More than half the population will receive a concussion at some point in their life. Adolescents, especially girls, are at the highest risk for persisting and late-onset symptoms. Persisting symptoms include headaches, dizziness, vision problems, fatigue, confusion, cognitive “fog,” and light and noise sensitivity. Late-onset symptoms are new ones that develop weeks to years post-concussion, including irritability and other personality changes, decreased resilience to stress, neuropsychiatric diagnosis, lower cognitive or physical endurance, infertility, problems regulating temperature, diabetes and sleep disruption. The inability to distinguish between typical adolescent volatile emotions, boundary-testing and impulsivity with post-concussion symptoms can delay intervention and exacerbate symptoms, leading to decisions and neurochemical disruptions that can impede meeting neurodevelopmental milestones. In addition to these post-concussion symptoms, concussion can potentially cause damage to brain cells that initiate long-lasting neurodegenerative processes, increasing the risk for age-related neurodegenerative diseases like dementia, Alzheimer’s, chronic traumatic encephalopathy (CTE) and Parkinson’s disease.1,2
The enduring consequences of concussion, especially the long-term impacts of aging after concussion, are not well studied. Additionally, there are few therapies available that prevent or reduce symptoms. Several reports indicate adolescent boys and girls, and women have increased severity and number of post-concussion symptoms.3 To date, few longitudinal clinical or preclinical studies target this age group and include both sexes, where females (girls/women) are treated based on data predominantly from males (boys/men). Since concussion-induced pathology is temporal and highly dynamic, detailed time courses of pathology and molecular processes are necessary to improve the knowledge base, inform clinicians, educate patients and families, identify novel therapeutic targets and improve current therapeutic interventions.
Our Translational Neurotrauma and Neurochemistry laboratory, under the direction of Theresa Currier Thomas, PhD, is the first to directly address these topics while working closely with Phoenix Children’s physicians to accurately identify therapeutic targets and test long-term efficacy to improve the long-term trajectory of patients.
Pathological Evaluation Using Histology
Our laboratory is currently evaluating a time course relationship of neuropathology, neuroinflammation and astrocyte activation after experimental concussion in male and female rats out to an equivalent of 10-15 years post-injury. Our key findings indicate neuropathology for the duration of the experiments in injured rats and the development of neuropathology as a function of age in control, non-injured rats. Neuropathology is associated with evidence of chronic neuroinflammation (Figure 1) and astrocyte activation (Figure 2) after injury where all forms of pathology are present as a function of age in control groups (@168 days post-injury). Neuroinflammatory cells, microglia, carry out many functions in the brain, often associated with specific cell shapes (morphology). The distribution of neuropathology and morphological profiles of neuroinflammatory cells differed, despite evidence of similar activation levels, indicating different biochemical roles at the latest time-point. In the cortex, hippocampus and hypothalamus, neuroinflammation was less robust in females compared to males.4 These altered and unique pathological trajectories will be further explored to distinguish between pathological or neuroprotective/neuroreparative phenotypes, associated secondary signaling cascades and test the ability to modulate these to reduce post-concussive symptoms and neurodegeneration.
1. Thalamus: Chronic microglial activation with sex differences in morphological profile. Increased soma (cell) counts indicate increased infiltrating and proliferating microglia. Decreased endpoints/soma are indicative of morphological changes representing a neuroinflammatory response. [A] 40x representative images of Iba-1 immunostaining for microglia in male and female, sham (control) and injured (FPI) rats at 7, 56 and 168 days post-injury (DPI). Scale bar = 100 µm. [B] For the soma count, there was an overall effect for FPI, DPI, an FPI*DPI interaction and an FPI*DPI*Sex interaction, where females had fewer soma at 7 DPI compared to males, and soma in female FPI remained significantly elevated in comparison to sex- and age-matched controls at 168 DPI. [C] There was an overall effect of FPI, with endpoints decreased at all DPI. [D] Ramified microglia are in a resting state (normal). [E-I] When activated, their morphology changes, and different morphologies indicate different functions. Activated morphologies quantified include hyper-ramified, reactive, amoeboid, bush and rod-shaped. All activated microglial morphologies were increased at 7 DPI (P<0.001). Ameboid and rod microglia were predominant at 7 DPI, where FPI males had greater numbers of reactive, bushy and rod morphologies compared to FPI females. At 168 DPI, reactive microglia were still significantly elevated in FPI males, while ameboid were significantly elevated in FPI females. Bar graphs represent the mean+standard error from the mean. PKrishna et al., in progress.
