Neurological Effects of Repeated Blast Exposure in Special Operations Personnel

Exposure to blast overpressure has been a pervasive feature of combat-related injuries. Studies exploring the neurological correlates of repeated low-level blast exposure in career "breachers" demonstrated higher levels of tumor necrosis factor alpha (TNFα) and interleukin (IL)-6 and decreases in IL-10 within brain-derived extracellular vesicles (BDEVs). The current pilot study was initiated in partnership with the U.S. Special Operations Command (USSOCOM) to explore whether neuroinflammation is seen within special operators with prior blast exposure. Data were analyzed from 18 service members (SMs), inclusive of 9 blast-exposed special operators with an extensive career history of repeated blast exposures and 9 controls matched by age and duration of service. Neuroinflammation was assessed utilizing positron emission tomography (PET) imaging with [18F]DPA-714. Serum was acquired to assess inflammatory biomarkers within whole serum and BDEVs. The Blast Exposure Threshold Survey (BETS) was acquired to determine blast history. Both self-report and neurocognitive measures were acquired to assess cognition. Similarity-driven Multi-view Linear Reconstruction (SiMLR) was used for joint analysis of acquired data. Analysis of BDEVs indicated significant positive associations with a generalized blast exposure value (GBEV) derived from the BETS. SiMLR-based analyses of neuroimaging demonstrated exposure-related relationships between GBEV, PET-neuroinflammation, cortical thickness, and volume loss within special operators. Affected brain networks included regions associated with memory retrieval and executive functioning, as well as visual and heteromodal processing. Post hoc assessments of cognitive measures failed to demonstrate significant associations with GBEV. This emerging evidence suggests neuroinflammation may be a key feature of the brain response to blast exposure over a career in operational personnel. The common thread of neuroinflammation observed in blast-exposed populations requires further study.

Temporal Alterations in Cerebrovascular Glycocalyx and Cerebral Blood Flow after Exposure to a High-Intensity Blast in Rats

The glycocalyx is a proteoglycan-glycoprotein structure lining the luminal surface of the vascular endothelium and is susceptible to damage due to blast overpressure (BOP) exposure. The glycocalyx is essential in maintaining the structural and functional integrity of the vasculature and regulation of cerebral blood flow (CBF). Assessment of alterations in the density of the glycocalyx; its components (heparan sulphate proteoglycan (HSPG/syndecan-2), heparan sulphate (HS), and chondroitin sulphate (CS)); CBF; and the effect of hypercapnia on CBF was conducted at 2-3 h, 1, 3, 14, and 28 days after a high-intensity (18.9 PSI/131 kPa peak pressure, 10.95 ms duration, and 70.26 PSI·ms/484.42 kPa·ms impulse) BOP exposure in rats. A significant reduction in the density of the glycocalyx was observed 2-3 h, 1-, and 3 days after the blast exposure. The glycocalyx recovered by 28 days after exposure and was associated with an increase in HS (14 and 28 days) and in HSPG/syndecan-2 and CS (28 days) in the frontal cortex. In separate experiments, we observed significant decreases in CBF and a diminished response to hypercapnia at all time points with some recovery at 3 days. Given the role of the glycocalyx in regulating physiological function of the cerebral vasculature, damage to the glycocalyx after BOP exposure may result in the onset of pathogenesis and progression of cerebrovascular dysfunction leading to neuropathology.

Exposure to Low-Intensity Blast Increases Clearance of Brain Amyloid Beta

The long-term effects of exposure to blast overpressure are an important health concern in military personnel. Increase in amyloid beta (Aβ) has been documented after non-blast traumatic brain injury (TBI) and may contribute to neuropathology and an increased risk for Alzheimer's disease. We have shown that Aβ levels decrease following exposure to a low-intensity blast overpressure event. To further explore this observation, we examined the effects of a single 37 kPa (5.4 psi) blast exposure on brain Aβ levels, production, and clearance mechanisms in the acute (24 h) and delayed (28 days) phases post-blast exposure in an experimental rat model. Aβ and, notably, the highly neurotoxic detergent soluble Aβ42 form, was reduced at 24 h but not 28 days after blast exposure. This reduction was not associated with changes in the levels of Aβ oligomers, expression levels of amyloid precursor protein (APP), or increase in enzymes involved in the amyloidogenic cleavage of APP, the β- and ϒ-secretases BACE1 and presenilin-1, respectively. The levels of ADAM17 α-secretase (also known as tumor necrosis factor α-converting enzyme) decreased, concomitant with the reduction in brain Aβ. Additionally, significant increases in brain levels of the endothelial transporter, low-density related protein 1 (LRP1), and enhancement in co-localization of aquaporin-4 (AQP4) to perivascular astrocytic end-feet were observed 24 h after blast exposure. These findings suggest that exposure to low-intensity blast may enhance endothelial clearance of Aβ by LRP1-mediated transcytosis and alter AQP4-aided glymphatic clearance. Collectively, the data demonstrate that low-intensity blast alters enzymatic, transvascular, and perivascular clearance of Aβ.

Metabotropic Glutamate Receptor 2 Expression Is Chronically Elevated in Male Rats With Post-Traumatic Stress Disorder Related Behavioral Traits Following Repetitive Low-Level Blast Exposure

Many military veterans who experienced blast-related traumatic brain injuries in the conflicts in Iraq and Afghanistan currently suffer from chronic cognitive and mental health problems that include depression and post-traumatic stress disorder (PTSD). Male rats exposed to repetitive low-level blast develop cognitive and PTSD-related behavioral traits that are present for more than 1 year after exposure. We previously reported that a group II metabotropic receptor (mGluR2/3) antagonist reversed blast-induced behavioral traits. In this report, we explored mGluR2/3 expression following blast exposure in male rats. Western blotting revealed that mGluR2 protein (but not mGluR3) was increased in all brain regions studied (anterior cortex, hippocampus, and amygdala) at 43 or 52 weeks after blast exposure but not at 2 weeks or 6 weeks. mGluR2 RNA was elevated at 52 weeks while mGluR3 was not. Immunohistochemical staining revealed no changes in the principally presynaptic localization of mGluR2 by blast exposure. Administering the mGluR2/3 antagonist LY341495 after behavioral traits had emerged rapidly reversed blast-induced effects on novel object recognition and cued fear responses 10 months following blast exposure. These studies support alterations in mGluR2 receptors as a key pathophysiological event following blast exposure and provide further support for group II metabotropic receptors as therapeutic targets in the neurobehavioral effects that follow blast injury.

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.

Late chronic local inflammation, synaptic alterations, vascular remodeling and arteriovenous malformations in the brains of male rats exposed to repetitive low-level blast overpressures

In the course of military operations in modern war theaters, blast exposures are associated with the development of a variety of mental health disorders associated with a post-traumatic stress disorder-related features, including anxiety, impulsivity, insomnia, suicidality, depression, and cognitive decline. Several lines of evidence indicate that acute and chronic cerebral vascular alterations are involved in the development of these blast-induced neuropsychiatric changes. In the present study, we investigated late occurring neuropathological events associated with cerebrovascular alterations in a rat model of repetitive low-level blast-exposures (3 × 74.5 kPa). The observed events included hippocampal hypoperfusion associated with late-onset inflammation, vascular extracellular matrix degeneration, synaptic structural changes and neuronal loss. We also demonstrate that arteriovenous malformations in exposed animals are a direct consequence of blast-induced tissue tears. Overall, our results further identify the cerebral vasculature as a main target for blast-induced damage and support the urgent need to develop early therapeutic approaches for the prevention of blast-induced late-onset neurovascular degenerative processes.

