International Journal of Molecular Sciences Review Microglia and Neuroinflammation: What Place for P2RY12? Albert Gómez Morillas , Valérie C. Besson † and Dominique Lerouet *,† UMR-S1144–Optimisation Thérapeutique en Neuropsychopharmacologie (OTeN), Faculté de Pharmacie de Paris, 4 Avenue de l’Observatoire, Université de Paris, 75006 Paris, France; [email protected] (A.G.M.); [email protected] (V.C.B.) * Correspondence: [email protected]; Tel.: +33-1-53-73-97-86 † These authors contributed equally to this work. Abstract: Microglia are immune brain cells involved in neuroinflammation. They express a lot of proteins on their surface such as receptors that can be activated by mediators released in the microglial environment. Among these receptors, purinergic receptor expression could be modified depending on the activation status of microglia. In this review, we focus on P2Y receptors and more specifically on P2RY12 that is involved in microglial motility and migration, the first step of neuroinflammation process. We describe the purinergic receptor families, P2RY12 structure, expression and physiological functions. The pharmacological and genetic tools for studying this receptor are detailed thereafter. Last but not least, we report the contribution of microglial P2RY12 to neuroinflammation in acute and chronic brain pathologies in order to better understand P2RY12 microglial role. Keywords: microglia; neuroinflammation; P2RY12; purinergic receptor 1. Introduction Citation: Gómez Morillas, A.; Besson, V.C.; Lerouet, D. Microglia Microglia are the main immune cells in the brain. These plastic cells display a variety and Neuroinflammation: What Place of morphological and functional states in both healthy and pathologic conditions. Numer- for P2RY12? Int. J. Mol. Sci. 2021, 22, ous studies demonstrated the involvement of microglial activation in different cerebral 1636. https://doi.org/10.3390/ neuroinflammatory pathologies [1]. Microglia express a variety of cell-surface proteins that ijms22041636 mediate their functions. Among these proteins, one can find some purinergic receptors. The latter are membrane-bound, ligand-gated ion channel (P2X) and G protein-coupled Academic Editor: Javier receptor (GPCR) (P2Y) for extracellular nucleotides involved in purinergic signaling. Glial Conde Aranda cells release and respond to ATP and other purinergic molecules, released by injured cells Received: 15 January 2021 or secreted, under both physiological and pathological conditions. Purinergic signaling Accepted: 3 February 2021 plays an important role in regulating microglial activity. As microglia are involved in Published: 6 February 2021 neuroinflammation, increasing studies have described the role of microglial purinergic receptors in brain inflammatory processes. In this review, we focus on P2Y receptors and Publisher’s Note: MDPI stays neutral more specifically on P2RY12. After a brief description of purinergic receptor families, we with regard to jurisdictional claims in then review P2RY12 structure, expression and functions, especially the microglial one. We published maps and institutional affil- also summarize the pharmacological and genetic tools developed to study this purinergic iations. receptor. Then, we highlight the contribution of microglial P2RY12 to neuroinflammation in nervous system pathologies. 2. P2RY12 Copyright: © 2021 by the authors. 2.1. Purinergic Receptor Families Licensee MDPI, Basel, Switzerland. Purinergic signaling and receptors were first described in the 1970s [2,3]. Purinergic This article is an open access article signaling is involved in neurodevelopment and pathophysiological processes, such as cell distributed under the terms and proliferation, differentiation, neuron–glia cross-talking and inflammation [4]. conditions of the Creative Commons The purinergic receptor family is divided into two subfamilies, P1 and P2 receptors, Attribution (CC BY) license (https:// depending on their endogenous agonists [4,5]. On the one hand, the P1 receptor subfamily creativecommons.org/licenses/by/ is constituted of four subtypes that are metabotropic receptors sensitive to adenosine. On 4.0/). Int. J. Mol. Sci. 2021, 22, 1636. https://doi.org/10.3390/ijms22041636 https://www.mdpi.com/journal/ijms Int. J. Mol. Sci. 2021, 22, x FOR PEER REVIEW 2 of 16 The purinergic receptor family is divided into two subfamilies, P1 and P2 receptors, depending on their endogenous agonists [4,5]. On the one hand, the P1 receptor subfamily is constituted of four subtypes that are metabotropic receptors sensitive to adenosine. On the other hand, P2 receptors are activated by nucleoside di- and triphosphates (denosine Int. J. Mol. Sci. 2021 22 , , 1636 diphosphate (ADP) and ATP; uridine di-(UDP) and triphosphate (UTP); 