Genetic influences on cellular reactions to spinal cord injury: a wound-healing response present in normal mice is impaired in mice carrying a mutation (WldS) that causes delayed Wallerian degeneration

Rabphilin3A reduces integrin-dependent growth cone signaling to restrict axon regeneration after trauma

Neural repair after traumatic spinal cord injury depends upon the restoration of neural networks via axonal sprouting and regeneration. Our previous genome wide loss-of-function screen identified Rab GTPases as playing a prominent role in preventing successful axon sprouting and regeneration. Here, we searched for Rab27b interactors and identified Rabphilin3A as an effector within regenerating axons. Growth cone Rabphilin3a colocalized and physically associated with integrins at puncta in the proximal body of the axonal growth cone. In regenerating axons, loss of Rabphilin3a increased integrin enrichment in the growth cone periphery, enhanced focal adhesion kinase activation, increased F-actin-rich filopodial density and stimulated axon extension. Compared to wild type, mice lacking Rabphilin3a exhibited greater regeneration of retinal ganglion cell axons after optic nerve crush as well as greater corticospinal axon regeneration after complete thoracic spinal cord crush injury. After moderate spinal cord contusion injury, there was greater corticospinal regrowth in the absence of Rph3a. Thus, an endogenous Rab27b - Raphilin3a pathway limits integrin action in the growth cone, and deletion of this monomeric GTPase pathway permits reparative axon growth in the injured adult mammalian central nervous system.

Co-targeting B-RAF and PTEN Enables Sensory Axons to Regenerate Across and Beyond the Spinal Cord Injury

Primary sensory axons in adult mammals fail to regenerate after spinal cord injury (SCI), in part due to insufficient intrinsic growth potential. Robustly boosting their growth potential continues to be a challenge. Previously, we showed that constitutive activation of B-RAF (rapidly accelerated fibrosarcoma kinase) markedly promotes axon regeneration after dorsal root and optic nerve injuries. The regrowth is further augmented by supplemental deletion of PTEN (phosphatase and tensin homolog). Here, we examined whether concurrent B-RAF activation and PTEN deletion promotes dorsal column axon regeneration after SCI. Remarkably, genetically targeting B-RAF and PTEN selectively in DRG neurons of adult mice enables many DC axons to enter, cross, and grow beyond the lesion site after SCI; some axons reach ∼2 mm rostral to the lesion by 3 weeks post-injury. Co-targeting B-RAF and PTEN promotes more robust DC regeneration than a pre-conditioning lesion, which additively enhances the regeneration triggered by B-RAF/PTEN. We also found that post-injury targeting of B-RAF and PTEN enhances DC axon regeneration. These results demonstrate that co-targeting B-RAF and PTEN effectively enhances the intrinsic growth potential of DC axons after SCI and therefore may help to develop a novel strategy to promote robust long-distance regeneration of primary sensory axons.

Central nervous system regeneration

Neurons of the mammalian central nervous system fail to regenerate. Substantial progress has been made toward identifying the cellular and molecular mechanisms that underlie regenerative failure and how altering those pathways can promote cell survival and/or axon regeneration. Here, we summarize those findings while comparing the regenerative process in the central versus the peripheral nervous system. We also highlight studies that advance our understanding of the mechanisms underlying neural degeneration in response to injury, as many of these mechanisms represent primary targets for restoring functional neural circuits.

Cell transplantation and secretome based approaches in spinal cord injury regenerative medicine

The axonal growth-restrictive character of traumatic spinal cord injury (SCI) makes finding a therapeutic strategy a very demanding task, due to the postinjury events impeditive to spontaneous axonal outgrowth and regeneration. Considering SCI pathophysiology complexity, it has been suggested that an effective therapy should tackle all the SCI-related aspects and provide sensory and motor improvement to SCI patients. Thus, the current aim of any therapeutic approach for SCI relies in providing neuroprotection and support neuroregeneration. Acknowledging the current SCI treatment paradigm, cell transplantation is one of the most explored approaches for SCI with mesenchymal stem cells (MSCs) being in the forefront of many of these. Studies showing the beneficial effects of MSC transplantation after SCI have been proposing a paracrine action of these cells on the injured tissues, through the secretion of protective and trophic factors, rather than attributing it to the action of cells itself. This manuscript provides detailed information on the most recent data regarding the neuroregenerative effect of the secretome of MSCs as a cell-free based therapy for SCI. The main challenge of any strategy proposed for SCI treatment relies in obtaining robust preclinical evidence from in vitro and in vivo models, before moving to the clinics, so we have specifically focused on the available vertebrate and mammal models of SCI currently used in research and how can SCI field benefit from them.

Developmental and Neurotoxicity of Acrylamide to Zebrafish

Acrylamide is a commonly used industrial chemical that is known to be neurotoxic to mammals. However, its developmental toxicity is rarely assessed in mammalian models because of the cost and complexity involved. We used zebrafish to assess the neurotoxicity, developmental and behavioral toxicity of acrylamide. At 6 h post fertilization, zebrafish embryos were exposed to four concentrations of acrylamide (10, 30, 100, or 300 mg/L) in a medium for 114 h. Acrylamide caused developmental toxicity characterized by yolk retention, scoliosis, swim bladder deficiency, and curvature of the body. Acrylamide also impaired locomotor activity, which was measured as swimming speed and distance traveled. In addition, treatment with 100 mg/L acrylamide shortened the width of the brain and spinal cord, indicating neuronal toxicity. In summary, acrylamide induces developmental toxicity and neurotoxicity in zebrafish. This can be used to study acrylamide neurotoxicity in a rapid and cost-efficient manner.

Benefits of Enhancing Nicotinamide Adenine Dinucleotide Levels in Damaged or Diseased Nerve Cells

Three unbiased lines of research have commonly pointed to the benefits of enhanced levels of nicotinamide adenine dinucleotide (NAD⁺) to diseased or damaged neurons. Mice carrying a triplication of the gene encoding the culminating enzyme in NAD⁺ salvage from nicotinamide, NMNAT, are protected from a variety of insults to axons. Protection from Wallerian degeneration of axons is also observed in flies and mice bearing inactivating mutations in the SARM1 gene. Functional studies of the SARM1 gene product have revealed the presence of an enzymatic activity directed toward the hydrolysis of NAD⁺ Finally, an unbiased drug screen performed in living mice led to the discovery of a neuroprotective chemical designated P7C3. Biochemical studies of the P7C3 chemical show that it can enhance recovery of NAD⁺ from nicotinamide by activating NAMPT, the first enzyme in the salvage pathway. In combination, these three unrelated research endeavors offer evidence of the benefits of enhanced NAD⁺ levels to damaged neurons.

Baicalin effects on rats with spinal cord injury by anti-inflammatory and regulating the serum metabolic disorder

Baicalin had neuroprotective effects on inhibiting neuronal cell apoptosis induced by spinal cord ischemic injury. This study aimed to explore the protective effects of Baicalin on rats with spinal cord injury (SCI) and its mechanism of action. The recovery of spinal cord nerve function in rats was evaluated by the Basso, Beattie, and Bresnahan (BBB) score and the combine behavioral score (CBS). The expressions of cytokines tumor necrosis factor α (TNF-α), interleukin-1β (IL-1β), and IL-6 were detected by the enzyme-linked immunosorbent assay method. Expressions of inflammation-related proteins were detected by Western blot. Multivariate statistical analysis was performed for serum metabolites. The BBB and CBS score results showed that Baicalin had a certain improvement on rats with SCI. SCI symptoms were significantly improved in low-dose and high-dose groups. The levels of TNF-α, IL-1β, and IL-6 in the SCI group were significantly increased. The expressions of NF-κB p65, NF-κB p50, p-IκBα, and IKKα in the SCI group showed the opposite trend compared with the low-dose and high-dose groups. Compared with the sham group, glutamine, levels of 3-OH-butyrate, N-acetylaspartate, and glutathione were significantly reduced, and the levels of glutamate and betaine were significantly increased in the SCI group. When Baicalin was administered, the contents of glutamine synthase (GS) and glutaminase (GLS) were significantly reduced, indicating that Baicalin had the effect of improving GS and GLS. Baicalin has protective effects on improving SCI and lower extremity motor function, has a significant anti-inflammatory effect, and regulates the serum metabolic disorder caused by SCI in rats.

