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`P H A R M A C O C H E M I S T R Y
`LIBRARY - V O L U M E 31
`
`RECEPTOR CHEMISTRY
`RECEPTOR CHEMISTRY
`TOWARDS THE THIRD MILLENNIUM
`TOWARDS THE THIRD MILLENNIUM
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`PHARMACOCHEMISTRY LIBRARY
`Editor: H. Timmerman
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`Volume 31
`
`RECEPTOR CHEMISTRY
`TOWARDS THE THIRD MILLENNIUM
`Proceedings of the 12th Camerino-Noordwijkerhout Symposium
`Camerino, Italy, 5-9 September 1999
`
`Edited by
`Ugo Gulini, Mario Gianella, Wilma Quaglia
`and Gabriella Marucci
`Department of Chemical Sciences, University of Camerino
`1 Via San Agostino, 62032 Camerino (MC), Italy
`
`2000
`
`ELSEVIER
`Amsterdam - Lausanne - New York- Oxford - Shannon - Singapore - Tokyo
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`ELSEVIER SCIENCE B.V.
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`ELSEVIER
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`Pharmaceutica Acta Helvetiae 74 (2000) v
`
`Preface
`
`PHARMACEUTICA
`ACTA HELVETIAE
`
`www.elsevier.com/locate/pharmactahelv
`
`Despite Langley's brilliant intuition way back in 1878,
`it was still a long time before the receptor could be called
`a reality; in fact, even in the early 1960s De Jongh
`described this molecule as a woman whose lovely seduc-
`tive image could be deduced only from the type of answer
`received to the many "messages" sent to her. Thus, when
`we met together for the first time in Camerino back in
`1978, we were rightly inspired by that enthusiasm typical
`of pioneers attracted by the fascination of a discipline still
`all to be discovered.
`Over the last twenty years our joints may well have
`begun to creak due to increasing age, but our enthusiasm
`has by no whit been dulled; indeed, even if we now know
`almost all there is to be known about the way ligands
`"mate" with receptors, which have by now been isolated,
`characterized, and cloned, many other secrets still remain
`to tease our curiosity. In particular, differentiation into
`distinct subpopulations and the multiplicity of transduction
`processes seem to offer us unhoped for, and even more
`specific targets in our search for new drugs. And so, that
`
`dream of Ehrlich in 1908 to design for each individual
`pathology a highly selective "charmed bullet", and thus
`one with reduced toxicity, now appears increasingly less
`utopistic.
`It is with these ideas that the 12th Camerino-Noord-
`wijkerhout Symposium has seen us into the third millen-
`nium with an awareness that the only success for convert-
`ing our hopes into reality is a multidisciplinary study based
`on a wakeful and critical comparison between experiences
`that have been reaching maturity through different ap-
`proaches to the problematic - as has always been the case
`in our Symposia.
