New Comprehensive Biochemistry

359 Pages·1993·20.509 MB·ii-viii, 3-369\359

by  Ferdinand Hucho (Eds.)

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Neurotransmitter receptors / editor Ferdinand Hucho.
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1. Neurotransmitter receptors. I. Hucho, Ferdinand, 1939-
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Neurotransmitter Receptors
Editor
FERDINAND HUCHO
Institut fur Biochemie
Freie Universitat Berlin,
Thielallee 63,
14195 Berlin, Germany
1993
ELSEVIER
Amsterdam London New York Tokyo
New Comprehensive Biochemistry
Volume 24
General Editors
A. NEUBERGER
London
L.L.M. van DEENEN
Utr ech t
ELSEVIER
Amsterdam London New York Tokyo
vii
List of contributors
Barnard, Eric A. Molecular Neurobiology Unit, Royal Free Hospital School of
-
Medicine, University of London, London NW3 2PF, U.K. Tel. [phone].
ext. 5445; Fax: [phone].
Bobker, Daniel H. Vollum Institute for Advanced Biomedical Research, Oregon
-
Health Sciences University, L-474, 3 18 1 S.W . Sam Jackson Park Road, Portland,
Oregon 97201-3098, USA. Tel. [phone]; Fax: [phone].
BGhme, Eycke Institut fur Pharmakologie, Freie Universitat Berlin, Thielallee 67-
-
73, D-14195 Berlin 33, Germany. Tel. [phone]; Fax: [phone].
Bouvier, Michef - DCpartement de Biochimie, UniversitC de MontrCal, C.P. 6128,
Succ. A, Montreal, Quebec, H3C 357, Canada. Tel: [phone]; Fax: +01-
5 14-343-2210.
Darlison, Mark G. - Institut fur Zellbiochemie und Klinische Neurobiologie, Univer-
sitatskrankenhaus Eppendorf, Martinistr. 52, D-2025 1 Hamburg, Germany. Tel:
[phone]; Fax: [phone].
Fagg, Graham E. CIBA-Geigy Ltd., Building K-147.2.14, CH-4002 Basel, Switzer-
-
land. Tel: [phone]; Fax: [phone].
Foster, Alan C. - Merck Sharp and Dohme Research Laboratories, Terlings Park,
Harlow, Essex CM20 2QR, U.K. Current address: Gensia Pharmaceuticals, 11025
Roselle Street, San Diego, CA 92121, USA
Helmreich, Ernst J. M. Physiologisch-Chemisches Institut, Universitat Wurzburg,
-
Koellikerstr. 2, D-97070 Wurzburg, Germany.
Hucho, Ferdinand Institut fur Biochemie der Freien Universitat Berlin, Thielallee
-
63, D-14195 Berlin, Germany. Tel: [phone]; Fax: [phone].
Jurv, Jaak - Laboratory of Bioorganic Chemistry, University of Tartu, 2 Jakobi
Street, 202 400 Tartu, Estonia. Tel: [phone] or 32884; Fax: [phone]-
35440 or 33427; Telex: 173243 TAUN.
Koesling, Doris Institut fur Pharmakologie, Freie Universitat Berlin, Thielallee 67-
-
73, D-14195 Berlin, Germany. Tel: [phone]; Fax: [phone].
...
Vlll
Lohse, Martin J. - Genzentrum der Ludwig-Maximilians-UniversitatA, m Klopfer-
spitz, D-82152 Martinsried, Germany. Tel: [phone]; Fax: [phone]-
3795.
MeyerhoJ Woygang - Institut fur Zellbiochemie und Klinische Neurobiologie,
Universitatskrankenhaus Eppendorf, Martinistr. 52, D-2025 1 Hamburg, Germany,
Tel: [phone]; Fax: [phone].
Nantel, Francois - DCpartement de Biochimie, UniversitC de MontrCal, C.P. 6128,
Succ. A, MontrCal, Quebec, H3C 357, Canada. Tel: [phone]; Fax: +01-
514-343-2210.
Otto, Henning - Institut fur Biochemie der Freien Universitat Berlin, Thielallee 63,
D-14195 Berlin, Germany. Tel: [phone]; Fax: [phone].
Richter, Dietmar - Institut fur Zellbiochemie und Klinische Neurobiologie, Univer-
sitatskrankenhaus Eppendorf, Martinistr. 52, D-2025 1 Hamburg, Germany. Tel:
[phone]; Fax: [phone].
