Influences of neurotrophins on mammalian motoneurons in vivo

Growth factors and molecular-driven plasticity in neurological systems

It has been almost 70 years since the discovery of nerve growth factor (NGF), a period of a dramatic evolution in our understanding of dynamic growth, regeneration, and rewiring of the nervous system. In 1953, the extraordinary finding that a protein found in mouse submandibular glands generated a halo of outgrowing axons has now redefined our concept of the nervous system connectome. Central and peripheral neurons and their axons or dendrites are no longer considered fixed or static "wiring." Exploiting this molecular-driven plasticity as a therapeutic approach has arrived in the clinic with a slate of new trials and ideas. Neural growth factors (GFs), soluble proteins that alter the behavior of neurons, have expanded in numbers and our understanding of the complexity of their signaling and interactions with other proteins has intensified. However, beyond these "extrinsic" determinants of neuron growth and function are the downstream pathways that impact neurons, ripe for translational development and potentially more important than individual growth factors that may trigger them. Persistent and ongoing nuances in clinical trial design in some of the most intractable and irreversible neurological conditions give hope for connecting new biological ideas with clinical benefits. This review is a targeted update on neural GFs, their signals, and new therapeutic ideas, selected from an expansive literature.

Cross-talk between motor neurons and myotubes via endogenously secreted neural and muscular growth factors

Neuromuscular junction (NMJ) research is vital to advance the understanding of neuromuscular patho‐physiology and development of novel therapies for diseases associated with NM dysfunction. In vivo, the micro‐environment surrounding the NMJ has a significant impact on NMJ formation and maintenance via neurotrophic and differentiation factors that are secreted as a result of cross‐talk between muscle fibers and motor neurons. Recently we showed the formation of functional NMJs in vitro in a co‐culture of immortalized human myoblasts and motor neurons from rat‐embryo spinal‐cord explants, using a culture medium free from serum and neurotrophic or growth factors. The aim of this study was to assess how functional NMJs were established in this co‐culture devoid of exogenous neural growth factors. To investigate this, an ELISA‐based microarray was used to compare the composition of soluble endogenously secreted growth factors in this co‐culture with an a‐neural muscle culture. The levels of seven neurotrophic factors brain‐derived neurotrophic factor (BDNF), glial‐cell‐line‐derived neurotrophic factor (GDNF), insulin‐like growth factor‐binding protein‐3 (IGFBP‐3), insulin‐like growth factor‐1 (IGF‐1), neurotrophin‐3 (NT‐3), neurotrophin‐4 (NT‐4), and vascular endothelial growth factor (VEGF) were higher (p < 0.05) in the supernatant of NMJ culture compared to those in the supernatant of the a‐neural muscle culture. This indicates that the cross‐talk between muscle and motor neurons promotes the secretion of soluble growth factors contributing to the local microenvironment thereby providing a favourable regenerative niche for NMJs formation and maturation.

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

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

Neurotrophin-3 stimulates stem Leydig cell proliferation during regeneration in rats

Neurotrophin‐3 (NT‐3) acts as an important growth factor to stimulate and control tissue development. The NT‐3 receptor, TRKC, is expressed in rat testis. Its function in regulation of stem Leydig cell development and its underlying mechanism remain unknown. Here, we reported the role of NT‐3 to regulate stem Leydig cell development in vivo and in vitro. Ethane dimethane sulphonate was used to kill all Leydig cells in adult testis, and NT‐3 (10 and 100 ng/testis) was injected intratesticularly from the 14th day after ethane dimethane sulphonate injection for 14 days. NT‐3 significantly reduced serum testosterone levels at doses of 10 and 100 ng/testis without affecting serum luteinizing hormone and follicle‐stimulating hormone levels. NT‐3 increased CYP11A1‐positive Leydig cell number at 100 ng/testis and lowered Leydig cell size and cytoplasmic size at doses of 10 and 100 ng/testis. After adjustment by the Leydig cell number, NT‐3 significantly down‐regulated the expression of Leydig cell genes (Lhcgr, Scarb1, Star, Cyp11a1, Hsd3b1, Cyp17a1, Hsd17b3, Hsd11b1, Insl3, Trkc and Nr5a1) and the proteins. NT‐3 increased the phosphorylation of AKT1 and mTOR, decreased the phosphorylation of 4EBP, thereby increasing ATP5O. In vitro study showed that NT‐3 dose‐dependently stimulated EdU incorporation into stem Leydig cells and inhibited stem Leydig cell differentiation into Leydig cells, thus leading to lower medium testosterone levels and lower expression of Lhcgr, Scarb1, Trkc and Nr5a1 and their protein levels. NT‐3 antagonist Celitinib can antagonize NT‐3 action in vitro. In conclusion, the present study demonstrates that NT‐3 stimulates stem Leydig cell proliferation but blocks the differentiation via TRKC receptor.

Stroke Recovery in Rats after 24-Hour-Delayed Intramuscular Neurotrophin-3 Infusion

Objective

Neurotrophin‐3 (NT3) plays a key role in the development and function of locomotor circuits including descending serotonergic and corticospinal tract axons and afferents from muscle and skin. We have previously shown that gene therapy delivery of human NT3 into affected forelimb muscles improves sensorimotor recovery after stroke in adult and elderly rats. Here, to move toward the clinic, we tested the hypothesis that intramuscular infusion of NT3 protein could improve sensorimotor recovery after stroke.

Methods

Rats received unilateral ischemic stroke in sensorimotor cortex. To simulate a clinically feasible time to treatment, 24 hours later rats were randomized to receive NT3 or vehicle by infusion into affected triceps brachii for 4 weeks using implanted catheters and minipumps.

Results

Radiolabeled NT3 crossed from the bloodstream into the brain and spinal cord in rodents with or without strokes. NT3 increased the accuracy of forelimb placement during walking on a horizontal ladder and increased use of the affected arm for lateral support during rearing. NT3 also reversed sensory impairment of the affected wrist. Functional magnetic resonance imaging during stimulation of the affected wrist showed spontaneous recovery of peri‐infarct blood oxygenation level–dependent signal that NT3 did not further enhance. Rather, NT3 induced neuroplasticity of the spared corticospinal and serotonergic pathways.

Interpretation

Our results show that delayed, peripheral infusion of NT3 can improve sensorimotor function after ischemic stroke. Phase I and II clinical trials of NT3 (for constipation and neuropathy) have shown that peripheral high doses are safe and well tolerated, which paves the way for NT3 as a therapy for stroke. ANN NEUROL 2019;85:32–46.

The many disguises of the signalling endosome

Neurons are highly complex and polarised cells that must overcome a series of logistic challenges to maintain homeostasis across their morphological domains. A very clear example is the propagation of neurotrophic signalling from distal axons, where target-released neurotrophins bind to their receptors and initiate signalling, towards the cell body, where nuclear and cytosolic responses are integrated. The mechanisms of propagation of neurotrophic signalling have been extensively studied and, eventually, the model of a 'signalling endosome', transporting activated receptors and associated complexes, has emerged. Nevertheless, the exact nature of this organelle remains elusive. In this Review, we examine the evidence for the retrograde transport of neurotrophins and their receptors in endosomes, outline some of their diverse physiological and pathological roles, and discuss the main interactors, morphological features and trafficking destinations of a highly flexible endosomal signalling organelle with multiple molecular signatures.

