Purinergic signalling

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Purinergic signalling (or signaling: see American and British English differences) is a form of extracellular signalling mediated by purine nucleotides and nucleosides such as adenosine and ATP. It involves the activation of purinergic receptors in the cell and/or in nearby cells, thereby regulating cellular functions. [1]

Contents

The purinergic signalling complex of a cell is sometimes referred to as the “purinome”. [2]

Background

Evolutionary origins

Exogenously applied ATP stimulates the closure of the Venus flytrap Digested fly.JPG
Exogenously applied ATP stimulates the closure of the Venus flytrap

Purinergic receptors, represented by several families, are among the most abundant receptors in living organisms and appeared early in evolution. [4]

Among invertebrates, the purinergic signalling system has been found in bacteria, amoeba, ciliates, algae, fungi, anemones, ctenophores, platyhelminthes, nematodes, crustacea, molluscs, annelids, echinoderms, and insects. [5] In green plants, extracellular ATP and other nucleotides induce an increase in the cytosolic concentration of calcium ions, in addition to other downstream changes that influence plant growth and modulate responses to stimuli. [6] In 2014, the first purinergic receptor in plants, DORN1, was discovered. [7]

The primitive P2X receptors of unicellular organisms often share low sequence similarity with those in mammals, yet they still retain micromolar sensitivity to ATP. The evolution of this receptor class is estimated to have occurred over a billion years ago. [8]

Molecular mechanisms

Generally speaking, all cells have the ability to release nucleotides. In neuronal and neuroendocrinal cells, this mostly occurs via regulated exocytosis. [1] Released nucleotides can be hydrolyzed extracellularly by a variety of cell surface-located enzymes referred to as ectonucleotidases. The purinergic signalling system consists of transporters, enzymes and receptors responsible for the synthesis, release, action, and extracellular inactivation of (primarily) ATP and its extracellular breakdown product adenosine. [9] The signalling effects of uridine triphosphate (UTP) and uridine diphosphate (UDP) are generally comparable to those of ATP. [10]

Purinergic receptors

Homology modeling of the P2RX2 receptor in the open channel state P2X2R receptor.jpg
Homology modeling of the P2RX2 receptor in the open channel state

Purinergic receptor s are specific classes of membrane receptors that mediate various physiological functions such as the relaxation of gut smooth muscle, as a response to the release of ATP or adenosine. There are three known distinct classes of purinergic receptors, known as P1, P2X, and P2Y receptors. Cell signalling events initiated by P1 and P2Y receptors have opposing effects in biological systems. [11]

NameActivationClass
P1 receptors adenosine G protein-coupled receptors
P2Y receptors nucleotides G protein-coupled receptors
P2X receptors ATP ligand-gated ion channel

Nucleoside transporters

Nucleoside transporters (NTs) are a group of membrane transport proteins which transport nucleoside substrates including adenosine across the membranes of cells and/or vesicles. NTs are considered to be evolutionarily ancient membrane proteins and are found in many different forms of life. [12] There are two types of NTs:

The extracellular concentration of adenosine can be regulated by NTs, possibly in the form of a feedback loop connecting receptor signaling with transporter function. [12]

Ectonucleotidases

Released nucleotides can be hydrolyzed extracellularly by a variety of cell surface-located enzymes referred to as ectonucleotidases that control purinergic signalling. Extracellular nucleoside triphosphates and diphosphates are substrates of the ectonucleoside triphosphate diphosphohydrolases (E-NTPDases), the ectonucleotide pyrophosphatase/phosphodiesterases (E-NPPs) and alkaline phosphatases (APs). Extracellular AMP is hydrolyzed to adenosine by ecto-5'-nucleotidase (eN) as well as by APs. In any case, the final product of the hydrolysis cascade is the nucleoside. [13] [14]

Pannexins

The Pannexin-1 channel (PANX1) is an integral component of the P2X/P2Y purinergic signaling pathway and the key contributor to pathophysiological ATP release. [15] For example, the PANX1 channel, along with ATP, purinergic receptors, and ectonucleotidases, contribute to several feedback loops during the inflammatory response. [16]

