Expression and characterisation of the heavy chain of tetanus toxin: reconstitution of the fully-recombinant dichain protein in active form.

Tetanus toxin, composed of a disulphide-linked heavy (HC) and light (LC) chain, preferentially blocks the release of inhibitory neurotransmitters in the spinal cord by Zn2+-dependent proteolytic cleavage of synaptobrevin. This intoxication involves binding via HC to ecto-acceptors on peripheral nerve endings, followed by internalisation and retrograde transportation to its prime site of action in central neurons. To facilitate exploitation of the toxin's unique activities, HC was expressed at a high level in Escherichia coli as a fusion with maltose binding protein; after cleavage by thrombin, free HC was isolated and its identity confirmed by Western blotting and N-terminal microsequencing. The expressed and native HC gave very similar circular dichroism spectra, excluding any gross differences in their folded structures. Recombinant HC antagonised the neuromuscular paralysing activity of the native toxin, by competing for binding to neuronal ecto-acceptors. The HC was reconstituted with bacterially-expressed LC to create disulphide-bridged dichain toxin that blocked neuromuscular transmission. The fully-recombinant toxin produced spastic paralysis in mice characteristic of the blockade of central inhibitory synapses, revealing that it undergoes axonal transport to the spinal cord, like the native toxin but with a reduced efficacy. This first report of the large-scale production of recombinant tetanus toxin in active form should facilitate studies on the use of engineered innocuous forms of the toxin as neuronal transport vehicles.     

Botulinum Neurotoxins A and E Undergo Retrograde Axonal Transport in Primary Motor Neurons

The striking differences between the clinical symptoms of tetanus and botulism have been ascribed to the different fate of the parental neurotoxins once internalised in motor neurons. Tetanus toxin (TeNT) is known to undergo transcytosis into inhibitory interneurons and block the release of inhibitory neurotransmitters in the spinal cord, causing a spastic paralysis. In contrast, botulinum neurotoxins (BoNTs) block acetylcholine release at the neuromuscular junction, therefore inducing a flaccid paralysis. Whilst overt experimental evidence supports the sorting of TeNT to the axonal retrograde transport pathway, recent findings challenge the established view that BoNT trafficking is restricted to the neuromuscular junction by highlighting central effects caused by these neurotoxins. These results suggest a more complex scenario whereby BoNTs also engage long-range trafficking mechanisms. However, the intracellular pathways underlying this process remain unclear. We sought to fill this gap by using primary motor neurons either in mass culture or differentiated in microfluidic devices to directly monitor the endocytosis and axonal transport of full length BoNT/A and BoNT/E and their recombinant binding fragments. We show that BoNT/A and BoNT/E are internalised by spinal cord motor neurons and undergo fast axonal retrograde transport. BoNT/A and BoNT/E are internalised in non-acidic axonal carriers that partially overlap with those containing TeNT, following a process that is largely independent of stimulated synaptic vesicle endo-exocytosis. Following intramuscular injection in vivo, BoNT/A and TeNT displayed central effects with a similar time course. Central actions paralleled the peripheral spastic paralysis for TeNT, but lagged behind the onset of flaccid paralysis for BoNT/A. These results suggest that the fast axonal retrograde transport compartment is composed of multifunctional trafficking organelles orchestrating the simultaneous transfer of diverse cargoes from nerve terminals to the soma, and represents a general gateway for the delivery of virulence factors and pathogens to the central nervous system.     

Multiple Domains of Tetanus Toxin Direct Entry into Primary Neurons

Tetanus toxin elicits spastic paralysis by cleaving VAMP‐2 to inhibit neurotransmitter release in inhibitory neurons of the central nervous system. As the retrograde transport of tetanus neurotoxin (TeNT) from endosomes has been described, the initial steps that define how TeNT initiates trafficking to the retrograde system are undefined. This study examines TeNT entry into primary cultured cortical neurons by total internal reflection fluorescence (TIRF) microscopy. The initial association of TeNT with the plasma membrane was dependent upon ganglioside binding, but segregated from synaptophysin1 (Syp1), a synaptic vesicle (SV) protein. TeNT entry was unaffected by membrane depolarization and independent of SV cycling, whereas entry of the receptor‐binding domain of TeNT (HCR/T) was stimulated by membrane depolarization and inhibited by blocking SV cycling. Measurement of the incidence of colocalization showed that TeNT segregated from Syp1, whereas HCR/T colocalized with Syp1. These studies show that while the HCR defines the initial association of TeNT with the cell membrane, regions outside the HCR define how TeNT enters neurons independent of SV cycling. This provides a basis for the unique entry of botulinum toxin and tetanus toxin into neurons.      

