Neuroinflammation is considered to be an important underlying process in the pathology of major depressive disorder (MDD) within a subpopulation of patients. MDD is associated with increased levels of proinflammatory cytokines in the blood, and cytokine-based treatments can induce depression. In mice, the induction of systemic inflammation with lipopolysaccharide (LPS) can induce depressive-like behaviours that are associated with symptoms of MDD. Microglia mediate the neuroinflammatory response within the brain and have a critical role in inflammation-induced depressive- like behaviours. Microglia within the brain exist in low O2 conditions (~5 %), though experimentation in vitro is typically carried out in high O2 conditions (20 %). The NLRP3 inflammasome is a molecular complex central to the production of the proinflammatory cytokine IL-1β and the propagation of the inflammatory response. NLRP3 inflammasome activity has been implicated in chronic stress and inflammation-based models of depressive-like behaviours in mice.The aims of this thesis were to study LPS-induced depressive-like behaviour in C57BL/6J mice, the role of NLRP3 in the behavioural output and the influence of oxygen (O2) availability on NLRP3 inflammasome activity in microglia cell cultures. Acute LPS induced depressive-like behaviours were observed in hedonia-based tasks but not in the forced swim test (FST). However, acute LPS induces a brief period of inflammation that does not address the sustained nature of depression. A FST depressive-like behaviour was observed in a novel 3-day increasing dose LPS model of sustained inflammation, whilst circumventing the development of LPS tolerance. The LPS-induced sickness was partially dependent upon NLRP3, though the resulting depressive-like behaviour was not. NLRP3 inflammasome signalling in microglia was studied in 5 % O2 conditions to replicate the hypoxic environment within the brain. Primary microglia isolated from mixed glial cultures by mild trypsinisation exhibited functional NLRP3 inflammasome expression and activity. When exposed to 5 % O2 (24 hours), NLRP3 inflammasome activity and adenosine triphosphate (ATP)-induced cell death was attenuated, whilst the production of other proinflammatory cytokines were unaffected. These data demonstrate the O2 sensitivity of NLRP3 inflammasome signalling in microglia.This thesis demonstrates a novel model of sustained inflammation and that inhibiting NLRP3 signalling may provide a target for attenuating neuroinflammation and the resulting behavioural changes. The importance of understanding the influence of O2 in microglia function and neuroinflammation was highlighted by the sensitivity of NLRP3 inflammasome activity to low O2.
|Date of Award
|1 Mar 2017
|Medical Research Council & Janssen Pharmaceutica NV
|Amanda Mackenzie (Supervisor) & Sarah Bailey (Supervisor)