Enhanced Voluntary Exercise Improves Functional Recovery following Spinal Cord Injury by Impacting the Local Neuroglial Injury Response and Supporting the Rewiring of Supraspinal Circuits

Exercise Volume Can Modulate the Regenerative Response to Spinal Cord Injury in Mice

Traumatic spinal cord injury (SCI) causes debilitating motor and sensory deficits that impair functional performance, and physical rehabilitation is currently the only established therapeutic reality in the clinical setting. In this study, we aimed to assess the effect of exercise of different volume and timing of intervention on functional recovery and neuromuscular regeneration in a mouse model of compressive SCI. Mice were assigned to one of four groups: laminectomy only (SHAM); injured, without treadmill training (SCI); injured, treadmill trained for 10 min until day 56 postinjury (TMT1); and injured, treadmill trained for two 10-min cycles with a 10-min pause between them until day 28 postinjury followed by the TMT1 protocol until day 56 postinjury (TMT3). On day 7 postinjury, animals started an eight-week treadmill-training exercise protocol and were trained three times a week. TMT3 mice had the best results in terms of neuroregeneration, functional recovery, and muscle plasticity as measured by functional and morphometric parameters. In conclusion, the volume of exercise can modulate the quality of the regenerative response to injury, when started in the acute phase and adjusted according to the inflammatory window.

The effect of early mobilisation (< 14 days) on pathophysiological and functional outcomes in animals with induced spinal cord injury: a systematic review with meta-analysis

INTRODUCTION

The optimum time to mobilise (standing, walking) following spinal cord injury (SCI) is unknown but may have implications for patient outcomes. There are no high-quality experimental studies that examine this issue, with a paucity of guidance for clinicians. Pre-clinical studies lead research in this field and can contribute to knowledge and support future clinical practice.

OBJECTIVE

to evaluate the effect of early compared to no mobilisation on pathophysiological and functional outcomes in animals with induced SCI.

METHODS

A systematic review with meta-analysis was conducted by searching pre-clinical literature in MEDLINE (PubMed), Embase (Ovid), Web of Science, OpenGrey, and EThOS (June 2023). Studies were included of any research method giving numerical results comparing pathophysiological and functional outcomes in rats and mice mobilised within 14-days of induced SCI to those that did not mobilise. Data were synthesised using random-effects meta-analyses. The quality of the evidence was assessed using the CAMARADES checklist. The certainty of findings was reported using the GRADE approach. This study is registered on PROSPERO (CRD42023437494).

RESULTS

Seventeen studies met the inclusion criteria. Outcomes found that Brain Derived Neurotrophic Factor levels were greater in those that initiated mobilisation within 14-days of SCI compared to the groups that did not. Mobilisation initiated within 14-days of SCI was also associated with statistically significant functional gains: (Basso, Beattie and Bresnahan locomotor rating score (BBB) = 2.13(0-21), CI 1.43, 2.84, Ladder Rung Walking Task = - 12.38(0-100), CI 20.01, - 4.76). Meta-analysis identified the greatest functional gains when mobilisation was initiated within 3 days of SCI (BBB = 3.00, CI 2.31-3.69, p < 0.001), or when delivered at low intensity (BBB = 2.88, CI 2.03-3.70, p < 0.001). Confidence in the findings from this review was low to moderate due to the risk of bias and mixed methodological quality.

CONCLUSION

Mobilisation instigated within 14-days of injury, may be an effective way of improving functional outcomes in animal models following SCI, with delays potentially detrimental to recovery. Outcomes from this study support further research in this field to guide future clinical practice.