Tetanus neurotoxin: conformational plasticity as an adaptive strategy

Tetanus neurotoxin (TeNT) secreted by Clostridium tetani is the causative agent of the spastic paralysis distinctive of human tetanus. TeNT is structurally related to the family of botulinum neurotoxins (BoNTs) produced by Clostridium botulinum that cause flaccid paralysis by disabling synaptic exocytosis at peripheral cholinergic neurons. By contrast, TeNT targets the central nervous system (CNS) by hijacking receptors for neurotrophic factors to enter peripheral neurons thereby being sorted into non‐acidifying endosomes, trafficking via retrograde axonal transport organelles, and entering spinal inhibitory interneurons after transcytosis (Fig 1 A). In this issue of EMBO Reports, Masuyer et al 1 describe the structural plasticity of individual TeNT domains in the context of the holotoxin in response to environmental pH, a key factor modulating TeNT fate and action. Through the concerted use of X‐ray crystallography, single particle cryo‐EM, and small angle X‐ray scattering (SAXS), the authors provide snapshots of conformational transitions that may underlie the productive path of TeNT from its entry in the peripheral nervous system (PNS) to its ultimate site of action on central glycinergic synapses.     

The structure of the tetanus toxin reveals pH‐mediated domain dynamics

The tetanus neurotoxin (TeNT) is a highly potent toxin produced by Clostridium tetani that inhibits neurotransmission of inhibitory interneurons, causing spastic paralysis in the tetanus disease. TeNT differs from the other clostridial neurotoxins by its unique ability to target the central nervous system by retrograde axonal transport. The crystal structure of the tetanus toxin reveals a “closed” domain arrangement stabilised by two disulphide bridges, and the molecular details of the toxin's interaction with its polysaccharide receptor. An integrative analysis combining X‐ray crystallography, solution scattering and single particle electron cryo‐microscopy reveals pH‐mediated domain rearrangements that may give TeNT the ability to adapt to the multiple environments encountered during intoxication, and facilitate binding to distinct receptors.     

Analysis of mutants of tetanus toxin Hc fragment: ganglioside binding, cell binding and retrograde axonal transport properties

Tetanus toxin binds neuronal tissue prior to internalization and trafficking to the central nervous system. Binding of the carboxy-terminal 50 kDa HC fragment of tetanus toxin to polysialogangliosides is important for this initial cell binding step. Using the three-dimensional structure of HC, mutants were designed to investigate the role of individual residues in ganglioside binding. Mutant proteins were tested for binding to GT1b gangliosides, to primary motoneurons and for their ability to undergo retrograde transport in mice. Two classes of mutant were obtained: (i) those containing deletions in loop regions within the C-terminal beta-trefoil domain which showed greatly reduced ganglioside and cell binding and did not undergo retrograde transport and (ii) those that showed reduced ganglioside binding, but retained primary neuronal cell binding and retrograde transport. The second class included point mutants of Histidine-1293, previously implicated in GT1b binding. Our deletion analysis is entirely consistent with recent structural studies which have identified sugar-binding sites in the immediate vicinity of the residues identified by mutagenesis. These results demonstrate that ganglioside binding can be severely impaired without abolishing cell binding and intracellular trafficking of tetanus toxin.     

Molecular basis for tetanus toxin coreceptor interactions

Tetanus toxin (TeNT) elicits spastic paralysis through the cleavage of vesicle-associated membrane protein-2 (VAMP-2) in neurons at the interneuronal junction of the central nervous system. While TeNT retrograde traffics from peripheral nerve endings to the interneuronal junction, there is limited understanding of the neuronal receptors utilized by tetanus toxin for the initial entry into nerve cells. Earlier studies implicated a coreceptor for tetanus toxin entry into neurons: a ganglioside binding pocket and a sialic acid binding pocket and that GT1b bound to each pocket. In this study, a solid phase assay characterized the ganglioside binding specificity and functional properties of both carbohydrate binding pockets of TeNT. The ganglioside binding pocket recognized the ganglioside sugar backbone, Gal-GalNAc, independent of sialic acid-(5) and sialic acid-(7) and GM1a was an optimal substrate for this pocket, while the sialic acid binding pocket recognized sialic acid-(5) and sialic acid-(7) with "b"series of gangliosides preferred relative to "a" series gangliosides. The high-affinity binding of gangliosides to TeNT HCR required functional ganglioside and sialic acid binding pockets, supporting synergistic binding to coreceptors. This analysis provides a model for how tetanus toxin utilizes coreceptors for high-affinity binding to neurons.     

