Chapter 13
Structural and Functional Characterization of Insect Genes Encoding Ligand-Gated Chloride-Channel Subunits
Downloaded by UNIV LAVAL on July 13, 2016 | http://pubs.acs.org Publication Date: May 9, 1995 | doi: 10.1021/bk-1995-0591.ch013
Douglas C . Knipple, Joseph E . Henderson, and David M. Soderlund Department of Entomology, New York State Agricultural Experiment Station, Cornell University, Geneva, N Y 14456
Ligand-gated chloride channels mediate synaptic inhibition in animal nervous systems and are sites of action of important drugs and toxins. Recent studies of vertebrate G A B A - and glycine-gated chloride channels have revealed their heteromultimeric organization and the large number of genes encoding unique but structurally related subunits that comprise them. In insects, physiological studies have demonstrated the existence of diverse ligand-gated chloride channels, but their underlying structural and functional properties remain poorly characterized. This paper describes our isolation of members of this gene family from Drosophila melanogaster by PCR-based homology probing and summarizes investigations in this genetic model system of the only two insect subunit genes isolated to date to which functional properties have been ascribed. The implications of these findings for target-based insecticide discovery efforts are discussed in the context of the major paradigms established from the study of homologous vertebrate receptors. Inhibitory neurotransmission plays a fundamental role in animal nervous systems by counterbalancing and modulating the excitatory inputs into postsynaptic cells. This process is mediated by amino acid neurotransmitter receptors, which, when activated, permit the selective flow of chloride ions across the postsynaptic membrane resulting in hyperpolarization of the postsynaptic cell (1). In mammals, glycine-gated chloride channels are the principal inhibitory receptors in the spinal cord and brainstem whereas GABA-gated chloride channels are the predominant inhibitory receptors elsewhere in the brain. These receptors are sites of action of important drugs and neurotoxins and have complex pharmacologies that are believed to reflect an underlying structural diversity owing to their heteromultimeric nature and variable subunit composition (reviewed in ref. 2). In contrast to the situation in vertebrates, the underlying structural basis of inhibitory neurotransmission in insects is poorly understood. Physiological studies provide evidence for the existence of several classes of functionally distinct chloride channels in insect nerve and muscle, but to date the complete structures of only two subunits having homology to vertebrate G A B A and glycine receptors have been deduced (3,4). This paper provides an overview of the current state of knowledge of 0097-6156/95/0591-0205$12.00/0 © 1995 American Chemical Society Clark; Molecular Action of Insecticides on Ion Channels ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
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MOLECULAR ACTION OF INSECTICIDES ON ION CHANNELS
ligand-gated chloride channels in insects and discusses the implications of recent functional studies of vertebrate receptors for ongoing investigations of insect ligandgated chloride channels.
Downloaded by UNIV LAVAL on July 13, 2016 | http://pubs.acs.org Publication Date: May 9, 1995 | doi: 10.1021/bk-1995-0591.ch013
Structural and Functional Properties of Vertebrate GABA and Glycine Receptors The Ligand-Gated Chloride Channel Gene Family of Vertebrates. Molecular cloning and expression studies have revealed an unexpectedly large number of genes encoding G A B A - and glycine-gated chloride channels. More than 20 individual members of the ligand-gated chloride channel gene family in mammals have been characterized to date, including five classes of G A B A receptor subunits (a, p, y, 8, and p) (5) and two classes of glycine receptor subunits (a and (J) (6). The diversity of this family of genes is further increased by the existence of multiple subunit isoforms in some of these classes (e.g., ccl,
