Blood-Brain Barrier Dysfunction after Primary Blast Injury in vitro

The injury progression in acute blast-induced mTBI in rats reflected by DTI and immunohistochemical examination

Diffusion tensor imaging (DTI) has emerged as a promising neuroimaging tool for detecting blast-induced mild traumatic brain injury (bmTBI). However, lack of refined acute-phase monitoring and reliable imaging biomarkers hindered its clinical application in early diagnosis of bmTBI, leading to potential long-term disability of patients. Here, we used DTI in a rat model of bmTBI generated by exposing to single lateral blast waves (151.16 and 349.75 kPa, lasting 47.48 ms) released in a confined bioshock tube (BST-I) to investigate whole-brain DTI changes in the acute-phase of bmTBI at 1, 3, 7 days after injury. Combined assessment of immunohistochemical analysis, transmission electron microscopy (TEM) and behavioral readouts allowed for linking DTI changes to synchronous cellular damages and identifying stable imaging biomarkers. The corpus callosum (CC) and brainstem were identified as predominantly affected regions, in which reduced fractional anisotropy (FA) was detected as early as the first day after injury, with a maximum decline occurring at 3 days after injury before returning to near normal levels by 7 days. Axial diffusivity (AD) values within the CC and brainstem also significantly reduced at 3 days after injury. In contrast, the radial diffusivity (RD) in the CC showed acute elevation, peaking at 3 days after injury before normalizing by the 7-day time point. Damages to nerve fibers, including demyelination and axonal degeneration, progressed in lines with changes in DTI parameters, supporting a real-time macroscopic reflection of microscopic neuronal fiber injury by DTI. The most sensitive biomarker was identified as a decrease in FA, AD and an increase in RD within the CC on the third day after injury, supporting the diagnostic utility of DTI in cases of bmTBI in the acute phase.

Twenty Years of Blast-Induced Neurotrauma: Current State of Knowledge

Blast-induced neurotrauma (BINT) is an important injury paradigm of neurotrauma research. This short communication summarizes the current knowledge of BINT. We divide the BINT research into several broad categories-blast wave generation in laboratory, biomechanics, pathology, behavioral outcomes, repetitive blast in animal models, and clinical and neuroimaging investigations in humans. Publications from 2000 to 2023 in each subdomain were considered. The analysis of the literature has brought out salient aspects. Primary blast waves can be simulated reasonably in a laboratory using carefully designed shock tubes. Various biomechanics-based theories of BINT have been proposed; each of these theories may contribute to BINT by generating a unique biomechanical signature. The injury thresholds for BINT are in the nascent stages. Thresholds for rodents are reasonably established, but such thresholds (guided by primary blast data) are unavailable in humans. Single blast exposure animal studies suggest dose-dependent neuronal pathologies predominantly initiated by blood-brain barrier permeability and oxidative stress. The pathologies were typically reversible, with dose-dependent recovery times. Behavioral changes in animals include anxiety, auditory and recognition memory deficits, and fear conditioning. The repetitive blast exposure manifests similar pathologies in animals, however, at lower blast overpressures. White matter irregularities and cortical volume and thickness alterations have been observed in neuroimaging investigations of military personnel exposed to blast. Behavioral changes in human cohorts include sleep disorders, poor motor skills, cognitive dysfunction, depression, and anxiety. Overall, this article provides a concise synopsis of current understanding, consensus, controversies, and potential future directions.

Sleep, inflammation, and hemodynamics in rodent models of traumatic brain injury

Traumatic brain injury (TBI) can induce dysregulation of sleep. Sleep disturbances include hypersomnia and hyposomnia, sleep fragmentation, difficulty falling asleep, and altered electroencephalograms. TBI results in inflammation and altered hemodynamics, such as changes in blood brain barrier permeability and cerebral blood flow. Both inflammation and altered hemodynamics, which are known sleep regulators, contribute to sleep impairments post-TBI. TBIs are heterogenous in cause and biomechanics, which leads to different molecular and symptomatic outcomes. Animal models of TBI have been developed to model the heterogeneity of TBIs observed in the clinic. This review discusses the intricate relationship between sleep, inflammation, and hemodynamics in pre-clinical rodent models of TBI.

Low-intensity open-field blast exposure effects on neurovascular unit ultrastructure in mice

Mild traumatic brain injury (mTBI) induced by low-intensity blast (LIB) is a serious health problem affecting military service members and veterans. Our previous reports using a single open-field LIB mouse model showed the absence of gross microscopic damage or necrosis in the brain, while transmission electron microscopy (TEM) identified ultrastructural abnormalities of myelin sheaths, mitochondria, and synapses. The neurovascular unit (NVU), an anatomical and functional system with multiple components, is vital for the regulation of cerebral blood flow and cellular interactions. In this study, we delineated ultrastructural abnormalities affecting the NVU in mice with LIB exposure quantitatively and qualitatively. Luminal constrictive irregularities were identified at 7 days post-injury (DPI) followed by dilation at 30 DPI along with degeneration of pericytes. Quantitative proteomic analysis identified significantly altered vasomotor-related proteins at 24 h post-injury. Endothelial cell, basement membrane and astrocyte end-foot swellings, as well as vacuole formations, occurred in LIB-exposed mice, indicating cellular edema. Structural abnormalities of tight junctions and astrocyte end-foot detachment from basement membranes were also noted. These ultrastructural findings demonstrate that LIB induces multiple-component NVU damage. Prevention of NVU damage may aid in identifying therapeutic targets to mitigate the effects of primary brain blast injury.

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.

Chronic effects of blast injury on the microvasculature in a transgenic mouse model of Alzheimer's disease related Aβ amyloidosis

BACKGROUND

Altered cerebrovascular function and accumulation of amyloid-β (Aβ) after traumatic brain injury (TBI) can contribute to chronic neuropathology and increase the risk for Alzheimer's disease (AD). TBI due to a blast-induced shock wave (bTBI) adversely affects the neurovascular unit (NVU) during the acute period after injury. However, the chronic effects of bTBI and Aβ on cellular components of the NVU and capillary network are not well understood.

METHODS

We exposed young adult (age range: 76-106 days) female transgenic (Tg) APP/PS1 mice, a model of AD-like Aβ amyloidosis, and wild type (Wt) mice to a single bTBI (~ 138 kPa or ~ 20 psi) or to a Sham procedure. At 3-months or 12-months survival after exposure, we quantified neocortical Aβ load in Tg mice, and percent contact area between aquaporin-4 (AQP4)-immunoreactive astrocytic end-feet and brain capillaries, numbers of PDGFRβ-immunoreactive pericytes, and capillary densities in both genotypes.

