Cytochemical evidence for redistribution of membrane pump calcium-ATPase and ecto-Ca-ATPase activity, and calcium influx in myelinated nerve fibres of the optic nerve after stretch injury

Determinants of spinal cord stress and strain in degenerative cervical myelopathy: a patient-specific finite element study

Degenerative cervical myelopathy (DCM) is the commonest cause of spinal cord dysfunction in older adults and is characterized by chronic cervical spinal cord compression. Spinal cord stress and strain during neck motion are also known contributors to the pathophysiology of DCM, yet these factors are not routinely assessed for surgical planning. The aim of this study was to measure spinal cord stress/strain in DCM using patient-specific 3D finite element models (FEMs) and determine whether spinal cord compression is the primary determinant of spinal cord stress/strain. Three-dimensional patient-specific FEMs were created for six DCM patients (mild [n = 2], moderate [n = 2] and severe [n = 2]). Flexion and extension of the cervical spine were simulated with a pure moment load of 2 Nm. Segmental spinal cord von Mises stress and maximum principal strain were measured. Measures of spinal cord compression and segmental range of motion (ROM) were included in a regression analysis to determine associations with spinal cord stress and strain. Segmental ROM in flexion-extension and axial rotation was independently associated with spinal cord stress (p < 0.001) and strain (p < 0.001), respectively. This relationship was not seen for lateral bending. Segmental ROM had a stronger association with spinal stress and strain as compared to spinal cord compression. Compared to the severity of spinal cord compression, segmental ROM is a stronger determinant spinal cord stress and strain. Surgical procedures that address segmental ROM in addition to cord compression may best optimize spinal cord biomechanics in DCM.

Oxidative Stress: A Suitable Therapeutic Target for Optic Nerve Diseases?

Optic nerve disorders encompass a wide spectrum of conditions characterized by the loss of retinal ganglion cells (RGCs) and subsequent degeneration of the optic nerve. The etiology of these disorders can vary significantly, but emerging research highlights the crucial role of oxidative stress, an imbalance in the redox status characterized by an excess of reactive oxygen species (ROS), in driving cell death through apoptosis, autophagy, and inflammation. This review provides an overview of ROS-related processes underlying four extensively studied optic nerve diseases: glaucoma, Leber's hereditary optic neuropathy (LHON), anterior ischemic optic neuropathy (AION), and optic neuritis (ON). Furthermore, we present preclinical findings on antioxidants, with the objective of evaluating the potential therapeutic benefits of targeting oxidative stress in the treatment of optic neuropathies.

Traumatic axonal injury: neuropathological features, postmortem diagnostic methods, and strategies

Traumatic brain injury (TBI) has high morbidity and poor prognosis and imposes a serious socioeconomic burden. Traumatic axonal injury (TAI), which is one of the common pathological changes in the primary injury of TBI, is often caused by the external force to the head that causes the white matter bundles to generate shear stress and tension; resulting in tissue damage and leading to the cytoskeletal disorder. At present, the forensic pathological diagnosis of TAI-caused death is still a difficult problem. Most of the TAI biomarkers studied are used for the prediction, evaluation, and prognosis of TAI in the living state. The research subjects are mainly humans in the living state or model animals, which are not suitable for the postmortem diagnosis of TAI. In addition, there is still a lack of recognized indicators for the autopsy pathological diagnosis of TAI. Different diagnostic methods and markers have their limitations, and there is a lack of systematic research and summary of autopsy diagnostic markers of TAI. Therefore, this study mainly summarizes the pathological mechanism, common methods, techniques of postmortem diagnosis, and corresponding biomarkers of TAI, and puts forward the strategies for postmortem diagnosis of TAI for forensic cases with different survival times, which is of great significance to forensic pathological diagnosis.

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.

Sports Related Brain Injury and Neurodegeneration in Athletes

Sports deserve a special place in human life to impart healthy and refreshing wellbeing. However, sports activities, especially contact sports, renders athlete vulnerable to brain injuries. Athletes participating in a contact sport like boxing, rugby, American football, wrestling, and basketball are exposed to traumatic brain injuries (TBI) or concussions. The acute and chronic nature of these heterogeneous injuries provides a spectrum of dysfunctions that alters the neuronal, musculoskeletal, and behavioral responses of an athlete. Many sports-related brain injuries go unreported, but these head impacts trigger neurometabolic disruptions that contribute to long-term neuronal impairment. The pathophysiology of post-concussion and its underlying mechanisms are undergoing intense research. It also shed light on chronic disorders like Parkinson's disease, Alzheimer's disease, and dementia. In this review, we examined post-concussion neurobehavioral changes, tools for early detection of signs, and their impact on the athlete. Further, we discussed the role of nutritional supplements in ameliorating neuropsychiatric diseases in athletes.

RGC and Vision Loss From Traumatic Optic Neuropathy Induced by Repetitive Closed Head Trauma Is Dependent on Timing and Force of Impact

Purpose

Traumatic optic neuropathy (TON) is often caused by blunt head trauma and has no currently effective treatment. Common animal models of TON induced by surgical crush injury are plagued by variability and do not mimic typical mechanisms of TON injury. Traumatic head impact models have recently shown evidence of TON, but the degree of head impact necessary to consistently induce TON is not well characterized, and it is examined here.

Methods

Traumatic skull impacts to C57BL/6J mice were induced using an electromagnetic controlled impact device. One impact performed at two depths (mild and severe), as well as three and five repetitive impacts with an interconcussion interval of 48 hours, were tested. Optokinetic responses (OKRs) and retinal ganglion cell (RGC) loss were measured.

Results

Five repetitive mild impacts significantly decreased OKR scores and RGC numbers compared with control mice 10 weeks after initial impact, with maximal pathology observed by 6 weeks and partial but significant loss present by 3 weeks. One severe impact induced similar TON. Three mild impacts also induced early OKR and RGC loss, but one mild impact did not. Equivalent degrees of TON were induced bilaterally, and a significant correlation was observed between OKR scores and RGC numbers.

