Impairment of retrograde neuronal transport in oxaliplatin-induced neuropathy demonstrated by molecular imaging

BACKGROUND AND PURPOSE

The purpose of our study was to utilize a molecular imaging technology based on the retrograde axonal transport mechanism (neurography), to determine if oxaliplatin-induced neurotoxicity affects retrograde axonal transport in an animal model.

MATERIALS AND METHODS

Mice (n = 8/group) were injected with a cumulative dose of 30 mg/kg oxaliplatin (sufficient to induce neurotoxicity) or dextrose control injections. Intramuscular injections of Tetanus Toxin C-fragment (TTc) labeled with Alexa 790 fluorescent dye were done (15 ug/20 uL) in the left calf muscles, and in vivo fluorescent imaging performed (0-60 min) at baseline, and then weekly for 5 weeks, followed by 2-weekly imaging out to 9 weeks. Tissues were harvested for immunohistochemical analysis.

RESULTS

With sham treatment, TTc transport causes fluorescent signal intensity over the thoracic spine to increase from 0 to 60 minutes after injection. On average, fluorescence signal increased 722%+/-117% (Mean+/-SD) from 0 to 60 minutes. Oxaliplatin treated animals had comparable transport at baseline (787%+/-140%), but transport rapidly decreased through the course of the study, falling to 363%+/-88%, 269%+/-96%, 191%+/-58%, 121%+/-39%, 75%+/-21% with each successive week and stabilizing around 57% (+/-15%) at 7 weeks. Statistically significant divergence occurred at approximately 3 weeks (p≤0.05, linear mixed-effects regression model). Quantitative immuno-fluorescence histology with a constant cutoff threshold showed reduced TTc in the spinal cord at 7 weeks for treated animals versus controls (5.2 Arbitrary Units +/-0.52 vs 7.1 AU +/-1.38, p<0.0004, T-test). There was no significant difference in neural cell mass between the two groups as shown with NeuN staining (10.2+/-1.21 vs 10.5 AU +/-1.53, p>0.56, T-test).

CONCLUSION

We show-for the first time to our knowledge-that neurographic in vivo molecular imaging can demonstrate imaging changes in a model of oxaliplatin-induced neuropathy. Impaired retrograde neural transport is suggested to be an important part of the pathophysiology of oxaliplatin-induced neuropathy.

Fluorescence imaging of fast retrograde axonal transport in living animals

Our purpose was to enable an in vivo imaging technology that can assess the anatomy and function of peripheral nerve tissue (neurography). To do this, we designed and tested a fluorescently labeled molecular probe based on the nontoxic C fragment of tetanus toxin (TTc). TTc was purified, labeled, and subjected to immunoassays and cell uptake assays. The compound was then injected into C57BL/6 mice (N = 60) for in vivo imaging and histologic studies. Image analysis and immunohistochemistry were performed. We found that TTc could be labeled with fluorescent moieties without loss of immunoreactivity or biologic potency in cell uptake assays. In vivo fluorescent imaging experiments demonstrated uptake and retrograde transport of the compound along the course of the sciatic nerve and in the spinal cord. Ex vivo imaging and immunohistochemical studies confirmed the presence of TTc in the sciatic nerve and spinal cord, whereas control animals injected with human serum albumin did not exhibit these features. We have demonstrated neurography with a fluorescently labeled molecular imaging contrast agent based on the TTc.

Immunization does not interfere with uptake and transport by motor neurons of the binding fragment of tetanus toxin

The nontoxic binding domain of tetanus toxin (fragment C or TTC) readily undergoes retrograde axonal transport from an intramuscular injection site. This property has led to investigation of TTC as a possible vector for delivering therapeutic proteins to motor neurons. However, the vast majority of individuals in the developed world have been vaccinated with tetanus toxoid and have circulating antitetanus antibodies that cross-react with TTC and may block the delivery of a TTC-linked therapeutic protein. However, it is uncertain whether the immune response is capable of completely neutralizing an intramuscular depot of protein prior to its internalization by presynaptic nerve terminals, where it is inaccessible to antibody. We have evaluated uptake of rhodamine-labeled TTC following intramuscular injection in normal animals and animals vaccinated with tetanus toxoid prior to injection of fluorescently labeled TTC. All animals demonstrated uptake of TTC, with fluorescence appropriately localized to the hypoglossal nerve and nucleus. The distribution and intensity of fluorescence within neurons and processes were indistinguishable between the two groups and were characteristic of TTC. Vaccinated animals showed levels of uptake of TTC into the brain comparable to those of immunologically naïve animals as measured by quantitative fluorimetry. All vaccinated animals had protective levels of antitetanus antibodies as measured by ELISA. Uptake of TTC by nerve terminals from an intramuscular depot is an avid and rapid process and is not blocked by vaccination associated with protection from tetanus toxin.

