Plasticity in Sublesionally Located Neurons Following Spinal Cord Injury

Central pattern generators in the brainstem and spinal cord: an overview of basic principles, similarities and differences

Central pattern generators (CPGs) are generally defined as networks of neurons capable of enabling the production of central commands, specifically controlling stereotyped, rhythmic motor behaviors. Several CPGs localized in brainstem and spinal cord areas have been shown to underlie the expression of complex behaviors such as deglutition, mastication, respiration, defecation, micturition, ejaculation, and locomotion. Their pivotal roles have clearly been demonstrated although their organization and cellular properties remain incompletely characterized. In recent years, insightful findings about CPGs have been made mainly because (1) several complementary animal models were developed; (2) these models enabled a wide variety of techniques to be used and, hence, a plethora of characteristics to be discovered; and (3) organizations, functions, and cell properties across all models and species studied thus far were generally found to be well-preserved phylogenetically. This article aims at providing an overview for non-experts of the most important findings made on CPGs in in vivo animal models, in vitro preparations from invertebrate and vertebrate species as well as in primates. Data about CPG functions, adaptation, organization, and cellular properties will be summarized with a special attention paid to the network for locomotion given its advanced level of characterization compared with some of the other CPGs. Similarities and differences between these networks will also be highlighted.

Intravenous Infusion of Mesenchymal Stem Cells Alters Motor Cortex Gene Expression in a Rat Model of Acute Spinal Cord Injury

Recent evidence has demonstrated that remote responses in the brain, as well as local responses in the injured spinal cord, can be induced after spinal cord injury (SCI). Intravenous infusion of mesenchymal stem cells (MSCs) has been shown to provide functional improvements in SCI through local therapeutic mechanisms that provide neuroprotection, stabilization of the blood-spinal cord barrier, remyelination, and axonal sprouting. In the present study, we examined the brain response that might be associated with the functional improvements induced by the infused MSCs after SCI. Genome-wide RNA profiling was performed in the motor cortex of SCI rats at 3 days post-MSC or vehicle infusion. Then, quantitative reverse transcription-polymerase chain reaction (qRT-PCR) data revealed that the "behaviorally-associated differentially expressed genes (DEGs)" were identified by the Pearson's correlation analysis with the behavioral function, suggesting that the "behaviorally-associated DEGs" may be related to the functional recovery after systemic infusion of MSCs in SCI. These results suggested that the infused MSCs alter the gene expression signature in the brain and that these expression changes may contribute to the improved function in SCI.

Neurotrophin-3 released from implant of tissue-engineered fibroin scaffolds inhibits inflammation, enhances nerve fiber regeneration, and improves motor function in canine spinal cord injury

Spinal cord injury (SCI) normally results in cell death, scarring, cavitation, inhibitory molecules release, etc., which are regarded as a huge obstacle to reconnect the injured neuronal circuits because of the lack of effective stimulus. In this study, a functional gelatin sponge scaffold was used to inhibit local inflammation, enhance nerve fiber regeneration, and improve neural conduction in the canine. This scaffold had good porosity and modified with neurotrophin‐3 (NT‐3)/fibroin complex, which showed sustained release in vitro. After the scaffold was transplanted into canine spinal cord hemisection model, hindlimb movement, and neural conduction were improved evidently. Migrating host cells, newly formed neurons with associated synaptic structures together with functional blood vessels with intact endothelium in the regenerating tissue were identified. Taken together, the results demonstrated that using bioactive scaffold could establish effective microenvironment stimuli for endogenous regeneration, providing a potential and practical strategy for treatment of spinal cord injury. © 2018 The Authors Journal of Biomedical Materials Research Part A Published by Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 2158‐2170, 2018.

Spinal Cord Stimulation and Augmentative Control Strategies for Leg Movement after Spinal Paralysis in Humans

Severe spinal cord injury is a devastating condition, tearing apart long white matter tracts and causing paralysis and disability of body functions below the lesion. But caudal to most injuries, the majority of neurons forming the distributed propriospinal system, the localized gray matter spinal interneuronal circuitry, and spinal motoneuron populations are spared. Epidural spinal cord stimulation can gain access to this neural circuitry. This review focuses on the capability of the human lumbar spinal cord to generate stereotyped motor output underlying standing and stepping, as well as full weight-bearing standing and rhythmic muscle activation during assisted treadmill stepping in paralyzed individuals in response to spinal cord stimulation. By enhancing the excitability state of the spinal circuitry, the stimulation can have an enabling effect upon otherwise "silent" translesional volitional motor control. Strategies for achieving functional movement in patients with severe injuries based on minimal translesional intentional control, task-specific proprioceptive feedback, and next-generation spinal cord stimulation systems will be reviewed. The role of spinal cord stimulation can go well beyond the immediate generation of motor output. With recently developed training paradigms, it can become a major rehabilitation approach in spinal cord injury for augmenting and steering trans- and sublesional plasticity for lasting therapeutic benefits.

