Nogo-A antibodies and training reduce muscle spasms in spinal cord-injured rats

The dual role of microglia in neuropathic pain after spinal cord injury: Detrimental and protective effects

Spinal cord injury (SCI) is a debilitating condition that is frequently accompanied by neuropathic pain, resulting in significant physical and psychological harm to a vast number of individuals globally. Despite the high prevalence of neuropathic pain following SCI, the precise underlying mechanism remains incompletely understood. Microglia are a type of innate immune cell that are present in the central nervous system (CNS). They have been observed to have a significant impact on neuropathic pain following SCI. This article presents a comprehensive overview of recent advances in understanding the role of microglia in the development of neuropathic pain following SCI. Specifically, the article delves into the detrimental and protective effects of microglia on neuropathic pain following SCI, as well as the mechanisms underlying their interconversion. Furthermore, the article provides a thorough overview of potential avenues for future research in this area.

Delayed viral vector mediated delivery of neurotrophin-3 improves skilled hindlimb function and stability after thoracic contusion

Intramuscular injection of an Adeno-associated viral vector serotype 1 (AAV1) encoding Neurotrophin-3 (NT3) into hindlimb muscles 24 h after a severe T9 spinal level contusion in rats has been shown to induce lumbar spinal neuroplasticity, partially restore locomotive function and reduce spasms during swimming. Here we investigate whether a targeted delivery of NT3 to lumbar and thoracic motor neurons 48 h following a severe contusive injury aids locomotive recovery in rats. AAV1-NT3 was injected bilaterally into the tibialis anterior, gastrocnemius and rectus abdominus muscles 48-h following trauma, persistently elevating serum levels of the neurotrophin. NT3 modestly improved trunk stability, accuracy of stepping during skilled locomotion, and alternation of the hindlimbs during swimming, but it had no effect on gross locomotor function in the open field. The number of vGlut1⁺ boutons, likely arising from proprioceptive afferents, on gastrocnemius α-motor neurons was increased after injury but normalised following NT3 treatment, suggestive of a mechanism in which functional benefits may be mediated through proprioceptive feedback. Ex vivo MRI revealed substantial loss of grey and white matter at the lesion epicentre but no effect of delayed NT3 treatment to induce neuroprotection. Lower body spasms and hyperreflexia of an intrinsic paw muscle were not reliably induced in this severe injury model suggesting a more complex anatomical or physiological cause to their induction. We have shown that delayed intramuscular AAV-NT3 treatment can promote recovery in skilled stepping and coordinated swimming, supporting a role for NT3 as a therapeutic strategy for spinal injuries potentially through modulation of somatosensory feedback.

Restoration of spinal cord injury: From endogenous repairing process to cellular therapy

Spinal cord injury (SCI) disrupts neurological pathways and impacts sensory, motor, and autonomic nerve function. There is no effective treatment for SCI currently. Numerous endogenous cells, including astrocytes, macrophages/microglia, and oligodendrocyte, are involved in the histological healing process following SCI. By interfering with cells during the SCI repair process, some advancements in the therapy of SCI have been realized. Nevertheless, the endogenous cell types engaged in SCI repair and the current difficulties these cells confront in the therapy of SCI are poorly defined, and the mechanisms underlying them are little understood. In order to better understand SCI and create new therapeutic strategies and enhance the clinical translation of SCI repair, we have comprehensively listed the endogenous cells involved in SCI repair and summarized the six most common mechanisms involved in SCI repair, including limiting the inflammatory response, protecting the spared spinal cord, enhancing myelination, facilitating neovascularization, producing neurotrophic factors, and differentiating into neural/colloidal cell lines.

Mapping the long rocky road to effective spinal cord injury therapy - A meta-review of pre-clinical and clinical research

Spinal cord injury (SCI) is a rare condition, which even after decades of research, to date still presents an incurable condition with a complex symptomatology. SCI can result in paralysis, pain, loss of sensation, bladder and sexual dysfunction, and muscle degeneration to name but a few. The large number of publications makes it difficult to keep track of current progress in the field and of the many treatment options, which have been suggested and are being proposed with increasing frequency. Scientific databases with user-oriented search options will offer possible solutions, but they are still mostly in the development phase. In this meta-analysis, we summarize and narrow down SCI therapeutic approaches applied in pre-clinical and clinical research. Statistical analyses of treatment clusters - assorted after counting annual publication numbers in PubMed and ClinicalTrials.gov databases - were performed to allow the comparison of research foci and of their translation efficacy into clinical therapy. Using the example of SCI research, our findings demonstrate the challenges that come with the accelerating research progress - an issue, which many research fields are faced with today. The analyses point out similarities and differences in the prioritization of SCI research in pre-clinical versus clinical therapy strategies. Moreover, the results demonstrate the rapidly growing importance of modern (bio-)engineering technologies.

