Intramuscular AAV delivery of NT-3 alters synaptic transmission to motoneurons in adult rats

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.

Administration of intramuscular AAV-BDNF and intranasal AAV-TrkB promotes neurological recovery via enhancing corticospinal synaptic connections in stroke rats

Stroke causes long-term disability in survivors. BDNF/TrkB plays an important role in synaptic plasticity and synaptic transmission in the central nervous system (CNS), promoting neurological recovery. In this study, we performed non-invasive treatment methods focused on intramuscular injection into stroke-injured forelimb muscles, or intranasal administration using adeno-associated virus (AAV) vectors carrying genes encoding BDNF or TrkB. In a permanent rat middle cerebral artery occlusion (MCAO) model, we assessed the effects of combination therapy with AAV-BDNF and AAV-TrkB on motor functional recovery and synaptic plasticity of the corticospinal connections. Our results showed that BDNF or TrkB gene transduced in the spinal anterior horn neurons and cerebral cortical neurons. Compared to AAV vector treatment alone, behavioral and electrophysiological results showed that the combination therapy significantly improved upper limb motor functional recovery and neurotransmission efficiency after stroke. BDA tracing, immunofluorescence staining, qRT-PCR, and transmission electron microscopy of synaptic ultrastructure results revealed that the combination therapy not only potently increased the expression of Synapsin I, PSD-95, and GAP-43, but also promoted the axonal remodeling and restoration of abnormal synaptic structures. These findings provide a new strategy for enhancing neural plasticity and a potential means to treat stroke clinically.

The Overexpression of Insulin-Like Growth Factor-1 and Neurotrophin-3 Promote Functional Recovery and Alleviate Spasticity After Spinal Cord Injury

Objective

This study was conducted to investigate the effects of the exogenous overexpression of nerve growth factors NT-3 and IGF-1 on the recovery of nerve function after spinal cord injury (SCI) and identify the potential mechanism involved.

Methods

Sixty-four female SD rats were randomly divided into four groups: an SCI group, an adeno-associated viral (AAV)-RFP and AAV-GFP injection group, an AAV-IGF-1 and AAV-NT-3 injection group, and a Sham group. After grouping, the rats were subjected to a 10-week electrophysiological and behavioral evaluation to comprehensively evaluate the effects of the intervention on motor function, spasticity, mechanical pain, and thermal pain. Ten weeks later, samples were taken for immunofluorescence (IF) staining and Western blot (WB) detection, focusing on the expression of KCC2, 5-HT2A, and 5-HT2C receptors in motor neurons and the spinal cord.

Results

Electrophysiological and behavioral data indicated that the AAV-IGF-1 and AAV-NT-3 groups showed better recovery of motor function (P < 0.05 from D14 compared with the AAV-RFP + AAV-GFP group; P < 0.05 from D42 compared with SCI group) and less spasticity (4-10 weeks, at 5 Hz all P < 0.05 compared with SCI group and AAV- RFP + AAV-GFP group) but with a trend for more pain sensitivity. Compared with the SCI group, the von Frey value result of the AAV-IGF-1 and AAV-NT-3 groups showed a lower pain threshold (P < 0.05 at 4-8 weeks), and shorter thermal pain threshold (P < 0.05 at 8-10 weeks). IF staining further suggested that compared with the SCI group, the overexpression of NT-3 and IGF-1 in the SCI-R + G group led to increased levels of KCC2 (p < 0.05), 5-HT2A (p < 0.05), and 5-HT2C (p < 0.001) in motor neurons. WB results showed that compared with the SCI group, the SCI-R + G group exhibited higher expression levels of CHAT (p < 0.01), 5-HT2A (p < 0.05), and 5-HT2C (p < 0.05) proteins in the L2-L6 lumbar enlargement.

Conclusion

Data analysis showed that the overexpression of NT-3 and IGF-1 may improve motor function after SCI and alleviate spasms in a rat model; however, these animals were more sensitive to mechanical pain and thermal pain. These behavioral changes may be related to increased numbers of KCC2, 5-HT2A, and 5-HT2C receptors in the spinal cord tissue. The results of this study may provide a new theoretical basis for the clinical treatment of SCI.

