Increased A-to-I RNA editing of the transcript for GABAA receptor subunit α3 during chick retinal development

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 alpha 3 was found to be subjected to RNA editing. The aim of this study was to determine if editing of the chicken alpha 3 subunit transcript occurs in the retina and if the editing is temporally regulated during development. We also raised the question if editing of the alpha 3 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 GABA(A) receptor subunits and chloride pumps, known to be involved in the switch. The results showed that the chick alpha 3 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 alpha 3 transcript in chick retina gives rise to an amino acid change that may be of importance in the switch from excitatory to inhibitory receptors.