Internalization of a GFP-tetanus toxin C-terminal fragment fusion protein at mature mouse neuromuscular junctions

The distribution, dynamics, internalization, and retrograde axonal traffic of a fusion protein composed of green fluorescent protein (GFP) and the atoxic C-terminal fragment of tetanus toxin (TTC) were studied after its in vivo injection. Confocal microscopy and immunogold electron microscopy revealed that the fusion protein (GFP-TTC) rapidly clustered in motor nerve terminals of the neuromuscular junction. Clathrin-coated pits, and axolemma infoldings located between active zones appeared to be involved in the internalization of the fusion protein. Biochemical analysis of detergent-extracted neuromuscular preparations showed that the GFP-TTC fusion protein was associated with lipid microdomains. We suggest that GFP-TTC clustering in these lipid microdomains favors the recruitment of other proteins involved in its endocytosis and internalization in motor nerve terminals. During its retrograde trafficking, GFP-TTC accumulated in different axonal compartments than those used by cholera toxin B-subunit suggesting that these two proteins are transported by different pathways and cargos.

Internalization and Mechanism of Action of Clostridial Toxins in Neurons

Botulinum toxins are metalloproteases that act inside nerve terminals and block neurotransmitter release via their activity directed specifically on SNARE proteins. This review summarizes data on botulinum toxin modes of binding, sites of action, and biochemical activities. Their use in cell biology and neuroscience is considered, as well as their therapeutic utilization in human diseases characterized by hyperfunction of cholinergic terminals.

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.

In vivo neuronal tracing with GFP-TTC gene delivery

The retrograde transport and transynaptic transfer properties of the nontoxic tetanus toxin C-fragment (TTC) can be used to visualize specific neural pathways or to deliver biomolecules in the central nervous system (CNS). Here we tested different delivery techniques to explore the potential use of a new GFP-TTC fusion construct for use as a genetic tracer in vivo. Plasmids encoding GFP-TTC were targeted to brain regions using intracerebral grafted transfected cells or adenoviral transduction. Transport was monitored using GFP fluorescence. We show that following GFP-TTC synthesis in grafted transfected cells, the TTC fragment alone, with no signal peptide, is necessary and sufficient to provide secretion and uptake of the fusion protein into neighboring neurons around the injection site. Using an adenoviral vector to express the fusion protein into brain neurons, we show that transduced neurons can deliver the fusion protein specifically into connected neurons, demonstrating that synaptic transfer in the CNS can be visualized with GFP-TTC.

Interaction of tetanus toxin derived hybrid proteins with neuronal cells

The non-toxic ganglioside binding domain of tetanus toxin (Hc fragment C or TTC) has been studied as a vector for delivering therapeutic proteins to neurons. There is little information on the cellular processing of proteins delivered by linkage to TTC. We have evaluated the cellular handling of a multi-domain hybrid protein containing TTC and both the human enzyme superoxide dismutase and the maltose binding protein from E. coli. Binding, internalization, and cleavage of this protein during prolonged incubation with fetal cortical neurons or cells of the N18-RE-105 line was evaluated by immunoblot analysis, ELISA, and immunocytochemistry. Hybrid proteins were bound and internalized in a manner very similar to TTC. Internalized proteins showed long-term stability within cells, and were degraded into predictable large protein fragments in both cell types. Fragments that were cleaved away from the TTC domain were released into extracellular fluid after internalization. Proteins coupled to TTC share its long-term stability after cellular internalization. After internalization, dissociation of proteins linked to TTC facilitates their release from the cell, but not into other cellular compartments such as the cytosol. TTC linked proteins are probably enclosed within a stable endosomal compartment throughout their cellular lifetime.

Tetanus and botulinum neurotoxins: mechanism of action and therapeutic uses

The clostridial neurotoxins responsible for tetanus and botulism are proteins consisting of three domains endowed with different functions: neurospecific binding, membrane translocation and proteolysis for specific components of the neuroexocytosis apparatus. Tetanus neurotoxin (TeNT) binds to the presynaptic membrane of the neuromuscular junction, is internalized and transported retroaxonally to the spinal cord. The spastic paralysis induced by the toxin is due to the blockade of neurotransmitter release from spinal inhibitory interneurons. In contrast, the seven serotypes of botulinum neurotoxins (BoNTs) act at the periphery by inducing a flaccid paralysis due to the inhibition of acetylcholine release at the neuromuscular junction. TeNT and BoNT serotypes B, D, F and G cleave specifically at single but different peptide bonds, of the vesicle associated membrane protein (VAMP) synaptobrevin, a membrane protein of small synaptic vesicles (SSVs). BoNT types A, C and E cleave SNAP-25 at different sites located within the carboxyl-terminus, while BoNT type C additionally cleaves syntaxin. The remarkable specificity of BoNTs is exploited in the treatment of human diseases characterized by a hyperfunction of cholinergic terminals.

