Blast exposure causes early and persistent aberrant phospho- and cleaved-tau expression in a murine model of mild blast-induced traumatic brain injury

Cumulative Blast Impulse Is Predictive for Changes in Chronic Neurobehavioral Symptoms Following Low Level Blast Exposure during Military Training

INTRODUCTION

Cumulative low-level blast exposure during military training may be a significant occupational hazard, increasing the risk of poor long-term outcomes in brain function. US Public Law 116-92 section 717 mandates that US Department of Defense agencies document the blast exposure of each Service member to help inform later disability and health care decisions. However, which empirical measures of training blast exposure, such as the number of incidents, peak overpressure, or impulse, best inform changes in the neurobehavioral symptoms reflecting brain health have not been established.

MATERIALS AND METHODS

This study was approved by the US Army Special Operations Command, the University of North Carolina at Chapel Hill, and the VA Puget Sound Health Care System. Using methods easily deployable across different organizational structures, this study sought to identify and measure candidate risk factors related to career occupational blast exposure predictive of changes in neurobehavioral symptom burden. Blast dosimetry-symptom relationships were first evaluated in mice and then tested in a military training environment. In mice, the righting time neurobehavioral response was measured after exposure to a repetitive low-level blast paradigm modeled after Special Operations training. In the military training environment, 23 trainees enrolled in a 6-week explosive breaching training course, 13 instructors, and 10 Service member controls without blast exposure participated in the study (46 total). All participants provided weekly Neurobehavioral Symptom Inventory (NSI) surveys. Peak blast overpressure, impulse, total number of blasts, Time in Low-Level Blast Occupation, and Time in Service were analyzed by Bayesian analysis of regression modeling to determine their probability of influence on the post-training symptoms reported by participants.

RESULTS

We tested the hypothesis that cumulative measures of low-level blast exposure were predictive of changes in neurobehavioral symptoms. In mice, repetitive blast resulted in reduced righting times correlated with cumulative blast impulse. In Service members, peak blast overpressure, impulse, total number of blasts, Time in Low-Level Blast Occupation, and Time in Service all showed strong evidence of influence on NSI scores after blast exposure. However, only models including baseline NSI scores and cumulative blast impulse provided significant predictive value following validation.

CONCLUSIONS

These results indicate that measures of cumulative blast impulse may have utility in predicting changes in NSI scores. Such paired dosimetry-symptom measures are expected to be an important tool in safely guiding Service members' occupational exposure and optimizing force readiness and lethality.

Macroscopic changes in aquaporin-4 underlie blast traumatic brain injury-related impairment in glymphatic function

Mild traumatic brain injury (mTBI) has emerged as a potential risk factor for the development of neurodegenerative conditions such as Alzheimer's disease and chronic traumatic encephalopathy. Blast mTBI, caused by exposure to a pressure wave from an explosion, is predominantly experienced by military personnel and has increased in prevalence and severity in recent decades. Yet the underlying pathology of blast mTBI is largely unknown. We examined the expression and localization of AQP4 in human post-mortem frontal cortex and observed distinct laminar differences in AQP4 expression following blast exposure. We also observed similar laminar changes in AQP4 expression and localization and delayed impairment of glymphatic function that emerged 28 days following blast injury in a mouse model of repetitive blast mTBI. In a cohort of veterans with blast mTBI, we observed that blast exposure was associated with an increased burden of frontal cortical MRI-visible perivascular spaces, a putative neuroimaging marker of glymphatic perivascular dysfunction. These findings suggest that changes in AQP4 and delayed glymphatic impairment following blast injury may render the post-traumatic brain vulnerable to post-concussive symptoms and chronic neurodegeneration.

Association of Blast Exposure in Military Breaching with Intestinal Permeability Blood Biomarkers Associated with Leaky Gut

Injuries and subclinical effects from exposure to blasts are of significant concern in military operational settings, including tactical training, and are associated with self-reported concussion-like symptomology and physiological changes such as increased intestinal permeability (IP), which was investigated in this study. Time-series gene expression and IP biomarker data were generated from "breachers" exposed to controlled, low-level explosive blast during training. Samples from 30 male participants at pre-, post-, and follow-up blast exposure the next day were assayed via RNA-seq and ELISA. A battery of symptom data was also collected at each of these time points that acutely showed elevated symptom reporting related to headache, concentration, dizziness, and taking longer to think, dissipating ~16 h following blast exposure. Evidence for bacterial translocation into circulation following blast exposure was detected by significant stepwise increase in microbial diversity (measured via alpha-diversity p = 0.049). Alterations in levels of IP protein biomarkers (i.e., Zonulin, LBP, Claudin-3, I-FABP) assessed in a subset of these participants (n = 23) further evidenced blast exposure associates with IP. The observed symptom profile was consistent with mild traumatic brain injury and was further associated with changes in bacterial translocation and intestinal permeability, suggesting that IP may be linked to a decrease in cognitive functioning. These preliminary findings show for the first time within real-world military operational settings that exposures to blast can contribute to IP.

Recent insights from non-mammalian models of brain injuries: an emerging literature

Traumatic brain injury (TBI) is a major global health concern and is increasingly recognized as a risk factor for neurodegenerative diseases including Alzheimer's disease (AD) and chronic traumatic encephalopathy (CTE). Repetitive TBIs (rTBIs), commonly observed in contact sports, military service, and intimate partner violence (IPV), pose a significant risk for long-term sequelae. To study the long-term consequences of TBI and rTBI, researchers have typically used mammalian models to recapitulate brain injury and neurodegenerative phenotypes. However, there are several limitations to these models, including: (1) lengthy observation periods, (2) high cost, (3) difficult genetic manipulations, and (4) ethical concerns regarding prolonged and repeated injury of a large number of mammals. Aquatic vertebrate model organisms, including Petromyzon marinus (sea lampreys), zebrafish (Danio rerio), and invertebrates, Caenorhabditis elegans (C. elegans), and Drosophila melanogaster (Drosophila), are emerging as valuable tools for investigating the mechanisms of rTBI and tauopathy. These non-mammalian models offer unique advantages, including genetic tractability, simpler nervous systems, cost-effectiveness, and quick discovery-based approaches and high-throughput screens for therapeutics, which facilitate the study of rTBI-induced neurodegeneration and tau-related pathology. Here, we explore the use of non-vertebrate and aquatic vertebrate models to study TBI and neurodegeneration. Drosophila, in particular, provides an opportunity to explore the longitudinal effects of mild rTBI and its impact on endogenous tau, thereby offering valuable insights into the complex interplay between rTBI, tauopathy, and neurodegeneration. These models provide a platform for mechanistic studies and therapeutic interventions, ultimately advancing our understanding of the long-term consequences associated with rTBI and potential avenues for intervention.

The Neurovascular Unit as a Locus of Injury in Low-Level Blast-Induced Neurotrauma

Blast-induced neurotrauma has received much attention over the past decade. Vascular injury occurs early following blast exposure. Indeed, in animal models that approximate human mild traumatic brain injury or subclinical blast exposure, vascular pathology can occur in the presence of a normal neuropil, suggesting that the vasculature is particularly vulnerable. Brain endothelial cells and their supporting glial and neuronal elements constitute a neurovascular unit (NVU). Blast injury disrupts gliovascular and neurovascular connections in addition to damaging endothelial cells, basal laminae, smooth muscle cells, and pericytes as well as causing extracellular matrix reorganization. Perivascular pathology becomes associated with phospho-tau accumulation and chronic perivascular inflammation. Disruption of the NVU should impact activity-dependent regulation of cerebral blood flow, blood-brain barrier permeability, and glymphatic flow. Here, we review work in an animal model of low-level blast injury that we have been studying for over a decade. We review work supporting the NVU as a locus of low-level blast injury. We integrate our findings with those from other laboratories studying similar models that collectively suggest that damage to astrocytes and other perivascular cells as well as chronic immune activation play a role in the persistent neurobehavioral changes that follow blast injury.

Differential effects on TDP-43, piezo-2, tight-junction proteins in various brain regions following repetitive low-intensity blast overpressure

Introduction

Mild traumatic brain injury (mTBI) caused by repetitive low-intensity blast overpressure (relBOP) in military personnel exposed to breaching and heavy weapons is often unrecognized and is understudied. Exposure to relBOP poses the risk of developing abnormal behavioral and psychological changes such as altered cognitive function, anxiety, and depression, all of which can severely compromise the quality of the life of the affected individual. Due to the structural and anatomical heterogeneity of the brain, understanding the potentially varied effects of relBOP in different regions of the brain could lend insights into the risks from exposures.

Methods

In this study, using a rodent model of relBOP and western blotting for protein expression we showed the differential expression of various neuropathological proteins like TDP-43, tight junction proteins (claudin-5, occludin, and glial fibrillary acidic protein (GFAP)) and a mechanosensitive protein (piezo-2) in different regions of the brain at different intensities and frequency of blast.

Results

Our key results include (i) significant increase in claudin-5 after 1x blast of 6.5 psi in all three regions and no definitive pattern with higher number of blasts, (ii) significant increase in piezo-2 at 1x followed by significant decrease after multiple blasts in the cortex, (iii) significant increase in piezo-2 with increasing number of blasts in frontal cortex and mixed pattern of expression in hippocampus and (iv) mixed pattern of TDP-3 and GFAP expression in all the regions of brain.

Discussion

These results suggest that there are not definitive patterns of changes in these marker proteins with increase in intensity and/or frequency of blast exposure in any particular region; the changes in expression of these proteins are different among the regions. We also found that the orientation of blast exposure (e.g. front vs. side exposure) affects the altered expression of these proteins.