RESULTS

The astroglia AQP4-capillary contact area in the Tg-bTBI group was significantly lower than in the Tg-Sham group at 3-months survival. No significant changes in the AQP4-capillary contact area were observed in the Tg-bTBI group at 12-months survival or in the Wt groups. Capillary density in the Tg-bTBI group at 12-months survival was significantly higher compared to the Tg-Sham control and to the Tg-bTBI 3-months survival group. The Wt-bTBI group had significantly lower capillary density and pericyte numbers at 12-months survival compared to 3-months survival. When pericytes were quantified relative to capillary density, no significant differences were detected among the experimental groups, for both genotypes.

CONCLUSION

In conditions of high brain concentrations of human Aβ, bTBI exposure results in reduced AQP4 expression at the astroglia-microvascular interface, and in chronic capillary proliferation like what has been reported in AD. Long term microvascular changes after bTBI may contribute to the risk for developing chronic neurodegenerative disease later in life.

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.

Xiao-Yao-San reduces blood-brain barrier injury induced by chronic stress in vitro and vivo via glucocorticoid receptor-mediated upregulation of Occludin

Blood-brain barrier (BBB) is a barrier which maintains the material exchange balance of brain microenvironment and could be destroyed by chronic stress (CS). Glucocorticoids (GCs) can mimic the chronic stress induced damage to BBB. GCs induced BBB trauma models in vitro and in vivo to explore the effects of the traditional medicine Xiao-Yao-San (XYS). In this research, we found CS could injure the BBB to change the biochemical index, which could be reversed by XYS in vitro. The abilities of cell proliferation, invasion, and the expression of tight junction related genes (Occludin, Claudin, JAM-1 and ZO-1) were suppressed by CS and the trauma could be reversed by XYS partly. It was showed that GRs interacted with Occludin directly and inhibited Occluding expression. In rats BBB trauma model, the GC content was deceased and BBB permeability was repaired by XYS. The expression of Occludin, Claudin, JAM-1 and ZO-1 were increased in the treatment of XYS. In our research, it shown that XYS affect the content of the GC and GR which interacted with Occludin directly for the first time. In addition, we also found that XYS could reduce BBB injury induced by CS via GR in BBB model in vitro. Therefore, it proves that XYS is a potential BBB repair medicine and may help to elucidate mechanism of brain pathology.

Sleep in the United States Military

The military lifestyle often includes continuous operations whether in training or deployed environments. These stressful environments present unique challenges for service members attempting to achieve consolidated, restorative sleep. The significant mental and physical derangements caused by degraded metabolic, cardiovascular, skeletomuscular, and cognitive health often result from insufficient sleep and/or circadian misalignment. Insufficient sleep and resulting fatigue compromises personal safety, mission success, and even national security. In the long-term, chronic insufficient sleep and circadian rhythm disorders have been associated with other sleep disorders (e.g., insomnia, obstructive sleep apnea, and parasomnias). Other physiologic and psychologic diagnoses such as post-traumatic stress disorder, cardiovascular disease, and dementia have also been associated with chronic, insufficient sleep. Increased co-morbidity and mortality are compounded by traumatic brain injury resulting from blunt trauma, blast exposure, and highly physically demanding tasks under load. We present the current state of science in human and animal models specific to service members during- and post-military career. We focus on mission requirements of night shift work, sustained operations, and rapid re-entrainment to time zones. We then propose targeted pharmacological and non-pharmacological countermeasures to optimize performance that are mission- and symptom-specific. We recognize a critical gap in research involving service members, but provide tailored interventions for military health care providers based on the large body of research in health care and public service workers.

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.

Temporal and Spatial Effects of Blast Overpressure on Blood-Brain Barrier Permeability in Traumatic Brain Injury

Blast-induced traumatic brain injury (bTBI) is a “signature wound” in soldiers during training and in combat and has also become a major cause of morbidity in civilians due to increased insurgency. This work examines the role of blood-brain barrier (BBB) disruption as a result of both primary biomechanical and secondary biochemical injury mechanisms in bTBI. Extravasation of sodium fluorescein (NaF) and Evans blue (EB) tracers were used to demonstrate that compromise of the BBB occurs immediately following shock loading, increases in intensity up to 4 hours and returns back to normal in 24 hours. This BBB compromise occurs in multiple regions of the brain in the anterior-posterior direction of the shock wave, with maximum extravasation seen in the frontal cortex. Compromise of the BBB is confirmed by (a) extravasation of tracers into the brain, (b) quantification of tight-junction proteins (TJPs) in the brain and the blood, and (c) tracking specific blood-borne molecules into the brain and brain-specific proteins into the blood. Taken together, this work demonstrates that the BBB compromise occurs as a part of initial biomechanical loading and is a function of increasing blast overpressures.

Primary Blast Causes Delayed Effects without Cell Death in Shell-Encased Brain Cell Aggregates

Previous work in this laboratory used underwater explosive exposures to isolate the effects of shock-induced principle stress without shear on rat brain aggregate cultures. The current study has utilized simulated air blast to expose aggregates in suspension and enclosed within a spherical shell, enabling the examination of a much more complex biomechanical insult. Culture medium-filled spheres were exposed to single pulse overpressures of 15-30 psi (∼6-7 msec duration) and measurements within the sphere at defined sites showed complex and spatially dependent pressure changes. When brain aggregates were exposed to similar conditions, no cell death was observed and no changes in several commonly used biomarkers of traumatic brain injury (TBI) were noted. However, similarly to underwater blast, immediate and transient increases in the protein kinase B signaling pathway were observed at early time-points (3 days). In contrast, the oligodendrocyte marker 2',3'-cyclic nucleotide 3'-phosphodiesterase, as well as vascular endothelial growth factor, both displayed markedly delayed (14-28 days) and pressure-dependent responses. The imposition of a spherical shell between the single pulse shock wave and the target brain tissue introduces greatly increased complexity to the insult. This work shows that brain tissue can not only discriminate the nature of the pressure changes it experiences, but that a portion of its response is significantly delayed. These results have mechanistic implications for the study of primary blast-induced TBI and also highlight the importance of rigorously characterizing the actual pressure variations experienced by target tissue in primary blast studies.

