Opinion
Exploring the pharmacological properties of insect nicotinic acetylcholine receptors

https://doi.org/10.1016/j.tips.2006.11.006Get rights and content

Insect nicotinic acetylcholine (nACh) receptors are molecular targets of insecticides such as neonicotinoids that are used to control disease-carrying insects and agricultural pests. To date, several insect nACh receptor subunits have been identified, indicating different nACh receptor subtypes and pharmacological profiles. Because of the difficulty in expressing functional insect nACh receptors in heterologous systems, new research tools are needed. Studies on insects resistant to the insecticide imidacloprid and on laboratory-generated hybrid and chimaeric nACh receptors in vitro have provided information about the molecular basis of receptor diversity, neonicotinoid resistance and selectivity. Additionally, recent results indicate that the sensitivity of insect nACh receptors to imidacloprid can be modulated by intracellular phosphorylation mechanisms, which offers a new approach to studying insect nACh receptor pharmacology.

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

References (61)

  • S. Halevi

    Conservation within the RIC-3 gene family. Effectors of mammalian nicotinic acetylcholine receptor expression

    J. Biol. Chem.

    (2003)
  • E.M. Jeanclos

    The chaperone protein 14-3-3eta interacts with the nicotinic acetylcholine receptor alpha 4 subunit. Evidence for a dynamic role in subunit stabilization

    J. Biol. Chem.

    (2001)
  • M.E. Williams

    Ric-3 promotes functional expression of the nicotinic acetylcholine receptor alpha7 subunit in mammalian cells

    J. Biol. Chem.

    (2005)
  • M. Ihara

    Diverse actions of neonicotinoids on chicken alpha7, alpha4beta2 and Drosophila-chicken SADbeta2 and ALSbeta2 hybrid nicotinic acetylcholine receptors expressed in Xenopus laevis oocytes

    Neuropharmacology

    (2003)
  • M. Shimomura

    Roles of loop C and the loop B-C interval of the nicotinic receptor alpha subunit in its selective interactions with imidacloprid in insects

    Neurosci. Lett.

    (2004)
  • M. Shimomura

    Insect–vertebrate chimeric nicotinic acetylcholine receptors identify a region, loop B to the N-terminus of the Drosophila Dalpha2 subunit, which contributes to neonicotinoid sensitivity

    Neurosci. Lett.

    (2005)
  • C. Bass

    Molecular characterisation of nicotinic acetylcholine receptor subunits from the cat flea, Ctenocephalides felis (Siphonaptera: Pulicidae)

    Insect Biochem. Mol. Biol.

    (2006)
  • A.K. Jones

    The nicotinic acetylcholine receptor gene family of the malaria mosquito, Anopheles gambiae

    Genomics

    (2005)
  • S.H. Thany

    Apisalpha2, Apisalpha7-1 and Apisalpha7-2: three new neuronal nicotinic acetylcholine receptor alpha-subunits in the honeybee brain

    Gene

    (2005)
  • W. Marszalec

    Desensitization of nicotine acetylcholine receptors: modulation by kinase activation and phosphatase inhibition

    Eur. J. Pharmacol.

    (2005)
  • V.L. Salgado et al.

    Desensitizing and non-desensitizing subtypes of alpha-bungarotoxin-sensitive nicotinic acetylcholine receptors in cockroach neurons

    J. Insect Physiol.

    (2004)
  • P. Deglise

    The insecticide imidacloprid is a partial agonist of the nicotinic receptor of honeybee Kenyon cells

    Neurosci. Lett.

    (2002)
  • A. Vermehren

    The nicotinic alpha subunit MARA1 is necessary for cholinergic evoked calcium transients in Manduca neurons

    Neurosci. Lett.

    (2001)
  • R.E-K. Nauen

    Thiamethoxam is a neonicotinoid precursor converted to clothianidin in insects and plants

    Pestic. Biochem. Physiol.

    (2003)
  • M. Grauso

    Novel putative nicotinic acetylcholine receptor subunit genes, Dalpha5, Dalpha6 and Dalpha7, in Drosophila melanogaster identify a new and highly conserved target of adenosine deaminase acting on RNA-mediated A-to-I pre-mRNA editing

    Genetics

    (2002)
  • J.A. Dent

    Evidence for a diverse Cys-loop ligand-gated ion channel superfamily in early bilateria

    J. Mol. Evol.

    (2006)
  • A. Fayyazuddin

    The nicotinic acetylcholine receptor Dalpha7 is required for an escape behavior in Drosophila

    PLoS Biol.

    (2006)
  • K. Chamaon

    Nicotinic acetylcholine receptors of Drosophila: three subunits encoded by genomically linked genes can co-assemble into the same receptor complex

    J. Neurochem.

    (2002)
  • S.J. Lansdell et al.

    Molecular characterization of Dalpha6 and Dalpha7 nicotinic acetylcholine receptor subunits from Drosophila: formation of a high-affinity alpha-bungarotoxin binding site revealed by expression of subunit chimeras

    J. Neurochem.

    (2004)
  • A.B. Elgoyhen

    alpha10: a determinant of nicotinic cholinergic receptor function in mammalian vestibular and cochlear mechanosensory hair cells

    Proc. Natl. Acad. Sci. U. S. A.

    (2001)
  • Cited by (0)

    View full text