Cytoskeletal protein α-II spectrin degradation in the brain of repeated blast exposed mice

Actomyosin-II protects axons from degeneration induced by mild mechanical stress

Whether, to what extent, and how the axons in the central nervous system (CNS) can withstand sudden mechanical impacts remain unclear. By using a microfluidic device to apply controlled transverse mechanical stress to axons, we determined the stress levels that most axons can withstand and explored their instant responses at nanoscale resolution. We found mild stress triggers a highly reversible, rapid axon beading response, driven by actomyosin-II-dependent dynamic diameter modulations. This mechanism contributes to hindering the long-range spread of stress-induced Ca2+ elevations into non-stressed neuronal regions. Through pharmacological and molecular manipulations in vitro, we found that actomyosin-II inactivation diminishes the reversible beading process, fostering progressive Ca2+ spreading and thereby increasing acute axonal degeneration in stressed axons. Conversely, upregulating actomyosin-II activity prevents the progression of initial injury, protecting stressed axons from acute degeneration both in vitro and in vivo. Our study unveils the periodic actomyosin-II in axon shafts cortex as a novel protective mechanism, shielding neurons from detrimental effects caused by mechanical stress.

Blast Exposure Alters Synaptic Connectivity in the Mouse Auditory Cortex

Blast exposure can cause auditory deficits that have a lasting, significant impact on patients. Although the effects of blast on auditory functions localized to the ear have been well documented, the impact of blast on central auditory processing is largely undefined. Understanding the structural and functional alterations in the central nervous system (CNS) associated with blast injuries is crucial for unraveling blast-induced pathophysiological pathways and advancing development of therapeutic interventions. In this study, we used electrophysiology in combination with optogenetics assay, proteomic analysis, and morphological evaluation to investigate the impairment of synaptic connectivity in the auditory cortex (AC) of mice following blast exposure. Our results show that the long-range functional connectivity between the medial geniculate nucleus (MGN) and AC was impaired in the acute phase of blast injury. We also identified impaired synaptic transmission and dendritic spine alterations within 7 days of blast exposure, which recovered at 28 days post-blast. Additionally, proteomic analysis identified a few differentially expressed proteins in the cortex that are involved in synaptic signaling and plasticity. These findings collectively suggest that blast-induced alterations in the sound signaling network in the auditory cortex may underlie hearing deficits in the acute and sub-acute phases after exposure to shockwaves. This study may shed light on the perturbations underlying blast-induced auditory dysfunction and provide insights into the potential therapeutic windows for improving auditory outcomes in blast-exposed individuals.

Peripherin: A proposed biomarker of traumatic axonal injury triggered by mechanical force

Traumatic axonal injury (TAI) is one of the most common pathological features of severe traumatic brain injury (TBI). Our previous study using proteomics suggested that peripherin (PRPH) should be a potential candidate as a biomarker for TAI diagnosis. This study is to further elucidate the role and association of PRPH with TAI. In the animal study, we performed immunohistochemistry, ELISA and morphological analysis to evaluate PRPH level and distribution following a severe impact. PRPH-positive regions were widely distributed in the axonal tract throughout the whole brain. Axonal injuries with PRPH inclusion were observed post-TBI. Besides, PRPH was significantly increased in both cerebral spinal fluid and plasma at the early phase post-TBI. Colocalization analysis based on microscopy revealed that PRPH represents an immunohistological biomarker in the neuropathological diagnosis of TAI. Brain samples from patients with TBI were included to further test whether PRPH is feasible in the real practice of neuropathology. Immunohistochemistry of PRPH, NFH, APP and NFL on human brain tissues further confirmed PRPH as an immunohistological biomarker that could be applied in practice. Collectively, we conclude that PRPH mirrors the cytoskeleton injury of axons and could represent a neuropathological biomarker for TAI.

