Focal axonal injury: the early axonal response to stretch

Traumatic Axonal Injury in the Optic Nerve: The Selective Role of SARM1 in the Evolution of Distal Axonopathy

Traumatic axonal injury (TAI), thought to be caused by rotational acceleration of the head, is a prevalent neuropathology in traumatic brain injury (TBI). TAI in the optic nerve is a common finding in multiple blunt-force TBI models and hence a great model to study mechanisms and treatments for TAI, especially in view of the compartmentalized anatomy of the visual system. We have previously shown that the somata and the proximal, but not distal, axons of retinal ganglion cells (RGC) respond to DLK/LZK blockade after impact acceleration of the head (IA-TBI). Here, we explored the role of the sterile alpha and TIR-motif containing 1 (SARM1), the key driver of Wallerian degeneration (WD), in the progressive breakdown of distal and proximal segments of the optic nerve following IA-TBI with high-resolution morphological and classical neuropathological approaches. Wild type and Sarm1 knockout (KO) mice received IA-TBI or sham injury and were allowed to survive for 3, 7, 14, and 21 days. Ultrastructural and microscopic analyses revealed that TAI in the optic nerve is characterized by variable involvement of individual axons, ranging from apparent early disconnection of a subpopulation of axons to a range of ongoing axonal and myelin perturbations. Traumatic axonal injury resulted in the degeneration of a population of axons distal and proximal to the injury, along with retrograde death of a subpopulation of RGCs. Quantitative analyses on proximal and distal axons and RGC somata revealed that different neuronal domains exhibit differential vulnerability, with distal axon segments showing more severe degeneration compared with proximal segments and RGC somata. Importantly, we found that Sarm1 KO had a profound effect in the distal optic nerve by suppressing axonal degeneration by up to 50% in the first 2 weeks after IA-TBI, with a continued but lower effect at 3 weeks, while also suppressing microglial activation. Sarm1 KO had no evident effect on the initial traumatic disconnection and did not ameliorate the proximal optic axonopathy or the subsequent attrition of RGCs, indicating that the fate of different axonal segments in the course of TAI may depend on distinct molecular programs within axons.

Traumatic axonopathy in spinal tracts after impact acceleration head injury: Ultrastructural observations and evidence of SARM1-dependent axonal degeneration

Traumatic axonal injury (TAI) and the associated axonopathy are common consequences of traumatic brain injury (TBI) and contribute to significant neurological morbidity. It has been previously suggested that TAI activates a highly conserved program of axonal self-destruction known as Wallerian degeneration (WD). In the present study, we utilize our well-established impact acceleration model of TBI (IA-TBI) to characterize the pathology of injured myelinated axons in the white matter tracks traversing the ventral, lateral, and dorsal spinal columns in the mouse and assess the effect of Sterile Alpha and TIR Motif Containing 1 (Sarm1) gene knockout on acute and subacute axonal degeneration and myelin pathology. In silver-stained preparations, we found that IA-TBI results in white matter pathology as well as terminal field degeneration across the rostrocaudal axis of the spinal cord. At the ultrastructural level, we found that traumatic axonopathy is associated with diverse types of axonal and myelin pathology, ranging from focal axoskeletal perturbations and focal disruption of the myelin sheath to axonal fragmentation. Several morphological features such as neurofilament compaction, accumulation of organelles and inclusions, axoskeletal flocculation, myelin degeneration and formation of ovoids are similar to profiles encountered in classical examples of WD. Other profiles such as excess myelin figures and inner tongue evaginations are more typical of chronic neuropathies. Stereological analysis of pathological axonal and myelin profiles in the ventral, lateral, and dorsal columns of the lower cervical cord (C6) segments from wild type and Sarm1 KO mice at 3 and 7 days post IA-TBI (n = 32) revealed an up to 90% reduction in the density of pathological profiles in Sarm1 KO mice after IA-TBI. Protection was evident across all white matter tracts assessed, but showed some variability. Finally, Sarm1 deletion ameliorated the activation of microglia associated with TAI. Our findings demonstrate the presence of severe traumatic axonopathy in multiple ascending and descending long tracts after IA-TBI with features consistent with some chronic axonopathies and models of WD and the across-tract protective effect of Sarm1 deletion.

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.

Mechanosensation in traumatic brain injury

Traumatic brain injury (TBI) is distinct from other neurological disorders because it is induced by a discrete event that applies extreme mechanical forces to the brain. This review describes how the brain senses, integrates, and responds to forces under both normal conditions and during injury. The response to forces is influenced by the unique mechanical properties of brain tissue, which differ by region, cell type, and sub-cellular structure. Elements such as the extracellular matrix, plasma membrane, transmembrane receptors, and cytoskeleton influence its properties. These same components also act as force-sensors, allowing neurons and glia to respond to their physical environment and maintain homeostasis. However, when applied forces become too large, as in TBI, these components may respond in an aberrant manner or structurally fail, resulting in unique pathological sequelae. This so-called "pathological mechanosensation" represents a spectrum of cellular responses, which vary depending on the overall biomechanical parameters of the injury and may be compounded by repetitive injuries. Such aberrant physical responses and/or damage to cells along with the resulting secondary injury cascades can ultimately lead to long-term cellular dysfunction and degeneration, often resulting in persistent deficits. Indeed, pathological mechanosensation not only directly initiates secondary injury cascades, but this post-physical damage environment provides the context in which these cascades unfold. Collectively, these points underscore the need to use experimental models that accurately replicate the biomechanics of TBI in humans. Understanding cellular responses in context with injury biomechanics may uncover therapeutic targets addressing various facets of trauma-specific sequelae.

Wallerian degeneration as a therapeutic target in traumatic brain injury

PURPOSE OF REVIEW

Diffuse or traumatic axonal injury is one of the principal pathologies encountered in traumatic brain injury (TBI) and the resulting axonal loss, disconnection, and brain atrophy contribute significantly to clinical morbidity and disability. The seminal discovery of the slow Wallerian degeneration mice (Wld) in which transected axons do not degenerate but survive and function independently for weeks has transformed concepts on axonal biology and raised hopes that axonopathies may be amenable to specific therapeutic interventions. Here we review mechanisms of axonal degeneration and also describe how these mechanisms may inform biological therapies of traumatic axonopathy in the context of TBI.

RECENT FINDINGS

In the last decade, SARM1 [sterile a and Toll/interleukin-1 receptor (TIR) motif containing 1] and the DLK (dual leucine zipper bearing kinase) and LZK (leucine zipper kinase) MAPK (mitogen-activated protein kinases) cascade have been established as the key drivers of Wallerian degeneration, a complex program of axonal self-destruction which is activated by a wide range of injurious insults, including insults that may otherwise leave axons structurally robust and potentially salvageable. Detailed studies on animal models and postmortem human brains indicate that this type of partial disruption is the main initial pathology in traumatic axonopathy. At the same time, the molecular dissection of Wallerian degeneration has revealed that the decision that commits axons to degeneration is temporally separated from the time of injury, a window that allows potentially effective pharmacological interventions.

