Stereology and Ultrastructure of Chronic Phase Axonal and Cell Soma Pathology in Stretch-Injured Central Nerve Fibers

Review: Emerging Eye-Based Diagnostic Technologies for Traumatic Brain Injury

The study of ocular manifestations of neurodegenerative disorders, Oculomics, is a growing field of investigation for early diagnostics, enabling structural and chemical biomarkers to be monitored overtime to predict prognosis. Traumatic brain injury (TBI) triggers a cascade of events harmful to the brain, which can lead to neurodegeneration. TBI, termed the "silent epidemic" is becoming a leading cause of death and disability worldwide. There is currently no effective diagnostic tool for TBI, and yet, early-intervention is known to considerably shorten hospital stays, improve outcomes, fasten neurological recovery and lower mortality rates, highlighting the unmet need for techniques capable of rapid and accurate point-of-care diagnostics, implemented in the earliest stages. This review focuses on the latest advances in the main neuropathophysiological responses and the achievements and shortfalls of TBI diagnostic methods. Validated and emerging TBI-indicative biomarkers are outlined and linked to ocular neuro-disorders. Methods detecting structural and chemical ocular responses to TBI are categorised along with prospective chemical and physical sensing techniques. Particular attention is drawn to the potential of Raman spectroscopy as a non-invasive sensing of neurological molecular signatures in the ocular projections of the brain, laying the platform for the first tangible path towards alternative point-of-care diagnostic technologies for TBI.

Neuroprotective Role of Nutritional Supplementation in Athletes

BACKGROUND

Neurodegenerative disorders belong to different classes of progressive/chronic conditions that affect the peripheral/central nervous system. It has been shown through studies that athletes who play sports involving repeated head trauma and sub-concussive impacts are more likely to experience neurological impairments and neurodegenerative disorders in the long run.

AIMS

The aim of the current narrative review article is to provide a summary of various nutraceuticals that offer promise in the prevention or management of sports-related injuries, especially concussions and mild traumatic brain injuries.

METHODS

This article reviews the various potential nutraceutical agents and their possible mechanisms in providing a beneficial effect in the injury recovery process. A thorough survey of the literature was carried out in the relevant databases to identify studies published in recent years. In the present article, we have also highlighted the major neurological disorders along with the associated nutraceutical(s) therapy in the management of disorders.

RESULTS

The exact pathological mechanism behind neurodegenerative conditions is complex as well as idiopathic. However, mitochondrial dysfunction, oxidative stress as well as intracellular calcium overload are some common reasons responsible for the progression of these neurodegenerative disorders. Owing to the multifaceted effects of nutraceuticals (complementary medicine), these supplements have gained importance as neuroprotective. These diet-based approaches inhibit different pathways in a physiological manner without eliciting adverse effects. Food habits and lifestyle of an individual also affect neurodegeneration.

CONCLUSION

Studies have shown nutraceuticals (such as resveratrol, omega-3-fatty acids) to be efficacious in terms of their neuroprotection against several neurodegenerative disorders and to be used as supplements in the management of traumatic brain injuries. Protection prior to injuries is needed since concussions or sub-concussive impacts may trigger several pathophysiological responses or cascades that can lead to long-term complications associated with CNS. Thus, the use of nutraceuticals as prophylactic treatment for neurological interventions has been proposed.

An insight into the vision impairment following traumatic brain injury

Traumatic brain injury (TBI) is one of the major cause of morbidity and mortality and it affects more than 1.7 million Americans each year. Depending on its location and severity, TBI leads to structural and functional damage in several parts of the brain such as cranial nerves, optic nerve tract or other circuitry involved in vision, and occipital lobe. As a result, the function associated with vision processing and perception are significantly affected and cause blurred vision, double vision, decreased peripheral vision and blindness. In this mini-review, we will focus the recent progress made to understand the pathology and underlying cellular/molecular mechanisms involved in the impairment of the integrity of visual systems following TBI.