Transient activation of Wnt/β-catenin signaling reporter in fibrotic scar formation after compression spinal cord injury in adult mice

After traumatic spinal cord injury (SCI), a scar may form with a fibrotic core (fibrotic scar) and surrounding reactive astrocytes (glial scar) at the lesion site. The scar tissue is considered a major obstacle preventing regeneration both as a physical barrier and as a source for secretion of inhibitors of axonal regeneration. Understanding the mechanism of scar formation and how to control it may lead to effective SCI therapies. Using a compression-SCI model on adult transgenic mice, we demonstrate that the canonical Wnt/β-catenin signaling reporter TOPgal (TCF/Lef1-lacZ) positive cells appeared at the lesion site by 5 days, peaked on 7 days, and diminished by 14 days post injury. Using various representative cell lineage markers, we demonstrate that, these transiently TOPgal positive cells are a group of Fibronectin(+);GFAP(-) fibroblast-like cells in the core scar region. Some of them are proliferative. These results indicate that Wnt/β-catenin signaling may play a key role in fibrotic scar formation after traumatic spinal cord injury.

Acute Axonal Degeneration Drives Development of Cognitive, Motor, and Visual Deficits after Blast-Mediated Traumatic Brain Injury in Mice

Axonal degeneration is a prominent feature of many forms of neurodegeneration, and also an early event in blast-mediated traumatic brain injury (TBI), the signature injury of soldiers in Iraq and Afghanistan. It is not known, however, whether this axonal degeneration is what drives development of subsequent neurologic deficits after the injury. The Wallerian degeneration slow strain (WldS) of mice is resistant to some forms of axonal degeneration because of a triplicated fusion gene encoding the first 70 amino acids of Ufd2a, a ubiquitin-chain assembly factor, that is linked to the complete coding sequence of nicotinamide mononucleotide adenylyltransferase 1 (NMAT1). Here, we demonstrate that resistance of WldS mice to axonal degeneration after blast-mediated TBI is associated with preserved function in hippocampal-dependent spatial memory, cerebellar-dependent motor balance, and retinal and optic nerve–dependent visual function. Thus, early axonal degeneration is likely a critical driver of subsequent neurobehavioral complications of blast-mediated TBI. Future therapeutic strategies targeted specifically at mitigating axonal degeneration may provide a uniquely beneficial approach to treating patients suffering from the effects of blast-mediated TBI.

Review : Apoptosis and Spinal Cord Injury

Apoptosis is the morphological counterpart of active, genetically programmed cell death and is important in development, immune function, and carcinogenesis. Recent data suggest that apoptosis may be important in neurodegenerative disorders, ischemic brain injury, and neurotrauma as well. Here we review very recent data from our laboratory and others that show that at least some of the pronounced secondary injury that follows spinal cord injury (SCI) may be caused by apoptosis and associated intracellular death pathways. Both neurons and glia seem to die by apoptosis; the response of oligodendrocytes in long tracts undergoing Wallerian degeneration is particularly long lived and may be responsible for chronic demyelination and some of the dysfunction in chronic SCI. These findings suggest that the therapeutic window for treatment of acute SCI may extend into the chronic phase. In addition, proliferation of ependymal cells occurs in concert with cell death, suggesting that both degeneration and repair may occur at the same time. Therapies aimed at altering the balance between these cellular events may be useful for future treatments of SCI. NEURO SCIENTIST 4:163-171, 1998

Rodent spinal cord injury models for studies of axon regeneration

For over a century, axon regeneration has been considered the Holy Grail for spinal cord injury (SCI) repair. Although there are other factors that could contribute to improving function, restoring the long motor and sensory tracts that are interrupted by SCI has the greatest potential for actually reversing paralysis, restoring the brain's control of autonomic functions mediated by sympathetic and parasympathetic circuits of the spinal cord and restoring sensation. Accordingly and in keeping with the overall theme of this special issue, this review focuses narrowly on rodent SCI models for studies of axon regeneration.

Behavioral and Histopathological Study of Changes in Spinal Cord Injured Rats Supplemented with Spirulina platensis

Spinal cord injury (SCI) is a devastating disease that leads to permanent disability and causes great suffering. The resulting neurological dysfunction and paralysis is proportional to the severity of the trauma itself. Spirulina is widely used as a nutritional supplement due to its high protein and antioxidant content. In the present study, the protective effect of the Spirulina treatment on locomotor function and morphological damage after SCI was investigated. Seventy Sprague-Dawley (SD) rats were divided into three groups: Sham (laminectomy alone), Control (laminectomy with SCI), and Experimental (laminectomy with SCI +180 mg/kg per day Spirulina platensis). A laminectomy was performed at T12 and an Inox No.2 modified forceps was used to perform a partial crush injury on the spinal cord. The rats were then perfused at 3, 7, 14, 21, and 28 days after injury for morphological investigations. The injured rat spinal cord indicated a presence of hemorrhage, cavity, and necrosis. Pretreatment with Spirulina significantly improved the locomotor function and showed a significant reduction on the histological changes. The experimental results observed in this study suggest that treatment with Spirulina platensis possesses potential benefits in improving hind limb locomotor function and reducing morphological damage to the spinal cord.

Combinatorial treatment of acute spinal cord injury with ghrelin, ibuprofen, C16, and ketogenic diet does not result in improved histologic or functional outcome

Because of the complex, multifaceted nature of spinal cord injury (SCI), it is widely believed that a combination of approaches will be superior to individual treatments. Therefore, we employed a rat model of cervical SCI to evaluate the combination of four noninvasive treatments that individually have been reported to be effective for acute SCI during clinically relevant therapeutic time windows. These treatments included ghrelin, ibuprofen, C16, and ketogenic diet (KD). These were selected not only because of their previously reported efficacy in SCI models but also for their potentially different mechanisms of action. The administration of ghrelin, ibuprofen, C16, and KD several hours to days postinjury was based on previous observations by others that each treatment had profound effects on the pathophysiology and functional outcome following SCI. Here we showed that, with the exception of a modest improvement in performance on the Montoya staircase test at 8-10 weeks postinjury, the combinatorial treatment with ghrelin, ibuprofen, C16, and KD did not result in any significant improvements in the rearing test, grooming test, or horizontal ladder. Histologic analysis of the spinal cords did not reveal any significant differences in tissue sparing between treatment and control groups. Although single approaches of ghrelin, ibuprofen, C16, and KD have been reported to be beneficial after SCI, our results show that the combination of the four interventions did not confer significant functional or histological improvements in a cervical model of SCI. Possible interactions among the treatments may have negated their beneficial effects, emphasizing the challenges that have to be addressed when considering combinatorial drug therapies for SCI.