Rinken, Ago - Laboratory of Bioorganic Chemistry, University of Tartu, 2 Jakobi
Street, 202 400 Tartu, Estonia. Tel: [phone] or 32884; Fax: [phone]-
35440 or 33427; Telex: 173243 TAUN.
Schultz, Giinter - Institut fur Pharmakologie, Freie Universitat Berlin, Thielallee 67-
73, D-14195 Berlin, Germany. Tel: [phone]; Fax: [phone].
Simon, Joseph - Molecular Neurobiology Unit, Royal Free Hospital School of Me-
dicin, University of London, London NW 3 2PF, U.K. Tel. [phone]; Fax:
[phone].
Stephenson, l? Anne - The School of Pharmacy, University of London, 29/39 Bruns-
wick Square, GD-London WClN IAX, U.K. Tel: [phone]; Fax: +44-71-
278-1 939.
Strange, Philip G. - Biological Laboratory, The University, Canterbury, Kent, CT2
7NJ, U.K.
Strasser,, Ruth H. Medizinische Klinik der Universitat Heidelberg, D-69117 Hei-
-
delberg, Germany.
Williams, John 2: - Vollum Institute for Advanced Biomedical Research, Oregon
Health Sciences University, L-474, 3181 S.W.S am Jackson Park Road, Portland,
Oregon 97201-3098, USA. Tel. [phone]; Fax: [phone].
V
Preface
This is a good time for a book on transmitter receptors: the cloning boom is over.
More than one hundred receptor sequences are known and recombinant DNA tech-
niques, used in combination with classical biochemistry, have provided us with a
wealth of information which can now be compiled in one volume of this series.
With the primary structures of most transmitter receptors at hand, it is safe to
predict that the next major step will have to wait for breakthroughs in biophysical
methods (spectroscopy, X-ray crystallography, ultramicroscopy). We can expect
principally new insights into receptor structure-function relationships only from sec-
ondary and tertiary structures of receptor proteins. For the time being we have to live
with the state reached after the major leap ahead made possible by molecular neuro-
biology.
It is useless to include chapters on all known receptors into a book of this kind,
although the series title - New Comprehensive Biochemistry - may suggest this. To
minimize redundancies only a few receptors (some of which are typical for a whole
group of similar receptors, others which are presently of special interest) are dealt
with in a full-size chapter. Others are represented in the TIPS Receptor Nomencla-
ture Supplement which is included as a special feature in this book and which makes
this volume more useful as a receptor handbook.
A major problem is the receptor nomenclature: no attempts have been made to
urge the chapter authors to use a consistent system of names and terms. At this stage
of ‘receptorology’ a logical nomenclature is not obvious. Therefore, the reader is
asked to be flexible when looking in the index for ‘his’ receptor. As a case in point,
one can take the terms 4TM and 7TA4, which are used by some neurochemists to
classify receptors into those having four and seven hydrophobic transmembrane do-
mains, respectively. This classification is based on the assumption that receptors span
the membrane with alpha helices. It may turn out that some of these transmembrane
domains are actually B-strands (evidence for this is accumulating in the literature).
This would mean that the twenty-two or so amino acids of the hydrophobic sequence
are too long for spanning the membrane only once and the 4TM and 7TM might
have to be renamed 6TM or IOTM.
It is a nice custom to use this page of a book for thanking: I have to thank the
colleagues who contributed a chapter despite their many other duties, the publisher
and his staff for the efficient professionality in producing this book and, last but not
least, Mary Wurm, who kept a close eye on my manuscripts, making them hope-
-
fully - readable.
Ferdinand Hucho
Berlin, Germany
June, 1993
E Hucho (Ed.) Neurotransmitter Receptors
8 1993 Elsevier Science Publishers B.V. All rights reserved.