IL-34 exacerbates pathogenic features of Alzheimer's disease and calvaria osteolysis in triple transgenic (3x-Tg) female mice

Hallmark features of Alzheimer's disease (AD) include elevated accumulation of aggregated Aβ40 and Aβ42 peptides, hyperphosphorylated Tau (p-Tau), and neuroinflammation. Emerging evidence indicated that interleukin-34 (IL-34) contributes to AD and inflammatory osteolysis via the colony-stimulating factor-1 receptor (CSF-1r). In addition, CSF-1r is also activated by macrophage colony-stimulating factor-1 (M-CSF). While the role of M-CSF in bone physiology and pathology is well addressed, it remains controversial whether IL-34-mediated signaling promotes osteolysis, neurodegeneration, and neuroinflammation in relation to AD. In this study, we injected 3x-Tg mice with mouse recombinant IL-34 protein over the calvaria bone every other day for 42 days. Then, behavioral changes, brain pathology, and calvaria osteolysis were evaluated using various behavioral maze and histological assays. We demonstrated that IL-34 administration dramatically elevated AD-like anxiety and memory loss, pathogenic amyloidogenesis, p-Tau, and RAGE expression in female 3x-Tg mice. Furthermore, IL-34 delivery promoted calvaria inflammatory osteolysis compared to the control group. In addition, we also compared the effects of IL-34 and M-CSF on macrophages, microglia, and RANKL-mediated osteoclastogenesis in relation to AD pathology in vitro. We observed that IL-34-exposed SIM-A9 microglia and 3x-Tg bone marrow-derived macrophages released significantly elevated amounts of pro-inflammatory cytokines, TNF-α, IL-1β, and IL-6, compared to M-CSF treatment in vitro. Furthermore, IL-34, but not M-CSF, elevated RANKL-primed osteoclastogenesis in the presence of Aβ40 and Aβ42 peptides in bone marrow derived macrophages isolated from female 3x-Tg mice. Collectively, our data indicated that IL-34 elevates AD-like features, including behavioral changes and neuroinflammation, as well as osteoclastogenesis in female 3x-Tg mice.

Self-Reported Symptoms in U.S. Marines Following Blast- and Impact-Related Concussion

INTRODUCTION

Recent research on traumatic brain injury (TBI) has suggested that the mechanism of injury (i.e., whether the TBI was caused by high-level blast [HLB] vs. direct physical impact to the head) may be an important factor in injury severity, symptomology, and recovery because of differences in physiological effects of each type of injury on the brain. However, differences in self-reported symptomology resulting from HLB- vs. impact-related TBIs have not been thoroughly examined. This study tested the hypothesis that HLB- and impact-related concussions result in different self-reported symptoms in an enlisted Marine Corps population.

MATERIALS AND METHODS

All records of 2008 and 2012 Post-Deployment Health Assessment (PDHA) forms completed by enlisted active duty Marines between January 2008 and January 2017 were examined for self-reported concussion, mechanism of injury, and self-reported symptoms experienced during deployment. Concussion events were categorized as either blast- or impact-related; individual symptoms were categorized as neurological, musculoskeletal, or immunological. A series of logistic regressions were performed to examine associations between self-reported symptoms experienced by healthy controls and Marines who endorsed (1) any concussion (mTBI), (2) a probable blast-related concussion (mbTBI), and (3) a probable impact-related concussion (miTBI); analyses were also stratified by PTSD. To determine if there were significant differences between odds ratios (ORs) for mbTBIs vs. miTBIs, 95% CIs were examined for overlap.

RESULTS

Marines with a probable concussion, regardless of the mechanism of injury, were significantly more likely to report all symptoms (OR range: 1.7-19.3). Overall, mbTBIs, compared with miTBIs, resulted in higher odds of symptom reporting for eight symptoms on the 2008 PDHA (tinnitus, trouble hearing, headache, memory problems, dizziness, dim vision, trouble concentrating, and vomiting) and six symptoms on the 2012 PDHA (tinnitus, trouble hearing, headaches, memory problems, balance problems, and increased irritability), all of which were in the neurological symptom category. Conversely, odds of symptom reporting were higher for Marines experiencing miTBIs (vs. mbTBIs) for seven symptoms on the 2008 PDHA (skin diseases or rashes, chest pain, trouble breathing, persistent cough, red eyes, fever, and other) and one symptom on the 2012 PDHA (skin rash and/or lesion), all of which were in the immunological symptoms category. mbTBI (vs. miTBI) was consistently associated with greater odds of reporting tinnitus, trouble hearing, and memory problems, regardless of PTSD status.

CONCLUSIONS

These findings support recent research suggesting that the mechanism of injury may play an important role in symptom reporting and/or physiological changes to the brain after concussion. The results of this epidemiological investigation should be used to guide further research on the physiological effects of concussion, diagnostic criteria for neurological injuries, and treatment modalities for various concussion-related symptoms.

Timing matters: Sex differences in inflammatory and behavioral outcomes following repetitive blast mild traumatic brain injury

BACKGROUND

Repetitive blast-related mild traumatic brain injury (mTBI) caused by exposure to high explosives is increasingly common among warfighters as well as civilians. While women have been serving in military positions with increased risk of blast exposure since 2016, there are few published reports examining sex as a biological variable in models of blast mTBI, greatly limiting diagnosis and treatment capabilities. As such, here we examined outcomes of repetitive blast trauma in female and male mice in relation to potential behavioral, inflammatory, microbiome, and vascular dysfunction at multiple timepoints.

METHODS

In this study we utilized a well-established blast overpressure model to induce repetitive (3x) blast-mTBI in both female and male mice. Acutely following repetitive exposure, we measured serum and brain cytokine levels, blood-brain barrier (BBB) disruption, fecal microbial abundance, and locomotion and anxiety-like behavior in the open field assay. At the one-month timepoint, in female and male mice we assessed behavioral correlates of mTBI and PTSD-related symptoms commonly reported by Veterans with a history of blast-mTBI using the elevated zero maze, acoustic startle, and conditioned odorant aversion paradigms.

RESULTS

Repetitive blast exposure resulted in both similar (e.g., increased IL-6), and disparate (e.g., IL-10 increase only in females) patterns of acute serum and brain cytokine as well as gut microbiome changes in female and male mice. Acute BBB disruption following repetitive blast exposure was apparent in both sexes. While female and male blast mice both exhibited acute locomotor and anxiety-like deficits in the open field assay, only male mice exhibited adverse behavioral outcomes that lasted at least one-month.

DISCUSSION

Representing a novel survey of potential sex differences following repetitive blast trauma, our results demonstrate unique similar yet divergent patterns of blast-induced dysfunction in female vs. male mice and highlight novel targets for future diagnosis and therapeutic development.

Increased Risk of Aging-Related Neurodegenerative Disease after Traumatic Brain Injury

Traumatic brain injury (TBI) survivors frequently suffer from chronically progressive complications, including significantly increased risk of developing aging-related neurodegenerative disease. As advances in neurocritical care increase the number of TBI survivors, the impact and awareness of this problem are growing. The mechanisms by which TBI increases the risk of developing aging-related neurodegenerative disease, however, are not completely understood. As a result, there are no protective treatments for patients. Here, we review the current literature surrounding the epidemiology and potential mechanistic relationships between brain injury and aging-related neurodegenerative disease. In addition to increasing the risk for developing all forms of dementia, the most prominent aging-related neurodegenerative conditions that are accelerated by TBI are amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD), Parkinson's disease (PD), and Alzheimer's disease (AD), with ALS and FTD being the least well-established. Mechanistic links between TBI and all forms of dementia that are reviewed include oxidative stress, dysregulated proteostasis, and neuroinflammation. Disease-specific mechanistic links with TBI that are reviewed include TAR DNA binding protein 43 and motor cortex lesions in ALS and FTD; alpha-synuclein, dopaminergic cell death, and synergistic toxin exposure in PD; and brain insulin resistance, amyloid beta pathology, and tau pathology in AD. While compelling mechanistic links have been identified, significantly expanded investigation in the field is needed to develop therapies to protect TBI survivors from the increased risk of aging-related neurodegenerative disease.

Review: Emerging Eye-Based Diagnostic Technologies for Traumatic Brain Injury

The study of ocular manifestations of neurodegenerative disorders, Oculomics, is a growing field of investigation for early diagnostics, enabling structural and chemical biomarkers to be monitored overtime to predict prognosis. Traumatic brain injury (TBI) triggers a cascade of events harmful to the brain, which can lead to neurodegeneration. TBI, termed the "silent epidemic" is becoming a leading cause of death and disability worldwide. There is currently no effective diagnostic tool for TBI, and yet, early-intervention is known to considerably shorten hospital stays, improve outcomes, fasten neurological recovery and lower mortality rates, highlighting the unmet need for techniques capable of rapid and accurate point-of-care diagnostics, implemented in the earliest stages. This review focuses on the latest advances in the main neuropathophysiological responses and the achievements and shortfalls of TBI diagnostic methods. Validated and emerging TBI-indicative biomarkers are outlined and linked to ocular neuro-disorders. Methods detecting structural and chemical ocular responses to TBI are categorised along with prospective chemical and physical sensing techniques. Particular attention is drawn to the potential of Raman spectroscopy as a non-invasive sensing of neurological molecular signatures in the ocular projections of the brain, laying the platform for the first tangible path towards alternative point-of-care diagnostic technologies for TBI.

The dynorphin/kappa opioid receptor mediates adverse immunological and behavioral outcomes induced by repetitive blast trauma

BACKGROUND

Adverse pathophysiological and behavioral outcomes related to mild traumatic brain injury (mTBI), posttraumatic stress disorder (PTSD), and chronic pain are common following blast exposure and contribute to decreased quality of life, but underlying mechanisms and prophylactic/treatment options remain limited. The dynorphin/kappa opioid receptor (KOR) system helps regulate behavioral and inflammatory responses to stress and injury; however, it has yet to be investigated as a potential mechanism in either humans or animals exposed to blast. We hypothesized that blast-induced KOR activation mediates adverse outcomes related to inflammation and affective behavioral response.

METHODS

C57Bl/6 adult male mice were singly or repeatedly exposed to either sham (anesthesia only) or blast delivered by a pneumatic shock tube. The selective KOR antagonist norBNI or vehicle (saline) was administered 72 h prior to repetitive blast or sham exposure. Serum and brain were collected 10 min or 4 h post-exposure for dynorphin A-like immunoreactivity and cytokine measurements, respectively. At 1-month post-exposure, mice were tested in a series of behavioral assays related to adverse outcomes reported by humans with blast trauma.

