The neurotrophins BDNF, NT-3, and NGF display distinct patterns of retrograde axonal transport in peripheral and central neurons.

The pattern of retrograde axonal transport of the target-derived neurotrophic molecule, nerve growth factor (NGF), correlates with its trophic actions in adult neurons. We have determined that the NGF-related neurotrophins, brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3), are also retrogradely transported by distinct populations of peripheral and central nervous system neurons in the adult. All three 125I-labeled neurotrophins are retrogradely transported to sites previously shown to contain neurotrophin-responsive neurons as assessed in vitro, such as dorsal root ganglion and basal forebrain neurons. The patterns of transport also indicate the existence of neuronal populations that selectively transport NT-3 and/or BDNF, but not NGF, such as spinal cord motor neurons, neurons in the entorhinal cortex, thalamus, and neurons within the hippocampus itself. Our observations suggest that neurotrophins are transported by overlapping as well as distinct populations of neurons when injected into a given target field. Retrograde transport may thus be predictive of neuronal types selectively responsive to either BDNF or NT-3 in the adult, as first demonstrated for NGF.     

Expression of neurotrophin receptors trkB and trkC and their ligands in rat adrenal gland and the intermediolateral column of the spinal cord

Neurotrophins and their trk receptors constitute major classes of signaling molecules with important actions in the developing and adult nervous system. With regard to the sympathoadrenal cell lineage, which gives rise to sympathetic neurons and chromaffin cells, neurotrophin-3 (NT-3) and nerve growth factor (NGF) are thought to influence developing sympathetic neurons. Neurotrophin requirements of chromaffin cells of the adrenal medulla are less well understood than those for NGF. In order to provide the bases for understanding of putative functions of neurotrophins for the development and maintenance of chromaffin cells and their preganglionic innervation, in situ hybridization has been used to study the expression of brain-derived neurotrophic factor (BDNF) and NT-3, together with their cognate receptors trkB and trkC, in the adrenal gland and in the intermediolateral column (IML) of the spinal cord. BDNF is highly expressed in the embryonic adrenal cortex and later in cells of the cortical reticularis zone. Adrenal medullary chromaffin cells fail to express detectable levels of mRNAs for BDNF, NT-3, and their cognate receptors trkB and trkC. Neurons in the IML express BDNF and trkB, and low levels of NT-3 and trkC. Our data make it unlikely that BDNF and NT-3 serve as retrograde trophic factors for IML neurons but suggest roles of BDNF and NT-3 locally within the spinal cord and possibly for sensory nerves of the adrenal cortex.     

Integrins and the extracellular matrix: key mediators of development and regeneration of the sensory nervous system

The somatosensory nervous system is responsible for the transmission of a multitude of sensory information from specialized receptors in the periphery to the central nervous system. Sensory afferents can potentially be damaged at several sites: in the peripheral nerve; the dorsal root; or the dorsal columns of the spinal cord; and the success of regeneration depends on the site of injury. The regeneration of peripheral nerve branches following injury is relatively successful compared to central branches. This is largely attributed to the presence of neurotrophic factors and a Schwann cell basement membrane rich in permissive extracellular matrix (ECM) components which promote axonal regeneration in the peripheral nerve. Modulation of the ECM environment and/or neuronal integrins may enhance regenerative potential of sensory neurons following peripheral or central nerve injury or disease. This review describes the interactions between integrins and ECM molecules (particularly the growth supportive ligands, laminin, and fibronectin; and the growth inhibitory chondroitin sulfate proteoglycans (CSPGs)) during development and regeneration of sensory neurons following physical injury or neuropathy.     

Peripherally-derived BDNF promotes regeneration of ascending sensory neurons after spinal cord injury

Background

The blood brain barrier (BBB) and truncated trkB receptor on astrocytes prevent the penetration of brain derived neurotrophic factor (BDNF) applied into the peripheral (PNS) and central nervous system (CNS) thus restrict its application in the treatment of nervous diseases. As BDNF is anterogradely transported by axons, we propose that peripherally derived and/or applied BDNF may act on the regeneration of central axons of ascending sensory neurons.

