Traumatic axonal injury induces proteolytic cleavage of the voltage-gated sodium channels modulated by tetrodotoxin and protease inhibitors

Focal increases of axoplasmic Ca2+, aggregation of sodium-calcium exchanger, N-type Ca2+ channel, and actin define the sites of spheroids in axons undergoing oxidative stress

Axonal spheroids occur as part of the pathology of a variety of neurologic diseases. Reactive oxygen species (ROS) trigger formation of spheroids, axonal severing, and Ca(2+) overload. The mechanisms by which ROS lead to the spheroid formation at specific axonal sites remain elusive. Here, using adult mouse primary neurons, we investigate the role of Ca(2+), its regulating systems, and cytoskeletal changes in formation of axonal spheroids triggered by ROS. The results reveal that dramatically higher axoplasmic Ca(2+) levels occur at the sites of axonal spheroids than in the rest of the axon. High focal axoplasmic Ca(2+) levels correlate with focal aggregation of the reverse Na(+)/Ca(2+) exchanger 1, voltage-gated N-type Ca(2+) channel α1B subunit, and actin at the sites of spheroids in individual axons. This study provides new insights into the mechanism of a spheroid formation at specific sites along axons undergoing oxidative stress and a basis for new neuroprotective strategies.

Neuroprotection and repair in multiple sclerosis

Multiple sclerosis (MS) is an inflammatory demyelinating disease that is considered by many people to have an autoimmune aetiology. In recent years, new data emerging from histopathology, imaging and other studies have expanded our understanding of the disease and may change the way in which it is treated. Conceptual shifts have included: first, an appreciation of the extent to which the neuron and its axon are affected in MS, and second, elucidation of how the neurobiology of axon-glial and, particularly, axon-myelin interaction may influence disease progression. In this article, we review advances in both areas, focusing on the molecular mechanisms underlying axonal loss in acute inflammation and in chronic demyelination, and discussing how the restoration of myelin sheaths via the regenerative process of remyelination might prevent axon degeneration. An understanding of these processes could lead to better strategies for the prevention and treatment of axonal loss, which will ultimately benefit patients with MS.

Calpastatin overexpression protects axonal transport in an in vivo model of traumatic axonal injury

Traumatic brain injury (TBI) causes substantial morbidity and mortality worldwide. A key component of both mild and severe TBI is diffuse axonal injury. Except in cases of extreme mechanical strain, when axons are torn at the moment of trauma, axonal stretch injury is characterized by early cytoskeletal proteolysis, transport disruption, and secondary axotomy. Calpains, a family of Ca(2+)-dependent proteases, have been implicated in this pathologic cascade, but direct in vivo evidence is lacking. To test the hypothesis that calpains play a causal role in axonal stretch injury in vivo, we used our rat optic nerve stretch model following adeno-associated viral (AAV) vector-mediated overexpression of the endogenous calpain inhibitor calpastatin in optic nerve axons. AAV vectors were designed for optimal expression of human calpastatin (hCAST) in retinal ganglion cells (RGCs). Calpain inhibition by the expressed protein was then confirmed in primary cortical cultures. Finally, we performed bilateral intravitreal injections of AAV vectors expressing hCAST or the reporter protein ZsGreen 3 weeks prior to unilateral optic nerve stretch. Immediately after stretch injury, Fluoro-Gold was injected into the superior colliculi for assessment of retrograde axonal transport. Rats were euthanized 4 days after stretch injury. Both hCAST and ZsGreen were detected in axons throughout the optic nerve to the chiasm. Calpastatin overexpression partially preserved axonal transport after stretch injury (58.3±15.6% reduction in Fluoro-Gold labeling relative to uninjured contralateral controls in ZsGreen-expressing RGCs, versus 33.8±23.9% in hCAST-expressing RGCs; p=0.038). These results provide direct evidence that axonal calpains play a causal role in transport disruption after in vivo stretch injury.

DOR activation inhibits anoxic/ischemic Na+ influx through Na+ channels via PKC mechanisms in the cortex

Activation of delta-opioid receptors (DOR) is neuroprotective against hypoxic/ischemic injury in the cortex, which is at least partially related to its action against hypoxic/ischemic disruption of ionic homeostasis that triggers neuronal injury. Na(+) influx through TTX-sensitive voltage-gated Na(+) channels may be a main mechanism for hypoxia-induced disruption of K(+) homeostasis, with DOR activation attenuating the disruption of ionic homeostasis by targeting voltage-gated Na(+) channels. In the present study we examined the role of DOR in the regulation of Na(+) influx in anoxia and simulated ischemia (oxygen-glucose deprivation) as well as the effect of DOR activation on the Na(+) influx induced by a Na(+) channel opener without anoxic/ischemic stress and explored a potential PKC mechanism underlying the DOR action. We directly measured extracellular Na(+) activity in mouse cortical slices with Na(+) selective electrodes and found that (1) anoxia-induced Na(+) influx occurred mainly through TTX-sensitive Na(+) channels; (2) DOR activation inhibited the anoxia/ischemia-induced Na(+) influx; (3) veratridine, a Na(+) channel opener, enhanced the anoxia-induced Na(+) influx; this could be attenuated by DOR activation; (4) DOR activation did not reduce the anoxia-induced Na(+) influx in the presence of chelerythrine, a broad-spectrum PKC blocker; and (5) DOR effects were blocked by PKCβII peptide inhibitor, and PKCθ pseudosubstrate inhibitor, respectively. We conclude that DOR activation inhibits anoxia-induced Na(+) influx through Na(+) channels via PKC (especially PKCβII and PKCθ isoforms) dependent mechanisms in the cortex.

Mechanisms of calpain mediated proteolysis of voltage gated sodium channel α-subunits following in vitro dynamic stretch injury

Although enhanced calpain activity is well documented after traumatic brain injury (TBI), the pathways targeting specific substrate proteolysis are less defined. Our past work demonstrated that calpain cleaves voltage gated sodium channel (NaCh) α-subunits in an in vitro TBI model. In this study, we investigated the pathways leading to NaCh cleavage utilizing our previously characterized in vitro TBI model, and determined the location of calpain activation within neuronal regions following stretch injury to micropatterned cultures. Calpain specific breakdown products of α-spectrin appeared within axonal, dendritic, and somatic regions 6 h after injury, concurrent with the appearance of NaCh α-subunit proteolysis in both whole cell or enriched axonal preparations. Direct pharmacological activation of either NMDA receptors (NMDArs) or NaChs resulted in NaCh proteolysis. Likewise, a chronic (6 h) dual inhibition of NMDArs/NaChs but not L-type voltage gated calcium channels significantly reduced NaCh proteolysis 6 h after mechanical injury. Interestingly, an early, transient (30 min) inhibition of NMDArs alone significantly reduced NaCh proteolysis. Although a chronic inhibition of calpain significantly reduced proteolysis, a transient inhibition of calpain immediately after injury failed to significantly attenuate NaCh proteolysis. These data suggest that both NMDArs and NaChs are key contributors to calpain activation after mechanical injury, and that a larger temporal window of sustained calpain activation needs consideration in developing effective treatments for TBI.