(2R,6R)-Hydroxynorketamine Treatment of Rats Exposed to Repetitive Low-Level Blast Injury

Many military veterans who experienced blast-related traumatic brain injuries (TBIs) in the conflicts in Iraq and Afghanistan suffer from chronic cognitive and mental health problems, including post-traumatic stress disorder (PTSD). Male rats subjected to repetitive low-level blast exposure develop chronic cognitive and PTSD-related traits that develop in a delayed manner. Ketamine has received attention as a treatment for refractory depression and PTSD. (2R,6R)-hydroxynorketamine [(2R,6R)-HNK] is a ketamine metabolite that exerts rapid antidepressant actions. (2R,6R)-HNK has become of clinical interest because of its favorable side-effect profile, low abuse potential, and oral route of administration. We treated three cohorts of blast-exposed rats with (2R,6R)-HNK, beginning 7-11 months after blast exposure, a time when the behavioral phenotype is established. Each cohort consisted of groups (n = 10-13/group) as follows: 1) Sham-exposed treated with saline, 2) blast-exposed treated with saline, and 3) blast-exposed treated with a single dose of 20 mg/kg of (2R,6R)-HNK. (2R,6R)-HNK rescued blast-induced deficits in novel object recognition (NOR) and anxiety-related features in the elevated zero maze (EZM) in all three cohorts. Exaggerated acoustic startle was reversed in cohort 1, but not in cohort 3. (2R,6R)-HNK effects were still present in the EZM 12 days after administration in cohort 1 and 27 days after administration in NOR testing of cohorts 2 and 3. (2R,6R)-HNK may be beneficial for the neurobehavioral syndromes that follow blast exposure in military veterans. Additional studies will be needed to determine whether higher doses or more extended treatment regimens may be more effective.

Progressive Transcriptional Changes in the Amygdala Implicate Neuroinflammation in the Effects of Repetitive Low-Level Blast Exposure in Male Rats

Chronic mental health problems are common among military veterans who sustained blast-related traumatic brain injuries. The reasons for this association remain unexplained. Male rats exposed to repetitive low-level blast overpressure (BOP) exposures exhibit chronic cognitive and post-traumatic stress disorder (PTSD)-related traits that develop in a delayed fashion. We examined blast-induced alterations on the transcriptome in four brain areas (anterior cortex, hippocampus, amygdala, and cerebellum) across the time frame over which the PTSD-related behavioral phenotype develops. When analyzed at 6 weeks or 12 months after blast exposure, relatively few differentially expressed genes (DEGs) were found. However, longitudinal analysis of amygdala, hippocampus, and anterior cortex between 6 weeks and 12 months revealed blast-specific DEG patterns. Six DEGs (hyaluronan and proteoglycan link protein 1 [Hapln1], glutamate metabotropic receptor 2 [Grm2], purinergic receptor P2y12 [P2ry12], C-C chemokine receptor type 5 [Ccr5], phenazine biosynthesis-like protein domain containing 1 [Pbld1], and cadherin related 23 [Cdh23]) were found altered in all three brain regions in blast-exposed animals. Pathway enrichment analysis using all DEGs or those uniquely changed revealed different transcription patterns in blast versus sham. In particular, the amygdala in blast-exposed animals had a unique set of enriched pathways related to stress responses, oxidative phosphorylation, and mitochondrial dysfunction. Upstream analysis implicated tumor necrosis factor (TNF)α signaling in blast-related effects in amygdala and anterior cortex. Eukaryotic initiating factor eIF4E (EIF4e), an upstream regulator of P2ry12 and Ccr5, was predicted to be activated in the amygdala. Quantitative polymerase chain reaction (qPCR) validated longitudinal changes in two TNFα regulated genes (cathepsin B [Ctsb], Hapln1), P2ry12, and Grm2. These studies have implications for understanding how blast injury damages the brain and implicates inflammation as a potential therapeutic target.

Elevations in Tumor Necrosis Factor Alpha and Interleukin 6 From Neuronal-Derived Extracellular Vesicles in Repeated Low-Level Blast Exposed Personnel

Objective

The purpose of this pilot study was to determine if military service members with histories of hundreds to thousands of low-level blast exposures (i. e., experienced breachers) had different levels of serum and neuronal-derived extracellular vesicle (EV) concentrations of interleukin (IL)-6, IL-10, and tumor necrosis factor alpha (TNFα), compared to matched controls, and if these biomarkers related to neurobehavioral symptoms.

Methods

Participants were experienced breachers (n = 20) and matched controls without blast exposures (n = 14). Neuronal-derived EVs were isolated from serum and identified with mouse anti-human CD171. Serum and neuronal-derived EVs were analyzed for IL-6, IL-10, and TNFα using an ultra-sensitive assay.

Results

Serum TNFα concentrations were decreased in breachers when compared to control concentrations (p < 0.01). There were no differences in serum concentrations of IL-6, IL-10, or the IL-6/IL-10 ratio between breachers and controls (p's > 0.01). In neuronal-derived EVs, TNFα and IL-6 levels were increased in breachers compared to controls (p's < 0.01), and IL-10 levels were decreased in the breacher group compared to controls (p < 0.01). In breachers the IL-6/IL-10 ratio in neuronal-derived EVs was higher compared to controls, which correlated with higher total Rivermead Post-concussion Questionnaire (RPQ) scores (p's < 0.05).

Conclusions

These findings suggest that exposure of personnel to high numbers of low-level blast over a career may result in enduring central inflammation that is associated with chronic neurological symptoms. The data also suggest that peripheral markers of inflammation are not necessarily adequate surrogates for central neuroinflammation.

Impact of ketogenic diet and ketone diester supplementation on body weight, blood glucose, and ketones in Sprague Dawley rats fed over two weeks

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Ketogenic diet influenced ketones, weight, and glucose in rats.

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Ketone diester supplement (20% by weight) was similar to ketogenic diet, but its effects on ketones and weight was smaller and it did not affect glucose.

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Changeover from standard diet to ketogenic diet resulted in sex-specific glucose changes.

Ketogenic diets consist of low carbohydrate/high fat, shifting energy reliance from glucose to ketone bodies. Ketone diester supplement to a standard diet (ketone ester) increases ketone bodies by adding a substance without altering other consumed foods. We evaluated weight, glucose, and ketone concentrations in rats fed ketogenic diet and ketone ester feeds. We hypothesized that these feeds would increase ketones and decrease glucose and weight. We tested 16 male and 16 female Sprague Dawley rats randomly assigned to standard diet, ketogenic diet, or ketone ester for two weeks. Weight and blood glucose and ketones were measured daily. Group means were compared by analysis of variance. Ketogenic diet and ketone ester both increased ketones and decreased weight compared to standard diet (p < 0.001). Glucose decreased only in ketogenic diet (p = 0.010), driven by a decrease from higher starting concentrations observed in standard diet males. Sex interacted with weight, with male gains impacted more by both ketogenic diet and ketone ester than female gains. Ketogenic diet had a larger effect size than ketone ester with regard to increased ketones and decreased weight. Ketogenic diet glucose significantly decreased over time because standard diet concentrations in males were high prior to initializing ketogenic diet. This suggests sex differences in energy substrate utilization. Ketogenic diet ketones peaked at 72 h then decreased to near basal levels at about 10 days, suggesting “fat adaption.” While this work is part of a larger project examining blast exposure, these results are relevant to any military forces considering ketone-increasing foods.