Exercise therapy guides cortical reorganization after midthoracic spinal contusion to enhance control of lower thoracic muscles, supporting functional recovery

Postural control is critical for locomotion, allowing for gait changes, obstacle avoidance and navigation of rough terrain. A major problem after spinal cord injury (SCI) is regaining the control of balance to prevent falls and further injury. While the circuits for locomotor pattern generation reside in the spinal cord, postural control consists of multiple, complex networks that interact at the spinal, brainstem and cortical levels. After complete SCI, cortical reorganization establishes novel control of trunk musculature that is required for weight-supported stepping. In this study, we examined the impact of exercise therapy on cortical reorganization in the more clinically relevant models of both moderate and severe midthoracic contusion injury in the rat. Results demonstrate that both spontaneous recovery and therapy induced recovery of weight-supported stepping utilize cortical reorganization. Moreover, exercise therapy further improves outcome by enhancing cortical control of lower thoracic muscles enabling improvements in interlimb coordination associated with improved balance that increases weight-supported stepping. The outcome of this study suggest that cortical control of posture is key to functional improvement in locomotion. This information can be used to improve the timing and type of therapy after SCI by considering changes along the entire neural axis.

Rehabilitation enhances epothilone-induced locomotor recovery after spinal cord injury

Microtubule stabilization through epothilones is a promising preclinical therapy for functional recovery following spinal cord injury that stimulates axon regeneration, reduces growth-inhibitory molecule deposition and promotes functional improvements. Rehabilitation therapy is the only clinically validated approach to promote functional improvements following spinal cord injury. However, whether microtubule stabilization can augment the beneficial effects of rehabilitation therapy or act in concert with it to further promote repair remains unknown. Here, we investigated the pharmacokinetic, histological and functional efficacies of epothilone D, epothilone B and ixabepilone alone or in combination with rehabilitation following a moderate contusive spinal cord injury. Pharmacokinetic analysis revealed that ixabepilone only weakly crossed the blood-brain barrier and was subsequently excluded from further investigations. In contrast, epothilones B and D rapidly distributed to CNS compartments displaying similar profiles after either subcutaneous or intraperitoneal injections. Following injury and subcutaneous administration of epothilone B or D, rats were subjected to 7 weeks of sequential bipedal and quadrupedal training. For all outcome measures, epothilone B was efficacious compared with epothilone D. Specifically, epothilone B decreased fibrotic scaring which was associated with a retention of fibronectin localized to perivascular cells in sections distal to the lesion. This corresponded to a decreased number of cells present within the intralesional space, resulting in less axons within the lesion. Instead, epothilone B increased serotonergic fibre regeneration and vesicular glutamate transporter 1 expression caudal to the lesion, which was not affected by rehabilitation. Multiparametric behavioural analyses consisting of open-field locomotor scoring, horizontal ladder, catwalk gait analysis and hindlimb kinematics revealed that rehabilitation and epothilone B both improved several aspects of locomotion. Specifically, rehabilitation improved open-field locomotor and ladder scores, as well as improving the gait parameters of limb coupling, limb support, stride length and limb speed; epothilone B improved these same gait parameters but also hindlimb kinematic profiles. Functional improvements by epothilone B and rehabilitation acted complementarily on gait parameters leading to an enhanced recovery in the combination group. As a result, principal component analysis of gait showed the greatest improvement in the epothilone B plus rehabilitation group. Thus, these results support the combination of epothilone B with rehabilitation in a clinical setting.

Synaptogenic gene therapy with FGF22 improves circuit plasticity and functional recovery following spinal cord injury

Functional recovery following incomplete spinal cord injury (SCI) depends on the rewiring of motor circuits during which supraspinal connections form new contacts onto spinal relay neurons. We have recently identified a critical role of the presynaptic organizer FGF22 for the formation of new synapses in the remodeling spinal cord. Here, we now explore whether and how targeted overexpression of FGF22 can be used to mitigate the severe functional consequences of SCI. By targeting FGF22 expression to either long propriospinal neurons, excitatory interneurons, or a broader population of interneurons, we establish that FGF22 can enhance neuronal rewiring both in a circuit-specific and comprehensive way. We can further demonstrate that the latter approach can restore functional recovery when applied either on the day of the lesion or within 24 h. Our study thus establishes viral gene transfer of FGF22 as a new synaptogenic treatment for SCI and defines a critical therapeutic window for its application.