UBE4B: a promising regulatory molecule in neuronal death and survival

Neuronal survival and death of neurons are considered a fundamental mechanism in the regulation of the nervous system during early development of the system and in adulthood. Defects in this mechanism are highly problematic and are associated with many neurodegenerative diseases. Because neuronal programmed death is apoptotic in nature, indicating that apoptosis is a key regulatory process, the p53 family members (p53, p73, p63) act as checkpoints in neurons due to their role in apoptosis. The complexity of this system is due to the existence of different naturally occurring isoforms that have different functions from the wild types (WT), varying from apoptotic to anti-apoptotic effects. In this review, we focus on the role of UBE4B (known as Ube4b or Ufd2a in mouse), an E3/E4 ligase that triggers substrate polyubiquitination, as a master regulatory ligase associated with the p53 family WT proteins and isoforms in regulating neuronal survival. UBE4B is also associated with other pathways independent of the p53 family, such as polyglutamine aggregation and Wallerian degeneration, both of which are critical in neurodegenerative diseases. Many of the hypotheses presented here are gateways to understanding the programmed death/survival of neurons regulated by UBE4B in normal physiology, and a means of introducing potential therapeutic approaches with implications in treating several neurodegenerative diseases.

Concepts and methods for the study of axonal regeneration in the CNS

Progress in the field of axonal regeneration research has been like the process of axonal growth itself: there is steady progress toward reaching the target, but there are episodes of mistargeting, misguidance along false routes, and connections that must later be withdrawn. This primer will address issues in the study of axonal growth after central nervous system injury in an attempt to provide guidance toward the goal of progress in the field. We address definitions of axonal growth, sprouting and regeneration after injury, and the research tools to assess growth.

Spinal cord injury with unilateral versus bilateral primary hemorrhage--effects of glibenclamide

In spinal cord injury (SCI), block of Sur1-regulated NC(Ca-ATP) channels by glibenclamide protects penumbral capillaries from delayed fragmentation, resulting in reduced secondary hemorrhage, smaller lesions and better neurological function. All published experiments demonstrating a beneficial effect of glibenclamide in rat models of SCI have used a cervical hemicord impact calibrated to produce primary hemorrhage located exclusively ipsilateral to the site of impact. Here, we tested the hypothesis that glibenclamide also would be protective in a model with more extensive, bilateral primary hemorrhage. We studied the effect of glibenclamide in 2 rat cervical hemicord contusion models with identical impact force (10 g, 25 mm), one with the impactor positioned laterally to yield unilateral primary hemorrhage (UPH), and the other with the impactor positioned more medially, yielding larger, bilateral primary hemorrhages (BPH) and 6-week lesion volumes that were 45% larger. Functional outcome measures included: modified (unilateral) Basso, Beattie, and Bresnahan scores, angled plane performance, and rearing times. In the UPH model, the effects of glibenclamide were similar to previous observations, including a functional benefit as early as 24h after injury and 6-week lesion volumes that were 57% smaller than controls. In the BPH model, glibenclamide exerted a significant benefit over controls, but the functional benefit was smaller than in the UPH model and 6-week lesion volumes were 33% smaller than controls. We conclude that glibenclamide is beneficial in different models of cervical SCI, with the magnitude of the benefit depending on the magnitude and extent of primary hemorrhage.

The challenges of long-distance axon regeneration in the injured CNS

Injury to the central nervous system (CNS) that results in long-tract axonal damage typically leads to permanent functional deficits in areas innervated at, and below, the level of the lesion. The initial ischemia, inflammation, and neurodegeneration are followed by a progressive generation of scar tissue, dieback of transected axons, and demyelination, creating an area inhibitory to regrowth and recovery. Two ways to combat this inhibition is to therapeutically target the extrinsic and intrinsic properties of the axon-scar environment. Scar tissue within and surrounding the lesion site can be broken down using an enzyme known as chondroitinase. Negative regulators of adult neuronal growth, such as Nogo, can be neutralized with antibodies. Both therapies greatly improve functional recovery in animal models. Alternatively, modifying the intrinsic growth properties of CNS neurons through gene therapy or pharmacotherapy has also shown promising axonal regeneration after injury. Despite these promising therapies, the main challenge of long-distance axon regeneration still remains; achieving a level of functional and organized connectivity below the level of the lesion that mimics the intact CNS.

Protein deiminases: new players in the developmentally regulated loss of neural regenerative ability

Spinal cord regenerative ability is lost with development, but the mechanisms underlying this loss are still poorly understood. In chick embryos, effective regeneration does not occur after E13, when spinal cord injury induces extensive apoptotic response and tissue damage. As initial experiments showed that treatment with a calcium chelator after spinal cord injury reduced apoptosis and cavitation, we hypothesized that developmentally regulated mediators of calcium-dependent processes in secondary injury response may contribute to loss of regenerative ability. To this purpose we screened for such changes in chick spinal cords at stages of development permissive (E11) and non-permissive (E15) for regeneration. Among the developmentally regulated calcium-dependent proteins identified was PAD3, a member of the peptidylarginine deiminase (PAD) enzyme family that converts protein arginine residues to citrulline, a process known as deimination or citrullination. This post-translational modification has not been previously associated with response to injury. Following injury, PAD3 up-regulation was greater in spinal cords injured at E15 than at E11. Consistent with these differences in gene expression, deimination was more extensive at the non-regenerating stage, E15, both in the gray and white matter. As deimination paralleled the extent of apoptosis, we investigated the effect of blocking PAD activity on cell death and deiminated-histone 3, one of the PAD targets we identified by mass-spectrometry analysis of spinal cord deiminated proteins. Treatment with the PAD inhibitor, Cl-amidine, reduced the abundance of deiminated-histone 3, consistent with inhibition of PAD activity, and significantly reduced apoptosis and tissue loss following injury at E15. Altogether, our findings identify PADs and deimination as developmentally regulated modulators of secondary injury response, and suggest that PADs might be valuable therapeutic targets for spinal cord injury.

Apolipoprotein E epsilon 4 allele and outcomes of traumatic spinal cord injury in a Chinese Han population

The association between apolipoprotein E (APOE) epsilon 4 (ε4) allele and outcomes of traumatic spinal cord injury (SCI) is still controversial and ambiguous. The objective of this study was to test the hypothesis that APOE polymorphisms are associated with outcomes after SCI in Chinese Han patients. APOE polymorphisms were determined in 100 patients with cervical SCI (C3-C8). The genotype frequency of this polymorphism was determined by using a polymerase chain reaction-restriction fragment length polymorphism assay. Patients with an APOE ε4 allele had significantly less motor recovery during rehabilitation than did patients without an APOE ε4 allele (mean 3.7 vs. 6.1; P = 0.04) and a longer rehabilitation length of stay (LOS) (mean 117.4 vs. 94.5; P = 0.02), but better sensory-pinprick recovery (mean 6.1 vs. 4.0; P = 0.03). There were no significant differences by APOE ε4 allele status in sensory-light touch recovery or acute LOS. This study suggests that the APOE ε4 allele is associated with outcomes after SCI and longer rehabilitation LOS in Chinese Han patients.

Regional heterogeneity in astrocyte responses following contusive spinal cord injury in mice

Astrocytes and their precursors respond to spinal cord injury (SCI) by proliferating, migrating, and altering phenotype. This contributes to glial scar formation at the lesion border and gliosis in spared gray and white matter. The present study was undertaken to evaluate astrocyte changes over time and determine when and where interventions might be targeted to alter the astrocyte response. Bromodeoxyuridine (BrdU) was administered to mice 3 days after SCI, and cells expressing BrdU and the astrocyte marker, glial fibrillary acidic protein (GFAP), were counted at 3, 7, and 49 days post-injury (DPI). BrdU-labeled cells accumulated at the lesion border by 7 DPI and approximately half of these expressed GFAP. In spared white matter, the total number of BrdU+ cells decreased, while the percentage of BrdU+ cells expressing GFAP increased at 49 DPI. Phenotypic changes were examined using the progenitor marker nestin, the radial glial marker, brain lipid binding protein (BLBP), and GFAP. Nestin was upregulated by 3 DPI and declined between 7 and 49 DPI in all regions, and GFAP increased and remained above naïve levels at all timepoints. BLBP increased early and remained high along the lesion border and spared white matter, but was expressed transiently by cells lining the central canal and in a unique population of small cells found within the lesion and in gray matter rostral and caudal to the border. The results demonstrate that the astrocyte response to SCI is regionally heterogeneous, and suggests astrocyte populations that could be targeted by interventions.

