trauma and cancer. In particular, the evaluation of adenosine derivatives as adjunctive therapy promises to have a significant impact on the treatment of certain cancers, although the transfer of these results to clinical practice requires a deeper understanding of how adenosine regulates the process of tumorigenesis.

Aeromonas caviae alters the activities of ecto-enzymes that hydrolyze adenine nucleotides in fish thrombocytes

It is recognized that the purinergic system, through the activities of ectonucleoside triphosphate diphosphohydrolase (E-NTPDase), ecto-5'-nucleotidase (E-5'-nucleotidase), and ecto-adenosine deaminase (E-ADA), is involved in the regulation and modulation of the physiological and pathological events linked to hemostasis. This occurs due to the role of adenosine diphosphate (ADP) in the activation and recruitment of platelets, and the role of adenosine (Ado) in the inhibition of platelet activation. Thus, here we aimed to evaluate whether Aeromonas caviae infection impairs the ecto-enzymes of the purinergic system in fish thrombocytes and the involvement of this system in the hemorrhagic septicemia. The total number of fish thrombocytes decreased in infected animals compared to uninfected animals. Regarding the ecto-enzymes of the purinergic system, the E-NTPDase and E-5'-nucleotidase activities increased in infected animals compared to uninfected animals, while the E-ADA activity decreased. These findings show that adenine nucleotide hydrolysis is modified in the thrombocytes of fish experimentally infected with A. caviae, which impairs the coagulation process due the excessive hydrolysis of ADP, a molecule linked with activation and recruitment of thrombocytes at the site of vascular injury, and augmentation on Ado levels, a molecule linked with inhibitory effects on platelet activation and aggregation. In summary, the purinergic system might contribute to the occurrence of hemorrhagic frames in fish infected with A. caviae.

Changes in inflammatory/cardiac markers of HIV positive patients

HIV replication promotes atherogenesis and participates in the immune response to the virus, thereby influencing the inflammatory profile. These changes may, in turn, contribute to the risk of cardiovascular diseases with involvement of platelets. However, adenine nucleotides and nucleosides involved in thromboregulation and modulation of immune response may therefore be affected by these alterations.

OBJECTIVES

This study sought to evaluate the profile of pro and anti-inflammatory cytokines (IL-10, IL-6, IL-17, TNF, IL-4, IL-2 and IFN-gamma), cardiac markers (troponin, CK, CK MB, LDH, CRP) in HIV-positive patients and assess the in vitro effect of antiretroviral therapy on the activities of ectonucleotidases (E-NTPDase and E-5'-nucleotidase) in human platelets.

DESIGN AND METHODS

Blood samples were obtained from ten HIV positive patients at the Infectious Disease Clinic of the University Hospital of Santa Maria, Brazil and ten HIV negative individuals (control group) for this study.

RESULTS

The results revealed that there were significant (P < 0.05) increases in serum levels of IL-6 and IFN-gamma with no significant (P > 0.05) changes in the serum levels of the cardiac markers investigated (CK, CK-MB, troponin, LDH and CRP). In addition, the ectonucleotidases (E-NTPDase and E-5'-nucleotidase) activities were not altered (P > 0.05) in human platelets when incubated with different antiretroviral drugs in vitro.

CONCLUSIONS

The results of this study suggest that, despite successful treatment, a proinflammatory state is not altered in HIV patients, and that antiretroviral therapy per se does not change the purinergic profile.

Anticoagulant Activity of Low-Molecular Weight Compounds from Heterometrus laoticus Scorpion Venom

Scorpion venoms are complex polypeptide mixtures, the ion channel blockers and antimicrobial peptides being the best studied components. The coagulopathic properties of scorpion venoms are poorly studied and the data about substances exhibiting these properties are very limited. During research on the Heterometrus laoticus scorpion venom, we have isolated low-molecular compounds with anticoagulant activity. Determination of their structure has shown that one of them is adenosine, and two others are dipeptides LeuTrp and IleTrp. The anticoagulant properties of adenosine, an inhibitor of platelet aggregation, are well known, but its presence in scorpion venom is shown for the first time. The dipeptides did not influence the coagulation time in standard plasma coagulation tests. However, similarly to adenosine, both peptides strongly prolonged the bleeding time from mouse tail and in vitro clot formation in whole blood. The dipeptides inhibited the secondary phase in platelet aggregation induced by ADP, and IleTrp decreased an initial rate of platelet aggregation induced by collagen. This suggests that their anticoagulant effects may be realized through the deterioration of platelet function. The ability of short peptides from venom to slow down blood coagulation and their presence in scorpion venom are established for the first time. Further studies are needed to elucidate the precise molecular mechanism of dipeptide anticoagulant activity.

Ischemic Conditioning Attenuates Platelet-Mediated Thrombosis

Data obtained in both preclinical models and humans have revealed that the favorable cardiac consequences of ischemic conditioning extend beyond a direct effect on the cardiomyocyte. In the current review, we summarize our as-yet limited understanding of the complex relationships between ischemic conditioning, platelet activation–aggregation, and cardioprotection.

Platelets and von Willebrand factor in atherogenesis

The role of platelet adhesion, activation, and aggregation in acute atherothrombotic events such as myocardial infarction and stroke is well established. There is increasing evidence that platelet-endothelial interactions also contribute to early atherosclerotic plaque initiation and growth. Through these interactions, platelet-derived factors can contribute to the proinflammatory and mitogenic status of resident mural cells. Among the many putative mechanisms for platelet-endothelial interactions, increased endothelial-associated von Willebrand factor, particularly in a multimerized form, which interacts with platelet glycoproteins and integrins, is a major factor and represents a therapeutic target in early atherogenesis.