Neural mechanobiology and neuronal vulnerability to traumatic loading

Influence of phospholipid head and tail molecular structures on cell membrane mechanical response under tension

Biological cell membranes are primarily comprised of a diverse lipid bilayer with multiple phospholipid (lipid) types, each of which is comprised of a hydrophilic headgroup and two hydrophobic hydrocarbon tails. The lipid type determines the molecular structure of head and tail groups, which can affect membrane mechanics at nanoscale and subsequently cell viability under mechanical loading. Hence, using molecular dynamics simulations, the current study investigated seven membrane phospholipids and the effect of their structural differences on physical deformation, mechanoporation damage, and mechanical failure of the membranes under tension. The inspected phospholipids showed similar yield stresses and strains, as well as pore evolution and damage, but significantly different failure strains. In general, failure occurred at a lower strain for lipids with a larger equilibrium area per lipid. The obtained results suggest that larger headgroup structure, greater degree of unsaturation, and tail-length asymmetry influenced the phospholipids' ability to pack against each other, increased the fluidity and equilibrium area per lipid of the membrane, and resulted in lower failure strain. Overall, this study provides insights on how different phospholipid structures affect membrane physical responses at the molecular level and serves as a reference for future studies of more complex membrane systems with intricate biophysical properties.

Effect of Force Field Resolution on Membrane Mechanical Response and Mechanoporation Damage under Deformation Simulations

Damage induced by transient disruption and mechanoporation in an intact cell membrane is a vital nanoscale biomechanical mechanism that critically affects cell viability. To complement experimental studies of mechanical membrane damage and disruption, molecular dynamics (MD) simulations have been performed at different force field resolutions, each of which follows different parameterization strategies and thus may influence the properties and dynamics of membrane systems. Therefore, the current study performed tensile deformation MD simulations of bilayer membranes using all-atom (AA), united-atom (UA), and coarse-grained Martini (CG-M) models to investigate how the damage biomechanics differs across atomistic and coarse-grained (CG) simulations. The mechanical response and mechanoporation damage were qualitatively similar but quantitatively different in the three models, including some progressive changes based on the coarse-graining level. The membranes yielded and reached ultimate strength at similar strains; however, the coarser systems exhibited lower average yield stresses and failure strains. The average failure strain in the UA model was approximately 7% lower than the AA, and the CG-M was 20% lower than UA and 27% lower than AA. The CG systems also nucleated a higher number of pores and larger pores, which resulted in higher damage during the deformation process. Overall, the study provides insight on the impact of force field-a critical factor in modeling biomolecular systems and their interactions-in inspecting membrane mechanosensitive responses and serves as a reference for justifying the appropriate force field for future studies of more complex membranes and more diverse biomolecular assemblies.

Dynamic biophysical responses of neuronal cell nuclei and cytoskeletal structure following high impulse loading

Cells are continuously exposed to dynamic environmental cues that influence their behavior. Mechanical cues can influence cellular and genomic architecture, gene expression, and intranuclear mechanics, providing evidence of mechanosensing by the nucleus, and a mechanoreciprocity between the nucleus and environment. Force disruption at the tissue level through aging, disease, or trauma, propagates to the nucleus and can have lasting consequences on proper functioning of the cell and nucleus. While the influence of mechanical cues leading to axonal damage has been well studied in neuronal cells, the mechanics of the nucleus following high impulse loading is still largely unexplored. Using an in vitro model of traumatic neural injury, we show a dynamic nuclear behavioral response to impulse stretch (up to 170% strain per second) through quantitative measures of nuclear movement, including tracking of rotation and internal motion. Differences in nuclear movement were observed between low and high strain magnitudes. Increased exposure to impulse stretch exaggerated the decrease in internal motion, assessed by particle tracking microrheology, and intranuclear displacements, assessed through high-resolution deformable image registration. An increase in F-actin puncta surrounding nuclei exposed to impulse stretch additionally demonstrated a corresponding disruption of the cytoskeletal network. Our results show direct biophysical nuclear responsiveness in neuronal cells through force propagation from the substrate to the nucleus. Understanding how mechanical forces perturb the morphological and behavioral response can lead to a greater understanding of how mechanical strain drives changes within the cell and nucleus, and may inform fundamental nuclear behavior after traumatic axonal injury. STATEMENT OF SIGNIFICANCE: The nucleus of the cell has been implicated as a mechano-sensitive organelle, courting molecular sensors and transmitting physical cues in order to maintain cellular and tissue homeostasis. Disruption of this network due to disease or high velocity forces (e.g., trauma) can not only result in orchestrated biochemical cascades, but also biophysical perturbations. Using an in vitro model of traumatic neural injury, we aimed to provide insight into the neuronal nuclear mechanics and biophysical responses at a continuum of strain magnitudes and after repetitive loads. Our image-based methods demonstrate mechanically-induced changes in cellular and nuclear behavior after high intensity loading and have the potential to further define mechanical thresholds of neuronal cell injury.

Tissue-Scale Biomechanical Testing of Brain Tissue for the Calibration of Nonlinear Material Models

Brain tissue is one of the most complex and softest tissues in the human body. Due to its ultrasoft and biphasic nature, it is difficult to control the deformation state during biomechanical testing and to quantify the highly nonlinear, time-dependent tissue response. In numerous experimental studies that have investigated the mechanical properties of brain tissue over the last decades, stiffness values have varied significantly. One reason for the observed discrepancies is the lack of standardized testing protocols and corresponding data analyses. The tissue properties have been tested on different length and time scales depending on the testing technique, and the corresponding data have been analyzed based on simplifying assumptions. In this review, we highlight the advantage of using nonlinear continuum mechanics based modeling and finite element simulations to carefully design experimental setups and protocols as well as to comprehensively analyze the corresponding experimental data. We review testing techniques and protocols that have been used to calibrate material model parameters and discuss artifacts that might falsify the measured properties. The aim of this work is to provide standardized procedures to reliably quantify the mechanical properties of brain tissue and to more accurately calibrate appropriate constitutive models for computational simulations of brain development, injury and disease. Computational models can not only be used to predictively understand brain tissue behavior, but can also serve as valuable tools to assist diagnosis and treatment of diseases or to plan neurosurgical procedures. © 2022 The Authors. Current Protocols published by Wiley Periodicals LLC.

