The survival-promoting effect of glial cell line-derived neurotrophic factor on axotomized corticospinal neurons in vivo is mediated by an endogenous brain-derived neurotrophic factor mechanism

Corticospinal tract: a new hope for the treatment of post-stroke spasticity

Stroke is the third leading cause of death and disability worldwide. Post-stroke spasticity (PSS) is the most common complication of stroke but represents only one of the many manifestations of upper motor neuron syndrome. As an upper motor neuron, the corticospinal tract (CST) is the only direct descending motor pathway that innervates the spinal motor neurons and is closely related to the recovery of limb function in patients with PSS. Therefore, promoting axonal remodeling in the CST may help identify new therapeutic strategies for PSS. In this review, we outline the pathological mechanisms of PSS, specifically their relationship with CST, and therapeutic strategies for axonal regeneration of the CST after stroke. We found it to be closely associated with astroglial scarring produced by astrocyte activation and its secretion of neurotrophic factors, mainly after the onset of cerebral ischemia. We hope that this review offers insight into the relationship between CST and PSS and provides a basis for further studies.

Interplay of BDNF and GDNF in the Mature Spinal Somatosensory System and Its Potential Therapeutic Relevance

The growth factors BDNF and GDNF are gaining more and more attention as modulators of synaptic transmission in the mature central nervous system (CNS). The two molecules undergo a regulated secretion in neurons and may be anterogradely transported to terminals where they can positively or negatively modulate fast synaptic transmission. There is today a wide consensus on the role of BDNF as a pro-nociceptive modulator, as the neurotrophin has an important part in the initiation and maintenance of inflammatory, chronic, and/or neuropathic pain at the peripheral and central level. At the spinal level, BDNF intervenes in the regulation of chloride equilibrium potential, decreases the excitatory synaptic drive to inhibitory neurons, with complex changes in GABAergic/glycinergic synaptic transmission, and increases excitatory transmission in the superficial dorsal horn. Differently from BDNF, the role of GDNF still remains to be unraveled in full. This review resumes the current literature on the interplay between BDNF and GDNF in the regulation of nociceptive neurotransmission in the superficial dorsal horn of the spinal cord. We will first discuss the circuitries involved in such a regulation, as well as the reciprocal interactions between the two factors in nociceptive pathways. The development of small molecules specifically targeting BDNF, GDNF and/or downstream effectors is opening new perspectives for investigating these neurotrophic factors as modulators of nociceptive transmission and chronic pain. Therefore, we will finally consider the molecules of (potential) pharmacological relevance for tackling normal and pathological pain.

Combined with anti-Nogo-A antibody treatment, BDNF did not compensate the extra deleterious motor effect caused by large size cervical cord hemisection in adult macaques

In spinal cord injured adult mammals, neutralizing the neurite growth inhibitor Nogo‐A with antibodies promotes axonal regeneration and functional recovery, although axonal regeneration is limited in length. Neurotrophic factors such as BDNF stimulate neurite outgrowth and protect axotomized neurons. Can the effects obtained by neutralizing Nogo‐A, inducing an environment favorable for axonal sprouting, be strengthened by adding BDNF? A unilateral incomplete hemicord lesion at C7 level interrupted the main corticospinal component in three groups of adult macaque monkeys: control monkeys (n = 6), anti‐Nogo‐A antibody‐treated monkeys (n = 7), and anti‐Nogo‐A antibody and BDNF‐treated monkeys (n = 5). The functional recovery of manual dexterity was significantly different between the 3 groups of monkeys, the lowest in the control group. Whereas the anti‐Nogo‐A antibody‐treated animals returned to manual dexterity performances close to prelesion ones, irrespective of lesion size, both the control and the anti‐Nogo‐A/BDNF animals presented a limited functional recovery. In the control group, the limited spontaneous functional recovery depended on lesion size, a dependence absent in the combined treatment group (anti‐Nogo‐A antibody and BDNF). The functional recovery in the latter group was significantly lower than in anti‐Nogo‐A antibody‐treated monkeys, although the lesion was larger in three out of the five monkeys in the combined treatment group.

Resveratrol Regulates BDNF, trkB, PSA-NCAM, and Arc Expression in the Rat Cerebral Cortex after Bilateral Common Carotid Artery Occlusion and Reperfusion

The polyphenol resveratrol (RVT) may drive protective mechanisms of cerebral homeostasis during the hypoperfusion/reperfusion triggered by the transient bilateral common carotid artery occlusion followed by reperfusion (BCCAO/R). This immunochemical study investigates if a single dose of RVT modulates the plasticity-related markers brain-derived neurotrophic factor (BDNF), the tyrosine kinase trkB receptor, Polysialylated-Neural Cell Adhesion Molecule (PSA-NCAM), and Activity-regulated cytoskeleton-associated (Arc) protein in the brain cortex after BCCAO/R. Frontal and temporal-occipital cortical regions were examined in male Wistar rats randomly subdivided in two groups, sham-operated and submitted to BCCAO/R. Six hours prior to surgery, half the rats were gavage fed a dose of RVT (180 mg·kg⁻¹ in 300 µL of sunflower oil as the vehicle), while the second half was given the vehicle alone. In the frontal cortex of BCCAO/R vehicle-treated rats, BDNF and PSA-NCAM decreased, while trkB increased. RVT pre-treatment elicited an increment of all examined markers in both sham- and BCCAO/R rats. No variations occurred in the temporal-occipital cortex. The results highlight a role for RVT in modulating neuronal plasticity through the BDNF-trkB system and upregulation of PSA-NCAM and Arc, which may provide both trophic and structural local support in the dynamic changes occurring during the BCCAO/R, and further suggest that dietary supplements such as RVT are effective in preserving the tissue potential to engage plasticity-related events and control the functional response to the hypoperfusion/reperfusion challenge.

Structural alterations in fast-spiking GABAergic interneurons in a model of posttraumatic neocortical epileptogenesis

Electrophysiological experiments in the partial cortical isolation ("undercut" or "UC") model of injury-induced neocortical epileptogenesis have shown alterations in GABAergic synaptic transmission attributable to abnormalities in presynaptic terminals. To determine whether the decreased inhibition was associated with structural abnormalities in GABAergic interneurons, we used immunocytochemical techniques, confocal microscopy and EM in UC and control sensorimotor rat cortex to analyze structural alterations in fast-spiking parvalbumin-containing interneurons and pyramidal (Pyr) cells of layer V. Principle findings were: 1) there were no decreases in counts of parvalbumin (PV)- or GABA-immunoreactive interneurons in UC cortex, however there were significant reductions in expression of VGAT and GAD-65 and -67 in halos of GABAergic terminals around Pyr somata in layer V. 2) Consistent with previous results, somatic size and density of Pyr cells was decreased in infragranular layers of UC cortex. 3) Dendrites of biocytin-filled FS interneurons were significantly decreased in volume. 4) There were decreases in the size and VGAT content of GABAergic boutons in axons of biocytin-filled FS cells in the UC, together with a decrease in colocalization with postsynaptic gephyrin, suggesting a reduction in GABAergic synapses. Quantitative EM of layer V Pyr somata confirmed the reduction in inhibitory synapses. 5) There were marked and lasting reductions in brain derived neurotrophic factor (BDNF)-IR and -mRNA in Pyr cells and decreased TrkB-IR on PV cells in UC cortex. 6) Results lead to the hypothesis that reduction in trophic support by BDNF derived from Pyr cells may contribute to the regressive changes in axonal terminals and dendrites of FS cells in the UC cortex and decreased GABAergic inhibition.

SIGNIFICANCE

Injury to cortical structures is a major cause of epilepsy, accounting for about 20% of cases in the general population, with an incidence as high as ~50% among brain-injured personnel in wartime. Loss of GABAergic inhibitory interneurons is a significant pathophysiological factor associated with epileptogenesis following brain trauma and other etiologies. Results of these experiments show that the largest population of cortical interneurons, the parvalbumin-containing fast-spiking (FS) interneurons, are preserved in the partial neocortical isolation model of partial epilepsy. However, axonal terminals of these cells are structurally abnormal, have decreased content of GABA synthetic enzymes and vesicular GABA transporter and make fewer synapses onto pyramidal neurons. These structural abnormalities underlie defects in GABAergic neurotransmission that are a key pathophysiological factor in epileptogenesis found in electrophysiological experiments. BDNF, and its TrkB receptor, key factors for maintenance of interneurons and pyramidal neurons, are decreased in the injured cortex. Results suggest that supplying BDNF to the injured epileptogenic brain may reverse the structural and functional abnormalities in the parvalbumin FS interneurons and provide an antiepileptogenic therapy.

