Survival Following Traumatic Brain Injury in <i>Drosophila</i> Is Increased by Heterozygosity for a Mutation of the NF-κB Innate Immune Response Transcription Factor Relish

Abstract

Traumatic brain injury (TBI) pathologies are caused by primary and secondary injuries. Primary injuries result from physical damage to the brain, and secondary injuries arise from cellular responses to primary injuries. A characteristic cellular response is sustained activation of inflammatory pathways commonly mediated by nuclear factor-κB (NF-κB) transcription factors. Using a <i>Drosophila melanogaster</i> TBI model, we previously found that the main proximal transcriptional response to primary injuries is triggered by activation of Toll and Imd innate immune response pathways that engage NF-κB factors Dif and Relish (Rel), respectively. Here, we found by mass spectrometry that Rel protein level increased in fly heads at 4-8 hr after TBI. To investigate the necessity of Rel for secondary injuries, we generated a null allele, <i>Rel</i>^<i>del</i> , by CRISPR/Cas9 editing. When heterozygous but not homozygous, the <i>Rel</i>^<i>del</i> mutation reduced mortality at 24 hr after TBI and increased the lifespan of injured flies. Additionally, the effect of heterozygosity for <i>Rel</i>^<i>del</i> on mortality was modulated by genetic background and diet. To identify genes that facilitate effects of <i>Rel</i>^<i>del</i> on TBI outcomes, we compared genome-wide mRNA expression profiles of uninjured and injured +/+, +/<i>Rel</i>^<i>del</i> , and <i>Rel</i>^<i>del</i> /<i>Rel</i>^<i>del</i> flies at 4 hr following TBI. Only a few genes changed expression more than twofold in +/<i>Rel</i>^<i>del</i> flies relative to +/+ and <i>Rel</i>^<i>del</i> /<i>Rel</i>^<i>del</i> flies, and they were not canonical innate immune response genes. Therefore, Rel is necessary for TBI-induced secondary injuries but in complex ways involving <i>Rel</i> gene dose, genetic background, diet, and possibly small changes in expression of innate immune response genes.