Ligand-Gated Receptor-Ion Channels (2023-24) PDF

WEEK 30 MPharm Programme Receptor Superfamilies Ligand-Gated Ion Channels Dr Gabriel Boachie-Ansah [email protected] Dale 113 ext. 2617 MPharm PHA112 Receptor Superfamilies - LGICs WEEK Receptor Superfamilies 30 Receptor Superfamily A group of receptors with a similar basic mole...

WEEK 30 MPharm Programme Receptor Superfamilies Ligand-Gated Ion Channels Dr Gabriel Boachie-Ansah [email protected] Dale 113 ext. 2617 MPharm PHA112 Receptor Superfamilies - LGICs WEEK Receptor Superfamilies 30 Receptor Superfamily A group of receptors with a similar basic molecular structure and that use the same signal transduction pathway 4 Major Receptor Superfamilies Ligand-gated / Ion channel linked receptors G-protein-coupled receptors Kinase-linked receptors Intracellular / Nuclear receptors Slide 2 of 39 MPharm PHA112 Receptor Superfamilies - LGICs WEEK 30 Receptor Superfamilies  Slide 3 of 39 MPharm PHA112 Receptor Superfamilies - LGICs WEEK 30 Ligand-Gated Ion Channels (LGICs) (Ionotropic Receptors) Slide 4 of 39 MPharm PHA112 Receptor Superfamilies - LGICs WEEK Outline of Lecture 30 Introduction to ligand-gated ion channel superfamily Subclassification of ligand-gated ion channels Key characteristics of the major ligand-gated ion channel families Molecular structure, subtypes, cellular expression and functional role of nicotinic acetylcholine receptors (nAChRs) Molecular structure, subcellular expression and functional role of GABAA receptors (GABAARs) Slide 5 of 39 MPharm PHA112 Receptor Superfamilies - LGICs Learning Outcomes WEEK 30 At the end of this lecture, you should be able to: Describe the unique features/characteristics of the ligand-gated ion channel superfamily Describe the basis for the subclassification of ligandgated ion channels Describe the key characteristic features of the major ligand-gated ion channel families Describe the molecular structure, subtypes, cellular expression & functional role of nicotinic acetylcholine receptors (nAChRs) Describe the molecular structure, subcellular expression & functional role of GABAA receptors (GABAARs) Slide 6 of 39 MPharm PHA112 Receptor Superfamilies - LGICs WEEK 30 Ligand-Gated Ion Channels (LGICs) Diverse superfamily of multimeric integral membrane proteins that incorporate extracellular ligand-binding sites and a central transmembrane ion-permeable channel/pore Binding of the ligand (neurotransmitter)  conformational change in receptor protein  opening of the ion-permeable pore  flow of permeant ions into/out of the cell down their electrochemical gradient Ion flow through the activated channel  modulation of cellular function LGICs involved in a wide range of important biological functions – e.g. synaptic transmission & neural communication, cell excitation, muscle contraction and intracellular signalling Slide 7 of 39 MPharm PHA112 Receptor Superfamilies - LGICs WEEK 30 Signal Transduction via LGICs Slide 8 of 39 MPharm PHA112 Receptor Superfamilies - LGICs WEEK 30 Ligand-Gated Ion Channels (LGICs) Receptor Subclassification Based on the effect on membrane potential & cellular electrical activity induced by ion flow through the open channel Excitatory, cation-selective LGICs – nAChR, 5-HT3R, ionotropic glutamate (NMDA, AMPA & kainate) receptors & P2X receptors Mediate influx of cations  membrane depolarisation Inhibitory, anion-selective LGICs – GABAAR & glycine receptors Mediate influx of anions  membrane hyperpolarisation Slide 9 of 39 MPharm PHA112 Receptor Superfamilies - LGICs WEEK 30 Ligand-Gated Ion Channels Inhibitory (anion-selective) & Excitatory (cation-selective) Ion Channels Slide 10 of 39 MPharm PHA112 Receptor Superfamilies - LGICs WEEK 30 Ligand-Gated Ion Channels (LGICs) Receptor Subclassification Based on molecular structure & subunit stoichiometry Cys-loop Receptor Family Pentameric – composed of 5 subunits nAChR, 5-HT3R, GABAAR, glycine receptors Ionotropic Glutamate Receptor Family Tetrameric – composed of 4 subunits NMDA, AMPA & kainate receptors P2X Receptor Family Trimeric – composed of 3 subunits P2X1, P2X2, P2X3, P2X4, P2X5 & P2X7 receptors Slide 11 of 39 MPharm PHA112 Receptor Superfamilies - LGICs WEEK 30 Ligand-Gated Ion Channels (LGICs) Receptor Subclassification Based on Molecular Architecture Slide 12 of 39 MPharm PHA112 Receptor Superfamilies - LGICs WEEK 30 Ligand-Gated Ion Channels (LGICs) Receptor Subclassification Cys-loop Receptor Family Functional receptors are composed of 5 subunits arranged around a central ion-conducting pore Subunits share a common structure consisting of a large, N-terminal extracellular domain – contains the ligand-binding site & the signature disulphide Cys-loop 4 transmembrane α-helices (M1-M4) that line the ion conducting pore a short extracellular C-terminal domain Examples: nAChR, 5-HT3R, GABAAR, Glycine receptors Slide 13 of 39 MPharm PHA112 Receptor Superfamilies - LGICs Cys-loop Ion Channel WEEK 30 Subunit Structure & Membrane Topology Slide 14 of 39 MPharm PHA112 Receptor Superfamilies - LGICs Cys-loop Ion Channel WEEK 30 Subunit Structure & Membrane Topology Slide 15 of 39 MPharm PHA112 Receptor Superfamilies - LGICs WEEK 30 Ligand-Gated Ion Channels (LGICs) Receptor Subclassification Ionotropic Glutamate Receptor Family Functional receptors are composed of 4 subunits arranged around a central ion-conducting pore Each subunit is an integral membrane protein with A large extracellular N-terminal domain 3 membrane-spanning domains (M1, M3 and M4) a hydrophobic hairpin domain (M2) that forms/lines the channel pore 3 intracellular domains: Loop1, Loop2 & the carboxyl tail Non-selective cation channels gated by glutamate Slide 16 of 39 MPharm PHA112 Receptor Superfamilies - LGICs WEEK 30 Ionotropic Glutamate Receptor (IGluR) Subunit Structure & Membrane Topology Slide 17 of 39 MPharm PHA112 Receptor Superfamilies - LGICs WEEK 30 Ionotropic Glutamate Receptor (IGluR) Subunit Structure & Membrane Topology Slide 18 of 39 MPharm PHA112 Receptor Superfamilies - LGICs WEEK 30 Ligand-Gated Ion Channels (LGICs) Receptor Subclassification P2X Receptor Family Functional receptors are composed of 3 subunits arranged around a central ion-conducting pore Each subunit is an integral membrane protein with 2 transmembrane domains (TM1 & TM2) separated by a large extracellular ligand-binding domain (‘ectodomain’) intracellular amino (N-) & carboxyl (C-) termini 7 receptor subunits coded in mammals (P2X1-7) – that co-assemble to form homo- or hetero-trimeric channels Non-selective cation channels gated by extracellular ATP Slide 19 of 39 MPharm PHA112 Receptor Superfamilies - LGICs WEEK 30 Slide 20 of 39 P2X Receptor – Membrane Topology MPharm PHA112 Receptor Superfamilies - LGICs WEEK 30 Ligand-Gated Ion Channels The Nicotinic Acetylcholine Receptor (nAChR) Slide 21 of 39 MPharm PHA112 Receptor Superfamilies - LGICs WEEK 30 Nicotinic Acetylcholine Receptor (nAChR) Prototypical Cys-loop ligand-gated receptors – activated endogenously by the neurotransmitter, acetylcholine Also activated by nicotine – hence, the name, nicotinic AChR Occur largely in the central & peripheral nervous system – mediate fast synaptic transmission in the nervous system & at the somatic neuromuscular junction (NMJ) Occur pre- and post-synaptically Binding of ACh  channel opening   influx of Na+ ions  membrane depolarisation   influx of Ca++ & neurotransmitter release (CNS), or fast post-synaptic excitation Play key roles in control of skeletal muscle contraction, and modulation of neuronal excitability & integration, gene expression, learning & memory formation, neuroprotection Slide 22 of 39 MPharm PHA112 Receptor Superfamilies - LGICs Nicotinic Acetylcholine Receptors WEEK 30 Molecular Structure & Function 2 distinct receptor subtypes exist Muscle (NM) subtype – occurs at neuromuscular junction (NMJ) Neuronal (NN) subtype – occurs at autonomic ganglia & in the CNS Both are pentameric complexes – composed of 5 subunits 17 nAChR subunits cloned: 1-α10, 1-4, , , and  Subunits co-assemble to form functional homo- or heteropentameric receptors Muscle (NM) subtype typically comprises 21, 1,  & / subunits Neuronal (NN) subtype typically comprises 2 & 3 subunits (hetero-pentamers) or 5α7 subunits (homo-pentamers) Slide 23 of 39 MPharm PHA112 Receptor Superfamilies - LGICs WEEK 30 Structure of Muscle Type (NM) nAChR M2 α-helices from all 5 subunits form & line the ion conducting pore There are 2 ACh binding sites - at the interface between the two α subunits and neighbouring  & / subunits Both must bind ACh molecules for the receptor to be activated Slide 24 of 39 MPharm PHA112 Receptor Superfamilies - LGICs WEEK 30 Structure of Neuronal Type (NN) nAChR Slide 25 of 39 MPharm PHA112 Receptor Superfamilies - LGICs WEEK Functional Roles of nAChRs 30 Autonomic Nervous System (PNS) Muscle (NM) subtype occurs at neuromuscular junction (NMJ) – mediates fast excitatory synaptic transmission  skeletal muscle contraction Neuronal (NN) subtype occurs post-synaptically at autonomic ganglia – mediates fast excitatory synaptic transmission & autonomic control of peripheral organs, e.g. CV & GI systems Central Nervous System (CNS) Neuronal (NN) subtype occurs widely – pre- & post-synaptically Key functional