CHAPTER 1
Transmitter receptors general principles
-
and nomenclature
FERDINAND HUCHO
Institut fur Biochemie, Freie Universitat Berlin, Thielallee 63. 0-14195 Berlin, Germany
1. Historical aspects and definition
1.1. History
Concepts evolve, and so did the receptor concept. Concepts are based on observa-
tions, on special experimental data that are generalized when somebody overlooks
enough of them and is wise enough to see the general principle in the flood of num-
bers, plots, and descriptions. The receptor concept emerged in the last quarter of the
19th century. The one name associated with the birth of ‘receptorology’i s John New-
port Langley. In 1878 he published his observations on the mutually exclusive action
of atropine and pilocarpin on saliva excretion by the submaxillary gland. He wrote
[l]: “... we may, I think, without much rashness, assume that there is a substance or
substances in the nerve endings or gland cells with which both atropine and pilo-
carpin are capable of forming compounds’’ In the same volume of the journal he
wrote [2]: “On this assumption then the atropine or pilocarpin compounds are
formed according to some law of which their relative mass and affinity for the sub-
stance are factors” The notion of a ‘substance’ forming a ‘compound’ was only a
quarter century later turned into the more general term ‘receptive substance’ [3]. This
concept of drug receptors evolved in parallel with Paul Ehrlich’s ‘Chemoreceptor
theory’ based on his immunological studies [4]. Actually both Langley and Ehrlich
give much credit to each other and both probably would not have come to the general
receptor concept without each other’s thinking and special experimental lines of evi-
dence. The third contemporary having contributed to the concept was Emil Fischer
with his ‘lock and key’ description of proteins interacting with their substrates.
Nevertheless, ‘receptor’ is a technical term, just a word. The idea of substances
specifically interacting with other substances dates back to Claude Bernard [5], who
around the middle of the 19th century described the exact localization of the arrow
poison curare interrupting (in modern terms) the nerve impulse transmission to the
muscle. And to coin a word is not the end of scientific research: up to the beginning
3
4
of the 1970s people were not sure whether a receptor really was more than a word.
They thought it might turn out to be “...something that happens, but not a substance
that can be isolated”[6]. In 1972 [7] the first receptor, the nicotinic acetylcholine re-
ceptor, was isolated. Nowadays, receptor research is one of the most active fields in
molecular biology. A detailed history of receptorology is not, however, the aim of
this Chapter. It would have to include milestones like the characterization of recep-
tors as allosteric proteins [8,9], their first imaging by electron microscopy [lO,ll], and
the introduction of recombinant DNA technology to receptor research [12,13], espe-
cially in connection with the introduction of Xenopus oocytes as expression systems
[14]. The reader will find much of this in the chapters on special receptors. One mile-
stone is still being awaited: crystallization and X-ray analysis of a receptor. We still
have no authentic three dimensional structure of a transmitter receptor. Taking this
not as a negative statement but rather as a challenge, we now turn to a summary of
some principles which have emerged from almost exactly two decades of biochemical
receptor research.
1.2. DeJinition
Receptors are proteins interacting with extracellular physiological signals and con-
verting them into intracellular effects. Neurotransmitter receptors are integral mem-
brane proteins; their physiological signals are neurotransmitters and neuromodula-
tors.
Writing down this definition, problems and exceptions spring to mind, but for the
time being I would like to stay with it. One should point out that it avoids discrimi-
nating against physiological signal molecules not yet defined unambiguously as
transmitters. For example, many of the neuropeptides discussed in this chapter have
not been classified as transmitters by the criteria summarized below. By including
neuromodulators in our definition, we can react flexibly to newly published experi-
mental data. On the other hand, it tacitly includes the so-called orphan receptors,
receptors discovered by reverse genetics (the art of ‘pulling out’ clones with consensus
probes homologous to sequences of well-known members of receptor families and
superfamilies). Orphan receptors are proteins (actually in most cases just DNA se-
quences) for which the endogenous ligand, the physiological signal, has not yet been
found.
The molecular definition given here is different from a more biological receptor
concept that names cellular entities like the rods and cones of the retina ‘photorecep-
tors’ and the cells of the muscle spindle ‘stretch receptors’. It is also different from the
not-quite-past habit of some pharmacologists to call every binding site of a drug or
toxin a receptor.
5
1.3. Criteria for calling a binding site a receptor
Most transmitter receptors are discovered by recombinant DNA and cloning tech-
niques. Their physiological and neurochemical characterization requires expression
of the protein and a detailed investigation of their biochemistry and pharmacology in
the expression system as well as in the nervous system. The initial step in investigating
a receptor therefore is studying its binding behavior (Chapter 2). Binding studies of-
ten are also the primary access to a receptor to be discovered in an animal. However,
in this case several criteria have to be observed to call a ligand-binding site a true
receptor [ 151:
1. The ligand binding site has to be saturable. Half-maximal saturation should be in
the same concentration range as the physiological concentration of the ligand.