Transcranial Laser Therapy Does Not Improve Cognitive and Post-Traumatic Stress Disorder-Related Behavioral Traits in Rats Exposed to Repetitive Low-Level Blast Injury

Many military veterans who experienced blast-related traumatic brain injuries (TBIs) in the conflicts in Iraq and Afghanistan suffer from chronic cognitive and mental health problems, including post-traumatic stress disorder (PTSD). Transcranial laser therapy (TLT) uses low-power lasers emitting light in the far- to near-infrared ranges. Beneficial effects of TLT have been reported in neurological and mental-health-related disorders in humans and animal models, including TBI. Rats exposed to repetitive low-level blast develop chronic cognitive and PTSD-related behavioral traits. We tested whether TLT treatment could reverse these traits. Rats received a 74.5-kPa blast or sham exposures delivered one per day for 3 consecutive days. Beginning at 34 weeks after blast exposure, the following groups of rats were treated with active or sham TLT: 1) Sham-exposed rats (n = 12) were treated with sham TLT; 2) blast-exposed rats (n = 13) were treated with sham TLT; and 3) blast-exposed rats (n = 14) were treated with active TLT. Rats received 5 min of TLT five times per week for 6 weeks (wavelength, 808 nm; power of irradiance, 240 mW). At the end of treatment, rats were tested in tasks found previously to be most informative (novel object recognition, novel object localization, contextual/cued fear conditioning, elevated zero maze, and light/dark emergence). TLT did not improve blast-related effects in any of these tests, and blast-exposed rats were worse after TLT in some anxiety-related measures. Based on these findings, TLT does not appear to be a promising treatment for the chronic cognitive and mental health problems that follow blast injury.

Chronic Effects of Breaching Blast Exposure on Sensory Organization and Postural Limits of Stability

OBJECTIVE

The goal of this effort to investigate if experienced breachers, professionals with a career history of exposure to repeated low-level blasts, exhibited postural instability.

METHODS

Postural data were examined using traditional tests of means and compared to normative data.

RESULTS

Breachers had significantly lower NeuroCom Sensory Organization Test (SOT) visual scores (within normative limits), prolonged Limits of Stability (LOS) test reaction time (30% of breachers and 7% of controls testing abnormal), and slower LOS movement velocity (21% of breachers and 0% of controls testing abnormal) compared to controls.

CONCLUSION

Our LOS test findings are like those previously reported for students in the military breacher training course and seem to indicate that while acute effects of blasts on sensory control of balance fade away, effects on postural LOS persist over time.

Low-level blast exposure induces chronic vascular remodeling, perivascular astrocytic degeneration and vascular-associated neuroinflammation

Cerebral vascular injury as a consequence of blast-induced traumatic brain injury is primarily the result of blast wave-induced mechanical disruptions within the neurovascular unit. In rodent models of blast-induced traumatic brain injury, chronic vascular degenerative processes are associated with the development of an age-dependent post-traumatic stress disorder-like phenotype. To investigate the evolution of blast-induced chronic vascular degenerative changes, Long-Evans rats were blast-exposed (3 × 74.5 kPa) and their brains analyzed at different times post-exposure by X-ray microcomputed tomography, immunohistochemistry and electron microscopy. On microcomputed tomography scans, regional cerebral vascular attenuation or occlusion was observed as early as 48 h post-blast, and cerebral vascular disorganization was visible at 6 weeks and more accentuated at 13 months post-blast. Progression of the late-onset pathology was characterized by detachment of the endothelial and smooth muscle cellular elements from the neuropil due to degeneration and loss of arteriolar perivascular astrocytes. Development of this pathology was associated with vascular remodeling and neuroinflammation as increased levels of matrix metalloproteinases (MMP-2 and MMP-9), collagen type IV loss, and microglial activation were observed in the affected vasculature. Blast-induced chronic alterations within the neurovascular unit should affect cerebral blood circulation, glymphatic flow and intramural periarterial drainage, all of which may contribute to development of the blast-induced behavioral phenotype. Our results also identify astrocytic degeneration as a potential target for the development of therapies to treat blast-induced brain injury.

Neuronally-derived tau is increased in experienced breachers and is associated with neurobehavioral symptoms

Military and law enforcement breachers are exposed to many low-level blasts during their training and occupational experiences in which they detonate explosives to force entry into secured structures. There is a concern that exposure to these repetitive blast events in career breachers could result in cumulative neurological effects. This study aimed to determine concentrations of neurofilament light (NF-L), tau, and amyloid-beta 42 (Aβ42) in serum and in neuronal-derived extracellular vesicles (EVs) in an experienced breacher population, and to examine biomarker associations with neurobehavioral symptoms. Thirty-four participants enrolled in the study: 20 experienced breachers and 14 matched military or civilian law enforcement controls. EV tau concentrations were significantly elevated in experienced breachers (0.3301 ± 0.5225) compared to controls (-0.4279 ± 0.7557; F = 10.43, p = 0.003). No statistically significant changes were observed in EV levels of NF-L or Aβ42 or in serum levels of NF-L, tau, or Aβ42 (p's > 0.05). Elevated EV tau concentrations correlated with increased Neurobehavioral Symptom Inventory (NSI) score in experienced breachers (r = 0.596, p = 0.015) and predicted higher NSI score (F(1,14) = 7.702, p = 0.015, R² = 0.355). These findings show that neuronal-derived EV concentrations of tau are significantly elevated and associated with neurobehavioral symptoms in this sample of experienced breachers who have a history of many low-level blast exposures.

Roadmap for Advancing Pre-Clinical Science in Traumatic Brain Injury

Pre-clinical models of disease have long played important roles in the advancement of new treatments. However, in traumatic brain injury (TBI), despite the availability of numerous model systems, translation from bench to bedside remains elusive. Integrating clinical relevance into pre-clinical model development is a critical step toward advancing therapies for TBI patients across the spectrum of injury severity. Pre-clinical models include in vivo and ex vivo animal work-both small and large-and in vitro modeling. The wide range of pre-clinical models reflect substantial attempts to replicate multiple aspects of TBI sequelae in humans. Although these models reveal multiple putative mechanisms underlying TBI pathophysiology, failures to translate these findings into successful clinical trials call into question the clinical relevance and applicability of the models. Here, we address the promises and pitfalls of pre-clinical models with the goal of evolving frameworks that will advance translational TBI research across models, injury types, and the heterogenous etiology of pathology.

Blast-induced temporal alterations in blood-brain barrier properties in a rodent model

The consequences of blast-induced traumatic brain injury (bTBI) on the blood–brain barrier (BBB) and components of the neurovascular unit are an area of active research. In this study we assessed the time course of BBB integrity in anesthetized rats exposed to a single blast overpressure of 130 kPa (18.9 PSI). BBB permeability was measured in vivo via intravital microscopy by imaging extravasation of fluorescently labeled tracers (40 kDa and 70 kDa molecular weight) through the pial microvasculature into brain parenchyma at 2–3 h, 1, 3, 14, or 28 days after the blast exposure. BBB structural changes were assessed by immunostaining and molecular assays. At 2–3 h and 1 day after blast exposure, significant increases in the extravasation of the 40 kDa but not the 70 kDa tracers were observed, along with differential reductions in the expression of tight junction proteins (occludin, claudin-5, zona occluden-1) and increase in the levels of the astrocytic water channel protein, AQP-4, and matrix metalloprotease, MMP-9. Nearly all of these measures were normalized by day 3 and maintained up to 28 days post exposure. These data demonstrate that blast-induced changes in BBB permeability are closely coupled to structural and functional components of the BBB.

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.