Peripherally delivered Adeno-associated viral vectors for spinal cord injury repair

Via the peripheral and autonomic nervous systems, the spinal cord directly or indirectly connects reciprocally with many body systems (muscular, intengumentary, respiratory, immune, digestive, excretory, reproductive, cardiovascular, etc). Accordingly, spinal cord injury (SCI) can result in catastrophe for multiple body systems including muscle paralysis affecting movement and loss of normal sensation, as well as neuropathic pain, spasticity, reduced fertility and autonomic dysreflexia. Treatments and cure for an injured spinal cord will likely require access of therapeutic agents across the blood-CNS (central nervous system) barrier. However, some types of repair within the CNS may be possible by targeting treatment to peripherally located cells or by delivering Adeno-Associated Viral vectors (AAVs) by peripheral routes (e.g., intrathecal, intravenous). This review will consider some future possibilities for SCI repair generated by therapeutic peripheral gene delivery. There are now six gene therapies approved worldwide as safe and effective medicines of which three were created by modification of the apparently nonpathogenic Adeno-Associated Virus. One of these AAVs, Zolgensma, is injected intrathecally for treatment of spinal muscular atrophy in children. One day, delivery of AAVs into peripheral tissues might improve recovery after spinal cord injury in humans; we discuss experiments by us and others delivering transgenes into nerves or muscles for sensorimotor recovery in animal models of SCI or of stroke including human Neurotrophin-3. We also describe ongoing efforts to develop AAVs that are delivered to particular targets within and without the CNS after peripheral administration using capsids with improved tropisms, promoters that are selective for particular cell types, and methods for controlling the dose and duration of expression of a transgene. In conclusion, in the future, minimally invasive administration of AAVs may improve recovery after SCI with minimal side effects.

Effects of methylprednisolone and treadmill training on spinal cord injury in experimental rats

Methylprednisolone (MP) is widely used to treat clinical spinal cord injury (SCI). Treadmill training is also considered an important treatment after SCI to improve motor function in patients, resulting in an evident improvement. Therefore, the present study was designed to evaluate and contrast the effects of MP and treadmill training administered in combination or alone after SCI in adult rats. A rat spinal cord T10 contusion model was induced in Sprague-Dawley rats using an impact device. A total of 40 rats were divided into four groups (n=10 rats/group): the MP, MP + treadmill training, SCI and sham group. At 30 min after injury, MP sodium succinate was injected into the rats of the MP and MP + treadmill training groups. Treadmill training began on the second week post-trauma and was performed for 8 weeks. The results showed that MP therapy combined with treadmill training significantly ameliorated several parameters of hind limb function compared with those by MP treatment alone (all P<0.05). A significantly reduced immunopositive area of Nogo receptor and chondroitin sulfate proteoglycans and reduced relative expression of these mRNAs were found in the MP + treadmill training group (P<0.05) compared with the findings in the MP group. In conclusion, the present study indicated that combined MP and treadmill training treatment improved the recovery of hind limb function in rats with SCI, thus potentially representing a promising strategy to cure SCI.

Early escitalopram administration as a preemptive treatment strategy against spasticity after contusive spinal cord injury in rats

