Intra-axonal neurofilament compaction does not evoke local axonal swelling in all traumatically injured axons

Traumatic axonal injury (TAI) contributes to morbidity and mortality following traumatic brain injury (TBI). Single-label immunocytochemical studies employing antibodies to neurofilament compaction (NFC), RM014, and antibodies to APP, a marker of impaired axonal transport (AxT), have shown that TAI involves both NFC and disruption of AxT. Although it may be hypothesized that both events occur within the same injured axon, this has not been confirmed. To determine the relationship between NFC and impaired AxT, dual-label immunofluorescence was employed. To compare and contrast specific changes associated with these two markers of TAI, single-label electron microscopy was also used. Rats were subjected to an impact acceleration injury (30 min-6 h survival), and their brains were prepared for dual-label immunofluorescence and single-label electron microscopy. APP and RM014 were consistently found in two distinct classes of TAI. One, which showed only RM014 immunoreactivity, was thin and elongate, was sometimes vacuolated, and revealed little progressive change over time. The second was distinguished by focal axonal swellings containing APP immunoreactivity alone in small-caliber axons or in combination with RM014 immunoreactivity in large-caliber axons. These swellings were localized to either nodal or internodal loci and underwent progressive swelling over time, ultimately leading to secondary axotomy. Ultrastructural examination of these two classes of TAI revealed NFC together with mitochondrial dilation without organelle pooling in the RM014 single-labeled axons. However, the APP single-labeled small-caliber axons and APP/RM014 dual-labeled large-caliber axons revealed a progressive accumulation of organelles associated with increased axonal swelling over time. In contrast to previous thought, it now appears that NFC may occur independent of impaired AxT in TAI. This finding underscores the complexity of TAI, suggesting the need for multiple immunocytochemical approaches to fully assess the overall axonal response to TBI.