Our data shows a novel mechanism whereby chronic stress or depression regulates the transcriptional profile of Rac1 in the NAc of rodents and humans through an epigenetic mechanism. Intriguingly, Rac1 transcriptional tone is normalized by chronic imipramine treatment in mice following chronic social defeat and is positively correlated with treatment response, suggesting that this adaptation may be necessary for effective antidepressant treatment. However, in the majority of MDD subjects found to be on antidepressant at the time of suicide, normal Rac1 expression and altered chromatin modifications along the Rac1 promoter and upstream regions were not restored, suggesting a need for more direct Rac1 targeting strategies to achieve therapeutic effects in this patient population. Functionally, in mice, selective genetic deletion of Rac1 and viral mediated gene-transfer of Rac1 mutants, or epigenetic modulation through class 1 HDAC inhibition, reveals that accumbal Rac1 signaling is both sufficient and necessary for social avoidance and anhedonia behavioral responses and synaptic structural plasticity. Together, these data suggest that small RhoGTPase signaling pathways, and Rac1 specifically, are viable candidates for future development as antidepressant therapeutic agents when targeted to the nucleus accumbens.
Indeed, small RhoGTPases have been heavily implicated in the neuropathogenesis of several psychiatric disorders including fragile X mental retardation32
, Rett syndrome33
. Under non-pathological conditions, Rac1 plays an important role in neuronal development35
, axon guidance36
and learning and memory processes37
. In regard to dendritic spines, as a general rule, increased synaptogenesis and functional synaptic plasticity is tightly correlated with the size and shape of a dendritic spine; as spines shift along a continuum from immature (stubby and thin) to mature (mushroom) they also shift towards greater synaptic strength and stability38,39
. Within neuronal populations, there is wealth of evidence supporting a role for Rac1 in dendritic spine morphogenesis and maintenance34,40–42
via modulation of cofilin spatiotemporal dynamics43,44
This signaling cascade is therefore exceptionally positioned to regulate the delicate interface between extra-cellular stimuli and dynamic reorganization of the actin cytoskeleton to accommodate neuronal transduction. The transcriptional downregulation of this system consequently plays an important role in setting the threshold for subsequent experience-dependant plasticity within NAc. Our data suggest that stress-induced decreases in Rac1 expression result in concurrent increases in immature stubby spine formation and cofilin localization within these spines. Recent findings have confirmed that decreased p21-activated kinase 3 (PAK3) activity, an immediate downstream effecter of Rac1, leads to both an increase in the growth of new, immature spines and an impairment of plasticity-mediated spine stabilization that interferes with the formation of persistent stable spines45
. A feature of chronic stress exposure is a shift towards synaptic instability17,46
. Such synaptic instability is relieved through the application of low-doses of ketamine14,46
, which has rapid antidepressant efficacy. Ketamine treatments normalize stress-induced immature spine formation resulting in a greater proportion of mature dendritic spines in pyramidal neurons of the prefrontal cortex46
. This “plasticity consolidation” treatment strategy11
may be occurring through a RhoGTPase-dependent mechanism, however, the specifics of this need to be teased apart in future studies.
It is noteworthy that within the NAc both cold water forced swim47
and chronic social defeat stress17,48
result in enhanced glutamatergic synaptic plasticity. In the case of social defeat, this coincides with the formation of immature stubby spines with smaller post-synaptic densities (PSDs) and a higher frequency of mini excitory postsynaptic currents (mEPSCs). The finding of increased excitatory drive in NAc is somewhat surprising since previous work shows that deep brain stimulation (DBS) in the NAc of treatment-resistant depression subjects produces a profound antidepressant response49,50
. However, DBS stimulates both excitatory and inhibitory neuronal populations, as well as fibers of passage that transit through the generalized region of stimulation. Further, the stimulation parameters used in such treatments are extremely super-physiological in frequency and amplitude that, based on Channel Rhodopsin studies, may result in de-sensitization of excitatory responses51
. How this ultimately regulates the diverse cell populations within the NAc is still unknown.
While there are clearly many acute biochemical events acting directly in the synapse that may mediate NAc plasticity, data here points to a more sustained long-lasting mechanism through downregulation of Rac1, as a result of stressful experience, to cause social avoidance and anhedonia. Thus, epigenetic mechanisms of sustained transcriptional disruptions provide a logical basis for such long-lasting synaptic restructuring events. While more general strategies of epigenetic regulation, through administration of histone deactelyase inhibitors20
or histone demethylases52
, may show significant antidepressant efficacy by enhancing Rac1 expression, still further refinement is necessary. A recent advance in targeting gene specific chromatin states using zinc finger artificial transcription factors that bind specific consensus sequences53
allow for targeted epigenetic therapeutics to act on specific genes to reverse long-term chromatin disruptions, for example, on Rac1
. However, until these tools are more widely available for testing in humans, a more feasible therapeutic strategy may be to screen compounds that target either Rac1 activity, or more generally, excitatory synapse consolidation.
In summary, we provide strong evidence that chronic stress induces long-term transcriptional downregulation of Rac1 through an epigenetic mechanism, robustly regulating NAc MSN synaptic structural and behavioral plasticity. Reversal of this endophenotype may present a novel therapeutic venue for exploration, and guide future therapeutics to target adaptive intracellular plasticity mechanisms.