International Journal of Molecular Sciences Review P2 Receptors in Cardiac Myocyte Pathophysiology and Mechanotransduction Sun-Hee Woo * and Tran Nguyet Trinh Laboratory of Pathophysiology, College of Pharmacy, Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon 34134, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-(0)42-821-5924 Abstract: ATP is a major energy source in the mammalian cells, but it is an extracellular chemical messenger acting on P2 purinergic receptors. A line of evidence has shown that ATP is released from many different types of cells including neurons, endothelial cells, and muscle cells. In this review, we described the distribution of P2 receptor subtypes in the cardiac cells and their physiological and pathological roles in the heart. So far, the effects of external application of ATP or its analogues, and those of UTP on cardiac contractility and rhythm have been reported. In addition, specific genetic alterations and pharmacological agonists and antagonists have been adopted to discover specific roles of P2 receptor subtypes including P2X4-, P2X7-, P2Y2- and P2Y6-receptors in cardiac cells under physiological and pathological conditions. Accumulated data suggest that P2X4 receptors may play a beneficial role in cardiac muscle function, and that P2Y2- and P2Y6-receptors can induce cardiac fibrosis. Recent evidence further demonstrates P2Y1 receptor and P2X4 receptor as important mechanical signaling molecules to alter membrane potential and Ca2+ signaling in atrial myocytes and their uneven expression profile between right and left atrium. Keywords: cardiac myocyte function; P2X receptors; P2Y receptors; extracellular ATP; mechanical signaling; pathohysiological roles Citation: Woo, S.-H.; Trinh, T.N. P2 1. Introduction Receptors in Cardiac Myocyte ATP has long been recognized as an intracellular energy source. ATP is now widely Pathophysiology and Mechanotransduction. Int. J. Mol. Sci. accepted as a key extracellular chemical messenger released from many cell types including 2021, 22, 251. https://doi.org/ neuronal cells, endothelial cells, muscle cells, and it significantly regulates different cell 10.3390/ijms22010251 functions via P2 purinergic receptors [1]. Extracellular ATP exerts several important effects in cardiac myocytes, such as negative and positive inotropic effects, negative or positive Received: 4 December 2020 chronotropic effects as well as antihypertrophic effects [2]. It is also known that ATP inhibits Accepted: 22 December 2020 glucose transport in the heart [3]. Cardiac cells from different heart regions and from Published: 29 December 2020 different species have different contexts of purinergic receptor subtypes. Such context of P2 receptor subtypes appears to determine ATP-mediated cellular responses, such as inotropy Publisher’s Note: MDPI stays neu- and chronotrophy. Use of transgenic and knock-out animals as well as pharmacological tral with regard to jurisdictional claims agonists and antagonists enabled understanding of specific function of each P2 receptor in published maps and institutional subtype in the heart under physiological and pathological conditions. The present review affiliations. concentrates on the effects of ATP on cardiac functions at the cellular levels and whole heart under physiological and pathological conditions and recent advances in discovering role of certain P2 receptors in atrial myocytes in mechanotransduction and Ca2+ regulation. Copyright: © 2020 by the authors. Li- 2. ATP as an Extracellular Chemical Messenger censee MDPI, Basel, Switzerland. This article is an open access article distributed Early studies have demonstrated ATP exocytosis using new bioluminescence methods under the terms and conditions of the with cell surface attached firefly luciferase [4,5], or atomic force microscopy [6]. Cellular Creative Commons Attribution (CC BY) ATP release can be detected by luciferin-luciferase assay at the multicellular levels [7–9], license (https://creativecommons.org/ and also by reporter cells at the single cell level [10,11]. Using a reporter cell expressing a licenses/by/4.0/). P2X receptor or P2Y receptor one can measure ATP release from nearby single target cells Int. J. Mol. Sci. 2021, 22, 251. https://doi.org/10.3390/ijms22010251 https://www.mdpi.com/journal/ijms Int. J. Mol. Sci. 2021, 22, 251 2 of 18 in real-time as P2X receptor currents or as P2Y receptor-mediated cytosolic Ca2+ increase that are activated by ATP [10–13]. In the mammalian cells there are multiple pathways of ATP release from intracellular space to external space. They include gap junction channels, connexins [9,11,14] and pannexins [8], cystic fibrosis transmembrane conductance regulator- linked pathway [15–21], maxi anion channels [12], volume-regulated Cl− channel [22], and exocytosis [23,24]. The ATP release processes in the cells are regulated by different types of stimuli including mechanical stimuli [9,25,26]. A line of evidence strongly support that ATP is a co-neurotransmitter in sympathetic nerves around the blood vessel [27]. In the atrial myocardium of the human heart, the nerve terminal varicosities form a dense network innervating the cardiac muscle, coming into close apposition with the cardiac myocytes [28]. In addition to neuron [29–31], cardiac myocytes [8,9,11,32–35], endothelial cells [36–38], smooth muscle cells [36,39,40], platelets [41–43], and other cell types have been shown to release ATP from cytosol to extracellular space under a stimulus via one or two type(s) of ATP release pathways. Extracellular ATP concentrations are thought to be about 1–40 nM and intracellular ATP concentrations are about 10 mM. In the coronary artery in the heart, the levels of ATP are physiologically very low (1 nM; [44]), mainly because ATP is rapidly degraded to ADP,AMP and adenosine by soluble and membrane bound ectonucleotidases (ecto-ATPases) [45,46]. However, in the interstitial fluid in the heart higher levels (40 nM) of ATP can be measured [47]. ATP level in the coronary artery in the heart significantly increases under electrical stimulation, application of cardiotonic agents [44,48–50], mechanical stretch [51], higher blood flow [48,52], high workload [53] and hypoxia/ischemia [32–35,41,47,50]. In addition, a line of evidence shows significant ATP release from cardiac ventricular and atrial myocytes under mechanical stresses. Stretch has been demonstrated using luciferin-luciferase assay to induce ATP release from ventricular myocytes of mouse heart through the pannexin-1 [7] and from atrial cells via pannexin-2 gap junction channels [8]. Recently, it has been shown using P2X7 receptor- expressing human embryonic kidney (HEK) 293 cells as a reporter that shear stress elicits immediate ATP release from isolated adult rat atrial myocytes, thereby inducing two different types of global Ca2+ waves [11]. It has also been shown that left atrial (LA) cells release ATP more than right atrial (RA) cells under the same shear force, and that the generations of different types of Ca2+ waves depend on P2 receptor subtypes (see below; [9,11]). Atrial ATP release under shear stress is known to be mediated by connexin 43 (Cx43) hemichannels [9]. 3. P2 Receptors in Cardiac Muscle and Their Pharmacological Properties Extracellular ATP and its analogs initiate large effects via cell surface P2 purinergic receptors in the cardiovascular system [1,2,53–57]. The P2 purinergic receptors are further divided in P2X ionotropic receptors and metabotropic P2Y receptors [58–63]. The P2X receptors are ligand-gated channels made of proteins with 379–472 amino acids and have two transmembrane domains with a large extracellular loop [64]. These receptors share a trimer topology, and they can assemble as both homomeric and heteromeric trimers of two transmembrane domain subunits that form non-selective cation channels [65]. P2X receptors open in response to micromolar ATP binding, resulting in the flow of cations such as Na+,K+, and Ca2+ across the cell membrane (see review [2]). There are seven P2X receptor subtypes (P2X receptor-1, -2, -3, -4, -5, -6, and -7) expressed in mammalian tissues [56,57]. Specialized functions are achieved by different P2X receptor subtypes in different cell types depending on the subtype expression profile, subcellular distributions, and their biophysical properties [2,56,57]. The P2Y receptors are G-protein coupled receptors, which, in turn, activates intra- cellular second messenger systems to modulate the physiological function of the cells. In addition to ATP and ADP, P2Y receptors bind to the pyrimidines UTP and UDP. A group of P2Y receptor subtypes (P2Y receptor-1, -2, -4, -6, and -11) are metabotropic receptors mainly coupled with Gq proteins to stimulate phospholipase C (PLC)-β followed by inosi- tol 1,4,5-trisphosphate (IP3) generation from phosphatidylinositol 4,5-bisphosphate (PIP2) and Ca2+ mobilization from intracellular stores [63,66]. In particular, the P2Y11 receptor Int. J. Mol. Sci. 2021, 22, 251 3 of 18 only additionally activate adenylate cyclase [63]. Remaining P2Y receptors, P2Y receptor- 12, -13, and -14 are coupled to Gi proteins that inhibit adenylate cyclase followed by a reduction of cAMP level in the cytosol [63,67]. Studies have demonstrated several subtypes of P2X receptors in the heart [61,68–70]. Early study has shown using immunohistochemical methods that P2X1 receptors in the heart localized to cardiac myocytes [71]. Among the seven subtypes of P2X receptors P2X4 receptors have been shown