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

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

In vitro models of traumatic brain injury

In vitro models of traumatic brain injury (TBI) are helping elucidate the pathobiological mechanisms responsible for dysfunction and delayed cell death after mechanical stimulation of the brain. Researchers have identified compounds that have the potential to break the chain of molecular events set in motion by traumatic injury. Ultimately, the utility of in vitro models in identifying novel therapeutics will be determined by how closely the in vitro cascades recapitulate the sequence of cellular events that play out in vivo after TBI. Herein, the major in vitro models are reviewed, and a discussion of the physical injury mechanisms and culture preparations is employed. A comparison between the efficacy of compounds tested in vitro and in vivo is presented as a critical evaluation of the fidelity of in vitro models to the complex pathobiology that is TBI. We conclude that in vitro models were greater than 88% predictive of in vivo results.

Left-shifted nav channels in injured bilayer: primary targets for neuroprotective nav antagonists?

Mechanical, ischemic, and inflammatory injuries to voltage-gated sodium channel (Nav)-rich membranes of axon initial segments and nodes of Ranvier render Nav channels dangerously leaky. By what means? The behavior of recombinant Nav1.6 (Wang et al., 2009) leads us to postulate that, in neuropathologic conditions, structural degradation of axolemmal bilayer fosters chronically left-shifted Nav channel operation, resulting in E(Na) rundown. This "sick excitable cell Nav-leak" would encompass left-shifted fast- and slow-mode based persistent I(Na) (i.e., I(window) and slow-inactivating I(Na)). Bilayer-damage-induced electrophysiological dysfunctions of native-Nav channels, and effects on inhibitors on those channels, should, we suggest, be studied in myelinated axons, exploiting I(Na)(V,t) hysteresis data from sawtooth ramp clamp. We hypothesize that (like dihydropyridines for Ca channels), protective lipophilic Nav antagonists would partition more avidly into disorderly bilayers than into the well-packed bilayers characteristic of undamaged, healthy plasma membrane. Whereas inhibitors using aqueous routes would access all Navs equally, differential partitioning into "sick bilayer" would co-localize lipophilic antagonists with "sick-Nav channels," allowing for more specific targeting of impaired cells. Molecular fine-tuning of Nav antagonists to favor more avid partitioning into damaged than into intact bilayers could reduce side effects. In potentially salvageable neurons of traumatic and/or ischemic penumbras, in inflammatory neuropathies, in muscular dystrophy, in myocytes of cardiac infarct borders, Nav-leak driven excitotoxicity overwhelms cellular repair mechanisms. Precision-tuning of a lipophilic Nav antagonist for greatest efficacy in mildly damaged membranes could render it suitable for the prolonged continuous administration needed to allow for the remodeling of the excitable membranes, and thus functional recovery.

Short-duration treatment with the calpain inhibitor MDL-28170 does not protect axonal transport in an in vivo model of traumatic axonal injury

Traumatic axonal injury is characterized by early cytoskeletal proteolysis and disruption of axonal transport. Calpain inhibition has been shown to protect axons in rodent models of traumatic brain injury. However, in these models, both white and gray matter are injured, making it difficult to determine if calpain inhibitors are directly protecting injured axons. To address this issue, we used our rat optic nerve stretch model to test the hypothesis that early calpain inhibition directly protects central nervous system (CNS) axons following stretch injury. Rats were given an intravenous bolus of the calpain inhibitor MDL-28170 (30 mg/kg) 30 min prior to unilateral optic nerve stretch, followed by a 15 mg/kg/h intravenous infusion over the next 2.5 h. Immunohistochemical analysis of optic nerves 30 min after stretch injury revealed variable increases of calpain-cleaved α-spectrin that appeared less evident in stretched nerves from drug-treated rats, although this difference was not statistically significant. Retrograde axonal transport measured by Fluorogold® labeling of retinal ganglion cells was significantly impaired after stretch injury. However, there was no difference in the number of Fluorogold-labeled cells in the vehicle vs. drug treatment groups. These results suggest that early short-duration calpain inhibitor therapy with MDL-28170 is not an effective strategy to prevent disruption of axonal transport following isolated axonal stretch injury in the CNS.

Partial interruption of axonal transport due to microtubule breakage accounts for the formation of periodic varicosities after traumatic axonal injury

Due to their viscoelastic nature, white matter axons are susceptible to damage by high strain rates produced during traumatic brain injury (TBI). Indeed, diffuse axonal injury (DAI) is one of the most common features of TBI, characterized by the hallmark pathological profiles of axonal bulbs at disconnected terminal ends of axons and periodic swellings along axons, known as "varicosities." Although transport interruption underlies axonal bulb formation, it is unclear how varicosities arise, with multiple sites accumulating transported materials along one axon. Recently, axonal microtubules have been found to physically break during dynamic stretch injury of cortical axons in vitro. Here, the same in vitro model was used in parallel with histopathological analyses of human brains acquired acutely following TBI to examine the potential role of mechanical microtubule damage in varicosity formation post-trauma. Transmission electron microscopy (TEM) following in vitro stretch injury revealed periodic breaks of individual microtubules along axons that regionally corresponded with undulations in axon morphology. However, typically less than a third of microtubules were broken in any region of an axon. Within hours, these sites of microtubule breaks evolved into periodic swellings. This suggests axonal transport may be halted along one broken microtubule, yet can proceed through the same region via other intact microtubules. Similar axonal undulations and varicosities were observed following TBI in humans, suggesting primary microtubule failure may also be a feature of DAI. These data indicate a novel mechanism of mechanical microtubule damage leading to partial transport interruption and varicosity formation in traumatic axonal injury.

Differential protection of neuromuscular sensory and motor axons and their endings in Wld(S) mutant mice

Orthograde Wallerian degeneration normally brings about fragmentation of peripheral nerve axons and their sensory or motor endings within 24-48 h in mice. However, neuronal expression of the chimaeric, Wld(S) gene mutation extends survival of functioning axons and their distal endings for up to 3 weeks after nerve section. Here we studied the pattern and rate of degeneration of sensory axons and their annulospiral endings in deep lumbrical muscles of Wld(S) mice, and compared these with motor axons and their terminals, using neurone-specific transgenic expression of the fluorescent proteins yellow fluorescent protein (YFP) or cyan fluorescent protein (CFP) as morphological reporters. Surprisingly, sensory endings were preserved for up to 20 days, at least twice as long as the most resilient motor nerve terminals. Protection of sensory endings and axons was also much less sensitive to Wld(S) gene-copy number or age than motor axons and their endings. Protection of γ-motor axons and their terminals innervating the juxtaequatorial and polar regions of the spindles was less than sensory axons but greater than α-motor axons. The differences between sensory and motor axon protection persisted in electrically silent, organotypic nerve-explant cultures suggesting that residual axonal activity does not contribute to the sensory-motor axon differences in vivo. Quantitative, Wld(S)-specific immunostaining of dorsal root ganglion (DRG) neurones and motor neurones in homozygous Wld(S) mice suggested that the nuclei of large DRG neurones contain about 2.4 times as much Wld(S) protein as motor neurones. By contrast, nuclear fluorescence of DRG neurones in homozygotes was only 1.5 times brighter than in heterozygotes stained under identical conditions. Thus, differences in axonal or synaptic protection within the same Wld(S) mouse may most simply be explained by differences in expression level of Wld(S) protein between neurones. Mimicry of Wld(S)-induced protection may also have applications in treatment of neurotoxicity or peripheral neuropathies in which the integrity of sensory endings may be especially implicated.