EphA4 deficient mice maintain astroglial-fibrotic scar formation after spinal cord injury

One important aspect of recovery and repair after spinal cord injury (SCI) lies in the complex cellular interactions at the injury site that leads to the formation of a lesion scar. EphA4, a promiscuous member of the EphA family of repulsive axon guidance receptors, is expressed by multiple cell types in the injured spinal cord, including astrocytes and neurons. We hypothesized that EphA4 contributes to aspects of cell-cell interactions at the injury site after SCI, thus modulating the formation of the astroglial-fibrotic scar. To test this hypothesis, we studied tissue responses to a thoracic dorsal hemisection SCI in an EphA4 mutant mouse line. We found that EphA4 expression, as assessed by beta-galactosidase reporter gene activity, is associated primarily with astrocytes in the spinal cord, neurons in the cerebral cortex and, to a lesser extent, spinal neurons, before and after SCI. However, we did not observe any overt reduction of glial fibrillary acidic protein (GFAP) expression in the injured area of EphA4 mutants in comparison with controls following SCI. Furthermore, there was no evident disruption of the fibrotic scar, and the boundary between reactive astrocytes and meningeal fibroblasts appeared unaltered in the mutants, as were lesion size, neuronal survival and inflammation marker expression. Thus, genetic deletion of EphA4 does not significantly alter the astroglial response or the formation of the astroglial-fibrotic scar following a dorsal hemisection SCI in mice. In contrast to what has been proposed, these data do not support a major role for EphA4 in reactive astrogliosis following SCI.

A bilateral cervical contusion injury model in mice: assessment of gripping strength as a measure of forelimb motor function

Here, we describe a bilateral cervical contusion model for mice. Adult female mice received graded bilateral contusion injuries at cervical level 5 (C5) using a commercially available impactor (the IH device). Three separate experiments were carried out to define conditions that produce impairments in forelimb function without unacceptable impairment of general health. A grip strength meter (GSM) was used to assess gripping ability as a measure of forelimb motor function; lesion size was assessed histologically by staining cross sections for H&E and GFAP. In Experiment 1, mice received injuries of 30 kilodynes (kdyn); these produced minimal deficits on grip strength. In Experiment 2, mice received injuries of 75 kdyn and 100 kdyn. Injuries of 75 kdyn produced transient deficits in gripping that recovered between 3 and 15 days post-injury (dpi) to about 90% of control; injuries of 100 kdyn produced deficits that recovered to about 50% of control. In Experiment 3, none of the mice that received injuries of 100 kdyn recovered gripping ability. Histological assessment revealed graded injuries that ranged from damage limited primarily to the dorsal column (DC) to damage to the DC, grey matter, ventral column and lateral column. Most lesions filled in with a fibrous tissue matrix, but fluid-filled cystic cavities were found in 13% of the 100 kdyn injury group and a combination of fibrous-filled/fluid-filled cystic cavities were found in 22% and 38% of the 75-kdyn and 100-kdyn injury groups, respectively. There was minimal urine retention following cervical contusion injuries indicating preservation of bladder function. Our results define conditions to produce graded bilateral cervical contusion injuries in mice and demonstrate the usefulness of the GSM for assessing forelimb motor function after cervical contusions.

Anatomical and functional outcomes following a precise, graded, dorsal laceration spinal cord injury in C57BL/6 mice

To study the pathophysiology of spinal cord injury (SCI), we used the LISA-Vibraknife to generate a precise and reproducible dorsal laceration SCI in the mouse. The surgical procedure involved a T9 laminectomy, dural resection, and a spinal cord laceration to a precisely controlled depth. Four dorsal hemisection injuries with lesion depths of 0.5, 0.8, 1.1, and 1.4 mm, as well as normal, sham (laminectomy and dural removal only), and transection controls were examined. Assessments including the Basso Mouse Scale (BMS), footprint analysis, beam walk, toe spread reflex, Hargreaves' test, and transcranial magnetic motor-evoked potential (tcMMEP) analysis were performed to assess motor, sensorimotor, and sensory function. These outcome measures demonstrated significant increases in functional deficits as the depth of the lesion increased, and significant behavioral recovery was observed in the groups over time. Quantitative histological examination showed significant differences between the injury groups and insignificant lesion depth variance within each of the groups. Statistically significant differences were additionally found in the amount of ventral spared tissue at the lesion site between the injury groups. This novel, graded, reproducible laceration SCI model can be used in future studies to look more closely at underlying mechanisms that lead to functional deficits following SCI, as well as to determine the efficacy of therapeutic intervention strategies in the injury and recovery processes following SCI.

ADAM8 is selectively up-regulated in endothelial cells and is associated with angiogenesis after spinal cord injury in adult mice

Endothelial cell (EC) loss and subsequent angiogenesis occur over the first week after spinal cord injury (SCI). To identify molecular mechanisms that could be targeted with intravenous (i.v.) treatments, we determined whether transmembrane "a disintegrin and metalloprotease" (ADAM) proteins are expressed in ECs of the injured spinal cord. ADAMs bind to integrins, which are important for EC survival and angiogenesis. Female adult C57Bl/6 mice with a spinal cord contusion had progressively more ADAM8 (CD156) immunostaining in blood vessels and individual ECs between 1 and 28 days following injury. Uninjured spinal cords had little ADAM8 staining. The increase in ADAM8 mRNA and protein was confirmed in spinal cord lysates, and ADAM8 mRNA was present in FACS-enriched ECs. ADAM8 colocalized extensively and exclusively with the EC marker PECAM and also with i.v.-injected lectins. Intravenous isolectin B4 (IB4) labels a subpopulation of blood vessels at and within the injury epicenter 3-7 days after injury, coincident with angiogenesis. Both ADAM8 and the proliferation marker Ki-67 were present in IB4-positive microvessels. ADAM8-positive proliferating cells were seen at the leading end of IB4-positive blood vessels. Angiogenesis was confirmed by BrdU incorporation, binding of i.v.-injected nucleolin antibodies, and MT1-MMP immunostaining in a subset of blood vessels. These data suggest that ADAM8 is vascular selective and plays a role in proliferation and/or migration of ECs during angiogenesis following SCI.

Robust axonal growth and a blunted macrophage response are associated with impaired functional recovery after spinal cord injury in the MRL/MpJ mouse

Spinal cord injury (SCI) in mammals leads to a robust inflammatory response followed by the formation of a glial and connective tissue scar that comprises a barrier to axonal regeneration. The inbred MRL/MpJ mouse strain exhibits reduced inflammation after peripheral injury and shows true regeneration without tissue scar formation following an ear punch wound. We hypothesized that following SCI, the unique genetic wound healing traits of this strain would result in reduced glial and connective tissue scar formation, increased axonal growth, and improved functional recovery. Adult MRL/MpJ and C57BL/6J mice were subjected to a mid-thoracic spinal contusion and the distribution of axon profiles and selected cellular and extracellular matrix components was compared at 1, 2, 4 and 6 weeks post-injury. Recovery of hind-limb locomotor function was assessed over the same time period. The MRL/MpJ mice exhibited robust axon growth within the lesion, beginning at 4 weeks post-injury. This growth was accompanied by reduced macrophage staining at 1, 2, 4 and 6 weeks post-injury, decreased chondroitin sulfate proteoglycan staining at 1-2 weeks and increased laminin staining throughout the lesion at 2-6 weeks post-injury. Paradoxically, the extent of locomotor recovery was impaired in the MRL/MpJ mice. Close examination of the chronic lesion site revealed evidence of ongoing degeneration both within and surrounding the lesion site. Thus, the regenerative genetic wound healing traits of the MRL/MpJ mice contribute to the evolution of a lesion environment that supports enhanced axon growth after SCI. However, this response occurs at the expense of meaningful functional recovery.

