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 in nearby cells, thereby regulating cellular functions.
It was proposed after adenosine triphosphate (ATP) was identified in 1970 as the transmitter responsible for non-adrenergic, non-cholinergic neurotransmission. Nowadays, it is known that ATP acts as a co-transmitter in most, if not all, nerves in the central and peripheral nervous system.
Receptors for adenosine (called P1) and for ATP and ADP (called P2) were distinguished in 1978. Later, the P2 receptors were subdivided into P2X and P2Y families based on their different mechanisms. In the early 1990s, when the receptors to purines and pyrimidines were cloned and characterized, numerous subtypes of P1 and P2 receptors were discovered.
The purinergic signalling complex of a cell is sometimes referred to as the "purinome".
Background
Evolutionary origins
Purinergic receptors, represented by several families, are among the most abundant receptors in living organisms and appeared early in evolution.
Among invertebrates, the purinergic signalling system has been found in bacteria, amoeba, ciliates, algae, fungi, sea anemones, ctenophores, platyhelminthes, nematodes, crustacea, molluscs, annelids, echinoderms, and insects. 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. In 2014, the first purinergic receptor in plants, DORN1, was discovered.
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.
Molecular mechanisms
Generally speaking, all cells have the ability to release nucleotides. In neuronal and neuroendocrinal cells, this mostly occurs via regulated exocytosis. 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. The signalling effects of uridine triphosphate (UTP) and uridine diphosphate (UDP) are generally comparable to those of ATP.
Purinergic receptors
Purinergic receptors 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.
Name Activation Class P1 receptors adenosine G protein-coupled receptors P2Y receptors nucleotides ATP ADP UTP UDP UDP-glucose 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. There are two types of NTs:
Concentrative nucleoside transporters (CNTs): Na+-dependent symporters Equilibrative nucleoside transporters (ENTs): Na+-independent passive transporters
The extracellular concentration of adenosine can be regulated by NTs, possibly in the form of a feedback loop connecting receptor signaling with transporter function.
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.
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. For example, the PANX1 channel, along with ATP, purinergic receptors, and ectonucleotidases, contribute to several feedback loops during the inflammatory response.
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. 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. From the 1980s onwards, these effects of adenosine have been used in the treatment of patients with supraventricular tachycardia.
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.
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.
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. P2Y receptors in the enteric nervous system and at intestinal neuromuscular junctions modulate intestinal secretion and motility.
Endocrine system
Cells of the pituitary gland secrete ATP, which acts on P2Y and P2X purinoreceptors.
Immune system
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.
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. 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. Large increases in extracellular ATP that are associated with cell death serve as a "danger signal" in the inflammatory processes.
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. Hypoxia-inducible factors also influence adenosine signalling.
Nervous system
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. 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.
In the peripheral nervous system, Schwann cells respond to nerve stimulation and modulate the release of neurotransmitters through mechanisms involving ATP and adenosine signalling. 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. These purinergic somatic-junctions have also been shown to be important for microglia to control neuronal development. Calcium signaling evoked by purinergic receptors contributes to the processing of sensory information.
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.
Purinergic signalling, and in particular tissue-injury induced ATP-release is very important for the rapid and robust phenotype changes of microglia.
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.
Respiratory system
ATP and adenosine are crucial regulators of mucociliary clearance. The secretion of mucin involves P2RY2 receptors found on the apical membrane of goblet cells. Extracellular ATP signals acting on glial cells and the neurons of the respiratory rhythm generator contribute to the regulation of breathing.
Skeletal system