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

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

Engineering a high throughput axon injury system

Several key biological mechanisms of traumatic injury to axons have been elucidated using in vitro stretch injury models. These models, however, are based on the experimentation of single cultures keeping productivity slow. Indeed, low yield has hindered important and well-founded investigations requiring high throughput methods such as proteomic analyses. To meet this need, we engineered a multi-well high throughput injury device to accelerate and accommodate the next generation of traumatic brain injury research. This modular system stretch injures neuronal cultures in either a 24-well culture plate format or 6 individual wells simultaneously. Custom software control allows the user to accurately program the pressure pulse parameters to achieve the desired substrate deformation and injury parameters. Analysis of the pressure waveforms showed that peak pressure was linearly related to input pressure and valve open times and that the 6- and 24-well modules displayed rise times, peak pressures, and decays with extremely small standard deviations. Data also confirmed that the pressure pulse was distributed evenly throughout the pressure chambers and therefore to each injury well. Importantly, the relationship between substrate deformation and applied pressure was consistent among the multiple wells and displayed a predictable linear behavior in each module. These data confirm that this multi-well system performs as well as currently used stretch injury devices and can undertake high throughput studies that are needed across the field of neurotrauma research.

Mild traumatic brain injury and diffuse axonal injury in swine

Until recently, mild traumatic brain injury (mTBI) or "concussion" was generally ignored as a major health issue. However, emerging evidence suggests that this injury is by no means mild, considering it induces persisting neurocognitive dysfunction in many individuals. Although little is known about the pathophysiological aspects of mTBI, there is growing opinion that diffuse axonal injury (DAI) may play a key role. To explore this possibility, we adapted a model of head rotational acceleration in swine to produce mTBI by scaling the mechanical loading conditions based on available biomechanical data on concussion thresholds in humans. Using these input parameters, head rotational acceleration was induced in either the axial plane (transverse to the brainstem; n=3), causing a 10- to 35-min loss of consciousness, or coronal plane (circumferential to the brainstem; n=2), which did not produce a sustained loss of consciousness. Seven days following injury, immunohistochemical analyses of the brains revealed that both planes of head rotation induced extensive axonal pathology throughout the white matter, characterized as swollen axonal bulbs or varicosities that were immunoreactive for accumulating neurofilament protein. However, the distribution of the axonal pathology was different between planes of head rotation. In particular, more swollen axonal profiles were observed in the brainstems of animals injured in the axial plane, suggesting an anatomic substrate for prolonged loss of consciousness in mTBI. Overall, these data support DAI as an important pathological feature of mTBI, and demonstrate that surprisingly overt axonal pathology may be present, even in cases without a sustained loss of consciousness.

Biomechanics of concussion

The rising awareness of the long-term health problems associated with concussions re-emphasizes the need for understanding the mechanical etiology of concussions. This article reviews past studies defining the common mechanisms for mild traumatic brain injury and summarizes efforts to convert the external input to the head (force, acceleration, and velocity) into estimates of motions and deformations of the brain that occur during mild traumatic brain injury. Studies of how these mechanical conditions contribute to the cellular mechanisms of damage in mild traumatic brain injury are reviewed. Finally, future directions for improving understanding concussion biomechanics are discussed.

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.

Anti-GD1a antibodies activate complement and calpain to injure distal motor nodes of Ranvier in mice

The motor axonal variant of Guillain-Barré syndrome is associated with anti-GD1a immunoglobulin antibodies, which are believed to be the pathogenic factor. In previous studies we have demonstrated the motor terminal to be a vulnerable site. Here we show both in vivo and ex vivo, that nodes of Ranvier in intramuscular motor nerve bundles are also targeted by anti-GD1a antibody in a gradient-dependent manner, with greatest vulnerability at distal nodes. Complement deposition is associated with prominent nodal injury as monitored with electrophysiological recordings and fluorescence microscopy. Complete loss of nodal protein staining, including voltage-gated sodium channels and ankyrin G, occurs and is completely protected by both complement and calpain inhibition, although the latter provides no protection against electrophysiological dysfunction. In ex vivo motor and sensory nerve trunk preparations, antibody deposits are only observed in experimentally desheathed nerves, which are thereby rendered susceptible to complement-dependent morphological disruption, nodal protein loss and reduced electrical activity of the axon. These studies provide a detailed mechanism by which loss of axonal conduction can occur in a distal dominant pattern as observed in a proportion of patients with motor axonal Guillain-Barré syndrome, and also provide an explanation for the occurrence of rapid recovery from complete paralysis and electrophysiological in-excitability. The study also identifies therapeutic approaches in which nodal architecture can be preserved.

Dendritic alterations after dynamic axonal stretch injury in vitro

Traumatic axonal injury (TAI) is the most common and important pathology of traumatic brain injury (TBI). However, little is known about potential indirect effects of TAI on dendrites. In this study, we used a well-established in vitro model of axonal stretch injury to investigate TAI-induced changes in dendrite morphology. Axons bridging two separated rat cortical neuron populations plated on a deformable substrate were used to create a zone of isolated stretch injury to axons. Following injury, we observed the formation of dendritic alterations or beading along the dendrite shaft. Dendritic beading formed within minutes after stretch then subsided over time. Pharmacological experiments revealed a sodium-dependent mechanism, while removing extracellular calcium exacerbated TAI's effect on dendrites. In addition, blocking ionotropic glutamate receptors with the N-methyl-d-aspartate (NMDA) receptor antagonist MK-801 prevented dendritic beading. These results demonstrate that axon mechanical injury directly affects dendrite morphology, highlighting an important bystander effect of TAI. The data also imply that TAI may alter dendrite structure and plasticity in vivo. An understanding of TAI's effect on dendrites is important since proper dendrite function is crucial for normal brain function and recovery after injury.