A New Framework for Investigating the Biological Basis of Degenerative Cervical Myelopathy [AO Spine RECODE-DCM Research Priority Number 5]: Mechanical Stress, Vulnerability and Time

STUDY DESIGN

Literature Review (Narrative).

OBJECTIVE

To propose a new framework, to support the investigation and understanding of the pathobiology of DCM, AO Spine RECODE-DCM research priority number 5.

METHODS

Degenerative cervical myelopathy is a common and disabling spinal cord disorder. In this perspective, we review key knowledge gaps between the clinical phenotype and our biological models. We then propose a reappraisal of the key driving forces behind DCM and an individual's susceptibility, including the proposal of a new framework.

RESULTS

Present pathobiological and mechanistic knowledge does not adequately explain the disease phenotype; why only a subset of patients with visualized cord compression show clinical myelopathy, and the amount of cord compression only weakly correlates with disability. We propose that DCM is better represented as a function of several interacting mechanical forces, such as shear, tension and compression, alongside an individual's vulnerability to spinal cord injury, influenced by factors such as age, genetics, their cardiovascular, gastrointestinal and nervous system status, and time.

CONCLUSION

Understanding the disease pathobiology is a fundamental research priority. We believe a framework of mechanical stress, vulnerability, and time may better represent the disease as a whole. Whilst this remains theoretical, we hope that at the very least it will inspire new avenues of research that better encapsulate the full spectrum of disease.

Plasma membrane disruption (PMD) formation and repair in mechanosensitive tissues

Mammalian cells employ an array of biological mechanisms to detect and respond to mechanical loading in their environment. One such mechanism is the formation of plasma membrane disruptions (PMD), which foster a molecular flux across cell membranes that promotes tissue adaptation. Repair of PMD through an orchestrated activity of molecular machinery is critical for cell survival, and the rate of PMD repair can affect downstream cellular signaling. PMD have been observed to influence the mechanical behavior of skin, alveolar, and gut epithelial cells, aortic endothelial cells, corneal keratocytes and epithelial cells, cardiac and skeletal muscle myocytes, neurons, and most recently, bone cells including osteoblasts, periodontal ligament cells, and osteocytes. PMD are therefore positioned to affect the physiological behavior of a wide range of vertebrate organ systems including skeletal and cardiac muscle, skin, eyes, the gastrointestinal tract, the vasculature, the respiratory system, and the skeleton. The purpose of this review is to describe the processes of PMD formation and repair across these mechanosensitive tissues, with a particular emphasis on comparing and contrasting repair mechanisms and downstream signaling to better understand the role of PMD in skeletal mechanobiology. The implications of PMD-related mechanisms for disease and potential therapeutic applications are also explored.

SIRT1 Inhibits High Shear Stress-Induced Apoptosis in Rat Cortical Neurons

INTRODUCTION

Sirtuin1 (SIRT1), one of NAD+-dependent protein deacetylases, is proved to be neuroprotective in aging diseases, but its effect on neuronal apoptosis has not been clarified. To investigate the role of SIRT1 in inhibiting neuronal apoptosis, SIRT1 was interfered or overexpressed in cortical neurons.

METHODS

We exerted overloading laminar shear stress with 10 dyn/cm2 for 4, 8, and 12 h on neurons to cause cortical neuronal apoptosis, and the apoptosis percentage was tested by TUNEL assay. The adenovirus plasmids containing SIRT1 RNA interference or SIRT1 wild type gene were transfected into neurons before shear stress loading. SIRT1 mRNA and protein level were tested by Real-time PCR, immunofluorescence and western blots assay.

RESULTS

SIRT1 was primarily expressed in nucleus of cortical neurons, and its mRNA level was significantly increased after 4 h stimulation. SIRT1 RNAi cortical neurons had higher TUNEL positive cells, while SIRT1 overexpression significantly decreased the percentage of died cells induced by shear stress compared to control group.

CONCLUSIONS

SIRT1 plays a neuroprotective role in shear stress induced apoptosis and could be as potential pharmacological targets against neuronal degeneration in future.

Neuronal Plasma Membrane Integrity is Transiently Disturbed by Traumatic Loading

The acute response of neurons subjected to traumatic loading involves plasma membrane disruption, yet the mechanical tolerance for membrane compromise, time course, and mechanisms for resealing are not well understood. We have used an in vitro traumatic neuronal injury model to investigate plasma membrane integrity immediately following a high-rate shear injury. Cell-impermeant fluorescent molecules were added to cortical neuronal cultures prior to insult to assess membrane integrity. The percentage of cells containing the permeability marker was dependent on the molecular size of the marker, as smaller molecules gained access to a higher percentage of cells than larger ones. Permeability increases were positively correlated with insult loading rate. Membrane disruption was transient, evidenced by a membrane resealing within the first minute after the insult. In addition, chelation of either extracellular Ca²⁺ or intracellular Ca²⁺ limited membrane resealing. However, injury following chelation of both extracellular and intracellular Ca²⁺ caused diminished permeability as well as a greater resealing ability compared to chelation of extracellular or intracellular Ca²⁺ alone. Treatment of neuronal cultures with jasplakinolide, which stabilizes filamentous actin, reduced permeability increases, while latrunculin-B, an actin depolymerizing agent, both reduced the increase in plasma membrane permeability and promoted resealing. This study gives insight into the dynamics of neuronal membrane disruption and subsequent resealing, which was found to be calcium dependent and involve actin in a role that differs from non-neuronal cells. Taken together, these data will lead to a better understanding of the acute neuronal response to traumatic loading.