Wallerian degeneration in the optic nerve stretch-injury model of traumatic brain injury: a stereological analysis

Patients with chronic traumatic encephalopathy (CTE) show loss of central white matter, central gray matter, and cortical gray matter with increasing post-traumatic survival. The majority of experimental studies using animals have, however, discussed only the ultrastructural pathophysiology of injured central white matter leading to secondary axotomy and the formation of axonal terminal bulbs. Using the stretch-injured optic nerve model in adult guinea pigs, the present study provides novel quantitative data concerning Wallerian degeneration of disconnected axonal fragments following secondary axotomy out to 12 weeks after injury to an optic nerve. The time course of Wallerian degeneration at the level of an individual nerve fiber is comparable to that reported in earlier studies over 48 h to two weeks after secondary axotomy. But only a relatively small proportion of nerve fibers within the optic tract degenerate via Wallerian degeneration during the first two weeks. Rather, examples of each of the three stages of Wallerian degeneration-acute axonal degeneration, latency of the distal axonal segment, and granular fragmentation-occur within the optic tract across the entire experimental survival of 12 weeks used in the present study. This data suggests that some nerve fibers initiate Wallerian degeneration days and weeks after the initial time of mechanical injury to an optic nerve. The number of intact nerve fibers continues to fall over at least three months after injury in the stretch-injury model of traumatic axonal injury. It is suggested that these novel findings relate to the mechanism(s) whereby central white matter volume decreases over months and years in CTE patients.

Repetitive mild traumatic brain injury causes optic nerve and retinal damage in a mouse model

There is increasing evidence that long-lasting morphologic and functional consequences can be present in the human visual system after repetitive mild traumatic brain injury (r-mTBI). The exact location and extent of the damage in this condition are not well understood. Using a recently developed mouse model of r-mTBI, we assessed the effects on the retina and optic nerve using histology and immunohistochemistry, electroretinography (ERG), and spectral-domain optical coherence tomography (SD-OCT) at 10 and 13 weeks after injury. Control mice received repetitive anesthesia alone (r-sham). We observed decreased optic nerve diameters and increased cellularity and areas of demyelination in optic nerves in r-mTBI versus r-sham mice. There were concomitant areas of decreased cellularity in the retinal ganglion cell layer and approximately 67% decrease in brain-specific homeobox/POU domain protein 3A-positive retinal ganglion cells in retinal flat mounts. Furthermore, SD-OCT demonstrated a detectable thinning of the inner retina; ERG demonstrated a decrease in the amplitude of the photopic negative response without any change in a- or b-wave amplitude or timing. Thus, the ERG and SD-OCT data correlated well with changes detected by morphometric, histologic, and immunohistochemical methods, thereby supporting the use of these noninvasive methods in the assessment of visual function and morphology in clinical cases of mTBI.

The neuroinflammatory response in humans after traumatic brain injury

AIMS

Traumatic brain injury is a significant cause of morbidity and mortality worldwide. An epidemiological association between head injury and long-term cognitive decline has been described for many years and recent clinical studies have highlighted functional impairment within 12 months of a mild head injury. In addition chronic traumatic encephalopathy is a recently described condition in cases of repetitive head injury. There are shared mechanisms between traumatic brain injury and Alzheimer's disease, and it has been hypothesized that neuroinflammation, in the form of microglial activation, may be a mechanism underlying chronic neurodegenerative processes after traumatic brain injury.

METHODS

This study assessed the microglial reaction after head injury in a range of ages and survival periods, from <24-h survival through to 47-year survival. Immunohistochemistry for reactive microglia (CD68 and CR3/43) was performed on human autopsy brain tissue and assessed 'blind' by quantitative image analysis. Head injury cases were compared with age matched controls, and within the traumatic brain injury group cases with diffuse traumatic axonal injury were compared with cases without diffuse traumatic axonal injury.

RESULTS

A major finding was a neuroinflammatory response that develops within the first week and persists for several months after traumatic brain injury, but has returned to control levels after several years. In cases with diffuse traumatic axonal injury the microglial reaction is particularly pronounced in the white matter.

CONCLUSIONS

These results demonstrate that prolonged microglial activation is a feature of traumatic brain injury, but that the neuroinflammatory response returns to control levels after several years.