Methodology/Principal Findings

The present study aimed to test the hypothesis by using conditioning lesion of the sciatic nerve as a model to increase the expression of endogenous BDNF in sensory neurons and by injecting exogenous BDNF into the peripheral nerve or tissues. Here we showed that most of regenerating sensory neurons expressed BDNF and p-CREB but not p75NTR. Conditioning-lesion induced regeneration of ascending sensory neuron and the increase in the number of p-Erk positive and GAP-43 positive neurons was blocked by the injection of the BDNF antiserum in the periphery. Enhanced neurite outgrowth of dorsal root ganglia (DRG) neurons in vitro by conditioning lesion was also inhibited by the neutralization with the BDNF antiserum. The delivery of exogenous BDNF into the sciatic nerve or the footpad significantly increased the number of regenerating DRG neurons and regenerating sensory axons in the injured spinal cord. In a contusion injury model, an injection of BDNF into the footpad promoted recovery of motor functions.

Conclusions/Significance

Our data suggest that endogenous BDNF in DRG and spinal cord is required for the enhanced regeneration of ascending sensory neurons after conditioning lesion of sciatic nerve and peripherally applied BDNF may have therapeutic effects on the spinal cord injury.     

Trophic interactions between sensory nerves and their targets

Neurotrophins are target-derived trophic factors essential for the survival and maintenance of neurons. Among these, nerve growth factor (NGF) and neurotrophin-3 (NT-3) are particularly important for sensory neurons. The actions of neurotrophins are through the p75 low-affinity receptor and the high-affinity receptor tyrosine kinase(trk). Each neurotrophin has its preferred receptor, i.e.trkA for NGF, andtrkC for NT-3. The primary sensory neurons in the dorsal root ganglion are classified into two categories, namely, the large and small sensory neurons based on their size. The large sensory neurons with the expression oftrkC depend on NT-3 for development and subserve the function of position sensations. Some of the small sensory neurons expresstrkA and are NGF-dependent. They are responsible for nociceptive sensation, the detection of painful and thermal stimuli. A more intriguing observation is the bidirectional interactions between nociceptive nerves and their target, the skin. The peripheral processes of small sensory neurons innervate the epidermis of the skin as free nerve endings. In denervated skin, there is a drastic reduction in the epidermal thickness, a finding corroborated by the phenomenon of trophic change, the shining and thinning of the skin, in the disorders of peripheral nerves. The performance of animals with peripheral nerve disorders improved after administration of neurotrophic factors. Based on these results, the therapeutic potentials of neurotrophic factors in human are under investigation.     

Differential involvement of trigeminal transition zone and laminated subnucleus caudalis in orofacial deep and cutaneous hyperalgesia: the effects of interleukin-10 and glial inhibitors

Background

Understanding the underlying mechanisms of neuropathic pain caused by damage to the peripheral nervous system remains challenging and could lead to significantly improved therapies. Disturbance of homeostasis not only occurs at the site of injury but also extends to the spinal cord and brain involving various types of cells. Emerging data implicate neuroimmune interaction in the initiation and maintenance of chronic pain hypersensitivity.

Results

In this study, we sought to investigate the effects of TGF-β1, a potent anti-inflammatory cytokine, in alleviating nerve injury-induced neuropathic pain in rats. By using a well established neuropathic pain animal model (partial ligation of the sciatic nerve), we demonstrated that intrathecal infusion of recombinant TGF-β1 significantly attenuated nerve injury-induced neuropathic pain. TGF-β1 treatment not only prevents development of neuropathic pain following nerve injury, but also reverses previously established neuropathic pain conditions. The biological outcomes of TGF-β1 in this context are attributed to its pleiotropic effects. It inhibits peripheral nerve injury-induced spinal microgliosis, spinal microglial and astrocytic activation, and exhibits a powerful neuroprotective effect by preventing the induction of ATF3⁺ neurons following nerve ligation, consequently reducing the expression of chemokine MCP-1 in damaged neurons. TGF-β1 treatment also suppresses nerve injury-induced inflammatory response in the spinal cord, as revealed by a reduction in cytokine expression.

