Connecting the dots: trafficking of neurotrophins, lectins and diverse pathogens by binding to the neurotrophin receptor p75NTR

Chronological brain lesions after SARS-CoV-2 infection in hACE2-transgenic mice

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) causes respiratory disease, but it can also affect other organs including the central nervous system. Several animal models have been developed to address different key questions related to Coronavirus Disease 2019 (COVID-19). Wild-type mice are minimally susceptible to certain SARS-CoV-2 lineages (beta and gamma variants), whereas hACE2-transgenic mice succumb to SARS-CoV-2 and develop a fatal neurological disease. In this article, we aimed to chronologically characterize SARS-CoV-2 neuroinvasion and neuropathology. Necropsies were performed at different time points, and the brain and olfactory mucosa were processed for histopathological analysis. SARS-CoV-2 virological assays including immunohistochemistry were performed along with a panel of antibodies to assess neuroinflammation. At 6 to 7 days post inoculation (dpi), brain lesions were characterized by nonsuppurative meningoencephalitis and diffuse astrogliosis and microgliosis. Vasculitis and thrombosis were also present and associated with occasional microhemorrhages and spongiosis. Moreover, there was vacuolar degeneration of virus-infected neurons. At 2 dpi, SARS-CoV-2 immunolabeling was only found in the olfactory mucosa, but at 4 dpi intraneuronal virus immunolabeling had already reached most of the brain areas. Maximal distribution of the virus was observed throughout the brain at 6 to 7 dpi except for the cerebellum, which was mostly spared. Our results suggest an early entry of the virus through the olfactory mucosa and a rapid interneuronal spread of the virus leading to acute encephalitis and neuronal damage in this mouse model.

Neuronal delivery of nanoparticles via nerve fibres in the skin

Accessing the peripheral nervous system (PNS) by topically applied nanoparticles is a simple and novel approach with clinical applications in several PNS disorders. Skin is richly innervated by long peripheral axons that arise from cell bodies located distally within ganglia. In this study we attempt to target dorsal root ganglia (DRG) neurons, via their axons by topical application of lectin-functionalized gold nanoparticles (IB4-AuNP). In vitro, 140.2 ± 1.9 nm IB4-AuNP were found to bind both axons and cell bodies of DRG neurons, and AuNP applied at the axonal terminals were found to translocate to the cell bodies. Topical application of IB4-AuNP on rat hind-paw resulted in accumulation of three to fourfold higher AuNP in lumbar DRG than in contralateral control DRGs. Results from this study clearly suggest that topically applied nanoparticles with neurotropic targeting ligands can be utilized for delivering nanoparticles to neuronal cell bodies via axonal transport mechanisms.

Viruses in connectomics: Viral transneuronal tracers and genetically modified recombinants as neuroscience research tools

Connectomic studies have become 'viral', as viral pathogens have been turned into irreplaceable neuroscience research tools. Highly sensitive viral transneuronal tracing technologies are available, based on the use of alpha-herpesviruses and a rhabdovirus (rabies virus), which function as self-amplifying markers by replicating in recipient neurons. These viruses highly differ with regard to host range, cellular receptors, peripheral uptake, replication, transport direction and specificity. Their characteristics, that make them useful for different purposes, will be highlighted and contrasted. Only transneuronal tracing with rabies virus is entirely specific. The neuroscientist toolbox currently include wild-type alpha-herpesviruses and rabies virus strains enabling polysynaptic tracing of neuronal networks across multiple synapses, as well as genetically modified viral tracers for dual transneuronal tracing, and complementary viral tools including defective and chimeric recombinants that function as single step or monosynaptically restricted tracers, or serve for monitoring and manipulating neuronal activity and gene expression. Methodological issues that are crucial for appropriate use of these technologies will be summarized. Among wild-type and genetically engineered viral tools, rabies virus and chimeric recombinants based on rabies virus as virus backbone are the most powerful, because of the ability of rabies virus to propagate exclusively among connected neurons unidirectionally (retrogradely), without affecting neuronal function. Understanding in depth viral properties is essential for neuroscientists who intend to exploit alpha-herpesviruses, rhabdoviruses or derived recombinants as research tools. Key knowledge will be summarized regarding their cellular receptors, intracellular trafficking and strategies to contrast host defense that explain their different pathophysiology and properties as research tools.

