Virus-Triggered ATP Release Limits Viral Replication through Facilitating IFN-β Production in a P2X7-Dependent Manner

Implications of COVID-19 in Parkinson's disease: the purinergic system in a therapeutic-target perspective to diminish neurodegeneration

The pathophysiology of Parkinson's disease (PD) is marked by degeneration of dopaminergic neurons in the substantia nigra. With advent of COVID-19, which is closely associated with generalized inflammation and multiple organ dysfunctions, the PD patients may develop severe conditions of disease leading to exacerbated degeneration. This condition is caused by the excessive release of pro-inflammatory markers, called cytokine storm, that is capable of triggering neurodegenerative conditions by affecting the blood-brain barrier (BBB). A possible SARS-CoV-2 infection, in serious cases, may compromise the immune system by triggering a hyperstimulation of the neuroimmune response, similar to the pathological processes found in PD. From this perspective, the inflammatory scenario triggers oxidative stress and, consequently, cellular dysfunction in the nervous tissue. The P2X7R seems to be the key mediator of the neuroinflammatory process, as it acts by increasing the concentration of ATP, allowing the influx of Ca2+ and the occurrence of mutations in the α-synuclein protein, causing activation of this receptor. Thus, modulation of the purinergic system may have therapeutic potential on the effects of PD, as well as on the damage caused by inflammation of the BBB, which may be able to mitigate the neurodegeneration caused by diseases. Considering all the processes of neuroinflammation, oxidative stress, and mitochondrial dysfunction that PD propose, we can conclude that the P2X7 antagonist acts in the prevention of viral diseases, and it also controls purinergic receptors formed by multi-target compounds directed to self-amplification circuits and, therefore, may be a viable strategy to obtain the desired disease-modifying effect. Thus, purinergic system receptor modulations have a high therapeutic potential for neurodegenerative diseases such as PD.

Adenosine Triphosphate Release From Influenza-Infected Lungs Enhances Neutrophil Activation and Promotes Disease Progression

BACKGROUND

Adenosine triphosphate (ATP) enhances neutrophil responses, but little is known about the role of ATP in influenza infections.

METHODS

We used a mouse influenza model to study if ATP release is associated with neutrophil activation and disease progression.

RESULTS

Influenza infection increased pulmonary ATP levels 5-fold and plasma ATP levels 3-fold vs healthy mice. Adding ATP at those concentrations to blood from healthy mice primed neutrophils and enhanced CD11b and CD63 expression, CD62L shedding, and reactive oxygen species production in response to formyl peptide receptor stimulation. Influenza infection also primed neutrophils in vivo, resulting in formyl peptide receptor-induced CD11b expression and CD62L shedding up to 3 times higher than that of uninfected mice. In infected mice, large numbers of neutrophils entered the lungs. These cells were significantly more activated than the peripheral neutrophils of infected mice and pulmonary neutrophils of healthy mice. Plasma ATP levels of infected mice and influenza disease progression corresponded with the numbers and activation level of their pulmonary neutrophils.

CONCLUSIONS

Findings suggest that ATP release from the lungs of infected mice promotes influenza disease progression by priming peripheral neutrophils, which become strongly activated and cause pulmonary tissue damage after their recruitment to the lungs.

ATP-P2X7 signaling mediates brain pathology while contributing to viral control in perinatal Zika virus infection

Zika virus (ZIKV), the causative agent of Zika fever, is a flavivirus transmitted by mosquitoes of the Aedes genus. Zika virus infection has become an international concern due to its association with severe neurological complications such as fetal microcephaly. Viral infection can induce the release of ATP in the extracellular environment, activating receptors sensitized by extracellular nucleotides, such as the P2X7 receptor. This receptor is the primary purinergic receptor involved in neuroinflammation, neurodegeneration, and immunity. In this work, we investigated the role of ATP-P2X7 receptor signaling in Zika-related brain abnormalities. Wild-type mice (WT) and P2X7 receptor-deficient (P2X7-/-) C57BL/6 newborn mice were subcutaneously inoculated with 5 × 106plaque-forming units of ZIKV or mock solution. P2X7 receptor expression increased in the brain of Zika virus-infected mice compared to the mock group. Comparative analyses of the hippocampi from WT and P2X7-/-mice revealed that the P2X7 receptor increased hippocampal damage in CA1/CA2 and CA3 regions. Doublecortin expression decreased significantly in the brains of ZIKV-infected mice. WT ZIKV-infected mice showed impaired motor performance compared to P2X7-/- infected mice. WT ZIKV-infected animals showed increased expression of glial markers GFAP (astrocytes) and IBA-1 (microglia) compared to P2X7-/- infected mice. Although the P2X7 receptor contributes to neuronal loss and neuroinflammation, WT mice were more efficient in controlling the viral load in the brain than P2X7 receptor-deficient mice. This result was associated with higher induction of TNF-α, IFN-β, and increased interferon-stimulated gene expression in WT mice than P2X7-/-ZIKV-infected. Finally, we found that the P2X7 receptor contributes to inhibiting the neuroprotective signaling pathway AKT/mTOR while stimulating the caspase-3 activation, possibly two distinct pathways contributing to neurodegeneration. These findings suggest that ATP-P2X7 receptor signaling contributes to the antiviral response in the brain of ZIKV-infected mice while increasing neuronal loss, neuroinflammation, and related brain abnormalities.

Adenosine triphosphate induced cell death: Mechanisms and implications in cancer biology and therapy

Adenosine triphosphate (ATP) induced cell death (AICD) is a critical cellular process that has garnered substantial scientific interest for its profound relevance to cancer biology and to therapeutic interventions. This comprehensive review unveils the intricate web of AICD mechanisms and their intricate connections with cancer biology. This review offers a comprehensive framework for comprehending the multifaceted role of AICD in the context of cancer. This is achieved by elucidating the dynamic interplay between systemic and cellular ATP homeostasis, deciphering the intricate mechanisms governing AICD, elucidating its intricate involvement in cancer signaling pathways, and scrutinizing validated key genes. Moreover, the exploration of AICD as a potential avenue for cancer treatment underscores its essential role in shaping the future landscape of cancer therapeutics.

Purinergic signaling in the battlefield of viral infections

Purinergic signaling has been associated with immune defenses against pathogens such as bacteria, protozoa, fungi, and viruses, acting as a sentinel system that signals to the cells when a threat is present. This review focuses on the roles of purinergic signaling and its therapeutic potential for viral infections. In this context, the purinergic system may play potent antiviral roles by boosting interferon signaling. In other cases, though, it can contribute to a hyperinflammatory response and disease severity, resulting in poor outcomes, such as during flu and potentially COVID-19. Lastly, a third situation may occur since viruses are obligatory intracellular parasites that hijack the host cell machinery for their infection and replication. Viruses such as HIV-1 use the purinergic system to favor their infection and persistence within the host cell. Therefore, understanding the particular nuances of purinergic signaling in each viral infection may contribute to designing proper therapeutic strategies to treat viral diseases.