Distribution Heterogeneity of Muscle Spindles Across Skeletal Muscles of Lower Extremities in C57BL/6 Mice

Muscle spindles, an important proprioceptor scattered in the skeletal muscle, participate in maintaining muscle tension and the fine regulation of random movement. Although muscle spindles exist in all skeletal muscles, explanations about the distribution and morphology of muscle spindles remain lacking for the indetermination of spindle location across muscles. In this study, traditional time-consuming histochemical technology was utilized to determine the muscle spindle anatomical and morphological characteristics in the lower extremity skeletal muscle in C57BL/6 mice. The relative distance from spindles to nerve-entry points varied from muscles in the ventral-dorsal direction, in which spindles in the lateral of gastrocnemius were not considered to be close to its nerve-entry point. In the longitudinal pattern, the domain with the highest abundance of spindles corresponded to the nerve-entry point, excluding the tibialis anterior. Spindles are mainly concentrated at the middle and rostral domain in all muscles. The results suggest a heterogeneity of the distribution of spindles in different muscles, but the distribution trend generally follows the location pattern of the nerve-entry point. Histochemical staining revealed that the spindle did not have a symmetrical structure along the equator, and this result does not agree with previous findings. Exploring the distribution and structural characteristics of muscle spindles in skeletal muscle can provide some anatomical basis for the study of muscle spindles at the molecular level and treatment of exercise-related diseases and provide a comprehensive understanding of muscle spindle morphology.

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.

Conditional RAC1 knockout in motor neurons restores H-reflex rate-dependent depression after spinal cord injury

A major complication with spinal cord injury (SCI) is the development of spasticity, a clinical symptom of hyperexcitability within the spinal H-reflex pathway. We have previously demonstrated a common structural motif of dendritic spine dysgenesis associated with hyperexcitability disorders after injury or disease insults to the CNS. Here, we used an adeno-associated viral (AAV)-mediated Cre-Lox system to knockout Rac1 protein expression in motor neurons after SCI. Three weeks after AAV9-Cre delivery into the soleus/gastrocnemius of Rac1-“floxed” adult mice to retrogradely infect spinal alpha-motor neurons, we observed significant restoration of RDD and reduced H-reflex excitability in SCI animals. Additionally, viral-mediated Rac1 knockdown reduced presence of dendritic spine dysgenesis on motor neurons. In control SCI animals without Rac1 knockout, we continued to observe abnormal dendritic spine morphology associated with hyperexcitability disorder, including an increase in mature, mushroom dendritic spines, and an increase in overall spine length and spine head size. Taken together, our results demonstrate that viral-mediated disruption of Rac1 expression in ventral horn motor neurons can mitigate dendritic spine morphological correlates of neuronal hyperexcitability, and reverse hyperreflexia associated with spasticity after SCI. Finally, our findings provide evidence of a putative mechanistic relationship between motor neuron dendritic spine dysgenesis and SCI-induced spasticity.

Targeted tPA overexpression in denervated spinal motor neurons promotes stroke recovery in mice

Our previous studies demonstrated that axonal remodeling of the corticospinal tract (CST) contributes to neurological recovery after stroke in rodents. The present study employed a novel non-invasive peripheral approach, to over-express tPA in denervated spinal motor neurons via recombinant adeno-associated virus (AAV) intramuscular injection in transgenic mice subjected to permanent middle cerebral artery occlusion (MCAo), in which the CST axons are specifically and completely labeled with yellow fluorescent protein (YFP). One day after surgery, mice were randomly selected to receive saline, AAV5-RFP, or tPA (1 × 10¹⁰ viral particles) injected into the stroke-impaired forelimb muscles (n = 10/group). Functional deficits and recovery were monitored with foot-fault and single pellet reaching tests. At day 28 after MCAo, mice received intramuscular injection of PRV-614-mRFP (1.52 × 10⁷ pfu) as above, and were euthanized four days later. Compared with saline or AAV-RFP-treated mice, AAV-tPA significantly enhanced behavioral recovery (p < 0.01, both tests), as well as increased CST axonal density in the denervated gray matter of the cervical cord (p < 0.001), and RFP-positive pyramidal neurons in both ipsilesional and contralesional cortices (p < 0.001). Behavioral outcomes were significantly correlated to neural remodeling (p < 0.05). Our results provide a fundamental basis for the development of therapeutic approaches aimed at promoting corticospinal innervation for stroke treatment.

Cerebrolysin enhances spinal cord conduction and reduces blood-spinal cord barrier breakdown, edema formation, immediate early gene expression and cord pathology after injury

