Division of Neurobiology, Barrow Neurological Institute, Phoenix,
Arizona (R.J.L.); Neurobiologie Moleculaire, Institut Pasteur, Paris,
France (J-P.C., N.L.N.); Department of Pharmacology and Experimental
Therapeutics, University of Maryland School of Medicine, Baltimore,
Maryland (E.X.A.); Department of Pharmacology, University Medical
School, University of Dundee, Dundee, United Kingdom (D.J.K.B.);
Department of Biology, University of California at San Diego, La Jolla,
California (D.K.B.); Department of Physiology, University of
Geneva/Central Medical University, Geneva, Switzerland (D.B.);
Department of Pharmacology, George Washington University School of
Medicine, Washington, D.C. (V.A.C.); Department of Pharmacology and
Therapeutics, McGill University, Montreal, Quebec, Canada (P.B.S.C.);
Institute for Behavioral Genetics, University of Colorado, Boulder,
Colorado (A.C.C., S.R.G., M.J.M.); Division of Neuroscience, Baylor
College of Medicine, Houston, Texas (J.A.D.); Department of
Pharmacology, Georgetown University School of Medicine, Washington,
D.C. (K.J.K.); Institute for Neurological Sciences, University of
Pennsylvania, Philadelphia, Pennsylvania (J.M.L.); Parkinson's
Institute, Sunnyvale, California (M.Q.); Department of Pharmacology,
School of Medicine, University of California at San Diego, La Jolla,
California (P.W.T.); Department of Biology and Biochemistry, University
of Bath, Bath, United Kingdom (S.W.)
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I. Introduction |
Nicotinic acetylcholine receptors
(nACh receptors2) in jawed
vertebrates are prototypical members of the multisubunit,
neurotransmitter-gated superfamily of ion channels (ionotropic
neurotransmitter receptors; see Section IV for selected
seminal papers, reviews, and collections). nACh receptors mediate some
of the effects of the endogenous neurotransmitter, acetylcholine, and
they are principal biological targets of the tobacco alkaloid,
nicotine, which generally (although exceptions exist) mimics acute
actions of acetylcholine at nACh receptors. nACh receptors play
critical physiological roles throughout the brain and body, mediating
excitatory neurotransmission at the vertebrate neuromuscular junction,
across autonomic ganglia, and at selected synapses in the brain and
spinal cord. Roles have also been suggested for nACh receptors in the
modulation of neurotransmitter release as well as in neurotrophism.
nACh receptors exist as a variety of types. At least one nACh receptor
type is found in developing skeletal muscle, and another is expressed
at the mature neuromuscular junction. There is evidence that several
nACh receptor types exist in peripheral neurons of autonomic and
sensory ganglia and in the brain and spinal cord. Evidence also exists
for expression of nACh receptor types in other tissues and cell types,
including lymphocytes, fibroblasts, pulmonary neuroendocrine cells,
spermatozoa, keratinocytes, granulocytes, chondrocytes, and placenta,
as well as in several sensory organs.
Classical pharmacological distinctions between some nACh receptor types
endure. For example, decamethonium is a selective inhibitor of nACh
receptor types in muscle, and hexamethonium is a selective antagonist
of nACh receptor types in autonomic ganglia. More contemporary studies
are beginning to yield distinctive pharmacological profiles for some
nACh receptor types. However, these findings are not yet clearly
integrated with molecular definitions of those receptors. This
precludes a pharmacological description of nACh receptor types in this
report, which instead focuses on matters of nomenclature and on
structural bases of nACh receptor diversity.
Heterogeneity or diversity in nACh receptor types is derived in part
from diversity in genes that encode nACh receptor subunits. Diverse
nACh receptor types are formed as pentamers from different combinations
of genetically distinct subunits, but not from all possible
combinations of subunits (Fig. 1 and
Table 1). Subunits confer distinctive
functional and structural properties to the nACh receptor types that
they form. With few exceptions, subunit compositions, stoichiometries,
and arrangements of naturally expressed nACh receptor types are not
known with absolute certainty.
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Fig. 1.
