Interactions between Stress-induced Inflammation, Glycogen Synthase Kinase 3 (GSK3), and Depression

Major depressive disorder is a prevalent disease that is inadequately treated with available interventions, and recovery is difficult, particularly in patients refractory to antidepressant treatments. Studying the learned helplessness depression model in mice, we examined mechanisms regulating its initiation and regulating its reversibility. Stress increases susceptibility to depression in patients and rodent models, and depression is associated with aberrant activation of inflammation. We found multiple inflammatory cytokines were increased in mouse plasma and hippocampus after a paradigm of inescapable foot shock stress that causes learned helplessness. A 24 hr prior pre-conditioning stress accelerated the rate of stress-induced plasma and hippocampal cytokine increases, often without altering the maximal levels of the cytokines. Blocking toll-like receptor 4 (TLR4), inhibiting glycogen synthase kinase-3 (GSK3), or administration of the antidepressant fluoxetine diminished the stress-induced neuroinflammation. Stress increased hippocampal levels of the danger-associated molecular pattern (DAMP) protein high mobility group box 1 (HMGB1), activated the inflammatory transcription factor NF-κB, and the NLRP3 inflammasome. Knockdown of HMGB1 blocked the acceleration of cytokine increases in the hippocampus caused by two successive stresses. Antagonism of TNFα modestly reduced susceptibility to learned helplessness, whereas TLR4 knockout mice were resistant to learned helplessness. Thus, stress-induces a broad inflammatory response in mouse hippocampus that involves TLR4, GSK3, and downstream inflammatory signaling, and these stress responses contribute to susceptibility to learned helplessness in mice. To examine factors regulating the recovery from depression, learned helplessness was induced and mice were separated into groups that recovered or did not recover within 4 weeks. Compared with mice that recovered and control mice, non-recovered mice displaying prolonged learned helplessness had greater hippocampal activation of GSK3, higher levels of HMGB1, and increased TNFα, IL-17A, and IL-23 levels. These changes were associated with increased permeability of the BBB and lower levels of the tight junction proteins occludin, ZO1, and claudin-5. Inhibition of GSK3 reduced inflammatory cytokine levels, and increased tight junction protein levels, and reversed impaired recovery, demonstrating that prolonged learned helplessness is reversible and is maintained by abnormally active GSK3. Stimulation of sphingosine 1-phosphate receptors by Fingolimod promoted production of tight junction proteins, promoted BBB integrity, and reversed impaired recovery from learned helplessness. In conclusion, this project demonstrates that a single stress induces the expression of many cytokines, and that sequential daily stresses accelerates the rate of cytokine production. Stress induced a broad inflammatory response in mouse hippocampus, which involves TLR4, GSK3, and inflammatory cytokines, and these stress responses contribute to susceptibility to depression-like behavior in mice. This project also provides a new model to begin to identify mechanisms that impair recovery from a depression-like behavior and the results raise the novel hypothesis that stress-induced impaired BBB integrity contributes to prolonged depression-like behavior