Persistently “Leaky” Nav Channels In Traumatized Axons: Lowered Barriers To Nav1.6 Voltage Sensor Motions In Blebbed Plasma Membrane As A Possible Explanation

Mechanical trauma of CNS nodes of Ranvier generates axolemmal blebs. The nodal Na+-channel, Nav1.6, leaks Na+ after traumatic brain injury but its molecular “lesion” is not understood. We found that traumatic stretch of Na+-dye loaded HEK-Nav1.6 cells causes an immediate TTX-sensitive Na+-leak. Also, using oocytes, we measured Nav1.6 current in cell-attached patches before and after pipette aspiration (which causes blebs) and observed irreversibly left-shifted g(V) and availability(V). To determine if intact cell Nav1.6 operation left-shifts with trauma, within-cell comparisons of HEK-Nav1.6 cell INa before and after traumatic stretch would be ideal, but this is impracticable. Instead, perforated patch recordings (multi-pulse protocols) of INa were obtained in a given HEK-Nav1.6 cell which was then swollen grossly (distilled water, 90-180 s) and returned to normal saline. After 5 min (for cell morphology and [Na+]ext to regularize) INa was re-measured. The resulting pattern of INa changes at various voltages showed that, post-osmotrauma, both g(V) and availability(V) were left-shifted at least 5 mV and this was irreversible (10 min experiments). Time controls showed no left-shift. A simple explanation is that, post-trauma, abnormally fluid disorganized bilayer of blebbed membrane presents abnormally low energy barriers to Nav1.6 voltage sensor motions. Smaller depolarizations are thus required to elicit sensor repacking than in stiffer pre-bleb bilayer. To the extent that trauma-induced blebbing was non-uniform, left-shift would be “smeared”. For mildly traumatized axons of the traumatic penumbra, such left-smeared Nav1.6 window current, by leaving no “safe” voltage, should prove even more excitotoxic than maximal left-shifting. Positive feedback in free-running axons would persistently elicit Na+-leak as window currents from variously traumatized areas triggered each other. Supported by CIHR and HSFO.