Dephosphorylated neurofilaments are vulnerable following focal traumatic brain injury in mice

Introduction: Neurofilaments (NFs), the intermediate filaments of the neuronal cytoskeleton, provide mechanical stability to the cell. NFH (200 kDa), one of the three NF protein subunits, comprises an α-helical rod domain and a multiphosphorylated carboxy-terminal ("sidearm") domain. When phosphorylated, NFH sidearms project radially from the filament core thereby influencing inter-filament spacing. 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 TBI in mice. Methods: Male, C57BL/6 mice were anesthetized and subjected to controlled cortical impact brain injury, which produces contusion in the underlying gray matter. Groups of mice were sacrificed at 5 min, 15 min, 90 min, 4 hr, or 1 day following injury (n=4 injured, n= 1 sham-injured per time point). Immunohistochemistry was performed using antibodies which recognize epitopes on either dephosphorylated (d-NFH) or phosphorylated (p-NFH) NFH sidearms. Results: As early as 5 min postinjury, immunoreactivity for d-NFH decreased dramatically in the injured cortex. At 4 hr and 1 day postinjury, areas in the injured core showed return of d-NFH staining compared to earlier time points. In contrast, only a mild decrease in immunostaining for p-NFH was observed in the injured cortex. In addition, scattered areas exhibited increased immunoreactivity and better preservation of cell morphology, particularly in layers II, III, and V of the cortex. Throughout all the time points, the ipsilateral dentate hilus showed decreased immunoreactivity for both d-NFH and p-NFH. Conclusion: Our preliminary studies indicate that dephosphorylated NFH in the cortex is more susceptible to damage following focal TBI in mice, suggesting that phosphorylation of this important cytoskel-et al protein may have a protective role in the setting of CNS injury.