Conclusions

Repetitive, mild closed head trauma in mice induces progressive RGC and vision loss that worsens with increasing impacts.

Translational Relevance

Results detail a reproducible model of TON that provides a reliable platform for studying potential treatments over a 3- to 6-week time course.

Traumatic axonal injury (TAI): definitions, pathophysiology and imaging-a narrative review

Introduction

Traumatic axonal injury (TAI) is a condition defined as multiple, scattered, small hemorrhagic, and/or non-hemorrhagic lesions, alongside brain swelling, in a more confined white matter distribution on imaging studies, together with impaired axoplasmic transport, axonal swelling, and disconnection after traumatic brain injury (TBI). Ever since its description in the 1980s and the grading system by Adams et al., our understanding of the processes behind this entity has increased.

Methods

We performed a scoping systematic, narrative review by interrogating Ovid MEDLINE, Embase, and Google Scholar on the pathophysiology, biomarkers, and diagnostic tools of TAI patients until July 2020.

Results

We underline the misuse of the Adams classification on MRI without proper validation studies, and highlight the hiatus in the scientific literature and areas needing more research. In the past, the theory behind the pathophysiology relied on the inertial force exerted on the brain matter after severe TBI inducing a primary axotomy. This theory has now been partially abandoned in favor of a more refined theory involving biochemical processes such as protein cleavage and DNA breakdown, ultimately leading to an inflammation cascade and cell apoptosis, a process now described as secondary axotomy.

Conclusion

The difference in TAI definitions makes the comparison of studies that report outcomes, treatments, and prognostic factors a daunting task. An even more difficult task is isolating the outcomes of isolated TAI from the outcomes of severe TBI in general. Targeted bench-to-bedside studies are required in order to uncover further pathways involved in the pathophysiology of TAI and, ideally, new treatments.

Three dimensional electron microscopy reveals changing axonal and myelin morphology along normal and partially injured optic nerves

Following injury to the central nervous system, axons and myelin distinct from the initial injury site undergo changes associated with compromised function. Quantifying such changes is important to understanding the pathophysiology of neurotrauma; however, most studies to date used 2 dimensional (D) electron microscopy to analyse single sections, thereby failing to capture changes along individual axons. We used serial block face scanning electron microscopy (SBF SEM) to undertake 3D reconstruction of axons and myelin, analysing optic nerves from normal uninjured female rats and following partial optic nerve transection. Measures of axon and myelin dimensions were generated by examining 2D images at 5 µm intervals along the 100 µm segments. In both normal and injured animals, changes in axonal diameter, myelin thickness, fiber diameter, G-ratio and percentage myelin decompaction were apparent along the lengths of axons to varying degrees. The range of values for axon diameter along individual reconstructed axons in 3D was similar to the range from 2D datasets, encompassing reported variation in axonal diameter attributed to retinal ganglion cell diversity. 3D electron microscopy analyses have provided the means to demonstrate substantial variability in ultrastructure along the length of individual axons and to improve understanding of the pathophysiology of neurotrauma.

Diffuse Axonal Injury and Oxidative Stress: A Comprehensive Review

Traumatic brain injury (TBI) is one of the world’s leading causes of morbidity and mortality among young individuals. TBI applies powerful rotational and translational forces to the brain parenchyma, which results in a traumatic diffuse axonal injury (DAI) responsible for brain swelling and neuronal death. Following TBI, axonal degeneration has been identified as a progressive process that starts with disrupted axonal transport causing axonal swelling, followed by secondary axonal disconnection and Wallerian degeneration. These modifications in the axonal cytoskeleton interrupt the axoplasmic transport mechanisms, causing the gradual gathering of transport products so as to generate axonal swellings and modifications in neuronal homeostasis. Oxidative stress with consequent impairment of endogenous antioxidant defense mechanisms plays a significant role in the secondary events leading to neuronal death. Studies support the role of an altered axonal calcium homeostasis as a mechanism in the secondary damage of axon, and suggest that calcium channel blocker can alleviate the secondary damage, as well as other mechanisms implied in the secondary injury, and could be targeted as a candidate for therapeutic approaches. Reactive oxygen species (ROS)-mediated axonal degeneration is mainly caused by extracellular Ca²⁺. Increases in the defense mechanisms through the use of exogenous antioxidants may be neuroprotective, particularly if they are given within the neuroprotective time window. A promising potential therapeutic target for DAI is to directly address mitochondria-related injury or to modulate energetic axonal energy failure.

Newfound sex differences in axonal structure underlie differential outcomes from in vitro traumatic axonal injury

Since traumatic axonal injury (TAI) is implicated as a prominent pathology of concussion, we examined potential sex differences in axon structure and responses to TAI. Rat and human neurons were used to develop micropatterned axon tracts in vitro that were genetically either male or female. Ultrastructural analysis revealed for the first time that female axons were consistently smaller with fewer microtubules than male axons. Computational modeling of TAI showed that these structural differences place microtubules in female axons at greater risk of failure during trauma under the same applied loads than in male axons. Likewise, in an in vitro model of TAI, dynamic stretch-injury to axon tracts induced greater pathophysiology of female axons than male axons, including more extensive undulation formations resulting from mechanical breaking of microtubules, and greater calcium influx shortly after the same level of injury. At 24h post-injury, female axons exhibited significantly more swellings and greater loss of calcium signaling function than male axons. Accordingly, sexual dimorphism of axon structure in the brain may also contribute to more extensive axonal pathology in females compared to males exposed to the same mechanical injury.

Axo-myelinic neurotransmission: a novel mode of cell signalling in the central nervous system

It is widely recognized that myelination of axons greatly enhances the speed of signal transmission. An exciting new finding is the dynamic communication between axons and their myelin-forming oligodendrocytes, including activity-dependent signalling from axon to myelin. The oligodendrocyte-myelin complex may in turn respond by providing metabolic support or alter subtle myelin properties to modulate action potential propagation. In this Opinion, we discuss what is known regarding the molecular physiology of this novel, synapse-like communication and speculate on potential roles in disease states including multiple sclerosis, schizophrenia and Alzheimer disease. An emerging appreciation of the contribution of white-matter perturbations to neurological dysfunction identifies the axo-myelinic synapse as a potential novel therapeutic target.