In the majority of spinal cord injury (SCI) patients, spasticity develops in the subacute phase and chronically persists with muscle hypertonia. Among various pathological conditions underlying spasticity, upregulated expression of 5-HT receptors (5-HTR) on the spinal motor neurons due to 5-HT denervation is considered one of crucial factors for hyperexcitability of the spinal circuit. As a 5-HT signal modulator, selective serotonin re-uptake inhibitors (SSRIs) are ordinarily prescribed for diseases associated with 5-HT in the CNS, and are known for their ability to increase 5-HT levels as well as to desensitize 5-HTR. Here, we hypothesized that early SSRI administration as a preemptive treatment strategy would effectively prevent the onset of spasticity. We used a rat model of contusive SCI and administered escitalopram during the first 4 weeks after injury, which is the period required for spasticity development in rodent models. We performed a swimming test to quantify spastic behaviors and conducted the Hoffman reflex test as well as histological analyses for 5-HT_2AR and KCC2 expressions. Four weeks of escitalopram administration suppressed spastic behaviors during the swimming test and reduced the population of spasticity-strong rats. Moreover, the treatment resulted in decreased immunoreactivity of 5-HT_2AR in the spinal motor neurons. Result of the H-reflex test and membrane expression of KCC2 were not significantly altered. In summary, early escitalopram administration could prevent the onset of spastic behaviors via regulation of 5-HT system after SCI, but could not modulate exaggerated spinal reflex. Our results suggest a novel application of SSRIs for preventative treatment of spasticity.

Botulinum Toxin Type A Possibly Affects Cav3.2 Calcium Channel Subunit in Rats with Spinal Cord Injury-Induced Muscle Spasticity

INTRODUCTION

Spinal cord injury (SCI) often causes muscle spasticity, which can be inhibited by using calcium channel blocker. Botulinum toxin type A (BoT-A) shows therapeutic efficacy on spasticity and may exert inhibitory effects on the calcium channel.

METHODS

A rat model with muscle spasticity was established after SCI via contusion and compression. Different concentrations (0, 1, 3 and 6 U/kg) of BoT-A Botox were injected in the extensor digitorum longus (EDL) muscles of the right hindlimb in the muscle spasticity model. The changes of muscle spasticity and calcium level in EDL muscles were measured after the establishment of SCI-induced spasticity. Cav3.2 calcium channel subunit and its mutant (M1560V) were analyzed using Western blot before (input) or after immunoprecipitation with anti-FLAG antibody, and their currents were measured in motoneurons by using whole-cell voltage clamp recordings.

RESULTS

SCI induced muscle spasticity, whereas calcium level in EDL muscles and expression of Cav3.2 was increased in the SCI model when compared with the sham group (p < 0.05). BoT-A Botox treatment significantly reduced muscle spasticity and calcium level in EDL muscles and Cav3.2 expression in a dose-dependent way (p < 0.05). The ratio of biotinylated to total Cav3.2 was reduced in the mutant (M1560V) of Cav3.2 and lower than that in the wild Cav3.2. BoT-A Botox intervention also reduced the current values of calcium channel and the ratio in a dose-dependent way (p < 0.05).

DISCUSSION

BoT-A Botox possibly attenuates SCI-induced muscle spasticity by affecting the expression of Cav3.2 calcium channel subunit in the rat models. There may be multiple mechanisms for the function of BoT-A Botox. Further work is needed to be done to address these issues.

Evaluating the effectiveness of anti-Nogo treatment in spinal cord injuries

As humans, we cannot regenerate axons within the central nervous system (CNS), therefore, making any damage to it permanent. This leads to the loss of sensory and motor function below the site of injury and can be crippling to a person’s health. Spontaneous recovery can occur from plastic changes, but it is minimal. The absence of regeneration is due to the inhibitory environment of the CNS as well as the inherent inability of CNS axons to form growth cones. Amongst many factors, one of the major inhibitory signals of the CNS environment is the myelin-associated Nogo pathway. Nogo-A, Nogo-B and Nogo-C (Nogo), stimulate the Nogo receptor, inhibiting neurite outgrowth by causing growth cones to collapse through activation of Rho Kinase (ROCK). Antibodies can be used to target this signalling pathway by binding to Nogo and thus promote the outgrowth of neuronal axons in the CNS. This use of anti-Nogo antibodies has been shown to upregulate CNS regeneration as well as drastically improve sensory and motor function in both rats and primates when coupled with adequate training. Here, we evaluate whether the experimental success of anti-Nogo at improving CNS regeneration can be carried over into the clinical setting to treat spinal cord injuries (SCI) and their symptoms successfully. Furthermore, we also discuss potential methods to improve the current treatment and any developmental obstacles.

