Synaptic targeting of retrogradely transported trophic factors in motoneurons: comparison of glial cell line-derived neurotrophic factor, brain-derived neurotrophic factor, and cardiotrophin-1 with tetanus toxin

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.

Complementary actions of BDNF and neurotrophin-3 on the firing patterns and synaptic composition of motoneurons

Neurotrophins, as target-derived factors, are essential for neuronal survival during development, but during adulthood, their scope of actions widens to become also mediators of synaptic and morphological plasticity. Target disconnection by axotomy produces an initial synaptic stripping ensued by synaptic rearrangement upon target reinnervation. Using abducens motoneurons of the oculomotor system as a model for axotomy, we report that trophic support by brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3) or a mixture of both, delivered to the stump of severed axons, results in either the prevention of synaptic stripping when administered immediately after lesion or in a promotion of reinnervation of afferents to abducens motoneurons once synaptic stripping had occurred, in concert with the recovery of synaptic potentials evoked from the vestibular nerve. Synaptotrophic effects, however, were larger when both neurotrophins were applied together. The axotomy-induced reduction in firing sensitivities related to eye movements were also restored to normal values when BDNF and NT-3 were administered, but discharge characteristics recovered in a complementary manner when only one neurotrophin was used. This is the first report to show selective retrograde trophic dependence of circuit-driven firing properties in vivo indicating that NT-3 restored the phasic firing, whereas BDNF supported the tonic firing of motoneurons during eye movement performance. Therefore, our data report a link between the synaptotrophic actions of neurotrophins, retrogradely delivered, and the alterations of neuronal firing patterns during motor behaviors. These trophic actions could be responsible, in part, for synaptic rearrangements that alter circuit stability and synaptic balance during plastic events of the brain.

A reappraisal of the central effects of botulinum neurotoxin type A: by what mechanism?

Botulinum neurotoxin A (BoNT/A) is a metalloprotease that enters peripheral motor nerve terminals and blocks the release of acetylcholine via the specific cleavage of the synaptosomal-associated protein of 25-kDa. Localized injections of BoNT/A are widely employed in clinical neurology to treat several human diseases characterized by muscle hyperactivity. It is generally assumed that the effects of BoNT/A remain localized to the injection site. However, several neurophysiological studies have provided evidence for central effects of BoNT/A, raising the issue of how these actions arise. Here we review these data and discuss the possibility that retrograde axonal transport of catalytically active BoNT/A may explain at least some of its effects at the level of central circuits.

Toll-like receptors in peripheral nerve injury and neuropathic pain

Peripheral nerve injury triggers a series of responses in the injured nerve, such as the dissolution of distal axons, the activation of Schwann cells, the production of various proinflammatory mediators, and the infiltration of circulating immune cells. These orchestrated events regulate the degeneration and subsequent regeneration of the injured nerve. In addition, peripheral nerve injury often accompanies chronic pain. Studies in this field have revealed that spinal cord microglia activation plays a critical role in the development of pain hypersensitivity. Recent studies using genetically modified mice indicate that Toll-like receptors (TLRs) are involved in nerve degeneration (Wallerian degeneration) and chronic pain (neuropathic pain) development after nerve injury. Here, we review studies that have implicated TLRs in mediating nerve degeneration/regeneration and neuropathic pain following nerve injury. In addition, we discuss possible mechanisms underlying the roles of TLRs in these neurological disorders.

