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  • Perspective

    GABA-Activated Ligand Gated Ion Channels: Medicinal Chemistry andMolecular Biology

    Mary Chebib and Graham A. R. Johnston*

    Adrien Albert Laboratory of Medicinal Chemistry, Department of Pharmacology, University of Sydney, Sydney,New South Wales 2006, Australia

    Received August 24, 1999

    1. Introduction

    GABA (1, 4-aminobutanoic acid, -aminobutyric acid)is the major inhibitory neurotransmitter in the brainand is essential for the overall balance between neu-ronal excitation and inhibition. GABA influences neu-rons via a large number of receptor subtypes which aregrouped on the basis of their pharmacology under threemajor classes of receptors: GABAA, GABAB, and GABACreceptors. GABAA and GABAC receptors are ligandgated ion channels, while GABAB receptors are G-protein coupled receptors. This Perspective comparesand contrasts aspects of the medicinal chemistry andmolecular biology of GABAA and GABAC receptors. Themolecular diversity of these ligand gated ion channelsrepresents important challenges for medicinal chemistsin the design of subunit-specific therapeutic agents.

    GABAA receptors were reviewed in the Journal ofMedicinal Chemistry by Krogsgaard-Larsen and col-leagues1 in 1994 in a Perspective titled GABAA Recep-tor Agonists, Partial Agonists, and Antagonists: Designand Therapeutic Prospects. There continues to beextensive investigations of GABAA receptors, particu-larly with respect to their modulation by importantdrugs, such as the benzodiazepines, ethanol, generalanesthetics, barbiturates, and a wide range of chemi-cally diverse substances. Johnston2 has described GABAAreceptors as the most complicated of the superfamilyof ligand-gated ion channels in terms of the largenumber of receptor subtypes and also the variety of

    ligands that interact with specific sites on the receptors.There are many excellent, extensive reviews on aspectsof GABAA receptors.1,3-10 GABAC receptors appear tobe much simpler than GABAA receptors, but they havenot been studied anywhere nearly as intensively asGABAA receptors.11-16

    1.1. Ligand Gated Ion Channels. Ligand gated ionchannels (LGICs) mediate fast synaptic transmissionvia the movement of ions through channels gated byneurotransmitters such as acetylcholine and GABA.These receptors are very different from G-protein coupledreceptors (GPCRs) that were the subject of a recentPerspective in the Journal of Medicinal Chemistry.17The nicotinicoid superfamily of LGICs encompassesseveral families of receptors including nicotinic acetyl-choline receptors, 5-HT3 receptors, GABAA receptors,GABAC receptors, strychnine-sensitive glycine receptors,and some invertebrate anionic glutamate receptors.Other superfamilies of LGICs include the excitatoryglutamate receptors (with subfamilies of NMDA, AMPA,and kainate receptors) and the ATP receptors (whichcurrently consist of a single family). From the nicotini-coid superfamily of LGICs, the most studied and bestunderstood of this class of receptors is the nicotinicacetylcholine receptors, while the GABAA receptorsappear to be the most complex and the GABAC receptorsthe simplest.18

    Members of the nicotinicoid superfamily are consid-ered to be pentamers composed of protein subunitslikely to have been derived from a common ancestor.18-20As illustrated in Figure 1A, each subunit has a largeextracellular N-terminal domain which incorporates

    * To whom correspondence should be addressed. Tel: 61 2 93516117. Fax: 61 2 9351 3868. E-mail: grahamj@mail.usyd.edu.au.

    Copyright 2000 by the American Chemical Society

    Volume 43, Number 8 April 20, 2000

    10.1021/jm9904349 CCC: $19.00 2000 American Chemical SocietyPublished on Web 03/30/2000

  • part of the agonist/antagonist-binding site, followed bythree membrane spanning domains (M1-3), an intra-cellular loop of variable length and a fourth membranespanning domain (M4), and the C-terminal end beingextracellular. Each subunit arranges itself such that thesecond membrane spanning domain (M2) forms the wallof the channel pore (Figure 1B) and the overall chargeof the domain determines whether the channel conductsanions or cations. Both GABAA and GABAC receptorsare GABA-gated chloride ion channels causing inhibi-tion of neuronal firing, with GABAA receptors beingheteromeric, i.e., composed of different subunits (e.g. R1,2, and 2 subunits as illustrated in Figure 1C) andGABAC receptors being homomeric (e.g. composed ex-clusively of F1 subunits, Figure 1D). The cytoplasmicloop (Figure 1A), between the third and fourth trans-membrane domains (M3 and M4), is believed to be thetarget for protein kinases, required for subcellulartargeting and membrane clustering of the receptor.