Diffuse traumatic axonal injury in the optic nerve does not elicit retinal ganglion cell loss

Much of the morbidity after traumatic brain injury (TBI) is associated with traumatic axonal injury (TAI). Although most TAI studies focus on corpus callosum white matter, the visual system has received increased interest. To assess visual system TAI, we developed a mouse model of optic nerve TAI. It is unknown, however, whether this TAI causes retinal ganglion cell (RGC) death. To address this issue, YFP (yellow fluorescent protein)-16 transgenic mice were subjected to mild TBI and followed from 2 to 28 days. Neither TUNEL (terminal deoxynucleotidyl transferase dUTP nick end labeling)-positive or cleaved caspase-3-immunoreactive RGCs were observed from 2 to 28 days after TBI. Quantification of immunoreactivity of Brn3a, an RGC marker, demonstrated no RGC loss; parallel electron microscopic analysis confirmed RGC viability. Persistent RGC survival was also consistent with the finding of reorganization in the proximal axonal segments after TAI, wherein microglia/macrophages remained inactive. In contrast, activated microglia/macrophages closely enveloped the distal disconnected, degenerating axonal segments at 7 to 28 days after injury, thereby confirming that this model consistently evoked TAI followed by disconnection. Collectively, these data provide novel insight into the evolving pathobiology associated with TAI that will form a foundation for future studies exploring TAI therapy and its downstream consequences.

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.

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.

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.

Traumatic white matter injury and toxic leukoencephalopathies

White matter injury may be secondary to a range of neurodegenerative disorders, such as the common dementing disorders of the elderly, or may be a consequence of specific white matter disorders, such as multiple sclerosis and the rare leukodystrophies. This article will focus on two relatively common primary groups of disorders of the white matter, traumatic white matter injury and toxic leukoencephalopathies. Traumatic axonal injury may be focal or diffuse, and is associated with a clinical spectrum ranging from concussion through to coma and death. The molecular mechanisms underlying axonal degeneration secondary to traumatic axonal degeneration are being elucidated and may give an insight into potential therapeutic targets. Toxic leukoencephalopathy may be secondary to exposure to a wide range of compounds, including chemotherapeutic drugs. These toxins may produce white matter injury through a range of mechanisms, and the potential toxic effects of compounds need to be considered when assessing a patient with a nonspecific leukoencephalopathy.

Traumatic axonal injury in the optic nerve: evidence for axonal swelling, disconnection, dieback, and reorganization

Traumatic axonal injury (TAI) is a major feature of traumatic brain injury (TBI) and is associated with much of its morbidity. To date, significant insight has been gained into the initiating pathogenesis of TAI. However, the nature of TAI within the injured brain precludes the consistent evaluation of its specific anterograde and retrograde sequelae. To overcome this limitation, we used the relatively organized optic nerve in a central fluid percussion injury (cFPI) model. To improve the visualization of TAI, we utilized mice expressing yellow fluorescent protein (YFP) in their visual pathways. Through this approach, we consistently generated TAI in the optic nerve and qualitatively and quantitatively evaluated its progression over a 48-h period in YFP axons via confocal microscopy and electron microscopy. In this model, delayed axonal swelling with subsequent disconnection were the norm, together with the fact that once disconnected, both the proximal and distal axonal segments revealed significant dieback, with the proximal swellings showing regression and reorganization, while the distal swellings persisted, although showing signs of impending degeneration. When antibodies targeting the C-terminus of amyloid precursor protein (APP), a routine marker of TAI were employed, they mapped exclusively to the proximal axonal segments without distal targeting, regardless of the survival time. Concomitant with this evolving axonal pathology, focal YFP fluorescence quenching occurred and mapped precisely to immunoreactive loci positive for Texas-Red-conjugated-IgG, indicating that blood-brain barrier disruption and its attendant edema contributed to this phenomenon. This was confirmed through the use of antibodies targeting endogenous YFP, which demonstrated the retention of intact immunoreactive axons despite YFP fluorescence quenching. Collectively, the results of this study within the injured optic nerve provide unprecedented insight into the evolving pathobiology associated with TAI.