`
`Ugo Gulini
`Mario Giannella
`Gabriella Marucci
`Wilma Quaglia
`
`Guest Editors
`
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`
`PHARMACOCHEMISTRY LIBRARY
`
`ADVISORY BOARD
`
`T. Fujita
`E. Mutschler
`N.J. de Souza
`F.J. Zeelen
`
`Department of Agricultural Chemistry, Kyoto University, Kyoto, Japan
`Department of Pharmacology, University of Frankfurt, Frankfurt, Germany
`Research Centre, Wookhardt Centre, Bombay, India
`Heesch, The Netherlands
`
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`
`Vol. 74 Nos. 2-3 PHA1LMACEUTICA ACTA HELVETIAE
`
`March 2000
`
`CONTENTS
`
`Contents list~Abstracts published in: Chemical Abstracts, Excerpta Medica, Intemational Pharmaceutical Abstracts
`
`Special Issue: Receptor Chemistry Towards the Third Millenium, Proceedings of the 12 th Camerino-Noordwijkerhout Symposium,
`Camerino, Italy, 5-9 September 1999
`
`Preface
`Acknowledgements
`Pharmacological receptors: a century of discovery- and more
`D.J. Triggle
`Cholinergic receptors and neurodegenerative diseases
`F. Gualtieri
`Nicotinic systems in central nervous systems disease: degenerative disorders and beyond
`P.A. Newhouse and M. Kelton
`Central nicotinic receptor ligands and pharmacophores
`R.A. Glennon and M. Dukat
`Structural aspects of high affinity ligands for the 0~4132 neuronal nicotinic receptor
`M.J. Dart, J.T. Wasicak, K.B. Ryther, M.R. Schrimpf, K.H. Kim, D.J. Anderson, J.P. Sullivan, M.D. Meyer
`Recombinant human receptors and functional assays in the discovery of altinicline (SIB-1508Y), a novel acetylcholine-gated ion
`channel (nAChR) agonist
`N.D.P. Cosford, L. Bleicher, J.-M. Vernier, L. Chavez-Noriega, T.S. Rao, R.S. Siegel, C. Suto, M. Washburn, G.K. Lloyd,
`I.A. McDonald
`Receptors in neurodegenerative diseases, muscarinic cholinergic receptors
`P. Angeli
`Design and development of selective muscarinic agonists for the treatment of Alzheimer's disease: characterization of tetrahydropyrimi-
`dine derivatives and development of new approaches for improved affinity and selectivity for M 1 receptors
`W.S. MesserJr., W.G. Rajeswaran, Y. Cao, H.-J. Zhang, A.A. EI-Assadi, C. Dockery, J. Liske, J. O'Brien, F.E. Williams,
`X.-P. Huang, M.E. Wroblewski, P.I. Nagy, S.M. Peseckis
`C1-1017, a functionally Ml-selective muscarinic agonist: design, synthesis, and preclinical pharmacology
`H. Tecle, R.D. Schwarz, S.D. Barrett, M.J. Cailahan, B.W. Caprathe, R.E. Davis, P. Doyle, M. Emmerling, D.J. Lauffer,
`T. Mirzadegan, D.W. Moreland, W. Lipiniski, C. Nelson, C. Raby, C. Spencer, K. Spiegel, A.J. Thomas, J.C. Jaen
`Ligands for the common allosteric site of acetylcholine M2-receptors: development and application
`U. Holzgrabe, W. Bender, H.M. Botero Cid, M. Staudt, R. Pick, C. Pfletschinger, E. Balatkov~, C. Tr&nkle, K. Mohr
`Receptors in cardiovascular disease: review and introduction
`A. Leonardi, G. Sironi, G. Motta
`Adrenoceptor subclassification: an approach to improved cardiovascular therapeutics
`J.P. Hieble
`The oLla and O~b-adrenergic receptor subtypes: molecular mechanisms of receptor activation and of drug action
`S. Cotecchia, O. Rossier, F. Fanelli, A. Leonardi, P.G. De Benedetti
`oL~-Adrenoreceptor antagonists bearing a quinazoline or a benzodioxane moiety
`C. Melchiorre, P. Angeli, M.L. Bolognesi, A. ChiarinL D. Giardin&, U. GulinL A. Leonardi, G. Marucci, A. Minarini, M. Pigini,
`W. Quaglia, M. Rosini, V. Tumiatti
`Selection, design and evaluation of new radioligands for PET studies of cardiac adrenoceptors
`V.W. Pike, M.P. Law, S. Osman, R.J. Davenport, O. Rimoldi, D. Giardin~, P.G. Camici
`Enigmatic receptors
`L. Brasili
`Imidazoline receptors: a challenge
`P. Bousquet, V. Bruban, S. Schann, J. Feldman
`
`v
`vi
`
`79
`
`85
`
`91
`
`103
`
`115
`
`125
`
`131
`
`135
`
`141
`
`149
`
`157
`
`163
`
`173
`
`181
`
`191
`
`201
`
`205
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`Sigma receptors: recent advances and new clinical potentials
`W.D. Bowen
`Excitatory amino acid receptors