Nanoparticle-assisted optical tethering of endosomes reveals the cooperative function of dyneins in retrograde axonal transport

Dynein-dependent transport of organelles from the axon terminals to the cell bodies is essential to the survival and function of neurons. However, quantitative knowledge of dyneins on axonal organelles and their collective function during this long-distance transport is lacking because current technologies to do such measurements are not applicable to neurons. Here, we report a new method termed nanoparticle-assisted optical tethering of endosomes (NOTE) that made it possible to study the cooperative mechanics of dyneins on retrograde axonal endosomes in live neurons. In this method, the opposing force from an elastic tether causes the endosomes to gradually stall under load and detach with a recoil velocity proportional to the dynein forces. These recoil velocities reveal that the axonal endosomes, despite their small size, can recruit up to 7 dyneins that function as independent mechanical units stochastically sharing load, which is vital for robust retrograde axonal transport. This study shows that NOTE, which relies on controlled generation of reactive oxygen species, is a viable method to manipulate small cellular cargos that are beyond the reach of current technology.

Stem cell treatment of degenerative eye disease

Stem cell therapies are being explored extensively as treatments for degenerative eye disease, either for replacing lost neurons, restoring neural circuits or, based on more recent evidence, as paracrine-mediated therapies in which stem cell-derived trophic factors protect compromised endogenous retinal neurons from death and induce the growth of new connections. Retinal progenitor phenotypes induced from embryonic stem cells/induced pluripotent stem cells (ESCs/iPSCs) and endogenous retinal stem cells may replace lost photoreceptors and retinal pigment epithelial (RPE) cells and restore vision in the diseased eye, whereas treatment of injured retinal ganglion cells (RGCs) has so far been reliant on mesenchymal stem cells (MSC). Here, we review the properties of non-retinal-derived adult stem cells, in particular neural stem cells (NSCs), MSC derived from bone marrow (BMSC), adipose tissues (ADSC) and dental pulp (DPSC), together with ESC/iPSC and discuss and compare their potential advantages as therapies designed to provide trophic support, repair and replacement of retinal neurons, RPE and glia in degenerative retinal diseases. We conclude that ESCs/iPSCs have the potential to replace lost retinal cells, whereas MSC may be a useful source of paracrine factors that protect RGC and stimulate regeneration of their axons in the optic nerve in degenerate eye disease. NSC may have potential as both a source of replacement cells and also as mediators of paracrine treatment.

Rabies virus envelope glycoprotein targets lentiviral vectors to the axonal retrograde pathway in motor neurons

Rabies pseudotyped lentiviral vectors have great potential in gene therapy, not least because of their ability to transduce neurons following their distal axonal application. However, very little is known about the molecular processes that underlie their retrograde transport and cell transduction. Using multiple labeling techniques and confocal microscopy, we demonstrated that pseudotyping with rabies virus envelope glycoprotein (RV-G) enabled the axonal retrograde transport of two distinct subtypes of lentiviral vector in motor neuron cultures. Analysis of this process revealed that these vectors trafficked through Rab5-positive endosomes and accumulated within a non-acidic Rab7 compartment. RV-G pseudotyped vectors were co-transported with both the tetanus neurotoxin-binding fragment and the membrane proteins thought to mediate rabies virus endocytosis (neural cell adhesion molecule, nicotinic acetylcholine receptor, and p75 neurotrophin receptor), thus demonstrating that pseudotyping with RV-G targets lentiviral vectors for transport along the same pathway exploited by several toxins and viruses. Using motor neurons cultured in compartmentalized chambers, we demonstrated that axonal retrograde transport of these vectors was rapid and efficient; however, it was not able to transduce the targeted neurons efficiently, suggesting that impairment in processes occurring after arrival of the viral vector in the soma is responsible for the low transduction efficiency seen in vivo, which suggests a novel area for improvement of gene therapy vectors.