Acute botulinum-like intoxication by tetanus neurotoxin in mice

Intravenous injection of purified tetanus toxin(1000-0.06 μg) killed mice within minutes(20–450 min), causing flaccid paralysis indistinguishable from that in botulinum intoxication: a linear relation was found between the log of the toxin dose and that of death time(survival time). The dose and route dependences of the manifestations of the spastic paralysis typical of classical tetanus and of the acute botulinum-like flaccid paralysis were studied in relation to the death time. Treatment of the toxin with trypsin or gangliosides did not affect its acute botulinum-like toxicity. Theophylline delayed the time of acute death due to the botulinum-like intoxication in mice caused by tetanus toxin and provided some protection.     

Endocytosis and retrograde axonal traffic in motor neurons

Spinal cord motor neurons control voluntary movement by relaying messages that arrive from upper brain centres to the innervated muscles. Despite the importance of motor neurons in human health and disease, the precise control of their membrane dynamics and its effect on motor neuron homoeostasis and survival are poorly understood. In particular, the molecular basis of the co-ordination of specific endocytic events with the axonal retrograde transport pathway is largely unknown. To study these important vesicular trafficking events, we pioneered the use of atoxic fragments of tetanus and botulinum neurotoxins to follow endocytosis and retrograde axonal transport in motor neurons. These neurotoxins bind specifically to pre-synaptic nerve terminals, where they are internalized. Whereas botulinum neurotoxins remain at the neuromuscular junction, tetanus toxin is retrogradely transported along the axon to the cell body, where it is released into the intersynaptic space and is internalized by adjacent inhibitory interneurons. The high neurospecificity and the differential intracellular sorting make tetanus and botulinum neurotoxins ideal tools to study neuronal physiology. In the present review, we discuss recent developments in our understanding of the internalization and trafficking of these molecules in spinal cord motor neurons. Furthermore, we describe the development of a reliable transfection method for motor neurons based on microinjection, which will be extremely useful for dissecting further the molecular basis of membrane dynamics and axonal transport in these cells.     

Limited Trafficking of a Neurotropic Virus Through Inefficient Retrograde Axonal Transport and the Type I Interferon Response

Poliovirus is an enteric virus that rarely invades the human central nervous system (CNS). To identify barriers limiting poliovirus spread from the periphery to CNS, we monitored trafficking of 10 marked viruses. After oral inoculation of susceptible mice, poliovirus was present in peripheral neurons, including vagus and sciatic nerves. To model viral trafficking in peripheral neurons, we intramuscularly injected mice with poliovirus, which follows a muscle–sciatic nerve–spinal cord–brain route. Only 20% of the poliovirus population successfully moved from muscle to brain, and three barriers limiting viral trafficking were identified. First, using light-sensitive viruses, we found limited viral replication in peripheral neurons. Second, retrograde axonal transport of poliovirus in peripheral neurons was inefficient; however, the efficiency was increased upon muscle damage, which also increased the transport efficiency of a non-viral neural tracer, wheat germ agglutinin. Third, using susceptible interferon (IFN) α/β receptor knockout mice, we demonstrated that the IFN response limited viral movement from the periphery to the brain. Surprisingly, the retrograde axonal transport barrier was equivalent in strength to the IFN barrier. Illustrating the importance of barriers created by the IFN response and inefficient axonal transport, IFN α/β receptor knockout mice with muscle damage permitted 80% of the viral population to access the brain, and succumbed to disease three times faster than mice with intact barriers. These results suggest that multiple separate barriers limit poliovirus trafficking from peripheral neurons to the CNS, possibly explaining the rare incidence of paralytic poliomyelitis. This study identifies inefficient axonal transport as a substantial barrier to poliovirus trafficking in peripheral neurons, which may limit CNS access for other viruses.     

