Impaired axonal transport and altered axolemmal permeability occur in distinct populations of damaged axons following traumatic brain injury.

Traumatic axonal injury (TAI) evolves within minutes to hours following traumatic brain injury (TBI). Previous studies have identified axolemmal disruption and impaired axonal transport (AxT) as key mechanisms in the evolution of TAI. While initially hypothesized that axolemmal disruption culminates in impaired AxT, previous studies employed single-label methodologies that did not allow for a full determination of the spatial-temporal relationships of these two events. To explore directly the relationship between impaired AxT and altered axolemmal permeability, the current investigation employed 40, 10, and 3 kDa fluorescently conjugated dextrans as markers of axolemmal integrity, with antibodies targeting the anterogradely transported amyloid precursor protein (APP) utilized as a marker of impaired AxT. Rats underwent impact acceleration TBI and were intrathecally administered 40 kDa, 40 + 10 kDa or 40 + 3 kDa fluorescently tagged dextrans, with brains subsequently prepared for APP immunofluorescence. Brainstem corticospinal tracts (CSpT), medial lemnisci (ML), and medial longitudinal fasciculi were examined for evidence of TAI. APP and all dextrans consistently localized to distinct classes of TAI. Dextrans were noted as early as 5 min following injury within axonal segments demonstrating an irregular/tortuous appearance, and were seen within thin and elongate/vacuolated axons by 30 min-6 h following injury. APP, first noted within swollen axons at 30 min following injury, was found within progressively swollen axons that showed no dextran colocalization within 3 h of injury. However, by 6 h, dextrans colocalized in disconnected axonal bulbs. At this time-point, dextrans also persisted within single-labeled, highly vacuolated/thin, and elongate axons. These studies confirm that axolemmal disruption and impaired AxT occur as distinct non-related events early in the pathogenesis of TAI. Further, these studies provide evidence that the process of impaired axonal transport and subsequent axonal disconnection leads to delayed axolemmal instability, rather than proceeding as a consequence of initial axolemmal failure. This finding underscores the need of multiple approaches to fully assess the axonal response to TBI.