Mapping of neurotrophins and their receptors in the adult mouse brain and their role in the pathogenesis of a transgenic murine model of bovine spongiform encephalopathy

Neurotrophins are a family of growth factors that act on neuronal cells. The neurotrophins include nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF) and neurotrophin (NT)-3, -4 and -5. The action of neurotrophins depends on two transmembrane-receptor signalling systems: (1) the tropomyosin-related kinase (Trk) family of tyrosine kinase receptors (Trk A, Trk B and Trk C) and (2) the p75 neurotrophin receptor (p75(NTR)). The interaction between neurotrophic factors and their receptors may be involved in the mechanisms that regulate the differential susceptibility of neuronal populations in neurodegenerative diseases. The aim of the present study was to evaluate the role of neurotrophins in the pathogenesis of bovine spongiform encephalopathy (BSE) using a transgenic mouse overexpressing bovine prnp (BoTg 110). Histochemistry for Lycopersicum esculentum agglutinin, haematoxylin and eosin staining and immunohistochemistry for the abnormal isoform of the prion protein (PrP(d)), glial fibrillary acidic protein (GFAP), NGF, BDNF, NT-3 and the receptors Trk A, Trk B, Trk C and p75(NTR) was performed. The lesions and the immunolabelling patterns were assessed semiquantitatively in different areas of the brain. No significant differences in the immunolabelling of neurotrophins and their receptors were observed between BSE-inoculated and control animals, except for p75(NTR), which showed increased expression correlating with the distribution of lesions, PrP(d) deposition and gliosis in the BSE-inoculated mice.

A novel in vivo method for isolating antibodies from a phage display library by neuronal retrograde transport selectively yields antibodies against p75(NTR.)

The neurotrophin receptor p75(NTR) is utilized by a variety of pathogens to gain entry into the central nervous system (CNS). We tested if this entry portal might be exploited using a phage display library to isolate internalizing antibodies that target the CNS in vivo. By applying a phage library that expressed human single chain variable fragment (scFv) antibodies on their surface to a transected sciatic nerve, we showed that (1) phage conjugated to anti-p75(NTR) antibody or phage scFv library pre-panned against p75(NTR) are internalized by neurons expressing p75(NTR); (2) subsequent retrograde axonal transport separates internalized phage from the applied phage; and, (3) internalized phage can be recovered from a proximal ligature made on a nerve. This approach resulted in 13-fold increase in the number of phage isolated from the injured nerve compared with the starting population, and isolation of 18 unique internalizing p75(NTR) antibodies that were transported from the peripheral nerve into the spinal cord, through the blood-brain barrier. In addition, antibodies recognizing other potentially internalized antigens were identified through in vivo selection using a fully diverse library. Because p75(NTR) expression is upregulated in motor neurons in response to injury and in disease, the p75(NTR) antibodies may have substantial potential for cell-targeted drug/gene delivery. In addition, this novel selection method provides the potential to generate panels of antibodies that could be used to identify further internalization targets, which could aid drug delivery across the blood-brain barrier.

Influence of glial-derived matrix molecules, especially chondroitin sulfates, on neurite growth and survival of cultured mouse embryonic motoneurons

Mechanisms controlling neuronal survival and regeneration play an important role during development, after birth, and under lesion conditions. Isolated embryonic mouse motoneurons have been a useful tool for studying such basic mechanisms. These cultured motoneurons depend on extracellular matrix (ECM) molecules, which are potent mediators of survival and axonal growth and guidance in the CNS and in vitro, exhibiting either attractive or repellent guidance cues. Additionally, ECM proteoglycans and glycoproteins are components of the glial scar acting as a growth barrier for regenerating axons. Compared with CNS axon outgrowth, less is known about the cues that guide motoneurons toward their peripheral targets. Because we are interested in the effects of glial-derived chondroitin sulfate proteoglycans (CSPGs), we have worked out a model system for investigating the influences of glial-derived matrix molecules on motoneuron outgrowth and survival. We used cultured embryonic mouse motoneurons to investigate axon growth effects of matrix molecules produced by the glial-derived cell lines A7, Neu7, and Oli-neu primary astrocytes as well as the immortalized Schwann cell line IMS32. The results indicate that molecules of the ECM, especially chondroitin sulfates, play an important role as axon growth-promoting cues. We could demonstrate a modifying effect of the matrix components on motoneuron survival and caspase3-induced apoptosis.

