ADAR2-dependent RNA editing of AMPA receptor subunit GluR2 determines vulnerability of neurons in forebrain ischemia

ADAR2 is a nuclear enzyme essential for GluR2 pre-mRNA editing at Q/R site-607, which gates Ca2+ entry through AMPA receptor channels. Here, we show that forebrain ischemia in adult rats selectively reduces expression of ADAR2 enzyme and, hence, disrupts RNA Q/R site editing of GluR2 subunit in vulnerable neurons. Recovery of GluR2 Q/R site editing by expression of exogenous ADAR2b gene or a constitutively active CREB, VP16-CREB, which induces expression of endogenous ADAR2, protects vulnerable neurons in the rat hippocampus from forebrain ischemic insult. Generation of a stable ADAR2 gene silencing by delivering small interfering RNA (siRNA) inhibits GluR2 Q/R site editing, leading to degeneration of ischemia-insensitive neurons. Direct introduction of the Q/R site edited GluR2 gene, GluR2(R607), rescues ADAR2 degeneration. Thus, ADAR2-dependent GluR2 Q/R site editing determines vulnerability of neurons in the rat hippocampus to forebrain ischemia.     

Editing of AMPA and Serotonin 2C Receptors in Individual Central Neurons,Controlling Wakefulness

(1) Pre-mRNA editing of serotonin 2C (5-HT_2C) and glutamate (Glu) receptors (R) influences higher brain functions and pathological states such as epilepsy, amyotrophic lateral sclerosis, and depression. Adenosine deaminases acting on RNA (ADAR1–3) convert adenosine to inosine on synthetic RNAs, analogous to 5-HT_2cR and GluR. The order of editing as well as mechanisms controlling editing in native neurons is unknown. (2) With single-cell RT-PCR we investigated the co-expression of ADAR genes with GluR and 5-HT_2CR and determined the editing status at known sites in the hypothalamic tuberomamillary nucleus, a major center for wakefulness and arousal. (3) The most frequently expressed enzymes were ADAR1, followed by ADAR2. The Q/R site of GluR2 was always fully edited. Editing at the R/G site in the GluR2 (but not GluR4) subunit was co-ordinated with ADAR expression: maximal editing was found in neurons expressing both ADAR2 splice variants of the deaminase domain and lacking ADAR3. (4) Editing of the 5-HT_2CR did not correlate with ADAR expression. The 5-HT_2CR mRNA was always edited at A, in the majority of cells at B sites and variably edited at E, C and D sites. A negative correlation was found between editing of C and D sites. The GluR4 R/G site editing was homogeneous within individuals: it was fully edited in all neurons obtained from 12 rats and under-edited in six neurons obtained from three rats. (5) We conclude that GluR2 R/G editing is controlled at the level of ADAR2 and therefore this enzyme may be a target for pharmacotherapy. On the other hand, further factors/enzymes besides ADAR must control or influence 5-HT_2CR and GluR pre-mRNA editing in native neurons; our data indicate that these factors vary between individuals and could be predictors of psychiatric disease.     

Developmentally regulated, combinatorial RNA processing modulates AMPA receptor biogenesis

The subunit composition determines AMPA receptor (AMPA-R) function and trafficking. Mechanisms underlying channel assembly are thus central to the efficacy and plasticity of glutamatergic synapses. We previously showed that RNA editing at the Q/R site of the GluR2 subunit contributes to the assembly of AMPA-R heteromers by attenuating formation of GluR2 homotetramers. Here we report that this function of the Q/R site depends on subunit contacts between adjacent ligand binding domains (LBDs). Changes of LBD interface contacts alter GluR2 assembly properties, forward traffic, and expression at synapses. Interestingly, developmentally regulated RNA editing within the LBD (at the R/G site) produces analogous effects. Our data reveal that editing to glycine reduces the self-assembly competence of this critical subunit and slows GluR2 maturation in the endoplasmic reticulum (ER). Therefore, RNA editing sites, located at strategic subunit interfaces, shape AMPA-R assembly and trafficking in a developmentally regulated manner.     

Dimerization of ADAR2 is mediated by the double-stranded RNA binding domain

Members of the family of adenosine deaminases acting on RNA (ADARs) can catalyze the hydrolytic deamination of adenosine to inosine and thereby change the sequence of specific mRNAs with highly double-stranded structures. The ADARs all contain one or more repeats of the double-stranded RNA binding motif (DRBM). By both in vitro and in vivo assays, we show that the DRBMs of rat ADAR2 are necessary and sufficient for dimerization of the enzyme. Bioluminescence resonance energy transfer (BRET) demonstrates that ADAR2 also exists as dimers in living mammalian cells and that mutation of DRBM1 lowers the dimerization affinity while mutation of DRBM2 does not. Nonetheless, the editing efficiency of the GluR2 Q/R site depends on a functional DRBM2. The ADAR2 DRBMs thus serve differential roles in RNA dimerization and GluR2 Q/R editing, and we propose a model for RNA editing that incorporates the new findings.     