Neuroprotective effects of selective N-type VGCC blockade on stretch-injury-induced calcium dynamics in cortical neurons

Acute elevation in intracellular calcium ([Ca(2+)](i)) following traumatic brain injury (TBI) can trigger cellular mechanisms leading to neuronal dysfunction and death. The mechanisms underlying these processes are not completely understood, but calcium influx through N-type voltage-gated calcium channels (VGCCs) appears to play a central role. The present study examined the time course of [Ca(2+)](i) flux, glutamate release, and loss of cell viability following injury using an in vitro neuronal-glial cortical cell-culture model of TBI. The effects of N-channel blockade with SNX-185 (e.g. omega-conotoxin TVIA) before or after injury were also examined. Neuronal injury produced a transient elevation in [Ca(2+)](i), increased glutamate release, and resulted in neuronal and glial death. SNX-185 administered before or immediately after cell injury reduced glutamate release and increased the survival of neurons and astrocytes, whereas delayed treatment did not improve cell survival but significantly facilitated the return of [Ca(2+)](i) to baseline levels. The new findings that N-type VGCCs are critically involved in injury-induced glutamate release and recovery of [Ca(2+)](i) argue for continued investigation of this treatment strategy for the clinical management of TBI. In particular, SNX-185 may represent an effective class of drugs that can significantly protect injured neurons from the secondary insults that commonly occur after TBI.

Voltage-gated sodium channels as therapeutic targets in epilepsy and other neurological disorders

Voltage-gated sodium channels (VGSCs) are key mediators of intrinsic neuronal and muscle excitability. Abnormal VGSC activity is central to the pathophysiology of epileptic seizures, and many of the most widely used antiepileptic drugs, including phenytoin, carbamazepine, and lamotrigine, are inhibitors of VGSC function. These antiepileptic drugs might also be efficacious in the treatment of other nervous system disorders, such as migraine, multiple sclerosis, neurodegenerative diseases, and neuropathic pain. In this Review, we summarise the structure and function of VGSCs and their involvement in the pathophysiology of several neurological disorders. We also describe the biophysical and molecular bases for the mechanisms of action of antiepileptic VGSC blockers and discuss the efficacy of these drugs in the treatment of epileptic and non-epileptic disorders. Overall, clinical and experimental data indicate that these drugs are efficacious for a range of diseases, and that the development of drugs with enhanced selectivity for specific VGSC isoforms might be an effective and novel approach for the treatment of several neurological diseases.

Sodium channelopathy induced by mild axonal trauma worsens outcome after a repeat injury

There is great concern that one mild traumatic brain injury (mTBI) predisposes individuals to an exacerbated response with a subsequent mTBI. Although no mechanism has been identified, mounting evidence suggests traumatic axonal injury (TAI) plays a role in this process. By using a cell culture system, a threshold of mild TAI was found where dynamic stretch of cortical axons at strains lower than 5% induced no overt pathological changes. However, the axons were found to display an increased expression of sodium channels (NaChs) by 24 hr. After a second, identical mild injury, pathologic increases in [Ca(2+)](i) were observed, leading to axon degeneration. The central role of NaChs in this response was demonstrated by blocking NaChs with tetrodotoxin prior to the second injury, which completely abolished postinjury increases in [Ca(2+)](i). These data suggest that mild TAI induces a form of sodium channelopathy on axons that greatly exaggerates the pathophysiologic response to subsequent mild injuries.

Mechanical breaking of microtubules in axons during dynamic stretch injury underlies delayed elasticity, microtubule disassembly, and axon degeneration

Little is known about which components of the axonal cytoskeleton might break during rapid mechanical deformation, such as occurs in traumatic brain injury. Here, we micropatterned neuronal cell cultures on silicone membranes to induce dynamic stretch exclusively of axon fascicles. After stretch, undulating distortions formed along the axons that gradually relaxed back to a straight orientation, demonstrating a delayed elastic response. Subsequently, swellings developed, leading to degeneration of almost all axons by 24 h. Stabilizing the microtubules with taxol maintained the undulating geometry after injury but greatly reduced axon degeneration. Conversely, destabilizing microtubules with nocodazole prevented undulations but greatly increased the rate of axon loss. Ultrastructural analyses of axons postinjury revealed immediate breakage and buckling of microtubules in axon undulations and progressive loss of microtubules. Collectively, these data suggest that dynamic stretch of axons induces direct mechanical failure at specific points along microtubules. This microtubule disorganization impedes normal relaxation of the axons, resulting in undulations. However, this physical damage also triggers progressive disassembly of the microtubules around the breakage points. While the disintegration of microtubules allows delayed recovery of the "normal" straight axon morphology, it comes at a great cost by interrupting axonal transport, leading to axonal swelling and degeneration.

Initial calcium release from intracellular stores followed by calcium dysregulation is linked to secondary axotomy following transient axonal stretch injury

Acute axonal shear and stretch in the brain induces an evolving form of axonopathy and is a major cause of ongoing motor, cognitive and emotional dysfunction. We have utilized an in vitro model of mild axon bundle stretch injury, in cultured primary cortical neurons, to determine potential early critical cellular alterations leading to secondary axonal degeneration. We determined that transient axonal stretch injury induced an initial acute increase in intracellular calcium, principally derived from intracellular stores, which was followed by a delayed increase in calcium over 48 h post-injury (PI). This progressive and persistent increase in intracellular calcium was also associated with increased frequency of spontaneous calcium fluxes as well as cytoskeletal abnormalities. Additionally, at 48 h post-injury, stretch-injured axon bundles demonstrated filopodia-like sprout formation that preceded secondary axotomy and degeneration. Pharmacological inhibition of the calcium-activated phosphatase, calcineurin, resulted in reduced secondary axotomy (p < 0.05) and increased filopodial sprout length. In summary, these results demonstrate that stretch injury of axons induced an initial substantial release of calcium from intracellular stores with elevated intracellular calcium persisting over 2 days. These long-lasting calcium alterations may provide new insight into the earliest neuronal abnormalities that follow traumatic brain injury as well as the key cellular changes that lead to the development of diffuse axonal injury and secondary degeneration.

Roles of channels and receptors in the growth cone during PNS axonal regeneration

Neurons in the peripheral nervous system (PNS) are known to maintain a regenerative capacity and will normally regenerate their axons within a permissive growth environment. The success of regeneration in the PNS largely depends on maintenance of the supportive basal lamina membrane, efficient removal of axonal and myelin debris by macrophages and Schwann cells, expression of neurotrophic factors by Schwann cells, and up-regulation of the intrinsic growth program in PNS neurons. The PNS regenerative process is well characterized through initial Wallerian degeneration followed by axonal sprouting, formation of neuronal growth cones, active axonal growth to the target, and finally sensory and motor functional recovery. The initiation and maintenance of active growth cones during peripheral nerve regeneration recapitulate many aspects of early neural development and are achieved through the activation of complex signaling cascades, involving various receptors, channels, cytoplasmic signaling cascades, as well as transcriptional and translational programs. This review focuses on roles of cell surface ion channels and receptors in the growth cone during Wallerian degeneration and axon regeneration in the PNS.

