The ADAR RNA editing enzyme controls neuronal excitability in Drosophila melanogaster

X Li,M A O'Connell,L P Keegan,R A Baines,I M Overton

doi:10.1093/nar/gkt909

Abstract

RNA editing by deamination of specific adenosine bases to inosines during pre-mRNA processing generates edited isoforms of proteins. Recoding RNA editing is more widespread in Drosophila than in vertebrates. Editing levels rise strongly at metamorphosis, and Adar5G1 null mutant flies lack editing events in hundreds of CNS transcripts; mutant flies have reduced viability, severely defective locomotion and age-dependent neurodegeneration. On the other hand, overexpressing an adult dADAR isoform with high enzymatic activity ubiquitously during larval and pupal stages is lethal. Advantage was taken of this to screen for genetic modifiers; Adar overexpression lethality is rescued by reduced dosage of the Rdl (Resistant to dieldrin), gene encoding a subunit of inhibitory GABA receptors. Reduced dosage of the Gad1 gene encoding the GABA synthetase also rescues Adar overexpression lethality. Drosophila Adar5G1 mutant phenotypes are ameliorated by feeding GABA modulators. We demonstrate that neuronal excitability is linked to dADAR expression levels in individual neurons; Adar-overexpressing larval motor neurons show reduced excitability whereas Adar5G1 null mutant or targeted Adar knockdown motor neurons exhibit increased excitability. GABA inhibitory signalling is impaired in human epileptic and autistic conditions, and vertebrate ADARs may have a relevant evolutionarily conserved control over neuronal excitability.

Highlights

RNA editing by ADARs has been proposed to diversify transcripts, in the brain, to meet the physiological needs of the organism [1,2]
To test whether the rescue of Adar 3/4S OE lethality by Rdl mutants was due to a reduction in GABA signalling, we examined the effect of decreasing GABAergic input on Adar 3/4S OE lethality
Taking advantage of the GAL4/UAS system, we placed the lethal Adar overexpression under temperature-dependent control, constructing a Drosophila strain that can be reared at 25C for the purpose of performing a genetic screen. When progeny from this line or from crosses of this line to wild-type are reared at 27C, all Adar 3/4S OE progeny die during pre-adult stages

Summary

Introduction

RNA editing by ADARs (adenosine deaminases acting on RNA) has been proposed to diversify transcripts, in the brain, to meet the physiological needs of the organism [1,2]. The ADAR enzymes convert specific adenosines to inosines within duplex regions of transcripts. Ribosomes decode inosine as guanosine, and editing events within open reading frames lead to production of edited isoforms of ion channel subunits and other proteins. Developmental and tissue-specific ADAR regulation is important, and both edited and unedited isoforms of channels have function; unedited GABAA (g-aminobutyric acid) receptor, for instance, is crucial for synapse formation in the developing vertebrate brain [12,13]

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