Multiple domains of tetanus toxin direct entry into primary neurons

High-Throughput IgG Epitope Mapping of Tetanus Neurotoxin: Implications for Immunotherapy and Vaccine Design

Tetanus is an acute, fatal disease caused by exotoxins released from Clostridium tetani during infections. A protective humoral immune response can be induced by vaccinations with pediatric and booster combinatorial vaccines that contain inactivated tetanus neurotoxin (TeNT) as a major antigen. Although some epitopes in TeNT have been described using various approaches, a comprehensive list of its antigenic determinants that are involved with immunity has not been elucidated. To this end, a high-resolution analysis of the linear B-cell epitopes in TeNT was performed using antibodies generated in vaccinated children. Two hundred sixty-four peptides that cover the entire coding sequence of the TeNT protein were prepared in situ on a cellulose membrane through SPOT synthesis and probed with sera from children vaccinated (ChVS) with a triple DTP-vaccine to map continuous B-cell epitopes, which were further characterized and validated using immunoassays. Forty-four IgG epitopes were identified. Four (TT-215-218) were chemically synthesized as multiple antigen peptides (MAPs) and used in peptide ELISAs to screen post-pandemic DTP vaccinations. The assay displayed a high performance with high sensitivity (99.99%) and specificity (100%). The complete map of linear IgG epitopes induced by vaccination with inactivated TeNT highlights three key epitopes involved in the efficacy of the vaccine. Antibodies against epitope TT-8/G can block enzymatic activity, and those against epitopes TT-41/G and TT-43/G can interfere with TeNT binding to neuronal cell receptors. We further show that four of the epitopes identified can be employed in peptide ELISAs to assess vaccine coverage. Overall, the data suggest a set of select epitopes to engineer new, directed vaccines.

Similarity and Diversity of Presynaptic Molecules at Neuromuscular Junctions and Central Synapses

Synaptic transmission is essential for controlling motor functions and maintaining brain functions such as walking, breathing, cognition, learning, and memory. Neurotransmitter release is regulated by presynaptic molecules assembled in active zones of presynaptic terminals. The size of presynaptic terminals varies, but the size of a single active zone and the types of presynaptic molecules are highly conserved among neuromuscular junctions (NMJs) and central synapses. Three parameters play an important role in the determination of neurotransmitter release properties at NMJs and central excitatory/inhibitory synapses: the number of presynaptic molecular clusters, the protein families of the presynaptic molecules, and the distance between presynaptic molecules and voltage-gated calcium channels. In addition, dysfunction of presynaptic molecules causes clinical symptoms such as motor and cognitive decline in patients with various neurological disorders and during aging. This review focuses on the molecular mechanisms responsible for the functional similarities and differences between excitatory and inhibitory synapses in the peripheral and central nervous systems, and summarizes recent findings regarding presynaptic molecules assembled in the active zone. Furthermore, we discuss the relationship between functional alterations of presynaptic molecules and dysfunction of NMJs or central synapses in diseases and during aging.

Neutralization of tetanus toxin by a novel chimeric monoclonal antibody

PURPOSE

Tetanus is a life-threatening disease characterized by muscle spasm caused by neurotoxin of Clostridium tetani. Given the current passive immunotherapy of tetanus with human anti-toxin polyclonal antibodies (PAbs) and the limitations of such preparations, neutralizing monoclonal antibodies (MAbs), especially chimeric or human antibodies with reduced immunogenicity might be considered as an alternative source.

METHODS

A mouse-human chimeric MAb, designated c-1F2C2, was generated and its binding specificities to various recombinant fragments of tetanus toxin, generated in E. coli, were determined. In vivo toxin neutralizing activity of c-1F2C2 was evaluated and compared with that of a commercially available human anti-toxin PAb in a mouse model. The possible mechanisms of toxin neutralizing activity of c-1F2C2 were investigated by assessing its inhibitory effects on toxin receptors binding, including GT1b ganglioside receptor and those expressed on PC12 cells.

