Diversity of neurotrophin action in the postnatal spinal cord

Planet of the AAVs: The Spinal Cord Injury Episode

The spinal cord injury (SCI) is a medical and life-disrupting condition with devastating consequences for the physical, social, and professional welfare of patients, and there is no adequate treatment for it. At the same time, gene therapy has been studied as a promising approach for the treatment of neurological and neurodegenerative disorders by delivering remedial genes to the central nervous system (CNS), of which the spinal cord is a part. For gene therapy, multiple vectors have been introduced, including integrating lentiviral vectors and non-integrating adeno-associated virus (AAV) vectors. AAV vectors are a promising system for transgene delivery into the CNS due to their safety profile as well as long-term gene expression. Gene therapy mediated by AAV vectors shows potential for treating SCI by delivering certain genetic information to specific cell types. This review has focused on a potential treatment of SCI by gene therapy using AAV vectors.

Effects of hindlimb unloading on neurotrophins in the rat spinal cord and soleus muscle

The aim of the present study was to investigate the effects of hindlimb unloading (HU) on the expression of neurotrophin-3 (NT-3) and brain-derived neurotrophic factor (BDNF), together with the expression of their high-affinity receptors tropomyosin receptor kinase C (TrkC) and tropomyosin receptor kinase B (TrkB), in lumbar (L4-6) segment of the spinal cord and in the soleus muscle. The mRNA and protein levels of the genes of interest were compared using quantitative PCR and western blot assays. Immunohistochemistry for NT-3 and BDNF was used to detect the levels of protein in the motoneurons in the lateral motor column. In this study, NT-3 and BDNF mRNA and protein expression were significantly increased in the spinal cord and soleus muscle after HU. NT-3 immunoreactivity, but not BDNF immunoreactivity, was significantly increased in the large motoneurons located in lateral motor column after 14 days of HU. The level of TrkC protein in the spinal cord and soleus muscle were significantly elevated after both 7 days and 14 days of HU. However, TrkC mRNA, TrkB mRNA and TrkB protein levels did not change significantly. Elevated BDNF, NT-3 and TrkC levels in the neuromuscular system indicate that neurotrophins are involved in HU-induced neuromuscular plasticity. NT-3 is a candidate to mediate the synaptic efficacy between alpha motoneurons and group Ia afferents.

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.

Nerve growth factor and nociception: from experimental embryology to new analgesic therapy

Nerve growth factor (NGF) is central to the development and functional regulation of sensory neurons that signal the first events that lead to pain. These sensory neurons, called nociceptors, require NGF in the early embryo to survive and also for their functional maturation. The long road from the discovery of NGF and its roles during development to the realization that NGF plays a major role in the pathophysiology of inflammatory pain will be reviewed. In particular, we will discuss the various signaling events initiated by NGF that lead to long-lasting thermal and mechanical hyperalgesia in animals and in man. It has been realized relatively recently that humanized function blocking antibodies directed against NGF show remarkably analgesic potency in human clinical trials for painful conditions as varied as osteoarthritis, lower back pain, and interstitial cystitis. Thus, anti-NGF medication has the potential to make a major impact on day-to-day chronic pain treatment in the near future. It is therefore all the more important to understand the precise pathways and mechanisms that are controlled by NGF to both initiate and sustain mechanical and thermal hyperalgesia. Recent work suggests that NGF-dependent regulation of the mechanosensory properties of sensory neurons that signal mechanical pain may open new mechanistic avenues to refine and exploit relevant molecular targets for novel analgesics.

Activity-dependent increase in neurotrophic factors is associated with an enhanced modulation of spinal reflexes after spinal cord injury

Activity-based therapies such as passive bicycling and step-training on a treadmill contribute to motor recovery after spinal cord injury (SCI), leading to a greater number of steps performed, improved gait kinematics, recovery of phase-dependent modulation of spinal reflexes, and prevention of decrease in muscle mass. Both tasks consist of alternating movements that rhythmically stretch and shorten hindlimb muscles. However, the paralyzed hindlimbs are passively moved by a motorized apparatus during bike-training, whereas locomotor movements during step-training are generated by spinal networks triggered by afferent feedback. Our objective was to compare the task-dependent effect of bike- and step-training after SCI on physiological measures of spinal cord plasticity in relation to changes in levels of neurotrophic factors. Thirty adult female Sprague-Dawley rats underwent complete spinal transection at a low thoracic level (T12). The rats were assigned to one of three groups: bike-training, step-training, or no training. The exercise regimen consisted of 15 min/d, 5 days/week, for 4 weeks, beginning 5 days after SCI. During a terminal experiment, H-reflexes were recorded from interosseus foot muscles following stimulation of the tibial nerve at 0.3, 5, or 10 Hz. The animals were sacrificed and the spinal cords were harvested for Western blot analysis of the expression of neurotrophic factors in the lumbar spinal cord. We provide evidence that bike- and step-training significantly increase the levels of brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and NT-4 in the lumbar enlargement of SCI rats, whereas only step-training increased glial cell-derived neurotrophic factor (GDNF) levels. An increase in neurotrophic factor protein levels that positively correlated with the recovery of H-reflex frequency-dependent depression suggests a role for neurotrophic factors in reflex normalization.

