CircFKBP8(5S,6)-encoded protein promotes stress susceptibility in mice by downregulating dopamine D3 receptor expression and its downstream AMPK/mTOR/ULK1 autophagy signalling

Major depressive disorder (MDD) is a serious mental disorder, yet the mechanism by which circular RNAs (circRNAs) are involved in the pathogenesis of MDD by encoding proteins is unknown. Our previous study found that circFKBP8 (5S, 6) relies on its encoded proteins, namely cFKBP8, to promote susceptibility to chronic unpredictable mild stress (CUMS) in mice, but the precise molecular mechanisms are unknown. Here we found that overexpression of circFKBP8 (5S, 6) or cFKBP8 in neurons of the prelimbic cortex (Prl) of CUMS mice downregulated the expression level of DRD3 and its downstream AMPK/ULK1 (Ser555) and AMPK/mTOR/ULK1 (Ser757) pathways, which resulted in downregulation of neuronal autophagy levels. Interestingly, both activation and overexpression of DRD3 ameliorated the exacerbation of depressive-like behaviors induced by circFKBP8 (5S, 6) or cFKBP8, activated both the AMPK/ULK1 (Ser555) pathway and the AMPK/mTOR/ULK1 (Ser757) pathway, and upregulated neuronal autophagy levels. In conclusion, circFKBP8 (5S, 6) or cFKBP8 promote susceptibility to CUMS in mice, at least in part, by down-regulating DRD3 expression and its downstream AMPK/mTOR/ULK1 signaling pathway-mediated neuronal autophagy. Overall design: 6-8 week male mice were randomly assigned to receive one of the following viral injections (bilateral prelimbic cortex tissues ; coordinates: AP +1.9 mm, ML ±0.3 mm, DV -2.5 mm from bregma): Vehicle control: rAAV-hSyn-bGH-polyA-CMV-EGFP (AAV/9), circFKBP8(5S,6) overexpression: rAAV-hSyn-circFKBP8-bGH-polyA-CMV-EGFP (AAV/9), cFKBP8 overexpression: rAAV-hSyn-peptide-3×Flag-P2A-EGFP-hGH-polyA (AAV/9). Neuronal overexpression of circFKBP8(5S,6) or its encoded protein cFKBP8 was confirmed 2 weeks post-injection. Mice subsequently underwent 5 weeks of chronic unpredictable mild stress, after which prelimbic cortex tissues were collected for RNA sequencing analysis.