Spinal cord evoked potentials (SCEP) are good indicators of spinal cord function in health and disease. Disturbances in SCEP amplitudes and latencies during spinal cord monitoring predict spinal cord pathology following trauma. Treatment with neuroprotective agents preserves SCEP and reduces cord pathology after injury. The possibility that cerebrolysin, a balanced composition of neurotrophic factors improves spinal cord conduction, attenuates blood-spinal cord barrier (BSCB) disruption, edema formation, and cord pathology was examined in spinal cord injury (SCI). SCEP is recorded from epidural space over rat spinal cord T9 and T12 segments after peripheral nerves stimulation. SCEP consists of a small positive peak (MPP), followed by a prominent negative peak (MNP) that is stable before SCI. A longitudinal incision (2mm deep and 5mm long) into the right dorsal horn (T10 and T11 segments) resulted in an immediate long-lasting depression of the rostral MNP with an increase in the latencies. Pretreatment with either cerebrolysin (CBL 5mL/kg, i.v. 30min before) alone or TiO₂ nanowired delivery of cerebrolysin (NWCBL 2.5mL/kg, i.v.) prevented the loss of MNP amplitude and even enhanced further from the pre-injury level after SCI without affecting latencies. At 5h, SCI induced edema, BSCB breakdown, and cell injuries were significantly reduced by CBL and NWCBL pretreatment. Interestingly this effect on SCEP and cord pathology was still prominent when the NWCBL was delivered 2min after SCI. Moreover, expressions of c-fos and c-jun genes that are prominent at 5h in untreated SCI are also considerably reduced by CBL and NWCBL treatment. These results are the first to show that CBL and NWCBL enhanced SCEP activity and thwarted the development of cord pathology after SCI. Furthermore, NWCBL in low doses has superior neuroprotective effects on SCEP and cord pathology, not reported earlier. The functional significance and future clinical potential of CBL and NWCBL in SCI are discussed.

The role of regulatory neuropeptides and neurotrophic factors in asthma pathophysiology

In the last decade, the attention of scientists in the field of biomedicine is focused on studying the relationship between the immunological and neurogenic components of the inflammatory response and their contribution to the pathophysiology of allergic inflammation in asthma. The review is devoted to detailing the mechanism of neurogenic inflammation involving regulatory neuropeptides (substance P, vasoactive intestinal peptide, calcitonin gene-related peptide) in the pathogenesis of bronchial hyperreactivity in asthma. The role of neurotrophic growth factors (nerve growth factor, brain-derived neurotrophic factor) in the regulation of remodeling of bronchi in asthma has been analyzed. The study of neuroimmune mechanisms in the pathophysiology of asthma will it possible to find new therapeutic targets in this research area.

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.

Translational Challenges of Rat Models of Upper Extremity Dysfunction After Spinal Cord Injury

There are approximately 17,500 new spinal cord injury (SCI) cases each year in the United States, with the majority of cases resulting from a traumatic injury. Damage to the spinal cord causes either temporary or permanent changes in sensorimotor function. Given that the majority of human SCIs occur in the cervical spinal level, the experimental animal models of forelimb dysfunction play a large role in the ability to translate basic science research to clinical application. However, the variation in the design of clinical and basic science studies of forelimb/upper extremity (UE) function prevents the ease of translation. This review provides an overview of experimental models of forelimb dysfunction used in SCI research with special emphasis on the rat model of SCI. The anatomical location and types of experimental cervical lesions, functional assessments, and rehabilitation strategies used in the basic science laboratory are reviewed. Finally, we discuss the challenges of translating animal models of forelimb dysfunction to the clinical SCI human population.

Remodeling of lumbar motor circuitry remote to a thoracic spinal cord injury promotes locomotor recovery

Retrogradely-transported neurotrophin signaling plays an important role in regulating neural circuit specificity. Here we investigated whether targeted delivery of neurotrophin-3 (NT-3) to lumbar motoneurons (MNs) caudal to a thoracic (T10) contusive spinal cord injury (SCI) could modulate dendritic patterning and synapse formation of the lumbar MNs. In vitro, Adeno-associated virus serotype two overexpressing NT-3 (AAV-NT-3) induced NT-3 expression and neurite outgrowth in cultured spinal cord neurons. In vivo, targeted delivery of AAV-NT-3 into transiently demyelinated adult mouse sciatic nerves led to the retrograde transportation of NT-3 to the lumbar MNs, significantly attenuating SCI-induced lumbar MN dendritic atrophy. NT-3 enhanced sprouting and synaptic formation of descending serotonergic, dopaminergic, and propriospinal axons on lumbar MNs, parallel to improved behavioral recovery. Thus, retrogradely transported NT-3 stimulated remodeling of lumbar neural circuitry and synaptic connectivity remote to a thoracic SCI, supporting a role for retrograde transport of NT-3 as a potential therapeutic strategy for SCI.