Schematic diagrams showing (upper) transmembrane
topology of nAChR subunits and (lower) features of nAChR subunit
assembly. Upper schematic illustrates the existence of an extensive,
extracellular N-terminal region, four transmembrane segments, a
cytoplasmic loop, and an extracellular C terminus for each nACh
receptor subunit (not drawn to scale; surrounding lipid bilayer is also
represented schematically). Lower schematic illustrates known
arrangements of subunits constituting a nACh receptor type predominant
in fetal skeletal muscle (left) and constituting a homooligomeric nACh
receptor composed of  7 subunits (right). Assembly of subunits is
thought to be regulated and limited by interactions between amino acid
residues at subunit interfaces. Ligand binding pockets are thought to
be formed at a subset of these assembly interfaces. For example, two
distinct classes of ligand binding sites are formed at interfaces
between 1 and  subunits or between 1 and  subunits of the
nACh receptor from fetal muscle. Five largely similar ligand binding
domains are thought to be formed at negative and positive faces of 7
subunits in homooligomeric 7-nACh receptors. All subunits contribute
to channel formation, reflected by influences of every nACh receptor
subunit identified to date on features of channel activity upon
interaction with nicotinic ligand (see Table 1).
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TABLE 1
Subunit compositions of well established, naturally expressed and
heterologously expressed nACh receptors
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To date, 16 nACh receptor subunit genes have been cloned from
vertebrates (Table 1). Each of the nACh receptor subunit proteins encoded by these genes is thought to have the same, general structural organization. The features of these integral membrane proteins include
an extensive N-terminal region positioned extracellularly, four
transmembrane segments (M1-M4), an extended cytoplasmic loop between
the third and fourth transmembrane segments containing amino acid
sequences absolutely unique to each subunit, and an extracellular C
terminus (Fig. 1). All nACh receptor subunits also contain a loop of 13 amino acids between two cysteine residues forming a disulfide bridge in
the N-terminal domain. These features are shared with families of
subunits that constitute ionotropic glycine receptors, ionotropic
-amino butyric acid receptors, invertebrate glutamate-gated chloride
channels, and ionotropic serotonin receptors (5-HT3
receptors). Invertebrate nACh receptor subunits can combine with
vertebrate nACh receptor subunits to form functional receptors,
suggesting that the former also belong to the superfamily. Comparisons
between rat and human amino acid sequences typically reveal over 80%
identity for a given nACh receptor subunit.
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II. Current Status of nACh Receptor Subunit Nomenclature and
Receptor Type Nomenclature and Classification |
The IUPHAR Subcommittee on Nicotinic Acetylcholine Receptors of
the International Union of Pharmacology Committee on Receptor Nomenclature and Drug Classification (NC-IUPHAR) recommends continued use of the present, Greek letter-based nomenclature for nACh receptor subunits, as is described in Section III, for the
foreseeable future. The subcommittee also recommends using the suffix
"gene" to distinguish the gene encoding a given subunit from the
subunit protein itself (e.g., the subunit gene encodes the subunit). In addition, the subcommittee recommends for the foreseeable
future a minimal nACh receptor type nomenclature/classification system based on the subunits that compose those receptor types to the extent
that such information is available. Examples of such a nACh receptor
type nomenclature and classification are also presented in
Section III.