A 29-amino acid fragment of Clostridium botulinum C3 protein enhances neuronal outgrowth, connectivity, and reinnervation

Small GTPases of the Rho family play versatile roles in the formation and development of axons and dendrites, effects often studied by the Rho-inactivating C3 transferase (C3bot) from Clostridium botulinum. Recently, we reported that transferase-deficient C3bot also exerted axonotrophic activity. Using overlapping peptides from the C3bot sequence, we identified a small peptide of 29 amino acids (covering residues 154-182) from the C-terminal region of C3bot that promotes both axonal and dendritic growth, as well as branching of hippocampal neurons, at submicromolar concentrations. Several C3bot constructs, including the short peptide, enhanced the number of axonal segments from mid- to higher-order segments. C3bot(154-182) also increased the number of synaptophysin-expressing terminals, up-regulated various synaptic proteins, and functionally increased the glutamate uptake. Staining against the vesicular glutamate and GABA transporters further revealed that the effect was attributable to a higher number of glutamatergic and GABAergic inputs on proximal dendrites of enhanced green fluorescent protein (EGFP)-transfected neurons. Using organotypical slice cultures, we also detected trophic effects of C3bot(154-182) on length and density of outgrowing fibers from the entorhinal cortex that were comparable to the effects elicited by full-length C3bot. In addition, an enhanced reinnervation was observed in a hippocampal-entorhinal lesion model. In summary, the neurotrophic effect of C3bot is executed by a C-terminal peptide fragment covering aa 154-182 of C3; it triggers dendritic and axonal growth and branching as well as increased synaptic connectivity. In contrast to full-length C3, this C3 peptide selectively acts on neurons but not on glial cells.

GDNF isoform affects intracellular trafficking and secretion of GDNF in neuronal cells

Glial cell line derived neurotrophic factor (GDNF) plays a critical role in central and peripheral neuron survival and function. In human and rodents, GDNF exists in an alternative spliced isoform (GDNF Delta 78), which has a 78 bp deletion in the pro-region of the GDNF encoding sequence. Whether the GDNF isoform affects GDNF function is unknown. Here, we investigated the secretion and intracellular localization of the GDNF Delta 78 isoform in neuronal cell populations. Our data indicate that a decreased secretion and an abnormal intracellular distribution of GDNF Delta 78 occurred in neuronal cells. The colocalization studies revealed much more localization of GDNF Delta 78 with Golgi marker-TGN38, which indicates that the accumulation of GDNF Delta 78 in the Golgi apparatus might in part account for its intracellular trafficking and secretion deficit. To our knowledge, it is reported for the first time that the GDNF Delta 78 isoform has a deficit in GDNF intracellular trafficking and secretion.

Long-distance retrograde effects of botulinum neurotoxin A

Botulinum neurotoxins (designated BoNT/A-BoNT/G) are bacterial enzymes that block neurotransmitter release by cleaving essential components of the vesicle fusion machinery. BoNT/A, which cleaves SNAP-25 (synaptosomal-associated protein of 25 kDa), is extensively exploited in clinical medicine to treat neuromuscular pathologies, facial wrinkles, and various types of pain. It is widely assumed that BoNT/A remains at the synaptic terminal and its effects are confined to the injection site. Here we demonstrate that catalytically active BoNT/A is retrogradely transported by central neurons and motoneurons and is then transcytosed to afferent synapses, in which it cleaves SNAP-25. SNAP-25 cleavage by BoNT/A was observed in the contralateral hemisphere after unilateral BoNT/A delivery to the hippocampus. Appearance of cleaved SNAP-25 resulted in blockade of hippocampal activity in the untreated hemisphere. Injections of BoNT/A into the optic tectum led to the appearance of BoNT/A-truncated SNAP-25 in synaptic terminals within the retina. Cleaved SNAP-25 also appeared in the facial nucleus after injection of the toxin into rat whisker muscles. Experiments excluded passive spread of the toxin and demonstrated axonal migration and neuronal transcytosis of BoNT/A. These findings reveal a novel pathway of BoNT/A trafficking in neurons and have important implications for the clinical uses of this neurotoxin.

