Worm Breeder's Gazette 14(3): 44 (June 1, 1996)

These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.

An extensive nicotinic acetylcholine receptor (nAChR) gene family in Caenorhabditis elegans

Nigel P Mongan, Howard A Baylis , David B Sattelle

The Babraham Institute Laboratory of Molecular Signalling, Department of Zoology, Downing St., Cambridge, CB2 3EJ, UK.

Although nicotinic acetylcholine receptors (nAChR) are one of the best
characterized membrane proteins, aspects of our understanding of
nicotinic acetylcholine receptor-mediated fast synaptic signalling in
neurons and on muscle remain superficial. For example, in no organism
studied to date is the full complement of nAChR subunits known.  The
nematode C. elegans is one of the simplest organisms to employ ACh as a
neurotransmitter and a number of genes for nAChR subunits have now been
identified1,2,3. Putative subunit genes have been identified by: (a)
cloning genes responsible for phenotypes such as resistance to the
anthelmintic drug levamisole (lev-1, unc-38, unc-29)1,2 and neuronal
degradation (deg-3)3; (b) screening libraries with probes from other
organisms (acr-2, acr-3)4; (c) identifying putative receptor subunits in
sequences from the C. elegans genome sequencing project (e.g. T09A5.3).
Sequence alignments reveal notable differences at the putative ACh
binding site, and have identified putative subunits (e.g.KO3F8.2, DEG-3)
which exhibit extreme divergence from previously characterised nAChRs. 
Viable mutants of nAChR subunit genes are available in C. elegans. The
use of comparative RT-PCR offers a powerful, simple means of exploring
the diversity of nAChR subunits expressed in C.elegans. This method may
make it possible to assign specific subunits to behavioural traits, by
correlating the absence of subunit expression in defined mutant
phenotypes. nAChR alpha subunits may be distinguished from other
ligand-gated ion channel subunits by the presence of a vicinal cysteine
motif at positions equivalant to 192,193 (numbering based on Torpedo 
alpha subunit). RT-PCR has been used to demonstrate the expression of
the largest family of expressed nAChR alpha subunits currently known for
any organism.  Evidence is presented for the existence of an nAChR alpha
subunit sub-family in C. elegans whose members have a YxxCC motif,
(rather than the more common YxCC motif) at the ACh binding site.  Such
sequence variations might be expected to influence the pharmacology of
nematode receptors, and may account for the insensitivity of certain
Ascaris suum and C. elegans nAChRs to the coral toxin, lophotoxin (or
its analogues)5. We have begun to study the in vivo expression of these
receptors using the green fluorescent protein reporter gene, and are
investigating the physiology of an alpha subunit (T09A5.3) which shows a
high degree of similarity to the homo-oligomer forming chick alpha7

1. Lewis J A et al. (1987)  Mol. Pharmacol. 31, 185-193

2. Fleming J T, Tornøe C, Riina H A, Coadwell J, Lewis J A and Sattelle
D B (1993) In Comparative Molecular Neurobiology (ed Pichon Y) 65-80,
Springer-Verlag, Berlin. 

3. Treinin M and Chalfie M (1994) Neuron 14;871-877

4. Squire M D, Tornøe C, Baylis H A, Fleming J T, Barnard E A and
Sattelle D B (1995) Receptors and Ion Channels 3, 107-115.

5. Tornøe C, Holden-Dye L, Bai D, Abramson S N and Sattelle D B (1994)
J. Physiol. (Lond.) 480, 96