Adenosine to inosine (A-to-I) pre-mRNA editing by the ADAR enzyme family has the potential to increase the variety of the proteome. This editing by adenosine deamination is essential in mammals for a functional brain. To detect novel substrates for A-to-I editing we have used an experimental method to find selectively edited sites and combined it with bioinformatic techniques that find stem-loop structures suitable for editing. We present here the first verified editing candidate detected by this screening procedure. We show that Gabra-3, which codes for the a3 subunit of the GABA A receptor, is a substrate for editing by both ADAR1 and ADAR2. Editing of the Gabra-3 mRNA recodes an isoleucine to a methionine. The extent of editing is low at birth but increases with age, reaching close to 100% in the adult brain. We therefore propose that editing of the Gabra-3 mRNA is important for normal brain development.

Recoding by adenosine-to-inosine RNA editing plays an important role in diversifying proteins involved in neurotransmission. We have previously shown that the Gabra-3 transcript, coding for the ␣3 subunit of the GABA A receptor is edited in mouse, causing an isoleucine to methionine (I/M) change. Here we show that this editing event is evolutionarily conserved from human to chicken. Analyzing recombinant GABA A receptor subunits expressed in HEK293 cells, our results suggest that editing at the I/M site in ␣3 has functional consequences on receptor expression. We demonstrate that I/M editing reduces the cell surface and the total number of ␣3 subunits. The reduction in cell surface levels is independent of the subunit combination as it is observed for ␣3 in combination with either the ␤2 or the ␤3 subunit. Further, an amino acid substitution at the corresponding I/M site in the ␣1 subunit has a similar effect on cell surface presentation, indicating the importance of this site for receptor trafficking. We show that the I/M editing during brain development is inversely related to the ␣3 protein abundance. Our results suggest that editing controls trafficking of ␣3-containing receptors and may therefore facilitate the switch of subunit compositions during development as well as the subcellular distribution of ␣ subunits in the adult brain. Adenosine to inosine (A-to-I)2 RNA editing is a mechanism used in the mammalian nervous system to provide alterations in the protein sequence by co-transcriptional modification of single nucleotides. This modification is catalyzed by adenosine deaminases that act on RNA (ADAR1 and ADAR2) that can selectively modify adenosine to inosine residues within double stranded pre-mRNAs. Within mRNA transcripts, inosine is read as guanosine by the translation machinery. Therefore, this mechanism has the potential to change the amino acid sequence and thereby the function of the protein. Several gene products encoding proteins involved in neurotransmission have been shown to be A-to-I edited, including ligand-and voltage-gated ion channels as well as a G-proteincoupled receptor and thereby creating diverse isoforms of proteins essential for balanced neuronal kinetics (reviewed in Ref. 1).One of the most well studied substrates for editing in the brain is the transcript coding for the AMPA glutamate receptor (GluA). AMPA receptors consist of four subunits (GluA1-GluA4) in different combinations. Changing a codon for glutamine to arginine in GluA2 is essential to the organism and required for a normal brain development (2, 3).We have previously found that the mouse Gabra-3 transcript, coding for the ␣3 subunit of the GABA A receptor undergoes site-selective A-to-I editing causing an isoleucine to methionine (I/M) change in the third transmembrane region (TM3) (4). The chloride-permeable (GABA A ) receptors are the main mediators of fast inhibitory neurotransmission in the mammalian central nervous system (reviewed in Ref. 5). These heteropentameric ligand-gated chloride ion channels can be f...

Adenosine deaminases acting on RNA (ADARs) catalyze the editing of adenosine residues to inosine (A-to-I) within RNA sequences, mostly in the introns and UTRs (un-translated regions). The significance of editing within non-coding regions of RNA is poorly understood. Here, we demonstrate that association of ADAR2 with RNA stabilizes a subset of transcripts. ADAR2 interacts with and edits the 3΄UTR of nuclear-retained Cat2 transcribed nuclear RNA (Ctn RNA). In absence of ADAR2, the abundance and half-life of Ctn RNA are significantly reduced. Furthermore, ADAR2-mediated stabilization of Ctn RNA occurred in an editing-independent manner. Unedited Ctn RNA shows enhanced interaction with the RNA-binding proteins HuR and PARN [Poly(A) specific ribonuclease deadenylase]. HuR and PARN destabilize Ctn RNA in absence of ADAR2, indicating that ADAR2 stabilizes Ctn RNA by antagonizing its degradation by PARN and HuR. Transcriptomic analysis identified other RNAs that are regulated by a similar mechanism. In summary, we identify a regulatory mechanism whereby ADAR2 enhances target RNA stability by limiting the interaction of RNA-destabilizing proteins with their cognate substrates.

Site-selective adenosine (A) to inosine (I) RNA editing by the ADAR enzymes has been found in a variety of metazoan from fly to human. Here we describe a method to detect novel site-selective A to I editing that can be used on various tissues as well as species. We have shown previously that there is a preference for ADAR2-binding to selectively edited sites over non-specific interactions with random sequences of double-stranded RNA. The method utilizes immunoprecipitation (IP) of intrinsic RNA–protein complexes to extract substrates subjected to site-selective editing in vivo, in combination with microarray analyses of the captured RNAs. We show that known single sites of A to I editing can be detected after IP using an antibody against the ADAR2 protein. The RNA substrates were verified by RT–PCR, RNase protection and microarray. Using this method it is possible to uniquely identify novel single sites of selective A to I editing.

Adenosine-to-inosine (A-to-I) RNA editing is a cotranscriptional or posttranscriptional gene regulatory mechanism that increases the diversity of the proteome in the nervous system. Recently, the transcript for GABA type A receptor subunit α3 was found to be subjected to RNA editing. The aim of this study was to determine if editing of the chicken α3 subunit transcript occurs in the retina and if the editing is temporally regulated during development. We also raised the question if editing of the α3 transcript was temporally associated with the suggested developmental shift from excitation to inhibition in the GABA system. The editing frequency was studied by using Sanger and Pyrosequencing, and to monitor the temporal aspects, we studied the messenger RNA expression of the GABAA receptor subunits and chloride pumps, known to be involved in the switch. The results showed that the chick α3 subunit was subjected to RNA editing, and its expression was restricted to cells in the inner nuclear and ganglion cell layer in the retina. The extent of editing increased during development (after embryonic days 8-9) concomitantly with an increase of expression of the chloride pump KCC2. Expression of several GABAA receptor subunits known to mediate synaptic GABA actions was upregulated at this time. We conclude that editing of the chick GABAA subunit α3 transcript in chick retina gives rise to an amino acid change that may be of importance in the switch from excitatory to inhibitory receptors.