Abstract WP101: Progesterone Counteracts Stress-Induced Microglial Priming by Reducing NLRP3 Inflammasome Activation After Ischemic Injury

Abstract

Introduction: Cerebral ischemia, a consequence of cardiac arrest and stroke, is a leading cause of death and disability in the U.S. To improve translational studies, STAIR recommends augmenting animal models with comorbidities that “better reproduce the pathophysiological state” of stroke patients. Therefore, we incorporated social stress into our ischemia model (Espinosa-Garcia et al., 2017). Prior exposure to social stress upregulates the inflammatory status of microglia. They become primed—sensitized to activation by subsequent ischemia—worsening outcome. Such potentiated activation may result from the release of danger signals in the brain, such as HMGB-1, which binds to TLR4 receptors and activates inflammasomes. The NLRP3 inflammasome consists of a sensor molecule, NLRP3, the ASC adaptor, and caspase-1, whose activation leads to pro-inflammatory IL-1β production. We hypothesized that progesterone (P4) administration will reduce NLRP3 inflammasome activation, one of the molecular mechanisms underlying P4’s neuroprotective effects against ischemia combined with stress. Methods: Adult male rats were exposed to social defeat stress for 8 days and then subjected to global ischemia. P4 was administered 2 and 6h after occlusion and then daily for 7 days. Animals were killed at 7 or 14 days post-ischemia and their hippocampi dissected for western blots. The hippocampus is highly sensitive to stress and selectively vulnerable to global ischemia. Results: Consistent with previous reports, stress primed microglia and increased ( p <0.05) HMGB1 and NLRP3 levels, with no changes in IL-1β production in non-ischemic controls. Stressed ischemic animals showed potentiated ( p <0.05) NLRP3 inflammasome activation and increased IL-1β production. P4 treatment robustly reduced ( p <0.05) HMGB1, TLR4, phospho-IκB, NLRP3, ASC, cleaved caspase-1, and IL-1β levels in both stressed and non-stressed ischemic animals. Conclusion: Our data show that P4 can modulate stress-induced microglial priming to subsequent ischemic injury by counteracting NLRP3 inflammasome activation in the hippocampus. P4 may thus prove a promising neuroprotective agent to reduce inflammation associated with poor outcome in stroke patients with comorbid stress.