Purinergic signalling in humans

Circulatory system

In the human heart, adenosine functions as an autacoid in the regulation of various cardiac functions such as heart rate, contractility, and coronary flow. There are currently four types of adenosine receptors found in the heart. [17] After binding onto a specific purinergic receptor, adenosine causes a negative chronotropic effect due to its influence on cardiac pacemakers. It also causes a negative dromotropic effect through the inhibition of AV-nodal conduction. [18] From the 1980s onwards, these effects of adenosine have been used in the treatment of patients with supraventricular tachycardia. [19]

The regulation of vascular tone in the endothelium of blood vessels is mediated by purinergic signalling. A decreased concentration of oxygen releases ATP from erythrocytes, triggering a propagated calcium wave in the endothelial layer of blood vessels and a subsequent production of nitric oxide that results in vasodilation. [20] [21]

During the blood clotting process, adenosine diphosphate (ADP) plays a crucial role in the activation and recruitment of platelets and also ensures the structural integrity of thrombi. These effects are modulated by the P2RY1 and the P2Y12 receptors. The P2RY1 receptor is responsible for shape change in platelets, increased intracellular calcium levels and transient platelet aggregation, while the P2Y12 receptor is responsible for sustained platelet aggregation through the inhibition of adenylate cyclase and a corresponding decrease in cyclic adenosine monophosphate (cAMP) levels. The activation of both purinergic receptors is necessary to achieve sustained hemostasis. [22] [23]

Digestive system

In the liver, ATP is constantly released during homeostasis and its signalling via P2 receptors influences bile secretion as well as liver metabolism and regeneration. [24] P2Y receptors in the enteric nervous system and at intestinal neuromuscular junctions modulate intestinal secretion and motility. [25]

Endocrine system

Cells of the pituitary gland secrete ATP, which acts on P2Y and P2X purinoreceptors. [26]

Immune system

As part of the inflammatory response, ATP activates the P2RX7 receptor, triggering a drop in intracellular potassium levels and the formation of inflammasomes Role of the P2X7 receptor in inflammation.png
As part of the inflammatory response, ATP activates the P2RX7 receptor, triggering a drop in intracellular potassium levels and the formation of inflammasomes

Autocrine purinergic signalling is an important checkpoint in the activation of white blood cells. These mechanisms either enhance or inhibit cell activation based on the purinergic receptors involved, allowing cells to adjust their functional responses initiated by extracellular environmental cues. [27]

Like most immunomodulating agents, ATP can act either as an immunosuppressive or an immunostimulatory factor, depending on the cytokine microenvironment and the type of cell receptor. [28] In white blood cells such as macrophages, dendritic cells, lymphocytes, eosinophils, and mast cells, purinergic signalling plays a pathophysiological role in calcium mobilization, actin polymerization, release of mediators, cell maturation, cytotoxicity, and apoptosis. [29] Large increases in extracellular ATP that are associated with cell death serve as a "danger signal" in the inflammatory processes. [30]

In neutrophils, tissue adenosine can either activate or inhibit various neutrophil functions, depending on the inflammatory microenvironment, the expression of adenosine receptors on the neutrophil, and the affinity of these receptors for adenosine. Micromolar concentrations of adenosine activate A2A and A2B receptors. This inhibits the release of granules and prevents oxidative burst. On the other hand, nanomolar concentrations of adenosine activate A1 and A3 receptors, resulting in neutrophilic chemotaxis towards inflammatory stimuli. The release of ATP and an autocrine feedback through P2RY2 and A3 receptors are signal amplifiers. [31] [32] Hypoxia-inducible factors also influence adenosine signalling. [19]

Nervous system

Microglial activation in the CNS via purinergic signalling Purinergic signalling Microglia.jpg
Microglial activation in the CNS via purinergic signalling