Survival and regeneration of cutaneous and muscular afferent neurons after peripheral nerve injury in adult rats

Peripheral nerve injury induces the retrograde degeneration of dorsal root ganglion (DRG) cells, which affects predominantly the small-diameter cutaneous afferent neurons. This study compares the time-course of retrograde cell death in cutaneous and muscular DRG cells after peripheral nerve transection as well as neuronal survival and axonal regeneration after primary repair or nerve grafting. For comparison, spinal motoneurons were also included in the study. Sural and medial gastrocnemius DRG neurons were retrogradely labeled with the fluorescent tracers Fast Blue (FB) or Fluoro-Gold (FG) from the homonymous transected nerves. Survival of labeled sural and gastrocnemius DRG cells was assessed at 3 days and 1-24 weeks after axotomy. To evaluate axonal regeneration, the sciatic nerve was transected proximally at 1 week after FB-labeling of the sural and medial gastrocnemius nerves and immediately reconstructed using primary repair or autologous nerve grafting. Twelve weeks later, the fluorescent tracer Fluoro-Ruby (FR) was applied 10 mm distal to the sciatic lesion in order to double-label sural and gastrocnemius neurons that had regenerated across the repair site. Counts of labeled gastrocnemius DRG neurons did not reveal any significant retrograde cell death after nerve transection. In contrast, sural axotomy induced a delayed loss of sural DRG cells, which amounted to 22% at 4 weeks and 43-48% at 8-24 weeks postoperatively. Proximal transection of the sciatic nerve at 1 week after injury to the sural or gastrocnemius nerves neither further increased retrograde DRG degeneration, nor did it affect survival of sural or gastrocnemius motoneurons. Primary repair or peripheral nerve grafting supported regeneration of 53-60% of the spinal motoneurons and 47-49% of the muscular DRG neurons at 13 weeks postoperatively. In the cutaneous DRG neurons, primary repair or peripheral nerve grafting increased survival by 19-30% and promoted regeneration of 46-66% of the cells. The present results suggest that cutaneous DRG neurons are more sensitive to peripheral nerve injury than muscular DRG cells, but that their regenerative capacity does not differ from that of the latter cells. However, the retrograde loss of cutaneous DRG cells taking place despite immediate nerve repair would still limit the recovery of cutaneous sensory functions.

Nonviral approaches for neuronal delivery of nucleic acids

The delivery of therapeutic nucleic acids to neurons has the potential to treat neurological disease and spinal cord injury. While select viral vectors have shown promise as gene carriers to neurons, their potential as therapeutic agents is limited by their toxicity and immunogenicity, their broad tropism, and the cost of large-scale formulation. Nonviral vectors are an attractive alternative in that they offer improved safety profiles compared to viruses, are less expensive to produce, and can be targeted to specific neuronal subpopulations. However, most nonviral vectors suffer from significantly lower transfection efficiencies than neurotropic viruses, severely limiting their utility in neuron-targeted delivery applications. To realize the potential of nonviral delivery technology in neurons, vectors must be designed to overcome a series of extra- and intracellular barriers. In this article, we describe the challenges preventing successful nonviral delivery of nucleic acids to neurons and review strategies aimed at overcoming these challenges.

Endosomal transport of neurotrophins: roles in signaling and neurodegenerative diseases

The internalization and retrograde axonal transport of neurotrophin receptors is important for their retrograde signal transduction supporting neuronal differentiation, plasticity, and survival. To influence transcription, neurotrophin signals initiated at synapses have to be conveyed retrogradely to the cell body. Signaling endosomes containing neurotrophin receptor signaling complexes mediate retrograde neurotrophin signaling from synapses to the nucleus. Interestingly, many neurodegenerative diseases, including Alzheimer's disease, Niemann Pick disease Type C, and Charcot-Marie-Tooth neuropathies, show alterations of vesicular transport, suggesting that traffic jams within neuronal processes may cause neurodegeneration. Although most of these diseases are complex and may be modulated by diverse pathways contributing to neuronal death, altered neurotrophin transport is emerging as a strong candidate influence on neurodegeneration. In this article, we review the mechanisms of internalization and endocytic trafficking of neurotrophin receptors, and discuss the potential roles of perturbations in neurotrophin trafficking in a number of neurodegenerative diseases.