RESULTS

Repetitive but not single blast exposure resulted in increased brain dynorphin A-like immunoreactivity. norBNI pretreatment blocked or significantly reduced blast-induced increase in serum and brain cytokines, including IL-6, at 4 h post exposure and aversive/anxiety-like behavioral dysfunction at 1-month post-exposure.

CONCLUSIONS

Our findings demonstrate a previously unreported role for the dynorphin/KOR system as a mediator of biochemical and behavioral dysfunction following repetitive blast exposure and highlight this system as a potential prophylactic/therapeutic treatment target.

Unraveling the Mechanobiology Underlying Traumatic Brain Injury with Advanced Technologies and Biomaterials

Traumatic brain injury (TBI) is a worldwide health and socioeconomic problem, associated with prolonged and complex neurological aftermaths, including a variety of functional deficits and neurodegenerative disorders. Research on the long-term effects has highlighted that TBI shall be regarded as a chronic health condition. The initiation and exacerbation of TBI involve a series of mechanical stimulations and perturbations, accompanied by mechanotransduction events within the brain tissues. Mechanobiology thus offers a unique perspective and likely promising approach to unravel the underlying molecular and biochemical mechanisms leading to neural cells dysfunction after TBI, which may contribute to the discovery of novel targets for future clinical treatment. This article investigates TBI and the subsequent brain dysfunction from a lens of neuromechanobiology. Following an introduction, the mechanobiological insights are examined into the molecular pathology of TBI, and then an overview is given of the latest research technologies to explore neuromechanobiology, with particular focus on microfluidics and biomaterials. Challenges and prospects in the current field are also discussed. Through this article, it is hoped that extensive technical innovation in biomedical devices and materials can be encouraged to advance the field of neuromechanobiology, paving potential ways for the research and rehabilitation of neurotrauma and neurological diseases.

A Novel In Vitro Platform Development in the Lab for Modeling Blast Injury to Microglia

Traumatic brain injury (TBI), which is mainly caused by impact, often results in chronic neurological abnormalities. Since the pathological changes in vivo during primary biomechanical injury are quite complicated, the in-depth understanding of the pathophysiology and mechanism of TBI depends on the establishment of an effective experimental in vitro model. Usually, a bomb explosive blast was employed to establish the in vitro model, while the process is complex and unsuitable in the lab. Based on water-hammer, we have developed a device system to provide a single dynamic compression stress on living cells. A series of amplitude (∼5.3, ∼9.8, ∼13.5 MPa) were generated to explore the effects of dynamic compression loading on primary microglia within 48 h. Apoptosis experiments indicated that primary microglia had strong tolerance to blast waves. In addition, the generation of intercellular reactive oxygen species and secretory nitric oxide was getting strongly enhanced and recovered within 48 h. In addition, there is a notable release of pro-inflammatory cytokine by microglia. Our work provides a reproducible and peaceable method of loading single dynamic compression forces to cells in vitro. Microglia showed an acute inflammatory response to dynamic loadings, while no significant cell death was observed. This insight delivers a new technological approach that could open new areas to a better understanding of the mechanism of cell blast injuries.

Interface astrogliosis in contact sport head impacts and military blast exposure

Exposure to military blast and repetitive head impacts (RHI) in contact sports is associated with increased risk of long-term neurobehavioral sequelae and cognitive deficits, and the neurodegenerative disease chronic traumatic encephalopathy (CTE). At present, the exact pathogenic mechanisms of RHI and CTE are unknown, and no targeted therapies are available. Astrocytes have recently emerged as key mediators of the multicellular response to head trauma. Here, we investigated interface astrogliosis in blast and impact neurotrauma, specifically in the context of RHI and early stage CTE. We compared postmortem brain tissue from former military veterans with a history of blast exposure with and without a neuropathological diagnosis of CTE, former American football players with a history of RHI with and without a neuropathological diagnosis of CTE, and control donors without a history of blast, RHI exposure or CTE diagnosis. Using quantitative immunofluorescence, we found that astrogliosis was higher at the grey-white matter interface in the dorsolateral frontal cortex, with mixed effects at the subpial surface and underlying cortex, in both blast and RHI donors with and without CTE, compared to controls. These results indicate that certain astrocytic alterations are associated with both impact and blast neurotrauma, and that different astroglial responses take place in distinct brain regions.

The role of protein phosphatase 2A tau axis in traumatic brain injury therapy

Background

Traumatic brain injury (TBI) is a debilitating disorder due to trauma caused by an external mechanical force eventually leading to disruption in the normal function of the brain, with possible outcomes including permanent or temporary dysfunction of cognitive, physical, and psychosocial abilities. There have been several studies focusing on the search and innovation of neuroprotective agents that could have therapeutic relevance in TBI management. Due to its complexity, TBI is divided into two major components. The first initial event is known as the primary injury; it is a result of the mechanical insult itself and is known to be irreversible and resistant to a vast variety of therapeutics. The secondary event or secondary brain injury is viewed as a cellular injury that does not manifest immediately after the trauma but evolved after a delay period of hours or several days. This category of injury is known to respond favorably to different pharmacological treatment approaches.

Main body

Due to the complexity in the pathophysiology of the secondary injury, the therapeutic strategy needs to be in a multi-facets model and to have the ability to simultaneously regulate different cellular changes. Several studies have investigated in deep the possible approaches relying on natural compounds as an alternative therapeutic strategy for the management of TBI. In addition, many natural compounds have the potential to target numerous different components of the secondary injury including neuroinflammation, apoptosis, PP2A, tau, and Aβ among others. Here, we review past and current strategies in the therapeutic management of TBI, focusing on the PP2A-tau axis both in animal and human subjects. This review uncovers, in addition, a variety of compounds used in TBI therapy.

Conclusion

Despite beneficial therapeutic effects observed in animals for many compounds, studies are still needed to be conducted on human subjects to validate their therapeutic virtues. Furthermore, potential therapeutic virtues observed among studies might likely be dependent on the TBI animal model used and the type of induced injury. In addition, specificity and side effects are challenges in TBI therapy specifically which site of PP2A dysfunction to be targeted.

The Emergence of Model Systems to Investigate the Link Between Traumatic Brain Injury and Alzheimer's Disease

Numerous epidemiological studies have demonstrated that individuals who have sustained a traumatic brain injury (TBI) have an elevated risk for developing Alzheimer's disease and Alzheimer's-related dementias (AD/ADRD). Despite these connections, the underlying mechanisms by which TBI induces AD-related pathology, neuronal dysfunction, and cognitive decline have yet to be elucidated. In this review, we will discuss the various in vivo and in vitro models that are being employed to provide more definite mechanistic relationships between TBI-induced mechanical injury and AD-related phenotypes. In particular, we will highlight the strengths and weaknesses of each of these model systems as it relates to advancing the understanding of the mechanisms that lead to TBI-induced AD onset and progression as well as providing platforms to evaluate potential therapies. Finally, we will discuss how emerging methods including the use of human induced pluripotent stem cell (hiPSC)-derived cultures and genome engineering technologies can be employed to generate better models of TBI-induced AD.

Perspectives on Primary Blast Injury of the Brain: Translational Insights Into Non-inertial Low-Intensity Blast Injury

Most traumatic brain injuries (TBIs) during military deployment or training are clinically "mild" and frequently caused by non-impact blast exposures. Experimental models were developed to reproduce the biological consequences of high-intensity blasts causing moderate to severe brain injuries. However, the pathophysiological mechanisms of low-intensity blast (LIB)-induced neurological deficits have been understudied. This review provides perspectives on primary blast-induced mild TBI models and discusses translational aspects of LIB exposures as defined by standardized physical parameters including overpressure, impulse, and shock wave velocity. Our mouse LIB-exposure model, which reproduces deployment-related scenarios of open-field blast (OFB), caused neurobehavioral changes, including reduced exploratory activities, elevated anxiety-like levels, impaired nesting behavior, and compromised spatial reference learning and memory. These functional impairments associate with subcellular and ultrastructural neuropathological changes, such as myelinated axonal damage, synaptic alterations, and mitochondrial abnormalities occurring in the absence of gross- or cellular damage. Biochemically, we observed dysfunctional mitochondrial pathways that led to elevated oxidative stress, impaired fission-fusion dynamics, diminished mitophagy, decreased oxidative phosphorylation, and compensated cell respiration-relevant enzyme activity. LIB also induced increased levels of total tau, phosphorylated tau, and amyloid β peptide, suggesting initiation of signaling cascades leading to neurodegeneration. We also compare translational aspects of OFB findings to alternative blast injury models. By scoping relevant recent research findings, we provide recommendations for future preclinical studies to better reflect military-operational and clinical realities. Overall, better alignment of preclinical models with clinical observations and experience related to military injuries will facilitate development of more precise diagnosis, clinical evaluation, treatment, and rehabilitation.

Repetitive Blast Exposure Increases Appetitive Motivation and Behavioral Inflexibility in Male Mice

Blast exposure (via detonation of high explosives) represents a major potential trauma source for Servicemembers and Veterans, often resulting in mild traumatic brain injury (mTBI). Executive dysfunction (e.g., alterations in memory, deficits in mental flexibility, difficulty with adaptability) is commonly reported by Veterans with a history of blast-related mTBI, leading to impaired daily functioning and decreased quality of life, but underlying mechanisms are not fully understood and have not been well studied in animal models of blast. To investigate potential underlying behavioral mechanisms contributing to deficits in executive functioning post-blast mTBI, here we examined how a history of repetitive blast exposure in male mice affects anxiety/compulsivity-like outcomes and appetitive goal-directed behavior using an established mouse model of blast mTBI. We hypothesized that repetitive blast exposure in male mice would result in anxiety/compulsivity-like outcomes and corresponding performance deficits in operant-based reward learning and behavioral flexibility paradigms. Instead, results demonstrate an increase in reward-seeking and goal-directed behavior and a congruent decrease in behavioral flexibility. We also report chronic adverse behavioral changes related to anxiety, compulsivity, and hyperarousal. In combination, these data suggest that potential deficits in executive function following blast mTBI are at least in part related to enhanced compulsivity/hyperreactivity and behavioral inflexibility and not simply due to a lack of motivation or inability to acquire task parameters, with important implications for subsequent diagnosis and treatment management.