In the central nervous system (CNS), ATP is released from synaptic terminals and binds to a plethora of ionotropic and metabotropic receptors. It has an excitatory effect on neurones, and acts as a mediator in neuronal–glial communications. [33] Both adenosine and ATP induce astrocyte cell proliferation. In microglia, P2X and P2Y receptors are expressed. The P2Y6 receptor, which is primarily mediated by uridine diphosphate (UDP), plays a significant role in microglial phagoptosis, while the P2Y12 receptor functions as a specialized pattern recognition receptor. P2RX4 receptors are involved in the CNS mediation of neuropathic pain. [34]

In the peripheral nervous system, Schwann cells respond to nerve stimulation and modulate the release of neurotransmitters through mechanisms involving ATP and adenosine signalling. [35] In the retina and the olfactory bulb, ATP is released by neurons to evoke transient calcium signals in several glial cells such as Muller glia and astrocytes. This influences various homeostatic processes of the nervous tissue including volume regulation and the control of blood flow. Although purinergic signaling has been connected to pathological processes in the context of neuron-glia communication, it has been revealed, that this is also very important under physiological conditions. Neurons possess specialised sites on their cell bodies, through which they release ATP (and other substances), reflecting their "well-being". Microglial processes specifically recognize these purinergic somatic-junctions, and monitor neuronal functions by sensing purine nucleotides via their P2Y12-receptors. In case of neuronal overactivation or injury, microglial processes respond with an increased coverage of neuronal cell bodies, and exert robust neuroprotective effects. [36] Calcium signaling evoked by purinergic receptors contributes to the processing of sensory information. [37]

During neurogenesis and in early brain development, ectonucleotidases often downregulate purinergic signalling in order to prevent the uncontrolled growth of progenitor cells and to establish a suitable environment for neuronal differentiation. [38]

Renal system

In the kidneys, the glomerular filtration rate (GFR) is regulated by several mechanisms including tubuloglomerular feedback (TGF), in which an increased distal tubular sodium chloride concentration causes a basolateral release of ATP from the macula densa cells. This initiates a cascade of events that ultimately brings GFR to an appropriate level. [39] [40]

Respiratory system

ATP and adenosine are crucial regulators of mucociliary clearance. [41] The secretion of mucin involves P2RY2 receptors found on the apical membrane of goblet cells. [41] Extracellular ATP signals acting on glial cells and the neurons of the respiratory rhythm generator contribute to the regulation of breathing. [42]

Skeletal system

In the human skeleton, nearly all P2Y and P2X receptors have been found in osteoblasts and osteoclasts. These receptors enable the regulation of multiple processes such as cell proliferation, differentiation, function, and death. [43] The activation of the adenosine A1 receptor is required for osteoclast differentiation and function, whereas the activation of the adenosine A2A receptor inhibits osteoclast function. The other three adenosine receptors are involved in bone formation. [44]

Pathological aspects

Alzheimer's disease

In Alzheimer's disease (AD), the expression of A1 and A2A receptors in the frontal cortex of the human brain is increased, while the expression of A1 receptors in the outer layers of hippocampal dentate gyrus is decreased. [38]

Asthma

In the airways of patients with asthma, the expression of adenosine receptors is upregulated. Adenosine receptors affect bronchial reactivity, endothelial permeability, fibrosis, angiogenesis and mucus production. [45]

Bone diseases

Purinergic signalling is involved in the pathophysiology of several bone and cartilage diseases such as osteoarthritis, rheumatoid arthritis, and osteoporosis. [46] Single-nucleotide polymorphisms (SNPs) in the P2RX7 receptor gene are associated with an increased risk of bone fracture. [43]

Cancer

The P2RX7 receptor is overexpressed in most malignant tumors. [47] The expression of the adenosine A2A receptor on endothelial cells is upregulated in the early stages of human lung cancer. [48]

Cardiovascular diseases

Formation of foam cells is inhibited by adenosine A2A receptors. [49]

Chronic obstructive pulmonary disease

Abnormal levels of ATP and adenosine are present in the airways of patients with chronic obstructive pulmonary disease. [50] [51]

Erectile disorders

The release of ATP increases adenosine levels and activates nitric oxide synthase, both of which induces the relaxation of the corpus cavernosum penis. In male patients with vasculogenic impotence, dysfunctional adenosine A2B receptors are associated with the resistance of the corpus cavernosum to adenosine. On the other hand, excess adenosine in penile tissue contributes to priapism. [52] [53]