Pre-differentiated embryonic stem cells promote neuronal regeneration by cross-coupling of BDNF and IL-6 signaling pathways in the host tissue

The mechanism of embryonic stem (ES) cell therapeutic action remains far from being elucidated. Our recent report has shown that transplantation of ES cells, predifferentiated into neuronal progenitors, prevented appearance of chronic pain behaviors in mice after experimentally induced spinal cord injury. In the current study, we tested the hypothesis that this beneficial effect is mediated by antiapoptotic and regenerative signaling pathways activated in the host tissue by transplanted ES cells. Spinal cord injury was induced by unilateral microinjections of quisqualic acid at spinal levels T12-L2. At 1 week after injury, the pre-differentiated towards neuronal phenotype ES cells were transplanted into the site of injury. Here we show that transplantation of pre-differentiated ES cells activate both brain-derived neurotrophic factor (BDNF) and interleukin-6 (IL-6) signaling pathways in the host tissue, leading to activation of cAMP/PKA, phosporylation of cofilin and synapsin I, and promoting regenerative growth and neuronal survival.

Muscle injection of AAV-NT3 promotes anatomical reorganization of CST axons and improves behavioral outcome following SCI

Here we propose the use of adeno-associated virus (AAV) vectors as a non-invasive vehicle for the nervous system to deliver genes to spinal motoneurons, based on their retrograde transport from muscle. Long-term protein expression in lower cervical motoneurons was achieved after injections of AAV into the triceps, independently of serotypes 1, 2, or 5. Muscle injections of AAV5-neurotrophin 3 (NT3) resulted in a significant increase in the levels of NT3 in the cervical enlargement, compared to those obtained after injections of AAV5-GFP. Following a dorsal lesion at C4/C5, animals injected with AAV5-NT3 made fewer errors (footslips) in the horizontal ladder test compared to those injected with AAV5-GFP. In parallel, the number and length of corticospinal tract (CST) fibers circumventing the injury site were significantly increased in rats injected with AAV5-NT3. Compared to controls, we observed less astrogliosis and less CST axonal retraction and/or enhanced sprouting in animals injected with AAV5-NT3. In sum, we demonstrate here that the delivery of nt3 via retrograde transport of AAV from triceps to cervical motoneurons leads to reduced functional loss and anatomical reorganization of the CST following injury, without introducing additional injury to the spinal cord.

Nerve growth factor regulates the firing patterns and synaptic composition of motoneurons

Target-derived neurotrophins exert powerful synaptotrophic actions in the adult brain and are involved in the regulation of different forms of synaptic plasticity. Target disconnection produces a profound synaptic stripping due to the lack of trophic support. Consequently, target reinnervation leads to synaptic remodeling and restoration of cellular functions. Extraocular motoneurons are unique in that they normally express the TrkA neurotrophin receptor in the adult, a feature not seen in other cranial or spinal motoneurons, except after lesions such as axotomy or in neurodegenerative diseases like amyotrophic lateral sclerosis. We investigated the effects of nerve growth factor (NGF) by retrogradely delivering this neurotrophin to abducens motoneurons of adult cats. Axotomy reduced the density of somatic boutons and the overall tonic and phasic firing modulation. Treatment with NGF restored synaptic inputs and firing modulation in axotomized motoneurons. When K252a, a selective inhibitor of tyrosine kinase activity, was applied to specifically test TrkA effects, the NGF-mediated restoration of synapses and firing-related parameters was abolished. Discharge variability and recruitment threshold were, however, increased by NGF compared with control or axotomized motoneurons. Interestingly, these parameters returned to normal following application of REX, an antibody raised against neurotrophin receptor p75 (p75(NTR)). In conclusion, NGF, acting retrogradely through TrkA receptors, supports afferent boutons and regulates the burst and tonic signals correlated with eye movements. On the other hand, p75(NTR) activation regulates recruitment threshold, which impacts on firing regularity. To our knowledge, this is the first report showing powerful synaptotrophic effects of NGF on motoneurons in vivo.