Single-session cortical electrical stimulation enhances the efficacy of rehabilitative motor training after spinal cord injury in rats

Low neuronal cAMP levels in adults and a further decline following traumatic central nervous system (CNS) injury has been associated with the limited ability of neurons to regenerate. An approach to increase neuronal cAMP levels post injury is electrical stimulation. Stimulation as a tool to promote neuronal growth has largely been studied in the peripheral nervous system or in spared fibers of the CNS and this research suggests that a single session of electrical stimulation is sufficient to initiate a long-lasting axonal growth program. Here, we sought to promote plasticity and growth of the injured corticospinal tract with electrical cortical stimulation immediately after its spinal injury. Moreover, given the importance of rehabilitative motor training in the clinical setting and in translating plasticity into functional recovery, we applied training as a standard treatment to all rats (i.e., with or without electrical stimulation). Our findings show that electrical cortical stimulation did improve recovery in forelimb function compared to the recovery in unstimulated animals. This recovery is likely linked to increased corticospinal tract plasticity as evidenced by a significant increase in sprouting of collaterals above the lesion site, but not to increased regenerative growth through the lesion itself.

Intramuscular Injection of Adenoassociated Virus Encoding Human Neurotrophic Factor 3 and Exercise Intervention Contribute to Reduce Spasms after Spinal Cord Injury

Limb spasms are phenomena of hyperreflexia that occur after spinal cord injury. Currently, the clinical treatment is less than ideal. Our goal is to develop a combination therapy based on individualized medicine to reduce spasticity after spinal cord injury. In this study, rats received a severe contusive injury at the T9 segment of the spinal cord, followed by gene therapy with adenoassociated virus encoding human neurotrophic factor 3 (AAV-NT3) and a 2-week exercise program starting at 4 weeks after injury. We quantified the frequency of spasms during a swimming test at 4 and 6 weeks after injury and confirmed the results of the swimming test by measuring the H-reflex of the plantar muscle. We obtained weekly hind limb exercise scores to assess the effect of the interventions in hind limb motor function improvement. Then, we used immunofluorescence to observe the immunoreactivity of spinal motor neurons, synaptophysin, cholinergic interneurons, and GABAergic interneurons. We also measured the expression of KCC2 in the spinal cord by western blot. We found that AAV-NT3 gene therapy, exercise, and combination therapy all attenuated the frequency of spasms in the swimming test conducted at 6 weeks after spinal cord injury and increased rate-dependent depression of H-reflex. Combination therapy was significantly superior to AAV-NT3 alone in protecting motor neurons. Recovery of KCC2 expression was significantly greater in rats treated with combination therapy than in the exercise group. Combination therapy was also significantly superior to individual therapies in remodeling spinal cord neurons. Our study shows that the combination of AAV-NT3 gene therapy and exercise can alleviate muscle spasm after spinal cord injury by altering the excitability of spinal interneurons and motor neurons. However, combination therapy did not show a significant additive effect, which needs to be improved by adjusting the combined strategy.

Effects of Treadmill Training Combined with Serotonergic Interventions on Spasticity after Contusive Spinal Cord Injury

Spasticity usually emerges during the course of recovery from spinal cord injury (SCI). While medications and physical rehabilitation are prescribed to alleviate spastic symptoms, the insufficiency of their effects remains an important problem to be addressed. Given the challenges associated with increasing the dose of medication, we hypothesized that a combination therapy with medication and physical rehabilitation can be effective. Therefore, we examined the effects of treadmill training (TMT) along with serotonergic medication using a spastic rat model after contusive injury. Spasticity-strong SCI rats were selected 4 weeks after SCI and received one of the following interventions for 2 weeks: only TMT, TMT with fluoxetine (a selective serotonin re-uptake inhibitor), TMT with cyproheptadine (a 5-HT₂ receptor antagonist), only fluoxetine, or only cyproheptadine. We performed the swimming test to quantify the frequency of spastic behaviors. We also evaluated hindlimb locomotor functions every week. At the end of the intervention, we examined the Hoffman reflex from the plantar muscle and the immunoreactivity of the 5-HT_2A receptor in spinal cord tissues. While the TMT group and cyproheptadine-treated groups showed decreased spastic behaviors and reduction in spinal hyperreflexia, the fluoxetine-treated group showed the opposite effect, even with TMT. Moreover, TMT suppressed the expression of the 5-HT_2A receptor in the lumbar spinal motor neurons, while cyproheptadine treatment did not change it. We did not observe any differences in locomotor functions between the groups. Taken together, our findings indicate that TMT and cyproheptadine significantly alleviated spastic symptoms, but did not show synergistic or additive effects.

