Strategies for regeneration and repair in spinal cord traumatic injury

Translating preclinical approaches into human application

In recent decades, several novel approaches of spinal cord repair have revealed promising findings in animal models. However, for a successful translation of these into a clinical trial in humans the specific conditions pertaining to human spinal cord injuries (SCI) have to be appreciated. Firstly, transection of the spinal cord is commonly applied in animal models, whereas spinal cord contusion is the predominant type of injury in humans, and generally leads to more extensive injury in two to three spinal cord segments. Secondly, the quadrupedal organization of locomotion in animals and the more complex autonomic functions in humans challenge the translation of animal behavior into recovery from human SCI. Thirdly, so far, no adequate animal model has been developed to resemble spastic movement disorder in human SCI. Fourthly, the extensive damage to spinal motor neurons and nerve roots in human cervical and thoracolumbar in spine trauma is but little addressed in current translational studies. This damage has direct implications for rehabilitation and repair strategies. Fifthly, there is increasing evidence for a neuronal dysfunction below the level of the lesion in chronic complete SCI. The relevance of this dysfunction for a regeneration-inducing treatment needs to be investigated. Lastly, an approach to facilitate an appropriate reconnection of regenerating tract fibers by functional training in the postacute stage has yet to be confirmed.

Strategies for spinal cord repair after injury: a review of the literature and information

INTRODUCTION

Thanks to the Internet, we can now have access to more information about spinal cord repair. Spinal cord injured (SCI) patients request more information and hospitals offer specific spinal cord repair medical consultations.

OBJECTIVE

Provide practical and relevant elements to physicians and other healthcare professionals involved in the care of SCI patients in order to provide adequate answers to their questions.

METHOD

Our literature review was based on English and French publications indexed in PubMed and the main Internet websites dedicated to spinal cord repair.

RESULTS

A wide array of research possibilities including notions of anatomy, physiology, biology, anatomopathology and spinal cord imaging is available for the global care of the SCI patient. Prevention and repair strategies (regeneration, transplant, stem cells, gene therapy, biomaterials, using sublesional uninjured spinal tissue, electrical stimulation, brain/computer interface, etc.) for the injured spinal cord are under development. It is necessary to detail the studies conducted and define the limits of these new strategies and benchmark them to the realistic medical and rehabilitation care available to these patients.

CONCLUSION

Research is quickly progressing and clinical trials will be developed in the near future. They will have to answer to strict methodological and ethical guidelines. They will first be designed for a small number of patients. The results will probably be fragmented and progress will be made through different successive steps.

Grafted human embryonic progenitors expressing neurogenin-2 stimulate axonal sprouting and improve motor recovery after severe spinal cord injury

Background

Spinal cord injury (SCI) is a widely spread pathology with currently no effective treatment for any symptom. Regenerative medicine through cell transplantation is a very attractive strategy and may be used in different non-exclusive ways to promote functional recovery. We investigated functional and structural outcomes after grafting human embryonic neural progenitors (hENPs) in spinal cord-lesioned rats.

Methods and Principal Findings

With the objective of translation to clinics we have chosen a paradigm of delayed grafting, i.e., one week after lesion, in a severe model of spinal cord compression in adult rats. hENPs were either naïve or engineered to express Neurogenin 2 (Ngn2). Moreover, we have compared integrating and non-integrating lentiviral vectors, since the latter present reduced risks of insertional mutagenesis. We show that transplantation of hENPs transduced to express Ngn2 fully restore weight support and improve functional motor recovery after severe spinal cord compression at thoracic level. This was correlated with partial restoration of serotonin innervations at lumbar level, and translocation of 5HT1A receptors to the plasma membrane of motoneurons. Since hENPs were not detectable 4 weeks after grafting, transitory expression of Ngn2 appears sufficient to achieve motor recovery and to permit axonal regeneration. Importantly, we also demonstrate that transplantation of naïve hENPs is detrimental to functional recovery.

Conclusions and Significance

Transplantation and short-term survival of Ngn2-expressing hENPs restore weight support after SCI and partially restore serotonin fibers density and 5HT1A receptor pattern caudal to the lesion. Moreover, grafting of naïve-hENPs was found to worsen the outcome versus injured only animals, thus pointing to the possible detrimental effect of stem cell-based therapy per se in SCI. This is of major importance given the increasing number of clinical trials involving cell grafting developed for SCI patients.

