Binding of botulinum neurotoxin to pure cholinergic nerve terminals isolated from the electric organ of Torpedo

Botulinum neurotoxin A activity is dependent upon the presence of specific gangliosides in neuroblastoma cells expressing synaptotagmin I

Botulinum neurotoxin A (BoNT/A) is the deadliest of all known biological substances. Although its toxicity makes BoNT/A a biological warfare threat, its biologic activity makes it an increasingly useful therapeutic agent for the treatment of muscular disorders. However, almost 200 years after its discovery, the neuronal cell components required for the activity of this deadly toxin have not been unequivocally identified. In this work, neuroblastoma cells expressing synaptotagmin I, a protein shown to be bound by BoNT/A, were used to determine whether specific gangliosides were necessary for BoNT/A activity as measured by synaptosomal-associated protein of 25 kDa (SNAP-25) cleavage. Ganglioside GT1b was found to support BoNT/A activity significantly more effectively than GD1a, which was far more effective than GM1 when added to ganglioside-deficient murine cholinergic Neuro 2a or to human adrenergic SK-N-SH neuroblastoma cells. Whereas both cell lines expressed synaptotagmin I, SNAP-25 cleavage was not observed in the absence of complex gangliosides. These results indicate that 1) gangliosides are required for BoNT/A activity, 2) synaptotagmin I in the absence of gangliosides does not support BoNT/A activity, and 3) Neuro 2a cells are an efficient model system for studying the biological activity of BoNT/A.

Localization of putative receptors for tetanus toxin and botulinum neurotoxin type A in rat central nervous system

Clostridial neurotoxins (tetanus and botulinum toxins) are potent blockers of neurotransmitter release. These toxins act specifically on the nervous system by interacting with still non-identified protein receptors together with gangliosides. Whereas many biochemical data are available on their binding properties to neuronal membranes in vitro, there is poor morphological evidence of their binding to mammalian central nervous system. In the present study, the binding of tetanus and botulinum neurotoxin type A to rat brain sections is reported. Both toxins bound to nerve terminals with a broad distribution in brain. Tetanus toxin additionally bound to nerve fibres. The staining patterns were clearly shown to be due to the interaction of the heavy chains, which contain the binding moiety, with the tissue. In an attempt to investigate the nature of the acceptors present in the tissue, some sections were pre-incubated with periodic acid. This treatment resulted in the additional binding of botulinum neurotoxin type A to nerve fibres. Since the extended staining of nerve terminals was not modified by this pretreatment, it is suggested that protein receptors of clostridial neurotoxins are located at the nerve terminals, which may be common constituents of the synapses.

Pharmacologic characterization of botulinum toxin for basic science and medicine

The use of Botulinum neurotoxin (BoNT) is increasing in both clinical and basic science. Clinically, intramuscular injection of nanogram quantities of BoNT is fast becoming the treatment of choice for a spectrum of disorders including movement disorders such as torticollis, blepharospasm, Meige Disease, and hemifacial spasm (Borodic et al., 1991, 1994a; Jankovic and Brin, 1991; Clarke, 1992). Neuroscientists are using BoNTs as tools to develop a better understanding of the mechanisms underlying the neurotransmitter release process. Consequently, our ability to accurately and reliably quantify the biologic activity of botulinum toxin has become more important than ever. The accurate measurement of the pharmacologic activity of BoNTs has become somewhat problematic with the most significant problems occurring with the clinical use of the toxins. The biologic activity of BoNTs has been measured using a variety of techniques including assessment of whole animal responses to in vitro effects on neurotransmitter release. The purpose of this review is to examine the approaches employed to characterize, quantify and investigate the actions of the BoNTs and to provide a guide to aid investigators in determining which of these methods is most appropriate for their particular application or use.

Productive and non-productive binding of botulinum neurotoxin A to motor nerve endings are distinguished by its heavy chain

Botulinum neurotoxin type A, a di-chain protein produced by Clostridium botulinum and responsible for botulism, blocks acetylcholine release from peripheral nerves by binding to the terminals, undergoing internalization and proteolyzing a protein essential for exocytosis. As butolinum neurotoxin is being used clinically for the treatment of dystonias and certain spasticities, deciphering the details of its specific targeting to cholinergic nerve endings has assumed great importance. Thus, interaction of butolinum neurotoxin type A with murine motor nerve terminals-a prime target in vivo-was investigated. Autoradiographic analysis revealed saturable, high-affinity interaction of radioiodinated toxin (0.4 nM) with two ecto-acceptor types, distinguished by an excess of the toxin's heavy chain which prevented only a fraction of this binding. Botulinum neurotoxin was also biotinylated through its free sulfhydryl groups, known not to be essential for neurotoxicity. Similar binding of this active derivative was, likewise, partially blocked by heavy chain, confirming the above results. This binding that is resistant to heavy chain equates to botulinum neurotoxin interacting with productive ecto-acceptors, leading to delivery to its cytosolic site of action, because heavy chain proved unable to antagonize toxin-induced neuromuscular paralysis. In contrast, it is deduced that botulinum neurotoxin bound to heavy chain-susceptible sites has a different fate, presumably due to trafficking via another route, and thus would be inefficient in causing neuroparalysis.