2. Thalamus: Chronic astrocyte activation after concussion and as a function of age. Increased intensity of staining (% black pixels) indicates activated astrocytes. Increased soma (cell) counts are indicative of proliferating astrocytes. Both are indications of astrocyte activation, which occurs when brain homeostasis is disrupted. [A] 40x representative images of GFAP immunostaining for astrocytes in male and female, sham (control) and injured (FPI) rats at 7, 56 and 168 days post-injury (DPI). Scale bar = 100 µm. [B] GFAP intensity increased as a function of FPI and FPI*DPI. [C and E] No sex differences were detected, so the sex differences were consolidated for statistical analysis to emphasize the increase in shams between 7 and 56 DPI and 168 DPI. [D] Soma counts changed as a function of FPI. Bar graphs represent the mean+standard error from the mean. PSabetta et al., under review.
Neurochemistry
The loss of neuronal connections, chronic neuroinflammation and astrogliosis creates an environment in the brain that can disrupt the way brain circuitry communicates/functions, culminating in the manifestation of post-concussion symptoms. In combination with the histological knowledge of structural changes within the brain, we use novel microelectrode arrays to measure the real-time communication of the brain, where we can draw more precise cause-and-effect relationships between the injury-induced deficits and therapeutic interventions on known molecular, structural and functional deficits.
Other Translational Neurotrauma and Neurochemistry Laboratory Interests
In collaboration with the University of Arizona College of Medicine - Phoenix, Phoenix VA Healthcare System and Midwestern University, we are evaluating the longitudinal effects of concussion on neuroendocrine dysregulation, the role of sex hormones, how changes in circulating hormones directly influence behaviorally relevant regions of the brain, the influence of the gut microbiome and mechanisms mediating post-injury cerebrovascular compromise, and are utilizing a fly (Drosophila) injury model to better understand specific pathological pathways that influence the trajectory of post-concussion injury pathology5 and sex-dependent differences in bioavailability of identified treatments.
Translational research provides evidence-based justification necessary to inform clinicians on how to improve diagnostic algorithms for late-onset symptoms and promote the utilization of multidisciplinary medical treatment for adolescent patients, where early intervention can influence patients’ long-term health and quality of life. The emerging sex differences raise awareness that gender plays an important role in the long-term consequences of concussion, where therapeutic approaches may need to diverge between males and females for optimal treatment. Improved knowledge regarding temporal molecular, pathological and circuit communication alterations can be utilized to identify novel therapeutic targets, ensure therapeutic interventions improve long-term outcomes and identify nuances between normal aging, neurodegenerative diseases and aging after a concussion.
Dr. Theresa Currier Thomas’ work is supported by the National Institutes of Health, Phoenix Children’s,
Arizona Biomedical Research Commission through the Arizona Department of Health Services and
Valley Research Partnership grants.
REFERENCES
- June D, Williams OA, Huang CW, et al. Lasting consequences of concussion on the aging brain: Findings from the Baltimore Longitudinal Study of Aging. NeuroImage. 2020;221:117182.
- Daglas M, Adlard PA. The Involvement of Iron in Traumatic Brain Injury and Neurodegenerative Disease. Front Neurosci. 2018;12:981.
- Currier Thomas T, Bromberg CE, Krishna G. Female sex in experimental traumatic brain injury research: forging a path forward. Neural regeneration research. 2022;17(3):550-552.
- Bromberg CE, Condon AM, Ridgway SW, et al. Sex-Dependent Pathology in the HPA Axis at a Sub-acute Period After Experimental Traumatic Brain Injury. Front Neurol. 2020;11:946.
- Buhlman LM, Krishna G, Jones TB, Thomas TC. Drosophila as a model to explore secondary injury cascades after traumatic brain injury. Biomed Pharmacother. 2021;142:112079.
