Evidence that adenosine triphosphate or a related nucleotide is the transmitter substance released by non-adrenergic inhibitory nerves in the gut

The Imperative for Innovative Enteric Nervous System-Intestinal Organoid Co-Culture Models: Transforming GI Disease Modeling and Treatment

This review addresses the need for innovative co-culture systems integrating the enteric nervous system (ENS) with intestinal organoids. The breakthroughs achieved through these techniques will pave the way for a transformative era in gastrointestinal (GI) disease modeling and treatment strategies. This review serves as an introduction to the companion protocol paper featured in this journal. The protocol outlines the isolation and co-culture of myenteric and submucosal neurons with small intestinal organoids. This review provides an overview of the intestinal organoid culture field to establish a solid foundation for effective protocol application. Remarkably, the ENS surpasses the number of neurons in the spinal cord. Referred to as the "second brain", the ENS orchestrates pivotal roles in GI functions, including motility, blood flow, and secretion. The ENS is organized into myenteric and submucosal plexuses. These plexuses house diverse subtypes of neurons. Due to its proximity to the gut musculature and its cell type complexity, there are methodological intricacies in studying the ENS. Diverse approaches such as primary cell cultures, three-dimensional (3D) neurospheres, and induced ENS cells offer diverse insights into the multifaceted functionality of the ENS. The ENS exhibits dynamic interactions with the intestinal epithelium, the muscle layer, and the immune system, influencing epithelial physiology, motility, immune responses, and the microbiome. Neurotransmitters, including acetylcholine (ACh), serotonin (5-HT), and vasoactive intestinal peptide (VIP), play pivotal roles in these intricate interactions. Understanding these dynamics is imperative, as the ENS is implicated in various diseases, ranging from neuropathies to GI disorders and neurodegenerative diseases. The emergence of organoid technology presents an unprecedented opportunity to study ENS interactions within the complex milieu of the small and large intestines. This manuscript underscores the urgent need for standardized protocols and advanced techniques to unravel the complexities of the ENS and its dynamic relationship with the gut ecosystem. The insights gleaned from such endeavors hold the potential to revolutionize GI disease modeling and treatment paradigms.

Extracellular nucleotides in smooth muscle contraction

Extracellular nucleotides and nucleosides are crucial signalling molecules, eliciting diverse biological responses in almost all organs and tissues. These molecules exert their effects by activating specific nucleotide receptors, which are finely regulated by ectonucleotidases that break down their ligands. In this comprehensive review, we aim to elucidate the relevance of extracellular nucleotides as signalling molecules in the context of smooth muscle contraction, considering the modulatory influence of ectonucleotidases on this intricate process. Specifically, we provide a detailed examination of the involvement of extracellular nucleotides in the contraction of non-vascular smooth muscles, including those found in the urinary bladder, the airways, the reproductive system, and the gastrointestinal tract. Furthermore, we present a broader overview of the role of extracellular nucleotides in vascular smooth muscle contraction.

Impairment of adenosine signaling disrupts early embryo development: unveiling the underlying mechanisms

Purinergic signaling has been implicated in many biological functions, including development. In this study, we investigate the functions of extracellular adenosine and adenosine receptors using a mouse embryonic stem cell (ESC) line and morula stages isolated from mouse embryos. Feeder-free mouse ESC was investigated in the absence and presence of the leukemia inhibitory factor (LIF), configuring undifferentiated cells and cells undergoing spontaneous differentiation. High alkaline phosphatase (ALPL) and low CD73 levels resulting in low adenosine (eADO) levels were characteristic for pluripotent cells in the presence of the LIF, while LIF deprivation resulted in augmented adenosine levels and reduced pluripotency marker expression, which indicated differentiation. Tracing ESC proliferation by BrdU labeling revealed that the inhibition of ALPL by levamisole resulted in a decrease in proliferation due to less eADO accumulation. Furthermore, caffeine and levamisole treatment, inhibiting adenosine receptor and eADO accumulation, respectively, reduced ESC migration, similar to that observed in the absence of the LIF. Pharmacological approaches of selective adenosine receptor subtype inhibition triggered specific adenosine receptor activities, thus triggering calcium or MAP kinase pathways leading to differentiation. In line with the in vitro data, mouse embryos at the morula stage were sensitive to treatments with A1 and A3 receptor antagonists, leading to the conclusion that A1 receptor and A3 receptor inhibition impairs proliferation and self-renewal and triggers inappropriate differentiation, respectively. The findings herein define the functions of eADO signaling in early development with implications for developmental disorders, in which adenosine receptors or ectonucleotidase dysfunctions are involved, and which could lead to malformations and miscarriages, due to exposure to caffeine.

The multifaceted role of extracellular ATP in sperm function: From spermatogenesis to fertilization

Extracellular adenosine 5'-triphosphate (ATP) is a vital signaling molecule involved in various physiological processes within the body. In recent years, studies have revealed its significant role in male reproduction, particularly in sperm function. This review explores the multifaceted role of extracellular ATP in sperm function, from spermatogenesis to fertilization. We discuss the impact of extracellular ATP on spermatogenesis, sperm maturation and sperm-egg fusion, highlighting the complex regulatory mechanisms and potential clinical applications in the context of male infertility. By examining the latest research, we emphasize the crucial role of extracellular ATP in sperm function and propose future research directions to further.

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.

Traditional Chinese medicine alleviating neuropathic pain targeting purinergic receptor P2 in purinergic signaling: A review

Past studies have suggested that Chinese herbal may alleviate neuropathic pain, and the mechanism might target the inhibition of purinergic receptor P2. This review discusses whether traditional Chinese medicine target P2 receptors in neuropathic pain and its mechanism in order to provide references for future clinical drug development. The related literatures were searched from Pubmed, Embase, Sinomed, and CNKI databases before June 2023. The search terms included"neuropathic pain", "purinergic receptor P2", "P2", "traditional Chinese medicine", "Chinese herbal medicine", and "herb". We described the traditional Chinese medicine alleviating neuropathic pain via purinergic receptor P2 signaling pathway including P2X2/3 R, P2X3R, P2X4R, P2X7R, P2Y1R. Inhibition of activating glial cells, changing synaptic transmission, increasing painful postsynaptic potential, and activating inflammatory signaling pathways maybe the mechanism. Purine receptor P2 can mediate the occurrence of neuropathic pain. And many of traditional Chinese medicines can target P2 receptors to relieve neuropathic pain, which provides reasonable evidences for the future development of drugs. Also, the safety and efficacy and mechanism need more in-depth experimental research.

Uridine Diphosphate Glucose (UDP-G) Activates Oxidative Stress and Respiratory Burst in Isolated Neutrophils