Editing independent effects of ADARs on the miRNA/siRNA pathways

Adenosine deaminases acting on RNA (ADARs) are best known for altering the coding sequences of mRNA through RNA editing, as in the GluR‐B Q/R site. ADARs have also been shown to affect RNA interference (RNAi) and microRNA processing by deamination of specific adenosines to inosine. Here, we show that ADAR proteins can affect RNA processing independently of their enzymatic activity. We show that ADAR2 can modulate the processing of mir‐376a2 independently of catalytic RNA editing activity. In addition, in a Drosophila assay for RNAi deaminase‐inactive ADAR1 inhibits RNAi through the siRNA pathway. These results imply that ADAR1 and ADAR2 have biological functions as RNA‐binding proteins that extend beyond editing per se and that even genomically encoded ADARs that are catalytically inactive may have such functions.     

Regulation of glutamate receptor B pre-mRNA splicing by RNA editing

RNA-editing enzymes of the ADAR family convert adenosines to inosines in double-stranded RNA substrates. Frequently, editing sites are defined by base-pairing of the editing site with a complementary intronic region. The glutamate receptor subunit B (GluR-B) pre-mRNA harbors two such exonic editing sites termed Q/R and R/G. Data from ADAR knockout mice and in vitro editing assays suggest an intimate connection between editing and splicing of GluR-B pre-mRNA.

By comparing the events at the Q/R and R/G sites, we can show that editing can both stimulate and repress splicing efficiency. The edited nucleotide, but not ADAR binding itself, is sufficient to exert this effect. The presence of an edited nucleotide at the R/G site reduces splicing efficiency of the adjacent intron facilitating alternative splicing events occurring downstream of the R/G site.

Lack of editing inhibits splicing at the Q/R site. Editing of both the Q/R nucleotide and an intronic editing hotspot are required to allow efficient splicing. Inefficient intron removal may ensure that only properly edited mRNAs become spliced and exported to the cytoplasm.

     

The role of RNA editing of kainate receptors in synaptic plasticity and seizures

The ionotropic glutamate receptor subunit GluR6 undergoes developmentally and regionally regulated Q/R site RNA editing that reduces the calcium permeability of GluR6-containing kainate receptors. To investigate the functional significance of this editing in vivo, we engineered mice deficient in GluR6 Q/R site editing. In these mutant mice but not in wild types, NMDA receptor-independent long-term potentiation (LTP) could be induced at the medial perforant path-dentate gyrus synapse. This indicates that kainate receptors with unedited GluR6 subunits can mediate LTP. Behavioral analyses revealed no differences from wild types, but mutant mice were more vulnerable to kainate-induced seizures. Together, these results suggest that GluR6 Q/R site RNA editing may modulate synaptic plasticity and seizure vulnerability.     

听原性惊厥易感大鼠下丘GluR2的表达及QR位点编辑水平

听原性惊厥易感大鼠是强直 -阵挛惊厥大发作的一种模型 .一般认为 ,下丘是听原性惊厥发作神经元网络的启动部位 .采用RT PCR、Western印迹、免疫组织化学等方法观察了听原性惊厥易感大鼠 (P77PMC)一次惊厥发作与惊厥点燃状态下AMPA受体亚基GluR2在下丘内表达的改变 ,并采用限制性酶切方法分析了GluR2Q R位点mRNA编辑水平的改变 .研究结果显示 ,一次惊厥发作后下丘内GluR2表达无明显改变 ,惊厥点燃后下丘内GluR2表达降低 ,一次惊厥发作及惊厥点燃状态下GluR2Q R位点处于编辑成熟状态 .提示 ,GluR2表达降低参与了点燃状态下的惊厥发作 ,在听原性惊厥易感大鼠惊厥发作机制中不涉及下丘内GluR2Q R位点编辑水平改变 .     

Censoring the editor in transient forebrain ischemia

A molecular explanation for why some neurons are more vulnerable than others to ischemic injury has long remained elusive. In this issue of Neuron, Peng et al. propose that CREB-dependent downregulation of the RNA editing enzyme ADAR2, resulting in defective Q/R editing of AMPA receptor GluR2 subunits and increased availability of calcium and zinc-permeable death-promoting AMPA receptors, underlies the vulnerability of some neuronal populations to ischemia.     