Pharmacological, Cell, and Gene Therapy Strategies to Promote Spinal Cord Regeneration

In this review, recent studies using pharmacological treatment, cell transplantation, and gene therapy to promote regeneration of the injured spinal cord in animal models will be summarized. Pharmacological and cell transplantation treatments generally revealed some degree of effect on the regeneration of the injured ascending and descending tracts, but further improvements to achieve a more significant functional recovery are necessary. The use of gene therapy to promote repair of the injured nervous system is a relatively new concept. It is based on the development of methods for delivering therapeutic genes to neurons, glia cells, or nonneural cells. Direct in vivo gene transfer or gene transfer in combination with (neuro)transplantation (ex vivo gene transfer) appeared powerful strategies to promote neuronal survival and axonal regrowth following traumatic injury to the central nervous system. Recent advances in understanding the cellular and molecular mechanisms that govern neuronal survival and neurite outgrowth have enabled the design of experiments aimed at viral vector-mediated transfer of genes encoding neurotrophic factors, growth-associated proteins, cell adhesion molecules, and antiapoptotic genes. Central to the success of these approaches was the development of efficient, nontoxic vectors for gene delivery and the acquirement of the appropriate (genetically modified) cells for neurotransplantation. Direct gene transfer in the nervous system was first achieved with herpes viral and E1-deleted adenoviral vectors. Both vector systems are problematic in that these vectors elicit immunogenic and cytotoxic responses. Adeno-associated viral vectors and lentiviral vectors constitute improved gene delivery systems and are beginning to be applied in neuroregeneration research of the spinal cord. Ex vivo approaches were initially based on the implantation of genetically modified fibroblasts. More recently, transduced Schwann cells, genetically modified pieces of peripheral nerve, and olfactory ensheathing glia have been used as implants into the injured spinal cord.

Layer specific and general requirements for ERK/MAPK signaling in the developing neocortex

Aberrant signaling through the Raf/MEK/ERK (ERK/MAPK) pathway causes pathology in a family of neurodevelopmental disorders known as 'RASopathies' and is implicated in autism pathogenesis. Here, we have determined the functions of ERK/MAPK signaling in developing neocortical excitatory neurons. Our data reveal a critical requirement for ERK/MAPK signaling in the morphological development and survival of large Ctip2⁺ neurons in layer 5. Loss of Map2k1/2 (Mek1/2) led to deficits in corticospinal tract formation and subsequent corticospinal neuron apoptosis. ERK/MAPK hyperactivation also led to reduced corticospinal axon elongation, but was associated with enhanced arborization. ERK/MAPK signaling was dispensable for axonal outgrowth of layer 2/3 callosal neurons. However, Map2k1/2 deletion led to reduced expression of Arc and enhanced intrinsic excitability in both layers 2/3 and 5, in addition to imbalanced synaptic excitation and inhibition. These data demonstrate selective requirements for ERK/MAPK signaling in layer 5 circuit development and general effects on cortical pyramidal neuron excitability.

DOI:http://dx.doi.org/10.7554/eLife.11123.001

In the nervous system, cells called neurons form networks that relay information in the form of electrical signals around the brain and the rest of the body. Typically, an electrical signal travels from branch-like structures at one end of the cell, through the cell body and then along a long fiber called an axon to reach junctions with another neurons.

The connections between neurons start to form as the nervous system develops in the embryo, and any errors or delays in this process can cause severe neurological disorders and intellectual disabilities. For example, genetic mutations affecting a communication system within cells known as the ERK/MAPK pathway can lead to a family of syndromes called the “RASopathies”. Abnormalities in this pathway may also contribute to certain types of autism. However, it is not clear how alterations to the ERK/MAPK pathway cause these conditions.

Xing et al. investigated whether ERK/MAPK signaling regulates the formation of connections between neurons and the activity of neurons in mouse brains. The experiments showed that the growth of axons that extend from an area of the brain called the cerebral cortex towards the spinal cord are particularly sensitive to changes in the level of signaling through the ERK/MAPK pathway. On the other hand, inhibiting the pathway has relatively little effect on the growth of axons within the cerebral cortex. Further experiments showed that many neurons in the cerebral cortex require the ERK/MAPK pathway to activate genes that alter neuronal activity and the strength of the connections between neurons.

Xing et al.’s findings suggest that defects in the connections between the cerebral cortex and different regions of the nervous system may contribute to the symptoms observed in patients with conditions linked to alterations in ERK/MAPK activity. Future studies will focus on understanding the molecular mechanisms by which ERK/MAPK pathway influences the organization and activity of neuron circuits during the development of the nervous system.

DOI:http://dx.doi.org/10.7554/eLife.11123.002

Biomaterial Approaches to Enhancing Neurorestoration after Spinal Cord Injury: Strategies for Overcoming Inherent Biological Obstacles

While advances in technology and medicine have improved both longevity and quality of life in patients living with a spinal cord injury, restoration of full motor function is not often achieved. This is due to the failure of repair and regeneration of neuronal connections in the spinal cord after injury. In this review, the complicated nature of spinal cord injury is described, noting the numerous cellular and molecular events that occur in the central nervous system following a traumatic lesion. In short, postinjury tissue changes create a complex and dynamic environment that is highly inhibitory to the process of neural regeneration. Strategies for repair are outlined with a particular focus on the important role of biomaterials in designing a therapeutic treatment that can overcome this inhibitory environment. The importance of considering the inherent biological response of the central nervous system to both injury and subsequent therapeutic interventions is highlighted as a key consideration for all attempts at improving functional recovery.

Experimental and clinical evidence for the protective role of progesterone in motoneuron degeneration and neuroinflammation

Far beyond its role in reproduction, progesterone exerts neuro-protective, promyelinating, and anti-inflammatory effects in the nervous system. These effects are amplified under pathological conditions, implying that changes of the local environment sensitize nervous tissues to steroid therapy. The present survey covers our results of progesterone neuroprotection in a motoneuron neurodegeneration model and a neuroinflammation model. In the degenerating spinal cord of the Wobbler mouse, progesterone reverses the impaired expression of neurotrophins, increases enzymes of neurotransmission and metabolism, prevents oxidative damage of motoneurons and their vacuolar degeneration (paraptosis), and attenuates the development of mitochondrial abnormalities. After long-term treatment, progesterone also increases muscle strength and the survival of Wobbler mice. Subsequently, this review describes the effects of progesterone in mice with induced experimental autoimmune encephalomyelitis (EAE), a commonly used model of multiple sclerosis. In EAE mice, progesterone attenuates the clinical severity, decreases demyelination and neuronal dysfunction, increases axonal counts, reduces the formation of amyloid precursor protein profiles, and decreases the aberrant expression of growth-associated proteins. These actions of progesterone may be due to multiple mechanisms, considering that classic nuclear receptors, extranuclear receptors, and membrane receptors are all expressed in the spinal cord. Although many aspects of progesterone action in humans remain unsolved, data provided by experimental models makes getting to this objective closer than previously expected.