Neurturin enhances the recovery of erectile function following bilateral cavernous nerve crush injury in the rat

Background

The molecular mechanisms responsible for the survival and preservation of function for adult parasympathetic ganglion neurons following injury remain incompletely understood. However, advances in the neurobiology of growth factors, neural development, and prevention of cell death have led to a surge of clinical interest for protective and regenerative neuromodulatory strategies, as surgical therapies for prostate, bladder, and colorectal cancers often result in neuronal axotomy and debilitating loss of sexual function or continence. In vitro studies have identified neurturin, a glial cell line-derived neurotrophic factor, as a neuromodulator for pelvic cholinergic neurons. We present the first in vivo report of the effects of neurturin upon the recovery of erectile function following bilateral cavernous nerve crush injury in the rat.

Methods

In these experiments, groups (n = 8 each) consisted of uninjured controls and animals treated with injection of albumin (blinded crush control group), extended release neurotrophin-4 or neurturin to the site of cavernous nerve crush injury (100 μg per animal). After 5 weeks, recovery of erectile function (treatment effect) was assessed by cavernous nerve electrostimulation and peak aortic pressures were measured. Investigators were unblinded to specific treatments after statistical analyses were completed.

Results

Erectile dysfunction was not observed in the sham group (mean maximal intracavernous pressure [ICP] increase of 117.5 ± 7.3 cmH₂O), whereas nerve injury and albumin treatment (control) produced a significant reduction in ICP elevation of 40.0 ± 6.3 cmH₂O. Neurturin facilitated the preservation of erectile function, with an ICP increase of 55% at 62.0 ± 9.2 cmH₂O (p < 0.05 vs control). Extended release neurotrophin-4 did not significantly enhance recovery of erectile function with an ICP change of 46.9 ± 9.6. Peak aortic blood pressures did not differ between groups. No significant pre- and post-treatment weight differences were observed between control, neurotrophin-4 and neurturin cohorts. All animals tolerated the five-week treatment course.

Conclusion

Treatment with neurturin at the site of cavernous nerve crush injury facilitates recovery of erectile function. Results support further investigation of neurturin as a neuroprotective and/or neuroregenerative agent facilitating functional recovery after cavernous or other pelvic autonomic nerve injuries.

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.

Human neural stem cell grafts ameliorate motor neuron disease in SOD-1 transgenic rats

BACKGROUND

Experimental therapeutics for degenerative and traumatic diseases of the nervous system have been recently enriched with the addition of neural stem cells (NSCs) as alternatives to fetal tissues for cell replacement. Neurodegenerative diseases present the additional problem that cell death signals may interfere with the viability of grafted cells. The adult spinal cord raises further challenges for NSC differentiation because of lack of intrinsic developmental potential and the negative outcomes of several prior attempts.

METHOD

NSCs from human fetal spinal cord were grafted into the lumbar cord of SOD1 G93A rats. The differentiation fate of grafted NSCs and their effects on motor neuron number, locomotor performance, disease onset, and survival trends/longevity were assessed. Trophic mechanisms of observed clinical effects were explored with molecular and cellular methodologies.

RESULT

Human NSCs showed extensive differentiation into neurons that formed synaptic contacts with host nerve cells and expressed and released glial cell line-derived neurotrophic factor and brain-derived neurotrophic factor. NSC grafts delayed the onset and progression of the fulminant motor neuron disease typical of the rat SOD1 G93A model and extended the lifespan of these animals by more than 10 days, despite the restricted grafting schedule that was limited to the lumbar protuberance.

CONCLUSION

NSC grafts can survive well in a neurodegenerative environment and exert powerful clinical effects; at least a portion of these effects may be related to the ability of these grafts to express and release motor neuron growth factors delivered to host motor neurons via graft-host connections.

