Simultaneous Reactive Change in Unmyelinated and Myelinated Axon Segments Following Experimental Diffuse Traumatic Brain Injury

Diffuse axonal injury (DAI) is a leading cause of traumatic brain injury (TBI) morbidity and has well-studied molecular pathobiology. Historically, white matter DAI studies indicated unmyelinated axons are more susceptible to injury than myelinated axons, with myelin posited to protect axons from diffuse TBI shear/tensile forces through unresolved mechanisms. Similarly, preclinical studies have also identified gray matter DAI localized to the perisomatic domain (i.e., the unmyelinated axon initial segment [AIS] and first one-to-two nodes of Ranvier). With these concepts in mind, we hypothesized unmyelinated segments are selectively vulnerable to TBI-mediated shear/tensile forces and serve as initiating sites for DAI pathobiology. Using murine midline fluid percussion injury, neocortical layer V pyramidal cell perisomatic domains at the gray-white matter interface were spatiotemporally examined for initiating pathology using antibodies to cytoskeletal proteins to demarcate unmyelinated segments and amyloid precursor protein (i.e., the gold-standard DAI marker) to identify injury. In cells expressing yellow fluorescent protein to enhance injury visualization, axonal swellings were observed simultaneously within perisomatic unmyelinated segments (e.g., AIS; nodes) as well as immediately adjacent myelinated segments, indicating concomitant reactive axonal changes. These data suggest non-selective axonal susceptibility and that myelin may not protect against diffuse injury forces. While expanding DAI topography to the gray-white matter junction, these findings also have implications for action potential initiation, axonal protein trafficking, and cortical circuit connectivity. Furthermore, studies are needed to determine if DAI pathological mechanisms are shared between white and gray matter axons, which have common and differentiating cytoarchitectural components.