Blast injury leads to chronic shifts in protein expression of distinct astrocyte populations

Abstract Blast traumatic brain injury results in chronic pathology, especially for those receiving repetitive injuries. To evaluate cellular changes induced by these pressure waves, we studied post-mortem prefrontal cortex of military personnel with a history of multiple blast exposures and military controls with no battlefield experience. Chronic increases in IBA1 (microglia) and GFAP (glial fibrillary acidic protein; astrocyte) immunoreactivity occurred in injured brains and also confirmed that GFAP-expressing astrocytes altered predominantly at interface regions of the brain: around blood vessels, the grey-white matter interface and in layer 1, consistent with the pattern of damage seen with blast exposure. We focused on pathologic implications of the astrocyte derived proteins GFAP, aquaporin-4, and connexin-43. Astrocyte morphology in injured samples altered significantly, revealing a disintegrated, beaded shape, with a loss of fine processes. We also observed a shift in astrocyte immunoreactivity, where control brains showed two dominant populations, labeling as either GFAP+ or aquaporin-4+ only, with a smaller portion of co-labeled cells. Samples from injured brains revealed the emergence of a third dominant population of cells with abnormal morphology co-labeled with GFAP and aquaporin-4; significant increases in astrocytes with abnormal morphology also occurred, including those both GFAP+ and aquaporin-4. Connexin-43, which helps maintain neural homeostasis, significantly co-labeled with aquaporin-4, and not GFAP, in both control and injured brains, suggesting the aquaporin-4 subtype to be homeostatic. Interlaminar astrocytes consistently showed abnormal morphology in injured brains, featuring extensive GFAP+ beaded processes. The GFAP+ beaded processes showed additional characteristics in the injured brains, being surrounded in a ring-like fashion by aquaporin-4 immunoreactivity as well as co-labeling with phosphorylated connexin-43, indicating an inflammatory phenotype. To investigate secondary pathology that might relate to immune dysfunction, immunoreactivity with IgG revealed the presence of autoantibody in injury samples, which primarily labeled neurons in layer 2-3 that also co-immunoreacted with complement C3. Half of the interlaminar astrocytes in the injured brains also showed immunoreactivity with C3. We conclude that control human cerebral cortex contains at least two distinct populations that express either GFAP or aquaporin-4, but not both. After military related blast traumatic brain injury, a third population of astrocytes emerges expressing both GFAP and aquaporin-4. Connexin-43 continues to be co-expressed with aquaporin-4, but shifts toward an abnormal morphology. We also find overall chronic alterations in expression of astrocytic proteins that coincide with induction of autoantibodies directed towards neurons and recruit complement.