A glial cell line-derived neurotrophic factor (GDNF):tetanus toxin fragment C protein conjugate improves delivery of GDNF to spinal cord motor neurons in mice

Glial cell line-derived neurotrophic factor (GDNF) has shown robust neuroprotective and neuroreparative activities in various animal models of Parkinson's Disease or amyotrophic lateral sclerosis (ALS). The successful use of GDNF as a therapeutic in humans, however, appears to have been hindered by its poor bioavailability to target neurons in the central nervous system (CNS). To improve delivery of exogenous GDNF protein to CNS motor neurons, we employed chemical conjugation techniques to link recombinant human GDNF to the neuronal binding fragment of tetanus toxin (tetanus toxin fragment C, or TTC). The predominant species present in the purified conjugate sample, GDNF:TTC, had a molecular weight of approximately 80 kDa as determined by non-reducing SDS-PAGE. Like GDNF, addition of GDNF:TTC to culture media of neuroblastoma cells expressing GFRalpha-1/c-RET produced a dose-dependent increase in cellular phospho-c-RET levels. Treatment of cultured midbrain dopaminergic neurons with either GDNF or the conjugate similarly promoted both DA neuron survival and neurite outgrowth. However, in contrast to mice treated with GDNF by intramuscular injection, mice receiving GDNF:TTC revealed intense GDNF immunostaining associated with spinal cord motor neurons in fixed tissue sections. That GDNF:TTC provided neuroprotection of axotomized motor neurons in neonatal rats further revealed that the conjugate retained its GDNF activity in vivo. These results indicate that TTC can serve as a non-viral vehicle to substantially improve the delivery of functionally active growth factors to motor neurons in the mammalian CNS.

Muscle-derived but not centrally derived transgene GDNF is neuroprotective in G93A-SOD1 mouse model of ALS

Glial cell line-derived neurotrophic factor (GDNF) is a potent survival factor for motoneurons (MNs), and is considered a potential agent for the treatment of amyotrophic lateral sclerosis (ALS) and other MN diseases. The effectiveness of GDNF may depend significantly upon its route of delivery to MNs. In this study we tested the neuroprotective effects of target-derived and centrally derived GDNF in the G93A-SOD1 mouse model of ALS using a transgenic approach. We found that overexpression of GDNF in the skeletal muscle (Myo-GDNF mice) significantly delayed the onset of disease and increased the life span of G93A-SOD1 mice by 17 days. The duration of disease also increased by 8.5 days, indicating that GDNF slowed down the progression of disease. Locomotor performance in Myo-GDNF/G93A-SOD1 mice was also significantly improved. The behavioral improvement correlated well with anatomical and histological data. We demonstrated that muscle-derived GDNF resulted in increased survival of spinal MNs, and twice as many MNs survived in end-stage double transgenic mice compared to end-stage G93A-SOD1 mice. Muscle-derived GDNF also had profound effects on muscle innervation and axonal degeneration. Significantly higher numbers of completely or partially innervated NMJs and large caliber myelinated axons were found in double transgenic mice. In contrast, we demonstrated that overexpression of GDNF in astrocytes in the CNS (GFAP-GDNF mice) failed to demonstrate any neuroprotective effects in G93A-SOD1 mice both on behavioral and histological levels. These data indicate that retrograde transport and signaling of GDNF is more physiological and effective for ALS treatment than anterogradely transported GDNF.

Altered balance of glutamatergic/GABAergic synaptic input and associated changes in dendrite morphology after BDNF expression in BDNF-deficient hippocampal neurons