The NC-IUPHAR subcommittee consensus is that it is premature to create
any alternative to the nomenclature for nACh receptor subunits or
receptor types already in place or to generate a classification of nACh
receptor types based on any of their properties other than their
subunit composition. This consensus is based partly on the
subcommittee's perspectives that there are probably more nACh receptor
subunit genes to be cloned, that there likely are more nACh receptor
types to be identified, and that the current understanding is deficient
about subunit compositions and other properties of many nACh receptor
types. Although ligand binding properties, functional characteristics,
and schemes for assembly of nACh receptor types differ, the
subcommittee's view is that more information is needed before these
features can be exploited as criteria for an enduring system of nACh
receptor type classification. The subcommittee's consensus also
recognizes the broad acceptance across the research community of the
nACh receptor subunit and receptor type nomenclature/classification
system already in place; there are very few deviations from this
nomenclature in the literature. Moreover, the subcommittee's opinion
is that the nomenclature/classification system currently in use is
reasonably sensible and understandable to the newly initiated,
particularly given the inherent complexity of multisubunit, ionotropic
neurotransmitter receptors relative to metabotropic neurotransmitter
receptors composed of a single polypeptide. The subcommittee
acknowledges limitations in the present system and that it is at
variance with several of the guidelines established by NC-IUPHAR. For
example, NC-IUPHAR prefers to avoid use of Greek letters but bows to
historical precedence in this instance. However, the subcommittee does
not see any obvious advantages to adoption of a new system at the
present time, and the current system is viewed as having flexibility
adequate to accommodate at least another series of advances in our
understanding of the nACh receptor system. Nevertheless, it is
anticipated that research progress over the next few years, including
completion of the human genome project and identification of all nACh
receptor subunit genes, will ultimately allow rational generation of
enduring nACh receptor subunit nomenclature and nACh receptor type
nomenclature/classification systems.
The NC-IUPHAR subcommittee recommends that commonly used prefixes such
as "neuronal nACh receptor", "muscle nACh receptor", or
"ganglionic/autonomic nACh receptor" not be adopted into nACh receptor nomenclature or classification. This is because there are many
possible nACh receptor subunits or receptor types that can be found in
any one of these tissues and because some nACh receptor subunits or
receptor types may be found in more than one of these tissues. Such
terminology applied to nACh receptor-directed, curaremimetic, snake
neurotoxins ("neuronal-bungarotoxin") instead of a neutral system
(such as one based on Greek letters) has been misleading at best.
Similarly, it is recommended that use of the prefix "structural"
not be propagated with regard to classification of nACh receptor subunits. This term currently has little use or meaning because evidence exists that each of the nACh receptor subunits identified to
date participates in formation of ligand binding pockets and/or influences nACh receptor functional properties. Continued use of the
term "non- subunit" is also discouraged.
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III. Present Definitions of nACh Receptor Subunits and Receptor
Types |
The 16 nACh receptor subunits identified to date are defined using
a Greek letter sometimes followed by an Arabic numeral (neither
subscripted nor superscripted). For example, there are nine alpha
subunits (1-9; 8 subunits have only been derived from chick),
four beta subunits (
1-4), and one each gamma (), delta (),
and epsilon (
) subunit. All nACh receptor subunits share
expression of a pair of tandem cysteine residues in the putative,
N-terminal, extracellular region. These residues are near to a site on
the 1, 4, or 7 subunits known to engage in agonist binding.
None of the other subunits (i.e., 1-4, , , or ) have
these tandem cysteines. Several approaches have been taken in attempts
to illuminate relationships between nACh receptor subunits and their
genes. At a low level of resolution, these analyses make it clear that
same nACh receptor subunits and their genes are most closely related
(e.g., the 1, , , and grouping; uniqueness of the
7/8 grouping and of the 9 subunit; grouping into an extended
family of 2/4 and 3/6 pairs). However, each of these
analyses is unique with respect to the range of amino acid or nucleic
acid sequences examined and the analytical/computational tools applied,
and each has produced somewhat different results, particularly with
regard to placement of the 1 subunit and the 2/4 and 5/3
subunit pairs. The NC-IUPHAR subcommittee consensus is that these
analyses are interesting and help guide those working in the field.
However, further evolution in these approaches and identification and
sequencing of nACh receptor subunits and their genes are needed to help
guide the subcommittee and NC-IUPHAR in future efforts to derive nACh
receptor subunit and receptor type classification/nomenclature.