Localized Axolemma Deformations Suggest Mechanoporation as Axonal Injury Trigger

Traumatic brain injuries are a leading cause of morbidity and mortality worldwide. With almost 50% of traumatic brain injuries being related to axonal damage, understanding the nature of cellular level impairment is crucial. Experimental observations have so far led to the formulation of conflicting theories regarding the cellular primary injury mechanism. Disruption of the axolemma, or alternatively cytoskeletal damage has been suggested mainly as injury trigger. However, mechanoporation thresholds of generic membranes seem not to overlap with the axonal injury deformation range and microtubules appear too stiff and too weakly connected to undergo mechanical breaking. Here, we aim to shed a light on the mechanism of primary axonal injury, bridging finite element and molecular dynamics simulations. Despite the necessary level of approximation, our models can accurately describe the mechanical behavior of the unmyelinated axon and its membrane. More importantly, they give access to quantities that would be inaccessible with an experimental approach. We show that in a typical injury scenario, the axonal cortex sustains deformations large enough to entail pore formation in the adjoining lipid bilayer. The observed axonal deformation of 10–12% agree well with the thresholds proposed in the literature for axonal injury and, above all, allow us to provide quantitative evidences that do not exclude pore formation in the membrane as a result of trauma. Our findings bring to an increased knowledge of axonal injury mechanism that will have positive implications for the prevention and treatment of brain injuries.

Neuronal Membrane Disruption Occurs Late Following Diffuse Brain Trauma in Rats and Involves a Subpopulation of NeuN Negative Cortical Neurons

The repercussions of traumatic brain injury (TBI) endure years following the initial insult and involve chronic impairments/disabilities. Studies indicate that these morbidities stem from diffuse pathologies, however, knowledge regarding TBI-mediated diffuse pathologies, and in particular, diffuse neuronal membrane disruption, is limited. Membrane disruption has been shown to occur acutely following injury, primarily within neurons, however, the progression of TBI-induced membrane disruption remains undefined. Therefore, the current study investigated this pathology over a longer temporal profile from 6 h to 4 w following diffuse TBI induced using the central fluid percussion injury (CFPI) model in rats. To visualize membrane disruption, animals received an intracerebroventricular infusion of tagged cell-impermeable dextran 2 h prior to experimental endpoints at 6 h, 1 d, 3 d, 1 w, 2 w, or 4 w post-CFPI. The percentage of total neurons demonstrating dextran uptake, indicative of membrane disruption, was quantified within the lateral neocortex layers V and VI from 6 h to 4 w post-injury. We found that membrane disruption displayed a biphasic pattern, where nearly half of the neurons were membrane disrupted sub-acutely, from 6 h to 3 d post-TBI. At 1 w the membrane disrupted population was dramatically reduced to levels indistinguishable from sham controls. However, by 2 and 4 w following CFPI, approximately half of the neurons analyzed displayed membrane disruption. Moreover, our data revealed that a subset of these late membrane disrupted neurons were NeuN negative (NeuN-). Correlative western blot analyses, however, revealed no difference in NeuN protein expression in the lateral neocortex at any time following injury. Furthermore, the NeuN- membrane disrupted neurons did not co-label with traditional markers of astrocytes, microglia, oligodendrocytes, or NG2 cells. Immunohistochemistry against NeuN, paired with a hematoxylin and eosin counter-stain, was performed to quantify the possibility of overall NeuN+ neuronal loss following CFPI. A NeuN- population was observed consistently in both sham and injured animals regardless of time post-injury. These data suggest that there is a consistent subpopulation of NeuN- neurons within the lateral neocortex regardless of injury and that these NeuN- neurons are potentially more vulnerable to late membrane disruption. Better understanding of membrane disruption could provide insight into the mechanisms of diffuse pathology and lead to the discovery of novel treatments for TBI.

Directing Neuronal Outgrowth and Network Formation of Rat Cortical Neurons by Cyclic Substrate Stretch

Neuronal mechanobiology plays a vital function in brain development and homeostasis with an essential role in neuronal maturation, pathfinding, and differentiation but is also crucial for understanding brain pathology. In this study, we constructed an in vitro system to assess neuronal responses to cyclic strain as a mechanical signal. The selected strain amplitudes mimicked physiological as well as pathological conditions. By subjecting embryonic neuronal cells to cyclic uniaxial strain we could steer the direction of neuronal outgrowth perpendicular to strain direction for all applied amplitudes. A long-term analysis proved maintained growth direction. Moreover, stretched neurons showed an enhanced length, growth, and formation of nascent side branches with most elevated growth rates subsequent to physiological straining. Application of cyclic strain to already formed neurites identified retraction bulbs with destabilized microtubule structures as spontaneous responses. Importantly, neurons were able to adapt to the mechanical signals without induction of cell death and showed a triggered growth behavior when compared to unstretched neurons. The data suggest that cyclic strain plays a critical role in neuronal development.

Mechanoporation is a potential indicator of tissue strain and subsequent degeneration following experimental traumatic brain injury

BACKGROUND

An increases in plasma membrane permeability is part of the acute pathology of traumatic brain injury and may be a function of excessive membrane force. This membrane damage, or mechanoporation, allows non-specific flux of ions and other molecules across the plasma membrane, and may ultimately lead to cell death. The relationships among tissue stress and strain, membrane permeability, and subsequent cell degeneration, however, are not fully understood.