Tetanus toxin is transported in a novel neuronal compartment characterized by a specialized pH regulation

Tetanus toxin binds specifically to motor neurons at the neuromuscular junction. There, it is internalized into vesicular carriers undergoing fast retrograde transport to the spinal cord. Despite the importance of this axonal transport pathway in health and disease, its molecular and biophysical characterization is presently lacking. We sought to fill this gap by determining the pH regulation of this compartment in living motor neurons using a chimera of the tetanus toxin binding fragment (TeNT HC) and a pH-sensitive variant of the green fluorescent protein (ratiometric pHluorin). We have demonstrated that moving retrograde carriers display a narrow range of neutral pH values, which is kept constant during transport. Stationary TeNT HC-positive organelles instead exhibit a wide spectrum of pH values, ranging from acidic to neutral. This distinct pH regulation is due to a differential targeting of the vacuolar (H+) ATPase, which is not present on moving TeNT HC compartments. Accordingly, inhibition of the vacuolar (H+) ATPase under conditions that completely abolish the intracellular accumulation of acidotrophic dyes does not affect axonal retrograde transport of TeNT HC. However, a functional vacuolar (H+) ATPase is required for early steps of TeNT HC trafficking following endocytosis, and it is localized to axonal vesicles containing TeNT HC. Altogether, these findings indicate that the vacuolar (H+ ATPase plays a specific role in early sorting events directing TeNT HC to axonal carriers but not in their subsequent progression along the retrograde transport route, which escapes acidification and targeting to degradative organelles.     

Neuronal binding of tetanus toxin compared to its ganglioside binding fragment (H(c))

The non-toxin 50 kD C-terminus peptide of the heavy chain of tetanus H(c) contains the ganglioside binding domain of tetanus toxin (TTX). H(c) retains much of the capacity of tetanus toxin for binding internalization and transport by neurons. For this reason tetanus H(c) has been studied as a vector for delivery of therapeutic proteins to neurons. We directly compared H(c) and TTX in the capacity to bind and be internalized by neurons by ELISA. Primary cultures of dissociated fetal cortical neurons were incubated with equimolar amounts of TTX or H(c). Neuronal associated tetanus protein was 4-8 fold greater on a molar basis with tetanus toxin compared to H(c) (1 h incubation). This increase in neuronal tetanus protein was evident with incubation in concentrations from 0.1 microM to 2 microM. There were greater amounts of TTX delivered to the cultured cells at both 0 degrees C (representing membrane bound tetanus protein) and 37 degrees C (bound and internalized tetanus protein). Unlike H(c), TTX showed significant continued accumulation of protein with increasing incubation durations. Neuronal associated TTX increased 2-3 fold over incubation times ranging from 1 to 8 h. Tetanus toxin appears to be clearly superior to the ganglioside binding fragment (H(c)) in the capacity for neuronal binding and internalization. Atoxic tetanus proteins containing additional molecular domains as well as H(c) may be more suitable vectors for linkage with therapeutic proteins and delivery to neurons.     

SV2 mediates entry of tetanus neurotoxin into central neurons

Tetanus neurotoxin causes the disease tetanus, which is characterized by rigid paralysis. The toxin acts by inhibiting the release of neurotransmitters from inhibitory neurons in the spinal cord that innervate motor neurons and is unique among the clostridial neurotoxins due to its ability to shuttle from the periphery to the central nervous system. Tetanus neurotoxin is thought to interact with a high affinity receptor complex that is composed of lipid and protein components; however, the identity of the protein receptor remains elusive. In the current study, we demonstrate that toxin binding, to dissociated hippocampal and spinal cord neurons, is greatly enhanced by driving synaptic vesicle exocytosis. Moreover, tetanus neurotoxin entry and subsequent cleavage of synaptobrevin II, the substrate for this toxin, was also dependent on synaptic vesicle recycling. Next, we identified the potential synaptic vesicle binding protein for the toxin and found that it corresponded to SV2; tetanus neurotoxin was unable to cleave synaptobrevin II in SV2 knockout neurons. Toxin entry into knockout neurons was rescued by infecting with viruses that express SV2A or SV2B. Tetanus toxin elicited the hyper excitability in dissociated spinal cord neurons - due to preferential loss of inhibitory transmission - that is characteristic of the disease. Surprisingly, in dissociated cortical cultures, low concentrations of the toxin preferentially acted on excitatory neurons. Further examination of the distribution of SV2A and SV2B in both spinal cord and cortical neurons revealed that SV2B is to a large extent localized to excitatory terminals, while SV2A is localized to inhibitory terminals. Therefore, the distinct effects of tetanus toxin on cortical and spinal cord neurons are not due to differential expression of SV2 isoforms. In summary, the findings reported here indicate that SV2A and SV2B mediate binding and entry of tetanus neurotoxin into central neurons.     