`G. Gaviraghi
`Excitatory amino acid agonists and antagonists: pharmacology and therapeutic applications
`D. G. Trist
`Metabotropic glutamate receptors: a structural view point
`R. Pellicciari, G. Costantino, A. Macchiarulo
`Synthesis and pharmacological properties of novel glycine antagonists
`D. Donati and R. Di Fabio
`Receptors in neurodegenerative diseases
`W. Froestl
`Neurotrophin receptor structure and interactions
`H. Yano and M.V. Chao
`The RET receptor tyrosine kinase: activation, signalling and significance in neural development and disease
`L Mason
`The ciliary neurotrophic factor and its receptor, CNTFRo~
`M.W. Sleeman, K.D. Anderson, P.D. Lambert, G.D. Yancopoulos, S.J. Wiegand
`Rediscovering good old friend IGF-I in the new millenium: possible usefulness in Alzheimer's disease and stroke
`S. Dor#, S. Kar, W.-H. Zheng, R. Quirion
`Apoptosis induced by death receptors
`P. Schneider and J. Tschopp
`Hijacked receptors
`D.J. Triggle
`ICAM-1 receptors and cold viruses
`J. Bella and M.G. Rossmann
`Viral-encoded G-protein coupled receptors: new targets for drug research?
`M.J. Smit, H. Timmerman, D. Verzijl, R. Leurs
`Chemokine receptors: interaction with HIV-1 and viral-encoded chemokines
`S. SozzanL P. Allavena, A. Vecchi, J. Van Damme, A. Mantovani
`General topics and perspectives
`H. Timmerman
`Pharmacological evidence of muscarinic receptor heterodimerization
`S. Chiacchio, M. Scarselli, M. Armogida, R. Maggio
`Constitutive activity of G protein coupled receptors and drug action
`R. Leurs, M.S.R. Pena, R.A. Bakker, A.E. Aiewijnse, H. Timmerman
`New dimensions in G protein signalling: G135 and the RGS proteins
`W.F. Simonds and J.-H. Zhang
`Kappa opioid agonists as targets for pharmacotherapies in cocaine abuse
`J.L. Neumeyer, N.K. Mello, S. Stevens Negus, J.M. Bidlack
`Author Index
`Keyword Index
`
`211
`
`219
`
`221
`
`231
`
`239
`
`247
`
`253
`
`261
`
`265
`
`273
`
`281
`
`287
`
`291
`
`299
`
`305
`
`313
`
`315
`
`327
`
`333
`
`337
`345
`347
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`
`E L S E V I E R
`
`Pharmaceutica Acta Helvetiae 74 (2000) 79-84
`
`PHARMAGEUTIGA
`ACTA HELVETIAE
`
`www.elsevier.com/locate/pharmactahelv
`
`Pharmacological receptors" a century of discovery
`
`and more
`
`D a v i d J. Triggle *
`The Graduate School, 562 Capen Hall, State University of New York, Buffalo, NY 14260, USA
`
`Abstract
`
`A brief survey of the history of the development of the concept of the pharmacological receptor is presented. From the pioneering
`concepts of Paul Ehrlich, John Langley and others, receptors are described in terms of their recognition properties, their structures,
`transducing abilities and the impact of genomics and their role in contributing to genetic diseases. The receptor concept has firmly
`underpinned our advances in drug development and molecular medicine of the latter half of this century and it is clear that it will continue
`to drive pharmaceutical developments in the 21st century. (cid:14)9 2000 Elsevier Science B.V. All rights reserved.
`
`Keywords: Receptors; Receptor history; Paul Ehrlich; John Newton Langley; Emil Fischer; Louis Pasteur; Drug-receptor interactions; Stereoselectivity;
`Receptor diseases; Receptor regulation; Receptor structure
`
`1. Introduction
`
`Separate, but complementary, lines of evidence led in
`the late 19th century to the establishment of the concept of
`"the receptor" as the cellular site at which drugs, toxins
`and antibodies mediated their physiological or pathological
`effects. These lines of evidence are particularly associated
`with Paul Ehrlich in Germany and John Newton Langley
`in England. However, their work built upon many cen-
`turies of work that attempted to define the action of
`naturally occurring materials on the body. These earlier
`contributions have been expertly summarized in the books,
`"Murder, Magic and Medicine" by John Mann (1992)
`and "In Search of a Cure" by M. Weatherall (1990).
