Nicotinic acetylcholine receptors (nAChRs)
mediate fast cholinergic synaptic transmission
apri il documento originale
Nicotinic acetylcholine receptors (nAChRs) mediate fast cholinergic synaptic
transmission and play roles in many
cognitive processes. They are under intense research as potential targets of
drugs used to treat neurodegenerative
diseases and neurological disorders such as Alzheimer's disease and
schizophrenia. Invertebrate nAChRs are targets of anthelmintics as well as a
major group of insecticides, the neonicotinoids. The honey bee, Apis mellifera,
is one of the most beneficial insects worldwide, playing an important role in
crop pollination, and is also a valuable model system for studies on social
interaction, sensory processing, learning, and memory. We have used the A.
mellifera genome information to characterize the complete honey bee nAChR gene
family. Comparison with the fruit fly Drosophila melanogaster and the malaria
mosquito Anopheles gambiae shows that the honey bee possesses the largest family
of insect nAChR subunits to date (11 members). As with Drosophila and Anopheles,
alternative splicing of conserved exons increases receptor diversity. Also, we
show that in one honey bee nAChR subunit, six adenosine residues are targeted
for RNA A-to-I editing, two of which are evolutionarily conserved in Drosophila
melanogaster and Heliothis virescens orthologs, and that the extent of editing
increases as the honey bee lifecycle progresses, serving to maximize receptor
diversity at the adult stage. These findings on Apis mellifera enhance our
understanding of nAChR functional genomics and provide a useful basis for the
development of improved insecticides that spare a major beneficial insect
The honey bee, Apis mellifera, is an important beneficial insect in agriculture.
In addition to producing honey and
beeswax, the contribution of A. mellifera to crop pollination is valued at more
than $14 billion dollars per year in
the U.S. alone (United States Department of Agriculture http://www.ars.usda.gov/main/main.htm).
Honey bees live in societies of considerable complexity and thus are studied as
models for social behavior (Robinson et al. 1997).
The neonicotinoids are the newest major group of insecticides, which includes
acetamiprid, clothianidin, dinotefuran,
imidacloprid, nitenpyram, thiacloprid, and thiamethoxam (Tomizawa and Casida
2005). The worldwide annual sales of neonicotinoids amounts to 1 billion dollars,
and they are used against piercing-sucking pests (aphids, leafhoppers, and
white-flies) of major crops. In France, the use of imidacloprid has been
suspended over concerns that it may be having a drastic effect on bee
populations (http://www.pan-uk.org/press/pr140604.htm), highlighting the
importance that effective insecticides should also show selectivity within
insects so that pollinators such as A. mellifera are spared. While the link
between imidacloprid use and bee population decline has yet to be proven,
studies have shown that imidacloprid is highly toxic to A. mellifera (Suchail et
al. 2004) and at sublethal doses can alter honey bee foraging and learning (Guez
et al. 2001; Lambin et al. 2001; Decourtye et al. 2004). Neonicotinoids act as
agonists on their molecular targets, nicotinic acetylcholine receptors (nAChRs)
(Matsuda et al. 2001), which are prototypical members of the cys-loop
ligand-gated ion channel (LGIC) superfamily (Karlin 2002). The fast actions of
acetylcholine (ACh) at synapses are mediated by nAChRs, which consist of five
homologous subunits arranged around a central ion channel (Corringer et al.
2000; Unwin 2005). Analyses of completed genomes have revealed diverse nAChR
gene families with mammals possessing 16 subunit genes, chicken, 17 (Millar
2003), Fugu rubripes, 28 (Jones et al. 2003), and Caenorhabditis elegans, at
least 27 (Jones and Sattelle 2004). In contrast, Drosophila melanogaster and
Anopheles gambiae have notably smaller nAChR gene families, each consisting of
10 subunits (Jones et al. 2005; Sattelle et al. 2005).
To date, four A. mellifera nAChR subunits (Apis2, Apis3, Apis7-1, and Apis7-2)
have been identified (Thany et al.
2003, 2005), which are expressed in brain structures that play roles in learning
and memory, olfactory signal
processing, mechanosensory antennal input, and visual processing. These findings
are consistent with ACh being a
major excitatory neurotransmitter in the insect nervous system (Breer and
Sattelle 1987; Lee and O'Dowd 1999). Patch clamp studies have demonstrated the
existence of a distinct nAChR subtype in the honey bee nervous system that is
blocked by the nAChR antagonists -bungarotoxin (-Btx), dihydroxy--erythroidine
and methyllycaconitine, while nicotine and imidacloprid acted as partial
agonists on this receptor (Goldberg et al. 1999; D?glise et al. 2002; Wustenberg
and Grunewald 2004). Another study has shown the presence of two nAChR
populations that differ in their responses to imidacloprid but not ACh (Nauen et
al. 2001). The involvement of nAChRs in honey bee behavior has also been
investigated. Injection of the nAChR agonist, nicotine, showed that potentiation
of the cholinergic system improves short term memory (Thany and Gauthier 2005)
and injection of the nAChR antagonist, mecamylamine, inhibited olfactory
learning or memory recall depending upon the site of injection (Lozano et al.