Cultured neurons from bdnf-/- mice display reduced densities of synaptic terminals, although in vivo these deficits are small or absent. Here we aimed at clarifying the local responses to postsynaptic brain-derived neurotrophic factor (BDNF). To this end, solitary enhanced green fluorescent protein (EGFP)-labeled hippocampal neurons from bdnf-/- mice were compared with bdnf-/- neurons after transfection with BDNF, bdnf-/- neurons after transient exposure to exogenous BDNF, and bdnf+/+ neurons in wild-type cultures. Synapse development was evaluated on the basis of presynaptic immunofluorescence and whole-cell patch-clamp recording of miniature postsynaptic currents. It was found that neurons expressing BDNF::EGFP for at least 16 h attracted a larger number of synaptic terminals than BDNF-deficient control neurons. Transfected BDNF formed clusters in the vicinity of glutamatergic terminals and produced a stronger upregulation of synaptic terminal numbers than high levels of ambient BDNF. Glutamatergic and GABAergic synapses reacted differently to postsynaptic BDNF: glutamatergic input increased, whereas GABAergic input decreased. BDNF::EGFP-expressing neurons also differed from BDNF-deficient neurons in their dendrite morphology: they exhibited weaker dendrite elongation and stronger dendrite initiation. The upregulation of glutamatergic synaptic input and the BDNF-induced downregulation of GABAergic synaptic terminal numbers by postsynaptic BDNF depended on tyrosine receptor kinase B activity, as deduced from the blocking effects of K252a. The suppression of dendrite elongation was also prevented by block of tyrosine receptor kinase B but required, in addition, glutamate receptor activity. Dendritic length decreased with the number of glutamatergic contacts. These results illuminate the role of BDNF as a retrograde synaptic regulator of synapse development and the dependence of dendrite elongation on glutamatergic input.

Loss of leukemia inhibitory factor receptor beta or cardiotrophin-1 causes similar deficits in preganglionic sympathetic neurons and adrenal medulla

Leukemia inhibitory factor (LIF) receptor beta (LIFRbeta) is a receptor for a variety of neurotrophic cytokines, including LIF, ciliary neurotrophic factor (CNTF), and cardiotrophin-1 (CT-1). These cytokines play an essential role for the survival and maintenance of developing and postnatal somatic motoneurons. CNTF may also serve the maintenance of autonomic, preganglionic sympathetic neurons (PSNs) in the spinal cord, as suggested by its capacity to prevent their death after destruction of one of their major targets, the adrenal medulla. Although somatic motoneurons and PSNs share a common embryonic origin, they are distinct in several respects, including responses to lesions. We have studied PSNs in mice with targeted deletions of the LIFRbeta or CT-1 genes, respectively. We show that LIF, CNTF, and CT-1 are synthesized in embryonic adrenal gland and spinal cord and that PSNs express LIFRbeta. In embryonic day 18.5 LIFRbeta (-/-) and CT-1 (-/-) mice, PSNs were reduced by approximately 20%. PSNs projecting to the adrenal medulla were more severely affected (-55%). Although LIFRbeta (-/-) mice revealed normal numbers of adrenal chromaffin cells and axons terminating on chromaffin cells, levels of adrenaline and numbers of adrenaline-synthesizing cells were significantly reduced. We conclude that activation of LIFRbeta is required for normal development of PSNs and one of their prominent targets, the adrenal medulla. Thus, both somatic motoneurons and PSNs in the spinal cord depend on LIFRbeta signaling for their development and maintenance, although PSNs seem to be overall less affected than somatic motoneurons by LIFRbeta deprivation.

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.

Patterns of expression of brain-derived neurotrophic factor and tyrosine kinase B mRNAs and distribution and ultrastructural localization of their proteins in the visual pathway of the adult rat

We have examined the cellular and subcellular distribution and the patterns of expression of brain-derived neurotrophic factor (BDNF), and of its high affinity receptor, tyrosine kinase B (TrkB), in retinorecipient regions of the brain, including the superior colliculus, the lateral geniculate nucleus and the olivary pretectal nucleus. In the retinorecipient layers of the superior colliculus, BDNF protein and mRNA were present in the cell bodies of a subpopulation of neurons, and BDNF protein was present in the neuropil as punctate or fiber-like structures. In the lateral geniculate nucleus, however, BDNF mRNA was not detected, and BDNF protein was restricted to punctate and fiber-like structures in the neuropil, especially in the most superficial part of the dorsal lateral geniculate nucleus, just below the optic tract. At the ultrastructural level, BDNF protein was localized predominantly to axon terminals containing round synaptic vesicles and pale mitochondria with irregular cristae, which made asymmetric (Gray type I) synaptic specializations (R-boutons). Enucleation of one eye was followed by loss of BDNF immunoreactivity and disappearance of BDNF-positive R-boutons in the contralateral visual centers, confirming the retinal origin of at least most of these terminals. TrkB was present in postsynaptic densities apposed to immunoreactive R-boutons in the superior colliculus and lateral geniculate nucleus, and was also associated with axonal and dendritic microtubules. These findings suggest that BDNF is synthesized by a subpopulation of retinal ganglion cells and axonally transported to visual centers where this neurotrophin is assumed to play important roles in visual system maintenance and/or in modulating the excitatory retinal input to neurons in these centers.

