The role of calpain-mediated spectrin proteolysis in traumatically induced axonal injury

In animals and man, traumatic brain injury (TBI) results in axonal injury (AI) that contributes to morbidity and mortality. Such injured axons show progressive change leading to axonal disconnection. Although several theories implicate calcium in the pathogenesis of AI, experimental studies have failed to confirm its pivotal role. To explore the contribution of Ca2+-induced proteolysis to axonal injury, this study was undertaken in an animal model of TBI employing antibodies targeting both calpain-mediated spectrin proteolysis (CMSP) and focal neurofilament compaction (NFC), a marker of intra-axonal cytoskeletal perturbation, at 15-120 minutes (min) postinjury. Light microscopy (LM) revealed that TBI consistently evoked focal, intra-axonal CMSP that was spatially and temporally correlated with NFC. These changes were seen at 15 min postinjury with significantly increasing number of axons demonstrating CMSP immunoreactivity over time postinjury. Electron microscopy (EM) demonstrated that at 15 min postinjury CMSP was confined primarily to the subaxolemmal network. With increasing survival (30-120 min) CMSP filled the axoplasm proper. These findings provide the first direct evidence for focal CMSP in the pathogenesis of generalized/diffuse AI. Importantly, they also reveal an initial subaxolemmal involvement prior to induction of a more widespread axoplasmic change indicating a spatial-temporal compartmentalization of the calcium-induced proteolytic process that may be amenable to rapid therapeutic intervention.