Inhibition of microglia priming by NLRP3 reduces the impact of early life stress and mild TBI.

Although most patients with mild traumatic brain injury (mTBI) experience rapid recovery, some report persistent chronic symptoms such as cognitive dysfunction. One risk factor for prolonged recovery after mTBI is early life stress (ELS). We hypothesized that ELS mediates prolonged cognitive dysfunction after mTBI by exacerbating the NRLP3 inflammasome signaling pathway, and that these effects could be reversed by inhibiting NLRP3.

Methods: To test this hypothesis, Sprague Dawley rat pups were maternally separated for 3 h daily from P2-P14. Subsequently, the rats underwent a mild-to-moderate fluid percussion brain injury (1.4 atm) or sham surgery during young adulthood and were then treated with either the NLRP3 inhibitor MCC950 or vehicle.

Results: We found that ELS significantly increased microglia and macrophage cell numbers within the hippocampus during mTBI recovery. Quantitative PCR demonstrated that the combination of ELS and mTBI significantly increased levels of HMGB1, TLR4, NLRP3, caspase 1, and IL-1β mRNA levels in the ipsilateral hippocampus at 24 h after injury. This upregulation was persistent and TLR4, NLRP3, caspase 1, and IL-1β levels remained elevated for up to 2 months after injury. Inhibition of the NLRP3 inflammasome with MCC950 reduced this upregulation both 24 h and 2 months after injury. Hippocampal microglia isolated by fluorescence-activated cell sorting demonstrated increased levels of NLRP3 after ELS alone, but not IL-1β. The upregulation in microglial IL-1β required the combination of ELS and mTBI and was ameliorated with MCC950. Additionally, MCC950 treatment improved glucocorticoid receptor downregulation in the hippocampus after ELS, mTBI alone and mTBI + ELS. The combinatory insult of ELS and mTBI also impaired associative fear memory which was prevented with MCC950 treatment.

Conclusion: In summary, ELS limits recovery after mTBI by upregulating the expression of NLRP3 inflammasome signaling molecules in microglia. Inhibition of NLRP3 is an effective therapeutic for treating chronic cognitive deficits after ELS and mTBI.