Cellular Exploration and REsolution of BRain Injury

SUMMARY:
The problem of traumatic brain injury (TBI) heterogeneity has been a critical barrier to successful translation of therapies in the field. TBI heterogeneity exists in the patient substrate pre-injury (genetics, sex, comorbidities), external injury characteristics (severity, mechanism), and resultant post-injury host response that is responsible for deleterious secondary injury processes (seizures, neuroinflammation, neurodegeneration) and repair/regeneration. Identification of final common molecular pathways and signatures that integrate this vast heterogeneity could be valuable for guiding biomarkers, therapeutic targets, and predictive enrichment. In this study, we present the first large-scale searchable murine single-cell atlas of the transcriptomic response to TBI in 339,357 cells across three levels of clinically relevant injury models, sex, distance and time from injury, as a foundational step in molecularly deconstructing TBI heterogeneity. We identify 23 cell types with massive heterogeneity in the single-cell response across these extrinsic (severity) and intrinsic (sex, brain region, time) factors, that has been underestimated. Majority of response to TBI was unique to individual cell populations with minimal overlap even within a single injury-model thus highlighting the importance of cell-level resolution. Through this effort, we report novel cell-specific targets and a previously unrecognized role for specific microglial and ependymal subtypes in post-TBI pathophysiology that is highly variable depending on the extrinsic and intrinsic factors studied. One ependymal subtype was a hub of neuroinflammatory signaling after contusional-TBI, particularly related to Il-1b. A single microglial-lineage along pseudotime (comprising 3 microglial subtypes) was a key mediator of host-response after TBI, and shared features with disease associated microglia noted in Alzheimer's disease and other neurodegenerative disorders, potentially providing a link between TBI and accelerated neurodegeneration. One microglial subtype within this lineage emerged as a key target – it was the only cell type of all 23 that retained persistent and marked gene expression changes 6 months post contusional-TBI. We identify sexually dimorphic gene expression and pathway vulnerabilities with cell-specific differences in both immune and non-immune biological processes. These likely contribute to sex-based outcome and warrant further study to facilitate discovery of cell- and sex-specific druggable targets. Active changes in brain regions distal from the site of primary TBI impact included infiltration of specific microglial populations as well as cell-specific transcriptomic changes in several genes and inflammatory processes distinct from both the peri-contusional and naïve signatures. This atlas validates several known contributors in TBI pathophysiology, and also identifies previously unrecognized targets and avenues for further research. Beyond our presented exemplar analyses (including pathways of clinical interest like sulfonylurea-receptor-1), the companion searchable atlas serves as a foundation for countless future efforts to understand cell-specific heterogeneity after TBI (https://shiny.crc.pitt.edu/cerebri/) as well as numerous other neurological diseases with overlapping pathophysiology.

-Jha et al. (in preparation, 2024)