Mechanoporation induced by diffuse traumatic brain injury: an irreversible or reversible response to injury?

Diffuse traumatic brain injury (DTBI) is associated with neuronal plasmalemmal disruption, leading to either necrosis or reactive change without cell death. This study examined whether enduring membrane perturbation consistently occurs, leading to cell death, or if there is the potential for transient perturbation followed by resealing/recovery. We also examined the relationship of these events to calpain-mediated spectrin proteolysis (CMSP). To assess plasmalemmal disruption, rats (n = 21) received intracerebroventricular infusion 2 h before DTBI of a normally excluded 10 kDa fluorophore-labeled dextran. To reveal plasmalemmal resealing or enduring disruption, rats were infused with another labeled dextran 2 h (n = 10) or 6 h (n = 11) after injury. Immunohistochemistry for the 150 kDa spectrin breakdown product evaluated the concomitant role of CMSP. Neocortical neurons were followed with confocal and electron microscopy. After DTBI at 4 and 8 h, 55% of all tracer-flooded neurons contained both dextrans, demonstrating enduring plasmalemmal leakage, with many demonstrating necrosis. At 4 h, 12.0% and at 8 h, 15.7% of the dual tracer-flooded neurons showed CMSP, yet, these demonstrated less advanced cellular change. At 4 h, 39.0% and at 8 h, 24.4% of all tracer-flooded neurons revealed only preinjury dextran uptake, consistent with membrane resealing, whereas 7.6 and 11.1%, respectively, showed CMSP. At 4 h, 35% and at 8 h, 33% of neurons demonstrated CMSP without dextran flooding. At 4 h, 5.5% and at 8 h, 20.9% of tracer-flooded neurons revealed only postinjury dextran uptake, consistent with delayed membrane perturbation, with 55.0 and 35.4%, respectively, showing CMSP. These studies illustrate that DTBI evokes evolving plasmalemmal changes that highlight mechanical and potential secondary events in membrane poration.