Aberrant histone H3 serotonylation dynamics in dorsal raphe nucleus contribute to maladaptive stress-induced gene expression programs and behavior [ChIP-seq]

Major depressive disorder (MDD) is a debilitating illness that affects millions of individuals worldwide. While chronic stress increases incidence levels of MDD, stress-mediated disruptions in brain function that precipitate the disorder remain elusive. Serotonin-associated antidepressants (ADs) remain the first line of therapy for many with MDD, yet low remission rates and delays between treatment and symptomatic alleviation have prompted skepticism regarding precise roles for serotonin in the precipitation of MDD. Our group recently demonstrated that serotonin epigenetically modifies histone proteins (H3K4me3Q5ser) to regulate transcriptional permissiveness in brain. However, this phenomenon has not yet been explored following stress and/or AD exposures. Here, we employed a combination of genome-wide (ChIP-seq, RNA-seq) and western blotting analyses in dorsal raphe nucleus (DRN) of male and female mice exposed to chronic social defeat stress to examine the impact of stress exposures on H3K4me3Q5ser dynamics in DRN, as well as associations between the mark and stress-induced gene expression. Stress-induced regulation of H3K4me3Q5ser levels were also assessed in the context of AD exposures, and viralmediated gene therapy was employed to manipulate H3K4me3Q5ser levels to examine the impact of reducing the mark in DRN on stress-associated gene expression and behavior. We found that H3K4me3Q5ser plays important roles in stress-mediated transcriptional plasticity in DRN. Mice exposed to chronic stress displayed dysregulated dynamics of These findings establish a neurotransmission-independent role for serotonin in stress-associated transcriptional and behavioral plasticity in DRN. To investigate the effects of chronic stress on gene expression in the mouse DRN, we performed CSDS in a cohort of male and female mice. We then performed gene expression profiling analysis using data obtained from RNA-seq of male and female mice that had underwent CSDS and handled control mice. To assess whether alterations in the genomic enrichment of H3K4me3Q5ser correspond with meaningful patterns of genomic regulation following chronic stress exposures, we performed CSDS in separate cohorts of male and female mice. We then performed Chromatin immunoprecipitation DNA-sequencing (ChIP-seq) for histone variants H3K4me3Q5ser and H3K4me3 (H3K4me3 was only used for the male cohort). Finally, to investigate the effect of depletion of H3K4me3Q5ser on chronic stress-induced gene expression, we performed gene expression profiling analysis using data obtained from RNA-seq of a separate cohort of stressed mice (and handled controls) injected with 3 different viruses (H3.3Q5A, H3.3 WT, and an empty vector control).