CNS axon regeneration inhibitors stimulate an immediate early gene response via MAP kinase-SRF signaling

Background

CNS axon regeneration inhibitors such as Nogo and CSPGs (Chondroitin Sulfate Proteoglycans) are major extrinsic factors limiting outgrowth of severed nerve fibers. However, knowledge on intracellular signaling cascades and gene expression programs activated by these inhibitors in neurons is sparse. Herein we studied intracellular signaling cascades activated by total myelin, Nogo and CSPGs in primary mouse CNS neurons.

Results

Total myelin, Nogo and CSPGs stimulated gene expression activity of the serum response factor (SRF), a central gene regulator of immediate early (IEG) and actin cytoskeletal gene transcription. As demonstrated by pharmacological interference, SRF-mediated IEG activation by myelin, Nogo or CSPGs depended on MAP kinase, to a lesser extent on Rho-GTPase but not on PKA signaling. Stimulation of neurons with all three axon growth inhibitors activated the MAP kinase ERK. In addition to ERK activation, myelin activated the IEG c-Fos, an important checkpoint of neuronal survival vs. apoptosis. Employing Srf deficient neurons revealed that myelin-induced IEG activation requires SRF. This suggests an SRF function in mediating neuronal signaling evoked by axon regeneration associated inhibitors. Besides being a signaling target of axon growth inhibitors, we show that constitutively-active SRF-VP16 can be employed to circumvent neurite growth inhibition imposed by myelin, Nogo and CSPGs.

Conclusion

In sum, our data demonstrate that axon regeneration inhibitors such as Nogo trigger gene expression programs including an SRF-dependent IEG response via MAP kinases and Rho-GTPases.

Cotransplantation of Olfactory Ensheathing Cells and Schwann Cells Combined with Treadmill Training Promotes Functional Recovery in Rats with Contused Spinal Cords

The present study investigated the ability of cotransplantation of Schwann cells (SCs) and olfactory ensheathing cells (OECs) combined with treadmill training in facilitating neuronal plasticity and promoting hindlimb function recovery of subacute moderate thoracic (T10) spinal cord contusion in rats. Two weeks postinjury, SCs were injected directly into the lesion, while OECs were injected into the adjacent tissues. The treadmill training with the rats began postinjury on day 7, with each session lasting 20 ± 10 min per day, 5 days per week, for 10 weeks. At the 11th week postinjury, OECs were found migrating longitudinally and laterally from the injection site to the injury site through the gray and white matter, while some traveled along the central canal or pia. The SCs remained densely packed and concentrated at the transplant site. The transplanted SCs supported ingrowth of numerous, densely populated neurofilament-positive (NF+), MBP+ axons. The OECs promoted elongation of moderate NF+, GAP-43+ axons and a few MBP+ axons in parallel with OEC processes. The GFAP immunoreactivity in the spared tissue surrounding the graft of SCs and OECs at the lesion site was less intense than that in the DMEM group. Treadmill training had no effect on GFAP immunoreactivity. Treadmill training increased the number of TH-immunoreactive neurons in the gray matter of L2 spinal cord. Moreover, cotransplantation of OECs and SCs significantly increased the BBB score during 5–8 weeks postinjury alongside treadmill training between 5 and 11 weeks. Cotransplantation of OECs and SCs combined with treadmill training resulted in the highest BBB score at 4 and 11 weeks. The study details the differential mechanisms of neuronal plasticity: ( 1 ) axon growth and remyelination induced by cotransplantation of OECs and SCs and ( 2 ) neuron plasticity below the lesion enhanced by treadmill training. The synergistic effects of the combined strategy enhance functional recovery. This manuscript is published as part of the International Association of Neurorestoratology (IANR) supplement issue of Cell Transplantation.

Cellular therapies for treating pain associated with spinal cord injury

Spinal cord injury leads to immense disability and loss of quality of life in human with no satisfactory clinical cure. Cell-based or cell-related therapies have emerged as promising therapeutic potentials both in regeneration of spinal cord and mitigation of neuropathic pain due to spinal cord injury. This article reviews the various options and their latest developments with an update on their therapeutic potentials and clinical trialing.