The extracellular purinergic agonist uridine diphosphate glucose (UDP-G) activates chemotaxis of human neutrophils (PMN) and the recruitment of PMN at the lung level, via P2Y14 purinergic receptor signaling. This effect is similar to the activation of PMN with N-formyl-methionyl-leucyl-phenylalanine (fMLP), a mechanism that also triggers the production of superoxide anion and hydrogen peroxide via the NADPH oxidase system. However, the effects of UDP-G on this system have not been studied. Defects in the intracellular phagocyte respiratory burst (RB) cause recurrent infections, immunodeficiency, and chronic and severe diseases in affected patients, often with sepsis and hypoxia. The extracellular activation of PMN by UDP-G could affect the RB and oxidative stress (OS) in situations of inflammation, infection and/or sepsis. The association of PMNs activation by UDP-G with OS and RB was studied. OS was evaluated by measuring spontaneous chemiluminescence (CL) of PMNs with a scintillation photon counter, and RB by measuring oxygen consumption with an oxygen Clark electrode at 37 °C, in non-stimulated cells and after activation (15 min) with lipopolysaccharides (LPS, 2 µg/mL), phorbol myristate acetate (PMA, 20 ng/mL), or UDP-G (100 μM). The stimulation index (SI) was calculated in order to establish the activation effect of the three agonists. After stimulation with LPS or PMA, the activated PMNs (0.1 × 106 cells/mL) showed an increase in CL (35%, p < 0.05 and 56%, p < 0.01, SI of 1.56 and 2.20, respectively). Contrariwise, the stimulation with UDP-G led to a decreased CL in a dose-dependent manner (60%, 25 μM, p < 0.05; 90%, 50-150 μM, p < 0.001). Nonetheless, despite the lack of oxidative damage, UDP-G triggered RB (SI 1.8) in a dose-dependent manner (38-50%, 100-200 μM, p < 0.0001). UDP-G is able to trigger NADPH oxidase activation in PMNs. Therefore, the prevention of OS and oxidative damage observed upon PMN stimulation with UDP-G indicates an antioxidant property of this molecule which is likely due to the activation of antioxidant defenses. Altogether, LPS and UDP-G have a synergistic effect, suggesting a key role in infection and/or sepsis.

Purine and purinergic receptors in health and disease

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

Neurochemical profile of the myenteric plexus in the rat colorectal region

The enteric nervous system, a major subdivision of the autonomic nervous system, is known for its neurochemical heterogeneity and complexity. The myenteric plexus, one of its two principal components, primarily controls peristalsis and its dysfunction may lead to a number of gastrointestinal motility disorders. The myenteric neurons have been described to use a wide variety of neurotransmitters although no evidence has been reported for the existence of adrenergic neurons in the hindgut. This study aims at elucidating the chemical coding of neurons in the myenteric plexus of the rat colon and anorectal region with particular emphasis on cholinergic and the so-called nonadrenergic, noncholinergic (NANC) transmitter systems. The immunostaining for choline acetyltransferase revealed an intense staining of the myenteric ganglia with clear delineation of their neuronal cell bodies and without local distributional differences in the colonic region. The myenteric ATPergic structures were mostly limited to fiber bundles surrounding unstained myenteric neurons and penetrating the two muscle layers. We also observed an abundance of intensely stained varicose substance P-immunopositive fibers, ensheathing the immunonegative myenteric neuronal cell bodies in a basket-like manner. Applying NADPH-diaphorase histochemistry and nitric oxide synthase immunohistochemistry, we were able to demonstrate numerous nitrergic somata of myenteric neurons with Dogiel Type I morphology. Apart from the observed nitrergic distributional patterns, no distinct variations were found in the staining intensity or distribution of myenteric structures in the colon and anorectal area. Our results suggest that myenteric neurons in the distal intestinal portion utilize a broad spectrum of enteric transmitters, including classical and NANC transmitters.

Emerging Role of Alarmins in Food Allergy: An Update on Pathophysiological Insights, Potential Use as Disease Biomarkers, and Therapeutic Implications

Food allergies are immuno-mediated adverse reactions to ingestion or contact with foods, representing a widespread health problem. The immune response can be IgE-mediated, non-IgE-mediated, or with a mixed mechanism. The role of innate immunity and alarmins in the pathogenesis of diseases such as asthma and atopic dermatitis is well known. Some authors have investigated the correlation between alarmins and food allergies, often obtaining interesting results. We analyzed articles published in English from the last 22 years present on PubMed concerning the role of alarmins in the pathogenesis of food allergies and their potential use as disease biomarkers, response biomarkers to therapy, or potential therapeutic targets. Nuclear alarmins (TSLP, IL-33, IL-25) appear to have a critical role in IgE-mediated allergies but are also implicated in entities such as eosinophilic esophagitis. Calprotectin and defensins may play a role as disease biomarkers and could help predict response to therapy, although results in the literature are often conflicting. Despite the promising results, more studies on humans still need to be conducted. Deepening our knowledge regarding alarmins and their involvement in food allergies could lead to the development of new biological therapies, significantly impacting patients' quality of life.

Hyoscine butylbromide mode of action on bowel motility: From pharmacology to clinical practice

BACKGROUND

Hyoscine butylbromide (HBB) has been available for use as an antispasmodic since 1951 and is indicated for the treatment of abdominal pain associated with cramps. A previous review in 2007 summarized the evidence on the mode of action of HBB in vitro and in vivo in both animal and human studies. However, since then, novel publications have appeared within the literature and also our knowledge of what represents normal motility in humans has evolved.

PURPOSE

This review is the result of the collaboration between a basic scientist and clinicians with the aim of providing an updated overview of the mechanisms of action of HBB and its clinical efficacy to guide not only use in clinical practice, but also future research.

P2X3 receptor antagonism attenuates the progression of heart failure

Despite advances in the treatment of heart failure, prognosis is poor, mortality high and there remains no cure. Heart failure is associated with reduced cardiac pump function, autonomic dysregulation, systemic inflammation and sleep-disordered breathing; these morbidities are exacerbated by peripheral chemoreceptor dysfunction. We reveal that in heart failure the carotid body generates spontaneous, episodic burst discharges coincident with the onset of disordered breathing in male rats. Purinergic (P2X3) receptors were upregulated two-fold in peripheral chemosensory afferents in heart failure, and when antagonized abolished these episodic discharges, normalized both peripheral chemoreceptor sensitivity and the breathing pattern, reinstated autonomic balance, improved cardiac function, and reduced both inflammation and biomarkers of cardiac failure. Aberrant ATP transmission in the carotid body triggers episodic discharges that via P2X3 receptors play a crucial role in the progression of heart failure and as such offer a distinct therapeutic angle to reverse multiple components of its pathogenesis.

P2X7R Mediates the Synergistic Effect of ATP and MSU Crystals to Induce Acute Gouty Arthritis

Activation of the nod-like receptor protein 3 (NLRP3) inflammasome by monosodium urate (MSU) crystals has been identified as the molecular basis for the acute inflammatory response in gouty arthritis. However, MSU crystals alone are not sufficient to induce acute gouty arthritis (AGA). Adenosine triphosphate (ATP) is an endogenous signaling molecule involved in the NLRP3 inflammasome activation. We aimed to explore the role of ATP in MSU crystal-induced AGA development. In peripheral blood mononuclear cell-derived macrophages obtained from gout patients, we observed a synergistic effect of ATP on MSU crystal-induced IL-1β release. Furthermore, in a rat model of spontaneous gout, we demonstrated that a synergistic effect of ATP and MSU crystals, but not MSU crystals alone, is essential for triggering AGA. Mechanistically, this synergistic effect is achieved through the purinergic receptor P2X7 (P2X7R). Blockade of P2X7R prevented AGA induction in rats after local injection of MSU crystals, and carrying the mutant hP2X7R gene contributed to the inhibition of NLRP3 inflammasome activation induced by costimulation of MSU crystals and ATP in vitro. Taken together, these results support the synergistic effect of ATP on MSU crystal-induced NLRP3 inflammasome activation facilitating inflammatory episodes in AGA. In this process, P2X7R plays a key regulatory role, suggesting targeting P2X7R to be an attractive therapeutic strategy for the treatment of AGA.