Autism and Clostridium tetani

Autism is a severe developmental disability believed to have multiple etiologies. This paper outlines the possibility of a subacute, chronic tetanus infection of the intestinal tract as the underlying cause for symptoms of autism observed in some individuals. A significant percentage of individuals with autism have a history of extensive antibiotic use. Oral antibiotics significantly disrupt protective intestinal microbiota, creating a favorable environment for colonization by opportunistic pathogens. Clostridium tetani is an ubiquitous anaerobic bacillus that produces a potent neurotoxin. Intestinal colonization by C. tetani, and subsequent neurotoxin release, have been demonstrated in laboratory animals which were fed vegetative cells. The vagus nerve is capable of transporting tetanus neurotoxin (TeNT) and provides a route of ascent from the intestinal tract to the CNS. This route bypasses TeNT's normal preferential binding sites in the spinal cord, and therefore the symptoms of a typical tetanus infection are not evident. Once in the brain, TeNT disrupts the release of neurotransmitters by the proteolytic cleavage of synaptobrevin, a synaptic vesicle membrane protein. This inhibition of neurotransmitter release would explain a wide variety of behavioral deficits apparent in autism. Lab animals injected in the brain with TeNT have exhibited many of these behaviors. Some children with autism have also shown a significant reduction in stereotyped behaviors when treated with antimicrobials effective against intestinal clostridia. When viewed as sequelae to a subacute, chronic tetanus infection, many of the puzzling abnormalities of autism have a logical basis. A review of atypical tetanus cases, and strategies to test the validity of this paper's hypothesis, are included.

Delivery of recombinant tetanus-superoxide dismutase proteins to central nervous system neurons by retrograde axonal transport

The nontoxic C fragment of tetanus toxin (TC) can transport other proteins from the circulation to central nervous system (CNS) motor neurons. Increased levels of CuZn superoxide dismutase (SOD) are protective in experimental models of stroke and Parkinson's disease, whereas mutations in SOD can cause motor neuron disease. We have linked TC to SOD and purified the active recombinant proteins in both the TC-SOD and SOD-TC orientations. Light microscopic immunohistochemistry and quantitative enzyme-linked immunosorbant assays (ELISA) of mouse brainstem, after intramuscular injection, demonstrate that the fusion proteins undergo retrograde axonal transport and transsynaptic transfer as efficiently as TC alone.

Retrograde, trans-synaptic and transneuronal transport of fragment C of tetanus toxin by sympathetic preganglionic neurons

The atoxic binding fragment of tetanus toxin, Fragment C, was injected into paravertebral ganglion 14, the avian homologue of the mammalian stellate ganglion. Postinjection survival intervals were varied from 2.5 h to 33 days. Experiments performed at the shortest survival time of 2.5 h showed that Fragment C was retrogradely transported by sympathetic preganglionic axons at a rate greater than or equal to 10 mm/h. At survival times ranging from 5 to 15 h. Fragment C-positive, retrogradely labeled sympathetic preganglionic neurons were observed within the last cervical spinal segment and throughout the first three thoracic spinal cord segments. Sporadic retrograde labeling of sympathetic preganglionic neurons was evident within the fourth and fifth thoracic spinal cord segments. Fragment C-labeled perikarya and dendrites exhibited both diffuse cytoplasmic immunostaining as well as intracellular, perinuclear accumulations of small. Fragment C-positive granules. Retrogradely labeled preganglionic neurons were found within both autonomic subnuclei within avian thoracic spinal cord; the column of Terni and the nucleus intercalatus spinalis. The distribution and numerical density of retrogradely labeled sympathetic preganglionic neurons indicated further that: (a) both myelinated and unmyelinated preganglionic axons appear to be capable of intra-axonally transporting Fragment C; and (b) it is unlikely that there is differential Fragment C labeling of a morphologically distinct population of sympathetic preganglionic neurons within or across subnuclei. Fragment C is transferred out of sympathetic preganglionic somas and dendrites into the surrounding neuropil at an aggregate rate greater than or equal to 5 mm/h. Trans-synaptic transport was evident at postinjection survival times as short as 5 h and continued to increase in density within the sympathetic preganglionic neuropil for 24 h. Fragment C-positive terminal labeling persisted for at least 20 days. At survival times greater than or equal to 1 day. Fragment C-positive puncta and weak intracellular labeling of neurons were evident in areas of the spinal gray outside of the nuclear boundaries of the column of Terni and nucleus intercalatus. The regions showing evidence of trans-synaptic and transneuronal labeling included: (a) a group of small cells dorsal to the column of Terni, (b) lamina V and (c) lamina VII. This expansion of Fragment C-labeled neuronal elements was segmental in organization and co-extensive with the retrograde labeling pattern of sympathetic preganglionic neurons. Spinal interneurons in these regions may provide segmental, monosynaptic input to sympathetic preganglionic neurons. Fragment C leaked into the systemic circulation from the site of injection in paravertebral ganglion 14.(ABSTRACT TRUNCATED AT 400 WORDS)