Conclusion

Our findings revealed that TGF-β1 is effective in the treatment of neuropathic by targeting both neurons and glial cells. We suggest that therapeutic agents such as TGF-β1 having multipotent effects on different types of cells could work in synergy to regain homeostasis in local spinal cord microenvironments, therefore contributing to attenuate neuropathic pain.     

Neuroprotective and growth-promoting effects of bone marrow stromal cells after cervical spinal cord injury in adult rats

Background aimsBone marrow stromal cells (BMSC) have been shown to provide neuroprotection after transplantation into the injured central nervous system. The present study investigated whether adult rat BMSC differentiated along a Schwann cell lineage could increase production of trophic factors and support neuronal survival and axonal regeneration after transplantation into the injured spinal cord.MethodsAfter cervical C4 hemi-section, 5-bromo-2-deoxyuridine (BrdU)/green fluorescent protein (GFP)-labeled BMSC were injected into the lateral funiculus at 1 mm rostral and caudal to the lesion site. Spinal cords were analyzed 2–13 weeks after transplantation.Results and ConclusionsTreatment of native BMSC with Schwann cell-differentiating factors significantly increased production of brain-derived neurotrophic factor in vitro. Transplanted undifferentiated and differentiated BMSC remained at the injection sites, and in the trauma zone were often associated with neurofilament-positive fibers and increased levels of vascular endothelial growth factor. BMSC promoted extensive in-growth of serotonin-positive raphaespinal axons and calcitonin gene-related peptide (CGRP)-positive dorsal root sensory axons into the trauma zone, and significantly attenuated astroglial and microglial cell reactions, but induced aberrant sprouting of CGRP-immunoreactive axons in Rexed's lamina III. Differentiated BMSC provided neuroprotection for axotomized rubrospinal neurons and increased the density of rubrospinal axons in the dorsolateral funiculus rostral to the injury site. The present results suggest that BMSC induced along the Schwann cell lineage increase expression of trophic factors and have neuroprotective and growth-promoting effects after spinal cord injury.     

Neurotrophin-4: a survival factor for adult sensory neurons

The nerve growth factor (NGF) family of neurotrophins provides a substantial part of the normal trophic support for sensory neurons during development. Although these neurotrophins, which include Brain-Derived Neurotrophic Factor (BDNF), Neurotrophin-3 (NT-3), and Neurotrophin-4 (NT-4), continue to be expressed into adulthood, there is little evidence that they are survival factors for adult neurons. Here we have examined the age-dependent neurotrophic requirements of a specialized type of mechanoreceptive neuron, called a D-hair receptor, in the dorsal root ganglion (DRG). Studies using knockout mice have demonstrated that the survival of D-hair receptors is dependent upon both NT-3 and NT-4. Here, we show that the time period when D-hair receptors require these two neurotrophins is different. Survival of D-hair receptors depends on NT-3 early in postnatal development and NT-4 later in the mature animal. The age-dependent loss of D-hair neurons in older NT-4 knockout mice was accompanied by a large reduction (78%) in neurons positive for the NT-4 receptor (trkB) together with neuronal apoptosis in the DRG. This is the first evidence that sensory neurons have a physiological requirement for a single neurotrophin for their continued survival in the adult.     