Systematic P2Y receptor survey identifies P2Y11 as modulator of immune responses and virus replication in macrophages

The immune system is in place to assist in ensuring tissue homeostasis, which can be easily perturbed by invading pathogens or nonpathogenic stressors causing tissue damage. Extracellular nucleotides are well known to contribute to innate immune signaling specificity and strength, but how their signaling is relayed downstream of cell surface receptors and how this translates into antiviral immunity is only partially understood. Here, we systematically investigated the responses of human macrophages to extracellular nucleotides, focusing on the nucleotide-sensing GPRC receptors of the P2Y family. Time-resolved transcriptomic analysis showed that adenine- and uridine-based nucleotides induce a specific, immediate, and transient cytokine response through the MAPK signaling pathway that regulates transcriptional activation by AP-1. Using receptor trans-complementation, we identified a subset of P2Ys (P2Y1, P2Y2, P2Y6, and P2Y11) that govern inflammatory responses via cytokine induction, while others (P2Y4, P2Y11, P2Y12, P2Y13, and P2Y14) directly induce antiviral responses. Notably, P2Y11 combined both activities, and depletion or inhibition of this receptor in macrophages impaired both inflammatory and antiviral responses. Collectively, these results highlight the underappreciated functions of P2Y receptors in innate immune processes.

P2X7 signaling influences the production of pro-resolving and pro-inflammatory lipid mediators in alveolar macrophages derived from individuals with asthma

Few new therapeutics exist to target airway inflammation in mild-to-moderate asthma. Alveolar macrophages regulate airway inflammation by producing proresolving eicosanoids. We hypothesized that stimulation of the purinergic receptor P2X7 in macrophages from individuals with asthma produces eicosanoids associated with airway inflammation and resolution, and that these responses are predicted, in part, by P2X7 pore function. Study subjects were recruited in an Institutional Review Board (IRB)-approved study. Alveolar macrophages were recovered from bronchoalveolar lavage fluid following bronchoscopy. Purinergic receptor classification was performed using flow cytometry and fluorescent cell assay. Macrophages were stimulated in vitro and eicosanoids were measured via ELISA or enzyme immunoassay (EIA) in the presence and absence of P2X7-specific agonist [2'(3')-O-(4-Benzoylbenzoyl)adenosine-5'-triphosphate tri(triethylammonium) salt (Bz-ATP)] and antagonist (AZD9056). Functional P2X7 pore status was confirmed in a live cell assay using P2X7-specific agonists and antagonists. Alveolar macrophages produced increased quantities of the oxylipins lipoxin A4 (LXA4), resolvin D1 (RvD1), and 15(S)-hydroxyeicosatetraenoic acid (15(S)-HETE) following stimulation with Bz-ATP compared with vehicle controls, responses that were attenuated in the presence of the P2X7-selective antagonist, AZD9056. LXA4 and RvD1 production was greatest at 1 h, whereas 15(S)-HETE was maximally produced 24 h. Prostaglandin E-2 and resolvin E1 were minimally produced by P2X7 activation, indicating differential signaling pathways involved in eicosanoid production in alveolar macrophages derived from individuals with asthma. The early production of the proresolving eicosanoids, LXA4 and resolvin D1, is regulated by P2X7, whereas generation of the proinflammatory eicosanoid, 15(S)-HETE, is only partially regulated through P2X7 signaling and reaches maximal production after the peak in proresolving eicosanoids.NEW & NOTEWORTHY Alveolar macrophages obtained from individuals with asthma produce soluble lipid mediators in response to P2X7 purinergic receptor signaling. Proinflammatory mediators may contribute to asthma exacerbations but proresolving mediators may help with resolution of asthma loss of control. These specialized proresolving lipid mediators may serve as future potential therapeutics for asthma exacerbation resolution and recovery.

Animal Models for the Investigation of P2X7 Receptors

The P2X7 receptor is a trimeric ligand-gated cation channel activated by extracellular adenosine 5'-triphosphate. The study of animals has greatly advanced the investigation of P2X7 and helped to establish the numerous physiological and pathophysiological roles of this receptor in human health and disease. Following a short overview of the P2X7 distribution, roles and functional properties, this article discusses how animal models have contributed to the generation of P2X7-specific antibodies and nanobodies (including biologics), recombinant receptors and radioligands to study P2X7 as well as to the pharmacokinetic testing of P2X7 antagonists. This article then outlines how mouse and rat models have been used to study P2X7. These sections include discussions on preclinical disease models, polymorphic P2X7 variants, P2X7 knockout mice (including bone marrow chimeras and conditional knockouts), P2X7 reporter mice, humanized P2X7 mice and P2X7 knockout rats. Finally, this article reviews the limited number of studies involving guinea pigs, rabbits, monkeys (rhesus macaques), dogs, cats, zebrafish, and other fish species (seabream, ayu sweetfish, rainbow trout and Japanese flounder) to study P2X7.

Dysfunctional purinergic signaling correlates with disease severity in COVID-19 patients

Ectonucleotidases modulate inflammatory responses by balancing extracellular ATP and adenosine (ADO) and might be involved in COVID-19 immunopathogenesis. Here, we explored the contribution of extracellular nucleotide metabolism to COVID-19 severity in mild and severe cases of the disease. We verified that the gene expression of ectonucleotidases is reduced in the whole blood of patients with COVID-19 and is negatively correlated to levels of CRP, an inflammatory marker of disease severity. In line with these findings, COVID-19 patients present higher ATP levels in plasma and reduced levels of ADO when compared to healthy controls. Cell type-specific analysis revealed higher frequencies of CD39+ T cells in severely ill patients, while CD4+ and CD8+ expressing CD73 are reduced in this same group. The frequency of B cells CD39+CD73+ is also decreased during acute COVID-19. Interestingly, B cells from COVID-19 patients showed a reduced capacity to hydrolyze ATP into ADP and ADO. Furthermore, impaired expression of ADO receptors and a compromised activation of its signaling pathway is observed in COVID-19 patients. The presence of ADO in vitro, however, suppressed inflammatory responses triggered in patients’ cells. In summary, our findings support the idea that alterations in the metabolism of extracellular purines contribute to immune dysregulation during COVID-19, possibly favoring disease severity, and suggest that ADO may be a therapeutic approach for the disease.

Melatonin: Regulation of Viral Phase Separation and Epitranscriptomics in Post-Acute Sequelae of COVID-19

The relentless, protracted evolution of the SARS-CoV-2 virus imposes tremendous pressure on herd immunity and demands versatile adaptations by the human host genome to counter transcriptomic and epitranscriptomic alterations associated with a wide range of short- and long-term manifestations during acute infection and post-acute recovery, respectively. To promote viral replication during active infection and viral persistence, the SARS-CoV-2 envelope protein regulates host cell microenvironment including pH and ion concentrations to maintain a high oxidative environment that supports template switching, causing extensive mitochondrial damage and activation of pro-inflammatory cytokine signaling cascades. Oxidative stress and mitochondrial distress induce dynamic changes to both the host and viral RNA m6A methylome, and can trigger the derepression of long interspersed nuclear element 1 (LINE1), resulting in global hypomethylation, epigenetic changes, and genomic instability. The timely application of melatonin during early infection enhances host innate antiviral immune responses by preventing the formation of "viral factories" by nucleocapsid liquid-liquid phase separation that effectively blockades viral genome transcription and packaging, the disassembly of stress granules, and the sequestration of DEAD-box RNA helicases, including DDX3X, vital to immune signaling. Melatonin prevents membrane depolarization and protects cristae morphology to suppress glycolysis via antioxidant-dependent and -independent mechanisms. By restraining the derepression of LINE1 via multifaceted strategies, and maintaining the balance in m6A RNA modifications, melatonin could be the quintessential ancient molecule that significantly influences the outcome of the constant struggle between virus and host to gain transcriptomic and epitranscriptomic dominance over the host genome during acute infection and PASC.