Enhanced transmission at a spinal synapse triggered in vivo by an injury signal independent of altered synaptic activity

Peripheral nerve crush initiates a robust increase in transmission strength at spinal synapses made by axotomized group IA primary sensory neurons. To study the injury signal that initiates synaptic enhancement in vivo, we designed experiments to manipulate the enlargement of EPSPs produced in spinal motoneurons (MNs) by IA afferents 3 d after nerve crush in anesthetized adult rats. If nerve crush initiates IA EPSP enlargement as proposed by reducing impulse-evoked transmission in crushed IA afferents, then restoring synaptic activity should eliminate enlargement. Daily electrical stimulation of the nerve proximal to the crush site did, in fact, eliminate enlargement but was, surprisingly, just as effective when the action potentials evoked in crushed afferents were prevented from propagating into the spinal cord. Consistent with its independence from altered synaptic activity, we found that IA EPSP enlargement was also eliminated by colchicine blockade of axon transport in the crushed nerve. Together with the observation that colchicine treatment of intact nerves had no short-term effect on IA EPSPs, we conclude that enhancement of IA-MN transmission is initiated by some yet to be identified positive injury signal generated independent of altered synaptic activity. The results establish a new set of criteria that constrain candidate signaling molecules in vivo to ones that develop quickly at the peripheral injury site, move centrally by axon transport, and initiate enhanced transmission at the central synapses of crushed primary sensory afferents through a mechanism that can be modulated by action potential activity restricted to the axons of crushed afferents.

BDNF and learning: Evidence that instrumental training promotes learning within the spinal cord by up-regulating BDNF expression

We have previously shown that the spinal cord is capable of learning a sensorimotor task in the absence of supraspinal input. Given the action of brain-derived neurotrophic factor (BDNF) on hippocampal learning, the current studies examined the role of BDNF in spinal learning. BDNF is a strong synaptic facilitator and, in association with other molecular signals (e.g. cAMP-response element binding protein (CREB), calcium/calmodulin activated protein kinase II (CaMKII) and synapsin I), important for learning. Spinally transected rats given shock to one hind leg when the leg extended beyond a selected threshold exhibited a progressive increase in flexion duration that minimized shock exposure, a simple form of instrumental learning. Instrumental learning resulted in elevated mRNA levels of BDNF, CaMKII, CREB, and synapsin I in the lumbar spinal cord region. The increases in BDNF, CREB, and CaMKII were proportional to the learning performance. Prior work has shown that instrumental training facilitates learning when subjects are tested on the contralateral leg with a higher response criterion. Pretreatment with the BDNF inhibitor TrkB-IgG blocked this facilitatory effect, as did the CaMKII inhibitor AIP. Intrathecal administration of BDNF facilitated learning when subjects were tested with a high response criterion. The findings indicate that instrumental training enables learning and elevates BDNF mRNA levels within the lumbar spinal cord. BDNF is both necessary, and sufficient, to produce the enabling effect.

Expression of neurotrophic factors in neonatal rats after peripheral inflammation

Neonatal peripheral inflammatory insult might result in the alteration of neuronal development in the nociceptive circuit. During early postnatal period, neurotrophins play important roles in neural development and sensory nerve innervation in the central and peripheral nervous systems. In this study, we investigated mRNA expression for neurotrophic factors and their receptors in the dorsal root ganglia of rat pups during postnatal life after peripheral inflammation induced by injection of complete Freund's adjuvant (CFA) into hind paw on postnatal day 1. Our results showed that mRNA expression levels of alpha-calcitonin gene-related peptides, tropomyosin-related kinase-A (trkA), p75 neurotrophin receptor (p75(NTR)), and brain-derived neurotrophic factor (BDNF) elevated significantly after CFA treatment. Such an increase began 1 day after CFA treatment and lasted 2 to 3 days for trkA, p75(NTR), and BDNF. In contrast, there was no change in mRNA expression levels for neurotrophin-4/5, beta-nerve growth factor (beta-NGF), trkB, glial cell line-derived neurotrophin factor, and receptor protein tyrosine kinase protein. Our study demonstrated that neonatal peripheral inflammatory insult might result in molecular changes of neurotrophic factors, particularly in NGF receptors and BDNF, in the process of neuronal development and plasticity in primary afferents during early neonatal period.

PERSPECTIVE

Neonatal peripheral inflammation model has been used for the exploration of neuropathic pain mechanism for years. This work provided further detailed information about possible neurotransmitters and peptides involved in this process. This might also lead to future clinical application.

Can regenerating axons recapitulate developmental guidance during recovery from spinal cord injury?