Neuropsin promotes oligodendrocyte death, demyelination and axonal degeneration after spinal cord injury

Previous studies indicated that the expression of neuropsin, a serine protease, is induced in mature oligodendrocytes after injury to the CNS. The pathophysiology of spinal cord injury (SCI) involves primary and secondary mechanisms, the latter contributing further to permanent losses of function. To explore the role of neuropsin after SCI, histochemical and behavioral analyses were performed in wild-type (WT) and neuropsin-deficient (neuropsin(-/-)) mice using a crush injury model, a well-characterized and consistently reproducible model of SCI. In situ hybridization revealed that neuropsin mRNA expression was induced in the spinal cord white matter from WT mice after crush SCI, peaking at day 4. Neuropsin(-/-) mice showed attenuated demyelination, oligodendrocyte death, and axonal damage after SCI. Although axonal degeneration in the corticospinal tract was obvious caudal to the lesion site in both strains of mice after SCI, the number of surviving nerve fibers caudal to the lesion was significantly larger in neuropsin(-/-) mice than WT mice. Behavioral analysis revealed that the recovery at days 10-42 was significantly improved in neuropsin(-/-) mice compared with WT mice in spite of the severe initial hindlimb impairments due to SCI in both strains. These observations suggest that neuropsin is involved in the secondary phase of the pathogenesis of SCI mediated by demyelination, oligodendrocyte death, and axonal degeneration.

Longitudinal magnetic resonance imaging of spinal cord injury in mouse: changes in signal patterns associated with the inflammatory response

Contusion-type spinal cord injury (SCI) in mice was followed longitudinally using in vivo magnetic resonance (MR) imaging along with neurobehavioral tests performed on postinjury Days 1, 7, 14 and 28. Magnetic resonance images were acquired from seven injured wild-type mice using a 9.4-T scanner and presented in sagittal and axial views to reflect the current state of the injured cord neuropathology on each day. The data were analyzed individually to gain more insights on the neuroinflammatory response unique to the mouse, to characterize the spatiotemporal evolution of the lesion and to quantify the changes in lesion volume and length with time. The MR intensity patterns on Day 1 showed acute injuries as focal in one group of three mice and as diffuse in the remaining group of four mice. The focal injuries appeared as a region of hypointensity with well-defined boundaries. These injuries first enlarged on Day 7, but then shrunk slightly by Days 14 and 28. In contrast, the diffuse injuries were initially obscure on Day 1, mainly because of loss of contrast between gray and white matters. On Day 7, lesions expanded asymptotically in both rostral and caudal directions with respect to the epicenter, and maintained its size on Days 14 and 28. Previous studies based on postmortem histological analysis have reported lesions behaving more like in the focal group. However, this new injury with diffuse characteristics may have important implications for SCI research carried out with mice. Unique experiments on genetically engineered mice with altered neuroinflammatory response should help clarify the origin of these differences in the lesion formation.

Strain-dependent recovery of spontaneous hindlimb movement in spinal cord transected mice (CD1, C57BL/6, BALB/c)

Reorganization and plasticity after spinal cord injury have been recently shown to take place in sublesional neuronal networks, but the possibility of strain-dependent changes at that level has never been explored. The authors studied the spontaneous return of hindlimb movement in low-thoracic spinal cord transected (Tx) mice from 3 commonly used strains. Without intervention, most CD1, C57BL/6, and BALB/c mice displayed some hindlimb movement recovery after Tx. Although all assessment methods unanimously reported that CD1 displayed higher recovery levels than did the C57BL/6 and BALB/c, higher scores were generally found with the Antri-Orsal-Barthe (M. Antri, D. Orsal, & J. Y. Barthe, 2002) and the Average Combined Score (P. A. Guertin, 2005a) methods. Such spontaneous recovery in low-thoracic Tx mice is likely the result of neuronal plasticity at the lumbosacral spinal cord level, suggesting that these sublesional changes are strain dependent.

Comparative analysis of lesion development and intraspinal inflammation in four strains of mice following spinal contusion injury

Susceptibility to neuroinflammatory disease is influenced in part by genetics. Recent data indicate that survival of traumatized neurons is strain dependent and influenced by polygenic loci that control resistance/susceptibility to experimental autoimmune encephalomyelitis (EAE), a model of CNS autoimmune disease. Here, we describe patterns of neurodegeneration and intraparenchymal inflammation after traumatic spinal cord injury (SCI) in mice known to exhibit varying degrees of EAE susceptibility [EAE-resistant (r) or EAE-susceptible (s) mice]. Spinal cords from C57BL/6 (EAE-s), C57BL/10 (EAE-r), BALB/c (EAE-r), and B10.PL (EAE-s) mice were prepared for stereological and immunohistochemical analysis at 6 hours or 3, 7, 14, 28, or 42 days following midthoracic (T9) spinal contusion injury. In general, genetic predisposition to EAE predicted the magnitude of intraparenchymal inflammation but not lesion size/length or locomotor recovery. Specifically, microglia/macrophage activation, recruitment of neutrophils and lymphocytes, and de novo synthesis of MHC class II were greatest in C57BL/6 mice and least in BALB/c mice at all times examined. However, lesion volume and axial spread of neurodegeneration were similar in C57BL/6 and BALB/c mice and were significantly greater than in C57BL/10 or B10.PL mice. Strains with marked intraspinal inflammation also developed the most intense lesion fibrosis. Thus, strain-dependent neuroinflammation was observed after SCI, but without a consistent relationship to EAE susceptibility or lesion progression. Only in C57BL/6 mice was the magnitude of intraspinal inflammation predictive of secondary neurodegeneration, functional recovery, or fibrosis.

Microglial phenotype: is the commitment reversible?

Microglia, the standby cells for immune defense in the CNS, have a reputation for exacerbating the neural damage that occurs in neurodegenerative diseases. However, research over the past few years has established that microglia do not constitute a single, uniform cell population, but rather comprise a family of cells with diverse phenotypes--some that are beneficial and others that the CNS can barely tolerate and that are therefore destructive. This finding raised several questions. What instructs microglia to acquire a particular phenotype, and how do these phenotypes differ? How committed are microglia to a specific phenotype? Can destructive microglia become protective, and can protective microglia retain their beneficial phenotype even when they encounter a destructive environment? Here, we address these questions, and the background of research that elicited them.

Genetic approaches to autonomic dysreflexia

Autonomic dysreflexia is a potentially life-threatening condition in which episodic hypertension occurs after injuries above the mid-thoracic segments of the spinal cord. Despite the seriousness of this condition, little is known of the molecular mechanisms that lead to its development. The completed sequencing of the mouse genome, its dense genetic map, and the large repository of engineered and spontaneous mouse mutants, make the mouse an ideal model organism in which to study the molecular mechanisms underlying autonomic dysreflexia. We subjected two wild-type strains of mice, 129Sv and C57BL/6, and one spontaneous mouse mutant, Wallerian degeneration slow (Wld s), to spinal cord transection and clip-compression injury. We found that the incidence of autonomic dysreflexia is greatly reduced, compared to spinal cord-transected wild-type mice, in Wld s mice after both injury paradigms and in 129Sv and C57BL/6 that have undergone the clip-compression injury. We also found that the amplitude of the dysreflexic response was greater in cord-compressed 129Sv than in C57BL/6 mice. These results implicate axonal degeneration as an important source of signals that trigger the development of autonomic dysreflexia and are discussed in the context of mouse genetics, interstrain differences and possible molecular mechanisms underlying autonomic dysreflexia after spinal cord injury.