Alterations in CD39/CD73 axis of T cells associated with COVID-19 severity

Purinergic signaling modulates immune function and is involved in the immunopathogenesis of several viral infections. This study aimed to investigate alterations in purinergic pathways in coronavirus disease 2019 (COVID‐19) patients. Mild and severe COVID‐19 patients had lower extracellular adenosine triphosphate and adenosine levels, and higher cytokines than healthy controls. Mild COVID‐19 patients presented lower frequencies of CD4⁺CD25⁺CD39⁺ (activated/memory regulatory T cell [mTreg]) and increased frequencies of high‐differentiated (CD27⁻CD28⁻) CD8⁺ T cells compared with healthy controls. Severe COVID‐19 patients also showed higher frequencies of CD4⁺CD39⁺, CD4⁺CD25⁻CD39⁺ (memory T effector cell), and high‐differentiated CD8⁺ T cells (CD27⁻CD28⁻), and diminished frequencies of CD4⁺CD73⁺, CD4⁺CD25⁺CD39⁺ mTreg cell, CD8⁺CD73⁺, and low‐differentiated CD8⁺ T cells (CD27⁺CD28⁺) in the blood in relation to mild COVID‐19 patients and controls. Moreover, severe COVID‐19 patients presented higher expression of PD‐1 on low‐differentiated CD8⁺ T cells. Both severe and mild COVID‐19 patients presented higher frequencies of CD4⁺Annexin‐V⁺ and CD8⁺Annexin‐V⁺ T cells, indicating increased T‐cell apoptosis. Plasma samples collected from severe COVID‐19 patients were able to decrease the expression of CD73 on CD4⁺ and CD8⁺ T cells of a healthy donor. Interestingly, the in vitro incubation of peripheral blood mononuclear cell from severe COVID‐19 patients with adenosine reduced the nuclear factor‐κB activation in T cells and monocytes. Together, these data add new knowledge to the COVID‐19 immunopathology through purinergic regulation.

The Potential of Purinergic Signaling to Thwart Viruses Including SARS-CoV-2

A long-shared evolutionary history is congruent with the multiple roles played by purinergic signaling in viral infection, replication and host responses that can assist or hinder viral functions. An overview of the involvement of purinergic signaling among a range of viruses is compared and contrasted with what is currently understood for SARS-CoV-2. In particular, we focus on the inflammatory and antiviral responses of infected cells mediated by purinergic receptor activation. Although there is considerable variation in a patient's response to SARS-CoV-2 infection, a principle immediate concern in Coronavirus disease (COVID-19) is the possibility of an aberrant inflammatory activation causing diffuse lung oedema and respiratory failure. We discuss the most promising potential interventions modulating purinergic signaling that may attenuate the more serious repercussions of SARS-CoV-2 infection and aspects of their implementation.

ENPP1's regulation of extracellular cGAMP is a ubiquitous mechanism of attenuating STING signaling

The metazoan innate immune second messenger 2′3′-cGAMP is present both inside and outside cells. However, only extracellular cGAMP can be negatively regulated by the extracellular hydrolase ENPP1. Here, we determine whether ENPP1’s regulation of extracellular cGAMP is a ubiquitous mechanism of attenuating stimulator of interferon genes (STING) signaling. We identified ENPP1H362A, a point mutation that cannot degrade the 2′-5′ linkage in cGAMP while maintaining otherwise normal function. The selectivity of this histidine is conserved down to bacterial nucleotide pyrophosphatase/phosphodiesterase (NPP), allowing structural analysis and suggesting an unexplored ancient history of 2′-5′ cyclic dinucleotides. Enpp1H362A mice demonstrated that extracellular cGAMP is not responsible for the devastating phenotype in ENPP1-null humans and mice but is responsible for antiviral immunity and systemic inflammation. Our data define extracellular cGAMP as a pivotal STING activator, identify an evolutionarily critical role for ENPP1 in regulating inflammation, and suggest a therapeutic strategy for viral and inflammatory conditions by manipulating ENPP1 activity.

Possible role of pannexin 1 channels and purinergic receptors in the pathogenesis and mechanism of action of SARS-CoV-2 and therapeutic potential of targeting them in COVID-19

Identifying signaling pathways and molecules involved in SARS-CoV-2 pathogenesis is pivotal for developing new effective therapeutic or preventive strategies for COVID-19. Pannexins (PANX) are ATP-release channels in the plasma membrane essential in many physiological and immune responses. Activation of pannexin channels and downstream purinergic receptors play dual roles in viral infection, either by facilitating viral replication and infection or inducing host antiviral defense. The current review provides a hypothesis demonstrating the possible contribution of the PANX1 channel and purinergic receptors in SARS-CoV-2 pathogenesis and mechanism of action. Moreover, we discuss whether targeting these signaling pathways may provide promising preventative therapies and treatments for patients with progressive COVID-19 resulting from excessive pro-inflammatory cytokines and chemokines production. Several inhibitors of this pathway have been developed for the treatment of other viral infections and pathological consequences. Specific PANX1 inhibitors could be potentially included as part of the COVID-19 treatment regimen if, in future, studies demonstrate the role of PANX1 in COVID-19 pathogenesis. Of note, any ATP therapeutic modulation for COVID-19 should be carefully designed and monitored because of the complex role of extracellular ATP in cellular physiology.

Molecular alterations in human milk in simulated maternal nasal mucosal infection with live attenuated influenza vaccination

Breastfeeding protects against mucosal infections in infants. The underlying mechanisms through which immunity develops in human milk following maternal infection with mucosal pathogens are not well understood. We simulated nasal mucosal influenza infection through live attenuated influenza vaccination (LAIV) and compared immune responses in milk to inactivated influenza vaccination (IIV). Transcriptomic analysis was performed on RNA extracted from human milk cells to evaluate differentially expressed genes and pathways on days 1 and 7 post-vaccination. Both LAIV and IIV vaccines induced influenza-specific IgA that persisted for at least 6 months. Regulation of type I interferon production, toll-like receptor, and pattern recognition receptor signaling pathways were highly upregulated in milk on day 1 following LAIV but not IIV at any time point. Upregulation of innate immunity in human milk may provide timely protection against mucosal infections until antigen-specific immunity develops in the human milk-fed infant.

Airway mucins promote immunopathology in virus-exacerbated chronic obstructive pulmonary disease

The respiratory tract surface is protected from inhaled pathogens by a secreted layer of mucus rich in mucin glycoproteins. Abnormal mucus accumulation is a cardinal feature of chronic respiratory diseases, but the relationship between mucus and pathogens during exacerbations is poorly understood. We identified elevations in airway mucin 5AC (MUC5AC) and MUC5B concentrations during spontaneous and experimentally induced chronic obstructive pulmonary disease (COPD) exacerbations. MUC5AC was more sensitive to changes in expression during exacerbation and was therefore more predictably associated with viral load, inflammation, symptom severity, decrements in lung function, and secondary bacterial infections. MUC5AC was functionally related to inflammation, as Muc5ac-deficient (Muc5ac-/-) mice had attenuated RV-induced (RV-induced) airway inflammation, and exogenous MUC5AC glycoprotein administration augmented inflammatory responses and increased the release of extracellular adenosine triphosphate (ATP) in mice and human airway epithelial cell cultures. Hydrolysis of ATP suppressed MUC5AC augmentation of RV-induced inflammation in mice. Therapeutic suppression of mucin production using an EGFR antagonist ameliorated immunopathology in a mouse COPD exacerbation model. The coordinated virus induction of MUC5AC and MUC5B expression suggests that non-Th2 mechanisms trigger mucin hypersecretion during exacerbations. Our data identified a proinflammatory role for MUC5AC during viral infection and suggest that MUC5AC inhibition may ameliorate COPD exacerbations.