Putative roles of soluble trophic factors in facial nerve regeneration, target reinnervation, and recovery of vibrissal whisking

It is well-known that, after nerve transection and surgical repair, misdirected regrowth of regenerating motor axons may occur in three ways. The first way is that the axons enter into endoneurial tubes that they did not previously occupy, regenerate through incorrect fascicles and reinnervate muscles that they did not formerly supply. Consequently the activation of these muscles results in inappropriate movements. The second way is that, in contrast with the precise target-directed pathfinding by elongating motor nerves during embryonic development, several axons rather than a single axon grow out from each transected nerve fiber. The third way of misdirection occurs by the intramuscular terminal branching (sprouting) of each regenerating axon to culminate in some polyinnervation of neuromuscular junctions, i.e. reinnervation of junctions by more than a single axon. Presently, "fascicular" or "topographic specificity" cannot be achieved and hence target-directed nerve regeneration is, as yet, unattainable. Nonetheless, motor and sensory reinnervation of appropriate endoneurial tubes does occur and can be promoted by brief nerve electrical stimulation. This review considers the expression of neurotrophic factors in the neuromuscular system and how this expression can promote functional recovery, with emphasis on the whisking of vibrissae on the rat face in relationship to the expression of the factors. Evidence is reviewed for a role of neurotrophic factors as short-range diffusible sprouting stimuli in promoting complete functional recovery of vibrissal whisking in blind Sprague Dawley (SD)/RCS rats but not in SD rats with normal vision, after facial nerve transection and surgical repair. Briefly, a complicated time course of growth factor expression in the nerves and denervated muscles include (1) an early increase in FGF2 and IGF2, (2) reduced NGF between 2 and 14days after nerve transection and surgical repair, (3) a late rise in BDNF and (4) reduced IGF1 protein in the denervated muscles at 28days. These findings suggest that recovery of motor function after peripheral nerve injury is due, at least in part, to a complex regulation of nerve injury-associated neurotrophic factors and cytokines at the neuromuscular junctions of denervated muscles. In particular, the increase of FGF2 and concomittant decrease of NGF during the first week after facial nerve-nerve anastomosis in SD/RCS blind rats may prevent intramuscular axon sprouting and, in turn, reduce poly-innervation of the neuromuscular junction.

Neurobiology of axonal transport defects in motor neuron diseases: Opportunities for translational research?

Intracellular trafficking of cargoes is an essential process to maintain the structure and function of all mammalian cell types, but especially of neurons because of their extreme axon/dendrite polarisation. Axonal transport mediates the movement of cargoes such as proteins, mRNA, lipids, membrane-bound vesicles and organelles that are mostly synthesised in the cell body and in doing so is responsible for their correct spatiotemporal distribution in the axon, for example at specialised sites such as nodes of Ranvier and synaptic terminals. In addition, axonal transport maintains the essential long-distance communication between the cell body and synaptic terminals that allows neurons to react to their surroundings via trafficking of for example signalling endosomes. Axonal transport defects are a common observation in a variety of neurodegenerative diseases, and mutations in components of the axonal transport machinery have unequivocally shown that impaired axonal transport can cause neurodegeneration (reviewed in El-Kadi et al., 2007, De Vos et al., 2008; Millecamps and Julien, 2013). Here we review our current understanding of axonal transport defects and the role they play in motor neuron diseases (MNDs) with a specific focus on the most common form of MND, amyotrophic lateral sclerosis (ALS).

Grafts of Fibroblasts Genetically Modified to Secrete Ngf, Bdnf, Nt-3, or Basic Fgf Elicit Differential Responses in the Adult Spinal Cord

Neuronal and axonal responses to neurotrophic factors in the developing spinal cord have been relatively well characterized, but little is known about adult spinal responses to neurotrophic factors. We genetically modified primary rat fibroblasts to produce either nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), or basic fibroblast growth factor (bFGF), then grafted these neurotrophic factor-secreting cells into the central gray matter of the spinal cord in adult rats. Spinal cord lesions were not made prior to grafting. From 2 wk to 6 mo later, sensory neurites of dorsal root origin extensively penetrated NGF-, NT-3-, and bFGF-producing grafts, whereas BDNF-secreting grafts elicited no growth responses. Putative noradrenergic neurites also penetrated NGF-secreting cell grafts. Local motor and corticospinal motor axons did not penetrate any of the neurotrophic factor-secreting grafts. These results indicate that unlesioned or minimally lesioned adult spinal cord sensory and putative noradrenergic populations retain significant neurotrophic factor responsiveness, whereas motor neurites are comparatively resistant even to those neurotrophic factors to which they exhibit survival dependence during development. Grafts of genetically modified cells can be a useful tool for characterizing neurotrophic factor responsiveness in the adult spinal cord and designing strategies to promote axonal regeneration after injury.

Functional Diversity of Neurotrophin Actions on the Oculomotor System

Neurotrophins play a principal role in neuronal survival and differentiation during development, but also in the maintenance of appropriate adult neuronal circuits and phenotypes. In the oculomotor system, we have demonstrated that neurotrophins are key regulators of developing and adult neuronal properties, but with peculiarities depending on each neurotrophin. For instance, the administration of NGF (nerve growth factor), BDNF (brain-derived neurotrophic factor) or NT-3 (neurotrophin-3) protects neonatal extraocular motoneurons from cell death after axotomy, but only NGF and BDNF prevent the downregulation in ChAT (choline acetyltransferase). In the adult, in vivo recordings of axotomized extraocular motoneurons have demonstrated that the delivery of NGF, BDNF or NT-3 recovers different components of the firing discharge activity of these cells, with some particularities in the case of NGF. All neurotrophins have also synaptotrophic activity, although to different degrees. Accordingly, neurotrophins can restore the axotomy-induced alterations acting selectively on different properties of the motoneuron. In this review, we summarize these evidences and discuss them in the context of other motor systems.

Prolongation of Relaxation Time in Extraocular Muscles With Brain Derived Neurotrophic Factor in Adult Rabbit

PURPOSE

We tested the hypothesis that short-term treatment with brain derived neurotrophic factor (BDNF) would alter the contractile characteristics of rabbit extraocular muscle (EOM).

METHODS

One week after injections of BDNF in adult rabbit superior rectus muscles, twitch properties were determined in treated and control muscles in vitro. Muscles were also examined for changes in mean cross-sectional areas, neuromuscular junction size, and percent of myofibers expressing specific myosin heavy chain isoforms, and sarcoendoplasmic reticulum calcium ATPases (SERCA) 1 and 2.

RESULTS

Brain derived neurotrophic factor-treated muscles had prolonged relaxation times compared with control muscles. Time to 50% relaxation, time to 100% relaxation, and maximum rate of relaxation were increased by 24%, 27%, and 25%, respectively. No significant differences were seen in time to peak force, twitch force, or maximum rate of contraction. Brain derived neurotrophic factor treatment significantly increased mean cross-sectional areas of slow twitch and tonic myofibers, with increased areas ranging from 54% to 146%. Brain derived neurotrophic factor also resulted in an increased percentage of slow twitch myofibers in the orbital layers, ranging from 54% to 77%, and slow-tonic myofibers, ranging from 44% to 62%. No significant changes were seen SERCA1 or 2 expression or in neuromuscular junction size.

CONCLUSIONS

Short-term treatment with BDNF significantly prolonged the duration and rate of relaxation time and increased expression of both slow-twitch and slow-tonic myosin-expressing myofibers without changes in neuromuscular junctions or SERCA expression. The changes induced by BDNF treatment might have potential therapeutic value in dampening/reducing uncontrolled eye oscillations in nystagmus.