Shock-Induced Damage Mechanism of Perineuronal Nets

The perineuronal net (PNN) region of the brain’s extracellular matrix (ECM) surrounds the neural networks within the brain tissue. The PNN is a protective net-like structure regulating neuronal activity such as neurotransmission, charge balance, and action potential generation. Shock-induced damage of this essential component may lead to neuronal cell death and neurodegenerations. The shock generated during a vehicle accident, fall, or improvised device explosion may produce sufficient energy to damage the structure of the PNN. The goal is to investigate the mechanics of the PNN in reaction to shock loading and to understand the mechanical properties of different PNN components such as glycan, GAG, and protein. In this study, we evaluated the mechanical strength of PNN molecules and the interfacial strength between the PNN components. Afterward, we assessed the PNN molecules’ damage efficiency under various conditions such as shock speed, preexisting bubble, and boundary conditions. The secondary structure altercation of the protein molecules of the PNN was analyzed to evaluate damage intensity under varying shock speeds. At a higher shock speed, damage intensity is more elevated, and hyaluronan (glycan molecule) is most likely to break at the rigid junction. The primary structure of the protein molecules is least likely to fail. Instead, the molecules’ secondary bonds will be altered. Our study suggests that the number of hydrogen bonds during the shock wave propagation is reduced, which leads to the change in protein conformations and damage within the PNN structure. As such, we found a direct connection between shock wave intensity and PNN damage.

Drosophila as a model to explore secondary injury cascades after traumatic brain injury

Drosophilae are emerging as a valuable model to study traumatic brain injury (TBI)-induced secondary injury cascades that drive persisting neuroinflammation and neurodegenerative pathology that imposes significant risk for long-term neurological deficits. As in mammals, TBI in Drosophila triggers axonal injury, metabolic crisis, oxidative stress, and a robust innate immune response. Subsequent neurodegeneration stresses quality control systems and perpetuates an environment for neuroprotection, regeneration, and delayed cell death via highly conserved cell signaling pathways. Fly injury models continue to be developed and validated for both whole-body and head-specific injury to isolate, evaluate, and modulate these parallel pathways. In conjunction with powerful genetic tools, the ability for longitudinal evaluation, and associated neurological deficits that can be tested with established behavioral tasks, Drosophilae are an attractive model to explore secondary injury cascades and therapeutic intervention after TBI. Here, we review similarities and differences between mammalian and fly pathophysiology and highlight strategies for their use in translational neurotrauma research.

NLRP3 Inflammasome-Dependent Increases in High Mobility Group Box 1 Involved in the Cognitive Dysfunction Caused by Tau-Overexpression

Tau hyperphosphorylation is a characteristic alteration present in a range of neurological conditions, such as traumatic brain injury (TBI) and neurodegenerative diseases. Treatments targeting high-mobility group box protein 1 (HMGB1) induce neuroprotective effects in these neuropathologic conditions. However, little is known about the interactions between hyperphosphorylated tau and HMGB1 in neuroinflammation. We established a model of TBI with controlled cortical impacts (CCIs) and a tau hyperphosphorylation model by injecting the virus encoding human P301S tau in mice, and immunofluorescence, western blotting analysis, and behavioral tests were performed to clarify the interaction between phosphorylated tau (p-tau) and HMGB1 levels. We demonstrated that p-tau and HMGB1 were elevated in the spatial memory-related brain regions in mice with TBI and tau-overexpression. Animals with tau-overexpression also had significantly increased nucleotide-binding oligomerization domain-like receptor pyrin domain-containing protein 3 (NLRP3) inflammasome activation, which manifested as increases in apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC), activating caspase-1 and interleukin 1 beta (IL-1β) levels. In addition, NLRP3^–/– mice and the HMGB1 inhibitor, glycyrrhizin, were used to explore therapeutic strategies for diseases with p-tau overexpression. Compared with wild-type (WT) mice with tau-overexpression, downregulation of p-tau and HMGB1 was observed in NLRP3^–/– mice, indicating that HMGB1 alterations were NLRP3-dependent. Moreover, treatment with glycyrrhizin at a late stage markedly reduced p-tau levels and improved performance in the Y- and T-mazes and the ability of tau-overexpressing mice to build nests, which revealed improvements in spatial memory and advanced hippocampal function. The findings identified that p-tau has a triggering role in the modulation of neuroinflammation and spatial memory in an NLRP3-dependent manner, and suggest that treatment with HMGB1 inhibitors may be a better therapeutic strategy for tauopathies.

Mouse closed head traumatic brain injury replicates the histological tau pathology pattern of human disease: characterization of a novel model and systematic review of the literature

Traumatic brain injury (TBI) constitutes one of the strongest environmental risk factors for several progressive neurodegenerative disorders of cognitive impairment and dementia that are characterized by the pathological accumulation of hyperphosphorylated tau (p-Tau). It has been questioned whether mouse closed-head TBI models can replicate human TBI-associated tauopathy. We conducted longitudinal histopathological characterization of a mouse closed head TBI model, with a focus on pathological features reported in human TBI-associated tauopathy. Male C57BL/6 J mice were subjected to once daily TBI for 5 consecutive days using a weight drop paradigm. Histological analyses (AT8, TDP-43, pTDP-43, NeuN, GFAP, Iba-1, MBP, SMI-312, Prussian blue, IgG, βAPP, alpha-synuclein) were conducted at 1 week, 4 weeks, and 24 weeks after rTBI and compared to sham operated controls. We conducted a systematic review of the literature for mouse models of closed-head injury focusing on studies referencing tau protein assessment. At 1-week post rTBI, p-Tau accumulation was restricted to the corpus callosum and perivascular spaces adjacent to the superior longitudinal fissure. Progressive p-Tau accumulation was observed in the superficial layers of the cerebral cortex, as well as in mammillary bodies and cortical perivascular, subpial, and periventricular locations at 4 to 24 weeks after rTBI. Associated cortical histopathologies included microvascular injury, neuroaxonal rarefaction, astroglial and microglial activation, and cytoplasmatic localization of TDP-43 and pTDP-43. In our systematic review, less than 1% of mouse studies (25/3756) reported p-Tau using immunostaining, of which only 3 (0.08%) reported perivascular p-Tau, which is considered a defining feature of chronic traumatic encephalopathy. Commonly reported associated pathologies included neuronal loss (23%), axonal loss (43%), microglial activation and astrogliosis (50%, each), and beta amyloid deposition (29%). Our novel model, supported by systematic review of the literature, indicates progressive tau pathology after closed head murine TBI, highlighting the suitability of mouse models to replicate pertinent human histopathology.

Oxidative Stress Signaling in Blast TBI-Induced Tau Phosphorylation

Traumatic brain injury caused by blast is associated with long-term neuropathological changes including tau phosphorylation and pathology. In this study, we aimed to determine changes in initial tau phosphorylation after exposure to a single mild blast and the potential contribution of oxidative stress response pathways. C57BL/6 mice were exposed to a single blast overpressure (BOP) generated by a compressed gas-driven shock tube that recapitulates battlefield-relevant open-field BOP, and cortical tissues were harvested at different time points up to 24 h after blast for Western blot analysis. We found that BOP caused elevated tau phosphorylation at Ser202/Thr205 detected by the AT8 antibody at 1 h post-blast followed by tau phosphorylation at additional sites (Ser262 and Ser396/Ser404 detected by PHF1 antibody) and conformational changes detected by Alz50 antibody. BOP also induced acute oxidative damage at 1 h post-blast and gradually declined overtime. Interestingly, Extracellular signal-regulated kinase (ERK) and c-Jun N-terminal kinase (JNK) were acutely activated in a similar temporal pattern as the rise and fall in oxidative stress after blast, with p38 showing a similar trend. However, glycogen synthase kinase-3 β (GSK3β) was inhibited at 1 h and remained inhibited for 24 h post blast. These results suggested that mitogen-activated protein kinases (MAPKs) but not GSK3β are likely involved in mediating the effects of oxidative stress on the initial increase of tau phosphorylation following a single mild blast.

Axonopathy precedes cell death in ocular damage mediated by blast exposure

Traumatic brain injuries (TBI) of varied types are common across all populations and can cause visual problems. For military personnel in combat settings, injuries from blast exposures (bTBI) are prevalent and arise from a myriad of different situations. To model these diverse conditions, we are one of several groups modeling bTBI using mice in varying ways. Here, we report a refined analysis of retinal ganglion cell (RGC) damage in male C57BL/6J mice exposed to a blast-wave in an enclosed chamber. Ganglion cell layer thickness, RGC density (BRN3A and RBPMS immunoreactivity), cellular density of ganglion cell layer (hematoxylin and eosin staining), and axon numbers (paraphenylenediamine staining) were quantified at timepoints ranging from 1 to 17-weeks. RNA sequencing was performed at 1-week and 5-weeks post-injury. Earliest indices of damage, evident by 1-week post-injury, are a loss of RGC marker expression, damage to RGC axons, and increase in glial markers expression. Blast exposure caused a loss of RGC somas and axons—with greatest loss occurring by 5-weeks post-injury. While indices of glial involvement are prominent early, they quickly subside as RGCs are lost. The finding that axonopathy precedes soma loss resembles pathology observed in mouse models of glaucoma, suggesting similar mechanisms.

