Rapid loss and partial recovery of neurofilament immunostaining following focal brain injury in mice

Neurofilaments (NF), the intermediate filaments of the neuronal cytoskeleton, provide mechanical stability to the cell. High-molecular-weight NF (NFH) comprises a heavily phosphorylated carboxyl terminal ("sidearm") domain which helps determine interfilament spacing distances. Experimental evidence suggests that dephosphorylation greatly increases the rate and extent of proteolysis of NFH. Because NF proteolysis has been implicated as one pathogenic mechanism underlying cell death following traumatic brain injury (TBI), we analyzed the patterns of acute NFH damage in relation to phosphorylation state following focal, concussive, controlled cortical impact (CCI) brain injury in mice. Brains from C57BL/6 male mice (n = 4 injured and n = 1 sham per time point) were evaluated 5 min, 15 min, 90 min, 4 h, and 24 h following CCI injury (1 mm depth, 5 m/s). Immunohistochemistry was performed using antibodies that recognize epitopes on either dephosphorylated (d-NFH) or phosphorylated (p-NFH) sidearms or on the core (c-NFH) domain. As early as 5-15 min postinjury, immunoreactivity for d-, p-, and c-NFH decreased in the ipsilateral cortex, and hippocampal CA3, CA1, and dentate areas. This marked decrease of NFH labeling occurred in the absence of notable cell loss. Furthermore, partial recovery of NFH labeling was observed as early as 90 min postinjury in the cortex and by 24 h postinjury in hippocampal CA3 and dentate. The results of this study suggest that both phosphorylated and dephosphorylated NFH are vulnerable almost immediately following focal brain injury in mice, but that injured neurons may have an adaptive capability to partially restore this important cytoskeletal protein.