Approaches to Peripheral Nerve Repair: Generations of Biomaterial Conduits Yielding to Replacing Autologous Nerve Grafts in Craniomaxillofacial Surgery

Peripheral nerve injury is a common clinical entity, which may arise due to traumatic, tumorous, or even iatrogenic injury in craniomaxillofacial surgery. Despite advances in biomaterials and techniques over the past several decades, reconstruction of nerve gaps remains a challenge. Autografts are the gold standard for nerve reconstruction. Using autografts, there is donor site morbidity, subsequent sensory deficit, and potential for neuroma development and infection. Moreover, the need for a second surgical site and limited availability of donor nerves remain a challenge. Thus, increasing efforts have been directed to develop artificial nerve guidance conduits (ANCs) as new methods to replace autografts in the future. Various synthetic conduit materials have been tested in vitro and in vivo, and several first- and second-generation conduits are FDA approved and available for purchase, while third-generation conduits still remain in experimental stages. This paper reviews the current treatment options, summarizes the published literature, and assesses future prospects for the repair of peripheral nerve injury in craniomaxillofacial surgery with a particular focus on facial nerve regeneration.

A novel synaptic plasticity rule explains homeostasis of neuromuscular transmission

Excitability differs among muscle fibers and undergoes continuous changes during development and growth, yet the neuromuscular synapse maintains a remarkable fidelity of execution. Here we show in two evolutionarily distant vertebrates (Xenopus laevis cell culture and mouse nerve-muscle ex-vivo) that the skeletal muscle cell constantly senses, through two identified calcium signals, synaptic events and their efficacy in eliciting spikes. These sensors trigger retrograde signal(s) that control presynaptic neurotransmitter release, resulting in synaptic potentiation or depression. In the absence of spikes, synaptic events trigger potentiation. Once the synapse is sufficiently strong to initiate spiking, the occurrence of these spikes activates a negative retrograde feedback. These opposing signals dynamically balance the synapse in order to continuously adjust neurotransmitter release to a level matching current muscle cell excitability.

DOI:http://dx.doi.org/10.7554/eLife.12190.001

Nerve cells communicate with each other, and with targets such as muscle cells, at junctions called synapses. The nerve cell before the synapses releases a chemical called a neurotransmitter, which binds to receptors on the cell after the synapses. However, the first cell cannot determine by itself whether it is releasing the correct amount of neurotransmitter to activate its partner. For this, it requires feedback from the second cell.

This feedback is particularly important at synapses between nerve cells and muscle cells, which are known as neuromuscular junctions. The likelihood that a given amount of transmitter will activate a muscle cell can vary with age and after exercise. Muscle cells must therefore be able to instruct their nerve cell partners to increase or decrease neurotransmitter release to accommodate these changes.

Ouanounou et al. have now identified the mechanism by which muscle cells determine whether nerve cells are releasing an appropriate amount of neurotransmitter. Experiments in two distantly related animals – mice and embryos from a frog called Xenopus – revealed that muscle cells use two calcium-based signals. The first is the flow of calcium ions into the muscle cell in response to binding of neurotransmitter to receptors at the synapses: this tells the muscle cell how active the nerve cell is. The second is the release of calcium ions from internal stores inside the muscle cell: this occurs whenever neurotransmitter release is sufficient to activate the muscle cell.

In response to the first calcium signal, the muscle cell sends positive feedback to the neuron, telling it to increase neurotransmitter release further. In response to the second signal, the muscle cell sends negative feedback to reduce neurotransmitter release. Thus, when neurotransmitter release is not enough to activate the muscle, positive feedback dominates and neurotransmitter release increases. However, when the muscle is activated, the two types of feedback act in balance to maintain efficient communication across the synapse.

The next steps are to identify the cell signaling cascades that are mobilized by the two calcium signals, including the specific molecule (or molecules) that regulate neurotransmitter release.

DOI:http://dx.doi.org/10.7554/eLife.12190.002

TrkB gene therapy by adeno-associated virus enhances recovery after cervical spinal cord injury

Unilateral cervical spinal cord hemisection at C2 (C2SH) interrupts descending bulbospinal inputs to phrenic motoneurons, paralyzing the diaphragm muscle. Recovery after C2SH is enhanced by brain derived neurotrophic factor (BDNF) signaling via the tropomyosin-related kinase subtype B (TrkB) receptor in phrenic motoneurons. The role for gene therapy using adeno-associated virus (AAV)-mediated delivery of TrkB to phrenic motoneurons is not known. The present study determined the therapeutic efficacy of intrapleural delivery of AAV7 encoding for full-length TrkB (AAV-TrkB) to phrenic motoneurons 3 days post-C2SH. Diaphragm EMG was recorded chronically in male rats (n=26) up to 21 days post-C2SH. Absent ipsilateral diaphragm EMG activity was verified 3 days post-C2SH. A greater proportion of animals displayed recovery of ipsilateral diaphragm EMG activity during eupnea by 14 and 21 days post-SH after AAV-TrkB (10/15) compared to AAV-GFP treatment (2/11; p=0.031). Diaphragm EMG amplitude increased over time post-C2SH (p<0.001), and by 14 days post-C2SH, AAV-TrkB treated animals displaying recovery achieved 48% of the pre-injury values compared to 27% in AAV-GFP treated animals. Phrenic motoneuron mRNA expression of glutamatergic AMPA and NMDA receptors revealed a significant, positive correlation (r(2)=0.82), with increased motoneuron NMDA expression evident in animals treated with AAV-TrkB and that displayed recovery after C2SH. Overall, gene therapy using intrapleural delivery of AAV-TrkB to phrenic motoneurons is sufficient to promote recovery of diaphragm activity, adding a novel potential intervention that can be administered after upper cervical spinal cord injury to improve impaired respiratory function.

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

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

Neurotrophic factors in spinal cord injury

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

Brain-derived neurotrophic factor G196A polymorphism predicts 90-day outcome of ischemic stroke in Chinese: a novel finding

BACKGROUND AND PURPOSE

Recovery after stroke varies considerably between individuals. An abundance of evidence suggests that genetic factors contribute to stroke recovery. The aim of this study was to determine whether or not the BDNF G196A polymorphism independently influences the occurrence, severity, and 90-day functional outcome in Chinese patients with ischemic stroke (IS).

METHODS

BDNF G196A genetic variants were investigated in 494 IS and 346 controls. Severity was assessed by the National Institutes of Health Stroke Scale at the time of admission. Three hundred and eight patients were assessed 90 days post-stroke using the Modified Rankin Scale to determine stroke outcome.

RESULTS

We showed that a significant association existed between the BDNF G196A AA genotype and the occurrence of IS (P=0.021), even after adjustment for covariates (P=0.028). The AA genotype of the BDNF G196A was associated with a poor outcome of recovery 3 months after stroke onset (P=0.008) was a novel finding, independent of other known predictors of poor outcome (P=0.012).

CONCLUSIONS

The BDNF G196A polymorphism was significantly associated with the occurrence and long-term outcomes of IS, thus BDNF G196A may be used as a prognostic biomarker and therapeutic target in IS.