RESULTS

In vivo neutralizing assay showed that c-1F2C2 was able to protect mice against tetanus toxin with an estimated potency of 7.7 IU/mg comparing with 1.9 IU/mg of the commercial human anti-toxin PAb for 10 MLD toxin and 10 IU/mg versus 1.9 IU/mg of the PAb for 2.5 MLD toxin. c-1F2C2 recognized fragment C of the toxin, which is responsible for binding of the toxin to its receptor on neuronal cells. Accordingly, the chimeric MAb partially prevented the toxin from binding to its receptors on PC12 cells (37% inhibition).

CONCLUSION

The chimeric MAb c-1F2C2 displayed similar structural and functional characteristics compared to its murine counterpart and might be useful for passive immunotherapy of tetanus.

Carboxypeptidase E Regulates Activity-Dependent TrkB Neuronal Surface Insertion and Hippocampal Memory

Activity-dependent insertion of the tropomyosin-related kinase B (TrkB) receptor into the plasma membrane can explain, in part, the preferential effect of brain-derived neurotrophic factor (BDNF) on active neurons and synapses; however, the underlying molecular mechanisms remain obscure. Here, we report a novel function for carboxypeptidase E (CPE) in controlling chemical long-term potentiation stimuli-induced TrkB surface delivery in hippocampal neurons. Total internal reflection fluorescence assays and line plot assays showed that CPE facilitates TrkB transport from dendritic shafts to the plasma membrane. The Box2 domain in the juxtamembrane region of TrkB and the C terminus of CPE are critical for the activity-dependent plasma membrane insertion of TrkB. Moreover, the transactivator of transcription TAT-CPE452-466, which could block the association between CPE and TrkB, significantly inhibited neuronal activity-enhanced BDNF signaling and dendritic spine morphologic plasticity in cultured hippocampal neurons. Microinfusion of TAT-CPE452-466 into the dorsal hippocampus of male C57BL/6 mice inhibited the endogenous interaction between TrkB and CPE and diminished fear-conditioning-induced TrkB phosphorylation, which might lead to an impairment in hippocampal memory acquisition and consolidation but not retrieval. These results suggest that CPE modulates activity-induced TrkB surface insertion and hippocampal-dependent memory and sheds light on our understanding of the role of CPE in TrkB-dependent synaptic plasticity and memory modulation.SIGNIFICANCE STATEMENT It is well known that BDNF acts preferentially on active neurons; however, the underlying molecular mechanism is not fully understood. In this study, we found that the cytoplasmic tail of CPE could interact with TrkB and facilitate the neuronal activity-dependent movement of TrkB vesicles to the plasma membrane. Blocking the association between CPE and TrkB decreased fear-conditioning-induced TrkB phosphorylation and led to hippocampal memory deficits. These findings provide novel insights into the role of CPE in TrkB intracellular trafficking as well as in mediating BDNF/TrkB function in synaptic plasticity and hippocampal memory.

Characterization and evaluation of the enzymatic activity of tetanus toxin submitted to cobalt-60 gamma radiation

BACKGROUND

Tetanus toxin blocks the release of the inhibitory neurotransmitters in the central nervous system and causes tetanus and its main form of prevention is through vaccination. The vaccine is produced by inactivation of tetanus toxin with formaldehyde, which may cause side effects. An alternative way is the use of ionizing radiation for inactivation of the toxin and also to improve the potential immunogenic response and to reduce the post-vaccination side effects. Therefore, the aim of this study was to characterize the tetanus toxin structure after different doses of ionizing radiation of 60Co.

METHODS

Irradiated and native tetanus toxin was characterized by SDS PAGE in reducing and non-reducing conditions and MALD-TOF. Enzymatic activity was measured by FRET substrate. Also, antigenic properties were assessed by ELISA and Western Blot data.

RESULTS

Characterization analysis revealed gradual modification on the tetanus toxin structure according to doses increase. Also, fragmentation and possible aggregations of the protein fragments were observed in higher doses. In the analysis of peptide preservation by enzymatic digestion and mass spectrometry, there was a slight modification in the identification up to the dose of 4 kGy. At subsequent doses, peptide identification was minimal. The analysis of the enzymatic activity by fluorescence showed 35 % attenuation in the activity even at higher doses. In the antigenic evaluation, anti-tetanus toxin antibodies were detected against the irradiated toxins at the different doses, with a gradual decrease as the dose increased, but remaining at satisfactory levels.