Axonal Degeneration in Tauopathies: Disease Relevance and Underlying Mechanisms

Tauopathies are a diverse group of diseases featuring progressive dying-back neurodegeneration of specific neuronal populations in association with accumulation of abnormal forms of the microtubule-associated protein tau. It is well-established that the clinical symptoms characteristic of tauopathies correlate with deficits in synaptic function and neuritic connectivity early in the course of disease, but mechanisms underlying these critical pathogenic events are not fully understood. Biochemical in vitro evidence fueled the widespread notion that microtubule stabilization represents tau's primary biological role and that the marked atrophy of neurites observed in tauopathies results from loss of microtubule stability. However, this notion contrasts with the mild phenotype associated with tau deletion. Instead, an analysis of cellular hallmarks common to different tauopathies, including aberrant patterns of protein phosphorylation and early degeneration of axons, suggests that alterations in kinase-based signaling pathways and deficits in axonal transport (AT) associated with such alterations contribute to the loss of neuronal connectivity triggered by pathogenic forms of tau. Here, we review a body of literature providing evidence that axonal pathology represents an early and common pathogenic event among human tauopathies. Observations of axonal degeneration in animal models of specific tauopathies are discussed and similarities to human disease highlighted. Finally, we discuss potential mechanistic pathways other than microtubule destabilization by which disease-related forms of tau may promote axonopathy.

Extent of impaired axoplasmic transport and neurofilament compaction in traumatically injured axon at various strains and strain rates

PRIMARY OBJECTIVE

Secondary axotomy is more prevalent than the primary axotomy and involves subtle intraaxonal changes in response to the injury leading to cytoskeletal disruptions including neurofilament (NF) misalignment and compaction, which is associated with the genesis of impaired axoplasmic transport (IAT). Recent studies have reported two differential axonal responses to injury, one associated with the cytoskeletal collapse and another with the IAT. The objective of this study was to determine the extent of IAT and early NF changes in axons that were subjected to a stretch of various degrees at different strain rates.

RESEARCH DESIGN AND METHODS

Fifty-six L5 dorsal spinal nerve roots were subjected to a predetermined strain at a specified displacement rate (0.01 and 15 mm/second) only once. The histological changes were determined by performing standard immunohistochemical procedures using beta amyloid precursor protein (β APP) and NF-68 kDa antibodies.

RESULTS AND CONCLUSIONS

No significant differences in the occurrence rate of either of the staining in the axons were observed when subjected to similar loading conditions, and the occurrence rate of both β APP and NF68 staining was strain and rate-dependent.

Effects of concussion on the blood-brain barrier in humans and rodents

Traumatic brain injury and the long-term consequences of repeated concussions constitute mounting concerns in the United States, with 5.3 million individuals living with a traumatic brain injury-related disability. Attempts to understand mechanisms and possible therapeutic approaches to alleviate the consequences of repeat mild concussions or traumatic brain injury on cerebral vasculature depend on several aspects of the trauma, including: (1) the physical characteristics of trauma or insult that result in damage; (2) the time “window” after trauma in which neuropathological features develop; (3) methods to detect possible breakdown of the blood–brain barrier; and (4) understanding different consequences of a single concussion as compared with multiple concussions. We review the literature to summarize the current understanding of blood–brain barrier and endothelial cell changes post-neurotrauma in concussions and mild traumatic brain injury. Attention is focused on concussion and traumatic brain injury in humans, with a goal of pointing out the gaps in our knowledge and how studies of rodent model systems of concussion may help in filling these gaps. Specifically, we focus on disruptions that concussion causes to the blood–brain barrier and its multifaceted consequences. Importantly, the magnitude of post-concussion blood–brain barrier dysfunction may influence the time course and extent of neuronal recovery; hence, we include in this review comparisons of more severe traumatic brain injury to concussion where appropriate. Finally, we address the important, and still unresolved, issue of how best to detect possible breakdown in the blood–brain barrier following neurotrauma by exploring intravascular tracer injection in animal models to examine leakage into the brain parenchyma.

Axonal Damage in Traumatic Brain Injury

Axonal damage is one of the most common and important pathologic features of traumatic brain injury. Severe diffuse axonal injury, resulting from inertial forces applied to the head, is associated with prolonged unconsciousness and poor outcome. The susceptibility of axons to mechanical injury appears to be due to both their viscoelastic properties and their highly organized structure in white matter tracts. Although axons are supple under normal conditions, they become brittle when exposed to rapid deformations associated with brain trauma. Accordingly, rapid stretch of axons can damage the axonal cytoskeleton, resulting in a loss of elasticity and impairment of axoplasmic transport. Subsequent swelling of the axon occurs in discrete bulb formations or in elongated varicosities that accumulate organelles. Calcium entry into damaged axons is thought to initiate further damage by the activation of proteases and the induction of mitochondrial swelling and dysfunction. Ultimately, swollen axons may become disconnected and contribute to additional neuropathologic changes in brain tissue. However, promising new therapies that reduce proteolytic activity or maintain mitochondrial integrity may attenuate progressive damage of injured axons following experimental brain trauma. Future advancements in the prevention and treatment of traumatic axonal injury will depend on our collective understanding of the relationship between the biomechanics and pathophysiology of various phases of axonal trauma.

