Control of kinetic properties of GluR2 flop AMPA‐type channels: impact of R/G nuclear editing

Abstract

The GluR2 flop subunit of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-type glutamate receptors greatly determines calcium permeability and kinetic properties of heteromeric AMPA subunit assemblies. Post-transcriptional editing of this subunit at the Q/R/N site controls calcium permeability whereas editing at the R/G site is involved in the regulation of biophysical properties. We used patch-clamp techniques with ultrafast solution exchange to examine the kinetics of recombinant human homomeric GluR2 flop channels transiently expressed in HEK293 cells [edited at the R/G site and Q/R/N site (GR), and unedited (RN) and edited (GN) at the R/G site both with asparagine (N) at the Q/R/N site]. The time constant of desensitization after application of 10 mm glutamate was 1.38 +/- 0.05 ms (n = 10), 5.53 +/- 0.57 ms (n = 7) and 1.33 +/- 0.06 ms (n = 12) for the GluR2 flop GR, RN and GN channels, respectively. The time constant of resensitization was 75 ms for the GluR2 flop RN and 30 ms for the GN channels. The dose-dependence of the peak current amplitude, kinetics of activation and deactivation, and peak open probability did not differ between RN and GN channels. The study shows that desensitization and resensitization kinetics of homomeric GluR2 flop channels are controlled by a single amino acid exchange (glycine by arginine) at the R/G site. Quantitative analysis by computer simulation using a circular kinetic scheme allows the prediction of the main experimental results.

Prolonged Positive Modulation of α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid (AMPA) Receptors Induces Calpain-Mediated PSD-95/Dlg/ZO-1 Protein Degradation and AMPA Receptor Down-Regulation in Cultured Hippocampal Slices

“…Together, our results argue against NMDArs being the principal source of increases in intracellular Ca 2ϩ needed for calpain activation with the manipulations described here. However, AMPArs are Na ϩ channels and are, by and large, impermeable to Ca 2ϩ ions (Lomeli et al, 1994;Jensen et al, 1998;Carlson et al, 2000;Iizuka et al, 2000;Krampfl et al, 2002;Kumar et al, 2002). Interestingly, elevated intracellular Na ϩ levels can increase Ca 2ϩ release from intracellular stores (Hoyt et al, 1998;Zhang and Lipton 1999), thus CX614-induced increases in AMPAr function and the resulting depolarization may contribute to increased intracellular levels of Ca 2ϩ .…”

Altered RNA Editing in Mice Lacking ADAR2 Autoregulation

“…The majority of well-characterized A-to-I editing events involve nonsynonymous codon changes in mRNA sequences, resulting in the production of proteins with altered functional properties. In mammals, the most prominent examples of A-to-I editing have been described for transcripts encoding ionotropic glutamate receptor subunits (GluR), a voltage-gated potassium channel subunit (K v 1.1), and the 2C subtype of the serotonin receptor (5-HT 2C R), which lead to the production of channels with altered electrophysiological and ion permeation properties (6,27,37,38,43,54) and receptors with decreased G-protein coupling efficiency (5,10,46). A-to-I modifications have also been described for nontranslated RNA species and noncoding regions of RNA transcripts, suggesting that such RNA modifications may also affect other aspects of RNA function, including splicing, trafficking, translation efficiency, and transcript stability (1,7,35,42,45).…”

Involvement of AMPA Receptor and Its Flip and Flop Isoforms in Retinal Ganglion Cell Death Following Oxygen/Glucose Deprivation

“…62,63 Editing of this site, allows for faster desensitization rate and a faster recovery period from desensitization. [36][37][38] Both ADAR1 and ADAR2 deaminases are able to edit at the R/G site, where an adenosine is converted to an inosine. 62,63 The inosine then is translated as a guanosine and, therefore, alters the translation of the R/G site codon to be read from an arginine (R) to glycine (G).…”

“…Electrophysiology recordings of the homomeric AMPAR composed of unedited Gria2 flop isoform produces a longer desensitization rate compared to the edited isoform. 38 There is a third ADAR deaminase, ADAR3, whose role and substrate are unknown. However, it is hypothesized that ADAR3 can compete and bind the same substrates as ADAR1 and 2 and, therefore, modulates or blocks ADAR1 and 2 activities.…”

“…34,35 Also, GLUA2-4 can be edited at the R/G site, where editing of the R/G site produces a faster desensitization and a faster recovery time from desensitization. [36][37][38] The complex heterogeneity of AMPARs isoforms may have a role in excitotoxicity during disease conditions, in which changes in AMPAR's posttranscriptional modifications (altering desensitization, recovery time, and ion permeability), may increase neuron susceptibility to AMPAR-mediated excitotoxicity. The involvement of the changes in AMPARs flip to flop ratios have been observed in clinical or experimental settings of neurodegenerative diseases and neuro trauma, such as amyotrophic lateral sclerosis (ALS), ischemia, retinitis pigmentosa, and Parkinson's disease.…”