Dramatic Increase of the RNA Editing for Glutamate Receptor Subunits During Terminal Differentiation of Clonal Human Neurons

Abstract: RNA editing plays an important role in determining physiological characteristics of certain glutamate-gated receptor (GluR) channels such as Ca²⁺ permeability and desensitization kinetics. In one case, the editing changes a gene-encoded glutamine (Q) to an arginine (R) codon located in the channel-forming domain of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor subunit GluR-B and also the kainate receptor subunits GluR5 and GluR6. Another case of RNA editing alters an arginine (R) to a glycine (G) codon at a position termed the "R/G" site of AMPA subunits GluR-B, C, and D. Double-stranded RNA-specific adenosine deaminases (DRADA) have been implicated as agents involved in the editing. By using a human teratocarcinoma cell line, NT2, we investigated the change of the RNA editing of GluR subunits in conjunction with the expression of two DRADA members, DRADA1 and DRADA2 genes, during neuronal differentiation. Whereas Q/R and R/G site RNA editing both become progressively activated in differentiating NT2 cells, the expression of the two DRADA genes can already be detected even in the undifferentiated NT2 cells. Development of the editing machinery appears to require, in addition to DRADA enzymes, a currently unidentified mechanism(s) that may become activated during neuronal differentiation.     

AMPA receptor tetramerization is mediated by Q/R editing

AMPA-type glutamate receptors (AMPARs) play a major role in excitatory synaptic transmission and plasticity. Channel properties are largely dictated by their composition of the four subunits, GluR1-4 (or A-D). Here we show that AMPAR assembly and subunit stoichiometry are determined by RNA editing in the pore loop. We demonstrate that editing at the GluR2 Q/R site regulates AMPAR assembly at the step of tetramerization. Specifically, edited R subunits are largely unassembled and ER retained, whereas unedited Q subunits readily tetramerize and traffic to synapses. This assembly mechanism restricts the number of the functionally critical R subunits in AMPAR tetramers. Therefore, a single amino acid residue affects channel composition and, in turn, controls ion conduction through the majority of AMPARs in the brain.     

ADAR2 A-->I editing: site selectivity and editing efficiency are separate events

ADAR enzymes, adenosine deaminases that act on RNA, form a family of RNA editing enzymes that convert adenosine to inosine within RNA that is completely or largely double-stranded. Site-selective A→I editing has been detected at specific sites within a few structured pre-mRNAs of metazoans. We have analyzed the editing selectivity of ADAR enzymes and have chosen to study the naturally edited R/G site in the pre-mRNA of the glutamate receptor subunit B (GluR-B). A comparison of editing by ADAR1 and ADAR2 revealed differences in the specificity of editing. Our results show that ADAR2 selectively edits the R/G site, while ADAR1 edits more promiscuously at several other adenosines in the double-stranded stem. To further understand the mechanism of selective ADAR2 editing we have investigated the importance of internal loops in the RNA substrate. We have found that the immediate structure surrounding the editing site is important. A purine opposite to the editing site has a negative effect on both selectivity and efficiency of editing. More distant internal loops in the substrate were found to have minor effects on site selectivity, while efficiency of editing was found to be influenced. Finally, changes in the RNA structure that affected editing did not alter the binding abilities of ADAR2. Overall these findings suggest that binding and catalysis are independent events.     

Amino acid substitutions in the pore helix of GluR6 control inhibition by membrane fatty acids

RNA editing at the Q/R site in the GluR5 and GluR6 subunits of neuronal kainate receptors regulates channel inhibition by lipid-derived modulators including the cis-unsaturated fatty acids arachidonic acid and docosahexaenoic acid. Kainate receptor channels in which all of the subunits are in the edited (R) form exhibit strong inhibition by these compounds, whereas wild-type receptors that include a glutamine (Q) at the Q/R site in one or more subunits are resistant to inhibition. In the present study, we have performed an arginine scan of residues in the pore loop of the GluR6(Q) subunit. Amino acids within the range from -19 to +7 of the Q/R site of GluR6(Q) were individually mutated to arginine and the mutant cDNAs were expressed as homomeric channels in HEK 293 cells. All but one of the single arginine substitution mutants yielded functional channels. Only weak inhibition, typical of wild-type GluR6(Q) channels, was observed for substitutions +1 to +6 downstream of the Q/R site. However, arginine substitution at several locations upstream of the Q/R site resulted in homomeric channels exhibiting strong inhibition by fatty acids, which is characteristic of homomeric GluR6(R) channels. Based on homology with the pore loop of potassium channels, locations at which R substitution induces susceptibility to fatty acid inhibition face away from the cytoplasm toward the M1 and M3 helices and surrounding lipids.