SUMMARY

Molecular signals initiating and triggering Wallerian degeneration appear to be playing an important role in traumatic axonopathy and recent advances in understanding their nature and significance is opening up new therapeutic opportunities for TBI.

Influence of Mechanical Stimuli on Schwann Cell Biology

Schwann cells are the glial cells of the peripheral nervous system (PNS). They insulate axons by forming a specialized extension of plasma membrane called the myelin sheath. The formation of myelin is essential for the rapid saltatory propagation of action potentials and to maintain the integrity of axons. Although both axonal and extracellular matrix (ECM) signals are necessary for myelination to occur, the cellular and molecular mechanisms regulating myelination continue to be elucidated. Schwann cells in peripheral nerves are physiologically exposed to mechanical stresses (i.e., tensile, compressive and shear strains), occurring during development, adulthood and injuries. In addition, there is a growing body of evidences that Schwann cells are sensitive to the stiffness of their environment. In this review, we detail the mechanical constraints of Schwann cells and peripheral nerves. We explore the regulation of Schwann cell signaling pathways in response to mechanical stimulation. Finally, we provide a comprehensive overview of the experimental studies addressing the mechanobiology of Schwann cells. Understanding which mechanical properties can interfere with the cellular and molecular biology of Schwann cell during development, myelination and following injuries opens new insights in the regulation of PNS development and treatment approaches in peripheral neuropathies.

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

Mild traumatic brain injury in the mouse induces axotomy primarily within the axon initial segment

Traumatic axonal injury (TAI) is a consistent component of traumatic brain injury (TBI), and is associated with much of its morbidity. Increasingly, it has also been recognized as a major pathology of mild TBI (mTBI). In terms of its pathogenesis, numerous studies have investigated the susceptibility of the nodes of Ranvier, the paranode and internodal regions to TAI. The nodes of Ranvier, with their unique composition and concentration of ion channels, have been suggested as the primary site of injury, initiating a cascade of abnormalities in the related paranodal and internodal domains that lead to local axonal swellings and detachment. No investigation, however, has determined the effect of TAI upon the axon initial segment (AIS), a segment critical to regulating polarity and excitability. The current study sought to identify the susceptibility of these different axon domains to TAI within the neocortex, where each axonal domain could be simultaneously assessed. Utilizing a mouse model of mTBI, a temporal and spatial heterogeneity of axonal injury was found within the neocortical gray matter. Although axonal swellings were found in all domains along myelinated neocortical axons, the majority of TAI occurred within the AIS, which progressed without overt structural disruption of the AIS itself. The finding of primary AIS involvement has important implications regarding neuronal polarity and the fate of axotomized processes, while also raising therapeutic implications, as the mechanisms underlying such axonal injury in the AIS may be distinct from those described for nodal/paranodal injury.

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.

Cytoskeletal transition at the paranodes: the Achilles' heel of myelinated axons

Myelination organizes axons into distinct domains that allow nerve impulses to propagate in a saltatory manner. The edges of the myelin sheath are sealed at the paranodes by axon-glial junctions that have a crucial role in organizing the axonal cytoskeleton. Here we propose a model in which the myelinated axons depend on the axon-glial junctions to stabilize the cytoskeletal transition at the paranodes. Thus paranodal regions are likely to be particularly susceptible to damage induced by demyelinating diseases such as multiple sclerosis.

Increased strain levels and water content in brain tissue after decompressive craniotomy

BACKGROUND

At present there is a debate on the effectiveness of the decompressive craniotomy (DC). Stretching of axons was speculated to contribute to the unfavourable outcome for the patients. The quantification of strain level could provide more insight into the potential damage to the axons. The aim of the present study was to evaluate the strain level and water content (WC) of the brain tissue for both the pre- and post-craniotomy period.

METHODS

The stretching of brain tissue was quantified retrospectively based on the computerised tomography (CT) images of six patients before and after DC by a non-linear image registration method. WC was related to specific gravity (SG), which in turn was related to the Hounsfield unit (HU) value in the CT images by a photoelectric correction according to the chemical composition of brain tissue.

RESULTS

For all the six patients, the strain level showed a substantial increase in the brain tissue close to the treated side of DC compared with that found at the pre-craniotomy period and ranged from 24 to 55 % at the post-craniotomy period. Increase of strain level was also observed at the brain tissue opposite to the treated side, however, to a much lesser extent. The mean area of craniotomy was found to be 91.1 ± 12.7 cm(2). The brain tissue volume increased from 27 to 127 ml, corresponding to 1.65 % and 8.13 % after DC in all six patients. Also, the increased volume seemed to correlate with increased strain level. Specifically, the overall WC of brain tissue for two patients evaluated presented a significant increase after the treatment compared with the condition seen before the treatment. Furthermore, the Glasgow Coma Scale (GCS) improved in four patients after the craniotomy, while two patients died. The GCS did not seem to correlate with the strain level.

CONCLUSIONS

We present a new numerical method to quantify the stretching or strain level of brain tissue and WC following DC. The significant increase in strain level and WC in the post-craniotomy period may cause electrophysiological changes in the axons, resulting in loss of neuronal function. Hence, this new numerical method provides more insight of the consequences following DC and may be used to better define the most optimal size and area of the craniotomy in reducing the strain level development.

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.

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.

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.

Electron tomographic analysis of cytoskeletal cross-bridges in the paranodal region of the node of Ranvier in peripheral nerves

The node of Ranvier is a site for ionic conductances along myelinated nerves and governs the saltatory transmission of action potentials. Defects in the cross-bridging and spacing of the cytoskeleton are a prominent pathological feature in diseases of the peripheral nerve. Electron tomography was used to examine cytoskeletal-cytoskeletal, membrane-cytoskeletal, and heterologous cell connections in the paranodal region of the node of Ranvier in peripheral nerves. Focal attachment of cytoskeletal filaments to each other and to the axolemma and paranodal membranes of the Schwann cell via narrow cross-bridges was visualized in both neuronal and glial cytoplasm. A subset of intermediate filaments associates with the cytoplasmic surfaces of supramolecular complexes of transmembrane structures that are presumed to include known and unknown junctional proteins. Mitochondria were linked to both microtubules and neurofilaments in the axoplasm and to neighboring smooth endoplasmic reticulum by narrow cross-bridges. Tubular cisternae in the glial cytoplasm were also linked to the paranodal glial cytoplasmic loop juxtanodal membrane by short cross-bridges. In the extracellular matrix between axon and Schwann cell, junctional bridges formed long cylinders linking the two membranes. Interactions between cytoskeleton, membranes, and extracellular matrix associations in the paranodal region are likely critical not only for scaffolding, but also for intracellular and extracellular communication.