Genetic control of neuronal activity enhances axonal growth only on permissive substrates

Background

Neural tissue has limited regenerative ability. To cope with that, in recent years a diverse set of novel tools has been used to tailor neurostimulation therapies and promote functional regeneration after axonal injuries.

Method

In this report, we explore cell-specific methods to modulate neuronal activity, including opto- and chemogenetics to assess the effect of specific neuronal stimulation in the promotion of axonal regeneration after injury.

Results

Opto- and chemogenetic stimulations of neuronal activity elicited increased in vitro neurite outgrowth in both sensory and cortical neurons, as well as in vivo regeneration in the sciatic nerve, but not after spinal cord injury. Mechanistically, inhibitory substrates such as chondroitin sulfate proteoglycans block the activity induced increase in axonal growth.

Conclusions

We found that genetic modulations of neuronal activity on both dorsal root ganglia and corticospinal motor neurons increase their axonal growth capacity but only on permissive environments.

Supplementary Information

The online version contains supplementary material available at 10.1186/s10020-022-00524-2.

Inositol Polyphosphate-5-Phosphatase K (Inpp5k) Enhances Sprouting of Corticospinal Tract Axons after CNS Trauma

Failure of CNS neurons to mount a significant growth response after trauma contributes to chronic functional deficits after spinal cord injury. Activator and repressor screening of embryonic cortical neurons and retinal ganglion cells in vitro and transcriptional profiling of developing CNS neurons harvested in vivo have identified several candidates that stimulate robust axon growth in vitro and in vivo Building on these studies, we sought to identify novel axon growth activators induced in the complex adult CNS environment in vivo We transcriptionally profiled intact sprouting adult corticospinal neurons (CSNs) after contralateral pyramidotomy (PyX) in nogo receptor-1 knock-out mice and found that intact CSNs were enriched in genes in the 3-phosphoinositide degradation pathway, including six 5-phosphatases. We explored whether inositol polyphosphate-5-phosphatase K (Inpp5k) could enhance corticospinal tract (CST) axon growth in preclinical models of acute and chronic CNS trauma. Overexpression of Inpp5k in intact adult CSNs in male and female mice enhanced the sprouting of intact CST terminals after PyX and cortical stroke and sprouting of CST axons after acute and chronic severe thoracic spinal contusion. We show that Inpp5k stimulates axon growth in part by elevating the density of active cofilin in labile growth cones, thus stimulating actin polymerization and enhancing microtubule protrusion into distal filopodia. We identify Inpp5k as a novel CST growth activator capable of driving compensatory axon growth in multiple complex CNS injury environments and underscores the veracity of using in vivo transcriptional screening to identify the next generation of cell-autonomous factors capable of repairing the damaged CNS.SIGNIFICANCE STATEMENT Neurologic recovery is limited after spinal cord injury as CNS neurons are incapable of self-repair post-trauma. In vitro screening strategies exploit the intrinsically high growth capacity of embryonic CNS neurons to identify novel axon growth activators. While promising candidates have been shown to stimulate axon growth in vivo, concomitant functional recovery remains incomplete. We identified Inpp5k as a novel axon growth activator using transcriptional profiling of intact adult corticospinal tract (CST) neurons that had initiated a growth response after pyramidotomy in plasticity sensitized nogo receptor-1-null mice. Here, we show that Inpp5k overexpression can stimulate CST axon growth after pyramidotomy, stroke, and acute and chronic contusion injuries. These data support in vivo screening approaches to identify novel axon growth activators.