Neuropathic pain and spasticity: intricate consequences of spinal cord injury

STUDY DESIGN

The 2016 International Spinal Cord Society Sir Ludwig Guttmann Lecture.

OBJECTIVES

The aim of this review is to identify different symptoms and signs of neuropathic pain and spasticity after spinal cord injury (SCI) and to present different methods of assessing them. The objective is to discuss how a careful characterization of different symptoms and signs, and a better translation of preclinical findings may improve our understanding of the complex and entangled mechanisms of neuropathic pain and spasticity.

METHODS

A MEDLINE search was performed using the following terms: 'pain', 'neuropathic', 'spasticity', 'spasms' and 'spinal cord injury'.

RESULTS

This review identified different domains of neuropathic pain and spasticity after SCI and methods to assess them in preclinical and clinical research. Different factors important for pain description include location, onset, pain descriptors and somatosensory function, while muscle tone, spasms, reflexes and clonus are important aspects of spasticity. Similarities and differences between neuropathic pain and spasticity are discussed.

CONCLUSIONS

Understanding that neuropathic pain and spasticity are multidimensional consequences of SCI, and a careful examination and characterization of the symptoms and signs, are a prerequisite for understanding the relationship between neuropathic pain and spasticity and the intricate underlying mechanisms.

The swimming test is effective for evaluating spasticity after contusive spinal cord injury

Spasticity is a frequent chronic complication in individuals with spinal cord injury (SCI). However, the severity of spasticity varies in patients with SCI. Therefore, an evaluation method is needed to determine the severity of spasticity. We used a contusive SCI model that is suitable for clinical translation. In this study, we examined the feasibility of the swimming test and an EMG for evaluating spasticity in a contusive SCI rat model. Sprague-Dawley rats received an injury at the 8th thoracic vertebra. Swimming tests were performed 3 to 6 weeks after SCI induction. We placed the SCI rats into spasticity-strong or spasticity-weak groups based on the frequency of spastic behavior during the swimming test. Subsequently, we recorded the Hoffman reflex (H-reflex) and examined the immunoreactivity of serotonin (5-HT) and its receptor (5-HT_2A) in the spinal tissues of the SCI rats. The spasticity-strong group had significantly decreased rate-dependent depression of the H-reflex compared to the spasticity-weak group. The area of 5-HT_2A receptor immunoreactivity was significantly increased in the spasticity-strong group. Thus, both electrophysiological and histological evaluations indicate that the spasticity-strong group presented with a more severe upper motor neuron syndrome. We also observed the groups in their cages for 20 hours. Our results suggest that the swimming test provides an accurate evaluation of spasticity in this contusive SCI model. We believe that the swimming test is an effective method for evaluating spastic behaviors and developing treatments targeting spasticity after SCI.

Direct, long-term intrathecal application of therapeutics to the rodent CNS

Systemic application of therapeutics to the CNS tissue often results in subtherapeutic drug levels, because of restricted and selective penetration through the blood-brain barrier (BBB). Here, we give a detailed description of a standardized technique for intrathecal drug delivery in rodents, analogous to the technique used in humans. The intrathecal drug delivery method bypasses the BBB and thereby offers key advantages over oral or intravenous administration, such as maximized local drug doses with minimal systemic side effects. We describe how to deliver antibodies or drugs over several days or weeks from a s.c. minipump and a fine catheter inserted into the subdural space over the spinal cord (20 min operative time) or into the cisterna magna (10 min operative time). Drug levels can be sampled by quick and minimally invasive cerebrospinal fluid (CSF) collection from the cisterna magna (5 min procedure time). These techniques enable targeted application of any compound to the CNS for therapeutic studies in a wide range of CNS disease rodent models. Basic surgery skills are helpful for carrying out the procedures described in this protocol.