The quantitative proteomic analysis of rare minnow, Gobiocypris rarus, infected with virulent and attenuated isolates of grass carp reovirus genotype Ⅱ

Grass carp reovirus genotype Ⅱ (GCRV II) causes severe hemorrhagic disease in grass carp and affects the aquaculture industry in China. GCRV Ⅱ isolates have been collected from different epidemic areas in China, and these isolates can lead to different degrees of hemorrhagic symptoms in grass carp. Rare minnow (Gobiocypris rarus) is widely used as a model fish to study the mechanism of hemorrhagic disease because of its high sensitivity to GCRV. In this study, the protein levels in the spleen of rare minnow after infection with GCRV virulent isolate JZ809 and attenuated isolate XT422 were investigated using isobaric tags for relative and absolute quantitation (iTRAQ)-based quantitative proteomics. 109 and 50 differentially expressed proteins (DEPs) in the virulent and attenuated infection groups were obtained, respectively, among which 40 DEPs were identified in both groups. Combining protein expression profiling with gene ontology (GO) annotation, the responses of rare minnow to the two genotypes GCRV Ⅱ in terms of upregulated proteins were similar, focusing on ATP synthesis, in which ATP can serve as a "danger" signal to activate an immunoreaction in eukaryotes. Meanwhile, the virulent genotype JZ809 induced more immunoproteins and increased the levels of ubiquitin-proteasome system members to adapt to virus infection. However, together with a persistent and excessive inflammatory response and declining carbon metabolism, rare minnow presented more severe hemorrhagic disease and mortality after infection with virulent JZ809 than with attenuated XT422. The results provide a valuable information that will increase our understanding of the pathogenesis of viruses with different levels of virulence and the mechanism of interaction between the virus and host. Furthermore, the 6 proteins that were only significantly upregulated in the XT422 infection group all belonged to cluster 2, and 28 of 30 proteins that were only upregulated in JZ809 infection group were clustered into cluster 1. For the downregulated proteins, all DEPs in the XT422 infection group were clustered into cluster 4, and 25 of 39 proteins that were only significantly downregulated in the JZ809 infection group belonged to cluster 3. The results indicated that the DEPs in the attenuated XT422 infection group might be sensitive and their abundance changed more quickly when fish experienced virus infection.

The Role of Purinergic P2X7 Receptor in Inflammation and Cancer: Novel Molecular Insights and Clinical Applications

The purinergic P2X7 receptor (P2X7R) is a transmembrane protein whose expression has been related to a variety of cellular processes, while its dysregulation has been linked to inflammation and cancer. P2X7R is expressed in cancer and immune system cell surfaces. ATP plays a key role in numerous metabolic processes due to its abundance in the tumour microenvironment. P2X7R plays an important role in cancer by interacting with ATP. The unusual property of P2X7R is that stimulation with low doses of ATP causes the opening of a permeable channel for sodium, potassium, and calcium ions, whereas sustained stimulation with high doses of ATP favours the formation of a non-selective pore. The latter effect induces a change in intracellular homeostasis that leads to cell death. This evidence suggests that P2X7R has both pro- and anti-tumour proprieties. P2X7R is increasingly recognised as a regulator of inflammation. In this review, we aimed to describe the most relevant characteristics of P2X7R function, activation, and its ligands, while also summarising the role of P2X7R activation in the context of inflammation and cancer. The currently used therapeutic approaches and clinical trials of P2X7R modulators are also described.

Purinergic modulation of the immune response to infections

Infectious diseases are caused by the invasion of pathogenic microorganisms such as fungi, bacteria, viruses, and parasites. After infection, disease progression relies on the complex interplay between the host immune response and the microorganism evasion strategies. The host's survival depends on its ability to mount an efficient protective anti-microbial response to accomplish pathogen clearance while simultaneously preventing tissue injury by keeping under control the excessive inflammatory process. The purinergic system has the dual function of regulating the immune response and triggering effector antimicrobial mechanisms. This review provides an overview of the current knowledge of the modulation of innate and adaptive immunity driven by the purinergic system during parasitic, bacterial and viral infections.

Purinergic signaling elements are correlated with coagulation players in peripheral blood and leukocyte samples from COVID-19 patients

For over a year, the coronavirus disease 2019 has been affecting the world population by causing severe tissue injuries and death in infected people. Adenosine triphosphate (ATP) and the nicotinamide adenine dinucleotide (NAD +) are two molecules that are released into the extracellular microenvironment after direct virus infection or cell death caused by hyper inflammation and coagulopathy. Also, these molecules are well known to participate in multiple pathways and have a pivotal role in the purinergic signaling pathway. Thus, using public datasets available on the Gene Expression Omnibus (GEO), we analyzed raw proteomics data acquired using mass spectrometry (the gold standard method) and raw genomics data from COVID-19 patient samples obtained by microarray. The data was analyzed using bioinformatics and statistical methods according to our objectives. Here, we compared the purinergic profile of the total leukocyte population and evaluated the levels of these soluble biomolecules in the blood, and their correlation with coagulation components in COVID-19 patients, in comparison to healthy people or non-COVID-19 patients. The blood metabolite analysis showed a stage-dependent inosine increase in COVID-19 patients, while the nucleotides ATP and ADP had positive correlations with fibrinogen and other coagulation proteins. Also, ATP, ADP, inosine, and hypoxanthine had positive and negative correlations with clinical features. Regarding leukocyte gene expression, COVID-19 patients showed an upregulation of the P2RX1, P2RX4, P2RX5, P2RX7, P2RY1, P2RY12, PANX1, ADORA2B, NLPR3, and F3 genes. Yet, the ectoenzymes of the canonical and non-canonical adenosinergic pathway (ENTPD1 and CD38) are upregulated, suggesting that adenosine is produced by both active adenosinergic pathways. Hence, approaches targeting these biomolecules or their specific purinoreceptors and ectoenzymes may attenuate the high inflammatory state and the coagulopathy seen in COVID-19 patients. KEY MESSAGES : Adenosinergic pathways are modulated on leukocytes from COVID-19 patients. Plasmatic inosine levels are increased in COVID-19 patients. ATP, ADP, AMP, hypoxanthine, and inosine are correlated with coagulation players. The nucleotides and nucleosides are correlated with patients' clinical features. The P2 receptors and ectoenzymes are correlated with Tissue factor in COVID-19.

The Pannexin 1/Purinergic Receptor P2X4 Pathway Controls the Secretion of MicroRNA-Containing Exosomes by HCV-Infected Hepatocytes

BACKGROUND AND AIMS

HCV infection is a major risk factor that can lead to chronic liver disease, including fibrosis, cirrhosis, and HCC. Progression of chronic liver disease by HCV infection is caused by a complex intercellular reaction. Especially, exosomes and microRNAs (miRNAs) from HCV-infected hepatocytes play a role in the pathogenesis of liver disease by facilitating cellular communication between parenchymal and nonparenchymal cells. However, the underlying mechanism of secretions of exosome and miRNAs during HCV infection is still open for study.

APPROACH AND RESULTS

In this study, we demonstrated a pathway for the release of exosome and exosomal miRNAs through caspase-3/pannexin 1 (Panx1)/P2X4 activation during HCV infection in hepatocytes. We found that HCV infection induced the stimulation of exosome release and activation of the caspase-3/Panx1/P2X4 pathway in Huh7.5.1 cells. In addition, miR-122 and miR-146a levels in extracellular exosomes from HCV-infected cells were dramatically increased whereas intracellular miR122 and miR-146a expression had no large changes. Notably, secretions of exosomes and exosomal miRNAs were decreased by inhibition of caspase 3, Panx1, and P2X4 whereas inhibition of ROCK-1 cleavage did not affect these during HCV infection in Huh7.5.1 cells.