Fates of neurotrophins after retrograde axonal transport: phosphorylation of p75NTR is a sorting signal for delayed degradation

Neurotrophins can mediate survival or death of neurons. Opposing functions of neurotrophins are based on binding of these ligands to two distinct types of receptors: trk receptors and p75NTR. Previous work showed that target-derived NGF induces cell death, whereas BDNF and NT-3 enhance survival of neurons in the isthmo-optic nucleus of avian embryos. To determine the fate of retrogradely transported neurotrophins and test whether their sorting differs between neurotrophins mediating survival- or death-signaling pathways, we traced receptor-binding, sorting, and degradation kinetics of target-applied radiolabeled neurotrophins that bind in this system to trk receptors (BDNF, NT-3) or only to p75NTR (NGF). At the ultrastructural level, the p75NTR-bound NGF accumulates with a significant delay in multivesicular bodies and organelles of the degradation pathway on arrival in the cell body when compared with trk-bound BDNF or NT-3. This delayed lysosomal accumulation was restricted to target-derived NGF, but was not seen when NGF was supplied to the soma in vitro. The kinase inhibitors K252a and Gö6976 alter the kinetics of organelle accumulation: phosphorylation of p75NTR is a sorting signal for delayed sequestering of p75NTR-bound NGF in multivesicular bodies and delayed degradation in lysosomes when compared with trk-bound neurotrophins. Mutagenesis and mass spectrometry studies indicate that p75NTR is phosphorylated by conventional protein kinase C on serine 266. We conclude that, in addition to the known phosphorylation of trks, the phosphorylation of p75NTR can also significantly affect neuronal survival in vivo by changing the intracellular sorting and degradation kinetics of its ligands and thus signaling duration.

Anterograde axonal transport of chicken cellular prion protein (PrPc) in vivo requires its N-terminal part

The cellular isoform of prion protein (PrP(c)) can exist in membrane-bound and secreted forms. Both forms of PrP(c) can be transported by retinal ganglion cell (RGC) axons along the optic nerve in the anterograde direction. In this study we determined which part of chicken PrP(c) is required for its anterograde axonal transport within the optic nerve of embryonic chicken. We intraocularly injected radio-iodinated fragments of recombinant chicken PrP(c) and then examined their anterograde axonal transport from retina into optic tectum. Using gamma-counting and different autoradiographic techniques we quantified anterograde axonal transport of the N-terminal part of chicken PrP(c) (amino acid residues 1-116) in this model system. The transport of the N-terminal part has similar properties as the anterograde transport of full-length chicken PrP(c) (Butowt et al., 2006) described previously (e.g., has similar efficiency, is microtubule-dependent, and is saturable). Moreover, the pattern of ultrastructural distribution of the N-terminal fragment within RGCs is similar to the distribution of full-length PrP(c). The C-terminal fragment of chicken PrP(c) (residues 118-246) and different PrP-derived peptides were not transported. Moreover, PrP(c)-derived peptides were sorted into different endocytotic pathways in neurons, indicating that they cannot substitute for full-length PrP(c) to study its internalization and trafficking. These data indicate that the N-terminal half of chicken PrP(c) contains the necessary information to drive the internalization and subsequent sorting of extracellular PrP(c) in RGCs soma into the anterograde axonal transport pathway.

Conventional kinesin-I motors participate in the anterograde axonal transport of neurotrophins in the visual system

Retinal ganglion cells (RGCs) anterogradely transport neurotrophins to the midbrain tectum/superior colliculus with significant downstream effects. The molecular mechanism of this type of axonal transport of neurotrophins is not well characterized. We identified kinesin-I proteins as a motor participating in the anterograde axonal movement of vesicular structures containing radiolabeled neurotrophins along the optic nerve. RT-PCR analysis of purified murine RGCs showed that adult RGCs express all known members of the kinesin-I family. After intraocular injection of (125)I-brain-derived neurotrophic factor (BDNF) into the adult mouse or (125)I-neurotrophin-3 (NT-3) into the embryonic chicken eye, radioactivity was efficiently immunoprecipitated from the optic nerve lysates by anti-kinesin heavy chain and anti-kinesin light chain monoclonal antibodies (H2 and L1). Immunoreactivity for the BDNF receptor trkB is also present in the immunoprecipitates obtained by the anti-kinesin-I antibodies. The delivery of the H2 antibody in vivo into the mouse RGCs substantially reduced anterograde axonal transport of (125)I-BDNF. Anterograde transport of BDNF was not diminished in kinesin light chain 1 (KLC1) knockout mice. However, this may be due to redundancy in functions between two different isoforms of KLC present in the RGCs, as it was described previously for kinesin heavy chains (Kanai et al. [ 2000] J Neurosci 20:6374-6384). These data indicate that kinesin-I is a protein motor that participates in the anterograde axonal transport of neurotrophins in the chicken and mouse visual pathways.