Intracerebroventricular administration of human umbilical cord blood cells delays disease progression in two murine models of motor neuron degeneration

The lack of effective drug therapies for motor neuron diseases (MND), and in general for all the neurodegenerative disorders, has increased the interest toward the potential use of stem cells. Among the cell therapy approaches so far tested in MND animal models, systemic injection of human cord blood mononuclear cells (HuCB-MNCs) has proven to reproducibly increase, although modestly, the life span of SOD1G93A mice, a model of familial amyotrophic lateral sclerosis (ALS), even if only few transplanted cells were found in the damaged areas. In attempt to improve the potential efficacy of these cells in the central nervous system, we examined the effect and distribution of Hoechst 33258-labeled HuCB-MNCs after a single bilateral intracerberoventricular injection in two models of motor neuron degeneration, the transgenic SOD1G93A and wobbler mice. HuCB-MNCs significantly ameliorated symptoms progression in both mouse models and prolonged survival in SOD1G93A mice. They were localized in the lateral ventricles, even 4 months after administration. However, HuCB-MNCs were not found in the spinal cord ventral horns. This evidence strengthens the hypothesis that the beneficial role of transplanted cells is not due to cell replacement but is rather associated with the production and release of circulating protective factors that may act both at the central and/or peripheral levels. In particular, we show that HuCB-MNCs release a series of cytokines and chemokines with antiinflammatory properties that could be responsible of the functional improvement of mouse models of motor neuron degenerative disorders.

Contribution of the delayed-rectifier potassium channel Kv2.1 to acute spinal cord injury in rats

Recent studies have reported that delayed-rectifier Kv channels regulate apoptosis in the nervous system. Herein, we investigated changes in the expression of the delayed-rectifier Kv channels Kv1.2, Kv2.1, and Kv3.1 after acute spinal cord injury (SCI) in rats. We performed RT-PCR analysis and found an increase in the level of Kv2.1 mRNA after SCI but no significant changes in the levels of Kv1.2 and Kv3.1 mRNA. Western blot analysis revealed that Kv2.1 protein levels rapidly decreased and then dramatically increased from 1 day, whereas Kv3.1b protein levels gradually and sharply decreased at 5 days. Kv1.2 protein levels did not change significantly. In addition, Kv2.1 clusters were disrupted in the plasma membranes of motor neurons after SCI. Interestingly, the expressional changes and translocation of Kv2.1 were consistent with the apoptotic changes on day 1. Therefore, these results suggest that Kv2.1 channels probably contribute to neuronal cell responses to SCI.

Predifferentiated embryonic stem cells promote functional recovery after spinal cord compressive injury

We tested the effects of mouse embryonic stem cells (mES) grafts in mice spinal cord injury (SCI). Young adult female C57/Bl6 mice were subjected to laminectomy at T9 and 1-minute compression of the spinal cord with a vascular clip. Four groups were analyzed: laminectomy (Sham), injured (SCI), vehicle (DMEM), and mES-treated (EST). mES pre-differentiated with retinoic acid were injected (8 x 10(5) cells/2 microl) into the lesion epicenter, 10 min after SCI. Basso mouse scale (BMS) and Global mobility test (GMT) were assessed weekly up to 8 weeks, when morphological analyses were performed. GMT analysis showed that EST animals moved faster (10.73+/-0.9076, +/-SEM) than SCI (5.581+/-0.2905) and DMEM (5.705+/-0.2848), but slower than Sham animals (15.80+/-0.3887, p<0.001). By BMS, EST animals reached the final phase of locomotor recovery (3.872+/-0.7112, p<0.01), while animals of the SCI and DMEM groups improved to an intermediate phase (2.037+/-0.3994 and 2.111+/-0.3889, respectively). White matter area and number of myelinated nerve fibers were greater in EST (46.80+/-1.24 and 279.4+/-16.33, respectively) than the SCI group (39.97+/-0.925 and 81.39+/-8.078, p<0.05, respectively). EST group also presented better G-ratio values when compared with SCI group (p<0.001). Immunohistochemical revealed the differentiation of transplanted cells into astrocytes, oligodendrocytes, and Schwann cells, indicating an integration of transplanted cells with host tissue. Ultrastructural analysis showed, in the EST group, better tissue preservation and more remyelination by oligodendrocytes and Schwann cells than the other groups. Our results indicate that acute transplantation of predifferentiated mES into the injured spinal cord increased the spared white matter and number of nerve fibers, improving locomotor function.

What neurobiology cannot tell us about addiction

Molecular neurobiological studies have yielded enormous amounts of valuable information about neuronal response mechanisms and their adaptive changes. However, in relation to addiction this information is of limited value because almost every cell function appears to be involved. Thus it tells us only that neurons adapt to 'addictive drugs' as they do to all sorts of other functional disturbances. This information may be of limited help in the development of potential auxiliary agents for treatment of addiction. However, a reductionist approach which attempts to analyse addiction at ever finer levels of structure and function, is inherently incapable of explaining what causes these mechanisms to be brought into play in some cases and not in others, or by self-administration of a drug but not by passive exposure. There is abundant evidence that psychological, social, economic and specific situational factors play important roles in initiating addiction, in addition to genetic and other biological factors. Therefore, if we hope to be able to make predictions at any but a statistical level, or to develop effective means of prevention, it is necessary to devise appropriate integrative approaches to the study of addiction, rather than pursue an ever-finer reductive approach which leads steadily farther away from the complex interaction of drug, user, environment and specific situations that characterizes the problem in humans.