Transplants of Neurotrophin-Producing Autologous Fibroblasts Promote Recovery of Treadmill Stepping in the Acute, Sub-Chronic, and Chronic Spinal Cat

Adult cats show limited spontaneous locomotor capabilities following spinal transection, but recover treadmill stepping with body-weight-supported training. Delivery of neurotrophic factors such as brain-derived neurotrophic factor (BDNF) and neurotrophic factor 3 (NT-3) can substitute for body-weight-supported training, and promotes a similar recovery in a shorter period of time. Autologous cell grafts would negate the need for the immunosuppressive agents currently used with most grafts, but have not shown functional benefits in incomplete spinal cord injury models and have never been tested in complete transection or chronic injury models. In this study, we explored the effects of autologous fibroblasts, prepared from the individual cats and modified to produce BDNF and NT-3, on the recovery of locomotion in acute, sub-chronic and chronic full-transection models of spinal injury. Fourteen female cats underwent complete spinal transection at T11/T12. Cats were separated into four groups: sham graft at the time of injury, and BDNF and NT-3 producing autologous fibroblasts grafted at the time of injury, 2 weeks after injury, or 6 weeks after injury. Kinematics were recorded 3 and 5 weeks after cell graft. Additional kinematic recordings were taken for some cats until 12 weeks post-graft. Eleven of 12 cats with neurotrophin-producing grafts recovered plantar weight-bearing stepping at treadmill speeds from 0.3 to 0.8 m/sec within 5 weeks of grafting, whereas control cats recovered poor quality stepping at low speeds only (≤ 0.4 m/sec). Further, kinematic measures in cats with grafts were closer to pre-transection values than those for controls, and recovery was maintained up to 12 weeks post-grafting. Our results show that not only are autologous neurotrophin-producing grafts effective at promoting recovery of locomotion, but that delayed delivery of neurotrophins does not diminish the therapeutic effect, and may improve outcome.

Non-Viral Nucleic Acid Delivery Strategies to the Central Nervous System

With an increased prevalence and understanding of central nervous system (CNS) injuries and neurological disorders, nucleic acid therapies are gaining promise as a way to regenerate lost neurons or halt disease progression. While more viral vectors have been used clinically as tools for gene delivery, non-viral vectors are gaining interest due to lower safety concerns and the ability to deliver all types of nucleic acids. Nevertheless, there are still a number of barriers to nucleic acid delivery. In this focused review, we explore the in vivo challenges hindering non-viral nucleic acid delivery to the CNS and the strategies and vehicles used to overcome them. Advantages and disadvantages of different routes of administration including: systemic injection, cerebrospinal fluid injection, intraparenchymal injection and peripheral administration are discussed. Non-viral vehicles and treatment strategies that have overcome delivery barriers and demonstrated in vivo gene transfer to the CNS are presented. These approaches can be used as guidelines in developing synthetic gene delivery vectors for CNS applications and will ultimately bring non-viral vectors closer to clinical application.

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

Delayed intramuscular human neurotrophin-3 improves recovery in adult and elderly rats after stroke

There is an urgent need for a therapy that reverses disability after stroke when initiated in a time frame suitable for the majority of new victims. We show here that intramuscular delivery of neurotrophin-3 (NT3, encoded by NTF3) can induce sensorimotor recovery when treatment is initiated 24 h after stroke. Specifically, in two randomized, blinded preclinical trials, we show improved sensory and locomotor function in adult (6 months) and elderly (18 months) rats treated 24 h following cortical ischaemic stroke with human NT3 delivered using a clinically approved serotype of adeno-associated viral vector (AAV1). Importantly, AAV1-hNT3 was given in a clinically-feasible timeframe using a straightforward, targeted route (injections into disabled forelimb muscles). Magnetic resonance imaging and histology showed that recovery was not due to neuroprotection, as expected given the delayed treatment. Rather, treatment caused corticospinal axons from the less affected hemisphere to sprout in the spinal cord. This treatment is the first gene therapy that reverses disability after stroke when administered intramuscularly in an elderly body. Importantly, phase I and II clinical trials by others show that repeated, peripherally administered high doses of recombinant NT3 are safe and well tolerated in humans with other conditions. This paves the way for NT3 as a therapy for stroke.

Mesenchymal stem cells secrete brain-derived neurotrophic factor and promote retinal ganglion cell survival after traumatic optic neuropathy

OBJECTIVE

The study aimed to investigate whether intravitreal injection of mesenchymal stem cells (MSCs) cultivated in vitro could increase the number of survived retinal ganglion cells (RGCs) after traumatic optic neuropathy and sought to identify potential mechanisms underlying such growth.

METHODS

The right eye of 24 cats in the MSC transplantation group accepted intravitreal injection of MSCs, and the other 24 cats in the phosphate buffer (PBS) control group received isotonic saline after traumatic optic neuropathy. The RGCs' survival rate in separated retinal and brain-derived neurotrophic factor (BDNF) expression were observed by Dil labeling and Enzyme-Linked Immuno Sorbent Assay (ELISA), respectively, at 3, 7, 14, and 28 days after transplantation.