The precise wiring of the adult mammalian CNS originates during a period of stunning growth, guidance and plasticity that occurs during and shortly after development. When injured in adults, this intricate system fails to regenerate. Even when the obstacles to regeneration are cleared, growing adult CNS fibres usually remain misdirected and fail to reform functional connections. Here, we attempt to fill an important niche related to the topics of nervous system development and regeneration. We specifically contrast the difficulties faced by growing fibres within the adult context to the precise circuit-forming capabilities of developing fibres. In addition to focusing on methods to stimulate growth in the adult, we also expand on approaches to recapitulate development itself.

Viral delivery of NR2D subunits reduces Mg2+ block of NMDA receptor and restores NT-3-induced potentiation of AMPA-kainate responses in maturing rat motoneurons

N-methyl-D-aspartate (NMDA) responsiveness of motoneurons declines during the initial 2 postnatal weeks due to increasing Mg2+ block of NMDA receptors. Using gene chip analyses, RT-PCR, and immunochemistry, we have shown that the NR2D subunit of the NMDA receptor (NMDAR), known to confer resistance to Mg2+ block, also declines in motoneurons during this period. We injected a viral construct (HSVnr2d) into the lumbar spinal cord on postnatal day 2 in an attempt to restore NMDAR function in motoneurons during the second postnatal week. Following HSVnr2d injection, we detected elevated levels of NR2D mRNA in spinal cord samples and NR2D protein specifically in motoneurons. These molecular changes were associated with marked functional alterations whereby NMDAR-mediated responses in motoneurons associated with both dorsal root (DR) and ventrolateral funiculus (VLF) inputs returned to values observed at E18 due to decreased Mg2+ blockade. Viruses carrying the beta-galactosidase gene did not induce these effects. NT-3 is known to potentiate AMPA-kainate responses in motoneurons if the response has an NMDAR-mediated component and thus is normally ineffective during the second postnatal week. Restoration of NMDAR-mediated responsiveness in the second postnatal week was accompanied by a return of the ability of neurotrophin-3 (NT-3) to potentiate the AMPA-kainate responses produced by both DR and VLF synaptic inputs. We conclude that delivery of the gene for a specific NMDA subunit can restore properties characteristic of younger animals to spinal cord motoneurons. This approach might be useful for enhancing the function of fibers surviving in the damaged spinal cord.

Involvement of collapsin response mediator proteins in the neurite extension induced by neurotrophins in dorsal root ganglion neurons

The pattern of sensory neuron extensions and connections is established during embryonic development through complex and varied guidance cues that control motility of growth cones and neurite morphogenesis. Semaphorins and neurotrophins are molecules that act as such cues. Collapsin response mediator proteins (CRMPs) are thought to be part of the semaphorin signal transduction pathway implicated in semaphorin-induced growth cone collapse. In this report, we present evidence that CRMPs are also involved in the neurite extension controlled by neurotrophins. We found that specific antibodies and the dominant-negative mutant protein for CRMP2 both potentiated the neurite extension induced by NGF, while specific antibodies and the corresponding mutant protein for CRMP1 both abolished the neurite extension induced by NT3. Our data suggest that CRMP2 has a negative effect on neurite extension induced by NGF and CRMP1 participates in the neurite formation/extension induced by NT3. These results point to a function for CRMPs in the regulation of neurite outgrowth induced by neurotrophins in sensory neurons.

Neurotrophins in spinal cord nociceptive pathways

Neurotrophins are a well-known family of growth factors for the central and peripheral nervous systems. In the course of the last years, several lines of evidence converged to indicate that some members of the family, particularly NGF and BDNF, also participate in structural and functional plasticity of nociceptive pathways within the dorsal root ganglia and spinal cord. A subpopulation of small-sized dorsal root ganglion neurons is sensitive to NGF and responds to peripheral NGF stimulation with upregulation of BDNF synthesis and increased anterograde transport to the dorsal horn. In the latter, release of BDNF appears to modulate or even mediate nociceptive sensory inputs and pain hypersensitivity. We summarize here the status of the art on the role of neurotrophins in nociceptive pathways, with special emphasis on short-term synaptic and intracellular events that are mediated by this novel class of neuromessengers in the dorsal horn. Under this perspective we review the findings obtained through an array of techniques in naïve and transgenic animals that provide insight into the modulatory mechanisms of BDNF at central synapses. We also report on the results obtained after immunocytochemistry, in situ hybridization, and monitoring intracellular calcium levels by confocal microscopy, that led to hypothesize that also NGF might have a direct central effect in pain modulation. Although it is unclear whether or not NGF may be released at dorsal horn endings of certain nociceptors in vivo, we believe that these findings offer a clue for further studies aiming to elucidate the putative central effects of NGF and other neurotrophins in nociceptive pathways.