Neuroprotective effects of NGF, BDNF, NT-3 and GDNF on axotomized extraocular motoneurons in neonatal rats

Neurotrophic factors delivered from target muscles are essential for motoneuronal survival, mainly during development and early postnatal maturation. It has been shown that the disconnection between motoneurons and their innervated muscle by means of axotomy produces a vast neuronal death in neonatal animals. In the present work, we have evaluated the effects of different neurotrophic factors on motoneuronal survival after neonatal axotomy, using as a model the motoneurons innervating the extraocular eye muscles. With this purpose, neonatal rats were monocularly enucleated at the day of birth (postnatal day 0) and different neurotrophic treatments (NGF, BDNF, NT-3, GDNF and the mixture of BDNF+GDNF) were applied intraorbitally by means of a Gelfoam implant (a single dose of 5 μg of each factor). We first demonstrated that extraocular eye muscles of neonatal rats expressed these neurotrophic factors and therefore constituted a natural source of retrograde delivery for their innervating motoneurons. By histological and immunocytochemical methods we determined that all treatments significantly rescued extraocular motoneurons from axotomy-induced cell death. For the dose used, NGF and GDNF were the most potent survival factors for these motoneurons, followed by BDNF and lastly by NT-3. The simultaneous administration of BDNF and GDNF did not increase the survival-promoting effects above those obtained by GDNF alone. Interestingly, the rescue effects of all neurotrophic treatments persisted even 30 days after lesion. The administration of these neurotrophic factors, with the exception of NT-3, also prevented the loss of the cholinergic phenotype observed by 10 days after axotomy. At the dosage applied, NGF and GDNF were revealed again as the most effective neuroprotective agents against the axotomy-induced decrease in ChAT. Two remarkable findings highlighted in the present work that contrasted with other motoneuronal types after neonatal axotomy: first, the extremely high efficacy of NGF as a neuroprotective agent and, second, the long-lasting effects of neurotrophic administration on cell survival and ChAT expression in extraocular motoneurons.

Deficits in axonal transport precede ALS symptoms in vivo

ALS is a fatal neurodegenerative disease characterized by selective motor neuron death resulting in muscle paralysis. Mutations in superoxide dismutase 1 (SOD1) are responsible for a subset of familial cases of ALS. Although evidence from transgenic mice expressing human mutant SOD1(G93A) suggests that axonal transport defects may contribute to ALS pathogenesis, our understanding of how these relate to disease progression remains unclear. Using an in vivo assay that allows the characterization of axonal transport in single axons in the intact sciatic nerve, we have identified clear axonal transport deficits in presymptomatic mutant mice. An impairment of axonal retrograde transport may therefore represent one of the earliest axonal pathologies in SOD1(G93A) mice, which worsens at an early symptomatic stage. A deficit in axonal transport may therefore be a key pathogenic event in ALS and an early disease indicator of motor neuron degeneration.

G-CSF protects motoneurons against axotomy-induced apoptotic death in neonatal mice

Background

Granulocyte colony stimulating factor (G-CSF) is a growth factor essential for generation of neutrophilic granulocytes. Apart from this hematopoietic function, we have recently uncovered potent neuroprotective and regenerative properties of G-CSF in the central nervous system (CNS). The G-CSF receptor and G-CSF itself are expressed in α motoneurons, G-CSF protects motoneurons, and improves outcome in the SOD1(G93A) transgenic mouse model for amyotrophic lateral sclerosis (ALS). In vitro, G-CSF acts anti-apoptotically on motoneuronal cells. Due to the pleiotrophic effects of G-CSF and the complexity of the SOD1 transgenic ALS models it was however not possible to clearly distinguish between directly mediated anti-apoptotic and indirectly protective effects on motoneurons. Here we studied whether G-CSF is able to protect motoneurons from purely apoptotic cell death induced by a monocausal paradigm, neonatal sciatic nerve axotomy.

Results

We performed sciatic nerve axotomy in neonatal mice overexpressing G-CSF in the CNS and found that G-CSF transgenic mice displayed significantly higher numbers of surviving lumbar motoneurons 4 days following axotomy than their littermate controls. Also, surviving motoneurons in G-CSF overexpressing animals were larger, suggesting additional trophic effects of this growth factor.

Conclusions

In this model of pure apoptotic cell death the protective effects of G-CSF indicate direct actions of G-CSF on motoneurons in vivo. This shows that G-CSF exerts potent anti-apoptotic activities towards motoneurons in vivo and suggests that the protection offered by G-CSF in ALS mouse models is due to its direct neuroprotective activity.

Brain plasticity and genetic factors

Brain plasticity refers to changes in brain function and structure that arise in a number of contexts. One area in which brain plasticity is of considerable interest is recovery from stroke, both spontaneous and treatment-induced. A number of factors influence these poststroke brain events. The current review considers the impact of genetic factors. Polymorphisms in the human genes coding for brain-derived neurotrophic factor (BDNF) and apolipoprotein E (ApoE) have been studied in the context of plasticity and/or stroke recovery and are discussed here in detail. Several other genetic polymorphisms are indirectly involved in stroke recovery through their modulating influences on processes such as depression and pharmacotherapy effects. Finally, new genetic polymorphisms that have not been studied in the context of stroke are proposed as new directions for study. A better understanding of genetic influences on recovery and response to therapy might allow improved treatment after stroke.

Trophic factor expression in phrenic motor neurons

The function of a motor neuron and the muscle fibers it innervates (i.e., a motor unit) determines neuromotor output. Unlike other skeletal muscles, respiratory muscles (e.g., the diaphragm, DIAm) must function from birth onwards in sustaining ventilation. DIAm motor units are capable of both ventilatory and non-ventilatory behaviors, including expulsive behaviors important for airway clearance. There is significant diversity in motor unit properties across different types of motor units in the DIAm. The mechanisms underlying the development and maintenance of motor unit diversity in respiratory muscles (including the DIAm) are not well understood. Recent studies suggest that trophic factor influences contribute to this diversity. Remarkably little is known about the expression of trophic factors and their receptors in phrenic motor neurons. This review will focus on the contribution of trophic factors to the establishment and maintenance of motor unit diversity in the DIAm, during development and in response to injury or disease.

Differential survival response of neurons to exogenous GDNF depends on the presence of skeletal muscle

Glial cell line-derived neurotrophic factor (GDNF) is known as a potent survival factor for neurons in vitro and in vivo. The current study investigated the effects of a single in utero injection with GDNF in both wild-type and Myf5-/-:MyoD-/- embryos. The embryos in the latter group, denoted double mutants (DM), do not contain skeletal muscle and associated neurotrophic factors due to lack of myogenesis and, therefore, neurons of the central and peripheral nervous system undergo excessively occurring programmed cell death (EPCD). We found that treatment with GDNF had no effect on wild type neuronal numbers in any of the anatomic locations investigated. However, GDNF rescued the neurons of the facial motor nucleus, the mesencephalic nucleus and the median motor column in the absence of skeletal muscle. The findings of the current study agree with previous reports that compromised mouse neurons have increased survival response to GDNF.

Neurotrophic activity of extracts from distal stumps of pre-degenerated peripheral rat nerves varies according to molecular mass spectrum

OBJECTIVE

We investigated neurotrophic activity of extracts from pre-degenerated and non-pre-degenerated peripheral nerves (complete extracts and extracts with fractions of narrower range of molecular weight) on the injured hippocampus.

METHODS

The experiment was carried out on male Wistar C rats. The complete extracts or fractions with different ranges of molecular weight were introduced to the site of injury with the autologous connective tissue chambers. We examined DiI-labeled hippocampal cell and AChE-positive nerve endings to assess the regeneration intensity.