`the
`From his extensive work on
`immunology and
`chemotherapy of parasitic infections, Ehrlich argued that
`cells must possess specific and defined protoplasmic side
`chains that, because of their unique chemistry and steric
`architecture, could interact specifically with the comple-
`mentary groups of a chemotherapeutic agent,
`toxin or
`antibody (Parascondola, 1981; Ehrlich, 1900):
`
`"For the sake of brevity in what follows we shall in
`general designate as receptor that binding group of the
`protoplasmic molecule to which a foreign, newly intro-
`duced group binds." P. Ehrlich, 1900
`
`* Tel.: +1-716-645-7315;
`triggle @ buffalo.edu
`
`fax: +1-716-645-2941;
`
`e-mail:
`
`Even prior to these speculations, Langley (1878) had
`observed:
`
`"We may, I think, without much rashness assume that
`there
`is some substance or substances
`in the nerve
`endings or gland cells with which both atropine and
`pilocarpine are capable of forming compounds. On this
`assumption,
`then,
`the atropine or pilocarpine com-
`pounds are formed according
`to some law of which
`their relative mass and chemical affinity for the sub-
`stance are factors." J.N. Langley, 1878
`
`But Langley (1906) also recognized the receptor as a
`transducing engine that:
`
`" . . . receives the stimulus and, by transmitting it causes
`contraction." J.N. Langley, 1906
`
`Langley, contemporaneously with the work of Ehrlich,
`used the term "receptive substance"
`for these specific
`entities and speculated that specific receptors must exist
`for curare, atropine, pilocarpine and the other autonomic
`drugs with which his research had been principally con-
`cerned. Certainly, the specificity of such drug-receptor
`interactions had been anticipated by Emil Fischer who
`wrote:
`
`" . . . I will say
`that enzyme and glucoside must fit
`together
`like lock and key in order
`to be able
`to
`exercise a chemical action on each other." Emil Fis-
`cher, 1894
`
`0031-6865/00/$ - see front matter (cid:14)9 2000 Elsevier Science B.V. All rights reserved.
`PII: S0031-6865(99)00046-1
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`80
`
`D.J. Triggle / Pharmaceutica Acta Helvetiae 74 (2000) 79-84
`
`Thus, by the beginning of this century, the conceptual
`foundation had been laid for the existence of pharmaco-
`logical receptors, albeit as "black boxes", that received
`input and translated it into a physiological, pharmacologi-
`cal or pathological output. The present century has been
`largely devoted to opening this box and defining its con-
`tents. It has been a spectacularly successful century that
`has culminated with the classification, isolation, characteri-
`zation and cloning of pharmacological receptors, with the
`identification of receptors - - "orphan receptors" - - for
`which ligands may not have been identified and with the
`determination of the detailed three-dimensional structure
`of a membrane receptor - - a bacterial potassium channel.
`
`2. Receptors as recognition entities
`
`The specificity of the drug-receptor recognition process
`has long been regarded as a critical feature of the receptor
`concept, even when the nature of receptors was entirely
`unknown. Indeed, the absence of such specificity, includ-
`ing stereoselectivity, is often a component of arguments
`that a receptor event is not involved in the action of a
`particular drug. These structure-activity relationships were
`originally qualitative in character, but were transformed
`first by the application of regression techniques that per-
`mitted
`the elucidation of one-dimensional quantitative
`structure-activity relationships (QSARs) and then by pro-
`tein sequence determination and
`the determination of
`three-dimensional protein structures and the mapping of
`receptor sites (Greer et al., 1994).