RESULTS

Quantitative analysis showed that RGCs were significantly attenuated at 3, 7, 14, and 28 days after transplantation in both areas of the retina (peripheral, P7d = 0.0011, P14d < 0.001, P28d < 0.001; central, P3d = 0.0437, P7d = 0.0067, P14d < 0.001, P28d < 0.001). Mean density of RGCs in the MSC transplantation group was significantly higher than that of the PBS control group after 14 days of treatment (P < 0.001). The homogenates BDNF (hBDNF) in the MSC transplantation group was obviously higher than that in the PBS control group at 14 and 28 days (P < 0.05).

CONCLUSIONS

The MSCs transplanted into the retina of cats can slow down RGC apoptosis and steadily express BDNF. The MSC-mediated neuroprotection after optic nerve injury may be related to BNDF.

Sustained delivery of bioactive neurotrophin-3 to the injured spinal cord

The sustained release of neurotrophin-3 from a nanoparticle/hydrogel composite resulted in functional and tissue benefit after compressive spinal cord injury.

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'.

Nanoparticles carrying neurotrophin-3-modified Schwann cells promote repair of sciatic nerve defects

Schwann cells and neurotrophin-3 play an important role in neural regeneration, but the secretion of neurotrophin-3 from Schwann cells is limited, and exogenous neurotrophin-3 is inactived easily in vivo. In this study, we have transfected neurotrophin-3 into Schwann cells cultured in vitro using nanoparticle liposomes. Results showed that neurotrophin-3 was successfully transfected into Schwann cells, where it was expressed effectively and steadily. A composite of Schwann cells transfected with neurotrophin-3 and poly(lactic-co-glycolic acid) biodegradable conduits was transplanted into rats to repair 10-mm sciatic nerve defects. Transplantation of the composite scaffold could restore the myoelectricity and wave amplitude of the sciatic nerve by electrophysiological examination, promote nerve axonal and myelin regeneration, and delay apoptosis of spinal motor neurons. Experimental findings indicate that neurotrophin-3 transfected Schwann cells combined with bridge grafting can promote neural regeneration and functional recovery after nerve injury.

Neurotrophins and spinal circuit function

Work early in the last century emphasized the stereotyped activity of spinal circuits based on studies of reflexes. However, the last several decades have focused on the plasticity of these spinal circuits. These considerations began with studies of the effects of monoamines on descending and reflex circuits. In recent years new classes of compounds called growth factors that are found in peripheral nerves and the spinal cord have been shown to affect circuit behavior in the spinal cord. In this review we will focus on the effects of neurotrophins, particularly nerve growth factor (NGF), brain derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3), on spinal circuits. We also discuss evidence that these molecules can modify functions including nociceptive behavior, motor reflexes and stepping behavior. Since these substances and their receptors are normally present in the spinal cord, they could potentially be useful in improving function in disease states and after injury. Here we review recent findings relevant to these translational issues.

Acute exercise prevents the development of neuropathic pain and the sprouting of non-peptidergic (GDNF- and artemin-responsive) c-fibers after spinal cord injury

Spinal cord injury (SCI) impaired sensory fiber transmission leads to chronic, debilitating neuropathic pain. Sensory afferents are responsive to neurotrophic factors, molecules that are known to promote survival and maintenance of neurons, and regulate sensory neuron transduction of peripheral stimuli. A subset of primary afferent fibers responds only to the glial cell-line derived neurotrophic factor (GDNF) family of ligands (GFLs) and is non-peptidergic. In peripheral nerve injury models, restoration of GDNF or artemin (another GFL) to pre-injury levels within the spinal cord attenuates neuropathic pain. One non-invasive approach to increase the levels of GFLs in the spinal cord is through exercise (Ex), and to date exercise training is the only ameliorative, non-pharmacological treatment for SCI-induced neuropathic pain. The purpose of this study was 3-fold: 1) to determine whether exercise affects the onset of SCI-induced neuropathic pain; 2) to examine the temporal profile of GDNF and artemin in the dorsal root ganglia and spinal cord dorsal horn regions associated with forepaw dermatomes after SCI and Ex; and 3) to characterize GFL-responsive sensory fiber plasticity after SCI and Ex. Adult, female, Sprague-Dawley rats received a moderate, unilateral spinal cord contusion at C5. A subset of rats was exercised (SCI+Ex) on automated running wheels for 20min, 5days/week starting at 5days post-injury (dpi), continuing until 9 or 37dpi. Hargreaves' and von Frey testing was performed preoperatively and weekly post-SCI. Forty-two percent of rats in the unexercised group exhibited tactile allodynia of the forepaws while the other 58% retained normal sensation. The development of SCI-induced neuropathic pain correlated with a marked decrease in the levels of GDNF and artemin in the spinal cord and DRGs. Additionally, a dramatic increase in the density and the distribution throughout the dorsal horn of GFL-responsive afferents was observed in rats with SCI-induced allodynia. Importantly, in SCI rats that received Ex, the incidence of tactile allodynia decreased to 7% (1/17) and there was maintenance of GDNF and artemin at normal levels, with a normal distribution of GFL-responsive fibers. These data suggest that GFLs and/or their downstream effectors may be important modulators of pain fiber plasticity, representing effective targets for anti-allodynic therapeutics. Furthermore, we highlight the potent beneficial effects of acute exercise after SCI.

