miR-34 regulates stress-induced depression-like state through the WDR26 ortholog melancholy in Drosophila

Major depressive disorder (MDD) is a complex and recurrent neuropsychiatric disorder with poorly understood molecular underpinnings. Psychosocial stress is a major environmental risk factor, yet the mechanisms linking stress exposure to behavioral outcomes remain unclear. MicroRNAs (miRNAs) have emerged as key regulators linking environmental stress to changes in gene expression in mood disorders. Here, using stress-induced rodent and Drosophila models, we identify the conserved, brain-enriched miRNA, miR-34, as a critical mediator of stress responses. Our expression analysis revealed significant downregulation of miR-34 in brain tissues from stress-induced depressed rodent and fruit fly models. We showed that this stress-induced downregulation of miR-34 is due to the upregulation of its transcriptional inhibitor Broad. Loss of miR-34 in flies induces depression-like behavior even in the absence of stress, whereas its overexpression in serotonin-sensing neurons confers resilience. We identify a conserved regulatory axis between miR-34 and CG7611, encoding the WDR26 ortholog Melancholy (Mel). Reducing mel levels mimics miR-34 overexpression, while its upregulation induces depression-like phenotypes. Proteomic analysis shows that mel overexpression recapitulates a stress-like state, characterized by downregulation of cytoskeletal, nuclear pore, chromatin, and transcriptional proteins, alongside upregulation of mitochondrial, metabolic, and detoxification pathways. These coordinated changes indicate a shift toward a low-activity, neuroprotective neuronal state driven by nuclear reprogramming and metabolic adaptation. Together, our findings reveal a conserved, adult-specific role for the CTLH complex in stress responses and establish miR-34 as a dosage-sensitive regulator of depression-like states.