METHODS

Fluorescent molecules of different sizes were introduced to the cerebrospinal fluid space prior to injury and animals were sacrificed at either 10 min or 24 h after injury. We compared the spatial distribution of plasma membrane damage following controlled cortical impact in the rat to the stress and strain tissue patterns in a 3-D finite element simulation of the injury parameters.

FINDINGS

Permeable cells were located primarily in the ipsilateral cortex and hippocampus of injured rats at 10 min post-injury; however by 24 h there was also a significant increase in the number of permeable cells. Analysis of colocalization of permeability marker uptake and Fluorojade staining revealed a subset of permeable cells with signs of degeneration at 24 h, but plasma membrane damage was evident in the vast majority of degenerating cells. The regional and subregional distribution patterns of the maximum principal strain and shear stress estimated by the finite element model were comparable to the cell membrane damage profiles following a compressive impact.

INTERPRETATION

These results indicate that acute membrane permeability is prominent following traumatic brain injury in areas that experience high shear or tensile stress and strain due to differential mechanical properties of the cell and tissue organization, and that this mechanoporation may play a role in the initiation of secondary injury, contributing to cell death.

Repeatability of a Dislocation Spinal Cord Injury Model in a Rat-A High-Speed Biomechanical Analysis

Dislocation is the most common, and severe, spinal cord injury (SCI) mechanism in humans, yet there are few preclinical models. While dislocation in the rat model has been shown to produce unique outcomes, like other closed column models it exhibits higher outcome variability. Refinement of the dislocation model will enhance the testing of neuroprotective strategies, further biomechanical understanding, and guide therapeutic decisions. The overall objective of this study is to improve biomechanical repeatability of a dislocation SCI model in the rat, through the following specific aims: (i) design new injury clamps that pivot and self-align to the vertebrae; (ii) measure intervertebral kinematics during injury using the existing and redesigned clamps; and (iii) compare relative motion at the vertebrae-clamp interface to determine which clamps provide the most rigid connection. Novel clamps that pivot and self-align were developed based on the quantitative rat vertebral anatomy. A dislocation injury was produced in 34 rats at C4/C5 using either the existing or redesigned clamps, and a high-speed X-ray device recorded the kinematics. Relative motion between the caudal clamp and C5 was significantly greater in the existing clamps compared to the redesigned clamps in dorsoventral translation and sagittal rotation. This study demonstrates that relative motions can be of magnitudes that likely affect injury outcomes. We recommend such biomechanical analyses be applied to other SCI models when repeatability is an issue. For this dislocation model, the results show the importance of using clamps that pivot and self-align to the vertebrae.

Region and species dependent mechanical properties of adolescent and young adult brain tissue

Traumatic brain injuries, the leading cause of death and disability in children and young adults, are the result of a rapid acceleration or impact of the head. In recent years, a global effort to better understand the biomechanics of TBI has been undertaken, with many laboratories creating detailed computational models of the head and brain. For these models to produce realistic results they require accurate regional constitutive data for brain tissue. However, there are large differences in the mechanical properties reported in the literature. These differences are likely due to experimental parameters such as specimen age, brain region, species, test protocols, and fiber direction which are often not reported. Furthermore, there is a dearth of reported viscoelastic properties for brain tissue at large-strain and high rates. Mouse, rat, and pig brains are impacted at 10/s to a strain of ~36% using a custom-built micro-indenter with a 125 μm radius. It is shown that the resultant mechanical properties are dependent on specimen-age, species, and region, under identical experimental parameters.

Therapeutic Potentials of Synapses after Traumatic Brain Injury: A Comprehensive Review

Massive studies have focused on the understanding of the pathobiology of cellular and molecular changes and injury mechanisms after traumatic brain injury (TBI), but very few studies have specially discussed the role of synapses in the context of TBI. This paper specifically highlights the role and therapeutic potentials of synapses after TBI. First, we review and conclude how synapses interact with constant structural, metabolic, neuroendocrine, and inflammatory mechanisms after TBI. Second, we briefly describe several key synaptic proteins involved in neuroplasticity, which may be novel neuronal targets for specific intervention. Third, we address therapeutic interventions in association with synapses after TBI. Finally, we concisely discuss the study gaps in the synapses after TBI, in hopes that this would provide more insights for future studies. Synapses play an important role in TBI; while the understandings on the synaptic participation in the treatments and prognosis of TBI are lacking, more studies in this area are warranted.

Dynamic association between perfusion and white matter integrity across time since injury in Veterans with history of TBI

OBJECTIVE

Cerebral blood flow (CBF) plays a critical role in the maintenance of neuronal integrity, and CBF alterations have been linked to deleterious white matter changes. Although both CBF and white matter microstructural alterations have been observed within the context of traumatic brain injury (TBI), the degree to which these pathological changes relate to one another and whether this association is altered by time since injury have not been examined. The current study therefore sought to clarify associations between resting CBF and white matter microstructure post-TBI.

METHODS

37 veterans with history of mild or moderate TBI (mmTBI) underwent neuroimaging and completed health and psychiatric symptom questionnaires. Resting CBF was measured with multiphase pseudocontinuous arterial spin labeling (MPPCASL), and white matter microstructural integrity was measured with diffusion tensor imaging (DTI). The cingulate cortex and cingulum bundle were selected as a priori regions of interest for the ASL and DTI data, respectively, given the known vulnerability of these regions to TBI.

RESULTS

Regression analyses controlling for age, sex, and posttraumatic stress disorder (PTSD) symptoms revealed a significant time since injury × resting CBF interaction for the left cingulum (p < 0.005). Decreased CBF was significantly associated with reduced cingulum fractional anisotropy (FA) in the chronic phase; however, no such association was observed for participants with less remote TBI.