Regenerative medicine for the treatment of spinal cord injury: more than just promises?

Spinal cord injury triggers a complex set of events that lead to tissue healing without the restoration of normal function due to the poor regenerative capacity of the spinal cord. Nevertheless, current knowledge about the intrinsic regenerative ability of central nervous system axons, when in a supportive environment, has made the prospect of treating spinal cord injury a reality. Among the range of strategies under investigation, cell-based therapies offer the most promising results, due to the multifactorial roles that these cells can fulfil. However, the best cell source is still a matter of debate, as are clinical issues that include the optimal cell dose as well as the timing and route of administration. In this context, the role of biomaterials is gaining importance. These can not only act as vehicles for the administered cells but also, in the case of chronic lesions, can be used to fill the permanent cyst, thus creating a more favourable and conducive environment for axonal regeneration in addition to serving as local delivery systems of therapeutic agents to improve the regenerative milieu. Some of the candidate molecules for the future are discussed in view of the knowledge derived from studying the mechanisms that facilitate the intrinsic regenerative capacity of central nervous system neurons. The future challenge for the multidisciplinary teams working in the field is to translate the knowledge acquired in basic research into effective combinatorial therapies to be applied in the clinic.

Neurotrophic factors in spinal cord injury

A major challenge in repairing the injured spinal cord is to assure survival of damaged cells and to encourage regrowth of severed axons. Because neurotrophins are known to affect these processes during development, many experimental approaches to improving function of the injured spinal cord have made use of these agents, particularly Brain derived neurotrophic factor (BDNF) and Neurotrophin-3 (NT-3). More recently, neurotrophins have also been shown to affect the physiology of cells and synapses in the spinal cord. The effect of neurotrophins on circuit performance adds an important dimension to their consideration as agents for repairing the injured spinal cord. In this chapter we discuss the role of neurotrophins in promoting recovery after spinal cord injury from both a structural and functional perspective.

Intraspinal AAV Injections Immediately Rostral to a Thoracic Spinal Cord Injury Site Efficiently Transduces Neurons in Spinal Cord and Brain

In the vast majority of studies utilizing adeno-associated virus (AAV) in central nervous system applications, including those published with spinal cord injury (SCI) models, AAV has been administered at the level of the cell body of neurons targeted for genetic modification, resulting in transduction of neurons in the vicinity of the injection site. However, as SCI interrupts many axon tracts, it may be more beneficial to transduce a diverse pool of supraspinal neurons. We determined if descending axons severed by SCI are capable of retrogradely transporting AAV to remotely transduce a variety of brain regions. Different AAV serotypes encoding the reporter green fluorescent protein (GFP) were injected into gray and white matter immediately rostral to a spinal transection site. This resulted in the transduction of thousands of neurons within the spinal cord and in multiple regions within the brainstem that project to spinal cord. In addition, we established that different serotypes had disparate regional specificity and that AAV5 transduced the most brain and spinal cord neurons. This is the first demonstration that retrograde transport of AAV by axons severed by SCI is an effective means to transduce a collection of supraspinal neurons. Thus, we identify a novel, minimally invasive means to transduce a variety of neuronal populations within both the spinal cord and the brain following SCI. This paradigm to broadly distribute viral vectors has the potential to be an important component of a combinatorial strategy to promote functional axonal regeneration.