Comparison of the pharmacological properties of GK11 and MK801, two NMDA receptor antagonists: towards an explanation for the lack of intrinsic neurotoxicity of GK11

Over-stimulation of NMDA receptors (NMDARs) is involved in many neurodegenerative disorders. Thus, developing safe NMDAR antagonists is of high therapeutic interest. GK11 is a high affinity uncompetitive NMDAR antagonist with low intrinsic neurotoxicity, shown to be promising for treating CNS trauma. In the present study, we investigated the molecular basis of its interaction with NMDARs and compared this with the reference molecule MK801. We show, on primary cultures of hippocampal neurons, that GK11 exhibits neuroprotection properties similar to those of MK801, but in contrast with MK801, GK11 is not toxic to neurons. Using patch-clamp techniques, we also show that on NR1a/NR2B receptors, GK11 totally blocks the NMDA-mediated currents but has a six-fold lower IC(50) than MK801. On NR1a/NR2A receptors, it displays similar affinity but fails to totally prevent the currents. As NR2A is preferentially localized at synapses and NR2B at extrasynaptic sites, we investigated, using calcium imaging and patch-clamp approaches, the effects of GK11 on either synaptic or extrasynaptic NMDA-mediated responses. Here we demonstrate that in contrast with MK801, GK11 better preserve the synaptic NMDA-mediated currents. Our study supports that the selectivity of GK11 for NR2B containing receptors accounts contributes, at least partially, for its safer pharmacological profile.

Coordinated network functioning in the spinal cord: an evolutionary perspective

The successful achievement of harmonious locomotor movement results from the integrated operation of all body segments. Here, we will review current knowledge on the functional organization of spinal networks involved in mammalian locomotion. Attention will not simply be restricted to hindlimb muscle control, but by also considering the necessarily coordinated activation of trunk and forelimb muscles, we will try to demonstrate that while there has been a progressive increase in locomotor system complexity during evolution, many basic organizational features have been preserved across the spectrum from lower vertebrates through to humans. Concerning the organization of axial neuronal networks that control trunk muscles, it has been found across the vertebrate range that during locomotor movement a motor wave travels longitudinally in the spinal cord via the coupling of rhythmic segmental networks. For hindlimb activation it has been found in all species studied that the rostral lumbar segments contain the key elements for pattern generation. We also showed that rhythmic arm movements are under the control of cervical forelimb generators in quadrupeds as well as in human. Finally, it is highlighted that the coordination of quadrupedal movements during locomotion derives principally from an asymmetrical coordinating influence occurring in the caudo-rostral direction from the lumbar hindlimb networks.

Neurological aspects of spinal-cord repair: promises and challenges

During the past few years, several approaches to spinal-cord repair have been successfully established in animal models. For their use in trials of spinal-cord injury (SCI) in human beings, specific difficulties that affect the success of clinical trials have to be recognised. First, transection of the spinal cord is commonly applied in animal models, whereas contusion, which generally leads to injury in two to three segments, represents the typical injury mechanism in human beings. Second, the quadrupedal organisation of locomotion in animals and the more complex autonomic functions in human beings, challenge translation of animal behaviour into recovery from SCI in people. Third, the extensive damage of motor neurons and roots associated with spinal-cord contusion is not addressed in current translational studies. This damage has direct implications for rehabilitation strategies and functional outcome. Fourth, there is increasing evidence for a degradation of neuronal function below the level of the lesion in chronic complete SCI. The relevance of this degradation for a regeneration-inducing treatment needs to be investigated. Fifth, the prerequisites to enable appropriate reconnection of regenerating tract fibres in a postacute stage have still to be established.