CONCLUSIONS

These results suggested that HCV infection caused secretions of exosomes and exosomal miRNAs dependent on the caspase 3/Panx1/P2X4 pathway. Our study provides a possible therapeutic intervention using Panx1 suppression for liver disease development mediated by exosomes from HCV-infected hepatocytes.

The role of P2X7 receptor in infection and metabolism: Based on inflammation and immunity

The P2X7 receptor (P2X7R) is a ligand-gated receptor belonging to the P2 receptor family. It is distributed in various tissues of the human body and is involved in regulating the physiological functions of tissues and cells to affect the occurrence and development of diseases. Unlike all other P2 receptors, the P2X7 receptor is mainly expressed in immune cells and can be activated not only by extracellular nucleotides but also by non-nucleotide substances which act as positive allosteric modulators. In this review, we comprehensively describe the role of the P2X7 receptor in infection and metabolism based on its role as an important regulator of inflammation and immunity, and briefly introduce the structure and general function of the P2X7 receptor. These provide a clear knowledge framework for the study of the P2X7 receptor in human health. Targeting the P2X7 receptor may be an effective method for the treatment of inflammatory and immune diseases. And its role in microbial infection and metabolism may be the main direction for in-depth research on the P2X7 receptor in the future.

Targeting purinergic receptors to suppress the cytokine storm induced by SARS-CoV-2 infection in pulmonary tissue

The etiological agent of coronavirus disease (COVID-19) is the new member of the Coronaviridae family, a severe acute respiratory syndrome coronavirus 2 virus (SARS-CoV-2), responsible for the pandemic that is plaguing the world. The single-stranded RNA virus is capable of infecting the respiratory tract, by binding the spike (S) protein on its viral surface to receptors for the angiotensin II-converting enzyme (ACE2), highly expressed in the pulmonary tissue, enabling the interaction of the virus with alveolar epithelial cells promoting endocytosis and replication of viral material. The infection triggers the activation of the immune system, increased purinergic signaling, and the release of cytokines as a defense mechanism, but the response can become exaggerated and prompt the so-called “cytokine storm”, developing cases such as severe acute respiratory syndrome (SARS). This is characterized by fever, cough, and difficulty breathing, which can progress to pneumonia, failure of different organs and death. Thus, the present review aims to compile and correlate the mechanisms involved between the immune and purinergic systems with COVID-19, since the modulation of purinergic receptors, such as A2A, A2B, and P2X7 expressed by immune cells, seems to be effective as a promising therapy, to reduce the severity of the disease, as well as aid in the treatment of acute lung diseases and other cases of generalized inflammation.

Cystic fibrosis improves COVID-19 survival and provides clues for treatment of SARS-CoV-2

Systemic pools of ATP are elevated in individuals homozygous for cystic fibrosis (CF) as evidenced by elevated blood and plasma ATP levels. This elevated ATP level seems to provide benefit in the presence of advanced solid tumors (Abraham et al., Nature Medicine 2(5):593-596, 1996). We published in this journal a paper showing that IV ATP can elevate the depleted ATP pools of advanced cancer patients up to levels found in CF patients with subsequent clinical, biochemical, and quality of life (QOL) improvements (Rapaport et al., Purinergic Signalling 11(2): 251-262, 2015). We hypothesize that the elevated ATP levels seen in CF patients may be benefiting CF patients in another way: by improving their survival after contracting COVID-19. We discuss here the reasoning behind this hypothesis and suggest how these findings might be applied clinically in the general population.

Generation of plasma cells and CD27-IgD- B cells during hantavirus infection is associated with distinct pathological findings

OBJECTIVE

Human hantavirus infections can cause haemorrhagic fever with renal syndrome (HFRS). The pathogenic mechanisms are not fully understood, nor if they affect the humoral immune system. The objective of this study was to investigate humoral immune responses to hantavirus infection and to correlate them to the typical features of HFRS: thrombocytopenia and transient kidney dysfunction.

METHODS

We performed a comprehensive characterisation of longitudinal antiviral B-cell responses of 26 hantavirus patients and combined this with paired clinical data. In addition, we measured extracellular adenosine triphosphate (ATP) and its breakdown products in circulation and performed in vitro stimulations to address its effect on B cells.

RESULTS

We found that thrombocytopenia was correlated to an elevated frequency of plasmablasts in circulation. In contrast, kidney dysfunction was indicative of an accumulation of CD27-IgD- B cells and CD27-/low plasmablasts. Finally, we provide evidence that high levels of extracellular ATP and matrix metalloproteinase 8 can contribute to shedding of CD27 during human hantavirus infection.

CONCLUSION

Our findings demonstrate that thrombocytopenia and kidney dysfunction associate with distinctly different effects on the humoral immune system. Moreover, hantavirus-infected individuals have significantly elevated levels of extracellular ATP in circulation.

Sex-Related Overactivation of NLRP3 Inflammasome Increases Lethality of the Male COVID-19 Patients

The COVID-19 pandemic, caused by SARS-CoV-2 infection, remains a dramatic threat to human life and economic well-being worldwide. Significant heterogeneity in the severity of disease was observed for patients infected with SARS-CoV-2 ranging from asymptomatic to severe cases. Moreover, male patients had a higher probability of suffering from high mortality and severe symptoms linked to cytokine storm and excessive inflammation. The NLRP3 inflammasome is presumably critical to this process. Sex differences may directly affect the activation of NLRP3 inflammasome, impacting the severity of observed COVID-19 symptoms. To elucidate the potential mechanisms underlying sex based differences in NLRP3 activation during SARS-CoV-2 infection, this review summarizes the reported mechanisms and identifies potential therapeutic targets.

GS-441524 inhibits African swine fever virus infection in vitro

African swine fever virus (ASFV) is a highly infectious and lethal swine pathogen that causes serious socio-economic consequences in endemic countries for which no safe and effective vaccine is currently available. GS-441524, a 1-cyano-substituted adenine C-nucleoside ribose analogue, inhibits viral RNA transcription by competing with natural nucleosides (ATP, TTP, CTP, and GTP) and effectively inhibits viral RNA-dependent RNA polymerase activity. However, whether GS-441524 can inhibit the replication of DNA viruses is unknown. In this study, we confirmed that GS-441524 inhibits ASFV infection in porcine alveolar macrophages (PAMs) in a dose-dependent manner; GS-441524 significantly inhibited ASFV replication at different time points after ASFV infection, particularly at the early stages of viral replication. Notably, GS-441524 did not increase the levels of antiviral cytokines or ATP in PAMs. However, an increase in the concentration of natural ATP in PAMs promoted the replication of ASFV and attenuated the inhibitory effect of GS-441524 in a dose-dependent manner. Our results suggest that GS-441524 is an effective antiviral against ASFV.