Shedding of the p75NTRneurotrophin receptor is modulated by lipid rafts

Protein ectodomain shedding is the proteolytic release of the extracellular domain of membrane-bound proteins. Neurotrophin receptor p75(NTR) is known to be affected by shedding. The present work provides evidence, in rat brain synaptosomes, that p75(NTR) is present in detergent-resistant membranes (DRM), also known as lipid rafts, only in its full-length form. Disrupting the integrity of lipid rafts causes solubilization of p75(NTR) after detergent treatment and enhancement of the shedding. Analyses of the enzymes described as being responsible for p75(NTR) shedding, i.e. tumor necrosis factor alpha convertase (TACE) and presenilin-1 (PS1), revealed that TACE is absent in DRM, while variable proportions of the C-terminal and N-terminal fragments of PS1 are found. In summary, our results point to a role of lipid rafts in the modulation of the shedding of the p75(NTR) receptor.

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.

Pathogenesis of rabies

Rabies is a central nervous system (CNS) disease that is almost invariably fatal. The causative agent is rabies virus (RV), a negative-stranded RNA virus of the rhabdovirus family. RV pathogenesis, like that of other viruses, is a multigenic trait. Recent findings indicate that in addition to the RV G protein viral elements that regulate gene expression, especially expression of the L gene, are also likely to play a role in RV pathogenesis. In vivo, RV infects almost exclusively neurons, and neuroinvasiveness is the major defining characteristic of a classical RV infection. A key factor in the neuroinvasion of RV is transsynaptic neuronal spread. While the ability of RV to spread from the post-synaptic site to the pre-synaptic site is mediated by the RV G protein, the RV P protein might be an important determinant of retrograde transport of the virus within axons. Although the mechanism(s) by which an RV infection cause(s) a lethal neurological disease are still not well understood, the most significant factor underlying the lethal outcome of an RV infection appears to be the neuronal dysfunction due to drastically inhibited synthesis of proteins required in maintaining neuronal functions.

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.

Comparative pathogenesis of the SAD-L16 strain of rabies virus and a mutant modifying the dynein light chain binding site of the rabies virus phosphoprotein in young mice

Recent reports have suggested that rabies virus phosphoprotein (P) interaction with dynein minus-end-directed microtubule motor proteins may be of fundamental importance in the axonal transport of rabies virus. A deletion of 11 amino acids was introduced into recombinant rabies virus SAD-L16 (L16) that modified the dynein light chain (LC8) binding site of the rabies virus P, producing mutant L-DeltaP11. This mutant is a useful tool for determining the role of P-LC8 interaction in viral spread and pathogenesis. Seven-day-old ICR mice were inoculated into a hindlimb thigh muscle with L16 or L-DeltaP11. Histopathological and immunohistochemical analyses of their brains were performed at serial time points in order to determine the pattern of viral spread. L16 spread to the brain and caused a severe encephalitis with apoptotic neuronal changes. L-DeltaP11 infected specific brain areas (brainstem and hippocampus) 1-2 days later than L16 and involved a smaller number of neurons in some brain regions. However, the neuronal apoptotic changes produced by both viruses were similar in most brain regions. Following peripheral inoculation, deletions modifying the LC8 binding site had an effect on delaying viral spread, but did not significantly alter the pattern of rabies virus encephalitis. The precise role of the rabies virus P-dynein interaction in the axonal transport of rabies virus, particularly the importance of this interaction during natural infection, merits further study.

Rabies virus receptors

There is convincing in vitro evidence that the muscular form of the nicotinic acetylcholine receptor (nAChR), the neuronal cell adhesion molecule (NCAM), and the p75 neurotrophin receptor (p75NTR) bind rabies virus and/or facilitate rabies virus entry into cells. Other components of the cell membrane, such as gangliosides, may also participate in the entry of rabies virus. However, little is known of the role of these molecules in vivo. This review proposes a speculative model that accounts for the role of these different molecules in entry and trafficking of rabies virus into the nervous system.

Multi-tasking by the p75 neurotrophin receptor: sortilin things out?