Time course of acute phase in mouse spinal cord injury monitored by ex vivo quantitative MRI

During the acute phase of spinal cord injury (SCI), major alterations of white and grey matter are a key issue, which determine the neurological outcome. The present study with ex vivo quantitative high-field magnetic resonance microimaging (MRI) was intended in order to identify sensitive parameters of tissue disruption in a well-controlled mouse model of ischemic SCI. MR imaging evidenced changes as early as the second hour after the lesion in the dorsal horns, which appear swollen. After 4 h, alterations of the white matter of dorsal and lateral funiculi were reflected by a progressive loss of white/grey matter contrast with further ventral extension by the 24th hour. Diffusion tensor imaging and multi-exponential T2 measurements permitted to quantify these physicochemical, time-related, alterations during the 24-h period. This characterization of spatial and temporal evolution of SCI will contribute to better define both the most appropriate targets for future therapies and more accurate therapeutic windows. Upcoming directions include the use of these parameters on in vivo animal models and their application to clinics. Indeed, magnetic resonance techniques appear now as a major non-invasive translation tool in CNS pathologies based on the development of more appropriate pre-clinical models.

Glutathione monoethyl ester improves functional recovery, enhances neuron survival, and stabilizes spinal cord blood flow after spinal cord injury in rats

Secondary damage after spinal cord (SC) injury remains without a clinically effective drug treatment. To explore the neuroprotective effects of cell-permeable reduced glutathione monoethyl ester (GSHE), rats subjected to SC contusion using the New York University impactor were randomly assigned to receive intraperitoneally GSHE (total dose of 12 mg/kg), methylprednisolone sodium succinate (total dose of 120 mg/kg), or saline solution as vehicle. Motor function, assessed using the Basso-Beattie-Bresnahan scale for 8 weeks, was significantly better in GSHE (11.2+/-0.6, mean+/-S.E.M., n=8, at 8 weeks) than methylprednisolone (9.3+/-0.6) and vehicle (9.4+/-0.7) groups. The number of neurons in the red nuclei labeled with FluoroRuby placed caudally to the injury site was significantly higher in GSHE (158+/-9.3 mean+/-S.E.M., n=4) compared with methylprednisolone (53+/-14.7) and vehicle (46+/-16.4) groups. Differences in the amount of spared SC tissue at the epicenter and neighboring areas were not significant among experimental groups. In a second series of experiments, using similar treatment groups (n=6), regional changes in microvascular SC blood flow were evaluated for 100 min by laser-Doppler flowmetry after clip compression injury. SC blood flow fell in vehicle-treated rats 20% below baseline and increased significantly with methylprednisolone approximately 12% above baseline; changes were not greater than 5% in rats given GSHE. In conclusion, GSHE given to rats early after moderate SC contusion/compression improves functional outcome and red nuclei neuron survival significantly better than methylprednisolone and vehicle, and stabilizes SC blood flow. These results support further investigation of reduced glutathione supplementation after acute SC injury for future clinical application.

Retinoic acid synthesis by a population of NG2-positive cells in the injured spinal cord

Retinoic acid (RA) promotes growth and differentiation in many developing tissues but less is known about its influence on CNS regeneration. We investigated the possible involvement of RA in rat spinal cord injury (SCI) using the New York University (NYU) impactor to induce mild or moderate spinal cord contusion injury. Changes in RA at the lesion site were determined by measuring the activity of the enzymes for its synthesis, the retinaldehyde dehydrogenases (RALDHs). A marked increase in enzyme activity occurred by day 4 and peaked at days 8-14 following the injuries. RALDH2 was the only detectable RALDH present in the control or injured spinal cord. The cellular localization of RALDH2 was identified by immunostaining. In the noninjured spinal cord, RALDH2 was detected in oligodendroglia positive for the markers RIP and CNPase. Expression was also intense in the arachnoid membrane surrounding the spinal cord. After SCI the increase in RALDH2 was independent of the RIP- and CNPase-positive cells, which were severely depleted. Instead, RALDH2 was present in a cell type not previously identified as capable of synthesizing RA, that expressed NG2 and that was negative for markers of astrocytes, oligodendroglia, microglia, neurons, Schwann cells and immature lymphocytes. We postulate that the RALDH2- and NG2-positive cells migrate into the injured sites from the adjacent arachnoid membrane, where the RALDH2-positive cells proliferate substantially following SCI. These findings indicate that close correlations exist between RA synthesis and SCI and that RA may play a role in the secondary events that follow acute SCI.