4-Aminopyridine derivatives enhance impulse conduction in guinea-pig spinal cord following traumatic injury

4-Aminopyridine (4-AP), a potassium channel blocker, is capable of restoring conduction in the injured spinal cord. However, the maximal tolerated level of 4-AP in humans is 100 times lower than the optimal dose in in vitro animal studies due to its substantially negative side effects. As an initial step toward the goal of identifying alternative potassium channel blockers with a similar ability of enhancing conduction and with fewer side effects, we have synthesized structurally distinct pyridine-based blockers. Using isolated guinea-pig spinal cord white matter and a double sucrose gap recording device, we have found three pyridine derivatives, N-(4-pyridyl)-methyl carbamate (100 microM), N-(4-pyridyl)-ethyl carbamate (100 microM), and N-(4-pyridyl)-tertbutyl (10 microM) can significantly enhance conduction in spinal cord white matter following stretch. Similar to 4-AP, the derivatives did not preferentially enhance conduction based on axonal caliber. Unlike 4-AP, the derivatives did not change the overall electrical responsiveness of axons to multiple stimuli, indicating the axons recruited by the derivatives conducted in a manner similar to healthy axons. These results demonstrate the ability of novel constructs to serve as an alternative to 4-AP for the purpose of reversing conduction deficits.

Contusion, dislocation, and distraction: primary hemorrhage and membrane permeability in distinct mechanisms of spinal cord injury

Object

In experimental models of spinal cord injury (SCI) researchers have typically focused on contusion and transection injuries. Clinically, however, other injury mechanisms such as fracture–dislocation and distraction also frequently occur. The objective of the present study was to compare the primary damage in three clinically relevant animal models of SCI.

Methods

Contusion, fracture–dislocation, and flexion–distraction animal models of SCI were developed. To visualize traumatic increases in cellular membrane permeability, fluorescein–dextran was infused into the cerebrospi-nal fluid prior to injury. High-speed injuries (approaching 100 cm/second) were produced in the cervical spine of deeply anesthetized Sprague–Dawley rats (28 SCI and eight sham treated) with a novel multimechanism SCI test system. The animals were killed immediately thereafter so that the authors could characterize the primary injury in the gray and white matter.

Sections stained with H & E showed that contusion and dislocation injuries resulted in similar central damage to the gray matter vasculature whereas no overt hemorrhage was detected following distraction. Contusion resulted in membrane disruption of neuronal somata and axons localized within 1 mm of the lesion epicenter. In contrast, membrane compromise in the dislocation and distraction models was observed to extend rostrally up to 5 mm, particularly in the ventral and lateral white matter tracts.

Conclusions

Given the pivotal nature of hemorrhagic necrosis and plasma membrane compromise in the initiation of downstream SCI pathomechanisms, the aforementioned differences suggest the presence of mechanism-specific injury regions, which may alter future clinical treatment paradigms.

Kinetics of ATP release following compression injury of a peripheral nerve trunk

Compression and/or contusion of a peripheral nerve trunk can result in painful sensations. It is possible that release of ATP into the extracellular space may contribute to this symptom. In the present study, we used real-time measurements of ATP-induced bioluminescence together with electrophysiological recordings of compound action potentials to follow changes in the extracellular ATP concentration of isolated rat spinal roots exposed to mechanical stimuli. Nerve compression for about 8 s resulted in an immediate release of ATP into the extracellular space and in a decrease in the amplitude of compound action potentials. On average, a rise in ATP to 60 nM was observed when nerve compression blocked 50% of the myelinated axons. After the compression, the extracellular concentration of ATP returned to the resting level within a few minutes. The importance of ecto-nucleotidases for the recovery period was determined by exposure of isolated spinal roots to high concentrations of ATP and by use of inhibitors of ecto-nucleotidases. It was observed that spinal roots have a high capacity for ATP hydrolysis which is only partially blocked by betagamma-methylene ATP and ARL 67156. In conclusion, acute nerve compression produces an increase in the extracellular concentration of ATP and of its metabolites which may be sufficient for activation of purinergic P2 and/or P1 receptors on axons of nociceptive afferent neurons.

Quantitative analysis of the relationship between intra- axonal neurofilament compaction and impaired axonal transport following diffuse traumatic brain injury

Traumatic axonal injury (TAI) following traumatic brain injury (TBI) contributes to morbidity and mortality. TAI involves intra-axonal changes assumed to progress to impaired axonal transport (IAT), disconnection, and axonal bulb formation. Immunocytochemical studies employing antibodies to amyloid precursor protein (APP), a marker of IAT and RMO14, a marker of neurofilament compaction (NFC), have shown that TAI involves both NFC and IAT, with the suggestion that NFC leads to IAT. Recently, new data has suggested that NFC may occur independently of IAT. The objective of this study was to determine quantitatively the precise relationship between NFC and IAT. Following TBI, rats were studied at 30 min, 3 h, and 24 h. Using single-label immunocytochemistry employing the antibodies RM014, APP, or a combined labeling strategy targeting APP/RMO14 in aggregate, the immunoreactive (IR) profiles were counted in the corticospinal tract (CSpT) and medial lemniscus (ML). In the CSpT, the number of axons demonstrating RMO14-IR approximated the number of axons showing APP-IR, with the APP-IR population showing a significant increase over 24 h (p < 0.05). The sum of both single-label counts equaled the aggregate APP/RMO14 numbers, demonstrating little relationship between NFC and IAT. In the ML, 75% of fibers demonstrated a separation of APP-IR and NFC-IR; however, 25% of the ML fibers showed co-localization of APP-IR and RMO14. The results of these studies indicate that, in the majority of damaged axons, NFC is not associated with IAT. Our findings argue for the use of multiple markers when evaluating the extent of TAI or the efficacy of therapies targeting the treatment of TAI.