    The hierarchy within the nicotinicoid superfamily ofLGICs as described by Le Novere and Changeux18 is asfollows: the first rank is the family defined by theendogenous ligand responsible for channel opening; the

    second rank is the subfamily defined by the ability toform endogenous receptors together; and the last rankis the tribe. In the present context of the GABA-activated LGIC family, GABAA and GABAC receptorsare subfamilies, while the tribes are the protein sub-units that show substantial sequence identity, e.g., thesix R subunits, the four subunits, etc., and the threeF subunits.

    The optimized alignment of the human GABAA 1 andGABAC F1 amino acid sequences from Le Novere andChangeux18 shows overall 172 matching amino acidsbetween these proteins equating to an optimized se-quence identity of 36%. Similar matching betweenhuman GABAA R1 and human GABAC F1 sequencesshows significantly less sequence identity: overall 128matching amino acids, 28% optimized sequence identity,about 50% identity in the transmembrane spanningdomains, and 44% identity in a 71-amino acid sequenceof the N-terminal extracellular domain.

    1.2. Recombinant Receptors: ComplementaryStructure-Activity Studies. The ability to expressfunctional receptors composed of proteins of knownamino acid sequence in suitable biological cells, e.g., frog

    Figure 1. Representations of GABA-activated ligand gated ion channels: A, general structure of one of the five protein subunitsshowing four membrane spanning regions; B, pentameric arrangement of the protein subunits showing the second membranespanning region lining the pore of the ion channel; C, heteromeric makeup of a GABAA receptor with two R1, two 2, and one 2protein subunits; and D, homomeric makeup of a GABAC receptor with five F1 protein subunits.

    1428 Journal of Medicinal Chemistry, 2000, Vol. 43, No. 8 Perspective

  • oocytes or human embryonic kidney (HEK) cells, thatdo not normally express such receptors has revolution-ized our understanding of LGICs. Cloning studies haveyielded a variety of sequence-related nucleotides thatcode for a variety of closely related membrane-boundLGIC receptors. When injected into appropriate cells,these cells will express functional LGIC recombinantreceptors that can be studied by conventional electro-physiological recording techniques, e.g., two-electrodevoltage clamp studies, or by radioligand binding studies.This technology enables structure-activity studies ona series of related compounds to be carried out on clearlydefined receptor subtypes.

    The native nucleotides coding for these receptors maybe subjected to site-directed mutagenesis or cutting andsplicing techniques to produce mutants or chimeras. Oninjection into appropriate cells, these nonnative nucleo-tides may produce mutant or chimeric receptors thathave different functional properties to the native recep-tors. This technology thus enables systematic variationof the receptors themselves and can be used to delineatewhich amino acid residues are responsible for thepharmacological and physiological properties of thereceptors.

    Structure-activity relationship (SAR) studies involv-ing structural variations of both the ligands and theirreceptor targets (termed complementary structure-activity studies)21 play important roles in drug develop-ment, being vital to the discovery of drugs that interactselectively with particular native and mutant receptorsubtypes. Inheritable mutations in the nicotinicoidsuperfamily of LGICs, including GABAA receptors, havebeen identified.22

    1.3. How GABA Activates LGICs. Ligand bindingto LGICs has been generally considered to be diffusion-limited, i.e., fast, with binding affinity being primarilydetermined by the rate of ligand dissociation. Thisappears to be the case for nicotinic LGICs where thebinding of acetylcholine is almost as fast as thatpredicted if the rate-limiting step is the diffusion ofacetylcholine into its binding site. Two recent studieson GABA-activated LGICs suggest, however, that GABAbinds orders of magnitude more slowly to GABAA andGABAC receptors than expected for relatively freediffusion.

    Jones et al.23 used patch clamping of rat hippocampalneurons in culture to study what defines ligand affinityat these native GABAA receptors. Their results indicatethat a ligand-specific energy barrier between unboundand bound states determines GABAA receptor selectiv-ity. They quantitatively model this barrier by requiringthe participation of movable elements within a flexiblebinding site. Their flexible binding-site model envisagesthe binding site behaving as a pair of mobile armsattached to fixed anchor sites by spring-like tethers.Binding is diffusion-limited only if the ligand is longenough to span the distance between these wells.Shorter agonists bind more slowly because the armsmust move to accommodate them, which requiresactivation energy. The model explicitly requires theligand to perform thermodynamic work and implies thatonly nondiffusion-limited ligands can be agonists be-cause otherwise they cause no movement of the receptor.Successful agonist binding involves the coordinated

    motion of separate parts of the re