Geranylgeranylacetone limits secondary injury, neuronal death, and progressive necrosis and cavitation after spinal cord injury

This study evaluates the neuroprotective effects of geranylgeranylacetone (GGA), which is known as an antiulcer agent and more recently as a heat-shock and other neuroprotective protein inducer, on secondary degeneration after spinal cord injury in rats. Crush injuries were produced at the T8 level using an extradural approach. Optimal administration conditions of GGA were established in an initial experiment by evaluating the appearance of lesions 24 h after injury in sections stained with H-E. Then, in a second experiment, animals treated with the optimal condition (600 mg/kg, 24 h before injury and thereafter every 24 h) were allowed to survive for 6 and 24 h and 1, 3, and 8 weeks after injury, and spinal cords were prepared for histological evaluation by staining for H-E for general histopathology and by silver staining for axons. There was a significant reduction (46%) in lesion volume 24 h after injury in animals treated with optimal administration conditions. The increase in tumor necrosis factor-alpha (TNF-alpha) and the accumulation of neutrophils in the damaged segment of the spinal cord 4 h after injury were significantly inhibited in animals that received GGA. Lesion size and cavitation area remained smaller in treated animals throughout the post-injury survival interval. These results suggest that GGA administration significantly reduces the secondary degeneration that would otherwise occur. The mechanism by which GGA exerts its beneficial effect is unknown but may involve reduction of TNF-alpha activation at the injured cord and/or inhibition of inflammation.

Lipopolysaccharide-induced up-regulation of activated macrophages in the degenerating taste system

Unilateral chorda tympani (CT) nerve section and maintenance on a sodium-restricted diet leads to a rapid decrease in neurophysiological taste responses to sodium in the contralateral, intact CT nerve. Up-regulation of immune function with lipopolysaccharide (LPS; 100 microg i.p.) induces a recovery of normal sodium taste responses, suggesting that the sodium-deficient diet is immunosuppressive. In fact, there is a bilateral increase in the number of lingual, activated macrophages in control-fed rats receiving CT nerve section that does not occur in sodium-deficient rats after sectioning. In the current study, we hypothesized that the LPS-induced recovery of normal taste function in sodium-deficient rats is based on an increase in the activated macrophage response to denervation. Rats receiving a unilateral CT nerve section, a sodium-restricted diet, and/or an injection of LPS (100 microg; i.p.) were overdosed with pentobarbital at day 2 postsectioning, and tongues were rapidly dissected and frozen. Cryosections were then immunohistochemically stained to determine the percentage of ED1 staining for activated macrophages or the number of alphabeta or gammadelta T cells. Activated macrophage levels were significantly increased in sodium-restricted rats that received LPS following unilateral CT nerve section, supporting our hypothesis. These novel findings suggest that LPS overcomes the immunosuppression induced by the sodium-restricted diet and also indicate that the immune system plays a role in regulating taste function after neural injury.

Differential responses of spinal axons to transection: influence of the NG2 proteoglycan

Spinal cord transections were performed in wild type and NG2 proteoglycan null mice in order to study penetration of regenerating axons into the scar that forms in response to this type of injury. Aside from the presence or absence of NG2, the features of the transection scar did not differ between the two genotypes. In both cases, the rostral and caudal spinal cord stumps were separated by collagenous connective tissue that was continuous with the spinal cord meninges. In wild type mice, oligodendrocyte progenitors, macrophages, and microvascular pericytes contributed to up-regulation of NG2 expression in and around the scar. Substantial amounts of non-cell associated NG2 were also observed in the scar. The abilities of two classes of spinal axons to penetrate the transection scar were examined. Serotonergic efferents and calcitonin gene-related peptide-positive sensory afferents both were observed within the lesion, with calcitonin gene-related peptide-positive axons exhibiting a greater capability to penetrate deeply into the scar tissue. These observations demonstrate inherent differences in the abilities of distinct types of neurons to penetrate the scar. Significantly, growth of serotonergic axons into the transection scar was observed twice as frequently in wild type mice as in NG2 knockout mice, suggesting a stimulatory role for the proteoglycan in regeneration of these fibers. These findings run counter to in vitro evidence implicating NG2 as an inhibitor of nerve regeneration. This work therefore emphasizes the importance of including in vivo models in evaluating the responses of specific types of neurons to spinal cord injury.

The cellular distribution of the Wlds chimeric protein and its constituent proteins in the CNS

The C57BL/Wld s mouse is a mutant strain of mouse that shows greatly slowed Wallerian degeneration both in the central and peripheral nervous system. Using immunohistochemistry, immunofluorescence and Western blotting, we have investigated the distribution of the chimeric Wld s protein and its different components in neurons of the CNS of Wld s mice and wild-type C57BL/6J mice. The expression of the Wld s protein is restricted to the nucleus in Wld s mice. Wld s was not detected in axons. The Wld s mice express both the normal and chimeric forms of ubiquitination factor E4 (Ube 4b) and nicotinamide mononucleotide adenylyltransferase-1 (Nmnat-1). The normal forms were expressed both in the cytoplasm and the nuclei of neurons in Wld s mice and wild-type mice, and were also present in the axon. The normal form of Ube4b, mono- and poly-ubiquitin and IkappaBalpha, a substrate of Ube4b, were not differentially expressed in Wld s mice compared with wild-type mice. However, the expression of both the normal and mutant forms of Nmnat-1 was higher in the nuclei of Wld s mice compared with wild-type mice. Therefore, axon protection in Wld s mice does not appear to be controlled by expression of Wld s protein in the axons per se and also is unlikely to be related to the different activity of Ube4b either in general ubiquitination or toward this particular substrate. The increased Nmnat-1 activity in the nucleus of Wld s mice compared with wild-type mice seems to be a significant factor in the axon protection. It is not known whether the expression of the Nmnat-1 in the axon is significant.

Enhanced axonal growth into a spinal cord contusion injury site in a strain of mouse (129X1/SvJ) with a diminished inflammatory response

After injury in the adult central nervous system, invading and intrinsic cells contribute to the formation of a lesion site that is refractory to axonal growth. To test the hypothesis that the inflammatory response to trauma dictates the extent of axonal growth after spinal cord injury, the time course of lesion evolution was compared in two mouse strains with contrasting cellular responses to peripheral inflammatory challenge. Adult C57Bl/6 and 129X1/SvJ mice received identical contusion injuries to the mid-thoracic spinal cord and were allowed to recover for 6 hours to 9 weeks. Both strains responded with a rapid, transient increase in chemokine expression, but the magnitude of this early response was slightly reduced in the 129X1/SvJ mice. Morphological indicators of inflammation were similar during the first week postinjury. After 7 days postinjury, however, the cellular responses differed between strains. The C57Bl/6 lesion core was chronically occupied by macrophages, devoid of astrocytes, and contained few axonal profiles. In contrast, as the macrophage density decreased a network of astrocytic processes and axons of central and peripheral origin invaded the center of the lesion site in 129X1Sv/J mice. Growth of axons in the 129X1Sv/J mice was accompanied by increased extravascular laminin in the lesion core and a reduced expression of chondroitin sulfate proteoglycan glycosaminoglycan sidechains in the periphery of the lesion. These results demonstrate that the diminished chronic inflammatory response in 129X1/SvJ mice is associated with enhanced cellular repair and increased axonal growth after spinal cord injury.