Fibrosis

The bronchoalveolar lavage (BAL) fluid of patients with idiopathic pulmonary fibrosis contains a higher concentration of ATP than that of control subjects. [54] Persistently elevated concentrations of adenosine beyond the acute-injury phase leads to fibrotic remodelling. [55] Extracellular purines modulate fibroblast proliferation by binding onto adenosine receptors and P2 receptors to influence tissue structure and pathologic remodeling. [54]

Graft-versus-host disease

Following tissue injury in patients with Graft-versus-host disease (GVHD), ATP is released into the peritoneal fluid. It binds onto the P2RX7 receptors of host antigen-presenting cells (APCs) and activates the inflammasomes. As a result, the expression of co-stimulatory molecules by APCs is upregulated. The inhibition of the P2X7 receptor increases the number of regulatory T cells and decreases the incidence of acute GVHD. [56]

Therapeutic interventions

Current

Clopidogrel (Plavix), an inhibitor of the P2Y12 receptor, was formerly the second best-selling drug in the world Plavix 2007-04-19.jpg
Clopidogrel (Plavix), an inhibitor of the P2Y12 receptor, was formerly the second best-selling drug in the world
Acupuncture

Mechanical deformation of the skin by acupuncture needles appears to result in the release of adenosine. [58] [59] A 2014 Nature Reviews Cancer review article found that the key mouse studies that suggested acupuncture relieves pain via the local release of adenosine, which then triggered close-by A1 receptors "caused more tissue damage and inflammation relative to the size of the animal in mice than in humans, such studies unnecessarily muddled a finding that local inflammation can result in the local release of adenosine with analgesic effect." [60] The anti-nociceptive effect of acupuncture may be mediated by the adenosine A1 receptor. [61] [62] [63] Electroacupuncture may inhibit pain by the activation of a variety of bioactive chemicals through peripheral, spinal, and supraspinal mechanisms of the nervous system. [64]

Anti-inflammatory drugs

Methotrexate, which has strong anti-inflammatory properties, inhibits the action of dihydrofolate reductase, leading to an accumulation of adenosine. On the other hand, the adenosine-receptor antagonist caffeine reverses the anti-inflammatory effects of methotrexate. [65]

Anti-platelet drugs

Many anti-platelet drugs such as Prasugrel, Ticagrelor, and Ticlopidine are adenosine diphosphate (ADP) receptor inhibitors. Before the expiry of its patent, the P2Y12 receptor antagonist Clopidogrel (trade name: Plavix) was the second most prescribed drug in the world. In 2010 alone, it generated over US$9 billion in global sales. [66]

Bronchodilators

Theophylline was originally used as a bronchodilator, although its usage has declined due to several side effects such as seizures and cardiac arrhythmias caused by adenosine A1 receptor antagonism. [67]

Herbal medicine

Several herbs used in Traditional Chinese medicine contain drug compounds that are antagonists of P2X purinoreceptors. [68] The following table provides an overview of these drug compounds and their interaction with purinergic receptors.

HerbDrug compoundPhysiologic effects on purinergic receptors
Many
Ligusticum wallichii
  • Reduction of thermal and mechanical hyperalgesia via P2RX3 antagonism [68]
Kudzu
  • Reduction of chronic neuropathic pain via P2RX3 and P2X2/3 antagonism [72]
Rheum officinale
Rhubarb
  • Induction of necrosis in human liver cancer cells via a decrease in ATP levels. [75]
Vasodilators

Regadenoson, a vasodilator which acts on the adenosine A2A receptor, was approved by the United States Food and Drug Administration in 2008 and is currently widely used in the field of cardiology. [76] [77] Both adenosine and dipyridamole, which act on the A2A receptor, are used in myocardial perfusion imaging. [78]

Proposed

Purinergic signalling is an important regulatory mechanism in a wide range of inflammatory diseases. It is understood that shifting the balance between purinergic P1 and P2 signalling is an emerging therapeutic concept that aims to dampen pathologic inflammation and promote healing. [11] The following list of proposed medications is based on the workings of the purinergic signalling system:

