Gender Differences in Acute Axonal Pathology Following Experimental Concussion

Academic Background

Each year, approximately 50 million people worldwide suffer from concussions, also known as mild traumatic brain injuries (TBI). However, for more than 15% of patients, this “mild” brain injury can lead to lasting neurocognitive dysfunction. The existing consensus indicates that concussions induce acute structural and physiological disruptions in brain network connectivity and function, particularly axonal pathology disseminated in white matter, also known as Diffuse Axonal Injury (DAI). Recent studies have shown that a key pathological substrate of concussion is damage to the white matter axons, and that the pathological outcomes post-concussion may differ between males and females.

Although males dominate the concussion cases in emergency rooms, this phenomenon is primarily because males are more likely to engage in activities that involve a risk of head impact. However, within the same sports, female athletes have a higher incidence of concussions and poorer outcomes, suggesting that gender differences might influence the extent of axonal pathology. The hypothesis that females have more small-diameter axons has been confirmed through autopsy and magnetic resonance imaging (MRI) studies, indicating that females may experience more extensive axonal injury following a concussion.

Research Source

This paper was jointly written by Hailong Song, Alexandra Tomasevich, Andrew Paolini, Kevin D. Browne, Kathryn L. Wofford, Brian Kelley, Eashwar Kantemneni, Justin Kennedy, Yue Qiu, Andrea L. C. Schneider, Jean-Pierre Dolle, D. Kacy Cullen, and Douglas H. Smith. The authors are affiliated with the Department of Neurosurgery at the University of Pennsylvania, Michael J. Crescenz Veterans Affairs Medical Center, and the Penn Center for Brain Injury and Repair, among other institutions. The article was published in the journal “Acta Neuropathologica.”

Research Process

Experimental Design and Model Construction

To investigate potential gender differences in axonal pathology following a concussion, the study used a clinically relevant pig concussion model that simulates human head rotational acceleration. This model induces selective axonal pathological changes through rapid head rotational acceleration without involving neuronal death or significant pathological changes. A total of 16 Hanford breed pigs aged approximately 6 to 8 months were used in the experiment, randomly divided into four groups: a sham-operated female group of 3 pigs, a sham-operated male group of 3 pigs, an injured female group of 5 pigs, and an injured male group of 5 pigs.

Experimental Steps

In the injury groups, the pigs underwent experimental concussion through rapid head rotational acceleration. The experiment employed a Hyge pneumatic actuator that converted linear motion to angular motion, producing a 110-degree head rotation within 20 milliseconds. This precise control of head movement ensured that both injured female and male pigs were examined 24 hours post-injury (a time point expected to exhibit extensive axonal pathology).

Tissue Preparation and Staining

At 24 hours post-injury, all animals were perfused transcardially to fix the brain, followed by brain removal, sectioning, and fixation in 10% formalin for one week. Subsequently, brain tissues were sectioned into 8-micron thick continuous slices and subjected to immunohistochemistry (IHC) for detecting amyloid precursor protein (APP) and myelin basic protein (MBP). Additionally, transmission electron microscopy (TEM) was used to evaluate potential gender differences in axon diameter and changes in average axonal diameter post-injury.

Main Research Findings

Differences in AXON Swelling and SODIUM CHANNEL Loss

First, the study found that the number of APP immunoreactive axon profiles in the brains of injured female pigs was significantly higher than in male pigs, particularly in the deep white matter and subcortical bundles adjacent to the lateral ventricles. This difference was consistent across consecutive coronal sections of the brain, and the extent of APP axonal pathology correlated positively with recovery time post-injury.

Secondly, the study observed that the loss of the major sodium ion channel Nav1.6 in the brains of injured female pigs was more extensive compared to male pigs. The loss of sodium ion channels was more widespread relative to the APP immunoreactive axon profiles. This suggests that beyond the physical network disconnection caused by axonal swelling, the loss of sodium ion channels may represent physiological disruptions in functional connectivity, potentially contributing to the prolonged recovery period in female pigs.

Differences in Axonal Ultrastructure

Cross-validation through TEM revealed that, regardless of pre-injury or post-injury status, the average axon diameter in female pigs was significantly smaller than in male pigs. Further analysis indicated that small-diameter axons were more susceptible to degeneration and degradation post-injury, whereas male pigs had a dominance of larger-diameter axons. Additionally, the study found that axon size increased post-injury in both genders, suggesting selective degeneration of small-diameter fibers, with this change being more pronounced in female pigs.

Conclusion

This study is the first to reveal significant gender differences in white matter axonal pathology following a concussion. The research indicates that tissue deformation induced by rotational acceleration of the brain leads to greater disruption in axonal transmission in females than in males, causing protein accumulation at swelling sites, further leading to fracture and degradation. The selective vulnerability of small-diameter axons may be a crucial mechanism behind these gender differences. The results explain why females may have poorer recovery and clinical outcomes post-concussion. The study recommends further exploration of non-invasive methods, such as blood biomarkers and advanced neuroimaging techniques, to identify potential gender differences post-concussion.

Research Value

This study provides scientific evidence for the role of gender in the pathology of concussions, significantly contributing to the understanding of concussion mechanisms and the development of personalized treatment plans. Additionally, the research points to future directions such as the application of advanced imaging technologies and blood biomarkers in pathological detection, which holds promise for providing more effective diagnostic and therapeutic tools for concussions.