Repetitive mild traumatic brain injury induces hippocampal neural immune signaling linked to astrocyte reactivity

Abstract

AbstractBackgroundRepeated mild traumatic brain injuries (mTBIs) sustained within a “window of vulnerability” can lead to a cumulative severity of outcomes such as memory impairment, depression, and long‐term development of amyloid beta (Aβ) pathology and neurofibrillary tangles. We previously reported that acute increases in neural immune signaling within the MAPK pathway and expression of pro‐inflammatory cytokines were associated with low cerebral blood flow (CBF) and worse long‐term spatial learning and memory in a weight‐drop mouse model of repetitive mTBI. We hypothesized that an increasing number of injuries would lead to an increase in neural immune signaling, pathological markers, and glial activation in the hippocampus.MethodFemale 3xTg mice at 3‐4mo of age were subjected to 1x, 3x, or 5x weight‐drop closed head injuries (CHI) spaced once‐daily. Animals were sacrificed and brains harvested at 30min, 4h, 24h, or 1‐month after the final CHI (n = 7‐10/time point/injury group). 32 cytokines, 10 MAPK phospho‐proteins, Aβ42, Aβ40, phospho‐tau (pThr181), and total tau were measured by Luminex in hippocampal tissues. GFAP was measured by ELISA. CBF was non‐invasively measured immediately prior to sacrifice.ResultWe observed a steady increase in both total tau and phospho‐tau and a trend of increased GFAP in the hippocampus after repeated injuries. We then identified groups of cytokines and MAPK phospho‐proteins which co‐vary with either GFAP or phospho‐tau. GFAP‐associated cytokines showed a rapid acute increase after injury then a return to basal levels by 24h, while GFAP‐associated MAPK dynamics changed with repeated injury. Cytokines and MAPK phospho‐proteins showed a notably high degree of cross‐correlation 4h after repeated injury, when GFAP levels were at their highest. Multivariate analysis revealed that high levels of GFAP were associated with low CBF while increased phospho‐tau in the frontal cortex was associated with high CBF.ConclusionTogether, our data indicate an increase in hippocampal neural immune signaling and astrocyte reactivity after repetitive brain injury. Immune signaling changes may be causally linked to spatial learning and memory deficits after repetitive mTBI.