Voltage gated sodium channels (NaChs) are essential for action potential initiation and propagation. As seen with genetic mutations, modifications altering the kinetic properties of NaChs have serious repercussions within neuronal network communication and underlie multiple disease states. This work evolved from the identification of a nongenetic NaCh modification within a neurodegenerative disorder:​ the finding that NaChs are proteolyzed following in vitro diffuse axonal injury, a model of traumatic brain injury (TBI), and that a broad spectrum inhibition of proteolysis leads to a significant reduction in Ca²⁺ deregulation after injury. Ca²⁺ deregulation is a prominent effect of TBI, therefore, if NaCh proteolysis contributes to this pathology, inhibition of proteolysis or inhibition of the downstream effects of proteolysis becomes a viable therapeutic option in treating patients with TBI, eliminating the need to directly inhibit NaChs that are essential to physiological neuronal transmission.

Therefore, to identify potential therapeutic candidates, we investigated the conditions leading to proteolysis of NaChs, the protease mediating proteolysis, and the functional significance of NaCh proteolysis. We found that proteolysis occurs within in vitro and in vivo models of TBI, requires elevated intracellular [Ca²⁺], and is mediated by the Ca²⁺ dependent protease calpain. NMDA receptors and NaChs, both direct and indirect initiation points for Ca²⁺ entry following a mechanical insult, are the major driving force for calpain activation and targeting of NaChs. Proteolyzed NaChs remain at the membrane surface prior to neuronal death and are located along, but not restricted to, unmyelinated axonal tracts. Six hours following a stretch injury, at the time of robust NaCh proteolysis, somatic neuronal responses to the NaCh agonist, veratridine, are altered. Inhibition of calpain activation and NaCh proteolysis, through overexpression of the endogenous calpain inhibitor calpastatin, fails to prevent the injury response to veratridine. This suggests that axonal, rather than somatic, proteolyzed sodium channels are the underlying contributors to the pathological changes seen after diffuse axonal injury and that therapies should be designed to specifically target this population of NaChs.