C fragment of tetanus toxin hybrid proteins evaluated for muscle-specific transsynaptic mapping of spinal motor circuitry in the newborn mouse

We investigated whether the non-toxic C fragment of tetanus toxin (TTC) fused to either beta-galactosidase or green fluorescent protein could be utilized to transsynaptically trace muscle-specific spinal circuitry in the neonatal mouse after i.m. injection into a single hindlimb muscle. We found that even with careful low volume injection (0.2-1.0 microl) into a single muscle (medial gastrocnemius), the TTC hybrid proteins spread rapidly to many other hindlimb muscles and to trunk musculature such that retrograde labeling of motoneurons could not be constrained to a single motoneuron pool. Retrogradely labeled motoneurons in the lower lumbar segments harboring the medial gastrocnemius motoneuron pool were first observed two hours after the medial gastrocnemius injection. Within the next 10 h, additional lumbar and lower thoracic motoneurons became labeled, and punctate labeling in the neuropil surrounding the motoneurons appeared. Many of the TTC hybrid protein-labeled puncta in the neuropil co-localized synaptotagmin, indicating that they represent presynaptic axon terminals onto motoneurons. Although this is consistent with retrograde transsynaptic passage, we found no evidence that the TTC hybrid proteins were transported further along premotor axons to label interneuron somata. The i.m. TTC injection procedure described here therefore provides an important tool for the study of presynaptic terminals onto motoneurons. However, additional technical modifications will be required to utilize TTC tracers for transsynaptic mapping of muscle-specific spinal motor circuitry in the neonatal mouse. We provide here a set of criteria for assessing the i.m. delivery of TTC tracers as a basis for future improvements in this technique.

Anterograde axonal transport of the exogenous cellular isoform of prion protein in the chick visual system

The cellular isoform of endogenous, newly synthesized prion protein (PrPc) can be transported by axons in the anterograde direction. To determine whether a mechanism exists for secreted PrPc to be internalized and then axonally transported, we analyzed internalization and anterograde axonal transport of radiolabeled recombinant PrPc after its intraocular injection in chick embryos. Internalization and axonal transport of exogenous PrPc to the midbrain by retinal ganglion cells (RGCs) is efficient, saturable and likely receptor-mediated. Ultrastructural quantitative localization of radiolabeled PrPc within RGC soma showed significant labeling of vesicular/endosomal compartments and much less labeling present over the Golgi apparatus and lysosomes, which indicates slow degradation of exogenous PrPc in this system. These data show that a mechanism exists to internalize a secreted form of PrPc and then to axonally transport such PrPc in an anterograde direction. This may provide an additional, novel mechanism for prion protein to spread among neurons.

Synaptic targeting of retrogradely transported trophic factors in motoneurons: comparison of glial cell line-derived neurotrophic factor, brain-derived neurotrophic factor, and cardiotrophin-1 with tetanus toxin