NK1 antagonists attenuate tau phosphorylation after blast and repeated concussive injury

Exposure to repeated concussive traumatic brain injury (TBI) and to blast-induced TBI has been associated with the potential development of the neurodegenerative condition known as chronic traumatic encephalopathy (CTE). CTE is characterized by the accumulation of hyperphosphorylated tau protein, with the resultant tau tangles thought to initiate the cognitive and behavioral manifestations that appear as the condition progresses. However, the mechanisms linking concussive and blast TBI with tau hyperphosphorylation are unknown. Here we show that single moderate TBI, repeated concussive TBI and blast-induced mild TBI all result in hyperphosphorylation of tau via a substance P mediated mechanism. Post-injury administration of a substance P, NK1 receptor antagonist attenuated the injury-induced phosphorylation of tau by modulating the activity of several key kinases including Akt, ERK1/2 and JNK, and was associated with improvement in neurological outcome. We also demonstrate that inhibition of the TRPV1 mechanoreceptor, which is linked to substance P release, attenuated injury-associated tau hyperphosphorylation, but only when it was administered prior to injury. Our results demonstrate that TBI-mediated stimulation of brain mechanoreceptors is associated with substance P release and consequent tau hyperphosphorylation, with administration of an NK1 receptor antagonist attenuating tau phosphorylation and associated neurological deficits. NK1 antagonists may thus represent a pharmacological approach to attenuate the potential development of CTE following concussive and blast TBI.

Repetitive blast mild traumatic brain injury increases ethanol sensitivity in male mice and risky drinking behavior in male combat veterans

BACKGROUND

Mild traumatic brain injury (mTBI) is common in civilians and highly prevalent among military service members. mTBI can increase health risk behaviors (e.g., sensation seeking, impulsivity) and addiction risk (e.g., for alcohol use disorder (AUD)), but how mTBI and substance use might interact to promote addiction risk remains poorly understood. Likewise, potential differences in single vs. repetitive mTBI in relation to alcohol use/abuse have not been previously examined.

METHODS

Here, we examined how a history of single (1×) or repetitive (3×) blast exposure (blast-mTBI) affects ethanol (EtOH)-induced behavioral and physiological outcomes using an established mouse model of blast-mTBI. To investigate potential translational relevance, we also examined self-report responses to the Alcohol Use Disorders Identification Test-Consumption questions (AUDIT-C), a widely used measure to identify potential hazardous drinking and AUD, and used a novel unsupervised machine learning approach to investigate whether a history of blast-mTBI affected drinking behaviors in Iraq/Afghanistan Veterans.

RESULTS

Both single and repetitive blast-mTBI in mice increased the sedative properties of EtOH (with no change in tolerance or metabolism), but only repetitive blast potentiated EtOH-induced locomotor stimulation and shifted EtOH intake patterns. Specifically, mice exposed to repetitive blasts showed increased consumption "front-loading" (e.g., a higher rate of consumption during an initial 2-h acute phase of a 24-h alcohol access period and decreased total daily intake) during an intermittent 2-bottle choice condition. Examination of AUDIT-C scores in Iraq/Afghanistan Veterans revealed an optimal 3-cluster solution: "low" (low intake and low frequency), "frequent" (low intake and high frequency), and "risky" (high intake and high frequency), where Veterans with a history of blast-mTBI displayed a shift in cluster assignment from "frequent" to "risky," as compared to Veterans who were deployed to Iraq/Afghanistan but had no lifetime history of TBI.

CONCLUSIONS

Together, these results offer new insight into how blast-mTBI may give increase AUD risk and highlight the increased potential for adverse health risk behaviors following repetitive blast-mTBI.

Laterality and region-specific tau phosphorylation correlate with PTSD-related behavioral traits in rats exposed to repetitive low-level blast

Military veterans who experience blast-related traumatic brain injuries often suffer from chronic cognitive and neurobehavioral syndromes. Reports of abnormal tau processing following blast injury have raised concerns that some cases may have a neurodegenerative basis. Rats exposed to repetitive low-level blast exhibit chronic neurobehavioral traits and accumulate tau phosphorylated at threonine 181 (Thr181). Using data previously reported in separate studies we tested the hypothesis that region-specific patterns of Thr181 phosphorylation correlate with behavioral measures also previously determined and reported in the same animals. Elevated p-tau Thr181 in anterior neocortical regions and right hippocampus correlated with anxiety as well as fear learning and novel object localization. There were no correlations with levels in amygdala or posterior neocortical regions. Particularly striking were asymmetrical effects on the right and left hippocampus. No systematic variation in head orientation toward the blast wave seems to explain the laterality. Levels did not correlate with behavioral measures of hyperarousal. Results were specific to Thr181 in that no correlations were observed for three other phospho-acceptor sites (threonine 231, serine 396, and serine 404). No consistent correlations were linked with total tau. These correlations are significant in suggesting that p-tau accumulation in anterior neocortical regions and the hippocampus may lead to disinhibited amygdala function without p-tau elevation in the amygdala itself. They also suggest an association linking blast injury with tauopathy, which has implications for understanding the relationship of chronic blast-related neurobehavioral syndromes in humans to neurodegenerative diseases.

Epigenetic Blockade of Hippocampal SOD2 Via DNMT3b-Mediated DNA Methylation: Implications in Mild Traumatic Brain Injury-Induced Persistent Oxidative Damage

The recurrent events of mild trauma exacerbate the vulnerability for post-traumatic stress disorder; however, the underlying molecular mechanisms are scarcely known. The repeated mild traumatic brain injury (rMTBI) perturbs redox homeostasis which is primarily managed by superoxide dismutase 2 (SOD2). The current study investigates the role of DNA methylation in SOD2 gene regulation and its involvement in rMTBI-induced persistent neuropathology inflicted by weight drop injury paradigm. The oxidative damage, neurodegenerative indicators, and SOD2 function and its regulation in the hippocampus were analyzed after 48 h and 30 days of rMTBI. The temporal and episodic increase in ROS levels (oxidative stress) heightened 8-hydroxyguanosine levels indicating oxidative damage after rMTBI that was concomitant with decline in SOD2 function. In parallel, occupancy of DNMT3b at SOD2 promoter was higher post 30 days of the first episode of rMTBI causing hypermethylation at SOD2 promoter. This epigenetic silencing of SOD2 promoter was sustained after the second episode of rMTBI causing permanent blockade in SOD2 response. The resultant oxidative stress further culminated into the increasing number of degenerating neurons. The treatment with 5-azacytidine, a pan DNMT inhibitor, normalized DNA methylation levels and revived SOD2 function after the second episode of rMTBI. The release of blockade in SOD2 expression by DNMT inhibition also normalized the post-traumatic oxidative consequences and relieved the neurodegeneration and deficits in learning and memory as measured by novel object recognition test. In conclusion, DNMT3b-mediated DNA methylation plays a critical role in SOD2 gene regulation in the hippocampus, and the perturbations therein post rMTBI are detrimental to redox homeostasis manifesting into neurological consequences.

Glucose metabolism: A link between traumatic brain injury and Alzheimer's disease

Traumatic brain injury (TBI), a growing public health problem, is a leading cause of death and disability worldwide, although its prevention measures and clinical cares are substantially improved. Increasing evidence shows that TBI may increase the risk of mood disorders and neurodegenerative diseases, including Alzheimer's disease (AD). However, the complex relationship between TBI and AD remains elusive. Metabolic dysfunction has been the common pathology in both TBI and AD. On the one hand, TBI perturbs the glucose metabolism of the brain, and causes energy crisis and subsequent hyperglycolysis. On the other hand, glucose deprivation promotes amyloidogenesis via β-site APP cleaving enzyme-1 dependent mechanism, and triggers tau pathology and synaptic function. Recent findings suggest that TBI might facilitate Alzheimer's pathogenesis by altering metabolism, which provides clues to metabolic link between TBI and AD. In this review, we will explore how TBI-induced metabolic changes contribute to the development of AD.

Progressive Cognitive and Post-Traumatic Stress Disorder-Related Behavioral Traits in Rats Exposed to Repetitive Low-Level Blast

Many military veterans who experienced blast-related traumatic brain injuries (TBI) in the conflicts in Iraq and Afghanistan currently have chronic cognitive and mental health problems including post-traumatic stress disorder (PTSD). Besides static symptoms, new symptoms may emerge or existing symptoms may worsen. TBI is also a risk factor for later development of neurodegenerative diseases. In rats exposed to repetitive low-level blast overpressure (BOP), robust and enduring cognitive and PTSD-related behavioral traits develop that are present for at least one year after blast exposure. Here we determined the time-course of the appearance of these traits by testing rats in the immediate post-blast period. Three cohorts of rats examined within the first eight weeks exhibited no behavioral phenotype or, in one cohort, features of anxiety. None showed the altered cued fear responses or impaired novel object recognition characteristic of the fully developed phenotype. Two cohorts retested 36 to 42 weeks after blast exposure exhibited the expanded behavioral phenotype including anxiety as well as altered cued fear learning and impaired novel object recognition. Combined with previous work, the chronic behavioral phenotype has been observed in six cohorts of blast-exposed rats studied at 3-4 months or longer after blast injury, and the three cohorts studied here document the progressive nature of the cognitive/behavioral phenotype. These studies suggest the existence of a latent, delayed emerging and progressive blast-induced cognitive and behavioral phenotype. The delayed onset has implications for the evolution of post-blast neurobehavioral syndromes in military veterans and its modeling in experimental animals.

Brain and blood biomarkers of tauopathy and neuronal injury in humans and rats with neurobehavioral syndromes following blast exposure

Traumatic brain injury (TBI) is a risk factor for the later development of neurodegenerative diseases that may have various underlying pathologies. Chronic traumatic encephalopathy (CTE) in particular is associated with repetitive mild TBI (mTBI) and is characterized pathologically by aggregation of hyperphosphorylated tau into neurofibrillary tangles (NFTs). CTE may be suspected when behavior, cognition, and/or memory deteriorate following repetitive mTBI. Exposure to blast overpressure from improvised explosive devices (IEDs) has been implicated as a potential antecedent for CTE amongst Iraq and Afghanistan Warfighters. In this study, we identified biomarker signatures in rats exposed to repetitive low-level blast that develop chronic anxiety-related traits and in human veterans exposed to IED blasts in theater with behavioral, cognitive, and/or memory complaints. Rats exposed to repetitive low-level blasts accumulated abnormal hyperphosphorylated tau in neuronal perikarya and perivascular astroglial processes. Using positron emission tomography (PET) and the [¹⁸F]AV1451 (flortaucipir) tau ligand, we found that five of 10 veterans exhibited excessive retention of [¹⁸F]AV1451 at the white/gray matter junction in frontal, parietal, and temporal brain regions, a typical localization of CTE tauopathy. We also observed elevated levels of neurofilament light (NfL) chain protein in the plasma of veterans displaying excess [¹⁸F]AV1451 retention. These findings suggest an association linking blast injury, tauopathy, and neuronal injury. Further study is required to determine whether clinical, neuroimaging, and/or fluid biomarker signatures can improve the diagnosis of long-term neuropsychiatric sequelae of mTBI.