`With these approaches, it is increasingly possible to
`interpret the actions of drugs at their receptors and to
`facilitate the design of drugs for new receptor sites. Thus,
`the design of the HIV-protease inhibitors, a critically avail-
`able class of drugs for the treatment of this lethal disease,
`was greatly facilitated by the resolution of the structure of
`the enzyme. The dimeric, essentially symmetric, structure
`composed of two identical aspartate protease-like domains,
`was critical to the development of the first protease in-
`hibitors.
`Stereochemistry of interaction has long been recognized
`in drug-receptor interactions and Pasteur very explicitly
`recognized that different stereoisomers could have very
`different physiological properties:
`
`"There cannot be the slightest doubt that the only and
`exclusive cause of this difference in the fermentation of
`the two tartaric acids is caused by the opposite molecu-
`lar arrangements of the tartaric acids. In this way, the
`idea of the influence of the molecular asymmetry of
`natural organic products
`is introduced
`into physio-
`logical studies, this important characteristic being per-
`haps the only distinct line or demarcation which we can
`draw today between dead and living matter. I have in
`fact set up a theory of molecular asymmetry, one of the
`
`most important and wholly surprising chapters of the
`science, which opens up a new, distant but definite
`horizon for physiology." Louis Pasteur, 1860
`
`The stereochemical basis of drug actions was early
`investigated by Arthur Cushny at the beginning of this
`century (Cushny, 1926). These pioneering investigations
`on atropine and related compounds revealed the quantita-
`tive differences that can occur between drug enantiomers.
`Today, the issue of the chirality of drug-receptor interac-
`tions has assumed both scientific and regulatory signifi-
`cance. Scientific and clinical significance derives from
`consideration of the efficacy of a single enantiomer versus
`a
`racemate,
`from considerations of
`stereoselective
`metabolism and disposition, and from the impact of the
`route of administration and patient variability. Regulatory
`issues derive from considerations that racemic drugs may
`represent separate agents in fixed combinations: develop-
`ment issues derive from considerations of the costs, includ-
`ing those for chemical synthesis, of pursuing a single
`enantiomer or a racemic mixture.
`Recent developments in stereochemistry have focused
`upon the gaseous general anesthetics,
`long a topic of
`discussion concerning their potential interactions with re-
`ceptors. The long-standing assumption that these agents
`interacted non-specifically through partitioning into lipid
`membranes has been challenged from a variety of sources,
`including the stereospecificity of interaction of isoflurane
`and halothane (Moody et al., 1994; Sedensky et al., 1994).
`The differences, though small, are potentially significant
`from the perspective of clinical safety.
`
`3. Receptors as transduction machines
`
`As realized by Langley, recognition is a necessary, but
`not sufficient, characteristic of a receptor. Receptors are
`also biological machines translating the information of the
`interaction with the ligand into the cellular response. And
`with this translation comes the necessary amplification of
`the input information by several orders of magnitude.
`Several types of physical and biochemical process are
`involved in this biological transduction. Of particular sig-
`nificance, because of their widespread occurrence are the
`transduction events mediated by G protein-coupled recep-
`tors and by ion channels. The G proteins are a large group
`or superfamily of GTP hydrolases and the interaction of an
`activated receptor with the heteromeric G protein releases
`bound GDP and replaces it with GTP with concomitant
`liberation of the activated GTP-associated GoL subunit.
`This activated subunit then interacts with a number of
`effector systems, including phospholipase C, adenylyl cy-
`clase and ion channels.
`Similar amplification events occur during the opening
`or closing of ion channels mediated by chemical (ligand-
`gated channels) or physical (potential-dependent channels).
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`D.J. Triggle / Pharmaceutica Acta Helvetiae 74 (2000) 79-84
`
`81
`
`To a first approximation, ion channel opening is an all-or-
`none stochastic event and the effect of a stimulus, chemi-
`cal or physical, is to alter the probability of channel
`opening. Patch clamp techniques make it possible to ob-
`serve the opening or closing of single channels and thus to
`measure single molecular events. This ability, coupled
`with recent structural information on the K + channel,
`makes possible to a first approximation a molecular de-
`scription of ion channel function.