Remote limb ischemic conditioning enhances motor learning in healthy humans

Brief bouts of sublethal ischemia have been shown to protect exposed tissue (ischemic conditioning) and tissues at remote sites (remote ischemic conditioning) against subsequent ischemic challenges. Given that the mechanisms of this protective phenomenon are multifactorial and epigenetic, we postulated that remote limb ischemic conditioning (RLIC) might enhance mechanisms responsible for neural plasticity, and thereby facilitate learning. Specifically, we hypothesized that conditioning of the nervous system with RLIC, achieved through brief repetitive limb ischemia prior to training, would facilitate the neurophysiological processes of learning, thus making training more effective and more long-lasting. Eighteen healthy adults participated in this study; nine were randomly allocated to RLIC and nine to sham conditioning. All subjects underwent seven consecutive weekday sessions and 2-wk and 4-wk follow-up sessions. We found that RLIC resulted in significantly greater motor learning and longer retention of motor performance gains in healthy adults. Changes in motor performance do not appear to be due to a generalized increase in muscle activation or muscle strength and were not associated with changes in serum brain-derived neurotrophic factor (BDNF) concentration. Of note, RLIC did not enhance cognitive learning on a hippocampus-dependent task. While future research is needed to establish optimal conditioning and training parameters, this inexpensive, clinically feasible paradigm might ultimately be implemented to enhance motor learning in individuals undergoing neuromuscular rehabilitation for brain injury and other pathological conditions.

Alterations in BDNF (brain derived neurotrophic factor) and GDNF (glial cell line-derived neurotrophic factor) serum levels in bipolar disorder: The role of lithium

OBJECTIVE

Brain-derived neurotrophic factor (BDNF) has been consistently reported to be decreased in mania or depression in bipolar disorders. Evidence suggests that Glial cell line-derived neurotrophic factor (GDNF) has a role in the pathogenesis of mood disorders. Whether GDNF and BDNF act in the same way across different episodes in bipolar disorders is unclear.

METHOD

BDNF and GDNF serum levels were measured simultaneously by enzyme-linked immunosorbent assay (ELISA) method in 96 patients diagnosed with bipolar disorder according to DSM-IV (37 euthymic, 33 manic, 26 depressed) in comparison to 61 healthy volunteers. SCID- I and SCID-non patient version were used for clinical evaluation of the patients and healthy volunteers respectively. Correlations between the two trophic factor levels, and medication dose, duration and serum levels of lithium or valproate were studied across different episodes of illness.

RESULTS

Patients had significantly lower BDNF levels during mania and depression compared to euthymic patients and healthy controls. GDNF levels were not distinctive. However GDNF/BDNF ratio was higher in manic state compared to euthymia and healthy controls. Significant negative correlation was observed between BDNF and GDNF levels in euthymic patients. While BDNF levels correlated positively, GDNF levels correlated negatively with lithium levels. Regression analysis confirmed that lithium levels predicted only GDNF levels positively in mania, and negatively in euthymia.

LIMITATIONS

Small sample size in different episodes and drug-free patients was the limitation of thestudy.

CONCLUSION

Current data suggests that lithium exerts its therapeutic action by an inverse effect on BDNF and GDNF levels, possibly by up-regulating BDNF and down-regulating GDNF to achieve euthymia.

Neurotrophins in the regulation of cellular survival and death

The neurotrophins play crucial roles regulating survival and apoptosis in the developing and injured nervous system. The four neurotrophins exert profound and crucial survival effects on developing peripheral neurons, and their expression and action is intimately tied to successful innervation of peripheral targets. In the central nervous system, they are dispensable for neuronal survival during development but support neuronal survival after lesion or other forms of injury. Neurotrophins also regulate apoptosis of both peripheral and central neurons, and we now recognize that there are regulatory advantages to having the same molecules regulate life and death decisions. This chapter examines the biological contexts in which these events take place and highlights the specific ligands, receptors, and signaling mechanisms that allow them to occur.

Expression of full-length and truncated trkB in human striatum and substantia nigra neurons: implications for Parkinson's disease

Brain derived neurotrophic factor (BDNF) is a potent mediator of cell survival and differentiation and can reverse neuronal injury associated with Parkinson's disease (PD). Tropomyosin receptor kinase B (trkB) is the high affinity receptor for BDNF. There are two major trkB isoforms, the full-length receptor (trkB.tk(+)) and the truncated receptor (trkB.t1), that mediate the diverse, region specific functions of BDNF. Both trkB isoforms are widely distributed throughout the brain, but the isoform specific distribution of trkB.t1 and trkB.tk(+) to human neurons is not well characterized. Therefore, we report the regional and neuronal distribution of trkB.tk(+) and trkB.t1 in the striatum and substantia nigra pars compacta (SNpc) of human autopsy tissues from control and PD cases. In both PD and control tissues, we found abundant, punctate distribution of trkB.tk(+) and trkB.t1 proteins in striatum and SNpc neurons. In PD, trkB.tk(+) is decreased in striatal neurites, increased in striatal somata, decreased in SNpc somata and dendrites, and increased in SNpc axons. TrkB.t1 is increased in striatal somata, decreased in striatal axons, and increased in SNpc distal dendrites. We believe changes in trkB isoform distribution and expression levels may be markers of pathology and affect the neuronal response to BDNF.

Truncated TrkB: beyond a dominant negative receptor

BDNF activates trkB receptors to regulate neuronal survival, differentiation, and proliferation. Mutations in the BDNF gene, altered BDNF expression, and altered trkB expression are associated with degenerative and psychiatric disorders. The full-length trkB receptor (trkB.tk(+)) undergoes autophosphorylation to activate intracellular signaling pathways. The truncated trkB receptor (trkB.t1) is abundantly expressed in the brain but lacks the catalytic tyrosine kinase domain. TrkB.t1 is a dominant-negative receptor that inhibits trkB.tk(+) signaling. While this is an important function of trkB.t1, it is only one of its many functions. TrkB.t1 sequesters and translocate BDNF, induces filopodia and neurite outgrowth, stimulates intracellular signaling cascades, regulates Rho GTPase signaling, and modifies cytoskeletal structures. TrkB.t1 is an active signaling molecule with regulatory effects on neurons and astrocytes.

An in vivo characterization of trophic factor production following neural precursor cell or bone marrow stromal cell transplantation for spinal cord injury

Cellular transplantation strategies for repairing the injured spinal cord have shown consistent benefit in preclinical models, and human clinical trials have begun. Interactions between transplanted cells and host tissue remain poorly understood. Trophic factor secretion is postulated a primary or supplementary mechanism of action for many transplanted cells, however, there is little direct evidence to support trophin production by transplanted cells in situ. In the present study, trophic factor expression was characterized in uninjured, injured-untreated, injured-treated with transplanted cells, and corresponding control tissue from the adult rat spinal cord. Candidate trophic factors were identified in a literature search, and primers were designed for these genes. We examined in vivo trophin expression in 3 paradigms involving transplantation of either brain or spinal cord-derived neural precursor cells (NPCs) or bone marrow stromal cells (BMSCs). Injury without further treatment led to a significant elevation of nerve growth factor (NGF), leukemia inhibitory factor (LIF), insulin-like growth factor-1 (IGF-1), and transforming growth factor-β1 (TGF-β1), and lower expression of vascular endothelial growth factor isoform A (VEGF-A) and platelet-derived growth factor-A (PDGF-A). Transplantation of NPCs led to modest changes in trophin expression, and the co-administration of intrathecal trophins resulted in significant elevation of the neurotrophins, glial-derived neurotrophic factor (GDNF), LIF, and basic fibroblast growth factor (bFGF). BMSCs transplantation upregulated NGF, LIF, and IGF-1. NPCs isolated after transplantation into the injured spinal cord expressed the neurotrophins, ciliary neurotrophic factor (CNTF), epidermal growth factor (EGF), and bFGF at higher levels than host cord. These data show that trophin expression in the spinal cord is influenced by injury and cell transplantation, particularly when combined with intrathecal trophin infusion. Trophins may contribute to the benefits associated with cell-based repair strategies for spinal cord injury.