Repeated Low-Level Blast Acutely Alters Brain Cytokines, Neurovascular Proteins, Mechanotransduction, and Neurodegenerative Markers in a Rat Model

Exposure to the repeated low-level blast overpressure (BOP) periodically experienced by military personnel in operational and training environments can lead to deficits in behavior and cognition. While these low-intensity blasts do not cause overt changes acutely, repeated exposures may lead to cumulative effects in the brain that include acute inflammation, vascular disruption, and other molecular changes, which may eventually contribute to neurodegenerative processes. To identify these acute changes in the brain following repeated BOP, an advanced blast simulator was used to expose rats to 8.5 or 10 psi BOP once per day for 14 days. At 24 h after the final BOP, brain tissue was collected and analyzed for inflammatory markers, astrogliosis (GFAP), tight junction proteins (claudin-5 and occludin), and neurodegeneration-related proteins (Aβ40/42, pTau, TDP-43). After repeated exposure to 8.5 psi BOP, the change in cytokine profile was relatively modest compared to the changes observed following 10 psi BOP, which included a significant reduction in several inflammatory markers. Reduction in the tight junction protein occludin was observed in both groups when compared to controls, suggesting cerebrovascular disruption. While repeated exposure to 8.5 psi BOP led to a reduction in the Alzheimer’s disease (AD)-related proteins amyloid-β (Aβ)40 and Aβ42, these changes were not observed in the 10 psi group, which had a significant reduction in phosphorylated tau. Finally, repeated 10 psi BOP exposures led to an increase in GFAP, indicating alterations in astrocytes, and an increase in the mechanosensitive ion channel receptor protein, Piezo2, which may increase brain sensitivity to injury from pressure changes from BOP exposure. Overall, cumulative effects of repeated low-level BOP may increase the vulnerability to injury of the brain by disrupting neurovascular architecture, which may lead to downstream deleterious effects on behavior and cognition.

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.

Development of a Blast Exposure Estimator from a Department of Defense-Wide Survey Study on Military Service Members

Long-term, repeated exposure to low-intensity blast overpressure is a potential causal factor of lasting outcomes reminiscent of post-concussion syndrome. Wearable blast sensor engineers are exploring elements of blast that are associated with outcomes. Currently, however, there are no devices that can truly record all blasts experienced by an individual. Military service members (n = 984) were surveyed about their lifelong exposure and behavioral health. Using heavy-arms-associated target outcomes, we calculated a generalized blast exposure value (GBEV) for each participant. A threshold of 200,000 GBEV units was established at which a participant was likely to report more intense symptomology. If repetitive, low-intensity blast exposure has even a subtle effect over time, operational readiness could be negatively impacted. A threshold of exposure can inform decisions about how to reduce detrimental exposure. The GBEV can be used to track ongoing exposure and potentially identify those who may be at risk for developing blast-related outcomes.

Hearing Loss and Irritability Reporting Without Vestibular Differences in Explosive Breaching Professionals

Background: Blast exposure is a potential hazard in modern military operations and training, especially for some military occupations. Helmets, peripheral armor, hearing protection, and eye protection worn by military personnel provide some acute protection from blast effects but may not fully protect personnel against cumulative effects of repeated blast overpressure waves experienced over a career. The current study aimed to characterize the long-term outcomes of repeated exposure to primary blast overpressure in experienced career operators with an emphasis on the assessment of hearing and vestibular outcomes.

Methods: Participants included experienced “breachers” (military and law enforcement explosives professionals who gain entry into structures through controlled detonation of charges) and similarly aged and experienced “non-breachers” (non-breaching military and law enforcement personnel). Responses to a clinical interview and performance on audiological and vestibular testing were compared.

Results: Hearing loss, ringing in the ears, irritability, and sensitivity to light or noise were more common among breachers than non-breachers. Breachers reported more combat exposure than non-breachers, and subsequently, memory loss and difficulty concentrating were associated with both breaching and combat exposure. Vestibular and ocular motor outcomes were not different between breachers and non-breachers.

Conclusion: Hearing-related, irritability, and sensitivity outcomes are associated with a career in breaching. Future studies examining long-term effects of blast exposure should take measures to control for combat exposure.

Pulmonary injury risk curves and behavioral changes from blast overpressure exposures of varying frequency and intensity in rats

At present, there are no set guidelines establishing cumulative limits for blast exposure numbers and intensities in military personnel, in combat or training operations. The objective of the current study was to define lung injury, pathology, and associated behavioral changes from primary repeated blast lung injury under appropriate exposure conditions and combinations (i.e. blast overpressure (BOP) intensity and exposure frequency) using an advanced blast simulator. Male Sprague Dawley rats were exposed to BOP frontally and laterally at a pressure range of ~ 8.5-19 psi, for up to 30 daily exposures. The extent of lung injury was identified at 24 h following BOP by assessing the extent of surface hemorrhage/contusion, Hematoxylin and Eosin staining, and behavioral deficits with open field activity. Lung injury was mathematically modeled to define the military standard 1% lung injury threshold. Significant levels of histiocytosis and inflammation were observed in pressures ≥ 10 psi and orientation effects were observed at pressures ≥ 13 psi. Experimental data demonstrated ~ 8.5 psi is the threshold for hemorrhage/contusion, up to 30 exposures. Modeling the data predicted injury risk up to 50 exposures with intensity thresholds at 8 psi for front exposure and 6psi for side exposures, which needs to be validated further.

Blast-Related Mild TBI Alters Anxiety-Like Behavior and Transcriptional Signatures in the Rat Amygdala

The short and long-term neurological and psychological consequences of traumatic brain injury (TBI), and especially mild TBI (mTBI) are of immense interest to the Veteran community. mTBI is a common and detrimental result of combat exposure and results in various deleterious outcomes, including mood and anxiety disorders, cognitive deficits, and post-traumatic stress disorder (PTSD). In the current study, we aimed to further define the behavioral and molecular effects of blast-related mTBI using a well-established (3 × 75 kPa, one per day on three consecutive days) repeated blast overpressure (rBOP) model in rats. We exposed adult male rats to the rBOP procedure and conducted behavioral tests for anxiety and fear conditioning at 1-1.5 months (sub-acute) or 12-13 months (chronic) following blast exposure. We also used next-generation sequencing to measure transcriptome-wide gene expression in the amygdala of sham and blast-exposed animals at the sub-acute and chronic time points. Results showed that blast-exposed animals exhibited an anxiety-like phenotype at the sub-acute timepoint but this phenotype was diminished by the chronic time point. Conversely, gene expression analysis at both sub-acute and chronic timepoints demonstrated a large treatment by timepoint interaction such that the most differentially expressed genes were present in the blast-exposed animals at the chronic time point, which also corresponded to a Bdnf-centric gene network. Overall, the current study identified changes in the amygdalar transcriptome and anxiety-related phenotypic outcomes dependent on both blast exposure and aging, which may play a role in the long-term pathological consequences of mTBI.