`
`4. Receptor classification
`
`Receptor structure, the linear and ultimately the three-
`dimensional representation of the sequence, provides a
`definitive classification and basis for the classification of
`receptors. This permits the identification of "families"
`and of "super-families" of receptors and, in recent years,
`has made possible the isolation and characterization of
`so-called "orphan receptors", for which physiological lig-
`ands or physiological function may not have been identi-
`fied. Earlier classification schemes that used the identity of
`the physiological ligand that interacts with the receptor, the
`nature of the physiological or pharmacological response
`induced by receptor activation or the nature of the antago-
`nist drug all have significant limitations. Multiple systems
`and receptors control blood pressure and similarly many
`receptors share a common biochemical cascade
`adeny-
`lyl cyclase or phospholipase C activation or the opening
`and closing of K + channels. To further complicate matters,
`many receptors are pleiotropic, initiating multiple conse-
`quences that may differ according to cell type and even
`agonist quality. Similarly, many physiological ligands may
`interact with multiple receptors that are of fundamentally
`different classes: acetylcholine interacts with both mus-
`carinic and nicotinic receptors, the former being members
`of the G protein-coupled family (Bikker et al., 1998) and
`the latter a member of the ligand-gated ion channel family
`(Holladay et al., 1997). Finally, many receptors are het-
`eromeric assemblies of multiple types of subunits: the
`
`pharmacological specificity and the actions induced can be
`very dependent upon subunit composition. Despite this
`complexity, it is convenient to recognize four principal
`families of chemically sensitive pharmacological receptors
`(Table 1).
`
`5. Receptor structure
`
`The majority of receptors under discussion are integral
`membrane proteins and have not, until recently, yielded to
`three-dimensional
`structural determination. However,
`progress is now being made in three principal areas
`the
`structure of rhodopsin as a model for the very large G
`protein-coupled receptor family, the role of the nicotinic
`acetylcholine receptor as a model for ligand-gated ion
`channels and a bacterial K + channel from Streptomyces
`lividans that will materially define the ionic conductivity,
`selectivity and gating processes of ion channels. Addition-
`ally, powerful molecular biological approaches including
`selective mutagenesis and the use of chimeric constructs
`have served to define the roles of particular sequences or
`residues in receptor recognition and activation processes.
`The very large G protein-coupled receptor family has
`provided many examples of the definition of residue roles
`in drug interactions. Thus, for the beta-adrenoceptor, criti-
`cal interacting residues have been determined to be aspar-
`tate-113 on helix III, serine-204 and -207 on helix V and
`phenylalanine-290 on helix VI. Such studies have defined
`a "homologous" binding pocket on this receptor family
`that is shared by the cationic neurotransmitters, acetyl-
`choline, histamine, norepinephrine etc., and related small
`ligands.
`Perhaps the most recent dramatic advance has been the
`determination of the three-dimensional structure of a bacte-
`rial K + channel from S. lividans (Doyle et al., 1998). This
`channel is composed of four identical subunits, each with
`two trans-membrane sequences and a "pore" region, that
`associate in "tepee" shape to form the functional ion
`channel containing within it the selectivity filter that dis-
`criminates K + from other ions. The selectivity filter con-
`tains a so-called "signature" sequence, highly conserved
`residues that characterize K + ion channels and the Gly-
`Tyr-Gly components of this sequence in the four subunits
`bind K + through their carbonyl residues and are responsi-
`ble for the ionic selectivity of the channel.