In vitro characterization of trophic factor expression in neural precursor cells

In cellular transplantation strategies for repairing the injured central nervous system, interactions between transplanted neural precursor cells (NPCs) and host tissue remain incompletely understood. Although trophins may contribute to the benefits observed, little research has explored this possibility. Candidate trophic factors were identified, and primers were designed for these genes. Template RNA was isolated from 3 NPC sources, and also from bone marrow stromal cells (BMSCs) and embryonic fibroblasts as comparative controls. Quantitative polymerase chain reaction was performed to determine the effect of cell source, passaging, cellular differentiation, and environmental changes on trophin factor expression in NPCs. Results were analyzed with multivariate statistical analyses. NPCs, BMSCs, and fibroblasts each expressed trophic factors in unique patterns. Trophic factor expression was similar among NPCs whether harvested from rat or mouse, brain or spinal cord, or their time in culture. The expression of neurotrophin NT-3, NT-4/5, glial-derived neurotrophic factor, and insulin-like growth factor-1 decreased with time in culture. Induced differentiation of NPCs led to a marked and statistically significant increase in the expression of trophic factors. Culture conditions and environmental changes were also associated with significant changes in trophin expression. These results suggest that trophins could contribute to the benefits associated with transplantation of NPCs as well as BMSCs. Trophic factor expression changes with NPC differentiation and environmental conditions, which could have important implications with regard to their behavior after in vivo transplantation.

Differential regulation of the expression of neurotrophin receptors in rat extraocular motoneurons after lesion

Neurotrophins acting through high-affinity tyrosine kinase receptors (trkA, trkB, and trkC) play a crucial role in regulating survival and maintenance of specific neuronal functions after injury. Adult motoneurons supplying extraocular muscles survive after disconnection from the target, but suffer dramatic changes in morphological and physiological properties, due in part to the loss of their trophic support from the muscle. To investigate the dependence of the adult rat extraocular motoneurons on neurotrophins, we examined trkA, trkB, and trkC mRNA expression after axotomy by in situ hybridization. trkA mRNA expression was detectable at low levels in unlesioned motoneurons, and its expression was downregulated 1 and 3 days after injury. Expression of trkB and trkC mRNAs was stronger, and after axotomy a simultaneous, but inverse regulation of both receptors was observed. Thus, whereas a considerable increase in trkB expression was seen about 2 weeks after axotomy, the expression of trkC mRNA had decreased at the same post-lesion period. Injured extraocular motoneurons also experienced an initial induction in expression of calcitonin gene-related peptide and a transient downregulation of cholinergic characteristics, indicating a switch in the phenotype from a transmitter-specific to a regenerative state. These results suggest that specific neurotrophins may contribute differentially to the survival and regenerative responses of extraocular motoneurons after lesion.

Combining glial cell line-derived neurotrophic factor gene delivery (AdGDNF) with L-arginine decreases contusion size but not behavioral deficits after traumatic brain injury

Our laboratory has previously demonstrated that viral administration of glial cell line-derived neurotrophic factor (AdGDNF), one week prior to a controlled cortical impact (CCI) over the forelimb sensorimotor cortex of the rat (FL-SMC) is neuroprotective, but does not significantly enhance recovery of sensorimotor function. One possible explanation for this discrepancy is that although protected, neurons may not have been functional due to enduring metabolic deficiencies. Additionally, metabolic events following TBI may interfere with expression of therapeutic proteins administered to the injured brain via gene therapy. The current study focused on enhancing the metabolic function of the brain by increasing cerebral blood flow (CBF) with l-arginine in conjunction with administration of AdGDNF immediately following CCI. An adenoviral vector harboring human GDNF was injected unilaterally into FL-SMC of the rat immediately following a unilateral CCI over the FL-SMC. Within 30min of the CCI and AdGDNF injections, some animals were injected with l-arginine (i.v.). Tests of forelimb function and asymmetry were administered for 4weeks post-injury. Animals were sacrificed and contusion size and GDNF protein expression measured. This study demonstrated that rats treated with AdGDNF and l-arginine post-CCI had a significantly smaller contusion than injured rats who did not receive any treatment, or injured rats treated with either AdGDNF or l-arginine alone. Nevertheless, no amelioration of behavioral deficits was seen. These findings suggest that AdGDNF alone following a CCI was not therapeutic and although combining it with l-arginine decreased contusion size, it did not enhance behavioral recovery.

Labeling of olfactory ensheathing glial cells with fluorescent tracers for neurotransplantation

Development of cell-based treatment strategies for repair of the injured nervous system requires cell tracing techniques to follow the fate of transplanted cells and their interaction with the host tissue. The present study investigates the efficacy of fluorescent cell tracers Fast Blue, PKH26, DiO and CMFDA for long-term labeling of olfactory ensheathing glial cells (OEC) in culture and following transplantation into the rat spinal cord. All tested dyes produced very efficient initial labeling of p75-positive OEC in culture. The number of Fast Blue-positive cells remained largely unchanged during the first 4 weeks but only about 21% of the cells retained tracer 6 weeks after labeling. In contrast, the number of cells labeled with PKH26 and DiO was reduced to 51-55% after 2 weeks in culture and reached 8-12% after 4-6 weeks. CMFDA had completely disappeared from the cells 2 weeks after labeling. AlamarBlue assay showed that among four tested tracers only CMFDA reduced proliferation rate of the OEC. After transplantation into spinal cord, Fast Blue-labeled OEC survived for at least 8 weeks but demonstrated very limited migration from the injection sites. Additional immunostaining with glial and neuronal markers revealed signs of dye leakage from the transplanted cells resulted in weak labeling of microglia and spinal neurons. The results show that Fast Blue is an efficient cell marker for cultured OEC. However, transfer of the dye from the transplanted cells to the host tissue should be considered and correctly interpreted.

Transgenic inhibition of astroglial NF-kappa B leads to increased axonal sparing and sprouting following spinal cord injury

We previously showed that Nuclear Factor kappaB (NF-kappaB) inactivation in astrocytes leads to improved functional recovery following spinal cord injury (SCI). This correlated with reduced expression of pro-inflammatory mediators and chondroitin sulfate proteoglycans, and increased white matter preservation. Hence we hypothesized that inactivation of astrocytic NF-kappaB would create a more permissive environment for axonal sprouting and regeneration. We induced both contusive and complete transection SCI in GFAP-Inhibitor of kappaB-dominant negative (GFAP-IkappaBalpha-dn) and wild-type (WT) mice and performed retrograde [fluorogold (FG)] and anterograde [biotinylated dextran amine (BDA)] tracing 8 weeks after injury. Following contusive SCI, more FG-labeled cells were found in motor cortex, reticular formation, and raphe nuclei of transgenic mice. Spared and sprouting BDA-positive corticospinal axons were found caudal to the lesion in GFAP-IkappaBalpha-dn mice. Higher numbers of FG-labeled neurons were detected immediately rostral to the lesion in GFAP-IkappaBalpha-dn mice, accompanied by increased expression of synaptic and axonal growth-associated molecules. After transection, however, no FG-labeled neurons or BDA-filled axons were found rostral and caudal to the lesion, respectively, in either genotype. These data demonstrated that inhibiting astroglial NF-kappaB resulted in a growth-supporting terrain promoting sparing and sprouting, rather than regeneration, of supraspinal and propriospinal circuitries essential for locomotion, hence contributing to the improved functional recovery observed after SCI in GFAP-IkappaBalpha-dn mice.

Progesterone neuroprotection in traumatic CNS injury and motoneuron degeneration

Studies on the neuroprotective and promyelinating effects of progesterone in the nervous system are of great interest due to their potential clinical connotations. In peripheral neuropathies, progesterone and reduced derivatives promote remyelination, axonal regeneration and the recovery of function. In traumatic brain injury (TBI), progesterone has the ability to reduce edema and inflammatory cytokines, prevent neuronal loss and improve functional outcomes. Clinical trials have shown that short-and long-term progesterone treatment induces a significant improvement in the level of disability among patients with brain injury. In experimental spinal cord injury (SCI), molecular markers of functional motoneurons become impaired, including brain-derived neurotrophic factor (BDNF) mRNA, Na,K-ATPase mRNA, microtubule-associated protein 2 and choline acetyltransferase (ChAT). SCI also produces motoneuron chromatolysis. Progesterone treatment restores the expression of these molecules while chromatolysis subsided. SCI also causes oligodendrocyte loss and demyelination. In this case, a short progesterone treatment enhances proliferation and differentiation of oligodendrocyte progenitors into mature myelin-producing cells, whereas prolonged treatment increases a transcription factor (Olig1) needed to repair injury-induced demyelination. Progesterone neuroprotection has also been shown in motoneuron neurodegeneration. In Wobbler mice spinal cord, progesterone reverses the impaired expression of BDNF, ChAT and Na,K-ATPase, prevents vacuolar motoneuron degeneration and the development of mitochondrial abnormalities, while functionally increases muscle strength and the survival of Wobbler mice. Multiple mechanisms contribute to these progesterone effects, and the role played by classical nuclear receptors, extra nuclear receptors, membrane receptors, and the reduced metabolites of progesterone in neuroprotection and myelin formation remain an exciting field worth of exploration.