Rethinking purinergic concepts and updating the emerging role of P2X7 and P2X4 in amyotrophic lateral sclerosis

The topic of the present review regards the ubiquitous and phylogenetically most ancient prototype of intercellular signaling, the one mediated by extracellular nucleosides and nucleotides, bearing a strong influence on pathophysiological processes in the nervous system. Not by chance, purine and pyrimidine molecules are the most prevalent and ubiquitous chemical messengers in the animal and plant kingdoms, operating through a large plethora of purinergic metabolizing enzymes, P1 and P2 receptors, nucleoside and nucleotide channels and transporters. Because ectonucleotidases degrade the agonists of P2 receptors while simultaneously generate the agonists for P1 receptors, and because several agonists, or antagonists, simultaneously bind and activate, or inhibit, more than one receptor subtype, it follows that an all-inclusive "purinergic network" perspective should be better considered when looking at purinergic actions. This becomes particularly crucial during pathological conditions as for instance amyotrophic lateral sclerosis, where the contribution of purinergic signaling has been demonstrated to differ according to each target cell phenotype and stage of disease progression. Here we will present some newly updated results about P2X7 and P2X4 as the most thoroughly investigated P2 receptors in amyotrophic lateral sclerosis, being aware that the comprehension of their actions is still in progress, and that the purinergic rationale for studying this disease must be however wide-ranging and all-inclusive. This article is part of the Special Issue on 'Purinergic Signaling: 50 years'.

Delineating Purinergic Signaling in Drosophila

Simplistic models can aid in discovering what is important in the context of normal and pathological behavior. First recognized as a genetic model more than 100 years ago, to date, fruit flies (Drosophila melanogaster) still remain an astonishingly good laboratory stand-in for scientists to study development and physiology and to investigate the molecular mechanisms of human diseases. This is because fruit flies indeed represent a simplistic model. Furthermore, about 75% of human disease-related genes have their counterparts in the Drosophila genome, added to the fact that fruit flies are inexpensive and extremely easy to maintain, being invertebrates and, moreover, lacking any ethical concern issues. Purinergic signaling is, by definition, mediated by extracellular purinergic ligands, among which ATP represents the prototype molecule. A key feature that has progressively emerged when dissecting the purinergic mechanisms is the multilayer and dynamic nature of the signaling sustained by purinergic ligands. Indeed, these last are sequentially metabolized by several different ectonucleotidases, which generate the ligands that simultaneously activate several different purinergic receptors. Since significant purinergic actions have also been described in Drosophila, the aim of the present work is to provide a comprehensive picture of the purinergic events occurring in fruit flies.

Altered purinergic receptor expression in the frontal cortex in schizophrenia

ATP functions as a neurotransmitter, acting on the ubiquitously expressed family of purinergic P2 receptors. In schizophrenia (SCZ), the pathways that modulate extracellular ATP and its catabolism to adenosine are dysregulated. However, the effects of altered ATP availability on P2 receptor expression in the brain in SCZ have not been assessed. We assayed P2 receptor mRNA and protein expression in the DLPFC and ACC in subjects diagnosed with SCZ and matched, non-psychiatrically ill controls (n = 20-22/group). P2RX7, P2RX4 and male P2RX5 mRNA expression were significantly increased (p < 0.05) in the DLPFC in SCZ. Expression of P2RX7 protein isoform was also significantly increased (p < 0.05) in the DLPFC in SCZ. Significant increases in P2RX4 and male P2RX5 mRNA expression may be associated with antipsychotic medication effects. We found that P2RX4 and P2RX7 mRNA are significantly correlated with the inflammatory marker SERPINA3, and may suggest an association between upregulated P2XR and neuroinflammation in SCZ. These findings lend support for brain-region dependent dysregulation of the purinergic system in SCZ.

Distribution and Function of Platelet-derived Growth Factor Receptor Alpha-positive Cells and Purinergic Neurotransmission in the Human Colon: Is It Different Between the Right and Left Colon?

Background/Aims

Platelet-derived growth factor receptor alpha-positive (PDGFRα⁺) cells function in the purinergic regulation of gastrointestinal motility, and purines are reportedly inhibitory neurotransmitters in the enteric nervous system. We explore the distribution and function of PDGFRα⁺ cells related to purinergic inhibitory neurotransmission in human right and left colons.

Methods

Human colonic segments were prepared with mucosa and submucosa intact, and the circular muscle tension and longitudinal muscle tension were recorded. Purinergic neurotransmitters were administered after recording the regular contractions. Immunohistochemistry was performed on the circular muscle layers. Intracellular recording was performed on the colonic muscular layer. SK3, P2RY1, and PDGFR-α mRNA expression was tested by quantitative real-time polymerase chain reaction (qPCR).

Results

Adenosine triphosphate (ATP) treatment significantly decreased the frequency and area under the curve (AUC) of the segmental contraction in right and left colons. Beta-nicotinamide adenine dinucleotide (β-NAD) decreased the frequency in the right colon and the amplitude, frequency and AUC in the left colon. Apamin significantly increased frequency and AUC in the left colon, and after apamin pretreatment, ATP and β-NAD did not change segmental contractility. Through intracellular recordings, a resting membrane potential decrease occurred after ATP administration; however, the degree of decrease between the right and left colon was not different. PDGFRα⁺ cells were distributed evenly in the circular muscle layers of right and left colons. SK3, P2RY1, and PDGFRα expression was not different between the right and left colon.

Conclusion

Purines reduce right and left colon contractility similarly, and purinergic inhibitory neurotransmission can be regulated by PDGFRα⁺ cells in the human colon.

ATP Secretion and Metabolism in Regulating Pancreatic Beta Cell Functions and Hepatic Glycolipid Metabolism

Diabetes (DM), especially type 2 diabetes (T2DM) has become one of the major diseases severely threatening public health worldwide. Islet beta cell dysfunctions and peripheral insulin resistance including liver and muscle metabolic disorder play decisive roles in the pathogenesis of T2DM. Particularly, increased hepatic gluconeogenesis due to insulin deficiency or resistance is the central event in the development of fasting hyperglycemia. To maintain or restore the functions of islet beta cells and suppress hepatic gluconeogenesis is crucial for delaying or even stopping the progression of T2DM and diabetic complications. As the key energy outcome of mitochondrial oxidative phosphorylation, adenosine triphosphate (ATP) plays vital roles in the process of almost all the biological activities including metabolic regulation. Cellular adenosine triphosphate participates intracellular energy transfer in all forms of life. Recently, it had also been revealed that ATP can be released by islet beta cells and hepatocytes, and the released ATP and its degraded products including ADP, AMP and adenosine act as important signaling molecules to regulate islet beta cell functions and hepatic glycolipid metabolism via the activation of P2 receptors (ATP receptors). In this review, the latest findings regarding the roles and mechanisms of intracellular and extracellular ATP in regulating islet functions and hepatic glycolipid metabolism would be briefly summarized and discussed.

Signalling by extracellular nucleotides in health and disease

Nucleotides are released from all cells through regulated pathways or as a result of plasma membrane damage or cell death. Outside the cell, nucleotides act as signalling molecules triggering multiple responses via specific plasma membrane receptors of the P2 family. In the nervous system, purinergic signalling has a key function in neurotransmission. Outside the nervous system, purinergic signalling is one of the major modulators of basal tissue homeostasis, while its dysregulation contributes to the pathogenesis of various disease, including inflammation and cancer. Pre-clinical and clinical evidence shows that selective P2 agonists or antagonists are effective treatments for many pathologies, thus highlighting the relevance of extracellular nucleotides and P2 receptors as therapeutic targets.