Diffuse Axonal Injury: An Important Form of Traumatic Brain Damage

Diffuse axonal injury (DAI) is a frequent form of traumatic brain injury in which a clinical spectrum of in creasing injury severity is paralleled by progressively increasing amounts of axonal damage in the brain. When less severe, DAI is associated with concussive syndromes; when most severe, it causes prolonged traumatic coma that is not related to mass lesions, increased intracranial pressure, or ischemia. Pathological investigations of DAI demonstrate widespread but heterogeneous microscopic damage to axons throughout the white matter of the cerebral and cerebellar hemispheres and brainstem. There is a propensity for injury to occur in the central third of the brain, and the corpus callosum and brain stem are especially prone to injury. In these locations, traumatic axonal damage can occur in several degrees of severity, ranging from transient disturbances of ionic homeostasis to swelling, impairment of axoplasmic transport with secondary (delayed) axotomy and primary axotomy (tearing). A more detailed understanding of the processes involved in axonal damage is crucial to the development of effective treatment for the clinical syndromes of DAI. NEUROSCIENTIST 4:202-215, 1998

Changes in Neurofilament and Microtubule Distribution following Focal Axon Compression

Although a number of cytoskeletal derangements have been described in the setting of traumatic axonal injury (TAI), little is known of early structural changes that may serve to initiate a cascade of further axonal degeneration. Recent work by the authors has examined conformational changes in cytoskeletal constituents of neuronal axons undergoing traumatic axonal injury (TAI) following focal compression through confocal imaging data taken in vitro and in situ. The present study uses electron microscopy to understand and quantify in vitro alterations in the ultrastructural composition of microtubules and neurofilaments within neuronal axons of rats following focal compression. Standard transmission electron microscopy processing methods are used to identify microtubules, while neurofilament identification is performed using antibody labeling through gold nanoparticles. The number, density, and spacing of microtubules and neurofilaments are quantified for specimens in sham Control and Crushed groups with fixation at <1min following load. Our results indicate that the axon caliber dependency known to exist for microtubule and neurofilament metrics extends to axons undergoing TAI, with the exception of neurofilament spacing, which appears to remain constant across all Crushed axon diameters. Confidence interval comparisons between Control and Crushed cytoskeletal measures suggests early changes in the neurofilament spatial distributions within axons undergoing TAI may precede microtubule changes in response to applied loads. This may serve as a trigger for further secondary damage to the axon, representing a key insight into the temporal aspects of cytoskeletal degeneration at the component level, and suggests the rapid removal of neurofilament sidearms as one possible mechanism.

Bone marrow-derived endothelial progenitor cells protect postischemic axons after traumatic brain injury

White matter sparing after traumatic brain injury (TBI) is an important predictor of survival and outcome. Blood vessels and axons are intimately associated anatomically and developmentally. Neural input is required for appropriate vascular patterning, and vascular signaling is important for neuron development and axon growth. Owing to this codependence between endothelial cells and axons during development and the contribution of endothelial progenitor cells (EPCs) in ischemic injury, we hypothesized that EPCs are important in axonal survival after TBI. We examined the effects of allogenic-cultured EPCs on white matter protection and microvascular maintenance after midline fluid percussion injury in adult Sprague-Dawley rats. We used two in vitro models of injury, mechanical stretch and oxygen-glucose deprivation (OGD), to examine the effects of EPCs on the mechanical and ischemic components of brain trauma, respectively. Our results indicate that EPCs improve the white matter integrity and decrease capillary breakdown after injury. Cultured cortical neurons exposed to OGD had less axon degeneration when treated with EPC-conditioned media, whereas no effect was seen in axons injured by mechanical stretch. The results indicate that EPCs are important for the protection of the white matter after trauma and represent a potential avenue for therapy.

Role of subconcussion in repetitive mild traumatic brain injury

Research now suggests that head impacts commonly occur during contact sports in which visible signs or symptoms of neurological dysfunction may not develop despite those impacts having the potential for neurological injury. Recent biophysics studies utilizing helmet accelerometers have indicated that athletes at the collegiate and high school levels sustain a surprisingly high number of head impacts ranging from several hundred to well over 1000 during the course of a season. The associated cumulative impact burdens over the course of a career are equally important. Clinical studies have also identified athletes with no readily observable symptoms but who exhibit functional impairment as measured by neuropsychological testing and functional MRI. Such findings have been corroborated by diffusion tensor imaging studies demonstrating axonal injury in asymptomatic athletes at the end of a season. Recent autopsy data have shown that there are subsets of athletes in contact sports who do not have a history of known or identified concussions but nonetheless have neurodegenerative pathology consistent with chronic traumatic encephalopathy. Finally, emerging laboratory data have demonstrated significant axonal injury, blood-brain barrier permeability, and evidence of neuroinflammation, all in the absence of behavioral changes. Such data suggest that subconcussive level impacts can lead to significant neurological alterations, especially if the blows are repetitive. The authors propose “subconcussion” as a significant emerging concept requiring thorough consideration of the potential role it plays in accruing sufficient anatomical and/or physiological damage in athletes and military personnel, such that the effects of these injuries are clinically expressed either contemporaneously or later in life.

Damage to myelin and oligodendrocytes: a role in chronic outcomes following traumatic brain injury?

There is increasing evidence in the experimental and clinical traumatic brain injury (TBI) literature that loss of central myelinated nerve fibers continues over the chronic post-traumatic phase after injury. However, the biomechanism(s) of continued loss of axons is obscure. Stretch-injury to optic nerve fibers in adult guinea-pigs was used to test the hypothesis that damage to the myelin sheath and oligodendrocytes of the optic nerve fibers may contribute to, or facilitate, the continuance of axonal loss. Myelin dislocations occur within internodal myelin of larger axons within 1–2 h of TBI. The myelin dislocations contain elevated levels of free calcium. The volume of myelin dislocations increase with greater survival and are associated with disruption of the axonal cytoskeleton leading to secondary axotomy. Waves of Ca²⁺ depolarization or spreading depression extend from the initial locus injury for perhaps hundreds of microns after TBI. As astrocytes and oligodendrocytes are connected via gap junctions, it is hypothesized that spreading depression results in depolarization of central glia, disrupt axonal ionic homeostasis, injure axonal mitochondria and allow the onset of axonal degeneration throughout an increasing volume of brain tissue; and contribute toward post-traumatic continued loss of white matter.

