Effects of repetitive mild traumatic brain injury on neural immune signaling and Alzheimer's disease-like pathology.

Abstract

Repeated 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 worse long-term cognitive outcome in a weight-drop mouse model of repetitive mTBI. We hypothesized that an increasing number of injuries would lead to changes in the kinetics of neural immune signaling and that expression of cytokines would correlate with increased Aβ and tau pathology in the 3xTg mouse model. 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=4-6/time point/injury group). 32 cytokines and 10 MAPK phospho-proteins were measured by Luminex in cortical tissues. Immunohistochemistry was conducted for phospho-tau (S262/T263) and Aβ (6E10 antibody). We observed distinct temporal dynamics in MAPK pathway signaling and cytokine expression between 1x, 3x, and 5xCHI mice. 1xCHI mice showed maximal MAPK pathway activation at 30min (e.g, phospho-p38, pStat1) which then decreased concomitantly with increased expression of pro-inflammatory cytokines (e.g., RANTES, IL-17) at 24h. In contrast, 3xCHI yielded MAPK activation similar to 1xCHI at 30min, but continued to increase until 24h. 5xCHI showed an accelerated signaling response peak at 4h. Further, multiple cytokines were increased at 24h after 3x and/or 5xCHI compared to 1xCHI (e.g., VEGF, IL-9, IL-17; p<0.05). In terms of pathology, Aβ was increased at 4,24h (p<0.05) and phospho-tau was increased after 1mo in 5xCHI animals compared to sham controls (p<0.01). Moreover, positive trends were observed between phospho-tau at 1mo and expression of pro-inflammatory cytokines IL-15 and M-CSF (p=0.13, FDR-corrected). Repetitive mTBI yielded amplified and sustained MAPK pathway signaling at short time points and a relationship between cytokine expression and tau phosphorylation after 1mo. These findings suggest that modulation of the MAPK pathway may be a promising therapeutic target for rmTBI.