Mild Axonal Stretch Injury In Vitro Induces a Progressive Series of Neurofilament Alterations Ultimately Leading to Delayed Axotomy

We report a new model of transient axonal stretch injury involving pressurized fluid deflection of bundles of axons, resulting in a transient 1-6% increase in original axon length to investigate the slow progression of axonal alterations that are characteristic of diffuse axonal injury (DAI). We found no discernable difference in axon bundle morphology or cytoskeletal neurofilament protein arrangement between unstretched and stretched axonal bundles at 24 h post-injury. However, by 48 h post-injury, there was a stereotypical response of stretched axons involving characteristic neurofilament alterations that bear similarities to in vivo neuronal responses associated with DAI that have been reported previously. For instance, neurofilament protein immunoreactivity (SMI-312) was increased in axons contained within 51% of all injured axon bundles at 48 h compared to surrounding unstretched axon bundles, suggestive of neurofilament compaction. Furthermore, axonal bundle derangement occurred in 25% of injured axon bundles, with individual fibres segregating from each other and becoming undulating and wavy. By 72 h post-stretch, 70% of injured axon bundles underwent secondary axotomy, becoming completely severed at the site of initial stretch injury. While these results suggest a temporal series of stereotypical responses of axons to injury, we were able to distinguish very clear differences between mildly (100-103% increase in original axonal length) injured and strongly injured (106%+) axons. For instance, mildly injured axons developed increased neurofilament immunoreactivtity (SMI-312) within 48 h, and the marked development of ring-like neurofilament immunoreactive structures within axonal bundles, which were rarely axotomized. Conversely, at more severe strain levels increased neurofilament immunoreactivity was less apparent, while axons often became distorted and disorganised within axonal bundles and eventually became completely disconnected. Almost no ring-like neurofilament structures were observed in these severely injured axonal bundles. This suggests that axons do not respond in a stereotypical manner to a transient stretch insult, and indeed that variable degrees of stretch injury activate different responses within axons, with dramatically different outcomes. Hence, it is possible that the cytoskeletal characteristics that we have used in this study may be useful parameters for discriminating between mildly and severely injured axons following TBI.

Development of a model for microphysiological simulations: small nodes of ranvier from peripheral nerves of mice reconstructed by electron tomography

The node of Ranvier is a complex structure found along myelinated nerves of vertebrate animals. Specific membrane, cytoskeletal, junctional, extracellular matrix proteins and organelles interact to maintain and regulate associated ion movements between spaces in the nodal complex, potentially influencing response variation during repetitive activations or metabolic stress. Understanding and building high resolution three dimensional (3D) structures of the node of Ranvier, including localization of specific macromolecules, is crucial to a better understanding of the relationship between its structure and function and the macromolecular basis for impaired conduction in disease. Using serial section electron tomographic methods, we have constructed accurate 3D models of the nodal complex from mouse spinal roots with resolution better than 7.5 nm. These reconstructed volumes contain 75-80% of the thickness of the nodal region. We also directly imaged the glial axonal junctions that serve to anchor the terminal loops of the myelin lamellae to the axolemma. We created a model of an intact node of Ranvier by truncating the volume at its midpoint in Z, duplicating the remaining volume and then merging the new half volume with mirror symmetry about the Z-axis. We added to this model the distribution and number of Na+ channels on this reconstruction using tools associated with the MCell simulation program environment. The model created provides accurate structural descriptions of the membrane compartments, external spaces, and formed structures enabling more realistic simulations of the role of the node in modulation of impulse propagation than have been conducted on myelinated nerve previously.

Light and electron microscopic assessment of progressive atrophy following moderate traumatic brain injury in the rat

The presence of progressive white matter atrophy following traumatic brain injury (TBI) has been reported in humans as well as in animal models. However, a quantitative analysis of progressive alterations in myelinated axons and other cellular responses to trauma has not been conducted. This study examined quantitative differences in myelinated axons from several white and gray matter structures between non-traumatized and traumatized areas at several time points up to 1 year. We hypothesize that axonal numbers decrease over time within the structures analyzed, based on our previous work demonstrating shrinkage of tissue in these vulnerable areas. Intubated, anesthetized male Sprague-Dawley rats were subjected to moderate (1.8-2.2 atm) parasagittal fluid-percussion brain injury, and perfused at various intervals after surgery. Sections from the fimbria, external capsule, thalamus and cerebral cortex from the ipsilateral hemisphere of traumatized and sham-operated animals were prepared and. estimated total numbers of myelinated axons were determined by systematic random sampling. Electron micrographs were obtained for ultrastructural analysis. A significant (P<0.05) reduction in the number of myelinated axons in the traumatized hemisphere compared to control in all structures was observed. In addition, thalamic and cortical axonal counts decreased significantly (P<0.05) over time. Swollen axons and macrophage/microglia infiltration were present as late as 6 months post-TBI in various structures. This study is the first to describe quantitatively chronic axonal changes in vulnerable brains regions after injury. Based on these data, a time-dependent decrease in the number of myelinated axons is seen to occur in vulnerable gray matter regions including the cerebral cortex and thalamus along with distinct morphological changes within white matter tracts after TBI. Although this progressive axonal response to TBI may include Wallerian degeneration, other potential mechanisms underlying this progressive pathological response within the white matter are discussed.

Effects of 4-aminopyridine on stretched mammalian spinal cord: the role of potassium channels in axonal conduction

Axonal conduction deficit is a major contributor to various degrees of disability after spinal cord injury. 4-aminopyridine (4-AP), a potassium channel blocker, has been shown to restore some conduction and improve neurological function in both animal and human studies. Using a double sucrose-gap recording device, we have examined the effects of 4-AP on isolated guinea pig spinal cord white matter after stretch injury. At a concentration of 100 microM, 4-AP increased the amplitude of the compound action potential by 100% while 1 microM 4-AP increased it by 43%, a larger response than seen following compression injury. The length of affected tissue is suggested as a potential explanation of this differential sensitivity to 4-AP. Plastic sections taken from the injury site revealed severe myelin damage, especially in the paranodal area, which may also partially explain why 4-AP has more effect on conduction after stretch injury than compression. In addition, we have shown that while enhancing conductivity in some axons, 4-AP significantly reduced the overall responsiveness to multiple stimuli, as evidenced by increase of the refractory period in response to dual stimuli and the decreased ability to follow repetitive stimuli. This increased refractoriness may be largely attributed to residual deficits in fibers newly recruited by 4-AP treatment.

Traumatic axonal injury results in biphasic calpain activation and retrograde transport impairment in mice

Traumatic axonal injury (TAI) is one of the most important pathologies associated with closed head injury, and contributes to ensuing morbidity. The authors evaluated the potential role of calpains in TAI using a new model of optic nerve stretch injury in mice. Male C57BL/6 mice were anesthetized, surgically prepared, and subjected to a 2.0-mm optic nerve stretch injury (n = 34) or sham injury (n = 18). At various intervals up to 2 weeks after injury, optic nerves were examined for neurofilament proteins and calpain-mediated spectrin breakdown products using immunohistochemistry. In addition, fluorescent tracer was injected into the superior colliculi of mice 1 day before they were killed, to investigate the integrity of retrograde axonal transport to the retina. Optic nerve stretch injury resulted in persistent disruption of retrograde axonal transport by day 1, progressive accumulation and dephosphorylation of neurofilament protein in swollen and disconnected axons, and subsequent loss of neurofilament protein in degenerating axons at day 14. Calpains were transiently activated in intact axons in the first minutes to hours after stretch injury. A second stage of calpain-mediated proteolysis was observed at 4 days in axonal swellings, bulbs, and fragments. These data suggest that early calpain activation may contribute to progressive intraaxonal structural damage, whereas delayed calpain activation may be associated with axonal degeneration.