Intramuscular Neurotrophin-3 normalizes low threshold spinal reflexes, reduces spasms and improves mobility after bilateral corticospinal tract injury in rats

Brain and spinal injury reduce mobility and often impair sensorimotor processing in the spinal cord leading to spasticity. Here, we establish that complete transection of corticospinal pathways in the pyramids impairs locomotion and leads to increased spasms and excessive mono- and polysynaptic low threshold spinal reflexes in rats. Treatment of affected forelimb muscles with an adeno-associated viral vector (AAV) encoding human Neurotrophin-3 at a clinically-feasible time-point after injury reduced spasticity. Neurotrophin-3 normalized the short latency Hoffmann reflex to a treated hand muscle as well as low threshold polysynaptic spinal reflexes involving afferents from other treated muscles. Neurotrophin-3 also enhanced locomotor recovery. Furthermore, the balance of inhibitory and excitatory boutons in the spinal cord and the level of an ion co-transporter in motor neuron membranes required for normal reflexes were normalized. Our findings pave the way for Neurotrophin-3 as a therapy that treats the underlying causes of spasticity and not only its symptoms.

DOI:http://dx.doi.org/10.7554/eLife.18146.001

Injuries to the brain and spinal cord cause disability in millions of people worldwide. Physical rehabilitation can restore some muscle control and improve mobility in affected individuals. However, no current treatments provide long-term relief from the unwanted muscle contractions and spasms that affect as many as 78% of people with a spinal cord injury. These spasms can seriously hamper a person’s ability to carry out day-to-day tasks and get around independently. A few treatments can help in the short term but have side effects; indeed while Botox injections are used to paralyse the muscle, these also reduce the chances of useful improvements. As such, better therapies for muscle spasms are needed; especially ones that reduce spasms in the arms.

Rats with injuries to the spinal cord between their middle to lower back typically develop spasms in their legs or tail, and rat models have helped scientists begin to understand why these involuntary movements occur. Now, Kathe et al. report that cutting one specific pathway that connects the brain to the spinal cord in anesthetised rats leads to the development of spasms in the forelimbs as well. Several months after the surgery, the rats had spontaneous muscle contractions in their forelimbs and walked abnormally. Further experiments showed that some other neural pathways in the rats became incorrectly wired and hyperactive and that this resulted in the abnormal movements.

Next, Kathe et al. asked whether using gene therapy to deliver a protein that is required for neural circuits to form between muscles and the spinal cord (called neurotrophin-3) would stop the involuntary movements in the forelimbs. Delivering the gene therapy directly into the forelimb muscles of the disabled rats a day after their injury increased the levels of neurotrophin-3 in these muscles. Rats that received this treatment had fewer spasms and walked better than those that did not. Further experiments confirmed that this was because the rats’ previously hyperactive and abnormally wired neural circuits became more normal after the treatment.

Together these results suggest that neurotrophin-3 might be a useful treatment for muscle spasms in people with spinal injury. There have already been preliminary studies in people showing that treatment with neurotrophin-3 is safe and well tolerated. Future studies are needed to confirm that it could be useful in humans.

DOI:http://dx.doi.org/10.7554/eLife.18146.002

Central nervous system regenerative failure: role of oligodendrocytes, astrocytes, and microglia

Animal studies are now showing the exciting potential to achieve significant functional recovery following central nervous system (CNS) injury by manipulating both the inefficient intracellular growth machinery in neurons, as well as the extracellular barriers, which further limit their regenerative potential. In this review, we have focused on the three major glial cell types: oligodendrocytes, astrocytes, and microglia/macrophages, in addition to some of their precursors, which form major extrinsic barriers to regrowth in the injured CNS. Although axotomized neurons in the CNS have, at best, a limited capacity to regenerate or sprout, there is accumulating evidence that even in the adult and, especially after boosting their growth motor, neurons possess the capacity for considerable circuit reorganization and even lengthy regeneration when these glial obstacles to neuronal regrowth are modified, eliminated, or overcome.