It is useful to point out that the grouping of 1, , , and subunits is relevant to their contributions in formation of nACh
receptor types in muscle, in combination with the 1 subunit. nACh
receptors in vertebrate fetal muscle contain two 1 subunits and one
each 1, , and subunit, whereas the subunit substitutes for the subunit in nACh receptors found in adult muscle. Thus, in
vertebrates, one form of nACh receptor found in fetal muscle can be
defined as the 11-nACh receptor, and one form of nACh receptor found in adult muscle can be defined as the
11-nACh receptor. Subscript and parenthetical notation is
used to define subunit stoichiometries [e.g.,
(1)21-nACh receptors for nACh receptors
containing two copies of the 1 subunit and single copies of 1,
, and subunits]. Note the caveat here with respect to nACh
receptor subunits and types found in muscle, as perhaps will be the
case with regard to other nACh receptor subunits and types, that there
is evidence for alternatively spliced products of 1 and subunit
genes. Thus, there may be more than one kind of nACh receptor found in
fetal or adult muscle, and a formal nACh receptor subunit and receptor
type nomenclature will need to accommodate and define features of
subunit variants and the nACh receptor types containing them.
The grouping of the 7/8 subunit pair and the separation of
the 9 subunit from the others is relevant in that nACh receptors containing these subunits happen to be capable of interaction with
longer forms of curaremimetic, snake neurotoxins. These subunits also
have the capacity to combine as homooligomers to create functional nACh
receptor types. nACh receptors known to form only as homooligomers are
defined, for example, as 7-nACh receptors, 8-nACh receptors, or
9-nACh receptors, with stoichiometries noted in subscript if known
[e.g., (7)5-nACh receptors]. nACh receptors containing both 7 and 8 subunits have been isolated from chick brain and/or retina and can be defined as 78-nACh receptors. Moreover, there is additional evidence that some nACh receptors might contain 7 plus
other kinds of subunits. If, for example, nACh receptors known to
contain 7 subunits possibly contain or are known to contain
additional kinds of subunits as a pure or mixed population of
receptors, the subcommittee recommends a notation employing an asterisk
as a "wild card" to define those entities as 7*-nACh receptors.
There is strong evidence that 3 and 4 subunits coexist in
naturally expressed nACh receptors found in autonomic ganglia and
derived cell lines. There is additional evidence that these naturally
expressed nACh receptors in autonomic neurons also contain 5
subunits, consistent with the fact that genes encoding 3, 5, and
4 subunits are clustered. Therefore, a more specific definition of
these receptors would be 354-nACh receptors. Moreover, there
is evidence that a subset of nACh receptors containing 3, 5, and
4 subunits also contain 2 subunits; these receptors can be
defined as 3524-nACh receptors. If a pure or mixed population of nACh receptors is known to contain 3 or 3 plus 4
subunits, but the identity of coassembled subunits is not certain, the
subcommittee recommends use of 3*-nACh receptors or 34*-nACh receptors, respectively, to describe those entities.
A numerically abundant nACh receptor type in the central nervous system
(CNS) is composed of 4 and 2 subunits, and data supporting a 2:3
stoichiometry exists. Thus, these receptors can be defined as
42-nACh receptors [or as
(4)2(2)3-nACh receptors as information
warrants]. However, minor populations of nACh receptors containing
4 and 2 subunits known to or possibly containing additional
subunits can be defined employing the asterisk/wild card notation as
42*-nACh receptors.
The subunit compositions of nACh receptors naturally expressed at lower
abundance in brain are not known, although there must be some roles for
2, 3, 5, 6, 3, and 4 subunits in formation of other
nACh receptor types. Considerable evidence exists suggesting natural
expression of functional nACh receptors in brain and spinal cord other
than those containing just 7 or just 4 and 2 subunits, but
further work is needed to determine the subunit compositions and
stoichiometries of these entities. From heterologous expression studies, it is known that unique, ligand binding pockets can be formed
at interfaces between 2, 3, 4, or 6 subunits and 2 or
4 subunits. Presence of 5 or 3 subunits has also been shown to
influence ligand recognition of heterologously expressed nACh receptors
containing other subunits. Heterologously expressed nACh receptors
containing these subunits can be defined in a straightforward fashion,
as knowledge about contributions of subunits to expressed nACh receptor
warrants (e.g., 64-nACh receptors if composed only of 6 and
4 subunits). Similarly, at least provisional definitions of
naturally expressed nACh receptors can be generated as information about the subunits constituting those nACh receptor is progressively obtained (e.g., 2*-nACh receptors for receptors known to contain 2 subunits).
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