tRNA epitranscriptomic alterations associated with opioid-induced reward-seeking and long-term opioid withdrawal

DNA cytosine methylation has been documented as a potential epigenetic mechanism of transcriptional regulation underling opioid use disorder. However, methylation of RNA cytosine residues, which would drive another level of biological influence as an epitranscriptomic mechanism of gene and protein regulation has not been studied. Here, we probed whether chronic morphine exposure could alter tRNA cytosine methylation (m5C) and resulting expression levels in the medial prefrontal cortex (mPFC), a brain region crucial for reward processing and executive function that exhibits opioid-induced molecular restructuring. We identified dynamic changes in glycine tRNA (tRNAGlyGCC) cytosine methylation, corresponding to altered expression levels of this tRNA at multiple timepoints following 15 days of daily morphine. Strikingly, a robust increase in methylation, coupled with decreased expression, was present after 30 days of withdrawal, suggesting that repeated opioid administration produces changes to the tRNA regulome long after discontinuation. Furthermore, forebrain-wide knockout of neuronal Nsun2, a tRNA methyltransferase, was associated with disruption of reward-seeking in the conditioned place preference paradigm, and this effect was recapitulated by regional mPFC Nsun2 knockout. Taken together, these studies provide a foundational link between the regulation of tRNA cytosine methylation and OUD and highlight the tRNA machinery as a potential therapeutic target in addiction.