Nerve terminals are specific sites of action of a very large number of toxins produced by many different organisms. The mechanism of action of three groups of presynaptic neurotoxins that interfere directly with the process of neurotransmitter release is reviewed, whereas presynaptic neurotoxins acting on ion channels are not dealt with here. These neurotoxins can be grouped in three large families: 1) the clostridial neurotoxins that act inside nerves and block neurotransmitter release via their metalloproteolytic activity directed specifically on SNARE proteins; 2) the snake presynaptic neurotoxins with phospholipase A(2) activity, whose site of action is still undefined and which induce the release of acethylcholine followed by impairment of synaptic functions; and 3) the excitatory latrotoxin-like neurotoxins that induce a massive release of neurotransmitter at peripheral and central synapses. Their modes of binding, sites of action, and biochemical activities are discussed in relation to the symptoms of the diseases they cause. The use of these toxins in cell biology and neuroscience is considered as well as the therapeutic utilization of the botulinum neurotoxins in human diseases characterized by hyperfunction of cholinergic terminals.

https://www.physiology.org/doi/pdf/10.1152/physrev.2000.80.2.717

Neurotoxins Affecting Neuroexocytosis

Regulated exocytosis of secretory granules or dense-core granules has been examined in many well-characterized cell types including neurons, neuroendocrine, endocrine, exocrine, and hemopoietic cells and also in other less well-studied cell types. Secretory granule exocytosis occurs through mechanisms with many aspects in common with synaptic vesicle exocytosis and most likely uses the same basic protein components. Despite the widespread expression and conservation of a core exocytotic machinery, many variations occur in the control of secretory granule exocytosis that are related to the specialized physiological role of particular cell types. In this review we describe the wide range of cell types in which regulated secretory granule exocytosis occurs and assess the evidence for the expression of the conserved fusion machinery in these cells. The signals that trigger and regulate exocytosis are reviewed. Aspects of the control of exocytosis that are specific for secretory granules compared with synaptic vesicles or for particular cell types are described and compared to define the range of accessory control mechanisms that exert their effects on the core exocytotic machinery.

Secretory Granule Exocytosis

Clostridial neurotoxins inhibit neurotransmitter release by selective and specific intracellular proteolysis of synaptobrevin/VAMP, synaptosomal-associated protein of 25 kDa (SNAP-25) or syntaxin. Here we show that in binary reactions synaptobrevin binds weakly to both SNAP-25 and syntaxin, and SNAP-25 binds to syntaxin. In the presence of all three components, a dramatic increase in the interaction strengths occurs and a stable sodium dodecyl sulfate-resistant complex forms. Mapping of the interacting sequences reveals that complex formation correlates with the presence of predicted alpha-helical structures, suggesting that membrane fusion involves intermolecular interactions via coiled-coil structures. Most toxins only attack the free, and not the complexed, proteins, and proteolysis of the proteins by different clostridial neurotoxins has distinct inhibitory effects on the formation of synaptobrevin-syntaxin-SNAP-25 complexes. Our data suggest that synaptobrevin, syntaxin and SNAP-25 associate into a unique stable complex that functions in synaptic vesicle exocytosis.

https://www.embopress.org/doi/pdf/10.1002/j.1460-2075.1994.tb06834.x

Synaptic vesicle membrane fusion complex: action of clostridial neurotoxins on assembly.

How the widely used botulinum neurotoxin A (BoNT/A) recognizes and enters neurons is poorly understood. We found that BoNT/A enters neurons by binding to the synaptic vesicle protein SV2 (isoforms A, B, and C). Fragments of SV2 that harbor the toxin interaction domain inhibited BoNT/A from binding to neurons. BoNT/A binding to SV2A and SV2B knockout hippocampal neurons was abolished and was restored by expressing SV2A, SV2B, or SV2C. Reduction of SV2 expression in PC12 and Neuro-2a cells also inhibited entry of BoNT/A, which could be restored by expressing SV2 isoforms. Finally, mice that lacked an SV2 isoform (SV2B) displayed reduced sensitivity to BoNT/A. Thus, SV2 acts as the protein receptor for BoNT/A.

https://science.sciencemag.org/content/sci/312/5773/592.full.pdf

SV2 is the protein receptor for botulinum neurotoxin A.

Blockade of acetylcholine release by botulinum neurotoxin type A at the neuromuscular junction induces the formation of an extensive network of nerve-terminal sprouts. By repeated in vivo imaging of N-(3-triethyl ammonium propyl)-4-(4-(dibutylamino)styryl) pyridinium dibromide uptake into identified nerve endings of the mouse sternomastoid muscle after a single intramuscular injection of the toxin, inhibition of stimulated uptake of the dye at the terminals was detected within a few days, together with an increase in staining of the newly formed sprouts. After 28 days, when nerve stimulation again elicited muscle contraction, regulated vesicle recycling occurred only in the sprouts [shown to contain certain soluble N-ethylmaleimide-sensitive factor attachment proteins (SNAREs) and to abut acetylcholine receptors] and not at the parent terminals. Therefore, only these sprouts could be responsible for nerve–muscle transmission at this time. However, a second, distinct phase of the rehabilitation process followed with a return of vesicle turnover to the original terminals, accompanied by an elimination of the by then superfluous sprouts. This extension and later removal of “functional” sprouts indicate their fundamental importance in the repair of paralyzed endplates, a finding with ramifications for the vital process of nerve regeneration.

https://www.pnas.org/content/pnas/96/6/3200.full.pdf

Functional repair of motor endplates after botulinum neurotoxin type A poisoning: Biphasic switch of synaptic activity between nerve sprouts and their parent terminals

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1016/0014-5793%2893%2980448-4

Jaime Jontes

Papers

Botulinum neurotoxins serotypes A and E cleave SNAP-25 at distinct COOH-terminal peptide bonds