Supplementary Therapeutic Possibilities to Alleviate Myocardial Damage Due to Microvascular Dysfunction in Coronavirus Disease 2019 (COVID-19)

Myocardial damage with a consequent rise in cardio-specific troponin level is a frequent phenomenon in severe cases of coronavirus disease 2019 (COVID-19). Its causes are capillary endothelial cell dysfunction, associated carditis, low oxygenization, and increased sympathetic tone, which all worsen myocardial stiffness and microvascular dysfunction (MD). They lead to severe myocardial dysfunction, arrhythmia, acute congestive heart failure, and a significant rise in death cases. During COVID-19, no specific cardiological treatment is started. As adjuvant therapy, anxiolytics in COVID-19 are widely used, but not in all anxious patients who had been infected with coronavirus. Anxiolytics can be useful to moderate MD and immunosuppressive effect of anxiety. The favorable effects of trimetazidine (TMZ) and Coenzyme Q₁₀ (CoQ₁₀) in the treatment of myocardial ischemia and heart failure had previously been proven, and also their anti-inflammatory effects had been suspected; however, they have not yet been used in COVID-19 cases. TMZ promotes glucose-mediated ATP production, which requires less oxygen, which explains its advantageous cardiac effects. Since it lowers serum and myocardial tissue proinflammatory cytokine levels and inhibits myocardial macrophage infiltration, it was suspected that TMZ might represent a novel therapeutic agent to prevent and treat sepsis-induced myocardial dysfunction. CoQ₁₀ plays an important role in cellular ATP production; however, its concentration is decreased in cardiovascular diseases and in influenza patients. Due to its anti-inflammatory effect, CoQ₁₀ has been suspected to have a key therapeutic target in influenza infection. We suggest considering these medicines to alleviate myocardial damage and inflammation in COVID-19.

SARS-CoV-2 early infection signature identified potential key infection mechanisms and drug targets

BACKGROUND

The ongoing COVID-19 outbreak has caused devastating mortality and posed a significant threat to public health worldwide. Despite the severity of this illness and 2.3 million worldwide deaths, the disease mechanism is mostly unknown. Previous studies that characterized differential gene expression due to SARS-CoV-2 infection lacked robust validation. Although vaccines are  now available, effective treatment options are still out of reach.

RESULTS

To characterize the transcriptional activity of SARS-CoV-2 infection, a gene signature consisting of 25 genes was generated using a publicly available RNA-Sequencing (RNA-Seq) dataset of cultured cells infected with SARS-CoV-2. The signature estimated infection level accurately in bronchoalveolar lavage fluid (BALF) cells and peripheral blood mononuclear cells (PBMCs) from healthy and infected patients (mean 0.001 vs. 0.958; P < 0.0001). These signature genes were investigated in their ability to distinguish the severity of SARS-CoV-2 infection in a single-cell RNA-Sequencing dataset. TNFAIP3, PPP1R15A, NFKBIA, and IFIT2 had shown bimodal gene expression in various immune cells from severely infected patients compared to healthy or moderate infection cases. Finally, this signature was assessed using the publicly available ConnectivityMap database to identify potential disease mechanisms and drug repurposing candidates. Pharmacological classes of tricyclic antidepressants, SRC-inhibitors, HDAC inhibitors, MEK inhibitors, and drugs such as atorvastatin, ibuprofen, and ketoconazole showed strong negative associations (connectivity score < - 90), highlighting the need for further evaluation of these candidates for their efficacy in treating SARS-CoV-2 infection.

CONCLUSIONS

Thus, using the 25-gene SARS-CoV-2 infection signature, the SARS-CoV-2 infection status was captured in BALF cells, PBMCs and postmortem lung biopsies. In addition, candidate SARS-CoV-2 therapies with known safety profiles were identified. The signature genes could potentially also be used to characterize the COVID-19 disease severity in patients' expression profiles of BALF cells.

P2RX7 at the Host-Pathogen Interface of Infectious Diseases

The P2X7 receptor (P2RX7) is an important molecule that functions as a danger sensor, detecting extracellular nucleotides from injured cells and thus signaling an inflammatory program to nearby cells. It is expressed in immune cells and plays important roles in pathogen surveillance and cell-mediated responses to infectious organisms. There is an abundance of literature on the role of P2RX7 in inflammatory diseases and the role of these receptors in host-pathogen interactions. Here, we describe the current knowledge of the role of P2RX7 in the host response to a variety of pathogens, including viruses, bacteria, fungi, protozoa, and helminths. We describe in vitro and in vivo evidence for the critical role these receptors play in mediating and modulating immune responses. Our observations indicate a role for P2X7 signaling in sensing damage-associated molecular patterns released by nearby infected cells to facilitate immunopathology or protection. In this review, we describe how P2RX7 signaling can play critical roles in numerous cells types in response to a diverse array of pathogens in mediating pathogenesis and immunity to infectious agents.

Increased Extracellular ATP in Plasma of Rheumatoid Arthritis Patients Activates CD8+T Cells

BACKGROUND

Rheumatoid arthritis (RA) is an autoimmune disorder with genetic and environmental causes often linked with the disease etiology. A disrupted metabolism has often been a characteristic of RA and an altered metabolic state of immune cells has been associated with their phenotypic and functional changes. The energy in the form of ATP produced by the metabolically active cells may thus initiate a cascade of immune responses there by influencing the disease pathogenesis or progression.

AIM OF THE STUDY

Through this study we have focused on determining the role of ATP in etiology of RA and aberrant cellular functions.

METHODS

Blood samples of 80 healthy controls (HC) and 95 RA patients were screened for extracellular ATP concentration, transcriptome analyses, an inflammatory mediator and the results were statistically analysed.

RESULTS

In this study, ATP is shown to be excessive in the plasma of RA patients (453.5 ± 16.09% in RA vs. 233.9 ± 10.07% in HC, p <0.0001) and significantly increases with the disease severity. The abundant extracellular ATP could activate circulating cytotoxic CD8⁺T cells in RA patients to produce Granzyme B.

CONCLUSION

Plasma ATP is thus identified to have a significant potential in progression and prognosis of RA and may thus be studied further to design better therapeutic approaches for the disease.

Purinergic signalling in host innate immune defence against intracellular pathogens

Purinergic signalling is an evolutionarily conserved signalling pathway mediated by extracellular nucleotides and nucleosides. Tri- and diphosphonucleotides released from host cells during intracellular pathogen infections activate plasma membrane purinergic type 2 receptors (P2 receptors) that stimulate microbicidal mechanisms in host innate immune cells. P2X ion channels and P2Y G protein-coupled receptors are involved in activating host innate immune defence mechanisms, phagocytosis, phagolysosomal fusion, production of reactive species, acidification of parasitophorous vacuoles, inflammasome activation, and the release of cytokines, chemokines, and other inflammatory mediators. In this review, as part of a special issue in tribute to Geoffrey Burnstock, we discuss advances in understanding the importance of P2 receptors in the host antimicrobial innate mechanisms against intracellular pathogen infections.

Hyperactivation of P2X7 receptors as a culprit of COVID-19 neuropathology

Scientists and health professionals are exhaustively trying to contain the coronavirus disease 2019 (COVID-19) pandemic by elucidating viral invasion mechanisms, possible drugs to prevent viral infection/replication, and health cares to minimize individual exposure. Although neurological symptoms are being reported worldwide, neural acute and long-term consequences of SARS-CoV-2 are still unknown. COVID-19 complications are associated with exacerbated immunoinflammatory responses to SARS-CoV-2 invasion. In this scenario, pro-inflammatory factors are intensely released into the bloodstream, causing the so-called "cytokine storm". Both pro-inflammatory factors and viruses may cross the blood-brain barrier and enter the central nervous system, activating neuroinflammatory responses accompanied by hemorrhagic lesions and neuronal impairment, which are largely described processes in psychiatric disorders and neurodegenerative diseases. Therefore, SARS-CoV-2 infection could trigger and/or worse brain diseases. Moreover, patients with central nervous system disorders associated to neuroimmune activation (e.g. depression, Parkinson's and Alzheimer's disease) may present increased susceptibility to SARS-CoV-2 infection and/or achieve severe conditions. Elevated levels of extracellular ATP induced by SARS-CoV-2 infection may trigger hyperactivation of P2X7 receptors leading to NLRP3 inflammasome stimulation as a key mediator of neuroinvasion and consequent neuroinflammatory processes, as observed in psychiatric disorders and neurodegenerative diseases. In this context, P2X7 receptor antagonism could be a promising strategy to prevent or treat neurological complications in COVID-19 patients.

