Repetitive mild traumatic brain injury elicits a reactive microglial morphology and elevates serum neurofilament light levels, independent of NLRP3 inflammasome inhibition

Abstract

There is growing evidence that a second mild traumatic brain injury (mTBI) sustained prior to cellular and neurological recovery from the first increases risk of cumulative pathological and neurological deficits. Despite this, the pathophysiology of single and repeated mTBI (rmTBI) is not well understood, and as such, no therapeutic interventions are available to mitigate the cumulative deficits induced. The Nod-like receptor pyrin domain containing 3 (NLRP3) inflammasome is an intracellular protein complex that is critical to the innate immune response. Although neuroinflammation is a key feature of mTBI pathophysiology, little is known about the role of the inflammasome in rmTBI. To investigate this, we used a clinically relevant rat model of mTBI and assessed whether pharmacologically inhibiting NLRP3 with the specific NRLP3 inhibitor MCC950, reduced inflammasome activation in the brain and thereby improved acute behavioural and molecular outcomes. We hypothesised that rmTBI would result in up-regulation of inflammasome-associated genes, a reactive microglial phenotype, axonal injury, and neurobehavioural deficits, and that these factors would be attenuated by NLRP3 inflammasome inhibition with MCC950. We found that rmTBI increased hippocampal apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC) mRNA expression, increased microglia cell number in the motor cortex, sensorimotor cortex, hippocampus, corpus callosum and thalamus, and elevated serum neurofilament light (NfL) levels that correlated with both the number of microglia in the corpus callosum, and sensorimotor deficits. These changes were not mitigated by MCC950 treatment. This study provides evidence that the NLRP3 inflammasome does not play a significant role in the acute neuroinflammatory response following rmTBI; however further studies are required to investigate its role chronically.