In-vitro approaches for studying blast-induced traumatic brain injury

Traumatic brain injury caused by explosive or blast events is currently divided into four phases: primary, secondary, tertiary, and quaternary blast injury. These phases of blast-induced traumatic brain injury (bTBI) are biomechanically distinct, and can be modeled in both in-vivo and in-vitro systems. The purpose of this review is to consider the mechanical phases of bTBI, how these phases are reproduced with in-vitro models, and to review findings from these models to assess how each phase of bTBI can be examined in more detail. Highlighted are some important gaps in the literature that may be addressed in the future to better identify the exact contributing mechanisms for bTBI. These in-vitro models, viewed in combination with in-vivo models and clinical studies, can be used to assess both the mechanisms and possible treatments for this type of trauma.

Calpain mediates proteolysis of the voltage-gated sodium channel alpha-subunit

Alterations in the expression, molecular composition, and localization of voltage-gated sodium channels play major roles in a broad range of neurological disorders. Recent evidence identifies sodium channel proteolysis as a key early event after ischemia and traumatic brain injury, further expanding the role of the sodium channel in neurological diseases. In this study, we investigate the protease responsible for proteolytic cleavage of voltage-gated sodium channels (NaChs). NaCh proteolysis occurs after protease activation in rat brain homogenates, pharmacological disruption of ionic homeostasis in cortical cultures, and mechanical injury using an in vitro model of traumatic brain injury. Proteolysis requires Ca(2+) and calpain activation but is not influenced by caspase-3 or cathepsin inhibition. Proteolysis results in loss of the full-length alpha-subunits, and the creation of fragments comprising all domains of the channel that retain interaction even after proteolysis. Cell surface biotinylation after mechanical injury indicates that proteolyzed NaChs remain in the membrane before noticeable evidence of neuronal death, providing a mechanism for altered action potential initiation, propagation, and downstream signaling events after Ca(2+) elevation.

Disruption of the ubiquitin proteasome system following axonal stretch injury accelerates progression to secondary axotomy

The ubiquitin proteasome system (UPS) plays a vital role in the regulation of protein degradation. Ubiquitination of proteins has been implicated in the pathological cascade associated with neuronal degeneration in both neurodegenerative disease and following acquired central nervous system (CNS) injury. In the present study, we have investigated the role of the UPS following mild to moderate in vitro axonal stretch injury to mature primary cortical neurons, a model of the evolving axonal pathology characteristic of diffuse axonal injury following brain trauma. Transient axonal stretch injury in this model does not involve primary axotomy. However, delayed accumulation of ubiquitin in neuritic swellings at 48 h post-injury (PI) was present in axonal bundles, followed by approximately 60% of axonal bundles progressing to secondary axotomy at 72 h PI. This delayed accumulation of ubiquitin was temporally and spatially associated with cytoskeletal damage. Pharmacological inhibition of the UPS with both MG132 and lactacystin prior to axonal injury resulted in a significant (p < 0.05) increase in the number of axonal bundles progressing to secondary axotomy at 48 and 72 h PI. These results demonstrate that, following mild to moderate transient axonal stretch injury, UPS activity may assist structural reorganization within axons, potentially impeding secondary axotomy. Protein ubiquitination in the axon may therefore have a protective role relative to the diffuse axonal changes that follow traumatic brain injury.

Membrane trauma and Na+ leak from Nav1.6 channels

During brain trauma, white matter experiences shear and stretch forces that, without severing axons, nevertheless trigger their secondary degeneration. In central nervous system (CNS) trauma models, voltage-gated sodium channel (Nav) blockers are neuroprotective. This, plus the rapid tetrodotoxin-sensitive Ca2+ overload of stretch-traumatized axons, points to "leaky" Nav channels as a pivotal early lesion in brain trauma. Direct effects of mechanical trauma on neuronal Nav channels have not, however, been tested. Here, we monitor immediate responses of recombinant neuronal Nav channels to stretch, using patch-clamp and Na+-dye approaches. Trauma constituted either bleb-inducing aspiration of cell-attached oocyte patches or abrupt uniaxial stretch of cells on an extensible substrate. Nav1.6 channel transient current displayed irreversible hyperpolarizing shifts of steady-state inactivation [availability(V)] and of activation [g(V)] and, thus, of window current. Left shift increased progressively with trauma intensity. For moderately intense patch trauma, a approximately 20-mV hyperpolarizing shift was registered. Nav1.6 voltage sensors evidently see lower energy barriers posttrauma, probably because of the different bilayer mechanics of blebbed versus intact membrane. Na+ dye-loaded human embryonic kidney (HEK) cells stably transfected with alphaNav1.6 were subjected to traumatic brain injury-like stretch. Cytoplasmic Na+ levels abruptly increased and the trauma-induced influx had a significant tetrodotoxin-sensitive component. Nav1.6 channel responses to cell and membrane trauma are therefore consistent with the hypothesis that mechanically induced Nav channel leak is a primary lesion in traumatic brain injury. Nav1.6 is the CNS node of Ranvier Nav isoform. When, during head trauma, nodes experienced bleb-inducing membrane damage of varying intensities, nodal Nav1.6 channels should immediately "leak" over a broadly left-smeared window current range.

Mechanical membrane injury induces axonal beading through localized activation of calpain

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

Role of Ca(2+) in injury-induced changes in sodium current in rat skeletal muscle

Characteristics of voltage-dependent sodium current recorded from adult rat muscle fibers in loose patch mode were rapidly altered following nearby impalement with a microelectrode. Hyperpolarized shifts in the voltage dependence of activation and fast inactivation occurred within minutes. In addition, the amplitude of the maximal sodium current decreased within 30 min of impalement. Impalement triggered a sustained elevation of intracellular Ca(2+). However, buffering Ca(2+) by loading fibers with AM-BAPTA did not affect the hyperpolarized shifts in activation and inactivation, although it did prevent the reduction in current amplitude. Surprisingly, the rise in intracellular Ca(2+) occurred even in the absence of extracellular Ca(2+). This result indicated that the injury-induced Ca(2+) increase came from an intracellular source, but it was not blocked by an inhibitor of release from the sarcoplasmic reticulum, which suggested involvement of mitochondria. Ca(2+) release from mitochondria triggered by carbonyl cyanide 3-chlorophenylhydrazone was sufficient to cause a reduction in sodium current amplitude but had little effect of the voltage dependence of activation and fast inactivation. Our data suggest the effects of muscle injury can be separated into a Ca(2+)-dependent reduction in amplitude and a largely Ca(2+)-independent shift in activation and fast inactivation. Together, the impalement-induced changes in sodium current reduce the number of sodium channels available to open at the resting potential and may limit further depolarization and thus promote survival of muscle fibers following injury.

Retrograde injury signaling in lesioned axons

The cell body of a lesioned neuron must receive accurate and timely information on the site and extent of axonal damage, in order to mount an appropriate response. Specific mechanisms must therefore exist to transmit such information along the length of the axon from the lesion site to the cell body. Three distinct types of signals have been postulated to underlie this process, starting with injury-induced discharge of axon potentials, and continuing with two distinct types of retrogradely transported macromolecular signals. The latter includes, on the one hand, an interruption of the normal supply of retrogradely transported trophic factors from the target, and, on the other hand, activated proteins originating from the injury site. This chapter reviews the progress on understanding the different mechanistic aspects of the axonal response to injury, and how the information is conveyed from the injury site to the cell body to initiate regeneration.

