Putative role of miR-9 in the remodelling of hippocampal dendritic arborisation induced by chronic stress and ketamine

Stress-based animal models of depression have shown dendritic remodelling in the hippocampus (HPC), suggesting a primary role of stress-induced maladaptive changes in etiopathogenesis. Antidepressant treatment with ketamine was shown to rapidly reverse the dendritic simplification induced by stress. Using the Chronic Mild Stress (CMS) animal model of depression, we have shown a reduction of CA3 pyramidal neurons apical dendritic length and complexity selectively in vulnerable animals, which was completely rescued by acute ketamine (Tornese et al, 2019). Here we aimed at studying putative molecular mechanisms underlying the morphological alterations induced by stress and ketamine. We found that dendritic simplification in CMS vulnerable rats was accompanied by changes in microRNAs known to play a role in neuroplasticity and stress response. In particular, miR-9-5p was selectively reduced in the HPC of vulnerable animals, while KET restored its levels to basal condition. To assess whether miR-9-5p was able to directly induce neuronal tree remodelling, we used in vitro expression modulation studies on primary HPC cultures and found that miR-9-5p downregulation significantly decreased total dendritic length and spine number, while miR-9-5p overexpression exerted the opposite effect. Bioinformatic analysis highlighted a number of putative target genes, and the top-hit ones (Gsk-3β, REST, Sirt1) were validated with Luciferase assay. Changes in protein expression levels of validated targets were measured in the HPC of CMS animals by Western-blot. Our results suggest that alterations in miR-9-5p levels may be involved both in mechanisms of stress resilience/vulnerability and in the fast antidepressant effect.