Synaptic rearrangement following axonal injury: Old and new players

Following axotomy, the contact between motoneurons and muscle fibers is disrupted, triggering a retrograde reaction at the neuron cell body within the spinal cord. Together with chromatolysis, a hallmark of such response to injury is the elimination of presynaptic terminals apposing to the soma and proximal dendrites of the injured neuron. Excitatory inputs are preferentially eliminated, leaving the cells under an inhibitory influence during the repair process. This is particularly important to avoid glutamate excitotoxicity. Such shift from transmission to a regeneration state is also reflected by deep metabolic changes, seen by the regulation of several genes related to cell survival and axonal growth. It is unclear, however, how exactly synaptic stripping occurs, but there is substantial evidence that glial cells play an active role in this process. In one hand, immune molecules, such as the major histocompatibility complex (MHC) class I, members of the complement family and Toll-like receptors are actively involved in the elimination/reapposition of presynaptic boutons. On the other hand, plastic changes that involve sprouting might be negatively regulated by extracellular matrix proteins such as Nogo-A, MAG and scar-related chondroitin sulfate proteoglycans. Also, neurotrophins, stem cells, physical exercise and several drugs seem to improve synaptic stability, leading to functional recovery after lesion. This article is part of a Special Issue entitled 'Neuroimmunology and Synaptic Function'.

Nogo limits neural plasticity and recovery from injury

The expression of Nogo-A and the receptor NgR1 limits the recovery of adult mammals from central nervous system injury. Multiple studies have demonstrated efficacy from targeting this pathway for functional recovery and neural repair after spinal cord trauma, ischemic stroke, optic nerve injury and models of multiple sclerosis. Recent molecular studies have added S1PR2 as a receptor for the amino terminal domain of Nogo-A, and have demonstrated shared components for Nogo-A and CSPG signaling as well as novel Nogo antagonists. It has been recognized that neural repair involves plasticity, sprouting and regeneration. A physiologic role for Nogo-A and NgR1 has been documented in the restriction of experience-dependent plasticity with maturity, and the stability of synaptic, dendritic and axonal anatomy.

Combination treatment with chondroitinase ABC in spinal cord injury--breaking the barrier

After spinal cord injury (SCI), re-establishing functional circuitry in the damaged central nervous system (CNS) faces multiple challenges including lost tissue volume, insufficient intrinsic growth capacity of adult neurons, and the inhibitory environment in the damaged CNS. Several treatment strategies have been developed over the past three decades, but successful restoration of sensory and motor functions will probably require a combination of approaches to address different aspects of the problem. Degradation of the chondroitin sulfate proteoglycans with the chondroitinase ABC (ChABC) enzyme removes a regeneration barrier from the glial scar and increases plasticity in the CNS by removing perineuronal nets. its mechanism of action does not clash or overlap with most of the other treatment strategies, making ChABC an attractive candidate as a combinational partner with other methods. in this article, we review studies in rat SCI models using ChABC combined with other treatments including cell implantation, growth factors, myelin-inhibitory molecule blockers, and ion channel expression. We discuss possible ways to optimize treatment protocols for future combinational studies. To date, combinational therapies with ChABC have shown synergistic effects with several other strategies in enhancing functional recovery after SCI. These combinatorial approaches can now be developed for clinical application.

Models of CNS injury in the nonhuman primate: A new era for treatment strategies

Central nervous system (CNS) injuries affect all levels of society indiscriminately, resulting in functional and behavioral deficits with devastating impacts on life expectancies, physical and emotional wellbeing. Considerable literature exists describing the pathophysiology of CNS injuries as well as the cellular and molecular factors that inhibit regrowth and regeneration of damaged connections. Based on these data, numerous therapeutic strategies targeting the various factors of repair inhibition have been proposed and on-going assessment has demonstrated some promising results in the laboratory environ. However, several of these treatment strategies have subsequently been taken into clinical trials but demonstrated little to no improvement in patient outcomes. As a result, options for clinical interventions following CNS injuries remain limited and effective restorative treatment strategies do not as yet exist. This review discusses some of the current animal models, with focus on nonhuman primates, which are currently being modeled in the laboratory for the study of CNS injuries. Last, we review the current understanding of the mechanisms underlying repair/regrowth inhibition and the current trends in experimental treatment strategies that are being assessed for potential translation to clinical applications.