A deep learning-based toolbox for Automated Limb Motion Analysis (ALMA) in murine models of neurological disorders

In neuroscience research, the refined analysis of rodent locomotion is complex and cumbersome, and access to the technique is limited because of the necessity for expensive equipment. In this study, we implemented a new deep learning-based open-source toolbox for Automated Limb Motion Analysis (ALMA) that requires only basic behavioral equipment and an inexpensive camera. The ALMA toolbox enables the consistent and comprehensive analyses of locomotor kinematics and paw placement and can be applied to neurological conditions affecting the brain and spinal cord. We demonstrated that the ALMA toolbox can (1) robustly track the evolution of locomotor deficits after spinal cord injury, (2) sensitively detect locomotor abnormalities after traumatic brain injury, and (3) correctly predict disease onset in a multiple sclerosis model. We, therefore, established a broadly applicable automated and standardized approach that requires minimal financial and time commitments to facilitate the comprehensive analysis of locomotion in rodent disease models.

Presenting ALMA toolbox, an open source Python repository for automatic analysis of mouse locomotion using bodypart coordinates from markerless pose estimation tools. ALMA is capable of analyzing both healthy and CNS-injured mice. ALMA is also capable of predicting onset of disease in a multiple sclerosis model.

Treatment with Pulsed Extremely Low Frequency Electromagnetic Field (PELF-EMF) Exhibit Anti-Inflammatory and Neuroprotective Effect in Compression Spinal Cord Injury Model

Background: Spinal cord injury (SCI) pathology includes both primary and secondary events. The primary injury includes the original traumatic event, and the secondary injury, beginning immediately after the initial injury, involves progressive neuroinflammation, neuronal excitotoxicity, gliosis, and degeneration. Currently, there is no effective neuroprotective treatment for SCI. However, an accumulating body of data suggests that PELF-EMF has beneficial therapeutic effects on neurotrauma. The purpose of this study was to test the efficacy of the PELF-EMF SEQEX device using a compression SCI mouse model. Methods: C57BL/6 mice were exposed to PELF-EMF for 4 h on a daily basis for two months, beginning 2 h after a mild-moderate compression SCI. Results: The PELF-EMF treatment significantly diminished inflammatory cell infiltration and astrocyte activation by reducing Iba1, F4/80, CD68+ cells, and GAFP at the lesion borders, and increased pro-survival signaling, such as BDNF, on the neuronal cells. Moreover, the treatment exhibited a neuroprotective effect by reducing the demyelination of the axons of the white matter at the lesion’s center. Conclusions: Treatment with SEQEX demonstrated significant anti-inflammatory and neuroprotective effects. Considering our results, this safe and effective rehabilitative device, already available on the market, may provide a major therapeutic asset in the treatment of SCI.

Perspectives in the Cell-Based Therapies of Various Aspects of the Spinal Cord Injury-Associated Pathologies: Lessons from the Animal Models

Traumatic injury of the spinal cord (SCI) is a devastating neurological condition often leading to severe dysfunctions, therefore an improvement in clinical treatment for SCI patients is urgently needed. The potential benefits of transplantation of various cell types into the injured spinal cord have been intensively investigated in preclinical SCI models and clinical trials. Despite the many challenges that are still ahead, cell transplantation alone or in combination with other factors, such as artificial matrices, seems to be the most promising perspective. Here, we reviewed recent advances in cell-based experimental strategies supporting or restoring the function of the injured spinal cord with a particular focus on the regenerative mechanisms that could define their clinical translation.

Chemogenetic approaches to unravel circuit wiring and related behavior after spinal cord injury

A critical shortcoming of the central nervous system is its limited ability to repair injured nerve connections. Trying to overcome this limitation is not only relevant to understand basic neurobiological principles but also holds great promise to advance therapeutic strategies related, in particular, to spinal cord injury (SCI). With barely any SCI patients re-gaining complete neurological function, there is a high need to understand how we could target and improve spinal plasticity to re-establish neuronal connections into a functional network. The development of chemogenetic tools has proven to be of great value to understand functional circuit wiring before and after injury and to correlate novel circuit formation with behavioral outcomes. This review covers commonly used chemogenetic approaches based on metabotropic receptors and their use to improve our understanding of circuit wiring following spinal cord injury.