2. The other essential criterium is specificity: the binding site should bind primarily
the physiological ligand and structurally closely related compounds. It should
definitely discriminate against the ‘wrong’ stereoisomer.
3. A third criterium is the location of the binding site under investigation within the
organism. There are reports in the literature about high-affinity saturable insulin
binding to glass beads, opiates binding to talcum (from the rubber gloves of the
experimenter), and cholinergic ligands binding to macrocyclic organic com-
pounds, interesting artifacts but obviously no receptors. Synaptic localization in
the pre- or postsynaptic membrane may be a strong indication of a binding site
being a true receptor, although one should be open-minded for extrasynaptic lo-
calizations as well.
I.4 . Agonists - antagonists
In many cases it is feasible to investigate a receptor using not its physiological signal
molecule, but rather an artificial ligand that may be more preferred because of its
stability, selectivity, high affinity, or its being available in a radioactively labeled
form. Historically, several receptors have been discovered by means of such ligands
before the physiological ligand was even known. For example, the opiate receptors
were detected before the discovery of the enkephalins, the endogenous opiates.
This pharmacological approach adds to our tools the large group of antagonists.
All neurotransmitters discovered so far are agonists, molecules triggering an effect in
the target cell after binding to its receptor. Antagonists, on the other hand, are com-
pounds that bind to the same receptor, with a similar or even (in most cases) higher
affinity, without a consequence other than preventing the agonist from exerting its
effect. Competitive antagonists compete with the agonist for the same or an overlap-
ping binding site on the receptor protein. Noncompetitive antagonists also block the
effect of the agonist, but bind to a site distinct from the agonist-binding site. They do
not inhibit and they may even promote agonist binding. A fourth class of receptor
ligands is the inverse agonists discovered with the GABAhenzodiazepine receptors.
6
They exert effects opposite to those of the agonist. The transmitter GABA acts as an
anxiolytic and muscle relaxant, while inverse agonists cause, depending on the recep-
tor occupancy, arousal, anxiety, and seizures (see chapter 6). Some of these com-
pounds are presently under investigation as nootropics. Of interest for pharmacolo-
gists are the partial agonists (or mixed agonists/antagonists). These are compounds
that bind with high affinity but exert only weak effects. Being (weak) agonists they
antagonize the effect of strong agonists.
For a detailed discussion of the criteria classifying an endogenous substance as a
transmitter, the reader is referred to standard textbooks of neurochemistry [15] and
neurobiology [ 161. Briefly, transmitters should be synthesized, stored within, and re-
leased upon stimulation from the neuron were its action is observed. Furthermore, its
subsequent removal from the synaptic cleft should be secured. Here I wish to point
out again that all neurotransmitters detected so far are agonists. One could envision
antagonistic neurotransmitters, a possibility indicated by cGMP, the ‘transmitter’
conveying the signal from rhodopsin in the photoreceptor’s disk membrane to so-
dium channels of the rod’s plasma membrane. The ‘signal’ (a photon) causes cleavage
of cGMP. The ‘message’ arriving at the plasma membrane is negative, antagonistic
in the exact definition of the word: its effect is to stop rather than to trigger an ion
flux. (This action is not to be confused with inhibitory chemical transmission. GABA
and glycine are inhibitory transmitters that act agonistically on anion instead of cat-
ion fluxes).
It remains to be seen whether or not there are antagonistic neurotransmitters. An-
other open question pivotal to our understanding of the mechanism of transmitter
receptor functioning is: what is the structural and chemical difference between an
agonist and an antagonist Perhaps the pharmacologists working with muscarinic
cholinergic receptors are closest to answering this question. But, in general, we can
state: by looking at the structural formula of a compound, we cannot yet predict
whether it will act as an agonist or antagonist.
1.5. Mechanism of receptor function
1.5.1. The triune receptor model
Transmitter receptors are signal converters, operationally containing three func-
tional moieties (Fig. 1): a signal-receiving (ligand-binding) part (‘R’), an effector
(‘E’), and a transducer (‘T’), coupling the former two and transducing the signal from
the binding site to the effector.
The description of receptors as signal converters implies that the extracellular sig-
nal is different from the signal released from the membrane to the intracellular space.
Transmitters are recognized and bound at the extracellular surface of the receptor
protein. They do not enter the cell. Being ‘first messengers’ their ‘message’ is con-
verted into an intracellular effect conveyed by ‘second messengers’, produced by the
effector moiety of the receptor.