RNA editing: a molecular mechanism for the fine modulation of neuronal transmission

The term "RNA editing" is used to identify any mechanism responsible for producing mRNA molecules with sequence information not specifically encoded in the DNA. RNA editing is therefore an important event in gene modification, which takes place at a post-transcriptional level. The molecular mechanism of RNA editing involves site-selective deamination of adenosine to inosine in pre-mRNA, which leads to altering translation codons and splicing in nuclear transcripts, whereby functionally distinct proteins can be produced from a single gene. The mammalian editing enzymes ADARs (adenosine deaminases acting on RNA) are widely expressed in brain and other tissues: however, up until now their substrates have mainly been found in the Central Nervous System (CNS). Of particular relevance in the CNS is the editing occurring at the ionotropic glutamate receptors (GluRs) level. Three AMPA and two Kainate receptors are subject to RNA editing. The consequence of this process is the substitution of specific amino acids in functionally critical positions of the receptors. Depending on the GluR involved, the consequences of editing will involve: activation and/or inhibition of splicing sites; modulation of the trafficking of the receptor to the plasma membrane; the process of tetramerization of the receptor subunits; modification of the ions passage through the receptor channel; modulation of the desensitization and action potential recovery times. All these events are specific to the different GluRs and are genetically and developmentally controlled. RNA editing is therefore a crucial event involved in controlling transmission of the action potential at the postsynaptic level. This modulation involves the transmission of all sensory stimuli to the CNS and gives rise to the "Sensotype". The Sensotype therefore defines the "way" in which the information acquired from the environment by the sensory systems is transmitted to the brain. The signals and inputs deriving from the Sensotype are transmitted to the brain, which processes and stores these signals thus generating the "Brainotype". Brainotype and Sensotype are genetically and environmentally determined; they are individually unique and specific to every living organism with a nervous system. Their characteristics are, at least in part, dependent on the modulation of the "RNA editing" process since glutamate receptors represent the main neurotransmitter system in the CNS.