Purinergic signaling in the male reproductive tract

Purinergic receptors are ubiquitously expressed throughout the body and they participate in the autocrine and paracrine regulation of cell function during normal physiological and pathophysiological conditions. Extracellular nucleotides activate several types of plasma membrane purinergic receptors that form three distinct families: P1 receptors are activated by adenosine, P2X receptors are activated by ATP, and P2Y receptors are activated by nucleotides including ATP, ADP, UTP, UDP, and UDP-glucose. These specific pharmacological fingerprints and the distinct intracellular signaling pathways they trigger govern a large variety of cellular responses in an organ-specific manner. As such, purinergic signaling regulates several physiological cell functions, including cell proliferation, differentiation and death, smooth muscle contraction, vasodilatation, and transepithelial transport of water, solute, and protons, as well as pathological pathways such as inflammation. While purinergic signaling was first discovered more than 90 years ago, we are just starting to understand how deleterious signals mediated through purinergic receptors may be involved in male infertility. A large fraction of male infertility remains unexplained illustrating our poor understanding of male reproductive health. Purinergic signaling plays a variety of physiological and pathophysiological roles in the male reproductive system, but our knowledge in this context remains limited. This review focuses on the distribution of purinergic receptors in the testis, epididymis, and vas deferens, and their role in the establishment and maintenance of male fertility.

Chronic cough: new insights and future prospects

Chronic cough is defined in adults as a cough that lasts for ≥8 weeks. When it proves intractable to standard-of-care treatment, it can be referred to as refractory chronic cough (RCC). Chronic cough is now understood to be a condition of neural dysregulation. Chronic cough and RCC result in a serious, often unrecognized, disease burden, which forms the focus of the current review.

The estimated global prevalence of chronic cough is 2–18%. Patients with chronic cough and RCC report many physical and psychological effects, which impair their quality of life. Chronic cough also has a significant economic burden for the patient and healthcare systems. RCC diagnosis and treatment are often delayed for many years as potential treatable triggers must be excluded first and a stepwise empirical therapeutic regimen is recommended.

Evidence supporting most currently recommended treatments is limited. Many treatments do not address the underlying pathology, are used off-label, have limited efficacy and produce significant side-effects. There is therefore a significant unmet need for alternative therapies for RCC that target the underlying disease mechanisms. Early clinical data suggest that antagonists of the purinergic P2X3 receptor, an important mediator of RCC, are promising, though more evidence is needed.

Chronic cough exerts a considerable burden on patients and healthcare systems. In addition to effective targeted therapies, further data are needed to understand the pathophysiology, epidemiology and disease burden.https://bit.ly/3Be9JZI

Functional Implications of Neurotransmitter Segregation

Here, we present and discuss the characteristics and properties of neurotransmitter segregation, a subtype of neurotransmitter cotransmission. We review early evidence of segregation and discuss its properties, such as plasticity, while placing special emphasis on its probable functional implications, either in the central nervous system (CNS) or the autonomic nervous system. Neurotransmitter segregation is a process by which neurons separately route transmitters to independent and distant or to neighboring neuronal processes; it is a plastic phenomenon that changes according to synaptic transmission requirements and is regulated by target-derived signals. Distant neurotransmitter segregation in the CNS has been shown to be related to an autocrine/paracrine function of some neurotransmitters. In retinal amacrine cells, segregation of acetylcholine (ACh) and GABA, and glycine and glutamate to neighboring terminals has been related to the regulation of the firing rate of direction-selective ganglion cells. In the rat superior cervical ganglion, segregation of ACh and GABA to neighboring varicosities shows a heterogeneous regional distribution, which is correlated to a similar regional distribution in transmission strength. We propose that greater segregation of ACh and GABA produces less GABAergic inhibition, strengthening ganglionic transmission. Segregation of ACh and GABA varies in different physiopathological conditions; specifically, segregation increases in acute sympathetic hyperactivity that occurs in cold stress, does not vary in chronic hyperactivity that occurs in hypertension, and rises in early ages of normotensive and hypertensive rats. Given this, we propose that variations in the extent of transmitter segregation may contribute to the alteration of neural activity that occurs in some physiopathological conditions and with age.

Purinergic signaling in the modulation of redox biology

Purinergic signaling is a cell communication pathway mediated by extracellular nucleotides and nucleosides. Tri- and diphosphonucleotides are released in physiological and pathological circumstances activating purinergic type 2 receptors (P2 receptors): P2X ion channels and P2Y G protein-coupled receptors. The activation of these receptors triggers the production of reactive oxygen and nitrogen species and alters antioxidant defenses, modulating the redox biology of cells. The activation of P2 receptors is controlled by ecto-enzymes named ectonucleotidases, E-NTPDase1/CD39 and ecto-5'-nucleotidase/CD73) being the most relevant. The first enzyme hydrolyzes adenosine triphosphate (ATP) and adenosine diphosphate (ADP) into adenosine monophosphate (AMP), and the second catalyzes the hydrolysis of AMP to adenosine. The activity of these enzymes is diminished by oxidative stress. Adenosine actives P1 G-coupled receptors that, in general, promote the maintenance of redox hemostasis by decreasing reactive oxygen species (ROS) production and increase antioxidant enzymes. Intracellular purine metabolism can also contribute to ROS generation via xanthine oxidase activity, which converts hypoxanthine into xanthine, and finally, uric acid. In this review, we describe the mechanisms of redox biology modulated by purinergic signaling and how this signaling may be affected by disturbances in the redox homeostasis of cells.

Expression Pattern of Purinergic Signaling Components in Colorectal Cancer Cells and Differential Cellular Outcomes Induced by Extracellular ATP and Adenosine

The purine nucleotide adenosine triphosphate (ATP) is known for its fundamental role in cellular bioenergetics. However, in the last decades, different works have described emerging functions for ATP, such as that of a danger signaling molecule acting in the extracellular space on both tumor and stromal compartments. Beside its role in immune cell signaling, several studies have shown that high concentrations of extracellular ATP can directly or indirectly act on cancer cells. Accordingly, it has been reported that purinergic receptors are widely expressed in tumor cells. However, their expression pattern is often associated with contradictory cellular outcomes. In this work, we first investigated gene expression profiles through "RNA-Sequencing" (RNA Seq) technology in four colorectal cancer (CRC) cell lines (HT29, LS513, LS174T, HCT116). Our results demonstrate that CRC cells mostly express the A2B, P2X4, P2Y1, P2Y2 and P2Y11 purinergic receptors. Among these, the P2Y1 and P2Y2 coding genes are markedly overexpressed in all CRC cells compared to the HCEC-1CT normal-like colonic cells. We then explored the cellular outcomes induced by extracellular ATP and adenosine. Our results show that in terms of cell death induction extracellular ATP is consistently more active than adenosine against CRC, while neither compound affected normal-like colonic cell survival. Intriguingly, while for the P2Y2 receptor pharmacological inhibition completely abolished the rise in cytoplasmic Ca²⁺ observed after ATP exposure in all CRC cell lines, Ca²⁺ mobilization only impacted the cellular outcome for HT29. In contrast, non-selective phosphodiesterase inhibition completely abolished the effects of extracellular ATP on CRC cells, suggesting that cAMP and/or cGMP levels might determine cellular outcome. Altogether, our study provides novel insights into the characterization of purinergic signaling in CRC.

To inhibit or to boost the ATP/P2RX7 pathway to fight cancer-that is the question

Despite new biological insights and recent therapeutic advances, many tumors remain at baseline during treatments. Therefore, there is an urgent need to find new therapeutic strategies to improve the care of patients with solid tumors. P2RX7 receptor (P2XR7), an ATP-gated ion channel characterized by its ability to form large pore within the cell membrane, is described by most of the investigators as a "chef d'orchestre" of the antitumor immune response. The purpose of this review is to detail the recent information concerning different cellular mechanisms linking P2RX7 to hallmarks of cancer and to discuss different progresses in elucidating how activation of the ATP/P2RX7/NLRP3/IL-18 pathway is a very promising approach to fight cancer progression by increasing antitumor immune responses.