Dorsal root compression produces myelinated axonal degeneration near the biomechanical thresholds for mechanical behavioral hypersensitivity

Increased sensitivity to mechanical stimuli produced by transient cervical nerve root compression is dependent on the severity of applied load. In addition, trauma in the nervous system induces local inflammation, Wallerian degeneration, and a host of other degenerative processes leading to axonal dysfunction. Here, axonal degeneration and inflammation were assessed following transient dorsal root compression to establish a relationship between conditions for dorsal root axonal changes and those previously established for the onset and maintenance of mechanical behavioral hypersensitivity (26.3 mN and 38.2 mN, respectively). Compression loads were applied over a range (0-110 mN) known to produce sustained behavioral hypersensitivity. CD68- and NF200-immunoreactivity, as well as axonal pathological changes, were assessed in the dorsal root to investigate the load thresholds requisite for inducing macrophage infiltration and axonal degeneration relative to those thresholds for producing the onset and persistence of behavioral hypersensitivity. Neurofilament accumulation and the depletion of NF200-immunoreactivity in the region of compressed tissue were produced for loads that produce mechanical behavioral hypersensitivity. A 50th-percentile load threshold was determined (31.6 mN) governing the onset of NF200 depletion. However, CD68-immunoreactivity was increased for nearly all loads, suggesting that macrophage recruitment may not be directly related to nerve root-mediated behavioral hypersensitivity. This study provides new evidence for threshold-mediated degenerative changes in the context of behavioral hypersensitivity following nerve root compression.

The multiple sclerosis degradome: enzymatic cascades in development and progression of central nervous system inflammatory disease

An array of studies implicate different classes of protease and their endogenous inhibitors in multiple sclerosis (MS) pathogenesis based on expression patterns in MS lesions, sera, and/or cerebrospinal fluid (CSF). Growing evidence exists regarding their mechanistic roles in inflammatory and neurodegenerative aspects of this disease. Proteolytic events participate in demyelination, axon injury, apoptosis, and development of the inflammatory response including immune cell activation and extravasation, cytokine and chemokine activation/inactivation, complement activation, and epitope spreading. The potential significance of proteolytic activity to MS therefore relates not only to their potential use as important biomarkers of disease activity, but additionally as prospective therapeutic targets. Experimental data indicate that understanding the net physiological consequence of altered protease levels in MS development and progression necessitates understanding protease activity in the context of substrates, endogenous inhibitors, and proteolytic cascade interactions, which together make up the MS degradome. This review will focus on evidence regarding the potential physiologic role of those protease families already identified as markers of disease activity in MS; that is, the metallo-, serine, and cysteine proteases.

Tensile stretching of cervical facet joint capsule and related axonal changes

This study examines axonal changes in goat cervical facet joint capsules (FJC) subjected to low rate loading. Left C5-C6 FJC was subjected to a series of tensile tests from 2 mm to failure using a computer-controlled actuator. The FJC strain on the dorsal aspect was monitored by a stereo-imaging system. Stretched (n = 10) and unstretched (n = 7) capsules were harvested and serial sections were processed by a silver impregnation method. The mean peak actuator displacement was 21.3 mm (range: 12-30 mm). The average peak strain encompassing various regions of the capsule was 72.9 +/- 7.1%. Complete failure of the capsule was observed in 70% of the stretched capsules. Silver impregnation of the sections revealed nerve fibers and bundles in all the regions of the capsule. A blinded analysis of digital photomicrographs of axons revealed a statistically significant number of swollen axons with non-uniform caliber in stretched FJCs. Axons with terminal retraction balls, with occasional beaded appearance or with vacuolations were also observed. Stretching the FJC beyond physiological range could result in altered axonal morphology that may be related to secondary or delayed axotomy changes similar to those seen in central nervous system injuries where axons are subjected to stretching and shearing. These may contribute to neuropathic pain and are potentially related to neck pain after whiplash events.

Linking impact to cellular and molecular sequelae of CNS injury: modeling in vivo complexity with in vitro simplicity

Traumatic brain injury (TBI) represents one of most common disorders to the central nervous system (CNS). Despite significant efforts, though, an effective clinical treatment for TBI is not yet available. The complexity of human TBI is modeled with a broad group of experimental models, with each model matching some aspect of the human condition. In the past 15 years, these in vivo models were complemented with a group of in vitro models, with these in vitro models allowing investigators to more precisely identify the mechanism(s) of TBI, the different intracellular events that occur in acute period following injury, and the possible treatment of this injury in vitro. In this paper, we review the available in vitro models to study TBI, discuss their biomechanical basis for human TBI, and review the findings from these in vitro models. Finally, we synthesize the current knowledge and point out possible future directions for this group of models, especially in the effort toward developing new therapies for the traumatically brain injured patient.

Sodium channel expression within chronic multiple sclerosis plaques

Multiple sclerosis (MS) is characterized by focal destruction of myelin sheaths, gliotic scars, and axonal damage that contributes to the accumulation of nonremitting clinical deficits. Previous studies have demonstrated coexpression of sodium channel Nav1.6 and the sodium-calcium exchanger (NCX), together with beta-amyloid precursor protein (beta-APP), a marker of axonal damage, in degenerating axons within acute MS lesions. Axonal degeneration is less frequent within chronic MS lesions than in acute plaques, although current evidence suggests that axonal loss in chronic lesions ("slow burn") is a major contributor to accumulating disability. It is not known, however, whether axonal degenerations in chronic and acute lesions share common mechanisms, despite radically differing extracellular milieus. In this study, the expression of sodium channels Nav1.2 and Nav1.6 and of NCX was examined in chronic MS plaques within the spinal cord. Nav1.2 immunostaining was not observed along demyelinated axons in chronic lesions but was expressed by scar and reactive astrocytes within the plaque. Nav1.6 immunoreactivity, which was intense at nodes of Ranvier in normal appearing white matter in the same sections, was present in approximately one-third of the demyelinated axons within these plaques in a patchy rather than continuous distribution. NCX was not detected in demyelinated axons within chronic lesions, although it was clearly present within the scar astrocytes surrounding the demyelinated axons. beta-APP accumulation occurred in a small percentage of axons within chronic lesions within the spinal cord, but beta-APP was not preferentially present in axons that expressed Nav1.6. These observations suggest that different mechanisms underlie axonal degeneration in acute and chronic MS lesions, with axonal injury occurring at sites of coexpression of Nav1.6 and NCX in acute lesions but independent of coexpression of these 2 molecules in chronic lesions.