Axonal pathology in traumatic brain injury

Over the past 70years, diffuse axonal injury (DAI) has emerged as one of the most common and important pathological features of traumatic brain injury (TBI). Axons in the white matter appear to be especially vulnerable to injury due to the mechanical loading of the brain during TBI. As such, DAI has been found in all severities of TBI and may represent a key pathologic substrate of mild TBI (concussion). Pathologically, DAI encompasses a spectrum of abnormalities from primary mechanical breaking of the axonal cytoskeleton, to transport interruption, swelling and proteolysis, through secondary physiological changes. Depending on the severity and extent of injury, these changes can manifest acutely as immediate loss of consciousness or confusion and persist as coma and/or cognitive dysfunction. In addition, recent evidence suggests that TBI may induce long-term neurodegenerative processes, such as insidiously progressive axonal pathology. Indeed, axonal degeneration has been found to continue even years after injury in humans, and appears to play a role in the development of Alzheimer's disease-like pathological changes. Here we review the current understanding of DAI as a uniquely mechanical injury, its histopathological identification, and its acute and chronic pathogenesis following TBI.

Therapy development for diffuse axonal injury

Diffuse axonal injury (DAI) remains a prominent feature of human traumatic brain injury (TBI) and a major player in its subsequent morbidity. The importance of this widespread axonal damage has been confirmed by multiple approaches including routine postmortem neuropathology as well as advanced imaging, which is now capable of detecting the signatures of traumatically induced axonal injury across a spectrum of traumatically brain-injured persons. Despite the increased interest in DAI and its overall implications for brain-injured patients, many questions remain about this component of TBI and its potential therapeutic targeting. To address these deficiencies and to identify future directions needed to fill critical gaps in our understanding of this component of TBI, the National Institute of Neurological Disorders and Stroke hosted a workshop in May 2011. This workshop sought to determine what is known regarding the pathogenesis of DAI in animal models of injury as well as in the human clinical setting. The workshop also addressed new tools to aid in the identification of this axonal injury while also identifying more rational therapeutic targets linked to DAI for continued preclinical investigation and, ultimately, clinical translation. This report encapsulates the oral and written components of this workshop addressing key features regarding the pathobiology of DAI, the biomechanics implicated in its initiating pathology, and those experimental animal modeling considerations that bear relevance to the biomechanical features of human TBI. Parallel considerations of alternate forms of DAI detection including, but not limited to, advanced neuroimaging, electrophysiological, biomarker, and neurobehavioral evaluations are included, together with recommendations for how these technologies can be better used and integrated for a more comprehensive appreciation of the pathobiology of DAI and its overall structural and functional implications. Lastly, the document closes with a thorough review of the targets linked to the pathogenesis of DAI, while also presenting a detailed report of those target-based therapies that have been used, to date, with a consideration of their overall implications for future preclinical discovery and subsequent translation to the clinic. Although all participants realize that various research gaps remained in our understanding and treatment of this complex component of TBI, this workshop refines these issues providing, for the first time, a comprehensive appreciation of what has been done and what critical needs remain unfulfilled.

Differential effects of FK506 on structural and functional axonal deficits after diffuse brain injury in the immature rat

Diffuse axonal injury is a major component of traumatic brain injury in children and correlates with long-term cognitive impairment. Traumatic brain injury in adult rodents has been linked to a decrease in compound action potential (CAP) in the corpus callosum, but information on trauma-associated diffuse axonal injury in immature rodents is limited. We investigated the effects of closed head injury on CAP in the corpus callosum of 17-day-old rats. The injury resulted in CAP deficits of both myelinated and unmyelinated fibers in the corpus callosum between 1 and 14 days postinjury (dpi). These deficits were accompanied by intra-axonal dephosphorylation of the 200-kDa neurofilament subunit (NF200) at 1 and 3 dpi, a decrease in total NF200 at 3 dpi and axonal degeneration at 3 and 7 dpi. Although total phosphatase activity decreased at 1 dpi, calcineurin activity was unchanged. The calcineurin inhibitor, FK506, significantly attenuated the injury-induced NF200 dephosphorylation of NF200 at 3 dpi and axonal degeneration at 3 and 7 dpi but did not affect the decrease in NF200 protein levels or impaired axonal transport. FK506 had no effect on CAP deficits at 3 dpi but exacerbated the deficit in only the myelinated fibers at 7 dpi. Thus, in contrast to adult animals, FK506 treatment did not improve axonal function in brain-injured immature animals, suggesting that calcineurin may not contribute to impaired axonal function.

Untangling the role of tau in Alzheimer’s disease: A unifying hypothesis

Recent investigations into the etiology and pathogenesis of Alzheimer’s disease (AD) in the past few years have expanded to include previously unexplored and/or disconnected aspects of AD and related conditions at both the cellular and systemic levels of organization. These include how AD-associated abnormalities affect the cell cycle and neuronal differentiation state and how they recruit signal transduction, membrane trafficking and protein transcytosis mechanisms to produce a neurotoxic syndrome capable of spreading itself throughout the brain. The recent expansion of AD research into intercellular and new aspects of cellular degenerative mechanisms is causing a systemic re-evaluation of AD pathogenesis, including the roles played by well-studied elements, such as the generation of Aβ and tau protein aggregates. It is also changing our view of neurodegenerative diseases as a whole. Here we propose a conceptual framework to account for some of the emerging aspects of the role of tau in AD pathogenesis.

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.