Riluzole protects Huntington disease patients from brain glucose hypometabolism and grey matter volume loss and increases production of neurotrophins

PURPOSE

Huntington disease (HD) mutation increases gain-of-toxic functions contributing to glutamate-mediated excitotoxicity. Riluzole interferes with glutamatergic neurotransmission, thereby reducing excitotoxicity, enhancing neurite formation in damaged motoneurons and increasing serum concentrations of BDNF, a brain cortex neurotrophin protecting striatal neurons from degeneration.

METHODS

We investigated metabolic and volumetric differences in distinct brain areas between 11 riluzole-treated and 12 placebo-treated patients by MRI and (18)F-fluoro-2-deoxy-D-glucose (FDG) PET scanning, according to fully automated protocols. We also investigated the influence of riluzole on peripheral growth factor blood levels.

RESULTS

Placebo-treated patients showed significantly greater proportional volume loss of grey matter and decrease in metabolic FDG uptake than patients treated with riluzole in all cortical areas (p<0.05). The decreased rate of metabolic FDG uptake correlated with worsening clinical scores in placebo-treated patients, compared to those who were treated with riluzole. The progressive decrease in metabolic FDG uptake observed in the frontal, parietal and occipital cortex correlated linearly with the severity of motor scores calculated by Unified Huntington Disease Rating Scale (UHDRS-I) in placebo-treated patients. Similarly, the rate of metabolic changes in the frontal and temporal areas of the brain cortex correlated linearly with worsening behavioural scores calculated by UHDRS-III in the placebo-treated patients. Finally, BDNF and transforming growth factor beta-1 serum levels were significantly higher in patients treated with riluzole.

CONCLUSION

The linear correlation between decreased metabolic FDG uptake and worsening clinical scores in the placebo-treated patients suggests that FDG-PET may be a valuable procedure to assess brain markers of HD.

Trophic factors in the carotid body

The aim of the present study is to provide a review of the expression and action of trophic factors in the carotid body. In glomic type I cells, the following factors have been identified: brain-derived neurotrophic factor, glial cell line-derived neurotrophic factor, artemin, ciliary neurotrophic factor, insulin-like growth factors-I and -II, basic fibroblast growth factor, epidermal growth factor, transforming growth factor-alpha and -beta1, interleukin-1beta and -6, tumour necrosis factor-alpha, vascular endothelial growth factor, and endothelin-1 (ET-1). Growth factor receptors in the above cells include p75LNGFR, TrkA, TrkB, RET, GDNF family receptors alpha1-3, gp130, IL-6Ralpha, EGFR, FGFR1, IL1-RI, TNF-RI, VEGFR-1 and -2, ETA and ETB receptors, and PDGFR-alpha. Differential local expression of growth factors and corresponding receptors plays a role in pre- and postnatal development of the carotid body. Their local actions contribute toward producing the morphologic and molecular changes associated with chronic hypoxia and/or hypertension, such as cellular hyperplasia, extracellular matrix expansion, changes in channel densities, and neurotransmitter patterns. Neurotrophic factor production is also considered to play a key role in the therapeutic effects of intracerebral carotid body grafts in Parkinson's disease. Future research should also focus on trophic actions on carotid body type I cells by peptide neuromodulators, which are known to be present in the carotid body and to show trophic effects on other cell populations, that is, angiotensin II, adrenomedullin, bombesin, calcitonin, calcitonin gene-related peptide, cholecystokinin, erythropoietin, galanin, opioids, pituitary adenylate cyclase-activating polypeptide, atrial natriuretic peptide, somatostatin, tachykinins, neuropeptide Y, neurotensin, and vasoactive intestinal peptide.

Progesterone modulates brain-derived neurotrophic factor and choline acetyltransferase in degenerating Wobbler motoneurons

Progesterone (PROG) shows neuroprotective effects in nervous system diseases. The Wobbler mouse, a model of motoneuron degeneration, suffers a mutation of the Vsp154 gene on chromosome 11 leading to motoneuron vacuolation and astrocytosis of the spinal cord. Previous work has demonstrated beneficial effects of PROG in the Wobbler mouse. As an extension of this work, we now studied steroid effects on neuronal brain-derived neurotrophic factor (BDNF) mRNA and protein, on choline acetyltransferase (ChAT) immunoreactivity (IR) and activity in the spinal cord, and on recovery of muscle atrophy. Wobbler mice received implants of PROG pellets (20 mg) at 6 and 10 weeks of age and were killed at 14 weeks. In situ hybridization for BDNF mRNA demonstrated that grain density in large (>600 microm2) and medium size (<600 microm2) ventral horn neurons was decreased in untreated Wobblers, whereas PROG treatment increased BDNF mRNA in both neuronal types. PROG also induced a subcellular redistribution of BDNF protein, which in controls and steroid-naive Wobblers showed a predominant perinuclear and nucleolar location, whereas after PROG treatment, it was detected in cytoplasmic aggregates. ChAT activity was reduced by 55.3% in muscles of untreated Wobbler mice, whereas a significant increment was obtained after PROG treatment. Wobblers also showed reduced number of ChAT positive motoneurons, but this number was restored to normal by PROG. Finally, the pronounced biceps atrophy of steroid-naive Wobbler mice was slightly but significantly increased by PROG-treatment. Considering the important role played by neurotrophins on neuronal function, changes in BDNF might be part of the PROG activated-pathways to provide neuroprotection and re-establish neurotransmission and neuromuscular function in this degeneration model.

Instrumental learning within the spinal cord: underlying mechanisms and implications for recovery after injury

Using spinally transected rats, research has shown that neurons within the L4-S2 spinal cord are sensitive to response-outcome (instrumental) relations. This learning depends on a form of N-methyl-D-aspartate (NMDA)-mediated plasticity. Instrumental training enables subsequent learning, and this effect has been linked to the expression of brain-derived neurotrophic factor. Rats given uncontrollable stimulation later exhibit impaired instrumental learning, and this deficit lasts up to 48 hr. The induction of the deficit can be blocked by prior training with controllable shock, the concurrent presentation of a tonic stimulus that induces antinociception, or pretreatment with an NMDA or gamma-aminobutyric acid-A antagonist. The expression of the deficit depends on a kappa opioid. Uncontrollable stimulation enhances mechanical reactivity (allodynia), and treatments that induce allodynia (e.g., inflammation) inhibit learning. In intact animals, descending serotonergic neurons exert a protective effect that blocks the adverse consequences of uncontrollable stimulation. Uncontrollable, but not controllable, stimulation impairs the recovery of function after a contusion injury.

Neurotrophin ligand-receptor systems in somatosensory cortex of adult rat are affected by repeated episodes of ethanol

Ethanol exposure profoundly affects learning and memory and neural plasticity. Key players underlying these functions are neurotrophins. The present study explored the effects of ethanol on the distribution of neurotrophins in the cerebral cortex of the adult rat. Age- and weight-matched pairs of adult male, Long-Evans rats were fed a liquid, ethanol-containing (6.7% v/v) diet or pair-fed an isocaloric control diet three consecutive days per week for 6, 12, 18, or 24 weeks. Brains were processed immunohistochemically for nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) expression and for the expression of three neurotrophin receptors, p75, trkA, and trkB. Total numbers of immunolabeled neurons in specific layers of somatosensory cortex of ethanol- and control-fed animals were determined stereologically. Ethanol exposure induced an increase in the numbers of NGF- or BDNF-expressing neurons and in neurotrophin content per somata. These changes were (a) time and (b) laminar dependent. In contrast, the number of receptor-expressing neurons did not change due to ethanol exposure or to length of time on the ethanol diet. Thus, ethanol induces the recruitment of cortical neurons to express neurotrophins and an increase in the amount of neurotrophin expression per neuron.