Reactive astrocytes protect tissue and preserve function after spinal cord injury

Reactive astrocytes are prominent in the cellular response to spinal cord injury (SCI), but their roles are not well understood. We used a transgenic mouse model to study the consequences of selective and conditional ablation of reactive astrocytes after stab or crush SCI. Mice expressing a glial fibrillary acid protein-herpes simplex virus-thymidine kinase transgene were given mild or moderate SCI and treated with the antiviral agent ganciclovir (GCV) to ablate dividing, reactive, transgene-expressing astrocytes in the immediate vicinity of the SCI. Small stab injuries in control mice caused little tissue disruption, little demyelination, no obvious neuronal death, and mild, reversible functional impairments. Equivalent small stab injuries in transgenic mice given GCV to ablate reactive astrocytes caused failure of blood-brain barrier repair, leukocyte infiltration, local tissue disruption, severe demyelination, neuronal and oligodendrocyte death, and pronounced motor deficits. Moderate crush injuries in control mice caused focal tissue disruption and cellular degeneration, with moderate, primarily reversible motor impairments. Equivalent moderate crush injuries combined with ablation of reactive astrocytes caused widespread tissue disruption, pronounced cellular degeneration, and failure of wound contraction, with severe persisting motor deficits. These findings show that reactive astrocytes provide essential activities that protect tissue and preserve function after mild or moderate SCI. In nontransgenic animals, crush or contusion SCIs routinely exhibit regions of degenerated tissue that are devoid of astrocytes. Our findings suggest that identifying ways to preserve reactive astrocytes, to augment their protective functions, or both, may lead to novel approaches to reducing secondary tissue degeneration and improving functional outcome after SCI.

High frequency electrical stimulation attenuates progressive necrosis and cavitation following spinal cord injury

This study evaluates the influence of preconditioning by following electrical stimulation on primary and secondary lesion formation following spinal cord injury in rats. The dorsal surface of the spinal cord was stimulated (500 Hz. 10 pulses/train, inter train interval of 10 sec. for 2 hrs) at the T7 level 24 hrs before a right side hemisection, carried out immediately after injury and maintained every 24 hrs for 7 days. Preconditioning by electrical stimulation of the spinal cord activates reactive astrocytes and significantly attenuates edema and progressive necrosis and cavitation, concerning especially the primary (1, 3 weeks post injury) and secondary (24 hrs, 1, 3 weeks post injury) lesion volume. The results suggest that pre-conditioning by electrical stimulation prevents spinal cord secondary lesion formation after injury, and that the beneficial effect is provided by astroglial cells with regard to their ability to attenuate trauma induced cellular cascades.

Neuroprotection after transient global cerebral ischemia in Wld(s) mutant mice

The Wld(s) mouse mutant demonstrates a remarkable phenotype of delayed axonal and synaptic degeneration after nerve lesion. In this study, the authors tested the hypothesis that expression of Wld protein is neuroprotective in an in vivo mouse model of global cerebral ischemia. This model is associated with selective neuronal degeneration in specific brain regions such as the caudate nucleus and CA2 hippocampal pyramidal cell layer. The extent of neuronal damage was quantified in Wld(s) compared to wild-type mice after an identical episode of global cerebral ischemia. The results demonstrated a significant and marked reduction in the extent of neuronal damage in Wld(s) as compared to wild-type C57Bl/6 mice. In the caudate nucleus, Wld expression significantly reduced the percentage of ischemic neuronal damage after global ischemia (Wld(s), 27.7 +/- 16.8%; wild-type mice, 58.7 +/- 32.3%; P = 0.036). Similarly, in the CA2 pyramidal cell layer, there was a significant reduction of neuronal damage in the Wld(s) mice as compared to wild-type mice after ischemia (Wld(s), 17.7 +/- 23.0%; wild-type mice, 41.9 +/- 28.0%; P < 0.023). Thus, these results clearly demonstrate that the Wld gene confers substantial neuroprotection after cerebral ischemia, and suggest a new role to that previously described for Wld(s).

Up-regulation of activated macrophages in response to degeneration in the taste system: Effects of dietary sodium restriction

Dietary sodium restriction combined with unilateral chorda tympani nerve section leads to a rapid and specific decrease in neurophysiological taste responses to sodium in the contralateral, intact chorda tympani (Hill and Phillips [1994] J. Neurosci. 14:2904-2910). Previous work demonstrated that dietary sodium restriction may induce these early functional deficits by inhibiting immune activity after denervation (Phillips and Hill [1996] Am. J. Physiol. 271:R857-R862). However, little is known about the leukocyte response to denervation of taste buds in fungiform papillae. In the current study, it was hypothesized that T cells and macrophages are increased in the tongue after unilateral denervation in control-fed but not sodium-restricted animals. Adult, specified pathogen-free rats received unilateral chorda tympani nerve section or sham section followed by dietary sodium restriction or maintenance on control diet. At day 1, 2, 5, 7, or 50 postsectioning, immunostaining was used to detect the percentage of staining for activated macrophages, the number of alpha beta T cells, and the number of delta gamma epithelial T cells in the tongue. The number of lingual T cells did not significantly differ between treatment groups following denervation. However, there was a dramatic bilateral increase in ED1(+) staining for activated macrophages in control-fed rats that peaked at day 2 postsectioning. In contrast, sodium-restricted rats did not show an increase in activated macrophages above baseline at any time postsectioning. Further analysis of extralingual macrophages indicated that the deficit in immune activity in sodium-restricted rats is localized to the tongue and is not widespread. A model for immune modulation of taste receptor cell function is proposed based on these novel findings.

Enhanced oligodendrocyte survival after spinal cord injury in Bax-deficient mice and mice with delayed Wallerian degeneration

Mechanisms of oligodendrocyte death after spinal cord injury (SCI) were evaluated by T9 cord level hemisection in wild-type mice (C57BL/6J and Bax+/+ mice), Wlds mice in which severed axons remain viable for 2 weeks, and mice deficient in the proapoptotic protein Bax (Bax-/-). In the lateral white-matter tracts, substantial oligodendrocyte death was evident in the ipsilateral white matter 3-7 mm rostral and caudal to the hemisection site 8 d after injury. Ultrastructural analysis and expression of anti-activated caspase-3 characterized the ongoing oligodendrocyte death at 8 d as primarily apoptotic. Oligodendrocytes were selectively preserved in Wlds mice compared with C57BL/6J mice at 8 d after injury, when severed axons remained viable as verified by antereograde labeling of the lateral vestibular spinal tract. However, 30 d after injury when the severed axons in Wlds animals were already degenerated, the oligodendrocytes preserved at 8 d were lost, and numbers were then equivalent to control C57BL/6J mice. In contrast, oligodendrocyte death was prevented at both time points in Bax-/- mice. When cultured oligodendrocytes were exposed to staurosporine or cyclosporin A, drugs known to stimulate apoptosis in oligodendrocytes, those from Bax-/- mice but not from Bax+/+ or Bax+/- mice were resistant to the apoptotic death. In contrast, the three groups were equally vulnerable to excitotoxic necrosis death induced by kainate. On the basis of these data, we hypothesize that the Wallerian degeneration of white matter axons that follows SCI removes axonal support and induces apoptotic death in oligodendrocytes by triggering Bax expression.

Autonomic dysreflexia after spinal cord transection or compression in 129Sv, C57BL, and Wallerian degeneration slow mutant mice

To study plasticity of central autonomic circuits that develops after spinal cord injury (SCI), we have characterized a mouse model of autonomic dysreflexia. Autonomic dysreflexia is a condition in which episodic hypertension occurs after injuries above the midthoracic segments of the spinal cord. As synaptic plasticity may be triggered by axonal degeneration, we investigated whether autonomic dysreflexia is reduced in mice when axonal degeneration is delayed after SCI. We subjected three strains of mice, Wld(S), C57BL, and 129Sv, to either spinal cord transection (SCT) or severe clip-compression injury (CCI). The Wld(S) mouse is a well-characterized mutant that exhibits delayed Wallerian degeneration. The CCI model is an injury paradigm in which significant the axonal degeneration is due to secondary events and therefore delayed relative to the time of the initial injury. We herein demonstrate that the incidence of autonomic dysreflexia is reduced in Wld(S) mice after SCT and in all mice after CCI. To determine if differences in afferent arbor sprouting could explain our observations, we assessed changes in the afferent arbor in each mouse strain after both SCT and CCI. We show that independent of the type of injury, 129Sv mice but not C57BL or Wld(S) mice demonstrated an increased small-diameter CGRP-immunoreactive afferent arbor after SCI. Our work thus suggests a role for Wallerian degeneration in the development of autonomic dysreflexia and demonstrates that the choice of mouse strain and injury model has important consequences to the generalizations that may be drawn from studies of SCI in mice.