2UDP-glucose) of 16 and classified as P2X and P2Y receptors. The P2X receptor group is composed of seven subtypes that are ATP-gated ion channels permeable to cations (Na+, K+ and Ca2+). P2Y thereceptors other hand, are P2metabotropic receptors are receptors activated bycoupled nucleoside with di- a andG-protein triphosphates that are (denosine activated by pu- diphosphate (ADP) and ATP; uridine di-(UDP) and triphosphate (UTP); UDP-glucose) and rines and pyrimidines [4,6]. P2RY12 is one of the eight members of P2Y receptor group ex- classified as P2X and P2Y receptors. The P2X receptor group is composed of seven subtypes thatpressed are ATP-gated by humans ion [7]. channels Indeed, permeable P2RY are to divided cations (Na+, into two K+ and subgroups Ca2+). P2Y based receptors on ligand bind- areing metabotropic and the selectivity receptors of coupledG-protein with binding. a G-protein The first that one are consists activated of by P2RY1, purines P2RY2, and P2RY4, pyrimidinesP2RY6 and [4 ,P2RY116]. P2RY12 that is oneare ofcoupled the eight with members Gq protein, of P2Y receptorwhich stimulates group expressed phospholipase by C humans(PLC), [resulting7]. Indeed, in P2RY calcium are dividedrelease from into two intrac subgroupsellular stores based and on ligand protein binding kinase and C (PKC) ac- thetivation. selectivity One of of G-protein these receptors, binding. TheP2RY11 first onecan consists also be ofcoupled P2RY1, with P2RY2, Gs P2RY4, protein, P2RY6 which stimu- andlates P2RY11 adenylate that are cyclase coupled (AC) with and Gq increases protein, which the production stimulates phospholipaseof cyclic adenosine C (PLC), monophos- resultingphate (cAMP). in calcium On release the contrary, from intracellular the members stores of and the protein second kinase subgroup, C (PKC) P2RY12, activation. P2RY13 and OneP2RY14, of these are receptors, coupled P2RY11with Gi canprotein, also beresult coupleding in with a decreased Gs protein, cAMP which production stimulates [6]. adenylate cyclase (AC) and increases the production of cyclic adenosine monophosphate (cAMP). On the contrary, the members of the second subgroup, P2RY12, P2RY13 and P2RY14,2.2. Structure are coupled with Gi protein, resulting in a decreased cAMP production [6]. P2RY12 is a Gi-coupled receptor containing 342 amino acids and with a molecular 2.2.weight Structure of 39 kilodaltons. Its structure is composed of seven hydrophobic transmembrane regionsP2RY12 of α is-helice a Gi-coupled connected receptor by three containing extra- 342and amino intracellular acids and loops, with and a molecular a carboxy-termi- weightnal helix of 39 VIII kilodaltons. that is parallel Its structure to the is composedmembrane of bilayer seven hydrophobic on the cytoplasmic transmembrane side (Figure 1) regions of α-helice connected by three extra- and intracellular loops, and a carboxy-terminal [8]. All P2Y receptors possess four cysteine residues (Cys 17, 97, 175, 270) at their extracel- helix VIII that is parallel to the membrane bilayer on the cytoplasmic side (Figure1)[ 8]. Alllular P2Y domain. receptors Regarding possess four P2RY12, cysteine residuesthese four (Cys cysteine 17, 97, 175, residues 270) at form their extracellulartwo disulfide bonds: domain.the first Regarding one between P2RY12, the N-terminal these four cysteine domain residues and the form third two extracellular disulfide bonds: loop, theand the sec- firstond one one between between the the N-terminal first and domain the second and the extracellular third extracellular loop. These loop, and disulfide the second bonds play a onekey between role in thethe first stimulation/inhibition and the second extracellular of this receptor. loop. These disulfide bonds play a key role in the stimulation/inhibition of this receptor. FigureFigure 1. Schematic1. Schematic secondary secondary structure structure of P2RY12. of P2RY12. P2RY12 P2RY12 is composed is composed of seven α -transmembraneof seven α-transmem- domainsbrane domains connected connected by three extra- by three and ex intracellulartra- and intracellular loops. There loops. are four Th extracellularere are four cysteinesextracellular cyste- (Cys)ines at (Cys) positions at positions 17, 97, 175, 17, and 97, 270175, that and form 270 twothat disulfide form two bonds. disulfide bonds. It is noteworthy that there is a marked difference between the agonist and the antag- It is noteworthy that there is a marked difference between the agonist and the antag- onist bound receptor structure. Indeed, when P2RY12 is activated, the receptor binding pocketonist isbound contracted, receptor suggesting structure. large-scale Indeed, rearrangements when P2RY12 in extracellular is activated, regions the receptor during binding the binding process.