A Bi-Directional Competitive Interaction between <i>CircHomer1</i> and <i>Homer1b</i> within the Orbitofrontal Cortex Regulates Reversal Learning

Although circular RNAs (circRNAs) are abundantly expressed in the mammalian brain and preferentially-derived from synaptic genes, their relevance for brain function and psychiatric disorders and their interactions with neuronal protein-coding gene expression still remain poorly understood. Here we show that alterations in the levels of circHomer1, a neuronal-enriched circRNA altered in psychiatric disease that has been shown to influence OFC-mediated cognitive flexibility, are inversely associated with the relative expression of the long linear HOMER1B mRNA isoform in both the orbitofrontal cortex (OFC) and stem cell-derived neuronal cultures of subjects with psychiatric disorders. Using in vivo mature circHomer1-specific knockdown (KD) in adult mouse OFC, we further demonstrate that downregulation of circHomer1 results in increased synaptic expression of Homer1b mRNA. Furthermore, we utilize circRNA antisense purification to show that circHomer1 directly binds to Homer1b mRNA. We then demonstrate that in vivo Homer1b-specific KD increases synaptic circHomer1 levels within the OFC and improves OFC-mediated behavioral flexibility. Importantly, we show that double circHomer1 and Homer1b in vivo co-KD results in a complete rescue in circHomer1-associated alterations in both chance reversal learning and synaptic gene expression. Lastly, we demonstrate that circHomer1 is an experience-dependent circRNA that is significantly altered during chance reversal learning and that variability in its baseline expression within the OFC is significantly inversely correlated to behavioral performance during chance reversal learning. Taken together our data provide novel mechanistic insights into the importance of circRNAs in brain function and disease.