ROLE OF CELLULAR STRESS RESPONSES IN THE PATHOPHYSIOLOGY AND TREATMENT OF PSYCHIATRIC DISORDERS

Major Depressive Disorder (MDD) is one of the most serious conditions classified among the stress-related psychiatric disorders. Not only it is one of the leading causes of disability worldwide but is also associated with poor treatment response and high rate of relapse, which dramatically increase the risk of death of affected individuals. Stressful life events contribute to MDD insurgence in the presence of an underlying genetic susceptibility. However, only a small percentage of the general population who experiences adverse events develops the full-blown disorder. These individuals, defined as vulnerable, fail to appropriately resolve a physiological stress response upon cessation of a stressful event. Conversely, most individuals are resilient, able to respond in a proper and non-pathological manner to stressful stimuli. Over the past decades, it has been reported that stress might differently affect the molecular asset of vulnerable and resilient subjects. Our lab has also reported alterations of neuroinflammation and redox balance following stress-exposure. These mechanisms are part or consequences of cellular stress responses, namely processes that are activated when an insult disrupts homeostasis to promote either survival or death. However, the precise mechanisms by which stress contributes to the derangements of these systems and how this can be implicated in MDD still needs to be completely elucidated. In this context, the aim of this project is to evaluate, through both gene candidate and unbiased approaches, if and how specific cellular stress responses are elicited in vulnerable and resilient individuals to MDD when exposed to stressors of different intensities and durations. Also, we investigated whether any alteration could be mitigated by the effect of the chronic pharmacological treatment with venlafaxine. To this end, we took advantage of a preclinical model of stress-induced MDD, the Chronic Mild Stress (CMS), articulated in multiple designs. These included a short-term (2 weeks) and a prolonged (6 weeks) stress exposure, the latter being followed by a brief and intense acute challenge (1h of acute restraint stress). In this design, a part of Vulnerable animals was also subjected to chronic treatment with the SNRI Venlafaxine for three consecutive weeks, concomitantly with the CMS exposure and prior to ARS. In both designs, the exposure to chronic stress successfully resulted in the stratification of animals in vulnerable and resilient to anhedonia, core symptom of MDD, thus exemplifying the real-life scenario. In the first part of the project, we assessed through gene-candidate analyses the effects of prolonged CMS on redox balance by evaluating the levels of cardinal mediators in brain areas of interest at the end of the stress exposure and after the ARS to mimic a more 5 dynamic context. We observed that despite CMS alone did not induce any modulation, it was able to affect the acute responsiveness of vulnerable animals. Specifically, they showed upregulation of the antioxidant component after ARS exposure, a potential compensatory mechanism reflecting a higher need to rebalance an oxidative state. Moreover, chronic venlafaxine treatment was able to normalize the observed alterations, suggesting a role for the drug in re-establishing the normal redox balance when needed. While prolonged CMS exposure did not result in measurable antioxidant modulation for the targets assessed, previous data from our lab showed that a shorter stress exposure was able to induce upregulation of some antioxidant markers, such as Heme-Oxygenase-1 (HO1) only in resilient subjects. We thus took advantage of HO1 to investigate which kinds of brain cells were mainly responsible for the observed modulations, exploiting immunofluorescence technique. We found that HO1-expressing cells were mainly located across the dentate gyrus (DG) of the hippocampus, and we observed that most of the signal was attributable to microglial cells (IBA1+). However, a considerable amount of the HO1 signal specifically located in the hilus of the DG was ascribable to cells with neuron-like morphology. To further explore this hypothesis, we stained slices with GAD65 (inhibitory interneurons marker) and Parvalbumin (marker of a specific subtype of inhibitory interneurons), confirming their partial involvement in the HO1-mediated antioxidant response. As we are aware that the complexity of the molecular alterations associated with stress response in the context of MDD cannot be limited to redox balance, we adopted a wider perspective on which other systems could intervene in the stress response, specifically examining how would vulnerability and resilience differ. Thus, we moved from a gene-candidate towards a more unbiased approach with proteomics, which eventually led us to assess the time-dependent effects of stress on protein homeostasis alterations and more in detail on the Unfolded Protein Response (UPR). Our findings revealed that vulnerable and resilient individuals do not differ in whether the response occurs, but rather in the specific pathways and timing of their activation. Moreover, venlafaxine treatment was able to normalize the main UPR alterations, indicating that it could ameliorate the vulnerability-associated phenotype also by alleviating ER stress. While further research is needed to fully elucidate the effects of stress on these processes, the present work contributes to refine our understanding of the molecular mechanisms underlying both vulnerability and resilience to stress-induced anhedonia, providing potential insights of therapeutic relevance.