Alteration of the neurofilament sidearm and its relation to neurofilament compaction occurring with traumatic axonal injury.

Traumatic injury evokes two characteristic forms of focal axonal injury, one of which involves focal perturbation of axolemmal permeability associated with rapid compaction of the underlying axonal neurofilament lattice and microtubular loss. In this process, the neurofilament sidearms have been the subject of intense scrutiny in relation to their role in this NF compaction, with the suggestion that the sidearms, thought to maintain interfilament distance, are proteolytically cleaved and degraded at the time of injury. The current communication addresses the fate of the NF sidearms in such injured axons. Adult cats were subjected to moderate/severe fluid percussion brain injury after intrathecal administration of horseradish peroxidase (HRP). This tracer, excluded by the intact axolemma of uninjured axons, was used to recognize injured axons via HRP intra-axonal uptake/flooding with HRP. Animals were perfused and processed for light microscopic and electron microscopic study of both HRP-containing and non-HRP-containing axons from the same field. HRP-containing axons consistently displayed evidence of traumatically-induced (NF) cytoskeletal collapse. Electron micrographs of HRP-containing axons as well as uninjured, non-HRP-containing axons from the same fields were videographically captured, digitized, enlarged and analysed for NF sidearm length and NF density. HRP-containing axons were found to have increased NF density. Surprisingly, this increased NF density occurred despite the retention of the NF sidearms, which now, however, were reduced in height in comparison to the non-HRP-containing uninjured axons. These observations are not consistent with previously published reports suggesting that overt proteolytic degradation of sidearms was responsible for NF compaction. Based on our findings, we suggest that the NF compaction associated with traumatically-induced axolemmal permeability changes may have its genesis in more subtle sidearm modification, perhaps involving a change in phosphorylation state.