▪ Abstract ADARs are RNA editing enzymes that target double-stranded regions of nuclear-encoded RNA and viral RNA. These enzymes are particularly abundant in the nervous system, where they diversify the information encoded in the genome, for example, by altering codons in mRNAs. The functions of ADARs in known substrates suggest that the enzymes serve to fine-tune and optimize many biological pathways, in ways that we are only starting to imagine. ADARs are also interesting in regard to the remarkable double-stranded structures of their substrates and how enzyme specificity is achieved with little regard to sequence. This review summarizes ongoing investigations of the enzyme family and their substrates, focusing on biological function as well as biochemical mechanism.

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RNA Editing by Adenosine Deaminases That Act on RNA

Excitotoxicity is one of the most extensively studied processes of neuronal cell death, and plays an important role in many central nervous system (CNS) diseases, including CNS ischemia, trauma, and neurodegenerative disorders. First described by Olney, excitotoxicity was later characterized as an excessive synaptic release of glutamate, which in turn activates postsynaptic glutamate receptors. While almost every glutamate receptor subtype has been implicated in mediating excitotoxic cell death, it is generally accepted that the N-methyl-D-aspartate (NMDA) subtypes play a major role, mainly owing to their high calcium (Ca2+) permeability. However, other glutamate receptor subtypes such as 2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl) propionate (AMPA) or kainate receptors have also been attributed a critical role in mediating excitotoxic neuronal cell death. Although the molecular basis of glutamate toxicity is uncertain, there is general agreement that it is in large part Ca2+-dependent. The present review is aimed at summarizing the molecular mechanisms of NMDA receptor and AMPA/kainate receptor-mediated excitotoxic neuronal cell death.

Molecular mechanisms of glutamate receptor-mediated excitotoxic neuronal cell death.

The kainic acid model of temporal lobe epilepsy

The progressive maturation and functional plasticity of the nervous system in health and disease involve a dynamic interplay between the transcriptome and the environment. There is a growing awareness that the previously unexplored molecular and functional interface mediating these complex gene-environmental interactions, particularly in brain, may encompass a sophisticated RNA regulatory network involving the twin processes of RNA editing and multifaceted actions of numerous subclasses of non-protein-coding RNAs. The mature nervous system encompasses a wide range of cell types and interconnections. Long-term changes in the strength of synaptic connections are thought to underlie memory retrieval, formation, stabilization, and effector functions. The evolving nervous system involves numerous developmental transitions, such as neurulation, neural tube patterning, neural stem cell expansion and maintenance, lineage elaboration, differentiation, axonal path finding, and synaptogenesis. Although the molecular bases for these processes are largely unknown, RNA-based epigenetic mechanisms appear to be essential for orchestrating these precise and versatile biological phenomena and in defining the etiology of a spectrum of neurological diseases. The concerted modulation of RNA editing and the selective expression of non-protein-coding RNAs during seminal as well as continuous state transitions may comprise the plastic molecular code needed to couple the intrinsic malleability of neural network connections to evolving environmental influences to establish diverse forms of short- and long-term memory, context-specific behavioral responses, and sophisticated cognitive capacities.

https://www.physiology.org/doi/pdf/10.1152/physrev.00036.2006

Noncoding RNAs and RNA Editing in Brain Development, Functional Diversification, and Neurological Disease

RNA editing by adenosine deamination has been shown to generate multiple isoforms of several neural receptors, often with profound effects on receptor function. However, little is known about the regulation of editing activity during development. We have developed a large-scale RNA sequencing protocol to determine adenosine-to-inosine (A-to-I) editing frequencies in the coding region of genes in the mammalian brain. Using the 454 Life Sciences (Roche) Amplicon Sequencing technology, we were able to determine even low levels of editing with high accuracy. The efficiency of editing for 28 different sites was analyzed during the development of the mouse brain from embryogenesis to adulthood. We show that, with few exceptions, the editing efficiency is low during embryogenesis, increasing gradually at different rates up to the adult mouse. The variation in editing gave receptors like HTR2C and GABAA (gamma-aminobutyric acid type A) a different set of protein isoforms during development from those in the adult animal. Furthermore, we show that this regulation of editing activity cannot be explained by an altered expression of the ADAR proteins but, rather, by the presence of a regulatory network that controls the editing activity during development.

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Large-scale mRNA sequencing determines global regulation of RNA editing during brain development

Kainate (KA) is a potent neuroexcitatory agent in several areas of the adult brain, with convulsant and excitotoxic properties that increase as ontogeny proceeds. Besides its depolarizing actions, KA may enhance intracellular accumulation of Ca2+ to promote selective neuronal damage. The effects of KA are mediated by specific receptors recently considered to be involved in fast neurotransmission and that can be activated synaptically. KA receptors, e.g. GluR5 and GluR6 have been characterized by molecular cloning. Structure-function relationships indicate that in the MII domain of these KA receptors, a glutamine (Q) or arginine (R) residue determines ion selectivity. The arginine stems from post-transcriptional editing of the GluR5 and GluR6 pre-RNAs, and the unedited and edited versions of GluR6 elicit distinct Ca2+ permeability. Using a PCR-based approach, we show that in vivo, Q/R editing in the GluR5 and GluR6 mRNAs is modulated during ontogeny and differs substantially in a variety of nervous tissues. GluR5 editing is highest in peripheral nervous tissue, e.g. the dorsal root ganglia, where GluR6 expression is barely detectable. In contrast, GluR6 editing is maximal in forebrain and cerebellar structures where GluR5 editing is lower. Intra-amygdaloid injections of KA provide a model of temporal lobe epilepsy, and we show that following seizures, the extent of GluR5 and GluR6 editing is altered in the hippocampus. However, in vitro, high levels of glutamate and potassium-induced depolarizations have no effect on GluR5 and GluR6 Q/R editing. GluR6 editing is rapidly enhanced to maximal levels in primary cultures of cerebellar granule neurons but not in cultured hippocampal pyramidal neurons. Finally, we show that cultured glial cells express partially edited GluR6 mRNAs. Our results indicate that Q/R editing of GluR5 and GluR6 mRNAs is structure-, cell type- and time-dependent, and suggest that editing of these mRNAs is not co-regulated.

Q/R editing of the rat GluR5 and GluR6 kainate receptors in vivo and in vitro: evidence for independent developmental, pathological and cellular regulation.

Frédéric Dessi

Papers

Q/R editing of the rat GluR5 and GluR6 kainate receptors in vivo and in vitro: evidence for independent developmental, pathological and cellular regulation.