Evidence for the Transient Presence of Atypical Astrocytes in Mice Following a Single, Closed-Head Mild Traumatic...

Mild traumatic brain injury (mTBI) affects roughly 42 million people each year, causes a variety of physical, behavioral, and cognitive symptoms, and increases the risk for developing neurological disorders, including post-traumatic headache (PTH) and Alzheimer's disease (AD). Multiple molecular and cellular changes occur following mTBI; here we focus on astrocytes - cells that respond to brain injury and are critical to maintaining neuronal and circuit homeostasis. While some astrocytes become reactive after mTBI, others adopt an 'atypical' state characterized by the loss of multiple functional astrocyte proteins, including glutamate transporters (GLT-1, GLAST) and ion channels (Kir4.1), without upregulation of prototypical reactive astrocyte markers (glial fibrillary acidic protein [GFAP]). Previous studies have shown that repeated mTBI causes atypical astrocytes (AtAs) that can persist for months, but we know much less about whether a single mTBI causes similar astrocyte phenotypes. To address this, we employed a closed-head mild traumatic brain injury (chmTBI) model in male and female mice and quantified the abundance of AtAs both acutely (3-days) and chronically (1-month) after a single injury. We found that 3-days after chmTBI, AtAs were present in areas subject to blunt force trauma (BFT), consistent with previous reports, as well as in other brain regions presumably affected by diffuse injury. One month after chmTBI, however, the proportion of AtAs was similar between chmTBI and sham injured mice, thereby suggesting AtAs do not persist long term in this model. Consistent with previous studies, this chmTBI model did not induce significant GFAP-positive reactive astrocytes as assayed using immunohistochemistry, at either timepoint. Overall, we show an increase in AtAs 3-days after a single chmTBI that returns to sham levels when examined 1-month after injury. This suggests that after a single mTBI, AtAs are present but do not persist long term, unlike in repeated mTBI where AtAs persist for months after injury.