Phosphodiesterase-4 inhibition restored hippocampal long term potentiation after primary blast

Due to recent military conflicts and terrorist attacks, blast-induced traumatic brain injury (bTBI) presents a health concern for military and civilian personnel alike. Although secondary blast (penetrating injury) and tertiary blast (inertia-driven brain deformation) are known to be injurious, the effects of primary blast caused by the supersonic shock wave interacting with the skull and brain remain debated. Our group previously reported that in vitro primary blast exposure reduced long-term potentiation (LTP), the electrophysiological correlate of learning and memory, in rat organotypic hippocampal slice cultures (OHSCs) and that primary blast affects key proteins governing LTP. Recent studies have investigated phosphodiesterase-4 (PDE4) inhibition as a therapeutic strategy for reducing LTP deficits following inertia-driven TBI. We investigated the therapeutic potential of PDE4 inhibitors, specifically roflumilast, to ameliorate primary blast-induced deficits in LTP. We found that roflumilast at concentrations of 1nM or greater prevented deficits in neuronal plasticity measured 24h post-injury. We also observed a therapeutic window of at least 6h, but <23h. Additionally, we investigated molecular mechanisms that could elucidate this therapeutic effect. Roflumilast treatment (1nM delivered 6h post-injury) significantly increased total AMPA glutamate receptor 1 (GluR1) subunit expression, phosphorylation of the GluR1 subunit at the serine-831 site, and phosphorylation of stargazin at the serine-239/240 site upon LTP induction, measured 24h following injury. Roflumilast treatment significantly increased PSD-95 regardless of LTP induction. These findings indicate that further investigation into the translation of PDE4 inhibition as a therapy following bTBI is warranted.

Glibenclamide pretreatment protects against chronic memory dysfunction and glial activation in rat cranial blast traumatic brain injury

Blast traumatic brain injury (bTBI) affects both military and civilian populations, and often results in chronic deficits in cognition and memory. Chronic glial activation after bTBI has been linked with cognitive decline. Pharmacological inhibition of sulfonylurea receptor 1 (SUR1) with glibenclamide was shown previously to reduce glial activation and improve cognition in contusive models of CNS trauma, but has not been examined in bTBI. We postulated that glibenclamide would reduce chronic glial activation and improve long-term memory function after bTBI. Using a rat direct cranial model of bTBI (dc-bTBI), we evaluated the efficacy of two glibenclamide treatment paradigms: glibenclamide prophylaxis (pre-treatment), and treatment with glibenclamide starting after dc-bTBI (post-treatment). Our results show that dc-bTBI caused hippocampal astrocyte and microglial/macrophage activation that was associated with hippocampal memory dysfunction (rapid place learning paradigm) at 28days, and that glibenclamide pre-treatment, but not post-treatment, effectively protected against glial activation and memory dysfunction. We also report that a brief transient time-window of blood-brain barrier (BBB) disruption occurs after dc-bTBI, and we speculate that glibenclamide, which is mostly protein bound and does not normally traverse the intact BBB, can undergo CNS delivery only during this brief transient opening of the BBB. Together, our findings indicate that prophylactic glibenclamide treatment may help to protect against chronic cognitive sequelae of bTBI in warfighters and other at-risk populations.

Protection against Blast-Induced Traumatic Brain Injury by Increase in Brain Volume

Blast-induced traumatic brain injury (bTBI) is a leading cause of injuries in recent military conflicts and it is responsible for an increased number of civilian casualties by terrorist attacks. bTBI includes a variety of neuropathological changes depending on the intensity of blast overpressure (BOP) such as brain edema, neuronal degeneration, diffuse axonal damage, and vascular dysfunction with neurological manifestations of psychological and cognitive abnormalities. Internal jugular vein (IJV) compression is known to reduce intracranial compliance by causing an increase in brain volume and was shown to reduce brain damage during closed impact-induced TBI. We investigated whether IJV compression can attenuate signs of TBI in rats after exposure to BOP. Animals were exposed to three 110 ± 5 kPa BOPs separated by 30 min intervals. Exposure to BOP resulted in a significant decrease of neuronal nuclei (NeuN) together with upregulation of aquaporin-4 (AQP-4), 3-nitrotyrosine (3-NT), and endothelin 1 receptor A (ETRA) expression in frontal cortex and hippocampus one day following exposures. IJV compression attenuated this BOP-induced increase in 3-NT in cortex and ameliorated the upregulation of AQP-4 in hippocampus. These results suggest that elevated intracranial pressure and intracerebral volume have neuroprotective potential in blast-induced TBI.

Blast Scaling Parameters: Transitioning from Lung to Skull Base Metrics

Neurotrauma from blast exposure is one of the single most characteristic injuries of modern warfare. Understanding blast traumatic brain injury is critical for developing new treatment options for warfighters and civilians exposed to improvised explosive devices. Unfortunately, the pre-clinical models that are widely utilized to investigate blast exposure are based on archaic lung based parameters developed in the early 20th century. Improvised explosive devices produce a different type of injury paradigm than the typical mortar explosion. Protective equipment for the chest cavity has also improved over the past 100 years. In order to improve treatments, it is imperative to develop models that are based more on skull-based parameters. In this mini-review, we discuss the important anatomical and biochemical features necessary to develop a skull-based model.

Acute death of astrocytes in blast-exposed rat organotypic hippocampal slice cultures

Blast traumatic brain injury (bTBI) affects civilians, soldiers, and veterans worldwide and presents significant health concerns. The mechanisms of neurodegeneration following bTBI remain elusive and current therapies are largely ineffective. It is important to better characterize blast-evoked cellular changes and underlying mechanisms in order to develop more effective therapies. In the present study, our group utilized rat organotypic hippocampal slice cultures (OHCs) as an in vitro system to model bTBI. OHCs were exposed to either 138 ± 22 kPa (low) or 273 ± 23 kPa (high) overpressures using an open-ended helium-driven shock tube, or were assigned to sham control group. At 2 hours (h) following injury, we have characterized the astrocytic response to a blast overpressure. Immunostaining against the astrocytic marker glial fibrillary acidic protein (GFAP) revealed acute shearing and morphological changes in astrocytes, including clasmatodendrosis. Moreover, overlap of GFAP immunostaining and propidium iodide (PI) indicated astrocytic death. Quantification of the number of dead astrocytes per counting area in the hippocampal cornu Ammonis 1 region (CA1), demonstrated a significant increase in dead astrocytes in the low- and high-blast, compared to sham control OHCs. However only a small number of GFAP-expressing astrocytes were co-labeled with the apoptotic marker Annexin V, suggesting necrosis as the primary type of cell death in the acute phase following blast exposure. Moreover, western blot analyses revealed calpain mediated breakdown of GFAP. The dextran exclusion additionally indicated membrane disruption as a potential mechanism of acute astrocytic death. Furthermore, although blast exposure did not evoke significant changes in glutamate transporter 1 (GLT-1) expression, loss of GLT-1-expressing astrocytes suggests dysregulation of glutamate uptake following injury. Our data illustrate the profound effect of blast overpressure on astrocytes in OHCs at 2 h following injury and suggest increased calpain activity and membrane disruption as potential underlying mechanisms.