1996, 2001). Recently it has been demonstrated that one distinct nAChR sub-type,
which is -Btx sensitive, is involved in long-term memory, whereas a second
subtype, which is -Btx insensitive, but is affected by mecamylamine, plays a
role in retrieval processes (Dacher et al. 2005). Interestingly, this mirrors to
a certain extent the mammalian central nervous system, where there are two
predominant nAChR subtypes, the 7 and 42 receptors, that are -Btx sensitive and
insensitive, respectively, and both receptor subtypes play a role in memory (for
review, see Hogg et al. 2003). Since individual nAChR subunits can confer
distinct pharmacological properties on a receptor (Romanelli and Gualtieri
2003), the multiple nAChR subtypes present in the honey bee nervous system are
likely to be determined by their subunit composition. Identifying the full
complement of honey bee nAChR subunits represents a critical step in
understanding the variety of roles played by nAChRs in the honey bee nervous
system and the exquisite repertoire of bee behavior, as well as in identifying
particular targets of chemical compounds. Here we have used the A. mellifera
genome to describe the complete honey bee nAChR gene family.
Splice variants increase Apis nicotinic receptor diversity
Two Apis nAChR subunits, Amel4 and Amel6, have alternatively spliced exons most
likely arising from tandem exon
duplication (Kondrashov and Koonin 2001). As with D4 and Agam4 (Lansdell and
Millar 2000; Jones et al. 2005), Amel4 possesses two alternatives for exon 4 (denoted
exon4 and exon4') (Fig. 4A). However, whereas D6 and Agam6 have two alternatives
for exon 3 (Grauso et al. 2002; Jones et al. 2005), Amel6 has only a single exon.
For 6 exon 8, both Apis and Anopheles have two alternatives, while Drosophila
has three, although the mosquito possesses exons analogous to D6 8b and 8c (Jones
et al. 2005), while the honey bee clearly possesses 8a and 8b-like exons.
We have used the available A. mellifera genome information to complete the
characterization of the honey bee nAChR gene family, thus describing the first
complete set of Hymenoptera nAChR subunits and the third insect nAChR gene
family following those of the two Diptera, A. gambiae (Jones et al. 2005) and D.
melanogaster (Sattelle et al. 2005). The three insect species represent 280
million years of evolution (Carpenter and Burnham 1985; De Gregorio and Lemaitre
2002) where the nAChR gene family has remained compact with A. mellifera having
11 genes encoding nAChR subunits, whereas both D. melanogaster and A. gambiae
possess 10 genes (Jones et al. 2005; Sattelle et al. 2005). The nAChR subunit
composition of Apis most closely resembles that of Anopheles in that both
possess nine and one subunit, while Drosophila has seven and three . The extra
honey bee subunit is a subunit (Amel2) making A. mellifera only the second
insect known to possess more than one non- type subunit.
The characterization of the full complement of honey bee nAChR subunits presents
an important basis for associating particular nAChR subtypes with key aspects of
behavior, identifying receptor subtypes targeted by neonicotinoids as well as
developing insecticides with improved selectivity. Indeed, comparison of
complete insect nAChR gene families has identified a highly divergent subunit
group (the D3 group) as well as species-specific proteome diversification
arising from alternative splicing and RNA editing, all of which represent
promising subunit differences to target for future rational insecticide design.
While studies using heterologous expression systems such as Xenopus laevis
oocytes have proven instructive in characterizing vertebrate nAChRs (Corringer
et al. 2000) and low levels of functional expression of an insect subunit, L1,
have been observed in Xenopus oocytes (Marshall et al. 1990), expression of
functional insct nAChRs has so far proven elusive (Sattelle et al. 2005).
Nevertheless, Drosophila nAChR subunits cn form robust functional channels when
coexpressed with a vertebrate 2 subunit (Bertrand et al. 1994) and studies on
such hybrid receptors have provided insights into the selectivity of
neonicotinoids for insect nAChRs over those of vertebrates (Matsuda et al. 1998;
Ihara et al. 2003), regions of subunit proteins involved in neonicotinoid
interactions (Shimomura et al. 2002, 2003, 2004), and the actions of different
neonicotinoids (Ihara et al. 2004). Also, computer models of insect nAChRs have
been recently generated, which permit docking experiments to assess interactions
with compounds of interest (Sattelle et al. 2005). Similar studies combining
functional expression with molecular modeling of Apis nAChRs are likely to prove
useful in screening for novel compounds that show low selectivity for honey bee
receptors and in dissecting the mechanisms of insecticide actions and
selectivity on nAChRs.
We are indebted to the A. mellifera Genome Project (Human Genome Sequencing
Center), which provided the starting point for this study. We thank Sandrine
Paute for technical support. We also thank Ryszard Maleszka and Chris Ponting
for encouragement, support, and helpful comments on the manuscript.
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