Effects of concussive impact injury assessed in a new murine neurotrauma model
Tagge, Chad Alan
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Postmortem brains from young athletes with a history of repetitive concussive head injury and military service personnel with history of blast neurotrauma revealed evidence of parenchymal contusion, myelinated axonopathy, microvasculopathy, neuroinflammation, neurodegeneration, and phosphorylated tauopathy consistent with chronic traumatic encephalopathy (CTE) (L. E. Goldstein et al., 2012). The mechanisms by which head trauma induces acute concussion and chronic sequelae are unknown. To elucidate the mechanistic connection between traumatic brain injury (TBI), acute concussion and chronic sequelae, including CTE, require the use of animal models. This doctoral dissertation investigated the hypothesis that closed-head impact injury in mice triggers acute neurological signs associated with sport-related concussion as well as brain pathologies and functional sequelae associated with CTE. To test this hypothesis, we developed a mouse model of impact neurotrauma that utilizes a momentum transfer device to induce non-skull deforming head acceleration, triggering transient neurological signs consistent with acute concussion and traumatic brain injury (TBI) in unanesthetized C57BL/6 mice. The Boston University Concussion Scale (BUCS) was developed to assess neurological signs that are consistent with acute concussion in humans. Mice exhibited contralateral circling and limb weakness, locomotor abnormalities, and impaired gait and balance that recapitulate acute concussion in humans. Concussed mice recovered neurological function within three hours, but demonstrated persistent myelinated axonopathy, microvasculopathy, neuroinflammation, and phosphorylated tauopathy consistent with early CTE. Concussive impact injury also induced blood-brain barrier disruption, neuroinflammation (including infiltration peripheral monocytes and activation microglia), impaired hippocampal axonal conduction, and defective long-term potentiation (LTP) of synaptic transmission in medial prefrontal cortex. Kinematic analysis during impact injury revealed head acceleration of sufficient intensity to induce acute concussion, traumatic brain injury (TBI), early CTE-linked pathology, and related chronic sequelae. Surprisingly, the presence or degree of concussion measured by BUCS did not correlate with brain injury. Moreover, concussion was observed following impact injury but not blast exposure under conditions that induce comparable head kinematics. Empirical pressure measurements and dynamic modeling revealed greater pressure on the head and compression wave loading in the brain during impact compared to blast neurotrauma. These findings suggest acute concussion is triggered by focal loading of energy that transit the brain before onset of macroscopic head motion. By contrast, the forces associated with rapid head motion is sufficient to induce CTE-linked pathology. Our results indicate that while acute concussion and chronic sequelae may be triggered by the same insult, the pathophysiological responses underpinning these effects are engaged through distinct mechanisms and time domains. Our results indicate that concussion is neither necessary nor sufficient to induce acute brain injury or chronic sequelae, including CTE.