In post-traumatic stress disorder (PTSD), individuals experience intrusive memories of trauma accompanied by hyperarousal and stress dysregulation. Treatments for PTSD are largely ineffective, representing an enormous area of unmet need. PTSD is differentiated from acute stress disorder (ASD) by the duration of symptom presentation; more or less than one month, respectively. Further, while nearly everyone experiences at least one traumatic event in their lifetime, only a subset develop PTSD. Taken together, this makes a strong argument for the importance of mechanistic research efforts aimed at incorporating the persistent nature of traumatic memories into their design, as well as variance in susceptibility.

To this end, we adapted a stress-enhanced fear memory (SEFM) protocol [1] to induce long-lasting, PTSD-like behaviors and molecular changes in mice with differential susceptibility to the stressor [2]. We used this protocol to investigate molecular pathways supporting persistent stress susceptibility versus resilience, focusing on the basolateral amygdala (BLA), an “emotional” memory center of the brain that is hyperactive in PTSD. First, we reported a transcriptional profile in the BLA associated with susceptibility that included differential expression of genes with known polymorphisms in PTSD subjects present for at least one month after SEFM training [2].

We next shifted focus to microRNAs (miRNA), small non-coding RNAs that act as translational repressors. miRNAs are attractive therapeutic targets for PTSD because evidence has accrued over the past decade demonstrating their role in memory regulation. miRNAs can have very long half-lives, consistent with the potential to influence memory persistence. Further, they achieve complex molecular regulation through their wide range of translational targets, and recent developments in RNA-based therapeutics indicate miRNAs are druggable [3].

Interestingly, acute stress alone is sufficient to induce a profound, long-lasting shift in the BLA miRNAome [4]. When incorporated with delayed learning (SEFM), we identified miRNA profiles present in the BLA one month after training that were unique to susceptible and resilient male mice [5]. Using viral-based overexpression or sequence-specific inhibitors at this delayed time point, we found manipulation of mir-135b-5p, a miRNA selectively upregulated in susceptible mice, bidirectionally regulates susceptible and resilient remote memory phenotypes [5]. mir-135b-5p may also function as a biomarker of PTSD symptomology, as levels are persistently regulated in serum from combat-exposed military veterans that developed PTSD [5]. We employed a similar approach to demonstrate that another miRNA, mir-598-3p, also regulates PTSD-like susceptibility one month after SEFM in a sex-dependent manner [6].

This body of work highlights several critical points for PTSD research. Focusing research on identification of mechanisms present long after a stressful event is relevant to developing PTSD treatments. Second, animal protocols that induce differential stress susceptibility [2], preferably with a method that does not require post hoc phenotyping to identify susceptible animals, avoid inadvertent obscuring of relevant brain changes. Third, miRNAs are potential therapeutic targets for psychiatric disease. These translational repressors are expressed with specificity at the level of whole organism, brain subregions and cell type, resulting in differential modulation of their protein targets. Finally, RNA-based small molecule inhibitors that directly regulate miRNAs can be developed into therapeutics.

Funding and disclosures

This work was funded by R00DA01469 and DP1DA051550 to S.E.S. and R01MH105400 to C.A.M. The authors have no financial disclosures.