RESULTS

The highest number of labeled hippocampal cells was observed in the group treated with fraction of molecular weight 10-100 kDa (72.5 +/- 13.7) obtained from pre-degenerated nerves. We observed the presence of AChE-positive fibers inside all examined chambers.

DISCUSSION

These results demonstrate that suitable modification of CNS environments by introducing the protein fractions obtained from peripheral nerves can initiate the regeneration of the damaged hippocampal structure in adult rats. Moreover, it is possible to intensify their neurotrophic effect by former pre-degeneration of peripheral nerves and extraction from the entire extract proteins of molecular weight of 10-100 kDa.

Spinal adenosine A2a receptor activation elicits long-lasting phrenic motor facilitation

Acute intermittent hypoxia elicits a form of spinal, brain-derived neurotrophic factor (BDNF)-dependent respiratory plasticity known as phrenic long-term facilitation. Ligands that activate G(s)-protein-coupled receptors, such as the adenosine 2a receptor, mimic the effects of neurotrophins in vitro by transactivating their high-affinity receptor tyrosine kinases, the Trk receptors. Thus, we hypothesized that A2a receptor agonists would elicit phrenic long-term facilitation by mimicking the effects of BDNF on TrkB receptors. Here we demonstrate that spinal A2a receptor agonists transactivate TrkB receptors in the rat cervical spinal cord near phrenic motoneurons, thus inducing long-lasting (hours) phrenic motor facilitation. A2a receptor activation increased phosphorylation and new synthesis of an immature TrkB protein, induced TrkB signaling through Akt, and strengthened synaptic pathways to phrenic motoneurons. RNA interference targeting TrkB mRNA demonstrated that new TrkB protein synthesis is necessary for A2a-induced phrenic motor facilitation. A2a receptor activation also increased breathing in unanesthetized rats, and improved breathing in rats with cervical spinal injuries. Thus, small, highly permeable drugs (such as adenosine receptor agonists) that transactivate TrkB receptors may provide an effective therapeutic strategy in the treatment of patients with ventilatory control disorders, such as obstructive sleep apnea, or respiratory insufficiency after spinal injury or during neurodegenerative diseases.

Cholesterol loss enhances TrkB signaling in hippocampal neurons aging in vitro

Binding of the neurotrophin brain-derived neurotrophic factor (BDNF) to the TrkB receptor is a major survival mechanism during embryonic development. In the aged brain, however, BDNF levels are low, suggesting that if TrkB is to play a role in survival at this stage additional mechanisms must have developed. We here show that TrkB activity is most robust in the hippocampus of 21-d-old BDNF-knockout mice as well as in old, wild-type, and BDNF heterozygous animals. Moreover, robust TrkB activity is evident in old but not young hippocampal neurons differentiating in vitro in the absence of any exogenous neurotrophin and also in neurons from BDNF -/- embryos. Age-associated increase in TrkB activity correlated with a mild yet progressive loss of cholesterol. This, in turn, correlated with increased expression of the cholesterol catabolic enzyme cholesterol 24-hydroxylase. Direct cause-effect, cholesterol loss-high TrkB activity was demonstrated by pharmacological means and by manipulating the levels of cholesterol 24-hydroxylase. Because reduced levels of cholesterol and increased expression of choleseterol-24-hydroxylase were also observed in the hippocampus of aged mice, changes in cellular cholesterol content may be used to modulate receptor activity strength in vivo, autonomously or as a way to complement the natural decay of neurotrophin production.

Characterization and use of the NSC-34 cell line for study of neurotrophin receptor trafficking

This study addressed the suitability of the NSC-34 cell line as a motor neuron-like model for investigating neurotrophin receptor trafficking and associated subcellular processes. Initially, culture conditions were optimized for the use of NSC-34 cells in confocal microscopy. Cell surface markers, as well as markers associated with the regulated endosomal pathway thought to be associated with neurotrophin receptor transport, were identified. The study revealed the presence of a number of molecules previously not described in the literature, including the tropomyosin-like receptor kinase C (TrkC), sortilin, the vesicular acetylcholine transporter (VAChT), and the lipid raft-associated ganglioside GT1b. The presence of both sortilin and Gt1b was of special interest, insofar as these markers have been implicated in direct relationships with the p75NTR receptor. Evidence is provided for neurotrophin-dependent internalization of p75NTR and TrkB. Both nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) increased the rate of internalization of p75NTR, with internalization dynamics comparable to those described for other cell lines. Thus, these studies not only describe components of the regulatory process governing the trafficking of this important receptor but also clearly demonstrate the value of NSC-34 cells as a suitable motor neuron model for the study of internalization and trafficking of cell surface molecules.

Neurotrophins Redirect p75NTRfrom a Clathrin‐Independent to a Clathrin‐Dependent Endocytic Pathway Coupled to Axonal Transport

The p75 neurotrophin receptor (p75(NTR)) plays multiple roles in neuronal physiology through interactions with many ligands and coreceptors. However, its intracellular neuronal trafficking prior to and after neurotrophin activation is still poorly characterized. We have previously shown that in response to nerve growth factor (NGF), p75(NTR) is retrogradely transported along the axons of motor neurons (MNs) in carriers shared with NGF, brain-derived neurotrophic factor and the tyrosine kinase receptor TrkB. Here, we report that NGF does not enhance the internalization or degradation of p75(NTR), which undergoes a rapid dynamin-dependent and clathrin-independent recycling process in MNs. Instead, incubation of cells with NGF leads to the redirection of a pool of plasma membrane p75(NTR) into clathrin-coated pits. The subsequent internalization of p75(NTR) via clathrin-mediated endocytosis, as well as the activity of Rab5, are essential for the sorting of the p75(NTR)-containing endosomes to the axonal retrograde transport pathway and for the delivery of p75(NTR) to the soma. Our findings suggest that the spatial regulation of p75(NTR) signalling is controlled by these ligand-driven routes of endocytosis.

Mitochondrial superoxide production and nuclear factor erythroid 2-related factor 2 activation in p75 neurotrophin receptor-induced motor neuron apoptosis

Nerve growth factor (NGF) can induce apoptosis by signaling through the p75 neurotrophin receptor (p75(NTR)) in several nerve cell populations. Cultured embryonic motor neurons expressing p75(NTR) are not vulnerable to NGF unless they are exposed to an exogenous flux of nitric oxide (*NO). In the present study, we show that p75(NTR)-mediated apoptosis in motor neurons involved neutral sphingomyelinase activation, increased mitochondrial superoxide production, and cytochrome c release to the cytosol. The mitochondria-targeted antioxidants mitoQ and mitoCP prevented neuronal loss, further evidencing the role of mitochondria in NGF-induced apoptosis. In motor neurons overexpressing the amyotrophic lateral sclerosis (ALS)-linked superoxide dismutase 1(G93A) (SOD1(G93A)) mutation, NGF induced apoptosis even in the absence of an external source of *NO. The increased susceptibility of SOD1(G93A) motor neurons to NGF was associated to decreased nuclear factor erythroid 2-related factor 2 (Nrf2) expression and downregulation of the enzymes involved in glutathione biosynthesis. In agreement, depletion of glutathione in nontransgenic motor neurons reproduced the effect of SOD1(G93A) expression, increasing their sensitivity to NGF. In contrast, rising antioxidant defenses by Nrf2 activation prevented NGF-induced apoptosis. Together, our data indicate that p75(NTR)-mediated motor neuron apoptosis involves ceramide-dependent increased mitochondrial superoxide production. This apoptotic pathway is facilitated by the expression of ALS-linked SOD1 mutations and critically modulated by Nrf2 activity.

