Mild traumatic brain injury/ concussion initiates an atypical astrocyte response caused by blood-brain barrier dysfunction

Mild traumatic brain injury/ concussion (mTBI) account for 70-90% of all reported TBI cases and cause long lasting neurological consequences in 10 to 40% of patients. Recent clinical studies revealed increased blood-brain barrier (BBB) permeability in mTBI patients, which correlated with secondary damage after mTBI. However, the cascade of cellular events initiated by exposure to blood-borne factors resulting in sustained damage are not fully resolved. We previously reported that astrocytes respond atypically to mTBI rapidly downregulating many proteins essential to their homeostatic function while classic scar formation does not occur. Here, we tested the hypothesis that mTBI -induced BBB damage causes atypical astrocytes through exposure to blood-borne factors. Using a mTBI mouse model, 2-photon imaging, an endothelial cell-specific genetic ablation approach, and serum-free primary astrocyte cultures, we demonstrated that areas with atypical astrocytes coincide with BBB damage and that exposure of astrocytes to plasma proteins is sufficient to initiate downregulation of astrocyte homeostatic proteins. While mTBI resulted in frequent impairment of both physical and metabolic BBB properties and leakage of small-sized blood-borne factors, deposition of the coagulation factor fibrinogen or vessel rupture were rare. Surprisingly, even months after mTBI BBB repair did not occur in areas with atypical astrocytes. Together, these findings implicate that even relatively small BBB disturbances are sustained long-term and render nearby astrocytes dysfunctional, likely at the cost of neuronal health and function.