Mural cell dysfunction leads to altered cerebrovascular tau uptake following repetitive head trauma

A pathological characteristic of repetitive traumatic brain injury (TBI) is the deposition of hyperphosphorylated and aggregated tau species in the brain and increased levels of extracellular monomeric tau are believed to play a role in the pathogenesis of neurodegenerative tauopathies. The pathways by which extracellular tau is eliminated from the brain, however, remains elusive. The purpose of this study was to examine tau uptake by cerebrovascular cells and the effect of TBI on these processes. We found monomeric tau interacts with brain vascular mural cells (pericytes and smooth muscle cells) to a greater extent than other cerebrovascular cells, indicating mural cells may contribute to the elimination of extracellular tau, as previously described for other solutes such as beta-amyloid. Consistent with other neurodegenerative disorders, we observed a progressive decline in cerebrovascular mural cell markers up to 12 months post-injury in a mouse model of repetitive mild TBI (r-mTBI) and human TBI brain specimens, when compared to control. These changes appear to reflect mural cell degeneration and not cellular loss as no difference in the mural cell population was observed between r-mTBI and r-sham animals as determined through flow cytometry. Moreover, freshly isolated r-mTBI cerebrovessels showed reduced tau uptake at 6 and 12 months post-injury compared to r-sham animals, which may be the result of diminished cerebrovascular endocytosis, as caveolin-1 levels were significantly decreased in mouse r-mTBI and human TBI cerebrovessels compared to their respective controls. Further emphasizing the interaction between mural cells and tau, similar reductions in mural cell markers, tau uptake, and caveolin-1 were observed in cerebrovessels from transgenic mural cell-depleted animals. In conclusion, our studies indicate repeated injuries to the brain causes chronic mural cell degeneration, reducing the caveolar-mediated uptake of tau by these cells. Alterations in tau uptake by vascular mural cells may contribute to tau deposition in the brain following head trauma and could represent a novel therapeutic target for TBI or other neurodegenerative disorders.

Repetitive Blast Promotes Chronic Aversion to Neutral Cues Encountered in the Peri-Blast Environment

Repetitive mild traumatic brain injury (mTBI) has been called the "signature injury" of military service members in the Iraq and Afghanistan wars and is highly comorbid with post-traumatic stress disorder (PTSD). Correct attribution of adverse blast-induced mTBI and/or PTSD remains challenging. Pre-clinical research using animal models can provide important insight into the mechanisms by which blast produces injury and dysfunction-but only to the degree by which such models reflect the human experience. Avoidance of trauma reminders is a hallmark of PTSD. Here, we sought to understand whether a mouse model of blast reproduces this phenomenon, in addition to blast-induced physical injuries. Drawing on well-established work from the chronic stress and Pavlovian conditioning literature, we hypothesized that even while one is anesthetized during blast exposure, environmental cues encountered in the peri-blast environment could be conditioned to evoke aversion/dysphoria and re-experiencing of traumatic stress. Using a pneumatic shock tube that recapitulates battlefield-relevant open-field blast forces, we provide direct evidence that stress is inherent to repetitive blast exposure, resulting in chronic aversive/dysphoric-like responses to previous blast-paired cues. The results in this report demonstrate that, although both single and repetitive blast exposures produce acute stress responses (weight loss, corticosterone increase), only repetitive blast exposure also results in co-occurring aversive/dysphoric-like stress responses. These results extend appreciation of the highly complex nature of repetitive blast exposure; and lend further support for the potential translational relevance of animal modeling approaches currently used by multiple laboratories aimed at elucidating the mechanisms (both molecular and behavioral) of repetitive blast exposure.

An update on the association between traumatic brain injury and Alzheimer's disease: Focus on Tau pathology and synaptic dysfunction

L.P. Li, J.W. Liang and H.J. Fu. An update on the association between traumatic brain injury and Alzheimer's disease: Focus on Tau pathology and synaptic dysfunction. NEUROSCI BIOBEHAV REVXXX-XXX,2020.-Traumatic brain injury (TBI) and Alzheimer's disease (AD) are devastating conditions that have long-term consequences on individual's cognitive functions. Although TBI has been considered a risk factor for the development of AD, the link between TBI and AD is still in debate. Aggregation of hyperphosphorylated tau and intercorrelated synaptic dysfunction, two key pathological elements in both TBI and AD, play a pivotal role in mediating neurodegeneration and cognitive deficits, providing a mechanistic link between these two diseases. In the first part of this review, we analyze the experimental literatures on tau pathology in various TBI models and review the distribution, biological features and mechanisms of tau pathology following TBI with implications in AD pathogenesis. In the second part, we review evidences of TBI-mediated structural and functional impairments in synapses, with a focus on the overlapped mechanisms underlying synaptic abnormalities in both TBI and AD. Finally, future perspectives are proposed for uncovering the complex relationship between TBI and neurodegeneration, and developing potential therapeutic avenues for alleviating cognitive deficits after TBI.

Applying a novel 3D hydrogel cell culture to investigate activation of microglia due to rotational kinematics associated with mild traumatic brain injury

Many investigations on mild traumatic brain injury (mTBI) aim to further understand how cells in the brain react to the mechanical forces associated with the injury. While it is known that rapid head rotation is a mechanism contributing to mTBI, establishing definitive thresholds for head rotation has proved challenging. One way to advance determining mechanisms and thresholds for injury is through in vitro models. Here, an apparatus has been designed that is capable of delivering rotational forces to three-dimensional (3D) hydrogel cell cultures. Using an in vitro model, we test the hypothesis that rotational kinematics can activate microglia suspended in a 3-dimensional mixed glia environment (absent neurons). The impact apparatus was able to deliver peak angular velocities of approximately 45 rad/s, a magnitude for angular velocity that in select literature is associated with diffuse brain injury. However, no measurable glial cell reactivity was observed in response to the rotational kinematics through any of the chosen metrics (nitric oxide, pro-inflammatory cytokine release and proportion of amoeboid activated microglia). The results generated from this study suggest that rotation of the glia alone did not cause activation - in future work we will investigate the effect of neuronal contributions in activating glia.

Prion-Like Propagation Mechanisms in Tauopathies and Traumatic Brain Injury: Challenges and Prospects

The accumulation of tau protein in the form of filamentous aggregates is a hallmark of many neurodegenerative diseases such as Alzheimer's disease (AD) and chronic traumatic encephalopathy (CTE). These dementias share traumatic brain injury (TBI) as a prominent risk factor. Tau aggregates can transfer between cells and tissues in a "prion-like" manner, where they initiate the templated misfolding of normal tau molecules. This enables the spread of tau pathology to distinct parts of the brain. The evidence that tauopathies spread via prion-like mechanisms is considerable, but work detailing the mechanisms of spread has mostly used in vitro platforms that cannot fully reveal the tissue-level vectors or etiology of progression. We review these issues and then briefly use TBI and CTE as a case study to illustrate aspects of tauopathy that warrant further attention in vivo. These include seizures and sleep/wake disturbances, emphasizing the urgent need for improved animal models. Dissecting these mechanisms of tauopathy progression continues to provide fresh inspiration for the design of diagnostic and therapeutic approaches.

DNA Methylation Patterns of Chronic Explosive Breaching in U.S. Military Warfighters

Background: Injuries from exposure to explosions rose dramatically during the Iraq and Afghanistan wars, which motivated investigation of blast-related neurotrauma. We have undertaken human studies involving military "breachers" -exposed to controlled, low-level blast during a 3-days explosive breaching course. Methods: We screened epigenetic profiles in peripheral blood samples from 59 subjects (in two separate U.S. Military training sessions) using Infinium MethylationEPIC BeadChips. Participants had varying numbers of exposures to blast over their military careers (empirically defined as high ≥ 40, and conversely, low < 39 breaching exposures). Daily self-reported physiological symptoms were recorded. Tinnitus, memory problems, headaches, and sleep disturbances are most frequently reported. Results: We identified 14 significantly differentially methylated regions (DMRs) within genes associated with cumulative blast exposure in participants with high relative to low cumulative blast exposure. Notably, NTSR1 and SPON1 were significantly differentially methylated in high relative to low blast exposed groups, suggesting that sleep dysregulation may be altered in response to chronic cumulative blast exposure. In comparing lifetime blast exposure at baseline (prior to exposure in current training), and top associated symptoms, we identified significant DMRs associated with tinnitus, sleep difficulties, and headache. Notably, we identified KCNN3, SOD3, MUC4, GALR1, and WDR45B, which are implicated in auditory function, as differentially methylated associated with self-reported tinnitus. These findings suggest neurobiological mechanisms behind auditory injuries in our military warfighters and are particularly relevant given tinnitus is not only a primary disability among veterans, but has also been demonstrated in active duty medical records for populations exposed to blast in training. Additionally, we found that differentially methylated regions associated with the genes CCDC68 and COMT track with sleep difficulties, and those within FMOD and TNXB track with pain and headache. Conclusion: Sleep disturbances, as well as tinnitus and chronic pain, are widely reported in U.S. military service members and veterans. As we have previously demonstrated, DNA methylation encapsulates lifetime exposure to blast. The current data support previous findings and recapitulate transcriptional regulatory alterations in genes involved in sleep, auditory function, and pain. These data uncovered novel epigenetic and transcriptional regulatory mechanism underlying the etiological basis of these symptoms.