Immunodeficiency impairs re-injury induced reversal of neuronal atrophy: relation to T cell subsets and microglia

Following facial nerve resection in the mouse, a substantial number of neurons reside in an atrophied state (characterized by cell shrinkage and decreased ability to uptake Nissl stain), which can be reversed by re-injury. The mechanisms mediating the reversal of neuronal atrophy remain unclear. Although T cells have been shown to prevent neuronal loss following peripheral nerve injury, it was unknown whether T cells play a role in mediating the reversal of axotomy-induced neuronal atrophy. Thus, we used a facial nerve re-injury model to test the hypothesis that the reversal of neuronal atrophy would be impaired in recombinase activating gene-2 knockout (RAG-2 KO) mice, which lack functional T and B cells. Measures of neuronal survival were compared in the injured facial motor nucleus (FMN) of RAG-2 KO and wild-type (WT) mice that received a resection of the right facial nerve followed by re-injury of the same nerve 10 weeks later ("chronic resection+re-injury") or a resection of the right facial nerve followed by sham re-injury of the same nerve 10 weeks later ("chronic resection+sham"). We recently demonstrated that prior exposure to neuronal injury elicited a marked increase in T cell trafficking indicative of a T cell memory response when the contralateral FMN was injured later in adulthood. We examined if such a T cell memory response would also occur in the current re-injury model. RAG-2 KO mice showed no reversal of neuronal atrophy whereas WT mice showed a robust response. The reversal of atrophy in WT mice was not accompanied by a T cell memory response. Although the number of CD4(+) and CD8(+) T cells in the injured FMN did not differ from each other, double-negative T cells appear to be recruited in response to neuronal injury. Re-injury did not result in increased expression of MHC2 by microglia. Our findings suggest that T cells may be involved in reversing the axotomy-induced atrophy of injured neurons.

Development of neurons expressing estrogen receptor α transiently in facial nucleus of prenatal and postnatal rat brains

The transient expression of estrogen receptor alpha (ERalpha) in the facial nucleus of rats during development was already reported. However, how and whether the receptor functions physiologically in the nucleus of developing rats are as yet unclear. In this study, we applied a retrograde tracer into one of the possible target muscles of the motoneurons in the nucleus, that is, the transverse auricular muscle (Mta), and examined whether ERalpha-immunopositive neurons take up the tracer. Because it is probable that neurogenesis, apoptosis, and maturation may be associated with the transient expression of ERalpha, we attempted to analyze the neurons expressing the receptor in the nucleus. We found that ERalpha-immunopositive neurons in the medial facial subnucleus innervate mostly the Mta. Quantitative analyses showed that the number of motoneurons projecting to the Mta remained the same throughout the ages examined, whereas that of ERalpha-immunopositive neurons decreased between postnatal days 6 and 11. Apoptosis and neurogenesis in the nucleus were not affected by the expression of ERalpha during development. ERalpha expression coincided with the maturation of neurons in the nucleus. Thus, it is possible that ERalpha expression in the facial nucleus during development plays important roles in the development of motoneurons and/or external pinna muscles.

Temporal changes in the expression of some neurotrophins in spinal cord transected adult rats

Functional recovery of neurons in the spinal cord after physical injury is essentially abortive in clinical cases. As neurotrophins had been reported to be responsible, at least partially, for the lesion-induced recovery of spinal cord, it is not surprising that they have become the focus of numerous studies. Studies on endogenous neurotrophins, especially the three more important ones, nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) in injured spinal cord might provide some important clues in clinical treatment. Here we investigate the immunohistological expression of the above three factors at lower thoracic levels of the spinal cord as well as changes in the motor functions of the adult rat hindlimbs after cord transection. The injured rats were allowed to survive 3, 7, 14 and 21 days post operation (dpo). Flaccid paralysis was seen at 3 dpo following cord transection, however, hindlimb function showed partial recovery from 7 dpo to 21 dpo. The numbers of NGF, BDNF and NT-3 immunopositive neurons and their optical densities all increased in the lesion-induced cord. The immuno-expression of NGF and BDNF peaked at 7 dpo, while that of NT-3 peaked at 7 dpo and remained so at least up to 14 dpo. These results suggested that neurotrophins might play essential roles in functional recovery of after spinal cord injury, but the time points for the expression of the three factors differed somewhat.

Localization of TrkC to Schwann cells and effects of neurotrophin-3 signaling at neuromuscular synapses

Neurotrophins and their receptors, the Trks, are differentially expressed among the cell types that make up neuromuscular and other synapses, but the function and directionality of neurotrophin signaling at synapses are poorly understood. Here we demonstrate, via immunostaining, Western blotting, and RT-PCR analyses, that TrkC, the receptor for neurotrophin-3 (NT3), is expressed by mouse perisynaptic and myelinating Schwann cells from birth through adulthood and is unaltered after denervation. Analyses of transgenic mice in which the NT3 coding sequence is replaced by lacZ showed that NT3 is expressed in motor neurons and Schwann cells during perinatal development, but not in adult mice. In muscle, NT3 is expressed by intrafusal muscle fibers within spindles, as has been previously reported. Surprisingly, NT3 is also expressed in extrafusal muscle fibers during perinatal life and in adults. Genetic approaches were used to explore the roles of NT3 and TrkC signaling at neuromuscular synapses. Overexpression of NT3 in muscle fibers during development resulted in an increased number of perisynaptic Schwann cells at neuromuscular synapses, without altering synaptic size, suggesting that muscle-derived NT3 might act as a mitogen or trophic factor for Schwann cells. Conditional deletion of NT3 from motor neurons did not alter the number of Schwann cells or other aspects of neuromuscular synaptic structure, suggesting that motor-neuron-derived NT3 is not required for normal development of perisynaptic Schwann cells or synapses. Together, these results demonstrate that NT3 expression is developmentally regulated in skeletal muscle and may modulate the number of Schwann cells at neuromuscular synapses.

Neurotrophic factors in neurodegeneration

Neurotrophic factors (NTFs) have the unique potential to support neuronal survival and to augment neuronal function in the injured and diseased nervous system. Numerous studies conducted over the last 20 years have provided evidence for the potent therapeutic potential of NTFs in animal models of neurodegenerative diseases. However, major obstacles for the therapeutic use of NTFs are the inability to deliver proteins across the blood-brain-barrier, and dose-limiting adverse effects resulting from the broad exposure of nontargeted structures to NTFs. Two recent developments have allowed NTFs' promise to be truly tested for the first time: first, recent improvements in viral vectors that allow the targeted delivery of NTFs while providing a long-lasting supply and sufficient therapeutic doses of NTFs; and second, improved animal models developed in recent years. In this review, we will discuss some of the potential therapeutic applications of NTFs in neurodegenerative diseases and the potential contribution of disturbed neurotrophic factor signaling to neurodegenerative diseases.