Rotavirus induces intercellular calcium waves through ADP signaling

Rotavirus causes severe diarrheal disease in children by broadly dysregulating intestinal homeostasis. However, the underlying mechanism(s) of rotavirus-induced dysregulation remains unclear. We found that rotavirus-infected cells produce paracrine signals that manifested as intercellular calcium waves (ICWs), observed in cell lines and human intestinal enteroids. Rotavirus ICWs were caused by the release of extracellular adenosine 5'-diphosphate (ADP) that activated P2Y1 purinergic receptors on neighboring cells. ICWs were blocked by P2Y1 antagonists or CRISPR-Cas9 knockout of the P2Y1 receptor. Blocking the ADP signal reduced rotavirus replication, inhibited rotavirus-induced serotonin release and fluid secretion, and reduced diarrhea severity in neonatal mice. Thus, rotavirus exploited paracrine purinergic signaling to generate ICWs that amplified the dysregulation of host cells and altered gastrointestinal physiology to cause diarrhea.

The potential similarities of COVID-19 and autoimmune disease pathogenesis and therapeutic options: new insights approach

Cytokine pathways and their signaling disorders can be the cause of onset and pathogenesis of many diseases such as autoimmune diseases and COVID-19 infection. Autoimmune patients may be at higher risk of developing infection due to the impaired immune responses, the use of immunosuppressive drugs, and damage to various organs. Increased secretion of inflammatory cytokines and intolerance of the patient's immune system to COVID-19 infection are the leading causes of hospitalization of these patients. The content used in this paper has been taken from English language articles (2005-2020) retrieved from the PubMed database and Google Scholar search engine using "COVID-19," "Autoimmune disease," "Therapeutic," "Pathogenesis," and "Pathway" keywords. The emergence of COVID-19 and its association with autoimmune disorders is a major challenge in the management of these diseases. The results showed that the use of corticosteroids in the treatment of autoimmune diseases can make diagnosis and treatment of COVID-19 more challenging by preventing the fever. Due to the common pathogenesis of COVID-19 and autoimmune diseases, the use of autoimmune drugs as a possible treatment option could help control the virus. KEY POINTS: • Inflammatory cytokines play an essential role in the pathogenesis of COVID-19 • ACE2 dysfunctions are related to the with COVID-19 and autoimmune diseases • The use autoimmune diseases drugs can be useful in treating COVID-19.

ADP/P2Y1 aggravates inflammatory bowel disease through ERK5-mediated NLRP3 inflammasome activation

Inflammasomes are essential for inflammation and pathogen elimination in response to microbial infection and endogenous danger signals. However, the mechanism of inflammasome activation by endogenous danger signals mediated posttranslational modification and the connection between inflammasomes and inflammatory diseases remains elusive. In this study, we found that ADP was highly released from injured colonic tissue as a danger signal during inflammatory bowel disease. Consequently, extracellular ADP activated the NLRP3 inflammasome through P2Y₁ receptor-mediated calcium signaling, which led to the maturation and secretion of IL-1β and further aggravation of experimental colitis. Genetic ablation or pharmacological blockade of the P2Y₁ receptor significantly ameliorated DSS-induced colitis and endotoxic shock through reducing NLRP3 inflammasome activation. Moreover, ERK5-mediated tyrosine phosphorylation of ASC was essential for activation of the NLRP3 inflammasome. Thus, our study provides a novel theoretical basis for posttranslational modification of ASC in NLRP3 inflammasome activation and revealed that ADP/P2Y₁ is a potential drug target for inflammatory bowel disease.

Blocking P2X7-Mediated Macrophage Polarization Overcomes Treatment Resistance in Lung Cancer

P2X7, a crucial sensor of extracellular ATP, is widely distributed in different immune cells as a potent stimulant of inflammation and immunity. P2X7 is also highly expressed in immunosuppressive cells such as tumor-associated macrophages (TAM) and even tumor cells. However, the function and potential applications of P2X7-mediated immunosuppressive responses in the tumor microenvironment remain unclear. Here, we demonstrated that P2X7 was highly expressed in TAMs and that P2X7 deficiency impaired the "M2-like" polarization of TAMs via downregulation of STAT6 and IRF4 phosphorylation both in vivo and in vitro P2X7 deficiency restricted the progression of urethane-induced lung carcinogenesis and Lewis lung cancer by decreasing tumor cell proliferation and angiogenesis, promoting T-cell mobilization, and reversing M2-like TAM polarization. Thus, deletion or blockade of P2X7 was therapeutic for lung cancer. Furthermore, resistance to both immunotherapy (anti-PD-1 antibody) and chemotherapy (cisplatin) was overcome by coadministration of the P2X7 inhibitors O-ATP, A-438079 hydrochloride, and A-740003. Therefore, our data revealed a vital role of P2X7 in tumor formation through regulating TAM polarization, suggesting the therapeutic potential of P2X7 blockade in patients with lung cancer.

P2 Purinergic Signaling in the Distal Lung in Health and Disease

The distal lung provides an intricate structure for gas exchange in mammalian lungs. Efficient gas exchange depends on the functional integrity of lung alveoli. The cells in the alveolar tissue serve various functions to maintain alveolar structure, integrity and homeostasis. Alveolar epithelial cells secrete pulmonary surfactant, regulate the alveolar surface liquid (ASL) volume and, together with resident and infiltrating immune cells, provide a powerful host-defense system against a multitude of particles, microbes and toxicants. It is well established that all of these cells express purinergic P2 receptors and that purinergic signaling plays important roles in maintaining alveolar homeostasis. Therefore, it is not surprising that purinergic signaling also contributes to development and progression of severe pathological conditions like pulmonary inflammation, acute lung injury/acute respiratory distress syndrome (ALI/ARDS) and pulmonary fibrosis. Within this review we focus on the role of P2 purinergic signaling in the distal lung in health and disease. We recapitulate the expression of P2 receptors within the cells in the alveoli, the possible sources of ATP (adenosine triphosphate) within alveoli and the contribution of purinergic signaling to regulation of surfactant secretion, ASL volume and composition, as well as immune homeostasis. Finally, we summarize current knowledge of the role for P2 signaling in infectious pneumonia, ALI/ARDS and idiopathic pulmonary fibrosis (IPF).

Targeting adenosinergic pathway and adenosine A2A receptor signaling for the treatment of COVID-19: A hypothesis

The most serious health issue today is the rapid outbreak of Coronavirus Disease 2019 (COVID-19). More than 6,973,427 confirmed cases were diagnosed in nearly 213 countries and territories around the world and two international conveyances, causing globally over 400,000 deaths. Epidemiology, risk factors, and clinical characteristics of COVID-19 patients have been identified, but the factors influencing the immune system against COVID-19 have not been well established. Upon infection or cell damage, high amounts of adenosine triphosphate (ATP) are released from damaged cells, which serve as mediators of inflammation through purinergic cell surface receptor signaling. As a protective mechanism to prevent excessive damage to host tissue, adenosine counteracts ATP's effects by adenosine receptor stimulation to suppress the pro-inflammatory response. Adenosine is seen as a major obstacle to the efficacy of immune therapies, and the adenosinergic axis components are critical therapeutic targets for cancer and microbial infections. Pharmacologic inhibitors or antibodies specific to adenosinergic pathway components or adenosine receptors in microbial and tumor therapy have shown efficacy in pre-clinical studies and are entering the clinical arena. In this review, we provide a novel hypothesis explaining the potential for improving the efficiency of innate and adaptive immune systems by targeting adenosinergic pathway components and adenosine A_2A receptor signaling for the treatment of COVID-19.