Preventing flow-metabolism uncoupling acutely reduces axonal injury after traumatic brain injury

We have previously presented evidence that the development of secondary traumatic axonal injury is related to the degree of local cerebral blood flow (LCBF) and flow-metabolism uncoupling. We have now tested the hypothesis that augmenting LCBF in the acute stages after brain injury prevents further axonal injury. Data were acquired from rats with or without acetazolamide (ACZ) that was administered immediately following controlled cortical impact injury to increase cortical LCBF. Local cerebral metabolic rate for glucose (LCMRglc) and LCBF measurements were obtained 3 h post-trauma in the same rat via ¹⁸F-fluorodeoxyglucose and ¹⁴C-iodoantipyrine co-registered autoradiographic images, and compared to the density of damaged axonal profiles in adjacent sections, and in additional groups at 24 h used to assess different populations of injured axons stereologically. ACZ treatment significantly and globally elevated LCBF twofold above untreated-injured rats at 3 h (p<0.05), but did not significantly affect LCMRglc. As a result, ipsilateral LCMRglc:LCBF ratios were reduced by twofold to sham-control levels, and the density of β-APP-stained axons at 24 h was significantly reduced in most brain regions compared to the untreated-injured group (p<0.01). Furthermore, early LCBF augmentation prevented the injury-associated increase in the number of stained axons from 3-24 h. Additional robust stereological analysis of impaired axonal transport and neurofilament compaction in the corpus callosum and cingulum underlying the injury core confirmed the amelioration of β-APP axon density, and showed a trend, but no significant effect, on RMO14-positive axons. These data underline the importance of maintaining flow-metabolism coupling immediately after injury in order to prevent further axonal injury, in at least one population of injured axons.

Unmyelinated axons show selective rostrocaudal pathology in the corpus callosum after traumatic brain injury

Axonal injury is consistently observed after traumatic brain injury (TBI). Prior research has extensively characterized the post-TBI response in myelinated axons. Despite evidence that unmyelinated axons comprise a numerical majority of cerebral axons, pathologic changes in unmyelinated axons after TBI have not been systematically studied. To identify morphologic correlates of functional impairment of unmyelinated fibers after TBI, we assessed ultrastructural changes in corpus callosum axons. Adult rats received moderate fluid percussion TBI, which produced diffuse injury with no contusion. Cross-sectional areas of 13,797 unmyelinated and 3,278 intact myelinated axons were stereologically measured at survival intervals from 3 hours to 15 days after injury. The mean caliber of unmyelinated axons was significantly reduced at 3 to 7 days and recovered by 15 days, but the time course of this shrinkage varied among the genu, mid callosum, and splenium. Relatively large unmyelinated axons seemed to be particularly vulnerable. Injury-induced decreases in unmyelinated fiber density were also observed, but they were more variable than caliber reductions. By contrast, no significant morphometric changes were observed in myelinated axons. The finding of a preferential vulnerability in unmyelinated axons has implications for current concepts of axonal responses after TBI and for development of specifically targeted therapies.

Stuck at the bench: Potential natural neuroprotective compounds for concussion

Background:

While numerous laboratory studies have searched for neuroprotective treatment approaches to traumatic brain injury, no therapies have successfully translated from the bench to the bedside. Concussion is a unique form of brain injury, in that the current mainstay of treatment focuses on both physical and cognitive rest. Treatments for concussion are lacking. The concept of neuro-prophylactic compounds or supplements is also an intriguing one, especially as we are learning more about the relationship of numerous sub-concussive blows and/or repetitive concussive impacts and the development of chronic neurodegenerative disease. The use of dietary supplements and herbal remedies has become more common place.

Methods:

A literature search was conducted with the objective of identifying and reviewing the pre-clinical and clinical studies investigating the neuroprotective properties of a few of the more widely known compounds and supplements.

Results:

There are an abundance of pre-clinical studies demonstrating the neuroprotective properties of a variety of these compounds and we review some of those here. While there are an increasing number of well-designed studies investigating the therapeutic potential of these nutraceutical preparations, the clinical evidence is still fairly thin.

Conclusion:

There are encouraging results from laboratory studies demonstrating the multi-mechanistic neuroprotective properties of many naturally occurring compounds. Similarly, there are some intriguing clinical observational studies that potentially suggest both acute and chronic neuroprotective effects. Thus, there is a need for future trials exploring the potential therapeutic benefits of these compounds in the treatment of traumatic brain injury, particularly concussion.

The axo-myelinic synapse

Axons have evolved to acquire myelination, enabling denser packing and speedier transmission. Although myelin is considered a passive insulator, recent reports suggest a more dynamic role. Axons, in turn, are endowed with neurotransmitter release and uptake systems along their trunks. Based on these observations, I argue that there may exist a new type of chemical synapse between axon and myelin, one that supports activity-dependent communication between the two. This raises intriguing possibilities of dynamic fine-tuning of the myelin sheath even in adulthood, efficient recruitment of resources for myelin maintenance and bi-directional signaling, whereby the axon informs its myelinating cell of its metabolic needs proportionally to the electrical traffic it is transmitting. This would also have implications for de- and dysmyelinating diseases should this axo-myelinic synapse become dysfunctional.

The molecular pathophysiology of concussive brain injury

Concussion or mild traumatic brain injury (mTBI) is a condition that affects hundreds of thousands of patients worldwide. Understanding the pathophysiology of this disorder can help manage its acute and chronic repercussions. Immediately following mTBI, there are several metabolic, hemodynamic, structural, and electric changes that alter normal cerebral function. These alterations can increase the brain's vulnerability to repeat injury and long-term disability. This review evaluates current studies from the bench to the bedside of mTBI. Acute and chronic effects of concussion are measured in both animal and clinical studies. Also, the effect of repeat concussions is analyzed. Concussion-induced pathophysiology with regards to glucose metabolism changes, mitochondrial dysfunction, axonal injury, and structural damage are evaluated. Translational studies such as functional magnetic resonance imaging, magnetic resonance spectroscopy and diffusion tensor imaging prove to be effective clinical tools for both prognostic and treatment parameters. Understanding the neurobiology of concussion will lead to development and validation of physiological biomarkers of this common injury. These biomarkers (eg, laboratory tests, imaging, electrophysiology) will then allow for improved detection, better functional assessment and evidence-based return to play recommendations.