`These structural studies have also revealed the impor-
`tance that very minor changes, frequently a single residue,
`can have on the drug-receptor interaction. Thus, the 5-
`HT~B receptor in the rodent and man is pharmacologically
`quite distinct, a differentiation that is provided by residue
`355, threonine in the human and asparagine in the rat
`(Oksenberg et al., 1992). Similarly, the interaction of
`barbiturates and other anesthetics with the GAB A p l
`receptor depends upon the presence of a single isoleucine
`residue: replacement of this residue by serine confers
`
`Table 1
`Classification of pharmacological receptors
`Class Type
`Characteristics
`1
`Ion channels
`Integral membrane; subunit composition;
`each subunit has two or more membrane
`inserts as a pore region and four or more
`form the central pore of the channel
`Seven-transmembrane integral proteins
`that couple to the G protein family of proteins
`One-transmembrane integral proteins
`that have kinase activity; may dimerize
`during receptor activation
`Non-membrane, cytosolic proteins with
`DNA binding domains; transcriptional
`regulators
`
`4
`
`Nuclear receptors
`
`2
`
`3
`
`G protein-coupled
`
`Enzyme-associated
`
`ANACOR EX. 2019 - 12/15
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`D.J. Triggle / Pharmaceutica Acta Helvetiae 74 (2000) 79-84
`
`anesthetic sensitivity (Belelli et al., 1999). Such changes
`are of extreme importance in the determination of individ-
`ual human sensitivity to drugs where single nucleotide
`polymorphisms (SNPs) may determine clinically signifi-
`cant drug responses and interactions (Kleyn and Vessell,
`1998). Thus, there are a number of polymorphisms in the
`human beta-adrenoceptors and these have been associated
`with bronchodilation in response to beta z-agonists and
`with the development of hypertension (Buscher et al.,
`1999). The P450-mediated drug metabolism process is
`highly polymorphic leading to extensive inter-individual
`variation in drug metabolism (Ingelman-Sundberg et al.,
`1999). Exploitation of
`this knowledge, now possible
`through gene-array technologies, will increasingly alter
`both drug development and drug prescribing.
`
`6. Orphan receptors
`
`The classical route to the receptor concept has always
`been the existence first of a drug and an associated family
`structure with defined physiological and pharmacological
`effects. A classic example is morphine and the opiates and
`the subsequent discovery of the endogenous ligands and
`subsequently the G protein-coupled opiate receptor. The
`isolated and expressed receptor could then be used as a
`screen for novel structures that might have more desirable
`therapeutic properties. Advances in molecular biology now
`permit the reverse of this process. DNA sequences are
`identified that are analogs of known receptors. These
`sequences can be expressed to yield novel or "orphan"
`receptors for which the endogenous ligand can now be
`hunted (Soontjens et al., 1996; Robertson and Willy, 1997;
`Civelli et al., 1998; Wilson et al., 1998).
`Both the G protein family and the steroid hormone
`family have yielded many orphan receptors. At least 140 G
`protein receptors have been identified from the human
`genome and since this class has generated major drugs for
`many therapeutic targets, the status of orphan receptors
`here has attracted much attention. The identification of the
`opiod receptor ORL1 is but one example, interacting with
`a specific endogenous ligand nociceptin that appears to
`have widespread roles in the mediation of nociception and
`stress reduction. Over 70 orphan receptors have thus far
`been identified in the steroid receptor family and for most
`of these, neither endogenous ligand nor physiological ac-
`tion has yet been defined.
`
`7. Receptors and genetic diseases
`
`in the structure and expression of receptor
`Defects
`proteins are increasingly known to be associated with
`specific disease states. As one of the largest families of
`receptors, the G protein-coupled receptors exhibit a variety
`of mutations and associated functional changes, including
`both "loss of function" and "gain of function" (Spiegel,
`
`1995; Farfel et al., 1999). These defects can lie in the
`actual receptors or in the associated G proteins. Similarly,
`mutations in ion channels are being associated with a
`variety of diseases from cardiac abnormalities to cystic
`fibrosis.
`Loss-of-function mutations in G protein-coupled recep-
`tors are quite common, with approximately 100 having
`been described, and include nephrogenic diabetes insipidus
`(V2 vasopressin receptor), familial hypothyroidism (TSH
`receptor), Hirschprung disease (endothelin B receptor)
`hypercalcemia and neonatal hyperparathyroidism (Ca 2+
`sensing receptor). These loss of functio