Ultrastructural analysis of the paranodal junction of myelinated fibers in 31-month-old-rats

Recent studies have revealed a significant decrease in white matter volume, including loss of myelin, with age but minimal decrease in gray matter volume (Guttmann et al., [1998] Neurology 50:972-978). Myelin is necessary for the rapid conduction of impulses along axons. Myelinated nerve includes various domains, the node of Ranvier, the paranodal region, the juxtaparanodal region and the internode. The paranodal junction may serve to anchor the myelin sheath to the axon. We analyzed the ultrastructure of the paranodal region in myelinated fibers from the aged rat brain. Severe alterations of myelinated fibers were observed in 31-month-old rats, resulting in the appearance of macrophages, splitting of the myelin sheath, myelin balloon formation and separation from the axon. Many paranodal retractions of myelinated axons occurred in the aged rats. It should be noted that the paranodal junction is functionally important, serving to anchor the myelin to the axon and that there is a diffusion barrier in the paranodal region. We analyzed myelin-related proteins from young and aged rat brains. The 21.5-kDa isoform of myelin basic protein (MBP) almost disappeared in the 31-month-old rats, whereas other myelin proteins were not significantly changed between young and aged rats. These results suggest that this isoform, a highly cationic charged major dense component protein that binds lipid bilayer in the membrane, may participate in the formation of a paranodal diffusion barrier at the myelin/noncompact membrane border.

Pathological changes of isolated spinal cord axons in response to mechanical stretch

White matter strips extracted from adult guinea-pig spinal cords were maintained in vitro and studied physiologically using a double sucrose gap technique and anatomically using a horseradish peroxidase assay. The amplitude of compound action potentials was monitored continuously before, during, and after elongation. Three types of conduction blocks resulting from stretch injury were identified: an immediate, spontaneously reversible component, which may result from a transient increase in membrane permeability and consequent disturbance of ionic distribution; a second component that was irreversible within 30-60 min of recording, perhaps resulting from profound axolemmal disruption; and a third component, which may be due to perturbation of the myelin sheath, that was reversible with application of 100 microM of the potassium channel blocker, 4-aminopyridine. The intensity of the conduction deficits correlated with the extent of initial stretch over a full range of severity. Stimulus-response data indicate that mechanical damage to axons in stretch was evenly distributed across the caliber spectrum. Morphological examinations revealed that a small portion of axons exhibited membrane damage at 2 min following stretch and appeared to be largely sealed at 30 min after injury. Further, in the entire length of the cord strip subjected to stretch, axons closer to the surface were found to be more likely to suffer membrane damage, which distinguished stretch injury from compression injury. In summary, we have developed an in vitro model of axonal stretch that provides the ability to monitor changes in the properties of central myelinated axons following stretch injury in the absence of pathological variables related to vascular damage. This initial investigation found no evidence of secondary deterioration of axons in the first 30 min after stretch in vitro, although there was evidence of both transient and lasting physiological and anatomical damage to axons and their myelin sheaths.

Protective effect of nimodipine on behavior and white matter of rats with hydrocephalus

OBJECT

Hydrocephalus, a pathological dilation of the ventricles of the brain, causes damage to periventricular white matter, at least in part, through chronic ischemia. The authors tested the hypothesis that treatment with nimodipine, an L-type calcium channel-blocking agent with demonstrated efficacy in a range of cerebral ischemic disorders, would ameliorate the adverse effects of experimental hydrocephalus.

METHODS

Hydrocephalus was induced in 3-week-old rats by injection of kaolin into the cisterna magna. The rats were treated by continuous administration of nimodipine or control vehicle for 2 weeks, beginning 2 weeks after induction of hydrocephalus. During the treatment period, the animals underwent repeated tests of motor and cognitive behavior. At the end of the treatment period, the rat brains were analyzed by histopathological and biochemical means. Nimodipine treatment prevented the declines in motor and cognitive behavior that were observed in untreated control rats. During the treatment period, ventricular enlargement, determined by magnetic resonance imaging, was equal in the two groups, although the corpus callosum was thicker in the treated rats. Myelin content in white matter and synaptophysin content in gray matter, an indicator of synapses, did not differ.

CONCLUSIONS

The protective effect of nimodipine is most likely based on improved blood flow, although prevention of calcium influx-mediated proteolytic processes in axons cannot be excluded. Adjunctive pharmacological therapy may be beneficial to patients with hydrocephalus.

Dynamic stretch correlates to both morphological abnormalities and electrophysiological impairment in a model of traumatic axonal injury

In this investigation, the relationships between stretch and both morphological and electrophysiological signs of axonal injury were examined in the guinea pig optic nerve stretch model. Additionally, the relationship between axonal morphology and electrophysiological impairment was assessed. Axonal injury was produced in vivo by elongating the guinea pig optic nerve between 0 and 8 mm (Ntotal = 70). Morphological damage was detected using neurofilament immunohistochemistry (SMI 32). Electrophysiological impairment was determined using changes in visual evoked potentials (VEPs) measured prior to injury, every 5 min for 40 min following injury, and at sacrifice (72 h). All nerves subjected to ocular displacements greater than 6 mm demonstrated axonal swellings and retraction bulbs, while nerves subjected to displacements below 4 mm did not show any signs of morphological injury. Planned comparisons of latency shifts of the N35 peak in the VEPs showed that ocular displacements greater than 5 mm produced electrophysiological impairment that was significantly different from sham animals. Logit analysis demonstrated that less stretch was required to elicit electrophysiological changes (5.5 mm) than morphological signs of damage (6.8 mm). Moreover, Student t tests indicated that the mean latency shift measured in animals exhibiting morphological injury was significantly greater than that calculated from animals lacking morphological injury (p < 0.01). These data show that distinct mechanical thresholds exist for both morphological and electrophysiological damage to the white matter. In a larger context, the distinct injury thresholds presented in the report will aid in the biomechanical assessment of animate models of head injury, as well as assist in extending these findings to predict the conditions that cause white matter injury in humans.

Calcium-mediated proteolytic damage in white matter of hydrocephalic rats?