Pathological vascular and inflammatory biomarkers of acute- and chronic-phase traumatic brain injury

Given the demand for developing objective methods for characterizing traumatic brain injury (TBI), research dedicated to evaluating putative biomarkers has burgeoned over the past decade. Since it is critical to elucidate the underlying pathological processes that underlie the higher diverse outcomes that follow neurotrauma, considerable efforts have been aimed at identifying biomarkers of both the acute- and chronic-phase TBI. Such information is not only critical for helping to elucidate the pathological changes that lead to poor long-term outcomes following TBI but it may also assist in the identification of possible prevention and interventions for individuals who sustain head trauma. In the current review, we discuss the potential role of vascular dysfunction and chronic inflammation in both acute- and chronic-phase TBI, and we also highlight existing studies that have investigated inflammation biomarkers associated with poorer injury outcome.

The Effect of Underwater Blast on Aggregating Brain Cell Cultures

Although the deleterious effects of primary blast on gas-filled organs are well accepted, the effect of blast-induced shock waves on the brain is less clear because of factors that complicate the interpretation of clinical and experimental data. Brain cell aggregate cultures are comprised of multiple differentiated brain cell types and were used to examine the effects of underwater blast. Suspensions of these cultures encased in dialysis tubing were exposed to explosive-generated underwater blasts of low (∼300 kPa), medium (∼2,700 kPa), or high (∼14,000 kPa) intensities and harvested at 1-28 days post-exposure. No changes in gross morphology were noted immediately or weeks after blast wave exposure, and no increases in either apoptotic (caspase-3) or necrotic (lactate dehydrogenase) cell death were observed. Changes in neuronal (neurofilament H, acetylcholinesterase, and choline acetyltransferase) and glial (glial fibrillary acidic protein, glutamine synthetase) endpoints did not occur. However, significant time- and pressure-related increases in Akt (protein kinase B) phosphorylation were noted, as well as declines in vascular endothelial growth factor levels, implicating pathways involved in cellular survival mechanisms. The free-floating nature of the aggregates during blast wave exposure, coupled with their highly hydrolyzed dialysis tubing containment, results in minimized boundary effects, thus enabling accurate assessment of brain cell response to a simplified shock-induced stress wave. This work shows that, at its simplest, blast-induced shock waves produce subtle changes in brain tissue. This study has mechanistic implications for the study of primary blast-induced traumatic brain injury and supports the thesis that underwater blast may cause subtle changes in the brains of submerged individuals.

Effects of Exposure to Blast Overpressure on Intracranial Pressure and Blood-Brain Barrier Permeability in a Rat Model

Exposure to blast overpressure (BOP) activates a cascade of pathological processes including changes in intracranial pressure (ICP) and blood-brain barrier (BBB) permeability resulting in traumatic brain injury (TBI). In this study the effect of single and multiple exposures at two intensities of BOP on changes in ICP and BBB permeability in Sprague-Dawley rats was evaluated. Animals were exposed to a single or three repetitive (separated by 0.5 h) BOPs at 72 kPa or 110 kPa. ICP was monitored continuously via telemetry for 6 days after exposure to BOP. The alteration in the permeability of BBB was determined by extravasation of Evans Blue (EB) into brain parenchyma. A significant increase in ICP was observed in all groups except the single 72 kPa BOP group. At the same time a marked increase in BBB permeability was also seen in various parts of the brain. The extent of ICP increase as well as BBB permeability change was dependent on intensity and frequency of blast.

Electroporation of Brain Endothelial Cells on Chip toward Permeabilizing the Blood-Brain Barrier

The blood-brain barrier, mainly composed of brain microvascular endothelial cells, poses an obstacle to drug delivery to the brain. Controlled permeabilization of the constituent brain endothelial cells can result in overcoming this barrier and increasing transcellular transport across it. Electroporation is a biophysical phenomenon that has shown potential in permeabilizing and overcoming this barrier. In this study we developed a microengineered in vitro model to characterize the permeabilization of adhered brain endothelial cells to large molecules in response to applied pulsed electric fields. We found the distribution of affected cells by reversible and irreversible electroporation, and quantified the uptaken amount of naturally impermeable molecules into the cells as a result of applied pulse magnitude and number of pulses. We achieved 81 ± 1.7% (N = 6) electroporated cells with 17 ± 8% (N = 5) cell death using an electric-field magnitude of ∼580 V/cm and 10 pulses. Our results provide the proper range for applied electric-field intensity and number of pulses for safe permeabilization without significantly compromising cell viability. Our results demonstrate that it is possible to permeabilize the endothelial cells of the BBB in a controlled manner, therefore lending to the feasibility of using pulsed electric fields to increase drug transport across the BBB through the transcellular pathway.

Primary Blast Injury Depressed Hippocampal Long-Term Potentiation through Disruption of Synaptic Proteins

Blast-induced traumatic brain injury (bTBI) is a major threat to United States service members in military conflicts worldwide. The effects of primary blast, caused by the supersonic shockwave interacting with the skull and brain, remain unclear. Our group has previously reported that in vitro primary blast exposure can reduce long-term potentiation (LTP), the electrophysiological correlate of learning and memory, in rat organotypic hippocampal slice cultures (OHSCs) without significant changes to cell viability or basal, evoked neuronal function. We investigated the time course of primary blast-induced deficits in LTP and the molecular mechanisms that could underlie these deficits. We found that pure primary blast exposure induced LTP deficits in a delayed manner, requiring longer than 1 hour to develop, and that these deficits spontaneously recovered by 10 days following exposure depending on blast intensity. Additionally, we observed that primary blast exposure reduced total α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) glutamate receptor 1 (GluR1) subunit expression and phosphorylation of the GluR1 subunit at the serine-831 site. Blast also reduced the expression of postsynaptic density protein-95 (PSD-95) and phosphorylation of stargazin protein at the serine-239/240 site. Finally, we found that modulation of the cyclic adenosine monophosphate (cAMP) pathway ameliorated electrophysiological and protein-expression changes caused by blast. These findings could inform the development of novel therapies to treat blast-induced loss of neuronal function.

