SSTCre:γ2f/f mice are resilient to CVS-induced changes in motivated behavior independent of sex
We previously reported that SSTCre:γ2f/f mice are resilient to the anxiogenic effects of uncontrolled chronic mild stress exposure 8. However, this protocol had failed to reliably induce anhedonia-like changes in rewarding behavior and thereby prevented us from testing the mice for resilience in this behavioral domain. Here we adopted a CVS protocol (Fig. 1A), which in SSTCre control mice results in both anxiety-like and anhedonia-like changes in motivated behavior. Weekly measurements of body weight during CVS exposure revealed similar stress-induced attenuation of body weight gain in SSTCre and SSTCre:γ2f/f male and female mice (Fig. 1B, F). Thus, with respect to whole body physiology all mice seemed to experience stress similarly, independent of genotype and sex. Separate cohorts of SSTCre, SSTCre:γ2f/+, SSTCre:γ2f/f mice were then subjected to CVS or NS control conditions to test for stress-induced changes in negatively (NSFT) and positively regulated motivated behavior (FUST, SSPT, SPT).
In male SSTCre mice, CVS exposure resulted in an increased latency to feed in the NSFT (Fig. 1C), a decreased time spent sniffing female urine in the FUST (Fig. 1D), and a reduced time spent in the center in the OFT (Figure S1A), indicating both anxiety-like increases in negatively regulated motivated behavior and anhedonia-like reductions in positively regulated motivated behavior. The behavioral measures in the SSPT and SPT were not informative as they were largely unaffected by CVS (Figure S1B, C). In striking contrast to SSTCre controls, SSTCre:γ2f/+ and SSTCre:γ2f/f mice showed a CVS-induced reduction in the latency to feed in the NSFT, indicating that they were not only resilient to CVS, but that the stress effect was opposite to that observed in SSTCre controls (Fig. 1C), with the animals seemingly becoming less anxious following stress. In the FUST, SSTCre:γ2f/f but not SSTCre:γ2f/+ male mice were resilient to CVS-induced reductions in female urine sniffing duration, indicating that stress resilience extends to positively regulated motivated behavior and that the degree of resilience scales with the level of disinhibition of SST neurons (Fig. 1D). In the OFT, neither the behavior of SSTCre:γ2f/+ mice nor of SSTCre:γ2f/f mice was significantly affected by stress (Figure S1A), which is again consistent with stress resilience, even though in this case the mutants did not differ from the SSTCre controls.
Female littermate mice were tested analogously, a couple of weeks after the males for practical reasons. In the NSFT, SSTCre female mice failed to show a CVS effect, in contrast to males. However, CVS of SSTCre:γ2f/+ and SSTCre:γ2f/f female mice resulted in a reduced latency to feed (Fig. 1G), which is indicative of stress resilience similar to males. In the OFT, female SSTCre:γ2f/+ and SSTCre:γ2f/f mice showed a CVS effect similar to SSTCre controls (Figure S1D). As for males, CVS of female mice had no effect on behavior in the SSPT and SPT (Figure S1E, F). In summary, the data suggest that female mice with disinhibited SST neurons are resilient to CVS-induced changes in negatively motivated behavior assessed in the NSFT. Changes in positively motivated behavior could not be assessed as behavior in the SSPT and SPT was unaffected by stress and the FUST is not applicable to females.
SSTCre:γ2f/f male but not female mice are resilient to CVS-induced changes in the mPFC transcriptome
We next assessed whether disinhibition of SST neurons results in stress resilience at the transcriptome level (Fig. 2A). We focused on the medial prefrontal cortex (mPFC) as a brain region known to control both positively and negatively regulated forms of motivated behavior. The brains of CVS-exposed mice were harvested 24 h after the last stressor and the mPFC was dissected and processed for RNA-Seq. In male mice, quantitation of CVS-induced DEGs (p < 0.01) from subsamples of SSTCre and SSTCre:γ2f/f mice revealed significantly fewer DEGs in the SSTCre:γ2f/f mice compared to SSTCre controls (Fig. 2B), indicating that stress resilience is reflected in fewer stress-induced DEGs. Volcano plots of differential expression analyses showed similar numbers of CVS-induced downregulated and upregulated DEGs (p < 0.01) in both genotypes (Fig. 2C, D). The number of DEGs determined based on all samples of each genotype confirmed the lower number of DEGs in SSTCre:γ2f/f (81) vs SSTCre controls (437). Importantly, heat maps of the DEGs showed that the 437 CVS-induced DEGs observed in the stress-vulnerable SSTCre mice (top row of heat map in Fig. 2C) were randomly affected by CVS in the stress-resilient SSTCre:γ2f/f mice (bottom row of that heat, note the lack of correspondence in color between the two genotypes). Similarly, the 81 CVS-induced DEGs from the stress-resilient SSTCre:γ2f/f mice (top row of heat map in Fig. 2D) were randomly affected by CVS in the stress-vulnerable SSTCre mice (bottom row of that heat map). Collectively the data indicate that stress resilience of male SSTCre:γ2f/f mice is reflected in both fewer and qualitatively different CVS-induced DEGs.