Sodium channels and multiple sclerosis: roles in symptom production, damage and therapy

Our understanding of the potential role of sodium channels in multiple sclerosis (MS) has grown substantially in recent years. The channels have long had a recognized role in the symptomatology of the disease, but now also have suspected roles in causing permanent axonal destruction, and a potential role in modulating the intensity of immune activity. Sodium channels might also provide an avenue to achieve axonal and neuronal protection in MS, thereby impeding the otherwise relentless advance of permanent neurological deficit. The symptoms of MS are largely determined by the conduction properties of axons and these, in turn, are largely determined by sodium channels. The number, subtype and distribution of the sodium channels are all important, together with the way that channel function is modified by local factors, such as those resulting from inflammation (eg, nitric oxide). Suspicion is growing that sodium channels may also contribute to the axonal degeneration primarily responsible for permanent neurological deficits. The proposed mechanism involves intra-axonal sodium accumulation which promotes reverse action of the sodium/calcium exchanger and thereby a lethal rise in intra-axonal calcium. Partial blockade of sodium channels protects axons from degeneration in experimental models of MS, and therapy based on this approach is currently under investigation in clinical trials. Some recent findings suggest that such systemic inhibition of sodium channels may also promote axonal protection by suppressing inflammation within the brain.

Lipid stress at play: mechanosensitivity of voltage-gated channels

Membrane stretch modulates the activity of voltage-gated channels (VGCs). These channels are nearly ubiquitous among eukaryotes and they are present, too, in prokaryotes, so the potential ramifications of VGC mechanosensitivity are diverse. In situ traumatic stretch can irreversibly alter VGC activity with lethal results but that is pathology. This chapter discusses the reversible responses of VGCs to stretch, with the general relation of stretch stimuli to other forms of lipid stress, and briefly, with some irreversible stretch effects (=stretch trauma). A working assumption throughout is that mechanosensitive (MS) VGC motions-that is, motions that respond reversibly to bilayer stretch-are susceptible to other forms of lipid stress, such as the stresses produced when amphiphilic molecules (anesthetics, lipids, alcohols, and lipophilic drugs) are inserted into the bilayer. Insofar as these molecules change the bilayer's lateral pressure profile, they can be termed bilayer mechanical reagents (BMRs). The chapter also discusses the MS VGC behavior against the backdrop of eukaryotic channels more widely accepted as "MS channels"--namely, the transient receptor potential (TRP)-based MS cation channels.

Preferential neuroprotective effect of tacrolimus (FK506) on unmyelinated axons following traumatic brain injury

Prior investigations of traumatic axonal injury (TAI), and pharmacological treatments of TAI pathology, have focused exclusively on the role of myelinated axons, with no systematic observations directed towards unmyelinated axon pathophysiology. Recent electrophysiological evidence, however, indicates that unmyelinated axons are more vulnerable than myelinated axons in a rodent model of experimental TAI. Given their susceptibility to TAI, the present study examines whether unmyelinated axons also respond differentially to FK506, an immunophilin ligand with well-established neuroprotective efficacy in the myelinated fiber population. Adult rats received 3.0 mg/kg FK506 intravenously at 30 min prior to midline fluid percussion injury. In brain slice electrophysiological recordings, conducted at 24 h postinjury, compound action potentials (CAPs) were evoked in the corpus callosum, and injury effects quantified separately for CAP waveform components generated by myelinated axons (N1 wave) and unmyelinated axons (N2 wave). The amplitudes of both CAP components were suppressed postinjury, although this deficit was 16% greater for the N2 CAP. While FK506 treatment provided significant neuroprotection for both N1 and N2 CAPs, the drug benefit for the N2 CAP amplitude was 122% greater than that for the N1 CAPs, and improved postinjury strength-duration and refractoriness properties only in N2 CAPs. Immunocytochemical observations, of TAI reflected in intra-axonal pooling of amyloid precursor protein, indicated that FK506 reduced the extent of postinjury impairments to axonal transport and subsequent axonal damage. Collectively, these studies further substantiate a distinctive role of unmyelinated axons in TAI, and suggest a highly efficacious neuroprotective strategy to target this axonal population.

Axonal conduction and injury in multiple sclerosis: the role of sodium channels

Multiple sclerosis (MS) is the most common cause of neurological disability in young adults. Recent studies have implicated specific sodium channel isoforms as having an important role in several aspects of the pathophysiology of MS, including the restoration of impulse conduction after demyelination, axonal degeneration and the mistuning of Purkinje neurons that leads to cerebellar dysfunction. By manipulating the activity of these channels or their expression, it might be possible to develop new therapeutic approaches that will prevent or limit disability in MS.

Cellular and subcellular change evoked by diffuse traumatic brain injury: a complex web of change extending far beyond focal damage

Until recently, our understanding of the cellular and subcellular changes evoked by diffuse traumatic brain injury has been framed in the context of primary focal injury. In this regard, the ensuing cell death cascades were linked to contusional-mediated changes associated with frank hemorrhage and ischemia, and these were assumed to contribute to the observed apoptotic and necrotic neuronal death. Little consideration was given to the potential that other non-contusional cell death cascades could have been triggered by the diffuse mechanical forces of injury. While the importance of these classical, contusion-related apoptotic and necrotic cell death cascades cannot be discounted with diffuse injury, more recent information suggests that the mechanical force of injury itself can diffusely porate the neuronal plasmalemma and its axolemmal membranes, evoking other forms of cellular response that can contribute to cell injury or death. In this regard, the duration of the membrane alteration appears to be a dependent factor, with enduring membrane change, potentially leading to irreversible damage, whereas more transient membrane perturbation can be followed by cell membrane resealing associated with recovery and/or adaptive change. With more enduring mechanical membrane perturbation, it appears that some of the traditional death cascades involving the activation of cysteine proteases are at work. Equally important, non-traditional pathways involving the lysosomal dependent release of hydrolytic enzymes may also be players in the ensuing neuronal death. These mechanically related factors that directly impact upon the neuronal somata may also be influenced by concomitant and/or secondary axotomy-mediated responses. This axonal injury, although once thought to involve a singular intraaxonal response to injury, is now known to be more complex, reflecting differential responses to injuries of varying severity. Moreover, it now appears that fiber size and type may also influence the axon's reaction to injury. In sum, this review explicates the complexity of the cellular and subcellular responses evoked by diffuse traumatic brain injury in both the neuronal somata and its axonal appendages. This review further illustrates that our once simplistic views framed by evidence based upon contusional and/or ischemic change do not fully explain the complex repertoire of change evoked by diffuse traumatic brain injury.

