Abstracts

RNA Editing of GABAA Receptors: A Potential Mechanism for Shaping Developing Synapses.

Abstract number : IW.46;
Submission category : 1. Translational Research
Year : 2007
Submission ID : 8179
Source : www.aesnet.org
Presentation date : 11/30/2007 12:00:00 AM
Published date : Nov 29, 2007, 06:00 AM

Authors :
A. H. Lagrange1, E. Y. Rula2, R. B. Emeson2, R. L. Macdonald1

Rationale: The α3 subunit is a predominant GABAA receptor isoform expressed during brain development, a period when GABA evokes depolarizing currents that may play an important role in synaptogenesis. Recent studies by Ohlson et al (2007) reported that editing of the α3 subunit mRNA occurs at about the same time. This process involves a site-selective deamination of a single adenosine in transmembrane domain 3, thereby converting a genomically encoded isoleucine (I) to a methionine (M) codon. Expanding on this previous work, we found that RNA transcripts encoding the adult mouse α3 subunit, but not the α1, 2, 4, 5 or 6 subunits, are edited to 90% completion in most brain regions, although 30% of the unedited form persists in the adult hippocampus. By contrast, the extent of editing is very low at embryonic day 15 (E15) and increases during development, reaching maximal levels by postnatal day 7 (P7). Methods: To explore the functional effects of editing, cDNAs encoding either non-edited α3(I) or edited α3(M) human subunits were cotransfected with human β3 and γ2L subunit cDNAs into HEK 293T cells. Whole cell voltage clamp of lifted cells was used to record the currents elicited by GABA applied with a rapid drug delivery system.Results: Compared to α1β2γ2 receptors (the predominant combination in the mature brain), α3(I/M)β3γ2L receptors were insensitive to low levels of GABA and activated very slowly but also desensitized and deactivated slowly. After excluding the possibility that RNAs encoding the non-edited α3(I) subunit were modified in HEK cells, we found that currents from receptors containing the non-edited α3 subunit [α3(I)β3γ2L] activated more rapidly, responded to slightly lower GABA concentrations and deactivated much more slowly than currents from edited α3(M)β3γ2L receptors. Since the non-edited α3(I) subunit is prevalent at a developmental period during which GABA-evoked currents are depolarizing, ramp current-voltage (IV) plots were generated before and during exposure to a low concentration of GABA. The edited form of the receptor conducted current in a nearly linear manner, whereas the non-edited form had strong outward rectification, consistent with enhanced influx of Cl- anions. Conclusions: Taken together, these results suggest that regulation of GABAA receptor α3 subunits allows a developmental period during which a predominant receptor isoform is particularly responsive to GABA. Since this occurs when GABA evokes depolarizing currents, the non-edited α3(I) form expressed in developing brain would allow for robust excitatory responses that may trigger sodium- and/or calcium-dependent action potentials. However, once the membrane potential exceeds the chloride reversal potential, GABAA receptors would conduct a strong influx of Cl-, thereby shunting their depolarizing response. Therefore, RNA editing of α3 GABAA proteins may provide a developmental window in which GABA is stimulatory, while at the same time maintaining inhibition during excessive excitation.
Translational Research