The role of enteric inhibitory neurons in intestinal motility

The enteric nervous system controls much of the mixing and propulsion of nutrients along the digestive tract. Enteric neural circuits involve intrinsic sensory neurons, interneurons and motor neurons. While the role of the excitatory motor neurons is well established, the role of the enteric inhibitory motor neurons (IMNs) is less clear. The discovery of inhibitory transmission in the intestine in the 1960's in the laboratory of Geoff Burnstock triggered the search for the unknown neurotransmitter. It has since emerged that most neurons including the IMNs contain and may utilise more than one transmitter substances; for IMNs these include ATP, the neuropeptide VIP/PACAP and nitric oxide. This review distinguishes the enteric neural pathways underlying the 'standing reflexes' from the pathways operating physiologically during propulsive and non-propulsive movements. Morphological evidence in small laboratory animals indicates that the IMNs are located in the myenteric plexus and project aborally to the circular muscle, where they act by relaxing the muscle. There is ongoing 'tonic' activity of these IMNs to keep the intestinal muscle relaxed. Accommodatory responses to content further activate enteric pathways that involve the IMNs as the final neural element. IMNs are activated by mechanical and chemical stimulation induced by luminal contents, which activate intrinsic sensory enteric neurons and the polarised interneuronal ascending excitatory and descending inhibitory reflex pathways. The latter relaxes the muscle ahead of the advancing bolus, thus facilitating propulsion.

P2X3 receptors participate in purinergic inhibition of gastrointestinal smooth muscle

The ATP analogue α,β-meATP is a potent relaxant of gastrointestinal smooth muscle, but its molecular target is uncertain inside the gut. α,β-meATP relaxed the carbachol-precontracted guinea-pig taenia coli in a concentration-dependent manner (EC₅₀, 2.0 ± 0.1 μM). A luciferase-based assay confirmed that α,β-meATP solutions were minimally contaminated with ATP. α,β-meATP-evoked relaxations were inhibited by the competitive P2Y1 antagonist MRS2179 (pA₂ = 5.36), but also by the competitive P2X3 antagonist, A-317491 (pA₂ = 5.51). When MRS2179 and A-317491 were applied together, residual α,β-meATP responses converted from brief to prolonged relaxations. Sodium nitroprusside (a nitric oxide donor) also caused prolonged relaxations. Immunohistochemistry revealed that P2X3 receptors were present in myenteric ganglion cells and their varicose nerve terminals. The amplitude of α,β-meATP responses was not inhibited by TTX (NaV channel blocker) and ωCgTx (N-type CaV channel blocker). However, responses to α,β-meATP were inhibited by TEA (non-selective K⁺-channel blocker), indicating that relaxations involved opening K⁺-channels. The findings of this study are consistent with the conclusion that α,β-meATP stimulates Ca²⁺-permeable P2X3 receptors on varicose nerve terminals to release inhibitory nucleotides: 1) ATP and β-NAD release results in P2Y1-mediated brief relaxations; 2) another released transmitter (possibly NO) results in prolonged relaxations. Prejunctional P2X3 receptors represent a purinergic feed-forward mechanism to augment the action of inhibitory nerves on gut motility. This positive feed-forward mechanism may counter-balance the known negative feedback mechanism caused by adenosine and prejunctional A1 receptors on inhibitory motor nerves.

Neurotransmitters responsible for purinergic motor neurotransmission and regulation of GI motility

Classical concepts of peripheral neurotransmission were insufficient to explain enteric inhibitory neurotransmission. Geoffrey Burnstock and colleagues developed the idea that ATP or a related purine satisfies the criteria for a neurotransmitter and serves as an enteric inhibitory neurotransmitter in GI muscles. Cloning of purinergic receptors and development of specific drugs and transgenic mice have shown that enteric inhibitory responses depend upon P2Y₁ receptors in post-junctional cells. The post-junctional cells that transduce purinergic neurotransmitters in the GI tract are PDGFRα⁺ cells and not smooth muscle cells (SMCs). PDGFRα⁺ cells express P2Y₁ receptors, are activated by enteric inhibitory nerve stimulation and generate Ca²⁺ oscillations, express small-conductance Ca²⁺-activated K⁺ channels (SK3), and generate outward currents when exposed to P2Y₁ agonists. These properties are consistent with post-junctional purinergic responses, and similar responses and effectors are not functional in SMCs. Refinements in methodologies to measure purines in tissue superfusates, such as high-performance liquid chromatography (HPLC) coupled with etheno-derivatization of purines and fluorescence detection, revealed that multiple purines are released during stimulation of intrinsic nerves. β-NAD⁺ and other purines, better satisfy criteria for the purinergic neurotransmitter than ATP. HPLC has also allowed better detection of purine metabolites, and coupled with isolation of specific types of post-junctional cells, has provided new concepts about deactivation of purine neurotransmitters. In spite of steady progress, many unknowns about purinergic neurotransmission remain and require additional investigation to understand this important regulatory mechanism in GI motility.

The extraordinary partnership of Geoff Burnstock and Mollie Holman

Here, we recognise some of the extraordinary accomplishments of the partnership between Geoff Burnstock and Mollie Holman, and the everlasting impact they both made in autonomic neuroscience in Australia. Much of strength today in autonomic neuroscience can be traced back to a time when Geoff and Mollie commenced their seminal studies on autonomic neuroscience, initially at Oxford, then at The University of Melbourne in the mid 1960's. Mollie and Geoff published their first paper together, at Oxford, with their then mentor, and doyenne of smooth muscle, Professor Edith Bülbring. They did not always agree on the interpretation of their own scientific findings. Geoff was convinced early on that Adenosine triphosphate (ATP), or a related purine, was an excitatory neurotransmitter at peripheral sympathetic neuroeffector junctions. Mollie was reticent for decades. However, she began to take the notion seriously that ATP maybe a neurotransmitter, when receptors for purines were identified in the 1990's. What the partnership between Mollie and Geoff taught us in Australia was to not fear respectful criticism, but rather to be receptive to and embrace objective, collegial and constructive scientific peer-review. One of the many great legacies of Geoff and Mollie was the large number of researchers, who were fortunate disciples of their supervision, and who have now themselves gone on to make significant discoveries in autonomic and visceral neuroscience. This review summarizes some of their major legacies and represents a very personal historical perspective of the two authors, pupils respectively of Mollie and Geoff.

Metabolic and behavioral features of acute hyperpurinergia and the maternal immune activation mouse model of autism spectrum disorder

Since 2012, studies in mice, rats, and humans have suggested that abnormalities in purinergic signaling may be a final common pathway for many genetic and environmental causes of autism spectrum disorder (ASD). The current study in mice was conducted to characterize the bioenergetic, metabolomic, breathomic, and behavioral features of acute hyperpurinergia triggered by systemic injection of the purinergic agonist and danger signal, extracellular ATP (eATP). Responses were studied in C57BL/6J mice in the maternal immune activation (MIA) model and controls. Basal metabolic rates and locomotor activity were measured in CLAMS cages. Plasma metabolomics measured 401 metabolites. Breathomics measured 98 volatile organic compounds. Intraperitoneal eATP dropped basal metabolic rate measured by whole body oxygen consumption by 74% ± 6% (mean ± SEM) and rectal temperature by 6.2˚ ± 0.3˚C in 30 minutes. Over 200 metabolites from 37 different biochemical pathways where changed. Breathomics showed an increase in exhaled carbon monoxide, dimethylsulfide, and isoprene. Metabolomics revealed an acute increase in lactate, citrate, purines, urea, dopamine, eicosanoids, microbiome metabolites, oxidized glutathione, thiamine, niacinamide, and pyridoxic acid, and decreased folate-methylation-1-carbon intermediates, amino acids, short and medium chain acyl-carnitines, phospholipids, ceramides, sphingomyelins, cholesterol, bile acids, and vitamin D similar to some children with ASD. MIA animals were hypersensitive to postnatal exposure to eATP or poly(IC), which produced a rebound increase in body temperature that lasted several weeks before returning to baseline. Acute hyperpurinergia produced metabolic and behavioral changes in mice. The behaviors and metabolic changes produced by ATP injection were associated with mitochondrial functional changes that were profound but reversible.

