Vulnerable Neurons Lack RNA Editor

Abstract

Glutamate receptors of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) type are composed of four subunits, and the GluR2 subunit dictates the calcium permeability of the channel. In the adult brain, mRNA editing converts GluR2 subunits into a calcium-impermeable form by converting a codon for glutamine (Q) to one for arginine (R). Previous reports demonstrated that in CA1 pyramidal neurons, calcium and zinc permeability through AMPA receptors increased substantially following transient ischemia, a condition that also causes glutamate to accumulate extracellularly. Peng et al . used single-cell reverse transcription polymerase chain reaction (RT-PCR) to show that transient ischemia reduced GluR2 subunit mRNA editing. RT-PCR was performed on RNA from cells for which the relative permeability of Ca 2+ and Na + (P Ca /P Na ) for AMPA receptors had been recorded. After 12 hours of ischemia, neurons from the CA1 region exhibited reduced GluR2 RNA editing and decreased abundance of adenosine deaminase acting on RNA 2 (ADAR2) mRNA (which encodes a nuclear enzyme involved in RNA editing), whereas other neurons in the hippocampus that are not as sensitive to ischemia-induced cell injury did not show any reduction in GluR2 editing or ADAR2 mRNA abundance. Forced expression of ADAR2 in rat brain prevented the decrease in GluR2 editing, the increase in calcium permeability, and, most important, postischemic neuronal injury. Knockdown of ADAR2 using RNAi inhibited GluR2 editing, increased calcium permeability, and accelerated the rate of neuronal death in CA1 neurons post-ischemic insult, and converted the normally resistant neurons of the dentate gyrus to ischemia-sensitive neurons. Neuronal survival after ischemia was rescued by forced expression of the GluR2(R) subunit. Ischemia-sensitive CA1 neurons exhibited decreased activation (phosphorylation) of the transcription factor CREB following 12 hours of ischemia, whereas neurons in the resistant regions did not. Forced expression of a constitutively active form of CREB prevented the decrease in ADAR2 abundance caused by ischemia, the decrease in GluR2 editing, the increase in P Ca /P Na , and loss of the CA1 neurons. Thus, differential sensitivity to ischemic insult appears to lie in the inactivation of CREB in sensitive neurons, which in turn prevents expression of ADAR2, thereby blocking AMPA receptor editing, allowing a calcium- and zinc-permeable form of the channel to contribute to cell injury and death (see Kittler for commentary). P. L. Peng, X. Zhong, W. Tu, M. M. Soundarapandian, P. Molner, D. Zhu, L. Lau, S. Liu, F. Liu, Y. Lu, ADAR2-dependent RNA editing of AMPA receptor subunit GluR2 determines vulnerability of neurons in forebrain ischemia. Neuron 49 , 719-733 (2006). [PubMed] J. T. Kittler, Censoring the editor in transient forebrain ischemia. Neuron 49 , 646-648 (2006). [PubMed]