Hydrocephalus is a pathological dilatation of the cerebrospinal fluid (CSF)-containing ventricles of the brain. Damage to periventricular white matter is multifactorial with contributions by chronic ischemia and gradual physical distortion. Acute ischemic and traumatic brain injuries are associated with calcium-dependent activation of proteolytic enzymes. We hypothesized that hydrocephalus is associated with calcium ion accumulation and proteolytic enzyme activation in cerebral white matter. Hydrocephalus was induced in immature and adult rats by injection of kaolin into the cisterna magna and several different experimental approaches were used. Using the glyoxal bis (2-hydroxyanil) method, free calcium ion was detected in periventricular white matter at sites of histological injury. Western blot determinations showed accumulation of calpain I (mu-calpain) and immunoreactivity for calpain I was increased in periventricular axons of young hydrocephalic rats. Proteolytic cleavage of a fluorogenic calpain substrate was demonstrated in white matter. Immunoreactivity for spectrin breakdown products was detected in scattered callosal axons of young hydrocephalic rats. The findings support the hypothesis that periventricular white matter damage associated with experimental hydrocephalus is due, at least in part, to calcium-activated proteolytic processes. This may have implications for supplemental drug treatments of this disorder.

Traumatic axonal injury: practical issues for diagnosis in medicolegal cases

In the 25 years or so after the first clinicopathological descriptions of diffuse axonal injury (DAI), the criterion for diagnosing recent traumatic white matter damage was the identification of swollen axons ('bulbs') on routine or silver stains, in the appropriate clinical setting. In the last decade, however, experimental work has given us greater understanding of the cellular events initiated by trauma to axons, and this in turn has led to the adoption of immunocytochemical methods to detect markers of axonal damage in both routine and experimental work. These methods have shown that traumatic axonal injury (TAI) is much more common than previously realized, and that what was originally described as DAI occupies only the most severe end of a spectrum of diffuse trauma-induced brain injury. They have also revealed a whole field of previously unrecognized white matter pathology, in which axons are diffusely damaged by processes other than head injury; this in turn has led to some terminological confusion in the literature. Neuropathologists are often asked to assess head injuries in a forensic setting: the diagnostic challenge is to sort out whether the axonal damage detected in a brain is indeed traumatic, and if so, to decide what - if anything - can be inferred from it. The lack of correlation between well-documented histories and neuropathological findings means that in the interpretation of assault cases at least, a diagnosis of 'TAI' or 'DAI' is likely to be of limited use for medicolegal purposes.

Neuronal degeneration in the gerbil brainstem is associated with spongiform lesions

Spongiform lesions arise in dendrites and glia in the brainstem of domestic Mongolian gerbils. Most pronounced within the cochlear nucleus (CN), this disorder is dynamic and progressive; the lesions increase in number, size, and extent with age. It has not been clear whether these spongioid lesions either cause or are associated with significant neural degeneration. In contrast, feral Mongolian gerbils (wild-trapped in Tuva) and their offspring show few spongiform lesions. The Tuvan gerbils provide an appropriate within-species control. We compared degeneration in the brainstem of domestic and Tuvan gerbils using the amino-cupric-silver (ACS) stain of de Olmos et al. [(1994) Neurotoxicol. Teratol., 16:545-561]. Positive histologic controls were provided by cerebellar stab wounds in domestic gerbils and by unilateral kainic acid injections into the CN of Tuvan gerbils. The ACS stain revealed extensive degeneration of axons, terminals, dendrites, and neurons in the brainstem of domestic gerbils. Neurodegeneration was most pronounced in the CN and was coextensive with spongiform lesions. Neurodegeneration was also seen in the trapezoid body, lateral lemniscus, and inferior colliculus, but was less pronounced than in the CN. The cerebellar stab wounds resulted in silver-stained Purkinje cells restricted to the stab wound local region. Kainic acid produced extensive neuronal and spongiform degeneration of the injected CN that was very similar to that spontaneously occurring in domestic gerbils. In contrast, the non-injected CN of Tuvan gerbils showed no neuronal or spongiform degeneration with the ACS stain. We conclude that, in domestic gerbils, the naturally occurring spongiform lesions of the CN and the accompanying neurodegeneration are both results of a common mechanism, most probably excitotoxic.

Axonal structure and function after axolemmal leakage in the squid giant axon

Membrane leakage is a common consequence of traumatic nerve injury. In order to measure the early secondary effects of different levels of membrane leakage on axonal structure and function we studied the squid giant axon after electroporation at field strengths of 0.5, 1.0, 1.6, or 3.3 kV/cm. Immediately after mild electroporation at 0.5 kV/cm, 40% of the axons had no action potentials, but by 1 h all of the mildly electroporated axons had recovered their action potentials. Many large organelles (mitochondria) were swollen, however, and their transport was reduced by 62% 1 h after this mild electroporation. One hour after moderate electroporation at 1.0 kV/cm, most of the axons had no action potentials, most large organelles were swollen, and their transport was reduced by 98%, whereas small organelle transport was reduced by 75%. Finally at severe electroporation levels of 1.65-3.0 kV/cm all conduction and transport was lost and the gel-like axoplasmic structure was clumped or liquefied. The structural damage and transport block seen after severe and moderate poration were early secondary injuries that could be prevented by placing the porated axons in an intracellular-type medium (low in Ca2+, Na+, and Cl-) immediately after poration. In moderately, but not severely, porated axons this protection of organelle transport and structure persisted, and action potential conduction returned when the axons were returned to the previously injurious extracellular-type medium. This suggests that the primary damage, the axolemmal leak, was repaired while the moderately porated axons were in the protective intracellular-type medium.

Loss of axonal microtubules and neurofilaments after stretch-injury to guinea pig optic nerve fibers

Axonal swellings, characterized by focal accumulations of membranous organelles at presumed sites of interrupted axonal transport, occur in diffuse axonal injury (DAI) in human, blunt head injury and in animal models of nondisruptive axonal injury. Membranous organelles are transported by fast axonal transport in association with microtubules. Although loss of microtubules has been documented at levels of injury severe enough to result in permeabilization of the axolemma to tracers such as horseradish peroxidase, there has been no detailed analysis of responses by microtubules in less severe or milder forms of nondisruptive axonal injury. To test the hypothesis that in less severe forms of axonal injury there is a rapid response by axonal microtubules that might provide an explanation for loss of fast axonal transport, we have carried out a morphometric analysis of microtubules in CNS axons after stretch-injury. There is loss of microtubules at nodes of Ranvier with nodal blebs within 15 min of injury, and in internodal axonal swellings between 2 and 4 h. There is a return to control values at nodes of Ranvier by 4 h, and at the internode by 24 h. There is no loss of microtubules at paranodes, although there is a reduction in their density in the first 2 h after injury. The greatest loss of microtubules occurs at sites of axolemma infolding. Hypothetical mechanisms that might lead to this loss resulting in focal disruption of fast axonal transport and the formation of axonal swellings are discussed.