Axons-on-a-chip for mimicking non-disruptive diffuse axonal injury underlying traumatic brain injury

Diffuse axonal injury (DAI) is the most severe pathological feature of traumatic brain injury (TBI). However, how primary axonal injury is induced by transient mechanical impacts remains unknown, mainly due to the low temporal and spatial resolution of medical imaging approaches. Here we established an axon-on-a-chip (AoC) model for mimicking DAI and monitoring instant cellular responses. Integrating computational fluid dynamics and microfluidic techniques, DAI was induced by injecting a precisely controlled micro-flux in the transverse direction. The clear correlation between the flow speed of injecting flux and the severity of DAI was elucidated. We next used the AoC to investigate the instant intracellular responses underlying DAI and found that the dynamic formation of focal axonal swellings (FAS) accompanied by Ca²⁺ surge occurs during the flux. Surprisingly, periodic axonal cytoskeleton disruption also occurs rapidly after the flux. These instant injury responses are spatially restricted to the fluxed axon, not affecting the overall viability of the neuron in the acute stage. Compatible with high-resolution live microscopy, the AoC provides a versatile system to identify early mechanisms underlying DAI, offering a platform for screening effective treatments to alleviate TBI.

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

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

A Comprehensive Review of Experimental Rodent Models of Repeated Blast TBI

We reviewed the relevant literature delineating advances in the development of the experimental models of repeated blast TBI (rbTBI). It appears this subject is a relatively unexplored area considering the first work published in 2007 and the bulk of peer-reviewed papers was published post-2011. There are merely 34 papers published to date utilizing rodent rbTBI models. We performed an analysis and extracted basic parameters to capture the characteristics of the exposure conditions (the blast intensity, inter-exposure interval and the number of exposures), the age and weight of the animal models most commonly used in the studies, and their endpoints. Our analysis revealed three strains of rodents are predominantly used: Sprague Dawley and Long Evans rats and wild type (C57BL/6J) mice, and young adult animals 8 to 12-week-old are a preferred choice. Typical exposure conditions are the following: (1) peak overpressure in the 27–145 kPa (4–21 psi) range, (2) number of exposures: 2 (13.9%), 3 (63.9%), 5 (16.7%), or 12 (5.6%) with a single exposure used for a baseline comparison in 41.24% of the studies. Two inter-exposure interval durations were used: (1) short (1–30 min.) and (2) extended (24 h) between consecutive shock wave exposures. The experiments included characterization of repeated blast exposure effects on auditory, ocular and neurological function, with a focus on brain etiology in most of the published work. We present an overview of major histopathological findings, which are supplemented by studies implementing MRI (DTI) and behavioral changes after rbTBI in the acute (1–7 days post-injury), subacute (7–14 days), and chronic (>14 days) phases post-injury.

iTRAQ-based proteomic analysis discovers potential biomarkers of diffuse axonal injury in rats

Diffuse axonal injury (DAI) is one of the most common and severe pathological consequences of traumatic brain injury (TBI). The molecular mechanism of DAI is highly complicated and still elusive, yet a clear understanding is crucial for the diagnosis, treatment, and prognosis of DAI. In our study, we used rats to establish a DAI model and applied isobaric tags for relative and absolute quantitation (iTRAQ) coupled with liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis to identify differentially expressed proteins (DEPs) in the corpus callosum. As a result, a total of 514 proteins showed differential expression between the injury groups and the control. Among these DEPs, 14 common DEPs were present at all seven time points postinjury (1, 3, 6, 12, 24, 48, and 72 h). Next, bioinformatic analysis was performed to elucidate the pathogenesis of DAI, which was found to possibly involve calcium ion-regulatory proteins (e.g., calsenilin and ryanodine receptor 2), cytoskeleton organization (e.g., peripherin, NFL, NFM, and NFH), apoptotic processes (e.g., calsenilin and protein kinase C delta type), and inflammatory response proteins (e.g., complement C3 and C-reactive protein). Moreover, peripherin and calsenilin were successfully confirmed by western blotting to be significantly upregulated during DAI, and immunohistochemical (IHC) analysis revealed that their expression increased and could be observed in axons after injury, thus indicating their potential as DAI biomarkers. Our experiments not only provide insight into the molecular mechanisms of axonal injury in rats during DAI but also give clinicians and pathologists important reference data for the diagnosis of DAI. Our findings may expand the list of DAI biomarkers and improve the postmortem diagnostic rate of DAI.