Effects of Repetitive Transcranial Magnetic Stimulation (rTMS) and Treadmill Training on Recovery of Motor Function in a Rat Model of Partial Spinal Cord Injury

BACKGROUND This study aimed to investigate the effects of repetitive transcranial magnetic stimulation (rTMS) and treadmill training (TT) on motor function recovery in rats with partial spinal cord injury (SCI). MATERIAL AND METHODS Sixty rats with moderate partial SCI at the 9th thoracic vertebral level induced by a Louisville Injury System Apparatus impactor were randomly allocated to 5 groups: Sham surgery (Intact); Sham rTMS without TT (S-rTMS/Non-TT); Sham rTMS with TT (S-rTMS/TT); rTMS without TT (rTMS/Non-TT); and rTMS with TT (rTMS/TT). Interventions commenced 8 days after SCI and continued for 8 weeks. Outcomes studied were Basso, Beattie, and Bresnahan locomotor scale scores, grid walking test, and biochemical analysis of the brain-derived neurotrophic factor (BDNF), synapsin I (SYN), and postsynaptic density protein-95 (PSD-95) in the motor cortex and spinal cord. RESULTS The rTMS/TT contributed to greater Basso, Beattie, and Bresnahan scores compared with the S-rTMS/Non-TT (P<0.01), S-rTMS/TT (P<0.05), and rTMS/Non-TT (P<0.05), and showed obviously reduced numbers of foot drops compared with the S-rTMS/Non-TT (P<0.05). The rTMS/TT significantly increased the expressions of BDNF, SYN, and PSD-95 compared with the S-rTMS/Non-TT, both in the motor cortex (P<0.01, P<0.01, P<0.001, respectively) and spinal cord (P<0.001, P<0.01, P<0.05, respectively). CONCLUSIONS In a modified rat model of SCI, combined rTMS with TT improved motor function, indicating that this combined approach promoted adaptive neuroplasticity between the motor cortex and the spinal cord. A combined app roach to improving motor function following SCI requires further evaluation to determine the possible clinical applications.

Exercise-Induced Plasticity in Signaling Pathways Involved in Motor Recovery after Spinal Cord Injury

Spinal cord injury (SCI) leads to numerous chronic and debilitating functional deficits that greatly affect quality of life. While many pharmacological interventions have been explored, the current unsurpassed therapy for most SCI sequalae is exercise. Exercise has an expansive influence on peripheral health and function, and by activating the relevant neural pathways, exercise also ameliorates numerous disorders of the central nervous system (CNS). While the exact mechanisms by which this occurs are still being delineated, major strides have been made in the past decade to understand the molecular underpinnings of this essential treatment. Exercise rapidly and prominently affects dendritic sprouting, synaptic connections, neurotransmitter production and regulation, and ionic homeostasis, with recent literature implicating an exercise-induced increase in neurotrophins as the cornerstone that binds many of these effects together. The field encompasses vast complexity, and as the data accumulate, disentangling these molecular pathways and how they interact will facilitate the optimization of intervention strategies and improve quality of life for individuals affected by SCI. This review describes the known molecular effects of exercise and how they alter the CNS to pacify the injury environment, increase neuronal survival and regeneration, restore normal neural excitability, create new functional circuits, and ultimately improve motor function following SCI.