History

The earliest reports of purinergic signalling date back to 1929, when the Hungarian physiologist Albert Szent-Györgyi observed that purified adenine compounds produced a temporary reduction in heart rate when injected into animals. [11] [81]

In the 1960s, the classical view of autonomic smooth muscle control was based upon Dale's principle, which asserts that each nerve cell can synthesize, store, and release only one neurotransmitter. It was therefore assumed that a sympathetic neuron releases noradrenaline only, while an antagonistic parasympathetic neuron releases acetylcholine only. Although the concept of cotransmission gradually gained acceptance in the 1980s, the belief that a single neuron acts via a single type of neurotransmitter continued to dominate the field of neurotransmission throughout the 1970s. [82]

Beginning in 1972, Geoffrey Burnstock ignited decades of controversy after he proposed the existence of a non-adrenergic, non-cholinergic (NANC) neurotransmitter, which he identified as ATP after observing the cellular responses in a number of systems exposed to the presence of cholinergic and adrenergic blockers. [83] [84] [85]

Burnstock's proposal was met with criticism, since ATP is an ubiquitous intracellular molecular energy source [86] so it seemed counter-intuitive that cells might also actively release this vital molecule as a neurotransmitter. After years of prolonged scepticism, however, the concept of purinergic signalling was gradually accepted by the scientific community. [1]

Today, purinergic signalling is no longer considered to be confined to neurotransmission, but is regarded as a general intercellular communication system of many, if not all, tissues. [1]

See also

Related Research Articles

<span class="mw-page-title-main">Adenosine receptor</span> Class of four receptor proteins to the molecule adenosine

The adenosine receptors (or P1 receptors) are a class of purinergic G protein-coupled receptors with adenosine as the endogenous ligand. There are four known types of adenosine receptors in humans: A1, A2A, A2B and A3; each is encoded by a different gene.

In the physiology of the kidney, tubuloglomerular feedback (TGF) is a feedback system inside the kidneys. Within each nephron, information from the renal tubules is signaled to the glomerulus. Tubuloglomerular feedback is one of several mechanisms the kidney uses to regulate glomerular filtration rate (GFR). It involves the concept of purinergic signaling, in which an increased distal tubular sodium chloride concentration causes a basolateral release of adenosine from the macula densa cells. This initiates a cascade of events that ultimately brings GFR to an appropriate level.

<span class="mw-page-title-main">Purinergic receptor</span> Family of cell membrane receptors in almost all tissues

Purinergic receptors, also known as purinoceptors, are a family of plasma membrane molecules that are found in almost all mammalian tissues. Within the field of purinergic signalling, these receptors have been implicated in learning and memory, locomotor and feeding behavior, and sleep. More specifically, they are involved in several cellular functions, including proliferation and migration of neural stem cells, vascular reactivity, apoptosis and cytokine secretion. These functions have not been well characterized and the effect of the extracellular microenvironment on their function is also poorly understood.

<span class="mw-page-title-main">P2X purinoreceptor</span>

The P2X receptors, also ATP-gated P2X receptor cation channel family, is a protein family that consists of cation-permeable ligand-gated ion channels that open in response to the binding of extracellular adenosine 5'-triphosphate (ATP). They belong to a larger family of receptors known as the ENaC/P2X superfamily. ENaC and P2X receptors have similar 3-D structures and are homologous. P2X receptors are present in a diverse array of organisms including humans, mouse, rat, rabbit, chicken, zebrafish, bullfrog, fluke, and amoeba.

<span class="mw-page-title-main">P2Y receptor</span> Subclass of purinergic P2 receptors

P2Y receptors are a family of purinergic G protein-coupled receptors, stimulated by nucleotides such as adenosine triphosphate, adenosine diphosphate, uridine triphosphate, uridine diphosphate and UDP-glucose.To date, 8 P2Y receptors have been cloned in humans: P2Y1, P2Y2, P2Y4, P2Y6, P2Y11, P2Y12, P2Y13 and P2Y14.