Steroid synthesis and metabolism in the nervous system: Trophic and protective effects

Steroids influence the activity and plasticity of neurons and glial cells during early development, and they continue to exert trophic and protective effects in the adult nervous system. Steroids are produced by the gonads and adrenal glands and reach the brain, the spinal cord and the peripheral nerves via the bloodstream. However, some of them, named “neurosteroids”, can also be synthesized within the nervous system. They include pregnenolone, progesterone, dehydroepiandrosterone and their reduced metabolites and sulfate esters. Little is known concerning the regulation of steroid synthesis in the nervous system, which involves interactions between different cell types. For example, the synthesis of progesterone by Schwann cells in peripheral nerves is regulated by a diffusible neuronal signal. Neurotrophic and neuroprotective effects of steroids have been documented both in cell culture and in vivo. PROG plays an important role in the neurological recovery from traumatic injury of the brain and spinal cord by mechanisms involving protection from excitotoxic cell death, lipid peroxydation and the induction of specific enzymes. After transection of the rat spinal cord, PROG increases the number of nitric oxide synthase expressing astrocytes immediately above and below the lesion. PROG also plays an important role in the formation of new myelin sheaths. This has been shown in the regenerating mouse sciatic nerve after lesion and in cocultures of sensory neurons and Schwann cells. PROG promotes myelination by activating the expression of genes coding for myelin proteins. The modulation of neurostransmitter receptors, in particular the type A γ-aminobutyric acid, the N-methyl-D-aspartate and the sigma 1 receptors, is involved in the psychopharmacological effects of steroids and allows to explain their anticonvulsant, anxiolytic, antidepressive and sedative effects as well as their influence on memory. Pregnenolone sulfate has been shown to reverse age-related deficits in spatial memory performance and to have protective effects on memory in different models of amnesia.     

Distribution of the 75-kD Low-Affinity Nerve Growth Factor Receptor in the Primate Peripheral Nervous System

Disruption of the 75-kD low-affinity nerve growth factor (NGF) receptor (p75) has been shown to result in sensory and sympathetic nervous system deficits (Lee et al., 1992a,b). In order to establish precisely which subsets of neurons are capable of responding to neurotrophins (NTs) through the low-affinity NGF receptor, p75 was localized in the primate autonomic and somatic sensory nervous systems. In the autonomic system, cell bodies of some parasympathetic and enteric neurons expressed detectable levels of p75, whereas all sympathetic neurons expressed the protein. In the sensory system, some, but not all, cell bodies were labeled in cranial and spinal sensory ganglia and in the mesencephalic nucleus. Some peripheral and central projections of the sensory neurons were also labeled. Centrally, most of the labeled processes were found in regions containing primarily small unmyelinated fibers, including lamina II of Rexed and areas of the solitary tract and nucleus. Peripherally, labeled processes were associated with unmyelinated nerves and specialized structures such as taste buds and Meissner corpuscles, but not with myelinated processes. This study indicates that the subset of neurons in the autonomic nervous system likely to be capable of responding to neurotrophins is broader than generally thought, and that p75-ex-pressing neurons tend to be clustered. Moreover, in the sensory nervous system p75 is expressed by most cell bodies, but expression in their projections is restricted both peripherally and centrally to unmyelinated processes and nerve terminals.     

Combining Peripheral Nerve Grafting and Matrix Modulation to Repair the Injured Rat Spinal Cord