Wallerian-like axonal degeneration in the optic nerve after excitotoxic retinal insult: an ultrastructural study

Background

Excitotoxicity is involved in the pathogenesis of a number neurodegenerative diseases, and axonopathy is an early feature in several of these disorders. In models of excitotoxicity-associated neurological disease, an excitotoxin delivered to the central nervous system (CNS), could trigger neuronal death not only in the somatodendritic region, but also in the axonal region, via oligodendrocyte N-methyl-D-aspartate (NMDA) receptors. The retina and optic nerve, as approachable regions of the brain, provide a unique anatomical substrate to investigate the "downstream" effect of isolated excitotoxic perikaryal injury on central nervous system (CNS) axons, potentially providing information about the pathogenesis of the axonopathy in clinical neurological disorders.

Herein, we provide ultrastructural information about the retinal ganglion cell (RGC) somata and their axons, both unmyelinated and myelinated, after NMDA-induced retinal injury. Male Sprague-Dawley rats were killed at 0 h, 24 h, 72 h and 7 days after injecting 20 nM NMDA into the vitreous chamber of the left eye (n = 8 in each group). Saline-injected right eyes served as controls. After perfusion fixation, dissection, resin-embedding and staining, ultrathin sections of eyes and proximal (intraorbital) and distal (intracranial) optic nerve segments were evaluated by transmission electron tomography (TEM).

Results

TEM demonstrated features of necrosis in RGCs: mitochondrial and endoplasmic reticulum swelling, disintegration of polyribosomes, rupture of membranous organelle and formation of myelin bodies. Ultrastructural damage in the optic nerve mimicked the changes of Wallerian degeneration; early nodal/paranodal disturbances were followed by the appearance of three major morphological variants: dark degeneration, watery degeneration and demyelination.

Conclusion

NMDA-induced excitotoxic retinal injury causes mainly necrotic RGC somal death with Wallerian-like degeneration of the optic nerve. Since axonal degeneration associated with perikaryal excitotoxic injury is an active, regulated process, it may be amenable to therapeutic intervention.

Proteolysis of submembrane cytoskeletal proteins ankyrin-G and αII-spectrin following diffuse brain injury: a role in white matter vulnerability at Nodes of Ranvier

A high membrane-to-cytoplasm ratio makes axons particularly vulnerable to traumatic injury. Posttraumatic shifts in ionic homeostasis promote spectrin cleavage, disrupt ankyrin linkages and destabilize axolemmal proteins. This study contrasted ankyrin-G and αII-spectrin degradation in cortex and corpus callosum following diffuse axonal injury produced by fluid percussion insult. Ankyrin-G lysis occurred preferentially in white matter, with acute elevation of all fragments and long-term reduction of a low kD form. Calpain-generated αII-spectrin fragments increased in both regions. Caspase-3 lysis of αII-spectrin showed a small, acute rise in cortex but was absent in callosum. White matter displayed nodal damage, with horseradish peroxidase permeability into the submyelin space. Ankyrin-G-binding protein neurofascin and spectrin-binding protein ankyrin-B showed acute alterations in expression. These results support ankyrin-G vulnerability in white matter following trauma and suggest that ankyrin-G and αII-spectrin proteolysis disrupts Node of Ranvier integrity. The time course of such changes were comparable to previously observed functional deficits in callosal fibers.

Blood-brain barrier Ca2+-ATPase cytochemistry: incubation media and fixation methods for differentiating Ca2+-specific ATPase from ecto-ATPase

Ca2+-ATPase cytochemistry frequently uses the incubation medium of Ando et al. that was introduced in 1981. Some studies, however, have suggested that this medium localizes ecto-ATPase in addition to Ca2+-ATPase and that Ca2+-ATPase is sensitive to fixation. Strong activity of the enzyme on the luminal surface of the blood-brain barrier (BBB) also is considered indicative of immature or pathological microvessels. We address here five questions. 1) Is the incubation medium of Ando et al. specific for BBB Ca2+-ATPase or does it also localize ecto-ATPase? 2) How are the two enzymes distributed in the BBB? 3) How would data interpretation be prone to error if the cytochemical study does not use controls identifying ecto-ATPase? 4) Does the amount of reaction product of both enzymes vary significantly when the cortical tissue is exposed to different fixatives? 5) Does the presence of Ca2+-ATPase on the luminal membrane of the BBB necessarily indicate immature or abnormal brain endothelial cells? Adult male Sprague-Dawley rats were perfused with one of two different fixatives and vibratome slices of the brain cortex were incubated in the medium of Ando et al. The controls used were those demonstrating the ecto-ATPase and those that do not. The results indicate that the incubation medium is not specific for Ca2+-ATPase, because it also localizes the ecto-ATPase. Ca2+-ATPase appears to be localized primarily on the luminal surface of the BBB, while ecto-ATPase is localized on both the luminal and abluminal surfaces. The portion of the reaction product contributed by Ca2+-ATPase would not have been identified if the controls uniquely identifying the ecto-ATPase had not been used. The amount of reaction product formed by Ca2+-ATPase is strongly dependent on the type of fixative used. The strong localization of Ca2+-ATPase on the luminal surface of the BBB is not only normal, but also better accounts for the physiological homeostasis of Ca2+ across the blood-brain interface and should not be interpreted as indicative of immature or pathological microvessels.

Chronic traumatic encephalopathy in athletes: progressive tauopathy after repetitive head injury

Since the 1920s, it has been known that the repetitive brain trauma associated with boxing may produce a progressive neurological deterioration, originally termed dementia pugilistica, and more recently, chronic traumatic encephalopathy (CTE). We review 48 cases of neuropathologically verified CTE recorded in the literature and document the detailed findings of CTE in 3 profession althletes, 1 football player and 2 boxers. Clinically, CTE is associated with memory disturbances, behavioral and personality changes, parkinsonism, and speech and gait abnormalities. Neuropathologically, CTE is characterized by atrophy of the cerebral hemispheres, medial temporal lobe, thalamus, mammillary bodies, and brainstem, with ventricular dilatation and a fenestrated cavum septum pellucidum. Microscopically, there are extensive tau-immunoreactive neurofibrillary tangles, astrocytic tangles, and spindle-shaped and threadlike neurites throughout the brain. The neurofibrillary degeneration of CTE is distinguished from other tauopathies by preferential involvement of the superficial cortical layers, irregular patchy distribution in the frontal and temporal cortices, propensity for sulcal depths, prominent perivascular, periventricular, and subpial distribution, and marked accumulation of tau-immunoreactive astrocytes. Deposition of beta-amyloid, most commonly as diffuse plaques, occurs in fewer than half the cases. Chronic traumatic encephalopathy is a neuropathologically distinct slowly progressive tauopathy with a clear environmental etiology.