CONCLUSION

Ionizing radiation promoted structural changes in the tetanus toxin such as fragmentation and/or aggregation and attenuation of enzymatic activity as the dose increased, but antigenic recognition of the toxin remained at good levels indicating its possible use as an immunogen. However, studies of enzymatic activity of tetanus toxin irradiated with doses above 8 kGy should be further analyzed.

Glial Synaptobrevin mediates peripheral nerve insulation, neural metabolic supply, and is required for motor function

Peripheral nerves contain sensory and motor neuron axons coated by glial cells whose interplay ensures function, but molecular details are lacking. SNARE-proteins mediate the exchange and secretion of cargo by fusing vesicles with target organelles, but how glial SNAREs contribute to peripheral nerve function is largely unknown. We, here, identify non-neuronal Synaptobrevin (Syb) as the essential vesicular SNARE in Drosophila peripheral glia to insulate and metabolically supply neurons. We show that tetanus neurotoxin light chain (TeNT-LC), which potently inhibits SNARE-mediated exocytosis from neurons, also impairs peripheral nerve function when selectively expressed in glia, causing nerve disintegration, defective axonal transport, tetanic muscle hyperactivity, impaired locomotion, and lethality. While TeNT-LC disrupts neural function by cleaving neuronal Synaptobrevin (nSyb), it targets non-neuronal Synaptobrevin (Syb) in glia, which it cleaves at low rates: Glial knockdown of Syb (but not nSyb) phenocopied glial TeNT-LC expression whose effects were reverted by a TeNT-LC-insensitive Syb mutant. We link Syb-necessity to two distinct glial subtypes: Impairing Syb function in subperineurial glia disrupted nerve morphology, axonal transport, and locomotion, likely, because nerve-isolating septate junctions (SJs) could not form as essential SJ components (like the cell adhesion protein Neurexin-IV) were mistargeted. Interference with Syb in axon-encircling wrapping glia left nerve morphology and locomotion intact but impaired axonal transport, likely because neural metabolic supply was disrupted due to the mistargeting of metabolite shuffling monocarboxylate transporters. Our study identifies crucial roles of Syb in various glial subtypes to ensure glial-glial and glial-neural interplay needed for proper nerve function, animal motility, and survival.

Epitope Mapping of Tetanus Toxin by Monoclonal Antibodies: Implication for Immunotherapy and Vaccine Design

Tetanus as a life-threatening disease is characterized by muscle spasm. The disease is caused by the neurotoxin of Clostridium tetani. Active form of tetanus neurotoxin is composed of the light chain (fragment A) and the heavy chain. Fragment A is a zinc metalloprotease, which cleaves the neuronal soluble N-ethylmaleimide-sensitive attachment receptor (SNARE) protein, leading to the blockade of inhibitory neurotransmitter release and subsequent generalized muscular spasm. Two functional domains of the heavy chain are fragment C, which is required for neuronal cell binding of the toxin and subsequent endocytosis into the vesicles, and fragment B, which is important for fragment A translocation across the vesicular membrane into the neuronal cytosol. Currently, polyclonal immunoglobulins against tetanus neurotoxin obtained from human plasma of hyper-immunized donors are utilized for passive immunotherapy of tetanus; however, these preparations have many disadvantages including high lot-to-lot heterogeneity, possibility of transmitting microbial agents, and the adverse reactions to the other proteins in the plasma. Neutralizing anti-tetanus neurotoxin monoclonal antibodies (MAbs) lack these drawbacks and could be considered as a suitable alternative for passive immunotherapy of tetanus. In this review, we provide an overview of the literature discussing epitope mapping of the published neutralizing MAbs against tetanus toxin.