Astrocyte and muscle-derived secreted factors differentially regulate motoneuron survival

During development, motoneurons (MNs) undergo a highly stereotyped, temporally and spatially defined period of programmed cell death (PCD), the result of which is the loss of 40-50% of the original neuronal population. Those MNs that survive are thought to reflect the successful acquisition of limiting amounts of trophic factors from the target. In contrast, maturation of MNs limits the need for target-derived trophic factors, because axotomy of these neurons in adulthood results in minimal neuronal loss. It is unclear whether MNs lose their need for trophic factors altogether or whether, instead, they come to rely on other cell types for nourishment. Astrocytes are known to supply trophic factors to a variety of neuronal populations and thus may nourish MNs in the absence of target-derived factors. We investigated the survival-promoting activities of muscle- and astrocyte-derived secreted factors and found that astrocyte-conditioned media (ACM) was able to save substantially more motoneurons in vitro than muscle-conditioned media (MCM). Our results indicate that both ACM and MCM are significant sources of MN trophic support in vitro and in ovo, but only ACM can rescue MNs after unilateral limb bud removal. Furthermore, we provide evidence suggesting that MCM facilitates the death of a subpopulation of MNs in a p75(NTR) - and caspase-dependent manner; however, maturation in ACM results in MN trophic independence and reduced vulnerability to this negative, pro-apoptotic influence from the target.

Modulation of p75-dependent motor neuron death by a small non-peptidyl mimetic of the neurotrophin loop 1 domain

The p75 neurotrophin receptor (p75NTR) is expressed by degenerating spinal motor neurons in amyotrophic lateral sclerosis (ALS). The mature and pro-form of nerve growth factor (NGF) activate p75NTR to trigger motor neuron apoptosis. However, attempts to modulate p75NTR-mediated neuronal death in ALS models by downregulating or antagonizing p75NTR with synthetic peptides have led to only modest results. Recently, a novel ligand of p75NTR, compound LM11A-24, has been identified. It is a non-peptidyl mimetic of the neurotrophin loop 1 domain that promotes hippocampal neuron survival through p75NTR and exerts protection against p75NTR-mediated apoptosis of oligodendrocytes induced by proNGF. Thus, LM11A-24 appears to activate p75NTR-linked survival but not death mechanisms, and may interfere with the ability of neurotrophins to induce apoptosis. Given these findings, we hypothesized that LM11A-24 might be a particularly potent inhibitor of motor neuron degeneration. We examined the effects of LM11A-24 on apoptosis of cultured rat embryonic motor neurons. Interestingly, in contrast to the effects observed in hippocampal cultures, LM11A-24 was unable to prevent motor neuron apoptosis induced by trophic factor deprivation. However, picomolar concentrations of LM11A-24 prevented p75NTR-dependent motor neuron death induced by either exogenous addition of NGF or spinal cord extracts from symptomatic superoxide dismutase-1G93A mice, in the presence of low steady-state concentrations of nitric oxide. LM11A-24 also inhibited motor neuron death induced by NGF-producing reactive astrocytes in co-culture conditions. These studies suggest that modulation of p75NTR by small molecule ligands targeting this receptor might constitute a novel strategy for preventing motor neuron degeneration.

Rab5 and Rab7 control endocytic sorting along the axonal retrograde transport pathway

Vesicular pathways coupling the neuromuscular junction with the motor neuron soma are essential for neuronal function and survival. To characterize the organelles responsible for this long-distance crosstalk, we developed a purification strategy based on a fragment of tetanus neurotoxin (TeNT H(C)) conjugated to paramagnetic beads. This approach enabled us to identify, among other factors, the small GTPase Rab7 as a functional marker of a specific pool of axonal retrograde carriers, which transport neurotrophins and their receptors. Furthermore, Rab5 is essential for an early step in TeNT H(C) sorting but is absent from axonally transported vesicles. Our data demonstrate that TeNT H(C) uses a retrograde transport pathway shared with p75(NTR), TrkB, and BDNF, which is strictly dependent on the activities of both Rab5 and Rab7. Therefore, Rab7 plays an essential role in axonal retrograde transport by controlling a vesicular compartment implicated in neurotrophin traffic.

Brain-derived neurotrophic factor facilitates in vivo internalization of tetanus neurotoxin C-terminal fragment fusion proteins in mature mouse motor nerve terminals

In a previous study it was reported that fusion proteins composed of the atoxic C-terminal fragment of tetanus toxin (TTC) and green fluorescent protein or beta-galactosidase (GFP-TTC and beta-gal-TTC, respectively) rapidly cluster at motor nerve terminals of the mouse neuromuscular junction (NMJ). Because this traffic involves presynaptic activity, probably via the secretion of active molecules, we examined whether it is affected by brain-derived neurotrophic factor (BDNF). Quantitative confocal microscopy and a fluorimetric assay for beta-gal activity revealed that co-injecting BDNF and the fusion proteins significantly increased the kinetics and amount of the proteins' localization at the NMJ and their internalization by motor nerve terminals. The observed increases were independent of synaptic vesicle recycling because BDNF did not affect spontaneous quantal acetylcholine release. In addition, injecting anti-BDNF antibody shortly before injecting GFP-TTC, and before co-injecting GFP-TTC and BDNF, significantly reduced the fusion protein's localization at the NMJ. Co-injecting GFP-TTC with neurotrophin-4 (NT-4) or glial-derived neurotrophic factor (GDNF), but not with nerve growth factor, neurotrophin-3 or ciliary neurotrophic factor, also significantly increased the fusion protein's localization at the NMJ. Thus, TTC probes may use for their neuronal internalization endocytic pathways normally stimulated by BDNF, NT-4 and GDNF binding. Different tyrosine kinase receptors with similar signalling pathways are activated by BDNF/NT-4 and GDNF binding. Thus, activated components of these signalling pathways may be involved in the TTC probes' internalization, perhaps by facilitating localization of receptors of TTC in specific membrane microdomains or by recruiting various factors needed for internalization of TTC.

Controlled release of nerve growth factor from fibrin gel

Nerve growth factor (NGF) is known to promote the axonal regeneration in injured nerve system. Delivery of NGF for a long period in a controlled manner may enhance the regeneration efficacy. In this study, we investigated whether NGF can be released from fibrin gel for a long period in a controlled manner. We also investigated whether sustained delivery of NGF using fibrin gel can enhance the efficacy of NGF in vitro. The addition of heparin to fibrin gel decreased the rate of NGF release from the fibrin gel. As the concentrations of thrombin and fibrinogen in fibrin gel increased, the NGF release rate decreased significantly, and the initial release burst decreased. NGF was released for up to 14 days in vitro. The bioactivity of NGF released from fibrin gel was assessed by morphological changes of pheochromocytoma (PC12) cells cultured in the presence of NGF-containing fibrin gel. NGF released from fibrin gel exhibited significantly higher degrees of PC12 cell viability and differentiation than NGF added in a free form daily into the culture medium. This study demonstrates that fibrin gel can release NGF in a sustained, controlled manner and in a bioactive form.

Neurotrophic factors and amyotrophic lateral sclerosis

The cause of motor neuron death in amyotrophic lateral sclerosis (ALS) remains a mystery. Initial implications of neurotrophic factor impairment involved in disease progression causing selective motor neuron death were brought forward in the late 1980s. These implications were based on several in vitro studies of motor neuron cultures in which a near to complete rescue of axotomized neonatal motor neurons in the presence of supplementary neurotrophic factors were revealed. These findings pawed the way for extensive investigations in experimental animal models of ALS. Neurotrophic factor administration in rodent ALS models demonstrated a remarkable effect on survival of degenerating motor neurons and rescue of axotomized motor neurons, both in vivo and in vitro. In the absence of efficient therapy for ALS, some of these promising neurotrophic factors have been administered to groups of ALS patients, as they appeared available for clinical trials. Up to date, none of tested factors has lived up to expectations, altering the outcome of the disease. This review summarizes current findings on neurotrophic factor expression in ALS tissue and these factors' potential/debatable clinical relevance to ALS and the treatment of ALS. It also discusses possible interventions improving clinical trial design to obtain efficacy of neurotrophic factor treatment in patients suffering from ALS.