Promotion of corticospinal tract growth by KLF6 requires an injury stimulus and occurs within four weeks of treatment

Axons in the corticospinal tract (CST) display a limited capacity for compensatory sprouting after partial spinal injuries, potentially limiting functional recovery. Forced expression of a developmentally expressed transcription factor, Krüppel-like factor 6 (KLF6), enhances axon sprouting by adult CST neurons. Here, using a pyramidotomy model of injury in adult mice, we confirm KLF6's pro-sprouting properties in spared corticospinal tract neurons and show that this effect depends on an injury stimulus. In addition, we probed the time course of KLF6-triggered sprouting of CST axons and demonstrate a significant enhancement of growth within four weeks of treatment. Finally, we tested whether KLF6-induced sprouting was accompanied by improvements in forelimb function, either singly or when combined with intensive rehabilitation. We found that regardless of rehabilitative training, and despite robust cross-midline sprouting by corticospinal tract axons, treatment with KLF6 produced no significant improvement in forelimb function on either a modified ladder-crossing task or a pellet-retrieval task. These data clarify important details of KLF6's pro-growth properties and indicate that additional interventions or further optimization will be needed to translate this improvement in axon growth into functional gains.

Effects of aerobic exercise training on muscle plasticity in a mouse model of cervical spinal cord injury

Cervical spinal cord injury (SCI) results in permanent life-altering motor and respiratory deficits. Other than mechanical ventilation for respiratory insufficiency secondary to cervical SCI, effective treatments are lacking and the development of animal models to explore new therapeutic strategies are needed. The aim of this work was to demonstrate the feasibility of using a mouse model of partial cervical spinal hemisection at the second cervical metameric segment (C2) to investigate the impact of 6 weeks training on forced exercise wheel system on locomotor/respiratory plasticity muscles. To measure run capacity locomotor and respiratory functions, incremental exercise tests and diaphragmatic electromyography were done. In addition, muscle fiber type composition and capillary distribution were assessed at 51 days following chronic C2 injury in diaphragm, extensor digitorum communis (EDC), tibialis anterior (TA) and soleus (SOL) muscles. Six-week exercise training increased the running capacity of trained SCI mice. Fiber type composition in EDC, TA and SOL muscles was not modified by our protocol of exercise. The vascularization was increased in all muscle limbs in SCI trained group. No increase in diaphragmatic electromyography amplitude of the diaphragm muscle on the side of SCI was observed, while the contraction duration was significantly decreased in sedentary group compared to trained group. Cross-sectional area of type IIa myofiber in the contralateral diaphragm side of SCI was smaller in trained group. Fiber type distribution between contralateral and ipsilateral diaphragm in SCI sedentary group was affected, while no difference was observed in trained group. In addition, the vascularization of the diaphragm side contralateral to SCI was increased in trained group. All these results suggest an increase in fatigue resistance and a contribution to the running capacity in SCI trained group. Our exercise protocol could be a promising non-invasive strategy to sustain locomotor and respiratory muscle plasticity following SCI.

Semaphorin 7A restricts serotonergic innervation and ensures recovery after spinal cord injury

Descending serotonergic (5-HT) projections originating from the raphe nuclei form an important input to the spinal cord that control basic locomotion. The molecular signals that control this projection pattern are currently unknown. Here, we identify Semaphorin7A (Sema7A) as a critical cue that restricts serotonergic innervation in the spinal cord. Sema7A deficient mice show a marked increase in serotonergic fiber density in all layers of the spinal cord while the density of neurons expressing the corresponding 5-HTR2α receptor remains unchanged. These alterations appear to be successfully compensated as no obvious changes in rhythmic locomotion and skilled stepping are observed in adult mice. When the system is challenged with a spinal lesion, serotonergic innervation patterns in both Sema7A-deficient and -competent mice evolve over time with excessive innervation becoming most pronounced in the dorsal horn of Sema7A-deficient mice. These altered serotonergic innervation patterns correlate with diminished functional recovery that predominantly affects rhythmic locomotion. Our findings identify Sema7A as a critical regulator of serotonergic circuit formation in the injured spinal cord.