Traumatic injury to the spinal cord (SCI) causes death of neurons, disruption of motor and sensory nerve fiber (axon) pathways and disruption of communication with the brain. One of the goals of our research is to promote axon regeneration to restore connectivity across the lesion site. To accomplish this we developed a peripheral nerve (PN) grafting technique where segments of sciatic nerve are either placed directly between the damaged ends of the spinal cord or are used to form a bridge across the lesion. There are several advantages to this approach compared to transplantation of other neural tissues; regenerating axons can be directed towards a specific target area, the number and source of regenerating axons is easily determined by tracing techniques, the graft can be used for electrophysiological experiments to measure functional recovery associated with axons in the graft, and it is possible to use an autologous nerve to reduce the possibility of graft rejection. In our lab we have performed both autologous (donor and recipient are the same animal) and heterologous (donor and recipient are different animals) grafts with comparable results. This approach has been used successfully in both acute and chronic injury situations. Regenerated axons that reach the distal end of the PN graft often fail to extend back into the spinal cord, so we use microinjections of chondroitinase to degrade inhibitory molecules associated with the scar tissue surrounding the area of SCI. At the same time we have found that providing exogenous growth and trophic molecules encourages longer distance axonal regrowth into the spinal cord. Several months after transplantation we perform a variety of anatomical, behavioral and electrophysiological tests to evaluate the recovery of function in our spinal cord injured animals. This experimental approach has been used successfully in several spinal cord injury models, at different levels of injury and in different species (mouse, rat and cat). Importantly, the peripheral nerve grafting approach is effective in promoting regeneration by acute and chronically injured neurons.Download video file.^{(224M, mp4)}     

The Beneficial Effect of Chitooligosaccharides on Cell Behavior and Function of Primary Schwann Cells is Accompanied by Up-Regulation of Adhesion Proteins and Neurotrophins

Chitosan-based tissue engineered nerve grafts are successfully used for bridging peripheral nerve gaps. The biodegradation products of chitosan are water-dissolvable chitooligosaccharides (COSs), which have been shown to support peripheral nerve regeneration. In this study, we aimed to examine in vitro interactions between COSs and Schwann cells (SCs), the principal glial cells in the peripheral nervous system. Treatment of primary SCs with COSs enhanced cell survival and promoted cell proliferation in a dose-dependent manner (0.25–1.0 mg/ml), as determined by real-time cell analyzer-based assay, cell growth assay, cell cycle analysis, and EdU incorporation. Western blot analysis and immunocytochemistry with antibodies against MBP and MAG (two myelin-specific markers) showed that COSs enhanced axonal myelination in a co-culture system consisting of SCs and dorsal root ganglia (DRGs). Furthermore, we observed that COSs enhanced the protein expression of N-cadherin and β-catenin in primary SCs, and also increased the release of BDNF and NGF in co-culture of SCs with DRGs. And we also noted that knockdown of N-cadherin in primary SCs reduced COSs-induced increase in cell proliferation. Our findings suggested that beneficial effects of COSs on cell behavior and functions of primary SCs might be accompanied by up-regulation of adhesion proteins and neurotrophins, thus providing a new insight into the supportive role of COSs during peripheral nerve regeneration.     

Neurotrophin-3 as an essential signal for the developing nervous system

Rapid advances in characterization of the biological actions mediated by the third member of the neurotrophin family, neurotrophin-3 (NT-3), have been made recently in vitro as well asin situ. These have been made possible by the cloning of the genes for NT-3 and for its transducing receptor tyrosine kinase TrkC. This article will focus on the roles of NT-3 in the nervous system.In situ localization of NT-3 consistent with that of its receptor is manifested at all developmental stages studied and into adulthood. Through TrkC, NT-3 signals a number of trophic effects, ranging from mitogenesis, promotion of survival, or differentiation, depending on the developmental stage of the target cells. The sites of action of NT-3 reside primarily in the peripheral nervous system (PNS), various areas of the central nervous system (CNS), and in the enteric nervous system (ENS). Analyses of the phenotypes of transgenic mice lacking NT-3 or injection of embryos with a blocking antibody have so far revealed the essential role of NT-3 in development of specific populations of the PNS, and in particular of proprioceptive, nodose, and auditory sensory neurons and of sympathetic neurons. The actions of NT-3 also extend to modulation of transmitter release at several types of synapses in the periphery as well as in the adult CNS. In addition, NT-3 may play a role in the development of tissues other than the nervous system, such as the cardiovascular system. Future investigations will widen the understanding of the many roles of NT-3 on both neuronal and nonneuronal cells.     