Neuronal sensitivity to tetanus toxin requires gangliosides.

Tetanus toxin produces spastic paralysis in situ by blocking inhibitory neurotransmitter release in the spinal cord. Although di- and trisialogangliosides bind tetanus toxin, their role as productive toxin receptors remains unclear. We examined toxin binding and action in spinal cord cell cultures grown in the presence of fumonisin B(1), an inhibitor of ganglioside synthesis. Mouse spinal cord neurons grown for 3 weeks in culture in 20 microM fumonisin B(1) develop dendrites, axons, and synaptic terminals similar to untreated neurons, even though thin layer chromatography shows a greater than 90% inhibition of ganglioside synthesis. Absence of tetanus and cholera toxin binding by toxin-horseradish peroxidase conjugates or immunofluorescence further indicates loss of mono- and polysialogangliosides. In contrast to control cultures, tetanus toxin added to fumonisin B(1)-treated cultures does not block potassium-stimulated glycine release, inhibit activity-dependent uptake of FM1-43, or abolish immunoreactivity for vesicle-associated membrane protein, the toxin substrate. Supplementing fumonisin B(1)-treated cultures with mixed brain gangliosides completely restores the ability of tetanus toxin to bind to the neuronal surface and to block neurotransmitter release. These data demonstrate that fumonisin B(1) protects against toxin-induced synaptic blockade and that gangliosides are a necessary component of the receptor mechanism for tetanus toxin.     

Selective retrograde transsynaptic transfer of a protein, tetanus toxin, subsequent to its retrograde axonal transport

The fate of tetanus toxin (mol wt 150,000) subsequent to its retrograde axonal transport in peripheral sympathetic neurons of the rat was studied by both electron microscope autoradiography and cytochemistry using toxin-horseradish peroxidase (HRP) coupling products, and compared to that of nerve growth factor (NGF), cholera toxin, and the lectins wheat germ agglutinin (WGA), phytohaemagglutinin (PHA), and ricin. All these macromolecules are taken up by adrenergic nerve terminals and transported retrogradely in a selective, highly efficient manner. This selective uptake and transport is a consequence of the binding of these macromolecules to specific receptive sites on the nerve terminal membrane. All these ligands are transported in the axons within smooth vesicles, cisternae, and tubules. In the cell bodies these membrane compartments fuse and most of the transported macromolecules are finally incorporated into lysosomes. The cell nuclei, the parallel golgi cisternae, and the extracellular space always remain unlabeled. In case the tetanus toxin, however, a substantial fraction of the labeled material appears in presynaptic cholinergic nerve terminals which innervate the labeled ganglion cells. In these terminals tetanus toxin-HRP is localized in 500-1,000 A diam vesicles. In contrast, such a retrograde transsynaptic transfer is not at all or only very rarely detectable after retrograde transport of cholera toxin, NGF, WGA, PHA, or ricin. An atoxic fragment of the tetanus toxin, which contains the ganglioside-binding site, behaves like intact toxin. With all these macromolecules, the extracellular space and the glial cells in the ganglion remain unlabeled. We conclude that the selectivity of this transsynaptic transfer of tetanus toxin is due to a selective release of the toxin from the postsynaptic dendrites. This release is immediately followed by an uptake into the presynaptic terminals.     

Prion disease incubation time is not affected in mice heterozygous for a dynein mutation

A mechanism for transmission of the infectious prions from the peripheral nerve ends to the central nervous system is thought to involve neuronal anterograde and retrograde transport systems. Cytoplasmic dynein is the major retrograde transport molecular motor whose function is impaired in the Legs at odd angles (Loa) mouse due to a point mutation in the cytoplasmic dynein heavy chain subunit. Loa is a dominant trait which causes neurodegeneration and progressive motor function deficit in the heterozygotes. To investigate the role of cytoplasmic dynein in the transmission of prions within neurons, we inoculated heterozygous Loa and wild type littermates with mouse-adapted scrapie prions intracerebrally and intraperitonially, and determined the incubation period to onset of clinical prion disease. Our data indicate that the dynein mutation in the heterozygous state does not affect prion disease incubation time or its neuropathology in Loa mice.     