Multimodal treatment for spinal cord injury: a sword of neuroregeneration upon neuromodulation

Spinal cord injury is linked to the interruption of neural pathways, which results in irreversible neural dysfunction. Neural repair and neuroregeneration are critical goals and issues for rehabilitation in spinal cord injury, which require neural stem cell repair and multimodal neuromodulation techniques involving personalized rehabilitation strategies. Besides the involvement of endogenous stem cells in neurogenesis and neural repair, exogenous neural stem cell transplantation is an emerging effective method for repairing and replacing damaged tissues in central nervous system diseases. However, to ensure that endogenous or exogenous neural stem cells truly participate in neural repair following spinal cord injury, appropriate interventional measures (e.g., neuromodulation) should be adopted. Neuromodulation techniques, such as noninvasive magnetic stimulation and electrical stimulation, have been safely applied in many neuropsychiatric diseases. There is increasing evidence to suggest that neuromagnetic/electrical modulation promotes neuroregeneration and neural repair by affecting signaling in the nervous system; namely, by exciting, inhibiting, or regulating neuronal and neural network activities to improve motor function and motor learning following spinal cord injury. Several studies have indicated that fine motor skill rehabilitation training makes use of residual nerve fibers for collateral growth, encourages the formation of new synaptic connections to promote neural plasticity, and improves motor function recovery in patients with spinal cord injury. With the development of biomaterial technology and biomechanical engineering, several emerging treatments have been developed, such as robots, brain-computer interfaces, and nanomaterials. These treatments have the potential to help millions of patients suffering from motor dysfunction caused by spinal cord injury. However, large-scale clinical trials need to be conducted to validate their efficacy. This review evaluated the efficacy of neural stem cells and magnetic or electrical stimulation combined with rehabilitation training and intelligent therapies for spinal cord injury according to existing evidence, to build up a multimodal treatment strategy of spinal cord injury to enhance nerve repair and regeneration.

Anatomical Plasticity of Rostrally Terminating Axons as a Possible Bridging Substrate across a Spinal Injury

Transfer of information across a spinal lesion is required for many aspects of recovery across diverse motor systems. Our understanding of axonal plasticity and which subpopulations of neurons may contribute to bridging substrates following injury, however, remains relatively incomplete. Most recently, attention has been directed to propriospinal neurons (PSNs), with research suggesting that they are capable of bridging a spinal lesion in rodents. In the current study, subpopulations of both long (C5) and short (T6, T8) PSNs-as well as a supraspinal system, the rubrospinal tract (RST)-were assessed following low thoracic (T9) hemisection in the cat using the retrograde tracer Fluoro-Gold. Acutely, within 2 weeks post-hemisection, the numbers of short and long PSNs, as well as contralateral RST neurons, with axons crossing the lesion were significantly decreased relative to uninjured controls. This decrease persisted bilaterally and was permanent in the long PSNs and the contralateral red nucleus (RN). However, by 16 weeks post-hemisection, the numbers of ipsilesional and contralesional short PSNs bridging the lesion were significantly increased. Further, the number of contralesional contributing short PSNs was significantly greater in injured animals than in uninjured animals. A significant increase over uninjured numbers also was seen in the ipsilateral (non-axotomized) RN. These findings suggest that a novel substrate of undamaged axons, which normally terminates rostral to the lesion, grows past a thoracic lesion after injury. This rostral population represents a major component of the bridging substrate seen and may represent an important anatomical target for evolving rehabilitation approaches as a substrate capable of contributing to functional recovery.

Therapeutic repair for spinal cord injury: combinatory approaches to address a multifaceted problem

The recent years saw the advent of promising preclinical strategies that combat the devastating effects of a spinal cord injury (SCI) that are progressing towards clinical trials. However, individually, these treatments produce only modest levels of recovery in animal models of SCI that could hamper their implementation into therapeutic strategies in spinal cord injured humans. Combinational strategies have demonstrated greater beneficial outcomes than their individual components alone by addressing multiple aspects of SCI pathology. Clinical trial designs in the future will eventually also need to align with this notion. The scenario will become increasingly complex as this happens and conversations between basic researchers and clinicians are required to ensure accurate study designs and functional readouts.

