Trends in Pharmacological Sciences
OpinionExploring the pharmacological properties of insect nicotinic acetylcholine receptors
Section snippets
Molecular diversity of insect nicotinic acetylcholine receptor subunits
Insect nicotinic acetylcholine (nACh) receptor subunits, like vertebrate nACh receptor subunits, consist of a large N-terminal extracellular domain involved in agonist binding, followed by three transmembrane regions (TM1–TM3, with TM2 lining the channel), a large intracellular loop, a fourth transmembrane domain (TM4) and a C-terminal extracellular region (Figure 1). The presence of two vicinal cysteine residues, equivalent to Cys192 and Cys193 in the electric organ (Torpedo marmorata) α1
Evidence for different nACh receptor subtypes
The subunit composition of native insect nACh receptors remains unclear, largely because the heterologous expression of functional insect nACh receptors has proved difficult. Nevertheless, behavioural studies using different nicotinic agonists and antagonists 5, 6 or Drosophila Dα7 mutants [7] have established that the insect central nervous system expresses different nACh receptor subtypes. This finding is reinforced by electrophysiological studies showing that, as in vertebrates, insect
nACh receptor-associated proteins: possible roles in function and assembly
Molecular cloning followed by functional expression in either Xenopus laevis oocytes or cultured mammalian cell lines has enabled studies of the physiology and pharmacology of vertebrate nACh receptors of defined subunit composition, which can reflect native nACh receptor subtypes 12, 13, 14, 15. Unfortunately, these approaches have failed in insect nACh receptor subunit expression studies, except for the Schistocerca nACh receptor Sgα1 16, 17. In vertebrates, crucial control steps for
Selectivity of neonicotinoids for insect nACh receptors
The neonicotinoid insecticides, such as imidacloprid, show sensitivity for both native and recombinant insect nACh receptors, attributable in part to the imidazolidine ring 26, 27. Chemical modification of this imidazolidine ring can lead to greater affinity, as is the case with clothianidin [27], for example. In studies of Drosophila–chicken Dα2–β2 and Dα1–β2 hybrid receptors expressed in Xenopus laevis oocytes, the neonicotinoid ligands tested [e.g. des-nitro-imidacloprid, nitempyram and the
Intracellular regulation of insect neuronal nACh receptors
Another novel and interesting feature is the recent characterization of the intracellular regulation of insect nACh receptors. An initial search for patterns of conserved amino acid residues associated with phosphorylation sites in nACh receptor subunits shows that they possess different potential phosphorylation sites for cAMP-dependent protein kinase A (PKA), protein kinase C (PKC), calcium-calmodulin-dependent protein kinase (CaM kinase) and endogenous protein tyrosine kinase 35, 36. Thus,
Concluding remarks
Coexpression of insect α subunits with vertebrate β subunits (hybrid nACh receptors), in addition to studies of chimaeric subunits, has provided important new insights into the pharmacological properties of insect nACh receptors 11, 28, 33, 44. But, in view of the problems associated with heterologous expression, other approaches such as RNA interference and studies of nACh receptor mutants in combination with functional studies will help to resolve the contribution of individual nACh receptor
Glossary
- α-Bungarotoxin (α-Bgt)
- toxin from snake venom. α-Bgt binding is considered to represent the distribution of α7-subunit-containing nACh receptors.
- DEG
- degeneration of certain neurons. In Caenorhabditis elegans, there are 42 different nACh receptor subunits, including the deg-3 group. des-2 is another gene in this group.
- DES
- degeneration suppressor. Mutations in the gene encoding this protein suppress the degeneration caused by deg-3.
- Drosophila Dα7 mutant
- excisions of P elements in the Dα7 subunits
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