Functional and Structural Neuroimaging Correlates of Repetitive Low-Level Blast Exposure in Career Breachers

Combat military and civilian law enforcement personnel may be exposed to repetitive low-intensity blast events during training and operations. Persons who use explosives to gain entry (i.e., breach) into buildings are known as “breachers” or dynamic entry personnel. Breachers operate under the guidance of established safety protocols, but despite these precautions, breachers who are exposed to low-level blast throughout their careers frequently report performance deficits and symptoms to healthcare providers. Although little is known about the etiology linking blast exposure to clinical symptoms in humans, animal studies demonstrate network-level changes in brain function, alterations in brain morphology, vascular and inflammatory changes, hearing loss, and even alterations in gene expression after repeated blast exposure. To explore whether similar effects occur in humans, we collected a comprehensive data battery from 20 experienced breachers exposed to blast throughout their careers and 14 military and law enforcement controls. This battery included neuropsychological assessments, blood biomarkers, and magnetic resonance imaging measures, including cortical thickness, diffusion tensor imaging of white matter, functional connectivity, and perfusion. To better understand the relationship between repetitive low-level blast exposure and behavioral and imaging differences in humans, we analyzed the data using similarity-driven multi-view linear reconstruction (SiMLR). SiMLR is specifically designed for multiple modality statistical integration using dimensionality-reduction techniques for studies with high-dimensional, yet sparse, data (i.e., low number of subjects and many data per subject). We identify significant group effects in these data spanning brain structure, function, and blood biomarkers.

Assessing a Blast-Related Biomarker in an Operational Community: Glial Fibrillary Acidic Protein in Experienced Breachers

Mild traumatic brain injury (mTBI) is a risk for military personnel due to blast overpressures, which may result from a variety of sources, including artillery and improvised explosive devices. Much research has gone into the search for a biomarker to identify patients with a TBI. The FDA recently identified two proteins, glial fibrillary acidic protein (GFAP) and ubiquitin C-terminal hydrolase-L1 (UCH-L1), as biomarkers to evaluate suspected brain injury. Our group previously observed changes in UCH-L1 in a military population exposed to repeated blast. In our current study we assessed GFAP protein levels in a military population exposed to repeated blast during a 2-week training protocol. We observed GFAP levels were reduced in the moderate blast cases on days 6 and 7 during the training. Specifically, moderate blast cases showed a 24.07% reduction from baseline on day 6 and a 29.61% reduction on day 7. Further, GFAP levels were negatively correlated with cumulative blast experienced during training and with duration of military service. We observed that repeated blast exposure at low levels may impact acute changes in GFAP. Additionally subacute cumulative blast exposure or duration of service was also a factor in influencing GFAP levels.

Blast exposure interacts with genetic variant 5HTTLPR to predict posttraumatic stress symptoms in military explosives personnel

The first of its kind, this study determined whether blast exposure interacts with genetic variant 5HTTLPR to predict posttraumatic stress (PTS) symptoms in 78 military explosives operators. In all models, blast-exposed 5HTTLPR S carriers registered definitively higher PTS symptoms in comparison to non-exposed S carriers, as well as exposed and non-exposed LL carriers (all p < 0.01). All findings were robust to confounding influences of age and traumatic brain injury diagnosis. Not only is blast exposure prevalent in EOD personnel, but it also interacts with genetic predisposition to predict trauma symptoms in this unique, at-risk military population.

Exposure to Blast Overpressure Impairs Cerebral Microvascular Responses and Alters Vascular and Astrocytic Structure

Exposure to blast overpressure may result in cerebrovascular impairment, including cerebral vasospasm. The mechanisms contributing to this vascular response are unclear. The aim of this study was to evaluate the relationship between blast and functional alterations of the cerebral microcirculation and to investigate potential underlying changes in vascular microstructure. Cerebrovascular responses were assessed in sham- and blast-exposed male rats at multiple time points from 2 h through 28 days after a single 130-kPa (18.9-psi) exposure. Pial microcirculation was assessed through a cranial window created in the parietal bone of anesthetized rats. Pial arteriolar reactivity was evaluated in vivo using hypercapnia, barium chloride, and serotonin. We found that exposure to blast leads to impairment of arteriolar reactivity >24 h after blast exposure, suggesting delayed injury mechanisms that are not simply attributed to direct mechanical deformation. Observed vascular impairment included a reduction in hypercapnia-induced vasodilation, increase in barium-induced constriction, and reversal of the serotonin effect from constriction to dilation. A reduction in vascular smooth muscle contractile proteins consistent with vascular wall proliferation was observed, as well as delayed reduction in nitric oxide synthase and increase in endothelin-1 B receptors, mainly in astrocytes. Collectively, the data show that exposure to blast results in delayed and prolonged alterations in cerebrovascular reactivity that are associated with changes in the microarchitecture of the vessel wall and astrocytes. These changes may contribute to long-term pathologies involving dysfunction of the neurovascular unit, including cerebral vasospasm.

Repeated Low-Level Blast Overpressure Leads to Endovascular Disruption and Alterations in TDP-43 and Piezo2 in a Rat Model of Blast TBI

Recent evidence linking repeated low-level blast overpressure exposure in operational and training environments with neurocognitive decline, neuroinflammation, and neurodegenerative processes has prompted concern over the cumulative deleterious effects of repeated blast exposure on the brains of service members. Repetitive exposure to low-level primary blast may cause symptoms (subclinical) similar to those seen in mild traumatic brain injury (TBI), with progressive vascular and cellular changes, which could contribute to neurodegeneration. At the cellular level, the mechanical force associated with blast exposure can cause cellular perturbations in the brain, leading to secondary injury. To examine the cumulative effects of repetitive blast on the brain, an advanced blast simulator (ABS) was used to closely mimic “free-field” blast. Rats were exposed to 1–4 daily blasts (one blast per day, separated by 24 h) at 13, 16, or 19 psi peak incident pressures with a positive duration of 4–5 ms, either in a transverse or longitudinal orientation. Blood-brain barrier (BBB) markers (vascular endothelial growth factor (VEGF), occludin, and claudin-5), transactive response DNA binding protein (TDP-43), and the mechanosensitive channel Piezo2 were measured following blast exposure. Changes in expression of VEGF, occludin, and claudin-5 after repeated blast exposure indicate alterations in the BBB, which has been shown to be disrupted following TBI. TDP-43 is very tightly regulated in the brain and altered expression of TDP-43 is found in clinically-diagnosed TBI patients. TDP-43 levels were differentially affected by the number and magnitude of blast exposures, decreasing after 2 exposures, but increasing following a greater number of exposures at various intensities. Lastly, Piezo2 has been shown to be dysregulated following blast exposure and was here observed to increase after multiple blasts of moderate magnitude, indicating that blast may cause a change in sensitivity to mechanical stimuli in the brain and may contribute to cellular injury. These findings reveal that cumulative effects of repeated exposures to blast can lead to pathophysiological changes in the brain, demonstrating a possible link between blast injury and neurodegenerative disease, which is an important first step in understanding how to prevent these diseases in soldiers exposed to blast.

N-acetylcysteine Amide Ameliorates Blast-Induced Changes in Blood-Brain Barrier Integrity in Rats

Traumatic brain injury resulting from exposure to blast overpressure (BOP) is associated with neuropathology including impairment of the blood-brain barrier (BBB). This study examined the effects of repeated exposure to primary BOP and post-blast treatment with an antioxidant, N-acetylcysteine amide (NACA) on the integrity of BBB. Anesthetized rats were exposed to three 110 kPa BOPs separated by 0.5 h. BBB integrity was examined in vivo via a cranial window allowing imaging of pial microcirculation by intravital microscopy. Tetramethylrhodamine isothiocyanate Dextran (TRITC-Dextran, mw = 40 kDa or 150 kDa) was injected intravenously 2.5 h after the first BOP exposure and the leakage of TRITC-Dextran from pial microvessels into the brain parenchyma was assessed. The animals were randomized into 6 groups (n = 5/group): four groups received 40 kDa TRITC-Dextran (BOP-40, sham-40, BOP-40 NACA, and sham-40 NACA), and two groups received 150 kDa TRITC-Dextran (BOP-150 and sham-150). NACA treated groups were administered NACA 2 h after the first BOP exposure. The rate of TRITC-Dextran leakage was significantly higher in BOP-40 than in sham-40 group. NACA treatment significantly reduced TRITC-Dextran leakage in BOP-40 NACA group and sham-40 NACA group presented the least amount of leakage. The rate of leakage in BOP-150 and sham-150 groups was comparable to sham-40 NACA and thus these groups were not assessed for the effects of NACA. Collectively, these data suggest that BBB integrity is compromised following BOP exposure and that NACA treatment at a single dose may significantly protect against blast-induced BBB breakdown.