Astrocyte-selective AAV-ADAMTS4 gene therapy combined with hindlimb rehabilitation promotes functional recovery after spinal cord injury

Chondroitin sulphate proteoglycans (CSPGs) are inhibitors to axon regeneration and plasticity. A disintegrin and metalloproteinase with thrombospondin motifs-4 (ADAMTS4) is a human enzyme that catalyses the proteolysis of CSPG protein cores. Infusion of ADAMTS4 into the damaged spinal cord was previously shown to improve functional recovery SCI, however, this therapy is limited in its enzyme form. Adeno-associated viral (AAV) vector gene therapy has emerged as the vector of choice for safe, robust and long-term transgene expression in the central nervous system. Here, an AAV expression cassette containing ADAMTS4 under the control of the astrocytic GfaABC₁D promoter was packaged into an AAV5 vector. Sustained expression of ADAMTS4 was achieved in vitro and in vivo leading to degradation of CSPGs. Compared to a contusion only group, AAV-ADAMTS4 resulted in significantly decreased lesion size, increased sprouting of hindlimb corticospinal tract axons, increased serotonergic fiber density caudal to a contusive spinal cord injury. Hindlimb-specific exercise rehabilitation was used to drive neuroplasticity towards improving functional connections. The combination of hindlimb rehabilitation with AAV-ADAMTS4 led to functional recovery after SCI compared to a contusion only group. Thus, long-term degradation of CSPGs through AAV-ADAMTS4 gene therapy in a combinational approach with rehabilitation represents a candidate for further preclinical development.

Mesenchymal stem cells and treadmill training enhance function and promote tissue preservation after spinal cord injury

Spinal cord injury (SCI) is considered a serious neurological disorder that can lead to severe sensory, motor and autonomic deficits. In this work, we investigated whether cell therapy associated with physical activity after mouse SCI could promote morphological and functional outcomes, using a lesion model established by our group. Mesenchymal stem cells (8 × 10⁵ cells/2 µL) or DMEM (2 µL), were injected in the epicenter of the lesion at 7 days after SCI, and the mice started a moderate treadmill training 14 days after injury. Functional assessments were performed weekly up to 8 weeks after injury when the morphological analyses were also performed. Four injured groups were analyzed: DMEM (SCI plus DMEM injection), MSCT (SCI plus MSC injection), DMEM + TMT (SCI plus DMEM injection and treadmill training) and MSCT + TMT (SCI plus MSC injection and treadmill training). The animals that received the combined therapy (MSCT + TMT) were able to recover and maintained the better functional results throughout the analyzed period. The morphometric analysis from MSCT + TMT group evidenced a larger spared white matter area and a higher number of preserved myelinated fibers with the majority of them reaching the ideal G-ratio values, when compared to other groups. Ultrastructural analysis from this group, using transmission electron microscopy, showed better tissue preservation with few microcavitations and degenerating nerve fibers. Also, this group exhibited a significantly higher neurotrophin 4 (NT4) expression as compared to the other groups. The results provided by this study support the conclusion that the association of strategies is a potential therapeutic approach to treat SCI, with the possibility of translation into the clinical practice.

Rehabilitation following spinal cord injury: how animal models can help our understanding of exercise-induced neuroplasticity

Spinal cord injury is a devastating condition that is followed by long and often unsuccessful recovery after trauma. The state of the art approach to manage paralysis and concomitant impairments is rehabilitation, which is the only strategy that has proven to be effective and beneficial for the patients over the last decades. How rehabilitation influences the remodeling of spinal axonal connections in patients is important to understand, in order to better target these changes and define the optimal timing and onset of training. While clinically the answers to these questions remain difficult to obtain, rodent models of rehabilitation like bicycling, treadmill training, swimming, enriched environments or wheel running that mimic clinical rehabilitation can be helpful to reveal the axonal changes underlying motor recovery. This review will focus on the different animal models of spinal cord injury rehabilitation and the underlying changes in neuronal networks that are improved by exercise and rehabilitation.