The dark side of neuroplasticity

Whether dramatic or modest, recovery of neurological function after spinal cord injury (SCI) is greatly due to neuroplasticity--the process by which the nervous system responds to injury by establishing new synaptic connections or by altering the strength of existing synapses. However, the same neuroplasticity that allows locomotor function to recover also produces negative consequences such as pain and dysfunction of organs controlled by the autonomic nervous system. In this review we focus specifically on structural neuroplasticity (the growth of new synaptic connections) after SCI and on the consequent development of pain and autonomic dysreflexia, a condition of episodic hypertension. Neuroplasticity after SCI is stimulated by the deafferentation of spinal neurons below the lesion and by the expression of growth-promoting neurotrophins such as nerve growth factor (NGF). A broad range of therapeutic strategies that affect neuroplasticity is being developed for the treatment of SCI. At one end of the spectrum are therapeutic strategies that directly or indirectly increase NGF in the injured spinal cord, and have the most robust effects on neuroplasticity. At the other end of the spectrum are neuroprotective strategies focused on supporting and rescuing uninjured, or partially injured, axons; these might limit the deafferentation stimulus for neuroplasticity. In the middle of this spectrum are strategies that block axon growth inhibitors without necessarily providing a growth stimulus. The literature supports the view that the negative consequences of neuroplasticity develop more commonly with therapies that directly stimulate nerve growth than they develop in the untreated injured cord. Compared to these conditions, neuroplasticity with negative outcomes is less prevalent after treatments that that neutralize axon growth inhibitors, and least apparent after strategies that promote neuroprotection.

Anti-Nogo-A and training: can one plus one equal three?

Following spinal cord injury (SCI) the adult central nervous system (CNS) has a limited but substantial capacity for repair and plastic reorganisation. The degree of reorganisation is determined by a number of factors such as the extent and location of the lesion, the remaining circuit activity within the CNS and the age at injury. However, even in the best cases this spontaneous reorganisation does not lead to full recovery of the affected behaviour but instead often results in a functionally successful but compensatory strategy. Current SCI research focuses on enhancing fibre tract (re-)growth and recovery processes. Two currently promising approaches are the neutralisation of CNS growth inhibitory factors, and rehabilitative training of remaining networks. Independently, both approaches can lead to substantial functional recovery and anatomical reorganisation. In this review we focus on Nogo-A, a neurite growth inhibitory protein present in the adult CNS, and its role in regenerative and plastic growth following SCI. We then discuss the efforts of rehabilitative training and the potential combination of the two therapies.

Plasticity after spinal cord injury: relevance to recovery and approaches to facilitate it

Motor, sensory, and autonomic functions can spontaneously return or recover to varying extents in both humans and animals, regardless of the traumatic spinal cord injury (SCI) level and whether it was complete or incomplete. In parallel, adverse and painful functions can appear. The underlying mechanisms for all of these diverse functional changes are summarized under the term plasticity. Our review will describe what is known regarding this phenomenon after traumatic SCI and focus on its relevance to motor and sensory recovery. Although it is still somewhat speculative, plasticity can be found throughout the neuraxis and includes various changes ranging from alterations in the properties of spared neuronal circuitries, intact or lesioned axon collateral sprouting, and synaptic rearrangements. Furthermore, we will discuss a selection of potential approaches for facilitating plasticity as possible SCI treatments. Because a mechanism underlying spontaneous plasticity and recovery might be motor activity and the related neuronal activity, activity-based therapies are being used and investigated both clinically and experimentally. Additional pharmacological and gene-delivery approaches, based on plasticity being dependent on the delicate balance between growth inhibition and promotion as well as the basic intrinsic growth ability of the neurons themselves, have been found to be effective alone and in combination with activity-based therapies. The positive results have to be tempered with the reality that not all plasticity is beneficial. Therefore, a tremendous number of questions still need to be addressed. Ultimately, answers to these questions will enhance plasticity's potential for improving the quality of life for persons with SCI.

Rehabilitative training and plasticity following spinal cord injury

Rehabilitative training is currently one of the most successful treatments to promote functional recovery following spinal cord injury. Nevertheless, there are many unanswered questions including the most effective and beneficial design, and the mechanisms underlying the training effects on motor recovery. Furthermore, rehabilitative training will certainly be combined with pharmacological treatments developed to promote the "repair" of the injured spinal cord. Thus, insight into training-induced mechanisms will be of great importance to fine tune such combined treatments. In this review we address current challenges of rehabilitative training and mechanisms involved in promoting motor recovery with the focus on animal models. These challenges suggest that although rehabilitative training appears to be a relatively straight forward treatment approach, more research is needed to optimize its effect on functional outcome in order to enhance our chances of success when combining pharmacological treatments promoting axonal growth and rehabilitative training in the clinic.