<span class="mw-page-title-main">Ectonucleotidase</span>

Ectonucleotidases consist of families of nucleotide metabolizing enzymes that are expressed on the plasma membrane and have externally oriented active sites. These enzymes metabolize nucleotides to nucleosides. The contribution of ectonucleotidases in the modulation of purinergic signaling depends on the availability and preference of substrates and on cell and tissue distribution.

<span class="mw-page-title-main">P2RX7</span> Protein-coding gene in the species Homo sapiens

P2X purinoceptor 7 is a protein that in humans is encoded by the P2RX7 gene.

<span class="mw-page-title-main">P2RY1</span> Protein-coding gene in the species Homo sapiens

P2Y purinoceptor 1 is a protein that in humans is encoded by the P2RY1 gene.

<span class="mw-page-title-main">P2RX1</span> Protein-coding gene in the species Homo sapiens

P2X purinoceptor 1, also ATP receptor, is a protein that in humans is encoded by the P2RX1 gene.

<span class="mw-page-title-main">P2RY2</span> Protein-coding gene in the species Homo sapiens

P2Y purinoceptor 2 is a protein that in humans is encoded by the P2RY2 gene.

<span class="mw-page-title-main">ENTPD1</span> Mammalian protein found in Homo sapiens

Ectonucleoside triphosphate diphosphohydrolase-1 also known as CD39, is a typical cell surface enzyme with a catalytic site on the extracellular face.

<span class="mw-page-title-main">GPR17</span> Protein-coding gene in the species Homo sapiens

Uracil nucleotide/cysteinyl leukotriene receptor is a G protein-coupled receptor that in humans is encoded by the GPR17 gene located on chromosome 2 at position q21. The actual activating ligands for and some functions of this receptor are disputed.

<span class="mw-page-title-main">P2RY11</span> Protein-coding gene in the species Homo sapiens

P2Y purinoceptor 11 is a protein that in humans is encoded by the P2RY11 gene.

<span class="mw-page-title-main">P2RY14</span> Protein-coding gene in the species Homo sapiens

P2Y purinoceptor 14 is a protein that in humans is encoded by the P2RY14 gene.

<span class="mw-page-title-main">LPAR6</span> Protein-coding gene in the species Homo sapiens

Lysophosphatidic acid receptor 6, also known as LPA6, P2RY5 and GPR87, is a protein that in humans is encoded by the LPAR6 gene. LPA6 is a G protein-coupled receptor that binds the lipid signaling molecule lysophosphatidic acid (LPA).

<span class="mw-page-title-main">P2RY10</span> Protein-coding gene in the species Homo sapiens

Putative P2Y purinoceptor 10 is a protein that, in humans, is encoded by the P2RY10 gene.

<span class="mw-page-title-main">P2RY4</span> Protein-coding gene in the species Homo sapiens

P2Y purinoceptor 4 is a protein that in humans is encoded by the P2RY4 gene.

P2 receptor may refer to:

<span class="mw-page-title-main">P2RX4</span> Protein-coding gene in the species Homo sapiens

P2X purinoceptor 4 is a protein that in humans is encoded by the P2RX4 gene. The product of this gene belongs to the family of purinoceptors for ATP. Multiple alternatively spliced transcript variants have been identified for this gene although their full-length natures have not been determined.

<span class="mw-page-title-main">Rostral ventromedial medulla</span> Group of neurons in medulla of brain

The rostral ventromedial medulla (RVM), or ventromedial nucleus of the spinal cord, is a group of neurons located close to the midline on the floor of the medulla oblongata. The rostral ventromedial medulla sends descending inhibitory and excitatory fibers to the dorsal horn spinal cord neurons. There are 3 categories of neurons in the RVM: on-cells, off-cells, and neutral cells. They are characterized by their response to nociceptive input. Off-cells show a transitory decrease in firing rate right before a nociceptive reflex, and are theorized to be inhibitory. Activation of off-cells, either by morphine or by any other means, results in antinociception. On-cells show a burst of activity immediately preceding nociceptive input, and are theorized to be contributing to the excitatory drive. Neutral cells show no response to nociceptive input.

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