Comprehensive Characterization of Cerebrovascular Dysfunction in Blast Traumatic Brain Injury Using Photoacoustic Microscopy

Blast traumatic brain injury (bTBI) is a leading contributor to combat-related injuries and death. Although substantial emphasis has been placed on blast-induced neuronal and axonal injuries, co-existing dysfunctions in the cerebral vasculature, particularly the microvasculature, remain poorly understood. Here, we studied blast-induced cerebrovascular dysfunctions in a rat model of bTBI (blast overpressure: 187.8 ± 18.3 kPa). Using photoacoustic microscopy (PAM), we quantified changes in cerebral hemodynamics and metabolism-including blood perfusion, oxygenation, flow, oxygen extraction fraction, and the metabolic rate of oxygen-4 h post-injury. Moreover, we assessed the effect of blast exposure on cerebrovascular reactivity (CVR) to vasodilatory stimulation. With vessel segmentation, we extracted these changes at the single-vessel level, revealing their dependence on vessel type (i.e., artery vs. vein) and diameter. We found that bTBI at this pressure level did not induce pronounced baseline changes in cerebrovascular diameter, blood perfusion, oxygenation, flow, oxygen extraction, and metabolism, except for a slight sO₂ increase in small veins (<45 μm) and blood flow increase in large veins (≥45 μm). In contrast, this blast exposure almost abolished CVR, including arterial dilation, flow upregulation, and venous sO₂ increase. This study is the most comprehensive assessment of cerebrovascular structure and physiology in response to blast exposure to date. The observed impairment in CVR can potentially cause cognitive decline due to the mismatch between cognitive metabolic demands and vessel's ability to dynamically respond to meet the demands. Also, the impaired CVR can lead to increased vulnerability of the brain to metabolic insults, including hypoxia and ischemia.

Low-level blast exposure disrupts gliovascular and neurovascular connections and induces a chronic vascular pathology in rat brain

Much concern exists over the role of blast-induced traumatic brain injury (TBI) in the chronic cognitive and mental health problems that develop in veterans and active duty military personnel. The brain vasculature is particularly sensitive to blast injury. The aim of this study was to characterize the evolving molecular and histologic alterations in the neurovascular unit induced by three repetitive low-energy blast exposures (3 × 74.5 kPa) in a rat model mimicking human mild TBI or subclinical blast exposure. High-resolution two-dimensional differential gel electrophoresis (2D-DIGE) and matrix-assisted laser desorption/ionization (MALDI) mass spectrometry of purified brain vascular fractions from blast-exposed animals 6 weeks post-exposure showed decreased levels of vascular-associated glial fibrillary acidic protein (GFAP) and several neuronal intermediate filament proteins (α-internexin and the low, middle, and high molecular weight neurofilament subunits). Loss of these proteins suggested that blast exposure disrupts gliovascular and neurovascular interactions. Electron microscopy confirmed blast-induced effects on perivascular astrocytes including swelling and degeneration of astrocytic endfeet in the brain cortical vasculature. Because the astrocyte is a major sensor of neuronal activity and regulator of cerebral blood flow, structural disruption of gliovascular integrity within the neurovascular unit should impair cerebral autoregulation. Disrupted neurovascular connections to pial and parenchymal blood vessels might also affect brain circulation. Blast exposures also induced structural and functional alterations in the arterial smooth muscle layer. Interestingly, by 8 months after blast exposure, GFAP and neuronal intermediate filament expression had recovered to control levels in isolated brain vascular fractions. However, despite this recovery, a widespread vascular pathology was still apparent at 10 months after blast exposure histologically and on micro-computed tomography scanning. Thus, low-level blast exposure disrupts gliovascular and neurovascular connections while inducing a chronic vascular pathology.

Blast-induced "PTSD": Evidence from an animal model

A striking observation among veterans returning from the recent conflicts in Iraq and Afghanistan has been the co-occurrence of blast-related mild traumatic brain injury (mTBI) and post-traumatic stress disorder (PTSD). PTSD and mTBI might coexist due to additive effects of independent psychological and physical traumas experienced in a war zone. Alternatively blast injury might induce PTSD-related traits or damage brain structures that mediate responses to psychological stressors, increasing the likelihood that PTSD will develop following a subsequent psychological stressor. Rats exposed to repetitive low-level blasts consisting of three 74.5 kPa exposures delivered once daily for three consecutive days develop a variety of anxiety and PTSD-related behavioral traits that are present for at least 9 months after blast exposure. A single predator scent challenge delivered 8 months after the last blast exposure induces additional anxiety-related changes that are still present 45 days later. Because the blast injuries occur under general anesthesia, it appears that blast exposure in the absence of a psychological stressor can induce chronic PTSD-related traits. The reaction to a predator scent challenge delivered many months after blast exposure suggests that blast exposure in addition sensitizes the brain to react abnormally to subsequent psychological stressors. The development of PTSD-related behavioral traits in the absence of a psychological stressor suggests the existence of blast-induced "PTSD". Findings that PTSD-related behavioral traits can be reversed by BCI-838, a group II metabotropic glutamate receptor antagonist offers insight into pathogenesis and possible treatment options for blast-related brain injury. This article is part of the Special Issue entitled "Novel Treatments for Traumatic Brain Injury".

Supervised learning technique for the automated identification of white matter hyperintensities in traumatic brain injury

BACKGROUND

White matter hyperintensities (WMHs) are foci of abnormal signal intensity in white matter regions seen with magnetic resonance imaging (MRI). WMHs are associated with normal ageing and have shown prognostic value in neurological conditions such as traumatic brain injury (TBI). The impracticality of manually quantifying these lesions limits their clinical utility and motivates the utilization of machine learning techniques for automated segmentation workflows.

METHODS

This study develops a concatenated random forest framework with image features for segmenting WMHs in a TBI cohort. The framework is built upon the Advanced Normalization Tools (ANTs) and ANTsR toolkits. MR (3D FLAIR, T2- and T1-weighted) images from 24 service members and veterans scanned in the Chronic Effects of Neurotrauma Consortium's (CENC) observational study were acquired. Manual annotations were employed for both training and evaluation using a leave-one-out strategy. Performance measures include sensitivity, positive predictive value, [Formula: see text] score and relative volume difference.

RESULTS

Final average results were: sensitivity = 0.68 ± 0.38, positive predictive value = 0.51 ± 0.40, [Formula: see text] = 0.52 ± 0.36, relative volume difference = 43 ± 26%. In addition, three lesion size ranges are selected to illustrate the variation in performance with lesion size.

CONCLUSION

Paired with correlative outcome data, supervised learning methods may allow for identification of imaging features predictive of diagnosis and prognosis in individual TBI patients.