Mechanical membrane injury induces axonal beading through localized activation of calpain

Diffuse axonal injury (DAI), a major component of traumatic brain injury, is characterized by a sequence of neurochemical reactions initiated at the time of trauma and resulting in axonal degeneration and cell death. Calcium influx through mechanically induced axolemmal pores and subsequent activation of calpains are thought to be responsible for the cytoskeletal damage leading to impaired axonal transport. Focal disruption of cytoskeleton accompanied by the accumulation of transported membranous cargo leads to axonal beading which is the characteristic morphology of DAI. By applying fluid shear stress injury on cultured primary neurons, acute calcium (Ca(2+)) and calpain responses of axons to mechanical trauma were investigated. Intracellular Ca(2+) concentration ([Ca(2+)](i)) shows a steady increase following injury that can be blocked by sealing membrane pores with Poloxamer 188 and by chelating intra- or extracellular Ca(2+). Calpain activity increases in response to mechanical injury and this increase depends on Ca(2+) availability and on axolemmal permeability. Both the [Ca(2+)](i) increase and calpain activity exhibit focal peaks along the axons which co-localize with mitochondria and predict future axonal bead locations. These findings suggest that mechanoporation may be the initiating mechanism resulting in ensuing calcium fluxes and subsequent calpain activity and that post-injury membrane repair may be a valid therapeutic approach for acute intervention in DAI.

Traumatic brain injury: can the consequences be stopped?

Traumatic brain injury is a leading cause of morbidity and death in both industrialized and developing countries. To date, there is no targeted pharmacological treatment that effectively limits the progression of secondary injury. The delayed progression of deterioration of grey and white matter gives hope that a meaningful intervention can be applied in a realistic timeframe following initial trauma. In this review we discuss new insights into the subcellular mechanisms of secondary injury that have highlighted numerous potential targets for intervention.

Poloxamer 188 reduces axonal beading following mechanical trauma to cultured neurons

Diffuse axonal injury (DAI), a major component of traumatic brain injury, is a progressive event that may lead to secondary neuronal death. DAI is thought to be initiated by mechanically-induced increases in axolemmal permeability resulting in disruption of the cytoskeleton and blockade of axonal transport. We report an in vitro model that mimics important features of DAI observed in vivo. We induced fluid shear stress injury (FSSI) on cultured primary chick forebrain neurons and characterized the resulting structural and morphological changes. In addition, we tested the post-injury effect of Poloxamer 188 (P188), a tri-block co-polymer that is known to promote resealing membrane pores. We found that FSSI induces axonal beading, the "hallmark" morphology of DAI. Furthermore, beads co-localized with microtubule disruption, also characteristic of DAI. P188 reduced axonal beading to control levels indicating that axolemma integrity is an excellent target for therapeutic interventions.

Calcium overloading in traumatic axonal injury by lateral head rotation: a morphological evidence in rat model

The study investigated morphologically axonal calcium overloading and its relationship with axonal structural changes. Twelve SD rats were divided into an injury and a sham group. The rat model of traumatic axonal injury (TAI) by lateral head rotation was produced. The oxalate-pyroantimonate technique for calcium localization was used to process the rat's medulla oblongata tissues with thin sections observed electron-microscopically for axonal structure and calcium precipitates on it. The axonal damage in medulla oblongata appeared at 2 h post-injury, gradually became diffuse and severe, and continued to exist at 24 hours. At 2 hours, calcium precipitates were deposited on separated lamellae and axolemma, but were rarely distributed in the axoplasm. At 6 hours, calcium precipitates occurred on separated lamellae and axolemma in much higher density, but on axoplasm in extremely small amounts. Some axons, though lacking structural changes of the myelin sheath, sequestered plenty of calcium deposits on their swollen mitochondria. At 24 hours, damaged axons presented with much more severe lamellae separation and calcium deposits. Axonal calcium overloading developed in rat TAI model using lateral head rotation. This was significantly related to structural damage in the axons. These findings suggest the feasibility of using calcium antagonists in cope the management of human DAI in its very early stage.

Minor traumatic brain injury in sports: a review in order to prevent neurological sequelae

Minor traumatic brain injury (mTBI) is caused by inertial effects, which induce sudden rotation and acceleration forces to and within the brain. At less severe levels of injury, for example in mTBI, there is probably only transient disturbance of ionic homeostasis with short-term, temporary disturbance of brain function. With increased levels of severity, however, studies in animal models of TBI and in humans have demonstrated focal intra-axonal alterations within the subaxolemmal, neurofilament and microtubular cytoskeletal network together with impairment of axoplasmic transport. These changes have, until very recently, been thought to lead to progressive axonal swelling, axonal detachment or even cell death over a period of hours or days, the so-called process of "secondary axotomy". However, recent evidence has suggested that there may be two discrete pathologies that may develop in injured nerve fibers. In the TBI scenario, disturbances of ionic homeostasis, acute metabolic changes and alterations in cerebral blood flow compromise the ability of neurons to function and render cells of the brain increasingly vulnerable to the development of pathology. In ice hockey, current return-to-play guidelines do not take into account these new findings appropriately, for example allow returning to play in the same game. It has recently been hypothesized that the processes summarized above may predispose brain cells to assume a vulnerable state for an unknown period after mild injury (mTBI). Therefore, we recommend that any confused player with or without amnesia should be taken off the ice and not be permitted to play again for at least 72h.