Experimental bypass surgery between the spinal cord and caudal nerve roots for spinal cord injuries

Spinal cord injuries often cause permanent neurological deficits and are still considered as inaccessible to efficient therapy. Injured spinal cord axons are unable to spontaneously regenerate in adult mammalians. Re-establishing functional activity especially in the lower limbs by reinnervating the caudal infra-lesional territories could represent an attractive therapeutic strategy. For several years, we have studied and developed surgical bypasses using peripheral nerve grafts bridging the supra-lesional rostral spinal cord to the caudal infra-lesional lumbar roots. Main objectives were: 1- to overcome the spinal cord lesion and the consecutive glial barrier blocking the axonal regeneration; 2- to find and bring an alternative source of regenerating axons; 3- to guide those axons toward precisely definite targets (for example, lower limb muscles). We report here the results of our experimental research, which led us from animal experimental models (rodents, primates) to the first human experimentation. Limitations of the method (especially technical pitfalls) are numerous. However, we have obtained encouraging results in our attempts to "repair" the motor pathway. Functional recovery with strong evidence of centrifugal axonal regeneration from the spinal cord to the periphery has been observed. Regarding the sensory pathway, we have found evidence of centripetal axonal regeneration from the periphery toward the spinal cord. Further studies are obviously advocated, but our experimental model of spinal cord - nerve roots bypasses may be integrated in future "repair" strategies of both motor and sensory pathways following spinal cord injury.

Entorhinal cortex lesion in the mouse induces transsynaptic death of perforant path target neurons

Entorhinal cortex lesion (ECL) is a well described model of anterograde axonal degeneration, subsequent sprouting and reactive synaptogenesis in the hippocampus. Here, we show that such lesions induce transsynaptic degeneration of the target cells of the lesions pathway in the dentate gyrus. Peaking between 24 and 36 hours post-lesion, dying neurons were labeled with DeOlmos silver-staining and antisera against activated caspase 3 (CCP32), a downstream inductor of programmed cell death. Within caspase 3-positive neurons, fragmented nuclei were co-localized using Hoechst 33342 staining. Chromatin condensation and nuclear fragmentation were also evident in semithin sections and at the ultrastructural level, where virtually all caspase 3-positive neurons showed these hallmarks of apoptosis. There is a well-described upregulation of the apoptosis-inducing CD95/L system within the CNS after trauma, yet a comparison of caspase 3-staining patterns between CD95 (Ipr)- and CD95L (gld)-deficient with non-deficient mice (C57/bl6) provided no evidence for CD95L-mediated neuronal cell death in this setting. However, inhibition of NMDA receptors with MK-801 completely suppressed caspase 3 activation, pointing to glutamate neurotoxicity as the upstream inducer of the observed cell death. Thus, these data show that axonal injury in the CNS does not only damage the axotomized neurons themselves, but can also lethally affect their target cells, apparently by activating glutamate-mediated intracellular pathways of programmed cell death.

Myosin II activity is required for severing-induced axon retraction in vitro

Understanding the mechanistic basis of the response of neurons to injury is directly relevant to the development of effective therapeutic approaches aimed at the amelioration of nervous system damage. Axons retract in response to severing. We investigated the mechanism of axon retraction in response to severing in vitro, testing the hypothesis that actomyosin contractility drives severing-induced axon retraction. Axon retraction commenced within 5 min following severing and correlated with actin filament accumulation at the site of severing. Depolymerization of actin filaments prevented retraction, demonstrating that actin filaments are required for severing-induced axon retraction. Direct inhibition of myosin II, using blebbistatin, minimized axon retraction in response to severing. Blocking RhoA-kinase (ROCK), a modulator of myosin II activity, inhibited axon retraction. Similarly, inhibiting myosin light chain kinase (MLCK) with a cell-permeable pseudo-substrate peptide also inhibited axon retraction. These data demonstrate that myosin II activity is required for severing-induced axon retraction in vitro, and suggest myosin II as a target for therapeutic interventions aimed at minimizing retraction following severing in vivo.

Astrocytes as support for axonal regeneration in the central nervous system of mammals

Reactive astrocytes are one of the main impediments for axonal regeneration in the central nervous system of mammals. Using mice KO for GFAP and vimentin, we show that reinnervation occurs after an hemisection of the spinal cord, mainly through sprouting of controlateral intact serotoninergic and cortico-spinal axons, thanks to the absence of glial reactivity. This reinnervation is paralleled by the restoration of impaired locomotion of the ipselateral hindleg. Future applications to spinal cord injured patients are discussed.