We next repeated the same analyses for female mice. A comparison of batch normalized male and female transcriptomes by PCA revealed an overt separation of male and female samples. By contrast, the CVS and NS samples of the SSTCre and SSTCre:γ2f/f mice of each sex were co-clustered (Fig. 2E). Thus, sex differences in the transcriptomes are much larger than the differences induced by CVS and genotype. The quantitation of CVS-induced DEGs from subsamples of female mice revealed similar number of CVS-induced DEGs in the stress-resilient SSTCre:γ2f/f mice compared to the stress-vulnerable SSTCre controls (Fig. 2F), as is also evident in the volcano blots, showing 402 CVS-induced DEGs across all samples for the stress-vulnerable SSTCre female controls (Fig. 2G) and 434 CVS-induced DEGs for the SSTCre:γ2f/f stress-resilient female mice (Fig. 2H). Moreover, the CVS-induced directional gene expression changes of DEGs in the stress-vulnerable SSTCre female mice (top row of heat map in Fig. 2G) were largely conserved in the stress-resilient SSTCre:γ2f/f female mice (bottom row of heatmap). Similarly, the directional gene expression changes of CVS-induced DEGs seen in the stress-resilient SSTCre:γ2f/f female mice were largely conserved in the stress-vulnerable SSTCre female controls (heatmap in Fig. 2H). In summary, SSTCre:γ2f/f male mice but not SSTCre:γ2f/f female mice are resilient to CVS-induced changes in the mPFC transcriptome. Therefore, for our further analyses of transcriptomes associated with stress resilience we focused on male mice. We elaborate on sex differences in the brain substrate of stress resilience in the Discussion.
The CVS-induced transcriptome changes of stress-resilient mice are distinct from the CVS-induced transcriptome changes of stress-vulnerable mice.
We argued that putative stress resilience genes should be uniquely affected by CVS in the stress-resilient mice or show opposite CVS effects in the stress-vulnerable compared to stress-resilient mice. A Venn diagram of CVS-induced DEGs of stress-vulnerable (SSTCre) mice and CVS-induced DEGs of stress-resilient (SSTCre:γ2f/f) mice revealed 427 CVS-induced DEGs that are uniquely observed in stress-vulnerable mice (for gene lists see Table S1), while 71 DEGs were specific for stress-resilient mice (Fig. 3A, Table S2). A mere 10 CVS-induced DEGs passed the threshold of p < 0.01 in both strains of mice, and nine of these were affected by CVS in the same direction (Fig. 3B). Notably, Etnk2 showed opposite responses to CVS in stress-vulnerable and stress-resilient mice, which is consistent with a contribution to stress resilience.
To more comprehensively compare the CVS-induced transcriptome changes of the two strains of mice, we performed a correlational analysis of CVS-induced Log2 FCs of the 437 DEGs observed in the SSTCre mice compared to the Log2 FCs of the same genes in the SSTCre:γ2f/f stress-resilient mice (Fig. 3C). We then analogously compared the Log2 FCs of the 81 CVS-induced DEGs observed in the SSTCre:γ2f/f stress-resilient mice with the CVS-induced Log2 FCs of the same genes in the SSTCre stress-vulnerable mice (Fig. 3D). Both of these contrasts revealed negligeable correlation (r = 0.3 and − 0.11, respectively), which confirms that the CVS-induced DEGs of the stress-vulnerable and stress-resilient mice are distinct. Putative stress resilience genes include the genes that show opposite CVS effects in the stress-vulnerable vs stress-resilient mice, highlighted by the four red quadrants of Fig. 3C and D (Table S3).