Signalling by the p75 neurotrophin receptor has been implicated in diverse neuronal responses, including increased differentiation or survival, inhibition of regeneration, and initiation of apoptotic cell death. These numerous roles are matched by, but are not yet correlated with, a multiplicity of extracellular ligands and intracellular interactors. Membrane proteins such as sortilin, a member of the Vps10p family of sorting receptors, and the glycosylphosphatidylinositol-linked Nogo receptor (NgR) and the associated adaptor lingo 1 have recently been added to the list of p75-interacting modulators. Other studies have described intramembranal cleavage of p75 and the potential nuclear targeting of cleavage fragments or of the complete receptor after it has been internalized into a putative signalling endosome. These findings suggest that some of the diversity in p75 activities might be due to differential subcellular localization and transport of p75 receptor complexes. We therefore argue that cell-biology-driven approaches are now required to make sense of p75 signalling.

Myosin Va and microtubule-based motors are required for fast axonal retrograde transport of tetanus toxin in motor neurons

Using a novel assay based on the sorting and transport of a fluorescent fragment of tetanus toxin, we have investigated the cytoskeletal and motor requirements of axonal retrograde transport in living mammalian motor neurons. This essential process ensures the movement of neurotrophins and organelles from the periphery to the cell body and is crucial for neuronal survival. Unlike what is observed in sympathetic neurons, fast retrograde transport in motor neurons requires not only intact microtubules, but also actin microfilaments. Here, we show that the movement of tetanus toxin-containing carriers relies on the nonredundant activities of dynein as well as kinesin family members. Quantitative kinetic analysis indicates a role for dynein as the main motor of these carriers. Moreover, this approach suggests the involvement of myosin(s) in retrograde movement. Immunofluorescence screening with isoform-specific myosin antibodies reveals colocalization of tetanus toxin-containing retrograde carriers with myosin Va. Motor neurons from homozygous myosin Va null mice showed slower retrograde transport compared with wild-type cells, establishing a unique role for myosin Va in this process. On the basis of our findings, we propose that coordination of myosin Va and microtubule-dependent motors is required for fast axonal retrograde transport in motor neurons.

The journey of tetanus and botulinum neurotoxins in neurons

Anaerobic bacteria of the genus Clostridia are a major threat to human and animal health, being responsible for pathologies ranging from food poisoning to gas gangrene. In each of these, the production of sophisticated exotoxins is the main cause of disease. The most powerful clostridial toxins are tetanus and botulinum neurotoxins, the causative agents of tetanus and botulism. They are structurally organized into three domains endowed with distinct functions: high affinity binding to neurons, membrane translocation and specific cleavage of proteins controlling neuroexocytosis. Recent discoveries regarding the mechanism of membrane recruitment and sorting of these neurotoxins within neurons make them ideal tools to uncover essential aspects of neuronal physiology in health and disease.

Dependence receptors: what is the mechanism?

Receptors of diverse primary structure and with diverse ligands have been reported to be capable of stimulating apoptosis in the absence of ligand binding. These receptors are called dependence receptors, and the newest member of this family appears to be the Sonic hedgehog receptor Patched, which has been reported to stimulate apoptosis when expressed in the absence of its ligand. The signaling mechanisms that account for this type of receptor activity are unknown. Several theories behind how dependence receptors may trigger cell death are described.

Axonal transport and neuronal transcytosis of trophic factors, tracers, and pathogens

Neurons can specifically internalize macromolecules, such as trophic factors, lectins, toxins, and other pathogens. Upon internalization in terminals, proteins can move retrogradely along axons, or, upon internalization at somatodendritic domains, they can move into an anterograde axonal transport pathway. Release of internalized proteins from neurons after either retrograde or anterograde axonal transport results in transcytosis and trafficking of proteins across multiple synapses. Recent studies of binding properties of several such proteins suggest that pathogens and lectins may utilize existing transport machineries designed for trafficking of trophic factors. Specific pathways may protect trophic factors, pathogens, and toxins from degradation after internalization and may target the trophic or pathogenic cargo for transcytosis after either retrograde or anterograde transport along axons. Elucidating the molecular mechanisms of sorting steps and transport pathways will further our understanding of trophic signaling and could be relevant for an understanding and possible treatment of neurological diseases such as rabies, Alzheimer's disease, and prion encephalopathies. At present, our knowledge is remarkably sparse about the types of receptors used by pathogens for trafficking, the signals that sort trophins or pathogens into recycling or degradation pathways, and the mechanisms that regulate their release from somatodendritic domains or axon terminals. This review intends to draw attention to potential convergences and parallels in trafficking of trophic and pathogenic proteins. It discusses axonal transport/trafficking mechanisms that may help to understand and eventually treat neurological diseases by targeted drug delivery.