ROLE OF ATM IN THE PATHOPHYSIOLOGY OF MAJOR DEPRESSIVE DISORDER (MDD)

Ataxia-telangiectasia mutated (ATM) is a serine/threonine protein kinase involved in the DNA damage repair (DDR) and many other cell processes. In neurons, ATM contributes to vesicles trafficking, neurotransmitter release, excitatory/inhibitory balance maintenance, the development of GABAergic system and neuroinflammation. Mutations in ATM cause Ataxia-telangiectasia (A-T), a syndrome characterized by devastating neurological symptoms ascribable primarily to cerebellar neurodegeneration. A-T patients have been reported to experience neuropsychiatric symptoms, including mood disturbances. Major depressive disorder (MDD) is a debilitating neuropsychiatric disease, characterized by affective, neurovegetative and cognitive symptoms and affecting about 300 million people worldwide with severe social and economic consequences. MDD is a complex disorder, and its aetiology is generally modelled on the “diathesis-stress hypothesis” that considers it to be the result of the interaction between the individual’s inherent vulnerability and the environmental stressors to which they are exposed. MDD’s pathophysiology features dysfunctions in the monoaminergic and glutamatergic systems, dysfunction of the hypothalamic-pituitary-adrenal (HPA) axis, altered neuroplasticity and neurogenesis and increased oxidative stress and inflammation. Notably, many of these mechanisms have been found to be controlled also by ATM. Given these premises and considering that in our previous study the antidepressant drug fluoxetine was sufficient to counteract cognitive defects of ΔAtm mice, the aim of this thesis was to investigate the involvement of ATM in major depressive disorder. To achieve this, we approached the research from multiple perspectives, using MDD models and ATM-deficient models both in vivo and in vitro. First, we demonstrated that rats vulnerable to chronic mild stress (CMS) are characterized by a significant reduction of ATM protein in the hippocampus, compared to control and resilient animals. Such reduction was independent of transcriptional mechanisms and was partially rescued by an acute administration of ketamine. Concurrently, we found that ATM-deficient mice feature depressive-like behaviours already in basal conditions and that Atmy/y females appear to be more vulnerable to chronic restrain stress than WT mice. While setting up our in vitro model of MDD, we unveiled that chronic exposure to corticosterone causes a significant decrease of ATM in hippocampal astrocytes but not in neurons. Thus, to assess how neuronal transmission could be affected by corticosterone-treated astrocytes, we used their conditioned medium (ACM) to treat mature hippocampal neurons. ACM treatment resulted in the impairment of excitatory pre-synaptic function and structure – as evidenced by the reduction of mEPSCs frequency and of VGLUT1 expression – after 1 hr, but not after 24 hr indicating an acute effect of factors release by astrocytes. Remarkably, neurons treated with the ACM of astrocytes where Atm mRNA was silenced via siRNA, exhibited the same functional defects, reinforcing ATM’s role in corticosterone-induced dysfunctions in the hippocampus. Finally, to identify proteins released by astrocytes that could be responsible for the synaptic defects encountered, we performed liquid chromatography tandem with mass spectrometry on all ACMs produced. We isolated 31 proteins significantly varied in a coherent way to our results. Further studies will be necessary to pinpoint the specific factors released by astrocytes that are causative of the synaptic defects unveiled in this work. This is the first time, to our knowledge, that ATM has been directly investigated in the context of neuropsychiatric disorders, specifically in major depressive disorder. Our findings provide new insights into ATM’s role in MDD pathophysiology, potentially opening new avenues for future research and treatment approaches.