Stress resilience involves chronic stress-induced enhancement of mRNA translation, while stress vulnerability is associated with impairment of diverse signal transduction pathways and reduced translation
To elucidate the function of putative stress resilience genes we performed Ingenuity Pathway analysis (IPA) using DEGs in a default setting. We compared the CVS-induced pathways affected by the 437 CVS-induced DEGs in the stress-vulnerable (SSTCre) mice to those affected by the 81 CVS-induced DEGs of the stress-resilient (SSTCre:γ2f/f) mice (Fig. 3E). IPA revealed 98 CVS-affected pathways (p < 0.05) in stress-vulnerable mice and 48 CVS-affected pathways in stress-resilient mice, with only six pathways affected by CVS in both strains of mice. We then compared the CVS-affected pathways between the two strains using IPA’s integrated comparison analysis tool. The top 14 CVS-regulated pathways (first ranked by Z-score after elimination of pathways related to coronavirus pathogenesis, cancer, autism and pancreatic secretion, and then ranked by p value) revealed a striking segregation of CVS-induced pathways between stress-vulnerable and stress-resilient mice. The seven pathways with the highest Z-scores were all selectively activated in the stress-resilient mice but not in stress-vulnerable mice (Fig. 3F). The next seven pathways were all selectively inhibited by CVS in the stress-vulnerable mice but not stress-resilient mice. The pathways activated by CVS in stress-resilient mice were all related to mRNA translation and ribosomal RNA processing, and they were principally driven by CVS-induced expression of the same four ribosomal proteins (RPS26, RPS28, RPS29, RPSA) (Fig. 3F, Figure S2). The pathways inhibited by CVS in stress-vulnerable mice were broadly related to inter- and intracellular signal transduction and cell adhesion.
In an attempt to further corroborate these findings, we performed pathway analyses of the 180 genes that showed opposite CVS effects in stress-resilient (SSTCre:γ2f/f) vs stress-vulnerable (SSTCre) mice, i.e. the genes that mapped to the four quadrants marked in red in Fig. 3C, D. These 180 genes were significantly altered by CVS (p < 0.01) in one of the two strains except for Etnk2 which was significantly affected in both strains (Fig. 3B). Based on these DEGs there were a total of 140 pathways that were differentially affected by CVS in the two strains of mice. The top 10 among these pathways ranked by Z-scores included the same translation- and rRNA processing-related pathways activated by CVS in stress-resilient mice (Figure S3), confirming that stress resilience is mediated by CVS-induced mRNA translation (for genes underlying pathway changes see Figure S2). These same pathways were downregulated by CVS in stress-vulnerable mice (Figure S3), although the genes underlying these pathways in this case were only nominally affected by CVS. An additional two pathways related to antigen presentation and gustation were activated in stress-vulnerable mice and inhibited in stress-resilient mice (Figure S3). Collectively these data strongly suggest that stress resilience induced by increased activity of SST neurons in the mPFC of male mice involves stress-activated mRNA translation, while stress vulnerability involves stress induced reductions in mRNA translation along with downregulation of diverse inter- and intra-cellular signaling pathways.
A key finding from analyses of the CSDS model was that stress resilience is an active process as evidenced by more numerous stress induced gene expression changes in resilient compared to vulnerable mice and by increased activity of dopaminergic neurons of the ventral tegmental area that project to the nucleus accumbens 19, 20. To assess whether corresponding features also apply to the resilience mechanism studied here we used subsampling to compare CVS-induced DEGs of SSTCre:γ2f/f vs NS SSTCre controls (corresponding to stress resilient vs NS control mice of the CSDS model) and of CVS SSTCre vs NS SSTCre controls (corresponding to stress susceptible vs NS control mice of the CSDS model). Indeed, CVS-exposed stress-resilient (SSTCre:γ2f/f) mice showed significantly greater number of DEGs than CVS exposed stress-vulnerable (SSTCre) controls (Figure S4) Thus, resilience driven by increased activity of SST neurons in the mPFC fits the definition of an ‘active’ process, analogous to that driven by dopaminergic neurons in the reward circuit.
Stress-resilient mice mimic transcriptomic and pathway changes of stress exposure but without stress axis activation
Acute stress of mice is known to increase the excitability of SST neurons in the mPFC 14, which raised the question whether the inverse is also the case: Does increased excitability of SST neurons due to disinhibition of SST neurons mimic the effects of stress exposure? To address this question experimentally we compared the CVS-induced DEGs of stress-vulnerable (SSTCre) mice with the genotype-induced DEGs of stress-resilient vs stress-vulnerable mice (Fig. 4A-D). A Venn diagram of the two sets of DEGs revealed an overlap of 33 DEGs that showed very similar fold changes with just one gene showing opposite directional effects (Fig. 4B). Therefore, the DEGs of NS stress-resilient vs stress-vulnerable mice are similarly induced by CVS in stress-vulnerable mice. It stands to reason, therefore, that these DEGs represent putative stress resilience genes that are naturally induced by chronic stress exposure even though these gene expression changes are insufficient to induce stress resilience. A broader correlational analysis of the Log2 FCs of the 437 CVS-induced DEGs in stress-vulnerable mice with the Log2 FCs of the same genes in NS stress-resilient vs stress-vulnerable mice revealed a strong correlation between CVS-induced and genotype-induced gene expression changes (r = 0.78, Fig. 4C) that was confirmed by a similarly strong correlation of Log2 FCs of the 270 genotype-induced DEGs and the CVS-induced Log2 FCs of the same genes in the stress-vulnerable mice (r = 0.67, Fig. 4D).