Tetanus toxin fragment C binds to a protein present in neuronal cell lines and motoneurons

Tetanus Toxin Fragment C Binds to a Protein Present in Neuronal Cell Lines and Motoneurons Tetanus neurotoxin is one of the most powerful protein toxins known, acting in vivo at femtomolar doses. Two main factors determine its high potency: a protease activity restricted to a single intracellular substrate and its absolute neurospecificity. Whereas the enzymatic properties of tetanus toxin have been thoroughly defined, the nature of its neuronal receptor(s) and their involvement in the intracellular trafficking of tetanus toxin are poorly understood. Using binding and crosslinking experiments, we report here on the characterisation of an N-glycosylated 15-kDa interacting protein, which behaves as an integral membrane protein. This putative receptor specifically interacts with the binding domain (fragment C) of tetanus toxin and not with several related botulinum neurotoxins in spinal cord motoneurons and neuronal-like cell lines. Sialic acid-specific lectins antagonise the binding of tetanus toxin to the cell surface and to the 15-kDa protein, supporting the central role of sialic acid residues in the recognition process. Altogether, these results indicate the existence of a neuronal protein receptor for tetanus toxin whose identification is likely to constitute a key step in the analysis of the molecular machinery involved in the toxin internalisation and retrograde transport.     

CAR-Associated Vesicular Transport of an Adenovirus in Motor Neuron Axons

Axonal transport is responsible for the movement of signals and cargo between nerve termini and cell bodies. Pathogens also exploit this pathway to enter and exit the central nervous system. In this study, we characterised the binding, endocytosis and axonal transport of an adenovirus (CAV-2) that preferentially infects neurons. Using biochemical, cell biology, genetic, ultrastructural and live-cell imaging approaches, we show that interaction with the neuronal membrane correlates with coxsackievirus and adenovirus receptor (CAR) surface expression, followed by endocytosis involving clathrin. In axons, long-range CAV-2 motility was bidirectional with a bias for retrograde transport in nonacidic Rab7-positive organelles. Unexpectedly, we found that CAR was associated with CAV-2 vesicles that also transported cargo as functionally distinct as tetanus toxin, neurotrophins, and their receptors. These results suggest that a single axonal transport carrier is capable of transporting functionally distinct cargoes that target different membrane compartments in the soma. We propose that CAV-2 transport is dictated by an innate trafficking of CAR, suggesting an unsuspected function for this adhesion protein during neuronal homeostasis.     

Abnormal neurofilament transport caused by targeted disruption of neuronal kinesin heavy chain KIF5A

To test the hypothesis that fast anterograde molecular motor proteins power the slow axonal transport of neurofilaments (NFs), we used homologous recombination to generate mice lacking the neuronal-specific conventional kinesin heavy chain, KIF5A. Because null KIF5A mutants die immediately after birth, a synapsin-promoted Cre-recombinase transgene was used to direct inactivation of KIF5A in neurons postnatally. Three fourths of such mutant mice exhibited seizures and death at around 3 wk of age; the remaining animals survived to 3 mo or longer. In young mutant animals, fast axonal transport appeared to be intact, but NF-H, as well as NF-M and NF-L, accumulated in the cell bodies of peripheral sensory neurons accompanied by a reduction in sensory axon caliber. Older animals also developed age-dependent sensory neuron degeneration, an accumulation of NF subunits in cell bodies and a reduction in axons, loss of large caliber axons, and hind limb paralysis. These data support the hypothesis that a conventional kinesin plays a role in the microtubule-dependent slow axonal transport of at least one cargo, the NF proteins.     

Analysis of retrograde transport in motor neurons reveals common endocytic carriers for tetanus toxin and neurotrophin receptor p75NTR.

Axonal retrograde transport is essential for neuronal growth and survival. However, the nature and dynamics of the membrane compartments involved in this process are poorly characterized. To shed light on this pathway, we established an experimental system for the visualization and the quantitative study of retrograde transport in living motor neurons based on a fluorescent fragment of tetanus toxin (TeNT HC). Morphological and kinetic analysis of TeNT HC retrograde carriers reveals two major groups of organelles: round vesicles and fast tubular structures. TeNT HC carriers lack markers of the classical endocytic pathway and are not acidified during axonal transport. Importantly, TeNT HC and NGF share the same retrograde transport organelles, which are characterized by the presence of the neurotrophin receptor p75NTR. Our results provide the first direct visualization of retrograde transport in living motor neurons, and reveal a novel retrograde route that could be used both by physiological ligands (i.e., neurotrophins) and TeNT to enter the central nervous system.