Ascending sensory, but not other long-tract axons, regenerate into the connective tissue matrix that forms at the site of a spinal cord injury in mice

Mice exhibit a unique wound healing response following spinal cord injury in which the lesion site fills in with a connective tissue matrix. Previous studies have revealed that axons grow into this matrix, but the source of the axons remained unknown. The present study assesses whether any of these axons were the result of long tract regeneration. C57Bl/6 mice received crush injuries and were allowed to survive for 6 weeks to 7 months. Biotinylated dextran amine (BDA) was injected into the somato-motor cortex to trace descending corticospinal tract (CST) axons, into the midbrain to label descending brainstem pathways including the rubrospinal and reticulospinal tracts, or into the L5 dorsal root ganglion to trace ascending projections of first-order sensory neurons. Spinal cords from other mice were prepared for immunocytochemistry using antibodies against neurofilament protein (NF), 5-HT to reveal descending serotonergic axons, calcitonin gene-related protein (CGRP) to reveal ascending sensory axons, and chondroitin sulfate proteoglycan (CSPG) to assess the distribution of molecules that are inhibitory to axon growth. NF immunostaining revealed axons in the connective tissue matrix at the lesion site, confirming previous studies that used protargol staining. CST axons did not enter the connective tissue matrix, but did sprout extensively in segments adjacent to the injury site. Rubrospinal and reticulospinal tract axons also did not grow into the lesion site. 5-HT-positive axons extended to the edge of the lesion, and a few axons followed astrocyte processes into the margins of the lesion site. In contrast to the other pathways, BDA-labeled ascending sensory axons did extend into and arborized extensively within the connective tissue matrix, although the subgroup of ascending axons that are positive for CGRP did not. These results indicate that the connective tissue matrix is permissive for regeneration of some classes of ascending sensory axons but not for other axonal systems.

Rats and mice exhibit distinct inflammatory reactions after spinal cord injury

Spinal contusion pathology in rats and mice is distinct. Cystic cavities form at the impact site in rats while a dense connective tissue matrix occupies the injury site in mice. Because inflammatory cells coordinate mechanisms of tissue injury and repair, we evaluated whether the unique anatomical presentation in spinally injured rats and mice is associated with a species-specific inflammatory response. Immunohistochemistry was used to compare the leukocytic infiltrate between rats and mice. Microglia/macrophage reactions were similar between species; however, the onset and magnitude of lymphocyte and dendritic cell (DC) infiltration were markedly different. In rats, T-cell numbers were highest between 3 and 7 days postinjury and declined by 50% over the next 3 weeks. In mice, significant T-cell entry was not evident until 14 days postinjury, with T-cell numbers doubling between 2 and 6 weeks. Dendritic cell influx paralleled T-cell infiltration in rats but was absent in mouse spinal cord. De novo expression of major histocompatability class II molecules was increased in both species but to a greater extent in mice. Unique to mice were cells that resembled lymphocytes but did not express lymphocyte-specific markers. These cells extended from blood vessels within the fibrotic tissue matrix and expressed fibronectin, collagen I, CD11b, CD34, CD13, and CD45. This phenotype is characteristic of fibrocytes, specialized blood-borne cells involved in wound healing and immunity. Thus, species-specific neuroinflammation may contribute to the formation of distinct tissue environments at the site of spinal cord injury in mice and rats.

Neurites undergoing Wallerian degeneration show an apoptotic-like process with Annexin V positive staining and loss of mitochondrial membrane potential

Wallerian degeneration, the disintegration of the distal part of an injured axon, is an important event in many neurodegenerative diseases. We studied Wallerian degeneration in dorsal root ganglion (DRG) explants in culture by separating neurites from their cell bodies with a scalpel. The severed neurites showed Annexin V positive staining, that spreads distally with a rate comparable to that of slow axonal transport in intact neurons in vivo. Moreover, the injured neurites showed loss of mitochondrial membrane potential. These features resemble those seen when cells undergo apoptosis. These data contribute to a new understanding of the mechanism of axonal degeneration, have implications for the response of stromal cells in central nervous system (CNS) and raise the prospect of new pharmacological treatments for those neurodegenerative pathologies where the protection of the cell body alone does not alleviate the disease.

False resurrections: distinguishing regenerated from spared axons in the injured central nervous system

Several recent studies report that axon regeneration can be induced in the mature mammalian nervous system by novel treatments or genetic manipulations. In assessing these reports, it is important to be mindful of the history of regeneration research, which is littered with the corpses of studies that reported regeneration that later proved incorrect. One important reason is the "spared axon conundrum," in which axons that survive a lesion are mistakenly identified as having regenerated. Here, we illustrate the problem and propose criteria that may be used to identify regenerated vs. spared axons, focusing on the injured spinal cord.

Therapeutic vaccination for spinal cord injury: helping the body to cure itself

Inflammation is thought to exacerbate the outcome of spinal cord injury. However, our findings have led us to view inflammation as a healing response that needs the help of a systemic immune response mediated by T helper 1 (Th1) cells that are specific to the abundant antigens residing in the lesion site. Strains differ in their ability to manifest, at the right time and intensity, a spontaneous T-cell response to antigens at the lesion site and therefore in their ability to generate a local inflammatory response whose outcome is beneficial (maintenance and repair). All strains, however, can benefit from immune intervention that boosts and regulates the inflammatory response. Because recovery comprises multi-step processes, pharmacological intervention will be less effective than well-synchronized, self-healing immune activity. Risk-free neuroprotective intervention might be achieved by post-traumatic vaccination with a weak, non-pathogenic, auto-antigen, causing autoimmune T cells to home to the lesion site where they become activated and therefore activate local phagocytic cells to remove hostile elements and provide growth factors.

Genetic influences on secondary degeneration and wound healing following spinal cord injury in various strains of mice

Various inbred strains of mice exhibit dramatic differences in sensitivity to excitotoxic cell death induced by systemic injections of kainic acid (KA). The present study evaluates whether the same strains are also differentially sensitive to secondary degeneration after spinal cord injury, in which excitotoxic cell death is thought to play a pathogenic role. Spinal cord crush injuries were produced at T9 in two inbred strains that are resistant to KA-induced excitotoxic cell death (C57Bl/6 and Balb/c) and four strains that are sensitive (CD-1, FVB/N, 129T2 Sv/EMS, and C57Bl/10). The spinal cord was prepared for light microscopy at intervals from 1 to 56 days postinjury, and the area of damaged tissue (termed lesion size) and amount of cavitation were determined by quantitative image analysis. Lesion size increased between 1 and 7 days in all strains and then decreased steadily in a wound-healing process that occurs uniquely in mice. The extent of cavitation also gradually decreased from 7 to 56 days in all strains. Although lesion area and cavitation decreased in all strains, there were significant differences in lesion size and cavitation across strains. Specifically, lesion areas in the KA-sensitive strains FVB/N, 129T2 Sv/EMS, and CD-1 were significantly larger at 56 days postinjury than in the KA-resistant strains C57Bl/6 and Balb/c. We conclude that the genetic differences that confer resistance and sensitivity to KA-induced neurotoxicity also modify the secondary degenerative processes that occur after spinal cord injury, so that resistance to excitotoxic injury leads to smaller overall lesions and a more effective wound-healing response.