The adenosinergic pathway in mesenchymal stem cell fate and functions

Mesenchymal stem cells (MSCs) play an important role in tissue homeostasis and damage repair through their ability to differentiate into cells of different tissues, trophic support, and immunomodulation. These properties made them attractive for clinical applications in regenerative medicine, immune disorders, and cell transplantation. However, despite multiple preclinical and clinical studies demonstrating beneficial effects of MSCs, their native identity and mechanisms of action remain inconclusive. Since its discovery, the CD73/ecto-5'-nucleotidase is known as a classic marker for MSCs, but its role goes far beyond a phenotypic characterization antigen. CD73 contributes to adenosine production, therefore, is an essential component of purinergic signaling, a pathway composed of different nucleotides and nucleosides, which concentrations are finely regulated by the ectoenzymes and receptors. Thus, purinergic signaling controls pathophysiological functions such as proliferation, migration, cell fate, and immune responses. Despite the remarkable progress already achieved in considering adenosinergic pathway as a therapeutic target in different pathologies, its role is not fully explored in the context of the therapeutic functions of MSCs. Therefore, in this review, we provide an overview of the role of CD73 and adenosine-mediated signaling in the functions ascribed to MSCs, such as homing and proliferation, cell differentiation, and immunomodulation. Additionally, we will discuss the pathophysiological role of MSCs, via CD73 and adenosine, in different diseases, as well as in tumor development and progression. A better understanding of the adenosinergic pathway in the regulation of MSCs functions will help to provide improved therapeutic strategies applicable in regenerative medicine.

Purinergic transmission in depressive disorders

Purinergic signaling involves the actions of purine nucleotides and nucleosides (such as adenosine) at P1 (adenosine), P2X, and P2Y receptors. Here, we present recent data contributing to a comprehensive overview of the association between purinergic signaling and depression. We start with background information on adenosine production and metabolism, followed by a detailed characterization of P1 and P2 receptors, with an emphasis on their expression and function in the brain as well as on their ligands. We provide data suggestive of altered metabolism of adenosine in depressed patients, which might be regarded as a disease biomarker. We then turn to considerable amount of preclinical/behavioral data obtained with the aid of the forced swim test, tail suspension test, learned helplessness model, or unpredictable chronic mild stress model and genetic activation/inactivation of P1 or P2 receptors as well as nonselective or selective ligands of P1 or P2 receptors. We also aimed to discuss the reason underlying discrepancies observed in such studies.

The effect of Xenin25 on spontaneous circular muscle contractions of rat distal colon in vitro

Xenin25 has a variety of physiological functions in the Gastrointestinal (GI) tract, including ion transport and motility. However, the motility responses in the colon induced by Xenin25 remain poorly understood. Therefore, the effect of Xenin25 on the spontaneous circular muscle contractions of the rat distal colon was investigated using organ bath chambers and immunohistochemistry. Xenin25 induced the inhibition followed by postinhibitory spontaneous contractions with a higher frequency in the rat distal colon. This inhibitory effect of Xenin25 was significantly suppressed by TTX but not by atropine. The inhibitory time (the duration of inhibition) caused by Xenin25 was shortened by the NTSR1 antagonist SR48692, the NK1R antagonist CP96345, the VPAC2 receptor antagonist PG99-465, the nitric oxide-sensitive guanylate-cyclase inhibitor ODQ, and the Ca2+ -dependent K+ channel blocker apamin. The higher frequency of postinhibitory spontaneous contractions induced by Xenin25 was also attenuated by ODQ and apamin. SP-, NOS-, and VIP-immunoreactive neurons were detected in the myenteric plexus (MP) of the rat distal colon. Small subsets of the SP-positive neurons were also Calbindin positive. Most of the VIP-positive neurons were also NOS positive, and small subsets of the NK1R-positive neurons were also VIP positive. Based on the present results, we propose the following mechanism. Xenin25 activates neuronal NTSR1 on the SP neurons of IPANs, and transmitters from the VIP and apamin-sensitive NO neurons synergistically inhibit the spontaneous circular muscle contractions via NK1R. Subsequently, the postinhibitory spontaneous contractions are induced by the offset of apamin-sensitive NO neuron activation via the interstitial cells of Cajal. In addition, Xenin25 also activates the muscular NTSR1 to induce relaxation. Thus, Xenin25 is considered to be an important modulator of post prandial circular muscle contraction of distal colon since the release of Xenin25 from enteroendocrine cells is stimulated by food intake.

Diabetes and hypertension: Pivotal involvement of purinergic signaling

Diabetes mellitus (DM) and hypertension are highly prevalent worldwide health problems and frequently associated with severe clinical complications, such as diabetic cardiomyopathy, nephropathy, retinopathy, neuropathy, stroke, and cardiac arrhythmia, among others. Despite all existing research results and reasonable speculations, knowledge about the role of purinergic system in individuals with DM and hypertension remains restricted. Purinergic signaling accounts for a complex network of receptors and extracellular enzymes responsible for the recognition and degradation of extracellular nucleotides and adenosine. The main components of this system that will be presented in this review are: P1 and P2 receptors and the enzymatic cascade composed by CD39 (NTPDase; with ATP and ADP as a substrate), CD73 (5′-nucleotidase; with AMP as a substrate), and adenosine deaminase (ADA; with adenosine as a substrate). The purinergic system has recently emerged as a central player in several physiopathological conditions, particularly those linked to inflammatory responses such as diabetes and hypertension. Therefore, the present review focuses on changes in both purinergic P1 and P2 receptor expression as well as the activities of CD39, CD73, and ADA in diabetes and hypertension conditions. It can be postulated that the manipulation of the purinergic axis at different levels can prevent or exacerbate the insurgency and evolution of diabetes and hypertension working as a compensatory mechanism.

Purinergic signaling in peripheral nervous system glial cells

To facilitate analyses of purinergic signaling in peripheral nerve glia, we review recent literature and catalog purinergic receptor mRNA expression in cultured mouse Schwann cells (SCs). Purinergic signaling can decrease developmental SC proliferation, and promote SC differentiation. The purinergic receptors P2RY2 and P2RX7 are implicated in nerve development and in the ratio of Remak SCs to myelinating SCs in differentiated peripheral nerve. P2RY2, P2RX7, and other receptors are also implicated in peripheral neuropathies and SC tumors. In SC tumors lacking the tumor suppressor NF1, the SC pathway that suppresses SC growth through P2RY2‐driven β‐arrestin‐mediated AKT signaling is aberrant. SC‐released purinergic agonists acting through SC and/or neuronal purinergic receptors activate pain responses. In all these settings, purinergic receptor activation can result in calcium‐independent and calcium‐dependent release of SC ATP and UDP, growth factors, and cytokines that may contribute to disease and nerve repair. Thus, current research suggests that purinergic agonists and/or antagonists might have the potential to modulate peripheral glia function in development and in disease.