roles include: modulation of transmitter release, neuronal excitability & integration, gene expression, differentiation & survival cognition, learning & memory formation and neuroprotection Slide 26 of 39 MPharm PHA112 Receptor Superfamilies - LGICs WEEK 30 Role of Muscle Type (NM) nAChRs in Skeletal Muscle Contraction Slide 27 of 39 MPharm PHA112 Receptor Superfamilies - LGICs WEEK 30 Role of Muscle Type (NM) nAChRs in Skeletal Muscle Contraction Slide 28 of 39 MPharm PHA112 Receptor Superfamilies - LGICs WEEK 30 Nicotinic Acetylcholine Receptor Subtypes Tissue Distribution, Function & Pharmacology Slide 29 of 39 MPharm PHA112 Receptor Superfamilies - LGICs WEEK 30 Ligand-Gated Ion Channels The GABAA Receptor Slide 30 of 39 MPharm PHA112 Receptor Superfamilies - LGICs GABAA Receptors WEEK 30 A class of Cys-loop ligand-gated receptors that are activated endogenously by the amino acid transmitter, -aminobutyric acid (GABA) Binding of GABA  channel opening   influx of Cl- ions  membrane hyperpolarisation  fast post-synaptic inhibition Gating is also allosterically modulated by a host of drugs & chemicals – benzodiazepines, barbiturates, neurosteroids, volatile & intravenous anaesthetics, alcohol, etc Occur widely in the CNS, and mediate fast (phasic) and tonic post-synaptic inhibition of neuronal activity Play a key role in mediating the actions of GABA as the main inhibitory transmitter in the brain, and in mediating the actions of many anxiolytic, antiepileptic & general anaesthetic drugs Slide 31 of 39 MPharm PHA112 Receptor Superfamilies - LGICs The GABAA Receptors WEEK 30 Molecular Structure & Function Receptor proteins are pentamers – composed 5 subunits 8 subunit classes cloned: 1-6, 1-3, 1-3, , , ,  and 1-3 Subunits co-assemble to form functional homo- or heteropentameric receptors Functional receptors are generally formed from two α-subunits, two -subunits, and either a - or -subunit (2α/2/1 or ) Subunit composition is related to synaptic or extrasynaptic location of receptor Two GABA binding sites located at the α/ subunit interfaces Slide 32 of 39 MPharm PHA112 Receptor Superfamilies - LGICs WEEK 30 Structure & Subunit Composition of GABAARs Slide 33 of 39 MPharm PHA112 Receptor Superfamilies - LGICs The GABAA Receptors WEEK 30 Localisation, Subtypes & Function Synaptic receptors Occur post-synaptically at synaptic sites (within the synaptic cleft) Typically contain a -subunit and α1, α2 or α3 subunits Activated by synaptically released GABA & mediate phasic inhibition Have lower affinity for GABA & high benzodiazepine sensitivity Extrasynaptic receptors Occur post-synaptically, but outside synaptic sites Typically contain a -subunit and α4, α5 or α6 subunits Activated by GABA spillover & mediate tonic inhibition Have higher affinity for GABA & low benzodiazepine sensitivity Preferentially modulated by propofol, neurosteroids, ethanol Slide 34 of 39 MPharm PHA112 Receptor Superfamilies - LGICs WEEK 30 The GABAA Receptor Localisation Synaptic & Extrasynaptic Slide 35 of 39 MPharm PHA112 Receptor Superfamilies - LGICs WEEK GABAA Receptors 30 Cellular Distribution & Functional Roles widespread distribution in the brain – high densities in cerebral cortex, hippocampus & cerebellum CNS effects & possible functional roles the major inhibitory receptor in CNS mediate fast (phasic) & slow (tonic) neuronal inhibition involved in control of feeding behaviour, circadian rhythm, sleep-wake cycle, vigilance, learning & memory implicated in anxiety/sleep disorders, depression, mania, autism, psychosis, alcoholism Slide 36 of 39 MPharm PHA112 Receptor Superfamilies - LGICs GABAA Receptors As Drug Targets WEEK 30 Allosteric Modulation GABAARs have multiple ligand binding sites, and are major targets for a large class of therapeutic & recreational drugs Targeting drugs include benzodiazepines, barbiturates, neurosteroids, general anaesthetics, and alcohol They bind to specific binding sites on GABAARs (allosteric binding sites) and enhance/potentiate the effects of the endogenous agonist, GABA (positive allosteric modulation) Binding to allosteric site  conformational change   affinity for GABA   GABA binding   channel opening & Cl- influx   membrane hyperpolarization & neuronal inhibition Effect is typically reflected as increased frequency and/or duration of opening of the receptor ion (Cl-) channel   Cl- influx Slide 37 of 39 MPharm PHA112 Receptor Superfamilies - LGICs WEEK 30 Slide 38 of 39 MPharm PHA112 Receptor Superfamilies - LGICs WEEK 30 Allosteric Modulation of GABAAR Potentiation By Benzodiazepines Slide 39 of 39 MPharm PHA112 Receptor Superfamilies - LGICs