Elucidating Mechanisms Underlying Cellular, Tissue Level, And Behavioral Resilience To The Detrimental Effects Of Chronic Stress Exposure
Restricted (Penn State Only)
- Author:
- Shao, Meiyu
- Graduate Program:
- Biology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- August 26, 2024
- Committee Members:
- Elizabeth Mcgraw, Program Head/Chair
Douglas Cavener, Chair of Committee
Yingwei Mao, Major Field Member
Anirban Paul, Outside Unit & Field Member
Bernhard Luscher, Major Field Member & Dissertation Advisor
Nicole Lazar, Minor Field Member - Keywords:
- stress resilience
GABAergic
RNA-seq
FACS - Abstract:
- Analyses of postmortem human brains and preclinical studies of rodents have identified somatostatin (SST)-positive interneurons as key elements that regulate the vulnerability to stress-related psychiatric disorders, including major depressive disorder (MDD). Conversely, genetically induced disinhibition of SST neurons through deletion of GABAA receptors from SST neurons (SSTCre:γ2f/f mice) or directly activating SST neurons in specific brain regions by chemogenetics results in behavioral changes that mimic the antidepressant drug action, including increased resilience to chronic stress-induced anxiety-like behaviors. This thesis aimed to further elucidate the mechanisms underlying stress resilience, including whether SSTCre:γ2f/f mice are resilient to stress-induced anhedonia, as well as possible sex differences in the brain substrate of stress resilience. Using a chronic variable stress (CVS) protocol, I showed that both male and female mice with disinhibited SST neurons (SSTCre:γ2f/f mice ) are resilient to stress-induced anxiety-like behaviors. In addition, testing of male SSTCre:γ2f/f mice in the female urine sniffing test (FUST) revealed resilience to chronic stress-induced anhedonia, a core symptom of MDD. To evaluate whether the stress resilience of SSTCre:γ2f/f mice extends to physiological measures, I quantified stress-induced defecation frequency as a possible indicator of gut motility, as well as serum corticosterone (Cort) levels as a parameter regulated by the hypothalamus-pituitary-adrenal (HPA) axis. I found that male, but not female, SSTCre:γ2f/f mice were resilient to CVS-induced changes in gut motility. However, male, but not female SSTCre:γ2f/f mice showed a stress-independent reduction in serum Cort levels, which may explain why male SSTCre:γ2f/f mice exhibited a greater level of stress resilience than their female littermates. We hypothesized that molecular changes in SST neurons induced by disinhibition of these neurons, along with molecular changes in other cell types in the mPFC, may underlie the stress resilience phenotype of SSTCre:γ2f/f mice. To elucidate molecular mechanisms underlying GABAergic control of stress resilience, we used RNA sequencing to characterize the stress-induced transcriptome changes at two levels: at the level of bulk tissue of the medial prefrontal cortex (mPFC) and at the level of purified SST neurons from the mPFC. At the tissue level, we found that stress resilience in the mPFC of male but not female SSTCre:γ2f/f mice is characterized by resilience to chronic stress-induced transcriptome changes. This sex specificity is consistent with separate data from our lab using chemogenetic mapping of the brain substrate of stress resilience. Therefore, our further studies focused on analyses of the mPFC of male mice. Interestingly, the transcriptome of non-stressed stress-resilient (SSTCre:γ2f/f) mice resembled the transcriptome of chronic stress-exposed stress-vulnerable (SSTCre control) mice. However, despite this stress-like transcriptome signature of non-stressed SSTCre:γ2f/f mice, the behavior and the serum Cort levels of these mice showed no signs of physiological stress. Most strikingly, chronic stress exposure of stress-resilient (SSTCre:γ2f/f) mice was associated with almost complete normalization of the chronic stress-like transcriptome signature, along with pathway changes indicating stress-induced enhancement of mRNA translation. Behaviorally, the mice with disinhibited SST neurons were not only resilient to chronic stress-induced anhedonia — they also showed an inversed anxiolytic-like response to chronic stress exposure that mirrored the chronic stress-induced reversal of the chronic stress-like transcriptome signature. To further evaluate stress resilience at the level of SST neurons, I developed a fluorescence-activated cell sorting (FACS) protocol to purify SST neurons from freshly dissociated brain tissue. Using transcriptomic analyses of purified SST neurons from stress-resilient (SSTCre:γ2f/f) and stress-vulnerable (SSTCre) mice, I found that stress-resilient SSTCre:γ2f/f male, but not female, mice were resilient to CVS-induced transcriptome changes in SST neurons, consistent with observations at the bulk tissue level. The transcriptome of SST neurons in stress-resilient SSTCre:γ2f/f male mice mimicked changes in oxidative and apoptotic pathways known to be associated with stress exposure, suggesting that the disinhibition of SST neurons may precondition the mice to become less susceptible to stress-induced detrimental changes. On the other hand, SST neurons in stress-resilient SSTCre:γ2f/f male mice exhibited greater stability of neuronal cell fate, strengthened synapse function, and increased energy production. These factors may all serve to protect SST cells and hence the mice from chronic stress-induced detrimental effects. However, in contrast to our bulk tissue findings, purified SST cells from stressed SSTCre:γ2f/f male mice showed no evidence of enhanced translation, indicating that these changes map to cell types other than SST neurons. Moreover, some transcriptome changes in mPFC SST neurons of stress resilient (SSTCre:γ2f/f) male mice, such as enhanced synaptic function, were not detectable at the tissue level, most likely due to dilution of the transcriptomes by other cell types. Ultimately, the data show that tissue level transcriptomic stress resilience was driven by stress resilience at the level of SST neurons. This thesis provides profound new insights into the molecular and cellular mechanisms underlying stress resilience and should be informative for the development of next generation antidepressants.