CONCLUSIONS

Our results showed that reduced CBF was associated with poorer white matter integrity in those who were further removed from their brain injury. Findings provide preliminary evidence of a possible dynamic association between CBF and white matter microstructure that warrants additional consideration within the context of the negative long-term clinical outcomes frequently observed in those with history of TBI. Additional cross-disciplinary studies integrating multiple imaging modalities (e.g., DTI, ASL) and refined neuropsychiatric assessment are needed to better understand the nature, temporal course, and dynamic association between brain changes and clinical outcomes post-injury.

Tissue Strain Reorganizes Collagen With a Switchlike Response That Regulates Neuronal Extracellular Signal-Regulated Kinase Phosphorylation In Vitro: Implications for Ligamentous Injury and Mechanotransduction

Excessive loading of ligaments can activate the neural afferents that innervate the collagenous tissue, leading to a host of pathologies including pain. An integrated experimental and modeling approach was used to define the responses of neurons and the surrounding collagen fibers to the ligamentous matrix loading and to begin to understand how macroscopic deformation is translated to neuronal loading and signaling. A neuron-collagen construct (NCC) developed to mimic innervation of collagenous tissue underwent tension to strains simulating nonpainful (8%) or painful ligament loading (16%). Both neuronal phosphorylation of extracellular signal-regulated kinase (ERK), which is related to neuroplasticity (R2 ≥ 0.041; p ≤ 0.0171) and neuronal aspect ratio (AR) (R2 ≥ 0.250; p < 0.0001), were significantly correlated with tissue-level strains. As NCC strains increased during a slowly applied loading (1%/s), a "switchlike" fiber realignment response was detected with collagen reorganization occurring only above a transition point of 11.3% strain. A finite-element based discrete fiber network (DFN) model predicted that at bulk strains above the transition point, heterogeneous fiber strains were both tensile and compressive and increased, with strains in some fibers along the loading direction exceeding the applied bulk strain. The transition point identified for changes in collagen fiber realignment was consistent with the measured strain threshold (11.7% with a 95% confidence interval of 10.2-13.4%) for elevating ERK phosphorylation after loading. As with collagen fiber realignment, the greatest degree of neuronal reorientation toward the loading direction was observed at the NCC distraction corresponding to painful loading. Because activation of neuronal ERK occurred only at strains that produced evident collagen fiber realignment, findings suggest that tissue strain-induced changes in the micromechanical environment, especially altered local collagen fiber kinematics, may be associated with mechanotransduction signaling in neurons.

Effects of repetitive low-pressure explosive blast on primary neurons and mixed cultures

Repetitive mild traumatic brain injury represents a considerable health concern, particularly for athletes and military personnel. For blast-induced brain injury, threshold shock-impulse levels required to induce such injuries and cumulative effects with single and/or multiple exposures are not well characterized. Currently, there is no established in vitro experimental model with blast pressure waves generated by live explosives. This study presents results of primary neurons and mixed cultures subjected to our unique in vitro indoor experimental platform that uses real military explosive charges to probe the effects of primary explosive blast at the cellular level. The effects of the blast on membrane permeability, generation of reactive oxygen species (ROS), uptake of sodium ions, intracellular calcium, and release of glutamate were probed 2 and 24 hr postblast. Significant changes in membrane permeability and sodium uptake among the sham, single-blast-injured, and triple-blast-injured samples were observed. A significant increase in ROS and glutamate release was observed for the triple-blast-injured samples compared with the sham. Changes in intracellular calcium were not significant. These results suggest that blast exposure disrupts the integrity of the plasma membrane, leading to the upset of ion homeostasis, formation of ROS, and glutamate release. Published 2016. †This article is a U.S. Government work and is in the public domain in the USA.

Synaptic Mechanisms of Blast-Induced Brain Injury

Blast wave-induced traumatic brain injury (TBI) is one of the most common injuries to military personnel. Brain tissue compression/tension due to blast-induced cranial deformations and shear waves due to head rotation may generate diffuse micro-damage to neuro-axonal structures and trigger a cascade of neurobiological events culminating in cognitive and neurodegenerative disorders. Although diffuse axonal injury is regarded as a signature wound of mild TBI (mTBI), blast loads may also cause synaptic injury wherein neuronal synapses are stretched and sheared. This synaptic injury may result in temporary disconnect of the neural circuitry and transient loss in neuronal communication. We hypothesize that mTBI symptoms such as loss of consciousness or dizziness, which start immediately after the insult, could be attributed to synaptic injury. Although empirical evidence is beginning to emerge; the detailed mechanisms underlying synaptic injury are still elusive. Coordinated in vitro-in vivo experiments and mathematical modeling studies can shed light into the synaptic injury mechanisms and their role in the potentiation of mTBI symptoms.

Alteration in synaptic junction proteins following traumatic brain injury

Extensive research and scientific efforts have been focused on the elucidation of the pathobiology of cellular and axonal damage following traumatic brain injury (TBI). Conversely, few studies have specifically addressed the issue of synaptic dysfunction. Synaptic junction proteins may be involved in post-TBI alterations, leading to synaptic loss or disrupted plasticity. A Synapse Protein Database on synapse ontology identified 109 domains implicated in synaptic activities and over 5000 proteins, but few of these demonstrated to play a role in the synaptic dysfunction after TBI. These proteins are involved in neuroplasticity and neuromodulation and, most importantly, may be used as novel neuronal markers of TBI for specific intervention.