Differential expression of mRNAs for neurotrophins and their receptors after axotomy of the sciatic nerve

The neurotrophin family includes NGF, brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4 (NT-4). Previous studies have demonstrated that expression of NGF and its low-affinity receptor is induced in nonneuronal cells of the distal segment of the transected sciatic nerve suggesting a role for NGF during axonal regeneration (Johnson, E. M., M. Taniuchi, and P. S. DeStefano. 1988. Trends Neurosci. 11:299-304). To assess the role of the other neurotrophins and the members of the family of Trk signaling neurotrophin receptors, we have here quantified the levels of mRNAs for BDNF, NT-3, and NT-4 as well as mRNAs for trkA, trkB, and trkC at different times after transection of the sciatic nerve in adult rats. A marked increase of BDNF and NT-4 mRNAs in the distal segment of the sciatic nerve was seen 2 wk after the lesion. The increase in BDNF mRNA was mediated by a selective activation of the BDNF exon IV promoter and adrenalectomy attenuated this increase by 50%. NT-3 mRNA, on the other hand, decreased shortly after the transection but returned to control levels 2 wk later. In Schwann cells ensheathing the sciatic nerve, only trkB mRNA encoding truncated TrkB receptors was detected with reduced levels in the distal part of the lesioned nerve. Similar results were seen using a probe that detects all forms of trkC mRNA. In the denervated gastrocnemius muscle, the level of BDNF mRNA increased, NT-3 mRNA did not change, while NT-4 mRNA decreased. In the spinal cord, only small changes were seen in the levels of neutrophin and trk mRNAs. These results show that expression of mRNAs for neurotrophins and their Trk receptors is differentially regulated after a peripheral nerve injury. Based on these results a model is presented for how the different neurotrophins could cooperate to promote regeneration of injured peripheral nerves.     

ErbB transmembrane tyrosine kinase receptors are expressed by sensory and motor neurons projecting into sciatic nerve.

Adult spinal cord motor and dorsal root ganglion (DRG) sensory neurons express multiple neuregulin-1 (NRG-1) isoforms that act as axon-associated factors promoting neuromuscular junction formation and Schwann cell proliferation and differentiation. NRG-1 isoforms are also expressed by muscle and Schwann cells, suggesting that motor and sensory neurons are themselves acted on by NRG-1 isoforms produced by their peripheral targets. To test this hypothesis, we examined the expression of the NRG-1 receptor subunits erbB2, erbB3, and erbB4 in rat lumbar DRG and spinal cord. All three erbB receptors are expressed in these tissues. Sciatic nerve transection, an injury that induces Schwann cell expression of NRG-1, alters erbB expression in DRG and cord. Virtually all DRG neurons are erbB2- and erbB3-immunoreactive, with erbB4 also detectable in many neurons. In spinal cord white matter, erbB2 and erbB4 antibodies produce dense punctate staining, whereas the erbB3 antibody primarily labels glial cell bodies. Spinal cord dorsal and ventral horn neurons, including alpha-motor neurons, exhibit erbB2, erbB3, and erbB4 immunoreactivity. Spinal cord ventral horn also contains a population of small erbB3+/S100beta+/GFAP- cells (GFAP-negative astrocytes or oligodendrocytes). We conclude that sensory and motor neurons projecting into sciatic nerve express multiple erbB receptors and are potentially NRG-1 responsive.     

Effect of Electroacupuncture on Neurotrophin Expression in Cat Spinal Cord after Partial Dorsal Rhizotomy