Blast waves from detonated military explosive reduce GluR1 and synaptophysin levels in hippocampal slice cultures

Explosives create shockwaves that cause blast-induced neurotrauma, one of the most common types of traumatic brain injury (TBI) linked to military service. Blast-induced TBIs are often associated with reduced cognitive and behavioral functions due to a variety of factors. To study the direct effects of military explosive blasts on brain tissue, we removed systemic factors by utilizing rat hippocampal slice cultures. The long-term slice cultures were briefly sealed air-tight in serum-free medium, lowered into a 37°C water-filled tank, and small 1.7-gram assemblies of cyclotrimethylene trinitramine (RDX) were detonated 15cm outside the tank, creating a distinct shockwave recorded at the culture plate position. Compared to control mock-treated groups of slices that received equal submerge time, 1-3 blast impacts caused a dose-dependent reduction in the AMPA receptor subunit GluR1. While only a small reduction was found in hippocampal slices exposed to a single RDX blast and harvested 1-2days later, slices that received two consecutive RDX blasts 4min apart exhibited a 26-40% reduction in GluR1, and the receptor subunit was further reduced by 64-72% after three consecutive blasts. Such loss correlated with increased levels of HDAC2, a histone deacetylase implicated in stress-induced reduction of glutamatergic transmission. No evidence of synaptic marker recovery was found at 72h post-blast. The presynaptic marker synaptophysin was found to have similar susceptibility as GluR1 to the multiple explosive detonations. In contrast to the synaptic protein reductions, actin levels were unchanged, spectrin breakdown was not detected, and Fluoro-Jade B staining found no indication of degenerating neurons in slices exposed to three RDX blasts, suggesting that small, sub-lethal explosives are capable of producing selective alterations to synaptic integrity. Together, these results indicate that blast waves from military explosive cause signs of synaptic compromise without producing severe neurodegeneration, perhaps explaining the cognitive and behavioral changes in those blast-induced TBI sufferers that have no detectable neuropathology.

Primary blast causes mild, moderate, severe and lethal TBI with increasing blast overpressures: Experimental rat injury model

Injury severity in blast induced Traumatic Brain Injury (bTBI) increases with blast overpressure (BOP) and impulse in dose-dependent manner. Pure primary blast waves were simulated in compressed gas shock-tubes in discrete increments. Present work demonstrates 24 hour survival of rats in 0–450 kPa (0–800 Pa∙s impulse) range at 10 discrete levels (60, 100, 130, 160, 190, 230, 250, 290, 350 and 420 kPa) and determines the mortality rate as a non-linear function of BOP. Using logistic regression model, predicted mortality rate (PMR) function was calculated, and used to establish TBI severities. We determined a BOP of 145 kPa as upper mild TBI threshold (5% PMR). Also we determined 146–220 kPa and 221–290 kPa levels as moderate and severe TBI based on 35%, and 70% PMR, respectively, while BOP above 290 kPa is lethal. Since there are no standards for animal bTBI injury severity, these thresholds need further refinements using histopathology, immunohistochemistry and behavior. Further, we specifically investigated mild TBI range (0–145 kPa) using physiological (heart rate), pathological (lung injury), immuno-histochemical (oxidative/nitrosative and blood-brain barrier markers) as well as blood borne biomarkers. With these additional data, we conclude that mild bTBI occurs in rats when the BOP is in the range of 85–145 kPa.

Primary blast injury causes cognitive impairments and hippocampal circuit alterations

Blast-induced traumatic brain injury (bTBI) and its long term consequences are a major health concern among veterans. Despite recent work enhancing our knowledge about bTBI, very little is known about the contribution of the blast wave alone to the observed sequelae. Herein, we isolated its contribution in a mouse model by constraining the animals' heads during exposure to a shockwave (primary blast). Our results show that exposure to primary blast alone results in changes in hippocampus-dependent behaviors that correspond with electrophysiological changes in area CA1 and are accompanied by reactive gliosis. Specifically, five days after exposure, behavior in an open field and performance in a spatial object recognition (SOR) task were significantly different from sham. Network electrophysiology, also performed five days after injury, demonstrated a significant decrease in excitability and increase in inhibitory tone. Immunohistochemistry for GFAP and Iba1 performed ten days after injury showed a significant increase in staining. Interestingly, a threefold increase in the impulse of the primary blast wave did not exacerbate these measures. However, we observed a significant reduction in the contribution of the NMDA receptors to the field EPSP at the highest blast exposure level. Our results emphasize the need to account for the effects of primary blast loading when studying the sequelae of bTBI.

The effect of explosive blast loading on human neuroblastoma cells

Diagnosis of mild to moderate traumatic brain injury is challenging because brain tissue damage progresses slowly and is not readily detectable by conventional imaging techniques. We have developed a novel in vitro model to study primary blast loading on dissociated neurons using nitroamine explosives such as those used on the battlefield. Human neuroblastoma cells were exposed to single and triple 50-psi explosive blasts and single 100-psi blasts. Changes in membrane permeability and oxidative stress showed a significant increase for the single and triple 100-psi blast conditions compared with single 50-psi blast and controls.