Cerebrospinal Fluid Biomarkers Are Associated With Glial Fibrillary Acidic Protein and αII-spectrin Breakdown Products in Brain Tissues Following Penetrating Ballistic-Like Brain Injury in Rats

Treatments to improve outcomes following severe traumatic brain injury (TBI) are limited but may benefit from understanding subacute-chronic brain protein profiles and identifying biomarkers suitable for use in this time. Acute alterations in the well-known TBI biomarkers glial fibrillary acidic protein (GFAP), αII-spectrin, and their breakdown products (BDPs) have been well established, but little is known about the subacute-chronic post-injury profiles of these biomarkers. Thus, the current study was designed to determine the extended profile of these TBI-specific biomarkers both in brain tissue and cerebral spinal fluid (CSF). Protein abundance was evaluated in brain tissue samples taken from regions of interest and in CSF at 24 h, 3 days, 7 days, 1 month, and 3 months following severe TBI in rats. Results showed increased full length GFAP (GFAP-FL) and GFAP-BDPs starting at 24 h that remained significantly elevated in most brain regions out to 3 months post-injury. However, in CSF, neither GFAP-FL nor GFAP-BDPs were elevated as a consequence of injury. Regional-specific reduction in αII-spectrin was evident in brain tissue samples from 24 h through 3 months. In contrast, SBDP-145/150 was robustly elevated in most brain regions and in CSF from 24 h through 7 days. Correlation analyses revealed numerous significant relationships between proteins in CSF and brain tissue or neurological deficits. This work indicates that TBI results in chronic changes in brain protein levels of well-known TBI biomarkers GFAP, αII-spectrin, and their BDPs and that SBDP-145/150 may have utility as an acute-chronic biomarker.

Understanding blast-induced neurotrauma: how far have we come?

Blast injuries, including blast-induced neurotrauma (BINT), are caused by blast waves generated during an explosion. Accordingly, their history coincides with that of explosives. Hence, it is intriguing that, after more than 1000 years of using explosives, our understanding of the pathological consequences of blast and body/brain interactions is extremely limited. Postconflict recovery mechanisms seemingly include the suppression of painful experiences, such as explosive injuries. Unfortunately, ignoring the knowledge generated by previous generations of scientists retards research progress, leading to superfluous and repetitive studies. This article summarizes clinical and experimental findings published about blast injuries and BINT following the wars of the 20th and 21th centuries. Moreover, it offers a personal view on potential factors interfering with the progress of BINT research working toward providing better diagnosis, treatment and rehabilitation for military personnel affected by blast exposure.

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.

Degradomics in Neurotrauma: Profiling Traumatic Brain Injury

Degradomics has recently emerged as a subdiscipline in the omics era with a focus on characterizing signature breakdown products implicated in various disease processes. Driven by promising experimental findings in cancer, neuroscience, and metabolomic disorders, degradomics has significantly promoted the notion of disease-specific "degradome." A degradome arises from the activation of several proteases that target specific substrates and generate signature protein fragments. Several proteases such as calpains, caspases, cathepsins, and matrix metalloproteinases (MMPs) are involved in the pathogenesis of numerous diseases that disturb the physiologic balance between protein synthesis and protein degradation. While regulated proteolytic activities are needed for development, growth, and regeneration, uncontrolled proteolysis initiated under pathological conditions ultimately culminates into apoptotic and necrotic processes. In this chapter, we aim to review the protease-substrate repertoires in neural injury concentrating on traumatic brain injury. A striking diversity of protease substrates, essential for neuronal and brain structural and functional integrity, namely, encryptic biomarker neoproteins, have been characterized in brain injury. These include cytoskeletal proteins, transcription factors, cell cycle regulatory proteins, synaptic proteins, and cell junction proteins. As these substrates are subject to proteolytic fragmentation, they are ceaselessly exposed to activated proteases. Characterization of these molecules allows for a surge of "possible" therapeutic approaches of intervention at various levels of the proteolytic cascade.