Neuroplasticity of the spinal cord following electroacupuncture (EA) has been demonstrated although little is known about the possible underlying mechanism. This study evaluated the effect of EA on expression of neurotrophins in the lamina II of the spinal cord, in cats subjected to dorsal rhizotomy. Cats received bilateral removal of L1–L5 and L7–S2 dorsal root ganglia (DRG, L6 DRG spared) and unilateral EA. They were sacrificed 7 days after surgery, and the L6 spinal segment removed and processed by immunohistochemistry and in situ hybridization histochemistry, to demonstrate the expression of neurotrophins. Significantly greater numbers of nerve growth factor (NGF) and neurotrophin-3 (NT-3) positive neurons, brain-derived neurotrophic factor (BDNF) immunoreactive varicosities and NT-3 positive neurons and glial cells were observed in lamina II on the acupunctured (left) side, compared to the non-acupunctured, contralateral side. Greater number of neurons expressing NGF mRNA was also observed on the acupunctured side. No signal for mRNA to BDNF and NT-3 was detected. The above findings demonstrate that EA can increase the expression of endogenous NGF at both the mRNA and protein level, and BDNF and NT-3 at the protein level. It is postulated that EA may promote the plasticity of the spinal cord by inducing increased expression of neurotrophins.     

Strategies to repair lost sensory connections to the spinal cord

Failure of injured axons to regenerate in the central nervous system (CNS) is the main obstacle for repair of stroke and traumatic injuries to the spinal cord and sensory roots. This regeneration failure is high-lighted at the dorsal root transitional zone (DRTZ), the boundary between the peripheral (PNS) and central nervous system where sensory axons enter the spinal cord. Injured sensory axons regenerate in the PNS compartment of the dorsal root but are halted as soon as they reach the DRTZ. The failure of regenerating dorsal root axons to re-enter the mature spinal cord is a reflection of the generally nonpermissive nature of the CNS environment, in contrast to the regeneration supportive properties of the PNS. The dorsal root injury paradigm is therefore an attractive model for studying mechanisms underlying CNS regeneration failure in general and how to overcome the hostile CNS environment. Here we review the main lines that have been pursued to achieve growth of injured dorsal root axons into the spinal cord: (i) modifying the inhibitory nature of the DRTZ by breaking down or blocking the effect of growth repelling molecules, (ii) stimulate elongation of injured dorsal root axons by a prior conditioning lesion or administration of specific growth factors, (iii) implantation of olfactory ensheathing cells to provide a growth supportive cellular terrain at the DRTZ, and (iv) replacing the regeneration deficient adult dorsal root ganglion neurons with embryonic neurons or neural stem cells.     

Rho-associated kinase (ROCK) inhibitor, Y27632, promotes neurite outgrowth in PC12 cells in the absence of NGF

Neurotrophins support neuronal survival and axonal regeneration after injury. To test whether local expression of Neurotrophin-3 (NT-3) would elicit axonal regeneration we lesioned the corticospinal tract (CST) at the level of the hindbrain and measured the number of axons that would grow from the unlesioned CST to the contralateral side where NT-3 was over expressed at the lumbar level of the spinal cord. An adenoviral vector that carried the rat NT-3 gene and the NGF signal peptide driven by the EF1α promoter (Adv.EF-NT-3) was used. This model enabled us to test the effects of NT-3 on axonal regeneration without confounding injury processes. Biotinylated dextran amine (BDA) was injected into the rat cortex on unlesioned side to mark CST axons 10 days postlesion. Adenoviral vectors (1 × 10⁹ pfu, Adv.EF-NT-3 or Adv.EF-LacZ) were delivered to lumbar spinal cord by retrograde transport from the sciatic nerve 4 days later. Histological examination 3 weeks later revealed that more BDA-labelled axons had grown from the unlesioned CST to the denervated side at the lumbar level. Morphometric measurements showed that a significantly larger number of BDA-labelled CST axons ( p  < 0.001) were present in the animals that were treated with Adv.EF-NT-3 than those treated with Adv.EF-LacZ. These data demonstrate that local expression of NT-3 will support axonal regeneration in the injured spinal cord without adverse effects and suggest that gene delivery of neurotrophins may be an effective strategy for nervous system repair after injury.
Acknowledgements:   Funded by NIH Grant NS35280 and by Mission Connect of the TIRR Foundation.