Traumatic brain injury (TBI) is caused by physical damage, often resulting from accidents, including traffic accidents and fall accidents. TBI is classified into two types: penetrating brain injury, which occurs when a sharp object perforates the skull as well as the brain, and closed brain injury, which is caused by violent shaking of the brain inside without a break in the skull1.
The causes of TBI are very diverse, including concussions, brain contusions, hematomas, and skull fractures; therefore, TBI mouse models have been developed using various protocols to replicate these different causes. For example, a repetitive concussive TBI model involves brain shaking, where mice are stuck several times using an electromagnetically controlled rubber impactor2. Additionally, in the weight-drop TBI model, a strong external force is exerted on the head by a standardized weight-drop device, causing focal blunt injury with an intact skull3. Furthermore, the stab-wound TBI model is prepared by puncturing the skull and brain using a needle4 (Figure 1A). Since several TBI models have been developed, it is important to choose a model based on the specific pathology that needs to be observed.
Brain injury caused by physical damage leads to primary and secondary brain injuries, which further exacerbate neuronal loss. Primary injury occurs immediately after the damage, resulting from the breakdown of the blood-brain barrier (BBB), hemorrhage, and hematoma. Therefore, minimizing hemorrhage and hematoma expansion is crucial, as these factors can exacerbate the severity of TBI symptoms. Secondary injury is triggered by intraparenchymal blood components, which subsequently lead to inflammation around the lesion5. The prognosis after brain injury depends on the inflammatory dynamics; therefore, it is crucial to rapidly mitigate both primary and secondary injuries for a favorable prognosis6,7,8.
The BBB is composed of pericytes, tight junctions between endothelial cells, and the endfeet of astrocytes, which work together to restrict the leakage of substances from the blood vessels in healthy brains9. In the presented stab-wound system, the BBB is physically disrupted. Common methods for evaluating the BBB integrity include staining for immunoglobulin G (IgG) and assessing the leakage of fluorescence tracers, such as Evans blue and dextran10,11. IgG staining labels blood components that leak from the lesion site and deposit in the brain. As the BBB recovers, leakage of blood components into the brain decreases, and these deposits are gradually degraded. Therefore, IgG staining is used to assess the extent of BBB recovery after brain injury. Additionally, the level of leakage of intravenously administered tracer into the brain parenchyma reflects the recovery of BBB. This method provides a clearer evaluation of the BBB dynamics, as tracer leakage directly indicates the transition of blood components from the bloodstream to the brain parenchyma. Furthermore, minimizing the hemorrhage leads to a milder primary injury, which is supported by prompt blood coagulation and timely fibrinolysis. Therefore, quantifying the expression of blood coagulation and fibrinolysis regulators is an effective way to analyze this process. Regarding the molecular mechanism underlying coagulation, hemorrhage after brain injury is stopped by fibrin formation. Subsequently, the fibrin-rich thrombus is degraded by tissue plasminogen activator (tPA) and urokinase plasminogen activator (uPA). In the stab-wound TBI mouse model, fibrin formation peaks on 1 day after the injury and reduces thereafter10. Thus, the recovery level of the BBB can be predicted by quantifying blood components and tracer extravasation into the brain parenchyma, as well as the expression of blood coagulation factors.
Quantification methods for inflammation in the secondary injury process include glial activation and inflammatory cytokine expression. Prolonged inflammation is mainly induced by excessive microglia and astrocyte accumulation around the lesion site. For example, in a stab-wound TBI model, stab-wounds stimulate the reactivation of glial cells around the lesion to remove the cell debris and blood components. This glial reactivation typically peaks 3 days after the stab-wound12,13. In addition to their phagocytosis function, reactivated glial cells secrete excessive inflammatory cytokines, resulting in neuronal loss around the lesion14. It has been reported that the attenuation of glial inflammation contributes to a favorable prognosis after brain injury12,14. Determining the level of inflammation is useful for evaluating the severity and prognosis. Therefore, it is essential to develop a TBI model suitable for assessing hemorrhage extension and inflammatory severity. This study introduces a stab-wound mouse model that mimics penetrating brain injury, with the aim of studying the mechanisms of hemorrhage, inflammation, and neuronal loss in TBI pathology.