A novel closed-body model of spinal cord injury caused by high-pressure air blasts produces extensive axonal injury and motor impairments

Diffuse axonal injury is thought to be the basis of the functional impairments stemming from mild traumatic brain injury. To examine how axons are damaged by traumatic events, such as motor vehicle accidents, falls, sports activities, or explosive blasts, we have taken advantage of the spinal cord with its extensive white matter tracts. We developed a closed-body model of spinal cord injury in mice whereby high-pressure air blasts targeted to lower thoracic vertebral levels produce tensile, compressive, and shear forces within the parenchyma of the spinal cord and thereby cause extensive axonal injury. Markers of cytoskeletal integrity showed that spinal cord axons exhibited three distinct pathologies: microtubule breakage, neurofilament compaction, and calpain-mediated spectrin breakdown. The dorsally situated axons of the corticospinal tract primarily exhibited microtubule breakage, whereas all three pathologies were common in the lateral and ventral white matter. Individual axons typically demonstrated only one of the three pathologies during the first 24h after blast injury, suggesting that the different perturbations are initiated independently of one another. For the first few days after blast, neurofilament compaction was frequently accompanied by autophagy, and subsequent to that, by the fragmentation of degenerating axons. TuJ1 immunolabeling and mice with YFP-reporter labeling each revealed more extensive microtubule breakage than did βAPP immunolabeling, raising doubts about the sensitivity of this standard approach for assessing axonal injury. Although motor deficits were mild and largely transient, some aspects of motor function gradually worsened over several weeks, suggesting that a low level of axonal degeneration continued past the initial wave. Our model can help provide further insight into how to intervene in the processes by which initial axonal damage culminates in axonal degeneration, to improve outcomes after traumatic injury. Importantly, our findings of extensive axonal injury also caution that repeated trauma is likely to have cumulative adverse consequences for both brain and spinal cord.

Opposite rheological properties of neuronal microcompartments predict axonal vulnerability in brain injury

Although pathological changes in axonal morphology have emerged as important features of traumatic brain injury (TBI), the mechanical vulnerability of the axonal microcompartment relative to the cell body is not well understood. We hypothesized that soma and neurite microcompartments exhibit distinct mechanical behaviors, rendering axons more sensitive to a mechanical injury. In order to test this assumption, we combined protein micropatterns with magnetic tweezer rheology to probe the viscoelastic properties of neuronal microcompartments. Creep experiments revealed two opposite rheological behaviors within cortical neurons: the cell body was soft and characterized by a solid-like response, whereas the neurite compartment was stiffer and viscous-like. By using pharmacological agents, we demonstrated that the nucleus is responsible for the solid-like behavior and the stress-stiffening response of the soma, whereas neurofilaments have a predominant contribution in the viscous behavior of the neurite. Furthermore, we found that the neurite is a mechanosensitive compartment that becomes softer and adopts a pronounced viscous state on soft matrices. Together, these findings highlight the importance of the regionalization of mechanical and rigidity-sensing properties within neuron microcompartments in the preferential damage of axons during traumatic brain injury and into potential mechanisms of axonal outgrowth after injury.

Computational analysis of fluid flow within a device for applying biaxial strain to cultured cells

In vitro systems for applying mechanical strain to cultured cells are commonly used to investigate cellular mechanotransduction pathways in a variety of cell types. These systems often apply mechanical forces to a flexible membrane on which cells are cultured. A consequence of the motion of the membrane in these systems is the generation of flow and the unintended application of shear stress to the cells. We recently described a flexible system for applying mechanical strain to cultured cells, which uses a linear motor to drive a piston array to create biaxial strain within multiwell culture plates. To better understand the fluidic stresses generated by this system and other systems of this type, we created a computational fluid dynamics model to simulate the flow during the mechanical loading cycle. Alterations in the frequency or maximal strain magnitude led to a linear increase in the average fluid velocity within the well and a nonlinear increase in the shear stress at the culture surface over the ranges tested (0.5-2.0 Hz and 1-10% maximal strain). For all cases, the applied shear stresses were relatively low and on the order of millipascal with a dynamic waveform having a primary and secondary peak in the shear stress over a single mechanical strain cycle. These findings should be considered when interpreting experimental results using these devices, particularly in the case when the cell type used is sensitive to low magnitude, oscillatory shear stresses.

Mechanical compression insults induce nanoscale changes of membrane-skeleton arrangement which could cause apoptosis and necrosis in dorsal root ganglion neurons

In a primary spinal cord injury, the amount of mechanical compression insult that the neurons experience is one of the most critical factors in determining the extent of the injury. The ultrastructural changes that neurons undergo when subjected to mechanical compression are largely unknown. In the present study, using a compression-driven instrument that can simulate mechanical compression insult, we applied mechanical compression stimulation at 0.3, 0.5, and 0.7 MPa to dorsal root ganglion (DRG) neurons for 10 min. Combined with atomic force microscopy, we investigated nanoscale changes in the membrane-skeleton, cytoskeleton alterations, and apoptosis induced by mechanical compression injury. The results indicated that mechanical compression injury leads to rearrangement of the membrane-skeleton compared with the control group. In addition, mechanical compression stimulation induced apoptosis and necrosis and also changed the distribution of the cytoskeleton in DRG neurons. Thus, the membrane-skeleton may play an important role in the response to mechanical insults in DRG neurons. Moreover, sudden insults caused by high mechanical compression, which is most likely conducted by the membrane-skeleton, may induce necrosis, apoptosis, and cytoskeletal alterations.

Powerful partnership: crosstalk between pannexin 1 and the cytoskeleton

The roles of pannexin 1 (Panx1) large-pore ion and metabolite channels are becoming recognized in many physiological and pathophysiological scenarios. Recent evidence has tightly linked Panx1 trafficking and function to the cytoskeleton, a multi-component network that provides critical structural support, transportation, and scaffolding functions in all cell types. Here we review early work demonstrating the mechanosensitive activation of Panx1 channels, and expand on more recent evidence directly linking Panx1 to the cytoskeleton. Further, we examine the reciprocal regulation between Panx1 and the cytoskeleton, and discuss the involvement of Panx1 in cytoskeletal-regulated cell behaviors. Finally, we identify important gaps in the current knowledge surrounding this emerging Panx1-cytoskeleton relationship.