Glial cell line-derived neurotrophic factor (GDNF), brain-derived neurotrophic factor (BDNF), and cardiotrophin-1 (CT-1) are the most potent neurotrophic factors for motoneurons, but their fate after retrograde axonal transport is not known. Internalized trophic factors may be degraded, or they may be recycled and transferred to other neurons, similar to the known route of tetanus toxin. We tested whether neonatal rat hypoglossal motoneurons target retrogradely transported trophic factors to synaptic sites on their dendrites within the brainstem and subsequently transfer these trophins across the synaptic cleft to afferent synapses (transsynaptic transcytosis). Motoneurons retrogradely transport from the tongue radiolabeled GDNF, BDNF, and CT-1 as well as tetanus toxin. Quantitative autoradiographic electron microscopy showed that GDNF and BDNF were transported into motoneuron dendrites with labeling densities similar to those of tetanus toxin. Although tetanus toxin accumulated rapidly (within 8 h) at presynaptic sites, GDNF accumulated at synapses more slowly (within 15 h), and CT-1 never associated with synapses. Thus, some retrogradely transported neurotrophic factors are trafficked similarly but not identically to tetanus toxin. Both GDNF and BDNF accumulate at the external (limiting) membrane of multivesicular bodies within proximal dendrites. We conclude that tetanus toxin, GDNF, and BDNF are released from postsynaptic sites and are internalized by afferent presynaptic terminals, thus demonstrating transsynaptic transcytosis. CT-1, however, follows a strict degradation pathway after retrograde transport to the soma. Synaptic and transcytotic trafficking thus are restricted to particular neurotrophic factors such as GDNF and BDNF.

Anterograde axonal transport of BDNF and NT-3 by retinal ganglion cells: roles of neurotrophin receptors

Retinal ganglion cells (RGCs) transport exogenous neurotrophins anterogradely to the midbrain tectum/superior colliculus with significant downstream effects. We determined contributions of neurotrophin receptors for anterograde transport of intraocularly injected radiolabeled neurotrophins. In adult rodents, anterograde transport of brain-derived neurotrophic factor (BDNF) was receptor-mediated, and transport of exogenous BDNF and neurotrophin-3 (NT-3) was more efficient, per RGC, in rodents than chicks. RT-PCR and Western blot analysis of purified murine RGCs showed that adult RGCs express the p75 receptor. Anterograde transport of BDNF or NT-3 was not diminished in p75 knock-out mice (with unaltered final numbers of RGCs), but BDNF transport was substantially reduced by co-injected trkB antibodies. In chick embryos, however, p75 antisense or co-injected p75 antibodies significantly attenuated anterograde transport of NT-3 by RGCs. Thus, neither BDNF nor NT-3 utilizes p75 for anterograde transport in adult rodent RGCs, while anterograde NT-3 transport requires the p75 receptor in embryonic chicken RGCs.

Transynaptic effects of tetanus neurotoxin in the oculomotor system

The question whether general tetanus arises from the independent sum of multiple local tetani or results from the actions of the transynaptic tetanus neurotoxin (TeNT) in higher brain centres remains unresolved. Despite the blood-borne dissemination of TeNT from an infected wound, the access to the central nervous system is probably prevented by the blood-brain barrier. However, several long-term sequelae (e.g. autonomic dysfunction, seizures, myoclonus, and sleep disturbances) present after the subsidence of muscle spasms might be indicative of central actions that occur farther away from lower motoneurons. Subsequently, the obvious entry route is the peripheral neurons followed by the transynaptic passage to the brain. We aimed at describing the pathophysiological correlates of TeNT translocation using the oculomotor system as a comprehensive model of cell connectivity and neuronal firing properties. In this study, we report that injection of TeNT into the medial rectus muscle of one eye resulted in bilateral gaze palsy attributed to firing alterations found in the contralaterally projecting abducens internuclear neurons. Functional alterations in the abducens-to-oculomotor internuclear pathway resembled in part the classically described TeNT disinhibition. We confirmed the transynaptic targeted action of TeNT by analysing vesicle-associated membrane protein2 (VAMP2) immunoreactivity (the SNARE protein cleaved by TeNT). VAMP2 immunoreactivity decreased by 94.4% in the oculomotor nucleus (the first synaptic relay) and by 62.1% presynaptic to abducens neurons (the second synaptic relay). These results are the first demonstration of physiological changes in chains of connected neurons that are best explained by the transynaptic action of TeNT on premotor neurons as shown with VAMP2 immunoreactivity which serves as an indicator of TeNT activity.