We next compared the pathways affected in NS stress-resilient vs stress-vulnerable mice with those induced by CVS exposure of stress-vulnerable mice. Among the top 15 pathways ranked by Z-score and p values, nine pathways were inhibited under both conditions, and only three pathways had opposing Z-scores (activation under one condition and inhibition in the other) (Figure S5). The majority of pathways affected were related to aspects of signal transduction. Therefore, the pathway changes of NS stress-resilient (SSTCre:γ2f/f) mice compared to NS stress-vulnerable mice mimic aspects of CVS exposure of stress-vulnerable (SSTCre) mice.
The notion that NS stress-resilient mice show transcriptome changes that are correlated with transcriptome changes of CVS-exposure of stress-vulnerable mice raised the question of whether the stress-resilient mice show evidence of constitutive activation of the hypothalamus-pituitary-adrenal (HPA) axis. To address this possibility, we compared the serum Cort levels of NS and CVS-exposed SSTCre (stress-vulnerable) and SSTCre:γ2f/f (stress-resilient) mice nine days after 21 days of CVS exposure. The stress-resilient (SSTCre:γ2f/f) mice showed a strong trend towards reduced serum Cort, independent of prior CVS exposure, and there were no lasting effects of CVS on Cort levels, independent of genotype (Figure S6). Therefore, although the transcriptome changes of the stress-resilient mice mimic those of chronic stress exposure of stress-vulnerable mice, stress resilience does not involve constitutive activation of the HPA axis.
Chronic stress exposure of stress-resilient mice results in reversal of constitutive gene expression changes of stress-resilient mice
One of the above 33 putative natural stress resilience genes, Etnk2, that is downregulated both in NS stress-resilient vs stress-vulnerable mice and in CVS-exposed stress-vulnerable mice (Fig. 4B) stood out already earlier as a candidate stress resilience gene that is upregulated by stress in stress-resilient mice (Fig. 3B), indicating that CVS exposure of stress-resilient mice reversed the downregulation of Etnk2 seen in NS stress-resilient vs stress-vulnerable mice. To address whether similar gene expression changes were associated more broadly with stress resilience, we compared the genotype-induced DEGs of stress-resilient vs stress-vulnerable mice with the CVS-induced DEGs of stress-resilient mice (Fig. 4E–H, Table S2, S4). There were 12 overlapping DEGs (including Etnk2) that passed the significance threshold for both contrasts. They all showed directional changes induced by CVS in stress-resilient mice that were opposite to those induced by genotype in the absence of stress (Fig. 4F). To further compare the effect sizes of all DEGs of the two contrasts we first compared the Log2 FCs of the 270 genotype-induced DEGs to the Log2 FCs of the same genes induced by CVS in SSTCre:γ2f/f mice. Strikingly, these two factors were almost perfectly anticorrelated (r = -0.71) (Fig. 4G). Similarly, the Log2 FCs of the 81 CVS-induced DEGs of SSTCre:γ2f/f mice were strongly anticorrelated with the Log2 FCs of the same genes in the genotype comparison (r = -0.95) (Fig. 4H). Thus, CVS exposure of the stress-resilient mice results in reversal of the baseline/constitutive transcriptome changes of the stress-resilient mice.
Comparison of the pathways induced by the above DEGs confirmed that CVS exposure of stress-resilient mice involved strong activation of pathways related to mRNA translation as seen earlier (Fig. 4I, J compared to Fig. 3F). Six of the next seven pathways were selectively inhibited in NS stress-resilient vs stress-vulnerable mice. As also noted earlier, the pathways inhibited in NS stress-resilient vs stress vulnerable mice were largely the same as the ones that were inhibited by CVS of stress-vulnerable mice (Figure S5).
Stress-induced DEGs of stress-vulnerable but not stress-resilient mice are prominently associated with risk genes of human stress-related psychiatric disorders
To examine the relevance of our findings for stress related psychiatric disorders we compared the CVS-induced DEGs of stress-vulnerable and stress-resilient mice with the DisGeNET human gene-disease associations library. Out of 437 CVS-induced DEGs of stress-vulnerable (SSTCre) mice, 75 (17.16%) were present in the DisGeNET library for MDD, BP, PTSD, and/or SCZ (Fig. 5). By contrast, a mere 6 out of 81 (7.4%) CVS-induced DEGs of the stress-resilient (SSTCre:γ2f/f) mice were associated with these disorders, and all these remaining DEGs were implicated in MDD. The greater enrichment of disease-associated CVS-induced DEGs of SSTCre compared to SSTCre:γ2f/f mice suggests that the mechanism underlying stress resilience of SSTCre:γ2f/f mice may have therapeutic utility for human stress-associated mental disorders.