Dynamics of early locomotor network dysfunction following a focal lesion in an in vitro model of spinal injury

It is unclear how a localized spinal cord injury may acutely affect locomotor networks of segments initially spared by the lesion. To investigate the process of secondary damage following spinal injury, we used the in vitro model of the neonatal rat isolated spinal cord with transverse barriers at the low thoracic-upper lumbar region to allow focal application of kainate in hypoxic and aglycemic solution (with reactive oxygen species). The time-course and nature of changes in spinal locomotor networks downstream of the lesion site were investigated over the first 24 h, with electrophysiological recordings monitoring fictive locomotion (alternating oscillations between flexor and extensor motor pools on either side) and correlating any deficit with histological alterations. The toxic solution irreversibly suppressed synaptic transmission within barriers without blocking spinal reflexes outside. This effect was focally associated with extensive white matter damage and ventral gray neuronal loss. Although cell losses were < 10% outside barriers, microglial activation with neuronal phagocytosis was detected. Downstream motor networks still generated locomotor activity 24 h later when stimulated with N-methyl-d-aspartate (NMDA) and serotonin, but not with repeated dorsal root stimuli. In the latter case, cumulative depolarization was recorded from ventral roots at a slower rate of rise, suggesting failure to recruit network premotoneurons. Our data indicate that, within the first 24 h of injury, locomotor networks below the lesion remained morphologically intact and functional when stimulated by NMDA and serotonin. Nevertheless, microglial activation and inability to produce locomotor patterns by dorsal afferent stimuli suggest important challenges to long-term network operation.

Exercise-induced motor improvement after complete spinal cord transection and its relation to expression of brain-derived neurotrophic factor and presynaptic markers

BACKGROUND

It has been postulated that exercise-induced activation of brain-derived neurotrophic factor (BDNF) may account for improvement of stepping ability in animals after complete spinal cord transection. As we have shown previously, treadmill locomotor exercise leads to up-regulation of BDNF protein and mRNA in the entire neuronal network of intact spinal cord. The questions arise: (i) how the treadmill locomotor training, supplemented with tail stimulation, affects the expression of molecular correlates of synaptic plasticity in spinal rats, and (ii) if a response is related to BDNF protein level and distribution. We investigated the effect of training in rats spinalized at low thoracic segments on the level and distribution of BDNF immunoreactivity (IR) in ventral quadrants of the lumbar segments, in conjunction with markers of presynaptic terminals, synaptophysin and synaptic zinc.

RESULTS

Training improved hindlimb stepping in spinal animals evaluated with modified Basso-Beattie-Bresnahan scale. Grades of spinal trained animals ranged between 5 and 11, whereas those of spinal were between 2 and 4. Functional improvement was associated with changes in presynaptic markers and BDNF distribution. Six weeks after transection, synaptophysin IR was reduced by 18% around the large neurons of lamina IX and training elevated its expression by over 30%. The level of synaptic zinc staining in the ventral horn was unaltered, whereas in ventral funiculi it was decreased by 26% postlesion and tended to normalize after the training. Overall BDNF IR levels in the ventral horn, which were higher by 22% postlesion, were unchanged after the training. However, training modified distribution of BDNF in the processes with its predominance in the longer and thicker ones. It also caused selective up-regulation of BDNF in two classes of cells (soma ranging between 100-400 microm2 and over 1000 microm2) of the ventrolateral and laterodorsal motor nuclei.

CONCLUSION

Our results show that it is not BDNF deficit that determines lack of functional improvement in spinal animals. They indicate selectivity of up-regulation of BDNF in distinct subpopulations of cells in the motor nuclei which leads to changes of innervation targeting motoneurons, tuned up by locomotor activity as indicated by a region-specific increase of presynaptic markers.

Finger force enslaving and surplus in spinal cord injury patients

This study investigated the phenomena of finger enslaving, involuntary finger actions by non-intended fingers, and force deficit, smaller maximum force by all four fingers than the sum of individual finger maximum forces in individuals with cervical spinal cord injuries (SCI). A total of 16 subjects participated in this study: 8 with a cervical spinal cord injury and 8 controls. Each of the injured subjects had one paralyzed finger. The results showed that the efforts to produce force using any individual finger induced force production in all other fingers, suggesting finger force enslaving. The maximum force during the four-finger task was greater than the sum of the individual finger forces during single-finger tasks in the SCI group, which was reflected by positive force deficit, "force surplus". One may utilize these findings for rehabilitation of paralyzed fingers caused by cervical spinal injuries.