Tyrosine kinase receptor immunoreactivity in trigeminal mesencephalic and motor neurons following transection of masseteric nerve of the rat

Neurotrophins are known to promote survival after neural injury. To determine the relative importance of tyrosine kinase receptors on the survival of axotomized trigeminal nuclear neurons, we examined the temporal expression profile of tyrosine kinase A, tyrosine kinase B and tyrosine kinase C receptors in the mesencephalic trigeminal nucleus and the motor trigeminal nucleus following transection of the masseteric nerve in rats. Axotomized neurons in these nuclei were retrogradely identified with FluoroGold. We found increase in tyrosine kinase A-immunoreactive mesencephalic trigeminal nucleus neurons in the second week after axotomy but no change in the number of tyrosine kinase A-immunoreactive motor trigeminal nucleus neurons. There was no change in the number of tyrosine kinase B-immunoreactive mesencephalic trigeminal nucleus neurons but the significant increase of tyrosine kinase B-immunoreactive motor trigeminal nucleus neurons throughout the period of observation (3 weeks) peaked at approximately 1 week after axotomy. There was no alteration in the number of tyrosine kinase C-immunoreactive mesencephalic trigeminal nucleus neurons but significant increase in tyrosine kinase C-immunoreactive motor trigeminal nucleus neurons observable by 4 days post-axotomy was followed by decline to levels lower than the control in 2 weeks. Temporal changes in the expression of individual tyrosine kinase receptors in mesencephalic trigeminal nucleus and motor trigeminal nucleus neurons following transection of the masseteric nerve suggest differential contribution of tyrosine kinase-specific neurotrophins to the survival of these neurons after axotomy.

Therapeutic potential of neurotrophic factors in neurodegenerative diseases

There is a vast amount of evidence indicating that neurotrophic factors play a major role in the development, maintenance, and survival of neurons and neuron-supporting cells such as glia and oligodendrocytes. In addition, it is well known that alterations in levels of neurotrophic factors or their receptors can lead to neuronal death and contribute to the pathogenesis of neurodegenerative diseases such as Parkinson disease, Alzheimer disease, Huntington disease, amyotrophic lateral sclerosis, and also aging. Although various treatments alleviate the symptoms of neurodegenerative diseases, none of them prevent or halt the neurodegenerative process. The high potency of neurotrophic factors, as shown by many experimental studies, makes them a rational candidate co-therapeutic agent in neurodegenerative disease. However, in practice, their clinical use is limited because of difficulties in protein delivery and pharmacokinetics in the central nervous system. To overcome these disadvantages and to facilitate the development of drugs with improved pharmacotherapeutic profiles, research is underway on neurotrophic factors and their receptors, and the molecular mechanisms by which they work, together with the development of new technologies for their delivery into the brain.

Light promotes regeneration and functional recovery and alters the immune response after spinal cord injury

BACKGROUND AND OBJECTIVES

Photobiomodulation (PBM) has been proposed as a potential therapy for spinal cord injury (SCI). We aimed to demonstrate that 810 nm light can penetrate deep into the body and promote neuronal regeneration and functional recovery.

STUDY DESIGN/MATERIALS AND METHODS

Adult rats underwent a T9 dorsal hemisection, followed by treatment with an 810 nm, 150 mW diode laser (dosage = 1,589 J/cm2). Axonal regeneration and functional recovery were assessed using single and double label tract tracing and various locomotor tasks. The immune response within the spinal cord was also assessed.

RESULTS

PBM, with 6% power penetration to the spinal cord depth, significantly increased axonal number and distance of regrowth (P < 0.001). PBM also returned aspects of function to baseline levels and significantly suppressed immune cell activation and cytokine/chemokine expression.

CONCLUSION

Our results demonstrate that light, delivered transcutaneously, improves recovery after injury and suggests that light will be a useful treatment for human SCI.

Differential mRNA expression of neurotrophic factors GDNF, BDNF, and NT-3 in experimental herpes simplex virus encephalitis

Glial cell line-derived neurotrophic factor (GDNF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3) mRNA levels were studied in the course of murine herpes simplex virus encephalitis. Induction of GNDF and NT-3 (both P < 0.05) was found during acute encephalitis. Despite absence of clinical impairment, both neurotrophic factors were overexpressed 2 months (NT-3) and 6 months (GDNF) following infection (both P < 0.05). Neurotrophic factors play an important role in neuronal survival and recovery after acute injury to the central nervous system (CNS) and may represent an additional therapeutic target for treatment of viral encephalitis.

Endogenous brain-derived neurotrophic factor protects dopaminergic nigral neurons against transneuronal degeneration induced by striatal excitotoxic injury

Injury to the central nervous system causes atrophy or death of connecting neurons and can modify the expression of neurotrophic factors. We observed transneuronal upregulation of brain-derived neurotrophic factor (BDNF) expression in the rat ipsilateral substantia nigra pars compacta after a striatal lesion induced by kainate. This effect is developmentally regulated because the enhancement of nigral BDNF expression was only observed when striatal lesion was performed on postnatal day (P) 15 and in adulthood, but not at P7. Interestingly, the lack of regulation of BDNF was coincident with the transynaptic degeneration of nigral neurons after striatal excitotoxic injury. Hence, the number of tyrosine hydroxylase-positive neurons in the substantia nigra pars compacta decreased when the lesion was performed at P7, but not at P15 or at P30. The analysis of the functional significance of this BDNF upregulation was done using trkB-IgG fusion proteins. After striatal injury, blockade of endogenous BDNF by trkB fusion proteins induced an atrophy of the dopaminergic neurons of the pars compacta. The injection of trkB-IgG fusion proteins did not modify the effects of kainate in the substantia nigra pars reticulata. Thus, our results show that BDNF exerts an autocrine/paracrine protective effect selectively on dopaminergic neurons against the loss of trophic support from the target striatum.

Axonal responses to cellularly delivered NT-4/5 after spinal cord injury

Neurotrophic factors delivered to the injured spinal cord have been shown to enhance axonal growth, prevent neuronal degeneration and partially improve sensorimotor function. The present study examined the effects of NT-4/5 on growth of spinal and supraspinal axons, glia, and functional outcome after spinal cord injury. Adult Fischer 344 rats received spinal cord dorsal hemisections or complete transections at the midthoracic level. Fibroblasts modified to secrete NT-4/5 or green fluorescent protein as controls were immediately grafted to the lesion site. Axonal growth responses were determined between 3 and 6 months postinjury by retrograde and anterograde tracing and immunohistochemistry. Motor axons, coerulospinal, reticulospinal, and propriospinal axons responded to NT-4/5 delivery after thoracic spinal cord injury with significantly increased axonal penetration into NT-4/5 secreting grafts compared to GFP-expressing control grafts. Axonal growth beyond NT-4/5-producing grafts and functional recovery were not observed. Numerous Schwann cells, but not oligodendrocytes, were present within NT-4/5-secreting grafts and remyelinated axons inside the graft. Thus, NT-4/5 and BDNF appear to be interchangeable to elicit substantial axonal growth in the injured spinal cord.

Secreted proNGF is a pathophysiological death-inducing ligand after adult CNS injury

The unprocessed precursor of the neurotrophin nerve growth factor (NGF), proNGF, has been suggested to be a death-inducing ligand for the neurotrophin receptor p75. Whether proNGF is a true pathophysiological ligand that is secreted, binds p75, and activates cell death in vivo, however, has remained unknown. Here, we report that after brain injury, proNGF was induced and secreted in an active form capable of triggering apoptosis in culture. We further demonstrate that proNGF binds p75 in vivo and that disruption of this binding results in complete rescue of injured adult corticospinal neurons. These data together suggest that proNGF binding to p75 is responsible for the death of adult corticospinal neurons after lesion, and they help to establish proNGF as the pathophysiological ligand that activates the cell-death program by means of p75 after brain injury. Interference in the binding of proNGF to p75 may provide a therapeutic approach for the treatment of disorders involving neuronal loss.