Explosive-driven double-blast exposure: molecular, histopathological, and behavioral consequences

Traumatic brain injury generated by blast may induce long-term neurological and psychiatric sequelae. We aimed to identify molecular, histopathological, and behavioral changes in rats 2 weeks after explosive-driven double-blast exposure. Rats received two 30-psi (~ 207-kPa) blasts 24 h apart or were handled identically without blast. All rats were behaviorally assessed over 2 weeks. At Day 15, rats were euthanized, and brains removed. Brains were dissected into frontal cortex, hippocampus, cerebellum, and brainstem. Western blotting was performed to measure levels of total-Tau, phosphorylated-Tau (pTau), amyloid precursor protein (APP), GFAP, Iba1, αII-spectrin, and spectrin breakdown products (SBDP). Kinases and phosphatases, correlated with tau phosphorylation were also measured. Immunohistochemistry for pTau, APP, GFAP, and Iba1 was performed. pTau protein level was greater in the hippocampus, cerebellum, and brainstem and APP protein level was greater in cerebellum of blast vs control rats (p < 0.05). GFAP, Iba1, αII-spectrin, and SBDP remained unchanged. No immunohistochemical or neurobehavioral changes were observed. The dissociation between increased pTau and APP in different regions in the absence of neurobehavioral changes 2 weeks after double blast exposure is a relevant finding, consistent with human data showing that battlefield blasts might be associated with molecular changes before signs of neurological and psychiatric disorders manifest.

Chronic elevation of plasma vascular endothelial growth factor-A (VEGF-A) is associated with a history of blast exposure

Mounting evidence points to the significance of neurovascular-related dysfunction in veterans with blast-related mTBI, which is also associated with reduced [18F]-fluorodeoxyglucose (FDG) uptake. The goal of this study was to determine whether plasma VEGF-A is altered in veterans with blast-related mTBI and address whether VEGF-A levels correlate with FDG uptake in the cerebellum, a brain region that is vulnerable to blast-related injury 72 veterans with blast-related mTBI (mTBI) and 24 deployed control (DC) veterans with no lifetime history of TBI were studied. Plasma VEGF-A was significantly elevated in mTBIs compared to DCs. Plasma VEGF-A levels in mTBIs were significantly negatively correlated with FDG uptake in cerebellum. In addition, performance on a Stroop color/word interference task was inversely correlated with plasma VEGF-A levels in blast mTBI veterans. Finally, we observed aberrant perivascular VEGF-A immunoreactivity in postmortem cerebellar tissue and not cortical or hippocampal tissues from blast mTBI veterans. These findings add to the limited number of plasma proteins that are chronically elevated in veterans with a history of blast exposure associated with mTBI. It is likely the elevated VEGF-A levels are from peripheral sources. Nonetheless, increasing plasma VEGF-A concentrations correlated with chronically decreased cerebellar glucose metabolism and poorer performance on tasks involving cognitive inhibition and set shifting. These results strengthen an emerging view that cognitive complaints and functional brain deficits caused by blast exposure are associated with chronic blood-brain barrier injury and prolonged recovery in affected regions.

Improvement in cognitive dysfunction following blast induced traumatic brain injury by thymosin α1 in rats: Involvement of inhibition of tau phosphorylation at the Thr205 epitope

Cognitive impairment is a significant sequela of traumatic brain injury (TBI) especially blast induced traumatic brain injury (bTBI), which is characterized by rapid impairments of learning and memory ability. Although several neuroprotective agents have been postulated as promising drugs for bTBI in animal studies, very few ideal therapeutic options exist to improve cognitive impairment following bTBI. Thymosin α1(Tα1), a 28-amino-acid protein that possesses immunomodulatory functions, has exhibited beneficial effects in the treatment of infectious diseases, immunodeficiency diseases and cancers. However, it remains unclear whether Tα1 has a therapeutic role in bTBI. Thus, we hypothesized that Tα1 administration could reverse the outcomes of bTBI. The blast induced TBI (bTBI) rat model was established with the compressed gas driven blast injury model system. A consecutive Tα1 therapy (in 1 ml saline, twice a day) at a dose of 200 µg/kg or normal saline (NS) (1 ml, twice a day) for 3 days or 2 weeks was performed. Utilizing our newly designed bTBI model, we investigated the beneficial effects of Tα1 therapy on rats exposed to bTBI including: cognitive functions, general histology, regulatory T (Treg) cells, edema, inflammation reactions and the expression and phosphorylation level of tau via Morris Water Maze test (MWM test), HE staining, flow cytometry, brain water content (BWC) calculation, IL-6 assay and Western blotting, respectively. Tα1 treatment seemed to reduce the 24-hour mortality, albeit with no statistical significance. Moreover, Tα1 treatment markedly improved cognitive dysfunction by decreasing the escape latency in the acquisition phase, and increasing the crossing numbers in the probe phase of MWM test. More interestingly, Tα1 significantly inhibited tau phosphorylation at the Thr205 epitope, but not at the Ser404 and Ser262 epitopes. Tα1 increased the percentage of Treg cells and inhibited plasma IL-6 production on 3d post bTBI. Moreover, Tα1 suppressed brain edema as demonstrated by decrease of BWC. However, there was a lack of obvious change in histopathology in the brain upon Tα1 treatment. This is the first study showing that Tα1 improves neurological deficits after bTBI in rats, which is potentially related to the inhibition of tau phosphorylation at the Thr205 epitope, increased Treg cells and decreased inflammatory reactions and brain edema.

Repetitive Head Trauma Induces Chronic Traumatic Encephalopathy by Multiple Mechanisms

Exposure to repetitive neurotrauma increases lifetime risk for developing progressive cognitive deficits, neurobehavioral abnormalities, and chronic traumatic encephalopathy (CTE). CTE is a tau protein neurodegenerative disease first identified in boxers and recently described in athletes participating in other contact sports (notably American football, ice hockey, rugby, and wrestling) and in military veterans with blast exposure. Currently, CTE can only be diagnosed by neuropathological examination of the brain after death. The defining diagnostic lesion of CTE consists of patchy perivascular accumulations of hyperphosphorylated tau protein that localize in the sulcal depths of the cerebral cortex. Neuronal abnormalities, axonopathy, neurovascular dysfunction, and neuroinflammation are triggered by repetitive head impacts (RHIs) and likely act as catalysts for CTE pathogenesis and progression. However, the specific mechanisms that link RHI to CTE are unknown. This review will explore two important areas of CTE pathobiology. First, we will review what is known about the biomechanical properties of RHI that initiate CTE-related pathologies. Second, we will provide an overview of key features of CTE neuropathology and how these contribute to abnormal tau hyperphosphorylation, accumulation, and spread.

Nitric oxide synthase mediates cerebellar dysfunction in mice exposed to repetitive blast-induced mild traumatic brain injury

We investigated the role of nitric oxide synthase (NOS) in mediating blood-brain barrier (BBB) disruption and peripheral immune cell infiltration in the cerebellum following blast exposure. Repetitive, but not single blast exposure, induced delayed-onset BBB disruption (72 hours post-blast) in cerebellum. The NOS inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME) administered after blast blocked BBB disruption and prevented CD4⁺ T-cell infiltration into cerebellum. L-NAME also blocked blast-induced increases in intercellular adhesion molecule-1 (ICAM-1), a molecule that plays a critical role in regulating blood-to-brain immune cell trafficking. Blocking NOS-mediated BBB dysfunction during this acute/subacute post-blast interval (24-71 hours after the last blast) also prevented sensorimotor impairment on a rotarod task 30 days later, long after L-NAME cleared the body. In postmortem brains from Veterans/military Servicemembers with blast-related TBI, we found marked Purkinje cell dendritic arbor structural abnormalities, which were comparable to neuropathologic findings in the blast-exposed mice. Taken collectively, these results indicate that blast provokes delayed-onset of NOS-dependent pathogenic cascades that can later emerge as behavioral dysfunction. These results also further implicate the cerebellum as a brain region vulnerable to blast-induced mTBI.

Blast Exposure Leads to Accelerated Cellular Senescence in the Rat Brain

Blast-induced traumatic brain injury (bTBI) is one of the major causes of persistent disabilities in Service Members, and a history of bTBI has been identified as a primary risk factor for developing age-associated neurodegenerative diseases. Clinical observations of several military blast casualties have revealed a rapid age-related loss of white matter integrity in the brain. In the present study, we have tested the effect of single and tightly coupled repeated blasts on cellular senescence in the rat brain. Isoflurane-anesthetized rats were exposed to either a single or 2 closely coupled blasts in an advanced blast simulator. Rats were euthanized and brains were collected at 24 h, 1 month and 1 year post-blast to determine senescence-associated-β-galactosidase (SA-β-gal) activity in the cells using senescence marker stain. Single and repeated blast exposures resulted in significantly increased senescence marker staining in several neuroanatomical structures, including cortex, auditory cortex, dorsal lateral thalamic nucleus, geniculate nucleus, superior colliculus, ventral thalamic nucleus and hippocampus. In general, the increases in SA-β-gal activity were more pronounced at 1 month than at 24 h or 1 year post-blast and were also greater after repeated than single blast exposures. Real-time quantitative RT-PCR analysis revealed decreased levels of mRNA for senescence marker protein-30 (SMP-30) and increased mRNA levels for p21 (cyclin dependent kinase inhibitor 1A, CDKN1A), two other related protein markers of cellular senescence. The increased senescence observed in some of these affected brain structures may be implicated in several long-term sequelae after exposure to blast, including memory disruptions and impairments in movement, auditory and ocular functions.

Beneficial association of angiotensin-converting enzyme inhibitors and statins on the occurrence of possible Alzheimer's disease after traumatic brain injury

BACKGROUND

Pathological analysis of brain tissue from animals and humans with a history of traumatic brain injury (TBI) suggests that TBI could be one of the risk factors facilitating onset of dementia with possible Alzheimer's disease (AD), but medications to prevent or delay AD onset are not yet available.

METHODS

This study explores four medication classes (angiotensin-converting enzyme inhibitors (ACEI), beta blockers, metformin, and statins) approved by the Food and Drug Administration (FDA) for other indications and evaluates their influence when used in combination on the risk of possible AD development for patients with a history of TBI. We identified patients with history of TBI from an existing Department of Veterans Affairs (VA) national database. Among 1,660,151 veterans who used VA services between the ages of 50 to 89 years old, we analyzed 733,920 patients, including 15,450 patients with a history of TBI and 718,470 non-TBI patients. The TBI patients were followed for up to 18.5 years, with an average of 7.7 ± 4.7 years, and onset of dementia with possible AD was recorded based on International Statistical Classification of Diseases (ICD) 9 or 10 codes. The effect of TBI on possible AD development was evaluated by multivariable logistic regression models adjusted by age, gender, race, and other comorbidities. The association of ACEI, beta blockers, metformin, statins, and combinations of these agents over time from the first occurrence of TBI to possible AD onset was assessed using Cox proportional hazard models adjusted for demographics and comorbidities.