Lack of chronic neuroinflammation in the absence of focal hemorrhage in a rat model of low-energy blast-induced TBI

Blast-related traumatic brain injury (TBI) has been a common cause of injury in the recent conflicts in Iraq and Afghanistan. Blast waves can damage blood vessels, neurons, and glial cells within the brain. Acutely, depending on the blast energy, blast wave duration, and number of exposures, blast waves disrupt the blood-brain barrier, triggering microglial activation and neuroinflammation. Recently, there has been much interest in the role that ongoing neuroinflammation may play in the chronic effects of TBI. Here, we investigated whether chronic neuroinflammation is present in a rat model of repetitive low-energy blast exposure. Six weeks after three 74.5-kPa blast exposures, and in the absence of hemorrhage, no significant alteration in the level of microglia activation was found. At 6 weeks after blast exposure, plasma levels of fractalkine, interleukin-1β, lipopolysaccharide-inducible CXC chemokine, macrophage inflammatory protein 1α, and vascular endothelial growth factor were decreased. However, no differences in cytokine levels were detected between blast-exposed and control rats at 40 weeks. In brain, isolated changes were seen in levels of selected cytokines at 6 weeks following blast exposure, but none of these changes was found in both hemispheres or at 40 weeks after blast exposure. Notably, one animal with a focal hemorrhagic tear showed chronic microglial activation around the lesion 16 weeks post-blast exposure. These findings suggest that focal hemorrhage can trigger chronic focal neuroinflammation following blast-induced TBI, but that in the absence of hemorrhage, chronic neuroinflammation is not a general feature of low-level blast injury.

Repeated Low Intensity Blast Exposure Is Associated with Damaged Endothelial Glycocalyx and Downstream Behavioral Deficits

Current clinical research into mild traumatic brain injury (mTBI) has focused on white matter changes as identified by advanced MRI based imaging techniques. However, perivascular tau accumulation in the brains of individuals diagnosed with mTBI suggests that the vasculature plays a key role in the pathology. This study used a rat model to examine whether the endothelial glycocalyx, a layer of the vasculature responsible for sensing luminal shear forces, is damaged by exposure to repeated low intensity blast, and whether this layer is associated with observed behavioral deficits. The blast exposure used consisted of 12, 40 kPa blast exposures conducted with a minimum of 24 h between blasts. We found that repeated blast exposure reduced glycocalyx length and density in various brain regions indicating damage. This blast exposure paradigm was associated with a mild performance decrement in the Morris water maze (MWM) which assesses learning and memory. Administration of hyaluronidase, an enzyme that binds to and degrades hyaluronan (a major structural component of the glycocalyx) prior to blast exposure reduced the observed behavioral deficits and induced a thickening of the glycocalyx layer. Taken together these findings demonstrate that the endothelial glycocalyx degradation following repeated blast is associated with behavioral decrements which can be prevented by treatment with hyaluronidase.

Exposure to a Predator Scent Induces Chronic Behavioral Changes in Rats Previously Exposed to Low-level Blast: Implications for the Relationship of Blast-Related TBI to PTSD

Blast-related mild traumatic brain injury (mTBI) has been unfortunately common in veterans who served in the recent conflicts in Iraq and Afghanistan. The postconcussion syndrome associated with these mTBIs has frequently appeared in combination with post-traumatic stress disorder (PTSD). The presence of PTSD has complicated diagnosis, since clinically, PTSD and the postconcussion syndrome of mTBI have many overlapping symptoms. In particular, establishing how much of the symptom complex can be attributed to the psychological trauma associated with PTSD in contrast to the physical injury of traumatic brain injury has proven difficult. Indeed, some have suggested that much of what is now being called blast-related postconcussion syndrome is better explained by PTSD. The relationship between the postconcussion syndrome of mTBI and PTSD is complex. Association of the two disorders might be viewed as additive effects of independent psychological and physical traumas suffered in a war zone. However, we previously found that rats exposed to repetitive low-level blast exposure in the absence of a psychological stressor developed a variety of anxiety and PTSD-related behavioral traits that were present months following the last blast exposure. Here, we show that a single predator scent challenge delivered 8 months after the last blast exposure induces chronic anxiety related changes in blast-exposed rats that are still present 45 days later. These observations suggest that in addition to independently inducing PTSD-related traits, blast exposure sensitizes the brain to react abnormally to a subsequent psychological stressor. These studies have implications for conceptualizing the relationship between blast-related mTBI and PTSD and suggest that blast-related mTBI in humans may predispose to the later development of PTSD in reaction to subsequent psychological stressors.

Pre-Clinical Traumatic Brain Injury Common Data Elements: Toward a Common Language Across Laboratories

Traumatic brain injury (TBI) is a major public health issue exacting a substantial personal and economic burden globally. With the advent of "big data" approaches to understanding complex systems, there is the potential to greatly accelerate knowledge about mechanisms of injury and how to detect and modify them to improve patient outcomes. High quality, well-defined data are critical to the success of bioinformatics platforms, and a data dictionary of "common data elements" (CDEs), as well as "unique data elements" has been created for clinical TBI research. There is no data dictionary, however, for preclinical TBI research despite similar opportunities to accelerate knowledge. To address this gap, a committee of experts was tasked with creating a defined set of data elements to further collaboration across laboratories and enable the merging of data for meta-analysis. The CDEs were subdivided into a Core module for data elements relevant to most, if not all, studies, and Injury-Model-Specific modules for non-generalizable data elements. The purpose of this article is to provide both an overview of TBI models and the CDEs pertinent to these models to facilitate a common language for preclinical TBI research.

Characterization of acute stress reaction following an IED blast-related mild traumatic brain injury

PRIMARY OBJECTIVE

To characterize an acute stress reaction (ASR) following an improvised explosive device (IED) blast-related mild traumatic brain injury (mTBI).

RESEARCH DESIGN

Participants were male, US military personnel treated in Afghanistan within 4 days following an IED-related mTBI event (n = 239).

METHODS AND PROCEDURES

Demographics, diagnosis of ASR, injury history and self-reported mTBIs, blast exposures and psychological health histories were recorded.

MAIN OUTCOMES AND RESULTS

In total, 12.5% of patients met ASR criteria. Patients with ASR were significantly younger and junior in rank (p < 0.05). Patients with ASR were more likely to experience the IED-blast while dismounted, report a loss of consciousness (LOC) and higher pain levels (p < 0.05). Adjusting for age and rank, multivariate logistic regression showed an association between mTBI history and ASR (AOR = 1.405; 95% CI = 1.105-1.786, p < 0.01). Adjusting for mechanism of injury (dismounted vs. mounted), LOC and pain, multivariate logistic regression showed an association between mTBI history and ASR (AOR = 1.453; 95% CI = 1.132-1.864, p < 0.01). Prior blast exposure and past psychological health issues were not associated with ASR.

CONCLUSIONS

A history of multiple mTBIs is associated with increased risk of ASR. Future research is warranted.

Neuronal DNA Methylation Profiling of Blast-Related Traumatic Brain Injury

Long-term molecular changes in the brain resulting from blast exposure may be mediated by epigenetic changes, such as deoxyribonucleic acid (DNA) methylation, that regulate gene expression. Aberrant regulation of gene expression is associated with behavioral abnormalities, where DNA methylation bridges environmental signals to sustained changes in gene expression. We assessed DNA methylation changes in the brains of rats exposed to three 74.5 kPa blast overpressure events, conditions that have been associated with long-term anxiogenic manifestations weeks or months following the initial exposures. Rat frontal cortex eight months post-exposure was used for cell sorting of whole brain tissue into neurons and glia. We interrogated DNA methylation profiles in these cells using Expanded Reduced Representation Bisulfite Sequencing. We obtained data for millions of cytosines, showing distinct methylation profiles for neurons and glia and an increase in global methylation in neuronal versus glial cells (p<10(-7)). We detected DNA methylation perturbations in blast overpressure-exposed animals, compared with sham blast controls, within 458 and 379 genes in neurons and glia, respectively. Differentially methylated neuronal genes showed enrichment in cell death and survival and nervous system development and function, including genes involved in transforming growth factor β and nitric oxide signaling. Functional validation via gene expression analysis of 30 differentially methylated neuronal and glial genes showed a 1.2 fold change in gene expression of the serotonin N-acetyltransferase gene (Aanat) in blast animals (p<0.05). These data provide the first genome-based evidence for changes in DNA methylation induced in response to multiple blast overpressure exposures. In particular, increased methylation and decreased gene expression were observed in the Aanat gene, which is involved in converting serotonin to the circadian hormone melatonin and is implicated in sleep disturbance and depression associated with traumatic brain injury.