Cellular GDNF delivery promotes growth of motor and dorsal column sensory axons after partial and complete spinal cord transections and induces remyelination

Glial cell line-derived neurotrophic factor (GDNF) is the prototypical member of a growth factor family that signals via the cognate receptors ret and GDNF-receptor alpha-1. The latter receptors are expressed on a variety of neurons that project into the spinal cord, including supraspinal neurons, dorsal root ganglia, and local neurons. Although effects of GDNF on neuronal survival in the brain have previously been reported, GDNF effects on injured axons of the adult spinal cord have not been investigated. Using an ex vivo gene delivery approach that provides both trophic support and a cellular substrate for axonal growth, we implanted primary fibroblasts genetically modified to secrete GDNF into complete and partial mid-thoracic spinal cord transection sites. Compared to recipients of control grafts expressing a reporter gene, GDNF-expressing grafts promoted significant regeneration of several spinal systems, including dorsal column sensory, regionally projecting propriospinal, and local motor axons. Local GDNF expression also induced Schwann cell migration to the lesion site, leading to remyelination of regenerating axons. Thus, GDNF exerts tropic effects on adult spinal axons and Schwann cells that contribute to axon growth after injury.

Restriction of axonal retraction and promotion of axonal regeneration by chronically injured neurons after intraspinal treatment with glial cell line-derived neurotrophic factor (GDNF)

The response of supraspinal neurons to acute or delayed treatment with GDNF following a spinal cord injury was examined. A cervical level 3 hemisection lesion cavity was created by tissue aspiration in adult, female rats. In one experiment gel foam saturated with GDNF was placed into the lesion cavity immediately after injury to determine if the extent of axonal retraction was affected by neurotrophic factor treatment. One week prior to sacrifice animals received a microinjection of biotinylated dextran amine (BDA) into the red nucleus and reticular formation to label descending spinal pathways by anterograde transport mechanisms. Animals were sacrificed 1 or 4 weeks after injury and treatment with GDNF. The terminal end of injured BDA-labeled rubrospinal and reticulospinal tract axons was identified and the distance from the lesion was measured. In comparison to PBS-treated animals, GDNF-treatment resulted in a significant decrease in the extent of axonal retraction of both rubrospinal and reticulospinal tract axons at 1 week after spinal cord injury for both tracts. At 4 weeks after injury the mean distance from the lesion was less than 240 microm following GDNF-treatment for both tracts, compared to over 480 microm following PBS-treatment. In the second experiment injured supraspinal neurons were labeled by retrograde transport of True Blue that had been placed into the lesion cavity. One month later scar tissue was removed from the cavity by aspiration to enlarge the cavity by approximately 500 microm in a rostral direction. GDNF-saturated gel foam was placed into the cavity for 60 min prior to apposition of an autologous peripheral nerve (PN) graft to the rostral cavity wall. One month later Nuclear Yellow was applied to the distal end of the PN graft and animals were sacrificed after 2 days. The number of supraspinal neurons containing both True Blue and Nuclear Yellow was counted as a measure of axonal regeneration by chronically injured neurons. There was a seven-fold increase in the number of regenerating neurons after GDNF-treatment, with the majority (65%) of dual-labeled neurons located within the reticular formation. These results indicate that GDNF has neuroprotective effects when provided acutely after injury and promotes axonal regeneration when provided in a chronic injury situation.

Progesterone up-regulates neuronal brain-derived neurotrophic factor expression in the injured spinal cord

Progesterone (PROG) provides neuroprotection to the injured central and peripheral nervous system. These effects may be due to regulation of myelin synthesis in glial cells and also to direct actions on neuronal function. Recent studies point to neurotrophins as possible mediators of hormone action. Here, we show that the expression of brain-derived neurotrophic factor (BDNF) at both the mRNA and protein levels was increased by PROG treatment in ventral horn motoneurons from rats with spinal cord injury (SCI). Semiquantitative in situ hybridization revealed that SCI reduced BDNF mRNA levels by 50% in spinal motoneurons (control: 53.5+/-7.5 grains/mm(2) vs. SCI: 27.5+/-1.2, P<0.05), while PROG administration to injured rats (4 mg/kg/day during 3 days, s.c.) elicited a three-fold increase in grain density (SCI+PROG: 77.8+/-8.3 grains/mm(2), P<0.001 vs. SCI). In addition, PROG enhanced BDNF immunoreactivity in motoneurons of the lesioned spinal cord. Analysis of the frequency distribution of immunoreactive densities (chi(2): 812.73, P<0.0001) showed that 70% of SCI+PROG motoneurons scored as dark stained whereas only 6% of neurons in the SCI group belonged to this density score category (P<0.001). PROG also prevented the lesion-induced chromatolytic degeneration of spinal cord motoneurons as determined by Nissl staining. In the normal intact spinal cord, PROG significantly increased BDNF inmunoreactivity in ventral horn neurons, without changes in mRNA levels. Our findings suggest that PROG enhancement of endogenous neuronal BDNF could provide a trophic environment within the lesioned spinal cord and might be part of the PROG activated-pathways to provide neuroprotection.

Neurotrophic factors expressed in both cortex and spinal cord induce axonal plasticity after spinal cord injury

We reported recently that overexpression of neurotrophin-3 (NT-3) by motoneurons in the spinal cord of rats will induce sprouting of corticospinal tract (CST) axons (Zhou et al. [2003] J. Neurosci. 23:1424-1431). We now report that overexpression of brain-derived neurotrophic factor (BDNF) or glial cell-derived neurotrophic factor (GDNF) in the rat sensorimotor cortex near the CST neuronal cell bodies together with overexpression of NT-3 in the lumbar spinal cord significantly increases axonal sprouting compared to that induced by NT-3 alone. Two weeks after unilaterally lesioning the CST at the level of the pyramids, we injected rats with saline or adenoviral vectors (Adv) carrying genes coding for BDNF (Adv.BDNF), GDNF (Adv.GDNF) or enhanced green fluorescent protein (Adv.EGFP) at six sites in the sensorimotor cortex, while delivering Adv.NT3 to motoneurons in each of these four groups on the lesioned side of the spinal cord by retrograde transport from the sciatic nerve. Four days later, biotinylated dextran amine (BDA) was injected into the sensorimotor cortex on the unlesioned side to mark CST axons in the spinal cord. Morphometric analysis of axonal sprouting 3 weeks after BDA injection showed that the number of CST axons crossing the midline in rats treated with Adv.BDNF or Adv.GDNF were 46% and 52% greater, respectively, than in rats treated with Adv.EGFP or PBS (P < 0.05). These data demonstrate that sustained local expression of neurotrophic factors in the sensorimotor cortex and spinal cord will promote increased axonal sprouting after spinal cord injury, providing a basis for continued development of neurotrophic factor therapy for central nervous system damage.

GDNF and IGF-I trophic factors delay hereditary Purkinje cell degeneration and the progression of gait ataxia

Neurotrophic factors GDNF and/or IGF-I were chronically infused into shaker mutant rats to rescue cerebellar Purkinje neurons from adult-onset heredodegeneration. The natural expression of the shaker mutation is characterized by spatially restricted degeneration of Purkinje cells that occurs earlier and faster in an anterior vermal compartment and slightly later and more slowly in a posterior vermal compartment. Gait ataxia and whole body tremor develop concomitant with the degeneration of Purkinje neurons. The number and spatial distribution of surviving Purkinje neurons, identified by cell-specific calbindin immunoreactivity, were quantitatively analyzed in mid-sagittal sections and correlated with quantitative movement analysis of hindlimb gait patterns. Compared to the number of surviving Purkinje cells in age-matched, non-infused, or saline-infused control mutants, 4 weeks of infusion of GDNF or IGF-I rescued many anterior compartment Purkinje cells from early degeneration. However, 2 and 4 weeks after cessation of GDNF or IGF-I infusion, respectively, the number and spatial distribution of surviving Purkinje cells was comparable to that observed in age-matched controls. Eight weeks of infusion of trophic factors did not support the continued survival of most anterior compartment Purkinje cells and was partially, and probably only transiently, neuroprotective for some posterior compartment Purkinje cells. When GDNF and IGF-I were infused together for 4 weeks the number of surviving Purkinje cells was additively greater than with either factor alone. Behaviorally, 4 weeks of infusion of trophic factors delayed the development of gait ataxia. Infused GDNF appeared to preserve hip stability, whereas IGF-I stabilized step length. Tremor was attenuated with 8 weeks of infusion of GDNF or IGF-I. GDNF-infused animals showed low power tremor frequencies, whereas IGF-I infusion resulted in a single large power peak with decreased numbers of low-amplitude frequencies. Collectively these findings indicate that exogenous trophic factors can delay the onset of hereditary Purkinje cell degeneration and gait ataxia. Quite surprisingly, GDNF and IGF-I appeared to act on disparate populations of mutant Purkinje cells, whose differential survival affected different aspects of locomotion.

