Reduced editing of low-affinity kainate receptor subunits in optic nerve glial cells

We have analyzed the degree of editing of adult optic nerve mRNAs encoding the low-affinity kainate receptor subunits, GluR5 and GluR6, the two major constituents of native receptors in this family. To this end, we used reverse transcription (RT) followed by polymerase chain reaction (PCR), and subsequent cloning and sequencing of the amplified fragments. Our results revealed that the GluR5 subunit is unedited at the Q/R site of cismembrane domain 2 (M2), whereas the GluR6 subunit is edited to a low extent at this site. These findings are in contrast to those reported by others using mRNAs from the adult brain in which GluR5 and GluR6 are edited at the Q/R site of M2 to a larger extent. In addition, we found that the adenosine deaminases, DRADA, RED1 and RED2, which edit ionotropic glutamate receptors in the brain, are expressed in the adult optic nerve and in oligodendrocytes, the major cell type in this structure. It thus appears that editing of kainate receptors in optic nerve cells is not limited by the availability of editing enzymes but rather by other, as yet unidentified factors. Overall, reduced editing of kainate receptor subunits in glial cells may result in different functional responses of the native receptors present in these cells with respect to those in neurons.