Enhanced CNS uptake of systemically administered proteins through conjugation with tetanus C-fragment

No other exogenous protein enters the central nervous system from the circulation as readily as tetanus toxin. We examined the capability of the non-toxic binding fragment of tetanus toxin (C-Fragment) so serve as a vehicle for transport of other proteins into the mouse CNS. Using periodate oxidation of the enzyme horseradish peroxidase (HRP), we synthesized two separate macromolecular complexes, one containing C-fragment and HRP, and the other C-fragment, HRP and a third "test" protein-human IgG. The distribution of C-fragment-HRP was typical of blood borne proteins including native C-fragment, with labeling of all neurons with known projections outside the blood-brain barrier, particularly large spinal motoneurons. C-fragment-HRP conjugates showed superior neuronal labeling to over 100-fold greater quantities of free HRP. Complexes containing C-fragment, HRP and human IgG were internalized by neurons from both intramuscular and intraperitoneal injections. The efficiency of neuronal uptake of IgG in the C-fragment conjugated form was enhanced over 40-fold compared to free IgG. Linkage of a large protein to C-fragment probably leads to enhanced endocytosis of that protein by neuronal terminals projecting outside the blood-brain barrier. C-fragment can serve as a vehicle to allow selected proteins to bypass the barrier and enter the CNS.

Retrograde transneuronal transport properties of fragment C of tetanus toxin

The atoxic fragment C of tetanus toxin reliably undergoes retrograde direct and transneuronal transport, but the full extent of its transport capabilities has not been examined. The primary visual pathways provide an excellent system for investigating, for the first time, the possibility of anterograde direct and transneuronal transport of fragment C. Following injection into the eye of the rat and rabbit, fragment C, localized with a monoclonal antibody, underwent anterograde direct transport to all retinorecipient areas. From these areas, fragment C exhibited retrograde transneuronal transport, strongly and reliably labeling regions that project to retinorecipient areas, including layers V and VI of visual cortex, the parabigeminal nucleus, the suprageniculate nucleus, and the reticular thalamus. In contrast, the absence of fragment C in regions receiving only input from, but not projecting to, retinorecipient areas, most notably layer IV of visual cortex, provides strong evidence for the lack of anterograde transneuronal transport. Thus, while fragment C can be induced to undergo anterograde direct transport by injection into the eye, it exhibits only retrograde transneuronal transport. These characteristics suggest that fragment C of tetanus toxin is a consistent and reliable retrograde transneuronal marker for the elucidation of central nervous pathways.

Selective uptake of C-fragment of tetanus toxin by sympathetic preganglionic nerve terminals

Immunohistochemical techniques have shown that the C-fragment of tetanus toxin injected into medial gastrocnemius muscle in the guinea-pig and rat, in addition to its retrograde transport in the axons of the somatomotor, sympathetic and sensory neurons supplying this muscle, is taken up and concentrated by terminal varicosities within pre- and paravertebral sympathetic ganglia at all thoracolumbar levels. Staining was absent in chronically denervated ganglia, demonstrating the specific association of the antigen with preganglionic varicosities. Preganglionic varicosities at all levels were also labelled after C-fragment injection into the peritoneal cavity or into denervated medial gastrocnemius; both of these procedures failed to label somatomotor or sensory neurons. Although retrograde trans-synaptic transport could be demonstrated, sympathetic labelling was sparse and non-specific, so that the C-fragment of tetanus toxin is unsuitable for the identification of sympathetic pathways. The selective and widespread uptake of tetanus toxin by sympathetic preganglionic terminals could explain the diverse autonomic symptoms observed in tetanus intoxication.

Motoneuron uptake from the circulation of the binding fragment of tetanus toxin

Tetanus toxin enters the central nervous system from the systemic circulation after it is internalized by motoneuron terminals at the neuromuscular junction. We have demonstrated that the atoxic binding fragment (C-fragment) of tetanus toxin is internalized preferentially by motoneurons. We examined the distribution of C-fragment after intravenous injection in the nervous systems of mice by immunohistochemical methods. All animals remained asymptomatic until killed one to two days after injection. C-fragment was found only within neurons with processes outside the blood-brain barrier. Large motoneurons of the spinal cord showed the greatest accumulation of C-fragment. Motoneurons of brain-stem nuclei (particularly facial and trigeminal), also showed substantial label of C-fragment. Small amounts of C-fragment were detected in dorsal root ganglion cells. Affinity of a systemically distributed substance for synaptic components, as well as an inability to cross the blood-brain barrier, may lead to its preferential localization in motoneurons.