Paranodin, a glycoprotein of neuronal paranodal membranes

Ranvier nodes are flanked by paranodal regions, at the level of which oligodendrocytes or Schwann cells interact closely with axons. Paranodes play a critical role in the physiological properties of myelinated nerve fibers. Paranodin, a prominent 180 kDa transmembrane neuronal glycoprotein, was purified and cloned from adult rat brain, and found to be highly concentrated in axonal membranes at their junction with myelinating glial cells, in paranodes of central and peripheral nerve fibers. The large extracellular domain of paranodin is related to neurexins, and its short intracellular tail binds protein 4.1, a cytoskeleton-anchoring protein. Paranodin may be a critical component of the macromolecular complex involved in the tight interactions between axons and myelinating glial cells characteristic of the paranodal region.

A mechanistic analysis of nondisruptive axonal injury: a review

Axons are particularly at risk in human diffuse head injury. Use of immunocytochemical labeling techniques has recently demonstrated that axonal injury (AI) and the ensuing reactive axonal change is, probably, more widespread and occurs over a longer posttraumatic time in the injured brain than had previously been appreciated. But the characterization of morphologic or reactive changes occurring after nondisruptive AI has largely been defined from animal models. The comparability of AI in animal models to human diffuse AI (DAI) is discussed and the conclusion drawn that, although animal models allow the analysis of morphologic changes, the spatial distribution within the brain and the time course of reactive axonal change differs to some extent both between species and with the mode of brain injury. Thus, the majority of animal models do not reproduce exactly the extent and time course of AI that occurs in human DAI. Nonetheless, these studies provide good insight into reactive axonal change. In addition, there is developing in the literature considerable variance in the terminology applied to injured axons or nerve fibers. We explain our current understanding of a number of terms now present in the literature and suggest the adoption of a common terminology. Recent work has provided a consensus that reactive axonal change is linked to pertubation of the axolemma resulting in disruption of ionic homeostatic mechanisms within injured nerve fibers. But quantitative data for changes for different ion species is lacking and is required before a better definition of this homeostatic disruption may be provided. Recent studies of responses by the axonal cytoskeleton after nondisruptive AI have demonstrated loss of axonal microtubules over a period up to 24 h after injury. The biochemical mechanisms resulting in loss of microtubules are, hypothetically, mediated both by posttraumatic influx of calcium and activation of calmodulin. This loss results in focal accumulation of membranous organelles in parts of the length of damaged axons where the axonal diameter is greater than normal to form axonal swellings. We distinguish, on morphologic grounds, between axonal swellings and axonal bulbs. There is also a growing consensus regarding responses by neurofilaments after nondisruptive AI. Initially, and rapidly after injury, there is reduced spacing or compaction of neurofilaments. This compaction is stable over at least 6 h and results from the loss or collapse of neurofilament sidearms but retention of the filamentous form of the neurofilaments. We posit that sidearm loss may be mediated either through proteolysis of sidearms via activation of microM calpain or sidearm dephosphorylation via posttraumatic, altered interaction between protein phosphatases and kinase(s), or a combination of these two, after calcium influx, which occurs, at least in part, as a result of changes in the structure and functional state of the axolemma. Evidence for proteolysis of neurofilaments has been obtained recently in the optic nerve stretch injury model and is correlated with disruption of the axolemma. But the earliest posttraumatic interval at which this was obtained was 4 h. Clearly, therefore, no evidence has been obtained to support the hypothesis that there is rapid, posttraumatic proteolysis of the whole axonal cytoskeleton mediated by calpains. Rather, we hypothesize that such proteolysis occurs only when intra-axonal calcium levels allow activation of mM calpain and suggest that such proteolysis, resulting in the loss of the filamentous structure of neurofilaments occurs either when the amount of deformation of the axolemma is so great at the time of injury to result in primary axotomy or, more commonly, is a terminal degenerative change that results in secondary axotomy or disconnection some hours after injury.

What's new in the diagnosis of head injury?

The diagnosis of DAI is not always easy, and should be based on adequate sampling of appropriate anatomical areas from a sliced, fixed brain. It is now recognised that there is a continuum of traumatic white matter damage, and that DAI represents only the severe end of the scale. Such damage may be detected from very shortly after a head injury-a fact that may give rise to some challenging diagnostic problems. Early axonal injury detected by means of beta APP immunostaining should be interpreted with caution. The most useful tools currently available for detecting axonal damage are antisera to beta APP, PG-M1, and GFAP, used in conjunction with a routine haematoxylin and eosin stain, but even with immunocytochemistry precise dating of histological changes may not be possible.

Molecular composition of the node of Ranvier: identification of ankyrin-binding cell adhesion molecules neurofascin (mucin+/third FNIII domain-) and NrCAM at nodal axon segments

Neurofascin, NrCAM, L1, and NgCAM are a family of Ig/FNIII cell adhesion molecules that share ankyrin-binding activity in their cytoplasmic domains, and are candidates to form membrane-spanning complexes with members of the ankyrin family of spectrin-binding proteins in a variety of cellular contexts in the nervous system. Specialized forms of ankyrin, 270 kD and/or 480 kD ankyrinG are components of the membrane undercoat of axons at the node of Ranvier. This paper focuses on definition of the isoforms of ankyrin-binding cell adhesion molecules localized with ankyrinG at the nodal axon segment. The exon usage of two major forms of neurofascin was determined by isolation of full-length cDNAs and used to prepare isoform-specific antibodies. An isoform of neurofascin containing a mucin-like domain and lacking the third FNIII domain was concentrated at axon initial segments and colocalized at nodes of Ranvier with ankyrinG and the voltage-dependent sodium channel. An alternative form of neurofascin lacking the mucin-like domain and containing the third FNIII domain was present in unmyelinated axons. The antibody initially raised against neurofascin was used to screen a rat brain cDNA expression library. In addition to neurofascin, this screen yielded a clone with 80% sequence identity to NrCAM from chicken. The sequences of two full-length cDNAs are presented. NrCAM is most closely related to neurofascin among the other members of the L1/neurofascin/NgCAM family, with over 70% identity between cytoplasmic domains. NrCAM, visualized with antibodies specific for the ecto-domain, also was found to be coexpressed with neurofascin at nodes of Ranvier and at axon initial segments. This is the first characterization of defined neuronal cell adhesion molecules localized to axonal membranes at the node of Ranvier of myelinated axons.