Botulinum and Tetanus Neurotoxins

Botulinum neurotoxins (BoNTs) and tetanus neurotoxin (TeNT) are the most potent toxins known and cause botulism and tetanus, respectively. BoNTs are also widely utilized as therapeutic toxins. They contain three functional domains responsible for receptor-binding, membrane translocation, and proteolytic cleavage of host proteins required for synaptic vesicle exocytosis. These toxins also have distinct features: BoNTs exist within a progenitor toxin complex (PTC), which protects the toxin and facilitates its absorption in the gastrointestinal tract, whereas TeNT is uniquely transported retrogradely within motor neurons. Our increasing knowledge of these toxins has allowed the development of engineered toxins for medical uses. The discovery of new BoNTs and BoNT-like proteins provides additional tools to understand the evolution of the toxins and to engineer toxin-based therapeutics. This review summarizes the progress on our understanding of BoNTs and TeNT, focusing on the PTC, receptor recognition, new BoNT-like toxins, and therapeutic toxin engineering.

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.

Effects of membrane properties on the binding activities of the HN and HC heavy-chain domains of botulinum neurotoxin A

Binding behaviors of the H_N and the H_C domains of BoNT/A were investigated individually to identify if there exist any differences in their interaction with the cell membrane. Recombinant fragments corresponding to both BoNT/A H_N and H_C regions were prepared (H_N519-845 and H_C967-1296) and their binding to synaptic proteins was verified. The binding behaviors of these heavy-chain domains were analyzed by treating the Neuro 2a, a murine neuroblastoma cell line, with compounds known to alter membrane properties. Cholesterol depletion and lipid raft inhibition increased the binding of H_N519-845 to Neuro 2a cells without affecting H_C967-1296-cell interaction. Sphingolipid depletion decreased the binding of cells to both H_C967-1296 and H_N519-845 whereas, loading exogenous GD1a, on to the Neuro 2a cells, increased the binding of both the peptides to cells. Microtubule disruption of the Neuro 2a cells by nocodazole decreased the binding of both H_C967-1296 and H_N519-845 to the treated cells. Inhibition of the clathrin-mediated endocytosis using dynasore, chlorpromazine or potassium (K⁺) depletion buffer lowered the binding of both H_C967-1296 and H_N519-845 to the cells, but seemed to exert a more pronounced effect on the binding of H_C967-1296 than on the binding of H_N519-845. Results indicate that while both the H_N and H_C domains are involved in the binding of the toxin to neuronal cells there are differences in their behavior which probably stem from their respective amino acid composition and structural location in the toxin three-dimensional structure along with their intended role in translocation and internalization into the cells.

A Heterologous Reporter Defines the Role of the Tetanus Toxin Interchain Disulfide in Light-Chain Translocation

Botulinum neurotoxins (BoNTs) and tetanus toxin (TeNT) are the most potent toxins for humans and elicit unique pathologies due to their ability to traffic within motor neurons. BoNTs act locally within motor neurons to elicit flaccid paralysis, while retrograde TeNT traffics to inhibitory neurons within the central nervous system (CNS) to elicit spastic paralysis. BoNT and TeNT are dichain proteins linked by an interchain disulfide bond comprised of an N-terminal catalytic light chain (LC) and a C-terminal heavy chain (HC) that encodes an LC translocation domain (HCT) and a receptor-binding domain (HCR). LC translocation is the least understood property of toxin action, but it involves low pH, proteolysis, and an intact interchain disulfide bridge. Recently, Pirazzini et al. (FEBS Lett 587:150-155, 2013, http://dx.doi.org/10.1016/j.febslet.2012.11.007) observed that inhibitors of thioredoxin reductase (TrxR) blocked TeNT and BoNT action in cerebellar granular neurons. In the current study, an atoxic TeNT LC translocation reporter was engineered by fusing β-lactamase to the N terminus of TeNT [βlac-TeNT(RY)] to investigate LC translocation in primary cortical neurons and Neuro-2a cells. βlac-TeNT(RY) retained the interchain disulfide bond, showed ganglioside-dependent binding to neurons, required acidification to promote βlac translocation, and was sensitive to auranofin, an inhibitor of thioredoxin reductase. Mutation of βlac-TeNT(RY) at C439S and C467S eliminated the interchain disulfide bond and inhibited βlac translocation. These data support the requirement of an intact interchain disulfide for LC translocation and imply that disulfide reduction is a prerequisite for LC delivery into the host cytosol. The data also support a model that LC translocation proceeds from the C to the N terminus. βlac-TeNT(RY) is the first reporter system to measure translocation by an AB single-chain toxin in intact cells.