Regulation of natural cell death in dopaminergic neurons of the substantia nigra by striatal glial cell line-derived neurotrophic factor in vivo

Dopamine (DA) neurons of the substantia nigra undergo a developmental cell death event that is biphasic, with peaks just after birth and at postnatal day 14. As envisioned by neurotrophic theory, this cell death is likely to be regulated by target interactions because it is augmented by their disruption. However, the nature of the trophic molecules mediating this regulation are unknown. We showed in vitro that glial cell line-derived neurotrophic factor (GDNF) is able to suppress apoptotic death in DA neurons in postnatal primary culture. We now demonstrate in vivo that administration of GDNF into the striatal target is able to suppress apoptosis. Consistent with a possible physiologic role for endogenous striatal GDNF in regulating this event, two anti-GDNF neutralizing antibodies augment cell death. These antibodies augment cell death only during the first (immediately postnatal) phase of the biphasic death event. We conclude that GDNF is the leading candidate for a target-derived neurotrophic factor for the regulation of the early phase of natural cell death in DA neurons.

Neurotrophic cross-talk between the nervous and immune systems: implications for neurological diseases

Inflammatory reactions in the central nervous system usually are considered detrimental, but recent evidence suggests that they also can be beneficial and even have neuroprotective effects. Intriguingly, immune cells can produce various neurotrophic factors of various molecular families. The concept of "neuroprotective immunity" will have profound consequences for the pathogenesis and treatment of neuroinflammatory diseases such as multiple sclerosis. It also will prove important for neurodegenerative disorders, in which inflammatory reactions often occur. This review focuses on recent findings that immune cells produce brain-derived neurotrophic factor in multiple sclerosis lesions, whereas neurons and astrocytes express the appropriate tyrosine kinase receptor TrkB. Together with functional evidence for the neuroprotective effects of immune cells, these observations support the concept of "neuroprotective immunity." We next examine current and future therapeutic strategies for multiple sclerosis and experimental autoimmune encephalomyelitis in light of neuroprotective immunity and finally address the broader implications of this new concept for other neuroinflammatory and neurodegenerative diseases.

Glial cell line-derived neurotrophic factor and chronic electrical stimulation prevent VIII cranial nerve degeneration following denervation

As with other cranial nerves and many CNS neurons, primary auditory neurons degenerate as a consequence of loss of input from their target cells, the inner hair cells (IHCs). Electrical stimulation (ES) of spiral ganglion cells (SGCs) has been shown to enhance their survival. Glial cell line-derived neurotrophic factor (GDNF) has also been shown to increase survival of SGCs following IHC loss. In this study, the combined effects of the GDNF transgene delivered by adenoviral vectors (Ad-GDNF) and ES were tested on SGCs after first eliminating the IHCs. Animal groups received Ad-GDNF or ES or both. Ad-GDNF was inoculated into the cochlea of guinea pigs after deafening, to overexpress human GDNF. ES-treated animals were implanted with a cochlear implant electrode and chronically stimulated. A third group of animals received both Ad-GDNF and ES (GDNF/ES). Electrically evoked auditory brainstem responses were recorded from ES-treated animals at the start and end of the stimulation period. Animals were sacrificed 43 days after deafening and their ears prepared for evaluation of IHC survival and SGC counts. Treated ears exhibited significantly greater SGC survival than nontreated ears. The GDNF/ES combination provided significantly better preservation of SGC density than either treatment alone. Insofar as ES parameters were optimized for maximal protection (saturated effect), the further augmentation of the protection by GDNF suggests that the mechanisms of GDNF- and ES-mediated SGC protection are, at least in part, independent. We suggest that GDNF/ES combined treatment in cochlear implant recipients will improve auditory perception. These findings may have implications for the prevention and treatment of other neurodegenerative processes. .

Developmental alteration of nerve injury induced glial cell line-derived neurotrophic factor (GDNF) receptor expression is crucial for the determination of injured motoneuron fate

Axotomy-induced neuronal death occurs in neonatal motoneurons, but not in adult rat. Here we demonstrated that during the course of postnatal development, nerve injury induced down-regulation of the glial cell line-derived neurotrophic factor (GDNF) receptor GFRalpha1 in axotomized hypoglossal motoneurons of rat are gradually converted to the adult up-regulation pattern of response. The compensatory expression of GFRalpha1 specifically in the injured motoneurons of neonates by adenovirus succeeded in rescuing the injured neurons without an application of growth factors. To the contrary, the nuclear antisense RNA for GFRalpha1 expression accelerates the axotomy-induced neuronal death in pups. These findings suggest that the receptor expression response after nerve injury is critical for the determination of injured motoneuron fate.

Fibroblast growth factor-2 requires glial-cell-line-derived neurotrophic factor for exerting its neuroprotective actions on glutamate-lesioned hippocampal neurons

FGF-2 is a potent neurotrophic factor for several populations of CNS neurons and has been shown to protect hippocampal neurons from glutamate-induced cell death in vitro and in vivo. Mechanisms underlying the neurotrophic and protective actions of FGF-2 have been resolved only in part. Using glutamate-treated cultured hippocampal neurons we show that FGF-2 shares its neuroprotective capacity with GDNF. Hippocampal neurons express glial-cell-line-derived neurotrophic factor (GDNF), its receptors c-Ret and the lipid-anchored GDNF family receptor-alpha1 (GFRalpha-1), and the FGF receptor 1 (FGFR I). Neutralizing antibodies to GDNF abolish the neuroprotective effect of FGF-2. In support of the notion that GDNF is required to permit the protective effects of FGF-2 we find that FGF-2 up-regulates GDNF and GFRalpha-1 in hippocampal neurons. Furthermore, FGF-2-induced GDNF causes enhanced phosphorylation of c-Ret and the signaling components Akt and Erk. A putative downstream target of FGF-2 and GDNF are bcl-2 gene family members, whose mRNAs are differentially up-regulated by the two factors. Together, these data suggest that GDNF is an important protective factor for glutamate-lesioned hippocampal neurons and an essential mediator of the neuroprotective actions of FGF-2.

Promoting axonal regeneration in the central nervous system by enhancing the cell body response to axotomy

Neurons projecting into the peripheral nervous system (PNS) regenerate their axons after injury, in contrast to those confined to the central nervous system (CNS). Both neuronal and nonneuronal factors contribute to the lack of CNS regeneration. In this review we concentrate on the differential gene expression response to axotomy in PNS vs. CNS neurons. In general CNS neurons fail to up-regulate or sustain the expression of regeneration-associated proteins (RAGs), including trophic factors and their receptors. The presumed lack of trophic support of axotomized CNS neurons provided the rationale for the exogenous application of trophic factors, either to the lesion site or to the cell bodies. Here, we review our data on the application of trophic factors to rubrospinal and corticospinal neurons. Cell body treatment of axotomized rubrospinal neurons with brain-derived neurotrophic factor (BDNF) reversed atrophy, increased GAP-43 and Talpha-1 tubulin mRNA expression, and promoted axonal regeneration into peripheral nerve grafts. Importantly, BDNF cell body treatment was still effective in the chronic setting, i.e., when initiated 1 year after injury, but BDNF had no effect when applied to the chronic spinal cord injury site. The ability to promote regeneration in chronically injured neurons will hopefully contribute to the development of treatment strategies for chronic spinal injuries.