Preemptive analgesia with lidocaine prevents Failed Back Surgery Syndrome

Failed Back Surgery Syndrome (FBSS) is commonly encountered in pain-treatment settings in the United States. We tested whether potential key factors in this syndrome, such as extracellular concentrations of excitatory amino acids (EAAs), are increased in the dorsal horn by synaptic release due to unintentional stretch and/or deformation/compression/transection of dorsal spinal structures during surgery. We hypothesized that pharmacological nerve block as a form of preemptive analgesia prior to any insult to dorsal root neurons will prevent an abnormally high increase in extracellular concentrations of EAAs in the dorsal horn and ultimately the establishment of central sensitization during back surgery. The L4 and L5 dorsal roots were cut bilaterally near the spinal cord to provide an adequate model to test for preemptive analgesia. Amino acid concentrations were measured by dorsal horn microdialysis sampling; EAAs aspartate and glutamate were significantly increased by 80% and 65% respectively, as were other amino acids compared to sham control values. Topical application of 1% Lidocaine, a voltage-gated Na(+) channel blocker, for 10 min prior to L4 and L5 bilateral dorsal rhizotomy (BDR) significantly attenuated the increase in EAA concentrations such that their values were not different from sham controls. Behavioral tests demonstrated significant hindlimb mechanical allodynia after BDRs that was significantly attenuated by Lidocaine pretreatment. Thus, Lidocaine pretreatment could offer a safe measure for prevention of chronic pain for back surgical procedures if given by intramuscular injection, topical administration onto spinal nerves and/or the dorsal spinal surface during surgical procedures that include nerve entrapment release, intervertebral disc modification and laminectomies.

Mitochondrial trafficking and morphology in healthy and injured neurons

Mitochondria are the primary generators of ATP and are important regulators of intracellular calcium homeostasis. These organelles are dynamically transported along lengthy neuronal processes, presumably for appropriate distribution to cellular regions of high metabolic demand and elevated intracellular calcium, such as synapses. The removal of damaged mitochondria that produce harmful reactive oxygen species and promote apoptosis is also thought to be mediated by transport of mitochondria to autophagosomes. Mitochondrial trafficking is therefore important for maintaining neuronal and mitochondrial health while cessation of movement may lead to neuronal and mitochondrial dysfunction. Mitochondrial morphology is also dynamic and is remodeled during neuronal injury and disease. Recent studies reveal different manifestations and mechanisms of impaired mitochondrial movement and altered morphology in injured neurons. These are likely to cause varied courses toward neuronal degeneration and death. The goal of this review is to provide an appreciation of the full range of mitochondrial function, morphology and trafficking, and the critical role these parameters play in neuronal physiology and pathophysiology.

Degenerative and spontaneous regenerative processes after spinal cord injury

Spinal cord injury results in acute as well as progressive secondary destruction of local and distant nervous tissue through a number of degenerative mechanisms. Spinal cord injury also initiates a number of endogenous neuroprotective and regenerative responses. Understanding of these mechanisms might identify potential targets for treatments after spinal cord injury in humans. Here, we first discuss recent developments in our understanding of the immediate traumatic and subsequent secondary degeneration of local tissue and long projecting pathways in animal models. These include the inflammatory and vascular responses during the acute phase, as well as cell death, demyelination and scar formation in the subacute and chronic phases. Secondly, we discuss the spontaneous axonal regeneration of injured and plasticity of uninjured systems, and other repair-related responses in animals, including the upregulation of regeneration-associated genes in some neurons, increases in neurotrophic factors in the spinal cord and remyelination by oligodendrocyte precursors and invading Schwann cells. Lastly, we comment on the still limited understanding of the neuropathology in humans, which is largely similar to that in rodents. However, there also are potentially important differences, including the reduced glial scarring, inflammation and demyelination, the increased Schwannosis and the protracted Wallerian degeneration in humans. The validity of current rodent models for human spinal cord injury is also discussed. The emphasis of this review is on the literature from 2002 to early 2005.

Spatiotemporal distribution of spectrin breakdown products induced by anoxia in adult rat optic nerve in vitro

Hypoxic/ischemic and traumatic injury to central nervous system myelinated axons is heavily dependent on accumulation of Ca ions in the axoplasm, itself promoted by Na influx from the extracellular space. Given the high density of nodal Na channels, we hypothesized that nodes of Ranvier might be particularly vulnerable to Ca overload and subsequent damage, as this is the expected locus of maximal Na influx. Adult rat optic nerves were exposed to in vitro anoxia and analyzed immunohistochemically for the presence of spectrin breakdown. Cleavage of spectrin became detectable between 15 and 30 mins of anoxia, and increased homogeneously along the lengths of fibers; localized breakdown was not observed at nodes of Ranvier at any time point analyzed. Spectrin breakdown was also found in glial processes surrounding axons. Confocal imaging of axoplasmic Ca also revealed a gradual and nonlocalized increase as anoxia progressed, without evidence of Ca 'hot-spots' anywhere along the axons at any time between 0 and 30 mins of anoxic exposure in vitro. Calculations of Ca diffusion rates indicated that even if Ca entered or was released focally in axons, this ion would diffuse rapidly into the internodes and likely produce diffuse injury by activating Ca-dependent proteases. Western blot analysis for voltage-gated Na channel protein revealed that key functional proteins such as these are also degraded by anoxia/ischemia. Thus, proteolysis of structural and functional proteins will conspire to irreversibly injure central axons and render them nonfunctional, eventually leading to transection, degradation, and Wallerian degeneration.

Roles of Voltage-Dependent Sodium Channels in Neuronal Development, Pain, and Neurodegeneration

Besides initiating and propagating action potentials in established neuronal circuits, voltage-dependent sodium channels sculpt and bolster the functional neuronal network from early in embryonic development through adulthood (e.g., differentiation of oligodendrocyte precursor cells into oligodendrocytes, myelinating axon; competition between neighboring equipotential neurites for development into a single axon; enhancing and opposing functional interactions with attractive and repulsive molecules for axon pathfinding; extending and retracting terminal arborization of axon for correct synapse formation; experience-driven cognition; neuronal survival; and remyelination of demyelinated axons). Surprisingly, different patterns of action potentials direct homeostasis-based epigenetic selection for neurotransmitter phenotype, thus excitability by sodium channels specifying expression of inhibitory neurotransmitters. Mechanisms for these pleiotropic effects of sodium channels include reciprocal interactions between neurons and glia via neurotransmitters, growth factors, and cytokines at synapses and axons. Sodium channelopathies causing pain (e.g., allodynia) and neurodegeneration (e.g., multiple sclerosis) derive from 1) electrophysiological disturbances by insults (e.g., ischemia/hypoxia, toxins, and antibodies); 2) loss-of-physiological function or gain-of-pathological function of mutant sodium channel proteins; 3) spatiotemporal inappropriate expression of normal sodium channel proteins; or 4) de-repressed expression of otherwise silent sodium channel genes. Na(v)1.7 proved to account for pain in human erythermalgia and inflammation, being the convincing molecular target of pain treatment.

Axon degeneration mechanisms: commonality amid diversity

A wide range of insults can trigger axon degeneration, and axons respond with diverse morphology, topology and speed. However, recent genetic, immunochemical, morphological and pharmacological investigations point to convergent degeneration mechanisms. The principal convergence points - poor axonal transport, mitochondrial dysfunction and an increase in intra-axonal calcium - have been identified by rescuing axons with the slow Wallerian degeneration gene (Wld(S)) and studies with blockers of sodium or calcium influx. By understanding how the pathways fit together, we can combine our knowledge of mechanisms, and potentially also treatment strategies, from different axonal disorders.

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.