Detection of acute pathologic changes following experimental traumatic brain injury using diffusion-weighted magnetic resonance imaging

Standard magnetic resonance imaging (MRI) has been shown to be remarkably insensitive to acute changes following traumatic brain injury. Because diffusion-weighted MRI has recently demonstrated excellent sensitivity to acute ischemic injury and other CNS abnormalities, we evaluated the use of diffusion MRI for the detection of pathologic changes in the rat brain during the first hours following parasagittal fluid percussion brain injury. Diffusion MRI was able to demonstrate a significant diffusion decrease in the primary cortical contusion injury and a comparable decrease in the ipsilateral thalamus. Tissue damage in the thalamus region is much weaker than in the cortex, but the thalamus is a primary site of axonal and dendritic injury in this model. T2 imaging in the same subjects showed slight enhancement in the neighborhood of the injured cortex but was unable to demonstrate injury elsewhere. Diffusion imaging was superior to T2 at demonstrating injury and the prominent diffusion decrease in the thalamus suggests that diffusion MRI is preferentially sensitive to axonal or dendritic injury.

Histopathological changes at central nodes of Ranvier after stretch-injury

While the brain readily deforms when exposed to rotational loads as experienced in violent movements of the head, axons are able only to sustain tensile loads. Two discrete classes of axonal injury have been posited: disruptive axonal injury, where axons are physically torn or fragmented at the time of the insult, and nondisruptive axonal injury, where there is a hypothesised "perturbation" of the axolemma which leads to a cascade of pathobiological changes which result in axotomy over a period between 2 and 24 h after the initial insult. In the latter, it is posited that the node of Ranvier is that part of the axon which is the initial locus of axonal damage/ histopathological change. This paper describes the ultrastructure of nodal blebs, axolemma limited protrusions of the nodal axoplasm into the perinodal space, in which the nodal dense undercoating has been lost and aggregates of membranous profiles occur within the axoplasm. In addition, this paper provides novel data for disruption of the axonal cytoskeleton in nodes where blebs occur within 15 min of stretch-injury. The cytoskeletal disruption is visualised in thin sections as an almost total loss of microtubules together with a reduced density of neurofilaments within the nodal axoplasm. The loss of microtubules is posited to result in a disruption of fast axonal transport which results in the focal accumulation of membranous organelles in adjacent paranodal regions of the axon to form so-called "axonal swellings." Cytochemical and freeze-fracture studies provide evidence for structural reorganisation of the nodal axolemma after stretch-injury, and it is posited that these changes provide a route for uncontrolled influx of calcium which leads to loss of axonal integrity which potentiates axotomy. It is suggested that increased understanding of regulatory mechanisms that control ion channel activity will greatly increase our understanding of responses of neurones to trauma.

Optic nerve elongation: does it exist?

The length of the optic nerves is a reflection of normal postnatal cranio-orbital development. Unilateral elongation of an optic nerve has been observed in two patients with orbital and skull base neoplasms. In the first case as compared to the patient's opposite, normal optic nerve, an elongated length of the involved optic nerve of 45 mm was present. The involved optic nerve in the second patient was 10 mm longer than the normal opposite optic nerve. The visual and extraocular function was preserved in the second patient. The first patient had only light perception in the affected eye. In this paper, the embryology, anatomy, and physiology of the optic nerve and its mechanisms of stretch and repair are discussed.

Traumatically induced axonal damage: evidence for enduring changes in axolemmal permeability with associated cytoskeletal change

Recent studies have demonstrated that delayed or secondary axotomy is a consistent feature of traumatic brain injury in both animals and man. Moreover, these studies have shown that the pathogenesis of this secondary axotomy involves various forms of initiating pathology, with the suggestion that, in some cases, only the axonal cytoskeleton is perturbed, while, in other cases, both the axonal cytoskeleton and related axolemma manifest traumatically induced perturbations. In the current communication, we continue in our investigation of the significance of these traumatically induced alterations in axolemmal permeability and their relation to any related intra-axonal cytoskeletal change. This was accomplished in cats which received intrathecal infusions of peroxidase, an agent normally excluded by the intact axolemma. These animals were subjected to traumatic brain injury, and sites showing altered axolemmal permeability to the peroxidase were assessed at the light and electron microscopic level. Through this approach, we recognized that a traumatic episode of moderate severity evoked changes in axolemmal permeability which surprising endured for up to 5 hrs postinjury. At such focal sites of altered permeability, the related cytoskeleton showed a statistically significantly neurofilament compaction, with the strong suggestion of concomitant neurofilament sidearm loss, microtubular dispersion, and mitochondrial abnormality. Over time, these events led to further disorganization of the axonal cytoskeleton which translated into impaired axoplasmic transport and secondary axotomy. Most likely, these alterations in axolemmal permeability result in either the direct or indirect effects upon the axonal cytoskeleton that precipitate the damaging sequences resulting in delayed axotomy.

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

There has been controversy for some time as to whether a posttraumatic influx of calcium ions occurs in stretch/nondisruptively injured axons within the central nervous system in both human diffuse axonal injury and a variety of models of such injury. We have used the oxalate/pyroantimonate technique to provide cytochemical evidence in support of such an ionic influx after focal axonal injury to normoxic guinea pig optic nerve axons, a model for human diffuse axonal injury. We present evidence for morphological changes within 15 min of injury where aggregates of pyroantimonate precipitate occur in nodal blebs at nodes of Ranvier, in focal swellings within axonal mitochondria, and at localized sites of separation of myelin lamellae. In parallel with these studies, we have used cytochemical techniques for localization of membrane pump Ca(2+)-ATPase and ecto-Ca-ATPase activity. There is loss of labelling for membrane pump Ca(2+)-ATPase activity on the nodal axolemma, together with loss of ecto-Ca-ATPase from the external aspect of the myelin sheath at sites of focal separation of myelin lamellae. Disruption of myelin lamellae and loss of ecto-Ca-ATPase activity becomes widespread between 1 and 4 h after injury. This is correlated with both infolding and retraction of the axolemma from the internal aspect of the myelin sheath to form periaxonal spaces which are characterized by aggregates of pyroantimonate precipitate, and the development of myelin intrusions into invaginations of the axolemma such that the regular profile of the axon is lost. There is novel labelling of membrane pump Ca(2+)-ATPase on the cytoplasmic aspect of the internodal axolemma between 1 and 4 h after injury. There is loss of an organized axonal cytoskeleton in a proportion of nerve fibres by 4-6 h after injury. We suggest that these changes demonstrate a progressive pathology linked to calcium ion influx after stretch (non-disruptive) axonal injury to optic nerve myelinated fibres. We posit that calcium influx, linked to or correlated with changes in Ca(2+)-ATPase activities, results in dissolution of the axonal cytoskeleton and axotomy between 4 and 6 h after the initial insult to axons.