Targeting ectonucleotidases to treat inflammation and halt cancer development in the gut

CD39 and CD73 control cell immunity by hydrolyzing proinflammatory ATP and ADP (CD39) into AMP, subsequently converted into anti-inflammatory adenosine (CD73). By regulating the balance between effector and regulatory cells, these ectonucleotidases promote immune homeostasis in acute and chronic inflammation; while also appearing to limit antitumor effector immunity in gut cancer. This manuscript focuses on the pivotal role of CD39 and CD73 ectonucleotidase function in shaping immune responses in the gut. We focus on those mechanisms deployed by these critical and pivotal ectoenzymes and the regulation in the setting of gastrointestinal tract infections, inflammatory bowel disease and tumors of the gastrointestinal tract. We will highlight translational and clinical implications of the latest and most innovative basic research discoveries of these important players of the purinergic signaling. Immunotherapeutic strategies that have been developed to either boost or control ectonucleotidase expression and activity in important disease settings are also reviewed and the in vivo effects discussed.

Salient brain entities labelled in P2rx7-EGFP reporter mouse embryos include the septum, roof plate glial specializations and circumventricular ependymal organs

The purinergic system is one of the oldest cell-to-cell communication mechanisms and exhibits relevant functions in the regulation of the central nervous system (CNS) development. Amongst the components of the purinergic system, the ionotropic P2X7 receptor (P2X7R) stands out as a potential regulator of brain pathology and physiology. Thus, P2X7R is known to regulate crucial aspects of neuronal cell biology, including axonal elongation, path-finding, synapse formation and neuroprotection. Moreover, P2X7R modulates neuroinflammation and is posed as a therapeutic target in inflammatory, oncogenic and degenerative disorders. However, the lack of reliable technical and pharmacological approaches to detect this receptor represents a major hurdle in its study. Here, we took advantage of the P2rx7-EGFP reporter mouse, which expresses enhanced green fluorescence protein (EGFP) immediately downstream of the P2rx7 proximal promoter, to conduct a detailed study of its distribution. We performed a comprehensive analysis of the pattern of P2X7R expression in the brain of E18.5 mouse embryos revealing interesting areas within the CNS. Particularly, strong labelling was found in the septum, as well as along the entire neural roof plate zone of the brain, except chorioidal roof areas, but including specialized circumventricular roof formations, such as the subfornical and subcommissural organs (SFO; SCO). Moreover, our results reveal what seems a novel circumventricular organ, named by us postarcuate organ (PArcO). Furthermore, this study sheds light on the ongoing debate regarding the specific presence of P2X7R in neurons and may be of interest for the elucidation of additional roles of P2X7R in the idiosyncratic histologic development of the CNS and related systemic functions.

P2Y receptors for extracellular nucleotides: Contributions to cancer progression and therapeutic implications

Purinergic receptors for extracellular nucleotides and nucleosides contribute to a vast array of cellular and tissue functions, including cell proliferation, intracellular and transmembrane ion flux, immunomodulation and thrombosis. In mammals, the purinergic receptor system is composed of G protein-coupled P1 receptors A₁, A_2A, A_2B and A₃ for extracellular adenosine, P2X1-7 receptors that are ATP-gated ion channels and G protein-coupled P2Y_{1,2,4,6,11,12,13 and 14} receptors for extracellular ATP, ADP, UTP, UDP and/or UDP-glucose. Recent studies have implicated specific P2Y receptor subtypes in numerous oncogenic processes, including cancer tumorigenesis, metastasis and chemotherapeutic drug resistance, where G protein-mediated signaling cascades modulate intracellular ion concentrations and activate downstream protein kinases, Src family kinases as well as numerous mitogen-activated protein kinases. We are honored to contribute to this special issue dedicated to the founder of the field of purinergic signaling, Dr. Geoffrey Burnstock, by reviewing the diverse roles of P2Y receptors in the initiation, progression and metastasis of specific cancers with an emphasis on pharmacological and genetic strategies employed to delineate cell-specific and P2Y receptor subtype-specific responses that have been investigated using in vitro and in vivo cancer models. We further highlight bioinformatic and empirical evidence on P2Y receptor expression in human clinical specimens and cover clinical perspectives where P2Y receptor-targeting interventions may have therapeutic relevance to cancer treatment.

Purinergic Signaling in Liver Pathophysiology

Extracellular nucleosides and nucleotides activate a group of G protein-coupled receptors (GPCRs) known as purinergic receptors, comprising adenosine and P2Y receptors. Furthermore, purinergic P2X ion channels are activated by ATP. These receptors are expressed in liver resident cells and play a critical role in maintaining liver function. In the normal physiology, these receptors regulate hepatic metabolic processes such as insulin responsiveness, glycogen and lipid metabolism, and bile secretion. In disease states, ATP and other nucleotides serve as danger signals and modulate purinergic responses in the cells. Recent studies have demonstrated that purinergic receptors play a significant role in the development of metabolic syndrome associated non-alcoholic fatty liver disease (NAFLD), non-alcoholic steatohepatitis (NASH), fibrosis, hepatocellular carcinoma (HCC) and liver inflammation. In this concise review, we dissect the role of purinergic signaling in different liver resident cells involved in maintaining healthy liver function and in the development of the above-mentioned liver pathologies. Moreover, we discuss potential therapeutic strategies for liver diseases by targeting adenosine, P2Y and P2X receptors.

Purinergic signaling in diabetes and metabolism

Purinergic signaling, a concept originally formulated by the late Geoffrey Burnstock (1929-2020), was found to modulate pathways in every physiological system. In metabolic disorders there is a role for both adenosine receptors and P2 (nucleotide) receptors, of which there are two classes, i.e. P2Y metabotropic and P2X ionotropic receptors. The individual roles of the 19 receptors encompassed by this family have been dissected - and in many cases the effects associated with specific cell types, including adipocytes, skeletal muscle, liver cells and immune cells. It is suggested that ligands selective for each of the four adenosine receptors (A₁, A_2A, A_2B and A₃), and several of the P2 subtypes (e.g. P2Y₆ or P2X7 antagonists) might have therapeutic potential for treating diabetes and obesity. This is a developing story with some conflicting conclusions relevant to drug discovery, which we summarize here.

Molecular pharmacology of P2Y receptor subtypes

Professor Geoffrey Burnstock proposed the concept of purinergic signaling via P1 and P2 receptors. P2Y receptors are G-protein-coupled receptors (GPCRs) for extracellular adenine and uracil nucleotides. Eight mammalian P2Y receptor subtypes have been identified. They are divided into two subgroups (P2Y₁, P2Y₂, P2Y₄, P2Y₆, and P2Y₁₁) and (P2Y₁₂, P2Y₁₃, and P2Y₁₄). P2Y receptors are found in almost all cells and mediate responses in physiology and pathophysiology including pain and inflammation. The antagonism of platelet P2Y₁₂ receptors by cangrelor, ticagrelor or active metabolites of the thienopyridine compounds ticlopidine, clopidogrel and prasugrel reduces the ADP-induced platelet aggregation in patients with thrombotic complications of vascular diseases. The nucleotide agonist diquafosol acting at P2Y₂ receptors is used for the treatment of the dry eye syndrome. Structural information obtained by crystallography of the human P2Y₁ and P2Y₁₂ receptor proteins, site-directed mutagenesis and molecular modeling will facilitate the rational design of novel selective drugs.

History of Geoff Burnstock's research on P2 receptors

Geoffrey Burnstock is a purinergic signalling legend who's discoveries and conceptualisation created and shaped the field. His scientific achievements were extraordinary and sustained. They included his demonstration that ATP can act as a neurotransmitter and hence extracellular signalling molecule, which he championed despite considerable initial opposition to his proposal that ATP acts outside of its role as an energy source inside cells. He led on purine receptor classification: initially of the P1 and P2 receptor families, then the P2X and P2Y receptor families, and then subtypes of P2X and P2Y receptors. This was achieved across several decades as he conceptualised and made sense of the emerging and growing evidence that there were multiple receptor subtypes for ATP and other nucleotides. He made discoveries about short term and long term/trophic purinergic signalling. He was a leader in the field for over 50 years. He inspired many and was a great colleague and mentor. I had the privilege of spending over 10 years (from 1985) with Geoff at the Department of Anatomy and Developmental Biology, University College London. This review is a personal perspective of some of Geoff's research on P2 receptors carried out during that time. It is a tribute to Geoff who I regarded with enormous respect and admiration.