Biomarkers for the diagnosis and prognosis of mild traumatic brain injury/concussion

Mild traumatic brain injury (mTBI) results from a transfer of mechanical energy into the brain from traumatic events such as rapid acceleration/deceleration, a direct impact to the head, or an explosive blast. Transfer of energy into the brain can cause structural, physiological, and/or functional changes in the brain that may yield neurological, cognitive, and behavioral symptoms that can be long-lasting. Because mTBI can cause these symptoms in the absence of positive neuroimaging findings, its diagnosis can be subjective and often is based on self-reported neurological symptoms. Further, proper diagnosis can be influenced by the motivation to conceal or embellish signs and/or an inability of the patient to notice subtle dysfunctions or alterations of consciousness. Therefore, appropriate diagnosis of mTBI would benefit from objective indicators of injury. Concussion and mTBI are often used interchangeably, with concussion being primarily used in sport medicine, whereas mTBI is used in reference to traumatic injury. This review provides a critical assessment of the status of current biomarkers for the diagnosis of human mTBI. We review the status of biomarkers that have been tested in TBI patients with injuries classified as mild, and introduce a new concept for the discovery of biomarkers (termed symptophenotypes) to predict common and unique symptoms of concussion. Finally, we discuss the need for biomarker/biomarker signatures that can detect mTBI in the context of polytrauma, and to assess the consequences of repeated injury on the development of secondary injury syndrome, prolongation of post-concussion symptoms, and chronic traumatic encephalopathy.

Mathematical Models of Blast-Induced TBI: Current Status, Challenges, and Prospects

Blast-induced traumatic brain injury (TBI) has become a signature wound of recent military activities and is the leading cause of death and long-term disability among U.S. soldiers. The current limited understanding of brain injury mechanisms impedes the development of protection, diagnostic, and treatment strategies. We believe mathematical models of blast wave brain injury biomechanics and neurobiology, complemented with in vitro and in vivo experimental studies, will enable a better understanding of injury mechanisms and accelerate the development of both protective and treatment strategies. The goal of this paper is to review the current state of the art in mathematical and computational modeling of blast-induced TBI, identify research gaps, and recommend future developments. A brief overview of blast wave physics, injury biomechanics, and the neurobiology of brain injury is used as a foundation for a more detailed discussion of multiscale mathematical models of primary biomechanics and secondary injury and repair mechanisms. The paper also presents a discussion of model development strategies, experimental approaches to generate benchmark data for model validation, and potential applications of the model for prevention and protection against blast wave TBI.

Galanin protects against nerve injury after shear stress in primary cultured rat cortical neurons

The neuropeptide galanin and its receptors (GalR) are found to be up-regulated in brains suffering from nerve injury, but the specific role played by galanin remains unclear. This study aimed to explore the neuroprotective role of galanin after shear stress induced nerve injury in the primary cultured cortical neurons of rats. Our results demonstrated that no significant changes in cell death and viability were found after galanin treatment when subjected to a shear stress of 5 dyn/cm(2) for 12 h, after increasing magnitude of shear stress to 10 dyn/cm(2) for 12 h, cell death was significantly increased, while galanin can inhibit the nerve injury induced by shear stress with 10 dyn/cm(2) for 12 h. Moreover, Gal2-11 (an agonist of GalR2/3) could also effectively inhibit shear stress-induced nerve injury of primary cultured cortical neurons in rats. Although GalR2 is involved in the galanin protection mechanism, there was no GalR3 expression in this system. Moreover, galanin increased the excitatory postsynaptic currents (EPSCs), which can effectively inhibit the physiological effects of shear stress. Galanin was also found to inhibit the activation of p53 and Bax, and further reversed the down regulation of Bcl-2 induced by shear stress. Our results strongly demonstrated that galanin plays a neuroprotective role in injured cortical neurons of rats.

Neuropathology of mild traumatic brain injury: relationship to neuroimaging findings

Neuroimaging identified abnormalities associated with traumatic brain injury (TBI) are but gross indicators that reflect underlying trauma-induced neuropathology at the cellular level. This review examines how cellular pathology relates to neuroimaging findings with the objective of more closely relating how neuroimaging findings reveal underlying neuropathology. Throughout this review an attempt will be made to relate what is directly known from post-mortem microscopic and gross anatomical studies of TBI of all severity levels to the types of lesions and abnormalities observed in contemporary neuroimaging of TBI, with an emphasis on mild traumatic brain injury (mTBI). However, it is impossible to discuss the neuropathology of mTBI without discussing what occurs with more severe injury and viewing pathological changes on some continuum from the mildest to the most severe. Historical milestones in understanding the neuropathology of mTBI are reviewed along with implications for future directions in the examination of neuroimaging and neuropathological correlates of TBI.