Enhancing proprioceptive input to motoneurons differentially affects expression of neurotrophin 3 and brain-derived neurotrophic factor in rat hoffmann-reflex circuitry

The importance of neurotrophin 3 (NT-3) for motor control prompted us to ask the question whether direct electrical stimulation of low-threshold muscle afferents, strengthening the proprioceptive signaling, could effectively increase the endogenous pool of this neurotrophin and its receptor TrkC in the Hoffmann-reflex (H-reflex) circuitry. The effects were compared with those of brain-derived neurotrophic factor (BDNF) and its TrkB receptor. Continuous bursts of stimuli were delivered unilaterally for seven days, 80 min daily, by means of a cuff-electrode implanted over the tibial nerve in awake rats. The H-reflex was recorded in the soleus muscle to control the strength of stimulation. Stimulation aimed at activation of Ia fibers produced a strong increase of NT-3 protein, measured with ELISA, in the lumbar L3-6 segments of the spinal cord and in the soleus muscle. This stimulation exerted much weaker effect on BDNF protein level which slightly increased only in L3-6 segments of the spinal cord. Increased protein level of NT-3 and BDNF corresponded to the changes of NT-3 mRNA and BDNF mRNA expression in L3-6 segments but not in the soleus muscle. We disclosed tissue-specificity of TrkC mRNA and TrkB mRNA responses. In the spinal cord TrkC and TrkB transcripts tended to decrease, whereas in the soleus muscle TrkB mRNA decreased and TrkC mRNA expression strongly increased, suggesting that stimulation of Ia fibers leads to sensitization of the soleus muscle to NT-3 signaling. The possibility of increasing NT-3/TrkC signaling in the neuromuscular system, with minor effects on BDNF/TrkB signaling, by means of low-threshold electrical stimulation of peripheral nerves, which in humans might be applied in non-invasive way, offers an attractive therapeutic tool.

Construction of a eukaryotic expression plasmid for human retina-derived neurotrophin-3

Neurotrophin-3 (NT-3) can promote the repair of central nervous system and retinal damage. In previous reports, NT-3 has been expressed by viral vectors. However, plasmid vectors have a safer profile compared with viral vectors in clinical studies. This study recombined amplified human retinal NT-3 with a eukaryotic expression plasmid containing green fluorescent protein (GFP) to construct an NT-3 expression plasmid, pEGFP-N1-NT-3. The transfection efficiency 48 hours after pEGFP-N1-NT-3 transfection to 293T cells was 50.06 ± 2.78%. Abundant NT-3-GFP was expressed in 293T cells as observed by fluorescence microscopy, suggesting the construct pEGFP-N1-NT-3 effectively expressed and secreted NT-3-GFP. Secretory vesicles containing NT-3-GFP were observed in a constant location in cells by laser scan confocal microscopy, indicating the expression and secretion processes of NT-3 in eukaryotic cells were in accordance with the physical synthesis processes of secreted proteins. Western blot assay showed that pro-NT-3-GFP had a molecular weight of 56 kDa, further confirming NT-3-GFP expression. At 48 hours after transfection, the concentration of NT-3 in culture medium was 22.3 ng/mL, suggesting NT-3 produced by pEGFP-N1-NT-3 was efficiently secreted. This study constructed a human retinal-derived NT-3 eukaryotic expression plasmid that efficiently expressed and secreted NT-3.

The Transcriptional Response of Neurotrophins and Their Tyrosine Kinase Receptors in Lumbar Sensorimotor Circuits to Spinal Cord Contusion is Affected by Injury Severity and Survival Time

Traumatic spinal cord injury (SCI) results in changes to the anatomical, neurochemical, and physiological properties of cells in the central and peripheral nervous system. Neurotrophins, acting by binding to their cognate Trk receptors on target cell membranes, contribute to modulation of anatomical, neurochemical, and physiological properties of neurons in sensorimotor circuits in both the intact and injured spinal cord. Neurotrophin signaling is associated with many post-SCI changes including maladaptive plasticity leading to pain and autonomic dysreflexia, but also therapeutic approaches such as training-induced locomotor improvement. Here we characterize expression of mRNA for neurotrophins and Trk receptors in lumbar dorsal root ganglia (DRG) and spinal cord after two different severities of mid-thoracic injury and at 6 and 12 weeks post-SCI. There was complex regulation that differed with tissue, injury severity, and survival time, including reversals of regulation between 6 and 12 weeks, and the data suggest that natural regulation of neurotrophins in the spinal cord may continue for months after birth. Our assessments determined that a coordination of gene expression emerged at the 12-week post-SCI time point and bioinformatic analyses address possible mechanisms. These data can inform studies meant to determine the role of the neurotrophin signaling system in post-SCI function and plasticity, and studies using this signaling system as a therapeutic approach.