Mechanical Injury Induces Brain Endothelial-Derived Microvesicle Release: Implications for Cerebral Vascular Injury during Traumatic Brain Injury

It is well established that the endothelium responds to mechanical forces induced by changes in shear stress and strain. However, our understanding of vascular remodeling following traumatic brain injury (TBI) remains incomplete. Recently published studies have revealed that lung and umbilical endothelial cells produce extracellular microvesicles (eMVs), such as microparticles, in response to changes in mechanical forces (blood flow and mechanical injury). Yet, to date, no studies have shown whether brain endothelial cells produce eMVs following TBI. The brain endothelium is highly specialized and forms the blood-brain barrier (BBB), which regulates diffusion and transport of solutes into the brain. This specialization is largely due to the presence of tight junction proteins (TJPs) between neighboring endothelial cells. Following TBI, a breakdown in tight junction complexes at the BBB leads to increased permeability, which greatly contributes to the secondary phase of injury. We have therefore tested the hypothesis that brain endothelium responds to mechanical injury, by producing eMVs that contain brain endothelial proteins, specifically TJPs. In our study, primary human adult brain microvascular endothelial cells (BMVEC) were subjected to rapid mechanical injury to simulate the abrupt endothelial disruption that can occur in the primary injury phase of TBI. eMVs were isolated from the media following injury at 2, 6, 24, and 48 h. Western blot analysis of eMVs demonstrated a time-dependent increase in TJP occludin, PECAM-1 and ICAM-1 following mechanical injury. In addition, activation of ARF6, a small GTPase linked to extracellular vesicle production, was increased after injury. To confirm these results in vivo, mice were subjected to sham surgery or TBI and blood plasma was collected 24 h post-injury. Isolation and analysis of eMVs from blood plasma using cryo-EM and flow cytometry revealed elevated levels of vesicles containing occludin following brain trauma. These results indicate that following TBI, the cerebral endothelium undergoes vascular remodeling through shedding of eMVs containing TJPs and endothelial markers. The detection of this shedding potentially allows for a novel methodology for real-time monitoring of cerebral vascular health (remodeling), BBB status and neuroinflammation following a TBI event.

Isolated Primary Blast Inhibits Long-Term Potentiation in Organotypic Hippocampal Slice Cultures

Over the last 13 years, traumatic brain injury (TBI) has affected over 230,000 U.S. service members through the conflicts in Iraq and Afghanistan, mostly as a result of exposure to blast events. Blast-induced TBI (bTBI) is multi-phasic, with the penetrating and inertia-driven phases having been extensively studied. The effects of primary blast injury, caused by the shockwave interacting with the brain, remain unclear. Earlier in vivo studies in mice and rats have reported mixed results for primary blast effects on behavior and memory. Using a previously developed shock tube and in vitro sample receiver, we investigated the effect of isolated primary blast on the electrophysiological function of rat organotypic hippocampal slice cultures (OHSC). We found that pure primary blast exposure inhibited long-term potentiation (LTP), the electrophysiological correlate of memory, with a threshold between 9 and 39 kPa·ms impulse. This deficit occurred well below a previously identified threshold for cell death (184 kPa·ms), supporting our previously published finding that primary blast can cause changes in brain function in the absence of cell death. Other functional measures such as spontaneous activity, network synchronization, stimulus-response curves, and paired-pulse ratios (PPRs) were less affected by primary blast exposure, as compared with LTP. This is the first study to identify a tissue-level tolerance threshold for electrophysiological changes in neuronal function to isolated primary blast.

Time Course and Size of Blood-Brain Barrier Opening in a Mouse Model of Blast-Induced Traumatic Brain Injury

An increasing number of studies have reported blood-brain barrier (BBB) dysfunction after blast-induced traumatic brain injury (bTBI). Despite this evidence, there is limited quantitative understanding of the extent of BBB opening and the time course of damage after blast injury. In addition, many studies do not report kinematic parameters of head motion, making it difficult to separate contributions of primary and tertiary blast-loading. Detailed characterization of blast-induced BBB damage may hold important implications for serum constituents that may potentially cross the compromised barrier and contribute to neurotoxicity, neuroinflammation, and persistent neurologic deficits. Using an in vivo bTBI model, systemic administration of sodium fluorescein (NaFl; 376 Da), Evans blue (EB; 69 kDa when bound to serum albumin), and dextrans (3-500 kDa) was used to estimate the pore size of BBB opening and the time required for recovery. Exposure to blast with 272 ± 6 kPa peak overpressure, 0.69 ± 0.01 ms duration, and 65 ± 1 kPa*ms impulse resulted in significant acute extravasation of NaFl, 3 kDa dextran, and EB. However, there was no significant acute extravasation of 70 kDa or 500 kDa dextrans, and minimal to no extravasation of NaFl, dextrans, or EB 1 day after exposure. This study presents a detailed analysis of the time course and pore size of BBB opening after bTBI, supported by a characterization of kinematic parameters associated with blast-induced head motion.

Enduring deficits in memory and neuronal pathology after blast-induced traumatic brain injury

Few preclinical studies have assessed the long-term neuropathology and behavioral deficits after sustaining blast-induced neurotrauma (BINT). Previous studies have shown extensive astrogliosis and cell death at acute stages (<7 days) but the temporal response at a chronic stage has yet to be ascertained. Here, we used behavioral assays, immmunohistochemistry and neurochemistry in limbic areas such as the amygdala (Amy), Hippocampus (Hipp), nucleus accumbens (Nac), and prefrontal cortex (PFC), to determine the long-term effects of a single blast exposure. Behavioral results identified elevated avoidance behavior and decreased short-term memory at either one or three months after a single blast event. At three months after BINT, markers for neurodegeneration (FJB) and microglia activation (Iba-1) increased while index of mature neurons (NeuN) significantly decreased in all brain regions examined. Gliosis (GFAP) increased in all regions except the Nac but only PFC was positive for apoptosis (caspase-3). At three months, tau was selectively elevated in the PFC and Hipp whereas α-synuclein transiently increased in the Hipp at one month after blast exposure. The composite neurochemical measure, myo-inositol+glycine/creatine, was consistently increased in each brain region three months following blast. Overall, a single blast event resulted in enduring long-term effects on behavior and neuropathological sequelae.

Opening of the blood-brain barrier tight junction due to shock wave induced bubble collapse: a molecular dynamics simulation study

Passage of a shock wave across living organisms may produce bubbles in the blood vessels and capillaries. It was suggested that collapse of these bubbles imposed by an impinging shock wave can be responsible for the damage or even destruction of the blood-brain barrier. To check this possibility, we performed molecular dynamics computer simulations on systems that contained a model of tight junction from the blood-brain barrier. In our model, we represent the tight junction by two pairs of interacting proteins, claudin-15. Some of the simulations were done in the absence of a nanobubble, some in its presence. Our simulations show that when no bubble is present in the system, no damage to tight junction is observed when the shock wave propagates across it. In the presence of a nanobubble, even when the impulse of the shock wave is relatively low, the implosion of the bubble causes serious damage to our model tight junction.

