Excitement of fall sports season tempered by risks of traumatic brain injury
Football season brings with it a lot of excitement, but also the specter of concussions and traumatic brain injury (TBI) that are endemic to contact sports. With millions of new dollars being allocated for brain trauma research, new animal models are needed to provide insights into the pathology of TBI and as a platform for testing new therapeutics and treatments.
Novel closed-skull murine model replicates features of TBI in humans
Roth, et. al. at the NIH devised a novel closed-skull mouse model of TBI that replicates some features associated with mild TBI in humans and is amenable to imaging by two-photon laser scanning microscopy (TPM). The authors used TPM to monitor progression and pathology immediately following compression brain injury in laboratory mice. Shortly after injury, blood leaks from vessels into the meninges. Cell death in the meninges increased over time (up to 12 hours after injury), but was not seen in the parenchyma until 9-12 hours after injury. Meningeal damage is observed in ~50% of patients with mild head trauma, indicating a common pathology. Astrocyte cell death in the glial limitans of FVB/N-Tg(GFAPGFP)14Mes/J (003257), which displays astrocyte –specific GFP fluorescence, occurs 5 minutes after injury.
Reactive oxygen species (ROS) appear in the meninges within 30 minutes of injury and are a primary mediator of cell death following compression injury. Administration of a reduced form of glutathione (GSH), a ROS scavenger, prior to injury resulted in a 50% reduction of meningeal cell death, whereas post-injury application had no effect. Post-injury GSH application resulted in reduced parenchymal cell death, survival of meningeal macrophages, and glial limitans preservation.
Inflammatory responses following compression injury
The NIH researchers next investigated the inflammatory responses to TBI in B6.129P-Cx3cr1tm1Litt/J (005582) mice, which express EGFP in monocytes, dendritic cells, NK cells and brain microglia, and in a LysMgfp strain that expresses EGFP in neutrophils, monocytes and macrophages. These strains allowed the team to visualize the cellular dynamics at the site of injury. These investigators found that:
- Meningeal macrophages die within 30 minutes of injury.
- Microglia fortify the glial limitans by:
- Extending 2-3 processes through compromised glial limitans into meninges to form a stable "honeycomb" network.
- Retracting ramified processes and extending a non-branching, phagocytotic "jellyfish"-like process toward glial limitans.
- Neutrophils are recruited to the site of injury.
Transcranial administration is a viable delivery route for small compounds
The researchers then demonstrated that skull bone is permeable to small molecular weight compounds. A range of differently sized fluorescent dextrans were applied transcranially. Dextrans of 40,000 MW and smaller were able to pass through the intact skull bone into the meninges. This delivery route was used to administer GSH to therapeutically ameliorate the brain injury, as described above, and represents a novel delivery route for testing new therapeutics for traumatic brain injury.