Treadmill training stimulates brain-derived neurotrophic factor mRNA expression in motor neurons of the lumbar spinal cord in spinally transected rats

Brain-derived neurotrophic factor (BDNF) induces plasticity within the lumbar spinal circuits thereby improving locomotor recovery in spinal cord-injured animals. We examined whether lumbar spinal cord motor neurons and other ventral horn cells of spinally transected (ST) rats were stimulated to produce BDNF mRNA in response to treadmill training. Rats received complete spinal cord transections as neonates (n=20) and one month later, received four weeks of either a low (100 steps/training session; n=10) or high (1000 steps/training session; n=10) amount of robotic-assisted treadmill training. Using combined non-radioactive in situ hybridization and immunohistochemical techniques, we found BDNF mRNA expression in heat shock protein 27-labeled motor neurons and in non-motor neuron cells was greater after 1000 steps/training session compared to the 100 steps/training session and was similar to BDNF mRNA labeling in untrained Intact rats. In addition, there were significantly more motor neurons that contained BDNF mRNA labeling within processes in the ST rats that received the higher amount of treadmill training. These findings suggested that motor neurons and other ventral horn cells in ST rats synthesized BDNF in response to treadmill training. The findings support a mechanism by which postsynaptic release of BDNF from motor neurons contributed to synaptic plasticity.

Research advances in gene therapy approaches for the treatment of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disease of motor neurons that causes progressive muscle weakness, paralysis, and premature death. No effective therapy is available. Research in the motor neuron field continues to grow, and recent breakthroughs have demonstrated the possibility of completely achieving rescue in animal models of spinal muscular atrophy, a genetic motor neuron disease. With adeno-associated virus (AAV) vectors, gene transfer can be achieved with systemic non-invasive injection and minimal toxicity. In the context of this success, we review gene therapy approaches for ALS, considering what has been done and the possible future directions for effective application of the latest generation of vectors for clinical translation. We focus on recent developments in the areas of RNA/antisense-mediated silencing of specific ALS causative genes like superoxide dismutase-1 and other molecular pathogenetic targets, as well as the administration of neuroprotective factors with viral vectors. We argue that gene therapy offers new opportunities to open the path for clinical progress in treating ALS.

Differential effects of brain-derived neurotrophic factor and neurotrophin-3 on hindlimb function in paraplegic rats

We compared the effect of viral administration of brain-derived neurotrophic factor (BDNF) or neurotrophin 3 (NT-3) on locomotor recovery in adult rats with complete thoracic (T10) spinal cord transection injuries, in order to determine the effect of chronic neurotrophin expression on spinal plasticity. At the time of injury, BDNF, NT-3 or green fluorescent protein (GFP) (control) was delivered to the lesion via adeno-associated virus (AAV) constructs. AAV–BDNF was significantly more effective than AAV–NT-3 in eliciting locomotion. In fact, AAV–BDNF-treated rats displayed plantar, weight-supported hindlimb stepping on a stationary platform, that is, without the assistance of a moving treadmill and without step training. Rats receiving AAV–NT-3 or AAV–GFP were incapable of hindlimb stepping during this task, despite provision of balance support. AAV–NT-3 treatment did promote the recovery of treadmill-assisted stepping, but this required continuous perineal stimulation. In addition, AAV–BDNF-treated rats were sensitized to noxious heat, whereas AAV–NT-3-treated and AAV–GFP-treated rats were not. Notably, AAV–BDNF-treated rats also developed hindlimb spasticity, detracting from its potential clinical applicability via the current viral delivery method. Intracellular recording from triceps surae motoneurons revealed that AAV–BDNF significantly reduced motoneuron rheobase, suggesting that AAV–BDNF promoted the recovery of over-ground stepping by enhancing neuronal excitability. Elevated nuclear c-Fos expression in interneurons located in the L2 intermediate zone after AAV–BDNF treatment indicated increased activation of interneurons in the vicinity of the locomotor central pattern generator. AAV–NT-3 treatment reduced motoneuron excitability, with little change in c-Fos expression. These results support the potential for BDNF delivery at the lesion site to reorganize locomotor circuits.

Exercise modulates microRNAs that affect the PTEN/mTOR pathway in rats after spinal cord injury

We investigated microRNAs (miRs) associated with PTEN/mTOR signaling after spinal cord injury (SCI) and after hind limb exercise (Ex), a therapy implicated in promoting spinal cord plasticity. After spinalization, rats received cycling Ex 5 days/week. The expression of miRs, their target genes and downstream effectors were probed in spinal cord tissue at 10 and 31 days post injury. Ex elevated expression of miR21 and decreased expression of miR 199a-3p correlating with significant change in the expression of their respective target genes: PTEN mRNA decreased and mTOR mRNA increased. Western blotting confirmed comparable changes in protein levels. An increase in phosphorylated-S6 (a downstream effector of mTOR) within intermediate grey neurons in Ex rats was blocked by Rapamycin treatment. It thus appears possible that activity-dependent plasticity in the injured spinal cord is modulated in part through miRs that regulate PTEN and mTOR signaling and may indicate an increase in the regenerative potential of neurons affected by a SCI.