Dexamethasone potentiates in vitro blood-brain barrier recovery after primary blast injury by glucocorticoid receptor-mediated upregulation of ZO-1 tight junction protein

Owing to the frequent incidence of blast-induced traumatic brain injury (bTBI) in recent military conflicts, there is an urgent need to develop effective therapies for bTBI-related pathologies. Blood-brain barrier (BBB) breakdown has been reported to occur after primary blast exposure, making restoration of BBB function and integrity a promising therapeutic target. We tested the hypothesis that treatment with dexamethasone (DEX) after primary blast injury potentiates recovery of an in vitro BBB model consisting of mouse brain endothelial cells (bEnd.3). DEX treatment resulted in complete recovery of transendothelial electrical resistance and hydraulic conductivity 1 day after injury, compared with 3 days for vehicle-treated injured cultures. Administration of RU486 (mifepristone) inhibited effects of DEX, confirming that barrier restoration was mediated by glucocorticoid receptor signaling. Potentiated recovery with DEX treatment was accompanied by stronger zonula occludens (ZO)-1 tight junction immunostaining and expression, suggesting that increased ZO-1 expression was a structural correlate to BBB recovery after blast. Interestingly, augmented ZO-1 protein expression was associated with specific upregulation of the α(+) isoform but not the α(-) isoform. This is the first study to provide a mechanistic basis for potentiated functional recovery of an in vitro BBB model because of glucocorticoid treatment after primary blast injury.

In vitro studies of primary explosive blast loading on neurons

In a military setting, traumatic brain injury (TBI) is frequently caused by blast waves that can trigger a series of neuronal biochemical changes. Although many animal models have been used to study the effects of primary blast waves, elucidating the mechanisms of damage in a whole-animal model is extremely complex. In vitro models of primary blast, which allow for the deconvolution of mechanisms, are relatively scarce. It is largely unknown how structural damage at the cellular level impacts the functional activity at variable time scales after the TBI event. A novel in vitro system was developed to probe the effects of explosive blast (ranging from ∼25 to 40 psi) on dissociated neurons. PC12 neurons were cultured on laminin-coated substrates, submerged underwater, and subjected to single and multiple blasts in a controlled environment. Changes in cell membrane permeability, viability, and cell morphology were evaluated. Significant increases in axonal beading were observed in the injured cells. In addition, although cell death was minimal after a single insult, cell viability decreased significantly following repeated blast exposure.

Mechanics of the brain: perspectives, challenges, and opportunities

The human brain is the continuous subject of extensive investigation aimed at understanding its behavior and function. Despite a clear evidence that mechanical factors play an important role in regulating brain activity, current research efforts focus mainly on the biochemical or electrophysiological activity of the brain. Here, we show that classical mechanical concepts including deformations, stretch, strain, strain rate, pressure, and stress play a crucial role in modulating both brain form and brain function. This opinion piece synthesizes expertise in applied mathematics, solid and fluid mechanics, biomechanics, experimentation, material sciences, neuropathology, and neurosurgery to address today’s open questions at the forefront of neuromechanics. We critically review the current literature and discuss challenges related to neurodevelopment, cerebral edema, lissencephaly, polymicrogyria, hydrocephaly, craniectomy, spinal cord injury, tumor growth, traumatic brain injury, and shaken baby syndrome. The multi-disciplinary analysis of these various phenomena and pathologies presents new opportunities and suggests that mechanical modeling is a central tool to bridge the scales by synthesizing information from the molecular via the cellular and tissue all the way to the organ level.

Measuring direct current trans-epithelial electrical resistance in organ-on-a-chip microsystems

Trans-epithelial electrical resistance (TEER) measurements are widely used as real-time, non-destructive, and label-free measurements of epithelial and endothelial barrier function. TEER measurements are ideal for characterizing tissue barrier function in organs-on-chip studies for drug testing and investigation of human disease models; however, published reports using this technique have reported highly conflicting results even with identical cell lines and experimental setups. The differences are even more dramatic when comparing measurements in conventional Transwell systems with those obtained in microfluidic systems. Our goal in this work was therefore to enhance the fidelity of TEER measurements in microfluidic organs-on-chips, specifically using direct current (DC) measurements of TEER, as this is the most widely used method reported in the literature. Here we present a mathematical model that accounts for differences measured in TEER between microfluidic chips and Transwell systems, which arise from differences in device geometry. The model is validated by comparing TEER measurements obtained in a microfluidic gut-on-a-chip device versus in a Transwell culture system. Moreover, we show that even small gaps in cell coverage (e.g., 0.4%) are sufficient to cause a significant (~80%) drop in TEER. Importantly, these findings demonstrate that TEER measurements obtained in microfluidic systems, such as organs-on-chips, require special consideration, specifically when results are to be compared with measurements obtained from Transwell systems.

The blood-brain barrier as a target in traumatic brain injury treatment

Traumatic brain injury (TBI) is one of the most frequent causes of death in the young population. Several clinical trials have unsuccessfully focused on direct neuroprotective therapies. Recently immunotherapeutic strategies shifted into focus of translational research in acute CNS diseases. Cross-talk between activated microglia and blood-brain barrier (BBB) could initiate opening of the BBB and subsequent recruitment of systemic immune cells and mediators into the brain. Stabilization of the BBB after TBI could be a promising strategy to limit neuronal inflammation, secondary brain damage and acute neurodegeneration. This review provides an overview on the pathophysiology of TBI and brain edema formation including definitions and classification of TBI, current clinical treatment strategies, as well as current understanding on the underlying cellular processes. A summary of in vivo and in vitro models to study different aspects of TBI is presented. Three mechanisms proposed for stabilization of the BBB, myosin light chain kinases, glucocorticoid receptors and peroxisome proliferator-activated receptors are reviewed for their influence on barrier-integrity and outcome after TBI. In conclusion, the BBB is recommended as a promising target for the treatment of traumatic brain injury, and it is suggested that a combination of BBB stabilization and neuroprotectants may improve therapeutic success.