Functional reconstitution of KCl-evoked, Ca(2+)-dependent acetylcholine release system in Xenopus oocytes microinjected with presynaptic plasma membranes and synaptic vesicles

We have developed a new method for the generation of functionally active presynaptic chimeras in Xenopus laevis oocytes. Frog oocytes injected with presynaptic subcellular fractions extracted from the electric organ of Torpedo marmorata release acetylcholine in a calcium-dependent manner upon chemical stimulation. Neither oocytes injected without presynaptic plasma membranes nor oocytes injected with ghost erythrocyte plasma membrane instead of presynaptic plasma membrane release acetylcholine. This suggests that specific presynaptic components necessary for KCl-evoked, Ca(2+)-dependent acetylcholine release become functionally integrated in the Xenopus laevis oocytes. Moreover, rhodaminated presynaptic plasma membranes and the synaptic vesicle protein synaptophysin are detected on the oocyte surface by fluorescence or immunofluorescence, respectively, showing that the injected presynaptic components are incorporated into the membrane of the frog oocyte. Furthermore, Botulinum neurotoxin type A, a specific blocker of acetylcholine release in the neuromuscular junction, inhibits the neurotransmitter release from the chimerical oocytes. This suggests that targets for toxin action are also functionally incorporated in the oocyte upon injection of membranous presynaptic components. Our results show that oocytes injected with presynaptic components behave as cholinergic nerve ending chimeras, at least in terms of neurotransmitter release and toxin targets. The system bypasses some problems associated with messenger RNA expression because not only proteins, but native presynaptic components are incorporated. This new technique may provide a useful approach for electrophysiological and pharmacological studies in order to characterize the synaptic transmission.

Characterization of a rabbit serum raised against a botulinum toxin type A binding protein from presynaptic plasma membranes from Torpedo electric organ

Botulinum neurotoxin type A blocks acetylcholine release from the peripheral nervous system. We have previously described a putative botulinum neurotoxin type A receptor of presynaptic plasma membranes from Torpedo. The electric organ of Torpedo, which is largely enriched in cholinergic nerve endings, is homologous to the neuromuscular junction, allowing us to isolate large scale of presynaptic components. In order to characterize this protein we have raised a polyclonal antibody (a-P140) against this receptor. The antiserum a-P140 recognizes a 140,000 mol. wt band in non-reducing conditions and an 80,000 band in reducing conditions. The immunohistochemistry assay reveals the P140 protein on the ventral face of the electrocytes where the nerve terminals are localized. Moreover, a-P140 antiserum recognizes the P140-BoNT/A complex after binding and cross-linking experiments. In addition, we have immunoprecipitated an in vitro translated product which is closely coincident in mol. wt to the 80,000 band of the receptor.

High resolution labeling of cholinergic nerve terminals using a specific fully active biotinylated botulinum neurotoxin type A

We report here on the synthesis and characterization of a fully active biotinylated derivative of the botulinum neurotoxin type A. Different ratios of biotin: botulinum toxin were tested to optimize derivatizing conditions and a ratio of 35:1 was selected for further experiments. The average number of biotin groups per toxin molecule was estimated to be 7.8, occurring at both heavy and light chains, and almost all externally located and easily accessible to recognition by streptavidin. The modified toxin retained its toxicity and its ability to interact with biological membranes. Apart from its suitability for detection in Western blots and in microtiter well plates, biotinylated botulinum toxin proved to be adequate for morphological labeling studies at both light and electron microscopy. Peroxidase histochemistry in cryostat sections of intoxicated rat hemidiaphragm muscles showed a distinct labeling of end-plates. Electron microscopy studies were performed on the electric organ of Torpedo marmorata using colloidal gold-conjugated streptavidin for detection. After intoxication of electric organ fragments with the modified toxin, gold labels were found associated with the presynaptic plasma membrane of nerve terminals and with the membrane of synaptic vesicles. Moreover, the distribution of biotinylated botulinum toxin binding sites over the membrane of synaptosomes isolated from the electric organ of Torpedo and their relationship with intramembrane particles were analyzed using the replica-staining label-fracture technique. It was found that the toxin is never associated with intramembrane particles.