RESULTS

Veterans with at least two TBI occurrences by claims data were 25% (odds ratio (OR) = 1.25, 95% confidence intervals (CI) (1.13, 1.37)) more likely to develop dementia with possible AD, compared to those with no record of TBI. In multivariable logistic regression models (propensity score weighted or adjusted), veterans taking a combination of ACEI and statins had reduced risk in developing possible AD after suffering TBI, and use of this medication class combination was associated with a longer period between TBI occurring and dementia with possible AD onset, compared to patients who took statins alone or did not take any of the four target drugs after TBI.

CONCLUSIONS

The combination of ACEI and statins significantly lowered the risk of development of dementia with possible AD in a national cohort of people with a history of TBI, thus supporting a clinical approach to lowering the risk of dementia with possible AD.

Progressive long-term spatial memory loss following repeat concussive and subconcussive brain injury in mice, associated with dorsal hippocampal neuron loss, microglial phenotype shift, and vascular abnormalities

There is considerable concern about the long‐term deleterious effects of repeat head trauma on cognition, but little is known about underlying mechanisms and pathology. To examine this, we delivered four air blasts to the left side of the mouse cranium, a week apart, with an intensity that causes deficits when delivered singly and considered “concussive,” or an intensity that does not yield significant deficits when delivered singly and considered “subconcussive.” Neither repeat concussive nor subconcussive blast produced spatial memory deficits at 4 months, but both yielded deficits at 14 months, and dorsal hippocampal neuron loss. Hierarchical cluster analysis of dorsal hippocampal microglia across the three groups based on morphology and expression of MHCII, CX3CR1, CD68 and IBA1 revealed five distinct phenotypes. Types 1A and 1B microglia were more common in sham mice, linked to better neuron survival and memory, and appeared mildly activated. By contrast, 2B and 2C microglia were more common in repeat concussive and subconcussive mice, linked to poorer neuron survival and memory, and characterized by low expression levels and attenuated processes, suggesting they were de‐activated and dysfunctional. In addition, endothelial cells in repeat concussive mice exhibited reduced CD31 and eNOS expression, which was correlated with the prevalence of type 2B and 2C microglia. Our findings suggest that both repeat concussive and subconcussive head injury engender progressive pathogenic processes, possibly through sustained effects on microglia that over time lead to increased prevalence of dysfunctional microglia, adversely affecting neurons and blood vessels, and thereby driving neurodegeneration and memory decline.

Acute and Chronic Molecular Signatures and Associated Symptoms of Blast Exposure in Military Breachers

Injuries from exposure to explosions rose dramatically during the Iraq and Afghanistan wars, which motivated investigations of blast-related neurotrauma and operational breaching. In this study, military “breachers” were exposed to controlled, low-level blast during a 10-day explosive breaching course. Using an omics approach, we assessed epigenetic, transcriptional, and inflammatory profile changes in blood from operational breaching trainees, with varying levels of lifetime blast exposure, along with daily self-reported symptoms (with tinnitus, headaches, and sleep disturbances as the most frequently reported). Although acute exposure to blast did not confer epigenetic changes, specifically in DNA methylation, differentially methylated regions (DMRs) with coordinated gene expression changes associated with lifetime cumulative blast exposures were identified. The accumulative effect of blast showed increased methylation of PAX8 antisense transcript with coordinated repression of gene expression, which has been associated with sleep disturbance. DNA methylation analyses conducted in conjunction with reported symptoms of tinnitus in the low versus high blast incidents groups identified DMRS in KCNE1 and CYP2E1 genes. KCNE1 and CYP2E1 showed the expected inverse correlation between DNA methylation and gene expression, which have been previously implicated in noise-related hearing loss. Although no significant transcriptional changes were observed in samples obtained at the onset of the training course relative to chronic cumulative blast, we identified a large number of transcriptional perturbations acutely pre- versus post-blast exposure. Acutely, 67 robustly differentially expressed genes (fold change ≥1.5), including UFC1 and YOD1 ubiquitin-related proteins, were identified. Inflammatory analyses of cytokines and chemokines revealed dysregulation of MCP-1, GCSF, HGF, MCSF, and RANTES acutely after blast exposure. These data show the importance of an omics approach, revealing that transcriptional and inflammatory biomarkers capture acute low-level blast overpressure exposure, whereas DNA methylation marks encapsulate chronic long-term symptoms.

Blast-Mediated Traumatic Brain Injury Exacerbates Retinal Damage and Amyloidosis in the APPswePSENd19e Mouse Model of Alzheimer's Disease

Purpose

Traumatic brain injury (TBI) is a risk factor for developing chronic neurodegenerative conditions including Alzheimer's disease (AD). The purpose of this study was to examine chronic effects of blast TBI on retinal ganglion cells (RGC), optic nerve, and brain amyloid load in a mouse model of AD amyloidosis.

Methods

Transgenic (TG) double-mutant APPswePSENd19e (APP/PS1) mice and nontransgenic (Non-TG) littermates were exposed to a single blast TBI (20 psi) at age 2 to 3 months. RGC cell structure and function was evaluated 2 months later (average age at endpoint = 4.5 months) using pattern electroretinogram (PERG), optical coherence tomography (OCT), and the chromatic pupil light reflex (cPLR), followed by histologic analysis of retina, optic nerve, and brain amyloid pathology.

Results

APP/PS1 mice exposed to blast TBI (TG-Blast) had significantly lower PERG and cPLR responses 2 months after injury compared to preblast values and compared to sham groups of APP/PS1 (TG-Sham) and nontransgenic (Non-TG-Sham) mice as well as nontransgenic blast-exposed mice (Non-TG-Blast). The TG-Blast group also had significantly thinner RGC complex and more optic nerve damage compared to all groups. No amyloid-β (Aβ) deposits were detected in retinas of APP/PS1 mice; however, increased amyloid precursor protein (APP)/Aβ-immunoreactivity was seen in TG-Blast compared to TG-Sham mice, particularly near blood vessels. TG-Blast and TG-Sham groups exhibited high variability in pathology severity, with a strong, but not statistically significant, trend for greater cerebral cortical Aβ plaque load in the TG-Blast compared to TG-Sham group.

Conclusions

When combined with a genetic susceptibility for developing amyloidosis of AD, blast TBI exposure leads to earlier RGC and optic nerve damage associated with modest but detectable increase in cerebral cortical Aβ pathology. These findings suggest that genetic risk factors for AD may increase the sensitivity of the retina to blast-mediated damage.

A Mouse Model of Repetitive Blast Traumatic Brain Injury Reveals Post-Trauma Seizures and Increased Neuronal Excitability

Repetitive blast traumatic brain injury (TBI) affects numerous soldiers on the battlefield. Mild TBI has been shown to have long-lasting effects with repeated injury. We have investigated effects on neuronal excitability after repetitive, mild TBI in a mouse model of blast-induced brain injury. We exposed mice to mild blast trauma of an average peak overpressure of 14.6 psi, repeated across three consecutive days. While a single exposure did not reveal trauma as indicated by the glial fibrillary acidic protein indicator, three repetitive blasts did show significant increases. As well, mice had an increased indicator of inflammation (Iba-1) and increased tau, tau phosphorylation, and altered cytokine levels in the spleen. Video-electroencephalographic monitoring 48 h after the final blast exposure demonstrated seizures in 50% (12/24) of the mice, most of which were non-convulsive seizures. Long-term monitoring revealed that spontaneous seizures developed in at least 46% (6/13) of the mice. Patch clamp recording of dentate gyrus hippocampus neurons 48 h post-blast TBI demonstrated a shortened latency to the first spike and hyperpolarization of action potential threshold. We also found that evoked excitatory postsynaptic current amplitudes were significantly increased. These findings indicate that mild, repetitive blast exposures cause increases in neuronal excitability and seizures and eventual epilepsy development in some animals. The non-convulsive nature of the seizures suggests that subclinical seizures may occur in individuals experiencing even mild blast events, if repeated.

Blast Preconditioning Protects Retinal Ganglion Cells and Reveals Targets for Prevention of Neurodegeneration Following Blast-Mediated Traumatic Brian Injury

Purpose

The purpose of this study was to examine the effect of multiple blast exposures and blast preconditioning on the structure and function of retinal ganglion cells (RGCs), to identify molecular pathways that contribute to RGC loss, and to evaluate the role of kynurenine-3-monooxygenase (KMO) inhibition on RGC structure and function.

Methods

Mice were subjected to sham blast injury, one single blast injury, or three blast injuries separated by either 1 hour or 1 week, using a blast intensity of 20 PSI. To examine the effect of blast preconditioning, mice were subjected to sham blast injury, one single 20-PSI injury, or three blast injuries separated by 1 week (5 PSI, 5 PSI, 20 PSI and 5 PSI, 5 PSI, 5 PSI). RGC structure was analyzed by optical coherence tomography (OCT) and function was analyzed by the pattern electroretinogram (PERG). BRN3A-positive cells were quantified to determine RGC density. RNA-seq analysis was used to identify transcriptional changes between groups.

Results

Analysis of mice with multiple blast exposures of 20 PSI revealed no significant differences compared to one 20-pounds per square inch (PSI) exposure using OCT, PERG, or BRN3A cell counts. Analysis of mice exposed to two preconditioning 5-PSI blasts prior to one 20-PSI blast showed preservation of RGC structure and function. RNA-seq analysis of the retina identified multiple transcriptomic changes between conditions. Pharmacologic inhibition of KMO preserved RGC responses compared to vehicle-treated mice.

Conclusions

Preconditioning protects RGC from blast injury. Protective effects appear to involve changes in KMO activity, whose inhibition is also protective.