Role of α-II-spectrin breakdown products in the prediction of the severity and clinical outcome of acute traumatic brain injury

αII-spectrin breakdown products are regarded as potential biomarkers for traumatic brain injury (TBI). The aim of the present study was to further evaluate these biomarkers by assessing their clinical utility in predicting the severity of injury and clinical outcome of patients with TBI. Eligible patients with acute TBI (n=17), defined by a Glasgow Coma Scale (GCS) score of ≤8, were enrolled. Ventricular cerebrospinal fluid (CSF) was sampled from each patient at 24, 72 and 120 h following TBI. An immunoblot assay was used to determine the concentrations of SBDPs in the CSF samples. The concentrations of SBDPs combined with the GCS score at 24 h after injury and the Glasgow Outcome Score (GOS) at 30 days after injury were compared and analyzed. The levels of SBDPs in CSF were markedly increased following acute TBI in comparison with those in the control group. In the early period after TBI, the levels of SBDPs were closely associated with GCS score. Comparisons of the SBDP levels with the severity of injury revealed significant differences between patients with the most severe brain injury and patients with severe brain injury in the first 24 h post-injury (P<0.05). The levels and dynamic changes of SBDPs in CSF exhibited a close association with GOS at 30 days after injury. The levels of SBDPs differed significantly between patients grouped according to prognosis (P<0.05). These results suggest that in the early period after TBI, the levels and dynamic changes of SBDPs in CSF can be useful in the prediction of the severity of injury and clinical outcome of patients.

Cellular Mechanisms and Behavioral Outcomes in Blast-Induced Neurotrauma: Comparing Experimental Setups

Blast-induced neurotrauma (BINT) has increased in incidence over the past decades and can result in cognitive issues that have debilitating consequences. The exact primary and secondary mechanisms of injury have not been elucidated and appearance of cellular injury can vary based on many factors, such as blast overpressure magnitude and duration. Many methodologies to study blast neurotrauma have been employed, ranging from open-field explosives to experimental shock tubes for producing free-field blast waves. While there are benefits to the various methods, certain specifications need to be accounted for in order to properly examine BINT. Primary cell injury mechanisms, occurring as a direct result of the blast wave, have been identified in several studies and include cerebral vascular damage, blood-brain barrier disruption, axonal injury, and cytoskeletal damage. Secondary cell injury mechanisms, triggered subsequent to the initial insult, result in the activation of several molecular cascades and can include, but are not limited to, neuroinflammation and oxidative stress. The collective result of these secondary injuries can lead to functional deficits. Behavioral measures examining motor function, anxiety traits, and cognition/memory problems have been utilized to determine the level of injury severity. While cellular injury mechanisms have been identified following blast exposure, the various experimental models present both concurrent and conflicting results. Furthermore, the temporal response and progression of pathology after blast exposure have yet to be detailed and remain unclear due to limited resemblance of methodologies. This chapter summarizes the current state of blast neuropathology and emphasizes the need for a standardized preclinical model of blast neurotrauma.

Alpha II Spectrin breakdown products in immature Sprague Dawley rat hippocampus and cortex after traumatic brain injury

After traumatic brain injury (TBI), proteolysis of Alpha II Spectrin by Calpain 1 produces 145 Spectrin breakdown products (SBDPs) while proteolysis by Caspase 3 produces 120 SBDPs. 145 and 120 SBDP immunoblotting reflects the relative importance of caspase-dependent apoptosis or calpain-dependent excitotoxic/necrotoxic cell death in brain regions over time. In the adult rat, controlled cortical impact (CCI) increased 120 SBDPs in the first hours, lasting a few days, and increased 145 SBDPs within the first few days lasting up to 14 days after injury. Little is known about SBDPs in the immature brain after TBI. Since development affects susceptibility to apoptosis after TBI, we hypothesized that CCI would increase 145 and 120 SBDPs in the immature rat brain relative to SHAM during the first 3 and 5 days, respectively. SBDPs were measured in hippocampi and cortices at post injury days (PID) 1, 2, 3, 5, 7 and 14 after CCI or SHAM surgery in the 17 day old Sprague Dawley rat. 145 SBDPs increased in both brain tissues ipsilateral to injury during the first 3 days, while changes in contralateral tissues were limited to PID2 cortex. 145 SBDPs elevations were more marked and enduring in hippocampus than in cortex. Against expectations, 120 SBDPs only increased in PID1 hippocampus and PID2 cortex. 145 SBDPs elevations occurred early after CCI, similar to previous studies in the adult rat, but resolved more quickly. The minimal changes in 120 SBDPs suggest that calpain-dependent, but not caspase-dependent, cell death predominates in the 17 day old rat after CCI.