Extracellular ATP: A Feasible Target for Cancer Therapy

Adenosine triphosphate (ATP) is one of the main biochemical components of the tumor microenvironment (TME), where it can promote tumor progression or tumor suppression depending on its concentration and on the specific ecto-nucleotidases and receptors expressed by immune and cancer cells. ATP can be released from cells via both specific and nonspecific pathways. A non-regulated release occurs from dying and damaged cells, whereas active release involves exocytotic granules, plasma membrane-derived microvesicles, specific ATP-binding cassette (ABC) transporters and membrane channels (connexin hemichannels, pannexin 1 (PANX1), calcium homeostasis modulator 1 (CALHM1), volume-regulated anion channels (VRACs) and maxi-anion channels (MACs)). Extracellular ATP acts at P2 purinergic receptors, among which P2X7R is a key mediator of the final ATP-dependent biological effects. Over the years, P2 receptor- or ecto-nucleotidase-targeting for cancer therapy has been proposed and actively investigated, while comparatively fewer studies have explored the suitability of TME ATP as a target. In this review, we briefly summarize the available evidence suggesting that TME ATP has a central role in determining tumor fate and is, therefore, a suitable target for cancer therapy.

History of ectonucleotidases and their role in purinergic signaling

Ectonucleotidases are key for purinergic signaling. They control the duration of activity of purinergic receptor agonists. At the same time, they produce hydrolysis products as additional ligands of purinergic receptors. Due to the considerable diversity of enzymes, purinergic receptor ligands and purinergic receptors, deciphering the impact of extracellular purinergic receptor control has become a challenge. The first group of enzymes described were the alkaline phosphatases - at the time not as nucleotide-metabolizing but as nonspecific phosphatases. Enzymes now referred to as nucleoside triphosphate diphosphohydrolases and ecto-5'-nucleotidase were the first and only nucleotide-specific ectonucleotidases identified. And they were the first group of enzymes related to purinergic signaling. Additional research brought to light a surprising number of ectoenzymes with broad substrate specificity, which can also hydrolyze nucleotides. This short overview traces the development of the field and briefly highlights important results and benefits for therapies of human diseases achieved within nearly a century of investigations.

Purinergic signalling in the cardiovascular system-a tribute to Geoffrey Burnstock

Geoffrey Burnstock made groundbreaking discoveries on the physiological roles of purinergic receptors and led on P2 purinergic receptor classification. His knowledge, vision and leadership inspired and influenced the international scientific community. I had the privilege of spending over 10 years (from 1985) with Geoff at the Department of Anatomy and Developmental Biology, initially as a PhD student and then as a postdoctoral research fellow. I regarded him with enormous admiration and affection. This review on purinergic signalling in the cardiovascular system is a tribute to Geoff. It includes some personal recollections of Geoff.

Nucleotide P2Y1 receptor agonists are in vitro and in vivo prodrugs of A1/A3 adenosine receptor agonists: implications for roles of P2Y1 and A1/A3 receptors in physiology and pathology

Rapid phosphoester hydrolysis of endogenous purine and pyrimidine nucleotides has challenged the characterization of the role of P2 receptors in physiology and pathology. Nucleotide phosphoester stabilization has been pursued on a number of medicinal chemistry fronts. We investigated the in vitro and in vivo stability and pharmacokinetics of prototypical nucleotide P2Y₁ receptor (P2Y₁R) agonists and antagonists. These included the riboside nucleotide agonist 2-methylthio-ADP and antagonist MRS2179, as well as agonist MRS2365 and antagonist MRS2500 containing constrained (N)-methanocarba rings, which were previously reported to form nucleotides that are more slowly hydrolyzed at the α-phosphoester compared with the ribosides. In vitro incubations in mouse and human plasma and blood demonstrated the rapid hydrolysis of these compounds to nucleoside metabolites. This metabolism was inhibited by EDTA to chelate divalent cations required by ectonucleotidases for nucleotide hydrolysis. This rapid hydrolysis was confirmed in vivo in mouse pharmacokinetic studies that demonstrate that MRS2365 is a prodrug of the nucleoside metabolite AST-004 (MRS4322). Furthermore, we demonstrate that the nucleoside metabolites of MRS2365 and 2-methylthio-ADP are adenosine receptor (AR) agonists, notably at A₃ and A₁ARs. In vivo efficacy of MRS2365 in murine models of traumatic brain injury and stroke can be attributed to AR activation by its nucleoside metabolite AST-004, rather than P2Y₁R activation. This research suggests the importance of reevaluation of previous in vitro and in vivo research of P2YRs and P2XRs as there is a potential that the pharmacology attributed to nucleotide agonists is due to AR activation by active nucleoside metabolites.

Burnstock and the legacy of the inhibitory junction potential and P2Y1 receptors

The synaptic event called the inhibitory junction potential (IJP) was arguably one of the more important discoveries made by Burnstock and arguably one of his finer legacies. The discovery of the IJP fundamentally changed how electromechanical coupling was visualised in gastrointestinal smooth muscle. Its discovery also set in motion the search for novel inhibitory neurotransmitters in the enteric nervous system, eventually leading to proposal that ATP or a related nucleotide was a major inhibitory transmitter. The subsequent development of purinergic signalling gave impetus to expanding the classification of surface receptors for extracellular ATP, not only in the GI tract but beyond, and then led to successive phases of medicinal chemistry as the P2 receptor field developed. Ultimately, the discovery of the IJP led to the successful cloning of the first P2Y receptor (chick P2Y1) and expansion of mammalian ATP receptors into two classes: metabotropic P2Y receptors (encompassing P2Y1, P2Y2, P2Y4, P2Y6, P2Y11–14 receptors) and ionotropic P2X receptors (encompassing homomeric P2X1–P2X7 receptors). Here, the causal relationship between the IJP and P2Y1 is explored, setting out the milestones reached and achievements made by Burnstock and his colleagues.

Purinergic signaling in infectious diseases of the central nervous system

The incidence of infectious diseases affecting the central nervous system (CNS) has been increasing over the last several years. Among the reasons for the expansion of these diseases and the appearance of new neuropathogens are globalization, global warming, and the increased proximity between humans and wild animals due to human activities such as deforestation. Neurotropism affecting normal brain function is shared by organisms such as viruses, bacteria, fungi, and parasites. Neuroinfections caused by these agents activate immune responses, inducing neuroinflammation, excitotoxicity, and neurodegeneration. Purinergic signaling is an evolutionarily conserved signaling pathway associated with these neuropathologies. During neuroinfections, host cells release ATP as an extracellular danger signal with pro-inflammatory activities. ATP is metabolized to its derivatives by ectonucleotidases such as CD39 and CD73; ATP and its metabolites modulate neuronal and immune mechanisms through P1 and P2 purinergic receptors that are involved in pathophysiological mechanisms of neuroinfections. In this review we discuss the beneficial or deleterious effects of various components of the purinergic signaling pathway in infectious diseases that affect the CNS, including human immunodeficiency virus (HIV-1) infection, herpes simplex virus type 1 (HSV-1) infection, bacterial meningitis, sepsis, cryptococcosis, toxoplasmosis, and malaria. We also provide a description of this signaling pathway in emerging viral infections with neurological implications such as Zika and SARS-CoV-2.