Patterns of developmental expression of the RNA editing enzyme rADAR2

To date, two structurally related RNA-editing enzymes with adenosine deamin ase activity have been identified in mammalian tissue: ADAR1 and ADAR2 [Bas s B. I. et al. (1997) RNA 3, 947-949]. In rodents, ADAR2 undergoes alternat ive RNA splicing, giving rise to two splice variants that differ by the pre sence or absence of a 10-amino-acid insert in the carboxy-terminal catalyti c domain. However, the physiological significance of the splicing and its r egional and developmental regulation are as yet unknown. The present study examined spatial and temporal patterns of ADAR2 gene transcripts within spe cific neuronal populations of rat brain. The two rodent ADAR2 isoforms were expressed at comparable levels at all ages examined. rADAR2 messenger RNA expression was first detectable in the thalamic nuclei formation at embryon ic day E19. The rADAR2b insert and rADAR2a splice probes produced images si milar to that of the rADAR2 pan probe. At birth, rADAR2a messenger RNA spli ce variants were abundantly expressed in the thalamic nuclei. No signal for any probe was detectable in other brain regions, including neocortex, hipp ocampus, striatum and cerebellum at this stage of development. During the f irst week of postnatal life, rADAR2 messenger RNA expression (detected with the pan probe) increased gradually in several brain regions, with low expr ession detected at postnatal day P7 in the olfactory bulb, inferior collicu lus, and within the pyramidal and granule cell layers of the hippocampus. H ybridization pattern of the rADAR2a variant probe reached peak expression a t about the second week of life, while peak expression of the rADAR2b probe was reached at about the third week of life. At the end of the first week of life (P7), expression of both splice variants was strongest in the thala mic nuclei. By P14, rADAR2 messenger RNA expression was more consolidated i n the deeper structures, including the thalamic nuclei and the granule cell layer of the cerebellum. By P21, maximal levels of rADARb expression were observed in the thalamic nuclei, inferior colliculus, cerebellum and pontin e nuclei. In the adult, rADAR2 messenger RNA expression was of highest inte nsity in the thalamic nuclei, with high levels of expression in the olfacto ry bulb, inferior colliculus, cerebellum and pontine nuclei. At the level o f the hippocampus, positive labelling was restricted to the CA3 region of t he Ammon's horn and the dentate gyrus, with weak signals in the CA1 subfiel d. rADAR2 pan expression was at near background levels throughout the neoco rtex and caudate putamen. In summary, our study shows that ADAR2 messenger RNA expression is regulate d in a cell-specific manner throughout development. At early ages, ADAR2 me ssenger RNA is expressed only within (and restricted to) the thalamic nucle i. By the third postnatal week, expression of the editase enzyme is more wi dely distributed throughout the olfactory bulb, CA3 and dentate gyrus of th e hippocampus, thalamus, inferior colliculus and the molecular cell layer o f the cerebellum. ADAR2 is thought to act at specific nucleotide positions in primary transcripts encoding glutamate receptor subunits, thereby alteri ng gating and ionic permeability properties of AMPA- and kainate-activated channels. ADAR2 also acts at pre-messenger RNA encoding the serotonin 5HT-2 C receptor to alter G-protein coupling. Thus, RNA editing may be an importa nt mechanism for fine-tuning of the physiological and pharmacological prope rties of transmitter receptors of the central nervous system. (C) 1999 IBRO . Published by Elsevier Science Ltd.