Vascular and inflammatory factors in the pathophysiology of blast-induced brain injury

Blast-related traumatic brain injury (TBI) has received much recent attention because of its frequency in the conflicts in Iraq and Afghanistan. This renewed interest has led to a rapid expansion of clinical and animal studies related to blast. In humans, high-level blast exposure is associated with a prominent hemorrhagic component. In animal models, blast exerts a variety of effects on the nervous system including vascular and inflammatory effects that can be seen with even low-level blast exposures which produce minimal or no neuronal pathology. Acutely, blast exposure in animals causes prominent vasospasm and decreased cerebral blood flow along with blood-brain barrier breakdown and increased vascular permeability. Besides direct effects on the central nervous system, evidence supports a role for a thoracically mediated effect of blast; whereby, pressure waves transmitted through the systemic circulation damage the brain. Chronically, a vascular pathology has been observed that is associated with alterations of the vascular extracellular matrix. Sustained microglial and astroglial reactions occur after blast exposure. Markers of a central and peripheral inflammatory response are found for sustained periods after blast injury and include elevation of inflammatory cytokines and other inflammatory mediators. At low levels of blast exposure, a microvascular pathology has been observed in the presence of an otherwise normal brain parenchyma, suggesting that the vasculature may be selectively vulnerable to blast injury. Chronic immune activation in brain following vascular injury may lead to neurobehavioral changes in the absence of direct neuronal pathology. Strategies aimed at preventing or reversing vascular damage or modulating the immune response may improve the chronic neuropsychiatric symptoms associated with blast-related TBI.

Effects of low-level blast exposure on the nervous system: is there really a controversy?

High-pressure blast waves can cause extensive CNS injury in human beings. However, in combat settings, such as Iraq and Afghanistan, lower level exposures associated with mild traumatic brain injury (mTBI) or subclinical exposure have been much more common. Yet controversy exists concerning what traits can be attributed to low-level blast, in large part due to the difficulty of distinguishing blast-related mTBI from post-traumatic stress disorder (PTSD). We describe how TBI is defined in human beings and the problems posed in using current definitions to recognize blast-related mTBI. We next consider the problem of applying definitions of human mTBI to animal models, in particular that TBI severity in human beings is defined in relation to alteration of consciousness at the time of injury, which typically cannot be assessed in animals. However, based on outcome assessments, a condition of "low-level" blast exposure can be defined in animals that likely approximates human mTBI or subclinical exposure. We review blast injury modeling in animals noting that inconsistencies in experimental approach have contributed to uncertainty over the effects of low-level blast. Yet, animal studies show that low-level blast pressure waves are transmitted to the brain. In brain, low-level blast exposures cause behavioral, biochemical, pathological, and physiological effects on the nervous system including the induction of PTSD-related behavioral traits in the absence of a psychological stressor. We review the relationship of blast exposure to chronic neurodegenerative diseases noting the paradoxical lowering of Abeta by blast, which along with other observations suggest that blast-related TBI is pathophysiologically distinct from non-blast TBI. Human neuroimaging studies show that blast-related mTBI is associated with a variety of chronic effects that are unlikely to be explained by co-morbid PTSD. We conclude that abundant evidence supports low-level blast as having long-term effects on the nervous system.

Select small nucleolar RNAs in blood components as novel biomarkers for improved identification of comorbid traumatic brain injury and post-traumatic stress disorder in veterans of the conflicts in Afghanistan and Iraq

BACKGROUND

The present study was designed to validate the ability of our recently identified set of small noncoding RNA candidate mild traumatic brain injury (mTBI) biomarkers to diagnose mTBI in the presence or absence of post-traumatic stress disorder (PTSD) comorbidity. Using qPCR, we explored the regulation of the candidate biomarkers in peripheral blood mononuclear cells (PBMC) from 58 veterans.

RESULTS

We confirmed that 4 small nucleolar RNAs (snoRNAs), ACA48, U35, U55, and U83A, are significantly down-regulated in PBMC from veterans with mTBI and PTSD compared to non-TBI, control subjects with PTSD only. We found that the snoRNA biomarkers are able to dissect subjects with comorbid mTBI and PTSD from PTSD subjects without mTBI with 100% sensitivity, 81% accuracy, and 72% specificity. No significant differential expression of snoRNA biomarkers was found in mTBI subjects without comorbid PTSD. However, we found significantly lower U55 contents in subjects with PTSD. We explored the regulation of ACA48 in rodent models of PTSD or blast-induced mTBI to gather proof-of-concept evidence that would connect the regulation of the biomarkers and the development of mTBI or PTSD. We found no change in the regulation of ACA48 in the mTBI rat model. We did, however, find significant down-regulation of ACA48 in the PTSD mouse model 24 hours following psychological trauma exposure. This may reflect a short-term response to trauma exposure, since we found no change in the regulation of ACA48 in veteran PTSD subjects 3.6 years post-deployment.

CONCLUSIONS

Additional application of the 4 snoRNA biomarker to current diagnostic criteria may provide an objective biomarker pattern to help identify veterans with comorbid mTBI and PTSD. Our observations suggest that biological interactions between TBI and PTSD may contribute to the clinical features of veterans with comorbid mTBI and PTSD. Future investigations on mTBI mechanisms or TBI biomarkers should consider their interactions with PTSD.

Chronic traumatic encephalopathy: clinical-biomarker correlations and current concepts in pathogenesis

Background

Chronic traumatic encephalopathy (CTE) is a recently revived term used to describe a neurodegenerative process that occurs as a long term complication of repetitive mild traumatic brain injury (TBI). Corsellis provided one of the classic descriptions of CTE in boxers under the name “dementia pugilistica” (DP). Much recent attention has been drawn to the apparent association of CTE with contact sports (football, soccer, hockey) and with frequent battlefield exposure to blast waves generated by improvised explosive devices (IEDs). Recently, a promising serum biomarker has been identified by measurement of serum levels of the neuronal microtubule associated protein tau. New positron emission tomography (PET) ligands (e.g., [¹⁸ F] T807) that identify brain tauopathy have been successfully deployed for the in vitro and in vivo detection of presumptive tauopathy in the brains of subjects with clinically probable CTE.

Methods

Major academic and lay publications on DP/CTE were reviewed beginning with the 1928 paper describing the initial use of the term CTE by Martland.

Results

The major current concepts in the neurological, psychiatric, neuropsychological, neuroimaging, and body fluid biomarker science of DP/CTE have been summarized. Newer achievements, such as serum tau and [¹⁸ F] T807 tauopathy imaging, are also introduced and their significance has been explained.

Conclusion

Recent advances in the science of DP/CTE hold promise for elucidating a long sought accurate determination of the true prevalence of CTE. This information holds potentially important public health implications for estimating the risk of contact sports in inflicting permanent and/or progressive brain damage on children, adolescents, and adults.