Continuum modeling of a neuronal cell under blast loading

Traumatic brain injuries have recently been put under the spotlight as one of the most important causes of accidental brain dysfunctions. Significant experimental and modeling efforts are thus underway to study the associated biological, mechanical and physical mechanisms. In the field of cell mechanics, progress is also being made at the experimental and modeling levels to better characterize many of the cell functions, including differentiation, growth, migration and death. The work presented here aims to bridge both efforts by proposing a continuum model of a neuronal cell submitted to blast loading. In this approach, the cytoplasm, nucleus and membrane (plus cortex) are differentiated in a representative cell geometry, and different suitable material constitutive models are chosen for each one. The material parameters are calibrated against published experimental work on cell nanoindentation at multiple rates. The final cell model is ultimately subjected to blast loading within a complete computational framework of fluid-structure interaction. The results are compared to the nanoindentation simulation, and the specific effects of the blast wave on the pressure and shear levels at the interfaces are identified. As a conclusion, the presented model successfully captures some of the intrinsic intracellular phenomena occurring during the cellular deformation under blast loading that potentially lead to cell damage. It suggests, more particularly, that the localization of damage at the nucleus membrane is similar to what has already been observed at the overall cell membrane. This degree of damage is additionally predicted to be worsened by a longer blast positive phase duration. In conclusion, the proposed model ultimately provides a new three-dimensional computational tool to evaluate intracellular damage during blast loading.

An in vitro injury model for SH-SY5Y neuroblastoma cells: effect of strain and strain rate

There is a great need to have in vitro cell injury model wherein a wide range of strain (ɛ) and strain rate (ε˙) can be precisely and independently applied. Such a model will enable exploration of various biomechanical loading conditions cells normally encounter during either blunt or blast impact-induced traumatic brain injuries (TBIs). In combination with a highly automated data acquisition and analysis system, this method can quickly generate a large data set of experimental results to yield identification of bio-mechanical and chemical sequelae following injury. A proper understanding of these sequelae will enable the discovery of the time window of opportunity available for pharmacological interventions. In this study we present such an injury model, a modified version of the Cultured Axonal Injury (CAI) device, and demonstrate its efficacy through viability of SH-SY5Y cells at different ranges of strain (0-140%) and strain rate (15-68 s⁻¹). We identified three different regimes in the stretch-induced dose-response of curves of SH-SY5Y cells, with a very sharp decline from live to dead in a narrow range of strain (30-55%). The effect of strain rate is minimal when the final strain in the cells was fixed at 50%. The model further shows that time-after-injury plays a vital role in the determination of recovery-deterioration pathways and the biological selection depends on the severity of initial injury. These data point out the initial strain level is vital to the cell fate and emphasize the need to study the various mechanisms triggered by different magnitudes of initial injuries.

Dynamic changes in neural circuit topology following mild mechanical injury in vitro

Despite its enormous incidence, mild traumatic brain injury is not well understood. One aspect that needs more definition is how the mechanical energy during injury affects neural circuit function. Recent developments in cellular imaging probes provide an opportunity to assess the dynamic state of neural networks with single-cell resolution. In this article, we developed imaging methods to assess the state of dissociated cortical networks exposed to mild injury. We estimated the imaging conditions needed to achieve accurate measures of network properties, and applied these methodologies to evaluate if mild mechanical injury to cortical neurons produces graded changes to either spontaneous network activity or altered network topology. We found that modest injury produced a transient increase in calcium activity that dissipated within 1 h after injury. Alternatively, moderate mechanical injury produced immediate disruption in network synchrony, loss in excitatory tone, and increased modular topology. A calcium-activated neutral protease (calpain) was a key intermediary in these changes; blocking calpain activation restored the network nearly completely to its pre-injury state. Together, these findings show a more complex change in neural circuit behavior than previously reported for mild mechanical injury, and highlight at least one important early mechanism responsible for these changes.

Adaptive movable neural interfaces for monitoring single neurons in the brain

Implantable microelectrodes that are currently used to monitor neuronal activity in the brain in vivo have serious limitations both in acute and chronic experiments. Movable microelectrodes that adapt their position in the brain to maximize the quality of neuronal recording have been suggested and tried as a potential solution to overcome the challenges with the current fixed implantable microelectrodes. While the results so far suggest that movable microelectrodes improve the quality and stability of neuronal recordings from the brain in vivo, the bulky nature of the technologies involved in making these movable microelectrodes limits the throughput (number of neurons that can be recorded from at any given time) of these implantable devices. Emerging technologies involving the use of microscale motors and electrodes promise to overcome this limitation. This review summarizes some of the most recent efforts in developing movable neural interfaces using microscale technologies that adapt their position in response to changes in the quality of the neuronal recordings. Key gaps in our understanding of the brain-electrode interface are highlighted. Emerging discoveries in these areas will lead to success in the development of a reliable and stable interface with single neurons that will impact basic neurophysiological studies and emerging cortical prosthetic technologies.

Rotational acceleration closed head flexion trauma generates more extensive diffuse brain injury than extension trauma

Our aim was to investigate if seemingly identical head and neck trauma would generate differing types of brain damage. We experimentally evaluated induced brain injuries immediately after trauma exposure, and at 1 week post-injury. Anesthetized rabbits were exposed once to a sagittal rotational acceleration head and neck injury at either a high or a low load level, using either flexion or extension. A high-load extension trauma induced scattered meningeal petechial hemorrhages and no deaths, in contrast to a flexion trauma of the same level, which resulted in extensive parenchymal and meningeal hemorrhages, and all animals succumbed immediately. A low-level flexion trauma induced scattered meningeal petechiae, but no gross damage, while extension at the same force generated no macroscopically visible acute brain injury. Immunohistochemical investigations carried out at 7 days disclosed that a low-level flexion trauma, as well as both low- and high-level extension exposures, all induced diffuse brain injuries in the cerebral cortex and white matter, corpus callosum, hippocampus, brainstem, and cerebellum, as revealed by abnormal distribution of neurofilaments, a prevalence of β-amyloid precursor protein, and astrogliosis. The diffuse brain injury seen after a low-level flexion trauma was equal to or more extensive than that seen after a high-level extension trauma. A low-level extension trauma induced only minor histopathological abnormalities. We conclude that a sagittal rotational acceleration trauma of the head and neck induced diffuse brain injury, and that flexion caused more extensive damage than extension at the same applied load.