To Inhibit or Enhance? Is There a Benefit to Positive Allosteric Modulation of P2X Receptors?

The family of ligand-gated ion channels known as P2X receptors were discovered several decades ago. Since the cloning of the seven P2X receptors (P2X1-P2X7), a huge research effort has elucidated their roles in regulating a range of physiological and pathophysiological processes. Transgenic animals have been influential in understanding which P2X receptors could be new therapeutic targets for disease. Furthermore, understanding how inherited mutations can increase susceptibility to disorders and diseases has advanced this knowledge base. There has been an emphasis on the discovery and development of pharmacological tools to help dissect the individual roles of P2X receptors and the pharmaceutical industry has been involved in pushing forward clinical development of several lead compounds. During the discovery phase, a number of positive allosteric modulators have been described for P2X receptors and these have been useful in assigning physiological roles to receptors. This review will consider the major physiological roles of P2X1-P2X7 and discuss whether enhancement of P2X receptor activity would offer any therapeutic benefit. We will review what is known about identified compounds acting as positive allosteric modulators and the recent identification of drug binding pockets for such modulators.

The powerful immune system against powerful COVID-19: A hypothesis

On March 11, 2020, the World Health Organization declared the coronavirus outbreak a pandemic. Since December 2019, the world has experienced an outbreak of coronavirus disease 2019 (COVID-19). Epidemiology, risk factors, and clinical characteristics of patients with COVID-19 have been reported but the factors affecting the immune system against COVID-19 have not been well described. In this article, we provide a novel hypothesis to describe how an increase in cellular adenosine triphosphate (c-ATP) can potentially improve the efficiency of innate and adaptive immune systems to either prevent or fight off COVID-19.

Increased host ATP efflux and its conversion to extracellular adenosine is crucial for establishing Leishmania infection

Intracellular survival of Leishmania donovani demands rapid production of host ATP for its sustenance. However, a gradual decrease in intracellular ATP in spite of increased glycolysis suggests ATP efflux during infection. Accordingly, upon infection, we show here that ATP is exported and the major exporter was pannexin-1, leading to raised extracellular ATP levels. Extracellular ATP shows a gradual decrease after the initial increase, and analysis of cell surface ATP-degrading enzymes revealed induction of the ectonucleotidases CD39 and CD73. Ectonucleotidase-mediated ATP degradation leads to increased extracellular adenosine (eADO), and inhibition of CD39 and CD73 in infected cells decreased adenosine concentration and parasite survival, documenting the importance of adenosine in infection. Inhibiting adenosine uptake by cells did not affect parasite survival, suggesting that eADO exerts its effect through receptor-mediated signalling. We also show that Leishmania induces the expression of adenosine receptors A2AR and A2BR, both of which are important for anti-inflammatory responses. Treating infected BALB/c mice with CD39 and CD73 inhibitors resulted in decreased parasite burden and increased host-favourable cytokine production. Collectively, these observations indicate that infection-induced ATP is exported, and after conversion into adenosine, propagates infection via receptor-mediated signalling.

Characteristics and the role of purinergic receptors in pathophysiology with focus on immune response

The nucleotide adenosine-5'-triphosphate (ATP) is mostly thought to be energy carrier, but evidence presented in multiple studies proves ATP involvement into variety of processes, due to its neuromodulatory capabilities. ATP and its metabolite-adenosine, bind to the purinergic receptors, which are divided into two types: adenosine binding P1 receptor and ADP/ATP binding P2 receptor. These receptors are expressed in different tissues and organs. Recent studies report their immunomodulatory characteristics, connected with varying immunological processes, such as immunological response or antigen presentation. Besides, they seem to play an important role in medical conditions such as bronchial asthma or variety of cancers. In this article, we would like to review recent discoveries on the field of purinergic receptors research focusing on their role in immunological system, and shed a new light upon the importance of these receptors in modern medicine development.

Polymorphisms in the P2X7 receptor, and differential expression of Toll-like receptor-mediated cytokines and defensins, in a Canadian Indigenous group

Canadian Indigenous peoples (First Nations and Inuit) exhibit a high burden of infectious diseases including tuberculosis influenced by societal factors, and biological determinants. Toll-like receptor (TLR)-mediated innate immune responses are the first line of defence against infections. We examined the production of a panel of 30 cytokines in peripheral blood-derived mononuclear cells (PBMC) isolated from Indigenous and non-Indigenous participants, following stimulation with five different TLR ligands. The levels of TLR-induced pro-inflammatory cytokines such as IL-12/23p40, IL-16, and IFN-γ, and chemokines (MCP-4, MDC and eotaxin) were different between Indigenous compared to non-Indigenous participants. Antimicrobial cationic host defence peptides (CHDP) induced by TLR activation are critical for resolution of infections and modulate the TLR-to-NFκB pathway to alter downstream cytokine responses. Therefore, we examined the expression of human CHDP defensins and cathelicidin in PBMC. mRNA expression of genes encoding for def-A1 and def-B1 were significantly higher following stimulation with TLR ligands in Indigenous compared to non-Indigenous participants. The purinergic receptor P2X7 known to be activated by ATP released following TLR stimulation, is a receptor for CHDP. Therefore, we further examined single nucleotide polymorphisms (SNP) in P2X7. Indigenous participants had a significantly higher percentage of a P2X7 SNP which is associated with reduced function and lower ability to clear infections. These results suggest that a higher frequency of non-functional P2X7 receptors may influence the activity of downstream immune mediators required for resolution of infections such as pro-inflammatory cytokines and CHDP defensins, thus contributing to higher burden of infections in Indigenous population.

P2Y2 purinergic receptor modulates virus yield, calcium homeostasis, and cell motility in human cytomegalovirus-infected cells

Human cytomegalovirus (HCMV) manipulates many aspects of host cell biology to create an intracellular milieu optimally supportive of its replication and spread. Our study reveals that levels of several components of the purinergic signaling system, including the P2Y2 and P2X5 receptors, are elevated in HCMV-infected fibroblasts. Knockdown and drug treatment experiments demonstrated that P2Y2 enhances the yield of virus, whereas P2X5 reduces HCMV production. The HCMV IE1 protein induces P2Y2 expression; and P2Y2-mediated signaling is important for efficient HCMV gene expression, DNA synthesis, and the production of infectious HCMV progeny. P2Y2 cooperates with the viral UL37x1 protein to regulate cystolic Ca2+ levels. P2Y2 also regulates PI3K/Akt signaling and infected cell motility. Thus, P2Y2 functions at multiple points within the viral replication cycle to support the efficient production of HCMV progeny, and it may facilitate in vivo viral spread through its role in cell migration.

How Many Mammalian Reovirus Proteins are involved in the Control of the Interferon Response?

As with most viruses, mammalian reovirus can be recognized and attacked by the host-cell interferon response network. Similarly, many viruses have developed resistance mechanisms to counteract the host-cell response at different points of this response. Reflecting the complexity of the interferon signaling pathways as well as the resulting antiviral response, viruses can-and often have-evolved many determinants to interfere with this innate immune response and allow viral replication. In the last few years, it has been evidenced that mammalian reovirus encodes many different determinants that are involved in regulating the induction of the interferon response or in interfering with the action of interferon-stimulated gene products. In this brief review, we present our current understanding of the different reovirus proteins known to be involved, introduce their postulated modes of action, and raise current questions that may lead to further investigations.