Friday, May 05, 2017

Histone variants promote vulnerability to depressive behaviors

Lepack et al. find that a particular histone protein variant in the nucleus accumbens contributes to stress susceptibility in mice (histones are highly alkaline proteins found in eukaryotic cell nuclei that package and order the DNA into structural units called nucleosomes.) The work suggests that compounds that block its action might be sought as potential therapies for human stress and depressive disorders.

Human major depressive disorder is a chronic remitting syndrome that affects millions of individuals worldwide; however, the molecular mechanisms mediating this syndrome remain elusive. Here, using a unique combination of epigenome-wide and behavioral analyses, we demonstrate a role for histone variant dynamics in the nucleus accumbens (NAc)—a critical brain center of reward and mood—contributing to stress susceptibility in mice. These studies, which also demonstrate that molecular blockade of aberrant dynamics in the NAc promotes resilience to chronic stress, promise to aid in the identification of novel molecular targets (i.e., downstream genes displaying altered expression as the result of stress-induced histone dynamics) that may be exploited in the development of more effective pharmacotherapeutics.
Human major depressive disorder (MDD), along with related mood disorders, is among the world’s greatest public health concerns; however, its pathophysiology remains poorly understood. Persistent changes in gene expression are known to promote physiological aberrations implicated in MDD. More recently, histone mechanisms affecting cell type- and regional-specific chromatin structures have also been shown to contribute to transcriptional programs related to depressive behaviors, as well as responses to antidepressants. Although much emphasis has been placed in recent years on roles for histone posttranslational modifications and chromatin-remodeling events in the etiology of MDD, it has become increasingly clear that replication-independent histone variants (e.g., H3.3), which differ in primary amino acid sequence from their canonical counterparts, similarly play critical roles in the regulation of activity-dependent neuronal transcription, synaptic connectivity, and behavioral plasticity. Here, we demonstrate a role for increased H3.3 dynamics in the nucleus accumbens (NAc)—a key limbic brain reward region—in the regulation of aberrant social stress-mediated gene expression and the precipitation of depressive-like behaviors in mice. We find that molecular blockade of these dynamics promotes resilience to chronic social stress and results in a partial renormalization of stress-associated transcriptional patterns in the NAc. In sum, our findings establish H3.3 dynamics as a critical, and previously undocumented, regulator of mood and suggest that future therapies aimed at modulating striatal histone dynamics may potentiate beneficial behavioral adaptations to negative emotional stimuli.

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