Immune-mediated neuroprotection of axotomized mouse facial motoneurons is dependent on the IL-4/STAT6 signaling pathway in CD4+ T cells

The CD4(+) T lymphocyte has recently been found to promote facial motoneuron (FMN) survival after nerve injury. Signal Transducer and Activator of Transcription (STAT)4 and STAT6 are key proteins involved in the CD4(+) T cell differentiation pathways leading to T helper type (Th)1 and Th2 cell development, respectively. To determine which CD4(+) T cell subset mediates FMN survival, the facial nerve axotomy paradigm was applied to STAT4-deficient (-/-) and STAT6-/- mice. A significant decrease in FMN survival 4 weeks after axotomy was observed in STAT6-/- mice compared to wild-type (WT) or STAT4-/- mice. Reconstituting STAT6-/- mice with CD4(+) T cells obtained from WT mice promoted WT levels of FMN survival after injury. Furthermore, rescue of FMN from axotomy-induced cell death in recombination activating gene (RAG)-2-/- mice (lacking T and B cells) could be achieved only by reconstitution with CD4(+) T cells expressing functional STAT6 protein. To determine if either the Th1 cytokine, interferon-gamma (IFN-gamma) or the Th2 cytokine IL-4 is involved in mediating FMN survival, facial nerve axotomy was applied to IFN-gamma-/- and IL-4-/- mice. A significant decrease in FMN survival after axotomy occurred in IL-4-/- but not in IFN-gamma-/- mice compared to WT mice, indicating that IL-4 but not IFN-gamma is important for FMN survival after nerve injury. In WT mice, intracellular IFN-gamma vs. IL-4 expression was examined in CD4(+) T cells from draining cervical lymph nodes 14 days after axotomy, and substantial increase in the production of both CD4(+) effector T cell subsets was found. Collectively, these data suggest that STAT6-mediated CD4(+) T cell differentiation into the Th2 subset is necessary for FMN survival. A hypothesis relevant to motoneuron disease progression is presented.

Neuregulins: Versatile growth and differentiation factors in nervous system development and human disease

The neuregulins are a family of growth and differentiation factors with a wide range of functions in the nervous system. The power and diversity of the neuregulin signaling system comes in part from a large number of alternatively-spliced forms of the NRG1 gene that can produce both soluble and membrane-bound forms. The soluble forms of neuregulin are unique from other factors in that they have a structurally distinct heparin-binding domain that targets and potentiates its actions. In addition, a finely tuned, bidirectional mechanism regulates when and where neuregulin is released from neurons in response to neurotrophic factors produced by both neuronal targets and supporting glial cells. Together, this produces a balanced intercellular signaling system that can be localized to distinct regions for both normal development and maintenance of the mature nervous system. Recent evidence suggests that neuregulin signaling plays important roles in many neurological disorders including multiple sclerosis, traumatic brain and spinal cord injury, peripheral neuropathy, and schizophrenia. Here, we review the basic biology of neuregulins and relate this to research suggesting their involvement with and potential therapeutic uses for neurological disorders.

BDNF is expressed in skeletal muscle satellite cells and inhibits myogenic differentiation

In skeletal muscle, brain-derived neurotrophic factor (BDNF) has long been thought to serve as a retrograde trophic factor for innervating motor neurons throughout their lifespan. However, its localization in mature muscle fibers has remained elusive. Given the postulated roles of BDNF in skeletal muscle, we performed a series of complementary experiments aimed at defining the localization of BDNF and its transcripts in adult muscle. By reverse transcription-PCR, in situ hybridization, and immunofluorescence, we show that BDNF, along with the receptor p75NTR, is not expressed at significant levels within mature myofibers and that it does not accumulate preferentially within subsynaptic regions of neuromuscular junctions. Interestingly, expression of BDNF correlated with that of Pax3, a marker of muscle progenitor cells, in several different adult skeletal muscles. Additionally, BDNF was expressed in Pax7+ satellite cells where it colocalized with p75NTR. In complementary cell culture experiments, we detected high levels of BDNF and p75NTR in myoblasts. During myogenic differentiation, expression of BDNF became drastically reduced. Using small interfering RNA (siRNA) technology to knock down BDNF expression, we demonstrate enhanced myogenic differentiation of myoblasts. This accelerated rate of myogenic differentiation seen in myoblasts expressing BDNF siRNA was normalized by administration of recombinant BDNF. Collectively, these findings show that BDNF plays an important regulatory function during myogenic differentiation. In addition, the expression of BDNF in satellite cells is coherent with the notion that BDNF serves a key role in maintaining the population of muscle progenitors in adult muscle.

Long-term adeno-associated viral vector-mediated expression of truncated TrkB in the adult rat facial nucleus results in motor neuron degeneration

Adult facial motor neurons continue to express full-length TrkB tyrosine kinase receptor (TrkB.FL), the high-affinity receptor for the neurotrophins BDNF and neurotrophic factor-4/5 (NT-4/5), suggesting that they remain dependent on target-derived and locally produced neurotrophins in adulthood. Studies on the role of TrkB signaling in the adult CNS have been hampered by the early lethality of bdnf, nt-4/5, and trkB knock-out mice. We disrupted TrkB.FL signaling in adult facial motor neurons using adeno-associated viral vector-mediated overexpression of a naturally occurring dominant-negative TrkB receptor, TrkB.T1. Expression of TrkB.T1 resulted in neuronal atrophy and downregulation of NeuN (neuronal-specific nuclear protein) and ChAT expression in facial motor neurons. A subset of transduced neurons displayed signs of motor neuron degeneration that included dendritic beading and rounding of the soma at 2 months of TrkB.T1 expression. Cell counts revealed a significant reduction in motor neuron number in the facial nucleus at 4 months after onset of expression of TrkB.T1, suggesting that a proportion of TrkB.T1-expressing motor neurons became undetectable as a result of severe atrophy or was lost because of cell death. In contrast, overexpression of TrkB.FL did not result in a decrease in facial motor neuron number. Our results indicate that a subset of facial motor neurons remains dependent on TrkB ligands for the maintenance of structural and molecular characteristics in adulthood.

Relevance of motoneuron specification and programmed cell death in embryos to therapy of ALS

The molecular cues that generate spinal motoneurons in early embryonic development are well defined. Motoneurons are generated in excess and consequently undergo a natural period of programmed cell death. Although it is not known exactly how motoneurons compete for survival in embryonic development, it is hypothesized that they rely on the ability to access limited amounts of trophic factors from peripheral tissues, a process that is tightly regulated by skeletal muscle activity. Attempts to elucidate the molecular mechanisms that underlie motoneuron generation and programmed cell death in embryos have led to various effective strategies for treating injury and disease in animal models. Such studies provide great hope for the amelioration of human amyotrophic lateral sclerosis (ALS), a devastating progressive motoneuron degenerative disease. Here we review the clinical relevance of studying motoneuron specification and death during embryonic development.