Blood brain barrier dysfunction and delayed neurological deficits in mild traumatic brain injury induced by blast shock waves

Mild traumatic brain injury (mTBI) resulting from exposure to blast shock waves (BSWs) is one of the most predominant causes of illnesses among veterans who served in the recent Iraq and Afghanistan wars. Such mTBI can also happen to civilians if exposed to shock waves of bomb attacks by terrorists. While cognitive problems, memory dysfunction, depression, anxiety and diffuse white matter injury have been observed at both early and/or delayed time-points, an initial brain pathology resulting from exposure to BSWs appears to be the dysfunction or disruption of the blood-brain barrier (BBB). Studies in animal models suggest that exposure to relatively milder BSWs (123 kPa) initially induces free radical generating enzymes in and around brain capillaries, which enhances oxidative stress resulting in loss of tight junction (TJ) proteins, edema formation, and leakiness of BBB with disruption or loss of its components pericytes and astrocyte end-feet. On the other hand, exposure to more intense BSWs (145-323 kPa) causes acute disruption of the BBB with vascular lesions in the brain. Both of these scenarios lead to apoptosis of endothelial and neural cells and neuroinflammation in and around capillaries, which may progress into chronic traumatic encephalopathy (CTE) and/or a variety of neurological impairments, depending on brain regions that are afflicted with such lesions. This review discusses studies that examined alterations in the brain milieu causing dysfunction or disruption of the BBB and neuroinflammation following exposure to different intensities of BSWs. Furthermore, potential of early intervention strategies capable of easing oxidative stress, repairing the BBB or blocking inflammation for minimizing delayed neurological deficits resulting from exposure to BSWs is conferred.

Effect of amplitude and duration of impulsive pressure on endothelial permeability in in vitro fluid percussion trauma

Background

Intracranial pressure changes during head impact cause brain injuries such as vasogenic edema and cerebral contusion. However, the influence of impulsive pressure on endothelial function has not yet been fully studied in vitro. In this study, we developed a pressure loading device that produced positive and negative pressures by modifying an in vitro fluid percussion model and examined the effects of the amplitude and duration of the pressures on endothelial permeability.

Methods

Human umbilical vein endothelial cells were subjected to three types of positive pressure (average amplitude/average duration of 352 kPa/23 ms, 73 kPa/27 ms, and 70 kPa/44 ms) and three types of negative pressure (−72 kPa/41 ms, −67 kPa/104 ms, and −91 kPa/108 ms), and the transendothelial electrical resistance (TEER) was measured between 15 min and 24 h after pressure loading for quantifying the formation of an integral monolayer of endothelial cells. After loading, vascular endothelial- (VE-) cadherin, an endothelium-specific cell-cell adhesion molecule involved in endothelial barrier function, was stained and observed using fluorescence microscopy.

Results

The pressure loading device could produce positive pressure pulses with amplitudes of 53–1348 kPa and durations of 9–29.1 ms and negative pressure pulses with amplitudes of −52–−93 kPa and durations of 42.9–179.5 ms. The impulsive pressure reduced the TEER associated with the change in VE-cadherin localization. Additionally, TEER decreased considerably at 15 min and 6 h post-loading, with these changes being significant in positive pressure with larger amplitude and shorter duration and in all types of negative pressures compared to pre-loading.

Conclusions

The changes in intracranial pressure during head impact impair endothelial barrier function by the disruption of the integrity of endothelial cell-cell junctions, and the degree of increase in endothelial permeability depends on the amplitude, duration, and direction (compressive and tensile) of the impulsive pressure.

Repeated primary blast injury causes delayed recovery, but not additive disruption, in an in vitro blood-brain barrier model

Recent studies have demonstrated increased susceptibility to breakdown of the cerebral vasculature associated with repetitive traumatic brain injury. We hypothesized that exposure to two consecutive blast injuries would result in exacerbated damage to an in vitro model of the blood-brain barrier (BBB) compared with exposure to a single blast of the same severity. Contrary to our hypothesis, however, repeated mild or moderate primary blast delivered with a 24 or 72 h interval between injuries did not significantly exacerbate reductions in transendothelial electrical resistance (TEER) across a brain endothelial monolayer compared with sister cultures receiving a single exposure of the same intensity. Permeability of the barrier to a range of different-sized solutes remained unaltered after single and repeated blast, supporting that the effects of repeated blast on BBB integrity were not additive. Single blast exposure significantly reduced immunostaining of ZO-1 and claudin-5 tight junction proteins, but subsequent exposure did not cause additional damage to tight junctions. Although repeated blast did not further reduce TEER, the second exposure delayed TEER recovery in BBB cultures. Similarly, recovery of hydraulic conductivity through the BBB was delayed by a second exposure. Extending the interinjury interval to 72 h, the effects of multiple injuries on the BBB were found to be independent given sufficient recovery time between consecutive exposures. Careful investigation of the effects of repeated blast on the BBB will help identify injury levels and a temporal window of vulnerability associated with BBB dysfunction, ultimately leading to improved strategies for protecting warfighters against repeated blast-induced disruption of the cerebral vasculature.

The mechanics of traumatic brain injury: a review of what we know and what we need to know for reducing its societal burden

Traumatic brain injury (TBI) is a significant public health problem, on pace to become the third leading cause of death worldwide by 2020. Moreover, emerging evidence linking repeated mild traumatic brain injury to long-term neurodegenerative disorders points out that TBI can be both an acute disorder and a chronic disease. We are at an important transition point in our understanding of TBI, as past work has generated significant advances in better protecting us against some forms of moderate and severe TBI. However, we still lack a clear understanding of how to study milder forms of injury, such as concussion, or new forms of TBI that can occur from primary blast loading. In this review, we highlight the major advances made in understanding the biomechanical basis of TBI. We point out opportunities to generate significant new advances in our understanding of TBI biomechanics, especially as it appears across the molecular, cellular, and whole organ scale.

Smooth muscle phenotype switching in blast traumatic brain injury-induced cerebral vasospasm

Due to increased survival rates among soldiers exposed to explosive blasts, blast-induced traumatic brain injury (bTBI) has become much more prevalent in recent years. Cerebral vasospasm (CVS) is a common manifestation of brain injury whose incidence is significantly increased in bTBI. CVS is characterized by initial vascular smooth muscle cell (VSMC) hypercontractility, followed by prolonged vessel remodeling and lumen occlusion, and is traditionally associated with subarachnoid hemorrhage (SAH), but recent results suggest that mechanical injury during bTBI can cause mechanotransduced VSMC hypercontractility and phenotype switching necessary for CVS development, even in the absence of SAH. Here, we review the mechanisms by which mechanical stimulation and SAH can synergistically drive CVS progression, complicating treatment options in bTBI patients.